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How to Control Kiln Shell Corrosion Report

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  • 1. SUMMER IN-PLANT TRAINING REPORT Shree Cement Ltd., Beawar(Rajasthan) Starting From 7th May 2012-1st July 2012 Project Title:- How to Control Kiln Shell CorrosionSubmitted to: - Submitted by:-Shri Sanjay Jain Ankit KarwaHOD,Mechanical Dept. 4th year UG, MSEShree Cement Ltd.,Beawar IIT KanpurHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 1
  • 2. AcknowledgmentI take immense pleasure in thanking Shri Vinay Saxena, Plant head, Shri Sanjay Jain, HODMechanical Department, and Shri Atul Sharma for having permitted me to carry out thisproject work.I wish to express my deep sense of gratitude to Shri R.P.Pareek and Shri S.Hawa for theirable guidance, useful suggestions and providing me necessary data which helped me incompleting the project work, in time.Needless to mention Shri Manoj Sharma, who has been a source of inspiration and for histimely guidance in the conduct of project work. I would also like to thank Shri sanjayBaldwa, Shri Harshwardhan, Shri Manish Purohit for all their valuable and timely assistancein the project work.I’d also like to express my gratitude towards the Mechanical Library and Shri PankajSharma, Librarian for helping me to make available different references during the project.Words are inadequate in offering my thanks to Shri Gopal Tripathi, Sr. Manager HRD forproviding us such good facilities during the project.I would like to express my heartfelt thanks to Shri Vijay Vyas, Officer HRD, for his sincerehelps throughout the project without which it seemed impossible to complete the project.Finally, yet importantly, I would like to express my heartfelt thanks to my institute, INDIANINSTITUTE OF TECHNOLOGY, KANPUR and its Material Science and EngineeringDepartment for providing me this opportunity to interact with this organization andunderstand the intricacies of the corporate world. Ankit Karwa, 4th year under graduate, Material science and Engineering, IIT KanpurHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 2
  • 3. Index1) Acknowledgment ………..22) Abstract ………..43) Introduction ………..54) About Shree Cement ………..65) Manufacturing Process ………146) General Chemistry of Cement Manufacturing ………197) The Kiln System ………228) Literature Review ………30 A) What is Corrosion ………30 B) Types of Corrosion ………31 C) What are Refractories ………38 D) Types of Refractories ………419) Type and Composition of refractory used at SCL, Beawar ………4510) Full details of Refractory linings, Coating and SS Plate Used ………4811) Corrosion of Kiln shell ………51 A) Introduction ………51 B) Corrosion of Cement Kiln ………52 C) Mechanism of Cement Kiln shell Corrosion ………53 D) Role of Refractories in tackling shell corrosion ………55 E) Role of Process Parameters on Shell Corrosion ………6012)Recommendations ………6213)References ………72How to control kiln shell corrosion, SCL Beawar (Raj.) Page 3
  • 4. AbstractCorrosion damage is a major issue in cement plants. The serious consequences of thecorrosion process in cement plants have become a problem of worldwide significance.Corrosion causes plant shutdowns, waste of valuable resources, loss or contamination ofproduct, reduction in efficiency, costly maintenance, and expenses over design canjeopardise safety.Typically, once a plant or any piece of equipment is put into service, maintenance isrequired to keep it operating safely and efficiently. This is particularly true for aging systemsand structures, many of which may operate beyond the original design life.The type of corrosion mechanism and its rate of attack depend on the nature of theatmosphere in which corrosion takes place. The first step in preventing corrosion is tounderstand its specific mechanism. The second and most important as well as most difficultstep is to design an effective type of protection mechanism.The Cement kilns are operating at higher thermal and volumetric loadings and utilizingalternate raw materials and fuels which are rich in volatiles creating thereby severe serviceconditions inside the rotary kiln. Such conditions cause the corrosion of the rotary kiln shellto take place in hot running conditions. Therefore investigations reported in the paper wereunique in nature with a very specific target to understand the mechanism of such corrosion,the role of various service conditions and process parameters on corrosion phenomena andestablish such remedial measures which could impede / reduce corrosion of rotary kiln shellin running conditions.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 4
  • 5. Introduction:-Cement is a common construction material; a binder in mortars and concretes that hardensin the presence of water. Cement is called hydraulic, when the hardened product is stablein an aqueous environment. The most widespread hydraulic cement today is portlandcement – a finely ground gray-to-white powder composed primarily of calcium silicates,calcium aluminates, and calcium ferrites, derived from mineral ingredients (Figure below). Fig 1:Cement PowderCement is made by heating limestone (calcium carbonate) with some other materials suchas clay to about 1400°C in a kiln, where a molecule of carbon dioxide is liberated calciumoxide, or quicklime, is formed, which is then blended with the other materials that havebeen included in the mix. The resulting hard substance, called clinker, is then ground witha small amount of gypsum into a powder to make Ordinary Portland Cement or PortlandCement, often referred to as OPC, the most commonly used type of cement in the world.Portland cement or clinker can be blended or interground with other materials to achievecertain properties. There are five classes of blended cement commonly used. They are asfollows:- • Portland blast-furnace slag cement • Portland-Pozzolan Cement(PPC) • Pozzolan-modified CementHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 5
  • 6. • Slag Cement • Slag-modified Portland The blended cement are gaining popularity because they require less energy to manufacture, they can be made with by-product materials that would normally be disposed in a landfill, thus reducing solid waste, and offer performance benefits for certain applications. When mixed with water, portland cement sets (stiffens) in a few hours and hardens over a period of weeks and months. These phenomena are caused by chemical reactions associated with hydration between the components of cement and water. The most common use of portland cement is in the production of concrete. Concrete is a composite consisting of aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement is also used in mortars (with sand and water only) for plasters and screeds, and in grouts (cement water mixes) placed into gaps to consolidate brick walls, foundations, etc.About Shree Cement:-Cement industry falls in the category of manufacturing industry. With the growth ofeconomy, cement industry is also taking substantial leaps. One amongst the companies,helping the cement industry to achieve its fast growth, is Shree Cement Ltd. It is located incentral Rajasthan, catering to the entire Rajasthan market with the most economic logisticscost.An ISO 9001:2000 Company, established in the Year 1984 & the Commercial Production ofUnit-I started in the Year 1985. Shree Cement Limited has its registered office located atBeawar (Raj) & Corporate Office at Kolkata (W.B.). “JO SOCHE VOH PAAVE”SHREE CEMENT LIMITED is an energy conscious & environment friendly businessorganization. Having ten Directors on its board under the chairmanship of Shri B.G. Bangur,the policy decisions are taken under the guidance of Shri H.M. Bangur, Managing Director.Shri M.K. Singhi, Executive Director of the company, is looking after all day to day affairs.The company is managed by qualified professionals with broad vision who are committedto maintain high standards of quality & leadership to serve the customers to their fullestsatisfaction.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 6
  • 7. The largest cement manufacturing plant at a single location in Northern India, under theflagship of Executive Chairman Shri B.G. Bangur & Managing Director Shri H.M. Bangur. TheCompany is aiming for 20 Million Ton Annual production by the year 2015.The plant is located near the city of Beawar, Dist. Ajmer in Central Rajasthan, sate ling theBeawar city at radius of 10 Kms. However the Beawar subdivision is will connected throughRail and Road both, situated on National Highway No. 8. Fig 2: Shree Cement Unit I & II, Beawar(Raj.)Shree Cement is manufacturing 33, 43, 53 & Shree ultra Red Oxide grade Cement inOrdinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC). Pozzolana used inthe manufacture of Portland cement is burnt clay of fly-ash generated at thermal powerplants. While Ordinary Portland cement is grey fine powder which is the result fromcrushing a dry mix made of clinker and gypsum. Shree cement manufactures both kinds ofcement.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 7
  • 8. Cement Industry in India poised for healthy growth supported by following factors:- • Growth of the Indian Cement industry directly influenced by GDP which is expected to grow at the rate of 7%. • With various infrastructure projects like roads and highways, railways, port power projected and real estate being implemented, the demand for cement is expected to grow at a fast rate. • Tax relief against interest on housing loans, stable interest rates and increasing competition in housing finance would significantly help in growth of this sector. • 16% Share in Rajasthan Cement Production. Some of another plant located in RAS. The RAS plants are far from Beawar at least 35 kms. All the same activities are doing in this plant & this plant is uses High grade material our compression in BEAWAR plant and this material send in BEAWAR plant & mix both the Material & get the superior quality product. One of newly grinding unit started in KHUSHKHERA –Distt-Bhiwadi, the plant is far from the BEAWAR plant almost 500 KM.UNIT -1 at Beawar Distt: Ajmer UNIT -2 at Beawar Distt: AjmerIncorporated in 1979, Put up in 1997 Cement Production -2.10 million tonesEstablished in 1985 Cement Production(Expected)-1.20 million tonesUNIT -3 at Ras Distt: Pali UNIT -4 at Ras Distt: PaliIncorporated in 2005 Cement Production - 1.0 million tonesCement Production - 1.0 million tonesUNIT -5,6,7 & 8 at Ras Distt: Pali Khuskheda Grinding unit (Distt. Alwar)Production – 1.00 million tons each Suratgarh , Jaipur & Roorkee Grinding units: one grinding unit each(also having a world record of 367 days)Green Power Project of 43 MW at Beawar & RAS, Thermal Power Plant of 100 MW at RASHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 8
  • 9. Production with Efficiency It has a track record of over 100% capacity utilization in the 18th yr of its existence. Against the national industry average of 84% it has registered the highest record production of 3.02MT with 116% of capacity utilization. The commercial production of Unit I started in the year 1985. The production augmented exponentially from the capacity of 0.6 MTPA in 1985 to around 4.8 million ton presently through modification, capacity enhancement and continuous improvement. The revised target is 25 MTPA by 2015.Innovative & Cost Conscious Management • Leadership in the use of alternative waste fuel. • Initiatives for Global warming reduction. • Partial utilization of waste heat. • Initiator in the use of pet coke for power generation in India.PROGRAMES: • Celebration of National Safety Day. • Celebration of National Safety Week. • Staff Training in the subject of Safety. • Environment Safety Day.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 9
  • 10. Policies of the CompanyHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 10
  • 11. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 11
  • 12. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 12
  • 13. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 13 Fig 4: Typical process flow diagram of a cement plant
