US2394874A

US2394874A - Electrorefining of nickel

Info

Publication number: US2394874A
Application number: US472471A
Authority: US
Inventors: Renzoni Louis Secondo
Original assignee: International Nickel Co Inc
Current assignee: Huntington Alloys Corp
Priority date: 1942-11-05
Filing date: 1943-01-15
Publication date: 1946-02-12
Anticipated expiration: 1963-02-12

Description

Feb. 12, 1946.

L. S. RENZONI ELECTRO REFINING OF NI CKEL Filed Jan. 15, 1943 i ia".

HNOLY TE CEMENT/7770A THNK No.

CEMENT CU, /w' 7-/0Z CEMENTHT/ON Cu eo-soZ CEMENITH T/ON CEME/VTHT/ON THN/r No.4-

IV/ PC WD ER DORR TEN/1 9 Sheets-Sheet 5 CEME/V 7' 6'0 AND U/VREHC TED /v/' POWDER CEMENT/ T/ON TEN/f No.6

CEMEN 7-H T/ON TH/wr No. 7

CEMENT 60 FROM TEN/1'8 N065? Fl; TRH TE F/L TER PRESS us .574 TEMP/55F LOU/SSRENZON/ INVENTOR.

HTTORNEY Feb. 12, 1946. L. s. RENZONI 2,394,874

' ELECTRO-REFINING 0F NICKEL Filed Jan. 15, 1943 a Sheets-Sheet 9 lNl/E'N TOR 4 OU/S 6.RENZO/V/ HTTORNEY sulfaltlebt biite, The prbqqss as usually on? I it4nvQ Y Jt ilsel ia e lha i iz phragm' separating the,anode cpmpgi tn ent min a s niq" ea andiqopne i1-111 not. n .10 n practicgl inejal'iogi for the xcon tinuong :rqmoiaL 91 them-rare .ext enis i fl ulnf nera f an non .imnu i iei l rsti. for 911.; ligri ibg drkp yid u me ze c yt whi h ntrpduqq lfiioa lle thqd QQWWE WP -SM m w cells atsu i itbi 0 his *pro ded y lead, Y1 a. reducing agent followed by addition or acid H carried out on an industrial scale. It has been 56 in whichiron, lead, arsenic and*cobalt are remov eci by bxidiitipn n v "precipitationisrior wine QW- QlfJFQP-PI ic ema i i The present invention also contemplates a proqes by 'whici 'piiimary slimes containing irjon,

senic; coppei and. cobalt iare it'reatedzwith for flcation of the impure anolyte and for the electro-deposition of cathode nickel or electro-nickel from purified electrolyte in accordance with the principles of the present invention;

Fig. 8 is a photomicrograph taken at 500 magnifications of electro-nickel produced in an allsulfate ion bath at 12 amperes per square foot; and

Fig. 9 is a. photomicrograph taken at 500 magnifications of electro-nickel produced in a sulfate-chloride electrolyte at 16 amperes per square foot.

Briefly stated, the electro-refining of nickel involves the production of impure nickel anodes which are cast in any suitable manner and generally have a nickel content of about 90% to about 96%. Dependent upon the source of the nickel from which the impure nickel anodes are produced, such anodes will contain greater or lesser amounts of the impurities present in the ore. Thus, for example, in one industrial operation, the impure nickel anodes contain about 94% to about 96% of nickel, about 2.5% to about 3.5% of copper, about 0.7% to about 0.9% of cobalt, about 0.5% to about 1.0% of iron, about 0.002% to about 0.004% of lead, about 0.05% to about 0.07% of arsenic and about 0.5% to about 0.8% of sulfur.

The electro-refining of nickel involves solution of the metallic constituents of the impure anode in the anolyte followed by purification of the anolyte to remove iron, copper, cobalt, lead, arsenic, etc. to obtain a purified electrolyte from which electro-nickel can be produced.

Electrolytic nickel produced in the all-sulfate electrolyte has the following composition:

The general features of the electrolytic cell preferably employed in conjunction with the Hybinette bag are described more or less in detail, together with the process employed in conjunction therewith, in the Hybinette U. S. Patent No. 805,969. In the prior art all-sulfate process, the electro-refining of nickel is generally and preferably carried out employing Hybinette bags. As those skilled in the art know, the Hybinette bag generally comprises a wooden frame of spruce or the like covered with a canvas of such weight that a slight hydrostatic pressure maintained on the cathode side of the canvas substantially preventsthe migration of copper, cobalt, nickel, iron and similar cations from the impure electrolyte through the canvas diaphragm into the purified electrolyte present as a catholyte in the cathode compartment of the cell.

As clearly set forth in U. S. Patent No. 805,969, a hydrostatic pressure is maintained in the cathode compartment and impure anolyte withdrawn from the anode compartment continuously. The impure anolyte is first subjected to purification to remove copper and. in the all-sulfate ion electrolyte, the copper is removed by cementation through addition of nickel, preferably in the form of freshly reduced nickel powder, before the iron, lead and arsenic are removed. After the copper has been removed by cementation, the iron, lead and arsenic are then removed. When desired,

the cobalt is removed in the all-sulfate ion. electrolyte by the addition of nickel, preferably in the form of an oxide or hydrated oxide prepared outside the electrolyte system by the action of a strong oxidizing agent upon nickellous hydrate. whereby the nickel goes into solution and the cobalt is precipitated. This, however, is a difiicult and costly operation. In order to derive the advantages of lower tank voltage, higher anode efliciency, lower steam consumption, better purification of impure anolyte, the production of a more highly purified electro-nlckel, and other advantages of the present invention, it was found that the sequence of steps for the purification of the impure anolyte which had provided satis factory results when employed in conjunction with the all-sulfate-ion electrolyte was inefiective when employed in conjunction with the novel electrolyte of the present invention containing chloride ions equivalent to up to about 50 grams of sodium chloride per liter.

