Leather & Textiles

They do two different jobs. Sodium sulphide attacks the keratin of hair and epidermis so it can be removed, while lime keeps the liquor strongly alkaline, swells the hide and opens the collagen fibre bundles so tanning chemicals later penetrate evenly. The limed pelt is then delimed with ammonium salts or acids. Liming liquors carry much of a tannery's BOD and COD load, and sulphide must be kept away from acids, which release hydrogen sulphide.

Basicity is the share of chromium's bonding capacity taken up by hydroxyl groups rather than sulphate, given as a percentage. Standard basic chromium sulphate is about 33% basic with roughly 25% Cr₂O₃. Low basicity keeps chrome complexes small so they penetrate the pelt; during basification, sodium bicarbonate or a self-basifying agent raises pH slowly, typically to about 3.8-4.0, so chrome fixes to collagen. Raising pH too fast fixes chrome on the surface.

Salt drives exhaustion and soda ash drives fixation. In water, cotton carries a negative surface charge that repels anionic reactive dyes; common salt or sodium sulphate suppresses that charge so dye moves onto the fibre. Soda ash, alone or with caustic soda for some dyes and deep shades, then raises pH so the dye bonds covalently with cellulose. Alkali added too early or in excess hydrolyses dye in the bath, wasting colour and adding to wash-off.

Peroxide bleaching runs hot and alkaline, typically around pH 10.5-11, where traces of iron, copper or manganese make peroxide decompose too fast, wasting bleach and causing pinholes or fibre damage. Sodium silicate, together with magnesium from the water or added salts, buffers the bath and stabilises the peroxide, while chelants such as EDTA or phosphonates bind the metal ions. Silicate can leave deposits and a harsh handle, so many mills use silicate-free stabilisers.