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A common misconception is that increasing the air flow to the hot end of the cooler will cool the clinker rapidly and recover more useful heat. In fact, contact time between cooling air and hot clinker is reduced with consequent lowering of secondary air temperature. Good clinker granulation is essential as fine, sandy clinker results in uneven air distribution and, commonly, a red river of hot clinker extending well down the cooler.
Secondary air temperature has been difficult to measure unless there is a hot-gas take off from the hood for tertiary or coal mill air. The availability of reliable secondary temperature offers potential for cooler grate speed control to be, at least partly, directed to maintaining constant secondary air temperature rather than the less important function of maximizing clinker cooling.
Fluctuating secondary air temperature will inevitably cause cycling of kiln operation. This serves to minimize the size of the rotary kiln and its heat loading; it does not reduce specific fuel consumption. Kiln operation is noticeably more stable and brick life is extended. This is also a useful means of consuming low grade fuels or waste materials.
The limit to fuel injection at the riser depends upon its size and consequent gas retention time, and upon fuel-air mixing characteristics; over-fuelling results in preheater operating problems, an increase in exit 15 gas temperature, and CO in the exh aust. All kilns, by definition, have a capacity limitation or bottleneck Sec Increasing fan capacity is always possible but may lead to excessive pressure drop or inadequate dust collection.
An alternative which may well be cost effective, especially for short-term production increase to meet peak market demand, is oxygen enrichment. Fig 4. This involves a maintenance-free injection port and does not cause refractory stress. The economics will depend on the cost of cryogenic oxygen or, for more permanent systems, the installation of an on-site Vacuum Swing Adsorption unit which can greatly reduce oxygen cost.
The vortex finders dip tubes of lower stage cyclones were for many years prone to collapse and, frequently, were not replaced. However, these may still be subject to failure and the effectiveness of vortex finders in lower cyclones should be carefully assessed by review of preheater temperature and pressure profile and of specific fuel efficiency both before and after the tubes are remo ved or fall out; in many cases there is scant justification for reinstallation. For kilns with grate coolers, the burner tip should be in the plane of the kiln nose hot or slightly inside the kiln providing it does not suffer damage from falling clinker.
The burner should normally be concentric with, and on the axis of, the kiln. Some 16 operators prefer to hold the burner horizontal and even tilted into the load.
Such orientation may result in reducing conditions and should be avoided. Clinker produced under reducing conditions causes reduced cement strength and abnormal setting. It should be appreciated that both burner position and tip velocity are intimately related to hood aerodynamics and can not be considered in isolation see Section 9.
Kiln rings are sections of heavy coating, usually in the burning zone, though sometimes also near the back of the kiln, which can grow to restrict both gas and material flow and eventually force shut-down.
Alternatively, ring collapse causes a flush of unburn ed material. Ring formation in the burning zone is commonly attributed to operational fluctuations though a low coal ash-fusion temperature or high mix liquid phase will increase the risk Bhatty; Proc ICS; , pg Early detection is possible with a shell scanner and rapid reaction is essential.
Such ring growth may be countered by varying kiln speed or by small movements 10cm of the burner in and out. Rings at the back of the kiln are usually associated with volatiles cycles, particularly excessive sulphur at the kiln inlet. It is evident, though of little help, that rings are structurally more stable in small diameter kilns. To prevent the resulting pollution, and to make use of the heat potential, kerogen-containing materials should be injected at higher temperatures; usually to a 1- or 2-stage preheater if the hydrocarbons are present in the limestone.
If the hydrocarbons occur in a minor constituent, this component may be ground separately and fed to the kiln riser. Distributed control systems DCSs have likewise replaced control systems once made up of numerous electronic or pneumatic analog loop controllers. The differences between systems lie mainly in their architecture. DCSs also come with high level programming software which automatically takes care of common programming tasks and greatly facilitates system configuration and maintenance.
Such interfaced PLCs continue to be favoured for discrete control due to speed, ease of programming, and reliability. The primary advantage of the PC system is the ease and economy of u pgrading speed and memory. However, while hardware costs are lower than with proprietary DCSs, programming costs are usually higher because automated high level programming software is not yet available. Also, to obtain the same level of redundancy, additional PCs must be incorporated. Various expert systems Linkman, Fuzzy Logic, etc can be overlaid to the computerized control platform and can give dramatic improvements in kiln stability, fuel efficiency, clinker quality and, consequently, in production rate.
Before implementation, however, they do require that adequate and reliable instrumentation is in place and that kiln operation is basically stable. These systems depend on characterising the dynamics of kiln operation and require regular retuning to maintain their usefulness. Process alarms should be carefully designed and maintained. Certain events are critical and require immediate attention but, if visual or aural signals are excessive, important alarms may be missed.
Critical alarms eg excess CO in exhaust should be designed so that cancellation is impossible until the problem is corrected. Interlocks are not uncommonly jumpered either by hard wiring or by programming to allow maintenance to cope with a temporary abnormality; such jumpering must be recorded and active jumpers frequently reviewed to prevent inadvertent permanence. Typical Aim o C o C 2. Whether normal operation is manual or automated, most kilns are still liable to upset periods due to ring building, coating loss, etc and, while every effort should in any case be made to minimize such instability, effective computer control must be able to cope with the situation.
Kiln feed and speed are usually controlled with a fixed linear relationship and unilateral variation of kiln speed should, at most, be used only as a short-term expedient eg to control a kiln flush. Typically cyclone preheater kilns rotate at Precalciner kilns rotate at 3. Retention in the preheater is secs. Keeping the same kiln retention time with increasing degree of calcination of the material entering the kiln resulted in extending this transition and there is evidence that the introduction since of short, two-pier, kilns has led to the reduction of material residence time before entering the burning zone from some 15 minutes to 6 minutes with resulting improvement in clinker mineralogy and grindability.
