Innovative, energy-efficient manufacture of cement by means of controlled mineral formation – Part 1
Summary: The mechanism of the formation of alite and belite with extremely coarse raw mixes may proceed kinetically more energy-efficient according to investigations using ESMA, DTA and XRD via three models. According to model 1 the reaction takes place due to the crystallization of the melt rich in SiO2 diluted with easily melting phases. Model 2 starts from centres supersaturated with CaO, which are used as crystal nuclei and crystallization centres for the formation of alite and belite and that are originated at the reaction interface between the melt rich in SiO2 and the aggregates of the solid phase of CaO. Model 3 refers to the absorption of the melt rich in SiO2 by the conventional melt. The alite formation is completed via the conventional melt.
Cement making is a very energy-intensive process. The overall energy costs result from the consumption of electric energy and fuel energy of approximately equal parts. In spite of the achieved reduction in heat consumption down to approximately 3000 to 3200 kJ needed to burn 1 kg of clinker, the theoretical heat consumption with about 1600 kJ is still significantly lower. However, the efficiency of approx. 50% thus achieved, which is extraordinarily favourable for technical burning processes, still involves a lot of reserves to further save fuel. The total consumption of electric...
Cement making is a very energy-intensive process. The overall energy costs result from the consumption of electric energy and fuel energy of approximately equal parts. In spite of the achieved reduction in heat consumption down to approximately 3000 to 3200 kJ needed to burn 1 kg of clinker, the theoretical heat consumption with about 1600 kJ is still significantly lower. However, the efficiency of approx. 50% thus achieved, which is extraordinarily favourable for technical burning processes, still involves a lot of reserves to further save fuel. The total consumption of electric energy, i.e. 96 to 111 kWh/t of cement, is also very high. Primarily, electric energy is needed for raw material preparation (about 33–37%), for clinker burning and cooling (approx. 20–24%) and for finish grinding (approx. 36–40%). The high energy consumption during raw material grinding is mainly due to the very high fineness. However, today the raw material preparation and the burning process of the conventional modern dry process have been optimized so far that known process engineering measures would not lead to any further considerable reductions.
To determine the optimization potentials in grinding, a kinetically controlled clinker formation with an enormous increase in reactivity in the liquid phase due to an optimization of the grain class conditions and the grain size range of the raw materials was investigated. This is due to the possibility that a higher raw meal reactivity may also be achieved by means of extremely coarse grinding. This is due to the thus released controlled formation of the easily melting minerals CAS2, CS, C3S2 and C2AS before entering the burning zone, which release, in addition to the conventional Al2O3 and Fe2O3, also the melt rich in SiO2. Thus, the amount of melt in the burning zone is enormously increased temporarily, which decisively accelerates the incorporation of CaO.
A comprehensive analysis [1] covering the properties of the melts suitable for clinker making, in particular covering their composition and the minimum temperature required for their formation, was carried out for the melt equilibriums in the corresponding zones rich in SiO2 and Al2O3 of the ternary system CaO-SiO2-Al2O3. It follows that some melts rich in SiO2 could be formed in raw mixes with increasing alkalinity and melting temperature in much larger amounts than conventional melts, based on four consecutive eutectics or forked points, respectively, in the SiO2-CaO corner (Fig. 1 and 2). Because of the continuous dissolution of CaO, the melts rich in SiO2, which are formed due to the extremely coarse grinding of the raw mix and the resulting controlled mineral formation, are unstable and exist only temporarily.
The investigations and calculations carried out [1–7] show that the liquid content in the burning zone for white cement
clinker can be temporarily increased from about 11–14% to 21–39% (Fig. 2) and for grey cement clinker from about 16–21% to 30–49% due to the additionally, newly-formed eutectics or quintuple points, respectively, rich in SiO2 and the adjacent ones with conventional Fe2O3 and/or rich in Al2O3.
The rate of mineral formation due to the melt is about 10000 to 100000 times higher compared to solid-state reactions. The conclusion is that a delay of the solid-state reactions is often due to the degree of the reactions in the melt. It follows that the formation of clinker can be accelerated efficiently by maintaining as long as possible the clinker melts rich in SiO2 but which are unstable and, consequently, exist only temporarily, and which are due to the controlled mineral formation when optimizing the C, S, A and F conditions according to raw meal fractions.
