P/0o/oil Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Method for refining aqueous nickel chloride solution The following statement is a full description of this invention, including the best method of performing it known to us: METHOD FOR REFINING AQUEOUS NICKEL CHLORIDE SOLUTION BACKGROUND OF THE INVENTION FIELD OF THE INVENTION 5 The present invention relates to a method for refining an aqueous nickel chloride solution, more specifically a method for efficiently removing an impurity element, in particular zinc, which can form a complex with chlorine from the solution by a simple system at a low cost. DESCRIPTION OF THE PRIOR ART 10 The common starting material for nickel refining has been nickel matte, which is concentrated nickel sulfide produced from a nickel ore by a smelting process. Recently, new nickel-containing materials have been used as starting materials for nickel refining, as various scraps and intermediates produced in refining processes have been extensively recycled, and hydrometallurgical 15 processes, e.g., sulfuric acid leaching for treating a laterite ore containing nickel at a low content, have been commercialized. A nickel matte as the common starting material contains zinc as an impurity element in trace quantities, because zinc can be removed by a smelting process. On the other hand, new nickel starting materials contain zinc normally at several 20 hundreds ppm to several per cents by weight together with other impurity elements, e.g., copper, cobalt and iron, since they are produced by a wet separation process, e.g., neutralization or sulfidation, as precipitates settled in a solution. A hydrometallurgical process treating a new nickel starting material 25 containing zinc and other impurity elements needs an additional step for removing zinc, when it involves leaching a nickel starting material in the presence of chlorine 1A gas, refining the resultant aqueous nickel chloride solution and electrolysis to produce electrodeposited nickel, because zinc contained in the starting material cannot be sufficiently removed by the conventional nickel refining process. In other words, zinc can form a complex with chlorine, e.g., ZnC1 4 i, in an 5 aqueous nickel chloride solution. Zinc is removed in the form of sulfide in the presence of hydrogen sulfide gas blown as a sulfidation agent into an aqueous nickel salt solution. However, removal of zinc from an aqueous nickel chloride solution is less efficient than from an aqueous nickel sulfate solution, and is accompanied by massive coprecipitation of nickel when zinc is to be completely 10 removed. Moreover, it involves another problem of increased investment cost for an aeration step needed to treat the mother liquor containing hydrogen sulfide. Zinc forms the hydroxide at a lower pH level than nickel, and can be removed by neutralization selectively to some extent. However, massive coprecipitation of nickel is inevitable also in this case when zinc is to be removed 15 very deeply, because the solution should be kept at a pH level above 6. Therefore, neutralization is not a desirable process. Zinc may be separated by solvent extraction. However, it is also not a desirable process, because it needs a large-size system and hence high investment cost. 20 As discussed above, the conventional techniques for removing zinc involve the problems resulting from a high investment cost required or greatly reduced nickel production yield. Some methods which use an ion-exchange resin have been proposed to solve these problems. For example, JP-A-2001-20021 (pages 1 and 2) discloses 25 a method which passes an aqueous cobalt chloride solution over an anion exchange resin to remove metallic ions capable of forming a chloride complex, e.g., cobalt, zinc and iron ions, by adsorption while allowing nickel and the 2 like to flow out. Another method removes cobalt from an aqueous nickel chloride solution by oxidation/neutralization and then brings the treated solution into contact with an anion-exchange resin to remove zinc and chromium simultaneously by adsorption. This method can deeply remove zinc from an 5 aqueous nickel chloride solution. However, it may involve problems of shortened service life of the anion-exchange resin and hence increased treatment cost, depending on type and concentration of an impurity element which can form a complex with chlorine, when it is present in the solution. Under these circumstances, there have been demands for methods for 10 efficiently removing an impurity element, in particular zinc, which can form a complex with chlorine from an aqueous nickel chloride solution by a simple system at a low cost. SUMMARY OF THE INVENTION It is an object of the present invention to overcome or at least ameliorate the 15 above discussed problems recognised in the prior art. The inventors of the present invention have found, after having extensively studied a method for refining an aqueous nickel chloride solution containing zinc and one or more other impurity elements which can form a complex with chlorine to achieve the above object, that impurity elements, in particular zinc, which can 20 form a complex with chlorine can be efficiently removed by treating the solution adjusted under specific