WO2014040138A1

WO2014040138A1 - Resin scavenging of nickel and cobalt

Info

Publication number: WO2014040138A1
Application number: PCT/AU2013/001050
Authority: WO
Inventors: James Vaughan; Patrick LITTLEJOHN
Original assignee: The University Of Queensland
Priority date: 2012-09-14
Filing date: 2013-09-16
Publication date: 2014-03-20

Abstract

A method for the selective recovery of nickel and cobalt from an ore processing stream is provided which includes the steps of contacting the ore processing stream with a resin presenting tertiary amine groups, separating the loaded resin from the ore processing stream and eluting the nickel and cobalt from the resin with an ammoniacal solution.

Description

RESIN SCAVENGING OF NICKEL AND COBALT

FIELD OF THE INVENTION

[0001] The invention relates to the field of ore processing. More particularly, this invention relates to a method of selectively recovering nickel and cobalt from an ore processing stream using an ion exchange resin.

BACKGROUND TO THE INVENTION

[0002] Any reference to background art herein is not to be construed as an admission that such art constitutes common general knowledge in Australia or elsewhere.

[0003] Ion exchange resin has been used to recover value metal in the uranium and gold industry through resii -in-pulp/leach and similar carbon-in- leach/pulp processes for decades. More recently, resin-in-pulp processes have gained attention as a potential method to improve the efficiency of nickel operations. While every nickel laterite operation is unique, many involve an acid leach, neutralization and oxidative precipitation of impurities followed by counter current decantation (CCD) to separate valuable liquor from the unwanted metal residue and precipitate. Counter current decantation of this material is challenging at best, with large CCD tanks having a large plant footprint and requiring high capital investment. Depending on the settling characteristics of the precipitate, 5% or more of the leached nickel and cobalt can be lost to the slurry underflow through solution entrainment, co-precipitation, and sorption processes on the high surface area solids. For a site producing 40,000 tonnes per annum nickel and 2,500 tonnes per annum cobalt this represents yearly losses of approximately US$40 million.

[0004] Resin-in-pulp (RIP) scavenging involves contacting ion exchange resin with nickel laterite tailings under conditions in which the valuable metals load onto the resin. As the resin beads are larger than the fine slurry particles, they can be separated from the slurry using vibration sieving. Following this, the resin is washed to remove residual slurry and solution, and then eluted to recover metal recover metal value. While exact values vary, typical Caron process tails contain roughly 300 mg/L nickel and 50 mg/L cobalt in slurry. High pressure acid leach tailings may contain 200 mg/L nickel and 35 mg/L cobalt in slurry. With efficient resin-in-pulp contact, upwards of 50% of this otherwise lost metal value could potentially be recovered.

[0005] Although the chelating ion exchange resins proposed for use in nickel laterite RIP are generally selective for nickel and cobalt over most other unwanted metals, laterite tailings solutions contain a relatively small amount of these metals of interest. Depending on the composition of the original ore and the method of leaching, the neutralized slurry can contain large amounts of solution phase magnesium and manganese (in the case of acid leaching) and vast amounts of ferric iron, silica, aluminium, and chromium in the solid phase. The presence of other cations that compete with nickel and cobalt for resin loading sites complicates resin-slurry equilibrium. In general, there is a trade off between recovery of nickel and cobalt and purity of loaded resin. To recover a high amount of the nickel and cobalt value, one must accept the presence of impurity metals on the resin. When resin is eluted, these impurity metals can follow the nickel and cobalt into the eluate.

[0006] The loaded resin is typically stripped by contact with strong acid, such as sulphuric acid. When a resin loaded with a high fraction of value metals has been attained, quantitative elution in this fashion is attractive. Using strong acid, metal is recovered in a small volume of eluent with rapid kinetics. However, as more impurities are loaded onto a resin, strong acid elution becomes less attractive as quantitative elution of a low purity resin produces a low purity eluate. Often functional groups will be filled with undesirable impurities such as ferric iron, aluminium, manganese, and magnesium.

[0007] Further, this acidic liquor is difficult to integrate into the flow sheets of existing nickel refineries. Residual acid makes refining of the contained nickel into mixed hydroxide precipitate costly, and high impurity content makes the liquor inappropriate for downstream integration close to final metal production. Due to the incompatibility of acidic eluate with most downstream nickel refining methods, methods, scavenged metal value must be integrated back into existing flow sheets through upstream recycle. Downstream integration of scavenged metal value, if possible, would improve the commercial viability of resin-in-pulp scavenging.

[0008] There is therefore a need for an ore processing approach allowing for the selective loading and subsequent selective elution of nickel and cobalt from an ion exchange resin which produces an eluate of good purity which is amenable to downstream integration within existing refinery process flow sheets.

OBJECT OF THE INVENTION

[0009] It is an aim of this invention to provide a method of separating nickel and cobalt from an ore processing stream using an ion exchange resin which overcomes or ameliorates one or more of the disadvantages or problems described above, or which at least provides a useful alternative.

[0010] Other preferred objects of the present invention will become apparent from the following description.

SUMMARY OF INVENTION

[0011] In one broad form the invention resides in a method of separating nickel and cobalt from an ore including the steps of solubilising the nickel and cobalt from the ore, contacting the solution containing the nickel and cobalt with an ion exchange resin presenting a functional group comprising at least two tertiary nitrogens to selectively load the nickel and cobalt onto the resin in preference to one or more impurity metals, separating the resin from the ore solutioh and eluting the nickel and cobalt from the resin, in preference to one or more impurity metals, in an ammoniacal solution.

[0012] According to a first aspect of the invention, there is provided a method for the selective recovery of nickel and cobalt from an ore processing stream including the steps of: (i) contacting the ore processing stream, said stream comprising solubilised nickel and cobalt, with an ion exchange resin of formula I;

formula I

wherein, U, if present, is a linking group;

Ri and F¾ are selected from the group consisting of heterocyclic, heteroalkyl, heteroalkenyl, carboxyl and sulfonic acid and at least one of Ri or f¾ is a group having a tertiary nitrogen;

if the dashed line is a bond then W is present and is

wherein L₂, if present, is a linking group and Ri and R₂ are as described;

to thereby load the nickel and cobalt onto the resin;

(ii) separating the ion exchange resin, with loaded nickel and cobalt, from the ore processing stream; and

(iii) contacting the nickel and cobalt loaded ion exchange resin with an ammoniacal solution to elute the nickel and cobalt into the ammoniacal solution to thereby recover the nickel and cobalt.

