High-temperature removal of metal vapors by solid sorbents.

Abstract

Emissions of metal vapor compounds during incineration and combustion is becoming an increasingly important problem. The kinetics and mechanism of high temperature removal of various metal vapors by solid sorbents has been investigated in this study. The kinetics experiments were performed in a high temperature microbalance reactor system under simulated flue gas atmosphere. Scanning electron microscopy, X-ray diffraction analysis, atomic absorption/emission spectrophotometry, Energy dispersive X-ray analysis, mercury porosimetry, and BET surface area analysis were used for characterization of the fresh and reacted sorbents. The results show that the process of metal vapor capture is not just physical condensation, but rather a complex combination of various chemical and physical processes. There are some similarities in the sorption process. For all the sorbents the rate of metal vapor sorption decreases with time and there is a final limit beyond which no more metal vapor gets captured. However, there are differences in the rate and reaction mechanism of metal vapor removal. Kaolinite and bauxite are suitable sorbents for lead and cadmium capture. The melting point of the lead aluminosilicate product formed after reaction of lead chloride with kaolinite and bauxite has a low melting point. Therefore, these sorbents are more suitable for downstream fixed bed removal of lead compounds. Removal of cadmium by bauxite occurs due to chemical reaction to form a cadmium aluminum silicate and a cadmium aluminate. Removal of cadmium by kaolinite occurs due to the formation of only the cadmium aluminosilicate. The final products of cadmium sorption have a higher water solubility as compared to that of the corresponding products for lead. Chlorine is not retained by the sorbents during the sorption process. Kaolinite, bauxite and emathlite are suitable sorbents for removal of alkali compounds. In adsorbing alkali chloride vapors, kaolinite and emathlite release all the chlorine back to the gas phase while bauxite retains some of the chlorine. Moreover, the products of reaction with emathlite have a melting point significantly lower than those for kaolinite and bauxite. At lower alkali concentrations, NaCl reacts irreversibly with kaolinite to form a sodium aluminosilicate product. When the local metal vapor concentration in the sorbent pores becomes higher than the saturation concentration for condensation, the metal vapor physically condenses in the sorbent pores and may subsequently react with the solid. The theoretical models developed were used to extract kinetic parameters from experimental data and for parametric studies. The kinetic data obtained can be used in design of practical metal removal systems.