US3755113A

US3755113A - Method for electrorefining of nickel

Info

Publication number: US3755113A
Application number: US00191040A
Authority: US
Inventors: I Entsev; N Kuntshev; G Haralamplev; D Petrov
Original assignee: Niizm
Current assignee: Niizm
Priority date: 1971-10-20
Filing date: 1971-10-20
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28

Abstract

Method for electrorefining of nickel at high current densities wherein the process is carried out by use of reverse current with a duration of the reversal electrode polarity of 3.0 to 8.0 percent of the total current period and a frequency of reversal of the poles of two to six times in a minute. The process is preferably carried out at a temperature of the solution of 50* to 80*C and with an electrolyte consisting substantially of nickel from 70 to 100 gr/l, as NiSO4; sodium chloride from 40 to 80 gr/l; sodium sulphate from 40 to 70 gr/l; and boric acid from 3 to 20 gr/l. The electrolyte is continuously agitated during the process; the process may advantageously be practiced at high current densities by apparatus capable of thoroughly and continuously circulating the electrolyte.

Description

United States Patent 1191 Entshev et al.

[ Aug. 28, 1973 METHOD FOR ELECTROREFINING OF NICKEL Inventors: lvan Dimitrov Entshev; Nikola Tzanov Kuntshev; Gueorgui Alexandrov Haralamplev, all of Plovdiv; Dimitar Andreev Petrov, Slatitza, all of Bulgaria Assignee: NnzM, Plovdiv, Bulgaria Filed: Oct. 20, 1971 Appl. No.: 191,040

Related US. Application Data Continuation-impart of Ser. No. 798,772, Feb. 12,

I969, abandoned.

US. Cl. 204/1'13, 204/DIG. 9 Int. Cl C22d l/14 Field of Search 204/112, 113, DIG. 9,

204/DIG. 10

References Cited UNITED STATES PATENTS 10/1948 Jernstedt 204/44 lll l ll l ll l Primary Exami'ner-John H. Mack Assistant Examiner-R. L. Andrews Attorney-Arthur O. Klein ABSTRACT Method for; electrorefining of nickel at high current densities wherein the process is carried out by use of reverse current with a duration of the reversal electrode polarity of 3.0 to 8.0 percent of the total current period and a frequency of reversal of the poles of two to six times in a minute. The process is preferably car- 8 Claims, 2 Drawing Figures IN VE N TORS Attorney Patented Aug. 28, 1973 FIG. I

IVAN DIMITROV ENTSHEV NIKOLA TZANOV KUNTSHEV GUEORGUI ALEXANDROV HARALAMPIEV DIMITAR ANDREEV PETROV METHOD FOR ELECTROREFINING OF NICKEL This application is a continuation-in-part of application Ser. No. 798,772, filed Feb. 12, 1969 and now abandoned.

The processes of electroplating with nickel and the electrorefining of nickel of applying the relatively simple method of electrolysis both come under the heading of electrodeposition of nickel. These processes both function by reason of. the changes which take place when electric current is applied by means of suitable electrodes immersed in solutions of a nickel salt. The salt is appreciably ionized, the nickel ions migrating to the cathode and being deposited there, while at the anode the salt is reformed, passing into the ionic form. If an insoluble anode is used, secondary reactions take place which lead to a depreciation of the metal content of the solution and the need, therefore, for other means of maintaining the metal ion concentration. Under the usual conditions of deposition, the ionic migration of the nickel ions is relatively slow. In any case, the transport number of the nickel ion is only a fraction of the total current which is responsible for deposition: hence the need, in practical application of electrodeposition, for some type of movement.

In recent years the deposition of nickel has undergone considerable transformation as the result of specialized research into the conditions which control the properties of the nickel deposit. The deposition process follows the known laws. The ampere-hour is thus associated with definite amounts of nickel which cannot be exceeded; such value may not be attained through failure to achieve optimum quantitative conditions, i.e. that the quantities of substances liberated in electrolysis are l proportional to the coulombs passed per second, including thus the conditions of current and time, and (2) the chemical equivalents of those substances. in fact, the physical character of the deposit is far more important than its theoretical quanity. Many circumstances operate to depreciate these physical properties.

