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Fuels, Furnaces & Refractories

3. Refractories

Dr. Eng. Yazan Al-ZainDepartment of Industrial Engineering

University of Jordan

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Classification of Refractories

• There is no general definition of a refractory. Essentially, it is a material of high melting point.

– This is a relative term and melting point is not the only criterion of usefulness.

• Most refractories are ceramic materials made from high-melting- point oxides, particularly SiO2 , Al2 O3 and MgO.

• Carbon is an important refractory, as well as carbides, borides and nitrides (being developed for high-temperature work).

• Metals of high melting points such as W & Mo are refractory metals and find uses in research apparatus.

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Classification of Refractories

• The most useful classification that depends on the behavior of refractories toward metallurgical slags is as follows:

– Acid refractories: based on SiO2 and include (1) silica and the fireclay series with 30-42% Al2 O3 ,and (2) sillimanite and andalusite with about 60% Al2 O3 .

– Basic refractories: based on MgO and include magnesite and dolomite, chrome magnesite and magnesite chrome.

– Neutral refractories: include carbon, chromite (FeO.Cr2 O3 ) and forsterite (2MgO.SiO2 ) bricks.

• As a general rule, acid refractories react readily with basic slags, basic refractories are attacked by acid slags, and neutral refractories are relatively inert to both types of slags.

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Properties and Testing

• Properties the refractory material may possess include:

– Rigidity and maintenance of size, shape and strength at the operating temperature.

– An ability to withstand thermal shock.

– Resistance to chemical attack by whatever gas, slag or metal is likely to be encountered.

• As there is no supreme material capable of standing up to every possible condition, the choice must be made to meet the requirements of the job to be done.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– Visual examination: should include general uniformity, texture, etc.

– Dimensional accuracy: the furnace has to be the correct size and shape when bricks are placed together. Edges should be straight, and faces should be flat. This can be done using a ruler or a caliper.

• If the shape is good and surfaces are flat, bricks can be set dry, i.e. without cement.

– After expansion (contraction): determined by heating a sample of bricks for a prolonged time at the proposed working temperature. Dimensions are measured before and after heating.

• If the expansion is too great, more thorough firing is needed or different bricks should be used. High values lead to sever cracking of furnace walls during use.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– Refractoriness: a test done to determine the melting point (or range) of the bricks used.

• A material maybe partly liquid yet appear solid on heating. At a higher temperature where the proportion of liquid exceeds a critical value, the material will collapse or appear liquid or pasty. The temperature at which the material collapses is considered as its refractoriness.

– Cold strength: measured by a simple compression test called the Cold Crushing Strength test. Needed to assure the user that the brick:

• Will not fail in compression when in structure.• Has been properly fired.• Would transport readily without damage to corners and edges.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– Hot strength: very important in high quality bricks but is not measured directly.

• Instead, it is a temperature that is determined; the temperature at which deformation under a standard load is rapid.

• The test is called the Refractoriness Under Load (R.U.L) and is rather like a short time creep test.

– Slag resistance: there are many types of tests, one is as follows:

• Done by drilling holes in the brick and packing these with samples of typical slag, likely to be encountered.

• The assembly is then heated to the working temperature for about an hour, cooled and sectioned.

• The extent of the penetration of the slag into the brick is noted and compared with others.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– True porosity: a popular test on bricks and is generally considered to give a good index of quality.

• Its measurement involves the determination of real and apparent densities by standard methods.

• Low porosity bricks have good resistance to abrasion, slagging and attack gases such as those of the blast furnace.

• Porosity values are usually about 20 – 25%, but lower values down to 10% can also be attained with some difficulty.

– Real density: determined on crushed and ground material by the density bottle method.

• Value gives indication on the degree of firing.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– Apparent density: evacuation in a vacuum desiccators and flooding with paraffin to fill the unsealed pore space, followed by weighing in air and in paraffin is a common method.

• Its value is of little use, unless perhaps to calculate the loads for transport.

– Permeability: measured in a simple apparatus in which air is blown at a measured rate through a cylinder of brick, the pressure drop being measured by a manometer.