conditions in two steps, oxidation/neutralization and ion exchange with an anion-exchange resin, achieving the present invention. A method for refining an aqueous nickel chloride solution containing zinc and one or more other impurity elements which can form a complex with chlorine, 25 comprising: 3 a first step for oxidation/neutralization of the solution adjusted beforehand under given conditions with respect to Ni concentration of 90 to 130g/L, oxidation reduction potential of 600 to 1200mV (determined using a silver/silver chloride reference electrode) and pH of 4.0 to 6.0 to remove the impurity elements, and 5 a second step for ion-exchanging the solution refined by the first step with an anion-exchange resin to remove zinc by adsorption. The third aspect of the present invention is the method of the first aspect for refining an aqueous nickel chloride solution, wherein an additional step for treating the solution refined by the first step with active carbon is included prior to the 10 second step. The method of the present invention for refining an aqueous nickel chloride solution can efficiently remove an impurity element, in particular zinc, which can form a complex with chlorine from the solution by a simple system at a low cost, and is of very high industrial value. 15 DETAILED DESCRIPTION OF THE INVENTION The method of the present invention for refining an aqueous nickel chloride solution is described in more detail. The method of the present invention for refining an aqueous nickel chloride solution is for refining an aqueous nickel chloride solution containing zinc and one 20 or more other impurity elements which can form a complex with chlorine, comprising a first step for oxidation/neutralization of the solution adjusted beforehand under given conditions with respect to Ni 4 concentration, oxidation-reduction potential and pH to remove the impurity elements, and second step for ion-exchanging the solution refined by the first step with an anion-exchange resin to remove zinc by adsorption. It is of essential significance for the present invention to remove by 5 oxidation/neutralization an impurity element, other than zinc, which can form a complex with chlorine from an aqueous nickel chloride solution as a starting material adjusted beforehand under given conditions with respect to Ni concentration, oxidation-reduction potential and pH in the first step, prior to the second step which removes zinc by ion-exchange with an anion-exchange resin. 10 This can greatly improve service life of the anion-exchange resin for the second step, and efficiently remove zinc. In other words, an impurity element, other than zinc, which can form a complex with chlorine is adsorbed on the anion-exchange resin during the ion-exchange step together with zinc to cause a breakthrough point in a shorter time, when it is present in the solution. Removal of such an 15 element prior to the second step prevents the above problem. The technical backgrounds of breakthrough point at an ion-exchange resin are described in detail. Two types of test solutions were prepared using a base solution containing Ni at 120g/L, and cobalt, copper, iron and zinc each at 0.001g/L or less; one was the base solution adjusted to contain zinc at 0.005g/L, 20 and cobalt, copper and iron each at 0.001g/L or less, and the other was the base solution adjusted to contain zinc at 0.005g/L, and cobalt and iron each at 0.5g/L, to follow removal of zinc from these test solutions by ion-exchange with an anion exchange resin. Zinc chloride, cobalt chloride and ferric chloride, all of reagent grade, were used for these solutions to adjust the zinc, cobalt and iron 25 concentrations. Each of these test solutions (250mL each) was separately put in a glass beaker, incorporated with 15mL of an anion-exchange resin (Amberlite IRA400, supplied by Organo Corp.) and stirred by a stirrer at 50 0 C, to analyze zinc concentration of the final solution. It was found that zinc was removed to 5 0.0001g/L or less with the solution of lower cobalt and iron concentrations, and only to 0.001g/L with the solution of higher cobalt and iron concentrations. In order to keep zinc concentration at 10ppm or less in electrodeposited nickel produced by electrolysis, it is necessary to decrease zinc concentration at 5 0.0001g/L or less in the aqueous nickel chloride solution to be treated by the electrolysis. In other words, it is essential to remove an impurity element, other than zinc, which can form a complex with chlorine prior to ion-exchange with anion-exchange resin. (1) Aqueous nickel chloride solution 10 The aqueous nickel chloride solution for the present invention is not limited, and an aqueous nickel chloride solution containing zinc and one or more other impurity elements is used. Of aqueous nickel chloride solutions useful for the present invention, one of the preferable ones is an aqueous nickel chloride solution produced by a hydrometallurgical process which treats a nickel starting 15 material by leaching with chlorine gas, refining the resultant aqueous nickel chloride solution and electrolysis to produce electrodeposited nickel. The aqueous solution produced contains impurity elements, e.g., zinc, cobalt, copper, iron and a noble