GB2108480A - Acid leach process for treating magnetic and non-magnetic nickel-copper mattes - Google Patents

Abstract

Magnetic and non-magnetic nickel- copper mattes are treated by an acid pressure leach in which the initial phase is leached out under oxidizing conditions until up to 70% of nickel values are extracted, at which time the leach is continued under non-oxidizing conditions. Copper values are converted from a basic copper sulphate formed during the oxidizing phase to a less acid-soluble form as digenite such that a resulting nickel-rich leach solution has reduced copper values permitting purification of the said leach solution by an atmospheric leach in which non- magnetic as well as magnetic nickel- copper mattes can be added for precipitation of dissolved copper and iron values. A further acid pressure leach is effected under oxidising conditions on the Cu-containing residue, copper recovered from this leach solution and the latter recycled to the first stage pressure leach to maintain the pH therein at 1.8 to 5.0. <IMAGE>

Description

SPECIFICATION Acid leach process for treating magnetic and non-magnetic nickel-copper mattes The present invention relates to a process for the treatment of nickel-copper sulphide mattes and, more particularly relates to a process for selectively leaching nickel and copper values from said mattes and for producing purified aqueous solutions containing said metal values suitable for recovery of the nickel and copper values.

The hydrometallurgical treatment of nickel-copper sulphide mattes has been extensively developed.

Known processes generally involve leaching of the matte to extract substantially all nickel and copper values to produce a leach solution which contains concentrations of both nickel and copper and treating the solutions for separate recovery of nickel and copper values. The substantial removal of copper from the nickel-bearing solution for subsequent recovery of nickel values as pure product metal, and conversely the substantial removal of nickel from the copper-bearing solutions for subsequent recovery of pure copper metal, are desired.While numerous methods are available for copper removal, such as cementation, electrowinning, precipitation with a soluble sulphide or solvent extraction, such known techniques are subject to various technical, operating and economical problems which adversely affect the overall economics of the processes, particularly where relatively large amounts of copper must be removed from solution.

U.S. Patent 3,741,752 discloses an acid leaching process for treating semi-metallic nickel-copper mattes produced by the smelting of high grade nickel and copper sulphide ores in a two-stage pressure leach process in which the first-stage pressure leach is conducted under oxidizing conditions until at least 70%, and preferably at least 80 to 90%, of the nickel is extracted, at which time the first-stage pressure leach is continued at an elevated temperature, preferably in the range of 130 - 150or, in the absence of oxygen for replacement of dissolved copper by nickel.The precipitation of copper from solution may not be complete, depending on the form and concentration of nickel in the matte, and the addition of a soluble sulphide reagent such as NaHS or H2S may be necessary to precipitate residual dissolved copper to the level required for cobalt and nickel recovery.

U.S. Patent 4,093,526 discloses a proces for refining a nickel-copper matte or concentrate in which at least about 20% by weight of the total nickel is present in an acid-soluble metalic form. The concentrate is first treated in an atmospheric leach in which 30 to 60% nickel extraction takes place, followed by two serially-coupled high pressure oxidation leaching steps to complete the extraction of nickel, cobalt and copper. The process also depends upon the rpesence of metallic nickel in the concentrate to precipitate copper quantitatively from the atmospheric leach solution by a cementation reaction, thereby provided a purified solution of nickel and cobalt sulphates, from which the metal values may be readily recovered by conventional techniques (e.g. electrowinning or hydrogen reduction).The presence of metallic nickel in the concentrate is ensured by reducing the sulphur content of the matte by oxidation during a preliminary pyrometallurgical conversion process. Alternatively, metalic nickel is povided by adding nickel scrap or recycled powder.

Both the aforementioned processes are particularly suited to the treatment of nickel-copper concentrates and mates which contain a significant proportion of the nickel in the form of metallic alloy in combination with copper and iron, rather than as a nickel sulphide. This metallic nickel is particularly effective in cementing copper from solution, and this factor is essential for the sucessful operation of both these processes.

However, it is not always feasible, economically or technically, to treat nickel-copper mattes containing metallic nickel. For example, some nickel-copper ores contain significant concentrations of the platinum group metals. These values can be recovered most economically by concentrating and smelting the metal sulphide to a sulphur-deficient nickel-copper matte in which the platinum group metals are largely concentrated in the nickel copper alloy. If the matte is cooled slowly, crushed and ground, and subjected to magnetic separation, the nickel-copper alloy, which is magnetic, can be separated as a platinum group metal concentrate, for separate treatment, leaving the bulk of the nickel and copper in a non-magnetic nickel-copper matte, low in platinum metal values, and consisting essentially of nickel and copper sulphides.

This non-magnetic fraction is already finely ground and thus forms a suitable feed for a hydrometallurgical treatment for nickel and copper recovery. However, since it contains no metallic nickel, it cannot be treated directly by either of the established processes described above.

The present invention seeks to provide a hydrometallurgical process for the treatment of non-magnetic and magnetic nickel-copper mattes to produce a substantially copper- and iron-free nickel sulphate solution suitable for recovery of nickel, and a copper-bearing solution suitable as feed to a copper electrowinning process.

It has surprisingly been found that a nickel sulphate solution containing up to about 10 g/L copper can be rendered substantially copper-free (10 mg/L Cu2+) by contacting it with finely-divided non-magnetic nickel-copper matte in an atmospheric purification leach at 60 to 900C. The copper precipitated in this reaction is a cuprous sulphide (CU1.96 S). No metallic copper is formed, as is the case when a matte or concentrate containing metalic nickel is used.It is believed that the principal mechanism of the copper precipitaiton is essentially the metathesis reaction:

It has also been found that when a nickel-copper matte is treated in a first-stage pressure leach of a two-stage pressure leach, said first stage consisting of an initial oxidizing phase and a secondary non-oxidizing phase, to be described, a solution containing less than 10 glL, and generally less than 5 g/L copper can be produced in a leach in which in excess of 80% of the nickel is extracted.

We have found that the need for reagents for copper and iron removal from the nickel-rich leach solution can be substantially obviated by conducting the first-stage pressure leach sequentially under controlled oxidizing and non-oxidizing conditions such that at the end of the oxidizing leach phase the major components of the leach residue and cupric sulphide (CuS) and basic copper sulphate (CuSO4.CU(OH)2) and that at the end of the non-oxidizing leach phase the major components of the leach residue are cupric sulphide and digenite (CUeS5). To acquire this desired state of the copper values, the oxidizing phase of the first-stage pressure leach is terminated when up to about 70% of the nickel values contained in the matte, preferably between 60 to 70% of the said nickel values, is solublized.Subsequent leaching of the matte under non-oxidizing conditions converts copper values to a less acid-soluble form with substantially reduced copper values in a nickel-rich leach solution.

The nickel-rich leach solution recovered from the first-stage pressure leach is passed to a purification leach conducted under mild atmospheric conditions in which it may be contacted with nickel-copper matte to precipitate copper values by metathesis and hydrolysis and iron values by oxidation and hydrolysis. The copper and the remainder of the nickel values in the first stage pressure leach residue are dissolved in the second-stage pressure leach and passed in a copper-rich leach solution to a copper electrowinning step with recycle of spent electrolyte containing the nickel values to the pressure leaches.

According to an embodiment of the process of the present invention, the major portion of nickel values are preferentially extracted from the matte in the first-stage acid pressure oxidation leach which is conducted to produce a solution which contains a substantial amount of dissolved nickel values and a low concentration of dissolved copper. The copper thus is not only precipitated but also selectively converted to an acid insoluble form (Cu9S5) which permits the attainment of lower levels of copper in solution at the end of the first-stage pressure leach.

Non-magnetic nickel-copper matte can be treated by a combination of the said two-stage pressure leach with an atmospheric purification leach to provide a substantially copper-free nickel-enriched leach solution suitable for nickel recovery.

