Removal of impurities from waste tire pyrolysis char using the molten salt thermal treatment

Thermo-chemical recycling of waste tire to generate pyrolytic char as alternative carbon black is a promising route for waste disposal and environmental sustainability. However, impurities in waste tire (like Zn, Si, Ca and S) are probably enriched in the pyrolytic char, and removal of those impurities is of great importance for char recycling. To avoid generating large amount of waste acids and polluted gases in the traditional impurities removing process, molten salt (Na2CO3-NaOH) thermal treatment was creatively proposed in the current work. The influence of operational temperatures (in the range of 350 to 450 °C) was investigated in relation to the efficiency of impurity removal. Results show that the removal efficiency of impurity components was firstly increased but then weakened by increasing treating temperature from 350 to 450 °C, while the highest removal rate was obtained at 400 °C. Generally, the current system could effectively remove more than 80% of ash and 70% of sulfur at 400 °C, with the recovery rate of char between 71% and 83%. Specifically, calcium carbonate/sulfate was removed from the char due to its good solubility in the molten salts. In addition, molten salts showed good activity to convert char contained inorganic sulfides and ·SH radicals into salts-soluble Na2S. Similarly, silicon and zinc were tended to be transformed into soluble silicate and zincate, which had higher solubility in the used molten salt. In addition, the agglomerated char particles were separated into smaller and more uniform size during molten salt thermal treatment.