Two-pier kilns have length:diameter ratios of vs for three-pier kilns. Kilns are frequently operated to the limit of the ID fan. In this case, low oxygen must be corrected by reducing both fuel and feed. Precalciner kilns burn fuel at the kiln hood using combustion air mainly drawn from the hot end of the grate cooler, and in the calciner using combustion air drawn from either the hood or the mid-section of the clinker cooler via a tertiary duct. Most precalciner kilns have dampers in the tertiary duct, and some have fixed or adjustable orifices in the riser, to control relative air flows to the two burners in order to maintain the desired fuel split.
Frequently these dampers fail and it is then essential to adjust the fuel flows to the resultant air flows. CO should, and NOx may, also be measured at the kiln inlet. In a precalciner kiln, an additional gas analyzer is required in the outlet duct from the bottom cyclone and, again, this should be operated at as low an oxygen level as is consistent with less than ppm of CO.
Note that traditional O2 operating levels must be modified if staged combustion Sec 9. Useful information on kiln operation can be obtained from frequent 2-hourly analysis of clinker for SO3, and periodic 8-hourly sampling of the underflow from the bottom cyclone stage s for SO3, Cl, and alkali determination.
Normal SO 3 levels typically 0. In precalciner kilns, retention time and heat loading are particularly low and alkalis K,Na tend to pass through to clinker while sulphur is volatilized and builds a cycle at the back of the kiln exacerbated by the deficiency of alkalis. If the kiln is burned too hot or if the flame impinges on the load, this cycle increases excessively until build-up or cyclone plugging occurs.
This is matched by an abnormally low SO3 and freelime contents in the clinker which should be taken as a warning. Eventually, if the kiln is allowed to cool, this sulphur is released and transient high clinker SO3 results. Such variation in clinker SO3 will also give rise to varying grindability in the finish mill. In order to minimize volatile cycles, hard burning mixes should be avoided, the sulphur:alkali ratio should be maintained between 0. It cannot be over-emphasized that kiln stability, fuel efficiency, finish grinding power consumption, and cement quality all depend greatly upon the provision of kiln feed and fuel with minimal variation both of chemistry and feed rate.
Healthy suspicion should be nurtured towards both instrument signals and manually reported data. Particular areas for mistrust are: False instrument signals of which pressure sensors and gas sampling probes are particularly liable to failure. Short term variations masked by electronically damped signals. Feeder variations especially when the material is either sticky or fine and dry.
Chemical variations hidden by faulty analytical methods, statistical mistreatment, or outright fraud.
Variations in kiln behaviour always have a cause; any variations which cannot be explained by observed feed deviation or known operational disturbance should alert to the possibility of faulty data. Automated kiln control seems, unfortunately, to have reduced operators' habits of looking in the kiln and inspecting the clinker produced.
The appearance of clinker can also be instructive; preferably black with surface glitter, dense but not dead burned, dark grey cores, and absence of excessive fines. Brown clinker is associated with increased heat consumption, reduced grindability, cement strength loss, and rapid setting. Certain alarms on the kiln control system are critical. Apart from normal mechanical alarms and the routine monitoring of kiln shell for refractory failure, the potential for explosion requires particular care.
Gas analysis is conventional at the feed end of the kiln, at the down-comer, and at the dust collector entrance. Flame detection is particularly vital during warm up of the kiln and fuel should be shut off by interlock if the flame is lost. When the kiln is up to temperature it is common to deactivate the flame detector but it should be impossible to start a kiln without this protection. The light-up of kilns is potentially dangerous as there is insufficient temperature in the system to ensure continuous ignition.
Unburned gas, either natural or volatile hydrocarbons from solid fuels, accumulates rapidly in the kiln and, if then re-ignited, will probably explode.
It is important that ignition be achieved as soon as the fuel is injected and, if the flame fails during warm-up, the kiln should be purged with 5 times the volume of kiln, pre-heater, ducting, and dust collector probably some minutes before re-ignition is attempted. Warm-up follows agreement by production and maintenance management that all work is completed, that all tools and materials have been removed, and that all doors are closed.
Work may, with discretion, continue in the cooler during warm but no -up workers should remain in the cooler at the time of ignition.
Commonly, warm-up from cold takes 24 hours from ignition to feed-on, but may be increased if extensive refractory work requires curing.
Uncomfortably for those of us who have always numbered from the top, it must be asserted that, with the proliferation of cyclone preheaters of other than 4 stages, numbering from the bottom allows more meaningful correla- tion from kiln to kiln. The designation "Portland cement" was originally due to a resemblance in colour and character to limestone found on the Isle of Portland off the south coast of England. The recognition that high temperature clinkering C3S formation greatly improved performance is attributed to Isaac Johnson in Joseph Ransome patented the rotary kiln in England in and the air-suspension cyclone preheater was patented by Vogel- Jorgensen in Czechoslovakia in No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical or otherwise, without the prior permission of Tradeship Publications Ltd.
This publication is intended solely for use by professional personnel who are competent to evaluate the significance and limitations of the information provided herein, and who will accept total responsibility for the application of the information. Mr Serge Brison 8. Portland cement is a calcined material comprising lime and sili- cates which is mixed with sand and stone and, upon hydration, forms a plastic material which sets and hardens to a rock-like material, con- crete.
