CA1197490A - Purification of nickel electrolyte - Google Patents

Abstract

ABSTRACT
An electrolytic process for purifying a nickel-containing solution as provided comprising forming an electrolytic system comprised of at least one electrolytic cell containing at least one anode, at least one cathode, and an aqueous electrolyte, said electrolyte being substantially copper-free and containing in solution nickel, an alkali metal, and chloride ions and at least one of said impurities, the improvement comprising: establishing in the electrolytic cell an anode compartment and cathode compartment separated by a chlorine-resistant diaphragm through which electrolytic contact is established between the anode and cathode compartments; flowing the electrolyte through the anode compartment; maintaining a non-circulating catholyte in the cathode compartment, said catholyte being an aqueous solution containing sufficient hydroxyl ions such that under operating conditions nickel ions are prevented from migrating to the cathode compartment and substantially only the decomposition ofwater and evolution of hydrogen occurs in the cathode compartment impressing an electrical current in the cell to generate chlorine at the anode, said chlorine in the aqueous electrolyte reacting with at least one impurity present in the electrolyte to provide a precipitate containing the impurity; and separating theprecipitate from the electrolyte. The process can be used to recover cobalt with reduced power consumption from a nickel refining electrolyte.

Description

79~

-1- PC-ll9~
PURIFICATION OF NICKEL ELECTROLYTE

TECHNICAL FIELD
The present invention relates to an electrolytic system and an electrochemical process for removing impurities îrom nickel-containing solu-tions. More particularly, it relates to an improved electrochemical process for removing contaminants such as cobalt, iron, arsenie and lead from a nickel refinery electrolyte.

BACKGROUND OF THE IMVENTION
In the electrorefining of nickel, crude nickel flnodes are corroded electrolytically, resulting in the dissoluffon of nickel and some OI the impurities in the electrolyte. The electrolyte is purified and then nickel is deposited cathodic&lly from the purified electrolyte. In nickeI refinery practice, the electrolyte is usually a slightly acidic aqueous solution containing nickel, sulfate, sodium and chloride ions, and boric acid, plus impurities. The impurities vary but may include one or more of the metal values cobalt, copper, iron, arsenic, and lead. To prevent or lower deposition of such impurities with nickel at the c&thode, it is necessary to remove them ~rom the electrolyte or to reduce lthem to an acceptable lcvel. The extent of removal will affect the purity oî the nickel deposited, and the degree of purity desired will depend on such factors as ultimate use of the nickel, cost of purification, diîficulty of removal OI the particular contaminant, and the vfllue of the impurity. For example, cobQlt is not considered a harmful contamillant in nickel for many uses of nickel because of the similarity of properties of nickel and cobalt. With the increased value of cobalt, it is now desir&ble to separate and recover the cobalt.

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-2- PC-l l 90 To remove contaminants from the electrolyte, th0 imp~lre electrolyte is usually pumped from the anode dissolution cell and the impurities are removedby chemical, physical or electrical techniques or fl combination thereof. Copper, for example, can be removed from solution to very low levels by cementation with metallic nickel or by precipitation with H2S. Iron is removed by aeration and hydrolysis. Cobalt, arsenic and lead are removed by addition OI chlorine gas.
Nickel carbonate is used to neutralize the acidifying effect that occurs as cob~lt and other impurities are preeipitated fronn solution. The purified electrolyte is then returned to the cell for plating of niclcel at the cathode.
An electrolyffc process for removing cobalt, iron, arsenic and/or lead contaminants from impure, decopperized nickel refinery electrolytes is disclosedin U.~. Patent 3,983,018. In such process nickel hydroxide and hydrogen form at the cathode of an electrolytic cell, and elemental chlorine, which is the agent to remove cobalt, iron, arsenic and lead impurities, is generated in-situ at the anode. Cobalt, iron, arsenic and lefld precipitates begin to form in the electr~lytic tank in time-dependent reactions, and precipitation is generally completedin a retention tank following treatment of the electrolyte in ths electrolytic tank. The precipitates are removed from the electrolyte by a conventional technique, e.g. filtration, and the purified electrolyte can be used for deposit of high purity nickel. The oxidative electrolytic process disclosed in the afore-mentioned patent is an improvement over the art in that iron, lead, arsenic and cobalt can be removed concurrently and effectively, without the need for complex stages required by chemical methods for the removal of such contam-inants. Furthermore, the in-situ generation of C12 permits easy handling and control of a difficult reagent. No chlorine gas handling equipment is required, and the cost of in-situ generated chlorine is lower than the cost of liquid chlorine currently used for chemical purification. With the proper controls, i.e. adjust-ment of anodic current density to generate only the chlorine required to oxidizethe undesired impurities, pollution due to excessive chlorine can be prevented and reagent economy can be insured. In addition, no chloride ion imbalance in the electrolyte i~ created because the in-situ generated chlorine will be completely reconverted to chloride ion. The present process, which also involvesthe advRntageous in-situ generaffon of chlorine, offers a further improvement over the aforementioned electrolytic purification process in that the precipita-tion CRn be achieved with lower energy requirements and with more efficient nickel recovery. 'rhe cathode can be operated at low current density, resulting ~g741~

