Ceramic Manufacturing Processes

8.05 Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues E Monfort, A Mezquita, E Vaquer, I Celades, V Sanfelix, and A Escrig, Universitat Jaume I, Castellón, Spain Ó 2014 Elsevier Ltd. All rights reserved.

8.05.1 Introduction 8.05.1.1 Ceramics 8.05.1.1.1 Ceramic Tiles 8.05.1.1.2 Structural Ceramics 8.05.1.1.3 Porcelain and Earthenware 8.05.1.1.4 Refractories 8.05.1.1.5 Other Products 8.05.1.2 Ceramic Manufacturing Processes 8.05.1.2.1 Raw Materials Preparation 8.05.1.2.2 Forming 8.05.1.2.3 Drying 8.05.1.2.4 Glazing 8.05.1.2.5 Firing 8.05.2 Energy Consumption and CO2 Emissions 8.05.2.1 Fuel and Energy Consumption Data 8.05.2.1.1 Ceramic Tiles 8.05.2.1.2 Bricks and Roof Tiles 8.05.2.1.3 Porcelain and Earthenware 8.05.2.1.4 Refractory Products 8.05.2.1.5 Other Products 8.05.2.2 CO2 Emission Factors and Emission Data 8.05.2.3 Techniques for CO2 Emission Abatement 8.05.2.3.1 Spray Dryers 8.05.2.3.2 Shaped Product Dryers 8.05.2.3.3 Kilns 8.05.3 Environmental Issues 8.05.3.1 Atmospheric Pollution 8.05.3.1.1 Particulate Matter Emissions (TSP, PM10, and PM2.5) Control 8.05.3.1.2 Gas Emissions Data and Control 8.05.3.2 Noise Emissions 8.05.3.3 Water Consumption and Wastewater Generation 8.05.3.3.1 Water Consumption 8.05.3.3.2 Emission Data and Recycling of Wastewater 8.05.3.3.3 Wastewater Treatments 8.05.3.4 Waste 8.05.3.4.1 Waste Characterization 8.05.3.4.2 Recycling, Reuse, and Disposal 8.05.4 Occupational Health Issues 8.05.4.1 General Overview 8.05.4.2 Prevention of Specific Occupational Risks in Ceramic Processes 8.05.4.2.1 Dust 8.05.4.2.2 Respirable Crystalline Silica 8.05.4.2.3 Lead 8.05.5 Summary Acknowledgments References

Comprehensive Materials Processing, Volume 8

http://dx.doi.org/10.1016/B978-0-08-096532-1.00809-8

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

8.05.1

Introduction

8.05.1.1

Ceramics

The term ceramics is generally used for inorganic materials, mostly obtained from naturally occurring minerals, which are hardened by a firing process. A wide range of raw materials and manufacturing techniques are used in the different branches of the ceramics industry. This chapter basically deals with the following types of ceramic products: ceramic tiles, structural ceramics, porcelain and earthenware, and refractories. These products are known as traditional ceramics. Technical or advanced ceramics are made by similar processes, but as they display specific, product-related features, they lie beyond the scope of this chapter.

8.05.1.1.1

Ceramic Tiles

Ceramic tiles have traditionally been used as wall cladding and flooring materials, primarily because of their technical characteristics, but also because of their aesthetic qualities. High-porosity tiles (10–18% water absorption) are typically used for wall cladding: these are known as earthenware tiles. In contrast, tiles with a low-porosity body (< 3% water absorption), known as stoneware tiles, are generally used for flooring. In recent years, moreover, the ISO 13006 standard, which sets out ceramic tile classification criteria, has incorporated a new group of tiles with very low porosity (<0.5% water absorption), called porcelain tiles.

8.05.1.1.2

Structural Ceramics

The term structural ceramics essentially encompasses ceramic materials that are used in building structures, such as bricks, roofing tiles, blocks, and joist-to-joist filler blocks. The products included in this group are sometimes also just called roof tiles and bricks. In terms of use, ceramic flooring products should also be included in the ceramic tile group; however, as these ceramic materials are usually made by extrusion, from a manufacturing process standpoint they are closer to actual structural ceramic products and are often included in this group.

8.05.1.1.3

Porcelain and Earthenware

Depending on the characteristics of the ceramic body, several products may be differentiated: l

Porcelain: The term refers to fully vitrified products with practically zero apparent porosity, high whiteness, and a variable degree of translucency. l Fine earthenware: Glazed products are involved with a white-colored porous body. l Majolica: These are glazed products with a red-colored coarse earthenware body. These products may also be classified in terms of their functionality, as the compositions used, though certainly alike, are formulated in terms of targeted service performance. In this sense, two subgroups may basically be distinguished: Household ceramics: Household ceramics include tableware, artificial and fancy goods made of porcelain, earthenware, and fine stoneware. Typical products are, for example, plates, dishes, cups, bowls, and jugs. l Sanitary ware: These are ceramic goods used for sanitary purposes. Typical vitreous china sanitary ware includes lavatory bowls, bidets, wash basins, and cisterns. l

8.05.1.1.4

Refractories

Refractories are products that are used in high-temperature applications. Refractories encompass a broad range of formed as well as nonformed products, which may be included in both the traditional ceramics and the advanced ceramics groups. Commercial refractories are usually divided into acidic, basic, and neutral refractories, though a precise distinction can often not be made. They may also be classified in terms of the main product component (e.g., alumina, silica, and magnesium refractories).

8.05.1.1.5

Other Products

In addition to the foregoing major products, other ceramic products are also manufactured in particular geographic areas or for specific applications and, though these are globally less widespread, they are of great importance. Such products include, for example, expanded clay aggregates, technical ceramics, and inorganic bonded abrasives: Expanded clay aggregates are porous ceramics with a uniform pore structure of fine, closed cells. They are used as loose or cement-bound material in the construction industry and also as loose material in garden and landscape design. l Technical ceramics are applied in many industries and include established products, such as insulators, and new ones, such as engine parts, catalyst carriers, bone replacement, filters, and many others. l Inorganic bonded abrasives or vitrified bonded grinding wheels are tools in which a synthetic abrasive is blended with a vitrified bond. l

8.05.1.2

Ceramic Manufacturing Processes

The traditional ceramic manufacturing process usually takes place in several successive stages: l

Raw materials preparation: Preparation basically occurs in either of two forms: by ‘dry milling’ or ‘wet milling.’

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues l

l l l l

73

Forming (shaping) of the piece: Forming methods may be broken down into three large groups, in rising order of moisture content: (a) forming by uniaxial or isostatic semi-dry pressing of a granulate material with low moisture content and/or binder addition; (b) plastic forming by extrusion, plastic pressing, wheel throwing, and so on, of plastic masses; and (c) forming by air slip casting or pressure casting of suspensions. Drying: The water that was required for forming is removed before the glazing and/or firing stage. Glazing: This is performed when a glazed product is to be made. Note that the order of the glazing and firing stages differs, depending on whether the product is made using one firing or multiple firings. Firing: The product may be subjected to one firing (single firing) or several firings (double firing, third firing, etc.). Sorting and packaging: Final characteristics of the ceramic product are controlled before packaging in order to detect any possible defects, classify the ceramic products according to trade categories, or discard the products. In certain cases, it may be of interest to repair the defects, if this is possible, by an additional firing. The general processing scheme of the main shaped ceramic products is depicted in Figure 1.

8.05.1.2.1

Raw Materials Preparation

The ceramic process begins with selection of the appropriate raw materials for the starting ceramic composition. In traditional ceramics, the following raw materials are essentially used: clays (which may be either red- or white-firing), kaolins, feldspars, quartz, carbonates, and other minor raw materials that depend on the type of product made. As natural raw materials are involved, preliminary homogenization is required in certain cases in order to assure consistent materials characteristics. This process may be performed before the materials are supplied to the ceramics manufacturer or at the ceramic production facility. In the latter case, the materials are deposited in heaps or piles, which may be left in the open air or entirely or partly covered, where they are homogenized and partially dried. The raw materials are usually subjected to either dry or wet milling. Milling of the individual raw materials or of the raw materials mixture (ceramic composition) making up the ceramic body may be performed. In ceramic tile manufacture, the raw materials are usually wet milled. The compositions are either wet milled in ball mills or directly dispersed in high-speed dispersers, yielding an aqueous suspension (slurry). Slurry water content is reduced by spray drying in order to obtain a granulated powder with the appropriate moisture content for the pressing process. In contrast, the ceramic composition is sometimes ground by dry milling; in this case, the resulting powder is subsequently moistened in order to form the tiles by semi-dry pressing. In structural ceramics manufacture, the raw materials composition is usually dry milled in hammer or pendulum mills. Water is then added to the dry-milled material in a mixer, thus producing a plastic mass for subsequent extrusion and/or plastic pressing. In the fabrication of porcelain and earthenware products, many of which are formed by slip casting, the raw materials are wet milled to obtain a suspension with appropriate properties for the slip casting operation. When forming is not done by slip casting, the slurry water content is reduced by filter pressing in order to produce a plastic mass with appropriate characteristics for the forming stage. In spray drying, the aqueous suspension (slurry) is atomized into a spray of fine droplets that are directed into a stream of hot air, yielding solid granular products with a low water content. The contact time between the sprayed suspension and the hot gas (from a burner or from the exhaust gases of a cogeneration turbine) is short, giving rise to violent evaporation of the water contained in each drop. Granules are obtained with an appropriate shape and size for semi-dry pressing, at moisture contents that vary depending on the process and end product involved. In the filter-pressing operation, water is removed from the slurry by applying pressure to the suspension, which is forced through a filtering fabric, separating the solids from the liquid. The resulting cake has a high residual moisture content (typically about 20%), which is appropriate for plastic forming processes. This approach is preferred in plastic forming processes, whereas spray drying is used for products formed by semi-dry pressing.

8.05.1.2.2

Forming

The raw materials preparation process is followed by the forming (shaping) process. This also exhibits certain differences, depending on the geometric shape or desired final appearance of the end product. Structural ceramics are usually formed by extrusion, sometimes followed by plastic pressing, whereas plastic pressing is typically used in forming roof tiles and vases, and to a lesser extent in forming bricks and special pieces. In contrast, ceramic tiles are usually formed by uniaxial semi-dry pressing (5–8% moisture content on a dry basis), using uniaxial hydraulic presses. However, some ceramic tiles are molded by extrusion to provide the end product with a particular aesthetic appearance, such as so-called rustic floor tile. Refractory bricks are usually formed by either uniaxial or isostatic pressing, sometimes at very high pressures. In tableware and decorative ceramics manufacture, the traditionally most widespread forming method is air slip casting in plaster molds. The method consists, first, of filling a plaster mold with a concentrated suspension of particles; part of the water is then removed by suction from the mold, yielding a product that reproduces the inner surface of the mold. Plastic molding is also used in tableware manufacture, particularly because of the ease of automating the throwing process, which provides higher productivity than slip casting. At present, however, high production volumes of thin products (plates, platters, etc.) are obtained in many plants by pressing. In this case, as such porcelain tableware exhibits a lower degree of symmetry than ceramic tiles, isostatic

74

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Figure 1

Traditional ceramic manufacturing process.

pressing is required. In this forming mode, the spray-dried powder is held in a rubber mold to which pressure is isostatically applied by a fluid. In the forming of vitreous china sanitary ware, though the traditional air slip casting method with plaster molds is still used, the most common forming process at present is pressure slip casting with resin molds. This forming mode produces the required thickness more rapidly, and the mold need not be dried.

8.05.1.2.3

Drying

The forming process is necessarily followed by drying in order to reduce the moisture content sufficiently for the subsequent glazing or firing operation to be appropriately conducted. There may also sometimes be an additional drying phase after the glazing stage.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

75

Many types of dryers are used, involving continuous, batch, or semicontinuous dryers, with highly varying drying times, depending on whether the products are individually dried or dried in stacks. The maximum drying temperature largely depends on the type of dryer and energy source, but usually lies between 100 and 200
C.

