17 - Columbia University in the City of New Yorkps24/PDFs/Biomodification of Mineral Surfaces and... · Biomodification of Mineral Surfaces ... identified a bacterium, now called

Lazaridis et al.

1paration Techniques75.flotation from dilute

17

Biomodification of Mineral Surfacesand Flotation

nd flotation of com-Ilogy (P. Mavros andp.307.i, Separation of toxic14: 441 (1979).:ipitate flotation for~velopments in Sepa-'L(I979),p.43.[avros, Dissolved-air(1992).

M. K. Yelloji Rao and P. SomasundaranHenry Krumb School of Mines, Columbia University, New York, New York

~

I. INTRODUCTION

Association of microorganisms with the formation and solubilization ofmineral deposits since geological times is well known. Mining operationshave long benefited from the activities of such naturally occurring mi-crobes, especially from the ability of some bacteria to leach metals frominsoluble ores. In spite of the fact that biooxidation of minerals has beenoCcurring in nature for centuries, it was believed until recently to be apurely chemical process mediated by water and dissolved oxygen. The con-tribution of the bacteria to metal leaching was recognized in 1947 whenColmer and Hinkle (1) identified a bacterium, now called Thiobacil/usfer-rooxidans, from the acid drainage of bituminous coal mines. The presenceof bacteria in the leach waters of Rio Tinto mines was first confIrmedin 1963 (2). Today, many countries throughout the world are adapting

bioleaching processes to recover metals from a wide variety of ores. ThePrincipal metals recovered microbiologically on an industrial scale includecopper, uranium, and gold, although other metals could also be recovered.Apart from Thiobacil/us ferrooxidans, innumerable bacterial species areknown to be associated with leaching operations. All these organisms ingeneral are acidophilic chemolithotrophs.

Several review articles and symposium proceedings summarize variousaspeCts of bacterial leaching and provide excellent background information

455

456Yelloji Rao and Somasundaran

Biomodification of Mineral Sur:

trophic, capable of using bsulfur) and simple organic c(sources. Sulfate-reducing baof anaerobic bacteria capalthem to hydrogen sulfide, fo

III. INFLUENCE OF SACPRODUCTS ON MI~

A. Biomodification of Su

Solojenken (20,21) reportedthe type SRB, microbe fat, ~and nonsulfide minerals. Thboth chalcopyrite and sphal((Fig. 1) (20). Studies with difdesorb the xanthogenate coa;In the case of bulk concentrthough control experiments (

bacteria of relevance to mineral beneficiationinfluence of bacteria and their products on flotationfactors that influence biomodification process

23

II. BACTERIA OF RELEVANCE TOMINERAL BENEFICIATION . .

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Figure 1 Effect of conditioning{Previously published in Proceecmission from Indian Institute of t-

Yelloji Rao and Somasundaran Biomodification of Mineral Surfaces 457

misms could also be used withial leaching generally requiresbstrate, while biomodificationl few minutes. Applications ofbeneficiation of sulfide miner-tk flotation, removal of pyriticcious metals, encapsulated inulfide concentrate followed byI sulfide concentrate. Reagentsites and bacterial debris couldand fluorspar minerals. Therecent information on bioflo-

ased technologies for mineral

trophic, capable of using both inorganic substrate (ferrous and reducedsulfur) and simple organic compounds (glucose and yeast extract) as energysources. Sulfate-reducing bacteria (SRB), desu/jovibrio, is a specific groupof anaerobic bacteria capable of using the oxygen of sulfates, reducingthem to hydrogen sulfide, for performing anaerobic respiration (19).

