Leather/Skin/Hide Processing Industry - Environmental Clearance

Leather / Skin / Hide Processing Industry (Tannery)current method, analogous to vegetable tanning. Here, the pelts progress into cleaner floatand the contaminated floats move towards the dirt soak. Only the dirt soak liquor, inwhich dirt and salt are accumulated are discharged to waste and treatment. This decreasesthe amount of water to be evaporated, when salinity is restricted, and reduces the presenceof biocides in effluent. However it does not solve the problem of what to do with the dirtsoak solution. Lagooning where feasible reduces the volume, but salt remains.The unhairing-liming float can also be reused for the next process. It must be taken intoaccount that the recovery rate of the liming float should not exceed 75 % in order to limitthe nitrogen concentration. Besides recycling materials (pumps, fine screening, storagetanks), it is sometimes necessary to warm the float before reuse and also to screen or skimit in order to eliminate undesirable floating solids and to remove hair and grease from thesurface. Without any sedimentation, an industrial recycling process can save 35 to 40 %of sodium sulphide and 40 to 45 % of the lime (with classical process quantities of 2.5%). Excessive quantities of lime should be avoided during the process. It is worth to recallin this regard that the theoretical requirement for bovine hide is about 1.2%.3.4.2.3 Tanning processa) Degreasing floatWhen sheepskins are solvent degreased, recycling of the residual solvent after distillationis currently operated. Furthermore, the extraction brine is also easy to reuse for saving ofsodium chloride.b) Pickling floatRecycling of pickling float has been proven to be highly satisfactory in terms of saltsavings and partly for acid savings. There is no great difficulty if density and acidity ofthe float can be regularly controlled.c) Tanning floatThe most common practice is to carefully collect the residual tanning float, to filter it, toadjust its acidity, and to reuse it as a new tanning float before adding fresh chromiumsalts. The recovered volume may be more than required for subsequent tanningoperations, but it is possible to reuse the liquor in post-tanning processes.Another possibility is to use the tanning float for a pre-tanning process. In this case, 60 %of the residual chromium can be recovered.When pickling and tanning are carried out in the same float, it is also possible to collectthe residual tanning float, to filter and acidify it and reuse it as a pickling float.3.4.2.4 Post-tanning processIt is much less feasible to recycle post-tanning floats since the chemical conditionrequired for the steps may be different and steps tend to be conducted sequentially in thesame float. Therefore, the problem of contamination is compounded, especially sincethese steps vary greatly, even in a single tannery. Thus recycling technology cannot berecommended.TGM for Tanneries August 20103-21