Refractories of cupola furnaces


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Refractories of cupola furnaces

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  2. 2. 2 CUPOLA FURNACES For many years, the cupola was the primary method of melting used in iron foundries. René-Antoine Ferchault de Reaumur built the first cupola furnace in France, about 1720. It is obtained in different sizes and can be operated for long a time. As we all know that Cupola furnace is basically used for the production of cast iron. The reason for using Cupola furnace includes:  Its continuous operating quality.  High melt rates.  Relatively low operating costs.  Ease of operation. It is an economical furnace for the production of grey cast iron, nodular cast iron and some malleable iron castings. APPLICATIONS:  It is highly used in iron foundries for melting purpose.  It is used for melting scrap metal or pig iron.  It is also used for duplexing and triplexing operations for making steels.  It is also used for melting some copper base alloys etc.  It is mostly used in the production of cast iron. The following diagram shows the basic working principle of Cupola Furnace:
  3. 3. 3 OPERATION: • The charge consisting of Iron, Coke, flux and possible alloying elements, is loaded through a charging door located less than halfway up the height of the cupola. Coke is the fuel used to heat the furnace.The iron is usually a mixture of pig iron and scrap. The flux is a basic compound such as limestone that reacts with coke ash and other impurities to form slag. The slag serves to cover the melt, protecting it from reaction with the environment inside the cupola and reducing heat loss. • The blast is blown through the tuyeres • These tuyeres are arranged in one or more row around the cupola body. • Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone. • A slag hole is provided to remove the slag from the melt • Through the tap hole molten metal is poured into the ladle • At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside. .
  4. 4. 4 REFRACTORIES IN CUPOLA FURNACE: The cupola consists of a vertical cylindrical steel sheet lined inside with acid refractory bricks. The lining is generally thicker in the lower portion of the cupola as the temperatures are higher than in upper portion. In Cupola Furnace, there are different zones having different temperatures and specific purpose. Keeping it in mind, we use different refractories for lining which fulfills the requirement. Therefore, we have to fond knowledge about different zones. Cupola Zones:  Well  Superheating, Combustion or Oxidizing Zone.  Reducing or Protective Zone.  Melting Zone.  Preheating Zone.  Stack 1. Well: It is the space between the bottom of the Tuyeres and the sand bed. Molten metal collected in this portion. 2. Combustion or Oxidizing zone: It is the zone where combustion takes place. The temperature in this zone is about 1800°C. It extends from the top of the tuyeres to a surface boundary below which all the Oxygen of air is consumed by combustion, chemical reaction that takes place in the zone is C(coke) + 02 (from air) → C02 + Heat 3. Reducing zone: It extends from the top of the combustion zone to the top of the initial coke bed. The CO2 produced in the combustion zone moves up and is reduced to CO. The temperature also drops to 1650°C. C02 + C2 → CO - Heat 4. Melting zone: It includes the first layer of pig iron above the initial coke bed. In this zone, the pig iron is melted. The following reaction takes place. 3Fe + 2CO → Fe3C + C02 5. Preheating zone: It includes all the layers of cupola charges placed above the melting zone to the top of the last charge. The layers of charges are heated by the out-going gases. The temperature in the zone up to 1050°C. 6. Stack: It is the zone beyond the pre-heating zone, through which the hot gases go to the atmosphere. As it is clear that all the Zones i.e. Combustion or Oxidizing zone, Reducing zone, Melting zone and Preheating zone have different temperatures, that is why different refractory is used in different zones. There are two places where refractories are required in Cupola furnace, One at the side of walls and the other at the bottom because of molten metal, when slag moves downward there are two possibilities that slag will come in contact with walls and as well as bottom also.
