Coal waste: handling, pollution impacts and utilization

3 Coal waste: handling, pollution impacts and utilization P. FECKO, Formerly with VSB-TU, Czech Republic, B. TORA, AGH University of Science and Technology, Poland, and M. TOD, RecyCoal, UK

DOI: 10.1533/9781782421177.1.63 Abstract: In 2008, 84 million tons (Mt) of hard coal was extracted in Poland and although there has been a subsequent decline, it remains a major source of high quality coal for domestic power generators and steel producers and its neighbours in Europe and beyond. In the extraction of the valuable coal products, the production of 1 tonne of hard coal generates 0.4 tonne of extractive ‘waste material’, comprising waste rock including lost coal, and washery rejects and tailings, both containing economically recoverable coal. This chapter focuses on the situation in Poland and neighbouring countries, describing efforts to recover this lost coal, but also serves to provide a good insight into similar situations in other major coal producing countries. A case study for a successful project recently completed by the RecyCoal company in the UK is included as the conclusion to this chapter. Key words: coal wate utilisation, waste to fuel, land reclaimation, coal waste management

3.1

Introduction

Poland is still one of the major producers of hard (bituminous) coal in the world and is currently still the largest producer in the EU.1 In the 1980s, annual production was about 200 Mt, but from 1990 onwards, hard coal production in Poland started to decrease, so that by 2008 it was only 84 Mt. At the present it is close to 80 Mt and there are only 40 hard coal mines in Poland, while in 1980s there were 70 mines. Observed at present, the dynamic of the decline of coal prices and the increasing production costs have led to deterioration of financial outcomes for coal producers resulting in a corresponding decline in hard coal extraction. However, coal remains a key contributor to the EU’s security of energy supply and will probably remain so for decades to come. Coal represents the fossil fuel with by far the 1

Sections 3.1 to 3.4 were contributed by Peter Fečko and Barbara Tora. Section 3.5 was contributed by Mike Tod.

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largest and most widely distributed global reserves, estimated to last for at least 100 years in Poland (Poland, 2007). Coal is expected to continue supplying about a quarter of global primary energy needs. As global primary energy consumption increases by 60% in the next 20 years, so will the use of coal (European Commission, 2006a). In the extraction of the valuable coal products, typically the production of 1 tonne of hard coal generates 0.4 tonne of extractive ‘waste material’, comprising waste rock including lost coal, and washery rejects and tailings, both containing economically recoverable coal. This chapter focuses on the situation in Poland and neighbouring coal-producing countries, describing efforts to recover this lost coal, but also serves to provide a good insight into similar situations in other major coal-producing countries such as Australia, the US, South Africa and the UK as well as the emerging economies of India and China where research and development in this field is intensifying. The chapter illustrates the issues, problems and potential solutions that are applicable to the wider arena for recovering ‘lost energy’ and other commercially usable materials from waste coal sources. In the Polish case, the focus is on fine coal losses which represent the major source of loss. Several of the treatment/processing approaches have been described in other chapters in this book and these sources are referenced as appropriate. In coal mining and utilization operations, the amount of fine-grained waste containing potentially recoverable coal, coke, coke breeze, etc., despite being significant in quantity, is currently not being visibly reduced. The group of fine-grain fuel sources includes the following: • coal slurries (i.e. suspensions of coal flotation tailings or other types of coal fines in water), including sediments; • dust arising from brown coal waste; • petroleum coke breeze from gasification processes of residues from crude oil processing; • fine coal residue from fluidized bed gasification of solid fuels; • coke breeze from decarburization of fly ash; • coke dust waste from production and processing of carbon and graphite products. Particularly significant are the quantities of coal slurries generated from the current hard coal mines and in-ground settling ponds, or settlers previously handed over to local authorities as reclaimed land. The available reported quantities of coal slurries vary widely and range from 0.5 million in situ tonnes at recently reported (2011) production levels to over 20 million in situ tonnes (mainly accumulated in operational and reclaimed settlers). The use of these fine-grain carbon sources and associated waste materials often encounters serious difficulties, inter alia due to such things as

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high moisture content, heterogeneity, fragmentation, and low and variable energy characteristics. In recent years, with the objective of rationalizing the use of such materials, some new technologies have been extensively developed and implemented for the economical recovery of fine-grain materials and waste to produce commercially viable fuels. This chapter will explain some of these initiatives and provide evidence that such practices can be economically implemented provided an appropriate approach is adopted. Options for the use of coil spoil are also discussed. A case study of a successful project recently completed by the RecyCoal company in the UK is included as the conclusion to this chapter.

3.1.1

Classification of mining wastes

Waste from the coal mining industry is generally divided into three groups taking into account their technical characteristics, and the operational and technological processes employed, i.e. • • •

Mining wastes, Tailings, and Secondary processing wastes.

Mining wastes, also known as extraction wastes, overburden or coal spoil (CS), are rocks and minerals generated from the mining and preparation plants resulting from opening up the main mineral deposit, composed mainly of cap and interlayer rocks. They represent an average of about 20% of the total mass of waste (Hycnar and Bugajczyk, 2004). Tailings include finely sized rock and mineral material extracted along with the coal and separated in the beneficiation processes (e.g. sorting, crushing, washing, flotation), and their share in the total mass of waste is on an average ~80%. Tailings, in other words, is the collective term for a slurry containing remnant material from processed fine coal, usually sized below 0.25 mm, after most of the valuable coal components have been extracted. The third group includes secondary processing wastes, i.e. the remnants of the main processing treatment, generated in the production processes of commercial products.

