REAGENTS OF BIOLOGICAL ORIGIN IN MEfALLURGY

P. Somasundaran and M.K. Yelloji Rao

Henry Knunb School of Mines, Columbia University, New York, NY 10027

ABSTRACTReagents of biological origin are becoming more and more important in the field of

metallurgy, especially in the areas of mineral processing and hydrometallurgy. The advantage of loweroperating cost over physical and chemical processes and the capacity to operate with low grade oresmake the microbial processes further attractive. In this paper, the role of microbes directly as areagent and microbially produced reagents or secreted metabolites in mineral processing andhydrometallurgy will be discussed. Adhesion of microbes to surfaces is known to alter thehydrophobicity of minerals. Applications include surface modification to impart hydrophobicity orhydrophilicity on sulfide or non-sulfide minerals and dissolution of precious metals. Microbes can alsoperform the role of flocculating agents. Biosorption of toxic and heavy metal ions by microbes arefmding application in treating tailing ponds. Mechanisms associated with the use for microbiallyproduced reagents are discussed along with some recent results.

INTRODUCTIONThe ever increasing demand for the metals on the one hand and the decreasing availability

of resources on the other are stimulating work across the world to look for better reagents and newtechniques for the processing of low grade ores. What is expected of these reagents is betterselectivity with respect to the ooUector, the depressant, the activator or the modifier property.Reagents of biological origin are of interest in this regard since they do have specific naturalinteractions with minerals. Microbes, their debris, and the secreted metabolites can act as reagentsin mineral processing and hydrometallurgy due to direct or indirect interaction with minerals. Directinteraction involves adhesion or attachment to minerals, and thereby modification of the surfaces.Indirect interaction refers to the biological products acting as surface active reagents. Theseinteractions can cause changes in the hydrophobicity of the minerals and, in the case of fine particles,dispersion or flocculation of their suspensions. Also, toxic and heavy metal ions in tailing ponds,ground and surface waters can be removed by microbes due to biosorptionfbioaccumulationprocesses. Purpose of this review is to discuss recent microbial-based processes and technologies formineral beneficiation and extraction.

In this review, a aitical discussion of the available literature is conducted with special

256 REAGENTS FOR BETTER METALLURGY

reference to the following aspects.i) Microbially induced flotation processes

ii) Secreted metabolites as notation reagentsill) Biosorption of metal ions by microbes

MICROBIALLY INDUCED FUYrATION PROC&~SESSOlojenken and his group (1976, 1979) were the first to report the use of microorganisms of

the type sulfate reducing bacteria (SRB), microbe fat and biomass in the flotation of several sulfideand nonsulfide minerals. SRB was found to depress the flotation of both chalcopyrite and sphaleritebut not those of molybdenite and galena (FIgUre 1) (Solojenken, 1976). Studies with different sulfideconcentrates show that SRB can desorb xanthogenate coatings causing them to lose their flotationactivily. In the case of bulk concentrates containing both sphalerite and galena, although controlexperiments do not show any selectivity in their separation, treatment with SRB yielded about 95%recovery of galena while sphalcrite recovery undcr these conditions was only 4.5%.

Thiobaci/Jus fem>oxidalls is the most widely studied bacterium and is currently the majorleaching microorganism of economic importance. 7: fem>oxidalls can directly oxidize sulfide mineralsthrough prior bacterial attachment or indirectly through ferric sulfate generated as a metabolicproduct (Bryner et al, 1954; Berry and Muff, 1978; Berry et al, 1978; Tonna; 1986). Yelloji Rao,Natarajan and Somasundaran (1992 a & b) have reported the effect of bacterial conditioning withThiobacillus ferrooxidans on the floatability or sulfide minerals. F"lgUre 2 shows the effect of bacterialconditioning on sphalerite recovery under different flotation conditions (Yelloji Rao et al, 1992b).While pretreatment with sul£uric acid solution at pH 2 without any bacteria itsel£ improved sphaleriteflotation significantly with and without flotation reagents, conditioning at the same pH withThiobaci//us ferrooxidans (108 ~Us/ml) caused some further improvement in the floatability.However, bacterial treatment did not show any effect when flotation was carried out afterconditioning with both the activator (CuS°4) and the collector (sodium isopropyl xanthate). It is alsoshown that when the cell dosage was increased to 109 cells/ml, the floatability of sphalerite wasreduced drastically, even when the flotation was carried out after conditioning with the flotationreagents (Yelloji Rao et ai, 1992b). In the case of galena also, natural floatability was enhancedappreciably upon pretreatment with sulfuric acid solution (F"tgure 3) (Yelloji Rao et al, 1992b).However, when Thiobaci//us ferrooxidans (108 ceUs/ml) was also used for the conditioning, the

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reducing bacteria on sulfide flotation

u . , . , , 7"/ ,0 2 4 6 8 24 48 72

Conditioning Time. hoursFig. 2. Effect of conditioning with ThiobaciJlus

lenooxidons on sphalerite Ootation

REAGENTS OF BIOLOGICAL ORIGIN IN METALLURGY

enhancement of natural floatability obtained wasminimal. The floatability of collector treated galenawas reduced by biopretreatment and an increase ofcell dosage to 109 cells/ml further depressed theflotation drastically.

