tanning industry processes, pollution and pollution controlby abu khairul bashar

8/9/2019 Tanning Industry Processes, Pollution and Pollution Controlby Abu Khairul Bashar

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Tanning Industry Processes, Pollution and Pollution

Control


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Content

Topic Name Page No

Introduction 01

Forms of Leather 02-03

Types of Leather 04-05

The Leather Production Process 05-10

Chemicals Used in Leather Processing 11-13

Chrome Tanning 14

Tannery Wastewater Characteristics 15

Environmental Hazards of Leather 16

Pollution Control 17-19

Conclusion 20

References 20


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Introduction

Leather is a durable and flexible material created by the tanning of animal rawhide and skin,

often cattle hide. It can be produced through manufacturing processes ranging from cottage

industry to industry. Leather is used for various purposes including clothing (e.g. shoes, hats,

jackets, skirts, trousers and belts),bookbinding,leather wallpaper,and as a furniture covering. It

is produced in a wide variety of types and styles and is decorated by a wide range of techniques.

Leather sector significantly contributes in foreign exchange earnings through exports of a large

volume of leather, leather goods, and footwear. Leather is a material created through the tanning

of hides and skins of animals, primarily cattle hide. The tanning process converts the putrescible

skin into a durable, long-lasting and versatile natural material for various uses. Leather is an

important material with many uses.Together with wood, leather formed the basis of much

ancient technology. The leather industry and the fur industry are distinct industries that are

differentiated by the importance of their raw materials. In the leather industry the raw materials

are by-products of the meat industry, with the meat having higher value than the skin. The fur

industry uses raw materials that are higher in value than the meat and hence the meat is classified

as a by-product. Taxidermy also makes use of the skin of animals, but generally the head and

part of the back are used. Hides and skins are also used in the manufacture of glue and gelatin.
http://en.wikipedia.org/wiki/Tanninghttp://en.wikipedia.org/wiki/Rawhide_%28textile%29http://en.wikipedia.org/wiki/Cottage_industryhttp://en.wikipedia.org/wiki/Cottage_industryhttp://en.wikipedia.org/wiki/Bookbindinghttp://en.wikipedia.org/wiki/Leather_wallpaperhttp://en.wikipedia.org/wiki/Leather_wallpaperhttp://en.wikipedia.org/wiki/Bookbindinghttp://en.wikipedia.org/wiki/Cottage_industryhttp://en.wikipedia.org/wiki/Cottage_industryhttp://en.wikipedia.org/wiki/Rawhide_%28textile%29http://en.wikipedia.org/wiki/Tanning


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Forms of Leather

There are a number of processes whereby the skin of an animal can be formed into a supple,

strong material commonly called leather.

Vegetable-tanned leather is tanned using tannin (hence the name "tanning") and other

ingredients found in vegetable matter, tree bark, and other such sources. It is supple and

brown in color, with the exact shade depending on the mix of chemicals and the color of

the skin. Vegetable-tanned leather is not stable in water; it tends to discolor, and if left to

soak and then dry it will shrink and become less supple and harder. In hot water, it will

shrink drastically and partly gelatinize, becoming rigid and eventually brittle. Boiled

leather is an example of this where the leather has been hardened by being immersed in

hot water, or in boiled wax or similar substances. Historically, it was occasionally used as

armour after hardening, and it has also been used for book binding. This is the only form

of leather suitable for use in leather carving or stamping.

Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and othersalts of chromium. It is more supple and pliable than vegetable-tanned leather, and does

not discolor or lose shape as drastically in water as vegetable-tanned. It is also known as

wet-blue for its color derived from the chromium. More esoteric colors are possible using

chrome tanning.

Aldehyde-tanned leather is tanned using glutaraldehyde or oxazolidine compounds.

This is the leather that most tanners refer to as wet-white leather due to its pale cream or

white color. It is the main type of leather used in chrome-free leather often seen in

infant's shoes and in automobiles that prefer a chrome-free leather. Formaldehyde tanning

(being phased out due to its danger to workers and the sensitivity of many people to

formaldehyde) is another method of aldehyde tanning. Brain-tanned leathers fall into this

category and are exceptionally water absorbent. Brain tanned leathers are made by a


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labor-intensive process which uses emulsified oils often those of animal brains. They are

known for their exceptional softness and their ability to be washed. Chamois leather also

falls into the category of aldehyde tanning and like brain tanning produces a highly water

absorbent leather. Chamois leather is made by using oils (traditionally cod oil) that

oxidise easily to produce the aldehydes that tan the leather.

Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or

Neradol types. This leather is white in color and was invented when vegetable tannins

were in short supply, i.e. during the Second World War. Melamine and other amino-

functional resins fall into this category as well and they provide the filling that modern

leathers often require. Urea-formaldehyde resins were also used in this tanning method

until dissatisfaction about the formation of free formaldehyde was realised.

Alum-tanned leather is tanned using aluminium salts mixed with a variety of binders

and protein sources, such as flour, egg yolk, etc. Purists argue that alum-tanned leather is

technically "tawed" and not tanned, as the resulting material will rot in water. Very light

shades of leather are possible using this process, but the resulting material is not as supple

as vegetable-tanned leather.

Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it

while it dries. Like alum-tanning, rawhide is not technically "leather", but is usually

lumped in with the other forms. Rawhide is stiffer and more brittle than other forms of

leather, and is primarily found in uses such as drum heads where it does not need to flex

significantly; it is also cut up into cords for use in lacing or stitching, or for making many

varieties of dog chews.


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Types of Leather

In general, leather is sold in three forms:

Full-Grain leatheror Top-Grain leather is referring to the upper section of a hide that

contains the epidermis or skin layer. It refers to hides that have not been sanded, buffed

or snuffed(otherwise known as Corrected) in order to remove imperfections on the

surface of the hide. Only the hair has been removed from the epidermis. The grain

remains in its natural state which will allow the best fiber strength, resulting in greater

durability. The natural grain also has natural breathability, resulting in greater comfort for

clothing. The natural Full-Grain surface will wear better than other leather. Rather than

wearing out, it will develop a natural "Patina" and grow more beautiful over time. The

finest leather furniture and footwear are made from Full-Grain leather. For these reasons

only the best raw hide are used in order to create Full-Grain or Top-Grain leather. Full

grain leathers can mainly be bought as two finish types: aniline and semi-aniline.

Corrected-Grain leather is any Top-Grain leather that has had its surfaces sanded,

buffed or snuffed in order to remove any imperfection on the surface due to insect bites,

healed scars or brands. Top-Grain leather is often wrongly referred to as Corrected-Grain.

Although Corrected-Grain leather is made from Top-Grain as soon as the surface is

corrected in any way the leather is no longer referred to asTop-Grain leather. The hides

used to create corrected leather are hides of inferior quality that do not meet the high

standards for use in creating aniline or semi-aniline leather. The imperfections are

corrected and an artificial grain applied. Most Correct leather is used to make Pigmented

leather as the solid pigment helps hide the corrections or imperfections. Corrected grain

leathers can mainly be bought as two finish types: semi-aniline and pigmented.

Split leatheris leather that is created from the fiberous part of the hide left once the Top-

Grain of the raw hide has been separated from the hide. During the splitting operation the

grain and drop split are separated. The drop split can be further split (thickness allowing)

into a middle split and a flesh split. In very thick hides the middle split can be separated

into multiple layers until the thickness prevents further splitting. Split leather then has an


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artificial layer applied to the surface of the split and is embossed with a leather grain.

Splits can are also used to create Suede. The strongest suedes are usually made from

grain splits (that have the grain completely removed) or from the flesh split that has been

shaved to the correct thickness. Suede is "fuzzy" on both sides. Suede is less durable than

top-grain. Suede is cheaper because many pieces of suede can be split from a single

thickness of hide, whereas only one piece of top-grain can be made. However,

manufacturers use a variety of techniques to make suede appear to be full-grain. For

example, in one operation, glue is mixed with one side of the suede, which is then

pressed through rollers; these flatten and even out one side of the material, giving it the

smooth appearance of full-grain. Latigo is one of the trade names for this product. A

reversed suede is a grained leather that has been designed into the leather article with the

grain facing away from the visible surface. It is not a true form of suede.

The Leather Production Process

Leather tanning is without a doubt one of the oldest human activities. Immediately after killing

the animal, in order to avoid degradation processes in the tissues, the skin is salted, dried or

refrigerated before the production process of tanning starts. Skins are salted with common

marine salt, which penetrates very quickly into the fibers, helping to a partial removal of water.

