WO2010135819A1 - Processes for extraction of nickel with iron-complexing agent - Google Patents

Abstract

The invention provides, in part, a process for extracting nickel from a source material including iron and nickel, by contacting the source material (e.g, an ultramafic ore) with an aqueous ammonia solution containing an iron-complexing agent (e.g., citrate) under suitable conditions, thereby extracting the nickel. The aqueous ammonia solution may optionally contain a sulfur-containing reductant, such as thiosulfate.

Description

PROCESSES FOR EXTRACTION OF NICKEL WITH IRON-COMPLEXING AGENT

FIELD OF INVENTION

[0001] The present invention relates to processes for extracting base metals. More specifically, the invention relates to processes for extracting nickel from a nickel- containing source material.

BACKGROUND OF THE INVENTION

[0002] Current technology for extracting nickel from various sources includes pyrometallurgical smelting of sulfide mineral concentrates, hydrometallurgical leaching of lateritic deposits in sulfuric acid, and chemical reduction of nickel in lateritic deposits at high temperature followed by leaching in ammonia, originally proposed by Caron, US Patent No. 1,487,145. Each of these processes suffer from technical or commercial drawbacks. For example, smelting involves fine milling and flotation to obtain a sulfide mineral concentrate, which is generally shipped to an off-site smelter, resulting in large transportation costs. Smelting is also generally not useful for ores containing high levels of magnesia. Leaching in sulfuric acid is not economic for highly acid consuming ore types, such as ultramafic deposits, due to high acid consumption and the generation of large amounts of magnesium and aluminium salts that require disposal. The Caron process is highly energy-intensive, due to the requirement for drying and milling the ore. [0003] Previous efforts to extract nickel and other base metals alloyed with iron have employed alkaline ammoniacal solutions (e.g., US Patent Nos. 3,845,189; 4,069,294; 4,187,281; 4,200,455; 4,229,213; 4,312,841; 4,322,390; 4,328,192; and 6,524,367). Generally, the alloys were artificially generated from more oxidized minerals by reduction at high temperature, rather than from naturally occurring alloy minerals, such as awaruite. US Patent, No. 3,984,237, discloses a process for leaching of "low-grade nickel complex ore" in alkaline ammoniacal solution at high temperature and pressure in an autoclave in the presence of sulfite and carbonate.

[0004] US Patent No. 3,645,454 by Fowler discloses a physical method in which an awaruite-rich concentrate is produced from asbestos tailings by magnetic means, and the particle size of the awaruite grains increased by ball milling. The awaruite grains are then recovered from magnetite and gangue minerals by size-separation. A subsequent US Patent by Fowler (No. 3,677,919) describes leaching of a magnetically-produced awaruite concentrate by iodine in a suitable solvent (e.g. methanol) and electrowinning of nickel from the same solvent as an alloy with iron.

[0005] US Patent Nos. 2,556,215 and 2,478,942 disclose processes for the recovery of iron or nickel, respectively, under high temperatures.

[0006] Niinae et al. disclose ammoniacal leaching of Ni from cobalt-rich ferromanganese crusts using ammonium thiosulfate and ammonium sulfite as reducing agents (Niinae et al., Preferential Leaching Of Cobalt, Nickel And Copper From Cobalt- Rich Ferromanganese Crusts With Ammoniacal Solutions Using Ammonium Thiosulfate And Ammonium Sulfite As Reducing Agents, Hydrometallurgy 40: 111-121, 1996) and Tzeferis et al. disclose microbial leaching of non-sulfide nickel ores (Mineral leaching of non-sulphide nickel ores using heterotrophic micro-organisms; Letters in Applied Microbiology 18:209-213,1994).

[0007] In many known processes, significant amounts of nickel and cobalt are not recovered due to co-precipitation with iron(III), e.g. as Fe(OH)₃.

SUMMARY OF THE INVENTION

[0008] In one aspect, the invention provides a process for extracting nickel from a source material including nickel and iron (the "nickel-containing source material"), by contacting the nickel-containing source material with an aqueous ammonia solution that includes an iron-complexing agent under suitable conditions, thus solubilizing the iron and extracting a sufficient quantity of the nickel from the nickel-containing source material. [0009] The nickel-containing source material may include one or more of an ultramafic material, an iron-nickel alloy, a nickel sulfide, or an industrial material. For example, the nickel-containing source material may include awaruite, josephinite or serpentinite, or may include an industrial by-product such as asbestos tailings. The source material may be milled to a suitable particle size, for example, up to 300 microns or at least 80% passing of milled product using a 48-mesh sieve (P-80 48 mesh). It is to be understood that, in general, a particle size that may be stirred in a stirred tank is suitable for use in a process according to the invention.

[0010] The iron-complexing agent may include a hydroxy-carboxylic acid, including but not limited to citric, tartaric, oxalic, glycolic, lactic and malic acid. In one embodiment, the iron-complexing agent may include citric acid or a salt thereof. In alternative embodiments, the iron-complexing agent may include tartaric or malic acid or a salt thereof. It is understood that the concentration of the iron-complexing agent may be varied by a person of skill in the art. Accordingly, the amount of the iron-complexing agent may be adjusted as necessary up to its solubility limit. The iron-complexing agent may be capable of solubilizing and/or complexing a substantial portion of the iron present in the nickel-containing source material.

[0011] In alternative embodiments, the process may also include a sulfur-containing reductant, such as thiosulfate, which may be present at a concentration of at least 0.1 mM (0.01 g/L S₂O₃ ^2").

[0012] In alternative embodiments, the suitable conditions may include a pH that is weakly alkaline e.g., ranging from about 7.0 to about 9.0 or any value therebetween, such as from about 7.5 to about 8.5, or about 8.0. It is understood that the suitable pH would depend on the temperature, and may be varied as appropriate by a person of skill in the art. [0013] In alternative embodiments, the suitable conditions may include a temperature ranging from about 2O⁰C to about 9O⁰C or any value therebetween. The process may be carried out at atmospheric pressure.

