Iron Sulphides and Oxides

Iron sulphides
Pyrite, marcasite and pyrrhotite have the tendency of decomposing with cyanidation. Pyrrhotite from this point of view presents the most serious problem; its dissolution carries a supplementary consumption effect of oxygen and a diminishing of the kinetic of gold extraction. Besides, a protective layer of gold sulphide, AuS can deposit itself in the surface of the grains of gold and the ferrous ions re-precipitate the dissolved gold. One stage of oxidation in the basic environment up towards the cyanidation, allows this problem to be resolved somewhat. Pyrite, very frequently associated with gold, has the fact of a different problem, since it is not that soluble. Gold can and is frequently finely mixed; it has a flotation concentration followed by a cyanidation, allowing the treatment of such minerals.

Iron oxides
Some iron hydroxides can some times shape a protecting layer around the grains of gold in the course of cyanidation decreasing in this way the extraction kinetic. This layer of iron oxide can also exist in a natural state. This dull gold concentrates with a lot of difficulty by flotation. Activation by the sulphide dioxide or a treatment of the diluted acid sometimes allows this obstacle to be overcome.

Carbonaceous materials
The carbonaceous materials can present themselves under a number of ways in the minerals: graphite, organic remains, humic acids, etc. These substances have the property of making the gold precipitate after it has been placed in the solution, conducing in this way the loss of the metal in the wastes of the treatment. The specific treatments allow these losses to be somewhat remedied (oxidation, carbon separation). Certain clay can also precipitate the gold cyanides. This precipitation finds its origin in the presence of hydro carbonated absorbed in its surface. But the correct precipitation due to the chemical composition of the clay influences notably on the nature of the ions that is not excluded. It can be pointed out phenomenon of gold precipitation has been put in evidence on certain pyretic minerals. It is useful to remember that exterior pollutions in the same minerals can be the origin of these treatment problems: roots, wooden pieces, plastic, diverse vegetation, flotation reagents, greasy oils etc.