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Contact electrification of coal and minerals
_•
Journal of
ELECTROSTATICS Journal of Electrostatics 32 (1994) 271-276
ELSEVIER
Contact electrification of coal and minerals B.A. K w e t k u s ABB Corporate Research, 5405 Baden-Dattwil and E T H Zurich, Laboratory of Solid State Physics, 8093 Zurich, Switzerland
Abstract The contact electrification of coals from France, the Great Britain and the United States has been studied. Additionally, the behavior of the most abundant minerals in coal (quartz, calcite and pyrite) has been investigated. The charge transfer upon repeated contacts with a copper plate has been measured. The study covers the influence of the mineral content, relative humidity, temperature and gas pressure on the electrification behavior.
1. Introduction The tribo- and contact electrification of solids is known for a long time but still the basic mechanisms leading to the transfer of electrical charges upon contact and separation of two solids are not well understood. The contact electrification of coal is of considerable technical importance for two reasons: (i) sparks may occur due to gas discharges and can ignite a coal dust mixture to cause severe hazards in coal mines and coal processing [1], (ii) electrostatic separation of the sulfurcontaining and ash-forming minerals from coal is often based on the tribocharging effect [2].
2. Experimental The setup used for the experiments has been described elsewhere [3]. Powdered samples (sieved 60-90 ~tm) were fixed on an insulating holder. The holder can be moved to repeatedly contact a metal plate in a controlled environment (vacuum chamber). Electrometers were used to measure the charges on the powder sample indirectly in a Faraday-cylinder and directly on the metal plate. The results were contact electrification curves representing the dependence of the total net charge on the samples on the number of contacts. 0304-3886/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0304-3886(93) E0036-H
272
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
Table 1 Analysis of the coal samples Coal
FR coal FR coal MG 1.3-1.35" FR coal < 1.3" US coal US coal MG < 1.3" US coal < 1.3 GB coal
Ash (dry) % weight
Volatile matter (dry and ash-free) % weight
Mineral analysis Pyrite
18.89 5.30 5.80 10.52 6.30 3.40 2.05
55.47
14.80
46.68
67.30
7.00
4.59
0.50
9.10
quartz % weight
calcite
53.70
9.20
Table 1 presents an analysis of the coals from France (FR), the Great Britain (GB) and the United States (US) used in the experiments. Some of the samples (*) were specially prepared by flotation to reduce the ash content.
3. Results and discussion
Figure 1 shows, as an example, the contact electrification curves for the US coal and pyrite. The coal sample revealed charge accumulation with subsequent contacts and a net charge in the 10 -9 C range. This behavior is typical for good insulators [4]. However, the pyrite sample showed a saturation tendency and a significantly lower net charge in the 10-12C range. The different behavior can be explained with the difference in electrical conductivity, which is approximately 10-12 (f~ cm)-1 for coal and 10-5 (f~ cm)-1 for pyrite. The charges on the pyrite spread rapidly over the particle surface. Thermodynamic equilibrium between the contact partners is reached after a few contacts, i.e. the Fermi levels of the solids in contact equalize. The same behavior was observed before with other well conducting materials (oxidized metal powders) [5]. Figure 2 gives the maximum total net charges on the powder samples after 50 repeated contacts with a copper plate. The standard deviation of the data is given by the error bars. The highest charge values, either positive or negative have been measured on the FR- and US-coal samples. The electrical conductivity of coal depends on the coal rank [6]. Low-ash and low-volatile coal like the GB coal generally reveal a high conductivity and therefore a low electrification tendency as confirmed by the experiments. The polarity of charge could be altered reducing the ash content of the coal by flotation. Two US- and FR-coal types prepared in this way showed positive net charges. However, no conclusive dependence of the total net charge on the ash content can be established.
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
273
0 US-coal/Copper -0.5 ~"
-1.0
g
-1.5
¢")
-2.0
%
O~
I
I
J
Pyrite/Copper -2
6" 04 '7
-4
%
•
•.'. "..............
-8
tO
-10
i
0
I
i
I
10
a
20
3o
Number of contacts
Fig. 1. Contact electrification curves of US coal and pyrite.
! :
US-coal M G < I . 3
!
FR-coal <1.3 GB-coal
Pyrite Calcite
14
Quartz FR-coal MG 1.3-1.35
I=1 I=1 I-IH
FR-coal US-coal <1.3
:
=
I
US-coal
-2
-1
0
Max.
