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Examination of single and repetitive impact breakage
Minerals Engineering, Vol. 7, No. 4, pp. 479-490, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892--6875/94 $6.00+0.00
Pergamon 0892-6875(93)E0033-T
EXAMINATION OF SINGLE AND REPETITIVE IMPACT BREAKAGE
A.M.E. RIZK, H.A.A. EL-SAGEER and M.A. DOHEIM Mining & Metall. Dept., Faculty of Engng., Assiut University, Egypt (Received 30 June 1993; accepted 8 November 1993)
ABSTRACT Impact breakage is experienced in many size reduction machines. The crushing behavior of a single bed under single and repetitive hnpact is studied. Materials with widely differing physical and mechanical properties and varied feed size were used to elucidate their behavior under varied values of impact energy. It is shown that with an increase in the feed size of the materials, the crushability increases at the same applied energy for both single and repetitive impacts. The repetitive #npact shows more energy eJ~ciency than the single hnpact with materials that show differences in compressive strength, depending upon the orientation of the applied impact. On the other hat~ single impact crushing is more effective than the repetitive onefor materials that have no bedding planes or cleavages. Keywords Impact breakage; crushing; stamp mill; fracture energy; breakage ratio
INTRODUCTION The energy required for size reduction of materials is greatly dependent on the employed type of loading. The impact breakage of single and multiple particles has been investigated previously [ I-6]. The breakage of particles by slow compression has also been studied [7-13]. Compressive and impact stresses have been applied on particle layers of narrow initial size [1]. The comparison of both types of loading revealed a principle congruence of tendencies as to the fragment size distribution, selection function and energy utilization. It was also found that the percentage of energy losses was higher in the ease of impact stress due to the kinetic energy of the fragments. In comparison to compressive stress, the product of eomminution was less fine. Pauw and Mare [2] broke single ore particles in an impact breakage device and determined that the optimum amount of impact energy to particles in size classes close to but larger than the required product size can lead to over-breakage [3]. Single particle crushing experiments were then done[7] to investigate whether impact or compression crushing is more suitable for liberation. It was found that the degree of liberation depends mainly on the degree of size reduction, and not on the manner of loading, which has no or only a weak influence. Klotz and Schuber [8] found that the parameters of the experimental size distribution depend on the comminution energy. They applied their work on single irregular shaped particles by slow compression. The stress conditions in roll, roller, ball and rod mills were simulated by slow compression of particle beds between flat, cylindrical and spherical pistons [9]. This showed that the parameters of the progeny 479
480
A.M.E.
RIZK et al.
size distribution are essentially not influenced by stress conditions and comminution energy. However, the mass fractions of the constituents assemblies strongly depend on the comminution energy. Specific fracture energy has a tendency to increase with decreasing particle size. Particles under a few micrometers deformed to a considerable extent without fracture [10]. This may suggest that even brittle materials deform plastically. The deformation and fracture behavior of a single spherical particle compressed between two rigid plates has been examined by means of finite element analyses. The influence of the behavior of different materials was investigated and the consequences with respect to processes discussed [I I]. The purpose of the present work is to compare the crushing results of single and repetitive impacts applied on single particle layers of different materials, by making use of the stamp mill as an impact machine. It should be noted that the layer is exposed to the same value of energy in both single and repetitive impacts, but this value of energy is exerted once, in the case of single impact and in the form of equal partitions in the case of repetitive impact. The materials to be tested are widely different in their physical and mechanical properties. Consequently, the present work is also intended to clarify the suitability of each of the mentioned impact types for each material.
EXPERIMENTAL WORK Materials Five materials namely feldspar, siliceous phosphate, limestone, marl and coal were used for the tests. Each material was crushed in a jaw crusher, then screened thoroughly to (-10+ 8), (-8 +6.3), (-6.3 +5), (-5+4) and (-4+3.15) mm fractions, as initial sizes for experimentations. The necessary weights for constructing a single particle layer were different from one size to the other depending upon their apparent densities. The weights were also different from one material to the other according to their mineral densities. Some physical and mechanical properties of the tested materials, which affect the crushing characteristics, are shown in Table 1. These properties include density, porosity, abrasion, compressive strength and modulus of elasticity. The moduli of elasticity were obtained from the stress -strain diagrams shown in Figure 1. TABLE 1 Some physical and mechanical properties of the studied materials Mechanical properties
Physical proper. Material type
Feldspar Phosphate Coal Limestone Marl
Densl~y gm/cm 2.495
2.674 1.272 2.613 2.630
Poros. %
Abrasion
value
1.3 17.9 2.1
0.012 0.175
24.8 28.9
0.150 0.237
0.014
Comp.str~ngth kN/cm along across
1.632 0.476 0.785
4.145 3.940 3.204 0.749 1.084
glast.~odulus kN/cm-1960 1600 1366 958 660
Apparatus and Procedure A stamp mill designed by Mehrim [14] as illustrated in Figure 2 was used for the experimental work. The dropping weight was 24.05 kg, and the free falling distance was varied from 4, 8, 12, 16 and 20 era. For each size of any studied material eight samples were tested. The conditions for carrying out these experiments are listed in Table 2. Each crushed sample was sized from its top size down to .-0.63 ram.
