In-furnace capture of cadmium and other semi-volatile metals by sorbents

Abstract

An 18 kW, cylindrical (inside diameter = 15-cm), 6-m high, laminar, downflow furnace was used to investigate the mechanisms governing reactive scavenging of cadmium vapors from combustion environments, by hydrated lime and kaolinite powder sorbents. Two major physical mechanisms dominate the reaction rate and sorbent utilization during the capture of semi-volatile metal by kaolinite, temperature-activated eutectic-melting-enhanced capture and excessive-melt deactivation, initiated at a higher temperature than the enhancement. However, cadmium forms higher temperature eutectics than other semi-volatile metals such as lead and sodium, which have a transition to a deactivating melt below 1100 °C. The network modifier responsible for the reaction enhancement is the metal/sorbent reaction product itself, which forms a eutectic with the remaining, unreacted sorbent. Global rate models were developed to describe the reaction of kaolinite with lead, sodium, and cadmium metal vapors. The following reaction scheme was valid for all three metal vapors, Na/Pb/Cd: Na 2 O / PbO / CdO + 1 2 ( Al 2 O 3 · 2 SiO 2 ) ⇒ P Na / Pb / Cd . Followed by the substrate (melting) deactivation step for Na and Pb, but not for Cd: P Na / Pb + 1 2 ( Al 2 O 3 · 2 SiO 2 ) ⇒ D + P Na / Pb . Eutectic-melting enhancement of cadmium capture by both kaolinite and hydrated lime was initiated between 1160 and 1280 °C. Cadmium is more effectively captured at higher temperatures (i.e., ∼1300 °C) than other semi-volatile metals, such as lead and sodium, and requires even higher temperatures to cause sorbent deactivation by excessive melting. Unlike cadmium, lead and sodium were poorly scavenged by hydrated lime.