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Zeta potential of coal fine-particles in aqueous suspension
Powder
Technology,
40 (1964)
161
161 - 165
Zeta Potential
of Coal Fine-Particles
S MOW,
K
T
Department
HARA,
of Mming.
in Aqueous
Suspension
AS0 Kyushu
Unwersity,
Hal:ozakl,
Fukuoka.
812
(Japan)
Umuers~ty.
Hakoraki.
Fukuoka,
812
(Japan)
and H. OKAMOTO Depa+nent
of Physics,
Kyushu
SUMMARY
The relation between the zeta potential of coal fine-partrcles and pH ualue of suspension in aqueous suspenston for different kmds of coal samples was determined by using a rectangular micro-electrophoresis cell. The empirical equation of the relation between the zeta potential of the particles and the chemrcal components of coal samples was determined in aqueous suspension of drfferent pH values The equation is represented as followsZ=BcX(C)+BnX(H)+BoX(0)+BNX X(N)+BsX(S)+B,X(A) where Z is the zeta potentual of the particles, (C). (H), (O), (N), (S) and (Ash) arc weight percent of carbon, hydrogen, oxygen, nltrogen, sulphur and ash.. res_pectiuely. and Bc, Bu, Bo, BN, Bs and BA are coeffmrents of each of the respectrue components of coal samples The above coefficients were calculated numerrcally by the least-square method in suspensions with different pH values The values of the coefficients. Bc, Bn, etc., (mV/%), indicate the zeta potential of each respective single component; viz. carbon, hydrogen, etc., of the coal samples When the chemrcal components of a coal sample are known, the zeta potential of the coal fineparticles can be determined by the equation with the coefficients obtained in suspensions with different pH values
INTRODUCTION
The zeta potential of mmeral particles in suspension plays a significant role in the
flotation behaviour of mineral ores and the sedimentation behavlour of colloidal systems [ 1, 2]_ Zeta potential measurements on different types of coal are useful for the coal fineparticles processing technology A rectangular micro-electrophoresis cell is commonly used for measunng the zeta potential of mineral particles in suspensron. We have proposed an improved method for determining the precise electrophoretic velocity of mineral particles [3]. By our method, we determmed the relation between the zeta potential of coal fineparticles and the pH value of aqueous suspension for different kinds of coal samples. It is important to find out why the different types of coal have different values of zeta potential at the same pH values of suspension. We attempted to relate the zeta potential of the coal particles to their elemental components of the coal samples. The empirical equation of the relation between the zeta potential and the chemical components of coal samples was determined in suspensions with different pH values, and coefficients OF the equation were calculated numerically by the least-square method
EXPERIMENTS
Coal samples The chemical compositions of the coal samples used m the zeta potential measurements are shown m the table The analysis of the samples was performed by K. Koike, and his colleagues, Chofu Lab , Mitsui Coal Mining Co Ltd , Japan. These coals were plotted on the H/C - O/C (atomic ratio) diagram proposed by Krevelen [4], as shown in Fig. 1. The numbers in the fi,qes correspond to the same numbers tabulated in the table. Brown @ Elsevler Sequoia/Prmted
in The Netherlands
TABLE Chemical composition
(wt_%) of dried coal samples
No.