In general, the depositing solution should (1) be of high metal content, (2) have good conducting power, (3) be stable in contact with the metal on which the metal'is deposited, (4) be stable against changes in atmospheric conditions, (5) have a constant metal content during the process, (6) yield compact deposits, and (7) possess the power of throwing. Substances which usually assume the colloidal form, the so-called addition agents, migrate toward the cathode and there'deposit in minute quantities, to induce fine crystalline structure.

In the modern electroplating of nickel the nickel depositing solution comprises nickel sulfate, nickel chloride, and boric acid. Most of the nickel ion content is contributed by the nickel sulfate. This salt is used because it is the least expensive salt of nickel with a stable the cathode potential curve. The boric acid serves as a buffer in the nickel plating solution.

It was recognized, however, that the inherent nature of electroplating baths necessitates limitation of current densities within a certain specific range to secure acceptable deposits. The limiting cathode current density for sound nickel deposits is a function of the nickel ion concentration in the cathode film, which in turn depends on the metal ion concentration of the bath itself. If the cathode current density exceeds the specified maximum which is commonly designated in good practice, the resultant is a coating which is generally dark, flaky, loosely adherent and not acceptable. In fact, the larger amount of nickel sulfate now used in the modern electroplating of nickel not only raises the limiting cathode current density but also lowers the resistivity of the electrolyte, thus reducing power costs and improving plating distribution.

Furthermore, in order to overcome the above-noted limiting factors to an extent that will materially increase production with plated nickel surfaces rated acceptable or better than accomplished in usual practice, and more specifically to decrease, for a given thickness and character of deposit, the time involved in producing the result desired, many processes for electrodepositing have been provided by using predetermined current cyclesaseffective plating currents. Thus, the object of the invention disclosed in Jemstedt, US. Pat. No. 2,524,912, is to provide for depositing a sound and homogeneous electroplate on a base member. The metal is electroplated by applying consecutively and separately, first, a conventional direct current which renders the member cathodic to deposit an increment of metal from the electrolyte, followed by an altemating current alone at a much heavier current density for a brief interval to smooth out and otherwise improve the previously deposited increment of thickness of metal. Repetition of this cycle will build up the electrodeposit to its required thickness. Plating time can be reduced to 25 percent to 50 percent of the time required with direct current alone. It is critical that the voltage and current density of the alternating current be much higher than that of the direct current portion of the cycle. The electrodeposition disclosed is typical for a metal from the group consisting of copper, silver, and brass.

Metal electrodeposits have been produced with highly desirable uniformity and brightness by applying the process disclosed in .lernstedt, U.S. Pat. No. 2,575,712. The electroplating process disclosed in such patent consists in first applying a direct current to the base member and thereafter discharging a condenser through the base member. The electric current to make the base member cathodic is applied with a current density varying from 50 to amperes per square foot; during the discharge of the condenser, which is charged to a potential of from 30 to 1,000 volts, the current density reaches a value much greater than with the cathodic current. It is to be noted, however, that the process of electrodeposit-ing disclosed is typical for a metal from the group consisting of copper, brass, silver, and zinc. Furthermore, the invention is not particularly effective in the case of chromium, since the base metal tends to deplate when subjected to the condenser discharge, rather than deplating the chromium.

Electrorefining has proved to be the most economical process for securing the high purity required for many commercial non-ferrous metals. Impure metal slabs are cast in varying sizes and shapes depending upon the particular metal being refined, to permit vertical hanging of the slabs in the refining cells. Purification of impure nickel is achieved by electrorefining by the electrolytic method. The hot nickel-bearing solution from the anode compartments is pumped over nickel powder to precipitate the copper, and passed through Dorr thickeners to eliminate solids, while air is blown through to oxidize the iron, which is precipitated as ferric hydroxide. The acid set free by the hydrolysis of ferric iron is neutralized by adding a suspension of nickel carbonate in water to the solution. After filtering, the solution flows to the cathode compartments and the nickel is deposited on starting sheets from which it is stripped and cut up to form the marketable product. The precious metals are recovered from the anode slimes from the electrolytic operation. It will be noted that nickel does not behave reversibly at an electrode, and therefore difficulties are encountered in nickel refining since copper and nickel both dissolve at the anode potential required for the nickel purification process to take place at an economical rate. Usually a cell divided into anode and cathode compartments by a porous diaphragm is employed, and the electrolyte in the cathode compartment is known as the catholyte. A process of this kind in which anodes of impure nickel metal are used is described in Bristish Specification No. 569,444, and one in which anodes of nickel sulphide matte are used in U.S. Pat. No. 2,839,461. The electrolyte used is a sulfate-chloride electrolyte.

