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Flotation of resinite from high ash coal
Minerals Engineering, Vol. 12, No. 2, pp. 229-234, 1999
Pergamon 0892--6875(98)00134-4
© 1999 Elsevier Science Ltd All rights reserved 0892--6875/99/$--see front matter
TECHNICAL NOTE FLOTATION OF RESINITE FROM HIGH ASH COAL
A.A. ATIA Chemistry Department, Faculty of Science, Menofeya University, Shibin E1-Kom, Menofeya, Egypt (Received 13 March 1998; accepted 20 October 1998)
ABSTRACT
Selective resinite flotation was studied for high ash coal under different conditions. It was found that both the grade and recovery of the obtained resinite depend on the type of frother used. The grade was improved to reach 80% with a recovery of 70% through wet screening, grinding and pH control. © 1999 Elsevier Science Ltd. All rights reserved
Keywords Coal; flotation frothers; pH control; screening; mineral processing
~TRODUCTION Resins are used in many industrial applications [1,2]. They are found in nature as secretions from various plants or as fossil deposits in the coal. Coal macerals include three main groups called resinite, vitrinite and intertinite [3,4]. The close similarity between resinite and coal particles results in difficulties in their separation [5,6]. In this paper detailed investigation of the effects of different factors on the flotation of resinite from high ash coal is presented.
EXPERIMENTAL Sample preparation The coal sample was obtained from University of Utah, USA and found to contain 5% resin and 22% ash. The fraction below 595 ~anawas used for the flotation experiments. All chemicals were Sigma Products and were used as received.
Bench scale flotation Selective resinite flotation from coal was carried out in a Denver flotation cell at 10% solids by weight. After conditioning the suspension, the frother was then added at the desired level. A recovery of resinite
229
230
A.A . Atia
from coal was accomplished where the resinite concentrate and the coal tailing were filtered, dried and analyzed.
Electrokinetic measurements The zeta potential measurements were performed using pure materials. About 0.1 g of wet-ground resinite or coal sample was mixed with 200 ml of l x l 0 -3 M KC1 solution. The suspension was then adjusted to the specified pH and left for 24 hours to achieve equilibrium. Measurements were performed by a Laser-Doppler electrophoresis technique using the zetasizer3 (Malvern Instruments Ltd., London, UK). Contact angle measurements The resinite surface was prepared by solvent evaporation of a resinite solution in toluene to deposit on an aluminum sheet. The polished surface of the coal was prepared by polishing the surface with Buehler #600 polishing paper. The obtained surfaces were used for contact angle measurements using a Rame-Hart contact angle goniometer [3].
RESULTS AND DISCUSSIONS Selective resinite flotation from coal The separation strategy is based on the natural differences in the hydrophobicity between resinite and coal species. Frothers were employed to stabilize the generated bubbles by lowering the surface tension of the medium. Figure 1 gives the results of flotation of resinite from coal for various frothers. Inspection of the Figure reveals that as the chain length of the frother increases the recovery increases. Figure 2 depicts the resinite grade as a function of dosage. Clearly, at the same dosage, for short chain frothers the grade is higher than for long chain ones. It is evident from the data presented in Figures 1 and 2 that both the grade and the recovery are dependent upon the type and quantity of the frother used. For the same frother, as the dosage increases more bubbles will be available for bubble-particles interaction. In this case, coal particles could float along with resinite particles, leading to a recovery increase and to a grade decrease [7,8]. The same conclusion could explain the depression of the resinite grade as the chain length of the frother increases. It is worth mentioning that the effects of n-butanol, iso-butanol and ter-butanol are quite similar. This implies that branching has no significant effect on the froth characteristics. 100
anol
00
f
//-j
/t/ r,.-j
0~8 40 //~IF J / , ~ r~ , ~ / ~ ~ ~ 1 1
i$o-propanol
;etanol
20 l~/'lB~
I$o-amyl -MIBC
i/ 200
400
600
Dosage, g/ton
800
1000
Fig. 1 Effect of dosage for various frothers on resinite recovery at natural pH.
