GB2097369A - Microbial leaching of sulphide ores - Google Patents

Abstract

The extraction of metals from ores and the upgrading of ores by a process of bacterial leaching is improved when the material to be treated contains a sulphide and, prior to the leaching, is roasted to remove part of the sulphur present in the sulphide. Preferably the material to be treated contains 10 to 70% by weight of sulphide, sulphide being added if necessary, and preferably from 10 to 25% of the sulphur in the sulphide is removed by the roasting.

Description

SPECIFICATION Microbial leaching of sulphide-containing ores This invention relates to an improved method of extracting metals from ores or concentrates containing sulphide.

It is known to extract metals from such materials by a microbial process using certain species of bacteria of the genus Thiobacillus, particularly T. ferro oxidans. These bacteria, which obtain their energy from oxidation of low-valency inorganic sulphur compounds in an acid environment, are capable of leaching many metals in addition to iron which is present in most sulphide ores.

Various modifications have been proposed to improve the efficiency of the process, either by speeding it up or by improving the degree of extraction of metals. These include treating fine particles of ore (which term is used hereinafter to include concentrate and mixtures of ores and/or concentrates) in suspension in a medium comprising the bacteria, enriching with carbon dioxide the air used to maintain the suspension and adding a surfactant to the medium.

We have found that the efficiency of the process is improved when the sulphide ore is given a preliminary roasting to remove part of the sulphur present as sulphide. Since the sulphide is the source of energy for the bacteria, this improvement on removing part of the source is surprising.

The improvement not only increases the degree of extraction of metals such as copper, zinc and tin, but also make it possible to extract precious metal such as silver, even when present in minor proportions, and to effectively treat ores containing impurities such as arsenic which have previously been untreatable by ordinary processes of extraction. The improved process may make it unnecessary to prepare concentrates since it can be applied to ores containing a major proportion of gangue. It can also be applied to processes of upgrading ores by leaching out some of the undesired associated constituents such as iron, sulphur and arsenic.

Another feature of the improvement in efficiency is the higher rate of extraction of metals.

Most ores contain a proportion of sulphide and though bacterial leaching can be carried out on ores containing as little as 1% sulphide, it is preferred to apply the process of this invention to ores containing at least 10% sulphide. When the ore initially contains less than 10% sulphide the proportion of sulphide can be raised by a preliminary upgrading or by adding a sulphide-rich ore. Addition of sulphide ore is particularly effective when initially the ore to be treated contains ores which do not provide a good growth substrate for the bacteria, e.g. oxides such as tin oxide.

The preliminary roasting of the ore involves heating it to a temperature of not more than 9000 C, preferably in the absence of oxygen or under reducing conditions, to remove part of the sulphur present in the sulphide. The minimum temperature required to do this depends on the form in which the sulphide is present. For some ores it may be as low as 4500C but more usually a temperature in the range 600-9000C is suitable.

Normally, roasting of sulphide ores takes place at a-temperature of 800 to 9000C under oxidising conditions and has as its objective the conversion of sulphide to oxide so that, for example, ferrous sulphide (pyrite) contained therein is converted to ferric oxide by substantially complete removal of sulphur. In contrast the primary objective of the roasting stage in the process of the present invention is to remove only part of the sulphur without, so far as is possible, producing metal oxide. For example, if the sulphide ore is pyrite, the objective is to convert part of it to pyrrhotite (FeS+x where x is in the range 0.1 to 0.7).

Many metal ores contain sulphide, often pyrite, in sufficient proportion to'achieve the improvement of this invention. When the ore of interest is an oxide which itself provides a poor growth substrate for the bacteria, it may be associated with sulphide in such a proportion that its treatment, either by way of extraction of the metal or by way of upgrading the ore, may be improved by preliminary roasting in accordance with this invention. If the sulphide content is low or negligible, then as described above, sulphide may be added, either as such or in the form of a sulphide-rich ore, to raise the sulphide content to at least the required overall proportion in the charge to be roasted.

The proportion of sulphide in the ore to be roasted is preferably in the range 1 0-70% of the ore, the optimum proportion depending on the nature and proportion of the metals in the ore.

It is essential, as described above, that the roasting should not eliminate all the sulphur present in the sulphide. Preferably at least 10% of the sulphur in the sulphide should be removed. A suitable upper limit depends on the nature of the sulphide ore and the roasting conditions: for example, in the case of pyrite up to 50% of the sulphur could be removed by carefully controlled roasting without significant production of oxide. As a useful approximate guide a weight loss of sulphur corresponding to 10 to 25% by weight of the sulphide is adequate in most cases. The roasting may be carried out under mild oxidising conditions but preferably, in order to reduce the risk of excess oxidation, the roasting may be carried out under a blanket of non-oxidising gas, or a reducing agent such as coal or carbon may be added to the roasting charge.

Iron is readily extractable from sulphide ores, and more quickly be the present invention but a further advantage is that the extraction of other metals from ores such as those containing copper, zinc, tin and silver is improved.

