Magnesium Oxide
Properties
| State | Solid (white powder) |
| Color | White |
| Solubility | Slightly soluble in water (0.086 g/L at 30 °C); reacts slowly to form Mg(OH)2 |
| Melting Point | 2852 °C |
| Boiling Point | 3600 °C |
About Magnesium Oxide
Magnesium oxide is the high-temperature workhorse of the refractories industry. The crystal sits in the same rock-salt structure as NaCl but with both ions doubly charged, so the lattice energy is enormous (roughly 3800 kJ/mol versus 786 for NaCl) and the melting point lands at 2852 °C — the highest of any common binary oxide other than the rare-earth and group-IV oxides. That single number is the reason the basic-oxygen steelmaking furnaces, cement clinker kilns rotating at 1450 °C, and copper-flash smelters all line their hot zones with MgO-based refractory bricks: nothing else this cheap survives the chemistry and the heat at the same time. The mineral form is periclase, found in marble contact-metamorphism aureoles and in the Earth's lower mantle as one of the dominant phases by volume. Production is straightforward in principle — calcine MgCO3 (magnesite) or Mg(OH)2 (brucite) at 700 to 1000 °C for 'caustic' MgO, or at 1500 to 2000 °C for the dense 'dead-burned' grade with low residual reactivity. The reactivity gap is huge: caustic MgO will set with MgCl2 brine into Sorel cement in minutes and is the active ingredient in pharmaceutical-grade antacid tablets, while dead-burned MgO is essentially inert and used as kiln-furniture refractory. MgO also has the rare combination of being a good electrical insulator and a respectable thermal conductor (about 50 W/m.K), which is why it fills the gap between the resistance wire and the steel sheath in mineral-insulated heating cables.
Where you'll encounter it
If you have ever climbed with grip chalk on your hands, watched a steel mill pour a heat from a basic-oxygen furnace, or peeled back the steel sheath of a tubular oven heating element, you have been in contact with magnesium oxide. A BOF refractory crew gunning fresh MgO-based lining onto a hot vessel is replacing brick that survived 1700-degree-C steel and basic CaO/MgO slag for thousands of heats — anything acidic like silica brick would dissolve in the slag within hours. A heating-element manufacturer fills the gap between the nichrome resistance wire and the Incoloy outer sheath with compacted dead-burned MgO powder, exploiting one of the few materials that simultaneously insulates electrically and conducts heat at about 50 W/m.K. Pharmaceutical-grade caustic MgO tablets at 250-500 mg neutralize stomach acid stoichiometrically.
Common Uses
- Refractory brick and gunning mix for steelmaking BOF and EAF furnaces, cement kilns, and copper smelters
- Mineral-insulated heating cable filler combining electrical insulation with thermal conductivity
- Pharmaceutical antacid (caustic-grade MgO tablets, 250-500 mg)
- Sorel cement binder mixed with MgCl2 brine for industrial flooring
- Soil amendment for acidic, magnesium-deficient pasture and arable ground
- Filler in dough conditioners and processed-cheese emulsifying salt blends
- Climbing and gymnastics chalk (often blended with MgCO3)
Safety Information
Very low acute toxicity; LD50 (rat, oral) above 3000 mg/kg. Inhaled dust irritates the upper airway and can produce metal-fume fever-like symptoms in welders working with magnesium alloys. Oral doses above 2 g produce osmotic diarrhea (caustic-grade MgO converts in vivo to MgCl2 then Mg(OH)2). Dead-burned MgO is biologically inert and not classified under GHS. The hot dust from calcination operations can cause thermal burns; standard refractory-handling PPE (dust mask, goggles, long sleeves) is the norm.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.