Various process instruments have made the job of quality control relatively easy in cement plants. Dr Bibekananda Mohapatra takes into the details of various instruments and the parameters that can be controlled.
In general, the process and quality control are the two important aspects of the manufacturing of Portland clinker/cement. The cement manufacturing process is normally controlled at each operational unit by monitoring the oxide constituents of raw meal, kiln feed as well as resultant clinker. Chemical composition, although plays an important role in controlling the manufacturing process, sometimes it does not reveal changes in the process which could ultimately lead to unsteady kiln operation, affects the formation of clinker mineral phases, which could ultimately affect the cement quality.
In present scenario, the cement industry has been undergoing transformational changes from chemistry-based quality control to mineralogy, petrology, thermal and minor oxide constituents based quality control due to the use of alternative raw materials, industrial by-products and alternative fuel (hazardous as well as non-hazardous) as partial replacement of conventional fuel like coal in pyro-processing to bring down the carbon emissions in atmosphere and to achieve environmental sustainability.
In last few decades, the cement industry has seen significant changes in the type of fuel utilised in clinkering process, leading to changes in the quality of end product. Therefore, in view of above, chemical composition, mineralogy and morphology of input materials, in process materials, and finished product needs to be closely monitored to optimise the cement manufacturing process and to ensure a consistent quality of finished product, Portland clinker.
The cement industry presently monitor process parameters LSF, SM, AM, liquid content and quality of clinker through potential clinker phases C3S, C2S, C3A and C4AF, estimated by Bogue calculations on the basis of estimation of oxide constituents by XRF, but the monitoring of chemico-mineralogical composition of raw mix and in-process materials, and clinker at each production stage, starting from raw material, raw mix design, pyro-processing, cooling profile, comminution, type of set controller in cement, is the most effective way to maintain good quality clinker in trouble free environment. It is not the chemistry of raw meal, but the clinker phases assemblages and their crystallography that determine the ultimate cement properties.
No doubt, potential phases of the clinker could be estimated from chemical analysis, but it is well known that percentage of actual clinker phases, as determined by physical methods, always differ from the chemically estimated potential phase contents. Both microscopy and XRD are widely used for physical estimation of clinker phases, and clinker microscopy has been used to investigate the clinker burning condition and prediction of cement strength. However, the drawbacks of clinker microscopy lie in method of sample preparation and in the microscopic visual evaluation. The role of advanced analytical technique like XRD becomes very crucial in case, where there is a wide variation in their raw material quality to produce consistent quality of clinker. Therefore, XRD becomes an indispensable tool not only for characterisation of materials, but also proved to be highly reliable tool in solving process and application problems.
The introduction of advanced mineralogical phase quantification software has further led to worldwide application of XRD to assess process and quality control in cement manufacture. Reitveld refinement software coupled with advanced detectors not only minimise the measurement time but also proved to be an effective quality and process control tool for qualitative as well as quantitative estimation of different mineral constituents and phases, encountered during cement manufacturing process.
Major areas in cement manufacturing process, starting from evaluation of raw materials, homogeneity of raw meal, burnability characteristics, clinker mineralogy, comminution process and impact of gypsum on cement properties, where XRD can play a crucial role are:
The present paper highlights the application of XRD in cement industry and how effectively, it could be used in cement industry along with microscopic and thermal analytical techniques for both quality and process optimisation.
Areas of applications of advanced analytical techniques
X-ray diffractometery has been widely used and has proved to be highly reliable tool for raw materials characterization, pyro-processing, clinker mineralogy, that ultimately affects the cement properties, including hydration characteristics and diagnostic studies on kiln coating formation, lump formation in cement silos and bags. Determination of free CaO content in the cement clinker is still regarded as an index of clinker quality. DTA/TGA study has been conducted for all types of raw materials, raw mix, clinker, in process samples like coating, lump formation, finished cement product and hydration mechanism. The main features of a cement quality control system involve: the raw mix composition, burning of the cement clinker, and the fineness and strength of the cement produced.
