Zinc Oxide, ZnO

Zinc Oxide occurs naturally as zincite or red zinc ore crystallised in the hemimorphic group of the hexagonal system, with a hardness of 4-4.5 and a density of 5.5-5.55.

Zinc oxide is prepared on the large scale by heating the metal to near its boiling-point. The vapours burn in the air and a white woolly mass of oxide condenses. It has been known under various names: pompholix and nix alba, or philosophical wool, in ancient times; zinc white, and occasionally flowers of zinc, in modern.

Proposals have been made to prepare it commercially by heating zinc sulphate or various zinc minerals with carbon, and by electrolysing a solution of sodium sulphate between zinc electrodes. The zinc hydroxide formed in the latter process is finally calcined to the oxide.

Amorphous zinc oxide is also obtained by igniting zinc hydroxide or carbonate.

A crystalline oxide results from igniting zinc nitrate, and zinc oxide also sublimes in crystals when (a) zinc is heated to redness in a current of steam; (b) zinc chloride is heated in a current of steam; (c) zinc oxide is heated in a slow current of hydrogen; (d) zinc oxide is heated in a current of oxygen; (e) zinc oxide is volatilised in the electric furnace.

Gorgeu, by heating zinc sulphate with an alkaline sulphate, obtained transparent, nacreous, greenish-yellow or greenish hexagonal prisms that belonged to the hexagonal system and were identical with zincite in hardness and density. He also obtained good crystals by heating equal parts of zinc fluoride and potassium fluoride in the presence of steam.

The artificial crystals are usually hexagonal, though some crystals, artificially produced during the distillation of zinc, belong to the regular system. Moissan obtained his crystals in long transparent needles.

According to Brugelmann, the density of amorphous zinc oxide, prepared by igniting the hydroxide or carbonate, is 5.47, and of the crystals obtained by igniting the nitrate, 5.78.

The specific heat of zinc oxide has been given as 0.12480. Ordinary amorphous zinc oxide is a white powder with a creamy tint, that goes yellow when hot and white again on cooling. The yellow tint in some commercial samples has been traced to cadmium sulphide. Zinc oxide has been said to lose part of its oxygen on heating, and the yellow lower oxide to remain stable if no oxygen is present during cooling. Crystalline samples often have a yellow tinge. The yellow colour is said to appear whether the oxide is heated in an atmosphere of oxygen or carbon dioxide or nitrogen, or in a partial vacuum.

At 18° C., 236,000 parts of water dissolve 1 part of zinc oxide. The precipitated oxide retains small quantities of moisture at high temperatures. Even after ignition it dissolves in ammonium chloride solution or in a solution of ammonia and ammonium carbonate. Acids readily dissolve it.

Hydrogen begins to reduce zinc oxide at 454° C., and the reduction can be carried to completion in dry hydrogen, though the reversible character of the reaction

H₂+ZnO ⇔ Zn+H₂O

renders this difficult. Zinc oxide is also reduced by carbon at a red heat and by heating with magnesium.

The reduction of zinc oxide by carbon is metallurgically important.

According to Boudouard, carbon monoxide is the immediate agent of reduction. The reduction of zinc oxide by carbon monoxide begins at a lower temperature than the reduction by carbon in an atmosphere of nitrogen. According to Lemarchands, zinc oxide can be reduced directly by carbon, and the reduction of the oxide is normally effected simultaneously by carbon and carbon monoxide. The amount of carbon entering into the reaction is intermediate between the quantities required by

ZnO+C = Zn+CO
and
2ZnO+C = 2Zn+CO₂,

because the reduction of zinc oxide by carbon monoxide

ZnO+CO = Zn+CO₂

is more rapid than the reduction of carbon dioxide by carbon. The reaction

ZnO+C ⇔ Zn+CO

proceeds from left to right at 600° C. The reverse reaction is only observable at much higher temperatures. Carbon monoxide can reduce zinc oxide at temperatures as low as 350°-400° C.

Zinc oxide and carbon are converted into zinc vapour and carbon monoxide at 1033° C.

Sulphur dioxide has no action on heated zinc oxide, but chlorine converts it into the chloride.

It is said to dissociate when heated with metallic zinc.

The heat of formation of zinc oxide has been frequently determined, and, according to de Forcrand, it varies from 80.29 to 84.7 Cal. for solid zinc and gaseous oxygen. Apparently the values are higher, as the temperatures at which the oxide is prepared are greater. This may be connected with increasing polymerisation as the temperature of the oxide is raised. Dilute sulphuric acid dissolves zinc oxide much less rapidly when it has been raised to a red heat than when it has been prepared at 350° C. The strongly heated oxide also dissolves relatively slowly in alkaline hydroxides.

Its heat of solution in aqueous hydrofluoric acid has been given as 21.820 Cal.

Zinc oxide is used as a polishing powder, especially for glass. It is employed in the manufacture of dental cements, crystalline glazes, glass, and porcelain. As a pigment it has the advantage over lead compounds of not darkening under the action of hydrogen sulphide.

Zinc ointment, which contains zinc oxide, is a familiar pharmaceutical product.