P. Somasundaran,* Namita Deo,* and K.A. Natarajant

ABSTRACT

Biological processes have recently become more attractive in hydrometallurgy and mineral processingdue to their lower operating costs, potential for processing low-quality ores, andflexibUity. Adhesionof microbes and their metabolic products can modify the particles to render them hydrophobic orhydrophilic. Such modifications of the surface properties are used in flotation and flocculation of min-erals. Microbes are also able to dissolve different precious metals from their ores. In this paper, the roleof microbes and their metabolic products as surface modifying reagents in extraction processes is dis-cussed. In addition, the ability of certain microbes to adsorb toxic metal ions or to degrade toxic chem-

icals discharged by industries is examined.

INTRODUCTION

The increasing demand for metals and the decreasing av~ilability of high-grade ores have ledto numerous investigations to find better processing techniques and reagents. Reagents ofbiological origin are of special interest because they exhibit specific interactions with miner-als. Microbes and their metabolic products can modify the mineral surfaces, either directly orindirectly. The direct mechanism involves adhesion of microbes to mineral particles, whilethe indirect mechanism refers to the biological reagents acting as surface-active reagents.Both types of interactions can lead to alteration of the mineral surfaces causing flocculationor dispersion, depending upon the nature of alterations. Also, bacteria can degrade or adsorbpollutants including toxic metal ions from tailing ponds. Biodegradation represents a poten-tially important and effective treatment for many of the organic industrial wastes producedby the various mineral industries.

Recent progress on these microbial processes in both hydrometallurgy and mineral pro-cessing is reviewed in this paper. In addition, the role of the mechanisms of interactionsfound in different biological processes is discussed.

Biomineral beneficiation can be defined as a beneficiation process brought about bymicroorganisms that can mediate various surface chemical processes, including:

. alteration of the surface characteristics of minerals;~-* Langmuir Center for Colloid and (nterfaces, Henry Krumb School of Mines, Columbia University,

New York, NY.t Depanment of Metallurgy, Indian Institute of Science, Bangalore, India.

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. generation of surface-active reagents; and. sorption, accumulation, and precipitation of ions and compounds.

Several types of autotrophic and heterotrophic bacteria, fungi yeasts, and algae have beenimplicated in mineral-beneficiation processes. Selective leaching, flotatipn, and flocculation aresome of the processes involved in biomineral processing (Natarajan, 1995).

BIOLEACHING

ThiobaciUus ferrooxidans is the most widely applied microorganism in the oxidation of sev-eral sulfide minerals. The following three major mechanisms have been proposed for thebioleaching of sulfide minerals by ThiobadUus ferrooxidans, (Gottschalk and Buchler, 1912;Dutrizac et al., 1971; Berry and Murr, 1978; Metha and Murr, 1983):

. indirect. direct, and

. galvanic interaction.

In most of the leaching processes, all three mechanisms may operate simultaneously. Theindirect attack mechanism involves the leaching of sulfides by ferric sulfate, which is a meta-bolic product of the bacteria, according to

Fe~ + 31fJ. °2 + H2O z FeS04 + H~04(f;Qp

and

2FeS04 + 'h °2 + H2SO4 ~ Fe2(S04)3 + 2H20 (EQ 2)

The ferric sulfate produced is an efficient oxidant capable of dissolving most of the sulfideminerals.

Several autotrophic and heterotrophic bacteria, algae, and fungi also promote dissolutionreactions. Fungi such as Aspergillus niger and Penicillum notatum degrade bauxite and clays.Bacillus mucilaginous excretes polysaccharides that could interact with silica, silicates, ironoxides, and calcium oxides (Groudev et al., 1983). Another approach for processing low-grade bauxites is through selective dissolution of iron and calcium using Aspergillus niger,Bacillus circulans, Bacillus poiymyxa, or Pseudomonu,s aeroginosa.

Soil bacteria and fungi play a significant role in the solubilization of phosphates. Rockphosphates can be solubilized by strains of Rhizobium and Bradyrhizobium. Hypomicrobiumspp. and Aspergillus niger are also efficient dephosphorisers (Halder et al., 1990). Biodegra-dation of carbonates through direct and indirect biological activity is of relevance in theremoval of calcareous gangue from ores

CaCO3 + W --t Ca.. + HCO3- (EQ 3)

Metabolic acid generation aids the above dissolution processes. In addition, CO2 generatedduring bactelial respiration can produce similar effects

(EQ4)CO2 + H2O ~ H2CO3

CaCO3 + H2CO3 ~ Ca++ + 2HCO3- (EQ 5)

Both organic and inorganic reagents are produced through bacterial metabolism. Differenttypes of reagents, such as mineral acids, fatty acids, polymers, and chelating agents, are gen-erated by bacteria, fungi, and yeasts. All these can markedly alter the efficiency of processingof minerals as well as pollutants.

