The History Of The Tanning Operation

Published Date: 02 Nov 2017

The process of tanning is the midpoint and the basis of leathermaking. Tanning is the stabilization of the collagen structure of the hide, using natural or synthetic chemicals. The stabilization is mainly an increase in resistance against water and it leads to a restricted swelling.

Resistance against water means that tanned material can no more underwent changements, which are caused by an aqueous medium: putrification, swelling and drying off to an inflexible solid mass. Tanning also leads to a changement of the appearance and of the handle or feel of skin or other kinds of connected tissues. The object of converting pelt into leather by tanning is to

stabilize it against enzymatic degrading and increase its resistance to chemicals,

raise its shrinking temperature and increase its resistance to hot water

reduce or eliminate its ability to swell

enhance its strength properties

lower its density by isolating the fibers

reduce its deformability

reduce its shrinkage in volume, area and thickness,

enhance the porosity of its fiber texture.

These effects are achieved by cross-linking the collagen chains with various tanning agents. A variety of chemical agents for tanning is available, with chrome being the most commonly used (Butler and Flynn, 2006).

What is chromium?

Chromium is a naturally occurring element found in rocks, animals, plants, soil, and in volcanic dust and gases. Chromium is present in the environment in several different forms. The most common forms are chromium(0), trivalent (or chromium(III)), and hexavalent (or chromium(VI)). Chromium(III) occurs naturally in the environment and is an essential nutrient required by the human body to promote the action of insulin in body tissues so that sugar, protein, and fat can be used by the body. Chromium(VI) and chromium(0) are generally produced by industrial processes. No known taste or odor is associated with chromium compounds. The metal chromium, which is the chromium(0) form, is a steel-gray solid with a high melting point. It is used mainly for making steel and other alloys. The naturally occurring mineral chromite in the chromium(III) form is used as brick lining for high-temperature industrial furnaces, for making metals and alloys (mixtures of metals), and chemical compounds. Chromium compounds, mostly in chromium(III) or chromium(VI) forms, produced by the chemical industry are used for chrome plating, the manufacture of dyes and pigments, leather tanning, and wood preserving. Smaller amounts are used in drilling muds, rust and corrosion inhibitors, textiles, and toner for copying machines.

(retrieved on : http://www.eoearth.org/article/Health_effects_of_chromium )

Chromium in Tanning Industry:

The industries with the largest contribution to chromium levels include leather tanning industries.

Tanning is the stage in which raw leather is processed and made more durable so that it can be made into various products in the subsequent segment of the industry. Tanning is composed of three stages as well: the pretreatment of animal hides, the application of a tanning agent, and finishing the material with treatments such as drying and shining (Correia; Stephenson, and Judd, 1994).

Chromium is one of the most widely used chemicals throughout this process. The wastewater effluent from tanneries can be a dangerous source of pollutants, and often contains dissolved and suspended organic and inorganic solids, potentially toxic metal salts, chrome, and electrolytes such as sodium chloride and sulfide. In general, these effluents can cause environmental problems related to their high chemical oxygen demand and elevated chromium concentrations (Jenkins, Barton, and Hesselberg, 2004).

Tanneries discharge contaminated wastewater into rivers, the pipelines and canals that transport wastewaters away from these industrial facilities often run through villages, where they occasionally leak into surrounding soils or are used to irrigate crops. People can be exposed to the contaminants produced by tanning through various pathways. The most common occupational hazard is inhalation of chromium at the work site. However, the populations near a tannery are often exposed to pollutants through contaminated water. People use the water from contaminated rivers and streams for several purposes, including irrigation, swimming, bathing, and washing dishes and clothing. There is also a high risk of this hazardous waste water mixing with the ground water, which is extracted for drinking water. The direct discharge of these wastes has contaminated the ground and surface water with dangerously high concentrations of chromium (Bhuiyan, 2010).

