The industrial potential of materials recovered from municipal solid waste

Conservation

& Recycling,

Pergamon

Press Ltd..

Vol.

3, pp.

1980. Pm&

259 - 213. in Great Britain

THE INDUSTRIAL POTENTIAL FROM MUNICIPAL

OF MATERIALS RECOVERED SOLID WASTE* t

P. R. BIRCH and D. V. JACKSON Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Herts, U.K.

Abstract - Waste sorting technology is reviewed briefly and the pilot scale development work at Warren Spring Laboratory, leading to the design of the Doncaster sorting plant, is described. Product marketing trials are discussed and the importance of a systematic approach in planning resource recovery systems is emphasized.

INTRODUCTION The growing concern over the need to conserve raw materials and energy has focused attention on the reclamation potential of municipal or household wastes, particularly, in the industrialised countries where better living standards have generated wasteful societies. In the U.K. the quantities of municipal waste generated in different areas range from 200 to 350 kg/capita/yr. The main recoverable constituents are paper and board, metals, glass and plastic but, in addition, vegetable and food wastes can be processed to produce animal food or fermented to produce compost either aerobically or anaerobically, the latter also producing methane. Recovery of constituents does not necessarily imply recycling. For example, paper and board can be recycled as pulp to the board mills, it can be used with the vegetable matter to make compost, it can be processed chemically and microbiologically to produce ethanol or single cell protein or it can be separated together with plastics and other combustible material to prepare refuse derived fuels. Similarly, glass can be recycled to bottle making, used in production of decorative tiles or finely ground and used in reflective paints. These are only some of the ways in which recovered materials may be marketable. In any particular situation, the recovery strategy adopted must depend on the average composition and total tonnage of the waste and the local markets for recovered products. At the risk of stating the obvious, there is no merit in recovering a product unless there is a market or user for it. The work on the development of household waste sorting at Warren Spring Laboratory has been based on d systems approach so that, as far as possible, the unit sorting operations can be adapted to suit particular situations both with regard to waste composition and to final products.

WASTE SORTING TECHNOLOGY Extensive developments in waste sorting are being pursued in the U.S.A.[l] and Europe[2] (Italy, France, Spain, Germany, Holland, Sweden and the U.K.) and, with one exception, all are based on dry sorting. The exception is the Black Clawson process[3] in which the whole of the waste input is pulped in a hydropulper, the main objective being to recover paper fibre. There appears to be little advantage to be gained from use of a wet system and it can be ‘0 Crown copyright. f Paper presented at the Second Recycling World Congress, Manila, Philippines, 20 - 22 March 1979. 259

260

P. R. BIRCH and D. V. JACKSON

disadvantageous if a main product from sorting is a waste derived fuel. Although there is a wide variety of flow sheets in existence for dry sorting, there are marked similarities in the primary separation procedures. In particular, all include magnetic separation for recovery of ferrous metals (mostly tin cans) and some form of air classification for separation of light materials (paper, plastic etc.) from the heavier fractions (glass, stones, non-ferrous metals etc.). A major difference is that the American approach and several of the European systems are based on front end shredding to reduce incoming waste to a convenient size for subsequent handling, whereas in the U.K. (Warren Spring Laboratory) and Italy (Sorain Cecchini), and also in a system being developed in France, shredding has been rejected in favour of primary sizing. The disadvantage of primary shredding, whether by hammer mill or flail mill, is that it results in additional cross contamination of the constituents of the waste with possible adverse effects on the quality of recovered products and represents a very energy and capital-intensive unit operation. Selection of the primary separation procedures must be based on the products that are aimed at for recovery. It is notable that in the U.S.A. the main products are waste derived fuel and ferrous metal together with aluminium and glass. There is little interest in recovery of paper and board for repulping presumably because of the availability of woodpulp and also because of the growing concern over future energy resources. In Europe, there is much greater emphasis on recovery of paper for recycling and also on plastics. Aluminium is of less interest largely because the all-aluminium can has not penetrated the European market to the extent that it has in America. Other factors affecting selection of sorting systems are composition of waste, scale of operations and local factors such as collection frequency and transport costs. American systems are aimed, largely, at plant capacities of 1 000 - 2 000 tons/day whereas in Europe, the average capacity will be in the range 300-600 tonnes/day. The influence of local factors is exemplified at the Rome plants operated by Sorain Cecchini[4], the only fully commercial sorting plants in operation in Europe at this time. Refuse collection is daily, 7 days/week, all refuse is in plastic bags and collection vehicles do not compact the waste. Thus degeneration of organic wastes, 50% by weight, is minimised and recovery of an animal feed product is possible. Products from the plants are compost from organic fines, animal feed, ferrous metal, paper pulp and plastics. The latter are used in the manufacture of refuse collecting sacks in admixture with virgin polymer. An incinerator is integrated with the sorting plant for combustion of residues and to raise steam for sterilising the animal feed product. Although animal feed can represent a major source of revenue for a sorting plant, it is unlikely to be a factor in many other countries because of concern about possible health hazards and contamination with toxic constituents.

THE DEVELOPMENT

OF WASTE SORTING AT W.S.L.

