Bacterial leaching of ores and other materials

 R. Näveke, Institut für Mikrobiologie, Technische Universität 
Braunschweig, Fed. Rep. Germany

 
Summary 

In nature sulfidic ores are decayed by weathering under the influence of oxygen and water. Microbiological investigations reveal that certain bacteria are the main agent in this process. Several bacteria, especially Thiobacilli, are able to solubilize heavy metal minerals by oxidizing ferrous to ferric iron as well as elemental sulfur, sulfide and 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 bacteria. 

The principal bacterium in ore leaching is Thiobacillus ferrooxidans, which is capable of oxidizing ferrous iron as well as sulfur and sulfur compounds. But there are some other bacteria which may also be involved. For example the thermophilic Sulfolobus plays a role in leaching at elevated temperatures. Thiobacillus thiooxidans, which oxidizes merely sulfur and sulfur compounds but not iron, and Leptospirillum 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 industrial scale by different procedures: dump leaching, in situ leaching, tank leaching, leaching in suspension. Sulfidic copper and uranium ores are the principle ores leached in several countries. So 20% to 25% of the copper production in the U.S.A. and about 5% of the world copper production is obtained by bacterial leaching. This process is a very slow 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 recover metals, desulfurizing of coal, developing methods for in situ leaching and using other microorganisms than those used until now. Basic microbiological research focuses on the biochemistry, physiology and genetics of the involved 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 sooner or later these resources will be exhausted. But how great are our resources in naturally occurring deposits? Before we answer this question we have to define what a metal ore deposit is. A metal ore deposit is a naturally occurring concentration of a metal or some metals 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 or not 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 expected with 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 processes which are the essential causes of natural weathering of sulfidic minerals. If sulfidic heavy metal minerals come into contact with air and 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 secondary minerals formed by oxidation of the primary ore minerals and in most cases a remarkable enrichment of ferric iron minerals (limonite and others).  
(b) an underlying cementation zone just below the groundwater level, in which minerals, formed by the reaction of primary ore minerals with the constituents of the leaching solution descending from the oxidation zone, are accumulated.  
(c) A zone in which the primary ore minerals are unchanged. 

So we have to look at these phenomena to understand what exactly happens in this process and to get an idea of how to apply 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. 
 
 

Thiobacilli 

The principal bacteria which play the most important role in solubilizing sulfidic metal minerals at moderate temperatures are species of the genus Thiobacillus. They are gramnegative rods, either polarly or nonflagellated. Most species are acidotolerant, some even extremely acidotolerant and acidophilic. Some grow best at pH 2 and may 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 only source of carbon and they derive their energy from a chemical transformation 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 + 1½O2 à S04-- + 2 H+ (2)

  

Thiobacillus ferrooxidans 

In addition to the oxidation of sulfur and sulfur compounds Thiobacillus ferrooxidans is able to oxidize ferrous to ferric iron and so derive its energy from this exergonic reaction. In this reaction hydrogen ions are consumed and so the pH of the medium should rise. But at pH values higher than 2 the ferric iron precipitates as ferric hydroxide, jarosites or similar compounds and this results in the formation of hydrogen ions, so that the pH of the medium is lowered as is the case with oxidation of sulfur compounds: 
 

2Fe++ + 2H+ + ½O2  ---->    2Fe+++ + H2 (3)
2Fe+++ + 6H20       ---->    2Fe(OH)3 + 6H+ (4)
2Fe++ + 5H20 + ½O2   ---->    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 oxidize sulfide 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 are of very low solubility in water, practically insoluble. These oxidations result in a solubilization of the minerals. This is often seen in the case of pyrite or marcasite, both FeS2, minerals which are oxidized very easily by Thiobacilli: 

FeS2 + H20 + 3½O2 à Fe++ + 2 SO4-- + 2 H+ (6)

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

MeIIS + 2O2 à Me++ + SO4--   (7)
 
Direct solubilization of sulfidic heavy metal minerals by Thiobacilli 
 

In the solubilization of sulfidic minerals there are several reactions involved which are not fully understood in all details nor in their relative importance. But some mechanisms are clear: 
 

(a) The oxidation of sulfide ions and of metal ions disturb the solubility equilibrium and so the sulfide mineral may dissolve slowly.

