Leather Processing, Chromium recovery

In the tannage operation tanning agents are used in order to prevent the leather from chemical and thermal degradation. The most common tanning agent is the chromium basic sulfate. It enters the pores of the skin by a diffusion process to react with the collagen carboxyl groups and to form inter- and intramolecular cross-linking which results in physical, chemical, and biological stability. The exhausted bath coming from the chromium tannage contains about 30 % of the initial salt content and it is normally sent to a cleaningup plant. Here chromium salts end up into the sludges creating serious problems for their disposal. Chromium recovery from tanning exhausted baths represents a significant economical advantage for the leather industry in terms of its reuse and for the simplification of the depolluting process of global effluents. An integrated process based on a preliminary ultrafiltration (UF) of the spent liquor followed by a nanofiltration (NF) treatment of the UF permeate has been proposed as a technically viable method for recovering chromium salts from spent tanning liquors (Cassano et al. 1996, 2007). The UF process allows a marked reduction of suspended solids and fat substances. The concentrated chromium solution obtained in the NF process can be reused for the preparation of new tanning baths. The NF permeate can be reused in the pickling step because of its high content of chlorides (Fig. 1).