8.05.1.2.4

Glazing

Glazing involves the application, by different methods, of one or more glaze coatings on the surface of the piece. Glazing serves to provide the end product with a series of targeted technical and aesthetic properties, such as impermeability, cleanability, gloss, color, texture, chemical resistance, and mechanical strength. Glazes are usually prepared by milling the glaze components and additives in ball mills, in order to obtain aqueous suspensions with appropriate characteristics for the particular application method to be used. Ceramic tiles are usually glazed by waterfall glazing or spraying. Spraying is also the usual method of glazing vitreous china sanitary ware and decorative porcelain. In porcelain tableware, and sometimes also in decorative porcelain manufacture, the items are dipped into the glaze suspension. A series of decorations are often applied to the base glaze layer, using different techniques. The most common techniques are screen printing, rotogravure, flexography, decals, or inkjet printing. Hand decoration is also still found in some subsectors.

8.05.1.2.5

Firing

As noted previously, depending on the type of product and desired finish, one or more firings may be performed. The ceramic substrate or body is sometimes subjected to a first firing or biscuiting, subsequently followed by one or more additional firings of the glaze and any decoration. In contrast, single firing, that is, simultaneous firing of the body and glaze(s) – generally prevails in the manufacture of glazed ceramic tile and vitreous china sanitary ware. In structural ceramics manufacture, the products are generally fired in continuous kilns, most commonly in tunnel kilns, though Hoffmann-type kilns are also still used. In certain cases, fast-firing roller kilns are used in manufacturing thin products (such as small flat slabs of fired clay or roof tiles), particularly when the product is subjected to a second firing. The peak firing temperatures are usually quite low compared with those used in manufacturing other ceramic products and range from about 850
C to a little over 1000
C. In the case of ceramic tiles, single firing in roller kilns has become widespread in both vitrified and porous products. Fast cycles of 30–60 min are used, with peak temperatures of 1000–1200
C. Ceramic tableware is produced by several firings, using low-profile tunnel kilns for the base product. Glaze (glost) firing is carried out in tunnel kilns or in roller kilns without extra kiln furniture at peak temperatures that may reach up to 1400
C: Glost firing temperatures are higher than those used to fire the body. Firing cycles last from 15 to 30 h. The decoration is fired in tunnel kilns or batch kilns in cycles ranging from 30 min to 4 h, at temperatures that hardly exceed 800
C. Vitreous china sanitary ware is predominantly manufactured by single firing in tunnel kilns at peak temperatures of 1150– 1280
C, using a traditional firing cycle of 17–30 h or a fast firing cycle of 9–14 h. Products are involved that it may be of interest to refire in order to correct defects, owing to their high value. Refiring is performed in batch kilns with kiln cars or in bell kilns. Ceramic artware is fired in highly varying cycles in continuous or intermittent kilns, which may be of the electric or combustion type. One or more firings are performed at peak temperatures of about 1100–1300
C, the temperature of the first firing being lower when there are several firings. Refractories are usually sintered in tunnel kilns in a wide range of peak firing temperatures, from 1250 to 1850
C, depending on the product being made.

8.05.2

Energy Consumption and CO2 Emissions

The ceramic manufacturing process uses a lot of energy, mainly involving thermal and to a lesser extent electric energy. In ceramic manufacturing, energy is primarily used for kiln firing; however, drying of semiprocessed or shaped ware is also often energy intensive. Most drying and firing operations use natural gas, liquefied petroleum gas (LPG), or fuel oil, though solid fuels, electricity, liquefied natural gas (LNG), and biogas/biomass are also used. The ceramic tile and the brick and roof tile manufacturing sectors are the biggest energy consumers. Although energy costs depend on many factors (type of product, geographic location, etc.), in general, total (electric and thermal) energy costs in traditional ceramics account for 15–25% of direct manufacturing costs. Fossil fuel combustion, such as natural gas, LPG, and fuel oil combustion, produces carbon dioxide emissions, CO2 being one of the greenhouse gases responsible for climate change and global warming.

8.05.2.1

Fuel and Energy Consumption Data

The main fuels used in manufacturing glazed ceramic materials are natural gas and LPG; unglazed products may be manufactured using other fuels, such as gas oil, fuel oil, petroleum coke, and biomass. The heating value and CO2 emission factor of the main fuels used in manufacturing ceramic materials are detailed in Table 1 (1). The specific thermal and electric energy consumption values of the various ceramics industry subsectors are set out below, based on European industry data (2,4).

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 1

Heating value and CO2 emission factor of the main fuels used in the ceramic industry

Parameter 1

Net heating value (GJ t ) Emission factor (t CO2 T J1)

Pet coke

Heavy oil

Gas oil

LPG

Natural gas

32.5 98.3

40.18 76

42.4 73

45.5 65

48.59 56

Data drawn from: Intergovernmental Panel on Climate Change (IPCC). Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories. Online: http://www.ipcc-nggip.iges.or.jp/public/gl/invs1.html.

Table 2

Specific energy consumption in ceramic tile manufacture Thermal energya 1

Electric energy 2

Product

MJ kg

MJ m

Ceramic tile

3.46

68.41

1

MJ kg

MJ m2

0.39

7.67

a

Referred to the net heating value of natural gas.

Data drawn from: Monfort, E; Mezquita, A; Granel, R.; Vaquer, E.; Escrig, A.; Miralles, A.; Zaera, V. Analysis of Energy Consumption and Carbon Dioxide Emissions in Ceramic Tile Industry Manufacture. In Qualicer 2010: XI World Congress on Ceramic Tile Quality ; Cámara oficial de comercio, industria y navegación: Castellón, 2010, www.qualicer.org.

8.05.2.1.1

Ceramic Tiles

Table 2 details the specific energy consumption involved in ceramic tile manufacture, taken from a study conducted in the Spanish tile industry in 2008–2009 (3). The values are broken down into thermal energy and electric energy, expressed in terms of fired product mass and surface area (commercial unit). Ceramic tiles are mainly manufactured by wet milling the raw materials that make up the tile body. However, dry milling is also used, particularly in Brazil, where it accounts for more than 70% of production. The energy consumption and carbon dioxide emissions of the ceramic tile manufacturing sectors in Spain and Brazil were compared in a recent study (4). The main findings in that study are summarized in Table 3, together with the thermal consumption data of the European ceramic tile industry (2). The table shows that specific thermal energy consumption in ceramic tile manufacturing by the dry route is significantly lower than that by the wet route. However, the products made by the dry process exhibit technical limitations compared with those made by the wet process, so that the products are not fully comparable. On the other hand, the total energy consumption in the wet process is practically identical in Spain and Brazil, these being two very different geographic areas in which different raw materials are used and products with notable differences in porosity are made. Overall energy consumption is thus largely determined by the manufacturing process involved when similar products are manufactured. The energy consumption data of the processes and products made in the European Union have been bundled together in the BREF document (2). Though they include practically every type of traditional ceramic product and process, very wide energy consumption ranges are obtained (as shown in the table), making it difficult to compare their thermal energy consumption data.

8.05.2.1.2

Bricks and Roof Tiles

In Europe the main fuel used to make bricks and roof tiles is natural gas, which accounts for about 90% of total energy consumption, though other fuels such as petroleum coke, fuel oil, or LPG are also used. The available European energy consumption data exhibit noticeable variations, depending on the type of product made. Although product type depends on intended use or on architectural criteria, in practice considerable differences may also be observed in relation to geographic area, because different products are made for the same use: Thus there are countries that traditionally manufacture low-density bricks (Italy, Austria, or Germany), high-density bricks (Northern Europe), barrel roof tiles

Table 3

Breakdown of average specific thermal energy consumption in the ceramic tile manufacturing process Specific thermal consumption (MJ kg1 fired product)

Country

Forming

Milling process

Spray drying

Drying

Firing

Total

Spain Brazil

Pressing Pressing

European Union

Pressing and extrusion

Wet method Wet method Dry method Wet and dry method

1.8 1.9 – 1.2–2.4

0.5 0.6 0.8 0.3–0.9

2.9 2.5 2.1 2.1–5.3

5.1 5.0 2.9 3.7–8.7

Data drawn from: IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es and Monfort, E.; Mezquita, A.; Vaquer, E.; Mallol, G.; Alves, H. J., Boschi, A. O. Brasil x Espanha: Consumo de energia térmica e emissões de CO2 envolvidos na fabricação de revestimentos cerâmicos. Ceraˆmica Ind. 2011, 16 (4), 13–20 (in Portuguese).

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 4

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Energy consumption data per kilogram fired brick and roof tile

Country

Energy source 1

Austria

Thermal energy (MJ kg ) Electric energy (MJ kg1) Total energy (MJ kg1)

Spain

Masonry brick

Facing brick

Roof tile

1.02–1.87 0.08–0.22 1.50–2.50

2.87 0.27 2.50–3.00

1.97–2.93 0.23–0.41 1.90–2.95

Data drawn from reference: IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

(Southern Europe), flat roof tiles (Northern Europe), and so on (2). Table 4 details the average energy consumption in brick and roof tile manufacture in two European countries (Austria and Spain) for which data are available.

8.05.2.1.3

Porcelain and Earthenware

This group of products includes tableware, ornamental ware, and vitreous china sanitary ware. The specific electric and thermal energy consumption in the manufacture of tableware and ornamental ware is detailed in Table 5, while Table 6 shows specific energy consumption in the manufacture of vitreous china sanitary ware at three facilities.

8.05.2.1.4

Refractory Products

The total specific energy consumption ranges in different refractories’ manufacturing process stages are listed in Table 7.

8.05.2.1.5

Other Products

This group comprises vitrified clay pipes and technical ceramics. The total energy consumption in the vitrified clay pipe manufacturing process of the German vitrified clay pipe industry is detailed in Table 8. The mineral raw materials are clays, opening agents, and glazes, and the fuel used is natural gas. The electricity consumption encompasses the entire manufacturing process, in addition to secondary plant units, lighting, and the like. The data come from about 90% of German vitrified clay pipe production. The specific (thermal and electric) energy consumption data from two electro-porcelain manufacturing plants are given, as examples, in Table 9. Energy consumption data from the manufacturing processes of other technical ceramics are unavailable.

Table 5 Specific energy consumption in the manufacture of tableware and ornamental ware Parameter

Value

Electric energy consumption (MJ kg1) Thermal energy consumption (MJ kg1)

4.5 70

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

Table 6

Specific energy consumption in the manufacture of vitreous china sanitary ware

Parameter

Plant 1

Plant 2

Plant 3

Production capacity (ton per year) Electric energy consumption (MJ kg1) Thermal energy consumption (MJ kg1)

10 000 3.60 30

5120 3.32 22

2900 3.16 28

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

Table 7

Total specific energy consumption in refractories manufacture (MJ kg1)

Process stage

Specific energy (MJ kg1)

Preparation, screening Weighing, proportioning, mixing Forming Drying, firing Subsequent treatment, packaging

0.35–0.50 0.045–0.070 0.13–0.20 3.0–6.3 0.08

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 8

Total specific energy consumption in vitrified clay pipe manufacture Specific energy (MJ kg1 fired product)

Vitrified clay pipe manufacture

Small vitrified clay pipes

Medium vitrified clay pipes

Large vitrified clay pipes

Electric energy consumption Thermal energy consumption

1.20 5.1

5.8

6.7

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

Table 9

Specific energy consumption in the manufacture of technical ceramics (electro porcelain)

Parameter

Specific energy (MJ kg1 fired product)

Electric energy consumption Thermal energy consumption

0.94 15–25

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es.

