- 1 Everything you need to know about cement Raw Materials Selection
- 1.1 LIMESTONE AND CARBONATE ROCKS
- 1.2 Genesis of Limestone Rock
- 1.3 Basic mechanism.
- 1.4 Diagenesis
- 1.5 Distribution of Limestone in Space and Time
- 1.6 Calcium Carbonate Minerals and Their Solubility Effects
- 1.7 Classification of Limestones
- 1.8 Geological Characteristics of Limestone Deposits from Exploitation Perspective
- 1.9 Advances in exploration and exploitation software
- 1.10 TECHNOLOGICAL ASSESSMENT OF LIMESTONES
- 1.11 Broad Specification of Cement-Grade Run-of-Mine Limestone
- 1.12 Additional Desirable Technological Characteristics
- 1.13 Associated Minerals and Impurities
- 1.14 Physico-Textural Features
- 1.15 Dependence of Decarbonation Behavior on Limestone Characteristics
- 1.16 ARGILLACEOUS MATERIALS
- 1.17 Genesis and Occurrence of Clays
- 1.18 Rock Association of Clay Minerals
- 1.19 Physical Properties of Clays
- 1.20 Thermochemical Reactivity
- 1.21 ROLE OF RAW MATERIALS IN THE PROCESS AND HARDWARE SELECTION
- 1.22 CONCLUDING OBSERVATIONS
- 1.23 ACKNOWLEDGEMENTS
Everything you need to know about cement Raw Materials Selection
by A. K. Chatterjee*
Dispersed and homogenized raw mixes for the manufacture of portland cement clinker consist basically of two generically different natural raw materials – calcium carbonate (or limestone) and aluminosilicates (or argillaceous substances) – that are complementary in nature in meeting the stoichiometric needs of forming the clinker phase assemblage. At times, when such stoichiometric needs cannot be met by the above two primary components, certain corrective materials such as bauxite, laterite, iron ore or blue dust, sand or sandstone, etc. are used to compensate the specific chemical shortfalls in the raw mix composition. For certain processing advantages a few chemical reagents are used as slurry thinners in the wet process and raw grinding operation, as granulation activators in the process of nodulization in semi-dry plants, as mineralizers in clinker formation, and as grinding aids in the cement milling process. These are essentially surface-active materials and are used in very small quantities, when required. Further, in the context of environmental ameliora-tion and resource conservation, a large number of industrial wastes or byproducts are used as basic or corrective raw materials in the cement industry.
Fig. 2.1.1. Limestone quarry for use in cement operation.
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A fairly comprehensive review of chemico-mineralogical characteristics of cement raw materials was published by Chatterjee (1983). This chapter is an attempt to focus on the issues relating to the selection of natural raw materials. The blending materials like ground granulated blast furnace slag, volcanic ash, calcined clay or metakaolin, pulverized fuel ash, and condensed silica fume on one hand, and the set retarders like gypsum on the other, have not been dealt with in this chapter.
LIMESTONE AND CARBONATE ROCKS
It has been broadly estimated by Oates (1998) that about 4500 million tons of limestone are used per annum worldwide. The major uses are in two areas: as aggregate in construction and as the primary raw material for cement making. The quantity of limestone used in the manufacture of cement is estimated at about 1500 million tonnes or one-third of what is generally mined every year. A lime-stone quarry for use in cement manufacturing is shown in Figure 2.1.1.
It is claimed that limestone has been in use for construction purposes since the Stone Age, and apparently the early records mention that Giza Pyramids used limestone for construction some 5800 years ago. There is evidence that by about 1000 B.C. there was widespread use of lime for buildings by many civilizations across the globe. There are also indications in the literature about extensive use of limestone as aggregate in lime-concrete since the Roman times. Obviously, the development of portland cement in the 19th century caused the major expansion in demand for limestone.
Genesis of Limestone Rock
Limestone is classified as a sedimentary rock and its genesis in the geological time frame has depended on the following pre-requisites:
1. Availability of calcium ion and carbon dioxide gas in the terrestrial system for their chemical interaction
2. A basin with conducive physico-chemical conditions for deposition
3. An appropriate mechanism of deposition
4. Suitable conditions for diagenesis
5. Geotectonic movements leading to global distribution of limestone deposits
Although each deposit of limestone has a unique history in terms of the above conditions, certain common facts and features are presented below.
It is widely believed that in order of abundance in the earth’s crust, calcium holds the fifth position after oxygen, silicon, aluminum, and iron. On the other hand, carbon diox-ide, according to Oates (1998) makes up about 0.03% by volume of the earth’s atmosphere. The combination of dissolved calcium ions and carbon dioxide results in the sedimentary deposition of calcium carbonate, which is subsequently converted into limestone rock.
Raw Materials Selection
The deposition takes place primarily in two different environments: marine and inland waters, the descriptions of which have been given by Scoffin (1987) as well as by Tucker and Wright (1990). The primary marine environment, of course, has its own variety that includes beaches, tidal, sub-tidal flats, lagoons, reefs, shelves, slopes, and deep basins. Whenever the physico-chemical condi-tions in the basins had been favorable in terms of concentration, temperature, salinity, water levels, turbidity, etc. for deposition, thick carbonate deposits formed in shallow seas within a 30° band on either side of the equator.
