BACKGROUND ART Copper is most commonly present in the earth's crust as copper-iron sulphide and copper sulphide minerals. The main minerals are chalcopyrite (CuFeS2), covellite (CuS), chalcocite (CuS2), bornite (Cu5 Fe), and enargite (Cu5 AsS). Chalcopyrite is the most common ore and most important ore from which copper is extracted. Copper also occurs in oxidised minerals, including carbonates, oxides, and hydroxy-silicates.

Copper-iron sulphides are not readily amenable to hydrometallurgical treatment in that they are not easily dissolved in acids and alkalis, so copper extraction from these minerals by pyrometallurgical techniques predominates.

This is particularly true of concentrates containing the mineral chalcopyrite. This extraction entails production of a concentrate containing 25%-35% copper, smelting of the concentrate to molten high copper matte,"converting"the matte to molten blister copper and refining to ultra pure copper. Normally, the sulphide concentrates are produced at the minesite and have to be sent to a smelter.

Smelting involves melting of the concentrate at temperatures of about 1200iC, resulting in the formation of sulphur dioxide and if there is arsenic (As) in the concentrate, arsenic trioxide. If smelters treat concentrates with high zinc content, the zinc becomes a fume which must be treated.

Smelters require a feed containing high amounts of sulphur to make the process autogenous otherwise the process requires external heating which is cost prohibitive. Smelting creates S02 emissions, which generally require conversion to sulphuric acid. This is expensive, generally uneconomic, and in many parts of the world, selling of the sulphuric acid is difficult because of the over capacity of production. In addition, increasingly stringent sulphuric acid specifications will mean that substantial additional capital will be inverted in plant installation to meet the standards. Furthermore certain countries have commenced issuing licences for any emissions to the atmosphere and this is further affecting the operation of smelters.

Roasting followed by leaching has been practised for many years as a alternative method of treating high grade concentrates. Also, pressure leaching using oxygen at elevated pressures and temperatures has been developed more recently as an alternative for treating high grade copper sulphide concentrates.

However, the technologies are not generally applicable for lower grade copper sulphide concentrates.

Chloride based systems, such as the Cuprex and INTEC processes are alternative leaching systems for treating copper sulphide concentrates. These processes, although technically feasible, have not progressed to commercial operations, mainly because of their complexity and operating difficulties. They are not yet considered to be viable alternatives to existing process technologies.

Factors common to existing technologies are: The high capital and operating costs entailed for a commercial operation.

The high degree of complexity and high maintenance requirements of existing processes means that plant availabilities are often low.

It is known that certain microorganisms (ie Thiobacillus Ferrooxidans, Thiobacillus Thiooxidans, Leptospirillum Ferrooxidans and Sulfolobus), under acidic conditions, can solubilise copper by converting copper sulphide, in the presence of oxygen, to the copper anion and the sulphate cation. This bioleach technique can be used at or slightly above room temperature, and at atmospheric pressure (ie no pressure vessels required). Under controlled conditions, up to 30gp1 of copper can be extracted into solution. The bioleach technique is not selective to copper, and will also solubilise iron, zinc, nickel and cobalt. These metals, if present in sufficient quantity, can be economically extracted.

The bioleach method has been used in heap leaching of copper ores on a commercial basis but not for copper concentrates. In particular, no commercial operation exists involving treatment of a concentrate containing chalcopyrite.

SUMMARY OF THE INVENTION It is the object of the present invention to provide a method by which metals, especially copper, can be separated or obtained from copper bearing minerals.

It is a particular object of the present invention to provide a treatment method for copper bearing materials, particularly those containing chalcopyrite, and/or those containing lower grade copper sulphide concentrates, in a manner which minimises input costs, and unusable waste products.

With this object in view, the present invention provides, in a first aspect, a method for separating copper from copper bearing materials comprising the steps of (a) subjecting a copper bearing material to a bioleach step involving iron/sulphur oxidising microorganisms to form a bioleach solution containing copper; (b) at least partially removing separated copper from the solution by contacting the solution with a copper extracting organic solvent; and (c) recycling at least part of the copper depleted solution to the bioleach step.

By combining the bioleach step with the copper removal step, and returning the solution to the bioleach step after copper has been removed copper can be removed from the copper concentrate in a low cost, energy efficient and effective manner.