Tanning Industry is considered to be a major source of pollution and tannery wastewater in particular, is a potential environmental concern (Ros and Ganter 1998) . Tanning industry wastes poses serious environmental impact on water (with its high oxygen demand, discolouration and toxic chemical constituents (Song et al. 2000), terrestrial and atmospheric systems. Tannery waste characteristically contains a complex mixture of both organic and inorganic pollutants. For example, in related studies, chlorinated phenols and chromium were found to be closely associated with the tannery waste (Reemste and Jekel 1997; Mwinyihija et al. 2005, 2006). Chromium as inorganic pollutant is a transition metal and exists in several oxidation states, with trivalent Cr3+ and

hexavalent Cr6+ species being the most common forms (KotaÅ› and Stasicka 2000). Furthermore when the two species of chromium (trivalent and hexavalent) are compared, differences in their chemical properties are observed (Andersen 1998). Indeed chlorinated phenols (e.g. 3, 5-dichlorophenol) as an organic pollutant associated with the tanning industry have been found to be highly toxic and affect the cellular compounds of organisms (Pasco et al. 2000) exposed to such waste.

Other pollutants of concern within the tanning industry include Azodyes, Cadmiumcompounds, Cobalt, Copper, Antimony, Barium, Lead, Selenium, Mercury, Zinc,Arsenic, Polychlorinated Biphyenls (PCB), Nickel, Formaldehyde resins andPesticides residues. Because tannery wastewater contains a complexity of pollutants including chromium and chlorinated phenols as indicated earlier, it is vital to dissect the toxic nature of such wastewater both to understand its environmental impacts and identify potential remediation strategies. Furthermore, there are strict regulations imposed for the environmental control of pollutants such as heavy metals and persistent organic pollutants (Tunay et al. 1994). Tannery wastewater is generally treated by various physico-chemical and biological methods and by a combination of both (Reemste and Jekel 1997). Physical and chemical processes are frequently employed to treat contaminated sites, but often do not destroy contaminants (Bouwer et al. 1994).

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Leather Sector

To comprehend on the essentials of ecotoxicology it is important to review the basis of the leather sector and explore the production chain. Thus the transformation of animal hides and skins into leather primarily involves five main phases; flaying, preservation (whenever applicable) to arrest putrefaction; removal of hair and flesh, and tanning depending on the final leather products targeted through the application of chemical agents. The aim eventually is to render the raw material to be non-putrescible and enhance its durability (Mwinyihija, 2010).

Pollution from the leather processing industries has a negative long-term impact on both the ecosystem health and functionality; and economic growth potential of a country irrespective of the immediate profit accruals intended. Development of the industry in Africa for example does not match the technical know-how and capacity in protecting and predicting the environmental impacts related to the industry. Cleaning up of such environment will require expenditure of funds, which could have promoted positive and sustainable development. Tanning has thus been deemed as one of the largest polluters in the world.

Tanning pollutants and its effect to the ecosystem

Tanning Industry involves the following processes in brief, linked closely with pollution: (retrieved on: http://www.academicjournals.org/jece/pdf/pdf2011/22%20Nov/Mwinyihija.pdf)

Soaking

The main aim at this stage is to wash the rawstock from physically bound materials mostly insecticides, salts (NaCl2) and other preservatives. Moreover rehydration of hides and skins occurs during soaking to replace lost moisture during the preservationphases (also referred to as curing) to allow permeation of chemicals during subsequent processes (Cassano et al. 2001). It is within this stage that high volume of water used consequently results to high discharge of effluent with high pollutant load. The current trends in the tanning industry especially

in the developing world preferably use salted hides and skins rather than air dried due to their high rehydration potential. This approach of curing emit high load of salt which predisposes arid conditions to the environment.

Liming

Liming involves the use of alkaline medium (e.g. lime) to condition raw hidesand skins. The aim is to remove the hair, flesh and splitting up of the fibre bundles by chemical and physical means (Ramasami and Prasad 1991). In this

process, Na2S is added to facilitate de-hairing (Flaherty et al. 1959). It is estimated that for processing 1 ton of raw skins (weight of skins before soaking),

De-liming, Bating and Pickling

Weak organic acids, digestive enzymes and inorganic acids, respectively, are used to remove lime, digest and remove the non-structural proteins and eventually bring the pH to a level that will enhance the tanning process (Cassano et al. 2001;Thanikaivelan et al. 2003).