So far as the mechanism of deposition is concerned, two main types are recognized – inorganic and organic. The inorganic route involves the direct precipitation or crystallization of carbonate. This mechanism had occurred in the environments of both marine and inland waters, and had resulted in some commercially significant deposits.
But it has generally been observed that most of the commercially viable deposits of carbonate rocks were formed by the organic route. This happened because of the fact that carbonate-secret-ing organisms such as bivalves, gastropods, brachiopods, corals, echinoderms, foraminifera, vari-ous algae, etc. had existed in seawaters quite extensively. A few illustrative scanning electron micrographs of some fossils associated with limestones are given in Figures 2.1.2 to 2.1.7. Many such organisms, such as algae, are known to remove carbon dioxide from water, causing precipita-tion of fine chemical carbonate. Certain other organisms directly remove calcium carbonate from marine water to form their shells, the accumulation of which gives rise to carbonate deposits. The carbonate sedimentation by the organic route also had taken place in inland waters but the result-ant deposits are generally not as extensive nor commercially as important as those produced in marine environments.
Conversion of carbonate sediments into rocks is geologically known as diagenesis and involves the steps briefly outlined here. After the carbonates form from chemical and mechan-ical breakdown of preexisting rocks and are, thereafter, transported as detrital particles or in chem-ical solution for deposition or precipitation in standing bodies of water in a layered sequence, the process of lithification starts under low-temperature low-pressure conditions.
Figure 2.1.2. Gastropod.
Figure 2.1.3. Cephalopod of cretaceous period.
Figure 2.1.4.Asilina of ecocene period.
Figure 2.1.5. Foraminifera of oligocene period.
Figure 2.1.6. Diatoms in a miocene limestone.
Figure 2.1.7. Surface striations on the pelycepods.
Simple compaction, cementation, neomorphism, microbial micritization, etc., are the specific vari-ants of the diagenetic processes, some details of which have been furnished by Tucker (1991). For example, compaction occurs during the burial process when the pebbles are crushed, the particles are closely packed, and pressure-induced dissolution and recrystallization take place. In the cemen-tation process, water supersaturated with calcite passes through porous carbonate layers resulting in the growth of calcite crystals in pores. The most common cement in medium-to-coarse grained limestone is “sparite” or “calcite spar.” Silica as quartz crystals also acts as a cement in some lime-stones. In microbial micritization many organisms bore into carbonate deposits and the holes become filled with calcium carbonate, called “micrite,”that typically forms an envelope around the skeletal grains. Neomorphism refers to the progressive transformation of aragonite to calcite or recrystallization of calcite into coarser crystals.
Distribution of Limestone in Space and Time
According to Wiersma (1990), limestone deposits cover about 10% of earth’s land surface and are found in many countries. This has happened, despite the genetic history of limestones that the deposits were essentially formed in shallow seas in the band 30°S to 30°N due to the effective growth of carbonate-producing organisms in clear water with less inflow of terrigenous material washed from the land. It is evident, therefore, that those parts of the continental plate, which have remained south of 30°S generally, have little limestone. Conversely, those continents that drifted into, and in some cases across the band 30°S to 30°N, are relatively rich in limestone deposits.
So far as the distribution of limestone deposits in time is concerned, despite very precise physico-chemical and climatic prerequisites for the carbonate deposition as mentioned earlier, the lime-stone deposits are encountered almost in all the geological eras and periods that span over 600 million years as indicated below:
Limestones are not common in the Precambrian shields, although where shallow seas had formed some carbonate deposition took place. It is believed that such Precambrian limestones were de-posited either as inorganic precipitates or as a result of the biochemical activity of very simple organisms such as bacteria. It may also be mentioned here that most of the Precambrian carbon-ates have been subjected to metamorphism and dolomitization during the long post-deposition period. The Palaeozoic to Quaternary Eras are all fossiliferous and have favored the deposition and formation of carbonate rocks although, according to Strakhov and others (1954), rocks of certain geological periods (Ordovician or Cretaceous) show predominance of carbonates, while those of certain other periods (viz. Upper Carboniferous or Triassic) are relatively poor in carbonates. Further, the limestone deposits in different geographical regions and geological ages had different environments of deposition. For example, the Carboniferous limestone deposits in England were formed in equatorial and subtropical conditions, while the later Jurassic limestones and the Creta-ceous chalk deposits were formed in the warm temperate zone, apparently at progressively higher latitudes. From geotectonic evidence it has been seen that where there was progressive subsidence of the sea bed over prolonged periods, very thick deposits accumulated, an illustration of which are the Triassic deposits in the Western Dolomites which have a thickness of more than 2000 m.
It is the general view of geologists that the sea levels were higher in the past than at present and the area of shallow seas capable of sustaining carbonate-producing organisms was considerably greater then. As a result, the environmental condusiveness for carbonate deposition apparently was more extensive in the Mesozoic and Tertiary periods.
Calcium Carbonate Minerals and Their Solubility Effects
Crystallo-chemically there are three broad groups of calcium-bearing carbonates: calcite, arago-nite, and dolomite. Based on the data furnished by Boynton (1980) and Chilingar and others (1967), the relevant thermo-chemical characteristics of these carbonates are given in Table 2.1.1. In the formation of carbonate deposits there could be different mechanisms by which the surface layers of the sea become super-saturated with respect to aragonite, calcite and dolomite. The rate of formation of dolomite is much slower than that of calcite and aragonite. The aragonite struc-ture is generally very low in magnesium (typically less than 0.5% MgCO3). Calcite structure could either be low in magnesium with less than 4% MgCO3 or high with typically 11% to 19% MgCO3.