Flotation of resinite from high ash coal
231
80 -1~70
n-butanol -~-
I,
60
iso-Cutanol
50
%
~
~
~_propanol
40
(_9 3O
1-~,
~.~..~N,,N._ ~
.~,ntano,
20 10
0
I
0
I
200
400
I
l
600
800
Dosage,gffon
1000
Fig.2 Effect of dosage for various frother on resinite grade at natural pH. Changing the pH in the flotation process is applied to increase the extent of dispersion of the feed particles. Figure 3 depicts the flotation of resinite from coal using n-butanol at pH 11.5-11.8. From the Figure, it can be concluded that the improvement of the grade at high levels of recovery is limited. This implies that pH control by itself is not sufficient to achieve high resinite grade at high recovery for such a coal sample.
80 .-gg_
70
pH 7.8
--#l.-
pH 11.5 n-butanol
60 o
50 40
3O 20 lO 0
I
o
20
I
I
40 60 Recovery, %
I
80
100
Fig.3 Effect of pH on the recovery and grade of resinite. For improving the resinite grade at high recovery, the feed was subjected to wet screening to remove the fines (fraction below 37 larn). After wet screening, the coarse fraction was wet ground, then subjected to flotation at pH 11.5-11.8. The results are shown in Figure 4 where a grade of >80% could be obtained
232
A . A . Atia
while maintaining about 70% recovery. Generally, to obtain both high resinite grade and recovery from high ash feed, three requirements have to be met. These are desliming, wet grinding and pH control. Desliming minimizes the fines which depress the grade. Wet grinding results in more liberation of resinite from coal. The pH control gives high dispersion and the resinite-coal interaction decreases. 90 80 70 60
°~.50 "0
(.9
40 30
original feed --II--
20
feed, grinding and pH
10
feed, screening, grinding and pH
0
[
10
20
I
30
l
I
40 50 Recovery, %
I
I
60
70
80
Fig.4 Flotation of resinite with n-butanol from coal sample subjected to various treatments.
Kinetics of resin extraction Heptane was used to extract the fossil resin from the original coal sample. Complete extraction of resinite was achieved after about 10 hours. Electrokinetic behaviour For investigation of the surface charge of both resinite and coal particles, zeta potential measurements were performed. A plot of zeta potential against the suspension pH for resinite and coal is given in Figure 5. The Figure shows little difference in the surface charge between resinite and coal. These differences result in a slight change in the hydrophobicity of resinite and coal, which reflects on their flotation behaviour. It was found that the frother addition has no effect on the zeta potential of either resinite or coal. This confirms that the frother used does not adsorb at the solid-liquid interface. Based on these results one can conclude that separation of resinite from coal with frothers is controlled by the adsorption of these reagents at the air-liquid interface. Contact angle measurements Contact angle has been adopted as a useful tool to evaluate the degree of wettability of the surfaces. Table 1 lists the contact angle data of resinite and coal measured by both captive bubble and sessile drop techniques. Clearly, the contact angle for resinite is greater than that for coal. This indicates that the resinite is slightly more hydrophobic than coal. This may be attributed to the possibility of oxidation of the coal surface, which reduces its hydrophobicity and consequently lowers its floatability.
Rotation of resinite f r o m high ash coal
233
40 20
-Q
\
>
E E
•
o
\
-20
0 Q. O~
,\~
--o- coal - n - resin
40
-60
-80
t
*
r
I
2
4
6
8
I
10
I"
12
14
pH Fig.5 Electrokinetic behaviour of coal and resinite.
T A B L E 1 The contact angles for resinite and coal
Sample Resinite Coal
Contact Sessile-Drop 81 58
Angle (degrees) Captive-Bubble 60 33
SUMMARY AND CONCLUSIONS Different factors affecting selective resinite flotation from high ash coal were investigated in this work. Frothers were employed to achieve the resinite separation, in order to increase and stabilize the generated bubbles, resulting in an efficient bubble-particle interaction. The difference in floatability between resinite and coal particles was attributed to the differences in their natural hydrophobicity. Wet screening, grinding and pH control were necessary to achieve a high resinite grade at satisfactory levels of recovery.
