Refractory

The salient properties of these materials include the capacity to with stand high temperatures without melting or decomposing and the capacity to re main unreactive and inert when exposed to severe environments. In addition, the ability to provide thermal insulation is often an important consideration. Refractory materials are marketed in a variety of forms, but bricks are the most common. Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation.
Of course, the performance of a refractory ceramic depends to a large degree on its composition. On this basis, there are several classifications—fireclay, silica, basic, and special refractories. Compositions for a number of commercial refractories are listed in Table 13.2. For many commercial materials, the raw ingredients consist of both large (or grog) particles and fine particles, which may have different compositions. Upon firing, the fine particles normally are involved in the formation of a bonding phase, which is responsible for the increased strength of the brick; this phase may be predominantly either glassy or crystalline. The service temperature is normally below that at which the refractory piece was fired.

Porosity is one microstructural variable that must be controlled to produce a suitable refractory brick. Strength, load-bearing capacity, and resistance to attack by corrosive materials all increase with porosity reduction. At the same time, thermal insulation characteristics and resistance to thermal shock are diminished. Of course, the optimum porosity depends on the conditions of service.

The general requirements of a refractory material can be summed up as :



Fireclay Refractories
The primary ingredients for the fireclay refractories are high-purity fireclays, alumina and silica mixtures usually containing between 25 and 45 wt% alumina.According to the SiO2-Al2O3 phase diagram, Figure 12.27, over this composition range the highest temperature possible without the formation of a liquid phase is 1587C (2890F). Below this temperature the equilibrium phases present are mullite and silica (cristobalite). During refractory service use, the presence of a small amount of a liquid phase may be allowable without compromising mechanical integrity. Above 1587C the fraction of liquid phase present will depend on refractory composition. Upgrading the alumina content will increase the maximum service temperature, allowing for the formation of a small amount of liquid.
Fireclay bricks are used principally in furnace construction, to confine hot atmospheres, and to thermally insulate structural members from excessive temperatures. For fireclay brick, strength is not ordinarily an important consideration, because support of structural loads is usually not required. Some control is normally maintained over the dimensional accuracy and stability of the finished product.

Silica Refractories
The prime ingredient for silica refractories, sometimes termed acid refractories, is silica. These materials, well known for their high-temperature load-bearing capacity, are commonly used in the arched roofs of steel- and glass-making furnaces; for these applications, temperatures as high as 1650C (3000F) may be realized. Under these conditions some small portion of the brick will actually exist as a liquid. The presence of even small concentrations of alumina has an adverse influence on the performance of these refractories, which may be explained by the silica–alumina phase diagram, Figure 12.27. Because the eutectic composition (7.7 wt% Al2O3) is very near the silica extremity of the phase diagram, even small additions of Al2O3 lower the liquidus temperature significantly, which means that substantial amounts of liquid may be present at temperatures in excess of 1600C (2910F). Thus, the alumina content should be held to a minimum, normally to between 0.2 and 1.0 wt%.
These refractory materials are also resistant to slags that are rich in silica (called acid slags) and are often used as containment vessels for them. On the other hand, they are readily attacked by slags composed of a high proportion of CaO and/or MgO (basic slags), and contact with these oxide materials should be avoided.

Basic Refractories
The refractories that are rich in periclase, or magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. The presence of silica is deleterious to their high-temperature performance. Basic refractories are especially resistant to attack by slags containing high concentrations of MgO and CaO and find extensive use in some steel-making open hearth furnaces.

Special Refractories
Yet other ceramic materials are used for rather specialized refractory applications. Some of these are relatively high-purity oxide materials, many of which may be produced with very little porosity. Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2), and mullite (3Al2O3-2SiO2). Others include carbide compounds, in addition to carbon and graphite. Silicon carbide (SiC) has been used for electrical resistance heating elements, as a crucible material, and in internal furnace components. Carbon and graphite are very refractory, but find limited application because they are susceptible to oxidation at temperatures in excess of about 800C (1470F). As would be expected, these specialized refractories are relatively expensive.
Properties of Refractories

Some of the important properties of refractories are:
Melting point:
Pure substances melt sharply at a definite temperature. Most refractory materials consist of high melting particles bonded together. At high temperature, glass fuses and as the temperature rises, the resulting slag increases in quantity by partial solution of the refractory particles. The temperature at which this action results in failure of a test pyramid (cone) to support its own weight is called, for convenience, the melting point of the refractory. Table 5.4 shows the melting point of some pure compounds used as refractories.
Size:
The size and shape of the refractories is a part of the design feature. It is an important feature in design since it affects the stability of any structure. Accuracy and size is extremely important to enable proper fitting of the refractory shape and to minimize the thickness and joints in construction.

Bulk density:
A useful property of refractories is bulk density, which defines the material present in a given volume. An increase in bulk density of a given refractory increases its volume stability, its heat capacity, as well as resistance to slag penetration.

Porosity:
The apparent porosity is a measure of the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume. This is an important property in cases where the refractory is in contact with molten charge and slags. A low apparent porosity is desirable since it would prevent easy penetration of the refractory size and continuity of pores will have important influences on refractory behaviour. A large number of small pores is generally preferable to an equivalent number of large pores.

Cold crushing strength:
The cold crushing strength, which is considered by some to be of doubtful relevance as a useful property, other than that it reveals little more than the ability to withstand the rigors of transport, can be used as a useful indicator to the adequacy of firing and abrasion resistance in consonance with other properties such as bulk density and porosity.