The copper bearing material may particularly be a concentrate, especially a concentrate containing chalcopyrite. Additionally, or alternatively, the concentrate may include low-grade copper concentrates containing levels of copper and sulphur which to date have been difficult or uneconomic to process.

Typically, the copper concentrate is a copper sulphide concentrate, which may contain copper-containing minerals.

The copper sulphide concentrates may also contain: liron containing sulphides, such as pyrite, marcasite, and pyrrhotite), Nickel sulphides (such as pentlandite, violarite, millerite), Zinc containing sulphides, (such as sphalerite and marmatite) Cobalt containing sulphides, in particular the iron sulphide pyrite containing cobalt in the crystal structure.

Other minerals (such as silicates), Precious metals, particularly native gold, silver and gold and silver containing minerals (such as argentite).

The method of the invention may be employed for treating the above concentrates to produce non-ferrous base metals such as nickel, zinc and cobalt and/or their compounds. The method may be used to recover gold and silver, for example, in the form of a concentrated residue containing gold and silver. Using the present method, a readily disposable, compact iron and silicate-containing residue may be the only waste product, therefore potentially eliminating or substantially reducing toxic waste products.

The copper concentrate, in particular, a concentrate containing chalcopyrite, may initially be ground to a small particle size, to improve its surface area and copper metal extraction rates. This may be accomplished in any type of grinding mill. In particular, the grinding may be accomplished in a vertically or horizontally stirred mill. The product from the mill may have a size such that 90% of the product is finer than between 5 and 45 micrometres.

The process may take as feed, concentrates with a copper content from 10% to 50% by mass, sulphur content 10% to 50%, iron content 10 to 50% up to a maximum limit of all products combined of 100%, Any quantity of gold and silver may be contained in the concentrate. The concentrate may also contain zinc, nickel and/or cobalt. The process is of particular value when the copper bearing material contains quantities of zinc greater than 5% by weight. Such a material or concentrate is difficult to treat by smelting.

The concentrate may be contacted with one or more micro-organisms in a vessels or tanks arranged in one or more bioleach stages. The bioleach step may be conducted under aerobic conditions, and suitably with agitation. The bioleach step may also be conducted under acidic conditions, and a preferred range of pH is from about 0.5 to about 3.0. The bioleach step may be conducted over a range of temperatures, and residence times. Suitably, the bioleach stage temperature can range between 20°C-90°C. The residence time depends on the feed and solids concentration and may be in the range of 1-8 days. The feed is preferably mixed with recycled solution from the solvent extraction step during, or alternatively before, the bioleach step.

The microorganisms may preferably be iron/sulphur oxidising microorganisms referred to as moderate thermophiles, or thermotolerant which operate at a preferred temperature of 40°C-55°C. This temperature range corresponds substantially with the optimum growth temperature range of the microorganisms. The moderate thermophiles are sometimes referred to as Sulfabacillus thermooxidans and may contain other mixed heterotrophic organisms. Alternatively, the mixture may be of aerobic bacteria Sulfolobus and similar bacteria having an operating temperature of 65-90°C these bacteria are referred to as extreme thermophiles. Alternatively, the microorganisms may be a mixture of the aerobic autotrophs Thiobacillus Ferrooxidans and Leptospirillum Ferrooxidans.

The method may also be conducted such that different microorganisms are contacted with the concentrate at different stages of the process. In a preferred embodiment, the first stage involves microorganisms referred to as moderate thermophiles and the second stage involves extreme thermophiles operating at a temperature of 70-80°C. In an alternative embodiment, the first stage may use extreme thermophiles and the second stage moderate themophiles. Any desired number of stages and arrangement of stages may be employed.

The reactor units may consist of one or more agitated, aerated reactors which can be connected in series or parallel. Aeration, with air or oxygen, should be sufficient to provide adequate oxygen for the bacterial oxidation, such that the dissolved oxygen level in the slurry can be maintained at a minimum of 1 parts per million but, preferably, a level of 2 parts per million. In order to enhance the bacterial leaching, a carbon dioxide supplement is added with the air. This can be in the range 300-10,000 ppm but typically to maintain a level of 1,500 parts of carbon dioxide in air. In order to reduce costs, this carbon dioxide can be supplied from the carbon dioxide liberated during an iron precipitation stage, if limestone or magnesium carbonate is used for neutralisation. Alternatively, the carbon dioxide at least partly can be obtained from the burning of limestone. If desired, part of the product slurry, (solids in solution) may be recycled to enhance bioleaching, and improve metal value recoveries.

