WO2015061836A1

WO2015061836A1 - Metal recovery process

Info

Publication number: WO2015061836A1
Application number: PCT/AU2014/001024
Authority: WO
Inventors: Paul Christopher Freeman; Mark Daniel Urbani
Original assignee: Strategic Metallurgy Pty Ltd
Priority date: 2013-11-01
Filing date: 2014-10-30
Publication date: 2015-05-07

Abstract

A process to recover a target metal from hydrometallurgical solutions comprising the method steps of: i. Adding a calcium based neutralising agent such as lime or limestone to a solution containing aluminium sulphate at moderate temperature thereby forming an amount of soluble basic aluminium sulphate; and precipitating excess sulphate as gypsum, ii. After removing gypsum, passing the solution formed in step i to a target metal recovery step in which the target metal, that is complexed to an amine functionality, is stripped from the amine functional group by the soluble basic aluminium sulphate in the aqueous solution, and iii. precipitating the target metal from the aqueous solution, which contains the target metal and basic aluminium sulphate by the addition of hydrogen peroxide. whereby the target metal enters the aqueous solution in step ii without substanial precipitation or pH control.

Description

"Metal Recovery Process" Field of the Invention

[0001] The present invention relates to a metal recovery process.

[0002] In one form, the process of the present invention is applicable to metal recovery processes that employ metal cation exchange or precipitation processes incorporating an acid equilibrium whilst extracting a metal.

[0003] In another form, the process of the present invention relates to the recovery of metals from loaded solvent extraction solvents or ion exchange resins, which consist of anion exchange functionalities, such as amines, by use of soluble basic aluminium salts as stripping agent. Particularly, uranium may be recovered from acidic leach liquors. Uranium, present in the loaded strip solution, is recovered by precipitation using hydrogen peroxide without the need for pH control.

Background Art

[0004] In typical hydrometallurgical processing of uranium ores the target metal is first extracted from the host mineral. Once in solution the solubilised metals are then separated from the barren solids. As this extraction process normally extracts more than one metal ion, the solutions need to be further processed to separate the extracted metals. There are several standard techniques for separating these metals. These techniques include, for example, selective precipitation, solvent extraction and ion exchange.

[0005] Some of these separation techniques result in the generation of acid. This is true of many metal separation processes, including reduction, precipitation, solvent extraction and ion exchange processes. The production of this acid often affects the metal extraction reaction(s) equilibrium and has detrimental effects on the efficiency of the extraction process reaction(s).

[0006] In the reduction of metals from aqueous solution the target metal is reduced to its metallic state by the addition of a reducing agent. In the case of metal reduction using hydrogen, the reaction proceeds according to the following general reaction:

MeS0₄ + H₂ = Me + H₂S0₄

[0007] As the concentration of acid increases the reaction kinetics and the extent of the reaction are retarded. The term "extent of reaction" refers to how

completely the reaction moves to the 'right'. Ammonia is generally required to neutralise the acid and ensure high metal recovery. Ammonia is hazardous to transport and handle and can be expensive if a market for ammonium sulfate, formed in the neutralising step, is not available.

[0008] Metals can also be reduced from aqueous solution via electrowinning using insoluble anodes. Here the target metal is reduced to its metallic state by applying an electric current to an aqueous solution within an electrochemical cell. Acid is again produced as a by-product according to the following general reaction:

MeS0₄ + H₂0 + current = Me + H₂S0₄ + 7₂0₂ where the metal is formed at the cathode and acid and oxygen are formed at the anode. For target metals with lower standard reduction potentials than the hydrogen ion, this increased hydrogen activity can result in hydrogen reduction at the cathode. This can lead to reduced electrical efficiency and cathode quality. As a result, the cell is divided into compartments to reduce the quantity of acid in contact with the cathode. Examples include nickel and cobalt electrowinning processes. This acid is eventually neutralised with neutralising agents such as lime, sodium alkalis or ammonia.

[0009] In both solvent extraction and ion exchange, the normal reaction in extracting the metal cation is to replace the original hydrogen ion(s) on the organic with the metal cation according to the following general reaction:

M²⁺ + 2RH ^ MR₂ + 2H⁺ where M = metal cation and R = organic chain. [0010] Often solvent extraction and ion exchange are equilibrium reactions in which the reaction moves to the right under low acid conditions and moves to the left under conditions of high acidity. Reduced metal recovery can occur as a result. An example of this is in copper solvent extraction.

[001 1] Metal can be precipitated in many forms, but commercially proven methods include precipitation as sulfides, hydroxides and oxides. These processes are summarised by the following reactions:

MeSO₄ + H₂S - MS + H₂SO₄ Sulfide Precipitation

MeSO₄ + H₂O = Me(OH)₂ + H₂SO₄ Hydroxide Precipitation

[0012] Metals have an optimum pH for precipitation kinetics and for maximum reaction extent. A neutralisation reagent is usually added to maintain the optimum pH for reaction kinetics and completion.

[0013] In the case of uranium, when uranium is extracted from uranium bearing ores with acidic solutions, the uranium can be separated from impurities by use of solvent extraction, whereby uranium is extracted as an anionic complex, such as UO₂(SO₄)3^4" or UO₂(SO₄)₂ ^2", with an amine functional component in the organic solvent. The extraction mechanism is via anion exchange and can be

represented by the following general reactions:

Protonation:

2R₃N + H₂SO₄ ^ [(R₃NH⁺)₂SO₄ ²-]_org

Extraction of UO_?(S0₄k^4':

[2(R₃H⁺)₂ SO₄ ²-]_org + UO₂(SO₄)₃ ^4" - [(R₃NH⁺)₄UO₂(SO₄)₃ ⁴-]_org + 2SO₄ ^2"

[(R₃NH⁺)₂ SO₄ ^2"]org + UO₂(SO₄)₂ ²- ^ [(R3NH⁺)₂UO₂(SO₄)₂ ²-]_org + SO₄ ^2"

[0014] There are two routes which can be used to recover uranium from the loaded solvent. Either a de-protonation route, in which the pH is increased so as to de-protonate the amine functional group, or an ion exchange route whereby the uranium containing anion is exchanged with a different anion, such as sulfate or chloride.

