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I n d . Eng. Chem. Res. 1987,26, 1214-1217
Difference in the Optimum Conditions for Size Reduction and Mechanical Activation by Wet Vibro -milling Hideyuki Ootani and Mamoru Senna* Faculty of Science and Technology, Keio University, Hiyoshi, Yokohama 223, Japan
Optimum charge and powder/liquid ratio were compared for conventional size reduction and mechanical activation of calcium carbonate on wet vibro-milling in xylene with and without silicone oil. Main parameters varied were the amount of charge (5-60 g), the solid volume fraction of the suspension (0.02-0.25), and the total volume of suspension (20-120 cm3) as well as the viscosity of the medium (5.5 X 10-4-1.4 X 10-1 Pas). Efficiency of mechanical activation, as estimated from the fractional amorphization and the rate of decomposition, was maximum a t the intermediate solid concentration of the suspension, with the total amount of solid volume being kept constant. The maximum values were higher when the total powder volume was smaller. For the conventional size reduction, in contrast, the higher loading of the solid favored. This kind of difference might be related to the difference in the form of deformation, as well as in the amount of the fraction of smaller particles. Comminution or fine grinding is one of the key operations in many industries. It is therefore of vital importance to improve the efficiency and to minimize the energy consumption for comminution. There are lots of basic studies in this direction as far as size reduction per se is concerned (Bown, 1966; Rumpf, 1973). As a result of fine grinding, however, not only does the average particle size decrease, but also the material is very often activated, resulting in the enhanced reactivity of solids. I t is thus feasible to produce or modify various kinds of fine particulate materials by grinding, in order to tailor the material for its subsequent application to a specific production process. One of the most important factors is thereby a change in the reactivity of solids as the result of stressing during grinding (Heinicke, 1984). Although it is generally believed that the energy efficiency of mechanical activation is quite low, light was seldom shed on its improvement. As far as the size and the number of grinding balls for vibro-milling are concerned, their effects on the size reduction seem to parallel that for mechanical activation (Shinozaki and Senna, 1981). There are many other factors, however, e.g., rheological or chemical additives (Klimpel, 1982),which affect the efficiency of wet grinding. It is thus possible that the optimum condition for the size reduction is not always identical with that for mechanical activation. The purpose of the present study is to find a guideline for the optimum condition of mechanical activation by wet vibro-milling. Since the effects of number and size, as well as the density of the grinding balls, were studied already to some extent (Senna and Kuno, 1971; Shinozaki and Senna, 1981), the main parameters varied in the present study were the amount of charge and the concentration of the suspension, or the ratio between solid material and the medium. Effects of viscosity of the medium or the whole suspension were also studied.
Experimental Section Vibro-milling. Natural limestone of coarse particle (average particle size 1700 pm) was used as a starting material. Vibro-milling was carried out by using a commercial vibro-mill (MB-1, Chuo Kakoki, Nagoya) operated * To whom all correspondence should be addressed.
with amplitude 8 mm and frequency 15.7 Hz. A cylindrical steel vessel with i.d. 5.7 cm and effective capacity 204 cm3was filled with a predetermined amount of limestone and xylene as well as grinding media, i.e., 120 pieces of 9.5" steel balls. For the purpose of examining the effect of the medium viscosity, silicone oil was added to the medium in some runs. Grinding time was kept constant at 1 h throughout the present work. Specific Surface. Specific surface area, S,, was measured by an air-permeability method under the constant pressure difference (Shimadzu, H100, Kyoto). The void fraction of the plug was kept constant between 0.40 and 0.45. Structural Defect. As a measure of mechanochemical effect due to vibro-milling, the relative intensity ratio of the X-ray diffraction peak, If,was used. Calcium fluoride was used as an internal standard. The fraction of X-ray amorphous part, FA, was defined as 1 - If. Rate of Decomposition. The rate of decompositionof vibro-milled CaC03was measured as another criterion of mechanical activation. Isothermal decomposition was carried out at 961 f 1 K in nitrogen flow by using an automatic thermobalance equipped with a line-focused infrared furnace. Kinetic data were analyzed by applying the first-order rate law to obtain the rate constant, kd. Viscometry. Viscosity of the medium, qo, was measured with a conventional Ostwald capillary viscometer. Rotational viscometer (Type B, Tokyo Keiki) was also used to measure the apparent viscosity of the suspension as a function of rotational speed of the rotor. The apparent viscosity of the suspension at the rotor speed 60 rpm, v60, was used for practical purposes.
