On the selective flocculation of coal using polystyrene latex

International Journal of Mineral Processing, 17 (1986) 187--203

187

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ON THE SELECTIVE F L O C C U L A T I O N O F COAL USING POLYSTYRENE LATEX

M.J. LITTLEFAIR and N.R.S. LOWE

Department of Mining and Mineral Engineering, University of Leeds, Leeds LS2 9JT, U.K. (Received July 18, 1984; revised and accepted December 13, 1985)

ABSTRACT Littlefair, M.J. and Lowe, N.R.S., 1986. On the selective flocculation of coal using polystyrene latex. Int. J. Miner. Process., 17: 187--203. Selective flocculation tests were conducted on coal/shale mixtures using polystyrene latex produced by emulsion polymerisation. These tests were carried out on coal/shale mixtures nominally --45 # m particle size. Studies were undertaken varying the dispersant dosage, polymer dosage, solids concentration, pulp pH, particle size distribution of the solids, temperature and the conditioning times and shear rates. It was observed that crude polystyrene latex is an effective flocculant for coal. De,entrapment of shale from the flocculated concentrate by mechanical agitation was necessary for substantial reductions in final ash content.

INTRODUCTION

The recovery of valuable minerals from slimes b y selective flocculation has been under investigation for many years, particularly on a laboratory scale {Read and Hollick, 1980). Their work has shown that under certain conditions, selective flocculation o f one mineral from a mixed mineral slurry can be achieved by adding small quantities of polymeric flocculants. There is one industrial application of the selective flocculation process at the Tilden Mine in the U.S.A. at which iron ore fines are recovered (Paananean and Turcotte,

1980). Selective flocculation is particularly suited to slimes treatment, as these can present problems in the processing of coarser products, and should be removed at an early stage in the processing circuit. For example, during flotation, slimes often produce excessive, non-selective adsorption of reagents due to the high surface energy of the particles (Plaksin, 1963). Also because of increased solubility and dissolution rates compared with coarser material, slimes often release ions into solution that can activate gangue species and cause a reduction in collector selectivity (Collins and Read, 1971). Discarding these slimes can result in significant losses of valuable

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188 minerals, especially if they are friable (Kaseman, 1951) and up to twenty percent of the total ore values can be lost at some plants (Friend and Kitchener, 1973). Large quantities of sub-flotation size coal are discharged every year into lagoons, and selective flocculation offers a means of recovering this useful material in the discard. In the work described, polystyrene latex has been used as a selective flocculant for coal and possesses a number of inherent advantages over polyacrylamides for this use which are outlined below. SELECTIVE FLOCCULATION OF COAL The selective flocculation of coal from shale has been studied in the past by a number of authors from which it is clear, with one exception, that coal is always the flocculated component (Read and Whitehead, 1973). Polyacrylamide flocculants have been used to selectively flocculate coal using sodium pyrophosphate as a dispersant (Blagov, 1970). In particular, low molecular weight non-ionic polyacrylamide flocculants have been found to function selectively for Pittsburg coals with most dispersants, especially at high pH (Hucko, 1977). Corn starch has also been used as a selective flocculant for coal in conjunction with carboxymethyl-cellulose as a dispersant, although using this natural polymer, high levels of shale entrapment were found in the coal flocs (Hucko, 1977). Polyacrylamides have also been used as selective flocculants for coal in conjunction with sulphonated polystyrene as a dispersant for shale (Mozgovoi, 1969). Various other attempts have been made to selectively flocculate coal from shale (Mieleki et al., 1963; Korczagin et al., 1969), and Blaschke (1972) has selectively flocculated the shale component using carboxymethylcellulose as a dispersant and 'Gigtar' as the flocculant, although the a m o u n t of coal remaining in dispersion was low. Other substances that have proved to be good flocculants for coal include polyethylene oxide (Kaminskii et al., 1976), highly anionic polyacrylamines (Miller and Deurbrouck, 1973). Ultrasonic waves (Han et al., 1977) and latexes (Lyndov et al., 1979). Modified polyacrylamides have been developed to enhance selectivity of the polymers to the coal surface. The addition of cresol into a non-ionic polyacrylamide has been shown to improve selectivity of the flocculant towards the coal surface, and therefore reduce the ash content of the coal flocs (Brookes et al., 1982). Also polyxanthate dispersants have been developed that selectively disperse pyrite in mixtures with coal, and on the addition of an anionic polyacrylamide to the suspension, the coal was selectively flocculated (Attia, 1982). The same author has shown that anionic polyacrylamides are selective flocculants for coal in mixtures with shale. However, the selectivity of flocculants for coal can be considerably reduced by the presence of inorganic ions (especially Ca2+), which lower the zeta potential of the shale (Hucko, 1977).

