miDTH INSTITUTE Operation and Optimis: ion of Kiln Systems Kiln Refractory Asocem ‘August 2006 237Gism DTH (INSTITUTE Table of Contents 6.1 INTRODUCTION 3 6.2 REFRACTORY PROPERTIES AND THEIR APPLICATION IN DIFFERENT ZONES .. peeannicets eocneee 62.1 Application of Different Refractory Shapes 6 62.2 Application of Refractory in the Different Zones . sia? 6.2.2.1 Preheater tower... 7 6.2.2.2 Rotary kiln 8 63 BRICK MOUNTING PROCEDURES AND PRACTICES... 10 6.3.1 Removal of Old Lining..... 7 10 6.3.2. Installation of New Lining. wea : a 10 64 GOOD KILN START-UP AND STOPPAGE PRACTICES... 16 6.4.1 Procedure for Drying out of Lining and Initial Start-Up bei 16 64.2 Starting and Stoppage Procedures During Normal Operation. 19 6.5. INSPECTION AND MAINTENANCE OF KILN LINING 2 Asocem Page 2 August 2006 238Gism DTH INSTITUTE KILN REFRACTORY 6.1 INTRODUCTION In the major part of the kiln system, unprotected steel structures have a very low lifetime due to high temperatures and exposure to aggressive compounds. Therefore it must be pro- tected by a lining made from a material that can withstand the high temperatures without fusion and decomposition, This is the refractory material, Refractory material is also used to protect the preheater cyclone, riser pipes and the cooler. The service life of the refractory lining is a function of many different factors. The princi- ple factors affecting the life time of the tining can be summarised as below: = mechanical load and wear ~ thermal load - the grade of brick chosen - mounting procedure ~ start-up and stoppage procedures The last three factors are under the direct influence of the kiln operating staff and therefore of special importance andi will be explained in more detail in this module A relining of the kiln system is expensive both in material, time and lost production. It is therefore essential that the refractory material lasts as long as possible through proper in- stallation and use. This module describes the properties of the refractory material and its application in the dif- ferent zones of the kiln system. Further, mounting procedures and practices, as well as start-up and stoppage procedures, will be described. ‘The last section will include inspec- tion of lining and criteria for replacement, Asocer Page 3 August 2006 239Gsm DTH INSTITUTE 6.2 REFRACTORY PROPERTIES AND THEIR APPLICATION IN DIFFERENT ZONES Refractory materials comes from a large number of producers, in many shapes and of many compositions. In general the properties of refractory material depends on raw materials, chemical and mineralogical composition and manufacturing process. The chemical analysis of refractory brick does not tell the whole story about its characteris- tics as each brick consists of particles of varying crystal structure sintered together, and of melt phase (bonding phase). Porosity also has an important part to play. The strength char- acteristics of refractory is generally speaking dependent on the properties of the bonding phase. The properties of the particles themselves become irrelevant if the bond between them gives away. Due to the importance of having good refractory material, manufactures continuously ex- periment with new materials as information is gathered on existing products. For instance the use of chrome based compounds in some magnesia type bricks have proven to have se~ rious negative environmental side affects e.g, leaching of poisonous salts to the ground wa- ter when disposed of and research has therefore concentrated on suitable alternatives such as magnesite-spinel bricks ‘The specific type of refractory material chosen will depend on the kiln zone in which it is to bbe used, but in general refractory material should have the following properties: + Resistance to abrasion © Resistance to high temperatures «Resistance to thermal shock «Resistance to spalling and chemical influence + Elasticity © Insulation ability Resistance to abrasion Resistance to abtasion is important in the burning zone and cooling zone of the kiln, How- ever, the exposure of the lining depends very much upon the thickness and stability of the clinker coating. In the cooling zone where the coating may be weak and unstable the wear may be correspondingly higher. High resistance to abrasion is normally linked to high den- sity. Asovem, Page August 2006 240Gsm DTH INSTITUTE Resistance to high temperature For a refractory material the maximum service temperature is given and should not be ex- ceeded. The service temperature is related to the refractoriness, which is the temperature at which the material softens. Resistance to thermal shock The damaging effect of thermal shock is mainly due to an unevenly distributed expansion and contraction of the lining material. This expansion may cause internal stresses in the re- fractory material for the following reasons: - the kiln shell restricts lining