244
Energy & Fuels 1993, 7, 244-248
Role of Electrical Double Layer Forces and Hydrophobicity in Coal Flotation in NaCl Solutions Chin Li and P. Somasundaran* Langmuir Center for Colloids and Interfaces, Henry Krumb School of Mines, Columbia University, New York, New York 10027 Received December 8,1992. Revised Manuscript Received December 10,1992
It is well-known that coal floatability can be enhanced by the addition of inorganic salts. Although considerable effort has been made in the past to investigate the role of these salts, their function in the coal flotation system is not yet clear. In this study, the floatability of a bituminous coal in NaCl solutions were evaluated using a modified Hdimond tube to delineate the role of the electrostatic interaction between bubbles and particles and the coal hydrophobicity. Zeta potential measurements showed that bubbles are negatively charged in the entire pH range tested, while coal particles are negatively charged only above pH 6.0. Also, both sodium and chloride ions were confirmed to act as indifferent ions for both coal particles and bubbles. Floatability of coal measured as a function of pH exhibited a maximum and minimum at low salt concentrations which is shown to be determined mainly by the coal hydrophobicity as measured by a new film levitation technique. At high salt concentrations (above 0.1 mol/dm3),the floatability without such maximum or minimum is controlled by the electrostatic interaction between the bubble and the particle.
Introduction Froth flotation is becoming increasingly recognized as a potential process for the removal of pyritic sulfur and ash impurities to produce ultraclean coal. It has been reported that the floatability of naturally hydrophobic coal can be increased significantly by the addition of inorganic electrolytes.14 Several attempts made in the past to discern the role of the salts in the flotation process have led to a considerable degree of controversy on this topic. Klassen and Makrousov' and Kitchener and his c o - ~ o r k e r stated s ~ ~ ~ that the addition of salt destabilizes the hydrated layers surrounding the particles and hence increases the floatability of coal. Marrucci and Nicodemo? on the other hand, proposed the addition of electrolytes to cause an increase in the surface potential of bubbles which results in reduced bubble coalescence and in turn to improve the flotation rate. Compression of the double layer by the added electrolyte, which can subsequently cause thinning and rupture of the wetting film between bubbles and particles, has also been proposed as a reason for the salt effects on f l o t a t i ~ n . ~ This ? ~ argument has been further supported by the results of several other studies showing flotation recovery to reach a maximum at minimum zeta ~0tential.lO-l~This view, on the other hand, has been criticized by some other researchers who dem(1)Klassen, V. I.; Mokrousov, V. A. An Introduction to the Theory
of Flotation; Translated by J. Leja and G. W. Poling; Butterworths:
London, 1963;pp 338-342. (2)Laskowski, J. Colliery Guardian 1965,211,361-366. (3)Yoon, R. H.Mining Congr. J. 1982,68,76-80. (4)Yoon, R. H.;Sabey, J. B. In Interfacial Phenomena in Coal Technology; Botsaris, G. D., Glazman, Y. M., Eds.; Marcel Dekker: New York, 1989;pp 87-114. (5)Blake, T. D.; Kitchener, J. A. J. Chem. SOC.,Faraday Trans. 1 1972,68,1435-1442. (6)Read, A. D.; Kitchener, J. A. J. Colloid Interface Sci. 1969,30, 391-398. (7)Marrucci, G.; Nicodemo, L. Chem. Eng. Sci. 1967,22,1257-1265. (8)Brown, D. J. In Froth Flotation 50th Anniversary Volume; Fuerstenau, D. W., Ed.; AIME New York, 1962;pp 518-538. (9)Laskowski, J.; Iskra, J. Trans. IMM 1977,79, C6-C10. (10)Fuerstenau, D. W. Trans. AIME 1957,208,1365-1367.
onstrated that the flotation maximum does not necessarily occur at the isoelectric potential of the coa1.14J5 None 01 the above studies, however, has attempted to isolate the electrostatic interaction effect from the hydrophobicity effect. This is likely due to a lack of information on the charge of bubbles which is essential in estimating the effect of electrostatic interaction in flotation. A systematic approach to understand the role of both the electrostatic interaction between bubbles and particles and the hydrophobicity of the coal particles by isolating the two effects is used in this study. The effect of the salt addition on the bubble charge and its contribution to the electrostatic interaction is also discussed.
Experimental Section Materials. Pittsburgh seam pgh bituminous coal, obtained from Bruceton mines in Alleghany county, PA, was used in this study. Large chunks of as-mined coal samples were broken into small pieces using a hammer and subsequently crushed in a Quaker mill in open atmosphere. The crushed coal was then dry ground in a closed mill with ceramic balls for 5 min. The ground product was sieved into different size fractions which were stored in plastic bags under an argon atmosphere to minimize ambient oxidation. Floatability and hydrophobicity experiments were conducted using the 35 X 80 mesh size fraction. The sample used for zeta potential determination was prepared by grinding the 35 X 80 mesh coal sample down tominus 200 mesh. Proximate and ultimate analyses of the coal sample are shown in Table I. ACS certified grade sodium chloride (NaCl)and pH modifying reagents, hydrochloric acid (HCl)and sodium hydroxide (NaOH), were purchased from Fisher Scientific Inc. Triply distilled water was used in all the experiments. Flotation Experiment. One gram of 35 X 80meshcoalsample was conditioned for 5 min in 100 g of sodium chloride solution at the desired salt concentration in a 150-cm3 beaker. The pH (11)Jaycock, M. J.; Ottewill, R. H. Trans. IMM 1963,72,497-506. (12)Chander, S.;Fuerstenau, D. W. Trans. AIME 1972,252,62-69. (13)Wen, W. W.; Sun, S. C. Trans. AIME 1977,262,174-180. (14)Celik, M. S.;Somasundaran, P. Colloids Surf. 1980,I, 121-124. (15)Jessop, R. R.;Stretton, J. L. Fuel 1969,48,317-320.