[0013] Suitably, Li and L₂ may be independently selected from the group consisting of alkyl, alkenyl, aryl, heteroaryl and benzyl linking groups. [0014] Preferably, at least one of Ri or R₂ is independently selected from the group consisting of tertiary N-alkyl, tertiary N-alkenyl, nitrogen heterocyclic, carboxyl and sulfonic acid.

[0015] More preferably, at least one of Ri or R₂ are or form part of a picolyl, methylquinoline, acetic acid or methylpiperidine group.

[0016] In one embodiment, the ion exchange resin is a mixed bis- picolylamine/iminodiacetic acid (IDA) resin.

[0017] In another embodiment, the ion exchange resin is a bis-picolylamine (BPA) resin.

[0018] Suitably, the ore processing stream is a nickel laterite ore processing stream.

[00 9] The contacting of the ore processing stream with the resin may take place at a resin-in-leach stage, resin-in-pulp stage, heap leach permeate or a decanted tailings stage of the stream.

[0020] Preferably, the resin will be separated from the ore processing stream by size screening when the processing stream is a slurry.

[0021] When the processing stream is a decanted tailings or other solution stream then the separation step may occur when the stream is passed over the resin.

[0022] The resin may be contacted with the ammoniacal solution in a batch vessel or within a column environment.

[0023] The ammoniacal solution is preferably ammonia in water optionally further comprising ammonium sulphate and/or ammonium carbonate.

[0024] It is preferred that the ammoniacal solution does not comprise any metal-containing compound.

[0025] The method may further include the step of introducing the ammoniacal eluate containing the nickel and cobalt directly back into the ore processing stream.

[0026] Preferably, the purity of the ammoniacal eluate is such that it is suitable suitable for introduction at a processing stream stage selected from the group consisting of an ore intermediate leach stage, a nickel/cobalt separation stage, a CCD tailings stage, a pH increasing stage and a stage where one of the metals is selectively recovered as an intermediate or final product.

[0027] Thus, in one embodiment, the eluate will comprise nickel and cobalt as the major portion of metals. That is, the eluate will contain a higher combined amount or concentration of nickel and cobalt compared with typical problem impurity metals selected from the group consisting of iron, magnesium, manganese, aluminium, calcium and chromium due to the selectivity of the loading and/or ammoniacal stripping steps.

[0028] Preferably, the metal content of the eluate is substantially comprised of nickel and cobalt.

[0029] A second aspect of the invention resides in the use of a resin of formula I in the selective recovery of nickel and cobalt from an ore processing stream including the steps of contacting the ore processing stream with the resin, separating the resin with loaded nickel and cobalt from the ore processing stream and contacting the resin with an ammoniacal solution to elute the nickel and cobalt from the resin.

[0030] Further features and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order that the invention may be readily understood and put into practical effect, preferred embodiments will now be described by way of example with reference to the accompanying figures wherein:

[0032] FIG 1 is a schematic flow sheet of an ore processing stream indicating sampling points for collected material later used in tests;

[0033] FIG 2 is a graphical representation of the ammoniacal column recovery of pure nickel and pure cobalt, loaded from a synthetic solution, from BPA resin; [0034] FIG 3 is a graphical representation of the kinetics of stripping of nickel, loaded from a synthetic solution, from BPA resin using ammonia versus sulphuric acid;

[0035] FIG 4 is a graphical representation of the ammonia elution response of resin-in-pulp loaded BPA resin;

[0036] FIG 5 a graphical representation of the column loading of BPA resin after a first contact with tailings decant liquor; and

[0037] FIG 6 a graphical representation of the column loading of BPA resin after a second contact with tailings decant liquor.

DETAILED DESCRIPTION OF THE DRAWINGS

[0038] The present invention is predicated, at least in part, on the finding that a bis-picolylamine (BPA) resin is not only advantageously selective in terms of loading nickel and cobalt from a solution or slurry comprising these value metals alongside metal impurities typically present during ore processing but is also highly selective in terms of stripping off nickel and cobalt, in preference to impurity metals which may have loaded, when stripping is performed in an ammoniacal solution. The realisation of this dual selectivity for both nickel and cobalt in resin loading and stripping allows, for the first time, for a commercially attractive resin- based recovery of these metals from processing and waste streams without which this significant portion of metal value would not be realised.

[0039] A further important consideration is that the use of an ammoniacal eluant and the selectivity of the stripping step mean that a relatively pure eluate is produced which is suitable for integration into downstream processing steps thereby adding value to the existing process flow sheet without the need for additional purification steps and without placing further burden on upstream steps with that ensuing economic cost. [0040] In this patent specification, adjectives such as first and second, left and right, front and back, top and bottom, etc., are used solely to define one element or method step from another element or method step without necessarily requiring a specific relative position or sequence that is described by the adjectives. The terms 'comprises', 'comprising', 'includes', 'including', or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

[0041] As used herein, the term "ore processing stream" refers to a number of steps occurring in a process flow sheet starting with the leaching of metals of value from an ore and ending with the metals recovered in substantially pure form. Different refineries will have different process streams depending on their favoured approach and/or the main ore type being processed. Such processes and the steps involved are well known to those of skill in the art.

[0042] The term "alkyr refers to optionally substituted linear and branched hydrocarbon groups having 1 to 20 carbon atoms. Where appropriate, the alkyl group may have a specified number of carbon atoms, for example, C1-C12 alkyl or Ci-C₈ alkyl or Ci-C₆ alkyl which includes alkyl groups having 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 carbon atoms in linear or branched arrangements. Non-limiting examples of alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, s- and t- butyl, pentyl, 2-methylbutyl, 3-methylbutyl, hexyl, heptyl, 2-methylpentyl, 3- methylpentyl, 4-methylpentyl, 2-ethylbutyl, 3-ethylbutyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl. The alkyl group may have a heteroatom within or at the end of the alkyl chain.

