| 摘要 |
Raw materials containing besides aluminium oxide mainly silica and/or titanium oxide, for instance bauxite, clays or laterites, are melted with lead oxide, and the melt slowly cooled, whereafter the crystals of alumina are separated from the mass as by gravity concentration or flotation. The lead oxide must be present in sufficient amount to form the compound PbO.SiO2 with any silica present and the compound PbO.TiO2 with titanium, and advantageously enough may be used to form the compounds 2PbO.SiO2 and 2PbO.TiO2. The aluminium oxide content of the melt should be less than 35 per cent, and may advantageously be reduced to 10--20 per cent as by adding further lead oxide or lead silicate to the charge. The melting point and viscosity of the charge may be lowered by adding sulphides such as PbS or Cu2S or lead chloride, and the alumina may be segregated into the upper portion of the melt by initial slow cooling to separate the alumina from the melt followed by rapid heating. The melt may also be centrifuged to assist separation of the grains of alumina. By treating the cooled and crushed melt with acid, a lead salt is obtained from which lead oxide for re-use may be recovered, or the lead oxide may be regenerated by first forming metallic lead and subsequently oxidizing it. By treating the cooled and crushed melt with acetic acid, a mixture of aluminium oxide, silica, and lead acetate is obtained, and by melting the lead acetate, alumina sinks to the bottom and silica rises to the top of the melt, whereby separation may be effected. The separation of titanium oxide and silica obtained in admixture by acid treatment of the cooled mass may be similarly effected. If the initial material, for instance red bauxite, contains iron, this should be removed, as by reduction to FeO followed by leaching with acid, by reduction to Fe3O4 followed by magnetic separation, or by chlorination with chlorine. Titanium may also be removed if desired, by treatment with chlorine. |
