By A. Clare
The term ‘by-product’ forms the core of the leather industry – the hides/skins from which leather is made are, economically, a 1% leftover from the meat industry. The leather-making processes upcycles these hides/skins, increasing their value whilst simultaneously reducing waste. Value statements concerning product, co-product (“by-product”), or waste are made at the point the hide is separated from the carcass.
The leather industry is continuously pushing towards more sustainable practice, and reducing waste plays a big part in this. All industries produce waste, but by working together, there is an opportunity for resources to be conserved and environmental pollution to be reduced. The leather-making process presents a good opportunity for this.
Utilizing by-products during preservation:
Preservation may be required at the slaughterhouse to prevent putrefaction (rotting), preserving the hide/skin, and allowing successful movement (or storage) to the tannery. The preservation process usually uses salts, which prevents bacterial growth; however, this results in high levels of total dissolved solids (TDS) in the soak liquor and solid waste streams. Consequently, a lot of research has been directed towards finding an alternative, more environmentally friendly method.
Coffee husk has demonstrated to be a potential replacement for salt during leather preservation. Coffee husk is the dried skin of a coffee bean and is a by-product generated during the processing of coffee seed pulp. Chlorogenic acid, a component known for its ability to prevent microorganism growth, successfully prevented microbial growth on goatskin, allowing 15 days of preservation when left soaking for 30 minutes. Furthermore, the resulting wastewater had significantly lower amount of TDS and there was no significant change in the resulting leather properties.
Biowaste produced by the Bael tree, Aegle marmelos, may also potentially be utilized during leather preservation. Dried leaves from the Bael tree, a tree with high antimicrobial activity, can be mixed with ethanol to produce an extract. A concentration of 5% applied to raw goatskins successfully preserved the skins for one month. The resulting leather was comparable to conventional methods; however, there was a significant reduction in water usage and pollution loads, compared to industrial processes. These two examples demonstrate how convergence of multi-industry waste streams can minimise inputs and beneficiate wastes.
Utilizing by-products in Beamhouse processes:
Preparation of hides/skins for tanning occurs in the beamhouse. The traditional liming and unhairing process uses chemicals, such as lime and sodium sulfide, which generate a lot of pollution; therefore, enzyme-assisted unhairing has been adopted as an environmentally friendly alternative method.
Enzymes derived from fish waste have demonstrated success during unhairing of goatskins. Protease extracted from fish waste caused complete removal of goatskin hair, supported by scanning electron microscopy evidence. Furthermore, the shrinkage temperature and physical strength of resulting leather were comparable to the control. Fish waste enzymatic unhairing also considerably reduced pollution loads in the waste stream.
After unhairing, hides/skins are fleshed, delimed, bated and pickled to prepare for tanning. Chrome tannage is the dominant practice, however, concerns regarding environmental pollution are pushing towards finding greener, more sustainable tanning methods. Utilizing kraft lignin, a by-product of the paper manufacturing processes has shown to be a potential eco-friendlier chromium-tannage substitute. Degradation of kraft lignin produces a phenolic compound, which, when applied to sheepskin, results in tanned leather with similar properties to conventional chromium-tanned leather.
Low-quality leather may need upgrading using fillers to improve the final feel and appearance. Whey protein, a by-product of the cheese industry, has shown to be a successful replacement for conventional petroleum-based filler products. Whey-filled leather showed good physical properties, with significantly higher tensile strength and young’s modulus compared to conventional fillers.
Utilizing by-products in Post-tanning processes:
After tanning, a dyeing step takes place. Agricultural wastes (i.e., wheat bran and rice bran) have successfully been used to cultivate fungi, for bio-pigment production. Water-soluble bio-colourants, extracted from fungi (Figure 1), have demonstrated to be successful when used as a dyeing agent on wet leather. A yellow bio-colourant illustrated good dyeing ability, presenting good results during dry- and wet-rub tests. Furthermore, leather dyed yellow showed high heat stability and excellent fastness to light.
Figure 1 - Extraction of bio-colourants from fungi for use in leather dyeing.
Fatliquoring is performed to improve leather elasticity and to increase water- and wear resistance. During alcohol production, fusel oil, an esterification by-product, is produced. Fusel oil has demonstrated success as a fatliquor when combined with soapstock, a waste alkaline refining of vegetable oils. After fattening, leather was soft, filled, and had high tensile strength.
Fish oil has also shown success as a fatliquor. Fish waste contains high organic content and, therefore, its disposal cost is often high. Extracting the oil from fish waste (Figure 2), and converting it to a fatliquor, has produced leather with soft grain and good flexibility. Furthermore, the leather had improved physical and mechanical properties, compared to leathers fattened with commercial fatliquors.
Figure 2 - Extraction of fish oil for use as a leather fatliquor.
After the leather has dried, a surface coating is usually applied to finish the product. Surface coatings are usually cellulose derivatives such as nitrocellulose and ethyl cellulose. Cellulose waste from groundnut and sugar cane industries have shown to be good leather finishing chemicals. Groundnut shells and sugarcane bagasse contain approximately 38% and 35%, respectively, making these wastes a viable, renewable cellulose resource. The extracted cellulose displays high similarity to commercial products and can upgrade low-quality leathers by enhancing grain smoothness.
Conclusion:
In this first part of a focus on waste reduction in the leather industry, the discussion has turned to how can greener chemistry be sourced through waste streams from other industries. Beneficiation of waste streams is not a new concept to the leather industry – with the hide being the starting example of the industry’s waste appetite. The leather industry has also used waste-derived chemistry from the paper, dairy, and petrochemical industry for over 100 years. Going back further in history, Roman and Greek tanners used blood, mucilage, and dairy wastes for valuable chemicals in the preparation of their leathers – an example of how steeped in tradition using waste is for tanners.