BACKGROUND OF THE INVENTION

[0001] This invention relates to a method of purifying a value metal-containing solution.

[0002] Due to its cost effectiveness, precipitation is often a preferred method of purifying a metal-containing solution. There are, however, limitations and disadvantages associated with this method which include the following:

(1 ) the addition of a precipitating reagent to the metal-containing solution can introduce impurities such as sodium, calcium and magnesium;

(2) some of these impurities, e.g. sodium and magnesium, are highly soluble cations which go into solution when the precipitation reagent dissolves into the metal-containing solution. Additional treatment is required to remove these impurities from the value metal-containing stream;

(3) the addition of the precipitating reagent, in a solution or slurry form, dilutes the metal-containing solution;

(4) a conventional precipitation process calls for stringent pH control to prevent localized precipitation of a target metal from the metal-containing solution; and

(5) multiple purification stages, which may require a separation technique such as an ion-exchange process, may be needed to remove all of the impurities. [0003] An object of the invention is to provide a method of recovering a value metal from a value metal-containing solution, using a precipitation process which, at least to some extent, addresses the aforementioned issues.

SUMMARY OF THE INVENTION [0004] The invention provides a method of purifying a value metal-containing solution which includes the steps of: a) dividing the value metal-containing solution into a minor feed stream and a major feed stream; b) treating the minor feed stream with a first reagent to generate a first minor suspension containing a first value metal-containing precipitate; c) recovering the first precipitate from the first minor suspension to form a first treatment stream; d) treating the major feed stream with the first precipitate to form a first major suspension containing a first impure precipitate that includes impurities with a lower solubility than the value metal; and e) removing the second precipitate from the first major suspension to form an at least partially purified stream.

[0005] The minor feed stream may be split into, at least, a first minor split stream and a second minor split stream. The first minor split stream may be treated in accordance with steps a) to e). [0006] The second minor split stream may be treated with a second reagent to form a second minor suspension which includes a second value metal-containing precipitate. This principle could be extended to a third, fourth etcetera number of minor split streams as required. [0007] The second value metal-containing precipitate may be recovered from the second minor suspension to form a second treatment stream.

[0008] The first major suspension may be treated with the second value metal- containing precipitate to form a second major suspension that contains a second impure precipitate. [0009] The first impure precipitate and the second impure precipitate may be recovered simultaneously to produce the at least partially purified stream.

[0010] The size of the minor feed stream, relative to the major feed stream, may be determined using a stoichiometric relationship taking into account the quantity of impurities in the value metal-containing solution. [0011] The nature of the first and second reagents may be dependent on one or more of the following: the solubility of the value metal in the value metal containing solution; the type of each impurity in the value metal containing solution; and a target pH-value of the first major suspension. [0012] The first reagent or second reagent may be a hydroxide, a carbonate or a sulphide. Thus the first or second value metal-containing precipitate, respectively, may be a hydroxide, a carbonate or a sulphide of the value metal.

[0013] A sulphide-containing precipitate may cause impurities to precipitate as sulphides.

[0014] The first reagent and second reagent may be of different compositions relative to each other. Preferably, the first reagent is selected to precipitate the value metal and impurities that have a lower solubility than the value metal. The second reagent may be selected to selectively precipitate impurities with similar properties to the value metal (or which have a lower solubility than the value metal) while the value metal remains in solution.

[0015] An oxidant may be added, according to requirement, to at least one of the minor feed stream, the first minor split stream, the second minor split stream, the major feed stream, the first major suspension and the second major suspension, to alter the solubility or the oxidation state of the value metal or the impurities. The first value metal-containing precipitate may contain impurities depending, at least, on the first reagent. These impurities may precipitate at pH-values which are lower than the pH-value at which the value metal is precipitated.

[0016] The precipitates may be recovered by means of any suitable solid/liquid separation process. BRIEF DESCRIPTION OF THE DRAWING

[0017] The attached drawing is a process flow sheet illustrating the use of a method of the invention to treat a nickel sulphate solution.

DESCRIPTION OF PREFERRED EMBODIMENT [0018] The invention is further described by way of example with reference to the accompanying drawing which is a diagrammatic representation of a method according to the invention for treating a value metal-containing solution 10, such as a nickel sulphide- containing solution. A target metal (value metal) is nickel (Ni) and impurities are iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), arsenic (As) and cobalt (Co). The impurities depend on the composition of the value metal-containing solution 10 and the aforementioned impurities are by way of example only.

[0019] The solution 10 is divided into a minor feed stream 12 and a major feed stream 14. Typically the minor feed stream 12 is substantially smaller (of the order of 10%) than the major feed stream 14. The size of the minor feed stream 12 is stoichiometrically determined by the extent (quantity) of impurities (Cu, Fe, etc.) to be removed from the solution 10.

[0020] The minor feed stream 12 is split into a first minor feed split stream 16 and a second minor feed split stream 18.

