FIELD OF THE INVENTION

[0001] The present invention relates to a process for the precipitation of nickel and/or cobalt from solution.

BACKGROUND TO THE INVENTION

[0002] In hydrometallurgical plants cobalt and nickel are leached into solution, either as the primary economic metal or as by-products in other metals processing (such as copper).

[0003] The most common industrial methods for recovery for nickel and cobalt from solution to a saleable form is either as metal or a hydroxide precipitate. In processing routes where the nickel and cobalt are precipitated from solution this can be done separately in sequence (to obtain a mainly nickel containing precipitate, such as nickel hydroxide and a mainly cobalt containing precipitate such as cobalt hydroxide) or as a mixed hydroxide precipitate (MHP). The neutralising agent used in this process is magnesia (MgO). Magnesia is an effective precipitation agent since it is cheaper than some neutralising agents like sodium hydroxide, readily available and as the nickel or cobalt is precipitated the magnesium passes from solid to solution meaning there is limited contamination of the final product with the neutralising agent. Magnesia, however is expensive relative to calcium based neutralisation agents such as Lime or Limestone.

[0004] A previous patent was filed for the recovery of zinc from sulphate solution using limestone or lime as a neutralising agent. The problem with such a process is that the precipitated zinc is contaminated with co-precipitated gypsum which arises from using calcium based neutralising agents. In US20140105797 Al (US patent number 8974753) a process is described where the resulting gypsum is removed from the zinc oxide/carbonate precipitate by exploiting the size difference between the zinc precipitate and the gypsum to render a commercially valuable high grade zinc product. The gypsum crystals are grown by recycling some product as seed. The zinc precipitate of interest and the gypsum are separated from each other by physical means such as a screen or hydrocyclone.

[0005] The current invention reports a similar process where surprisingly, a similar process can be applied to nickel and cobalt to produce a nickel and/or cobalt oxide/hydroxide or a carbonate precipitate. The precipitate type will depend on the operating conditions but for simplicity will be referred to as nickel precipitate or cobalt precipitate or a nickel containing material or a cobalt containing material. The metals can either be precipitated singly, separately, separately in sequence or together as a mixed precipitate. The advantages of this process are use of lime or limestone which is more abundant and significantly cheaper than magnesia. It also prevents magnesium accumulating in the system process water causing solubility limitations and contamination from co-precipitation. In some embodiments, other deleterious elements in solution such as magnesium can remain in solution and not be co-precipitated resulting in a higher grade nickel and/or cobalt precipitate.

[0006] The applicant does not concede that the prior arts discussed in this process forms part of the common general knowledge in Australia or elsewhere.

[0007] Throughout this specification, the term "comprising" or its grammatical equivalents are to be taken to have an inclusive meaning unless the context of use indicates otherwise.

BRIEF DESCRIPTION OF THE INVENTION

[0008] It is an object to the present invention and to provide an improved process for precipitating nickel or cobalt from an acidic solution containing dissolved nickel or cobalt. The solution may also contain one or more of dissolved magnesium, iron, copper, silicon, cadmium and zinc. The process can be used to recover a high grade and saleable precipitate containing nickel and/or cobalt.

[0009] In a first aspect, the present invention provides a method of precipitating a nickel or cobalt containing solid from an acidic solution containing dissolved nickel and/or cobalt comprising: a) contacting the solution with a calcium containing neutralising agent to raise the pH of the solution to 5.0 to 9.0 at a temperature of from 40 to 95 °C to thereby precipitate a solid cobalt containing material and gypsum, and separating the cobalt containing material from the gypsum; or b) contacting the solution with a calcium containing neutralising agent to raise the pH of the solution to 7.0 to 9.0 at a temperature of from 40 to 95 °C to thereby precipitate a solid nickel containing material and gypsum, and separating the nickel containing material from the gypsum; or c) contacting the solution with a calcium containing neutralising agent to raise the pH of the solution to 5.0 to 9.0 at a temperature of from 40 to 95 °C to thereby precipitate a solid nickel and cobalt containing material and gypsum, and separating the nickel and cobalt containing material from the gypsum; or d) performing steps a) and b) in a sequence, at different pH to recover separate cobalt and nickel products wherein the cobalt is precipitated in step (a) at a pH between 5.0 to 7.5 and nickel is precipitated in step (b) at a higher pH between 8.0 and 9.0.

