Recovery and re-use of chromium from the waste generated by leather industry

Abstract

Leather industry processes hides of slaughtered and game animals; meaning by-products of the meat industry. The main output is stabilized leather. The conversion of raw hide into leather comprises a complex series of chemical, physical-chemical and mechanical operations. Approximately 80% of leather is stabilized (tanned) by trivalent chromium (CrIII) salts. The processing of 1,000 kg of raw hide into leather results in the creation of 200 kg of leather with chromium content of 3 kg, 250 kg of non-tanned protein waste and 200 kg of tanned waste with chromium content of 3 kg. The tanning process generates up to 50,000 kg of waste water containing 5 kg of chromium. Consequently, only 20 % of collagen raw material is valorized by leather manufacturing processes and about 73 % of chromium ends as waste (primary leather waste). Another kind of leather waste is the so-called manipulation waste (secondary waste) generated during processing of the stabilized leather into final leather products, e.g. shoes and other leather goods. These products, when worn-out and disposed of, represent the final stage of leather waste (tertiary waste). Due to the content of chromium, all the above named kinds of waste are potentially hazardous. While chromium III is harmless, chromium VI is highly toxic and carcinogenic. It is considered a causative agent of cancer of urinary bladder, renal pelvis and kidney. The main problem of all kind of chrome-tanned leather waste lies in a possibility of spontaneous oxidation from CrIII to CrVI. The answer is the free enthalpy change of the oxidation reaction. In the case of the oxidation from CrIII to CrVI the value of free enthalpy is negative, which implies that the spontaneous oxidation can occur. While there are many ways of disposal of the primary waste and we can consider this problem solved, disposal or further processing of tertiary waste, especially old shoes, still represent a technological challenge. In co-operation with the USDA Eastern Regional Research Center, Wyndmoor and the Texas Technical University in Lubbock we have developed chemical methods for separation of chromium from the collagen part in worn-out leather products. We have also found applications for both the protein part and separated chromium. ©2011 Nova Science Publishers, Inc. All rights reserved.