Question

Write the chemical reactions involved in the process of extraction of gold. Explain the role of dilute nacn and zn in this process.

Answer 1

The ore is first finely ground and may be concentrated by flotation; if it contains certain impurities, it may be roasted. It is then mixed with a dilute solution of sodium cyanide (or potassium or calcium cyanide) while air is bubbled through it. The gold is oxidized and forms the soluble aurocyanide complex ion, Au(CN)2 - 1. The solution is separated from the ore by methods such as filtration, and the gold is precipitated by adding powdered zinc. The precipitate usually contains silver, which is also precipitated, and unreacted zinc.

The process of extracting gold from ore with cyanide is called cyanidation. The reaction, known as Elsner's Equation, is:

4 Au + 8 CN- + O2 + 2 H2O = 4 Au(CN)2- + 4 OH--