[0014] This summary of the invention does not necessarily describe all features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings related to embodiments of the invention wherein:

[0016] FIGURE 1 is a schematic diagram of the chemical processes involved in awaruite leaching in alkaline ammonia-citrate-thiosulfate solution;

[0017] FIGURE 2 is a graph showing extraction of nickel (circles) and iron

(squares) from milled josephinite (pH 8.00, 50°C, / = 3.0 M, 1.5 M [NH₃]total, 75 mM citrate, 2.0 mM S₂O₃ ^2");

[0018] FIGURE 3 is a graph showing the dependence of the initial rate for nickel extraction from josephinite at 50°C on thiosulfate concentration (note: logarithmic abscissa scale; error bars are one standard deviation in the fit);

[0019] FIGURE 4 is a graph showing the effect of citrate concentration on final extraction of nickel (circles) and iron (squares) from milled josephinite at 50°C; [0020] FIGURE 5 is a graph showing the extraction of nickel (circles) and iron

(squares) from milled josephinite at 25°C (pH 8.50, 1 = 3.0 M, 1.5 M [NH₃]total, 100 mM citrate, 50 mM S₂O₃ ^2");

[0021] FIGURE 6 is a graph showing a summary of nickel extraction from milled josephinite at 25°C as a function of citrate and thiosulfate concentration (pH 8.50,1.5 M

[NH₃]total, 7= 3.0 M);

[0022] FIGURE 7 is a graph showing the extraction of nickel (circles) and iron

(squares) from milled serpentinite-awaruite ore (Sample FPOOl) (pH 8.00, 50°C, / = 3.0

M, 1.5 M [NH₃]total, 50 mM citrate, 2.0 mM thiosulfate);

[0023] FIGURE 8 is a graph showing nickel extraction from Sample FP226, indicated as Ni % with respect to total nickel in the Sample, measured by solution XRF; standard conditions (pH 8.00, 50°C, 1.5 M [NH₃]_totai, 50 mM citrate, 2.0 mM S₂O₃ ^2"; pulp density 86 g L^"1); based on total nickel

[0024] FIGURE 9 is a graph showing extraction of nickel from Sample FP226; aggressive conditions (3.0 M NH₃, 500 mM citrate, 2.0 mM S₂O₃ ^2", pH initially 8.9, 60°C,

160 g L^"1 pulp density);

[0025] FIGURE 10 is a graph showing extraction of iron from Sample FP226; aggressive conditions (3.0 M NH₃, 500 mM citrate, 2.0 mM S₂O₃ ^2", pH initially 8.9, 60°C,

16O g L^"1 pulp density);

[0026] FIGURE 11 is a graph showing rate constants (double exponential fit) of nickel (white) and iron (grey) leaching for Sample FP226; aggressive conditions;

[0027] FIGURE 12 is a graph showing extraction of nickel from Sample FP226; aggressive conditions without citrate (3.0 M NH₃, 2.0 mM S₂O₃ ^2", pH initially 8.6, 60°C,

130 g L^"1 pulp density)

[0028] FIGURE 13 is a graph showing extraction of nickel from Sample FP226; high pH conditions (1.5 M NH₃, 2.0 mM S₂O₃ ^2", pH 9.00, 50°C, 134 g L^"1 pulp density)

[0029] FIGURE 14 is a graph showing the comparison of the effect of different leaching conditions on nickel extraction from Sample FP226 (see Table 4 for conditions); standard conditions (circles, solid line), aggressive conditions (squares, dashed line), aggressive conditions, no citrate (diamonds, dotted line), high pH (triangles, dashed and dotted line);

[0030] FIGURE 15a-d are graphs showing the effect of (a) thiosulfate (b) citrate and temperature (5O⁰C - circles, 6O⁰C - squares, 7O⁰C - triangles; 8O⁰C - diamonds) (c) ammonia (250 raM citrate) and (d) pH on nickel extraction (1.5 M [Ntbj_totai, pH 9, 5 mM thiosulfate, 50 mM citrate, 50⁰C except where noted) for Sample FP226, flask tests;

[0031] FIGURE 16a-d are graphs showing the effect of (a) thiosulfate (b) citrate and temperature (5O⁰C - circles, 6O⁰C ~ squares, 7O⁰C -- triangles; 8O⁰C - diamonds) (c) ammonia (250 mM citrate) and (d) pH on iron extraction (1.5 M [NHsj_tota_U pH 9, 5 mM thiosulfate, 50 mM citrate, 50°C except where noted) for Sample FP226, flask tests;

[0032] FIGURE 17 is a graph showing the effect of milling time on nickel extraction for 50 mM citrate (grey) and 250 mM citrate (black);

[0033] FIGURE 18 is a graph showing the effect of milling time on iron extraction for 50 mM citrate (grey) and 250 mM citrate (black) for Sample FP226, flask tests;

[0034] FIGURE 19a-b are graphs showing the effects of citrate and thiosulfate on extraction of (a) nickel and (b) iron (3.0 M [NH₃]_totai), (100 mM ~ circles, 250 mM ~ squares, 500 mM ~ triangles) for Sample FP226; aggressive conditions;

[0035] FIGURE 20a-b are graphs showing the effects of citrate and [NH₃]_tot_ai on extraction of (a) nickel and (b) iron (2.0 mM S₂O₃ ^2"), (1.5M [NH₃]_totøi - circles, 3.0 M

[NH₃]_totai ~ squares) for Sample FP226; aggressive conditions;

[0036] FIGURE 21 is a graph comparing extraction using different iron-complexing agents (black: nickel, grey: iron); line is nickel extraction from 'blank' test for Sample

FP226, flask tests;

[0037] FIGURE 22 is a graph showing nickel extraction from Sample FP226, measured by solution XRF; 7O⁰C (pH 9.00 at RT, 1.5 M [NH₃]_totai, 100 mM citrate, 5.0 mM S₂O₃ ^2", pulp density 136 g L^"1); based on total nickel;

[0038] FIGURE 23 is a graph showing iron extraction from Sample FP226, measured by solution XRF; 70⁰C (pH 9.00 at RT, 1.5 M [NH₃]_totai, 100 mM citrate, 5.0 mM S₂O₃ ^2", pulp density 136 g L^"1); based on total iron;

[0039] FIGURE 24 is a graph showing pH change during leaching of Sample FP226

(70°C, pH 9.00 at RT, 1.5 M [NH₃]_totai, 100 mM citrate, 5.0 mM S₂O₃ ^2", pulp density 136 g

L-¹);

[0040] FIGURE 25 is a graph showing nickel extraction from the Brazilian nickel sulfide ore with 100 mM citrate (circles), 50 mM citrate (diamonds) and without citrate

(squares). All other conditions excepting the citrate concentration were identical in the three tests. DETAILED DESCRIPTION

[0041] The invention provides, in part, a process for extracting nickel from a nickel source or feedstock that includes iron i.e., the nickel-containing source material, using an aqueous ammonia solution containing a suitable iron-complexing agent (e.g., citrate) and, optionally, a suitable sulfur-containing reductant (e.g., thiosulfate). [0042] Without being bound to any particular theory, the processes thought to be involved in leaching of the nickel-containing source material, such as a nickel-iron alloy particle, are shown in Figure 1 where citrate (or any agent capable of complexing and/or solubilizing Fe(III) in an aqueous ammonia solution) may be used to minimize precipitation of Fe(III), which may inhibit the leaching of nickel, and thiosulfate (or any suitable sulfur-containing reductant) may be optionally used to facilitate leaching of the nickel. The sulfur-containing reductant may alter the surface of the alloy particles and thus enable leaching to occur and the complexing agent may inhibit the precipitation of the iron. Silicate minerals are generally inert to this leaching chemistry, and as such, the costly dissolution of magnesium and aluminium may be avoided.