3.2

Potential uses of recoverable materials

The qualitative research conducted in various research centres shows that coal wastes have the quality features appropriate for many applications (Hycnar and Józefiak, 2007), such as:

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• For recovery of coal, as low-energy material (possibly in slurry form) for combustion in power plants; • Manufacture of construction products and refractories; • As a filling and sealing material in various types of engineering works; • In agricultural applications as fertilizer or substrate; • Hydro construction and engineering (e.g. building river dams and embankments of settlers and to strengthen the shipping channels); • Marine engineering (e.g. the construction of embankments, coastal protection and of wharves); • Road engineering (e.g for building road and railway embankments).

3.2.1

Coal spoil

A characteristic feature of mineral waste from hard coal mining, i.e. coal spoil (CS), is its large mineral-petrographic variation. The mineral composition of these waste materials broken down by the dominant petrographic groups is given below in Table 3.1. The main rock types in coal mine spoil are usually clayey rocks (claystone), mudstone and sandstone often with siderite inclusions. Individual Table 3.1 Brief petrographic description of rock spoil Rock

Description

Slate

Sedimentary, detrital rock of Different thickness of black colour, less frequently layers relatively low dark grey colour, with a mechanical strength, slate layered (slated) siltstone and separateness pelite texture Sedimentary, detrital rock of Rocks of varying dark grey to black colour, mechanical strength, solid, with non-directional, depending on the compact, massive, siltstone content of coal texture, occasionally the rock contains siderite nodules (spherosiderite – sedimentary carbonate rock, of grey-brown to brownishyellow colour with highdensity, compact, massive, non-directional texture Sedimentary, detrital rock of Rock with a fairly high light to dark grey, solid, with mechanical strength, non-directional texture, less depending largely on frequently slightly layered, sandstone binder (the compact, massive, psammitic more clayey, the less sometimes mixed with resistance) psephitic fractions

Mudstone

Sandstone

Comments

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rocks are characterized by different physical and chemical properties, which mainly determine their usability. Depending on where CS is obtained, and the per cent share of individual rocks in the total mass of waste, its physical and chemical parameters vary (Hycnar and Bugajczyk, 2004). CS is characterized by the heterogeneous nature of the various rocks of which it is composed. Since in practice the methods of separation of individual lithological forms are not feasible, the properties of CS are determined by the type of mechanical processing used (other methods are not practicable because of their high cost). To improve the quality parameters of coal spoil for its commercial utilization, it is necessary to use coal preparation processes. The purpose of mechanical processing is to eliminate from the product rock veins and impurities, including mainly coal, characterized by worse physical and chemical properties. A characteristic feature of the preparation process is the alternate crushing and sieving of fine fractions. Typically, at least two treatment steps are used for crushing and screening in order to achieve a noticeable improvement of the product properties relative to the raw material (CS). The most extensive CS technological line would consist of three processing steps for crushing and screening processes, i.e., in the first step coal impurities are separated (this product is subject to recovery of coal); in the second step, weaker rocks and mineral veins are separated, which are waste products, while the third step will produce useful aggregate materials. The end result of beneficiation should be that the product meets the requirements of European Standard EN 13043:2002 – Aggregates for bituminous mixtures and surface treatments for roads, airfields and other areas for traffic. The presented results (Hycnar and Bugajczyk, 2004) indicate that, despite thorough mechanical processing, most products are characterized by a low resistance to weathering (mainly frost) and resistance to abrasion. At the same time, a quite high (compared with traditional aggregates) strength to crushing, mainly for sandstones and mudstones, indicates the possibility to use them with limited environmental impact. The results also indicate the appropriateness of mechanical processing in more than one preparation node. A drawback of repeated crushing and sieving is that more waste is generated, which can be used only for reclamation works. CS processing technology must have a sound commercial basis, i.e., be applied in a way that ensures that saleable products are generated. Therefore the selection of raw material to be processed is important, perhaps processing only the material containing fine-grained sandstones and mudstones to recover any coal, aggregates and other useful components. A good example follows with the UK example. Aggregates obtained after processing can be used as road sub-base in its no-frosting zones (or if enriched with other frost-resistant aggregates, such as slags and natural aggregates – both broken and natural – in frost zones) provided they are protected against moisture. Depending on the type of

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lithological material, aggregates obtained can be used in different categories of roads. Processing of deposits with parent rocks such as slate or claystone, contaminated with coal inclusions, currently is not economically viable. Such waste without mechanical processing can be used in auxiliary engineering structures.