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ADSORPTION OF POLYMERS ONTO COAL AND SHALE

The adsorption of polyacrylamide onto clays in weakly acidic media is a chemisorption phenomenon (Stutzman and Siffert, 1977), where the polyelectrolyte is adsorbed onto the external surfaces of the clay. The polyacrylamide adsorbs by replacing adsorbed water from the clay surface (Wadsworth and Peck, 1958; Parfitt and Greenland, 1970). Since the dissociation of these sites is pH dependent, this gives rise to a wide variation in adsorption capacity with pH. Also adsorption of polyacrylamides onto clays may be 'entropy
190 the adsorbed polymer is higher than that in solution, and this effect has been shown to occur with polystyrenes of different molecular weights adsorbing onto glass (Furusawa et al., 1982). Also polymers of lower molecular weight adsorb more rapidly onto particulate surfaces, but are replaced by higher molecular weight polymers which show preferential adsorption (Kolthoff and Gutmacher, 1952). Polystyrene adsorbed onto carbon black is held by a number of anchor segments, each occupying an area roughly corresponding to an adsorption site capable of binding one solvent molecule (Frish et al., 1959). Also polymers adsorbed onto glass surfaces at low solution concentrations have more adsorbed segments than polymer layers formed at high solution concentrations (Furusawa and Yamamoto, 1983), since polymers adsorbed from dilute solution tend to adopt a flat conformation, whereas those absorbed from concentrated solution adopt an extended loop formation with fewer points of attachment (Stromberg et al., 1964). Thus uncharged polymers appear to adsorb onto mineral surfaces by hydrogen bonding or hydrophobic interactions, and charged polymers adsorb mainly by electrostatic attraction, although hydrogen bonding particularly with polyacrylamide plays a major role (Littlefair and Lowe, 1984). ORIGIN OF NON-SELECTIVITY IN POLYACRYLAMIDE POLYMERS Polyacrylamide polymers normally have some selectivity towards either coal or shale surfaces, although there is inevitably some adsorption of the flocculant onto unwanted mineral by virtue of the powerful hydrogen bonding capacity of the amide group (Emsley, 1981). This group is present in all polyacrylamides and consequently leads to higher gangue contents in the flocculated sediment than would otherwise be the case. Even the addition of chelating groups into the polymer and of selective surfactants to pre-treat surfaces prior to flocculation does not entirely overcome this problem. Since hydrogen bonding is a major attractive force between polymer and hydrated surface in aqueous suspensions, the only method of overcoming this problem of non-selectivity of adsorption would be to use polymers that have no hydrogen bonding capabilities. The use of hydrophobic polymers as selective flocculants presents an alternative to the conventional polyacrylamide flocculants, with the possibility of high selectivity. HYDROPHOBIC POLYMERS 1N SELECTIVE FLOCCULATION H y d r o p h o b i c polymers of high molecular weight which have been solubilised in water have an affinity for hydrophobic surfaces and cause flocculation of such minerals (Littlefair and Lowe, 1984). Minerals not naturally hydrophobic may be made so by treating the surface with a selective surfactant that renders it hydrophobic. This produces a suitable surface for adsorption by a hydrophobic polymer thereby causing it to selectively flocculate. There are no groups in such hydrocarbon molecules capable of hydrogen

191 bonding which means the polymer only has an affinity for the mineral possessing the hydrophobic surface and entrapment of gangue in the floccules is reduced to a minimum. The problem of solubilising these hydrophobic polymers in water for use as selective flocculants can be approached in three ways: (1) The polymer can be dissolved in an organic solvent and the mixture emulsified in water by means of an emulsifying chemical. (2) Dissolution of the polymer in a surfactant solution by formation of a soluble ionic complex o f surfactant and organic polymer. This adsorption of surfactant ions by the polymer is a result of interaction of the non-polar parts of both molecules by hydrophobic bonding (Isemura and Imanishi, 1958). (3) Synthesis of the polymer by emulsion polymerisation which produces a colloidal suspension of polymer spheres (Roe and Brass, 1957). This can then be used as a flocculant. This last approach was chosen to produce polystyrene for use as a selective flocculant for coal. THE PROBLEM

