High-temperature interactions between multiple-metals and kaolinite

Abstract

High-temperature combustion flue gases provided the environments for this investigation of multiple-metals interactions with dispersed kaolinite. Whereas previous research was restricted to interactions of single-metals with kaolinite powders, this work focuses on understanding how kaolinite transformations induced by reaction with one metal species can affect the mechanisms of interaction with other metal species. Experiments were performed in an 18 kW downflow furnace, by doping combinations of semi-volatile metals through a natural gas flame, and injecting supermicron sized powder kaolinite sorbent into the post flame. An aerosol size fractionation method was used to quantify the fraction of metal captured. It was found that the melt-associated restructuring of the meta-kaolinite crystal, caused by eutectics formed with lead and sodium reaction products (self-enhancing for lead and sodium capture), enhances the capture of cadmium by kaolinite at 1160°C. Cadmium reaction products by themselves fail to initiate a self-enhancing eutectic-melt at this temperature, because cadmium forms a higher temperature eutectic with the meta-kaolinite crystal than does lead or sodium. In fact, at temperatures in the range 1000°C to 1300°C, cadmium enhances the capture of lead and sodium by slowing down the deactivating excessive-melt driven by the lead or sodium/sorbent eutectic at these temperatures. Hence, total metal capture is enhanced in this temperature range for the Cd/Pb and Cd/Na systems by the formation of an optimum eutectic-melt, whereby significant melt-enhancement is induced without sorbent deactivation. Lead and sodium reaction products form similar high-temperature eutectics with kaolinite. However, sodium capture dominates over lead capture with a slightly higher reaction rate than lead, thus achieving capture before the most significant sorbent deactivation occurs. Sodium also dominates by effectively displacing lead previously captured on sorbent sites. These observations are quantified using models that extend those developed for single-metal capture to multiple-metal situations.