[0043] The nickel-containing source material for use in processes according to the invention include, without limitation, nickel present in an ore or concentrate thereof, or in an industrial concentrate. In alternative embodiments, the nickel-containing source material may be obtained from naturally occurring terrestrial sources, such as ores, or from extra-terrestrial sources, such as meteorites, or from non-naturally occurring sources such as industrial materials. In alternative embodiments, the nickel-containing source material or feedstock may also include iron, cobalt, magnesium, manganese, copper or mixtures thereof. The nickel content of the ore may vary widely in type and amount, depending on the source of the ore. In alternative embodiments, the nickel can be present both as awaruite and in silicate minerals, and the nickel fraction in silicate minerals may not be extracted according to the processes according to the invention. In alternative embodiments, laterites are specifically excluded from sources of nickel for use according to the invention.

[0044] In alternative embodiments, the nickel-containing source material can be a nickel-iron alloy, a nickel sulfide or a nickel oxide. In alternative embodiments, the nickel sulfide can be present in mafic-ultramafic (also known as ultrabasic) rocks, or as naturally- occurring nickel-iron alloys.

[0045] In alternative embodiments, the nickel-iron alloy may be awaruite, also known as josephinite or souesite. In general, awaruite has a variable composition around Ni₃Fe, with an isometric crystal system. Awaruite may be present in serpentinite/asbestos deposits or in ultramafic rocks. Accordingly, serpentinite/asbestos deposits or ultramafic rocks may be used as sources for awaruite. In alternative embodiments, other nickel-iron alloys such as kamacite, taenite or tetrataenite may be used. In alternative embodiments, wairauite may be used. In general, any nickel-containing material that is alloyed with iron and capable of being sulfidized with, for example, thiosulfate to form ammine complexes, may be used in processes according to the invention.

[0046] In alternative embodiments, the nickel-containing source material for use in processes according to the invention may be an industrial by-product such as tailings (e.g., asbestos tailings).

[0047] In alternative embodiments, the nickel-containing material may be milled to a suitable particle size, for example, up to 300 microns or at least 80% passing of milled product using a 48-mesh sieve (P-80 48 mesh). It is to be understood that, in general, a particle size that may be stirred in a stirred tank is suitable for use in a process according to the invention.

[0048] In alternative embodiments, the nickel-containing source material may be subjected to known processes for extraction of materials prior to use in processes according to the invention.

[0049] The ammonia may be added as a salt, e.g. ammonium sulfate, ammonium thiosulfate, ammonium citrate, ammonium chloride, ammonium carbonate, ammonium phosphate, ammonium bromide, ammonium iodide, ammonium sulfite, ammonium fluoride, ammonium sulfide, etc., and partially converted to ammonia by the addition of a base, e.g. ammonia, ammonium hydroxide, sodium hydroxide, etc. In alternative embodiments, the ammonia may be added as liquid or aqueous ammonia and may be partially neutralized by citric acid or sulfuric acid.

[0050] A suitable iron-complexing agent may be any agent, such as a di- or tri- chelating ligand, that is capable of complexing and solubilizing Fe(III) in an aqueous ammonia solution, hi general, a suitable iron-complexing agent is capable of solubilizing and/or complexing a substantial portion of the iron present in the source material. By "substantial portion " is meant at least about 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%. 80%, 85%, 90 %, 95%, 98%, 99% or about 100% of the iron alloyed with nickel in the nickel-containing source material, or any value between about 10% to about 100% of the iron alloyed with nickel in the nickel-containing source material. [0051] Exemplary iron-complexing agents include, without limitation citric acid, glycolic acid, lactic acid, malic acid, tartaric acid, or other αr-hydroxy carboxylic acid- based agents or siderophores dihydroxyphenylalanine (DOPA); 1-amino-ω- (hydroxyamino)alkanes, ω-JV-hydroxy amino acids, etc., or salts thereof. It is understood that the concentration of the iron-complexing agent may be varied by a person of skill in the art, depending on a number of factors, including the amount of iron alloyed with nickel in the nickel-containing source material, or the amount of solubilizable iron in the nickel- containing source material. Accordingly, the amount of the iron-complexing agent may be adjusted as necessary up to its solubility limit, as known in the art or determined by standard assays.

[0052] The iron-complexing agent may be added as a salt or in the acidic form (e.g., citric acid); for citrate, a suitable form may be as an ammonium salt, e.g. the dibasic, (NH₄)₂C₆O₇H₆, or tribasic, (NH₄)₃C₆O₇H₅ form, or may be as a sodium salt. The solubility of exemplary citrate salts are set forth in Table 1.

Table 1.

Molar

Salt CAS Watei r solubility Ref MW solubility sodium citrate tribasic g/L at Sigma- dihydrate [6132-04-3] 29.4 20⁰C Aldrich 294.1 0.1 ammonium g/L at Sigma- citrate dibasic [3012-65-5] 226 20⁰C Aldrich 226.19 1.0 potassium citrate g/L at Sigma- monobasic [866-83-1] 115 20°C Aldrich 230.2 0.5 potassium citrate tribasic g/L at Sigma- monohydrate [6100-05-6] 324 20⁰C Aldrich 324.42 1.0 sodium citrate dibasic g/L at Sigma- sesquihydrate [6132-05-4] 263 2O⁰C Aldrich 263.1 1.0 sodium citrate [18996-35- g/L at Sigma- monobasic 5] 53.5 20°C Aldrich 214.11 0.2 ammonium g/L at citrate tribasic [3458-72-8] N/A 20⁰C

g/L at citric acid [77-92-9] 590 20⁰C 192.12 3.1 citric acid g/L at monohydrate [5949-29-1] 651 20°C 210.14 3.1 [0053] In alternative embodiments, citrate may be present in a concentration of about 10 mM to about 3 M or greater, or any value therebetween such as about 50 mM, 75 mM, 100 mM, 200 mM, 500 mM, 750 mM etc.

