Silica: Pure silica melts at a very high temperature, namely 1713°C, but when it comes in contact with alkaline substances, it forms silicates, some of which melt easily. Therefore, the presence of alkali in silica used as a refractory material should be avoided. Silica occurs in the form of quartzite, Dinas rock and ganister. Ganister is a rock with a high content of natural silicon (98% SiO2). In the ready state, it is used in the form of bricks.Silica bricks are made of dense, hard, fine-grained quartzite (called ganister). After crushing, the rock is ground with water and mixed with approximately 2% lime. From this mixture, bricks can be molded by hand or by a mechanical press; after drying with hot air, they are baked at the right time and at the right temperature. Lime, which is an alkali, chemically reacts with acidic SiO2 and fuses at various positions of the brick to give it strength. The brick expanded by 3.5% after firing.
roofs of Acid and Basic
Refractory clay: The main components of refractory clay are kaolinite and Al2O3. 2SiO2. 2H2O. However, refractory clay contains different proportions of alumina and silica. Impurities in refractory clay, such as alkali, sand, iron oxide, calcium silicate and magnesium silicate, can adversely affect their performance. At high temperatures, refractory clay loses its water of hydration and is mainly composed of alumina and silica. The method of making refractory bricks is the same as that of silica bricks. The finely divided clay is mixed with a specified amount of water in a clay mill. The mixture is then pressed into a mold, dried and finally burned. In order to control shrinkage and speed up production, 20-80% of burnt or calcined clay called Grog is mixed into the clay, while flint clay and plastic clay are used as binders. By changing the properties of flint clay and plastic clay in the brick mixture, the properties of the manufactured bricks can be controlled.
Linings of blast furnaces for the melting of iron, copper, lead ores etc.
The linings of flues and shocks.
Heat treatment furnaces.
Checker-work of regenerative furnaces.
Magnesite: This is the most important basic refractory material. It is made by "burning to death" the mineral magnesite, which is essentially magnesium carbonate. At any temperature above about 700°C, magnesite decomposes into magnesium oxide and carbon dioxide, but the properties of the resulting product vary greatly depending on the combustion temperature. The product below 900°C is "caustic magnesium oxide", which is easily hydrated into magnesium hydroxide and used in cement. Microscopic examination revealed no crystals, which is why it is often referred to as amorphous magnesium oxide. Cooking for a long time at 1800°C will produce insoluble and non-reactive "burned" magnesite, which is easily recognized as a crystalline mineral.
Hearths of basic open-hearth and copper reverberatory furnaces.
Electric arc and induction furnaces.
Lining of L.D. crucible (in the manufacture of steel).
Dolomite: It is a bicarbonate of calcium and magnesium, which is abundant in India. It is generally fired in a shaft kiln. Compared with magnesite bricks, dolomite bricks have lower conductivity and refractoriness. The calcium oxide present in the brick tends to combine with the water and carbon dioxide present in the atmosphere and cause the brick to collapse. They are cheaper than magnesite bricks, but they have poor performance. It is widely used to repair basic open hearth stove tops and furnace bottoms.
Chromite: Chromite refractories are made of core minerals mainly containing chromite and FeO. Alumina. The minerals are finely crushed and mixed with small amounts of binder materials such as clay, bauxite and magnesium oxide. The mixture is cast into bricks, dried and burned in a kiln at a temperature of approximately 1500°C to 1700°C.
Used in steel industry for lining open hearths and also in the bottom of soaking pits.
Sillimanite- Its composition is Al2O3. Si2 (flbrolite) appears in the form of long needle-like crystals. It has a high softening point (above 1800°C). Its conductivity is very low.
Used for making blocks, bricks, crucibles, refractory fittings for utensils and pipes for surface combustion.