Name
(Country)
1 2 3 4 5 6 7 8 9 1G 11 12 13
Miike (Japan) Beluga (US America) Banco (Indonesia) M~llrnerran (Australia) Yallourn (Australia) Quintette (Canada) Matsuzawa (Japan) Madagascar (Malgache) Sunagawa (Japan) Ashibeku (Japan) Mourn (Australia) Miller (Austraha) Datong (Chma)
C
H
0
N
77.3 65-4 71.4 67.5 67.5 73 9 81.5 80 2 ‘78.4 77-1 i6.3 71.1 78.5
5.5 5.1 5.1 5.6 4.6 3-Q 24 0.3 5.6 5-6 43 48 4.5
7.4 20.6 19.8 10-i 25.9 6.9 2.4 10 5 83 6.8 6.0 7.2 81
1.2 O-7 l-2 1.0 0.6 0.9 1.2 0.0 2.1 19 1.6 1.6 0.8
Ash 08 0.0 0.2 0.4 0.2 0.2 1.5 O-0 03 03 0.4 0.8 05
7.8 8.2 23 14.8 12 14 2 11.0 91 53 83 11-4 14.6 76
velocities of the particles (partrcle diameter: l5 ,um) were observed microscoprcally in the rectangular micro-electrophoresis cell. Zeta potential measurellient was performed by the method previously reported [3] _
Atomic
G/c ratlo
Fig. 1. H/C - O/C (atomic ratio) diagram_ (Numbers in the figure correspond to the same numbers tabulated
Experirr ental results The rAation between the zeta potential of the coal samples and the pH value of suspension was determined for thirteen coal samples. _4n example of the results LS represented by that obtained for Miller Coal (No. 12) as illustratrrd in Fig. 2 The value of the negative charge c-f the coal particles increases with the pH of the suspension_ Similar curves were observed wrth the other coal samples except Nos. 2, 3 and 5. The results for Banco Coal (No. 3) illustrated in Fig 3 exarnplify the
in the table_)
coal, low-rank bituminous coal, medium-rank bituminous coal, high-rank bituminous coal, semi-anthracite and anthracite are respectively represented as V, VI, VII, VIII, M and X, in the diagram used by Klevelen [4]_ Experimental method Two grams of coal powder ground by a vibrating mill were added into 1000 ml of distilled water and the desired pH value obtamed by either adding HCI or NaOH. After adequate stirring, the suspension was kept in a constant temperature bath at 20 “C for 30 min. The rest of the experiment was conducted in a room whose temperature was regulated to between 20 and 22 “C. The
pH
Value
Frg. 2. The relatron between the zeta potentral of coal particles and the pH value of suspension for Miller Coal (No. 12)
pH
Value
the zeta potential of coal particles and the pI1 value of suspension for Banco Coal (No 3). Fig_ 3 The relation between
exceptions_ The value of the negative charge of the coal particles for Banco Coal decreases with increase of the pH in the alkaline region. A wde scatter of the data is noticeable m the alkaline region for Banco Coal as shown in Fig. 3. The results of thirteen coal samples are illustrated in Figs. 4 and 5. The numbers in the figures correspond to the same numbers tabulated in the table
I[, , , , , , , , , , , , , 0
2
4
6 pH
8
10
12
Value
Fig. 4 The relation between the zeta potenclal of coal particles and the pH value of suspension for dlfferent kmds of coal samples
DISCUSSIONS Although the zeta potential is the nature of the surface of the mineral particles in suspension, we attempted to relate the zeta potential to the elemental components on the basis of the simplest idea that the chemical component ratio at the mineral surface without any surface oxidation effect is proportional to that of the bulk chemical composition. The empirical equation of the relation between tbe zeta potential and the chemical components of coal samples was determined in aqueous suspension with different pH values The equation is as follows: Z=BcX(C)+BuX(N)+BoX(O)+ +BNX(N-)+BsX(S)+BAX(A) where 2 is the zeta potential
(1) (mV)
of the par-
ticles, (0, (II), (O), (N), (S) and (A) are weight percent of carbon, hydrogen, oxygen, nitrogen, sulphur and ash, respectively, and Bc, BH, B,, BN, Bs and BA are coefficients (mV/%) of each of the respective components of the coal samples. The weight percent of
