Green Matters2 February 2009
For good or for bad, chromium remains the tannage we all have to work with: its properties and performance have, as yet, been unmatched by any single tanning agent. Yet it is not the only effluent concern tanners face. Graham Lampard reports on the green issues.
The formalities of opening a conference usually demand that the president says something along the lines of: ‘Hello, good evening and welcome' and having heard that many times before I usually switched off. However, at the 1995 IULTCS Congress then president, Dr David Bailey, began by espousing the benefits of chromium tanning at a time when this wasn't the done thing. And he ended by suggesting that ‘nothing was greener than chromium'. Thirteen years later, Dr Heinz-Peter Germann, director, Lederinstitut Gerberschule, Reutlingen, in the Procter Memorial Lecture at the SLTC Conference this year, agreed1.
He said that although there has been a change in the production of leathers, Table 1, around 80% of all leather is still chromium tanned. Of the remaining 20%, about a third of that was for automotive leather: the car manufacturers being the major driving force behind the ‘need' for chrome-free leathers.
Table 1: tanning methods
80 - 85%
Chrome free tannages
(of which 30% is chrome free
15 - 20%
The lecture was an objective overview of the ecology of the tanning process and Dr Germann reviewed the alternatives that have been, and are being, sought to chromium. These include wet-white, oxazolidines, wasserglass, enzymatic crosslinking and natural plant crosslinkers. However, he ended on what most tanners, I hope, would say was a positive note: this being with the comment that ‘chrome tanning with best technique can be classed as environmentally competitive'.
This raises a number of questions. For instance, what is the ‘best technique?' As I've said before, back in the 60s, the long gone Wades of Nottingham tannery used a chrome tannage that had zero chromium emissions. This they achieved through deliming the elephant hides they were using, adding 0.2% formic acid, diluted 1:10, as a chaser pickle - no other float - and after ten minutes adding 42% or 50% basic chromium.
The reduction in pH was enough to stop the chromium precipitating and more, importantly, ensured penetration while the high basicity and inherent alkalinity ensured basification without the need for additional chemicals. The result was no chromium in the effluent, a reduction in chemical usage, lower total dissolved salts and reduced water usage.
Secondly, what is classed as ‘environmentally competitive'? At a recent ASTM D31 committee on leather, reported in JALCA2, Nick Cory, head of the LIA in Washington, reported on the latest actions of ISO. This concerned the reduction of the allowable amount of hexavalent chromium from ten ppm to three ppm. The reduction is not based on toxicology testing but purely on test methodology and detection limits.
Although D31 voted against the reduction the ballot was passed and, therefore, the official maximum for chrome (VI) is now three ppm. In the same piece, the American Apparel & Footwear Association has ‘published a list of recommended restricted substances, and again the term ‘free', as in chrome free, begs for a definition. Many metals are found in soil and water, and it is unrealistic to think that absolute zero can be achieved for certain restricted metals when processing hides from a living animal.'1
Although, biodiesels have been developed from feedstocks, because of their source they are expensive and as well as driving up feedstock prices, the need for feed has limited their use as a fuel. Although alternative base materials, such as tallow and tannery waste, have been investigated over the years, problems such as purity and water content have limited their appeal.
In a lecture to the ALCA 2007 conference3, Karel Kolomaznik from the faculty of applied informatics, Zlin, Czech Republic, reviewed ways of transforming tannery waste into useful biodiesel. He said that waste fat produced by tanneries during the processes in which raw hide is transformed into leather could represent a very important raw material for biodiesel production.
Under laboratory and pilot-scale conditions, biodiesel samples were prepared from waste fat. His team has developed a process of biodiesel production that has three stages:
- 1) Refining: the melting of the raw material to separate the fats from other undesirable components
- 2) organic base-catalyzed methanolysis
- 3) separation of glycerine layer
Where his team have advanced the practicality of using tannery fats is shown by the mathematical model of zonal refining and melting, and the use of organic bases4. This innovative approach produces a high quality biodiesel and glycerine containing practically no ash.
A rejuvenated IULTCS has written a number of position papers, including one on the environmental aspects of leather production, the early stages of which are elucidated here:
Rawstock: Preservation of fresh or cooled hides and skins
Fresh or uncured rawstock is available to tanneries in many countries. Whenever possible, it says, the treatment of fresh hides and skins is the best solution to reduce salt pollution and the time elapsing between slaughter and further treatment must not exceed a few hours. When an abattoir and a tannery are operationally linked, fresh rawstock may be used, but excess above the capacity of the tannery must be handled differently.
Beyond this short period, the IULTCS suggest it is necessary to cool the hides and skins, either in ice or with cold air.
Reviewing the drying possibilities for preservation, the paper outlines the alternatives suggesting that shade drying of small skins is a low cost and environmentally acceptable process in some climates, while dry salting, combining salt curing and shade drying, can minimise the amount of salt used for preservation of skins and hides
It says the use of antiseptics with low environmental impact and toxicity can help to increase storage time of fresh or chilled hides and skins. However, their use must be regularly reviewed, to reflect changing legislation, and some have both bactericidal and fungicidal properties making them appropriate for soaking, pickle and wet blue preservation.