metal, which can form a complex with chlorine. (2) First step 20 The first step for the present invention treats an aqueous nickel chloride solution by oxidation/neutralization, after it is adjusted under given conditions with respect to Ni concentration, oxidation-reduction potential and pH to remove impurity elements in the form of hydroxides. Ni concentration of the aqueous nickel chloride solution for the first step is not 25 limited. However, it is preferably 90 to 130g/L, more preferably 100 to 120g/L. An aqueous nickel chloride solution produced by leaching a nickel starting material with chlorine gas normally contains nickel at around 170g/L. It is treated by 6 oxidation/neutralization after being diluted to a chlorine ion concentration at which chlorine complexes of impurity elements, e.g., cobalt, copper and iron, become unstable. This is to utilize difference among the impurity elements in stability while they 5 form a chlorine complex in an aqueous nickel chloride solution. Decreasing chlorine ion concentration makes cobalt, copper and iron complexes with chlorine less stable and more easily removed by the oxidation/neutralization. It should be noted, however, that increasing extent of dilution increases the diluent quantity and hence system capacity. Therefore, the Ni concentration is set at 90g/L, which 10 corresponds to the minimum chlorine concentration at which zinc can form a complex with the chlorine ion, or more. At above 130g/L, on the other hand, cobalt, copper and iron concentrations cannot be removed to a level, e.g., 0.01g/L or less, at which the adverse effects of retarding zinc adsorption in the second step can be controlled to a minimum acceptable level. It is preferable to recycle 15 the spent solution discharged from the electrolysis step as the diluent for the aqueous nickel chloride solution. Oxidation-reduction potential of the aqueous nickel chloride solution for the first step is not limited. It is however preferably 600 to 1200mV (determined using a silver/silver chloride reference electrode), more preferably 1000 to 1200mV. At 20 below 600mV, the oxidation proceeds insufficiently to satisfactorily remove cobalt, copper and iron. At above 1200mV, on the other hand, oxidation of nickel is accelerated to increase coprecipitated Ni quantity. The oxidant for the first step is not limited. One of the preferable ones is chlorine gas, which causes little accumulation of the impurity elements. 25 In the first step, pH level of the aqueous nickel chloride solution is not limited. However, it is preferably 4.0 to 6.0, more preferably 4.0 to 5.0. At below 4.0, the neutralization proceeds insufficiently to satisfactorily remove cobalt, copper and iron. At above 6.0, on the other hand, neutralization of nickel is accelerated to increase coprecipitated Ni quantity. The pH adjusting agent for the first step is 7 not limited, but an alkali salt is normally used. The preferable alkali salts include nickel hydroxide, basic nickel carbonate and nickel carbonate, which cause little accumulation of the impurity elements. (3) Second step 5 The second step for the present invention is for ion-exchanging the solution refined by the first step with an anion-exchange resin to remove zinc by adsorption. The aqueous nickel chloride solution is refined in the first step to remove the impurity elements, e.g., cobalt, copper and iron, to 0.01g/L or less, in order to minimize their adverse effects of retarding adsorption of zinc in the 10 second step. The resin which adsorbs zinc is cleaned with a hydrochloric acid solution, and then subjected to an elution procedure with water, after the aqueous nickel chloride solution deposited thereon is recovered. In the second step, pH level is not limited. However, it is set at 6.0 or less, at which neutralization of nickel tends to be controlled. In other words, the solution 15 refined in the first step is directly used for the second step without being adjusted. Adsorption temperature in the second step is not limited, but preferably 30 to 70 0 C. The procedure for the second step is not limited. It is however preferably carried out in a column packed with a commercial anion-exchange resin, because 20 of its high efficiency resulting from the liquid being continuously passed over the resin at a given rate until the adsorption breakthrough occurs. (4) Treatment with active carbon In the present invention, the aqueous nickel chloride solution may be treated with active carbon, as required, after being refined in the first step and before 25 being charged to the second step. This removes dissolved chlorine or trace quantities of a noble metal by adsorption on active carbon, when present in the 8 refined solution, and thereby more efficiently controls deterioration of zinc adsorption capacity in the second step, because dissolved chlorine and a noble metal have an adverse effect on zinc adsorption capacity of the ion-exchange resin. 