Table 2.1.1. Thermo-Chemical Characteristics of Calcium Carbonate Minerals
Aragonite is pseudo-stable with respect to calcite under ambient conditions and has a higher solubility in water by about 7% at all temperatures. It has been reported that aragonite slowly re-crystallizes into calcite in the presence of water. However, in the absence of water this transfor-mation takes places at 400°C to 500°C. Calcite is metastable with respect to dolomite in seawater that contains dissolved magnesium. The process of dolomitization is a slow geological process, and it is slowly reversed in the presence of fresh water.
According to Ghosh (1983), rise in temperature of seawater in the proximity of shelf or equator decreases the solubility of CO2 in water and favors precipitation of CaCO3.Agitation of water by breakers releases CO2 and favors carbonate precipitation. The solubility of CO2 decreases with decrease in atmospheric pressure. Evaporation in restricted basins results in a decrease in CO2 and an increase of salinity, facilitating carbonate deposition.
Notwithstanding what has been stated above regarding the solubility of calcium carbonate miner-als, it may be mentioned here that the high calcium and dolomitic limestones are among the most chemically stable substances. Decomposition never occurs at ordinary temperatures and these minerals are also unaffected by CO2 -free water, except for very negligible dissolution as mentioned by Boynton (1980).
Classification of Limestones
Limestone is treated as an omnibus term and is often imprecisely used to represent any calcium carbonate-bearing rock. However, it can be classified on the basis of various criteria, depending on the purpose for which a given classification is deemed necessary. A fairly detailed treatise on the classification of limestones has been furnished by Ghosh (1983). Most classifications obviously are for geological needs. For the purposes of industrial applications like cement making, one may derive one’s own version from such classifications and distinguish one limestone from the other in terms of their genesis, lithology, mineralogy, texture, and microstructure.
The major platform of distinction is the presence or absence of fossils related to the organic or inorganic genesis. Having recognized this differentiation, one may further classify the types based on broad lithology. This mode of distinction can be developed as follows:
1. Inorganic sedimentary limestones – typical examples include “travertine” deposited from natural hot springs; “tufa,” a soft porous rock also associated with natural springs; “stalactites” or “stalagmites,” columnar calcitic deposits inside the caves; “micrites” originating from calcitic mud or silt, etc.
2. Fossiliferous sedimentary limestones – rocks of widely differing textural and microstructural characteristics and fossil content broadly differentiated as “biosparites” or “biomicrites.” While the former represent the more extensively occurring massive well-bedded limestones consist-ing of fossil skeletons or small shells in a recrystallised calcitic matrix, the latter refer to the limestones having organic debris in a micrite matrix. Within this broad frame one may further distinguish the following types:
i. Algal limestone resulting from the action of algae
ii. Oolitic or pisolitic limestone showing round calcitic grains
iii. Reef or coral limestone characterized by nonbedded large fossiliferous deposits
iv. Chalk representing a white soft rock with very little land-derived silty material, generally thick (50 m to 400 m) in deposition
v. Marl designating an impure soft rock of marine origin in which varying amounts of clay and sand are present in a loosely knit crystalline structure
vi. Sea sand, essentially of quaternary age, representing calcareous sand carried on to sea beaches, occasionally with a mixed mineralogy of aragonite and calcite
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3. Metamorphic carbonate rock – “marble” which is highly crystalline and often highly dolomitic, although marbles derived from pure limestones through metamorphism without dolomitization, consist simply of coarse white calcite grains
4. Igneous carbonate rock – “Carbonatite,” which is effusive in nature and rare in occurrence
All of the above limestones can be further classified on the basis of certain textural and composi-tional yardsticks, some illustrations of which are given in Table 2.1.2 and 2.1.3.
The micro-structural variations encountered in different limestones are illustrated in Figures 2.1.8 to 2.1.16.
Table 2.1.2. Impurity Based Classification of Limestones
Table 2.1.3. Classification Based on Crystal Size
It may be relevant to recapitulate here a few of the classical approaches made in the past to differ-entiate the limestones. A classification often referred to by the geologists is by Pettijohm (1957) which distinguishes “calcirudite,” “calcarenite,” and “calcilulite” based on gravel, sand, and silt-sized carbonate particles respectively.The classifications based on calcite and dolomite content as proposed by Carozzi (1960), based on Ca/Mg ratio as proposed by Chilingar (1957), or based on calcite, dolomite, and clay content as reported by Bissell and Chilingar (1967) are often useful for specific purposes. Fitting a given limestone resource to one or more classifications ultimately helps in understanding its behavioral pattern by comparison with parallels.
Geological Characteristics of Limestone Deposits from Exploitation Perspective
While the classification of limestones as explained in the previous section is important for a broad understanding of this important natural resource, the selection of a limestone deposit for explor-ation and use demands looking at each limestone deposit from different perspectives as the user is ultimately concerned with specific features of its suitability for an end use.
Based on the geological complexities, age, basin configuration, amenability to prospecting, and proving operations as well as minability and post-mining treatment, the limestone deposits can be sub-divided into different types as reported earlier by Ghosh and Chatterjee (1979). All limestone deposits in India, for instance, have been brought under three categories: simple, complex, and intricate.
