Stepwise sedimentation of calcium and magnesium ions by CO2 absorption in concentrated seawater

During the precipitation of calcium and magnesium ions by absorbing CO2 in concentrated seawater, MgO was used to regulate the pH of the solution at 6.5-7.2 to precipitate calcium ions from concentrated seawater. After the equilibrium of calcium ion precipitation was reached, NaOH was used as the pH regulator for precipitating magnesium ions in the solution, so as to obtain two pure carbonate precipitates. The effects of MgO addition, gas flow, temperature in the first step, and gas flow, temperature, and pH in the second step on precipitation were studied through single factor experiments, and the suitable experimental conditions were initially determined. Then, the first and second step reaction conditions were optimized using response surface methodology. The optimal experimental conditions for the first step precipitation were determined to be MgO dosage of 7.43 g/L, temperature of 42 ℃, and gas flow time of 23 min. The optimal conditions for the second step precipitation were determined to be temperature of 45 ℃, pH of 9.0, and gas flow time of 25 min. The predicted calcium and magnesium ion concentrations after the concentrated seawater reaction were basically consistent with the simulated results under the optimal experimental conditions obtained by response surface methodology. The actual calcium and magnesium ion removal rates were slightly lower than the simulated results, but the calcium ion and magnesium ion removal rates were still 90.2% and 85.6%, respectively. XRD analysis of the precipitated sediment showed that the purity of the calcium salt and magnesium salt was high. The results provided a new method for utilizing concentrated seawater to reduce CO2 emissions and recover resources.