Mercury emission characteristics and mechanism in the raw mill system of cement clinker production

Mercury pollution has attracted worldwide attention due to its toxicity, bioaccumulation and persistence. Cement clinker production is the top emitter of atmospheric mercury in China and the emissions from raw mill systems account for about 85% of all emissions. However, the mercury emission characteristics and mechanisms as a function of time during an operation cycle are still unclear. This study aims to reveal the mercury emission characteristics and mechanisms in cement plants by comprehensively using offline and online field measurements, control experiments and heat transfer analysis. Research results indicated that an intermediate temperature (300-500 °C) desorption and the heterogeneous oxidation of mercury in the precalciner, the selective adsorption of oxidized gaseous mercury (Hg²⁺) to raw meal, and Hg²⁺ re-vaporization in the conditioning tower jointly caused an increase in the Hg²⁺ ratio (15.3%-83.6%) during the mill-off mode. In addition, mercury concentrations remained at approximately 6.5 μg/Nm³ during the mill-on mode while the values reached a peak of 1835.4 μg/Nm³ during the mill-off mode. Thus, atmospheric mercury emissions during the mill-off mode accounted for 35.0%- 71.7% of the emissions during the entire cycle, although the mill-off period only lasted for 5%- 17% of the whole cycle. Our results therefore suggest that supervisory monitoring of mercury in cement clinker production should specify the operating status of raw mills. Mercury control technologies targeting a relatively short period for the mill-off mode can substantially reduce mercury emissions from cement clinker production, and thus, the related impacts on ecosystems and human health.