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

Calcium Molybdate (CaMoO₄)

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 MoO₄^2-. 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.