A further embodiment comprises the situation where slurry is removed from the reactor, filtered to separate the solids and the solution, the solids recycled back to the bioleach and the solution retained for recovery of copper as described below. An additional step in which the solids recycled to the bioleach are subjected to further grinding is also a preferred option. The slurry may be removed from any reactor.

However it is desirable that if the circuit is operating using two types of bacterial culture in different stages that slurry be removed and recycled from a reactor using one type of bacteria to a reactor using the same type of bacteria. It may also be preferable to selectively recycle only unleached sulphides by separating the unleached sulphides from the residue to recycle, and disposing of the flotation tailing. Cooling/heating arrangements may be employed as necessary.

Providing additional nutrients with the solid feed, particularly nitrogen, phosphates and potassium, can further enhance the bioleach step. The level of addition depends on the extent of such nutrients in the original feed solids.

Addition of up to 0.5 gpl of ammonium and phosphate containing salts has been found to accelerate bioleaching. An additional enhancer for certain feedstocks is the addition of ferrous ion. Ferrous ion, when added to the bioleach feed as the chemical ferrous sulphate or contained in the raffinate recycled from the solvent extraction step, may improve the bioleaching by reducing the redox potential and thus assist overall metal extractions.

It has been found by one of the inventors that the redox potential of the slurry may be an important parameter in enhancing the bioleaching of the copper concentrate. Accordingly, redox potential may be controlled as part of the process. This is described in South African Patent Application No. ZA 971307,"A Process for the leaching of Chalcopyrite", the contents of which are hereby incorporated herein by reference.

The preferred redox potential range in each bioleach reactor is 400-430mv (versus Ag/AgCI). The redox potential required for leaching chalcopyrite is lower than that for other sulphides. The maintenance of low redox potential is a key step in the process. The redox potential can be modified by a number of alternative mechanisms.

(a) reducing the quantity of air added to the system such that the redox potential does not increase; (b) increase the pH to an alkaline level at which ferric iron hydroxides are precipitated, which increases the ratio of ferrous to ferric ion and reduces the redox potential; (c) by adding ferrous sulphate, either directly as a salt or by allowing levels of ferrous sulphate to build up in the discharge solution from the solvent extraction stage, such that an increased level is added. The redox potential decreases with increasing ferrous ion; (d) by adding additional concentrate feedstock such that amounts of ferrous ion are formed; and/or (e) by manipulation of the rate at which minerals such as pyrrhotite or sphalerite, which are readily and rapidly oxidised by ferric iron and consuming ferric iron in the process, are added to the reactors, be it contained in the feed material or fed as separate feed streams.

In one particular embodiment, a mixture of microorganisms which have a preferred growth temperature of between 20°C and 55°C is advantageously mixed with the initial stages of the bioleaching circuit. The temperature of the slurry in the reactors is maintained within the required temperature range by heating or cooling as necessary.

The latter reactors used in the bioleach reactor arrangement may be operated with a mixture of micro-organisms called thermophiles which operate in the temperature range of 60-80°C. The most commonly known of these organisms is Sulfolobus.

In another embodiment, the thermophile (Sulfolobus) operating in the temperature range of 60-80°C can be used in all stages of the bioleaching circuit.

If limestone is added to the reactors to adjust the pH, then iron hydroxides may be contained in the bioleach residue. In addition, calcium sulphate will also be present in the residue when limestone has been added.

Where the copper concentrate contains gold and silver, these elements will normally not be dissolved during the bioleach and will remain in the residue. The gold and silver may be recovered from the residue using cyanidation or other standard gold recovery method. To carry out the cyanidation it is necessary to remove all acidic copper liquor from the solids, wash with water and to increase the pH to 9 or above.

The bioleach solution contains copper, and typically, iron (mainly ferric but can also include some unoxidised ferrous), plus minor amounts of zinc, calcium, and magnesium. A typical concentration of this solution can be 10-50 g/i copper, 20-50 g/I iron. The amount of zinc may vary depending on the feed levels.

