CA1214748A - Process for nickel electroreplenishment for nickel refinery electrolyte - Google Patents

Abstract

ABSTRACT

A process is provided for the electrolytic dissolution of impure nickel refinery anodes to replenish nickel in a nickel-containing electrolyte, which comprises establishing an electrolytic system comprised of at least one electrolytic cell, said cell containing at least one impure refinery anode and at least one cathode compartment containing at least one cathode, said anode and cathode compartments being separated by chloride-resistant diaphragm through which electrolytic contact can be established between the anode and cathode compartments; flowing an aqueous anolyte through the anode compartment, said anolyte comprising nickel ions, alkali metal ions and at least one of the ions chloride and sulfate inns; maintaining a non-circulating catholyte in the cathode compartment, said catholyte being an aqueous alkaline solution containing sufficient hydroxyl ions such that under operating conditions of the cell nickel ion migration to the cathode compartment is prevented and substantially only the decomposition of water takes place in the cathode compartment, hydrogen being liberated at the cathode; and applying a direct current in the cell to dissolve the impure nickel refinery anode in the anode compartment and to dissociate water in the catholyte and generate hydrogen at the cathode.

Description

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~LECTROREFINING PROCE~
, . . .
TEC}lNICAL FI~LD
The present process relates to an electrolytic system and an electrochemical process for refininF metals. More particularly, it relate~ to 5 an improved diaphraæm cell for dissolution of nickel re~mery anodss.

BACKGROUND OF THE INVENTION
In the electroreIinin~ OI nickel, crude nickel anodes ~re corroded electrolvtically, resulting in the dissolution of nickel and æome o~ the impurities in the electrolvte. The electrolvte i~ puriffed and then nickel 10 is deposited cathodicallv from the puriiEied electrolyte. Typically, the crude - soluble anode i8 used only to replenish nickel in the electrolyte, the electrolyte beinF derived from previous l~rocessing operations, e.g. in the treatment of cres, mattes, concentrates, calcines, residues, scrap, etc. In the normal cr~urse of recoverin~ nickel bv electrorefinin~, usinF, ~or example, 15 nickel sulffde anodes a deficiency of nickel in the anolyte arises caused b~
the depositinF of more nickel on the cathode than is dissolvin~ from the anode, and there is a corrssponding increase in acid content in the anolyte which lowers the pH, e.~. to about 1.5 to 1.9. It is desirable to operate the electroreffninF cell at a p~l of at least 3 æince below p~ 3 Ni plating 0 i8 inefficient due to hydro~en evolution. Above a pH of about 5 nickel will precipitate .
The present invention has amon~ its objectives providinF a method which will cure the nickel dissolution imbalance in the cell. This and other obiects are achieved bv usinF an electrolytic diaphraFm cell in which the ~5 anode compartment contains an irQpure soluble nickel anode, and the impure nickel anode is dissolved electrol~Ttically essentially independently of the cathode at which nickel i8 deposited. In the cathode compartment the cathode . immersed in an alkaline catholyte permits hydrogen evolution and the~kaline - catholvte prevents mi~ration of nickel ions to the cathode. ~

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BRIEF DESCRIPTION OF DRAWIN~S
.. ... ... . _ . . _ The accompanying figure is a schernatic version of a diaphragm cell ~or the electrodissolution o-f impure nickel refinery anodes.

