FIELD OF THE INVENTION

The present invention relates to a process for the treatment of sulphide minerals and relates particularly, though not exclusively, to a process for the conversion of insoluble sulphides into soluble compounds or other compounds resulting from the chemical breakdown of the sulphide minerals, for subsequent separation and recovery of component metals. The present invention also relates more particularly, though not exclusively, to a process for the breakdown of sulphide grains to enable the recovery of gold and other valuable constituents such as silver which are so finely distributed so as to preclude their economic separation and recovery by conventional grinding, cyanidation and CIP processing.

BACKGROUND TO THE INVENTION

Sulphide minerals such as arsenopyrite, chalcopyrite and pentlandite, are found in the sulphide zone of many ore bodies. The sulphide zone underlies the leached zone containing oxide minerals near the surface of an earth formation. The concentration of sulphide minerals tends to increase the deeper the ore body due to the distance from the surface of the earth formation, where oxidation occurs more readily. Some secondary enrichment may occur in the sulphide zone where there has been redeposition of values oxidised from the leached zone by penetrating water.

The ores of many valuable metals occur as sulphides. For example, the primary copper bearing mineral is chalcopyrite, CuFeS ₂ , and a common nickel mineral is pentlandite, (Fe,Ni) _U S ₁₀ . Other metals which are refined from sulphide minerals include lead, zinc, molybdenum, vanadium, cadmium, cobalt and silver. The stable, insoluble nature of most of these sulphides is a major

factor determining the processes used for the refining of such sulphide minerals. The pyrometallurgical processes of roasting, smelting and converting involve selective high temperature reactions of the sulphides with oxygen, which enable separation and conversion of the constituent sulphides to oxides, which are subsequently reduced to metals.

A disadvantage of these known pyrometallurgical processes is that they also liberate significant quantities of gaseous sulphur dioxide which is subject to increasingly more stringent environmental regulations requiring its removal from exhaust gases so as not to enter the environment. Alternative, hydrometallurgical processes where applicable often involve the use of elevated temperatures and high pressure leaching to extract the metal from the sulphides. Such processes employ vessels capable of withstanding high temperatures and pressures and require complex control systems to ensure containment, maintenance of optimum operating conditions and prevention of dangerous emissions. An example of hydrometallurgical process is the Sheritt-Gordon process which involves the use of a high temperature, high pressure ammonia leach to breakdown nickel sulphides for subsequent conversion into nickel metal. Sulphidic gold containing minerals may exhibit a highly refractory nature, such that they are not amenable to winning. The refractory nature is often associated with an ultra-fine distribution of gold within sulphide grains and the existence of the gold in solid solution in the sulphides. With such highly refractory ores, recovery of the gold can only be achieved by first chemically altering or breaking-down the sulphide minerals.

Methods currently known for the breaking-down of the sulphide include roasting, pyrolysis, oxidative leaching in acid or alkaline media and bacterial oxidation. Of these methods roasting is often preferred, because it is fast, well understood and energetically self-supporting.

However, many refractory ores contain arsenopyrite and the evolution of volatile As ₂ 0 ₃ , as well as S0 ₂ , during the roasting process presents significant environmental problems. In addition, recovery of gold after roasting is often incomplete due to containment within the calcine.

Pyrolysis is also a pyrometallurgical process, involving heating of the ore to temperatures of approximately 600°C in a vacuum or inert atmosphere to prevent oxidation from occurring. Elemental sulphur and arsenic vapours are formed from the conversion of pyrite

(FeS ₂ ) and arsenopyrite (FeAsS) to pyrrhotite (FeS) and the gold is subsequently recovered more easily from the pyrrhotite matrix. However, this process has the disadvantage of being endothermic, requiring an energy input, and the pyrrhotite is well known to be a cyanicide.

The application of oxidative leaching to the recovery of refractory gold generally requires severe conditions of high temperature and pressures to achieve acceptable rates of recovery. Such processes require the use of costly vessels capable of withstanding the severe operating conditions, and the chemical reactions may consume large quantities of oxygen. While ultra-fine grinding of the concentrate may be used to activate the sulphide mineral and improve recovery rates, the additional cost may be prohibitive. Bacterial oxidation and leaching is a very slow process which is prone to a number of problems including cyanide poisoning of the micro-organisms and nutrient starving.

