Tanneries are often associated with the characteristic and obnoxious "sulfide smell", which is in fact caused by low concentrations of sulfhydric gas, also known as hydrogen sulfide. Levels as low as 0.2 ppm of H2S are already unpleasant for humans and a concentration of 20 ppm is unbearable. As a result, tanneries might be forced to close down beamhouse operations or are forced to re-locate away from populated areas.
As beamhouse and tanning are often done in the same facility, smell is actually the lesser problem. Through human errors, this always holds the danger of mixing acidic floats with the sulfide containing beamhouse float and releasing higher amounts of H2S. At a level of 500 ppm all olfactory receptors are blocked and the gas, therefore, becomes unnoticeable and an exposure for 30 min results in a life threatening intoxication. At a concentration of 5,000 ppm (0.5%), the toxicity is so pronounced that a single breath is enough to cause immediate death within seconds.
Despite all these problems and risks, sulphide has been the preferred chemical for unhairing for more than a century. This can be attributed to unavailable workable alternatives: the use of organic sulphides has shown to be practicable but not really accepted due to the extra costs involved. Unhairing solely by proteolytic and keratolytic enzymes has been tried over and over again but for the lack of selectiveness was difficult in practice to control. A lot of work has also been invested in oxidative unhairing, but until today it is very limited in its use as it is hard to get consistent results.

 

The unhairing process

Covington has calculated the theoretical required amount of sodium sulphide of industrial grade (60-70%) for a hair burn process to be just 0.6%, relative to hide weight. In practice, the typical amounts employed for a reliable process are much higher, namely 2-3%. The main reason for this is the fact that the rate of unhairing is depending on the concentration of sulphide ions (S2-) in the float. Short floats are commonly used to obtain a high concentration of sulphide. Nevertheless reducing sulphide levels negatively affects complete hair removal in an acceptable time frame.
Looking more closely at how the rate of unhairing depends on the concentration of the employed chemicals, it is quite obvious that a high concentration is especially needed directly at the point of attack for a particular process. In a hair burn process, this point of attack is the keratin of the hair cortex, which is degraded by sulphide due to the breaking-down of cystine bridges.
In a hair safe process, where the keratin is protected by the immunisation step, the point of attack is mainly the protein of the hair bulb which is hydrolysed either solely due to the alkaline conditions or by proteolytic enzymes, if present. A second and equally important point of attack is the pre-keratin that is situated above the hair bulb; it can be degraded by proteolytic hydrolysis combined with the keratolytic effect of sulphide.
Whatever process is used for unhairing, it is of utmost importance that these points of attack are easily accessible for the process chemicals, allowing for a high local concentration of sulphide which will in turn result in a high rate of unhairing. This also means that if easy access of active process chemicals (eg lime, sulphide, enzyme etc) to the crucial locations can be provided, it will be possible to use significantly lower amounts of these chemicals.

Soaking is a key factor for efficient unhairing

All chemicals employed in the unhairing process are water soluble and water is the process medium. Grease is therefore a natural barrier reducing the effectiveness of any unhairing chemical. The removal of grease can significantly improve the performance of the subsequent unhairing process. Consequently, the basis for an effective unhairing with a significantly reduced offer of chemicals needs to be laid in the soaking step.
The target is an efficient degreasing of the hair and the hide surface and a removal of sebaceous grease. On the other hand one needs to avoid removing too much grease in general, especially from the flesh, because it is often not possible to keep it in emulsion and fat smearing will be the result. This leads to a greasy surface rather than the desired "dry" one, which impairs the effectiveness of the unhairing process.
While the selective removal of grease from certain structural elements of the hide exposes them to the subsequent attack of the unhairing chemicals, other parts of the hide can at the same time be protected from it. Experience shows that soaking under alkaline conditions provided by earth-alkali compounds finally results in leathers with improved fullness of flanks and bellies and a higher useable area. So far there is no fully conclusive explanation for this well proven fact, but analytical figures show that indeed soaking with earth alkalines results in a very different distribution of fatty substances within the hide compared to soaking with soda ash.
While the degreasing effect with soda ash is quite uniform, using earth alkalines results in a higher content of fatty substances in loose structured areas of the pelt, ie in the flanks. Whether this is due to a selective removal of fat from other parts or to a re-deposition of fatty substances cannot be said at this moment. Whatever the exact reason is, the beneficial effect on cutting yield is undeniable.
A new selective soaking agent makes use of the described effects; it provides the optimal pre-conditions for good hair-root and fine-hair removal with reduced sulphide offer, and at the same time it preserves the integrity of bellies and flanks.

 

Low sulphide enzymatic assisted unhairing

After the hide is properly prepared in soaking, unhairing is most effectively achieved with a process employing a combination of an enzymatic proteolytic formulation and the keratolytic effect of sulphide. However, in a hair safe process, the sulphide offer can now be drastically reduced to levels of only 1% relative to hide weight on bigger bovine hides. This can be done without any compromise regarding the rate and effectiveness of unhairing or the cleanliness of the pelt. The lower offer also results in significantly reduced levels of sulphide in the liming float as well as in the hide (it will release less H2S in later deliming and pickling!). Even a traditional hair burn process can be performed at same low sulphide offer.
Apart from the keratolytic effect of sulphide, proteolytic hydrolysis is always required for unhairing. The hair bulb, which consists of protein, and the pre-keratin situated above it need to be attacked. This is accomplished by alkalinity and optionally also by proteolytic enzymes.
Collagen is more prone to hydrolysis than keratin, and after lime addition the native collagen is chemically modified and hence becomes more sensitive. In addition, the alkaline swelling also makes the pelt susceptible to physical damage. It is, therefore, much safer to accomplish the proteolytic attack on hair bulb and pre-keratin at a lower pH before the addition of lime.
This can be achieved by a new proteolytic enzymatic unhairing formulation that has its highest activity around pH 10.5. At the typical pH of a liming process of around 13, the activity is substantially lower. This means that the pelt is less exposed to hydrolytic degradation when it is in its most sensitive state.

 

A low sulphide, low lime hair safe process

A soaking agent protecting the loose structured areas of the hide and an enzymatic unhairing formulation that is deactivitated at high pH guarantee optimal conditions to obtain best quality and the maximum possible usable area of leather. At the same time, the new unhairing system allows a significant reduction of sulphide offer, even in a hair burn process. But the highest benefits are obtained if it is used in a hair safe process. The combined effects of a highly efficient soaking and the selective proteolytic effect of a special enzyme formulation result in an extremely reliable unhairing without problems of fine hair and hair roots and with improved cleanliness of the pelt.

The system improves the opening-up of the hide which leads to softer leather if not compensated for by a reduction of the lime offer. This, in combination with a screening of the hair by a filter, leads to a substantial sludge reduction.

 

Conclusion

A low sulfide, low lime process with good epidermis, hair-root and fine-hair removal is possible with the proper preparation of the hide in soaking. A selective enzymatic auxiliary can be used in unhairing without affecting the integrity of grain, bellies and flanks.
Combining both products, the technology provides the following benefits over a traditional way of working:

  • – improved safety
  • – much less obnoxious smells
  • – substantially reduced load on the environment – sulphide, nitrogen, COD, sludge
  • – optimised and more consistent yield in lay-out, cutting and leather quality
  • – lower chemical, process and waste costs