TREATMENT OF WASTES FROM TANNERIES IN INDIA presentation
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16-05-2010, 01:47 PM



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TREATMENT OF WASTES FROM TANNERIES IN INDIA

Presented By:
Anshumali Padhi
M.Tech (Environmental Engg.)
VNIT, Nagpur


¢ INTRODUCTION

The tanning industry is one of the oldest industries in India.
Tanning is the process of making leather which does not readily decompose from the skins of animals.
It involves chemically binding of the organic or inorganic materials to the protein structure of the hide & 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.
Different types of hides (or skins) are used for leather production including those of the cow, sheep, goat, ox, horse, pig and other animals.
There are approximately 113 leather tanning facilities in India.
The number of tanneries in India has significantly decreased in the last 40 years due to the development of synthetic substitutes for leather, increased leather imports, and environmental regulations.


¢ INDUSTRY DESCRIPTION & PRACTICES

Leather production usually involves three distinct phases:
(a) preparation (in the beamhouse);
(b) tanning (in the tanyard); and
© finishing, including dyeing and surface treatment. A wide range of processes and chemicals (including chrome salts) are used in the tanning and finishing processes.



¢ TANNING & FINISHING OF LEATHER GENERALLY CONSISTS OF :

soaking and washing to remove salt, restore the moisture contents of the hides, and remove any foreign material such as dirt and manure;
liming to open up the collagen structure by removing interstitial material;
fleshing to remove excess tissue from the interior of the hide;
dehairing/dewooling to remove hair/wool either by mechanical or chemical means;
bating and pickling to delime the skins, and condition the hides to receive the tanning agents;
tanning to stabilize the hide material and impart basic properties to the hides;
retanning, dyeing, and fat-liquoring to impart special properties to the leather, increase penetration of tanning solution, replenish oils in the hides and impart color to the leather; and
finishing to final product specification.


¢ TYPES OF TANNING

Tanning is essentially the reaction of collagen fibers in the hide with chromium, alum, or other chemical agents. The most common tanning agents used are the trivalent chromium and vegetable tannins extracted from specific tree barks. Alum, syntans (man-made chemicals), formaldehyde, glutaraldehyde, and heavy oils are other tanning agents.

Mainly there are two types of tanning:
Vegetable Tanning
Chrome Tanning



VEGETABLE TANNING

The hides are first trimmed and soaked to remove salt and other solids and to restore moisture lost during curing.
Hides are then dehaired to ensure that the grain is clean and the hair follicles are free of hair roots. Liming is the most common method of hair removal, but thermal, oxidative, and chemical methods also exist.
Bating, an enzymatic action for the removal of unwanted hide components after liming, is performed to impart softness, stretch, and flexibility to the leather.
Bating and deliming are usually performed together by placing the hides in an aqueous solution of an ammonium salt and proteolytic enzymes at 27-32oC.
Pickling may also be performed by treating the hide with a brine solution and sulfuric acid to adjust the acidity for preservation or tanning.
It usually takes 3 weeks for the tanning material to penetrate to the center of the hide. The skins or hides are then wrung and may be cropped or split; heavy hides may be retanned and scrubbed.
For sole leather, the hides are commonly dipped in vats or drums containing sodium bicarbonate or sulfuric acid for bleaching and removal of surface tannins. Materials such as lignosulfate, corn sugar, oils, and specialty chemicals may be added to the leather.

¢ CHROME TANNING

Chrome-tanned leather tends to be softer and more pliable than vegetable-tanned leather, has higher thermal stability, is very stable in water, and takes less time to produce than vegetable-tanned leather.
Chrome tanning is performed using the reaction between the hide and a trivalent chromium salt, usually a basic chromium sulfate. In this process, the hides are in a pickled state at a pH of 3 or lower, the chrome tanning materials are introduced, and the pH is raised.
The grain leathers are then separated for retanning, dyeing, and fatliquoring. Fatliquoring is the process of introducing oil into the skin before the leather is dried to replace the natural oils lost in beamhouse and tanyard processes.



¢ TANNERY WASTE CHARACTERISTICS

Usually the tannery wastes are characterized by strong color & odor, high BOD, high pH & high dissolved salts.
This industry can cause irreversible damage to the water environment in the vicinity.
It consists of acidic and alkaline liquors, with chromium levels from 100 to 400 mg/L, sulfide levels from 200 to 800 mg/L, nitrogen levels from 200 to 1,000 mg/L.
Biochemical oxygen demand (BOD5) levels usually ranges from 160 to 24,000 mg/l. Chemical oxygen demand (COD) ranges from 800 to 43,000 mg/L and chloride is found in the range of 200 to 70,000 mg/L in individual streams and 5,600 to 27,000 mg/L in the combined stream and also high levels of fat.



Some chemicals of concern commonly used in Leather Industry:

¢ PCP (Pentachlorophenol)
¢ Hexavalent Chromium
¢ Azo Dyes (banned in Europe)
¢ Resorcinol Dyes
Formaldehyde




POLLUTION PREVENTION & CONTROL IN TANNERIES

Use salt or chilling methods to preserve hides instead of persistent insecticides and fungicides.
Inject tanning solution in the skin using high pressure nozzles and implement chrome recovery from chrome containing wastewaters which should be kept segregated from other wastewaters. Recycle chrome after precipitation and acidification. Improve fixation of chrome by addition of dicarboxylic acids.
Consider the use of carbon dioxide in deliming to reduce ammonia in wastewater.
Use only trivalent chrome when required for tanning.
Use non organic solvents for dyeing and finishing.
Recover hide trimmings for use in the manufacture of glue, gelatin, and similar products.
Recycle wastes to the extent feasible in the manufacture of fertilizer, animal feed, and tallow provided the quality of these is not compromised.
Use tanned shavings in leather board manufacture.
Control odor problems by good housekeeping, such as minimal storage of flesh trimmings and organic material.
Recover energy from the drying process to heat process water.



REFERENCES:

Danish Technological Institute. 1992. Possibilities for a Reduction of the Pollution Load from Tanneries. Final Report, Nordic Council of Ministers.
Indian Standards Institution. 1977. Guide for Treatment and Disposal of Effluents of Tanning Industry. New Delhi, India.
United Nations Environment Programme (UNEP). 1991. Tanneries and the Environment: A Technical Guide to Reducing the Environmental Impact of Tannery Operations.
World Bank, Environment Department. 1996. "Pollution Prevention and Abatement: Tanning and Leather Finishing." Technical Background Document.
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bernnebez
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01-08-2010, 04:14 PM

It is a good thing to know about this tanning seminar and presentation. It would be a great place to attend and learn more about ways and things about tanning.
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