This is a very efficient and economical process, easy to apply and widespread. The drying

system is to eliminate as much water as possible from the skin, so to avoid the development of

micro-organisms and bacteria. The drying system is more suitable for sheep and goat

skins, while less to preserve cowhides. The leather manufacturing process can be basically

divided into three major phases:

A. The preparatory stages

B. Tanning

C. Crusting

D. surface coating


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Preparatory stages

The preparatory stages are when the hide/skin is prepared for tanning. During the preparatory

stages many of the unwanted raw skin components are removed. Many options for pretreatment

of the skin exist. Not all of the options may be performed. Preparatory stages may include

preservation-the hide/skin is treated with a method which renders it temporarily unputrescible.

Soaking - water for purposes of washing or rehydration is reintroduced.

Liming - unwanted proteins and "opening up" is achieved.

Unhairing - the majority of hair is removed.

Fleshing - subcutaneous material is removed.

Splitting - the hide/skin is cut into two or more horizontal layers.

Reliming - the hide/skin is further treated to achieve more "opening up" or more protein

removal.

Deliming - liming and unhairing chemicals are removed from the pelt.

Bating - proteolytic proteins are introduced to the skin to remove further proteins and to

assist with softening of the pelt.

Degreasing - natural fats/oils are stripped or as much as is possible from the hide/skin.

Frizing - physical removal of the fat layer inside the skin. Also similar to Slicking.

Bleaching - chemical modification of dark pigments to yield a lighter coloured pelt.

Pickling - lowering of the pH value to the acidic region. Must be done in the presence of

salts. Pickling is normally done to help with the penetration of certain tanning agents,

e.g., chromium (and other metals), aldehydic and some polymeric tanning agents

Depickling - raising of the pH out of the acidic region to assist with penetration of certain

tanning agents
http://en.wikipedia.org/w/index.php?title=Preservation_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Soakinghttp://en.wikipedia.org/wiki/Liming_%28leather_processing%29http://en.wikipedia.org/w/index.php?title=Unhairing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fleshing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Splitting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Reliming_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Deliming_%28leather_processing%29http://en.wikipedia.org/w/index.php?title=Bating_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Degreasing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Frizing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Bleaching_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Pickling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Depickling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Depickling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Pickling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Bleaching_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Frizing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Degreasing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Bating_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Deliming_%28leather_processing%29http://en.wikipedia.org/w/index.php?title=Reliming_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Splitting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fleshing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Unhairing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Liming_%28leather_processing%29http://en.wikipedia.org/wiki/Soakinghttp://en.wikipedia.org/w/index.php?title=Preservation_%28leather_processing%29&action=edit&redlink=1


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Tanning

Tanning is the process that converts the protein of the raw hide or skin into a stable materialwhich will not putrefy and is suitable for a wide variety of end applications. The principal

difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible

material that can putrefy when re-wetted (wetted back), while tanned material dries out to a

flexible form that does not become putrid when wetted back. A large number of different tanning

methods and materials can be used; the choice is ultimately dependent on the end application of

the leather. The most commonly used tanning material is chromium,which leaves the leather,

once tanned, a pale blue colour (due to the chromium), this product is commonly called wet

blue. Theacidity of hides once they have finished pickling will typically be between pH of 2.8-

3.2. At this point the hides are loaded in a drum and immersed in a float containing the tanning

liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the

tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be

made to see the penetration by cutting the cross section of a hide and observing the degree of

penetration. Once an even degree of penetration is observed, the pH of the float is slowly raised

in a process called basification. This basification process fixes the tanning material to the leather,

and the more tanning material fixed, the higher the hydrothermal stability and increased

shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned

would typically finish somewhere between3.8-4.2.

There are several types of tanning:

Chrome tanning is the most widespread. The duration of the Chromium tanning is around

2 or 3 hours for small and thin skins, up to a maximum of 24 hours for thicker ones. At

the end of the tanning the skins appear blue-green. This is called wet-blue and at this

stage it can be sold.