[0054] A suitable sulfur-containing reductant may be any agent capable of facilitating or accelerating leaching of nickel, for example, by accelerating corrosion of the nickel-containing source material and may include, without limitation, inorganic sulfur compounds such as thiosulfate, dithionite, bisulfide, sulfide, or elemental sulfur, or may include organic sulfur compounds such as thiols. It is understood that the concentration of the sulfur-containing reductant may be varied by a person of skill in the art. Accordingly, the amount of the sulfur-containing reductant may be adjusted as necessary up to its solubility limit, as known in the art or determined by standard assays. [0055] The suitable sulfur-containing reductant, if used, may be added as a salt. For example, thiosulfate, if used, may also be added as an ammonium salt, e.g. ammonium thiosulfate, (NH₄)₂S₂θ₃, or as a sodium salt. In alternative embodiments, the thiosulfate may be present at a concentration of at least 0.1 mM (0.01 g/L S₂O₃ ^2"). In alternative embodiments, the thiosulfate may be present at a concentration of about 0.1 mM to about 100 mM or any value therebetween, such as about 2mM to about 50 mM. In alternative embodiments, the thiosulfate may be present up to its solubility limit. [0056] In alternative embodiments, non-ionic organic compounds, for example, thiourea, thioacetamide, or thiols, may be used as suitable sulfur-containing reductants. [0057] In one exemplary embodiment, where nickel is present as awaruite and citrate and thiosulfate are used as the iron-complexing agent and sulfur-containing reductant, respectively, the leaching process may proceed as follows:

Ni₃Fe + ⁹UO₂ + 2(NH4>3cit + 3NH₃ + 3(NHt)₂SO₄ → 3[Ni(NH₃J₄]SO₄ + (NH₄J₃[Fe(CIt)₂] + ⁹/₂H₂O

[0058] The nickel is dissolved as an ammine complex, and the iron as a citrate complex. Thiosulfate does not appear in the above equation because it has a non- stoichiometric role. Without being bound to a particular theory, thiosulfate may interact with the surface of the alloy mineral, and may either break down the oxide passive layer, forming a non-protective sulfur passive layer, or convert the mineral surface from an alloy to a sulfide phase, which is amenable to leaching in an alkaline ammonium citrate medium. [0059] The nickel-containing source material may be subjected to leaching by contacting the nickel-containing source material with an aqueous ammonia solution that includes an iron-complexing agent under suitable conditions, thus extracting a sufficient quantity of the nickel from the nickel-containing source material. A sulfur-containing reductant may be optionally used.

[0060] It is to be understood that the pressure, pH, temperature, time, etc. may have an effect on the amounts and concentrations of iron-complexing agent and sulfur- containing reductant required to achieve solubilization and/or complexation of a substantial portion of the iron and extraction of a sufficient quantity of the nickel from the nickel-containing source material - such conditions may be termed "suitable conditions" and may be determined by the skilled person based on the knowledge in the art and the teachings herein. In addition, the nickel-containing source material may determine the conditions used in processes according to the invention. By "sufficient quantity" of the nickel is meant at least at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90 %, 95%, 98%, 99% or about 100% of nickel present in the nickel-containing source material, or any value between about 5% to about 100% of nickel present in the nickel-containing source material, as determined by assays described herein or known in the art. In alternative embodiments, a "sufficient quantity" of the nickel is meant at least at least about 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90 %, 95%, 98%, 99% or about 100% of the nickel alloyed with iron in the nickel-containing source material, or any value between about 5% to about 100% of the nickel alloyed with iron in the nickel- containing source material, as determined by assays described herein or known in the art. [0061 ] The leaching may be carried out at a temperature range of about 2O⁰C to about 90⁰C, or any value therebetween, for example, about 25⁰C, 3O⁰C, 35⁰C, 4O⁰C, 45⁰C, 5O⁰C, 55°C, 6O⁰C, 65⁰C, 7O⁰C, 75⁰C, 8O⁰C, or about 85⁰C. In some embodiments, the leaching temperature may be as high as possible to achieve efficient leaching of a sufficient quantity of nickel present in the nickel-containing source material, as determined by assays described herein or known in the art, at atmospheric pressure. In alternative embodiments, the leaching temperature may be as high as possible to achieve efficient leaching of a sufficient quantity of nickel alloyed with iron in the nickel-containing source material, as determined by assays described herein or known in the art, at atmospheric pressure. In alternative embodiments, the leaching temperature may be at least about 5O⁰C. In alternative embodiments, the leaching temperature may be no greater than about 8O⁰C. In alternative embodiments, the temperature may not exceed the boiling point of the leaching solution (e.g., 100⁰C at sea level in a non-pressurized vessel) although it is to be understood that the presence of salts, changes in pressure, etc. may alter the boiling point.

[0062] The leaching may be carried out for a period of time sufficient to release a sufficient quantity of nickel present in the nickel-containing source material, or release a sufficient quantity of nickel alloyed with iron in the nickel-containing source material, as determined by assays described herein or known in the art, into solution. In alternative embodiments, the leaching may be carried out for about 24 hours, 48 hours, 72 hours, or longer. In alternative embodiments, the leaching may be carried out for a period of weeks or months, for example, in heap leaching processes.

[0063] The leaching may be carried out at a suitable pH, for example a weakly alkaline pH. In alternative embodiments, the pH may range from about 7.0 to about 9.0 or any value therebetween, such as about 7.25, 7.5, 7.75, 8.0, 8.25, 8.5, or about 8.75. It is understood that the suitable pH would depend on the temperature, and may be varied as appropriate by a person of skill in the art. In alternative embodiments, the pH may be maintained at levels suitable for containment of the ammonia, to minimize loss of the ammonia.

[0064] In alternative embodiments, a suitable source of air or oxygen may be streamed into the leaching tank.

[0065] The leaching may be carried out at atmospheric pressure. In alternative embodiments, pressures higher than atmospheric pressure may be used and the leaching process may be carried out in a pressurized vessel.

[0066] The leaching may be carried out using conventional procedures known in the art. In general, the leaching may be carried out while agitating the leach solution, for example, in a stirred tank, hi alternative embodiments, the leaching may be carried out on heaps.

[0067] The leaching may be carried out without substantial pre-treatment of the nickel-iron source material. For example, in the case of a nickel-containing ore, the leaching may be carried out without subjecting the ore to elevated temperatures i.e., roasting. In some embodiments, the nickel-containing ore may be milled prior to leaching.

In alternative embodiments, the nickel-containing ore may be in the form of a concentrate.

In alternative embodiments, gravity and/or magnetic separations may be carried out on the source material, and the residue leached according to the processes according to the invention.

[0068] Recovery of nickel from the leach solution can be by any of the conventional means such as solvent extraction and electrowinning, precipitation as a salt, reduction with hydrogen to the metallic form, etc. In alternative embodiments, the nickel need not be separately recovered from the solubilized iron or other components of the leachate such as copper or cobalt, which may be present in the leachate in significant amounts. In alternative embodiments, the leachate may be precipitated for example for processing at a remote site. In alternative embodiments, direct electrowinning from the ammonia-citrate-

(thiosulfate) solution to form a ferronickel product may be desirable.

[0069] The present invention will be further illustrated in the following examples.

[0070] EXAMPLES

[0071] Example I (Josephinite tank test at 50⁰O

[0072] In this example, a milled josephinite sample (nickel present primarily as an iron alloy) was leached in a stirred-tank at 50⁰C. The following salts were added to 1.3 kg of deionized water: 148.7 g of (NH₄)₂SO₄, 20.9 g of Na₂SO₄, 33.1 g of Na₃cit.2H₂O

(where cit = C₆H₅O₇ ^3") and 0.5 g OfNa₂S₂O₃. The pH was adjusted to -8.6 with 43 mL of

2 M NaOH solution, and a further 157 g of water was added to bring the total to 1.5 kg.

The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 50⁰C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was adjusted to 8.00 with further addition of NaOH solution (105 mL).

