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Influence of surfactants on the mechanical properties and comminution of wet-milled calcite
403
Powder Technology, 65 (1991) 403-410
Influence of surfactants of wet-milled calcite
G. M. Carter, J. L. Henshall
on the mechanical properties and comminution
and R. J. Wakeman
Separation Processes Centre, University of Exeter, Exeter, Devon (U.K.)
Abstract This paper reports the initial findings of an investigation to relate the influence of surfactants on the particle size distribution and rate of milling of calcite during wet milling. The results for calcite, milled with and without surfactants for a maximum of 72 h, are presented. Particle size distributions are correlated with hardness, indentation fracture toughness and scanning electron microscope observations. It is proposed that the mechanism of comminution is brittle cleavage of the particles in all cases. The most effective grinding aids are those which enhance the propensity to cleavage and which maximise branching and secondary cracking.
Introduction The grinding of minerals is widely used as a processing step for the production of ceramics, glass, cement, pigments, ores for refinement, etc. [ 11. However, within this process only an estimated 1% of the total energy input is accounted for in the creation of new surface area. Increased rates of milling have been sought through optimisation of mill designs, by providing favourable physical conditions within the mill and by the use of grinding aids. Initially, an enhanced rate of comminution may be achieved by wet rather than dry grinding. Thus, the dispersing medium itself may be considered to be an aid. However, the term ‘grinding aid’ usually refers to some chemical additive, introduced into the feedstock of a mill, that produces more efficient grinding of the charge [2-71. Typically, a grinding aid will be some organic compound that modifies the surface chemistry of the mineral. Grinding aids are, in the extreme, believed to enhance grinding efficiency either by modifying the rheological characteristics of the mill charge or by assisting the fracture process itself. The latter theory is often condemned because of the large difference in the velocity of a propagating crack and that of diffusing molecules [5], assuming that any chemomechanical effects are restricted to the immediate vicinity of a propagating crack tip. However, the influence of surface active species on mechanical properties of
0032-5910/91/%3.50
inorganic solids, the Rehbinder effect, has received increasing attention following Rehbinder’s work on drilling fluids [8,9]. At the other ex-
treme, surfactants have been suggested to function by reducing viscosity, by preventing agglomeration of small particles or by alteration of the enthalpy content of the charge. This paper reports the preliminary findings of an investigation to quantify a range of commercially available surfactants as potential grinding aids, with a view to identifying the mechanisms for any increase in efficiency, whether by modification to the rheological properties, by assisting fracture through surface free energy changes, or by whatever means.
Experimental
procedures
Grinding
The calcite used in these grinding experiments was precipitated reagent grade and was washed with single- and then double-distilled water to give a constant value of ionic conductance measured on the supernatant prior to grinding. All laboratory ware used in the preparation of the mill charges was soaked overnight in an anionic detergent and then washed with single- and then double-distilled water to give a constant value of ionic conductance. The mill jars used were porcelain, 2.5 litre capacity. These were charged with 1 kg of cleansed 12-mm diameter porcelain balls,
0
Elsevier
Sequoia/Printed
in The Netherlands
404 TABLE 1 Summary of aqueous surfactant additives used in grinding experiments Surfactant
Classification
Supplier
Dispex N40
Sodium salt of polycarboxylic acid
Allied Colloids
FC171
Fluorinated alkyl alkoxylate
3M
FC170c
Fluorinated alkyl polyoxethylene ethanols
3M
250 g of washed calcite, 700 g of double-distilled
water and 2.5 g of surfactant. In addition, one jar was washed, dried and then loaded with 250 g of dry calcite. The jars were rotated at 96 rpm and small samples withdrawn at intervals of 1, 2, 4, 8 and 72 h. The three surfactants used are all commercially available and are listed in Table 1. The fluorocarbons were selected for their ability to decrease the aqueous interfacial surface energy of materials to very low levels. The polycarboxylic acid salt is a surfactant recommended by the supplier as a grinding aid for calcite. Particle size measurements were performed using a Malvern Mastersizer laser diffraction system with an appropriate lens fitted to allow measurements in the range 0.1 to 80 pm. The data were analysed using the supplied software with values of 1.66 for the refractive index, 0.1 for the absorption and ‘presentation value’ of 1 207 as recommended by Malvern Instruments. Indentation Hardness and Toughness
Parallel-faced calcite single crystal specimens were cleaved from a large parent crystal and indented either in air, or after immersion in double-distilled water or surfactant solutions. The solutions used were of the same concentration as those used in the grinding experiments. Initially, specimens were immersed in the surfactant solutions for several hours prior to testing. However, etching of the surface by the Dispex N40 solution made subsequent measurement of the crack length and half-diagonal impossible. Consequently, immersion times were reduced to 15 min. A Leitz Miniload 2 microhardness tester with a Vickers diamond pyramid was used for the indentation tests with an applied load of 15 g. In each case, a minimum of ten impressions was made on the as-cleaved plane with one of the indentor diagonals aligned parallel to the edge of the crystal. The lengths of the two half-diagonals, a, and the lengths of the two longest cracks, c, from the centre of the impressions to the crack tip, along the diagonal, were measured, using an optical
microscope with reflected light at x 125 magnification, with a calibrated eyepiece. Electron microscopy
The indented crystals and the dried powders were mounted on aluminium stubs and then gold coated prior to examination with a Hitachi S520 scanning electron microscope. Small samples of the powder ground in Dispex solution for 72 h were dispersed on a carbon grid and examined with a transmission electron microscope at 80 kV.