The vegetable tanning is the oldest, made with the use of tannins which give the

vegetable tanned leather shades of brown, more or less intense.
http://en.wikipedia.org/wiki/Chromiumhttp://en.wikipedia.org/wiki/Chromium


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Crusting

Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation isincluded in the crusting sub-process. The chemicals added during crusting have to be fixed in

place. The culmination of the crusting sub-process is the drying and softening operations.

Crusting may include the following operations:

Wetting back - semi-processed leather isrehydrated.

Sammying - 45-55%(m/m) water is squeezed out the leather.

Splitting - the leather is split into one or more horizontal layers.

Shaving - the leather is thinned using a machine which cuts leather fibres off.

Neutralization - the pH of the leather is adjusted to a value between 4.5 and 6.5.

Retanning - additional tanning agents are added to impart properties.

Dyeing - the leather is coloured.

Fat liquoring - fats/oils and waxes are fixed to the leather fibres.

Filling - heavy/dense chemicals that make the leather harder and heavier are added.

Stuffing - fats/oils and waxes are added between the fibres.

Stripping - superficially fixed tannins are removed. Whitening - the colour of the leather is lightened.

Fixation - all unbound chemicals are chemically bonded/trapped or removed from the

leather

Setting - area, grain flatness are imparted and excess water removed.

Drying - the leather is dried to various moisture levels (commonly 14-25%).

Conditioning - water is added to the leather to a level of 18-28%.

Softening - physical softening of the leather by separating the leather fibres.

Buffing - abrasion of the surfaces of the leather to reduce nap or grain defects.
http://en.wikipedia.org/w/index.php?title=Wetting_back_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Rehydratedhttp://en.wikipedia.org/w/index.php?title=Sammying_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Splitting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Shaving_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Neutralisation_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Retanning_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Dyeing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fatliquoring_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Filling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Stuffing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Stripping_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Whitening_%28leather_processing%29http://en.wikipedia.org/w/index.php?title=Fixation_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Setting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Drying_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Conditioning_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Softening_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Buffing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Buffing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Softening_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Conditioning_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Drying_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Setting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fixation_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Whitening_%28leather_processing%29http://en.wikipedia.org/w/index.php?title=Stripping_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Stuffing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Filling_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fatliquoring_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Dyeing_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Retanning_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Neutralisation_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Shaving_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Splitting_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Sammying_%28leather_processing%29&action=edit&redlink=1http://en.wikipedia.org/wiki/Rehydratedhttp://en.wikipedia.org/w/index.php?title=Wetting_back_%28leather_processing%29&action=edit&redlink=1


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Surface coating

For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing

operations may include:

oiling

brushing

padding

impregnation

buffing

spraying

roller coating

curtain coating

polishing

plating

embossing

ironing

combing (hair-on)

glazing
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Figure: General Flowchart of leather Processing


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Chemicals Used in Leather Processing

The following is a list of chemicals commonly used in leather making:

Beamhouse and Tanyard

Biocides -Biocides prevent the growth of bacteria which can damage the hides or skins

during the soaking process

Surfactants-Surfactants are used to help with the wetting back of the hides or skins

Degreasers -Degreasers help with the removal of natural fats and greases from the hides

or skins

Swell regulating agents -Swell regulating agents help prevent uneven swelling of the

hides or skins during liming

Lime-Lime is used to swell the hides or skins

Sodium sulphide -Sodium sulphide chemically destroys the hair on hides or skins

Sodium hydrosulphide- Sodium hydrosulphide chemically destroys the hair on hides or

skins. It does not create as much swelling as sodium sulphide

Low sulphide unhairing agents-Low sulphide unhairing agents help to reduce the amount

of sulphides used in a tannery thus reducing the environmental impact of tanneries

Caustic soda-Caustic soda is used during the liming process to help swell the hides or

skins

Soda ash-Soda ash is used during the soaking or liming processes to help raise the pH of

the hides or skins

Ammonium sulphate-Ammonium sulphate is used during the deliming process and helps

remove lime from the hides or skins

Ammonium chloride-Ammonium chloride is used during the deliming process and helps

remove lime from the hides or skins

Sodium metabisulphite -Sodium metabisulphite is used during the deliming process and

helps prevent the formation of toxic hydrogen sulphide gas during deliming. It also acts

as a bleaching agent

Formic acid-Formic acid is used during the pickling process to lower the pH of the hides

or skins


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Sulphuric acid-Sulphuric acid is used during the pickling process to lower the pH of the

hides or skins

Salt-Salt is used during the pickling process to prevent acid swelling of the hides or skins

