Preparation of Cobalt

The metallurgy of cobalt is complicated by the fact that cobalt ores invariably contain a certain amount of nickel. Since these two metals closely resemble one another in their chemical properties it will be evident that their complete separation on a commercial scale is a matter of considerable difficulty. It is not usually required, however. The details of the actual methods employed in the commercial production of cobalt are kept fairly secret, more particularly as regards the initial stages of the preparation of the crude oxide. We shall, therefore, content ourselves by giving in outline accounts of a few different methods that may be employed. It is convenient to discuss the subject in three sections, namely:

1. Preparation of cobalt oxide from cobalt ores.
2. Purification of cobalt oxide.
3. Preparation of metallic cobalt from its oxide.

Preparation of Cobalt from Arsenical Ores

Wohler's method consists in fusing the finely divided arsenical ores such as smaltite, skutterudite, and cobaltite with three parts by weight of potassium carbonate and three of sulphur. An impure cobalt sulphide results, together with a sulpho-arsenate of potassium, which latter is readily extracted with water. The insoluble residue is again treated in the same manner, and the resulting cobalt sulphide, after extraction with water, is free from arsenic, but it still contains nickel, iron, lead, copper, and bismuth in the form of sulphides. Prolonged roasting in air or treatment with nitric acid converts all the metals into their sulphates, in which condition they are dissolved in water. Passage of hydrogen sulphide through the acidulated solution precipitates the lead, copper, and bismuth as insoluble sulphides, and the solution containing iron, nickel, and cobalt is filtered off. Addition of nitric acid and calcium carbonate at the boiling-point effects the oxidation and precipitation of the iron in a basic condition, leaving the sulphates of cobalt and nickel in solution.

Liebig's procedure consists in heating the finely divided ore to redness with potassium hydrogen sulphate until fumes cease to be evolved. On cooling, the mass contains a soluble double sulphate of cobalt and potassium, which can be extracted with water, leaving a residue containing insoluble compounds, mainly oxides and arsenic derivatives of iron, nickel, and some cobalt.

It is doubtful if these methods have ever been employed for manufacturing purposes. European arsenical cobalt ores have been worked for cobalt oxide by a process similar to that described later for working up arsenical nickel ores. Moreover, a certain amount of cobalt oxide is also derived from the arsenical nickel ores, since these usually contain appreciable quantities of cobalt.

The European arsenical ores have now been largely displaced by those from Cobalt District, Ontario, which have been worked for some years by the Canadian Copper Company, who have employed the following process:

The ore is crushed and ground in ball mills to pass through a 30-mesh sieve. It is mixed with suitable fluxes (limestone and quartz) and smelted in small blast-furnaces having a capacity of 25 to 30 tons per twenty-four hours. The products obtained are (i) flue dust, which is returned to the furnaces, and crude arsenious oxide, which is resublimed and sold; (ii) a silicate slag, which is thrown away unless it contains more than 10 ounces of silver per ton; (iii) crude silver bullion, which is mechanically detached and cupelled to a fineness of 994 before it is sold to silver refiners; and (iv) a speiss of cobalt, nickel, iron, and copper arsenides, containing considerable amounts of silver. The crude silver bullion contains about three-fourths of the silver present in the ore.

The speiss is crushed and ground with 20 per cent, of sodium chloride till it passes a 30-mesh sieve, and then roasted in reverberatory furnaces. The chloridised product is extracted with water to remove unchanged salt and soluble compounds of cobalt, nickel, and copper. The copper is removed from the liquor by treatment with scrap-iron, and the cobalt and nickel are then precipitated with caustic soda, and the precipitate washed, dried, calcined, and ground. It contains about 40 per cent, of cobalt to 3 per cent, of nickel, since the latter is not attacked so readily as the former in chloridising the speiss; the mixed oxides also contain about 15 ounces of silver per ton.

The residue from the chloridised speiss, after extraction of soluble cobalt and nickel salts, is extracted with sodium thiosulphate, to dissolve out silver chloride, which is recovered as the sulphide and reduced to metal. The residue is dried, ground, and smelted with quartz to remove most of the iron as a slag. This slag is reworked with more ore in the blast-furnaces, as it contains silver and cobalt. The new speiss simultaneously produced is treated as described above for recovering cobalt and nickel, copper, and silver. The final residue is dried, mixed with 20 per cent, of sodium nitrate and 10 per cent, of sodium carbonate, and roasted in reverberatory furnace to convert the arsenic into sodium arsenate, which is extracted with hot water. The dried residue has the following average composition:

Cobalt	30.7%
Nickel	28.5%
Arsenic	1.1%
Silver	34.6 oz. per ton

and is sold to cobalt and nickel refiners. Between December 1905 and February 1913, it is stated that more than

40,000,000 ounces of silver, 2,200,000 pounds of cobalt, 1,500,000 pounds of nickel, 4,500,000 pounds of pure white arsenic (As₂O₃),

were produced by the foregoing method.

