Process for extraction of nickel and copper values from a polymetallic sulfide concentrate

Abstract

The invention provides a process for extraction of nickel and copper values from a polymetallic sulfide concentrate, the process comprising: providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution, wherein the polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals and wherein the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 5 wt.%; subjecting the slurry to pressure oxidation in one or more continuous oxidation stages to leach nickel and copper into the aqueous leach solution; separating one or more streams of pregnant leach solution from the slurry; and recovering nickel and copper from the one or more streams of pregnant leach solution, wherein at least nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and an oxidation-reduction potential of below 520 mV vs Ag / AgCl, and wherein at least a portion of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage.

Description

[0001] Process for extraction of nickel and copper values from a polymetallic sulfide concentrate

[0002] Technical Field

[0003] [1] The invention relates to a process for extraction of nickel and copper values from a polymetallic sulfide concentrate, comprising subjecting a polymetallic sulfide concentrate comprising nickel, copper and iron to pressure oxidation in one or more continuous oxidation stages to leach nickel and copper into an aqueous leach solution, wherein at least nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage.

[0004] Background of Invention

[0005] [2] High yield extraction of nickel, copper and other value metals from polymetallic sulphide concentrates has conventionally relied on at least one pyrometallurgical process step. The concentrate is roasted to remove sulphur and produce a calcined product which is susceptible to leaching. The roasting process generates sulfur dioxide and trioxide off-gas which needs to be captured, usually in the form of sulphuric acid. Sulphuric acid disposal is costly and the roasting process can also release toxic metals to the environment such as arsenic, mercury and thallium.

[0006] [3] Hydrometallurgical processes competing with the roasting approach include biological oxidation processes using tank or heap leaching, which are slow and provide poor metal recoveries, and thermochemical oxidative leaching processes. Various efforts have been made to develop ambient pressure, low temperature oxidative leach processes which avoid the need for an autoclave. The Albion process, for example as disclosed in US patent 5,993,635, enhances open tank leaching of a sulfide concentrate by recycling ferric ions from downstream processing of the pregnant leach solution to the open tank.

[0007] [4] The Activox process, for example as disclosed in the US5,917,116, is a low temperature (<100°C) pressure oxidation process which enhances copper recovery by including chloride ions in the sulfuric acid leach solution. The chloride is said to mitigate the passivation of copper sulfide minerals (e.g. chalcopyrite) which otherwise severely limits the extraction of copper under low temperature oxidation conditions.

[0008] [5] Despite some technical advances, open tank and low temperature processes are generally unable to achieve sufficiently high recoveries of value metals for a commercial process. The use of an autoclave is thus required to accommodate the elevated reaction temperatures and oxidant pressures needed for acceptable value metal recovery, despite the greater process complexity and cost associated with this approach. [6] Medium temperature pressure oxidation processes previously investigated have typically employed oxygen partial pressures of greater than 1000 kPa. For example, Pandey et al (Mineral Processing and Extractive Metallurgy 2002, 111 (2): 106-09) conducted extensive test-work on a sulfide concentrate analysing 10.85% Ni and 15% Cu. Much of the nickel was present as millerite, violarite and siegenite and the copper was hosted in chalcopyrite. Under single-stage pressure oxidation conditions of 150°C and oxygen partial pressure of 1085 kPa, unsatisfactory recoveries of nickel (64%) and copper (36%) were obtained. Somewhat improved metal recoveries were obtained at higher oxygen partial pressure (2170 kPa oxygen partial pressure, 70% nickel recovery, 71% copper recovery) and such conditions were subsequently applied in a two-stage process where maximum nickel and copper recoveries of 78% and 79% were obtained. The poor nickel extraction was attributed to passivation effects caused in part by elemental sulfur coating of the nickel sulfide minerals.

[0009] [7] The international patent application published as WO 2024 / 103106 A1 (WO’ 106) discloses pressure oxidation of metal sulfide concentrates to preferentially leach various electronegative value metals, such as copper, nickel, cobalt and / or zinc, by manipulating the oxygen partial pressure to control the oxidation-reduction potential in the leach. This is said to minimise passivation of value metal-containing minerals, providing improved recoveries. For nickel-rich sulfide concentrates, WO’ 106 discloses leaching at a temperature of 165°C to achieve acceptable nickel and copper recovery. Examples 5 and 6 disclose pressure oxidation leaching of sulfide concentrates with high pyrite concentration. By controlling the oxidationreduction potential at the leach temperature of 165°C, oxidation of value metal sulfides is kinetically favoured over pyrite. Pyrite that does oxidise under these conditions is fully oxidised with high selectivity to sulfate.

[0010] [8] To improve value metal recoveries and address problems with the passivation of sulfide minerals, industry has generally moved towards total oxidative pressure leach technology in which the polymetallic sulphide concentrate is subjected to pressure oxidation under high pressure, high temperature oxidation conditions where sulfide is near-quantitatively oxidised to sulfate. However, this approach has several technical, economic and environmental drawbacks including the high energy demand associated with high temperature and high pressure processing, the cost of high pressure autoclaves, the consumption of large amounts of oxygen gas, and the generation of large quantities of sulfuric acid by-product which must typically be neutralised and disposed of.

[0011] [9] Much of the research focus in the area of pressure oxidation leaching of sulfide concentrates has been on copper recovery, in particular from chalcopyrite-rich sulfide concentrates, or precious metal recovery. However, some of the advancements made to enhance such processes, such as increased ferric and chloride concentrations in the leach, may be ineffective or even detrimental for nickel recoveries from some nickeliferous sulfide concentrates.

[0012]

[0010] There is therefore an ongoing need for new processes for extraction of nickel and copper values from polymetallic sulfide concentrates, particularly concentrates containing significant amounts of passivation-susceptible nickel sulfide minerals, which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative.

[0013]

[0011] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

[0014] Summary of Invention

[0015]

[0012] In an aspect, the process disclosed herein relies on the surprising finding that very high recoveries of one or more value metals, particularly nickel, from a polymetallic sulfide concentrate containing significant amounts of pyrrhotite may be achieved via a pressure oxidation leach process including at least one low-intensity continuous pressure oxidation stage conducted at a leach temperature less than 150°C, despite significant selectivity to elemental sulfur under conditions where the elemental sulfur exists in the molten state.

[0016]

[0013] In the low-intensity stage or stages, the oxygen partial pressure is maintained between 50 kPa and 500 kPa and the leach temperature between 120°C and 150°C, conditions which are considerably less demanding than either a conventional medium temperature pressure oxidation or a total-oxidative pressure leach approach. By manipulating the reaction conditions appropriately within these ranges, the oxidation-reduction potential can be controlled at a value just sufficient to leach nickel and optionally also copper, and to oxidise pyrrhotite, but sufficiently low that passivation of nickel and / or copper sulfides does not unacceptably inhibit value metal recoveries, despite the production of elemental sulfur.

[0017]

[0014] The process disclosed herein has been found particularly useful for nickeliferous sulfide concentrates comprising nickel to a significant extent in refractory nickel sulfide minerals such as millerite and / or violarite. Counter-intuitively, a reduction in oxidation intensity has been found to provide higher nickel recoveries from such concentrates than can be obtained under medium temperature pressure oxidation conditions or even in some high temperature total oxidative pressure leach conditions. Lower-intensity pressure oxidation conditions provide significant further benefits such as reduced process complexity and cost. Furthermore, desirable metal recoveries can be obtained despite producing elemental sulfur in significant quantities, particularly from pyrrhotite in the concentrate.

[0015] Incomplete oxidation of sulfide to form elemental sulfur is desirable as it reduces the amount of sulfuric acid by-product, the oxygen consumption and the exothermic heat generation. However, the inventors have found that this is advantageously done at temperatures below about 150°C, where the elemental sulfur product is molten but low in viscosity and readily dispersible within the slurry. By contrast, at temperatures higher than 150°C, molten sulfur undergoes a phase transition causing a dramatic increase in viscosity and a tendency to foul the process equipment. Without limitation by any theory, it is also believed that maintaining the elemental sulfur product below 150°C may limit or avoid passivation of value sulfide minerals by sulfur coating mechanisms, thus facilitating higher extractions of nickel and other value metals.

[0018]

[0016] In accordance with a first aspect, disclosed herein is a process for extraction of nickel and copper values from a polymetallic sulfide concentrate, the process comprising: providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution, wherein the polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals and wherein the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 5 wt.%; subjecting the slurry to pressure oxidation in one or more continuous oxidation stages to leach nickel and copper into the aqueous leach solution; separating one or more streams of pregnant leach solution from the slurry; and recovering nickel and copper from the one or more streams of pregnant leach solution, wherein at least nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and an oxidation-reduction potential of below 520 mV vs Ag / AgCI, and wherein at least a portion of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage.

[0019]

[0017] In some embodiments, the polymetallic sulfide concentrate comprises at least 1 wt.% nickel, or at least 2 wt.% nickel, such as at least 5 wt.% nickel, for example at least 10 wt.% nickel.

[0020]

[0018] In some embodiments, the polymetallic sulfide concentrate comprises nickel present in one or more nickel sulfide minerals selected from millerite, violarite and siegenite, optionally wherein at least 1%, or at least 5 %, such as at least 10%, of the nickel in the polymetallic sulfide concentrate is present in the one or more nickel sulfide minerals selected from millerite, violarite and siegenite. In some embodiments, at least 1%, or at least 5 %, such as at least 10%, of the nickel in the polymetallic sulfide concentrate is present in millerite.

[0019] In some embodiments, the polymetallic sulfide concentrate comprises copper present in chalcopyrite. In some embodiments, at least 10%, or at least 25 %, such as at least 50%, of the copper in the polymetallic sulfide concentrate is present in chalcopyrite.

[0021]

[0020] In some embodiments, the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 10 wt.%, such as greater than 20 wt.%, or greater than 30 wt.%.

[0022]

[0021] In some embodiments, at least 5%, or at least 10%, or at least 20%, such as at least 30%, for example at least 50%, of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage.

[0023]

[0022] In some embodiments, nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage at an oxidation-reduction potential of below 500 mV vs Ag / AgCI, such as below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, for example below 400 mV vs Ag / AgCI.

[0024]

[0023] In some embodiments, continuous pressure oxidation in at least one low-intensity continuous oxidation stage is conducted at an oxygen partial pressure of between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa.

[0025]

[0024] In some embodiments, continuous pressure oxidation in at least one low-intensity continuous oxidation stage is conducted at a temperature of between 130°C and 147°C, or between 130°C and 140°C.

[0026]

[0025] In some embodiments, the polymetallic sulfide concentrate has a P80 of less than 30 micron, or less than 20 micron, such as less than 15 micron, for example less than about 10 micron.

[0027]

[0026] In some embodiments, at least 95%, preferably at least 96%, more preferably at least 97%, most preferably at least 98%, of nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0028]

[0027] In some embodiments, at least 90%, preferably at least 95%, or at least 97%, most preferably at least 98% of copper in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0029]

[0028] In some embodiments, the polymetallic sulfide concentrate further comprises cobalt and at least 60%, preferably at least 80%, more preferably at least 90%, yet more preferably at least 95%, most preferably at least 98%, of cobalt in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0030]

[0029] In some embodiments, at least two continuous oxidation stages, or each continuous oxidation stage of the pressure oxidation is a low-intensity continuous oxidation stage performed at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and an oxidation-reduction potential of below 520 mV vs Ag / AgCl. In some embodiments, the oxygen partial pressure in each of these low-intensity continuous oxidation stages is between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa. In some embodiments, the oxidation-reduction potential in each of these low-intensity continuous oxidation stages is below 500 mV vs Ag / AgCl, such as below 450 mV vs Ag / AgCl.

[0031]

[0030] In other embodiments, one or more initial continuous oxidation stages of the pressure oxidation is a low-intensity continuous oxidation stage as defined here, and one or more subsequent (downstream) continuous oxidation stages has higher intensity oxidation conditions. In some such embodiments, the oxidation-reduction potential in the subsequent continuous oxidation stages is higher than 520 mV vs Ag / AgCl, for example between 520 mV vs Ag / AgCl and 600 mV vs Ag / AgCl, but the temperature remains below about 150°C, such as below 147°C. This may advantageously retain the elemental sulfur in a desirably low viscosity state through the entire pressure oxidation process.

[0032]

[0031] In some embodiments, less than 90%, or less than 80%, such as less than 70%, for example less than 60%, of the sulfur content of the polymetallic sulfide concentrate is oxidised to sulfate in the pressure oxidation.

[0033]

[0032] In some embodiments, at least 10%, or at least 20%, such as at least 30%, for example at least 50%, of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the pressure oxidation.

[0034]

[0033] In some embodiments, the aqueous leach solution comprises less than 10 g / L chloride, or less than 4 g / L chloride, such as less than about 2 g / L chloride, for example less than 1 g / L chloride.

[0035]

[0034] In some embodiments, the slurry is subjected to pressure oxidation in at least two sequential continuous oxidation stages comprising: a low-intensity first continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and a first oxidation-reduction potential of below 520 mV vs Ag / AgCl, wherein the first oxidation-reduction potential is sufficiently high to leach at least a portion of the nickel in the polymetallic sulfide concentrate into the aqueous leach solution in the low-intensity first continuous oxidation stage; and a second continuous oxidation stage conducted at a second oxygen-reduction potential sufficiently high to leach copper and residual nickel from the polymetallic sulfide concentrate into the aqueous leach solution.

[0036]

[0035] In some embodiments, at least 10%, such as at least 30%, at least 40%, at least 50%, or at least 60%, e.g. between 20 % and 80 % or between 20% and 50%, of the nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the first oxidation stage.

[0037]

[0036] In some embodiments, the first oxidation-reduction potential is sufficiently low that copper in the polymetallic sulfide concentrate remains substantially insoluble. In some embodiments, the first oxidation-reduction potential is sufficiently low that at least a portion of any dissolved copper introduced to the low-intensity first continuous oxidation stage precipitates therein.

[0038]

[0037] In some embodiments, the aqueous leach solution comprises dissolved copper, and the first oxidation-reduction potential is sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage. In some embodiments, the aqueous leach solution comprises at least 1 g / L copper, or at least 2 g / L copper, or at least 3 g / L copper, or at least 4 g / L copper, or at least 5 g / L copper, such as at least 6 g / L copper. In some embodiments, the aqueous leach solution comprises dissolved copper in an amount equivalent to at least 30%, or at least 40%, such as at least 50%, of the copper in the polymetallic sulfide concentrate in the slurry (prior to leaching).

[0039]

[0038] In some embodiments, the first oxidation-reduction potential is less than 425mV vs Ag / AgCI, or less than 400 mV vs Ag / AgCI, or less than 380 mV vs Ag / AgCI, for example less than 370 mV vs Ag / AgCI.

[0040]

[0039] In some embodiments, the second oxidation-reduction potential is greater than 380 mV vs Ag / AgCI, or greater than 400 mV vs Ag / AgCI, or greater than 425 mV vs Ag / AgCI, for example greater than 450 mV vs Ag / AgCI.

[0041]

[0040] In some embodiments, the process comprises separating a nickel-rich pregnant leach solution from slurry in the low-intensity first continuous oxidation stage, and recovering nickel from the nickel-rich pregnant leach solution. In some embodiments, the nickel-rich pregnant leach solution has a copper concentration of less than 2 g / L, or less than 1 g / L, or less than 0.5 g / L, or less than 0.3 g / L, such as less than 0.1 g / L. Typically, the nickel-rich pregnant leach solution is sent for nickel recovery without first recovering a copper product therefrom in a copper recovery unit.

