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Suspension rheology during wet comminution in planetary ball mill
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Suspension rheology during wet comminution in planetary ball mill ˇ zek, ˇ c ∗ , Krunoslav Zi ˇ Gordana Matijasi´ Antun Glasnovi´c University of Zagreb, Faculty of Chemical Engineering and Technology, Department of Mechanical and Thermal Process Engineering, Maruli´cev trg 20, 10000 Zagreb, Croatia.
a r t i c l e
i n f o
a b s t r a c t
Article history:
Obtaining and controlling desirable system behavior in terms of rheology is important issue
Received 20 July 2007
in many engineering problems.
Accepted 14 November 2007
This study monitors rheological behavior of dolomite suspensions produced by wet comminution in planetary ball mill. Dolomite has a number of uses, in agriculture, steel making, glass making, ceramic industry, pharmaceutical industry, etc. Therefore, it is very interesting
Keywords: Planetary ball mill
for investigation. Rheological behavior of attained suspensions was characterized as strongly pseudoplastic
Suspension rheology
whereas the pseudoplasticity degree was connected to particle size distribution. Rheological
Wet comminution
properties of the product become equal after the effective comminution time regardless of primary particle properties before comminution. Therefore, the comminution in a highly energetic mill such as planetary ball mill, within short time interval will induce development of systems with the same granulometric and consequently the same rheological properties regardless of the primary system properties before the comminution. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Comminution has been used extensively in ceramic, mineral, pharmaceutical, cosmetic, food and other industries for a quite some time (Choi et al., 2001; Kotake et al., 2002; Ipek et al., 2005; He et al., 2004; Fuerstenau and Abouzeid, 2002; Vital et al., 2008). Because of greater process efficiency, comminution in tumbling mill is usually conducted with the presence of liquid medium (Tangsathitkulchai, 2002, 2003; Yekeler et al., 2001). Controlling wet comminution process through application of versatile process parameters could yield product of the desired properties, e.g. suspension viscosity, rheological behavior, which is a crucial for further suspension handling. The rheology and processing properties of suspensions depends on many factors such as: process conditions (mill rotational speed, number of balls, pot volume), solid content, temperature, pH, dispersant concentration, particle size dis-
tribution (Ferguson and Kembłowski, 1991; Barnes et al., 1989; He et al., 2004; Bhattacharya et al., 1998) and so on. Actually, all these influences can be categorized into three groups: liquid, particle and mill parameters. Many studies are focused on the influence of one of these groups of parameters on rheology or product quality (Olhero and Ferreira, 2004; Yekeler et al., 2001; He et al., 2006.). This work deals with so called “particle group parameters”. In order to investigate rheology of suspensions produced during wet comminution in planetary ball mill, five polydisperse samples and twelve monodisperse dolomite samples were used. Besides that, to eliminate other effects (liquid and mill group) experiments were performed using one volume solid content, same ball diameter and material, number of balls and rotational speed of planetary ball mill. pH, temperature and the purity of dolomite were monitored during comminution to insure that there were no changes in chemical composition that may affect suspension rheology.
∗ ´ trg 20, 10000 Corresponding author at: University of Zagreb, Faculty of Chemical Engineering and Technology, Marulicev Zagreb, Croatia. ´ E-mail address: [email protected] (G. Matijaˇsic). 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.11.013
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Nomenclature dv/dy K n nRRSB q3 (x) t T x x63 p 0
shear rate (s−1 ) consistency index (Pa sn ) flow behavior index parameter of RRSB function particle size distribution time (min) temperature (◦ C) particle diameter (m) parameter of RRSB function (m) plastic viscosity from Bingham model (Pa s) shear stress (Pa) extrapolated Bingham yield stress (Pa)
Table 1 – RRSB function parameters for polydisperse dolomite samples before comminution
P1 P2 P3 P4 P5
2.