I
LO 0
2
4
pH
6
I
I
6
Value
I
I
IO
I
1
12
I
I
Fig 5 The relation between the zeta potential of coal particles and the pH value OF susgension for different kmds of coal samples
each component is uniquely convertible into atomic ratio. It should be noted that a term which represents the effect caused by surface oxidation is neglected for simplicity. How to mtroduce the surface oxidation effect in order to get a more realistic expression for the zeta potential is still food for thought The above coefficients were calculated numencally by the least-square method
164 with a high accuracy of 32 digits (quadruple precision) in suspensions with c’ifferent pH values. In the case of a suspensio I with pH = 3, the following equation is obtaraed: Z,,,
=s, = 0.77
x (C) + 3.00
x (0) -
1.78
-
11.97 X (A)
X (H) -
x (N) -
5.t2
2.86
X
x (S) + (2)
The relation between observed values of the zeta potential of coal particles and those calculated by putting values of the chemical components of coal samples into eqn. (2) is illustrated in Fig. 6 The numbers in the figure correspond to the same numbers m the table. The coefficient of correlation between the above two values was 0.972 There is good agreement between both values. The behaviours of the above coefficients as a function of the FH are Illustrated m Fig. 7. The coefficients of correlation between the observed and calculated values of the zeta potential of coal particles in suspensions of pH 4, 5, 6, 7, 8, 9, 10 and 11 were O-911, 0.858, O.SlS,O 837,0.770, 0.774, OS29 and O.SS3, respectively. Whereas the coefficient of the equation for carbon content Bc of the coal samples decreases, the r?efficients of oxygen B,, sulphur Bs and nitrogen BN mcrease with increase of the pH of the suspension The coefficients for hydrogen Bn and ash B, do not show any significant tendency to decrease or inuease with the increase of pH. By putting values of the chemical components of coal samples into eqn. (1) with the coefficients shown in Fig. 7, the zeta potentials of the thirteen samples were calculated at different pH values of the suspension_ Typical exar..ples are the results obtained for Miller Coal (No. 12) and Banco Coal (No. 3) shown in Fig. 8 and Fig. 9, respectively_ The solid lines in Figs. S and 9 are the same lmes shown in Figs. 2 and 3, respectively. The broken lines with the cross marks in Figs. S and 9 represent the calculated values. As shown in Fig. 8, the observed and calculated lines agree well mth each other in the case of Miller Coal (No. 12) In the case of Banco Coal (No. 3), the calculated curve is similar to that or‘ the observed one as shown in Fig_ 9_ Larger discrepancies are noticed between the observed and calculated values as compared with the results for other coal samples.
(pH=J)
v
*30
I
I
I
I
0
-10
-20
-30
I
1
-30
-20
*lO
Calculated
Value
(mV
1
Fig 6. The relation between the observed flues of the zeta potential of coal particles and those values calculated by the empirxal eqn. (2)_
-15’
0
’
’ 2
’
’ 4
’ pH
’ 6
’
’ a
’
’ ’ 10
’ I2
’
Value
Fig. 7. The values of the coefficients of each of the components in the empirical eqn (1) obtained at different pH values of the suspension.
The zeta potentials are affected by the state of surface oxidation of the coal [ 5,6]. Samples Nos. 2, 3 and 5 which have greater oxygen content might be sublected to a s~llar phenomenon. Although the nnportant problems on such surface oxidation, maceral composition of the coal types and the type of sulphm (orgamc or inorganic) are left pendmg, we present a theoretical approach for estimating the zeta potential of coal fines in water using empirical relations.
