Innovative, energy-efficient manufacture
of cement by means of controlled mineral
formation – Part 2

Since the portion of the energy costs of the cement production is naturally very high, the cement industry is extremely interested in reducing its energy consumption by means of permanent process innovations and a continuous increase in efficiency. The following enormous technical and economic benefits can be achieved according to the new process by extremely coarse grinding of the raw mix by means of an innovative new design of the wet and dry grinding plants [1-3]:

1. The energy consumption for raw material grinding is reduced by 60 to 80 %.

2. The fuel required for clinker burning is reduced by up to 15 %.

3. The performance of the raw grinding plant is increased 2.5 up to 4.5 times.

4. The kiln output is increased by 15–30 % and the emission of CO₂, CO and NO_x is minimized.

5. The emission of fine dust from the kiln and the grinding plants is reduced by 25 up to 50 %.

6. The energy consumption of the ESP for waste gas dedusting of the grinding plants and of the kilns is reduced.

7. The specific metal abrasion of grinding media during raw material grinding is reduced by 50 to 80 %.

8. The noise level of tube and ball mills is reduced by 50 to 80 %.

9. The usage properties and the grindability of cement are improved by 5–20 %.

10. The brightness of white and bright cements is increased by 3–6 %.

11. The capital expense for raw grinding plants and filter equipment is decreased.

In order to clarify the reasons for the mentioned technical and economic benefits of the energy saving process, the changes in the burning process have been analysed.

Industrial experience gained with the dry and wet process has shown that a higher speed during burning of the extremely coarse raw mix leads to a clearly increasing stability of the kiln operation. This can be explained as follows: Mechanical stress is very helpful for an accelerated mineral formation with an increased portion of the clinker melt, because, compared to the decreased portion of the clinker melt with the usual eutectic, the melt leads to an effective decrease of the saturated zone and a clearly more complete saturation and splitting up of the solid phase of free lime. Thus, the slowest reaction stages are accelerated by the melt. Consequently, the formation of clinker is considerably accelerated, in particular with raw meal containing the coarsely ground limestone. The following model of clinker formation in the burning zone of the rotary kiln due to the increasingly formed melt rich in SiO₂ according to the new process can be assumed: The rolling process in the rotary kiln with its large mass movement exerts infinite pressure shocks on every agglomerate formed in the burning bed, i.e. small ones at the upper edge and strong ones at the lower edge, which

– due to the diffusion of the melt into the internal crystal beds of the coarse-grained lime particles via structure and crystal defects including the accompanying splitting make them fine aggregates as well as

– distribute the wetting and agglutinating melt over the entire aggregate still in bulk thus accelerating the saturation of the solid phase of free lime with the melt,

– lead to an acceleration of the mutual dissolution due to a closer moving together of the conventional melts and those rich in SiO₂, and

– lead to relatively homogeneous and dense grains because the solids move closer together.

After the controlled clinker formation due to the optimization of the grain class conditions of the raw mix, it should be expected that the amount of melt will clearly be increased, in particular at the beginning of the burning zone, and that, consequently, the melt cover over all particles will be considerably improved right from the start. In this case the cavities of the nodules can be completely filled with a free, non pseudo-solid or too viscous, moist melt rich in SiO₂. The free, i.e. non pseudo-solid or not too viscous, melt in the cavities of the nodules can be pressed by pressure shocks into the pores of the coarse CaO particles if the nodules are in an excessively moist condition. With the lack of clinker melt, formed according to the conventional process, this is obviously very limited.

With the increased temperature the melt rich in SiO₂ can be kept sufficiently liquid despite the increase in basicity caused by the dissolution of the solid phase of free lime. At the same time the rolling process counteracts the adhesive capacity of the melt in so far as it abrades the tenacious particles of the solid phase and simultaneously incorporates the melt in the interior of the CaO and clinker grains. With an increased portion of melt, favourable conditions are created to saturate the coarse solid grains with the melt via structure defects (pores, cracks) and crystal defects and then to split them to become very small aggregates.

The total amount of the melt is decreased due to the dissolution of a temperature-dependent portion of the aggregates, formed from the coarse-grained solid phase of free lime, in the increasingly formed melt rich in SiO₂ according to the new process and its subsequent topochemical integration due to the nucleation and accretive crystallization on the surface of the residual solid phase of CaO towards the solid phase of alite. The free melt rich in Al₂O₃ and Fe₂O₃ remaining at the burning temperature remains droppable and free-flowing, and the melt rich in SiO₂ on the surface of the solid phase not yet chemically integrated, as a very thin layer, will certainly behave similarly pseudo-solid like water as thin cover on solids or in capillaries. This increasingly complements the integration of the residual phase of CaO.