[0042]

[0041] In some embodiments, the process comprises separating a copper-rich pregnant leach solution from slurry subjected to pressure oxidation in the second continuous oxidation stage to produce a copper-rich pregnant leach solution, and recovering copper from the copper- rich pregnant leach solution to produce a copper-lean raffinate containing nickel.

[0042] In some embodiments, the process further comprises recycling the copper-lean raffinate and optionally a portion of the copper-rich pregnant leach solution to the low-intensity first continuous oxidation stage. Typically, the copper-lean raffinate is recycled to the low- intensity first continuous oxidation stage without first recovering a nickel product therefrom in a nickel recovery unit.

[0043]

[0043] In some embodiments, the process comprises recycling a portion of the copper in the copper-rich pregnant leach solution to the low-intensity first continuous oxidation stage, thereby providing dissolved copper in the aqueous leach solution. In some embodiments at least 50%, or at least at least 67%, such as at least 75%, for example about 80%, of copper in the copper-rich pregnant leach is recycled to the low-intensity first continuous oxidation stage.

[0044]

[0044] In some embodiments, the process comprises removing a slurry portion from the low-intensity first continuous oxidation stage, flash cooling the slurry portion, dividing the slurry portion into a solids-rich fraction and a nickel-rich pregnant leach solution, for example by thickening, and recycling the solids-rich fraction to the low-intensity first continuous oxidation stage to provide cooling therein. The nickel-rich pregnant leach solution may be sent for nickel recovery therefrom as disclosed herein.

[0045]

[0045] In some embodiments, at least the first and second continuous oxidation stages, and preferably each of the at least two sequential continuous oxidation stages, is performed in a reaction compartment of one autoclave. Optionally, the reaction compartment of each sequential continuous oxidation stage in the same autoclave shares a common gas headspace.

[0046]

[0046] In some embodiments, the process further comprises adjusting one or more process parameters in at least one low-intensity continuous oxidation stage, for example the oxygen partial pressure, in response to an analytically determined measurement of oxidationreduction potential in said at least one continuous oxidation stage.

[0047]

[0047] In some embodiments, providing the slurry comprises (i) grinding the polymetallic sulfide concentrate, for example to a P80 of less than 15 microns, or to a P80 of less than about 10 microns, and (ii) combining the ground polymetallic sulfide concentrate with one or more aqueous fluids.

[0048]

[0048] In some embodiments, providing the slurry comprises combining the polymetallic sulfide concentrate with a barren recycle stream produced when recovering nickel and / or copper from a stream of pregnant leach solution. In some embodiments, the barren recycle stream comprises dissolved nickel and / or copper remaining after recovering the nickel and / or copper.

[0049] In some embodiments, the process comprises removing a slurry portion from a low- intensity continuous oxidation stage, flash cooling the slurry portion, and recycling at least a portion of the flash cooled slurry portion to the low-intensity first continuous oxidation stage to provide cooling therein.

[0049]

[0050] In some embodiments, each continuous oxidation stage is performed in a reaction compartment of one autoclave. Optionally the reaction compartment of each continuous oxidation stage shares a common gas headspace.

[0050]

[0051] In another aspect, the process disclosed herein relies on the surprising finding that value metals, particularly copper, can be extracted with exceptionally high recoveries from a nickel- and copper-bearing polymetallic sulfide concentrate into an aqueous leach solution via a pressure oxidation leach process including at least one low-intensity continuous pressure oxidation stage when a significant fraction of the leached copper is recycled to provide dissolved copper in the aqueous leach solution.

[0051]

[0052] In the low-intensity stage or stages, the oxygen partial pressure is maintained between 50 kPa and 500 kPa and the leach temperature between 120°C and 150°C. By manipulating the reaction conditions appropriately within these ranges, the oxidation-reduction potential can be controlled at a value sufficient to leach nickel and optionally also to oxidise an unstable iron sulfide such as pyrrhotite, but sufficiently low that the value metals are recovered in high yields in a subsequent continuous oxidation stage conducted under more intensive oxidising conditions. It is proposed that the dissolved copper reacts, via a metathesis reaction, with one or more metal sulfide minerals that contain a more reducible metal such as iron, cobalt or nickel, thereby leaching the more reducible metal into solution in the low-intensity continuous pressure oxidation stage. This metathesis reaction may enhance subsequent value metal extraction by increasing particle porosity and / or mitigating passivation effects, particularly of chalcopyrite.

[0052]

[0053] The process disclosed herein has been found particularly useful for nickeliferous sulfide concentrates comprising copper to a significant extent in refractory copper sulfide minerals such as chalcopyrite.

[0053]

[0054] In accordance with a second aspect, disclosed herein is a process for extraction of nickel and copper values from a polymetallic sulfide concentrate, the process comprising: providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution, wherein the polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals; subjecting the slurry to pressure oxidation in at least two sequential continuous oxidation stages to leach nickel and copper into the aqueous leach solution; separating one or more streams of pregnant leach solution from the slurry; recovering nickel and copper from the one or more streams of pregnant leach solution; and recycling a portion of the copper in the one or more streams of pregnant leach solution to provide dissolved copper in the aqueous leach solution, wherein nickel is leached into the aqueous leach solution in a low-intensity first continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa and a first oxidationreduction potential of below 520 mV vs Ag / AgCI, and wherein copper and residual nickel are leached from the polymetallic sulfide concentrate into the aqueous leach solution in a second continuous oxidation stage.

[0054]

[0055] In some embodiments, the first oxidation-reduction potential is below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, such as below 400 mV vs Ag / AgCI.

[0055]

[0056] The first oxidation-reduction potential may be sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage.

[0056]

[0057] In some embodiments, at least 50%, or at least at least 67%, such as at least 75%, for example about 80%, of copper in the one or more streams of pregnant leach solution is recycled to provide dissolved copper in the aqueous leach solution.

[0057]

[0058] In some embodiments, the aqueous leach solution comprises recycled copper in an amount of at least 1 g / L copper, or at least 2 g / L copper, or at least 3 g / L copper, or at least 4 g / L copper, or at least 5 g / L copper, such as at least 6 g / L copper. In some embodiments, the aqueous leach solution comprises dissolved copper in an amount equivalent to at least 30%, or at least 40%, such as at least 50%, of the copper in the polymetallic sulfide concentrate in the slurry (prior to leaching)

[0058]

[0059] In some embodiments, the polymetallic sulfide concentrate comprises pyrrhotite, such as in an amount greater than 5 wt.%, or greater than 10 wt.%, or greater than 20 wt.%, for example greater than 30 wt.%.

[0059]

[0060] In some embodiments, at least a portion of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the low-intensity first continuous oxidation stage. In some embodiments, at least 5%, or at least 10%, or at least 20%, such as at least 30%, for example at least 50%, of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the low-intensity first continuous oxidation stage. A further portion of sulfide in the polymetallic sulfide concentrate may be oxidised to form elemental sulfur in the second continuous oxidation stage.

[0061] In some embodiments, the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the pressure oxidation such that less than 90%, or less than 80%, or less than 70%, such as less than 60%, of the sulfur content of the polymetallic sulfide concentrate is converted to sulfate.

[0060]

[0062] In some embodiments, the polymetallic sulfide concentrate comprises at least 1 wt.% nickel, or at least 2 wt.% nickel, such as at least 5 wt.% nickel, for example at least 10 wt.% nickel. In some embodiments, at least a portion of the nickel is present in one or more nickel sulfide minerals selected from millerite, violarite and siegenite. In some embodiments, at least 1%, or at least 5 %, such as at least 10%, of the nickel in the polymetallic sulfide concentrate is present in the one or more nickel sulfide minerals selected from millerite, violarite and siegenite. In some embodiments, at least 1%, or at least 5 %, such as at least 10%, of the nickel in the polymetallic sulfide concentrate is present in millerite.

[0061]

[0063] In some embodiments, at least a portion of the copper in the polymetallic sulfide concentrate is present in chalcopyrite. In some embodiments, at least 10%, or at least 25 %, such as at least 50%, of the copper in the polymetallic sulfide concentrate is present in chalcopyrite.

[0062]

[0064] In some embodiments, nickel is leached into the aqueous leach solution in the low- intensity first continuous oxidation stage at an oxidation-reduction potential of below 520 mV vs Ag / AgCI, or below 500 mV vs Ag / AgCI, such as below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, for example below 400 mV vs Ag / AgCI.

[0063]

[0065] In some embodiments, continuous pressure oxidation in the low-intensity first continuous oxidation stage is conducted at an oxygen partial pressure of between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa.

[0064]

[0066] In some embodiments, continuous pressure oxidation in the low-intensity first continuous oxidation stage is conducted at a temperature of between 130°C and 147°C, or between 130°C and 140°C. In some embodiments, continuous pressure oxidation in each continuous oxidation stage is conducted at a temperature of between 130°C and 147°C, or between 130°C and 140°C.

[0065]

[0067] In some embodiments, the polymetallic sulfide concentrate has a P80 of less than 30 micron, or less than 20 micron, such as less than 15 micron, for example less than about 10 micron.

[0068] In some embodiments, at least 95%, preferably at least 96%, more preferably at least 97%, most preferably at least 98%, of nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0066]

[0069] In some embodiments, at least 90%, preferably at least 95%, or at least 97%, most preferably at least 98%, of copper in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0067]

[0070] In some embodiments, the polymetallic sulfide concentrate further comprises cobalt and at least 60%, preferably at least 80%, more preferably at least 90%, yet more preferably at least 95%, most preferably at least 98%, of cobalt in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

[0068]

[0071] In some embodiments, at least two continuous oxidation stages, or each continuous oxidation stage of the pressure oxidation is a low-intensity continuous oxidation stage performed at a temperature of between 120°C and 150°C and an oxygen partial pressure of between 50 kPa and 500 kPa. In some embodiments, the oxygen partial pressure in each of these low-intensity continuous oxidation stages is between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa. In some embodiments, the oxidation-reduction potential in each of these low-intensity continuous oxidation stages is below 500 mV vs Ag / AgCI, such as below 450 mV vs Ag / AgCl.

[0069]

[0072] In other embodiments, the second continuous oxidation stage has higher intensity oxidation conditions. In some such embodiments, the second continuous oxidation stage is conducted with a second oxidation-reduction potential higher than 520 mV vs Ag / AgCI, for example between 520 mV vs Ag / AgCI and 600 mV vs Ag / AgCI, but the temperature remains below about 150°C, such as below 147°C. This may advantageously retain elemental sulfur in a desirably low viscosity state through the entire pressure oxidation process.

[0070]

[0073] In some embodiments, at least 10%, or at least 20%, such as at least 30%, for example at least 50%, of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the pressure oxidation.

[0071]

[0074] In some embodiments, the aqueous leach solution comprises less than 10 g / L chloride, or less than 4 g / L chloride, such as less than about 2 g / L chloride, for example less than 1 g / L chloride.

[0072]

[0075] In some embodiments, at least 10%, such as at least 30%, at least 40%, at least 50%, or at least 60%, e.g. between 20 % and 80 % or between 20% and 50%, of the nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the low- intensity first oxidation stage.

[0076] In some embodiments, the first oxidation-reduction potential is sufficiently low that copper in the polymetallic sulfide concentrate remains substantially insoluble.

[0073]

[0077] In some embodiments, the first oxidation-reduction potential is less than 380 mV vs Ag / AgCI, for example less than 370 mV vs Ag / AgCl.

[0074]

[0078] In some embodiments, the process comprises separating a nickel-rich pregnant leach solution from slurry in the low-intensity first continuous oxidation stage, and recovering nickel from the nickel-rich pregnant leach solution. In some embodiments, the nickel-rich pregnant leach solution has a copper concentration of less than 2 g / L, or less than 1 g / L, or less than 0.5 g / L, or less than 0.3 g / L, such as less than 0.1 g / L. In some embodiments, the nickel-rich pregnant leach solution is sent for nickel recovery without first recovering a copper product therefrom in a copper recovery unit.

[0075]

[0079] In some embodiments, the process comprises separating a copper-rich pregnant leach solution from slurry subjected to pressure oxidation in the second continuous oxidation stage to produce a copper-rich pregnant leach solution. The process may comprise recovering copper from the copper-rich pregnant leach solution to produce a copper-lean raffinate containing nickel. In some embodiments, the copper-lean raffinate and a portion of the copper- rich pregnant leach solution is recycled to the low-intensity first continuous oxidation stage. Typically, the copper-lean raffinate is recycled to the low-intensity first continuous oxidation stage without first recovering a nickel product therefrom in a nickel recovery unit. In some embodiments, at least 50%, or at least at least 67%, such as at least 75%, for example about 80%, of the copper in the copper-rich pregnant leach solution is recycled to the low-intensity first continuous oxidation stage, thereby providing dissolved copper in the aqueous leach solution. In some embodiments, the aqueous leach solution comprises dissolved copper in an amount equivalent to at least 30%, or at least 40%, such as at least 50%, of the copper in the polymetallic sulfide concentrate in the slurry (prior to leaching)

[0076]

[0080] In some embodiments, the process comprises removing a slurry portion from the low-intensity first continuous oxidation stage, flash cooling the slurry portion, dividing the slurry portion into a solids-rich fraction and a nickel-rich pregnant leach solution, for example by thickening, and recycling the solids-rich fraction to the low-intensity first continuous oxidation stage to provide cooling therein. The nickel-rich pregnant leach solution may be sent for nickel recovery therefrom as disclosed herein.

[0077]

[0081] In some embodiments, at least the first and second continuous oxidation stages, and preferably each of the at least two sequential continuous oxidation stages, is performed in a reaction compartment of one autoclave. Optionally, the reaction compartment of each sequential continuous oxidation stage in the same autoclave shares a common gas headspace.

[0082] In some embodiments, the process further comprises adjusting one or more process parameters in at least the low-intensity first continuous oxidation stage, for example the oxygen partial pressure, in response to an analytically determined measurement of oxidation-reduction potential in said at least one continuous oxidation stage.

[0078]

[0083] In some embodiments, providing the slurry comprises (i) grinding the polymetallic sulfide concentrate, for example to a P80 of less than 15 microns, or to a P80 of less than about 10 microns, and (ii) combining the ground polymetallic sulfide concentrate with one or more aqueous fluids.

[0079]

[0084] In some embodiments, providing the slurry comprises combining the polymetallic sulfide concentrate with a barren recycle stream produced when recovering nickel and / or copper from a stream of pregnant leach solution. In some embodiments, the barren recycle stream comprises dissolved nickel and / or copper remaining after recovering the nickel and / or copper.

[0080]

[0085] In some embodiments, the process comprises removing a slurry portion from the low-intensity first continuous oxidation stage, flash cooling the slurry portion, and recycling at least a portion of the flash cooled slurry portion to the low-intensity first continuous oxidation stage to provide cooling therein.

[0081]

[0086] In some embodiments, each continuous oxidation stage is performed in a reaction compartment of one autoclave. Optionally the reaction compartment of each continuous oxidation stage shares a common gas headspace.

[0082]

[0087] Where the terms “comprise”, “comprises” and “comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[0083]

[0088] Further aspects of the invention appear below in the detailed description of the invention.

[0084] Brief Description of Drawings

[0085]

[0089] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0086]

[0090] Figure 1 is block flow diagram of a process for extraction of nickel and copper values from a polymetallic sulfide concentrate according to some embodiments of the present disclosure.

[0091] Figure 2 schematically depicts an autoclave for performing the pressure oxidation step of a process for extraction of nickel and copper values from a polymetallic sulfide concentrate, according to some embodiments of the present disclosure .

[0087]

[0092] Figure 3 is block flow diagram of a process for extraction of nickel and copper values from a polymetallic sulfide concentrate according to some embodiments of the present disclosure.