Singleand repetitiveimpactbreakage
NE
o z
v
t/)
481
t,.0 [---a Limestone 3 . 6 -0 Phosphate • Feldspar 3.2 -o Coal 2.8 -• Marl 2.4 2.0 1.6
1.2 0.1 .
Oil 0
3
6
9
12
15
18 .21
2/,
2"/
S t r a / n s C m l C m X 10 u
Fig. 1 Stress strain diagram for the studied materials.
_•30 ~z ~eS.O0
141.00
St,m ~ ' ~ , e l ~ - - , ~ BOSS
~
~z ,,s.oo
"~l'~
~5
ho.
24.0SKg
.JI
E,L." Dims in C m s .
~
~ZG,L.O,00
Fig.2 The stamp mill used in the work [14].
30
482
A.
M.
E.
RIzK
etal.
TABLE 2 Experimental program of each size of the tested materials gxpet. No.
Palling dist a n c e , m.
No.of drops
Ccushingveight,kg
2 3
0.04 0.04 0.04 0.04 0.08 0.12 0.16 0.20
consumed en~cg¥ kghxlO
ff
4
S
H ff
1 1 1 1
:epetit.
5.2402064 7.8603096
24.05 n
Event type
ff H H
W
10.4804120 13.1005160
N
5.2402064 7.8603096 10.4804120
single
N
#
II ee
13.1005160
RESULTS AND DISCUSSION The specific surface areas of the feed and crushed samples were calculated using the following equation
[15]: (1)
[6,~ / .T
The specific surface area of each crushed sample (Sp) was divided by that of the feed (St-) to obtain the breakage ratio. The consumed energy for crushing was calculated for each tested sample using the following equation [14]: E
=
(2)
W . L . F . 2.7236 x 10.6 kWh
The specific fracture energy in kWh/ton was obtained by dividing the E-values by the weight of each sample in tons. The basic data for the calculations, other than the measured quantities, are shown in Table 3. An example of the breakage ratio calculations for the size (-10+8)mm of marl is shown in Table 4. TABLE 3 Weight and specific surface of the feed s,'unples of the studied materials ( Feed Hate:
ia 1 type
Feldspa: Phosphate Coal Limestone Hat1
-lo+e
I
-8+6.3
size
,
m
)
I -6.3+5
-5+4
v,q. 108 95.5
4.68 4.36
48
9.17
72 83
4.46 4.44
80 68 41 67 65
5.89 5.49 11.55 5.62 5.58
72 59 33
50 57
7.45 6.95 14.61 7.11 7.07
]
lSf
47 52 24
9.35 8.73 18.34
42
8.93 8.87
43
-4+3.15 V, gm I 8f: 39 40.5 23
11.77 10.98 23.09
32
11.24 11.17
33
Figures 3 to 7 show the effect of the specific fracture energy on the breakage ratio for the five feed sizes of the five studied materials making use of single or repetitive impacts. From these Figures, it can be seen that as the specific fracture energy increases, the breakage ratio increases for both single and repetitive impacts. The plots of the Figures also indicate that the breakage ratio increases slightly with increasing specific fracture energy for the hard materials (feldspar and phosphate), moderately for the medium hard materials (limestone and coal) and sharply for the soft materials (marl).
Single and repetitive impactbreakage TABLE 4 Breakage ratio calculations for marl at Particle
size,am
Dn, c a
-8+6.3 -6.3+5 -5+4
-4+3.15 -3.15+2 -2+1.6 -1.6+1.25 -1.25+0.63 -0.63+0
(-10+8 mm) feed size under two drops ~4b / Dn
A~
0.1722 0.2028 0.2885 0.2756 0.2014
0.155 0.145
0,9000 0.7150 0.5650 0.4500 0.3575 0.2575 0.1800 0,1425 0.0940 0.0315
-10+8
483
0.163 0.124 0.072 0.108
0.4194
0.1222 0.1825
0.022 0.026 0.044
0.4681
4.4762
0.14X
.8089 3 .9920
6 ~ I Pp
Product spec.surface, (cm2~qn) Feed s p e c i f i c s u r f a c e , ( c m ' / g n ) The b r e a k a q e r a e t o
27 . 1 8 1 4 .44 6 .12
I0 ALI
&MI
9 - aUFl
oC!
o PI
iI
~
I
7
/
1/
_
I
O ,m
m 6
L
J
&s L
4
m
,~T
-----
Repetitive impact Single impact
01 0
I 0.05
I 0.10
Specific
I Od5
I
O. 20
I
0.25
0.30
fracture encrgy, k W h l t o n
Fig.3 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 1 (-10+8 ram). The relationship between the breakage ratio and the specific fracture energy for the different feed sizes of the same material is shown in Figures 8 to 12 for the studied materials for repetitive impact. The Figures indicate that as the initial size increases the breakage ratio increases at the same applied energy, which is in a good agreement with that obtained by Sikong et al [10]. From Figures 3 to 7, it can be seen that the repetitive impact is more suitable than the single one for the same value of the applied energy with respect to the coal for the all used feed sizes. This is also observed in marl having feed sizes [(-10 + 8) and (-8 + 6.3) mm]. This behaviour may be attributed to the presence
484
A.M.E.
