P/00/01 1 Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Process for purification of aqueous solution of nickel sulfate containing cobalt and calcium The following statement is a full description of this invention, including the best method of performing it known to us: 004450409vl SPECIFICATION PROCESS FOR PURIFICATION OF AQUEOUS SOLUTION OF NICKEL SULFATE CONTAINING COBALT AND CALCIUM BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a process for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, more particularly a process of industrially high efficiency for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, the former being present at a high concentration, which can increase cobalt treatment capacity and produce a purified aqueous solution of nickel sulfate of reduced cobalt and calcium concentration by improving cobalt extraction efficiency in the exchange reaction. DESCRIPTION OF THE PRIOR ART A process for purification of an aqueous solution of nickel sulfate containing cobalt and calcium purifies a crude aqueous solution of nickel sulfate containing a variety of impurities to produce the high-purity solution at industrially high efficiency. A purified aqueous solution of nickel sulfate of high purity can be treated by various processes, adopted as required, into high-purity nickel salts, e.g., nickel sulfate by concentration (e.g., crystallization), nickel oxide by roasting, and nickel carbonate by neutralization with sodium carbonate. Nickel salts, e.g., nickel sulfate, oxide and carbonate, have various industrial applications. These include materials for general electrolytic plating, electroless plating for computer hard disks, and for catalysts and batteries. Nickel sulfate, in particular, has been extensively used as a 2 004450409v1 material for plating and secondary batteries. One of the raw materials for nickel salts, e.g., nickel sulfate, is crude nickel sulfate containing cobalt at a relatively low concentration. A crude aqueous solution of nickel sulfate, produced by treating a raw material containing cobalt at several percent, is also used fairly extensively. These raw materials include nickel matte, and compound hydroxide or sulfate of nickel and cobalt. Therefore, the process for producing a purified aqueous solution of nickel sulfate from the crude solution or the like generally includes a step for separating nickel and cobalt from each other. Cobalt is rarer and more expensive than nickel, and is separated and purified into various products, e.g., electrolytically purified cobalt, and cobalt chloride and carbonate, to improve economic efficiency of the above process. The industrial applications of nickel salts often require impurities present in the aqueous solution of nickel sulfate to be minimized. These impurities include ammonia, sodium, iron, zinc, copper, calcium and magnesium, in addition to cobalt. Solvent extraction is a normal choice for purifying a crude aqueous solution of nickel sulfate containing impurities. The solvent extraction processes fall into two general concepts for purifying the solution; (1) extraction of impurities in an organic phosphate-based extractant, e.g., acidic phosphonic or phosphinic acid ester, and (2) extraction of nickel in an organic extractant, followed by stripping of the nickel-loading organic phase with sulfuric acid. However, each concept involves problems resulting from release of the hydrogen ion while impurities or nickel present in a crude aqueous solution is being extracted with an acid extractant, which invariably needs use of sodium hydroxide or ammonia as a neutralizer. When impurities are to be extracted from a crude aqueous solution of 3 004450409v1 nickel sulfate with an acid extractant, for example, impurities (e.g., cobalt, calcium, iron, zinc, copper or the like), which are generally extracted at a lower pH level than nickel, can be separated in the extractant by adjusting pH of the system to purify the solution. However, this involves problems resulting from contamination of the purified aqueous solution with the sodium or ammonium ion, incorporated as a neutralizer necessary for the extraction process. On the other hand, when nickel is to be extracted with an acid extractant from a crude aqueous solution of nickel sulfate in the extractant, it is loaded together with impurities, which are extracted at a lower pH level than nickel. Stripping with sulfuric acid to recover nickel from the above loaded extractant is difficult to remove all of these impurity elements. Some processes have been proposed to solve these problems, where a crude aqueous solution of nickel sulfate containing cobalt and other impurities is brought into contact with an acid extractant treated beforehand to extract nickel therein (such an extractant is hereinafter sometimes referred to as nickel-loading, acid extractant) to exchange (substitute) nickel in the nickel-loading, acid extractant with impurities, beginning with cobalt, - which are extracted in an acid extractant in preference to nickel, in order to produce a purified aqueous solution of nickel sulfate and, at the same time, the organic extractant with concentrated cobalt. These processes may be represented by the ones described below: (1) A solvent extraction process for separating impurities, e.g., cobalt, calcium magnesium and iron, from a crude aqueous solution of nickel sulfate with a nickel-containing, alkyl phosphonic acid ester or alkyl phosphinic acid as an extractant (disclosed by, e.g., Japanese Patent Laid-open Publication No.10-30135 (pages 1 and 2)). 