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Clean Technology in the Dyehouse - Sustainable Leather Production – Part 26

Published: 22nd May 2020
Author: Dr Clive Jackson-Moss; head; International School of Tanning Technology
The previous 25 articles in this series have discussed the different clean technology options that are available to the tanner in the beamhouse and tanyard. This article will focus on the clean technology that is available in the dyehouse that can help the tanner reduce the environmental impact of the dyehouse processes.
Many of the clean technologies available to the tanner in the beamhouse and tanyard are not dependent on the chemical supply companies for the technology. For instance, a tannery can implement recycling of various floats, and this does not require any new chemicals supplied by the chemical companies. However, when it comes to clean technology in the dyehouse, this is principally driven by chemical developments within the chemical companies that are then made available to the tanneries for implementation. 
Some of the clean dyehouse technology options available are discussed below.
 
Compact Methods in the Dyehouse
In a compact process, all of the chemicals are added in the same float. In a classical compact process, the wet blue is first wet back, then neutralized, and then the retanning, dyeing and fatliquoring processes are all carried out at the same time in the same float. In ultra-compact processes, the wet blue is first wet back, and then the remaining processes are all carried out simultaneously i.e. the netralization, retanning, dyeing and fatliquoring processes are carried out in the same float. Special products that have been developed by the chemical companies have allowed this compact processing to be possible. 
The advantages of using compact methods in the dyehouse are as follows:
Reduced water usage – most of the different processing stages take place in the same float, without the need to drain floats and then add fresh floats to the drum.
Shorter processing times – this leads to a savings in electricity consumption, as well as a higher throughput through the tannery as more batches can be processed each day
Fewer production mistakes – with fewer process stages, there is less chance of a mistake being made in the process
Reduced chemical oxygen demand (COD) discharge from the dyehouse
Simplified product handling and storage
More efficient administration of chemical stock
 
The Replacement of Nonylphenol ethoxylates (NPEs) Degreasers
Surfactants and degreasers are often used in the wetting back stage of the dyehouse. In leather degreasing, the objective is to extract either natural fat or machine grease (from splitting and shaving) from the hide or skin and move this into the surrounding float. For many years, NPEs were the main chemical product used, due to their excellent attraction for fats, wetting ability, emulsifying ability and low cost. 
The problem with NPEs is that they are poorly degraded in the effluent. These surfactants are degraded into smaller chain NPEs and nonyl phenol, both of which are toxic. These molecules are persistent metabolites i.e. they cannot be further broken down within the environment. They have been shown to be endocrine disruptors. They produce an estrogenic response in a variety of aquatic organisms, resulting in a change in the organism’s sexual nature.
Most chemical suppliers now supply NPE-free degreasers and surfactants which are more environment-friendly.
 
Ecologically Improved Syntans and Retanning Chemicals
Although polymers can be used on their own for the retanning of leathers, there will always be a need for the use of syntans, resins and vegetable tannins. The reason for this is that there is a wide variety of properties that different leathers are expected to have, and these can only be achieved by the use of different types and concentrations of syntans, resins and vegetable tannins.
Vegetable tannins in particular, lead to a very high COD in tannery wastewater. One of the problems that syntans and resins cause is that they reduce the biodegradability of dyehouse effluents as these products are not easily degraded by bacteria in effluent treatment plants. 
Combining polymers with the traditional retanning chemicals results in a much more environment-friendly process. Due to the high exhaustion of the polymers, the concentration of the other retanning chemicals offered can be reduced. This leads to a better uptake of dyes and fatliquors by the leather. Because of the lower concentration of retanning chemicals that are applied to the leather, the leather becomes less anionic than would normally be the case, resulting in the better uptake of dyes and fatliquors. This in turn leads to a large overall reduction in the COD of the dyehouse effluent.  
Newer generation syntans have less residual phenol and formaldehyde present in them. They also contain less inorganic salts. These salts are not taken up by the leather, with the result that they are discharged in the effluent. Liquid retanning chemicals often contain less salts than the corresponding product in powder form. An added advantage of liquid retanning products is that they do not produce any dust in the tannery working environment. 
 

Azo dyestuffs. Image source: www.leatherusa.org

Environment-friendly Dyestuffs
If dyestuffs are not exhausted properly, this can lead to a slight increase in the COD of the tannery wastewater. However, what is far more important is the discolouration of the receiving water by the dyes.
Newer generation dyes have better exhaustion due to improved dye chemistry that improves dye build-up on the leather. 
The use of liquid dyes instead of powder dyes also has many advantages. They usually have a lower salt content, have a higher dye build-up and better exhaustion. They are usually easier to handle and add to a drum, and the dyehouse effluent contains less salt. 
Certain azo dyes have been banned due to the fact that they can be broken down into carcinogenic amines.
 
 

Polymeric fatliquor. Image source: www.indiamart.com

Polymeric Fatliquors
Fatliquors are added to leather to ensure that the leather is soft and supple, and has a pleasant smell. They also play a role in the tensile and tear strength of the leather. When producing very soft leathers, high concentrations of fatliquor are added to the leather to provide the lubrication necessary to achieve the softness. Unfortunately, these fatliquors contribute to a high COD in the effluent from the dyehouse. 
Polymeric fatliquors are formulated from polymers with a softening effect. The use of polymeric fatliquors has a marked reduction in the COD of the dyehouse effluent. The reason is that virtually all of the polymer is taken up by the leather. They have a very high affinity for the leather fibres, and this results in a negligible contribution to the COD of the wastewater. Other advantages are that they usually have a high lightfastness and heat resistance.
Some trials have shown that the use of polymeric fatliquors can virtually eliminate the need to use retanning chemicals as the polymer itself gives a good filling effect to the leather. This can lead to a 60 – 65% reduction in the COD levels of the dyehouse effluent.
 
From the discussion above, it should be evident that the drivers in terms of clean dyehouse technology have been the chemical supply companies and the development of far more environment-friendly chemicals that can be used by the tanner.
 
References
Dix, J P. Chemical developments leading to cleaner production. Part 2. Wet post-tanning operations and finishing. World Leather, August/September 2000, p 48 – 54.
Puntener, A G. Compact methods for the wet end. JALCA, Vol 94, p 96 – 103 (1999).
Stockman, G B, Rangarajan, R and Didato, D. The replacement of nonylphenol ethoxylates (NPEs) as degreasing agents in wet blue manufacture. World Leather, October 2005, p 25 – 27.
 
In the next issue: The next article will be the last one in this series on clean technology in the tannery, and will discuss the clean technology options in the finishing department of a tannery.
 
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