III. INFLUENCE OF BACTERIA AND THEIRPRODUCTS ON MINERAL FLOTATION

A. Biomodification of Sulfide Mineral Surfaces

Solojenken (20,21) reported for the first time the use of microorganisms ofthe type SRB, microbe fat, and biomass in the flotation of several sulfideand nonsulfide minerals. The SRB was found to depress the flotation ofboth chalcopyrite and sphalerite but not those of molybdenite and galena(Fig. I) (20). Studies with different sulfide concentrates show that SRB candesorb the xanthogenate coatings making them lose th;o" flotation activity.In the case of bulk concentrate containing both sphalerite and galena, al-though control experiments do not show any selectivity in their separation,

Ie available literature is made

iciationts on flotationprocess

I

'QQ

studied bacterium and is cur-economic importance. It is aletabolism through the oxida-ganic sulfur compounds. Thisassociated with sulfide miner-ecies widely involved in leach-rospirul/um ferrooxidans, andferrooxidans can directly oxi-'ial attachment or indirectlycal product (11-14). Galvanicre than one sulfide mineral is1 a case is to remove passive,15-17). Thiobacil/us thiooxi-oxidizing sulfur and reduced-iron (6). Leptospiral/um fer-lobus acidocaldarious species1m activity at higher tempera-Irganism Sulfolobus is mixo-

80

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Time. min

Figure 1 Effect of conditioning with sulfate-reducing bacteria on sulfide flotation.(Previously published in Proceedings of ICCM-79, BARC, India. Reprinted with per-mission from Indian Institute of Metals.)

liltBiomodification of Mineral SUI458 Yelloji Rao and Somasundaran

1 00 ~ ..., p'

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treatment with SRB yielded about 9.50/0 recovery of galena while sphaleriterecovery under these conditions was only 4..50/0.

Yelloji Rao et al. (22,23) have reported the effect of bacterial condition-ing with T. ferroox;dans on the floatability of sulfide minerals. Figure 2shows the effect of bacterial conditioning on sphalerite recovery underdifferent flotation conditions (23). While pretreatment with sulfuric acidsolution at pH 2 without any bacteria itself improved sphalerite flotationsignificantly (with and without flotation reagents), conditioning at the samepH with T. ferrooxidans (loa cells/mL) further improved the floatability.However, bacterial treatment did not show any effect when flotation wascarried out after conditioning with both activator and collector. On theother hand, when the cell dosage was increased to 10' ceUs/mL, the float-ability of sphalerite was reduced drastically (Fig. 3), even when flotationwas carried out after conditioning with flotation reagents (23). In the caseof galena also, natural floatability was enhanced appreciably upon pretreat-ment with sulfuric acid solution (Fig. 4) (23). However, when T. ferrooxi-dons (10' ceUs/mL) was also used for conditioning, such enhancement ofnatural floatability was not observed. The floatability of collector-treated

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Fipre 3 Effect of bacterial conmL con.centration of T. ferrooxiMinerals and Metallurgical Pro<:Society of Mining Engineers, In(I OO~~~::~~::::e: ~

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galena was reduced by biopicells/mL further depressed tl

During the conditioning'face oxidation of the minesphalerite and galena can be

znS -+ Zn2+ + S + 2e

PbS -+ Pb2+ + S + 2e

Elemental sulfur thus generahence can increase the natu:Thiobacil/us jerrooxidans is Jsulfate (14). At the acidic pHite surface is soluble, lead sPresence of oxidized insolulknown to interfere with the arecovery of galena was signifhigh cell dosage of 1~ cellsmainly by the enhanced aUac

20

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. 6 8 2. 48 72

Conditioning Time, h

Figure 2 Effect of bacterial conditioning on the floatability of sphalerite at 10' cells!mL concentration of T. ferrooxidans at 1 % pulp density. (Previously published inMinerals and Metallurgical Processing, Vol. 9, 1992. Reprinted with permission fromSociety of Mining Engineers, Inc.)

~~

Yelloji Rao and Somasundaran Biomodification of Mineral Surfaces 459

very of galena while sphalerite0/0.e effect of bacterial condition-of sulfide minerals. Figure 2on sphalerite recovery underretreatment with sulfuric acidimproved sphalerite flotation

ents). conditioning at the same:her improved the floatability.any effect when flotation was:tivator and collector. On thesed to l~ cells/mL. the float-(Fig. 3). even when flotation

ltion reagents (23). In the caseced appreciably upon pretreat-~. However. when T. jerrooxi-:tioning. such enhancement ofloatability of collector-treated

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0 . 8 2. .8 72 96 120 ,.. 168 192

Conditioning Time. h

Figure 3 Effect of bacterial conditioning on the floatability of sphalerite at 109 cells!mL concentration of T. ferrooxidans at 20% pulp density. (Previously published inMinerals and Metallurgical Processing, Vol. 9, 1992. Reprinted with permission fromSociety of Mining Engineers, Inc.)