[0010] In some embodiments, the acidic solution also contains dissolved magnesium and the precipitation step is operated without causing substantial precipitation of magnesium. In this manner, dissolved magnesium stays in solution and does not report to the nickel and/or cobalt precipitate. The precipitation process suitably involves a precipitation step that is operated in a range where nickel and/or cobalt will be precipitated and magnesium will remain in solution. The key operating conditions are pH from 5.0 - 9.0, or 6.0 to 9.0, for cobalt and/or nickel. In some embodiments, in steps (a), (b) or (c) the pH is from 7.5 to 8.5 or about 7.5, and an operating temperature of from 40 to 95 °C, preferably about 60°C, is used. The calcium containing neutralising agent is suitably in the form of lime (CaO) or hydrated lime (Ca(OH) ₂ ). In some embodiments, limestone (CaCO ₃ ) may be used as the neutralising agent. A mixture of different calcium- containing neutralizing agents may be used. [0011] In some embodiments, the solution containing dissolved nickel and/or cobalt may also contain other dissolved metals, such as iron, copper, silicon, cadmium and zinc, however other metals may also be present. In this embodiment, the process may also include the step of (d) contacting the solution with a calcium containing neutralising agent to raise the pH of the solution to 4.5 to 6.0 at a temperature of from 40 to 95°C to precipitate other metals (Cu and/or Zn) to form a solid Cu and/or Zn containing material and gypsum, and separating the Cu and/or Zn containing material from the gypsum. This step will suitably be conducted before precipitation of nickel and/or cobalt. Removing the dissolved copper and/or zinc from the solution enables the copper and/or zinc to be recovered and sold or processed further to form copper metal and/or zinc metal. In embodiments where the solution also contains dissolved magnesium, this precipitation step may be operated without causing substantial precipitation of magnesium. The precipitated copper and zinc material and precipitated gypsum will normally be separated from the liquor, with the liquor (still containing dissolved nickel and/or cobalt) being sent for further processing to recover nickel and/or cobalt therefrom.

[0012] In one embodiment of the present invention, the process comprises contacting the solution with a calcium containing neutralising agent to raise the pH of the solution to 5.0 to 9.0, or from 6.0 to 9.0, more preferably 6.5 to 7.5 at a temperature of from 40 to 95°C, preferably about 60°C, to thereby precipitate a solid nickel and/or cobalt containing material and gypsum, in some embodiments without causing substantial precipitation of magnesium, and separating the nickel and/or cobalt containing material from the gypsum. In this embodiment, the calcium containing neutralising agent is suitably in the form of lime (CaO) or hydrated lime (Ca(OH) ₂ ). Nickel and/or cobalt in solution will be precipitated in the form of nickel oxide and/or cobalt oxide in this embodiment. Throughout this specification, the term "nickel oxide" will be used to refer to a combination of nickel oxide, nickel hydroxide and nickel sulphate since depending on the operating conditions will dictate the proportions of each compound formed. Similarly, the term "cobalt oxide" will be used to refer to a combination of cobalt oxide, cobalt hydroxide and cobalt sulphate since depending on the operating conditions will dictate the proportions of each compound formed. [0013] In embodiments of the present invention where the nickel and/or cobalt in solution is precipitated as nickel and/or cobalt oxide, the process involves a precipitation step that is operated in a range where nickel and/or cobalt will be precipitated and, in some embodiments, dissolved magnesium will remain in solution. In some embodiments, the operating conditions include pH from 5.0 - 9.0, preferably about 7.5 to about 8.5, and an operating temperature of from 40 to 95 °C, preferably about 60°C. The calcium containing neutralising agent is suitably in the form of lime (CaO) or hydrated lime (Ca(OH) ₂ ).

[0014] In one embodiment, in the precipitation stage, the pH of solution containing nickel and/or cobalt is raised, for example by the addition of neutralising agent such as lime, to a pH in the range of 5.0 to 9.0, or from 6.0 to 9.0, to precipitate nickel and/or cobalt as oxide forms and the whole or a major part of lime is converted to gypsum. This stage may be operated successfully at temperature up to boiling points of the solution at atmospheric pressure, preferably in the range of 40 to 95°C. The pulp leaving this stage may be sent to a separation stage with or without solid-liquid separation. In some circumstance, it may be desirable to re-circulate portions of pulp or thickened pulp from this stage to the precipitation stage.

[0015] In some circumstances, it may be desirable to purify the liquor before the precipitation stage to remove other metals. Known processes may be used to remove other metals from solution prior to the nickel and/or cobalt precipitation step.

