In nature, there is no such refractory which meets all these properties. From among the available refractories, three types meet the important properties for specific applications. These refractories are commonly used to form induction furnace linings. These are:
- Silica (Si02) ——- (acidic)
- Alumina (Al2O3) ——- (neutral)
- Magnesia (MgO) ——- (basic)
Silica (Acidic Lining):
The silica lining (Si02) is acidic in nature. It is produced by crushing and grading of a good quality of quartzite having very high purity (silica content above 99%). It is used in the powdered condition. Correct grading is important for uniform and hard ramming. Typical size grading of silica ramming mass is given in table-2. Boric acid (or boric oxide) is the most commonly used binder. Pure silica has a melting point of 17230 C. the rammed lining is sufficiently refractory for use up to 1550 – 16500 C. Between 6000 C and 16000 C, silica expands very little and is superior to both alumina and magnesia to resist thermal shock. Being available abundantly in nature, the cost is very low in comparison to alumina and magnesia.
- Application: It is most widely used in iron-foundries. It is also used in some steel-melting.
- Limitations: It cannot be used for temperature above 16500 C. With high manganese melt, silica forms low viscosity glass and is washed out Very fast. At high temperatures, silica dissociates into silicon and oxygen, and silicon And oxygen and silicon level in metal go up. Si02 + 2C= 2 Si + 2CO