Institute of Solid State Chemistry of Ural Department of Russian Academy of Sciences, Ekaterinburg, Russia:

A. V. Vaylert, Post-Graduate Student, e-mail: vailert@mail.ru
I. N. Pyagay, Senior Researcher
V. L. Kozhevnikov, Director
L. A. Pasechnik, Senior Researcher
S. P. Yatsenko, Head of Laboratory

This article shows the results of high temperature autoclave and sintering leaching of alumina and alkali from alumina production red mud by concentrated solutions of sodium hydroxide in the presence of different amounts of lime additions. The studies were carried out in the temperature range of 210–280 ^oС with exposure time from 1 to 4 h. Sintered or autoclave-treated sludge was leached with 10% alkali solution or with water at the temperature of ~100 ^oС during 2 h (including heating and cooling of pulp). The pulp was cooled to room temperature with following separation by centrifuging or vacuum filtration for obtaining of alkalinealuminate solution. The maximum extraction of alumina from red mud into alkalinealuminate solution (>70%) is achieved in the following conditions:
— introduction of additional amount of lime, ensuring the ratio of CaO/SiO₂ = 0.3;
— autoclave treatment temperature, equal to 250 ^oС;
— autoclave storaging during 1 h.
When the temperature is raised to 280 ^oС, extraction of alumina is increased additionally by 7%. Sintering of the sludge at the CaO/SiO₂ ratio, equal to 1.8 and 2.1 during 3 h, made possible to extract 54.1 and 53.1% of alumina into the solution, respectively. The chemical processes of decomposition and transformation of aluminum-containing phases of sludge are described as functions of the charge composition, temperature and treatment time. With large additions of lime, formation of calcium-containing compounds in sludge residue defines fixation and exclusion of aluminum from the reaction medium. At the same time, small additions of lime lead to liberation of aluminate and sodium ions due to decomposition of zeolites. There were selected the conditions of maximal recovery of alumina to solution and reduction of residual sodium content in sludge to less than 1%.

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