Partitioning effect of mercury content and speciation in gypsum slurry as a function of time

For gypsum slurry samples taken from wet flue gas desulfurization (WFGD) systems, a mercury partitioning effect was observed between the solid and liquid components. This partitioning effect, which appears to be pseudo-second-order, is a function of time which results in an increase in mercury content for the solid portion of the gypsum slurry as time increases. Moreover, the mercury content for the solid portion of the slurry reaches a plateau after 24 h. Thus, the sampling method for the gypsum slurry is an important factor in determining an accurate mercury mass balance in the WFGD systems. Temperature-controlled decomposition techniques were employed to acquire an understanding of the mercury species in WFGD gypsum. It was determined that the primary mercury species in WFGD gypsum was mercuric sulfide (HgS). Moreover, the content of HgS as well as mercuric oxide (HgO) in the solid portion of the slurry was susceptible to the partitioning effect and steadily increased with separation time. Other mercury compounds present were mercury sulfate and mercury chloride; however, these mercury species exhibited less change with separation time. Additionally, a comparison of the mercury present in WFGD gypsum from five different coal-fired power plants using thermal decomposition techniques showed that the distribution of mercury species was not the same; however, HgS was primary and mercury chloride compounds or HgO secondary. HgO varied between 15 and 25 % of the total mercury in gypsum. No clear correlation between mercury speciation and sulfur and chlorine content in the coal was determined. Thermogravimetric analysis was employed to compare the effects of settling time with gypsum quality. Data show that longer settling times results in lower quality gypsum.