Ferrous sulphide, FeS

Ferrous sulphide, FeS, occurs in nature as the mineral troilite, which is found in nodules in the majority of meteorites containing iron. When crystalline it appears to belong to the hexagonal system, and has probably been formed in the presence of excess of iron. It may be obtained by the direct union of iron and sulphur at red heat. If the iron is in the form of filings and is intimately mixed with the sulphur, the mass becomes incandescent when once the reaction has been started. Synthetic iron disulphide, heated above 700° C., is converted into ferrous sulphide.

When iron pyrites, FeS₂, is heated to bright redness in the absence of air or in hydrogen, it yields ferrous sulphide. In crystalline form ferrous sulphide is produced by passing hydrogen sulphide over ferrous oxide at high temperatures or over metallic iron at dull red heat.

If iron wire is used exposed in a bundle to the hydrogen sulphide, it readily becomes encrusted with tiny crystals, silver-white in appearance when first prepared. The crystals are regarded as belonging to the hexagonal system.

The same reaction appears to take place at the ordinary temperature when iron and sulphur are brought into contact under enormous pressures, namely of the order of 6500 atmospheres. The product resembles ordinary ferrous sulphide in that it is homogeneous under the microscope, and evolves a continuous stream of hydrogen sulphide when immersed in dilute sulphuric acid.

Ferrous sulphide has a bluish black appearance, reminiscent of that of magnetic oxide, but it is not magnetic. Density 4.67. It is stable when heated in hydrogen or in the absence of air, but when heated in air it readily oxidises to ferrous sulphate, whilst at red heat all the sulphur is expelled, red ferric oxide remaining.

When exposed to steam at red heat ferrous sulphide is decomposed, yielding hydrogen, hydrogen sulphide, and ferroso-ferric oxide. Thus: -

3FeS + 4H₂O = Fe₃O₄ + 3H₂S + H₂.

At higher temperatures sulphur dioxide and sulphur are also formed.

When heated in a current of chlorine, ferric chloride and sulphur chloride distil over.

When heated in a sealed tube at 150° to 200° C. with thionyl chloride, ferrous sulphide is oxidised to ferric chloride. Thus: -

6FeS + 16SOCl₂ = 6FeCl₃ + 8SO₂ + 7S₂Cl₂.

Ferrous sulphide is reduced when heated with manganese, yielding metallic iron: -

FeS + Mn = MnS + Fe,

the reaction being exothermic. This reaction is of great practical importance in connection with the desulphurisation of steel. Liquid ferrous sulphide freezes at 1171° C., and melts at 1187° C. The heat of formation of ferrous sulphide from iron and sulphur has been determined as: -

[Fe] + [S] = [FeS] + 23,070 calories.
[Fe] + [S] = [FeS] + 18,800 calories.

When heated to about 130° C., both ordinary commercial ferrous sulphide and meteoric troilite undergo a polymorphic change, and, on cooling, a break in the cooling curve is observed at this point. Synthetic ferrous sulphide which does not contain any excess of free iron exhibits no such break, although with 7 per cent, of free iron the transition point is very marked at 138° C., and further addition of iron does not change it. It appears, therefore, that the excess of iron catalytically assists the change in the case of synthetic ferrous sulphide. Troilite, on the other hand, does not contain excess of iron, but possibly its carbon content behaves catalytically in an analogous manner. Ferrous sulphide, stable at ordinary temperatures, is thus known as the a variety, that above 130° C. being termed the β variety. When heated to 298° C. a second polymorphic transformation occurs.

As ordinarily prepared, ferrous sulphide readily dissolves in dilute sulphuric or hydrochloric acid, evolving hydrogen sulphide - a reaction that affords a convenient laboratory method of preparing the gas. When pure, however, ferrous sulphide dissolves extremely slowly in the cold acids. The presence of free iron acts as an accelerator of the reaction, the nascent hydrogen produced by its solution in acid effecting the reduction of the adjacent particles of ferrous sulphide to hydrogen sulphide and metallic iron.

