RECOVERY OF LIME, SULFUR, AND IRON FROM GYPSUM AND PYRITE WASTES

Currently, gypsum and pyrite, two major wastes generated by the coal c ombustion industry, have to be land-filled. The feasibility of a new a pproach of recovery of lime, iron, and sulfur products from these wast es is discussed in the present work. By thermal decomposition of pyrit e, 42% of the sulfur is recovered and pyrrhotite is produced. A thermo dynamic analysis shows that the recovery of iron and lime from pyrrhot ite and gypsum in a single-stage process is not feasible. Instead, the process has to consist of two subsequent stages. The first is the red uction of pyrrhotite with carbon in the presence of lime to produce ir on, calcium sulfide, and carbon monoxide, followed by reaction of calc ium sulfide with gypsum to produce Lime and a concentrated sulfur diox ide stream. The kinetics of the reactions involved in the two-stage pr ocess is studied using thermogravimetric analysis. The reaction rates are favorable above 950 degrees C and depend on initial reagent concen trations. The effectiveness of magnetic separation of the Fe/CaS mixtu re is studied. The amount of sulfur in the magnetic fraction depends o n the reaction temperature.