Module: 6 Lecture: 28

Transcription

1 Module: 6 Lecture: 28 CLAY PRODUCTS AND REFRACTORIES 2. STRUCTURAL CLAY PRODUCTS Water Raw Clay Grinding Screening Mixing Soaking Maturation Moulding Firing Clay Product Dryer Figure: Manufacturing of Clay Products Block diagram of manufacturing process Diagram with process equipment Animation Unglazed pipes, tiles, terracotta and building bricks are manufactured from locally clays without glaze, which contain the fluxes needed for binding. For building and face bricks, red burning clay is used, which is cheap and durable. Bricks can be manufactured from Soft mud process Stiff mud process Dry press process Soft mud process The clay mixture contain 30% water is moulded in moulds. This mould bricks are dried in tunnel drier. While common bricks are burnt in scove kiln, in which coal is used as fuel. The kiln is built from the green bricks with the outside walls daubed or scoved with clay. It is a variation of undraft type of kiln. After burning the kiln is completely dismantled. N P T E L 182

2 Stiff and mud process Clay mixture contains 12-15% water, which is just enough to stick together. The clay is forced out through a die in a screw or auger machine. The extruded clay bar passes long or short belt conveyor on to a cutting table on which a frame with number of wires which automatically cuts the bar into appropriate lengths. These bricks may be repressed to make face bricks. Dry process The water content of the clay is reduced to 4-7% which make the clay nonplastic. The brick unit is moulded at high temperature. PROPERTIES USES Depending on the content of the soil, clay can appear in various colours, from a dull gray to a deep orange-red. Clays exhibit plasticity when mixed with water in certain proportions, when dry, it becomes firm and when fired in a kiln, permanent physical and chemical changes occur which converted clay into a ceramic material. Because of these properties, clay is used for making pottery items, both utilitarian and decorative. Different types of clay, when used with different minerals and firing conditions, producing earthenware, stoneware, and porcelain. Clay is the starting raw material for manufacturing bricks, tiles, terracotta, pottery, earthenwares, sewer, drain pipes, and covers for electrical cables Clay is one of the oldest building materials on Earth, among other ancient, naturally-occurring geologic materials such as stone and organic materials like wood. Between one-half and two-thirds of the world's population, in traditional societies as well as developed countries, still live or work in a building made with clay as an essential part of its load-bearing structure. Also a primary ingredient in many natural building techniques Used to create adobe, cob, cordwood, and rammed earth structures and building elements such as wattle and daub, clay plaster, clay render case, clay floors and clay paints A traditional use of clay as medicine goes back to prehistoric times. Kaolin clay and attapulgite have been used as anti-diarrheal medicines Used where natural seals are needed, such as in the cores of dams, or as a barrier in landfills against toxic seepage Clay tablets were used as the first known writing medium, inscribed with cuneiform script through the use of a blunt reed called a stylus. N P T E L 183

3 Clays sintered in fire were the first form of ceramic, Bricks, cooking pots, art objects, dishware, and even musical instruments such as the ocarina can all be shaped from clay before being fired. Used in many industrial processes, such as paper making, cement production, and chemical filtering. Used in the manufacture of pipes for smoking tobacco. 3. REFRACTORY MATERIALS Refractories are ceramic materials that can withstand unusually high heat as well as abrasion and the corrosive effects of acids and alkalis. They are used in furnaces, stills for the cracking of petroleum, ceramic kilns, boilers, incinerators, electrolytic cells for aluminum production etc. Refractories are an important constituent of nuclear reactors. Jet engines would not last very long without refractory parts. Fire clay is an important raw material for refractories. In recent years non-clay refractories of alumina, zirconia, silicon carbide, chromia, magnesite, graphite and other less common materials are developed. The cost of these refractories is much higher than that of fireclay. However, their use in critical parts of a furnace will keep it in operating condition for longer periods, with less time taken out for repairs. CLASSIFICATION Acid refractories Basic refractories Neutral refractories or special refractories Acid refractories The prime ingredient for acid refractory is silica. Acid refractory have high temperature loadbearing capacity and are used in the arched roofs of steel and glass making furnaces at temperatures as high as C. At this temperature small portion of the brick will actually exist as a liquid. Alumina should be kept at 0.2 to 1.0% by weight because it adversely effect on the performance of these refractories. These refractory materials are resistant to acid slags that are rich in silica. But they are readily attacked by basic slags composed of CaO and / or MgO, there so contact with these oxides should be avoided. N P T E L 184

