Extraction of Crude Metal from Concentrated Ore

Extraction of crude metals from concentrated ore is the process through which raw metals are obtained from their ores. The mixture in which metals are found in their original state is called ore. These metals are obtained through extraction from their ores. The metals are needed to be refined from their ores using the required reducing agent by converting the concentrated ores to oxides. The procedure requires three important steps that are-

1. Concentration of an ore
2. Metal Isolation from the concentrated ore
3. Purifying the metal

This process of the isolation of a metal from its ore is scientifically termed as metallurgy.

Key Takeaways: Ore, metal, oxide, metal oxide, reducing agent, electrolysis

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Conversion of the Concentrated Ores

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The concentrated ore which is found is either a carbonate or a hydrated oxide or a Sulfide. The two major processes through which the concentrated ores are converted into its oxides are:

• Calcination
• Roasting

Calcination

Calcination can be termed as a method by which the ore is heated below its melting point in absence of air due to which calcination takes place. There are few changes which takes place during the calcination process:

• Impurities are removed
• Removal of moisture
• Ores becomes porous
• From the hydrate oxides, water is removed
• Decomposition of carbonates into oxides

The following reactions can take place:

Carbonate ores get converted to oxide ores.

ZnCO₃ → ZnO (s) + CO₂

Here, the ore is heated strongly to convert it into zinc oxide in complete absence of air. During the process, carbon dioxide is emitted.

Hydrate ores become anhydrous in nature. 

Al₂O₃.H₂O → Al₂O₃ + 2H₂O

Here, water vapour is released from the bauxite ore during the process and aluminum oxide in anhydrous form is obtained.

Roasting

In this process, the ore in the presence of excess air is heated strongly with a temperature which is below the melting point of that metal. This results in the conversion of the ore into oxide and removal of impurities. This process is termed as Roasting.

Roasting is used for metals like sulphur, arsenic and phosphorus.

For Example:

(i) S + O₂ → SO₂

(ii) 4 As + 3 O₂ → 2 As₂O₃

(iii) P₄ + 5 O₂ → 2 P₂ O₅

The sulphide ores of lead, copper and zinc are also treated with the similar process.

• 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂
• 2 PbS + 3 O₂ → 2 PbO + 2 SO₂
• 2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂

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Reduction of Oxides to Metal

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The process in which metal oxides are reduced to free metals using reducing agents like- carbon monoxide, carbon or other metals like Hydrogen, aluminum. The reduction process takes place in low temperatures.

Smelting

The roasted ore is first mixed with carbon and then heated to a temperature above the melting point inside a furnace where the metal is reduced to molten form. The process gives out carbon monoxide gas. In case, CO is used as a reducing agent, then carbon dioxide gas will be released. This process is called Smelting.

Ex- ZnO + C →  Zn + CO

Fe₂O₃ + 3 CO → 2 Fe + 3CO₂

Alumino Thermite Process

Aluminum is very effective when used for the isolation of several metals like iron, manganese, chromium, etc. the process is exothermic and also produces huge amounts of energy as aluminum is highly electropositive.

Example- Cr₂O₃ + 2 Al → 2 Cr

Reduction by Hydrogen

As hydrogen has an inflammable nature, it is used in various metal oxides like tungsten, molybdenum as a reducing agent at very high temperatures.

Example: MoO₃ + 3 H₂ → Mo + 3 H₂O

Reduction by Water Gas

Current of water gas (CO + H₂) is passed to nickel oxide for reduction of nickel using heat up to 600K.

Example - NiO + CO → Ni + CO₂

NiO + H₂ → Ni + H₂O

Auto- Reduction

Auto-Reduction or Self Reduction is a process for sulphide ore reduction of Hg, Cu and Pb. Here, there is no need for any reducing agent. By simply roasting the metal is obtained.

Example– HgS + O₂ →  3 Hg + SO₂

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Types of Converters

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The process of roasting is done in furnaces or converters which are of two types:

• Bessemer converter: large Pear-shaped converters known as Bessemer converter. Copper and pig-iron are usually purified here.
• Pierce-Smith converter: A converter consisting of large horizontal steel drums on rollers. This process needs its own heat and the temperature goes up to 2673K

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Other Methods of Reduction

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Apart from the previous methods, there are other methods used for reduction of metals. They are:

Hydro-metallurgy

Metals like silver, gold, etc are extracted with the help of a more reactive metal like Zinc, which is more electropositive in nature.

Example: Ag₂S + 4NaCN → 2Na [Ag (CN)₂] + Na₂S

Electrolytic Reduction or Electrometallurgy

Some alkali metals which are highly electropositive in nature are reduced using the electrolysis process by fusing them with salts.

Example: NaCl → Na⁺ + Cl^–

Na⁺ + e^– → Na (at cathode)

Cl^– → Cl + e^– (at anode)

Cl + Cl → Cl₂

Froth Floatation

Dual medium of water and oil is used here to separate the pure metal from its ore. Air in compressed form is sent in the mixture, which forms froth in oil medium. The metal gets collected in the oil medium, on the top section and the impurities settle down below with the water. This process is called froth floatation.

Magnetic Separation

This is the easiest procedure in which wheels with magnets in a conveyor belt run and magnetic particles stick to it leaving the impurities below. This way the metals are easily separated from their concentrated ores. This method is called magnetic separation.

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Things to Remember

• Thermit welding is generally used for the welding of iron.
• ‘Magnetic separation’ is another method to extract metal from concentrated ore where wheels with magnets in a conveyor belt run and magnetic particles stick to it leaving the impurities below.
• Metal Extraction is always based on the chemical or physical properties of the ores and the metals.
• Ores are always contaminated with undesired or earthly substances which are called gangue.
• Sand, clays etc. are removed from the ore by the processes known as benefaction, dressing and concentration.

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