RIZK et al.
of cleavages and bedding planes. The variation of the compressive strength as shown in Table 1 supports this explanation. The high energy of the single impact loses a great amount of its value through the cleavages. Consequently, the stresses can be relieved and microcrack propagation of the crack tips deteriorates. These results support the finding of Pauw [3]. 10 9
;L
8
w
5
•F2
•C2
m
---•
O[ 0
0.05
0. K}
Specific
0.15
Repetitive impact Single impact 0. Z0
0.25
0.30
0,35
fracture energy, kWh/ton
Fig.4 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 2 (-8+6.3 mm). 8
AL3
AM3 o P 3 7 , ~
n F3
• C3 /
/
.4
O
O) J¢ L
co
3
- - ~ - - Repetitve impact Single impact Ot 0
0.05 0.10 0.IS 0.20 0.25 0.30 Specific fracture energy, kWh/ton
0.35
0.40
Fig.5 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 3 (-6.3+5 ram).
485
Single and repetitive impact breakage
The use of repetitive impact leads to the combination of the superseded and preceded stress waves as a result of the repetitive low amounts of energy, so the fractured zone propagates due to fatigue.
,
?
~L4
AM/, o P/,
• F4
a C4
.,,4
./"" 4 r
/ -/
6
4 3
.jr-"
2
_,
V/'j ----
II 0
I
0
!
I
I
I
Rel~itive impact S;ngle impact l
i
L
i
o
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0./.5 0.50 O.SS Specific
fracture energy,kWhlton
Fig.6 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 4 (-5 +4 ram).
1.5 &M$ o P5 BF5 eC5
.2 5 ~4 el
,~3
----
t
Repetitive impact S;ngte impact
0
0
O.OS 0.10
0.15
O.ZO 0.2S 0.30 0.3S
0.40
0.45 O.SO O.5S
Spec;fi¢ fracture energy, kWh/ton Fig.7 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 5 (-4+3.15 ram).
486
A.M.E.
R]ZK et al.
8
~L,!
&L2
oL3
71- • L/, o L 5
6 O
10
a
4
NI
3
OI 0
I 0.05
I 0.10
I O.IS
I 0.20
I 0.25
I 0.30
i 0.35
I 0.40
0.45
Specific fracture energy, kWh/ton
Fig.8 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Limestone in repetitive impact. IO &M!
AM2
IM4
oM5
oM3
8
el J¢ al
P 4
re,
0
0
0.05
0.1}
O.tS
Specific
0.20
0.25
0.30
0.35
(].tO
O.d;5
fracture energy,kWh/ton
Fig.9 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Marl in repetitive impact. Figures 3 and 4 show that, for the same specific fracture energy value (1 kWh/t as example), the bmakap ratios for marl and limestone have high values (from 3.1 to 8.5) compared with feldspar (from 1.5 to
Single and repetitiveimpact breakage
487
2.1). The main reason for this can be explained from the obtained compressive strength of the three materials in Table 1, i.e. the higher the compressive strength the less the breakage ratio obtained. In other words, the energy required to extend away from the elastic limit to reach the breakage for feldspar is greater than that required for marl or limestone. The breakage ratios for coal and phosphate are similar for the same reason.
4"01 Apt
i e z o r3
._o 3.0
//l
~
o
2.5 2.0 n% 1.5 I.O 0.5
O/ 0
I
I
I
I
I
I
0.05 0 .I) 0.15 0 . 2 0 0.25 0.30 Sp~fic fracture energy,kWh/ton
0.3S
Fig. 10 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Phosphate in repetitive impact.
3.0~ L~FI &F2 oF3 I
I uF4 2.5
oF5 r 0
•~ 2.0
! m I
0.5
0
i I I I 0.05 O. K) 0.15 O. 20 0.25 Specific fracture energy, kWh/ton
0<10
0.35
Fig. 11 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Feldspar in repetitive impact.
488
A.M.E.
RIZK et al.
The results for feldspar and phosphate reveal that crushing by the high amount of energy due to single impact produces a closer sized product than crushing by the low energy repetitive type. Consequently, the product surface area as well as the breakage ratio become low. The resultant trend agrees with that obtained by Goll and Hanisch [1] although their results were obtained under compressive stress conditions. 7 ACt
AC2
IC/,
oC5
nC3
6
s
/ ,A
4
3 I
2
1
I
0
I
i
I
I
i
I
I
I
0.05 O.lO 0.15 0.20 0.25 0.20 0.35 02.0 0.65 0.50 0.55 1160 Specific fracture energy~ kWh/ton
Fig. 12 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Coal in repetitive impact. The materials which have no bedding planes or cleavages (feldspar and phosphate) do not show variation in compressive strength with orientation. Regarding these materials, the single impact is more suitable, from an energy efficiency point of view, since a shorter time is needed to transmit the high impact energy away from the point of impact, which leads to quick propagation of the cracks. These cracks would propagate through the impacted material, along with the reflected tensile stress waves created from the compressive impact waves. This finding agrees with that obtained by Gross and Zimmerley as reported by Harris [16] and Pauw [3]. Gross and Zimmerley found that a lower value of new specific surface area was produced by the two impact procedure, when employing two impacts producing the same total gross energy as one impact. On the other hand, Pauw concluded that the product fines increase with increasing energy per impact. From Table 1 it is clear that the limestone and marl have high porosity values as well as low moduli of elasticity, which indicate that these materials are brittle and porous. This explains why both marl and limestone give higher breakage ratios than the other three materials used. The coal has a low porosity but a high modulus of elasticity, so a high portion of the single impact energy may be consumed in the elastic range.