~

galena was reduced by biopretreatment, and increase of cell dosage to 1~cells/mL further depressed the flotation drastically (Fig. 5) (23).

During the conditioning with sulfuric acid solution, dissolution or sur-face oxidation of the mineral is possible. The dissolution reactions forsphalerite and galena can be represented as follows:

ZnS -+ Zn2+ + S + 2e (I)

PbS -+ Pb2+ + S + 2e (2)

Elemental sulfur thus generated on the mineral surfaces is hydrQj)hobic andhence can increase the natural floatability of both sphalerite and galena.Thiobacil/usjerrooxidans is known to oxidize such elemental sulfur to formsulfate (14). At the acidic pH of 2, while the zinc sulfate formed on sphaler-ite surface is soluble, lead sulfate species formed on galena is insoluble.Presence of oxidized insoluble products on the sulfide mineral surface isknown to interfere with the action of the collector (24). Hence, the flotationrecovery of galena was significantly decreased after biopretreatment. At thehigh cell dosage of 109 cells/mL, floatability is proposed to be governedmainly by the enhanced attachment of the bacteria. Recovery of both sphal-

oatability of sphalerite at 108 cells!density. (Previously published in

12. Reprinted with permission from

Biomodification of Mineral Surfaces 461

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&: 20

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4 8 24 48 72 96 120 144 168 192

Conditioning Time, h

lility of galena at 10- cells/mlreviously published in Miner-rinted with permission from

Figure 5 Effect of baderial conditioning on the floatability of galena at 10' cells/mlconcentration of T. ferrooxidans at 20% pulp density. (Previously published in Miner-als and Metallurgical Processing, Vol. 9, 1992. Reprinted with permission fromSociety of Mining Engineers, Inc.)

the microbes and their

d the possibility of using:ury sulfides by flotation.produced no change ~th10nite recovery decreasedreparation. It is suggested,ntimonite crYstals leading:mains unaffected ~th no

With 10 mgIL of microbe fat, fluorite was selectively floated while theassociated minerals, calcite and barite, floated little, and quartz floatedpractically none. The optimum flotation of fluorite with oleic acid, a con-ventional collector for nonsulfide ores, is obtained in the pH range of7-10while with microbe fat is in the range 4-10. The expanded pH range isconsidered to be due to the fact that microbe fat contains a number ofsaturated and unsaturated fatty acids, with the former fixed more rapidlyto the fluorite surface~ Infrared spectra for collector and microbe fat inter-actions with minerals were identical. The use of microbe fat (in I: I ratio ofoleic acid) yields practically the same quality scheelite concentrate withincreased WO3 recovery and reduced cost.

The use of biomass as a flotation agent has been demonstrated also forcelestine and associated minerals, calcite, barite, and quartz (20). Flotationrecoveries of these minerals using biomass are shown in Figure 6 as afunction of pH. Both calcite and barite were depressed with 10 mg/L ofbiomass while about 20'1. of celestine could be selectively floated with it.With an increase of biomass concentration to 20 mg/L, the selectivity couldbe increased to about 800/. around pH 10. With about 50-75 mg/L, all the

agents of biological origin;ents in the case of nonsul-s investigation showed thethe flotation of fluorspar.

-

Biomodification of Mineral Sur462 Velloji Rao and Somasundaran

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Dil8Ct t8:I8Fisure 6 Effect of biomass on the floatability of celestine and associated mineral!(1,2 3: 20 p.g/L; 4: 5 p.g/l; 5: 10 p.g/l). (Previously published in Proceedings 01ICCM-79, BARC, India. Reprinted with permission from Indian Institute of Metals.) Ba:t8ia~

(1010 ceIs/nt6,u.-,v fi

.." ~associated minerals were practically depresss. The depressing action ofbiomass was further compared with dextrine. a conventional depressant:300 g/ton of dextrine yielded a 96.2070 CaF.2 with 84.9070 recovery to becompared with 96.3070 of CaF2 with a recmery of 8M9lG'-8tain-..ith 50g/ton of biomass. The ability of biomass nw:rom~!ItO .hydrate inaqueous solution and to more selectively adsorb OK": t1m)gaDgue_mineralsmade it possible to use them as depressors in nonsulrlde Ba.-flotation.