The aim has been to develop a sorting system using low-cost and readily available technology and incorporating sufficient flexibility into the system to enable the full recovery potential of municipal waste to be explored and evaluated. Initially, studies were on a 3 - 5 tonne/hr pilot plant, details of which have been described[5] and have been widely shown in a film “The New Prospectors”[6]. Based on these pilot plant studies, a commercial scale plant has been designed in conjunction with Motherwell Bridge Taco1 Ltd. and is currently under construction at Doncaster by the South Yorkshire County Council with financial support from the Department of the Environment. All of the development work has also been funded by the Department of the Environment. The flowsheet for the Doncaster plant is given in Fig, 1. Details of the plant design and

THE INDUSTRIAL

POTENTIAL

OUSTBlN WASTE r- _ -,_ BAG FROM * BURSTER TIPPING FLOORL

-1Smm

1 1 i

OF MATERIALS

r---I

rfiti

HAMMER

~__-__.+_--_--_--_

L._M’LL_.i

i

DISCARD

4

4

MODIFIED STONER 1

l4;:4;;;

I

_+_A_i

k

I $iEJI@

RECOVERED FROM MUNICIPAL SOLID WASTE 261

40mm +lSmm ‘--I

‘GLASS-RICH’ .-.

TtRTb"g

AIR CLASSIFIER

‘LIGHTS’,

KNIFE MILL

‘HEAVIES’

RECOVERY

BALER

DRIER

I NOTE.

I 1

CHAIN DOTTED ARE EXPECTED ADDED AFTER OF THE PLANT

1

ITEMS TO BE START-UP

I

l

NOM-FERROUS CONCENTRATE

,t,,

COLWREO GLASS

1 +4onHn a-15mm DISCARM

1 P%!s =

Fig. 1. Doncaster refuse processing plant: outline flowsheet.

associated development work leading to it will be published shortly. As can be seen, all items of equipment, with the exception of the air classifier, are available on the open market and have simply been adapted for the particular use. A number of detailed modifications have been made, however, to adapt this equipment. For example, a wider range of screening facilitated by use of a ragger. The flowsheet given is for single stream operation at an initial capacity of 10 tonnes/hr. However, the design permits the introduction of a second stream to extend the capacity to 20 tonnes/hr, and site facilities and much of the equipment for the higher throughput are being provided from the outset. It is planned to operate the plant on a 2 shift basis, 16 hr/day, 5 days/week with an 80% availability to allow for maintenance and public holidays giving a total capacity of approximately 67 000 tonnes/annum of refuse.

THE AIR CLASSIFIER Between 1972 and 1975, a simple air classifier was developed and tested at Warren Spring Laboratory (W.S.L.). This unit was used by Motherwell Bridge Taco1 Ltd. as the starting point for the design of a full-scale unit which ultimately became the subject of a joint Motherwell Bridge Taco1 Ltd.1W.S.L. patent application[7]. The prototype machine has been tested at W.S.L. prior to its installation in the Doncaster refuse processing plant. Figure 2 shows schematically its main features. Preliminary results obtained using this separator to process refuse collected originally in Doncaster have shown that combustibles can be separated from sized refuse with minimal dust

262

P. R. BIRCH and D. V. JACKSON

Fig. 2. Air classifier.

I

collection equipment and relatively low specific energy consumption. The ash content of the ‘light’ or less dense, combustible fraction, which varied in the range 7 - 13% by weight, was particularly encouraging in view of the relatively high ash content of Doncaster refuse. PRODUCTS

AND PRODUCT

MARKET ASSESSMENT

Based on pilot plant studies at W.S.L. using samples of Doncaster waste, the anticipated materials balance for the plant has been calculated assuming 67 000 tonnes/yr throughput. Over 250 tonnes of waste have been processed in some 50 trials. The materials balance is shown in Table 1. Table 1.

Materials balance for the sorting plant (results based on several runs treating refuse collected in the Doncaster area)

Product

Magnetics ‘Fuel’ Paper-rich Glass-rich Dense reject Coarse reject Organic-rich reject ‘Fines’ (-16 mm)

Weight %

8 23 3 5 6 5 21 29 100

Magnetics ‘Fuel’ Paper-rich Glass-rich Dense reject Coarse reject Oraanic-rich reject ‘F&s’ (-16 mm)

Paper and cardboard

Plastics

5 67 90 29 34 -

2 11 5 4 5 -

Apparent assay (‘as received’) (wt. Yo) Textiles Glass Magnetics Organics 1 7 3 39 <) -


27 4 4 7 Distribution (Percentage, weight basis).

-

2 9 1
3 2 1 <1 38 20 10 -

7

15

6

1 14
4 8

100

loo

loo

2 57 10 5 26 _

4 61 4 _ 5 26 _

2 44 2 _ 52 _

75 20 3 _

94 2 4 _

100

loo

100

100

100

100

2

Fines

87 <1 5 -

8 23 3 5 6 5 21 29

-

Misc.

3 1 1 100 30 2

1 97

Assays determined by hand sorting of moist as received material. Results are a rough guide only - no account has been taken e.g. of labels and food wastes in tin cans or bi-metallic cans.

THE INDUSTRIAL POTENTIAL OF MATERIALS RECOVERED FROM MUNICIPAL SOLID WASTE 263

Initially, marketable products from the Doncaster plant will be ferrous metal (mostly tin cans), glass, densified waste derived fuel and a paper-rich product for fibre recovery. Tonnage quantities of these products have been prepared and accumulated over the course of many pilot plant runs to enable realistic trials to be made by potential industrial users. As a result, it has been possible to assign values to the products and these are shown in Table 2. Current primary raw material prices are depressed due to low industrial demand and consequently both demand and price for secondary materials are at very low levels. Thus for certain products a price range is given, as changes can occur rapidly when industrial demand picks up.