[0015] In hydrometallurgical plants that operate using the de-protonation route, in most cases, ammonia is used as the base, either as an ammonia/ammonium solution, or by direct injection of ammonia gas into the mixer inlet. The stripping reaction is as follows:

[(R₃NH⁺)4U02(S0₄)3^4"]org + 4NH₄OH→· [4R₃N]_org + (NH₄)₂S0₄ + (NH₄)₂U0₂(S0₄)₂ + 4H₂0

[0016] Uranium is recovered from the strip liquor as ammonium di-uranate by precipitation with ammonia. The precipitation reaction is as follows:

2(NH₄)₂UO₂(SO₄)₂ + 6NH₄OH ^ (NH₄)₂U₂O₇ + 4(NH₄)₂SO₄ +3H₂O

[0017] After solid-liquid separation of the precipitated ammonium di-uranate ("ADU"), the liquid is returned to the solvent extraction strip stages.

[0018] Operators of the de-protonation route using ammonia typically experience several issues. Strict pH control with a gradual increasing pH gradient between adjacent strip stages is a necessity. Excursion of pH can result in ADU

precipitation in strip mixers, which has an adverse effect on SX plant operation. Precipitated solids create crud, which can result in mixer continuity upsets.

Common U SX operating practice is to operate mixers with organic continuous mixing. This is to prevent the formation of stable emulsions, which in turn minimises organic reagent losses. Crud entering a mixer can flip the mixing continuity from organic continuous to aqueous continuous. This can result in stable emulsions in the settlers. This could affect adjacent mixers, whereby emulsions entering the mixers can flip the mixing continuity to aqueous

continuous. If this problem is not dealt with early, then all settlers could contain stable emulsions resulting in high organic reagent losses, low uranium recovery and eventual plant shut down.

[0019] In some circumstances, acid addition is required to redissolve ADU in order to minimise the adverse affects on plant performance. This adds to reagent costs as acid is consumed during dissolution and ammonia is required to recover the re- dissolved uranium. A large number of strip stages (3-4 in most cases) are required to allow for a steady increase in pH during stripping.

[0020] The by-product of the stripping process is ammonium sulfate. Impurities that enter the strip solution must be controlled by removing a portion of the strip solution. Ammonium sulfate is also present in this solution. This can become an environmental issue as some operations are governed by strict regulations in regards to ammonium sulfate contamination of ground water.

[0021] For some plants, ammonia gas or solution can be difficult to import or transport to site.

[0022] The anionic exchange strip mechanism, using sulfuric acid as an example, is as follows:

[(R₃NH⁺)₄U02(S04)3^4"]org + 2H₂S0₄ ^ [2(R₃NH⁺)₂S0₄ ²-]_org + U0₂(S0₄)₃ ^4" + 4H⁺

[0023] After neutralisation of the free acid, uranium is recovered from the strip liquor as uranyl peroxide by precipitation with hydrogen peroxide. The

precipitation reaction is as follows:

U0₂S0₄ + H₂0₂ + 2H₂0 - U0_4.2H₂0 + H₂S0₄

[0024] The free acid generated by the above reaction must be neutralised to drive the reaction to completion. After filtration, the filtrate is re-acidified and recycled to the stripping stage.

[0025] Stripping loaded solvent by an anion exchange method requires a large number of stages and a high concentration of the anion in the strip solution. When using sulfuric acid, 3-4M sulfuric acid is required with 4-6 strip stages to obtain acceptable stripping efficiency. Reagent costs are high as the acid used for stripping can not be recycled and must be neutralised by addition of base. Generally, a cheap base such as lime (CaO) is used to reduce costs and remove sulfate. During the precipitation process, addition of base is also required to maximise precipitation efficiency of the uranyl peroxide. A more expensive base, other than lime or limestone, such as sodium hydroxide, magnesia or ammonia is required. The use of calcium rich bases will result in gypsum precipitation, which will contaminate the uranium rich product.

[0026] Generally a two stage precipitation is implemented in which the first stage utilises a cheap base such as lime or limestone to precipitate the sulfate as gypsum. Due to the high sulfate concentration of the liquor, the majority in the form of sulfuric acid, a significant mass of gypsum is produced. Two issues arise; the first is the necessity to store the gypsum waste and the second is some uranium is entrained in the gypsum cake, resulting in an overall reduction in uranium recovery.

[0027] US Patent 4388208 describes the production of an aqueous solution of aluminium sulfate containing poly-nuclear complexes of the kind:

Al_m(OH)_n ^(3m-ⁿ⁾⁺

In which m and n are positive integers, characterised in that said composition has a total aluminium content of between 0.2 and 2.0 mol/L, and that it is a clear, stable solution comprising poly-nuclear complexes determined by the associated values: Al (mol/L) Mole ratio

0.2 1 .5

0.5 1 .1 -1 .6

1 .0 0.8-1 .7

1 .9 0.35-1.5

2.0 0.3-1 .4

50-70% of the aluminium content is present as said aluminium poly-nuclear complex. The said solutions are prepared by bringing a carbonate or hydrogen carbonate of alkali metal and an aluminium sulfate into aqueous solution;

selecting the amount of aluminium sulfate so as to obtain in solution a given total aluminium content of between 0.2 and 2.0 mol /L; selecting the amount of carbonate or hydrogen carbonate used in relation to the amount of aluminium sulfate used so that the ratio between the numbers of moles of OH and the total number of moles of aluminium in the resultant solution does not substantially lie outside the values listed in the abovementioned table.

