bacterial leaching of ores and other materials

7/27/2019 Bacterial Leaching of Ores and Other Materials

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Bacterial leaching of oresand

other materials

R. Nveke, Institut fr Mikrobiologie, Technische UniversittBraunschweig, Fed. Rep. Germany

Summary

In nature sulfidic ores are decayed by weathering under the influenceof oxygen and water. Microbiological investigations reveal thatcertain bacteria are the main agent in this process. Several bacteria,especially Thiobacilli, are able to solubilize heavy metal minerals byoxidizing ferrous to ferric iron as well as elemental sulfur, sulfideand other sulfur compounds to sulfate. So they enhance leaching ofheavy metals from sulfidic ores under aerobic conditions about 104

fold or more compared with weathering without bacteria.

The principal bacterium in ore leaching is Thiobacillus ferrooxidans,which is capable of oxidizing ferrous iron as well as sulfur andsulfur compounds. But there are some other bacteria which may also beinvolved. For example the thermophilic Sulfolobus plays a role inleaching at elevated temperatures. Thiobacillus thiooxidans, whichoxidizes merely sulfur and sulfur compounds but not iron, andLeptospirillum ferrooxidans, which contrarily oxidizes only ferrous

iron, may play a role if they work together or with other bacteria.

Bacterial ore leaching can be applied to extract heavy metals from low

grade ores, industrial wastes and other materials on an industrialscale by different procedures: dump leaching, in situ leaching, tankleaching, leaching in suspension. Sulfidic copper and uranium ores arethe principle ores leached in several countries. So 20% to 25% of thecopper production in the U.S.A. and about 5% of the world copperproduction is obtained by bacterial leaching. This process is a veryslow one and needs a long time (years) for good recovery, but its main

advantages are low investment costs and low operating costs.

Current investigations deal with the leaching of ores other than thosementioned, leaching industrial wastes to recover metals, desulfurizingof coal, developing methods for in situ leaching and using othermicroorganisms than those used until now. Basic microbiologicalresearch focuses on the biochemistry, physiology and genetics of theinvolved microorganisms and on the complex interrelationships in the

microbial community of leaching biotopes.

Introduction

It is a fact that resources of metal ores are limited and that sooneror later these resources will be exhausted. But how great are ourresources in naturally occurring deposits? Before we answer thisquestion we have to define what a metal ore deposit is. A metal ore

deposit is a naturally occurring concentration of a metal or somemetals from which this metal can be obtained in an economic way. So,


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whether or not a deposit of metal ore can be considered are source ornot depends on the costs we have to pay for extracting the metal fromthe ore and on the price we can get for the pure metal on the market.In other words: If the price of a metal rises -as is to be expectedwith depletion of the resources- and the costs of extraction are

lowered, the amount of resources in the world rises.

Microbial leaching of ores depends primarily on bacterial processeswhich are the essential causes of natural weathering of sulfidicminerals. If sulfidic heavy metal minerals come into contact with airand water they begin to decay with the formation of sulfate, sometimes

sulfuric acid, and water soluble heavy metal cations.

Weathering of an ore body results in a typical picture:

a) An upper oxidation zone,being in contact withatmospheric oxygen and rain

water, which contains secondaryminerals formed by oxidation ofthe primary ore minerals and inmost cases a remarkableenrichment of ferric iron

minerals (limonite and others).(b) an underlying cementationzone just below the groundwaterlevel, in which minerals, formedby the reaction of primary oreminerals with the constituentsof the leaching solutiondescending from the oxidation

zone, are accumulated.

(c) A zone in which the primary

ore minerals are unchanged.

So we have to look at thesephenomena to understand whatexactly happens in this processand to get an idea of how toapply these natural processes toore leaching on an industrialscale.

Microbiology of ore leaching

Microbiological investigations revealed that certain bacteria are the

main agent in natural weathering of sulfidic heavy metal minerals.

Thiobacilli

The principal bacteria which play the most important role insolubilizing sulfidic metal minerals at moderate temperatures are


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species of the genus Thiobacillus. They are gramnegative rods, eitherpolarly or nonflagellated. Most species are acidotolerant, some evenextremely acidotolerant and acidophilic. Some grow best at pH 2 andmay grow at pH 1 or even at pH 0.5. Most species are tolerant against

heavy metal toxicity.