• If taken into conjunction with porosity, texture maybe deduced; e.g. low porosity and high permeability may indicate cracks; high porosity and low permeability, closed or very small pores.

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Properties and Testing

• The following tests are commonly carried out on refractories:

– Chemical analysis: carried out by standard classical methods, such as X-ray diffraction.

• Results of analyses indicate primarily the type of brick.

• Details of the analysis may also indicate the quality of the brick within its class.

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Alumino-Silicate Refractories

• The Al2 O3 –SiO2 phase diagram is shown below, with the compositions of some refractories marked on it.

• The composition changes from super silica at very low Al2 O3 content to alumina at very low SiO2 content.

• It can be seen that the liquid phase is present at 1587 ºC when the content of Al2 O3 is below 60 wt.%, while solid phases are stable up to 1890 ºC when the content of Al2 O3 exceeds 60 wt.%.

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Alumino-Silicate Refractories

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3 5

6

Super Silica

Silica

Firebrick

Kaolinite

Mullite

Alumina

1

2

3

4

5

4

6

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Alumino-Silicate Refractories Firebricks & Origin of Fireclay

• Firebricks are the most common class of refractory, the common brick of the furnace designer.

– They are made from fireclay which usually occurs in association with coal measures.

– Clays are produced by the decomposition of igneous rocks by geological agencies.

– The kind of clay depends on the kind of parent rock and on the treatment it has had.

– Fireclay is derived from acid rocks like granite, in which felspars of the type K2 O.Al2 O3 .6SiO2 are decomposed by H2 O and CO2 (possibly at high temp. and press.) to give K2 CO3 , SiO2nH2 O, and Al2 O3 .SiO2 .2H2 O. The last formula is that of kaolinite.

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Alumino-Silicate Refractories Firebricks- Structure of Fireclays

• Structure of clays is similar in that each is made of alternate layers of “silica” and “gibbsite” (Al(OH)3 ). The latter has itself a layered structure, a sheet of aluminum ions (Al3+) being sandwiched between similar layers of hydroxyl ions (OH–), each Al3+ ion being associated with 6(OH–) ions, each of which has a share in two Al3+

ions.

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Alumino-Silicate Refractories Firebricks- Structure of Fireclays

• The silica layers are also made up of three sheets of ions. The middle sheet is of hexagonally arranged Si4+ with O2–

ions tetrahedrally arranged around them so that on one side there are three O2–

ions, each sharing two Si4+ ions, and on the other side one O2–

ion per Si4+ ion, this one unshared. The net effect is that there are too many O2–

ions for electrical stability and the silica sheet achieves its stability by incorporating its unshared O2–

ions into the gibbsite layer where they replace some of the OH–

ions.

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Alumino-Silicate Refractories Manufacture of Firebricks

• Manufacture of firebricks follows three stages, as follows:

– Smoking or Steaming of the starting raw material (clay), starting from 20 ºC up to 300 ºC for 12 to 48 hours under reducing conditions. Moisture held within the clay particles will be expelled.

– Decomposition: done at temperatures up to 900 ºC in an oxidizing flame for 10 to 24 hours. Clays decompose over 500 ºC, combined water being driven off leaving an amorphous residue called “meta- kaolinite”.

• Any free a-quartz is converted to b-quartz above 573 ºC.

• Carbon and sulfur will burn out.

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Alumino-Silicate Refractories Manufacture of Firebricks

• Manufacture of firebricks follows three stages, as follows:

– Full Firing: done up to 1200 to 1400 ºC for 12 to 18 hours. The formation of silicates proceeds from 1000 ºC onwards.

• Silica and alumina are converted to high temperature modifications and combine to form mullite at temperatures above about 1100 ºC.

• Under-firing leaves the center fragile and weak.

• Over-firing may induce slumping (collapse) and causes high susceptibility to thermal shock.

• The whole operation takes 3 to 5 days and the final structure will be mullite needles in a matrix of glass (amorphous silica).

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Alumino-Silicate Refractories Microstructure of Mullite

Mullite needles in a matrix of amorphous silica.