 (b) Hydrogen ions formed in connection with sulfide and ferrous iron oxidation by the bacteria attack the mineral and release metal ions and hydrogen sulfide or elemental sulfur:   
NiS + 2 H+ à Ni++ + H2 (8)
FeS2 + 2 H+ à 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+ + ½O2 -> Me++ + H20 + S° (10)

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

Cu2S + 2 H+ + ½O2 à 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+++ à Me++ + 2F++ + S°
The iron is thereby reduced to ferrous iron which is oxidized to ferric iron again by Thiobacillus ferrooxidans: 
2Fe++ + 2H+ + ½O2 à 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): 

S° + H20 + 1½O2 -+ 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 not accessible to the direct mechanisms, especially whose metals which can not be oxidized by the bacteria. Moreover some non-sulfidic heavy metal 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 oxidized by ferric iron to uranium(VI) and so soluble uranyl ions UO2 are formed: 

UO2 + 2Fe+++ à (UO2)++ + 2 Fe++ (15)

Thiobacillus thiooxidans, an extremely acidophilic but not ferrous iron oxidizing species of the Thiobacilli, is not able to solubilize sulfidic heavy metal minerals in pure culture. Nevertheless T. thiooxidans plays a role in metal leaching. The solubilization of sulfidic minerals by Thiobacillus ferrooxidans is increased by cooperation with T. thiooxidans as compared with the effect of T. ferrooxidans alone. We can assume that the cause of this enhancement is the oxidation of elemental sulfur and hydrogen sulfide which is formed as a result of the oxidation by ferric iron according to equation (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 be effective in solubilizing sulfidic minerals. In hot biotopes containing sulfur or oxidisable sulfur compounds, such as hydrothermal vents and self heating brown coal dumps, one can find an archaebacterium 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 from oxidation of ferrous iron like Thiobacillus ferrooxidans. Its pH-range of 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 85°C, its optimum 70 75°C. 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 gramnegative spirillum, facultatively chemolithoautotroph, deriving its energy from oxidizing ferrous iron like Thiobacillus ferrooxidans. But in contrast to this latter bacterium it cannot oxidize sulfur or sulfur compounds and is incapable of utilizing the iron of sulfidic minerals. 

Leptospirillum ferrooxidans alone cannot solubilize sulfidic ferrous iron containing minerals. But in cooperation with Thiobacillus thiooxidans, which, for its part alone, is also unable to dissolve sulfidic minerals, it can; both bacteria together disintegrate sulfidic 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 reactions catalyzed by bacteria are the essential processes which lead to decay of sulfidic heavy metal minerals and some other minerals and that abiotic reactions play a negligible role. If sulfidic ores are percolated with simple water or diluted salt solutions under aeration in laboratory percolators in parallel sets, one set not sterilized or inoculated with natural acid mine effluent, another set under sterile conditions, it can be seen that disintegration of ore and leaching of metals proceeds in the not sterilized or inoculated percolators very much quicker than in the sterilized ones, the ratio being about 104 or higher. 
 

In such percolator experiments it is observed that almost all the bacteria adhere to the pieces of ore and especially to the surfaces of the sulfidic minerals. Only a small amount of bacteria is floating free in the medium. So the bacteria are in close contact to the almost insoluble substrate which they oxidize to yield energy. This seems to be necessary because we can assume, that solubilization of the minerals by some direct mechanisms requires direct contact. 

The rate of dissolution of the metal minerals is essentially limited by the accessible surface of the minerals and can be enhanced by grinding the minerals or the pieces of ore resp. to smaller grains. If the sulfidic minerals are not freely exposed, but are embedded in rock, as is normally the case with heavy metal ores, the rate of leaching is limited above all by the diffusion rates of solutes through fissures. Oxygen, ferric ions and hydrogen ions have to diffuse from the outside of the piece of ore, to the metal minerals inside and, conversely, metal, sulfate and hydrogen 

ions have to diffuse out to the surrounding medium, regardless of whether 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 scale for many years, almost solely to leach copper and uranium. Actually it was used for extracting copper from sulfidic ores long ago and long before bacteria were recognized as the cause of natural weathering. In some places ore leaching was operated some centuries ago, for instance at Rio Tinto in Spain. In the last decades bacterial ore leaching was 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 size has to be so that on the one hand the leaching liquor can percolate through the dump and air may enter from the sides, and on the other hand the distances for mass diffusion inside the grains are as short as possible. 