3.2.2

Coal tailings

The hard coal beneficiation process in mechanical preparation plants generates coarse, small or fines rejects and coal tailings slurries. The tailings are the finest grain size, with the majority below ~0.25 mm, whereby material sized below 0.035 mm makes up to 60% share in the slurry composition. Depending on the quality parameters (ash and sulphur content, calorific value, etc.), such slurries can be transferred as an ingredient to energy mixtures, or are dumped in earth settlers of individual mines. Most slurries to date have been collected in settlers, as there were no customers interested in buying them at the time they were produced. Dumped slurries were therefore treated as waste from coal preparation processes. Most of this waste is actually a potentially viable energy source. For this reason, in recent years, the interest in combustion options has increased as other fossil energy sources have increased in delivered cost. There is also interest in using coal tailings in construction products and engineering projects. Some coal tailings are transferred to preparation plants for recovery of coal contained in the waste. Currently about 9% of generated waste is utilized in this way. The residue after the recovery of coal is re-dumped or used, for example in hydraulic backfilling or the building construction materials industry. Energy generation from coal tailings is covered in more detail in the sub-section below. Coal tailings are quite commonly used in the manufacture of construction products for the building industry – as an essential raw material for obtaining slate aggregate, i.e., a lightweight building construction aggregate used in the manufacture of lightweight concrete, as well as an essential raw material or component for the production of various building construction elements, such as bricks or roofing tiles. Currently, only about 0.5% of generated waste is utilized in this way. The waste is also added to the charge in the production of cement, in order to adjust the main module of cement clinkers. Coal tailings may also be useful for the production of refractory materials, but only if they have a high content of Al2O3. Attempts have been made to recover metal concentrates from coal tailings, including aluminium, iron, titanium, germanium and gallium. Fine coal waste can also, after mixing with a compound fertilizer and peat, be used for biological reclamation and restoration of the fertility of devastated land, or reclamation of soil.

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Flotation tailings wastes, a specific type of tailings, have not yet found an industrial application due to a number of factors including significant thixotropy, high humidity and difficulties in transport. However, such wastes can be used as a material for filling abandoned workings in mines or to seal the surface stockpiles. Post-flotation wastes from beneficiation of coking coals with calorific value more than 5 000 kJ/kg can be used as fuel for the production of building construction ceramics, and after further beneficiation as an additive to energy fuel.

3.2.3

Energy generation from coal slurries

The most common model for the management of coal slurries is their supply to coal-fired power plants and, to a lesser extent, to industrial energy generation plants. Such a solution, though very simple and practical, is not optimal for environmental as well as economic reasons. If they are supplied in this way, approximately 30% of water is transported, often with heavy contamination of transport routes and user sites. In the case of fluidized bed combustion, a water-slurry with a water content well above 30% is often used. Studies, research and trials undertaken for optimizing the management of fine-grain materials and waste for fuel production include: 1. Qualitative assessment of fine-grain materials as a source of heat energy. 2. Technologies for beneficiation of fine-grain materials as a fuel source for energy processes. While the qualitative assessment processes enable identification of the possibilities and conditions for direct management, beneficiation technologies indicate the possibilities and conditions for their use to generate high-quality stand-alone fuels or components for their production. Technological processes generally provide conditions to influence the quality of fine-grain materials. An example might be the generation of coal slurries from water suspensions; for instance only by replacing the belt presses with chamber filter presses, a greater moisture removal from the slurry is achieved together with an improved energy performance of slurries – Fig. 3.1. Poor quality of coal slurries is also due to the lack of selective separation of water and slurry suspensions in mines, and also often unoptimized operation and emptying of slurry settlers. The dewatering processes selected for fine-grain materials and coal tailings for energy utilization purposes are also a critical step in achieving a suitable end product. In recent years, considerable progress has been achieved in the filtration and subsequent agglomeration of coal slurries, coke dust, coal and biomass blends and alternative fuels for effective briquetting.

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Andritz belt press

Chamber filter press

Qir (kJ/kg)

9000 8000 7000 6000 5000 4000 64

66

68

72 70 Wtr + Ar (%)

74

76

Qir = f(Wtr + Ar); Ar = constant

3.1 Impact of dewatering method of water-slurry suspension on coal slurry properties. (Source: Giemza et al., 2009.)

Regardless of the direction and method of processing, in some cases significant problems are caused by the loosened structure of coal slurries, which determines the performance of coal blending and biomass blends, achieved durability of granulated materials and effectiveness of slurry dewatering processes. This problem has not been fully resolved and still requires further testing and trials.

3.3 3.3.1

Size enlargement, dewatering and drying of coal waste Briquetting and agglomeration

A number of processes have been applied to the treatment and size enlargement of filtered coal and waste slurries. These include briquetting, granulation and agglomeration treatments and have been adapted from successful development and use in other similar industrial activities. Chapter 15 (by Andrew Vince) in Volume 1 of The Coal Handbook includes a more detailed description of these processes and Chapter 13 (by GotzBickert) of Volume 1 includes useful details of the dewatering processes that are used to prepare the slurries for subsequent treatment (see Section 13.3.2). The differences in the form of products obtained are given in Fig. 3.2. The process for caking fine-grain materials and coal tailings should obviously be economically viable and must therefore be selected mainly based on market requirements. Briquetting Process: The most durable and geometrically adjustable caking of fine-grain materials is achieved by the briquetting process. It is important to properly prepare the feed and adjust the type and amount of binder added. As a result of blending coal slurries, sediment and flotation

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Coal waste: handling, pollution impacts and utilization Briquette