3 PC~ 0 in low cell voltage and low power consumption. Also, the cells can be operated with essentiallg no nickel platinF on the cathodes, re~ultin~ in efficient H2 evolution and NaOH nroduction in the cathocle compartments.

BRIEF l)ESCRIPTION OF DRAWINaS
Fi,~ure 1 i8 a schematic version o~ a diaphragm electrolytic cell in accordance with the present inventinn. The cell ia ~hown in lsometric ~A) nnd front (B) views.
Fi~ure 2 i8 a simplified flowsheet illustrating a nickel refinerv purification process in accordance with the present invention.

ln T~IE INVRNTION
In an electrolytic process for purifvin~ a nickel-containinF ~olution bv removal of at leaYt one impuritv selected from the group con~istlng of cobalt, iron, araenic (if iron i~ present3, and lead, which proce~s comprises forming anelectrolvtic s~stem comprised of at least one electrolytic cell containinR~ at least one anode, at least one cathode, and an aqueous acidic electrolyte, said electro-lvte beinF substantially copper-free and containinF in solution nickel, alkali metal, and chloride ions and at least one of sflid impurities, the impro~ement comprisin~: establishlng in the electrolytic cell an anode compartment and cathode compartment aeparated by a chlorine-resistant diaphragm through which electrolvtic contact is established between the anode and cathode compartment~;
flowing the electrolvte through the anode compartment; maintainin~ a non-cir~!ulatinF catholvte in the cathode compartment, ~aid catholvte being an aqueous alkaline solution containing su~ffcient h~droxyl ions such that under operatin~ conditions nickel ions are prevented from migrating to the cathode compartment and substantiallv onlv the decomposition of water and evolution of hvdro~en can occur in the cathode compartment; impressin~ an electrical current in the cell to generate chlorine at the anode; said chlorine in the aqueous elec-trolvte reacting with at least one impurity present in the electrolyte to provide a precipitflte containin~ the impuritv; and separatin~ the precipitate from the electrolvte. The process can be used to recover cobalt with reduced power c- naumption from a nickel refinery electrolyte.
Operation of' the proces~ depends on the in-~itu generation of chlorine, which is derived from chloride in the electrolyte. The chloride ion may be present in the re~inery electrolyte a~ a result of previous process steps or it mav be added, e.g. a~ an alkali metal chloride or nickel chloride. A typical decopperized nickel refinery electrolyte may contain the followin~ components in~rams per liter (~pl~.

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-4- PC-l 1 9o Ni++ = 40-80 Co++ = 0.05-5 Na~ = 10-50 As~ = 0.001-2 (or AS~++) Cl- = 15-90 Pb++ = 0.0001-0.01 SO4= = 2-150 Cu++ = 0.~01-0.01 H3BO3 = 5-20 Fe++ = 0.01-1 Typically, the pH of the impure electrolyte! will vary from about 2.5 to about 5.2.
The composition of the electrolyte will, of course, vary with the ore and the various processes for treating said ores. The electrolyte is the anolyte component of the electrolytie cell, thus the components of the electrolyte, apart from the chloride ion and nickel refining electrolyte are selected to satisfy the electrical as well as chemical requirements of the system. The desired concentration of the essential components in the electrolyte is dependent on thefunction of each. The pH of the impure electrolyte feed should be about 3.8 to