Experimental Results Effect of the Amount of Powder. When the amount of the medium, xylene, was kept constant at 66 cm3, the specific area decreased quasi-linearly with increasing solid volume fraction, 4p,as shown in Figure 1. The amount of X-ray amorphous portion, F A , on the other hand, decreased sharply with increasing solid volume fraction, as shown also in Figure 1, the rate of decrease being leveled off with increasing @p. The rate constant of thermal decomposition, kd, decreased also monotonically with increasing solid content as shown in Figure 2. The manner of decrease in the
0888-5885/87/ 2626-1214$01.50/0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987 1215 I
I
10.8 0.L
-
VI
- 0.7; 0.3 -
0;
up 0.2
- 0.6
-
1 0
I
.I
01
0.2
03
I
QP
Figure 1. Variation of specific surface area obtained from the airpermeability method, s,, and fractional amorphization, F A , with the solid volume fraction of the suspension, q$. The amount of the medium was kept constant at 66 cm3.
Figure 4. Variation of fractional amorphization, F A , with the total volume of the suspension, V,. The broken lines connect the point of equal solid volume of the entire suspension. For symbols, see Figure 3. - 6
b - ._ 'cL El\
n
Y
2-
t - 0
Figure 2. Variation of decomposition rate constant, kd,with solid volume fraction of the suspension, The amount of the medium was kept constant a t 66 cm3.
I
0
Figure 5. Variation of decomposition rate constant, k d , with the total volume of the suspension, V,. The broken lines connect the point of equal solid volume of the entire suspension. For symbols, see Figure 3.
,
-8 50
100 Vs /cm3
Figure 3. Variation of specific surface area obtained from the airpermeability method, s,, with the total volume of the suspension, V,. The solid lines connect the suspension of equal solid volume fraction; (0) bP = 0.112, (0) bP = 0.223, (A)bP = 0.335. The broken lines connect the point of equal amount of solid volume in the entire suspension.
decomposition rate was a kind of hybrid between those of and S,. It is to be anticipated, since the rate of decomposition is a function of surface area and structural defects (Criado and Gonzalez, 1984). Effect of Total Amount of Suspension. Variation of specific surface area with total volume of suspension is shown in Figure 3. Solid lines in Figure 3 connect the data with equal solid volume fraction, whereas broken lines correspond to the values a t equal volume of the powder. Since both solid and broken lines decline from upper left
FA
150
V s /cm
OL
0
JOO
50
-6
-L
in[?, /Pa.s 1
-2
Figure 6. Variation of specific surface area obtained from the airpermeability method, s,, with medium viscosity, vo For symbols, see Figure 3.
to lower right, it is obvious that comminution is simply more effective when the total volume of the suspension is smaller. Effects of suspension volume on the mechanochemical effect are a little more complex. Whereas the fraction of X-ray amorphous portion decreased monotonically with increasing suspension volume, V,, as shown in Figure 4, there were maxima in F A when V , was varied by changing the concentration of the suspension, the amount of powder being kept constant, as shown by broken lines in Figure 4. The appearence of maxima was even more noticeable when one takes the rate constant of isothermal decomposition as a criterion of the mechanochemical activation,
1216 Ind. Eng. Chem. Res., Vol. 26, No. 6, 1987
O3
02
t
I
O ' t
I 0 1 -8
'
-6
-4
-2
In [ ?,/Po s I
Figure 7. Variation of fractional amorphization, FA, with the medium viscosity, vo. For symbols, see Figure 3.
O!
P
Figure 8. Relationship between the viscosities of the medium, qo, and the suspension, qeo, the latter being measured by a Type B rotational viscometer a t 60 rpm.
as shown in Figure 5. As shown in Figures 4 and 5, the maxima were more predominant with their higher values when the total amount of the solid was smaller. Effect of Viscosity. The specific surface decreased with increasing viscosity of the medium, tlo, as shown in Figure 6. The rate of decrease was larger at higher viscosity range. FA decreased almost linearly with logarithm of the medium viscosity, as shown in Figure 7 . Since the apparent viscosity, 760, of the whole suspension varied almost linearly with qo on a double logarithm scale, as shown in Figure 8, the above-mentioned properties were equally well correlated with 760.
