We are familiar with the metallurgical processes which are generally termed as pyrometallurgy (that is, pyro means at high temperatures). Pyrometallurgy has been used in producing metals from their ores as ancient times and needs large quantities of energy. Such poses two serious problems namely atmospheric pollution (mostly by sulphur-dioxide) and relatively high costs which make treatment of low grade ore economically unfeasible. Toward the end of the 19th century, a different process termed as hydrometallurgy was introduced. Hydrometallurgy is now employed to produce some metals, comprising gold and silver, and also salt of other metals. This method now competes with pyrometallurgy in the production of copper, zinc and nickel.
Hydrometallurgy (that is, hydro meaning water) has been used to extract metals from their ores by the utilization of aqueous chemical solutions at moderate temperatures. Hydrometallurgical methods generally need less energy and produce less air pollution and are well suited particularly to low-grade ores and ores which have some metals with identical properties. The first two uses of this method were for the extraction of gold from low-grade ores and for the production of aluminum oxide, or aluminum from bauxite and aluminum bearing ore.
Processes of Hydrometallurgy:
Hydrometallurgy comprises two different steps. First is the selective leaching of a given metal ion from the solution via selective precipitation as an ionic compound. To acquire a clearer understanding of such methods, consider these illustrations:
Gold is at times found in ores in the elemental state however takes place generally in comparatively small concentration. To extract or acquire the gold metal, a method termed as cyanidation is used. In this case the crushed ore is treated by an aqueous cyanide solution in the presence of air thus making the complex ion, Au(CN)2- as illustrated in the equation below:
4Au(s) + 8CN-(aq) + O2 + 2H2O(l) → 4Au(CN)2-(aq) + 4OH-(aq)
Pure gold is then recovered via the reaction of Au(CN)2- by zinc powder to decrease Au+ to Au
2Au(CN)2-(aq) + Zn(l) → 2Au(s) + 2Zn(CN)2-(aq)
Consider as well the Bayer method for the extraction of alumina from bauxite. The ore is leached by sodium hydroxide to dissolve the amphoteric aluminum oxide.
Al2O3(s) + 2OH-(aq) → 2AlO2-(aq) + H2O(l)
This method leaves behind solid impurities like SiO2, Fe2O3 and TiO2 which are not appreciably soluble in the basic solution. After the solid impurities are eliminated, the pH of the solution is lowered via addition of carbon-dioxide. The pure aluminum oxide reforms and is then electrolyzed to generate aluminum metal.
There are different types of leaching agents employed. A leaching agent can simply be water if the metal containing compound is a water soluble chloride or sulphate. Though, most generally, the metal is present in a water-insoluble substance that should be dissolved. In such cases, leaching agents employed are generally aqueous solutions having acids, bases, oxidizing agents, salts or some combination of these. The leaching or dissolving method often comprises the formation of complex ions. For illustration, an ore having water-insoluble lead sulphate is treated by an aqueous sodium chloride solution and then a soluble complex ion, PbCl42- is formed therefore:
PbSO4(s) + 4Na+(aq)4Cl-(aq) → 4Na+(aq) + PbCl42-(aq) + SO42-(aq)
Likewise, the cyanidation process for the recovery of gold yields in the formation of the complex ion. The gold, since it is present in the ore as particles of metal, it should first be oxidized via oxygen to produce Au+ which then reacts by CN- to form soluble Au(CN)2- species. Therefore, in this case, the leaching method comprises a combination of oxidation and Complexation.
Recovering the metal ions from the leaching solution comprises making an insoluble solid including the metal ion to be recovered. This step might comprise addition of an anion to make an insoluble salt, reduction to the solid metal (that is, chemically or electrolytically), or a combination of reduction and precipitation of salt. Illustrations are as described below:
Precipitate of a salt Cu2+(aq) + S2-(aq) → CuS(s)
Au+(aq) + Fe2+(aq) → Au(s) + Fe3+(aq)
Reduction (a) Chemical Cu2+(aq) + Fe(s) → Cu + Fe2+(aq)
Ni2+(aq) + H2(g) → Ni(s) + 2H+(aq)
Reduction (b) Electrolytic Al3+(aq) + 3e- → Al(s)
Cu2+(aq) + 2e- → Cu(s)
Reduction plus precipitation 2Cu2+(aq) + 2Cl-(aq) + H2SO3(aq) + H2O(l)
→ 2CuCl(s) + 3H+(aq) + HSO4-(aq)
Advantages of Hydrometallurgy:
In comparison of hydrometallurgy and its traditional counterpart (that is, pyrometallurgy), hydrometallurgy is seen to have such benefits:
a) Leaching agent can be pumped directly to ore deposits in the earth.
b) Economically appropriate for low grade ores.
c) The method produces little or no pollution.
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