OF ENTRAPMENT

OF GANGUE

Gangue material not intended to flocculate can become bound to the floccules, chemically or physically and be carried down with them. Solutions have been suggested (Yarar et al., 1979) for freeing entrapped particles without destroying the floccules themselves. Yarar et al. (1979) tested a rotating cylinder, an elutriating column and a tilted-plate elutriator. Others have found it beneficial to apply low-frequency vibrations to the lower part of the tilted-plate elutriator, to free entrapped shale particles from coal flocs. EXPERIMENTAL

Preparation of coal and shale samples Plus 1" samples of coal and shale obtained from a nearby North Yorkshire colliery were crushed and ground to --600 pm in a rod mill. Having bagged the samples into 500-g bags, the average ash content of the coal and shale samples was determined by incineration at 815°C for 2~ hours. Samples were ground to approximately 5% + 45pm, though accurate size distributions are given.

Production of polystyrene Polystyrene has frequently been produced by emulsion polymerisation (Piirma et al., 1975). The reaction was carried out in glass bottles clipped onto a rotating wheel in a water bath, held at constant temperature. They

192 were held securely in position with Terry clips but easily removeable. The rotating wheel was driven by a chain from an electric motor with a variable speed control. Approximately 35 r.p.m, provided sufficient agitation for emulsion polymerisation. Emulsion polymerisation, being a free radical chain reaction, is highly sensitive to the presence of impurities. Precautions to deoxygenate the reactants and ensuring reasonable purity were taken. Important variables of polymerisation, monomer/water ratio, soap concentration, initiator concentration, time of reaction and temperature of reaction were standardised. The following conditions were used: Distilled water 150 cm 3 Styrene 60 cm 3 Potassium oleate (soap) 0.5 g Potassium persulphate (initiator) 0.1 g Temperature 60 ° C Reaction time 2 hrs.

Selective flocculation of coal and shale Experiments were conducted on 1 : 1 mixtures of coal and shale with an ash content of 47%. The apparatus used in the selective flocculation experiments consisted of a modified two-litre separating funnel, fitted with a variable speed stirrer. The flocculated sediment was removed from the bottom of the flask by opening the screw clip. The mixture was conditioned with dispersant for ten minutes prior to flocculant additions to ensure effective dispersion o f solids.

De-entrapment of flocculated sediment The elutriation column, similar to the one used previously (Clauss et al., 1976) but of simple design, was used w i t h o u t success. Therefore, as an alternative method the sediment was simply returned to the modified separating funnel in one litre of water, agitated and allowed to resettle. All de-entrapment experiments were carried out in tap water (approx. pH 7.5) and in about 4 kg per tonne dispersant (calgon), except where otherwise stated. MATERIALS Commercial BDH styrene, potassium oleate (prepared in the laboratory) and potassium persulphate were used for emulsion polymerization. The coal and shale used had ash contents of 4% and 89.5%, respectively.

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RESULTS

Preliminary trials showed that crude polystyrene latex had a strong affinity towards the coal surface and flocculated it well, but showed no tendency to flocculate shale. De-entrapment reduced the ash content of the final sediment significantly, while acceptable values for percent combustible recovery and percent weight yield were still obtained. Experiments were repeated three times (except experiments involving temperature variation, which were repeated twice.) An estimated average error is given as + 0.8% for ash content, + 3% for weight yield and + 5% for combustible recovery. A number of variables were considered important in determining the op-

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CONDITIONS: 2% solids concentration,

solids size class C, 3.9kg per tonne flocculant dosage, pH 6.0 KEY.' 1, % Combustible recovery before de-entrappment; 2 % Ash before de-entrappment; 3. % Weight yield before de-entrappment; 4. % Combustible recovery after de-entrappment; 5. % Weight yield after deentrappment; 6. % Ash after de-entrappment.

Fig. I. Variation of ash, combustible recovery and weight yield with dispersant (calgon) concentration.

