That cyanide solutions in works practice do not remain pure in a chemical sense is to be expected, having in view their contact during treatment with the many contaminants found in gold or silver ore amenable to cyanidation. Impurities cause a chemical cyanide loss and at times detrimentally affect the extraction of the valuable metals sought. It is fortunate that in the practical application of the process, impurities do not usually accumulate to a prohibitive degree; this is to a large extent due to dilution of the solutions by the replacement by fresh water of the moisture leaving the plant with the residue.

With regard to visual signs of impurities, ferrocyanide colors solutions a brownish yellow of varying intensity. Other impurities are generally insufficient to create a characteristic color.

Impurities have their principal source in and are traceable to the ore constituents, to contamination underground and to secondary reactions in the treatment at the surface. On the Witwatersrand the oxidation of the pyrite content of the banket reef and wall rock is the chief source. The first place, in order of importance, may, therefore, be given to iron and sulphur in their many combinations and to reactions occurring during exposure in stopes through contact with water (containing dissolved oxygen) so freely applied in the course of dust-preventive measures. Pyrite and marcasite (FeS2) and pyrrhotite (Fe7S8), where present, are converted (but to a relatively small extent) into soluble ferrous sulphate (FeSO4), ferric sulphate, Fe2 (SO4)3, and free sulphuric acid (H2SO4), while colloidal sulphur may be set free. These, in their turn, to avoid excessive corrosion of iron and steel equipment underground, are precipitated by the addition of neutralizing lime as hydrated oxides of iron, both in the ferric (Fe2O3.H2O) and ferrous (Fe(OHO)2) state, varying according to the degree of completeness of such neutralization and oxidation. The hydroxide in the ferrous state is particularly soluble in a cyanide solution. When these find their way to the cyanide-treatment plant, either with the ore or through the medium of mine make-up water, ferrocyanide (Na4Fe(CN)6) and thiocyanate (NaCNS) are formed. This tendency to form acid ferrous salts and thus to destroy the oxygen necessary for gold solution is also latent in the ore undergoing cyanide treatment. As a measure of protection, an alkali, such as lime, is therefore provided and is available throughout; also corrective oxidizing treatment is applied.

An oxidation product of the sulphur, sodium thiosulphate (Na2S2O3) plays a part. It is often present in the first effluent solution from sand treatment, and its general effect is, by decomposition in passing through the zinc-precipitation boxes, to form an insulating film of sulphide on the zinc which lowers precipitating efficiency. Complete oxidation of this compound to a sulphate appears difficult to attain in practice. Sodium sulphocyanide (NaCNS) is present in practically all solutions in relatively small quantities. It has not been definitely proved to have a deleterious effect on gold extraction.

The alkaline sulphide, sodium sulphide (Na2S), resulting from the reaction between the Cyanide and ferrous sulphide (FeS) and generally supposed to have a retarding effect on gold and silver dissolution, is not often observed in solutions on the Witwatersrand, a fact which is due undoubtedly to its precipitation as zinc sulphide (ZnS) by the sodium zincocyanide [Na2Zn(CN)4] present in all solutions where zinc is used as a precipitant and also to its oxidation to thiosulphate. In silver extraction its incidence is more pronounced, as the silver itself is often in direct combination with sulphide, as Ag2S in the mineral argentite, pyrargyrite (Ag3SbS3), and in proustite (Ag3AsS3). Lead reagents, such as lead acetate, lead nitrate, or lead oxide, are generally used as a safeguard, acting as precipitants of the sulphide.

Resulting from the abrasion and fracture of steel and iron in ore crushing and grinding, metallic iron is found in all mill pulps. Oxidation of this takes place to a certain extent through dissolved oxygen in the water and solutions employed and the aeration of the sand and slime incidental to the treatment process. Any ferrous oxide thus formed is attacked by cyanide solution and is a cyanicide, since the resultant ferrocyanide is practically useless as a gold solvent.

With the use of zinc as the precious-metal precipitant, various reactions between this and cyanide take place, the principal compounds being zinc hydrate (ZN(OH)2), sodium zincocyanide (Na2Zn(CN)4), and sodium zinc ferrocyanides (Na2ZnFe(CN)6)² and (Na2Zn3Fe2(CN)12). Sodium zincocyanide is a solvent of gold, as shown by Julian and Smart. The amount of zinc dissolved is considerable, but its retention in solution is not cumulative, as it is precipitated by reaction with the sulphide constituents of the ore in the ordinary course of treatment and by the ferrocyanide. Sodium cyanide is regenerated in the same reaction, the cyanide loss, therefore, being much less than at first would be expected.

Calcium is introduced in the form of lime (CaO) for the purpose of providing a neutralizing agent. Its use results in the formation of calcium carbonate (CaCO3) and of calcium sulphate (CaSO4). On the Witwatersrand, the treated mine water used as water supply probably introduces the greater portion of the CaSO4 content of the solutions.

With favorable temperature or saturation conditions it crystallizes out over the entire plant, including the interior of pipes, and may become troublesome. As a physical obstacle and an insulator of zinc, it may be considered objectionable. The use of sufficiently clean water is the best preventive. Sodium carbonate is sometimes used to remove it as precipitated calcium carbonate. Magnesia is introduced to a small extent from underground sources, and finds its way into the solutions as magnesium sulphate (MgSO4) and magnesium carbonate (MgCO3).

Gelatinous silica is often found in cyanide solutions. Its effect in practice is more physical than chemical, in clogging filters and extractor boxes. Its source is the action of acid mine waters on the ore constituents. Silica may also be introduced in the form of calcium silicate as an impurity in lime.

Organic matter is a common source of impurities in cyanide solutions, its reducing effect being notorious. It is usually regarded as having its origin in mine timber, sewage and sacking, coming from underground and from the surface in the form of vegetal matter and sewage contamination of water used in milling. In the self¬decomposition of cyanide solution, organic compounds such as formates are formed. Prevention is the soundest remedy. Failing this, oxidation by means of chlorine oxidizers has proved efficacious where these can be applied directly or in a separate circuit before cyanide treatment. When strong oxidizers are used on cyanide solutions, free cyanide will be lost by conversion to cyanate.

In cyaniding ores containing copper minerals, it is found that the carbonate, oxide, and sulphate minerals particularly are attacked by cyanide with avidity, causing a heavy cyanide consumption by the formation of cuprosocyanide [KCu(CN)2]. In practice this is minimized by the use of extremely weak solutions. Provided that the copper content of the ore is not excessive, it is found that copper does not accumulate in the solution, as it is constantly being precipitated by sodium sulphide.