Precious metals are refined using various techniques to separate them from impurities, including electrolytic refining and chemical processes like aqua regia.
Refining Methods for Precious Metals
Refining precious metals is crucial to increase their purity for various applications, from jewelry to electronics. Here are the primary methods:
1. Electrolytic Refining
- Process: Electrolytic refining uses an electric current to separate the desired metal (e.g., gold, silver) from impurities.
- How it works:
- The impure metal acts as the anode (positive electrode) in an electrolytic cell.
- A thin sheet of the pure metal acts as the cathode (negative electrode).
- An electrolyte solution containing a salt of the metal is used.
- When current is applied, the metal at the anode dissolves into the electrolyte, while pure metal is deposited on the cathode. Impurities either settle as "anode mud" or remain in the electrolyte.
- Example: Silver refining commonly uses an electrolytic process, where silver is transferred from an impure anode to a pure silver cathode in a silver nitrate solution.
2. Chemical Refining (Aqua Regia)
- Process: Aqua regia involves dissolving the precious metal in a highly corrosive mixture of nitric acid and hydrochloric acid.
- How it works:
- Aqua regia is capable of dissolving gold and platinum, which are resistant to most other acids.
- Gold dissolves to form tetrachloroaurate(III) anions ($AuCl_4^−$) in solution.
- After dissolving, the gold can be selectively precipitated by adding a reducing agent or neutralized to resolidify.
- Reaction: The equation for the dissolution of gold in aqua regia is: $Au + 3 HNO_3 + 4 HCl \rightleftharpoons [AuCl_4]^- + 3NO_2 + H_3O^+ + 2 H_2O$
3. Smelting
- Process: Smelting involves heating ore or concentrate to a high temperature to melt the desired metal and separate it from impurities.
- How it works:
- The high temperature causes the metal to melt and separate from the slag (impurities).
- The molten metal is then collected and further refined.
- Application: Often used as an initial step in refining gold, silver, and platinum group metals.
4. Cupellation
- Process: Cupellation is a process used to refine precious metals, particularly gold and silver, from base metals such as lead, copper, zinc, and other impurities.
- How it works: The precious metal is alloyed with lead and heated to a high temperature. The lead oxidizes and forms litharge (lead oxide), which is either absorbed by the cupel (a shallow dish made of bone ash or magnesia) or skimmed off, leaving the refined precious metal behind.
- Application: Historically important and still used for assaying and refining gold and silver.
Summary
Precious metals undergo refining through various methods, including electrolytic processes, chemical treatments (like aqua regia), smelting, and cupellation, each leveraging different principles to separate the desired metal from impurities, achieving high purity levels.