Processes of electrorefining involve problems that do not arise in the production of decorative electrodeposits of nickel. In contrast to the very thin layers of nickel deposited in decorative electroplating, those formed in the electrorefining of nickel must be thick enough to be self-supporting. Preferably they are at least onetwentieth inch thick, and commonly they are up to 0.4 inch thick.

One of the requirements in electrorefining is that the nickel shall be of very high purity, and for this reason the usual addition agents employed to produce bright nickel deposits must be avoided.

Another requirement in electrorefining is the production of deposits which are stress-free as far as possible, as stressed heavy deposits are prone to crack and warp, thus seriously interfering with the normal production of cathode nickel.

In practice it is found that the surface of the cathode nickel becomes progressively more nodular as the thickness of the cathode increases or-as the current density is increased. This nodularity effectively limits the current density at which nickel cathodes can be grown and the thickness of cathode nickel which can be produced by means of the electrorefining process, and many attempts have been made to produce smoother and thicker electrolytic nickel at higher current densities, because of the inherent economic advantages at the refinery and because such nickel would be more acceptable as an article of commerce.

Although many solutions have been proposed to overcome the foregoing difficulties and to provide smoother, thicker electrolytic nickel, none was entirely successful when carried out on a commerical scale.

Thus, according to the invention disclosed in British Specification 941,103 to International Nickel Company, the nickel is electrodeposited from an aqueous nickel-refining catholyte that is free from reduciblesul phur compounds and that contains in solution from 0.01 to 0.10 gram/liter of an organic cyanide. The process and catholyte may be used in nickel electrorefining processes employing impure nickel anodes, nickel matte anodes, or insoluble anodes, and may be used in the production not only of commercial cathode nickel but also of the thin nickel cathode starting sheets employed in producing commercial cathode nickel.

In the method for electrorefining impure nickel material disclosed in Brandt, U.S. Pat No. 3,114,687, the composition of the aqueous catholyte has a pH of about I to 5 and consists of nickel, sodium, chloride ions, sulfate ions, and boric acid in determined concentrations, as well as a water soluble organic cyanide compound in a concentration of about 0.01 to 0.10 gram per liter. The temperature of the catholyte is maintained between and F. The catholyte is electrolized by passing electric current at a current density between about 5 to about 25 amperes per square foot. The method is applicable in all types of nickel electrorefining electrolytes, including the all-chloride and the allsulfate electrolytes, as well as the sulfate-chloride electrolyte.

The method of another invention disclosed in Le Nickel, British Patent Specification 1,045,630, is applicable with both low and high current density, the latter being (e.g. 2,000 to 3,000 Amp/sq.ft). It proposes the use of an aqueous electrolyte affording a satisfactory anode corrosion and cathode deposit. The ferro-nickel anodes used mainly contain nickel and iron. The method is applicable irrespective of the nickel contents of the anode; however, for the sake of economy, it is desirable to use ferro-nickel anodes having a nickel content higher than 80 percent by weight. If the method is applied by using a high-density current, a fraction of the catholyte must be recirculated at a high rate, and the nickel concentration must be relatively high in order to yield a satisfactory homogeneous deposit.

Many inventions deal particularly with the apparatuses for the thorough and continual agitation of the solution so as to insure a complete uniformity of mixture of the different materials incorporated therein. Siphons are generally used to move liquids from a higher to a lower level when their containers cannot be conveniently moved or tipped. They are also used whenone wises to draw off the upper layers of the liquid without disturbing the sediment at its bottom. In the apparatus disclosed in Gauss, U.S. Patent No. 3,561,602, the tank containing the solution is provided with a centrifugal pump for continually drawing off a portion of the solution, means for driving said pump, a filter, and discharge means leading from said filter directly to the tank and terminating in a horizontal direction below the surface of the solution in the tank. A form of apparatus adapted for carrying out a method of electroplating is disclosed in Main Et Al, U.S. Patent No. 2,576,998, wherein the turbulence of the solution adjacent to the cathodes is maintained by projecting jets of the solution through the body thereof toward the cathodes.