Chrome Tanning

New techniques for improving the recycling of chromium to reduce its impacts to the environment are unavailable in the developing world because of continual use of traditional methods of chrome tanning. The main assumption in such systems relies on the fact that Cr salts precipitate with NaOH followed by the dissolution of Cr(OH)3 in sulphuric acid. However, the quality of the recovered solution is not always optimal due to the presence of the toxic state of the metal, lipidic substances and other impurities (Cassano et al. 2001). Therefore, whatever method is used to reduce the amount of Cr salts in the final discharge, it will portend chromium as a potentially toxic source to the environment. Nearly 90% of all leather produced is tanned using Cr salts (Stein and Schwedt 1994). Generally 8% of the basic chromium sulphate salt is used for conventional tanning. It binds with the collagenous protein to convert to leather. The mentioned processes are a few of the total that are found in processing of leather but they form

the vital point of intervention in pollution control. The main pollutants found during leather processing include, NaCl2 and pesticides, strong alkalines and sulphides, inorganic residual compounds, dissolved matter and chromium salts. Chlorinated phenols are important compounds to be investigated due to the various mixtures used in the tanning industry and their ecotoxicity potential (UNEP 1994). Vegetable tannins are also used to retan leather to impart certain specific desired properties or could be used alone in producing leather especially at the rural tanning level by the use of plant material (tree barks and pods are commonly used in Africa). Mostly, the vegetable tannin materials are derived from plants and consist of condensed or hydrolysable tannins (Zywicki et al. 2002). In the East African region, wattle is extensively used as a tannin material. Efforts to study the polyphenolic structures of condensed tannins have been hampered by the fact that the structure rapidly transforms during the tanning process to yet unknown products (Zywicki et al. 2002). However the use of chromium salts during leather tanning is

still common in most of the developing world.

Chromium in the Environment

Water Systems

In this system, Cr originates from weathering or rock, wet precipitation and dry fallout from the atmosphere and run off from the terrestrial systems (Kotaś and Stasicka 2000). In rivers and lakes, the Cr concentration is usually limited to 0.5–100 nM (Handa 1988; Kaczynski and Kieber 1993). In seawaters it varies from 0.1 to 16 nM (Dejong and Brinkman 1978). The tanning industry can contribute significantly to the increase in Cr concentration if located near the water systems. In nature Cr exists in its two stable oxidation states, Cr3+ and Cr6+. The presence and ratio between these two forms depend on various processes; chemical and photochemical, Redox transformation, precipitation/dissolution (Kotaś and Stasicka 2000). Campanella (1996) described the Cr3+ in oxygenated aqueous solution as predicted by thermodynamic calculations on the stable species at pH £ 6 whereaspH ³ 7 the CrO4 2− ion as predominate under anoxic and suboxic condition, trivalent Cr should be the only form (Kotaś and Stasicka 2000).

The nature and behaviour of various Cr forms found in wastewatercan be quite different from those present in natural water because of altered physico-chemical condition of the effluents originating from various industrial sources.

The presence and concentration of Cr forms in effluents depends on Cr compounds applied during processing, pH, organic and/or inorganic waste coming from the material processing (KotaÅ› and Stasicka 2000).

Cr3+ dominates from tanneries, textile (printing, dyeing) and decorative plating industry waste (Nriagu 1988). With reference to the tanning industry, Cr3+ in the effluents is the most expected form but with Redox reactions occurring in the sludge, an increase in the hexavalent form can occur. Under slightly acidic or neutral pH conditions in this type of wastewater, the poorly soluble Cr(OH3).aq should be the preferred form, but a high content of organic matter originated from hide/ skin material processing is effective in forming soluble Cr3+ complexes (Stein and

Schwedt 1994; Walsh and O’Haloran 1996a, b).

Soil Systems

An increase in local chromium concentration in soils (Table 2.3) originates from fallout and wash out of atmospheric chromium containing particles as well as from the chrome bearing sludge and refuse from industrial activity (KotaÅ› and Stasicka 2000). Dechromification is thought of as being of vital importance because without

it, theoretically all atmospheric oxygen could be a threat to life on earth (James and Bartlett 1983).

Cr3+ adsorption into humic acids renders it insoluble, immobile and unreactive. This process is most effective within the pH range of 2.7–4.5 (Walsh and O’Haloran 1996b). Other macromolecular ligands behave similarly. In contrast, mobile ligands such as citric acid, diethylene triamine pentaacetic acid (DTPA) and fulvic acid form soluble Cr3+ complexes, which mediate its relocation and oxidation to Cr6+ in soils (James and Bartlett 1983; James 1996).

