BACTERIAL BENEFICIATION OF MINERALS

Abstract: Process for the benificiation of ores, comprising the dissolution of pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds by a strain of Thiobacillus ferrooxidans capable of oxidising iron and sulfur. The valuable metal is not solubilised by the micro-organism. The process is especially applicable to the treatment of tin ores, before, during or after beneficiation and also tailings from tin beneficiation which can contain up to 14 % of the tin originally present in the ore.

Description: BACTERIAL BENEFICATION OF MINERALS This invention relates to a novel process and in particular to biological leaching and to the beneficiation o mineral ores. BACKGROUND ART Bacterial leaching is traditionally concerned with the dissolution of valuable metals from within mineral assemblies. However, this invention aims at specific removal of undesirable metal components. Gangue or contaminating lo value metals can be biologically separated from valuable mineral ore components with which they are associated. Such bacterial leaching is disclosed in Australian Patents 274 690, 279 538, 284 782, 292 916 and 474 361, U.S. Patents 2 829 964, 3 272 621, 3 347 661, 3 607 235 and 3 679 397, and Canadian Patent 1 023 947. It is generally considered that autotrophic bacteria appear to be most important in the field of extractive metallurgy. The chemosynthetic autotrophic iron bacteria that are implicated in biological leaching are widely distributed in nature where iron salts, sulfur and hydrogen sulfide are present, particularly in an acid environment. The most important member of this group is Thiobacillus ferrooxidans (Colmer, A.R. and Hinkle, M.E. (1974), Science 106. 253). T. ferrooxidans derives energy for growth from the oxidation of ferrous iron, sulfur and sulfides (Tuovinen, O.H. and Kelly, D.P. (1974), Z. Allg. Mikrobiol. 12., 311-346) 4FeS0 4 + 0 2 + 2H 2 S0 4 ■> 2Fe 2 (S0 4 ) 3 + 2H 2 0 S 8 + 120 2 + 8H 2 0 H> 8H 2 S0 4 H 2 S + 20 2 ^ H 2 S0 4 This rod-shaped bacterium is aerobic and requires an acid environment between pH 2 and 3.5. Carbon for growth is obtained from atmospheric CO. and nitrogen from dissolved ammonia or nitrates. Valuable metals are often present in ores as insoluble metal sulfides. The leaching process is the end result of the bacteria acting upon the metal sulfide, which serves as an energy source in the presence of other nutrients. The mineralogy of the ore minerals and associated gangue are extremely important in establishing the feasibility of leaching. Bacterial leaching is associated with the presence of pyrite (FeS 2 ), pyrrhotite or other gangue reduced iron and/or sulfur compounds, representing ubiquitous growth substrates which often occur in association with other more valuable minerals, e.g., copper, uranium, tin. One mechanism of bacterial leaching is of an indirect nature and is reliant on the presence of pyrite. In the leaching environment, ferrous sulfate, produced by the chemical oxidation of pyrite - 2FeS 2_ + 702. + 2H.20 > 2FeSO4. + 2H2-SO4. is oxidised to ferric sulfate by T. ferrooxidans (see equation 1). The ferric sulfate then reacts with metallic sulfide minerals as follows - FeS 2 + Fe 2 (S0 4 ) 3 3FeS0 4 + 2S Q The bacteria also oxidise elemental sulfur to sulfuric acid 2S + 30 2 + 2H20 * > 2H-2S04. The presence of iron-oxidizing bacteria also ensures that ferrous iron is oxidised back to ferric iron. The products of these reactions, ultimately dependent on the presence of pyrite or other iron sulfides, are ferric sulfate and sulfuric acid, a mixture capable of oxidising and dissolving many otherwise insoluble minerals. DISCLOSURE OF INVENTION This invention relates to ore types where the valuable metal in question is associated with pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds but is present in a highly oxidised state or other recalcitrant state and as such is recalcitrant to the normal mechanisms of bacterially assisted metal solubilization as described above. Examples of suitable ores which can be treated according to the invention are cassiterite; pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compound associated laterite deposits, e.g. nickel laterites; and heavy mineral sands, e.g., ilmenite. The invention is also applicable to the removal of pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds associated with gold etc., or other inert, valuable metals thus, bacterial dissolution of unwanted pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds allows beneficiation