A study on the action mechanisms of grinding aids used for clinker comminution Elodie Roblot a,*, Philippe Grosseau a, Bernard Guilhot a, Bruno Classen b, Claude Haehnel b, Eric Gaffet c a Laboratoire des Procédés en Milieux Granulaires UMR CNRS 5148, Ecole des Mines de Saint Etienne, 158 cours Fauriel, F42023 Saint Etienne Cedex 2 b CTG Italcementi Group, rue des Technodes, BP 01, F78931 Guerville Cedex c NanoMaterials Research Group NRG - UMR 5060 CNRS / UTBM, Site de Sévenans, F90010 Belfort Cedex *
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Abstract In cement industry, clinker grinding in steel tumbling mills has a very low energy efficiency. The use of a small quantity of products called grinding aids can improve the energy efficiency of this operation. However, the grinding mechanism with such products has not been explained precisely yet. In order to understand it, we study the effect of two grinding aids on clinker grinding using a planetary mill. First, we have underlined the fact that the two grinding aids have distinct effects on clinker grinding. Second, we have found that they improve not only the clinker comminution, but also the one of other materials like limestone and slag. Third, some experiments using three different values of the rotation-to-revolution ratio for the planetary mill have shown that the efficiency of both grinding aids is influenced by this ratio. Finally, their mode of action could be linked to the improvement of the powder dispersion in the mill. Keywords : Industrial minerals, Dry grinding, Process optimisation
1. Introduction Clinker grinding in ball mills is a very expensive operation in cement processing. This operation needs a great amount of electric energy for a low energetic efficiency of around 1%. Some grinding systems are designed to improve the grinding operation efficiency. It involves complex grinding systems including pre-grinding and finishing grinding facilities and very efficient classifiers (Jankovic et al., 2004). Another way to improve the grinding efficiency is to use a low amount (0.05 wt% to 0.25 wt%) of liquid additives called grinding aids. However, their use is based mainly upon empirical background, and no precise knowledge is established on their precise mechanism of action. Several studies show the role of organic or mineral products used as grinding aids on various minerals grinding. They underline the role of certain phenomena on minerals grinding improvement. First, the prevention of agglomeration of the ground mineral inside the mill is viewed as an efficient mechanism (Ikazaki et al., 1996; Moothedath and Ahluwalia, 1992; Nair and Paramasivam, 1999). Secondly, the powder dispersion implies the modification of the minerals flow in the mill. Indeed, the powder fluidity is favoured, which allows for a better material availability in the effective grinding zone. However, this fluidity improvement should not affect the grinding efficiency brought about by the dramatic decrease of the powder friction coefficient (Moothedath and Ahluwalia, 1992). The first part of the study underlines that the two grinding aids used, called A and B, act differently on clinker grinding. Then, their effects on limestone and slag grinding are tested in order to verify if they also improve the grinding efficiency of those different minerals. Finally, a grinding parameter which sets the grinding energy involved during the operation is changed to observe the effect on the efficiency of A and B. 2. Operating method 2.1. Grindings The grindings are realized with two planetary ball mills, the Retsch “PM400” and the Fritsch “Pulverisette 4”, equipped with two stainless steel pots (Fig. 1). Each pot is filled with 27 vol.% stainless steel balls and then with crushed clinker, or slag or limestone. The two other last minerals chosen are useful in cement making, as limestone is the major raw material included in the first part of the process, before the heat treatment. And blast furnace slag is added with clinker at the grinding to make a cement type (blast furnace cement). The ground powder without grinding aid is called reference.
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Fig. 1 : Principle of the planetary ball mill
The sun wheel and pots rotation speeds with regard to the mill frame are noted and , respectively. The ratio / is called rotation-to-revolution speed ratio. In the third set of grindings, / is set to three different values. The gradual increase of / allows to modify the balls movements and the grinding energy they supply to the powder during the operation (Abdellaoui and Gaffet, 1995). Thus it shows the influence of the energy supplied to the powder on the efficiency of grinding aids A and B. Table 1 : Grinding parameters for each set of grindings.
2.2. Measurements The specific surface area, called fineness, is measured on ground clinker and slag with Blaine method. The evolution of the fineness with grinding time gives the relative grinding efficiencies of A and B. The fineness also gives an idea of the relative grindability of clinker (Elwan et al., 2002) or slag. To measure the high surface areas of ground limestone, the BET method is applied, using an ASAP 2010 Micromeritics apparatus with nitrogen as adsorptive gas. Like for clinker and slag, the relative effectiveness of A and B on limestone grinding is evaluated by the evolution of the specific surface with grinding time. Like the specific surfaces, the particle sizes distributions are measured in order to get an indication upon the grinding efficiency. These measurements were achieved with a laser particle sizer, a Malvern Mastersizer 2000 with the wet and dry dispersion accessories. On the one hand, in the wet condition the powders are dispersed in water. Prior to the measurement, the powder suspensions are efficiently dispersed using an external ultrasonic probe. On the other hand, the dry condition allows to evaluate the state of dispersion of the powder in the same environment as during the grindings, that is in air.