In the operation of the all-sulfate process, anode efliciency is lower than cathode efiiciency which results in anolyte flowing from the plating tanks at pH 3.0 to pH 3.2. This value is too low for iron hydrolysis and must be raised to a minimum of pH 5.0 before attempting iron precipitation. The pH rise is brought about by carrying out copper cementation before iron hydrolysis. The nickel powder functions in cementing copper and in neutralizing excess acid. Thus, in one operation using anodes containing 2.65% copper. the cementation of pounds of copper requires the dissolution of 1'73 pounds of reduced nickel powder. Of this amount, 92 pounds are required for cementing copper and 81 pounds for neutralizing acid. In the sulfate-chloride process, anode efiiciency is practically equal to cathode efficiency and anolyte pH is 4.6. This allows for the removal of iron in the presence of cupric ions. During copper cementation the amount of excess acid present is small. Thus, in the operation usin anodes containing 2.65% copper, the cementation of 100 pounds of copper requires 133 pounds of nickel powder, of which 41 pounds are consumed for acid neutralization. The operation of the sulfate-chloride process, therefore, shows a clear savingin nickel powder consumed for copper cementation.

In order that those skilled in the art may have a clearer understanding of the present invention, the process will first be described in conjunction with the simplified flow sheets, Figures 1 to 6, and then the process will be described in conjunction with the drawings, Figs. 7 and 7a.

The refining of nickel-containing material by electro-deposition of nickel from a purified electrolyte involves the use of a number of electrolytic cells employing a diaphragm to separate impure anolyte from the catholyte. The number of cells required is, of course, dependent upon the amount of nickel to be electro-deposited in a unit of time, most of the cells being operated with a soluble anode while a few of the cells are operated employing insoluble anodes. The soluble anodes are obtained by casting or otherwise forming from impure metallic material containing a relatively high percentage of nickel, but also containing so much of the impurities, copper, iron. cobalt. lead, arsenic and sulfur, that the impure anodes are not acceptable, per se. to the trade as pure nickel. The impure anodes are immersed in the anode compartment of the diaphragm cell in contact with lean anolyte obtained from previous operations, 1. e., catholyte, the nickel content of Thus, an iron slime, which may be'called secondary iron slime, is formed and is separated from the fluids containing nickel, cobalt and copper in the form of soluble salts. The secondary iron slimes contain about 40% iron and small amounts of copper, cobalt, and nickel which may be treated in any suitable manner for the recovery of copper, cobalt and nickel therein contained. The fluid portion containing the nickel, cobalt and coppe as sulfates, preferably, is treated with nickel powder at a pH of about 5.2 to precipitate the copper as cement-copper and the cement-copper is removed in any suitable manner such as, for example, by a filter press. The fluid portion containing the nickel and cobalt as sulfates is treated with an alkaline oxidizing agent to preferentially oxidize and hydrolyze the cobalt. Preferably, an alkaline solution of sodium hypochlorite is employed as the oxidizing and precipitating agent. The cobalt is precipitated as a hydrated oxide or hydroxide containing from 53 to 55% cobalt, 2 to 4% nickel, and traces of copper and iron. The precipitate is allowed to settle and the supernatant nickel solution, now containing onlyapproximately 0.2 gram per liter cobalt is removed by decantation. The nickel values are recovered from this solution by precipitation as basic nickel carbonate which is then collected in a filter press, washed, resuspended in water and added to oxidation tank No. 3 of the nickel electrolyte purification system to supply a portion of the alkali required for cobalt hydrolysis. The first cobalt precipitate is resuspended in water and redissolved by the addition of sulfur dioxide and sulfuric acid. Th resulting solution, containing approximately 30 grams per liter cobalt, and 1 to 2 grams per liter nickel is again treated with an alkaline sodium hypochlorite solution and the cobalt is again precipitated. The second cobalt precipitate contains from 55 to 57% cobalt, from 0.4 to 0.6% nickel and traces of copper and iron. The fluid portion from the second cobalt precipitation contains approximately 0.5 gram per liter nickel and approximately 0.2 gram per liter cobalt and is used for washing the first cobalt precipitate; It is believed that the reduction of the iron from the ferric state to the ferrous state and the oxidation of the'reduced iron to the ferric state fol-- lowed by hydrolysis and precipitation as ferric hydroxide is illustrated by the following equation:

2Fe(OH)3+2Co(OH)3+2Ni(OH)3+ In Fig. 4 there is provided in a more or less diagrammatic manner a simplified flow sheet illustrative of the removal of arsenic and lead. After the oxidation and precipitation of the iron has been completed, chlorine is introduced into the impure electrolyte and the arsenic, lead and cobalt oxidized to the higher states of oxidation. Instead of the chlorine, other oxidizing agents, such as hydrogen peroxide, persulfates, and the like. could be employed, but it is preferred to use chlorine in view of the fact that an ample supply is available at the insoluble anode of the electrorefining cells. The arsenic, lead, and, if desired, cobalt are oxidized. The arsenic and lead precipitate, together with the iron, may be removed as primary iron slimes. When it is desirable to remove the cobalt at the same time as the iron,

arsenic and lead are removed, it is necessary to maintain the. solution at a pH of about 4.0 to about 5.0 and preferably at a. pH of about 4.5 and to augment the chlorine produced at the insoluble anodes with chlorine from an outside source. In order to establish and maintain the desirable pH for precipitation of cobalt, an alkali is added, preferably basic nickel carbonate, to the impure electrolyte in sufilcient quantity to maintain a pH between the aforesaid limits.