Thiobacilli are chemolithoautotrophs, that means CO2 may be the onlysource of carbon and they derive their energy from a chemicaltransformation of inorganic matter. All Thiobacilli oxidize sulfur or

sulfur compounds to sulfate or sulfuric acid.

Oxidation of hydrogen sulfide

by ThiobacilliOxidation of elemental Sulfur

by Thiobacilli

If they oxidize hydrogen sulfide, thiosulfate, polythionates or

elemental sulfur they produce hydrogen ions and so they lower the pH of

the medium, often below pH 2, in some cases below pH 1.HS- + 2O2 --> S04

-- + H+ (1)S + H20 + 1O2 S04

-- + 2 H+ (2)

Thiobacillus ferrooxidans

In addition to the oxidation of sulfur and sulfur compoundsThiobacillus ferrooxidans is able to oxidize ferrous to ferric ironand so derive its energy from this exergonic reaction. In thisreaction hydrogen ions are consumed and so the pH of the medium shouldrise. But at pH values higher than 2 the ferric iron precipitates asferric hydroxide, jarosites or similar compounds and this results inthe formation of hydrogen ions, so that the pH of the medium is

lowered as is the case with oxidation of sulfur compounds:

2Fe++ + 2H+ + O2

----> 2Fe+++ + H20 (3)

2Fe+++ + 6H20 ----> 2Fe(OH)3 + 6H+ (4)


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2Fe++ + 5H20 + O2 ----> 2Fe(OH)3 + 4H+ (5)

Oxidation of ferrous iron

by T. ferrooxidans

Oxidation of ferrous iron by T.

ferrooxidans with subsequent

precipitation of ferric

hydroxide

As will be shown later, owing to its ability to oxidize ferrous iron,T. ferrooxidans is the principal agent of bacterial ore leaching at

moderate temperatures.

Thiobacilli and sulfidic minerals

Some Thiobacilli, especially T. ferrooxidans, are able to oxidizesulfide and some heavy metals -mainly iron but also copper, zinc,molybdenum and presumable some other metals - in the form of sulfidicheavy metal minerals which are of very low solubility in water,practically insoluble. These oxidations result in a solubilization ofthe minerals. This is often seen in the case of pyrite or marcasite,

both FeS2, minerals which are oxidized very easily by Thiobacilli:

FeS2 + H20 + 3O2 Fe++ + 2 SO4

-- + 2 H+ (6)

but also in the case of other minerals. Oxidation of the sulfide of a

divalent metal:

MeIIS + 2O2 Me++ + SO4

-- (7)


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Direct solubilization of

sulfidic heavy metal minerals by

Thiobacilli

In the solubilization ofsulfidic minerals there areseveral reactions involved whichare not fully understood in alldetails nor in their relativeimportance. But some mechanisms

are clear:

(a) The oxidation of sulfideions and of metal ions disturbthe solubility equilibrium andso the sulfide mineral may

dissolve slowly.(b) Hydrogen ions formed in connection with sulfide and ferrous ironoxidation by the bacteria attack the mineral and release metal ions

and hydrogen sulfide or elemental sulfur:

NiS + 2 H+ Ni++ + H2S (8)

FeS2 + 2 H+ Fe++ + H2S + S (9)

Hydrogen sulfide and elemental sulfur are then oxidized by the

bacteria to sulfuric acid, which gives rise to more hydrogen ions.

(c) The combination of hydrogen ion attack and oxidation with oxygen

releases metal ions and elemental sulfur:

MeIIS + 2 H+ + O2 -> Me++ + H20 + S (10)

in the case of chalcocite (Cu2S) it forms covellite (CuS), and copper

ions:

Cu2S + 2 H+ + O2 CuS + Cu

++ + H20 (11)

These processes are called the direct mechanisms of bacterial mineral

solubilization to distinguish them from an indirect mechanism:

(d) Ferric ions, - formed by oxidation of ferrous iron by T.ferrooxidans, are a strong oxidant and may oxidize sulfidic bound

metals so that soluble metal cations are formed:

MeIIS + 2F+++ Me++ + 2F++ + SThe iron is thereby reduced to ferrous iron which is oxidized to

ferric iron again by Thiobacillus ferrooxidans:2Fe++ + 2H+ + O2 2F

+++ + H2O (13)

The elemental sulfur may be oxidized by Thiobacilli to sulfuric acid

which supports the dissolution of the mineral according to equations

(8) to (11):


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S + H20 + 1O2 -+ SO4-- + 2H+ (14)

Indirect solubilization of

sulfidic heavy metal minerals

by

Thiobacillus ferrooxidans

Indirect solubilization of

uraninite by Thiobacillus

ferrooxidans

By this indirect mechanism of bacterial dissolution of sulfidicminerals also heavy metal minerals can be attacked which are notaccessible to the direct mechanisms, especially whose metals which cannot be oxidized by the bacteria. Moreover some non-sulfidic heavymetal minerals can be brought into solution through oxidation mediated

by the ferric/ferrous iron system.

This latter fact is of particular importance in leaching uranium ores:uranium(IV) for example as uranium dioxide UO2, uraninite, is oxidizedby ferric iron to uranium(VI) and so soluble uranyl ions UO2 are

formed:

UO2 + 2Fe+++ (UO2)

++ + 2 Fe++ (15)

Thiobacillus thiooxidans, an extremely acidophilic but not ferrousiron oxidizing species of the Thiobacilli, is not able to solubilizesulfidic heavy metal minerals in pure culture. Nevertheless T.thiooxidans plays a role in metal leaching. The solubilization ofsulfidic minerals by Thiobacillus ferrooxidans is increased bycooperation with T. thiooxidans as compared with the effect of T.ferrooxidans alone. We can assume that the cause of this enhancementis the oxidation of elemental sulfur and hydrogen sulfide which isformed as a result of the oxidation by ferric iron according toequation (12), for this oxidation produces hydrogen ions which in turn

attack the minerals according to equations (8) and (9).


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Direct solubilization of pyrite

or marcasite by Thiobacillus

ferrooxidans

Other bacteria

In addition to Thiobacilli there are some other bacteria known to beeffective in solubilizing sulfidic minerals. In hot biotopes

containing sulfur or oxidisable sulfur compounds, such as hydrothermalvents and self heating brown coal dumps, one can find anarchaebacterium named Sulfolobus. This is a bacterium without a rigid

cell wall, round shaped, about 0.8 to 1.0 m in diameter.

Like Thiobacilli it is acidophilic, chemolithoautotroph and derivesits energy from oxidation of sulfur and sulfur compounds and fromoxidation of ferrous iron like Thiobacillus ferrooxidans. Its pH-rangeof growth is pH 1.0 - 6.0 and its optimum at about pH 2. A salient

characteristic is its thermophily: its growth

range is 45 85C, its optimum 70 75C. Species of this genus,especially S. brierleyi seem to be the main agent in metal leaching at

high temperatures.

Leptospirillum

Often one can see in acid metal leaching biotopes spirilloid bacteria.They belong to the species Leptospirillum ferrooxidans, a gramnegativespirillum, facultatively chemolithoautotroph, deriving its energy fromoxidizing ferrous iron like Thiobacillus ferrooxidans. But in contrastto this latter bacterium it cannot oxidize sulfur or sulfur compounds

and is incapable of utilizing the iron of sulfidic minerals.


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Leptospirillum ferrooxidans alone cannot solubilize sulfidic ferrousiron containing minerals. But in cooperation with Thiobacillusthiooxidans, which, for its part alone, is also unable to dissolvesulfidic minerals, it can; both bacteria together disintegratesulfidic ferrous iron containing minerals by oxidation and bringing

them into solution (Balashova et al., 1974).

Bacterial leaching versus abiotic leaching

Simple laboratory experiments can show, that chemical reactionscatalyzed by bacteria are the essential processes which lead to decayof sulfidic heavy metal minerals and some other minerals and thatabiotic reactions play a negligible role. If sulfidic ores arepercolated with simple water or diluted salt solutions under aerationin laboratory percolators in parallel sets, one set not sterilized orinoculated with natural acid mine effluent, another set under sterile

conditions, it can be seen that disintegration of ore and leaching ofmetals proceeds in the not sterilized or inoculated percolators verymuch quicker than in the sterilized ones, the ratio being about 104 or

higher.

