Potential danger of chromium tanned wastes

Abstract

This contribution deals with the waste from the processes in which raw hide is transformed into leather, as well as with the waste generated during further leather processing and the final waste of leather industry, the worn-out shoes. All this waste, if deposited in the open-air dumps, represents a potential threat to human health, because it contains trivalent chromium (Cr III), which can oxidise to its hexavalent form (Cr VI) under various conditions. The authors discuss the possibilities of spontaneous transformation of Cr III into Cr VI and propose several technologies of the waste treatment to eliminate the health and environmental risks. Chrome-tanned waste, especially the chrome shavings, is processed with the use of modified enzymatic hydrolysis. The chromium cake obtained after filtration has proved to serve as a suitable precipitator of chromium ions from chromium containing spent liquor. Simultaneously, the chromium cake is enriched by those chromium ions and can be re-used in the tanning process. The method has been successfully tested in pilot and industrial scale. It enables us to possess the total control of solid and liquid chrome-tanned waste, which has shown a new approach towards the issue of chromium recycling in the tanning industry. A two-step technology has been proposed for processing leather scraps, for which it is not possible to apply the same method as in the case of chrome shavings. One of the products of this technology is a protein hydrolysate containing a high portion of nitrogen, phosphorus and potassium. This hydrolysate has been already successfully used as a good-quality NPK fertilizer in agriculture. The authors also warn against the classic ways of worn-out shoes treatment, i.e. disposal in the open-air dumps or combustion, which leads to potentially dangerous products. The separate collection of worn-out shoes is recommended and it is assumed that the hybrid hydrolysis technology used for leather scraps could be also successful at solving the problem of old shoes.