This magnesium oxide powder contains secondary particles in which a plurality of primary particles of magnesium oxide having a crystal phase and a grain boundary phase are at least partially fused together by the grain boundary phase, and a median diameter obtained by a laser diffraction scattering method is 300 .Math.m or less.

Provided are magnesium oxide particles which are good in dispersibility in a resin or rubber, can function sufficiently as an acid acceptor or scorch retarder, and do not lower, even after combined with a resin or rubber, material properties thereof; a resin composition; a rubber composition; and a shaped body. The magnesium oxide particles satisfying the following (A) to (C): (A) an average particle size is 5 μm or less; (B) a BET specific surface area is 20 m.sup.2/g or more and 200 m.sup.2/g or less; and (C) a screen residue by a sieve opening of 45 μm is 0.1% by weight or less.

Provided are magnesium oxide particles which are good in dispersibility in a resin or rubber, can function sufficiently as an acid acceptor or scorch retarder, and do not lower, even after combined with a resin or rubber, material properties thereof; a resin composition; a rubber composition; and a shaped body. The magnesium oxide particles satisfying the following (A) to (C): (A) an average particle size is 5 μm or less; (B) a BET specific surface area is 20 m.sup.2/g or more and 200 m.sup.2/g or less; and (C) a screen residue by a sieve opening of 45 μm is 0.1% by weight or less.

A thermally conductive resin filler including a sintered body containing at least magnesium oxide, calcium oxide, and silicon oxide, the thermally conductive filler being characterized in that, when the molar number of calcium element contained in the total composition of the sintered body in terms of calcium oxide (CaO) is defined as MCa, and the molar number of silicon element contained in the total composition of the sintered body in terms of silicon oxide (SiO.sub.2) is defined as MSi, the molar ratio of the calcium oxide (CaO) to the silicon oxide (SiO.sub.2) represented by MCa/MSi is in the range of 0.1 or more and less than 2.0.

A thermally conductive resin filler including a sintered body containing at least magnesium oxide, calcium oxide, and silicon oxide, the thermally conductive filler being characterized in that, when the molar number of calcium element contained in the total composition of the sintered body in terms of calcium oxide (CaO) is defined as MCa, and the molar number of silicon element contained in the total composition of the sintered body in terms of silicon oxide (SiO.sub.2) is defined as MSi, the molar ratio of the calcium oxide (CaO) to the silicon oxide (SiO.sub.2) represented by MCa/MSi is in the range of 0.1 or more and less than 2.0.

A process for producing a highly calcined and uniformly calcined product from a feedstock. The process comprising the steps of grinding the feedstock to powder, preheating the powder, and calcining the powder in a reactor plant that comprises a number of reactor segments in which a flash calciner is used in each progressive reactor segment to incrementally react the powder by raising the temperature in each segment. The last segment may be a high-temperature reactor that has a controlled residence time and temperature that may allow controlled finishing of the calcination process to achieve a desired degree of calcination and sintering of the product; and cooling of the product.

Disclosed is a technique for capturing, refining and storing byproduct hydrogen generated by a seawater electrolyzer, using the byproduct hydrogen in an energy system, and producing high-purity magnesium oxide from alkali byproducts additionally produced after seawater electrolysis. An energy system 100 may include a seawater electrolyzer 110 generating a chlorine substance by electrolyzing seawater, a hydrogen storage unit 120 capturing, refining, and storing byproduct hydrogen generated in the electrolysis process by the seawater electrolyzer, a fuel cell 130 using, as fuel, the byproduct hydrogen stored in the hydrogen storage unit, an MgO acquisition unit 140 converting, into magnesium oxide, magnesium hydroxide additionally generated from the seawater in the seawater electrolyzer, a hydrogen capture pipe 150 having one side coupled to the seawater electrolyzer and other side coupled to the hydrogen storage unit and transferring the byproduct hydrogen from the seawater electrolyzer to the hydrogen storage unit.

A magnesium oxide powder containing magnesium oxide particles, wherein an average sphericity of a magnesium oxide particle having a projected area equivalent circle diameter of a particle cross section by microscopy of 10 μm or more among the magnesium oxide particles is 0.73 or more and 0.95 or less, a proportion of a magnesium oxide particle having a sphericity of 0.65 or less among the magnesium oxide particles is 20% or less on a number basis, and a proportion of a magnesium oxide particle having a sphericity of 0.55 or less among the magnesium oxide particles is 15% or less on a number basis.

Provided are: a spherical magnesium oxide having not only high sphericity but also smooth surface and having excellent moisture resistance and excellent filling properties, and a method producing the same. In the present invention, by controlling the boron and iron contents of the calcined magnesium oxide to be in the respective predetermined ranges, there is provided a spherical magnesium oxide having a volume-based cumulative 50% particle diameter (D50), as measured by a laser diffraction/scattering particle size distribution measurement, in the range of from 3 to 200 μm, which is the range for a relatively large particle diameter, and a high sphericity of 1.00 to 1.20, as measured from viewing a SEM photomicrograph, as well as smooth surface, and having excellent moisture resistance and excellent filling properties. A predetermined spherical magnesium oxide is provided by virtue of the synergies obtained from the boron content of 300 to 2,000 ppm and the iron content of 100 to 1,500 ppm.

Provided are: a spherical magnesium oxide having not only high sphericity but also smooth surface and having excellent moisture resistance and excellent filling properties, and a method producing the same. In the present invention, by controlling the boron and iron contents of the calcined magnesium oxide to be in the respective predetermined ranges, there is provided a spherical magnesium oxide having a volume-based cumulative 50% particle diameter (D50), as measured by a laser diffraction/scattering particle size distribution measurement, in the range of from 3 to 200 μm, which is the range for a relatively large particle diameter, and a high sphericity of 1.00 to 1.20, as measured from viewing a SEM photomicrograph, as well as smooth surface, and having excellent moisture resistance and excellent filling properties. A predetermined spherical magnesium oxide is provided by virtue of the synergies obtained from the boron content of 300 to 2,000 ppm and the iron content of 100 to 1,500 ppm.