In such percolator experiments it isobserved that almost all the bacteriaadhere to the pieces of ore andespecially to the surfaces of thesulfidic minerals. Only a small amountof bacteria is floating free in the

medium. So the bacteria are in closecontact to the almost insolublesubstrate which they oxidize to yieldenergy. This seems to be necessarybecause we can assume, thatsolubilization of the minerals by somedirect mechanisms requires direct

contact.

The rate of dissolution of the metalminerals is essentially limited by theaccessible surface of the minerals andcan be enhanced by grinding theminerals or the pieces of ore resp. tosmaller grains. If the sulfidicminerals are not freely exposed, butare embedded in rock, as is normallythe case with heavy metal ores, therate of leaching is limited above allby the diffusion rates of solutesthrough fissures. Oxygen, ferric ionsand hydrogen ions have to diffuse fromthe outside of the piece of ore, tothe metal minerals inside and,conversely, metal, sulfate andhydrogen

ions have to diffuse out to the surrounding medium, regardless ofwhether the bacteria are within the fissures on the sulfidic minerals


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or on the outside of the piece of ore.

Bacterial leaching of a piece of

ore with imbedded sulfidic ore

minerals

Technical application

Bacterial disintegration of ores has been applied on a technical scalefor many years, almost solely to leach copper and uranium. Actually itwas used for extracting copper from sulfidic ores long ago and longbefore bacteria were recognized as the cause of natural weathering. Insome places ore leaching was operated some centuries ago, for instanceat Rio Tinto in Spain. In the last decades bacterial ore leaching wascarried out in many countries: Canada, U.S.A., Mexico, Australia,India, U.S.S.R., Turkey, Yugoslavia, Romania, Hungary, Spain and some

other countries.

Dump leaching

The most commonly applied method is that of the percolator principle.Big dumps of ore are set up on an impermeable ground. The grain sizehas to be so that on the one hand the leaching liquor can percolatethrough the dump and air may enter from the sides, and on the otherhand the distances for mass diffusion inside the grains are as short

as possible.

The leaching liquor is distributed on the top of the dumps bysprinklers or by intermittent flooding of ponds. At the bottom theliquor is collected, in some cases by a drainage system, and conducted

to a collecting reservoir from which it is pumped back on top of thedump. Before pumping back to the dump the whole liquor or a part of itmay be conditioned, that means extracting the dissolved metal (forinstance copper by cementation with iron scrap), addition of sulfuric

acid if the pH is too high and addition of nutrient salts if desired.


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Copper from ores which contain sulfides are leached on the whole bydump leaching. Chiefly copper ores of the porphyric type (disseminatedcopper ores) with low concentrations of copper (below 0.6% Cu) areleached in this way. For instance in some states of the U.S.A. at someopen pit mines, in which low grade copper ores are excavated, big dump

leaching facilities are operated. The height of the dumps ranges from20 m to about 200 m and they may contain up to 10 9 t of ore at onemine. The grain size is up to 1 m3, the copper concentration is 0.1 to

0.6%.

The pH of the circulating liquor is about 2.0 - 3.5, its ironconcentration about 35 - 60 mmol/l. In the on-flowing liquor the ironis almost completely ferrous iron, whereas in the outflow only 108 to40%, sometimes 70%, of the iron is ferrous iron. So we can conclude,that iron is oxidized by the bacteria almost exclusively inside thedump. This fits with the observation, that almost all bacteria adhereon the ore and only a small amount is free in the fluid as mentionedabove. Therefore a good aeration of the dumps is necessary, but this

occurs unaided at least in their outer and upper parts by thermic airbuoyancy for the temperatures in the dumps are elevated by thereaction heat up to 30 40C, and in some spots temperatures near60C were measured. By the way: out-streaming air at the top of, the

dumps contains much less oxygen than does normal air.