8.05.2.2

CO2 Emission Factors and Emission Data

The direct CO2 emissions produced in ceramic tile manufacture basically stem from two sources (5): l

Combustion emissions: These stem from the exothermic combustion reaction between the fuel and the oxidizer.  y  y H2 O þ x CO2 Cx Hy þ x þ O2 / 4 2

l

Process emissions: These stem from the decomposition of carbonates present in the raw materials during the firing stage CaCO3 /CaO þ CO2 [ MgCO3 /MgO þ CO2 [

Total direct CO2 emissions will, therefore, depend on the fossil fuel and on the characteristics of the raw materials used in the manufacturing process. Direct CO2 emissions may also occur as a result of the combustion of organic matter present in the raw materials or of organic admixtures in the manufacturing process. However, except in certain very specific compositions and products, these CO2 emissions are usually negligible compared with those described above. On the other hand, there are also indirect CO2 emissions, which stem from electricity consumption, raw materials and product transport, and the like. Although such indirect CO2 emissions must be taken into account in product life-cycle assessment studies, they have not been included here because they depend on numerous specific facility-related factors. The quantity of carbonates present in the raw materials depends on the type of ceramic composition involved: l

In earthenware (i.e., porous) tile manufacture, the tile compositions usually have a 10–15% carbonate content by weight, while the carbonate content in stoneware tile compositions usually does not exceed 5%, that of porcelain tile being below 0.5%. l The carbonate content in ceramic compositions used in brick and roof tile manufacture tends to vary considerably, ranging from compositions free of carbonates to compositions with 25% carbonate content, depending on the raw materials source and targeted end-product porosity. l Porcelain compositions contain no carbonates, while the carbonate content in earthenware compositions is about 10–20%. The specific CO2 emissions produced in ceramic tile manufacturing in Spain and Brazil are detailed in Table 10. These values include processing emissions as well as combustion emissions, the fuel used being natural gas in both cases. The emissions were calculated according to the methodology proposed by the European Commission (5). The table gives a range of specific emissions for the firing stage, owing to the great variation in carbonate content in the raw materials. The specific CO2 emissions for several ceramic subsectors envisaged in the BREF are detailed in Table 11. The values have been obtained from the specific consumption data (assuming that the fossil fuel used is natural gas) and carbonate content. The table shows that the products fired at a higher temperature and with a greater number of firings produce greater emissions, as the CO2 emissions are more closely related to energy consumption than to raw materials carbonate content.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 10

79

Specific CO2 emissions produced in ceramic tile manufacture kg CO2 per ton

Country

Process

Spray drying

Drying

Firing

Total

Spain Brazil

Wet method Wet method Dry method

93  4 99  5 –

23  2 29  2 42  4

147–222 124–196 110  6

263–338 252–324 152

Data drawn from: Monfort, E.; Mezquita, A.; Vaquer, E.; Mallol, G.; Alves, H. J., Boschi, A. O. Brasil x Espanha: Consumo de energia térmica e emissões de CO2 envolvidos na fabricação de revestimentos cerâmicos. Ceraˆmica Ind. 2011, 16 (4), 13–20 (in Portuguese).

Table 11

Specific CO2 emissions Firing

Subsector

Type

Temperature (
C)

Emissions (kg CO2 per ton)

Ceramic tiles Bricks and roof tiles Porcelain and earthenware products (tableware and vitreous china sanitary ware)

Single firing Single firing Double firing

1150–1200 850–1100 1180–1270

152–338 60–275 750–4000

Source: Own elaboration.

8.05.2.3

Techniques for CO2 Emission Abatement

Since direct CO2 emissions are related to fossil fuel combustion and raw materials carbonate contents, one of the major CO2 emission abatement strategies entails reduction of energy consumption, applying the best available techniques (BATs). The BREF document for the ceramics industry (2) sets out some of the best available techniques for enhancing energy efficiency: l

Improved kiln and dryer design Recovery of kiln surplus heat, especially from the kiln cooling zone l Application of a fuel changeover in the kiln firing process l Modification of ceramic bodies l

In addition to the above approaches, other measures can also be applied to the different process facilities to reduce thermal energy consumption and, hence, decrease CO2 emissions. A number of the energy-saving measures applied to process facilities are described below (6).

8.05.2.3.1

Spray Dryers

8.05.2.3.1.1 Increasing suspension solids contents Increasing the solids content of a suspension involves reducing its water content. If the suspension is to be subjected to a drying process, it is of interest, from an energy viewpoint, to maximize suspension density and thus minimize the amount of water to be evaporated. For example, raising suspension solids content from 62 to 68% provides an energy saving of 25% (6). 8.05.2.3.1.2 Raising drying-gas temperature Hot drying-gas energy depends on gas flow rate and temperature. If the energy input into the spray dryer is to be kept constant, a rise in the temperature of the gases entering the spray dryer needs to be accompanied by a reduction in the gas flow rate. If the drying-gas input flow rate is lowered, the gas flow rate in the spray dryer stack is reduced, for the same quantity of evaporated water, so that energy losses through the exhaust flue are decreased. This holds provided that the spray dryer gas output temperature is kept constant. An increase in drying-gas temperature and a decrease in drying-gas flow rate thus lead to lower stack energy losses, consequently raising spray dryer efficiency. The enhanced efficiency can be used either to evaporate a greater amount of water, that is, to raise production, or to reduce the energy input into the spray dryer, keeping production constant. The energy saving attained in a spray dryer by raising drying-gas temperature is shown in Figure 2. 8.05.2.3.1.3 Heat recovery facilities in spray dryers 8.05.2.3.1.3.1 Cogeneration systems Cogeneration is defined as the joint production, in a sequential process, of electricity (or mechanical energy) and useful thermal energy, from a single fossil energy source.

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90

0,50

0,48

88

0,46

86

84

0,44 Consumption

Efficiency (%)

Energy consumption (kWh kg–1)

Efficiency

82

0,42

0,40 400

450

500

550

600

650

80 750

700

Figure 2 Energy saving and enhancement of spray dryer efficiency on raising drying-gas temperature. Reproduced from Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

Table 12

Comparison of the energy efficiency of a conventional system and of a cogeneration system

Parameter

Units

Conventional system

Cogeneration system

Electric efficiency Thermal efficiency Total overall efficiency Primary energy consumption Energy losses

(%)

33 90 55 1.8 0.8

32 55 87 1.18 0.18

(kW (kW generated useful energy)1)

Data drawn from: Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

The primary energy used may come from a fossil fuel (in a solid, liquid, or gaseous state), energy valorization of waste, or waste energy recovery from industrial processes. The joint recovery of electricity and heat in the same facility allows high cogeneration plant efficiencies to be obtained and, therefore, a primary energy saving. The energy efficiency of a conventional system and of a cogeneration system is compared in Table 12. Spray dryers are facilities that run continuously and have a very high thermal demand. The cogeneration systems implemented in spray dryers are gas turbines. A gas turbine’s working life is considerably shortened by stops and starts: this therefore needs to be minimized to lengthen the gas turbine’s life span. The range of electric power in the gas turbines installed in the ceramic tile manufacturing sector is quite wide, from 3.5 to 10 MW. With temperatures close to 500
C and an oxygen content above 16%, the resulting gas-turbine exhaust gases are appropriate for use as an oxidizer in the spray-dryer burner. A schematic illustration is shown in Figure 3 of a spray-drying facility that uses gas turbine exhaust gases. These turbine cogeneration systems linked to spray dryers are widely implemented in the European ceramic tile manufacturing sector (particularly in Spain and to a lesser extent in Italy). 8.05.2.3.1.3.2 Heat recovery from other process facilities Spray-dryer drying gases usually come from a burner that raises ambient air temperature to working temperature. However, there are also alternative systems that enable surplus hot gases from other process facilities to be recovered, saving spray-dryer fuel consumption by partly or entirely replacing ambient air as an oxidizer, provided that the oxygen content in the recovered gas exceeds 16%. The greater the temperature difference between the ambient air and the recovered gases, and the larger the amount of replaced air, the greater will be the fuel saving in the spray dryer.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Output gas bypass

81

Air Cleaning system Burner Spray dryer

Air filters

Flue gases Natural gas Solids suspension Gas turbine

Figure 3 Scheme of a spray dryer using exhaust gases from a gas turbine. Reproduced from Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

8.05.2.3.2

Shaped Product Dryers

8.05.2.3.2.1 Optimization of drying-gas flow rate After the hot gases have entered into contact with the shaped products, they are extracted from the dryer and exhausted into the atmosphere through the stack. In many cases, however, part of the drying-gas flow is not exhausted through the stack but is recirculated again to the dryer as a drying agent. These recirculated gases can be used as oxidizers at the burners because they have a high temperature and an oxygen content above 15%. The rest are exhausted via the stack, which thus purges the system. Energy balances drawn up in dryers have shown that most of the energy losses occur through dryer stacks. Therefore, one of the options for enhancing dryer energy efficiency consists of reducing stack gas enthalpy. Stack gas stream enthalpy can be reduced by lowering the gas flow rate, acting upon the stack valve or the fan that extracts the gases from the dryer, using a frequency inverter. Decreasing the stack gas flow rate lowers gas temperature and raises gas humidity. Reducing stack gas temperature and flow rate thus lowers energy losses and, hence, burner fuel consumption. 8.05.2.3.2.2 Reduction of water content in the shaped product Practically all the energy consumed in the dryers is basically used in evaporating the water contained in the products exiting the forming stage. A reduction in product water content means less water needs to be evaporated; that is, less energy is needed to dry formed products in the drying stage. 8.05.2.3.2.3 Heat recovery facilities in dryers Heat recovery installations in dryers enable part of the drying air to be replaced with hotter gases from other manufacturing process facilities. These gases can come from the kiln or from cogeneration engines. 8.05.2.3.2.3.1 Use of cogeneration systems with internal combustion engines Cogeneration engine exhaust gases have a high temperature, ranging from 400 to 500
C. These gases are diluted with preheated air (at 60–70
C) from the water-cooling circuit used for internal engine cooling (cylinders, oil, etc.), which reaches temperatures of about 95
C. The result is a gas stream that usually has a temperature of 200–300
C. These gases are suitable for use as drying gases, though their incorporation into the dryers needs to be carefully studied in each particular case, in order to impact the programmed drying cycle as little as possible. With a view to regulating dryer operation, dryers are also fitted with gas burners. Figure 4 schematically illustrates how a cogeneration engine works. 8.05.2.3.2.3.2 Kiln heat recovery Depending on the origin of the recovered gases, these can be used in the driers either directly or after treatment. Kiln cooling gases usually have a high flow rate and temperature of about 100–250
C. These gases contain no pollutants, as they come from the kiln cooling zone. They can therefore be used directly in other process facilities, such as dryers.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Dryer

Burner Engine

Figure 4 Schematic illustration of gas engine operation. Reproduced from Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

Output gases KILN 1 GAS CLEANING SYSTEM

KILN 2

Dryers Flue gases

Heat exchanger

DRYER 1 DRYER 2 DRYER 3

Cold fluid

Hot fluid DRYER 4

Figure 5 Schematic illustration of combustion gas heat recovery using an air–air heat exchanger. Reproduced from Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

Kiln combustion gases are exhausted through the stack. The temperature of this gas stream is about 200
C. The gas stream has a variable composition, as it contains combustion products from the fuel used and products from the chemical reactions that occur in the ceramics being made. As a result, the recovery of the stream in other process facilities makes it necessary previously to clean these gases or to use a heat exchanger in which part of the energy contained in the stream is transferred to a fluid (either air or a thermal fluid), in order to be able to use the transferred heat in other process facilities. At present, air–air heat exchangers of thermal fluid heat exchangers may be used. Figure 5 schematically illustrates combustion gas heat recovery from two kilns to several dryers, using a heat exchanger. This arrangement enables hot kiln combustion gases to transfer part of their heat to the ambient air by means of a heat exchanger: This clean hot air can thus be used in the dryers. If heat is to be exchanged via a thermal fluid, heat exchangers need to be installed at each facility in which combustion gas heat is to be recovered, as the thermal fluid cannot be directly used in other process facilities. Once the thermal fluid has transferred its heat, this returns to the kiln heat exchanger through a closed circuit. The energy saving attained by this measure depends on the replaced amount of gas, dryer working temperature, and so forth. Reference (7) reports that savings of up to 70% have been attained recently in dryer energy consumption.

8.05.2.3.3

Kilns

8.05.2.3.3.1 Optimization of the ceramic body composition The firing of ceramic materials requires a great energy input in the form of hot gases. Key firing parameters affecting kiln energy consumption include peak firing temperature and firing cycle duration, as well as the programmed kiln temperature–time curve. Peak firing temperature is determined by the body composition and the characteristics of the finished product, therefore making it of interest to optimize the composition in order not to work at excessively high firing temperatures. In addition, the body composition determines the chemical reactions that occur during firing. This makes it convenient to minimize reactions that slow down the firing of the material, such as the oxidation of organic material or carbonate decomposition.

