When the pyrites occur in tailings which have been subjected to the action of the weather for some time before treatment, compounds are formed which are more prejudicial to the solution than the sulphides. Sulphide of iron, FeS2, is oxidised by air and water, ferrous sulphate and free sulphuric acid being formed, thus:

FeS2 + H2O + 70 = FeSO4 + H2SO4

The protosulphate suffers further oxidation, and normal ferric sulphate (Fe2 . 3SO4) is produced, which eventually loses acid and becomes a soluble basic sulphate, Fe2O3 . 2SO3. Other basic salts of complex and unknown compositions appear to be formed also.

W. A. Caldecott has investigated the products of decomposition of iron pyrites contained in slimes accumulated in dams or pits in the Transvaal, with the following results. His view is that the main stages in the oxidation of pyrites or marcasite are as follows:

Whether the decomposition proceeds in this order or not, it is certain that all the products named are found in weathered tailings.

In the presence of such oxidised copper and iron pyrites, the following reactions take place:

• The free sulphuric acid liberates hydrocyanic acid.

H2SO4 + 2KCy = K2SO4 + 2HCy

• Ferrous sulphate reacts on the cyanide, forming ferrous cyanide, which dissolves in the excess of potassium cyanide, so that it does not appear in the free state.

FeSO4 + 2KCy = FeCy2 + K2SO4
FeCy2 + 4KCy = K4FeCy6

The potassic ferrocyanide, if sufficient acid be present, reacts with fresh ferrous sulphate forming a bluish-white precipitate.

FeSO4 + K4FeCy6 = K2Fe2Cy6 + K2SO4

This precipitate oxidises in the air to Prussian blue if free acid is present:

4K2Fe2Cy6 + O2 + 2H2SO4 = 3FeCy2. 2Fe2Cy6 (Prussian blue) +K4FeCy6  + 2K2SO4 + 2H2O

Both these precipitates are decomposed by potash or soda and therefore cannot be formed in their presence. The reactions may be represented as follows:

K2Fe2Cy6 + 2KOH = K4FeCy6 + Fe(OH)2
3FeCy2. 2Fe2Cy6 + 12NaOH = 3Na4FeCy6 + 2Fe2(OH)6

Consequently, if free acid is not present Prussian blue is hardly formed at all, as the solution soon becomes alkaline, and the precipitate is decomposed as fast as it is formed. It follows from these reactions that if the blue colour of Prussian blue is visible in the vats or on the surface of the tailings heaps, an enormous waste of cyanide must have taken place, and the matter should be at once investigated.

• Ferric sulphates are decomposed by potassium cyanide, hydrocyanic acid being evolved and ferric hydrate precipitated.
• A mixture of ferrous and ferric sulphates produce Prussian blue by reacting with potassium cyanide, ferrocyanide of potassium being formed at first as above; the equation is:

3K4FeCy6 + 2Fe2(SO4)3 = 3FeCy2. 2Fe2Cy6 + 6K2SO4

Here again the waste of cyanide is prevented by keeping the solutions alkaline.

• Sulphate of copper, CuSO4, acts differently from FeSO4, cuprous cyanide, Cu2Cy2, being formed, soluble in excess of KCy to K2Cu2Cy4, a compound very prone to decomposition. Copper sulphate also gives a precipitate with potassium ferrocyanide, thus:

K4FeCy6 + CuSO4 = K2CuFeCy6 + K2SO4

• Ferrous hydrate, when formed as above, is instantly dissolved in KCy, thus:

Fe(OH)2 + 6KCy = K4FeCy6 + 2KOH

Ferric hydrate, however formed, does not act on potassium cyanide. Its only action is mechanical, as it collects in a gelatinous mass on the filters and checks the flow of liquid.

Copper and zinc in the condition of hydrates or carbonates are quickly dissolved in preference to the precious metals. If sulphates of these metals are formed in an ore containing limestone or clay, double decomposition occurs with the production of sulphate of lime or alumina, and oxides or carbonates of the heavy metals, which are dissolved by the cyanide, thus:

ZnSO4 + CaCO3 = ZnCO3 + CaSO4
ZnCO3 + 2 KCy = ZnCy2 + K2CO3