4 Acid refractories Fusion temperature 1. Silica(SiO2) C 2. Aluminium silica (46% Al2O3 + 54% SiO2) C 3. Alumina (Al2O3) C 4. Silmanite (63%Al2O3 + 37%SiO2) C Basic refractories The main constituent of basic refractories is magnesia (MgO), along with calcium, chromium and iron compounds. The presence of silica is harmful for use in 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. Basic refractories Fusion temperature 1. Magnesia C 2. Bauxite C 3. Dolomite C Special refractories or neutral refractories The special refractories are relatively expensive due to use of high-purity oxide materials and very little porosity. Alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2) and mullite (3AI2O3.2SiO2), as well as carbide compounds, in addition to carbon and graphite are used for manufacture of special refractories. Silicon carbide (SiC2) has been used for electrical resistance elements, as a crucible material, and in internal furnace components. As carbon and graphite are susceptible to oxidation at temperatures in excess of about C, they find limited application in spite of refractory characteristics. Neutral refractories Fusion temperature 1. Chromite C 2. Graphite C 3. Silicon carbide C 4. Zirconia C Silica and high alumina refractories Refractories are obtained from ores of silica or those of silica and alumina. After mining or chemical production and calcining, refractory materials are crushed, ground and prepared to size. They are then mixed with other materials and shaped as bricks. Bricks are used for lining-melting and other applications. Bricks of acid, N P T E L 185

5 basic and neutral refractories having different shapes are available for furnace construction. Bricks during construction work are boned and cushioned with the help of a mortar consisting of chrome, silicon carbide, silica and alumina. Brick work may be coated with thin mortar for further protecting the same during furnace operation. MANUFACTURE Composition of silica and basic refractory bricks Type of Brick Silica (SiO2) Alumina (Al2O3) Lime (CaO) Magnesia (MgO) Iron oxide (Fe2O3) Chromic oxide (Cr2O3) Other Oxides 1. Silica Basic Chrome Magnesite Forsterite Composition of fireclays and high alumina bricks Type of Brick 1. Fireclay Silica (SiO2) Alumina (Al2O3) Titania TiO2 Other oxides Super duty Semi-Silica Medium duty Low duty High Alumina 60% Alumina % Alumina % Alumina % Alumina Trace 0.6 Water Clay Hot air out Rotary kiln Grinding Dry Press Refractory Bricks Screening Mixing Dryer Hot air in Figure: Manufacturing of Refractory Block diagram of manufacturing process N P T E L 186

6 Diagram with process equipment Animation It involves the following steps Crushing The clay is crushed to 25 mm in size. Grinding The crushed materials are ground in suitable grinding machine down to 200 mesh size. Screening Screening is carried out to separate fine and coarse materials. The desired size material is used for brick making and oversize is recycled to grinding machine. Mineral-dressing Mineral dressing is used to purify the raw materials and producing better refractories. Purification is carried out by settling, magnetic separation and by chemical methods. Storage Pure materials are stored in storage bins using bucket elevators,. Mixing Proper distribution of plastic material throughout the mass is necessary for easy moulding. Moulding It can be accomplished either manually or mechanically at high pressure. Refractories of low density and low strength can be produce by hand-moulding. While mechanical-moulding produces refractories of high density and strength. Deairing is essential in order to increase the density and strength of refractory by mechanical moulding. Deairing It is carried out by Allowing air inside the void space in the refractory to go out by decreasing the rate of pressure application and release N P T E L 187

7 Double-pressing the material viz. first pressed and allowed to crack. Then it is repressed again so as to close the voids. Applying vacuum through vents in the moulds Drying Drying is carried out to remove the moisture from refractories. Drying is carried out in tunnel dryer usually very slowly and under well-set conditions of humidity and temperature, depending upon the type of refractory. Rack cars are placed in a long tunnel and heat is supplied by steam, which passes through the tunnel. Firing To stabilize and strengthen the structures of refractories firing is carried out in tunnel kilns or shaft kilns or rotary kilns. The bricks are generally fired at a temperature as high as or higher than their arc temperature. The firing temperature of different bricks is as follows High-fired super duty bricks : C Kaolin bricks : C Some basic bricks : C PROPERTIES The following properties need to be considered while selecting a refractory material. Refractoriness Refractoriness is indicated in terms of pyrometric cone equivalent (PCE) value which indicates the temperature of softening the refractory. PCE should be well above the operating temperature. As inner end of the refractory wall is at higher temperature compare to the outer end, therefore, unless the brick melts away completely, it can often be used to withstand a temperature higher than its softening temperature. The outer end will be at a lower temperature and still in a solid state, thus giving strength. Strength Strength under the combined effect of temperature and load is an important factor, particularly in taller furnaces, the refractory has to support a heavy load. Refractory must be strong enough to resist physical wetting away and to take load. Refractoriness under load is an important consideration because usually a refractory fails at a lower temperature when subjected to load because some crystals which have become fluidic act as a lubricant and deformation becomes easier when subjected to load. N P T E L 188

