Introduction to Refractories

Introduction to Refractories

Refractories are defined in ASTM C71 as non metallic materials having those chemical and physical properties that make them applicable for structures or as components of systems that are exposed to environments above 538 deg C. Refractories are inorganic, nonmetallic, porous and heterogeneous materials composed of thermally stable mineral aggregates, a binder phase and additives. These materials have ability to retain its physical shape and chemical identity when subjected to high temperatures. Refractories perform four basic functions namely (i) act as a thermal barrier between a hot medium and the wall of the containing vessel, (ii) represent a chemical protective barrier against corrosion, (iii) ensure a physical protection, preventing the erosion of walls by the circulating hot medium and (iv) act as thermal insulation for heat retention. Refractories are classified in the different following ways.

Classification based on chemical composition –  Refractories are classified on the basis of their chemical behaviour into following three classes.

  • Acid refractories – These are those refractories which are attacked by alkalis or basic slags. These are used in acidic atmosphere or where slags are acidic. Example of these refractories are silica and zirconia.
  • Basic refractories – These refractories are attacked by acid slags but stable to alkaline slag, dust and fumes at the elevated temperatures. These refractories are used in alkaline atmospheres. Example of these refractories are magnesia, dolomite and chromite.
  • Neutral refractories – These refractories are chemically stable to both acids and bases and used in the areas where slag and environment are either acidic or basic. Examples are carbon graphite, chromites and alumina. Grphite is the least reactive and is extensively used in the furnaces where the process of oxidation can be controlled.

Classification based on physical form – Refractories are classified according to their physical form in the following two ways.

  • Shaped refractories – These are commonly known as refractory bricks and are those which have fixed shape. The shapes can be standard shape or special shape. Standard shaped bricks have dimensions that are conformed to by most refractory manufacturers and are generally applicable to kilns and furnaces of the same type. On the other hand, special shaped refractories are specifically made for particular furnace. These refractory shapes may not be applicable to another furnace of the same type. Standard shaped refractories are always machine pressed and thus have uniformity of properties. Special shapes are usually hand molded and are normally associated with slight variation in properties.
  • Unshaped refractories – These refractories are without definite form and are only given shape after application. It forms joint less lining and are also known as monolithic refractories. These refractories are further categorized as plastic refractories, ramming mixes, castables, gunning mixes, fettling mixes and mortars.

Classification based on method of manufacture – Refractories can be classified by the method of their manufacture. The usual methods of manufacture are (i) dry press process, (ii) fused cast, (iii) hand molded, (iv) formed (normal, fired or chemical bonded) and (v) unformed (monolithic, plastics, ramming masses, gunning, castables and spray masses).

Classification according to refractoriness – Refractories are classified into following four types according to their refractoriness.

  • Low heat duty refractories – These refractories have refractoriness in the range of 1520 deg C to 1630 deg C and have pyrometric cone equivalent (PCE) value in the range of 19 to 28. Example of these refractories is silica bricks.
  • Intermediate heat duty refractories – These refractories have refractoriness in the range of 1630 deg C to 1670 deg C and have pyrometric cone equivalent (PCE) value in the range of 28 to 30. Example of these refractories is fire clay bricks.
  • High heat duty refractories – These refractories have refractoriness in the range of 1670 deg C to 1730 deg C and have pyrometric cone equivalent (PCE) value in the range of 30 to 33. Example of these refractories is chromite bricks.
  • Super heat duty refractories – These refractories have refractoriness greater than 1730 deg C and have pyrometric cone equivalent (PCE) value greater than 33. Example of these refractories is magnesite bricks.

Classification based on the oxide content – Refractories are classified as (i) single oxide refractories such as alumina, magnesia, and zirconia, (ii) mixed oxide refractories such as spinel and mullite and (iii) non oxide refractories such as borides, carbides and silicates.

Classification based on the density of the refractory – Refractories are classified as dense or insulating types. The most high temperature refractories are high density and these high density bricks have resistance to slags of different chemical compositions, fumes, dust and gases. On the other hand insulating refractories are of low densities and provide insulating properties besides offering resistance to corrosion and chemical reactions with the operating environment.

Important refractories

The following are some of the important refractories.

  • Fireclay refractories – Fireclay refractories are essentially hydrated aluminum silicates with minor proportion of other minerals. These refractories have maximum share of the production of refractories on a volume basis. The composition of fireclay refractories consists of SiO2 less than 78 % and Al2O3 less than 44 %. Fireclay refractories are least costly and are used extensively. ASTM subdivides fireclay refractories into four major classifications depending primarily upon fusion temperature (PCE). Four standard classes of fireclay refractories are super duty, high duty, medium duty and low duty. These classes cover the range from around 18 % alumina to 44 % alumina and from about 50 % silica to 80 % silica. Characteristically, fireclay bricks begin to soften far below their fusion temperature and under load actual deformation take place. The amount of deformation depends upon the load and once started this deformation is slow but continuous process unless the load or the temperature is reduced. Due to this reason fire clay bricks are not being used in wide sprung arches in furnaces operating continuously at high temperatures.
  • Alumina refractories – Alumina refractories contain alumina (Al2O3) which is one of the most chemically stable oxides. Alumina offers excellent hardness, strength and spalling resistance. It is insoluble in water, superheated steam and in most inorganic acids and alkalis. Alumina refractories carry all purpose characteristics of fireclay refractories into higher temperature ranges which make these refractories suitable for lining furnaces up to 1850 deg C. Alumina refractories have high resistance in oxidizing and reducing atmospheres. With increase in alumina content, the refractoriness of the high alumina refractories increases. These refractories are specified by the amount of alumina in it. The 50 %, 60 %, 70 % and 80 % classes contain their respective alumina content with an allowable range of +/- 2.5 %. Alumina bricks with 72 % alumina and 28 % silica are known as mullite bricks. These bricks have excellent volume stability and strength at high temperatures. Alumina refractories with 99 % alumina are called corundum refractories. These refractories contain single phase poly crystalline and alpha alumina. A stack of standard alumina refractory bricks is shown in Fig 1.

