leaching of ores and other materials

8/6/2019 Leaching of Ores and Other Materials

1/18

Bacterial leaching of ores and

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 b y

oxidizing ferrous to ferric iron as well as elemental sulfur, sulfideand other sulfur compounds to sulfate. So they enhance leaching of

heavy metals from sulfidic ores under aerobic conditions about 104

fold or more compared with weathering without bact eria.

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 Sulf olobus plays a role inleaching at elevated temperatures. Thiobacillus thiooxidans, whichoxidizes merely sulfur and sulfur compounds but not iron, and

Leptospirillum ferrooxidans, which contrarily oxidizes only ferrous

iron, may play a role if they work t ogether 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 v eryslow 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 those

mentioned, leaching industrial wastes to recove r 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 the

involved microorganisms and on the complex interrelationships in themicrobial 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 our


2/18

resources in naturally occurring deposits? Before we answer thisquestion we have to define what a metal ore deposit is. A metal oredeposit is a naturally occurring concentration of a metal or somemetals from which this metal can be obtained in an economic way. So,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 from

the 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 with

atmospheric 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 groundwater

level, in which minerals,formed by the reaction ofprimary ore minerals with theconstituents of the leachingsolution descending from theoxidation 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 process

and to get an idea of how toapply these natural processes

to ore leaching on an

industrial scale.

Microbiology of ore leaching

Microbiological investigations revealed that certain bacteria are the

main agent in natural weathering of sulfidic heavy metal minerals.


3/18

Thiobacilli

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

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 CO 2 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 Thiobacilli Oxidation 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 iron

and so derive its energy from this exergonic reaction. In thisreaction hydrogen ions are consumed and so the pH of the medium s houldrise. But at pH values higher than 2 the ferric iron precipitates as


4/18

ferric 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)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 sulfidic

heavy metal minerals which ar e 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


5/18

divalent metal:

MeIIS + 2O2 Me++ + SO4

-- (7)

Direct solubilization of

sulfidic heavy metal minerals

by Thiobacilli

In the solubilization of

sulfidic minerals there areseveral reactions involvedwhich are not fully understoodin all details nor in their

relative importance. But some

mechanisms are clear:

(a) The oxidation of sulfideions and of metal ions disturbthe solubility equilibrium and

so the sulfide mineral maydissolve 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 (Cu 2S) it forms covellite (CuS), and copper

ions:

Cu2S + 2 H+ + O2 CuS + Cu

++ + H20 (11)

These processes are called the direct mechanisms of bacterial mineralsolubilization 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


6/18

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

+++ + H2O (13)

The elemental sulfur may be oxidized by Thiobacilli to sulfuric acidwhich supports the dissolution of the mineral according to equations

(8) to (11):

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 sulfidic

minerals 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 heavy

metal minerals can be brought into solution through o xidation 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 UO 2, uraninite, is oxidized

by ferric iron to uranium(VI) and so soluble uranyl ions UO 2 areformed:

UO2 + 2Fe+++ (UO2)

++ + 2 Fe++ (15)

Thiobacillus thiooxidans, an extremely acidophilic but not ferrous

iron 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 of


7/18

sulfidic 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 enha ncementis 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).

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 biotopescontaining sulfur or oxidisable sulfur compounds, such as hydrothermal

vents 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 derives

its 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.


8/18

Leptospirillum

Often one can see in acid metal leaching biotopes spirilloid bacteria.They belong to the species Leptospirillum ferrooxidans, a gramnegative

spirillum, 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.

Leptospirillum ferrooxida ns alone cannot solubilize sulfidic ferrousiron containing minerals. But in cooperation with Thiobacillus

thiooxidans, 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 proce sses which lead to decay

of 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 aeration

in laboratory percolators in par allel sets, one set not sterilized orinoculated with natural acid mine effluent, another set under sterileconditions, 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 10 4 or

higher.


9/18

In such percolator experiments it is

observed that almost all the bacteriaadhere to the pieces of ore andespecially to the surfaces of thesulfidic minerals. Only a small amoun tof bacteria is floating free in themedium. So the bacteria are in close

contact to the almost insolublesubstrate which they oxidize to yield

energy. 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 sulfidic

minerals are not freely exposed, butare embedded in rock, as is normallythe case with heavy metal ores, the

rate of leaching is limited above allby the diffusion rates of solutesthrough fissures. Oxygen, ferric ions

and 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

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 o f natural weathering. Insome places ore leaching was operated some centuries ago, for instanceat Rio Tinto in Spain. In the last decades bacterial ore leaching was


10/18

carried 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 the

liquor is collected, in some cases by a drainage system, and conductedto a collecting reservoir from which it is pumped back on top of the

dump. Before pumping back to the dump the whole liqu or 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.

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. a t 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 m 3, the copper concentration is 0.1 to

0.6%.