71 103 g

103 Granulated coal

50 x 50 mm

Coal agglomerate 104

106

Eco-pea coal

3.2 Caking types of coal slurries granulation and agglomeration. (Source: Giemza et al., 2009.)

concentrate and addition of a specific binder, production process has been established for coal briquettes of calorific value of 16–24 MJ/kg. The size of briquettes is adjusted to the requirements of customers. Similar work has been performed for the briquetting of brown coal, biomass, and selected municipal waste with and without binders. The quality of briquettes obtained was affected most by the pressures applied; this also should explain the highest mechanical strength of briquettes obtained from briquetting stamp presses. Particularly interesting results were obtained by pressing chopped straw; as a result of friction and release of heat from the feed, water evaporates and the concentrated feed is subject to a strong curing with released resin substances. During the period when the price differences between coke dust and metallurgical coke were quite interesting, two technologies were developed for briquetting coke dust, both dry and wet. Coke briquettes of lower mechanical strength (e.g. for lime kilns) were obtained using suitable binders. Coke briquettes suitable for a copper smelting process required not only a binder but also an additional heat treatment step. Granulation Process: A granulation process has been applied to graphite dust caking. Due to the hyrophobicity of the graphite grains it was necessary to test and choose the appropriate binder. Granules with the highest mechanical strengths were obtained as a result of additional thermal treatment. Depending on the binder used, hardening of granules can occur within 180–240°C and at higher temperatures. Granulation of coal slurries and their blends with coal and/or sawdust is a simple process; however, the granulation installations are characterized by relatively low yields.

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Agglomeration Process: In recent years there has been a huge interest in caking coal slurries; the simplest solution has appeared to be the agglomeration process. In cases where a high mechanical strength of grain agglomerates or their specific shape is not required, it is sufficient to intensely mix the feed to cause formation, merging and cross-sticking of grains to form an agglomerated product. Mechanical stability of the agglomerate can be raised through the proper preparation of the feed and the addition of selected binders and additional granulation. The effect of different binders on the grain size of agglomerates, after ageing, is illustrated by the results in Table 3.1. With very damp, clayey coal slurries full relaxation of their structure is not readily achieved and proper mixing with a binder, due to which the resulting agglomerate is characterized by larger grains and lower compressive strength. Agglomerated material likely to be subjected to ageing by weathering is, in most cases, subject to further curing. The year-round experience from stockpiling agglomerates in the stock yard has shown that under the influence of rain and snow and the impact of the sun and changing temperatures the surface of the agglomerate heap develops a shell (2–8 cm thick) that serves to insulate the agglomerate (Fig. 3.3). Under the shell there is a much looser agglomerate, and while the shell layer can be easily crushed it helps to protect the integrity of the agglomerate. Physical and chemical properties of agglomerates depend mainly on the coal slurry used and the type and amount of the binder. Apart from having acquired a ‘gravel’ structure and showing resistance to water and wind erosion, slurries usually display improved energy characteristics. Depending on the slurry used, agglomerates were obtained with the calorific value from 7.6 to 18.2 MJ/kg. Agglomerated material can include coal dusts produced and/or constitute a new independent coal fuel. Binders: Out of the many substances used as binders-adhesives for coal slurries, particularly useful is quicklime (used for decades for briquetting and granulating coal). The addition of quicklime to wet coal slurry influences the process in many ways: First comes the formation of calcium hydroxide, which practically means dehydration of slurry and adhesive bonding of grains: CaO + H2O = Ca(OH)2

[3.1]

Then the carbonization reaction occurs, leading to hardening of the agglomerate structure: Ca(OH)2 + CO2 = CaCO3 + H2O

[3.2]

While in the process of combustion of agglomerates, calcium compounds contained in the product react with sulphur compounds and reduce emissions of SO2 into the atmosphere, as a result of reactions:

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3.3 Dumping ground surface after 1 year of agglomerate storage. (Source: Giemza et al., 2009.)

CaCO3 + SO2 = CaSO3 + CO2

[3.3]

2CaSO3 + O2 = 2CaSO4

[3.4]

In the case of use of hydrated lime, reactions [3.2] to [3.4] occur; however, hydrated lime readily takes in water and creates a ‘lime cake’ causing adhesion of the grains. To obtain a more durable agglomerate, an agglomeration process is required that ensures the formation of agglomerates that are not prone to disintegration during transport, optional storage and addition to other fuels. This stage of the agglomeration process basically relies on creating sufficiently strong adhesion forces. The full mechanical strength of agglomerate is obtained in the process of ageing, during which a full carbonization of calcium hydroxide [3.2] occurs. The process of hardening of the agglomerate (carbonization) depends on many factors, among which time plays a significant role. Often, calcium-bonded agglomerates, granulates and briquettes gain half of the final mechanical strength after several days of contact with air. The speed of the carbonization process (reaction [3.2]) depends, inter alia, on the grain size of reagents, their specific surface area, ambient temperature and degree of aeration of the agglomerate. In practice, the process of carbonization is accelerated by contacting the agglomerates with exhaust gases (an increase of CO2) or adding ‘catalysts’ in the process of agglomeration. For this reason, a third type of binder that can be used is hydrated lime with addition of molasses, which accelerates the reaction of carbon dioxide with calcium hydroxide and increases the adhesion forces. Based on the studies and tests so far carried out, the technology was developed for batch and continuous production of agglomerate using typical industrial mixers and one from a palette of several binders. Depending on local conditions and technological solution, two-shaft, planetary and

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turbulence mixers, etc. are useful for agglomerating coal slurries. In this respect, mobile concrete plants that ensure agglomerate output of up to ~ 400 tonnes/h are very useful. Cost analyses carried out for commissioning of agglomerating plants and economic effects obtained indicate that the payback period ranges from several months to two years. The developed technical and technological assumptions for agglomerating coal slurries are the subject of the investment process for the construction of a plant with a capacity of ~80 tonnes/h [5]. Coal slurry agglomerating technology is suitable to use not only in mined and fuel processing plants, but also in power plants, power and heating plants and heating plants.

