Transcript MS PowerPoint - Catalysis Eprints database

Pollution control in leather industry (Tannery)
S. Chandravathanam
Research Scholar,
National Centre for catalysis Research,
Department of Chemistry,
Indian Institute of Technology Madras,
Chennai-36
Children’s Club Lecture Series, 28th Apr. 09
Leather is a material produced from animal skins (or hides).
- is used for making shoes, upholstery, clothing, gloves, hats,
books, handbags, etc
 Tanning is the process of making leather, which does not easily
decompose, from the skins of animals.
 It involves chemically binding of the organic or inorganic materials
to the protein structure of the hide and preserve it from deterioration.
 The substances generally used to accomplish the tanning process are
chromium or tannin, an acidic chemical compound which is extracted
from the bark of trees, such as chestnut, oak, etc.
 These processes prevent putrefaction and rotting of hide under humid
conditions and permanently alters the protein structure of skin to leather,
so that it can not ever return to rawhide or raw skin.
 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
Preservation of hides - drying, salting

Cleaning and Soaking

Unhairing – Liming

Deliming and Bating

Tanning- Vegetable tanning, Chrome Tanning

Finishing and Coating
Unhairing
Structure of the skin
Structure of the border area between
the epidermis and the corium
- 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
Type of tanning agents give rise to different
types of tanning operations –
 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.
• Water soluble phenolic polymers or oligomers (M, 500 – 5000) of
natural and synthetic origin exhibit tanning properties. They bind to
collagen by forming hydrogen bonds with its carbonyl, hydroxyl and
carboxyl groups, engaging with the peptide groups of the collagen
backbone.
• 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.
Condensation tanning agents form
condensation products in solution and
form small insoluble particles; example
is catechin which condenses to form
dicatechin.
- Synthetic tanning agents, or syntans, are sulfonated condensation
products of formaldehyde and phenols.
- The tanning action of synthetic phenolic tannins resembles that of the
natural polyphenols.
Chrome Tanning
 Tanning is mainly performed with
chromium (III) sulfates.
 Chromium (III) salt has
remarkable complex forming
capacity.
 Binding of chromium to collagen
takes place at the carboxyl groups
of the amino acid side chains.
 The hydroxyl bridges are very
stable, and the complex binding
forces of the carboxyl groups
strongly resist dissociation; these
are the reasons for the excellent
tanning action of Chromium (III) in
comparison to other tanning agents.
Finishing and Coating
 The finishing operation seals the leather surface to make it tougher and
more resistant, reduces surface defects and nonuniform coloration.
 Oils and fats are applied for this purpose.
Tannery Waste Characteristics
Gaseous emissions
- the most important emission to the atmosphere consists in volatile organic
compounds (VOC’s) from the finishing department. Solvent based finish
systems have been substituted by water based systems.
Process Solid Wastes
- pieces of leather in various stages of processing, and wastewater
treatment sludge constitute the bulk of the process solid waste from
tanneries.
- 35% of all tannery solid waste is trimmings and shavings of various
types.
- Waste finishes account for about 2% of tannery solid waste.
- Wastewater screenings and sludge account for about 60% of tannery
solid waste.
- 3% of tannery solid waste is floor sweepings.
Process Liquid Wastes
- average total water use is 2,200 l/100 kg of hides processed.
- 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
- tannery waste treatment plants include operations like,
- screening,
- flow equalization,
- pH adjustment,
- primary sedimentation
- biological treatment and
- secondary sedimentation.
special treatments
- spent chrome tanning solution,
- spent vegetable tanning solution and
- spent dye solutions.
Spent chrome tanning liquor treatment
- the spent chrome tanning liquor is a concentrated, acidic solution
containing a high concentration of chromium, chlorides, and other
contaminants.
- commonly used method of chromium removal is conversion of all
chromium to the trivalent form and then precipitation of the trivalent ions as
chromium hydroxide.
- 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.
- dosages of granular activated carbon up to 5,000 mg/1 were used in the
preliminary color removal.
- 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
- the automation and computerization of the tanneries, has entailed a
reduction of water and chemicals dosage and consequently of the waste
water load.
- in the 1970’s, the average chemicals consumption was approximately
400 kg per ton rawhide, whereas in the 1990’s this was reduced to
approximately 250 kg/ton.
- a typical example is the consumption of lime in the beamhouse. The
dosage used nowadays is something like 2-3% of the hide weight, against
5% earlier.