[0016] Following precipitation, the pulp is sent to a separation process in order to separate the nickel and/or cobalt precipitate from gypsum. In one embodiment, nickel and/or cobalt precipitate is separated from the gypsum using a flotation step. Flotation is a process for separating finely ground minerals from their associated gangue. This process is usually used to separate one solid from another by using the affinity of air bubbles to solids. In this stage, nickel and/or cobalt oxide including metal oxides or hydroxides is recovered as flotation concentrate and gypsum is recovered as tailing. Cationic collectors, such as dodecylamine hydrochloride, are used as flotation agents. Potassium amyl xanthate or dodecylamine sulphate may be used for this purpose. Any frother could be used, but a Dowfroth 250 was found useful. The flotation process may be operated at ambient temperature and also successfully at temperature up to 90.degree. C. and the pulp from the third stage may be used without any heating or cooling stages. The gypsum from the flotation step can be recycled to either of the leaching or neutralization steps.

[0017] In an alternative separation process, the precipitation step results in the nickel and/or cobalt precipitate forming a finer fraction of particles and the gypsum forming a coarser fraction of particles. The finer particles (which contain most of the precipitated nickel and/or cobalt) may be separated from the coarser particles (which contain most of the gypsum) using alternative separation techniques. This operation may be used in conjunction with, or as an alternative to, flotation. The sizing separation may be performed by any method, either individually or in combination. For example, sizing may be performed by using one or more of the following techniques: classifier, elutriation, settling, screening, tables, and cyclones. A classifier is a device for subjecting comminuted ore to the action of water either in such a way that a division of the ore particle is made into two or more products according to relative settling powers. Cyclones are devices primarily used for separation of solids from fluids. Cyclones oppose centrifugal forces collinear to fluid drag, substantially at right angles to a rapid carrying current. Since such separation depends on relative particle size and specific gravity, it can be used for separation of solids from each other.

[0018] In one embodiment, the precipitation step results in most of the nickel and/or cobalt precipitate having a particle size of less then 30 μπι and most of the gypsum having a particle size of greater than 30 μm. In other embodiments, the particle size of the precipitated gypsum in the precipitated nickel and/or cobalt may be controlled such that the cut point between the coarser particles and the finer particles is different to 30 μm. For example, in another embodiment, most of the gypsum may report to a +50 μπι fraction and most of the nickel and/or cobalt precipitate may report to a- 50 μm fraction. In some embodiments, the cut point between the course fraction and the fine fraction is around 20 μm. Other size cutpoints may also be used. [0019] Control of the size of the gypsum particles may be achieved by feeding gypsum seed particles to the precipitation stage. The gypsum seed particles may comprise a recycled stream of gypsum fines that are recovered from the precipitation stage (for example, this stream may be obtained by recycling part of the fines stream from the separator that is used to separate the course gypsum particles from the fine particles after the precipitation stage). The recycle ratio can be controlled such that the gypsum particles are not too large to impact the process.

[0020] In another embodiment, the nickel and/or cobalt oxide in the precipitated slurry can be separated from gypsum by a screening technology that uses appropriate sieves. In this stage, the precipitated slurry is wet-screened with a series of sieves and the undersized materials are collected for the final product. The oversized materials are collected and recycled partly or wholly to the leaching or neutralization step. Also, the oversized materials can be leached with acid, such as sulphuric acid, to dissolve nickel and/or cobalt. Then, the nickel and/or cobalt bearing solution can be recycled to the leaching, neutralization or precipitation stage to recover nickel and/or cobalt. The leached residue can be washed and recovered as pure gypsum. The undersized materials also can be treated further by flotation or granulometric/size separation operation, as described in the previous sections, to increase nickel and/or cobalt grade. In another embodiment, the process comprises contacting the solution with a calcium carbonate containing neutralising agent to raise the pH of the solution to 5.0 to 7.5, or from 6.0 to 7.5, preferably 6.5 to 7.5, at a temperature of from 40 to 95 °C preferably about 60°C, to precipitate a solid nickel and/or cobalt containing material and gypsum without causing substantial precipitation of magnesium, and separating the solid nickel and/or cobalt containing material from the gypsum. In this embodiment, the calcium containing neutralising agent is suitably limestone (CaCO ₃ ) and the nickel and/or cobalt in solution is precipitated as nickel and/or cobalt carbonate.