Sodium formate-Sodium formate is used during the tanning process to assist with the

penetration of chromium tanning salts into the hides or skins

Chromium sulphate -Chromium sulphate is the tanning agent used to make wet blue

Aldehyde tanning agents-Aldehydes are tanning agents used to make wet white

Magnesium oxide-Magnesium oxide is used during basification and raises the pH of the

hide or skin to allow the chromium or aldehyde to chemically bind to the skin protein

Fungicide-Fungicides are chemicals that are used to prevent the growth of moulds or

fungi on tanned hides or skins

Dyehouse

Surfactants / Wetting agents-Surfactants help in the wetting back of the wet blue in the

dyehouse

Degreasers-Degreasers help remove grease or fats that may be present on the wet blue as

a result of the wet blue coming into contact with machinery

Sodium formate-Sodium formate helps raise the pH during the neutralization process

Sodium bicarbonate-Sodium bicarbonate helps raise the pH during the neutralization

process

Formic acid-Formic acid reduces the pH for the rechroming process or helps with

chemically fixing dyehouse chemicals to the leather at the end of the dyehouse processes

Chrome syntans-Chrome syntans are used during rechroming to improve the softness of

the final leathet

Chromium sulphate-Chromium sulphate is used during rechroming to improve the

softness of the final leather

Syntans-Syntans are used to give properties such as softness, fullness, roundness to the

leather

Resins -Resins are used to give fullness and a tight grain to the leather

Polymers -Polymers are used to give fullness and a tight grain to the leather


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Dyes-Dyes are used to give the leather a colour desired by the customer

Dyeing auxiliaries-Dyeing auxiliaries help disperse the dyes evenly

Fatliquors- Fatliquors are oils that are added to leather to give softness to the final leather

Finishing

Acrylic resins-Acrylic resins give specific properties to the leather finish such as

adhesion, water resistance

Butadiene resins-Butadiene resins give specific properties to the leather finish such as

good coverage

Polyurethane resins-Polyurethane resins give specific properties to the leather finish such

as good toughness and good lightfastness

Fillers-Fillers help fill small blemishes on the leather surface

Dullers-Dullers help reduce the gloss of the finish

Crosslinkers-Crosslinkers are used to toughen the leather finish and improve the water

resistance properties of polyurethanes

Handle modifiers-Handle modifiers are used to give the leather surface a waxy or

slippery feel

Nitrocellulose lacquers -Nitrocellulose lacquers are used in the top coat of a leather finish

Acrylic lacquers-Acrylic lacquers are used in the top coat of a leather finish

Polyurethane lacquers -Polyurethane lacquers are used in the top coat of a leather finish

Viscosity modifiers -Viscosity modifiers are used to increase the viscosity of a finish

mixture

Pigments-Pigments are colouring agents that help hide defects on the leather surface

Dyes-Dyes are colouring agents that are used to slightly change the colour of the leather

finish or to give the leather finish a more natural look

Defoamers- Defoamers are used to prevent bubbles from forming in the finish mixture


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Chrome Tanning

The most common tanning method in the world. Today, 80-90% of leathers in the world are

tanned by chrome tanning. Chrome tanning uses a solution of chemicals, acids and salts(including chromium sulphate) to tan the hide. Its a very quick process, taking about a day to

produce a piece of tanned leather. First hide are limed to remove hair and then are pickled by

being left in the acid salt mixture, before being placed in the chromium sulphate. All hides then

come out looking light blue (known as wet blue).In 2008, about 24 million tonnes of

chromium was produced. About 2% of it has been used for the production of chromium sales,

such as chromium sulphate, for the making of leather tanning materials but also for the

production of dyestuffs and plastics. Worldwide approximately 480,000 tons of chromium

tannins are produced per year.The most important chrome deposits are found in South Africa

accounting for 33% of production, while India and Kazakhstan provided 20% and 17%

respectively. Brazil, Finland, Oman, Russia, and Turkey together contributed a further 21%,

while some 12 smaller producer countries brought the balance of 9%.

