Magnesia, acted as a refractory material, is widely used in industrial metallurgical furnaces owing to its high refractoriness and excellent corrosion resistance to basic slag and metal melt. Generally, the fabrication of high-quality magnesia aggregates with high purity and high bulk densities (>3.40 g/cm³) plays a critical role in producing MgO-based refractories that show satisfying mechanical properties and corrosion resistance. In China, magnesite is mainly used as a raw material to produce sintered magnesia at high-temperature. while, as the generally coarse-grained of magnesite, it is difficult to produce high density magnesia even sintered at high temperatures, being due to the poor sintering property. As reported that the introduction of oxides additives such as TiO₂, Al₂O₃ ZrO₂, etc. could effectively promote the sintering of magnesia. However, the additives also resulted in a low melting phase formation, which significantly reduced the high-temperature performance of magnesia. In fact, besides magnesite, China also has abundant magnesium resources in salt lakes. Compared to magnesite, magnesia derived from salt lake owns the advantage of higher purity and without CO₂ emissions during the production process. In this work, magnesium hydroxide produced from salt lake was used as raw material to prepare the high-purity dense magnesia via a two-step method. Effects of calcination temperature and grinding time on the morphology and sintering activity of MgO were studied, microstructure and densification behavior of magnesia sintering at high-temperature were investigated also.

Magnesium hydroxide powder [Mg(OH)₂] was used as raw material. Firstly, it was lightly-burned to produce MgO when calcined at 650, 750, 850 ℃ and 950 ℃ with a holding time of 1 h and a heating rate of 5 ℃/min, repetitively. Secondly, the obtained powder was finely ground using mechanical ball milling for 0.5, 1.0, 3.0, 5.0, 12.0 ℃ and 24.0 h, respectively, here, zirconia balls acted as the media and anhydrous ethanol as solvent. Thirdly, the finely ground and dried powders were pressed into a green body with a size of Φ20 mm × 20 mm under a pressure of 100 MPa. Finally, the green body was sintered at 1 600 ℃ for 3 h to prepare the magnesia samples.

Mg(OH)₂ would decompose into MgO agglomerates with crystallite and nano-pores when calcined at 650-850 ℃. The crystal size of MgO continuously increased with the increase of calcination temperature, and rapidly increased to 320 nm when temperature reached to 950 ℃. Correspondingly, the specific surface area of calcined MgO decreased with the increase of the temperature. In addition, a small amount of undecomposed Mg(OH)₂ still existed in lightly-burned MgO even treated at 750 ℃. When calcined at 850 ℃, the Mg(OH)₂ decomposed completely and the obtained MgO crystallite possessed high activity and best sintering properties. With the increase of grinding time, the number of MgO agglomerates decreased continuously and well disappeared when the grinding time reached to 12 h, and the average size of MgO agglomerates decreased to 1.72 μm. However, by further increasing the grinding time to 24 h, the average size of MgO agglomerates only decreased to 1.70 μm. The destruction of MgO aggregates could reduce the number of large pores and improve the contact between agglomerates in the green specimens, both could enhance the densification of the sintered specimens.

When the calcination temperature increased from 650 ℃ to 850 ℃, the sintering activity of crystallite in MgO agglomerations reduced slightly due to the increase of average size of MgO crystallite from 138 nm to 180 nm, while the density of sintered magnesia exhibited an increasing trend as content of residual Mg(OH)₂ in MgO agglomerations reduced. When calcined at 950 ℃, the sintering activity of the crystallite reduced dramatically and average size sharply increased to 320 nm, which obviously decreased the sintering activity and reduced the grain size and density of the specimens. The MgO agglomerations can be effectively crushed via the ball milling process, whose average size sharply reduced from 55.90 to 2.69 μm when ball milling for 0.5 h, and then this value decreased slowly to 1.72 μm when ball milling increasing to 12 h. However, a further increase of ball milling time would not produce the average size. The crush of MgO agglomerations could reduce the large pores in green compacts and enhanced the bonding between MgO crystallites, therefore enhancing the grain growth and densification of the specimens. When calcination temperature and milling time were 850 ℃ and 12 h, respectively, a dense magnesia with an average grain size of 30.4 μm and bulk density of 3.42 g/cm³ could be prepared when sintered at 1 600 ℃ for 3 h.