[0043] The term "alkeny refers to optionally substituted unsaturated linear or branched hydrocarbon groups, having 2 to 20 carbon atoms and having at least one carbon-carbon double bond. Where appropriate, the alkenyl group may have a specified number of carbon atoms, for example, C₂-C ₂ alkenyl, C₂-C₈ alkenyl or C2-C₆ alkenyl which includes alkenyl groups having 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 carbon atoms in linear or branched arrangements. Non-limiting examples of alkenyl groups include, ethenyl, propenyl, isopropenyl, butenyl, s- and t-butenyl, butenyl, s- and t-butenyl, pentenyl, hexenyl, hept-l,3-diene, hex-l,3-diene, non- 1,3.5-triene and the like. The alkenyl group may have a heteroatom within or at the end of the alkyl chain.

[0044] The term "aryf means a C₆-Ci₄ membered monocyclic, bicyclic or tricyclic carbocyclic ring system having up to 7 atoms in each ring, wherein at least one ring is aromatic. Examples of aryl groups include, but are not limited to, phenyl, naphthyl, tetrahydronaphthyl, indanyl and biphenyl. The aryl may comprise 1-3 benzene rings. If two or more aromatic rings are present, then the rings may be fused together, so that adjacent rings share a common bond.

[0045] The term "heterocyclic? refers to an aromatic or non-aromatic ring having 1 to 4 heteroatoms said ring being isolated or fused to a second ring selected from 3- to 7-membered alicyclic ring containing 0 to 4 heteroatoms, aryl and heteroaryl, wherein said heteroatoms are independently selected from O, N and S. Heterocyclic systems maybe attached to another moiety via any number of carbon atoms or heteroatoms of the radical and are both saturated and unsaturated, which also includes all forms of carbohydrate moieties. Non-limiting examples of heterocyclic include pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl, tetrahydorfuranyl, imidazolinyl, thiomorpholinyl, and the like.

[0046] The terms "tertiary N-alkyl, tertiary N-alkenyl and nitrogen heteroaromatic" refer to such groups when they contain a tertiary nitrogen atom. The groups may be connected to the remaining functionality of the linker via a direct attachment to their tertiary nitrogen atom or via connection to the alkyl group, alkenyl group or aromatic ring.

[0047] According to a first aspect of the invention, there is provided a method for the selective recovery of nickel and cobalt from an ore processing stream including the steps of :

(i) contacting the ore processing stream, said stream comprising solubilised nickel and cobalt, with an ion exchange resin of formula I;

formula I

wherein, L_{1 (} if present, is a linking group;

Ri and R₂ are selected from the group consisting of heterocyclic, heteroalkyl, heteroalkenyl, carboxyl and sulfonic acid and at least one of Ri or R₂ is a group having a tertiary nitrogen;

if the dashed line is a bond then W is present and is

wherein L₂, if present, is a linking group and Ri and R₂ are as described;

to thereby load the nickel and cobalt onto the resin;

[0048] It will be understood that the W and hence the L₂ -branch' from the resin matrix may not be present as a separate and different functionality. If W and hence L₂ is present then it is preferred that at least one of the Ri or R₂ attached thereto will present a different functionality to the corresponding R-_\ and R₂ connected to Li to thereby provide a variety of functionalities or at least dual dual functionality to the resin. For example, in one embodiment the Li branch could end in a BPA functionality while the L₂ branch may end in an acetic acid/picolyl mixed functionality. In a further embodiment, one branch could end in a BPA functionality while the other ends in an iminodiacetic acid (IDA) functionality. This embodiment can provide advantages in terms of useful selectivity coupled with a reduced cost for the resin compared with a similar resin showing only BPA functionality at both branches. Preferably, U and L₂, if present, are benzyl-containing linking groups and more preferably are benzyl.

[0049] Preferably, at least one of Ri or R₂ is selected from the group consisting of tertiary N-alkyl, tertiary N-alkenyl, nitrogen heterocyclic, carboxyl and sulfonic acid.

[0050] More preferably, at least one of Ri or R₂ are or form part of a picolyl, quinoline, acetic acid or piperidine group. For example, if Ri and R₂ were both pyridine attached to the N-CH₂ at the 2-position of the ring then a BPA functionality would be formed.

[0051] In one embodiment the resin of formula I is a resin of formula II:

wherein, Ri and R₂ may be independently selected as described above and R3 is hydrogen, alkyl, hydroxyl or alkoxy.

In one embodiment, the R₂ extending from the nitrogen connected to the heteroaryl ring bearing the R3 substituent may be such as to form a BPA functionality i.e. it will provide a further pyridine ring. The Ri and R₂ extending from the other resin branch may be such as to together form an iminodiacetic acid functionality.

[0052] In a further embodiment, the resin is of formula III:

wherein, R₂ is selected from the group consisting of pyridine, carboxyl and sulfonic acid and R₃ is as defined above.

[0053] Preferably, the resin is selected from the group consisting of BPA resin, acetic acid/picolyl resin, iminodiacetic/BPA and sulfonic acid/picolyl resin.

[0054] In one embodiment, the ion exchange resin is a bis-picolylamine resin. This means that the resin presents only BPA functionality. In a further embodiment, the ion exchange resin presents a mix of iminodiacetic acid functionality and BPA functionality.

[0055] A second aspect of the invention resides in the use of a resin of formula I in the selective recovery of nickel and cobalt from an ore processing stream including the steps of contacting the ore processing stream with the resin, separating the resin with loaded nickel and cobalt from the ore processing stream and contacting the resin with an ammoniacal solution to elute the nickel and cobalt from the resin.

[0056] Although not wishing to be bound by any theory it may be that the success of a resin in selectively binding nickel and cobalt and allowing their selective removal in an ammoniacal solution depends upon the presence of at least two tertiary nitrogens in the functionality available for metal complexation. One such tertiary nitrogen is provided by the nitrogen attached directly to the linking group and so the other will be provided by the functionality defined by one of Ri or R₂. Preferably, the resin will present three tertiary nitrogens available for complexation, such as with BPA resin. [0057] Cost versus functionality may be a relevant consideration and so the resin may display dual functionality. For example, the resin of formula I may display both a BPA and iminodiacetic acid (IDA) functionality, one from the l_i chain and one from the L₂ chain. BPA resin is relatively more expensive than IDA resin and so a mixed BPA/IDA resin may provide a sufficient portion of the advantages discussed herein with the IDA functionality providing some assistance in metal recovery but primarily being present to significantly lower the cost of the resin compared to a pure BPA resin.

