R.P.Daniels, D.Cutner, A.Sadeghi, Qualus, UK – Indian Leather Magazine
Cost savings can be made by reducing the quantities of chemicals used in retanning, dyeing and fatliquoring. These can be considerable, but for success – that is the “same” leather at the end of the process -three quite different events must take place at the same time.
- The conditions within the established process must change to get a better uptake of the retanning and fatliquoring products. If this is done as a single event, the chemical uptake will be greater, but the leather will have too much filling and be over lubricated.
- There must be a reduction in chemical offer, but the total uptake must be the same as the original process. This is where the cost savings from lower chemical use becomes clear.
- The retanning and fatliquoring products must be distributed throughout the leather structure in the same way as the original process. If this is not achieved, the leather will be “different”.
This is not an easy matter. Changes within an established process – managing smaller amounts of the same chemicals to create the same products – are limited.
The chemical process
The techniques that control these chemical reactions are fixed and of common knowledge to tanners. They can be summarised as:
- pH control, for manipulation of the charge and the rate of fixation.
- Temperature adjustment, as this directly affects the speed of the chemical reaction.
- Time management, as this optimises the uptake of chemicals in the time available.
- Concentration, as the rate and efficiency of the reaction is dependent upon strength of the chemicals in the float.
Changes to pH, temperature and time – in conjunction with a reduced chemical offer – can give the same uptake of products in total, but their locations throughout the leather structure will be different. Typically, there may be an acceptable uptake on the grain and flesh parts, but the inner most sections remain under processed. The leather will be too firm and under filled. Chemical control alone cannot make the same product from a lower chemical offer.
However, a lower chemical offer combined with less water in the float offers improved uptake, with deeper penetration into the fibre structure.
The role of mechanical action
A fast dispersion of products throughout the processing vessel is essential for a uniform uptake of chemicals. This is better suited to higher float levels, being especially important where light surface dyeing, surface lubrication or fixations are needed. Higher floats are also needed to ensure that leathers in process remain open and free-flowing, with good contact with chemicals in solution.
With low floats this movement can rapidly become that of a single mass, or several large bundles of leather. This raises issues of irregular uptake and tangling. Effects include variations in colouration, drawn grain and uneven penetration.
And greater mechanical action can over-stress the structure. This can cause a loss in break characteristics, and even tearing, and is especially clear with leathers more sensitive to mechanical action, or of lower substance.
Lower float conditions depend upon the physical properties of the leather at the time of process, the load weight in relation to the capacity of the processing vessel, speed of rotation, and internal configurations. A high concentration offers a route forward, but a different type of mechanical action is needed.
A new approach
Significant changes have taken place in the industrial washing of fabrics. Essentially, the weight of the fabrics loaded into the washing machine remains unchanged, but part of the water is replaced by inert spheres. This matches the original volume, but as less water is present, fewer detergents and washing aids are needed. In addition, the spheres maintain separation between the fabrics, and the gentle massaging or kneading action stretches out creases and folds.
In a similar way, spheres can be used to introduce chemicals into the leather structure. The total float volume (water and spheres) can be the same as an established process, but uses less water overall.
This enables fast distribution of chemicals throughout the processing vessel, and good separation of the leather in process. Additionally, the increase in concentration due to less water in the float enables a lowering of chemical offer. The uptake, distribution and penetration can match the conventional float situations, but the process becomes more efficient.
Moreover, products used in process are weighed with great accuracy, so cost savings are immediately clear. As leather is a very sensitive indicator of process success, meaningful judgements can be based on the final leather and its value, and monetary savings in chemicals.
Managing the technology
Sphere applications – Sfere technology – were developed by Qualus for six years before introduction to commercial leather manufacture.
The technique is fully established and offers advantage within all aspects of chemical uptake, mainly the high expense areas of retannage, dyeing and fatliquoring. On full industrial scale, savings of retanning agents and fat liquors range between 10% and 15%.
The technology includes the retention of spheres throughout float exchange and washing sequences. This is regardless of size, services, and internal construction of the processing vessel. The collection and cleaning of spheres after process, their storage and delivery to following processes are all part the bespoke packages available to the tanner.
The technology suits tanneries of any size and can be assessed I expanded one drum at a time. It suits individual tanneries, but can also be managed within clusters. Here, a single location can provide washing and cleaning of spheres before delivery back to the tanneries for subsequent reuse. There are few limitations within tanneries to implement this technology. It is a safe and long awaited means to reduce chemical offers, retain quality and reduce waste.
Reduced environmental impact.
The savings in process water and energy for heating, can be readily measured and costed. But manufacturing plant, process details, and the wastewater plant are individual and subject to change. Accordingly, it is not possible to provide “all encompassing” value on waste minimisation.
It is difficult to know the true efficiency of any retanning and fatliquoring processes. Over and above measurements at the end of process, unbound materials washed from the structure need quantifying. This means that volumes of water at the end of all processing steps and their residual content need taking into account.
These products need accurate measurement, and, as they are complex, this is beyond normal analysis. An effective technique is to measure the COD of the float against time throughout the process. This can show the uptake of organic content in the process from start to finish. But there are limitations, regardless of technique.
However, it is useful to consider a hypothetical situation where the chemical process was originally, say, 70% efficient, but where the same result is being provided by a 10% reduction in chemicals. Here, the original uptake would be 70 Kg for every 100Kg of product offered, with 30 Kg of unused product discharged from process. If the uptake remains at 70 Kg, but with a 10% reduction in offer, i.e.
90 Kg, the unused product is lowered to 20 Kg. The load for subsequent chemical treatment, energy for biological treatment, and management of solids is reduced by one third.
The values will vary with each process. If the process was 60% efficient, with 10% savings, the load would still be reduced by 25%. Regardless of these permutations, the saving in unused chemicals – and the costs for treating those pristine products – can be seen as significant.
Furthermore, there may be components within these products that are environmentally persistent and difficult to address within effluent treatment. Any reductions of these components at source makes end-of-pipe discharge to consent limits more secure. Neutral salts are a consideration too, as these are often contained within commercial products. A lowering of TDS has direct implications within reverse osmosis costings and management of residual dry solids.
There are few limitations within tanneries to implement Sfere technology and save complex and expensive materials.
- Designed to purpose, it is a safe and long awaited means to significantly reduce offers and waste in established processes.
- It can be viewed as a refinement of mechanical actions applied throughout retanning and fatliquoring processes.
- Quality is maintained, and scope is offered for better processing as new types of leathers are developed.
- Significant quantities of pristine products presently channelled directly to wastewater treatment can be eliminated.
- It offers a better use of resources – a major step towards a more sustainable future.