Magnesium oxide

Magnesium oxide, commonly termed magnesia, is a fundamental alkaline earth metal oxide with the molecular formula MgO. This white crystalline powder possesses an exceptionally high melting point of 2,852°C and boiling point of 3,600°C, with a specific gravity of 3.58 at 25°C. The compound demonstrates solubility in acidic media and ammonium salt solutions, while undergoing gradual hydration to form magnesium hydroxide in aqueous environments. When introduced into carbon dioxide-saturated aqueous solutions, it generates magnesium bicarbonate. The material exhibits hygroscopic characteristics, progressively absorbing atmospheric moisture and carbon dioxide, and emits irritant fumes upon thermal decomposition.

Magnesium oxide is predominantly sourced from magnesite (MgCO₃), dolomite (MgCO₃·CaCO₃), and marine resources. Industrial production typically employs calcination of magnesite or dolomite to facilitate thermal decomposition. An alternative seawater-based method involves precipitation of magnesium hydroxide through calcium hydroxide treatment, followed by calcination to yield the final product. Additional production routes utilize magnesium chloride brines—obtained from seawater or post-bromine extraction processes—where introduction of sodium hydroxide or sodium carbonate generates precipitates of magnesium hydroxide or basic magnesium carbonate, subsequently converted to magnesium oxide through thermal treatment. Contemporary Chinese production predominantly relies on magnesite, dolomite, natural brines, and consolidated salt blocks as primary raw materials.

Among magnesium-based compounds, magnesium oxide demonstrates the highest production scale, accounting for approximately 75% of total industrial magnesium chemical output. Magnesium oxide calcined at temperatures below 900°C is categorized as light-burned magnesia, characterized by reduced bulk density, extensive specific surface area, and prominent adsorption capabilities. This variant fulfills multiple industrial functions: serving as a catalytic agent, a reinforcing filler in rubber composites, and a key component in magnesium oxychloride cement when combined with magnesium chloride solutions. Additionally, it acts as a flame retardant incorporated into building materials, a therapeutic agent for gastric hyperacidity and peptic ulcers (frequently formulated with calcium carbonate to mitigate laxative effects), and a nutritional supplement in animal feeds and agricultural fertilizers.

Light-burned magnesia, calcined within the temperature range of 950–1050°C, exhibits increased bulk density and a defined particle size distribution, along with enhanced hydration characteristics. This variant functions as a separating agent for silicon steel sheets, where it reacts with surface silicon dioxide to form a magnesium silicate barrier layer, effectively preventing sintering during high-temperature processing operations. Heavy-burned magnesia, produced at elevated temperatures of 1500–1800°C, demonstrates high volumetric density, minimal specific surface area, exceptional thermal stability, and reduced chemical reactivity—manifested through decreased acid dissolution rates and slower hydration kinetics. These properties render it suitable for high-temperature refractory applications, including the fabrication of crucibles and furnace linings.