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During the conditioning with sulfuric acidsolution, dissolution or surface oxidation of the

mineral is possible.ZnS --> z.n2+ + S + 2e ...(1)PhS > Pb2+ + S + 2e ...(2)

Elemental sulfur thus generated on the mineralsurfaces is hydrophobic and hence can increase the

--~ ~." . natural floatability of both sphalerite and galena.0 ~ . . . , ',. ... . I Thiobacillus lerrooxidalls is known to oxidize such

0 2 4 6 8 24 .8 72 elemental sulfur to sulphate (Bryner et at, 1954).Conditioning Time. hours While the zinc sulfate fonned on the sphalerite

... . surface is soluble at the acidic pH of 2, lead sulfateFJ8;" 3.. Eft'ect .of conditionIng ~th species fonned on the galena is insoluble. OxidizedThiobaaUus ferrooxidalls on galena flotation insoluble products on the sulfide mineral surface

100 are known to interfere with the action of collector(Wark, 1938). The notation of galena was hencesignificantly decreased u~n biopretreatment. At

the high cell dosage of 109 cells/ml, floatability is

proposed to be governed mainly by the enhanced

attachment of the bacteria.

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Lyalikova and Lyubavina (1986) have discussedthe possibility of using Thiobacillus ferroaxidans toseparate antimony and mercury sulfides byflotation. Bacterial conditioning for ninety minutesproduced no change with respect to cinnabarrecovery (89%) while antimonite recoverydecreased from 89% to 62% leading to almostcomplete separation. It is suggested thatThiobacillus fenooxidans can oxidize the surface ofantimonite crystals leading to depression of thefloatability while cinnabar remains unaffected withno change in its floatability.

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~ Ba;taia SlJSpa'6Kr1 in I:ii 2distiled wa9" (1010 ceIshri) In contrast to the above in fuels area There is

- ~ filsed ba:terial tiq,u a dire need currently for advanced coal cleaning~ wi" ba:teria (1010 processes to treat pyritic sulfur coals in anceIshnI) environmentally acceptable and cost effective

Fig. 4. Comparison of etIect of different manner. Townsley et al (1987) have reported thetreatments on pyrite flotation etIects of bacterial conditioning with Thiobad/Jus

felTOOxidans on the suppression of pyritic sulfur asa part of coal the cleaning process. The effects of conditioning pyrite with bacterial suspension indirect bacterial liquor, membrane filtered liquor with and without bacteria at pH 2.Q are illustratedin Figure 4 (Townsley et a1, 1987). The natural floatability of 84.5% was found to decrease to about7.7% upon conditioning of with bacteria suspended at pH 2 for 2.5 minutes. Best suppression was

80

60

40

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258 REAGENTS FOR BETTER METALLURGY

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obtained with membrane filtered liquorsupplemented with bacteria wherein the recoverywas only 4%. This clearly shows the importance ofbacteria and the associated medium in altering thefloatability of pyrite. Attia and Elzeky (1985) haveshown that at natural pH, neither the nutrientmedium without bacteria nor the bacterialsuspension produced any depression of coal(Figure 5). In the case of pyrite, although nutrientmedium alone did not affect flotation, bacterialconditioning was found to affect the flotation andsuch an effect was severe when old culture wasused.

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0;; 3'0 6~ 9~ 1~0 1~ 1~0 2~ 240 Different mechanisms have been suggestedPreconditioning Time. min for pyrite suppression due to bacterial ~nditioning.