[0028] These observations were made solely in the context of generating a suitable solution for water purifying, paper sizing and plant dewatering purposes only. No disclosure is made of how a solution containing soluble basic aluminium sulfate might be utilised in the broader context of the recovery of uranium from hydrometallurgical solutions and similarly there is no disclosure of any of the advantages that might be realised thereby.

[0029] The present invention has as one object thereof to overcome substantially the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto.

[0030] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to formed part of common general knowledge as at the priority date of the application.

[0031] Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0032] The following description in part describes, by way of example, the formation of a basic aluminium sulfate. However, the principal underlying the present invention is not limited to sulfate as the anion. A similar chemistry is experienced when using chloride solutions. Still further, it is envisaged that a similar reaction will be achieved in the presence of nitrate ions as the anion. The same principal applies to any soluble basic metal salt, or mixtures thereof, the most common cations being ferric, aluminium and chromium that each form soluble basic salts.

[0033] The following description makes reference to various Figures. Thereafter description is provided relating to the specific Figure mentioned utilising certain numerals to denote parts, portions or areas of the process/method/apparatus described. That description is to be understood to relate to the Figure previously referred to or introduced unless the context demands otherwise. Accordingly, the numerals denoted in the description are to be understood to relate to the Figure previously referred to also, again unless the context demands otherwise.

Disclosure of the Invention

[0034] In accordance with the present invention there is provided a metal recovery process comprising the method steps of:

(i) Adding a neutralising agent to a solution containing one or more non-target metals at moderate temperature thereby forming an amount of a soluble basic salt; and (ii) Passing the solution formed in step (i) to a target metal recovery step in which acid generated in that target metal recovery step is consumed at least to some extent by the soluble basic salt from step (i).

[0035] Preferably, the non-target metal ions in step (i) are aluminium, ferric or chromium ions.

[0036] Still preferably, the soluble basic salt of the non-target metal is a soluble basic sulfate.

[0037] Still preferably, the target metal recovery step (ii) is a reduction, electrowinning, solvent extraction, ion exchange or precipitation step.

[0038] Yet still further preferably, in the target metal recovery step, metal recovery is more complete than would have been possible if some or all of the acid generated had not been consumed.

[0039] Preferably the barren solution, with respect to the target metal, is passed, at least in part, to a non-target metal ion removal step utilising a neutralising agent.

[0040] Still preferably, the moderate temperature of step (i) is between 0°C and about 90°C.

[0041] Still further preferably, the moderate temperature of step (i) is between 0°C and about 80°C.

[0042] In one form of the present invention the non target metal ion removal step produces either an aluminium hydroxide or mixed hydroxide precipitate.

[0043] Preferably, the aluminium hydroxide or mixed hydroxide formed in the non- target metal removal step is utilised as the neutralising agent in step (i), whereby aluminium tenor in the metal recovery circuit is increased in turn creating more soluble basic aluminium sulfate and further enhancing extraction of the target metal ions. [0044] In another form of the present invention, the neutralising agent in step (i) can be any base stronger than the basic aluminium sulfate. This base could include hydroxides of target metals that have been precipitated elsewhere in the flow sheet, or oxides of target metals.

[0045] In accordance with the present invention there is further provided a process to recover a target metal from hydrometallurgical solutions comprising the method steps of:

(i) Adding a calcium based neutralising agent such as lime or limestone to a solution containing aluminium sulfate at moderate temperature thereby forming an amount of a soluble basic aluminium sulfate; and precipitating excess sulfate as gypsum,

(ii) After removing gypsum, passing the solution formed in step (i) to a uranium recovery step in which the uranium, that is complexed to an amine functionality, is stripped from the amine functional group by the soluble basic aluminium sulfate in the aqueous solution, and

(iii) Precipitating uranium from the aqueous solution, which contains uranium and basic aluminium sulfate, by the addition of hydrogen peroxide, whereby uranium enters the aqueous solution in step (ii) without substantial precipitation or pH control.

[0046] Preferably, the target metal recovery step (ii) is either a solvent extraction stripping step or ion exchange elution step.

[0047] Still preferably, the amine functionality in a solvent extraction step can consist of commercially available amines tri-n-octylamine (Alamine 300), tri- isooctylamine (Alamine 308), tri-(C8Cio)amine (Alamine 336 or Armeen), tri- isodecyclamine (Alamine 310) or tri-laurylamine (Alamine 304).

[0048] The stripped extractant from step (ii) is preferably passed to a wash circuit to remove entrained aluminium then to an extraction circuit in which uranium is extracted by the anion exchanger extractant from an acid solution. In some cases the wash solution can be water or sodium hydroxide solution to improve uranium stripping efficiency. The wash raffinate, containing aluminium, is recycled to the stripping circuit. The loaded amine functionality as solvent or IX resin may be subject to a scrubbing stage to remove impurities prior to being passed to step (ii).

[0049] The aqueous solution generated in step (iii), which contains aluminium sulfate, is passed at least in part, to step (i).

[0050] Preferably, the moderate temperature of step (i) is between 0°C and about 80°C.

[0051] Still preferably, the moderate temperature of step (i) is between 0°C and about 60°C.

[0052] In a further form of the present invention part of the aluminium sulfate solution generated from the uranium precipitation step can be subject to an aluminium precipitation step in order to remove aqueous solution.

[0053] Alternatively, part of the solution generated in step (i) can be subject to an aluminium precipitation step in order to remove aqueous solution.

[0054] Preferably, the aluminium hydroxide or mixed hydroxide formed in the aluminium precipitation step is utilised as the neutralising agent in step (i), whereby the aluminium tenor in the solvent extraction circuit is not diminished significantly by the aqueous solution removal step.