Table 2. Product Waste derived fuel Ferrous metal Glass Paper

Reclaimed products expected from Doncaster refuse

Weight in tonnes/yr

14 000 4000 3ooo 2000

Price/tonne w 1- 10 12-24 8-10 12-20

Revenue/yr w 98-140000 48-% Ooo 24-30000 24-40000

Revenue/tonne

of refuse

1X-2.09 0.72- 1.44 0.36-0.45 0.36-0.60 2.90- 3.95

Latest estimates of capital and operating costs for the Doncaster plant handling 67 000 tonnes of refuse/yr give a cost of f9.70/tonne of refuse with no allowance for residue disposal (see later). With revenues as shown in Table 2, this is equivalent to a net disposal cost of f6.00-f7.OO/tonne. The capital cost figures may be high due to the innovative and experimental nature of the Doncaster plant but, nevertheless, there is clearly a need to maximise income by attention to product quality and yield and to efficient marketing.

FERROUS METALS The recovery of ferrous metals (mostly tin cans) from refuse has been practised by some local authorities for many years, both from raw refuse and from incinerator residues but generally with poor efficiency as recoveries have seldom exceeded 50%. This is despite the apparently simple technology involved in the magnetic separation. Work at W.S.L. and elsewhere[8] has shown that with proper attention to belt loadings, strength and configuration of magnetic field etc., recoveries of around 90% can be achieved readily and consistently. Tin contamination produces adverse effects (hot shortness) in steel and therefore ferrous scrap containing in excess of 0.02 - 0.03% Sn is unacceptable to steel makers. Some municipal ferrous scrap does end up in steel making, probably mixed with higher grade scrap so that the effect of contaminants is minimised by dilution. This is undesirable and there is increasing concern in the industry over the growth of residuals in steel (particularly copper and tin) resulting from scrap recycling. The preferred outlets for municipal ferrous scrap are in production of cast or refined irons where a proportion of tin is beneficial in the product or in detinning for recovery of pure tin metal and high grade steel for recycling. The problems involved in the re-use of magnetic metals from municipal solid waste have been discussed in detail by Duckett[9], although essentially based on the situation in the U.S.A. At least one company in the U.K.[lO] has recognised the importance of municipal scrap and has adapted their smelting facility accordingly. Baled bright cans are fed directly to a cupola and smelted. Contaminants such as food residues, labels etc. are burnt off in the cupola and contaminants,

264

P. R. BIRCH and D. V. JACKSON

such as aluminium lids, are lost by oxidation into the slag. Off-gases from the cupola are cleaned in a carefully designed gas-cleaning circuit to avoid air pollution problems. Molten metal from the cupola is collected in a shaking ladle where composition adjustments can be made, following rapid analysis of metal samples, to ensure products meet customer specifications. From a resource conservation point of view, detinning would appear to be the preferred outlet for municipal ferrous scrap as both tin metal and high grade steel are recoverable. The detinning process involves treating can scrap with a caustic soda solution, containing an oxidant, to dissolve the tin selectively leaving high grade steel. High purity tin is recovered from the cleaned solution by electro winning which simultaneously regenerates the caustic solution for re-use in the dissolution. Tin cans recovered from refuse are contaminated with food residues, labels and other undesirable materials which cause serious problems in contaminating the caustic liquors in the detinning process. Opening the seams is also a problem as it is difficult to get the tin content of steel low enough if the seams are not fully exposed to the caustic solution. Detinning of cans recovered from refuse is not a viable commercial operation in the U.K. at this time but is likely to be so in the near future. Trials at W.S.L. have shown that the ferrous concentrate can be upgraded by classification to remove massive iron followed by shredding and cleaning procedures to yield a product closer to the specification required by the detinners. Various alternative can cleaning procedures are also under investigation by the industry. It is worth noting that, whereas the smelters prefer baled scrap at high density, there will be a limit to the bale density acceptable for detinning as further costs may be incurred by the detinners in bale breaking and shredding to expose surfaces for chemical attack. At present, it is envisaged that the ferrous scrap recovered at the Doncaster sorting plant will be sold for iron-making but if, in future, detinning is likely to offer a better return, then minor modifications to the product circuit may be necessary to meet the requirements of the detinners.

GLASS The desirability of recovering glass in a refuse sorting circuit in the U.K. is debateable. The raw materials for glass making are readily available from indigenous sources at comparatively low cost. Although in-plant scrap or cullet is normally recycled, there is little economic incentive for the industry to use post-consumer cullet. Although the use of a proportion of cullet in the feed mix to glass making has advantages in controlling the melt and reducing energy consumption, the cullet required is often available from in-house scrap and it is anticipated that more will become available in the future as the ‘bottle-bank’ scheme launched by the Glass Manufacturers Federation[ 1l] gets properly underway. Another difficulty is the target specification for foreign cullet laid down by the industry and reproduced in Table 3. On the other hand, apart from the well-aired arguments of resource recovery and conservation, there are several factors which favour separation of glass. In the primary sorting circuit, glass is recovered initially in an intermediate product consisting of putrescrible material together with ‘heavies’ such as stones, ceramics and non-ferrous metals. The putrescrible material has potential for compost making, with or without methane generation, and this requires the removal of glass. The ‘heavies’ can be worked up for non-ferrous recovery again requiring the separation of glass. Thus, although glass recovery alone may be marginal on an economic basis, it is essential to evaluate it if the full potential for refuse reclamation is to be adequately explored. At W.S.L. two approaches to glass recovery have been investigated. Initially a glass-rich

THE INDUSTRIAL POTENTIAL OF MATERIALS RECOVERED FROM MUNICIPAL Table 3.