The decrease of the temperature of melt formation down to less than 1200 °C leads to an essential extension of the burning zone with a simultaneous, proportional shortening of the length of the transition zone where the mineral formation takes place due to the solid-state reaction. Due to the later decom­position of the calcite particles, which are coarser than 0.2–3 mm, and the extension of the burning zone it will also be possible to achieved a shortening of the temperature and time intervals between the end of the calcination process and the beginning of the melt formation. This results in a reduction of the degree of the chemical passivation of the remaining free calcium oxide and of the belite in part formed due to solid-state reactions, which are caused by the growth and the compression of their crystals and as a consequence of the high temperatures. Thus, the reaction rate of the raw mix is increased, which also ensures an increase in the kiln productivity and a reduction of the fuel required.

 There are significant indications that the coating behaviour in industrial rotary kilns is essentially improved during burning of the extremely coarse raw mixes. A clear reduction of the shell temperature of the three-station kiln (Fig. 1) can be observed in the range between the kiln inlet and the first tyre as well as at the end of the usual coating zone between the second tyre and the normal coating layer, i.e. the coatings expand towards the kiln inlet. The analysis of the condition of coating formation during the kiln shutdown shows that the range of coating in the burning zone is extended by 40 to 50 %. Of course, this is due to the formation of the melt rich in SiO₂, which has a clearly lower formation temperature than the conventional one, i.e. by 100 to 200 °C. This is an industrial proof that an easier melting melt rich in SiO₂ is formed earlier in addition to the conventional one during burning of the extremely coarse raw mixes.

The desired porous coating layer, which can be clearly more stressed and which has a clearly stronger adhesive capacity, is formed in the extended range of coating of the lining. Thus, the thermally and chemically very heavily stressed, conventionally open range of the kiln lining is protected. Therefore, the stability of the kiln lining is extended and the energy loss due to the radiation from the surface of the kiln shell and the resulting fuel consumption are clearly reduced.

The reduction of the kiln shell temperature in the area between the kiln inlet and the first tyre is due to a higher thermal conductivity of the extremely coarse kiln charge and a better rolling process. Thus, more thermal energy is transferred from the kiln lining heated by kiln gases to the extremely coarse kiln charge.

The increased temperature in the narrow area between the first and the second tyre is due to the dissolution in the extremely coarse kiln charge of the ring rich in alkalis, which takes place earlier with the conventional, fine kiln charge.

Various results from operations according to the new process show that the formation of a too thin liquid melt could not be observed, which causes, under certain burning conditions, an inappropriate cascading and, consequently, a formation of spheres and coating due to an outflow of the melt on the surface of the clinker nodules. It happens essentially more rarely that the coating will fall down from the upper position of the kiln. The coating layer rich in SiO₂ is formed considerably more quickly at locally damaged coating and lining points than the conventional coating layer rich in Al₂O₃ and Fe₂O₃. The kiln operation shows that the formation of rings and spheres can also be eliminated. This can be explained as follows:

– The basicity and the formation temperature of the clinker melt rich in SiO₂ continuously increase with heating up.

– The viscosity of the melt rich in SiO₂ is higher than that of the conventional melt.

– The formation of a too thin liquid, conventional melt rich in Al₂O₃ and Fe₂O₃ is eliminated by the permanent absorption of the melt rich in SiO₂.

– The initial temperature of the burning zone is by at least 100 °C lower than according to the conventional process.

To confirm the behaviour of the melt rich in SiO₂ without rings, measures already recommended to eliminate an abundance of melts and the formation of rings should be mentioned [4-6]. If coal is used as fuel, it is practical to follow the proposal to add limestone meal to the kiln coal. Due to the compensation of the coal ash with limestone meal, it is impossible right from the start that thin liquid melt droplets will form. Thus, the formation of rings can be eliminated. This can be explained as follows:

The melting point of all mixes consisting of clinker and coal ash is lower than that of pure ash. Thus the melt promoting effect of the ash becomes clear. Furthermore, it was shown that the highly viscous, acid melt of the coal ash will essentially penetrate into pores and cracks of the clinker more slowly than the thin liquid, medium-basic melts of lignite ash. Since the formation of rings can be avoided by adding limestone meal to the coal ash very rich in SiO₂, it can be expected that a medium-basic melt rich in SiO₂, which is formed according to the new process, will not promote the formation of rings. This has been proved in industry.