The flow sheet of Fig. 5 is illustrative in a more or less diagrammatic manner in a simplified way of the removal of copper from the partially purified electrolyte by cementation. The partially purified anolyte is introduced into the first cementation tank into which likewise are introduced the solids from the settler or Dorr tank. The contents of the first cementation tank then pass to the second cementation tank from which the cement-copper is removed. This cementcopper contains about to copper and about 7% to 10% nickel and is further treated for the separation and recovery of the values contained therein. The impure electrolyte passes from cementation tank 2 (corresponding to tank I of Fig. '7) to cementation tank 3 (corresponding to tank l5 of Fig. 7) where nickel is added at intervals, preferably, and in amounts sufllcient to maintain the copper content of the overflow from the settling tank (or Dorr tank l8 as shown in Fig. '7) below a particular value of about 0.01 to about 0.02 gram per liter. Cement-copper containing some nickel is likewise introduced into cementation tank No. 3 (corresponding to tank l5 of Fig, 7). 'The precipitated copper and the partially purified electrolyte then pass to two additional copper cementation tanks to allow sufficient time for the reaction, and thence to a settling tank or Dorr tank where the solids are removed and conveyed to the first cementation tank and the overflow or solution of partially purified electrolyte passes to cementation tank No. 6 (corresponding to tank IQ of Fig. 7) where the nickel powder necessary to remove substantially all of the residual copper is added. To provide time for the reaction to reach completion, the contents of cementation tank No. 6 (corresponding to tank [9 of Fig. '7) are conveyed to cementation tank No. '7 (corresponding to tank 20 of Fig. '7) and then to a suitable means for removing cement-copper from the solution, such as fllter press 22 (Fig. 7) The filter press has been found to be satisfactory for this purpose. The filter cake comprises the cement-copper and unused nickel powder and is returned to cementation tank No. 3. When the cobalt has been removed with the iron, the filtrate contains about 0.001 gram to about 0.01 gram, preferably about 0.005 gram per liter of cobalt and is of sufficient purity to be electrolyzed in the electro-refining cells. When the cobalt has not been removed in the iron removal unit, the filtrate is passed to the cobalt removal system. The reaction by which the copper is cemented out of solution is believed to be illustrated by the following equation.

CllC12+Ni NiC1z+Cu pH 4.2 to 5.4 Temperature F During the cementation of the copper, it is preferable to maintain the temperature of the electrolyte at about 135 F. and to allow the pH to rise from about 4.2 to about 5.4. However, a temperature between about 120 F. and about F.

and a pH within the range of about 4.0 to about 5.5 are also satisfactory.

In the event that the removal of iron, arsenic and lead is carried out under such conditions that the cobalt is not precipitated, it is necessary to remove the cobalt after the iron, arsenic, lead and copper have been removed from the impure anolyte. Such an anolyte may be termed partially purified electrolyte. The removal of cobalt is illustrated in a simplified and more or less diagrammatic manner in Fig. 6. The partially purified electrolyte containing about 0.0002 to about 0.0005 gram per liter of copper and about 0.12 gram per liter to about 0.15 gramper liter of cobalt is conveyed to a reaction tank into which molecular chlorine and nickel carbonate are introduced in sufficient quantities to precipitate cobalt to yield a purified electrolyte containing a residual concentration of 0.001 gram per liter to 0.01 gram per liter cobalt. The cobalt precipitates as an oxide or hydrated oxide at a pH of about 4.5 to about 5.0. The suspension istreated to remove the cobalt precipitate in any suitable manner and preferably with a filter press to obtain a cobalt precipitate containing about 18% to about 20% cobalt, about 25% to about 30% nickel, about 0.005% to about 0.01% iron, which is then treated preferably in accordance with the process illustrated in Fig. 3 for the recovery of the cobalt. The filtrate which is now the purified electrolyte for direct introduction into the cathode compartments of the electro-refining cells contains about 0.001 to about 0.01 gram per liter of cobalt, about 0.0002 gram per liter to about 0.0005 gram per liter of copper, about 0.0001gram per liter of arsenic, about 0.0001 gram per liter of lead, and about 0.0001 gram per liter of iron. It is believed that the reaction to precipitate cobalt in the aforesaid described manneris illustrated by the following equation:

The impure anolyte is removed from the anode compartments I (Fig. 7) and transferred by means of conduit 2 to a holding tank 3. The impure anolyte as withdrawn from the anode compartments has a temperature of about 130 F. to about 140 F., a pH of the order of pH 4.6 and contains about 0.25 gram per liter to about 0.35 gram per liter of copper, about 007 gram per liter to about 0.10 gram per liter of iron, about 0.0002 gram per liter to about 0.0004 gram per liter of lead, about 0.004 gram per liter to about 0.006 gram per liter of arsenic, and about 0.12 gram per liter to about 0.15 gram per liter of cobalt. The impure electrolyte in holding tank 3 is then conveyed in any suitable manner, for example by means of pump 4 and conduit 5, to the unit for removal of iron, arsenic and lead which comprises four or more containers, such as tanks 6, I, 8 and 9, connected in series. The contents of tanks 6, l and 9 are agitated, preferably by air introduced therein by means of conduit 28. The removal of iron, arsenic and lead is based upon the oxidation of the iron, arsenic and lead.

and the precipitation of these materials as oxides or hydrated oxides or hydroxides or as insoluble iron and lead compounds of arsenic. In order to precipitate the arsenic and lead, it was found necessary to introduce an oxidizing agent, such as hydrogen peroxide, persulfates, sulfur dioxide and air or chlorine into the reaction mixture after the iron had been oxidized and precipitated.

' tates as the hydroxide.

This is quite contrary to usual practice when purifying an impure all sulfate-ion anolyte.

In-the impure anolyte, a portion of the copper is present as cupric ions. The iron is oxidized at the expense of the cupric ions and the cupric ions reoxidized by air passed into theoxidation tanks. The oxidized iron then hydrolyzes and precipi- The pH change during the oxidation of the iron and the precipitation thereof is very slight, being equivalent to a drop from pH 4.6 to pH 4.5. It would appear that the successful oxidation of the iron and precipitation thereof is dependent upon the ratio of cuprous the trivalent to the pentavalent stage and for the oxidation of lead from the bivalent to the tetravalent stage. Preferably, the chlorine is obtained from the anode compartments of the e1ectrorefining cells 84 having insoluble anodes. At the same time, more or less of the cobalt present in the impure anolyte is likewise oxidized and under proper conditions can be removed, together with the iron, lead, arsenic and some copper. However, when desirable, the removal of iron, lead and arsenic can be so controlled as to substantially eliminate the removal of cobalt at this place in the purification system and the cobalt can be removed at a later stage in greater purity. The oxidation of the iron, lead and arsenic is substantially completed in oxidation tankv 8 and,

oxidation tank 9 is only provided to insure the completion of the reaction. The arsenic content of the electrolyte is lowered from about 0.005

' tank I 3.

gram per liter to about 0.0001 gram per liter after passage through oxidizing tank 8. The iron, arsenic and lead precipitates, together with any cobalt and copper which are precipitated, are removed as primary slimes in any suitable manner, preferably by any suitable mean H to a. filter press 12. The filter cake (primary iron slimes) is treated for the recovery of cobalt as hereinafter described in conjunction with Figs. 3 and 7a, while the filtrate is further treated to remove copper and traces of cobalt.