Alumina refractories

Fig 1 A stack of standard alumina bricks.

  • Silica refractories – Silica refractories are those refractories which contain at least 93 % silica (SiO2). These refractories have second highest share of the production of refractories on a volume basis. Silica refractories have the outstanding property of excellent mechanical strength at temperatures approaching their actual fusion point. This property of silica refractories contrast that of many other refractories which begins to fuse and creep at temperatures considerably lower than their fusion points. The major drawback of silica refractories is that they are susceptible to spalling at temperatures below 650 deg C. Temperature fluctuations above 650 deg C donot affect silica refractory adversely and in this range it is classed as a good spalling resistant refractory. Silica refractories need special precaution during heating and during cooling since it undergoes phase changes. Silica refractories are not of practical use if the furnace is to cool down to room temperature frequently.
  • Magnesite refractories – Magnesite refractories are chemically basic refractories containing at least 85 % magnesium oxide. These are manufactured either from natural occurring magnesite or sea water magnesia. Raw magnesite is dead burnt to produce magnesia (MgO) for making these refractories. These refractories have excellent resistance to basic slags specially lime and iron rich slags but their physical properties are relatively poor. For steel making furnaces especially BOF (basic oxygen furnace) normally carbon is added to magnesia to produce magnesia carbon refractories. Magnesia carbon refractories have better resistance to highly basic slags at high temperatures.
  • Dolomite refractories – These are basic refractories made from dead burnt dolomite. Dolomite (CaCO3+MgO3) when dead burnt by high temperature firing produce CaO+MgO. For dolomite refractories CaO+MgO greater than 97 % is desirable. This percentage of CaO+MgO is usually obtained from high purity dolomite. Dolomite refractories have very good resistance to thermal shock and alkali attack. These refractories with zirconia enrichment are used for crack arresting.
  • Chromite refractories – In these refractories along with chromite, magnesite is present. There is diffrerence between chrome magnesite and magnesite chrome refractories. While chrome-magnesite refractories usually contain 15 % to 35 % Cr2O3 an 42 % to 50 % MgO, magnesite-chrome refractories contain at least 60 % MgO and 8 % to 18 % Cr2O3. Chrome magnesite refractories are used for building the critical paths of the high temperature furnaces. These refractories can withstand corrosive slags and gases and have high refractoriness. Magnesite refractories are suitable for service at the highest temperatures an in contact with more basic slags. These refractories has better spalling resistance than chrome magnesite refractories.
  • Carbon refractories – In these refractories the principle component is carbon. These refractories are characterized by a high refractoriness, high thermal conductivity and high chemical resistance but are highly susceptible to oxidation. Beccause of low interfacial tension between carbon and slag melts, there is little slag infiltration. Carbon refractories are extremely resistant to thermal shock because of high thermal conductivity and low thermal expansion. carbon refractories are susceptible to attack by oxygen, steam and CO2 in an oxidizing atmosphere above 400 deg C.
  • Zirconia refractories = Zirconia refractories contain Zirconium dioxide (ZrO2) which is a polymorphic material. It has certain difficuties in its usage  and fabrication as a refractoy material and hence it is stablized by incorporating small quantities of calcium, magnesium and cerium oxides. The properties of zerconia refractories are dependent on the degree of stabiliation and quantity of stabilizer as wll as the quantity of original raw material. Zirconia refractories have a very high strength at room temperature which is maintained up to temperatures as high as 1480 deg C. They are used for high temperature application. Since the thermal conductivity of zirconium dioxide is much lower than other refractory materials, it is, therefore, used as a high temperature insulating refractory. Zirconia refractories have very low thermal losses and does not reacts with liuid metals and hence it is useful making refractory crucibles.
  • Monolithic refractories – Monolithic refractories  is the name given to all unshaped refractory materials which are installed as some form of suspension that ultimately harden to form a solid mass. The advantges of these refractories are (i) elimination of joints, (ii) faster application, (iii) heat saving, (iv) better spalling resistance, (v) volume stability, (vi) easy to transport, handle and instal and (vii) reduced downtime for repairs. Different techniques used for the placement of these refractories are ramming, casting, gunniting and spraying etc. Types of these refractories are castable refractories, insulating castables, plastic refractories, ramming mixes, patching refractories, coating refractories, mortars, gunning and fettling mixes etc.
  • Insulating refractories – These are high porosity refractories with low thermal conductivity used to reduce the rate of heat flow and thus reduce heat losses by maximizing heat conservation within a furnace. These refractories are lighter with low densities. The three basic types of insulating refractories are (i) very thin low density fibres made from organic or inorganic materials, (ii) cellular material in closed or open cell made of organic or inorganic material (iii) Flaked or granular inorganic materials bonded in the desired form.