3.3.2

Dewatering and drying of fine-grain materials and coal tailings

Energy properties of fine-grain materials and coal tailings depend to a large extent on their moisture as shown earlier in Fig. 3.1; for these reasons, it is sought to minimize the water content in the fuels or fuel additives produced from them. As previously mentioned, the processes most frequently used for this purpose are the processes of filtration, gravity and evaporation under normal weather conditions. In these cases, depending on the type of material, moisture content can vary in a wide range from 20% to 60%. The traditional source of heat in the drying of flotation concentrates, slurries and their concentrates are exhaust gases from the combustion of coal dust, coke oven gas and coal bed methane. While until recently coal slurry drying seemed to be not economically viable, now taking into account the access to sources of waste heat this issue should be seriously considered and worked around. The technical progress made in the design of dryers of flotation concentrates and coals points to the possibility of rapid deployment of drying processes for coal slurries and their mixtures with additions of biomass. In a number of cases there is an interest and need to use installations for periodic drying of biomass. The viability of using combustion engines fired with coal bed methane used in mines was pointed out even earlier [6, 7]. Exhaust gases from the combustion of methane are characterized by a relatively high temperature of 400°C. Mines implementing the programme for coal bed methane management are increasingly turning to gas engines as a source of electricity. So far, the exhaust heat has not been utilized. Also system power plants increasingly often consider additional sources of electricity based on internal combustion engines fuelled by natural gas. In all these cases, the exhaust heat can be used for the drying of fine-grain materials and coal tailings. Drying of coal slurries, so far, is not very common. In Germany, for example, a rotary (kiln) dryer was used with fluidized bed combustion for drying coal concentrate obtained from flotation beneficiation of coal slurry and

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flotation tailings; obtained pulverized coal with a moisture content below 3% was supplied as fuel for cement plants and as a reducing agent for blast furnaces. In Russia, there was cited an example of coal slurry drying for the ceramics industry. As early as the 1960s, Miechowice power plant in Poland built a tubular drying kiln fired with coal slurries and since then similar plants have been built in other countries. For the selection of the type of drying kiln, taking into account sources of heat and the possibilities to dry coal slurries, flotation tailings and biomass and their mixtures, rotary dryers, air (tubular) and fluidized bed kilns have all been considered. The eventual solution often depends on the local conditions and preferences of the entity concerned.

3.3.3

Materials with ultra-fine particle size

There has been considerable difficulty in the commercial use of a number of fine-grain materials and coal tailings as fuels or sources of heat, due to their ultra-fine particle size, heterogeneity and high water content. The solution to these problems is offered by the processes of caking and drying of fine-grain materials and coal tailings. The research and plant testing that has been carried out has led to the development of technology for caking coke and coal dust, graphite, biomass, etc. As mentioned earlier, briquetting processes have been implemented, inter alia, for the production of coal fuel briquettes. For the purpose of smelting and foundry, granulated graphite is produced. Based on the assumptions developed in technological and technical terms, an installation for coal slurry agglomeration is being made, with a capacity of ~80 tonnes/h. The properties of coal slurry agglomerate are significantly influenced by the addition of binders. Out of the various binders tested the best results are guaranteed by the addition of quicklime, which on the one hand binds the water contained in the product and on the other hand, by reaction with carbon dioxide, preserves the structure of the agglomerate. The economic analyses conducted show that the payback period may be achieved within a period ranging from as little as a few months up to about two years. Optimization studies carried out on the management of fine-grain materials and coal tailings have been extended to include the viability of drying the material. If waste heat sources are available locally, such as from internal combustion engines, or a power plant, the resulting dried material is a more marketable product but this obviously increases the treatment cost. The designed and developed technologies to optimize the management of fine-grain materials and coal tailings are intended for use not only by mining companies and coal fuel distributors, but also by power plants and combined power and heating plants.