The simple deposits are large, continuous, bedded, horizontal-to-low dipping deposits that are geologically undisturbed with abundant outcrops and are uniform in quality. These kinds of deposits require the least intensity of prospecting for proving of reserves. The complex deposits include the ones that are moderately or steeply dipping, gently folded, consistent, medium to large deposits; deposits with frequent intercalations or variable thickness, coral limestone deposits, limeshell layers or sea sand deposits that are highly variable in shape and thickness;
Figure 2.1.8. Very coarse grained (>1.5 mm) calcite with euhedral quartz at calcite boundary.
Figure 2.1.9. Microcrystalline limestone with subhedral to euhedral calcite (10 to 40 micron).
Figure 2.1.10. Microcrystalline limestone with subhedral calcite (<10 microns) show-ing inter-crystalline porosity. Finer quartz grains embedded at the calcite boundary.
Figure 2.1.11. Microcrystalline limestone with calcites (40 to 80 microns) embedded in the fine anhedral matrix.
Figure 2.1.12. Matrix at places shows flaky morphology.
Figure 2.1.13. Microcrystalline limestone with subhedral to anhedral calcites, (10 to 30 microns) showing inter-crystalline porosity.
Figure 2.1.14. Calcite grains ( <10 microns) with calcareous cementing materials, high porosity.
Figure 2.1.15. Rhombic calcites in calcareous cementing materials. Overgrowth (authi-genic) on the rhombic calcites, with porous cementing material.
Figure 2.1.16. Coarse euhedral calcite embedded in fine grained matrix.
lenticular, folded, or structurally disturbed deposits; or deposits in hilly terrains with a little struc-tural disturbance. The intricate deposits include highly complicated, highly folded, faulted and dislocated, and highly lenticular deposits or those deposits that are interbedded with clay or shale, or show extreme variations in form, thickness, and assay values. Many Precambrian limestone deposits belong to this category. A high degree of intensity of prospecting is required in assessing deposits of this kind, particularly for mechanized mining.
It should be borne in mind that the adequacy of exploration and proving of a limestone deposit for setting up a cement plant ultimately has to be judged by the optimum expenditure incurred in the exploration program in order to avoid risky ventures of developing inadequately explored deposits or unprofitable propositions of setting up a cement plant on over explored deposits.
The most judicious course to assess the adequacy of an exploration program of a limestone deposit is to adopt the minimum risk factor method which is based on the reliability of reserves estimated in terms of quality and quantity.
The tentative target of an exploration program for a given deposit is to ensure the estimate of its reserves of appropriate reliability as indicated in Table 2.1.4. Arriving at the delineation of reserves of the reliability level of the “proved” category is considered a prerequisite for the preparation of a detailed project report and final investment decisions.
Table 2.1.4. Illustrative Targets of Limestone Reserves for Cement Plants at the Resource Proving Stage
Advances in exploration and exploitation software
The developments in planning of exploration as well as in methods of compilation, processing, and interpretation of data are phenomenal due to massive advances in computing power and software design. These develop-ments have influenced the process of raw materials exploration and planning for cement manufac-ture. McCaffrey (1999) postulates that a relational geological database allows storage and query of all geo-chemical quality parameters in addition to physical attributes such as mining thickness. Complex 3D geological and quality models can be developed to assist in deposit visualization. Statistical and multiple element compositing tools can be used to investigate the quality distribu-tion. It is possible to take advantage of available software for 3D visualization of the different stages of exploration, raw mix proportioning models, etc. Austroplan’s Geo-byte, Maptek’s Vulcan 3D, and Surpac 2000 of Surpac Software International are examples of a few popular software programs used around the world.
TECHNOLOGICAL ASSESSMENT OF LIMESTONES
Broad Specification of Cement-Grade Run-of-Mine Limestone
It has already been mentioned that the chemical composition of limestone for cement manufac-ture should be such that it gives in combination with an argillaceous component and, under unavoidable circumstance with corrective materials, a raw mix of appropriate composition. Hence, no rigorous specification can be drawn up for the cement-grade limestone. Notwithstanding this, according to Ghosh and Chatterjee (1979), an attempt was made several years back to specify the cement-grade limestone in a way that does not exceed the limiting oxide values given in Table 2.1.5. However, this is not a substitute for the comprehensive test to which every limestone under consideration for industrial use should be subjected in combination with supplementary and corrective materials.
Table 2.1.5. Broad Chemical Specification of Cement-Grade Run-of-Mine Limestone
* LSF = Lime saturation factor, MS = Silica modulus.
Additional Desirable Technological Characteristics
For use in cement making the most important technological requirement from limestone is its proper dissociation to release reactive lime in the kiln system. The salient aspects of the decarbona-tion reaction of the carbonate minerals had been dealt with by Chatterjee (1983). It may suffice to mention here that in the process of raw materials selection, one may have to be cognizant of the effects of two major factors of limestone on its calcination behavior viz. 1) the associated minerals and impurities and 2) textural features including grain size of calcite.
Associated Minerals and Impurities
The effect of the associated minerals and impurities is basically to lower the decomposition temperature as the dissociation pressure of calcite is increased by oxides like SiO2,Al2O3, and Fe2O3.It has also been observed that the lime crystals obtained from the dissociation of dolomite are 1.5 to 2.0 times smaller than those obtained from calcite. As a result, the reaction surface is larger when lime crystals are available from dolomites, resulting in faster reactions.