The maximum prolongation of the existence of melts rich in SiO2 can be achieved by extremely coarse grinding and the optimization of the particle size range of raw meals when forming all four eutectics rich in SiO2 (Fig. 1 and 2). This is only possible if the portion of the calcite particles in the raw meal, which are capable of solid-state reactions, are attuned to the eutectic in the CS-CAS2-S system rich in SiO2 with a CaO content of approx. 23.7 wt.-% and a melting temperature of 1170°C. Three further eutectic melts rich in SiO2 with increasing alkalinity can be formed due to heating up the raw mix to more than 1170°C and the accompanying dissolution of the coarse lime particles in the melt. First eutectic melts are formed in the range of the CS-C2AS-CAS2 system at 1265°C with a CaO content of approx. 38.7 wt.-%, then follow melts of the CS-C3S2-C2AS system at 1310–1318°C with a CaO content of approx. 47.2 wt.-% and finally the C2AS-C3S2-b-C2S system at 1335°C with a CaO content of approx. 48.2 wt.-%.
An extremely coarse-ground raw mix has a higher reactivity than a conventional, fine mix. Therefore, it should be expected that the major portion of coarse-grained lime with particle sizes of up to 3 mm will be split up first into fine aggregates due to the melt rich in SiO2 with the highest wetting energy before it is dissolved in the melt. The velocity and the completion of these consecutive processes limited in time depend on the structure of the limestone and on the properties and the time of existence, above all, of the melt rich in SiO2. The porosity and the spatial distribution of the minor minerals in the calcite rock are relevant characteristics of the structure. The properties and the time of existence of the melt depend on the fineness of the raw materials, on the burning conditions and on the content in the raw mixes, above all, of the alkalis and MgO, which form the more easily melting silicates such as akermanite (Ca2MgSi2O7), alumoakermanite (Ca,Na)2(Al, Mg,Fe2+)(Si2O7), bredigite (C7Mg[SiO4]4) and brownmillerite (CaFe1,2Mg0,4Si0,4O5 (Si,Mg). The calculations showed that the existence of the melts rich in SiO2 as long as possible requires an extremely coarse grinding of the raw mix with a residue of about 80 wt.-% on the 80 µm sieve, at least of the CaO carrier.
This means that the longest time of existence of the melt, which has to be achieved at least for the lime carrier with a maximum of very coarse grinding, will provide the better prerequisites for an optimally controlled mineral formation and the resulting accelerated and complete formation of clinker. The quicker the coarse-grained lime dissolves, the more time the formed fine aggregates have to dissolve as well, first in the clinker liquid rich in SiO2 and then, from 1300–1350°C, also in the clinker melt rich in Fe2O3 and/or Al2O3.
Three versions can be assumed for the mechanism of the alite formation:
Version 1: If you take as a starting point the fact that the formation of belite and particularly alite takes place due to the crystallization of the melt rich in SiO2 with an existing alkalinity between alite and belite, the burning temperature should be increased at least to 1800–2000°C according to the crystallization field of C2S existing in the C2S-C3A-C3A area. The first crystals precipitating are a-C2S. Then the crystals of a-C2S and C3S will simultaneously crystallize via the connecting boundary curve 4-3 [w1] when cooling down. When reaching the composition and the temperature of point 3, the composition of the solid phase is characterized by a point K, which can be defined as a melt rich in SiO2 with a continuously changing content of CaO and SiO2 as the continuation of a straight line up to the edge of C2S-C3S between point 3 and the point of the chemical composition. However, the burning temperatures of 1800–2000°C required for the described mechanism of clinker formation cannot be implemented for economic and technical reasons. If the melt rich in SiO2 is diluted by low-melting phases, the temperature of the start of the formation of the melt will be lowered.