Results
Figures l(a)-(e) show the nature and extent of the deformation and cracking around the indentations. In all cases, it is apparent that the crack initiation depends upon the twinning and twin/ twin and twin/slip interactions, which have been studied previously [ 10-151. The type of cracking formed, however, is very different for the differing environments. In air, Fig. l(a), the cracks emanating from the lower apex of the indentation are clearly crystallographic in nature, whereas the major one from the right hand apex is curved. In water, Fig. l(b), both cracks are largely crystallographic in nature, and there is a marked tendency for conchoidal cracking to occur as well. In the Dispex N40, Fig. l(c), cracking is also primarily crystallographic, but the extent of crack branching and secondary cracking is much greater. In both Figs. l(d) and (e), it is apparent that the cracking is not well aligned with the indentation diagonals, but the directions are more closely related to the twinning crystallography. Table 2 gives the values of Vickers hardness (H,) at 0.147 N load, the fracture toughness K,, calculated using eqn. ( 1) (Evans [ 161)with a value of 141.8 GPa [ 171for the elastic modulus (E), and the ratios of the crack to half-diagonal lengths lcja for the series of indentation tests made in differing environments. / c \2/5 f&=Hva”2(G) x lo” where y = - 1.59 - 0.342 - 2.02~~ + 11.23~~ -24.47~~ + 16.32~’ and z = log,, (c/a). The crack lengths used were the projected values parallel to the appropriate indentation diagonal. The Vickers hardness values for the calcite single crystals are very similar for the tests in air, water and FC 171 fluorocarbon, with an average
(4
Fig. 1. Vickers diamond pyramid indentations in single crystals of calcite using 15 g normal load. The ambient environments during testing were (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC171 solution and (e) FC170c solution. The two most widely separated dots on the scale bars correspond to the distance recorded below them. TABLE 2 Vickers hardness (H, 0.015), K,,, c/a ratios and calculated critical particle sizes for calcite single crystals immersed in different environments Property
Environment Air
Water
Dispex
FC171
FC17Oc
$Pa)
1.34
1.32
1.65
1.38
1.56
$N+)
4.2 0.18
4.8 0.15
0.15 5.0
0.14 5.3
4.55 0.17
dE”t
2.71
1.94
1.24
1.54
1.78
(pm)
value of 1.35 GPa. The results of the tests in fluorocarbon FC 170~ and Dispex N40 show an increase in hardness of 15% and 22%, respectively. Overall, the effect of an aqueous environ-
ment, with or without a surfactant, is to render the calcite less resistant to fracture by indentation, as reflected in the larger c/a ratios and the lower values of K,,. Figures 2-4 show the particle size distributions for calcite ground for 1, 4 and 72 h, respectively, in (a) air, (b) double-distilled water, (c) Dispex N40, (d) FC 171 and (e) FC 17Oc, and for the unground powder, Fig. 2(f). The data are presented as histograms of the volume fraction of calcite in a given size range and the related accumulated frequency curves. Table 3 gives the median particle sizes, span calculated as the difference in the upper and lower deciles divided by the median, and the specific surface areas assuming equivalent spherical diameters. Since for the tests performed in air, the calcite coats the balls and jars, thus preventing further comminution, the results are of academic interest only and are included for comparison with the
406
Particle
Particle
Size (microns)
Size (microns)
(W
1 0-l
100
Particle
10’
102
Particle
Size (microns)
(4
a 1 Particle
Size (microns)
Size (microns)
Particle
Size (microns)
(4 Fig. 2. Histograms of volume fraction of particles in a given size range and associated plots of cumulative volume fraction trs. particle size for calcite ground for 1 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution, (e) FC17Oc solution and (t) washed but unground calcite.
aqueous values. The main features of the data presented in Figs. 2-4 and Table 2 are: (i) There is a development of a secondary ‘peak’ in particle sizes in the range 0.1-1.0 pm. This represents a small volume fraction of the total charge, but represents a relatively large number of particles. (ii) In all cases, the main peak of the histogram decreases with increasing grinding time, except for air, where little further reduction occurs after 4 h grinding. (iii) The distributions for the water and fluorocarbon solutions are broadly similar at any given grinding time. The Dispex N40, however, enables the formation of a much greater proportion of fine particles, e.g., ~2 ,um, as shown by the relatively larger specific surfaces areas, with the effect being particularly pronounced after 72 h. (iv) The span of the distributions generally appears to be less for the fluorocarbon surfactant solutions, which again is a reflection of the smaller volume fraction of material that is in the submicron regime. Figures 5(a)-(f) are SEM micrographs of the unground calcite and samples after grinding for 1 h or 2 h in the various environments. For the ground specimens, the blocky angular nature of the particles shows that the comminution process is dominated by cleavage fracture rather than plastic deformation. The different effects of the
environment, as shown in Fig. 1 for the indentations, are shown again in the details of the particle fracturing process. The surface ledges and variations shown in Fig. 5(a) are typical of those for precipitated particles. Figure 5(b) shows predominately highly facetted fractured particles for dry ground calcite. Grinding in water only, Fig. 5(c), produces particles which are primarily facetted, but with a significant number of curved fracture surfaces. Figure 5(d) shows that in this case small sub-micron particles are commonly formed during the major fracturing process. This relates to the high incidence of secondary cracking shown in Fig. l(c). Figures 5(e) and (f) show that in some grains there is clear evidence of the fracture process being related to the twin formation process. Again, this correlates with the observations of cracking around the indentations i.e., Figs. l(d) and (e).
