Electrolytic processes for the extraction of nickel

The electrolytic extraction of nickel from its ores has not been successful: firstly, because the ores are too impure, and contain too little metal to be used as anodes or to be dissolved in any other way within the circuit; and secondly, because the preparation of nickel soln. from ores outside the circuit is difficult and expensive. The electrolytic method, however, can be used for nickel-copper mattes, and it has been profitably used in the separation of nickel-copper alloy since the operation is difficult when conducted by a dry process. In separating nickel from its mattes or alloys, the copper is deposited from an acidified electrolyte on the cathode whilst the nickel passes into soln. The soln. is then freed from other metals, and the nickel is deposited from its soln. by insoluble anodes.

The decomposition potential of nickel is greater than that of copper and many other metals. According to M. le Blanc, for instance, the decomposition voltage of nickel sulphate is 2.09 volts, and of nickel chloride, 1.85 volts. Nickel is more electropositive than hydrogen, and the overvoltage on nickel is not great, and accordingly nickel is deposited from feebly acidic soln. This subject was discussed by F. Forster, who showed that in order to get thick deposits of nickel, the electrolyte must be hot, say, 60° to 70°, and the strength of the current, between 3.5 and 8.5 amperes per sq. ft. at 3 to 6 volts according to the nature of the electrolyte. W. Eorchers found with insoluble anodes, using salts of cresolsulphonic acid, 5.5 amperes per sq. ft., and 2 to 2.5 volts were necessary; and K. Brand, using a soln. of the sulphate sat. with ammonia, and a carbon or iron anode, found 2.8 amperes per sq. ft., and 2.4 volts were necessary. According to C. Schnabel, a current of 914.9 amperes is needed to separate a kilogram of nickel per hour; and the energy for this is 24×914.9 watts, or 2.99 H.P.; or allowing for a 12 per cent, loss in converting mechanical into electric energy, and a 25 per cent, loss in the current through conversion into heat, etc., 4.48 H.P., or about 9 kgrms. of coal, are needed for the deposition of a kilogram of nickel per hour.

E. Wohlwill investigated the separation of copper and nickel using an alloy as the anode, and a soln. of nickel and copper sulphates as electrolyte. The separation depends on the soln. of both metals and the precipitation of copper at the anode only at a particular voltage and current density. More copper is deposited at one electrode than is dissolved at the other, for the copper deposited at the cathode is equivalent to the copper and nickel dissolved at the anode; consequently, during the deposition of the copper, the decrease in the conc. of the copper and the increase in the conc. of the nickel proceed rapidly, so that very soon hydrogen is liberated at the cathode. An amount of copper, as sulphate, equivalent to the amount of nickel dissolved at the anode must be added to the electrolyte at intervals. When about 12| lbs. of nickel sulphate per cubic foot has accumulated in the electrolyte, the electrolysis is stopped; insoluble anodes are substituted for the nickel-copper anodes. Lead anodes are used when there is no risk of the formation of lead peroxide as is the case when ferrous sulphate is present. When most of the nickel has been removed from the electrolyte, the mother-liquor is evaporated and crystallized for the metal sulphates. W. Borchers studied the treatment of an alloy of nickel, copper, and iron (2:1:1) used as anode with an acid soln. of copper sulphate as electrolyte.

As soon as the liquor had become sat. with the sulphates of iron and nickel, the copper was precipitated by scrap iron, and the nickel separated from the iron as ammonium nickel sulphate. The latter salt was converted into nickel sulphate.

E. F. Gunther found that the copper deposit is good so long as the conc. of the copper in the electrolyte does not fall below 1 per cent. Poor results were obtained with ammonium nickel sulphate as electrolyte owing to its low solubility. E. F. Gunther also studied the deposition of nickel from soln. of nickel sulphate, using insoluble and soluble anodes - lead, zinc, and copper. Diaphragm cells were employed; two salts were placed in the anode compartment, one which formed a soluble and the other an insoluble salt with the anode metal regenerating at the same time as the first salt. In the experiments with lead anodes, sodium chlorate was used along with sodium chloride, sulphate, or chromate, which form insoluble lead salts. The deposits were good in all cases, but the regeneration with sodium sulphate was not so good. With copper anodes, the solvent and precipitant were respectively sodium sulphate and carbonate. The nickel deposit was fair. With zinc anodes, the solvent salt was sodium chloride or sulphate, and the precipitant was sodium carbonate. The nickel deposits were good. The precipitated salts were not good as pigments. W. Borchers devised a cell suitable for the production of pigments in the anode compartment, and of nickel in the cathode compartment.

In 1877, E. Andre proposed extracting nickel from nickeliferous matte, speiss, or alloys, cast in the form of anode plates, and suspended in dil. sulphuric acid. The current was to be so regulated that the copper alone deposited on the cathode of carbon or copper, leaving a soln. of iron and nickel sulphates in the electrolyte. The iron was to be precipitated from the electrolyte by evaporating the ammoniacal soln. in a current of air. The soln. of nickel sulphate was to be decanted from the precipitated ferric hydroxide, and worked up for nickel sulphate, oxide, or metal. In the last case, the nickel could be obtained by electrodeposition. W. Stahl, and B. Neumann suggested a modification of the process. G. A. Guess added a little glue to the electrolyte for the deposition of nickel, and recommended lead anodes, and a current density of 250 amps, per sq. ft. He also recommended adding finely divided calcium carbonate in suspension to the electrolyte, and to suspend the cathode in a sack diaphragm. The calcium carbonate forms a double basic sulphate of copper which precipitates.