Preparation of Cobalt from Oxide Ores

Herrenschmidt's method is said to consist in mixing the powdered mineral (wad) to a thin paste with ferrous sulphate solution, and heating to boiling. Sulphates of cobalt, nickel, and manganese pass into solution, whilst iron oxide, silica, and alumina remain behind as an insoluble residue. Thus:

2FeSO₄ + MnO₂ + CoO = Fe₂O₃ + MnSO₄ + CoSO₄,
2FeSO₄ + Co₂)O₃ = Fe₂O₃ + 2CoSO₄.

After filtration, addition of sodium sulphide to the clear solution effects the precipitation of the three metals, cobalt, nickel, and manganese, as sulphides. Digestion with the calculated quantity of ferric chloride oxidises the manganese sulphide to sulphate, which passes into solution. The residue consists of cobalt and nickel sulphides, which are washed and converted into their soluble sulphates by roasting. The sulphates are extracted with water, and converted into chlorides by addition of calcium chloride solution. Their separation is effected as follows: The requisite fraction of the chloride solution is precipitated with milk of lime, and the insoluble hydroxides of nickel and cobalt thus obtained are oxidised to the black hydroxides by treatment with chlorine. The washed precipitate is then introduced into the remainder of the chloride solution and the whole is well stirred and heated, when the black hydrated oxide of nickel passes into solution, displacing the remainder of the cobalt from the solution into the precipitate. The final product is thus a suspension of hydrated peroxide of cobalt in a solution of nickel chloride, from which the cobalt precipitate is removed by filtration, washed, and ignited to the black oxide.

Cobalt oxide obtained by the foregoing methods always contains a little oxide of nickel besides small amounts of other impurities. The following analyses of two Canadian samples of the commercial oxide will serve to illustrate this point:

Co	Ni	Fe	S	As	SiO2	Ca	Ag	Insoluble residue
70.36	1.12	0.82	0.45	0.10	0.20	0.50	-	-
69.2	1.4	0.50	0.54	-	-	0.37	trace	1.46

Since nickel and cobalt closely resemble one another in their general properties, it is, as already stated, generally unnecessary for commercial purposes to effect a complete separation of the two metals. Should such be necessary, however, to obtain a purer oxide of cobalt than the ordinary commercial grade, several methods may be adopted.

The crude oxide is dissolved in hydrochloric acid. To the warm liquor finely divided calcium carbonate is added gradually, with stirring, until no further precipitate is obtained. The precipitate being removed by filtration, the solution is free from iron, arsenic, and silica. The solution is then precipitated with a solution of bleaching powder, added slowly with constant stirring until almost the whole of the cobalt is precipitated as black hydrated oxide. By this means practically none of the nickel is thrown down. The precipitate is washed, dried, and calcined to oxide. It is then boiled with sodium carbonate solution to convert any calcium sulphate into carbonate, and after thorough washing, is treated with very dilute hydrochloric acid to remove the calcium carbonate. Finally the oxide is washed, dried, and calcined.

By treating a crude oxide of composition (1) above in this manner, the product had the following composition:

Co	Ni	Fe	S	As	Ca	SiO2
71.99	0.041	0.11	0.02	none	0.02	none

It will be observed that practically the whole of the nickel may be removed by this simple process.

When it is necessary to remove the nickel completely, the following processes are available:

1. The preparation of cobalt chloropentammine chloride,
   
   
   
   from solutions of cobalt salts containing nickel has been adopted as a useful commercial method of isolating the" cobalt. It is a crystalline salt, reddish violet in colour, nearly insoluble in concentrated hydrochloric acid, and is readily obtained by adding 8 parts of concentrated aqueous ammonia, containing a little ammonium chloride, to 4 parts of crystallised cobalt chloride dissolved in a small quantity of water. After aspirating air through for several hours and then exposing to air for two to three days the mixture assumes a red tint, whereupon it is acidified with hydrochloric acid, and a voluminous precipitate of the cobaltammine is obtained. This is washed with aqueous hydrochloric acid, and may be worked up into any desired cobalt salt. Any nickel present in the original solution remains unaltered, a complete separation of the two metals being thus effected.
2. Cobalt chloride readily dissolves in ether saturated with hydrogen chloride, yielding a blue solution. Nickel chloride, on the contrary, is insoluble, being precipitated as the yellow, anhydrous salt. The separation of nickel from cobalt chloride may thus be effected as follows:
   
   A kilogram of crystallised cobalt chloride, already freed from metals other than nickel, is dissolved in 1600 grams of concentrated hydrochloric acid and poured into 6 litres of ether saturated with hydrogen chloride. After filtration the ether is evaporated off, leaving a residue of cobalt chloride.
3. Fischer's Nitrite Process hinges on the fact that when potassium nitrite is added in excess to a solution of a cobalt salt containing free acetic acid, a yellow precipitate of potassium cobalti-nitrite is obtained. Nickel does not yield an insoluble salt under these conditions, and an effective separation of the two metals may thus be made.
4. Mend's Process. - A very complete separation of nickel from cobalt may be effected by reducing to the metallic condition and removing the nickel in the form of volatile nickel carbonyl by heating in a current of carbon monoxide. Cobalt does not yield a carbonyl under the same conditions.

