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Minggu, 12 April 2009

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. Table 5.5 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.

TABLE 5.5 TYPICAL REFRACTORY PROPERTIES

Property

High Thermal Mass(High Density Refractories)

Low Thermal Mass (Ceramic Fibre)

Thermal Conductivity, W/m K

1.2

0.3

Specific Heat, J/kg K

1000

1000

Density ,kg/m3

2300

130

Typical Refractories in Industrial Use

Depending on the area of application such as boilers, furnaces, kilns, ovens etc, temperatures and atmospheres encountered different types of refractories are used. Typical installations of refractories are shown in Figure 5.6

Figure 5.6

Fireclay Refractories

Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminium silicates with various amounts of silica ranging from SiO2 content of less than 78% and containing less than 44% of Al2O3.

Table 5.6 shows that as the quantity of impurities increases and the amount of Al2O3

5. Insulation & Refractories

decreases, the melting point of fireclay brick decreases. Owing to its relative cheapness and widespread location of the raw materials used to manufacture firebricks, this material finds use in most furnaces, kilns, stoves, etc.

Firebrick is the most common form of refractory material. It is used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and by many others.

TABLE 5.6 PROPERTIES OF TYPICAL FIRECLAY BRICKS

Brick

Percent SiO2

Percent Al2O3

Other Constituents

PCE °C

Super Duty

49-53

40−44

5-7

1745-1760

High Duty

50-80

35-40

5-9

1690-1745

Intermediate

60-70

26-36

5-9

1640-1680

HighDuty (Siliceous)

65-80

18-30

3-8

1620-1680

Low Duty

60-70

23-33

6-10

1520-1595

High Alumina Refractories

Alumino silicate refractories containing more than 45% alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100%. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories includes the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.

Silica Brick

Silica brick (or Dinas) is a refractory material containing at least 93% SiO2. The raw material is quality rocks. Various grades of silica brick have found extensive use in the iron and steel melting furnaces. In addition to high fusion point multi-type refractories, the other important properties are their high resistance to thermal shock (spalling) and their high refractoriness. It finds typical use in glass making and steel industry.

The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. This behaviour contrasts with that of many other refracto­ries, for example alumino silicate materials, which begin to fuse and creep at temperatures con­siderably lower than their fusion points. Other advantages are flux and stag resistance, volume stability and high spalling resistance.

Magnesite

Magnesite refractories are chemically basic materials, containing at least 85% magnesium oxide. They are made from naturally occurring magnesite (MgCO3). The properties of magne­site refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio of less than 2 with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions. The slag resistance is very high particularly to lime and iron rich slags.

Chromite Refractories

Here, a distinction must be made between chrome-magnesite refractories and magnesite-chromite-refractories. Chromemagnesite material usually contain 15-35% Cr2O3 and 42-50% MgO whereas magnesite-chromite refractories contain at least 60% MgO and 8-18% Cr2O3.

Chrome-magnesite refractories are made in a wide range of qualities and are used for build­ing the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness. The magnesite-chromite products are suitable for service at the highest temperatures and in contact with the most basic slags used in steel melt­ing. Magnesite-chromite usually ahs a better spalling resistance than chrome-magnesite.

Zirconia Refractories

Zirconium dioxide (ZrO2) is a polymorphic, material. There are certain difficulties in its usage and fabrication as a refractory material. It is essential to stabilize it before application as a refractory. This is achieved by incorporating small quantities of calcium, magnesium and ceri¬um oxide, etc. Its properties depend mainly on the degree of stabilization and quantity of stabi¬lizer as well as the quality of the original raw material. Zirconia refractories have a very high strength at room temperature which is maintained upto temperatures as high as 1500 °C. They are, therefore, useful as high temperature constructional materials for furnaces and kilns. The thermal conductivity, of zirconium dioxide is found to be much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory.
Since Zirconia exhibits very low thermal losses and does not react readily with liquid metals, it is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Zirconia is a useful refractory material for glass furnaces primarily since it is not easily wetted by molten glasses and because of its low reaction with them.

Oxide Refractories (Alumina)

Alumina refractory materials which consist of aluminium oxide with little traces of impurities are often known as pure alumina. Alumina is one of the most chemically stable oxides known. It is mechanically very strong, insoluble in water and super heated steam, and in most inorgan­ic acids and alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodi­um carbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850 °C.

Monolithics


Monolithic refractories (single piece cast in the shape of equipment such as one for a ladle shown in Figure 5.7) are replacing the conventional type fired refractories at a much faster rate in many applications including those of industrial furnaces. The main advantages being:

· It eliminates joints which is an inherent weakness

· Method of application is faster and skilled measures in
large number are not required

· Transportation and handling are simple

· Offers better scope to reduce downtime for repairs

· Offers considerable scope to reduce inventory and eliminate special shapes

· It is a heat saver

· Has better spalling resistance

· Has greater volume stability

Various means are employed in the placement of monolithics like ramming, casting, gunniting, spraying, sand slinging, etc. Ramming masses are used mostly in cold applications where proper consolidation of the material is important. The same practice can be adopted with both air setting and heat setting materials. Proper ramming tools need to be selected.

Castables by name implies a material of hydraulic setting in nature. Calcium aluminate cement being the binder, it will have to be stored properly to prevent moisture absorption. Further its strength starts deteriorating after a period of 6 to 12 months.

Insulating Materials

Insulating materials greatly reduce the heat losses through walls. Insulation is effected by providing a layer of material having a low heat conductivity between the internal hot surface of a furnace and the external surface, thus causing the temperature of the external surface reduced.

The insulating materials may be classified into the following groups:

· Insulating bricks

· Insulating Castables

· Ceramic fibre

· Calcium silicate

· Ceramic coating

Insulating materials owe their low conductivity to their pores while their heat capacity depends on the bulk density and specific heat. Structure of air insulating material consists of minute pores filled with air which have in themselves very low thermal conductivity, excessive heat affects all insulation material adversely, but the temperatures to which the various materials can be heated before this adverse effect occurs differ widely. Clearly, therefore, the choice of an insulating material must depend upon its effectiveness to resist heat conductivity and upon the temperature that it will withstand.

One of the most widely used insulating materials is diatomite, also known as kiesel guhr which is made up of a mass of skeletons of minute aquatic plants deposited thousands of years ago on the beds of seas and lakes. Chemically this consists of silica contaminated with clay and organic matter. A wide range of insulating refractories with wide combinations of properties are now available.

The important physical properties of some insulating refractories are shown in the Table 5.7.

TABLE 5.7 PHYSICAL PROPERTIES OF INSULATING REFRACTORIES

Type

Thermal conductivity at 400 °C

Max. safe temperature °C

Cold Crushing Strength Kg/cm2

Porosity %

Bulk density Kg/m3

Diatomite Solid Grade

0.025

1000

270

52

1090

Diatomite Porous Grade

0.014

800

110

77

540

Clay

0.030

1500

260

68

560

High Aluminia

0.028

1500–1600

300

66

910

Silica

0.040

1400

400

65

830

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