Figure 7 Comparison of effecttion. {Previously published inReprinted with permission frorr

suppression was obtained wibacteria wherein the recoverbacteria and the associatedAttia and Elzeky (27) havemedium without bacteria ncsion of coal (Fig. 8). On thdid not affect pyrite flota!affect the flotation. and su<to the duration of culture pI

Pyrite suppression due tcdue to different reasons. Tcchanges in surface charge inbacterial metabolites. or ba

C. Removal of Pyritic Sulfur from Coal

Townsleyet al. (26) have reported the effect of bacteriaiconditioning withT. f~ooxidans on the suppression of pyritic sulfur ~a part of the COal-cleaning process. Advanced coal-cleaning processes are necessary to treatpyritic sulfur coals in an environmentally acceptable and cost-effectivemanner. The effect of conditioning pyrite with bacterial suspension in di-rect bacterial liquor and membrane-filtered liquor with and without bacte-ria at pH 2.0 is shown in Figure 7 (26). The natural floatability of 84.5'10decreased to about 7.70/. upon conditioning with bacteria suspended at pH2 for 2.5 min. Conditioning with membrane-filtered bacterial liquor gave32.5'10 recovery, while with direct bacterial liquor was about 17.40/.. Best

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Yelloji Rao and SomasundaranBiomodification of Mineral Surfaces

463

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lestine and associated mineralsy published in Proceedings of>m Indian Institute of Metals.)

Dilect b8:8;a ICJD' ~5 X 1010 ~

I. The depressing action ofa conventional depressant:with 84.90/0 recovery to beof 86.50/0 obtained with 50cromolecules to hydrate inIrb on the gangue mineralsnsulfide ore flotation.

Bacteri. ~ in ~ 2 ciltikj (1010 ~

~ fi ~ b8ct8ia Uq.xx ~, b8ct8ia (1010 c8hTj)

bacterial conditioning withulfur as a part of the coal-=sses are necessary to treat;eptable and cost-effectivebacterial suspension in di-or with and without bacte-tUfal floatability of 84.SOf81 bacteria suspended at pHtered bacterial liquor gave:Of was about 17.4'10. Best

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464 Biomodification of Mineral ~Yelloji Rao and Somasundaran

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periods have to be adequprocesses.

Dogan et al. (30) haverooxidans followed by fI(resulted in a coal with 10'leaching alone. However,as the removal of pyrite dl

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IV. FACTORS THAT IfBIOMODIFICA TIC

An understanding of the (oping appropriate biomodtation or suppression. Theconditioning are listed in 1fication process are discus

0:0 30 60 90 120 150 180 210 240

Preconditioning Time. min

Figure 8 Flotation of coal and pyrite at natural pH after preconditioning with nutri-ent medium and T. ferrooxidans. (a) Coal with aerated nutrient medium [0] and withT. ferrooxidans [']. (b) Pyrite with aerated nutrient medium. (c) Pyrite with 2-weekold T. ferrooxidans. (d) Pyrite with 6-week old T. ferrooxidans. (Previously publishedin Coal Science and Technology, Vol. 9, 1985. Reprinted with permission fromElsevier Science Publishers B. V.)

A. Particle Size

Use of microorganisms inthe surface involves bactewhere biomodification pr!few molecular layers to chavoid smaller particles as jcient surface to bring outtion may be necessary to a'coarser particles, althoughrequired is relatively less, tsubsequent flotation.

