Effects of CO2 fine bubble injection on reactive crystallization of dolomite from concentrated brine

In this study, we used the minute gas-liquid interfaces around CO2 fine bubbles as new reaction fields where the crystal nucleation proceeds dominantly and developed a crystallization technique to synthesize dolomite (CaMg(CO3)2) with the desired crystal quality. CaMg(CO3)2 has a crystal structure derived from calcite by ordered replacement of the Ca2+ in calcite by Mg2+. To improve the functionality of the crystal for better utilization of the CaMg(CO3)2, an effective method for an approach to a Mg/Ca ratio of 1.0 and downsizing is indispensable in the crystallization process. In the vicinity of the minute gas-liquid interfaces, the accumulation of Ca2+ and Mg2+ is caused by the negative electric charge on the fine bubbles surface; thus, CaMg(CO3)2 fine particles with a high Mg/Ca ratio can be expected to crystallize. At a reaction temperature (Tr) of 298 K and CO2 flow rate (FCO2) of 11.9 mmol/(L min), CO2 bubbles with an average bubble diameter (dbbl) of 40-2000 µm were continuously supplied to 300 mL of the concentrated brine coming from salt manufacture discharge and CaMg(CO3)2 crystallized within a reaction time of 120 min. Fine bubbles with dbbl of 40 µm were generated using a self-supporting bubble generator by increasing the impeller shear rate under reduced pressure. For comparison, the bubbles with dbbl of 300 or 2000 µm were obtained using a dispersing-type generator. Moreover, FCO2 and Tr were varied as operating parameters during the reactive crystallization supplying CO2 fine bubbles. Consequently, CO2 fine bubble injection is effective for the high-yield crystallization of CaMg(CO3)2 with a Mg/Ca ratio of 1.0 and downsizing of CaMg(CO3)2 particles owing to the acceleration of crystal nucleation caused by the local increase in the supersaturation at the minute gas-liquid interfaces.