The first-stage pressure leach solution is separated from the solids residue and may be passed directly to a nickel recovery stage or, if desired, in accordance with a preferred embodiment of the process of the present invention, treated for purification of the nickel sulphate solution by precipitation of any residual copper values present in the solution.

The solids residue from the first-stage pressure leach, which contains substantially all copper values as well as nickel and other metal values not extracted in the first-stage leach, is treated in a second-stage acid pressure oxidation leach which is conducted to extract substantially all copper values therefrom as well as residual nickel values which comprise a minor portion of the dissolved metal values in solution.

Copper can be recovered from the leach solution, for example, by electrowinning or by hydrogen reduction with concurrent regeneration of sulphuric acid and the spent solution from such copper recovery operation, which contains the nickel extracted in the second-stage leach as well as the regenerated sulphuric acid and residual copper values, is preferably recycled to the first pressure leaching stage.

In accordance with an embodiment of the process of out invention, the separation and recovery of copper and nickel values contained in nickel-copper mattes is effected by: leaching finely divided particles of said matte in a first-stage pressure leach under an initial oxygen overpressure of above about 10 psig (69 kPa) and at a temperature above about 100"C in an aqueous acid sulphate solution having a pH in the range of from about 1.8 to about 5.0 for dissolution of up to about 70%, preferably about 60 to 70%, of the nickel values contained in the matte, dissolution of a minor portion of copper values from the matte and conversion of residual copper values from said matte to cupric sulphide and basic copper sulphate; continuing said first-stage pressure leach under non-oxidizing conditions for continued dissolution of nickel values contained in the matte and for conversion of said basic copper sulphate to a copper sulphide in the form of digenite, whereby said copper values contained in said matte are converted to a less acid-soluble and more easily filtered form and the leach solution contains a relatively high concentration of dissolved nickel values and a relatively low concentration of copper values; separating said first-stage pressure leach solution from said first-stage pressure leach residue for recovery of nickel therefrom; reacting said first-stage pressure leach residue in a second-stage pressure leach at a temperature above about 135"C and under an oxygen overpressure of at least about 15 psig (103 kPa) with an aqueous acid sulphate solution containing sufficient sulphur values to combine as sulphates with substantially all copper values and a relatively low concentration of dissolved nickel values; recovering dissolved copper values from said secod-stage pressure leach solution in a copperwinning step while simultaneously generating sulphuric acid to produce a spent electrolyte containing sulphuric acid, dissolved nickel values and residual dissolved copper values; and recycling sufficient of said spent electrolyte from said copperwinning step to said first-stage pressure leach to maintain the pH of the first-stage leach solution within said range of about 1.8 to about 5.0 and to adjust the total sulphur content of the slurry in said first-stage pressure leach such that there is about 1 mole of sulphur in said slurry to each mole of nickel and copper.

A preferred embodiment of our invention comprises purifying said first-stage pressure leach solution by adding finely divided particles of a nickel-copper matte to said solution at atmospheric pressure, at a solution pH of up to about 4 and at a solution temperature in the range of about 60 to 900C while aerating said solution for precipitation of contained copper and iron values and simultaneous dissolution of minor amounts of the nickel contained in the matter whereby less than 10 mg/L of total copper and iron remains in solution at a final pH in the range of about 5 to about 6.5.

The process of our invention has particular utility in the treatment of nickel-copper concentrates and mattes which are substantially free of metallic nickel.

A detailed description of the process of our invention will now be described with reference to the accompanying drawings, in which: Figure lisa flowsheet illustrating an embodiment of the process of the invention for the recovery of copper and nickel values from a high grade nickel-copper sulphide matte; and Figure 2 is a flowsheet of another embodiment of the process of our invention.

