Recovery of metals from power plant fly ash by two‐stage preferential chlorination

Abstract

Preferential chlorination or pre-chlorination of iron oxide in fly ash was investigated as one means for reducing the iron in the final product. The chlorination behavior of pure Fe 2 O 3 , pure Al 2 O 3 , pure SiO 2 , a mixture corresponding in composition to that of non-magnetic fly ash, and oxides mixtures with various combinations of Fe 2 O 3 , Al 2 O 3 and SiO 2 were also studied to give a better understanding of the selective chlorination of the Fe 2 O 3 and of the effects of oxides upon the chlorination of other oxides.The results reveal that chlorine attacks preferentially upon iron oxide. In the presence of carbon at a moderate temperature, the addition of A1 2 O 3 and/or SiO 2 to Fe 2 O 3 generally decreases the chlorination extent of Fe 2 O 3 ; however, the addition of SiO 2 to Al 2 O 3 or to Fe 2 O 3 -Al 2 O 3 mixture increases the chlorination extent of Al 2 O 3 . The chlorination extents of Fe 2 O 3 and of Al 2 O 3 in the ash-carbon mixture and in the ignited ash are generally lower than those in the corresponding Fe 2 O 3 -Al 2 O 3 mixture. Comparison of the extents of chlorination for fly ash at low to moderate temperatures in the presence of carbon to those at high temperatures without adding carbon has been made. The results indicate that the chlorination of ash to produce AlCl 3 with low iron contamination without losing significant amounts of aluminum can be best accomplished in a two-stage process consisting of a selective chlorination of Fe 2 O 3 at moderate temperature (say, 700°C) and a high temperature (say, 850°C) chlorination of Al 2 O 3 , both with addition of carbon.