[0088]

[0093] Figure 4 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SC>42'), in the pressure leach experiment of Example 2.

[0089]

[0094] Figure 5 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the pressure leach experiment of Example 3.

[0090]

[0095] Figure 6 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2') to elemental sulfur (S°) and sulfate (SO42'), in the pressure leach experiment of Example 4.

[0091]

[0096] Figure 7 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the pressure leach experiment of Example 5.

[0092]

[0097] Figure 8 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the pressure leach experiment of Example 6.

[0093]

[0098] Figure 9 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the first continuous oxidation stage of Example 7.

[0094]

[0099] Figure 10 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the second continuous oxidation stage of Example 7.

[0095]

[0100] Figure 11 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the only continuous oxidation stage of Example 8.

[0096]

[0101] Figure 12 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the first continuous oxidation stage of Example 9.

[0102] Figure 13 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2') to elemental sulfur (S°) and sulfate (SC>42'), in the second continuous oxidation stage of Example 9.

[0097]

[0103] Figure 14 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2') to elemental sulfur (S°) and sulfate (SO42'), in the first continuous oxidation stage of Example 10.

[0098]

[0104] Figure 15 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the second continuous oxidation stage of Example 10.

[0099]

[0105] Figure 16 is a graph showing the ORP, the extraction of metals, and the % total oxidation of initial sulfide (S2j to elemental sulfur (S°) and sulfate (SO42'), in the pressure leach experiment of Example 11 .

[0100] Detailed Description

[0101]

[0106] The present invention relates to processes for extraction of nickel, copper and optionally other metal values from a polymetallic sulfide concentrate. In an aspect, the process comprises providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution. The polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals. The polymetallic sulfide concentrate may comprise pyrrhotite, for example in an amount greater than 5 wt.%.

[0102]

[0107] The process comprises subjecting the slurry to pressure oxidation in one or more continuous oxidation stages to leach nickel and copper into the aqueous leach solution. One or more streams of pregnant leach solution are separated from residual solids in the slurry, and nickel and copper are recovered from the one or more streams of pregnant leach solution.

[0103]

[0108] In at least one low-intensity continuous oxidation stage, at least nickel is leached into the aqueous leach solution at a temperature of between 120°C and 150°C and an oxygen partial pressure of between 50 kPa and 500 kPa. The oxidation-reduction potential in this low- intensity continuous oxidation stage may thus be maintained below 520 mV vs Ag / AgCI, such as below 500 mV vs Ag / AgCI, or below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, for example below 400 mV vs Ag / AgCI.

[0104]

[0109] At least a portion of sulfide in the polymetallic sulfide concentrate, which may include a substantial portion of the pyrrhotite sulfide, is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage. By oxidised to form elemental sulfur it is meant that sulfide is converted to elemental sulfur, and not further oxidised to sulfate, in the stage.

[0110] In some embodiments, the oxidation conditions in the pressure oxidation as a whole are such that the polymetallic sulfide concentrate is partially oxidised to produce elemental sulfur with significant selectivities.

[0105]

[0111] In another aspect, the process comprises providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution. The polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals.

[0106]

[0112] The process comprises subjecting the slurry to pressure oxidation in at least two sequential continuous oxidation stages to leach nickel and copper into the aqueous leach solution. One or more streams of pregnant leach solution are separated from the slurry, and nickel and copper are recovered from the one or more streams of pregnant leach solution. A portion of the copper in the one or more streams of pregnant leach solution may be recycled to provide dissolved copper in the aqueous leach solution.

[0107]

[0113] In a low-intensity first continuous oxidation stage, at least nickel is leached into the aqueous leach solution at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa and a first oxidation-reduction potential of below 520 mV vs Ag / AgCI, such as below 500 mV vs Ag / AgCI, or below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, for example below 400 mV vs Ag / AgCI. The first oxidation-reduction potential may be sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage.

[0108]

[0114] Copper and residual nickel are then leached from the polymetallic sulfide concentrate into the aqueous leach solution in a second continuous oxidation stage. The second continuous oxidation stage may be conducted at a second oxidation-reduction potential that is higher than the first oxidation-reduction potential and sufficiently high to leach nickel and copper into the aqueous leach solution with high efficiency. The metathesis reactions occurring in the low-intensity first continuous oxidation stage, via recycling of copper, are believed to facilitate exceptionally high value metal recoveries, particularly of copper, in the second continuous oxidation stage.

[0109] Polymetallic sulfide concentrate

[0110]

[0115] The processes disclosed herein extract metal values from a polymetallic sulfide concentrate comprising nickel, copper and iron present in a plurality of sulfide minerals. The polymetallic sulfide concentrate may comprise additional metals present in sulfide minerals, such as cobalt and / or zinc. As used herein, a polymetallic sulfide concentrate comprising nickel, copper and iron present in a plurality of sulfide minerals refers to a polymetallic sulfide concentrate comprising a plurality of sulfide minerals such that at least nickel, copper and iron are present in the concentrate. Each of nickel, copper and iron may in principle be present in a single sulfide mineral or in a plurality of sulfide minerals.

[0111]

[0116] Typically, the polymetallic sulfide concentrate is a flotation concentrate. In other words, it has been produced by froth flotation of a mined ore, typically after one or more preliminary processing steps including grinding. In principle, however, the concentrate may be produced by any means of concentrating value metal-containing sulfide minerals from gangue in an ore.

[0112]

[0117] The polymetallic sulfide concentrate is preferably a fine particulate to increase the effectiveness of leaching. In some embodiments, the polymetallic sulfide concentrate has a P80 of less than 30 micron, or less than 20 micron, or less than 15 micron, such as less than about 10 micron. In some embodiments, the process may comprise grinding the polymetallic sulfide concentrate to a desired particle size for the pressure oxidation leaching.

[0113]

[0118] In some embodiments, the polymetallic sulfide concentrate comprises nickel in an amount of at least 1 wt.%, such as at least 2 wt.%, at least 3 wt.%, at least 5 wt.%, at least 7 wt.%, or at least 10 wt.%, or at least 15 wt.%. The amount of nickel in some concentrates can be as high as 20 wt.%. In some embodiments, nickel is the most abundant value metal in the polymetallic sulfide concentrate. In particular, nickel may be present in a higher mass fraction than copper and any cobalt.

[0114]

[0119] The polymetallic sulfide concentrate comprises one or more nickel sulfide minerals. Non-limiting example of nickel sulfide minerals may include pentlandite ((Fe.NijgSs), heazlewoodite (NisS2), millerite (NiS), violarite (FeNi2S4), siegenite ((Ni, 80)384). In some embodiments, the concentrate comprises pentlandite which has historically been a major source of mined nickel. However, as high grade ores are mined out in many locations, there is an increasing need to extract nickel in high yields from concentrates comprising significant quantities of other nickel sulfide minerals, including some which are more refractory to oxidative leaching than pentlandite, e.g. due to increased susceptibility to passivation.

[0115]

[0120] In some embodiments, therefore, the polymetallic sulfide concentrate comprises nickel present in one or more of millerite, violarite and siegenite. For example, at least 1 %, or at least 2 %, or at least 5 %, or at least 10%, such as at least 20%, for example at least 30%, of the nickel in the polymetallic sulfide concentrate is present in one or more of these minerals. Millerite may be the dominant refractory nickel-sulfide mineral in the concentrate, present in amounts of up to 10 wt.% in some samples and thus representing a very substantial proportion of the nickel in the concentrate. In some embodiments, at least 1 %, or at least 2 %, or at least 5 %, or at least 10%, such as at least 20%, for example at least 30%, of the nickel in the polymetallic sulfide concentrate is present in millerite. The process disclosed herein has been found effective to achieve high nickel recoveries from such concentrates, despite the use of low-intensity oxidation conditions. In some scenarios, it is expected that higher nickel recoveries will be obtained according to the presently described process than are obtainable in a high-intensity total oxidation leach process.

[0116]

[0121] In some embodiments, the polymetallic sulfide concentrate comprises copper in an amount of at least 0.1 wt.%, such as at least 0.3 wt.%, at least 0.5 wt.%, at least 0.7 wt.%, at least 1 wt.%, at least 2 wt.%, or at least 3 wt.%.

[0117]

[0122] The polymetallic sulfide concentrate comprises one or more copper sulfide minerals. In some embodiments, the polymetallic sulfide concentrate comprises chalcopyrite (CuFeS2). At least 10%, or at least 25 %, such as at least 50%, of the copper in the polymetallic sulfide concentrate may be present in chalcopyrite. Chalcopyrite-rich sulfide concentrates are commonly processed by pyrometallurgical techniques (smelting and roasting), and direct hydrometallurgical extraction of copper from such resources is known to be challenging due to the refractory nature of this mineral. However, the process disclosed herein has been found effective to achieve high copper recoveries from concentrates comprising chalcopyrite, despite the use of low-intensity oxidation conditions.

[0118]

[0123] In some embodiments, the polymetallic sulfide concentrate further comprises cobalt. In some embodiments, the polymetallic sulfide concentrate comprises cobalt in an amount of at least 0.1 wt.%, such as at least 0.15 wt.%, at least 0.2 wt.%, at least 0.3 wt.%, or at least 0.5 wt.%. Cobalt is typically present in amount of about 10 to 20% of the nickel content, and may thus be present in amounts of up to about 2-3 wt.%.

[0119]

[0124] The polymetallic sulfide concentrate may comprise one or more cobalt sulfide minerals. In some embodiments, the polymetallic sulfide concentrate comprises carrolite (CUC02S4). Carrolite is conventionally processed by pyrometallurgical techniques (roasting) rather than direct hydrometallurgical extraction methods due to its refractory nature. In some embodiments, the polymetallic sulfide concentrate comprises cobalt present in linnaeite (C03S4) and / or cattierite (C0S2). Such cobalt sulfide minerals, which may be intergrown with iron sulfide, are conventionally processed by total pressure oxidative leaching or biological leaching techniques. The process disclosed herein has been found effective to achieve high cobalt recoveries despite the use of low-intensity oxidation conditions.

[0120]

[0125] The polymetallic sulfide concentrate comprises one or more iron sulfide minerals. Such minerals may include one or more of barren sulfides such as pyrite (FeS2), pyrrhotite (Fe(i-X)S; x = 0 to 0.125) and marcasite (FeS2), and value containing iron sulfides such as chalcopyrite. Oxidation of barren iron sulfide minerals to at least some extent is unavoidable in pressure oxidation, increasing consumption of oxygen and forming either additional sulfuric acid or elemental sulfur which has conventionally been considered undesirable due to its propensity for passivation of value metal sulfides.

[0121]

[0126] In some embodiments, the polymetallic sulfide concentrate comprises pyrrhotite, for example in an amount of at least 5 wt.%, or greater than 10 wt.%, or greater than 15 wt.%, such as greater than 20 wt.%. Due to its instability, pyrrhotite is susceptible to partial oxidation to form elemental sulfur whereas the more refractory pyrite tends to fully oxidise directly to sulfate. Under the oxidation conditions disclosed herein, pyrrhotite-containing concentrates can be effectively leached to extract nickel and copper despite oxidising sulfide to elemental sulfur with significant selectivities.

[0122]

[0127] The polymetallic sulfide concentrate may further comprise other metals, for example zinc which may be present in sphalerite (ZnS). Other metal sulfides may be present in amounts too low for recovery, but in principle zinc or other metals may be leached and recovered in the process disclosed herein if present in sufficient concentrations to make this attractive.

[0123] Pressure oxidation in one or more continuous oxidation stages

[0124]

[0128] The process disclosed herein comprises subjecting a slurry comprising the polymetallic sulfide concentrate in an aqueous leach solution to pressure oxidation to leach nickel and copper into the aqueous leach solution. The pressure oxidation is conducted in one or more continuous oxidation stages, and in some embodiments in at least two sequential continuous oxidation stages. After the final continuous oxidation stage, a stream of pregnant leach solution may be separated from residual solids and sent for value metals recovery. In some embodiments, another stream of pregnant leach solution may be separated from slurry in an earlier (upstream) continuous oxidation stage, and sent for value metals recovery.

[0125]

[0129] As used herein, a continuous oxidation stage refers to a pressure oxidation reaction stage conducted in a reaction compartment of an autoclave under continuous operating conditions, whereby polymetallic sulfide concentrate for leaching is fed continuously or semi- continuously to a slurry inventory in the reaction compartment, and leached slurry is removed continuously or semi-continuously from the slurry inventory in the reaction compartment for further leaching in another oxidation stage or for value metals recovery. The slurry inventory is typically well-mixed with individual particles of concentrate retained in the reaction compartment for a distribution of residence times, so that the continuous oxidation stage operates in a mode that approaches or approximates a continuous stirred-tank reactor. A continuous oxidation stage can thus be distinguished from a batch oxidation reaction where the entire slurry inventory is initially charged to an autoclave reaction compartment, subjected to pressure oxidation leaching, and then discharged at the end of the required reaction time. In a continuous oxidation stage, the composition of the slurry inventory, the oxidation conditions and the oxygen-reduction potential may if desired remain approximately constant or within a narrow operating window compared to a batch oxidation reaction.

[0126]

[0130] The aqueous leach solution in the slurry immediately prior to pressure oxidation comprises at least water, and optionally other components such as sulfuric acid, a dispersant such as lignosulfonate, and a defoamer. The aqueous solution may also comprise salts (e.g. sodium, calcium, magnesium, sulfate, chloride, etc) which accumulate in the process liquors to acceptable levels. Due to internal recycles, the aqueous leach solution immediately prior to pressure oxidation may comprise dissolved value metals including nickel, cobalt and copper.

[0127]

[0131] In some embodiments, the aqueous leach solution comprises significant quantities of dissolved copper, such as at least 1 g / L copper, or at least 2 g / L copper, or at least 3 g / L copper, or at least 4 g / L copper, or at least 5 g / L copper, such as at least 6 g / L copper. In some embodiments, the aqueous leach solution comprises dissolved copper in an amount equivalent to at least 30%, or at least 40%, such as at least 50%, of the copper in the polymetallic sulfide concentrate in the slurry (prior to leaching). In some embodiments, the aqueous leach solution comprises dissolved copper in an amount equivalent to at least 40% of copper present in chalcopyrite in the slurry (prior to leaching). The dissolved copper may be provided by recycling a portion of the copper in the one or more streams of pregnant leach solution.

[0128]

[0132] In some embodiments, pressure oxidation is conducted in a single continuous oxidation stage. In other embodiments, pressure oxidation is conducted in multiple successive continuous oxidation stages. In other words, the slurry flows successively through successive continuous oxidation stages in sequence.

[0129]

[0133] The continuous oxidation stages or stages of pressure oxidation are performed in one or more autoclaves. In some embodiments where pressure oxidation includes multiple successive continuous oxidation stages, each continuous oxidation stage is performed in a separate compartment of a single autoclave. In some cases the compartments share a common headspace, and thus may have a similar oxygen partial pressure. In other scenarios, the compartments are separated by a mechanical divider which allows separate control of the gas atmosphere and oxygen partial pressure in sequential compartments.

[0130]

[0134] The oxidant feed to pressure oxidation is typically an O2-containing gas and may suitably be selected from air, O2-enriched air or O2 gas such as cryogenic oxygen gas. Oxidant feeds with high concentrations of O2 may be preferred to minimise the total autoclave pressure and to increase oxygen utilisation, but in practice the O2 concentration may be limited by economic considerations.

[0135] In the processes disclosed herein, the autoclave may be operated at a total pressure of less than 1500 kPa (g), or less than 1000 kPa (g), such as less than 800 kPa (g), which is considerably lower than the operating pressures of many other pressure oxidation leach processes.