It is generally accepted that the influence of external environment on the mechanical properties of materials was first reported by Rehbinder and co-workers [8,9]. Despite much work in this field, there is no clear single explanation of this phenomenon, althogh Westwood and Goldheim [ 181 and McMillan et al. [ 19,201 showed that there is a marked effect of the zeta potential of a
407
101
10"
1
Particle
Size
10'
1 i i 0-'
IO!
100
Partlcle
(microns)
-0 .5
Size
I 02
(micnxx)
_......o.5
10-l
100 Particle
101 Size
1OL
0-
I
i
101
100
Particle
(microns)
----,0.5
Size
0
102
I
lo-!
(microns)
(4
101
100
Particle
Size
102
(microns)
(4
Fig. 3. Histograms of volume particle size for calcite ground FC 17Oc solution.
fraction of particles in a given size range and associated plots of cumulative volume fraction vs. for 4 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution and (e)
1 ,r-----/y5
10-L
lo!
100 Partde
Size
102 Particle
(microns)
Size
(rmcrons)
(4 1
0.5
/ i
0 10-l
100 Particle
lo? Size
(microns)
102
10’
102 Particle
Size
(microns)
Particle
Size
(microns)
(4
Fig. 4. Histograms of volume particle size for calcite ground FC 170~ solution.
fraction of particles in a given size range and associated plots of cumulative volume fraction vs. for 72 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution and (e)
solution on the hardness of immersed ceramics. In the present situation, the measured Vickers hardnesses are consistent with the standard Moh’s hardness of calcite, i.e., 3. There is no difference in the hardness values in air, water and FC 171 solution, but the hardnesses in FC 170~ and
Dispex N40 are significantly greater. The plastic deformation required to accommodate the pyramida1 indentation in calcite consists of a combination of dislocation generation and motion, and twinning. Therefore, the observed increase in hardness, particularly for the Dispex N40 solution, means
408 TABLE 3 Median particle sizes (in micrometres), span ((particle size at 90 vol.% - particle size at 10 vol.%)/median), specific surface areas SSA (m2/cm3) and volume per cent of charge with particle sizes less than 2 pm (V% < 2), for calcite ground in different environments Grinding time (h) 0
Environment Parameter Median Span SSA V%<2
Air
Water Dispex FCl71
,
20.2 Y 1.3 0.610 2.0
* (
FC170c
> . >
1
Median Span SSA V%<2
11.3 11.3 11.4 1.50 2.00 2.06 1.881 1.854 1.80 7.5 9.4 9.2
2
Median Span SSA V%<2
4
Median Span SSA V%<2
9.8 9.1 8.4 1.91 1.90 2.18 2.216 2.031 2.031 10.7 11.1 13.3 7.3 6.9 6.9 6.1 5.8 1.99 2.06 1.86 2.21 2.21 2.403 2.566 2.793 2.295 2.380 14.5 14.9 13.1 17.9 20.1
8
Median Span SSA V%<2
n.a.a n.a.a n.a.” n.a.a
72
Median Span SSA V%<2
11.9 ‘11.6 1.80 1.82 1.751 1.80 8.2 8.6
9.3 8.5 1.68 1.69 2.124 1.534 9.0 13.0
5.0 4.7 4.1 3.91 2.00 1.89 2.19 2.21 3.054 3.230 2.813 2.749 21.2 21.5 26.4 26.0 1.9 1.9 5.9 1.7 1.3 1.6 1.58 2.09 1.52 1.72 2.571 4.591 6.740 4.184 4.221 60.4 61.3 17.4 69.5 77.0
?r.a. = not available.
