Salt Removal From Black Dross By Heat Treatment

Abstract:

Recycling of aluminium scrap and white dross yields significant economic and energy savings, as well as environmental benefits. During the smelting process of recycled aluminium scraps fluxes consisting of NaCl and KCl are added on the molten bath to prevent metal oxidation and remove impurities from the charge, and thus black dross is formed. Black dross contains water-soluble compounds which may react with water or environmental humidity and release chlorides into the water and/or give off noxious gases (H2, CH4, NH3 and H2S). After the salt has been removed from the black dross it is considered to be non-hazardous, and can safely be landfilled. Black dross does also contain many relatively valuable compounds that may be worth recovering, i.e. alumina (Al2O3) and spinel (MgO-Al2O3).The aim of the present work is to experimentally investigate the salt removal from black dross by heat treatment under different conditions. The first and second part of the study covers heat treatment of secondary aluminium dross in an inductive furnace, and the third part in a rotary furnace to enhance stirring. In all cases the chemical composition of the black dross after heat treatment were investigated by SEM-EDS and XRD analyses.In the present study it was established that the salt contained in the black dross, i.e. 35wt%, starts to evaporate at approximately 1273 K in an inert atmosphere (argon), as well as in vacuum. The salt removal efficiency was found to increase in both cases as a function of temperature. When conducting the experiments in a vertical induction furnace in an atmosphere of argon, the highest salt removal efficiency was obtained after heat treatment of a 20 g sample for 10 hours at 1573K. A residual salt amount containing 0.33wt% chloride was obtained in this case. By sucking vacuum simultaneously as argon gas was introduced into the system lowered the chloride concentration in the residue to 0.2wt% when heat treated at 1473 K for 8 hours, and to 0.095wt% when heat treated at 1523 K for 10 hours. The improvement obtained by increasing the temperature by 50 K may, however, not be justified from an economical view point.When conducting the experiments in a rotary furnace in an atmosphere of argon it was, due to the enhanced stirring, possible to increase the sample size from 20 g to 750 g, and still reach results that were in good agreement with the results obtained in the induction furnace.It was also established in the present study that at temperatures above 1273 K the aluminium nitride present in the melt were partly oxidized to alumina, as well as to spinal. Based on the present results it is concluded that the optimum conditions for salt removal from black dross by heat treatment is in an atmosphere of argon in a rotary furnace.

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