The solubility of metal oxides and hydroxides at high temperatures. Results and implications of recent ORNL measurements

The results of high temperature solubility studies at ORNL are presented in which mainly direct pH measurements were made of aqueous solutions in contact with the crystalline solid phases: Al(OH)3, AlOOH, Fe3O4, Mg(OH)2, Nd(OH)3, and ZnO. Examples are highlighted of specific phenomena such as: the kinetics of gibbsite and boehmite dissolution and precipitation; the appearance of metastable equilibria in the dissolution of Fe3O4; the extremely rapid precipitation of crystalline brucite, Mg(OH)2; and anomalies in the apparent solubility profiles of AlO(OH) and ZnO. General trends associated with the effects of temperature and ionic strength are mentioned. Some of the potentiometric investigations were augmented by conventional batch {AlO(OH) and ZnO}, and flow-through column {ZnO} experiments. In the additional case of ZnCr2O4, the extremely low solubility of this spinel permitted application of only the latter technique and these results are discussed in terms of the measured chromium levels that resulted from incongruent dissolution