The process of the invention is based on the discovery that mechanical activation, either by itself or combined with thermal treatment, can induce chemical reactions between sulphide minerals and certain reactants at low temperatures which cause the chemical breakdown of the sulphide grains. Such reactions, while thermodynamically favoured, generally were previously not thought to occur at temperatures less than that required for the formation of S0 ₂ , because of kinetic limitations.

Mechanical activation involves the use of mechanical energy to increase the chemical reactivity of a system so as to induce mechanochemical reactions which involve changes in chemical composition as a consequence of the applied mechanical energy. For example, one form of mechanical activation, described in U.S. Patent No. 5,328,501, is concerned with a chemical reduction process involving mechanically activated chemical reduction of reducible metal compounds with a reductant during milling in a high energy ball mill, to refine and manufacture metals and alloys. During milling the energy imparted to the reactants through ball/reactant collision events enable the starting materials to react, causing the chemical reduction reaction to occur without the need for high temperatures or melting to increase reaction rates. Another form of mechanical activation is the process of mechanical alloying described in U.S. Patent No. 3,591,362 by which alloys are formed from pure starting materials by milling in a high energy ball mill.

SUMMARY OF THE INVENTION

The present invention was developed with a view to providing an improved, more environmentally acceptable process for the treatment of sulphide minerals. More particularly, the present invention was also developed with a view to providing an improved lower cost process for the treatment of refractory gold ores and concentrates, which facilitates maximum recovery of the gold.

According to one aspect of the present invention there is provided a process for the chemical breakdown of insoluble sulphide minerals into soluble compounds or other compounds, the process comprising: subjecting a mixture of a sulphide mineral and a suitable reagent to mechanical activation to increase the chemical reactivity of the reactants and/or reaction kinetics such that a chemical reaction will occur which produces a compound or compounds that can be more readily

processed to extract a metal from the sulphide mineral.

Advantageously the process according to the invention comprises the further step of subjecting the mixture to thermal treatment. In one form of the invention said thermal treatment may be performed simultaneously with said mechanical activation.

Typically at least one of the soluble compounds produced is a sulphate compound which is soluble in water. The soluble compound or compounds may be in a mixture with insoluble compounds, or in some instances the compounds resulting from the chemical breakdown of the sulphide minerals may all be insoluble in water.

Advantageously the insoluble sulphide minerals contain refractory gold and/or other precious metals which can be more readily recovered from the soluble compound or other compounds by conventional hydrometallurgical or other recovery methods. The sulphide mineral may be a metal sulphide of the kind known to contain gold such as pyrrhotite (FeS) or any of a number of other gold containing mineral sulphides such as arsenopyrite (FeAsS) pyrite (FeS ₂ ) or chalcopyrite (CuFeS ₂ ) . Refractory gold is typically bound up as pure sub-microscopic gold particles in the sulphide, or may be embedded in the sulphide matrix as gold atoms. Chemical breakdown of the refracting sulphide mineral according to the process of the invention liberates the sub-microscopic gold particles and atomic gold so that the gold can be recovered more easily using conventional recovery techniques.

Any reagent which is thermodynamically capable of reacting with the sulphide to breakdown the sulphide to a sulphate or other compounds may be suitable. The reagent may be a solid, liquid or gas, and two or more reagents may be employed if desired. A suitable reagent may typically be an oxide which can exist in more than one oxidation state. Suitable oxides include CuO, Mn0 ₂ , PbO, Ca0 ₂ , NaO, V ₂ 0 ₅ and others, including mineral oxide phases such as, for example, Fe ₂ 0 ₃ (Hematite) .

Advantageously the process of the invention can be used to selectively convert one or more sulphide minerals into a first soluble compound or compounds characterised by a first negative free energy change, and into a second soluble compound or compounds characterised by a second negative free energy change whereby two or more metals can be selectively extracted.

In a preferred form of the invention, mechanical activation is performed inside a mechanical mill, for example, a ball mill. Mechanical activation occurs in a ball mill when grinding media, typically steel or ceramic balls, are kept in a state of continuous relative motion with a feed material by the application of mechanical energy, such that the energy imparted to the feed material during ball-feed-ball and ball-feed-liner collisions is sufficient to cause mechanical activation.

Throughout the remainder of the specification reference will be made to mechanical activation being carried out inside a mechanical mill. Any of the commercially available mills of this type may be suitable. Examples of this type of mill are nutating mills, tower mills, planetary mills, vibratory mills, attritor mills and gravity-dependent-type ball mills.

