Preparation of Lamellar Mg(OH)₂ with Caustic Calcined Magnesia through Apparent Hydration of MgO

Abstract

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.

Preparation of Mg(OH)2 with caustic calcined magnesia through ammonium acetate circulation

“…The observations obtained from Figs. 2 and 3 are consistent with our previous studies using other circulation mediums (Guo et al, 2013). For the influence of reaction temperature (Fig.…”

“…If the concentration of Ca 2+ becomes too high and will affect the purity of the prepared MH product, the influence of Ca 2+ can be easily eliminated by adding nitrogen fertilizer (ammonium bicarbonate) to the ammonia-evaporation reaction solution before the ammonia-evaporation reaction starts. For example, the concentration of Ca 2+ can be reduced from 0.5 mol·L −1 to below 0.05 mol·L − 1 by introducing ammonium bicarbonate, that is, 95% Ca 2+ is removed (Guo et al, 2013).…”

“…While for the ammonia-evaporation unit in the present ammonium-circulation method, it probably also starts with the dissolution of MgO (column 2 in Table 1) like the direct hydration method. Previously, NH 4 Cl, NH 4 NO 3 and (NH 4 ) 2 SO 4 have shown the ability to be used as a circulation medium (Atashi et al, 2010;Li et al, 2010;Guo et al, 2013). If ammonium acetate (NH 4 Ac) is used as the circulation medium, the corresponding magnesium salt obtained from the ammonia-evaporation unit is Mg(Ac) 2 , which should be able to accelerate the ammonia-evaporation reaction by promoting the dissolution of MgO.…”

“…Compared to deadburned magnesia, CCM is produced by calcining magnesite at lower temperatures (usually 750-1100°C, but in some cases even b750°C) that allows one to produce a high porosity and reactive product (Birchal et al, 2000;Zhu et al, 2013). CCM can be used as magnesium source to prepare magnesium hydroxide (MH), an important fine chemical with a wide range of applications (Beall et al, 2013;Guo et al, 2013), such as in the neutralization of acid and treatment of flue gases, as a flame retardant, as precipitant for of heavy metals in wastewaters, as well as precursor for preparing MgO.…”

“…In view of the above problems, we developed ammonium-circulation method to prepare MH (Li et al, 2009(Li et al, , 2010Guo et al, 2013). The process contains two units, i.e., an ammonia-evaporation unit and a MH-precipitation (ammonia-absorption) unit (see Table 1).…”