] 94 timum conditions for selective flocculation and these are considered below Also the effect of de-entrapment has been exeunined for each variabl,,

1. Conditioning times and shear rates/'or selective flocculation The solids were conditioned for ten minutes in tap water at high shear (1200 rpm). The flocculant was conditioned for five minutes at low shear (350 rpm). Five minutes (-+ two minutes) was an optimum time for flocculant conditioning although selectivity was unaffected by shear rate (within this range of 350--1200 r p m )

2. Type and dosage o f dispersants Several dispersants were tested, including calgon (sodium hexametaphosphate), sodium silicate, sodium polystyrenesulphonate (VERSA TL-500 and NATROL 72); and polystyrene maleic anhydride sulphonate (VERSA TL-3 and VERSA TL-50). These were used with dosages ranging from 0 to 4 kg per tonne. Of these, only calgon proved satisfactory for the selective dispersion of shale. The results are shown in Fig. 1. At the calgon dosage of 4 kg per tonne an improvement of about 6% ash is observed. However, after deentrapment there is very little variation of final ash with calgon dosage. The per cent weight yield and per cent combustible recovery stayed constant and the latter only showed a small reduction after de-entrapment.

3. Polystyrene dosage The dosage of polystyrene was varied from 0 to 20kg per tonne. The results in Fig. 2 show a steady decrease in ash content of the flocculated product, with increase in polystyrene dosage, until about 5 kg per tonne beyond which no decrease in ash content was found. The weight yield and combustible recovery indicate no significant improvement beyond 2 kg per tonne. It is also clear that at all dosage levels tested, selective flocculation of the coal was occurring. De-entrapment is marginally more effective at higher polystyrene dosages. In practice, the floccules formed at higher dosages are observed to be larger. It is believed that t h e y are more strongly bound at higher concentrations of polystyrene since more polystyrene spheres are available for bonding. So these stronger floccules are able to survive the de,entrapment process better than smaller floccules formed at lower polystyrene concentrations.

4. Solids concentration It was found that the solids concentration had a significant effect on the ash content of the concentrate, at least between 1 to 5% solids. 2 per cent solids were found to produce the lowest ash content as illustrated in Fig. 3.

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FLOCCULANT DOSAGE/kg per tonne 2% solids concentration, 2-0 kg per tonne dispersant dosage, pH 6-0, solids size class C KEY: 1. % Combustible recovery before de-entrappment; 2. % Weight yield before de-entrappment; 3. % Ash before de-entrappment; 4. % Combustible recovery after de-entrappment; 5. %Weight yield after de-entrappment; 6. % Ash after de-entrappment.

CONDmONS:

Fig. 2. Variation of ash, combustible recovery and weight yield with flocculant dosage. The weight yield and combustible recovery curves indicate no improvement after 2 per cent solids. After de-entrapment similar trends are observed with the exception that weight yield increases with increasing solids concentration.

5. Pulp pH The pH of the pulp was adjusted before adding the polystyrene. Figure 4 illustrates the results. There is no significant change in ash content between pH 5 and 8 although an increase in ash is seen at pH 4. After de-entrapment, the ash content is seen to increase after pH 7. The decrease in combustible recovery and weight yield after pH 7 is noteworthy. It was observed that

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OE}NDITION$: 2.0kg per tonne dispersant dosage, pH 6.0, 13.7kg per tonne flocculant dosage, solids size class C KEY: 1 % Combustible recovery before de-entrappment; 2 % Weight yield beforede-entrappment; 3. % Ash before de-entrappment; 4.%Combustible recovery after de-entrappment; 5. %Weight yield after de-entrappment; 6 % Ash after de-entrappment

Fig. 3. Variation of ash, combustible recovery and weight yield with solids concentration. large floccules were formed at pH 4 and small floccules were formed at pH 8. This helps to explain the observed results, that is, high entrapment at pH 4 gives a high ash content, and at pH 8 weak floccules make de-entrapment less efficient. It seems that the pH is important in the bonding of polystyrene spheres to coal particles. 6. Particle size

The results are illustrated in Fig. 5 and the corresponding size distributions are given in Fig. 6. The samples were prepared by increasing the grinding time. Figure 6 shows that after one minute grinding the coal particles aggregate. However, the ash content of the concentrate decreases from class A through to class E in line with increased grinding time which serves to

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pH CONDnlONS: 2% solids concentration, 2.0kg per tonne flocculant dosage, 2.0kg per tonne dispersantdosage, solids size class C KEY: 1. % Combustible recovery before de-entrappment; 2. % Ash before de-entrappment; 3. % Weight yield before de-entrappment; 4. % Combustible recovery after de-entrappment; 5. %Ash after deentrappment; 6. %Weight yield afterde-entrappment.