In its broadest aspects the present invention has among its objects the overcoming of the abovementioned limiting factors to an-extent that will materially increase production of high purity nickel by electrolytic refining, such product being as acceptable as that secured in accordance with prior practice.

More specifically, an object of the invention is to decrease, for a given thickness and character of nickel deposit, the time involved in'producing the result desired.

A further object of this invention is to provide for applying nickel to an insoluble electrode employing direct current in combination with the same direct current with its polarity reversed at predetermined intervals.

Another object of the invention is to provide an apparatus for carrying outthe process of electrolytic refining, according to the main object of the invention.

The electrorefining process of the present invention consists in first applying a direct current to the first of the electrodes to render this electrode cathodic, whereby nickel is deposited thereon, and thereafter in reversing the polarity of the direct current applied to thefirst of the electrodes to render this electrode anodic during a predetermined interval, and repeating the cycle until a predetermined thickness of deposited nickel has been produced. Unusual and unexpected results have been obtained by reversing the polarity of the direct current through the electrodes in the cycle. High current densities can be used without disturbing the metallurgical characteristics of the nickel deposit, thereby increasing the effective rate 'of deposition.

The electrodes are supplied with current flowing from a direct current source. A suitable switching means is interposed in the circuit to reverse flow of current to the electrodes. Since the electrodes continuously change their polarity, the terms anode and cathode in the common sense are not appropriate in this case; they will be used only conditionally in this description. The electrode on which nickel is deposited will be called a cathode. It may be a stainless steel mother plate, the surfaces of which are properly prepared; it may also consist of a nickel mother sheet having a predetermined thickness, obtained by the electrodeposition of nickel on a stainless steel mother plate. Both types of cathodes are commonly used in the prior art. The other electrode, which is conditionally called the anode, may be soluble, consisting of impure nickel metal, or nickel matte, or nickel alloys (e.g. ferro-nickel). It may also be insoluble, consisting of lead alloys, graphite, or platinized titanium. Pure titanium cathodes could also be used, but they are too expensive.

At first the direct current is-applied to the cathode on which nickel is deposited, in such manner as to render this, electrode cathodic, at a predetermined current density, so that deposition occurs during a certain time interval. This time interval may vary from about to 30 seconds. In actual practice, it has been found that the current density used may be considerably higher than the current density which is employed for any given electrolyte using continuous direct current only. Thus, the current density according to the present invention may vary from to 100 amperes per square foot without undesirable results. It will be noted that the direct current during this period is constant.

After the period of from 10 to seconds, the switching means is actuated to reverse the polarity of the direct current supplied to the cathode, in such manner as to render this electrode anodic, employing the same current density as that first used in the process. In fact, the current density drops to the value zero and immediately thereafter it reaches a value not greater than with the cathodic current first applied. The electrode remains anodic during a predetermined interval. The period may vary from about 0.3 to 2.4 seconds. During this interval, the direct current also remains constant. Thereafter, the current cycle is repeated. ln other words, the electrodeposition process is conducted by applying reversed direct current flow at a duration of 3 to 8 percent of the whole cycle of the electric current and the frequency of changing the sign of the electrodes is from 2.0 to 6.0 times a minute.

The exact nature of the reaction that takes place during the reversed-current portion of the cycle is not known. It is believed that the reversed current, due to its parameters, reacts during the anodic portions of the cycle to cause a de-polarization of the cathodic electrode. Before electrolysis starts, the single potentials of the electrodes are alike, having a value that depends upon the nickel-ion concentration of the electrolyte. As electrolysis proceeds, it will be observed that the potential of the anode becomes more positive and that of the cathode more negative than the initial, or equilibrium potential. The change of potential of an electrode as a result of electrolysis is known as the polarization of that electrode. One obvious explanation of such a change in potential is the change in concentration that occurs near an electrode as a result of electrolysis. ln the case considered, nickel is deposited at the cathode, and the adjacent solution tends to become less concentrated in nickel ions and nickel solution. The polarization at the electrode thatis caused by the changes in local concentration of metal salts and ions is known as concentration polarization. Certain processes such as diffusion, ion migration, and convection (commonly through vigorous agitation) tend to equalize the concentration at the electrodes and in the bath but never produce entirely uniform concentrations. Thus, the change in potential of the cathodic electrode as the result of the reversal of electrode polarity provides an equilibrium potential and corresponds to a change in concentration of the film of solution at the electrode surface. Consequently, the reversal of electrode polarity equalizes the concentration at the electrodes and in the bath, i.e. uniform concentrations are produced.