Kuniyoshi (28) have attributed pyrite ~ssion to bacterial oxidation,leading to formation of jarosite or a jarosite-like insoluble sulfate (hydro-philic film) on the pyrite surface. Ata.. and Elzeky (27) have suggestedthat the bacteria could adsorb on the mineral s~and grow. Also, T.fe"ooxidans are capable of producing polymeric surface active substances(mainly polysaccharides and lipids) that can adsorb- on the pyrite surface.Growth of bacteria coupled with adsorption of cdl-excreted compoundscan be expected to make the mineral surface more' wettable thereby affect-ing the floatability. The bioadsorption process is believed to be rapidenough to be completed in a matter of a few minutes. In fact, in the case ofpyrite, 90"'0 of the inoculated cells attached to the surface within 2 min ofconditioning (29). Earlier work on pyrite flotation has shown that a shortconditioning for 2.5 min with T. fe"ooxidans can reduce the natural float-ability (26). It is unlikely that within such a short period enough bacterialmetabolites are produced to affect the floatability to any measurable extent.The only other possibility for flotation depression under these conditions isthe bacterial attachment onto the minerals, which will result in a hydro-philic surface. Therefore, unlike in bacterial leaching processes, the forma-tion of bacterial metabolites and their attachment to minerals during short

Table 1 Factors To Be Conttion Process.

Mineral 1. particle2. pulp del1. inoculur2. preadap

a. minerb. mediu

I. effect of2. presenCf3. composi

Bacteria

Medium

SO}

50}

40

30

20

Yelloji Rao and Somasundaran Biomodification of Mineral Surfaces 465

periods have to be adequately considered in any study of biomodificationprocesses.

Dogan et al. (30) have reported that bacterial conditioning with T. fer-rooxidans followed by flotation not only removed pyritic sulfur but alsoresulted in a coal with lower ash content than sulfur removal by bacterialleaching alone. However, the reasons for the removal of ash content as wellas the removal of pyrite due to bacterial conditioning were not presented.

IV. FACTORS THAT INFLUENCEBIOMODIFICATION PROCESS

An understanding of the effect of various parameters would help in devel-oping appropriate biomodification of the mineral surfaces for selective flo-tation or suppression. The factors that can be controlled prior to or duringconditioning are listed in Table 1, and their possible effects on the biomodi-fication process are discussed.

fter preconditioning with nutri-I nutrient medium [0] and withledium. (c) Pyrite with 2-weekloxidans. (Previously published~printed with permission from

A. Particle Size

Use of microorganisms in mineral beneficiation through biomodification ofthe surface involves bacterial conditioning followed by flotation. In caseswhere biomodification process is governed by surface oxidation to only afew molecular layers to change its surface characteristics, it is beneficial toavoid smaller particles as it would take longer periods for modifying suffi-cient surface to bring out the desired changes. Also, higher cell concentra-tion may be necessary to achieve the desired effect. On the other hand, withcoarser particles, although the cell concentration and the conditioning timerequired is relatively less, there could be practical difficulties in carrying outsubsequent flotation.

ion to bacterial oxidation,k.e insoluble sulfate (hydro-Slzeky (27) have suggested;urface and grow. Also, T.ic surface active substancessorb on the pyrite surface.If ceU-excreted compounds)re wettable thereby affect-5S is believed to be rapidlutes. In fact, in the case ofthe surface within 2 min ofion has shown that a shorta.n reduce the natural float-)rt period enough bacterialy to any measurable extent.In under these conditions islich will result in a hydro-:hing processes, the forma-nt to minerals during short

Table 1 Factors To Be Controlled for Better Biomodifica-tion Process.

Mineral

Bacteria

Medium

1. particle size2. pulp densityI. inoculum (cell) concentration2. preadaptation to

a. mineral substrateb. medium pH

I. effect of pH2. presence and absence of nutrients3. composition

466Yello;i Rao and Somasundaran

Biomodification of Mineral ~

recovery of sphalerite (T,although almost unaffectf1& to 108 cells/mL, wasinitial cell concentration t(ability of galena was founcconcentration from 1& ceexplained earlier for sphalccation is governed by bactit is determined by the adstudy is significant for th(from a complex sulfide a~associated with precious mflotation suppression was Itance of optimum cell corthe removal of pyritic sulfocidocoldorious (31). The Jcell concentrations up to 2increase in the cell concentfoaming was observed, andents, O2 and CO2, hindering

Since bacteria could gro\longed conditioning, even iIbe taken to control the cell Jtioning is involved. This mprocesses since time involv,minutes.