[0021] In a first precipitation step 20A the first minor feed split stream 16 is treated with a first reagent 22 containing sodium hydroxide to form a first minor suspension 24. The suspension 24 is subjected to a solid-liquid separation step 26 to produce a first treatment stream 28, containing sodium sulphateand a first value metal-containing precipitate 30, which includes nickel hydroxide.

[0022] The stream 28 is subjected to a downstream effluent treatment step 32.

[0023] In a second precipitation step 20B the second minor split stream 18 is treated with a second reagent 34 which includes sodium hydrogen sulphide (NaHS) to form a second minor suspension 36. The suspension 36 is subjected to a solid-liquid separation step 38 to produce a second treatment stream 40 and a second value metal-containing precipitate 42, which includes nickel sulphide.

[0024] The stream 40 is subjected to a downstream effluent treatment step 44. [0025] In a precipitation step 46, the precipitate 30 is added to the major feed stream 14. The nickel hydroxide in the precipitate 30 increases the pH of the major feed stream 14 to a value of about 5.0. This causes the nickel hydroxide to dissociate into nickel and hydroxide. The nickel re-dissolves into the major feed stream 14 and the hydroxide precipitates impurities (that are insoluble at a pH value lower than 5) contained in the major feed stream 14, forming a first major suspension 48.

[0026] After about 1 hour, in a step 50, the second value metal-containing precipitate 42 is added to the first major suspension 48 to form a second major suspension 52. The nickel sulphide in the precipitate 42 causes the selective precipitation of impurities still contained in the first major suspension 48. These impurities typically have similar properties to, or a lower solubility than, the value metal. In this case the impurity is copper. The selective precipitation could, for example, be achieved by means of metathesis. [0027] Oxygen, not shown, is continuously added as appropriate to the minor feed stream, the major feed stream, the first minor split stream, the second minor split stream, the first major suspension and the second major suspension to oxidize the ferrous iron to ferric iron. Ferric has a lower solubility than ferrous iron. [0028] The second major suspension 52 is subjected to a solid-liquid separation step 54 to form a partially purified stream 56 and an impurity-containing precipitate 58.

[0029] The partially purified stream 56 contains the value metal in solution and is subjected to further purification to recover the value metal as a metal product 64 (in this case a nickel product). [0030] The impurity-containing precipitate 58 may be discarded or subjected to further processing.

[0031] In alternative embodiments of the invention, precipitation reagents are, for example, selected from a hydroxide reagent (e.g. NaOH), a carbonate reagent (e.g. Na ₂ C0 ₃ ) and a sulphide reagent (e.g. Na ₂ S). These reagents are exemplary only and are non-limiting. The selection depends on the solubility of the impurities present in the feed solution 10 and, in certain cases, the quantity or availability of the reagent.

[0032] An accurate mass balance of each step is crucial. Table 1 represents an example of a suitable steady-state mass balance, generated using SysCAD software. [0033] Table 1

[0034] Table 1 sets out theoretical values on which a pilot plant or process plant for recovering nickel could be modelled. The values indicated are for a process carried out at ambient conditions (25°C and atmospheric pressure). [0035] The following specific description is made with reference to the attached drawing.

[0036] A nickel-containing solution 10 (column 1 ) is supplied at a mass flow rate of 134.14 tons/hour and a volumetric flow of 1 13.94 m ³ /hour. The nickel-containing solution consists of 877.63 g/L of water, copper(ll)sulphate in solution, ferric sulphate in solution and nickel sulphate in solution, each subject to the mass flow rates indicated in column 1.

[0037] The nickel-containing solution 10 is divided into a major feed stream 14 (column

3) and a minor feed stream 12 (column 6) subject to mass flow rates of 123.04 tons/hour and 1 1.10 tons/hour and volumetric flow rates of 104 m ³ /hour and 9.43m ³ /hour, respectively. The minor feed stream 12 (column 6) is further divided into a first minor split stream 16 (column 4) and a second minor split stream 18 (column 5). The mass flow rates of the copper(ll)sulphate, ferric sulphate and nickel sulphate, contained in solution in each stream, are as indicated in each respective column.

[0038] A first reagent 22 (column 12) is added to the first minor split stream 16 (column

4) . The mass flow rate of the first reagent 22 is 7.08 tons/hour and a volumetric flow rate is 6.47 m ³ /hour. The first reagent 22 constitutes a pure solution of sodium hydroxide (927.68 g/L water) at a mass flow rate of 1.08 ton/hour.

[0039] The first reagent 22 and the first minor split stream 16 form a first minor suspension 24 (column 8). The suspension 24 is subject to a mass flow rate of 15.18 tons/hour and a volumetric flow rate of 14.25 m ³ /hour. The suspension 24 constitutes a solution of 131.00 g/L sodium sulphate containing hematite and nickel hydroxide in suspension (subject to the indicated mass flow rates). Sodium hydroxide and copper(ll)sulphate remain in solution.