  5. 5. 5 The shell of the cupola, being usually made of steel, has refractory brick and plastic refractory patching material lining it. The bottom is lined in a similar manner but often a clay and sand mixture(bod) may be used, as this lining is temporary. Finely divided coal can be mixed with the clay lining so when heated the coal decomposes and the bod becomes slightly friable, easing the opening up of the tap holes. The bottom lining is compressed or 'rammed' against the bottom doors. Some cupolas are fitted with cooling jackets to keep the sides cool and with oxygen injection to make the coke fire burn hotter. CUPOLA LINING MATERIALS: For the lining and patching of Cupola Furnaces, following materials are used:  Standard Fire Bricks (High alumina brick).  Cupola Fire Bricks.  Ultra Gun 55.  Ultra Gun 1600.  Hydra-Max.  RAMWELL.  SURGUN.  CRITERION  Fire clay  Patching Mass 1) STANDARD FIRE BRICKS: Fire bricks are used for the lining of Cupola. Standard and special size bricks are also used for specific applications. According to the content of Al2O3 (alumina) Fire bricks can be divided into fire clay brick and high alumina brick. As we know that by increasing the percentage of alumina the refractoriness also increases. Features: o Resistance to high temperature, resistance to erosion, resistance to abrasion. o Excellent compression strength. o Resistance to acid as well as alkali. o Excellent heat stability.
  6. 6. 6 2) CUPOLA FIRE BRICKS This shape of brick is known as cupola fire brick as it specially designed for cupola furnace and cannot be used for other furnace. It can be easily lined on the circular walls of cupola furnace. Cupola bricks are acknowledged for their supreme quality.  They have precise dimensions.  They have long life.  They have high strength.  Light in weight.  Accurate dimensions. 3) ULTRA GUN 55: Ultragun55 is a gunning material used in the cupola furnace for the designing and applications in the melting zone of the cupola furnace. It can be used for lining in the cupola furnace. It is induced or installed into the blast furnace by a gunning machine called ROTOMAT. It is a high silicon carbide gunning material and the presence of silicon carbide in higher amount enables it to cause resistance to slag in the melting zone in which the first layer of pig iron is melted just above the initial coke bed.
  7. 7. 7 4) ULTRA GUN 1600: ULTRAGUN1600 is a low cement gunning material used in the cupola furnace and its purpose is to take off the hot gases from the cupola furnace which were fed into the cupola furnace with the help of tuyeres. It is a low cement gunning material which indicates that it has low cement content means it has low amount of mixing water. It is also installed on the cupola furnace with the help of gunning machine ROTOMAT. Difference Between Ultra Gun 55 And Ultra Gun 1600: The main and the basic difference between the UltraGun55 and UltraGun1600 is of the temperature. As clear from the name that the temperature of UltraGun55 is lower as compared to Ultragun1600. This is the reason that UltraGun55 is used in the zone of the Cupola Furnace where the temperature is lower and ULTRAGUN1600 is used in the higher temperature zone of the Cupola Furnace. WHAT IS ROTAMAT?? ROTAMAT is a gunning machine used to induce gunning materials into the cupola furnace. It is usually used by a popular gunmix system called “THE VELCO GUNMIX SYSYTEM”. The gunning material introduced to the gunning machine is first moistened by compressed air and water which allows the development of dust and rebound to reduce considerably. As a result we get better gunning results due to less dust and better moistening and also less cost due to less rebound. The ROTAMAT consist of a rotor by which we can control the capacity of the gunning material. GUNNING MATERIAL: Gunning materials are unshaped refractories commonly used in steel industry for the maintenance of converter linings.