[0055] In accordance with the present invention there is still further provided a hydrometallurgical process for the recovery of uranium from a uranium bearing ore or concentrate, the process comprising the following method steps:

Preparation of the uranium loaded solvent

(i) Passing the uranium bearing ore or concentrate to a leach step in which at least a proportion of the contained uranium is extracted into solution, forming a pregnant leach solution ("PLS"); (ii) Passing the PLS from step (i) to a solid liquid separation step producing a solid residue and a solution containing the bulk of the extracted uranium;

(iii) Subjecting the PLS from step (ii) to a uranium solvent extraction step whereby the uranium is extracted as an anion, and separated from impurities, generating a loaded uranium solvent;

Preparation of the stripping solution

(iv) Adding lime or limestone to a solution containing aluminium sulfate at moderate temperature in order to precipitate gypsum and aluminium from solution in the form of aluminium hydroxide;

(v) Passing the solution from step (iv) to a solid liquid separation step producing a solid containing aluminium and a solution containing impurities from the stripping process;

(vi) Contacting the solid from step (v) with solution containing aluminium sulfate at moderate temperature thereby forming an amount of a soluble basic aluminium salt;

(vii) Passing the solution from step (vi) to a solid liquid separation step producing a solid residue and solution containing basic aluminium sulfate;

(viii) Passing the solution formed in step (vii) to a uranium recovery step in which the solution is contacted with the loaded uranium solvent and the uranium is stripped from the amine functional group by the soluble basic aluminium salt in the aqueous solution, the uranium entering the aqueous solution without significant precipitation or pH control;

(ix) The aqueous solution formed in step (viii), which contains soluble uranium and basic aluminium sulfate, is contacted with hydrogen peroxide to precipitate uranium from solution, acid generated being consumed at least to some extent by the soluble basic aluminium salt, (x) Passing the solution from step (ix) to a solid liquid separation step, producing a solid uranium product and solution containing aluminium sulfate; and

(xi) The solution from step (x), which contains aluminium sulfate, being divided, whereby one portion is subject to steps (iv) and (v) and another portion is subject to step (vi) so as to generate more basic aluminium sulfate for use in step (viii).

[0056] In one form of the present invention the steps (iv) and (vi) may be achieved simultaneously whereby the basic aluminium sulfate is formed substantially in a single step. In such a form of the invention it is envisaged that step (v) would be unnecessary. In this case a bleed of the liquor generated in step (vii) to control the water balance would be required. The aluminium present in the bleed solution can be recovered by precipitation with lime or limestone, then recycled to step (iv).

Brief Description of the Drawings

[0057] The process of the present invention will now be described, by way of example only, with reference to three embodiments thereof and the

accompanying drawings, in which:-

Figure 1 is a flow sheet depicting a hydrometallurgical process for the recovery of copper by a conventional split circuit process in accordance with the prior art;

Figure 2 is a flow sheet depicting a metal recovery process for the recovery of copper in accordance with a first embodiment of the present invention;

Figure 3 is a flow sheet depicting a hydrometallurgical process for the recovery of uranium by acid leach, solvent extraction and recovery as ammonium di-uranate in accordance with the prior art;

Figure 4 is a flow sheet depicting a hydrometallurgical process for the recovery of uranium in accordance with a second embodiment of the present invention; Figure 5 is a flow sheet depicting a hydrometallurgical process for the recovery of uranium in accordance with a third embodiment of the present invention; and

Figure 6 is a McAbe Thiele diagram showing the uranium content results subsequent to the stripping of loaded organic with BAS stripping solution.

Best Mode(s) for Carrying Out the Invention

[0058] In most hydrometallurgical processes significant quantities of non-target metal ions, for example ferric ions, aluminium ions and chromium ions, report to the solution together with any targeted metal ions. The process of the present invention, in accordance with a first embodiment thereof, utilises the properties of soluble basic salts of the non-target metal ions, for example sulfates, within the processing of the target metals in solution.

[0059] In the case of aluminium and ferric ions, the ions are generally removed by precipitation with a neutralising agent. The inventors believe the first embodiment of the present invention has particular application to base metals, where invariably the relevant non-target metal ions that report to solution are aluminium and/or ferric ions.

[0060] Soluble basic aluminium sulfate can be formed by the addition of a neutralising agent to an aluminium solution at moderate temperature as follows:

AI₂(S0₄)₃ + CaC0₃ + H₂0 - 2AI(OH)S0₄ + CaS0₄ + C0₂

[0061] On the addition of acid the basic aluminium sulfate reverts back to aluminium sulfate consuming the acid in the process:

2AI(OH)SO₄ + H₂SO₄ - AI₂(SO₄)₃ + 2H₂O

[0062] Therefore, in the metal recovery step with the presence of sufficient basic aluminium sulfate all the acid generated by the reaction would be consumed as follows: 1 . For hydrogen reduction of metals:

MeS0₄ + H₂ + 2AI(OH)S0₄ - Me + AI₂(S0₄)₃ + 2H₂0

Examples of target metals are nickel and cobalt. In this embodiment of the process, traditionally used ammonia is replaced by a relatively cheap and less hazardous reagent (to transport and handle). The requirement to market the ammonium sulfate is no longer required.

2. For electrowinning of metals:

MeS0₄ + 2AI(OH)S0₄ + current ^ Me + AI₂(S0₄)₃ + H₂0 + ¹/₂0₂

Examples of target metals are nickel and cobalt. In this embodiment of the process, the solution pH is managed directly within the cell, potentially increasing electrical efficiency and also potentially reducing the recirculating flow rates required in some modes of practice.

3. For solvent extraction or ion exchange of metals:

MeS0₄ + 2RH + 2AI(OH)S0₄ - MeR₂ + AI₂(S0₄)₃ + 2H₂0

MeS0₄ + 2RH + 2Fe(OH)S0₄ - MeR₂ + Fe₂(S0₄)₃ + 2H₂0

An example target metal would be copper. As there is little or no free acid in solution the copper extraction proceeds to completion producing a copper free raffinate. Other examples are the solvent extraction of certain rare earth metals and transition metals that can be extracted within the natural pH range of the basic aluminium or ferric sulfate.

[0063] In Figure 1 there is shown a standard split circuit flow sheet for the processing of a copper containing ore or concentrate/feed material 1 to recover copper 6, as the target metal, in accordance with the prior art.