Advantages of Chrome Tanning

Quick and easy to produce, usually only taking up to a day

Water can roll off the surface easily with appropriate retanning and finishing processes

Soft and supple to the touch

It is possible to obtain leather with a stable colour

It is cheaper to buy than vegetable tanned leather, which means it is also easier to find

It has a high degree of thermal resistance

Disadvantages of Chrome Tanning

Chrome tanning is very bad for the environment

Its produced with little craftsmanship and very often mass produced

It doesnt wear well with time

Chrome tanning often smells of chemicals

It doesnt appear (neither is it) very natural


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Tannery Wastewater Characteristics

Tannery wastewaters are highly complex and are characterized by high contents of organic,

inorganic and nitrogenous compounds, chromium, sulfides, suspended solids and dissolved

solids. The characteristics of tannery wastewater vary considerably from tannery to tannery

depending upon the size of the tannery, chemicals used for a specific process, amount of water

used and type of final product produced by a tannery. Tannery wastewater is characterized

mainly by measurements of

Biochemical Oxygen Demand (BOD),

Chemical Oxygen Demand (COD),

suspended solids (SS) and

Total Dissolved Solids (TDS),

chromium and

Sulfides etc.

In general, tannery wastewaters are basic, have a dark brown color and have a high content of

organic substances that vary according to the chemicals used. The influents were characterized

by high alkalinity content with a resulting pH value of above 8 due to the chemicals used in

leather processing.


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Environmental Hazards of Leather

Raising animals for food and leather requires huge amounts of feed, pastureland, water, and

fossil fuels. Animals on factory farms produce 130 times as much excrement as the entire human

population, without the benefit of waste treatment plants. The U.S. Environmental Protection

Agency (EPA) has even acknowledged that livestock pollution is the greatest threat to our

waterways.Although some leathermakers deceptively tout their products as eco-friendly,

turning skin into leather also requires massive amounts of energy and dangerous chemicals,

including mineral salts, formaldehyde, coal-tar derivatives, and various oils, dyes, and finishes,

some of them cyanide-based. Most leather produced in the U.S. is chrome-tanned; all wastes

containing chromium are considered hazardous by the EPA.Tannery effluent contains large

amounts of pollutants, such as salt, lime sludge, sulfides, and acids. The process of tanning

stabilizes the collagen or protein fibers in skins so that they actually stop biodegrading

otherwise the leather would rot right off your feet.

People who work in and live near tanneries suffer too. Many die from cancer possibly caused by

exposure to toxic chemicals used to process and dye the leather. The Centers for Disease Control

and Prevention found that the incidence of leukemia among residents in an area near one tannery

in Kentucky was five times the U.S. average.Arsenic, a common tannery chemical, has long been

associated with lung cancer in workers who are exposed to it on a regular basis. Studies of

leather-tannery workers in Sweden and Italy found cancer risks between 20% a nd 50% above

[those] expected.Additionally, raising the animals whose skin eventually becomes leather

requires vast quantities of water and wide tracts of pastureland, which must be cleared of trees.

Runoff from feedlots and dairy farms also creates a major source of water pollution. Huge

amounts of fossil fuels are consumed in livestock production as well; by contrast, plastic

wearables account for only a fraction of the petroleum used in the U.S.The production of leather

hurts animals, the environment, and the workers who manufacture it. The only ones who benefit

are people who profit from the misery and suffering of others.


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Pollution Control

Water pollution control-

Untreated tannery wastes in surface waters can bring about a rapid deterioration of their physical,

chemical and biological properties. Simple end-of-pipe effluent treatment processes can remove

over 50% of suspended solids and biochemical oxygen demand (BOD) of effluent. More

sophisticated measures are capable of higher levels of treatment. As tannery effluents contain

several chemical constituents that need to be treated, a sequence of treatment processes in turn

must be used. Flow segregation is useful to allow separate treatment of concentrated waste

streams.

Pre-treatment settling Mechanical screening to remove coarse material

Flow equalization (balancing)

Primary treatment Sulphide removal from beamhouse effluents

Chromium removal from tanning effluents

Physical-chemical treatment for BOD removal and

neutralization

Secondary treatment Biological treatment

Activated sludge (oxidation ditch)

Activated sludge (conventional)

Lagooning (aerated, facultative or anaerobic)

Tertiary treatment Nitrification and denitrification

Sedimentation and sludge

handling

Different shapes and dimensions of tanks and basins

Table: Summarizes technological choices available for treatment of tannery effluents.