For example, the copper-bearing ore enargite is usually associated with pyrite from which it normally is separated before extracting copper. However, when using this invention the pyrite can be left In the ore and after the preliminary roasting, the efficiency of extraction of copper is improved. This extraction of copper from enargite (which also contain arsenic) is preferably carried out in the presence of more than 50% pyrite in the ores to be treated. Also by way of example, the extraction of zinc from sphalerite using this invention is preferably carried out in the presence of about 25% sulphide. Some sphalerite ores are rich in pyrite but others are poor and in the latter case, it may be desirable to add pyrite before the preliminary roasting.Further, the extraction of copper and zinc from chalcopyrite using this invention is preferably carried out in the presence of about 4050% of sulphide and the process may be used either to leach the copper and zinc or to upgrade the copper- and zinc-containing ore by removal of iron and other associated elements. In another upgrading process, the separation of cassiterite from associated sulphide, usually 1 5-30% of pyrite, is facilitated by the process of the present invention in that the iron content is more readily leached.

Where sulphide is added it is preferred to add pyrite since this ore is readily available and provides a good substrate for the bacteria. In fact, good use can be made of a high pyrite addition if it also carries precious metal such as silver.

The ore may be crushed and ground before the roasting treatment or the grinding step may be carried out after roasting, since the roasting of some ores may facilitate grinding.

The microbial leaching may be by any of the known methods, such as percolation, but is preferably carried out in a suspension of fine particles of the ore in an acidic aqueous medium containing the bacteria. For a percolation leach the ore should be ground to a particle size not greater than 2 mm and for a suspension leach to a particle size not greater than 0.125 mm.

in a suspension leach the medium containing preferably 10 to 40% by weight of solids may be stirred mechanically or by aeration at a temperature in the range 25 to 400C. The pH value of the medium is preferably in the range 1 to 3. The pH value may be adjusted by addition of sulphuric acid and this may have been produced from the sulphur removed in the preliminary roasting stage. If it is desired to remove from the leach liquid some of the iron which has been extracted, the pH of the liquid is preferably adjusted to be above 2.4 so that some of the iron is precipitated as hydroxide. An advantage of the improved process is that if the ore contains arsenic this need not be removed by roasting, but can be dealt with in the leaching stage where it can be precipitated in disposable form as ferric arsenate.

The medium should also contain the usual nutrients for the bacteria either derived from the ore or added as such. It is important that the bacteria should be multiplying during the leaching process which may be carried out usually for a period of between 1 80 and 300 hours.

The time, temperature and solids content of the leaching stage can be adjusted according to the product and the degree of extraction required from any particular ore. The leach may be carried out in a single stage or in multiple stages. The optimum leach conditions will vary with the scale of the operation.

The strain of T. ferro oxidans used in the process is preferably one which has been adapted to the metals, in addition to iron, which it is desired to leach.

At the end of a suspension leaching stage the stirred medium is run off into settling tanks, or preferably cones, where the coarse solids are allowed to settle as sludge. The liquor, which still contains bacteria and fines in suspension, is separated from the sludge and a portion may be retained for seeding further suspension leaching processes. The remainder is clarified to remove the fines, and the clarified liquor which may contain up to 95% of the copper or zinc present in the original ore may be subjected to the usual processes of recovering metals present in solution. The clarified liquor may also contain a substantial proportion of the iron from the ore together with arsenic if it was present.

The fines contain, in addition to fine particles of ore, the bacteria used in the leaching stage and it is an unexpected feature of the process of this invention as applied to silver-containing ores that the bacteria, which have been used in the leaching stage and recovered in the fines, carry in a surface film a high proportion of the silver contained in the ore. In the form of these fines, the silver is readily recoverable by normal cyanide processes. This can be done even when the original ore contained "poisons" for the cyanide which would have rendered impossible cyanide treatment of the ore itself, for example as in some tin-bearing ores such as cassiterite. Silver may also be present in the sludge in the form of particles of the metal and this also may be recovered by cyanide extraction.

The invention is illustrated by the following examples: EXAMPLE 1 A sulphide ore containing pyrite, tin oxide and silver upgraded by gravity concentration was treated by first mixing the ore with charcoal and roasting for one hour at 7500C to remove about 1 5% of the sulphur from the sulphide mineral present. The roasted ore was then subjected for 250 hours to a bacterial leach. The leach was started with 1 07 to 1 09 cells of Thiobacillus ferro oxidans per litre of leaching medium at a pH of 1.8 to 2.0 and was run for 250 hours at a temperature of 350C. The majority of the metals contained in the ore were leached into solution with the exception of tin and silver, which were enriched in the residue. Also contained in the residue was eiemental sulphur which was then removed by melting or solvent extraction or by an oxidising roast in the form of sulphur dioxide. The result of the treatment produced a final residue enriched in tin and silver. The silver was recovered from the fines and residue by a cyanide leach.

Approximate analysis of ore before treatment 35% Fe, 5% Zn, 0.5% Cu 9.5% Sn, Ag 1.8 Kg/TONNE Approximate analysis of solid residue after 30% Sn, Ag 5.4 Kg/TONNE treatment % of metal recovered in the bacterial leach Zn 92%, Cu 90%, Fe 90%, Sn 0% solution By this improved process, most of the zinc and copper is recovered in solution and the tin-bearing ore is substantially upgraded.