that the surface state of the calcite is altered sufficiently in a manner such that there is a reduction in the extent of twinning, Fig. l(c). The enhanced twinning observed around the indentations immersed in FC 171 and FC 17Oc, Figs. l(d) and (e), can be explained on the basis that the twins are surface initiated, with the controlling parameter being the interfacial energy [ 12, 131. Thus, the reduction in surface energy in the fluorocarbon solutions will enable the twins to be initiated more easily. The cracks that are formed around the indentations are clearly asymmetrical and are influenced by the plastic deformation processes, in particular the degree and extent of twinning and twin interactions. There are several approaches to the determination of fracture toughness parameters from measurements of cracking around indentations [21,22]. There is no clearly defined analysis that can be applied with the present geometry of deformation and cracking, but the formula of Evans [ 161 is most commonly utilised in the analysis of single crystal toughness data [ 16,23,24]. Although the absolute error may be
as high as 30% [25], the relative precision and significance are much better, and the very obvious differences in the ratios of crack to indentation half-diagonal lengths are reflected in the K,, values. There was no significant difference in projected lengths between the two major cracks which formed from the indentations. The absolute toughness values are very low and markedly anisotropic, as would be expected from a material which cleaves well. Several mechanisms have been proposed to explain the manner in which the surface state can influence so significantly the extent of crack propagation, but these remain largely speculative. However, the present results clearly demonstrate that the environment has a marked effect. Also of significance, but not reflected in the numerical analysis, is that the Dispex N40 is mildly aggressive with regard to plastically deformed calcite. This results in the copious secondary cracking which occurs around the indentations. There are, however, no signs of etching or other forms of chemical attack on the calcite particles ground in the Dispex N40 solution. This confirms that plastic deformation, and the associated strain fields, are required for the chemical interaction between the calcite and Dispex N40 to occur. Also, since no attack takes place on even the smallest calcite particles, this indicates that these are not formed by plastic deformation. This confirms the TEM observations, Fig. 5, showing the absence of any plastic deformation. Kendal [26] has developed an analysis of the fracture of single particles as occurs in the comminution process. Large particles will fracture by crack initiation and propagation, whereas small particles can only break as a result of plastic deformation induced rupture. The critical particle transition size, dcrit can be estimated as 2
This gives the values for dcrit listed in Table 2. This clearly shows that the Dispex N40 allows particle fracture by a brittle, and thus low-energy, mechanism to the smallest particle sizes. It is apparent therefore that for the present experimental conditions of wet milling of 20-pm calcite powder, the comminution mechanism is one of brittle cleavage fracture. The particles fracture as a result of impacts which typically result in the formation of a large remnant particle and several smaller ones. It would appear that the main function of the surfactant is to influence the mechanical properties of the calcite, and hence the mechanics of the fragmentation process. The
(4
(4
Fig. 5. Scanning electron micrographs of (a) unground calcite, (b) calcite ground for 2 h in double-distilled water, (c) calcite ground for 2 h in Dispex N40 solution, (d) calcite ground for 2 h in FC171 solution and (e) calcite ground for 1 h in FC170c solution. The two most widely separated dots on the scale bars correspond to the distance recorded below them.
Dispex N40 surfactant solution enhances the degree of secondary cracking, either at the time of impact or by mild chemical attack very shortly afterwards, which produces many more smaller particles. These particles, which are smaller than the critical size for cleavage fracture, do not appear to further reduce in size by plastic deformation and/or rupture. However, where fracture is preceded by extensive plastic deformation, as in the comminution of covalently bonded organic solids, then grinding aids appear to function by promoting interparticulate repulsive forces allowing either modified conditions within the mill prior to fracture (i.e., greater solids content) or by preventing agglomeration of particles. It would be anticipated that similar effects could be obtained for grinding of similar inorganic macroscopically brittle materials. The chemical nature of the surfactants which
would promote secondary cracking cannot, as yet be predicted for particular powders, but further work is in progress.
Conclusions The main conclusions to be drawn from this work are: ( 1) The near surface mechanical properties, i.e., hardness and fracture, of calcite can be affected by the external environment. (2) Comminution of calcite particles > 2 pm in size in aqueous solutions occurs virtually entirely by brittle cleavage fracture. (3) Surfactants which increase the extent of cracking, particularly the amount of secondary cracking, are most likely to be beneficial to the rate and extent of comminution.
410
Acknowledgements The authors would like to thank member companies of the Separation Processes Centre (Exeter University) and the Science and Engineering Research Council for financial support.
10 J. F. Bell, Amer. Mineralogist, 26( 1941) 247. 11 F. J. Turner, D. T. Griggs and H. Heard, Butt. Geol. Sot. Amer., 65 (1954) 883. 12 I. V. Obreimov and V. I. Startsev, Souier Physics JETP, 35 (1959) 743.
13 R. E. Cooper, Proc. Roy. Sot. A, 270 (1962) 525. 14 P. Braillon and J. Serughetti, Physica Status Soltdi (a), 36 (1976) 637.
15 P. Braillon, L. Kubin and J. Serughetti, Physica Status Solidi (a), 45 (1978) 453.
References G. C. Lowrison, Crushing and Grinding, Butterworth, London, 1974. P. Somasundaran and I. J. Lin, Ind. Eng. Process Des. Dev., 11 (1972) 321. R. H. Snow, Powder Technol., 7( 1973) 69. R. H. Snow, Powder Technol., 23 (1979) 31. R. R. Kimpel and L. G. Austin, Powder Technol., 31 (1982) 239.
D. W. Fuerstenau, K. S. Venkataraman and B. V. Velamakanni, Int. J. Miner. Process., 15 (1985) 251. K. Tkacova and N. Stevulova, Powder Technot., 52 (1987) 161.
P. A. Rehbinder and N. Kalinkovskaya, J. Tech. Phys. USSR, 2 (1932) 726. P. A. Rehbinder, L. A. Schreiner and K. F. Zhigach, Hardness Reducers in Drilling, Moscow Academy of Sciences, 1944.
16 A. G. ‘Evans, in Fracture Mechanics Applied to Brittle Materials. A.S.T.M. Soec. Tech. Pub. No. 678 (1978) . I. 112. 17 H. Kaga,‘Phys. Rev., ‘3 (1968) 900. 18 A. R. C. Westwood and D. L. Goldheim, J. Appl. Phys., 39 (1968) 3401.
19 N. H. MacMillan, R. D. Huntingdon and A. R. C. Westwood, Philos. Mag., 28 (1973) 923. 20 N. H. MacMillan, R. D. Huntingdon and A. R. C. Westwood, J. Mater. Sci., 9 (1974) 697. 21 C. B. Ponton and R. D. Rawlings, Mater. Sci. Technol., 5 (1989) 865.