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3.4

Waste storage

The coal mining waste materials deposited within the waste dumps or spoil heaps are very varied and because there was no perceived commercial value, disposal costs were minimized and, as a result, their contents are usually unknown and could also include things like mine timber, tramp iron, old rags, etc. The basic types of materials therefore have markedly different physical-mechanical properties and petrographic or chemical composition. In the body of such heaps there are igneous and sedimentary rocks with included coal, low grade coal, coal dust and sludges from cleaning cross cuts (Fečko et al., 2009), washery refuse, slag and often also rubble and municipal waste. Very old heaps, especially, may also contain high percentages of organic mass. Organic mass such as timber and other plant material is found accompanied by crumbling coal clays and silts, representing a considerable challenge to efforts at recovering useful material. Clay minerals and clastic quartz prevail in the groundmass and there is eaglestone, a concretionary nodule of clay ironstone, occurring in places. Accessory pyrite is common, which weathers quite rapidly into limonite and sulphates that are quickly washed away with rain creating acidity (Spudil, 1998). The waste rock is not mechanically sorted and hence can contain a wide range of sizes ranging from boulders to clayey-silty particles. Dominant are stone fractions with fragments of different sizes, from first centimetres to decimetres. Erosion factors cause further fragmentation (disintegration of sandstone into sand, laminated disintegration of aleuropelites along the cleavage plane). In connection with oxidation processes, the coal substance often ignites easily. Self-ignition of the refuse heap material occurs practically in all coal districts world-wide. The most prominent are the manifestations in the spoil heaps of underground mine and coal preparation plants. The main energy source of the processes is decomposition of fossil organic mass which starts already during coal extraction and is fastest in un-weathered coal. Most heat liberates during oxidation of unsaturated organic compounds into compounds of a humic acid type (Tvrdý and Sejkora, 1999). The process is accelerated by other exothermic reactions, primarily by oxidation of mineral sulphides – pyrite and marcasite. A significant role is played by the humidity – water molecules participate in the reactions between oxygen and coal and thus speed up the oxidation. Water vapour also condenses more easily than carbon dioxide that displaces as a result and leaves the coal substance surface for further oxidation. If the liberated energy is not concurrently removed, gas desorption from coal takes place under temperatures of up to 160°C. Oxidic complexes form at higher temperatures. If the temperature exceeds 300°C burning occurs; while heat is liberated during oxidation, energy is consumed in the reducing medium of intensely warmed through heap sections

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with no access of air. At temperatures of over 350°C, thermal decomposition of the coal substance takes place. At temperatures higher than 1000°C, high-temperature carbonization occurs in the reducing medium (Tvrdý and Sejkora, 1999). The liberated gas products of coal carbonization rise up through the refuse heap material and their ignition may take place in the aeration zone. The depth of the site of heating corresponds to a balanced ratio of two key external factors for coal substance self-ignition, oxygen access in required concentration and possible accumulation of oxidation heat. Fire in the refuse heaps most frequently originates below the surface layer of land waste, in the depth from 0.2 to 0.5 m, rarely as deep as 5 m (Králík, 1984). Despite thermally insulating properties of waste rock there are local centres of burning with maximum temperature exceeding 1000°C. From these centres, fire penetrates the sub-surface layers and thus spreads along the heap body. The calorific effect of coal substance alteration leads to a considerable heating of the surrounding sedimentary rocks and their gradual natural firing. Clay sediments, during heating of which the temperature of 600°C was not exceeded, are not much different in their mineral composition from grey sediments before alteration. The most prominent changes occur in the association of clay minerals, kaolinite and illite. Red colouring of the thermally altered sediments is characteristic and is caused by finely dispersed hematite. Clayey-silty rocks exposed to higher temperatures (900–1200°C) show more distinct variations in the mineral composition and their alteration is connected with the formation of porcelanite and vitreous phase (Králík, 1984). The overall character and colour of porcelanite can vary. Porcelanites fired in the reducing manner are black or grey; oxidically fired porcelanites are red. The structure of such rocks may be significantly porous. At running out of reddish-brown vitreous mass on the base of fired porcelanites, irregular cohesive and solid laminar bodies, which are placed on relatively loose material, form in the burnt-out waste rock (the so-called porcelanite sinters).

3.5

Coal recovery from colliery waste dumps in the United Kingdom

As an example of the foregoing, the following is a description of a very successful waste recovery operation in the UK, including a description of an innovative processing approach which has been developed specifically for the treatment of old mine waste piles and for the recovery of potentially saleable coal from reject material discarded from conventional coal preparation plants. The coalfields of the United Kingdom and in several other European countries where coal mining activities are no longer significant, are dotted

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with poorly rehabilitated colliery waste dumps originating from the nineteenth century industrial revolution through to the tail end of large-scale underground coal mining in the latter half of the twentieth century. Such waste piles are very similar in characteristics to those described earlier; almost all of them contain material from underground mining operations such as wastes from the mine and surface operations. This would include mine development stone, roof and floor material and a variety of non-coal waste such as timber, tramp metal and garbage as well as washery discards and in some cases tailings material. Much of the material was derived from state-controlled mining enterprises and the majority was emplaced prior to the stringent environmental and land use planning regulations that govern modern mining activity in Europe. For economic reasons many of these legacy sites have been subject to very light-touch rehabilitation with minimal reshaping and simple grass seeding, though some have had no rehabilitation at all and some have had very effective schemes to return them to beneficial use, usually funded through public sector urban or coalfields renewal or regeneration schemes. Some private sector rehabilitation work has been undertaken over the last 30 years or so. For the most part the economic drivers for this have been the redevelopment of the site as valuable development land, the recovery of low grade aggregates (particularly from waste dumps subject to past combustion) and the recovery of remnant coal. In the United Kingdom, a private sector company, RecyCoal Ltd, has become highly specialized in recovering remnant coal from abandoned colliery waste heaps to fund high quality, sustainable rehabilitation and beneficial re-use of poorly rehabilitated sites. Using a unique process to recover coarse coal (+0.5 mm), the company has profitably rehabilitated numerous waste dumps over the last 30 years and aims to expand its field of operations into a number of overseas territories.

