Patent GB-811859-A (GB)
Title: Improvements in the production of gases containing sulphur dioxide by stepwise roasting of iron sulphidic ores containing arsenic and/or antimony in fluidised layers with the recovery of roasting residues practically free from arsenic and antimony
IPC: C22B1/10
Filed: 1956-02-15
Abstract: In the production of gases containing sulphur dioxide by stepwise roasting of iron sulphidic ores containing arsenic and/or antimony, with free oxygen-containing gases in fluidized layers, while recovering roasting residues practically free from arsenic and antimony, the oxygen for roasting is supplied as a fresh gas separately and independently to the individual fluidized layers for preliminary and after-roasting, the oxygen is supplied to the first fluidized layer in such amount that a preliminarily roasted product containing at least 3 per cent sulphidic sulphur is obtained, the preliminarily roasted product is completely roasted in one or more subsequent fluidized layers and the roaster gas formed in the first fluidized layer is withdrawn separately from the roaster gas formed in the one or more subsequent fluidized layers.  Sufficient oxygen, e.g. as air, may be passed to the preliminary roasting to obtain a gas containing 16-20.5 per cent sulphur dioxide and a roasted product consisting of ferrous sulphide and ferrosoferric oxide of which the sulphidic S content is preferably at least 15 per cent.  The preliminarily roasted product should preferably contain a small proportion of Fe3O4, which is inversely proportional to the height of the first fluidized layer, and is after roasted with excess oxygen, when a roaster gas containing 8-13 per cent SO2 is obtained. Alternatively, pyrites and air are supplied to the preliminary roasting layer in such proportions as to obtain a roaster gas containing 20 per cent SO2 and a product consisting, besides gangue, only of ferrous sulphide and iron disulphide of which the sulphidic sulphur content is 30 per cent or more.  For materials containing arsenic the individual layers should be maintained at 600-850 DEG C., preferably 750-800 DEG  C., and for materials containing antimony, or arsenic and antimony, the individual layers should be maintained at 550-880 DEG  C., preferably 750-800 DEG  C. in the first layer, when lead present is volatilized, and 750-880 DEG  C. in sub-sequent layers.  Members to remove excess heat from the individual beds may be provided, and cooling may be controlled by varying the air supply to increase, or decrease, the depth of immersion of cooling surfaces.  The gas leaving the first layer may be cooled to 450-500 DEG  C. in the tubular vaporizer of a steam boiler, and passed to an electrical gas purification plant, whence the dust is returned to the next fluidized layer together with the preliminary roasted product. Dust may also be removed before the electrical purification in a cyclone, operated hot, and may be wholly, or partly, continuously, or discontinuously, returned to the first layer.  The gases from the layers may be purified and worked up separately, or united after purification, and worked up together. Addition of oxygen is necessary to work up to SO3 in a contact reaction.  The roasting may be carried out in separate fluidized layer furnaces, or in a single furnace subdivided into layer chambers.  The preliminarily roasted product may be withdrawn and stored, before after-roasting, under conditions such that oxidation is prevented, e.g. by cooling to below 250 DEG  C., preferably below 100 DEG  C., or, without cooling, in an atmosphere free from, or poor in, O2, such as N2, or roaster gas.  By this intermediate storage it is possible to carry out the preliminary and after-roasting consecutively in a single layer furnace. Specification 718,769 is referred to.