Xrd Kiln Control
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(2) •. The potential for improving clinker quality with improved heat flux profile, with subsequent savings in grinding energy, and marketing advantages The need to reduce gaseous emissions to meet compliance levels Potential for other operating cost reductions (refractory use, kiln stability etc). • •. Mineralogical Composition 60. 25. 50 20. 40. Percent. 15. Alite Belite-b Belite-g. 30. Ferrite Aluminate-o 10. Aluminate-c. 20. 5 10. 0 1/07/02 0:00. 0 1/07/02 8:00. 1/07/02 16:00. 2/07/02 0:00. Time. Fig 2 Continuous XRD Trend of clinker minerals in real time Because of the large potential savings, cement plant operators have invested heavily in improving kiln control, and over the decades very sophisticated expert systems have evolved to this end. However, these control the equipment (fans, fuel, feed rates, etc) without knowing what is happening to the material being processed in the kiln, except through circumstantial evidence. The continuous on-stream XRD is an invaluable tool for providing the missing information continuously and in real time, on which kiln control can be based. This provides the basis for a new level of kiln control, where the controlled parameter is the clinker mineralogy, including percentages of alite, belite, ferrite, aluminate, free lime, alkali sulfate, periclase or any other species of interest. Furthermore, by monitoring the various forms of the clinker minerals (α,β,γ, ortho-rhombic, cubic etc) the kiln operation could be adjusted (calciner operation, kiln speed, burner set up, cooler operation for example) to ensure production at the optimum quality and cost position.. The On-Stream XRD Analysis Process In cement product applications, the cement can be fed directly to the Continuous OnStream XRD without further treatment. For clinker analysis, the clinker must be ground to approximately 50% passing 45 micron. This requires a continuous sample taken at the cooler discharge, modified cooler grate or at the kiln nose ring, and further prepared for presentation to the XRD. (It should be noted however that taking a representative -2-. www.fctinternational.com.
(3) sample at the kiln discharge or even along a cooler somewhere could be extremely problematical because of the segregation that occurs there). A sample can also be taken from a clinker transfer point, as shown in Figure 3. A virtually continuous bulk sample is crushed and then sub-sampled to provide a smaller, continuous, representative sample of about 300 - 600grams/minute which is milled to provide a sample that is fed continuously through the XRD. The excess crushed clinker sample and the ground, analyzed sample is continuously returned to the clinker transport system. The Continuous On-Stream XRD analyses a moving bed of ground clinker, using its unique area detector to continuously collect the complete XRD pattern. In this way, the analyser is able to produce real time trends of all clinker minerals including free lime contained in the clinker. This trend displayed in the control room and laboratory is the basis for control actions. No other XRD analyser on the market today produces a complete mineralogical composition of a moving bed of material in real time. In addition the moving bed of material provides the best opportunity to ensure that the analysis returned is truly representative of the bulk stream. An automated laboratory based batch analyser would take many days to analyze the amount of sample that moves through a Continuous On-Stream XRD in one hour.. Clinker Mineralogy, Free Lime, NOx and Fuel Consumption Clinker mineralogy is a function of the elements present in the kiln feed AND the treatment they receive in the kiln. Bogue equations, historically used to “calculate” a hypothetical mineral composition ignore the heat treatment in the kiln, and assume an idealized equilibrium reaction. Consequently, as is now commonly known by cement chemists, the Bogue equations rarely agree with the real composition of clinker and cement, and can be very misleading. A continuous on-stream XRD analyzing clinker will report exactly what is being produced, and allow kiln adjustment to optimize the clinker composition. For example, one plant site was found to have relatively high proportions of γ-C2S in the clinker, pointing to a long slow cooling rate that could be addressed by cooler operation and burner set up. Other plants experience large swings in clinker mineralogy as a result of using alternative fuels, while others have large fluctuations in clinker reactivity and cement behaviour without any warning or indication. None of these above are indicated by the conventional XRF analysis that most plants still rely on. Free lime has been used as an indication of clinker burning. The presence of free lime in clinker may be due to a number of reasons. These include4: • inhomogeneous raw meal • raw meal too coarse • raw meal LSF too high • inadequate burning (such that the free lime is not combined with the other oxides). 4. D Knöfel, in Labahn and Kohlhaas, Cement Engineers’ Handbook, Bauverlag, 1983 -3www.fctinternational.com.