[0021] In one embodiment, milled limestone is contacted with nickel and/or cobalt sulphate solution at pH 5.0 - 7.0 or 6.0 to 7.0, at a temperature of from 40 to 95 °C, preferably about 60°C. The milled limestone may have a nominal size of less than 100 μm, such as about 75 μm . Nickel and/or cobalt carbonate and gypsum will precipitate. The slurry is then passed to a separator to separate the nickel and/or cobalt carbonate and gypsum from the solution. For example, the separator may be a thickener where the overflow (which may contain +30 μπι particles that include a significant proportion of the precipitated gypsum and unreacted limestone) is sent forward to nickel and/or cobalt polishing (as not all nickel and/or cobalt may be removed from solution) or for solid/liquid separation. The underflow (which may contain sub 30 μπι particles that include a significant proportion of the precipitated nickel and/or cobalt carbonate) from the first thickener may be partly recycled to another reactor or directly to a reaction vessel or thickener, preferably at about 60°C, to contact all or just a portion of the incoming fresh nickel and/or cobalt sulphate stream from the leach process. In this stage, any unreacted limestone from nickel and/or cobalt precipitation is converted to gypsum. Minimal nickel and/or cobalt would be precipitated from solution at this stage as reaction of unreacted limestone would dominate. The slurry from this stage is sent to a second separator and the liquid stream (overflow), which still contains dissolved nickel and/or cobalt, is sent for nickel and/or cobalt precipitation with limestone. The underflow from the second separator may be sent to nickel and/or cobalt solution polishing to remove any nickel and/or cobalt from solution or may be sent straight to gravity separation where any precipitated nickel and/or cobalt carbonate would report to the fine fraction and the coarse gypsum fraction would be recycled to the process. The concentrate could then be passed to a further separator, such as a thickener and filter. The cake may be washed to remove any magnesium in solution and also remove any nickel and/or cobalt should a nickel and/or cobalt polishing step be excluded. The nickel and/or cobalt polishing step may be conducted by any known method and could be a continuously stirred tank reactor (CSTR) contacting the slurry and hydrated lime at suitable conditions, such as pH 6.5, to precipitate any nickel and/or cobalt, from solution.

[0022] In one embodiment of the present invention, the precipitation step results in a slurry containing precipitated nickel and/or cobalt carbonate and gypsum being formed. This slurry may be separated into a nickel and/or cobalt carbonate -rich fraction and a gypsum-rich fraction, the gypsum-rich fraction containing precipitated gypsum and unreacted limestone. The nickel and/or cobalt carbonate- rich fraction may comprise fine particles, such as sub 30 μπι particles, that include a significant proportion of precipitated nickel and/or cobalt carbonate, and the gypsum-rich fraction may comprise coarse particles, such as +30 μπι particles that include a significant proportion of precipitated gypsum and unreacted limestone. The nickel and/or cobalt carbonate -rich fraction may be sent forward to nickel and/or cobalt polishing (as not all nickel and/or cobalt may be removed from solution) or for solid/liquid separation. The gypsum-rich fraction may be recycled to another reactor or sent directly to a reaction vessel or thickener, preferably at about 60°C, to contact all or a portion of fresh nickel and/or cobalt sulphate stream from a leach process to convert unreacted limestone to gypsum. The resulting slurry may be separated into a liquid stream and a solids stream. The liquid- stream, which contains dissolved nickel and/or cobalt, may be sent to nickel and/or cobalt precipitation with limestone and the solids stream may be sent to nickel and/or cobalt solution polishing to remove any nickel and/or cobalt from solution or sent straight to separation where any precipitated nickel and/or cobalt carbonate is separated from precipitated gypsum. The precipitated nickel and/or cobalt carbonate may report to a fine fraction and the precipitated gypsum may report to a coarse fraction. The precipitated gypsum may be recycled to the process. The precipitated nickel and/or cobalt carbonate may be subjected to a liquid/solid separation and the solid may be washed to remove any magnesium in solution. The washed solids may be recovered as a nickel and/or cobalt carbonate concentrate. In other embodiments, the particle size of the precipitated gypsum in the precipitated nickel and/or cobalt may be controlled such that the cut point between the coarser particles and the finer particles is different to 30 μm. For example, in another embodiment, most of the gypsum may report to a +50 μπι fraction and most of the nickel and/or cobalt precipitate may report to a- 50 μπι fraction. In some embodiments, the cut point between the course fraction and the fine fraction is around 20 μιη. Other size cutpoints may also be used.