[0058] Other than the functionalities described above the resin may be of a standard design which is well known in the art. For example polystyrene cross linked with divinyl benzene is common although the resins suitable for use in the present method are not so limited. Generally, the resin should be of good mechanical strength to minimise attrition during resin-in-pulp loading, reasonable loading capacity to deliver commercial value and of a suitable size to allow for easy filtering to remove it from a fine slurry environment.

[0059] The resin of choice for the metals industry is currently a large bead resin functionalised with IDA. Although IDA resin is relatively selective for nickel and cobalt over impurities, in scavenging resin-in-pulp the concentration of impurities is orders of magnitude higher than that of base metal value. As a result, IDA resin from scavenging circuits is typically only 30-50% loaded with nickel and cobalt. Many functional groups are filled with undesirable impurities such as ferric iron, aluminium, manganese, and magnesium.

[0060] Further, IDA resin is most readily stripped using strong sulphuric acid. The quantitative nature of this strip removes not only the nickel and cobalt, but also the significant amounts of impurities co-loaded onto the resin. The problem thus created is that this liquor is difficult to integrate into the flow sheets of existing nickel refineries. Residual acid makes refining of the contained nickel into mixed hydroxide precipitate costly, and high impurity content makes the liquor inappropriate for downstream integration close to final metal production. Due to the incompatibility of acidic eluate with most downstream nickel refining methods, scavenged value must be integrated back into existing flow sheets through into existing flow sheets through upstream recycle which has to date worked against the commercial viability of resin-in-pulp scavenging.

[0061] The use of BPA resin, and similarly functionalised resins, is shown herein to provide distinct advantages over IDA resin and strong acid stripping thereof. While the total capacity of BPA resin is roughly half that of IDA, BPA has been found to be orders of magnitude more selective for base metals such as nickel, cobalt, copper and zinc over important laterite impurities. As a result, the useful capacity of BPA resin under resin-in-pulp scavenging conditions is similar to that of IDA resin.

[0062] As can be seen from the results presented herein, copper and zinc tend to follow nickel and cobalt through the loading and stripping process. This may be an advantage in a refinery where the ore to be processed contains significant quantities of these metals. Thus, in one embodiment, the present method may include the recovery of copper and/or zinc. In typical nickel and cobalt refineries copper and zinc are treated as impurities. In this situation the eluate can be introduced into appropriate existing purification stages such as, for example, hydrogen sulphide precipitation for copper or solvent exchange for zinc. It may often be the case that the ore being processed does not contain significant enough quantities of copper and zinc to be problematic in the ammoniacal eluate.

[0063] BPA resin has, therefore, seen use in metals recovery processes but it has not previously been realised that BPA resin loaded with nickel, cobalt and a portion of metal impurities can have both the nickel and cobalt stripped in a highly selective manner by the use of an ammoniacal solution. This realisation provides distinct operational advantages for refineries in that it not only provides a nickel/cobalt solution which is relatively pure but also allows for its downstream integration due to its ammoniacal rather than strongly acidic nature.

[0064] Resin capacity and selectivity work in tandem to dictate the amount of resin required to load a given amount of nickel and cobalt. In a scavenging process only small amounts of nickel and cobalt are present against a complex background of unwanted impurities. As a result, the useful resin capacity is determined not only by the number of sorption sites on the resin, but also by the tendency of those sorption sites to load unwanted impurities instead of the desired nickel and cobalt. For this reason BPA resin is particularly favoured due to the high selectivity it demonstrates in the loading of nickel and cobalt over most impurity metals. The experimentally determined selectivity order of BPA resin is as follows: Cu » Ni > Fe(HI) > Co > n » K > Ca > Na > g > Al.

[0065] Suitably, the ore processing stream is a hydrometallurgical nickel laterite ore processing stream. The method is particularly applicable to the processing of laterite ores due to the relatively high levels of nickel and cobalt that remain in tailings and, particularly, due to the suitability of downstream integration of an ammoniacal eluate solution.

[0066] The contacting of the ore processing stream with the resin may take place at a resin-in-leach stage, a resin-in-pulp stage, a heap leach permeate stage or a decanted tailings stage of the stream. The consideration as to when the resin is introduced into the processing stream may be influenced by a number of factors including the environment for the resin e.g. solids size and amount, pH etc and engineering concerns such as the ease of processing a solution stream versus a slurry stream.

[0067] The method of the present invention has been shown to work equally well in an RIP or clarified tailings solution environment. Details are set out in the experimental section but FIG 4 sets out the ammonia elution response of RIP loaded BPA resin while table 4 details the recovery of nickel and cobalt from different RIP environments. In every case the recovery of nickel was greater than 95% while that of cobalt was at least 80%. The loading of nickel and cobalt from a clarified tailings liquor was also successfully performed with results shown in FIGs 5 and 6 as well as table 6.

[0068] In a particularly advantageous embodiment the process stream is heap leach permeate or tailings liquor i.e. substantially only that liquid component which has been separated from the heaped leached ore or from fine solids in a slurry via CCD or other means. The results described herein show that BPA resin can be show that BPA resin can be successfully loaded in a selective manner with base metals of value, such as nickel and cobalt, by passing the tailings solution, at a relatively high flow rate, through a column packed with the BPA resin. Subsequent elution with an ammoniacal solution in a small number of bed volumes, and again at relatively high flow rates, is also shown to result in high levels of recovery of nickel and cobalt with only minimal levels of impurities such as Mg, Mn, Al, Ca, Cr, Fe etc.

[0069] Such an approach is clearly attractive from an engineering and systems integration perspective and so is more likely to be taken up by existing refineries. A simple filtration step, for example through a bed of sand or other filter material, can remove any fine solids to thereby provide a solution amenable to simple passing through one or more columns to ensure maximum recovery of value metals.

[0070] Thus, the resin may be separated from the ore processing stream by size screening when the processing stream is a slurry but when the processing stream is a decanted tailings or other solution stream, as described, then the separation step may occur without need for a separate action by an operator when the stream is passed over the resin. That is, the separation in that instance is a physical separation achieved when the solution passes through the column or other container holding the resin. If an RIP approach is used then vibration screening and washing of the resin is suitable to remove solids material.