CONCLUSIONS The crushability of the studied materials increases with increase in the amount of applied energy in both single and repetitive impacts. However, the repetitive impact is more energy-efficient than the single one for the materials that show differences in compressive strength depending upon the orientation of the applied impact. Single impact crushing, accompanied with a high amount of energy, is more effective than repetitive impacts for the materials that have no bedding planes or cleavages.
Single and repetitive impact breakage
489
It is advised that crushing machines that apply abrasion and chipping with impact mechanisms be used for the materials having bedding planes or cleavages. Crushing machines which apply a pure impact mechanism, e.g. hammer mills, can be used mainly for the materials which have no bedding planes or cleavages.
LIST OF SYMBOLS
Dn E F L nT
pp S Sf Sp
arithmetic mean (average of the top and bottom screen size); cm. utilized energy; kWh. frequency (number of drops). free falling distance; cm. number of screens. density of the material; gm/em3. total surface of one unit mass of a sample (specific surface);cm2/gm. specific surface area of the feed sample; cm2/gm. specific surface area of the crushed sample; cm2/gm.
Sizes 1, 2, 3, 4 & 5 correspond to (-10+8), (-8+6.3), (-6.3+5), (-5+4), and (-4+3.15) size fractions respectively; mm. W
A4,
dropping weight; kg. mass fraction where: 4) = A4)l + Adp2 + ....... + A4m the value of 4~ for the entire sample is unity. shape factor = 1.75, [17].
REFERENCES .
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
liE 7..4-E
Goll, G. & Hanisch, J., Comparison of crushing results obtained by impact and compressive stresses acting upon particle layers. Aufbereitungs-Technik. Nr. 10 (1987). Pauw, e.G. & Mare, M.S., The determination of optimum impact breakage of an ore. Powder Technol., 54, 3-13 (1988). Pauw, e.G., The optimization of overbreakage during repetitive impact breakage. Powder Technol., 56, 251-257 (1988). Dan, C.C. & Schubert, H., Breakage probability, progeny size distribution and energy utilization of comminution by impact. 7th European Syrup. Comminution., 169-178 (1990). Vervoorn, P.M., Hoeksma, J.K. & Scarlett, B., Particle impact testing. 7th European Symp. Comminution. Edited by Schonert. K. 195-210 (1990). Leschonski, K. & Matsumura, S., A single impact against solid wall as a basis for impact grinding. 7th European S3,mp. Comm., Edited by Schonert K. 211-213 (1990). Kiss, L. & Schonert, K., Liberation of two component material by single particle compression and impact crushing. AuJbereitungs-Technik, Nr.5 (1980). Klotz, K. & Schubert, H., Crushing of single irregularly shaped particles by compression : Size distribution of progeny particles. Powder Technol., 32, 129-137 (1982). Hanisch, J. & Schubert, H., Compressive comminution of particle beds. AuJbereitungs-Technik., Nr.10 (1988). Sikong, L., Hashimoto, H. & Yashima, S., Breakage behaviour of fine particles of brittle mineral and coal. Powder TechnoL, 61, 51-57 (1990). Kienzeir, R. & Schmitt, W., On single particle comminution: Numerical analysis of compressed spheres. Powder TechnoL, 61, 29-38 (1990). Muller, H. & Weiehert, R., Probability of breakage of particles in compression tests upon single particle and packed bed conditions. 7th European Syrup. Comm., 159-168 (1990).
490
13. 14. 15. 16. 17.
A.M.E.
RIZK et al.
Muller, F. & Schonert, K., Influences of stressing velocity and interstitial liquids on interpartiele breakage. 7th European Syrup. Comm., 179-194 (1990). Mehrim, M.R., Energetics of comminution. M.Sc. Thesis. Mining and Metall. Dept. Assiut University, (1970). Mccabe, W. & Smith, J., Unit Operations of Chemical Engineering. McGraw-Hill Book Camp. Inc. New York. 810-814 (1967). Harris, C.C., On the role of energy in comminution: a review of physical and mechanical principles. Trans. 1MM Sec. C., 75. Bulletin No. 712. c 37-e 56 (1966). Gaudin, A.M., Principles of Mineral Dressing. McGraw-Hill Camp. Inc. New York & London. 131-134 (1935).
