Mumbai university > Civil > SEM 8 > Industrial Waste Treatment

Marks: 10M

Year: May 2014,

Vegetable tanning:

• This process uses tannins obtained from tree barks of chestnut, oak, quebracho, myroblaan or wattle.

• Vegetable tanning may be done for heavy as well as light leathers. Counter current method of tanning is commonly used in which hides come into contact first with weak liquors exhauted by use in other vats.

• When heavy leather is made, hides are bleached to remove excess tannin. It is done by the help of weak soda ash and weak sulphuric acid. The quantities of chemicals used are 0.75kg of soda ash and 2 kg of sulphuric acid per 100 kg of hide.

• Heavy leather is then subjected to finishing operations such as treatment with oils to give flexibility and addition of salts and sugar to add weight. The leather may be polished, rolled and given artificial grain.

• Light leather after splitting is subjected to thorough washing and dyeing. The desired degree of flexibility is achieved by fat liquoring.

Chrome tanning:

• The process of chrome tanning is of shorter duration than vegetable tanning and produces more resistant leather.

• After bating the hides are subjected to pickling in which NaCl and sulphuric acid are used. The reaction is allowed to go on for 10-16 hours.

• Tanning is done by charging the hides in drums containing basic chromium sulphate and sodium chloride.

• Towards the end of tanning period sodium carbonate is added to the drums to promote combination of the tanning material with the hides.

• The hides are then thoroughly washed and limed. They are then subjected to colouring, fat liquoring and finishing.