Mullite needles

Amorphous silica

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Alumino-Silicate Refractories Properties and Uses of Firebricks

• Firebricks are classified by alumina content as follows:

– Aluminous firebricks: contain 38 to 45% Al2 O3 , balance is SiO2 .

– Ordinary firebricks: contain 22 to 38% Al2 O3 , balance is SiO2 . Nevertheless, bricks with low than 32 % Al2 O3 are of low quality and referred to as siliceous firebricks.

– Semi-silica bricks: contain between 10 and 22% Al2 O3 .

• Other oxides maybe also found; e.g. titanium oxide may range between 0.2 to 1.5 %. Fe2 O3 may come up to 4%. K2 O between 0.2 and 1.5 % but should be less than 0.5% and CaO and MgO may account for 1 to 2%.

– They are important in forming silicate bond during firing. However, they act as slag in any attack on the brick. The lower their values the better.

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Alumino-Silicate Refractories Properties and Uses of Firebricks

• Refractoriness rises from 1550 ºC to 1750 ºC as alumina rises from 25 to 45%.

• Under load, softening is evident at about 1500 ºC or earlier depending on composition.

• Spalling (flaking) resistance is usually good, the higher the Al2 O3 the better.

• Resistance to acid slags is generally good while resistance to FeO, basic slags and alkalies poor.

• Slag resistance depends to some extent on texture and is better in high Al2 O3 grades, partly because of the Al2 O3 and party because the proportion of alkalies and other fluxes is often rather low in these more expensive bricks.

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Alumino-Silicate Refractories Properties and Uses of Firebricks

• Firebricks are the common brick of the furnace designer.

– They appear in various industrial furnaces where heat is applied.

– They also find uses in domestic fireplaces and stoves.

– They are moderately cheap and the material is indigenous and traditional.

– In blast furnace, they appear in the hearth, walls and ladles.

– They also appear in open hearth furnaces.

– They appear in molds for casting, particularly in runners for molten metal.

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Alumino-Silicate Refractories High Alumina Bricks

• Bricks can be made up to 70 to 73% alumina, called mullite bricks and contain very little silica after firing.

• Alumina bricks are made entirely of bauxite with small amount of clay added as a binder. (Bauxite is an aluminum ore and is the main source of aluminum).

• These bricks remain solid up to 1810 ºC. These bricks are too expensive and their use is limited to places where conditions are severe.

– They have excellent spalling resistance, excellent cold crushing strength and very good hot strength.

– Although they have very good high strength, they can be used where the bricks are liable to heavy mechanical wear and abrasion.

• Resistance to slagging is much better than normal firebricks, where resistance to acid slags, glasses, FeO and lime is very good, so they can be used in glass tanks.

• Mullite bricks are replacing firebricks in many applications such as in blast furnace and in the regenerators in open hearth furnaces.

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Silica Bricks

• Silica bricks contain not less than 95% SiO2 and with less than 1% Al2 O3and 0.3% alkalies.

• Ganister, the ore from which silica is made. It is clay-bonded sandstone and has been used in furnace construction for years.

• Ganister contains up to 98% SiO2 which is suitable for brick-making.

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Silica Bricks Allotropy in Silica

• There are said to be 15 modifications of silica, the important forms are as follows:

– The naturally occurring form is -quartz, which transforms rapidly and reversibly to β-quartz at 573 ºC.

– Volume change is 1.35%, large enough to shatter a piece of the material or an unfired bricks unless heating past the temperature is very slow.

– β-quartz is stable up to 867 ºC, but may persist to much higher temperatures.

– β-quartz transforms to tridymite if catalyzed by a mineralizer such as calcium tungstate.

– Above 1470 ºC, cristobalite is the stable form. It remains stable to the melting point at 1723 ºC.

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Silica Bricks Manufacture

• Crushing of ore should aim at angular particles and grinding to smaller sizes is important.

• Lime water (saturated solution of Ca(OH)2 in water) is added to about 1.7% CaO to (1) form temporary bond and (2) also gives some plasticity.