A similar result was obtained when the roasting was carried out at 6500C for 1 2 hours.

EXAMPLE 2 A complex copper sulphide flotation concentrate containing copper, zinc, iron and arsenic and also silver was treated first by roasting at 7500C under reducing conditions as described in Example 1 to remove about 1 5% of the sulphur and some arsenic. The roasted concentrate was then subjected to a bacterial leach as in Example 1 and this resulted in extraction of the majority of the metal with the exception of silver which was enriched in the residue, this silver being recovered by a cyanide leach.

Approximate analysis of the concentrate 20% Cu, 3.2% Zn, 13% Fe before treatment Approximate analysis of solid residue after 1.2% Cu, 0.85% Zn, 1 7% Fe treatment Ag 3.27 Kg/TONNE % of metal recovered in the bacterial leach Cu 97%, Zn 91%, Fe 50% solution EXAMPLE 3 A complex sulphide flotation concentrate containing zinc, copper and iron was mixed with 25% of iron sulphide (pyrite) and prepared by roasting under reducing conditions as in Example 1 to remove about 1 5% of the sulphur content. The roasted ore was then subjected to a bacterial leach as described in Example 1 which resulted in the extraction of the majority of the metals.

Approximate analysis of the concentrate 19.8% Zn, 7.9% Cu, 14.6% Fe before addition of pyrite Approximate analysis of solid residue after 4.8% Zn, 1.87% Cu, 9.5% Fe treatment % of metal recovered in the bacterial leach Zn 90%, Cu 90%, Fe 80% solution EXAMPLE 4 A similar concentrate to that used in Example 3 was roasted at 7500C, one portion of it under normal oxidising conditions and a second portion of it after mixing it with coal and 25% by weight of pyrite. In the roasting of the second portion about 1 5-20% of the sulphur content of the mixture was removed. Both portions were then separately subjected to a bacterial leach as described in Example 1.

The % extraction in the leach was as follows: % extraction Leach Rate Zn Cu Fe Zn Cu Roasted at 7500C 53.6 44.6 58.3 18.7 4.0 Roasted with added 79.0 85.0 70.7 45.0 10.0 pyrite and coal EXAMPLE 5 Bolivian sulphide ore having a head assay of Zn 0.13%, Cu 7.25%, Fe 35% was roasted at 7000C for one hour under non-oxidising conditions. The resulting weight loss was 13%. The roasted ore was leached for 250 hours by a process as described in Example 1. By way of comparison, an unroasted sample of the ore was leached in the same way.The result of the leaching was: % Extraction Zn Cu Fe Roasted ore 67.2 93.9 74.9 Un-roasted ore 53.8 19.0 11.7 EXAMPLE 6 A Dominican sulphide concentrate having a head assay of Fe 22.3%, Cu 0.8% and Zn 0.59% was mixed with coal and roasted for one hour at 6500C, the resulting weight loss being 5%. The roasted concentrate was leached for 1 80 hours by a process as described in Example 1. For the purpose of comparison, an un-roasted portion of the concentrate was leached in the same way but for 500 hours.

The result of the leaching was: % Extraction Zn Cu Fe Roasted concentrate 100 100 95.4 Un-roasted concentrate 65.6 100 82.7 EXAMPLE 7 A Bolivian sulphide ore having a head assay of Zn 4.86%, Cu 4.5% and Fe 27% was roasted under a blanket of non-oxidising gas for one hour at 6500C. The weight loss was 9.7%. The roasted ore was leached by a process as described in Example 1 and by way of comparison an un-roasted sample of the ore was leached in the same way. The result of the leaching was: % Extraction Zn Cu Fe Roasted ore 100 96.5 97.6 Un-roasted ore 95.7 29.1 89.8

Claims (11)

1. A process of extracting metals from ores (as hereinbefore defined) containing sulphide by microbial leaching in which the ore is roasted to remove part of the sulphur present in the sulphide before being leached.

2. A process as claimed in Claim 1 in which the ore contains from 10 to 70% by weight of sulphide.

3. A process as claimed in Claim 1 in which a sulphide ore, preferably pyrite, is mixed with the original ore to raise the sulphide content to from 10 to 70% by weight.

4. A process as claimed in Claim 1,2 or 3 in which at least 10% of the sulphur in the sulphide is removed.

5. A process as claimed in Claim 1, 2 or 3 in which the proportion of sulphur removed corresponds to from 10 to 25% by weight of the sulphide.

6. A process as claimed in any preceding claim in which substantially no oxide is produced from the sulphide by the roasting.

7. A process as claimed in any preceding claim in which the ore is mixed with coal or charcoal before roasting.

8. A process as claimed in any one of Claims 1 to 4 in which the roasting is carried out under a blanket of non-oxidising gas.

9. A process as claimed in any preceding claim in which copper and/or zinc is extracted in the leaching.

10. A process as claimed in any preceding claim in which a tin-bearing ore is upgraded by leaching and is recovered as a solid residue.

11. A process as claimed in any preceding claim in which the ore contains silver and after the leaching process, sludge and/or fines are separated and treated to extract silver therefrom.