nRRSB
x63 (m)
1.29 3.36 0.93 2.17 3.97
1412 1628 780 1320 253
Methods and materials
Dolomite samples were derived from Samoborka d.d., Croatia. Material mass was discretisized in 12 size intervals: 3350–2360 m (M1); 2360–1700 m (M2); 1700–1180 m (M3); 1180–850 m (M4); 850–600 m (M5); 600–425 m (M6); 425–300 m (M7); 300–212 m (M8); 212–150 m (M9); 150–106 m (M10); 106–75 m (M11) and 75–0 m (M12) with series of sieves (ASTM E11-95 standard). Rheological behavior of attained suspensions was also analyzed through comminution of five different polydisperse samples noted as P1, P2, P3, P4 and P5. Particle size distribution of those samples was characterized using RRSB distribution function (Table 1.) Polydisperse samples were prepared using different proportions of monodisperse fractions. Dolomite size fraction M1 was ground at different volume fractions (20, 30, 40 and 50 vol% of solid) using water as liquid medium. Twelve dolomite one-size fractions and five polydisperse samples were grounded separately in planetary ball mill at 200 rpm for 25 min at 40 vol% of solid. Polydisperse samples were also grounded at 300 rpm. Comminution was conducted in planetary ball mill, Fritsch GmbH, Pulverisette 6. Process data and mill specifications are shown in Table 2. Particle size distribution of ground samples was determined using laser diffraction method with Mastersizer 2000,
Table 2 – Process conditions and characteristics of planetary ball mill (Pulverisette 6, Fritsch GmbH) Pot and ball material Pot volume Revolution speed (operational) Pot diameter Disc diameter Ball diameter Number of balls Rotation/revolution ratio
Corundum, 99.7% Al2 O3 500 mL 200 rpm, 300 rpm 10 cm 13.5 cm 20 mm 25 0.82
385
Malvern Instruments. During the laser diffraction measurement, particles are passed through a focused laser beam. These particles scatter light at an angle that is inversely proportional to their size. The angular intensity of the scattered light is then measured by a series of photosensitive detectors that enable measurements in range of 0.02–2000 m. Particle size distribution is calculated according to Mie Theory (Cadle, 1965). Rheological behavior was determined on the basis of rheological diagrams obtained with Brookfield DV-III+ rheometer and expressed in parameters of corresponding rheological behavior model. The principle of operation of the DV-III+ rheometer is to drive a spindle immersed in the test suspension through a calibrated spring. Viscous drag of the fluid against the spindle causes the spring to deflect, and this deflection is correlated with torque. Conversion factors are needed to calculate viscosity from the measured torque, and are typically precalibrated for specific tool and container geometries. (Brookfield Engineering Laboratories, Inc., Manual No. M/98-211-A0701). The calculated shear rate depends on the rotation speed, the tool geometry, and the size and shape of the sample container. Rheological data was obtained using small sample adapter consisting a cylindrical sample chamber no. 13R and cylindrical spindle model SC4-21 suitable for measuring suspension rheology. Such combination provides a defined geometry system, and enables measurements of small sample volumes (8 mL). A powder diffractometer (Philips PW1830) was used for structure characterization. The X-ray diffractograms were obtained using Cu K␣ radiation (at 40 kV and 30 mA). Silicon (5 mass%) was added to all samples as internal standard correction.
3.
Results and discussion
Preliminary determination of, so called, optimal volume fraction of solid was conducted using only M1 dolomite size fraction. Suspensions with 20, 30, 40 and 50 vol% of solid were prepared and ground for maximal period of 30 min. During that period mill was stopped several times and particle size distribution was characterized. To avoid material loss and experimental error, for every period during those 30 min, a new batch (suspension with same solid volume content of monodisperse sample) was prepared. Particle size distribution of ground samples was expressed in terms of median size obtained using Process Tools program, Aspen Tech. Fig. 1 illustrates changes of sample median size during comminution process at 200 rpm with different solid volume fractions. Result comparison showed that the smallest median size was derived using 40 vol% of solid during 30 min comminution period. Within 15–25 min only, lower median size was achieved using 50 vol% of solid. Such trend was not expected but probable explanation was found in previous research of grinding kinetics. According to Tangsathitkulchai (2003) specific breakage rate of some materials is considerably influenced by solid content. That study showed that solid volume content around 45% is the inflection point and above that point specific breakage rate significantly decrease with higher solid volume content. Further comminution at 50% solid volume content resulted with rapid slowing down of process. Fuerstenau and Abouzeid (2002) explained why water content is significant parameter in wet comminution process. Water is adsorbed
386
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Fig. 1 – Change of median size of ground M1 sample with comminution time at different solid volume fractions.
on the newly created surface and prevents the fines from agglomerating. Smaller water content will result with earlier occurrence of agglomerating phenomena. On the other hand, at higher water content (20 and 30 vol% of solid) water will adsorb on the particle surface resulting uniform stress distribution across the surface and obstructive particle breakage. Therefore, further research was carried out at 40 vol% of solid. Suspensions with 12 monodisperse samples, denoted as M1 to M12, were prepared in order to determine rheological behavior and contribution of these one-size fractions to final rheology of polydisperse samples. Rheological behavior was characterized using power law model: =K
dv n dy
(1)
Bingham model was used to extrapolate yield stress: = 0 +
dv dy
(2)
Rheological diagrams for monodisperse samples are shown in Fig. 2. Ground polydisperse samples also showed pseudoplastic behavior (Fig. 3.) Rheological parameters obtained from power law model are shown in Table 3. Table 4 contains extrapolated values
Fig. 3 – Rheological diagrams of ground polydisperse samples.
