VOC emission from alumina calcination stacks caused by thermal decomposition of organic additives

Abstract

The final step of the Bayer process, involving the calcination of Al(OH)3 to alumina (Al2O3), is a significant source of Volatile Organic Compound (VOC) emission. In this paper we show that a typical dewatering agent (DWA), dioctyl sodiumsuccinate, decomposes to a series VOCs under conditions that are representative of an industrial calcination stack. At lower temperatures of 300–700°C, the predominant VOC released is 3-methyleneheptane (3-MH), due to cleavage of the alkyl-O bond in the additive precursor. Benzene is the predominate VOC generated at temperatures above 700°C, along with lesser quantities of toluene, styrene, p-xylene, ethylbenzene and traces of thiophene. In the temperature range 700–900°C, the benzene:toluene ratio steadily increases from 1:1 to approximately 6:1 under the conditions employed. The use of surfactants to minimise moisture content before calcination is thus shown to directly lead to the release of environmentally sensitive VOCs in alumina calcination stacks, and a more complete understanding of total VOC emission from all stages of the Bayer process is thus established.