Finely milled josephinite (1.49 g) grading 61.3% Ni was added. Solution samples were taken at various intervals, filtered, and analyzed for nickel and iron by XRF. The pH was readjusted to 8.00 after sampling, as required. Nickel extraction was complete after 23 hours (Figure 2). The final concentrations of nickel and iron were 662 and 171 ppm, respectively.

[0073] The effects of citrate and thiosulfate concentration were established at 50⁰C

(Figures 3 and 4). The citrate concentration affected the final nickel extraction, whereas the thiosulfate concentration strongly influenced the rate of leaching, and high concentrations were detrimental to the leaching rate.

[0074] Example II (Josephinite tank test at 25°O

[0075] In this example, a milled josephinite sample (nickel present as an iron alloy) was leached in a stirred-tank at 25°C. The following salts were added to 1.3 kg of deionised water: 128.8 g of (NH₄)₂SO₄, 42.6 g Of Na₂SO₄, 33.9 g of (NH₄)₂Hcit (where Hcit = C₆H₆O₇ ^2") and 11.9 g OfNa₂S₂O₃. The pH was adjusted to -8.6 with 123 mL of 2 M NaOH solution, and a further 178 g of water was added to bring the total to 1.5 kg. The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 25°C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was adjusted to 8.50 with further addition of NaOH solution (18 mL). Finely milled josephinite (1.48 g) grading 61.3% Ni was added. Solution samples were taken at various intervals, filtered, and analysed for nickel and iron by XRF. The pH was readjusted to 8.50 after sampling, as required. Nickel extraction reached 66% after 24 hours (Figure 5). The final concentrations of nickel and iron were 414 and 131 ppm, respectively.

[0076] The effects of varying the citrate and thiosulfate concentration on the leaching reaction were also determined at 25°C. Higher concentrations of both, compared to 50°C, were required for satisfactory leaching to occur (see Figure 6 and Table 2).

Table 2: Summary of nickel extraction (%) from josephinite at 25⁰C Citrate (mM) 50 100 300 750

S₂O₃ ²- 2 - - 53.3 49.2

(mM) 10 39.6

50 53.5 66.6

[0077] Example III (FPOOl/Ouebec sample tank test at 50⁰O

[0078] In this example, a milled serpentinite ore sample (FPOOl, Quebec sample), containing nickel only in awaruite and silicate minerals, was leached in a stirred-tank at 50°C. The elemental composition and mineralogy of Sample FPOOl, as determined by ICP-MS, were as follows: Table 3

FP001 composition (ICP-MS)*

Al 1096 ppm Pb 11 ppm

Ba 3 ppm Mg 262057 ppm

Ca 533 ppm Mn 704 ppm

Cr 489 ppm Ni 3142 ppm

Co 88 ppm Sc 3 ppm

Cu 12 ppm V 2 ppm

Fe 27659 ppm Zn 59 ppm

La 63 ppm

* analysed for but not detected:

Sb, As, Bi, Cd, Hg, Mo, P, K, Ag, Na, Sr, Tl, Ti, W, Zr

Table 4

FP001 Mineralogy

Ni-Fe Alloy 0.39

Ni-Sulphide 0.00

Serpentine 62.26

Olivine 30.89

Clinopyroxene 0.01

Orthopyroxene 0.10

Amphibole 1.03

Talc 0.49

Quartz 0.10

Feldspars 0.01

Epidote 0.00

Chlorite 0.46

Micas/Clays 0.05

Other Silicates 0.00

Fe-Oxides 0.94

Chromite 1.67

Sulphides 0.01

Other Oxides 0.01

Carbonate 0.00

Mg-Oxide/Hydroxide 1.58

Other 0.00

Total 100.00 [0079] The following salts were added to 1.3 kg of deionised water: 148.7 g of (NH₄)₂SO₄, 31.5 g of Na₂SO₄, 22.1 g of Na₃cit (where cit = C₆H₅O₇ ^3") and 0.5 g of Na₂S₂O₃. The pH was adjusted to -8.6 with 40 mL of 2 M NaOH solution, and a further 135 g of water was added to bring the total to 1.5 kg. The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 50°C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was adjusted to 8.00 with further addition of NaOH solution (26 mL). Finely milled serpentinite ore (95.3 g) grading 0.31% total Ni was added. Solution samples were taken at various intervals, filtered, and analysed for nickel and iron by XRF. The pH was readjusted to 8.00 after sampling, as required. Nickel extraction reached 71% after 48 hours (Figure 7) based on nickel concentration in solution. The final concentrations of nickel and iron were 142 and 170 ppm, respectively. Based on ICP-MS measurements of the head and tail grade, the nickel extraction was 78.9% (Table 5).

Table 5

Elemental cαmptnHioσ of serpentinite-awamKe ore before and after leaching (ppm; excluding oxygen, sulflu- and silicon)

Element Heed Tail

Na - 853

Mg 256700 253721

Al 1103 1268

Ca 535 195

V 2 .

Sc 3 4

Cr 509 994

Mn 709 552

Fe 2728« 29861

Co 89 30

Nt 3099 708

Cu 12 24 ^'

Zn 59 81

As - 9

Zr 48

Ag - 3

Sb - 9

Ba 3 5

U 63

Bi 12 8

Mass(g) 95.3 87.8 [0080] Example IV (FP226 standard condition tank test 50°C)

[0081] Sample FP226 - tank leaching

[0082] Hand specimen-sized pieces from Northern British Columbia, Canada, referred to as Sample FP226, were crushed in a series of jaw and gyratory crushers to fine gravel size and then milled for 60 s in ~100 g batches in a ring mill. This process resulted in a pale grey powder. The elemental composition of Sample FP226, as determined by

ICP-MS, was as follows:

Table 6

FP226 composition (ICP-MS)*

Cu 531 ppm La 28 ppm

Pb 37 ppm Sr 1 ppm

Zn 160 ppm Sc 8 ppm

Co 94 ppm Al 2700 ppm

Ni 2275 ppm Ca 5900 ppm

Ba 29 ppm Fe 55200 ppm

Cr 1726 ppm Mg 283100 ppm

V 21 ppm Na 100 ppm

Mn 799 ppm

* analysed for but not detected:

Ag, As, Sb, Hg, Mo, Tl, Bi, Cd, W, Zr (ppm); Ti, K, P (%)

[0083] A leaching test starting at the standard conditions (pH 8.00, 50°C, 1.5 M

[NH₃]_totai, 50 mM citrate, 2.0 mM S₂O₃ ^2"; pulp density 86 g L^"1) was conducted on this sample. After 47 hours had elapsed, the nickel extraction (based on 0.228% total Ni from

ICP-MS analysis) was 37%, and appeared to have peaked (Figure 8).