5.2 and can be adjusted, if required, with nickel carbonate or another suitable base. The nickel ion concentration should not exceed its maximum solubility in the electrolyte. The alkali metal level should be sufficient to ensure conduc-tivity of the electrolyte. The chloride ion must be present, and it should be present in sufficient quantity so that C12 rather than 2 is generated at the anode. The pararneters of the system can be designed so that the amount of chlorine generated is substantially not in excess over that amount reguired for the oxidation. Typically, at least S gpl of chloride and preferably greater than 15 or 20 gpl of chloride ions should be present in the electrolyte. The chloride ions may be provided by an aL"ali metal chloride such as sodium or potassium chloride. Sodium chloride is preferred because it is less expensive. The other metal values present may also be present as chloride salts. The use of other halides other than chloride are within the contemplation of the invention;
however, they are not practical. Therefore, the description of the invention is given herein in terms of the generation in-situ of chlorine from chloride ions in the electrolyte.
The impure nickel-containing solution (e.g. nickel refining electrolyte) fed to the anode compartment constitutes the anolyte. It is characterlstic of tlle cell of this invention that the catholyte can be maintained essentially separate from the anolyte and in essentially a steady state. In operation of the c~ell, hydrogen ions and alkali metal ions migrate from the anolyte into the caltholyte and are available to react with hydroxyl ions to form, -5- PC-l l 90 respectively, water and alkali metal hydroxide. The water decomposes cathodically and E12 is discharged. The ~lydroxyl ions remaining from the decomposition are available for the migrating hydrogen and alkali metal ions.
At the cathode, as indicated above, 3I2 is discharged. The catholyte is maintained separate from the anolyle and as an aqueous ~lkaline solution advantageously containing at least 0.5 moles of hydroxyl ions. For example, the sodium hydroxide level is maintained at about 20 or 4~ to 200 gpl, e.g. about lS0 gpl. Under the alkaline conditions in the cathode compartment, essentially no nickel migrates into the cathode compartment. To prevent a build-up of alkali hydroxide in the catholyte, a hydrostatic head is maintained at the catholyte, with water flow into the catholyte provided at a pressure designed to keep a positive flow through the diaphragm from the catholyte into the anolyte. In thisway, the catholyte can be maintained essentially in a steady state with H2 discharge at the cathode, and essentially no nickel plating at the cathode and essentially no precipitate of nickel in the alkaline catholyte.
The electrodes must be made of materials which are good conductors and resistant to their respective environments. They may take any appropriate form for the ce~ design, e.g., they can be sheets, rods, tubes, metal mesh, expanded metal, etc. The substrate materials for the electrodes can be surface materials on more conductive cores. The anodes are insoluble and any anodes appropriate for Cla production are useful. For example, graphite anodes or dimensionally stable anodes may be used. It is well known, for example, to use platinum group metal anodes or platinum group metals and/or platinum group metal oxide-coated valve metals. The cathodes must be of an alkali-resistant material, e.g., steel, stainless steel, or nickel. The spacing of the anodes andcathodes is not considered critical, however they should be placed such that there is no short circuiting between the electrode and the diaphragm.
The diaphragm separating the electrolyte and catholyte must be of a material which is resistant to the reactants and products on both sides of the cell and which has low electrical resistance. Since chloride is present and chlorine is generated in the cell, it is particularly important that the diaphragm be resistant to chloride and to chlorine, and this property is referred to herein as chlorine-resistant material. Additional features of the diaphragm are permeability and physical strength. In general, diaphragms of the type used in cells for chlorinemanu~acture would be suitable. Examples of suitable materials are asbestos and a fluorinated Ihydrocarbon such as polytetrafluoroethylene ~modified for ~l97~

