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The process of extracting a metal from a concentrated ore depends on the ore’s nature and the nature of the impurities present in the ore. Oxidation and reduction reactions are included in these processes. The concentrated ore must first be transformed into a reduction-ready state. Oxides are less difficult to eliminate. The extraction of crude metals from concentrated ores is accomplished in two steps:
(i) conversion of ore to oxides of the metal
(ii) reducing the metal oxides to elemental metals.
Conversion of Ore to Oxides of the Metal of Interest (oxidation)
The concentrated ore can be hydrated oxide, carbonate, or sulphide. It can be converted to its oxide form using one of two methods:
Roasting: Roasting is the process of strongly heating the ore in the presence of a large amount of air to a temperature lower than the melting point of the metal. Roasting is a common method for converting sulphide ores to oxides. The concentrated ore is oxidised in this method by heating it in an appropriate furnace with an excess of oxygen below the melting point of the metal.
2PbS + 3O2 → 2PbO + 2SO2↑
2Cu2S + 3O2 → 2Cu2O + 2SO2↑
2ZnS + 3O2 → 2ZnO + 2SO2↑
Roasting removes impurities like arsenic, sulphur, and phosphorus by converting them into volatile oxides.
4As + 3O2 → 2As2O3 ↑
S8 + 😯2 → 8SO2 ↑
P4 + 5O2 → P4O10 ↑
Sulphur, arsenic, and phosphorus impurities are removed as volatile oxides- as O, and P2O5. Metal ores are transformed into oxides.
The SO2 generated is used in the production of H2SO4.
Calcination: The procedure of heating an ore much below its melting point, either with a restricted supply of air or in the absence of air, is called calcination.
The hydrated oxide’s crystallisation water escapes as moisture during this process. Any present organic matter is also removed, leaving a porous ore behind. This method can also be used when only a limited air supply is available.
Hydrated oxide and carbonate ores are typically calcined to produce oxides.
The following changes happen during calcination:
(i) removal of moisture
(ii) removal of volatile impurities
(iii) obtain porous ore
(iv) removal of water from the hydrated oxide
(v) decomposition of carbonate to oxide
Example 1. Aluminium is found as Al2O3.H2O in bauxite ore. Water vapour is released, and anhydrous aluminium oxide is yielded when bauxite ore is calcined.
Al2O3.H2O → Al2O3 + 2H2O
Example 2. Removal of hydration water from limonite :
Fe2O3.3H2O → 2Fe2O3 + 3H2O
Reducing the Metal Oxides to Elemental Metals (reduction)
Typically, metal oxides are reduced to free metals by using an appropriate reducing agent such as carbon, carbon monoxide, or another metal. Electron gain or electronification is involved in the reduction process.
Thermal reduction pyrometallurgy refers to metal extraction by heating a metal oxide with an appropriate reducing agent.
Some metals are easy to reduce (at low temperatures), whereas others are difficult to reduce (at high temperatures). However, reduction necessitates the use of heat at all times.
The metal can be extracted using the following reducing agents, depending on the nature of the oxide and the metal:
Smelting: In this method, we add-flux and a reducing agent such as carbon, carbon monoxide (or) aluminium to the concentrated ore, and we melt the mixture in a smelting furnace at a high temperature (above the melting point of the metal).
Using Carbon: The metal’s oxide ore is mixed with coal (coke) and heated vigorously in a furnace in this method (usually in a blast furnace). This method applies to metals that do not form carbides with carbon at the decrease in temperature.
ZnO (s) + C (s) → Zn (s) + CO (g) ↑
3. Using Hydrogen: This process can be used with metal oxides (Fe, Pb, Cu) with a lower electro-positive character than Hydrogen.
Ag2O (s)+ H2 (g) → 2Ag (s) + H2O (l)
4. Using Metal: An alumino thermite process can reduce metallic oxides such as Cr2O3. The metal oxide is mixed with aluminium powder and put in a fire clay crucible.
BaO2 + Mg → BaO + MgO
Conclusion
The process of extracting a metal from a concentrated ore depends on the ore’s nature and the nature of the impurities present in the ore.
The extraction of crude metals from concentrated ores is accomplished in two steps:
(i) conversion ore to oxides of the metal of interest
It can be converted to its oxide form using one of two methods:
Roasting is the process of strongly heating the ore in the presence of a large amount of air to a temperature lower than the melting point of the metal.
Calcination involves heating an ore much below its melting point, either with a restricted supply of air or in the absence of air.
(ii) reducing the metal oxides to elemental metals.
Pyrometallurgy is the extraction of metal by heating a metal oxide with an appropriate reducing agent. The metal can be extracted using the following reducing agents, depending on the nature of the oxide and the metal: Smelting, using Hydrogen, carbon etc.