The operating results of kilns with cyclone preheaters show that the temperature and the degree of calcination of the kiln charge are increased by up to approx. 10 % in the feed end housing of the kiln despite a coarser raw mix with a residue of 30–60 % on the 90 µm sieve. For instance, the degree of calcination is increased from 60 % to 70 %. A missing formation of coating in the cyclone preheater is a good prerequisite not to install the bypass, thus additionally reducing the fuel consumption and increasing the early strength of cement.

The essentially improved heat exchange can be explained by the missing, electrostatically caused aggregate formation when extremely coarse raw mixes are passing the cyclone preheater, completely opposed to conventional, fine raw mixes. This is confirmed by the complete interruption of the covering of the cyclone outlets and the subsequent cyclone filling with kiln charge.

The transition zone is the narrowest stage of the kiln limiting its output. This is due to a very limited heat transfer between the kiln gases and the conventional, fine kiln charge. Completely opposed to the conventional, fine kiln charge, the extremely coarse kiln charge does not reach the fluidity state. This is due to the fact that, compared to the conventional, fine raw mix, the portion of oversize with particle sizes of 0.09 to 3 mm of the extremely coarse raw mix with approx. 30-60 % is extremely high, and that the bulk density of the extremely coarse raw mix amounts to approx. 1200–1400 g/l compared to approx. 900–1000 g/l of the conventional, fine one. Thus, the thermal conductivity in the burning bed is essentially improved and the rolling process of the kiln charge is clearly better. An improved rolling process in the transition zone of the kiln leads to an essentially more intensive renewal and a significant increase of the burning bed surface. The heat transfer between the gases and the kiln charge as well as between the lining and the kiln charge is clearly improved due to the described advantages caused by energy transfer. Increasing the kiln speed during burning of the extremely coarse raw mix, compared to the conventional, fine raw mix, leads to a clearly higher intensity of the renewal of the burning bed surface for the above reasons, which, on the one hand, has a positive effect on the heat transfer between the gases and lining, and, on other hand, between the gases and the kiln charge. It has also a positive effect on the kiln operation and the free lime bonding.

The kiln charge reaches the burning zone of the kiln thermally more homogeneous and with a higher temperature. Not to forget the infinite shocks between the particles and the nodules, which decisively intensify the course of the solid-state reactions and of the reactions via the melt. An extremely coarse raw mix is an excellent prerequisite for a higher kiln output of up to 30 % and a reduction of the fuels with emissions of CO and NO_x needed for clinker burning by up to 15 %.

So as to be able to arrive at quantitative statements as regards the effect of the extremely coarse raw material grinding on the redistribution of the foreign elements between the clinker minerals, industrially produced cement clinkers from extremely coarse and conventional, fine raw mixes were investigated by means of the electron-probe microanalysis (Table 1).

When comparing the values of the microanalysis, clear differences concerning the contents of foreign ions in the same main phases could be observed between clinkers of the extremely coarse and conventional, fine raw mix (Table 1), which can be taken as proof for the formation of alite from the melt rich in SiO₂.

Alite in the clinker of the extremely coarse raw mix is essentially poorer in Al₂O_3, Fe₂O₃, Na₂O and K₂O by 28 to 42 %, respectively, and richer in MgO and MnO by 27–34 % than that in the clinker of the conventional, fine raw mix. Details showed that the MgO content is considerably increased by 27.3 % in the alite from 1.34 % of the clinker of the conventional, fine raw mix to 1.7 % of the clinker of the extremely coarse raw mix. This leads to a relevant decrease of the periclase content in the clinker of the extremely coarse raw mix (Mg images in Figure 2 a, b). The higher MgO content in the alite of the clinker of the extremely coarse raw mix is due to the fact that the formation of the melt rich in SiO₂ takes place in the range of the topochemically formed systems of CS-CAS₂-S, CS-C₂AS-CAS₂ and CS-C₃S₂-C₂AS incorporating topochemically formed and MgO containing easily melting silicates, such as akermanite (Ca₂MgSi₂O₇), alumoakermanite (Ca,Na)₂(Al, Mg,Fe²⁺)(Si₂O₇), bredigite (C₇Mg[SiO₄]₄) and brownmillerite (CaFe_1,2Mg_0,4Si_0,4O₅ (Si,Mg). This has been proved by the quantitative diffraction investigations carried out on raw mixes burned at 1100–1200 °C.

