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:-.APPLICATIONS OF BIOLOGICALPROCESSES IN i"IET ALL URGY*

P. SOMASL'NDARM~. YELLOn RAO K. M~JKARI.YUNZh"l REN and K.A. NA T ARA1.-\.J.~ .

Lanamuir Cenb'e for Colloids and Interfaces.Henry Klumb School of Mines. Columbia Univ~ity. New York. NY 10027, USA

'Colgate-Palmolive Company. 909 River Road, Piscataway. NJ 08855, USA2Department of Metallurgy, Indian Institute of Science. Bangalore 560 012. India

The role of~rlanisms iD meta11\1r1Y ia reviewed iD this paper. ~icroorllnisms md theirbyptoducu have beeu sbown to ftmcuon u flotation reagentS and flocculantS (or tbe

processiDj md separ3Qon or low zndc ore panicles. The ability of mJCroorpnisms to adsorbtOxic heavy IDetal ioas md to break up tOxic c~tS to bamI1ess byp~ ia examiDedfrom the view point of fiDdiDl applications for environmental protection. TheiDrerxtioDiatlacbme1U ~Mft'- betWeen IDicroorPDiSIDS aDd miDCt21 panicles ~ also

discussed. The role of cbaDaes iD surface p r'~ brouabt QD(O microorpaisms aM meminerals due to IDicrobe-miDCraI iDfCtXtioas ia recopiz&d. tilM1entaDdiDl me ~baDiSmS

iDvolved iD these bioproceSS8S will prove useful for developiDg Dew and improved proccssiDg

techniques.

mRODL"cnoN

~iicroorganisms. exc:pt for th~ir detrini~ntal role ;n biocorrosion. pla:-predominantly a beneficial part in metallurgy. The area that has benetited th~ most ismineral processing and tl-.i:; ;- due to ;;h~ increasi~: OVU3ii"!3 "o~ts of ::onvr:~tio!'.:ltechniques tor processing low and lean iJ'ade ores. Biviogical processes are look~d upon asalternative techniques for the reason that they can not only handl~ low grade ores. but alsoprovide an economic alternative to today's mineral processing industries. Mostimportantly. biological processes have the least impact on the environm~nt. Theseprocesses appear attractive because microbes and th~ir by-products can pro...id~ speciticinteractions. in some cases selective. with minerals. These interactions ot- microbes andtheir agents with minerals can be indirect with biological products acting as surface acti..e~ents. or direct due to microbial adhesion or anachment to panicles bringing out desiredsurface modification. Both types ot- interactions can l~ad to alteration of mineralhydrophobicity and in some cases cause flocculation or dispersion of rnin~ral suspensions.Additionally. microbes can either remove toxic m~tal ions from ground and surt"ace waste

.Based on previous articles published by the authors in Colloids and Surfaces. 1993.1997, and Miocra1s and ~{eta11urgica1 Processing. 1993.

4

water by bioaccumulation or break.ini the toxic componentS into harmless byproductS.Other biological processes invol..;na becterialleacbing and biofouling are not discussed heresince both the areas are quite ~'ell studied and understood. Only recent progress in otherbiologi~l processes of relevance to mineral processing are reviewed in this paper. Suchmechani1ms for the microbe-mineral interactions as well as the impact of the microbialen\;ronment that can influence the kinetics of the biological processes are discussed.

BIOFLOTATIO:'f PROCESSES

Although many bioprocesses have been occurring in narure.for ages. the importanceof microorganisms and th~ir b.'oprodUCtS in mineral processing was first recognized only in1976 [1]. Sulfate reducing bacteria (SRB). microbial fat and other biomass were found toinfluence the flotation response of both sulfide and nonsulfide minerals. For example. SRBcan depress the flotation of both chalcopyrite and sphalerite but not that of molybdeniteand galena (Figure I) [I]. This has been attributed to the selective desorption ofxanthogenate coatinss on minerals by SRB. causing their depression. Since galena mostoften is mixed ~ith sphalerite in nature. an important implication is that SRB can pro"ideselective flotation of ialena from such mixnl.-es. In fact. tests on 3olch minemls usins SKEyielded a 95°/0 ~ove~' of galena ~ith only ~.5% recove~. of sphalerite ~.hiie control testsdid not show any selectivity.