Mechanical activation and thermal treatment can be combined through the use of a thermally insulated high energy mill, such as an attritor. With such high intensity mills, power inputs of the order of 100-200 kw/m ³ and higher can be achieved. The thermal energy generated can result in mill temperatures exceeding 400°C in insulated mills. The utilisation of this heat during milling will substantially increase the reaction kinetics through the combined effect of mechanical and thermal activation, thus reducing milling times and costs. A substantial increase in process efficiency is also achieved through the capture and use of otherwise wasted thermal energy generated during milling.

It will be appreciated that the mechanical

activation may also be achieved by any suitable means other than ball milling. For example, mechanical activation may also be achieved using jet mills, rod mills, roller mills or crusher mills.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a better understanding of the invention preferred embodiments of the process, and examples of reactions according to the process will now be described in detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a graph illustrating the percentage of zinc (Zn) metal recovered by milling ZnS and CuO powders followed by leaching, as a function of milling time;

Figure 2 is a graphical representation of the output curves from a differential thermal analyser (DTA) used for testing the as-milled powders prior to leaching;

Figure 3 is a graphical representation of the output curves from a thermal gravimetric analyser (TGA) used for testing the as-milled powders prior to leaching; Figure 4 is a graph illustrating the percentage of copper (Cu) metal recovered by milling CuS and CuO powders followed by leaching, as a function of milling time; and,

Figure 5 is a graph illustrating the percentage of zinc (Zn) metal recovered by milling ZnS (Concentrate) and ZnS (Ore) samples with CuO powder followed by heat treatment at 600°C and leaching, as function of milling time.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The process of the invention involves the use of mechanical activation with or without thermal treatment to cause chemical reactions to occur, typically at low temperatures, between sulphide minerals and a suitable reagent which selectively converts the sulphide mineral into a compound or compounds that can be more readily processed to extract a valuable metal. The chemical reactions that occur enable the valuable metals to be directly and selectively separated from unwanted gangue using hydrometallurgical leaching processes.

In the process of the invention the sulphide ores or minerals are typically placed inside a mechanical mill together with a suitable reagent or reagents, and subjected to milling action. As a consequence of mechanical activation associated with milling, collision events involving the reactants and the grinding media occur which may induce the sulphide minerals to enter into reaction with the reagent materials to form soluble compounds. The mechanical activation may increase the reactivity of the reactants to such an extent that a reduced amount of subsequent heating is required to cause the sulphπdes to enter into reaction with the reagent materials to form a sulphate or other soluble compound(s) . In the process of the invention, the reaction kinetics are increased by the action of a ball mill in providing mechanical activation with or without thermal treatment.

In the process of ball milling the reactants, including the sulphide minerals and suitable reagents, collide with each other and the grinding media. At least one of the reactants should be a solid and the reactivity of the reactants increases due to the increase in reaction area resulting from the decrease in particle size of the solid phase associated with fracture events. A welding, mixing of atoms and/or exchange of atoms occurs at the interfaces of colliding particles to promote reactivity.

If the reagent employed is an oxide, its function is to react with the sulphide in a manner in which the oxide provides oxygen atoms to the sulphide, resulting in the formation of a (soluble) sulphate from the sulphide. An example of such a reaction is the conversion of zinc sulphide to zinc sulphate using copper oxide as the reactant, by the reaction:

ZnS + 8CuO → ZnS0 ₄ + 4Cu ₂ 0

This reaction is characterised by a free energy change of -174.6kJ/mole at 300K and is thus thermodynamically favourable. However, the reaction is precluded from occurring at ambient conditions because of kinetic constraints associated with displacement of the reactant phases by the product phases and low diffusion coefficients.

Such displacement reactions generally require heating of the reactants to high temperatures to achieve acceptable reaction kinetics for the reaction to occur. However, with certain sulphides, the temperatures required for the displacement reaction to occur may be higher than that required for oxidation of the sulphide to form S0 ₂ , thus precluding the displacement reaction from occurring. In such instances, the role of mechanical activation is that it enables the reaction to occur either mechanochemically during the milling process or subsequently during thermal treatment. If the reaction occurs during subsequent heating the role of mechanical activation is that it enables the reaction to occur at a temperature less than that required for the oxidation of the sulphide to form S0 ₂ (i.e. in this case the role of mechanical activation is that it decreases the reaction temperature by a critical amount so that the oxidation reaction is precluded from occurring on heating) .