  8. 8. 8 5) HYDRA-MAX: HYDRAMAX is a high alumina, ultra -low cement castables, containing silicon carbide for superior slag resistance in the well and siphon box. It is a low cement castable refractory means it has low cement content and it a castable refractory so usually a cement binder is used like HAC( high alumina cement) which reacts with the low amount of water and form a strong ceramic bond. The advantage of using low cement castables are:  They are very dense and porous means they have high porosity.  They are highly wear resistant and abrasion resistant.  They have high resistance to infiltrations i.e. alkalis. Just like cement, HydraMax is usually supplied dry for mixing and placement on site, it must be installed behind suitable formwork by internal vibration. It has great importance in cupola furnace. Low-moisture castable products can be installed via vibration casting or concrete/refractory pump. Designed for rapid turnaround in hot trough and runner applications. Applications include trough and runner working linings, precast working and safety lining shapes, tilting runner linings and castable taphole facing. 6) RAMWELL: Ramwell linings are high alumina pitch free granular, clay bonded, graphitized ramming material containing between 10% and 20% silicon carbide used for installation in the well and syphon box where slag resistance is paramount. RAMWELL is supplied ready to use and is installed by pneumatic ramming behind robust formwork. Fused alumina based RAMWELL 90 or RAMWELL PS001 and calcined alumina based RAMWELL 75 ramming materials are recommended for the well, syphon and taphole of the cupola RAMWELL PS001 or RAMWELL 90 ramming materials are ideally suited to lining the well and syphon box. Alternatively a cast solution can be archieved with HYDRAMAX AT3A or HYDRAMAX VX 281M. Ultra low cement castable with SiC level from 18% to 31%. If a castable solution is preferred, then HYDRAMAX AT3A or HYDRAMAX VX281M ultra-low cement castables are recommended. Combinations such as a castable lining for the well together with rammables for the syphon box are also possible. What is Syphon Box?? A place in Cupola Furnace where slag layer is formed after melting is called a Syphon Box. 7) SURGUN It is a unique range of low cement, gunned castables ideal for the repair and maintenance of the cupola well and melting zone of the cupola furnaces. SURGUN is supplied dry for application by the VELCO Gunmix system.
  9. 9. 9 8) CRITERION It is a range of low cement castables supplied dry for mixing and placement on site, it must be installed behind suitable formwork by internal or external vibration. Chamotte based CRITERION 50ME is ideally suited for the cupola shaft. 9) FIRE CLAY Fire clay is the important refractory used in cupola furnace. It is used as binder during lining of Cupola with bricks. If you are not using fire clay cupola is not complete. In general fire clay joined two or more refractory bricks by this fire clay acts as mortars that joined two or more things which strengthen it and also act as refractory. Fire clay are made up of clay containing alumina silicates and impurities such as alkalis and ironoxide.The alumina content ranges from 25% to 45% depending upon impurities content and alumina and silicon ratio. Fire clay are classified as  low duty  medium duty  high duty  super duty depending upon the alumina range. Fire clay also act as a binder furnace between lining of Cupola gradesand bricks .fire clay in Cupola Furnace is approximately to 9D to 54D.They are available in IS- 7 and IS-8 clay is the most reliable and has a long shelf life. The main use of fireclay in cupola is to minimize heat resisting barriers between high and low temperatures zones. 10) PATCHING MASSES : Patching refractories are used where any damage is done or on the places where there is a chance of failure or damage. Patching refractories are made up of fused silica. Every time after the first heat cupola needs to be patched with patching masses. These materials are similar to plastic refractories though have a soft plasticity allowing them to be pounded into place. Patching refractories have some tremendous qualities that have high density, better thermal stability anti-erosion means they are resistant to basic slag layer because in cupola Cao and FeO acts as basic slag layer so patching refractories are also resistant to this. It also helps in increasing the life of refractory lining. Following is the table showing various refractories in Cupola Furnace with their sizes: Product Size Cupola Fire Bricks 9D to 54D with std. cross section of 4.5”X3”(Inches) Standard Fire Bricks Cross section of 9X4.5X1” 9X4.5X1.5” 9X4.5X2” 9X4.5X3” (Inches) Fire Clay Fine Clay of -22 mesh. Patching Mass Combination of Fine Clay, Special binders and refractory grits up to 7mm.