The process of our invention is applicable to magnetic and non-magnetic nickel-copper mattes containing relatively large proportions of both nickel and copper and in which the weight ratio of nickel to copper is less than about 2.5, preferably within a Ni/Cu weight ratio between about 0.75 and 2 when the nickel-copper matte is fed first-stage pressure leach. Magnetic mattes typically contain a Ni/Cu weight ratio of 0.75 to 2.5 and non-magnetic mattes typically contain a Ni/Cu weight ratio of 0.90 to 2.5. In the embodiments of our invention in which matte is subjected to an atmospheric leach as a step in the purification of nickel sulphate solutions prior to nickel recovery, a matte having a Ni/Cu weight ratio within the range of from 0.75 to 1.5 has been found satisfactory.

The matte, magnetic or non-magnetic, preferably contains less than 0.7 mole of sulphur for each mole of contained nickel, copper and other acid reactive metals such as iron and cobalt which may be present in the matte and capable of forming metal sulphates under the leaching conditions. Non-magnetic mattes tested indicated S/Ni + Cu mole less than 0.67, more specifically ratios of 0.59 to 0.64, while magnetic mattes tested indicated S/Ni + Cu mole ratios of 0.48 to 0.58. A non-magnetic matte for the Ni/Cu weight ratio of 0.90 to 2.5 would have a theoretical mole ratio of 0.578 to 0.62. Thus a non-magnetic matte is considered to have a S/Ni + Cu mole ratio of greater than about 0.57 and a magnetic matte has a S/Ni + Cu ratio of less than about 0.57.

The presence of excess sulphur in the matte can result in the production in the leaches of excess acid with resulting low pH and decreased selectivity of nickel extraction relative to copper, particularly in the first pressure leach, to be described.

With reference now to the flowsheet illustrated in Figure 1, a finely divided high grade nickel-copper sulphide matte, usually also containing significant amounts of cobalt and iron together with precious metals, is fed to first-stage pressure leach 10 which is conducted at a temperature in the range of about 120 - 140"C in an aqueous acid sulphate solution having a pH in the range of from about 1.8 to about 5 under an initial oxygen overpressure of above about 10 psig (69 kPa) for dissolution of a major portion of nickel values contained in the matte, dissolution of a minor portion of copper values from the matte, and conversion of residual copper values from said matte to cupric sulphide and basic copper sulphate.First-stage pressure leach 10 is conducted initially with an oxidizing leach phase under the oxidizing atmosphere for a length of time to permit the following reactions to occur for dissolution of up to about 70% of the nickel values contained in the matte:

X-ray diffraction analysis has shown that at the end of the oxidizing leach phase of the first stage pressure leach the major components of the residue are cupric sulphide and basic copper sulphate with residual nickel present in the form of polydymite (Ni3S4).

The first-stage pressure leach is continued during a non-oxidizing phase of the leach with reactions of the following type taking place:

Acid generated in reaction (5) redissolves copper from basic copper sulphate according to reaction (6) and the resulting copper sulphate apparently reacts with nickel sulphide (polydymite) according to reaction (5) with continuation of this cycle until all the basic copper sulphate is converted to digenite. X-ray diffraction analysis of the residue at the completion of the first-stage pressure leach shows that the major components of the leach residue are cupric sulphide and digenite with residual nickel present as polydymite and millerite (NiS).The combined oxidizing and non-oxidizing phases ofthefirst-stage pressure leach require a leaching time of about 1 - 3 hours to give an extraction of up to 80 and 90% of the nickel and precipitation of copper from solution to less than 10 g/L, preferably less than 5 g/L.

Acid requirements for reaction 1 are supplied by spent electrolyte from the copper electrowinning step and, if necessary, by fresh sulphuric acid added directly to the first-stage pressure leach. The pH of the leach solution is regulated to provide a final pH in the range of from about 1.8 to about 5, preferably in the range of between 3.5 - 4.5.