  • 14. Manufacturing Process:MinningLimestone is the main raw material of cement and is obtained from the mines. Bore holesare made in the mines at various locations, and samples are collected to test the CC%(CaCO3 percentage) in the mines. Limestone is retrieved by blasting the mines and then therock material obtained from the mines is crushed. This crushed limestone (raw material) issent to the plant with the help of conveyer belts and/or transportation.Stacking and ReclaimingA stacker is a large machine used in bulk material handling applications. It is mainly used toarrange the incoming feed in piles. It is important to maintain the homogenous anduniformity before discharging to further process. A stacker usually operates on a rail-likestructure with movable wheels, but the main operation is performed on a fixed place. Themain function of a Reclaimer is to recover the material and at the same time maintainuniformity. At this stage the material is collected in hoppers via conveyer belts. Reclaimersare volumetric machines and are rated in m3/h (cubic meters per hour) for capacity, whichis often converted to t/h (tonnes per hour) based on the average bulk density of thematerial being reclaimed. Reclaimers normally travel on a rail between stockpiles in thestockyard and are generally electrically powered by means of a trailing cable.Pile of limestone is made by horizontal stacking of different CC% limestone to get therequired CC%. To obtain homogenized limestone for cement production, vertical reclaimingis done.Raw Material GrindingRaw mill (Vertical Roller Mill) is a grinding equipment which is used to grind the incomingfeed fed through hoppers. There are three hoppers in each unit. The first two carryinglimestone and third hopper containing Laterite (zinc slag i.e. molten residue at the bottomof zinc smelter). Laterite has a high percentage of iron. Raw mill is tall unit containing ahorizontally movable disc and three vertical disc, assembled a way to stand the vibrationswith the virtue of the state of art suspension system. Raw mill grinds the feed to very smallsize and the output is sucked through the provided vents. The output is generally known asRaw Meal. This raw meal is stored in Silos.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 14
  • 15. Coal GrindingThe heat required for heating and conversion is obtained by burning of grinded pet coke.Pet coke is the residue of petroleum refining and is grinded again in a VRM (Coal mill). Thisgrinded pet coke is stored in a storage bin.ClinkerisationThe raw material powder is fed into a preheater in which suction is created with the help ofsuction gas pump. This provides a larger residence time in the cyclonic preheaters, whichare placed in a zigzag manner. The material reaches approximately 880oC at the end of thepreheater. In the preheater major reaction taking place is the decomposition of CaCO3 toproduce CaO. This material then enters the rotary kiln, in which hot air is blasted from theopposite direction at a temp of about 1500oC. CaO (C), Al2O3 (A), SiO2 (S), Fe2O3 (F) reactin various parts of the kiln to produce C3A (10%), C4AF (10%), C2S (30%), C3S (50%). Thismixture is called as clinker. Clinker formed above is cooled with the help of the air in Gratecoolers, and then stored in clinker silos.The key component of the gas-suspension pre-heater is the cyclone. A cyclone is a conicalvessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortexwithin the vessel. The gas leaves the vessel through a co-axial "vortex-finder". The solids arethrown to the outside edge of the vessel by centrifugal action, and leave through a valve inthe vertex of the cone. Cyclones were originally used to clean up the dust-laden gasesleaving simple dry process kilns. If, instead, the entire feed of raw meal is encouraged topass through the cyclone, it is found that a very efficient heat exchange takes place: the gasis efficiently cooled, hence producing less waste of heat to the atmosphere, and the rawmeal is efficiently heated. This efficiency is further increased if a number of cyclones areconnected in series.The number of cyclones stages used in practice varies from 1 to 6. Energy, in the form offan-power, is required to draw the gases through the string of cyclones, and at a string of 6cyclones, the cost of the added fan-power needed for an extra cyclone exceeds theefficiency advantage gained. It is normal to use the warm exhaust gas to dry the rawmaterials in the raw mill. The hot feed that leaves the base of the pre-heater string istypically 20% calcined, so the kiln has less subsequent processing to do, and can thereforeachieve a higher specific output.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 15
  • 16. Fig 6:Pyro ProcessCement kilns are used for the pyro-processing stage of manufacture of cement, in whichcalcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates.Cement kilns are the heart of the cement production process: their capacity usually definesthe capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of their efficiency has been thecentral concern of cement manufacturing technology.The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tubeslopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions perhour. Raw mix is fed in at the upper end, and the rotation of the kiln causes it gradually tomove downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, orpulverized solid fuel, is blown in through the "burner pipe", producing a large concentricHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 16
  • 17. flame in the lower part of the kiln tube. As material moves under the flame, it reaches itspeak temperature,before dropping out of the kiln tube into the cooler. Air is drawn firstthrough the cooler and then through the kiln for combustion of the fuel. In the cooler theair is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln,thus causing intense and rapid combustion of the fuel. Fig 7: Cement Rotary kilnNote: - The details of Cement Rotary kiln are discussed in detail in later part of the report.Cement grindingThe clinker leaving the cooler has a variable particle size, typically of diameter 3-25 mm.Thus it is necessary to grind the clinker to a more uniform particle size. This can either bedone directly after the cooler, or the clinker can be transferred to a silo until it is needed,see Figure 2-1. Cement clinker is, however, not a stable material: It reacts readily withwater/water vapour and CO2 from the air. This will result in pre-hydration and carbonation.Extended storage periods may therefore have an effect on the cement properties.In connection with the clinker grinding, 3-6 wt-% gypsum (CaSO4󲐀2H2O) is added to theclinker. The gypsum has a marked effect on both the strength and the setting of cement.Additives such as coal fly ash, sand or raw material may also be added at this point in orderto contribute positively to the strength-giving properties of the cement. Some of theseAdditives can replace a significant fraction of the clinker, thereby saving energy needed forcalcinations and clinker reactions. Thus the additives have a great potential to reduceenergy consumption and CO2-emissions per kg cement produced. For this reason, manycement companies seek to minimize their environmental impact by exploring the use ofnew Supplementary Cementitious Materials (SCM), examples being volcanic ashes orkaolinite clays.The grinding of clinker and additives can be performed with several different mill systems.When a sufficient fineness is reached, the final Portland cement product is transferred to acement silo. It is stored in this silo until it is being packed and shipped to the end users.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 17
  • 18. Fig 8: Detail flow sheet of whole processHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 18
  • 19. General Chemistry of cement manufacturingThe Portland cement is made by heating a mixture of limestone and clay, or other materialsof similar bulk composition and sufficient reactivity, ultimately to a temperature of about1450oC. Partial fusion occurs, and nodules of clinker are produced. The clinker is mixedwith a few per cent of calcium sulphate and finely ground, to make cement. The calciumsulphate controls the rate of set and influences the rate of strength development. It iscommonly described as gypsum, but this may be partly or wholly replaced by the otherforms of calcium sulphate. Some specifications allow the addition of other materials at thegrinding stage. The clinker typically has a composition in the region of 67% CaO, 22% SiO2,5% Al2O3, 3% Fe2O3 and 3% other components, and normally contains 4 major phases: 1. Alite 2. Belite 3. Aluminate 4. FerriteAlite:Alite is the most important constituent of all normal Portland cement clinkers, of which itconstitutes 50-70%. It is tricalcium silicate (Ca3SiO5) modified in composition and crystalstructure by ionic substitutions. Reaction of this phase with water is very quick and innormal Portland cements it is the most important of the constituent’s phases for strengthdevelopment; it ages up to 28 days, it is by far the most important.Belite:Belite constitutes 15-30% of normal Portland cement clinkers. It is di calcium silicate(Ca2SiO4) modified by ionic substitutions and normally present wholly or largely as the βpolymorph. It reacts slowly with water, thus contributing little to the strength during thefirst 28 days, but substantially to the further increase in strength that occurs at later ages.By one year, the strengths obtainable from pure alite and pure belite are about the sameunder comparable conditions.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 19
  • 20. Aluminate:Aluminate constitutes 5-10% of most normal Portland cement clinkers. Basically it is tricalcium aluminate (Ca3Al2O6) substantially modified in composition and sometimes also instructure by ionic substitutions. It has a rapid reaction with water. This phase causes anundesirable rapid setting so we need a controlling agent for this phase (like Gypsum).Ferrite:It is tetra calcium alumina ferrate i.e. Ca2AlFeO5, substantially modified in composition byvariation in Al/Fe ratio and ionic substitution. It has variable reaction rate withwater.Initially it reacts rapidly with water but reaction goes on slowing down with timewhich is due to the difference in the compositions or other characteristics.Clinker consists of MgO up to 4-5 %. If there is an excess of only 2% then it can causeexpansion of hardened concrete on reaction with water. This excess can occur as periclase(MgO). Same problem might happen with SO3 if it is not under the limit of 3.5 % .To produce white cement we increase the ratio of Al2O3 to Fe2O3. Dark color of the cementis due to ferrite.Reaction of cement with water is exothermic. This total heat can be reduced by loweringalite and aluminate content in coarser grinding. This can also be done by the addition of flyash.Clinker ReactionsAfter having passed the calciner, the calcined raw meal is admitted to the rotary kiln wherethe remaining cement clinker reactions take place. Calcined raw meal is also called hot mealin cement terminology in order to distinguish it from raw meal not yet calcined.The names, chemical compositions and abbreviations used in cement nomenclature for thefour main constituents of Portland cement clinker are shown in Table 1. Belite and alite arethe strength-giving minerals. Alite reacts fast with water during hydration and accounts forthe early strengths while belite reacts slower and gives the cement its late strengths.Ferrite and aluminate do not contribute to the strengths of the cement. But they possessimportant properties when burning clinker. These properties of ferrite and aluminate willbe explained later in this section.Name Chemical Composition NomenclatureBelite 2Cao.SiO2 C2SAlite 3Cao.SiO2 C3SAluminate 3CaO.Al2O3 C3AFerrite 4CaO.Al2O3.Fe2O3 C4AFTable 1: Main Constituents of Portland CementHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 20
  • 21. Free lime (CaO), free periclase (MgO), earth alkali sulfates (CaSO4, MgSO4), Alkali sulfates(Na2SO4, K2SO4) and other minor components may also be found in the clinker.The requirements for the Portland cement clinker are: a) > 67 wt.-% calcium silicates (C2S and C3S). The remainder must mainly be Fe2O3, Al2O3 and other oxides. b) < 5 wt.-% MgO c) The CaO/SiO2 ratio by mass shall not be less than 2.0.Clinker reactions occur at temperatures between 700-1450°C, see Figure 9. The reactionsforming the final clinker involve intermediate compounds, and the clinker reactions may beaffected by minor compounds. The overall clinker reactions are described, while a detailedoverview of the chemistry, phase relations etc. Fig 9: Phase Diagram for cement clinker productionHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 21
  • 22. The kiln systemThe Production of cement may be divided into three parts: 1) Preparation of rawmaterials,2)pyro-processing and 3)clinker processing, storage and shipment. Pyro-processing covers the thermal treatment of the raw materials necessary to obtain thecement clinker. Pyro-processing takes place in the preheater, calciner, rotary kiln andcooler. These sections are commonly referred to as the kiln system.Rotary kilnThe rotary kiln is often referred as heart of cement plant. This is where the chemical clinkerformation reactions take place. The rotary kiln is simply a long, cylindrical tube consisting ofan outer shell and an inner refractory lining. Typical lengths and diameters for modernrotary kilns are between 40-100 m, and 3-6m, respectively. Rotary kilns are inclined 1-4°and rotate 1-5rpm in order to facilitate mass transport and ensure clinker formingprocesses such as nodulization. Production capacity is typically 2000-4000 tonnes of clinkerper day (tpd), but may be as high as 12000 tpd.In the material outlet, the rotary kiln is equipped with a burner. The burner’s main functionis to form a flame to provide energy for clinker reaction to takes place. The flame of therotary kiln burner should be short, narrow and strongly radiant in order to achieve a goodheat transfer from the flame to materials in the bed.Modern rotary kiln burners are often designed to burn a variety of fuels. This is todayachieved by using multi-channel burners with separate channels for fuels and primary airwhich make it possible to adjust primary air amounts, injection velocities and momentumindependently of the fuel flows. Swirl may be used to enhance mixing and stabilize theflame. The recent trend in multiple fuel burners is to use a single common fuel channelwhich allows more flexibity towards fuel particle size and type. Fig 10: Left: Outer view of rotary kiln seen from above. Right: Inner view seen from burner end.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 22