Version 2: The coarse CaO grains could only be dispersed effectively and finely already at low temperatures starting with about 1200°C in the melt, if sufficient melt is available and if its wetting energy is sufficiently high. The first requirement will be met due to the formation of the melt rich in SiO2, which may be formed with an amount of 2 to 4 times larger compared to the melt rich in Fe2O3 and/orAl2O3. The wetting energy of the melt at the oxidic solid phase is increased with increasing electronegativity of the cations in the melt. Since the electronegativity of Al2O3 compared to SiO2 will increase from 1.5 to 1.8, the melt rich in SiO2 will wet the CaO grains more than the melts rich in Al2O3 and Fe2O3. Thus, it should be expected that the coarser CaO grains (larger than 3 mm) would be split up more finely and quickly by the melt rich in SiO2 with a higher wetting energy than by the conventional melt rich in Al2O3 and Fe2O3. For these reasons, a clearly larger contact reaction area and, consequently, a higher speed of the CaO dissolution, can be achieved in shorter time in the melt rich in SiO2 and later also in the melt rich in Al2O3 and Fe2O3. In particular in the melt rich in SiO2, numerous centres supersaturated with CaO may be formed at the reaction interface to the dispersed CaO phase serving as crystal nuclei and crystallization centres for the topochemical formation of alite and belite. Thus, the conclusion can be drawn that the melt rich in SiO2 may also topochemically release and accelerate the formation not only of belite but also, above all, of alite during clinker burning without being heated up to 2000°C. With an increase of the CaO content to more than approx. 48.2 wt.-%, the melting temperature is increased rapidly as can be seen at the forked point rich in SiO2 with a melting temperature of 1335°C in the triangle C2AS-C3S2-a-C2S of the constitution diagram C-S-A. This could cause the accelerated crystallization of belite and alite at the boundary to the CaO grains during the dissolution of CaO in the melt rich in SiO2.
Version 3: After splitting up the coarse lime particles of up to 3 mm to become fine aggregates, with the following formation of alite due to crystallization from the centres supersaturated with CaO of the melt rich in SiO2, alite may also be formed by the absorption of the melt rich in SiO2 in the directly adjacent conventional melt rich in Fe2O3 and/or Al2O3 with simulta-neous dissolution of CaO and final crystallization of alite.
However, the supersaturation of the melt rich in SiO2 with CaO until alite is formed is limited. Therefore, alite is formed via the melt rich in SiO2 predominantly due to the dissolution of the highly basic melt rich in SiO2 in the adjacent conventional, thermodynamically stable melt rich in Fe2O3 and/or Al2O3 with simultaneous dissolution of CaO thus limiting the topochemically formed belite.
The amount of the conventional melt rich in Fe2O3 and/or Al2O3 is determined at the beginning of the contact by the difference of the chemical stock between the adjacent, existing melt rich in Fe2O3 and/or Al2O3 and the ascending forked point in the triangle a-C2S-C3S-C3A adjacent to the field C3S as well as by the melting temperature of the fluid phase. A further absorption of the melt rich in SiO2 in the conventional melt rich in Fe2O3 and/or Al2O3 will depend on the rate of the alite crystallization from the melt. When the kiln charge is heated up from approx. 1300°C to the finishing burn temperature, the final formation of alite should be accelerated according to the mechanism described.
In the presence of the metastable melt rich in SiO2, belite is mainly formed due to the crystallization directly from the melts rich SiO2 as well as in Al2O3 and Fe2O3, and only marginally due to the solid-state reactions with subsequent over crystallization via the conventional melt rich in Al2O3 and Fe2O3. Thus, the slowest stage of the mineral formation, i.e. the dissolution of the solid phase at the surface of the melt supersaturated with Ca+2 ions, can be eliminated. In this way conditions are created where the formation of belite is almost impossible, which is required for the alite formation, due to the solid-state reactions according the conventional technology with subsequent dissolution in the conventional melt rich in Fe2O3 and/or Al2O3. The portion of the solid phase consisting of the topochemically formed belite according to the conventional technology amounts to approx. 60% at the beginning of the burning zone, and to approx. 80% of the amount of clinker with free CaO. Since the belite phase is almost completely formed from an extremely coarse raw mix directly from the melt rich in SiO2 and the conventional one, the portion of the solid phase of the clinker to be dissolved in the melt rich in Al2O3 and Fe2O3 is decreased from approx. 80% with the conventional, fine raw mix down to 20% with an extremely coarse one.
This strong decrease of the portion of the solid phase to be dissolved in the melt, with a simultaneously considerably increased portion of the clinker melt, theoretically up to approx. 40%, results in a proportional decrease, on average, of the degree of supersaturation of the melt surface towards Ca2+ ions. This accelerates the dissolution of the remaining free CaO in the clinker liquid. Thus, the reactivity of the coarsely ground raw mix is improved. At the end of burning with the completion of the dissolution of CaO and of the formation of alite, the melt rich in SiO2 is gradually transformed into a melt rich in Al2O3 and Fe2O3.
