Sulphur plays important role in paper industry. It is used in the chemical process which is used in the manufacture of paper pulp. Wood material is soften by using an acid bisulfite solution which helps in removing lignin from the cellulose. This sulfite liquor consists free sulphur dioxide, which is obtained either by burning the sulphur or by roasting of iron pyrites which is dissolved into the water in a concentration of 5 to 8%, containing 2-3% bisulfite.
This batch process of sulfite digestion is done through a pressure vessel containing steel shell with acid-resistant linings.
Paper pulp liquor is made by sulphur dioxide which is further absorbed into water to give sulphurous acid. sulphur must be burned by correct amount of oxygen, so that formation of suflur trioxide could be avoided which gives undesired sulphuric acid which promotes hydrolysis of cellulose with no contribution to delignification.
S + O2 → SO2
SO2 + H2O ⇌ H2SO3
2 SO2 + O2 → 2SO3
In the making of cooking liquor including addition of counter ions as carbonates or hydroxides. In the liquid, relative amount of each species, depends on the relative amount of sulphurous. Below is an example-
For divalent carbonates, MCO3
MCO3 + 2H2SO3 → M(HSO3)2 + CO2 + H2O
M(HSO3)2 + MCO3 → 2 MSO3 + CO2 + H2O