On completion of the reaction, the product phases are separated using hydrometallurgical techniques. In the

above example, ZnS0 ₄ is soluble in water, while Cu ₂ 0 is insoluble, enabling separation directly by the leaching of ZnS0 ₄ in water. Following removal of the ZnS0 ₄ the Cu ₂ 0 may be regenerated by heating in air to form CuO. Alternatively, regeneration of the Cu ₂ 0 may precede the removal of ZnS0 ₄ by the simultaneous oxidation of Cu ₂ 0 to CuO during the thermal treatment (in an oxidizing environment) being carried out to form the sulphate phase. It is also possible for in si tu regeneration of CuO to occur by mechanical activation during milling if the mechanochemical reaction is carried out under an oxidising atmosphere.

The present invention may also provide a method for selectively separating the individual metals in a mineral sulphide. For example, with chalcopyrite, (CuFeS ₂ ) it is thermodynamically possible to form both copper sulphate and iron sulphate. The free energy change and kinetics associated with the formation of iron sulphate differs from the free energy change and kinetics for the formation of copper sulphate. Therefore, when the reaction products are determined by thermodynamic considerations, the reaction of chalcopyrite with copper oxide will result in the preferential formation of iron sulphate by the reaction:

CuFeS ₂ + 8CuO → FeS0 ₄ + 4Cu ₂ 0 + CuS

On the other hand if kinetic factors control the reaction then adding the correct amount of CuO or milling for the correct period of time it may be possible for all of the copper to be selectively converted into CuS0 ₄ while the iron remains as iron sulphide according to the following reaction:

CuFeS ₂ + 8CuO → CuSo ₄ + 4Cu ₂ 0 + FeS

Since CuS0 ₄ is soluble in water, this would

enable the copper initially present in the CuFeS ₂ to be separated and removed from the iron sulphide. Following regeneration of the CuO, as described above, copper oxide may be recycled. The above process is not limited to the separation of two metals but can be applied to the separation of any number of valuable metals present in a mineral sulphide. The process can also be applied to the separation and recovery of metals in a mixture of naturally occurring or artificially produced sulphides, i.e. such as a mixture of sphalerite, ZnS and galena, PbS.

The separation may be effected by the sequential addition of the oxidiser as described above or, alternatively, sufficient oxidiser may be added to form sulphates of two or more, or all, of the sulphides present. The sulphates are then separated from the gangue by leaching and the metals subsequently refined by standard hydrometallurgical or electrochemical methods. The sulphates of all of the constituents need not be soluble in the same solution. The variable solubility of different sulphates may be used as a basis of the separation process.

When the process of the invention is applied to a sulphide mineral containing refractory gold, the function of mechanical activation and/or thermal treatment is to increase the reactivity of the reactants and/or reaction kinetics to such an extent that a chemical reaction occurs which breaks down the sulphide to produce a sulphate or other soluble or insoluble compounds, enabling liberation of entrapped, sub-microscopic gold during subsequent hydrometallurgical processing.

An example of the process is the conversion of pyrrhotite (FeS) to iron sulphate by reaction with hematite (Fe ₂ 0 ₃ ) as the reagent, by the reaction: FeS + 12Fe ₂ 0 ₃ → FeS0 ₄ + 8Fe ₃ 0 ₄ This reaction is characterised by a negative free energy at temperatures above 360°C, however, higher

temperatures are normally required to cause the reaction to occur because of slow reaction kinetics. By milling the reactants prior to heating, the reaction temperature may be significantly reduced enabling the reaction to proceed at a temperature less than the S0 ₂ formation temperature. The occurrence of the above reaction causes disintegration of the FeS phase enabling liberation of entrapped, sub- microscopic gold during subsequent leaching.

A second example of the process is the partial or complete conversion of a sulphide matrix to a sulphate phase by a mechanically and/or thermally activated reaction with an oxide to enable recovery of gold and other valuable metals locked within the sulphide matrix.

For example, the process can also be applied to the separation of gold and copper in a refractory pyrite/chalcopyrite mixture. For the case of kinetic control discussed above, the selective conversion of the copper in chalcopyrite to copper sulphate will result in breakup of the mineral phases. Following the removal of CuS0 ₄ by leaching, the residue may be treated with cyanide to recover the refractory gold previously locked in the sulphide matrix. Alternatively, the remaining sulphide phases can be further reacted with the oxide if further breakup of the sulphide matrix is required to liberate the gold or other valuable metal.