SOLID WASTE 265

Target specifications for ‘foreign’ cullet[l2] Target specification

1. 2. 3. 4. 5. 6. 7. 8.

Size Moisture Type of glass Organics Ferrous metals Non-ferrous metals Inorganic solids Colour Flint glass Amber glass Green glass

2 in. No dust No drainage Soda-lime-silica 0.05% loss on ignition 0.01% max. size Y4in. 0.01% max. size Y4in. 0.05% max. size 1/4in. No refractory materials 20/o amber 0.1% green 90% amber 90% green

fraction was obtained using the ‘thrower’* in conjunction with gravity separation in a brine tank to remove non-ferrous metals. Froth flotation was used to concentrate the glass in a sandsized product assaying a minimum of 99% glass. The impurity in this concentrate was mostly carbon (coal). On average, 80% of the product was flint glass and 20% coloured glass. Although the product fell far short of the target specification laid down by the industry, bench tests carried out by the British Glass Industries Research Association demonstrated that it would be suitable for production of green container glass, where the melt ingredients may contain up to 20% of cullet. In a further trial, quantities of ashtrays and,glasses were made using as much as 35% of the W.S.L. cullet. The overall recovery of glass from refuse using this approach was 50%. Despite the successful demonstration trials, the glass industry expressed reservations about large scale use of the glass flotation concentrate product. In particular, they were concerned at the difficulty of monitoring the material for the presence of unacceptable contaminants, the possible difficulty of obtaining consistent quality and adverse effects in the melt procedures due to the use of such fine cullet. An alternative approach involved the use of two stoners (basically an air table developed originally for separating stones from seeds), in place of the thrower, brine tank and selective comminution. The first stoner was adapted to discard the lighter putrescible materials whilst the second stoner, suitably modified, recovered the glass in the light fraction separately from the non-ferrous metals, stones etc. The glass-rich material, -50 mm + 15 mm in size, was upgraded by optical sorting on the basis of transparency, no attempt being made to colour sort at this stage. As the glass surfaces were frequently soiled, washing was necessary prior to optical sorting. An elutriater was therefore introduced to provide additional scavenging to remove residual light material while at the same time cleaning the glass surfaces. The grade of glass recovered depended on the number of stages of optical sorting used. A product assaying not less than 99% glass was obtained using two stages, a bulk sorter followed by final up-grading. Even higher qualities were achieved using a third stage. Overall recovery of mixed glass concentrate was again about 50% based on refuse input. Samples of the concentrate were submitted to the British Glass Industry Research Association for trials as before. Tests showed the optically sorted product suitable for use as cullet in the manufacture of green container glass, a considerable proportion of the opaque objects in the final product dissolve in molten glass. The optical sorting circuit has been selected for inclusion in the Doncaster plant because it *A device in which materialsare projected across the surface of an inclined steel plate. Separation is achieved by exploiting differences in sliding friction (see ref. [6]).

266

P. R. BIRCH and D. V. JACKSON

offers certain advantages compared with froth flotation. Process economics are more favourable, the product is expected to be more acceptable for use by the industry and, furthermore, colour sorting can be introduced at a later stage if the revenue from colour sorted products justifies the further capital cost involved. Also it is likely that effluent treatment costs will be lower than those associated with a froth flotation circuit. In the pilot trials to date, it has been assumed that the preferred market for the reclaimed glass will be in the manufacture of green container glass. There are, however, possible alternative markets, e.g. in the production of decorative tiles and other uses[l3]. The production of significant tonnages of glass concentrate at the Doncaster plant will enable the full market potential to be explored.

NON-FERROUS

METALS

Some of the flows rejected in the course of glass separation will contain a significant proportion of non-ferrous metal which can be recovered by additional stages of physical processing. Preliminary work carried out at W.S.L. resulted in the recovery of copper-rich and aluminium-rich fractions shown in Table 4. It is proposed to investigate the possibility of adding a non-ferrous metal module to the Doncaster plant circuit after the viability of the process core has been established.

Table 4.

Composition of non-ferrous metal products separated from Doncaster refuse by physical processing Per cent by weight Al-rich Product Cu-Rich Product

Component Al alloys Brass Composites (mainly rubber and brass) Copper Lead Nickel (Die cast) zinc Magnetics Miscellaneous (combustible) material (mainly rubber)

4

3 55 4 12 7 7 4 7

10

1

79

4 2 1 -

Note: the components in each fraction were determined by hand sorting.

REFUSE DERIVED FUEL The air classifier lights fraction from sorted household waste represents the highest tonnage of recovered, re-usable material. The main constituents of this product are paper and cardboard 60 - 859’0, plastics 5 - 15% and textiles 5 - 10% by weight. In addition, the product is contaminated with minor amounts of putrescibles, non-ferrous metals, glass, ash and clinker and miscellaneous combustible and non-combustible materials. From the range of analyses, it might be inferred that the most profitable line for recycling would be the recovery of fibre from the paper and cardboard fraction for reuse in paper or board making. However, the product is far too low grade to be acceptable to the paper industry as mixed waste paper and, although studies are being pursued to upgrade the product for fibre recovery, in the immediate future the greatest potential lies in its use as a fuel.

