Research On The Utilization Of Chrome Shaving By Pyrolysis

Chrome shaving is a kind of solid waste containing chrome compounds such as peels,leather powders and chrome leather scraps,which come from tanning leather processingusing chrome tanning agents. According to the related reports, there are more than0.7million tons of such chromium solid wastes produced in China every year, and most ofwhich are land filled or piled up directly eventually. Chrome shaving consists of more than80%collagens and3.5%~4.5%Cr2O3, thus it has high recycling values. Current researchfocuses on recycling by ordinary chemical or biochemical method, in which chromeshaving is separated to two parts by hydrolysis. One part is chrome which can reused in theleather tannning process.The other part is collagen which can be made into other products(such as retanning agents, finishing agents, pharmaceutical products, and cosmetics).However, due to the incomplete separation, the application of this collagen is subjected tothe limitations.Recycling of chrome shaving by means of thermochemical method is a feasible way.As the first stage of thermochemical process, pyrolysis is a potential pathway forrecycling. In this study, TG was combined with DTA to study the pyrolysis and combustioncharacteristics of chrome shaving, compared with three kinds of municipal wastes, whichwere paper, plastic and vegetable. The Coast-Redfern integral method was used to calculatepyrolysis and combustion dynamics parameters, as well as the calculation ofthermodynamic parameters like ΔG, ΔH and ΔS, to explore the behavior of pyrolysis andcombustion of chrome shaving. Pyrolysis process of chrome shaving took a longer time,whereas both the stage of the ignition of volatile precipitation and the combustion of fixedcarbon completed in a shorter period of time.Then a fixed bed was used to study the pyrolysis characteristics of chrome shaving,focusing on the characteristics of pyrolysis products. The distribution of pyrolysis productswas influenced by the carrier gas flow rate, final pyrolysis temperature and heating methods.The liquid yields of fast pyrolysis reached the highest level at550℃, which could be usedas raw materials of chemical extraction. The gas products pyrolyzed above550℃had a higher heating value, which met the requirements of the calorific value of burning gas. Thecalorific value of the solid products improved a little. But there is a problem that has to besolved, that is the chromium component. Thus the contents of the hexavalent and totalchromium in the pyrolysis char were detected. The results showed that the contents of thetotal chromium in pyrolysis char increased significantly with the increasing of temperature.Yet the contents of the hexavalent chromium decreased exponentially, and when thepyrolysis temperature reached550℃, the contents of the hexavalent chromium was verylimited.As a high N-containing substance, the migration and transformation of nitrogen in thepyrolysis progress should be worth of concern. A portable infrared gas analyzer combinedwith a tube furnace was used to detcet the release of N-containing gases. And finally thedistribution of nitrogen in gas and solid phases could be obtained.