A further increase in the chemical activity of the raw mix is also achieved by the decrease of the crystal growth, particularly, of the free CaO. In addition to an enormous increase in the amount of melt, coarse grinding, at least of the calcite carrier, releases two other relevant changes in clinker formation:
1. The calcination of the coarse calcite particles of more than 0.5 mm and more than 3 mm, which can be chemically incorporated only via the melt, will be finished by up to 50°C later.
2. The temperature of the melt formation will be decreased from 1300 to 1100°C, which is released due to the formation of low-basic, easily melting silicates.
This results in a decrease of the temperature and time interval between the formation of alite and the calcination and, due to the decrease of the crystal growth, leads to a significant reduction of the inhibiting effect on the chemical activity of the remaining free lime and on the belite in part still formed topochemically. Thus, the dissolution of the remaining free lime and of the belite in part formed topochemically is inhibited only a little by the crystal growth in the clinker liquid leading to an acceleration of the alite formation or admitting a reduction of the burning temperature.
Due to the mechanical stress in the rotary kiln
– the homogenization of the CaO particles dispersed in the melt,
– the contact between the melts rich in SiO2 and the conventional melts, and
– the abrasion of the supersaturated and saturated zone and of the jacket on the surface of the CaO grains generated by the topochemically crystallized alite and belite can be clearly intensified and
– the resulting alite formation can be significantly accelerated.
To prove the ideas described above regarding the formation of alite, a raw mix for cement making extremely rich in CaO and SiO2 with a lime standard of 98 to 110 and a silica ratio of 6 to 14 consisting of limestone (K) and phosphorus slag (PS) can be used. A much more energy-efficient formation of alite as opposed to the same conventional, fine raw mix can be achieved if the portion of the granulated PS in the raw mix is increased from 42–45% to 50–95% and is fed to the kiln unground and separated from the ground limestone and the remaining PS [7, 11, 12]. Based on many years of operating experience, the following technical and economic advantages can be achieved when using unground PS compared to the same, conventional, fine raw mix very rich in silica:
1. The throughput of the kiln is increased by approx. 40%.
2. The fuel required for clinker burning is reduced by approx. 30% due to a lower temperature of formation of the clinker liquid and a better heat transfer between the kiln charge and the kiln gases in the transition zone of the kiln.
3. The degree of whiteness of white cement is increased by 4–10% due to a reduced metal abrasion of the grinding media and the resulting decrease of the admixture of colouring metals in the raw mix as well as a redistribution of the colouring oxides of the silicates into the matrix, which is clearly less susceptible to discoloration.
4. The dust emission from the kiln is reduced by 35 to 55%.
5. The specific metal abrasion of the grinding media and mill lining is reduced by at least 45–70% during raw material grinding.
6. The energy consumption for raw meal grinding is reduced by 45–75%.
These improvements are especially due to the melt extremely rich in SiO2 but very poor in Al2O3 and almost free from Fe2O3, which is formed at the beginning of the burning zone due to the unground, granulated phosphorous slag and in which alite can be formed as described above.
The laboratory burning tests and burning during operation show that the finish burn temperature remains constant and amounts to about 1450°C, regardless of whether the PS in the raw mix is ground or unground. The melting point of the melts formed according the two processes is clearly different. This has the following reasons:
When using unground PS, the portion of the clinker liquid is clearly increased from 5–6% to 30–45%. The melting temperature of the melt formed very rich in SiO2, poor in Al2O3 and almost free from Fe2O3 corresponds to the melting temperature of the PS and amounts to approx. 1200–1230°C according to the DTA (Fig. 3 and 4a). This, in turn, leads to an essentially higher reaction rate via the melt rich in SiO2 than via the conventional melt rich in Al2O3.