THE INVENTION
In accordance with the present invention a process is provided for the electrolytic dissolution of impure nickel refinery anodes to replenish nickel in a nickel-containing electrolyte, which process comprises: establishing an elec-trolytic system comprised of at least one electrolytic cell, said cell containin~ at least one impure reinery anode and at least one cathode compartment containing at least one cathode, said anode and cathode compartments being separated by chloride-resistant diaphragm through which electrolytic contact can be estab-lished between the anode and cathode compartments; flowing an aqueous anolyte through the anode compartment, said anoly~e comprising nickel ions, alkali metalions and at least one of the ions chloride and sulfate ions; maintaining a non-1~ circulating catholyte in the cathode compartment, said catholyte being an aqueous alkaline solution containing suf~icient hydroxyl ions such that under operating conditions of the cell nickel ion migration to the cathode compartmentis prevented and substantially only the decomposition of water takes place in the cathode compartment, hydrogen being liberated at the cathode, and applying a direct current in the cell to dissolve the impure nickel refinery anode in the anode compartment, to dissociate water in the catholyte and generate hydrogen at the cathode.
The process depends on the independent operation of the anode and cathode compartments. The catholyte is maintained essentially in a steady state.
Hydrogen and alkali metal ions migrate from the anolyte and are available to react with hydroxyl ions to form water and alkali metal hydroxide respectively.
The water d~omposes cathodically and H2 is discharged at the cathode. The hydroxyl ions remaining from the decomposition are available for the migra~ing hydrogen ions and sodium ions. The catholyte is maintained as an aqueous alkaline solution. The hydroxide is provided by an alkali metal hydroxide and sufficient hydroxide is present to provide a concentration of alkali metal hydroxide equiva-lent to about 40 to 80 grams per liter (gpl) of sodium hydroxide. Migration of nickel ions to the catholyte is prevented by precipitation of nickel hydroxide on -' the anode side of the diaphragm. To prevent the build-up of alkali hydroxide in ~ . 35 the catholyte, the catholyte is permitted to bleed into the anolyte. This can be 7~

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accomplished by maintaining a hydrostatic head in the catholyte. The catholyte is replenished by addition of water.
The cathode must be a good conductor and resistant to the alkaline medium. Steel, stainless steel and nickel are examples of suitable cathode materials. The cathodes may take any suitable appropriate form for the eell design; e.g. they ean be in the form of sheets, rods, tubes.
In typieal nickel refinery practice erude niekel electrodes are dissolved in a sulfate-ehloride-borie aeid electrolyte. Tlle bath also will eontain alkali metals. The invention is not limited to the use of any particular electrolyte or erude anodes. For example, the anodes may be derived from nickel eoncen-trates, mattes, residues, serap, ete. The invention is particularly useful, however, for soluble nickel anodes having a relatively high sulfur content, e.g. above about 20%, ~.g. 22-30% S.
The diaphragm separating the anode and cathode compartments must be of a chloride-resistant material whieh has a low eleetrieal resistance, and which provides eleetrolytie eontact between the anolyte and catholyte. In general, diaphragms of the type used in cells for chlorine manufacture would be suitable. It is possible to use ion exchange membranes. However, they are not preferred for reasons o cost and on the laek of strength of those presently available. Examples of suitable materials are asbestos and fluorinated hydro-earbons sueh as polytetrafluoroethylene modified for wettability. Suitable materials for a diaphragm for the eleetrolytie cell of this invention are eommereially available, e.g., under the names KANEKALON* (a product of Kane~uchi Chemical Industry Co.) and DYNEL* (a product of Union Carbide).
The eell is operated, typically, with an anode current density of about 200 amperes per square meter (ASM), and it can be varied, e.g., from about 100 to about 500 ASM. The cell is maintained at about 55 to 65C, typically at about 60C.
It is a particular advantage of the present proeess that the cells ean be operated so that no nickel will plate out on the cathodes even under upset eondi-tions. Instead of permeating into the cathode compartment, nickel ions precipi-tate on the anodic side of the diaphragm. In terms of economics this can mean a considerable savings in energy and filtration steps when compared to chemical dissolution of niekel-containing concentrate outside the electrorefining cell.
* Trademark 7~