  • 23. In early cement plants, the kiln system consisted only of a rotary kiln: Raw materials weredried, preheated, calcined and burned to clinker on their way through the rotary kiln. Thisprocess required very long rotary kilns, often significantly longer than 100 m. The rawmaterials were either introduced as dry raw meal or as water/raw material slurry, and thistype of plants were therefore commonly referred to as dry long kilns or wet long kilns,respectively. Due to a low energy efficiency, this type of cement plants is very expensive tooperate, and are rarely constructed today.The rotary kiln consists of an outer steel shell and an inner refractory lining for thermalinsulation, in order to maintain and resist the high process temperatures. In a rotary kiln,the refractory usually consists of bricks of special composition and sizes, able to withstandhigh temperatures. However, the refractory may also be a cast lining of concrete. Therefractory lining is subject to a wide range of destructive influences through the mechanicaldynamics of the rotary kiln, the chemistry of the cement clinker process and the type offuels used. The intensity of these stresses varies according to the operating conditions andkiln sections. The rotary kiln is therefore equipped with a range of refractory bricks withdifferent properties to ensure appropriate kiln zone lining.The burning zone refractory lining usually suffers the greatest wear due to the highertemperatures in the burning zone. However, the burning zone lining is protected by acoating layer which prolong the lifetime of the refractory lining. The coating is a mass ofclinker or dust particles that adheres to the wall of the kiln, having changed from a liquid orsemiliquid to a solidified state.Generally, the burning zone refractory lifetime is 9 to 12 months depending on the specifickiln type and operating conditions. The refractory lifetime of the colder rotary kiln materialinlet zone is typically 12 to 48 months. Thus, these different rotary kiln zones do not have tobe replaced quite as frequently.The most used brick types today are chromium-free magnesia-alumina-spinel bricks withMgO content of 80-95 wt.-% and Al2O3 content of 3-18 wt.-%. Minor compounds aretypically Fe2O3, Mn2O3, SiO2, CaO and ZrO2. These brick types are regarded as having thelongest service life and the best price/performance ratio.The increased use of alternative fuels in the cement industry may lead to higher levels ofrecirculating alkali metals and sulfur within the kiln system.Recirculating alkali metals andsulfur cause large quantities of salts to condense in and on the refractory lining,predominantly in the temperature window 750°C to 1,100°C . Salt compounds enter intoHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 23reactions with refractory bricks that contain alumina and the bricks can be destroyed by salt
  • 24. crystallization and alkali spalling. Sulfur oxides make the reactions even worse, byformation of alkali sulfate salt.Various strategies are used by the refractories industry to counteract these wear processes.These include use of additives that produce low gas permeability and reduce theinfiltration tendency of alkalis. Other solutions are sealing or impregnating the refractorymaterial to form a protective zone.One of the most successful and widespread solutions is addition of 3-6 wt.-%Silicon carbide, SiC, which leads to an appreciable resistance to alkali attack. The additionof SiC leads in situ to formation of liquid phases, which seal the refractory surface andprotect against alkali infiltration.Increased circulation of inorganic volatiles such as sulfur and chlorine in the kiln system,due to increased alternative fuel utilization also entails a higher risk of kiln shell corrosion(Jøns and Østergaard, 2001). Efforts have been made to identify suitable refractory steelsfor cement rotary kilns, with characteristics that are a compromise between good creepresistance, high corrosion resistance in the presence of chlorine and sulfur, and strongresistance to abrasion when hot.Thermal profile and kiln subdivisionsThe rotary kiln thermal profile varies throughout its length, depending on the temperatureand chemical reactions involved during the process (see in Table 2).The rotary kiln can be subdivided into several zones or regions that are exposed not only tothermal and chemical wear but also to mechanical stresses. The influence of one or severalof these factors, to minor or greater proportion determines the refractory lining typerequired for each zone: A. Decarbonation zone: from 300ºC to 1000°C (+)This stage can occur either inside of the old wet process rotary kilns or in the preheatertower of modern units consisting of two steps: Firstly, between 300°C and 650°C where theraw meal heating occurs, accompanied by a dehydration reaction; Secondly, between 650°Cand 1000°C, when the limestone decarbonation takes place generating CO2 and CaO.The first step is characterized by the following aspects:How to control kiln shell corrosion, SCL Beawar (Raj.) Page 24 • Presence of raw meal (there are no new mineral phases development);
  • 25. • Erosion (due to raw meal flow at high velocities); • low temperature; • Evaporation and dehydration (of water) chemically bonded to the raw material.In this zone it is very important that the refractory products have the capability to protectthe rotary kiln drive (good insulation degree) and good resistance to impacts of anomalousbuild-ups. Bricks with less than 45% Al2O3 content are suitable. Besides that, when alkalinesalts are present, a vitreous glassy layer can develop with the alkali on the brick surface,preventing its propagation or later infiltration.In the second stage of these reactions, the development of new mineralogical phasesoccurs: • Formation of CaO and CO2; • Formation of CA, C12A7 and C2S; • Temperature variation; • Alkali attack.Usually, the use of bricks with a 70% Al2O3 content is recommended, which offers a highmechanical resistance, low porosity, and low thermal conductivity. However, the risk ofeutectic reactions formations on the Al2O3-CaO- SiO2 , system and alkali resistance is alimiting factor.B. Upper transition zone: from 1000ºC to 1238°C (+)It is the most unstable and difficult area for refractory specification. Although thetemperature range varies from 1000°C to 1338°C, incidences of thermal overloads arefrequent. This fact is linked on the flame shape, to the fuel type and to the design of the kilnmain burner. Therefore, it is in this area where coating starts to develop as soon as firstdrops of liquid phase appear. Coating becomes very unstable if the operational conditionspresent high variability.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 25
  • 26. Table 2C. Sintering zone: from 1338ºC to 1450°C (+)In this area a full development of coating at 1450ºC(+) is expected. The presence of someliquid phase facilitates the dissolution of C2S in the same what promotes the reaction thatgenerates C3S. The highest temperature in the kiln is reached at this area. Usually it shouldbe around 1450ºC for ordinary Portland Cements. Liquid phase is also around 25% at1450ºC. If process is under control, coating will be stable and able to protect the liningduring the whole campaign. However, if there is a big variability at ram meal controlparameters or uneven fuels types shifting, coating will be unstable and refractoriessubmitted to an enormous thermo-chemical wear. The refractory products must resist hightemperatures, infiltration of molten liquid calcium silicates, and/or alkaline sulfates, and beable to hold a stable coating.Usually at this kiln zone it is possible to find: • Presence of incipient liquid phase from 18 to 32%, free lime and C2S; • Development of C3S by the reaction of CaO and C2S. • Clinker liquid phase infiltration and coating formation; • Chemical attacks by alkaline sulfates; • High operational temperature.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 26
  • 27. D. Lower transition zone from: 1400ºC to 1200°C (+)This area usually operates between 1400°C and 1200°C. Around 1200ºC begins thecrystallization of the clinker the mineral phases, but not. Although the liquid phase can stillbe present, it is a stage of low chemical activity, considering that most of C3S has alreadybeen formed with a remaining amount of free lime around 1%. Nevertheless, it is a zonesubmitted to temperature variations since it is right under the influence of the secondaryair temperature coming from the cooler.This area is characterized by the following aspects: • Presence of the clinker liquid phase; • Chemical attacks by alkaline sulfates; • Frequent temperature variations when flame impinges over the brick; • Continuous thermal shock; • Redox atmosphere when using alternative fuels with poorly designed burner; • Mechanical stress imposed by the tire station and kiln shell ovality.In order to support the temperature variations under mechanical stress, this part of theprocess requires the use of basic bricks with high structural flexibility, low permeability togas, high hot modules of rupture and abrasion resistance.E. Pre-cooling zone from: 1200ºC to 1000°C (+)Originally, many kilns have been designed to promote the end of freezing andcrystallization of the just developed clinker phases. However, nowadays, the existence ofthis zone into the kiln depends of the clinker cooler type and the secondary airtemperature entering into the kiln. With old grate coolers it was around 700ºC, and for themodern high efficiency ones from 1150°C to 1100°C. In this zone at that temperaturerange, there is high abrasion (clinker nodules), accentuated discharge erosion (by theclinker dust carried by secondary and tertiary airs) and mechanic stresses (nose ring platesand retention ring for refractory products).The main characteristics of this kiln zone are: • High abrasion / erosion; • Frequent thermal shocks; • High mechanical stresses (compression/traction).In most of the modern furnaces equipped with high efficiency coolers, this zone is notinside the rotary kiln but in the first cooling compartment.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 27
  • 28. Wet ProcessThe original rotary cement kilns were called wet process kilns. In their basic form theywere relatively simple compared with modern developments. The raw meal was supplied atambient temperature in the form of slurry.A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because alot of water has to be evaporated and the process of heat transfer is not very efficient.The slurry may contain about 40% water. This takes a lot of energy to evaporate andvarious developments of the wet process were aimed at reducing the water content of theraw meal. An example of this is the filter press (imagine a musical accordion 10-20 metreslong and several metres across) - such adaptions were described as semi-wet processes.The wet process has survived for over a century because many raw materials are suited toblending as slurry. Also, for many years, it was technically difficult to get dry powders toblend adequately.Quite a few wet process kilns are still in operation, usually now with higher-tech bits boltedon. However, new cement kilns are of the dry process type.Dry ProcessIn a modern works, the blended raw material enters the kiln via the pre-heater tower. Here,hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are usedto heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.The dry process is much more thermally efficient than the wet process.Firstly, and most obviously, this is because the meal is a dry powder and there is little or nowater that has to be evaporated.Secondly, and less obviously, the process of transferring heat is much more efficient in a dryprocess kiln.An integral part of the process is a heat exchanger called a suspension preheater. This is atower with a series of cyclones in which fast-moving hot gases keep the meal powdersuspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal isat almost the same temperature as the gas.The basic dry process system consists of the kiln and a suspension preheater. The rawHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 28materials, limestone and shale for example, are ground finely and blended to produce the