[0131]

[0136] Oxidation of sulfide in pressure oxidation is exothermic and thus releases significant heat which must be removed. The exotherm may be controlled by addition of a quench fluid to one or more continuous oxidation stages or by other methods known to those of skill in the art, including internal cooling coils, external jacketed cooling or a flash-recycle step. In the latter scenario, a slurry portion is removed from a continuous oxidation stage, the slurry portion is flash cooled in a flash tank, removing steam from the flash tank, and at least a portion of the flash-cooled slurry portion is recycled to the same or different continuous oxidation stage of pressure oxidation to provide cooling therein. In some embodiments, the flash-cooled slurry portion is divided into solids-rich fraction and a solids-lean fraction, i.e. a clarified pregnant leach solution, preferably by thickening. The solids-rich fraction may be recycled to a continuous oxidation stage of pressure oxidation to provide cooling therein. The clarified pregnant leach solution may then be sent as another stream of pregnant leach solution for value metals recovery.

[0132] Low-intensity continuous oxidation stage

[0133]

[0137] The pressure oxidation includes at least one low-intensity continuous oxidation stage in which at least nickel is leached into the aqueous leach solution conducted at a temperature of between 120°C and 150°C and an oxygen partial pressure of between 50 kPa and 500 kPa. The oxidation-reduction potential may thus be maintained below 520 mV vs Ag / AgCI and typically well below this such as below 500 mV vs Ag / AgCI, or below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, such as below 400 mV vs Ag / AgCI. Typically, at least the first stage of oxidation is a low-intensity continuous oxidation stage.

[0134]

[0138] As is well known, the oxidation-reduction potential (also known as redox potential, ORP, electrochemical potential, or Eh) is a measure of the tendency of a chemical species in solution to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively. The oxidation-reduction potential can be measured with a commercially available ORP meter of the type routinely used in hydrometallurgical process monitoring, and is conventionally measured with reference to a silver-silver chloride (i.e. Ag / AgCI) reference electrode. In oxidative leaching of a slurry comprising iron sulfide minerals, the measured oxidation-reduction potential of the slurry is essentially determined by the ratio of ferric (Fe3+) and ferrous (Fe2+) ions in the aqueous phase. In turn, this ratio is dependent on - and can be empirically linked to - the partial pressure of oxygen in the autoclave compartment. If necessary, the oxidation-reduction potential in the oxidation stage may be analytically measured by sampling the slurry and measuring the oxidation-reduction potential of the liquid phase thereof in an analytical laboratory.

[0135]

[0139] In embodiments where pressure oxidation includes multiple successive continuous oxidation stages, each stage may optionally be a low-intensity continuous oxidation stage as defined herein, or alternatively a subsequent stage may be conducted under higher intensity conditions while preferably remaining at a temperature below 150°C. The oxidation conditions and particularly the oxidation-reduction potential in each stage may be different. For example, as will be further explained hereafter, a low-intensity first continuous oxidation stage may be conducted at a first oxidation-reduction potential sufficiently low that (i) copper in the polymetallic sulfide concentrate remains substantially insoluble (e.g. less than 0.3 g / L, preferably less than 0.1 g / L Cu in the aqueous leach solution) and / or (ii) dissolved copper in the aqueous leach solution, typically introduced by recycling, reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate. However, the first oxidationreduction potential may be sufficiently high that (i) at least nickel, and optionally cobalt (if present), are leached and / or (ii) a portion of barren iron sulfide, in particular pyrrhotite (if present) is oxidised. A subsequent, second continuous oxidation stage may then be conducted at an oxidation-reduction potential sufficiently high that copper and residual nickel and cobalt are leached, preferably to high levels of extraction.

[0136]

[0140] To leach nickel without solubilising copper, the oxygen-reduction potential in some embodiments is below 380 mV vs Ag / AgCI, or below 370 mV vs Ag / AgCI, such as below 360 mV vs Ag / AgCI, for example between 280 and 380 mV vs Ag / AgCI, or between 290 and 360 mV vs Ag / AgCI. To leach both copper and nickel, the oxygen-reduction potential in some embodiments is above 380 mV vs Ag / AgCI, such as between 380 and 520 mV vs Ag / AgCI, or between 400 and 450 mV vs Ag / AgCI.

[0137]

[0141] In some embodiments, as already disclosed herein, the aqueous leach solution that is contacted with the polymetallic sulfide concentrate prior to leaching comprises significant quantities of dissolved copper. It has been found that the presence of dissolved copper in the aqueous leach solution in a low-intensity first continuous oxidation stage may increase the recovery of value metals, including but not limited to copper, from the polymetallic sulfide concentrate in the pressure oxidation process as a whole.

[0138]

[0142] Without wishing to be limited by any theory, it is proposed that dissolved copper initially present in the aqueous leach solution reacts, via a metathesis reaction, with one or more metal sulfide minerals that contain a more reducible metal such as iron, cobalt or nickel, thereby leaching the more reducible metal into solution. As used herein, a metal which is more reducible than copper refers to a metal element with a lower (more negative) standard electrode potential than copper. This metathesis reaction is enabled by comparatively non-oxidising conditions in the low-intensity continuous oxidation stage, and a suitably low oxidationreduction potential may thus be selected. It is further proposed that the metathesis reaction may (i) render the particles of concentrate porous and / or (ii) avoid or mitigate the formation of passivating elemental sulfur on the metal sulfide mineral(s) containing the more reducible metal. One or both of these effects may beneficially enhance extraction of value metals, particularly copper, in a subsequent continuous oxidation stage conducted under more intensive oxidising conditions.

[0139]

[0143] In embodiments where the polymetallic sulfide concentrate comprises chalcopyrite, for example, dissolved copper(ll) in the aqueous leach solution may react with chalcopyrite to form the copper-rich mineral diginite and liberate iron(ll), according to equation (1). Notably, this reaction does not produce elemental sulfur. It is understood that copper metathesis of chalcopyrite thus avoids or mitigates the passivation problems that occur when chalcopyrite initially reacts under more oxidising conditions where iron leaching is accompanied by elemental sulfur formation.

[0140] 3CuFeS2(s) + 6CUSO4(aq) + 4H2O (I) 5CUI.8S(S) + 3FeSO4(aq) + 4 H2SO4(aq) (1)

[0141]

[0144] Similarly, it is proposed that overall nickel and / or cobalt recoveries may be enhanced when a portion of these metals is initially liberated from their respective host sulfide minerals by copper metathesis in the low-intensity continuous oxidation stage.

[0142]

[0145] The first oxidation-reduction potential may be sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage. This does not necessarily require that the first oxidation-reduction potential is sufficiently low to render copper insoluble, and indeed there may be a net positive extraction of copper from the polymetallic sulfide concentrate into the aqueous leach solution. Copper precipitated via metathesis may form easy-to-leach secondary copper sulfides which can then be leached via pressure oxidation reactions in the first stage.

[0143]

[0146] In some embodiments, therefore, the oxygen-reduction potential in the low-intensity continuous oxidation stage is below 450 mV vs Ag / AgCI, such as below 425 mV vs Ag / AgCI, or below 400 mV vs Ag / AgCI, for example between 360 and 425 mV vs Ag / AgCI, or between about 380 and 400 mV vs Ag / AgCI.

[0144]

[0147] The oxygen partial pressure in the low-intensity continuous oxidation stages may be chosen and optionally varied under operator control to achieve the desired oxidationreduction potential, or to operate within a desired oxidation-reduction potential range. In some embodiments, at least one low-intensity continuous oxidation stage is conducted at an oxygen partial pressure of between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, or between 80 kPa and 200 kPa, such as between 100 kPa and 150 kPa. The oxygen partial pressures in successive low-intensity continuous oxidation stages may be the same or different.

[0145]

[0148] The temperature of each low-intensity continuous oxidation stage is between 120°C and 150°C, and may be between 130°C and 147°C, or between 125°C and 135°C, for example between 130°C and 140°C. Under such conditions, elemental sulfur produced in the oxidation process may be molten but of low viscosity. The temperatures in successive low-intensity continuous oxidation stages may be the same or different.

[0146]

[0149] In some embodiments, nickel is leached into the aqueous leach solution in a low- intensity stage, or in each low-intensity stage, under conditions where sulfide in the polymetallic sulfide concentrate is oxidised at least partially to form elemental sulfur as a final product of pressure oxidation, preferably with significant selectivity for elemental sulfur relative to sulfate. The two-electron oxidation of sulfide to elemental sulfur consumes less oxygen, and produces less heat and sulfuric acid to be neutralised, compared to the six-electron oxidation of sulfide to sulfate.

[0147]

[0150] For example, at least 5%, or at least 10%, at least 20%, at least 30%, at least 50%, such as at least 60%, of the sulfide associated with the polymetallic sulfide concentrate may be converted to elemental sulfur (and not further to sulfate in that stage). The elemental sulfur is thus present in the slurry which leaves pressure oxidation and will report to the solid tailings.

[0148]

[0151] In such embodiments, the polymetallic sulfide concentrate may comprise significant amounts of pyrrhotite, for example amounts of greater than 5 wt.%, or greater than 10 wt.%, or greater than 15%, or greater than 20 wt.%, or even greater than 30 wt.%. Without wishing to be limited by any theory, pyrrhotite is believed to oxidise readily to elemental sulfur under the low-intensity oxidation conditions, due to the instability of pyrrhotite, whereas the more refractory pyrite tends to oxidise directly to sulfate. The processes disclosed herein have surprisingly been found particularly useful for recovering value metals from pyrrhotite-rich concentrates despite the high selectivity to elemental sulfur.

[0149]

[0152] The concentration of chloride in the aqueous leach solution (in the slurry immediately prior to pressure oxidation) is preferably low. In some embodiments, the aqueous leach solution comprises less than 4 g / L chloride, preferably less than 2 g / L chloride, most preferably less than 1 g / L chloride. Since low chloride concentrations are preferred, it is typically undesirable to introduce chlorine into the process intentionally, for example via pretreatment of the polymetallic sulfide concentrate with a chlorine-based lixiviant. In some embodiments, therefore, the polymetallic sulfide concentrate is not subjected to a pre-leach treatment with a chloride-containing lixiviant such as chlorine (Ch) or hydrochloric acid (HCI).

[0150] Embodiments

[0151]

[0153] A process 100 for extraction of nickel and copper values from a polymetallic sulfide concentrate, according to some embodiments of the present disclosure, will now be described with reference to Figure 1 .

[0152]

[0154] Polymetallic sulfide flotation concentrate 110 is provided for extraction of metal values therein in a pressure oxidation process. Concentrate 110 comprises nickel, copper, cobalt and iron present in a plurality of sulfide minerals. A portion of the nickel may be present in refractory or passivation-susceptible sulfide minerals such as millerite, violarite and siegenite. At least a portion of the copper may be present in chalcopyrite. At least a portion of the iron may be present in pyrrhotite. Concentrate 110 may comprise pyrrhotite in an amount greater than 5 wt.%, such as greater than 10 wt.% or greater than 20 wt.%.

[0153]

[0155] Concentrate 110 as provided may have a suitable particle size for pressure oxidation leaching, such as a P80 of less than 100 micron, preferably less than 20 micron, or less than 15 micron, for example less than about 10 micron. In some embodiments, however, concentrate 110 has an undesirably large particle size distribution as provided and process 100 comprises a step 112 of grinding concentrate 110 to desired particle size distribution, such as to a P80 of less than 20 micron, or less than 15 micron, for example less than 10 micron.

[0154]

[0156] The resultant finely ground concentrate 114 is then sent to repulping step 116 where it is combined with an aqueous phase to produce slurry 118 with a suitable pulp density for oxidative pressure leaching, such as between 5 wt.% solids and 40 wt.% solids, for example between 10 wt.% solids and 30 wt.% solids. The aqueous phase of slurry 118, which will become the aqueous leach solution in pressure oxidation, comprises at least water, for example supplied by water feed 120. Water feed 120 may be a recycle water stream from an on-site water recovery process and may thus contain dissolved ions such as sodium, calcium, magnesium, sulfate and chloride that accumulate in the water recovery process. Typically, however, a portion of the aqueous phase in slurry 118 is provided via a barren recycle stream produced downstream of pressure oxidation within process 100, for example copper-lean raffinate or spent electrolyte stream 122 from copper recovery or raffinate I barren liquor stream 124 from cobalt and nickel recovery. Water feed 120 may be added only as make-up water to replace water purged or lost from the process, typically with the solid leach residue.

[0155]

[0157] In some embodiments, the aqueous phase of slurry 118 comprises dissolved copper, for example at least 1 g / L copper, or at least 2 g / L copper, or at least 3 g / L copper, or at least 4 g / L copper, or at least 5 g / L copper, such as at least 6 g / L copper. The dissolved copper may be provided by recycling pregnant leach solution 144 via by-pass line 123.

[0156]

[0158] The aqueous phase of slurry 118 may be acidic prior to pressure oxidation. Acid present in the recycle streams may optionally be supplemented by addition of acid stream 126, preferably comprising sulfuric acid. However, in principle the aqueous phase of slurry 118 may be provided by mixing the concentrate with clean water. Slurry 118 will be acidified by in situ generation of sulfuric acid in the first continuous oxidation stage of pressure oxidation, thus providing a slurry comprising concentrate in an acidic leach solution even if the aqueous phase of slurry 118 were non-acidic prior to oxidative leaching.

[0157]

[0159] Preferably, the concentration of chloride in the aqueous phase of slurry 118 is low. The inventors have found that improved copper recoveries can be obtained with low chloride leach solutions in low-intensity pressure oxidation as disclosed herein, and it is proposed that chloride may also contribute to passivation of nickel sulfide minerals. In some embodiments, the aqueous phase of slurry 118 has a chloride concentration of less than 10 g / L chloride, or less than 4 g / L chloride, such as less than about 2 g / L chloride, or less than 1 g / L chloride. Some amount of chloride such as between 0 and 1 g / L may be unavoidable due to soluble chloride in the polymetallic sulfide concentrate, and in some embodiments higher chloride concentrations must be tolerated due to build-up in the recycle stream and the available water make-up. However, it may be preferred in some embodiments to reduce the chloride concentration as far as practically and economically possible.

[0158]

[0160] Since low chloride concentrations are preferred, it is typically undesirable to introduce chloride into process 100 intentionally, for example via pre-treatment of the polymetallic sulfide concentrate with a chlorine-based lixiviant. In some embodiments, therefore, the polymetallic sulfide concentrate is not subjected to a pre-leach treatment with a chlorine-containing lixiviant such as chlorine (CI2) or hydrochloric acid (HCI).

[0159]

[0161] Optionally, one or more functional additives 128 may be added to slurry 118 produced in repulping step 116. Alternatively, such additives may be added directly to a reaction compartment where one or more continuous oxidation stages of the pressure oxidation are conducted. In some embodiments, functional additives 128 comprise a dispersant such as lignosulfonate. In some embodiments, functional additives 128 comprise a defoamer.

[0160]

[0162] Slurry 118 is fed to pressure oxidation step 128 where it is subjected to pressure oxidation in one or more continuous oxidation stages, thereby producing a leached slurry 130 in which the aqueous phase comprises extracted nickel and copper. In some embodiments, pressure oxidation is conducted in a single continuous oxidation stage, for example in a singlecompartment autoclave comprising a mechanical agitator to mix the slurry and oxygen. In other embodiments, pressure oxidation is conducted in two or more sequential continuous oxidation stages as will be further described hereafter.

[0161]

[0163] Oxidant feed 132 is fed to pressure oxidation step 128 to provide oxygen (O2) for oxidising sulfide minerals. Oxidant feed 132 is typically an 02-containing gas and may suitably be selected from air, 02-enriched air or O2 gas such as cryogenic oxygen gas. Preferably, oxidant feed 132 is supplied continuously or semi-continuously as required to replenish O2 consumed in sulfide oxidation reactions, thereby maintaining the partial pressure of O2 (PO2) in the continuous oxidation stage(s) at a desired value or within a desired range.

[0162]

[0164] Oxidation of sulfide minerals in pressure oxidation step 128 may produce an exotherm, releasing significant heat in the continuous oxidation stage(s). The exotherm may be controlled by addition of a quench fluid 134 as required to one or more continuous oxidation stages, thereby maintaining the temperature in the quenched continuous oxidation stage(s) at a desired value or within a desired range. Quench fluid 134 may be a metal-containing recycle stream such as raffinate 122 or 124 and / or pregnant leach solution 144, to avoid dilution of the process liquors, but water may also be used. Alternatively or in addition, the exotherm may be controlled by other methods known to those of skill in the art, including internal cooling coils, external jacketed cooling or a flash-recycle step. In the latter scenario, a slurry portion is removed from a continuous oxidation stage, the slurry portion is flash cooled in a flash tank, removing steam from the flash tank, and at least a portion of the flash-cooled slurry portion is recycled to the same or different continuous oxidation stage of the pressure oxidation step 128 (either directly or by pre-mixing with slurry 118) to provide cooling therein.

[0163]

[0165] Advantageously, the amount of cooling required in pressure oxidation step 128 may be limited in comparison to total oxidation leach processes due to the lower intensity oxidation conditions and reduced conversion of sulfide and elemental sulfur to sulfate.

[0164]

[0166] In pressure oxidation step 128, the oxidative pressure leaching in at least one low- intensity continuous oxidation stage (hereafter, a “low-intensity stage”) is conducted at a temperature of between 120°C and 150°C and an oxygen partial pressure of between 50 kPa and 500 kPa. The reaction conditions within these ranges, together with other factors such as the pulp density, residence time and thus the extent of sulfide oxidation in the polymetallic sulfide concentrate, is controlled to maintain a desirably low oxidation-reduction potential of below 520 mV vs Ag / AgCI, and in some embodiments significantly lower such as below 500 mV vs Ag / AgCI, or below 450 mV vs Ag / AgCI, or below 425 mV vs Ag / AgCI, such as below 400 mV vs Ag / AgCI. In some embodiments, as will be described further hereafter, the oxidationreduction potential is sufficiently low in at least one low-intensity stage that only nickel is leached while copper remains substantially insoluble. In other embodiments, the oxidation- reduction potential is sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate.

[0165]

[0167] Preferably, the oxidation-reduction potential is maintained at a value just sufficient to leach nickel, or both nickel and copper. Without wishing to be limited by any theory, operating in this mode, i.e. avoiding excessively high oxidation-reduction potentials and oxygen partial pressures, is proposed to avoid or significantly mitigate the passivation of one or more nickel sulfides, such as millerite, violarite and siegenite, which limits the nickel extraction efficiency from certain concentrates under higher intensity oxidation conditions. The process disclosed herein may also exploit and amplify the higher extraction kinetics of nickel, cobalt and copper sulfides over the lower oxidation rate of gangue minerals such as pyrite, tellurides, selenides, arsenopyrite, arsenides, and the like. This may limit the leaching of undesired components into the acidic leach solution, with advantages to the overall size and intensity of the process.

[0166]

[0168] In some embodiments, nickel is leached into the aqueous leach solution in at least one low-intensity stage, or in each low-intensity stage, of pressure oxidation step 128, under conditions where sulfide in the polymetallic sulfide concentrate, e.g. pyrrhotite sulfide, is oxidised at least partially to form elemental sulfur as a final product of the pressure oxidation, preferably with significant selectivity for elemental sulfur relative to sulfate. The two-electron oxidation of sulfide to elemental sulfur consumes less oxygen, and produces less heat and sulfuric acid to be neutralised, compared to the six-electron oxidation of sulfide to sulfate.

[0167]

[0169] By producing elemental sulfur within a reaction temperature range of from 120°C to 150°C, the inventors have found that the elemental sulfur product is molten but low in viscosity and readily dispersible within the slurry, for example with the aid of a dispersant. By contrast, at temperatures higher than 150°C, molten sulfur undergoes a phase transition causing a dramatic increase in viscosity and a tendency to foul the process equipment. Without limitation by any theory, it is also believed that maintaining the elemental sulfur product below 150°C may limit or avoid passivation of value sulfide minerals by sulfur coating mechanisms, thus facilitating higher extractions of nickel and other value metals.

[0168]

[0170] In some embodiments, nickel is leached into the aqueous leach solution in at least one low-intensity stage of pressure oxidation step 128 under conditions where dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage.

[0169]

[0171] To achieve the required control of oxidation-reduction potential, lower temperatures and / or lower oxygen partial pressures within the wider ranges may be both sufficient and desirable for value metal extraction. In some embodiments, continuous pressure oxidation in at least one low-intensity stage is conducted at a temperature of below 140°C, such as between 130°C and 147°C, or between 130°C and 140°C. In some embodiments, continuous pressure oxidation in at least one low-intensity stage is conducted at an oxygen partial pressure of below 400 kPa, or between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa.

[0170]

[0172] In some embodiments, the oxidation-reduction potential in at least one low-intensity oxidation stage is analytically measured. The measurement may thus allow the process to be monitored, and if necessary controlled to maintain the oxidation-reduction potential in a desired range. In situ measurement of oxidation-reduction potential may not be feasible and thus the oxidation-reduction potential in the relevant low-intensity stage may be determined by a sampling approach. Suitably, a slurry sample is withdrawn from the reaction compartment, cooled, depressurised, and clarified to provide a substantially solids-free sample for analysis, typically using a commercially available ORP meter of the type routinely used in hydrometallurgical process monitoring. Preferably, the slurry sample is analysed immediately after sampling by rapidly cooling, depressurisation, centrifugation, filtration and ORP measurement, thus minimising changes in the measured oxidation-reduction potential due to continued reactions in the slurry sample. Because the oxidation-reduction potential of the slurry is essentially determined by the ratio of ferric (Fe3+) and ferrous (Fe2+) ions in the aqueous leach solution (and thus only indirectly controlled by PO2), the oxidation reaction potential inside the continuous oxidation stage can be determined with sufficient accuracy by such a sampling approach.

[0171]

[0173] In some embodiments, process 100 may comprise a step of adjusting one or more process parameters within the relevant low-intensity stage, such as oxygen partial pressure, pulp density, or residence time, in response to an analytically determined measurement of oxidation-reduction potential. The oxygen partial pressure impacts the oxidation-reduction potential and reaction kinetics, is readily measured in situ and is typically a variable within operator control. In some embodiments therefore the oxygen partial pressure in at least one low-intensity stage is adjusted in response to an analytically determined measurement of oxidation-reduction potential, for example to maintain the oxidation-reduction potential within a desired range.

[0172]

[0174] However, it should be appreciated that process 100 does not require that oxidationreduction potential is directly measured at all, since the oxidation-reduction potential may be maintained in a desirable range by setting, monitoring and / or controlling other process parameters, and particularly the oxygen partial pressure. Based on operating experience, there is a direct correlation between oxygen partial pressure and oxidation-reduction potential, and process control in some embodiments may therefore be suitably achieved by setting, monitoring and / or controlling oxygen partial pressure within the range of 50 to 500 kPa or preferred sub-ranges thereof. Oxygen partial pressure is routinely measured inside pressure leach autoclaves by commercially available gas sensors.

[0173]

[0175] In some embodiments, at least two continuous oxidation stages, or each continuous oxidation stage of the pressure oxidation is a low-intensity stage as disclosed herein, and thus performed at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa and an oxidation-reduction potential of below 520 mV vs Ag / AgCl. Particularly when the concentrate comprises significant quantities of pyrrhotite, such embodiments may be preferred to maintain and accommodate a significant overall sulfide oxidation selectivity to elemental sulfur, in contrast to total oxidation leach processes where sulfide is substantially oxidised to sulfate after the final stage of pressure oxidation. Thus, for example, sulfide in the polymetallic sulfide concentrate may be converted to elemental sulfur (and not further to sulfate) with a selectivity of at least 10%, or at least 30%, such as at least 50%. In some embodiments, less than 90%, or less than 80%, or less than 70%, such as less than 60%, for example less than 50%, of the sulfur content of the polymetallic sulfide concentrate is oxidised to sulfate in the pressure oxidation. The elemental sulfur product may report to the solid leach residue in the process.

[0174]

[0176] While such arrangements are preferred to avoid excessive production of sulfate, it will be appreciated that it is not essential that each sequential continuous oxidation stage in process 100 is a low intensity process as disclosed herein. For example, pressure oxidation step 128 may in principle comprise a first, low-intensity continuous oxidation stage performed at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa and an oxidation-reduction potential of below 520 mV vs Ag / AgCl, or preferred ranges therein as previously discussed, and a subsequent continuous oxidation stage conducted at higher intensity oxidation conditions, particularly with respect to oxidationreduction potential. Preferably, the temperature is maintained below 150°C in the subsequent stage, with the higher intensity conditions achieved via higher oxidation-reduction potential. One or more advantages, in particular a high extraction efficiency of nickel, or nickel and copper, may be obtained in the overall process due to the control of leaching conditions in the low-intensity first continuous oxidation stage even if the slurry is subsequently subjected to more intense oxidation conditions.

[0175]

[0177] Pressure oxidation step 128 may achieve a high extraction efficiency of nickel, which in some implementations may be higher than is attainable with a conventional medium pressure oxidation or total oxidation approach. In some embodiments, at least 95%, or at least 96%, such as at least 97%, for example about 98% of nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution and thus available for recovery downstream of pressure oxidation.

[0176]

[0178] Pressure oxidation step 128 may also achieve a high extraction efficiency of copper and / or cobalt. In some embodiments, at least 90%, such as at least 95%, or at least 97%, of copper in the polymetallic sulfide concentrate is leached into the aqueous leach solution. Higher copper recoveries may be favoured by using copper metathesis as disclosed herein. In some embodiments, at least 60%, or at least 80%, such as at least 90%, or at least 95%, or at least 98% of cobalt in the polymetallic sulfide concentrate is leached into the aqueous leach solution.

[0177]

[0179] Despite the control of oxidation intensity in pressure oxidation step 128, leached slurry 130 typically comprises excess sulfuric acid produced during oxidation and may thus be sent to partial neutralisation step 136 where base 138, typically a calcium base such as limestone, is added. A portion of the dissolved iron in the aqueous phase may be precipitated in this neutralisation step.

[0178]

[0180] Partially neutralised slurry 140 is then sent to solids separation step 142 where pregnant leach solution 144 is separated from solid leach residue 146 (including any precipitated iron). Separation may be conducted by one or more thickeners, filters or other separation techniques well known to those skilled in the art. The solid leach residue 146 may be washed, e.g. with water, with the washate returned to the slurry in separation to reduce value metal losses to the solid tailings.

[0179]

[0181] Alternately, partial neutralisation step 136 can take place after solids separation step 142. In this case a further solid-liquid separation step may be required to recover any precipitate arising from neutralisation prior to copper recovery.

[0180]

[0182] At least a portion of pregnant leach solution 144 is then sent, via line 121 , to copper recovery step 148 where copper product 150 is recovered, typically by a conventional approach such as solvent extraction followed by electrowinning, thereby producing copper-lean raffinate 152. A copper-lean stream 122, for example a portion of copper-lean raffinate 152, may be recycled to repulping step 116 and / or to form quench fluid 134.

[0181]

[0183] In some embodiments, another portion of pregnant leach solution 144 is recycled, via by-pass line 123, to repulping step 116 to provide dissolved copper in the aqueous leach solution of slurry 118. The desired concentration of copper may be achieved by controlling the ratio of pregnant leach solution 144 recycled via by-pass line 123 relative to that advanced via line 121. In some embodiments, at least 50%, or at least at least 67%, such as at least 75%, for example about 80%, of the copper in pregnant leach solution 144 is recycled.

[0184] Copper-lean raffinate 152 may then be sent to further neutralisation step 154 where base 156, typically a calcium base such as limestone, is added. The pH may be increased sufficiently to facilitate the removal of remaining impurities that may be of concern to the subsequent recovery of cobalt and nickel, such as iron, copper, aluminium, silicon, and / or chromium. Precipitate 157 is recovered by a conventional solid / liquid separation technique and may be at least partially recycled to partial neutralisation step 136 to re-dissolve any coprecipitated value metals. Iron in precipitate 157 will not completely re-dissolve and thus leaves the circuit with solid leach residue 146.

[0182]

[0185] The resultant clarified copper-lean raffinate 155 is sent to cobalt / nickel recovery step 158 where cobalt and nickel are recovered separately or together, for example as mixed cobalt and nickel product 160. This may be achieved by a conventional approach such as solvent extraction and precipitation as hydroxy-sulfate salts, sometimes referred to as Mixed Hydroxide Precipitate. The value metal-lean raffinate 124 may be recycled after suitable water treatment to repulping step 116 and / or to form quench fluid 134.

[0183]

[0186] As will be explained further hereafter, a further stream 164 of pregnant leach solution, e.g. a nickel-rich, copper-lean pregnant leach solution, may optionally be separated from the leach slurry within an early stage of pressure oxidation step 128, and sent directly to further neutralisation step 154 for subsequent nickel recovery in cobalt / nickel recovery step 158, thus by-passing copper recovery step 148.

[0184]

[0187] An autoclave 228 for use in the pressure oxidation step according to some embodiments of the present disclosure will now be described with reference to Figure 2. For example, autoclave 228 may suitably be used in process 100 as previously disclosed herein.

[0185]

[0188] Autoclave 228 comprises at least two reaction compartments configured for conducting sequential continuous oxidation stages of the pressure leaching. Adjacent reaction compartments may be separated by an internal dividing wall, allowing transfer of slurry by, for example, overflow across the dividing wall. As depicted, autoclave 228 comprises first reaction compartment 230 and second reaction compartment 232 separated by internal dividing wall 234. The reaction compartments may share a common gas headspace 236, as depicted, or the headspaces may be separated by a mechanical divider allowing independent control of the gas atmosphere in each compartment. Reaction compartments 230 and 232 are equipped with mechanical agitators 238 and 240, respectively, which are configured to suspend the slurry and mix the slurry with gas in the headspace, thus providing sufficient mass transfer of O2 during oxidative pressure leaching. In some instances agitator 238 and 240 may comprise multiple agitators within the said reaction compartment.

[0189] In use, slurry 118 comprising polymetallic sulfide flotation concentrate 110 is fed continuously or semi-continuously to first reaction compartment 230 where it is subjected to pressure oxidation in a first continuous oxidation stage. The first continuous oxidation stage is a low-intensity stage as disclosed herein. Oxidant feed 132, preferably 02-rich gas, is thus fed into one or more locations in the autoclave, typically into a high shear zone beneath the impeller of agitator 238 to permit design oxygen uptake and to maintain the oxygen partial pressure in first reaction compartment 230 at a desired pressure or pressure range between 50 kPa and 500 kPa. The slurry in first reaction compartment 230 is maintained at a desired temperature or within a desired temperature range between 120°C and 150°C, with the reaction exotherm controlled by providing cooling as required. For example, quench fluid 134 may be added to the slurry. The residence time in the first continuous oxidation stage may be in the range of 0.25 to 3 hours, for example about 1 hour.