ACKNOWLEDGMENT The author would like to thank Professor J. D. Miller, University of Utah, USA for providing the samples and the zeta potential facility used in this study.
REFERENCES 1.
2. .
Arnold, B. and Aplan, F., The hydrophobicity of coal macerals, Fuel, 68, 651--657 (1989). Yu, Q., Bukka, K., Ye, Y. and Miller, J.D., FTIR spectroscopic analysis of surface oxidation reactions during ozonation of fossil resin and coal, Coal Preparation 10, 77-92 (1992). Yu, Q., Selective resinite flotation from coal, Ph.D. Thesis, University of Utah, SLC, Utah, USA (1991).
234
4.
5. 6. 7. 8.
A.A. Atia
Teerman, S., Crelling, J. and Glass, G., fluorescence spectra analysis of resinite macerals from coals of the Hanna formation--Wyoming--USA, Int. J. of Coal Geology 7, 315-334 (1987). Gutierrez-Roduiguez, J.A., Estimating the hydrophobicity of coal, Colloids and Surfaces 12, 125 (1984). Yu, Q., Li, L. and Miller, J.D., Recovery and characterization of macroscopic fossil resins from western coals, Am. Chem. Soc. Syrup. Ser. 617, 279-287 (1995). Klimpel, R.R. & Hansen, R.D., Frothers demonstrating enhanced recovery of fine particles of coal in froth flotation, United states Patent, Patent Number 4,761,223 Aug. 2 (1988). Manev, E. and Pugh, R.J., Frothers/collector interactions in thin films and flotation, SME, Arizona, USA (1992).
C o r r e s p o n d e n c e on papers published in M i n e r a l s Engineering is invited, preferably by e-mail to [email protected], or by Fax to +44-(0)1326-318352
Pergamon 0892--6875(98)00134-4
© 1999 Elsevier Science Ltd All rights reserved 0892--6875/99/$--see front matter
TECHNICAL NOTE FLOTATION OF RESINITE FROM HIGH ASH COAL
A.A. ATIA Chemistry Department, Faculty of Science, Menofeya University, Shibin E1-Kom, Menofeya, Egypt (Received 13 March 1998; accepted 20 October 1998)
ABSTRACT
Selective resinite flotation was studied for high ash coal under different conditions. It was found that both the grade and recovery of the obtained resinite depend on the type of frother used. The grade was improved to reach 80% with a recovery of 70% through wet screening, grinding and pH control. © 1999 Elsevier Science Ltd. All rights reserved
Keywords Coal; flotation frothers; pH control; screening; mineral processing
~TRODUCTION Resins are used in many industrial applications [1,2]. They are found in nature as secretions from various plants or as fossil deposits in the coal. Coal macerals include three main groups called resinite, vitrinite and intertinite [3,4]. The close similarity between resinite and coal particles results in difficulties in their separation [5,6]. In this paper detailed investigation of the effects of different factors on the flotation of resinite from high ash coal is presented.
EXPERIMENTAL Sample preparation The coal sample was obtained from University of Utah, USA and found to contain 5% resin and 22% ash. The fraction below 595 ~anawas used for the flotation experiments. All chemicals were Sigma Products and were used as received.
Bench scale flotation Selective resinite flotation from coal was carried out in a Denver flotation cell at 10% solids by weight. After conditioning the suspension, the frother was then added at the desired level. A recovery of resinite
229
230
A.A . Atia
from coal was accomplished where the resinite concentrate and the coal tailing were filtered, dried and analyzed.