One obvious advantage in operation with uniform concentrations is the achievement of a more uniform electrocrystallization of the nickel, and consequently a finer crystalline and close-packed structure could be expected.

Generally, in increase in electrolyte concentration produces a decrease in zeta-potential of the electrical double layer which exists in the neighborhood of the charged electrode surface, and ions of high charge of sign opposite from that of the electrode can completely reverse the sign of the zeta-potential. Consequently, the electrokinetic phenomena associated with the movement of the ions, being related to one-another through the zeta-potential, are affected by the value and the sign of this potential. Excessive cathode polarization at high current densities is a well-known phenomenon in practice. Such a change in potential corresponds to a change in electrolyte concentration, and therefore, affects the movement of the ions. As stated above, the reversal of polarity provides an equilibrium potential and tends to equalize the zeta-potentials of both charged electrode surfaces.

Another obvious advantage in operating with equalized zeta-potentials is the improvement of the deposit which becomes smoother and non-nodular.

It has been observed that after the anodic period of the cycle in accordance with the present invention, the entire deposited nickel is rendered smoother than previous to this period. Also, any nickel dissolved now from'the anodic rendered electrode is driven into the solution immediately adjacent the electrode, so that a relatively great concentration of nickel ions is present when deposition is resumed during the next interval of direct cathodic current through the electrode.

Furthermore, the electrical power consumed to form the nickel crystal grating is released during the anodic period, the resistance between the electrodes drops, and therefore the potential in the bath decreases. The decomposition voltage of water is attained, and hydrogen is liberated at the anodic electrode. The atomic state of the hydrogen liberated at the anodic electrode causes the action de-polarization of that electrode.

It is clear that the reversal of polarity is beneficial to both the anodic and the cathodic processes, the surface of the anode is actively attacked, and deleterious passivation of the anode is avoided.

It should be emphasized that the upper limit of time during which the electrode is rendered anodic and the above-noted beneficial processes take place, is quite critical. It has been observed that employing a current cycle with a greater anodic period than 8 percent of the whole cycle of the electric current, is not effective for its intended purposes.

The invention is applicable in all-chloride and allsulfate electrolytes, as well as in sulfate-chloride electrolytes. In accordance with the invention it is advantageous to electrolyze an aqueous catholyte having a pH of about 1.0 to 4.0 and containing about 70 to about 100 grams per liter of nickel, about 40 to about 80 grams per liter of sodium chloride, about 40 to about 70 grams of sodium sulfate (in general more than 30 grams per liter sodium), and about 3 to about grams per liter of boric acid. The impurities, such as copper, iron, cobalt, carbon, arsenic, lead, etc. should not exceed 0.045 grams per liter.

As explained before, the anode may be either an impure nickel anode or a nickel matte anode. The cathode is usually a nickel starting sheet. The temperature of the electrolyte should be maintained between about 50C and about 80C. The process is also operable when utilizing insoluble anodes. It may be carried out without the addition of any surface-active agent.

Although the invention is not limited to the use of any particular apparatus for the circulation of the electrolyte within the electrorefining cell, it may advantageously be practiced, when electrorefining of nickel at high current densities is carried out, with the use of an apparatus capable of thoroughly and continuously circulating the electrolyte. Apparatus of that character is illustrated in the accompanying drawing, in which:

FIG. 1 is a vertical longitudinalsectional view of an electrorefining cell provided with an apparatus for circulating the electrolyte; and

FIG. 2 is a view in vertical transverse section through an anode compartment of the electrorefining cell in FIG. 1.

The electrorefining tank 1 employed is a compartmented cell divided into anode compartments 3 and cathode compartments 2, by means of permeable diaphragms 4. The tank 1 is filled to a point below the top with the solution 10. A collecting tube 7 leads from the purification plant to the longitudinal side 11 of the tank. Several pipes 6 are disposed on the tank side of the collecting tube 7 and lead down toward the cathode compartments where they discharge at the upper endside of the penneable diaphragms 4 a short distance above the free surface of the solution in the vertical direction. The opposite longitudinal side 12 of the tank is provided with an overflow arrangement 5.

