page 3
The big picture
Reactions between acids and bases
There are several reactions in the iron and steel making processes which involve acids and bases. One of the raw materials which is fed into the Blast Furnace is limestone, an almost pure form of calcium carbonate. Limestone decomposes in the hot furnace to give calcium oxide, which is a base.

calcium carbonatecalcium oxide+carbon dioxide

Calcium oxide joins with acidic impurities in the ore, to make a salt. The biggest impurity is silica, a weak acid, which combines with calcium oxide to make calcium silicate, a salt which is formed as a molten slag.

calcium oxide+silicacalcium silicate
acid or base?acid or base?acid or base or salt?

Calcium oxide also plays an important part in the Basic Oxygen Steelmaking furnace. Here it helps to remove impurities such as phosphorus and sulphur from the steel. Phosphorus is a non-metal and produces an acidic oxide, P2O5. In the BOS furnace P2O5 combines with the basic calcium oxide to make calcium phosphate. This forms part of a molten slag which collects on the surface of the molten metal.
Question 2.

Write equations to show how calcium oxide would react with the oxide of phosphorus, P2O5.