Effects of Impurities on Hydrothermal Reactions of Alumina

Abstract

Corundum (Al₂O₃) was obtained from boehmite (AlOOH) under hydrothermal conditions at temperatures above 400°C. The grain size of the crystals with definite habit was 10 to 100 microns.
In the present study impurity effects on the crystallization process of corundum in the above reaction have been investigated. It was found that the reaction rate, crystal size and habit, and incorporation of added ions were strongly influenced by temperature change, pH change, and addition of impurities into solution.
These observed data were explained as the results of impurity effects on: (1) solvent structure, (2) solution structure, (3) solubility, (4) diffusion process, (5) nucleation process, and (6) growth process.
In some cases a metastable phase was formed under the influence of certain impurity. Fluorine ion in the solution inhibited the corundum nucleation and caused the formation of tohdite in the corundum stable region. This effect was explained as a change of solution structure, that is, structure change of hydrated poly aluminate ion in the solution by the existence of fluorine ion. It must be noted that the solution structure is an important factor for the crystallization process.
Habit modification of corundum by adding iron group ions as sulfate, nitrate, chloride, hydroxide or oxide was determined. Valency and coordination number of these ions incorporated into the corundum lattice were determined from their visible reflection spectra. Copper ion is not solid soluble in corundum if sulfate ion coexists, while it is solid soluble substitutionally as Cu³⁺ in 6 coordinated site if it is added as nitrate, oxide or hydroxide. Judging from the formation mechanism of “Star” corundum, Copper ion seems not to be incorporated into the lattice through the basal plane (0001).