THE INDUSTRIAL

POTENTIAL

OF MATERIALS

RECOVERED

FROM

MUNICIPAL

SOLID

WASTE

267

There are two options for using this material as fuel: (a) shred to a suitable size and burn in suspension type burners either alone or as a supplement to pulverized coal; (b) densify into pellets or briquettes and fire on conventional solid fuel grate-type units, again alone or as a mixture with coal or other fuel. Suspension firing would appear to offer the least cost alternative particularly if, for example, the whole output from a sorting plant could be used in a pulverized fuel burning power station. However, there are difficulties with this approach. The shredded product, which has a very low bulk density, is difficult and costly to handle, store and transport. In preliminary firing trials, difficulties were experienced in attaining a controlled rate of feed, thermal efficiency was poor (41- 73%) and there was a large carry-over of unburnt or partially burnt material. Refractory walls suffered severe slagging, the ash fusion temperatures of RDF being typically in the range 1 lOO- 1 200°C compared with 1 300°C or more for most coals. It was concluded that suspension firing, although undoubtedly feasible, required extensive investigation and accordingly a research project has been initiated covering burner design, combustion air requirements, corrosion problems and materials of construction. Early firing trials with densified pellets on a chain grate stoker gave promising results and indicated that the handling and firing properties of the pellets may be little different from industrial solid fuels. It was therefore decided to concentrate on the development of methods for densifying the refuse derived fuel as this should offer an immediate wide market for use of the fuel in existing industrial solid fuel appliances. The main factors investigated were method of shredding, type of equipment best suited to production of a densified product and the moisture content to give an acceptable product in terms of strength and handling characteristics. Tests were made on a range of equipment that was readily available, the tests were not exhaustive but designed to identify, within a limited time-scale, the best options for equipment selection for installation in the commercial plant. (a) Shredding trials Tests were made on four types of shredding equipment, hammer mill, shear mill, flail mill (straw grinder) and knife mill. The results are summarised below and average size distributions for the various mill products are shown in Table 5.

Table 5. Size in mm + 50.8 -50.8 -25.4 -12.7 -8 + -5.6

+ 25.4 + 12.7 + 8 5.6

Size analysis

of product

from various

Shear mill A

Shear mill B

6 41 27 14 7 5

2 11 30 16 24 17

shredding

machines

Flail mill Knife mill 1 5 34 23 19 19

*1 4 28 25 20 23

The hammer mill exhibited poor performance in shredding plastics and textiles, demonstrating why other workers have found it necessary to employ two or even three hammer milling stages to obtain adequate comminution. Tests on two shear mills showed that both gave inadequate shredding of textiles and, generally, throughputs were low. Despite being used on the largest quantity of material, because of ready availability and with consequent opportunity to vary operating conditions, the flail mill (straw grinder) gave erratic results and the product size distribution was inconsistent. The throughput was low relative to the installed power and

268

P. R. BIRCH and D. V. JACKSON

the wear rate was unacceptably high. The knife mill was the only machine demonstrated to be capable of consistently producing material suitable for feeding to densification machinery described later. Product size distribution was satisfactory (Table 5), even with knife edges showing considerable wear and throughput and power consumption of the machine tested were acceptable, for example, the knife mill consumed approximately 40% less energy/tonne than a conventional hammer mill. However, during one trial, a piece of tramp metal inadvertently passed through the mill and the damaged caused demonstrated that, for use in the sorting plant, it will be necessary to incorporate a metal detection and rejection system between the air classifier discharge and the shredder inlet. These results point to knife mills as being most suitable and this type of shredder has been elected for incorporation in the commercial scale sorting plant. (b) Densfication If a densified refuse derived fuel is to be suitable for burning, either alone or mixed with coal, in combustion systems where coal is normally employed, and without major changes to such systems, it must fulfil certain requirements. (i) Handling. The product should have a bulk density similar to that of coal and sufficient mechanical strength to permit handling by conveying systems normally used for coal. It must be possible to transfer the product to storage and from there to the combustion equipment without fragmenting to produce large quantities of fines. (ii) Storage. The product should not consolidate for example, due to pressure from the weight of stored material or to bonding changes resulting from changes in humidity. (iii) Firing properties. The burning rate of the refuse derived fuel should be compatible with that of coal, i.e. it must be higher to compensate for density of RDF. In selecting densification equipment for test purposes, the emphasis has been on machines readily available that have been used for similar purposes and could be easily adapted for use with waste derived fuel. Suitable equipment has been found mainly in the field of animal feedstuffs manufacture, but machines designed to briquette peat and wood waste have also been considered. Tests were made on four annular ring-die pelleting machines and two reciprocating briquetting machines. The reciprocating presses generally tended to exhibit high wear rates and modifications to feeding arrangements would be required if high throughput rates are to be achieved. Annular ring-die pelleting machines gave the most satisfactory products although in selecting a particular machine attention needs to be paid to feed arrangements and die wear with the particular material to be used. Feed preparation is critical, in particular, large pieces of textiles must be excluded from the pellet mill feed. Moisture levels in excess of 25% resulted in unsatisfactory pellets. A lower limit for moisture has not been established; the results suggested that 15% is satisfactory although, in some cases, good pellets were made with only 6 - 10% moisture. Cooling of pellets after discharge from the mill, and before storage, permits some further loss of moisture (the pellets are often steaming on leaving the hot die) and also results in improved strength for subsequent handling. In cases where the moisture content of the air classifier lights is high, drying is essential before pelletisation. Conveniently, drying can be combined with conveying by ingesting hot air into a pneumatic conveying system. The drying process must be closely controlled to produce a material with consistent moisture content for densification and a fuel of acceptably low moisture content for subsequent efficient combustion. The possible danger of plastic film building up on the dryer walls remains to be investigated. (c) Combustion trials with densifed refuse-derived fuels Pelletised and briquetted RDF has been used in various trials on chain grate stokers and a

