January 17, 2021

Chemicals Used in Mining - Free Milling Gold Ores

The gold generally occurs as liberated due to oxidation and alteration of sulphide minerals due to chemical action, or native gold occurring in quartz veins or as electrum in silver rich ores. The major characteristic is the gold is easily liberated.

In these ores generally either a combination of Simple Gravity and Conventional Cyanidation

Cyanide is the general descriptive term applied usually to sodium cyanide, NaCN, but the basic formulae are in terms of potassium cyanide, KCN. However, either sodium or calcium cyanide is used in practice.

Typical Free Milling and Refractory Gold Ores Flow Sheet.

The calcium cyanide is sold in impure form analyzing close to 50 per cent. NaCN equivalent and sodium cyanide is sold in various grades from 85 to 98 per cent NacN. The main difference between these alkaline cyanides, aside from their cost, is their relative dissolving power. This depends entirely on the percentage of cyanide radical present.

The equation generally accepted as expressing the reaction of gold in dilute cyanide solutions is as follows:

4Au + 8NaCN + O2 + 2H2O = 4NaAu (CN)2 + 4NaOH

Thus when fresh surfaces of gold are exposed to the action of cyanide in an aqueous solution containing free oxygen, a gold cyanide compound and a hydroxide (alkali) will be formed.

It is still standard practice to express cyanide strengths in terms of potassium cyanide (KCN) even though sodium cyanide is actually used in the plant. The reason for this variation was the fact that the early work in cyanidation was based on the use of KCN.

Oxygen is recognized as an indispensable factor in the dissolution of gold and silver by cyanide solutions, atmospheric air being the customary source. The use of an alkali to provide "protective alkalinity" in the cyanide solution is essential. Fresh slaked lime (calcium hydroxide) is generally the alkali employed. In addition to providing protection for the cyanide, lime also helps with the precipitation of obnoxious salts and improves leaching and settlement and filtration rates.