B. Pulp Density

Pulp density would be an important controlling factor where biomodifica-tion process is governed by either bacterial attachment or surface ox.idationby bacterial activity. For the latter case, at low pulp densities, the modifica-tion rate is limited by the external surface area of the minerals, while athigh pulp densities, the important parameters are transfer and solubility ofgaseous nutrients necessary for bacterial activity. For a process governed bybacterial attachment, the controlling parameter is always the ratio of avail-able surface area to the cell concentration. Studies on the pyritic sulfurremoval rate from coal by the thermophilic microorganism S. acidoca/dari-ous showed an increase in the removal rate of up to 15% and then adecrease with further increase in the pulp density (Fig. 9) (31). This hasbeen attributed to particle agglomeration at relatively high pulp densitiesleading to a reduction in external surface area. The influence of bacterialconcentration as well as the absence of gaseous nutrients may also be thereasons for such a decrease in the removal rate.

C. Inoculum Concentration

Regardless of the mechanism of microbial action during biomodificationprocesses, cell concentration and their activity will playa dominant role inbringing out the desired changes. A change in cell concentration alone couldchange the floatability of the mineral to a larger extent. The flotation

?..j'" 15(/)~E.."i 10..

~>0Ef 5

g L -~--~~ iI0

0 5 10 15 20 25 30

Pulp density Ci coal/lOO ml)

~6---~

.

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Table 2 Effect of Initial Cell Coability of Sphalerite and Galena

~

SCuSI

Initial cellconcentration/mL,.0

I~I~10'10'10'Figure 9 Diagram showing pyritic sulfur removal as a function of coal pulp density.

(Previously published in Biotechnology and Bioengineering, Vol. 27, 1985. Re-printed with permission from John Wiley & Sons, Inc.) Previously published in Minerals an,

with permission from Society of Mini

Yelloji Rao and SomasundaranBiomodification of Mineral Surfaces

467

recovery of sphalerite (Table 2) after biotreatment with T. fe"ooxidans,although almost unaffected for an initial cell concentration in the range oflij2 to 101 cells/mL, was found to be decreased with further increase ininitial cell concentration to lij9 cells/mL (23). On the other hand, the float-ability of galena was found to decrease gradually with increase in initial cellconcentration from lij2 cells/mL to 109 cells/mL during biotreatment. Asexplained earlier for sphalerite, while at low cell concentrations, biomodifi-cation is governed by bacterial-induced oxidation; at high concentrations,it is determined by the adsorption of the bacteria on the minerals. Thisstudy is significant for the differential flotation of sphalerite and galenafrom a complex sulfide as well as for bioflotation of the sulfide matrixassociated with precious metals. In the case of pyritic sulfur removal also,flotation suppression was better at higher cell concentrations (26). Impor-tance of optimum cell concentration is further shown in Figure 10 forthe removal of pyritic sulfur from coal by the thermophilic organism S.acidocaldarious (31). The removal rate increased initially with increasingcell concentrations up to 2 x 101 cells/mL and decreasr-Yith any furtherincrease in the cell concentration. At very high cell concentrations, heavyfoaming was observed, and this may reduce the transfer of gaseous nutri-ents, O2 and CO2. hindering the desired bacterial activity (29).

Since bacteria could grow on mineral substrates alone (23) during pro-longed conditioning, even in the absence of nutrient medium, care shouldbe taken to control the cell population whenever longer duration of condi-tioning is involved. This may not be a critical factor in biomodificationprocesses since time involved in initial conditioning is limited to a fewminutes.