[0071] The resin may be contacted with the ammoniacal solution in a batch vessel or within a column environment. The choice of stripping environment may depend on the particular resin loaded and its eluting properties but, as described, in one embodiment the resin may be contacted with the eluant in a column or like chromatographic apparatus. For those process streams where the decant liquor contains only trace amounts or nickel or cobalt then an alternative option is to leach the tails with acid and perform a solid liquid separation to isolate the liquor separation to isolate the liquor for subsequent exposure to the resin. Also, the BPA, mixed BPA IDA or other resin as previously described may be used in a resin-in-pulp process.

[0072] The ammoniacal solution is preferably ammonia in water optionally further comprising ammonium sulphate and/or ammonium carbonate. It has been found that even relatively weak ammoniacal solutions can successfully strip nickel and cobalt from BPA resin. Table 1 sets out the recovery of nickel, which was loaded from a pure synthetic solution, from BPA resin in a series of different ammoniacal solutions while FIG 2 indicates the dynamics of nickel and cobalt recovery showing that stripping occurs quickly within a small number of bed volumes. A preferred ammoniacal solution comprises at least about 5% ammonia. Close to quantitative stripping is achieved at about 11% ammonia.

[0073] It is preferred that the ammoniacal solution does not comprise any metal-containing compound, such as magnesium sulphate. Ammoniacal solutions have been used to recover metals from ion exchange resins but often will include metal compounds within the stripping environment. It has been shown herein that this is unnecessary when stripping nickel and cobalt from BPA resin. The addition of metals, such as magnesium, which are considered impurities in nickel and cobalt refining, is undesirable as it results in an eluate which is not amenable to integration into downstream steps of a processing flow sheet such as, for example, a solvent exchange, hydrogen reduction or ammonium sulphate recrystallisation step.

[0074] The method may further include the step of introducing the ammoniacal eluate containing the nickel and cobalt directly back into the ore processing stream. The production of an eluate which can be easily and cost effectively integrated into an existing refinery process flow sheet into one or more downstream steps has not previously been realised. This, at least in part, explains the slow uptake of resin recovery technology in the base metals industry. [0075] The production of an eluate which contains significant impurities such as Mg, n, Al, Ca, Cr and Fe presents challenges in that it cannot be introduced into a processing stream of reasonably high purity as subsequent recovery of the pure metals through solvent exchange followed by hydrogen reduction, electrowinning or other processes would be compromised. The only option then is to introduce the eluate upstream into the leaching or subsequent neutralisation or decantation steps. This adds further volume to already limiting stages and increases the overall recovery cost.

[0076] Further, with a typical strong acid eluate the pH of the solution means it is difficult or at least costly, as discussed above, for it to be introduced into processing steps addressing key intermediate products or where pure metal is to be directly recovered. A key intermediate appearing on the flow sheets of a number of refinery types is a mixed hydroxide precipitate (MHP). Strong acid eluate will only add to the cost of the MHP processing route.

[0077] The present inventive method provides for a solution to this problem by the generation of a high purity, ammoniacal eluate. Preferably, the purity of the ammoniacal eluate is such that it is suitable for introduction at a processing stream stage selected from the group consisting of an ore intermediate leach stage, a nickel/cobalt separation stage, a CCD tailings stage, a pH increasing stage or a stage where one of the metals is selectively recovered as an intermediate or final product.

[0078] The particular stage at which the eluate is introduced will vary with the flow sheet used by the refinery. By way of example, if the site employs an ammonia leach then the eluate may be mixed with the liquid fraction exiting the CCD after the first stage leach. This would effectively upgrade the concentration of nickel, assuming use of a Caron type (ammonium carbonate) process, since the present eluate would be at approximately 15-20g/L Ni and would be added to a solution that typically sits at 10 g/L Ni. To be clear, in one embodiment, in a process flow sheet employing the Caron Process (ammonia+ammonium carbonate), the flow sheet may comprise a roast step followed by a leach step and then CCD with the slurry going to tails. This can be followed by a scavenging scavenging process which then produces a liquor phase. When the resin scavenging of the present invention is applied then this may be a suitable point at which to integrate the ammonia eluate. Alternatively, if the quantities of ammonia are appropriate then it may be suitable to first perform the steam strip and subsequently integrate the ammonia eluate. This will then be followed by solvent extraction to separate the nickel and cobalt.

[0079] In an ammonium sulphate system the leach solution produced will typically contain 100 g-Ni/L In this case it might be more desirable to feed this eluate into the leaching stage (instead of after). Alternatively, at a site which uses acid leaching followed by H₂ reduction to recover nickel, the eluate generated in the present method could be added into the system at the point where the pH of the acidic leach solution is to be raised, typically by the addition of ammonia i.e. in an ammonium sulphate type system. In this case, the ammoniacal eluate is used as the source of ammonia, but comes with the additional solubilised and relatively pure nickel (instead of using pure ammonia). Depending on how the load circuit is run, more or less Cu/Co/Zn can be collected in the eluate, making this eluate compatible earlier or later in the refining process. For example, an eluate which is relatively high in Cu would better be introduced with other material inputted into an appropriate solvent exchange purification step rather than further downstream where nickel metal is to be obtained.

[0080] When a Sherritt type process is employed at a refinery then the flowsheet and possible integration points for the nickel-ammonia eluate solution provided by ammonia stripping of the present resin can be summarized as follows. Firstly, a pressure oxidizing leach of the mixed nickel cobalt sulphide is performed followed by possible integration of the ammonia-nickel eluate (particularly if high in copper). The next step in the process may be copper removal using a copper boil. Once again, following this step, may be a suitable point for integration of the ammoniacal eluate (if low in copper). From here, nickel/cobalt separation using solvent extraction or selective precipitation may be appropriate to isolate nickel from cobalt. [0081] In one embodiment, the eluate will comprise nickel and cobalt as the major portion of metals. That is, the eluate will contain a higher combined amount or concentration of nickel and cobalt compared with typical problem impurity metals selected from the group consisting of iron, magnesium, manganese, aluminium, calcium and chromium due to the selectivity of the loading and/or ammoniacal stripping steps. Preferably, the metal content of the eluate is substantially comprised of nickel and cobalt. Clearly the proportion of nickel and cobalt in the eluate will depend greatly on the base metal content of the processing stream from which the resin was loaded as well as the concentration of impurities also contained therein. In most refinery processing streams, however, the above will be true.