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83

Figure 6 Side-view thermograph of a roller kiln (left) and a tunnel kiln (right). Reproduced from Monfort, E.; Mezquita, A.; Mallol, G.; Granel, R.; Vaquer, E. Guı´a de Ahorro Energe´tico en el Sector de Baldosas Cera´micas de la Comunidad Valenciana (in Spanish).

8.05.2.3.3.2 Reducing product thickness Thinner products provide energy savings, not just in the firing stage, but throughout the entire production process. In the first place, smaller quantities of raw materials (clays, nonplastics, deflocculants, etc.) are required, entailing energy savings in the drying processes, as less water needs to be evaporated. However, a reduction in thickness of the unfired material leads to lower mechanical strength of the products; this property is very important because unfired products need to withstand processing mechanical stresses as the products travel through the dryer to the kiln. In the kiln, a thinner product means that less material needs to be fired, with the ensuing energy saving. 8.05.2.3.3.3 Optimization of the kiln charge Optimization of the firing surface area in roller kilns and of the working charge in tunnel kilns raises kiln production, entailing lower energy consumption per unit processed product. In the case of tunnel kilns, the use of kiln cars made of materials with lower bulk density provides energy savings, as less energy is needed to raise the kiln car temperature. 8.05.2.3.3.4 Reduction of energy losses through the kiln surfaces Between 10 and 30% of the energy used in firing ceramics in continuous kilns (both roller and tunnel kilns) is lost through kiln walls, fan surfaces, flues, and other kiln surfaces. Figure 6 presents a side-view thermograph of a roller kiln and a tunnel kiln. The two thermographs show that the highest outside wall temperatures are found in the firing zone, thus producing the greatest energy losses through the kiln surface. Consequently, improved kiln insulation and good maintenance provide energy savings in the kiln. 8.05.2.3.3.5 Reduction of combustion air flow rate and increase in combustion air temperature Reducing the combustion air flow rate saves energy because a smaller gas volume then needs to be heated in the kiln. A 2% reduction in the combustion air flow rate at the burners thus yields an energy saving of about 5% (6). Fuel combustion takes place in the burners. The heat released in this reaction is used to raise the combustion gas temperature from the oxidizer input temperature to the programmed temperature. The higher the temperature at which the combustion air is fed in to the burners, the lower is the fuel consumption. A 50
C rise in combustion air temperature provides a 3% energy saving (6). The constraint on the allowable peak temperature in the combustion primary air feed stems from two factors: l l

The maximum temperature that the facility’s distribution, regulation, and control elements can withstand. Maintenance of the air:fuel ratio in order to avoid air (oxygen) shortage in the kiln atmosphere. When air temperature rises, air density decreases so that the same air volume contains less mass, therefore making it necessary to raise the air flow rate in order to hold the stoichiometric ratio. Several possibilities are available for raising oxidizing air temperature, all of which are based on kiln waste heat recovery.

8.05.3

Environmental Issues

The ceramics industry, just as every industrial process, has a series of related environmental impacts. This chapter examines the impacts deemed most important in the literature: air emissions of particles and gaseous compounds, water consumption and discharge, and waste production. A further potentially significant aspect, depending on company location and applicable local regulations, is noise emission. Noise control is basically performed by implementing the generally applicable preventive and corrective measures for every industrial facility, so that this issue will be addressed only briefly.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 13

Characterization of ceramic industry air emissions

Manufacturing process stage

Type

Temperature

Emitted pollutant

Raw materials handling Dry milling Wet milling Drying of solid materials or spray drying Forming Drying of formed products Glaze preparation and application Firing (one or more firings)

Fugitive Channeled Channeled Channeled Channeled Channeled Channeled Channeled

Ambient Ambient Ambient 80–120
C Ambient 80–150
C Ambient 150–250
C

Particles Particles Particles Particles and combustion gasesa Particles Particles and combustion gasesa Particles Particles and gas compoundsa,b

a

Combustion gases: CO2, CO, SOx, NOx, among others. Gas compounds: heavy metals, acid pollutants (SOx, NOx, HF, HCl, etc.).

b

Data drawn from: Blasco, A.; Escardino, A.; Busani, G.; Monfort, E.; Amorós, J. L.; Enrique, J. E.; Beltrán, V.; Negre, P. Tratamiento de Emisiones Gaseosas, Efluentes Lı´quidos y Residuos So´lidos de la Industria Cera´mica; Instituto de Tecnología Cerámica-Asociación de Investigación de las Industrias Cerámicas: Castellón, 1992 (in Spanish) and Instituto de Tecnología Cerámica. Guı´a de Mejores Te´cnicas Disponibles para el Sector de Fabricacio´n de Baldosas Cera´micas en la Comunitat Valenciana; Centro de Tecnologías Limpias: Valencia, 2010 (in Spanish).

8.05.3.1

Atmospheric Pollution

This section describes the main characteristics of the air emissions produced in the ceramic process. These emissions display certain specific characteristics, which depend on the process stage in which they are generated. Ceramics industry air emissions are divided into two types, depending on the point of emission: 1. Channeled emissions: emissions released into the atmosphere through stacks or flues. 2. Fugitive emissions: emissions not released into the atmosphere through stacks or flues, but fundamentally produced by handling bulk solid materials. By way of introduction, Table 13 briefly characterizes ceramics industry emissions (8,9).

8.05.3.1.1

Particulate Matter Emissions (TSP, PM10, and PM2.5) Control

Particulate matter (hereafter PM) emissions in the ceramics industry are deemed one of the major environmental impacts of the ceramic materials manufacturing process, in quantitative terms, as this type of industrial activity processes dusty materials (clays, kaolins, feldspars, sands, quartzes, etc.). In addition to other parameters, the potential impact of PM emissions on human health is determined by particle size. Particles are classified as follows: l

Settleable particles: particles in the air that are deposited by gravity or transported by rain. TSP: total suspended particulate matter not readily deposited by the action of gravity. l PM10: particles that pass through a selective size inlet for an aerodynamic diameter of 10 mm with a cutoff efficiency of 50%. l PM2.5: particles that pass through a selective size inlet for an aerodynamic diameter of 2.5 mm with a cut-off efficiency of 50%. l

8.05.3.1.1.1 Fugitive PM emissions data and control As mentioned above, the main raw materials used in the ceramics industry are of a dusty nature, so that the finest particle fractions may be suspended in the surrounding air during handling, particularly in transport, storage, mechanical treatment, and subsequent handling operations. When such operations are enclosed (in a specific installation housing or containment system), the suspended particles are usually collected by extraction systems and separated from the air by a cleaning system, the cleaned stream then being emitted through a channeled source. However, not all suspended particles are captured by the extraction systems, nor are all operations enclosed, hence resulting in nonchanneled or fugitive particle emissions into the atmosphere. Indeed, fugitive dust emissions in the ceramics industry are essentially produced in bulk particulate materials storage areas (in open or only partly enclosed building areas or yards). However, they also occur in the pretreatment operations of these materials, when such operations (grinding and milling) do not take place in confined areas, and in trucking (loading and unloading operations, and in the circulation of the trucks themselves). Studies on particle emissions in ceramic manufacturing processes indicate that particle emissions from fugitive sources, depending on the manufacturing process, may be as important as stack particle emissions. In order to reduce the environmental impact associated with this type of emission, individual measures or a combination of different techniques can be implemented. This assures an abatement of the fugitive dust emissions arising in the storage and handling operations of particulate materials such as raw materials (clays, feldspars, kaolins, etc.) and semiprocessed materials (spray-dried granules, dry-milled clay, etc.).

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85

The proposed corrective measures may be broken down as follows: measures for operations with dusty materials, measures for storage areas of these types of bulk materials, and measures for reducing particle resuspension owing to vehicle circulation: Measures for operations with dusty materials: l l l l l l l

Enclosure of operations performed with dusty materials, such as loading and unloading, grinding, milling, and mixing. Filtration of the air extracted from raw materials loading and proportioning. Use of silos with an appropriate capacity, equipped with filtration systems of the air extracted from filling operations. Dust covers for conveyor belts running with dusty raw materials. Use of pneumatic conveying systems (provided this is technically feasible). Performance of handling operations in closed industrial buildings with extraction systems and subsequent cleaning of the gas stream. Reduction of leaks and spillages, performing good maintenance of the facilities. Measures for storage areas of bulk particulate materials:

l

Fencing using windbreaks, walls (natural or artificial), plant barriers, and the like. Control of the height of materials unloading points, automatically whenever feasible, or reduction of the unloading rate. l Cleaning and wetting by spraying systems. l

Measures for dusty materials transport: l

Paving and cleaning of circulating areas. Limitation of circulating speed. l Wet cleaning systems for truck chassis and tires. l Dust covers for truck loads and avoidance of spillage and overflow. l Whenever feasible, transport shall be by tanker trucks, with pneumatic conveying systems for loading and unloading. l

Finally, the emission factors for PM10 for companies that apply a combination of corrective measures, in addition to the efficiency achieved through the implementation of the measures considered, are detailed in Table 14. The values in this table have been drawn from reference (10), obtained at ceramic tile, brick, and roof tile manufacturing facilities. 8.05.3.1.1.2 Channeled PM emissions data and control Channeled particle emissions have traditionally been the most widely studied particulate emissions from a legal and technical viewpoint. In these emissions a distinction may be made, as indicated in Table 13, between cold emissions or emissions at ambient temperature (originating in localized extractions) and hot emissions or combustion gas emissions (originating in dryers and kilns). When it comes to defining the characteristics of a given particulate matter, it is very important to identify the originating process stage. In this sense, the relationship between the different particle size fractions (TSP, PM10, and PM2.5) will depend on the type of process that produces the industrial emissions, as well as on the types of pollutants and other features that define the resulting emissions. Classification of the applied corrective measures and PM10 emission factors (expressed as g of PM10 per ton of finished product)

Table 14

Type of facility

Applied control measures

Efficiency (%)

PM10 emission factor (g per ton)

Closed

Transport is performed on paved areas Total enclosure of handling areas Suction systems with filtration

>95

7–11

Semi-closed

Transport is performed on paved areas Partial enclosure of handling areas Frequent irrigation of unpaved areas Transport is largely performed on paved areas (>75%)b Enclosure and suction systems in dusty operations (e.g., crushing) Windbreaks at the perimeter

75–80

71–84

25–50

220–240

Transport is largely performed on unpaved roads (>75%)a Few control measures in dusty operations

<20

270–350

Extensively implemented corrective measures Open Scantily implemented corrective measures a

Percentage of the operation occurring on unpaved roads. Percentage of the operation occurring on paved roads.

b

Data drawn from: Monfort, E.; Sanfelix, V.; Celades, I.; Gomar, S.; Martín, F.; Aceña, B.; Pascual, A. Diffuse PM10 Emission Factors Associated with Dust Abatement Technologies in the Ceramic Industry. Atmos. Environ. 2011, 45 (39), 7286–7292. Values obtained from experiments conducted at ceramic tile, brick, and roof tile manufacturing facilities.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Figure 7 Schematic view of a fabric filter baghouse with pressure pulse regeneration. Reproduced from IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007. http://eippcb.jrc.es

