Question

Explain the leaching process of silver and gold using chemical reactions?

Answer 1

The leaching process of Gold and silver is also called the Mac-Arthur Forrest cyanide process. The extraction process of these less reactive metals are made to react with cyanide salts and made into water soluble complexes. Then this solution formed is treated with a good suitable reducing agent and then metal is made free from solution.

Complete step by step answer:

Now, let us have a detailed look at the leaching process or this process is also called the Mac- Arthur Forrest Cyanide process.
In the leaching process, the Gold and Silver ore is leached with NaCN or KCN, cyanide ions. Let us take an example to understand this more clearly with reactions involved:
Let us take silver sulphide, $A{g_2}S$ and add NaCN, Sodium cyanide solution to it. The reaction can be written as:
$A{g_2}S + 4NaCN \to 2Na[Ag{(CN)_2}] + N{a_2}S$
Now the complex formed is water soluble, so we need a good reducing agent, like Zinc can be used. When Zinc is added with this water soluble complex, then we get Silver separated out.
$2Na[Ag{(CN)_2}] + Zn \to N{a_2}[Zn{(CN)_4}] + 2Ag \downarrow$
Thus, the precipitated silver obtained can be separated by filtration.
In the same way, we can obtain Gold, first by leaching with KCN, and making water soluble complex, and then using a reducing agent like Zinc, to precipitate Gold and then filtering it, we can get Gold from its respective ores.
Additional information: The leaching process can only be carried out, if ore is soluble in some particular solution, and impurities should be insoluble in that solution. The word leaching here means that the ore we are treating should become soluble.
The complex or cyanide ions obtained from these reactions have to be recycled or destroyed carefully as they can be harmful.

Note:
The note should be taken of which reducing agent to be added, as we know the electrochemical series. The metal which has higher reducing power and can reduce Gold and Silver, such metal can be added. One of the best examples is Zinc, which has good reducing power to reduce Gold and Silver, as Zinc itself will get oxidised very well.