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3. Results and discussions 3.1. Efficiency of A and B on clinker grinding The curves on Fig. 2 show the gap between reference clinker finenesses and clinker ground with A or B ones. In the grinding conditions tested, the grinding aids allow the fineness to increase further with grinding time compared to the reference clinker. For 10 min of grinding, the finenesses of the clinker ground with A and B are quite close, and are 20% higher than the one of the reference. Then as the grinding goes on, the finenesses with grinding aid A increases continuously, allowing the fineness to be 50% higher than the one of the reference at 40 min. On the other hand, after 20 min of grinding, the finenesses improvement allowed by B decreases, reaching a steady value of 14%.
Fig. 2 : Gap between reference clinker fineness and finenesses of clinker ground with A or B.
Finally, the effect of A on the fineness evolution with grinding time is different from the effect of B. Grinding aid A allows to increase constantly the fineness until the maximum grinding time while grinding aid B makes the fineness increase efficiently only up to 20 min. 3.2. Efficiency of grinding aids on slag and limestone grinding The evolution of the specific surface area of limestone with grinding time increases for all the grindings (Fig. 3). The curves obtained with grinding aids A and B show higher specific surfaces values than the reference. Consequently, the grinding aids are efficient on limestone grinding.
Fig. 3 : Evolution of BET specific surface area with grinding time for reference limestone and limestone ground with A or B.
The particle size distributions (PSD) were measured in water and in air on the limestone ground during 10 min. The three PSD show that there is a better size reduction of the particles for the grindings using A or B (Fig. 4a). Indeed, the reference limestone PSD shows a population peak at 50 µm, but not the limestone ground with a grinding aid. When the measure is achieved in air (Fig. 4b), under a dispersion pressure of 3.5 bar, the PSD of the reference shows a supplementary peak at 300 µm. As this population is missing when the reference is measured in water after a ultrasonic dispersion, this shows that it is composed of agglomerates. Consequently, the reference is subjected to agglomeration when dispersed in air, whereas the powders ground with A or B are
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well dispersed in that case. Those experiments involving limestone show that A and B could improve the grinding via a dispersing effect.
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Fig. 4 : Particle size distributions of the limestone ground during 10 min, measured in water (a) and in air (b).
For limestone grinding, the range of grinding time chosen is [5 min, 15 min], which is shorter than the range used for the slag grindings, that is [15 min, 60 min]. Indeed the grinding times were chosen taking into account the relative grindabilities of the minerals ground. So, as the grindability of limestone is by far better than the slag one (Opoczky, 1996), the grinding time necessary for the first material is shorter. The choice of the grinding times is based on experimental results. Thus, the final grinding time is set as the time at which the powder is stuck on the pots inner surfaces, due to the lack of fine particles disposal system. For the reference slag, the fineness increases between 15 min and 30 min until an asymptotic value (Fig. 5). When slag is ground with A or B, the finenesses increase until 60 min, and they both allow an improvement in slag grinding. For grinding aid A, the improvement is effective from 15 min. On the contrary, B is a hindrance before 30 min of grinding. Its positive grinding aid effect can be observed from 40 min. The negative effect of B on the first step of slag grinding can be due to its lubricating properties on the steel surfaces of the pots. In fact this lubricating effect can make the balls slide on the steel surfaces, preventing them from being involved in the breakage process. After 30 min, B is no longer stuck on the steel surfaces and it can interact with the slag powder to improve the grinding.
Fig. 5 : Finenesses evolution with grinding time for reference slag and slag ground with A and B.
As shown in Fig. 6, the PSD measured in air for the samples ground during 20 min and 60 min corroborate the results obtained with fineness measurements. Indeed, the PSD found for the powder ground 20 min with B (Fig. 6a) shows a higher fraction of coarse particles (above 16 µm) than the reference and the slag ground with A. On the other hand, for 60 min (Fig. 6b), the reference still has some particles coarser than 30 µm, whereas the clinker ground with B has only particles finer than 30 µm.
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(a)
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Fig. 6 : Dry particle sizing : particle size distributions of the slag ground during 20 min (a) and 60 min (b).
Finally, those grindings show that products A and B are not only efficient on clinker grinding but also on limestone and slag grindings. In other words, their mechanisms of action are not simply linked with the nature of the phases of clinker. Their mechanism of action could be linked with the prevention of the powder agglomeration in the grinding pots. 3.3. Influence of the rotation-to-revolution ratio on A and B efficiencies Clinker was ground with a Fritsch mill “Pulverisette 4”, allowing the decoupling of the sun wheel and the pots rotation speeds. We used this feature to achieve clinker grindings with three rotation-to-revolution ratio values, in order to see the influence on A and B efficiencies. Considering the reference clinker, it can be seen on the “ref.” curves on Fig. 7, that the fineness increases with /. Consequently, the grinding efficiency for the clinker ground without additive is improved when this ratio increases.