expansion as the kiln shell temperature is al- ways lower than that of the lining = the temperature of a brick is always higher on the side facing inwards than on the side facing the kiln shell, causing material to twist, ~The temperature on the hot side of a brick is highest when exposed to the heat radiation of the flame and lowest when submerged under the material. In the burning zone the temperature variations may be as much as 200°C per revolution. Resistance to spalting and chemical influence Where fining and kiln shell is exposed to chemical attack, the permeability of a material becomes of great importance. The permeability of a material is dependent on the porosity, such that the higher the porosity the higher the permeability. Aggressive vapours present in the kiln gasses, penetrates through the open pores of the refractory. Here, the vapour will condense at a depth where the temperature corresponds to the evaporation temperature. The liquid compounds will react with the brick minerals resulting in a layer within the brick with reduced strength and resistance to temperature fluctuations. When the brick fractures along this weakened layer, penetration and stratification will continue in accordance with the new temperature profile in the brick. Not only the lining may be damaged by chemical attack. There are examples of serious corrosion occurring on the kiln shell due to the pene tration of chlorine through the lining. To avoid chemical attacks of this nature, the lining must be dense and exhibit low perme- ability in the kiln zones where aggressive vapours and liquids are present. ‘Asocem Pages August 2006 241MIDTH INSTITUTE Elasticity Mechanical forces are exerted on the lining by the kiln shell as it flexes during rotation. To absorb this change in shape the lining must have a certain clasticity. This elasticity is in part achieved by the use of mortar in the joints between the individual bricks. Finally the bricks must have insulating properties to keep the kiln shell temperature within acceptable limits and to reduce the overall heat loss of the kiln system. 6.2.1 Application of Different Refractory Shapes Linings in cylindrical parts are commonly made from bricks of two different classifications, circular bricks or side-arch bricks. The latter is by far the most common duc to better lin lasticity and better possibility of correcting for manufacturing tolerances and kiln shel] it- regularities. The circular brick will therefore not be dealt with here. In conical parts, sections with internal fittings, comers and around planetary cooler inlets, unshaped refractories or monolithic lining is often used, These masses are moulded on the spot and locked to the sieel plate with heat resistant stee! anchors, Arch bricks are produced with tapers corresponding to standard kiln diameters, 2-3 — 4 — — 8 meters and any kiln diameter can be built up by a combination of two standard tapers. The bricks are laid in the kiln with the long side parallel to the kiln axis. Brick sizes are usually characterised by three numbers - e.g, 620. The first number refers to the diameter of the kiln expressed in meters, and the following two numbers denote the thickness of the lining in centimetres. For any kiin diameter, the mixing ratio and total number of bricks per Fing can be derived from graphs supplied by the manufacturer. The briek dimensions can vary ftom different suppliers. The international 1SO-standard has constant dimensions: 198 x [03 mm at the base of the brick facing the kiln shell. This so- called 3 shape has larger width (i.¢, 103 mm) than previously used and therefore better taper. As 103 mm + the joint equal 2/3 the number of bricks per ring is 30 x D in meters. Eg. a kiln with a diameter of 4 meters would require 120 brieks per ring, Unshaped refractories, or monolithic lining, are often used where odd shapes are required i.e. in conical parts, scetions with space cabins and corners. These masses are moulded on the spot and locked to the steel plate with heat resistant stecl anchors. The monolithic refractories comprise castables, ramming masses and gunning masses, some of which are hydraulic setting while others are chemicaily bonded. [n most eases, they have to be prepared with water or some other mixing liquid at the work site. Asocem Page 6 August 2006, 242Gsm DTH INSTITUTE Monolithic refractories do not acquire their final properties until the first heating-up of the Kiln to service temperature. It is therefore absolutely cssential that the plant operating and maintenance personnel are familiar with suppliers instructions and recommendations for the specific refractory used. Unshaped refractories is less uniform than that of bricks due to the influence of addition of water, mixing time and working practices. Financially, the normal monolithic lining costs, approximately 30% more than the corresponding bricks. 