INVESTIGATING THE PHYSICAL CHANGES OF CEMENT CLINKER PARTICLES BROKEN IN HIGH-PRESSURE GRINDING ROLLS

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1 INVESTIGATING THE PHYSICAL CHANGES OF CEMENT CLINKER PARTICLES BROKEN IN HIGH-PRESSURE GRINDING ROLLS Mahmut Camalan 1,a, Çetin Hoşten 2 1. Middle East Technical University, Mining Engineering Department, Turkey 2. Middle East Technical University, Mining Engineering Department, Turkey a. Corresponding Author (mcamalan@metu.edu.tr) ABSTRACT High-pressure grinding rolls () are used prior to ball mills to reduce the energy consumption in cement clinker grinding. One advantage of -ball mill systems over conventional ball mill systems is that the produces internal micro-cracks within particles which weaken the particles, providing faster breakage rates at the subsequent ballmill step. This study aims to find physical evidence for these induced micro-cracks. For that purpose, micro-hardness indentation tests were applied on three size fractions taken from and of an industrial in order to compare the mean and variance of the hardness values of each size fraction of the and. Also, microscopic images of mm size fraction of the and were obtained and compared to discuss physical changes effected by the on the particle surfaces. The results show that the mean micro-hardness values of the are significantly lower than those of the, indicating existence of micro-cracks. Besides, the seem to contain a severe porous network, which were eliminated in the due to compression. In terms of weakening, however, the micro-cracks in the are still more effective than the porous network in the. INTRODUCTION High-pressure grinding rolls () have been used prior to ball mills to reduce the energy consumption. consists of a pair of rotating rolls through which a bed of particles are nipped and ground with high external pressure exerted on the bed. The efficiency of -ball mill is two-fold: First, was found to consume lower energy at lower reduction ratios than ball mill due to its compressive loading mechanism [Fuerstenau et al., 1990]. This promoted the use of as a pre-grinder. Secondly, external stresses developed high inter-particle stresses through the particle-bed which, in turn, produced micro-cracks within particles. Then, these induced micro-cracks weaken the particles, and enhance breakage at subsequent ball mill stage [De, 1995; Fuerstenau et al., 1999; Fuerstenau and Vazquez-Favela, 1997]. The purpose of this work is to use a direct means of showing the effect of weakening. This is achieved by comparing the micro-hardness of three size classes taken from the and streams of an industrial. Reflected-microscopy images of the and were also used to compare relavant features of the particle surfaces. EXPERIMENTAL Chemical analyses of the of the and particles were analyzed by Spectro IQ model X-ray fluorescence (XRF) spectrometer. Pellets were used for these analyses. The chemical contents given in Table 1 reveal

2 Table 1. Chemical content of the and Chemical composition of the narrow-size fractions, % mm mm mm Feed Product Feed Product Feed Product CaO SiO Fe 2O Al 2O SO MgO K 2O Na 2O LOI that the chemical contents of the and are more or less identical. Micro-hardness tests were performed on , and mm size fractions of the and. For this experiment, polishedsection were prepared and loaded gradually under Vickers microindenter. The loading scheme were stopped when the force is around 5 N. Then, the were unloaded progressively. During this loadingunloading cycle, simultaneous measurement of force vs. penetration displacement allowed the measurement of the hardness by a procedure described by Oliver and Pharr (1992). In this method, micro-hardness is calculated by the following formula: smaller indent area in a hard material than in a soft one. Reflected microscope was used to obtain images from polished sections of the and. Only images of the mm particles were used in this study. Images were taken with 20x and 50x magnification. The loading-displacement curves of both the and reveal that cement clinker exhibited elastic behavior; however, these gave permanent deformation at the end of the cycle. For the sake of brevity, only the loading behavior of mm and were shown in Figure 1 and Figure 2. H= P max A (1) where H is the hardness (MPa), Pmax is the peak load (N) in the maximum of the loading-unloading curve and A is the projected area (mm 2 ) that the indenter produced at the end of the loadingunloading scheme. For a given load, Eq.1 reveals that the applied load will make a