165
calculated
numerically
by the least-square
method in suspensions wrth different pH values. The zeta potential of coal fines without any surface oxidation effect is estimated by eqn (1) with the coefficients, Bc, Bn, etc., (mV/%), which indicate the zeta potential of the respective components such as carbon, hydrogen, etc. of the coal samples
/ 201 0
’
’ 2
’
’ 4
e---z
Calculated
Value
-
Casewed
Value
’
’ 6
’
’ a
’
’ ’ ’ ’ IO 12 14
pH Value Frg. 8. The zeta potentral of coal parhcles calculated by puttrng the chemical components tabulated in the table into the empnxal eqn. (1) with the coefhc~ents shown in Fig. 7 for Miller Coal (No 12)
2
4
*---
Calculated
“due
-
Observed
Value
6
.9
IO
12
ACKNOWLEDGEMENTS
We are grateful to Mr K. Koike and his colleagues, Mltsui Mining Co. Ltd., who performed the chemical analyses of coal samples. We wish to thank Mr K. Tatsumoto, Mitsur Mining Co. Ltd., who helped us in getting the coal samples We also wish to thank Messrs. M. Takayanagl and R. Tsuru, who were graduate students of Kyushu University, for therr helpful assistance in our experiments We Would like to express sincere gratitude to Mr. Samuel Am Ndamukong (Foreign Research Student of Kyushu Uruverslty from the United Republic of Cameroon) for his critical reading of this paper It 1s a pleasure to acknowledge the important comments and the references suggested by the referees concerning this paper.
REFERENCES
pH Value Frg_ 9. The zeta potentml of coal particles calculated by putting the chemmal components tabulated m the table rnto the empirical eqn. (1) wrth the coefficients shown in Frg 7 for Ranco Coal (No. 3)
CONCLUSIONS
The empirical eqn. (1) was presented to relate the zeta potential measurements on dlfferent types of coal to their elemental compositions_ The coefficients of the equation were
1 D W. Fuerstenau, J. M Resenbsum and J. Laskowski, Colloids and Surfaces. 8 (1983) 153 2 R H Hunter, Zeta Potentml in ColIoid Scrence. Pnncrplcs and Applications, Academic Press, London, 1981, pp_ 6 - 7 and 219 - 257. 3 S. Mori, H. Okarnoto, T Hara and K Aso, m P Somasundaran (Ed ), Frne Partrcles Processzng. Society of Mining Engineers of AIME, New York, 1980, pp_ 632 - 651. 4 D. W van Krevelen, Fuel. 29 (1950) 269 5 M. S Cehc and P Somasundsran. Collolds and Surfaces, I (1980) 121 6 J. D. Mrller. J. S Laskowskr and S S Chang. Collolds and Surfaces. 8 (1983) 137
Technology,
40 (1964)
161
161 - 165
Zeta Potential
of Coal Fine-Particles
S MOW,
K
T
Department
HARA,
of Mming.
in Aqueous
Suspension
AS0 Kyushu
Unwersity,
Hal:ozakl,
Fukuoka.
812
(Japan)
Umuers~ty.
Hakoraki.
Fukuoka,
812
(Japan)
and H. OKAMOTO Depa+nent
of Physics,
Kyushu
SUMMARY
The relation between the zeta potential of coal fine-partrcles and pH ualue of suspension in aqueous suspenston for different kmds of coal samples was determined by using a rectangular micro-electrophoresis cell. The empirical equation of the relation between the zeta potential of the particles and the chemrcal components of coal samples was determined in aqueous suspension of drfferent pH values The equation is represented as followsZ=BcX(C)+BnX(H)+BoX(0)+BNX X(N)+BsX(S)+B,X(A) where Z is the zeta potentual of the particles, (C). (H), (O), (N), (S) and (Ash) arc weight percent of carbon, hydrogen, oxygen, nltrogen, sulphur and ash.. res_pectiuely. and Bc, Bu, Bo, BN, Bs and BA are coeffmrents of each of the respectrue components of coal samples The above coefficients were calculated numerrcally by the least-square method in suspensions with different pH values The values of the coefficients. Bc, Bn, etc., (mV/%), indicate the zeta potential of each respective single component; viz. carbon, hydrogen, etc., of the coal samples When the chemrcal components of a coal sample are known, the zeta potential of the coal fineparticles can be determined by the equation with the coefficients obtained in suspensions with different pH values
INTRODUCTION
The zeta potential of mmeral particles in suspension plays a significant role in the
