Preparation Of MgO Whisker Via Ammonia Precipitation-Hydrothermal Modification-Calcination Route

MgO whiskers are widely used as the refractory and reinforcing materials owing to their high intensity, perfect heat-resistant, low-density and the special one dimension morphology. MgO whiskers are usually synthesized by the Chemical Vapor Deposition (CVD) methods, the corresponding shortcomings such as the rigorous synthetical condition, the high cost due to the expensive raw materials and high energy cost limited the further scale-up of the CVD technology. The possibility to synthesize MgO whiskers via the ammonium precipitation–hydrothermal modification-calcination route was investigated in this paper, including the influence of process parameters on the properties of the Mg(OH)2 /MgO whiskers as well as the directional growth of Mg(OH)2 whiskers at hydrothermal conditions. The aim of this paper was to provide an alternative method for the utilization of the magnesium resource in the salt lake.The original MgCO3.3H2O whiskers were formed at room temperature, using MgCl2 as the Mg source, NH4HCO3 and NaOH as the precipitation agents. MgCO3.3H2O whiskers were converted to Mg5(CO3)4(OH)4H2O whiskers after aging for 30-120 minutes. Mg5(CO3)4(OH)4H2O whiskers were decomposed to porous MgO after heating at 600oC for 2 hours.MgO whiskers were synthesized via the coprecipitation-hydrothermal reaction–sintering route, using MgSO4 as the magnesium source and ammonia aqueous solution as the precipitation agent.Dispersive Mg(OH)2 particles, favoring the hydrothermal formation of Mg(OH)2 whiskers, were synthesized at the following condition: 20oC, 400 rpm, molar ratio of MgSO4 to NH3: 1:2.6, aging time 30-120 minutes. The high initial supersaturation was favorable for the formation of nuclei and fine particles, the later low supersaturation was favorable for the dispersion of particles.The morphology of Mg(OH)2 whiskers were influenced obviously by the hydrothermal process parameters such as the molar ratio of the raw materials, the hydrothermal temperature and reaction time, etc. Mg(OH)2 whiskers with a diameter of 0.1-0.2μm and a length of 50-60μm were synthesized after hydrothermal treatment of the Mg(OH)2 slurry formed at room temperature at 140-160oC for 10-14 hours.The following phase change occurred in the hydrothermal process: Mg(OH)2→5Mg(OH)2MgSO43H2O→Mg(OH)2The following precipitation reactions co-existed in the hydrothermal process: Mg2++2OH-= Mg(OH)2 5Mg(OH)2+Mg2++SO2-4=5 Mg(OH)2 MgSO43H2OThe precipitation of Mg(OH)2 was carried out quickly owing to its high supersaturation at the hydrothermal condition, leading to the quick consumption of the soluble Mg2+ and promoting the dissolution of 5 Mg(OH)2 MgSO43H2O.SO42- was inserted into the Mg(OH)2 crystal lattice after hydrothermal treatment, leading to the change of the growth direction of Mg(OH)2 from along (0001) planes to along [11 2? 4] direction.