Note that the nature or composition of the particulate matter emitted by channeled sources is also closely related to the composition and/or nature of the raw materials used. For example, in the case of emissions from extraction points in the raw materials preparation and forming stages, the composition of the emitted particles is dominated by clays, quartz, and feldspar, among other materials. However, when the emissions come from glazing line extractions, the emissions are characterized by mixtures of particles from the body raw materials and from the glaze composition (heavy metals, boron, etc.). This appears reasonable, as cold emissions are involved in which the processed material undergoes no significant physical or chemical transformations, the particles being formed by mechanical processes. However, in firing stage emissions, particles are formed by thermal processes (volatilization and condensation), which could give rise to significant sulfur compounds and minor heavy metal emissions, among others. Different cleaning technologies are available for the treatment of particulate matter emissions. The selection of the most appropriate system depends, among other factors, on the size of the particles to be separated, operating conditions, costs, capital outlay, waste recycling possibilities, and spatial requirements. In ceramic sector emissions, emitted particle size obviously depends on the process stage; however, owing to the types of materials used, particle size is usually below 20 mm. Cyclones allow larger-sized particles to be separated, but cyclone efficiency only makes cyclones serviceable as precleaning systems in stages that work with relatively coarse materials (granules). It may be noted that ceramic drying and firing stages do not emit a large quantity of PM, so that cleaning systems are generally not required for these pollutants, though these stages may require cleaning systems for gaseous pollutants (SOx, HF, etc.). Several separation and filtration systems may be used to minimize channeled particle emissions into the atmosphere: Fabric filter baghouses or bag filters (BF). In these systems, the gas stream to be cleaned passes through a fabric filter (which may be made of different materials, such as polyester, acrylic, or Teflon). The particles are thus deposited on the surface of the fabric, usually in the form of sleeves or bags. The type of fabric chosen will depend on gas stream characteristics (temperature, presence of moisture, acids, etc.). However, use of a suitable fabric for specific gas stream conditions enables high particle retentions to be attained, generally above 98% and even up to 99%, depending on particle size (Figure 7). l Plate filters. The main elements in these filters are the rigid filtering media, which are assembled as compact elements in the filter system; these filter elements usually consist of Teflon-coated sintered polyethylene. The main advantages of this gas-cleaning system are high particle removal efficiency, high resistance to abrasive wear caused by the particles, and the smaller volume of these devices compared to that of fabric filter baghouses, as they can run at higher filtering speeds (Figure 8). l Wet particle scrubbers. In these systems, the gas stream to be cleaned is put into contact with a liquid, generally water, which retains the particles carried in the gas stream. Several types may be differentiated in terms of scrubber design (packed and plate towers, spray towers, Venturi towers, etc.). However, the most widely implemented device in the ceramic sector is the Venturitype scrubber, because it provides greater efficiency (Figures 9 and 10). l Electrostatic precipitators (ESP). These devices are able to clean gas emissions that contain solid and/or liquid particles. They are particularly useful when large flow volumes containing small particles are to be treated with high efficiency. The working l

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

87

Figure 8 Schematic illustration of a plate filter. Reproduced from IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007. http://eippcb.jrc.es.

Figure 9 Scheme of a Venturi scrubber. Reproduced from Mallol, G.; Monfort, E.; Busani, G.; Lezaun, F. J. Depuracio´n de los gases de combustio´n en la industria cera´mica, 2nd ed., Instituto de Tecnología Cerámica: Castellón, 2001 (in Spanish).

principle of an ESP consists of the application between a discharge electrode and a collection electrode (usually known as plates) of a sufficiently high electric voltage difference (40–120 kV) to electrically charge the particles present in the gas stream to be cleaned. The charged particles are attracted by the collecting electrode, on which they are deposited, forming a layer. The collection plates are then cleaned by periodic ‘rapping’ (vibration), which dislodges the particles. If the cleaning system does not work properly, the particles can be dragged back into the gas stream with the ensuing diminished system efficiency (Figure 11). The average values of the total particle concentration (CTSP), expressed as mg m3 N (concentrations refer to normal conditions: temperature of 273 K and a pressure of 1013 hPa), and of the PM10/TSP and PM2.5/TSP fractions, expressed in (%), for the main ceramic process stages, with and without cleaning system, are detailed in Table 15.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Figure 10 Scheme of a spray tower. Reproduced from Mallol, G.; Monfort, E.; Busani, G.; Lezaun, F. J. Depuracio´n de los gases de combustio´n en la industria cera´mica, 2nd ed., Instituto de Tecnología Cerámica: Castellón, 2001 (in Spanish).

Figure 11 Scheme of an electrostatic precipitator. Reproduced from Mallol, G.; Monfort, E.; Busani, G.; Lezaun, F. J. Depuracio´n de los gases de combustio´n en la industria cera´mica, 2nd ed., Instituto de Tecnología Cerámica: Castellón, 2001 (in Spanish).

In the literature surveyed, emission values are usually expressed as TSP (i.e., total suspended particulate matter); values of PM10 have only been found in studies at ceramic tile manufacturing facilities (12–14), though similar PM10 values may be expected at other traditional ceramics facilities.

8.05.3.1.2

Gas Emissions Data and Control

These types of pollutants are characteristic of industrial activities or processes that run at high temperatures and/or in which combustion processes occur. The main gas pollutants may be divided into the following groups: l

Gases produced in combustion processes (such as CO2, CO, NOx, and SOx), for example, in spray drying, drying of shaped products, and firing. l Gases produced by raw materials decomposition (CO2 from carbonates, NOx from nitrates, HF from silicates, HCl from mineral impurities, SOx from sulfates and/or sulfides, among others). Such pollutants are typically found in high-temperature processes (firing, calcination stages, etc.) in which important physical and chemical transformations occur in the processed materials.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 15

89

TSP concentrations and PM10/TSP and PM2.5/TSP fraction percentages Average values a

Type of process

Process stage

Measured gas stream

Processes at ambient temperature (extractions)

Milling Semi-dry forming

After BF cleaning Raw gas stream After BF cleaning Raw gas stream After BF cleaning Raw gas stream After cyclone cleaning After BF cleaning Raw gas stream

Glazing Processes at medium temperature (drying)

Drying Spray drying

Processes at high temperature (firing)

Firing




Tgas ( C)

CTSP (mg mN

15–30 15–30

3 b

PM10 / TSP (%)

PM2.5 / TSP (%)

110–120 80 75–100

<5 109 <5 132 <5 <5 >1000 <5

74.8 21.0 75.3 51.8 74.5 84.5 73.4 81.7

53.4 2.1 28.9 20.1 41.7 66.9 41.3 53.5

160–210

11

99.4

93.9

18–40

)

a

The measured gas stream was either the raw gas stream exiting the process stage or this stream after it had been subjected to a cleaning system. mg m3 N : concentrations refer to normal conditions: 273 K and 1013 hPa.

b

Data drawn from: Monfort, E.; Celades, I.; Mestre, S.; Sanz, V.; Querol, X. PMx Data Processing in Ceramic Tile Manufacturing Emissions. Key Eng. Mater. 2004, 264–268, 2453– 2456; Monfort, E.; Celades, I.; Gomar, S.; Rueda, F.; Sanfelix, V.; Minguillón, M. C. Determination of PMx/TSP Fractions in Channelled Emissions of the Ceramic Industry. In Qualicer 2006: IX World Congress on Ceramic Tile Quality; Cámara oficial de comercio, industria y navegación: Castellón, 2006, pp P.BC97–P.BC110, www.qualicer.org and Minguillón, M. C.; Monfort, E.; Querol, X.; Alastuey, A.; Celades, I.; Miró, J. V. Effect of Ceramic Industrial Particulate Emission Control on Key Components of Ambient PM10. J. Environ. Manage. 2009, 90 (8), 2558–2567. Values obtained from measurements conducted at ceramic tile manufacturing facilities.

The origin, emission temperature, arising compounds, and so on, of the gas pollutants emitted during the ceramic process, on which such information is available, are set out in further detail below: l

l

l

l

l

Fluorine compounds: Fluorine compounds are essentially found in ceramics industry emissions as gas compounds in the firing stage. In general, the fluorine in ceramics firing kilns stems from the decomposition of clay minerals that contain fluorine impurities in their structure and begins at temperatures of about 400–600
C. The main compounds that form are hydrofluoric acid, silicon tetrafluoride, and, to a lesser extent, alkaline fluorides in particle form, the presence of these last compounds being practically negligible (2,15,16). Sulfur compounds (SOx): SOx stems mainly from: (1) oxidation of the sulfur contained in fossil fuels (such as coke or heavy oil) during the combustion process; and (2) the presence of sulfur in certain raw materials (basically clay) compositions owing to impurities, typically pyrite (FeS2) and gypsum (CaSO4$2H2O), which lead to sulfur compound (SOx) emissions, particularly SO2–SO3, during the firing stage (2,17,18). Sulfur emissions in the ceramics firing process may occur in two periods. A first emission, detected at about 450
C, may be assigned to pyrite oxidation (FeS2) and to sulfur emissions relating to organic matter. A second emission may occur above 750
C, up to the end of the firing cycle; in this case the emission is due to sulfate decomposition (anhydrite, CaSO4) and is limited by the vitrification process of the product (pore closing). If such sulfates are not fully removed during the firing process, they may lead to efflorescence (soluble salts) in the end product (17,18). Chlorine compounds (HCl): The origin of these emissions is mainly linked to the presence of the chlorine ion in the water used as raw material. The chlorine levels in this water can rise when the water used is treated industrial wastewater, as chlorine is commonly used in industrial wastewater treatment. It may be noted that most of the clays and additives used contain trace levels of chlorine; however, even though trace levels are involved, the processing of large quantities clay may lead to high chlorine compound emission levels. The chlorine present in shaped ceramics is found in the following forms: as inorganic compounds (salts) and as organic compounds. In addition, in certain cases, another possible source of chlorine is the presence of chlorine in the glaze composition. Chlorine compound emissions occur mainly during the firing stage, when most of the chlorine volatilizes at temperatures above 850
C, owing to decomposition of chlorine-bearing mineral salts. The decomposition of organic compounds that contain chlorine gives rise to HCl emission in the 450–550
C range (2). Nitrogen oxides (NOx): The term nitrous oxides (NOx) refers to the group of nitrogen oxides, basically NO and NO2, found in gas streams. Generally, the ratio in which they are found in ceramics industry emissions is 90% NO and 10% NO2 (9). The NOx present in these emissions can stem from: B Oxidation of nitrogen molecules in the fuel: These nitrous oxides are usually called fuel NOx. The arising quantity is low when the fuel used is natural gas (2). B The reaction, at combustion temperature, between nitrogen and the oxygen in the air, known as thermal NOx (2). B The decomposition of the nitrates present in the raw materials. NOx formation in combustion processes is encouraged at temperatures above 1400
C, so that NOx emissions of this pollutant are only significant in certain types of ceramic products (e.g., refractories and hard porcelain) (2). Volatile organic compounds (VOCs): Ceramic raw materials can contain organic matter. In addition, in the manufacturing process itself, materials of an organic nature are used that are added as bonding agents, pore formers, adhesives, and the like.

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During process stages that require heat (drying and firing), VOCs may be released as a result of the decomposition of the organic materials used. In the firing stage, depending on VOC emission temperature and kiln design, thermal breakup of these compounds may occur. l Heavy metals: The presence of heavy metals is mainly related to the composition of the raw materials used in the glaze preparation and application stages, though they can also stem from impurities in the raw materials and/or fuels. With a view to reducing gas compound emissions, two types of approaches are used: primary and secondary measures. 8.05.3.1.2.1 Primary measures A number of primary measures may be adopted to reduce gaseous compound emissions (e.g., HF, HCl, SOx, and heavy metals) resulting from combustion processes. Such measures include: l

Selecting raw materials that reduce the presence of pollutant precursors in the raw materials. Optimizing the firing curve and reducing water vapor in the kiln gases. l Selecting the fuel. l

8.05.3.1.2.2 Secondary measures The main gas-cleaning systems for reducing gas emissions of inorganic compounds (SOx, HF, and HCl) are as follows: cascade-type packed bed adsorbers, module adsorbers, and dry gas-cleaning systems by means of filtration. All these systems consist of placing the gases in contact with a substance or reagent that retains the pollutants and, thus, releasing a gas with a smaller pollutant concentration. These corrective measures are described below. l

Cascade-type packed bed adsorbers: In this type of system, the reaction between the adsorbent (e.g., CaCO3) and the pollutants (mainly HF, SOx, and HCl) in the gas stream takes place in a chamber in which the adsorbent falls by gravity and through which the gases circulate. In order to increase the reaction time and contact surface, walls are installed in this chamber. These walls assure effective distribution and circulation of the gas stream, in order to enhance contact with the adsorbent. In these systems, the consumed reagent is collected at the bottom of the device. Such adsorbers can withstand gas temperatures above 500
C, without it being necessary previously to cool the gases, and they efficiently remove HF, SOx, and HCl from kiln emissions. Specific adsorbents can be used for each situation (Figure 12). l Module adsorbers: This system is mainly used for removing HF. Dry adsorption is involved in which the gas stream passes through a steel reactor containing honeycomb modules that contain calcium hydroxide, which reacts chemically with the HF in the gas stream, forming calcium fluoride (CaF2). Module lifetime depends on plant operating time, gas flow volumes, and the fluorine concentration in the gas stream. Saturated modules are replaced with new ones (Figure 13). l Fabric filter baghouses or electrostatic precipitators with injected reagents: These systems consist of a chemical reaction step, in which a reagent is added to the gases, and a separation step, which can be performed with a fabric filter baghouse (BF) or an electrostatic precipitator (ESP).