flotation behaviour of mineral ores and the sedimentation behavlour of colloidal systems [ 1, 2]_ Zeta potential measurements on different types of coal are useful for the coal fineparticles processing technology A rectangular micro-electrophoresis cell is commonly used for measunng the zeta potential of mineral particles in suspensron. We have proposed an improved method for determining the precise electrophoretic velocity of mineral particles [3]. By our method, we determmed the relation between the zeta potential of coal fineparticles and the pH value of aqueous suspension for different kinds of coal samples. It is important to find out why the different types of coal have different values of zeta potential at the same pH values of suspension. We attempted to relate the zeta potential of the coal particles to their elemental components of the coal samples. The empirical equation of the relation between the zeta potential and the chemical components of coal samples was determined in suspensions with different pH values, and coefficients OF the equation were calculated numerically by the least-square method
EXPERIMENTS
Coal samples The chemical compositions of the coal samples used m the zeta potential measurements are shown m the table The analysis of the samples was performed by K. Koike, and his colleagues, Chofu Lab , Mitsui Coal Mining Co Ltd , Japan. These coals were plotted on the H/C - O/C (atomic ratio) diagram proposed by Krevelen [4], as shown in Fig. 1. The numbers in the fi,qes correspond to the same numbers tabulated in the table. Brown @ Elsevler Sequoia/Prmted
in The Netherlands
TABLE Chemical composition
(wt_%) of dried coal samples
No.
Name
(Country)
1 2 3 4 5 6 7 8 9 1G 11 12 13
Miike (Japan) Beluga (US America) Banco (Indonesia) M~llrnerran (Australia) Yallourn (Australia) Quintette (Canada) Matsuzawa (Japan) Madagascar (Malgache) Sunagawa (Japan) Ashibeku (Japan) Mourn (Australia) Miller (Austraha) Datong (Chma)
C
H
0
N
77.3 65-4 71.4 67.5 67.5 73 9 81.5 80 2 ‘78.4 77-1 i6.3 71.1 78.5
5.5 5.1 5.1 5.6 4.6 3-Q 24 0.3 5.6 5-6 43 48 4.5
7.4 20.6 19.8 10-i 25.9 6.9 2.4 10 5 83 6.8 6.0 7.2 81
1.2 O-7 l-2 1.0 0.6 0.9 1.2 0.0 2.1 19 1.6 1.6 0.8
Ash 08 0.0 0.2 0.4 0.2 0.2 1.5 O-0 03 03 0.4 0.8 05
7.8 8.2 23 14.8 12 14 2 11.0 91 53 83 11-4 14.6 76
velocities of the particles (partrcle diameter: l5 ,um) were observed microscoprcally in the rectangular micro-electrophoresis cell. Zeta potential measurellient was performed by the method previously reported [3] _
Atomic
G/c ratlo
Fig. 1. H/C - O/C (atomic ratio) diagram_ (Numbers in the figure correspond to the same numbers tabulated
Experirr ental results The rAation between the zeta potential of the coal samples and the pH value of suspension was determined for thirteen coal samples. _4n example of the results LS represented by that obtained for Miller Coal (No. 12) as illustratrrd in Fig. 2 The value of the negative charge c-f the coal particles increases with the pH of the suspension_ Similar curves were observed wrth the other coal samples except Nos. 2, 3 and 5. The results for Banco Coal (No. 3) illustrated in Fig 3 exarnplify the
in the table_)
coal, low-rank bituminous coal, medium-rank bituminous coal, high-rank bituminous coal, semi-anthracite and anthracite are respectively represented as V, VI, VII, VIII, M and X, in the diagram used by Klevelen [4]_ Experimental method Two grams of coal powder ground by a vibrating mill were added into 1000 ml of distilled water and the desired pH value obtamed by either adding HCI or NaOH. After adequate stirring, the suspension was kept in a constant temperature bath at 20 “C for 30 min. The rest of the experiment was conducted in a room whose temperature was regulated to between 20 and 22 “C. The
pH
Value
Frg. 2. The relatron between the zeta potentral of coal particles and the pH value of suspension for Miller Coal (No. 12)
pH
Value
the zeta potential of coal particles and the pI1 value of suspension for Banco Coal (No 3). Fig_ 3 The relation between