22 C. B. Ponton and R. D. Rawlings, Mater. Sci. Technol., 5 (1989) 961. 23 R. P. Ingel, Ph.D. Dissertation, Catholic University of America, Washington, D.C. (1982). 24 M. 0. Guillou. G. M. Carter. R. M. Hoooer and J. L. Henshall, J. Hard Mat., I (1990) 65. * 25 J. Lankford, J. Mater. Sci. Let., 1 (1982) 493. 26 K. Kendal, Proc. R. Sot. Lond. A, 361(1978) 245.
Powder Technology, 65 (1991) 403-410
Influence of surfactants of wet-milled calcite
G. M. Carter, J. L. Henshall
on the mechanical properties and comminution
and R. J. Wakeman
Separation Processes Centre, University of Exeter, Exeter, Devon (U.K.)
Abstract This paper reports the initial findings of an investigation to relate the influence of surfactants on the particle size distribution and rate of milling of calcite during wet milling. The results for calcite, milled with and without surfactants for a maximum of 72 h, are presented. Particle size distributions are correlated with hardness, indentation fracture toughness and scanning electron microscope observations. It is proposed that the mechanism of comminution is brittle cleavage of the particles in all cases. The most effective grinding aids are those which enhance the propensity to cleavage and which maximise branching and secondary cracking.
Introduction The grinding of minerals is widely used as a processing step for the production of ceramics, glass, cement, pigments, ores for refinement, etc. [ 11. However, within this process only an estimated 1% of the total energy input is accounted for in the creation of new surface area. Increased rates of milling have been sought through optimisation of mill designs, by providing favourable physical conditions within the mill and by the use of grinding aids. Initially, an enhanced rate of comminution may be achieved by wet rather than dry grinding. Thus, the dispersing medium itself may be considered to be an aid. However, the term ‘grinding aid’ usually refers to some chemical additive, introduced into the feedstock of a mill, that produces more efficient grinding of the charge [2-71. Typically, a grinding aid will be some organic compound that modifies the surface chemistry of the mineral. Grinding aids are, in the extreme, believed to enhance grinding efficiency either by modifying the rheological characteristics of the mill charge or by assisting the fracture process itself. The latter theory is often condemned because of the large difference in the velocity of a propagating crack and that of diffusing molecules [5], assuming that any chemomechanical effects are restricted to the immediate vicinity of a propagating crack tip. However, the influence of surface active species on mechanical properties of
0032-5910/91/%3.50
inorganic solids, the Rehbinder effect, has received increasing attention following Rehbinder’s work on drilling fluids [8,9]. At the other ex-
treme, surfactants have been suggested to function by reducing viscosity, by preventing agglomeration of small particles or by alteration of the enthalpy content of the charge. This paper reports the preliminary findings of an investigation to quantify a range of commercially available surfactants as potential grinding aids, with a view to identifying the mechanisms for any increase in efficiency, whether by modification to the rheological properties, by assisting fracture through surface free energy changes, or by whatever means.
Experimental
procedures
Grinding
The calcite used in these grinding experiments was precipitated reagent grade and was washed with single- and then double-distilled water to give a constant value of ionic conductance measured on the supernatant prior to grinding. All laboratory ware used in the preparation of the mill charges was soaked overnight in an anionic detergent and then washed with single- and then double-distilled water to give a constant value of ionic conductance. The mill jars used were porcelain, 2.5 litre capacity. These were charged with 1 kg of cleansed 12-mm diameter porcelain balls,
0
Elsevier
Sequoia/Printed
in The Netherlands
404 TABLE 1 Summary of aqueous surfactant additives used in grinding experiments Surfactant
Classification
Supplier
Dispex N40
Sodium salt of polycarboxylic acid
Allied Colloids
FC171
Fluorinated alkyl alkoxylate
3M
FC170c
Fluorinated alkyl polyoxethylene ethanols
3M
250 g of washed calcite, 700 g of double-distilled
water and 2.5 g of surfactant. In addition, one jar was washed, dried and then loaded with 250 g of dry calcite. The jars were rotated at 96 rpm and small samples withdrawn at intervals of 1, 2, 4, 8 and 72 h. The three surfactants used are all commercially available and are listed in Table 1. The fluorocarbons were selected for their ability to decrease the aqueous interfacial surface energy of materials to very low levels. The polycarboxylic acid salt is a surfactant recommended by the supplier as a grinding aid for calcite. Particle size measurements were performed using a Malvern Mastersizer laser diffraction system with an appropriate lens fitted to allow measurements in the range 0.1 to 80 pm. The data were analysed using the supplied software with values of 1.66 for the refractive index, 0.1 for the absorption and ‘presentation value’ of 1 207 as recommended by Malvern Instruments. Indentation Hardness and Toughness
Parallel-faced calcite single crystal specimens were cleaved from a large parent crystal and indented either in air, or after immersion in double-distilled water or surfactant solutions. The solutions used were of the same concentration as those used in the grinding experiments. Initially, specimens were immersed in the surfactant solutions for several hours prior to testing. However, etching of the surface by the Dispex N40 solution made subsequent measurement of the crack length and half-diagonal impossible. Consequently, immersion times were reduced to 15 min. A Leitz Miniload 2 microhardness tester with a Vickers diamond pyramid was used for the indentation tests with an applied load of 15 g. In each case, a minimum of ten impressions was made on the as-cleaved plane with one of the indentor diagonals aligned parallel to the edge of the crystal. The lengths of the two half-diagonals, a, and the lengths of the two longest cracks, c, from the centre of the impressions to the crack tip, along the diagonal, were measured, using an optical
microscope with reflected light at x 125 magnification, with a calibrated eyepiece. Electron microscopy
The indented crystals and the dried powders were mounted on aluminium stubs and then gold coated prior to examination with a Hitachi S520 scanning electron microscope. Small samples of the powder ground in Dispex solution for 72 h were dispersed on a carbon grid and examined with a transmission electron microscope at 80 kV.
