Types of Roasting

    The most common type of roast, the oxidizing roast is employed to convert sulphides to oxides and the typical reaction is 2MS + 3O2 = 2MO + 2SO2. The sulphating roast is used to convert sulphides to sulphates, usually used to process the calcine by leaching (dissolving) since many sulphates are water soluble. The reaction is MS + 2O2 = MSO4.
    Why do some conditions produce an oxidizing roast and other sulphating roast? It is important to compare next reactions,
    2MS + 3O2 = 2MO + 2SO2 ; O2/M = 1.5
    MS + 2O2 = MSO4 ; O2/M = 2
    The sulphating roast needs more oxygen, and controlling the oxygen content of roaster atmosphere will make the operator to generate oxide or sulphate as desired. Another controlling factor is temperature. It is important to mention that each sulphate has a temperature at which is decomposed. If the roaster is operated above this temperature, then sulphates will not be formed. Below this temperature, sulphates will be formed and not oxides. The presence of high concentrations of SO3 in the roaster promotes the formation of sulphates. This is because the overall sulphating reaction is made up of two part reactions,
     
    Oxidation…………………… 2MS + 3O2 = 2MO + 2SO2
    Oxidation of SO2…………………….. 2SO2 + O2 = 2SO3
    Sulphation…………………...….. 2MO + 2SO3 = 2MSO4
    Overall reaction……………………….MS + 2O2 = MSO4
     
    Basically oxygen and high sulphur dioxide concentrations help the reactions go forward. In other to produce metal chlorides for leaching or other process, the metal sulphide can be roasted with chloride salt like sodium chloride. For example, copper chloride is volatile and can be removed from iron concentrates by a chloridizing roast. The chlorination roast uses chlorine to get the same result as the chloridizing roast. Reducing gases can also be employed in roasting; the most common gases employed are CO and H2. The oxides of arsenic and antimony are volatile and can be eliminated from concentrates by an oxidizing roast. The impurities may be in the form of metal arsenides or antimonides.
    4MAs + 5O2 = 4MO + 2As2O3
    4MSb + 5O2 = 4MO + 2Sb2O3