[0023] Since cobalt and nickel can be precipitated from solution at different pH, on another embodiment, it is possible to treat a process solution containing both cobalt and nickel to produce separate cobalt and nickel final products. In this way cobalt would be precipitated first in a pH range of 5.0 to 7.5 at a temperature of around 60°C and nickel would be precipitated second in a slightly higher pH range of 7.0 to 9.0 at a temperature around 60°C. After the first precipitation step to recover cobalt, the processing steps described above would be used to recover a final cobalt product. The resulting solution containing nickel would be subjected to a precipitation step to precipitate nickel and the processing steps described above would be used to recover a final nickel product.

[0024] In another embodiment, the fraction containing cobalt and/or nickel containing material (that has been formed by precipitation) may be contacted with an acidic feed solution, such as a fresh acidic feed solution, and held at a pH that is lower than the pH at which the calcium and/or nickel containing material is first precipitated, followed by separating the cobalt and/or nickel containing material from a gypsum containing material. In this embodiment, these additional steps can upgrade the cobalt and/or nickel containing material. For example, the cobalt and/or nickel containing material that is recovered as a fine fraction following the initial cobalt and/or nickel precipitation step is contacted with fresh acidic solution and held at a lower pH than the pH used in the initial precipitation step. For example, if the cobalt and/or nickel containing material was initially precipitated at pH 8.5 and the cobalt and/or nickel containing material (such as the fine fraction recovered following the initial precipitation step) is contaminated with gypsum/lime/limestone or magnesium, then the cobalt and/or nickel containing material could be contacted with fresh acidic feed solution and held at a lower pH, such as a pH of 7.0, and subsequently treated to separate the solids into a gypsum containing fraction and a cobalt and/or nickel containing fraction. It is in found that this step results in precipitation of more cobalt and/or nickel from the fresh feed solution, re-solubilises some of the precipitated magnesium from the concentrate and allows removal of gypsum to the gypsum containing fraction.

[0025] Depending upon the relative amounts of cobalt and/or nickel containing material and fresh acidic solution that is used in this step, it may be necessary to add some additional calcium containing material to increase the pH to a level at which cobalt and/or nickel containing material will precipitate, although the pH in this step should be kept below the pH used in the initial precipitation step. It will also be appreciated that in a preferred embodiment, a sufficient amount of acidic feed solution is contacted with the cobalt and/or nickel containing material to cause the pH to decrease to the desired level at which unreacted lime or limestone in the cobalt and/or nickel containing material reacts to form additional gypsum, any precipitated magnesium resolubilises and cobalt and/or nickel in the acidic solution precipitates, without requiring the addition of any additional calcium containing material.

[0026] Although the pH used in the step of contacting the cobalt and/or nickel containing material with acidic feed solution should be conducted at a pH that is lower than the pH used in the initial precipitation step, the pH range used in the step of contacting the cobalt and/or nickel containing material acidic feed solution should still fall within the pH ranges given in (a), (b) or (c) above.

[0027] All other aspects of the process of step (a), (b) or (c) may be essentially the same as described in US20140105797 Al (the entire contents of which are herein incorporated by cross reference), such as residence time, seeding of gypsum crystals and separation of nickel and/or cobalt precipitate and gypsum by gravity or by other separation process that separates finer particles from coarser particles. The gypsum fraction from the gravity separation or other separation process may be at least partly recycled to the process. The nickel and or cobalt oxide fraction may be thickened and filtered.

[0028] The nickel or cobalt carbonate process is a variant again of US20140105797 Al where the process is operated at conditions where cobalt and nickel are precipitated and magnesium is not. The calcium containing neutralising agent is suitably limestone (CaC0 ₃ ). There are many differences between hydrated lime and limestone. The obvious one is the chemical composition but the important one in this application is the reactivity. Limestone is capable of precipitating nickel and cobalt from solution, but it isn't as reactive as hydrated lime and therefore significantly more limestone is required to precipitate the metals, for example up to 50% above the stoichiometric requirement due to unreacted limestone. One possible mechanism that causes this arises where the limestone particle becomes coated in a gypsum layer and remains inert. The other avenue for unreactive limestone is that the driving force for complete reaction is not as high with limestone compared to hydrated lime. Generally speaking, the case where gypsum coats the particle can be overcome by regrinding the material to liberate the limestone surfaces but this is not amenable to the process as grinding will break down the gypsum particles potentially rendering them less than 30μπι and reporting to the concentrate but likely interfering with the seeding/gypsum growth cycle. The driving force issue that limits limestone reactivity can be overcome by re-treating the solids with fresh feed.

