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

The Bayer process for the production of alumina consists of three main stag es: bauxite digestion with sodium hydroxide solution at high temperatures a nd pressures, precipitation of crystalline gibbsite from the aluminate liqu or under atmospheric conditions, and finally, calcination of the precipitat ed gibbsite. During the last decade, a new variation of the precipitation s tage is under development in the Laboratory of Metallurgy, NTUA, whereby bo chmite (monohydrate alumina, Al2O3 . H2O) is precipitated instead of gibbsi te under atmospheric conditions. Accurate knowledge of the solubility of boehmite in concentrated sodium hyd roxide solutions under mild temperature conditions is necessary in order to study the boehmite precipitation conditions. So far, the solubility of boe hmite in sodium hydroxide solutions has been studied experimentally in dilu ted sodium hydroxide solutions and at temperatures higher than 100 degreesC . These conditions are completely different from those applied in the new b oehmite precipitation process. Therefore, the aim of this work is on the on e hand, the development of a theoretical model predicting the solubility of boehmite in 2-4.5 M sodium hydroxide solutions and in the temperature regi on of 30-150 degreesC, and on the other hand, the experimental determinatio n of boehmite solubility in order to assess the model validity. The speciation diagram of aluminium ions in sodium hydroxide solution has b een developed taking into consideration all mononuclear and polynuclear hyd roxoaluminate ions. The mathematical model comprises 21 complex equations a nd its development was based on the hypothesis that the only existing alumi nium-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 f unction of the pH of the solution. Although this observation significantly simplified the mathematical model, it was not possible to obtain an analyti cal solution and for this reason the mathematical model was solved using "M athcad" 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 hydroxid e and the temperature. A very good agreement exists between the theoretical values of solubility o f boehmite and the experimental ones. The observed deviation between the th eoretical and the experimental values varies generally within the margins o f repeatability of experiments +/- 2 g/l. As a conclusion, the theoretical model can predict with accuracy +/- 2 g/l the solubility of boehmite in con centrated 2-4.5 M sodium hydroxide solutions and in the temperature region 30-150 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.