Degreasing of hides and skins
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Degreasing is most relevant in processing sheepskins, where the natural fat content is about 10 – 20 % of dry weight. Degreasing as a separate process step is not usual for bovine hides. The nature of this fat makes it difficult to remove because of the presence of glycerides and a high melting temperature.
Excessive amounts of grease in the skin may interfere with uniform penetration of tan or dye, causing difficulties in the finishing processes and creating dark and greasy patches on the finished leather. Degreasing of greasy skins is particularly important before chrome tannage as the chromium salts can react with the greases and form insoluble chromium soaps, which are very difficult to remove subsequently [50, Sharphouse 1983].
The three different methods commonly used for degreasing are:
1. degreasing in an aqueous medium with an organic solvent and a non-ionic surfactant
2. degreasing in an aqueous medium with a non-ionic surfactant
3. degreasing in a solvent medium.
Typical process parameters
|Heat transfer medium
|air and chemicals
|BASF. (2007). Pocket book for the leather technologist (4. Auflage).
Solvent degreasing using dimethyl ether
Degreasing is carried out in a closed-loop process using solvent recovery. The extraction agent used is dimethyl ether (DME).
The working pressure is approximately 686 kPa and the process temperature is between 15 and 30 °C. Fat and water are removed from the collagen matrix. Fat and water are separated from the DME by distillation at low temperatures, since the boiling point of DME is -25 oC. Fat and water can be collected separately. The process allows hides and skins to be dried resulting in a stabilised, low weight, easy-tohandle material. The dried hides or skins can be stored or transported.
Use of salt/acid pickling is avoided.
Pretanning is unnecessary.
Grease is recovered in a re-usable form. It is of high quality and has a high value in comparison with fat obtained from the traditional aqueous or organic solvent based degreasing processes. The fat will also not contain any traces of surfactants or other chemicals. Moreover, as the degreasing is conducted as relatively low temperatures, the thermal degradation and hydrolysis of the fat is minimal resulting in a low peroxides index.
DME is listed as an acceptable extraction solvent for the production of food additives.
Achieved environmental benefits
This technique replaces one or several polluting steps in the treatment of hides and skins requiring fat removal. These are especially polluting when additional solvent degreasing is required, as in most wool-on production.
This technique decreases the number of steps in the treatment of skins and hides, decreasing the total process time considerably compared to a traditional process. Furthermore, no water is consumed, no chemicals are added to the hides/skins prior to degrease, and other solvents, e.g. PER (an R40 substance), are not required to reach high levels of fat removal and removal efficiency.
A conservation/pickling/storage step is often required to achieve a high degree of degreasing of high-fat-content sheepskins, such as 'hogs' (English domestic types). This would not be necessary when applying this technique, as the degreasing process can be performed directly from pelt skins with no salt addition and at relatively low temperatures, making pretanning also superfluous.
This technique has low energy requirements; little mechanical energy is used, and it operates at low temperatures. This compares favourably with the relatively high energy used in the treatment of water from the aqueous degreasing process in order to break the emulsion and separate fat/surfactant mixtures. The energy requirements for the distillation of the extraction solvent DME, are low compared with the distillation of, e.g. PER (b.p. 120 ºC as against -25 ºC for DME). A low temperature energy source, e.g. steam condensates returning to a boiler, is sufficient to distil the DME from the water/fat mixture extracted from the skins. Hence the carbon foot-print as compared to that of the traditional leather manufacturing process is significantly improved.
This technique requires no water to perform the degreasing step, while in the conventional manufacturing process for sheepskins, degreasing is one of the water-consuming steps. The aqueous effluent from the DME process consists of the water which comes with the skins only. Since no surfactants are necessary for the degreasing step and the fat is recovered for further use, the COD load in the waste water will decrease.
The global salinity pollution derived from leather production can be reduced significantly by means of this technique. A significant part of the salt used in the beamhouse is avoided by the use of this technique compared to aqueous degreasing. Moreover, the preservation with salt can be avoided as well if this method is applied to raw skins to produce a dried material. The excellent preserving properties of the process are experimentally tested and reported. Useable fat/grease produced as opposed to a waste.
Although the technique operates in closed loop, there are residual emissions, quantified as a maximum of 3 g per kg of treated skin. That results in an estimation of emission level of 3 tonnes per year as a maximum for a tannery size of 1 million skins per year.
The implementation of the process requires the design and construction of a specific plant that uses currently available materials and operation devices. However, this equipment is not common in tanneries and thus, a separate location must be dedicated to apply the technique. Roughly, a sheltered surface of 200 m3 and a 100-m3 area outside for the DME storage and recovering unit is required. Local safety regulations may influence these sizes. These figures would apply to a mid-size tannery, or a production capacity of between 200000 and 500000 skins per year.
The technique has been suggested for use in degreasing of woolled and unwoolled sheepskins. It could be applied in the processing of bovine hides.
Operational costs are dependent on several factors and must be determined in every case. The same applies for the economical savings. Payback periods of the investment of less than 3 years are realistic.
Investment costs are estimated between EUR 1.5 and 2.5 million for a mid size tannery or a production capacity of between 200 000 and 500 000 skins per year. For a tannery processing 4 000 skins per day the use of this technique is calculated to save up to EUR 250 000 per year as compared to aqueous degreasing.
Driving force for implementation
The key-drivers to implement the technology are given below.
- No water is used in processing.
- No contaminated waste water is produced.
- Working conditions are improved for production people.
Status of development
A pilot plant of 120 litres' capacity with a complete recovery unit has been built in Avinyó, Spain able to treat up to 25 kg of material. Trials and semi-industrial production trials have been carried out since 2005 in this plant. Different sheepskin, leather, and other fatty hides/skins have been treated specifically for some of the main tanneries in Spain, France, Italy, UK, Holland, Norway, Turkey, and South Africa. Performance of the process has been extensively assessed by external leather institutes such as LGR-Germany, CTC-France, and AIICA-Spain.Reference literature[144, AkzoNobel 2005].
Source: Joint Research Centre, Best Available Techniques (BAT) Reference Document for the Tanning of Hides and Skins, 2013
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