It is known that MgO in limestone can be present in various forms: magnesium silicates, dolo-mites, magnesite, ankerite, brucite, etc. The growth of the periclase crystals in a clinker is obviously related to the temperature at which different magnesium minerals release magnesia on decomposi-tion (Table 2.1.6). The lower this temperature, the greater the chances of crystal growth, particu-larly in situations where magnesia does not react with other oxides to form any transitional phase.
Table 2.1.6. Decomposition Temperature of Magnesian Minerals (Volkinskii, 1972)
The presence of coarse grains of quartz or siliceous veins is undesirable as it may affect the grind-ability and reactivity of the limestone as explained later in this chapter. The presence of sulfur as sulfate is undesirable as it is difficult to decompose.
The physical properties of limestones show wide variation, apparently related to the prior history of rock formation and the resultant minerographic and textural features.
The color of limestones reflects the level and nature of associated minerals and impurities. High purity calcite limestones are white in color and other shades in the less pure varieties are essentially caused by carbonaceous and ferruginous materials.
Depending on the mineralogy and texture, the porosity of the limestones, according to Schwarzkopf (1994), varies from almost zero to 30%, while the upper value for chalks and marls often goes up to 40% and 50% respectively. For predominantly occurring compact limestones, it is in the range of 2% to 5%. Similarly, dolomites normally show relatively low porosity level (1% to 10%).
The water absorption in limestones follows the pattern of pores and level of porosity. While it may be negligible in dense varieties, it may go up to 20% in porous chalky varieties.
The Moh’s hardness of most limestones lie in the range of 2 to 4. The apparent density of lime-stones on drying at 110°C ranges from 1.5 to 2.6 g/cm3.
In addition to the above basic physical properties, a few characteristics like crushability and grind-ability as well as the physical behavior of crushed and screened limestones are important for the process of cement manufacture.
Crushability is a generalized parameter expressing the energy consumed in crushing a rock under a dynamic load. The property takes into account the elastic, plastic, and strength behavior of a given sample and is measured as a work index. Similarly, grindability is determined with the help of a specially designed ball mill according to standard practice and is expressed as Bond’s index. For details, one may refer to the article by Bond. The Bond grindability index for commonly occurring compact limestones lies in the range of 4 to 13 kWh/short ton.
Sometimes, for crusher selection purposes, the compressive strength of limestones, determined on sized cylindrical samples, is used. The compressive strength of limestones, like other properties, also shows wide variation from 10 to 200 MPa.
The bulk density largely depends on the apparent density of limestones, their particle size distribu-tion, and particle shape. In general, the reported range of variation of bulk density of limestones is 1400 to 2700 kg/m3, although typical values fall in the range of 1400 to 1600 kg/m3.
The external angle of repose for screened limestone is generally 35° – 45° but is affected by size distribution, fines level, and moisture content.
The abrasion resistance characteristic of crushed stone is often called for in the process of techno-logical assessment.
A broad framework of sequential steps and scope of technological assessment of limestones has been given by Ghosh and Chatterjee (1979). Some of the desirable characteristics are 1) the average size of calcite crystals (not particle size) should preferably be less than 0.25 mm, 2) presence of coarse grains of quartz and siliceous veins is undesirable, 3) limestone should have low moisture content (<5%), and 4) limestone should have low compressive strength (below 100 MPa).
For a general appreciation of the extent of variability of physical properties encountered in lime-stones and difficulties of correlating the physical properties with their chemical characteristics, a set of data for some limestone samples of different ages is furnished in Table 2.1.7.
The limestones shown here are all high in silica content. Reasons for the major differences in grindability probably include differences in the mineralogy of the siliceous contaminant.
Table 2.1.7. Physical and Chemical Characteristics of Some Limestones of Different Ages
Dependence of Decarbonation Behavior on Limestone Characteristics
It has already been indicated that the reactivity of a limestone is dependent on the crystal size of calcite and other associated minerals, and more particularly of quartz. This is evident from Table 2.1.8 in which the characteristics of two different limestones and their decarbonation behavior in air at 800°C have been shown. Limestones A and B are comparable in their chemical composition, while they significantly differ in their calcite crystal size. This difference is reflected in the slower decarbonation of limestone B, when both limestones were ground to the same level of 15% residue on 90 µm to eliminate the effect of differential particle size on the decarbonation reaction.
On the whole, it has been observed that the finer the crystal size or the finer the particle size and the more impure a limestone, the lower the activation energy of dissociation, which may vary in a wide range from 30 to 60 kcal/mole.
There is also a tentative inverse correlation between the activation energy and the peak tempera-ture of decomposition, as well as the activation energy and the rate of decomposition of a given limestone. It has also been experimentally established that the lower activation energy of limestone dissociation has a strong positive effect on the activation energy of clinker phase formation with higher lime combinability, as reported by Sinha and others (1980).
Table 2.1.8. Effect of Calcite Crystal Size on the Decarbonation Rate of Limestones
The expression “argillaceous materials” refers to all fine-grained natural earthy substances that are alternatively known as “clay.” It includes shales and argillites. Chemically, these materials are essentially hydrous aluminum silicates, with magne-sium or iron substitution wholly or in part for the aluminum in certain minerals, and with alkalies or alkaline earths also present as essential constituents in a few others. A shale deposit as a source of argillaceous materials in cement manufacturing is shown in Figure 2.1.17.