[0084] Example V (Aggressive tank test Sample FP226)

[0085] More aggressive conditions were used in a subsequent test (Figures 9 and 10).

Specifically, the ammonia concentration was doubled to 3.0 M, the citrate concentration was increased 10-fold to 500 mM, and the temperature was raised to 6O⁰C. The thiosulfate concentration was held constant at 2.0 mM. Rather than preparing the leaching solution from ammonium sulfate, sodium citrate and sodium hydroxide, citric acid and aqueous ammonia were used. The citric acid (144.1 g) was dissolved in ~1 L of water, and the ammonia (300 mL) was added. The total volume of the solution was then brought up to 1500 mL, without any adjustment of pH (which was 9.9). Note that this procedure differs from that used previously, in which -1500 g of water (less the sodium hydroxide solution) was added to the solid components. The two methods therefore resulted in different final volumes. A higher than normal pulp density (160 g L^"1) was used to ensure a higher nickel concentration in solution, in order to obtain more reliable XRF data. [0086] During the test, no attempt was made to maintain the pH at a fixed value. At 60°C, the starting pH was 8.92, and it decreased during the test at a decreasing rate. The initial leaching rate exceeded that of the FPOOl test, although the final extraction was lower (based on total nickel). A major difference between this sample and Example III (FPOOl /Quebec) is the iron content - 2.7% in Example III and 5.5% in this example (another difference could be the awaruite particle size). Rates of Ni and Fe leaching were indistinguishable (Figure 11).

[0087] Example VI (Aggressive, no citrate tank test. Sample FP226) [0088] A further test was run at similar conditions to Example V, in the absence of citrate. The solution was prepared by diluting aqueous ammonia with water and titrating to pH 10 using diluted sulfuric acid. The solution therefore contained sulfate instead of citrate. When the temperature was raised to 60°C, the solution pH prior to adding the solid was 8.58 rather than 8.92, which may be due to the lack of citrate in the solution changing the temperature dependence of the solution pH. The nickel extraction curve is shown in Figure 12. The maximum nickel extraction (41%) was obtained after 24 hours, which may indicate a non-awaruite source of nickel. The iron concentration during the test remained essentially zero, as would be expected in the absence of citrate. The lack of iron dissolution also led to only a small decrease in pH (from 8.58 to 8.48) during the test, as iron hydrolysis was minimal.

[0089] After the success of the 'aggressive' conditions, a more systematic investigation was initiated, starting with testing the effect of higher pH, keeping all other parameters at the standard conditions. Surprisingly, in this Sample, the nickel extraction at pH 9.00 (Figure 13) was worse than at the standard conditions (pH 8.00). [0090] Comparison of the nickel extraction based on analysis of the solids after leaching and the solution measurements carried out during leaching indicated that the extraction was lower based on the solid.

[0091] The four tests run on sample FP226 are compared below in Figure 14, and the conditions are summarised in Table 7. The aggressive conditions resulted in the best nickel extraction. The other two tests (aggressive without citrate and standard conditions) reached the approximately the same point (~40% by solution and -34% by solid), although the extraction from the aggressive (no citrate) test was slightly more rapid.

Table 7: Conditions used in FP226 tests

Aggressive High

Condition Standard Aggressive (no citrate) pH

PH 8.00 8.9 8.6 9.00 (initially) (initially)

T (⁰C) 50 60 60 50

[NHsW (M) 1.5 3.0 3.0 1.5

Citrate (mM) 50 500 0 50-100

S₂O₃ ^2" (mM) 2.0 2.0 2.0 2.0

Ni (solution 38.3 62.1 42.8 31.4 %)

Ni (solid %) 35.4 41.5 33.4 —

[0092] Example VII (Sample FP226. Various conditions)

[0093] A series of small scale tests were carried out in 250 mL conical flasks on Sample FP226, varying pH (7-10), temperature (50-80°C) and milling time (1-5 min) as well as thiosulfate (2-50 mM), citrate (50-250 mM) and ammonia (1.5-3.0 M) concentration. Pulp density and shaking speed were held constant. Each test was titrated at ambient temperature to the desired pH, and was not adjusted again. The tests were run for 24 hours, at which point the solution was filtered and nickel and iron measured by XRF. The details of each test and the results are given in Table 8. Oxygen limitation was not a problem, since the amount of oxygen in the flask was many times more than required to leach awaruite and any magnetite. Furthermore, gas-liquid mixing was adequate, since the initial amount of dissolved oxygen was insufficient to leach the amount of nickel and iron found in solution.

Table 8: Conditions and results of the flask tests (based on total Ni and Fe from ICP-MS of milled FP226)

[NH₃JT pH T S₂O₃ ²- Citrate Milling Ni Fe (M) CQ (mhi) (mM) (min) (%) (%)