(4) 1 2. 3. 4. 7 5. 6 7. XRD Figure 3 Schematic for clinker sampling at a transfer point (1). A sampler cuts a portion of the main clinker stream at the transfer point (2), and this is processed by the crusher (3), reduced to a smaller sample by the rotary divider (4) and milled to a powder (5). The powdered clinker is sent though the continuous XRD via a feedscrew (6). Excess sample and analysed sample is returned to the process (7).. -4-. www.fctinternational.com.
(5) By having a complete picture of variations in the free lime it should be possible to correlate variations in the free lime with the correct cause, and thereby take appropriate corrective action. The laboratory test typically takes some hours to produce a result by conventional wet methods, and almost as long with laboratory based XRD. The continuous on-stream XRD produces a result in real time giving the kiln operator relevant and timely information for kiln control. This free lime analysis together with other compositional information can also be automatically read by an expert system controlling the kiln. It should be noted here though that free lime on its own can be a dangerous measure if not taken in context with the complete composition. There have been many cases of severe damage done to kilns as a result of a kiln operator trying to achieve a free lime target that was impossible to reach because of chemical composition. Controlling free lime to a target though is normal for cement production. However, more often than not, clinker free limes are much lower than the nominal target. The reasons are that a hard burned kiln is more stable as it can cope with process variations better. It may be that the chemical variation is high or feed rate or fuel rate variable and the kiln is burned hard to prevent regular upsets. A continuous on-stream XRD would indicate all of this and provide precise information for kiln control.. NOx Levels as a function of kiln burning Hard burning is a very expensive way to operate, with the costs largely hidden. These costs are in fuel consumption, production rate, clinker quality and NOx emissions. NOx levels are primarily dependent on how hard a kiln is being burned, and figure 4 above is from a recent investigation of a calciner kiln. Clearly, better kiln control can reduce NOx emissions by 30 – 40%, and with the aid of a continuous on-stream XRD to achieve this, such technology should be of interest to those plants striving to minimize NOx emissions.. Fuel Consumption as a function of kiln burning It is well known that hard burning uses more fuel per kg of clinker. As a general rule, increasing free lime in clinker from about 0.3% to an average of 1% will reduce specific fuel consumption by around 5%, and will result in a 5% increase in production rate from that kiln. This equates to a considerable economic advantage through better kiln control possible now through the availability of continuous on-stream XRD.. Clinker Quality as a function of kiln burning Clinker quality, or reactivity, is also largely affected by the kiln heat treatment. While high free lime means that less alite and belite are formed, as well as introducing possible soundness issues with concrete, low free lime is also detrimental. The “hard burning” of clinker does form some of the less reactive phases leading to slow strength development.. -5-. www.fctinternational.com.
(6) 1000. NOx (lb/hr). 900. NOx. 800 700 600 500 400 0. 0.2. 0.4. 0.6. 0.8. 1. 1.2. 1.4. 1.6. Freelime, %. Figure 4 Correlation between free lime and NOx emissions. So generally, as depicted in figure 5 below, there will be an optimum free lime for optimal strength development in cement. Typically this lies in the 1.0% to 1.5% range. Hard burning can reduce 28 day cement strengths by 5 – 10 MPa, compared to the optimum potential strength of the cement. Figure 5 also indicates the clinker grindability as a function of free lime, and as expected harder burning leads to harder to grind clinker. Cement mill throughput can be affected by up to 10% depending on how hard the clinker is burned. The implications for this can be severe for a cement mill, especially where cement mill capacity is limiting. Compare clinker at 1% free lime with clinker at 0.3% free lime as an example. Firstly, a 10% drop in strength at 28 days will mean that finer grinding will be required in an effort to regain some of that strength, while at the same time, the clinker will be 6% harder to grind. The combined effect will be at least a 15% loss of cement mill throughput.. Complete Analysis In many plants a laboratory XRD is being used for free lime only. This is partly a legacy of older XRD technology and analysis software. In older XRD analysers collecting a iffraction pattern could be time consuming because the hardware required each angle to be collected sequentially, by scanning the detector over the angular range of interest. For this reason it became simpler to limit the data collection to the range of angles that were relevant to free lime measurement. In addition the data analysis is likely to be based simply on the count rate of one of the free lime peaks, which requires careful standardization to maintain accuracy.. -6-. www.fctinternational.com.