19 factor where biomodifica-Ichment or surface oxidationpulp densities, the modifica-ea of the minerals, while atire transfer and solubility ofy. For a process governed byr is always the ratio of avail-tudies on the pyritic sulfur:roorganism S. acidocaldari-of up to 15% and then a1sity (Fig. 9) (31). This has~latively high pulp densities. The influence of bacterials nutrients may also be the

ion during biomodificationvi1l playa dominant role inII concentration alone couldIfger extent. The flotation

Percent recovery

Sphalerite (10-6 MCuSO. + 10-' M NaIX)

Initial cellconcentration/rot Galena (10-' M NaIX)

0

IOZ

I~

10'

10*

10'

97.094.696.395.994.252.9

92.184.078.275.948.322.5

function of coal pulp density.Jeering, Vol. 27, 1985. Re-

468 Yelloji Rao and Somasundaran Biomodification of Mineral

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Initial cell concentration (cell.tal

. I . I

4 8 12

Bioleacbing c0

Figure 11 Diagram showingleaching time on the sphaleritEngineering Journal, Vol. 44, 1S.A.)

Figure 10 Diagram showing the effect of initial cell concentration on pyritic sulfurremoval. (Previously published in Biotechnology and Bioengineering, Vol. 27, 1985.Reprinted with permission from John Wiley & Sons, Inc.)

of T. jerrooxidans, whichhave been developed (35).

E. Medium pH and Con

As biomodification proces~tioning followed by flotatiooptimum bacterial activityconditions may be differeninitial conditioning or the b.less the same activity at the j

Effect of nutrient mediurstood in order to determinesubjecting them for the biofor the growth and activitypyrite flotation, nutrient metion (27) while membrane-markedly (26). Also membr.

D. Preadaptation of Bacteria

The major question that arises in using microbial p~8es is with respectto longer conditioning time. Preadaptation ~acteria'JD mineral substratecan reduce the duration of bacterial conditioDina.;pric-r - t~DD. FigureII shows the effect of bacterial adaptation o~the.;b~~~ching ofsphalerite (32). Although the extraction of zinc dlMSi"~sMdJ. increasedduration of bioleaching, for a given leaching time.extract~~ increaseswith the bacterial adaptation period. The growth.;and~y of bacteria,in general, follow a lag-log-stationary phase. !)be. to!.1'rcadaptation, the lagphase will be either reduced or diminished depending on the duration,and the bacteria would begin their biomodification process immediately.Adaptation of T. fe"ooxidans to various sulfide mineral substrates is awell-known phenomenon (32-34).

For the biomodification process wherein chemical oxidation is also in-volved, a higher acidic pH is advantageous for faster rate. The optimumgrowth and microbial activity for T. ferrooxidans is at pH 2.3 in the tem-perature range of 20-35°C. Preadaptation of these bacteria to acidic mediacould improve tolerance to lower pH values of 0.5 to 1.0. Modified strains

2.$

2.0

1.5 t

1.0

0.5

o;i Rao and Somasundaran 469Biomodification of Mineral Surfaces

p1%6.ee~/y

~~ 10

~C0 8~.a0 61VIVI:eC 4N

/0 4 Weeks

/A...~::::~ W..k.~~~::::: ..9'"

~

a;'

Q4 8 12 16 20

Bioleaching duration, days

24*Figure 11 Diagram showing the effect of the bacterial adaptation period and bio-leaching time on the sphalerite bioleaching. (Previously published in The ChemicalEngineering Journal, Vol. 44,1990. Reprinted with permission from Elsevier SequoisS.A.)

Icentration on pyritic sulfur~ng;neer;ng, Vol. 27,1985.

of T. ferrooxidans, which can withstand very acidic solutions (pH < 1),have been developed (35).

processes is with respecteria to mineral substratetrior to flotation. Figureile bacterial leaching ofs increase with increasedextraction also increasesa.nd activity of bacteria,0 preadaptation, the lagnding on the duration,>n process immediately.mineral substrates is a

E. Medium pH and Composition

As biomodification process is a two-stage process involving initial condi-tioning followed by flotation, the pH maintained has to be suitable for bothoptimum bacterial activity and flotation. In many cases, these two pHconditions may be different, and hence either pH has to be altered afterinitial conditioning or the bacteria has to be preadapted to possess more orless the same activity at the flotation pH. .