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Direct attack involves bacterial attachment to mineral substrates and break-up of the lat-tice by bacterial action. A simplified reaction describing the process is

~ have beenflocculation are

MeS + 2°2 -+ MeSO4

where Me represents a bivalent metal.(EQ 6)

The galvanic conversion mechanism, on the other hand, is based on electrochemical prin-ciples. When two sulfide minerals with different rest potentials are contacted in a leachingmedium, a galvanic cell would be formed. In such a cell, the mineral with the lower potentialserves as the anode undergoing oxidation, while the nobler sulfide acts as the cathode. Thepresence of ThiobaciUus ferrooxidans accelerates the electrochemical oxidation process.

Anodic oxidation of chalcopyrite in contact with nobler pyrite, with oxygen reductionoccurring on pyrite surfaces, can be represented as

dation of sev-osed for theBuchler, 1912;

CuFeS2 -+ Cu" + Fe" + 250 + 4e (EQ7)

02 + 4W + 4e ~ 2H20 (EQ 8)

Refractory gold, where gold occurs finely disseminated within host sulfide minerals such asarsenopyrite, pyrite, chalcopyrite, and sphalerite, is not amenable to direct cyanidation, andconventional gold extraction processes become uneconomical. To liberate the interlockedgold, host sulfide minerals are oxidized by bacterial pretreatment, leaving the native goldfree in the residue. The biochemical reactions involved in arsenopyrite and pyrite are repre-sented by

ltaneously. TheNhich is a meta-

(f.QJ)

(EQ 2)

of the sulfideFeAsS + 311J. °2 + H2O T.ferroo.rldans ;- FeAs°4 + H2SO4 (EQ 9)

2FeS2 + 7ih °2 + H2O T.ferrooxidans ~ Fe2(SO,.)3 + H2SO4 (EQ 10)

Access for cyanide to gold in many refractory ores can be enhanced by microbial leaching ofthe matrix material. There are also microbes that produce enzymes that can complex withgold. The potential for such direct microbial treatment of gold and other precious or raremetals is very clear (Polkin et al.1982).

note dissolutionuxite and clays.. silicates, ironIcessing low-ergillus niger,

Jhates. Rock

iypomicrobium)90). Biodegra-lance in the

(EQ 3)

CO2 generated

(EQ4)

(EQ 5)

JOlism. Different: agentS, are gen-icy of processing

BIOFLOTATION

In addition to leaching, bacterial conditioning with T. ferrooxidans can produce significantsurface modification of sulfide minerals to affect their flotation behavior. For example,T. ferrooxidans can enhance the flotation of pyrite in sulfuric acid medium at pH 2 with verylittle effect on the flotability of galena. The availability of initial biomass and the period ofbacterial mineral interaction where variables influencing the flotability of both galena andthe sphalerite. With an increase in the cell concentration and reaction time, the flotability ofboth galena and sphalerite were deleteriously affected. For sphalerite, 94% recovery wasobtained after treatment with 108 cells/mL This decreased to about 53% when the cell con-centration was increased ten fold. On the other hand, the notability of galena was observedto be affected more by an increase in the initial biomass (Yelloji Rao et al., 1991,1992). Itwould thus be possible to separate sphalerite from mixed zinc-lead sulfide ores through bac-terial pretreatment under controlled conditions.

Similar surface modification by T. ferrooxidans fmds application also in coal cleaning pro-cesses. Conditioning of coal with T. ferrooxidans can cause depression of pyrite withoutaffecting the notability of coal (Botang, 1997). In addition to pyrite suppression, bacterialconditioning with T. ferrooxidans can also help in the removal of ash from coal (Bos andKuenen, 1990).

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It has been shown that the natural flotability of pyrite dropped from 85% to less than 10%after conditioning with the bacteria. Traditional flotation reagents are known to increase theflotation recovery of coal, total sulfur content, and total ash content. In contrast, bacterialconditioning produces no effect on coal flotation but decreases the total sulfur content(Botang, 1997). .

Biomass can act as a flotation reagent for the beneficiation of nonsulfide minerals. Recentstudies have established that BaciUus POlymyxa, a soil bacterium indigenously associatedwith several iron ore and bauxite deposits, can significantly alter the surface chemistry ofminerals such as hematite, corundum, calcite, and quartz (Deo and Natarajan, 1997). Theflotability of quartz was found to increase significantly, attaining a steady-state recovery of60% after three hours. On the other hand, the flotability of corundum, hematite, and calcitewas deleteriously affected due to such interactions. Thus, it is clear that bacterial interactioncan render the quartz surface more hydrophobic and hematite, corundum, and calcite sur-faces more hydrophilic. It should then be possible to separate silica from iron oxide, alumina,or limestone efficiently through biotreatment. It has also been observed that, in the presenceof small dosages of collector, close to 100% separation of quartz from hematite and corun-dum is possible. The mechanisms for these interactions are, however, yet to be understood.Opportunity exists to explore these phenomena and to achieve effective separation throughthe control of cell biomass and flotation time.