[0064] The copper bearing feed material 1 is passed to a leach step 20 in which at least a proportion of the contained copper is extracted into solution, together with an amount of non-target metal, including aluminium. The leach discharge is passed from the leach step 20 to a primary thickener 21 producing thickened slurry 7, which contains some copper in solution and a solid residue, and a high grade PLS (pregnant leach solution) 3. The high grade PLS 3 from the primary thickener 21 is passed to the high grade solvent extraction step 22 in which copper is loaded onto an extractant producing a high grade raffinate 4, the loaded extractant subsequently being stripped of the loaded copper which is passed to a copper recovery step, for example electrowinning 23 from which the copper metal 6 is obtained.

[0065] The high grade raffinate 4, containing some copper and acid, is recycled to the leach step 20 to utilise the acid for leaching and extraction of more copper.

[0066] The thickened slurry 7 is passed to a solid liquid separation step, for example a counter-current decantation ("CCD") circuit 24, producing a solid residue 1 1 and a low grade PLS 8 containing the bulk of the remaining Cu in the thickened slurry. Wash return 14 is utilised to wash the contained copper from the residue 1 1 .

[0067] The low grade PLS 8 from the CCD circuit 24 is passed to the low grade solvent extraction step 25 in which copper is loaded onto an extractant producing a low grade raffinate 9, the loaded extractant subsequently being stripped of the loaded copper which is passed to the copper electrowinning stage 23 from which the copper metal 6 is obtained. The low grade raffinate 9 produced from the low grade solvent extraction step 25, which contains some copper, is passed to an acid and metal removal step 26.

[0068] In the acid and metal removal step 26 a neutralising agent 12 is added, raising the pH and precipitating an aluminium containing residue 13. The remaining solution 14 is recirculated to the CCD circuit 24. The residual copper present in the low grade raffinate is either precipitated in step 26 or remains in solution 14. Part of this copper will be lost to the residue 1 1. [0069] In Figure 2 there is shown a process in accordance with a first embodiment of the present invention. In as much as the process shares certain process steps with the prior art process of Figure 1 like numerals denote like parts.

[0070] A portion of the low grade raffinate 9 is passed to the impurity removal stage 26. A neutralising agent is added raising the pH and precipitating an aluminium containing residue 13. After solid/liquid separation the solution 14 is passed to the CCD stage 24 for washing and the aluminium containing residue is passed to a basic aluminium sulfate generation stage 27.

[0071] The remaining portion of the low grade raffinate 9 is passed to a basic aluminium sulfate stage 27. In the basic aluminium sulfate stage 27 the aluminium sulfate and acid (if any) present in the low grade raffinate 9 reacts with the aluminium hydroxide present in the residue 13 to produce a basic aluminium sulfate (BAS) solution 15.

[0072] After solid/liquid separation, the BAS solution 15 is passed to the low grade copper solvent extraction stage 25. In this stage the BAS neutralises the acid generated such that loading of the extractant with copper ions is more complete than would have been possible if some or all of the acid generated had not been consumed. In doing so, more copper reports to copper electrowinning 23 in preference to the residue 1 1 or the metal hydroxide residue 13.

[0073] This same principal is applicable to the formation of basic ferric sulfate. The relevant reaction is as follows:

Fe₂(S0₄)₃ + CaC0₃ + H₂0 - 2Fe(OH)S0₄ + CaS0₄ + C0₂

[0074] The basic ferric sulfate reverts back to ferric sulfate thereby consuming acid generated in the copper solvent extraction.

[0075] Further, the same principal is also applicable to the formation of basic chromium sulfate should such be present in sufficient quantities.

4. For sulfide precipitation of metals: MeS0₄ + H₂S + 2AI(OH)S0₄ - MeS + AI₂(S0₄)₃ + 2H₂0

MeS0₄ + H₂S + 2Fe(OH)S0₄ - MeS + Fe₂(S0₄)₃ + 2H₂0

Target metals would include copper, zinc, nickel and cobalt.

5. For hydroxide precipitation of metals:

MeS0₄ + 2AI(OH)S0₄ - Me(OH)₂ + AI₂(S0₄)₃

Examples of a target metal are iron and uranium. Traditionally iron is an impurity within hydrometallurgical circuits and is removed with limestone producing an iron oxide/gypsum waste product. Using basic aluminium sulfate, a pure saleable iron product can be formed. Additionally in the precipitation of iron from zinc Roast Leach Electrowin flowsheets, basic aluminium sulfate can replace zinc calcine as the precipitating agent meaning that all zinc calcine is eventually processed through a strong acid leach, increasing zinc recovery. The basic aluminium sulfate can be formed using zinc calcine.

In the precipitation of uranium from aqueous solution using peroxide, acid is generated:

U0₂ ²⁺ + H₂0₂ - U0₄ + 2H⁺ pH control is required. The use of basic salts can be used to neutralise acid without the need for pH control.

[0076] Even if there is insufficient basic aluminium sulfate present to consume all the acid present, at least some of the acid will be consumed. In this manner, the presence of any basic aluminium sulfate will improve metal recovery by

consuming acid.

[0077] In the aluminium or impurity removal step 26, the neutralising agent 12 is added, raising the pH and precipitating an aluminium hydroxide containing residue. As noted above, the precipitated aluminium hydroxide is utilised as the neutralising agent in the basic aluminium sulfate step 27, with any remaining aluminium/mixed hydroxide forming the aluminium containing residue 16.