The invention is further described and illustrated by the following examples. These examples are illustrative of a large number of candidate reactions and are not to be construed as limiting the invention in any way.

Example 1A

ZnS and CuO powders mixed together in a molar ratio of 1:8, corresponding to the reaction:

ZnS + 8CuO → ZnS0 ₄ + 4Cu ₂ 0 (1) were loaded into a hardened steel vial and mechanically

milled together with ten 12 mm diameter steel balls in a SPEX mixer/mill for various periods of up to 24 hours. Following milling the powder was removed from the vial and leached through a filter with 100 ml of water. The effect of milling time on the recovery of zinc determined from chemical analyses of the leach solution is shown in Figure 1.

Milling for 6 hours resulted in the extraction of 80% of the Zn originally present in the zinc sulphate. The leach solution contained 4.1μg/ml of copper, which represented only 0.03% of the original amount of Cu. X-ray diffraction measurements of the powder after milling for 12 hours showed the presence of Cu ₂ 0, inferring that the above sulphating reaction (1) had occurred during milling. No other phases were identified by x-ray diffraction, indicating that the zinc sulfate phase was amorphous.

Additional tests were carried out on the as- milled powders, prior to leaching, to determine the nature of the mechanochemical reaction occurring during milling. A Rigagu combined differential thermal analyser (DTA) and thermal gravimetric analyser (TGA) with a heating rate of 20°C/minute was used to study reactions occurring during heating the as-milled powder in air. The DTA and TGA measurements are shown in Figures 2 and 3 respectively for samples milled for 3, 6 and 12 hours [curves (a), (b) and (c) ] . The DTA measurements showed the occurrence of two exothermic reactions during heating. The magnitude of the DTA peak associated with the lower temperature reaction decreased with increasing milling time, while the temperature of the peak associated with the higher temperature react- "- also decreased with increasing milling time. The TGA me rements showed that an increase in mass accompanied both reactions.

On the basis of the present measurements the lower temperature thermal peak has been identified with the thermal reaction of zinc sulphide remaining after milling with CuO via the sulfating reaction (1) . The Cu ₂ 0 formed

by this reaction is assumed to immediately react with oxygen from the atmosphere to form CuO, ie 2Cu ₂ 0 + 0 ₂ —> 4Cu0, thus regenerating the reagent. The higher temperature reaction is believed to be associated with the regeneration of the Cu ₂ 0 formed during mechanical milling. The effect of prior mechanical milling in reducing the temperatures required for the reaction to be completed is clearly shown in Figures 2 and 3.

Example IB In this example ZnS and CuO were mixed together with a different starting stoichiometry and the milled powders were subjected to thermal treatment subsequent to mechanical activation.

ZuS and CuO powders mixed together in a molar ratio of 1:4 were located into a hardened steel vial and mechanically milled together with ten 12 mm diameter steel balls in a SPEX mixer/mill for 3 hours. Following milling the powder was removed from the vial and heat treated at 500°C for 2 hours. Following heat treatment the sample was leached through a filter with 100 ml of water. Chemical analysis of Zn in the leach solution showed an extraction of 78.4% of the Zn originally present as zinc sulphide. An identical experiment carried out with a ZnS:CuO molar ratio of 1:8 resulted in 88.3% recovery of Zn.

Example 1C

In this example a mixture of ZnS and CuO was subject to thermal treatment at the same time as mechanical activation.

ZnS and CuO powders mixed together in a molar ratio of 1:8, corresponding to the reaction:

ZnS + 8CuO → ZnS0 ₄ + 4Cu ₂ 0 were loaded into a hardened steel vial and mechanically milled together with 6 mm diameter steel balls in an insulated attritor mill for one hour. During milling the

mill temperature was found to reach 415°C. Following milling the powder was removed from the vial and leached through a filter with 100 ml of water. Chemical analysis of Zn in the leach solution showed an extraction of 72% of the Zn originally present as zinc sulphide.

Example 2■ ^•

CuS and CuO powders mixed together in a molar ratio of 1:8, corresponding to the reaction:

CuS + 8CuO -→ CuS0 ₄ + 4Cu ₂ 0 (2)

were loaded into a hardened steel vial and mechanically milled together with ten 12 mm diameter steel balls in a SPEX mixer/mill for various periods up to 24 hours. Following milling the powder was removed from the vial and leached through a filter with 100 ml of water, followed by leaching in 100 ml of dilute hydrochloric acid. The effect of milling time on the recovery of copper determined from chemical analyses of Cu and sulphur recovered from the leach solution is shown in Figure 4. It is seen that milling for 24 hours resulted in 100% extraction of the copper originally present as copper sulphide.