77Iiobacillus ferrooxidans rr f). the most widely StUdied leaching microorganism.can oxidize sulfide minerals not onlv directlv bv bacterial adhesion but also indirectlv bvits metabolite product- ferric sulfate'[2-5]. ~ addition to leaching, bacterial conditio~ingwith T ferrooxidans can produce significant surface modification of sulfide minerals [6-8].Figure 2. shows the flotation recovery of sphalerite w1der different conditions. including theeffect of bacterial conditioning. ~'hile conditioning ~ith sulfuric acid solution at pH ~accelerated sphalerite flotation in a shoTt range of time. the additional effect due to bacterialconditionini was minimal when flotation ~.as dont! after conditioning ~ith the activator andthe collector. Contrary t" thi~. flcatability of galena did not increase upon bacteri~!treat.rnent and in fact decreased drastically ~.hen such treatment ~.as followed byconditioning with the collector (Figure 3) [7). These results are important from the vie~.point of providing selectivity in the separation of sulfide minerals. T. fe,.,.ooxidans hasbeen tested also for separating antimony and mercury sulfides by bacterial conditioningfollowed by flotation [8). Conditionina with bacteria for 90 minutes showed no change incinnabar recovery while antimonite recovery dropped from 890/0 to 6.~~/0 leading to highselectivity. It was proposed that oxidization of antimonite surfaces by T .fe,,"ooxidansleads to the depression of floatabilit). while cinnaba.- remained unaffected. ~ith no changein its tluatability.

StUdy of mechanisms in...olved in providing selective separation of sulfide mineralsshowed that dissolution or surface oxidation of the mineral was possible duringconditlonina widt sulfuric acid.

ZnS -> zoz. + S + 2e ; ...:.(1)PbS ---> Pb2'" .;. S - 2e .;. (2)

The elemental sulfur thus produced on the mineral surface increases the hydrophobicityand therefore the floatability of both the minerals. Howe...er. during bacterial conditioning.

5

.,There have been some attempts to explain th~ pyrite suppression etTect due to

bacterial conditioning. It is claimed that the adsorption of intact ba~teria. bacterialm~t:&bolites or ba~terial debris is the main reason for the change in surface charg~.wenability and thus decreasing th~ rloatability of p~-rite [9.10]. Th~ results sho\\-n :nFigurc .. .>uggests tt-.ar the bacterial suspension in distilled \\"ater i'~lf depressed th~ p~ ri'~flotation to les.)' than 10"/0 recover:- ...t p:{ 2 in ... re\\' min"te~ ft is to ~ noted th;ltadsorption of intact bacteria can take place rapidl~'. in the order of minutes. In t3C:.Bagdigian and ~{yenon have sho~"D adsorption of~'o cells in two minutes [12]. Since itis unlikely that bacteria will produce enough metabolites to change floatability measurabl~.during such a short period, direct bacterial attachment can be suggested to generate 3hYdrophilic surface on these mineral particles.

6

the same as that with traditional collectors and hence did not give any indications of themechanisms.

Biomass can act also as a flotation reagent in the beneficiation of nonsulfideminerals. For example. \A.ithjust 10 mg;1 of biomass. about 2~/0 of celestine \A."3S selectivelyfloated ~uppres.sing the associated minerals (calcite. barite and quartz) [15]. Increase ofbiomass' dosage further increased the selectivit." and with 50 to 75 mg./} of biomass almostall associated minerals remain depressed. Interestingly. while 300 git(\n of the conventionaldextrine depressant provided a flotation recove~ of 85°0 CaF:. only 50 g:ton of biomassprovided 86% CaF2 recovery, The abilit." ofbiomacromolecules to be hydrated in aqueoussol~tion and to selectively adsorb onto gangue materials is considered to be the reason forthe depressant property. In spite of these promising results on microbiall~- inducedflotation processes. the cC\ncept still n~~ds to be tested at pilot !~'..els before comrr.e:-ciaiprACtice can become viable.