Confusion between cement and concrete is endemic among the uninitiated. Portland cement is manufactured in a series of processes which may be represented as shown: Limestone calcium carbonate and other materials containing appro- priate proportions of calcium, silicon, aluminum, and iron oxides are crushed and milled to a fine flour-like raw meal. The reaction prod- ucts leave the kiln as a black nodular material, clinker.
The clinker is finally interground with a small proportion of gypsum to control the rate of hydration and the fine product is cement. In general, however, an eutectic mix is sought which minimizes the heat input required for clinkering and the total cost of raw materials, while producing a cement of acceptable performance.
An approximate analysis for raw mix on ignited basis, or for clinker, is: Cement mixes vary from "cement rock", a single component which, as mined, contains appropriate proportions of the required minerals, to 4 or 5 component mixes comprising one or two grades of limestone, a shale or clay, and one or more additives to augment Si02, Al or Fe levels.
Excess alkalis K, Na affect both kiln opera- tion build-ups and product quality alkali-aggregate reactivity. Conventional wisdom suggests that the sto- ichiomatric ratio of alkalis to sulfur should be kept between 0. Excess Cl causes serious problems for preheater operation. Materials, as mined, therefore, are typically proportioned: Additives are usually brought in, albeit in small quantities.
Mining plans are developed according to the geology of the materials. If the limestone is not homogeneous, it may be necessary to blend rock from different areas in order to maximize recovery, and it may also be necessary to mine selectively in order to avoid low grade material or problems such as alkalis.
Mining and hauling are commonly moni- tored by: Apart from chemistry, grindability is also a factor in selecting raw mate- rials. In particular, silica additives containing large-grain quartz are very difficult to grind and can result in hard burning and high fuel con- sumption. If quartz silica is employed it should, preferably, have a nat- ural grain size of less than ntt. In recent years, cement kilns have been increasingly employed to utilize industrial by-products eg mill scale and to dispose of industrial waste materials eg water treatment sludge in return for disposal fees.
Such materials include: Ca agents: The use of waste materials for cement manu- facture has led to incorporation of a much wider range of trace elements and their effects are reviewed by Bhatty PCA Bulletin RDT. Apart from raw materials, gypsum and fuel are required for cement manufacture together with various pozzolanic materials both natural and by-product if interground cements are produced. Reserves are classed according to the detail in which they have been explored: Class A or proven reserves: Extensive drilling has confirmed quantity, mineralogy, variation, mining and legal availability.
Class B or probable reserves: Sufficient drilling to allow presumption of quality and availability. Class C or indicated reserves: Widely spaced drilling gives extent and some knowledge of quality. Class D or inferred reserves: Initial exploration and consideration of geology allow general assumption. Typically feed should be less than cm and, either the feed hopper should be pro- tected by an appropriate grizzly, or a hydraulic breaker may be installed to reduce oversized rock.
Commonly there are primary, secondary and, occasionally, tertiary crushers in series.
Most crushers are operated in open circuit though, frequently, they are also preceded by a screen or grizzly to bypass fine material direct to product. Hopper shapes and feed mechanisms are important considerations where material flow may be a problem. Crushers may operate by compression or by impact.
Compression machines comprise single and double jaw crushers and gyratory crush- ers Figure 1. Single Toggle Jaw Crusher also has a measure of vertical jaw motion which adds attrition to compression.
Sticky materials can be handled but wear rate is increased. Gyratory Crusher operates by pressure between a gyrating cone and a stationary or spring-loaded crushing ring. Hard, abrasive materials can be handled with reduction ratios of about 5: Wear is low but reduction ratios are usually less than 8: Impact machines may be either hammer mills or impact crushers. The feeder elevation and, therefore, the velocity with which the material enters the circle is critical; if too low, the material bounces on top of the hammers and if too high, it penetrates through the circle and can damage the rotor discs.
The discharge is partially or wholly screened by grates against which secondary reduction by attrition takes place. The grate slot size governs discharge top size but this configuration requires relatively dry material to avoid plugging. Impact Crushers Horizontal or Vertical are similar in operation to hammer mills. Some units involve attrition but relatively wet materials can be handled.
Monitoring of crusher operation requires: Drying is commonly effected either in combined crusher-dryers, in rotary dryers which can either use dedicated hot gas generators or waste heat from kiln exhaust, or in autogenous mills.
In each case, the air flows will involve dust collection while the high humidity of the used gas usually favors electrostatic precipitators over bag houses.
Monitoring includes: If raw materials are partially predried, they may cause serious handling problems. Steam evolution from the hot material causes build-up and can plug dust collectors.
Also the dry, fine fraction is liable to flush if held in intermediate storage. There are advantages in both handling and heat consumption if raw materials can be handled up to mill feed and dried in the raw mill.
More commonly, material variation can be mitigated, and buffer storage more economically provided, by a mixed preblend pile, either longitudinal or circular. There are two major stacking systems: Chevron - stacking in layers along a single axis with the feed conveyor sweeping backwards and forwards along the length of the pile Figure 2. Winrow - stacking in longitudinal strips side be side and then in suc- cessive layers; this avoids the segregation which characterizes chevron stacking Petersen; WC Bulk Materials Handling Review; , pg 30 but otherwise there is little difference in blending efficiency.
The win- row system requires a more complex and expensive stacking belt arrangement Figure 2. Chemical analysis of the material, especially if premixed, as fed to stor- age is essential. The latter method has the obvious advantage of rapidity and of avoiding a very difficult sampling problem. V Preblend effectiveness the ratio of estimated standard deviations [escl] for feed and product is roughly related to the number of layers IN] by: Recovery to mill feed is either by end or side reclaim: End Reclaim — various systems to scrape an entire end face with a transverse scraper at floor level moving material to a discharge con- veyor Figure 2.