Electrokinetic measurements The zeta potential measurements were performed using pure materials. About 0.1 g of wet-ground resinite or coal sample was mixed with 200 ml of l x l 0 -3 M KC1 solution. The suspension was then adjusted to the specified pH and left for 24 hours to achieve equilibrium. Measurements were performed by a Laser-Doppler electrophoresis technique using the zetasizer3 (Malvern Instruments Ltd., London, UK). Contact angle measurements The resinite surface was prepared by solvent evaporation of a resinite solution in toluene to deposit on an aluminum sheet. The polished surface of the coal was prepared by polishing the surface with Buehler #600 polishing paper. The obtained surfaces were used for contact angle measurements using a Rame-Hart contact angle goniometer [3].
RESULTS AND DISCUSSIONS Selective resinite flotation from coal The separation strategy is based on the natural differences in the hydrophobicity between resinite and coal species. Frothers were employed to stabilize the generated bubbles by lowering the surface tension of the medium. Figure 1 gives the results of flotation of resinite from coal for various frothers. Inspection of the Figure reveals that as the chain length of the frother increases the recovery increases. Figure 2 depicts the resinite grade as a function of dosage. Clearly, at the same dosage, for short chain frothers the grade is higher than for long chain ones. It is evident from the data presented in Figures 1 and 2 that both the grade and the recovery are dependent upon the type and quantity of the frother used. For the same frother, as the dosage increases more bubbles will be available for bubble-particles interaction. In this case, coal particles could float along with resinite particles, leading to a recovery increase and to a grade decrease [7,8]. The same conclusion could explain the depression of the resinite grade as the chain length of the frother increases. It is worth mentioning that the effects of n-butanol, iso-butanol and ter-butanol are quite similar. This implies that branching has no significant effect on the froth characteristics. 100
anol
00
f
//-j
/t/ r,.-j
0~8 40 //~IF J / , ~ r~ , ~ / ~ ~ ~ 1 1
i$o-propanol
;etanol
20 l~/'lB~
I$o-amyl -MIBC
i/ 200
400
600
Dosage, g/ton
800
1000
Fig. 1 Effect of dosage for various frothers on resinite recovery at natural pH.
Flotation of resinite from high ash coal
231
80 -1~70
n-butanol -~-
I,
60
iso-Cutanol
50
%
~
~
~_propanol
40
(_9 3O
1-~,
~.~..~N,,N._ ~
.~,ntano,
20 10
0
I
0
I
200
400
I
l
600
800
Dosage,gffon
1000
Fig.2 Effect of dosage for various frother on resinite grade at natural pH. Changing the pH in the flotation process is applied to increase the extent of dispersion of the feed particles. Figure 3 depicts the flotation of resinite from coal using n-butanol at pH 11.5-11.8. From the Figure, it can be concluded that the improvement of the grade at high levels of recovery is limited. This implies that pH control by itself is not sufficient to achieve high resinite grade at high recovery for such a coal sample.
80 .-gg_
70
pH 7.8
--#l.-
pH 11.5 n-butanol
60 o
50 40
3O 20 lO 0
I
o
20
I
I
40 60 Recovery, %
I
80
100
Fig.3 Effect of pH on the recovery and grade of resinite. For improving the resinite grade at high recovery, the feed was subjected to wet screening to remove the fines (fraction below 37 larn). After wet screening, the coarse fraction was wet ground, then subjected to flotation at pH 11.5-11.8. The results are shown in Figure 4 where a grade of >80% could be obtained
232
A . A . Atia
while maintaining about 70% recovery. Generally, to obtain both high resinite grade and recovery from high ash feed, three requirements have to be met. These are desliming, wet grinding and pH control. Desliming minimizes the fines which depress the grade. Wet grinding results in more liberation of resinite from coal. The pH control gives high dispersion and the resinite-coal interaction decreases. 90 80 70 60
°~.50 "0
(.9
40 30
original feed --II--
20
feed, grinding and pH
10
feed, screening, grinding and pH
0
[
10
20
I
30
l
I
40 50 Recovery, %
I
I
60
70
80
Fig.4 Flotation of resinite with n-butanol from coal sample subjected to various treatments.
