GB709858A - Roasting zinc plant residues - Google Patents

Abstract

<PICT:0709858/III/1> The material remaining undissolved when roasted zinc concentrate is leached by conventional processes with liquor containing sulphuric acid ("zinc plant residue," mainly zinc ferrite) is treated with concentrated sulphuric acid and roasted in fluidized suspension, preferably at 660-700 DEG C., by passing air or other oxygen-containing gas upwardly through the bed whilst imparting heat thereto. The zinc plant residue is treated with preferably strong sulphuric acid before or after being dried, and crushed to a suitable size, or the acid-treated zinc plant residue may be pumped directly into the hot bed. The sulphuric acid may be replaced by oleum. The roasting process is preferably carried out in the absence of reducing conditions. A "nominal holding time" of 1-10 hours has been found suitable at the preferred temperature. Heat is imparted to the bed through the walls of the reactor or tubes passing through the bed, or by the introduction of solid or liquid fuel into the bed. In the latter case it may be desirable to pass oxygen or oxygen-enriched air through the bed. In an embodiment, see Fig. 3, oil is burnt in a combustion chamber 23 surrounding the bed and the hot gases used to preheat the gases entering the bed through conduit 8 and distribution pipes 6, in heat exchanger 9. Air containing hot combustion products from a separate combustion chamber may also be used. Zinc is recovered from the roasted product by leaching, e.g. with dilute sulphuric acid: it is present mainly as sulphate, with a little oxide and passes into solution whilst iron remains insoluble. The mixture may be thickened, filtered, the filtrate purified and zinc obtained therefrom by electrolysis.