Results
Figures l(a)-(e) show the nature and extent of the deformation and cracking around the indentations. In all cases, it is apparent that the crack initiation depends upon the twinning and twin/ twin and twin/slip interactions, which have been studied previously [ 10-151. The type of cracking formed, however, is very different for the differing environments. In air, Fig. l(a), the cracks emanating from the lower apex of the indentation are clearly crystallographic in nature, whereas the major one from the right hand apex is curved. In water, Fig. l(b), both cracks are largely crystallographic in nature, and there is a marked tendency for conchoidal cracking to occur as well. In the Dispex N40, Fig. l(c), cracking is also primarily crystallographic, but the extent of crack branching and secondary cracking is much greater. In both Figs. l(d) and (e), it is apparent that the cracking is not well aligned with the indentation diagonals, but the directions are more closely related to the twinning crystallography. Table 2 gives the values of Vickers hardness (H,) at 0.147 N load, the fracture toughness K,, calculated using eqn. ( 1) (Evans [ 161)with a value of 141.8 GPa [ 171for the elastic modulus (E), and the ratios of the crack to half-diagonal lengths lcja for the series of indentation tests made in differing environments. / c \2/5 f&=Hva”2(G) x lo” where y = - 1.59 - 0.342 - 2.02~~ + 11.23~~ -24.47~~ + 16.32~’ and z = log,, (c/a). The crack lengths used were the projected values parallel to the appropriate indentation diagonal. The Vickers hardness values for the calcite single crystals are very similar for the tests in air, water and FC 171 fluorocarbon, with an average
(4
Fig. 1. Vickers diamond pyramid indentations in single crystals of calcite using 15 g normal load. The ambient environments during testing were (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC171 solution and (e) FC170c solution. The two most widely separated dots on the scale bars correspond to the distance recorded below them. TABLE 2 Vickers hardness (H, 0.015), K,,, c/a ratios and calculated critical particle sizes for calcite single crystals immersed in different environments Property
Environment Air
Water
Dispex
FC171
FC17Oc
$Pa)
1.34
1.32
1.65
1.38
1.56
$N+)
4.2 0.18
4.8 0.15
0.15 5.0
0.14 5.3
4.55 0.17
dE”t
2.71
1.94
1.24
1.54
1.78
(pm)
value of 1.35 GPa. The results of the tests in fluorocarbon FC 170~ and Dispex N40 show an increase in hardness of 15% and 22%, respectively. Overall, the effect of an aqueous environ-
ment, with or without a surfactant, is to render the calcite less resistant to fracture by indentation, as reflected in the larger c/a ratios and the lower values of K,,. Figures 2-4 show the particle size distributions for calcite ground for 1, 4 and 72 h, respectively, in (a) air, (b) double-distilled water, (c) Dispex N40, (d) FC 171 and (e) FC 17Oc, and for the unground powder, Fig. 2(f). The data are presented as histograms of the volume fraction of calcite in a given size range and the related accumulated frequency curves. Table 3 gives the median particle sizes, span calculated as the difference in the upper and lower deciles divided by the median, and the specific surface areas assuming equivalent spherical diameters. Since for the tests performed in air, the calcite coats the balls and jars, thus preventing further comminution, the results are of academic interest only and are included for comparison with the
406
Particle
Particle
Size (microns)
Size (microns)
(W
1 0-l
100
Particle
10’
102
Particle
Size (microns)
(4
a 1 Particle
Size (microns)
Size (microns)
Particle
Size (microns)
(4 Fig. 2. Histograms of volume fraction of particles in a given size range and associated plots of cumulative volume fraction trs. particle size for calcite ground for 1 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution, (e) FC17Oc solution and (t) washed but unground calcite.
aqueous values. The main features of the data presented in Figs. 2-4 and Table 2 are: (i) There is a development of a secondary ‘peak’ in particle sizes in the range 0.1-1.0 pm. This represents a small volume fraction of the total charge, but represents a relatively large number of particles. (ii) In all cases, the main peak of the histogram decreases with increasing grinding time, except for air, where little further reduction occurs after 4 h grinding. (iii) The distributions for the water and fluorocarbon solutions are broadly similar at any given grinding time. The Dispex N40, however, enables the formation of a much greater proportion of fine particles, e.g., ~2 ,um, as shown by the relatively larger specific surfaces areas, with the effect being particularly pronounced after 72 h. (iv) The span of the distributions generally appears to be less for the fluorocarbon surfactant solutions, which again is a reflection of the smaller volume fraction of material that is in the submicron regime. Figures 5(a)-(f) are SEM micrographs of the unground calcite and samples after grinding for 1 h or 2 h in the various environments. For the ground specimens, the blocky angular nature of the particles shows that the comminution process is dominated by cleavage fracture rather than plastic deformation. The different effects of the
environment, as shown in Fig. 1 for the indentations, are shown again in the details of the particle fracturing process. The surface ledges and variations shown in Fig. 5(a) are typical of those for precipitated particles. Figure 5(b) shows predominately highly facetted fractured particles for dry ground calcite. Grinding in water only, Fig. 5(c), produces particles which are primarily facetted, but with a significant number of curved fracture surfaces. Figure 5(d) shows that in this case small sub-micron particles are commonly formed during the major fracturing process. This relates to the high incidence of secondary cracking shown in Fig. l(c). Figures 5(e) and (f) show that in some grains there is clear evidence of the fracture process being related to the twin formation process. Again, this correlates with the observations of cracking around the indentations i.e., Figs. l(d) and (e).