of yield stress obtained according to Bingham model (Eq. (2)). Transition from pseudoplasticity to Bingham plasticity occurs above shear rate 40 s−1 (Fig. 4). Such extrapolation is common and reported by Ferguson and Kembłowski (1991), Barnes et al. (1989), Theng and Wells (1995), Tangsathitkulchai (2003) and other authors. Rheological diagrams with corresponding rheological parameters both induce that all the samples, except M1 (largest size) and M12 (the smallest one) under the same process conditions will produce suspensions of almost identical rheological properties. Finally, product properties are equal after the effective time regardless of primary particle properties before comminution. To analyze factors that may influence rheological behavior of suspensions, pH and temperature of suspensions before and after comminution were measured. Fig. 5 clearly shows that there were no significant changes in temperature during comminution period. Temperature of suspensions after 25 min will rise only 5 ◦ C at 200 rpm what is insufficient to
Table 3 – Parameters of pseudoplastic rheological model used to describe rheological behavior of obtained suspensions n
Fig. 2 – Rheological diagrams of 12 ground monodisperse samples.
K (Pa sn )
Monodisperse samples M1 0.26 M2 0.24 M3 0.23 M4 0.24 M5 0.23 M6 0.20 M7 0.21 M8 0.23 M9 0.21 M10 0.24 M11 0.22 M12 0.19
1.47 2.24 2.64 2.73 2.90 2.67 2.64 2.18 2.62 2.44 2.34 4.60
Polydisperse samples P1 0.21 P2 0.22 P3 0.23 P4 0.21 P5 0.21
2.90 2.86 2.64 2.89 2.94
Average values
n = 0.225; K = 2.54 Pa sn
n = 0.216; K = 2.85 Pa sn
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
387
Table 4 – Parameters of Bingham rheological model used to evaluate shear yield stress of obtained suspensions 0 (Pa)
p (Pa s)
Monodisperse samples M1 3.1 M2 4.4 M3 5.1 M4 5.6 M5 5.8 M6 4.8 M7 5.0 M8 4.3 M9 4.9 M10 5.1 M11 4.4 M12 8.1
0.016 0.021 0.022 0.023 0.023 0.018 0.019 0.018 0.019 0.022 0.018 0.024
Polydisperse samples P1 5.4 P2 5.4 P3 5.2 P4 5.4 P5 5.5
0.020 0.021 0.021 0.020 0.022
Average values
0 = 4.9 Pa; p = 0.020 Pa s
0 = 5.4 Pa; p = 0.021 Pa s
Fig. 6 – pH changes during wet comminution in planetary ball mill.
Fig. 4 – Extrapolation of Bingham yield stress.
Fig. 7 – X-ray diffractograms of the dolomite samples grounded for time indicated on graph.
Fig. 5 – Temperature changes during wet comminution in planetary ball mill.
cause the change in rheological behavior. To avoid even those small changes and also settling of suspension in rheometer, suspensions were left to achieve room temperature (24 ± 2 ◦ C), stirred and then the rheological diagrams were obtained. The change of suspension pH during comminution was significant. All suspensions after 25 min comminution period, irrespectively of starting particle size before comminution, resulted with pH around 9 (Fig. 6). Gradual increase of pH could be result of traces of mineral impurities, which were liberated during
388
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Fig. 10 – Particle size distribution given at different time and mill rotational speed for polydisperse sample P1.
Fig. 8 – Particle size distribution of ground samples: (a) monodisperse (b) polydisperse.
comminution as the specific surface is increasing. Characteristic odor would suggest traces of some sulphide ore minerals that were liberated in the form of hydrogen sulphide. It is well known that dolomite in water is subjected to incongruent dissolution with calcite precipitation and the release of magnesium into solution (dedolomitization). However, dedolomitization mostly occurs in groundwater as reported by many authors (Drewer, 1997; Stumm and Morgan, 1981; Singurindy and Berkowitz, 2004). Since the experiments were conducted in the water environment, X-ray diffractograms were provided using powder diffractometer in order
Fig. 9 – Rheological diagrams given at different time and mill rotational speed for polydisperse sample P1.