Copper is removed by cementation after the removal of iron, lead and arsenic has been completed and is carried out in the copper cementation system as illustrated in a diagrammatic manner in Fig. '7 and in the simplified flow-sheet Fig. 5. The copper cementation unit includes seven or more cementation and holding tanks l3, l4, l5, l5, l1, l9 and 20 and a settler l8, such as a Dorr tank. The filtrate from the primary slimes separation (accomplished in filter press 12) is introduced through conduit 2| into cementation Freshly reduced nickel powder is introduced into cementation tank l9 into which the overfiow of the settler or Dorr tank 18 is introduced. The eflluent from cementation tank 20 passes to a filter press 22 or other means of separating solids from liquids in which the cement copper, together with some nickel, is separated from the partially purified electrolyte. The

solids from this separation containing an appreciable amount of nickel are introduced into cementation tank l by suitable means 23. Additional nickel powder, together with the filter cake from press 22 is likewise introduced into cementation tank l5 in amounts sufficient to maintain the copper content of the overflow from the settler tank not greater than about 0.02 gram per liter. The solids settling out in the settler or Dorr tank are returned to the first cementation tank [3 of the system by suitable means, such as pump 24 and conduit 25, and the cement copper is removed from cementation tank l4 in any suitable manner.

Dependent upon the conditions existing in the unit for the removal of iron, arsenic and lead, the electrolyte flowing from the copper cementation system is more or less purified of cobalt. In the event that suiflcient cobalt has been removed in the iron, lead, arsenic system, the electrolyte is sufficiently purified to be introduced directly into the cathode compartments 3| of the electrolytic cells by means of conduit 26. On the other hand, if sufficient cobalt has not been removed, the electrolyte flowing from the copper cementation unit, filter press or the like 22, is transferred by means of conduit 21 to tank 28 and treated with chlorine plus nickel carbonate Constituents Grams per liter From about 0.001 to about 0.01. From about 00002 to about 0.0005. About 0.0001. About 0.0001.

About 0.0001.

It is preferred to maintain the electrolyte at a temperature between about 130 F. and about 140 F. during purification and electro-deposition. The pH of the catholyte during electrodeposition is preferably maintained at about 4.0 to about 5.0.

It is desirable to recover the cobalt whether precipitated with the iron, arsenic and lead or removed separately. Furthermore, in view of the fact that an appreciable amount of nickel is precipitated with the iron, lead and arsenic of the primary slimes, it is desirable to further treat the primary slimes, whether they contain cobalt or not, to recover the nickel contained therein and to reintroduce the nickel so recovered into the electro-refining system.

In the event that the primary slimes contain cobalt in amounts greater than 5% to 6%, the treatment of the primary slimes differs from the treatment to which the primary slimes are subjected when the primary slimes contain less than 5% cobalt. When the primary slimes contain more than 5% to 6% cobalt, the primary slimes from filter press l2 are introduced into container 33 by suitable means such as conduit 34 and suspended therein in water (Fig. 7a). Sulfur dioxide, or any other suitable reducing agent is introduced and the reduction reaction is allowed to proceed until all of the nickel, cobalt and copper, and from about 5% to about 10% of the iron are reduced from the oxidized to the reduced states. When sulfur dioxide is used as the reducing agent, the end point of the reaction is reached when the pH value of the suspension has been reduced to pH 4.3 to pH 4.5. Any suitable acid, preferably sulfuric acid, is then introduced in an amount required to reduce the pH to 1.5 to 2.0 at which value the slime is completely dissolved; During the reaction, the tem perature in container 33 rises from about F. to about F. and no external heat is required. When the reaction is completed, the cobalt and nickel compounds and preferably a part of the iron and copper compounds of the primary slimes have been reduced and brought into solution.

The solution of the values of the primary slimes is then transferred to container 35 (Fig. 7a) by means of conduit 36. Further amounts of primary slimes are. introduced into container 35 by means of channel 31. The primary slimes have an oxidizing effect on ferrous and cuprous ions and an acid neutralizing effect which may be expressed by the following reactions:

The reaction is complete when the pH rises to pH 4.3 to 4.5. The oxidation reaction is carried out at 180 F. By suitable control it is possible to carry out the reaction to yield a solution of cobalt, nickel, and copper essentially free of iron, and to produce a secondary iron slime containing about 1% to about 2% cobalt, about.2% to about 4% nickel, about 5% to about 7% copper and about 36% to about 40% iron, from 2.5% to 3.5% arsenic and about 0.2% to 0.3% lead.