Commercial oxide of cobalt, as obtained from the smelters, consists essentially of cobalto-cobaltic oxide, Co₃O₄, and a careful study has been made by Kalmus of the conditions under which this oxide may be reduced to metallic cobalt.

Reduction of the Oxide with Carbon

By heating cobalto-cobaltic oxide with carbon in a furnace reduction ensues, carbon monoxide or dioxide or a mixture of the two being evolved according to circumstances. Thus:

Co₃O₄ + 4C = 3Co + 4CO
Co₃O₄ + 4CO = 3Co + 4CO₂.

Charcoal and lampblack effect the reduction more easily than anthracite, complete reduction being effected with the former at 900° C., some 20 to 30 per cent, of charcoal in excess of that required in the above equation being desirable. At higher temperatures the reduction proceeds much more rapidly. By briquetting the charcoal and cobalt oxide, using some organic material, such as molasses, as binder, reduction may be effected at a slightly lower temperature.

In the commercial production of cobalt the oxide is usually heated with charcoal in the manner described for nickel.

Reduction of the Oxide with Hydrogen

This begins at 190° to 200° C., and proceeds easily at 250° C. Between 500° and 700° C. more than 90 per cent, of the oxide is reduced in a few minutes, but further reduction is slow. As the temperature rises the reduction becomes more rapid and complete, whilst at 1100° C. it is rapidly completed. The cobalt must be cooled in hydrogen as it is very susceptible to oxidation. For the production of a pure, carbon-free metal this method is recommended.

Reduction of the Oxide with Carbon Monoxide

Reduction of cobalto-cobaltic oxide to the metal takes place rapidly, and is quite complete at 900° C. Between 350° and 450° C. the reaction is very interesting. At first some oxide is reduced to metallic cobalt; after a time the finely divided metal decomposes the carbon monoxide, depositing solid carbon, presumably in the same way as its analogue, iron, namely:

Co + 2CO = Co + С + CO₂.

At about 600° C. the cobalto-cobaltic oxide is reduced to cobaltous oxide, CoO, and if this is removed from the furnace and exposed to the air it becomes incandescent in consequence of re-oxidation to the higher oxide:

6CoO + O₂ = 2Co₃O₄.

Reduction of the Oxide with Aluminium

This is known as Goldschmidt's process, and hinges upon the fact that the heat of formation of aluminium oxide is 392.6 kilogram calories, which is higher than that of most other metallic oxides - in other words, aluminium is capable of reducing such oxides to the free metal by reason of its greater avidity or attraction for their oxygen. The heat of formation of Co₃O₄ is 193.4 cals. Now to reduce three molecules of cobalto-cobaltic oxide to metal requires 3×193.4 = 580.2 cals. Thus:

3Co₃O₄ = 9Co + 6O₂ - 580.2 cals. (i)

By oxidising 8 atoms of aluminium to 4 molecules of oxide, 4×392.6 = 1570.4 cals. are liberated. Thus:

8Al + 6O₂ = 4Al₂O₃ + 1570.4 cals. (ii)

Combining equations (i) and (ii) we have:

3Co₃O₄ + 8Al = 9Co + 4Al₂O₃ + 990.2 cals.

In other words, the reaction is strongly exothermic and, when once started with an ignition powder such as a fuse of finely divided aluminium and potassium chlorate wrapped in paper (Kalmus), will proceed very vigorously to completion. The metal may be produced in this way in a very pure form containing less than 0.1 per cent, of aluminium and, of course, entirely free from carbon.

Cobalt may be obtained by the electrolysis of aqueous solutions of cobalt sulphate containing ammonium sulphate and hydroxide, using a platinum cathode and anode.

In preparing a specimen of the pure metal for atomic weight determinations, Winkler found the following solution useful:

• 100 c.c. cobalt sulphate solution, containing 11.2 grams of cobalt per litre,
• 30 grams ammonium hydroxide of density 0.905,
• 500 c.c. water,

employing a current density of 0.6 amperes.

The metal obtained in this way contains a small quantity of oxide, but this may be reduced by heating in a current of hydrogen gas.

For laboratory purposes cobalt may readily be obtained by reduction of its oxides by hydrogen at temperatures above 250° C.; by heating the chloride or purpureo salt to redness in a current of the same gas; and by calcining the oxalate either in a closed vessel or in an atmosphere of hydrogen. This third method invariably yields a slightly carburised metal. Cobalt may be obtained in the wet way by precipitation from faintly acid solutions of its salts by metallic magnesium, as also from ammoniacal solutions with metallic zinc.

Electrolytic reduction of the double cobalt ammonium sulphate in the presence of ammonium hydroxide, or of the double oxalate in the presence of excess of ammonium oxalate, yields a very pure metal.

Pyrophoric Cobalt

Pyrophoric Cobalt was described by Magnus in 1825, who prepared it by reduction of its oxides in a current of hydrogen. The best temperature of reduction appears to be 250° C. If the reduction is effected at 700° C. the cobalt is not pyrophoric. Pyrophoric cobalt is a black powder, which burns brilliantly when exposed to the air in consequence of rapid oxidation.