BIOSORPTION OF ~IET AL IONS BY MICROBES

Microorganisms can be emplo~'ed to recover metals from low gradc: ore .depositsand waste products. One v.-ay~ by bacterial leaching, in which microbes extract metaisdirectly from insoluble solids. Anothcr means of recovering metals involves biosorption.bioaccwnulation in which microbes adsorb metal ions from solutions. Although moSt metalions in surface and ground water and, when discharged in industrial and nuclear ~-aste sitesare in dilute form and do not pose any irnrnediate danger. they can become hannful once theconcentration reaches toxic levels. Considerable literature exists on the harmful effects ofheavy metal ions to humans and animals [16-22]. Biosorption is relevant here. from anenvironmental point of view, since the microbes can concenttate and remove hea,,"y or to¥icmetals from effluents prior to discharge. It is obvious that such concentration,accumulation processes are not necessaril~' controlled b~' one ~'pe of species but possibl~.by more than one species. OitTerent microorganisms and their abilit:-, to adsorb differenttypes of metal ions a.ooe S!IO~"D in Table 1. One noticeable example is bacterium C.vanidiumca/darium which can adsorb many hea,,: metal ions including Cd:., Cr::-. Cu:.. Fe:-, p~-and Zn:. [17. 24]. Other than the microorganisms. specie$ such as marine alga ccm alsoaccwnulate metals. such as C02- [18. 19j.

Many attempts have been made to understand the mechanisms of biosorption ofions. some of which are discussed below. Biopol~.mers are considered to be the keysubstance for uranium biosorption as their carboxylic residues can bind metal ions; neitherbiomass nor polysaccharides r.3'/e any affinity to uranium. Th~ 3!Z~! alginate with capsularpolysaccharides extracted from plasmid transformed bact~:ia had higher uranium sorptioncapacity than the whole cells [':'0). Role of biopolymers is further demonstrated in anexample involving uranium biosorption by .4. vine/andii. The adsorption isotherms withfresh and old cultures under the Same physiological conditions show measurable differences(Figure 6) [20]. This has been anributed to the reduced amounts of capsular slime in oldercultures. These changes in fresh and aged cultu*s are also reflected in the surfaceproperties. For example, zeta potential and hydroPhobicity of microbes are shown todepend on aging. freezing [23] as well as the presence of inorganic and organic species [14].

1

Other than in laboratory level ex~riments. one system invol"'ing immobilizedbiom3Ss of R. arrhi:us for uranium recovery was tested in a pilot pl3nt stUdy. It isworthwhile to note that the performance sho"'ed promise for industria! application andthat the biomass could function e~cientiy for 11 o~rating .:ycles [2l]. T:...e immobilizedbaderia retained hiih metal loa4s without any loss in ste3dy-state .1ctivity and theacctlmulated metal was recovered as a concentrated solution. Biosor?tion has made.:onsiderable progress in moving from theory to industrial practice J.5 it is not onl~'environmentally useful but also more economical than many other processes.

Biorem~diation is a process which breaks up the to:tic compone:1tS into ham-lcssproducu t."\Stead of adsorbina tOxic ions "ntO microbial surface. .o\n interesting eX3mple isthe action of the cyanide tOlerant bClcteria in the effluent from gold e:ttraction process(Homestake Mine's gold-processing Plant, South Dakota)[25}. There are alsomicrooraanisms which can break up nanlral pettoleum productS from i!'ound water mdcontaminated sites (26}. Currently. there are at least SO companies in l.nited States alon~and over hundred companies worldwide using this technology to clean up sites ranaingfrom gasoline soaked soil to superfund areas flooded Ylith carcinogens. This ~'3S possiblemainly due to the identification of appropriate microorganist':'.s and recognition of th~mechanisms involved.