If the feed contains zinc, and the zinc levels in the leached solution are about or above 1.0 g/l, the zinc-containing solution can be subjected to a process step to remove or extract the zinc. This may be achieved by solvent extraction, after the extraction of copper using a suitable commercial organic extractant; and the removal of iron and other impurities which may interfere with the zinc solvent extraction step.

The bioleach solution may be subjected to an iron precipitation step, as high levels of ferric iron can be detrimental to the copper solvent extraction step.

The iron may be precipitated by neutralising the bioleach solution and typically, the bioleach solution can be neutralised to a pH of between 2.4 and 3. The iron precipitation step can be conducted in a well agitated, aerated reactor, and a suitable neutralising agent may include lime, limestone, magnesium oxide or magnesium carbonate. The pH of the bioleach solution must be controlled so that minimal amounts of copper are co-precipitated with the ferric iron. The iron precipitation step may be conducted at room temperature or elevated room temperature and typically a temperature range of 20°C-90°C may be used. An advantage of the higher temperature is that the iron precipitates more rapidly as a readily compact iron hydroxide. However, to achieve this, external heating of the solution is needed, and this may not be economic. The residence time required in the iron precipitation step may be dependent on the reaction temperature. For instance, at a pH of 2.7, a residence time of 2-6 hours is usually required. At elevated temperatures (ie 70°C or above), only 1-2 hours may be required.

Alternatively, the ferric iron may be precipitated by operating the bioleach reactors at pH greater than 2.5. This particular alternative is a novel step for a continuous process because no reference has indicated that operating above a pH of 2.5 is possible.

Powdered limestone may be added to the first bioleach reactor to obtain a minimum pH of 2.5. The iron precipitates as a form of ferric hydroxide or ferrihydrite at this pH. A greater quantity of limestone may be added to the other reactors. Of course, neutralising agents other than limestone may be used.

The solution from the bioleach may be at any temperature between 30 and 70°C depending on the bacterial culture used in the process.

If the iron is precipitated with lime or limestone, gypsum may be formed as well as the iron precipitate. The gypsum can be used as an agricultural chemical, as the iron has no detrimental effect this product may be sold as a by-product.

Alternatively, the iron precipitate may be disposed of in a suitable tailings dam.

Final disposal may require increasing the pH to 7 or above, if environmental regulations so demand. The iron precipitation step may be conducted in a plurality of reactors, which may be connected in series, and each reactor may be fitted with aeration facilities. As mentioned above, the carbon dioxide liberated in the iron precipitation step, can be used in the initial bioleach step.

The copper content of the bioleach solution may be increased by a copper solvent extraction process. The copper solvent extraction process can also separate copper from remaining impurities (particularly any residual ferric and ferrous iron). The higher strength solution from the solvent extraction can be used for electrowinning to produce a high quality copper cathode of LME (London Metal Exchange) grade with a minimum of 99.9935% Cu. The solvent extraction step may use commercially available copper extractants such as LIX622 and LIX984 and other phenolic oxime reagents These extractants are preferably diluted with a fine cut kerosene such as Shellsol 2046 to concentrations of between 10% to 50% of the extractant.

The solvent extraction step may comprise one or more solvent extraction stages whereby copper can be removed from the aqueous feed solution. The aqueous feed solution is normally acidic to prevent or minimise copper loss, and typically a pH range of about 0.5 to about 2 is used. Adjustment of pH is done as necessary, prior to, or during each stage. After extraction of the copper into the solvent, the remaining solution (raffinate) is preferably returned to the bioleach circuit to maintain acidity and to recover residual contained copper.

If the bioleach solution contains zinc in sufficient quantity then a further extraction stage may be employed to recover the zinc; this may be by solvent extraction or by other means known in the art. In addition, if the bioleach solution contains nickel and/or cobalt these metals may be removed by bleeding part of the raffinate stream to a removal stage for their removal.

Excess raffinate may be neutralised using lime or limestone, as necessary, or used for heap leaching.

Using the solvent extraction step, it is found that about 10-15g/l of copper can be removed from the feed solution in each solvent extraction stage when LIX 984 or LIX 622 is used as an organic extractant at an organic: aqueous ratio of 3: 1 or higher. Therefore, if the feed solution contains more than 10-15 g/I of copper, a plurality of extraction steps may be required. For instance, a feed solution of 25 g/t copper may require at least two stages of extraction. More stages may be used as necessary.