Discussion Efficiency of Activation. The efficiency of grinding in an actual grinding machine is to be considered from two aspects, Le., (i) energy density applied to the material to be ground in the grinding zone, and (ii) probability of capture in the grinding zone. Since the mechanical activation also arises from the stress applied by the grinding media, the above-mentioned two factors seem likely to dominate in the case of mechanical activation as well. This will be examined in the following. Effect of the Amount of Powder and Total Suspension. When the solid volume fraction increases at constant amount of medium (66 cm3), the capture efficiency should increase, whereas the energy density decreases, not only because of the larger number of particles to be stressed but also because of the cushioning effect of the powder layers between grinding media. The similar tendency was also found in the case of mechanical acti-
vation of PbOz on dry grinding with increasing feeds (Senna and Schonert, 1982). As far as the specific surface area, S,, is concerned, the tendency that smaller amounts of samples lead to larger S, is quite similar to that found in Figure 1. One of the reasons for the nonlinearity of the change in FA and kd with &, as compared with S,, seems to be related with the effect of particle size distribution on the mechanical activation. The increase in the specific surface area can take place by the size reduction of each grade of particles, whereas the mechanical activation takes place predominantly for the nonelastically deformed particles, i.e., preferentially for those which are smaller than a certain critical size (Hess, 1980). Effect of Viscosity. The increase in the viscosity of the medium enhances the capture efficiency, for one thing, with more intimate encounter of the powder with grinding media. The movement of the grinding media, for another, is suppressed when the medium is viscous. In the present case, the latter factor should have dominated, since the increase in the surface area as well as the mechanochemical effect was more significant with smaller medium viscosity. The effect of the viscosity of the entire suspension seems to be similar.
Concluding Remarks-Existence of the Optimum Condition As shown in Figures 4 and 5, the existence of the maxima in the parameters for mechanical activation, Le., F A and kd, with respect to the total volume of suspension, the amount of the powder being kept constant, typifies the operating condition of wet grinding for the sake of mechanical activation. For the specific surface, S,, no such maximum was observed in the same kind of plots, as shown in Figure 3. This is of practical importance, since it suggests that the concentration of the suspension should be neither too high nor to low a t the predetermined charge of the solid mass. For the low efficiency, the wet grinding at too low suspension concentration is reasonably explained by the low capture efficiency, as is also the case in the conventional size reduction process (Sepulveda and Herbst, 1981). In these cases, the resulting specific surface was also small. In contrast, the low efficiency at too high concentration of the suspension seems to be related with a speciality of mechanical activation, since in latter cases the specific surface increases and nevertheless the extent of activation decreases. This could be explained when the difference in the manner of deformation is taken into account. Decreasing liquid medium, with the solid mass being kept constant, enriches the effective density of mechanical energy given by the grinding ball, since the hindrance of the liquid layer is smaller. This would favor the brittle fracture over the inelastic deformation. Further experimental works are necessary to explain and verify the above-mentioned speculative mechanisms. Acknowledgment We are grateful to Prof. H. Kuno for valuable discussions.
Nomenclature FA = fractional amorphization If = relative intensity ratio of X-ray diffraction kd = rate constant of decomposition, min-l S, = specific surface area from air permeability, m2g-' V, = total volume of suspension, cm3 Greek Symbols to= viscosity of the medium, Pa.s
I n d . Eng. C h e m . Res. 1987,26, 1217-1222
= apparent viscosity of suspension at rotational speed 60 rpm, Pes d p = solid volume fraction of suspension
Literature Cited Bown, R. W. Trans.-Znst. Min. Metall., Sect. C 1966, C75, C173. Criado, J. M.; Gonzalez, M. Thermochim. Acta 1984, 79, 91. Heinicke, G. Tribochemistry; Akademie-Verlag: Berlin, 1984. Hess, W. Dissertation TH Karlsruhe 1980, p 154.
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Klimpel, T. Powder Technol. 1982,32, 267. Rumpf, H. Aufbereitungstech. 1973, 14, 59. Senna, M.; Kuno. H. J. Am. Ceram. SOC.1971,54, 259. Senna, M.; Schonert, K. Powder Technol. 1982, 31, 269. Sepulveda,J. L.; Herbst, J. A. Proc. Tech. Program: Int. Powder Bulk Solids Handl. Process. 1981, 10. Shinozaki, M.; Senna, M. Znd. Eng. Chem. Fundam. 1981, 20, 59.