BIOFLOCC1;U not".. PROCESSES

Another important application of microbes and biological re3ientS in mineralprocessina is in the area of flocculation. Conventional approach for the processin~ of fmepanicle involves polymeric flocculantS and unfornmately. they are neither ve~. ~Iecti...enor ~.ork pert'ectly. One of the options to tackle this problem is by empl,;,:o"ing reagentS ot"biological origin. For eX:1D1ple. .\.i}cobactiriwn phlei is ShO~l1 to cause sel~ti.."e tlocculationot" fine coal over pyrite and ash [:71. This ~-as attributed to the existencc "f plcnty ot" tart:..CH: groups on bacterial surface which caused selective ~hesion [0 :he h~.drophobicparticulate~ with no adhesion to pyrite and ash.torming Ininer:J.1s. Fi,;,cculation of PranS~ coal \lrith .\./}"cobucterium pI. ':e: ("Figure i) show that. mor~ (~"1 80" ') vf p~.ritic suit.;,could be rejected &om coal \lrith a high recovery of combustible material usina this method.

Biomass. in addition to functioning as a flotation agent. can 3lso be a goodflocculant as the microbe itself. Flocculation system of bentonite particles. showed thepresence of desired extra cellular cyanobacterial bioflocculants from Pho,.",;dium speciesstrain J J (18]. These biom:lcromolecules are neg3tively ch.1ried sulfatedheteropolysaccharides. with bound fatty acids and proteins. Flocculation of bentoniteparticles is proposed to bc due to the combined etTects of bioflocculants 3nd CaCI!. Thelatter. by itself. can 3lso cause flocculation and ~'a5 found to be c:ssenti:ll tor the activity ofbioflocculants. Increase in CaCl: concentration led to a decrease in the bioflocculantconcentration needed for flocculation. This was attributed to the activation of bentoniteparticles by calcium chloride for the adsorption ofbioflocculants.

Fine panicles of coal can also be flocculated using biopolymcrs derived frommicrooraanisms. ,\l ph/ei [29]. These biopolymers made coal surfaces more hydrophobicand flocculated coal efficiently. A comparison of the flocculation efficien~' of biopolymers

11

(~Sl M.R. Pollock. ift I.C. a_Ius ~4 R. Y. Slaincr (Ed$.>. TIlt BKteria. Aeademie Press. London ~4

~cw York. 1962. 4:p.I~I.(391 A.R. Clcnn. ProductiOll of Exuxellular ;xoceins by BKteMa. Ann. Rev. ~ticrobiol- 30 (1976) 41.(~Ol J.R.. Sokatch. ift I.C. CunSAIus. J.R..Sokarch anct L.N. OrmStOn (Eds.). TIle BK:eria: A Treatise an

S.aucn&re Jnd Function. A~ie Press. t979. p.229-290.(411 P.E. Kolcbnncler an4 R..~. "An~. J. 8Ktcrio1~ 161 (1916) S51.(.i~1 J.O. Cisar. A.Eo Vaftef. W.B. CIrt S.H. Curl. S. Hunt.c~rOIIe an4 A.L. SUCbcrg. Infectioft ~4

ImmUftiry 56 (1988) 2943.(43} N. Ohmura. K. Tsugira. J. Koizumi ~4 H. Saika. J. ofBKteriol. 178 (1996) 576.(~I M. Steier and C.A. Jerez. Fems. ~ticrobiol. Left.. 108 (199.).S(4'1 5.K. Htl&cnoft and W.A. Crmlcr. J. Su~oleal\ar $UUCrure 5 (1976) 291.("1 S. T3kaku- T. Fujimori md H. Iwasaki. J. Qen. Appl. ~icrobiol..!S (1979) :1.:9.(.471 A. Koftda. S. Ok\! JI\c1 K. Hilashiani. L Colloid anct Intertx. Science. 14. (I" I) 21~.(~81 K.A. ~~an. Hydrometallllrl)'. 29 (1992) 161.

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