The organic solution may be stripped of copper by contacting it with spent electrolyte from a copper electrowinning stage. One or more stages of stripping may be required to remove substantially all of the copper from the organic solvent. The copper electrolyte can be provided with copper concentration of 45-65 g/l, and preferably 45-60 gpl, which is a suitable concentration for copper electrowinning.

An advantage of solvent extraction is that any acid which is generated may be recycled to the bioleach circuit or neutralised using inexpensive neutralising reagents (such as lime or limestone). The copper-depleted aqueous solution from the solvent extraction step or steps, may be recycled to the bioleach step.

The acidity of the recycled solution may be adjusted by adjusting the degree of neutralisation after the solvent extraction step. Thus, if the bioleach is a net acid consumer, the solvent extraction acid neutralisation may be adjusted to ensure that no additional acid is required for addition at the bioleach stage.

If the solution is recycled to the bioleach step, it may be necessary to periodically remove some of the impurities to prevent build up in the process liquor. Therefore, a bleed stream may be provided which will remove the impurities. After removal of the impurities, the bleed stream liquor may be recycled to the bioleach if it retains recoverable copper or useable acid.

Alternatively, the resultant solution may be neutralised to precipitate iron and other dissolved metals from the solution. This may be achieved by adding lime, limestone or magnesium hydroxide to the solution. The precipitate may then be recycled to the solvent extraction stage to recover the copper.

Still further, a portion of the recycled copper-depleted solution may be fed back to the solvent extraction step to dilute incoming bioleach solution to provide a copper concentration suitable for extraction.

The invention provides, in a second aspect, an apparatus in which the method as above described may be conducted.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will now be described with reference to the following drawings in which: Figure 1 is a general flow diagram of the process according to an embodiment of the invention; Figure 2 is a flow diagram of the bioleach process with the alternative of recycling residue from the final reactor when only one mixture type of bacteria is being used at one operating temperature; and Figure 3 is a flow diagram of the bioleach process with the alternative use of two types of bacteria operating at different temperatures; incorporating recycle of residue from the final reactor of the first bioleach stage.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Bioleach Stage The process can equally be carried out with the bioleach in different configurations, all of which are suitable for the integrated process.

The four main configurations were: (i) Using moderately thermophilic bacteria in the integrated circuit.

(ii) Using moderately thermophilic bacteria in conjunction with precipitation of iron in the reactors (Figure 2).

(iii) Using moderately thermophilic bacteria in conjunction with precipitation of iron in the reactors and recycling part of the residue to the feed.

(iv) Using moderately thermophilic bacteria as the first stage and extreme thermophilic bacteria for the second stage of bioleaching in conjunction with precipitation of iron in the reactors and recycling part of the residue to the first stage (Figure 3).

(v) Using extreme thermophilic bacteria in the integrated circuit.

Any combination of the configurations is also within the scope of the invention.

Referring to Figure 1, a copper sulphide concentrate stream 30 assaying at 25% copper, 30% iron, 1.0% zinc 0.1% cobalt and 40g/t silver was slurried in water 40, following regrind 10 (if necessary) to produce a 10% w/w slurry.

Additional nutrients 50 (with water and raffinatte recycle, as desired) were added to assist with bacterial growth, these were magnesium sulphate, ammonium sulphate and diothiophosphate. The quantities required may be varied as required For cases (i) to (iv), the slurry was fed to a fully continuous bioleach train, of four reactors R1-R4 (R1, R2 forming the first stage; R3 and R4 forming the second stage with the temperature controlled at 48-50°C and the pH at 1.6-1.8.

The redox potential was maintained between 380 and 430mV.

In case (iv), reactors R3 and R4 were operated at 70°C with pH 1.5-2.5.

The aeration was maintained at a level sufficient to maintain a level of dissolved oxygen in the slurry of 2 parts per million. In one embodiment, the addition of carbon dioxide to maintain a various level of 700-3000 parts per million in air as a carbon supplement was used.

In cases (ii), and (iv) shown schematically in Figures 2 and 3, part of the leached residue 115 is recycled to R1 solid/liquid ("S/L") separation step 1100 integrates the iron and residue removal stages.