However, C₃A and C₆A₂F are richer in Al₂O_3, Fe₂O₃ and, in particular, alkalis and poorer in MgO, TiO₂ and Cr₂O₃ than those of the conventional, fine raw mix. In particular, C₆A₂F is heavily enriched with alkalis by 308–349 % and additionally with SiO₂ by 100 %. These changes in the intermediate phase lead to a decrease of the melting point of the conventional melt rich in Al₂O₃ and Fe₂O₃ and to an increase of its quantity. An increase in the amount of melt with a low melting point not only at the beginning of the burning zone due to the formation of the melt rich in SiO₂ but also of the conventional melt rich in Al₂O₃ and Fe₂O₃ in the final stage of alite formation at the end of the burning zone leads to a further increase of the reactivity of the extremely coarse raw mix.

The brightness of white and bright cements is increased by 3–6 % due to the redistribution of the colouring oxides of the silicates into the matrix less sensitive to discolouration as well as due to a decrease of colouring oxides in the raw mix.

The negative effect of the increased alite content on the ­kinetics of the alite formation and its stability is reduced due to the redistribution of the alkalis from the alite into the intermediate phase during burning of the extremely coarse raw mix. This has an additional positive effect on the reactivity of the raw mix.

A stoichiometrically too low decrease of the MgO content in the C₃A and the C₆A₂F richer in SiO₂, compared to its increase in the alite, is due to the additional dissolution of the remaining periclase in the clinker. This ensues from a comparison of the Mg images in Figure 2 a, b of clinkers of the extremely coarse and conventional, fine raw mixes. The increase of the SiO₂ content in C₆A₂F is due to the formation of C₆A₂F from the residual melt that is formed during the alite formation from the melt rich in SiO₂. The increase of the alkali content is due to the decrease of alkalis in the alite.

The presence of C₆A₂F with the frequently increased and the usually low SiO₂ content indicates that it is formed from the melt rich in SiO₂ and from the portion of the extremely coarse raw mix that is capable of solid-state reaction and confirms the above described mechanism of clinker formation. A clearly lower MgO content in C₃A than in C₆A₂F is due to the fact that there is no ternary compound of the three components C, A and Mg in the C-A-Mg system. A selective increase of the K₂O volatilization should be expected since C₃A can only be enriched with Na₂O by 22 %. The K content of C₃A is decreased by 8.4 %.

The structure of prismatic alite crystals and occasionally rounded belite crystals in the clinker of the extremely coarse raw mix is clearly more distinct than in the clinker of the conventional, fine raw mix (Fig. 2 a, b). Alite crystals in the clinker of the conventional, fine raw mix are often intergrown in aggregates.

1.  Contrary to all ideas prevailing so far, a much coarser raw meal can very well be sintered and granulated if the corresponding prerequisites have been created for the formation of the melt near the quintuple points rich in SiO₂.

2.  The temperature and the degree of calcination of the kiln charge will increase by up to 10 % after the cyclone preheater due to the missing, electrostatically caused aggregate formation in the raw mix with a residue of 30-60 % on the 90 µm sieve. Consequently, the covering of cyclone outlets and the cyclone filling with kiln charge will not take place.

3. The thermal conduction in the burning bed is essentially increased since the extremely coarse kiln charge is not in a fluidity state but in a rolling process and since its bulk density is increased from 900-1000 g/l to 1200-1400 g/l.

4. The surface of the extremely coarse kiln charge is clearly more intensively renewed and increased due to the easily flowing rolling process in the transition zone of the kiln. Thus, the heat transfer between the gases and the kiln charge is considerably accelerated, the output of the kiln is increased by up to 30 % and the consumption of fuels with emissions of CO and NO_x is reduced by up to 15 %.

5. Due to the melt rich in SiO₂ formed at 1100–1200 °C, the range of coating in the burning zone of the kiln is extended by up to 40–50 % by a porous coating layer with a clearly stronger adhesive capacity that can clearly be more stressed. Thus, the thermally and chemically heavily loaded kiln lining is protected. The coating layer rich in SiO₂ is clearly formed more quickly at locally damaged coating and lining points.                   

6. A strong redistribution of the foreign ions between the clink-er minerals takes place in the clinker of the extremely coarse raw mix. Alite was enriched with MgO and was poor in alkalis and Al(Fe)₂O₃, which leads to a decrease of the periclase content in the clinker and admits a higher MgO content in the raw meal. Furthermore, the amount of the conventional melt and the reactivity are also increased.