Characterisation of raw materials: Quality of raw materials used in the cement manufacturing has a very significant role on the quality of clinker/cement as well as on selecting the various process parameters and conditions for optimal utilisation of materials. Evaluation of mineralogical and microscopic characteristics of different raw materials have an important role for the assessment of the quality of these materials. Normally, Ordinary Portland Cement (OPC) contains around 62 to 65 per cent CaO, 20 to 22 per cent SiO2, and 10 to 15 per cent combined Al2O3, Fe2O3, MgO and alkalis with four principal compounds; C3S, C2S, C3A and C4AF. These compounds are formed by a series of clinkerisation reactions at the temperature range in between 1300-1500oC, between lime on one side and silica, alumina, and iron oxides on other. In cement manufacturing process, calcareous material, preferably limestone bearing calcite (CaCO3) being a predominant component of the raw mix, along with clays and shales are used to provide the silica. The presence of Al2O3, Fe2O3, MgO, and alkalis in cement raw mix has facilitated the formation of clinker phases (calcium silicates).
Dolomite [CaCO3.MgCO3] is the principal magnesium bearing carbonate phase in limestone. The mineralogy of limestone and dolomite can easily be determined with ease in comparison to Optical microscopy. Similarly, phlogopite [KMg3(Al,Si3)O10(OH,F)2], which belongs to mica group of minerals, but serves as a very active mineraliser in sintering process of cement raw mix due to the fluorine bearing phase, that could easily be detected by XRD. Therefore, the mineralogy of raw materials, particularly unconventional one is very crucial in determining the reactivity of cement raw mixes.
DTA/TGA used extensively for the examination of the mineralogy of the cement raw materials. In addition to calcite, a number of accessory minerals, such as quartz, feldspar, mica group of minerals, dolomite etc., are present in limestone. Calcite, quartz, dolomite and clay minerals show distinct endothermic peaks and muscovite, phlogopite, and carbonaceous materials show exothermic peaks. Decarbonisation of pure calcite records in the range of 890 to 910 degree Celsius, in case of limestone it may be in between 850 to 950 degree, owing to the variation in degree of crystallisation (including metamorphism), grain size and also for associated impurities.
As the mass loss in TGA of calcite is due to the release of CO2, amount of CaCO3 in the limestone can be estimated from the TGA curve. Dissociation of MgCO3 (dolomite) at around 800¦C in DTA/TG curve can be easily distinguished from that of CaCO3 (calcite) and hence dolomite may be identified and estimated from the DTA of the limestone. Similarly, although muscovite and phlogopite belong to the same group of minerals with almost similar optical properties, can be distinguished from each other by their DTA peaks. Thermal characteristics of some common minerals present in cement raw materials are given in Table 1.
Optical microscopy is a powerful tool to examine the morphology of raw material, particularly limestone and have detrimental effect on the burning characteristics of raw mixes if calcite and quartz crystallite is larger in size. For example, coarser quartz grains will make kiln feed hard to burn and may leave a cluster of silicate crystals in the clinker, leading to harder clinker grindability. Therefore, the optimum grain size of calcite and quartz should be below 125 and 45-micron for optimum burning condition. The influence of quartz is approximately five times greater than that of calcite on the clinkered product.
Homogeneity of cement raw mix: Homogeneity of the cement raw mix plays an important role in determining the quality of product and in energy savings. Heterogeneity is a disadvantage during the burning process and causes reduced clinker quality, lower kiln output and an unstable situation in the kiln. It has been shown that heterogeneity inherent in the preparation of the raw mixes leads to the wide local variations in mineralogy and interstitial melt composition as well as in the micro texture of the clinker.
Usually the homogeneity of the raw mix is determined from the variation in the chemical composition in different size fraction samples. But this procedure is time consuming process and XRD provide a viable solution in determining the homogeneity of cement raw mixes by monitoring the diffraction lines of two major minerals (calcite and quartz) in raw mix. Distribution of calcite and quartz generally determines the degree of homogeneity and the relative quantities of then can easily be determined by XRD [Goswami et.al, 1990a].
Sintering (Hot meal analysis): In the sintering process, calcinations of raw mix proceeds with changes in mineralogy. Simple chemistry of sintered materials cannot provide adequate knowledge regarding the exact nature of final product and the XRD is the appropriate tool to examine the mineralogy of the sintered material i.e dissociation of material, degree of calcination and other phase transformations. Hot meal analysis helps to check degree of calcination, lime combinability in the kiln feed entering the kiln, monitor mineralising effect, and thus to adjust the fuel feed to the pre-calciner and also subsequently in the pyro processing zone.