Fig. 4. Variation of ash, combustible recovery and weight yield with pH.

show that the final ash is more dependent on the size of shale particles than coal particles. After de-entrapment an improvement in ash content is seen.

7. Temperature There is no significant variation in ash content of the sediment with temperature (5--30°C) though the weight yields and combustible recoveries are slightly improved at higher temperatures. Figure 7 shows this. De-entrapment gives better ash contents in the final sediment.

8. Conditions during de-entrapment The agitation time (0.5--15 minutes), the shear rate ( 3 0 0 - - 1 2 0 0 r.p.m.) and the dispersant dosage during de,entrapment did not have a significant

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PARTICI E SIZE CLAS!~ CONDITIONS: 2% solids concentration 3-7kg per tonne fiocculant dosage. 1.0kg per tonne d~spersantdosage, pH 6.0 K E Y : 1 % Combustible recovery before de-entrappr'nent; 2 % Ash be#ore de-entrappment; 3 %Wepght yield before de-entrappment; 4 % Combustible recovery after de-entrappment; 6 % Weight yield after de entrappment; 6 % Ash after de~ntrappment

Fig. 5. Variation of ash, combustible recovery and weight yield with particle size. effect on the ash of the final concentrate. The pH during de-entrapment showed an optimum at pH 6.0. DISCUSSION In the reaction to produce polystyrene the styrene was emulsified in water to produce micellar aggregates with the catalyst (or chain initiator) in the aqueous phase. The initial formation of free radicals also takes place in this phase (Harkins, 1947). As long as micelles are present new particles are formed, although as the particles grow, soap is continually adsorbed until eventually no more miceUes exist. The rate of new particle formation then decreases rapidly until the concentration of soap in the aqueous phase is so low that very few new particles form. The existing particles then tend to grow in size and the number o f particles remains constant (Ewart and Cart,

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Fig. 6. Size distributions of coal and shale samples measured by particle size analyser after 10 minutes mixing with calgon.

1954). The actual number of particles produced is a function of concentration of soap, temperature of polymerisation and initiator used (Smith, 1948). The production of polystyrene by this method tends to produce a bimodal distribution of particle sizes (Chao and Piirma, 1983). Polystyrene particles produced by emulsion polymerisation have 'hairs' protruding from the surface that are prevented from collapsing back onto the particle surface due to the presence of charged groups (from the initiator) and are 5--10 pm long (Van der Ven et al., 1983). These may allow the polystyrene particles to partake in a bridging mechanism, similar to polyacrylamide, onto the coal surfaces. There may also be a hydrophobic interaction between the coal and polystyrene surfaces causing mutual agglomeration. It is noted that cross-linked polystyrene consists of reactive aromatic regions separated by short aliphatic chains. It shows surface activity although it has no heteroatoms. This is very similar to the basic structure of coal. The use of hydrophobic polymers in selective flocculation offers a number of advantages over water soluble polyacrylamides. Since they contain no

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FEMPERATURE 'C CONDITIONS: 2% solnds concentration, 9.0kg per tonne flocculant dosage 1.0kg per tonne dispersant dosage, pH 6.0, solids s~zeclass C KEY: 1 % Combustible recovery before de-entrappment; 2 % Ash before de~entrappment; 3 %Wetght y~eld before de-entrappment 4 % Combustible recovery after de-entrappment 5 % Weqght yreid after du entrappment; 6 % Ash afterde-entrappment

Fig. 7. Variation of ash, combustible recovery and weight yield with temperature. amide groups, the hydrophobic polymers have no hydrogen bonding facilities and are, therefore, very selective towards hdyrophobic surfaces. Also, since polystyrene requires slightly higher dosages than polyacrylamides, the flocs are stronger and can be mechanically agitated without too much degradation. These polymers are also non-toxic and could be used safely where material may be released into water circuits. However, how hydrophobic polymers affect other processes in the processing circuit e.g. flotation is unknown. CONCLUSIONS The data presented clearly show that polystyrene latexes can be used as selective flocculants for coal and the physical conditions to achieve this have

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been optimised. De~ntrapment of the flocculated sediment produces significant decreases in ash. Under optimum conditions selective flocculation followed by de-entrapment produced a coal sediment of 14 per cent ash from a testing material of 47% ash. Further work is under way to test the polystyrene latex on different coals and to investigate the mechanism of flocculation. ACKNOWLEDGEMENTS

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