of the valuable metal required. Such beneficiation may effectively serve to concentrate the desired metal and may also facilitate conventional extractive procedures through reduced ore grinding and more efficient flotation and physical separation techniques. The invention therefore provides a process for increasing the concentration of wanted metal values in an ore, ore extract or other like material which contains pyrite pyrrhotite or other gangue reduced iron and/or sulfur compounds. The process of the invention comprises cultivating a strain of Thiobacillus ferrooxidans capable of oxidising iron and sulfur under acidic conditions in a medium containing the ore, ore extract or other like material and water under aerobic conditions, in the presence of a source of nitrogen thereby removing iron and sulfur. It is preferred that the medium be supplemented by ammonium, potassium, magnesium, calcium, phosphate and nitrate ions. Alternatively, these may be present in the water or the material being treated. If that material or water contains a source of nitrogen, then further nitrogen need not necessarily be added. Some forms of Thiobacillus ferrooxidans appear to fix atmospheric nitrogen. With such forms air may provide the nitrogen source. The material to be treated may be pre-ground if necessary, depending on the type of material. The process of the invention can be applied to several stages of the beneficiation process. The ore can be treated by the process of the invention in situ before mining. This would reduce both mining and conventional beneficiation costs by reducing the amount of material to be mined and beneficiated. Alternatively, the ore can be mined and stockpiled and treated according to the invention before the usual beneficiation. In certain cases this will facilitate subsequent grinding of the ore, especially cassiterite, thereby minimising losses through fines. In another aspect of the invention, the sulfide rύREAϋ flotation concentrates can be treated with a strain of Thiobacillus ferrooxidans capable of oxidising iron and sulfur to facilitate their further treatment. Concentrates from other intermediate stages of beneficiation processes may also be treated according to the invention. The tailings from beneficiation processes can also be treated according to the invention in order to obtain a feedstock with metal values in sufficient concentration to make further beneficiation economic. Some forms of Thiobacillus ferrooxidous may function at moderate or extreme temperatures and processes of the invention employing such organisms may be carried out at temperatures up to the maximum temperature at which the particular organism is viable. Generally, however, the processes will be performed at temperatures of 5°C to 42°C, preferably from 28°C to 32°C. In a particular preferred embodiment the invention is especially applicable to the treatment of tin ores such as cassiterite, the beneficiation of which can be effected by dissolution of pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds by a strain of Thiobacillus ferrooxidans capable of oxidising iron and sulfur. Tin is not solubilised by the micro-organism. The biological dissolution of pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds may favourably affect subsequent metallurgical extractive procedures by making the ore more amenable to grinding. Leaching of cassiterite jLn situ would simplify mining operations and provide feed to the mill with a lesser comminution energy requirement. This would reduce mill throughput for the same recoverable tin production. When the invention is applied to heap leaching of cassiterite, a higher grade feed is also provided to the mill thus reducing mill throughput requirements, improving tin recovery in the gravity circuit by requiring less grinding and thereby generating a lesser proportion of tin fines. The leaching of bulk sulfide flotation concentrate potentially improves tin recovery. By improving tin recovery in the flotation stage, subsequent leaching will permit a lower grade concentrate. Thus, the flotation process can be worked harder to achieve a higher tin recovery at the expense of a lower concentrate grade because the grade can subsequently b improved by the process of the present invention. MODES FOR CARRYING OUT THE INVENTION Suitable organisms have been isolated from surface waters and ore samples located within cassiterite containing dolomite host rock at a tin minesite. The cultures, which have been temporarily designated BA-MBW3, BA-MBW9, BA-MBS2 