Oxygen-rich and moderate/low temperature calcination treatment of chromium containing sludge for high efficient Cr recovery and toxicity reduction

The chromium (Cr) pollution of soil has recently gained a lot of attention for the demand of environmental protection. One of its main resources is from the landfill of the chromium containing sludge (CCS) that produces from reduction-precipitation treatment of the Cr(VI) wastewater. In this work we provide an improved alkaline oxidation calcination (AOC) treatment for CCS, which achieves high Cr recovery efficiency at relatively low calcination temperature and oxygen-rich conditions. The leaching risk of the final residue is much reduced compared to the conventional calcination. It reveals that by using air flow the oxidation efficiency is effectively improved to a value of near to similar to 90 % at 450 degrees C. The thermal analyses of the reaction suggest that Cr (III) in CCS transforms to chromite at the temperature range of 340-450 degrees C, which ensure the subsequent oxidation. On the other hand, it reveals that at higher temperature ( 500 degrees C) it will produce Ca-Fe oxides which are confirmed to be dissolved in the leachate of the toxicity test. The concentration of Cr (III) increases obviously, which could be associated with the release of Cr(III) that doped into Ca-Fe oxides as solid solution. The transformations of the CCS components during the AOC process are depicted in detail. The method proposed here enables high oxidation efficiency of CCS and meanwhile avoids the environmental risk of final residue.