[0186]

[0190] Addition of slurry 118 causes the agitated slurry in first reaction compartment 230 to continuously or semi-continuously flow into second reaction compartment 232, e.g. by overflowing internal dividing wall 234 with the slurry level in the second compartment maintained at a lower level to prevent back-mixing. Optionally, a portion of slurry 118 may also be added directly to second reaction compartment 232. The slurry in the second reaction compartment is subjected to pressure oxidation in a second continuous oxidation stage. Leached slurry 130 comprising extracted nickel and copper in the aqueous phase is then continuously or semi-continuously removed from second reaction compartment 232 for further processing, e.g. initially in partial neutralisation step 136. The residence time in the second continuous oxidation stage may be in the range of 0.25 to 5 hours.

[0187]

[0191] Optionally, though not essentially, the second continuous oxidation stage is also a low-intensity stage as disclosed herein. The oxygen partial pressure in the second reaction compartment 232 may be similar to that in the first reaction compartment when there is a common headspace. The temperature of the slurry in the second reaction compartment 232 may be controlled independently of the first reaction compartment 230, but is preferably also maintained at a desired temperature or within a desired temperature range between 120°C an 150°C, with the reaction exotherm controlled by providing cooling as required. For example, quench fluid 135 may be added to the slurry.

[0188]

[0192] The oxidation-reduction potential may be lower in first reaction compartment 230 than in second reaction compartment 232, which may at least in part be because of the lower extent of reaction (i.e. sulfide conversion) in the first continuous oxidation stage. The oxygenreduction potential is sufficiently high that nickel is leached into the acidic leach solution in the first oxidation stage, which may optionally be operated so as to extract at least 10%, such as at least 30%, at least 40%, at least 50%, or at least 60%, e.g. between 20 % and 80 %, of the nickel in the polymetallic sulfide concentrate. Without wishing to be limited by any theory, leaching a portion of nickel in a particularly low potential first continuous oxidation stage may avoid or limit passivation of certain nickel sulfides minerals, allowing a high overall nickel extraction to be achieved in the two-stage process.

[0189]

[0193] In some embodiments, the oxygen-reduction potential in first reaction compartment 230 is sufficiently low that copper in the polymetallic sulfide concentrate remains substantially insoluble. Under such conditions, dissolved copper entering first reaction compartment 230, e.g. in slurry 118 or quench fluid 134, may actually precipitate from solution (i.e. negative copper extraction). In such scenarios, the aqueous phase of slurry 118 in first reaction compartment 230 may be nickel-rich but copper-lean, providing the opportunity to separate and remove a nickel-rich pregnant leach solution to be sent more directly for nickel recovery as will be further described hereafter.

[0190]

[0194] In some embodiments, the oxygen partial pressure in first reaction compartment 230 is thus between 50 kPa and 300 kPa, or between 50 kPa and 200 kPa, such as between 80 kPa and 200 kPa, for example between 100 kPa and 150 kPa . The first oxygen-reduction potential may be below about 380 mV vs Ag / AgCI, or below 370 mV vs Ag / AgCI, such as below 360 mV vs Ag / AgCI, for example between 280 and 380 mV vs Ag / AgCI, or between 290 and 360 mV vs Ag / AgCI.

[0191]

[0195] In other embodiments, the oxidation-reduction potential in first reaction compartment 230 is sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt.

[0192]

[0196] In some embodiments, therefore, the oxygen-reduction potential in in first reaction compartment 230 is below 450 mV vs Ag / AgCI, such as below 425 mV vs Ag / AgCI, or below 400 mV vs Ag / AgCI, for example between 360 and 425 mV vs Ag / AgCI, or between about 380 and 400 mV vs Ag / AgCI.

[0193]

[0197] The oxidation-reduction potential in second reaction compartment 232 is sufficiently high to leach copper and residual nickel from the polymetallic sulfide concentrate into the acidic leach solution. Therefore, a high recovery of both nickel and copper can be obtained in the overall two-stage pressure oxidation step.

[0194]

[0198] In some embodiments, the oxygen-reduction potential in second reaction compartment 232 is above 380 mV vs Ag / AgCI, or above 400 mV vs Ag / AgCI, above 450 mV vs Ag / AgCI, above 500 mV vs Ag / AgCI, or above 520 mV vs Ag / AgCI such as between 400 and 600 mV vs Ag / AgCI, or between 450 and 550 mV vs Ag / AgCI. The oxygen partial pressure in second reaction compartment 232 may be similar to first reaction compartment 230 due to a shared headspace, or it may be higher with other design configurations.

[0195]

[0199] The oxidising conditions in first reaction compartment 230 and second reaction compartment 232 may be controlled according to the principles disclosed herein such that a significant overall sulfide oxidation selectivity to elemental sulfur is provided in pressure oxidation, particularly when the concentrate comprises a significant quantity of pyrrhotite. In some embodiments, less than 90%, or less than 80%, or less than 70%, such as less than 60%, for example less than 50%, of the sulfur content of the polymetallic sulfide concentrate is oxidised to sulfate in the pressure oxidation.

[0196]

[0200] As disclosed herein and depicted in Figure 2, autoclave 228 comprises two continuous oxidation stages. However, it will be appreciated that autoclave 228 may comprise more than two reaction compartments for conducting successive continuous oxidation stages, for example up to six. Autoclave 228 may comprise two or more first reaction compartments 230 operated under conditions to allow nickel extraction while copper remains insoluble and / or copper metathesis occurs, as described herein. Autoclave 228 may comprise two or more second reaction compartments 232 operated under conditions to allow simultaneous nickel and copper extraction, preferably to high extraction levels, as described herein.

[0197]

[0201] A process 300 for extraction of nickel and copper values from a polymetallic sulfide concentrate according to some embodiments of the present disclosure will now be described with reference to Figure 3.

[0198]

[0202] Polymetallic sulfide flotation concentrate 110 may optionally be ground in grinding step 112 and is then sent to repulping step 116 where it is combined with an aqueous phase to produce slurry 118 as described herein with reference to Figure 1. The aqueous phase of slurry 118 may comprise copper-lean raffinate 322 and optionally pregnant leach solution 344 to provide a desired concentration of dissolved copper, with make-up as required from water feed 120, acid 126 and / or one or more functional additives 128 (e.g. a dispersant).

[0199]

[0203] Slurry 118 is then fed to pressure oxidation step 328 where it is subjected to pressure oxidation in two or more continuous oxidation stages, thereby producing a leached slurry 337. In some embodiments, continuous oxidation stages 330 and 332 may be performed in the two reaction compartments of autoclave 228 as disclosed herein with reference to Figure 2. Oxidant gas 333 may be fed to provide O2 to both stages, and quench fluids 334, 335 may be added to one or both stages.

[0200]

[0204] In process 300, slurry portion 370 may be continuously or semi-continuously removed from first continuous oxidation stage 330 and sent to flash tank 372 where it is flashed, removing steam 374 (optionally recovering heat therefrom for use in the process) and producing flash-cooled slurry portion 376. Slurry portion 376 is then sent to separator 378, preferably a thickener, to produce nickel-rich pregnant leach solution 346, preferably as a thickener overflow, and solids-rich fraction 380, preferably as a thickener underflow. For example, solids-rich fraction 380 may have a solids concentration of from 20 to 60 wt.%. The solids-rich fraction is then recycled to first continuous oxidation stage 330 to provide cooling. It will be appreciated that it may be recycled either directly to stage 330 or indirectly, e.g. to repulp step 116, so that it re-enters stage 330 with slurry 118.

[0201]

[0205] Alternatively, flash-cooled slurry portion 376 may be recycled directly or indirectly to first continuous oxidation stage 330 to provide cooling, without thickening it first (dotted line in Figure 3, or indirectly with slurry 118). In either case, the exotherm in stage 330 may be controlled at least in part due to the external flash-cooling of slurry portion 370.

[0202]

[0206] In some embodiments, as already disclosed herein with reference to Figure 2, the oxygen-reduction potential in first continuous oxidation stage 330 is sufficiently low that copper in the polymetallic sulfide concentrate remains substantially insoluble, and any dissolved copper entering stage 330 (e.g. via copper-lean raffinate 322) may actually precipitate from solution (i.e. negative copper extraction). In this scenario, pregnant leach solution 346 comprises nickel and optionally also cobalt if present in the concentrate but is substantially free of copper (e.g. less than 0.1 g / L copper).

[0203]

[0207] In other embodiments, the aqueous leach solution initially comprises dissolved copper and the oxidation-reduction potential in first continuous oxidation stage 330 is sufficiently low that the dissolved copper reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate. However, it is not necessary that copper is insoluble in first continuous oxidation stage 330 and a portion of the copper in the polymetallic sulfide concentrate may in fact be leached into the aqueous leach solution (i.e. positive copper extraction). Typically, only a small fraction of the copper in the polymetallic sulfide concentrate, such as less than 20%, is extracted.

[0204]

[0208] Nickel is typically non-quantitatively leached in the first stage but a significant amount of nickel, such as at least 10%, at least 30%, at least 40%, at least 50%, or at least 60%, for example between 20% and 80% of the nickel in the polymetallic sulfide concentrate is leached in first continuous oxidation stage 330. This provides the opportunity to recover nickel and cobalt while by-passing copper recovery step 348. Moreover, without limitation by theory, it is understood that nickel leaching under the low-intensity conditions of first continuous oxidation stage 330 avoids or limits passivation of nickel sulfide minerals, ultimately facilitating a high recovery in the overall process.

[0209] Nickel-rich pregnant leach solution 346 is sent to neutralisation step 366 where base 338, typically a calcium base such as limestone, is added. Dissolved iron, dissolved copper (if present) and other impurities in the aqueous phase are precipitated in this neutralisation step. Solids in neutralised pregnant leach solution 367, including any slurry solids carried over from thickening and precipitated iron and other impurities, is then removed in solids separation step 382 by one or more thickeners, filters or other separation techniques well known to those skilled in the art. The separated solids may be washed, e.g. with water, with the washate returned to the slurry in separation step 382 to reduce value metal losses. Recovered solids 396 may be discarded to tailings. Alternatively, if recovered solids 396 comprises significant amounts of copper (e.g. when there is net positive extraction of copper in first continuous oxidation stage 330), recovered solids 396 may be sent via line 397 to partial neutralisation step 336 in the copper recovery circuit. The copper therein will re-dissolve into pregnant leach solution 340 for recovery.

[0205]

[0210] Clarified nickel-rich pregnant leach solution 384 is then sent to cobalt / nickel recovery step 386 for recovery of nickel and cobalt. Nickel and cobalt may be precipitated, e.g. by addition of magnesium oxide 388, to produce a Mixed Hydroxide Precipitate product 390. The value metal-lean raffinate 324 may then be further processed for release to process water and / or recycled within process 300 after suitable water treatment, e.g. to repulping step 116 and / or to form quench fluid 334 or 335. Alternatively, cobalt and nickel are recovered separately or together in cobalt / nickel recovery step 386 by another approach such as solvent extraction.

[0206]

[0211] The oxygen-reduction potential in second continuous oxidation stage 332 is sufficiently high to leach both residual nickel and copper in the concentrate, as already disclosed herein with reference to Figure 2. Preferably, at least 95%, or at least 96%, such as at least 97%, or at least 98% of nickel in the polymetallic sulfide concentrate 110 is leached into the aqueous leach solution by the time that leached slurry 337 flows out of pressure oxidation step 328. Preferably, at least 90%, or at least 95%, or at least 97% of copper in the polymetallic sulfide concentrate 110 is leached into the aqueous leach solution when leached slurry 337 flows out of pressure oxidation step 328. Higher copper recoveries may be favoured by using copper metathesis as disclosed herein.

[0207]

[0212] Leached slurry 337, which may comprise most of the leached copper and significant portions of the leached nickel and cobalt dissolved in the aqueous leach solution, is sent to partial neutralisation step 336 where base 338, typically a calcium base such as limestone, is added to neutralise sulfuric acid. A portion of the dissolved iron in the aqueous phase may be precipitated in this neutralisation step. Recovered solids 396 may be added to leach slurry 337, e.g. in partial neutralisation step 336, to re-dissolve copper therein for recovery.

[0213] Partially neutralised slurry 340 is then sent to solids separation step 342 where copper-rich pregnant leach solution 344 is separated from solid leach residue 346 (including any precipitated iron). The separation may be conducted by one or more thickeners, filters or other separation techniques well known to those skilled in the art. The solid leach residue 346 may be washed, e.g. with water, with the washate returned to the slurry in separation step 342 to reduce value metal losses to the solid tailings.

[0208]

[0214] At least a portion of clarified copper-rich pregnant leach solution 344 is then sent, via line 321 , to copper recovery step 348 where copper product 350 is recovered, typically by a conventional approach such as solvent extraction followed by electrowinning, thereby producing copper-lean raffinate 322. Copper-lean raffinate 322 is then recycled to first continuous oxidation stage 330, either directly or indirectly by combination with slurry 118.

[0209]

[0215] In some embodiments, another portion of pregnant leach solution 344 is recycled, via by-pass line 323, to repulping step 116 to provide dissolved copper in the aqueous leach solution of slurry 118. The desired concentration of copper may be achieved by controlling the ratio of pregnant leach solution 344 recycled via by-pass line 323 relative to that advanced via line 321. In some embodiments, at least 50%, or at least at least 67%, such as at least 75%, for example about 80%, of the copper in pregnant leach solution 344 is recycled. Apart from the beneficial effects of copper metathesis, as disclosed herein, copper recycling may advantageously allow copper to build up in copper-rich pregnant leach solution 344, allowing a smaller solvent exchange circuit to be used in copper recovery step 348. This may be particularly useful in embodiments where the polymetallic sulfide concentrate has a low copper content.

[0210]

[0216] The combination of copper-lean raffinate 322 and the recycled portion of pregnant leach solution 344 comprises substantially all of the extracted nickel and cobalt content present in leached slurry 337. All the leached nickel and cobalt present in the aqueous phase of second continuous oxidation stage 332, together with other dissolved impurities, is thus circulated back to first continuous oxidation stage 330 and ultimately removed via nickel-rich pregnant leach solution 346 for recovery in nickel / cobalt recovery step 386.

[0211] EXAMPLES

[0212]

[0217] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein. Materials and methods

[0213]

[0218] For Examples 2-6, the following pressure oxidation procedure was used. The as- received flotation concentrates were reduced in size to the specified P80 employing a ball mill or a stirred media mill or both. Aqueous slurries of the milled concentrates were charged to a 4 litre Parr type autoclave and heated anaerobically under agitation to the required temperature. At the start of the test when the required temperature had been achieved, the agitator tip speed was raised to approximately 4m / sec and a blend of cryogenic gaseous oxygen and nitrogen were admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. A vent control valve was modulated to establish the desired total pressure in the autoclave of 760 kPa(g). The purpose of the nitrogen was to provide a carrier gas to the analysers located on the vent, one of which was an oxygen analyser. The flow of oxygen was adjusted to establish the desired oxygen partial pressure.

[0214]

[0219] Timed kinetic slurry samples were abstracted from the autoclave iso-thermally and once in the sample receiver the isolation valves were closed and the contents of the sample received were rapidly cooled within 10 seconds, depressurised, presented to the centrifuge where the solids and liquids were separated, solids appropriately washed and the centrate clarified in a polishing filtration step. The ORP and pH was determined on the 25°C polished filtrate sample.

[0215] Example 1 (comparative).

[0216]

[0220] A polymetallic sulfide flotation concentrate was provided for leaching experiments in which the primary sulphide minerals comprised pyrite (3.3%), pyrrhotite (<0.5%), pentlandite (14.0%), millerite (4.4%), violarite (1.7%) with major gangue minerals being serpentine (50.5%) and magnesite (9.5%). The concentrate assay is shown in Table 1.

