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 occurs in several forms.

There are approximately twelve forms of crystalline silica. Quartz is the most common form of crystalline silica. Silica has a number of distinct crystalline forms (polymorphs) in addition to amorphous forms. With the exception of ‘stishovite’ and ‘fibrous silica’, all of the crystalline forms involve tetrahedral SiO4 units linked together by shared vertices in different arrangements. Silicon–oxygen bond lengths vary between the different crystal forms. For example in alpha quartz the bond length is 161 picometers (pm), whereas in alpha tridymite it is in the range 154 pm to171 pm. The Si-O-Si angle also varies between a low value of 140 deg in alpha tridymite and up to 180 deg in beta tridymite. In alpha quartz, the Si-O-Si angle is 144 deg.

The three most common forms of crystalline silica are as follows.

  • Quartz – Quartz is by far the most common form of crystalline silica found in nature and around 12 % of the earth’s crust is made up of quartz. All soils contain at least trace amounts of crystalline silica in the form of quartz.
  • Cristobalite – It is found when diatomaceous earth is heat treated. This category includes calcined (heated to a high temperature) and metamorphosed sandstones and some volcanic siliceous rocks.
  • Tridymite – It is found in volcanic siliceous rocks.

The distinction between crystalline and amorphous forms of silica is not always clear cut. The form of silica can change in a process depending on how the silica is treated. Chemical treatment, as well as high temperature processes or treatment of amorphous silica, can create crystalline silica. For example, when diatomaceous earth, an amorphous form of silica, is calcined (heated to a high temperature), crystalline silica can be formed. Additionally, crystalline silica can be formed if plants containing amorphous silica are burned at high enough temperatures. Conversely, treatment of crystalline silica can convert it into amorphous silica. This occurs in glass manufacture and in silica gel production.

Silica is a fairly widely used ceramic material both as a precursor to the fabrication of other ceramic products and as a material on its own. Silica has considerably low thermal expansion, a fairly high melting point and is resistant to creep, making it a good refractory material. It tends to be used in acid environments if used as such or used as a starting material for the synthesis of other refractory products.  Due to the fact that silica is insoluble in the majority of acids, it is used as a refractory material in acidic environments. Silica is classified as an acid refractory as it behaves like an acid at high temperatures reacting with bases.

Silica has a melting point of 1,713 deg C and a boiling point of 2,950 deg C. Alpha quartz has a density of 2.648 grams per cubic centimeter. Its molar mass is 60.08 grams per mol. The IUPAC ID of silica is ‘Silicon dioxide’. Other names of silica are ‘quartz’, ‘silicic oxide’, ‘silicon oxide’, and ‘crystalline silica’.

Silica is a group IV metal oxide, which has good abrasion resistance, electrical insulation and high thermal stability. It is insoluble in all acids with the exception of hydrogen fluoride (HF). Key properties of silica are given in Tab 1 and Tab 2.

 

Tab 1 Key properties of quartz and fused silica
Sl. No. Property Unit Value
  Quartz Fused silica
1 Density gm/cc 2.65 2.2
2 Thermal conductivity Wm-1 K 1.3 1.4
3 Thermal expansion coefficient 10-6 K-1 12.3 0.4
4 Tensile strength MPa 55 110
5 Compressive strength MPa 2070 690-1380
6 Poisson’s ratio 0.17 0.165
7 Fracture toughness MPa 0.79
8 Melting point deg C 1830 1830
9 Modulus of elasticity GPa 70 73
10 Thermal shock resistance Excellent Excellent
11 Permittivity* epsilon’ 3.8 – 5.4 3.8
12 Tan (delta x 104)* 3
13 Loss factor* epsilon” 0.0015
14 Dielectric field strength* kV/mm 15.0-25.0 15.0-40.0
15 Resistivity* omega m 1012 – 1016 >1018
* Dielectric properties at 1 MHz 25 deg C
Tab 2 Differences between the different crystal structures of silica
Phase   Density (gm/cc) Thermal expansion (10-6 K-1)
1 Quartz 2.65 12.3
2 Tridymite 2.3 21
3 Cristobalite 2.2 10.3

 

The only stable form under normal conditions is ‘alpha quartz’ in which crystalline silicon dioxide is usually encountered. In nature, impurities in crystalline alpha quartz can give rise to colours. The high-temperature minerals, ‘cristobalite’ and ‘tridymite’, have both lower densities and indices of refraction than quartz. Since the composition is identical, the reason for the discrepancies is due to the increased spacing in the high-temperature minerals. As is common with many substances, the higher the temperature, the farther apart the atoms are, due to the increased vibration energy.