Side Reclaim — a boom mounted scraper working end to end along the side of the pile. This gives less effective blending and recovery flow rate is not constant Figure 2. Two piles are normally operated with one being built while the other is recovered. Length to width ratio should be at least 5: Total capacity of days con- sumption is normal. Preblend piles are usually covered, both to prevent rainfall on the fine and, therefore, absorbent material, and to contain fugitive dust.
Circular preblends are of significantly lower construction cost than lon- gitudinal beds, but preclude subsequent expansion. Preblending should be monitored for compositional variation of feed and discharge to determine a blending ratio.
Normally drying is effected by ducting part of the kiln exhaust gas through the mill. Obviously a high drying requirement may be inconsistent with maxi- mizing the thermal efficiency of the kiln; generally five and six stage preheaters are only employed where subsequent drying by the exhaust gas is minimal.
Alternatively, but less efficiently, dedicated hot gas gen- erators can be used for drying in the raw mill. Most of the power con- sumed in a mill is converted to heat and this equates to drying of approximately It moisture per kWh.
Where preblended wet materials are fed for dry- ing in the raw mill, silos are unnecessary.
Virtually all modern raw mills incorporate closed circuits with classifi- cation in a cyclone or mechanical separator and return of coarse mater- ial for regrinding Figure 3.
Ball mill operation is described in more detail under finish milling Section 5. Roller mills have a lower specific power consumption than ball mills. Loesche mills comprise conical rollers which are hydraulically pressed onto a horizontal rotating grinding table.
Feed material is directed onto the center of the table and is thrown outward by rotation under the rollers and into a rising air current at the periphery which is directed by means of a louvre ring. Material drying occurs in air suspension between table and classifier. Smooth running depends upon the stability of the material bed; occasional vibra- tion may be due to abnormal feed, particularly dry or fine material.
The mill is started with the rollers lifted away from the table. When low- ered to their grinding position, actual metal to metal contact should be prevented by limit switches and consistent feed. Material which is not carried upwards by the air stream falls from the table to a rejects trap, but every effort should be made to exclude tramp metal which can damage the grinding surfaces.
Loesche mills are defined by grinding table diameter dM , number of grinding rollers, and roller size; eg L. M41A50 is 4. Primary roller mill controls are: Thus, con- trol response should be appropriately faster. Specific power consumption depends upon material hardness and mill efficiency. Mill product is monitored either by continuous, on-line analysis or by laboratory analysis of hourly grab or composite samples. Computer programs are employed to effect feed corrections in order to maintain the desired optimum composition of long-term say 6-hour average.
Automatic feed correction will probably be ineffective if any of the feed silos have been contaminated. Raw mills are monitored by: Equivalent tonnes Eqt are theoretical tonnes of cement pro- duced from given quantities of raw materials; see Plant Reporting, Section Separators are covered in more detail under finish milling Section 5.
Roller mills require high efficiency separators to avoid material bed problems and high pressure drop associated with the accumulation of excessive fines in the mill circuit.
The two major types involve turbulence in which the material is tumbled about by the injection of high volume air through air-pads on the silo floor and controlled flow where sequenced light aeration of segments of air-pads causes layers of material in the silo to blend by differential rates of descent within the silo.
Controlled flow silos may have multiple discharge chutes, or an inverted cone over a center discharge within which the meal is flu- idized. Compressor power consumption is approximately: The for- mer involves either a filling cycle corrected progressively to average the target mix, or a sequence of filling, mixing, sampling and analyzing, correcting, remixing, and then feeding to kiln.
Continuous blending involves simultaneous feeding of the silo, overflow to a second silo and discharge to kiln feed. Modern blending silos are generally of continuous, controlled flow type with each silo having capacity of more than 24 hours kiln feed and yielding a blending ratio of Blending silos should be monitored by: As raw meal is liable to solid- ify if left inactive during a kiln shut-down for example , blending silos should be emptied or recirculated when not in use. Short term feed fluctuation eg hunting of feeder control as well as average feed rate should be monitored.
Air-suspension preheater kilns lose a fraction of kiln feed by entrain- ment in exhaust gas. As this fine fraction is usually of atypical compo- sition, kiln feed analysis must be biased to yield the desired clinker composition.
Thus, even if the dust collector catch is returned directly to the kiln, it must still be compensated. Likewise, care must be exercised to minimize the chemical disturbance due to dust return.
Depending upon quantity and chemical variation, one or more holding tanks may be desirable to collect different dusts and allow constant metered return either to blend silo or to kiln feed. Such dust can also occasionally be employed as a cement intergrind or disposed of for agricultural or sludge stabilization purposes. If the kiln exhaust passes directly and continuously to dust collection, then the dust may be returned directly to the kiln with kiln feed or, sometimes, by insufflation at the hood or at the feed-end of the kiln which minimizes re-entrainment of the fines.
Kiln feed is monitored by: Chemical analysis on one- or two-hourly grab samples to deter- mine statistical variation. If oxides are measured, statistics should use C3S or LSE Kiln feed should typically have an estimated standard deviation for hourly grab samples of less than 0. Note 1. It should be born in mind that stan- dard deviation is not a perfect measure of variation as, simply applied, it does not distinguish between a steady trend and constant fluctuation.
One might also beware of the dubious practice of eliminating extreme values from a set before calculating standard deviation: Kiln feed is normally conveyed by bucket elevator to the top of the pre- heater to minimize power consumption.
If this conveying is effected pneumatically, de-aeration is desirable before injection as the entrain- ing air otherwise adds to the kiln ID fan load and may reduce kiln capacity. Kiln feed to clinker ratio is typically 1.