3.5.1

Natural medium dual density process

A natural medium process is used to treat the recovered material. This patented process, known as the Natural Medium Dual Density (NMDD) process, was developed in-house and has evolved over many iterations. The process is described later, but comprises a barrel washer operating in conjunction with cyclones as the cleaning method. At its heart is the recognition that the majority of any waste dump comprises shales, clays and mudstones and that coal will present to separators as a relatively small percentage. Under these circumstances, the conventional dense medium processes are expensive and usually ineffective, but in the NMDD process, the waste mineral materials themselves have a high enough specific density and are present in adequate amounts to form an aqueous suspension

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Coal waste: handling, pollution impacts and utilization Feed

Screens Dewatering sieves

Picking station Crusher Discard

Barrel

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Crusher

Coal and coal brg. liquid Discard Dirty water Clean water

Coal and dirt liquid primary

Cyclone P. Discard

Coal and 5% dirt liquid secondary

Cyclone S. Discard

Coal

Centrifuge Dir ty wate r

Thickeners

Discard

Fresh water

Slurry

Bore hole pump Press house Press cake

3.4 Flow diagram of the dual density natural medium process (RecyCoal process).

that will behave as a controllable flotation medium for separating the coal material. So clays and silts that can be a nuisance in a conventional dense medium process actually facilitate the effectiveness of separation in the NMDD process. The treatment plant is typically designed and constructed in modules with each module capacity of ~350 tonnes of feed material per hour. The plant is sited in a self-contained area, and, by incorporating plate and frame filter presses in the waste fines circuit, it operates a closed circuit system with no effluent discharge. The material excavated from the spoil heap is taken to the processing area from where it is fed into the plant. The material passes through a sizer to reduce the top size of the feedstock to less than 150 mm.

3.5.2 Treatment plant In Europe, a picking belt is normally required to remove metals, wood and other discarded components of past mining operations before the feedstock is flushed into the slightly inclined barrel, producing a primary separation by a water-based slurry medium, which consists of the fine dirt from the feedstock held in suspension in water; no chemicals or other additives are required in the washing process. An internal Archimedean scroll separates heavier sandstone, mudstone, clay and shale from lighter materials by

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transporting heavier material back up the barrel along the scroll and out to discard conveyor. Coal and the lighter dirt materials are washed over the scroll along the barrel and out of the separation zone and are then sized over a vibrating screen at −40 mm with the oversize being crushed and recirculated over the same screen. The remaining coal and dirt is maintained in suspension in a primary cyclone tank before being pumped to a primary and secondary autogenous cyclone system which further separates the remaining dirt from the coal. The discarded material is rinsed over vibrating screens to recover the washing medium and is then conveyed out to join the discard material from the barrel for stockpiling prior to transportation back to the excavation site to form the rehabilitation profile. Through the use of the natural medium and the operating pressure in the cyclones, separation is achieved at defined relative densities. The two banks of cyclones are operated at differing densities to accurately control the quality of the coal produced. This is achieved with an on-site laboratory ensuring that quality is maintained and losses recognized and minimized. The coal from the cyclone system is rinsed to recover the washing medium on a vibrating screen before passing to a dewatering centrifuge where the moisture level is further reduced to the minimum practical level. The coal is then conveyed to a final stockpile. In the UK, the product quality achieved using the Dual Density Natural Medium process, which recovers coarse coal (+0.5 mm), is on a par with imported thermal coals and recovery rates are in excess of 95%. Product coal is dispatched from the plant site by either rail or road depending on the infrastructure. Waste heaps in the United Kingdom tend to hold little fine coal in the sub 0.5 mm fraction. These fines are typically found in discreet locations such as old, abandoned settling ponds. Where present on a rehabilitation site these fines can be sold direct to power generators (normally at considerable discount) or blended with primary product where market specifications and fines’ quality permits. Surplus slurry from the washing process which has a high concentration of fine dirt in suspension is transferred to the effluent treatment plant where it is treated with flocculants to aid settlement of solids which settle to form sediment at the bottom of the thickener. The clarified water decants over the thickener weir and is recovered for re-use. The thickened slurry settling in the bottom of the thickener is continuously fed via buffer tanks to a number of recessed plate filter presses, housed in a stand-alone, totally enclosed filter press house. This method of dewatering negates the need for slurry lagoons and ensures that a stable waste product is produced which can be handled and transported by dump truck to be placed within the new contours of the rehabilitation site without compromising the integrity of the finished landform.

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3.5.3 The dump-site recovery process: The treatment process involves the following steps: 1. Feed material is excavated from the tip area and delivered to a stockpile by trucks. 2. Feed material is fed into the wash plant for processing. 3. Coal is separated from the dirt in the washing plant using the natural medium process. 4. Dewatered coarse discard material from the washing process is used to construct the approved new land formation. 5. Fine discard material from the washing process is put through filter presses to produce a disposable cake which is also used in the new land formation. 6. All discard material is used to produce the final landform ready for restoration. 7. The final landform is then rehabilitated to the agreed scheme. 8. All water in the system is recovered for re-use with only top-up water added.

