Zirconium Dioxide, ZrO₂

It has already been stated that zirconia occurs in various minerals, both free and combined with silica and other oxides. Zircon, ZrSiO₄, is the chief source of zirconia and other compounds of zirconium; and in order to prepare this mineral for chemical treatment it is ignited and quickly plunged into cold water; by this means it is rendered pulverisable. The powdered mineral may then be treated in various ways, the chief of which are:

1. Fusion with alkali carbonate or hydroxide.
2. Fusion with potassium hydrogen fluoride.
3. Fusion with potassium hydrogen sulphate.
4. Reduction with carbon in the electric furnace.

Fusion with Alkali Carbonate or Hydroxide

When powdered zircon is fused with alkali carbonate or hydroxide and the melt is treated with water, alkali zirconate remains as a crystalline powder. After washing, this powder is dissolved in dilute hydrochloric or sulphuric acid, and from the solution thus obtained hydrated zirconia is precipitated by ammonia.

Fusion with Potassium Hydrogen Fluoride

The powdered zircon is intimately mixed with about four times its weight of potassium hydrogen fluoride, and gently ignited in a platinum crucible till it is freed from water. The platinum crucible is then placed inside a covered Hessian crucible and the whole is heated in a wind-furnace for two hours. The melt, after cooling, is boiled with dilute hydrofluoric acid, and the silicon is thus eliminated as almost insoluble potassium silici-fluoride, which is filtered off. From the filtrate potassium zirconi-fluoride crystallises on cooling; this salt is recrystallised and then decomposed with strong sulphuric acid. From a solution of the zirconium sulphate thus formed, hydrated zirconia is precipitated by ammonia.

Fusion with Potassium Hydrogen Sulphate

When zircon is fused with potassium hydrogen sulphate and the mass is boiled with water containing sulphuric acid a residue of basic zirconium sulphate, 4ZrO₂.3SO₃.14H₂O, is obtained. This is then decomposed by fusion with caustic soda, so that after washing with water a residue of zirconia remains, which is purified by being dissolved in hot concentrated sulphuric acid and reprecipitated, after dilution, by ammonia.

Reduction with Carbon in the Electric Furnace

Troost endeavoured to eliminate silica from zircon by mixing the latter with carbon, compressing the mixture into a cylinder, and heating it in the electric arc surrounded with an atmosphere of carbon dioxide. Although most of the silica was reduced and vaporised, about 1 per cent, remained in the residue. Moissan and Lengfeld heated a mixture of zircon and sugar-charcoal in the electric furnace; the silicon was volatilised and the zirconium formed a carbide containing a very small amount of silicon. This carbide was decomposed at a dull red heat by chlorine and the resulting chlorides were boiled with concentrated hydrochloric acid, whence zirconium chloride separated almost pure. This was then decomposed in solution by ammonia, and pure hydrated zirconia obtained, free from iron and silicon.

A modification of this process consists in the addition of lime, whereby calcium carbide is formed and becomes the reducing agent according to the reaction:

ZrSiO₄ + 2CaC₂ = ZrC + 2CaO + 2CO + SiC.

According to Wedekind it is best to dissolve the zirconium carbide in aqua regia and then add tartaric acid and hydrogen peroxide, which precipitates the peroxide.

Zirconia may be obtained in a pure state by the ignition of hydrated zirconia, or of the nitrate, sulphate, or oxalate of the metal. Produced thus it is a fine white powder of density 5.489 or 5.4824. When zirconia is fused with borax and the borax is extracted with sulphuric acid the zirconia is obtained in tetragonal prisms, of density 5.71, and isomorphous with rutile and cassiterite, but not with thoria. Crystallised zirconia is also produced when the powdered oxide is heated with hydrogen chloride gas under pressure. Zirconia fuses in the electric furnace; its density at 15° C. after fusion is 5.75 (Lehmann). The coefficient of linear expansion of zirconia is 0.00000084, that of quartz being 0.0000007. The temperature to which it has been heated has much influence on the solubility of zirconia in acids, for, like alumina and some other oxides, it undergoes polymerisation on ignition. Thus after slight ignition zirconia is easily soluble in mineral acids, but after strong ignition it dissolves only in concentrated sulphuric and hydrofluoric acids.

It was observed by Berzelius, as early as 1825, that zirconia, when strongly heated, emits a bright light; and this oxide, when made into rods or plates, has been employed as a substitute for lime in the Drummond light. It was also used by Auer von Welsbach as a constituent of the earlier mantles for incandescent gas light, and by Nernst in the preparation of the rods for his electric glow-lamp. Its power of withstanding high temperature and chemical reagents, and its very low coefficient of expansion make zirconia a suitable material from which to manufacture crucibles. In a crucible made of zirconia mixed with 10 per cent, of magnesia, platinum and quartz can be melted.