  10. 10. 10 Why we use Acidic Refractories in Basic Atmosphere Instead of Basic Refractory? As we know that acidic refractories are used in places where slag and atmosphere are basic and on the other hand basic refractories are used in places where slag and atmosphere are acidic in nature, but this is not in the case of Cupola Furnace because there are two distinct types of slags in the cupola. In the slag layer in the cupola well, the slags contain mostly CaO and SiO2. Above the slag layer the predominant oxides are CaO and FeO. These slags are much more corrosive in nature and their corrosiveness is exacerbated by the high temperatures that exist in the regions above the tuyeres. The CaO-FeO slag above the slag layer is basic hence silica based refractory perform very poorly in this region. MgO refractory might be best suited for use in this region but due to cost factor, MgO refractories are not used. However, alumina refractory (which is a neutral refractory) is most often used in these conditions. Even alumina refractory is rapidly eroded by slag attack in the region above the tuyeres. This leads to the considerable loss of heat. Recently, alumina/silicon carbide refractories(which is a combination of neutral and special refractory) have been used in this region with good results. The improvement stems from the high thermal conductivity of the refractory. This sufficiently lowers the temperature on the hot side of the refractory to allow a frozen layer of slag to form which serves to protect the refractory. This slows the erosion process and leads to heat savings with ensuing benefits of lower fuel requirements, higher iron temperatures, improved alloy recovery and lower refractory maintenance, which offsets the extra cost for refractory. The slag above the taphole is much less corrosive than the slag above the tuyeres. Alumina refractory or silicon carbide/alumina refractory is most commonly used today. For basic slag operation, the refractory can be MgO or carbon. Almost every vessel that holds or produces liquid iron is lined with refractory materials and is susceptible to refractory erosion by slag. In other circumstances, slags can combine with refractory materials to form accretions that hamper production. The consequences of refractory problems, loss of production and the cost to replace the refractory can be serious. Thus, extending the life of a refractory lining is an important consideration. Iron oxide is present in large amounts in many of the slags found in foundry vessels and furnaces. Unfortunately, iron oxide is among the best solvents for refractories, and in particular it is a very good solvent for silica refractories. The vulnerability of refractories to FeO attack can be roughly judged by the amount of refractory that will dissolve in pure FeO at 15000 C. Solubility of Oxides in FeO @15000 C Al2O3 11% MgO 5% SiO2 40%
  11. 11. 11 MgO is the least soluble and silica is the most soluble. Thus, silica refractories are the most susceptible to attack by FeO. For this reason, silica is being used less and less in foundry applications despite of its very low cost. MgO has the lowest solubility in FeO. As a basic oxide it dissolves readily in acid slags, which unfortunately are the most common type of slags generated in the foundry. As a result MgO refractories are not often used. Alumina refractories have risen in popularity because they are less sensitive to FeO erosion than silica and are reasonably resistant to acid slags. A refractory that is gaining popularity is alumina containing silicon carbide . Although silicon carbide should react readily with FeO and dissolve in iron, the success of the refractory is attributed to its high thermal conductivity. This enables the temperature of the refractory at the iron interface to drop below the freezing point of the slag. The refractory becomes coated with solid slag which is much less corrosive than the liquid slag. Owning to the destruction of the refractory lining in cupola furnaces, it operate for periods of 12 to 18 hours and then undergo routine repairs i.e. patching and gunning. Two or three cupola furnaces are operated alternately when daily or 24 hour iron smelting is required. Now a days continuous operation water cooled cupolas for cast iron production is used, which made possible smelting for periods of 100-120 hours routine repair of the refractory lining. It is very important that in the bottom lining of furnace cement refractories i.e Hydramix – low moisture cement refractory were installed as the slag was basic and more high temperature resistant, slag penetration resistant, corrosive resistant and ultra high hot strength refractories were opted to be used. Moreover it is a common phenomenon that when water is allowed to be in contact with high temperature molten state materials a very devastating explosion occurs because of formation of Hydrogen Bomb.