[0029] In all embodiments of the present invention, it may be possible to separate the precipitated nickel and or cobalt containing compound from gypsum using flotation, granulometric sizing or a combination of flotation and granulometric sizing.

[0030] Throughout this specification, the expression "without causing substantial precipitation of magnesium" should be taken to mean that less than 10% of the magnesium in solution is precipitated, or less then 5% of the magnesium in solution is precipitated, or less than 3% of the magnesium in solution is precipitated, or less then 1% of the magnesium in solution is precipitated.

[0031] In order to further understand the present invention, a preferred embodiment will be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 shows the flow sheet of embodiments of the process of the present invention;

[0033] Figure 2 is a process flowsheet of the process used in Example 1 ;

[0034] Figure 3 shows the nickel and calcium content of the +20 μm and -20 μm particle size fractions obtained in example 4; and

[0035] Figure 4 shows the particle size distribution of the particles for the +20 μm and -20 μm fractions of the particles precipitated in example 4. DETAILED DESCRIPTION OF THE DRAWINGS

[0036] It will be appreciated that the attached drawing has been provided for the purposes of describing a preferred embodiment of the present invention. Thus, it will be understood that the present invention should not be considered to be limited to the features as shown in the attached drawing.

[0037] In the attached drawings, the pregnant leach solution containing cobalt and or nickel enters the precipitation process. In industrial practice, this may occur after other metals have been removed, such as copper, zinc, iron, aluminium and manganese. For the purposes of the example, cobalt only will be used although the process can be used for zinc, copper, cobalt and nickel to recover a valuable high grade precipitate.

[0038] Figure 1 shows cobalt recovery using cobalt precipitation. In Figure 1, a pregnant leach solution 1 is fed to precipitation vessel A as described in US20140105797 Al, the entire contents of which are herein incorporated by cross reference. The preferred process conditions are pH 7.5 at 40 - 95°C. In this step, lime 4 is added to the liquid. The lime may be hydrated lime (Ca(OH)2) or lime (CaO). Steam may also be required for heating (6 and 7), but if CaO is added, the heat released by the exothermic hydration reaction to form Ca(OH) ₂ may be sufficient to heat the solution to the desired temperature. Addition of the lime causes precipitation of cobalt oxide. Gypsum will also be formed. Careful control of the precipitation parameters results in the cobalt oxide forming with a crystal structure that allows for easy separation of the cobalt oxide from the precipitated gypsum by virtue of differences in the size of the cobalt oxide particles and the gypsum particles.

[0039] The slurry 3 of liquid and precipitated solids from cobalt oxide precipitation step A and B is sent to a thickener C and the underflow to a cobalt oxide separator E, which suitably may be in the form of a cyclone. The thickener overflow is process water. In this separator E, the solids are separated into a fine overflow stream (e.g. sub 30μηι particles or sub 20 μm particles) 21 (which contains approximately 50% cobalt and approximately 2 percent calcium, equating to 95 to 99% recovery of the cobalt oxide) and a coarse underflow stream (e.g. plus 30μπι particles or plus 20 μπι particles stream) (which contains approximately 2% cobalt and the bulk of the remainder being gypsum).

[0040] The cobalt oxide stream 11 is sent to a cobalt oxide filter G and the resulting cake may then be dried further depending on product requirements and put into bags for sale. A flocculating agent may be added. Wash water is used to wash the filter cake to remove any soluble contaminants such as chlorine.

[0041] Returning now to the cobalt oxide separator E, the underflow is split into recycle streams 16 and 17. Stream 17 is returned to an upstream process with excess acid requiring neutralisation before the cobalt precipitator to re-solubilise any cobalt in this stream as shown by process L which in a cobalt plant may be used to remove iron, aluminium and manganese (FAM). Stream 16 is recycled to permit seeding of the gypsum crystal facilitating improved separation of cobalt oxide and gypsum at the cobalt oxide separator. If hydrated lime slurry is used, stream 16 is recycled to the hydrated lime stock tank. If CaO is used, stream 16 is recycled to the cobalt oxide precipitator.

[0042] The cobalt oxide product may be recycled as well to grow cobalt oxide particle size so that filtration performance is improved but still allowing efficient separation from the gypsum.