THE INDUSTRIAL POTENTIAL OF MATERIALS RECOVERED FROM MUNICIPAL SOLID WASTE 269

multifuel package boiler. Briquettes gave difficulties in handling and charging and in the maintenance of satisfactory combusion; the main emphasis has therefore been on combustion of pellets produced in ring-die machines. Successful combustion of pelletised RDF has been achieved. Pellets of 9, 15 or 19 mm diameter by about 25 mm long were burnt alone or in admixture with coal in a Parkinson Cowan ‘Vekos Powermaster’ multifuel package boiler. When operating on 100% pellet feed, boiler efficiencies in the range 72 - 86% were obtained operating at up to 99% of rating. The makers estimate that about 80% thermal efficiency is normally attained when firing this boiler with coal. Gaseous emissions were acceptable, oxides of nitrogen showing a range of 100 - 260 ppm and hydrocarbons 50 ppm or less. Only two measurements each were made for sulphate and chloride giving 361 and 453 mg mm3SO,*- and 141 and 602 mg mm3Cl-. Concentration of vinyl chloride monomer was in the tens of parts per billion range and calculations showed that, even with a comparatively high proportion of PVC in the fuel, ground level concentrations of VCM should be well below safety limits. However, further testwork must be done to confirm these tentative findings to ensure that no toxic hazard problems will arise. This type of boiler has manual ash clearance and the high ash content of RDF, compared with coal, gave an unacceptable rate of build up of ash on the fixed grate. However, the manufacturers are satisfied that it would be relatively easy to equip the boiler with semi-automatic de-ashing equipment. The rate at which ash from RDF builds up appeared to be satisfactory when a 50:50 by weight mixture of pelletised RDF and coal was used as fuel. No problems of ash clinkering were encountered. Combustion trials were also conducted on several industrial chain grate stokers. A comparison of the analyses of coal and RDF used in one such trial is shown in Table 6 and emission data when burning coal or RDF alone or in 50:50 mixtures are given in Table 7. Table 6.

Analysis of fuel used in stoker tests Pellets Coal (Arkwright washed smalls)

Proximate Analysis Moisture Ash Volatile matter Fixed carbon Volatile matter Calorific values MJ kg-’ Ultimate analysis Moisture Ash Carbon Hydrogen Sulphur Chlorine Carbon dioxide Table 7.

(as analysed (as analysed (as analysed (as analysed (dry, ash-free

%) o/o) 070) %) %)

(dry, ash-free) (as (as (as (as (as (as (as

analysed analysed analysed analysed analysed analysed analysed

Concentrations

070) Vo) vo) W) 070) vo) vo)

12.5 10.2 65.4 11.9 84.6

6.1 6.5 34.0 53.4 38.9

21.8

34.4

12.5 11.7 49.8 6.1 0.2 0.24 0.22

6.1 6.9 76.8 5.1 1.65 0.22 0.08

of pollutants from stoker tests Total

Test no.

Fuel

1.

100% coal 100% RDF pellets 100% RDF pellets 50070RDF/50% Coal 50% RDF/SO% Coal

g;: 3(a). 3(b). ??

NO + NO,-

Acidity (ml 0.1 Nm-‘)

so=(mg’m-‘)

cl(mg rn+)

461.6 66.7 97.6 328.3 352.5

2366 188 200 1765 1757

12.1 25.3 104.7 54.5 40.6

(pE)

114 110 132 140

(~7%)

c%Zs (ppm C3) 80.7* 14 6 11 5 11 1 2 15 1 15 /

Particulates (mg m-3) -31.0 70.1 82.9

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P. R. BIRCH and D. V. JACKSON

Although these tests were of several hours duration, the tonnages of RDF available from the pilot plant have not been sufficient to investigate all the variables and more extensive trials are required to optimise grate speed, pellet depth, primary and secondary air requirements etc. The chain grate stoker trials showed that the smaller sizes of densified RDF could be burned satisfactorily, if not alone, then in 50:50 mixtures with coal. Emissions were satisfactory, and thermal efficiencies and the percentages of maximum boiler ratings achieved were considered very reasonable in view of the relative brevity of the tests and the lack of experience with these fuels. There is no doubt that with further experience, and possibly minor modifications to equipment, efficiencies and percentage boiler ratings can be improved and that densified waste derived fuel represents an acceptable supplementary fuel for use in a significant range of existing industrial combustion equipment.