[0040] The relevant reaction is as follows: 2NaOH + NiS0 ₄ = Na ₂ S0 ₄ + Ni(OH) ₂

[0041] The first minor suspension 24 (column 8) is subject to a solid/separation step 26 to produce a first value metal-containing precipitate 30 (column 16) which is separated in a first treatment step 30 (column 14). The first value metal containing-precipitate 30 contains hematite and nickel hydroxide in solid form and sodium hydroxide and copper(ll)sulphate in solution, each subject to flow rates as indicated in the respective columns.

[0042] A second reagent 34 (column 13) is added to the second minor split stream 18 (column 5). The mass flow rate of the first reagent 34 is 3.59 tons/hour and a volumetric flow rate is 3.25 m ³ /hour. The second reagent 34 constitutes a solution of sodium hydroxide and sodium hydrogen sulphide (NaHS) (921.74 g/L water) [See Table 1 for relevant mass flow rates.]

[0043] The second reagent 34 and the second minor split stream 18 form a second minor suspension 36 (column 9). The suspension 36 is subject to a mass flow rate of 6.59 tons/hour and a volumetric flow rate of 6.18 m ³ /hour. The suspension 36 comprises a solution of 1 1 1.89 g/L sodium sulphate containing hematite and nickel sulphide in suspension (subject to the indicated mass flow rates). Sodium hydroxide, NaHS and copper(ll)sulphate remain in solution (at indicated mass flow rates). [0044] The relevant reaction is as follows:

NaHS + NaOH + NiS0 ₄ = Na ₂ S0 ₄ + NiS + H ₂ 0

[0045] The second minor suspension 36 (column 9) is subjected to a solid/separation step 38 which produces a second value metal-containing precipitate 42 (column 17). The second value metal containing-precipitate 42 contains hematite and nickel sulphide in solid form and sodium hydroxide and NaHS in solution, each subject to flow rates as indicated in the respective columns.

[0046] The first value metal containing-precipitate 30 (column 16) is added to the major feed stream 14 (column 4) to form a first major suspension 48 (column 10). The first major suspension 45 comprises a solution of nickel sulphate at a mass flow rate of 30.89 tons/hour. Sodium hydroxide, copper(ll)sulphate and sodium sulphate are found in solution at the indicated flow rates. Hematite is suspended in the solution.

[0047] The relevant reaction is the dissociation of nickel hydroxide (due to the decrease in pH). The amount of nickel hydroxide is controlled so that the pH does not decrease to such an extent that impurities less soluble than nickel are dissolved yet is reduced to a sufficient extent that all the nickel hydroxide dissociates into solution.

[0048] The second value metal containing-precipitate 42 (column 17) is added to the first major suspension 48 (column 10) to form a second major suspension 52 (column 1 1). The second major suspension 52 is subject to a mass flow rate of 128.43 tons/hour and a volumetric flow rate of 109.38 m ³ /hour and comprises a solution of nickel sulphate containing, in solution, NaHS, sodium hydroxide and sodium sulphate. Solids in suspension are nickel sulphide, hematite and copper sulphide - See Table 1 for the relevant mass flow rates in each respective column.

[0049] The second major suspension 52 (column 1 1 ) is subjected to a solid/liquid separation step 54, to separate an impurity containing-precipitate 58 (column 2) from a partially purified stream 56 (column 7). The impurity containing-precipitate 58 mainly comprises the solids contained in the second major suspension 52 whereas the partially purified stream 56 comprises the nickel sulphate-containing solution 10 (including NaHS, sodium hydroxide and sodium sulphide, each in solution).

[0050] The method of the invention is suitable for multiple applications such as:

• purification of a nickel sulphate solution;

• purification of a cobalt sulphate solution prior to electrowinning;

• neutralisation of free acid prior to value metal recovery by processes such as crystalisation;

• rejection of impurities from a rare earth elements (REE) chloride solution;

• rejection of Fe from a titanium solution.

[0051] The method of the invention has the following advantages associated therewith:

• since the preferred precipitation reagent (22;34) is not added into the entire impure feed solution 10, the introduction of metal impurities associated with the precipitation reagent is avoided; • there is a concentrating effect of the value metal in the step 46 where the value metal dissociates from the anion, allowing an in situ formation of free anions to react with the impurities in the major feed stream 14 to form the impurity precipitate 58 - this simplifies process control as a strict pH control is not required due to the buffering effect of the value metal on the solution containing the anions which are formed in situ;

• the volume of the partially purified liquid stream 56 to be treated is small - this reduces capital and operational expenditure of further processing steps;

• the volumes of the effluents (32;44) to be treated are reduced significantly; · several of the impurities can be rejected in one step, minimising the number of process steps;

• the requirement for further concentration of the value metal, by other processes such as solvent extraction, may be avoided.