exceptions_ The value of the negative charge of the coal particles for Banco Coal decreases with increase of the pH in the alkaline region. A wde scatter of the data is noticeable m the alkaline region for Banco Coal as shown in Fig. 3. The results of thirteen coal samples are illustrated in Figs. 4 and 5. The numbers in the figures correspond to the same numbers tabulated in the table
I[, , , , , , , , , , , , , 0
2
4
6 pH
8
10
12
Value
Fig. 4 The relation between the zeta potenclal of coal particles and the pH value of suspension for dlfferent kmds of coal samples
DISCUSSIONS Although the zeta potential is the nature of the surface of the mineral particles in suspension, we attempted to relate the zeta potential to the elemental components on the basis of the simplest idea that the chemical component ratio at the mineral surface without any surface oxidation effect is proportional to that of the bulk chemical composition. The empirical equation of the relation between tbe zeta potential and the chemical components of coal samples was determined in aqueous suspension with different pH values The equation is as follows: Z=BcX(C)+BuX(N)+BoX(O)+ +BNX(N-)+BsX(S)+BAX(A) where 2 is the zeta potential
(1) (mV)
of the par-
ticles, (0, (II), (O), (N), (S) and (A) are weight percent of carbon, hydrogen, oxygen, nitrogen, sulphur and ash, respectively, and Bc, BH, B,, BN, Bs and BA are coefficients (mV/%) of each of the respective components of the coal samples. The weight percent of
I
LO 0
2
4
pH
6
I
I
6
Value
I
I
IO
I
1
12
I
I
Fig 5 The relation between the zeta potential of coal particles and the pH value OF susgension for different kmds of coal samples
each component is uniquely convertible into atomic ratio. It should be noted that a term which represents the effect caused by surface oxidation is neglected for simplicity. How to mtroduce the surface oxidation effect in order to get a more realistic expression for the zeta potential is still food for thought The above coefficients were calculated numencally by the least-square method
164 with a high accuracy of 32 digits (quadruple precision) in suspensions with c’ifferent pH values. In the case of a suspensio I with pH = 3, the following equation is obtaraed: Z,,,
=s, = 0.77
x (C) + 3.00
x (0) -
1.78
-
11.97 X (A)
X (H) -
x (N) -
5.t2
2.86
X
x (S) + (2)
The relation between observed values of the zeta potential of coal particles and those calculated by putting values of the chemical components of coal samples into eqn. (2) is illustrated in Fig. 6 The numbers in the figure correspond to the same numbers m the table. The coefficient of correlation between the above two values was 0.972 There is good agreement between both values. The behaviours of the above coefficients as a function of the FH are Illustrated m Fig. 7. The coefficients of correlation between the observed and calculated values of the zeta potential of coal particles in suspensions of pH 4, 5, 6, 7, 8, 9, 10 and 11 were O-911, 0.858, O.SlS,O 837,0.770, 0.774, OS29 and O.SS3, respectively. Whereas the coefficient of the equation for carbon content Bc of the coal samples decreases, the r?efficients of oxygen B,, sulphur Bs and nitrogen BN mcrease with increase of the pH of the suspension The coefficients for hydrogen Bn and ash B, do not show any significant tendency to decrease or inuease with the increase of pH. By putting values of the chemical components of coal samples into eqn. (1) with the coefficients shown in Fig. 7, the zeta potentials of the thirteen samples were calculated at different pH values of the suspension_ Typical exar..ples are the results obtained for Miller Coal (No. 12) and Banco Coal (No. 3) shown in Fig. 8 and Fig. 9, respectively_ The solid lines in Figs. S and 9 are the same lmes shown in Figs. 2 and 3, respectively. The broken lines with the cross marks in Figs. S and 9 represent the calculated values. As shown in Fig. 8, the observed and calculated lines agree well mth each other in the case of Miller Coal (No. 12) In the case of Banco Coal (No. 3), the calculated curve is similar to that or‘ the observed one as shown in Fig_ 9_ Larger discrepancies are noticed between the observed and calculated values as compared with the results for other coal samples.