4 004450409vl (2) A nickel sulfate purification process comprising series of steps: extraction step with an acid extractant for separating nickel from a crude aqueous solution of nickel sulfate containing sodium or ammonia at a high concentration to prepare a nickel-loading organic phase, step for washing the nickel-loading organic phase prepared in the preceding extraction step with a nickel-containing washing solution, and exchanging step for exchanging (substituting) nickel in the nickel-loading organic phase washed in the preceding step with impurities, e.g., cobalt, present in the crude aqueous solution of nickel sulfate, by reacting the organic phase with the crude solution containing cobalt at a high concentration, to produce a purified aqueous solution of nickel sulfate and, at the same time, organic phase in which cobalt is concentrated. This process further comprises steps; stripping step with diluted sulfuric acid for selectively stripping nickel from the impurity-loading organic extractant prepared in the preceding exchanging step, stripping step with hydrochloric acid for recovering cobalt from the organic phase prepared in the preceding stripping step for selective stripping of nickel, step for washing the organic phase prepared in the preceding cobalt recovery step, and stripping step with sulfuric acid for separating other impurities from the organic phase washed in the preceding step, where part of the impurity-free organic phase prepared in the preceding impurity stripping step is recycled back to that extraction step as an acid extractant, and the remainder is used for diluting the nickel-loading organic phase (disclosed by, e.g., Japanese Patent Laid-open Publication No.10-310437 (pages 1 to 5)). These proposals have been contributing to processes for producing high-purity, refined aqueous solution of nickel sulfate from a crude aqueous solution of nickel sulfate containing impurities and recovering cobalt. 5 004450409vl More recently, however, demands have been increasing for processes which can treat a crude aqueous solution of nickel sulfate containing cobalt and other impurities at a higher concentration to produce a highly pure aqueous solution of nickel sulfate and, at the same time, improve efficiency of recovering expensive cobalt. To satisfy these demands, application of the above processes (disclosed by, e.g., Japanese Patent Laid-open Publication No.10-310437 (pages 1 to 5) ) is expected, because of their potential for producing a highly pure aqueous solution of nickel sulfate from a crude aqueous solution of nickel sulfate and, at the same time, an organic solvent in which cobalt is concentrated. These processes can be of industrially high efficiency, when they can have increased capacity for treating a starting material of higher cobalt concentration, because an existing production system for purifying an aqueous solution of nickel sulfate can be positively utilized for nickel/cobalt separation to increase recovered cobalt production. Treatment of a starting material of higher cobalt concentration will give a crude aqueous solution of nickel sulfate of higher cobalt concentration. There are 2 concepts to increase cobalt treatment capacity for the step of exchanging nickel in the nickel-loading organic phase with cobalt and other impurities present in a crude aqueous solution of nickel sulfate in these processes; (1) increasing flow rate of the nickel-loading organic phase to increase amount of cobalt to be extracted per unit time, and (2) increasing cobalt concentration in the organic phase from the exchanging reaction step. The former concept of increasing treatment flow rate is a simple approach, but needs large investments to increase capacity of the solvent extraction reaction facilities, e.g., mixer settler, in proportion to the increased flow rate to keep a residence time required for separation of the nickel-loading organic phase from a crude aqueous solution of nickel sulfate, which may 6 004450409v1 harm economic efficiency. For the latter concept, on the other hand, it is effective to increase concentration of cobalt in the exchanged organic phase by increasing concentration of an acid extractant in the nickel-loading organic phase and cobalt extraction efficiency. When phosphoric or phosphonic acid is used as an acid extractant for the exchanging reaction, 1 mol of cobalt or calcium can be extracted per 2 mols of the extractant. It is the maximum attainable extraction rate of cobalt or calcium (hereinafter referred to as the stoichiometric extraction rate). With 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. as an extractant, the stoichiometric extraction rate of cobalt is 18.3g/L in the organic solvent containing the extractant at 20% by volume and a diluent. On a -commercial scale, however, cobalt extraction efficiency is generally low at 40 to 60% of the stoichiometric extraction rate. The low extraction rate results from intentional control of cobalt concentration to stabilize its extraction, because (1) driving force for the extraction reaction is decreased with an extractant having a larger number of functional groups bound to the nickel or cobalt ions than an extractant having a larger number of functional groups bound to the hydrogen ions, (2) utilization of nickel in the exchanging reaction is decreased, when pH level of the reaction system is increased to increase extraction capacity, which is accompanied by accelerated extraction of nickel to increase its concentration in the exchanged organic phase, and (3) increased cobalt concentration in the organic phase generally increases its viscosity to decreased efficiency for separating the organic phase from the aqueous solution phase. Therefore, there are problems to be solved also for increasing cobalt concentration in the exchanged organic phase to increase cobalt extraction capacity. 