The melt rich in Al2O3 and F and almost free from Fe2O3 is formed at approx. 1300°C in the raw mix with the conventional fine PS according to the endothermic effect with the maximum on the DTA curve at 1320°C (Fig. 4b) according to the solid-state reactions including the formation of predominantly belite. The temperature of the melt formation is reduced from 1455 to approx. 1300°C both in the conventional and the process presented here (Fig. 4a, b) at the ascending forked point 3 rich in Al2O3 (Fig. 1). In the new process, the PS melt is formed at approx. 1200°C with easily melting minerals containing F and MgO, such as kuspidine (3CaO·CaF2·2SiO2) before the melt rich in Al2O3 is formed. Analytical peaks of d 2,085; 2,84; 3,07 Å of fluorsilicate (C3S)3C2SCaF2 (Fig.5) with a thermally narrow stability interval can be seen in the conventional fine raw mix at 1100°C. The endothermic effect on the DTA curve at 1130-1350°C (Fig. 5 b) has obviously been released by the melting of fluorsilicate including the formation of trigonal C3S, a’-C2S and the fluid phase [13].
A clear decrease of the burnability of the raw mix with the one hundred per cent conventional, fine PS is due to the chemical incorporation of the PS before it melts at 1220°C to become difficultly melting clinker minerals due to the solid-state reactions. Since the portion of the melt of the conventional, fine mix extremely rich in SiO2, poor in Al2O3 and almost free from Fe2O3 only amounts to approx. 5–6% during burning, the reactivity may heavily be inhibited due to the supersaturation of CaO at the melt surface. Nevertheless, the mix consisting of the limestone and the PS has a relatively high reactivity. This is probably due to the lower surface tension and viscosity of the melt containing F and rich in Al2O3.
Thus, the temperature to complete the formation of white clinker is decreased from 1550–1600°C to 1450°C. When using the conventional, fine raw mix, the temperature of formation of the melt via the already mentioned forked point 3 rich in Al2O3 and diluted with easily melting minerals (Fig. 1) is reduced from 1455°C to 1300–1320°C. However, it is about 100 °C higher, i.e.1200–1230°C, than the melt based on unground PS rich in SiO2. The formation of alite described above may take place via the melt rich in SiO2 based on the unground PS with the final completion via the thinned, ascending forked point 3 with easily melting constituents in the triangle a-C2S-C12A7-C3A, adjacent to the crystallization field of C3A. Thus, the reactivity of the raw mix is essentially improved [7, 11, 12].
It follows from the investigations with the electron microprobe (Table 1) that alite is poorer in Al2O3, Fe2O3, P2O5, F and MnO and richer in MgO in the clinker of the raw mix consisting of limestone and phosphorous slag (KPS) with 50–95% of unground PS compared to the clinker based on the conventional, fine raw mix of limestone and phosphorous slag. However, the conventional intermediate phase predominantly consisting of C11A7CaF2 and C3A is richer in Al2O3, Fe2O3, P2O5, F and MnO and poorer in MgO and CaO than that of the conventional, fine raw mix. The decrease of the CaO content in the conventional melt should be caused, when using up to 95% unground PS, by an increase of the C11A7CaF2 content instead of the C3A due to the higher F content. The CAF phase included in some cases was enriched with SiO2. These changes in the intermediate phase lead to a decrease of the melting point of the conventional melt and to an increase in its amount including an even higher resulting reactivity of the mix with unground PS.
The increase of CaOfr. in the clinker from 0.3–0.5% to 1.7–2%, when increasing the portion of unground PS from 50-95% to 100%, shows that the reactivity of the raw mix decreases. Even so, these results prove that alite is formed in the viscous melt rich in SiO2. It should be assumed that the melt rich in Al2O3 will be formed increasingly from more than approx. 1320°C due to the dissolution of CaO in the viscous melt rich in SiO2 and to the alite crystallization. However, the thus formed amount of the melt rich in Al2O3 remains smaller than that directly formed by the ground portion of PS of the KPS mix. This limits the absorption capacity of the melt continuously decreasing as residue of SiO2 in the melt rich in Al2O3. Since the conventional melt rich in Al2O3 is thinner than the melt rich in SiO2, its decreasing amount leads to the kinetically slower formation of alite and, consequently, to an increase in the content of CaOfr. in the clinker.