Kiln feed is subject to successive reactions as its temperature increases Lea; The Chemistry of Cement and Concrete: The first units were 4-stage preheaters. Exit gas temperatures, static pressures, and specif- ic fuel consumptions for modern precalciner kilns are typically: The major cyclone preheater configurations are shown in Figure 5.
Other terms frequently encountered include: Healthy suspicion should be nurtured towards both instrument signals and manually reported data. It cannot be over-emphasised that kiln stability. In a precalciner kiln. Particular areas for mistrust are: This is matched by abnormally low SO3 and free lime contents in the clinker which should be taken as a warning.
Useful information on kiln operation can be obtained from frequent 2-hourly analysis of clinker for SO3 and periodic 8-hourly sampling of the underflow from the bottom cyclone stage s for SO3. Brown clinker is associated with increased heat consumption. The light-up of kilns is potentially dangerous as there is insufficient temperature in the system to ensure continuous ignition.
Automated kiln control seems. When the kiln is up to temperature it is common to deactivate the flame detector but it should be impossible to start a kiln without this protection. Flame detection is particularly vital during warm-up of the kiln and fuel should be shut off by interlock if the flame is lost.
Unburned gas. Variations in kiln behaviour always have a cause. The appearance of clinker can also be instructive. Gas analysis is conventional at the feed end of the kiln. Apart from normal mechanical alarms and the routine monitoring of kiln shell for refractory failure.
Modern kiln and cooler camera systems. Other causes have also been proposed Jakobsen. For management scrutiny it is useful to have either a 'read-only' CRT which can be interrogated without interfering with kiln operation. Certain alarms on the kiln control system are critical.
Warm-up follows agreement by production and maintenance management that all work is completed. A simple and reliable ignition system has been described by Davies ICR. For PC kilns. It is important that ignition be achieved as soon as the fuel is injected and. A typical chart is shown Figure 4. If the coal mill uses hot gas from the cooler. Work may. Warm-up from shorter stops where the kiln is still hot.
For coal fired kilns. Before and during warm-up. A useful practice is to drill through the brick every meter whenever the kiln is down and coating has been stripped wider spacing and lesser frequency is adequate in the low alumina brick area. A detailed historical record of refractory replacement and review thereof are important to minimise cost and service interruption.
The burner pipe is withdrawn. Suggested inching is as follows: Shut-down may be either: Warm areas of shell can be controlled by use of a fixed fan array or of movable fans which can be directed at the area. Emergency repairs. If the hot spot is a dull red and is in the burning zone it may be possible to recoat the area and continue operation.
PS Kiln shells should also be inspected visually. Heat curing. Then brick thickness can be measured in from the line at intermediate positions. Non-intrusive instruments to measure brick thickness are also available eg Hoganas Linometer. Changes in fuels. The extent of coating should be observed whenever the kiln is entered and. There is an extensive literature on kiln brick types and performance of which the following is a brief selection: Selecting refractories. Benchmarking refractory performance.
Such drilling requires discretion to locate the shell and to identify irregular circumpherential wear. Red spots on surfaces other than the kiln may be temporarily secured by building a steel box on the outside to cover the hot area and filling the box with castable refractory. In strongly seasonal market areas. In more uniform markets.
Magnesite Spinel Dolomite 3. There are two principal metric brick configurations. With considerable variation. Both are described by a three digit code. Comparison between gross and net figures indicates the wastage of potential refractory life.
For plant cost tracking. Brick Installation in Kotary Kilns. Castables may be 'heavy' or 'lightweight insulating' and are classed: They are classed: The low cement types are very sensitive to water content and water addition must be strictly minimised. The two major bricking techniques are the epoxy method and the 'ringjack' method Mosd.
The design and array of these anchors is critical to allow necessary movement of refractory against steel. They are mixed with water. Castables are concretes with refractory aggregate and a high-temperature resistant high Al2O3 hydraulic binder.
Castable and plastic refractories require steel anchors to hold them in place. RefrAmerica Excessive vibration. Plastics have to be heated at specified rates to cure which can allow immediate kiln warm-up to begin. Plastics have the consistency of modelling clay and are usually rammed into place though they can be gunned if so formulated.
Both have their place. Proc IKA. Large sections of castable refractory are frequently laid without expansion joints which depend upon anchors to accommodate expansion and contraction.
If further retaining rings are installed. While brick walls in coolers. Conventional rings are rolled plates mm high and slightly less than the width of one brick positioned such that the uphill brick is in tight contact. Brick walls are also prone both to dust infiltration and to heat distortion of the steel backing which can cause failure. Experience with large numbers of bricked kilns indicates that no additional retaining rings are necessary if the kiln is reasonably straight and does not have excessive ovality.
The kiln shell should be provided with a steel retaining ring 1M uphill of the nose casting to resist the thrust resulting from rotation and inclination of the kiln. Provision is normally made for expansion when installing refractories.
It should be recognised that material entering the kiln at the hood uses high-grade heat for preheating which is less efficient than conventional kiln feeding. The low profile of the ring results in massive loading of the contact surface with the uphill brick and consequent risk of failure.
The effects of insufflation are to: Although specially shaped bricks must be laid on the rings. With coal as fuel. The dust may be either metered into the primary air using a venturi.
Connection to the shell should be by plug welding Refratechnik Manual. Gortan et a] ZKG. Taiheiyo Cement has developed a new bypass system for chlorides which separates coarse dust in a cyclone for return to the kiln while the fine dust. Conventional bypasses generate relatively large quantities of dust which constitute a disposal problem. The criteria for bypass operation are complex depending upon input concentrations. Where volatile components are unavoidable. Heat penalty is approximately: Automated control of the bypass has been proposed based upon sampling and analysis of the hot meal Triebel et al.