Fig. 11 – Thixotropic behavior of suspension obtained after 25 min at 300 rpm (polydisperse sample P1).
to examine dolomite purity after the comminution process (Fig. 7a). The diffractogram of the original powders indicates that samples in all size fractions are composed of pure dolomite. Comminution of monosized fractions did not change purity of dolomite samples as there were no diffraction maximums of any other compounds but dolomite (Fig. 7b). Diffraction intensities of dolomite slightly decreased after 25 min of comminution and are mostly distinguishable for the (1 0 4) crystal lattice face as is increase in integral breadth of the peaks. These changes are attributed to the increase of lattice defaults since it is unlikely for amorphization to occur during wet comminution (Garcia et al., 2002). According to factors analyzed it is obvious that particle size distribution of ground samples influence and determine rheological behavior of suspension. Fig. 8a and b show particle size distribution of ground samples after 25 min. Those figures are analogue to Figs. 2 and 3 that are representing rheological diagrams. It can be seen (Fig. 8a) that particle size distribution of M1 and M12 samples are boundary while all other monodisperse samples gave particle size distribution within those margins as well as rheological behavior. On the other hand, all polydisperse samples resulted with same particle size distribution and same rheological behavior.
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Therefore, if there is demand for higher suspension viscosities it is necessary to increase comminution time or mill rotational speed rather than decrease initial particle size. For confirmation of these results polydisperse samples were ground for 30 and 35 min at 200 rpm and for 25 min at 300 rpm. Figs. 9 and 10 show comparison of those results in form of rheological diagrams and particle size distribution. It is evident that increase in revolution speed to 300 rpm (intensification of comminution conditions) would yield the product with smaller particle sizes and also higher viscosities. Nevertheless, if particle size distribution of such suspensions has a higher content of smaller particle sizes, it is essential to examine the stability of suspension and also tendency to thixotropic behavior (Bournonville and Nzihou, 2002; Usui et al., 2001). Fig. 11 clearly shows that suspension obtained after 25 min at 300 rpm for polydisperse sample P1 has a thixotropic behavior which can negatively effect suspension processing.
4.
Conclusions
Rheological behavior of attained suspensions can be described as pseudoplastic. Degree of suspension pseudoplasticity and their viscosities are consequence of achieved particle size distribution. Under given conditions, regardless of initial particle size before comminution, only negligible differences in rheological behavior can be obtained. To achieve higher values of suspension viscosities it is necessary to increase comminution time or mill rotational speed. X-ray diffractograms of ground samples showed that the purity of dolomite remained the same after 25 min of comminution, while only small changes in lattice defaults can be notices through changes of intensity and breadth of diffraction maximums.
references
Barnes, H.A., Hutton, J.F. and Walters, K., (1989). An Introduction to Rheology. (Elsevier Science Publisher B.V, Amsterdam, Netherlands). Bhattacharya, I.N., Panda, D. and Bandopadhyay, P., 1998, Rheological behaviour of nickel laterite suspensions. Int J Miner Process, 53: 251–263. Bournonville, B. and Nzihou, A., 2002, Rheology of non-Newtonian suspensions of fly ash: effect of concentration, yield stress and hydrodynamic interactions. Powder Technol, 128: 148–158.
389
Brookfield Engineering Laboratories, Inc., Manual No. M/98-211-A0701. Cadle, R.C., (1965). Particle size, Theory and Industrial Applications. (Reinhold Publishing Corporation, New York). Choi, W.S., Chung, H.Y., Yoon, B.R. and Kim, S.S., 2001, Applications of grinding kinetics analysis to fine grinding characteristics of some inorganic materials using a composite grinding media by planetary ball mill. Powder Technol, 115: 209–214. Drewer, J.I., (1997). The Geochemistry of Natural Waters. (Prentice-Hall Inc, Englewood Cliffs, New Jersey, USA). Ferguson, J. and Kembłowski, Z., (1991). Applied Fluid Rheology. (Elsevier Science Publisher Ltd, London, England). Fuerstenau, D.W. and Abouzeid, A.