It would suggest that the formation of alite predominantly takes place in the PS melt extremely rich in SiO2, but very low in Al2O3, containing F and almost free from Fe2O3, which is formed at the beginning of the burning zone at 1200–1225°C during the dissolution of CaO and the subsequent absorption of the residual melt increasingly decreasing in SiO2 in the conventional melt rich in Al2O3 formed by the portion of ground PS at 1320°C. A further and, above all, final formation of C3S via the melt rich in Al2O3 with accompanying dissolution of the remaining CaOfr. can be completed via the ascending forked point 3 adjacent to the crystallization field of C3A and C12A7 (C11A7CaF2) in the triangle a-C2S-C12A7-C3A (Fig. 1) with a melting point decreased to 1300-1320 °C in the presence of MgO, F, P2O5, Fe2O3 and MnO.
A reaction accelerating and mineralizing effect of the highly viscous ash melts already at 1200°C can be used as further proof of the described mechanism of the formation of alite. [14].
Summarizing it can be said that the alite formation, based on the extremely coarse raw mix, takes place, irrespective of the preceding mechanism, at the end of the burning zone via a conventional melt rich in Fe2O3 and/or Al2O3, which corresponds to the ascending forked point 3 in the triangle a-C2S-C3S-C3A adjacent to the field C3S, the real composition and melting temperature of which result from an existing portion of C4AF and other, easily melting phases.
The investigations carried out and the operating experience gained [1–6] show that the following relevant changes, as regards the energy efficiency, take place during the solid-state reactions and the reactions caused by the melt when burning the extremely coarse raw mixes.
1. Because the calcination of the coarse calcite particles of 0.2–3 mm is finished by about 50°C later and since the temperature of the melt formation is reduced from 1300°C to 1100°C, there is an essential decrease of the temperature and time interval between the formation of alite and the final calcination. Due to an essential decrease of the accretive crystallization, the coarse-grained CaO and the belite, in part topochemically formed, enter the burning zone finally crystalline and highly reactive, which leads to an acceleration of the formation of alite or a decrease of the burning temperature.
2. The reduction of the crystal growth of silicates due to the transformation of the topochemically formed C2S to CS and C3S2 with the attuned portion of CaO particles capable of solid-state reactions for eutectics rich in SiO2.
3. Due to a reduction of the CaO particles capable of solid-state reactions appropriate for eutectics rich in SiO2, the topo-chemical formation of belite, C3A and C12A7 at temperatures below 1300°C is avoided to a great extent, and easily melting CAS2, CS, C3S2 and C2AS ensure the formation of the additional melt rich in SiO2.
4. The mechanism of the formation of alite and belite during burning of the extremely coarse raw mix is described by the following three kinetic, energy-efficient models with a clearly increased portion of the melt in the burning zone:
a. by the crystallization of the melt rich in SiO2 of the triangle C2S-C3A-C3S diluted with easily melting phases;
b. by numerous centres formed in the melt rich in SiO2, supersaturated with CaO, at the reaction interface of the coarse-grained solid phase of CaO split up into aggregates, which are used as crystal nuclei and also as crystallization centres for the topochemical formation of alite and belite between the melt rich in SiO2 and the CaO aggregates;
c. by the absorption of the melt rich in SiO2 in the adjacent conventional melt rich in Fe2O3 and/or Al2O3 with sim-ultaneous dissolution of CaO.
5. Now alite is not only formed due to the dissolution of the topochemically formed belite and the residual free lime in the melt rich in Fe2O3 and/or Al2O3 with subsequent crystallization, but predominantly by topochemical reactions between the earlier formed melt rich in SiO2 and the coarse particles of CaO split up into aggregates and by the absorption of the melt rich in SiO2 in the melt rich in Fe2O3 and/or Al2O3 including dissolution of the residual CaO.
6. The formation of alite is completed at the end of the burning zone via a conventional melt rich in Fe2O3 and/or Al2O3, which corresponds to the ascending forked point 3 in the triangle a-C2S-C3S-C3A adjacent to the field C3S, the melting temperature of which is considerably reduced due to easily melting phases.
7. Since alite and belite may predominantly be formed directly from the melt rich in SiO2 and via its dissolution in the melt rich in Fe2O3 and/or Al2O3 during the incorporation of CaO, the portion of the solid phase of the clinker to be dissolved in the melt rich in Fe2O3 and/or Al2O3 is reduced from approx. 80% with the conventional fine raw mix down to 20% with the extremely coarse one. This minimizes the supersaturation of the surface of the melt and accelerates the formation of alite.
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