STUDY OF THE FORMATION OF NATROALUNITE BY THERMOHYDROLYSIS

Prolonged boiling (72 h) of a solution of aluminium sulfate in the pre sence of either NaNO3 or Na2SO4 leads to the formation and precipitati on of natroalunite. The pH of the solution during this process was mon itored. The pH decreased as the temperature increased, was stable whil e the solution was boiling, and increased when the solution was cooled . The solid was formed while the solution was boiling and the increase in pH upon cooling was found to depend on the quantity of precipitate d solid. The yield, in terms of aluminium ions, was found to depend on the composition of the solution. It was zero if no sodium ions were p resent, a few percent in a Pyrex reactor, and increased substantially when sodium ions were present in solution. Lithium ions do not give ri se to solid phase, although other results obtained with lithium compar e well with the sodium results. The composition of the solution was in vestigated using Al-27 NMR. which indicated that the concentration of the complex [AlSO4] increases with temperature. These results and the crystal structure of the compounds suggest a mechanism in which the an ion [(AlOH)3(H2O)(6)(OH)3(SO4)(2)](-) is formed first and, if the alka li cation is large enough, the anion can dehydrate and precipitates.