A process using plentiful and inexpensive caustic calcined magnesia (CCM; main component is MgO) to prepare lamellar MH of high purity is described. The process begins with the reaction of MgO in CCM with NH4NO3 at ∼100 °C to produce ammonia gas and Mg(NO3)2 solution and the subsequent filtration of Mg(NO3)2 solution to remove impurities (e.g., SiO2). Then, the obtained ammonia is introduced into the Mg(NO3)2 solution to produce MH precipitate and NH4NO3. After the MH precipitate is separated by filtration, the filtrate containing NH4NO3 is returned to the initial step. The overall reaction of this process is the hydration of MgO to MH. Therefore, this process is called the apparent-hydration method. This apparent-hydration method overcomes the disadvantages of the true hydration method, such as the inability to remove impurities. A crucial finding is that, because of the presence of trace SO4 2– in the obtained Mg(NO3)2 solution, a MH product with a narrow particle size distribution cannot be produced. After removal of the SO4 2– with Ba(NO3)2, lamellar MH with a narrow particle size distribution can be synthesized at 120 °C in one step. This work provides a promising route to the use of CCM to prepare MH.

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Aqua ammonia (AA) is frequently used to precipitate various metal hydroxide or oxide particles. Here, we replace AA with gaseous ammonia (GA) to precipitate Mg(OH) 2 particles in MgCl 2 aqueous solution under mild conditions. The results show that GA can give a higher consistency in the growth of Mg(OH) 2 particles although both AA and GA essentially produce Mg(OH) 2 by the reaction of Mg 2+ with OH À . This work provides results that suggest using ammonia as a precipitant.