[0082] The successful loading of a selected BPA resin with nickel and cobalt, their subsequent stripping in an ammoniacal solution and the selectivity demonstrated will now be set out in some detail. It will be appreciated that the present method is not limited to the use of any one resin type and so the following is by way of illustration only.

EXPERIMENTAL

Resin characteristics

[0083] The resin used in the following tests is Lewatit MonoPlus TP 220 (hereinafter referred to as TP 220) made by Lanxess ION in Germany. TP 220 presents a bis-picolylamine functional group and has a backbone of polystyrene cross linked with divinylbenzene. The mean particle diameter of the spherical shaped resin is 0.6 mm with 90% of the particles in the range of 0.55 to 0.65 mm. The capacity of the resin depends on solution composition, but previous research has shown the operational capacity of TP 220 to be 0.9-1.1 moles of equivalent charge per litre of tapped wet settled resin (all future resin volumes refer to tapped wet settled resin). Resin was received protonated with sulphuric acid and was not treated prior to use aside from a de-ionized water rinse. Resin handling and resin loading in synthetic solution

[0084] To explore the resin stripping chemistry, samples of TP 220 were loaded with a synthetically produced nickel and cobalt solution. This was carried out by contacting a known volume of tapped wet settled resin in the protonated form with 10 times the volume of resin of solution containing 5 g/L Ni or Co as dissolved sulphate salts. The resin and solution were gently agitated at room temperature. The pH was then adjusted through addition of 2M NaOH until it was stable at pH 4, typically taking 4-6 hours in total. Once loaded, resin was placed in a column and washed with 20-30 bed volumes of de-ionized water to remove any solution still entrained in pores. This process yielded resin loaded with 30 g Ni or 19 g of Co per litre of resin.

[0085] Determination of the loaded resin composition was carried out by quantitatively stripping 2 to 5 mL of resin. This small volume of resin was wet filtered and contacted with 100 mL of 200 g/L H2SO4 in a baffled Erienmeyer flask. The flask was set in a shaking incubator at 25°C rotating at 150 rpm for 24 hours. The efficacy of different eluate compositions was tested in a similar fashion. To determine the kinetics of the stripping reaction, samples of the solution were taken at regular intervals.

Tailings material - industrial and synthetic

[0086] Resin-in-pulp tests were carried out in batch reactors using both synthetically created pulp and actual tailings sourced from an industrial nickel refinery. Synthetic pulp was made by starting with a solution of 20 g/L Fe(lll), 4 g/L Al, 2 g/L Mn, and 20 g/L Mg with 0.5-1 g/L Ni and 50-500 mg/L Co. This solution was heated to 80°C on a hotplate in a baffled batch reactor with vigorous stirring via an overhead impeller. The pH was increased to precipitate iron through addition of CaC0₃ as slurry (20% w/w). Iron precipitation was carried out over 5 hours with the terminal pH at 3.9-4 and an extent of reaction of greater than 99.5%. The composition of the resultant solution in the slurry (20% w/w solids) is shown in table 1. Resin-in-pulp contact with this slurry was carried out carried out immediately after precipitation was complete and the slurry had cooled to 60°C.

Table 1 : Composition of solution in synthetic pulp

[0087] The industrial plant samples used in the following experiments came from three points in a nickel laterite ore processing stream. The first was a sample of slurry (Slurry ) taken after the acid leach step but before final neutralization. The second was a sample of slurry (Slurry 2) taken from after final neutralization before discharge to tailings pond. The third was a sample of decant liquor (Decant 3) from the overflow of the tailings pond to the crystallization pond. FIG 1 shows the source point of each material in the process schematic. The composition of these materials is summarized in table 2. Slurries 1 and 2 are slurries of 35% w/w solids, while decant material 3 is clarified liquor.

Table 2: Composition of solution in industrial pulp slurries and decant liquor

Resin-in-pulp procedure

[0088] The same experimental method was used to test resin-in-pulp scavenging on both the industrial and synthetic material. Once the sample was at the chosen temperature, wet filtered TP 220 resin was added. Resin was contacted with pulp for 3 hours under moderate mixing. The pH of the reactor was monitored inline and was controlled through addition of 20% w/w CaC0₃ slurry. slurry. The slurry reached the pH set point of 4.0 within 20-30 minutes and was stable for the rest of the contact time. On completion, final slurry and solution were sampled while resin was separated from slurry via sieves and washed thoroughly with water. Small samples of resin were assayed using the acid stripping procedure described above while the rest was set aside for column elution or further reloading.

Column testing procedures testing decant liquor

[0089] The third industrial plant material tested was decant solution rather than a slurry and so metal value could be scavenged by simply passing the solution through a column packed with the resin. The experimental method for this was therefore quite simple. Tailings decant liquor was passed over the resin at a rate of 5 bed volumes per hour at 25°C. Samples of permeate were collected. Following this, the resin was rinsed in the column with de-ionized water.

[0090] Whether the resin was loaded through resin-in-pulp contact or column contact, the experimental method for elution was the same. Simply, eluent was pumped into the top of the column using a peristaltic pump, passed over the resin, and eluate was collected and sampled. As ammonia based elution was used, care was taken to keep the eluent reservoir, column, and eluate reservoir air tight to prevent excess volatilization of ammonia. Additionally, the column was water jacketed to allow temperature control when necessary.

Eluent Composition - Synthetic Batch Flasks and Resin Columns

[0091] To determine the optimal composition of the eluent, solutions containing various amounts of ammonia and ammonium sulphate/carbonate were contacted with the resin which had been loaded from the synthetic nickel solution and allowed to equilibrate. The results of these tests are presented in table 3. It can be seen that nickel is readily stripped by even dilute ammonia solutions, with or without ammonium sulphate or carbonate. Test NH₃ % (NH₄)₂S0₄ g/L (NH₄)₂C0₃ g/L % Ni recovery

1 2.80 - - 75

2 5.60 - - 84

3 8.43 - 92

4 11.24 - - 99

5 5.60 ^' 289 - 91

6 5.60 198 - 89

7 5.60 99 - 90

8 5.60 - 217.5 85

9 5.60 - 152 86

10 5.60 - 76 91

Table 3: Equilibrium recovery of nickel from resin loaded from synthetic nickel solution with various ammoniacal solutions, 5 mL resin, 100 ml_ solution, 25°C

[0092] Next, tests were carried out to determine the elution behaviour of nickel and cobalt in columns. Two columns were run, one containing resin loaded from the synthetic pure nickel solution and the other containing resin loaded from the synthetic pure cobalt solution. Samples of these resins were eluted with 7% NH₄OH solution at a rate of 10 BV/hour at 25°C. The results of these columns are shown in FIG 2.