1
charg~(lO "9C)
after 50 contacts
Fig. 2. Contact electrification of coal and mineral samples.
2
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271 276
274
The charge values of the mineral samples (calcite, quartz and pyrite) were orders of magnitude lower than on coal. This is an important result for the electrostatic separation of minerals from coal. However, this investigation was carried out under high vacuum conditions (<5 x 10-5 mbar). Other parameters influence the charge
(")
7(~
•
US.coal/Copper
6 l , -
0
FR-coal/Copper
0
gm
"0
o
0
4
~4
m :S
0 2
,
I
0
,
I
20
i
I
40
,
I
60
=
80
1O0
Relative humidity/*/. Fig. 3. Contact charge in dependence on the relative humidity.
10 -e •
FR-coal<1.3/Copper
0 US.coalMG
0 0
r-
10-10
0
o
IO 5
0
10 -11
10-12 . . . . . . . . . . . . . . .00001 .0001 .001
. . . . . .
a
.01
......
d
.I
......
~
. . . . . .
1
i
. . . . . .
10
Pressure/mbar Fig. 4. Contact charge in dependence on the air pressure.
J
100
......
1000
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
275
2.0 • 0
A
(J
Quartz/Copper Calcite/Copper
1.5
O T-
1.0 in
J¢
¢J e-
0.5
•
•
¢U o 20
0
•
,
,
9
,
30
40
50
60
o
I
70
80
Temperature/°C Fig. 5. Contact charge in dependence on the temperature.
transfer under conditions for practical applications, e.g. the relative humidity of the air. Figure 3 presents results obtained for the contact electrification of the coal samples in air of different relative humidity at 1000 mbar total pressure. It can be seen that the total net charge is significantly reduced by increasing the humidity. This is ascribed to the increase of electrical conductivity by the adsorption of water [7]. In Fig. 4 the maximum net charge is plotted as a function of the pressure of dry air (relative humidity <1%). Both coal samples show typical characteristics with a minimum for the net charge at medium pressures. The occurrence of this minimum indicates that the charge exchange is limited by gas breakdown phenomena and follows Paschen's law. Such discharges have been observed before with other materials [8]. Figure 5 shows the maximum negative charge on calcite and quartz depending on the temperature. The values have been measured under high vacuum conditions. An increase in the contact charge with increasing temperature could be due to water desorption or surface modification resulting in an enhanced number of surface states for electron transfer. The Fermi-level shift is negligible in the measured temperature range. Virtually no effect of a temperature increase was observed with the calcite sample but quartz showed a strong dependence of the contact charge on the temperature. The total net charge increased by almost two orders of magnitude between 20 °C and 80 °C.
4. Conclusion
The contact electrification behavior of coal samples revealed both positive and negative charging. Strong dependence on the chemical composition (especially the ash content), the ambient gas pressure and the air humidity was observed. Gas discharge
276
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
is the electrification-limiting mechanism at higher gas pressures. The charge levels on the mineral samples were generally much lower than on the coal samples. Contact electrification of quartz was influenced by the temperature.
Acknowledgements Prof. H.C. Siegmann and Prof. K. Sattler are gratefully acknowledged for their help, encouragement and advise throughout this work. The work was in part financially supported by the Kommission zur F/Srderung der Wissenschaftlichen Forschung
(KWF). References [1] K.L. Cashdollar and M. Hertzberg (Eds.), Industrial Dust Explosions, ASTM Special Technical Publication, 958, Pittsburgh, 1986. I-2] I.I. Incutet, Electrostatic Mineral Separation, Wiley, New York, 1984. I-3] B.A. Kwetkus, K. Sattler and H.C. Siegrnann, J. Phys. D: Appl. Phys., 25 (1992) 139. 1-41 J. Lowell and A.C. Rose-Innes, Adv. Phys., 29 (1980) 947. I5"1 B.A. Kwetkus and K. Sattler, Zeitschr. Fiir Phys. B, 82 (1991) 87. [61 E.I. Parkhomenko, Electrical Properties of Rocks, Plenum, New York, 1967. [7-1 S. Nieh and T. Nguyen, Part. Sci. Techn., 5 (1987) 115. 1'8-1 B.A. Kwetkus, B. Gellert and K. Sattler, Inst. Phys. Conf. Ser., 118 (1991) 229.