7 Therefore, there are demands for methods which can increase cobalt concentration in the exchanged organic phase, i.e., increase cobalt extraction efficiency, while controlling viscosity increase of the phase, without increasing plant capacity, in order to increase cobalt treatment capacity. 5 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 this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. 10 SUMMARY OF THE INVENTION It is an object of the present invention to provide a process of industrially high efficiency for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, the former being present at a high concentration, which can increase cobalt treatment capacity and produce a purified aqueous solution of nickel sulfate 5 of reduced cobalt and calcium concentration by improving cobalt extraction efficiency in the exchange reaction, in consideration of the above-described problems involved in the conventional techniques. The inventors of the present invention have found, after having extensively studied an aqueous nickel sulfate solution purification process based on solvent 20 extraction involving an exchanging reaction in a multi-stage, countercurrent reaction tank system to attain the above object, that cobalt extraction efficiency can be improved by bringing an aqueous solution phase of nickel sulfate containing cobalt and calcium into contact with a mixed organic solvent phase containing an extractant at a specific concentration, and keeping the aqueous phase at a specific 25 pH level, and calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase at a specific level, achieving the present invention. The first aspect of the present invention is a process for purification of 8 an aqueous solution of nickel sulfate, comprising solvent extraction involving an exchanging reaction in a multi-stage, countercurrent reaction tank system, wherein an aqueous solution of nickel sulfate containing cobalt and calcium is supplied as the aqueous phase to the last stage of the reaction tank system and a mixed organic 5 solvent containing an extractant of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester loading nickel, diluted with a hydrocarbon to an extractant concentration of 20 to 30% by volume, is supplied as the organic phase to the first stage of the reaction tank system, to bring these phases into contact with each other for an exchanging reaction, the aqueous phase being kept at a pH level of 4.5 to 5.5 in the last stage, o and calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase being kept at 0.4g/L or less and 25g/L or less, respectively, in the last stage. The second aspect of the present invention is the process of the first aspect for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, 15 wherein the mixed organic solvent contains the extractant at 25 to 30% by volume. The third aspect of the present invention is the process of the first aspect for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, wherein totaled nickel, cobalt and calcium concentration in the organic phase is 20g/L or less in the last stage. 20 The fourth aspect of the present invention is the process of the first aspect for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, wherein the aqueous phase is kept at a pH level of 4.8 to 5.2 in the last stage. As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and 25 "comprised", are not intended to exclude further additives, components, integers or steps. 9 004450409v1 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 plots viscosity of the organic phase against totaled nickel, cobalt and calcium concentration in the organic phase. Figure 2 plots totaled nickel, cobalt and calcium concentration in the Organic phase against pH level for the exchanging reaction. Figure 3 plots cobalt extraction efficiency (ratio of cobalt amount in the organic phase to the stoichiometric cobalt extraction rate) against calcium concentration in the organic phase. DETAILED DESCRIPTION OF THE INVENTION The process of the present invention for purification of an aqueous solution of nickel sulfate containing cobalt and calcium is described in detail. The process of the present invention for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, the former being present at a high concentration, is of industrially high efficiency, because it can increase cobalt treatment capacity without increasing plant capacity by improving. cobalt extraction efficiency in the exchange reaction between the aqueous solution and nickel-loading acid