[0217] Table 1.

[0218]

[0221] The flotation concentration was ground to a P80 of 12.5 microns with a rod mill and then a vertical stirred ball mill, repulped to 28% solids by mass in mixtures of tap water and process recycle water at ratios controlled to achieve different chloride concentrations (0.7-6.2 g / L chloride) in the slurry liquid phase. The slurry was then charged to a 3.4 litre Parr type autoclave together with an initial charge of sulfuric acid (c.a. 260 kg I ton) and heated anaerobically under agitation to the required temperature. When the required temperature had been achieved, the agitator tip speed was raised to approximately 4m / sec and gaseous oxygen was admitted to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The vent control valve was closed and oxygen added to achieve the specified operating pressure and oxygen partial pressure. The autoclave was run under pressure control via oxygen addition and no venting occurred during the test.

[0219]

[0222] The slurry was thus subjected to pressure oxidation leaching at 150°C with an oxygen partial pressure of 1000 kPa in the autoclave for two hours. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals is shown in Table 2 for the slurries with different chloride concentrations:

[0220] Table 2.

[0221]

[0223] Unsatisfactory recoveries of nickel and cobalt were obtained in these pressure oxidation leach experiments conducted with medium intensity leach conditions (150°C and 1000 kPa PO2). Increased levels of chloride in the leach were detrimental to Ni and Co recovery. The low nickel recoveries are attributed to passivation of millerite and violarite in the flotation concentrate.

[0222] Example 2 (comparative).

[0223]

[0224] A pyrite-rich polymetallic sulfide flotation concentrate was provided for leaching experiments in which the primary sulphide minerals comprised pyrite (8.6%), pyrrhotite (1.1 %), carrollite (0.6%), pentlandite (31.3%), millerite (2.2%), violarite (2.3%) with a major gangue mineral being magnesite (15.1%). The concentrate assay is shown in Table 3.

[0224] Table 3.

[0225]

[0225] The flotation concentration was ground to a P80 of 10 microns, repulped to 20% solids by mass in synthetic recycle water (pH neutral; assay: 15g / L sodium, 3 g / L magnesium, 40g / L sulfate and 2 g / L chloride) and then subjected to pressure oxidation at 140°C with an oxygen partial pressure of 120-130 kPa in the autoclave. The electrochemical potential increased to 475 mV (Ag / AgCI) over 3 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals and the sulfur deportment / speciation is shown in Figure 4 (results over time) and Table 4 (final results):

[0226] Table 4.

[0227]

[0226] Reasonable recoveries of nickel, cobalt and copper were obtained in this pressure oxidation leach experiment at low temperature and oxygen partial pressure conditions which resulted in low ORP. The nickel extraction was high despite the presence of millerite and violarite in the flotation concentrate. However, the oxidation of sulfur species to sulfate was near-quantitative (97%), despite the low-intensity conditions, due to the low pyrrhotite content in the starting concentrate. Without wishing to be limited by any theory, it is understood that pyrite tends to oxidise directly to sulfate when oxidised.

[0228] Example 3.

[0229]

[0227] A polymetallic sulfide flotation concentrate was provided for leaching experiments in which the primary sulphide minerals comprised pyrite (3.6%), pyrrhotite (38.6%), chalcopyrite (4.5%), pentlandite (31.1%), millerite (nil%), violarite (1.6%) with a major gangue mineral being magnesite (5.1%). The concentrate assay is shown in Table 5.

[0230] Table 5.

[0231]

[0228] The flotation concentration was ground to a P80 of 10 microns, repulped to 20% solids by mass in synthetic recycle water (pH neutral; assay: 15g / L sodium, 3 g / L magnesium, 40g / L sulfate and 2 g / L chloride) and then subjected to pressure oxidation at 135°C with an oxygen partial pressure of 160-180 kPa in an autoclave. The electrochemical potential increased to 497 mV (Ag / AgCI) over 2.5 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals and the sulfur deportment / speciation is shown in Figure 5 (results over time) and Table 6 (final results):

[0232] Table 6.

[0233]

[0229] Acceptable recoveries of nickel and cobalt and moderate recovery of copper were obtained in this pressure oxidation leach experiment at low temperature and oxygen partial pressure conditions which resulted in low ORP and low conversion of sulfur species to sulfate.

[0234] Example 4.

[0235]

[0230] A pressure oxidation leach experiment was performed with the same polymetallic sulfide flotation concentrate as used in Example 3.

[0236]

[0231] The flotation concentration was ground to a P80 of 10 microns, repulped to 20% solids by mass in synthetic recycle water (pH neutral; assay: 15g / L sodium, 3 g / L magnesium, 40g / L sulfate and 2 g / L chloride) and then subjected to pressure oxidation at 130°C with an oxygen partial pressure of 175-186 kPa in an autoclave. The electrochemical potential increased to 483 mV (Ag / AgCI) over 2.5 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals and the sulfur deportment / speciation is shown in Figure 6 (results over time) and Table 7 (final results):

[0237] Table 7.

[0238]

[0232] Acceptable recoveries of nickel and cobalt and moderate recovery of copper were obtained in this pressure oxidation leach experiment at low temperature and oxygen partial pressure conditions which resulted in low ORP and low conversion of sulfur to sulfate. Example 5.

[0239]

[0233] A pressure oxidation leach experiment was performed with the same polymetallic sulfide flotation concentrate as used in Example 3.

[0240]

[0234] The flotation concentration was ground to a P80 of 10 microns and repulped to 20% solids by mass in chloride-free synthetic recycle water (pH neutral; assay: 15g / L sodium, 3 g / L magnesium, 40g / L sulfate and 0 g / L chloride). The slurry was then subjected to pressure oxidation at 135°C with an oxygen partial pressure of 150-170 kPa in an autoclave. The electrochemical potential increased to 503 mV (Ag / AgCI) over 2.5 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The chloride concentration in the slurry liquid phase after the leach was 0.4 g / L. The extraction of metals and the sulfur deportment / speciation is shown in Figure 7 (results over time) and Table 8 (final results):

[0241] Table 8.

[0242]

[0235] Acceptable recoveries of nickel, cobalt and copper were obtained in this pressure oxidation leach experiment at low temperature and oxygen partial pressure conditions which resulted in low ORP and low conversion of sulfur species to sulfate.

[0243] Example 6.

[0244]

[0236] A polymetallic sulfide flotation concentrate was provided for leaching experiments in which the primary sulphide minerals comprised pyrite (2.7%), pyrrhotite (52.2%), chalcopyrite (1.2%), pentlandite (23.3%), millerite (0.2%), violarite (0.2%) with low magnesite (0.7%). The concentrate assay is shown in Table 9.

[0245] Table 9.

[0246]

[0237] The flotation concentration was ground to a P80 of 10 microns, repulped to 20% solids by mass in synthetic recycle water (pH neutral; assay: 22g / L sodium, 3 g / L magnesium, 60g / L sulfate and 2 g / L chloride) and then subjected to pressure oxidation at 135°C with an oxygen partial pressure of 165-180 kPa in an autoclave. The electrochemical potential increased to 495 mV (Ag / AgCI) over 2.5 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals and the sulfur deportment / speciation is shown in Figure 8 (results over time) and Table 9 (final results):

[0247] Table 9.

[0248]

[0238] Acceptable recoveries of nickel and cobalt and moderate recovery of copper were obtained in this pressure oxidation leach experiment at low temperature and oxygen partial pressure conditions which resulted in low ORP and low conversion of sulfur species to sulfate.

[0249] Example 1.

[0250]

[0239] A process 300 as disclosed herein was simulated in a batch-continuous experimental study using the same polymetallic sulfide flotation concentrate as used in Example 3. The experimental description below should be read with reference to the description of process 300 and Figure 3.

[0251]

[0240] The flotation concentrate was ground to a P80 of 10 microns using a ball mill followed by a stirred media mill. Slurry 118 was prepared by combining the ground concentrate at 20% solids by mass in an aqueous liquor simulating the composition obtained in repulping step 116 when recycling copper-lean raffinate 322 in process 300, and making up with process water. The chloride concentration was about 2 g / L in the aqueous phase of slurry 118.

[0252]

[0241] The slurry was initially charged and then flowed continuously into a 4 litre Parr type autoclave and subjected to pressure oxidation in a continuous oxidation stage simulating first continuous oxidation stage 330 of process 300. The slurry flow rate was set to provide a residence time of about 60 minutes over 760 minutes of continuous leaching, the agitator tip speed was approximately 4m / sec, the temperature was maintained at 135°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of 97-105 kPa in the autoclave at total pressure of 720 kPa (g). The oxidation reduction potential (determined by sampling and measuring the leached slurry as disclosed below) was thus maintained in the range of 360- 380 mV (Ag / AgCI). The pH was in the range of about 1.0 - 1 .8.

[0253]

[0242] Slurry continuously flowing out of the autoclave was periodically sampled and analysed to determine the extent of oxidation and metal extraction in simulated first continuous oxidation stage 330. The contents of the sample received were rapidly cooled within 10 seconds, depressurised, presented to the centrifuge where the solids and liquids were separated, solids appropriately washed and the centrate clarified in a polishing filtration step. The ORP and pH were determined on the 25°C polished filtrate sample. The results over time are shown in Figure 9 with steady state conditions summarised in Table 10:

[0254] Table 10. a negative Cu extraction indicates that Cu dissolved in the aqueous phase of slurry 118 precipitated during pressure oxidation.

[0255]

[0243] Much of the O2 consumption in simulated first continuous oxidation stage 330 was directed to oxidation of pyrrhotite, with only about 20-25 % nickel extraction. Nevertheless, nickel can be recovered from this stage per the process 300 design due to (i) recycling of copper-lean raffinate 322 containing nickel extracted in second continuous oxidation stage 332, and (ii) the very low copper concentration (<0.1 g / L) caused by copper precipitation under the low ORP conditions.

[0256]

[0244] The same slurry feed was used to simulate the second stage of oxidation. The slurry was initially charged and then flowed continuously into a 4 litre Parr type autoclave and subjected to pressure oxidation in a continuous oxidation stage simulating second continuous oxidation stage 332 of process 300. The slurry flow rate was set to provide a residence time of about 180 minutes (simulating a residence time of 120 minutes in second stage oxidation) over 760 minutes of continuous leaching (about 4 reactor turnovers, sufficient to approximate steady state), the agitator tip speed was approximately 4m / sec, the temperature was maintained at 135°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of about 105 kPa in the autoclave at total pressure of 720 kPa (g). The electrochemical potential (determined by sampling and measuring the leached slurry as disclosed herein) was thus maintained in the range of 470-480 mV (Ag / AgCI). The pH was in the range of about 0.9 - 1 .0.

[0257]

[0245] Leached slurry continuously flowing out of the autoclave was periodically sampled and analysed (as described above) to determine the extent of oxidation and metal extraction in simulated second continuous oxidation stage 332. The results over time are shown in Figure 10 with steady state conditions summarised in Table 11 :

[0258] Table 11.

[0259]

[0246] It is evident from the results that very high extractions of nickel and cobalt, and moderately high extraction of copper, was obtained in the two stage process, despite the incomplete oxidation of sulfide, resultant production of elemental sulfur and the low-intensity oxidation conditions.

[0260] Example 8 (comparative)

[0261]

[0247] A batch-continuous experimental study was conducted using the same polymetallic sulfide flotation concentrate as used in Example 3.

[0262]

[0248] The flotation concentrate was ground to a P90 of 10 microns using a ball mill followed by a stirred media mill. Slurry was prepared by combining the ground concentrate at 20% solids by mass in an aqueous liquor simulating recycled copper lean raffinate 322 after solvent extraction (containing nickel sulfate, no copper added), and making up with process water. No chloride was added to the aqueous phase.

[0249] The slurry was initially charged and then flowed continuously into a 4 litre Parr type autoclave and subjected to pressure oxidation in a continuous oxidation stage. The slurry flow rate was set to provide a residence time of about 120 minutes over 720 minutes of continuous leaching, the agitator tip speed was approximately 4m / sec, the temperature was maintained at 145°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of 70-90 kPa in the autoclave at total pressure of 820 kPa (g). The oxidation reduction potential (determined by sampling and measuring the leached slurry as disclosed below) was maintained in the range of 530-600 mV (Ag / AgCI). The pH was in the range of about 0.4 to 0.6.

[0263]

[0250] Slurry continuously flowing out of the autoclave was periodically sampled and analysed to determine the extent of oxidation and metal extraction. The contents of the sample received were rapidly cooled within 10 seconds, depressurised, presented to the centrifuge where the solids and liquids were separated, solids appropriately washed and the centrate clarified in a polishing filtration step. The ORP and pH were determined on the 25°C polished filtrate sample. The results over time are shown in Figure 11 with steady state conditions summarised in Table 12:

[0264] Table 12.

[0265]

[0251] Under these conditions, only 95% of nickel and 85% of copper in the polymetallic sulfide concentrate was recovered, while sulfide oxidation primarily formed sulfate.

[0266] Example 9.

[0267]

[0252] A process 300 as disclosed herein was simulated in a batch-continuous experimental study using the same polymetallic sulfide flotation concentrate as used in Example 3. The experimental description below should be read with reference to the description of process 300 and Figure 3.

[0268]

[0253] The flotation concentrate was ground to a P90 of 10 microns using a ball mill followed by a stirred media mill. Slurry 118 was prepared by combining the ground concentrate at 20% solids by mass in an aqueous liquor simulating the composition obtained in repulping step 116 when recycling a portion of pregnant leach solution 344 together with copper-lean raffinate 322, and making up with process water. The aqueous liquor contained 6.4 g / L copper, corresponding to just over 50% of the copper in the concentrate to be leached, plus nickel. No chloride was added to the aqueous phase of slurry 118.

[0269]

[0254] The slurry was initially charged and then flowed continuously into a 4 litre Parr type autoclave and subjected to pressure oxidation in a continuous oxidation stage simulating first continuous oxidation stage 330 of process 300. The slurry flow rate was set to provide a residence time of about 60 minutes over 760 minutes of continuous leaching, the agitator tip speed was approximately 4m / sec, the temperature was maintained at 145°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of 40-60 kPa in the autoclave at total pressure of 820 kPa (g). The oxidation reduction potential (determined by sampling and measuring the leached slurry as disclosed below) was thus maintained in the range of 380- 400 mV (Ag / AgCI). The pH was in the range of about 0.8 to 1.1.

[0270]

[0255] Slurry continuously flowing out of the autoclave was periodically sampled and analysed to determine the extent of oxidation and metal extraction in simulated first continuous oxidation stage 330. The contents of the sample received were rapidly cooled within 10 seconds, depressurised, presented to the centrifuge where the solids and liquids were separated, solids appropriately washed and the centrate clarified in a polishing filtration step. The ORP and pH were determined on the 25°C polished filtrate sample. The results over time are shown in Figure 12 with steady state conditions summarised in Table 13:

[0271] Table 13.

[0272]

[0256] Much of the O2 consumption in simulated first continuous oxidation stage 330 was directed to oxidation of pyrrhotite, mainly to elemental sulfur with only minor oxidation to sulfate. Only about 36 % of nickel, 13% of cobalt and 13% of copper was extracted from the feed concentrate.