-6- PC-l l 90 wettability). An otherwise suitable material may be made chlorine-resistant by coating it with a suitable polymeric material. Suitable materials are commercially available, e.g., under the names KANEKALON* (a product of Kanegafuchi Chemical lndustry Co.), and DYNEL* (a product of Union Carbide~.
The cell is operated with an anode current density, typically, OI about 200 Amperes per square meter (ASM), and varied between about l00 to 500 ASM.
The temperature in the cell is maintained bletween 50~ and 70.
The flow of electrolyte through the anolyte compartment is adjusted so that the desired reaction takes place at a suitable îixed current and in a desired number of cells. The numbers of cells is dictated in part by the system designed. A major advantage in the present process is that the electrolytic purification of a nickel electrolyte can be carried out with a lower power consumption, e.g. with a decrease of at least 30% over that required for diaphragmless cells. Another advantage is that the present process can be operated at a cell voltage below 5 V, e.~. about 3V. In addition to the lower cost of purification of the electrolyte, recovery of cobalt from the electrolyte can likewise be effected at a considerable saving in power. As indicated previously,the aforementioned diaphragmless electrode process of U.S. Patent No.
3,983,018 is an improvement over prior art non-electrolytic methods, and the present process provides a further improvement in the electrolytic system.
The process of the present invention is illustrated by reference to the accompanying drawings in which Figure 1 is a schematic cell and Figure 2 is a simplified flowsheet in accordance with the present invention. The flowsheet of ~igure 2 shows electrolytic cell l0 with a chlorine-resistant diaphragm ll separating the anode compartment 14 irom the cathode compartment 15. The schematic cell represented in Figure l shows anode 13, which is suitable for C12generation made of, e.g., graphite, and two cathodes 12 made of, e.g., stainlesssteel. Referring to Figure 2:
An impure electrolyte derived, for example, from the dissolution of crude nickel anodes in an electrolytic process (not shown) and decopperized, e.g., by cementation with metallic nickel (not shown) is fed to the anode compartmenb l4 of electrolytic cell l0. The composition of the electrolyte depen~; on the composition of the the ore and preceding treatments. However, in the present process -in addition to nickel and contaminants such as cobalt, arsenic, iron and ~Trademark ~ ~74~9~Ç

q PC-1190 lead - the electrolvt/? will contnin alkali metal ion~ and chlor~de ion~. Pref-erably, the chloride ion content will be sufffcient to Five high Cl~ e~ficiency at the anode. Sulfate ions are often pre3ent frnm prevlous proces~ ~tep~ and boric acid i~ a well-known additive to refinery electrolytes. To illustrate the invention an impure electrolyte i8 uaed which contain~ in solution, appro~lmately, 60 E~pl nickel, 95 gpl sulfate, 35 gpl ~odium, 16 ~pl boric acld, 35 ~pl chloride and as impurities n.1 ~pl cobalt, 0.05 gpl iron, 0.005 gpl ar~enic, and 0.002 gpl lead. The p~ of the impure electrolyte i8 about 2.0 to 5.2. Under an impressed current elemental chlcrine ~ormed &t anode 13 beFlns to react in the anode lQ compartment 14 with divalent cobalt ions, and similarly with the ions of iron, ar~enic and lead in the electrolvte. Preferabl~, air ig 6par~ed into the anolvte(not shown) to improve contact of cell-g~enerated C12 and of ba~e with impurities to be oxidized and to redissolve nickel hvdroxide precipitated on the diaphragm.The o7c{dation reactiona converting the contaminants cobalt, ~r~enic,` iron and lead 16 to in~oluble hvdroxides are time-dependent, and the reacting anolvte i8 tranaferred to a retention tank 16 for the oxidation reaction~ and h~ydroly~i~ to occur. TheoxidizinF power of the anolyte i~ governed by the current density at the anode, emciencv of the chlorine reaction, tank size and re3idence time for reacti~rity.In the cathode compartment 15, the catholyte, which i~ an aqueouc ~0 medium containin~ aodium h~rdroxide i~ a non-circulating electrolyte. The initial NaOTI content can be about 80 ~pl and sodium chloride about 0 to 40 gpl. A
hvdrostatic head ia ~aintalned in the cathode compartment by water addition to eatablish a bleed rate from the catholyte into the anol~te through the diaphragm80 that the ~:odium hydroxide level in the catholyte i8 maintalned at about 8û to ~5 ~00 ~pl.
enerallv, in the retention tank, hydrolysi~ take~ about 20 minute~ to about 1 hour, typicallv about 4n minutes. To maintain the pll at the de~ired value, depending on the aelectivitv desired, a small amount of alkaline material, e.F. NiCO3, i~ added to the electrolvte (i.e., the anolYte fed from the anode compartment) in retention tank 16. The oxidized electrolyte i8 pumped or ~ravit~r fed to filter 17 and the cobalt, iron, ar~enic and lead precipitates formed are removed from the liquid. PuriIied electrolyte may be returned to the reffning circuit or, alternati~rel~, sent through one or more addiffonal purification sequence~ ~r further reduction of impurity content, if necessary.
TJnder the influence of applied current chlorine is generated at anode 74gl~