(pH=J)
v
*30
I
I
I
I
0
-10
-20
-30
I
1
-30
-20
*lO
Calculated
Value
(mV
1
Fig 6. The relation between the observed flues of the zeta potential of coal particles and those values calculated by the empirxal eqn. (2)_
-15’
0
’
’ 2
’
’ 4
’ pH
’ 6
’
’ a
’
’ ’ 10
’ I2
’
Value
Fig. 7. The values of the coefficients of each of the components in the empirical eqn (1) obtained at different pH values of the suspension.
The zeta potentials are affected by the state of surface oxidation of the coal [ 5,6]. Samples Nos. 2, 3 and 5 which have greater oxygen content might be sublected to a s~llar phenomenon. Although the nnportant problems on such surface oxidation, maceral composition of the coal types and the type of sulphm (orgamc or inorganic) are left pendmg, we present a theoretical approach for estimating the zeta potential of coal fines in water using empirical relations.
165
calculated
numerically
by the least-square
method in suspensions wrth different pH values. The zeta potential of coal fines without any surface oxidation effect is estimated by eqn (1) with the coefficients, Bc, Bn, etc., (mV/%), which indicate the zeta potential of the respective components such as carbon, hydrogen, etc. of the coal samples
/ 201 0
’
’ 2
’
’ 4
e---z
Calculated
Value
-
Casewed
Value
’
’ 6
’
’ a
’
’ ’ ’ ’ IO 12 14
pH Value Frg. 8. The zeta potentral of coal parhcles calculated by puttrng the chemical components tabulated in the table into the empnxal eqn. (1) with the coefhc~ents shown in Fig. 7 for Miller Coal (No 12)
2
4
*---
Calculated
“due
-
Observed
Value
6
.9
IO
12
ACKNOWLEDGEMENTS
We are grateful to Mr K. Koike and his colleagues, Mltsui Mining Co. Ltd., who performed the chemical analyses of coal samples. We wish to thank Mr K. Tatsumoto, Mitsur Mining Co. Ltd., who helped us in getting the coal samples We also wish to thank Messrs. M. Takayanagl and R. Tsuru, who were graduate students of Kyushu University, for therr helpful assistance in our experiments We Would like to express sincere gratitude to Mr. Samuel Am Ndamukong (Foreign Research Student of Kyushu Uruverslty from the United Republic of Cameroon) for his critical reading of this paper It 1s a pleasure to acknowledge the important comments and the references suggested by the referees concerning this paper.
REFERENCES
pH Value Frg_ 9. The zeta potentml of coal particles calculated by putting the chemmal components tabulated m the table rnto the empirical eqn. (1) wrth the coefficients shown in Frg 7 for Ranco Coal (No. 3)
CONCLUSIONS
The empirical eqn. (1) was presented to relate the zeta potential measurements on dlfferent types of coal to their elemental compositions_ The coefficients of the equation were
1 D W. Fuerstenau, J. M Resenbsum and J. Laskowski, Colloids and Surfaces. 8 (1983) 153 2 R H Hunter, Zeta Potentml in ColIoid Scrence. Pnncrplcs and Applications, Academic Press, London, 1981, pp_ 6 - 7 and 219 - 257. 3 S. Mori, H. Okarnoto, T Hara and K Aso, m P Somasundaran (Ed ), Frne Partrcles Processzng. Society of Mining Engineers of AIME, New York, 1980, pp_ 632 - 651. 4 D. W van Krevelen, Fuel. 29 (1950) 269 5 M. S Cehc and P Somasundsran. Collolds and Surfaces, I (1980) 121 6 J. D. Mrller. J. S Laskowskr and S S Chang. Collolds and Surfaces. 8 (1983) 137