Dezincification and ash reduction of waste tire pyrolysis carbon black via combined microwave heating − NaOH treatment

The high-value utilization of waste tire pyrolysis carbon black (CBp) is hindered by its high ash content, especially high zinc, sulfur and silicon content. However, the existing removal method generates a large amount of waste acid and alkali and fails to directly recover zinc. Based on the high-efficiency and green characteristics of microwaves, without the need to add additional reagents, this study efficiently removed and recovered zinc and sulfur by using the spontaneous reaction of substances inside the raw materials as well as the dissociation and regeneration of minerals. Moreover, sodium hydroxide (NaOH) was used to remove silicon. In this process, 99.94 % of zinc and 89.82 % of sulfur can be removed and recovered, and silicon is removed simultaneously. The fixed carbon content of CBp increased from 80.22 % to 94.02 %. The mechanism of dezincification and desulfurization via microwave treatment was investigated. It was found that CBp exhibits excellent dielectric properties, with a dielectric constant as high as 36.3 F/M. In the microwave field, the heat generated by the conductive polarization of carbon powder serves as the primary heat source for zinc and sulfur removal. During the entire process, zinc is removed in the form of ZnS and ZnO, while sulfur is removed as ZnS. Compared to conventional heating, microwave heating can achieve superior sulfur-removal and Zinc-removal efficiency at lower temperatures. The process offers a scalable, acid-free route for UNSDG-12 compliant valorisation of CBp, which is conducive to the effective utilization of solid waste resources.