-Z.M., 2002, The energy efficiency of ball milling in comminution. Int J Miner Process, 67: 161–185. Garcia, F., Le Bolay, N. and Frances, C., 2002, Changes of surface and volume properties of calcite during a batch wet grinding process. Chem Eng J, 85: 177–187. He, M., Wang, Y. and Forssberg, E., 2004, Slurry rheology in wet ultrafine grinding of industrial minerals: a review. Powder Technol, 147: 94–112. He, M., Wang, Y. and Forssberg, E., 2006, Parameters effects on wet ultrafine grinding of limestone through slurry rheology in a stirred medial mill. Powder Technol, 161: 10–21. Ipek, H., Ucbas¸, Y., Yekeler, M. and Hos¸ten, C¸., 2005, Dry grinding kinetics of binary mixtures of ceramic raw materials by Bond milling. Ceram Int, 31: 1065–1071. Kotake, N., Suzuki, K., Asahi, S. and Kanda, Y., 2002, Experimental study on the grinding rate constant of solid materials in a ball mill. Powder Technol, 122: 101–108. Olhero, S.M. and Ferreira, J.M.F., 2004, Influence of particle size distribution on rheology and particle packing of silica-based suspensions. Powder Technol, 139: 69–75. Stumm, W. and Morgan, J.J., (1981). Aquatic Chemistry. (Wiley, New York, USA). Singurindy, O. and Berkowitz, B., 2004, Dedolomitization and flow in fractures. Geophys Res Lett, 31: L24501. doi:10.1029/2004GL021594 Tangsathitkulchai, C., 2003, The effect of slurry rheology on fine grinding in a laboratory ball mill. Int J Miner Process, 69: 29–47. Tangsathitkulchai, C., 2002, Acceleration of particle breakage rates in wet batch ball milling. Powder Technol, 124: 67–75. Theng, B.K.G. and Wells, N., 1995, The flow characteristics of halloysite suspensions. Clay Miner, 30: 99–106. Usui, H., Kishimoto, K. and Suzuki, H., 2001, Non-Newtonian viscosity of dense slurries prepared by spherical particles. Chem Eng Sci, 56: 2979–2989. Yekeler, M., Ozkan, A. and Austin, L.G., 2001, Kinetics of fine wet grinding in a laboratory ball mill. Powder Technol, 114: 224–228. ¨ Vital, A., Zurcher, S., Dittmann, R., Trottmann, M., Lienemann, P., ¨ Bommer, B., Graule, T., Apel, E. and Holand, W., 2008, Ultrafine comminution of dental glass in a stirred media mill. Chem Eng Sci, 63(2): 484–494.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Suspension rheology during wet comminution in planetary ball mill ˇ zek, ˇ c ∗ , Krunoslav Zi ˇ Gordana Matijasi´ Antun Glasnovi´c University of Zagreb, Faculty of Chemical Engineering and Technology, Department of Mechanical and Thermal Process Engineering, Maruli´cev trg 20, 10000 Zagreb, Croatia.
a r t i c l e
i n f o
a b s t r a c t
Article history:
Obtaining and controlling desirable system behavior in terms of rheology is important issue
Received 20 July 2007
in many engineering problems.
Accepted 14 November 2007
This study monitors rheological behavior of dolomite suspensions produced by wet comminution in planetary ball mill. Dolomite has a number of uses, in agriculture, steel making, glass making, ceramic industry, pharmaceutical industry, etc. Therefore, it is very interesting
Keywords: Planetary ball mill
for investigation. Rheological behavior of attained suspensions was characterized as strongly pseudoplastic
Suspension rheology
whereas the pseudoplasticity degree was connected to particle size distribution. Rheological
Wet comminution
properties of the product become equal after the effective comminution time regardless of primary particle properties before comminution. Therefore, the comminution in a highly energetic mill such as planetary ball mill, within short time interval will induce development of systems with the same granulometric and consequently the same rheological properties regardless of the primary system properties before the comminution. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Comminution has been used extensively in ceramic, mineral, pharmaceutical, cosmetic, food and other industries for a quite some time (Choi et al., 2001; Kotake et al., 2002; Ipek et al., 2005; He et al., 2004; Fuerstenau and Abouzeid, 2002; Vital et al., 2008). Because of greater process efficiency, comminution in tumbling mill is usually conducted with the presence of liquid medium (Tangsathitkulchai, 2002, 2003; Yekeler et al., 2001). Controlling wet comminution process through application of versatile process parameters could yield product of the desired properties, e.g. suspension viscosity, rheological behavior, which is a crucial for further suspension handling. The rheology and processing properties of suspensions depends on many factors such as: process conditions (mill rotational speed, number of balls, pot volume), solid content, temperature, pH, dispersant concentration, particle size dis-
tribution (Ferguson and Kembłowski, 1991; Barnes et al., 1989; He et al., 2004; Bhattacharya et al., 1998) and so on. Actually, all these influences can be categorized into three groups: liquid, particle and mill parameters. Many studies are focused on the influence of one of these groups of parameters on rheology or product quality (Olhero and Ferreira, 2004; Yekeler et al., 2001; He et al., 2006.). This work deals with so called “particle group parameters”. In order to investigate rheology of suspensions produced during wet comminution in planetary ball mill, five polydisperse samples and twelve monodisperse dolomite samples were used. Besides that, to eliminate other effects (liquid and mill group) experiments were performed using one volume solid content, same ball diameter and material, number of balls and rotational speed of planetary ball mill. pH, temperature and the purity of dolomite were monitored during comminution to insure that there were no changes in chemical composition that may affect suspension rheology.
