Solubility of boehmite in concentrated sodium hydroxide solutions: model development and assessment

The Bayer process for the production of alumina consists of three main stages: bauxite digestion with sodium hydroxide solution at high temperatures and pressures, precipitation of crystalline gibbsite from the aluminate liquor under atmospheric conditions, and finally, calcination of the precipitated gibbsite. During the last decade, a new variation of the precipitation stage is under development in the Laboratory of Metallurgy, NTUA, whereby bochmite (monohydrate alumina, Al2O3 . H2O) is precipitated instead of gibbsite under atmospheric conditions. Accurate knowledge of the solubility of boehmite in concentrated sodium hydroxide solutions under mild temperature conditions is necessary in order to study the boehmite precipitation conditions. So far, the solubility of boehmite in sodium hydroxide solutions has been studied experimentally in diluted sodium hydroxide solutions and at temperatures higher than 100 degreesC. These conditions are completely different from those applied in the new boehmite precipitation process. Therefore, the aim of this work is on the one hand, the development of a theoretical model predicting the solubility of boehmite in 2-4.5 M sodium hydroxide solutions and in the temperature region of 30-150 degreesC, and on the other hand, the experimental determination of boehmite solubility in order to assess the model validity. The speciation diagram of aluminium ions in sodium hydroxide solution has been developed taking into consideration all mononuclear and polynuclear hydroxoaluminate ions. The mathematical model comprises 21 complex equations and its development was based on the hypothesis that the only existing aluminium-bearing species in sodium hydroxide solutions at pH higher than 10 is the tetrahydroxoaluminate ion. This was deduced from the speciation diagram of aluminium ions in sodium hydroxide solutions which shows the stability regions of all the mononuclear and polynuclear hydroxoaluminate ions as a function of the pH of the solution. Although this observation significantly simplified the mathematical model, it was not possible to obtain an analytical solution and for this reason the mathematical model was solved using "Mathcad" software. Regression analysis performed on the data obtained from the solution of the mathematical model resulted in an easy-to-use regression equation relating the solubility of boehmite to the initial concentration of sodium hydroxide and the temperature. A very good agreement exists between the theoretical values of solubility of boehmite and the experimental ones. The observed deviation between the theoretical and the experimental values varies generally within the margins of repeatability of experiments +/- 2 g/l. As a conclusion, the theoretical model can predict with accuracy +/- 2 g/l the solubility of boehmite in concentrated 2-4.5 M sodium hydroxide solutions and in the temperature region 30-150 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.