BIOFLOCCULATIONThere are yet other possible uses of microorganisms in mineral engineering that havereceived little attention. Many microorganisms possess flocculent growth habit and produceextracellular polymers such as polysaccharides, polypeptides, and polyglycans. Such biologi-cally generated polymers can cause flocculation of the microorganisms as well as of the min-eral particles. Mainly, the molecular architectUre of the bacterial cell wall determines the

forces of dispersion and agglomeration.Mycobacterium phlei, a gram-positive, rod-shaped, prokaryotic cell has been observed to

be useful as a flocculent for phosphate slimes, hematite, and coal fines (Smith et al., 1991;Mishra et al., 1993). At mildly acidic pH values, this organism flocculated hematite but notquartz. Substantial settling of hematite fmes could be observed within a few minutes after

bacterial interaction.Selective flocculation of fine coals with respect to the separation of pyritic sulfur could

also be achieved in the presence of M. phlei. More than 85% of pyritic sulfur could beremoved from coal by bioflocculation. Biosurfactants secreted by such organisms may be pri-marily responsible for the flocculent action. The mechanisms involved in bioflocculation maybe similar to those encountered in other systems involving bridging, charge neutralization,hydrophobic bonding and particle-to-particle contacts but relative roles of various factors are

not well established.Bacillus polymyxa, a gram-positive rod, has been observed to enhance the flocculation of

hematite, corundum, and calcite (Deo and Natarajan, 1997). On the other hand, it dispersesquartz by making its surface hydrophobic. For example, while about 99% of hematite, cal-cite, and corundum could be settled within two minutes in the presence of 109 cells/mL inthe pH range of 4 to 7, only about 100/0 of the quartz particles could be settled under theabove conditions. Selective bioflocculation could prove to be useful for the separation of sil-ica from hematite, corundum, an~ calcite under the above conditions.

BIOENVIRONMENTAL CONTROLMicroorganisms are capable of concentrating metals from aqueous solutions through extra-cellular precipitation, volatilization, complexation and accumulation, cellular binding, andaccumulation in intracellular spaces. Biosorption and bioaccumulation can be advantageously

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:hanlO%:rease theIcterial:ent

Is. Recent:iatedistry of17). The:overy ofnd calcitetteractioncite sur-,alumina,~ presenced corun-:1erstood.1 through

ayed producelch biologi-of the min-toes the

served to:il., 1991;te but notltes after

used for the removal of toxic metals such as copper, lead, zinc, mercury, chromium, and ura-nium from mineral processing effluents. Both living and dead biomass can be used for theabove purpose. The use of dead biomass has advantages in that

. no nutrient supply is needed,. the ready availability at very low costs as a waste material, and .

. the absence of metal-toxicity problems.

The type and amount of biomass, as well as the solution chemistry of the effluents, affect

metal uptake by biomass.The following four major mechanisms have been identified for the removal of metal ions

from aqueous solutions by microbial actions (Kellyet al., 1979):. Bioadsorption consisting of the exocellular or pericellular binding of metals, which may

involve either:- cation adsorption, which can be complexed to the behavior of commercial carboxylic

cation exchange resins and- metal deposition precipitation at microbial surfaces, which may occur as surface com-

plexation or chelation or as entrapment by extracellular organelles.. Bioaccumulation consisting of intracellular uptake of metals, which may occur by:

- direct intracellular uptake of metals,

- uptake via monovalent cation transport systems,

- uptake via undefined processes, and

- uptake via anion transport pathways.. Production of insoluble compounds with precipitation of:

- metal sulfides and

- metal oxides

. Miscellaneous microbe-metal interactions

Biosorption and bioaccumulation are processes where metal removal is a direct consequenceof the physical contact of microorganisms with metal ions. However, the metals may also beremoved from solutions without the need for this physical contact. This phenomenon isencountered whenever metal ions combine with the direct or indirect products of microbialmetabolism and is, therefore, dependent on the existence of a viable biomass. In this case,microorganisms can be considered as mere low-cost producers of the chemical compoundsthat capture metal ions. These compounds can be either the products of microbial degrada-tion of organic matter or metabolites. Heavy metals can be effectively removed from solu-tions depleted of oxygen, for instance, by the activity of anaerobic, heterotrophic organisms.This, in effect, reduces the environment thus created and favors the development of a micro-flora of sulfate-reducers. The H2S produced by the microorganisms combines with heavy-metal cations to form insoluble sulfides, which precipitate out of solution according to the

simple reaction:

ur could:I bemay be pri-llation may:-alization,; factors are

:ulation ofit disperseslatite, ca!-~lls/mL in\der the"ation of sil-

Me+2 + H2S -+ MS + 2H+ (EQ 11)