and BA-MBS3 consist of gram-negative rods of varying sizes (0.5 - 2 micron) with rounded ends, which occur singly and frequently in pairs. The morphology of these cultures are consistent with the presence of both "typical" Thiobacillus ferrooxidans as described extensively in the literature and a typical "Thiobacillus ferrooxidans like" rods of greater length. The organisms designated BA-MBW3, BA-MBW9, BA-MBS2 and BA-MBS3 are mixtures of Thiobacillus ferrooxidans strains BA-MBW9 is capable of chemoautotrophic growth in the absence of an organic carbon substrate. In artificial minewater medium designated 9K salts medium (Silverman, M.L. and Lundgren, D.G. (1959), J. Bacteriol. 78., 326) under aerobic conditions, BA-MBW9 rapidly oxidizes ferrous suifate, oxidation being complete in approximately 72 hours. Medium 9K Addition per litre FeS0 4 .7H 2 0 40g (NH 4 ) 2 S0 4 3.0g KC1 O.lg K 2 HP0 4 0.5g MgS0 4 .7H 2 0 0.5g Ca(N0 3 ) 2 O.Olg The oxidation of ferrous to ferric iron results in the medium becoming amber and then progressively reddish-brown in colour and at the latter stages of growth is associated with precipitation of elemental sulfur making the medium opaque. BA-MBW9 was isolated from a site characterized by a pH of 2.45 and at 16°C. The culture grows rapidly within the range 20° to 32°C, however the limits of growth have not been determined. It is acidophilic, growth being most rapid in the region pH 2-2.5. Growth has also been observed at pH 1.7. Maintenance of BA-MBW9 on synthetic medium does not result in loss of ability to oxidize naturally occurring sulfides. Two conventional extractive processes for tin are illustrated in Figures 1 and 2. The areas where these processes can be improved by the process of the invention are indicated by asterisks. The following examples provide a description of beneficiation processes relating to cassiterite (SnO_), whereby contaminating pyrite gangue is biologically removed from the desired tin fraction. OMPI_ &NATiq EXAMPLE 1 This example illustrates the ability of a Thiobacillus ferrooxidans containing culture to remove pyrit from a cassiterite containing porphyry ore. The culture was isolated from the minesite. A cassiterite containing quartz porphyry ore composite was used in this example. The ore was crushed to -3.2mm and an elemented analysis conducted (Table 1). Table 1:- Analysis of cassiterite in quartz porphyry composite ore. Element Assay(% by Wt) Element Assay (% bv Wt) Sn 0.44 Zn 0.073 Sol Sn 0.03 Pb 0.009 S 4.5 Ag 1 Fe 5.7 F 0.40 AS 0.09 C0 2 1.1 Cu 0.03 WO, 0.01 Bi 0.006 Portions (5g) of the above ore were dispensed into Erlenmeyer flasks (500ml) and heat sterilised. The flasks were further supplemented with 100ml aliquots either sterile mineral salts medium (I9K) or acidified distilled water (pH 2.2). The aqueous mineral salts medium contained per litre of distilled water, ammonium sulfate (3g), potassium chloride (O.lg), dipotassium hydrogen phosphate (0.3g), magnesium sulfate (0.5g) and calcium nitrate (O.Olg). The pH was adjusted to pH 2.2 using concentrated sulfuric acid. The contents of the flask were inoculated (5% v/v/) with either sterile acid water or a T. ferrooxidans containing culture designated MBW-9. The flasks were incubated on an orbital shaker at 120opm at 28°C. Samples (5ml) were taken asceptically at appropriate time intervals. The samples were centrifuged (3,000 rpm) to remove debris and the supernatants retained for analyses. The following analyses were conducted. Soluble metals (tin, iron) were determined by Atomic Absorption Spectrophotometry. Soluble ferric iron was monitored by reaction with acid thiocyanate (20%, w/v) and spectrophotometric absorption at 480nm. The pH was followed using a standard laboratory pH meter. Microscopic examination of culture flasks was conducted using a standard research microscope. Ore residues remaining after leaching were analysed by standard chemical tests. X-ray diffraction and microscopic examination. In ύninoculated flasks, there was a slow chemical release of iron which reported largely as ferrous iron (Table 2). In flasks inoculated with culture MBW9 in mineral salts medium (I9K), a similar rate of iron release was observed (Table 2). However, when the ore was incubated with culture MBW9 in acidified distilled water (pH 2.2), almost complete iron solubilization was observed after 52 days. (Table 2). Iron release throughout the incubation period was associated with an increase in bacterial numbers. There was no solubilization of tin from the ore