This causes accretions of material in ducts and plugging of preheater cyclones with dire operating results. Appropriate operator protection and training are mandatory. Cl or by temperature excursions. SO3 found in the stack originates from pyrites or organic sulphur in the raw materials.
It is customary to clean build-up material during operation by using airlances. Refractory insulation should be maintained to avoid unnecessary cooling of hot surfaces. The hot meal at the back of the kiln and in the lower cyclone stages is particularly prone to freeze and build-up at cold spots. Cyclone clearing is a major operation requiring kiln shut-down and is normally effected through angled ports in the cyclone by long air lances. Air cannons are frequently installed in areas of persistent build-up with a discharge cycle which is optimised for coating removal Zimmer.
It should be noted that natural SO3 scrubbing in cyclone preheaters is very efficient. Smooth finishing of feed-chute and riser refractory is helpful. The difference varies with fuel: Coal Oil. It must be noted. In practice. The choice is normally based on price and availability. On the other hand. The nature of production and handling by major suppliers should minimize short-term fluctuation while long-term variation can be compensated by analysis and normal kiln control procedures.
A more precise determination of optimum fineness according to coal type has been described by Seidel ZKC. Stockpiling of coal requires vigilance as spontaneous combustion is common. Petroleum coke has certain advantages.
If long-term storage is necessary. In recent years the cost of fuel. This results in slower response to control changes which makes for more difficult control of the kiln.
Gas requires turbulent diffusion and its heat flux tends to be released more slowly than with oil or coal. Smouldering coal should be dug out. Primary air is not essential and the gas is injected as axial. Thermocouples embedded M below the surface allow monitoring for combustion. It should be noted that there are two main types of pet-coke: It should also be noted that.
Coal is usually dried. Fluid coke consists of small spherical particles resulting from a continuous coking process at about ". A more significant limit to use for many plants will be the sodium content. Typical analysis is: Liquid waste fuels: Spent potliners from the aluminium industry are another potentially valuable fuel source Kohnen. Tyres are potentially attractive though shredding or pyrolisis eliminates much of the cost benefit while fuels added discontinuously.
Delayed coke may be 'sponge' or 'shot'. Numerous other by-product and waste fuels have been used and many command disposal fees.
With such materials. Their use hitherto has been constrained by a typically neurotic fear of fluoride and cyanide residues. Coal can be ground in most types of mill but roller mills probably predominate. Note that significant volatile matter and. Waste fuels may be burned in the kiln. It may be noted that the Babcock E-Mill. Of more importance is the ability of an indirect system with a single mill to supply two or more burners where a pure direct system requires one mill per burner.
There are several variations on the two basic coal firing systems. There is a common assumption that indirect firing yields higher thermal efficiency by reducing primary air and by excluding the water vapour from coal drying. Similarly mills are normally designed for 55 Hardgrove index and harder coals lower HGI w ill result in de-rating.
Mills with common table and fan drives may be given separate drives. Fires in direct firing mills are extinguished by adding feed to act as a heat sink and lowering the mill inlet temperature. Mills in indirect firing systems conventionally employ CO monitoring to detect combustion thermocouples are too slow to respond. May Vendors specify a minimum airflow.
Rock and metal rejects fall from the table into the hut air plenum and are swept by a rotating scraper for discharge through an air-locked chute. Fires are usually the result of rags or wood lodging within the mill and may be detected by an increasing discharge air temperature unrelated to increased inlet temperature or reduced feed rate.
This airflow is required to ensure that coal does not remain above its ignition temperature long enough for auto-ignition. Typical specifications used by vendors for burners with indirect firing systems are as follows: For indirect firing.
In the United States all coal mil! Although most mill fires occur on start-up or just after shut-down. Hot air for drying coal can come from cooler exhaust normal air or preheater exhaust low oxygen. Tip velocity must always be substantially greater than the flame propagation velocity. The pipe is usually narrowed near the tip to minimise parasitic pipe losses and convert the flow into the desired static pressure.
The inlet temperature to the mill is controlled to maintain the outlet temperature as described above and de-dusted in a cyclone. One annulus is used for conveying pulverised fuel from the mill and one or more separate streams are used to supply primary air fur controlling the flame Figure 4. On pneumatic conveying systems.
Coal Fires and Explosions: Coal firing almost inevitably involves a normal operating condition where pulverised coal is in contact with air before reaching the burner. Coal dust explosive tendency increases with volatiles content and with fineness. Safety considerations are reviewed in a publication.
The design of a coal firing system is. Final Report. EPR1 Research Project Coal obviously can be handled safely but pulverised coal should always be considered as a potential explosive. A smouldering fire can become explosive if disturbed. This type of cooler allows l. Distortion of the support frame translates into uneven gaps and plate-to-plate contact which significantly reduces cooling effect. There may be up to 8 under-grate compartments and two or three separately driven grate sections.
Similar grate designs are also available for the non-recouperating zone but with lower resistance as required by plenum aeration. Under-grate pressures are approximately mm in the first compartment reducing progressively to about mm in the last compartment. Fuller coolers are defined by width and length of each grate in series.
The grates used with air beams. It should be confirmed that such ad hoc modifications do not cause significant deterioration such as fluidisation of clinker on the first grate. Air beam designs are now available from all cooler manufacturers which.
On traditional grate coolers. For conventional grates. This compares to approximately 0. Realignment minimally involves shimming of each support girder in turn starting from the discharge end. Older coolers. Undergrate air pressure is maintained constant by closed loop control of the grate speed using under-grate pressure as the process input variable.