The circulation of the electrolyte within the diaphragm compartment is achieved at a rate of flow of 40 to 300 ml/Ah, the higher rates corresponding to the heavier current densities. The electrolyte enters through the pipes 6, flowing in a downward movement into the cathodic diaphragm compartments 2, then out of the diaphragm walls 4 of the compartments and through a bottom longitudinal slot 8 in the opposite wall 12 of the tank with respect to the inlet pipes 6. Then in an upwards stream the electrolyte, now as anolyte, flows out through a top longitudinal passage 9, disposed outside of the tank wall 12 (on the principle of communicating vessels). In other words, the circulation of the electrolyte is actually achieved in a direction parallel to the cathodic surfaces and perpendicularly to the direction of the distribution of the electric force lines and the ions.

This mode of circulation of the electrolyte, along with the current reversal, has the advantage of decreas' ing the limitations due to the diffusion effects and of diminishing the concentration of the impurities in the solution, as well as in the cathodes.

Polyvinyl chloride is the material preferably used for the diaphragms. The difference between the levels of the catholyte and anolyte is maintained in the range from 8 20 mm.

The following examples will illustrate the advantages of the present invention.

EXAMPLE I Sulfate-chloride electrolyte consisting of about grams per liter of nickel, about 60 grams per liter of sodium chloride, about 60 grams per liter of sodium sulfate, about 8 grams per liter of boric acid, and about 0.03 grams per liter of impurities in total, was electrolyzed to deposit cathode nickel at a direct current density of about 50 amperes per square foot and by reversing the polarities of the direct current through the electrodes in a cycle comprising a time interval from 20 seconds to render the electrode on which nickel is deposited cathodic, followed by a period of reversed current flow at a duration of 5 percent of the whole cycle of the electric current, to render this electrode anodic. In other words, the frequency of changing the sign of the electrodes was 4.0 times a minute. The anodes were of impure nickel, consisting of about percent of nickel, about 4 percent of copper, about 2 percent of iron, about 0.8 percent of cobalt, about 1 percent of sulphur, a.o., all percentages by weight. The temperature of the electrolyte was maintained at about 65C. The cathodes were of titanium sheets. The circulation of the electrolyte into the cell was achieved in accordance with the construction shown in FIGS. 1 and 2, at a rate of flow of ml/Ah. After about 80 hours deposit under the above conditions, the titanium sheets coated on each face with a layer of electrolytic nickel were extracted from the bath, scoured and washed. The

nickel plates were separated from their supporting sheets and cut to standard dimensions to yield the final product. The nickel obtained was of high-purity of 98.97 percent by weight. The cathodic deposit takes place with an efficiency attaining 94 percent for the current applied. The technological electric energy consumption was about 2,200 kWh/ton Ni.

EXAMPLE 2 Sulfate-chloride electrolyte consisting of about 90 grams per liter of nickel, about 70 grams per liter of sodium chloride, about 80 grams per liter of sodium sulfate, about grams per liter of boric acid, and about 0.025 grams per liter of impurities in total, was electrolyzed to deposit cathode nickel at a direct current density of about 100 amperes per square foot and by reversing the polarity of the direct current through the electrodes in a cycle comprising a time interval of 12 seconds to render the electrode on which nickel is deposited cathodic, followed by a period of reversed current flow at a duration of 7 percent of the whole cycle of the electric current, to make the same electrode anodic, i.e. the frequency of changing the sign of the electrodes was 5.0 times a minute. The anodes of impure nickel have had the same composition as those used in Example 1. The cathodes were also of titanium sheets. The circulation of the electrolyte into the cell was achieved as shown in FIGS. 1 and 2, at a rate of 250 ml/Ah. After about 50 hours deposit, the nickel obtained was of high purity, being 99.95 percent nickel by weight. The efficiency attained was 92 percent for the current applied. The electric energy comsumption was 2,600 kWh/ton Ni.