In case (v), the slurry was fed to a fully continuous bioleach train with the temperature controlled at 70°C and the pH at 1.5-2.5. The redox potential was allowed to reach its natural level, normally in the range 550-600 mV. The aeration was maintained at a level of dissolved oxygen in the slurry of greater than 0.5 parts per million. In one embodiment, the addition of carbon dioxide to maintain a level of between 1700-3000 parts per million in air as a carbon supplement was used.

The volume of the bioleach circuit, in cases (i) to (iv), was 300 litres. This comprised four (or five as the case may be) reactors. The first reactor has a volume of 150 litres and the next three reactors 50 litres each. It is preferred that the first reactor R1 has a volume greater than the other reactors such that the time that the slurry remains in the first reactor is increased compared to the other reactors. In case (v), the volume of the bioleach circuit was 50 litres comprising a first reactor of 2.5 litres and the next two reactors 12.5 litres each.

In cases (i) to (iv), the slurry was initially inoculated with a mixture of moderately thermophilic bacteria. The mixture comprises bacteria with iron/sulphur oxidising bacillus rods and spherical cocci. The bioleach circuit ran continuously for more than a year and it was not necessary to reinoculate the reactors. The circuit conditions were altered from time to time to test various modifications to the integrated circuit. In case (v), the slurry was initially inoculated with a thermophilic culture comprising large round Sulfolobus-like microorganisms.

The tables below present the bioleach results found over overall residence time range of 3-8 days. The overall copper extraction was in the range 80%-97%, depending on residence time and bacterial type used. For the moderately thermophilic bacteria, specific copper extraction data shows that 70%-80% of the copper extraction occurred in the first reactor. For the thermophilic bacteria, the copper extraction in the first reactor was generally lower at around 50%.

The results show that bioleaching of the copper concentrate was an efficient and effective technique for solubilising the copper.

BIOLEACH EXTRACTION RESULTS COPPER CONCENTRATE (25% Cu, 30% Fe, 1% Zn, 0.1% Co) OPTION 1: MODERATE THERMOPHILIC BACTERIA (NO RECYCLE) PRECIPITATION OF IRON AFTER REACTORS Residence Time (days) Pulp Density (% Solids) Extraction (Cu %) 3 10 66 4 10 78 5 10 88 6 10 88 I I II OPTION 2: MODERATE THERMOPHILIC BACTERIA (NO RECYCLE) PRECIPITATION OF IRON IN REACTORS Residence Time (days) Pulp Density (% Solids) Extraction (Cu %) 3 10 73 4 10 89 51092 6 10 93 OPTION 3: RECYCLE AND MODERATE THERMOPHILIC BACTERIA Residence Time (days) Pulp Density (% Solids) Extraction (Cu %) 3 10 80 4 10 94 5 10 95 61096 OPTION 4: RECYCLE AND TWO BACTERIAL TYPES MODERATE THERMOPHILIC (STAGE 1) AND THERMOPHILIC (STAGE 2) Residence Time (days) Pulp Density (% Solids) Extraction (Cu %) 3 10 80 4 10 94 5 10 96 6 10 97 8 1 10 97 11 OPTION 5: THERMOPHILIC BACTERIA (NO RECYCLE) PRECIPITATION OF IRON AFTER REACTORS Residence Time (days) Pulp Density (% Solids) Extraction (Cu %) 3.3 7. 5 48 4.9 7. 5 89 6. 5 7. 5 96 Iron Precipitation Stage-Subsequent to Bioleachina Prior to any optional copper solvent extraction, leached solids 110 which are disposed of or treated for precious metal recovery are removed by S/L separation at 100, and the copper containing solution 120 is subjected to an iron removal step. In the iron removal step, the bioleach solution containing 10-30 gpl copper and 10-20 gpl iron was placed in a 100-litre reactor and aerated.

This reactor contained the complete solution from one day's operation of the bioleach section of the pilot plant. The temperature is maintained at 70°C and the pH is adjusted to about 2.8 using powdered limestone additions 150. The addition was made slowly to ensure no rapid changes in pH occurred.

Precipitation of iron occurred creating a slurry. After about 3 to 4 hours, floculant was added to the slurry and settling of the solids occurred in the same reactor.

