Bruno München Wenzel

Formation of PCDD and PCDF in the thermal treatment of footwear leather wastes

The leather waste generated by the footwear industry is considered dangerous due to the presence of trivalent chromium, derived from the salt utilized to tan hides. In Brazil, the majority of this waste is disposed on landfills and only about 3% are recycled. The thermal treatment is an alternative method for purification of such residues. By using this technique it is possible to generate energy and recover the chromium present in the ash for the production of basic chromium sulfate (tanning industry), high carbon ferrochromium or carbon-free ferrochromium (steel industry). In the last 10 years, the gasification and combustion of footwear leather waste have been intensively studied at the Federal University of Rio Grande do Sul. The research experiment for characterization of the emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) were carried out in a semi-pilot unit (350 kW(th)). From new investments the thermal capacity of the unit will increase to 600 kW(th). The unit will produce power from the heat generated in the combustion. The experimental results indicated that during the thermal treatment of footwear leather wastes, the formation mechanism of PCDD/F is the de novo synthesis. Most of PCDD/F were found in the particulate phase (>95%). A kinetic model was used for discussion of the achieved experimental results. The model is based in the carbon gasification, PCDD/F formation, desorption and degradation. From the conclusions obtained in this work will be possible minimize the PCDD/F formation in process of combustion of footwear leather wastes.

Aluminothermic reduction of Cr2O3 contained in the ash of thermally treated leather waste

In this study the viability of utilising ashes with high chromium oxide content, obtained by thermal treatment of footwear leather waste, in the production of low-carbon ferrochromium alloy (Fe-Cr-LC) by aluminothermic reduction was investigated. The following key-factors were selected for process modelling: the quantity of aluminium (Al) employed in the reaction, the iron amount added, the iron compound (Fe and/or Fe2O3) used, and the chromic acid addition. The process was investigated using a 24 full factorial design where the percentage of Cr2O3 reduced was used as the response. Variance analysis was employed to determine the significant effects and to validate the obtained model. The model was useful for finding the optimal operating conditions, including the maximisation of chromium conversion and the gross margin. Both resulted in similar process conditions, with 76.8±12.3% of chromium being reduced to the metallic phase, and 1.65±0.52 USD (kg ash)-1 as the gross margin. The qualities of some alloys obtained were investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis (SEM/EDS). The results showed that the main problem for these alloys in a standard specification was the P and S content, suggesting that a pre-treatment is required.