Synthesis of hydrotalcite using magnesium from seawater and dolomite

Three methods of synthesizing hydrotalcite (HT) have been developed using magnesium from seawater and dolomite (MgCa(CO₃)₂). In the first process, 1.0 M Na₂CO₃ solution was added to calcium ion free artificial seawater containing AlCl₃ with an initial Mg/Al molar ratio of 2.0 to approximately 3.7 until a pH of 10 was obtained. The solution was then continuously stirred for 1 h at 60 °C. CO₃^2--HT was precipitated as a single phase, and the initial Mg/Al molar ratio, which each recovery of Mg²⁺ and Al³⁺ from the solution was above 98%, was 2.0-2.3. In the second process, a Ca(OH)₂ slurry was added to artificial seawater containing AlCl₃ with an initial Mg/Al molar ratio of 1.0 to approximately 5.0 until a pH of 10.5 was obtained, and then was stirred for 1 h at 60 °C. HT was also precipitated as a single phase with initial Mg/Al molar ratio 2.0-4.0. The initial Mg/Al molar ratio, which each recovery of Mg²⁺ and Al³⁺ from the solution was above 98%, was 2.2 to approximately 3.3, but SO₄^2- and Cl^- were contained in the precipitated HT. When HT was produced using initial Mg/Al molar ratio of 3.0 at 25 °C, SO₄^2- and Cl^- in the HT were ion-exchanged with CO₃^2- in a 0.05 M Na₂CO₃ solution for 24 h at 25 °C, and SO₄^2- and Cl^- content of the HT were decreased to 0.5 and 0.05 wt%, respectively. In the third process, dolomite calcined at 1000 °C was added to an AlCl₃ solution with an initial Mg/Al molar ratio of 1.0 to approximately 2.0, and the solution was stirred for 1 to approximately 4 h at 25 to approximately 90 °C. HT was precipitated with the smallest amount of MgO and Mg(OH)₂ when the initial Mg/Al molar ratio was 1.5 and the solution was stirred for 4 h at 90 °C.