As mentioned above, the three crystalline forms of silica are (i) quartz, (ii) tridymite, and (iii) cristobalite. Each of these crystalline forms has a high and low temperature forms which can transform reversibly. Of all silica polymorphs, quartz is the only stable form at normal ambient conditions, and all other silica polymorphs eventually transform into quartz provided sufficient time is given. The other polymorphs are stable at different and sometimes very special conditions, mostly high temperatures and high pressures, but some of them may also form at low temperatures and pressures under conditions where quartz is stable. In theory, at normal pressure trigonal quartz (alpha quartz) transforms into hexagonal beta quartz at 573 deg C, upon further heating the SiO2 transforms into hexagonal beta tridymite at 870 deg C and later to cubic beta cristobalite at 1470 deg C (Fig 1).

Fig 1 Forms of silica

Formation of tridymite only occurs in the presence of impurities at temperatures between around 870 deg C and 1470 deg C. However, cristobalite nevertheless is formed above 1250 deg C, because the transformation velocity of quartz to cristobalite is much faster than to tridymite. Above around 1470 deg C, tridymite is converted to cristobalite which melts at 1713 deg C.

The transformation from ‘alpha quartz’ to ‘beta quartz’ which takes place abruptly at 573 deg C is accompanied by a significant change in volume and can easily induce fracturing of ceramics or rocks passing through this temperature limit. Modification and volume changes of silica are given in Tab 3.

Tab 3 Modification and volume changes of SiO2
Modification changes <—-> reversible —-> irreversible Transformation temperature in deg C Volume changes in %
Beta <—–> alpha quartz 573 0.8-1.3
Alpha quartz —-> alpha cristobalite 1250 (around 1050*) 17.4
Beta <—-> alpha cristobalite Around 260 2-2.8
Alpha quartz —-> alpha tridymite* Around 870 14.4
Gamma <—> beta <—> alpha  tridymite* 117-163 0.5
Alpha  tridymite —-> alpha cristobalite* 1470 0
Alpha cristobalite —-> melt 1713 +/- 10
Alpha  tridymite —-> melt** 1670 +/- 10
Fused silica —-> alpha cristobalite Above around 1150* Around 0.9
* in presence of impurities, ** rapid heating

 

Fused silica is a high purity grade of silica. It is around 99.4 % to 99.9 % SiO2 and is produced by carbon arc, plasma arc; gas fired continual extrusion or carbon electrode fusion. Fused silica is used as a refractory material or in investment casting.

In the majority of silicates, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central silicon atom. The most common instance for this is seen in the quartz polymorphs. As an example, in the unit cell of alpha quartz, the central tetrahedron shares all four of its corner oxygen atoms, the two face-centered tetrahedra share two of their corner oxygen atoms, and the four edge-centered tetrahedra share just one of their oxygen atoms with other SiO4 tetrahedra. This leaves a net average of 12 out of 24 total vertices for that portion of the seven SiO4 tetrahedra that are considered to be a part of the unit cell for silica.

Silicon dioxide in combination with basic oxides forms a large group of minerals known as silicates. Silica is also known in other forms such as chert, flint, chalsadony and opal, which are hydrated from of silica. Silica is one of the most important raw materials in the ceramics industry. Silica refractory bricks possess excellent thermal shock resistance, particularly in certain temperature ranges. There are temperature ranges where the resistance to thermal shock is very poor. This is due to the fact that large volume changes take place at elevated temperature during conversion from one modification to another. Modern silica bricks contain very little unconverted quartz. The presence of a liquid phase (glass) is another important aspect since it becomes fluid at operating temperature and thereby reduces refractoriness under load. The share of the crystalline high temperature modifications of quartz which exist at room temperature in silica bricks are in the following ranges, 12 % quartz, 33 % cristobalite, 42 % tridymite and 13 % glass matrix. In practice, the formation of these modifications is affected by the presence of impurities, so that silica bricks during heating and cooling may be transformed into a large number of combinations of these crystalline forms. The quartz modifications have relatively close-packed structures, and high density, where-as the tridymite and cristobalite forms are comparatively open structured. The great change in density between quartz and tridymite is responsible for the large expansion that occurs during the formation of tridymite.

SiO2 plays an important role in the production of hot metal (liquid iron) and liquid steel during primary steelmaking. In these slags it is present in the form of silicates and is an important component in determining the basicity (CaO/SiO2) of the slag. While during hot metal production the slag is to be neutral or mild basic, it is highly basic during the production of liquid steel. Silica content of slag during production of hot metal is in the range of 29 % to 38 % while it is in the range of 12 % to 19 % in the steelmaking slag.

Uses of silica

Silica has many uses. Uses connected with steel industry include the following.

  • Moulds and cores used to make metal castings
  • Refractory bricks and ramming masses used in steel plants, foundries, and cement plants
  • Filter media for water filtration systems
  • Sandblasting abrasives
  • Building materials such as concrete, grout and plaster