Physicochemical studies on dusts. VII. The effect of acids on the solution properties of silicious dusts

The rate of solution of a pure rock‐crystal dust (e.g. Madagascar quartz) in a borate buffer (pH 7–5) is reduced by prior extraction with acid, whereas similar treatment of impure silicious dusts (e.g. Lochaline sand, a sandstone, or orthoclase felspar) increases their solubility rates. The enhanced rate is not due to regeneration of a disturbed amorphous surface‐layer, of the type produced by crushing and grinding silica, and it can be reduced, sometimes back to its original value, by heat treatment within the temperature range of about 60–1150°. Silica gel, on the other hand, does not show a reduction in solubility rate below about 700°; hence, the observed enhanced rates cannot be attributed to hydration of the silica surfaces during acid extraction. Infra‐red spectrometry leads to the same conclusion. During reduction of the enhanced solubility rates by heat treatment and by other methods, the silica surfaces become inactivated, and further extraction with acid does not raise the solubility rate again. Possible reasons are advanced. Alumina is removed from two impure silicious dusts during acid extraction. Pretreatment of such dusts with sulphuric acid of various pH values causes a slight increase in all the initial pH values, indicating partial removal of hydrogen ions from the extracting medium. It is considered that these ions are adsorbed on the surface irregularities produced by the prior removal of impurities by acid extraction, and the various solubility effects observed are explained on the basis of this layer of adsorbed hydrogen ions. Reduction of solubility by heat treatment shows the characteristics of a second‐order reaction, with a rate constant independent of temperature. It is suggested that the reaction consists of desorption of hydrogen ions; a tentative mechanism is discussed.