[0093] These results show that a high recovery of nickel and cobalt from TP 220 resin can be achieved in a small number of bed volumes at a relatively high elution flow rate. These results demonstrate that both nickel and cobalt can be recovered efficiently with a mild ammoniacal elution.

Elution Kinetics

[0094] To investigate the kinetics of ammonia stripping, samples of the resin loaded with nickel from the synthetic solution were contacted with one of two different strengths of ammonia solution or a sulphuric acid solution using the stripping methodology described above, with frequent samples taken for the first 2 hours and a final sample after 24 hours to show equilibrium behaviour. The kinetic curves generated are shown in FIG 3.

[0095] It is clear from the results that the ammonia elution reaction is considerably faster than acidic stripping of the same resin, though at equilibrium all three solutions recover greater than 95% of the nickel on the resin. Increasing the concentration of ammonia improved reaction kinetics, but not drastically so. The efficacy of ammonia stripping is surprising. Although nickel and cobalt are known to form ammonium complexes it would not be expected that the dynamics of stripping would outperform that of strong acid utilisation.

Resin-tn-Pulp Loading

[0096] Resin-in-pulp scavenging of nickel and cobalt was then tested on the synthetic and industrial materials described in tables 1 and 2. The results of these tests are shown in table 4 and table 5. In every case, recovery of nickel was greater than 95% and recovery of cobalt greater than 80%. While not shown in these tables, resin-in-pulp contact at 25°C yielded similarly high recovery. However, the higher temperature improved the rheology of the slurry and allowed for easier mixing. In each test, resin-slurry contact time was 3 hours. Solution samples were taken at the 0.5 and 1.5 hour mark and in each test the solution concentration of nickel at 30 minutes was 20-40 ppm, indicating that the vast majority of nickel was recovered in the first 30 minutes of contact. Cobalt followed a similar profile. This kind of rapid loading of the metals of interest assists in integration of the RIP scavenging into the existing processes of a working refinery.

Table 4: Resin-in-pulp scavenging o Ni and Co from synthetic and industrial materials, pH 4, 60°C, 3 hour contact time

Table 5: Composition of loaded resin from resin-in-pulp tests in table 4 (g/L)

[0097] Evidence of the high selectivity of TP 220 for nickel and cobalt can be seen in Table 5, which shows that the useful capacity of the resin (defined as mass of nickel and cobalt per litre resin over mass of all loaded metals) ranges from 75-90%. The most significant impurity loaded by TP 220 was ferric iron. However, this loading is not irreversible and it can be seen from the resin selectivity order that nickel is higher than ferric iron. As a result, iron is exchanged for nickel as the resin undergoes successive loading stages. This was demonstrated with test 4, which began with resin loaded in a previous contact. This initial resin was loaded with 10.3 g/L Ni and 2.94 g/L Fe. After contact with another batch of tailings slurry, the loaded resin contained 15.2 g/L Ni and 1.3 g/L Fe. These results indicate that iron will not accumulate on the resin beyond a certain point which is a distinct advantage during subsequent metals purification steps.

[0098] After being loaded through resin-in-pulp contact, resin was eluted using aqueous ammonia. FIG 4 shows the elution response of the combined resin from tests 1 and 2. Only Ni, Co, Cu and Zn are shown in the figure, as the elution of other metals was minimal. It is notable that peak concentrations of Al, Fe, Cr, Mg, Mn, and Ca in the eluate were 1 ppm or less. This provides a relatively high purity eluate which therefore opens up a number of options as to which downstream point in the process flow sheet the eluate can be introduced. After 4 bed volumes of eluent, elution recovery of nickel and cobalt was 94% and 83% respectively. and 83% respectively. This level of selectivity and nickel and cobalt recovery was achieved in all columns of resin-in-pulp loaded resin.

Column Loading from Tailings Decant

[0099] Recovery of nickel and cobalt value from tailings pond decant liquor was investigated. The industrial sample used was taken from tailings decant en route to crystallization ponds, and as such contained virtually no solids. As a result, nickel and cobalt could be recovered onto BPA resin from this liquor using much simpler column ion exchange as opposed to resin-in-pulp. The column contacting took the following form: TP 220 resin in the proton form was first contacted with decant liquor, then resin was eluted using aqueous ammonia, followed by a second contact with decant liquor. FIG 5 and FIG 6 show breakthrough curves for Ni, Co, and Fe for each contact. Breakthrough curves for Al, Ca, Cr, Mg, Mn and Na are neglected, as these elements effectively showed no affinity for the resin in these tests. Once again this indicates an eluate of excellent purity can be achieved based upon the selective loading and ammoniacal elution of BPA resin. The small amount of Zn and Cu in solution followed the same curve as that of Ni.

[00100] The results of these tests show that nickel and cobalt load onto the BPA resin in a highly preferential manner even in spite of the low pH of the liquor and subsequent ammonia elution showing the same high selectivity and recovery as the resin loaded from pulp. It can be seen that the resin is less selective for cobalt than nickel, indicated by the earlier breakthrough curve of cobalt. Towards the end of each column, cobalt concentration in the permeate was greater than that of the head solution, indicating that cobalt loads early and then is pushed off the resin in exchange for nickel.

[00101] The chief impurity concern in these tests was iron. The total concentration of iron in the head solution was 5.8 g/L, however, most of this was present as ferrous iron, indicated by the relatively low E_h of the solution (373 mVAg/Agci)- Ferrous iron does not bond strongly with BPA resin, but ferric iron is second only to nickel and copper in the selectivity order. As a result, it can be can be seen from iron levels in the permeate in FIG 5 and FIG 6 that iron gradually loads onto the resin. However, by the end of the second contact with resin the amount of iron in the permeate is greater than that of the head, indicating that iron is actually being exchanged in favour of nickel. This result suggests that the amount of iron on the resin will plateau rather than increase at the expense of available useful capacity.

