Three basic grades of “burned” magnesium oxide can be obtained from the calcination step with the differences between each grade being related to the degree of reactivity remaining after exposure to a range of extremely high temperatures.
The original or “parent” magnesium hydroxide particle is usually a large and loosely bonded particle. Exposure to thermal degradation causes this particle to alter its structure so that the surface pores are slowly filled in while the particle edges become more rounded.
Thermal alteration dramatically affects the reactivity of magnesium oxide since less surface area and pores are available for reaction with other compounds. This change in particle structure is demonstrated in the photographs below.
Dead Burned Magnesium Oxide
Temperatures used when calcining magnesia to produce refractory grade magnesium oxide will range between 1500ºC – 2000ºC. The magnesium oxide is referred to as “dead burned” or “periclase” since most, if not all, of the reactivity has been eliminated. Refractory grade magnesium oxide is used extensively in steel production to serve as both protective and replaceable linings for equipment used to handle molten steel.
Hard Burned Magnesium Oxide
A second grade of magnesium oxide produced from calcining magnesia at temperatures ranging from 1000ºC – 1500ºC is termed “hard-burned.” Due to its narrow range of reactivity, this grade is typically used in applications where slow degradation or chemical reactivity is required such as with leather tanning and phosphate cements.
Light Burned Magnesium Oxide
The third grade of magnesium oxide is produced by calcining magnesia at temperatures ranging from 700ºC – 1000ºC. This grade is termed “light burned” or “caustic” magnesia. Due to the material’s wide reactivity range, industrial applications are quite varied and include plastics, rubber, paper and pulp processing, steel boiler additives, adhesives, and acid neutralization to name just a few.
Magnesium hydroxide is available as a naturally-occurring mineral called brucite; as a concentrated aqueous suspension (slurry) of magnesium hydroxide solids (synthetic or naturally-occurring); and as a dried powder. In slurry form, the magnesium hydroxide suspension should have good stability for transport in drums, totes, bulk tank trucks and railcars, as well as for long term storage in agitated tanks without excessive solids settling.
Magnesium hydroxide slurry can also be produced by reacting magnesium oxide with water:
MgO + H2O —-> Mg(OH)2 (slurry)
The length of time for full hydration, which is an exothermic process, will depend on the starting concentration and surface area of the magnesium oxide in addition to temperature.
Magnesium hydroxide typically has moderate reactivity and is used in wastewater treatment, flame retardants, fuel additives, utility boiler treatment, pulp bleaching, and specialty applications.