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The process of the present invention is illustrated by reference to the accompanying Figure, a schematic electrolytic cell 10 having a chloride-resistant diaphragm 11 separating the anode compartment 14 from the cathode compartment 15. Electrolyte is fed to the anode compartment 14 through input conduit 16 and discharged through discharge conduit 17. E lectrical connections are not shown. In the electrolytic cell 10 anode 13 is an impure nickel refinery anode and the cathodes 12 are made of stainless steel rods.
In operation, a feed electrolyte typically composed of 70-90 gpl Ni~+, about 30 to 55 gpl, Cl-, about 50 to 100 gpl SO~ and about 15 to 35 gpl Na~~ is fed 1~ at a pH of about 1.5 to the anode compartment lds of cell 10 through input pipe 16.
In the anode compartment the pH of the electrolyte is maintained on the acid side, advantageously at about 3 but no higher than about 5. The electrolyte is caused to flow through the anode compartment, and the anode compartment of the cell is discharged through discharge pipe 17. The anolyte and catholyte, separated by the diaphragm are in electrolytic contact and the anode and cathodeare electrically connected (not shown). The cell, for example, is o,oerated at an anode current density of about 10~-500 ASM, e.g. 200 ASM. The effluent at a pH
of 3 or higher is discharged from the cell. A number of cells can be used in thesystem. The effluent from each of the cells is combined as an impure electrolytewhich can be purified.
ln the cathode compartment 15, the catholyte which is an aqueous medium containing, e.g., sodium hydroxide is a non-circulatin~ electrolyte whichis maintained so as to keep the pH greater than about 12. The initial catholyte can be, for example, an aqueous solution containing 40 gpl NaOH and about 58 gplNaCl. A hydrostatic head is maintained in the cathode compartment to establish a bleed rate from the catholyte to the anolyte through the diaphragm so that theproper alkalinity in the catholyte is maintained, and water is added to the catholyte to make up the bleed to the anolyte.
Impurities in the electrolyte will vary. An anolyte, may contain impurities such as copper, lead, arsenic, cobalt and iron in solution. As indicated above, the anolyte may be fed to a purification system (not shown) for removal of such impurities. The impurities can be removed by conventional chemical, physical and/or electrical means. 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 of chlorine gas. Nickel carbonate is used to neutralize 79~

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the acidifying effect that occurs as cobalt and other impurities are precipitated from solution. Electrolytic processes for removing eobalt, iron, arsenic and/or lead contaminants from impure, decopperized nickel refinery electrolytes are disclosed in ~.S. Patent 3J983,018 and in co-pending Canadian Patent ApplicationSerial No. 409,241, filed simultaneously herewith. ln such processes, niclcel hydroxide and hydrogen form at the cathode of an electrolytic cell and elementalchlorine, which is the agent to remove cobalt, iron, arsenic and lead impurities, is generated in-situ. The precipitation of such impurities are time-dependent reactions which can be completed in a separate tankO After removal of the precipitates, the purified electrolyte is returned to the eell for deposit of nickel at the cathode. Precipitated impurities can be removed, e.g., by filtration, andthe purified anolyte is suitable, e.g. for deposition as highly pure nickel, e.g. in an electrowinning cell.
In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given. In the tests power consumption is calculated by taking into account nickel dissolved (determinsd byanode weight differences and nickel content of the anode) and the nickel effectively dissolved in the anolyte (obtained by subtracting from the former value the nickel present in precipitated nickel hydrate). All tests in the Examples are carried out in an electrolytic cell similar to that shown schematically in the Figure. In each of the tests a nickel sulfide refinery anode is immersed in R
flowing anolyte and two nickel cathodes in the form of sheets are immersed in a non-flowing sodium hydroxide. The anode and cathode compartments are sepa-rated by a diaphragm made of heat treated KA~EKALON. Anode and/or cathode bags are used where indicated.
l~pon passage of current through the cell (at anode current densities from 200 to 500 ASM), the main reaction at the anode is nickel dissolution resulting in the replenishment of the flowing anolyte. Water decomposition with H2 evolution takes place at the cathode with the formation of NaOH, the Na~
being provided from the anolyte across the diaphragm. The NaOH concentration is expected to reach a constant level due to diffusion of NaOH to the anolyte where H2SOa~ (if present in the anolyte) is neutralized. The experimental apparatus is designed so that electrolyte is brought from a main reservoir to a constant level container where it is preheated to establish a temperature of about 55C in the electrolytic cell.