[00102] After each contact with decant liquor, resin was eluted with 6.8% aqueous ammonia in the same fashion as the resin-in-pulp loaded resin. The results of elution were similar to that of the resin-in-pulp loaded resin. Nickel and cobalt recovery in 2 bed volumes of eluate was 90% and 75% respectively, with ppm levels of impurities. A summary of resin compositions at each stage of the sequential column loading is presented in table 6.

Table 6: Composition of resin after column loading from decant liquor and elution with aqueous ammonia

[00103] In summary, BPA functionalized TP 220 resin was found to be effective at recovering nickel and cobalt both in batch resin-in-pulp tests on industrial neutralized tails slurry as well as in column contact with clarified tailings decant liquor. The resin's high selectivity for nickel and cobalt over impurities leads to a useful resin capacity of 75-90%, considerably higher than that of iminodiacetic acid functionalized resin. Ferric iron was the most significant impurity to be loaded, but does not overwhelm the resin. Instead, iron is gradually pushed off by nickel due to its rank in the resin selectivity order. Resin-in-pulp loading kinetics were rapid, with over 95% of the metal ultimately loaded being taken up by the loaded being taken up by the resin in the first 30 minutes of contact. The scavenging of nickel and cobalt from tailings decant offers a potential method to recover value metals that would otherwise be lost to waste while obviating the need for more complicated resin-in-pulp contact.

[00104] Resin-in-pulp scavenging of nickel and cobalt from laterite tailings has not been commercialized to date, in large part due to the high cost of integrating acidic eluate from IDA resin into existing nickel refining flow sheets. The use of BPA resin and ammonia elution of nickel and cobalt offers new possibilities for integration of resin-in-pulp, and resin after tailings solution contact, with existing refining flow sheets. As the eluate contains high concentrations of nickel and cobalt and only trace amounts of impurities, downstream integration of value metal recovered from tailings is appropriate thereby offering hitherto unrealised efficiencies in the value chain.

[00105] The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. Accordingly, this patent specification is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

Claims

1. A method for the selective recovery of nickel and cobalt from an ore processing stream including the steps of:

formula I

wherein, Li, if present, is a linking group;

Ri and R₂ are independently selected from the group consisting of heterocyclic, heteroalkyl, heteroalkenyl, carboxyl and sulfonic acid and at least one of Ri or R₂ is a group having a tertiary nitrogen;

if the dashed line is a bond then W is present and is

L₂

wherein L₂, if present, is a linking group and Ri and R2 are as described; to thereby load the nickel and cobalt onto the resin;

(ii) separating the ion exchange resin, with loaded nickel and cobalt, from the ore processing stream; and (Hi) contacting the nickel and cobalt loaded ion exchange resin with an ammoniacal solution to elute the nickel and cobalt into the ammoniacal solution to thereby recover the nickel and cobalt.

2. The method of claim 1 wherein l_i and L₂ are independently selected from the group consisting of alkyl, alkenyl, aryl, heteroaryl and benzyl linking groups.

3. The method of claim 1 or claim 2 wherein Ri and R₂ are independently selected from the group consisting of tertiary N-alkyl, tertiary N-alkenyl, nitrogen heterocyclic, carboxyl and sulfonic acid.

4. The method of any one of the preceding claims wherein at least one of R, or R2 are or form part of a picolyl, methylquinoline, acetic acid or methylpiperidine group.

5. The method of any one of the preceding claims wherein the resin of formula I is a resin of formula II:

formula II

wherein, Ri and R₂, independently, are as described for claim 1 and R3 is selected from the group consisting of hydrogen, alkyl, hydroxyl and alkoxy.

6. The method of any one of claim 1 to claim 4 wherein the resin is of formula

formula Hi

wherein, F¾ is selected from the group consisting of pyridine, carboxyl and sulfonic acid and R₃ is selected from the group consisting of hydrogen, alkyl, hydroxyl and alkoxy.

7. The method of claim 1 wherein the resin is selected from the group consisting of BPA resin, acetic acid/picolyl resin and sulfonic acid/picolyl resin.

8. The method of claim 1 wherein the resin is a bis-picolylamine resin or a mixed bis-picolylamine/iminodiacetic acid resin.

9. The method of any one of the preceding claims wherein the ore processing stream is a nickel laterite ore processing stream.

10. The method of any one of the preceding claims wherein the contacting of the ore processing stream with the resin takes place at one or more of a resin-in- leach stage, resin-in-pulp stage, heap leach permeate or a decanted tailings stage of the stream.

11. The method of claim 10 wherein the resin is separated from the ore processing stream by size screening when the processing stream is a slurry.

12. The method of claim 10 wherein when the processing stream is a decanted tailings or other solution stream then the separation step occurs when the stream is passed over the resin.

13. The method of claim 12 wherein the tailings or solution stream is filtered to remove solid particles prior to being passed over the resin.

14. The method of any one of the preceding claims wherein the resin is contacted with the ammoniacal solution in a batch vessel or within a column packed with the resin.

15. The method of any one of the preceding claims wherein the ammoniacal solution is an aqueous ammonia solution.

16. The method of claim 15 wherein the aqueous ammonia solution further comprises ammonium sulphate and/or ammonium carbonate.

17. The method of any one of the preceding claims further including the step of introducing the ammoniacal eluate containing the nickel and cobalt directly back into the ore processing stream.

18. The method of claim 17 wherein the ammoniacal eluate is introduced into a stage of the processing stream selected from the group consisting of an ore intermediate leach stage, a nickel/cobalt separation stage, a CCD tailings stage, a pH increasing stage and a stage where one of the metals is selectively recovered as an intermediate or final product.

19. The method of any one of the preceding claims wherein when the ore processing comprises an ammonia leach step then the ammoniacal eluate is introduced into a liquid fraction exiting a counter current decantation step following a first stage leach.

20. The method of any one of claim 1 to claim 18 wherein when the ore processing comprises an acid leach followed by a hydrogen reduction step then the ammoniacal eluate is introduced into the process at a step where the pH of the acidic leach solution is to be raised.

21. The method of any one of the preceding claims wherein the metal content of the eluate is substantially comprised of nickel and cobalt.