Refractories are classified in multiple ways, based on: • Chemical composition • Method of manufacture • Size and shape • Fusion temperature • Refractoriness • Thermal conductivity
Chemical composition Acidic refractories Acidic refractories are generally impervious to acidic materials but easily attacked by basic materials, and are thus used with acidic slag in acidic environments. They include substances such as
silica,
alumina, and
fire clay brick refractories. Notable reagents that can attack both alumina and silica are hydrofluoric acid, phosphoric acid, and fluorinated gases (e.g. HF, F2). At high temperatures, acidic refractories may also react with limes and basic oxides. •
Silica refractories are refractories containing more than 93%
silicon oxide (SiO2). They are acidic, have high resistance to thermal shock, flux and slag resistance, and high spalling resistance. Silica bricks are often used in the iron and steel industry as furnace materials. An important property of silica brick is its ability to maintain hardness under high loads until its fusion point.
Basic refractories Basic refractories are used in areas where slags and atmosphere are basic. They are stable to alkaline materials but can react to acids, which is important e. g. when removing
phosphorus from
pig iron (see
Gilchrist–Thomas process). The main raw materials belong to the RO group, of which magnesia (MgO) is a common example. Other examples include dolomite and chrome-magnesia. For the first half of the twentieth century, the steel making process used artificial
periclase (roasted
magnesite) as a furnace lining material. •
Magnesite refractories are composed of ≥ 85%
magnesium oxide (MgO). They have high slag resistance to lime and iron-rich slags, strong abrasion and corrosion resistance, and high refractoriness under load, and are typically used in metallurgical furnaces. •
Dolomite refractories mainly consist of
calcium magnesium carbonate. Typically, dolomite refractories are used in converter and refining furnaces. •
Magnesia-chrome refractories mainly consist of magnesium oxide (MgO) and
chromium oxide (Cr2O3). These refractories have high refractoriness and have a high tolerance for corrosive environments.
Neutral refractories These are used in areas where slags and atmosphere are either acidic or basic and are chemically stable to both acids and bases. The main raw materials belong to, but are not confined to, the R2O3 group. Common examples of these materials are
alumina (Al2O3),
chromia (Cr2O3) and carbon. •
Alumina refractories are composed of ≥ 50% alumina (Al2O3).
Method of manufacture • Dry press process • Fused cast • Hand molded • Formed (normal, fired or chemically bonded) • Un-formed (monolithic-plastic, ramming and gunning mass, castables, mortars, dry vibrating cements.) • Un-formed dry refractories.
Size and shape Refractory objects are manufactured in standard shapes and special shapes. Standard shapes have dimensions that conform to conventions used by refractory manufacturers and are generally applicable to kilns or furnaces of the same types. Standard shapes are usually bricks that have a standard dimension of and this dimension is called a "one brick equivalent". "Brick equivalents" are used in estimating how many refractory bricks it takes to make an installation into an industrial furnace. There are ranges of standard shapes of different sizes manufactured to produce walls, roofs, arches, tubes and circular apertures etc. Special shapes are specifically made for specific locations within furnaces and for particular kilns or furnaces. Special shapes are usually less dense and therefore less hard wearing than standard shapes.
Unshaped (monolithic) These are without prescribed form and are only given shape upon application. These types are known as monolithic refractories. Common examples include plastic masses,
ramming masses, castables, gunning masses, fettling mix, and mortars. Dry vibration linings often used in
induction furnace linings are also monolithic, and sold and transported as a dry powder, usually with a magnesia/alumina composition with additions of other chemicals for altering specific properties. They are also finding more applications in blast furnace linings, although this use is still rare.
Fusion temperature Refractory materials are classified into three types based on
fusion temperature (melting point). •
Normal refractories have a fusion temperature of 15801780 °C (e.g. fire clay) •
High refractories have a fusion temperature of 17802000 °C (e.g. chromite) •
Super refractories have a fusion temperature of > 2000 °C (e.g. zirconia)
Refractoriness Refractoriness is the property of a refractory's multiphase to reach a specific softening degree at high temperature without load, and is measured with a
pyrometric cone equivalent (PCE) test. Refractories are classified as: •
Super duty: PCE value of 33–38 •
High duty: PCE value of 30–33 •
Intermediate duty: PCE value of 28–30 •
Low duty: PCE value of 19–28
Thermal conductivity Refractories may be classified by
thermal conductivity as either conducting, nonconducting, or insulating. Examples of conducting refractories are
silicon carbide (SiC) and
zirconium carbide (ZrC), whereas examples of nonconducting refractories are silica and alumina. Insulating refractories include
calcium silicate materials,
kaolin, and zirconia. Insulating refractories are used to reduce the rate of heat loss through furnace walls. These refractories have low thermal conductivity due to a high degree of porosity, with a desired porous structure of small, uniform pores evenly distributed throughout the refractory brick in order to minimize thermal conductivity. Insulating refractories can be further classified into four types: •
Heat-resistant insulating materials with application temperatures ≤ 1100 °C •
Refractory insulating materials with application temperatures ≤ 1400 °C •
High refractory insulating materials with application temperatures ≤ 1700 °C •
Ultra-high refractory insulating materials with application temperatures ≤ 2000 °C ==See also==