Study on the Recycling Technology of Unhairing-Liming and Tanning Wastewater

The recycling technology is one of the most important ways to realize the lean production in leather industry. In this study, a novel closed recycling technology of unhairing-liming and chrome tanning wastewater has been applied in mass production scale in a local tannery.

The aim of this research is to explore the technical points of the recycling technology of liming and tanning wastewater, and evaluate the effect of the process and the quality of the obtained products.

In the recycling process, the content of various ions and TOC of the wastewater were measured. The collagen fiber dispersion and the distribution of ions in pelts were characterized. The shrinkage temperature and mechanical properties of leather obtained by the recycling technology were also tested. All the trials shower that the novel closed recycling technology worked well without impairing the quality of the zero emission of unhairing-liming and chrome tanning wastewater was realized.

The unhairing-liming and chrome tanning are two of the most important steps in the production process of leather making. The quality of unhairing-liming and tanning process will affect the following process as we as the properties of the finished leather. Therefore, the unhairing-liming and chrome tanning wastewater is the main source of pollution in the leather manufacture.

The recovery and reuse of wastewater is one of the effective ways to realize the clean production of liming and chrome tanning which has been paid more and more attention by researchers. Compared with the normal craft, the pelt obtained by this recycling technology was evenly swelled. The pelts also showed a clean surface and an obvious weight gain. The morphology of grain of limed pelt was investigated by super depth on field microscope. The results indicated that the hair of two pelts, limed pelt obtained by the normal process and the limed pelt obtained by fourth times recycling process, was removed thoroughly.

The hair pores were both clearly visible. So, the recycling technology of liming wastewater was safe and reliable. In order to assess the novel recycling technology, the physical mechanical properties of the finished leather samples obtained by conventional process and the recycling technology were compared. The results indicated that the physical mechanical properties of finished leather were not influenced by the recycling technology. On the contrary, the tensile strength and tear strength of leather sample from the recycling technology were better than that of the sample obtained by the conventional technology. Furthermore, the yield of leather in the recycling technology was 95%, while the conventional technology is about 93%. Compared with the normal process, the novel recycling technology could save more than 80.8% water, 33.3% of sulphide and 23.4% lime in liming process. While in the chrome tanning wastewater recycling craft, 20% of the chrome tanning agent as well as 70% of salt were saved.