In most cases the addition of nutrients is not necessary becauseThiobacilli are lithoautotrophs and need only some inorganic nutrientsbesides an energy source. The required inorganic nutrients may betaken from the ore. The nitrogen source may be an exception for oresusually contain only small amounts of nitrogen compounds. But it hasbeen found that strains of Thiobacillus ferrooxidans are able toreduce molecular nitrogen and so meet their demand for nitrogen

(Mackintosh, 1978).

Operating big dumps the circulation rate is about 5000 m3 of liquor per

hour (20 -30 l m-2 h-1). The copper concentration of the out-flowingliquor is about 8 mmol/l (500 g/m3). In the U.S.A. 200,000 to 250,000 tof copper are produced annually by bacterial leaching, equivalent to20 -25% of the total copper production. In the whole world about 5% of

the total copper production is obtained by bacterial leaching.

Bacterial leaching is a very slow process. Around 3 to 10% of thecopper content is leached out of a low grade sulfidic copper ore peryear. So dumps may be operated 10 to 20 years. But on the other handdump leaching is a simple and cheap method. It needs only a littlecapital investment, has low operating costs, requiring-little labor,


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and is well-suited to low grade ores if they contain the metal insulfidic minerals or if sulfides are contained in addition. A certainamount of pyrite in the ore is favourable because oxidation of pyriteby Thiobacilli releases enough hydrogen ions to lower the pH value and

enough ferric iron for the indirect oxidation mechanism.

Besides copper uranium is leached by bacteria from its ores on atechnical scale. This leaching depends wholly on indirect oxidation bymeans of the ferric/ferrous iron system according to equation (15). Sothe leaching of uranium ores which contain pyrite as an iron source ismost economical. Otherwise one has to add pyrite or another source of

iron.

The technical set-up of uranium ore leaching may be the dump method,but sometimes a variation of this, so-called heap or basin leaching isapplied. The ore is set up in basins. The mode of operation ispreferably a two stage leaching: the out-flowing liquor, in which theiron is largely in the ferrous form, is treated in an oxidation pond.In this the liquor is aerated to enable Thiobacillus ferrooxidans tooxidize ferrous iron and to obtain the ferric iron required foroxidation of uranium! The oxidized liquor is then pumped back to the

dump or basin.

In situ leaching

In a few cases it has been attempted to leach ores by means ofbacteria without excavating the ore prior to leaching. At first sightit seems advantageously to leach ores on the spot were they are, for

excavating costs can be saved. But difficulties arise if the ore bodyis impermeable or if there are only a few channels through which theleaching liquor would stream downwards without percolating the orebody entirely. In such cases the ore body has to be cracked by

explosions.


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In situ ore

leaching from

injection

wells to

producing

wells

Moreoverthere may besomedifficultiesconnectedwith thegeologicalsituationbecause it isnecessary tocollect theliquor after

it has passedthrough theore body.Unsuitablesiting maylead to largeamounts ofthe leachingfluidescaping

underground.To myknowledge

bacterialleaching insitu, in astrict sense,has not yetbeenperformed. Inthe U.S.A.uraniumdeposits wereleached insituunderground

as shown inthe picture.


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But these leachings weredone abiotic without usingbacteria. There are somebacterial leaching set-upswhich in a broader sensecan be called in situ

leaching. To this belongsthe percolating of aworked-out mine withresidues of ore as isschematically shown.InCanadian uranium minesafter they were worked-outthe walls, roofs andfloors were hosed down atintervals of severalmonths.The water wascollected and the uraniumextracted.

Other types of bacterial leaching plants

Some other types of bacterial ore leaching arrangements were set up ona laboratory scale as well as on a semi-technical scale. Big tanks maybe filled with pieces of ore like a laboratory percolator and then theore may be percolated. Such an pilot plant has been set up at the JohnD. Sullivan Centre for In-Situ Mining Research in Socorro, New Mexico.The advantage of such a tank leaching is that the process can beeasily controlled and regulated. The ore can be heated simply byinsulating the walls of the tank, so the reaction heat of the

oxidation is used for heating. Of course leaching in tanks is moreexpensive than dump leaching and could therefore be applied only to

special purposes.

