It is known that addition of lime to the Bayer circuit promotes improvement of unit processes such as digestion, thickening, desilication, filtration, etc. The lime is also required for generation of tricalcium aluminate hexahydrate (TCA) which is used as a filter-bed in polishing filtration of pregnant liquor.
Lime prepared from different limestone deposits differs considerably in chemical composition. With the aim of determining the applicability of using magnesian limestone in the Bayer process, investigations were conducted into the impact of magnesium salts on TCA – production, structure and filtration properties, as well as the behaviour of the processed TCA in the digestion circuit.
To examine the influence of composition, materials with MgO content from 0.75 % to 44.0 % were used as initial substances. These raw materials included limestone, magnesium limestone, dolomite and a hydrated magnesium carbonate – magnesium hydroxide double salt, MgCO3∙Mg(OH)2∙3H2O. All compounds were processed in three stages:
1. Roasting at temperature, either 1150 or 1200ºС
2. Water slaking;
3. Reaction with caustic-aluminate liquor.
The products obtained were compared by chemical and phase analysis. The influence of magnesium content on the particle size distribution (PSD) of the corresponding TCA and specific resistance of the resultant filter-beds also were investigated. In the course of this research, the basic possibility of using TCA prepared from magnesium limestone as a pre-coat for polishing filtration was confirmed. Additionally, the optimal conditions for TCA production, such as roasting temperature and condensate dosage, were also defined.
Further to this work, the influence of magnesium salts in the digestion of Sredny Timan bauxite, was studied using a variety of additives (burnt limestone, burnt magnesium limestone and TCA batches prepared from both of these materials). Major beneficial impacts associated with the use of magnesium in high-temperature digestion were identified and are presented in this paper.