Difference between revisions of "Information about hides and skins"

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Applied Processes and techniques

The possible steps in the production of leather are shown schematically in Figure 1. There is considerable variation between tanneries, depending on the type of leather being produced.

Figure 1: Process steps in leather making (chromium tanning)

Operations carried out in the beamhouse, the tanyard, and the post-tanning areas are performed in water or 'float'. After post-tanning, the leather is dried and subsequent operations are dry processes.

Wet processing was in the past carried out in fixed vessels or pits, with the hides or skins being moved from one pit to the next. This method is still used by some tanneries, particularly for vegetable tanning, but now mechanical agitation of either the hides and skins or the liquid is used [50, Sharphouse 1983]. The pits are not emptied between each batch of hides or skins. Several pits containing increasing concentrations of the same solution may be used for a single process step.

In most tanneries to which this BREF applies, rotating vessels are used. These may be drums (closed vessels rotating on horizontal axles), or mixers (open at one end and operating at an angle). In both types, several process steps may be carried out while the hides or skins remain in the same vessel, with the liquid being changed or recirculated.

Degreasing as a separate process step is applied only to sheepskins and pigskins and an effluent stream containing surfactants may be produced. Sheepskins may be processed without unhairing, to produce 'wool on' sheepskins.

All of the process steps might not be performed by a single entity or within one site. Some intermediate products are more stable than others, and where delays are possible, a transfer between processors is carried out at one of the stages shown with dotted lines in Figure 1. It is usual for sheepskins to be traded in the pickled state, whereas bovine hides are usually traded at the wet blue stage. Either may be traded as crust leather. Greater subdivision of processing is possible within a tannery cluster.

A person or company carrying out the processing of sheepskins up to the pickled stage is known as a fellmonger.

There is no standard pattern within the European Union of trading in hides and skins or in the form in which they are delivered to a tannery for processing. They may be:

• delivered directly from local slaughterhouses
• obtained from hide dealers
• obtained from hide markets
• traded internationally.

Energy consumption

Energy consumption in tanneries depends mainly on the following factors:

• the production methods, and the capacity and size of equipment
• the age and sophistication of electric motor controls
• the amount of mechanical handling used to move hides and skins
• the drying methods used
• heat losses from process vessels and from buildings
• air exchange rates to meet workplace safety conditions
• types of waste water treatment on site
• types of waste treatment and recovery of energy from waste on site.

Heat losses may be mitigated by thermal insulation, but may be exacerbated by a low external temperature. A high moisture content in the air may increase the energy consumed in drying. Energy use data from one climatic zone may not be an accurate guide to what may be achieved in another.

The age and efficiency of the combustion equipment and boiler plant will determine the proportion of the energy of fuel which is made available as thermal energy in the tannery. A larger central boiler may be more efficient, but if operations are dispersed on a large site, heat losses from pipework may eliminate the gains. Such considerations are not confined to tanneries and are covered more fully in the BREF document for energy efficiency (ENE) [109, EC 2008].

Table 1: Consumption of thermal and electrical energy

Table 1 gives an indication of energy consumption by type of energy input. The proportions can vary greatly between tanneries. For example, mechanical handling can be undertaken by fork lift trucks powered by internal combustion engines or by electrically powered conveyors. For this reason, the total energy input to the leather making process is a better comparator. It is necessary that data be compared for the same stages of the leather making process. Ideally, energy use should be monitored and reported separately for each process stage, and it is known that some of the most energy-efficient tanneries do so.

Where more detailed data for energy use are available it is important that comparisons between tanneries be made on the same basis. For example, 'effluent treatment' may or may not include biological treatment, which can account for more than 50 % of the total energy consumption in the treatment of tannery effluent.

Best available technologies (BATs)

1. In order to reduce energy consumed in drying, BAT is to optimise the preparation for drying by samming or any other mechanical dewatering.

2. In order to reduce energy consumption for wet processes, BAT is to use short floats.

Description

Reducing the energy used to heat water by reducing hot water use.

Applicability

The technique cannot be applied in the dyeing process step and for the processing of calfskins.

Applicability is also limited to:

• new processing vessels;
• existing processing vessels that allow the use of, or can be modified to use, short floats.

Table 2: BAT-associated energy consumption rates

Source: Joint Research Centre, Best Available Techniques (BAT) Reference Document for the Tanning of Hides and Skins, 2013, p.13-14

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