Magnesia – Magnesium Oxide (MgO)

Chemical Formula

MgO

Topics Covered

Background

Plasma Display Screens

Substrate for Thin Film Formation

Background

Magnesia or magnesium oxide is an alkaline earth metal oxide. The majority of magnesium oxide produced today is obtained from the calcination of naturally occurring minerals, magnesite, MgCO₃, being the most common. Other important sources of magnesium oxide are seawater, underground deposits of brine and deep salt beds from which magnesium hydroxide [Mg(OH)₂] is processed. Magnesium is the eighth most abundant element constituting about two per cent of the earth's crust and typically 0.12% of seawater. Both MgCO₃and Mg(OH)₂ are converted to MgO by calcination. The thermal treatment of the calcination process affects the surface area and pore size and hence the reactivity of magnesium oxide formed. The source largely determines the level and nature of impurities present in the calcined material. Caustic calcined magnesia which is used in a wide range of industrial applications e.g. plastics, rubber, adhesives and acid neutralisation is formed by calcining in the range 700 – 1000^oC. By calcining in the range 1000 – 1500^oC the magnesium oxide is used where its lower chemical activity is required e.g. fertiliser and animal feed. Dead-burned magnesia, which is produced in shaft and rotary kilns at temperatures over 1500^oC, has reduced chemical reactivity therefore is more suited to refractory applications. Finally fused magnesia which is produced in an electric arc furnace from caustic calcined magnesia at temperatures in excess of 2650^oC is used for a variety of refractory and electrical applications.

Key Properties

There are few dense engineering ceramics of the structural type made from pure magnesia. However there is a wide range of refractory and electrical applications where magnesia is firmly established. The properties of major interest are as follows:

· Good Refractoriness

· Good Corrosion Resistance e.g. Basic Steelmaking Slags, Sodium Hydroxide, Fe, Co, Ni

· High Thermal Conductivity

· Low Electrical Conductivity

· Transparency to Infrared

Applications

Refractories

Magnesia is widely used in the steel industry as a refractory brick often impregnated with carbon (tar, pitch, graphite) to give optimum properties for corrosion resistance in environments of basic slags, particularly in BOF furnaces or slag lines of treatment ladles. Magnesia bricks often in combination with spinel or chrome are also used in ferroalloy, non-ferrous, glass and cement industries. Castables and sprayables based on magnesia are widely used for basic refractory linings for steel transfer applications. The lime to silica ratio present in the magnesia has a major influence on its properties.

Crucibles

Magnesia crucibles have found application in the superalloy industry, nuclear industry and chemicals industry where corrosion resistance is required. Manufacture is by firing isostatically pressed or slip cast shapes made from fused magnesia. Various purity grades are commercially available. Additives are employed to promote sintering or restrict crystal growth. These range from clay to yttria and alumina depending on application. Crucibles with stability of 2400^oC in air, 1700^oC in reducing atmospheres, 1600^oC in vacuum and 1400^oC in hydrogen have been reported in the literature.

Cements

Magnesia (or Sorel) cement is a refractory binder based on a magnesium oxychloride formulation. It is fast-hardening and has a number of refractory and general repair applications. Magnesia is also used as a room temperature curing agent for phosphate cements.

Heating Elements

Magnesia powder is widely used as a filling for electrical heating elements for applications in contact with air or liquids such as electric cooker rings, storage heaters, washing machines, and diesel engine glow plugs. Fused magnesia has the ideal combination of electrical resistance and thermal conductivity. The MgO forms a layer between the element and the outer sheath. It is also used as mineral insulation in cables.

Thermocouple Tubes

Extruded magnesia protective sheaths have been used to house thermocouples in aggressive environments.

Brake Linings

Magnesia has been included in brake linings due to its thermomechanical properties. Its intermediate hardness gives sufficiently low wear on metal while conducting heat from the friction contact surfaces.

Plasma Display Screens

Magnesia has been used as a protective film for plasma display screens where its electro-optical properties are used to advantage.

Substrate for Thin Film Formation

High purity grades of magnesia have been used to grow thin film semiconductors. Other similar electronic applications have been investigated.