A very interesting method is leaching ground ore in suspension.Grinding ore down to particle size of below 0.1 mm increasesconsiderably the specific surface area and so increases the leachingrate substantially. But ore which is ground to low particle sizecannot be percolated, it has to be treated in suspension. Therefore areactor is required in which the suspension can be agitated andaerated. The pulp may contain 10% to 20% solids in suspension ("pulp

density").

Suspension leaching is a very effective method and has the advantagethat it can easily be controlled and regulated. So it may be possible


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to chose a favourable temperature and to add phosphate, ammonia,carbon dioxide, sulfuric acid, iron or other additives in order toaccelerate the leaching process. But on the other hand it is expensiveand its application is restricted to special purposes, for instance tothe leaching of concentrates Suspension leaching on a laboratory scalein agitated flasks is a convenient tool to investigate the

leachability of an ore and to reveal the optimal leaching conditions.

Problems

There are many possibilities for disturbing bacterial leaching. Lackof iron can be met in most cases by adding iron in some form,preferably as pyrite because by oxidation of pyrite not only iron ionsare formed but also hydrogen ions. Therefore addition of pyrite iswell suited if it is necessary to lower the pH. For this latter

purpose also elemental sulfur may be added instead of pyrite,Thiobacilli will then oxidize sulfur to sulfuric acid.

A large amount of carbonates may cause serious disruption becauseThiobacilli and other bacteria concerned with ore leaching areacidophilic. They are inactive and don't grow in a neutral or alkalinemilieu. If enough hydrogen ions are formed by bacterial oxidationactivity alkali of earth carbonates may be neutralized and decomposed.But then another problem arises: the alkali of earth ions precipitateas sulfates and these may disturb the leaching by plugging and by

covering the surfaces of the ore minerals.

In ponds on the top of dumps operated by the pond system ferric ironcompounds often precipitate. This hinders the infiltration of theliquor by plugging the upper layer of the ore dump. Fromtime to time

the precipitates have to be scraped off.

Toxic substances in the ore may inhibit or kill the bacteria.Thiobacilli, Sulfolobus and Leptospirillum ferrooxidans are verytolerant against dissolved heavy metals. The following limits of heavy

metal tolerance of T. ferrooxidans were observed:

Cu

Zn

Ni

U

Mo

0.87

mol/l

1.83

mol/l

0.85

mol/l

0.004

mol/l0.05

mol/l

55 g/l

120 g/l

l50 g/l

1 g/l without

adaptation12 g/l after

adaptation

0.08 g/l


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0.0008mol/l

Arsenic, molybdenum, silver and mercury may be toxic to Thiobacilli.

Noteworthy is the higher tolerance against molybdenum of Sulfolobusbrierleyi: this bacterium metabolizes without inhibition at amolybdenum concentration of 20 mmol/l or higher, whereas Thiobacillitolerate molybdenum only up to about 1 mmol/l (Brierley, Murr, 1973).In some cases the tolerance of leaching bacteria against toxic

substances may be developed by adaptation.

Further investigations

Many factors influence bacterial ore leaching:

properties of the microorganisms mineral species includingaccompanying minerals surface area of the minerals, particle sizewater availability temperature pH redox potential oxygen supply carbondioxide supply, supply of other nutrients e.g. nitrogen compounds,

phosphate toxic substances light formation of secondary minerals

Much work has been done on the influence of these factors,qualitatively and quantitatively. Further effort is needed tounderstand fully all dependencies in all cases of bacterial leaching.Much work has to be done in order to find new applications and new

methods. Many research groups in several countries work in this field.An interesting approach is genetical manipulation of leaching

bacteria.

But here I should confine myself to report what is done in the Federal

Republic of Germany.

(a) Dr. Bosecker in his laboratory at the Bundesanstalt frGeowissenschaften in Hanover investigates the application of bacterialleaching to new ores. In particular he has tried to leach copper fromcopper bearing black shale, nickel out of gabbro and other basicplutonic rocks, and zinc out of old dumps which were left by miners

some centuries ago in Germany.

(b) Bacterial leaching of industrial waste materials is done on anlaboratory scale and in pilot plants by the group of Prof. Onken inDortmund and the German mining company Preussag at the Harz Mountains(Goslar). Tailings from flotation plants, metal containing drosses andsimilar materials are leached, mainly as suspensions in different

bioreactors.