-8- PC-l 1 90 13 and H2 is generated at the cathodes 12. In general, the impurities are oxidized and precipitated by hydrolysis. The overall simplified reaction with respect to Co++ in the electrolyte and retention tanks can be represented as îollows:

Ci2 + 2Co~+ + 3NiCO3 + 3H20~ 2Co~OH).~ + 3Ni++ ~ 2Cl- t 3C02 Other reactions, e.g. NiC03 oxidation to Ni+++, are di~ficL~t to prevent. Iron behaves similar to cobalt and arsenic and lead precipitate out with the iron.
By careful control of the oxidation potential and pH of the electrolyte9 it is possible to selectively precipitate unwanted elements. In this way, lor example, a purer cobalt precipitate can be obtained. The selective process, which can be effected advantageously in a retention tank, involves two stages. In the first stage, the electrolyte is partially oxidized, adjusted to a pH
slightly below that required for cobalt precipitation, but high enough for precipitation of iron, arsenic and lead, e.g. a pH of about 3.5 to 4Ø After the iron, arsenic and lead precipitates are removed by filtration, in a second stagethe electrolyte is fully oxidized in an electrolytic tank and the pH in the retention tank is increased sufficiently, e.g. between about 4.3 and 4.8, to precipitate cobalt. Alternatively, or in combinatîon with retention tanks, it ispossible to adjust the number of cells and r~te of anolyte flow so that at a given current density, the electrolyte (anolyte) will emerge from the electrolytic system at a desired pH.
In order to give those skil1ed in the art a better understanding of the invention, the following illustrative example is given.

EXAMPLE
In the tests of this Example, a diaphragm-equipped Clg generating cell is compared with a diaphragm-free cell used to obtain the data of l~xamples1, 2 and 3 of the aforementioned U.S. Patent No. 3,983,081. In the tests shown in the patent, the impure electrolyte used is decopperized tank house electrolyte from a nickel refinery containing about 40-80 gpl nickel, about 33-45 gpl sodium, about 46-56 gpl chloride, about 100-150 gpl sulfate, 13-16 gpl H3BO3, less than 0.01 gpl of dissolved copper, and having a pH of 2.9. The cell contains a stainless steel wire cathode 2.4 mm diameter by 6.4 cm long and graphite anodes 1.3 cm x 6.4 cm x 9 cm. The temperature of the electrolytes and retention tanks are controlled between 54C and 60C. Upon leaving the electrolytic tank, the -9- PC-I l90 oxidizing power of the solution is equivalent to 0.290 gpl chlorine.
The comperative tests run according to the present process were essentially the same as described above ex~ept that a KANEKALON diaphragm separates th0 anolyte (impure electrolyte feed~ from the catholyte. The S catholyte compartment contains 20 gpl NaOH. Water is pumped into a cathode bag to maintain a head of l cm.
The accompanying TABLE: shows the general conditions under which the comparative experiments are carried out. It also gives the analysis of the impurities in the feed and the effluent.
The comparative results in the TABLE show that for the purific~tion of similar electrorefining electrolytes, the power consumption required in the cell of the present invention is only about one third to one half that needed with a diaphragm-free C12 generating cell.
Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modificaffons and variations may be resorted to without departing from the spirit and scope of the invention,as those skilled in the art will readily understand. Such modifications and variations are considered to be within the pUrYieW and scope of the in~ention and appended claims.

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Claims (20)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an electrolytic process for purifying a nickel-containing solution by removal of at least one impurity selected from the group consisting of cobalt, iron, arsenic (if iron is present), and lead, which process comprises forming anelectrolytic system comprised of at least one electrolytic cell containing at least one anode, at least one cathode, and an aqueous electrolyte, said electro-lyte being substantially copper-free and containing in solution nickel, an alkali metal, and chloride ions and at least one of said impurities, the improvement comprising: establishing in the electrolytic cell an anode compartment and cathode compartment separated by a chlorine-resistant diaphragm through which electrolytic contact is established between the anode and cathode compartments;
flowing the electrolyte through the anode compartment; maintaining a non-circulating catholyte in the cathode compartment, said catholyte being an aqueous solution containing sufficient hydroxyl ions such that under operating conditions nickel ions are prevented from migrating to the cathode compartment and substantially only the decomposition of water and evolution of hydrogen occurs in the cathode compartment impressing an electrical current in the cell to generate chlorine atthe anode, said chlorine in the aqueous electrolyte reacting with at least one impurity present in the electrolyte to provide a precipitate containing the impurity; and separating the precipitate from the electrolyte.