Pergamon 0892-6875(93)E0033-T
EXAMINATION OF SINGLE AND REPETITIVE IMPACT BREAKAGE
A.M.E. RIZK, H.A.A. EL-SAGEER and M.A. DOHEIM Mining & Metall. Dept., Faculty of Engng., Assiut University, Egypt (Received 30 June 1993; accepted 8 November 1993)
ABSTRACT Impact breakage is experienced in many size reduction machines. The crushing behavior of a single bed under single and repetitive hnpact is studied. Materials with widely differing physical and mechanical properties and varied feed size were used to elucidate their behavior under varied values of impact energy. It is shown that with an increase in the feed size of the materials, the crushability increases at the same applied energy for both single and repetitive impacts. The repetitive #npact shows more energy eJ~ciency than the single hnpact with materials that show differences in compressive strength, depending upon the orientation of the applied impact. On the other hat~ single impact crushing is more effective than the repetitive onefor materials that have no bedding planes or cleavages. Keywords Impact breakage; crushing; stamp mill; fracture energy; breakage ratio
INTRODUCTION The energy required for size reduction of materials is greatly dependent on the employed type of loading. The impact breakage of single and multiple particles has been investigated previously [ I-6]. The breakage of particles by slow compression has also been studied [7-13]. Compressive and impact stresses have been applied on particle layers of narrow initial size [1]. The comparison of both types of loading revealed a principle congruence of tendencies as to the fragment size distribution, selection function and energy utilization. It was also found that the percentage of energy losses was higher in the ease of impact stress due to the kinetic energy of the fragments. In comparison to compressive stress, the product of eomminution was less fine. Pauw and Mare [2] broke single ore particles in an impact breakage device and determined that the optimum amount of impact energy to particles in size classes close to but larger than the required product size can lead to over-breakage [3]. Single particle crushing experiments were then done[7] to investigate whether impact or compression crushing is more suitable for liberation. It was found that the degree of liberation depends mainly on the degree of size reduction, and not on the manner of loading, which has no or only a weak influence. Klotz and Schuber [8] found that the parameters of the experimental size distribution depend on the comminution energy. They applied their work on single irregular shaped particles by slow compression. The stress conditions in roll, roller, ball and rod mills were simulated by slow compression of particle beds between flat, cylindrical and spherical pistons [9]. This showed that the parameters of the progeny 479
480
A.M.E.
RIZK et al.
size distribution are essentially not influenced by stress conditions and comminution energy. However, the mass fractions of the constituents assemblies strongly depend on the comminution energy. Specific fracture energy has a tendency to increase with decreasing particle size. Particles under a few micrometers deformed to a considerable extent without fracture [10]. This may suggest that even brittle materials deform plastically. The deformation and fracture behavior of a single spherical particle compressed between two rigid plates has been examined by means of finite element analyses. The influence of the behavior of different materials was investigated and the consequences with respect to processes discussed [I I]. The purpose of the present work is to compare the crushing results of single and repetitive impacts applied on single particle layers of different materials, by making use of the stamp mill as an impact machine. It should be noted that the layer is exposed to the same value of energy in both single and repetitive impacts, but this value of energy is exerted once, in the case of single impact and in the form of equal partitions in the case of repetitive impact. The materials to be tested are widely different in their physical and mechanical properties. Consequently, the present work is also intended to clarify the suitability of each of the mentioned impact types for each material.
EXPERIMENTAL WORK Materials Five materials namely feldspar, siliceous phosphate, limestone, marl and coal were used for the tests. Each material was crushed in a jaw crusher, then screened thoroughly to (-10+ 8), (-8 +6.3), (-6.3 +5), (-5+4) and (-4+3.15) mm fractions, as initial sizes for experimentations. The necessary weights for constructing a single particle layer were different from one size to the other depending upon their apparent densities. The weights were also different from one material to the other according to their mineral densities. Some physical and mechanical properties of the tested materials, which affect the crushing characteristics, are shown in Table 1. These properties include density, porosity, abrasion, compressive strength and modulus of elasticity. The moduli of elasticity were obtained from the stress -strain diagrams shown in Figure 1. TABLE 1 Some physical and mechanical properties of the studied materials Mechanical properties
Physical proper. Material type
Feldspar Phosphate Coal Limestone Marl
Densl~y gm/cm 2.495
2.674 1.272 2.613 2.630
Poros. %
Abrasion
value
1.3 17.9 2.1
0.012 0.175
24.8 28.9
0.150 0.237
0.014
Comp.str~ngth kN/cm along across
1.632 0.476 0.785
4.145 3.940 3.204 0.749 1.084
glast.~odulus kN/cm-1960 1600 1366 958 660
Apparatus and Procedure A stamp mill designed by Mehrim [14] as illustrated in Figure 2 was used for the experimental work. The dropping weight was 24.05 kg, and the free falling distance was varied from 4, 8, 12, 16 and 20 era. For each size of any studied material eight samples were tested. The conditions for carrying out these experiments are listed in Table 2. Each crushed sample was sized from its top size down to .-0.63 ram.
