Hydrometallurgy, 5 (1980) 117--125 117 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

R E M O V A L O F P Y R I T E F R O M C O A L BY C O N D I T I O N I N G W I T H Thio- bacillus f e rroox idans F O L L O W E D BY O I L A G G L O M E R A T I O N

A.G. KEMPTON, NAYERA MONEIB

Department of Biology, University o f Waterloo, Waterloo, Ontario (Canada)

R.G.L. McCREADY*

Department of Energy, Mines and Resources, Ottawa, Ontario (Canada)

and C.E. CAPES

National Research Council of Canada, Ottawa, Ontario (Canada)

(Received April 3rd, 1979; accepted June 20th, 1979)

ABSTRACT

Kempton, A.G., Moneib, N., McCready, R.G.L. and Capes, C.E., 1980. Removal of pyrite from coal by conditioning with Thiobacillus ferrooxidans followed by oil agglomera- tion. Hydrometallurgy, 5:117--125.

A novel process that can reject as much as 90% of the pyrite from finely divided coal is described. Cells of Thiobacillus ferrooxidans are mixed with an aqueous slurry of the coal and the mixture is vigorously agitated for as little as 15 min. Pyrite is apparently rendered hydrophilic and is rejected with the tailings when the hydrophobic coal is recovered by oil agglomeration. The process is superior to bacteriological oxidation which requires several days to remove comparable amounts of pyrite from coal through conversion to sulphate.

It is shown that the bacterial cells can be recovered after agglomeration and used to treat at least six successive batches of coal. The minimum contact time, the minimum ino- culum size and the ultimate number of times that the bacteria can be recycled are related variables that could not be defined under the laboratory conditions used. It was found, however, that Thiobacillus ferrooxidans grown on rejected railings supplemented with in- expensive mineral salts are at least as effective in the process as cells grown in synthetic medium.

INTRODUC~ON

The e f f ec t o f Thiobacil lus f e r roox idans o n p y r i t e i n coa l has b e e n u n d e r

s t u d y s ince 1947 ( C o l m e r a n d H ink l e , 1947 ) w h e n t he m o t i v a t i o n fo r such research was t he r e a l i z a t i o n t h a t acid m i n e d r a i n a g e was t h e r e su l t o f ba c t e r i a l o x i d a t i o n of i r o n su l f ide m i n e r a l s t o su l fu r ic acid. T h e first i n d i c a t i o n t h a t b io log ica l o x i d a t i o n o f p y r i t e c o u l d be bene f i c i a l , i f i t r e d u c e d the s u l f u r con-

NRCC No. 17816.

*Present address: Research Station, Agriculture Canada, Lethbridge, Alberta, Canada.

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tent of coal, was probably made by Ashmead (1955). Subsequent reports con- tinued to regard T. ferrooxidans solely as an oxidizing agent. The biological reactions were elucidated (Leathen et al., 1953a, 1953b; Silverman, 1967) and various methods to utilize these reactions to solve the problem of high- sulfur coal have been proposed (Silverman et al., 1963; Dugan, 1977). Unfor- tunately, the bacterial oxidation of pyrite is a self-limiting first, order reaction, probably due to the precipitation of Fe(OH)3, which progressively inhibits intimate contact between the microorganisms and the surface of the sulfide mineral. Consequently, the best oxidation experiments reported to date re- quire 5 days to remove about 97% of the pyrite (Dugan and Apel, 1978) using mixed cultures of T. ferrooxidans and T. thiooxidans. After compiling the re- sults of others and performing some further experiments, Moneib (1978) con- cluded that bacterial oxidation of high-sulfur coal is not likely to be a practi- cal process because it is too slow and requires a high ratio of bacteria to coal.

