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2 Manufacture of Portland Cement Peter Del Strother 2 Manufacture of Portland Cement Peter del Strother 2.1 RAW MATERIALS FOR CLINKER MANUFACTURE 2.1.1 Chemical Targets for Raw Meal Lime saturation factor (LSF) is a measure of the ratio of limestone to other recipe components, normally expressed as a percentage. LSF ¼ 100∗CaO=ðÞ2:8∗SiO2 +1:18∗Al2O3 +0:65∗Fe2O3 When clinker LSF is 100, the proportion of alite, the principal strength giving calcium silicate, is maximised. In practice clinker LSF is typically in the range 94–98. Clinker LSF depends on the raw meal LSF and the contribution of fuel ash, which is normally low LSF. Silica modulus (SM) and alumina modulus (AM) influence both clinker quality and burnability, the ease with which alite forms in the hottest part of the kiln, the ‘burning zone’. SM ¼ SiO2=ðÞAl2O3 +Fe2O3 AM ¼ Al2O3=Fe2O3 2.1.1.1 Quarried Raw Materials Limestone The fundamental requirement for clinker manufacture is a source of lime (CaO) and sources of silica, alumina and iron oxide. In almost all cases the lime comes primarily from limestone. Limestone is a sedimentary rock composed of the hard parts of once living organisms. If raw material limestone has a LSF of more than about 200 it should not be difficult to devise a recipe with LSF, SM and AM suitable for clinker production. If LSF in limestone is lower than about 200 it becomes harder to find suitable non- calcareous components. Limestone is rarely pure calcium carbonate; deposits often incorporate siliciclastic components, such as silt from rivers or even volcanic ash. Limestone may also contain compounds of zinc, lead and fluorine, for example. Contaminants in limestone and other raw meal components may have a significant impact on clinker quality, kiln operation and emission to atmosphere. Of special relevance to quality and kiln operation are: MgO: Dolomite CaMg(CO3)2 in limestone may arise from the original depositional environment or from diagenesis, the process of change to the sediment since deposition. It may be dispersed very unevenly within a limestone deposit. The ratio of MgO to CaO in dolomite is about 70% by weight. The European Standard EN 197 stipulates that ‘The magnesium content (MgO) shall not exceed 5.0% by mass’ in cement. The US ASTM C150-07 allows 6% MgO. Only a small pro- portion of dolomite in the raw materials recipe would be required to breach these limits. Chlorides: Limestone, especially porous limestone found near the sea, may be contaminated with sodium chloride (NaCl). Chloride is commonly found in alternative fuels, such as domestic-refuse-derived fuels. Chloride in cement is limited by most national cement standards and a kiln bypass may be required to remove chloride from the kiln system. Fluorides: Limestone altered by mineralising solutions may contain a suite of minerals including fluorine as fluorite, CaF2. The presence of fluoride acts as ‘mineraliser’. It also decreases the viscosity of the liquid phase in the sintering section of the kiln and if present in sufficient quantity may result in what should be nodular clinker becoming more like lava, resulting in serious damage to the clinker cooler. Alkalis, compounds of Na and K: The alkalis are found both in the clay (or shale) and the calcareous components of the recipe, but at higher levels in the clay or shale. If alkali levels in clinker are higher than required, selective use of low alkali clay or shale may overcome the problem. Increasing SM by addition of quartz sand also reduces clinker alkalis by reducing the proportion of clay or shale in the recipe. If those options are not available, non-alkali chloride can be added Lea’s Chemistry of Cement and Concrete. https://doi.org/10.1016/B978-0-08-100773-0.00002-2 © 2019 Elsevier Ltd. All rights reserved. 31 32 Lea’s Chemistry of Cement and Concrete to the kiln system and alkali removed as potassium chloride (KCl) in bypass dust. For example with increased use of alternative fuels containing PVC (polyvinyl chloride), Na2Oequiv in clinker can be reduced by between 0.05% and 0.1%. 2.1.1.2 Non-Calcareous Components Quarried non-calcareous components include shale and clay, which are silicate minerals containing compounds of alumina and iron. Quartz sand, a source of silica, and iron ore are also used. 2.1.2 Secondary Raw Materials The use of secondary raw materials is usually driven by cost, to which transport is often a major contributor. Waste-derived raw materials may have sustainability benefits. PFA (pulverised fuel ash), arising from the burning of coal in coal-fired power stations is widely used; it usually contains unburnt carbon, which may give ‘free’ heat input to the kiln system but may also give rise to CO emissions. In countries with emission trading schemes the organic carbon in waste raw materials may provide a financial benefit as it is considered to reduce CO2 emissions through a concomitant reduction in the use of conventional fuels. 