Describe the principle controlling each of the following processes:

1. Preparation of cast iron from pig iron.

2. Preparation of pure alumina (Al₂O₃) from bauxite ore.

1. The iron obtained by smelting (extraction of metal by heating and melting) it in blast furnace is called pig iron. Pig iron (having carbon content from 3.8-4.7 %) is melt with scrap iron and coke using hot air blast. In this way, cast iron is prepared. Due to this, impurities such as C, S and P present in the pig iron are removed as CO₂, SO₂ and P₂O₅ and carbon content reduced to about 3%. It is extremely hard and brittle.

2. The principal ore of aluminium is bauxite Al₂O₃.xH₂O. Bauxite is concentrated by digesting the powdered ore with concentrated solution of NaOH at 473-523K. The Al₂O₃ is leached or drained out as sodium aluminate. The sodium aluminate is neutralized by passing CO₂ gas and hydrated Al₂O₃ is precipitated which is filtered, dried and heated to give pure Al₂O₃.

The basic principle lies that bauxite is soluble in concentrated NaOH solution whereas the impurities are not. The reactions are as follows: