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The Different Grades of Magnesium Oxide

Three basic types or grades of "burned" magnesium oxide can be obtained from the calcination step with the differences between each grade related to the degree of reactivity remaining after being exposed 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 to produce refractory grade magnesia will range between 1500ºC - 2000ºC and the magnesium oxide is referred to as "dead-burned" since most, if not all, of the reactivity has been eliminated. Refractory grade MgO 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 type of magnesium oxide produced from calcining at temperatures ranging from 1000ºC - 1500ºC is termed "hard-burned.” Due to it's narrow range of reactivity, this grade is typically used in applications where slow degradation or chemical reactivity is required such as with animal feeds and fertilizers.

Light burned magnesium oxide
The third grade of MgO is produced by calcining at temperatures ranging from 700ºC - 1000ºC and is termed "light-burn" 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.

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