Preparation of Calcium

The methods by which the problem of the separation of metallic calcium has been attacked may be divided into two classes -

1. Electrolysis of a fused compound.
2. The displacement of calcium from its compounds by a more electropositive metal.

Only the first has risen to the dignity of a commercial process, and that not to any great extent, owing to the limited use to which the metal has hitherto been put.

Davy electrolysed a paste of calcium oxide mixed with the red oxide of mercury, using a platinum anode and a mercury cathode. An amalgam was formed from which distillation in a current of hydrogen failed to remove the mercury completely. Several subsequent investigators attempted the separation by similar methods with more or less success.

The experiments of Bcfrchers and Stockem first made possible the production of metallic calcium on a large scale. They electrolysed used calcium chloride, avoiding a temperature above the melting-point of calcium at the cathode because the molten metal dissolves in calcium chloride, producing the subchloride Ca₂Cl₂. The product obtained round the cathode submerged below the surface of the liquid, was spongy, brittle, and not very pure.

Redlich and Suter obtained a much more compact product by a device which has been adopted in the commercial production of calcium at Bitterfeld by the firm of Siemens and Halske. Calcium chloride, sometimes mixed with calcium fluoride to reduce the temperature of fusion, is used. As calcium burns in air above 800° C., this forms the limit beyond which the temperature must not be allowed to rise. The cathode, a cooled iron rod just touching the surface of the liquid, is gradually raised as electrolysis proceeds, drawing with it the solid calcium protected by a layer of the fused salt. This contact cathode tends to prevent the formation of a " metallic fog," or suspension of finely divided metallic calcium. The current density required at the cathode is about 100 amp. per sq. cm. The metal is obtained in the form of irregular cylinders weighing 200 to 300 grm. The following percentage analysis of Bitterfeld calcium has been given: Ca 97.75, Mg 0.89, Si 0.84, Fe 0.14, Al 0.37. It may, however, reach a purity of 99.3 – 99.6 per cent. Further purification of the commercial product may be effected by distillation in a high vacuum.

Subsequent study of the electrolytic separation of the metal has been mainly concerned with the relative merits of the submerged and contact cathodes, and with the desirability or otherwise of adding another electrolyte to the calcium chloride. Goodwin 5 at first adopted a submerged cathode of wrought iron passing through the bottom of a furnace of Acheson graphite, the walls of which formed the anode. Eventually he replaced this by the contact electrode employed at Bitterfeld, and that appears to be the most satisfactory method.

Goodwin used fused calcium chloride alone for the bath, but Moldenhauer and Andersen used potassium chloride in place of the calcium fluoride of the commercial process, and with a current density of 60-110 amp. per sq. cm. obtained metallic calcium practically free from potassium, with a current efficiency of 75-90 per cent. Brace concluded that pure, completely dehydrated calcium chloride is better than any admixture with another electrolyte. He also found that careful control of the temperature is necessary, and can be secured by varying the separation and degree of immersion of the anodes. Almost the entire charge of the electrolyte should be molten, only a small portion at the sides and bottom of the graphite container being allowed to remain solid.

Wohler described the conditions under which the metal may be prepared on a laboratory scale. He used a mixture of 100 parts of calcium chloride with 12 parts of calcium fluoride, melting at 660° C., and contained in an iron vessel. The cathode was an iron wire of 8 mm. diameter, and a current of 40 amp. at 38 volts was supplied, giving a density of 100 amp. per sq. cm. of cathode surface.

It is also possible to obtain calcium by electrolysis of a concentrated solution of calcium chloride, instead of the fused salt, using a mercury cathode. The amalgam may then be distilled.

These consist essentially in the replacement of calcium in its salts by a more electropositive metal. In the first attempts, in which, for example, calcium iodide was heated with sodium, or with sodium and zinc, or calcium iodide was replaced by a mixture of potassium iodide and calcium chloride, the pure product was never obtained.

Moissan succeeded in getting rid of excess of sodium by utilising the fact that, although it dissolves calcium at red heat, the two metals are not miscible at crystallising-point, and the sodium can be removed by treatment with absolute alcohol, leaving the calcium untouched in the form of brilliant white crystals of 98.9 – 99.2 per cent, purity. It was necessary to avoid contact with oxygen, hydrogen, or nitrogen, and to use pure salts. The fact that the process required sodium and large quantities of absolute alcohol prevented its use commercially.

The difficulties of the displacement method are due largely to the circumstance that, although calcium is less electropositive than sodium at ordinary temperatures, at a temperature of about 800° C., or even lower, it will liberate sodium from its compounds, due either to the greater volatility of sodium at higher temperatures, or to calcium becoming more electropositive than sodium. It will not, however, replace potassium. At bright red heat a mixture of calcium and potassium is obtained.

Other reducing agents have been suggested in place of sodium - for example, magnesium, carbon, and aluminium. Methods have also been proposed for the direct production of calcium alloys, but these will be referred to later.