[0078] By utilising the precipitated aluminium hydroxide as neutralising reagent to create the basic aluminium sulfate this allows a build up in aluminium tenor in the circuit and create more basic aluminium sulfate, which would in turn enhance the effectiveness of the metal extraction. The reaction is as follows:

AI₂(S0₄)₃ + AI(OH)₃ - 3AI(OH)S0₄

[0079] The reaction to form the basic aluminium sulfate is very rapid and takes place in well under a minute. The inventors have determined that the lower the temperature the more stable the basic aluminium sulfate and envisage that temperatures of up to about 90°C will realise the benefits of the present invention. Ratios of OH^" to Al³⁺ ions of as high as 1 .7 to 2.0 are achievable. This equates to approximately 3.4-4.0M basic aluminium sulfate concentration. The maximum concentration of basic aluminium sulfate in solution will be reduced by the concentration of other ions and will depend on the temperature of the solution as per standard solubility laws. Higher concentrations are possible with the chloride or nitrate forms of the salt.

[0080] It is further to be understood that under certain conditions basic salts can precipitate to a limited extent with time. In these conditions the stream containing basic salts both in solution and in the solid form may still be utilised as an acid neutralising reagent as the precipitated basic salt reacts in the same manner as the soluble basic salt and will redissolve.

[0081] A similar chemistry is experienced when using solutions containing chloride ions, the relevant reaction being as follows:

2AICI₃ + CaCO₃ + H₂O 2AI(OH)CI₂ + CaCI₂ + CO₂

[0082] It is envisaged that a similar reaction will be achieved in the presence of nitrate ions as the anion. Uranium

[0083] For hydrometallurgical processes that employ an acid leach process to extract uranium, solvent extraction may be employed to recover the uranium. Stripping of the uranium from loaded solvents can be achieved by de-protonating the active component of the solvent. The process of the present invention utilises the properties of soluble basic aluminium salts, for example sulfates, as stripping agent to strip uranium by de-protonating the solvent.

[0084] The inventors believe the present invention to be advantageous over current methods as pH control is not required for stripping and there is no risk of precipitating yellow cake in the strip stages. It also eliminates the use of ammonia, which is hazardous and more expensive than the Ca based neutralising agents, lime and limestone.

[0085] Soluble basic aluminium sulfate can be formed by the addition of a neutralising agent to an aluminium solution at moderate temperature as follows:

AI₂(S0₄)₃ + CaC0₃ + H₂0 - 2AI(OH)S0₄ + CaS0₄ + C0₂

AI(OH)₃ + AI₂(S0₄)₃ - 3AIOHS0₄

[0086] The use of Ca based neutralising agents, such as lime and limestone, allow for sulfate precipitation as gypsum, which prevents the build-up of sulfate in the metal recovery circuit.

[0087] When contacted with loaded uranium solvent the basic aluminium sulfate de-protonates the amine functionality and reverts back to aluminium sulfate:

[3(R₃NH⁺)₄U0₂(S0₄)₃ ^4"]org + 12AIOHS0₄ -»

[12R₃N]_org + 5AI₂(S0₄)₃ + AI₂(U0₂(S0₄)₂)₃ + 12H₂0

[0088] Since the pH of basic aluminium sulfate lies in the range of pH 3.0 - 4.0, there is no precipitation of uranium. The basic aluminium sulfate can be in excess to maximise uranium stripping. [0089] The stripping process can be achieved without pH control since the pH of the basic aluminium sulfate would remain at pH 3.0-4.0 when in excess, which lies in the typical pH range for conventional stripping of loaded uranium solvent.

[0090] The chemistry of the loaded strip liquor, which contains basic aluminium sulfate, is ideal for uranium peroxide precipitation. The precipitation reaction is as follows:

UO2SO4 + H2O2 + 2H₂0 - U0₄.2H₂0 + H₂S0₄

[0091] Neutralisation of the generated acid is required to drive the reaction to completion. Conventional methods involve pH control in the range of pH 3.0-4.0 by the addition of neutralising agents. When excess basic aluminium sulfate is present in the strip solution, no pH control is required as the reaction becomes:

U0₂S0₄ + H₂0₂ + 2AI(OH)S0₄ + 2H₂0 ^ U0₄.2H₂0 + AI₂(S0₄)₃

[0092] In Figure 3 there is shown a standard flow sheet for the processing of a uranium containing ore or concentrate/feed material 1 to recover uranium as ammonium di-uranate 13, as the target metal, in accordance with the prior art.

[0093] The uranium bearing feed material 1 is passed to a leach step 20 in which at least a proportion of the contained uranium is extracted into solution forming a pregnant leach solution ("PLS"). The leach discharge 2 is passed from the leach step 20 to a solid liquid separation step, for example a counter-current

decantation ("CCD") circuit 21 , producing a solid residue 5 and a PLS 3 containing the bulk of the extracted uranium.

[0094] The PLS 3 from the CCD circuit 21 is passed to the first of 4 extraction stages of a solvent extraction step (22-25) in which it is contacted with stripped organic 8 in a counter current operation. The uranium in the PLS 3 is loaded onto an amine based extractant producing a uranium free raffinate 4 which exits extraction stage 4 25. The loaded extractant subsequently exits extraction stage 1 22 and is scrubbed of the loaded impurities and possibly some uranium in two scrubbing stages 26 and 27. A scrub solution 1 1 enters scrub stage 2 27 and exits scrub stage 1 26. The scrub raffinate 12 is then returned to the extraction stage 1 22 to recover uranium that was scrubbed from the solvent.

[0095] The scrubbed organic 7 is passed from the scrub stage 2 27 to the first of 4 stripping stages 28-31 of the solvent extraction circuit, in which it is contacted in a counter current operation with a barren strip liquor 10. Ammonia 35 is injected into each strip stage to control the pH. The stripped organic 8 exits strip stage 4

31 and is recycled to extraction stage 4 25 to recover more uranium. A loaded strip liquor 9 exits strip stage 1 28 and is fed to the first of two precipitation stages

32 and 33. Ammonia 35 is added in the precipitation stages 32 and 33 to raise and control the pH to allow for the precipitation of ammonium di-uranate 13.