As large clinker nodules and blocks of coating cannot effectively be cooled. Visual inspection of the cooler is important. This consists totally of static grates with clinker transport effected by reciprocating pusher bars above the grate surface.
As kiln discharge rate and clinker size can vary with kiln operation. Wear of the reciprocating bars is significant but is predictable. The static grate section has been found particularly effective and.
Numerous refinements include a pendulum frame for the moving grates which is claimed to minimise drive maintenance. This cooler also incorporates an ingenious flow regulator on each grate which maintains a constant air How through the clinker bed regardless of bed porosity. Size reduction is more effective if performed at a mid point in the cooler using a roll crusher. The tuning of under-grate pressure control requires skill and experience. Planetary coolers comprise a ring of tubes attached to the kiln shell and turning with the kiln.
Clinker production is not usually measured directly and is calculated from kiln feed with ultimate confirmation from cement shipment. Clinker coolers are monitored by: Other coolers are occasionally encountered which avoid the need for separate dust collection: Rotary coolers are simple rotating drums which lift the clinker to fall through the incoming combustion air stream effecting heat exchange.
These coolers can cause mechanical problems on the kiln. Acoustic horns have been suggested as a means to improve cooler heat recovery WC. Such coolers are. These coolers are limited to small kilns. Broken grates can allow excessive leakage of hot clinker to the undergrate compartment with risk of major damage. Thermocouples or level indicators placed below the grate drive permit an alarm for clinker filling up within a compartment. There is no direct contact between clinker and air so that no dust collection is necessary.
The shell also imparts rotary motion to the refractory lining in order to convey the raw meal through the kiln and discharge the clinker produced. Since refractory replacement is the major cause of kiln downtime in most cement plants. The G-Cooler comprises a number of standard modules stacked vertically as required for temperature reduction and horizontally according to throughput.
These units tend to require little maintenance' and little control once the column discharge gates have been adjusted to ensure smooth outflow and the filling of all columns. Peters G-Coolers are supplied by Babcock Materials Handling Division and are secondary systems usually installed in series with planetary coolers or with grate coolers which are being run beyond their design rating.
Rotary and planetary coolers are less effective than grate coolers. Although a kiln appears to be a straight. While the cross-section is generally considered elliptical. Replacement of planetary coolers by grate coolers is not uncommon Cohrs. For increasing cooler capacity.
Proper alignment of the kiln support rollers and maintenance of tyre pad clearances are the two primary precautions for minimising stress.
Evidence of excessive support roller thrusting can be detected by temperature comparisons of the thrust bearings and from bearing wear rates. The material strength of the individual refractory unit together with the strength of the joints between units enables the kiln lining to deform to some extent without failure. The lining absorbs these stresses through minute amounts of deflection within the individual refractory units bricks themselves and through relative motion between refractory units.
Shafts should be slightly skewed relative to the kiln shell's theoretical axis at each pier to generate some thrust on the kiln tyre that pushes it. Though it is extremely difficult to quantify refractory lining stresses. Roller shafts must be on the same slope as me kiln though they do deflect due to the kiln vertical load by as much as l-5mm.
All shafts on a given pier must be parallel to avoid generation of unnecessary thrust bearing loads. On a properly skewed set of bearings. These shell deflections impose significant and.
Although all kiln thrust rolls are designed to support the entire kiln's downward load. Slight changes in skew can relieve the rollers' downhill thrust.
When all rollers are properly skewed. The two most common methods are to monitor the temperature of the bearing housing where the thrust bearing 'button' is mounted. Even within this type there are many variations in design.
Many kiln designers have found that it is important to protect the internal surfaces of the seal plates from exposure to radiant heat. This is usually achieved by appropriate provision for radiant heat shielding and cooling air flow.
Kiln shell design has historically been based on consideration of the kiln shell as a beam of cylindrical cross section. There are numerous designs of kiln seal and most work reasonably well if they are properly maintained. Effective designs are characterised by tyre locations that balance the load uphill and downhill on mid-kiln tyres and yield shell overhangs between one and two.
This is often referred to as 'checking for fish-scales' since the roller surface will feel rough in one direction and smooth in the other. Apart from overheating. At the discharge end. Since wear between these components is unavoidable. The feed end seal must protect against cm WG differential pressure compared to 1cm or less for the discharge seal. The most common replacement seal design utilises flexible sheet metal plates arrayed around a steel ring mounted to the kiln shell Geiger.
Kiln seals are required at the inlet and discharge ends to exclude false air. The shell thickness is selected to maintain calculated material stress levels well within the steel's capabilities and manufacturers utilise historically proven stress limits that accommodate variation from design assumptions.
It is also important that the rotating component run out be held within the seal's capability. It should be remembered that roller and tyre surfaces can be very hot during kiln operation. Infrared imaging systems are used increasingly for this purpose and also to provide information on refractory condition.
Drilling a ' crack stopper' hole at the end of a crack is a common practice but it will generally not be effective unless it is at least 25mm in diameter. Each bead should overlap the one below it by about half the bead width to provide heat for relief of weld shrinkage stresses in the underlying bead. Welding should be performed with appropriate filler metals laid down in straight beads. Magnetic particle or ultrasonic inspection should be used to determine that the entire crack has been removed.
Heads should be laid in straight parallel lines. Continued operation at or above this temperature will generally result in permanent shell deformation or crack initiation. Generally there are two thicknesses of shell at each tyre. The plate between tyre sections is even thinner than the flanking plate. It is often acceptable simply to mark the extent of these cracks while continuing to operate until an opportune shutdown. Creep should never be zero and may typically be up to about 2cm per revolution.