Capes et al. (1973) proposed a method for the desulfurization of coal in- volving t reatment of the finely ground coal slurry with T. ferrooxidans fol- lowed by coal recovery through selective oil agglomeration. Of the order of 80% of the pyrite could be removed by their method and it was concluded that total pyrite oxidation and leaching were not involved because little sul- fate was found and because relatively short biological t reatment times (from 1 to 3 days) were sufficient. Instead, it was postulated that the pyrite parti- cles were rendered preferentially hydrophilic and were rejected in the agglo- meration step through surface oxidation by bacterial action.

In more recent experiments, McCready (unpublished) found that cells of T. ferrooxidans need only be in contact with finely ground coal in water slur- ry for 30 min under vigorous agitation to cause up to 80% of the pyrite to be rejected during subsequent agglomeration with oil. From the point of view of a large scale industrial process, such shortened biological treatment times would be much more attractive than the 1- to 3-day periods used by Capes et al. (1973). In fact, the short contact times used by McCready suggested that a strictly physical adsorption phenomenon was taking place since the cells of T. ferrooxidans cannot multiply and grow to any significant extent in a 30 min contact period. Such a process might be termed bioadsorption, a physical process by which the bacteria render pyrite in fine coal hydrophilic so that the pyrite is rejected to a large extent with the tailings when the hydrophobic coal is recovered by oil agglomeration. The work reported below was under- taken to learn more about this novel microbiological coal desulfurizing pro- cess occurring within short biological t reatment times.

MATERIALS AND METHODS

The strain of T. ferrooxidans isolated by McCready from abandoned pyritic uranium mine tailings near Elliot Lake, Ontario was shown to have an opti- mum pH near 2.3 and grew well in Tuovinen and Kelly's (1973) medium. Stock cultures were prepared in 250-ml Erlenmeyer flasks, each containing

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100 ml o f medium, and grown at 30°C on a gyra to ry shaker for 72 h. S tock cultures remain active at 4°C for at least one week. For mass culture, a 10-1 ca rboy conta in ing 8 1 o f med ium was inocula ted with 400 ml of s tock cul ture and incuba ted at r o o m t empera tu re for 72 h under forced aeration and stir- ring.

To obtain a cell mass relatively free f rom solid con taminants , the cul ture was s iphoned off, leaving the Fe(OH)3 precipitate in the cul ture vessel. The cells were col lected by centr i fuging the cul ture at 20 ,000 X g for 20 min in a Sorvall RC-2B centrifuge. T h e y were then resuspended and washed twice with sterile Tuovinen and Kelly's FeSO4-free medium at pH 2.3, and finally resus- pended in the FeSO4-free medium. Cell num:Jers in the final suspension were de te rmined by a direct c o u n t using a Pet roff - -Hausser count ing chamber un- der a phase con t ras t microscope.

The coal was representat ive o f that found in the Minto area o f New Bruns- wick, Canada. Two samples were used in this s tudy to test the effect o f par- ticle size dis t r ibut ion on pyr i te removal. Their characteristics are given in Ta- ble 1.

TABLE 1

Properties* of coals used

Coal no.1 Coal no. 2 ( ground ) (ball milled )

Ash 19.2 22.2 Total sulfur 6.5 10.1 Inorganic (sulfate) 0.3 0.2 Pyritic sulfur 5.1 8.0 Organic sulfur 1.4 1.5

Particle size distribution** 4 (%) > 100~m 1 (%) > 8~m 11 > 60~m 10 > 6am 20 > 40am 30 > 5am 29 > 30am 89 > 3am 43 > 20am 95 > 2am 69 > 10am 88 > 5am 98 > 2~m

*All values expressed as percent by weight, dry basis. **Particle sizes > 45urn were analyzed by wet screen analysis, while those <45urn were analyzed with a Coulter Counter.

Stepwise details o f the laboratory-scale b ioadsorp t ion and oil agglomera- t ion processes have been recorded elsewhere (Moneib, 1978). Briefly, 100 g of coal-water slurry conta in ing 35 g of coal were added to 400 ml o f FeSO4- free med ium in a 1-1itre flask. The desired a m o u n t o f cell suspension was ad- ded and then the flask was shaken well and incubated on a gy ra to ry shaker

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for the s t ipu la ted t ime . Agg lomera t ion wi th Varsol (Esso) was carr ied out in a Waring Blendor equ ipped wi th a 0 - - 1 2 0 V rheos ta t accord ing to the proce- dure descr ibed b y Capes et al. (1973) .

