|Magnesium was first prepared by acting on its chloride with potassium. Later, a mixture of magnesium chloride, calcium fluoride, sodium chloride, and potassium chloride, or of carnallite and fluorspar, was used - sodium being substituted for potassium. |
Proposals have been made to reduce the sulphate with iron or hydrocarbons; to reduce magnesium minerals, such as magnesite or dolomite, with coal and ferric oxide; and to reduce the double cyanide of sodium and magnesium with zinc.
The direct reduction of fused magnesia by carbon is violent at 2030° C., and potassium vapour at a strong red heat will also reduce it.
Magnesium is now prepared by electrolysis. Sir Humphrey Davy obtained an amalgam of magnesium by electrolysing a mixture of magnesia and red mercuric oxide between a platinum anode and a mercury cathode. Subsequent experimenters electrolysed solutions of magnesium salts: Becquerel obtained the metal from a concentrated solution of the chloride, and a solution of the double sulphate of ammonium and magnesium was electrolysed at 70°-100° C.
An attempt was made to electrolyse molten double sulphides of magnesium and the alkalies. In 1852 Bunsen electrolysed the molten anhydrous chloride; Matthiessen mixed three equivalents of potassium chloride with every four of magnesium chloride, and Fischer took the natural step of employing carnallite - the naturally occurring double chloride of magnesium and potassium. Magnesium is now chiefly manufactured by electrolysing this fused salt. Carnallite is less volatile than magnesium chloride, and more easily dehydrated without formation of magnesia.
|Gratzel electrolytic call for magnesium |
The electrolysis is performed in an iron pot which acts as cathode. The anode is of carbon, and holes in the porcelain pot surrounding it permit communication between the anodic and cathodic compartments. To maintain the molten condition of the liberated magnesium the cell is placed in a furnace. A number of such cells is usually connected in series with a source of current, so that the P.D. between each pair of electrodes is about 8 volts. A usual current density is about 1000 amp. per sq. metre. The chlorine is carried off by a side pipe, and the molten magnesium collects in the cathodic compartment on the surface of the molten electrolyte. It is protected from oxidation by an atmosphere of inert gas. The holes between the anodic and cathodic compartments are arranged to avoid any passage of the upper layer of magnesium into the anode compartment. The working temperature is apparently between 700° C. and 800° C.
The globules of magnesium do not coalesce in the presence of magnesium oxide or magnesium sulphate. The addition of ammonium chloride during the fusion of the magnesium chloride prevents the formation of oxide. Any sulphate is decomposed by adding enough carbon to the fused chlorides: the oxide thus produced is converted into chloride by ammonium chloride. Oettel found that the addition of calcium fluoride removed the disinclination of the magnesium globules to coalesce.
Swan could say in 1901 that the only company known to be manufacturing metallic magnesium was the Aluminium und Magnesium Gesellschaft, Hemelingen. Their electrolyte contained equimoleeular proportions of magnesium, potassium, and sodium chlorides. It was prepared from carnallite by adding sodium chloride. Anhydrous magnesium chloride was continuously added during the process (which was continuous) to keep the bath composition constant. The electrolyte was kept basic and contained calcium fluoride.
Molinari says that, prior to the Great European War, the world's consumption of metallic magnesium approached 100 tons. American companies are now producing considerable quantities.
Magnesium can be purified by distillation, but it is more usually purified by remelting with pure carnallite in an iron crucible. The floating metal is ladled off and poured into ingots. It is usually converted into wire by squeezing the semifluid metal through dies, and is then commonly rolled into ribbon. It is also handled commercially as powder.
Magnesium metal may contain from 6-17.5 c.c. of occluded hydrogen, and from 1.2-4.1 c.c. of occluded carbon monoxide, in every 20 grm. From 0.05-0.07 per cent, of silica, and about 0.08 per cent, of iron and aluminium, have been found in it. Samples containing small amounts of alumina and ferric oxide have been found to be free from carbon, alkali metals, and alkaline earth metals.