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ItemSulphate-oxide equilibria for copper, iron, cobalt, and nickel( 1961)Applications of the process of differential (or selective) sulphation have been extensively reported over the last ten years, coinciding with the adaptation of the fluidised bed reactor (or fluo-solids roaster) to metallurgical practice. With this technique a suitable combination of temperature and sulphur trioxide partial pressure is employed, so that metals with sulphate decomposition pressures higher than the chosen value (for the particular temperature), are converted to oxides, while the more stable sulphates are unaffected. The idea of such separations was first outlined by Marchal, and Wohler and Grunsweig (cited below). This method is particularly useful in the extraction of non-ferrous metals, where iron, the most common impurity, may be removed. In such processes reliance is placed on the instability of iron sulphates, which are converted to ferric oxide, while, for example, copper, nickel, cobalt and manganese, remain as sulphates. Hence, if roasting is followed by a water leach, the sulphates are dissolved, while the oxides remain in the residue. Table 1.1 lists a few of the many commercial separations achieved by suitable control with this process. As the table shows, a wide range of ores and concentrates may be treated by this technique which considerably shortens the steps necessary to achieve satisfactory separations and increases the efficiency of extraction of the valuable metals. A further advantage is that metal sulphides or oxides may be treated in this way. In the case of the former, air is admitted to give a gas of the required composition, while with the latter, air together with sulphur dioxide from an external source, such as a pyrites roaster is used.