It is generally accepted that the influence of external environment on the mechanical properties of materials was first reported by Rehbinder and co-workers [8,9]. Despite much work in this field, there is no clear single explanation of this phenomenon, althogh Westwood and Goldheim [ 181 and McMillan et al. [ 19,201 showed that there is a marked effect of the zeta potential of a
407
101
10"
1
Particle
Size
10'
1 i i 0-'
IO!
100
Partlcle
(microns)
-0 .5
Size
I 02
(micnxx)
_......o.5
10-l
100 Particle
101 Size
1OL
0-
I
i
101
100
Particle
(microns)
----,0.5
Size
0
102
I
lo-!
(microns)
(4
101
100
Particle
Size
102
(microns)
(4
Fig. 3. Histograms of volume particle size for calcite ground FC 17Oc solution.
fraction of particles in a given size range and associated plots of cumulative volume fraction vs. for 4 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution and (e)
1 ,r-----/y5
10-L
lo!
100 Partde
Size
102 Particle
(microns)
Size
(rmcrons)
(4 1
0.5
/ i
0 10-l
100 Particle
lo? Size
(microns)
102
10’
102 Particle
Size
(microns)
Particle
Size
(microns)
(4
Fig. 4. Histograms of volume particle size for calcite ground FC 170~ solution.
fraction of particles in a given size range and associated plots of cumulative volume fraction vs. for 72 h in (a) air, (b) double-distilled water, (c) Dispex N40 solution, (d) FC170 solution and (e)
solution on the hardness of immersed ceramics. In the present situation, the measured Vickers hardnesses are consistent with the standard Moh’s hardness of calcite, i.e., 3. There is no difference in the hardness values in air, water and FC 171 solution, but the hardnesses in FC 170~ and
Dispex N40 are significantly greater. The plastic deformation required to accommodate the pyramida1 indentation in calcite consists of a combination of dislocation generation and motion, and twinning. Therefore, the observed increase in hardness, particularly for the Dispex N40 solution, means
408 TABLE 3 Median particle sizes (in micrometres), span ((particle size at 90 vol.% - particle size at 10 vol.%)/median), specific surface areas SSA (m2/cm3) and volume per cent of charge with particle sizes less than 2 pm (V% < 2), for calcite ground in different environments Grinding time (h) 0
Environment Parameter Median Span SSA V%<2
Air
Water Dispex FCl71
,
20.2 Y 1.3 0.610 2.0
* (
FC170c
> . >
1
Median Span SSA V%<2
11.3 11.3 11.4 1.50 2.00 2.06 1.881 1.854 1.80 7.5 9.4 9.2
2
Median Span SSA V%<2
4
Median Span SSA V%<2
9.8 9.1 8.4 1.91 1.90 2.18 2.216 2.031 2.031 10.7 11.1 13.3 7.3 6.9 6.9 6.1 5.8 1.99 2.06 1.86 2.21 2.21 2.403 2.566 2.793 2.295 2.380 14.5 14.9 13.1 17.9 20.1
8
Median Span SSA V%<2
n.a.a n.a.a n.a.” n.a.a
72
Median Span SSA V%<2
11.9 ‘11.6 1.80 1.82 1.751 1.80 8.2 8.6
9.3 8.5 1.68 1.69 2.124 1.534 9.0 13.0
5.0 4.7 4.1 3.91 2.00 1.89 2.19 2.21 3.054 3.230 2.813 2.749 21.2 21.5 26.4 26.0 1.9 1.9 5.9 1.7 1.3 1.6 1.58 2.09 1.52 1.72 2.571 4.591 6.740 4.184 4.221 60.4 61.3 17.4 69.5 77.0
?r.a. = not available.