∗ ´ trg 20, 10000 Corresponding author at: University of Zagreb, Faculty of Chemical Engineering and Technology, Marulicev Zagreb, Croatia. ´ E-mail address: [email protected] (G. Matijaˇsic). 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.11.013
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Nomenclature dv/dy K n nRRSB q3 (x) t T x x63 p 0
shear rate (s−1 ) consistency index (Pa sn ) flow behavior index parameter of RRSB function particle size distribution time (min) temperature (◦ C) particle diameter (m) parameter of RRSB function (m) plastic viscosity from Bingham model (Pa s) shear stress (Pa) extrapolated Bingham yield stress (Pa)
Table 1 – RRSB function parameters for polydisperse dolomite samples before comminution
P1 P2 P3 P4 P5
2.
nRRSB
x63 (m)
1.29 3.36 0.93 2.17 3.97
1412 1628 780 1320 253
Methods and materials
Dolomite samples were derived from Samoborka d.d., Croatia. Material mass was discretisized in 12 size intervals: 3350–2360 m (M1); 2360–1700 m (M2); 1700–1180 m (M3); 1180–850 m (M4); 850–600 m (M5); 600–425 m (M6); 425–300 m (M7); 300–212 m (M8); 212–150 m (M9); 150–106 m (M10); 106–75 m (M11) and 75–0 m (M12) with series of sieves (ASTM E11-95 standard). Rheological behavior of attained suspensions was also analyzed through comminution of five different polydisperse samples noted as P1, P2, P3, P4 and P5. Particle size distribution of those samples was characterized using RRSB distribution function (Table 1.) Polydisperse samples were prepared using different proportions of monodisperse fractions. Dolomite size fraction M1 was ground at different volume fractions (20, 30, 40 and 50 vol% of solid) using water as liquid medium. Twelve dolomite one-size fractions and five polydisperse samples were grounded separately in planetary ball mill at 200 rpm for 25 min at 40 vol% of solid. Polydisperse samples were also grounded at 300 rpm. Comminution was conducted in planetary ball mill, Fritsch GmbH, Pulverisette 6. Process data and mill specifications are shown in Table 2. Particle size distribution of ground samples was determined using laser diffraction method with Mastersizer 2000,
Table 2 – Process conditions and characteristics of planetary ball mill (Pulverisette 6, Fritsch GmbH) Pot and ball material Pot volume Revolution speed (operational) Pot diameter Disc diameter Ball diameter Number of balls Rotation/revolution ratio
Corundum, 99.7% Al2 O3 500 mL 200 rpm, 300 rpm 10 cm 13.5 cm 20 mm 25 0.82
385
Malvern Instruments. During the laser diffraction measurement, particles are passed through a focused laser beam. These particles scatter light at an angle that is inversely proportional to their size. The angular intensity of the scattered light is then measured by a series of photosensitive detectors that enable measurements in range of 0.02–2000 m. Particle size distribution is calculated according to Mie Theory (Cadle, 1965). Rheological behavior was determined on the basis of rheological diagrams obtained with Brookfield DV-III+ rheometer and expressed in parameters of corresponding rheological behavior model. The principle of operation of the DV-III+ rheometer is to drive a spindle immersed in the test suspension through a calibrated spring. Viscous drag of the fluid against the spindle causes the spring to deflect, and this deflection is correlated with torque. Conversion factors are needed to calculate viscosity from the measured torque, and are typically precalibrated for specific tool and container geometries. (Brookfield Engineering Laboratories, Inc., Manual No. M/98-211-A0701). The calculated shear rate depends on the rotation speed, the tool geometry, and the size and shape of the sample container. Rheological data was obtained using small sample adapter consisting a cylindrical sample chamber no. 13R and cylindrical spindle model SC4-21 suitable for measuring suspension rheology. Such combination provides a defined geometry system, and enables measurements of small sample volumes (8 mL). A powder diffractometer (Philips PW1830) was used for structure characterization. The X-ray diffractograms were obtained using Cu K␣ radiation (at 40 kV and 30 mA). Silicon (5 mass%) was added to all samples as internal standard correction.
