Mechanisms and Models Describing Sodium and Lead Scavenging by a Kaolinite Aerosol at High Temperatures

Abstract

High temperature mechanisms of interaction between a dispersed kaolinite aerosol and vaporized sodium and lead were investigated using experimental data obtained from an 18 kW downflow furnace. The validity of using aerosol size fractionation to determine the extent of reaction between metal and substrate was confirmed through a series of specialized tests. The reaction stoichiometries for the Na/kaolinite and Pb/kaolinite systems were found to differ from literature values but were similar to each other. New light was shed on the nature of the reaction products for the Na/kaolinite and Pb/kaolinite systems, and for semivolatile metal/kaolinite sorbent systems in general. Modified reaction mechanisms were developed that accommodate key-observed processes (i.e., both enhancing and inhibitory substrate-melting processes). Rate coefficients for each system were quantitatively determined. Mechanisms are interpreted in the light of known information on the structure and nature of kaolin clay and based on observed phase transformations of the Na/kaolinite and Pb/kaolinite systems. The new models for high temperature Na/kaolinite and Pb/kaolinite interactions compared well to both new and literature data. These models can be used to predict and optimize the scavenging of vaporized sodium and lead by kaolinite mineral particles in fossil fuel-fired systems and waste incinerators.

Isolation of Metals from Liquid Wastes: Reactive Scavenging in Turbulent Thermal Reactors

“…This is in contrast to previous results for lead and sodium (37,45,46). Figure 4 indicates a maximum sorption of 80% was achieved for the test series examined…”

“…Global kinetics for the sorption of sodium, lead, cadmium, and mixtures of these metals, on kaolinite, have been proposed by Davis and Wendt (37), Yang et al, (44), and Gale and Wendt (45,46). For lead and sodium sorption, the proposed mechanism consists of an adsorption reaction with low or zero activation energy, followed by a sorbent deactivation step with high activation energy.…”

“…With sorbent deactivation parameters equivalent to those used by Davis and Wendt (37) and Gale and Wendt (46), best fit cesium sorption parameters [k 1 =5.2e+2 m 3 /(mol·s), and E 1 =8.7e+1 J/mol] produce the response curve presented in Figure 4 with an r 2 =0.95. Although undue fundamental significance of these global rate constants would be misplaced, it might be argued that kaolinite deactivation is a common result of adsorption of various metals on kaolinite.…”

“…Gale and Wendt (45,46,57), one can adapt their two-step model consisting of an adsorption reaction involving a reactive cesium species (CsOH) and activated kaolinite (assumed to be metakaolinite, (37) and Gale and Wendt (57) describe how in addition to the metal speciation, the phase transformations of kaolinite (Al 2 O 3 ·2SiO 2 ·2H 2 O) in the combustor are important. These transformations influence the substrate reactivity, and may be strongly temperature dependent (57).…”

Mercury Control with Calcium-Based Sorbents and Oxidizing Agents

“…In this state, the alumina layer is well disposed to accommodate metals into the open interstitial sites within its matrix, which then reverts back to a stable octahedral coordination, where the captured metal is trapped in a stable compound. Kaolinite has been shown to be effective at capturing lead, cadmium, and sodium [27][28][29][30], which are all semi-volatile metals.…”