  • 29. raw meal. The raw meal is fed in at the top of the preheater tower and passes through theseries of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinkercooler are blown through the cyclones. Heat is transferred efficiently from the hot gases tothe raw meal.The heating process is efficient because the meal particles have a very high surface area inrelation to their size and because of the large difference in temperature between the hotgas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before enteringthe kiln.A development of this process is the precalciner kiln. Most new cement plant is of thistype. The principle is similar to that of the dry process preheater system but with the majoraddition of another burner, or precalciner. With the additional heat, about 85%-95% of themeal is decarbonated before it enters the kiln. Fig 11: Preheater Precalciner KilnHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 29
  • 30. Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process),the kiln can be shorter and of smaller diameter for the same output. This reduces thecapital costs of a new cement plant. A dry process kiln might be only 70m long and 6m widebut produce a similar quantity of clinker (usually measured in tonnes per day) as a wetprocess kiln of the same diameter but 200m in length. For the same output, a dry processkiln without a precalciner would be shorter than a wet process kiln but longer than a dryprocess kiln with a precalciner.Literature ReviewCorrosion • What is corrosion • Types of corrosionRefractories • What is refractory • Types of refractories • What is the composition of refractories used at SCL,BeawarWhat is corrosionCorrosion is the gradual destruction of material, usually metals, by chemical reaction withits environment. In the most common use of the word, this means electro-chemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, theformation of iron oxides, is a well-known example of electrochemical corrosion. This type ofdamage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occurin materials other than metals, such as ceramics or polymers, although in this context, theterm degradation is more common. Corrosion degrades the useful properties of materialsand structures including strength, appearance and ability to contain a vessels contents.Many structural alloys corrode merely from exposure to moisture in the air, but the processcan be strongly affected by exposure to certain substances. Corrosion can be concentratedlocally to form a pit or crack, or it can extend across a wide area more or less uniformlycorroding the surface. Because corrosion is a diffusion controlled process, it occurs onHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 30exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such
  • 31. as passivation and chromate-conversion, can increase a materials corrosion resistance.However, some corrosion mechanisms are less visible and less predictable.Types of corrosion1) Uniform Corrosion or General CorrosionThis type of corrosion is chemical or electrochemical in nature. However, there are nodiscrete anode or cathode areas. This form of corrosion is uniform over the surface of themetal exposed to the environment. The metal gradually becomes thinner and eventuallyfails.The energy state of the metal is basically what causes this reaction. Referred to as the“dust-to-dust” process, high levels of energy are added to the raw mmaterial to producethe metal. This high energy level causes an unnaturally high electrical potential. One law ofchemistry is that all materials will tend to revert to its lowest energy level, or its naturalstate. After high levels of energy are added to the metal, when it is exposed to theenvironment (an electrolyte), it will tend to revert to its natural state. This process isnormally extremely slow, and is dependent on the ion concentration of the electrolyte thatit is exposed to. Only under very extreme conditions (acidic electrolyte) can this form ofcorrosion be significant. The corrosion rate for steel climbs drastically at a pH below 4, andat a pH of about 3 , the steel will dissolve.General corrosion tends to slow down over time because the potential gradually becomeslower. Failures of pipelines or tanks would not quickly occur from this type of corrosionsince no pitting or penetration of the structure occurs, just a general corrosion over theentire surface (except under very extreme circumstances where the metal could dissolve inan acid electrolyte). However, in nature, the metal is not completely uniform and theelectrolyte is not completely homogeneous, resulting in electrochemical corrosion cells thatgreatly overshadow this mild form of corrosion.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 31
  • 32. Fig 12: Uniform corrosion2) Concentration Cell CorrosionThis type of corrosion is caused by an electrochemical corrosion cell. The potentialdifference (electromotive force) is caused by a difference in concentration of somecomponent in the electrolyte. Any difference in the electrolyte contacting the metal formsdiscrete anode and cathode regions in the metal. Any metal exposed to an electrolyteexhibits a measurable potential or voltage. The same metal has a different electricalpotential in different electrolytes, or electrolytes with different concentrations of anycomponent. This potential difference forces the metal to develop anodic and cathodicregions. When there is also an electrolyte and a metallic path, the circuit is complete,current flows, and electrochemical corrosion will occur. Soil is a combination of manydifferent materials. There are also many different types of soil, and even the same type ofsoil varies greatly in the concentration of its constituents. Therefore, there is no such thingas truly homogeneous soil.These soil variations cause potential differences (electromotive force) on the metal surfaceresulting in electrochemical corrosion cells. Liquids tend to be more uniform, but can varyin the concentration of some components such asHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 32
  • 33. Fig 13: Concentration Cell Caused by Different Environmentsoxygen varies by depth and flow rates. Biological organisms are present in virtually all-natural aqueous environments, these organisms tend to attach to and grow on the surfaceof structural materials, resulting in the formation of a biological film, or biofilm. These filmsare different from the surrounding electrolyte and have many adverse effects. Followingare examples of common forms of concentration cell corrosion. I. Dissimilar Environment II. Oxygen Concentration III. Moist/Dry ElectrolyteIV. Non-Homogeneous Soil V. Concrete / Soil InterfaceVI. Backfill ImpuritiesVII. Biological Effects3) Galvanic CorrosionThis type of corrosion is caused by an electrochemical corrosion cell developed by apotential difference in the metal that makes one part of the cell an anode, and the otherpart of the cell the cathode. Different metals have different potentials in the sameelectrolyte. This potential difference is the driving force, or the voltage, of the cell. As withany electrochemical corrosion cell, if the electrolyte is continuous from the anode to thecathode and there is a metallic path present for the electron, the circuit is completed andcurrent will flow and electrochemical corrosion will occur.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 33
  • 34. I. Dissimilar Metals The most obvious form of this type of corrosion is when two different kinds of metal are in the electrolyte and metallically bonded or shorted in some manner. All metals exhibit an electrical potential; each metal has its distinctive potential or voltage (paragraph 2-4). When two different metals are connected, the metal with the most negative potential is the anode; the less negative metal is the cathode. An “active” metal is a metal with a high negative potential, which also means it is anodic when compared to most other metals. A “noble” metal is a metal with a low negative potential, which also means it is cathodic when compared to most other metals. Dissimilar metal corrosion is most severe when the potential difference between the two metals, or “driving voltage,” is the greatest. Fig 14: Galvanic Corrosion Cell Caused by Different Metals Examples of active metals are new steel, aluminum, stainless steel (in the active state), zinc, and magnesium. Examples of noble metals are corroded steel, stainless steel (in the passivated state), copper, bronze, carbon, gold, and platinum. One example of this type of corrosion occurs when coated steel pipelines are metallically connected to bare copper grounding systems or other copper pipelines (usually water lines) II. Dissimilar Alloys The most obvious example of this type of corrosion is different metal alloys. For example, there are over 200 different alloys of stainless steel. Also, metals are not 100 percent pure. They normally contain small percentages of other types of metals. Different batches of a metal vary in content of these other metals. Different manufacturers may use different raw materials and even the same manufacturer may use raw materials from different sources. Each batch of metal mayHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 34 be slightly different in electrical potential. Even in the same batch of metal, the
  • 35. concentration of these other materials may vary slightly throughout the finished product. All these differences will produce the electromotive force for this type of corrosion to occur.III. Impurities in Metal No manufacturing process is perfect. Small impurities may be mixed into the metal as it is produced or cooled. Impurities at the surface of the metal may become part of the electrolyte causing concentration cell corrosion, or if metallic, they may be anodic (corrodes and leaves a pit behind), or cathodic (corroding surrounding metal).IV. Temperature Metal that is at an elevated temperature becomes anodic to the same metal at a lower temperature. As previously discussed, a more active metal is anodic to a more noble metal. Since elevated temperature makes a metal more active, it becomes anodic to the rest of the metal. This electrochemical corrosion cell may cause accelerated corrosion on metals that are at elevated temperatures.4) Stray Current CorrosionThis type of electrochemical corrosion cell is caused by an electromotive force from anexternal source affecting the structure by developing a potential gradient in the electrolyteor by inducing a current in the metal, which forces part of the structure to become ananode and another part a cathode. This pickup and discharge of current occurs when ametallic structure offers a path of lower resistance for current flowing in the electrolyte.This type of corrosion can be extremely severe because of very high voltages that can beforced into the earth by various sources.The potential gradient in the electrolyte forces one part of the structure to pick up current(become a cathode) and another part of the structure to discharge current (become ananode).Stray current corrosion occurs where the current from the external source leaves the metalstructure and enters back into the electrolyte, normally near the external power sourcecathode. The external power source is the driving force, or the voltage, of the cell. Straycurrent corrosion is different from natural corrosion because it is caused by an externallyinduced electrical current and is basically independent of such environmental factors asconcentration cells, resistivity, pH and galvanic cells. The amount of current (corrosion)How to control kiln shell corrosion, SCL Beawar (Raj.) Page 35
  • 36. depends on the external power source, and the resistance of the path through the metallicstructure compared to the resistance of the path between the external source’s anode andcathode. Fig 15: Stray Current Corrosion Cell Caused byExternal Anode and CathodeAn example of stray current corrosion is caused by impressed current cathodic protectionsystems, where a “foreign” electrically continuous structure passes near the protectedstructures anodes and then crosses the protected structure (cathode). This corrosion isusually found after failures in the foreign structure occur. Stray current corrosion is themost severe form of corrosion because the metallic structure is forced to become an anodeand the amount of current translates directly into metal loss. If the amount of currentleaving a structure to enter the electrolyte can be measured, this can be directly translatedinto metallic weight loss. Different metals have specific amounts of weight loss whenexposed to current discharge. This weight loss is normally measured in pounds (orkilograms) of metal lost due to a current of one amp for a period of one year (one amp-year). For example, if a stray current of just two amps were present on a steel pipeline, theresult would be a loss of 18.2 kilo grams (40.2 pounds) of steel in one year. For a coatedpipeline, this could result in a penetration at a defect in the coating in an extremely shortperiod of time, sometimes only a few days.5) Crevice CorrosionCrevice Corrosion refers to the localized attack on a metal surface at, or immediatelyadjacent to, the gap or crevice between two joining surfaces. The gap or crevice can beformed between two metals or a metal and non-metallic material. Outside the gap orwithout the gap, both metals are resistant to corrosion.The damage caused by crevice corrosion is normally confined to one metal at localized areawithin or close to the joining surfaces.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 36