The leach liquor from the first-stage pressure leach enriched in nickel, is subjected to a purification leach for precipitation of copper and iron to less than about 10 mg/L to render the solution suitable for cobalt and nickel recovery. A portion of the matte is contacted with this solution which preferably contains not more than about 10 g/L copper, not more than about 0.5 g/L iron and not more than about 5 g/L sulphuric acid, under mild leaching conditions, whereby only a minor amount of the nickel present in the matte feed is dissolved, copper is precipitated by metathesis and iron is removed by oxidation and hydrolysis.

We have found that only sufficient dissolution of the nickel values in the matte occurs to consume the free acid present. The product solution normally contains about 100 g/L nickel and less than 10 mg/L copper and iron at a pH within the range of 5 to 6.

It is believed the principal reactions which take place in the purification leach 14 are as follows:

The solids residue from atmospheric purification leach 14 is passed to first-pressure leach 10.

The use of matte for reducing the copper concentration from 10 g/Lto less than 10 mg/L consumes about 10 kg of magnetic matte for every kg of copper precipitated and about 20 kg of non-magnetic matte for every kg of copper precipitated.

The solids residue from the first-stage pressure leach contains a significant amount of non-extracted nickel as well as substantially all the copper content of the matte rendered in a dilute-acid insoluble condition. This residue is passed to second-stage leach 16 in which it is further leached with sulphuric acid solution from copper electrowinning 12 at a temperature in the range of 135 - 1 500C, preferably at about 150'C under an oxygen overpressure of at least about 15 psig (103 kPa) with a final acid concentration within the range of from about 5 to 20 g/LH2SO4.We have found that operating the second-stage pressure leach at about 1 50"C results in the reduction of the iron content of the leach liquor, as compared to lower temperatures commonly used, to obviate the need for a separate iron removal step prior to copper electrowinning 12.

Essentially all the remaining nickel in the matte, which can comprise up to 45 g/L of the product leach solution from leach 16, but preferably less than 30 glL, is fed to copper electrowinning 12 in a copper-rich solution for production of copper cathodes and recycle of spent electrolyte containing nickel values to pressure leaches 10 and 16.

Figure 2 illustrates another embodiment of our invention in which the finely divided high grade nickel-copper sulphide matte is fed first to atmospheric leach 20 which is conducted under the conditions discussed hereinabove with reference to the flowsheet of Figure 1. The leach solution preferably contains not more than about 10 g/L copper, not more than about 0.5 g/L iron and not more than about 5 g/L sulphuric acid and is conducted under mild leaching conditions whereby minor amounts of the nickel present in the matte feed, normally less than 10% of the nickel, is dissolved and copper and iron precipitated by reactions 8 to 10.

The solids residue from leach 20 is passed to first-stage pressure leach 22 which is conducted under the conditions specified hereinabove for first-pressure leach 10 with recycle of the leach solution from pressure leach 22 back to purification leach 20 for preparation of the solution for cobalt and nickel recovery.

The solids residue from first-stage pressure leach 22 is passed to second-stage pressure leach 24 which is operated under the conditions dicussed above in connection with second-stage pressure leach 16. A selenium removal step 26, for removal of selenium to less than 1 mg/L by precipitation as cuprous selenide prior to electrowinning of copper in step 28, may be included. It has been found that the preferred temperature of about 150"C in the second-stage pressure leach converts the selenium preferentially to the selenite (IV) oxidation state, thereby facilitating its removal by reductive precipitation.

The embodiments of the process of our invention will now be described with reference to the following examples.

Example 1 This example demonstrates the pressure leaching of a non-magnetic nickel-copper matte having the following analysis by weight: 34.2% Ni, 35.2% Cu and 24.1% S. 750 g of the matte was comminuted to 90% minus 44 m, was slurried with 5 L of a leach solution containing 43 g/L Ni, 13.5 g/L Cu, 1.1 g/L Fe and 31.5 g/L H2SO4, and leached at 135"C under 20 psi (128 kPa) oxygen partial pressure for 180 min.

