Hydrometallurgy, is the process of extraction of metal from ore by preparing an aqueous solution of a salt of the metal and recovering the metal from the solution. i.e., metal is extracted from the metal ore by dissolving the metal ore in a suitable solvent.
Hydrometallurgy process is known to have originated in the 16th century itself, but its development took place only in the 20th century, stimulated partly by the desire to extract gold from low-grade ores. As the scientific technology advanced-the development of ion exchange, solvent extraction, and other processes were introduced which led to an extremely broad range of applications of hydrometallurgy. Hydrometallurgy process is now used to produce more than 70 metallic elements, isn’t it amazing?. More importantly, most of gold and much silver, large tons of copper and zinc are produced by the hydrometallurgy process.
Hydrometallurgy process typically consists of the following three stages:
1. Leaching
2. Solution concentration and purification
3. Metal or metal compound recovery
Hydrometallurgy process usually involves the concentration of the metal ore by leaching process or dissolution of the metal or metal compound in water, commonly with additional agents. It is then followed by separation of the waste and purification of the leach solution. Later, either the metal or one of its pure compounds is precipitated from the leach solution by chemical or electrolytic means. The most common leaching agent used are dilute sulfuric acid, Sodium hydroxide etc,.
Note: Hydrometallurgy process is also based on the principle that a more electropositive metal can displace a less electropositive metal from its solutions.
Leaching: Leaching is the chemical method of concentrating the ore. Leaching is often used when the ore is soluble in some suitable solvent or chemical reagent while the gangue particles are insoluble. Leaching is a widely used extractive metallurgy technique which converts metals into soluble salts in aqueous media. Compared to pyrometallurgical operations, leaching is easier to perform and much less harmful, because no gaseous pollution occurs.
Bauxite ore of Aluminium is concentrated by treating the ore with concentrated sodium hydroxide solution.
Bauxite ore contains Silica (SiO2), Iron oxides and Titanium oxides (TiO2) as impurities. When the powdered bauxite ore is digested with concentrated solution of NaOH at about 473K-523K temperature and 35–36 bars of pressure. Al2O3 dissolves in the solution as Sodium aluminate & SiO2 as Sodium silicate leaving the impurities behind.
The Solution if filtered to separate the insoluble impurities & the filterate is neutralized by passing CO2 gas to precipitate hydrated Al2O3. A small quantity of freshly prepared Al2O3 is seeded to induce precipitation.
The sodium silicate remains in the solution & hydrated alumina is filtered, dried & heated to give back pure Al2O3
Silver and Gold are leached from their native ores, with dilute solution of Sodium cyanide (NaCN) or Potassium cyanide (KCN) in presence of Oxygen (O2) from air, when their soluble complex cyanides are obtained. The method is called Mac-Arthur Forest Cyanide process.
Silver (Ag) & Gold (Au) are recovered from their solutions by adding zinc which displaces them.
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