Three methods of synthesizing hydrotalcite(HT) have been developed using magnesium from seawater and dolomite(MgCa(CO3)(2)). In the first process, 1.0M Na2CO3 solution was added to calcium ion free artificial seawater containing AlCl3 with an initial Mg/Al molar ratio of 2.0 similar to 3.7 until a pH of 10 was obtained. The solution was then continuously stirred for 1 h at 60 degrees C. CO32--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 similar to 5.0 until a pH of 10.5 was obtained, and thm was stirred for 1 h at 60 degrees C. HT was also precipitated as a single phase with initial Mg/Al molar ratio 2.0 similar to 4.0. The initial Mg/Al molar ratio, which each recovery of Mg2+ and Al3+ from the solution was above 98%, was 2.2 similar to 3.3, but SO42- and Cl- were contained in the precipitated HT. When HT was produced using initial Mg/Al molar ratio of 3.0 at 25 degrees C, SO42- and Cl-in the HT were ion-exchanged with CO32- in a 0.05M Na2CO3 solution for 24 h at 25 degrees C, and SO42- and Cl- content of the HT were decreased to 0.5 and 0.05wt%, respectively. In the third process, dolomite calcined at 1000 degrees C was added to an AlCl3 solution with an initial Mg/Al molar ratio of 1.0 similar to 2.0, and the solution was stirred for 1 similar to 4 h at 25 similar to 90 degrees 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 degrees C.