The process of dissolving gold from the ore with an oxygenated cyanide solution is called cyanide leaching. At present, it has been perfected in terms of process, equipment, management or operation. As mentioned earlier, the dissolution of gold in an oxygen-containing cyanide solution is essentially an electrochemical corrosion process.
The main agents used in the leaching process are cyanide and protective base.
1) Cyanide
The cyanide used in industrial leaching of gold by cyanidation mainly includes potassium cyanide (KCN), sodium cyanide (NaCN), calcium cyanide [Ca(CN)2] and ammonium cyanide (NH4CN).
The cyanide commonly used in the production is sodium cyanide, which is a highly toxic white powder, and commercial sodium cyanide is generally pressed into a spherical or massive form.
Industrially, cyanide melt is also used as a leaching agent. It is a mixture of calcium cyanide, salt and coke which are melted in an electric furnace. In addition to containing 40% to 45% of Ca(CN)2 and NaCN, it also contains some impurities harmful to the cyanidation process, such as soluble sulfides, carbon and some insoluble impurities. It is characterized by low price, but the amount is large, about 2 to 2.5 times that of sodium cyanide. In order to eliminate the effects of harmful impurities, pretreatment should be carried out when using cyanide melt. Through-air processing method is strongly stirred or added to the solution an appropriate amount of lead salts.
In theory, it takes only 0.5 g of sodium cyanide to dissolve 1 g of Au, but in actual production, the consumption of cyanide is 20 to 200 times the theoretical amount, or even higher. The amount of consumption depends mainly on the amount of other components in the ore that can react with cyanide.
2) Protection of alkali
The protection of the base is mainly to maintain the stability of the cyanide solution and reduce the hydrolysis loss of cyanide. The addition of the base in the cyanide leaching is maintained in the leaching tank or in the grinding process of the cyanide ore. When mineral complex composition, containing some, such as pyrrhotite cyanidation process or the like of harmful minerals, the protection of the base is added in the grinding process, beneficial or harmful mineral oxide formed precipitate was removed.
The protective base can be potassium hydroxide and sodium hydroxide, but more commonly inexpensive lime (calcium hydroxide). Should telluride gold ore processing ore strong base such as required degree, or with sodium hydroxide as well.
The amount of the protective base to be added should be an appropriate amount, and the pH of the slurry is generally maintained at 10 to 11. At this time, the CaO mass fraction in the slurry is about 0.01% to 0.02%. Too low is not conducive to the prevention of cyanide hydrolysis. Excessively high, although it can promote the flocculation of negatively charged silicon mud, is conducive to slurry precipitation and liquid purification, but has a significant adverse effect on the leaching rate of gold.
When lime is used as a protective base, it is preferably added in the form of lime milk to facilitate process control.
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