Preparation and uses of silica bricks:

Uses:

• The refractory properties of quartz sand are employed in molds for cast iron. Huge quantities of sand, with varying amounts of clay impurity to bond the grains are used for this purpose in the foundries of the world.
• Crystal quartz in piezoelectric oscillators have become indispensable for the control of radio transmitters, radar equipment and other electronic timing gear.
• Silica bricks are used in the glass furnaces, lining of acid converters and roofs of electric and hearth furnaces.

Preparation:

• Silica bricks are made from crushed quartzite rock, known as ganister, which is bonded with 1.5 to 3.0% of lime. They are molded smaller than the dimensions desired in the finished brick to allow for an expansion of about 3/8 in./ft as the quartz inverts to tridymite during firing.
• A firing schedule of about 20 days at 1450oC is required. Study of the phase diagram of the silica-lime system shows why considerable quantities of lime may be used to bond the quartzite in silica bricks without loss of refractoriness.
• The lime is taken up in an immiscible lime-rich glass phase of which only a small amount is formed because of its high lime content.
• In use, silica bricks are characterized by retention of rigidity and load-bearing capacity to temperatures above 1600oC, without the slow yield characteristic of fireclay brick.
• If a cold silica brick is heated suddenly, it spalls and disintegrates owing to the sudden volume changes taking place at the high-low inversions of tridymite and cristobalite.
• If heated cautiously through this sensitive temperature region, silica brick is very resistant to temperature shock.