Fig. 5. Effect of conditioning with nutrient Townsley et al (1987) have proposed It to be duemediUm and Thiobacillus ferrooxidans on to ch~es in ~urface cha~e in res~nse to eit~ercoal and pyrite flotation adsorption of mtact bactena, bacterial metabolites

or bacterial debris. Harada and Kuniyoshi (1985)have attributed pyrite depression to bacterial oxidation causing the formation of jarosite or a jarosite-like insoluble sulfate (hydrophilic film) on the pyrite surface. Attia and Elzeky (1985) havc pointedout that the bacteria could adsorb on the mineral surface in this case and grow. Also, Thiobadllusferrooxidims are capable of producing polymeric surface active substances (mainly polysaccharides andlipids) which can adsorb on the pyrite surface. Bacterial growth coupled with adsorption of cell-excreted compounds can be expected to make the mineral surface more wettable and thus affect itsfloatability. The bioadsorption process is belicved to be rapid enough to be completed in a mattcr offew minutcs. In fact, in the case of pyrite, Bagdigian and Myerson (1986) observed 90% of theinoculated cells to become attached to thc surface within two minutes of conditioning. Work on pyritcflotation has shown that a short conditioning for 2.5 minutes with Thiobacillus ferrooxidims candepress the natural floatability (Townsley et a1, 1987). It is unlikely that within such a short periodenough bacterial metabolites are produced to affect the floatability to a measurable extent. The otherpossibility then for flotation depression under these conditions is the bactcrial attachment onto theminerals which will result in a hydrophilic surface.

Dogan et al (1985) have reported that bacterial conditioning with Thiobaci/lus ferrooxidansfollowed by notation not only removed pyritic sulfur more than by bacterialleacbing alone but alsoresulted in a coal with a lower ash content. However, the reasons for the removal or ash content aswell as the removal of pyrite due to bacterial conditioning are not known.

BIOMODIFICATION OF NONFERROUS AND NONSULFIDE MINERALSMicrobial products such as secreted metabolites, microbe fat and biomass can also act as

flotation reagents. Solojenken (1979) has demonstrated the use of reagents of biological origin suchas microbe fat and biomass 3... flotation reagents for nonsulfide and fluorspar ores of different origin.Good selectivity was obtained with microbe fat as a collector for the flotation of fluorspar; associatedminerals, calcite and barite, floated little and quartz practically did not float. Optimum flotation offluorite with oleic acid, a conventional collector for nonsulfide ores, is obtained in the pH range of7 to 10 while with microbe fat the range is 4 to 10. The expanded pH range is considered to resultfrom the fact that microbe fat contains a number of saturated and unsaturated fatty acids with theformer attaching rapidly to the fluorite surface. Infrared spectra for the collector and the microbe fatinteractions with minerals were identical.

REAGENTS OF BIOLOGICAL ORIGIN IN METALLURGY 259

The use of biomass as a ftWtioo agent has been demonstrated also for celestine andassociated minerals, CAlcite, barite and quartz (Figure 6) (Solojenken, 19'79). Both CAlcite and baritewere depressed with 10 mg/1 of biomass while about W% of celestine could be selectively floated.Increase of biomass concentration further increased the seIedivity. With abold 50-75 mg/I, all theassociated minerak ~ pradically depressed. SoIojenken (1979) further compared die de~action of biomass with that of dextrine, a conventional depressant. With 300 g/ton of dextrine, 96.2%CaF2 with 84.9% recovery was obtained, in comparison with 96.3% of CaF2 with a recovery of 86.5%obtained with 50 g/ton of biomass. The ability or biomass macromolecules to hydrate in aqueoussolution and to more seJedi..-eJy adsorb on the gangue minerals made them potentiaJ depressors innonsuIfide ore ~ation.

BI~RPTION OF METAL IONS BY MICROB~Microorganisms are employed for recovering metals from solid wastes in two ways: extraction

of metals from insoluble solids by microorganisms through bioleaching and recovery of metal ionsfrom solutions by microorganisms through biosor~ion/bioaCC1Jmulation. The latter process is alsoused to concentrate metals from effiuents before discharge to streams, lakes 01" ground waters tominimize accompanying poDution problems. In the past two decades, research on biosOI"ption basadvanced rrom fundamental to applicd stage owing to its application for environmental protection andthe economical advantage over OODventional chemical proccsses. GeneraUy metal ions are present inthe effluents in dilute conccntratioos and do not pose any immediate danger. However. the metalscan get concentratcd by microorganisms due to the persistence of former in the environment andtheir sorption can become detrimental to microorganisms when the concentration of the metal ionsexceed the tolerance limits.

There are reports in literature on bio6orpriOD of aIm<8 aD heavy metal ions wbidI arehazardous to humans and animals (TSCUIS and Volesky, 1982; Ahlf. 1988; Kuyukak and Volesky,1989a & b; Scharer and Byerley, 1989, Tsezos ct al, 1989-, Ralyh, 1~. Adsorption by the bacterium~ cQ/d4riwn of heavy metal ioos which include ~+, 03+, Cu2+, Fe2+, Pb2+ and z.n2+has bceo demonstrated by Ah1f (1988). Also, cUcDSi\Ie work bas been done by Kuyubk and Vaiesky(1989a and b) 00 the aeeumulatioo of eobak by marine alga and the mechanisms inrol~.