• Molding is done using the dry press method (water less than 5%).

• Firing may take up to 2 weeks and should attain about 1500 ºC.

– Heating past 573 ºC must be slow to accommodate quartz and cooling between 300 and 100 ºC must also be very slow to avoid cracking as the high temperature forms of tridymite and cristobalite transform to the room temperature modofications.

– The resulting brick is a mixture of tridymite and cristobalite depending on the firing temperature and time

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Silica Bricks Manufacture

• Bonding of the particles in the bricks is partly by the formation of calcium silicate glass and partly through an interlocking action between tridymite needles.

• The most valuable property of silica bricks is a very high R. U. L. which is due to the tridymite bond, and rigidity can be maintained under load very close to the melting points.

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Silica Bricks Properties and Uses

• Refractoriness of silica bricks is around 1710 ºC.

• Very good spalling resistance above 300 ºC.

• Slagging resistance is extremely good, considering the extremely acidic nature of the brick.

• Permeability is very low due to low porosity (held below 15%).

• Due to its low density (1800 kg/m3), silica bricks are fairly light.

• Used as roofs of open hearth steelmaking furnaces and walls for coke ovens. This is due to its ability to withstand the compressive forces in arched roofs at temperatures up to 1600 ºC (as its lightness reduced these forces to minimum).

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Carbon Refractories

• Carbon bricks are made from coke, crushed, size-graded and bonded with tar or pitch.

• The mixture is molded and fired at about 1000 ºC to make the grade called “carbon” or up to 2500 ºC to produce graphite.

• These materials are stronger than natural graphite and more readily produced in the shapes and sizes required by industry.

• Graphite has higher electrical and thermal conductivities than carbon and is rather denser.

• Carbon bricks have excellent hot strength, and good resistance to thermal spalling.

• They are not subject to chemical attack except by oxidizing slags and oxidizing gases (air, CO2 and H2 O) where graphite is rather more resistant than carbon.

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Carbon Refractories

• One of the most important properties of these materials is their excellent machinability which enables complex shapes to be made.

• Both carbon and graphite are used as electrodes in arc furnaces and also in electrolysis plant such as that producing aluminum.

– Graphite is preferred where a high current density is involved for its lower electrical resistivity, but it is considerably more costly.

• When high resistivity is desired, such as electrical resistors in resistance furnaces, then carbon is preferred for its higher resistivity.

• Main application of carbon bricks is in blast furnace as linings.

• Also used as linings for furnaces making phosphorous, calcium carbide, aluminum and magnesium.

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Carbon Refractories

• Crucibles for melting cast iron and other metals are made from fireclay and natural graphite.

– It gradually oxidizes so that carbon is lost, but it is an economical refractory for use under moderately severe conditions.

• In powder metallurgy, graphite is used for molds and plungers for hot pressing.

• In laboratories, it appears as crucibles, as resistors and heating elements.

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Insulating Refractories

• All refractories are insulating in some degree, but when it is desirable to minimize the loss of heat without enlarging the structure careful selection from a small group of special materials is desirable.

• It is necessary to reduce heat flow by radiation, convection and conduction.

– Radiation is minimized by baffling: a single screen will reduce radiation transfer between two points by at least 50%. Two screens will reduce it by over 67%, three by over 75%, and so on.

– To reduce convective transfer, the free air space around the hot zone should be divided up into small compartments to that circulation in each is limited.

– To minimize conduction, the hot zone must be surrounded by a substance of low thermal conductivity at its operating temperature. Gases, except H and He, have very low conductivities and air is the obvious choice.

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Insulating Refractories

• The ideal insulation is then: some kind of honeycomb structure of minute cell dimensions, filled with air and constructed of something with very low conductivity and extremely thin walls. It should be rigid at the operating temperature.

– Up to about 1500 ºC, porous firebrick is available. It is made from fireclay, mixed with hard wood sawdust or chip. The wood burns out on firing so leaving a light porous brick.

– A thin blanket of wool can be put on the inside casing of the furnace to maximize the low temperature insulation effect.