After 120 min. the copper content of the leach solution was 20 g/L at pH 3, and 83% of the nickel had been solubilized.

After 180 min. the copper content of the leach solution had increased to 28.5 g/L at pH 3, and 94% of the nickel had been extracted. The leach rnsiduehadthefollowing composition: 3.1% Ni, 44.7% Cu, 5.6% Fe. The leach solution analyzed (g/L) 91 Ni, 28.5 Cu, 1.5 Co and 0.13 Fe.

Example 2 This example demonstrates the pressure leaching of a non-magnetic matte of the same composition as Example 1 under initial oxidizing conditions to less than 70% solubilization of the nickel values in the matte followed by pressure leaching under non-oxidizing conditions. 750 g of the matte was slurried with 5 L of a leach solution containing 43 g/L Ni, 13.5 g/L Cu, 0.9 g/L Fe and 32.5 g/L H2SO4, and leached for 75 min. at 135"C under 20 psi (138 kPa) oxygen partial pressure. After 75 min., at which time the nickel extraction was 68%, the oxygen supply was cut off, and the leach continued under steam pressure only at 135 C.

After 135 min. the copper content of the leach solution was 4.3 g/L at pH4 and 85% of the nickel had been extracted.

After 225 min. the copper content of the leach solution was 4.8 g/L at pH 3.8 and 94% of the nickel had been solubilized.

The leach residue analyzed by weight: 3.1% Ni, 60.4% Cu, 3.8% Fe. The leach solution analyzed: (g/L) 92 Ni, 4.8 Cu, 1.3 Co and 0.04 Fe.

The leach solution produced in Example 2 can be easily decopperized by treatment with non-magnetic nickel-copper matte in an atmospheric purification leach, while the solution produced in Example 1 contains more copper than can be precipitated by the non-magnetic matte at the solids/liquid ratio imposed by the overall process balance limitations.

Example 3 This example describes the embodiment of our invention illustrated in Figure 2. 7.2 kg of a non-magnetic nickel-copper matte contained by weight 42.6% Ni, 30.0% Cu, 0.44% Co, 1.22% Fe and 24.4% S, previously comminuted to 90% minus 44 m, was contacted with 50 L of a first stage pressure leach solution containing: (g/L) 66 Ni, 7.8 Cu, 0.66 Co, 0.7 Fe, at pH 4, in an atmospheric purification leach at 80 - 90 for 300 min. Air was sparged through the leach slurry at a rate of 75 Cumin.

After 300 min. the leach solution analyzed: (g/L) 72 Ni, 0.76 Co. 0.001 Cu, < 0.001 Fe at pH 6.6. This solution is a suitable feed to any conventional cobalt and nickel recovery process.

The matte residue recovered from the purification leach analysed by weight 38.2% Ni, 31.4% Cu, 0.49% Co and 23.8% S.

X-ray diffraction analysis of the original matte and leach residue samples indicated that both nickel and copper were present throughout the treatment as sulphides, and that no metallic copper was formed in the process.

The residue from the purification leach was washed and re-slurried with 50 L solution containing: (g/L) 33 Ni, 11 Cu, 0.34 Co, 0.5 Fe and 42 H2S04, and was pressure leached at 135"C under 20 psi (138 kPa) oxygen partial pressure (50 L/min. air sparge) for 95 min. After 95 min. the air supply was cut off and the leach was continued at 1 350C under steam pressure only for a further 120 min.

After the oxidizing phase of the leach, the solution contained 10.4 g/L copper, and after the non-oxidizing leach this had descreased to 6.5 g/L copper. Nickel extractions were 69% after the oxidizing leach and 86% after the non-oxidizing leach.

The leach solution, which forms the solution feed to the atmospheric purification leach, contained (g/L) 69 Ni, 0.66 Co, 6.5 Cu, 0.4 Fe, while the leach residue analyzed: (weight %) 8.8 Ni, 0.3 Co, 58.3 Cu, 2.2 Fe and 28.0 S.