FIBRE RECOVERY FOR RECYCLING

TO PAPER AND BOARD MAKING

As discussed earlier, two of the products from the sorting plant are expected to be relatively rich in cellulose fibre, the so-called ‘paper-rich fraction’ derived from material coarser than 200 mm in size, and the ‘fuel product’ derived from the - 200 mm + 50 mm size fraction by air classification. On a weight basis, paper and board are the major constituents of refuse and, based on average prices paid for mixed waste paper, the recovery of fibre for recycling can be a very significant factor in determining the revenue and hence the overall economics of a refuse sorting plant. It is frequently argued that waste paper is best recovered by at source segregation and separate collection but it should be noted that separation by householders is limited, generally, to newspapers and magazines, whereas recovery from mixed waste offers the opportunity of recycling more of the better grade, long-fibred material, such as Kraft wrapping paper, currently disposed of in refuse. Early pilot scale trials showed that the + 200 mm fraction of refuse could be readily upgraded to give a product containing as much as 25% of the paper and board in refuse at a grade of about 90%. Tonnage samples of this material were supplied to boardmakers for trials and their results showed that although the product could be utilized, it contained a higher proportion of contraries than normally acceptable in mixed waste paper and would be attractive only in times of exceptional demand. Subsequently, changes in refuse composition, in particular the increase in plastics and the trend towards use of plastic sacks for refuse collection, led to a deterioration of quality of the paper-rich product and encouraged investigation of upgrading procedures. Upgrading methods can be divided roughly into essentially dry methods which can be incorporated into refuse sorting systems or wet methods which involve recovery of a fibre pulp similar to that produced at the front end of paper and board making procedures. The latter are inappropriate to sorting plants and should be considered as supplementary processes at the front end of the paper or board making circuit at mills where extensive effluent treatment facilities are available. A number of dry processes has been considered. Work has been carried out elsewhere which suggests that thermal methods to shrink and densify plastic film[l4] or methods based on preferential uptake of water by paper[l5], followed in each case by further air classification, may bring about the required upgrading. However, bench scale work at W.S.L. suggested that such techniques were unlikely to be as effective as wet processing to produce a better quality pulp. A novel dry up-grading process has been evolved at W.S.L. which is based on laser light reflection[ 161. Paper generates random diffuse reflections whereas plastics, under the same

THE INDUSTRIAL POTENTIAL OF MATERIALS RECOVERED FROM MUNICIPAL SOLID WASTE 271

conditions of illumination, gives specular reflections. It is possible, with appropriate electronic equipment, to distinguish between these and generate a signal which can operate a blower so that either paper or plastic can be selectively removed. This approach has been used on the coarse, + 200 mm fraction of refuse sorted on the W.S.L. pilot plant. Latest work has shown that with the laser activated splitter operating at an infrared wavelength product grades of 95% fibre can be achieved. A suitable wet process was based on the ‘Fibreflow’ drum manufacturered in Finland by Ahlstrom Oy[ 171. This consists basically of a near-horizontal revolving drum divided into two sections (see Fig. 3). In the first section incoming feed material is agitated with hot water, at

SCREENING ZONE

Fig. 3. The Fibreflow system.

high consistency, causing paper and board to break down into fibre while leaving plastic film and textile contaminants virtually unaffected. The second section of the drum is fitted with perforated plates, Liberated fibre is washed through the apertures in these plates, leaving coarse contraries to be discharged separately from the end of the drum. Tonnage samples of both the paper-rich and ‘fuel’ products separated from Doncaster waste were processed using a pilotscale ‘Fibreflow’ drum made available for the purpose by the manufacturers. The drum operated well giving apparent pulp recoveries of 75% from the ‘fuel’ product and 85% from the paper-rich material. Analysis of pulp samples showed the pulp from the paper-rich material to be virtually identical to that from normal waste paper. Biological examination of the pulps showed the presence of organisms as would be expected in mixed waste paper but pathogenic organisms were not found and the results suggested that there was no particular health hazard in handling these pulps. Longer term pulping trials are, however, essential to confirm that no health hazard exists. A secondary paper mill is proposing to install a drum pulper so that long term pulping trials can be conducted on the products from the commercial sorting plant in Doncaster.

DISCUSSION Throughout this paper, discussion of cost and operating data for refuse sorting has been omitted deliberately. The intention has been to show that products recovered from household waste can be used effectively as raw materials for industry. The major factors in determining the economic viability of household waste sorting plants are: (a) capital and operating costs of the plant; (b) revenue from sale of products; (c) alternative waste disposal costs. For the U.K., reliable capital and operating costs will be available only when the first

