Silica and its role in the production of iron and steel...

Silica and its role in the production of iron and steel Silicon, the element, is the second most abundant element in the earth’s crust. Silica is the scientific name for a group of minerals made of silicon and oxygen. It is one of the most abundant oxide materials in the earth’s crust and is found in most mineral deposits found on the earth. It is the starting material for the production of ceramics and silicate glasses. Silica (from the Latin word ‘silex’), is an oxide of silicon. It is a compound made up of silicon and oxygen atoms and has the chemical formula SiO2. It occurs commonly in nature as sandstone, silica sand or quartzite. It is the most frequently found in nature as quartz (SiO4). It is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound in several minerals. Silica occurs in a variety of crystalline modifications and also as an under-cooled melt called quartz glass. The crystal structure of the individual SiO2 modifications can differ widely, so that distinct density changes occur during transformation. This is of great importance during heating and cooling because of the change in the volume. Silica can be a naturally occurring substance, like quartz, or it can result from human activities. It occurs in many forms. It can exist in an amorphous form (vitreous silica) or in a variety of crystalline forms. Amorphous silica is found in nature (e.g., diatomaceous earth and plants), as well as in synthetic materials. In amorphous silica, the silicon and oxygen atoms are not arranged in any particular pattern. Amorphous forms of silica have a random pattern while in crystalline silica, atoms of silicon and oxygen are arranged in a repeating, three dimensional pattern which is known as crystal lattice. Crystalline silica...

Silica Refractories

Silica Refractories Silica refractories were first produced in United Kingdom in 1822 from Ganister (caboniferous sandstone) or from so called Dinas sand. Silica occurs in a variety of crystalline modifications, e.g. quartz, tridymite, and cristobalite and also as an under-cooled melt called quartz glass. The crystalline modifications each have a high and low temperature forms which can transform reversibly. The crystal structure of the individual SiO2 modifications can differ widely, so that distinct density changes occur during transformation. This is of great importance during heating and cooling because of the change in the volume. Quartz requires the smallest volume and the quartz glass the largest. During firing above approximately 900 deg C, quartz transforms into the other modifications and melt completely at 1725 deg C. During slow cooling , reversible volume decreases take place  which are a result of the spontaneous transformation of the crystal structure from the high to the low temperature modification (Fig 1). The reversible and irreversible volume effects can cause considerable stress within the refractory brick structure. Fig 1 Calculated volume and density changes Production of silica refractories The silica refractories are manufactured as multiple asymmetric shapes, which are normally keyed or interlocked with each other by means of tongues and grooves. It is the objective of the manufacturer of silica refractory bricks to select the raw materials and the firing process in such a manner that the degree of quartz transformation is suitable for the intended application of the brick. The raw material for silica brick is naturally occurring quartzite which must meet certain requirements in order to achieve optimum brick properties. If refractoriness or thermal expansion under load (creep) are the main requirements, a quartzite of high chemical purity must be selected. Raw materials for volume stable products...