NO161181B - PROCEDURE FOR ELECTROLYTIC SOLUTION OF POLLUTANEIC NICKEL REFINING ANODES. - Google Patents

Description

The present invention relates to a method for the electrolytic dissolution of contaminated nickel refining anodes, preferably nickel sulphide anodes.

In the electrorefining of nickel, raw nickel anodes are electrolytically corroded, resulting in the dissolution of nickel and some of the impurities in the electrolyte. The electrolyte is cleaned,

and nickel is then cathodically deposited from the purified electrolyte. Typically, the soluble raw anode is only used to supplement

nickel in the electrolyte, the electrolyte being obtained from previous processing operations, e.g. from the treatment of ore, cut stone, concentrates, heated ores, residues, scrap etc. In the usual course of extraction of nickel by electro-refining using, for example, nickel sulphide anodes', a deficit of nickel occurs in the anolyte in relation to the amount that is deposited at the cathode, and there is a corresponding increase in acid content in the anolyte, which lowers the pH e.g. to 1.5-1.9. It is desirable to operate the electrorefining cell at a pH of at least 3 since nickel deposition is ineffective below pH 3

due to hydrogen evolution. Above a pH of approx. 5, nickel will precipitate.

The present invention has among its purposes to provide a method which will remedy the nickel dissolution imbalance in the cell.

The method according to the invention with preferred embodiments is stated in the claims, and reference is made to these.

The drawing schematically shows a diaphragm cell suitable for

use when carrying out the method according to the invention.

According to the invention, contaminated nickel refining anodes are dissolved anodically by passing electric direct current through an electrolysis cell with anode and cathode chambers separated by means of an electrolyte-permeable diaphragm. An aqueous acidic anolyte comprising nickel ions, alkali metal ions, and chloride or sulfate ions or both flows through the anode decariner, while a non-circulating catholyte is held in the cathode chamber, comprising an aqueous alkaline solution containing sufficient hydroxyl ions to prevent migration of nickel ions into the cathode chamber and to mainly only the splitting of water occurs in the cathode chamber, with the release of hydrogen at the cathode.

The procedure depends on the independent operation of the anode and cathode chambers. The catholyte is essentially maintained in a stable state. Hydrogen and alkali metal ions migrate from the anolyte and are available for reaction with hydroxyl ions to form water and alkali metal hydroxide respectively. The water is split cathodically and H2

is taken out at the cathode. The hydroxyl ions that remain from the splitting are available for the migrating hydrogen ions and sodium ions. The catholyte is maintained as an aqueous alkaline solution. The hydroxide is provided as an alkali metal hydroxide, and sufficient hydroxide is present to provide a concentration of alkali metal hydroxide equivalent to 40-80 grams per

liters (gpl) of sodium hydroxide. Migration of nickel ions to the catholyte is prevented by precipitation of nickel hydroxide on the anode side of the diaphragm. To prevent the accumulation of alkali hydroxide in the catholyte, catholyte can be led into the anolyte. This can be done by maintaining a hydrostatic pressure level in the catholyte. The catholyte is refilled by adding water.

The cathode must be a good conductor and resistant to the alkaline medium. Steel, stainless steel and nickel are examples of suitable cathode materials. The cathodes may have any suitable shape suitable for the cell design; they can e.g. be in the form of plates, rods, tubes.

In typical execution of nickel refining, raw nickel electrodes are dissolved in a sulfate-chloride-boric acid electrolyte. The bath will also contain alkali metals. The invention is not limited to the use of any particular electrolyte or raw anodes. The anodes can, for example, come from nickel concentrates, cutting stone, residues, scrap etc. However, the invention is particularly suitable for soluble nickel anodes with a relatively high sulfur content, e.g. over 20%, e.g. 22-30% S.

The diaphragm separating the anode and cathode chambers must be of electrolyte-resistant material having low electrical resistance and providing electrolytic contact between the anolyte and the catholyte. In general, diaphragms of the type used in cells for chlorine production will be suitable. Examples of suitable materials are asbestos and fluorinated hydrocarbons such as polytetrafluoroethylene modified for wettability. Suitable materials for a diaphragm for use in electrolytic cells for carrying out the method according to the invention are commercially available, e.g. under the names KANEKALON<®> (a product of Kanefuchi Chemical Industry Co.) and DYNEL® (a product of Union Carbide).

The cell is typically operated with an anode current density of approx. 200 amps per square meters (A/m<2>). The cell is kept at 55-65°C, typically at approx. 60°C.

