Author

Ahad Zabett

Date of Award

1999

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

W-K. Lu

Abstract

Steelmaking dust is generated in high temperature refining step at a rate of about 2% of the steel produced. Electric Arc Furnace (EAF) dust is classified as a hazardous material due to its content of leachable heavy metals, i.e., Pb, Cd, and Cr. Therefore, it must be treated before disposal to meet certain environmental regulations. Most processes for the treatment of EAF dust involve recovery of valuable metals, such as zinc. In most cases alkali halides and lead in the dust are problematic in both processing and product quality. In the present work a pre-treatment is proposed to separate "more volatile species" which include the alkali halides, lead compounds and cadmium oxide, from "less volatile species" which include iron, zinc and calcium oxides. Evaporation of volatile species in this process takes place at about 900° C in a virtually closed system under reduced pressure, and condensation of the vapors occurs at a lower temperature. The thermodynamic aspects of the process are considered. With the use of three different experimental apparatuses the kinetics of the system are studied. The rate of individual kinetic steps; evaporation, condensation, heat and mass transfer, are calculated and compared with the observed overall rate of reaction. A numerical model for the heat transfer inside the dust bed is developed. Applying the principles of mass and heat transfer to the system under investigation, and using the results of the experiments and the numerical model of heat transfer, it is shown that heat transfer across the porous dust bed is most likely the rate controlling step. A rotary reaction chamber is designed to facilitate the heat transfer to dust particles and eliminate the slow kinetic step of heat transfer across the stationary dust bed. With the use of the rotary reaction chamber the duration of treatment is significantly reduced under otherwise identical conditions. The benefit of the rotational movement may be appreciated by direct comparison of the treatment of twenty five grains EAF dust. At a furnace temperature of 1100°C for 95% removal of the volatile species the time required (from the introduction of the apparatus at 25°C to its withdrawal from the furnace) is 12 minutes for the stationary and 8 minutes for the rotational chamber. At a lower furnace temperature of 950°C , the degrees of removal of lead and potassium are about 51% and 27% for 12 minutes in the stationary reaction chamber and about 83% and 78% for 10 minutes in the rotary reaction chamber. A secondary incinerator dust is also investigated in the present work. In 6 minutes at a furnace temperature of 950°C about 99% of the volatile species including NaCl , KCl , and lead compounds are removed from twenty five grams dust. The residue has an enrichment of zinc from 27%wt to 78%wt . This environmentally friendly and energy efficient process may be applicable for the separation of "more volatile species" in dust generated from most high temperature processes such as steelmaking, incineration, nonferrous processes and cement manufacturing.

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