  • 37. Fig 16: a type 316 stainless steel tube and tube sheet from a heat exchanger in a seawater reverse osmosis (SWRO) desalination plant suffered crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet.6) Pitting CorrosionCertain conditions, such as low concentrations of oxygen or high concentrations of speciessuch as chloride which complete as anions, can interfere with a given alloys ability to re-form a passivating film. In the worst case, almost all of the surface will remain protected,but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion atthese points will be greatly amplified, and can cause corrosion pits of several types,depending upon conditions. While the corrosion pits only nucleate under fairly extremecircumstances, they can continue to grow even when conditions return to normal, since theinterior of a pit is naturally deprived of oxygen and locally the pH decreases to very lowvalues and the corrosion rate increases due to an auto-catalytic process. In extreme cases,the sharp tips of extremely long and narrow corrosion pits can cause stress concentrationto the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly smallhole can hide a thumb sized pit from view. These problems are especially dangerousbecause they are difficult to detect before a part or structure fails. Pitting remains amongthe most common and damaging forms of corrosion in passivated alloys, but it can beprevented by control of the alloys environment. Fig 17: Scheme of pitting corrosionHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 37
  • 38. What are RefractoriesAny material can be described as a ‘refractory,’ if it can withstand the action of abrasive orcorrosive solids, liquids or gases at high temperatures. The various combinations ofoperating conditions, in which refractories are used, make it necessary to manufacture arange of refractory materials with different properties. Refractory materials are made invarying combinations and shapes depending on their applications. General requirements ofa refractory material are: • Withstand high temperatures • Withstand sudden changes of temperatures • Withstand action of molten metal slag, glass, hot gases, etc • Withstand load at service conditions • Withstand load and abrasive forces • Conserve heat • Have low coefficient of thermal expansion • Should not contaminate the material with which it comes into contactDepending on the area of application such as boilers, furnaces, kilns, ovens etc,temperatures and atmospheres encountered different types of refractories are used.Some of the important properties of refractories are:a) Melting PointPure substances melt instantly at a specific temperature. Most refractory materials consistof particles bonded together that have high melting temperatures. At high temperatures,these particles melt and form slag. The melting point of the refractory is the temperature atwhich a test pyramid (cone) fails to support its own weight.b) SizeThe size and shape of the refractories is a part of the design of the furnace, since it affectsthe stability of the furnace structure. Accurate size is extremely important to properly fitthe refractory shape inside the furnace and to minimize space between construction joints.c) Bulk DensityThe bulk density is useful property of refractories, which is the amount of refractorymaterial within a volume (kg/m3). An increase in bulk density of a given refractoryincreases its volume stability, heat capacity and resistance to slag penetration.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 38
  • 39. d) PorosityThe apparent porosity is the volume of the open pores, into which a liquid can penetrate, asa percentage of the total volume of the refractory. This property is important when therefractory is in contact with molten charge and slag. A low apparent porosity preventsmolten material from penetrating into the refractory. A large number of small pores isgenerally preferred to a small number of large pores.e) Cold Crushing StrengthThe cold crushing strength is the resistance of the refractory to crushing, which mostlyhappens during transport. It only has an indirect relevance to refractory performance, andis used as one of the indicators of abrasion resistance. Other indicators used are bulkdensity and porosity.f) Pyrometric cones and Pyrometric cones equivalent (PCE)The ‘refractoriness’ of (refractory) bricks is the temperature at which the refractory bendsbecause it can no longer support its own weight. Pyrometric cones are used in ceramicindustries to test the refractoriness of the (refractory) bricks. They consist of a mixture ofoxides that are known to melt at a specific narrow temperature range. Cones with differentoxide composition are placed in sequence of their melting temperature alongside a row ofrefractory bricks in a furnace. The furnace is fired and the temperature rises. One cone willbends together with the refractory brick. This is the temperature range in oC above whichthe refractory cannot be used. This is known as Pyrometric Cone Equivalent temperatures. Fig 17: Pyrometric conesg) Creep at high TemperatureCreep is a time dependent property, which determines the deformation in a given time andat a given temperature by a refractory material under stress.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 39
  • 40. h) Volume stability, expansion, and shrinkage at high temperaturesThe contraction or expansion of the refractories can take place during service life. Suchpermanent changes in dimensions may be due to: • The changes in the allotropic forms, which cause a change in specific gravity • A chemical reaction, which produces a new material of altered specific gravity • The formation of liquid phase • Sintering reactions • Fusion dust and slag or by the action of alkalies on fireclay refractories, to form alkali-alumina silicates. This is generally observed in blast furnacesi) Reversible thermal expansionAny material expands when heated, and contracts when cooled. The reversible thermalexpansion is a reflection on the phase transformations that occur during heating andcooling.j) Thermal ConductivityThermal conductivity depends on the chemical and mineralogical composition and silicacontent of the refractory and on the application temperature. The conductivity usuallychanges with rising temperature. High thermal conductivity of a refractory is desirablewhen heat transfer though brickwork is required, for example in recuperators,regenerators, muffles, etc. Low thermal conductivity is desirable for conservation of heat,as the refractory acts as an insulator. Additional insulation conserves heat but at the sametime increases the hot face temperature and hence a better quality refractory is required.Because of this, the outside roofs of open-hearth furnaces are normally not insulated, asthis could cause the roof to collapse. Lightweight refractories of low thermal conductivityfind wider applications in low temperature heat treatment furnaces, for example in batchtype furnaces where the low heat capacity of the refractory structure minimizes the heatstored during the intermittent heating and cooling cycles. Insulating refractories have verylow thermal conductivity. This is usually achieved by trapping a higher proportion of air intothe structure. Some examples are: • Naturally occurring materials like asbestos are good insulators but are not particularly good refractories • Mineral wools are available which combine good insulating properties with good resistance to heat but these are not rigid • Porous bricks are rigid at high temperatures and have a reasonably low thermal conductivity.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 40
  • 41. Type of RefractoriesRefractories can be classified on the basis of chemical composition, end use and methods ofmanufacture as shown below.Classification method ExampleChemical CompositionACID, which readily combines with bases Silica, Semisilica, AluminosilicateBASIC, which consists mainly of metallic Magnesite, Chrome-magnesite, Magnesite-oxides that resist the action of bases chromite, DolomiteNEUTRAL, which does not combine with Fireclay bricks, Chrome, Pure Aluminaacids nor basesSpecial Carbon, Silicon Carbide, ZirconiaEnd use Blast furnace casting pitMethod of manufacture Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying) Table 3: Classification of refractories based on chemical composition a. Fireclay refractoriesFirebrick is the most common form of refractory material. It is used extensively in the ironand steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry,and many others.Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractoriesconsist of aluminum silicates with varying silica (SiO ) content of up to 78 percent and 2Al O content of up to 44 percent. Table 4 shows that the melting point (PCE) of fireclay 2 3brick decreases with increasing impurity and decreasing Al O . This material is often used in 2 3furnaces, kilns and stoves because the materials are widely available and relativelyinexpensive.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 41
  • 42. Brick Type Percentage Percentage Percentage other PCE °C SiO2 Al2O3 constituentsSuper Duty 49-53 40-44 5-7 1745-1760High Duty 50-80 35-40 5-9 1690-1745Intermediate 60-70 26-36 5-9 1640-1680High Duty 65-80 18-30 3-8 1620-1680SiliceousLow Duty 60-70 23-33 6-10 1520-1595 Table 4: Properties of fireclay Bricksb. High alumina refractoriesAlumina silicate refractories containing more than 45 percent alumina are generally termedas high alumina materials. The alumina concentration ranges from 45 to 100 percent. Therefractoriness of high alumina refractories increases with increase in alumina percentage.The applications of high alumina refractories include the hearth and shaft of blast furnaces,ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.c. Silica BrickSilica brick (or Dinas) is a refractory that contains at least 93 percent SiO2. The raw materialis quality rocks. Various grades of silica brick have found extensive use in the iron and steelmelting furnaces and the glass industry. In addition to high fusion point multi-typerefractories, other important properties are their high resistance to thermal shock (spalling)and their high refractoriness. The outstanding property of silica brick is that it does notbegin to soften under high loads until its fusion point is approached. This behavior contrastswith that of many other refractories, for example alumina silicate materials, which begin tofuse and creep at temperatures considerably lower than their fusion points. Otheradvantages are flux and stag resistance, volume stability and high spalling resistance.d. MagnesiteMagnesite refractories are chemically basic materials, containing at least 85 percentmagnesium oxide. They are made from naturally occurring magnesite (MgCO3). Theproperties of magnesite refractories depend on the concentration of silicate bond at theoperating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio ofless than two with a minimum ferrite concentration, particularly if the furnaces lined withthe refractory operate in oxidizing and reducing conditions. The slag resistance is very highparticularly to lime and iron rich slags.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 42
