The starting point of leather production is the
natural product skin. The finished material is the natural product leather.
Between the two lie a number of chemical processes, some of which are very
Water plays a special role in leather production,
for example, for washing of the hides and pelts which often bear various amount
of salts, dung, blood, fat and residual flesh and also as a means of transport
of chemicals used for depilation and for the actual tanning and drying
processes as well as the removal of contaminants in so far as they remain in
the waste water. More ever, water is also required for many intermediate
The substances of major chemical and
environmental importance are sulphur (in the form of sodium sulphide) and chromium
as used in chrome tanning. Both substances are classified as Dangerous
substances by the German water management law.
The nature of leather production has changed
fundamentally during the last 100 years. The former craftsmans workshops have
become gigantic, extensively mechanized and automated industrial operations.
Yet the raw and pelts are still the greatest cost factor, accounting from 60%
of the outlay.
The leather industry views itself as a recycling
industry because it contributes to the disposal of hides and pelts necessarily
occurring as by-products of the meat production and produces a valuable
material having many uses. Disposal problems do occur with those parts of hides
cannot be tanned to produce leather and some of its auxiliary materials.
Above all, it is important to ensure that a
holistic environment view is adopted. Any regulations must be well considered
and the interplay between waste water and solid waste (sludge) must receive due
consideration. For example, the chromium level in the aqueous effluent can be
further reduced at high cost by use of additional chemicals, but only to
produce additional amounts of sludge requiring expensive disposal. It is
therefore appropriate to take a closer look at the importance of chromium in leather
Chromium and Chrome Tanning:-
Chrome tanning was invented in 1858 by the
German chemist F.L. Knapp and come into general use about 100 years ago after
various improvements to process had been introduced in the USA. Since then, some 85 % of the leather produced worldwide is tanned with chromium salts, either
alone or in combination with other tanning agents.
Attacks on chrome tanning as toxic or highly toxic are usually made in the absence of a knowledge of the true situation, or half truth are knowing propagated. The view that heavy metal is synonymous with poison (chromium in metallic form is a heavy metal) is widely held, but it is not true in this general form.
What are missing from the assessment of chrome tanned leather are a balanced view and a distinction between the valence state and chemical form in which chromium is present. Only the trivalent form is active as a tanning agent. Now a day, tanning is usually performed with basic chromium sulphate, which accounts for the intrinsic blue-green colour. In contrast, compounds of the hexavalent form of chromium are rightly regarded as dangerous. This necessary differentiation is often overlooked in the case of leather.
In the 1968 edition of the leading German encyclopedia Brockhaus, It was written, The nocuousness of chromium and its components depends upon their chemical form and upon the nature and duration of their action on the human body. Metallic chromium and its trivalent compounds are regarded as almost non toxic, hexavalent chromic acid and its salt (chromates) are mainly toxic.
It is difficult to understand why this realization and the results of scientific research on chromium and chromium of leather have not received recognition and sufficient consideration in environment legislation. There is probably no other metal whose properties are affected to such a great extent by valancy as chromium. Thus it is all the more important that the normal consumer is not confused or deliberately mislead.
If a distinction between the two kinds of
chromium is made and the explanation in Brockhous encyclopedia is
supplemented by the words of Paracelsus, founder of iatrochemistry, that it is
solely the amount (dose) which decides whether a substance is a poison, it
becomes possible to reconcile such contradictory statements as chromium is a
poison and Chromium is an essential trace element. The first statement
refers to oxidation state that is present in leather.
Another example illustrating the need for distinction is that of oxygen. Human cannot exist without the diatomic form of this element O2 while the tratomic form, O3 known as ozone is poisonous.
Reports of allergies caused by chromium are generally attributable to the hexavalent form (or to another accompanying substance), moreover, then only occur rarely predisposed persons. Allergies are also caused by strawberries and by milk, yet nobody would consider banning those products.
The recognition that chromium-3 is an essential i.e. vital, trace element, which has to be supplied from outside the body, is a comparatively recent finding. The importance of chromium in controlling the human glucose balance and thus in preventing or reducing the extent of diabetes was discovered in 1959 by mertz and others.
At an international congress held in France 1991 on the topic of Trace Element in medicine, emphasis was placed on the importance of chromium in human nutrition. Studies performed, for example in the USA, have revealed that human require a daily intake of 50 to 200 micrograms of Cr-3, while the actual intake is usually only 25 to 33 micrograms. This shortfall in the natural diet has led to the present availability of chromium tablets containing 50 micrograms of chromium-3, known as Chromium GTF, in the same way as other preparations are available to counter other dietary deficiencies of essential substances.