extractant. As such, it is suitable for an exchanging reaction step in a nickel sulfate purification process comprising several steps; e.g., extraction step for producing the nickel-loading organic phase, step for exchanging with cobalt present in a crude aqueous solution of nickel sulfate, step for stripping of nickel from the exchanged organic phase in which cobalt is concentrated, cobalt recovering step, and impurity stripping step. The process of the present invention for purification of an aqueous solution of nickel sulfate containing cobalt and calcium uses a multi-stage, countercurrent reaction tank system, wherein the aqueous solution of nickel 10 004450409v1 sulfate is supplied as the aqueous phase to the last stage and a mixed organic solvent containing an extractant of 2-ethylhexylphosphonic acid/mono-2-ethylhexyl ester loading nickel, diluted with a hydrocarbon, is supplied as the organic phase to the first stage, to bring these phases into contact with each other for an exchanging reaction, the aqueous phase being kept at a given pH level in the last stage, and calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase being kept at a given concentration in the last stage. (1) Aqueous solution of nickel sulfate The aqueous solution of nickel sulfate to be treated by the present invention contains cobalt and calcium. The particularly preferable solution is a crude aqueous solution of nickel sulfate containing cobalt at a high concentration, produced by treating a raw material containing cobalt at several percent. It may be adjusted beforehand at a given pH level, to help adjust pH level for the exchanging reaction. (2) Mixed organic solvent containing a nickel-loading acid extractant In the present invention, a nickel-loading acid extractant is prepared beforehand for the exchanging reaction. In the present invention, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester is used as the acid extractant, which is diluted with a hydrocarbon to a given extractant concentration to prepare the mixed organic solvent. The extractant of 2-ethylhexylphosphonic acid mono- 2-ethylhexyl ester can efficiently extract cobalt, calcium and magnesium. It has a separation coefficient in a sulfuric acid solution of cobalt/nickel: 650, calcium/nickel: 110 and magnesium/nickel: 50. It has a still higher separation coefficient for zinc, iron and copper, by which is meant that these elements are preferentially extracted. 11 004450409v1 Acid extractants are generally viscous, and are mixed with a diluent before use. In the present invention, it is diluted with a hydrocarbon. The hydrocarbon is not limited for the present invention, and may be aliphatic or aromatic, of which an alkyl benzene as an aromatic hydrocarbon is particularly preferable. The mixed organic solvent for the present invention contains the extractant of 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester at 20 to 30% by volume, preferably 25 to 30%. At below 20% by volume, cobalt extraction rate per unit amount of the mixed organic solvent is insufficient. At above 30% by volume, on the other hand, the organic solvent may be excessively viscous to deteriorate organic solvent/aqueous phase separation efficiency to cause unstable operation and decreased productivity. The mixed organic solvent of the above composition for the present invention is suitable for exchanging of impurities present in the aqueous solution of nickel sulfate, e.g., cobalt and calcium which are extracted in preference to nickel, with nickel loaded beforehand by the extractant. It is an economically recommended practice to recycle the exchanged organic phase containing impurities back to a series of nickel stripping, cobalt recovery and impurity stripping step in the nickel sulfate purification process (disclosed by, e.g., Japanese Patent Laid-open Publication No.10-310437 (pages 1 to 5) ), in order to recover nickel remaining in the organic phase, recover cobalt and remove impurities other than cobalt in the respective step, and to clean and recover the organic phase for reuse. The mixed organic solvent can load nickel by common solvent extraction. For example, nickel can be extracted from an aqueous solution of nickel sulfate as the starting material with the acid extractant described above in 12 004450409vl a multi-stage, countercurrent reaction tank system, wherein the extractant is supplied to the first stage while the aqueous solution to the last stage for the extraction reaction at a pH of 5.0 to 7.0. It is preferable to keep a concentration of nickel loaded in the mixed organic solvent in excess of the stoichiometric level for impurity elements to be extracted by the exchanging reaction. Insufficient amount of nickel for impurity elements should leave these elements in the purified aqueous solution of nickel sulfate, even when the exchanging reaction proceeds completely. At the stoichiometric level, on the other hand, the exchanging reaction cannot proceed completely, because nickel in the organic phase is depleted as the reaction proceeds. (3) Purification system and method In the present invention, the reactor system for the exchanging reaction may be selected from various multi-stage, countercurrent reaction tank systems capable of efficiently separating the organic and aqueous phase from each other