It is a particular advantage of the present invention that the cell can be operated so that no nickel will be deposited on the cathodes even under disturbed conditions. Instead of penetrating into the cathode chamber, nickel ions are precipitated on the anode side of the diaphragm. Economically speaking, this can mean considerable savings in energy and filtration steps compared to chemical dissolution of nickel-containing concentrate outside the electrorefining cell.

The method according to the present invention is illustrated with reference to the accompanying Figure,

which is a schematic electrolysis cell 10 with a chlorine-resistant diaphragm 11 that separates the anode chamber 14 from the cathode chamber 15. Electrolyte is led to the anode chamber 14 through inlet line 16 and withdrawn through outlet pipe 17.

Electrical connections are not shown. In the electrolysis cell 10, anode 13 is a contaminated nickel refining anode, and the cathodes 12 are made of stainless steel rods.

In operation, an output electrolyte typically composed of 70-90 gpl Ni<++>, 30-55 gpl Cl", 50-100 gpl SO and 15-35 gpl Na at a pH of about 1.5 is passed to the anode chamber 14 of the cell 10 through inlet pipe 16. In the anode chamber, the pH of the electrolyte is kept on the acidic side, advantageously at about 3, but not higher than about 5. This causes the electrolyte to flow through the anode chamber, and the anode chamber in the cell is emptied through outlet pipe 17. The anolyte and the catholyte , separated by the diaphragm, is in electrolytic contact, and the anode and cathode are connected electrically (not shown).The cell is operated, for example, with an anode current density of 100-500 A/m<2>, eg 200 A/m<2 >.The effluent stream at a pH of 3 or higher is withdrawn from the cell.Several cells can be used in the system.The effluent stream from each of the cells is combined into a contaminated electrolyte that can be purified.

In the cathode chamber 15, the catholyte, which is an aqueous medium containing e.g. sodium hydroxide, a non-circulating electrolyte which is maintained so that the pH is kept higher than about 12. The starting catholyte can, for example, be an aqueous solution containing 40 gpl NaOH and approx. 58 gpl NaCl. A hydrostatic head is maintained in the cathode chamber to provide a drain rate from the catholyte to the anolyte through the diaphragm so that the proper alkalinity of the catholyte is maintained, and water is added to the catholyte to replace the drain to the anolyte.

Impurities in the electrolyte will vary. An anolyte can contain contaminants such as copper, lead, arsenic, cobalt and iron in solution. As indicated above, the anolyte may be directed to a purification system (not shown) for removal of such contaminants. The contaminants can be removed by conventional chemical, physical and/or electrical aids. Copper, for example, can be removed from solution to very low levels by cementation with metallic nickel or by precipitation with H2S. Iron is removed by air supply and hydrolysis. Cobalt, arsenic and/ lead are removed by adding chlorine gas.

Nickel carbonate is used to neutralize the acidic

effect that occurs when cobalt and other pollutants

ger is precipitated from the solution. Electrolytic processes for the removal of cobalt, iron, arsenic and/or lead contaminants from contaminated nickel refining electrolytes

ter where cups have been removed, is described in US patent 3 983 018 and in NO patent application no. 83.2872, submitted at the same time as this application. In such processes, nickel hydroxide and hydrogen are formed at the cathode of an electrolytic cell, and elemental chlorine, which is the agent that removes cobalt, iron, arsenic

and lead contaminants, are formed in situ. The precipitation of such pollutants are time-dependent reactions that can be carried out in a separate tank. After removal of the precipitates, the purified electrolyte is returned to a cell for

deposition of nickel at the cathode. Precipitated contaminants can be removed, e.g. by filtration, and the purified anolyte is suitable e.g. for disposal as very pure nickel, e.g. in an electroextraction cell.

In order to give those skilled in the field a better understanding of the invention, the following illustrative examples are given. IN

tests, power consumption is calculated by including dissolved nickel in the calculation (determined by anode weight difference

clay and nickel content in the anode) and the nickel that is effectively dissolved in the anolyte (obtained by subtracting the nickel

kel present in precipitated nickel hydrate, from the first

said value). All tests in the examples are carried out in an electrolysis cell similar to the one shown schematically in the figure.

In each of the tests, a nickel sulphide refining anode is immersed

in a flowing anolyte, and two nickel cathodes in the form of pla-

ter is immersed in a non-flowing sodium hydroxide.