[0043] The above process can be replicated for nickel recovery. Alternatively, a mixed nickel and cobalt precipitate may also be formed if the pregnant liquor contains dissolved nickel and dissolved cobalt. It will be appreciated that nickel and cobalt are often found together in ore bodies. There are many processes available that enable separation of dissolved nickel from dissolved cobalt in solutions containing both dissolved metals and these known processes may form part of the upstream processing of the solutions to obtain a solution containing dissolved nickel (containing only relatively minor amounts of cobalt) and a solution containing dissolved cobalt (containing only relatively minor amounts nickel). Alternatively, a solution containing appreciable quantities of both dissolved nickel and dissolved cobalt may be subjected to the present process to form a mixed precipitate containing both nickel and cobalt. [0044] Alternatively the process can be performed sequentially to recover separate cobalt and nickel products when cobalt and nickel are both present in the feed solution.

[0045] The process can be replicated for copper, zinc and nickel recovery and can also be applied with limestone.

Example 1

[0046] Two continuous precipitation tests were set up with the objective to test the concept of precipitation of cobalt hydroxide with lime.

[0047] Using lime to precipitate metal hydroxides is well known in the industry. The metal hydroxide is precipitated but insoluble gypsum co-product is also precipitated lowering the grade of the final product. In the present invention, the process is controlled such that the gypsum is rendered as a significantly different particle size than the metal hydroxide by seeding and recycling the gypsum. The metal hydroxide reports to the finer fraction (-50μm). The two products can be separated with a hydrocyclone with the underflow returning to the process and the overflow reporting to the final product.

[0048] The test conditions were:

• pH 7.5 and 8.5

• 60°C

• 10 gpl cobalt in acidified solution to pH 3.0 (this was higher to precipitate enough material for screening and analysis out of the test)

• Run for 20 hours with a residence time of 4h in the precipitators

• Using hydrated lime slurry with a gypsum seed material such that the gypsum ratio was 3: 1 by mass with hydrated lime.

[0049] A process flow sheet of the experimental process is shown in figure 2.

[0050] The main results from the tests were: Table 1 :

Example 2

[0051] The test conditions were:

10 gpl cobalt in acidified solution to pH 3.0 (this was higher to precipitate enough material for screening and analysis out of the test)

Run for 20 hours with a residence time of 4h in the precipitators

Using hydrated lime slurry with a gypsum seed material such that the gypsum ratio was 3: 1 by mass with hydrated lime

Re-dissolution of the +20 μm fraction with 50 gpl acid solution for 4 hours

[0052] The variables were:

pH 7.5, 8.0, 8.5 and 9.0

60°C and 85°C

Magnesium in solution at 2 gpl or 0 gpl

With or without seed gypsum in the lime slurry

Using a riser or a shroud for the reactor discharge

[0053] A summary of the initial results are presented in Table 2. Table 2: Initial experimental results

[0054] Initial results were counter-intuitive in that increasing pH resulted in a lower final cobalt grade. It was found that due to the size of the equipment, the riser could not operate effectively resulting in accumulation of coarse particles in the reactor and biasing results. A clear trend was that lower operating pH resulted in less magnesium precipitation and higher extent of leaching from the +20μηι size fraction.

[0055] Based on these results further test work was conducted to investigate the impacts of the presence of magnesium in solution, presence of gypsum in the lime slurry and different reactor arrangement to maximise cobalt grade. The round 2 of test work results are shown in Table 3.

[0056] Table 3:

[0057] PT05 and PT06 tests were conducted in the absence of magnesium in solution with and without gypsum seed in the lime slurry. The cobalt grade is high in both tests. PT06 grade is lower due to the contamination from gypsum in the seed material. The full benefit of gypsum recycle is difficult to obtain on this small scale experimentation. While cobalt grade was maximised, the re-dissolution efficiency remained around 90% which is likely to be uneconomic at full scale. PT07 and PT08 showed markedly lower cobalt grade with the presence of magnesium. At pH 8.5 all of the magnesium was precipitated causing contamination of the final concentrate. Temperature did not have an impact of cobalt grade.

[0058] A further round of test work was conducted to investigate the effect of contacting PT07 and PT08 -20 m fraction with fresh (acidic) cobalt solution and holding at a lower pH. The hypothesis being that cobalt will be precipitated out and magnesium leached into solution increasing the concentrate grade.

[0059] Table 4: Round 3 test work results

[0060] Table 4 shows that the PT07 and PT08 concentrate samples could be upgraded from 35% to over 45% cobalt. Of particular interest is PT08a where nearly all magnesium is leached out of the concentrate. This has implications for the final circuit design since a two stage contact could be performed to minimise magnesium and maximise cobalt grade. Due to the sample size, the leaching of cobalt in the +20 μπι could not be checked.

