Bacterial leaching of the metals

(Figure) The installation for the bacterial leaching of copper / The technological scheme of the experimental-industrial plant for the bacterial leaching of copper: 1 - the regenerator for the solutions; 2 - the pump for the circulating solutions; 3 - the pipeline for the leaching solution; 4 - the valves; 5 - the supply pipelines; 6 - the irrigation hoses; 7 - the boreholes-irrigators; 8 - the unit with the stowed ore; 9 - the working for the collection of the productive solutions; 10 - the pump for the productive solutions; 11 - the thickener; 12 - the cementation troughs; 13 - the drying of the cement copper; 14 - the transport tracks; 15 - the compressor station; 16 - the iron scrap.

BACTERIAL LEACHING OF THE METALS (EN: bacterial lixiviation, bacterial leaching; DE: bakterielle Auslaugung; FR: lessivation bacterienne, lessivage bacterien; ES: lixiviacion bacteriana; RU: бактериальное выщелачивание) is the extraction of the chemical elements from the ores, concentrates, and rocks with the help of the bacteria or their metabolites. The large part is combined with the leaching by the weak solutions of the sulfuric acid from the bacterial and chemical origin, and also by the solutions, which are containing the organic acids, proteins, peptides, polysaccharides, and so on.

The leaching of the metals from the ores is known since the old times. They performed during the 1566 within Hungary the complete cycle for the leaching with the usage of the system for irrigation, they practiced within Germany the leaching of copper from the dumps of the waste rock since the 16th century. They leached the copper ores during the 1725 within Spain at the Rio Tinto mine. These events were the first practical applications of the bacterial leaching, the mechanism of which (participation of bacteria) was not known. There has been isolated during the 1947 by the American microbiologists from the mining waters the previously unknown Thiobacillus (Th.) ferrooxidans microorganism, which oxidizes almost all sulfidic minerals, sulfur and the series of its reduced compounds, ferrous iron, and also the Cu+, Se2-, Sb3+, U4+ at the pH from 1.0 to 4.8 (the optimum is 2.0-3.0) and temperature from 5 to 35 degrees Celsius (the optimum is 30-35 degrees Celsius). The quantity of the cells of these bacteria within the zone of oxidation of the sulfidic deposits reaches 1 million - 1 billion within 1 gram of the ore or 1 millilitre of the water.

The leaching of copper with the help of the Th. ferrooxidans has been patented within the USA during the 1958 (C. Zimmerli, and others). Within the USSR, the researches have been started during the end of the 50-ies. Later it has been shown, that within the sulfidic ores, there are common also other bacteria, which are oxidising the Fe2+, S0, and sulfidic minerals, namely, the Leptospirillum (L.) ferrooxidans, Thiobacillus organopatus, Thiobacillus thiooxidans, Sulfobacillus (S.) thermosulfidooxidans, and others. The L. ferrooxidans oxidizes the Fe2+, and during the simultaneous presence with the Th. thiooxidans or Th. organoparus it oxidizes the sulfidic minerals at the pH of 1.5-4.5 (the optimum is 2.5-3.0) and the temperature of approximately 28 degrees Celsius. The S. thermosulfidooxidans oxidizes the Fe2+, S0, and sulfidic minerals at the pH of 1.9-3.5 and the temperature of 50 degrees Celsius. The series of the other thermophilic bacteria oxidizes the Fe, S, and sulfidic minerals at the pH of 1.4-3.0 and the temperature of 50-80 degrees Celsius. The processes of oxidation of the inorganic substrates serve as the single source of energy for these bacteria. They obtain the carbon for the synthesis of the organic compounds of the cells from the CO2, and they obtain other elements from the ores and solutions.

During the bacterial leaching for the ores of the non-ferrous metals, there are widely used the Th. ferrooxidans thiobacteria, which directly oxidise the sulfidic minerals, sulfur, and iron, and form the Fe3+ as the chemical oxidant, and the sulfuric acid as the solvent. Because of this, the consumption of the H2SO4 during the bacterial leaching decreases. The Fe3+ is the major oxidant during the leaching of the ores of uranium, vanadium, copper from the secondary sulfides and other elements. The greatest speed of the bacterial leaching is achieved with the fine grinding of the ore or concentrate (200 mesh and less), within the dense slurries (up to 20% of the solid substance), during the active stirring and aeration of the slurry, and also under the optimal for the bacteria pH, temperature, and with the high content of the cells of the bacteria (10^9-10^10 within 1 millilitre of the slurry). Under the favourable conditions, there transition during 1 hour from the concentrates into the solution: Cu up to 0.7 grams per litre, Zn up to 1.3, Ni up to 0.2, and so on. Up to 90% of As is extracted from the tin-and-gold-bearing concentrates during 70-80 hours. The speed of the oxidation of the sulfidic minerals in the presence of the bacteria increases by the hundreds and thousands times, and of the Fe2+ by approximately 2 • 10^5 times, in comparison with the chemical process. The selectivity of the process of the bacterial leaching for the non-ferrous metals is determined by both the crystal-chemical peculiarities of the sulfides, and their electrochemical interaction. The rare elements are contained within the crystal lattices of the sulfidic minerals or of the host rocks, and, during their destruction, transition into the solution and are leached. Consequently, bacteria play the indirect role for the leaching of the rare elements.

