Recovery of Tannins from Exhausted Baths in the Leather Industry by Nanofiltration

The use of a nanofiltration process is described in order to rationalise the vegetable tannage step in the leather industry through the recovery of tannins from the exhausted baths and their reuse as tanning agents. The results obtained on pilot scale using a membrane module, identified after a screening of various nanofiltration membranes, the operating and fluid-dynamic conditions and the mass balance of the nanofiltration process are reported and discussed. Skins treated with the recovered solutions revealed chemical and physical parameters very similar to those measured on control skins tanned with standard solutions. According to the obtained results, it is possible to suggest a process scheme that, starting from the exhausted tanning baths, through a nanofiltration membrane purification/concentration, permits to increase the tannin/non tannin ratio of the retentate solution. Advantages are in terms of: reduction of environmental impact, simplification of cleaning-up processes of wastewaters, decrease of disposal costs, saving of chemicals and water.