The treatment of smelter fume prior to segregation roasting

A method is described whereby smelter fume, containing lead and zinc as major components and copper as a valuable impurity, may be pretreated so that subsequently 80–90% of the lead can be segregated and separated as PbS on char, with 92–98% of the zinc and 80–85% of the copper remaining in the residue in a form that can be treated by processes such as the commercial Jarosite process for treating zinc residues.The pretreatment involves selective sulphate roasting of the fume, premixed with an iron-containing additive, substantially all lead compounds in the fume are converted to lead sulphate and all zinc and copper complexed as ferrites. Depending on the nature of the iron-containing additive, the sulphate-roasting atmosphere can be provided by feeding SO₂ and O₂ or air mixtures to the roaster, or it can be generated internally by the oxidation and/or decomposition of sulphur compounds in the fume—iron-additive mixture. In either case, 800°C is the optimum sulphate-roast temperature. The distribution of impurities in the fume is discussed.