The leaching liquor is distributed on the top of the dumps by sprinklers or by intermittent flooding of ponds. At the bottom the liquor is collected, in some cases by a drainage system, and conducted to a collecting reservoir from which it is pumped back on top of the dump. Before pumping back to the dump the whole liquor or a part of it may be conditioned, that means extracting the dissolved metal (for instance 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 by dump leaching. Chiefly copper ores of the porphyric type (disseminated copper ores) with low concentrations of copper (below 0.6% Cu) are leached in this way. For instance in some states of the U.S.A. at some open pit mines, in which low grade copper ores are excavated, big dump leaching facilities are operated. The height of the dumps ranges from 20 m to about 200 m and they may contain up to 109 t of ore at one mine. 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 iron concentration about 35 - 60 mmol/l. In the on-flowing liquor the iron is almost completely ferrous iron, whereas in the outflow only 108 to 40%, sometimes 70%, of the iron is ferrous iron. So we can conclude, that iron is oxidized by the bacteria almost exclusively inside the dump. This fits with the observation, that almost all bacteria adhere on the ore and only a small amount is free in the fluid as mentioned above. Therefore a good aeration of the dumps is necessary, but this occurs unaided at least in their outer and upper parts by thermic air buoyancy for the temperatures in the dumps are elevated by the reaction heat up to 30 40°C, and in some spots temperatures near 60¡C 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 because Thiobacilli are lithoautotrophs and need only some inorganic nutrients besides an energy source. The required inorganic nutrients may be taken from the ore. The nitrogen source may be an exception for ores usually contain only small amounts of nitrogen compounds. But it has been found that strains of Thiobacillus ferrooxidans are able to reduce 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-flowing liquor is about 8 mmol/l (500 g/m3). In the U.S.A. 200,000 to 250,000 t of copper are produced annually by bacterial leaching, equivalent to 20 -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 the copper 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 hand dump leaching is a simple and cheap method. It needs only a little capital investment, has low operating costs, requiring-little labor, and is well-suited to low grade ores if they contain the metal in sulfidic minerals or if sulfides are contained in addition. A certain amount of pyrite in the ore is favourable because oxidation of pyrite by 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 a technical scale. This leaching depends wholly on indirect oxidation by means of the ferric/ferrous iron system according to equation (15). So the leaching of uranium ores which contain pyrite as an iron source is most 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 is applied. The ore is set up in basins. The mode of operation is preferably a two stage leaching: the out-flowing liquor, in which the iron 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 for oxidation 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 of bacteria without excavating the ore prior to leaching. At first sight it seems advantageously to leach ores on the spot were they are, for excavating costs can be saved. But difficulties arise if the ore body is impermeable or if there are only a few channels through which the leaching liquor would stream downwards without percolating the ore body entirely. In such cases the ore body has to be cracked by explosions. 
 

In situ ore leaching from injection wells to producing wells 

Moreover there may be some difficulties connected with the geological situation because it is necessary to collect the liquor after it has passed through the ore body. Unsuitable siting may lead to large amounts of the leaching fluid escaping underground. 
To my knowledge bacterial leaching in situ, in a strict sense, has not yet been performed. In the U.S.A. uranium deposits were leached in situ underground as shown in the picture. 

 
But these leachings were done abiotic without using bacteria. There are some bacterial leaching set-ups which in a broader sense can be called in situ leaching. To this belongs the percolating of a worked-out mine with residues of ore as is schematically shown.In Canadian uranium mines after they were worked-out the walls, roofs and floors were hosed down at intervals of several months.The water was collected and the 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 may be filled with pieces of ore like a laboratory percolator and then the ore 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 be easily controlled and regulated. The ore can be heated simply by insulating the walls of the tank, so the reaction heat of the oxidation is used for heating. Of course leaching in tanks is more expensive 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 increases considerably the specific surface area and so increases the leaching rate substantially. But ore which is ground to low particle size cannot be percolated, it has to be treated in suspension. Therefore a reactor is required in which the suspension can be agitated and aerated. The pulp may contain 10% to 20% solids in suspension ("pulp density"). 

Suspension leaching is a very effective method and has the advantage that it can easily be controlled and regulated. So it may be possible to chose a favourable temperature and to add phosphate, ammonia, carbon dioxide, sulfuric acid, iron or other additives in order to accelerate the leaching process. But on the other hand it is expensive and 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 the leachability of an ore and to reveal the optimal leaching conditions. 