6.2.2 Application of Refractory in the Different Zones ‘The zone where the refractory is to function determines the specific demands on the mate- rial. For instance in the transition zone of the kiln the importance of heat resistance is a more determining factor than it would be in the kiln inlet zone where the temperatures are lower. ‘The following, will outline the application of refractory material in the preheater tower and the rotary kiln. 6.2.2.1 Preheater tower The lining in the kiln riser pipe is exposed to hot gases containing aggressive vapours such as alkalis and chloride from the kiln. Dense, low-alumina bricks are therefore required as high alumina bricks are exposed to alkali attacks. Complete monolithic linings may be sus- ceptible to anchor collapses and alkaline attacks and are therefore not recommended. In the remaining parts of the preheater the lining is not exposed to potential wear, but it is impor- tant to have a smooth and dense lining, a good insulation and a stable construction. The linings in riser pipes, ducts and cyclones are normally two-layer. The inner lining is preferably made from low-alumina firebricks and insulated with molar bricks, or block in- sulation, ‘The upper stage in 4-stage cyclones normally has one layer of firebrick lining as, tempera- tures are lower Asocem Page? Auguist 2006 243Gis DTH INSTITUTE The roofs of cyclones are usually flat, suspended roofs made from specially shaped fire- bricks, inserted on steel beams. In order to reduce the number of shapes, the comers and edges may be made as monolithic linings In the riser pipe and the lower most stage in 4-stage pre-heaters a more alkali resistant monolithic material is normally recommended due to potential concentration of aggressive alkali vapours, In the colder stages material need not be as alkali resistant. 6.2.2.2 Rotary kiln Inlet Cone In order to avoid the many types of special brick shapes, the cone is normally made as monolithic lining, anchored to the steel tube by means of Y-shaped heat resistant anchors. To ensure a strong dense lining, the castable must be vibrated with a poker vibrator. Caleining Zone In this zone chemical attacks from alkali vapours may be expected and moderate abrasion and temperature variations are encountered. As.a consequence of this a dense or otherwise alkali resistant brick must be used. A good insulation ability of the brick is preferable in order to reduce heat losses. However, a lower insulating ability must often be accepted in opting for a more dense brick which is more resistant to thermal shock. In dry processing and calcining kilns systems, the relative heat Joss from the kiln shell is small, Therefore brick strength often has higher priority than insulating abilities. In kilns with smaller diameters, i.e. less than 4 m, good briek lifetime and insulating abil ties can be obtained using a two-layer lining consisting of low-alumina bricks and insulat- ing molar backing bricks. Alternatively an insulating firebrick can be used in one layer as the fire brick gives a good insulation and can be used up to about 150°C lining tempera- ture. Towards the outlet of the calcining zone kiln conditions approach those of the follow- ing transition zone. Transition Zone In this zone, the lining temperature reaches 1150 ~ 1400°C, and the lining is exposed to chemical reactions with the charge as well as high and varying temperature. The transition Asocem Page 8 ‘August 2006 244Gism DTH INSTITUTE zone is the most troublesome bricking zone due to unstable coating in this area. A basic brick quality may better promote a stable clinker coating which will give a longer brick life. However, if there are many kiln stops coating will not be formed and a high alumina bricks, may in such a case provide a better alternative. Burning Zone In the buming zone very high temperatures are reached, and incipient melting of the charge takes place. Basic type bricks are therefore used as nap-basic bricks may react violently with the clinker liquid phase. The charge may react chemically with the lining material and alkali and sulphur compounds can penetrate the lining causing breakage and rapid break- down. There are three types of basic (non acid) brick qualities used: Magnesite-Chrome, Magne- site-Spinel, and Dotomit Magpesite-Chrome bricks are composed of Periclase and chromate grains in various pro- portions and grain sizes and are bonded by silica additions. The use of this brick type is in decline due to the environmental problems of disposing the toxic chrome compounds. As a result of this, the chrome-free magnesite bricks sugh as Magnesite-Spinel bricks are gaining a wider use for the lining of cement kilns. The magnesite-spinel brick is primarily composed of magnesia and alumina and has many similar properties 10 the magnesite- chrome brick. Dolomite is made of periclase and calcium oxide. In Europe