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Fig. 7 : Finenesses evolution with / for reference clinker and clinker ground with A (a) or B (b).
The clinker ground 20 min and 40 min with A shows higher finenesses than the reference sample (Fig.7a). At 20 min of grinding, the gap of fineness between the reference and the clinker ground with A, increases from 61 % to 70 % when / is switched from 1.0 to 2.2 (Table 2). When the clinker is ground further, i.e. 40 min, this gap of fineness decreases notably when / increases from 1.5 to 2.2. This could be explained by a clogging effect of the particles into the pots : after each grinding, we have observed the tendency of the powder to adhere on the steel surfaces. For the reference clinker, the same state of powder adherence on the grinding medium surfaces is obtained for each value of / at 40 min, e.g., a small adherence on the steel surfaces. On the other hand, for the clinker ground with A, the steel surfaces at 40 min become more and more covered by the clinker as / increases. At the value of 2.2, the powder has entirely clogged the bottom of the pot. This corresponds to the ratio where the gap of fineness between the reference clinker and clinker ground with A decreases. Indeed, when the powder is clogging into the pots, it can’t undergo further grinding. Table 2 : Fineness increase for the clinker ground with 0.2 wt% A, compared to the reference.
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Clinker ground 20 min with B at /=1.0 has the same fineness as the reference (Fig.7b). However, for the other grindings with B, the finenesses are higher than the reference. At 20 min, the gap of fineness between the reference and clinker ground with B increases up to 30% with / (Table 3). And, when the clinker is ground 40 min with B, this gap keeps quite identical for each / value. These evolutions are consistent with the fact that with B, little adherence of the powder is observed on the grinding surfaces. Table 3 : Fineness increase for the clinker ground with 0.03 wt% B, compared to the reference.
The power supplied by a single ball of each diameter, i.e. 10 mm or 20 mm, has been evaluated using numerical calculations based on a mathematical treatment of the planetary mill (Abdellaoui and Gaffet, 1995). Table 4 shows that the calculated values of the kinetic energy of collision per ball increase with /. On the other hand, the shock frequency first decreases and then increases with the ratio. Consequently, the power per ball also decreases and then increases with /. This is not consistent with the fineness increases observed for the clinker grindings. Indeed, the finenesses increase with /, which should correspond to an increase of the power supplied by the balls. However, the model used here does not take into account the balls interactions neither the pot filling rate. Now, these parameters have a prominent influence on the balls motion and on the power they supply to the powder during the grinding (Dallimore and McCormick, 1997). Then, we have to calculate the power using the model taking into account the balls interaction to verify if the incoherence can be solved. Table 4 : Calculated kinetic energies and shock frequencies of single balls during planetary grindings.
4. Conclusions This study on the efficiency of two grinding aids have underlined that : -A and B have different behaviours on clinker grinding improvement, -they both improve the grinding efficiency of materials with different grindabilities from clinker, namely slag and limestone. They prevent the agglomeration of limestone during grinding, -A and B increase the clinker finenesses when the rotation-to-revolution ratio of the planetary mill increases. Some more calculations should be done to evaluate the effective balls movements and powers involved for each value of the rotation-to-revolution ratio. References Abdellaoui, M., Gaffet, E., The physics of mechanical alloying in a planetary ball mill : mathematical treatment. Acta Metallurgica et Materialia, 1995, 43(3), 1087-1098. Dallimore, M.P., McCormick, P.G., DEM of mechanical alloying in a planetary ball mill. Materials Science Forum, 1997, 235-238, 5-14. Elwan, M.M., Mahmoud, G.A. and El-Didamony, H., Effects of some grinding aids on the grindability of Portland cement. Silicates Industriels, 2002, 67(11-12), 141-143. Ikazaki, F., Kamiya, K., Uchida, K., Kawai, A., Yoda, S., Gotoh, A., Chemically assisted dry comminution of an inorganic powder. Advanced Powder Technology, 1996, 7(2), 111-120. Jankovic, A., Valery, W., Davis, E., Cement grinding optimisation. Minerals Engineering, 2004, 17, 10751081. Moothedath, SK., Ahluwalia, SC., Mechanism of action of grinding aids in comminution. Powder Technology, 1992, 71(3), 229-237. Nair, PBR, Paramasivam, R., An analysis of the influence of grinding aids on the breakage process of calcite in media mills. Advanced Powder Technology, 1999, 10(3), 223-243. Opoczky, L., Grinding technical questions of producing composite cement. International Journal of Mineral Processing, 1996, 44-45, 395-404.
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