Higher degree of calcined kiln feed would require less input in the kiln. With the help of XRD, determination of degree of calcination is easier and more rapid, along with formation of intermediate and final phases and can take remedial measures to achieve desired mineralogy in the final clinkers. Thus, hot meal evaluations thus are predictive of the subsequent burning zine behavior of the kiln feed. These studies could be advantageous in optimising the kiln performance of the kiln feed and could be a good approach to monitor the mineralising effect using mineraliser. Hot meal analysis for alkalis, chlorides and sulphates helps to maintain these volatiles within permissible limit for trouble free processing without formation of undesired buildups and coating formation. Formation of salts like langbeinite are the indicators for build-up tendencies.
Reactivity and burnability of cement raw mix: The quality of Portland clinker/cement primarily depends on the quantity and characteristics of different mineral phases present in cement raw mix, which in turn influence the burning process. It has been reported by several researchers that raw mixes with identical composition and fineness cannot be presumed to show equal degree of burnability and identical raw mix compositions, even with more or less similar burning processes may not be expected to produce the same clinker mineral assemblages. Reactivity and burnability are two fundamental characteristics that affects the through put of the plant and ultimately specific power consumption. Therefore, in recent years, much attention has been given on the study of the inherent characteristics, particularly mineralogy of the raw materials. The factors that affecting the burnability of cement raw mix are:
The reactivity of a cement raw mix depends on the mineral phases of lime and clay components. Polymorphic form of CaCO3, calcite in limestone is more reactive than aragonite which in turns more reactive than dolomite and ankerite
(calcite>aragonite> dolomite> ankerite) (Fig 3). Similarly, for silica and alumina phases of clay components, kaolinite is more reactive than montomorillonite and chlorite is the least reactive component. Therefore, the mineralogy of raw materials plays an important role in determining the reactivity of cement raw mixes. Similarly, silica polymorphs/compounds and their order of reactivity shows that glassy slag is most reactive and quartz is least one.
The order of their reactivity is; quartz With XRD technique, the clinker free lime could be analysed rapidly, and could be an important feedback for process control strategies on frequent basis, leading to better efficiency of the pyro-processing operations. In addition to this, information on the clinker mineralogy (like C3S, C2S formation), will offer greater control on the clinker phase composition thus achieving consistency in the quality of clinker.
It is observed that chemically and physically identical raw mixes experienced different degree of burnability, that caused unsteady operation of kiln and thus adversely affected the quality of the product. To investigate the problem, three chemically and granulometrically identical raw mixes, based on limestone from a single deposit, showing different degree of burnability were selected for extensive inherent (mineralogical) characteristics. These mixes, RM-1, RM-2 and RM-3 showed identical size gradation. The chemistry of raw mixes and % free CaO estimated after firing these samples at the temperature of 1,400 degree Celsius for one hour is given in Table 2.
From Table 2, it is clear that LSF, SM, AM and C/S increase from RM-1 to RM-3, in contrary to that % free CaO at 1400oC decreased from RM-1 to RM-3, indicating that mix RM-3 shows higher degree of burnability in-spite of high LSF in comparison to other mixes. Chemical as well as granulometric composition of the raw mixes could not explain this phenomenon and therefore, the mineralogy of raw mixes plays an important role in characterising their burnability.
The most distinct feature amongst the unfired raw mixes was that the presence of fluorine bearing mineral phlogopite; entirely absent from RM-1, discernable in RM-2 and significantly high in RM-3, thus resulting in different degree of burnability characteristics of above raw mixes [Goswami et.al, 1985; Goswami et.al, 1990b; Goswami et al., 1991]. Therefore, it was possible to correlate the burnability of cement raw mixes by XRD as fluorine content in phlogopite worked as mineralizer and F- present inherently in the raw mix is more effective than the external F- added to the mix.
DTA/TGA technique helpful to investigate the thermal behavior of cement raw mixes, throughout the heating temperature range and provide crucial information about the burnability inside a kiln, by monitoring endothermic and exothermic reactions taking place even before the dissociation of carbonates, to offset the critical heat balance to a considerable extent [Rao et.al, 1977; Webb et.al, 1957]. Degree of burnability of cement raw mix is examined in terms of free CaO content and was found that a relationship exists between free CaO and the temperature of dissociation of carbonate component [Goswami, 1987]. Furthermore, DTA curve of the raw mix records the process of clinkerisation showing devitrification (exothermic peak at 1,270 degree Celsius) and melt formation (endothermic peak at 1,300 degree Celsius).
ABOUT THE AUTHOR: Dr Bibekananda Mohapatra is Director General of National Council for Cement and Building Materials, Ballabgarh, Haryana. This is Part I of the article. Part II will be published in the next issue.