Mechanisms associated with the adsorptioo of ioos by miaoorganisms and immobilizedbiomass are discussed below with special refereDce to uranium. Isotherms for biosorptiOD by A.villelandii at various pH values from uraDyI nitrate solutions contaiDiDg 2.4 g 1-1 sodium sulfate areshowu in F1gure 7 (Scharer aDd B~ley, 1989). Also shOWD are the ~ isotbem15 obtained with~UDg and old cukurcs uDder the same physiological coDditKJDS. The ~ difference wasattnouted to the reduced amount of capsular slime iD older cultures. Surface properties such as zetapotential and hydrophobicity have been showu to depend, iD addition to added inorganic and organicspecies, on such treatments as agiDg. or freeze/thaw (YeUoji Rao et aI, 1993). This iDdicates theimportance of coatroDing the hanoesting conditioos iD sorptioa processes. Studies carried out byScharer and Byerley (1989) with capsular poi)'Saccbarides extracted from plasmid transformed bacteriahave shown algal alginate to possess higher uraDium sorption capacity than the whole ceUs (SchareraDd Byerley, 1989). SiDce Deither the biomass Dor the poiysacchrides displayed sigDificaDt selectivityfor uranium, it was proposed that the ioos could biDd to carboxylic residues of the biopoiymer5. Apilot plant study of the immOOilized biomass of R. anIrizus for the recovery of uranium from thebioleach solutions has shown that the biomass can be effectively used for 12 cycles with consisteDtperrormaDce thus showing promise ror upgradiDg the process to industrial levels (fsezos et aI, 1989).

CONCWDING REMARKSReagents of biological origin can drastically alter the surface properties of the minerals. Such

changes can be exploited to separate minerals during notation for the purpose of beneficiation and

260 REAGENTS FOR BETTER METALLURGY

100

---80 " ?-.. ~;.:~'c'-.

\N. 60

>.....>0Q~ 40

100

w

~ 800:s~e 60

"'i-f40

!Q 20>::5'

, pH 4.0 (Old culture)~~

pH 3.0..~ ~ 4 \~

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0 20 40 60 80 100 120

Pi 6 c« f di . p~ with b. Equihoriwn U(VI). mg/i19. . cued 0 con ttomng 10mass on . . .the flotation of celestine and associated Fig. 7. lsothenns for uranium bisorptiOD byminerals Azotobacter vinelandii at various pH values

for the cleaning of coal. Uptake of metal ions by the microbes and the biomass can also be used forthe removal of hazardoDS heavy metal ions. Flocculation as well as dispersion, for example, for wastetreatment, can also be achieved by biological processes but mech~J1isms involved are not wellunderstood in this case. The cells can produce extracellular polysaccharides and polypeptide basedpolymers, which have the capacity to function as flocculating agents. The biopolymer segments canbridge not only the cells but also fine mineral particles. Microbial adhesion to minerals although inmany cases appears to depend on the specific property of the fimbriae present on the cell surface,adhesion does occur in their absence as welt. Presence of runbriae as welt as their removal has beenrecently shown to alter key surface properties of the microbes (Yeltoji Rao et at. 1993).

Although, preliminary studies on microbial interactions with minerals indicate some potentialapplications, for full exploitation of all the biological processes and for scaling up to large reactors,more information on factors that control microbe-mineral inteTactions, such as, role of fimbriae,effective permeability for the flow of nutrient/culture medium in the particle bed, microbial toleranceto high temperature and concentrations, design and optimization of the rate of biological processesand cost effectiveness is necessary. Genetk manipulation can be used to make microbes adapt toelevated temperatures, metal ion concentrations and extreme pH conditions. Wide use of microbesor their production on an industrial scale can be expected to occur with improvement in processefficiency and cost.

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Bagdigian R.M. and Myerson, A.s., 1986. The Adsorption of Thiobacillus ferrooxidans on CoalSurfaces, Biotech. and Bioengg., 28:467-479.

Berry, V.K. and Murr, LE., 1978. Direct Observations of Bacteria and Quantitative Studies of their

REAGENTS OF BIOLOGICAL ORIGIN IN METALLURGY 261

Catalytic Role in the Leadting of lAJW Grade Copper Bearing Waste, MctaDurgicalapplications of Bacterial Leaching and Related Microbiological Phenomena, (Eds.) Murr,LE., Torma, AE. and Brierley, A., Academic Press, New York, pp 103-136.

