Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials of the Russian Academy of Sciences Kola Science Center, Apatity, Murmansk Region, Russia:
V. A. Matveev, Head of Laboratory, e-mail: matveev@chemy.kolasc.net.ru
D. V. Mayorov, Senior Research Associate, e-mail: mayorov@chemy.kolasc.net.ru

This paper describes the results of a study that looked at the processing of ammonia alum extracted from nepheline to produce high-purity aluminium oxides. It is shown that the alkaline impurities in the inter-crystalline regions, which are a part of the aluminium hydroxide formed during the ammonation of ammonia alum, can be successfully removed when the hydroxide is transformed into a compound of different class, e.g. aluminium and ammonium bicarbonate NH₄AlCO₃(OH)₂. Moreover, such compound has a great potential to become a source of finely dispersed aluminium oxides. Initial ammonia alum was extracted from the sulfuric acid solutions of nepheline concentrate by crystallization at 15 ^оC, followed by recrystallization aimed at reducing the impurities. The ammonia alum with the following composition (on a dry matter basis), weight %: 0.05–0.06 K₂O and 0.06–0.07 Na₂O, was hydrolyzed with gaseous ammonia. The ammonation product, which is a mixture of aluminium hydroxide and ammonium sulfate, was leached with water. The resultant suspension was filtered, and the aluminium hydroxide precipitate – thoroughly washed. The aluminium hydroxide was then autoclaved with ammonium carbonate solutions at various concentrations, processing times and temperatures. The reaction product was subjected to IR spectroscopic, differential thermal and X-ray phase analysis, which showed that NH₄AlCO₃(OH)₂ was formed in all cases. А γ-form of aluminium oxide with less than 0.02 wt.% of K₂O and Na₂O was obtained through heat treatment of the NH₄AlCO₃(OH)₂ samples at 550 ^оC. The specific surface area and porosity of the γ-Al₂O₃ samples were estimated. They were 374–406 m²/g and 0.72–0.81 cm³/g, respectively. Based on their characteristics, such γ-Al₂O₃ samples can be used for the production of catalysts, adsorbents and desiccants. An X-ray phase analysis of the products resultant from the heat treatment of γ-Al₂O₃ at 1100–1250 ^оC for 1 hour showed that even at 1150 ^оC it almost completely transforms into corundum, which indicates a high sintering activity of the resulting alumina and predetermines the possibility of using it in alumina ceramics production. Using the data from the conducted research, the authors demonstrate how to obtain high-purity aluminum oxides through processing of ammonia alum extracted from nepheline. A process flow diagram is also included.

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