The suspension of primary slimes in container 35 is transferred to filter press 38 by any suitable means 39. The fluid portion of the suspension of the primary cobalt-containing iron slimes after separation from the secondary iron slime by any suitable means, such as filter 38, is transferred to container 40 by suitable means 4| and treated with reduced nickel powder in container 40 at a temperature of about F. to about F. and preferably at about 150 F. and at a pH of 4.5 to about 5.5, and preferably at about pH 5.2. Under these conditions, the copper contained in the primary iron slimes and present in the fluid portion as copper sulfate, or chloride, or both, is precipitated out as cement-copper by the addition of the nickel powder. The suspension containing the cement-copper is then transferred by any suitable means 42 to filter press or the like 43 and the cement-copper separated from the fluid portion containing nickel and cobalt as soluble nickel and cobalt sulfates and/or chlorides. The solution containing the nickel and cobalt as soluble salts is then transferred by suitable means 44 to container 45, to which alkaline sodium hypochlorite is added. This hypochlorite solution is made up preferably by chlorinating a soda ash solution containing 3.89 pounds of soda ash per pound of chlorine added. Sufficient hypochlorite solution is added to container 45 so that the cobalt is precipitated as a hydratedcobaltic oxide. The residual cobalt content of the solution is reduced to 0.2 gram per liter cobalt by this means. For the precipitation of the cobalt, the temperature of the solution should be maintained at about 80 F. to about 120 F., and preferably at about 100 F. and the pH should be maintained within the range of about pH 1.8 to about pH 2.4 and preferably at about pH 2.3. The precipitated cobalt contains 53 to 55% cobalt and 2% to 4% nickel after the initial separation from the nickel bearing solution. The precipitate 'formed in tank 45 is allowed to settle, the liquid portion decanted, the precipitate washed once with water, resuspended with water, and redissolved with sulfur dioxide plus sulfuric acid. By suitable control, a

solution free from sulfur dioxide at pH 2.0 is

obtained. The solution at this point contains approximately 30 grams per liter cobalt and 1 to 2 grams per liter nickel. The cobalt is again precipitated by the action of alkaline sodium hypochlorite to yield a precipitate containing 55% to 57% cobalt and-0.4% to 0.6% nickel, and a solution containing approximately 0.2 gram per liter cobalt and approximately 1 gram per liter nickel. The precipitate is allowed to settle, the fluid portion decanted or separated by any suitable means, and the precipitate washed twice by decantation and finally separated from the wash water by means of filter press 46 into which the suspension is introduced b suitable means 41. The fluid electrolyte, after the initial removal of cobalt to provide a nickel solution containing not more than about 0.2 gram of cobalt per liter, is.

then transferred by means of channel 48 to tank 49 and treated with soda ash to precipitate the nickel as basic nickel carbonate. This suspension is transferred by suitable means 50 to filter press or the like and the nickel carbonate separated. The precipitate is washed, re-emulsified with water in tank 52 and transferred to oxidation tank 8 by means of conduit 53 to assist in the precipitation of cobalt.

When the primary iron-slimes are substantially devoid of cobalt, the treatment to which the primary iron slimes is subjected is somewhat different from that to which primary iron slimes containing cobalt is subjected. This treatment of primary iron slimes substantially devoid of cobalt will be discussed in conjunction with the unit for recovery of nickel from primary iron slimes (Figs. 7 and 7a). The primary iron slimes substantially devoid of cobalt are transferred by means of channel 55 to container 56 (Fig. 7a). Anode slime wash water containing about 3 grams per liter to about 5 grams per liter of chloride ion is introduced into container 56 by means of conduit 51, When desired, anode slime wash water can be added by suitable means 58 to the 4 suspension of the primary iron slimes substantially devoid of cobalt in channel 55 in addition to introducing the anode slime wash water into container 56. The suspension is then conveyed by means of conduit 59 to container 60 where a temperature of about 170 F. to about 180 F. is maintained together with a pH of about 2.0 to about 3.5. Anolyte, from the anode compartments of the electro-refining cells having insoluble anodes, is likewise introduced into container 60 by means of conduit 6|. The anolyte so introduced has a pH of about 1.3 to about 1.5 and contains about 0.2 to about 0.4 gram per liter of molecular chlorine. Under these conditions, the major portion of the nickel of the primary iron slimes, together with some arsenic, lead, iron and copper pass into solution. The suspension is conveyed, as by pump 62 and pipe 63, to filter press 64, or any other suitable means, wherein the solids are separated from the fluid portion of the suspension. The fluid portion containing nickel, copper, some iron, arsenic and lead is conanodes, the precious metals, gold, silver and the metals of the platinum metal group remain on the corroded anodes as anode slimes. This anode slime is washed off the residual portion of the soluble anodes 66 in tank 61, preferably with impure electrolyte or anolyte in conduit 68, as illustrated in Figs. 7 and 7a (precious metals recovery unit) from which it flows into slime sump 69. The suspension of anode slimes is then transferred by suitable means such as pump 10 and pipe H to a holding tank 12 and. finally through channel 13 to a means 14 for separating the fluid portion from the solid portion. The fluid portion is returned to the anode slimes suspension sump 69 by means of conduit 15, while the solid portion is conveyed by suitable means I6 to a holding tank 11 where it is suspended in wash water at a temperature of about F. to about 160 F., and preferably about F. The slimes are then transferred by suitable means 18 to a washing filter press 19 and washed therein until the wash water has a chlorine content not greater than about 0.002 gram per liter, and the washed slime a residual chlorine content of 0.1% to 0.2%. This washing operation is quite necessary for the efllcient recovery of the precious metals in the anode slimes. In the presence of small amounts of chlorine a relatively large portion of the precious metals is lost in later steps of the purification process. The thoroughly washed primary slimes are then transferred directly to a calciner. The washed filter cake is transferred directly to a calciner 80 by any suitable means 8I. The slimes'are roasted to reduce the sulfur content to approximately 0.2% and to obtain oxides of the base metals and some of the platinum group metals. The resulting oxide is then charged into an electric anode furnace 82 along with the proper percentage of coke, and the metal is cast in regular anode moulds 83 to form secondary or precious metal anodes. These are returned to the electro-refining cells and are corroded in regular soluble anode cells set aside for the purpose. To prevent loss of slime during corrosion, these anodes are wrapped in closely woven cotton duck. After essentially complete corrosion, the anode scrap, together with the adhering slime, is transferred to the precious metal concentrating plant where the slime is carefully removed from the scrap and treated with acid for the removal of the base metals from gold and the metals of the platinum group. The secondary anode scrap is returned to the electric anode furnace 82 and returns to the electrolytic nickel refinery as secondary anodes.