Singleand repetitiveimpactbreakage
NE
o z
v
t/)
481
t,.0 [---a Limestone 3 . 6 -0 Phosphate • Feldspar 3.2 -o Coal 2.8 -• Marl 2.4 2.0 1.6
1.2 0.1 .
Oil 0
3
6
9
12
15
18 .21
2/,
2"/
S t r a / n s C m l C m X 10 u
Fig. 1 Stress strain diagram for the studied materials.
_•30 ~z ~eS.O0
141.00
St,m ~ ' ~ , e l ~ - - , ~ BOSS
~
~z ,,s.oo
"~l'~
~5
ho.
24.0SKg
.JI
E,L." Dims in C m s .
~
~ZG,L.O,00
Fig.2 The stamp mill used in the work [14].
30
482
A.
M.
E.
RIzK
etal.
TABLE 2 Experimental program of each size of the tested materials gxpet. No.
Palling dist a n c e , m.
No.of drops
Ccushingveight,kg
2 3
0.04 0.04 0.04 0.04 0.08 0.12 0.16 0.20
consumed en~cg¥ kghxlO
ff
4
S
H ff
1 1 1 1
:epetit.
5.2402064 7.8603096
24.05 n
Event type
ff H H
W
10.4804120 13.1005160
N
5.2402064 7.8603096 10.4804120
single
N
#
II ee
13.1005160
RESULTS AND DISCUSSION The specific surface areas of the feed and crushed samples were calculated using the following equation
[15]: (1)
[6,~ / .T
The specific surface area of each crushed sample (Sp) was divided by that of the feed (St-) to obtain the breakage ratio. The consumed energy for crushing was calculated for each tested sample using the following equation [14]: E
=
(2)
W . L . F . 2.7236 x 10.6 kWh
The specific fracture energy in kWh/ton was obtained by dividing the E-values by the weight of each sample in tons. The basic data for the calculations, other than the measured quantities, are shown in Table 3. An example of the breakage ratio calculations for the size (-10+8)mm of marl is shown in Table 4. TABLE 3 Weight and specific surface of the feed s,'unples of the studied materials ( Feed Hate:
ia 1 type
Feldspa: Phosphate Coal Limestone Hat1
-lo+e
I
-8+6.3
size
,
m
)
I -6.3+5
-5+4
v,q. 108 95.5
4.68 4.36
48
9.17
72 83
4.46 4.44
80 68 41 67 65
5.89 5.49 11.55 5.62 5.58
72 59 33
50 57
7.45 6.95 14.61 7.11 7.07
]
lSf
47 52 24
9.35 8.73 18.34
42
8.93 8.87
43
-4+3.15 V, gm I 8f: 39 40.5 23
11.77 10.98 23.09
32
11.24 11.17
33
Figures 3 to 7 show the effect of the specific fracture energy on the breakage ratio for the five feed sizes of the five studied materials making use of single or repetitive impacts. From these Figures, it can be seen that as the specific fracture energy increases, the breakage ratio increases for both single and repetitive impacts. The plots of the Figures also indicate that the breakage ratio increases slightly with increasing specific fracture energy for the hard materials (feldspar and phosphate), moderately for the medium hard materials (limestone and coal) and sharply for the soft materials (marl).
Single and repetitive impactbreakage TABLE 4 Breakage ratio calculations for marl at Particle
size,am
Dn, c a
-8+6.3 -6.3+5 -5+4
-4+3.15 -3.15+2 -2+1.6 -1.6+1.25 -1.25+0.63 -0.63+0
(-10+8 mm) feed size under two drops ~4b / Dn
A~
0.1722 0.2028 0.2885 0.2756 0.2014
0.155 0.145
0,9000 0.7150 0.5650 0.4500 0.3575 0.2575 0.1800 0,1425 0.0940 0.0315
-10+8
483
0.163 0.124 0.072 0.108
0.4194
0.1222 0.1825
0.022 0.026 0.044
0.4681
4.4762
0.14X
.8089 3 .9920
6 ~ I Pp
Product spec.surface, (cm2~qn) Feed s p e c i f i c s u r f a c e , ( c m ' / g n ) The b r e a k a q e r a e t o
27 . 1 8 1 4 .44 6 .12
I0 ALI
&MI
9 - aUFl
oC!
o PI
iI
~
I
7
/
1/
_
I
O ,m
m 6
L
J
&s L
4
m
,~T
-----
Repetitive impact Single impact
01 0
I 0.05
I 0.10
Specific
I Od5
I
O. 20
I
0.25
0.30
fracture encrgy, k W h l t o n
Fig.3 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 1 (-10+8 ram). The relationship between the breakage ratio and the specific fracture energy for the different feed sizes of the same material is shown in Figures 8 to 12 for the studied materials for repetitive impact. The Figures indicate that as the initial size increases the breakage ratio increases at the same applied energy, which is in a good agreement with that obtained by Sikong et al [10]. From Figures 3 to 7, it can be seen that the repetitive impact is more suitable than the single one for the same value of the applied energy with respect to the coal for the all used feed sizes. This is also observed in marl having feed sizes [(-10 + 8) and (-8 + 6.3) mm]. This behaviour may be attributed to the presence
484
A.M.E.
