Dynamic Behavior of Calcium and Mercury in Carbon Reduction of Gypsum

Abstract

In a series of studies on regeneration and cleaning of gypsum produced from flue gas desulfurization, change in chemical form of calcium and release behavior of mercury during reduction of gypsum with solid carbon were investigated. Calcium sulfate dihydrate as a model compound for gypsum was mixed with solid carbon and heated to a terminal temperature of 1100 °C under a nitrogen stream. CaS was produced by the reduction of CaSO₄ with carbon. CaO formed drastically above 900 °C via a reaction of CaSO₄ and CaS. An optimum amount of solid carbon to CaSO₄ was determined for the production of CaO. To remove mercury from FGD gypsum during the carbon reduction, release behavior of mercury from gypsum was investigated. Model compounds were prepared by addition of reagents of HgCl₂, HgO and HgSO₄ to calcium sulfate dihydrates. HgCl₂ in gypsum was released as Hg via HgO during heating in a nitrogen stream. HgO and HgSO₄ changed to elemental Hg and volatilized. Carbon addition accelerated the release of every mercury compounds to gas phase as elemental Hg from the matrix of gypsum.