Pyri t ic su l fur c o n t e n t o f coal was d e t e r m i n e d b y the p rocedu re o f van Hees and Ear ly (1949) excep t tha t the i ron was d e t e r m i n e d b y a tomic a b s o r p t i o n (Perkin-Elmer , Model 303, Norwalk , Connec t i cu t ) .

RESULTS AND DISCUSSION

The first series of expe r imen t s , the results o f which are comp i l ed in Tab le 2, was designed to over lap thc cond i t ions originally used b y McCready in his pre- l iminary work . Three levels o f cell loading and th ree c o n t a c t t imes were tes ted for each o f the coal samples descr ibed in Table 1. McCready ' s e x p e r i m e n t s c o r r e s p o n d e d to the mid- range cond i t i on wi th a cell loading o f 2 .66 X 101' cells/g coal and 30 min con t ac t t ime .

TABLE 2

Pyrite removal from different coals under variable conditions of inoculum size and bio- adsorption contact time

Treatment Coal no. 1 Coal nQ. 2

% pyrite % rejection % pyrite % rejection

Untreated 5.05 -- 8.01 -- Agglomeration alone 4.2 16.8 4.56 43.1 Variable inoculum size (contact time constant at 30 min) 5.32 x 10" cells/gcoal 2.90 42.6 2.48 69.9 (6 1 of medium used) 2.66 × 10 ' ' cells/g coal 2.80 44.6 2.50 68.9 (3 1 of medium used) 1.33 x 10 ~ cells/g coal 3.40 32.7 2.24 72.0 (1.51 of medium used) Variable contact time (inoculum size constant at 2.66 × 10 '~ celts/g of coal) 60 min 2.64 47.7 2.27 71.7 30 rain 2.80 44.6 2.50 68.9 15 rain 3.41 32.5 3.12 61.1

As expec ted , the resul ts in Table 2 show t h a t py r i t e re jec t ion increases as the coal impur i t ies are exposed m o r e ful ly b y finer gr inding (cf. Tab le 1). T h e tes t w i t h o u t i nocu l um agrees wi th the w o r k o f Capes et al. ( 1973) w h o f o u n d t h a t re jec t ion of grea ter t h a n a b o u t 50% of the pyr i t e was di f f icul t b y agglo- me ra t i on alone. With i n o c u l u m present , 70% or m o r e o f the py r i t e was rejec- ted using the f iner grind of coal no. 2. Doub l ing the i n o c u l u m c o n t a c t t i m e f r o m 30 min to 60 min had l i t t le e f fec t on the pyr i t e re jec t ion while a reduc- t ion to 15 min decreased pyr i t e r emova l to a b o u t the 60% level.

A d isadvantage o f the t echn ique used in these initial e x p e r i m e n t s is the

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large amount of chemicals needed for the nutrient medium. For example, in the experiment with a cell loading of 2.66 × 1011 cells/g coal, medium con- taining 99.9 g of FeSO4 • 7H20 was used to grow the bacteria required to de- sulfurize 35 g of coal (i.e. 2.85 g/g coal). In the results of Table 2, however, it is evident that pyrite rejection was independent of inoculum size over the range tested. Thus, although a considerable amount of FeSO4 • 7H20 was still required, these initial experiments demonstrated that cells from one-half the amount of medium originally used by McCready were sufficient to attain si- milar levels of pyrite rejection.

