Discuss measures for the control of SO_x pollution.

(i) Removal of sulphur from the fuel before combustion: A number of processes are employed. The pyrites from coal can be eliminated by physical techniques such as grinding followed by washing. Organic sulphur in coal can be removed by carbonization, liquefaction or gasification.
(ii) Removal of SO_x from flue gases: A number of methods are employed as.
(a) Limestone method: CaCO₃ is added directly to the boiler where quick lime is formed.

Quicklime (CaO) reacts with SO_x in the flue gases to form calcium sulphite and calcium sulphate. 

Calcium sulphite and calcium sulphate are removed and disposed of.
(b) Lime slurry method: Sulphur dioxide is passed through lime slurry where calcium sulphite is formed. 

The waste CaSO₃ can be used to prepare gypsum by oxidation of CaSO₃.

(c) Magnesium oxide slurry method: Sulphur dioxide may be removed by using magnesium oxide for scrubbing.

(iv) Sodium hydroxide method: Sulphur dioxide can be removed by first treating with sodium hydroxide and then with calcium hydroxide.