In carrying out the electro-refining of nickel in accordance with the aforedescribed process, it is essential that the pH of electro-deposition be not higher than about pH 5.0 and not lower than about pH 4.0. The amount of iron present in the impure anolyte should be equal to at least about 15 times the sum of the arsenic and lead present in the impure anolyte. The copper present in the i'mpure anolyte should be equal to about 3.0 to about 4.5 times the amount of iron present in the impure electrolyte and it is desirable to maintain the ratio of nickel to sodium in the electrolyte at not less than 1.50 to 1.00. However, the total sodium present should not exceed 30 grams per liter equivalent to about '15 grams per liter of sodium chloride. In practice it is preferred to maintain the sodium chloride content at about 50 grams per liter. If it is desirable to increase the chloride ion concentration, without further increasing the total sodium ion concentration, it may be accomplished by the addition of calcium chloride to the sulfate-chloride electrolyte followed by filtration to remove the precipitated calcium sulfate. In this manner the chloride ion replaces an equivalent amount of sulfate ion in the solution and there is no increase in the total cation content of the solution. This scheme is used to advantage in changing from an all-sulfate nickel electrolyte having a relatively large concentration of sodium sulfate to sulfate-chloride nickel electrolyte having a desirable maximum sodium ion concentration of 30 grams per liter. The chlorine necessary for the oxidation of the cobalt is about 2.5 to 3.5 times the theoretical amount. The nickel required for copper cementation is about 1.4 to about 1.5 times the theoretical amount. Nickel required for copper cementation from all-sulfate electrolyte is about 1.9 to about 2 times the theoretical amount. The electrolyte preferably has the following composition:

Constituents Grams per liter About 40 to about 60. About 27 to about 30. About 71 to about 120. About 10 to about 35. About 45 to about 50. About to about 25.

The impure anolyte prior to treatment to provide purified electrolyte has the following composition:

Constituents Grams per liter About 40 to about 60. About 27 to about 30. About 71 to about 120. About 10 to about 35. About 45 to about 50. About 15 to about 25. About 0.12 to about 0.15. About 0.07 to about 0.10. About 0.25 to about 0.45.

About 0.004 to about 0.006.

About 0.0002 to about 0.0004.

The purified electrolyte has the following composition:

Constituents Grams per liter The pH of the anolyte in the anode compartments of the cells having soluble anodes should be maintained between a pH of about 4.0 to about 5.0 and preferably at a pH of about 4.5. The pH of the catholyte in the cathode compartments of 'up to about 20 amperes per square foot.

the cells employing soluble anodes should be maintained within a range of about pH 4.0 to about pH 5.0. The anolyte in the cells havin insoluble anodes should be maintained at about pH 1.25 to about pH 1.5 and preferably at about pH 1.4 and the pH of the catholyte of the cells employing insoluble anodes should be about 4.0 to 5.0 and preferably about pH 4.5. The precipitation of the iron, arsenic and lead without substantial precipitation of the cobalt is carried out at a temperature of about F. to about F. and a pH of about 4.0 to about 4.5, and preferably at a temperature of about 135 F. and a pH of about 4.2. The removal of iron, arsenic and lead together with cobalt is carried out at a temperature of about 130 F. to about 135 F. and preferably at about 135 F. and a pH of about .0 to about 5.0 and preferably at about pH 4.5. The removal of copper by cementation is carried out at a temperature of about 130 F. to about 135 F. and preferably at about 135 F. while the pH is allowed to rise from about 4.2 to about 5.4.

When the cobalt present has not been removed simultaneously with the iron, lead and arsenic, it is preferred to carry out the precipitation of the cobalt at a pH of about 4.0 to about 5.0 and a temperature of about 135 F. although satisfactory results can be obtained employing a pH of about 4.0 to about 5.0 and a temperature of about 120 F. to about 180 F.

In the treatment of primary iron slimes containing cobalt, the pH during the reduction of the oxidized slimes to the reduced condition should be allowed to change from pH 6.5 to pH 4.5 by the action of sulfur dioxide. The pH is then reduced to 2.0 by sulfuric acid additions with the temperature of the reaction maintained within the range of 80 F. to about F. and preferably at about 120 F. The reoxidation of the reduced ions of the primary iron slimes should be carried out within a pH range of about 2.0 to about 4.5 and preferably finishing off at about pH 4.5 and at a temperature of about F. to about F. and preferably at about 180 F. The removal of copper by cementation from the primary iron slimes solution should be carried out at a temperature of about 120 F. to about 160 F. and preferably at about 130 F. and within the pH range of about. 4.0 to about 5.5 and preferably at about 5. It is preferred to maintain a pH of about 2.3 and a temperature of about 100 F. in precipitating the cobalt from the solution derived from the secondary iron slimes. However, this precipitation may be carried out within the range of about pH 1.8 to about pH 2.5 and at a temperature of about 80 F. to about 130 F. The electro-deposition of the nickel from the purified electrolyte is carried out at current densities of It will be noted that these current densities are appreciably greater than the current densities generally employed in electro-refining nickel from allsulfate electrolytes. In this connection it is to be noted particularly that in contrast with the tendency of the electro-nickel to form large berries when current densities in excess of 12 amperes per square foot are employed with an all sulfate ion electrolyte, electro-nickel produced in the sulfate-chloride electrolyte does not tend to produce large berries at even higher current densities for relatively long periods. Consequently, much higher current densities may be employed without encountering the difficulty arising from the tendency for electro-nickel produced in allsulfate ion electrolytes to produce large berries coarser grained as is recognized by comparison of Figs. 9 and 10. From the standpoint of purity, the sulfate-chloride electro-nickel is superior to the all-sulfate ion electro-nickel. For comparison, the following analysis of electro-nickel produced in the all-sulfate electrolyte as compared to the analysis of electro-nickel produced in the sulfate-ion chloride electrolyte in accordance with the process described hereinbefore is provided:

All-sulfate Sulfate-chloride electrolyte,

per cent Constituents Although the present invention has been described in conjunction with certain preferred embodiments thereof, those skilled in the art will readily recognize that variations and modifications can be made. Such variations and modiflcations are to be considered within the purview of the specification and the scope of the appended claims.