Example 3:

NiS and CuO powders were mixed together in a molar ratio of 1:8, corresponding to the reaction:

NiS + 8CuO → NiS0 ₄ + 4Cu ₂ 0 (3)

and were loaded into a hardened steel vial and mechanically milled together with ten 12mm diameter steel balls in a SPEX 8000 mixer/mill for 36 hours. The vial was loaded and sealed in an argon atmosphere. Following milling, the powder was removed from the vial and leached in 100ml of water for 2 hours and the leach solution separated from the residue by filtering. Chemical analyses of Ni recovered from the leach solution showed an extraction of 60% of the

nickel originally present as nickel sulphide.

Example 4:

Chalcopyrite (FeCuS ₂ ) and CuO powders were mixed together in a molar ratio of 1:8, corresponding to the reaction:

FeCuS ₂ + 8Cu0 → CuS0 ₄ + 8Cu ₂ 0 + FeS (4)

and were loaded into a hardened steel vial and mechanically milled together with ten 12mm diameter steel balls in a SPEX 8000 mixer/mill for 24 hours. The vial was loaded and sealed in an argon atmosphere. Following milling, the powder was removed from the vial and leached in 100ml of water for 2 hours and the leach solution separated from the reside by filtering. Chemical analyses of Cu recovered from the leach solution showed an extraction of 77.4% of the copper and 5.3% of the iron originally present in the chalcopyrite.

Example 5 :

ZnS and Galena (PbS) and CuO powders were mixed together in the molar ratio corresponding to the reaction:

0.1 PbS + ZnS + 8Cu0 → ZnS0 ₄ + 8Cu ₂ 0 + 0.lPbS (5)

and were loaded into a hardened steel vial and mechanically milled together with ten 12mm diameter steel balls in a SPEX 8000 mixer/mill for 24 hours. The vial was loaded and sealed in an argon atmosphere. Following milling, the powder was removed from the vial and leached in 100ml of water for 2 hours and the leach solution separated from the reside by filtering. Chemical analyses of the leach solution showed an extraction of 78.9% of the zinc originally present in the mixture. The leach solution contained only 0.5% of the Pb originally present in the mixture.

Example 6 :

ZnS and Fe ₂ 0 ₃ (Hematite) powders were mixed together in the ratio ZnS:Fe ₂ 0 ₃ of 1:3.66. The amount of Fe ₂ 0 ₃ was only 30.5% of the molar ratio required for Fe ₂ 0 ₃ to supply all of the oxygen needed for the formation of ZnS0 ₄ . The powders were loaded into a hardened steel vial and mechanically milled together with ten 12mm diameter steel balls in a SPEX 8000 mixer/mill for 24 hours. The vial was loaded and sealed in an argon atmosphere. Following milling, the powder was removed from the vial and annealed in air at 600°C for 1 hour. The powder was then leached in 100ml of water for 2 hours and the leach solution separated from the residue by filtering. Chemical analyses of the leach solution showed an extraction of 84.0% of the zinc originally present in the mixture. The leach solution contained only 0.42% of the Fe originally present in the mixture.

This example demonstrates that the process works with less than stoichiometric quantities of the reagent, so that the amount of reagent required may be reduced. This may be important where large quantities of reagent are employed and/or where it is not economically feasible to recover the reagent. It is also noted that ZnS/Fe ₂ 0 ₃ reaction has a positive free energy change below 500°C and hence will not occur below this temperature. In this example the role of mechanical milling is to increase the reaction kinetics in order that the reaction will occur rapidly on heating to above the equilibrium temperature.

Example 7 : A refractory gold concentrate containing pyrite and pyrrhotite was mixed together with Fe ₂ 0 ₃ in a weight ratio of 21.8:1. Ten grams of the mixture were loaded togecher with ten 12 mm diameter steel balls in SPEX 8000 mixer/mill and mechanically milled for one hour. Following milling the powder was heated at 500°C for one hour. The

powder was then leached in a sodium cyanide solution to recover the gold. Chemical analysis of the leach solution showed a gold recovery corresponding to 122 grams/tonne of ore. A sample of the concentrate subjected to the same cyanide leach conditions as the reacted sample showed a gold recovery of 36 grams per tonne of ore. The total gold content of the concentrate as determined by fire assay analysis was 127 grams per tonne. Therefore, the mechanochemical/thermal reaction process yielded a 96% recovery of the gold as compared with 28% recovery using the standard method.

