Experiments were performed to demonstrate the feasibility of vacuum roasting of copper concentrates to eliminate the impurities, As, Sb, Bi, Pb and Zn prior to their entry into the copper smelting circuit. One hundred gram samples of three copper concentrates of different chemical, mineralogicalcomposition and impurity content were roasted at temperatures between 400°C and 900°C under a vacuum of 80 Pa for a period of 30 to 90 minutes in a stainless steel cylindrical retort that was air-cooled on one end. It was found that almost complete removal of As, Sb, Pb and up to 60 % Zn and 90 % Bi elimination from the concentrates was achieved after 60 minutes of vacuum roasting at 900°C. Less elimination was observed at lower temperatures and shorter times and the rate and ultimate level of elimination was seen to vary with the mineralogical composition but in no obvious pattern. Scanning electron microscopy performed on samples of the copper concentrates prior to roasting revealed distinct sulfide particles of Pb and Zn but not of As, Sb nor Bi. X-ray diffraction analyses also failed to detect the presence of sulfides of either of As, Sb or Bi. Examination of the retort interior in the vicinity of the air-cooled region at the end of roasting trials showed segregated condensation of the volatized metallic components and the sulfur; this observation suggests that the separate, stage-wise recovery or separation of these metals may be possible.
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