They conduct the bacterial leaching of the non-ferrous metals from the dumps of the low-grade ore (heap) and from the ore body (underground). The technological scheme for the bacterial leaching is shown on the Figure.

The irrigation of the ore within the dump or ore body is performed by the aqueous solutions of the H2SO4, which are containing the Fe3+ and bacteria. The solution is supplied through the boreholes during the underground leaching, or by the method of sprinkling on the surface during the heap leaching. The processes of oxidation of the sulfidic minerals proceed within the ore with the presence of O2 and bacteria, and copper transitions from the insoluble compounds into the soluble ones. The solution, which is containing copper, arrives into the grouting or other installation (sorption, extraction) for the extraction of copper, then onto the dump or into the ore body (the scheme is of the closed-cycle type). The intensification of the leaching is achieved by the activation of the life activity of the thiobacteria and other sulfide-oxidising bacteria, which are existing within the ore itself, and are adapted to the specific conditions of the environment (type of the ore, chemical composition of the solutions, temperature, and so on). There are necessary for this purpose the pH of 1.5-2.5, high oxidation-reduction potential (Eh is 600-750 millivolts), favourable and stable chemical composition of the solutions, which is achieved by the method of their regeneration and regime of aeration and moistening (irrigation) of the ore. It is prescribed in certain cases to add the salts of nitrogen and phosphorus, and also the bacteria, which have been grown on the circulating solutions within the ponds-regenerators. The quantity of the cells of the bacteria within the leaching solution and ore must be at least 10^6-10^7 within 1 millilitre or 1 gram respectively. The cost of 1 tonne of the copper, which has been obtained by this method, is by 1.5-2 times lower, than with the usual hydrometallurgical or pyrometallurgical methods.

The bacterial leaching of the refractory sulphidic concentrates is conducted by the method of the direct flow within the series of the serially connected tanks, with the stirring and aeration by the aerolift at the temperature of 30 degrees Celsius, pH of 2.0-2.5, and the concentration of the Th. ferrooxidans cells of 10^10-10^11 within 1 millilitre of the slurry. The scheme for the processing of the sulphidic concentrates is closed-cycle. The circulating solutions, after the partial or complete regeneration, are used as the nutritional medium for the bacteria, and as the leaching solution. There are most active the cultures of the bacteria, which are adapted to the complex of the factors (pH, heavy metals, the type of the concentrate, and so on) under the conditions of the active process of the bacterial leaching. The examples of the bacterial leaching within the tanks are: during the 72-96 hours, there are extracted from the collective copper-zinc concentrates into the solution up to 90%-92% of Zn and Cd, with the extraction of Cu and Fe approximately 25% and 5% respectively; it is possible to extract Cu, Zn and Cd completely from the lead concentrates. There are achieved within the solutions the concentrations of the metals: Cu up to 50 grams per litre, Zn up to 100 grams per litre, and so on. The arsenopyrite within the tin-bearing and gold-bearing arsenic concentrates is destructed practically completely during the 120 hours, which permits in certain cases to clean the concentrates of the harmful impurity of the arsenic, in other cases to extract up to 90% of gold during the subsequent cyanidation.

Within the various countries, there are conducted also the researches on the bacterial leaching of the metals from the wastes of the beneficiation, dusts, slags, and so on. There are being developed the methods for the bacterial leaching of the gold, manganese, non-ferrous metals, and also for the beneficiation of bauxites with the usage of the heterotrophic microorganisms (microscopic fungi, yeasts, bacteria). These microorganisms use the organic substances as the source of the energy and carbon.

There play the leading importance (sic) during the leaching with the help of the heterotrophs the processes of the complex-forming of the organic compounds with the metals, and also the peroxides and humic acids.

The introduction of the bacterial leaching, as well as of the other hydrometallurgical methods for the extraction of metals, has the great economic significance. The reserves of the raw materials are expanding on the account of the usage of the ores, which are poor and lost within the depths, and so on. The bacterial leaching ensures the comprehensive and more complete utilization of the mineral raw materials, improves the culture of the production, does not require the creation of the complicated mining-extracting complexes, is favourable for the protection of the environment.

The bacterial leaching is used on the industrial scale for the extraction of copper from the discarded ores within the USA, Peru, Spain, Portugal, Mexico, Australia, Yugoslavia, and other countries. The bacteria are used within the series of the countries (USA, Canada, South Africa) for the leaching of uranium. The bacterial leaching of copper is being introduced within the USSR at the series of the deposits.

The technological scheme of the experimental-industrial plant for the bacterial leaching of copper:

  1. - the regenerator for the solutions;
  2. - the pump for the circulating solutions;
  3. - the pipeline for the leachiThe installation for the bacterial leaching of copper ng solution;
  4. - the valves;
  5. - the supply pipelines;
  6. - the irrigation hoses;
  7. - the boreholes-irrigators;
  8. - the unit with the stowed ore;
  9. - the working for the collection of the productive solutions;
  10. - the pump for the productive solutions;
  11. - the thickener;
  12. - the cementation troughs;
  13. - the drying of the cement copper;
  14. - the transport tracks;
  15. - the compressor station;
  16. - the iron scrap.