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Synthesis of magnesium aluminate spinel (MgAl2O4) was investigated by employing ball milling and co-precipitation methods. The starting materials (aluminum hydroxides) were obtained from the purified sodium hydroxide leaching solution of black dross. The characteristics of the synthesized spinel was analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) images. In this work, the effect of calcination temperature and time on the formation of spinel by the two methods was compared. Calcination temperature showed a great effect on the formation of spinel in both methods. The results showed that the co-precipitation method has many advantages over the ball milling method. In ball milling method, complete conversion of the starting materials to spinel was impossible even at 1500 °C, while complete conversion to spinel was accomplished at 1000 °C for 5 h by the co-precipitation method. The average size of the spinel synthesized at these optimum conditions of the co-precipitation method was about 17 nm. A process can be developed to synthesize spinel from the black dross which is regarded as hazardous materials.

Aluminum dross results from the re-melting of used aluminum cans, and is a valuable resource containing alumina. In general, aluminum dross contains oxides of Al, Ca, Fe, Mg, Si and Ti and some salts. As a continuation of the previous work to recover pure alumina from the dross, the effect of employing ultrasound wave in the NaOH solution was investigated to improve the leaching percentage of alumina from the mechanically activated dross. The use of ultrasound improved the leaching percentage of alumina from 35% to 60% and resulted in a leaching solution where the purity of aluminate ion was 99%. Among the optimum conditions (5 M NaOH, 50 o C, 4 h, pulp density of 100 g/L and ultrasonic power 100 W), leaching time had a remarkable effect on the dissolution of alumina in the presence of ultrasound. The combined effect of ultrasound and the addition of CaCl 2 and organic acids to the NaOH solution was also investigated. Roasting of the black dross with CaCl 2 followed by ultrasound leaching did not improve the leaching percentage of alumina. The presence of organic acids in the NaOH solution increased the leaching percentage of alumina as well as silica, and thus was not favorable for recovering pure aluminate solution from the black dross.

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