Example 8:

Samples of zinc sulphide (sphalerite) concentrate containing 54.9% Zn and ore containing 10.8% Zn were mechanically milled with CuO in a SPEX mixer/mill with ten 12 mm diameter steel balls. A Zn:Cu molar ratio of 1:8 was used for each sample. Following milling the powder was removed from the vial and heat treated at 600°C for 1 hour. The samples were subsequently leached through a filter with. 100 ml of water. The effect of milling time on the recovery of zinc determined from chemical analyses of the leach solution is shown in Figure 5. Milling for 4 hours resulted in 69.2% extraction of the zinc in the concentrate and 40.8% of the Zn in the ore. For the ore sample milled for one hour heat treatment at 600°C resulted in 42.3% recovery of Zn while heat treatment at 500°C resulted in a recovery of 73.6% of Zn. The lower Zn recovery at 600°C was due to the formation of the basic sulphate phase

(ZnO)2ZnS0 ₄ .

Example 9:

A sample of pyrite (FeS ₂ ) was mixed with CaO in a molar ratio of 1:3 and milled in an attritor mill at 400°C for 2 hours. Examination of the asmilled powder by x-ray diffraction showed the presence of CaS0 ₂ + Fe ₂ 0 ₃ indicating chemical breakdown of FeS ₂ .

Example 10 :

A sample of pyrite (FeS ₂ ) was mixed with CaO in a molar ratio of 1:3 and milled with water in an attritor mill for 2 hours. Examination of the asmilled powder showed that the phases CaS0 ₃ x 0.5 H ₂ 0, CaS0 ₄ x 0.5 H ₂ 0 and CaO x Fe ₂ 0 ₃ formed during milling as a consequence of chemical breakdown of FeS ₂ .

The above examples illustrate the manner in which insoluble sulphide minerals can be chemically broken down into other compounds, typically soluble metal sulphates, that can then be leached to recover a substantial proportion of the metal present in the original sulphide. The process typically involves mechanical activation followed by or simultaneous with, if necessary, thermal treatment. In some cases, mechanical activation alone may suffice.

The process of the invention can be readily applied on a commercial scale. A suitable mechanical mill of the kind commonly used in mineral processing can be used to perform the mechanical activation. Such a mill may be located at or near the mine site or mineral processing plant. Mechanical activation of the sulphides and the reagent(s) can be performed in a ball mill using a continuous or batch feed process which is linked to a conventional leaching circuit. Thermal treatment may be effected as a post-milling process step, or may be performed simultaneously with milling by heating the mill contents, for example, to 250°C. With a thermally insulated high energy mill, such as an attritor, power inputs of the order of 100-200 kw/m ³ and higher are possible, which can result in mill temperatures in excess of 400°C due to the thermal energy generated. In some cases the need for flotation may be eliminated and the mill contents passed directly to the leaching circuit. Post- milling recovery of insoluble compounds may be employed to regenerate the reagent materials.

The process for the treatment of sulphide minerals and ores described above has a number of significant advantages over conventional pyrometallurgical processes, including the following:

1. The process is simple and does not require the complex control systems associated with high temperature separation and conversion of sulphide minerals to soluble oxides.

2. The process can eliminate flotation if there is a high initial sulphide content in the ore, and therefore may be applied to the ore directly at the minesite.

3. The process can be operated at conditions close to ambient and does not present a high risk to the operators or the environment in the event of failure.

4. The process does not involve the production of significant quantities of pollutants such as gaseous sulphur dioxide (S0 ₂ ) , As ₂ 0 ₃ , S and As vapours, and thus does not present a high risk for catastrophic emission of harmful substances into the environment.

5. The process can achieve a higher recovery rate than conventional pyrometallurgical processes.

It will be apparent to persons skilled in the materials processing and chemical engineering arts that numerous variations and modifications can be made to the process without departing from the basic inventive concepts. All such modifications and variations are considered to be within the scope of the present invention,

the nature of which is to be determined from the foregoing description and the appended claims. Furthermore, the preceding examples are provided for illustrative purposes only, and are not intended to limit the scope of the process of the invention.