Quality improvement of recycled chromium in the tanning operations by membrane processes

Use of pressure-driven membrane processes integrated with conventional precipitation/dissolution operations is described in order to reduce chromium content in waste water from the tanning process and simultaneously improve the quality of the recycled chromium by means of elimination/reduction of organic lipolytic components, metals and other impurities that accompany the chromium recovered by traditional processes. The results obtained in the identification of types of membranes and modules, chemical-physical parameters, pretreatment of fluids, cleaning procedures and fluid dynamic conditions are reported and discussed. According to the obtained results, it is possible to suggest a process scheme that, starting from the exhausted chromium solution, through a nanofiltration membrane purification/concentration permits to obtain solutions that can be reused in tanning and/or retanning operations. The concentrate solution seems to guarantee improved characteristics of tanned and retanned skins with respect the skins treated with the traditional techniques as showed by chemical and physical essays on leather samples. The process permits also the reuse of the permeate from the nanofiltration in the pickling phase, considering the high content of chlorides in the solution.