Although the invention is illustrated and described with reference to a plurality of preferred embodiments thereof, it is to be expressly understood that it is no way limited to the disclosure of such a plurality of embodiments, but is capable of numerous modifications within the scope of the appended claims.

What is claimed is:

1. Method of electrorefining nickel at high direct current densities from to 100 amperes per square foot with an aqueous electrolyte solution, comprising carrying out the process by reversing the polarity of the direct current through the electrodes in a cycle comprising a time interval of 10 to 30 seconds corresponding to a frequency of reversing the poles of two to six times in a minute to render the electrode on which nickel is deposited cathodic followed by a period of reversed current flow at a duration of 3 to 8 percent of the total current period to make the same electrode anodic.

2. Method of electrorefining nickel as claimed in claim 1, wherein the process is carried out at a temperature of the solution of 50 to 80C and with an electrolyte solution consisting substantially of nickel from to 100 gr/l, sodium chloride from 40 to gr/l; sodium sulphate from 40 to 70 gr/l; and boric acid from 3 to 20 gr/l.

3. Method of electrorefining nickel as claimed in claim 2, comprising circulating the solution at a rate of flow of 40 to 300 ml/Ah.

4. Method of electrorefining nickel as claimed in claim 3, comprising feeding the solution on the top of the cathode, and flowing the solution out from the bath in a self-draining condition in an upward stream by the use of a siphon.

5. Method of electrorefining nickel, as claimed in claim 1, wherein the duration of the electrode polarity reversal is about 5 percent of the total current period and the frequency of reversing the poles is about four times in a minute.

6. Method of electrorefining nickel, as claimed in claim 5, herein the process is carried out at a temperature of the solution of about 50 to 80C, with an electrolyte solution consisting substantially of nickel of about 80 gr/l; sodium chloride of about 60 gr/l; sodium sulphate of about 60 gr/l; and boric acid of about 8 gr/l, and wherein the direct current density of about 50 amperes per square foot.

7. Method of electrorefining nickel, as claimed in claim 1, wherein the duration of the electrode polarity reversal duration is about 7 percent of the total current period and the frequency of reversing the poles is about five times in a minute.

8. Method of electrorefining nickel, as claimed in claim 7, wherein the process is carried out at a temper ature of the solution of about 50 to 80C, with an electrolytic solution consisting substantially of nickel of about g/l; sodium chloride of about 70 g/l; sodium sulphate of about 80 g/l; and boric acid of about 15 g/l, and wherein the direct current density is about amperes per square foot.

Claims

2. Method of electrorefining nickel as claimed in claim 1, wherein the process is carried out at a temperature of the solution of 50* to 80* C and with an electrolyte solution consisting substantially of nickel from 70 to 100 gr/l, sodium chloride from 40 to 80 gr/l; sodium sulphate from 40 to 70 gr/l; and boric acid from 3 to 20 gr/l.
3. Method of electrorefining nickel as claimed in claim 2, comprising circulating the solution at a rate of flow of 40 to 300 ml/Ah.
4. Method of electrorefining nickel as claimed in claim 3, comprising feeding the solution on the top of the cathode, and flowing the solution out from the bath in a self-draining condition in an upward stream by the use of a siphon.
5. Method of electrorefining nickel, as claimed in claim 1, wherein the duration of the electrode polarity reversal is about 5 percent of the total current period and the frequency of reversing the poles is about four times in a minute.
6. Method of electrorefining nickel, as claimed in claim 5, herein the process is carried out at a temperature of the solution of about 50* to 80*C, with an electrolyte solution consisting substantially of nickel of about 80 gr/l; sodium chloride of about 60 gr/l; sodium sulphate of about 60 gr/l; and boric acid of about 8 gr/l, and wherein the direct current density of about 50 amperes per square foot.
7. Method of electrorefining nickel, as claimed in claim 1, wherein the duration of the electrode polarity reversal duration is about 7 percent of the total current period and the frequency of reversing the poles is about five times in a minute.
8. Method of electrorefining nickel, as claimed in claim 7, wherein the process is carried out at a temperature of the solution of about 50* to 80*C, with an electrolytic solution consisting substantially of nickel of about 90 g/l; sodium chloride of about 70 g/l; sodium sulphate of about 80 g/l; and boric acid of about 15 g/l, and wherein the direct current density is about 100 amperes per square foot.