Figure 2.1.17. Shale pit as a source of argillaceous additives for raw mix in cement manufacturing.
Some argillaceous materials are composed of a single clay mineral but in many there is a mixture of them. In addition to the clay minerals, many argillaceous materials contain non-clay minerals like quartz, calcite, feldspar, sulphidic species, etc. Many others may contain organic substances as well as water-soluble salts. Some clay materials may contain phases that are amorphous.
Structurally, these clay minerals are part of the larger family of phyllosilicates and are characterized by interlinked tetrahedral and octahedral sheets. The structural configuration of clay minerals has been dealt with in fair detail by Grim (1962), Brindley and Brown (1989), and Velde (1992), a summary of which is presented below.
The structural variations among the clay minerals can be understood by considering various phys-ical combinations of tetrahedral and octahedral sheets and the electrostatic effect chemical substi-tution has on the structural units. The tetrahedral sheets are composed primarily of Si4+ and oxygen, but minor amounts of Al3+ or Fe3+ may substitute for Si4+.The substitution of Mg2+ for Si4+ leaves the tetrahedral sheet negatively charged. The cations of the octahedral sheet are composed primarily of Al3+,Fe3+,Mg2+, and Fe2+,but all other transition elements, except Sc, may be included. The anions of the octahedral sheet are O2-,OH1- and F1-.The smallest unit of the octahedral sheet contains three octahedra having an ideal net charge of negative six, i.e. three O2-. If the negative charge is balanced by two trivalent cations, the layer is referred to as dioctahedral layer; if balanced by three bivalent cations, the layer is referred to as a trioctahedral layer. Substitution of bivalent cations for trivalent cations, and univalent cations (Li1+) for bivalent cations or unfilled octahedral sites, leaves the octahedral layer a net negative charge. The tetrahe-dral apical oxygen is shared with the octahedral layer to join the two types of layers.
The least complicated clay minerals are the 1:1 clay minerals composed of one tetrahedral (T) layer and one octahedral (O) layer. These 1:1 clay minerals are also referred to as TO minerals. The TO package has a basal spacing (nominal thickness) of 0.7 nm (7Å), and the minerals are commonly referred to as 7Å minerals. Clay minerals that are composed of two tetrahedral layers and one octa-hedral layer are referred to as 2:1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2:1 structure has a basal spacing (nominal thickness) of 1.0 nm (10Å). The multitude of variation in clay minerals is caused by substitution in the octahedral and tetrahedral layers, resulting in charge deficits. The manner in which the charge deficit is balanced leads to many of the useful and unique properties of clay minerals.
As a result of such diversity of constitution of argillaceous materials, their technical assessment for specific applications is rather complex. Broadly speaking, the property controlling factors in argillaceous materials are the following:
1. Clay minerals
2. Non-clay minerals
3. Organic substances
4. Exchangeable ions and soluble salts
5. Particle characteristics including shape, size, and orientation
6. Structural assembling formed by 1:1 or 2:1 linkage of tetrahedral and octahedral sheets as well as neutralization of excess layer charge by various interlayer materials
Some relevant data regarding the structure and properties of clay minerals are furnished in Table 2.1.9.
Table 2.1.9. Structure and Properties of Commonly Encountered Clay Minerals
Genesis and Occurrence of Clays
According to Worrall (1968), there are mainly two types of clays by genesis – “residual” and “sedi-mentary.” Residual clays are those which have not been transported by natural agencies and are found side by side with the weathered igneous rocks from which they are formed. Sedimentary clays, in contrast, are those which have been removed from their place of origin by natural agencies.
Residual clays can be obtained in a comparatively pure state, while sedimentary clays are rarely obtained pure as these materials get contaminated during transportation. The transportation factor also makes the particle of residual clays smaller and irregular.
Grim (1962) had explained that argillaceous alteration products are often found around metallif-erous deposits due to hydrothermal action. In such residual clays there is zonal arrangement of the clay minerals around the source of alteration with mica and kaolinite being close to the sources and chlorite and montmorillonite being more distant.
All types of clay minerals, with the possible exception of attapulguite, have been identified in vari-ous types of soils. The character of the clay mineral found in a given soil depends on the nature of the parent material and also on the climate, topology, vegetation, duration of the alteration process, course of transportation, etc.
Illite and chlorite appear to be the dominant clay mineral constituents in sediments preferably accumulating in seas. Montmorillonite is much less abundant in sediments older than the Mesozoic as compared to the younger sediments. It is also seen that kaolinite is less abundant in very ancient sediments than in those deposited after about the Devonian period. Attapulgite and sepiolite have not been reported in sediments older than the Tertiary age.
Rock Association of Clay Minerals
Argillaceous rocks generally show the presence of multiple clay minerals. Shales generally contain illite and chlorite. Montmorillonite is a common constituent of many shales of Mesozoic and younger age. Kaolinite is a common mineral of some shales but usually in minor proportions.
Slates are also composed of illite and chlorites but with higher degree of crystallinity.
The carbonate rocks show the association of a wide range of clay minerals, with illite and chlorites more predominant than kaolinite and montmorillorite.