(7) Schematic Optimum Free Lime vs. 28 Day Strength and Grindability 150%. S T R E N G T H. 130% 55 MPa. 110% W.r.t. 3002/kg. G R I N D A B I L I T Y. O v e r. Inc. reas. b u r n I n g. i ng. C3 S (the o. reti. cal s. tren. gth. Optimum %Free Lime. Gri ). nda. C em ent. bili ty (. Stre ngt h. a c tu. al). (act ual). 50. 45 0.5. 1.0. 1.5 % Free Lime. 2.0. 3.0 Strength1-2.ppt. Figure 5 Cement Strengths as a function of clinker free lime. The FCT-ACTech Continuous On-Stream XRD analyser uses a patented full area detector, which is able to measure the entire diffraction pattern simultaneously without moving. This full area detector, combined with the latest fundamental parameter Rietveld pattern analysis, is ideal for continuous measurement and provides a complete picture of the cement mineralogy. Continuous On-Stream XRD analysis has the advantage of providing a much more complete picture of what is actually happening. Comparing laboratory XRD with continuous XRD is like comparing single photos to a complete video of the mineralogy emerging from the kiln. In the FCT-ACTech XRD analyser, the diffraction pattern analysis is performed using a fundamental parameter Rietveld analysis, which gives a complete quantitative and standardless analysis of the pattern. The accuracy of this method has been tested and proven 2,5. Rietveld analysis gives a complete breakdown of all of the minerals present in the clinker stream. This is highly relevant to proper quality control. For example it is possible to monitor the concentration of the gamma form of belite, which is an undesirable consequence of operating the kiln with slow heating, and or inadequate quenching of the hot clinker as it exits the kiln.. 5. Scarlett N, Madsen I, Manias C and Retallack D, Powder Diffraction, June 2001. -7www.fctinternational.com.
(8) Mineral Alite Belite – beta Belite – gamma Aluminate – cubic Aluminate – orthorombic Ferrite – alpha Ferrite – beta Lime Periclase Arcanite Quartz. Shorthand, or composition C3S C2S-β C2S-γ C3A-c C3A-o C4AF-α C4AF-β CaO MgO K2SO4 SiO2. Table 1 Summary of some mineral phases measured (also trended with time) FCT-ACTech’s XRD Rietveld analysis includes the clinker minerals shown in Table 1. From Table 1 it is clear that the analysis provided by a properly analysed XRD pattern is indeed extensive, and other minerals can be included in the analysis if desired. It is important to note that the values measured for the standard components (eg. C3S etc.) are true measures of the cement mineralogy, and not the Bogue values that are calculated from chemistry. As discussed earlier, there are usually large discrepancies between Bogue calculated hypothetical composition and the true mineralogy as given by XRD. XRD can also monitor any compound of interest in clinker such as the percentage of Periclase (free MgO) in the clinker, which can be just as important for expansion as free lime. If the plant is producing more than one type of clinker then the Continuous On-Stream XRD will give a clear indication of the type change, showing when to switch from one storage point to another, since a change to a different type will be clearly distinguished in a timely manner. This results in improved quality and an optimum usage of the higher value clinker. This can be particularly useful with oil well clinker production, where clinker quality and suitability for oil well cement can be difficult to establish.. Conclusion Most modern plants now use sophisticated expert control systems to optimise their kiln operation, but even the best control system cannot compensate for a lack of data. By using a Continuous On-Stream XRD the control system has access to extremely valuable information about the processes occurring in the kiln and clinker cooler. In particular the XRD provides detailed information about the trends in mineralogy, and these trends can be correlated with operating conditions. This information enables the control system and operators to achieve the optimum operating conditions, with all of the associated benefits, such as improved clinker quality, improved clinker grindability, reduced emissions and reduced fuel consumption.. -8-. www.fctinternational.com.
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