Atmospheric Systems

Chromium present in the atmosphere originates from anthropogenic sources, which account for 60–70%, as well as from natural sources, which account for the remaining 30–40% (Seigneur and Constantinou 1995). Industrial activities still remain the major source of pollution to the atmospheric systems. Others could be natural sources like volcano eruptions and erosion of soil and rocks (Kotaś and Stasicka 2000). Sea salt particles and forest wild fires do not seem to be important sources of chromium (Payna and Nriagu 1988). Average atmospheric concentrations of this metal are, 1 ng/m3 in rural to 10 ng/m3 in polluted urban areas (Nriagu 1988). The amount of chromium at any particular time depends on the intensity of industrial processes, proximity to the sources, the amount of chromium released and meteorological factors (Kotaś and Stasicka 2000).

Chromium Pathways in the Environment

The distance covered by a deposited metal in the environment depends on meteorological factors, topography and vegetation (Nriagu 1988). Transport within the terrestrial and water systems is greatly affected by chemical speciation; chemical forms of chromium and their affinity to chemical and photochemical Redox transformations, precipitation/dissolution and adsorption/desorption process e.g. occurring in individual compartments of the biogeochemical cycle of chromium (KotaÅ› and Stasicka 2000).

Cr6+ is known as the most mobile Cr form in soil and water and tends to dominate in these systems, whereas Cr3+ is generally not transported over great distances because of its low solubility and tendency to be adsorbed in the pH range typical of natural soils and water. Redox conversion of Cr3+ to Cr6+ can increase Cr dislocation from the soil into the water systems (Schroeder and Lee 1975; Bartlett and Kimble 1976; James and Bartlett 1983).

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Leather Tanning Industry in Pakistan

The tanning industry occupies an important place in Pakistan's economy. At the time of independence, there was no tanning worth the name in Pakistan except Bata. Few tanneries at Lahore and Multan were producing only vegetable sole leather. The entire production of hides and skins were being exported in a raw form. Thereafter the local tanning industry making at first semi-finished leather made rapid progress due to favourable raw material situation, cheap labour and the existence of growing demand and foreign market, and is now reckoned among the country's main industries. The present tanning industry consists of about 436 tanning units with an estimated installed capacity at around 47.58 million square metre of chrome leather and 28.18 million kgs. of vegetable tanned leather.

TANNERIES IN PAKISTAN

Punjab

301

Sindh

117

N.W.F.P

15

Baluchistan

1

Azad Kashmir

2

TOTAL

436

The availability of indigenous raw material to this industry depends upon the livestock population. Livestock population has increased at an annual average rate of 5.37 per cent during 1981-89. Availability of hides and skins with the break-up of slaughtered and fallen animals are 6.53 million hides and 36.23 million skins available for the tanning industry. Of these 46.6 per cent came from goat, 36 per cent from sheep, 10 per cent from buffalo and 66 per cent from cow. Of the 436 tanneries 50 are fully mechanised converts these hides and skins into leather.

Chromium Causing Diseases:

Chromium can exist in air, water, soil, and food, and common exposure pathways include ingestion, inhalation, or dermal contact. The primary health impacts from chromium are damage to the gastrointestinal, respiratory, and immunological systems, as well as reproductive and developmental problems. Chromium VI is a known human carcinogen, and depending on the exposure route, can increase the rate of various types of cancers. Occupational exposure to chromium VI, which often occurs through inhalation, has been linked to increased rates of cancer in the respiratory system. According to the WHO, over 8,000 workers in the tanneries suffer from gastrointestinal, dermatological, and other diseases, and 90% of this population dies before the age of 50 (Maurice, 2001).

Chromium (VI) trioxide, chromates and dichromates are highly toxic by all exposure routes

Chromium (VI) compounds are toxic and can cause cancer in humans

Chromium (VI) trioxide and chromates are corrosive and oxidising

Ingestion of chromium (VI) chromate/dichromate compounds may cause immediate burning of the mouth and throat. severe damage to the gut, heart, liver and kidneys, and possibly death

Short-term inhalation of chromium (VI) chromate/dichromate dust or mist cause irritation of the respiratory tract and lung damage

Long-term inhalation of chromium (III) salts causes swelling of the airways and lungs

Long-term inhalation of chromium (VI) may cause ulcers in the nose, irritation of the lungs, lung cancer and possible kidney effects

Skin exposure can result in skin ulcers if left untreated

Chromium (VI) compounds can cause cancer in humans. It is unknown if chromium (III) can cause cancer

Potassium dichromate may be toxic to the reproductive system and the unborn child.