The role pH has in affecting gold leaching rates by cyanide and the functions of calcium hydroxide in cyanidation are as follow:
1. For safety and to prevent loss of cyanide by hydrolysis.
2. To prevent loss of cyanide by the action of carbon dioxide in the air.
3. To decompose bicarbonates in mill water before using it in cyanidation.
4. To neutralize acidic compounds such as ferrous salts, ferric salts, and magnesium sulphate in mill water before adding it to the cyanide circuit.
5. To neutralize acidic constituents in the ore.
6. To neutralize acidic compounds resulting from the decomposition of various minerals in the ore, in cyanide solutions.
7. To aid in the settlement of fine ore particles so that clear pregnant solution can be separated from cyanided ore.
8. To improve extraction when treating ores containing, for example, tellurides, ruby silver, which decompose more readily at higher alkalinities.
In some instances, for specific purposes, protective alkalinity in the cyanide solution is maintained by the use of other alkalies such as sodium hydroxide and sodium carbonate. These alkalies, however, usually are not effective settling agents. Although the use of an alkali is essential in cyanidation, many investigators have stated that alkalies such as sodium hydroxide, and particularly calcium hydroxide, retard the dissolution of gold in cyanide solutions. Researchers investigated the effect of calcium hydroxide and sodium hydroxide on the rate of dissolution of gold in cyanide solutions containing 0.10% NaCN. Their work was done on gold discs having a surface area of 10 sq. cm.; 100 ml. of cyanide solution was used and this was aerated continuously throughout the tests. They found that when using calcium hydroxide, the rate of dissolution decreased rapidly when the pH of the cyanide solution was close to 11 and dissolution was practically negligible at a pH of 12.2.
The effect of sodium hydroxide was much less pronounced; the rate of dissolution did not begin to slow up until a pH of 12.5 was reached and even at a pH of 13.4 dissolution was much more rapid than dissolution in a solution of the same cyanide strength containing calcium hydroxide at a pH of 12.2. The effect of the calcium ion on the dissolution of gold was then investigated by adding calcium chloride and calcium sulphate to a cyanide solution. Neither of these salts affected the rate of gold dissolution to any appreciable extent. The solubility of oxygen in cyanide solutions containing various amounts of calcium hydroxide was determined, but no appreciable difference was found between one containing no calcium hydroxide and one containing 0.105 per cent. The investigators concluded that the great reduction in the rate of dissolution of gold in sodium cyanide solutions caused by the addition of calcium hydroxide is due neither to lower solubility of oxygen in the solutions nor simply to the presence of calcium ions. Apparently both calcium and hydroxyl ions must be present to produce the full effect. It should be pointed out, however, that in treating a relatively large piece of gold with a small volume of cyanide solution under theoretical conditions, an effect such as that of alkalinity may be greatly exaggerated as.compared with practical conditions.
https://www.academia.edu/292086/Kinetics_and_Mechanism_of_Gold_and_Silver_Dissolution_In_Cyanide_Solution