  • 43. e. Chromite RefractoriesTwo types of chromite refractories are distinguished: • Chrome-magnesite refractories, which usually contain 15-35 percent Cr O and 42-50 2 3 percent MgO. They 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. • Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent Cr O . They are suitable for service at the highest temperatures and for 2 3 contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite.f. Zirconia RefractoriesZirconium dioxide (ZrO2) is a polymorphic material. It is essential to stabilize it beforeapplication as a refractory, which is achieved by incorporating small quantities of calcium,magnesium and cerium oxide, etc. Its properties depend mainly on the degree ofstabilization, quantity of stabilizer and quality of the original raw material. Zirconiarefractories have a very high strength at room temperature, which is maintained up totemperatures as high as 15000C. They are therefore useful as high temperatureconstruction materials in furnaces and kilns. The thermal conductivity of zirconium dioxideis much lower than that of most other refractories and the material is therefore used as ahigh temperature insulating refractory. Zirconia exhibits very low thermal losses and doesnot react readily with liquid metals, and is particularly useful for making refractory cruciblesand other vessels for metallurgical purposes. Glass furnaces use zirconia because it is noteasily wetted by molten glasses and does not react easily with glass.g. Oxide Refractories(Alumina)Alumina refractory materials that consist of aluminium oxide with little traces of impuritiesare known as pure alumina. Alumina is one of the most chemically stable oxides known. It ismechanically very strong, insoluble in water, super heated steam, and most inorganic acidsand alkalies. Its properties make it suitable for the shaping of crucibles for fusing sodiumcarbonate, sodium hydroxide and sodium peroxide. It has a high resistance in oxidizing andreducing atmosphere. Alumina is extensively used in heat processing industries. Highlyporous alumina is used for lining furnaces operating up to 1850oC.h. MonolithicsMonolithic refractories are single piece casts in the shape of equipment, such as a ladle asshown in Figure 18. They are rapidly replacing the conventional type fired refractories inmany applications including industrial furnaces. The main advantages of monolithics are:How to control kiln shell corrosion, SCL Beawar (Raj.) Page 43
  • 44. • Elimination of joints which is an inherent weakness • Faster application method • Special skill for installation not required • Ease of transportation and handling • Better scope to reduce downtime for repairs • Considerable scope to reduce inventory and eliminate special shapes • Heat savings • Better spalling resistance • Greater volume stabilityMonolithics are put into place using various methods, such as ramming, casting, gunniting,spraying, and sand slinging. Ramming requires proper tools and is mostly used in coldapplications where proper consolidation of the material is important. Ramming is also usedfor air setting and heat setting materials. Because calcium aluminate cement is the binder,it will have to be stored properly to prevent moisture absorption. Its strength startsdeteriorating after 6 to 12 months. Fig 18: A monolithic lining for LadelHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 44
  • 45. Type and Composition of refractory used at SCL, Beawar (kiln 2)Brick Type TOPMAG A1 ALMAG 85 FERROMA PERILEX 83 G 90 Magnesia- Magnesia- Magnesia- Magnesia- Fused Spinel Fused Spinel Hercynite ChromiteCharacteristi 77-81 MgO 85-89MgO 87-92MgO 81-85MgOc Componentin %bulk density 2.9-3.05 2.85-3 2.85-3 2.9-3.05(g/cm3)Apparent 15-17 16-18 16-18 17-19porosity %Cold 65 50 50 55CrushingStrengthN/mm2Seger Cone >42 >42 >42 42Thermalexpansion 400°C 0.3 0.4 0.4 0.4Lin. % at 800°C 0.9 0.8 0.9 1.1 1200°C 1.4 1.4 1.5 1.7Thermal 100 100 100 80Shockresistance at950°C/airThermalConductivity 300°C 3.9 4 3.7 4W/m.K at 700°C 2.9 3 3 3 1000°C 2.6 2.7 2.6 2.8Typical field upper transition upper and lower burning burning Zonesof zone, tyre transition zones zone and as well as upperapplication section subject subject to severe upper and lower to high service transition transition Zones mechanical load conditions with zone, good subject to with redox alkali attack and coatability, severe service conditions and redox chrome ore conditions,low extreme alkali conditions, free Chrome Content attack, chrome chrome ore free ore freeHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 45
  • 46. Above mentioned refractories are imported bricks. These bricks are basic bricks. Currentlythese bricks are using in Kiln-2(at outlet where temperature is very high) with addition ofhigh Alumina bricks at kiln inlet where temperature is not so high. Where as in kiln-1 we areusing only High Alumina bricks.Currently High Alumina Bricks (made in India) using in Kiln-1 and in inlet of kiln-2.Product Maximum C.C.S. Chemical analysis Refractoriness Materialname Recommended (Kg/Cm2) Pyrometric Req. Temperature Cone.orton (°C) Al2O3 Fe2O3AC 40S 1420 395 40.5 2.15 30 2.21AC 60S 1480 545 59.2 2.5 35 2.42AC 70S 1460 720 69.8 3 36 2.61How to control kiln shell corrosion, SCL Beawar (Raj.) Page 46
  • 47. Full details of Refractory linings, Coating and SS Plate Used:For Kiln-1Fig 19: Refractory lining of kiln-1How to control kiln shell corrosion, SCL Beawar (Raj.) Page 47
  • 48. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 48
  • 49. For Kiln-2Fig 20: Refractory lining of Kiln-2How to control kiln shell corrosion, SCL Beawar (Raj.) Page 49
  • 50. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 50
  • 51. Corrosion of Kiln ShellIntroductionWith the ever increasing demand of cement due to the exponential growth of constructionindustry, Indian Cement Industry has been put to perform at its best than ever before. Withthe advances in understanding the cement chemistry and material behavior in side rotarykiln, lot many alternate raw materials and fuels have been either in use or beinginvestigated for their suitability. While these alternate raw materials and fuels proved to bebeneficial in terms of financial aspects associated with them, but the presence ofdeleterious volatile compounds posed equally serious threats to cause problems such askiln shell corrosion, build ups and rings besides attacking the refractory lining and reducingtheir campaign lives. Amongst these, kiln shell corrosion is most serious problem as it actssilently and reduces the shell thickness to below critical structural and mechanical limits ofstability of kiln shell.Corrosion can be defined as the destruction or deterioration of material due to the reactionwith its environment. The grades of steel used for kiln shell range from general engineeringsteel to low alloy steels. The minimum shell plate thickness is around 20 mm for the shellsection between two supports and those under the tyres (riding rings) may be between 60-80 mm thick. A reduction of the shell thickness due to corrosion can be presumed to becritical when the thickness of the shell becomes 15 mm or so. To overcome the problem ofkiln shell corrosion, the best way could be to prevent the volatiles to reach upto kiln shell.To prevent the passage of volatiles, refractories play a vital role and the selection of properquality, adequate installation measures and highly oxidizing conditions in hottest zone ofthe kiln, stable kiln operation at high speed proved as the key to reduce / minimize theextent of kiln shell corrosion. Here I will try to bring out the role of the refractories andprocess conditions to reduce / minimize the kiln shell corrosion based upon the studiescarried out in the past.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 51
  • 52. Corrosion of Cement KilnCorrosion of cement kiln shell is influenced by a number of factors such as composition ofthe metallic shell and its environment, temperature of the shell, cleanliness or roughnessof the shell surface, its contact with other materials and severe process conditions.Further, it is determined largely by the degree to which the scale formed under particularcondition blocks further action between the shell and environment. Each steel or alloybehaves more or less individually and forms its own characteristic type of scale whosecomposition and imperviousness are specific to the given alloy, atmosphere, temperatureand duration of exposure. Consequently, even a slight difference in composition of steel oratmosphere for instance, the presence of sulphur may have a substantial influence uponthe type and progress of corrosion.It has been reported that carbon dioxide and sulphur dioxide are active scaling agents ofiron and steel, carbon dioxide being less deterrent. The presence of sulphur dioxideincreases rate of scaling and often results in deep intergranular penetration of the steelthrough the formation of a liquid iron oxide – iron sulphide eutectic. The deleterious effectsof sulphur dioxide can be offset by providing excess oxygen. Alloying elements such aschromium, aluminium and silicon present in steel may greatly affect the rate of scaling.When present in significant concentration, they oxidize rapidly yielding a relativelyimpervious film which retards the rate of further attack on the underlying metal. On theother hand a high concentration of sulphur increases the rate of attack just as does thesulphur in the atmosphere. The influence of carbon is relatively small. The main reason ofshell corrosion can be attributed to alternate oxidation at high temperature and acidicreaction at low temperatures when the kiln is stopped for repairs. The corrosionphenomenon takes place mainly due to presence of oxides, chlorides and sulphide at hightemperature. Various types of corrosion that affect the kiln shell are:(i) Corrosion due to oxidation under high temperature(ii) Corrosion due to sulphide under high temperature(iii) Corrosion due to chloride under high temperature(iv) Corrosion due to hygroscopic materialThe rate of corrosion depends on the material, the surface condition, the corrosionmedium, the time available and the temperature. The resistance to scaling of steels alsodiminishes in consequence of frequent temperature changes. Investigations have shownthat in the majority of cases the corrosive attack is intensified by frequent changes inHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 52temperature. In most cases, the damage due to high temperature corrosion manifests
  • 53. itself in general removal of material or in superficial cracking. Oxidation in the kilnatmosphere is possible due to the presence of O2, H2O/ H2, CO2/CO.The occurrence of high temperature corrosion under surface deposits is also an importantfactor. The damage due to corrosion is always intensified if the surface of the affectedportion is covered with such deposits. During fairly long kiln shutdown for repairs, a rustingprocess is also presumed to be superimposed upon the high temperature corrosion(scaling) that has occurred during kiln operation. The deposit of salts containing potassiumchloride in particular on the shell becomes active because being hygroscopic it absorbsatmospheric moisture. It is observed that the chloride can reach the kiln shell in the formof gases, the same is not the case for alkali oxides.Alkalis can only penetrate the lining as a part of liquid potassium and / or Sodium saltmelts. If the corrosion products therefore contain substantial quantities of Potassium orSodium, the form of corrosion is termed as Hot Corrosion indicating that liquid phase takespart in the corrosion reactions. Mechanism of kiln shell corrosionThe reactions inside the kiln are different from reactions on the kiln shell surface since boththe temperature and atmosphere are different. One of the most important reactions in thelining is the oxygen consumption where SO2 consumes oxygen and condenses as SO3: 2 SO2 (g) + O2 = 2 SO3 (↓)The SO3 formed condenses as calcium or magnesium salts. The result can be that anoxidizing environment inside the kiln turns into a reducing environment at the kiln shell.OxidationIn an oxidizing atmosphere, the iron from the steel shell will react with oxygen to form anoxide scale. Generally, this oxide scale is formed by more or less firm layers of different ironoxides, the compound with the highest oxygen content, Fe2O3, being found at the scale-brick interface, and the compound with the highest iron content, FeO at the metal-scaleinterface. At normal kiln operating temperatures, the outer layer becomes relatively firm.SulphidizationWhen no oxygen is present, SO2 takes over as the oxygen donor and a different reactionoccurs. The reaction may be written as follows: 4 Fe + 2 SO2 (g) = Fe3O4 + FeS2How to control kiln shell corrosion, SCL Beawar (Raj.) Page 53
