The Use of Oxalic Acid as a Chelating Agent in the Dissolution Reaction of Calcium Molybdate

In this study, the dissolution behavior of calcium molybdate (CaMoO4) was investigated in oxalic acid (H2C2O4) solution. The effects of stirring speed, temperature, H2C2O4 concentration, and particle size on the dissolution reaction of CaMoO4 were determined. The dissolved quantities of molybdenum and calcium were analyzed quantitatively by ICP-OES. Fractional conversion of CaMoO4vs time and concentration of calcium vs time diagrams were plotted. It was observed that at constant temperatures and lower H2C2O4 concentrations, the dissolution increased by increasing H2C2O4 concentration, but at higher H2C2O4 concentrations, the effect of H2C2O4 concentrations was negligible. The dissolution reaction of CaMoO4 in H2C2O4 solution was performed in two steps as series–parallel type reaction. In the first step, CaMoO4 reacted with H2C2O4 to form the water-soluble calcium aqua oxalato molybdate (Ca[MoO3(C2O4)(H2O)]) intermediate chelate product. In the second step, the intermediate chelate, Ca[MoO3(C2O4)(H2O)], reacted with the reactant, H2C2O4, to yield water-soluble hydrogen oxalato dimolybdate chelate (H2[(MoO3)2(C2O4)]) and insoluble CaC2O4H2O as final products. It was found that 500 rpm was enough to eliminate the resistance of liquid film layer that surrounds the solid particles. It was concluded that the optimum temperature was 313 K (40 °C) and the optimum concentration of H2C2O4 was 1 kmol m−3 to obtain high conversion during the dissolution of CaMoO4.