Magnesia (Mg0) is a basic lining. It is made from either naturally occurring carbonate ore or is synthetically produced from seawater, It is graded in size from 0 to 5 mm, but somewhat coarser to silica ramming mass. Two types of bonds are used- either spinal bond (Mg0.Al203) or silicate bond (2Mg0.Si02).
Applications: Magnesia linings are widely used in steel melting as well as in the melting of high alloyed irons, for temperature up to 16500 C. Magnesia is resistant to basic slag and high-manganese melts.
Limitations: Magnesia has very poor resistance to thermal shock, hence it serves best only with continuous furnace operation.
Different Zones in Lining:
During sintering (and also during normal working) the temperature of the lining varies considerably from its hot inner face to the cold outer face next to the asbestos sheet. Three different zones are created in the lining.
- A sintered zone is formed at the hot face in which due to high temperature (approx 13000 C), the fusion of boro-silicate glasslike binder with silica ramming mass is complete. This zone has very low-porosity and is strong. Its thickness is approximately one-fifth of the lining thickness.
- The next zone, behind the sintered zone, is called a fritted zone, it is subjected to a somewhat lower temperature (900-13000 C), and hence the bond development is partial. This zone has higher porosity and less strength, It is about two-fifth of the lining thickness.
- The last outermost zone, which is next to the asbestos sheet, does not attain high temperature, there is no bond formation, and thee unbounded granular lining remains in its original form. It is highly porous and has low strength.
In case of any crack in the sintered zone or fritted zone, this outermost zone prevents metal penetration reaching to power coil by spreading the leaked metal in all directions.
With an increase in the use of the lining, the lining slowly becomes thin, and the interfaces between these zones move towards the outer coil. Finally, the unbounded zone is completely eliminated. If a crack develops in the lining at this stage, metal can penetrate to power coil.