C. Hoepfner suggested several modifications of a process for extracting nickel electrolytically. In one modification of the process, the nickel ore was partially roasted to make the iron insoluble; and it was then extracted with a soln. of calcium chloride containing cupric chloride in the form of anode liquor from a subsequent stage of the process. The copper and nickel dissolved as cuprous and nickel chlorides respectively. The silver and iron were removed chemically, and the purified soln. of calcium, cuprous and nickel chlorides was electrolyzed to precipitate the copper. The remaining copper was removed chemically. The soln. of calcium and nickel chlorides on electrolysis with a sheet nickel cathode, and graphite anode gave a good deposit of nickel. The process was not successful; it was modified by J. Savelsberg and G. Wannschaff. E. Basse and G. Selve added organic substances - acetic or citric acid, glycerol, or dextrose - to neutral or slightly acid soln. of nickel, cobalt, iron, and zinc so as to prevent the precipitation of the hydrated oxides by alkalies. On electrolysis of the soln., made alkaline by the addition of potassium or sodium hydroxide, iron, cobalt, and zinc are deposited on the cathode whilst nickel either remains in soln. or is precipitated as hydrated oxide according to the conc. of the soln., or if the passage of the current is prolonged. Ammonium carbonate is added to the soln. to convert the free alkali into carbonate, and the soln. is then electrolyzed for nickel. D. de P. Ricketts electrolyzed a soln. of nickel and copper sulphates mixed with the sulphate of an alkali metal. Copper is deposited on the cathode and a sparingly soluble double sulphate collects at the bottom of the bath. TJ. le Verrier separated iron and nickel by electrolyzing a soln. containing ammonium nickel sulphate or ammonium nickel chloride and sodium chloride. The soln. was kept alkaline by means of a soluble hypochlorite. The anode was an alloy of nickel and iron. Hydrated ferric oxide was precipitated in the bath, and nickel deposited on the cathode. M. Kugel used hot nickel salt soln. acidified with perchloric, bromic, or sulphuric acid; the anodes wore nickel matte. The acid content of the bath was maintained by the addition of a conc. soln. of a magnesium salt of the acid - e.g. magnesium sulphate for nickel sulphate. H. A. Frasch used for the anode a copper plate on which rested coarsely crushed matte; a layer of sand over the matte served as diaphragm; a soln. of sodium chloride just covered the diaphragm, and over that a dil. soln. of sodium carbonate served as a cathode compartment. On electrolysis the chlorine formed at the anode acted on the matte, forming a soln. of nickel and copper chlorides which were alternately deposited electrolytically on the cathode.

In T. Ullce's process anodes of cast cakes of the bessemerized matte, and sheets of copper as cathodes are employed. The electrolyte was prepared by dissolving granulated matte in sulphuric acid, and it contained about 8 per cent, excess acid. The acid required replenishing from time to time. The copper deposited on the cathode, and copper and nickel dissolved at the anode; the anode slime contained the precious metals. When the percentage of copper in the soln. is small, the liquor is syphoned off, and the residual copper precipitated by sodium sulphide or by filtering through nickel sulphide. The iron in the soln. can be precipitated by adding hydrated nickel oxide. The nickel in the soln. can be precipitated as carbonate, or it can be deposited electrolytically. The precious metals in the slimes are recovered by the process of B. Moebius - 3. 22, 3. In D. H. Browne's process, the copper-nickel alloy is cast into anode plates, and while copper is being deposited, copper sheets are used for the cathode. The electrolyte is a soln. of chlorides of copper and nickel, which is replenished by making it flow continuously through a tower where it comes in contact with nickel-copper matte, a soln. of sodium chloride and chlorine. The chlorine is generated from the nickel chloride soln. The copper is deposited first, and when the nickel in the electrolyte has attained a suitable concentration, the liquor is run off, the copper is first precipitated and then the iron. The soln. is then electrolyzed with carbon anodes and nickel sheet cathodes. The chlorine from the anodes passes up the tower indicated above. In N. V. Hybinette's process the copper-nickel matte is roasted, and the resulting oxides are leached with 10 per cent, sulphuric acid. This removes most of the copper but very little nickel. The residue is then heated with sulphuric acid to the temp, at which the sulphates become anhydrous, and again leached with dil. sulphuric acid. The residue is heated with hydrochloric acid and again leached with sulphuric acid. The solid residue is then smelted, and the metal is cast into anodes for electrolysis. A soln. of nickel sulphate is used as electrolyte. Cathodes consisting of iron plates thinly coated with graphite are employed, G. Haglund studied the process.

J. Gamier treated fused nickel matte in an earthenware tube provided with carbon electrodes, with a current of 23 amperes at 10 volts, and found that the conductivity of the mixture remained very regular, but the voltage gradually diminished, although the temp, of the furnace was practically constant. The following table gives the composition of the original substance and of the products round the anode and cathode after passing the current for an hour and then cooling slowly:

	Sulphur	Iron	Nickel	Copper
Original substance	21.10	33.30	16.30	29.00 per cent
Anode product	16.60	35.40	5.30	39.90
Cathode product	4.70	49.10	19.10	26.13

The sulphur is, to a large extent, eliminated probably as carbon disulphide, and the remainder is concentrated at the anode. The quantity of nickel increases from the anode to the cathode, whilst that of copper increases in the reverse direction. The iron, on the whole, tends to accumulate at the cathode. C. T. Henning discussed the electric smelting of nickel ores.