2.2 RECIPE In the cement industry the following abbreviations are commonly used: C for CaO S for SiO2 A for Al2O3 F for Fe2O3 In this chapter these abbreviations will be used sparingly. However, they will be used in the Bogue formulae, see Section 2.4. An example of a recipe made from a small selection of raw materials is given in Table 2.1. LSF, SM and AM are also indicated for each component as these give an indication of what that material contributes to the recipe. The data in Table 2.1 is set out with the bold cells being adjustable and the percentage limestone being 100 less the sum of the clay and sand percentages. An LSF of 100 has been set as a raw meal target. Clinker LSF would typically be at least 2% points lower because fuel ash is normally low in CaO. With three components it is possible to control only two variables. In this example if the SM target is reduced to 2.75, equivalent Na2O in raw meal rises from 0.41% to 0.44%, see Table 2.2. It is possible to calculate the exact percentage of each component using the inverse matrix method, but the spreadsheet method is easily used and the natural variability of raw materials does not justify greater precision. On cement production works substantial investments are made to homogenise raw materials in order to minimise vari- ability in raw meal chemistry. Variability in raw meal chemistry leads to kiln instability, variability in clinker quality and hence cement performance, especially strength, in concrete. 2.3 KILN SYSTEMS 2.3.1 Preparation of Feedstock and Grinding The quality of raw meal has a direct impact on clinker quality and on the production process. It is necessary to meet raw meal chemistry targets and minimise standard deviation. Methods of achieving these outcomes vary from slurry basins for the wet process to stacker-reclaimer stores and raw meal homogenising silos for the dry process. Representative sampling is a chal- lenge and for most dry process systems cumulative raw meal samples are collected over an hour or so and control decisions made following analysis of those samples. Increasingly online cross belt analysers are being used for raw meal chemistry control. A significant advantage is that deviations from target are detected in just a few minutes and automatic corrective action taken. The need for investment in raw meal homogenisation is thereby reduced. Cross belt analysers are able to measure chemistry of crushed rock in the 0–75mm size range. The technology used is Prompt Gamma Neutron Activation Analysis. The neutrons penetrate the full cross-section of the material on the belt so the analysis is of the bulk material, not just of the surface. Feedstock preparation technology tends to be site specific and individual manufacturers offer different solutions. The operational principles are introduced in a number of readily available cement making manuals, such as The Cement Plant TABLE 2.1 Example of Recipe (% Dry Basis) at LSF 100 and SM 2.9, Using Three Raw Materials TABLE 2.2 Example of Recipe (% Dry Basis) at LSF 100 and SM 2.75, Using Three Raw Materials Manufacture of Portland Cement 33 34 Lea’s Chemistry of Cement and Concrete Operations Handbook.1 Research has shown that for complete combination in the kiln, the particle size of calcium carbonate should be <125mm and that of quartz <45mm.2 Calcium carbonate loses 44% of its mass during calcination, resulting in a large specific surface area available for reaction. Quartz (SiO2) is resistant to both mechanical and chemical attack and has a much lower specific surface area than a lime particle of the same size. The melting point of SiO2 is about 1700°C and that of CaO >2500°C, both considerably above the temperature achieved in the kiln burning zone. Given an appropriate particle size and time, both CaO and quartz dissolve in the liquid phase.3 Silicate minerals have lower melting points than quartz and are readily incorporated into the liquid phase in the burning zone of the kiln. Coarse quartz in raw meal reduces burnability because belite clusters form around the quartz grains, inhibiting conversion of belite to alite. For explanation of alite and belite see Section 2.4. Quartz is harder to grind than limestone and it is not possible to grind both quartz finely and limestone less finely at the same time in a single raw mill. The target for raw meal fineness is a compromise typically in the range 10%–15% residue on a 90mm sieve. Grinding more finely increases raw mill electrical consumption and hence kiln specific power consumption.
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