Inhibiting Mercury Re-emission and Enhancing Magnesia Recovery by Cobalt-Loaded Carbon Nanotubes in a Novel Magnesia Desulfurization Process

Abstract

Mercury re-emission, because of the reduction of Hg²⁺ to form Hg⁰ by sulfite, has become a great concern in the desulfurization process. Lowering the concentrations of Hg²⁺ and sulfite in the desulfurization slurry can retard the Hg⁰ formation and, thus, mitigate mercury re-emission. To that end, cobalt-based carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg²⁺ removal and sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the Hg²⁺ adsorption and effect of Hg²⁺ adsorption on catalytic activity of Co-CNTs were investigated. Experimental results revealed that the Co-CNTs not only accelerated sulfite oxidation to enable the recovery of desulfurization by-products but also acted as an effective adsorbent of Hg²⁺ removal. The Hg²⁺ adsorption rate mainly depended on the structure of the adsorption material regardless of the cobalt loading and morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation was not significantly affected due to the Hg²⁺ adsorption. Additionally, the isothermal adsorption behavior was well-fitted to the Langmuir model with an adsorption capacity of 166.7 mg/g. The mercury mass balance analysis revealed that the Hg⁰ re-emission was decreased by 156% by adding 2.0 g/L of Co-CNTs. These results can be used as a reference for the simultaneous removal of multiple pollutants in the wet-desulfurization process.