Kinetics of resin extraction Heptane was used to extract the fossil resin from the original coal sample. Complete extraction of resinite was achieved after about 10 hours. Electrokinetic behaviour For investigation of the surface charge of both resinite and coal particles, zeta potential measurements were performed. A plot of zeta potential against the suspension pH for resinite and coal is given in Figure 5. The Figure shows little difference in the surface charge between resinite and coal. These differences result in a slight change in the hydrophobicity of resinite and coal, which reflects on their flotation behaviour. It was found that the frother addition has no effect on the zeta potential of either resinite or coal. This confirms that the frother used does not adsorb at the solid-liquid interface. Based on these results one can conclude that separation of resinite from coal with frothers is controlled by the adsorption of these reagents at the air-liquid interface. Contact angle measurements Contact angle has been adopted as a useful tool to evaluate the degree of wettability of the surfaces. Table 1 lists the contact angle data of resinite and coal measured by both captive bubble and sessile drop techniques. Clearly, the contact angle for resinite is greater than that for coal. This indicates that the resinite is slightly more hydrophobic than coal. This may be attributed to the possibility of oxidation of the coal surface, which reduces its hydrophobicity and consequently lowers its floatability.
Rotation of resinite f r o m high ash coal
233
40 20
-Q
\
>
E E
•
o
\
-20
0 Q. O~
,\~
--o- coal - n - resin
40
-60
-80
t
*
r
I
2
4
6
8
I
10
I"
12
14
pH Fig.5 Electrokinetic behaviour of coal and resinite.
T A B L E 1 The contact angles for resinite and coal
Sample Resinite Coal
Contact Sessile-Drop 81 58
Angle (degrees) Captive-Bubble 60 33
SUMMARY AND CONCLUSIONS Different factors affecting selective resinite flotation from high ash coal were investigated in this work. Frothers were employed to achieve the resinite separation, in order to increase and stabilize the generated bubbles, resulting in an efficient bubble-particle interaction. The difference in floatability between resinite and coal particles was attributed to the differences in their natural hydrophobicity. Wet screening, grinding and pH control were necessary to achieve a high resinite grade at satisfactory levels of recovery.
ACKNOWLEDGMENT The author would like to thank Professor J. D. Miller, University of Utah, USA for providing the samples and the zeta potential facility used in this study.
REFERENCES 1.
2. .
Arnold, B. and Aplan, F., The hydrophobicity of coal macerals, Fuel, 68, 651--657 (1989). Yu, Q., Bukka, K., Ye, Y. and Miller, J.D., FTIR spectroscopic analysis of surface oxidation reactions during ozonation of fossil resin and coal, Coal Preparation 10, 77-92 (1992). Yu, Q., Selective resinite flotation from coal, Ph.D. Thesis, University of Utah, SLC, Utah, USA (1991).
234
4.
5. 6. 7. 8.
A.A. Atia
Teerman, S., Crelling, J. and Glass, G., fluorescence spectra analysis of resinite macerals from coals of the Hanna formation--Wyoming--USA, Int. J. of Coal Geology 7, 315-334 (1987). Gutierrez-Roduiguez, J.A., Estimating the hydrophobicity of coal, Colloids and Surfaces 12, 125 (1984). Yu, Q., Li, L. and Miller, J.D., Recovery and characterization of macroscopic fossil resins from western coals, Am. Chem. Soc. Syrup. Ser. 617, 279-287 (1995). Klimpel, R.R. & Hansen, R.D., Frothers demonstrating enhanced recovery of fine particles of coal in froth flotation, United states Patent, Patent Number 4,761,223 Aug. 2 (1988). Manev, E. and Pugh, R.J., Frothers/collector interactions in thin films and flotation, SME, Arizona, USA (1992).
C o r r e s p o n d e n c e on papers published in M i n e r a l s Engineering is invited, preferably by e-mail to [email protected], or by Fax to +44-(0)1326-318352