Effect of nutrient medium on the flotation of minerals should be under-stood in order to determine whether the cells have to be separated prior tosubjecting them for the biomodification purpose. Nutrients are necessaryfor the growth and activity of bacteria. It is reported that in the case ofpyrite flotation, nutrient medium alone does not have any effect on flota-tion (27) while membrane-filtered bacterial liquor can reduce flotationmarkedly (26). Also membrane-filtered liquor supplemented with bacteria

cal oxidation is also in-Lster rate. The optimumis at pH 2.3 in the tem-bacteria to acidic mediato 1.0. Modified strains

470 Biomodification of MineriYelloji Rao and Somasundaran

brings out better suppression of pyrite than bacterial suspension at pH 2.0(26).

V. SUMMARY

Biomineral technology involving surface modification of minerals by bacte-rial conditioning prior to flotation could be successfully used in mineral,beneficiation. Applications of biomodification of mineral surfaces includebeneficiation of sulfide minerals from complex ore body by bulk or selec-tive flotation, removal of pyritic sulfur from coal, and the enrichment ofprecious metals encapsulated in sulfide matrix. In the case of flotation ofnonsulfide and nonferrous ore deposits, reagents of biological origin suchas microbe fat and biomass act as collector and depressant. These reagentsnot only provide better selectivity and recovery but also are less expensivethan conventional reagents. Mechanisms for surface modification can in-volve either adsorption of microbes and their products or surface oxidationto a few molecular layers to change its surface characteristics. Change inbacterial concentration alone plays a dual role in altering the floatability ofminerals. Presence and absence of nutrients, necessary for bacterial growthand activity, show different effects on the flotation of minerals. Preadapta-tion of desired bacteria to mineral substrates enhances the rate of biomodi-fication, thereby decreasing the period of conditioning required prior toflotation. This would be an important factor with respect to use of microor-ganisms in mineral beneficiation. Adaptatio.. of bacteria to medium pHand composition could also be useful for obtaining maximum selectivity inflotation. Most significant development in.. this area will come from devel-opment of microbes that have functional groups that.can.selectively interactwith minerals. It should also be possible to develop'miarDtes with flDlbriaewith selective recognition for mineral surface species when the precise roleof the fimbriae in attachment and adhesion to surfa~s;understood.

ACKNOWLEDGMENTSThe authors acknowledge Unilever Research, USA for partial support ofthis work.

4. D. Woods and D. E.tion in BiotechnoloK.York, pp. 81-93 (198

5. A. E. Torma, Leachiin Eight Volumes (H.Germany,6B:367-39

6. S. R. Hutchins, M. Sisms in reclamation 0

7. J. O. Gregory"and :Biotechnol. Progr., 2

8. L. E. Murr, Theoryin-situ, Min. Sci. Eng

9. K. A. Natarajan, I. 1\interactions of interesress in Biohydrometa,Mineraria Sarda, Ital)

10. K. A. Natarajan, "Bi,and Practice in Hydro

II. V. K. Berry and L. Estudies of their catalytMetallurgical ApplicaPhenomena (L. E. MuNew York (1978), pp.

12. V. K. Berry, L. E. t.chalcopyrite and pyritmetallurgy, 3: 309-32(

13. A. E. Torma, Biohydland Bioengineering Sy

14. L. C. Bryner, J. V. B(leaching mineral sulfic

15. K. A. Natarajan andProcesses, (J. A. Clun1-13.

16. N. Jyothi, "The rolesulfides," M.Sc. (Engg

17. B. E. Purkiss, CorrosAspects of Metallurgy128. (1970).

18. T. D. Brock, K. M. BIgenus of sulfur-oxidizArch. Mikrobiol., 84: :

19. J. R. Postgate and lcies, the nonspolrulati(1966).

20. P. M. Solojenken, "FIagents of Biological OrcaI Metallurgy, BARC,

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Biomodification of Mineral Surfaces 471'elloji Rao and Somasundaran

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28. I. INTRODUCTION

When one tries to derm(among other things, theother words, the start is ;creation of a froth of a l

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Nevertheless, referenceare the most important cl~particularly to the way th,classifications available fcsulfides) forms the thio c<mercapto group; common

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