1.5 9 50 50 50 1 27.6 1.93

1.5 9 50 25 50 1 28.1 1.93

Thiosulfate 1.5 9 50 10 50 1 26.9 1.96

1.5 9 50 5 50 1 28.7 1.98

1.5 9 50 2 50 1 27.3 1.96

1.5 9 50 5 50 1 21.6 1.86

1.5 9 50 5 100 1 23.1 2.36

Citrate 1.5 9 50 5 150 1 24.0 2.58

1.5 9 50 5 200 1 24.4 2.69

1.5 9 50 5 250 1 25.5 2.70

2.0 9 50 5 50 1 23.7 2.02

2.5 9 50 5 50 1 23.1 2.02

3.0 9 50 5 50 1 25.3 1.92

1.0 9 50 5 250 1 24.9 2.93

[NH₃]τ 1.5 9 50 5 250 1 25.2 2.82

2.0 9 50 5 250 1 25.1 2.80

2.5 9 50 5 250 1 25.0 2.74

3.0 9 50 5 250 1 25.2 2.58

1.5 7 50 5 50 1 28.4 2.53

PH 1.5 8 50 5 50 1 26.4 2.45

1.5 10 50 5 50 1 23.5 0.38

Temperature 1.5 9 60 5 50 1 25.1 2.12 1.5 9 60 5 100 1 26.0 2.83

1.5 9 60 5 250 1 27.9 3.32

1.5 9 70 5 50 1 37.9 2.70

1.5 9 70 5 100 1 40.6 3.37

1.5 9 70 5 250 1 41.0 4.05

1.5 9 80 5 50 1 33.9 2.44

1.5 9 80 5 250 1 39.5 3.96

1.5 9 50 5 50 2 26.2 1.89

1.5 9 50 5 250 2 32.7 3.34 Milling 1.5 9 50 5 50 5 32.9 1.51

1.5 9 50 5 250 5 33.0 3.65

[0094] Figures 15a-d show the effects of thiosulfate, citrate, ammonia, pH and temperature on nickel extraction from Sample FP226. Thiosulfate and ammonia had little influence on this sample, whereas the extraction increased slightly as citrate increases, and increasing pH inhibits nickel extraction. The biggest effect by far appeared to be temperature. Note that the data marked as 80°C were actually closer to 75°C, since the incubating shaker was not able to maintain 80°C. The 'standard condition' stirred-tank test (2 mM S₂O₃ ^2", 50 mM citrate, 1.5 M [NH₃]_totai, pH 9, 50°C) leached 35.4% (based on solids), and another test at 70°C (with 100 mM citrate and 5 mM thiosulfate) leached 38.9% (solution XRF); the equivalent shake-flask tests leached 27.3% and 40.6%. [0095] The effects on iron extraction are shown in Figures 16a-d. Thiosulfate had little effect on iron extraction, whereas increasing total ammonia slightly decreased iron extraction. Increasing citrate concentration and temperature both resulted in greater iron solubility. Increasing pH dramatically decreased iron solubility. Since the majority of the iron is not derived from awaruite (typical molar ratio of iron to nickel in solution is about two, compared to one third in awaruite), the lack of dependence on thiosulfate and ammonia was expected. Without being bound to any particular theory, much of the dissolved iron may be from dissolution of magnetite. Any change to the conditions that increases the stability of Fe(OH)₃ relative to citrate complexes of Fe(III), such as a pH increase, decreases the dissolved iron concentration. Hence, an increase in the concentration of citrate increases iron solubility, as citrate complexes become more stable. The effects of temperature may affect the pKa of ammonia, citrate and Fe(III); the net effect is increased iron solubility.

[0096] The effect of particle size was investigated using milling time as a surrogate for the actual particle size distribution (Figures 17 and 18). For both 50 and 250 mM citrate, five minutes of milling in a ring mill resulted in the best nickel extraction. Longer milling time decreased iron extraction for 50 mM citrate, whereas it was increased for 250 mM citrate. Without being bound to any particular theory, this could be due to adsorption of citrate on particle surfaces, and 50 mM citrate was no longer adequate to hold iron in solution when presented with a greater surface area. With sufficient citrate, as demonstrated by the 250 mM citrate conditions, iron extraction increased with increasing milling time (i.e. smaller particle size).

[0097] The 'aggressive' conditions used in the stirred-tank test (60°C, 3.0 M [NH₃]_totai, 0.5 M citrate, 2.0 mM S₂O₃ ^2", pH initially -10 at room temperature), were carried out in an equivalent shake-flask test and several variations on those conditions were also carried out (Table 9). Additionally, a 'blank' test was also carried out at otherwise 'aggressive' conditions, lacking thiosulfate and citrate. These tests were run for 24 hours and analysed as above, with the 'blank' and 'aggressive' tests providing a minimum and maximum extraction range for the shake-flask tests. The blank achieved just 12.3% nickel extraction, whereas the 'aggressive' conditions leached 40.1% nickel, along with 4.03% iron. Iron extraction in the 'blank' test was just 0.12%, as expected in the absence of citrate.

Table 9: Conditions and results of flask tests varying the 'aggressive' conditions (based on total Ni and Fe from ICP-MS of milled FP226)

[NH₃Jr T pH S₂O₃ ²- Citrate Milling Ni Fe (M) (⁰C) (mM) (mM) (min) (%) (%)

Blank 3.0 9 60 0 0 1 12.3 0.12

Aggressive 3.0 10 60 2 500 1 40.1 4.03

1.5 10 60 2 500 1 29.8 3.04

1.5 10 60 2 250 1 30.0 2.76

1.5 10 60 2 100 1 28.1 1.84

Variations 3.0 10 60 2 100 1 34.1 1.74

3.0 10 60 50 500 1 42.0 3.84

3.0 9 60 2 500 1 35.1 4.25

3.0 10 60 0 500 1 30.8 4.07

[0098] In Figure 19, the effects of thiosulfate and citrate (at 3.0 M [NH₃]_tot_a0 on nickel and iron extraction are plotted together. These figures show that in the absence of both thiosulfate and citrate (circle), very little nickel and no iron was leached. Increasing citrate resulted in both more nickel and more iron being extracted, whereas increasing thiosulfate had a minor effect. Substantial nickel leaching occured at high citrate, even in the absence of thiosulfate. Iron leaching remained independent of thiosulfate at the 'aggressive' conditions, as was the case at the 'standard' conditions. [0099] The effects of citrate and ammonia (at constant thiosulfate) on nickel and iron extraction are plotted in Figure 20. At the 'standard' total ammonia concentration (1.5 M), varying citrate had a negligible effect on nickel leaching. At high ammonia (3.0 M [NH₃]_totai), there was an increase in nickel extraction with an increase in citrate concentration. Increasing citrate resulted in greater extraction of iron, with a more dramatic effect at the higher ammonia concentration. Increasing the ammonia concentration, at 500 mM citrate, increased the iron extraction, whereas at 100 mM citrate, the iron extraction was unaffected by an increase in [NHj]_totai from 1.5 to 3.0 M. [00100] Additional iron-complexing agents, such as tartaric acid, oxalic acid, glycolic acid, lactic acid and malic acid, were compared at 100 mM complexant, 2 mM thiosulfate, 1.5 M [NH₃]_to_tai, pH 9 (initially), 50°C and 150 g/L pulp density. Figure 21 and Table 10 compare the effect of different complexants on nickel and iron extraction. Without being bound to any theory, the relatively poor performance of oxalate for nickel leaching may be because of poor solubility of oxalate complexes, thereby probably blocking the surface of the awaruite grains. Malic acid achieved almost as effective nickel extraction as citrate and tartrate, while dissolving significantly less iron.

Table 10: Nickel and iron extraction using different iron-complexing agents

Complexing Ni Fe agent (%) (%) citrate* 23.7 2.17 tartrate 22.3 1.57 oxalate 13.6 0.15 glycolate 19.7 0.17 lactate 18.7 0.19 malate 21.3 0.62

* 100 mM citrate, 2.5 mM thiosulfate, 1.5 M NH₃, pH 9, 50°C

[00101] Example VIII (Sample FP226 tank test at 70⁰Q

[00102] Based on the results of the shake- flask tests, which showed that temperature had a strong effect on the final nickel extraction, a tank test was carried out at 70°C (with 1.5 M [NH₃]_totai, 150 g/L pulp density and pH 9 initially), with slightly higher thiosulfate (5 mM) and citrate (100 mM). The nickel leaching curve (Figure 22) shows that the extraction reached 28% after 6 hours and 39% after 48 hours. Iron extraction followed a similar trajectory (Figure 23). In this test, the pH was titrated to 9 at ambient temperature and then not adjusted again - at the beginning of the test it was 7.35 (due to the temperature increase to 70°C), and decreased during the test to ~7 (Figure 24). The pH drop corresponded closely with the increase in nickel and iron concentration. [00103] Example IX - Brazilian nickel sulfide ore