3.5.4

Case study – Langton, Derbyshire/Nottinghamshire border, United Kingdom

The former Langton Colliery waste heap had previously been the subject of a poor restoration scheme which left the site unsightly and vulnerable to combustion and leachate. The waste material was largely arising from the original mine heading development and preparation plant rejects. RecyCoal undertook an environmentally driven coal recovery and reclamation scheme for the small site and adjoining industrial land, which comprised just over 30 hectares. The site was initially proved by drilling and sampling the material on a 100 m grid pattern. Each metre-length of drill recovery was sampled and analysed for yield, and composite samples were prepared for quality assessment. In-house laboratory testing indicated that 10.5% clean coal yield was available at typical power generation quality specification. The local permit application process was slow, in part because the site straddled the boundary of two permitting authorities, but mainly because of the nature of modern British permitting procedures and the location of the site in a built-up area (as is usually the case in the UK and in many coalfield districts in Europe). The excavation and washing of approximately 4 500 000 tonnes of waste recovered 475 000 tonnes of saleable coal, and rehabilitated the site over a 3-year period ending in early 2013. The project will ensure that a sustainable

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3.5 Langton processing plant.

3.6 Colliery waste heap with plant site (visible on the opposite side of the M1 motorway).

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long-term solution is provided by the high quality rehabilitation of the site. The coal produced from the site was transported by road to a nearby power station without incident. The site utilized a single module plant operating 24 h per day from Monday mornings to Saturday mornings. Downtime was used for scheduled plant maintenance tasks. The site was staffed by a total workforce of 35 including management and on-site laboratory. On completion, the plant was dismantled, refurbished and rebuilt at a site of similar scale approximately 30 miles away. The rehabilitation aim of the project was to provide sustainable open space to be used for agriculture and nature conservation purposes. One challenge to the project was the paucity of topsoil on the site, but this is not uncommon on projects of this nature and good quality soils were manufactured on-site using plant discard, treated sewage sludges and a variety of suitable industrial waste products such as paper mill waste and brewery waste. The final landform is pleasing to the eye, which is important not least because the site is adjacent to the main M1 motorway, but also provides 8.8 hectares of new woodland and scrub with 22 000 new trees, 2 km of new hedgerow planting, 7.2 hectares of dry grassland and wetland and 7.6 hectares of agricultural land. Improved rights of way for public amenity give greater public access to the land and the reinstatement and enlargement of the industrial provision of the site for future development will aid future economic development.

3.6

References

European Commission (2006a), Sustainable power generation from fossil fuels: aiming for near-zero emissions from coal after 2020. COM(2006) 843 final. Brussels. Giemza, H., Gruszka, G., Hycnar, J.J., Józefiak, T. and Kiermaszek, K. (2007), Optymalizacja zagospodarowania sedymentu węglowego. Technologia brykietowania sedymentu. Polityka Energetyczna t. 10, z. spec. 2. Giemza, H., Gruszka, G., Hycnar, J.J., Józefiak, T., Kiermaszek, K. and Pyc, A. (2008), Innowacyjne kierunki optymalizacji obiegów wodnych dla wydzielenia i wykorzystania sedymentów w ZPMW Jastrzębskiej Spółki Węglowej S.A. Innowacyjne i przyjazne dla środowiska systemy przeróbcze surowców mineralnych – KOMEKO. Giemza, H., Gruszka, G., Hycnar, J.J., Józefiak, T., Kiermaszek, K. (2009), Technologie odzysku drobnoziarnistych materiałów i odpadów węglowych na potrzeby produkcji paliw i energetyki, XXIII Conference Materials Issues of energy resources and energy in the domestic economy Zakopane, ISBN 978-83-6019557-4. Hycnar, J.J. and Bugajczyk, M. (2004), Kierunki racjonalnego zagospodarowania drobnoziarnistych odpadów węglowych. Polityka Energetyczna t. 7, z. spec.

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Hycnar, J.J. and Józefiak, T. (2007), Brykietowanie odpadów drobnoziarnistych. VIII Śląskie Seminarium Ochrony Środowiska. Bytom. Jelinek, J., Malis, J., Danek, T., Thomas, J. and Slivka, V. (2011), Old coal refuse heaps – possible sources of opening materials for the ceramic industry, Mineral Processing, Journal of the Polish Mineral Engineering Society, XII, nr 1(27), ISSN 1640–4920. Králík, J. (1984), Tepelné změny uhlonosných sedimentů při požárech důlních odvalů a přírodním hoření uhelných slojí. Ostrava: Sborník vědeckých prací VŠB, řada hornickogeologická, roč. 30, č. 1. VŠB, s. 171–198. Poland (2007), Strategy of hard coal mining industry activities in Poland for 2007– 015. Warsaw. Spudil, J. (1998), Studie možnosti využití odpadních hald po těžbě. Praha: MS GET. Szymkiewicz, A., Fraś, A. and Przystaś, R. (2009), Kierunki zagospodarowania odpadów wydobywczych w Południowym Koncernie Węglowym S.A. Wiadomości Górnicze, nr 8. Tvrdý, J. and Sejkora, J. (1999), Hořící uhelné haldy a redepozice toxických látek při samovolném termickém rozkladu uhelné hmoty. EKO- ekologie a společnost., 4, s. 11–15. Tvrdý, J. and Sejkora, J. (2000), Novotvořené minerální fáze na hořícím odvalu dolu Kateřina v Radvanicích. Uhlí, rudy, geologický průzkum, 7(3), s. 19–24.

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