Magnesium oxide is a material which has a high melting point of about 2800° C., and is excellent in corrosion resistance, and sintered magnesium oxide and electromelted magnesium oxide are widely used as raw materials for magnesium oxide ceramic sintered bodies to be used for refractory materials.
However, magnesium oxide has a large thermal expansion coefficient of about 13.5×10−6/° C., and particles of magnesium oxide themselves are easily broken by cleavage. Due to such a property, magnesium oxide has a disadvantage of poor spalling resistance. In addition, magnesium oxide itself is a substance having high reactivity, and therefore has the problem that magnesium oxide easily reacts with moisture and the like in air, has reduced water resistance, and is difficult to handle.
It is known that a ceramic sintered body is produced using magnesium oxide-containing spinel (MgAl2O4) as an alternative material for magnesium oxide for improving spalling resistance and water resistance while maintaining the corrosion resistance of magnesium oxide in view of the above-mentioned problems.
PTL 1 discloses a method for producing a magnesia spinel powder, the method including the step of mixing aluminum hydroxide, magnesium hydroxide and a mineralizer, then firing the resultant mixture until achievement of a specified specific surface area (claim 1), and suggests that a sintered body is produced by firing the powder (paragraph [0001]) PTL 1 suggests that the mixed amount of aluminum hydroxide and magnesium hydroxide is consistent with the stoichiometric ratio of magnesia spinel (paragraph [0019]).
PTL 2 suggests that MgO-rich electromelted spinel is added in an amount of 5 to 100% to a spinel-containing refractory material (claim 1), and the MgO-rich electromelted spinel is a material in which the surfaces of magnesia (MgO) particles solid-dissolved with Al2O3 are entirely or partially covered with a spinel composition (paragraph [0010]).
PTL 3 discloses a method for producing a granulated material mainly composed of spinel, the method including blending magnesium hydroxide and/or magnesium oxide with aluminum hydroxide and/or aluminum oxide to prepare a raw material, bringing the raw material into a state of a cake containing water, and firing the cake in a rotary kiln at a maximum temperature of 1300° C. or higher to obtain a granular material having a spherical shape or an angular shape (claim 1).
PTL 4 discloses a method for producing a high-density spherical material mainly composed of spinel, the method including the step of blending magnesium hydroxide and/or magnesium oxide with aluminum hydroxide and/or aluminum oxide, forming the mixture into a semi-dried material containing water, firing the thus-obtained raw material at 1000° C. to 1400° C., then press-molding the raw material, and firing the obtained molded product in a rotary kiln at a temperature of 1500° C. or higher (claim 1).