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An exploration of dry powder chromatography
Powder Technology, 50 (1987) 217 - 220
217
An Exploration of Dry Powder Chromatography ORR Micromeritics Instrument Corporation, Norcross, GA 30093 (U.S.A.) C.
SUMMARY
APPARATUS
Gravity flow o f p o w d e r s through a vibrated column o f loose glass spheres was explored to determine the influence o f particle size and shape on the rate o f passage. As might be expected, large, irregular particles pass more slowly than small, irregular ones; long, needleshaped particles pass more slowly than otherwise identical short ones; but, surprisingly, spherical particles required more time to pass than irregular ones. Closely sized fractions o f all such particles greater than 45 p m in diameter pass reproducible in order o f size, and any sized fraction in a mixture o f fractions makes its proportional contribution. Unfortunately, particles under about 45 p m obey no consistent pattern, and hence render overall p o w d e r behavior unpredictable when present.
The dry powder chromatograph of this study consisted basically of (i) a means for inserting a powder sample at the head of a column at zero time, (ii) the column, and (iii) a means for detecting the emergence of the sample from the column as a function of elapsed time. The column was m o u n t e d vertically and vibrated vigorously up and down. Gravity thus produced the powder migration through the column. Powder was introduced in a matter of a second or so upon an initiation signal by arranging a small hopper with a wide, solenoidopened throat directly at the head of the column. The hopper was attached to, and vibrated with, the column. The powder discharged directly from the column into the pan of a weighing transducer. This transducer was carefully m o u n t e d , cushioned, and electrically isolated so not to pick up vibratory energy. The o u t p u t of the transducer was fed to the Y-axis of an X - Y recorder to indicate linearly the recovered sample weight. Column vibration was started prior to test initiation at which time the weight indication of the sample recovery transducer was set to zero. A test was begun by depressing a switch which simultaneously tripped the solenoid of the powder hopper and activated the Y-axis travel of the recorder pen. X-axis pen travel was tied to column vibration frequency. This tie-in was adopted to eliminate the influence of any speed changes that might occur in the vibration drive motor. Each upward motion in the vibratory cycle of the column interrupted a light beam which created one pulse to drive the pin along the X-axis. An electronic converter, finally, was incorporated electronically to decrease logarithmically with time the actual magnitude of the pen steps. This resulted in a convenient linear-log plot depicting sample passage rate as shown in subsequent figures.
INTRODUCTION Whitby [ 1], in an investigation of the mechanics of sieving, showed that the rate of passage of particles through a sieve produced essentially two straight-line regions on a loglog plot, the first region representing those particles well below sieve opening size and the second near-opening size particles. Meloy et al. [2 - 5] characterized particle shape using the time of passage of closely sized particles through a stack of identical sieves, the particles having been previously sized just to pass the sieve stack. Sieves require careful handling and are difficult to keep free of blocked sections. This study explored the characteristics of a special column through which a wide range of particle sizes could freely pass. The time of passage as a function of both particle size and shape was assessed in the manner generally associated with gas and liquid chromatography techniques. 0032-5910/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
218
The column with which all results presented herein were obtained consisted of 7 aluminum sections each 1.5 in in internal diameter and 2.0 in high. The sections were clamped together with a disc of No. 20 U.S. Standard Series wire mesh (opening 841 pm) between each section. Each section was also approximately 85% filled with 1680 to 2000~pm diameter solid glass spheres. These spheres could thus neither pass nor lodge in the wire mesh. Instead, each of the seven beds of spheres presented a shifting, tortuous series of paths through which sample particles had to pass. There was also no blinding of the supporting wire discs as long as sample particle diameters (minimum dimension in the case of needle-shaped particles) were confined to well below mesh opening sizes. Pure vertical vibration of the column was provided by mounting it on a rigid slide mechanism. An off-center cam with cam follower lifted the column l/16 in with each revolution. Spring loading held the follower and cam in contact. The cam was driven by a D.C. motor turning at approximately 1000 rev. min-’ .
100
0-
0.1
0.5 Time,
1
2
345
min
Fig. 1. Passage rate of two sized fractions and of an equal mixture of the fractions.
of limestone
did the larger size trace, demonstrates the essential individual action of each particle size. Figures 2 and 3 reveal the same individual action for nickel powder fractions. In these cases, recovered fractions were combined after individual testing and retested, giving the trace labeled ‘combined’. The horizontal ending for each individual fraction trace is evidence of complete recovery even though the presentation on a plot with a
POWDERS
z
Agricultural limestone, nickel, crushed glass, and spherical glass as used in reflecting signs constituted the powders of this study. A portion of each material was separated with &in. U.S. Standard Sieves into p-sized fractions. Needle-like particles were prepared in three lengths by chopping multistrand, copper, hook-up wire in a special shear. Each strand was approximately 200 pm in diameter. Quantities with lengths averaging 850, 1850, and 2600 pm were produced. Sample quantities for tests typically were between 1 and 10 g.