I claim:

1. A process for electro-refining nickel in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel in said anode compartment with an aqueous solution having sulfate ions and chloride ions as the principal anions and containing about 27 grams to about 30 grams per liter of chloride ions, and about 71 to about 120 grams of sulfate ion per liter while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing iron, copper, cobalt, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, continuously removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize said copper to the cuprie state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to about 5.0, treating said impure anolyte with suflicient chlorine gas to oxidize and precipitate arsenic and lead, separating said precipitates of iron, arsenic and lead from said impure anolyte to obtain a partially purified anolyte, removing copper from said partially purified anolyte by treatment with nickel powder, separating said nickel powder from said impure anolyte to obtain partially purified electrolyte, passing additional quantities of chlo- 75 rine gas through said partially purified electrolyte while adding sufllcient nickel carbonate to maintain a pH of about 4.0 to about 5.0 whereby said cobalt is oxidized and precipitated, separatin said cobalt precipitate from said partially purified electrolyte to obtain purified electrolyte, introducing said purified electrolyte into said cathode compartment, and maintaining a sumcient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

2. 'I'heprocess as set forth and described in claim 1 in which the copper is precipitated at a pH of about 4.0 to about 5.5.

3. The process as set forth and described in claim 1 wherein the electro-deposition or nickel 01' high purity is carried out at current densities of about 12 to 20 amperes per square foot.

4. A process for electro-reiining nickel which comprises establishing an aqueous solution having a pH of about 4 to about 5 containing about 40 to about 60 grams of nickel per liter, about 20 to about 30 grams of sodium per liter, about 27 to about 30 grams of chlorin per liter, about 71 to about grams of sulfate ion per liter, about 15 to about 25 grams of boric acid per liter, not more than 0.01 gram of cobalt per liter, not more than about 0.0001 gram of iron per liter, not more than about 0.0005 gram of copper per liter, not more than about 0.0001 gram of arsenic per liter, and not more than about 0.0001 gram of lead per liter, introducing said electrolyte into the cathode compartment of an electrodepositing cell, said anode compartment being separated from said cathode compartment by a porous diaphragm, electro-depositing nickel at a cathode in said cathode compartment, forcing a portion of said catholyte through said porous diaphragm into said anode compartment under sufficient pressure to substantially prevent copper ions, cobalt ions and ions of other impurities, passing through said diaphragm into said cathode compartment, anodically corroding an impure nickel anode containing copper, iron, cobalt, arsenic and lead, as well as nickel in said electrolyte forced into said anode compartment thereby obtaining a regenerated electrolyte containing increased amounts of nickel and prohibitive amounts of the impurities cobalt, copper, iron, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, said regenerated electrolyte having a pH of about 4 to about 5, passing oxygen through said regenerated electrolyte to oxidize said copper to the cupric state, the cupric copper reacting on the said iron in the reduced state to oxidize substantially all of said iron to the ferric state whilst maintaining o the'aforesaid pH, passing chlorine into said regenerated electrolyte to oxidize said arsenic and lead, allowing suflicient time for the hydrolysis of saidoxidized iron salts to obtain a precipitate containing iron, arsenic and lead, separating said precipitate of iron, arsenic and lead from said regenerated electrolyte to obtain a partially purified electrolyte, adding metallic nickel to said partially purified electrolyte to precipitate copper, separating said precipitated copper from said partially purified electrolyte to obtain a further purified electrolyte substantially devoid of iron, copper, arsenic and lead impurities and containing nickel and cobalt, precipitating said cobalt by introducing chlorine and nickel carbonate into said further purified electrolyte, separating said precipitated cobalt from said further purified electrolyte to obtain a purified electrolyte containing about 40 to about 60 grams of nickel per liter, about 20 to about 30 grams of sodium per liter, about 2'7 to about 30 grams of chlorine per liter, about 71 to about 120 grams of sulfate ion per liter, about to about 25 grams of boric acid perliter, not more than 0.01 gram of cobalt per liter, not more than about 0.0001 gram of iron per liter, not more than about 0.0005 gram of copper per liter, not more than about 0.0001 gram of arsenic per liter, and not more than about 0.0001 gram of lead per liter, and introducing said purified electrolyte into the cathode compartment of an electro-depositing cell.

5. A process for electro-refining nickel in an electrolytic cell havingflan anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corrodingimpure nickel in said anode compartment with an aqueous solution comprising essentially about 27 to about 30 grams of chloride ion per liter, about 71 to about 120 grams of sulfate ion per liter, about to about 30 grams of sodium ion per liter, and about 15 to about grams of boric acid per liter while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing about 40 to about 60 grams per liter of nickel, about 0.25 to about 0.45 gram per liter of copper, about 0.12 to about 0.15 gram per liter of cobalt, about 0.07 to about 0.10 gram per liter of iron, about 0.0002 to about 0.0004 gram per liter of lead and about 0.004 to about 0.006 gram per liter of arsenic in the reduced state, continuously removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize said copper to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to about 5.0. treating said impure anolyte with sufilcient chlorine gas to oxidize and precipitate arsenic and lead, separating said precipitates of iron, arsenic and lead from said impure anolyte to obtain a partially purified anolyte. removing copper from said partially purified anolyte by treatment with nickel powder, separating said nicke1 powder from said impure anolyte to obtain partially purified electrolyte. passing additional quantities of chlorine gas through said partially purified electrolyte while adding sufficient nickel carbonate to maintain a pH of about 4.0 to about 5.0 whereby said cobalt is oxidized and precipitated, separating said cobalt precipitate from said partially purified electrolyte to obtain purified electrolyte, introducing said purified electrolyte into said cathode compartment, maintaining a sufiicient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment, and depositing nickel of high purity.