In an at tempt to reduce further the amount of nutrient needed for cell pro- duction, it was decided in the next series of experiments to reuse the railings water, which presumably contained the bacteria, to treat subsequent batches of coal. The initial experiments conducted with the ball-milled coal no. 2 were repeated and the waste water, containing the ash and the rejected pyrite from the first 35 g of coal, was used to treat a second 35 g sample by the bio- adsorption and agglomeration process. The washings from this second test were then used to treat a third, 35 g sample of coal. The results are given in Table 3, from which it was concluded that the bacterial inoculum did not lose effectiveness when reused twice. In fact, there was an apparent trend to in- creased efficiency but this was not tested for statistical significance. In addi- tion, the initial inoculum can be as little as 6.65 × 101°cells/g of coal and the bioadsorption contact time as little as 15 min without greatly affecting the amount of pyrite rejected.

These results suggested that multiple recycling of the tailings water should be studied further and the data in Table 4 show the effect of using an initial cell loading of 6.65 × 10 l° cells/g coal to treat six successive 35 g batches of

TABLE 3

Recycling bacterial i n o c u l u m to remove pyrite under variable cond i t ions o f inocu lum size and b ioadsorpt ion contact t i m e (coal no. 2)

Treatment Pyrite con ten t (per cent )

initial fresh inocu- inocu lum inocu- lum re- recycled lure cycled tw ice

once

Variable i n o c u l u m size ( contac t t ime cons tant at 30 rain) 5 .32 × 101~ cel ls /g coal 8 .01 2 .48 1 .72 1 .66 2 .66 × 10 ~1 cel ls /g coal 8 .01 2 .50 2 .13 1 .56 1 .32 × 1011 ce l l s /g coal 8 .01 2 .24 2 .04 2.07 6 .65 × 101°ce l l s /g coal 8 .01 2 .00 1 .84 1 .62

Variable c o n t a c t t ime ( i n o c u l u m size cons tant at 2 .66 × 1011 ce l l s /g o f coa l ) 60 rain 8 .01 2 .27 1 .56 1 .66 30 rain 8 .01 2 .50 2 .13 1 .56 15 min 8 .01 3 .12 2.47 1 .88

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TABLE4

Initial inoculum of 6.65 × 101° cells/g of coal recycled five times at 15 rain intervals. Pyrite content after bioadsorption and oil agglomeration (coal no. 2)

Treatment % Pyrite % Rejection

None 8.01 -- Agglomeration alone 4.56 43.07 Fresh inoculum 2.64 67.04 1st recycled inoculum 2.34 70.79 2nd 1.48 81.52 3rd 1.86 76.78 4th 1.98 75.28 5th 2.14 73.28

coal no. 2 a t 15 min intervals. The n u m b e r of successive ba tches tha t could be t rea ted in this way was l imi ted by the size of the l a b o r a t o r y e q u i p m e n t . Af te r each t r e a t m e n t , the to ta l v o l u m e o f l iquid increased b y the 65 ml of wa te r in which the 35 g o f coal was dispersed, plus the a c c u m u l a t e d v o l u m e of re jec ted py r i t e and ash. Af t e r six t r e a t m e n t s , the vo lume exceeded tha t o f the Waring Blendor . I t is evident in Tab le 4 t ha t recycl ing of the tailings liquid as a source of cells did no t decrease the e f f ic iency o f pyr i t e re ject ion, at least af- ter six stages of recycle . I t should be n o t e d tha t in the series o f expe r imen t s descr ibed in Table 4, a m e d i u m conta in ing on ly 25 g o f FeSO4 • 7 H : O was used to g row the cells needed to t r ea t 210 g of coal (i.e. 0 .12 g/g coal) . This co r r e sponds to a r educ t ion b y a f ac to r of 24 in the a m o u n t of added i ron nu- t r ien t c o m p a r e d wi th the a m o u n t no t ed above for the initial series of experi- ments .

