Pollution Control in Leather (Tannery)
Pollution Control in Leather (Tannery)
Pollution Control in Leather (Tannery)
Different types of hides (or skins) are used for leather production
including those of the cow, ox, pig, sheep, goat, horse, buffalo, crocodile
and other animals.
Tannery Processes
Unhairing – Liming
- During unhairing the hides are soaked in lime solution which swells the
fibers of the hide and loosens the hair at the follicle.
- the loosened hair is removed either manually or mechanically.
- small quantity of sodium sulfide is added with lime to facilitate unhairing.
Tanning
vegetable or
chrome tanning.
Vegetable tanning
• The tanning of hides and skins with plant extracts is one of the oldest
human skills.
• Many plants produce tanning agents, which occur in their wood, bark,
leaves, and roots of woods like, chestnut, oak, etc.
• Phenolic agents penetrate the pelt and collagen fibrils more slowly
than chrome tanning agents.
Composition of vegetable tanning agents
Vegetable tanning
Hydrolyzable tannins,
undergo hydrolysis to give
smaller components;
example is chebulinic acid,
which is hydrolyzed to
trigalloyl glucose and
chebulic acid.
The finishing operation seals the leather surface to make it tougher and
more resistant, reduces surface defects and nonuniform coloration.
Gaseous emissions
- the processes contributing the most wastewater were the de-lime and bate
wash, the lime wash, and the soak.
- the processes contributing the greatest pollution load, in terms of COD, were the
lime pits discharge, the de-lime and bate wash, the spent vegetable tanning liquor
and the spent dye solution.
- the spent chrome liquor contains between 400 and 600 mg/1 of chromium, of
COD 4,300 mg/1, and a chloride content of 72,000 mg/1. The pH of the spent
chrome tanning liquor ranges between 3.5 and 4.5, and has a characteristic light
blue color. The ratio of pounds of chrome tanned hides to gallons of spent chrome
liquor was about 10 to 1.
- the waste discharge from the vegetable tanning process is relatively small, but
quite troublesome because of very high solids content, color, and COD. Spent
vegetable tanning liquor sample shows that the solution is essentially a colloidal
suspension of the bark extracts, which are largely tannins.
Tannery Waste Management
special treatments
- spent chrome tanning solution,
- spent vegetable tanning solution and
- spent dye solutions.
Spent chrome tanning liquor treatment
- as the chromium ions in the spent chrome tanning solution are already in
the trivalent form, removal of the chrome can be accomplished by pH
adjustment and sedimentation.
- the results show that optimum removal of chromium from the spent
chrome liquor was attained at a pH of 11.5.
Spent vegetable tanning liquor treatment
- although the volume of this waste is not large, it represents a large fraction of
the pollutional load because of its strong concentration. The BOD and COD are
not removed easily by biological waste treatment because of the nature of the
tannin molecules, and their some toxic effect.
- briefly stated, the method requires equal additions of waste and solvent, pH
adjustment, slow mix, and sedimentation.
Reuse
Most lime fleshings of rawhide, pelt trimmings, and even some chrome
leather waste were sold to producers of animal glue.
Besides the amounts used for glue production, many chrome shavings
were sold to produce proteinaceous products and chrome tanning agents.
A biogas plant uses lime fleshings, trimmings, and hair from the
tanneries for biogas production as the fat content of the fleshings gives a
high gas yield.
Remarks