Preparation Of High Purity Magnesium Oxide By Crystallization-Calcination With Bischofite And Sodium Carbonate

High purity magnesium oxide has wide application and high added value. It used to be intermediate compound to prepare a series of special magnesium oxide and is concerned with its inexpensive production process. Magnesium resource is rich and of good quality in Qing-hai salt lake. The preparation of high purity magnesium was studied in the paper with the magnesium resource discharged from extraction of potassium resource. MgCO3-5H2O was firstly synthesized by reactive crystallization with brine-soda method, and then high purity magnesium oxide was prepared by calcining MgCO3·5H2O.The solubility and stability of MgCO3·5H2O were studied in pure water and sodium chloride solution. MgCO3·5H2O could be stable without crystal phase transition in0～10℃and0-0.5mol·L-1. The solubility of MgCO3·5H2O was measured, that increased with in-creases of temperature and concentration of sodium chloride. The relevant solubility model equations Inx=A+B/(T/k) was also built based on the solubility data of MgCO3·5H2O, and the one with high correlation coefficient was obtained by the computer fitting calculation.In this paper, the preparation of MgCO3·5H2O by reactive crystallization was discussed. The influences of the residence time, reaction temperature, reaction concentration and the amount of additive were investigated to obtain the optimum operating conditions. The MgCO3·5H2O particle was orthorhombic cluster in big size and uniformly distribution. In order to remove impurity, the washing process of MgCO3·5H2O was discussed as well. The influences of washing temperature, washing consumption and washing times were investi-gated. Based on the actual production situation and economic benefits, the optimum washing operating condition was obtained.The calcining process that MgO was synthesized by MgC03·5H2O was also studied. This process could be divided into two stages:dehydration of five crystal waters and decom-position of MgCO3. A globally optimized iterative iso-conversion method was proposed to calculated the activation energy of thermal decomposition. The calculated activation energies of MgCO3·5H2O at each thermal decomposition stage were29.30kJ·mol-1and63.88kJ·mol-1respectively. By globally optimized iterative isothermal method, the most probable dehydra-tion mechanism function was G(a)=1-(1-α)1/4at two stages, corresponding to solid phase boundary reaction mechanism. Pre-exponential factors at two stages were9.104×105s-1and1.159×102s-1.In the calcining process, the influences of conditions were investigated. With the stand-ard of magnesium oxide purity and activity, the optimum washing calcining conditions were obtained:600℃calcining temperature,2h calcining time and heating by temperature of muf-fle furnace. In the condition, high purity magnesium oxide belonged to tetragonal system was prepared, which purity was more than99.9%.