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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.