Physical Properties of Clays
Properties like plasticity, specific surface, water requirement, suspension stability, coagulation of clay particles, swelling, etc. are of considerable importance in raw meal preparation, particularly in the wet and semi-dry processes. Clays with at least 7 to 15 plasticity index and having 10% parti-cles of +0.2 mm size and cumulative 20% particles of 0.08 mm size with cation exchange capacity of 11 to 12 meq/100g are reportedly considered suitable as cement-making variety in situations where clay has a process role other than providing the stoichiometric requirements.
Experience has established that aluminosilicate, when present in raw mixes from the argillaceous component, turns out to be significantly more reactive than fine-ground silica. It has been gener-ally observed that the water vapor and hydroxyl group released from the argillaceous materials shows some catalytic effect on the dissociation of calcium carbonate and the subsequent solid-state oxidic reactions.
The trends of decomposition of a few clay minerals are shown in Table 2.1.10. It is evident that the temperature ranges over which different clay minerals release water and hydroxyl ions as well as transform into an amorphous state before stable phase formation are different. It is obvious, there-fore, that their reactivity with lime and other oxides would not be identical in different situations. However, still there is no unanimity in the views of different investigators regarding the order of reactivity of different clay minerals. As reported by Volkonskii and others (1973), in accordance with one investigation, the decreasing order of reactivity was as follows:
Another investigation reports the following decreasing order of reactivity:
Notwithstanding such differences, it is evident that micaceous minerals have a tendency to show relatively low reactivity.
It may also be borne in mind that the argillaceous materials have a high probability of containing titania, alkalies, sulphides, sulfates, and phosphates. The reactivity may be strongly influenced by the presence of the above constituents.
Table 2.1.10. Decomposition Behavior of Some Clay Minerals (Volkonskii and Others, 1973)
When the primary components of a raw mix do not jointly permit achieving the desired range of modulii values, a third or even a fourth component is added, known as corrective materials. It has been a practice to recognize that a material with more than 70% silica, 40% iron oxide, or 30%alumina can be termed as siliceous, ferruginous, or aluminous corrective materials.
Sand, sandstone, or quartzite acts as the source of siliceous corrective material. Other conditions being equal, the grain size and specific surface of silica in free form (and particularly of the least reactive forms like quartz and chalcedony), determine the rate of reaction in a kiln feed. According to Makashev (1976), the reactivity of different types of silica, free or combined, increases in the following order:
Quartz < chalcedony < opal < -cristobalite and -tridymite < silica from feldspars < silica from mica and amphibole < silica from clay minerals < silica from glassy slags.
On the whole, silica in amorphous state or derived from silicates or hydrosilicates is preferable to silica in other forms.
The correction of iron oxide in a raw mix is generally done with iron ore, which may either be magnetitic or haematitic. A haematite with colloidal texture or martitized magnetite is quite reac-tive with lime and alumina. Limonite (FeO·OH·H2O), often associated with laterite, is more reac-tive than the ferric oxide hydrate phases like goethite and lepidocrocite. It has been observed by Makashev (1976) that the reactivity of raw mat`erial is often favorably influenced by the presence of iron oxide in the ferrous state, the appearance of which is obviously dependent on the parent mineral. For example, chlorite and glauconite may release FeO below 500°C, while geothite and lepidocrocite yields Fe2O3 at about 300°C. The iron-bearing minerals thus play an important role
in shaping the reactivity of the kiln feed, although for alite formation the mineralogy is of little import since most of the iron has entered the clinker melt by this point.
The correction of alumina is done with the help of bauxite or aluminous laterite. Mineralogically these rocks contain such aluminous minerals as gibbsite (Al2O3·H2O), boehmite (Al2O3·H2O), and diaspore (Al2O3·H2O). Generally these phases show low crystallinity and high energy in the green state, dehydrate at 300°C – 500°C, and give rise to different forms of alumina that ultimately define their reactivity at higher temperatures.
ROLE OF RAW MATERIALS IN THE PROCESS AND HARDWARE SELECTION
In the cement production process, since there are quite a few unit operations employing different hardware systems, it is important to examine the role of raw materials in this perspective.
For example, a full understanding of the characteristics of the limestone to be used in a given situ-ation will help in selecting the right type of crusher. Some key requirements include:
• Crushing strength (kg/cm2) or crushability index
• Hardness value
• Abrasion value
• Identity of impurities
• Moisture content (minimum and maximum)
While the first three parameters are important for selection of type and materials of construction, the last two factors may strongly influence the process. It is understood that the handling of wet sticky materials may affect the throughput and increase the maintenance cost. Similarly, most materials to be crushed contain inclusions, which have totally different behavioral patterns from one another when introduced into a crusher or a screening system. The characteristics of these inclusions play an important part in the selection of the right equipment.
It may be relevant to mention here that the jaw crushers and gyratory crushers are designed to handle hard or abrasive materials. A gyratory crusher is generally chosen for the large feed size and capacity required in modern quarries, whereas jaw crushers are used in relatively small primary applications. Of all the types of crushers available for the primary and secondary crushing of lime-stone and clay, the double roll crusher is certainly the most versatile. It can handle dry dusty mate-rial to extremely wet and sticky material. According to McCater (1996), this type of crusher can handle limestones with a compressive strength of up to 140 MPa. The impact crushers are often preferred for their high reduction ratio. However, there are, limitations with its ability to handle sticky and abrasive materials.