Journal of
ELECTROSTATICS Journal of Electrostatics 32 (1994) 271-276
ELSEVIER
Contact electrification of coal and minerals B.A. K w e t k u s ABB Corporate Research, 5405 Baden-Dattwil and E T H Zurich, Laboratory of Solid State Physics, 8093 Zurich, Switzerland
Abstract The contact electrification of coals from France, the Great Britain and the United States has been studied. Additionally, the behavior of the most abundant minerals in coal (quartz, calcite and pyrite) has been investigated. The charge transfer upon repeated contacts with a copper plate has been measured. The study covers the influence of the mineral content, relative humidity, temperature and gas pressure on the electrification behavior.
1. Introduction The tribo- and contact electrification of solids is known for a long time but still the basic mechanisms leading to the transfer of electrical charges upon contact and separation of two solids are not well understood. The contact electrification of coal is of considerable technical importance for two reasons: (i) sparks may occur due to gas discharges and can ignite a coal dust mixture to cause severe hazards in coal mines and coal processing [1], (ii) electrostatic separation of the sulfurcontaining and ash-forming minerals from coal is often based on the tribocharging effect [2].
2. Experimental The setup used for the experiments has been described elsewhere [3]. Powdered samples (sieved 60-90 ~tm) were fixed on an insulating holder. The holder can be moved to repeatedly contact a metal plate in a controlled environment (vacuum chamber). Electrometers were used to measure the charges on the powder sample indirectly in a Faraday-cylinder and directly on the metal plate. The results were contact electrification curves representing the dependence of the total net charge on the samples on the number of contacts. 0304-3886/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0304-3886(93) E0036-H
272
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
Table 1 Analysis of the coal samples Coal
FR coal FR coal MG 1.3-1.35" FR coal < 1.3" US coal US coal MG < 1.3" US coal < 1.3 GB coal
Ash (dry) % weight
Volatile matter (dry and ash-free) % weight
Mineral analysis Pyrite
18.89 5.30 5.80 10.52 6.30 3.40 2.05
55.47
14.80
46.68
67.30
7.00
4.59
0.50
9.10
quartz % weight
calcite
53.70
9.20
Table 1 presents an analysis of the coals from France (FR), the Great Britain (GB) and the United States (US) used in the experiments. Some of the samples (*) were specially prepared by flotation to reduce the ash content.
3. Results and discussion
Figure 1 shows, as an example, the contact electrification curves for the US coal and pyrite. The coal sample revealed charge accumulation with subsequent contacts and a net charge in the 10 -9 C range. This behavior is typical for good insulators [4]. However, the pyrite sample showed a saturation tendency and a significantly lower net charge in the 10-12C range. The different behavior can be explained with the difference in electrical conductivity, which is approximately 10-12 (f~ cm)-1 for coal and 10-5 (f~ cm)-1 for pyrite. The charges on the pyrite spread rapidly over the particle surface. Thermodynamic equilibrium between the contact partners is reached after a few contacts, i.e. the Fermi levels of the solids in contact equalize. The same behavior was observed before with other well conducting materials (oxidized metal powders) [5]. Figure 2 gives the maximum total net charges on the powder samples after 50 repeated contacts with a copper plate. The standard deviation of the data is given by the error bars. The highest charge values, either positive or negative have been measured on the FR- and US-coal samples. The electrical conductivity of coal depends on the coal rank [6]. Low-ash and low-volatile coal like the GB coal generally reveal a high conductivity and therefore a low electrification tendency as confirmed by the experiments. The polarity of charge could be altered reducing the ash content of the coal by flotation. Two US- and FR-coal types prepared in this way showed positive net charges. However, no conclusive dependence of the total net charge on the ash content can be established.
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
273
0 US-coal/Copper -0.5 ~"
-1.0
g
-1.5
¢")
-2.0
%
O~
I
I
J
Pyrite/Copper -2
6" 04 '7
-4
%
•
•.'. "..............
-8
tO
-10
i
0
I
i
I
10
a
20
3o
Number of contacts
Fig. 1. Contact electrification curves of US coal and pyrite.
! :
US-coal M G < I . 3
!
FR-coal <1.3 GB-coal
Pyrite Calcite
14
Quartz FR-coal MG 1.3-1.35
I=1 I=1 I-IH
FR-coal US-coal <1.3
:
=
I
US-coal
-2
-1
0
Max.