the use of dolomite bricks the hottest part of the burning zone is very common. Dolomite can only be used in the mid- dle of the buing zone where it always is protected by coating as it has little abrasion resis- tance. Dolomite is not resistant to water vapour. Therefore it will deteriorate if the kiln is stopped for longer periods. Substantial heat losses from the kiln shell are found in the burning zone. Insulating proper ties of the lining would therefore seem to be valuable, but only in few cases have insulation behind the basic bricks given satisfactory results. However, in order to avoid damage to the shell the insulation properties of the lining plus the coating should always result in shell temperatures below 350°C. Cooling Zone This zone can be lined with cam lining to facilitate cooling. The lining in this area is ex- posed both to abrasion and to temperature changes. Therefore a temperature resistant high Asocem Pages August 2006 245fiism DTH INSTITUTE alumina bauxite based brick is used. For the dam ring the same type of bricks as for the other part of the cooling zone is used. 6.3 BRICK MOUNTING PROCEDURES AND PRACTICES 1 Removal of Old Lining Excessive time is spent on removal of old lining where this operation is carried out by hand, Furthermore, the working hazards involved are serious. By mechanising the breaking down of old lining, loading and transport, the time consumption can be reduced to '/3 oF “4 of the time required by manual operation thereby reducing total kiln down-time. By remote control of the equipment working hazards can be almost eliminated 6.3.2 Installation of New Lining Correct installation of refractory linings is an important factor in achieving long service life. In other words a high grade refractory product can easily be ruined by impropet instal- lation. Before starting the brick work it is very important to place guidelines on the kiln shell to ensure that the rings are laid perpendicular to the kiln axis. It is recommended that the bricks are laid in straight joint construction without overlap of bricks from one ring to the other. Overlapping of bricks in adjacent rings must be avoided as this inevitably will lead to damage by crushing of the brick comers. The refractory lining should fit as tightly as possible against the kiln shell. The more se- curely the brickwork rings fit, the less will be the amount of relative movement between the lining and the shell. If the rings are fitted too loose the bricks may fall out, or the bricks may be crushed when moving relatively to each other. If basic bricks are used, expansion joints will have to be provided in accordance with the manufactures instructions. The pur- pose of expansion joints is to reduce the stresses caused by thermal expansion and kiln shell ovality. Asocem Page 10 ‘August 2006 246fe ESmomrn INSTITUTE During installation of lining, many practical problems will arise. Depending on installation methods, the kiln cannot be rotated for long periods of time, or the kiln must be tumed at short intervals, It is therefore recommended to work out a time table with due regard (0 co- ordination of lining work and other repair activities inside the kiln. The building up of the lower half of the rings in the I manner. For the upper holf, three different methods are normally employed: Screw~ Method, Kiln-Rig-Method and Gluing- Method ing is always carried out in the same eke Screw-Jack Method This method is mostly used in kilns with diameter less than 4 meters. For larger diameters the screw-jacks required becomes too heavy and because of the larger lengths they are also more liable to buckle. The screw-jack-method has the advantage that the same set of jacks can be used in different diameters. Tapered sections and dam rings ate easily supported with jacks, which are also convenient in connection with smaller repair jobs. Figure 6.1 depicts the use of screw jacks for installation of lining, Figure 1.1 Lining wit Screw Jack Asocem Page 11 August 2006 247fism DTH INSTITUTE From an overall repair point of view the method has the big disadvantage that turning of the kiln is necessary at intervals. As placing of the jacks has to be carried out very carefillly, installation times are rather tong. In general the installation time depends on the type of bonding required. Laying with mor- tar is considerably slower than laying without bonding, material ‘The speed of installation, calculated as square meters installed per man hour (m?