Pyrometric cone equivalent (PCE):
Temperature at which a refractory will deform under its own weight is known as its softening temperature which is indicated by PCE. Refractories, due to their chemical complexity, melt progressively over a range of temperature. Hence refractoriness or fusion point is ideally assessed by the cone fusion method. The equivalent standard cone which melts to the same extent as the test cone is known as the pyrometric cone equivalent. Thus in the Figure 5.5 refractoriness of Sample A is much higher than B and C. The pyrometric cone equivalent indicates only the softening temperature. But, in service the refractory is subjected to loads which would deform the refractory at a much lower temperature than that indicated by PCE. With change in the environmental conditions, such as reducing atmosphere, the P.C.E. value changes drastically.

MELTING POINTS OF PURE COMPOUNDS
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Pure Compound Formula Melting Temperature ℃
Alumina Al2O3 2050
Lime CaO 2570
Chromite FeOCr2O3 2180
Chromium Oxide Cr2O2 2275
Megnesia MgO 2800
Silica SiO2O 1715
Titania TiO2O 1850

Refractoriness under load (RUL):
The refractoriness under load test (RUL test) gives an indication of the temperature at which the bricks will collapse, in service conditions with similar load.

Creep at high temperature:
Creep is a time dependent property which determines the deformation in a given time and at a given temperature by a material under stress.

Volume stability, expansion, and shrinkage at high temperatures:
The contraction or expansion of the refractories can take place during service. Such permanent changes in dimensions may be due to:
i) The changes in the allotropic forms which cause a change in specific gravity.
ii) A chemical reaction which produces a new material of altered specific gravity.
iii) The formation of liquid phase.
iv) Sintering reactions.
v) It may also happen on account of fluxing with dust and stag or by the action of alkalies on fireclay refractories, to form alkali-alumina silicates, causing expansion and disruption.
This is an example which is generally observed in blast furnaces.

Reversible Thermal Expansion:
Any material when heated, expands, and contracts on cooling. The reversible thermal expansion is a reflection on the phase transformations that occur during heating and cooling.

Thermal Conductivity:
Thermal conductivity depends upon the chemical and mineralogical compositions as well as the glassy phase contained in the refractory and the application temperature. The conductivity usually changes with rise in temperature. In cases where heat transfer is required though the brick work, for example in recuperators, regenerators, muffles, etc. the refractory should have high conductivity. Low thermal conductivity is desirable for conservation of heat by providing adequate insulation.
The provisions for back-up insulation, conserves heat but at the same time it increases the hot face temperature and hence the demand on the refractory quality increases.
Accordingly, insulation on the roof in open hearth furnaces is normally not provided, otherwise it would cause failure due to severe dripping. Depending on the characteristic of the refractory used in the hot face, such as the high temperature load bearing capacity, it may be required that the quality of the brick be increased to match the rise temperature caused by over insulation.
Light weight refractories of low thermal conductivity find wider applications in the moderately low temperature heat treatment furnaces, where its primary function is usually conservation of energy. It is more so in case of batch type furnaces where the low heat capacity of the refractory structure would minimize the heat storage during the intermittent heating and cooling cycles.

Classification of Refractories

Refractories can be classified on the basis of chemical composition and use and methods of manufacture as shown below:
Classification based on Chemical composition Examples
ACID → which readily combines with bases. Silica, Semisilica, Aluminosilicate.
BASIC → which consists mainly of metallic oxides which resist the action of bases. Magnesite, chromemagnesite, Dolomite.
NEUTRAL →which doesn't combine; neither with acids nor bases. Chrome, Pure. Alumina
Special Carbon, Silicon Carbide, Zirconia.
Classification based on end use. Blast furnace Casting Pit
Classification based on method of manufacture
  • Dry Press Process
  • Fused Cast
  • Hand Moulded
  • Formed Normal, (fired or Chemically bonded.)
  • Unformed (Monolithics-plastics,Ramming Mass, Gunning Castable,Spraying.)


Mineral-based refractories are classified according to their chemical composition:
i. Acid bricks contain at least 92%~ silicon oxide (SiO2).
ii. Semi-basic bricks contain at least 65% silicon oxide. but less than 30% alumina (A12O3).
iii. Neutral bricks contain at least 30% alumina.
iv. Basic bricks contain at least 60% magnesium oxide (MgO).
v. Synthetic refractories e.g. silicon carbide are produced by melting and casting processes.
The structure of the furnace consists mainly of refractory bricks and cement, which must be able to withstand the high furnace temperatures and must be carefully selected and constructed. The furnace structure may contain monolithic refractories, which can be shaped in situ, e.g. those used for burner quarls. There are three basic types of monolithic refractories:

• Castables
• Mouldables
• Ramming mixtures

Different furnace zones normally operate at different temperatures. The correct selection of refractory materials for the various parts of the furnace and for various components e.g. hearths, walls, etc, is important. This process is governed not only by properties like thermal conductivity, expansion, etc, but also by the experience of the furnace designer or builder.
The hearth is the most important and the most severely treated region of a furnace. It should be able to bear the required load and withstand chemical attack and mechanical wear. The selection of hearth refractories is less critical for top and bottom fired furnaces, than for top fired only pusher types.
For optimum strength and thermal insulation, the walls, roof and hearth of most furnaces are constructed using layers of refractory materials. Thermal insulation is determined by the thermal properties of the refractory, and these properties are important in minimising transmission and storage heat losses. compares the thermal properties of typical high density and low density refractory materials. Structural heat losses can be reduced by using low thermal mass refractory materials in the construction of the furnace.