Dissolution of Sparingly Soluble Inorganic Compound in Aqueous Suspension Containing Ion Exchange Resin (Part 9)

Abstract

Dissolution process of CaMoO4 in aqueous suspension containing H-R, a strongly acidic ion exchange resin with hydrogen form, was studied mainly by chemical analysis of the liquid phase. Several analytical methods such as titration using NaOH solution or H-R suspension, calorimetry, thermal analysis and XRD were carried out in order to confirm the dissolution mechanism. CaMoO4 does not dissolve according to the equation, CaMoO4+2(H-R)→Ca-R2+H2MoO4 owing to the aggregation to polyanion by the acidification of MoO42-. By considering the experimental results, it is concluded that CaMoO4 dissolves to yield an acid having a composition to be H4Mo8O26. The dissolution process of CaMoO4 and alkali titration for the resultant acid are summarized as follows: (1) The dissolution process of CaMoO4 as a function of H-R (g) is expressed by the following two-stages reactions, 8CaMoO4+12(H-R)→Ca2Mo8O26(aq)+6(Ca-R2)+6H2O Ca2Mo8O26(aq)+4(H-R)→H4Mo8O26(aq)+2(Ca-R2). With increasing the amount of H-R, the concentrations of Ca and Mo increase at the initial stage which is followed by the decrease in Ca2+ concentration at the final stage. The formation of the intermediate, Ca2Mo8O26(aq), is proved by the experimental fact of ca. 4 of Mo/Ca molar ratio. (2) The neutralization between the final acid product, H4Mo8O26(aq) and NaOH solution proceeds according to the following two-stage reactions, H4Mo8O26(aq)+4NaOH(aq)→Na4Mo8O26(aq)+4H2O, Na4Mo8O26(aq)+12NaOH(aq)→8Na2MoO4(aq)+6H2O.