/MH) is to some extent independent of the kiln diameter because @ greater number of people can work inside a larger kiln In the following m? /MH is provided to give an idea of some actually observed installation rates. Man hours refer to the entire crew ie. kiln liners and helpers. laid with mortar 0,15 m’/MH Complicated secti ‘MH Cylindrical sections laid without mortar: 0,35 - 0,40 m’/MH Cylindrical sections laid with mortar: 0,25 -0,30 r Kiln-Rig-Method In principle this method is based on a construction created to support the bricks one by one during installation of the upper half of the ring, One or two rings can be completed at the same time, The method is well suited for large diameters and has the big advantage that tuming of the kiln is not necessary, This means that repair jobs elsewhere in the kiln can proceed without interruption. Figure 6.2 shows a diagram of a kiln rig, ‘The method comprises several variations, ¢.g.: 2 simple kiln rig using wedges to press the bricks up against the kiln shell: Pogo-Sticks with spring loaded supports substituting the wedges or: highly mechanised kiln rigs with prefixed hydraulically operated supports. Each Fing is built up separately and before closing the rings, the brickwork is compressed with a hydraulic jack, From a safety point of view the kiln-rig method is superior to the jack method. With in- creasing mechanisation, installation times can be reduced considerably. Further, the quality of the lining work is usually superior to that of the jack method. However, for small repair Jobs kiln rigs are unsuitable, and simple template or jacks may be used instead, Asocem Page 12 August 2006 248fausmiptn INSTITUTE Figure 6.2 Lining with Kiln Rig With highly mechanised kiln rigs the following results have been observed: Complicated sections: not suitable Cylindrical sections laid with mortar: 0,30 - 0,50 m/MH Cylindrical sections laid without mortar: 0,50 ~ 0,75 m?/MH Asocem Page 13, ‘August 2006 249(eas DTH INSTITUTE Comparison with the corresponding performance of the jack method does not give full credit to the rig method, because a greater number of people can work effectively with the kiln rig and thus the overall kiln down time can be reduced. An example from an actual lining job will illustrate the rate of installation. Kiln diameter: 6,3 meter ‘Man power per shift: 16 men 5,9 miday (24 working hours) Average speed in sections without Mortar: 8,9 m/day Maximum speed: 10,4 miday Average speed in sections with mortar: Gluing-Method Using this method, brick in the lower part of the kiln and parallel to the kiln axis, are suc- cessively glued to the Kiln shell, alternating with sections laid in the usual way without glue. The glued sections will keep the inter-laying bricks in position. A suitable glue would be a two component adhesive, based on epoxy or polyacrylate resin. To ensure good bond- ing the kiln shell should be clean and without any dust, oil or moisture. ‘The number of glued strips of brickwork will depend on the diameter of the kiln. Generally speaking the following sequence may be employed: Six courses glued, six not glued, four glued, six not glued, four glued six not glued, ete. Please refer to Figure 6.3. Asocem Page 1 ‘August 2006 250i BD sree erick bricks tte ry BD riche ita ee a working stage 1 Asocem Page 15 August 2006 mp INSTITUTE 251Gasmuprn INSTITUTE Figure 6.3 Lining using Glue Method ‘The method can be used in large diameter kilns and long stretches can be lined at a time. A disadvantage is that the gluing to some extent is depending on climatic conditions (Frost and condensation) and, as with the case of the jack method, turning of the kiln is necessary. A supply problem is involved because of limited storage time of the glue. In new, clean kilns very short installation times have been achieved whereas in old kilns cleaning of the shell may take a long time, Generally, installation time lie between those of the jack method and those of the kiln rig method. 6.4 GOOD KILN START-UP AND STOPPAGE PRACTICES Kiln refractory life time is known to be directly related to the procedures adopted for the heating (up) and cooling (down) of the kiln, Incorrect heating and cooling procedures of the kiln may seriously weaken the refractories and can couse sudden briek failure which neces- sitates a time consuming and expensive kiln stop for repair work. Severe lining failure may even bring about serious damage to the kiln shell which requires extensive mechanical re- pairs. Although the lining will resist high temperatures for a long time, it suffers damage by the stresses induced by the internal expansion and contraction of the material during heating and cooling. Minimising these stresses and protecting the lining from local overheating is, of primary concern to the operational staf? during heating or cooling of the kiln. ‘This section will deal with the correct procedures for the drying out of a new lining, proce- dures for start-up and stoppage of the kiln, barring of the kiln during start up and stoppage, and the control of the lining condition that rests with the kiln operational staff. 