Recovery of calcium carbonate and hydrogen sulfide from waste calcium sulfide

The removal of H2S from hot coal gas using a limestone-based sorbent generates a large quantity of calcium sulfide waste (e.g., 67 tons/day from the gasification of 1000 tons/day of 3% S coal) that cannot be landfilled because of potential H2S evolution and sulfide leaching. In the process proposed here, the CaS is dissolved by reaction with H2S complexed with aqueous methyldiethanolamine (MDEA) or other alkanolamines. In the second step, the highly soluble Ca(HS)(2) reacts readily with CO2, also complexed with aqueous MDEA, to precipitate pure CaCO3 of uniform crystal size and to form MDEA-complexed H2S in solution. The solution from step 2 is split-half is recycled to step 1 and half is sent to a stripper to recover H2S and from there to a column where the needed CO2 is absorbed from a combustion gas.