Origins and Behavior of Dioxins and Furans in Zinc-Bearing Dust

Аннотация

The use of zinc-bearing (galvanized) scrap as a batch component in electrosmelting produces dust from which nonferrous metals may be extracted. However, the behavior of chlorine and its components in electrosmelting dust containing zinc and lead has not been adequately studied. Research shows that chlorine and its compounds in electrosmelting batch and hence in the arc-furnace emissions must be regarded as a hazard because they form highly toxic organic compounds: dioxins and furans. They are released to the atmosphere not only as gas but also as in adsorbed form on the surface of the dust particles. According to the available data, their concentration is 5–500 ng/kg of dust, depending on the smelting parameters. The formation of dioxins and furans in arc furnaces is analyzed, as well as their behavior in captured dust. With 1.3% chlorine in electrosmelting dust, 99.9% forms relatively safe compounds—mainly chlorides—but the remainder forms dioxins and furans. The quantity of dioxins and furans adsorbed on the surface of dust particles is 474 ng/kg of dust. Dioxins and furans are powerful environmental toxins (first hazard class) and therefore raise the hazard status of the dust from the fourth to the third class (or higher). That must be taken into account in handling the dust. In addition, the dioxins and furans are transported to the environment by dust particles, onto which the chemicals are sorbed. Therefore, electrosmelting dust whose surface contains adsorbed dioxins and furans may carry these toxic materials into living organisms. Means of decreasing dioxin and furan emissions in electrosteel production are considered. Methods of dust processing characterized by low environmental impact and resource conservation are also discussed. In particular, the possibility of using milk of lime to irrigate the exhaust gases from the arc furnace is analyzed. This method is found to lower the content of dioxins and furans to acceptable levels. The effectiveness of the proposed methods is assessed.