Zinc Sulphide, ZnS

Zinc Sulphide, ZnS, occurs naturally as the valuable ores wurtzite and zinc blende. Zinc blende (sphalerite) crystallises in the tetrahedral group of the isometric system, with a hardness of 3.5-4 and a density of 4.05. Wurtzite crystallises in the hemimorphic group of the hexagonal system, with a hardness of 3.54 and a density of 3.98. Both minerals vary in colour from dark brown to black - blende being often black like coal.

Crystalline zinc sulphide is prepared most directly by subliming the amorphous form in an electric furnace. Since the crystals of zinc sulphide obtained by sublimation are hexagonal, wurtzite apparently represents the more stable form at high temperatures. The transformation temperature of blende into wurtzite apparently varies with the associated impurities. The reverse change, when the temperature falls, is slow, and may occupy two or three days. The sublimation of zinc sulphide, amorphous or crystalline, has been performed in the presence of alumina, and in an atmosphere of hydrogen, nitrogen, sulphur dioxide, or hydrogen sulphide.

Other methods of obtaining crystalline zinc sulphide (wurtzite) are -

1. Heating zinc oxide or silicate in sulphur vapour at a red heat.
2. Interaction between zinc vapour and hydrogen sulphide.
3. Melting together zinc sulphate, barium sulphide, and calcium fluoride.
4. Action of zinc chloride vapour, diluted with carbon dioxide, on some metallic sulphides, especially tin sulphide.

Crystals of blende are said to be obtained by heating zinc sulphide in a solution of hydrogen sulphide under pressure, by subjecting zinc chloride vapour to the action of hydrogen sulphide, and by the action of carbon disulphide on zinc oxide at a white heat.

White amorphous zinc sulphide is precipitated by passing hydrogen sulphide through solutions of zinc salts. Since it is less soluble than the oxide or carbonate, suspensions of these in water are converted into sulphide by hydrogen sulphide. Zinc sulphide is perceptibly soluble in sodium hydrogen sulphide, freely in mineral acids, and somewhat soluble in ammonium chloride on boiling.

The precipitation of zinc sulphide by passing hydrogen sulphide through solutions of its salts is not usually quantitative: this is usually explained by the reversibility of the reaction

ZnCl₂+H₂S ⇔ ZnS+2HCl.
According to Glixelli, the reaction
ZnSO₄+H₂S = ZnS+SO₄

is not reversible, partial precipitation arising from false equilibria, and prolonged passage of hydrogen sulphide, at ordinary temperature, through ¼ and ½ molar solutions of zinc sulphate, precipitates the zinc completely.

When zinc sulphide is precipitated in media in which it is slightly soluble, as in the presence of weak acids, it is either crystalline or becomes so on standing. According to Villiers, it is soluble in sodium hydrogen sulphide at the moment of precipitation, and passes from this etat protomorphique into a crystalline insoluble form with a rapidity which varies with conditions. According to Glixelli, the β-sulphide precipitated from alkaline solutions is forty-six times as soluble as the a-sulphide precipitated from acid solutions.

When zinc sulphide is first precipitated in media in which it is very insoluble it readily goes into colloidal solution. A colloidal solution has been prepared by precipitation with hydrogen sulphide in ammoniacal or neutral solution and washing with hydrogen sulphide water, and by passing hydrogen sulphide into an aqueous suspension of zinc oxide. The addition of glycerine, or other substances increasing the viscosity of the solution, facilitates the formation of these colloidal solutions, which are milky by reflected and orange by transmitted light.

Seligmann obtains a readily filterable zinc sulphide by heating a strongly ammoniacal solution, containing 0.5 grm. zinc in 200 c.c., to 60° or 80° C. and adding a slight excess of ammonium sulphide.

Zinc sulphide is tinged light brown or grey by exposure to light or heating to 60°-70° C. This behaviour, which may be due to polymerisation, is promoted by various substances and inhibited by others, and is important for the use of zinc sulphide as a pigment, whether alone or, mixed with finely divided barium sulphate, in lithophone.

The ordinary sulphide, natural or artificial, phosphoresces after exposure to light. This phosphorescence is affected by traces of other metals, but the data seem to be somewhat contradictory. A similar phosphorescence is also stimulated by exposing the sulphide to the action of ozone. Apparently there is no phosphorescence in ordinary precipitated zinc sulphide, but if it is heated for about one and a half hours at 650°-900° C. it will phosphoresce under light, Becquerel rays, X-rays, cathode rays, and radioactive emanations. The favouring conditions for phosphorescent behaviour seem to be semi-crystalline condition and the presence of chlorine ions. The latter may strain the particles of zinc sulphide by coating them with zinc chloride. The luminescence of crystalline zinc sulphide under X-rays is increased by the presence of 1.30 per cent, of cadmium sulphide in solid solution.

The reaction
ZnS+H₂O = H₂S+ZnO

begins at a dull red heat and is rapid at higher temperatures.

Oxygen converts zinc sulphide into sulphate at 500° C., and the sulphate decomposes into oxide at higher temperatures. Carbon reduces zinc sulphide to metal at a high temperature - carbon disulphide being formed.

The specific heat of zinc blende is about 0.12. According to Regnault, the specific heat of zinc sulphide is 0.12303.

[Zn] + [S] = [ZnS]+43.0 Cal.
[Zn] + [S(rhombic)] = [ZnS(cryst)]+41.3 Cal.
The reaction
ZnS+3CO₂ = ZnO + SO₂+3CO

begins at 750° C. and is vigorous at 1000° C.

There are probably no definite hydrates of zinc sulphide, though some have been reported.

A white pentasulphide, ZnS₅, is said to be precipitated from solutions of zinc salts by potassium pentasulphide, which is decomposed by acids with the evolution of hydrogen sulphide and deposition of sulphur.

An unstable zinc hydrogen sulphide, Zn(HS)₂, may be produced during the action of hydrogen sulphide or sodium hydrogen sulphide on solutions of zinc salts.

The oxysulphide, 4ZnS.ZnO, occurs naturally as voltzite in globular masses with a hardness of 3-3.5 and a density of 4.9-5.0.

The double sulphides, Na₂S.3ZnS, K₂S.3ZnS, Ag₂S.3ZnS, and CuS.3ZnS, have been described.