A mathematical model is proposed for a precipitation process of magnesium carbonate in a heterogeneous stirred tank reactor. The model includes a description of dissolution of Mg(OH)2, absorption of CO2 and precipitation of MgCO3. The Nernst-Planck equation is used in the dissolution model to maintain the mass balance and electroneutrality. The van Krevelen-Hoftijzer expression is introduced to describe the enhancement effect of reaction between dissolved CO2 and OH- on the mass transfer rate of dissolution and absorption. In the precipitation model, a simplified population balance equation is solved by a moment method for both dissolving and precipitating particles. Unknown precipitation kinetics parameters for Mg(OH)2MgCO3 system are fitted against experimental data and compared with Ca(OH)2CaCO3 system. According to the present analysis, the liquid-solid and gas-liquid mass transfer dominate the global rate of precipitation. The precipitation kinetics and pH have strong influences on the concentration of reactants and the yield of precipitation.