Microwave Enhanced Reaction Crystallization Process Of Hydromagnesite

Hydromagnesite (hydrated magnesium carbonate hydroxide) is a kind of important and widely used magnesium inorganic materials. The new development direction for magnesium resources utilization is that the inexpressive and abundant magnesium chloride in salt lake is used as raw material to prepare the functional magnesium carbonate with excellent performance and high additional value, making the product serialization, functionalization and specialization.In this article, MgCl2 solution was used as magnesium source and Na2CO3 or NaHCO3 solution was used as precipitant, and hydromagnesite was directly synthesis by the reaction crystallization process when the two solutions were mixed under the microwave field. The prepared hydromagnesite was characterized by XRD, FT-IR, and SEM. The effects of the reaction conditions on the crystal product and the reaction course were investigated, where the reaction conditions included reaction temperature, reactant concentrations, NaCl concentration, pH value, stirring speed and so on.The results showed that the reaction crystallization process in this system involved the phase transition. That is, the amorphous material or metastable solution was formed firstly, and then it gradually transited to hydromagnesite crystal. Reaction temperature, reactant concentrations, pH value and stirring speed had great influences on the reaction crystallization process. Increase of reaction temperature, reactant concentrations, pH value and stirring speed could accelerate the reaction rate, while the effect of these reaction conditions on the morphology of hydromagnesite crystal became more complex. The higher the reaction temperature was, the larger the particle size of the prepared hydromagnesite crystal was. The higher the reactant concentration was, the smaller the crystal size of hydromagnesite was. With the increase of pH value in the solution, the layers of the crystal became thinner and the crystal particle became easy to agglomerate. With the decrease of stirring speed, the particle size of crystal increased. Based on the experimental optimization, a new method to prepare the spherical crystal of hydromagnesite with a large specific surface area was developed.Comparing the reaction crystallization processes carried out under microwave field and under water-bath, it was found that microwave could enhance the reaction crystallization process, especially for the nucleation, phase transfer, growth of hydromagnesite crystal. And it also could increase the reaction rate and make the particle size of crystal bigger under microwave field.Furthermore, the crystallization kinetics of hydromagnesite under microwave field was studied. Based on the PBE (population balance equation) and the population density data of hydromagnesite crystal, the growth rate model of crystal was determined by comparing the classical size-dependent growth rate models. It was found that the Bransom model could easily and accurately describe the crystal growth for this reaction system. Under the microwave field, the growth rate equation of hydromagnesite crystal was obtained as Gt=0.25 x L-0.2. In the water-bath, the relationship between the crystal growth rate and the particle size L of crystal was L-1.24. This result proved that the heating method affected the relationship between the crystal growth rate and the crystal particle size, and the crystal size had less effect on the crystal growth rate under the microwave field.