Leaching kinetics of ZN2+ and SO42- from tire pyrolytic char (TPC) using the shrinking core model

Abstract

Tire pyrolytic char has the potential to be used as a raw material/product in different applications. The only problem associated with the tire pyrolytic char is the presence of some impurities, which limits its potential for use as carbon black and as a precursor to producing activated carbon. Tire pyrolytic char contains high concentrations of Zn2+ and S, which are used as ingredients while manufacturing tires. Our main aim of this study was to perform leaching studies to remove of Zn2+ and SO42- from the tire pyrolytic char using a solution of H2O2 as a solvent and to perform kinetic studies further using the shrinking core model to determine the rate-limiting step for the leaching of Zn2+ and SO42-. Process variables such as stirring speed (rpm), solvent concentration (ppm), temperature (℃), and solid-to-liquid ratio (g/100 ml) were investigated. It was found that the leaching rate of Zn2+ and SO42- from the pyrolytic tire char increased with an increase in all the process variables except for the solid to liquid ratio, which had an opposite effect on the leaching rate. Fourier Transform Infrared Spectra (FTIR) analysis, SEM-EDS, and Xray diffraction (XRD) analysis were used to characterize the pyrolytic tire char before and after the leaching experiments. FTIR showed that sulfur-containing functional groups disappeared after leaching with a solution of H2O2. SEM-EDS also showed a reduction in both Zn and S presence from the pyrolytic tire char after the leaching. XRD results showed that the raw tire pyrolytic char contained diffraction peaks associated with ZnO and ZnS, which disappeared after exposure to leaching. The kinetic parameters obtained showed that the rate-limiting step for the leaching of Zn2+ and SO42- from tire pyrolytic char was diffusion through the product layer. The regression coefficients obtained for diffusion through the product layer for both Zn2+ and SO42- were closer to 1 than values obtained for the chemical reaction between the fluid reactant with the solid at the surface of the solid particle. This was further confirmed by the apparent activation energies obtained for the leaching of Zn2+ and SO42- -11 kJ.mol-1 and 36 kJ.mol-1, respectively. Semi-empirical models for the leaching of Zn2+ and SO42- were developed to describe the leaching process.

Keywords

• tire pyrolytic char
• leaching
• shrinking core model
• semi-empirical model