(c) Coal often contains considerable amounts of pyrite which oncombustion is oxidized to sulfur dioxide. To minimize the emission ofthis toxic and acid forming gas Dr. Ebner and his co-workers in a

laboratory of the Bergbau-Forschung in Essen try to desulfurize coalby bacterial leaching.


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(d) As was mentioned above in situ leaching is problematic. The miningcompany Preussag set up an underground pilot plant in ah old mine inwhich a complex sulfidic ore has been excavated for more than athousand years. In about 2 years it will be worked-out. In the upperand older parts of the mine the miners of past centuries left a lot ofore which is now slowly leached by natural bacterial oxidation and

which cannot be excavated economically. The company is investigatingwhether or not these residues can be recovered economically by

bacterial leaching.

(e) Prof. Stetter and his group at the University of Regensburg haveisolated about 350 new strains of bacteria, some of these, from hotsprings and other hot biotopes in areas with volcanic activity inItaly and Iceland, are thermophilic, some extremely so. Prof. Stetterand his co-workers have examined these new strains in respect of theirability to leach metals out of ore minerals. Some 170 out of the 350isolates are active in metal leaching. It may be that some of thethermophilic strains are suited to leaching at high temperatures with

high rates on a technical scale.

(f) The mining company "Uranerzbergbau" did some work on leachingspecial uranium ores and investigated conditions for leaching with

rates which make this operation economical.

(g) In our laboratory in Braunschweig we investigate the ecology ofthe microbial communities which develop in acid ore leaching biotopes.The main object of our work is an old mine in the Harz Mountains inwhich the mining company has installed an underground in situ leachingpilot plant. We have isolated a lot of microorganisms from this mine

biotope in which the pH is between 2 and 3.

We have been able to show that many neutrophilic, heterotrophicbacteria, isolated from the mine, become acidotolerant growing in aspent culture medium of Thiobacilli. This fact explains why so many

neutrophilic bacteria are found in acid leaching biotopes.

We wondered why so many heterotrophic bacteria live in an inorganicbiotope which is thought to be free of organic matter, forheterotrophic bacteria need organic matter as an energy source andnutrient. The answer is that the autotrophic Thiobacilli are primaryproducers of organic matter, because they can synthesize biomass fromcarbon dioxide. We found that many of the heterotrophic bacteriaisolated from this biotope can grow at the expense of organic matter

produced and partly excreted by the Thiobacilli.

Growth of heterotrophic bacteria at the

expense of organic matter, able to

reduce

molecular nitrogen

Bacterial leaching of heavy metals

from

sulfidic ore minerals


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It is known that the metabolic activity and the growth of Thiobacilliare inhibited by some organic compounds. We found that this is alsothe case with organic matter which is excreted by the Thiobacillithemselves. The consumption of this organic matter by heterotrophicbacteria therefore supports the metabolic activity and the growth of

Thiobacilli, as we were able to show.

Bacterial leaching of

heavy metals from sulfidic

ore minerals. The

interrelationship between

Thiobacilli andaccompanying heterotrophic

bacteria.

Almost all of the isolatedstrains of Thiobacillusferrooxidans, but none ofthe T. thiooxidansstrains,are able toutilize molecular nitrogenas a nitrogen source. The

T. thiooxidans strains cannot grow without additionof nitrogen compounds.Among the isolatedheterotrophic bacteriawere some strains able toreduce molecular nitrogen.In mixed cultures withthese heterotrophicstrains the T. thiooxidansstrains grow and leachmetals from ore withoutaddition of nitrogencompounds. So we know thatThiobacilli may beprovided with an


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utilizable nitrogen sourceby heterotrophic nitrogen

reducing bacteria.

In summary there are several interactions between the autotrophicThiobacilli and their heterotrophic companions, and we don't know yetall of them. So we hope to learn more about the interrelationships inthese interesting biocenoses of acid leaching biotopes, and we hopethat more detailed knowledge in this field can help influence

bacterial leaching methods towards greater efficiency.

February 1986

bacterial leaching of ores and other materials

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