[0257] The continually discharging slurry from the 4 litre Parr type autoclave was subjected to a thickening step and 60% of the thickener overflow was removed and replaced with process water such that the blend comprised 1.7 g / L magnesium and 15.7% solids. This slurry was initially charged to the 4 litre Parr type autoclave to commence the leach, and after 1 hour continuous flow of the same slurry commenced. The slurry was thus subjected to pressure oxidation in a continuous oxidation stage simulating second continuous oxidation stage 332 of process 300. The slurry flow rate was set to provide an arithmetic average residence time of about 120 minutes. This was continued for 840 minutes of continuous leaching (about 7 reactor turnovers, sufficient to approximate steady state). The agitator tip speed was approximately 4m / sec, the temperature was maintained at 145°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of about 200 kPa in the autoclave at total pressure of 820 kPa (g). The electrochemical potential (determined by sampling and measuring the leached slurry as disclosed herein) was thus maintained in the range of 560-660 mV (Ag / AgCI). The pH was in the range of about 0.25 - 0.35 corresponding to a free acid concentration of approximately 39 g / L H2SO4.

[0273]

[0258] Leached slurry continuously flowing out of the autoclave was periodically sampled and analysed (as described above) to determine the extent of oxidation and metal extraction in simulated second continuous oxidation stage 332. The results over time are shown in Figure 13 with steady state conditions summarised in Table 14:

[0274] Table 14.

[0275]

[0259] In this two-stage leach process, over 98% of each of nickel, cobalt and copper in the polymetallic sulfide concentrate was recovered, a significant improvement over Example 8 which was conducted at the same temperature and for a similar total leach time. The improvement is attributed to the low-intensity conditions in the first continuous oxidation stage and the presence of a significant quantity of dissolved copper in the aqueous leach solution prior to leaching.

[0260] Without wishing to be bound by any theory, it is understood that dissolved copper initially present in the aqueous leach solution reacts, via a metathesis reaction with metal sulfide mineral(s) that contain a more reducible metal such as iron, cobalt or nickel, thereby liberating the more reducible metal. In particular, dissolved copper(ll) may react with chalcopyrite to liberate iron(ll) via equation (1). The metathesis reaction, which is facilitated by the comparatively non-oxidising conditions (380-400 mV vs Ag / AgCI) in the first continuous oxidation stage, may (i) render the concentrate particles porous and / or (ii) avoid or reduce the formation of passivating elemental sulfur on the metal sulfide mineral(s) containing the more reducible metal, particularly chalcopyrite. One or both of these effects may beneficially enhance subsequent extraction of value metals, particularly copper, in the second continuous oxidation stage conducted under more intensive oxidising conditions.

[0276] Example 10.

[0277]

[0261] A process 300 as disclosed herein was simulated in a batch-continuous experimental study using the same polymetallic sulfide flotation concentrate as used in Example 3. The experimental description below should be read with reference to the description of process 300 and Figure 3.

[0278]

[0262] The flotation concentrate was ground to a P80 of less than 10 microns using a ball mill followed by a stirred media mill. Slurry 118 was prepared by combining the ground concentrate at 20% solids by mass in an aqueous liquor simulating the composition obtained in repulping step 116 when recycling a portion of pregnant leach solution 344 together with copper-lean raffinate 322 in process 300, and making up with process water. The aqueous liquor contained 3.2 g / L copper, corresponding to just under 50% of the copper in the concentrate to be leached, and 20-22 g / L nickel. No chloride was added to the aqueous phase of slurry 118.

[0279]

[0263] The slurry was initially charged and then flowed continuously into a 4 litre Parr type autoclave and subjected to pressure oxidation in a continuous oxidation stage simulating first continuous oxidation stage 330 of process 300. The slurry flow rate was set to provide a residence time of about 60 minutes over 720 minutes of continuous leaching, the agitator tip speed was approximately 4m / sec, the temperature was maintained at 145°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of about 80 kPa in the autoclave at total pressure of 1035 kPa (g). The oxidation reduction potential (determined by sampling and measuring the leached slurry as disclosed below) was thus maintained in the range of 380- 400 mV (Ag / AgCI). The pH was in the range of about 0.7 to 0.9.

[0264] Slurry continuously flowing out of the autoclave was periodically sampled and analysed to determine the extent of oxidation and metal extraction in simulated first continuous oxidation stage 330. The contents of the sample received were rapidly cooled within 10 seconds, depressurised, presented to the centrifuge where the solids and liquids were separated, solids appropriately washed and the centrate clarified in a polishing filtration step. The ORP and pH were determined on the 25°C polished filtrate sample. The results over time are shown in Figure 14 with steady state conditions summarised in Table 15:

[0280] Table 15.

[0281]

[0265] Much of the O2 consumption in simulated first continuous oxidation stage 330 was directed to oxidation of pyrrhotite, mainly to elemental sulfur with only minor oxidation to sulfate. Only about 41 % of nickel, 20% of cobalt and 16% of copper was extracted from the feed concentrate.

[0282]

[0266] The continually discharging slurry from the 4 litre Parr type autoclave was subjected to a thickening step and 60% of the thickener overflow was removed and replaced with process water such that the blend comprised 3.4 g / L magnesium and 7.2 g / L sodium and 12% solids. This slurry was initially charged to the 4 litre Parr type autoclave to commence the leach, and after 1 hour continuous flow of the same slurry commenced. The slurry was thus subjected to pressure oxidation in a continuous oxidation stage simulating second continuous oxidation stage 332 of process 300. The slurry flow rate was set to provide an arithmetic average residence time of about 60 minutes. This was continued for 600 minutes of continuous leaching (about 10 reactor turnovers, sufficient to approximate steady state). The agitator tip speed was approximately 4m / sec, the temperature was maintained at 145°C, and a blend of cryogenic gaseous oxygen and nitrogen was admitted under controlled mass flow conditions to the autoclave head space and inductively drawn into the slurry by the rotating impeller. The gas flows were regulated to provide an oxygen partial pressure of about 300 kPa in the autoclave at total pressure of 1050 kPa (g). The electrochemical potential (determined by sampling and measuring the leached slurry as disclosed herein) was thus maintained in the range of 525-540 mV (Ag / AgCI). The pH was in the range of about 0.3 - 0.5 corresponding to a free acid concentration of approximately 35 g / L H2SO4.

[0267] Leached slurry continuously flowing out of the autoclave was periodically sampled and analysed (as described above) to determine the extent of oxidation and metal extraction in simulated second continuous oxidation stage 332. The results over time are shown in Figure 15 with steady state conditions summarised in Table 16:

[0283] Table 16.

[0284]

[0268] This example demonstrates good recoveries of nickel and copper in a two-stage process with low-intensity first stage and copper recycle. However, compared to Example 9 the recoveries were slightly reduced. Without limitation by theory, this is attributed to lower total leach time of 2 hours (vs 3 hours in Example 9), slightly lower ORP in the second stage leach and / or lower amount of recycled copper in aqueous leach solution.

[0285] Example 11 (Comparative).

[0286]

[0269] This example used the same polymetallic sulfide flotation concentrate as used in Example 3. The ground flotation concentration was repulped to 20.5% solids by mass in synthetic recycle water (pH neutral; assay: 16g / L sodium, 3 g / L magnesium, 47g / L sulfate and 2 g / L chloride) and then subjected to pressure oxidation at 185°C with an oxygen partial pressure of 600-750 kPa in the autoclave, in a batch reaction. The electrochemical potential increased to 560 mV (Ag / AgCI) over 2.5 hours of leaching. The pregnant leach solution was then cooled and separated from the solid residue. The extraction of metals and the sulfur deportment / speciation is shown in Figure 16 (results over time) and Table 17 (final results):

[0287] Table 17.

[0270] Good recoveries of nickel, cobalt and copper were obtained in this pressure oxidation leach experiment conducted at high temperature (185°C). However, elemental sulfur is viscous and difficult to disperse with lignosulfonates at such temperatures. The solid residue included agglomerated lumps of unleached concentrate. Before complete oxidation to sulfate, the elemental sulfur agglomerates the minerals, forms sulfur pellets, and adheres to autoclave internals, which is problematic for continuous operation.

[0288]

[0271] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

Claims

Claims1. A process for extraction of nickel and copper values from a polymetallic sulfide concentrate, the process comprising: providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution, wherein the polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals and wherein the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 5 wt.%; subjecting the slurry to pressure oxidation in one or more continuous oxidation stages to leach nickel and copper into the aqueous leach solution; separating one or more streams of pregnant leach solution from the slurry; and recovering nickel and copper from the one or more streams of pregnant leach solution, wherein at least nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and an oxidationreduction potential of below 520 mV vs Ag / AgCI, and wherein at least a portion of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage.

2. The process according to claim 1 , wherein the polymetallic sulfide concentrate comprises at least 5 wt.% nickel.

3. The process according to claim 1 or claim 2, wherein the polymetallic sulfide concentrate comprises nickel present in one or more nickel sulfide minerals selected from millerite, violarite and siegenite.

4. The process according to any one of claims 1 to 3, wherein the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 20 wt.%.

5. The process according to any one of claims 1 to 4, wherein at least 10% of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the at least one low-intensity continuous oxidation stage.

6. The process according to any one of claims 1 to 5, wherein nickel is leached into the aqueous leach solution in at least one low-intensity continuous oxidation stage at an oxidation-reduction potential of below 450 mV vs Ag / AgCl.

7. The process according to any one of claims 1 to 6, wherein continuous pressure oxidation in at least one low-intensity continuous oxidation stage is conducted at a temperature of between 130°C and 147°C.

8. The process according to any one of claims 1 to 7, wherein at least 96% of nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

9. The process according to any one of claims 1 to 8, wherein at least 95% of copper in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

10. The process according to any one of claims 1 to 9, wherein at least 10% of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the pressure oxidation.

11. The process according to any one of claims 1 to 10, wherein the aqueous leach solution comprises less than 2 g / L chloride.

12. The process according to any one of claims 1 to 11 , wherein the slurry is subjected to pressure oxidation in at least two sequential continuous oxidation stages comprising: a low-intensity first continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa, and a first oxidation-reduction potential of below 520 mV vs Ag / AgCl, wherein the first oxidation-reduction potential is sufficiently high to leach at least a portion of the nickel in the polymetallic sulfide concentrate into the aqueous leach solution in the low-intensity first continuous oxidation stage; and a second continuous oxidation stage conducted at a second oxygen-reduction potential sufficiently high to leach copper and residual nickel from the polymetallic sulfide concentrate into the aqueous leach solution.

13. The process according to claim 12, wherein the first oxidation-reduction potential is sufficiently low that copper in the polymetallic sulfide concentrate remains substantially insoluble.

14. The process according to claim 12 or claim 13, wherein the aqueous leach solution comprises dissolved copper, and wherein the first oxidation-reduction potential is sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage.

15. The process according to any one of claims 12 to 14, comprising separating a nickel-rich pregnant leach solution from slurry in the low-intensity first continuous oxidation stage, and recovering nickel from the nickel-rich pregnant leach solution.

16. The process according to claim 15, comprising separating a copper-rich pregnant leach solution from slurry subjected to pressure oxidation in the second continuous oxidation stage to produce a copper-rich pregnant leach solution, and recovering copper from the copper-rich pregnant leach solution to produce a copper-lean raffinate containing nickel.

17. The process according to claim 16, comprising recycling the copper-lean raffinate and optionally a portion of the copper-rich pregnant leach solution to the low-intensity first continuous oxidation stage.

18. The process according to claim 16 or claim 17, comprising recycling at least 50% of the copper in the copper-rich pregnant leach solution to the low-intensity first continuous oxidation stage, thereby providing dissolved copper in the aqueous leach solution.

19. The process according to any one of claims 12 to 18, comprising removing a slurry portion from the low-intensity first continuous oxidation stage, flash cooling the slurry portion, dividing the slurry portion into a solids-rich fraction and a nickel-rich pregnant leach solution and recycling the solids-rich fraction to the low-intensity first continuous oxidation stage to provide cooling therein.

20. A process for extraction of nickel and copper values from a polymetallic sulfide concentrate, the process comprising:providing a slurry comprising a polymetallic sulfide concentrate in an aqueous leach solution, wherein the polymetallic sulfide concentrate comprises nickel, copper and iron present in a plurality of sulfide minerals; subjecting the slurry to pressure oxidation in at least two sequential continuous oxidation stages to leach nickel and copper into the aqueous leach solution; separating one or more streams of pregnant leach solution from the slurry; recovering nickel and copper from the one or more streams of pregnant leach solution; and recycling a portion of the copper in the one or more streams of pregnant leach solution to provide dissolved copper in the aqueous leach solution, wherein at least nickel is leached into the aqueous leach solution in a low-intensity first continuous oxidation stage conducted at a temperature of between 120°C and 150°C, an oxygen partial pressure of between 50 kPa and 500 kPa and a first oxidation-reduction potential of below 520 mV vs Ag / AgCI, and wherein copper and residual nickel are leached from the polymetallic sulfide concentrate into the aqueous leach solution in a second continuous oxidation stage.

21. The process according to claim 20, wherein the first oxidation-reduction potential is below 425 mV vs Ag / AgCI.

22. The process according to claim 20 or claim 21, wherein the first oxidation-reduction potential is sufficiently low that dissolved copper in the aqueous leach solution reacts by metathesis with one or more sulfide minerals in the polymetallic sulfide concentrate, thereby liberating one or more of iron, nickel or cobalt in the low-intensity first continuous oxidation stage.

23. The process according to any one of claims 20 to 22, wherein at least 50% of copper in the one or more streams of pregnant leach solution is recycled to provide dissolved copper in the aqueous leach solution.

24. The process according to any one of claims 20 to 23, wherein the aqueous leach solution comprises recycled copper in an amount of at least 2 g / L copper.

25. The process according to any one of claims 20 to 24, wherein the polymetallic sulfide concentrate comprises at least 5 wt.% nickel.

26. The process according to any one of claims 20 to 25, wherein at least a portion of the nickel is present in one or more nickel sulfide minerals selected from millerite, violarite and siegenite.

27. The process according to any one of claims 20 to 26, wherein at least a portion of the copper in the polymetallic sulfide concentrate is present in chalcopyrite.

28. The process according to any one of claims 20 to 27, wherein the polymetallic sulfide concentrate comprises pyrrhotite in an amount greater than 5 wt.%, and wherein at least a portion of sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the low-intensity first continuous oxidation stage.

29. The process according to any one of claims 20 to 28, wherein at least 10% of the sulfide in the polymetallic sulfide concentrate is oxidised to form elemental sulfur in the pressure oxidation.

30. The process according to any one of claims 20 to 29, wherein continuous pressure oxidation in the low-intensity first continuous oxidation stage is conducted at a temperature of between 130°C and 147°C.

31. The process according to any one of claims 20 to 30, wherein at least 96% of nickel in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

32. The process according to any one of claims 20 to 31, wherein at least 95% of copper in the polymetallic sulfide concentrate is leached into the aqueous leach solution in the pressure oxidation.

33. The process according to any one of claims 20 to 32, wherein the aqueous leach solution comprises less than 2 g / L chloride.

34. The process according to any one of claims 20 to 33, comprising separating a nickel-rich pregnant leach solution from slurry in the low-intensity first continuous oxidation stage, and recovering nickel from the nickel-rich pregnant leach solution.

35. The process according to claim 34, comprising removing a slurry portion from the low- intensity first continuous oxidation stage, flash cooling the slurry portion, dividing the slurryportion into a solids-rich fraction and the nickel-rich pregnant leach solution and recycling the solids-rich fraction to the low-intensity first continuous oxidation stage to provide cooling therein.

36. The process according to any one of claims 20 to 35, comprising separating a copper-rich pregnant leach solution from slurry subjected to pressure oxidation in the second continuous oxidation stage to produce a copper-rich pregnant leach solution, and recovering copper from the copper-rich pregnant leach solution to produce a copper-lean raffinate containing nickel.

37. The process according to claim 36, comprising recycling the copper-lean raffinate and a portion of the copper-rich pregnant leach solution to the low-intensity first continuous oxidation stage.

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