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A review of techniques to reduce salt in tannery wastewater: Sustainable leather production – Part 11

Published: 28th Apr 2017
Author: Dr Clive Jackson-Moss; head; International School of Tanning Technology

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In this series on sustainable leather production, the previous ten articles have discussed the steps that tanners could take to reduce the salt concentration in tannery effluents. Another major source of pollution in a tannery is the liming / unhairing process, and the following few articles in this series on sustainable processing will highlight what is being done to reduce pollution at this processing stage. Before moving on to the steps that can be taken to reduce pollution levels from the liming / unhairing process, it is worth summarizing the different options that have been discussed so far in this series to reduce salt usage in the tannery.
 
Part 1. Use of Green Hides
This article discussed the use of green hides in the tannery. Green hides are fresh from the abattoir and have not been salted, hence no salt is introduced into the tannery and the salt in the soaking effluent is removed. There are some disadvantages to the processing of green hides, one of which is that many of the unwanted skin components are not removed by the curing salt. This results in a leather that is harder, and steps need to be taken to remove these unwanted skin components, normally by using higher concentrations of soaking enzymes. 
 
Part 2. Irradiation of Hides
Irradiation of hides was discussed as a way of eliminating the use of salt for the curing of hides and skins. Hides and skins can be cured with this method and if wrapped in plastic, reasonably long storage periods can be achieved. However, the high cost of the irradiation equipment will prevent its commercial use in the future.
 
Part 3. Chilling of Hides and Skins
If a tannery is situated close to an abattoir, fresh hides that arrive at a tannery can be placed in a cold room and remain in the cold room for a few days before being processed. This eliminates the need for salt. The article did discuss some of the disadvantages of this method. The cost of the cold rooms is one disadvantage, but this method is also problematic for curing sheepskins with wool on them. The wool is a good insulator, and can prevent the temperature of the sheepskin being lowered to a temperature that will prevent bacterial growth, especially when the sheepskins are piled on top of each other. 
 
Part 4. Use of Biocides and Potassium Chloride
This article was the first to discuss adding a type of chemical to the hides or skins to cure or preserve them. Biocides are well known as they kill bacteria, but the disadvantage of using them as a curing agent is that they are only effective for 5 – 10 days. Potassium chloride (KCl) can be used to replace salt as the curing agent, and although it works well, the cost is much greater than that of using salt (NaCl). 
 
Part 5. LIRICURE
LIRI Technologies carried out research which successfully showed that the quantity of salt applied to hides and skins to cure them could be significantly reduced (by up to 50%) if EDTA was added to the salt. Although the LIRICURE process is not a salt-free curing process, it does result in a 50% reduction in salt usage for the curing process, and hence a significant reduction in salt ending up in tannery effluents from tanneries processing LIRICURE hides and skins.
 
Part 6. Use of Silicates for Curing
Various trials have shown that sodium silicate can be used to cure hides and skins for many months. The silicates are used in much the same way as salt. The silicate dewaters the hides and skins extremely well and results in a large reduction of the weight of the hides and skins. The main disadvantage is that the silicate is more expensive than salt.  
 
Part 7. Salt-free Pickling 
Most tanners think that the salt in tannery effluents is only due to the processing of salted hides. What is often over-looked is that a high percentage of the salt in the final effluent is due to the use of salt in the pickling process. The pickling process can contribute up to 30% of the salt load from a tannery. Aromatic sulfonic acids have been successfully used to replace the salt in the pickling process, and although they lead to some advantages in the wet blue and final leather, the cost of these chemicals relative to the cheap cost of salt makes their use extremely limited.
 
Part 8. The Thrublue or Modified Thrublue Process
Due to the high salt usage in the traditional pickling process, research has been carried out to investigate whether the pickling process could be eliminated from traditional wet blue processing. This work was carried out at LASRA, the New Zealand Leather and Shoe Research Association and resulted in the development of what is known as the Thrublue process. The tanning process starts immediately after the bating process, and thus the pickling process does not take place. No salt or pickling acids are used, resulting in reduced salt entering the tannery effluent. 
 
Part 9. EasyWhite Tan
The EasyWhite Tan process is similar to the Thrublue process in some ways in that no pickling process is carried out and hence no salt is used. The tanning process also starts after the bating process. The main difference is that the EasyWhite Tan process does not use chrome as the tanning agent, which is also an environmental advantage. 
 
Part 10. Mechanical Desalting, Pickle Liquor Recycling, Chrome Recycling
This was the last article that discussed possible salt reduction strategies in the tannery. If no steps have been taken by a tannery to process salt-free or reduced salt cured hides, or reduce salt usage in the processing stages such as the pickle, the use of a mechanical salt shaker can prevent quite significant quantities of salt from entering the liquid waste. This salt will be removed from the hides or skins as a solid waste prior to the start of processing, and can be disposed to a landfill site. A reduction in salt concentration in the effluent will be achieved.
 
1. This article also discussed the effect of recycling either the pickle liquor or the chrome tanning floats. If these floats are recycled, the salt that is dissolved in these floats will not enter the tannery effluent, but will be re-used in the tannery. – Dr Clive Jackson-Moss [+27 (0)46 622 7310, clive@tanschool.co.za, www.tanschool.co.za]
 
The following few articles will discuss the research that has been carried out to reduce pollution from the liming / unhairing processing stage. This single process stage accounts for up to 60% of the total pollution that comes from a tannery. It is therefore an important process stage to investigate if a reduction in the pollution load from a tannery is to be achieved. 
©2017 S&V Publications
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