Smelting

In chemical reduction process, the choice of reducing agent depends upon the chemical reactivity of the metal.

Smelting (Carbon reduction method)

This method is used for the extraction of lead, zinc, iron, copper, manganese and tin. In this method, the roasted oxide ore is mixed with carbon (charcoal, coal or coke) and a flux, and is heated to a very high temperature in a suitable furnace. Carbon reduces the oxide to metal.

Hydrogen reduction method

Hydrogen can reduce certain oxides to metals e.g.,

Carbon monoxide reduction method

In certain cases CO gas produced in the furnace itself can be used as a reducing agent. For example,

Magnesium reduction method

Oxides of certain metals are reduced by Mg e.g.,

Aluminium reduction method (Alumino Thermic Process)

Certain metal oxides cannot be reduced by carbon. Such metallic oxides can be reduced by aluminium powder. This process has been widely used to reduce TiO₂, Cr₂O₃ and Mn₃O₄ to get the corresponding metal.

Self-reduction method

When the sulphide ores of less electropositive metals like Hg, Cu, Pb, Sb etc., are heated in air, a part of the ore gets oxidized to oxide or sulphate, which then reacts with the remaining sulphide ore to give the metal and SO₂. This process is also known as self-reduction method.

Reduction by more electropositive metals (precipitation or hydrometallurgy)

This method is employed when leaching method had been used to concentrate the ore. The metals are obtained by reducing their ions in the solution as precipitates by a more electropositive metal. This method is also called as the hydrometallurgy method of reduction.

For example, when a heap of copper glance (Cu₂S) is exposed to air and water, it gets converted to copper sulphate. Copper is recovered from copper sulphate solution by adding some iron scrap to its solution.

Ag and Au are also recovered from the solutions of their complex cyanide salts by zinc scrap.

Problems

6. What is Goldschmidt thermite process?

Solution

The process of reduction of a metal oxide to the metal with the help of aluminium powder, which liberates large amount of heat to produce the metal in its molten state, is called Goldschmidt thermite process.

7. How is gold recovered?

Solution

Gold being a noble metal occurs in its native form. It is further purified by the process of leaching or precipitiation called hydrometallurgy. The native gold in its complex form is first precipitated with potassium cyanide solution. It is then recovered from the solution by adding scrap zinc.