Enhanced CaMg（CO₃）₂ Production by CO₂ Microbubble Injection into Concentrated Brine

Abstract

In order to build a utilization system of seawater resources based on the desalination and salt production process and to prevent scaling in reverse osmosis and electrodialysis units, a recovery and upgrading method for calcium（Ca） and magnesium （Mg） from the concentrated brine discharge from salt manufacture in Japan was investigated. The reactive crystallization technique of carbonate using carbon dioxide （CO₂） bubbling is effective for a separation/recovery method of the dissolved Ca^2＋ and Mg^2＋ in the concentrated brine, because the solubility of carbonate is lower than the solubility of hydroxide in the solution at a pH range below 8.0. Especially, dolomite （CaMg（CO₃）₂）, which is double salt of calcium carbonate and magnesium carbonate, has numerous applications as the manufacture of refractories, as neutralizer of soil acidity in agriculture, as mineral supplement for food and drug, etc.. CaMg（CO₃）₂ has crystal structure derived from that of calcite by ordered replacement Ca^2＋ in calcite by Mg^2＋. To improve the functionality of crystal for the better CaMg（CO₃）₂ utilization, it is essential to gain access to the Mg/Ca ratio of 1.0 and to reduce the particle size in the crystallization process. Generally, high concentrations of Ca^2＋, Mg^2＋ and CO₃^2－ are necessary for the production of CaMg（CO₃）₂ with a Mg/Ca ratio of 1.0, because the Mg/Ca ratio increases with increasing the supersolubility product in the bulk solution. In this study, the micron-scale bubble formation technique that enables the generation of regions with a higher ion concentration around the minute gas-liquid interfaces was applied to the reactive crystallization of CaMg（CO₃）₂. In the regions near the minute gas-liquid interfaces, Ca^2＋ and Mg^2＋ accumulate because of the negative electric charge on the microbubble surface, and the concentration of CO₃^2－ increases because of the acceleration of CO₂ mass transfer caused by minimizing the bubble diameter; hence, the fine particles of CaMg（CO₃）₂ with a high Mg/Ca ratio can be expected to crystallize. At a reaction temperature of 298 K and reaction pH of 6.8, CO₂ bubbles with an average diameter（d_bbl） of 40 - 2000 μm were continuously supplied to the concentrated brine coming from salt manufacture discharge and CaMg（CO₃）₂ was crystallized within the reaction time（t_r） of 120 min. Microbubbles with a d_bbl of 40 μm were generated using a self-supporting bubble generator by the shear of the impeller and a negative pressure owing to high-rotation. For comparison, the bubbles with a d_bbl of 200, 300, 800 or 2000 μm were obtained using a dispersing-type generator. Consequently, minimizing the bubble formation accelerated remarkably the crystallization of CaMg（CO₃）₂ fine particles with an average size of about 2.0 μm and decreased t_r necessary for the achievement of Mg/Ca ratio of 1.0.