Pollution control in leather industry (Tannery)

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

Tanning- Vegetable tanning, Chrome Tanning

Finishing and Coating

Deliming and Bating

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.

Unhairing

Type of tanning agents give rise to different

types of tanning operations –

vegetable or

chrome tanning.

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.

Vegetable tanning

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.

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.

Finishing and Coating

Tannery Waste Characteristics

- 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.

Gaseous emissions

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 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.

Tannery Waste Management

- 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 chrome 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.

Spent vegetable tanning liquor treatment

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.

Reuse

- 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.

Remarks