f
m
z 0
lz
0
0.1
0.5 Time.
1
2
345
min
Fig. 2. Passage rate of two sized fractions of nickel powder and of the combination of the fractions. 100 1
f RESULTS
Figures 1 through 8 reveal graphically the characteristics of the system. The three traces of Fig. 1 show individual rates of passage for two sized fractions of limestone and, superimposed, the rate of passage of a mixture prepared from equal weights of each fraction. The mixture trace, by starting as did the smaller individual fraction trace and ending as
E :
212-210
Cm
: :
0
0.1
0.6 Time.
1
2
346
mln
Fig. 3. Passage rate of three sized fractions of nickel powder and of the combination of the fractions.
219
Y-axis label of 100% recovery might suggest otherwise; the scale of this axis applies only to the ‘combined’ trace. Figure 4 presents the trace of a limestone powder having particles from 63 to 425 pm in diameter and then the trace of the same powder after particles between 212 and 250 pm were extracted by sieving. The deficiency in mid-range particles is clearly evident in the second trace. Figure 5 shows the result for equal size and equal sample weight fractions of crushed glass particles and glass spheres. The fact that the crushed fraction emerged sooner is somewhat surprising in that powders of spherical particles ordinarily flow much more readily than do irregular particle powders. This behavior is confirmed by the trace of Fig. 6, which is for a 25% crushed and 75% spherical by weight mixture of the two glasses. The dotted lines reproduce the traces of Fig. 5 for aid in revealing the comparison. The influence of length on the passage of cylindrical rod-like particles of copper wire all
,
_______ ,/- e\ :I’ ’
I
$
,’ ’
\*
,I’ ,’
y
,‘2 r
’
,’ ,’
b\
2
I
,’
I’ ; I’ 0 ,’
0
’
-1’
,’ ,’
I’
de
2/ ,,’
/
, ,’ I
0.1
I
0.5 Time,
2
1
345
min
Fig. 6. Passage rate for a mixture of the two glasses of Fig. 5.
of approximately 200 pm in diameter is shown in Fig. 7. Not surprisingly, longer rods require longer times for passage. The trace for the 2600 -pm length rods eventually attained loo%, but roughly 1 h was required as the approach was quite slow. Finally, the traces of Fig. 8 show the relative passage rates and recovery of different sized fractions but equal initial weights of nickel powder, including an under 45 pm
100
.:
0.1
0.5 Time.
2
1
345
min
Fig. 4. Passage rate of a wider distribution of limestone particles and a repeat with a mid-range fraction removed.
0.1
0.5 Time.
1
2
.I
0.1
0.6 Tlma
1
2
345
.mln
Fig. 7. Passage rates for copper rod-like particles of 200 pm diam. and different lengths.
345
min
Fig. 5. Passage rates for fractions of crushed and spherical glass powders of the same size range.
Time.
mln
Fig. 8. Relative passage rates for sized fractions of nickel powder including the -45 pm component.
J
220
fraction. The latter was never 100% recovered using only column vibration. Experiments with such fines were inevitably terminated by blowing the remaining particles from the column with a jet of compressed air aimed alternately into the top and bottom of the column. This behavior of fines was found with all the powder materials tested.
noted, the retained fines could be readily blown out with compressed air. Also, the column never required cleaning by disassembly as long as it was not exposed to particle sizes greater than the retaining screen opening dimensions. The system and technique described define what this author finds to be interesting phenomena. The work is reported in the hope that it will suggest to someone a practical use.
CONCLUSIONS
Were it not for the less than complete recovery of particles under about 45 pm in diameter, the system described might find practical utility in size or shape characterization. Various off-center cams and cams with stepped risers were explored to alter the vibrational intensity and, hence, the behavior of the fine particles. These steps shifted the curves but did not substantially change the patterns given. Powders were thoroughly dried and the column was flushed with dry nitrogen, again to no avail. But, as already
REFERENCES K. T. Whitby, Symposium on Particle Size MeaASTM Spec. Tech. Publ. No. 234, 1958, p. 3. T. P. Meloy, Int. Symp. on Recent Advances in Particulate Science and Technology, I.I.T, Madras, 1982, p. A-99. T. P. Meloy and K. Makino, Powder Technol., 36 (1983) 253. T. P. Meloy, N. Clark, T. E. Durney and B. Pitchumani, Part. Sci. Tech., 2 (1984) 259. S. K. Kennedy, T. P. Meloy and T. E. Durney, J. Sed. Petro., 55 (1985) 356.
surement,
217
An Exploration of Dry Powder Chromatography ORR Micromeritics Instrument Corporation, Norcross, GA 30093 (U.S.A.) C.