Berry, V.K., Murr, LE. and Hiskey, J.B., 1978. Galvanic Intcractioos Between CbaJcopyritc and PyriteDuring Bacterial Leaching of Low Grade Waste, Hydrometallurgy, 3:~.326.

Bryner, Lc., Beck, J. V.. Davis, D.B. and WtIson, D.G.. 1954. Microorgani.~ in Leaching MineralSuUidcs, lad. Eogg. <::hem. 46:2587-2592-

Dogan, MZ., 07bayoglu, G., Hicyilmaz, C., Sarikaya, M. and Or-ccogiz, G., 1985. Bacterial LeachingVersus Bacterial Conditioning and flotation in DesuHurization of Coal. XV InternationalMineral Processing Congress, Cannes, Jun 2-9, pp 304-313.

Harada T. and Kuniyoshi, N., 1985. Effects of Bacterial Oxidation on the F1oatabiIity of Pyrite, J. Min.Metall. lost. Japan, 101:719-724.

Kuyubk, N. and VoIcsky, B., ~. Accwnulatioa of CobaJt by Marine Alga, Biotech. and Biocngg.,33:~14.

Kuyukak, N. and Volcsky, B., 1989. The Mechanism of Cobalt Biosorption, Biotech. and Bioengg.,33:822-83L

Lyalikova, N.N. and Lyubavina, LL, 1986. On the Possibility of Using a Cukure of ThiobacillusfemJOxidans to Separate Antimony and Mercury Sulfides During F1otation. In: Fundamentalsand Biob~ometaUurgy, (cds) R.W. Lawrence, R.M.R. Branioo and H.G. Ebner, Elsevier,New Y <Kk, pp ~.

Ralph, BJ., 1985. Biotechnology Applied to Raw Minerals Processing. In: Comprehensivebiotechnology, Vol 4, (cditor-in-chief) M. Moo-Young, Pergamon Press, New York, pp 201-234.

Scharer, J.M. and Byerley, JJ., 1989. Aspects of Uranium Adsorption by Microorganisms,HydromctaDurgy, 21:319-329.

SoIojcnkcn, P.M.. fk>atabifdy aDd Lcacbing of Low Grade Ores with Reagr;ats of Biological Origin,Intcrnational Conference on Advances in Chemical Metallurgy, BARC, Bombay, India, pp37/1-37/13 (1979).

SoIojcnkcn, P .M., Lyubavina, LL, Larin, V.K., Bergctwn, LD. and DyatJOYitsbya, E. V., 1916. ANew ColJector in Nonsulfidc Ore F1otation, Bulletin Nonferrous Metallurgy. 16:21.

Torma, A.E., 1986. Biohydrometallurgy as an Emerging Technology, Biotechnology andBiocllgiDCeriDg Symposium 16:49-63.

Townsley, C.C., Atkins, A.S. and Davis, AJ., 1987. Suppression of Pyritic Sulfur During F1otationTests Using the Bacterium 1hiobaci/Jus fem)aljdllns, Biotech. Biocngg., 30:1-8.

Tsezos, M., McCready, R.G.L aDd BcD, J.P., 1989. The Continuous Recovery of Uranium fromBioi0gicaliy Leached SoIutioas Using Imm~ Biomass, Biotech. aDd Biocogg., 34:10-17.

Tsezos, M., and Volesky, B., 1982. The Mechanism of Thorium Biosorption By Rhizopus alThizus,Biotech. Biocngg.. 26:955-969.

Wart, I.W., 1938. Principles of f1otation, Aust. lost. MiD. MctalI., M~ pp 163-181,.Yelloji Rao, M.K., Natarajan, K.A. and Somasundaran, P., 1~. Effect of Bacterial Conditioning

of Sphalerite and Galena with 1hiobacillus ferrooridans on their F1oatability. In: R. W. Smithand M. MISra (Editors), EngiDCering Foundation Conference Mineral Bioprocessing, TMSPublication, pp 105-120.

Y eUoji Rao, M.K., Natarajan, K.A. and Somasundaran, p.. 1992b. Effect of Biotreatment withThiobllcilbls fe1rotuidans on the Floatability of Sphalerite and Galena, Minerals andMetallurgical Processing, 9:95-100.

Yelloji Rao, M.K., Somasundaran, P., Schilling. K.M., Carson, R. and Ananthapadmanabhan, K.P.,1993. (Unpublished work).

Y eUoji Rao, M.K.' Somasundaran, P., Schilling. K.M., Carson, R. and Ananthapadmanabhan, K.P.,1993. Interfacical Potential and Hydrophobicity Characteristics of Actinomyces viscosus andits F"lIDbriac-Deficicnt Mutants. (To be communicated).