  

Problems  

There are many possibilities for disturbing bacterial leaching. Lack of 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 is well 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 because Thiobacilli and other bacteria concerned with ore leaching are acidophilic. They are inactive and don't grow in a neutral or alkaline milieu. If enough hydrogen ions are formed by bacterial oxidation activity alkali of earth carbonates may be neutralized and decomposed. But then another problem arises: the alkali of earth ions precipitate as 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 iron compounds often precipitate. This hinders the infiltration of the liquor 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 very tolerant 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 
0.05 mol/l 

0.0008 mol/l

55 g/l 

120 g/l 

l50 g/l 

1 g/l without adaptation 
12 g/l after 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 a molybdenum concentration of 20 mmol/l or higher, whereas Thiobacilli tolerate 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 including accompanying minerals surface area of the minerals, particle size water availability temperature pH redox potential oxygen supply carbon dioxide 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 to understand 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 für Geowissenschaften in Hanover investigates the application of bacterial leaching to new ores. In particular he has tried to leach copper from copper bearing black shale, nickel out of gabbro and other basic plutonic 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 in Dortmund and the German mining company Preussag at the Harz Mountains (Goslar). Tailings from flotation plants, metal containing drosses and similar materials are leached, mainly as suspensions in different bioreactors. 

(c) Coal often contains considerable amounts of pyrite which on combustion is oxidized to sulfur dioxide. To minimize the emission of this toxic and acid forming gas Dr. Ebner and his co-workers in a laboratory of the Bergbau-Forschung in Essen try to desulfurize coal by bacterial leaching. 

(d) As was mentioned above in situ leaching is problematic. The mining company Preussag set up an underground pilot plant in ah old mine in which a complex sulfidic ore has been excavated for more than a thousand years. In about 2 years it will be worked-out. In the upper and older parts of the mine the miners of past centuries left a lot of ore which is now slowly leached by natural bacterial oxidation and which cannot be excavated economically. The company is investigating whether or not these residues can be recovered economically by bacterial leaching. 

(e) Prof. Stetter and his group at the University of Regensburg have isolated about 350 new strains of bacteria, some of these, from hot springs and other hot biotopes in areas with volcanic activity in Italy and Iceland, are thermophilic, some extremely so. Prof. Stetter and his co-workers have examined these new strains in respect of their ability to leach metals out of ore minerals. Some 170 out of the 350 isolates are active in metal leaching. It may be that some of the thermophilic strains are suited to leaching at high temperatures with high rates on a technical scale. 

(f) The mining company "Uranerzbergbau" did some work on leaching special uranium ores and investigated conditions for leaching with rates which make this operation economical. 

(g) In our laboratory in Braunschweig we investigate the ecology of the microbial communities which develop in acid ore leaching biotopes. The main object of our work is an old mine in the Harz Mountains in which the mining company has installed an underground in situ leaching pilot 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, heterotrophic bacteria, isolated from the mine, become acidotolerant growing in a spent 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 inorganic biotope which is thought to be free of organic matter, for heterotrophic bacteria need organic matter as an energy source and nutrient. The answer is that the autotrophic Thiobacilli are primary producers of organic matter, because they can synthesize biomass from carbon dioxide. We found that many of the heterotrophic bacteria isolated 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
 
It is known that the metabolic activity and the growth of Thiobacilli are inhibited by some organic compounds. We found that this is also the case with organic matter which is excreted by the Thiobacilli themselves. The consumption of this organic matter by heterotrophic bacteria 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 and accompanying heterotrophic bacteria. 
 

Almost all of the isolated strains of Thiobacillus ferrooxidans, but none of the T. thiooxidans strains,are able to utilize molecular nitrogen as a nitrogen source. The T. thiooxidans strains can not grow without addition of nitrogen compounds. Among the isolated heterotrophic bacteria were some strains able to reduce molecular nitrogen. In mixed cultures with these heterotrophic strains the T. thiooxidans strains grow and leach metals from ore without addition of nitrogen compounds. So we know that Thiobacilli may be provided with an utilizable nitrogen source by heterotrophic nitrogen reducing bacteria. 

In summary there are several interactions between the autotrophic Thiobacilli and their heterotrophic companions, and we don't know yet all of them. So we hope to learn more about the interrelationships in these interesting biocenoses of acid leaching biotopes, and we hope that more detailed knowledge in this field can help influence bacterial leaching methods towards greater efficiency. 

February 1986