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EXAMPLE I
The purpose of this example is to illustrate the advantage of the presence of H2SO~ in the anolyte.
A summary of experimental conditions and results for comparative tests A ~c ~3 are shown in Table I. To maintain a positive hydrostatic pressure in the catholyte, in both tests water is added to the cathode compartment at about 1.5% of the flow rate of the anolyte to the anode compartment. Both tests are carried out in accordance with the present invention. (However, the test conditions are not optimized.) In Test A the anolyte does not contain H2S04. In Test B the anolyte contains 5 gpl H2SO4.
In both tests, the current efficiency for nickel dissolution is 92% (by weight loss) and no nickel is plated at the cathode. The power consumption is about 3 kW per kg of nickel dissolved. The results in Table I show that H2SO4 must be added to the feed to prevent precipitation of nickel as mixed Ni(OH)~-NiSO4 in the vicinity of the diaphragm.
Acids other than H2S04 may be used. However, it is advantageous to use H2SO4 because it is less expensive, for example, than HCl. Also, with HCl there is a chance of formation of C12, if cr builds up in the system.

~0 EXAMPLE II
The purpose of this example is to illustrate the effects of recycling of electrolyte to the feed container and water addition to the cathode compartment and, also, to demonstrate that impure nickel anodes can be corroded in accord-ance with the present process with high electrical efficiency.
A summary of experimental conditions and results is shown in Table Il.
The tests are carried out at relatively high current densities and electrolyte flowrates in order to shorten the duration of each experiment. The composition of the nickel refinery anode for each of the tests is given in Table III. The catholyte eomposition for each of Tests D, E and F is given in Table IV. Analyses of the feed (anolyte) and effluent from the anode compartment are given in Table In test D of this example, electrolyte is passed through the cell without anolyte recycle, air sparging cr water addition to the catholyte. Under the conditions of Test D only 26% of the acid present in the feed electrolyte isneutralized, and paradoxically 63% of the dissolved nickel reports as hydrate, precipitated on the diaphragm and collected at the bottom of the cell. Increased : I

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TABI.E m ANODE COMPOSITIONS

Analysis (Wt.%) Test Ni Co Fe _Cu As S
C 6~.5 1.34 5.41 2.7S 0.26 17.4 D, E(a), E(b) 74.6 0.61 0.46 4.34 0.18 19.1 F(a), F(b), F(c) 71.2 0.92 0.78 5.08 0.12 20.6 _ TABLE IV
CAT~OL.YT~ COMPOSlTlON
Duration Catholyte NaOH (gp1) As~
Test(Hours) Initial Final (mg/l) E(a) 22 146 208 E(b) 22 200 236 F(a) 27 91 88 F(b) 22 76 70 - F(c) 25 76 56 50 _ * As collected in cathode bags during 168 h of testing, represents 0.3-0.4%
of total As input in cell.

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TABLE: V
ANOI~E CO~PAR~ ENT l~EI) AND FFFLUEN'r~
Ana1YS;S (~P1) TeSt N; CO CU = AS SO~ Na~ C1_ Feed C 70.0 0.12 0.20 0.026 NA 119 22.5 45.9 Eff1Uent C 70.5 0.19 0.35 0.084 NA 115 22.5 45.9 Feed D 71.0 0.17 0.35 0.093 .021 113 24.0 43.3 Eff1Uent D 78.2 0.19 0.49 0.088 .028 121 24.0 46.4 Feed E 75.0 0.18 0.46 D.086 .026 117 23.0 44.3 10Eff1Uen$ E(a) 78.0 0.20 û.58 0.079 .031 121 23.0 45.9 Feed Etb) 72.0 0.18 0.54 0.082 .028 112 21.0 40.8 Eff1Uent Etb~ 69.7 0.18 0.57 0.074 .038 109 21.0 40.7 Feed F(a) 71.0 0.25 0.53 0.070 .037 115 33.0 43.4 Eff1Uent F(a) 81.7 0.32 0.69 0.071 .043 128 23.0 48.4 Feed F(b) 72.0 0.28 0.60 0.061 .034 114 20.0 42.1 Eff1Uent F[b) 72.0 0.30 0.67 0.056 .035 117 20.0 42.2 Feed F(C) 72.0 0.30 0.67 0.056 .035 117 20.0 42.2 Eff1Uent F(C) 75.0 0.33 û.72 0.050 .034 120 20.0 43.1 .
* Eff1Uent COnCen$rat;OnS der;Ved bY nOrma1jZ;ng net COnCentrat;OnS.
20NA = NO Ana1YS;S.
TABLE YI
D~IED RESIDIJE COMPO~ ~N
ReS;dUe Ana1YS;S t~Yt.%j TeSt N; Fe AS SO4 CQ CU S
C 46.2 0.43 .052 8.48 0.22 0.58 5.97 D 44.7 0.47 .09 4.00 0.21 1.07 8.32 E 47.2 0.4S .10 4.86 0.221.S2 4.13

3~ 7~8 agitation of the anolyte ;s provided in the two subsequent experiments, Tests No.
E(a) and E(b) by recycling about 80% of the electrolyte. In Test No. E(b), the anolyte is also air sparged. The results indicate that increased agitation by recyc]ing and sparging of air did not improve substantially the acid neutralization or prevent the precipitation of hydrate.
As indicated in Table IV, during Tests E(a) and E(b), the concentration of NaOH in the cathode compartment is always increasing without reaching a steady level at which extensive anolyte neutralization is expected to take place.
To remedy this, in Test F(a) intermittent additions of small volumes of water (about 50 ml/hr) are made to the cathode compartment to force NaOH produced in the catholyte into the flowing anolyte, while maintaining all other forms of agitation. In Tests F(b) and F(c) the additions of water to the cathode compartments are made continuously at flow rates representing, respectively, 0.7and 1.4 percent of the anolyte flowrate. These additions are insignificant in the sense that losses of water through evaporation represent between 3 and 5 percentof the total volume of electrolyte after each run. In Tests F5a), F(b) and F(c) most of the feed acid is neutralized and the amount of nickel hydrate preeipitated in the cell is reduced in comparison with Tests D, E~a) and E(b).
In all the Tests of Table II, the anode dissolves with high electrical efficiency. The lower overall efficiency of nickel dissolution in Test F is attributed to the fact that the anode used is almost completely dissolved at theend of the test.
As shown in Table II, power consumptions are of approximately 3.5 }~Wlkg Ni in Test F (with water addition) but 9 to 12 kW/kg Ni in Tests D and E
(without water addition to the catholyte).
Reference to Table IV which gives catholyte analyses for the tests, shows that the concentration of sodium hydroxide increased steadily from 3.~5 to5.9 moles (M) during Tests D, E(a) and E(b) performed without water addition to the catholyte, without reaching a maximum. In Tests F(a), F(b) and F(c) (with addition of water to the catholyte) the NaOH hydroxide decreased from 2.3 to 1.4Nl. The arsenic concentration in the catholyte was ~r50 mg/l after 168 hours of operating representing about 0.3 to û.4% of the arsenic present in the electrolyte.
Feed and effluent solutions analyses are shown in Table V. For purpose of comparison the concentrations of each effluent has been derived assuming that the sodium concentration was constant. Although the Ni concen-tration should increase by about 1 to 3 gpl in the effluents the analytical precision - 12~ PC-2142/CAN

overlaps this increase and no significance can be attached to the Ni values. A
steady increase of Co and Cu and no increase of Fe concentration is observed throughout the tests. The ~s increases during Tests D, E(a) and E(b) with low pHeffluent and tended to remain constant in Test F run at higher pH.
5Assays of the oven dried hydrate precipitates obtained in each test are shown in TaMe VI. X-ray diffraction of residues from Tests D and E gave a pattern which was a close match to Ni(OH)2.
The tests of Example II demonstrated the advantage of adding water to the catholyte in the present process.