I t is well k n o w n (Capes et al., 1973) tha t T. ferrooxidans can grow on the pyr i t e in coal. A final series o f expe r imen t s , the results o f which are given in Table 5, was p e r f o r m e d to use re jec ted pyr i te as a source of i ron for cell growth. The tailings f r o m the p reced ing e x p e r i m e n t (see Table 4), con ta in ing bac t e r i a t h a t had been recyc led five t imes and the re jec ted pyr i t e and ash, were placed

TABLE 5

Pyrite content after bioadsorption and oil agglomeration using T. ferrooxidans inoculum grown on waste pyrite (coal no. 2)

Treatment % Pyrite % Rejection

None 6.01 -- Agglomeration alone 2.54 58.36 Fresh inoculum 3.56 41.64 1st recycled inoculum 0.34 94.42 2nd 0.72 88.20 3rd 0.40 93.44 4th 0.43 92.95

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in a 10 1 carboy and brought to 8 1 with FeSO4-free medium at pH 2.3. The culture was aerated and stirred at room temperature for 2 weeks, at which time the formation of "yellow b o y " and a decrease in pH indicated that T. ferroxidanes was growing on the waste pyrite. Four hundred milliliters of this culture were used to treat consecutive batches of another sample of coal No. 2. As shown in Table 5, cells grown on rejected pyrite can be used in the bioadsorption process, followed by oil agglomeration, to eliminate as much as 90% of the pyrite from successive batches of coal.

There are two complementary avenues for further development of this bio- adsorption process. The extent to which a bacterial inoculum can be recycled must be determined in a semi-closed system designed to permit excess water and tailings to be bled off. Depending on the minimum inoculum size required to sustain the process, T. ferrooxidans may be grown entirely on waste pyrite under opt imum conditions in continuous culture or some FeSO4-grown cells may be required. Bioadsorption contact time and inoculum size may prove to be covariable.

In addition to investigating such process parameters, the biological mecha- nism of the process must be determined. The present results do not indicate an enzymatic reaction with pyrite because the pyrite rejected was remarkably constant (within analytical error) over a range of contact times and inoculum sizes. Surface activation (Tributsch, 1976; Murr and Berry, 1976; Bennett and Tributsch, 1978) is more probable, although the specificity for pyrite could require either active cells, cell components or metabolic products. Microscop- ic observations (Moneib, 1978) showed that cells appeared to become attached to particles which could be pyrite; but the selective recognition of substrate sites by active cells involves the mechanism of chemotaxis which would have to be proven. It is possible that the pyrite is rendered hydrophilic by specific wetting agents excreted by T. ferrooxidans. This has been proposed (Jones and Starkey, 1961) and phosphotidylinositol has been identified as such a compound in cultures of T. thiooxidans (Schaeffer and Umbreit, 1963), but others have doubted the presence of this phospholipid to any great extent in the culture medium of T. ferrooxidans {Schnaitman and Lundgren, 1965). Determination of the biological mechanism may further reduce the cost of this process. Should the mechanism not involve intact active cells, the biologi- cal agent could be produced under laboratory conditions rather than incorpo- rating a culture vessel as an adjunct part of the operational plant.

CONCLUSIONS

T. ferrooxidans is capable of rendering the pyrite in coal hydrophilic by a surface-active process known as bioadsorption. In this state, pyrite can be readily rejected from coal when used in conjunction with a separation process such as selective oil agglomeration. Bioadsorption is rapid enough to be worthy of consideration for larger-scale operations whereas self-limiting bacteriologi- cal pyrite oxidation is not. Bioadsorption takes place in a matter of minutes

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rather than days. Since the bacter ia are no t inactivated by the oil agglomera- t ion step, at least when Varsol is used, t h e y can be recovered f rom the tailings and recycled an unde te rmined n u m b e r o f times. It appears possible to re- charge the sys tem as required by growing bacter ia on waste pyrite. The bio- logical mechanism of the process has no t ye t been de termined.

ACKNOWLEDGEMENTS

This work was done under con t rac t to the National Research Council o f Canada (Cont rac t Serial numbers OSU77-00220 and OSU78-00161) .

The authors acknowledge the help o f W. Sprung of the University of Water- loo and o f R.D. Coleman of N.R.C.C. in suppor t o f this work.

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