RIZK et al.
of cleavages and bedding planes. The variation of the compressive strength as shown in Table 1 supports this explanation. The high energy of the single impact loses a great amount of its value through the cleavages. Consequently, the stresses can be relieved and microcrack propagation of the crack tips deteriorates. These results support the finding of Pauw [3]. 10 9
;L
8
w
5
•F2
•C2
m
---•
O[ 0
0.05
0. K}
Specific
0.15
Repetitive impact Single impact 0. Z0
0.25
0.30
0,35
fracture energy, kWh/ton
Fig.4 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 2 (-8+6.3 mm). 8
AL3
AM3 o P 3 7 , ~
n F3
• C3 /
/
.4
O
O) J¢ L
co
3
- - ~ - - Repetitve impact Single impact Ot 0
0.05 0.10 0.IS 0.20 0.25 0.30 Specific fracture energy, kWh/ton
0.35
0.40
Fig.5 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 3 (-6.3+5 ram).
485
Single and repetitive impact breakage
The use of repetitive impact leads to the combination of the superseded and preceded stress waves as a result of the repetitive low amounts of energy, so the fractured zone propagates due to fatigue.
,
?
~L4
AM/, o P/,
• F4
a C4
.,,4
./"" 4 r
/ -/
6
4 3
.jr-"
2
_,
V/'j ----
II 0
I
0
!
I
I
I
Rel~itive impact S;ngle impact l
i
L
i
o
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0./.5 0.50 O.SS Specific
fracture energy,kWhlton
Fig.6 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 4 (-5 +4 ram).
1.5 &M$ o P5 BF5 eC5
.2 5 ~4 el
,~3
----
t
Repetitive impact S;ngte impact
0
0
O.OS 0.10
0.15
O.ZO 0.2S 0.30 0.3S
0.40
0.45 O.SO O.5S
Spec;fi¢ fracture energy, kWh/ton Fig.7 The effect of the specific fracture energy on the breakage ratio for the studied materials of feed size 5 (-4+3.15 ram).
486
A.M.E.
R]ZK et al.
8
~L,!
&L2
oL3
71- • L/, o L 5
6 O
10
a
4
NI
3
OI 0
I 0.05
I 0.10
I O.IS
I 0.20
I 0.25
I 0.30
i 0.35
I 0.40
0.45
Specific fracture energy, kWh/ton
Fig.8 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Limestone in repetitive impact. IO &M!
AM2
IM4
oM5
oM3
8
el J¢ al
P 4
re,
0
0
0.05
0.1}
O.tS
Specific
0.20
0.25
0.30
0.35
(].tO
O.d;5
fracture energy,kWh/ton
Fig.9 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Marl in repetitive impact. Figures 3 and 4 show that, for the same specific fracture energy value (1 kWh/t as example), the bmakap ratios for marl and limestone have high values (from 3.1 to 8.5) compared with feldspar (from 1.5 to
Single and repetitiveimpact breakage
487
2.1). The main reason for this can be explained from the obtained compressive strength of the three materials in Table 1, i.e. the higher the compressive strength the less the breakage ratio obtained. In other words, the energy required to extend away from the elastic limit to reach the breakage for feldspar is greater than that required for marl or limestone. The breakage ratios for coal and phosphate are similar for the same reason.
4"01 Apt
i e z o r3
._o 3.0
//l
~
o
2.5 2.0 n% 1.5 I.O 0.5
O/ 0
I
I
I
I
I
I
0.05 0 .I) 0.15 0 . 2 0 0.25 0.30 Sp~fic fracture energy,kWh/ton
0.3S
Fig. 10 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Phosphate in repetitive impact.
3.0~ L~FI &F2 oF3 I
I uF4 2.5
oF5 r 0
•~ 2.0
! m I
0.5
0
i I I I 0.05 O. K) 0.15 O. 20 0.25 Specific fracture energy, kWh/ton
0<10
0.35
Fig. 11 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Feldspar in repetitive impact.
488
A.M.E.
RIZK et al.
The results for feldspar and phosphate reveal that crushing by the high amount of energy due to single impact produces a closer sized product than crushing by the low energy repetitive type. Consequently, the product surface area as well as the breakage ratio become low. The resultant trend agrees with that obtained by Goll and Hanisch [1] although their results were obtained under compressive stress conditions. 7 ACt
AC2
IC/,
oC5
nC3
6
s
/ ,A
4
3 I
2
1
I
0
I
i
I
I
i
I
I
I
0.05 O.lO 0.15 0.20 0.25 0.20 0.35 02.0 0.65 0.50 0.55 1160 Specific fracture energy~ kWh/ton
Fig. 12 Effect of the specific fracture energy on the breakage ratio for the used sizes (1,2,3,4 & 5) of Coal in repetitive impact. The materials which have no bedding planes or cleavages (feldspar and phosphate) do not show variation in compressive strength with orientation. Regarding these materials, the single impact is more suitable, from an energy efficiency point of view, since a shorter time is needed to transmit the high impact energy away from the point of impact, which leads to quick propagation of the cracks. These cracks would propagate through the impacted material, along with the reflected tensile stress waves created from the compressive impact waves. This finding agrees with that obtained by Gross and Zimmerley as reported by Harris [16] and Pauw [3]. Gross and Zimmerley found that a lower value of new specific surface area was produced by the two impact procedure, when employing two impacts producing the same total gross energy as one impact. On the other hand, Pauw concluded that the product fines increase with increasing energy per impact. From Table 1 it is clear that the limestone and marl have high porosity values as well as low moduli of elasticity, which indicate that these materials are brittle and porous. This explains why both marl and limestone give higher breakage ratios than the other three materials used. The coal has a low porosity but a high modulus of elasticity, so a high portion of the single impact energy may be consumed in the elastic range.
