The migration and transformation of chromium during co-processing of cement raw meal mixed with chrome-polluted soil

Abstract To efficiently dispose of chrome-polluted soil, we tested the co-processing of raw meal mixed with chrome-polluted soil in a tube furnace (laboratory experiments) and a cement rotary kiln (field-scale experiments). The migration and transformation reactions of chromium were analyzed and the environmental risk was evaluated. The average mass balance value was 91% for the laboratory experiments. In field-scale experiments, the mass balance values were 110% for the control experiments and 84% when 1% soil was treated. Therefore, only a small amount of Cr was volatilized into the flue gas. The average total Cr concentration in the soil samples was 403.25 mg/kg, and the ratio of Cr(VI) to total Cr was 1.83% or less. On average, 45.15% of Cr(III) was oxidized to Cr(VI) in laboratory experiments, while 87.94% of Cr(III) was oxidized in field-scale experiments, and the difference could be a result of the different calcination conditions. The materials in the cement rotary kiln make full contact with oxygen, and in this high temperature and oxidizing atmosphere, abundant CaO and MgO promote the oxidation of Cr(III) to CaCrO4. SiO2, Al2O3, and Fe2O3 reduce CaCrO4, which inhibits Cr(III) oxidation. The Cr concentration in the cement products was well below the Chinese standard limits. Therefore, the treatment of 1% chrome-polluted soil with a cement rotary kiln is experimentally safe.