Figure 12 Process diagram of a cascade-type packed bed adsorber with peeling drum. Reproduced from IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007. http://eippcb.jrc.es.

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Figure 13 Schematic view of a honeycomb module adsorber system. Reproduced from IPTS European Commission. Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007. http://eippcb.jrc.es.

The reagent is injected into the flue that leads the gases from the kiln stack to the (BF or ESP) separator. The most common reagents are Ca(OH)2, NaHCO3, CaCO3, and Na2CO3 used in the form of micronized dry solids. This reagent is sprayed into the flue by a pneumatic system. It is important to feed the reagent in before the separator, in order to assure the necessary contact time between the gas phase and the solid phase. Baghouses (the most widely used system) need to be designed to work at a lower temperature than the kiln gas exit temperature. When required, these gases can be cooled to ambient temperature by dilution with air or by an air–air heat exchanger. Fabric filter baghouse design for this emission source needs to take into account, in particular, the high temperature of the gases to be treated. This will mainly condition the filter operating mode and type of filter bag used. The material of which the filter bag is made needs to be able to withstand alkali and acid conditions, while the gas input temperature into the system must also be defined. Electrostatic precipitators can be used instead of baghouses, using the same reagent injection system. This system has the advantage of being able to run at higher gas stream temperatures (above 400
C) without it being necessary to cool the gas stream before it is fed into the cleaning system. In addition, the system enables gas stream energy to be reused in other production process stages. Nevertheless, contact between the adsorbent and the pollutant is not as good as when fabric filter baghouses are used. As a result, baghouses are the most widely used systems and allow the dimensions of the input flue to be minimized Figures 14 and 15.

Figure 14 Schematic illustration of dry flue-gas cleaning with a fabric filter baghouse. Source: ITC. Reproduced from Blasco, A.; Escardino, A.; Busani, G.; Monfort, E.; Amorós, J. L.; Enrique, J. E.; Beltrán, V.; Negre, P. Tratamiento de Emisiones Gaseosas, Efluentes Lı´quidos y Residuos So´lidos de la Industria Cera´mica; Instituto de Tecnología Cerámica-Asociación de Investigación de las Industrias Cerámicas: Castellón, 1992 (in Spanish).

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Figure 15 Schematic illustration of dry flue-gas cleaning with an electrostatic precipitator. Source: ITC. Reproduced from Instituto de Tecnología Cerámica, Valencia. Guı´a de Mejores Te´cnicas Disponibles para el Sector de Fabricacio´n de Baldosas Cera´micas en la Comunitat Valenciana; Centro de Tecnologías Limpias, 2010 (in Spanish).

Table 16

HF, SO2, SO3, and particle cleaning efficiency in the firing stage using the systems described

Cleaning system

Packed bed

Packed bed

Baghouse

Baghouse/ESP

Adsorbent

CaCO3

CaCO3-based granulate

Micronized Ca(OH)2

Micronized NaHCO3

<99 <20 <80 <50 <99

<99 <85 <85 <50 <99

<99 <80 <90 <85 <99

<99 <99 <99 <89 <99

2.5 1

2.5 1.5–2.5

2.0 1.5–2

1.2 8–10

Cleaning efficiency (%)

HF SO2 SO3 HCl Particles

Stoichiometric ratio Relative cost

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es and own elaboration.

Table 17

Concentration levels of the main acid pollutants in the ceramics firing stage without flue-gas cleaning a C (mg m3 N )

Product

HF

HCL

SOx

NOx

Bricks and roof tiles Nonbasic refractory products Ceramic tiles Household ceramics Sanitary ware (tunnel kilns) Technical ceramics (shuttle kilns during firing of electrical insulators)

1–20 0.4–2.5 5–60 1–35 1–30 >120

1–120

10–500 260–490 1–300 – 1–100 –

20–200 25–200 5–150 13–150 10–50 20–120

20–150 – 1–25 –

mg m3 N : Concentrations refer to dry flue gas at 18 vol.% oxygen and normal conditions (273 K and 1013 hPa).

a

Data drawn from: Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry, 2007, http://eippcb.jrc.es. Values obtained from experiments conducted at different ceramic manufacturing facilities.

Table 16 summarizes the attainable efficiency of the different cleaning systems described above for different pollutants (HF, SO2, SO3, HCl, and particles) in the firing stage as a function of the type of adsorbent used, as well as the necessary stoichiometric ratio and the relative cost of the adsorbent. Finally, Table 17 details the concentration levels of the main acid pollutants (HF, HCl, SOx, and NOx) emitted during the firing stages of different ceramics.

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8.05.3.2

93

Noise Emissions

Noise emissions occur in various ceramic manufacturing process stages, basically from noise-producing machinery and noisy working procedures, though much noise is not really ceramic sector specific. Sound emissions can often be reduced by directly applying measures at the noise source. Major noise sources are, for example, pneumatic filter-cleaning systems, compressors, and preparation and handling unit engines. Noise protection can be achieved by enclosure of the noisy units or by buildup of noise protection walls. Double walls or sheathing in a double-shelled construction is very efficient, because the air between the first and the second leaf assures greater noise protection. Other noise abatement measures at the units include the use of silencers at the noise source and the replacement of fast-rotating fans with larger slowerrotating fans. The way workers operate also affects noise emissions. Gates should be closed if there is no steady through traffic, while cautious driving of trucks and forklifts at the facility also reduces noise emissions. Regular maintenance by greasing, as well as timely replacement of silencers, also leads to noise abatement. For further specific information, please see (2).

8.05.3.3

Water Consumption and Wastewater Generation

8.05.3.3.1

Water Consumption

Water plays a key role in the ceramics industry, as it is an indispensable element for a series of processes. Water use in the process may be divided into four large groups, each use in turn determining water’s ultimate destination: l

As a raw material in preparing ceramic compositions (wet milling, wetting when dry milling is involved), glaze preparation, and product wetting for subsequent glaze application. l In reducing fugitive particle emissions (produced by transport across unpaved areas and outdoor storage of dusty materials) and channeled emissions (wet gas-cleaning systems). l As a coolant, mainly for cooling hydraulic oils in the pressing operation, machining of fired ceramics, compressor cooling, and so on. l As a washing agent for raw materials preparation and glaze preparation and application facilities. Table 18 summarizes the main uses of water in each ceramic manufacturing process stage and water’s final destination.

8.05.3.3.2

Emission Data and Recycling of Wastewater

Water consumption in ceramic compositions preparation (wet milling, wetting after dry milling), glaze preparation, and product wetting for subsequent glaze application hardly produces any wastewater, as this is practically all released into the air by evaporation in the different process stages. The amount of washing water produced in the wet preparation section for ceramic compositions is relatively small and can be recirculated in the raw materials milling stage. In contrast, the washing water from the glaze preparation and application sections is the main source of wastewater in the ceramics industry. This water is usually turbid and colored, owing to the very fine suspended glaze and clay mineral particles. From a chemical viewpoint, such washing water is characterized by the presence of: l

Suspended solids: clays, frit residues, insoluble silicates in general. Dissolved anions: sulfates, chlorides, fluorides, and so on. l Dissolved and/or suspended heavy metals, mainly Pb and Zn. l

Table 18

Breakdown of the main uses of process water and water’s final destination in the ceramics industry

Process stage

Use of water

Wastewater destination

Raw materials handling

Irrigation of unpaved areas and outdoor raw materials

Discharge Air emission (evaporation)

Wet milling

Raw material Washing agent

Reuse in raw materials preparation

Dry milling

Raw material

Air emission (evaporation)

Raw materials dryers and spray dryers

Washing agent Wet gas-cleaning systems

Air emission (evaporation) Reuse in raw materials preparation

Forming

Hydraulic oil coolant

Reuse in closed circuit cooling towers

Glaze preparation and application

Raw material Washing agent

Reuse in raw materials preparation Washing agent after a physical and chemical treatment Discharge after cleaning

Machining of fired ceramics

Coolant

Reuse in closed circuits (after cleaning)

Data drawn from: Blasco, A.; Escardino, A.; Busani, G.; Monfort, E.; Amorós, J. L.; Enrique, J. E.; Beltrán, V.; Negre, P. Tratamiento de Emisiones Gaseosas, Efluentes Lı´quidos y Residuos So´lidos de la Industria Cera´mica; Instituto de Tecnología Cerámica-Asociación de Investigación de las Industrias Cerámicas: Castellón, 1992 (in Spanish) and Instituto de Tecnología Cerámica. Guı´a de Mejores Te´cnicas Disponibles para el Sector de Fabricacio´n de Baldosas Cera´micas en la Comunitat Valenciana; Centro de Tecnologías Limpias: Valencia, 2010 (in Spanish).

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Table 19 Average nontreated wastewater composition in glazed ceramic tile manufacturing facilities Characteristics

Variation range

pH Suspended material (g l1) Chlorides (mg l1) Sulfates (mg l1) Calcium (mg l1) Magnesium (mg l1) Sodium (mg l1) Potassium (mg l1) Iron (mg l1) Zinc (mg l1) Lead (mg l1) Boron (mg l1) Chromium (mg l1) COD (mg l1) BOD5 (mg l1)

7–9 1–150 100–1000 100–1000 50–500 10–100 50–500 1–50 <0.05–10 0.1–50 <5 0.5–15 <2 100–2000 10–200

Data drawn from: Blasco, A.; Escardino, A.; Busani, G.; Monfort, E.; Amorós, J. L.; Enrique, J. E.; Beltrán, V.; Negre, P. Tratamiento de Emisiones Gaseosas, Efluentes Lı´quidos y Residuos So´lidos de la Industria Cera´mica; Instituto de Tecnología Cerámica-Asociación de Investigación de las Industrias Cerámicas: Castellón, 1992 (in Spanish) and Instituto de Tecnología Cerámica. Guı´a de Mejores Te´cnicas Disponibles para el Sector de Fabricacio´n de Baldosas Cera´micas en la Comunitat Valenciana; Centro de Tecnologías Limpias: Valencia, 2010 (in Spanish). Values obtained from experiments conducted at different ceramic tile manufacturing facilities.

l l

Boron. Organic matter: screen printing vehicles, glues, fluidizers, and the like used in glazing operations.

The concentrations of these elements will depend on the type and composition of the glazes involved and on the volume of water used. Reuse of this wastewater in raw materials preparation and/or as a washing agent may require pretreatment. Table 19 presents general guidance on the average nontreated wastewater composition in glazed ceramic tile manufacturing facilities. The variation ranges of certain parameters are very wide, for the reasons indicated previously. Finally, the water used as a heat exchange vehicle in ceramics-forming operations and in mechanical treatments (polishing, edgegrinding, beveling, etc.) can be recycled in closed circuits, after simple cooling and/or cleaning operations.