Attention should be given to the weld joints between plates of different thickness. There is ample evidence that for acceptable fatigue life. The most common location for shell cracking is at the transition between the flanking plate and the thin shell plate that spans between piers. Failures generally occur at the toe of the weld joint on the thin plate side.
Tyre thrusting or excessive thrust loading on a tyre is indicated by hard contact between a kiln tire and its retaining mechanism. This should be measured with a device commonly known as the Obourg Pen Tester Chapman. The layers of weld bead should be built up until the toe of each weld bead lies on the line connecting the lips of the groove.
On tyres adjacent to the kiln gear. The most frequent cause of excessive tyre thrust loading is a slope difference between support rollers and the kiln axis through the tyre. Some creep is essential at all times and it should not normally exceed about 1cm per revolution. Tyre creep varies with shell temperature and the continuous measurement offered by some shell scanners is valuable.
More importantly. Fuller Company At the first opportunity. Though tyre creep should be logged daily for each tyre.
Figure Although the gear is able to hold the kiln shell with minimal ovality. When tyre pad clearance on these tires reaches a level which is associated with excessive ovality on the other tyres. Precalciner technology and the desire to minimise retention time of material between calcination and sintering have resulted in a trend to increased kiln rotation speed. Kiln drives generally utilise girth gears and pinions designed to give over 20 years of continuous service if lubrication and alignment are maintained.
It is generally ineffective to attempt to relieve tyre thrust by cutting support rollers. As older kilns are upgraded it is common for drive speeds to be increased and this is usually accomplished in one of three ways: Lubrication should be in accordance with the gear supplier's recommendations for viscosity at operating temperature.
Any changes in support roller position should be considered for their effect on gear alignment and it is. Motor designs provide for short term loading of up to about 2. Axial thrusting of a kiln tyre should be corrected before significant metal is removed from the tyre sides because the resulting undercut is difficult to remedy. The tip to root clearance between gear and pinion should be measured routinely and after any refractory failure which may have resulted in shell damage.
An infrared pyrometer is used to measure the gear and pinion tooth flank temperatures and a t least three measurements across the gear face should be recorded monthly.
This is actually done somewhat indirectly by measuring the position of the riding rings. These temporary. Kiln designs make generous allowances for the weight of the kiln charge.
Kiln alignment. To operate reliably the support roller journal surface and bearing clearances must be adequate as must the viscosity. Allowable bearing pressures are determined by the bearing materials used.
Variations in coating pattern resulting in non-uniform shell temperature distributions can cause temporary changes in the shell's theoretical axis of rotation. The lowest risk operating state then. If a motor is run close to its current limit. Then imaginary straight lines are drawn between each tyre axis to determine if the tyres are high. If the kiln shell is relatively straight. To achieve this state. It is essential to inspect the shaft surface routinely during shut downs andremachine before excessive circumpherential scoring may cause penetration of the oil film.
After the kiln speeds up. The support rollers on each pier should have their axes aligned parallel to the theoretical axis of the kiln between each pier and slightly cut to impart an upward thrust to the tyre on each pier. Note that shell expansion at operating temperature is approximately 20cm and it should be confirmed that the tyres are centred on the rollers when hot.
The generator is connected to a bus feeding power to selected equipment which would include: The best system is a stand-by diesel generator of ca 1MW which starts automatically upon failure of the main power supply. The vertical locations of the tyre axes relative to the theoretical straight line kiln axis may vary significantly from pier to pier among kilns.
Bearing temperatures also are indicators of the support roller loading and. Kiln support rollers are designed to bear the weight of the kiln as well as some of the downhill thrust acting along the axis of the inclined kiln. Corrosion of the kiln shell is not normally a serious problem unless high levels of sulphur or chloride are present. The support rollers should then be adjusted to give equal deflection between left and right rollers on a given pier as well as between different piers.
Refratechnik Symposium. This is not. This will result in the support rollers lightly touching against their thrust bearings.
The sum of all support roller thrusting should keep the kiln from contacting its thrust rollers continuously when internal coating is normal and uniformly distributed. The most effective means to make the final alignment adjustments is to utilise an ovality gauge to measure the shell deflections at each tyre. Serious distortion of the kiln shell will result if it is not turned within minutes of a crash stop. The more reliable the main power supply and the more infrequent the use of the emergency system.
Kiln inching drive Primary air fan Water supply pump for gas analyser probe Cooler first and second compartment fans Emergency lighting Control system monitors Pack-house and load-out Alternatively.
Certain manual procedures are then essential such as withdrawing the burner pipe from the kiln hood an d opening doors at the top of the preheater if there are no automatic vents. While total clinker capacity should be equivalent to at least 14 days of kiln production.
A maximum composite free lime for the cement should be established and used as a control parameter. The desirability of large capacity storage buildings has resulted in a range of structures including sheds. In combination with cement storage. Clinker storage serves also to blend the clinker and a silo should not be filled and discharged to milling at the same time unless the silo has multiple discharge points to avoid short-circuiting.
Peter ZKG. There are obvious economies of scale for clinker storage and single silos up to High free lime clinker must be blended into mill feed with circumspection to ensure that the cement is not expansive. It is inadvisable habitually to run clinker silos down to the steel cone as abrasion will eventually cause structural failure.
Increased milling capacity has often been achieved by adding pregrinding. Outside stockpiling of clinker is not usually worthwhile if subject to rainfall. Recovery from the stockpile should not contribute more then 1. In recent years there have been numerous plant capacity increase projects as well as the construction of new lines.
Ball mills however are still the most common and will be discussed here roller mills were covered in Section 3.