  • 54. Accordingly, a sulphidization reaction can be identified by the occurrence of either pyrite(FeS2) or pyrrhotite (FeS) in the corrosion products. The oxidation by O2 and by SO2alternates. As sulphide layers are more porous than oxide layers, the corrosion rate of theshell will increase. However, experience from different plants shows that, as long aschlorides are not present, the corrosion rate stays at an acceptably low level.Sulphidization is enhanced by the presence of chlorides, mainly because they affect themorphology of the corrosion scale, hindering the formation of a strong, protective oxidelayer. The total reaction is a chain process taking place at different temperatures. Atemperature gradient between the kiln atmosphere and the kiln shell is created by theporous deposit and the refractory lining. The first reaction of the chain process takes placein the kiln and can be described as high temperature hydrolysis of the thermally unstablealkali chlorides to form the more stable sulphates. This reaction step is followed by re-oxidation of hydrogen chloride gas (by oxygen or SO2) at lower temperatures to produceelemental chlorine, which attacks the kiln shell. The basic reactions (with potassium asalkali) are: 2 KCl (g) + H2O (g) + SO2 (g) + ½ O2 (g) = K2SO4 + 2 HCl (g) (T > 900oC) 2 HCl (g) + ½ O2 (g) = Cl2 (g) + H2O (T < 400oC)The formation of ‘free’ hydrochloric acid (HCl) gas in cement kilns is thus accompanied byformation of alkali sulphates. When this is the case, the formation is restricted to a quitenarrow temperature between 1100 oC and 1300oC. The formation of hydrochloric acid is aconsequence of the thermal instability of calcium sulphate and the thermal stability ofpotassium sulphate. The evaporation of alkali chlorides cannot begin until thesetemperatures are reached since gas and material move counter-current in the kiln.If most of the KCl in kiln feed can evaporate to KCl (g) at temperatures below 1000 to1150 oC, the formation of HCl (g) will be quite limited because the tendency of KCl (g) tohydrolyse at such temperatures is low. The low temperature evaporation of chloridesexplains why normal preheater kilns are less vulnerable to chloride-enhancedsulphidization.While, in most cases, chlorides in preheater kilns evaporate during or shortly aftercalcinations without substantial formation of hydrogen chloride gas, the case is different forkilns with tertiary air duct. Such kilns will show delayed alkali chloride evaporation and,consequently, evaporation will be followed by more extensive hydrolysis of the chlorides.Once Cl2 (g) is formed, it can reach the kiln shell through the refractory bricks or throughthe gaps/joints within and between rings and will react with either the oxide-sulphideHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 54
  • 55. layers or, most likely, directly with the kiln shell according to the following reactionsresulting in the corrosion of the kiln shell: • reaction with the oxide-sulphide layers: FeS + Fe3O4 + 4 Cl2 = 4 FeCl2 + SO2 + O2 • reaction with the kiln shell: Cl2 + Fe = FeCl2Role of Refractories in tackling shell corrosionThe role of refractories in cement kiln is primarily to protect the steel shell from the directattack of deleterious gases and clinker melt and to reduce the shell temperature so thatsteel of the shell does not loose its properties. The reduction in shell temperature also leadsto energy conservation besides providing a workable condition near kiln shell. The entireCRK system including, preheater, precalciner, rotary kiln and cooler is lined with suitablesize and quality of refractories to achieve the above mentioned advantages. Amongst allthe sections as mentioned above, the service conditions inside the rotary kiln are mostsevere thereby requiring special attention for the shape, type and quality of refractories tobe used and installation practices to be employed.Passage of Volatiles through BricksThe various studies carried out by NCB have established that volatile pass through the bodyof the refractory bricks and reach up to kiln shell. The samples of worn out refractory bricksas collected during visits were cut into three sections, top, middle and bottom and thesewere subjected to chemical and mineralogical investigations to find out the mineral phasespresent and formation of new phases. The results of chemical analysis and XRDinvestigations are given in Table 5 and Table 6 respectively for all the cases.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 55
  • 56. Brick Area Chemical Constituents % Al2O3 Fe2O3 SiO2 SO3 Na2O K2O ClFresh Brick 72.35 2.49 17.85 0.07 0.16 0.38 0.01Worn out Brick-Top LayerCase I 60.57 3.56 15.08 0.76 0.17 6.75 1.00Case II 69.40 2.33 16.39 0.96 0.29 3.35 0.41Case III 62.10 4.00 19.81 0.37 0.44 5.20 0.35Worn out Brick-Middle LayerCase I 65.86 3.47 14.21 0.24 0.19 6.86 1.30Case II 70.70 3.37 14.31 0.52 0.55 2.21 0.46Case III 63.20 3.77 19.48 0.21 0.22 3.70 0.24Worn out Brick-Bottom LayerCase I 74.40 5.31 16.32 0.22 0.18 1.49 0.06Case II 70.18 3.10 15.64 0.91 0.40 0.81 0.12Case III 65.22 5.60 18.60 0.30 0.31 0.90 0.32 Table 5: Chemical Analysis of Different Layers of Worn Out BricksThe results of chemical analysis of the top layer indicate that bricks have undergone verysevere chemical attack, which has resulted in decrease of Alumina content. Theconcentration of SO3, Na2O and K2O was in the range of 0.37 – 0.96, 0.17-0.44 and 3.35-6.75percent respectively. The concentration of chloride was in the range of 0.35-1.00 percent.The results of chemical analysis of middle layer indicate that the volatiles have traveledthrough the bricks and have reached upto the middle layer of the bricks. Concentration ofSO3, Na2O and K2O are in the range of 0.21-0.52, 0.19-0.55 and 2.21-6.86 percentrespectively. The concentration of chloride was in the range of 0.24-1.30 percent.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 56
  • 57. S. Plants Mineral compounds present inNo Top layer Middle layer Bottom layer1 Unused bricks Al2O3, Al6 Si2O13 Al2O3, Al6 Si2O13 Al2O3, Al6 Si2O132 Case I Al2O3, KAlSi3O8, Al6 Si2O13, Al2O3, KAlSi3O8, Al2O3, Al6 Si2O13, ,KCl KAlSiO4, Ca2.Al2SiO7 , KCl Al6 Si2O13, KAlSiO4, Ca2.Al2SiO7 ,KCl3 Case II Al2O3, KAlSi3O8, Al2 SiO5, Al2O3, KAlSi3O8, Al2O3, KAlSi3O8, Al2 SiO5, Na2SO4, Ca2.Al2SiO7 KCl Al2 SiO5, Na2SO4, Ca2.Al2SiO7 Ca3.Al6O12.CaSO4, KCl NaCl4 Case III Al2O3, Al6Si2O13, Al2O3, Al2O3, Al6Si2O13, Al2O3 KAlSi2O6,Al2 SiO5, Al6Si2O13, KCl, SiO2, SiO2 Fe2O3, KCl, Ca.Al2Si2O8 , KCl Al2O3SiO2 NaCl KAl.Si2O6 , , K2SO4, NaCl Table 6: XRD Investigations of different layers of Refractory BricksThe results of chemical analysis of bottom layer of refractory bricks indicate that theconcentration and reactivity of these volatiles is so high that these are able to travel uptothe bottom of the bricks thereby reaching upto the kiln shell. The concentration of SO3,Na2O and K2O are in the range of 0.22-0.91, 0.18-0.40 and 0.81-1.49 percent respectively.The concentration of chloride was in the range of 0.06-0.32 percent. The comparison withthe top and middle layer indicates that the concentration of volatiles has decreased.The results of XRD investigations of corresponding samples also indicate the interaction ofbricks with deleterious volatile oxides leading to formation of feldsphatic compounds likeAl6Si2O13, KAl.Si2O6 , Ca.Al2Si2O8, besides formation of most detrimental oxide i.e. KCl(sylvite). Formation of these compounds in the brick matrix led to volume expansion andHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 57breaking of ceramic bonds ultimately leading to breaking or loosening of bricks.
  • 58. The above investigations of worn out refractory bricks have provided very usefulinformation for the possible causes of kiln shell corrosion. The chemical analysis of toplayers of bricks indicate that these bricks have undergone severe interaction with calcinedkiln feed or clinker and have absorbed volatiles through the open pores in the bricks. Thebricks have three types of pores namely: • Through pores • Closed pores within brick body • One side open and one side closed poresOut of all the three types of pores, one side open and one side closed pores basically createproblem for brick failure and does not directly affect the phenomena of kiln shell corrosion.The closed pores are the ones which neither damage brick structure by way of interactionnor help in kiln shell corrosion.The third types of pores are very dangerous from the point of view of damaging the brickstructure as well as causing kiln shell corrosion. Because these thorough pores providepassage to volatiles to reach up to kiln shell. The best approach would be to check thepermeability of these bricks before dispatch from the site of manufacturers. Thepermeability of bricks gives a very clear picture of through pores present in the bricks.Passage of Volatiles through joints/gapsThe volatile rich kiln gases and clinker liquid are very prone to reach upto to kiln shellthrough the gaps between joints within individual rings or ring to ring joints as well. Thesegaps may occur due to various reasons such as: • Individual brick behavior at working temperature • Inadequate installation due to poor workmanship • Actual service conditions prevailing inside the kiln In order to confirm the possibility of volatiles traveling through the gaps within and between rings and reaching at the interface of bricks and kiln shell, another exercise was carried out. While collecting brick samples during kiln stoppage, some of the rings have shown shifting and gaps created between rings. Samples of worn out bricks were carefully taken out ensuring the coatings on the side surfaces of bricks to remain intact. These coatings were scrapped carefully and analyzed for their chemical constituents. The results of analysis indicate that two types of reactions are taking place.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 58
  • 59. In one case, the results indicate that severe interactions between brick, clinker liquid richin volatiles and material of kiln shell have taken place. The scrapping of coatings on brickside-top has composition which indicates severe reaction with clinker melt and brickmaterial. The enrichment of volatiles as high as 2.60 % SO3 and 7.11 % K2O and 0.39 %chloride confirm severe reaction at the interface resulting in formation of highly aggressiveliquid which entered through the gap and reaching up to kiln shell. At the interface ofbottom layer of brick and kiln shell, this liquid has reacted with kiln shell and complexcompounds have formed. This phenomenon is confirmed by increased Fe2O3 content (6.80%) in the scrapings obtained from side bottom of brick. Figure 21 is showing thephotograph of a brick having heavy coating on one of its side.In another case, the scrapping obtained from brick has shown totally different reactionmechanism. Here the volatile available at kiln surface have reacted with shell materialand intermediate compounds of Fe2O3 and sulfur are formed in liquid state. The presenceof chloride has further aggravated the situation and severe shell corrosion has taken place.These liquids then deposited on the gaps between and within rings. Figure 22 is showingthe brick with heavy metallic deposit on one of its side. Fig 21: Clinker melt deposit on side walls of brick Fig 22: Metallic Deposit on one side of brickHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 59
  • 60. The above clearly confirm that the passage of clinker liquids rich in volatiles and hot volatilegases is much more dangerous as their concentration at the place of corrosion becomesalarmingly high and severe reaction takes place between kiln shell and clinker rich involatiles / volatile gases leading to shell corrosion.Role of Process Parameters on Shell CorrosionThe kiln shell temperature is of major importance to the speed of shell corrosion. In all thestudies carried out by NCB, the shell temperature of the affected areas has been found tobe on the higher side reaching as high as 400 0C thus providing a very conducive conditionsat inner shell to form scale.As known, the carbon steel is poor in oxidation therefore rate of oxidation increase rapidlywith the increase in temperature. It has been reported that the rate of oxidation at 400OC isalmost fourteen times than that at 2000C and it increases manifolds with the increase intemperature. In a rotary kiln where the temperature of outside surface of shell is 300 -4000C, the inside temperature is expected to be much higher and the oxidation rate at thattemperature could be considerably high which is not desirable.Another factor influencing the problem of corrosion is the poor oxidizing conditions insidethe kiln. It is quite clear that O2 and SO2 act alternately as oxygen donors for most of thecorrosion with chloride gas acting as the main promoter, making the corrosion layersporous so that they offer no diffusion protection against further thermal and chemicalattack. The gas analysis measurements indicate severe oxygen deficient conditions at kilninlet in all the cases. The measurements of CO / O2 at kiln inlet were carried out andpresented in Table 7 below: S.No. Plant Gas Analysis O2% CO% 1 Case I 0.79-1.67 0.10-2.12 2 Case II 0.60-2.00 0.20-0.60 3 Case III 0.19-0.31 0.90-1.66 Table 7: Measurement of CO/O2 at kiln inletFrom the above table it is clear that the O2 and CO levels are far from the recommendedlimits of 1.5 and 0.1 percent respectively. Sometime, it was found that the CO/O2 analyzerinstalled at kiln inlet is showing incorrect readings and their levels could not be maintained/ controlled to the above recommended limits.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 60