Through their activity, microorganisms can also induce changes in the oxidation states ofsome elements, thus producing less soluble ionic species. A typical example is the oxidationof soluble ferrous to much less soluble ferric iron by T. ferrODxidans.

A number of different waste substances, both toxic and nontoxic, are discharged by vari-ous mineral-industry operations. Many of these substances are complex organic chemicalsused in flotation or hydrometallurgical plants. Others include various petroleum productsused in general mill and mine equipment operation and maintenance. The handling,

ugh extra-ding, and'antageously

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treatment and proper disposal of these substances can be a difficult and expensive task.Bioremediation represents a potentially effective treatment for many of the organic wastesproduced by the mineral industries.

Bioremediation is a process in which toxic components are degraded into hanDless prod-ucts rather than just adsorption of toxic ions on microbial surfaces. An interesting example isthe action of the cyanide-tolerant bacteria in the effluent from the gold-extraction processes(Whidock, 1989). There are also microorganisms that can break up natural petroleum prod-ucts from groundwater and contaminated sites (Foght et al., 1990). There are now at least50 companies in the United States alone and over 100 companies worldwide using this tech-nology to clean up sites ranging from gasoline-soaked soil to supervened areas flooded withcarcinogens. Solozhenkin and Lyubavina (1980) investigated the biodegradation of thiol col-lectors, such as potassium 2 21,6,61-tetramethyl-iminoxyl-xanthate (KTIX), potassium butylxanthate (KDX), and sodium diethyl dithiocarbamate (NaEC), by bacteria. In the presence ofbacteria, KTIX was 100% destroyed within 45 minutes, while, in the absence of bacteria, only45% of the compound was destroyed. In another study, biodegradation of different collec-tors, such as sodium oleate, sodium isopropyl xanthate, isododecyl oxypropyl aminopropylamine, and ammonium acetate by Bacillus polymyxa (Deo and Natarajan, 1998) was investi-gated. In the presence of bacteria, about 150 ppm of amine collector was destroyed withintwo hours, xanthate within five hours, and oleate in around six hours, while, in the absenceof bacteria, the above collectors remained unaltered. These studies showed that flotationcollectors can be effectively destroyed by bacterial degradation and the procedure may bea viable method for treating such wastes.

From the viewpoint of microbial-mediated removal of chemicals from wastewaters,manmade organic compounds can be classified into amenable and refractory to microbialdegradation. The organic compounds belonging to the first class can be eliminated by:

. biotransfonning them into innocuous fonDs,

. degrading them by mineralization to carbon dioxide and water,. anaerobically decomposing them to carbon dioxide and methane, and. volatilizing them.

The organic compounds falling into the second class are removed by sorption (Rossi, 1990).

CONCLUSIONS

It is clear that microbes and their metabolite products can markedly alter the surface proper-ties of mineral particles. Processes such as bioflotation and bioflocculation can take advan-tage of such surface alteration to achieve selective separations. Although preliminary studieshave shown potential applications of biological processes in mineral processing, very few ofthese are being practiced commercially. Furthermore, there is limited information availableon the mechanisms of the attachment of microbes and microbial products.

The wide use of microbes or their production on an industrial scale can be expected to occurwith improvement in the understanding of the mechanisms of bacteriaVmineral interactions.

Microorganisms could also be efficiently used in environmental control in mining and mineralprocessing. Biosorption, bioaccumulation, and biodegradation can be used with advantage todetoxify liquid and solid wastes. The potential for use of microbe-mediated processes for bothextraction of values and removal of pollutants is clear. It will be useful to develop an under-standing of the basic processes behind the microbial effects to fully realize such pOtential.

226

larmless prod-ring example is:tion processesetroleum prod-~ now at leastusing this tech-is flooded withtion of thiol col-)otassium butylthe presence of)f bacteria, onlyfferent collec-t aminopropyl~8) was investi-~stroyed withinin the absencethat flotation

ocedure may be

astewaters,ry to microbialminated by:

n (Rossi, 1990).

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e surface proper-an take advan-!liminary studies.ing, very few of!1ation available

~xpected to occurral interactions.ling and mineralith advantage to:'ocesses for both~lop an under-:h potential.

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