8 Specific gravity Specific gravity reflects true porosity of refractory. Porosity increases with increase in bulk density. It is also a criterion while buying the raw material because raw materials of higher specific gravity will give fewer bricks or article per unit weight. Eventually increases the production cost. Refractory material in powdered form is used for making furnace lining or for repairs. Lesser weight of a material with lower specific gravity will be required for this purpose. Materials of lower specific gravity are also preferable because bricks in the lower part of a tall structure will not be subjected to a heavy load Porosity and slag permeability If the refractory has open pores the gases and slags will enter the material more easily and to a greater depth and may react and reduce the life of the refractory. Thermal expansion In furnace design, allowance has to be made for thermal expansion. This becomes a very important factor if the refractory is subjected to rapid changes in temperature as a furnace door which is, on opening, is suddenly exposed to air at room temperature. Within a single brick in the wall there may be a temperature drop of a few hundred degrees from the hot face to the cold face. This causes differences in thermal expansion in different sections of the brick. There may be changes in the internal structures of the material with the result that there is expansion in volume. All these result in the development of high internal stresses. Thermal conductivity It is important factor if the refractory is subjected to rapid changes in temperature. Thermal conductivity should be low so that the heat of the furnace may not be lost. Exceptions to this rule are coke over walls, mulle furnaces and retorts which is heated form outside. They should be made of refractories of high thermal conductivity. To prevent heat loss sometimes a refractory is backed by an insulate material like asbestos. The life of refractories is shorter which are constantly maintained at high temperature. Electrical conductivity It should be low for electrical furnaces. Except graphite, all other refractories are bad conductors of electricity. The electrical resistivity of the refractories should drop rapidly with increase in temperature. Chemical composition The chemical composition of the refractory should be such that the surrounding do not chemically attack the refractory and corrode it. N P T E L 189

9 Characteristics of refractories Can withstand high temperatures without being fused Crucibles and furnace sides and bottoms containing molten metal are made up of refractories Use as ladles for pouring metal into the mould Constitute furnace walls and roof and thus minimize heat losses Properties of various refractory bricks Type Class Properties 1. High Alumina 50% Alumina to 90% Alumina 2. Fireclay Low, medium and high duty High refractoriness which is increasing with alumina content High mechanical strength at high temperatures Excellent to fair resistance to spilling Greater resistance to corrosion Good spilling resistance and thermal insulation value Fair resistance to fluxes and acid slags Lower resistance to basic slags and fluxes 3. Fireclay Semi-Silica Rigidity under load at high temperatures Resistance to structural spilling Volume stability Resistance to volatile alkalis or fumes 4. Silica Super duty Conventional 5. Chrome fired magnesite Chemically bonded High refractoriness and resistance to abrasion High mechanical strength at high temperatures Greater thermal conductivity as compared to high duty fireclay brick, at high temperatures High resistance to corrosion by acid slags Fair resistance to attack by oxides of lime, magnesia and iron Readily attacked by basic slags and fluorine Poor resistance to spilling at low temperatures Better resistance to spilling High resistance to corrosion by basic slags Mechanical strength and stability of volume at high temperatures 6. Chrome fired High-resistance to corrosion by basic and moderately acid slags and fluxes Basic slags do not adhere to chrome bricks Absorbed iron oxide may damage expansion Possess thermal conductivity lower than that of magnesite brick but higher than fireclay brick N P T E L 190

10 7. Magnesite fired -Magnesia 88-90% -above 90% -Chemically bonded metal encased Extremely high refractoriness and high thermal conductivity Poor resistance to slags containing high % silica Great resistance to corrosion by basic slags Chemically boned and metal encased bricks have marked resistance to spilling 8. Forsterite-Fired Excellent strength at high temperature High refractoriness Attacked by acid slags Fair resistance to most basic slags Good corrosion resistance to alkali compounds USES Application and selection of different refractories Type of refractory Name of the furnace used for melting ferrous metals /alloys 1. Fire bricks Bottom of induction furnace Sub-hearth of open hearth furnace Doors of open hearth furnace Spout of open hearth furnace Spout of direct arc furnace (acidic) 2. Silica brick Side walls and roof of direct arc furnace Roof of open hearth furnace 3. Magnesite-chrome brick Melting zone of basic cupola Direct arc furnace roof (for Ni and Cu melting) 4. Dolomite Backing of open hearth furnace Side walls of direct arc furnace(basic) 5. Chrome magnesite brick Side walls of direct arc furnace (basic) Side walls of open hearth furnace Blocks and ends of open hearth furnace 6. High Alumina fireclay Ladle refractories - as lining nozzle, stopper etc. 7. Magnesite Hearth and side walls of direct arc furnaces for melting nickel and copper Sub - hearth of direct arc furnace for melting steel (basic) N P T E L 191

11 Refractories used in various furnaces Furnace Refractory material 1. Iron blast furnace Entire lining consists of fireclay but of different qualities Hearth and Bosh Shaft or stock Top Hot blast stoves Hot blast main Bustle pipe 2. Acid open-hearth Furnace Portions above the working floor Regenerative walls 3. Basic Open-hearth Furnace Roof Side walls Hearth 4. Acid Bessemer Converter Body Tyres Coarsely ground fireclay Medium ground fireclay Finely ground fireclay Porous fireclay bricks Fireclay Fireclay Silica bricks Fireclay bricks Silica bricks Silica bricks Dolomite or magnesite Ganister Fireclay 5. Basic Bessemer Converter Calcined dolomite or magnesite 6. Basic Electric Furnace Roof Bottom and sides Silica bricks 7. Cupola Fireclay lining 8. Reheating Furnace Roof Hearth A layer of fireclay bricks next to shell and upon this a layer of magnesite Silica bricks Chromite or magnesite bricks N P T E L 192