The anode and cathode chambers are separated by a diaphragm

of heat-treated KANEKALON (. r1 Anode and/or cathode bags are used where indicated.

When current is passed through the cell (in the case of anode current-

densities from 200 to 500 A/m<z>) is the main reaction at the anode nickel solution, whereby the Ni content is supplemented in the current

mende anolyte. Water splitting with H» evolution takes place at the cathode with the formation of NaOH, Na being provided

from the anolyte through the diaphragm. The NaOH concentration is expected to reach a constant level due to diffusion of NaOH into the anolyte, where I^SO^ (if present in the anolyte) is neutralized. The experimental equipment is designed so that the electrolyte is brought from a main reservoir to a container with a constant level, where it is preheated to create a temperature of approx. 55°C in the electrolysis cell.

EXAMPLE I

The purpose of this example is to illustrate the advantage of the presence of I^SO^ in the anolyte.

A summary of experimental conditions and results

for comparison tests A and B are shown in Table I. To maintain a positive hydrostatic pressure in the catholyte, water is added in both tests to the cathode chamber at approx. 1.5% of the flow rate of the anolyte to the anode chamber. Both tests are performed according to the present invention. (However, the test conditions are not optimised). In Test A, the anolyte does not contain I^SO^. In Test B, the anolyte contains 5 gpl I^SO^.

In both tests, the litter yield for nickel dissolves

ning 92% (by weight loss), and no nickel is deposited at the cathode. The power consumption is approx. 3 kW per kg nickel dissolved. The results in Table I show that I^SO^ must be added to the starting material to prevent precipitation of nickel as mixed Ni(OH) 2 -NiSO^ in the vicinity of the diaphragm.

Acids other than I^SO^ may be used. However, it is advantageous to use ^SO^ because it is less expensive than, for example, HC1. Furthermore, with HC1 there is the possibility of Cl2 formation, if Cl accumulates in the system.

EXAMPLE II

The purpose of this example is to illustrate the effects of recycling electrolyte to the starting material container and adding water to the cathode chamber and also to show that contaminated nickel anodes can be corroded according to the present process with high electrical efficiency.

A summary of experimental conditions and results is

shown in Table II. The tests are performed at relatively high current densities and electrolyte flow rates to shorten the duration of each test. The composition of the nickel refining anode for each of the tests is shown in Table III. The catholyte composition for each of the Tests

D, E and F are shown in Table IV. Analyzes of starting material

the ale (anolyte) and the outlet current from the anode chamber are shown in Table V.

In test D according to this example, electrolyte is led again

nom the cell without anolyte recycling, air bubbling or water addition to the catholyte. Under the conditions of Test D, only 26% of the acid present in the starting electrolyte is neutralized, and paradoxically, 63% is found

of the dissolved nickel as hydrate, precipitated on the diaphragm and collected at the bottom of the cell. Increased agitation of anolyte

ten is provided in the two subsequent trials, Tests No. E(a) and E(b) by recycling approx. 80% of the electrolyte. IN

Test No. E(b) air is bubbled through the anolyte as well. The results

indicates that increased agitation by recirculation and air passage

blowing did not significantly improve acid neutralization or prevent precipitation of hydrate.

As indicated in Table IV, the concentration of NaOH in the cathode chamber is always increasing during Tests E(a) and E(b)

without it reaching a stable level at which some extensive anolyte neutralization is expected to take place. To remedy this, in Test F(a) intermittent additions of small volumes of water (approx. 50 ml per hour) are made to the cathode chamber to drive the NaOH produced in the catholyte into the flowing anolyte, while all other forms of agitation

ring is maintained. In Tests F(b) and F(c) the addi-

of water to the cathode chamber continuously at flow rates representing 0.7 and 1.4% respectively

of the anolyte flow rate. These additions are negligible in the sense that loss of water by evaporation represents between 3 and 5% of the total volume of electro-

listen after each run. In Tests F(a), F(b) and F(c), most of the starting acid is neutralized, and the amount of nickel hydrate

which is precipitated in the cell, is reduced compared to Tests D, E (a) and E (b).

In all Tests according to Table II, the anode dissolves with high electrical efficiency. The lower overall efficiency of nickel dissolution in Test F is due to the fact that the anode used is almost completely dissolved at the end of the test.

As shown in Table II, the power consumption is approx. 3.5 kW per kg of nickel in Test F (with water addition), but 9-12 kW per

kg of nickel in Tests D and E (without adding water to the catholyte) .

