Abstract
To efficiently dispose of chrome-polluted soil, 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) were tested. 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 CaCrO 4 . SiO 2 , Al 2 O 3 , and Fe 2 O 3 reduce CaCrO 4 , which inhibits Cr(III) oxidation. The Cr concentration in the cement products was well below the Chinese standard limits. The physical properties of clinker conformed to the Chinese standards. Therefore, the treatment of 1 wt% chrome-polluted soil with a cement rotary kiln is technically feasible and experimentally safe. • Chrome-polluted soil co-processed in cement kiln. • Most of the Cr was fixed in the solid products. • Most of the Cr(III) was oxidized to Cr(VI) after being calcined(45.15%–87.94%). • Water soluble Cr(VI) in the cement products was below Chinese standard limits. • The co-processing technology poses no environmental risk at lower mixing ratio.
Highlights
Chromium is widely used in steel, leather, and other industries, and the resulting chromium residue can cause serious pollution
Cement kilns have some advantages for the co-processing of solid waste, and can increase the capacity for solid waste disposal and reduce the consumption of raw materials and fuel by the cement industry(Aranda Usón et al 2013; Kosajan et al 2020)
The Si:Al atomic ratio in the raw materials was approximately 3:1, so Cr volatilization is reduced. These results indicate that only a small amount of Cr is volatilized into the flue gas and can be ignored in future experiments
Summary
Chromium is widely used in steel, leather, and other industries, and the resulting chromium residue can cause serious pollution. In China, the annual production of chromium exceeds 160,000 tonnes, the cumulative amount of chromium residue is nearly six million tonnes, and the amount of chrome-polluted soil exceeds 20 million tonnes(Wang et al 2011). Cement kilns have some advantages for the co-processing of solid waste, and can increase the capacity for solid waste disposal and reduce the consumption of raw materials and fuel by the cement industry(Aranda Usón et al 2013; Kosajan et al 2020). The migration and transformation of Cr and other heavy metals during co-processing can threaten environmental and human health. Most chromium residue is in the trivalent (Cr(III)) and the highly toxic hexavalent (Cr(VI)) forms.
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