[00104] A Brazilian nickel sulfide ore composed of ultramafic material, containing nickel in sulfide and silicate minerals, was used. The elemental composition is shown in Table 11. Table 11

Brazilian sulfide ore composition (ICP-MS)*

Sc 29 ppm Ba 96 ppm

V 151 ppm La 8 ppm

Cr 858 ppm Pb 40 ppm

1467

Mn Ppm Na 6400 ppm

114700

Co 146 ppm Mg ppm

3898 40000

Ni ppm Al ppm

1425

Cu ppm P 500 ppm

Zn 180 ppm K 2200 ppm

43800

Sr 133 ppm Ca ppm

Zr 15.5 ppm Ti 2800 ppm

115800

Mo 10 ppm Fe ppm

Ag 0.7 ppm

*Analysed for but not detected: As, Cd, Sb₁ W, Hg, Tl, Bi (ppm)

[00105] XRD analysis of Brazilian nickel-copper sulfide ore (pyrrhotite contains nickel as an impurity, -1%), was as shown in Table 12:

Table 12

[00106] The mineralogy was therefore about 92.3% silicate (shaded areas in Table 12), about 1.4 % Calcite (CaCO₃), about 0.9% Chalcopyrite (CuFeS₂), and about 5.2% Pyrrhotite (Fe_1-xS) (total about 99.8%).

[00107] The gravel sized ultramafic material, containing nickel in sulfide and silicate minerals, was milled for 30 s in a ring mill, and then leached in a stirred-tank at 50°C. The following salts were added to 1.3 kg of deionised water: 148.7 g of (NH₄)₂Sθ4 and 53.3 g OfNa₂SO₄. The pH was adjusted to -8.9 with 90 mL of 2 M NaOH solution, and a further 135 g of water was added to bring the total to 1.5 kg. The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 50°C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was adjusted to 8.00 with further addition of NaOH solution (8 mL). Finely milled ore (50.0 g) grading 0.39% total Ni was added. Solution samples were taken at various intervals, filtered, and analysed for nickel by AAS. The pH was readjusted to 8.00 after sampling, as required. Nickel extraction reached 65% after 54 hours (Figure 25) based on nickel concentration in solution. The final concentration of nickel was 85 ppm.

[00108] A subsequent test was carried out on the same material with 50 mM of citrate in solution. The following salts were added to 1.2 kg of deionised water: 148.7 g of (NH₄)₂SO₄, 32.0 g Of Na₂SO₄ and 22.1 g of Na₃cit.2H₂O (where cit = C₆H₅O₇ ^3"). The pH was adjusted to -8.6 with 34 mL of 2 M NaOH solution, and a further 266 g of water was added to bring the total to 1.5 kg. The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 5O⁰C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was 8.06 initially and was not adjusted down to 8.00. Finely milled ore (50.0 g) grading 0.39% total Ni was added. Solution samples were taken at various intervals, filtered, and analysed for nickel and iron by XRF. The pH was readjusted to 8.00 after sampling, as required. Nickel extraction reached 70% after 69 hours (Figure 25) based on nickel concentration in solution. The final concentrations of nickel and iron were 91 and 522 ppm, respectively. [00109] A further test was carried out on the same material with 100 mM of citrate in solution. The following salts were added to 1.2 kg of deionised water: 148.7 g of (NH₄)₂SO₄, 10.7 g Of Na₂SO₄ and 44.1 g Na₃cit.2H₂O. The pH was adjusted to 8.91 with 87 mL of 2 M NaOH solution, and a further 214 g of water was added to bring the total to 1.5 kg. The solution was transferred to a jacketed vessel fitted with an oxygen sparger, a pH probe and a temperature probe and was thermally equilibrated to 50°C. Agitation was achieved by an impeller operating at 1200 rpm. Oxygen was bubbled at a constant rate of 30 mL min^"1. The pH was adjusted to 8.00 with a further 7 mL of NaOH. Finely milled ore (50.0 g) grading 0.39% total nickel was added. Solution samples were taken at various intervals, filtered, and analysed for nickel by AAS. The pH was readjusted to 8.00 after sampling, as required. Nickel extraction reached 81% after 48 hours (Figure 25) based on nickel concentration in solution. The final concentration of nickel in solution was 105 ppm.

[00110] This example showed that the addition of citrate increased both the rate of nickel extraction and the final nickel recovery in a nickel sulfide containing source material (Figure 25).

[00111] AU citations are hereby incorporated by reference.

[00112] The present invention has been described with regard to one or more embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

Claims

WHAT IS CLAIMED IS:

1. A process for extracting nickel from a source material comprising nickel and iron, the process comprising:

a) providing a source material comprising nickel and iron;

b) providing an aqueous ammonia solution comprising an iron-complexing agent;

c) contacting the source material with the aqueous ammonia solution under suitable conditions, wherein a substantial portion of the iron is solubilized, thereby extracting a sufficient quantity of the nickel from the source material.

2. The process of claim 1 wherein the source material comprises one or more of an ultramafic material, an iron-nickel alloy, a nickel sulfide, or an industrial material.

3. The process of claim 1 wherein the source material comprises awaruite or josephinite.

4. The process of claim 1 wherein the source material comprises serpentinite.

5. The process of claim 2 wherein the industrial material is an industrial by-product.

6. The process of any one of claims 1 to 5 wherein the iron-complexing agent comprises citrate or a salt thereof.

7. The process of claim 4, wherein the citrate is present at a concentration of about 50 mM to about 500 mM.

8. The process of any one of claims 1 to 5 wherein the iron-complexing agent comprises tartrate, glycolate, oxalate, lactate, or malate, or salts thereof.

9. The process of any one of claims 1 to 8 wherein the iron-complexing agent is capable of solubilizing a substantial portion of the iron present in the source material.

10. The process of any one of claims 1 to 9 wherein the iron-complexing agent is capable of complexing a substantial portion of the iron present in the source material.

11. The process of any one of claims 1 to 10 wherein the process further comprises a sulfur-containing reductant.

12. The process of claim 11 wherein the sulfur-containing reductant is thiosulfate.

13. The process of claim 12 wherein the thiosulfate is present at a concentration of about 2 mM to about 50 mM.

14. The process of any one of claims 1 to 13 wherein the suitable conditions comprise a pH that is weakly alkaline.

15. The process of claim 14 wherein the pH ranges from about 7.5 to about 8.5.

16. The process of any one of claims 1 to 15 wherein the suitable conditions comprise a temperature ranging from about 2O⁰C to about 90⁰C.

17. The process of any one of claims 1 to 16 wherein the process is carried out at atmospheric pressure.

18. The process of any one of claims 1 to 17 wherein the source material is milled to a suitable particle size.

19. The process of any one of claims 1 to 18 further comprising precipitating a leachate comprising the iron and the nickel.