Hydrated ferrous sulphide, FeS.Aq., is readily obtained as a bulky black precipitate on adding an alkali sulphide to a solution of a ferrous salt. If a ferric salt is employed, it is reduced to the ferrous condition with simultaneous precipitation of sulphur. Thus: -

FeCl₂ + (NH₄)₂S = FeS + 2NH₄Cl, 2FeCl₃ + 3(NH₄)₂S = 2FeS + 6NH₄Cl + S.

Hydrated ferrous sulphide is slightly soluble in water, yielding a greenish solution. From electric conductivity measurements its solubility has been calculated as 70.1×10^-6 gram-molecules per litre.

Ferrous sulphide is insoluble in aqueous caustic soda or potash, although the mixture of ferrous sulphide and sulphur formed on adding ammonium sulphide to ferric chloride yields a dark green solution.

Addition of dilute acid causes the evolution of hydrogen sulphide, a ferrous salt passing into solution. For this reason ferrous sulphide cannot be completely precipitated by passage of hydrogen sulphide through a neutral solution of a ferrous salt of a mineral acid, as the reaction is reversible, according to the equation: -

FeX + H₂S ⇔ FeS + H₂X.

In acid solution - for example, sulphuric acid - no precipitate is obtained unless the pressure of the hydrogen sulphide is increased. The greater the concentration of the acid the higher must be the pressure of the hydrogen sulphide.

Ferrous sulphide may, however, be precipitated from solutions of ferrous salts in the presence of sodium acetate - a fact that was known to Gay-Lussac - and even from ferrous acetate in the presence of acetic acid, and from solutions of iron in citric or succinic acids.

Ferrous sulphide is oxidised by acidulated hydrogen peroxide solution, yielding ferric sulphate or hydrolysed products of this salt. With ammoniacal zinc chloride no reaction occurs at the ordinary temperature, but at 160° to 170° C. in a sealed tube ferrous hydroxide and zinc sulphide are produced.

Ferrous sulphide unites with other metallic sulphides to form stable double compounds. Many of these occur in nature as minerals, a few of the more important being pyrrhotite or magnetic pyrites, 5FeS.Fe₂S₃; Pentlandite, 2FeS.NiS; marmatite, FeS.4ZnS; and Daubreelite, FeS.Cr₂S₃.

These and other more or less stable sulphides have been prepared in the laboratory. Thus 3FeS.2MnS is formed when ferrous and manganous sulphides are fused together. It melts at 1362° C., and forms solid solutions in all proportions with manganous sulphide.

FeS.Cr₂S₃ results on heating a mixture of iron, chromium hydroxide, and sulphur, as a black insoluble compound.

FeS.Al₂S₃ results when ferrous sulphide or pyrites is reduced with metallic aluminium. Thus: -

4FeS + 2Al = FeS.Al₂S₃ + 3Fe,
and 2FeS₂ + 2Al = FeS.Al₂S₃ + Fe.

On melting gold and iron together in the presence of sulphur, FeS.Au₂S is obtained.

A study of the freezing-point curves for mixtures of ferrous and cuprous sulphides appears to indicate the existence of three compounds, namely: -

2Cu₂S.FeS, which is stable at all temperatures below the freezing- point;

3Cu₂S.2FeS, which undergoes a change at 180° to 230° C., metallic copper being set free, and a product rich in sulphur remaining; 2Cu₂S.5FeS, which breaks up into the first compound and free ferrous sulphide at temperatures between 500° and 600° C.

On calcining sodium thiosulphate with ferrous oxalate, Na₂S.2FeS is obtained as bronze-coloured prisms. The corresponding potassium compound, K₂S.2FeS, is formed on reducing potassium ferric sulphide, K₂S.Fe₂S₃, with hydrogen; or by heating iron (1 part) with sulphur (5 parts) and potassium carbonate (5 parts). It yields needle-shaped crystals or thick tablets, resembling potassium permanganate in appearance.

Ferrous thio-antimonite, 3FeS.Sb₂S₃, or Fe₃Sb₂S₆, is obtained on precipitation of a ferrous salt with potassium thio-antimonite.