The above examples show that the crusher selection strongly depends on full characterization of raw materials. Similarly, the selection of a grinding system also involves proper understanding of raw material characteristics, and particularly its
• Grindability index
• Moisture content as fed to the system
• Abrasiveness, often inferred from free silica content.
The ultimate objective is to achieve the targeted particle size distribution, average particle size, and specific surface with least consumption of energy and other operating costs. It has been reiterated by Schnatz (1999) that ball mills are cost-effective only when a high degree of wear is expected due to,say, high quartz content. High pressure grinding rolls can be profitably used for relatively dry raw materials. Roller mills are generally preferred for raw grinding due to their low energy consumption and the option of simultaneous drying.
Coming to the pyroprocessing, it may be mentioned that different raw mixes with more or less the same chemical composition and equal fineness may have greatly differing burnabilities. The reason for the differences lies in the variation off the mineral composition of the raw mix constituents, their crystal size, and particle size. According to the F. L. Smidth training modules (1997), world-wide experience has shown that poor burnability is primarily caused by the presence of coarse grains of calcite (+125 µm) and quartz (+45 µm). Deleterious effects of the presence of coarse grains of dolomite (+125 µm), feldspar (+63 µm), and shale (+50 µm) have also been reported.
The volatile matters, and more particularly alkalies and chlorides, present in the raw materials have a very strong bearing on the process of clinkerization in the present-day preheater-precalciner rotary kilns. The circulation of these volatiles in the system without any bypass imposes certain upper input limits for these constituents in raw mixes. On a loss-free basis, these limits are normally considered as follows:
The study of alkalies, and sulfur- and chloride-bearing minerals in the raw materials becomes essential in this context.
Cement production is dependent on a wide spectrum of sedimentary and metamorphic rocks, limestones being the most critical in terms of quality and quantity.
A limestone resource required for setting up a cement plant first needs to be examined from the point of view of its prospectivity. The geological history defines the complexity of a deposit and an appropriate exploration program is necessary for proving the deposit and arriving at a reliable esti-mate of the reserve in terms of quality and quantity. A rational approach is to classify the deposits in terms of their geological complexity and the reserves in terms of their reliability so that the quantum of exploration could be decided for a given deposit to arrive at “proved,” “probable,” and “possible” reserves required for setting up a cement plant.
After proving a limestone resource, it is essential to undertake its thorough compositional assess-ment. A limestone with a minimum of 44% to 45% CaO and maximum 3.0% to 3.5% MgO, 0.6%R2O, 0.6% to 0.8% SO3, and 0.015% to 0.05% Cl is regarded as a cement grade limestone, provided its SiO2,Al2O3, and Fe2O3 contents satisfy the ultimate modulii values of raw mixes. The compositional ranges of aluminosilicate materials cannot be defined as rigidly as they have to match the principal carbonate component. In general, an argillaceous component with more than 3% R2O and/or 1% SO3 may be considered, prima facie, unsuitable. For most of the minor constituents, 0.5% is regarded as a safe limit. A special examination is called for if some of the constituents exceed this limit.
The comminution and thermal behaviors of raw materials primarily depend on their mineral phases and microstructural features. The crystal size of calcite and the associated mineral assem-blage define the dissociation characteristics of limestone in a very effective manner. The mineral-ogy of argillaceous materials is quite complex due to their layered structure, frequent ionic substitutions in the layers, unbalanced charges, and inclusion of interlayer materials. Because of these complications, the choice of clay can be guided more by Si: (A,F) ratio, volatile contents, fusibility, granulometry, cation exchange capacity (CEC), etc. In the ultimate evaluation it is important to note that the concurrence of carbonate dissociation and thermal decomposition of aluminosilicate components is considered a basic necessity for high reactivity and proper burning.
The above concepts are equally applicable for all other siliceous, ferruginous, or aluminous correc-tive materials, when their use becomes unavoidable due to stoichiometric needs. Different forms of silica, aluminum hydrates, ferri/ferous minerals, etc., present in the above corrective materials influence the burning process quite significantly and hence demand careful evaluation in terms of overall impact on the raw mix burnability.
In cement production, size reduction is an important material preparation step. The amenability of limestones to size reduction processes is apparently controlled, inter alia, by the free and fixed silica content and the crystal size variations of calcite and quartz phases. The hardware selection for comminution is also dependent on the above mineralogical and microstructural features.
Since the ultimate burning process is involved with the prior size-reduction steps, there has been a progressive evolution of the limiting particle size for different mineral forms, supported by experi-mental findings. In raw materials selection the attainability of such particle size distribution patterns requires specific attention.
All in all, it should be realized that the production of cement is to a large extent dependent on natural raw materials, and more specifically on limestones.
Since the scale of cement operation is large, the location of a plant for its life is decided by the occurrence of a limestone deposit. The technological suitability and consistent supply of raw mate-rials are, therefore, of paramount significance. All efforts for proper selection of raw materials are absolutely essential.
The author is thankful to his colleagues at the Company’s Research and Consultancy Directorate
and more particularly to Mr. S. A. Khadilkar and Mr. P. G. Lele for providing test data on raw
materials from the Directorate’s database and also, to Dr. S. K. Jatty for helping with the scanning
electron micrographs of limestone samples.
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