6.4.1 Procedure for Drying out of Lining and Initial Start-Up After replacement of lining in the kiln, the new lining is dried by heating. It is of great im- portance both for monolithic and bricks that the heating is done very slovrly to avoid cracks and spalling. It is normal practice to allow 72 hours for the heating of a newly lined kiln to operating temperature. Asocem: Poge 16 ‘August 2006 252[ism DTH INSTITUTE The procedure for drying of new refractory material will ultimately depend on the recom- mendations by the supplier, but is normally accomplished by heating the kiln system with the main bumer or by temporarily installed heat generators, On smaller and localised repairs, heat gencrators can provide a good alternative to starting the main burner. Heat generators can be placed at the exact point of repair, and drying out can be done in a more controlled manner. In case of a new lining or larger repairs it is necessary to heat up the kiln system with the main bumer. In some cases it may be an advantage to install auxiliary burners for drying ‘out the lining in the cooler. ‘The castable material employed in the kiln and preheater will contain a considerable amount of water which is expelled as vapour through the pores of the material during the initial heating, If the heating process is too rapid, a high vapour pressure is formed in the material causing it to break apart, ‘New bricks will, during the initial heating process expand in the kiln shell. As they are not covered by a protective layer of clinker coating the surface temperature will rise quickly during heating, If heating is too fast, this could produce large temperature differences be- tween the brick surface and the interior, causing internal stresses in the new lining, Fur- thermore a rapid heating of the kiln may cause the lining to expand faster than the kiln shell, and the bricks will be crushed upon themselves. Principles for drying out: Controlling the drying process thus becomes important to the longevity of a new lining. ‘The most important factor to control is the rate of temperature increase in the lining and the surrounding kiln shell, An average figure suggests that the rate of temperature increase of the lining should not exceed approximately 25 °C per hour according to some manufac- tures. The object of this precaution is not only to avoid thermal shock but also to attain equilibrium of thermal expansion between brickwork and kiln shell, More particularly it is to allow for a slower rate of heating of the tires (live rings) in relation to the kiln shell. If the kiln shell is heated to rapidly it will be constricted by the live rings causing permanent deformation of the kiln shell, Such deformation will reduce the life time of both the lining and the kiln shell Figure 6.4 shows one example of a kiln heat up procedure following a major repair of the lining. The heat source is the kiln main burner and a small draft is maintained to avoid for- mation of CO gasses. Asocem Page 17 ‘August 2006 253Giism DTH INSTITUTE Figure 6.4 Heat up Procedure for Cold Kiln with New Lining The figure shows the recommenced lower cyclone stage temperature versus time. By regu- lating the kiln fuel the cyclone temperature is controlied to follow the curve in the figure. The kiln lining must be inspected from the burners platform every 30 minutes to ensure that the temperature of the lining remains below 1000 °C and to check for local overheating. In case of excessive kiln temperature in the burning zone, and if the temperature in the prehca- ter cyclone so permits, the heat must be drawn upstream in kiln by increasing the draught. However, changes to the procedure shown in the figure may be required if-a different prac- tice is recommended by the refractory supplier. The drying-out process must only be commenced at such time that the actual start-up of kiln can be continued immediately afterwards. If a new lining has been heated to service Asocem Page 18 August 2006 254Easmiorm INSTITUTE temperature and cooled down before material has been fed to the kiln, ihe lining may col- lapse. Therefore, when cooling down the kiln, it is important that clinker material is present to fill up the joints as the bricks contract, thus keeping the lining in place. Barring For preservation of the lining, reducing the barring of the cold kiln to a minimum is desi able. However, barring is necessary to alleviate thermally induced stress and to allow for the longitudinal expansion of the kiln shel. ‘The procedure for barring of kiln during 2 72 hour heating period is as follows: From 0 — 1 hours: No barring From I = 24 hours: 100° every 30 minutes. From 24 ~ 48 hours: 100° every 15 minutes From 48 - 70 hours: Continuously with the barring device From 70 hours: Continuously with main motor Throughout this period the longitudinal expansion of kiln and its position on the supporting, roliers must be kept under stringent supervision, Once the normal kiln temperature has been reached, it is imperative that the kiln does not stop for more than 10 minutes without barring. 