SUMMARY
APPARATUS
Gravity flow o f p o w d e r s through a vibrated column o f loose glass spheres was explored to determine the influence o f particle size and shape on the rate o f passage. As might be expected, large, irregular particles pass more slowly than small, irregular ones; long, needleshaped particles pass more slowly than otherwise identical short ones; but, surprisingly, spherical particles required more time to pass than irregular ones. Closely sized fractions o f all such particles greater than 45 p m in diameter pass reproducible in order o f size, and any sized fraction in a mixture o f fractions makes its proportional contribution. Unfortunately, particles under about 45 p m obey no consistent pattern, and hence render overall p o w d e r behavior unpredictable when present.
The dry powder chromatograph of this study consisted basically of (i) a means for inserting a powder sample at the head of a column at zero time, (ii) the column, and (iii) a means for detecting the emergence of the sample from the column as a function of elapsed time. The column was m o u n t e d vertically and vibrated vigorously up and down. Gravity thus produced the powder migration through the column. Powder was introduced in a matter of a second or so upon an initiation signal by arranging a small hopper with a wide, solenoidopened throat directly at the head of the column. The hopper was attached to, and vibrated with, the column. The powder discharged directly from the column into the pan of a weighing transducer. This transducer was carefully m o u n t e d , cushioned, and electrically isolated so not to pick up vibratory energy. The o u t p u t of the transducer was fed to the Y-axis of an X - Y recorder to indicate linearly the recovered sample weight. Column vibration was started prior to test initiation at which time the weight indication of the sample recovery transducer was set to zero. A test was begun by depressing a switch which simultaneously tripped the solenoid of the powder hopper and activated the Y-axis travel of the recorder pen. X-axis pen travel was tied to column vibration frequency. This tie-in was adopted to eliminate the influence of any speed changes that might occur in the vibration drive motor. Each upward motion in the vibratory cycle of the column interrupted a light beam which created one pulse to drive the pin along the X-axis. An electronic converter, finally, was incorporated electronically to decrease logarithmically with time the actual magnitude of the pen steps. This resulted in a convenient linear-log plot depicting sample passage rate as shown in subsequent figures.
INTRODUCTION Whitby [ 1], in an investigation of the mechanics of sieving, showed that the rate of passage of particles through a sieve produced essentially two straight-line regions on a loglog plot, the first region representing those particles well below sieve opening size and the second near-opening size particles. Meloy et al. [2 - 5] characterized particle shape using the time of passage of closely sized particles through a stack of identical sieves, the particles having been previously sized just to pass the sieve stack. Sieves require careful handling and are difficult to keep free of blocked sections. This study explored the characteristics of a special column through which a wide range of particle sizes could freely pass. The time of passage as a function of both particle size and shape was assessed in the manner generally associated with gas and liquid chromatography techniques. 0032-5910/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
218
The column with which all results presented herein were obtained consisted of 7 aluminum sections each 1.5 in in internal diameter and 2.0 in high. The sections were clamped together with a disc of No. 20 U.S. Standard Series wire mesh (opening 841 pm) between each section. Each section was also approximately 85% filled with 1680 to 2000~pm diameter solid glass spheres. These spheres could thus neither pass nor lodge in the wire mesh. Instead, each of the seven beds of spheres presented a shifting, tortuous series of paths through which sample particles had to pass. There was also no blinding of the supporting wire discs as long as sample particle diameters (minimum dimension in the case of needle-shaped particles) were confined to well below mesh opening sizes. Pure vertical vibration of the column was provided by mounting it on a rigid slide mechanism. An off-center cam with cam follower lifted the column l/16 in with each revolution. Spring loading held the follower and cam in contact. The cam was driven by a D.C. motor turning at approximately 1000 rev. min-’ .
100
0-
0.1
0.5 Time,
1
2
345
min
Fig. 1. Passage rate of two sized fractions and of an equal mixture of the fractions.
of limestone
did the larger size trace, demonstrates the essential individual action of each particle size. Figures 2 and 3 reveal the same individual action for nickel powder fractions. In these cases, recovered fractions were combined after individual testing and retested, giving the trace labeled ‘combined’. The horizontal ending for each individual fraction trace is evidence of complete recovery even though the presentation on a plot with a
POWDERS
z
Agricultural limestone, nickel, crushed glass, and spherical glass as used in reflecting signs constituted the powders of this study. A portion of each material was separated with &in. U.S. Standard Sieves into p-sized fractions. Needle-like particles were prepared in three lengths by chopping multistrand, copper, hook-up wire in a special shear. Each strand was approximately 200 pm in diameter. Quantities with lengths averaging 850, 1850, and 2600 pm were produced. Sample quantities for tests typically were between 1 and 10 g.