samples as determined by leachate and residue analysis. O FI Table 2;- Iron solubilization from composite porphyry cassiterite oren incubated in either mineral salts medium (I9K) or acidified water and inoculated with culture MBW9. LEACHING PERIOD (DAYS)* a) TOTAL IRON (mg/l) 0 10 14 18 25 32 39 45 52 1. Uninoculated 32 104 115 140 238 307 444 559 680 762 856 control 2. MBW9 culture in 266 342 361 - 568 592 860 894 1046 1076 1250 mineral salts 3. MBW9 cultive in 308 669 746 -932 1070 1432 1815 2022 2460 2650 2994 acid water b) FERRIC IRON (mg/l) 1. Uninoculated 22 87 95 112 129 166 183 215 230 252 263 con ol 2. MBW9 culture in 212 183 213 237 253 282 280 318 320 348 357 mineral salts 3. MBW9 cultive in 215 632 756 926 1170 1510 1810 2000 2250 2520 2700 acid water * Theoretical recovery = 2850 mg/l An elemental anaylsis was conducted on the ore residues remaining after the completion of this experiment. The results are shown in Table 3. Total iron and sulfur content was significantly reduced in the ore residue from the flask inoculated with culture MBW9. There was also a reduction in elemental sulfur and sulfide content in this residue. An increase in sulfate-S content indicated bacterial sulfide oxidation was probably occurring, and would explain the increase in acidity associated with this treatment. In contrast results of the analysis of the ore inoculated with culture MBW9 in I9K mineral salts medium approximated those of the uninoculated control (Table 3) with little reduction in sulfide, total iron or total sulfur. These results largely substantiate the findings based on leachate analysis. Table 3:- Analysis of ore residue after bacterial leaching. WT(%) TREATMENT Fe S S S S Total Elemental Sulfur Sulfide Residue 1-Mineral 3.80 3.85 1.01 0.07 2.77 salts, uninoculated Residue 2-Mineral 4.60 4.40 0. 88 0.09 3.43 salts, cultive MBW9 Residue 3-Acid 2. 80 1. 57 0.41 0.49 0. 67 water , culture MBW9 O PI ?t The ore residues were also examined by X-ray diffraction. The results are shown in Table 4. In agreement with the elemental analysis (Table 3), pyrite was not found in residue 3, which supported growth of culture MBW9 in acid water. Indeed, pyrite appears to have been largely replaced by ammonio-jarosite (NH 4 Fe 3 (S0 4 ) 2 (OH) 6 ) which represented an accessory component present at a level between 5 and 20%. In residues 1 and 2 where no bacterial leaching had taken place, pyrite was found to be present at levels semi-quantitated as accessory (Table 4). Table 4:- Ore residue analysis. X-ray diffraction. Component Residue 1 Residue 2 Residue 3 (No Bacteria) (No Bacteria) (Bacteria) Quartz D D D Topaz SD SD SD Pyrite A A A Ammonio-jarosite - _ • * A Sulfur Tr Tr - Geothite - - Tr Mica Tr Tr Tr D Dominant - most abundant component. SD Sub-dominant-next most abundant comporlent providing its percentage level is judged above about 20%. = Accessory. Components judged to be present between the levels of some 5 to 20%. Tr = Trace. Components judged to be below about 5%. = Absent. It is apparent from the above data that removal of pyrite from cassiterite containing porphyry ore can be achieved in acidified water inoculated with T. ferrooxidans containing cultures. OMPI /Λλ-m. WIPO EXAMPLE 2 This example demonstrates the ability of the same Thiobacillus ferrooxidans containing culture cited in Exampl 1 to remove pyrite from a different cassiterite containing quartz porphyry ore. Details of this ore were not supplied. Sterilised ore (lOg) was dispensed into Erlenmeyer flasks (500ml) containing either mineral salts medium or acidified distilled water. Details of medium addition and composition are outlined in Example 1. The flasks were inoculated (1% v/v) with a T. ferrooxidans containing culture designated MBW9. The conditions of flask incubation, content sampling and analysis are given above (Example 1). The results are shown in Table 5. Table 5:- Iron solubilization from porphyry ore by culture MBW9 in either mineral salts medium or acidified water. INCUBATION PERIOD (DAYS) METAL CONC.