Precipitated solids are separated from the majority of the liquor in a solid liquid separation stage 34 and are washed with potable water. A part of the barren strip liquor 14 is bled from the circuit to control the build-up of impurities and

ammonium sulfate. The remaining portion of the barren strip liquor 10 is recycled to strip stage 4 31 for further uranium recovery.

[0096] In Figure 4 there is shown a metal recovery process in accordance with a second embodiment of the present invention. In as much as the process shares certain process steps with the prior art process of Figure 3 like numerals denote like parts.

[0097] The scrubbed organic 7 is passed to the first of four stripping stages 28-31 of the solvent extraction circuit, in which it is contacted without pH control in a counter current operation with barren strip liquor 10 containing basic aluminium sulfate. The stripped organic 8 exits strip stage 4 31 and is contacted with wash water 18 in a wash stage 39. The wash raffinate is passed to strip stage 4 31 for aluminium and, if present, uranium recovery. The washed organic 19 is directed to extraction stage 4 25 for recovery of more uranium. The loaded strip liquor 9, containing soluble uranium, aluminium sulfate and basic aluminium sulfate, exits strip stage 1 28 and is fed to the first of two precipitation stages 32 and 33.

Hydrogen peroxide 40 is added in these stages to precipitate uranium as uranyl peroxide 16. No pH control is required. [0098] The precipitated solids are separated from the majority of the liquor in a solid liquid separation stage 34. The solids are washed with potable water to remove and recover aluminium sulfate. The wash liquor can be used to dilute the basic aluminium sulfate liquor 10 to reduce the calcium to below saturation. Part of the liquor from the solid liquid separation step 34 is fed to an aluminium hydroxide precipitation stage 35 in which the liquor is contacted with a lime or limestone slurry 15 used to raise the pH to greater than 4 in order to precipitate aluminium from solution as aluminium hydroxide. The solids are separated from the majority of the liquor in a solid liquid separation stage 36.

[0099] A bleed 14 is removed from the circuit at stage 36 to control the water balance and build-up of impurities. The solids are then contacted in a BAS generation stage 37 with the remaining liquor from the uranium peroxide solids liquid separation stage 34 to generate basic aluminium sulfate. The slurry is passed from the BAS generation stage 37 to a solid liquid separation step 38 to remove the gypsum residue 17. The solids are washed with potable water to recover aluminium sulfate and basic aluminium sulfate, and the resulting barren strip soluion 10 is fed to strip stage 4 31 for recovery of more uranium. Aluminium sulfate and wash water 41 can be added to the barren strip liquor 10 to maintain the aluminium concentration and reduce the Ca concentration.

[00100] In Figure 5 there is shown a variation of the flow sheet described in Figure 4. The flow sheet describes a metal recovery process in accordance with a third embodiment of the present invention. In as much as the process shares certain process steps with the prior art process of Figure 3 and the process of Figure 4 like numerals denote like parts.

[00101] All of the liquor from the solid liquid separation step 34 is fed to the BAS generation stage 37. Lime or limestone slurry 15 is added to step 37 to generate BAS. The slurry is passed from the BAS generation stage 37 to a solid liquid separation step 38 to remove the gypsum residue 17. The solids are washed with potable water to recover aluminium sulfate and basic aluminium sulfate, and the resulting barren strip liquor 10 is fed to strip stage 4 31 for recovery of more uranium. Aluminium sulfate and wash water 41 can be added to the barren strip liquor 10 to maintain the aluminium concentration and reduce the Ca concentration.

[00102] A bleed of the barren strip liquor 10, which contains BAS, is removed to control the water balance and build-up of impurities. The barren strip liquor bleed is passed to an aluminium hydroxide precipitation stage 35 in which the liquor is contacted with a lime or limestone slurry 15 used to raise the pH to greater than 4 in order to precipitate and recover aluminium from solution as aluminium hydroxide. The solids are separated from the majority of the liquor in a solid liquid separation stage 36 and recycled to the BAS generation stage 37.

[00103] A bleed 14, which does not contain aluminium, is removed from the circuit to control the water balance and build-up of impurities.

[00104] The present invention may be further understood by reference to the following non-limiting example.

Example

[00105] A solution containing 6 g/L U, as uranyl sulfate, at pH 1.0 was contacted with a synthetic organic solution containing 5% Armeen extractant and 2% isodecanol in Kermac 400 / 500 diluent at an organic to aqueous (O/A) ratio of 1 :1 . The resultant loaded organic solution was separated from the aqueous raffinate solution and filtered through phase separation paper. The loaded organic contained 5.58 g/L U.

[00106] An aqueous solution containing 30 g/L Al was prepared by dissolving AI₂(SO4)3.18H₂O in water. Approximately one third of this solution was mixed with limestone to pH 4.50 producing an Al and gypsum containing precipitate. The solids were filtered then mixed with the remaining two thirds of the Al containing solution for 5 minutes. The slurry was filtered and the filtrate collected. The filtrate was diluted by approximately 10% water to reduce the Ca concentration to below saturation. The liquor contained 176 g/L BAS.

[00107] The loaded organic was mixed with the BAS stripping solution at O/A ratios of 1 :3, 1 :1 , 3:1 , 8:1 and 15:1 for 4 minutes at 45°C without pH control. The phases were allowed to separate then filtered individually. The aqueous solutions were assayed for uranium content and the organic uranium

concentration was determined by difference. The results are presented as a McAbe Thiele diagram in Figure 6. The diagram indicates that >95% U can be stripped from the loaded organic solution in 3 stages, resulting in a loaded strip solution containing 20 g/L U.

[00108] A batch stripping test was conducted to generate sufficient loaded strip liquor for a precipitation test. This involved contacting a loaded organic solution containing 3.5% Armeen, 1 .4% isodecanol and 4.0 g/L U in Kermac 400 / 500 with the BAS solution at an O/A ratio of 10:1 . The loaded strip liquor was separated and filtered. It contained 28.3 g/L U.

