Leblanc Process - Chemistry

Chemistry

The Leblanc process was a batch process in which sodium chloride was subjected to a series of treatments, eventually producing sodium carbonate. In the first step, the sodium chloride was heated with sulfuric acid to produce sodium sulfate (called the salt cake) and hydrogen chloride gas according to the chemical equation

2 NaCl + H₂SO₄ → Na₂SO₄ + 2 HCl

This chemical reaction had been discovered in 1772 by the Swedish chemist Carl Wilhelm Scheele. Leblanc's contribution was the second step, in which the salt cake was mixed with crushed limestone (calcium carbonate) and coal and fired. This reaction happens in two parts. First the coal (carbon) was oxidized to carbon dioxide, reducing the sulfate to sulfide. Second, the calcium and sodium swap their ligands to leave the thermodynamically favorable combination of sodium carbonate and calcium sulfide. This mixture is called black ash.

Na₂SO₄ + 2 C → Na₂S + 2 CO₂

Na₂S + CaCO₃ → Na₂CO₃ + CaS

The soda ash was then separated from the black ash by washing it with water. The wash water was then evaporated to yield solid sodium carbonate. This extraction process was termed lixiviation.