5 The procedure for the treatment with active carbon is not limited. However, it is preferable to use commercial active carbon of wood, coal, coconut or the like packed in a column. An aqueous nickel chloride solution containing impurity elements which can form a complex with chlorine, e.g., cobalt, copper, iron and zinc, can be refined by 10 these steps to be suitable for electrolysis of nickel, substantially free of these impurity elements. EXAMPLES The present invention is described in more detail by EXAMPLES, which by no means limit the present invention. Metals were analyzed by atomic absorption 15 spectrometry in EXAMPLES. EXAMPLE 1 An aqueous nickel chloride solution produced by leaching a nickel starting material with chlorine was treated by the following first and second steps for a hydrometallurgical process involving oxidation/neutralization and then electrolysis 20 to produce electrodeposited nickel. (1) First step First, the aqueous nickel chloride solution was diluted with the spent solution discharged from the electrolysis step to have a nickel concentration of 120g/L before it was treated by oxidation/neutralization. Then, it was incorporated with a 25 given quantity of zinc chloride of reagent grade, to adjust the starting aqueous nickel chloride solution under given conditions, and treated by 9 oxidation/neutralization under the following conditions. The resulting precipitate was separated by filtration, and the filtrate as the refined solution was analyzed for its composition. The results are given in Table 1, which also shows composition of the starting aqueous nickel chloride solution. 5 [Oxidation/neutralization conditions] (1) Oxidation condition: Oxidation-reduction potential was set at 10OOmV (determined using a silver/silver chloride reference electrode) with chlorine gas as an oxidant blown into the system. (2) Neutralization condition: pH level was adjusted at 4.5 with nickel carbonate 10 (Sumitomo Metal Mining) as a pH adjusting agent. Table 1 Concentration (g/L) Ni Co Cu Fe Zn Starting aqueous nickel chloride 120 0.3 0.007 0.2 0.7 solution Solution refined in the first step 120 0.001 <0.001 <0.001 0.7 As shown in Table 1, cobalt, copper and iron were removed to 0.001g/L or less, whereas zinc was not. 15 (2) Second step The solution refined in the first step, i.e., the aqueous nickel chloride solution oxidation/neutralization-treated, was incorporated with zinc chloride of reagent grade to prepare the starting adsorption solution. Its composition is given in Table 2. 10 Table 2 Concentration (g/L) Ni Co Cu Fe Zn Solution for the second step 115 0.002 0.0001 0.0003 0.003 The solution was passed through a column packed with 200mL of an anion exchange resin (Amberlite IRA400, supplied by Organo Corp.). The adsorption 5 conditions were liquid space velocity (liquid volume/hour/resin volume): 5 and temperature: 50 0 C. The treated solution was analyzed for zinc concentration. The results are given in Table 3. Table 3 Liquid rate (BV) 5 100 200 253 306 Zinc concentration (mg/L) <0.1 <0.1 <0.1 <0.1 <0.1 10 As shown in Table 3, the solution was highly purified to contain zinc at 0.1mg/L or less until it was passed over the anion-exchange resin to 306 multiples of bed volume (BV). EXAMPLE 2 An aqueous nickel chloride solution produced by leaching a nickel starting 15 material with chlorine was treated by the first and second steps for a hydrometallurgical process involving oxidation/neutralization and then electrolysis to produce electrodeposited nickel, where the effect of treating the solution with 11 active carbon was verified. The solution refined in the first step was passed over active carbon. Each of the solutions, one treated with active carbon and the other not, was incorporated with zinc chloride of reagent grade at 0.003g/L as zinc. 5 Each of these solutions was passed through a column packed with 200mL of an anion-exchange resin (Amberlite IRA400, supplied by Organo Corp.) at a space velocity of 2(SV2) to about 300 multiples of BV. Next, the resin was washed with water at a space velocity of 2(SV2) to 5 multiples of BV to elute out the adsorbed zinc. The adsorption/elution cycles were repeated 50 times. 10 For the solution not treated with the active carbon, the adsorption-treated liquid volume until zinc was detected at 0.0001g/L or more (hereinafter referred to as breakthrough BV) decreased gradually, to about 70% of the initial level at the 50 cycles. For the solution treated with the active carbon, on the other hand, the anion 15 exchange resin exhibited the adsorption performance substantially unchanged throughout 50 cycles, with decreased breakthrough BV not confirmed. Therefore, treatment with active carbon controls decrease in breakthrough BV in the adsorption/elution cycles in the ion-exchange step, decreasing elution frequency, and hence both waste liquid treatment cost and waste liquid treatment investment 20 cost. The method of the present invention is useful for refining an aqueous nickel chloride solution for the nickel refining area, in particular suitable for refining an aqueous nickel chloride solution containing at a high content an impurity element which can form a complex with chlorine. 25 As used herein, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude 12 other additives, components, integers or steps. Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment, or any form of suggestion, that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that 5 this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. 13