[00109] The loaded strip liquor was mixed with a 105% stoichiometric excess of hydrogen peroxide. A yellow precipitate formed immediately. The slurry was allowed to mix for 1 hour. It was filtered and the filtrate was assayed for uranium. It contained 4 ppm U indicating >99.9% U precipitated from solution.

[001 10] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

1 . A metal recovery process comprising the method steps of:

(i) Adding a neutralising agent to a solution containing one or more non- target metals at moderate temperature thereby forming an amount of a soluble basic salt; and

(ii) Passing the solution formed in step (i) to a target metal recovery step in which acid generated in that target metal recovery step is consumed at least to some extent by the soluble basic salt from step

(^■)^■

2. A process according to claim 1 , wherein the non-target metal ions in step (i) are aluminium, ferric or chromium ions.

3. A process according to claim 1 or 2, wherein the soluble basic salt of the non- target metal is a soluble basic sulfate.

4. A process according to any one of claims 1 to 3, wherein the target metal recovery step (ii) is a reduction, electrowinning, solvent extraction, ion exchange or precipitation step.

5. A process according to any one of the preceding claims, wherein in the target metal recovery step, metal recovery is more complete than would have been possible if some or all of the acid generated had not been consumed.

6. A process according to any one of the preceding claims, wherein a barren solution, with respect to the target metal, is passed, at least in part, to a non- target metal ion removal step utilising a neutralising agent.

7. A process according to any one of the preceding claims, wherein the moderate temperature of step (i) is between 0°C and about 90°C.

8. A process according to claim 7, wherein the moderate temperature of step (i) is between 0°C and about 80°C.

9. A process according to any one of the preceding claims, the non target metal ion removal step produces either an aluminium hydroxide or mixed hydroxide precipitate.

10. A process according to claim 9, wherein the aluminium hydroxide or mixed hydroxide formed in the non-target metal removal step is utilised as the neutralising agent in step (i), whereby aluminium tenor in the metal recovery circuit is increased in turn creating more soluble basic aluminium sulfate and further enhancing extraction of the target metal ions.

1 1 . A process according to any one of the preceding claims, wherein the neutralising agent in step (i) is any base stronger than the basic aluminium sulfate.

12. A process according to claim 1 1 , wherein the base includes one or more hydroxides of target metals that have been precipitated elsewhere in the flow sheet, or oxides of target metals.

13. A process to recover a target metal from hydrometallurgical solutions comprising the method steps of:

14. A process according to claim 13, wherein the target metal recovery step (ii) is either a solvent extraction stripping step or ion exchange elution step.

15. A process according to claim 13 or 14, wherein the amine functionality in the solvent extraction step comprises one or more of the amines tri-n-octylamine (Alamine 300), tri-isooctylamine (Alamine 308), tri-(C8C10)amine (Alamine 336 or Armeen), tri-isodecyclamine (Alamine 310) or tri-laurylamine (Alamine 304).

16. A process according to any one of claims 13 to 15, wherein the stripped extractant from step (ii) is passed to a wash circuit to remove entrained aluminium then to an extraction circuit in which uranium is extracted by the anion exchanger extractant from an acid solution.

17. A process according to claim 16, wherein the wash solution is water or sodium hydroxide solution to improve uranium stripping efficiency.

18. A process according to claim 16 or 17, wherein a wash raffinate, containing aluminium, is recycled to the stripping circuit.

19. A process according to any one of claims 13 to 18, wherein the loaded amine functionality as solvent or IX resin is subject to a scrubbing stage to remove impurities prior to being passed to step (ii).

20. A process according to any one of claims 13 to 19, wherein the aqueous solution generated in step (iii), which contains aluminium sulfate, is passed at least in part, to step (i).

21 . A process according to any one of claims 13 to 20, wherein the moderate temperature of step (i) is between 0°C and about 80°C.

22. A process according to claim 21 , wherein the moderate temperature of step (i) is between 0°C and about 60°C.

23. A process according to any one of claims 13 to 20, wherein a part of the aluminium sulfate solution generated from the uranium precipitation step is subject to an aluminium precipitation step in order to remove aqueous solution.

24. A process according to any one of claims 13 to 20, wherein part of the solution generated in step (i) is subject to an aluminium precipitation step in order to remove aqueous solution.

25. A process according to any one of claims 13 to 24, wherein the aluminium hydroxide or mixed hydroxide formed in the aluminium precipitation step is utilised as the neutralising agent in step (i), whereby the aluminium tenor in the solvent extraction circuit is not diminished significantly by the aqueous solution removal step.

26. A hydrometallurgical process for the recovery of uranium from a uranium bearing ore or concentrate, the process comprising the following method steps:

(i) Passing the uranium bearing ore or concentrate to a leach step in which at least a proportion of the contained uranium is extracted into solution, forming a pregnant leach solution ("PLS");

(ii) Passing the PLS from step (i) to a solid liquid separation step producing a solid residue and a solution containing the bulk of the extracted uranium;

(vi) Contacting the solid from step (v) with solution containing aluminium sulfate at moderate temperature thereby forming an amount of a soluble basic aluminium salt; (vii) Passing the solution from step (vi) to a solid liquid separation step producing a solid residue and solution containing basic aluminium sulfate;

(ix) The aqueous solution formed in step (viii), which contains soluble uranium and basic aluminium sulfate, is contacted with hydrogen peroxide to precipitate uranium from solution, acid generated being consumed at least to some extent by the soluble basic aluminium salt,

(x) Passing the solution from step (ix) to a solid liquid separation step, producing a solid uranium product and solution containing aluminium sulfate; and

27. A process according to claim 26, wherein the steps (iv) and (vi) are achieved simultaneously whereby the basic aluminium sulfate is formed substantially in a single step.

28. A process according to claim 27, wherein step (v) is not present and a bleed of the liquor generated in step (vii) to control the water balance is present.

29. A process according to any one of claims 26 to 28, wherein the aluminium present in the bleed solution is recovered by precipitation with lime or limestone, then recycled to step (iv).