The pH of the circulating liquor is about 2.0 - 3.5, its iron

concentration 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 menti oned


11/18

above. Therefore a good aeration of the dumps is necessary, but thisoccurs 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 so me inorganic nutrientsbesides an energy source. The required inorganic nutrients may be

taken 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 Th iobacillus ferrooxidans are able toreduce molecular nitrogen and so meet their demand for nitrogen

(Mackintosh, 1978).

Operating big dumps the circulation rate is about 5000 m 3 of liquor per

hour (20 -30 l m-2 h-1). The copper concentration of the o ut-flowing

liquor is about 8 mmol/l (500 g/m 3). In the U.S.A. 200,000 to 250,000 tof copper are produced annually by bacterial leaching, equivalent to

20 -25% of the total copper production. In the whole world about 5% ofthe total copper production is ob tained 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 per

year. So dumps may be operated 10 to 20 years. But on the other handdump leaching is a s imple and cheap method. It needs only a littlecapital investment, has low operating costs, requiring -little labor,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). So

the 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 to

oxidize ferrous iron and to obtain the ferric iron required foroxidation of uranium! The oxidized l iquor 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 of


12/18

bacteria without excavating the ore prior to leaching. At first sightit seems advantageously to leach ores on the spot were they are, forexcavating 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 th e ore body has to be cracked by

explosions.

In situ ore

leaching from

injection

wells to

producing

wells

Moreoverthere may besome

difficultiesconnected

with thegeologicalsituationbecause it isnecessary tocollect theliquor afterit has passed

through theore body.Unsuitablesiting may

lead to large

amounts ofthe leachingfluidescaping

underground.To myknowledgebacterialleaching in

situ, in astrict sense,has not yetbeenperformed. In

the U.S.A.uraniumdeposits wereleached insituunderground

as shown in

the picture.


13/18

But these leachings were

done abiotic withoutusing bacteria. There aresome bacterial leachingset-ups which in abroader sense can becalled in situ leaching.

To this belongs thepercolating of a worked -

out mine with residues ofore as is schematicallyshown.In Canadian uraniummines after they were

worked-out the walls,roofs and floors werehosed down at intervalsof several months.Thewater was collected andthe uranium extracted.

Other types of bacterial leaching plants

Some other types of bacterial ore leaching arrangements were set up on

a 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 John

D. 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 by

insulating the walls of the tank, so the reaction heat of theoxidation 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 increas esconsiderably the specific surface area and so increases the leaching

rate 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 b e agitated andaerated. The pulp may contain 10% to 20% solids in suspension ("pulp

density").


14/18

Suspension leaching is a very effective method and has the advantagethat it can easily be controlled and regulated. So it may be possibleto chose a favourabl e 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 to

the leaching of concentrates Suspension leaching on a laboratory scale

in agitated flasks is a convenient tool to investigate theleachability 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 ions

are 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 be causeThiobacilli 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 oxidation

activity alkali of earth carbonates may be neutral ized 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 syst em ferric iron

compounds often precipitate. This hinders the infiltration of theliquor by plugging the upper layer of the ore dump. From time 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/l

55 g/l

120 g/l

l50 g/l

1 g/l without

adaptation12 g/l after


15/18

0.05

mol/l

0.0008mol/l

adaptation

0.08 g/l

Arsenic, molybdenum, silver and mercury may be toxic to Thiobacilli.Noteworthy is the higher tolerance against molybdenum of Sulfolobus

brierleyi: 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 inclu dingaccompanying minerals surface area of the minerals, particle sizewater availability temperature pH redox potential oxygen supply carbon

dioxide supply, supply of other nutrients e.g. nitrogen compounds,

phosphate toxic substances light formation of s econdary 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 newmethods. 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 German y.

(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 a nd 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 an

laboratory 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 on


16/18


17/18

expense of organic matter, able to

reduce

molecular nitrogen

from

sulfidic ore minerals

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 an d the growth of

Thiobacilli, as we were able to show.

Bacterial leaching of

heavy metals fromsulfidic ore minerals.

The interrelationship

between Thiobacilli and

accompanying

heterotrophic bacteria.

Almost all of theisolated strains of

Thiobacillus

ferrooxidans, but none ofthe T. thiooxidansstrains,are able to

utilize molecularnitrogen as a nitrogensource. The T.

thiooxidans strains cannot grow without additionof nitrogen compounds.Among the isolatedheterotrophic bacteriawere some strains able toreduce molecular


18/18

nitrogen. In mixed

cultures with theseheterotrophic strains theT. thiooxidans strainsgrow and leach metalsfrom ore without additionof nitrogen compounds. So

we know that Thiobacillimay be provided with an

utilizable nitrogensource by heterotrophicnitrogen reducing

bacteria.

In summary there are several interactions between the autotrophicThiobacilli and their heterotrophic companions, and we don't know yet

all 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 influen ce

bacterial leaching methods towards greater efficiency.

February 1986

leaching of ores and other materials

Documents