3.
Results and discussion
Preliminary determination of, so called, optimal volume fraction of solid was conducted using only M1 dolomite size fraction. Suspensions with 20, 30, 40 and 50 vol% of solid were prepared and ground for maximal period of 30 min. During that period mill was stopped several times and particle size distribution was characterized. To avoid material loss and experimental error, for every period during those 30 min, a new batch (suspension with same solid volume content of monodisperse sample) was prepared. Particle size distribution of ground samples was expressed in terms of median size obtained using Process Tools program, Aspen Tech. Fig. 1 illustrates changes of sample median size during comminution process at 200 rpm with different solid volume fractions. Result comparison showed that the smallest median size was derived using 40 vol% of solid during 30 min comminution period. Within 15–25 min only, lower median size was achieved using 50 vol% of solid. Such trend was not expected but probable explanation was found in previous research of grinding kinetics. According to Tangsathitkulchai (2003) specific breakage rate of some materials is considerably influenced by solid content. That study showed that solid volume content around 45% is the inflection point and above that point specific breakage rate significantly decrease with higher solid volume content. Further comminution at 50% solid volume content resulted with rapid slowing down of process. Fuerstenau and Abouzeid (2002) explained why water content is significant parameter in wet comminution process. Water is adsorbed
386
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Fig. 1 – Change of median size of ground M1 sample with comminution time at different solid volume fractions.
on the newly created surface and prevents the fines from agglomerating. Smaller water content will result with earlier occurrence of agglomerating phenomena. On the other hand, at higher water content (20 and 30 vol% of solid) water will adsorb on the particle surface resulting uniform stress distribution across the surface and obstructive particle breakage. Therefore, further research was carried out at 40 vol% of solid. Suspensions with 12 monodisperse samples, denoted as M1 to M12, were prepared in order to determine rheological behavior and contribution of these one-size fractions to final rheology of polydisperse samples. Rheological behavior was characterized using power law model: =K
dv n dy
(1)
Bingham model was used to extrapolate yield stress: = 0 +
dv dy
(2)
Rheological diagrams for monodisperse samples are shown in Fig. 2. Ground polydisperse samples also showed pseudoplastic behavior (Fig. 3.) Rheological parameters obtained from power law model are shown in Table 3. Table 4 contains extrapolated values
Fig. 3 – Rheological diagrams of ground polydisperse samples.
of yield stress obtained according to Bingham model (Eq. (2)). Transition from pseudoplasticity to Bingham plasticity occurs above shear rate 40 s−1 (Fig. 4). Such extrapolation is common and reported by Ferguson and Kembłowski (1991), Barnes et al. (1989), Theng and Wells (1995), Tangsathitkulchai (2003) and other authors. Rheological diagrams with corresponding rheological parameters both induce that all the samples, except M1 (largest size) and M12 (the smallest one) under the same process conditions will produce suspensions of almost identical rheological properties. Finally, product properties are equal after the effective time regardless of primary particle properties before comminution. To analyze factors that may influence rheological behavior of suspensions, pH and temperature of suspensions before and after comminution were measured. Fig. 5 clearly shows that there were no significant changes in temperature during comminution period. Temperature of suspensions after 25 min will rise only 5 ◦ C at 200 rpm what is insufficient to
Table 3 – Parameters of pseudoplastic rheological model used to describe rheological behavior of obtained suspensions n
Fig. 2 – Rheological diagrams of 12 ground monodisperse samples.
K (Pa sn )
Monodisperse samples M1 0.26 M2 0.24 M3 0.23 M4 0.24 M5 0.23 M6 0.20 M7 0.21 M8 0.23 M9 0.21 M10 0.24 M11 0.22 M12 0.19
1.47 2.24 2.64 2.73 2.90 2.67 2.64 2.18 2.62 2.44 2.34 4.60
Polydisperse samples P1 0.21 P2 0.22 P3 0.23 P4 0.21 P5 0.21
2.90 2.86 2.64 2.89 2.94
Average values
n = 0.225; K = 2.54 Pa sn
n = 0.216; K = 2.85 Pa sn
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
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Table 4 – Parameters of Bingham rheological model used to evaluate shear yield stress of obtained suspensions 0 (Pa)
p (Pa s)
Monodisperse samples M1 3.1 M2 4.4 M3 5.1 M4 5.6 M5 5.8 M6 4.8 M7 5.0 M8 4.3 M9 4.9 M10 5.1 M11 4.4 M12 8.1
0.016 0.021 0.022 0.023 0.023 0.018 0.019 0.018 0.019 0.022 0.018 0.024
Polydisperse samples P1 5.4 P2 5.4 P3 5.2 P4 5.4 P5 5.5
0.020 0.021 0.021 0.020 0.022
Average values
0 = 4.9 Pa; p = 0.020 Pa s
0 = 5.4 Pa; p = 0.021 Pa s
Fig. 6 – pH changes during wet comminution in planetary ball mill.