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Air pollution control-

Air emissions fall into three broad groups: odours, solvent vapours from finishing operations and

gas emissions from the incineration of wastes.Biological decomposition of organic matter as well

as sulphide and ammonia emissions from wastewaters are responsible for the characteristic

objectionable odours arising from tanneries. The siting of installations has been an issue because

of the odours that have historically been associated with tanneries. Reduction of these odours is

more a question of operational maintenance than of technology.

Solvent and other vapours from the finishing operations vary with the type of chemicals used and

the technical methods employed to reduce their generation and release. Up to 30% of the solvent

used may be wasted through emissions, while modern processes are available to reduce this to

around 3% in many cases.The practice by many tanneries of incinerating solid wastes and offcuts

raises the importance of adopting good incinerator design and following careful operating

practices.

Waste management

Treatment of sludge constitutes the largest disposal problem, apart from effluent. Sludges of

organic composition, if free from chrome or sulphides, have value as a soil conditioner as well as

a small fertilizer effect from nitrogenous compounds contained therein. These benefits are best

realized by ploughing immediately after application. Agricultural use of chrome-containing soils

has been a matter of controversy in various jurisdictions, where guidelines have determined

acceptable applications.Various markets exist for the conversion of trimmings and fleshings into

by-products used for a variety of purposes, including the production of gelatin, glue,

leatherboard, tallow grease and proteins for animal feed. Process effluents, subject to suitable

treatment and quality control, are sometimes used for irrigation where water is in short supply

and/or effluent disposal is severely restricted.

To avoid problems of leachate generation and odour, only solids and dewatered sludges should

be disposed of at landfill sites. Care must be taken to ensure that tannery wastes do not react with

other industrial residues, such as acidic wastes, which can react to create toxic hydrogen sulphide

gas. Incineration under uncontrolled conditions may lead to unacceptable emissions and is not

recommended.


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Pollution Prevention

Improving production technologies to increase environmental performance can achieve a number

of objectives, such as:

increasing the efficiency of chemical utilization

reducing water or energy consumption

Recovering or recycling rejected materials.

Water consumption can vary considerably, ranging from less than 25 l/kg of raw hide to greater

than 80 l/kg. Water use efficiency can be improved through the application of techniques such as

increased volume control of processing waters, batch versus running water washes, low float

modification of existing equipment; low float techniques using updated equipment, re-use of

wastewater in less critical processes and recycling of individual process liquors.

Traditional soaking and unhairing account for over 50% of the BOD and chemical oxygen

demand (COD) loads in typical tanning effluents. Various methods can be employed to substitute

for sulphide, to recycle lime/sulphide liquors and to incorporate hair-saving techniques.

Reduction in chromium pollution can be achieved through measures to increase the levels of

chrome that are fixed in the tanning bath and reduce the amounts that are bled out in

subsequent processes. Other methods to reduce release of chromium are through direct recycling

of used chrome liquors (which also reduces salinity of waste effluent) and the treatment of

collected chrome-bearing liquors with alkali to precipitate the chromium as hydroxide, which

can then be recycled


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Conclusion

The leather sector contributes substantially to the industrialization process of the country as well

as export earnings. The sector is passing through a transition period currently. Creation of new

products with markets has become imperative in view of the global compulsion. Tannery

industry is considered to be a major source of pollutant and tannery wastewater in particular, is a

potential environmental concern.

References

Doris Schubert, Assessment of the Environmental Release of Chemicals From the

Leather Processing Industry, IC-07 Leather Processing Industry 28 July 1998.

Reuters, Toxic Tanneries Drive Bangladesh Leather Exports: Report, 9 Oct. 2012.

Veyalkin I, Gerein V. Retrospective cohort of cancer mortality at the minsk leather

tannery. Industrial Health. 2006;44:6974 International School of Tanning Technology-https://sites.google.com/site/isttschool/

Srivastava, S., A.H. Ahmad and I.S. Thakur, 2007. Removal of chromium and

pentachlorophenol from tannery effluents. Biores. Technol., 98: 1128-1132.

Wastewater Engineering by Metcalf and Eddy-fourth edition

tanning industry processes, pollution and pollution controlby abu khairul bashar

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