The use of Langmuir–Hinshelwood mechanism to explain the kinetics of calcium molybdate leaching in oxalic acid solution

The effects of H2C2O4 concentration, temperature, stirring speed and particle size on the dissolution rate of synthetically prepared CaMoO4 were investigated. Dissolution of CaMoO4 in H2C2O4 solution took place as a series–parallel type reaction. In the first reaction step, which proceeded according to the “Langmuir–Hinshelwood mechanism”, calcium aqua oxalato molybdate (Ca[MoO3(C2O4)(H2O)]) was formed as a complex chelate intermediate product. In the second reaction step, together with insoluble CaC2O4H2O, soluble H2MoO4 was formed during the slow hydrolysis of Ca[MoO3(C2O4)(H2O)], and H2MoO4 reacted rapidly with H2C2O4 to form the water soluble hydrogen oxalato dimolybdate chelate (H2[(MoO3)2(C2O4)]). In the first step, the reaction rate was found to be first order with respect to H2C2O4 concentration at low concentrations while zero order at high concentrations. Activation energies were calculated for first and zero order reactions as 41.4 and 49.9 kJ mol− 1, respectively. A kinetic equation was derived that explains the relationship between fractional conversion of CaMoO4 and time. In the second step, the reaction rate was found to be first order with respect to H2C2O4 concentration and activation energy was calculated as 43.6 kJ mol− 1. Kinetic equations were derived that explain the relationship between the concentration of Ca[MoO3(C2O4)(H2O)] and time and the concentration of H2[(MoO3)2(C2O4)] and time. The diagrams plotted according to the model equations were in good agreement with the diagrams obtained experimentally where the protective behavior of CaC2O4H2O was not effective.

► CaMoO4 was dissolved in H2C2O4 solution to form water soluble H2[(MoO3)2(C2O4)].
► The reaction of CaMoO4 in H2C2O4 solution was under chemical reaction control.
► Langmuir–Hinshelwood mechanism was used to define the reaction rate equation.
► This mechanism was chosen due to the shifting of the order of H2C2O4 concentration.
► Formation of CaC2O4H2O layer ceased the dissolution reaction at high temperatures.