Example 3

[0061] This example was conducted to show precipitation of nickel in accordance with an embodiment of the present process. A synthetic acidic feed solution containing 19.0g/L nickel was prepared by mixing nickel sulphate hexahydrate reagent with tap water and then adjusting to pH 4.0 using sulphuric acid.

[0062] The nickel precipitation test (PT09) was operated at 60°C for a period of 6.17 hours. This was a batch test using a heated 5 L reactor with lime used to control the pH to 9.0. The reactor was started half full with a milk of lime slurry at pH 9.0 and 60°C. The synthetic nickel feed solution was slowly added to the reactor until the pH decreased to 7.0, at which point additional lime was added until pH was 9.0 again. This process was repeated until the reactor was filled. 5.17 hours of the time was used for incremental feed solution addition and another hour was used to allow the system to equilibrate. The final reactor slurry was collected, filtered and assayed for nickel and calcium for mass balance purposes. The collected solids were initially laser sized and then wet screen through a 20 μπι screen using processed liquors. The oversize (+20μιη) and undersize fractions (- 20μιτι) were each laser sized and then assayed for nickel and calcium for mass balance and product evaluation purposes. [0063] The totalised mass balance for the tests was 89% and the nickel mass balance was 109%. Lime consumption was 34.3 kg/m ³ of feed (1.81 kg/kg Ni), reported as hydrated lime (Ca(OH) ₂ ). The final precipitate from the tests was a green colour, indicative of Ni(OH) ₂ . The nickel precipitation test produced a fine product grading 44.6% nickel (by weight) at 98.5% recovery.

[0064] Table 5 shows the mass balance and assays arising from this test:

Table 5:

[0065] Table 6 shows data relating to particle size distribution and assays obtained in this example.

Table 6:

[0066] As can be seen, the -20μηι fraction contained 44.60% by weight nickel, which represents a recovery of 98.5% of the nickel in the feed. The -20μηι fraction contained 5.88% calcium, which represented 19.2% of the calcium added to the precipitation step. The +20μηι fraction contained only 0.60% nickel, which represented 1.5% recovery of the nickel in the feed. The coarse fraction contains 22.3% by weight calcium, which represented 80.8% of the calcium added to the precipitation step.

Example 4

[0067] A further experimental run was conducted in which a feed solution containing 20 g/L nickel (as nickel sulphate) dissolved in solution was fed to a precipitation tank. The precipitation tank was operated at pH 9.0, with pH control being achieved by addition of hydrated lime. The operating temperature was 60°C. This resulted in precipitation of particulate material. The particular material was separated into a -20 μπι fraction and a +20 μπι fraction by screening the particular material on the 20 μπι screen.

[0068] Figure 3 shows the grade of the +20 μπι and -20 μπι particle size fractions obtained in this example. The minus 20 μπι fraction contained 44.6% nickel with a 98.5% recovery of nickel to the -20 μπι fraction. The -20 μπι fraction contained 4.9% calcium. The +20 μπι fraction contained approximately 22% weight calcium and less than 1% by weight nickel. Figure 4 shows the particle size distribution for the +20μιτι and the -20μιτι fractions obtained in this example.

[0069] The recovery of nickel to the minus 20 μπι fraction could be improved with gypsum seeding to the lime. However, gypsum seeding was not conducted in this experiment.

[0070] It may also be possible that limestone could be used to precipitate some of the nickel. Other pH ranges and temperatures could also be used to optimise the grade and product type formed.

Comparative example

[0071] An experiment was run in which it was attempted to precipitate a solid lithium containing material from an acidic solution containing 18.3 g/L lithium at a pH of 4.0. Again, a milk of lime slurry having a pH of 9.0 was used. The acidic solution containing dissolved lithium was fed to the reactor containing the milk of lime slurry. The temperature was 60°C. Incremental addition of the feed solution over a two-hour period saw minimal reduction in the pH. The pH was then increased to 10.0 by the addition of lime but again there was minimal reduction in pH after incremental addition of the feed solution over a 30 minute period. The pH was then increased to 11.0 but the test appeared to buffer at pH 10.86, whether adding lime slurry or feed solution after another 15 minute period. There was no apparent lithium precipitate formed in the test. The only precipitate product that appeared to form had an off-white colour indicative of gypsum. Accordingly, the process of the present invention was unable to produce a lithium precipitate for recovery.

[0072] Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.