2015 Volume 69 Issue 4 Pages 262-269
In order to build up a utilization system of seawater resources based on the desalination and salt production process and to prevent scaling in reverse osmosis（RO）and electrodialysis（ED）units, a recovery and upgrading method for calcium（Ca）and magnesium（Mg）from brine solution was studied. From the viewpoint of solubility of salts, the synthesis of calcium carbonate（CaCO3）by reactive crystallization between the dissolved Ca2＋ in brine solution and carbon dioxide（CO2）can be considered as an effective separation/recovery method. The obtained CaCO3 is able to upgrade to hydroxyl apatite（HAP）with treatment of phosphoric acid and hydroxide. Moreover, the remaining Mg2＋ can be recovered from the Ca2＋ removed brine by reactive crystallization of magnesium carbonate（MgCO3）. From the results obtained in this study, at a solution pH of 8.0 and a solution temperature of 313 K, the crystallization phenomena of MgCO3 caused by CO2 bubble injection and minimizing bubble size can be summarized as follows: 1）when Na2CO3 solution was continuously supplied to the Ca2＋ removed brine, only basic magnesium carbonate（4MgCO3・Mg（OH）2・4H2O）with an average size of about 25 µm was produced; 2）in the case where CO2 bubbles with an average diameter of 2000 µm were injected, magnesium carbonate tri-hydrate（MgCO3・3H2O）was selectively crystallized within 30 min, and 4MgCO3・Mg（OH）2・4H2O was obtained at 60 min; 3）by minimizing bubble diameter to 20 µm, the produced moles of MgCO3・3H2O and 4MgCO3・Mg（OH）2・4H2O increased and the obtained particles were downsized to around 4.5 µm. Furthermore, the effects of solution pH and solution temperature on the reactive crystallization of MgCO3 were investigated under CO2 minute-bubble injection. Consequently, 5）at a solution pH of 7.0, the induction period for nucleation increased remarkably and only MgCO3・3H2O was produced, 6）when the solution temperature was increased to 333 K, 4MgCO3・Mg（OH）2・4H2O with high-selectivity was crystallized irrespective of crystallization time.