after contacting them. The particularly preferable one is a continuous, multi-stage, countercurrent mixer settler system, where the nickel-loading mixed solvent and crude aqueous solution of nickel sulfate come into contact with each other, the former supplied to the first stage and the latter to the last stage. Therefore, the purified aqueous solution of nickel sulfate is discharged from the first stage mixer settler, and the exchanged organic phase containing cobalt from the last stage mixer settler. The exchanged organic phase preferably contains nickel at 1 to 4.5g/L. Nickel in the organic phase is depleted as the exchanging reaction proceeds. When it is depleted to a concentration below 1g/L, the nickel/cobalt exchanging reaction proceeds insufficiently, to increase concentration of cobalt in the purified aqueous solution of nickel sulfate. At above 4.5g/L, on the other hand, recovery of nickel in the purified aqueous solution of 13 004450409vl nickel sulfate may be retarded. The nickel/cobalt exchanging reaction will proceed smoothly at a nickel concentration of 1 to 4.5 g/L in the organic phase. In the present invention, the aqueous phase in the last stage of the multi-stage, countercurrent reaction tank system is adjusted at a pH level of 4.5 to 5.5, preferably 4.8 to 5.2. Increasing the pH level accelerates extraction of nickel and cobalt in the organic phase. At a pH level below 4.5, cobalt concentration in the exchanged organic phase decreases to deteriorate cobalt extraction efficiency. At a pH level above 5.5, on the other hand, nickel concentration in the organic phase increases to deteriorate rate of recovery of nickel in the purified aqueous solution of nickel sulfate, but also cobalt extraction efficiency resulting from increased number of nickel atoms bound to functional groups in the extractant to substantially reduces number of functional groups to be bound to cobalt. A conventional exchanging step with an acid extractant loading nickel at an adequate concentration, releasing no hydrogen ion, needs no neutralizer to have an adequate pH level, because the exchanging stage can be kept at a pH level of 4 to 6, when the aqueous solution of nickel sulfate to be supplied to the stage is adjusted beforehand at a pH level in the above range. However, the present invention is characterized by finer adjustment of pH level, which may be realized with sulfuric acid. Table 1 gives the relationship between pH level of the aqueous phase and nickel concentration in the organic phase in the last stage settler in the multi-stage, countercurrent reaction tank system, where a crude aqueous solution of nickel sulfate is treated with a mixed solvent containing nickel-loading 2-ethylhexylphosphonic acid mono -2 -ethylhexyl ester as an extractant, diluted with an alkyl benzene to a given extractant 14 004450409vl concentration. Table 1 pH level of the aqueous Nickel concentration in the No- phase in the settler organic phase (g/L) 1 4.3 0.99 2 4.8 1.20 3 5.0 2.19 4 5.3 3.90 5 5.4 4.50 As shown in Table 1, nickel concentration in the organic phase decreases as pH level decreases, and the nickel/cobalt exchanging reaction proceeds smoothly at a nickel concentration in the organic phase in the above-described range from 1 to 4.5g/L, which is achieved at a pH level of 4.5 to 5.5. It is essential for the present invention to keep calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase each at a given level in the last stage of the multi-stage, countercurrent reaction tank system. This prevents viscosity increase of the organic phase and makes cobalt extraction efficiency more stable than the conventional operation without harming the nickel/cobalt exchanging reaction. Totaled nickel, cobalt and calcium concentration in the organic phase is adjusted at 25g/L or less, preferably 20g/L or less, in the last stage of the multi-stage, countercurrent reaction tank system for the present invention. At a totaled nickel, cobalt and calcium concentration in the organic phase above 25g/L, the mixed organic solvent may be excessively viscous to 15 004450409vl decrease efficiency for separating the organic phase from the aqueous phase, causing unstable operation and notably deteriorated productivity. Fig.1 plots viscosity of the organic phase against totaled nickel, cobalt and calcium concentration in the organic phase, the relationship being similar to that given in Table 1. As shown, viscosity of the organic phase starts to notably increase as totaled nickel, cobalt and calcium concentration in the organic phase increases beyond 25g/L. The totaled concentration also depends on pH level for the exchanging reaction. Fig.2 plots totaled nickel, cobalt and calcium concentration in the organic phase against pH level for the exchanging reaction in a range of high extractant concentration in the organic phase, the relationship being similar to that given in Table 1. As shown, totaled nickel, cobalt and calcium concentration in the organic phase increases as the pH level increases to accelerate extraction. It is also noted that totaled concentration of nickel, cobalt and calcium extracted in the organic phase increases as extractant concentration increases at the same pH level. Calcium concentration in the organic phase is adjusted at 0.4g/L or less in the last stage of the