Fig. 4 – Extrapolation of Bingham yield stress.
Fig. 7 – X-ray diffractograms of the dolomite samples grounded for time indicated on graph.
Fig. 5 – Temperature changes during wet comminution in planetary ball mill.
cause the change in rheological behavior. To avoid even those small changes and also settling of suspension in rheometer, suspensions were left to achieve room temperature (24 ± 2 ◦ C), stirred and then the rheological diagrams were obtained. The change of suspension pH during comminution was significant. All suspensions after 25 min comminution period, irrespectively of starting particle size before comminution, resulted with pH around 9 (Fig. 6). Gradual increase of pH could be result of traces of mineral impurities, which were liberated during
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c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Fig. 10 – Particle size distribution given at different time and mill rotational speed for polydisperse sample P1.
Fig. 8 – Particle size distribution of ground samples: (a) monodisperse (b) polydisperse.
comminution as the specific surface is increasing. Characteristic odor would suggest traces of some sulphide ore minerals that were liberated in the form of hydrogen sulphide. It is well known that dolomite in water is subjected to incongruent dissolution with calcite precipitation and the release of magnesium into solution (dedolomitization). However, dedolomitization mostly occurs in groundwater as reported by many authors (Drewer, 1997; Stumm and Morgan, 1981; Singurindy and Berkowitz, 2004). Since the experiments were conducted in the water environment, X-ray diffractograms were provided using powder diffractometer in order
Fig. 9 – Rheological diagrams given at different time and mill rotational speed for polydisperse sample P1.
Fig. 11 – Thixotropic behavior of suspension obtained after 25 min at 300 rpm (polydisperse sample P1).
to examine dolomite purity after the comminution process (Fig. 7a). The diffractogram of the original powders indicates that samples in all size fractions are composed of pure dolomite. Comminution of monosized fractions did not change purity of dolomite samples as there were no diffraction maximums of any other compounds but dolomite (Fig. 7b). Diffraction intensities of dolomite slightly decreased after 25 min of comminution and are mostly distinguishable for the (1 0 4) crystal lattice face as is increase in integral breadth of the peaks. These changes are attributed to the increase of lattice defaults since it is unlikely for amorphization to occur during wet comminution (Garcia et al., 2002). According to factors analyzed it is obvious that particle size distribution of ground samples influence and determine rheological behavior of suspension. Fig. 8a and b show particle size distribution of ground samples after 25 min. Those figures are analogue to Figs. 2 and 3 that are representing rheological diagrams. It can be seen (Fig. 8a) that particle size distribution of M1 and M12 samples are boundary while all other monodisperse samples gave particle size distribution within those margins as well as rheological behavior. On the other hand, all polydisperse samples resulted with same particle size distribution and same rheological behavior.
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 384–389
Therefore, if there is demand for higher suspension viscosities it is necessary to increase comminution time or mill rotational speed rather than decrease initial particle size. For confirmation of these results polydisperse samples were ground for 30 and 35 min at 200 rpm and for 25 min at 300 rpm. Figs. 9 and 10 show comparison of those results in form of rheological diagrams and particle size distribution. It is evident that increase in revolution speed to 300 rpm (intensification of comminution conditions) would yield the product with smaller particle sizes and also higher viscosities. Nevertheless, if particle size distribution of such suspensions has a higher content of smaller particle sizes, it is essential to examine the stability of suspension and also tendency to thixotropic behavior (Bournonville and Nzihou, 2002; Usui et al., 2001). Fig. 11 clearly shows that suspension obtained after 25 min at 300 rpm for polydisperse sample P1 has a thixotropic behavior which can negatively effect suspension processing.
4.
Conclusions
Rheological behavior of attained suspensions can be described as pseudoplastic. Degree of suspension pseudoplasticity and their viscosities are consequence of achieved particle size distribution. Under given conditions, regardless of initial particle size before comminution, only negligible differences in rheological behavior can be obtained. To achieve higher values of suspension viscosities it is necessary to increase comminution time or mill rotational speed. X-ray diffractograms of ground samples showed that the purity of dolomite remained the same after 25 min of comminution, while only small changes in lattice defaults can be notices through changes of intensity and breadth of diffraction maximums.
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