Received for review June 3, 1986 Accepted February 27, 1987
Solubilization of Cresols by 1-Hexadecylpyridinium Chloride Micelles and Removal of Cresols from Aqueous Streams by Micellar-Enhanced Ultrafiltration Subray N. Bhat,+$George A. Smith,+§Edwin E. Tucker,?§Sherril D. Christian,*?§and John F. ScamehornlL Department of Chemistry, Institute f o r Applied Surfactant Research, and School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
Willie Smith Halliburton Services, Duncan, Oklahoma 73536
In micellar-enhanced ultrafiltration (MEUF), solubilization of organic solutes in micelles, combined with ultrafiltration of these micellar solutions, leads to the removal of dissolved, low molecular weight organic compounds from water. T h e permeate from MEUF contains the organic solute a t concentrations equal to the unsolubilized solute concentration in the rejected (retentate) solution. Therefore, the equilibrium solubilization of the solute dictates the permeate purity or rejection of the solute by the membrane. T h e semiequilibrium dialysis method has been used to investigate the equilibrium solubilization of 0-,m-, and p-cresol by aqueous solutions of 1-hexadecylpyridinium chloride, throughout a range of concentrations of the cresols and the surfactant. The apparent solubilization constant, K = X,/ (concentration of unsolubilized cresol), has been correlated with X c , the mole fraction of cresol in the micelle. For each of the three cresol-1-hexadecylpyridinium chloride systems, K is found to vary nearly linearly with X c , throughout the range 0 < X c < 0.5. The removal of organic contaminants from aqueous streams, by the process called micellar-enhanced ultrafiltration (MEUF), has been shown to be effective for several types of organic compounds (Leung, 1979; Dunn et al., 1985,1986; Scamehorn and Harwell, 1986; Gibbs et al., 1986). In MEUF, a surfactant is added to the aqueous stream containing a dissolved organic contaminant, causing a large fraction of the solute to associate with the surfactant micelles. When the aqueous stream is passed through an ultrafilter having a molecular weight cutoff in the range 1000-20 000, most of the organic compound and the surfactant remain in the retentate solution. In a number of MEUF experiments, the effluent or permeate solution has been shown to contain organic solute at a very low concentration, approximately equal to the concentration of free organic molecules in the retentate solution. Research on the equilibrium solubilization of organic solutes by surfactant solutions is important, both in relation to practical problems such as detergency, enhanced oil recovery, or micelle-based separations and because detailed solubilization results can play a major role in increasing our understanding of the properties and structure of micellar solutions. There have been relatively Department of Chemistry. $On leave from the Department of Chemistry, North Eastern Hill University, Shillong 793003, India. f
Institute for Applied Surfactant Research. School of Chemical Engineering and Materials Science.
few solubilization studies yielding accurate information about the concentration dependence of the solubilization constant (or equivalently the activity coefficient of the solute in the intramicellar “solution”)(Doughertyand Berg, 1974; Goto and Endo, 1978; Valenzuela et al., 1984; Abuin et al., 1984). Typically, the extent of solubilization has been measured only at saturation of the surfactant solution by an organic solute, and it has commonly been assumed that the activity of the solute in aqueous solutions at fixed micellar concentration can be predicted by some form of ’ Henry’s law. However, several types of measurementsnotably solute vapor pressure results-have shown that adherence to Henry’s law in such systems is the exception, rather than the rule (Thomas and Christian, 1981; Christian et al., 1981, 1982, 1986; Tucker and Christian, 1982, 1985). Although vapor pressure studies (Christian et al., 1981; Tucker and Christian, 1982,1985)have probably provided the most accurate solubilization data for volatile organic solutes, methods for investigating the solubilization of solutes having vapor pressures of only a few torr or less are not so reliable. Recently, we reported the development of a new, generally applicable method for studying the equilibrium solubilization of almost any type of solute by aqueous surfactant solutions (Christian et al., 1985; Smith et al., 1986). Semiequilibrium dialysis (SED) is similar to MEUF in that it utilizes a membrane to separate a concentrated solution of solute and surfactant from a permeate solution containing only small amounts of either
0888-5885/87/2626-1217$01.50/00 1987 American Chemical Society