multi-stage, countercurrent reaction tank system for the present invention. At a calcium concentration above 0.4g/L, cobalt extraction efficiency may not be improved, but decreased to 40 to .60% of the stoichiometric extraction rate, which is on a level with that associated with the conventional operation. Fig.3 plots cobalt extraction efficiency (ratio of cobalt amount in the organic phase to the stoichiometric cobalt extraction rate) against calcium concentration in the organic phase, the relationship being similar to that 16 004450409vl given in Table 1. The solid line in Fig.3, obtained by the regression analysis, indicates that a cobalt extraction efficiency of 60% or more can be secured at a calcium concentration of 0.4g/L or less. The relationship between calcium concentration and viscosity of the organic phase in the last stage is not clear, conceivably resulting from locally increased calcium concentration in the aqueous phase as calcium concentration in the organic phase increases during the exchanging reaction, to cause separation of insoluble calcium sulfate and increased viscosity of the organic phase when contaminated with calcium sulfate. Totaled nickel, cobalt and calcium concentration in the organic phase depends on pH level of the exchanging reaction at a given extractant concentration. Concentration of calcium, which is distributed almost completely in the organic phase, can be adjusted by controlling flow rate of a crude aqueous solution of nickel sulfate or amount of the organic phase for dilution. EXAMPLES The present invention is described in more detail by EXAMPLES, which by no means limit the present invention. The following analytical procedures were used in EXAMPLES. (1) Analysis of metals: Determined by atomic absorption spectrometry (2) Viscosity of the organic phase: Determined by a B-type rotational viscometer EXAMPLE 1 The exchanging step was carried out by a 4-stage, countercurrent, mixer settler system (effective mixer volume: 300mL, effective settler volume: 3,OOOmL), where the organic phase was supplied to the first stage 17 004450409v1 of the system and aqueous phase .to the fourth stage. A mixed organic solvent was prepared as the organic phase by diluting 2-ethylhexylphosphonic acid mono -2 -ethylhexyl ester (PC-88A@, Daihachi Chemical Industry) with an alkyl benzene (Clean Sol G@, Nippon Oil Corporation) to 25% by volume and then treated to load nickel at 25g/L. The organic phase was incorporated, as required, with the organic phase for dilution, prepared by stripping step for cobalt/calcium separation. A crude aqueous solution of nickel sulfate containing nickel, cobalt and calcium at 50 to 60, 30 to 40 and 0.6g/L, respectively, was used as the aqueous phase, which was adjusted at a pH level of 4.5 to 5.0. These organic phase and aqueous phase were supplied at 90 and 30 to 40mL/minute to adjust calcium load. The aqueous phase was finely adjusted at a pH level of 4.8 to 5.2 with sulfuric acid in the settler in the fourth stage of the mixer settler system. The mixer settler system was continuously operated at 40 to 45*C for 8 hours or more while keeping calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase at 0.40g/L or less and 20g/L or less, respectively, in the fourth mixer settler. Samples were collected from the exchanged aqueous phase and organic phase, as required, to analyze nickel, cobalt and.calcium concentration. Table 2 gives nickel, cobalt and calcium concentration in the aqueous phase, i.e., purified aqueous solution of nickel sulfate, discharged from the first mixer settler and exchanged organic phase from the fourth mixer settler; totaled nickel, cobalt and calcium concentration in that organic phase; and efficiency of cobalt extracted in that organic phase. Table 2 18 004450409v1 Composition of the Composition of the exchanged Co - purified aqueous No. organic phase (g/L) extraction solution of nickel efficiency sulfate (g/L) (%) Ni Co Ca Total Ni Co Ca 1 3.43 14.40 0.40 18.23 63 100 0.006 0.018 2 4.00 14.50 0.37 18.87 63 100 0.007 0.006 3 2.06 16.20 0.39 18.65 71 98.7 0.007 0.006 4 1.12 16.10 0.25 17.47 70 99.4 0.007 <0.005 5 4.12 15.36 0.37 19.87 67 102 0.006 <0.005 As shown in Table 2, efficiency of cobalt extracted in the organic phase can be secured at 60% or more, cobalt is mostly extracted in the aqueous phase, and the purified crude solution of nickel sulfate containing impurities at a very low concentration (cobalt: 10mg/L or less, and calcium: 20mg/L or less) can be stably produced, when the aqueous phase in the settler of the fourth mixer settler is kept at a pH of 4.8 to 5.2, and calcium concentration and totaled nickel, cobalt and calcium concentration in the organic phase discharged from the fourth stage of the mixer settler are kept at 0.40g/L or less and 20g/L or less, respectively. As described above, the process of the present invention for purification of an aqueous solution of nickel sulfate containing cobalt and calcium, the former being present at a high concentration, is of industrially high efficiency, because it can increase cobalt treatment capacity and produce a purified aqueous solution of nickel sulfate of reduced cobalt and calcium concentration by improving cobalt extraction efficiency in the exchange reaction. As such, it is of very high industrial value. 19