1
charg~(lO "9C)
after 50 contacts
Fig. 2. Contact electrification of coal and mineral samples.
2
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271 276
274
The charge values of the mineral samples (calcite, quartz and pyrite) were orders of magnitude lower than on coal. This is an important result for the electrostatic separation of minerals from coal. However, this investigation was carried out under high vacuum conditions (<5 x 10-5 mbar). Other parameters influence the charge
(")
7(~
•
US.coal/Copper
6 l , -
0
FR-coal/Copper
0
gm
"0
o
0
4
~4
m :S
0 2
,
I
0
,
I
20
i
I
40
,
I
60
=
80
1O0
Relative humidity/*/. Fig. 3. Contact charge in dependence on the relative humidity.
10 -e •
FR-coal<1.3/Copper
0 US.coalMG
0 0
r-
10-10
0
o
IO 5
0
10 -11
10-12 . . . . . . . . . . . . . . .00001 .0001 .001
. . . . . .
a
.01
......
d
.I
......
~
. . . . . .
1
i
. . . . . .
10
Pressure/mbar Fig. 4. Contact charge in dependence on the air pressure.
J
100
......
1000
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
275
2.0 • 0
A
(J
Quartz/Copper Calcite/Copper
1.5
O T-
1.0 in
J¢
¢J e-
0.5
•
•
¢U o 20
0
•
,
,
9
,
30
40
50
60
o
I
70
80
Temperature/°C Fig. 5. Contact charge in dependence on the temperature.
transfer under conditions for practical applications, e.g. the relative humidity of the air. Figure 3 presents results obtained for the contact electrification of the coal samples in air of different relative humidity at 1000 mbar total pressure. It can be seen that the total net charge is significantly reduced by increasing the humidity. This is ascribed to the increase of electrical conductivity by the adsorption of water [7]. In Fig. 4 the maximum net charge is plotted as a function of the pressure of dry air (relative humidity <1%). Both coal samples show typical characteristics with a minimum for the net charge at medium pressures. The occurrence of this minimum indicates that the charge exchange is limited by gas breakdown phenomena and follows Paschen's law. Such discharges have been observed before with other materials [8]. Figure 5 shows the maximum negative charge on calcite and quartz depending on the temperature. The values have been measured under high vacuum conditions. An increase in the contact charge with increasing temperature could be due to water desorption or surface modification resulting in an enhanced number of surface states for electron transfer. The Fermi-level shift is negligible in the measured temperature range. Virtually no effect of a temperature increase was observed with the calcite sample but quartz showed a strong dependence of the contact charge on the temperature. The total net charge increased by almost two orders of magnitude between 20 °C and 80 °C.
4. Conclusion
The contact electrification behavior of coal samples revealed both positive and negative charging. Strong dependence on the chemical composition (especially the ash content), the ambient gas pressure and the air humidity was observed. Gas discharge
276
B.A. Kwetkus/Journal of Electrostatics 32 (1994) 271-276
is the electrification-limiting mechanism at higher gas pressures. The charge levels on the mineral samples were generally much lower than on the coal samples. Contact electrification of quartz was influenced by the temperature.
Acknowledgements Prof. H.C. Siegmann and Prof. K. Sattler are gratefully acknowledged for their help, encouragement and advise throughout this work. The work was in part financially supported by the Kommission zur F/Srderung der Wissenschaftlichen Forschung
(KWF). References [1] K.L. Cashdollar and M. Hertzberg (Eds.), Industrial Dust Explosions, ASTM Special Technical Publication, 958, Pittsburgh, 1986. I-2] I.I. Incutet, Electrostatic Mineral Separation, Wiley, New York, 1984. I-3] B.A. Kwetkus, K. Sattler and H.C. Siegrnann, J. Phys. D: Appl. Phys., 25 (1992) 139. 1-41 J. Lowell and A.C. Rose-Innes, Adv. Phys., 29 (1980) 947. I5"1 B.A. Kwetkus and K. Sattler, Zeitschr. Fiir Phys. B, 82 (1991) 87. [61 E.I. Parkhomenko, Electrical Properties of Rocks, Plenum, New York, 1967. [7-1 S. Nieh and T. Nguyen, Part. Sci. Techn., 5 (1987) 115. 1'8-1 B.A. Kwetkus, B. Gellert and K. Sattler, Inst. Phys. Conf. Ser., 118 (1991) 229.