  • 61. The high rate of corrosion in the burning/pre-burning zone can be due to the fact that thesulphate bearing compounds gets recycled along with feed material and travel up toburning zone where they get partially dissociated and vaporized once again. Due to this, theincrease in SO3 concentration takes place leading to the increase in partial pressure of thesevolatile gases over a period of time. As a result, they get diffused through the open pores ofrefractory lining reach upto the kiln shell and initiate the corrosion process. Manylaboratory experiments conducted elsewhere have reported that high levels of carbonmonoxide due to incomplete combustion of coal also increases the rate of corrosion. In oneof the kiln where the inlet CO formation is going as high as 2.12%, it can easily be expectedthat the CO content in the burning zone will be much higher. Thus, it is essential to burn thefuel completely and as fast as possible to avoid any CO formation or reducing conditions toexist in any part inside the kiln and to have proper and sufficient oxidizing conditions.The chemical analysis of Kiln Shell Flakes indicate that these flakes are formed from theoriginal material of kiln shell and heavy to severe interaction with volatile oxides have takenplace at the interface. The concentration of SO3, Na2O and K2O are in the range of 0.44-3.46, 0.21-0.72 and 0.79-2.49 percent respectively. The concentration of chloride was in therange of 2.90-5.53 percent and is considered to be very high. Oxides Plants Case I Case II Case III LOI 5.74 9.68 23.40 SiO2 3.17 2.10 1.62 Fe2O3 67.32 64.02 63.48 Al2O3 12.56 13.08 2.50 CaO 2.08 1.25 1.71 MgO 0.30 0.25 0.38 SO3 3.46 0.83 0.45 Na2O 0.61 0.72 0.21 K2O 1.60 2.49 0.80 Cl 2.90 5.18 5.63 Table 8: Chemical Analysis of Kiln Shell FlakesThe XRD investigations also confirms the formation of compounds FeS2, NaFeS2, NaCl,, KCl,FeS as major mineral phases in almost all the samples of kiln shell flakes and the othermineral phases present were Fe2O3, Fe3O4, Na2SO4,, MgFe2O4, Al0.5Si0.7O2.25, K6Na4Cl, etc. Thepresence of these compounds clearly indicates that the participating oxides for thesecompounds were Iron from kiln shell and volatiles reached to kiln shell surface throughbricks and gaps/joints.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 61
  • 62. RecommendationsThe possible causes for the occurrence of shell corrosion in cement kiln could therefore besummarized as under: • Penetration of refractories and attack of alkalis on kiln shell • Poorly designed Brick shape and size, • Poor Brick quality in terms of porosity and permeability • Inadequate Installation quality • Improper combustion of fuel leading to reducing conditions inside kilnThe possible preventive measures coming out from the above studies to minimize thetendency and rate of shell corrosion phenomenon are as discussed below: A) It seems to be more successful to apply different type of gas-tight coatings or paints on the inner kiln shell to lessen the chloride attack. The inner kiln shell should be coated with the available anti corrosive paint immediately after de- lining. This will also help in preventing the absorption of atmospheric moisture by hygroscopic compounds such as KCl. a) One coating “KilnGard-600SCW” which is manufactured by 3L&T Company has been proven a good Anti-Corrosive Coatings for cement kilns. This coating has been used in many cement plants of Mexico, Colombia and Brazil. The results obtained after using this coating in cement kilns are shown on next page. It shows that firstly they used SS plate to Combat Corrosion but just after 11 months there was a decrease of 1.3mm in thickness of Shell (Figure 1 on next page). After that they used KilnGard-600SCW and after 12 month the thickness of shell was practically same (Figure 6 on next page).How to control kiln shell corrosion, SCL Beawar (Raj.) Page 62
  • 63. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 63
  • 64. How to control kiln shell corrosion, SCL Beawar (Raj.) Page 64
  • 65. B) Silicon Carbide compounds are an effective solution for corrosion resistance. At operating temperature, decomposition and oxidation form specific glass phases—blocking the porosity and forming new phases with the alkalis without disruptive growth. There is little to no spalling of the hot face and alkali penetration is reduced. Our CALDE™ RAM and CALDE™ PATCH ranges, present very good resistance to alkali corrosion. The alkalis may penetrate the surface, but they will not develop new expanding compounds. C) Basic reason of corrosion is alkalis so to remove these alkalis we can use alkali by-pass system. Alkali Bypass is a duct between the feed end of the kiln and the preheater tower through which a portion of the kiln exit gas stream is withdrawn and quickly cooled by air or water to avoid excess buildup of alkali, chloride and/or sulfur on the raw feed. This may also be referred to as the “kiln exhaust gas bypass”. Currently at SCL we are using Rotary Kiln Manufactured by KHD.KHD also provide service of Bypass system. Bypass Systems of KHD The development of efficient KHD Bypass Systems significantly contributed to the success of the KHD preheater concept and, consequently, to a modern and energy-saving low alkali high quality clinker production system.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 65
  • 66. For many years KHD plants have initially been provided with KHD Bypass Systems which are operating with raw materials of high chloride or sulfate content. Especially for using secondary raw materials and secondary fuels of significant chlorine and/or sulfur contents the KHD Bypass System enables a flexible kiln operation at varying fuel and/or raw-material mixtures. A portion of the volatile components will be removed from in the area of the inlet chamber by means of a bypass and thus be withdrawn from the circuit. This will lower the level of the volatile circuit to such an extent that coatings remain controllable and operational reliability.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 66
  • 67. D) The kiln shell temperature from outside should not exceed 300°C. Additional cooling fans should be provided for effective external cooling. In addition, the selection of bricks in this area should be reviewed with respect to their thermal conductivity and wear rate pattern. If there is no coating on bricks in that area, high wear rate leads to reduced residual lining thickness leading to higher shell temperature. Therefore, Refractory bricks with relatively lower thermal conductivity and high wear resistance should be preferred in the corrosion affected area. Here is attached the kiln shell Temperature profile of Kiln-2 at SCL,Beawar. We can see that Maximum Temperature is around 390°C.which is very high compare to recommend above. So some extra cooling fans should be provided for effective external cooling. Note: - Shell Temperature Profile and max. min. Temp. of shell of Kiln-2 is attached at the end of this report. E) As the investigations have confirmed that the intrusions of volatiles have taken place not only through the bricks but also through joints and gaps created during the campaign. It is recommended that a thorough study of lining quality, design and installation be carried out with a view to ensure that the intrusion of volatiles through bricks and joints/gaps is reduced to minimal. In this connection, it is very important to mention here that besides other parameters, two characteristics should be given due attention namely, permeability and reversal thermal expansion of bricks, While permeability plays a role in allowing gases/liquids to reach up to kiln shell, the reversal thermal expansion gives an idea of extent of shrinkage a particular brick undergo when the kiln temperature is reduced from its normal operating temperature to any lower temperature. F) The shell corrosion is attributed to a large extent to poor oxidizing conditions prevailing in the kiln. The best measure against chloride promoted sulphidization is to keep a steady supply of fresh air to theHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 67
  • 68. surface of the kiln shell. The CO/O2 analyzer installed at the kiln inlet should be made working properly to avoid incomplete combustion of coal which otherwise could lead to uncontrolled/unnoticed formation of CO and low oxygen level at kiln inlet. The oxygen at kiln inlet should be maintained at 1.5 % minimum to ensure proper oxidizing conditions with CO not exceeding 0.1 percent at this point. Here at SCL the % Composition of O2 and CO at Kiln-2 inlet is 5-6% and 0.01- 0.15% respectively. So by looking at above composition we can say that here oxidizing conditions are not poor. These compositions are not aid in corrosion of kiln shell. So in future also company should maintain these conditions. G) Currently in plant we are using combination Basic Magnesia Refractory and high Alumina refractories in Kiln-2.but we are not using basic refractory in kiln-1.In kiln-1 we are using only high Alumina Refractory. In view of corrosion and heat loss use of Basic refractory is highly recommended. Although basic refractories are costlier than neutral refractories but they provide more service time and more resistance to corrosion and alkali attack. H) Ultra-ZCoat is the family name for a series of refractory coatings designed to protect the bricks, monolithics, castables, ceramic fibers and steel shells in furnaces, boilers and various high temperature vessels. Ultra-ZCoat Refractories are characterised by a very high content of zirconia and other special ceramics which imparts extremely high resistance to the aggressive environments typically encountered at temperatures up to 1900°C. Ultra- ZCoat has better adhesion than most conventional zirconia based refractories.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 68
  • 69. A 3mm protective layer of Ultra Z coat can typically increase the working life of any underlying refractory by a factor of two to four times. Ultra Z Coat is supplied in the form of a powder which is mixed with cold water to the consistency of a paste, cement or slurry prior to application by trowel, brush or spray. This Coating is Manufactured by Wearresist Technologies Pvt. Ltd.. Problems Faced: High temperature and Chemical corrosion. Alkali Gases: Sodium, Sulpher and Chlorides. Wear On Kiln: Reduction in shell thickness of the Kiln Remedial Solution Suggested: Ultra-ZCoat N Temperature Range: Upto 1900°C Finish: Matt, Buff colored, Non-Vitreous Fig 26: The Kiln shell is dismantled for remedial repair. Procedure for Application 1.Surface is prepared by sand blasting upto 40-60 microns. 2. Ultra-ZCoat concoction is diluted upto 20% by water. 3. First coat is applied 1 mm thick by spatula. 4. Second coat is applied 1 mm thick after an interval of 6 hours. 5. Coverage Ratio: 2mm thickness gained by application of 4 Kgs over a 1 Sq. meter area.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 69
  • 70. Fig 27: Application of 1st Coat of Ultra-ZCoat Benefits 1. Produced a gas tight surface and minimized energy losses. 2. Reduced the effects of thermal shock. 3. Protected the refractories against the aggressive effects of burning fuel oil, gas and solid fuels. 4. Reduced Spalling and eliminated cracking 5. Reduced Slag adhesion and increased resistance to corrosion 6. Increased the working life of kiln and reduced the costs of maintenance shutdowns. I) Application of spreader jack should be done in each ring at the time of closing the individual rings and due care should be taken so that bricks do not crush or develop internal cracks due to pressure applied by spreader jack. Further, jack should not be used on key bricks. By using Spreader jack we can fix the brick ring with very less gaps in between refractories. So chances of alkali attack through gap in between brick will be very less. In no case chiseling or hammering of bricks should be done. The ring should be closed using key bricks instead of cutting and fixing with the help of hammer.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 70
  • 71. Kiln Shell Temperature Profile with max.min. and avg. shell temperatureHow to control kiln shell corrosion, SCL Beawar (Raj.) Page 71
  • 72. References 1. http://en.wikipedia.org/wiki/Shree_Cement 2. http://www.khd.com/bypass-systems.html 3. http://www.calderys.com/refractory-focus/wear-resistant.html 4. http://www.azom.com/article.aspx?ArticleID=2382 5. http://www.projectsmonitor.com/detailnews.asp?newsid=13713 6. http://corrosion-doctors.org/index.htm 7. http://met-engineering.blogspot.in/2009/10/types-of-corrosion.html 8. http://www.docstoc.com/docs/97075944/PETROLEUM-COKE# 9. Kiln Shell Corrosion by Ricardo Mosci 10.Investigation of cement rotary Kiln corrosion by E. s. Jons, M. J. L. 0stergard 11.http://www.3l-t.com/ 12.Combustion of large solid fuels in cement rotary kilns by Anders Rooma Nielsen 13.Mechanism of Shell Corrosion caused by volatiles in Cement Kilns and remedial measures by D Yadav, S K Chaturvedi, Y P Sethi, M M Ali and M Vasudeva,NCB IndiaNote: The Refractory Lining Data is made available by Shri R.P.Pareek. The Data for Kiln thickness with and without using SS plate is made available by Shri Harshwardhan of PMS,Cell. The data of outer Shell Temperature of Kiln and Different Chemical Composition at inlet is available from CCR, SCL, Beawar.How to control kiln shell corrosion, SCL Beawar (Raj.) Page 72