that the surface state of the calcite is altered sufficiently in a manner such that there is a reduction in the extent of twinning, Fig. l(c). The enhanced twinning observed around the indentations immersed in FC 171 and FC 17Oc, Figs. l(d) and (e), can be explained on the basis that the twins are surface initiated, with the controlling parameter being the interfacial energy [ 12, 131. Thus, the reduction in surface energy in the fluorocarbon solutions will enable the twins to be initiated more easily. The cracks that are formed around the indentations are clearly asymmetrical and are influenced by the plastic deformation processes, in particular the degree and extent of twinning and twin interactions. There are several approaches to the determination of fracture toughness parameters from measurements of cracking around indentations [21,22]. There is no clearly defined analysis that can be applied with the present geometry of deformation and cracking, but the formula of Evans [ 161 is most commonly utilised in the analysis of single crystal toughness data [ 16,23,24]. Although the absolute error may be
as high as 30% [25], the relative precision and significance are much better, and the very obvious differences in the ratios of crack to indentation half-diagonal lengths are reflected in the K,, values. There was no significant difference in projected lengths between the two major cracks which formed from the indentations. The absolute toughness values are very low and markedly anisotropic, as would be expected from a material which cleaves well. Several mechanisms have been proposed to explain the manner in which the surface state can influence so significantly the extent of crack propagation, but these remain largely speculative. However, the present results clearly demonstrate that the environment has a marked effect. Also of significance, but not reflected in the numerical analysis, is that the Dispex N40 is mildly aggressive with regard to plastically deformed calcite. This results in the copious secondary cracking which occurs around the indentations. There are, however, no signs of etching or other forms of chemical attack on the calcite particles ground in the Dispex N40 solution. This confirms that plastic deformation, and the associated strain fields, are required for the chemical interaction between the calcite and Dispex N40 to occur. Also, since no attack takes place on even the smallest calcite particles, this indicates that these are not formed by plastic deformation. This confirms the TEM observations, Fig. 5, showing the absence of any plastic deformation. Kendal [26] has developed an analysis of the fracture of single particles as occurs in the comminution process. Large particles will fracture by crack initiation and propagation, whereas small particles can only break as a result of plastic deformation induced rupture. The critical particle transition size, dcrit can be estimated as 2
This gives the values for dcrit listed in Table 2. This clearly shows that the Dispex N40 allows particle fracture by a brittle, and thus low-energy, mechanism to the smallest particle sizes. It is apparent therefore that for the present experimental conditions of wet milling of 20-pm calcite powder, the comminution mechanism is one of brittle cleavage fracture. The particles fracture as a result of impacts which typically result in the formation of a large remnant particle and several smaller ones. It would appear that the main function of the surfactant is to influence the mechanical properties of the calcite, and hence the mechanics of the fragmentation process. The
(4
(4
Fig. 5. Scanning electron micrographs of (a) unground calcite, (b) calcite ground for 2 h in double-distilled water, (c) calcite ground for 2 h in Dispex N40 solution, (d) calcite ground for 2 h in FC171 solution and (e) calcite ground for 1 h in FC170c solution. The two most widely separated dots on the scale bars correspond to the distance recorded below them.
Dispex N40 surfactant solution enhances the degree of secondary cracking, either at the time of impact or by mild chemical attack very shortly afterwards, which produces many more smaller particles. These particles, which are smaller than the critical size for cleavage fracture, do not appear to further reduce in size by plastic deformation and/or rupture. However, where fracture is preceded by extensive plastic deformation, as in the comminution of covalently bonded organic solids, then grinding aids appear to function by promoting interparticulate repulsive forces allowing either modified conditions within the mill prior to fracture (i.e., greater solids content) or by preventing agglomeration of particles. It would be anticipated that similar effects could be obtained for grinding of similar inorganic macroscopically brittle materials. The chemical nature of the surfactants which
would promote secondary cracking cannot, as yet be predicted for particular powders, but further work is in progress.
Conclusions The main conclusions to be drawn from this work are: ( 1) The near surface mechanical properties, i.e., hardness and fracture, of calcite can be affected by the external environment. (2) Comminution of calcite particles > 2 pm in size in aqueous solutions occurs virtually entirely by brittle cleavage fracture. (3) Surfactants which increase the extent of cracking, particularly the amount of secondary cracking, are most likely to be beneficial to the rate and extent of comminution.
410
Acknowledgements The authors would like to thank member companies of the Separation Processes Centre (Exeter University) and the Science and Engineering Research Council for financial support.
10 J. F. Bell, Amer. Mineralogist, 26( 1941) 247. 11 F. J. Turner, D. T. Griggs and H. Heard, Butt. Geol. Sot. Amer., 65 (1954) 883. 12 I. V. Obreimov and V. I. Startsev, Souier Physics JETP, 35 (1959) 743.
13 R. E. Cooper, Proc. Roy. Sot. A, 270 (1962) 525. 14 P. Braillon and J. Serughetti, Physica Status Soltdi (a), 36 (1976) 637.
15 P. Braillon, L. Kubin and J. Serughetti, Physica Status Solidi (a), 45 (1978) 453.
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D. W. Fuerstenau, K. S. Venkataraman and B. V. Velamakanni, Int. J. Miner. Process., 15 (1985) 251. K. Tkacova and N. Stevulova, Powder Technot., 52 (1987) 161.
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19 N. H. MacMillan, R. D. Huntingdon and A. R. C. Westwood, Philos. Mag., 28 (1973) 923. 20 N. H. MacMillan, R. D. Huntingdon and A. R. C. Westwood, J. Mater. Sci., 9 (1974) 697. 21 C. B. Ponton and R. D. Rawlings, Mater. Sci. Technol., 5 (1989) 865.
22 C. B. Ponton and R. D. Rawlings, Mater. Sci. Technol., 5 (1989) 961. 23 R. P. Ingel, Ph.D. Dissertation, Catholic University of America, Washington, D.C. (1982). 24 M. 0. Guillou. G. M. Carter. R. M. Hoooer and J. L. Henshall, J. Hard Mat., I (1990) 65. * 25 J. Lankford, J. Mater. Sci. Let., 1 (1982) 493. 26 K. Kendal, Proc. R. Sot. Lond. A, 361(1978) 245.