Alternatives Are Necessary:-
In the light of the above knowledge the question arises whether chrome tanning, which is not dangerous according to scientific evidence should be replaced by other tanning methods, as repeatedly proposed. Are there in fact any viable alternatives if the leather is still desired which has all the properties produced by chrome tanning and which are necessary and valued for leather apparel? Vegetable tanning frequently mentioned as a replacement yields and different kind of leather. The production of vegetable tanning agents on the worldwide scale required would probably represent a greater interference with nature (the plant kingdom and the climate) than the retention of chrome tanning which has been used on global scale for over a century without, detrimental effects on the health of human kind and has been technologically mastered.
The reason given for the rigorous maximum levels for total chromium i.e. without distinction between Cr-3 and Cr-6 i.e. the fear that the nontoxic Cr-3 could be transferred into the poisonous form Cr-4, has been discounted by the results of research work. Conversion of Cr-3 into Cr-6 is possible only in the presence of strong oxidizing agents, which hardly occurs practice. More ever, Chromium-6 is quickly reduced to chromium-3 in presence of water. Transformation to added Cr-6 into Cr-3 has been unequivocally established in the waste water clarification sludge from leather factories, and has also been established in measurements on seepage water.
Great importance attaches to the use of the
correct analytical method in determining chromium in its different oxidation
states. Finding of Cr-6 in leather or in waste of water from leather factories
are almost always spurious and due to analytical errors, or Cr-6 comes from
materials other than the leather, a so called matrix effect. The oxidation of
Cr-3 to Cr-6 during leather production itself and in dumping sites can be ruled
out with a high degree of certainty.
Nevertheless, leather factories devote great effort to keep chromium out of their waste water, and also legally required to do so by waste water legislation. Chromium can be recovered by precipitation and reused for tanning. This recycling process keeps the chromium levels in the sludge at a minimum. Furthermore, the recovery of chromium in the tanning process is not new. It has been practiced for 70 years in some production plant.
The present maximum value for agricultural use of sewage sludges are currently 1200 mg Cr/kg of dry matter in the federal republic of Germany. The EC is considering a maximum value twice as high. The USA, known for its highly sensitive approach to environmental matters, is even considering abandoning the limit for Cr-3. This can be justified on the ground that chromium is hardly mobile. The chromium is bound so firmly to the fibres in leather that it can hardly be washed out. Thus there is no danger that the chromium present in old shoes, bags, leather apparel etc. on garbage dumps can be transported in significant quantities into the seepage water of the dump site.
Disposal of Leather:-
With regards to solid water disposal, the disposal of sewage sludge is of prime concern (alongside shavings and cutting waste) to most tanning plants, while it is the disposal of the leather remnants that concerns the leather working industry. And ultimately, the customer is forced by question of what to do with old shoes, handbags, briefcases, leather jackets tec. Prior to the present garbage crises, all these materials were deposited in municipal landfill sites.
Avoidance of waste generation has a little room for improvement in leather manufacture because the technological constraints dictate production of certain amounts of residues and sludges. These depends in turn upon waste water regulations. This principle is no longer applicable to the end product.
Thus there remains the question of re-utilization. Like the collections of old clothing in the textile sector, collection of leather clothing, shoes and other leather products are also conceivable. Even if the articles in good condition are sent to countries of the third world, this would be welcomed as alleviating the situation. However, such a measure cannot be regarded as true recycling in the technical sense.
Size reduction of leather article in a shredder as practiced with textiles, is conceivable, but the utilization of the leather fibers still faces technical problems. Until now, only leather shavings and non- dyed remnants, generally vegetable tanned, have been processed to form leather fiber material. Increase in the proportion of chrome leather and the use of coloured leather calls for further research. This also applies to the use of leather residues for fertilizers and animal feeds. One of the main topics is the separation of chromium and protein.
In all research projects, which are also supported by the federation of the German leather Industry, the question arises whether sufficient application, i.e. demand, exist or can be created for the products of utilization, whether they be leather fiber material, insulation boards or artificial fertilizer.
As long as the re-utilization of most leather articles has not been solved and constitutes a problem the only means of disposal is in the municipal waste. Since the behavior of chrome tanned leather goods in waste disposal sites, there is no reason to declare old shoes and leather clothing as special waste.
In the incineration of household waste containing leather particles, even in very small proportion. It might be feared that the oxidation of Cr-3 could lead to the dangerous Cr-6 oxidation state. This can be avoided by the use of special process steps (temperature and time control). Another process reduces any Cr-6 present in the ash to Cr-3. Research is currently being conducted into these questions.
The latest research results reassuring, however, surprises have to be expected in such a sensitive area of environmental protection. Decisions should only be made on the basis of reliable scientific evidence and not for emotional reasons.
About the Author
The Author is a senior technical faculty in FDDI, Fursatganj, Raebareli.