(mσ/l) METAL 14 21 27 32 Iron (total) 1. MBW9 + 180 387 689 967 1040 1115 Mineral Salts 2. MBW9 + 176 256 571 823 974 1115 Acid water Iron (Ferric) 1. MBW9 + 91 362 640 956 1076 1166 Mineral Salts 2. MBW9 + 80 242 523 808 984 1174 Acid water Tin 1. MBW9 + 4.4 0.5 0 0.3 0 0 Mineral Salts 2. MBW9 + 0.8 0 0 0.2 0 0 Acid water Uninoculated flasks showed no significant release of iron (results not shown) together with an absence of bacteria on microscopic examination. In the presence of bacteria, iron solubilization was observed in the presence and absence of added mineral salts. There was no solubilization of tin. EXAMPLE 3 This example shows the ability of a Thiobacillus ferrooxidans containing culture not isolated from the minesite to remove iron from a cassiterite containing quartz porphyry ore similar to the ore sample used in Example 2. Sterilized ore (4g) was dispensed into Erlenmeyer flasks (500ml) containing mineral salts medium. Details of the medium addition and composition are outlined in Example 1. The flasks were inoculated (5% v/v) with either sterile acid water or a culture containing Thiobacillus ferrooxidans. The conditions of flask incubation, content sampling and analysis are given above (Example 1). The results are shown in Table 6. Table 6:- Iron solubilization from porphyry ore by a culture containing Thiobacillus ferrooxidans not isolated from the minesite. INCUBATION PERIOD (DAYS) METAL CONC.(mg/l) METAL 0 6 11 18 25 28 34 Iron (total) 1. Uninoculated 14.5 36 29 29.5 27.6 34 18 control 2. Inoculated 181 179.5 177 136 369 547 954 Tin 1. Uninoculated 0.9 1.0 0.5 0.3 0.2 0.5 0. control 2. Inoculated 1.1 0.2 0.2 0 1.6 0.8 The uninoculated flask showed no release of iron. In the presence of bacteria however, considerable iron solubilization was observed after a lag period of about 18 days. This probably reflects culture adaptation to the ore. There was no solubilization of tin in either inoculated flasks or uninoculated controls. O PI ^St

Claims: CLAIMS 1. A process for increasing the concentration of wanted metal values in an ore, ore extract or other like material which contains pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds as in impurity, characterized in that a strain of Thiobacillus ferrooxidans capable of oxidising iron and sulfur is cultivated under acidic conditions in a medium containing said ore, ore extract or other like material, and water under aerobic conditions, thereby converting said pyrite, pyrrhotite or other gangue reduced iron and/or sulfur compounds into water soluble species. 2. A process according to claim 1, characterised in that said medium is supplemented by one or more of ammonium, potassium, magnesium, calcium, chloride, phosphate and nitrate ions. 3. A process according to claim 1 or claim 2, characterised in that said ore is, or that said ore extract or other like material is derived from cassiterite, a laterite deposit, nickel laterite, a heavy mineral sand or ilmenite. 4. A process according to claim 1 or claim 2, characterised in that said ore is treated in situ at a minesite. 5. A process according to claim 1 or claim 2, characterised in that said ore is treated in a stockpile prior to crushing. 6. A process according to claim 1 or claim 2, characterised in that said ore is treated after crushing and/or grinding. 7. A process according to claim 1 or claim 2, characterised in that said ore is treated prior to sulfide flotation. 8. A process according to claim 1 or claim 2, characterised in that said ore extract is a tailing from a sulfide flotation process which is treated immediately subsequent to flotation. 9. A process according to claim 1 or claim 2, characterised in that said ore extract is a final by-product of a mineral beneficiation process. 10. A process according to claim 1 or claim 2 , characterised in that said ore extract is a beneficiated mineral. 11. A process according to claim 1 or claim 2, characterised in that said strain is cultivated at a temperature between 5°C and 42°C. 12. A process according to claim 1 or claim 2, characterised in that said strain is cultivated at a temperature between 28°C and 32°C. 13. A process according to claim 1 or claim 2, characterised in that said strain is cultivated at a pH-value between 1.5 and 3.5. 14. A process according to claim 1 or claim 2, characterised in that said strain is cultivated at a pH-value between 2.0 and 2.5. 15. A process according to claim 1 or claim 2, characterised in that said strain is a strain isolated from a mineside from which said ore, pre extract or other like material is obtained. 16. A process according to claim 1 or claim 2, characterised in that said medium is inoculated with a leachate derived from a previously performed process according to claim 1 or claim 2.

Inventors: HOLDEN PETER JAMES, KELLEY BRUCE CHARLES, HOLDEN, PETER, JAMES, KELLEY, BRUCE, CHARLES
Applicants: BIOTECH AUSTRALIA PTY LTD, BIOTECHNOLOGY AUSTRALIA PTY. LIMITED
Application year: 1983
Main classification: C22B1
IPC codes: B03D1, C12P3, C12R1, C22B3