Reference to Table IV, showing catholyte analyzes for the tests, shows that the concentration of sodium hydroxide increased steadily from 3.75 to 5.9 mol (M) during Tests D, E(a) and E(b) conducted without water addition to the catholyte , without it reaching a maximum. In Tests F(a), F(b) and F(c) (with the addition of water to the catholyte) the NaOH hydroxide decreased from 2.3 to 1.4 M. The arsenic concentration in the catholyte was -50 mg/l after 168 hours operation, which represented approx. 0.3-0.4% of the arsenic that was present in the electrolyte.

Analyzes of feed material and effluent stream are shown in Table V. For comparison purposes, the concentration of each effluent stream has been derived, assuming that the sodium concentration was constant. Although the nickel concentration should increase by approx. 1-3 gpl in the outlet streams, the analytical precision overlaps this increase, and no significance can be attached to the nickel values. A stable increase of Co and Cu and no increase of the Fe concentration is observed throughout the tests. As increased during Tests D, E(a) and E(b) at low pH effluent and tended to remain constant in Test F, run at higher pH.

Test results from the oven-dried hydrate precipitates obtained in each test are shown in Table VI. X-ray diffraction of residues from Tests D and E gave a pattern that was an exact counterpart to Ni(OH)2.

The tests according to Example II showed the advantage of adding water to the catholyte in the present method.

EXAMPLE III

The purpose of this example is to demonstrate the beneficial effect of blowing air through the anolyte by a method according to the present invention.

The experiments of this example are carried out in a similar manner to those described in Example II, except that a cloth of Dynel is used to enclose the nickel sulfide anode to prevent mixing of the anode residue and Ni(OH) 2 formed in the anolyte on the treated KANEKALON diaphragm.

Test conditions and results are summarized in Table VII. The results indicate that nickel is dissolved at 90.6% anode current yield and that 94% of the nickel is found in the anolyte, the remaining 6% of the nickel being found in the nickel hydrate precipitate. About. 88% of r^SO^ in the starting material is neutralized.

The above test was continued for 100 hours without air bubbling (Test H). This resulted in precipitation of 74% of the dissolved nickel as nickel hydrate, and consequently only 39% of the starting I^SO^ is neutralized.

The invention is particularly suitable for replacing nickel in a nickel-containing electrolyte for use in an electrorefining process. It can be applied to any system for the recovery of nickel by electrorefining from an electrolyte containing nickel in solution, where at least a portion of the nickel deposited comes from soluble, contaminated anodes that are electrically corroded in a flowing electrolyte, with adjustments in processing conditions that will be clear to professionals in the field.

Claims (9)

1. Process for the electrolytic dissolution of contaminated nickel refining anodes, preferably nickel sulphide anodes, characterized in that the anodes are dissolved anodically by passing a direct current through an electrolysis cell (10) with an anode chamber (14) and a cathode chamber (15) separated by an electrolyte-permeable diaphragm ( 11), wherein an acidic aqueous anolyte comprising nickel ions, alkali metal ions and chloride or sulfate ions or both flows through the anode chamber while a non-circulating catholyte is maintained in the cathode chamber, comprising an aqueous alkaline solution containing sufficient hydroxyl ions to prevent migration of nickel ions to the cathode chamber and that mainly only the splitting of water takes place in the cathode chamber, with the release of hydrogen at the cathode. 2. Method according to claim 1, characterized by atpHi the anolyte is kept at approx. 3. 3. Method according to claim 2, characterized in that the pH of the anolyte is maintained by the addition of sulfuric acid. 4. Method according to one or more of the preceding claims, characterized in that the anolyte in the anode chamber is agitated by bubbling through with air. 5. Method according to one or more of the preceding claims, characterized in that a hydrostatic pressure level is maintained in the cathode chamber by adding water so that the catholyte flows through the diaphragm into the anode chamber and the pH in the cathode chamber is kept essentially constant. 6. Method according to one or more of the preceding claims, characterized in that the atpHi of the catholyte is kept above 12. 7. Method according to one or more of the preceding claims, characterized in that the anode current density is from 100 to 500 A/m<2>. 8. Method according to one or more of the preceding claims, characterized in that the anolyte comprises nickel ions, chloride ions, sulphate ions, sodium ions and boric acid. 9. Method according to one or more of the preceding claims, characterized in that the anolyte is a nickel electrorefining electrolyte, the nickel content therein being thereby supplemented.