Alumina recovery from material contg. aluminium and impurities of silicon, etc. - by selective removal of impurities by fluorination and chlorination to gaseous cpds., saving energy and avoiding waste disposal problems

摘要

Al2O3 recovery from natural or synthetic raw materials (IA) or residues (IB) contg. Al and contaminated with Si, Fe and/or Ti comprises selective removal of the impurities by fluorination and chlorination to gaseous cpds., leaving solid pure Al2O3. The pref. process involves: (a) mixing (I) with a solid fluorinating agent (II), pref. metal fluoride, esp. AlF3 and pref. in stoichiometric excess w.r.t. Si; (b) heating the mixt. to max. 1250 deg.C, so that the water present is expelled and Si is converted to SiF4 gas; (c) reducing the reaction temp. to under 800 or under 850 deg.C; and (d) adding a gaseous chlorinating agent (III), so that impurities forming gaseous chlorides are transferred to the gas phase, leaving pure Al2O3 as residue. ADVANTAGE - The process is more economical than usual, since the specific energy consumption in Al2O3 recovery from e.g. bauxite is lower than in the extn. (Bayer) process, whilst the impurities are converted to useful prods., rather than residues, esp. red mud, which cause difficulties in disposal or use.$ The process can be operated economically with bauxites contg. over 4-5 (wt.)% SiO2. The amt. of type of oxide-hydroxides in the bauxite has little effect on the process, whereas extn. with aq. NaOH soln. is more difficult with samples contg. boehmite (gamma-AlOOH) or diaspore (alpha-AlOOH) than those contg. mainly hydrargillite (Al(OH)3).