2. A process as defined in claim 1, wherein the concentration of hydroxyl ions in the catholyte is at least about 0.5 M.

3. A process as defined in claim 1, wherein the electrolyte in the anode compartment is maintained at a pH of up to about 5.

4. A process as defined in claim 1, wherein means are provided for catholyte flow through the diaphragm into the electrolyte.

5. A process as defined in claim 1, wherein the reacting electrolyte is removed to a retention tank for hydrolysis of the impurities, the pH in the reten-tion tank being adjusted to a value consistent with precipitation of desired impurities by hydrolysis.

6. A process as defined in claim 5, wherein the pH of the electrolyte in the retention tank is adjusted to a value between about 3.5 and 4.0 for precipitation of iron, arsenic and lead by hydrolysis.

7. A process as defined in claim 5, wherein the pH of the electrolyte in the retention tank is adjusted to about 4.3 to 4.8 during precipitation of the impurities, the pH being adjusted to precipitate cobalt.

8. A process as defined in claim 1, wherein the electrolytic purification cell is operated with an anode current density of about 100 to 500 ASM.

9. A process as defined in claim 1, wherein the electrolyte is derived from a nickel refinery process operated at about 50° to 70°C and the electrolytic purification process is operated at substantially the same temperature.

10. A process as defined in claim 1, wherein the concentration of chloride ions in the electrolyte is at least about 5 gpl.

11. A process as defined in claim 1, wherein the chloride content in the electrolyte is maintained at least at about 15 gpl.

12. A process as defined in claim 1, wherein the catholyte is an aqueous solution of an alkali metal hydroxide and the hydroxide is maintained equivalentto about 20 to about 200 gpl of NaOH.

13. A process as defined in claim 1, wherein water is permitted to flow into the catholyte to bleed catholyte into the electrolyte at a rate which will maintain the hydroxide level in the catholyte between about 40 to about 200 gpl.

14. A process as defined in claim 1, wherein the impure nickel-containing solution is derived from a nickel refinery electrolyte.

15. An electrolytic process defined as in claim 1, wherein the cell is operated at an average cell voltage of less than 5V.

16. An electrolytic process defined as in claim 1, wherein the cell is operated at a cell voltage between 3 and 5V.

17. In an electrolytic process for purifying a nickel-containing solution, which also contains cobalt and may contain one or more impurity selected from the group consisting of iron, arsenic (if iron is present), and lead, which comprises forming an electrolytic system comprised of at least one electrolytic cell containing at least one anode, at least one cathode, and an aqueous acidic electrolyte, said electrolyte being substantially copper-free and containing in solution nickel, cobalt, alkali metal, and chloride ions the improvement comprising:
establishing in the electrolytic cell an anode compartment and cathode compartment separated by a chlorine-resistant diaphragm through which electrolytic contact is established between the anode and cathode compartments; flowing the electrolyte through the anode compartment; maintaining a non-circulating catholyte in the cathode compartment, said catholyte being an aqueous alkaline solution containing about 20 to about 200 gpl of sodium hydroxide and said catholyte under operatingconditions yielding substantially only hydrogen at the cathode and hydroxyl ionsin solution, said hydroxyl ion concentration in the catholyte preventing the migration of nickel ions from the catholyte to the anolyte; maintaining a positive water pressure in the catholyte, thereby causing catholyte bleed into the electrolyte; impressing an electrical current in the cell to generate chlorine at the anode, said chlorine in the aqueous electrolyte reacting with cobalt and (if present) iron, arsenic and lead; removing the reacting electrolyte from the anode compartment to a retention tank for hydrolysis of cobalt and other impurities (if present), the pH being adjusted to a value consistent with precipitation of derived metal values.

18. A process as defined in claim 17, wherein the pH of the electrolyte in the retention tank is adjusted to and maintained between about 3.5 to 4.0 for the precipitation of iron, arsenic and lead, (if present), and after removal of the precipitate, the pH is adjusted to between about 4.3 and 4.8 to precipitate cobalt, and the cobalt is recovered from the purified electrolyte.

19. A process as defined in claim 17, wherein the nickel-containing solution is derived from a nickel refinery electrolyte.

20. A process as defined in claim 19, wherein the nickel-containing solution is derived from a nickel refinery electrolyte.