The purpose of this example is to demonstrate the beneficial effect of air sparging in the anolyte in a process of the present invention.
The experiments of this example are carried out in similar manner to those described in Example II, except that a cloth made of DYNEL is used to bag 15the nickel sulEide anode to prevent mixing of the anode residue and Ni(O~)2 formed in the anolyte on the treated ~ANEÇ~ALON diaphragm.
Test conditions and results are summarized as given in Table VII. The results indicate that nickel dissolves at 90.6% anode current efficiency and that 94% of the nickel reports in the anolyte, the remaining 6% of the nickel reports in 20the nickel hydrate precipitate. About 88% of the H2SO4 in the feed is neutralizedO
The above test was continued for 100 hours without air sparging (Test H). This resulted in precipitation of 74% of the dissolved nickel as nickel hydrate and consequently only 39% of the feed H2SO4 is neutralized.
25This example demonstrate the advantage of air sparging in the process of the present invention.
The present invention may be used for any system for the recovery of metals by electrorefining from an electrolyte containing said metal in solution,where at least a part of the metal deposited is derived from soluble impure anodes 30electrically corroded in a flowing electrolyte, with adjustments in processingconditions that will be obvious to those skilled in the art. It is also noted that although the present invention has been described in conjunction with preferred embodiments, modifications and variations may be resorted to wi$hout departing from the spirit and scope of the invention, as those skilled in the art will readily 35understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

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While in accordance with the provisions of the statute, there are illustrated and described herein specific embodiments of the invention. Tho6e slcilled in the art will understand that changes may b made in the form OI the invention covered b~r the claims and that certain 5 ~eature~ OI the invention ma~ ~ometimes be used to advantage without a corre~ponding u~e of the other feature~.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. A process for the electrolytic dissolution of impure nickel refinery anodes to replenish nickel in a nickel-containing electrolyte, which comprises establishing an electrolytic system comprised of at least one electrolytic cell,said cell containing at least one impure refinery anode and at least one cathodecompartment containing at least one cathode, said anode and cathode compart-ments being separated by chloride-resistant diaphragm through which electrolyticcontact can be established between the anode and cathode compartments; flowing an aqueous anolyte through the anode compartment, said anolyte comprising nickel ions, alkali metal ions and at least one of the ions chloride and sulfate ions;
maintaining a non-circulating catholyte in the cathode compartment, said catholyte being an aqueous alkaline solution containing sufficient hydroxyl ionssuch that under operating conditions of the cell nickel ion migration to the cathode compartment is prevented and substantially only the decomposition of water takes place in the cathode compartment, hydrogen being liberated at the cathode; and applying a direct current in the cell to dissolve the impure nickelrefinery anode in the anode compartment and to dissociate water in the catholyteand generate hydrogen at the cathode.
2. A process as defined in claim 1, wherein the pH in the anolyte is adjusted to and maintained at about 3.
3. A process as defined in claim 1, wherein means are provided for catholyte flow through the diaphragm into the anolyte.
4. A process as defined in claim 1 wherein the electrolytic cell is operated at an anode current density of about 100 to 500 ASM.
5. A process as defined in claim 1, wherein the pH of the catholyte is maintained above about 12.
5. A process as defined in claim 1, wherein the hydroxide in the catholyte is maintained at a level equivalent to about 40 to about 80 g/l of sodium hydroxide.
7. A process as defined in claim 1, wherein H2SO4 is used to adjust the pH of the anolyte.
8. A process as defined in claim 1, wherein air sparging is maintained in the anolyte.

9. A process as defined in claim 1, wherein impure-anodes are nickel sulfide anodes.

10. A process as defined in claim 1, wherein the anolyte comprises nickel ions, chloride ions, sulfate ions, sodium ions, and boric acid.

11. A process as defined in claim 1, wherein the impure effluent from the anode compartment is treated to remove impurities.