The problem of salt pollution can be partially eliminated by using hand shaking, mechanical brushes or a suitable drum, and the salt can be reused in pickling after dissolution and removal of solids. However, it must not be used for curing purposes because it is too contaminated with halophilic or halotolerant bacteria, which can cause red heat.
This method of salt recovery gives a partial answer to the salt pollution problem. Neither brine curing nor salt curing can be considered as cleaner technologies, even if pre-fleshing green hides reduces this waste. It is recognised that salt curing is one of the greater contributors to the environmental impact of tannery operations.
Even recovering some of it has limited benefits because its reuse is extremely limited, its ecological disposal is difficult to impossible and the cost of fresh salt is so low. Perhaps an omission here is the work of Bailey et al5, into the use of potassium chloride which, it was claimed, produced a leather that was equal to one preserved in sodium chloride but the effluent from which could be used on the land as a potassium source.
The paper reviews the new generation of drums and processors that now facilitate efficient draining and washing and allow for the routine use of low floats for processing, thereby resulting in significant savings in water consumption. It suggests the consumption of fresh water can be minimised by using a counter-current system of washing, to concentrate the salt and the other soluble materials, such as dirt and blood, and advocates the fleshing of green hides after soaking. As Kolomaznik shows, green fleshings are more valuable than limed fleshings with regard to tallow recovery.
A problem with green fleshing is the presence of dung on hides, which causes the fleshing blade to cut into hide, thereby damaging the pelt in an economically unacceptable way. Where available, it suggests, animals that have been reared through a quality assurance or clean hide scheme should be used, as these schemes generally require animal husbandry practices that minimise dung contamination.
The enzymatic treatment of hides and skins can be considered as a cleaner technology only if the amount of sodium sulfide is reduced substantially. However it is not yet possible to replace totally sodium sulfide in processing skins and hides.
There are other agents available that reduce the amount of sulfide in liming, eg organic sulfur compounds (mercaptoethanol, salts of thioglycolic acid, formamidinesulfinic acid) and amines based proprietary products. However, in these cases the amount of compound needed would probably be prohibitively expensive, and hair-dissolving processes contribute significantly to the COD and BOD.
Recovery of hair before dissolution, either when it is separated during the liming, or at the end of a hair saving process, can lead to a COD reduction of 15-20% for the mixed tannery effluent, and a total nitrogen decrease of 25-30%. It is an advantage to filter off the loosened hair as soon as possible and higher COD and nitrogen reduction can be obtained.
This process can be considered as a cleaner technology if the hair is utilised, even as a nitrogen source, and there are several established methods of hair saving, routinely used in industry. However, is it recognised that they do not provide a complete effect, since each incorporates a hair-dissolving step, to deal with residual short hairs.
The paper also reviews best practice through deliming, tanning and post-tanning processes and, along with the other position papers the IULTCS has written, is well worth reading.
In an extensive literature survey to be printed in JALCA6, Soufiane Tahiri, from the University Chouaïb Doukkali, Morocco, and Miguel de la Guardia, from Valencia University, Spain, review the different approaches available in the scientific literature for the treatment and valorization of solid wastes generated from the leather industry.
Tanning processes generate significant volumes of byproducts and waste materials and, because of that, tanneries can cause serious negative environment impacts. The advancement of laws protecting the environment have prompted industrialists, environmentalists and scientists to look for the decontamination of wastes and to transform tannery solid waste materials to valuable co-products useful to be recycled or employed in other industries.
They have examined examples of organic fertilizers, biomaterial gelatine and soluble collagen production for multiple applications, and investigated the production of energy by biomethanization, which has been proposed as a solution for untanned wastes.
For tanned wastes, several applications and treatments were investigated such as the recovery of proteins and chromium and their valorization, and incineration. As Redwood pointed out in a speech to the SLTC7, the idea of cradle-to-the-cradle production is gaining popularity, especially among car producers, where the disposal legislation is becoming tighter.
In many ways this is to the advantage of the tanner, and those using chromium, for if it can be shown that the tanner has the technology to extract the chromium from the used cars, it a) means chromium is not locked in the ground, and b) salves the concerns of the environmentalists who see chromium (VI) leeching into the ground at every opportunity. As Tahiri and de al Guardia write, with regard to solid waste, there are many answers available to the tanner to recycle their waste, and overall opportunities abound for those who are seeking to enhance the environmental reputation of tanneries.
- SLTC 2008 Annual Conference, Procter Memorial Lecture
- ALCA Annual Business Meeting, J. Amer. Leather Chem. Assn 2008 CIII (10) 357
- J. Amer. Leather Chem. Assn, 2009 in press
- D Bailey, IULTCS Congress, May 17, 1995, paper 2,
- J. Amer. Leather Chem. Assn, 2009 in press
- SLTC 2008 Annual Conference