Competition for sodium and toxic metals capture on sorbents

The removal of toxic heavy metals by sorbents has been the focus of much recent research. In single metal/single sorbent systems, laboratory experiments have demonstrated capture of lead and cadmium by kaolinite and lime at high temperatures. However, practical systems (i.e., pulverized coal fired boilers) generally involve sodium as well as toxic metals and the amount of sodium usually exceeds that of the toxic metals. The purpose of this paper is to explore the effect of sodium on the capture of lead and cadmium by sorbents. Experiments were conducted in a 16 kW, 0.15 m ID, 6 m tall laboratory downflow furnace. Aqueous solutions of toxic metals and metal/sodium pairs were introduced into the furnace through a natural gas flame via atomization and subsequent vaporization. For all experiments, the total metal/sorbent equivalence ratio was maintained at 0.7. Approximately 0.5 g/min of lime or kaolinite ponder (mean particle diameter congruent to 1.4 mu m) was injected along the furnace centerline in the post dame at a temperature of 1500 K. Further downstream, at 1200 K, particulate samples were isokinetically extracted and size segregated in a Berner low pressure impactor. Experimental results showed that cadmium and sodium were captured by lime. Furthermore, cadmium,vas captured by lime preferentially over sodium. Lead capture by lime occurred at a much slower rate than that for cadmium or sodium. Kaolinite, on the other hand, showed a preference for lead and sodium capture over cadmium. Consequently, for capture of multiple metals, a sorbent mixture of lime and kaolinite might be more effective than a single sorbent.