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commercial plant has been completed and operated under steady conditions for at least several months. Capital cost figures, or estimates, are available in the literature based on developments both in North America and Europe. Unfortunately, sufficient cost data are seldom available to make adequate comparisons, particularly taking local factors such as labour costs and land prices into full account. Nevertheless, studies at W.S.L. indicate that when such capital data as are available for various developments are adjusted or extrapolated to a common basis, differences in capital costs for the different systems are unlikely to make major differences in the overall operating costs of sorting plants. Their economic viability will depend on the efficiency with which reusable or recyclable products are recovered and the grade in relation to the industrial markets available. The quantity of residue and its disposal costs is also an important factor. The major problem in estimating revenues from any materials reclamation or recycling scheme is the notorious fluctuation in demand, and, therefore, the prices paid, for secondary raw materials. For example, the current price for baled cans separated from raw refuse appears to be in the range El0 - Elytonne whereas only about 2 yr ago some authorities were selling the same product for closer to f30/tonne. The major potential revenue earners from a refuse sorting plant are the fuel product and mixed waste paper for recycling to fibre recovery. In the W.S.L. approach to the development of a sorting system the variable demand for secondary raw materials has been considered, particularly with regard to fibre recovery. In times of high demand, mixed waste paper will command a price of f20/tonne or more and this will give a much greater revenue than merely selling the fibre for use as fuel. As shown in Table 2, the fuel product is expected to command a minimum price of &7/tonne based on the cost of industrial coal and making allowances for reduced calorific value of RDF and a cash incentive to promote industrial use. The sorting plant described has the flexibility to divert paper and board either to fibre recovery or to production of RDF. Furthermore, there is potential for increased paper and board recovery by suitable adjustment of screen sizes at the coarse end of the trommel. The potential return on fibre is also the incentive for research and development into methods of upgrading the paper or fibre-rich products. Generally, a major contaminant in this product is film plastic. Although at present there is little or no outlet for mixed plastic, this situation is changing as more effort is put into commercial exploitation of plastic recycling systems. The plastic content of refuse, although much lower in the U.K. compared with other countries such as Japan, can be expected to grow thus increasing the incentive for recovery. Taken together, waste paper and plastic represent a useful supplementary fuel but their potential values, at times of high market demand, are much higher if reclaimed separately. The problem of fluctuating markets for reclaimed materials is exacerbated in the case of refuse sorting plants because some of the products are ‘new’ in the sense that they have not been used previously by industry as raw materials. This is particularly the case with the fuel product. If industry is to be convinced of the value of the fuel potential of refuse it is essential that adequate information is available both on the nature of the fuel and how it can be used without incurring corrosion or erosion problems in the combustion equipment or penalties associated with unacceptable emissions of either solids or toxic or harmful gases. A specification is required for fuels derived from refuse in the same way that specifications exist for other solid fuels. The problem of specifications for reclaimed or recycled materials has already been recognised and in the US, in particular, extensive work is in progress[lS]. This initiative is wholly commendable but it can be argued that, for some products, it is too early in the development to write specifications adequately. There is no doubt that, if the industry or user view predominates, specifications are likely to be unduly severe as no industrial user will put highly expensive production equipment at risk by using inferior materials. Equally,

THE INDUSTRIAL POTENTIAL OF MATERIALS RECOVERED FROM MUNICIPAL SOLID WASTE 273

known that where specifications exist for recovered materials they are often more stringent than is demanded by the end use. At this point in time, much of the resource recovery work is at too early a stage of development to specify precisely the requirements for re-usable materials. It is only when recovery plants come on stream continuously that sufficient quantities of materials for long term trials will become available. The success of resource recovery depends on the willing collaboration between operators of resource recovery systems and the industrial users.

it is

Acknowledgemenrs - The authors gratefully adknowledge the dedicated work of the sorting team at W.S.L. whose efforts have made this paper possible. The work described forms part of a research programme which is funded by the Department of the Environment.

REFERENCES 1. U.S. Environmental Protection Agency, Fourth Report to Congress, Resource Recovery and Waste Reduction (1977). 2. H. Alter, Resource Recovery in Europe, NCRR Bulletin (1976). 3. W. Herbert, Solid waste recycling at Franklin, Ohio, Proc. 3rdMineral Waste Utilizafion Symp., Chicago (1972). 4. H. Orth, The refuse recycling plants at Rome, Miill und Abfail 1, 7 (1976). 5. P. R. Birch and E. Douglas, Recovery of potentially re-usable materials from domestic refuse by physical sorting, Resource Recovery and Conservation 1, 319 (1976). 6. “The New Prospectors”, Central Film Library Catalogue no: U.K. 3290, Film produced by the Central Office of Information, available with descriptive brochure from Central Film Library, Government Building Bromyard Avenue, London W3 7JB. 7. U.K. patent application No. 22875/78. 8. B. D. Linley, Tinplate recycling, Resource Recovery and Conservation 2, 225 (1976/1977). 9. J. E. Duckett, The influence of tin content on there-use of magnetic metals recovered from municipal solid waste, Resource Recovery and Conservation, 2, 301 (1976/1977). 10. D. K. Southwick, Melting of ferrous scrap from domestic refuse, Symp. - The Technology of Reclamation, University of Brimingham. 7 - 11 April (1975). 11. Anon, Materials Reclamation Week/y 13 May (1978). 12. Joint GMF/BGIRA Working Party. A specification for cullet. ENV/3, Feb. (1975). 13. R. J. Breakspere, P. J. Heath and R. J. Morgan, Waste glass - re-use or throw-away?, Ist World Recycling Congress, Basle, Switzerland, March (1978). 14. U.S. Patent 3814240 (1974). 15. F. J. Colon, The mechanical separation of paper from municipal refuse, Proc. Ist Int. Symp. on Materials and Energy from Refuse, Antwerp (1976). 16. U.S. patent application 98/77. 17. U.K. patent 1525947 (1978). A. B. Munksjo and A. Osakeyhtio, Ahlstrom, method and an apparatus for recovering t”rbre and fibrous material. 18. H. Alter, Development of specifications for recycled materials, 1st World Recycling Congress, Bask, Switzerland, March (1978).