8.05.3.3.3

Wastewater Treatments

The targets set with regard to water mainly involve optimizing water consumption in order to minimize wastewater production and, hence, minimize wastewater discharge. In order to achieve these objectives, primary measures (good practices) and/or secondary measures (wastewater treatment systems) can be applied. Primary measures for process optimization include: l

Acting on the water circuit, installing automatic valves that prevent water leaks. Installing in-plant high-pressure cleaning systems. l Separating water collection from the different processes. l Reusing wastewater in the same process, in particular, repeating cleaning-water reuse after appropriate treatment. l

When needed, secondary measures can be applied: An appropriate cleaning treatment must be chosen. The most common wastewater treatment techniques in the ceramics industry involve physical sedimentation and homogenization processes, followed by chemical neutralization, coagulation, and flocculation stages. The resulting type of facility is highly suited for wastewater treatment in the ceramics industry and provides great versatility in the order of the elements that compose it, yielding efficiencies above 90% for suspended material and the major heavy metals (such as Zn and Pb). However, depending on the type of product made, organic matter content and boron concentrations in the wastewater may sometimes be significant and, consequently, require specific treatments. Boron is the more troublesome of the two pollutants because it is expensive to remove owing to the high solubility of boron and boron compounds. Current options for boron removal are based on the use of ion exchange resins and reverse osmosis processes. These treatments can be applied in all ceramics industry subsectors, though this is not the only possible order and will change in order to match the nature of the water to be treated, as well as the type of process that is producing the discharge at issue and its flow volume. The following aspects need to be considered in regard to the ultimate destination of the wastewater: l

If the wastewater is to be reused in wet milling the raw materials, a previous cleaning treatment may not be needed, though it is recommended that the wastewater be homogenized in order to keep the main characteristics of the suspension. l If the wastewater is to be used as cleaning water or discharged, a pretreatment is required.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

8.05.3.4 8.05.3.4.1

95

Waste Waste Characterization

If urban-type refuse (from company dining facilities, administration services, garden upkeep, sanitary water treatment sludge, etc.) is excluded, the waste produced in the ceramic manufacturing process may be divided into two large groups: Nonhazardous waste The largest amount of waste produced in all ceramic manufacturing process stages is nonhazardous waste. The following types of waste may be noted: l l l l l

Residues of raw materials and additives, gas-cleaning waste, and unfired ceramic scrap containing no hazardous compounds (before the firing process). Finished product scrap of products nonconforming to specifications or standards (after the firing process). Waste from auxiliary services to the manufacturing process and from maintenance, such as scrap iron, rollers, refractory elements, and abrasives. Packaging waste (cardboard, plastics, wood, etc.). Sludge and aqueous suspensions that contain ceramic materials.

Hazardous waste Hazardous waste is waste that, owing to its characteristics, can entail risks for the environment or for individuals. It therefore requires special treatment, as well as continuous control with regard to transport and disposal. Hazardous waste is produced in process stages that use raw materials containing substances that can make waste hazardous. The following may be noted: l l l l l

Residues of raw materials and additives, gas-cleaning waste, and unfired ceramic scrap containing hazardous compounds (before the firing process). Materials containing hazardous substances that are deposited in water collection channels (inside the factory). Sludge containing hazardous substances from process wastewater treatment. Waste packaging that has contained hazardous products. Used oils and other general maintenance waste (spent batteries, used filters, etc.).

Table 20 summarizes the main types of waste that can originate in the ceramic manufacturing process, in terms of the different process stages.

8.05.3.4.2

Recycling, Reuse, and Disposal

Various approaches for preventing or reducing solid waste production in the ceramic manufacturing process are as follows: l

Recycling of raw materials recovered in different production stages in the same process, before the materials have undergone any type of treatment (heating, mixing with additives, etc.) that could modify their properties. l Reuse of unfired waste that can be readily recycled as raw material.

Table 20

Classification of the main types of waste produced in the ceramics industry Waste production

Process stage

Nonhazardous waste

Raw materials preparation Forming Glaze preparation Glaze application Firing Sorting Packing and palletizing Maintenance Laboratory

X X X X X X X X X

Hazardous waste Xa Xb Xb Xa

X X

a

Used oils. The classification as hazardous or nonhazardous depends on the nature and concentration of the raw materials used.

b

Data drawn from: Blasco, A.; Escardino, A.; Busani, G.; Monfort, E.; Amorós, J. L.; Enrique, J. E.; Beltrán, V.; Negre, P. Tratamiento de Emisiones Gaseosas, Efluentes Lı´quidos y Residuos So´lidos de la Industria Cera´mica; Instituto de Tecnología Cerámica-Asociación de Investigación de las Industrias Cerámicas: Castellón, 1992 (in Spanish) and Instituto de Tecnología Cerámica. Guı´a de Mejores Te´cnicas Disponibles para el Sector de Fabricacio´n de Baldosas Cera´micas en la Comunitat Valenciana; Centro de Tecnologías Limpias: Valencia, 2010 (in Spanish).

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Table 21

Specific waste production and degree of recycling in Italy and Spain

Waste

kg m2

kg ton1

Degree of internal recycling

Unfired waste Fired scrap Glazing sludge

0.60–1.10 0.35–0.80 0.1–0.2

30–55 18–40 6–12

100% 30–50% 100%

Data drawn from: Enrique, J. E.; Monfort, E. Situación actual y perspectivas de futuro de los residuos de la industria azulejera. Cera´mica Informacio´n 1996, 221, 20–34 (in Spanish); Moreno, A.; Enrique, J. E.; Bou, E.; Monfort, E. Sludge Reuse in Glazes and Engobes. Cfi Ber. DKG, 1996, 73 (4), 209–214; and Assopiastrelle, Rapporto integrato 1998: ambiente, energia, sicurezza-salute, qualita`, Sassuolo, 1998 (in Italian).

l l

Reuse of fired ceramic scrap (in the same process or in other industries). Reduction of fired product losses: introducing automatic control measures in the kilns and optimizing charge distribution.

On the other hand, the sludge and suspensions produced as a result of the cleaning operations in the glaze preparation and application sections may be treated in different ways: They can be valorized either in the same system or in the fabrication of other products. Much of this waste is valorized by use as raw material in certain production process stages, particularly in the preparation of ceramic compositions by dry or wet milling or in the mixing stage. An important reduction in raw materials consumption is thus achieved, while simultaneously decreasing industrial impact. Sludge and suspension valorization consists of reusing the aqueous suspensions and sludge that contain ceramic materials in the ceramic manufacturing process, adding them to the raw materials when the raw materials are wet milled, as they can be used either directly or after a physical or physicochemical treatment. When a dry raw materials preparation process is involved, the sludge needs to be dried beforehand. The average specific production of each type of waste and the degree of waste reuse in the same manufacturing process are detailed in Table 21 (19–21). The data come from the Italian and Spanish ceramic industries. As the table shows, the greatest difficulties lie in recycling fired ceramic scrap, as this poses technical as well as economic difficulties in the milling stage because of scrap characteristics (abrasion, hardness, etc.). This is particularly pronounced in recycling low-porosity fired ceramic scrap, as this requires the use of special milling systems. When ceramics are formed by extrusion, the ensuing fired scrap can also be used as grog.

8.05.4

Occupational Health Issues

8.05.4.1

General Overview

The most important health risks in the traditional ceramics industries (possibly in decreasing order of prevalence of professional diseases) are as follows (22): Ergonomic risks relating to the manual lifting or handling of loads, such as bags of raw materials or boxes of finished products. Physical risks from the handling and circulation of kiln cars or motorized forklifts or pallet trucks, work at great heights, entrances to small spaces, and contact with electric, pneumatic, or mechanical energy sources such as cutting machines, revolving items, gears, shafts, belts, and pulleys. l Exposure to noise at levels that, in certain sections, may reach up to 100 dB(A). Major sources of noise include mills and crushers, fans, compressors, servovalves, vibrators, and other motorized devices. l Respiratory exposure to chemical agents, usually in particulate form, used in production. l l

This chapter briefly describes the specific occupational health risks associated with the traditional ceramics industry in regard to dust and respirable crystalline silica, and lead.

8.05.4.2 8.05.4.2.1

Prevention of Specific Occupational Risks in Ceramic Processes Dust

8.05.4.2.1.1 Exposure In the traditional ceramics industries, raw materials are used that are usually found in particulate form. Raw materials handling or processing can, therefore, give rise to airborne particulates and ensuing worker exposure. The most important operations in which exposure can occur are as follows (23): l

Raw materials preparation. The handling of particulate raw materials can produce airborne dust: for example, unloading and transfer of raw materials, stockpiling and hopper loading with power shovels, primary grinding, and automated raw materials transport (conveyor belts, bucket elevators, etc.) to subsequent production stages. l Milling/dispersion of the compositions. The finest materials tend to disperse in the ambient during the grinding/disperser millcharging step in wet processes, and in charging and discharging in dry milling.

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Semi-dry forming. Airborne dust can be produced during the various forming steps (sieving, die filling, demolding, etc.) and in the brushing of shaped products. l Glazing. The glazing techniques that can produce most airborne particulates are airbrushing and disk glazing, dry glaze application, and fettling (removal) of unfired glaze from parts of the product that are not to be glazed. These operations are sometimes performed by hand. l Cleaning and maintenance of the facilities. l

Airborne particulates are usually classified in terms of particle fractions that tend to be deposited in certain areas of the respiratory tract: l

Inhalable fraction: mass fraction of the total aerosol inhaled through the nose and mouth. Thoracic fraction: mass fraction of the inhaled particles that penetrate beyond the larynx. l Respirable fraction: mass fraction of the inhaled particles that penetrate into the nonciliated respiratory system. l

Samplers of the respective fractions shall conform to the specifications established by agreement and defined in standard ISO 7708:1995. These specifications are used to define the occupational exposure limits to particulate chemical agents. Occupational exposure limits are reference values for the assessment and control of risks inherent to exposure, mainly by inhalation, to chemical agents in the workplace. Every country or region usually has its own exposure limits to chemical agents. However, the threshold limit values (TLVs) proposed by the American Conference of Governmental Industrial Hygienists (ACGIH) enjoy worldwide prestige and are usually used as references in this sense. The TLV 8-h time-weighted average (TWA) proposed by the ACGIH for inhalable dust is 10 mg m3, and for respirable dust 3 mg m3 (24). 8.05.4.2.1.2 Health effects The airborne particles considered to be of particular relevance for health are those that can reach the pulmonary cavities (respirable fraction). If the inhaled particles reach the lung cavities, they will be captured by so-called macrophage cells, this being a particular case of a fundamental biological process known as phagocytosis. While some particles are quickly solubilized, others are practically insoluble, leading to particle buildup in the lung tissue and the formation of fibrous tissue: that is, walling-off of the pulmonary region in which gas exchange takes place with the cells, with the ensuing breathing difficulties. The pulmonary fibrotic reaction caused by exposure to dust is clinically termed pneumoconiosis. Crystalline silica is the main cause of most pneumoconiosis, owing to its pathogenic power and abundance in the earth’s crust. As a result, the term silicosis is often inappropriately used in referring to any pneumoconiosis, independently of the actual cause. However, particulates other than silica, such as coal dust, can also produce pneumoconiosis (25). Chronic obstructive pulmonary diseases (COPDs) are also dust-related occupational lung diseases (26). Simple pneumoconiosis can degenerate further into progressive massive fibrosis (PMF) (27). 8.05.4.2.1.3 Control Control techniques of exposure to airborne particulates in traditional ceramics industries essentially involve measures aimed at preventing exposure to any chemical agent. This section briefly sets out the most important preventive measures. A more extensive compilation may be found in reference (23). 8.05.4.2.1.3.1 Containment of operations Many operations are automated in the traditional ceramics industries and require no operators. All automatic operations that produce airborne particles (e.g., elevators, conveyor belts, and glaze sprayers) should be enclosed. Enclosure may sometimes entail technical difficulties, such as moisture condensation when hot, moist particulates are being conveyed (e.g., freshly produced spray-dried granules). 8.05.4.2.1.3.2 Localized extractions All operations in which airborne particulates are produced need to be fitted with localized dust extractions. This involves installing dust extraction networks in the raw materials preparation, forming, glaze preparation, and glazing sections. The particles present in the extraction streams need to be cleaned before emission, as set out in foregoing sections. 8.05.4.2.1.3.3 General ventilation Together with the localized dust extraction systems, it is advisable to have good general ventilation, using forced ventilation if necessary. The ventilation system must not cause settled dust to be stirred up and must ensure that contaminated air does not spread to clean areas. Dust suppression sprays (sprinklers or fine mist) may be used to prevent the generation of airborne dust and promote dust settling. 8.05.4.2.1.3.4 Wet processes From an occupational health viewpoint, wet processes are preferred to dry processes because they usually entail less dust exposure. Consequently, when a given ceramic product can be made by either process route, this should be an additional factor to be taken into account in process design.

