Tanning Industry Processes, Pollution and Pollution Control
Content Topic Name
Forms of Leather
Types of Leather
The Leather Production Process
Chemicals Used in Leather Processing
Tannery Wastewater Characteristics
Environmental Hazards of Leather
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.
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. ďƒ› 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. ďƒ› Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other salts 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. ďƒ› 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
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. ďƒ› 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 aminofunctional 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. ďƒ› 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. ďƒ› 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.
Types of Leather In general, leather is sold in three forms: ď‚ˇ
Full-Grain leather or 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 leather is leather that is created from the fiberous part of the hide left once the TopGrain 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 4|Page
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
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
Tanning Tanning is the process that converts the protein of the raw hide or skin into a stable material which 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‖. The acidity of hides once they have finished pickling will typically be between pH of 2.83.2. 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.
Crusting Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included 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 is rehydrated.
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.
Surface coating For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include:
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 11 | P a g e
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 12 | P a g e
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. It’s 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
It’s produced with little craftsmanship and very often mass produced
It doesn’t wear well with time
Chrome tanning often smells of chemicals
It doesn’t appear (neither is it) very natural 14 | P a g e
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% and 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 ď ż Water pollution controlUntreated 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)
Sulphide removal from beamhouse effluents Chromium removal from tanning effluents Physical-chemical treatment for BOD removal and neutralization
Biological treatment Activated sludge (oxidation ditch) Activated sludge (conventional) Lagooning (aerated, facultative or anaerobic)
Tertiary treatment Sedimentation
Nitrification and denitrification and
sludge Different shapes and dimensions of tanks and basins
handling Table: Summarizes technological choices available for treatment of tannery effluents.
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ď ż Air pollution controlAir 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. ď ż 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. 18 | P a g e
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.
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;44:69–74
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
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