6.4.2. Starting and Stoppage Procedures During Normal Operation Start-up After a short shutdown where the kiln burning zone temperature is still above 300 °C, the kiln may be te-heated to operating temperatures fairly rapidly. In such a case, the required heating period is around 4 ~ 6 hours. If the burning zone temperature has reached ambient temperature the heating process must be stretched over a 24 hour period to allow for thermal expansion of brickwork and kiln shell, The recommended procedure is shown on Figure 6.5 Asocem Page 19 August 2006 255mipTH INSTITUTE f " ‘stage 4 erctono Figure 6.5 Steriing Procedure for Cold Kiln with Old Lining Asocem Page 20 August 2006 256Gism DTH INSTITUTE During the heating up period the kiln is barred as follows: From 0 ~ I hour: No barring From | ~8 hours: 100° every 30 minutes From 8 ~ 16 hours: 100° every 15 minutes From 16 ~ 22 hours Continuously with barring device ; From 22 ~ 24 hours: Continuously with main motor. Shutdown The kiln must always be cooled down as slowly as possible in order to prevent the lining, from being cooled too rapidly in relation to the Kiln shell. Otherwise, there is a risk that the lining may work loose, with simultaneous risk of twisting and drop-out of bricks. ‘The cooling must take place with a minimum amount of draft through the kiln to ensure a slow cooling process. The amount of primary air must be reduced to a minimura although some primary air is necessary to cool the burner Recommended barring during shutdown, From 0~ 1 hours: 100° every 10 minutes From 1 ~2 hours: 100° every 15 minutes From 2~6 hours: 100° every 20 minutes, From 6 ~ 24 hours: 100° every 30 minutes From 24 48 hours As needed. During the final 24-hour period, kiln barring will be required if the axial kiln movement exceeds the maximum limit for normal kiln operation. In the event of heavy rainfall, the kin must be barred continuously by means of the barring device to prevent excess mechanical stress in the kiln shell Asocem Page 21 August 2006 257Giism DTH INSTITUTE 6.5 INSPECTION AND MAINTENANCE OF KILN LINING Proper maintenance of the kiln lining is of crucial importance. This applies especially to the lining in the burning zone. Poor maintenance may cause thermal damage of the kiin shell as hot spots appear, Whenever the inside of the kiln is accessible the lining should be inspected and a record made of the observations. It is recommended to make a drawing showing the extension and thickness of the coating and remaining lining. ‘The record should include the location of the different refractory materials. Finally a note should be made of the nature of observed dete- rioration. By means of a thin spring cord which is pressed through joints or eracks down to the shell, it is possible to measure the thickness of the lining. Care must be taken to choose measur- ing points in areas, where the lining appears thinner than the surrounding lining to avoid overestimating lining thickness and expected lifetime. If the lining is so dense that the cord cannot enter to the shell, a small hale can be drilled. Comparing the lining record with previous records gives an indication of the rate of wear throughout the kiln zones. Proper records will enable the operating staff to monitor refrac- tory performance and schedule major repairs. When to repair and how much to repair are very difficult questions to answer along general Lines. Usually it is recommendable to replace burning zone linings of less than half of the original thickness. [fon the other hand a tong kiln stoppage is planned for the near future, it may be an advan- tage to just do a quick repair job and leave otherwise dubious lining for the upcoming planned stoppage. It is certain, however, that every stop with cooling down and heating up of the lining has a deteriorating affect on the refractory. From this point of view it pays to plan the replace- ments with regard to Jong running periods without lining failures. Once the lifetime of a certain part of the lining is established within reasonable limits it of ten pays to plana "systematic replacement programme". Such a programme ean only be established after a period of systematic recording of lining performance. Refractory lifetime For comparison of refractory performance from one kiln to another or from one material to another the specific refractory consumption may be used, This figure is calculated as kg Asocem Page 22 August 2006 258Gism DTH INSTITUTE refractory material used per ton clinker produced. Literature will sometimes refer to the to- tal consumption of refractory material in the kiln system and sometimes it is reported for the different zones. For burning zones the brick consumption will normally be between 0,5 and 1 kg/ton clinker, but figures as low as 0,2 kg/ton and as‘high as 2,5 kg/ton have been reported socom Page 23 ‘August 2006 259