Leather Industry, Unhairing-Liming

The aim of the unhairing-liming step is to remove from the raw skin all the components which are not transformed into leather, such as the superficial epidermis including the hair and the subcutaneous adipose layer. The liming step introduces chemicals such as lime (Ca(OH)2) and sodium sulfide (Na2S) or sodium hydrosulfide (NaHS) which open the fibrous structure of the skin. Consequently, exhausted effluents are highly polluted for the presence of sulfide, amines, organic matter coming from degradation of hairs and epidermis, and high concentration of alkalis. The COD of exhausted effluents ranges between 20,000 and 40,000 mg/L of consumed oxygen. The UF treatment of the exhausted unhairing liquor has been one of the first membrane approaches introduced in the leather industry (Molinari 1995). It can be exploited to recover sulfide and solubilized lime together with low molecular weight proteic substances in the permeate stream in order to reuse the purified solution for the preparation of a new liming bath. High molecular weight proteic components, coming from chemical degradation of hairs and epidermis, are concentrated in the retentate stream (Fig. 1). Considering that 60-65 % of the initial sulfide remains in the exhausted liquor and 5-10 % is lost in the retentate, the quantity of sulfide that is possible to recycle with an UF system is 55-60 %. An innovating system based on the use of enzymes and small quantities of sodium sulfide (1.5 % instead of 10 %) has been proposed and tested on industrial scale. In this approach the enzymatic unhairing was combined to the continuous cross-flow UF of the bath permitting to keep constant the sulfide concentration in the unhairing bath due to its permeation through the UF membrane. The organic components (products of degradation of the keratin and of the interfibrillar proteins, fat substances, etc.) are concentrated in the retentate stream. This new approach is safer for workers and characterized by a reduced environmental impact because hairs are not dissolved and can be removed separately (Cassano et al. 1998).