3 Table 1. Statistics of the hardness values of mm and Fig 1. The load-displacement curve for mm (y-axis: penetration load up to 5 mn, x-axis: penetration depth up to 3*10 4 nm) mm Sample Size 4 6 Mean Variance Table 2. Statistics of the hardness values of mm and Fig 2. The load-displacement curve for mm (y-axis: penetration load up to 5 mn, x-axis: penetration depth up to 5*10 4 nm) RESULTS AND DISCUSSION Tables 1-3 show that the micro-hardness values of the are significantly lower than those of the. For, these values reveal that there were some surface discontinuties around the indenter which enhanced plastic deformation so that higher indentation areas, i.e. lower hardness values, were obtained. Therefore, it is possible that these features were micro-cracks since stresses are mainly concentrated at the tips of these cracks, giving plastic deformation initially mm Sample Size 6 8 Mean Variance Table 3. Statistics of the hardness values of mm and mm Sample Size 6 6 Mean Variance

4 In addition to the existence of microcracks, the deviation among the hardness values of the (Tables 1 to 3) are quite lower than the one among. The higher deviation in the indicates that the clinker has a wide distribution of surface textures, most probably due to its porous nature. In fact, Figs. 3-4 show that the are quite porous. However, these pores could have been eliminated by high compression field inside the (Figs. 5 and 6). In fact, Walsh (1966) studied the effect of pores and cracks on physical properties of rocks, and discussed that hydrostatic pressures (not exactly, but close to the situation inside ) could have higher effect on pores than on cracks, since the increasing pressure on pore walls could eventually fail the particle, whereas cracks will merely close under increasing hydrostatic pressure. Still, the induced micro-cracks, which were not differentiated in the reflected microscope, seemed to be quite effective than the pore network in terms of weakening. Figure 4. A microscopic image from (alite and belite in dark grey or pale brown color, undifferentiated ferrite and aluminate matrix, pores in bright form, severe number of pores, magnification: 50x) Figure 5. A microscopic image from (alite and belite in pale blue, undifferentiated ferrite and aluminate matrix, scarce number of pores, magnification: 20x) Figure 3. A microscopic image from (alite and belite in pale blue color, undifferentiated ferrite and aluminate matrix, pores in bright form, severe number of pores, magnification: 20x) Figure 6. A microscopic image from (alite and belite in blue or dark grey color, undifferentiated ferrite and aluminate matrix, pores in bright

5 form, scarce number of pores, magnification: 50x) CONCLUSIONS The micro-hardness of clinker particles stressed in were quite lower than that of non-compressed clinker. This indicates the existence of micro-cracks. Oliver, W. C., & Pharr, G. M. (1992). An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Journal of Materials Research, 7(6), Walsh, J. B. (1966). Cracks and pores in rocks. In Proceedings of the First Congress of the International Society of Rock Mechanics (pp ). Lisbon. might eliminate severe porous network in the clinker ; yet, the generated micro-cracks are more effective than these porous network of clinker in terms of weakening. ACKNOWLEDGEMENT The authors would like to acknowledge the Central Lab and Mining Engineering Department at METU; and also Dr. Akın Geven for his contribution to this study. REFERENCES De, A. K. (1995). Modeling and optimization of fine grinding of minerals in high-pressure roll mill - ball mill hybrid comminution circuits. ProQuest Dissertations and Theses. University of California, Berkeley, Ann Arbor. Retrieved from 521?accountid=13014 Fuerstenau, D. W., Kapur, P. C., Schoenert, K., & Marktscheffel, M. (1990). Comparison of energy consumption in the breakage of single particles in a rigidly mounted roll mill with ball mill grinding. International Journal of Mineral Processing, 28((1-2)), Fuerstenau, D. W., Lutch, J. J., & De, a. (1999). The effect of ball size on the energy efficiency of hybrid high-pressure roll mill/ball mill grinding. Powder Technology, 105(1-3), Fuerstenau, D. W., & Vazquez-Favela, J. (1997). On assessing and enhancing the energy efficiency of comminution processes. Minerals and Metallurgical Processing,, 14(1),