6. A process for electro-refining nickel in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel. in said anode compartment with an aqueous solution having sulfate ions and chloride ions as the principal anions and containing about 27 grams to about 30 grams per liter of chloride ions and about 71 to about 120 grams of sulfate ion per liter while maintainin a pH- of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing iron, copper, cobalt, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, removing said impure anolyte from said anode compartment, passing a relatively weak oxidizing agent comprising oxygen-containing gases through said impure anolyte to oxidize said copper to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4 to about 5. oxidizing said reduced arsenic and lead of said impure anolyte to precipitate arsenic and lead, separating said precipitates of iron, arsenic and lead from said impure anolyte to obtain a partially purified anolyte, removing copper from said partially purified anolyte by treatment with nickel powder, separating said nickel powder from said impure anolyte to obtain partially purified electrolyte, adding additional quantities of a relatively strong oxidizing agent to said partially purified electrolyte together with suflicient alkali to maintain a pH of about 4.0 to about 5.0 whereby said cobalt is oxidized and precipitated, separating said cobalt precipitate from said partially purified electrolyte to obtain purified electrolyte, introducing said purified electrolyte into said cathode compartment, and maintaining a sufilcient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

7. A process for electro-refining nicke1 in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel in said anode compartment with an aqueous solution comprising essentially about 2'7 to about 30 grams of chloride ion per liter, about '71 to about 120 grams of sulfate ion per liter, about 20 to about 30 grams of sodium ion per liter, and about 15 to about 25 rams of boric acid per liter while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing about 40 to about 60 grams per liter of nickel, about 0.25 to about 0.45 gram per liter of copper, about 0.12 to about 0.15 gram per liter of cobalt, about 0.07 to about 0.10 gram per liter of iron, about 0.0002 to about 0.0004 gram per liter of lead and about 0.004 to about 0.006 gram per liter of arsenic in the reduced state, continuously removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize said copper to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to about 5.0, treating said impure anolyte with to obtain a purified electrolyte, adjusting'the pH of the purified electrolyte to about 4.0 to about 5.0 by addition of'acid thereto to obtain a catholyte, introducing said catholyte into said cathode compartment, and maintaining a sufiicient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

8. A process for electro-reflning nickel in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel in said anode compartment with an aqueous solution having sulfate ions, chloride ions and borate ions as the principal anions and containing about 27 grams to about 30 grams per liter of chloride ions and about 71 to about 120 grams of sulfate ion per liter while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing iron, copper, cobalt, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, continuously removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize said copper to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to'about 5.0, treating said impure anolyte with sufiicient chlorine gas to oxidize and precipitate arsenic, lead and cobalt while adding enough nickel carbonate to maintain a pH of about 4.0 to about. 5.0, separating said precipitates of iron, arsenic, lead and cobalt from said impure anolyte to obtain a partially purified anolyte, removing copper from said partially purified anolyte by successive treatments with nickel powder of progressively increasing purity, separating said nickel powder from said partially purified anolyte to obtain a purified electrolyte, adjusting the pH of said purified electrolyte to about 4.0 to about 5.0 by addition of acid thereto to obtain a catholyte, introducing said catholyte into said cathode compartment, and maintaining a sufficient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

9. In a process for continuously purifying an impure nickel electrolyte comprising an aqueous solution having sulfate ions and chloride ions as the principal anions and containing about 27 to about 30 grams per liter of chloride ions and about '71 to 120 grams of sulfate ion per liter together with nickel. iron, copper, cobalt, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, the improvement which consists in removing iron in said impure electrolyte by passing air through said impure electrolyte to oxidize said reduced copper of the impure electrolyte to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated within a pH range of about 4.0 to about 5.0 and filtering the solution to remove said iron precipitate.

10. A process for electro-refining nickel in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corrodingimpure nickel in said anode compartment with an aqueous solution having sulfate ions and chloride ions as the principal anions and containing about 27 grams to about 30 grams per liter of chloride ions and about 71 to about grams per liter of sulfate ions while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing iron, copper, cobalt, arsenic and lead in the reduced state, said copper being present in an amount equal to about 3.0 to about 4.5 times the amount of iron, removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize said copper to the cupric state, the cupric copper reacting with said iron in the reduced state to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to 5.0, oxidizing said reduced arsenic, lead and cobalt of said impure. anolyte to precipitate arsenic, lead and cobalt, separating said precipitates of iron, arsenic, 1ead and cobalt from said impure anolyte to obtain a partially purified anolyte, removing copper from said partially purified anolyte by treatment with nickel powder, separating said nickel powder from said partially purified anolyte to obtain a purified electrolyte, introducing said purified electrolyte into said cathode compartment and maintaining a sufflcient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

11. A process for electro-refining nickel in an electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel in said anode compartment with an aqueous solution having sulfate ions and chloride ions as the principal anions and containing about 2'7 to 30 grams per liter of chloride ions and about '71 to 120 grams per liter of sulfate ion, while maintaining a pH of about 4.0 to about 5.0 to thereby obtain an impure anolyte containing iron, copper, cobalt, arsenic and lead as impurities, said copper being present in an amount equal to about 3.0 to 4.5 times the amount of iron, removing said impure anolyte from said anode compartment, passing air through said impure anolyte to oxidize aid copper to the cupric state, the cupric copper reacting with said iron to oxidize substantially all of said iron to the ferric state whereby said iron is hydrolyzed and precipitated without substantial change in said pH of about 4.0 to 5.0, oxidizing said arsenic and lead of the impur anolyte to precipitate arsenic and lead, removing copper from said impure anolyte by treatment with nickel powder, introducing the thus-purified electrolyte into said cathode compartment, and maintaining a sufficient hydrostatic head in said cathode compartment to substantially prevent the flow of anions from said anode compartment to said cathode compartment.

12. A process for electro-refining nickel in an .electrolytic cell having an anode compartment and a cathode compartment separated by a pervious diaphragm which comprises anodically corroding impure nickel in said anode compartment with an aqueous solution containing about 2'7