Gunning Mix

The raw materials used for the production of magnesia refractories consist essentially of the basic oxide magnesium (MgO). These raw materials are (i) chemically precipitated magnesium hydroxide, ii) the mineral magnesite, which is naturally occurring magnesium carbonate, and to a minor extent (iii) the mineral brucite, naturally occurring magnesium hydroxide. Chemically precipitated magnesium hydroxide, prepared by causing slaked lime or slaked calcined dolomite to react with magnesium-bearing brine, contains small quantities of accessory minerals derived from the limestone or dolomite, and from the brine. The natural mineral magnesite contains small quantities of accessory minerals, such as dolomite, serpentine, talc, chalcedony, and quartz. Iron carbonate, in solid solution with the magnesium carbonate, is present in several magnesites.

Raw magnesia containing materials are ‘dead-burned’ in rotary kiln or shaft kiln to prepare them for use. The dead-burning consists of a high temperature heat treatment which drives off chemically combined water and / or carbon dioxide, and converts the remaining product into dense grains or lumps resistant to atmospheric moisture and carbon dioxide. In dead-burning, additions such as iron oxide, alumina, silica, or lime can be made to achieve desired compositions.

The temperature of dead-burning varies from around 1,540 deg C to 1,850 deg C, depending upon the type and purity of the product. In the process of dead-burning, magnesia hydroxide dissociates to form magnesia and water vapour, magnesium carbonate dissociates to form magnesia and carbon dioxide gas. The water vapour and the carbon dioxide gas escape with the kiln gases. The dead-burned product, which is known commercially as ‘dead burned magnesia’ consists mainly of aggregates of periclase crystals, with a fine-grained crystalline ground mass normally composed of silicates of magnesium and to a minor extent of calcium.

The colour of dead-burned magnesia low in iron oxide varies from white to buff or tan. Dead-burned magnesia which contains several percent of iron oxide is normally chocolate-brown in colour, and its microscopic examination shows that the periclase crystals normally contain dark moss-like inclusions. These have been identified as magnesio-ferrite (MgO.Fe2O3).

The most desirable quantities of the accessory constituents, and their relative proportions, depend upon the purposes for which the product is to be used. Dead-burned magnesia used in furnace hearths in general has a higher content of accessory constituents than that used for making bricks. In recent years, ultra high purity dead-burned magnesia containing 99 % have gained wide spread use.