The solution ferric iron level was reduced to between less than 1 gpl with less than 1% copper loss. Although some ferrous ion remains this does not affect the solvent extraction.

Supernatant solution was removed from the reactor and collected for solvent extraction. The settled solids 190 were filtered in a pressure filter 180 and washed with water in the filter 180 prior to disposal 200. The filtrate was added to the supernatant liquor for feeding to solvent extraction. The combined solution is referred to as PLS (pregnant leach solution). In the iron precipitation step, carbon dioxide evolves. Although this was not collecte and used on the pilot plant, the C02 gas can be recycled to the bioleach reactors to assist the bioleaching.

Iron Precipitation Stage-Integral with Bioleachinq Prior to any optional copper solvent extraction step, leached solids which are disposed of or treated for precious metal recovery are removed by S/L separation and the copper containing solution must be subjected to an iron removal step. In the iron removal step, the bioleach solution contained 10-30 gpl copper. Powdered limestone stream 260 was added to the first bioleach reactor R1 to obtain a minimum pH of 2.5. The iron precipitates at this pH, some copper also precipitates and the losses are from 0.5 to 2.0% of the total copper.

In cases (ii) and (iv), stage 140 may accordingly be omitted, though the circuit shown in Figure 3 may incorporate an additional iron precipitation stage for liquor 170 from reactors R3 and R4, if necessary. Liquor 170 also passes ultimately to solvent extraction.

Solvent Extraction of Copper After the iron precipitation step, the filtered bioleach solution is subjected to a continuous copper solvent extraction train 300. Prior to solvent extraction, the bioleach solution assays at 10-20 g/I copper. The copper solvent extraction train comprises a plurality of separate copper solvent extraction steps, the number of which will depend primarily upon the concentration of copper in the bioleach solution (copper tenor). In the first step E1, the bioleach solution is subjected to extraction with an organic solvent comprising 33% LIX622 in Shellsol 2046. In the first step, the copper tenor was reduced from 12 g/I copper in the feed to 4 g/t and then in the second stage, E2, to less than 1gpl in the raffinate (the remaining bioleach solution). The organic solvent is separated from the raffinate due to the difference in specific gravity of the solvent and the solution and due to the fact that the organic does not mix with the solution (aqueous) phase. The loaded organic solvent L5 is stripped in strip stages S1 and S2, the stripping liquid being the copper electrolyte returned 400 from the copper electrow in step. The stripped solvent is recycled back through the copper solvent extraction train.

The resultant neutralised bioleach solution (ie raffinate 310,50) is recycled through back to the bioleach step. As recycling of the raffinate will result in increased impurity levels, a bleed stream 315 is provided. The bleed stream 315 may be treated to remove dissolved metals from solution; it is also suitable for the recovery of nickel and cobalt if these are present in the raffinate in economically recoverable quantities.

The solvent extraction process operated for between ten and twenty four hours per day for more than six months.

The operating conditions for the period were two parallel circuits operating with two extract (E1, E2) and two strip stages (S1, S2). The PLS flowrate was 100 litres/day. Other conditions were Operating temperature: 40°C Organic 33% LIX 622 extractant Strip Electrolyte 35-40gpt copper 160-180 gpl H2SO4 Residence Time: 2 minutes per stage Operating mode: Organic continuous pH: 0.5 to 2 (adjustment prior to each stage as necessary) The results indicate that the copper tenor was sequentially reduced from 10-30 gpl in the feed to 1 gpl in two stages. The copper solvent extraction of bioleach solutions, provides an economically attractive method for recovering the copper in bioleach solutions into a copper electrolyte suitable for copper electrowinning.

Electrowinnina of Copper Strong electrolyte solutions 330 from the solvent extraction stage containing 40-50 g/L Cu and 140-180 g/L H2SO4 were stored in heated vessels prior to electrowinning by conventional methodology in electrowin stage 500.

Product copper 510 is exported.

The solutions were pumped through the electrowinning cell on a continuous basis for 10 hours. The operation was stopped when the spent electrolyte copper tenor reached 35 gpl copper.

Cobalt sulphate was added to the electrolyte to reduce lead corrosion In summary, copper electro winning provided an efficient method for recovering copper from electrolyte solutions produced from copper solvent extraction unit operations.

Various other changes and modifications can be made without departing from the spirit and scope of the present invention.