Neutralization Circuit

After the autoclaving circuit, the slurry is sent to neutralization to increase the pH from 1.5 to 10.5 in preparation for cyanide leaching. The slurry on discharge from the autoclave has 15 to 20 grams per liter of sulphuric acid. The acid is neutralized with milk of lime. Cyanidation is practiced above a pH of 9.5, otherwise the cyanide is volatized, thereby increasing costs. Also, below pH 9.5 copious amounts of HCN is released which is a worker health and safety concern.

During neutralization pure oxydgen and/or air is added to convert as much as possible ferous iron to ferric iron. (??) Ferrous iron amines cyanide, level increasing costs (??). Also the addition of oxygen and/or air increases the dissolved oxygen content in the solution. The dissolved oxygen is necessary for the dissolution (??) of gold according to the (??) equation.

From neutralization the slurry proceeds to the carbon-in-leach (CIL) process for gold dissolution and adsorbtion.