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Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

8.05.4.2.1.3.5 Maintenance and cleaning Good maintenance and cleaning are basic measures that can minimize dust emissions in the workplace. Cleaning should be carried out regularly using either dry cleaning methods, which involve vacuum cleaning of the dry dust, or wet cleaning methods, which prevent fine dust from becoming airborne by trapping it in water. 8.05.4.2.1.3.6 Personal protection The use of individual protective gear to control dust exposure (filter masks) should be limited to cases in which it has not been possible to control exposure by any other means. Such cases may involve certain manual operations, such as the unloading of bags, in which personal protection may be needed to complement localized dust extraction. Certain auxiliary maintenance operations may also require the use of respiratory protection.

8.05.4.2.2

Respirable Crystalline Silica

8.05.4.2.2.1 Exposure In the traditional ceramics industries, quartz is widely used as a raw material, either as a natural clay constituent or as an added individual raw material. Quartz plays a key role in all ceramic manufacturing process stages (plasticity control, green mechanical strength, firing shrinkage, etc.). It is, therefore, an irreplaceable raw material. As a result, occupational environments of traditional ceramics industries can contain appreciable quantities of quartz. Respirable crystalline silica (RCS) is the term used to refer to the silica in crystalline form (quartz or cristobalite) in respirable dust. The TLVs for RCS have varied considerably in recent years. In 1986 the ACGIH adopted a value of 0.1 mg m3 for quartz and 0.05 mg m3 for cristobalite as TLV–TWA for RCS exposure (28). In 2001 a single TLV–TWA was put forward for quartz and cristobalite of 0.05 mg m3 (29). Finally, in 2006 the TLV–TWA for quartz and cristobalite was further reduced to 0.025 mg m3 (30). If these values are compared to those indicated previously for respirable dust, and it is taken into account that quartz is an essential component in ceramic compositions, the TLVs for RCS are clearly much more restrictive values for the traditional ceramics industries. The results of the evaluation of RCS exposure obtained in European traditional ceramics industries in the period 2003–2007 are detailed for guidance in Table 22 (31). The table shows that the most critical stages with regard to RCS exposure are those in which ceramic compositions with powdered quartz are processed: namely, raw materials preparation and semi-dry forming. It is precisely in these stages that the adoption of such low RCS exposure limits as those proposed by the ACGIH would entail a greater technical and economic impact (32). 8.05.4.2.2.2 Health effects As noted above, the main health effect of exposure to crystalline silica of respirable size is silicosis. A milestone was reached in 1997 when the International Agency for Research on Cancer (IARC) classified crystalline silica in the form of cristobalite or quartz as ‘category 1 carcinogens.’ In making this evaluation, the IARC working group also noted that carcinogenicity was not detected in all industrial circumstances studied and may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity (33). This observation by the IARC has been a source of controversy. Epidemiological data usually use silicosis as an indicator of RCS exposure, a relative risk of lung cancer being found in this case between 1.5 and 6.0. However, in the studies in which it has been attempted to obtain a direct relationship, the association is not as clear (34). Donaldson and Borm (35) reflected on the difficulties encountered by the IARC working group in evaluating the carcinogenicity of crystalline silica, focusing on the differences existing between molecular and particulate toxicants. Particulates never act as a constant entity. Their reactivity in a biological medium depends on the mechanical, thermal, and chemical history of a given dust, as well as on the frequent presence of surface contaminants. Therefore, in contrast for example to benzene, which is a toxicant with

Table 22

Results of the evaluation of RCS exposure in the European traditional ceramics industries RCS concentration (mg m3)

Process stage

Number of samples

Median

Upper quartile

Raw materials preparation Wet forming Dry forming Glazing Firing Sorting, packaging Maintenance

53 43 16 70 30 31 11

0.060 0.037 0.119 0.026 0.008 0.011 0.004

0.110 0.058 0.145 0.052 0.036 0.027 0.073

Data drawn from: Monfort, E.; Ibañez, M. J.; Escrig, A.; Jackson, P.; Cartlidge, D.; Gorbunov, B.; Creutzenberg, O.; Ziemann, C. Respirable Crystalline Silica in the Ceramic Industries. Sampling, Exposure and Toxicology. cfi/Ber. DKG 2008, 85 (12), 36–42. Personal samplings were performed at European ceramic tile, brick and roof tile, tableware, sanitary ware, and refractories manufacturers.

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

99

only one form, each source of crystalline silica dust has its own carcinogenic potential, which can be largely modified by even slight alterations of the state of its surface (36). Samples of respirable dust collected in the workplace environments of traditional ceramic industries were shown to display lower biological activity than the positive control, which consisted of DQ12 quartz (with proven fibrogenic activity) (37). Independently of the controversy regarding RCS carcinogenicity, there is generally deemed to be sufficient information to conclude that the relative risk of developing lung cancer increases in people who have silicosis. Therefore, preventing silicosis reduces the risk of lung cancer (38). 8.05.4.2.2.3 Control Control measures for RCS exposure, as an airborne dust constituent is involved, are basically the same as those described for airborne dust.

8.05.4.2.3

Lead

8.05.4.2.3.1 Exposure Lead exposure in the ceramics industry is mainly associated with glazing/decoration of ceramic ware, as lead is principally used in frit formulation, though it is also used for certain ceramic pigments. Lead frits are characterized by their high fusibility, which enables fired glaze coatings to be obtained with a characteristic gloss. This property allows lead to be used in fritted glaze compositions for low-temperature decorations (third fire, flat and hollow glass, etc.), as well as in developing particular colorations and finishes (leather-type glazes, embossings, etc.). As a result, lead is still extensively used in the ceramic sector, though its use has decreased in recent years owing to the development of alternative, lead-free formulas (39). As noted, the workplaces with the greatest risk are those associated with glaze and ink preparation and application operations. The main entry path is the respiratory route, though in this case the oral route can be significant if good habits of personal hygiene are not kept. A TLV–TWA of 0.05 mg m3, measured as Pb, is recommended for occupational exposure to elemental lead and its inorganic compounds (24). In the case of lead, environmental control is usually accompanied by biological control: for lead, blood concentration levels are specified that must not be exceeded. Some countries have specific regulations for lead. 8.05.4.2.3.2 Health effects Lead poisoning or saturnism, caused by occupational exposure to lead, is characterized by numerous symptoms (40): l l l l l

Neurological effects. Lead exposure can damage the central nervous system, resulting in neurological disorders and injuries to the peripheral nervous system, mostly involving motor effects. Hematological effects. Lead inhibits the body’s ability to make hemoglobin, by interfering with several enzymatic steps in the metabolic pathway of the heme group. This causes anemia, altering oxygen transport to the blood and, hence, to other body organs. Endocrine effects. There is a clear anticorrelation between blood lead levels and vitamin D levels. Lead is thus likely to impair growth, cellular maturity, and bone and teeth development. Renal effects. One direct effect of prolonged exposure to lead is renal insufficiency. Studies have demonstrated the link between lead exposure and hypertension, an effect that can be mediated by renal mechanisms. Effects on reproduction. Lead can be transmitted from the mother to the fetus by placental transfer, which can cause miscarriage as well as increased mortality and disease in newborn babies.

In addition, exposure to lead has been shown to cause cancer in laboratory animals. Moreover, epidemiological studies report a significant increase in several types of cancer (stomach, lung, and bladder cancer). The foregoing led the IARC to classify inorganic lead compounds as probably carcinogenic to humans (Category 2A) 2006 (41). 8.05.4.2.3.3 Control The most efficient way of preventing exposure to lead is, obviously, by avoiding the use of lead-containing glaze compositions and pigments, using lead solely for finishes unattainable by other means. As noted, this tends to be the approach adopted in the traditional ceramics industries. The formulation of lead-free frits has involved replacing lead with other vigorous fluxes, in particular, boron (42). Recently, however, studies have shown that boron compounds can be toxic for reproduction (43), so that the coming years will foreseeably witness the implementation of measures to control the risks associated with the use of boron. In the traditional ceramics industries, lead compounds are also found in particulate form. The measures required to control exposure to lead are, therefore, the same as those for airborne dust. In addition, with a view to preventing exposure by swallowing lead dust, the prohibition to eat, drink, or smoke in occupational environments where lead compounds are handled is a basic preventive measure, as is keeping good habits of personal hygiene.

8.05.5

Summary

The energy-saving measures, best available techniques, and occupational risk controls described in this review have been summarized in Tables 23–25, with a view to facilitating their application and making the ceramics industry more environmental and health friendly.

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Summary of energy-saving measures

Table 23

Process facility

Energy-saving measure

Type of technology

Efficiency

Cost

Spray dryers

Increasing suspension solids contents Raising drying-gas temperature Heat recovery facilities in spray dryers from cogeneration systems Heat recovery facilities in spray dryers from other process facilities Optimization of drying-gas flow rate Reduction of water content in the shaped product Heat recovery facilities in dryers Electric dryers Optimization of the ceramic body composition Reducing product thickness Optimization of the kiln charge Reduction of energy losses through kiln surfaces Reduction of combustion air flow rate Raising combustion air temperature Electric kilns New compositions to reduce both firing temperature and firing cycle Heat recovery from kilns to produce electricity New fuels from biomass: biogas

Widespread Widespread Not widespread Not widespread Widespread Widespread Not widespread Breakthrough technology Widespread Not widespread Widespread Widespread Not widespread Not widespread Breakthrough technology Breakthrough technology Breakthrough technology Breakthrough technology

*** ** *** ** ** ** *** * * *** *** ** *** *** * ** * ***

* * *** *** ** * ** *** * * * ** * ** *** ** *** ***

Shaped product dryers

Kilns

Legend: * low; ** medium; *** high.

Summary of best available techniques

Table 24 Emissions

Best available techniques

Efficiency (%) or C (mg m3 N )

Dust

Reducing diffuse dust emissions by applying a combination of the following techniques: l Measures for dusty operations l Measures for bulk storage areas l Measures for dusty materials transport Bag filters Primary measures and techniques: l Reduction of the input of pollutant precursors l Optimization of the firing curve l Reduction of water vapor in the kiln gases l Selection of the fuel Secondary measures and techniques: l Cascade-type packed bed adsorber (reagent: CaCO3) l Bag filter or electrostatic precipitator (reagent: Ca(OH)2/NaHCO3) l Reducing water consumption by applying process optimization measures l Cleaning process wastewater by applying process wastewater treatment systems l Reducing the emission load of pollutants in wastewater discharges Recycling and reuse of solid waste by applying one or a combination of the following techniques: l Recycling systems l Reusing waste in the same process or in others l Reducing fired product losses

Closed facilities and paved areas: >95% Semi-closed facilities and paved areas: 75–80%

Diffuse

Channeled Gaseous Compounds

Wastewater

Waste

<5 mg m3 N Cascade-type packed bed adsorbers: HF: <99%; SO2: 20–85%; SO3: 80–85%; HCl < 90%. Bag filter or electrostatic precipitator: HF: <99%; SO2: 80–99%; SO3: 90–99%; HCl <90%; HCl 85–89%.

Applying process wastewater treatment systems: Suspended material and the major heavy metals (such as Zn and Pb): >90%

Unquantified

Ceramic Manufacturing Processes: Energy, Environmental, and Occupational Health Issues

Table 25

101

Summary of the control of specific occupational risks in ceramic processes

Risk agent

Control measure

Efficiency

Cost

Dust, respirable crystalline silica, and lead

Containment of operations Localized extractions General ventilation Wet processes Maintenance and cleaning Personal protection Good personal habits Substitution of lead compounds

*** *** * Variable * ** ** ***

*** *** ** Variable * * * Variable

Lead Legend: * low; ** medium; *** high.

Acknowledgments This study was supported by the Instituto Valenciano de las Pequeñas y Medianas Empresas (IMPIVA), through the ‘Programa de Desarrollo Estratégico,’ by funding project IMDEEA/2011/109 – IMDEEA/2012/138.

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