X-ray diffraction analysis of residue samples indicated that at the end of the oxidizing leach phase the major solid species present were covellite (CuS) antlerite (CuSO4. 2Cu(OH)2) and polydymite Ni3S4), while at the end of the non-oxidizing leach the major species were covellite, digenite (Cu9S6) and polydymite.

The first-stage pressure leach residue (5.7 kg) was washed and re-slurried with 40 L solution containing: (g/L 12 Ni, 10 Cu, 0.3 Co. 0.3 Fe, and 40 H2S04 and was subjected to a second-stage pressure leach at 1500C under 20 psig (138 kPa) oxygen partial pressure (68 Cumin. air sparge) for 360 min.

The final leach solution contained (g/L) 96 Cu, 28 Ni, 0.8 Co, 0.3 Fe, 14 His04, and the leach residue (0.27 kg) analyzed: (weight %) 1.5 Ni, 005 Co, 6.8 Cu, 29.2 Fe and 16.9 S.

The overall metal extractions from the nickel-copper matte in the process were 99.9% Ni, 99.6% Co and 99.2% Cu.

A second stage pressure leach solution of this composition is a very suitable feed to a conventional copper electrowinning process, from which spent electrolyte can be recycled to the leaching circuit to meet the requirements for acid, and to bleed nickel and cobalt from the copperwinning circuit.

The final leach residue may be discarded or, where warranted economical, treated further for the recovery of precious metal values.

Example 4 This example illustrates the treatment of a magnetic nickel-copper matte, i.e. matte containing nickel-copper metal alloy, by the embodiment of the process of our invention shown in Figure 1.

0.8 kg of a magnetic nickel-copper matte containing by weight 37.6% Ni, 38.6% Cu, 0.6% Co. 0.66% Fe and 19.7% S, was comminuted to 70% minus 44 m, and pressure leached in 5L solution containing Ag/L) 10 Ni, 10 Cu, 0.4 Co, 0.5 Fe and 60 H2SO4, at 135"C under 20 psi (138 kPa) oxygen overpressure for 30 min. After 30 min., at which time the nickel extraction was 70.6%, the oxygen supply was cut off, and the leach was continued at 135"C under steam pressure only for a further 120 min.

The final leach solution contained (g/L) 60 Ni, 1.2 Cu, 1.1 Co. 0.002 Fe at pH 4.8.

The leach residue (0.54 kg) analysed (%) 58.4 Cu, 9.9 Ni, 0.32 Co and 25.5 S, corresponding to a nickel extraction of 82%.

The leach solution (5 L) from the first stage pressure leach was contacted with 0.5 kg of the same nickel-copper matte, as was used in the first-stage leach, in a purification leach at 80 C -85'Cwith a 2Umin.

air sparge for 120 min.

The final solution contained (g/L) 70 Ni, 1.3 Co, < 0.001 Cu, and 0.001 Fe. The purification leach residue, which is recycled to the first-stage pressure leach, analysed 36.6% Ni, 39.9% Cu and 19.1% S.

0.75 kg of a composite first-stage pressure leach residue containing by weight 7.8% Ni, 55.8% Cu, 0.25% Cu, 0.25% Co and 24.3% S, which had bee generated in a series of first-stage pressure leaches of magnetic nickel-copper matte under the conditions described above, was pressure leached at 150"C under 20 psi (138 kPa) oxygen overpressure for 240 min. The initial leach solution (5 L) analysed (g/L) 12 Ni, 10 Cu, 0.25 Co. 0.3 Fe and 40H2SO4.

The final leach solution contained (g/L) 82 Cu, 25 Ni, 0.8 Co, 0.2 Fe and 5H2SO4, and the final residue analysed: (weight %) 0.5 Ni, 0.02 Co, 14.6 Cu, 38.2 Fe and 14.2 S.

Overall metal extractions from the nickel-copper matte in the two-stage leach process corresponded to 99.97% Ni, 99.95% Co and 98.6% Cu.

