Rapid synthesis of magnetic zeolite materials from fly ash and iron-containing wastes using supercritical water for elemental mercury removal from flue gas

Abstract

A novel one-step supercritical hydrothermal synthesis method was developed in this study to prepare magnetic zeolites from fly ash (FA) and iron-containing wastes (red mud (RM) and steel slag (SS)) for removing elemental mercury (Hg0) from flue gas. The efficiency of these magnetic zeolites for Hg0 removal and their regeneration and recyclability were evaluated using a fixed-bed reactor. The crystal structure, chemical compositions, surface chemistry, and Brunauer–Emmett–Teller specific surface area of the prepared magnetic zeolites were investigated by X-ray diffraction, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, and nitrogen adsorption experiments. The main crystalline phase of the synthetic magnetic zeolite is cancrinite. SS is rich in iron oxide; therefore, the mixture of FA and SS containing 11–15% iron oxide was found to be a better precursor for preparing magnetic zeolite with better performance for Hg removal. The efficiency of Hg0 removal over magnetic zeolites was above 80% in the presence of hydrochloric acid (HCl, 10–100 ppm). This study indicates that the mechanism of Hg0 removal over magnetic zeolite in simulated atmosphere containing HCl follows the Eley–Rideal mechanism. Moreover, pre-adsorption of HCl leads to the generation of the active Cl*, which is the main reason that the Hg0 removal efficiency of the HCl-modified zeolite within the temperature range of 50–300 °C was above 80%.