CONCLUSIONS The crushability of the studied materials increases with increase in the amount of applied energy in both single and repetitive impacts. However, the repetitive impact is more energy-efficient than the single one for the materials that show differences in compressive strength depending upon the orientation of the applied impact. Single impact crushing, accompanied with a high amount of energy, is more effective than repetitive impacts for the materials that have no bedding planes or cleavages.
Single and repetitive impact breakage
489
It is advised that crushing machines that apply abrasion and chipping with impact mechanisms be used for the materials having bedding planes or cleavages. Crushing machines which apply a pure impact mechanism, e.g. hammer mills, can be used mainly for the materials which have no bedding planes or cleavages.
LIST OF SYMBOLS
Dn E F L nT
pp S Sf Sp
arithmetic mean (average of the top and bottom screen size); cm. utilized energy; kWh. frequency (number of drops). free falling distance; cm. number of screens. density of the material; gm/em3. total surface of one unit mass of a sample (specific surface);cm2/gm. specific surface area of the feed sample; cm2/gm. specific surface area of the crushed sample; cm2/gm.
Sizes 1, 2, 3, 4 & 5 correspond to (-10+8), (-8+6.3), (-6.3+5), (-5+4), and (-4+3.15) size fractions respectively; mm. W
A4,
dropping weight; kg. mass fraction where: 4) = A4)l + Adp2 + ....... + A4m the value of 4~ for the entire sample is unity. shape factor = 1.75, [17].
REFERENCES .
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
liE 7..4-E
Goll, G. & Hanisch, J., Comparison of crushing results obtained by impact and compressive stresses acting upon particle layers. Aufbereitungs-Technik. Nr. 10 (1987). Pauw, e.G. & Mare, M.S., The determination of optimum impact breakage of an ore. Powder Technol., 54, 3-13 (1988). Pauw, e.G., The optimization of overbreakage during repetitive impact breakage. Powder Technol., 56, 251-257 (1988). Dan, C.C. & Schubert, H., Breakage probability, progeny size distribution and energy utilization of comminution by impact. 7th European Syrup. Comminution., 169-178 (1990). Vervoorn, P.M., Hoeksma, J.K. & Scarlett, B., Particle impact testing. 7th European Symp. Comminution. Edited by Schonert. K. 195-210 (1990). Leschonski, K. & Matsumura, S., A single impact against solid wall as a basis for impact grinding. 7th European S3,mp. Comm., Edited by Schonert K. 211-213 (1990). Kiss, L. & Schonert, K., Liberation of two component material by single particle compression and impact crushing. AuJbereitungs-Technik, Nr.5 (1980). Klotz, K. & Schubert, H., Crushing of single irregularly shaped particles by compression : Size distribution of progeny particles. Powder Technol., 32, 129-137 (1982). Hanisch, J. & Schubert, H., Compressive comminution of particle beds. AuJbereitungs-Technik., Nr.10 (1988). Sikong, L., Hashimoto, H. & Yashima, S., Breakage behaviour of fine particles of brittle mineral and coal. Powder TechnoL, 61, 51-57 (1990). Kienzeir, R. & Schmitt, W., On single particle comminution: Numerical analysis of compressed spheres. Powder TechnoL, 61, 29-38 (1990). Muller, H. & Weiehert, R., Probability of breakage of particles in compression tests upon single particle and packed bed conditions. 7th European Syrup. Comm., 159-168 (1990).
490
13. 14. 15. 16. 17.
A.M.E.
RIZK et al.
Muller, F. & Schonert, K., Influences of stressing velocity and interstitial liquids on interpartiele breakage. 7th European Syrup. Comm., 179-194 (1990). Mehrim, M.R., Energetics of comminution. M.Sc. Thesis. Mining and Metall. Dept. Assiut University, (1970). Mccabe, W. & Smith, J., Unit Operations of Chemical Engineering. McGraw-Hill Book Camp. Inc. New York. 810-814 (1967). Harris, C.C., On the role of energy in comminution: a review of physical and mechanical principles. Trans. 1MM Sec. C., 75. Bulletin No. 712. c 37-e 56 (1966). Gaudin, A.M., Principles of Mineral Dressing. McGraw-Hill Camp. Inc. New York & London. 131-134 (1935).