f
m
z 0
lz
0
0.1
0.5 Time.
1
2
345
min
Fig. 2. Passage rate of two sized fractions of nickel powder and of the combination of the fractions. 100 1
f RESULTS
Figures 1 through 8 reveal graphically the characteristics of the system. The three traces of Fig. 1 show individual rates of passage for two sized fractions of limestone and, superimposed, the rate of passage of a mixture prepared from equal weights of each fraction. The mixture trace, by starting as did the smaller individual fraction trace and ending as
E :
212-210
Cm
: :
0
0.1
0.6 Time.
1
2
346
mln
Fig. 3. Passage rate of three sized fractions of nickel powder and of the combination of the fractions.
219
Y-axis label of 100% recovery might suggest otherwise; the scale of this axis applies only to the ‘combined’ trace. Figure 4 presents the trace of a limestone powder having particles from 63 to 425 pm in diameter and then the trace of the same powder after particles between 212 and 250 pm were extracted by sieving. The deficiency in mid-range particles is clearly evident in the second trace. Figure 5 shows the result for equal size and equal sample weight fractions of crushed glass particles and glass spheres. The fact that the crushed fraction emerged sooner is somewhat surprising in that powders of spherical particles ordinarily flow much more readily than do irregular particle powders. This behavior is confirmed by the trace of Fig. 6, which is for a 25% crushed and 75% spherical by weight mixture of the two glasses. The dotted lines reproduce the traces of Fig. 5 for aid in revealing the comparison. The influence of length on the passage of cylindrical rod-like particles of copper wire all
,
_______ ,/- e\ :I’ ’
I
$
,’ ’
\*
,I’ ,’
y
,‘2 r
’
,’ ,’
b\
2
I
,’
I’ ; I’ 0 ,’
0
’
-1’
,’ ,’
I’
de
2/ ,,’
/
, ,’ I
0.1
I
0.5 Time,
2
1
345
min
Fig. 6. Passage rate for a mixture of the two glasses of Fig. 5.
of approximately 200 pm in diameter is shown in Fig. 7. Not surprisingly, longer rods require longer times for passage. The trace for the 2600 -pm length rods eventually attained loo%, but roughly 1 h was required as the approach was quite slow. Finally, the traces of Fig. 8 show the relative passage rates and recovery of different sized fractions but equal initial weights of nickel powder, including an under 45 pm
100
.:
0.1
0.5 Time.
2
1
345
min
Fig. 4. Passage rate of a wider distribution of limestone particles and a repeat with a mid-range fraction removed.
0.1
0.5 Time.
1
2
.I
0.1
0.6 Tlma
1
2
345
.mln
Fig. 7. Passage rates for copper rod-like particles of 200 pm diam. and different lengths.
345
min
Fig. 5. Passage rates for fractions of crushed and spherical glass powders of the same size range.
Time.
mln
Fig. 8. Relative passage rates for sized fractions of nickel powder including the -45 pm component.
J
220
fraction. The latter was never 100% recovered using only column vibration. Experiments with such fines were inevitably terminated by blowing the remaining particles from the column with a jet of compressed air aimed alternately into the top and bottom of the column. This behavior of fines was found with all the powder materials tested.
noted, the retained fines could be readily blown out with compressed air. Also, the column never required cleaning by disassembly as long as it was not exposed to particle sizes greater than the retaining screen opening dimensions. The system and technique described define what this author finds to be interesting phenomena. The work is reported in the hope that it will suggest to someone a practical use.
CONCLUSIONS
Were it not for the less than complete recovery of particles under about 45 pm in diameter, the system described might find practical utility in size or shape characterization. Various off-center cams and cams with stepped risers were explored to alter the vibrational intensity and, hence, the behavior of the fine particles. These steps shifted the curves but did not substantially change the patterns given. Powders were thoroughly dried and the column was flushed with dry nitrogen, again to no avail. But, as already
REFERENCES K. T. Whitby, Symposium on Particle Size MeaASTM Spec. Tech. Publ. No. 234, 1958, p. 3. T. P. Meloy, Int. Symp. on Recent Advances in Particulate Science and Technology, I.I.T, Madras, 1982, p. A-99. T. P. Meloy and K. Makino, Powder Technol., 36 (1983) 253. T. P. Meloy, N. Clark, T. E. Durney and B. Pitchumani, Part. Sci. Tech., 2 (1984) 259. S. K. Kennedy, T. P. Meloy and T. E. Durney, J. Sed. Petro., 55 (1985) 356.
surement,