Mineralogical changes occurring during the fluid-bed roasting of zinc sulfide concentrates

During the fluid-bed roasting of zinc sulfide concentrates, the sulfur in the sphalerite (Zn,Fe)S diffuses out of the particles, whereas the associated zinc and iron are converted to (Zn,Fe)O. The iron from the (Zn,Fe)O phase migrates outward to the peripheries of the particles, forming ZnFe2O4. The resulting porous ZnO+ZnFe2O4 particles agglomerate to form large, spherical masses. A compact shell subsequently develops on the surface of the agglomerates and continues to grow inward; eventually, the particles become compact. Rhythmic bands consisting of intergrowths of ZnO, ZnFe2O4, Zn2SiO4, and lead oxide/oxysulfate are often present in the agglomerated masses, and the Zn2SiO4 and lead oxide/oxysulfate phases seem to form by a vapor phase reaction. Occasionally, defluidization occurs during roasting. The defluidization agglomerates consist of calcine particles that are cemented by zinc oxysulfate and zinc sulfate.