Leather production processes
Encyclopedia
The leather manufacturing process is divided into three sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating may be added into the sequence. The list of operations that leathers undergo vary with the type of leather.
making process
is in general restricted to batch
processing, but if the surface coating sub-process is added then some continuous processing can be included. The operation
flow has to follow the preparatory → tanning → crusting → surface coating sub-process order without deviation, but some of the sub-processes can be omitted to make certain leathers (or partially tanned/ untanned products).
, 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 2.8-3.2. 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 between 3.8-4.2.
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of turbid and foul-smelling wastewater, including chromium levels of 100–400 mg/L, sulfide levels of
200–800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport.
With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.
Tanning is especially polluting in countries where environmental norms are lax, such as in India - the world's 3d largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg. VOC emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. Very clearly, the process remains highly polluting all the same. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization
posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that « Even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries. »
In Kanpur, the self-proclaimed "Leather City of World" and a city of 3 million people on the banks of the river Ganges, pollution levels were so high that, despite an industry crisis, the pollution control board has decided to seal 49 high-polluting tanneries out of 404 in July 2009 . In 2003 for instance, the main tannery's effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open .
The higher cost associated to the treatment of effluents as compared to untreated effluent discharging leads to environmental dumping to reduce costs. For instance, in Croatia
in 2001, proper pollution abatment cost 70-100 USD/t of raw hides processed against 43 USD/t for irresponsible behaviour.
No general study seems to exist, but the current news is rife with documented examples of untreated effluent discharge. In November 2009 for instance, it was discovered that one of Uganda's main leather producing companies directly dumped its waste water in a wetland adjacent to Lake Victoria
.
Production management
The leatherLeather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
making process
Industrial process
Industrial processes are procedures involving chemical or mechanical steps to aid in the manufacture of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry....
is in general restricted to batch
Batch
Batch may refer to:Food and drink*Batch , an alcoholic fruit beverage*Batch loaf, a type of bread popular in Ireland*A dialect term for a bread roll used in Nuneaton and Coventry, England*Small batch, bourbon whiskey blended from selected barrels...
processing, but if the surface coating sub-process is added then some continuous processing can be included. The operation
Manufacturing operations
Manufacturing operations concern the operation of a facility, as opposed to maintenance, supply and distribution, health, and safety, emergency response, human resources, security, information technology and other infrastructural support organizations....
flow has to follow the preparatory → tanning → crusting → surface coating sub-process order without deviation, but some of the sub-processes can be omitted to make certain leathers (or partially tanned/ untanned products).
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.
- limingLiming (leather processing)In Liming for parchment or leather processing, the hides are soaked in an alkali solution by a drum/paddle or pit-based operation where four main objectives are met...
- 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 chromiumChromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
, 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 2.8-3.2. 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 between 3.8-4.2.
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.
- neutralisation - 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.
- fatliquoring - 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:- oiling
- brushing
- padding
- impregnation
- buffing
- spraying
- roller coating
- curtain coating
- polishing
- plating
- embossing
- ironing
- combing (hair-on)
- glazing
Environmental impact
In addition to the other environmental impacts of leather, the production processes have a high environmental impact, most notably due to:- the heavy use of polluting chemicals in the tanning process
- air pollution due to the transformation process (hydrogen sulfideHydrogen sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
during dehairing and ammoniaAmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
during deliming, solvent vapours).
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of turbid and foul-smelling wastewater, including chromium levels of 100–400 mg/L, sulfide levels of
200–800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport.
With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.
Tanning is especially polluting in countries where environmental norms are lax, such as in India - the world's 3d largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg. VOC emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. Very clearly, the process remains highly polluting all the same. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization
United Nations Industrial Development Organization
The United Nations Industrial Development Organization , French/Spanish acronym ONUDI, is a specialized agency in the United Nations system, headquartered in Vienna, Austria...
posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that « Even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries. »
In Kanpur, the self-proclaimed "Leather City of World" and a city of 3 million people on the banks of the river Ganges, pollution levels were so high that, despite an industry crisis, the pollution control board has decided to seal 49 high-polluting tanneries out of 404 in July 2009 . In 2003 for instance, the main tannery's effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open .
The higher cost associated to the treatment of effluents as compared to untreated effluent discharging leads to environmental dumping to reduce costs. For instance, in Croatia
Croatia
Croatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...
in 2001, proper pollution abatment cost 70-100 USD/t of raw hides processed against 43 USD/t for irresponsible behaviour.
No general study seems to exist, but the current news is rife with documented examples of untreated effluent discharge. In November 2009 for instance, it was discovered that one of Uganda's main leather producing companies directly dumped its waste water in a wetland adjacent to Lake Victoria
Lake Victoria
Lake Victoria is one of the African Great Lakes. The lake was named for Queen Victoria of the United Kingdom, by John Hanning Speke, the first European to discover this lake....
.