The Kinetics of Volatile Lead Compound Formation During Simulated Hazardous Waste Incineration

Air pollution from fine metal-containing particles (and vapors) formed during hazardous waste incineration has attracted less attention than other incinerator emissions. Recently, however, metal pollution has become subject to more stringent regulations. The U.S. Environmental Protection Agency has announced limitations on the emissions of ten toxic metals. Pollution control systems that will effectively remove fine (submicron) metal-containing particles from flue gases are difficult to construct. Incinerators have not been designed or operated to minimize the formation of such particles. Industrial-scale incineration testing has produced anomalous results for air emissions of lead and other metals.

To gain a better fundamental understanding of the generation of metal emissions, the kinetics of the formation of volatile pollutants in simulated incinerator kilns were experimentally examined as a function of temperature and acid gas concentration. The chemical reactions between lead oxide, a common toxic metal found in hazardous waste streams, and hydrogen chloride, commonly produced in hazardous waste incineration are the specific focus. Descriptive models were constructed of the kinetics and mass transport of the lead dichloride and oxychloride intermediate compounds that were found to be generated. Fine particulate matter composed of lead dichloride was produced.

At temperatures between 260 and 310°C, the hydrochloridation reaction kinetics were measured, and an apparent activation energy of 22 kcal/mol was obtained. At 300°C and apparent reaction rate of 10-7 mol/(cm^2.s) was found at 2000 ppm of HCl. At higher temperatures, the formation of lead dichloride was diffusionally controlled, principally in the liquid or solid ash phases, and, until 600°C was reached, changed little from the 10-7 mol/(cm^2.s) rate in approximately 30- to 60-min batch hydrochloridation experiments. An interesting phenomenon was found in that at temperatures greater than 590°C, a glassy surface ash phase was formed while at temperatures between 450 and 590°C, distinctly liquid ash phase was formed. The glassy ash phase grew rapidly in thickness and exhibited a lower volatility of lead dichloride than did the liquid phase formed at lower temperatures. This has interesting implications for incineration because this research has revealed a situation in which a higher volatility product is formed at lower temperature, contrary to most expectations and to purely equilibrium considerations.