Claims (8)

1. A continuous process for separating and recovering copper and nickel values contained in nickel-copper matte which comprises: leaching finely divided particles of said matte in a first-stage pressure leach under an initial oxygen overpressure of above about 10 psig (69 kPa) and at a temperature above about 100"C in an aqueous acid sulphate solution having a pH in the range of from 1.8 to 5.0 for dissolution of up to about 70% of the nickel values contained in the matte, dissolution of a minor portion of copper values from the matte and conversion of residual copper values from said matte to cupric sulphide and basic copper sulphate;; continuing said first-stage pressure leach under non-oxidizing conditions for continued dissolution of nickel values contained in the matte and for conversion of said basic copper sulphate to a copper sulphide as digenite, whereby said copper values contained in said matte are converted to a less acid-soluble form and the leach solution contains a high concentration of dissolved nickel values and a low concentration of copper values; separating said first-stage pressure leach solution from said first-stage pressure leach residue for recovery of nickel therefrom;; reacting said first-stage pressure leach residue in a second-stage pressure leach at a temperature above about 135"C under an oxygen overpressure of at least about 15 psig (103 kPa) with an aqueous acid sulphate solution containing sufficient sulphur values to combine as sulphates with substantially all copper and nickel values contained therein to provide a second-stage pressure leach residue and a second-stage pressure leach solution containing a high concentration of dissolved copper values and a low concentration of dissolved nickel values; recovering dissolved copper values from said second-stage leach solution in a copperwinning step whilst simultaneously generating sulphuric acid to produce a spent solution containing sulphuric acid, dissolved nickel values and residual dissolved copper values; and recycling sufficient of said spent solution from said copperwinning step to said first-stage pressure leach to maintain the pH of the first-stage pressure leach solution within said range of from 1.8 to 5.0.

2. A process as claimed in claim 1, which additionally comprises purifying the nickel-rich leach solution from said first-stage pressure leach containing copper and iron values and having a pH of up to about 5 by adding finely divided particles of nickel-copper matte to said solution at a temperature in the range of 60"C to 90"C under atmospheric pressure in the presence of oxygen for precipitation of said copper and iron values and simultaneous dissolution of minor amounts of the nickel contained in the matte whereby less than about 10 mg/L of total copper and iron remains in solution at a final pH in the range of from 5 to 6.5; separating the purified leach solution from the residue for recovery of nickel therefrom; and feeding the residue to the first-stage pressure leach.

3. A process as claimed in claim 1, wherein the nickel-copper matte is fed to an atmospheric purification leach prior to subjecting said matte to the first-stage pressure leach in which the matte is subjected to the atmospheric purification leach at a temperature in the range of 60"C to 90"C in the presence of oxygen in a nickel-rich leach solution from the first-stage pressure leach for precipitation of contained copper and iron to less than 10 mg/L and simultaneous dissolution of minor amounts of nickel contained in the matte at a final pH in the range of from 5 to 6.5; separating the purified leach solution from the residue for recovery of nickel therefrom; and feeding the said residue to the first-stage pressure leach.

4. A process according to claim 3, wherein the nickel-rich leach solution used in the purification leach contains not more than 10 g/L copper, not more than 0.5 g/L iron and not more than 5 g/L sulphuric acid at a pH in the range of from 1.8 to 5.0.

5. A process as claimed in any one of claims 1-4, wherein said nickel-copper matte is substantially non-magnetic.

6. A process as claimed in any one of claims 1-5, wherein the first-stage pressure leach is conducted at a temperature of from 120 - 140"C at a pH in the range of from 3.5 to 4.5 and the second-stage pressure leach is conducted at a temperature of about 150"C.

7. A process as claimed in any one of claims 1-6, wherein the first-stage leach is continued under oxidizing conditions until from 60 to 70% of the nickel in the matte or residue is extracted.

8. A process according to claim 1, when carried substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.