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The use of an aqueous solution to extract metals from their ores is known as Hydrometallurgy. One early example of hydrometallurgy is a process used to obtain gold. Gold occurs in its elemental state, but often as very small particles mixed with other substances. The gold can be separated out of the mixture by selectively dissolving it into solution, a process called leaching. Hydrometallurgy use for extraction of Cu, Ag, Au etc. Hydrometallurgy used two famous processes (1) MacArthur-Forrest Process (2) Leaching

(1) MacArthur-Forrest Process:
(A) Extraction of Gold:
In the Mac Arthur process, solid gold reacts with sodium cyanide to form a soluble gold complex. The impurities are filtered out of the solution and the gold is reduced back to elemental gold with a reactive metal such as zinc.
(B) Extraction of Silver:
Metallic Ag is dissolved from its ore in dilute NaCN solution, and the solute so obtained is treated with scrap Zn when Ag is precipitated. Air is blown into the solution oxidize Na2S. Leaching the metals like silver, gold with CN- is an oxidation reaction
Here Zn act as reducing agent
Leaching has been practiced for many years and often results in the contamination of streams and rivers with cyanide. New alternatives, using the thiosulfate ion (S2O3-) , are being investigated to replace it.
(C) Extraction of Copper:
Different acid, base, and salt solutions are sometimes used to selectively separate out metal-bearing minerals. For example, sulfuric acid is used to separate the copper and iron from the mineral chalcopyrite, CuFeS2, and a sodium chloride solution is used to separate the lead from the insoluble mineral anglesite, PbSO4.
Hydrometallurgy is often more economical than Pyrometallurgy, due to the high energy costs associated with the elevated temperatures needed for calcination and roasting.

(2) Leaching:
Leaching is often used if the ore is soluble in some suitable solvent like acids, bases and suitable chemical reagents. For example Al, Ag, and Au ore and low grade copper ore.
Leaching of alumina from bauxite:
The principal ore of aluminium, bauxite, usually contains SiO2, iron oxide and titanium oxide (TiO2) as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. This way, Al2O3 is leached out as sodium aluminate (and also SiO2 as sodium silicate) leaving behind the impurities, iron oxide and titanium oxide.
The aluminate in solution is neutralised by passing CO2 gas and hydrated Al2O3 is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated Al2O3 which induces the precipitation
The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure Al2O3:

This step comprises the Bayer’s process.

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