0SS7-062~/93/2507-0244$04.00/0 0 1993 American Chemical Society
Coal Flotation in NaCl Solutions
Energy & Fuels, Vol. 7, No. 2, 1993 246
Table I. Proximate, Ultimate, and Forms of Sulfur Analyses for the Alleghany Coal description percent Ultimate Analysis: As Received moisture 1.79 carbon 80.65 hydrogen 5.30 nitrogen 1.74 chlorine 0.10 sulfur 1.12 ash 3.29 oxygen (by diff) 7.80 Proximate Analysis: As Received volatile matter 36.29 fixed carbon 58.60 caloric value 14000 BTU/lb pyritic sulfate organic
Sulfur Forms: As Received 0.28 0.0083 0.83
of the slurry was then adjusted to the desired value and the slurry conditioned for five more minutes. The slurry was then transferred to a modified Halliiond tube and the flotation carried out for 10 min using nitrogen at a flow rate of 10 cm3/min. Float and sink fractions were filtered, dried, and weighed. Zeta Potential Measurement. Coal slurries used in the zeta potential measurements were prepared using a procedure similar to that used in flotation experiments except that a 0.1 w t ?6 coal slurry was used here. After conditioning, approximately 25 cm3 of the slurry was transferred into the sample cell of a Lazer Zee Meter for the zeta potential measurement. Zeta potential of bubbles was measured in a setup designed for monitoring the electrokinetic behavior of gas bubbles; a detailed description of the experimental setup and procedure has been presented earlier.16 Bubble Size Determination. The bottom portion of the Hdimond tube was used to generate bubbles which were pumped into a specially constructed cell similar in design to the sample cell of the Lazer Zee Meter. The procedure for obtaining stationary bubbles was similar to that used in zeta potential measurements. The bubble size distribution was determined using an Omnicon Model 3000 image analyzer. Hydrophobicity Evaluation. Hydrophobicity of the coal sampleswas evaluatedusing a film levitation technique developed specifically for the present purpose with a modified 350-cm3 Buchner funnel provided with a coarse glass frit at the bottom (40-60 pm in pore size). A detailed description of the experiA 0.25-g sample mental setup and procedure is given e1se~here.I~ of 35 X 80 mesh coal was first conditioned in a beaker following the same procedure as in the flotation experiments. The conditioned slurry was then transferred into the funnel from which the excess solution was pumped out through the pores of the glass frit using a peristaltic pump. The pump was reversed as soon as air bubbles were seen breaking through the glass frit. The hydrophobic particles, carried by the air-water interface, were separated from the hydrophilic ones which remained on the glass frit. These two fractions were then filtered, dried, and weighed. Hydrophobicityindex,numerically equivalentto weight percent float in these tests, is presented on a scale of 0-100.
Results and Discussion Effect of Salt Concentration on the Charge of Bubble. According to the earlier paper,ls bubbles in the NaCl solution are negatively charged above -pH 1.5 (the isoelectric point of bubble), and an increase in salt concentration causes a decrease in the magnitude of the (16)Li, C.; Somasundaran,P. J. Colloids Surf. 1991, 146, 215-218. (17)Li, C.; Somasundaran,P. submitted for publication in Colloids Surf.
0
-20
x
A
a
1
E 0
._
-40
4-
C P) Y
0
a
-60
0
+
0 N
-80
I
I
6
8
-100 2
4
0
1
I
10
12
PH
Figure 1. Zeta potential of bubbles as a function of pH in NaCl solutions. I
40t Th
NaCl Conc. A
M
1X10-’
-40
2
4
6
8
10
12
PH
Figure 2. Zeta potential of coal as a function of pH in NaCl solutions.
zeta potential (Figure 1). This suggests that Na+ and Clions do not specificallyadsorb at the gas-liquid interface. Effect of Salt Concentration on the Charge of Coal. Zeta potential of coal was determined as a function of pH in triply distilled water and in NaCl solutions of three different concentrations (1X lO-l, 1 X le2, and 1 X 106 mol/dm3). Zeta potential-pH curves for coal shown in Figure 2 yield an isoelectric point of 6.0 f 0.1. Also, increase in NaCl concentration caused only a reduction in the magnitude of the zeta potential with no effect on the isoelectric point. This indicates that neither sodium nor chloride ions specifically adsorb on the coal surface. Effect of Salt Concentration on Coal Flotation. Coal flotation was conducted in triply distilled water as well as in NaCl electrolyte solutions. It is found that changes in solution pH and salt concentration have a very complex effect on the flotation behavior of coal (Figures 3 and 4). The floatability of coal can be seen to decrease with increase in salt concentration at low salt concentrations and increase at high concentrations. Figure 3 shows that in triply distilled water and 1C6 mol/dm3 NaCl solution around 75% of the coal can be floated at and below 4.5 (i.e., the natural pH of the coal slurry after 10min conditioning). The floatability of coal then drops precipitously with increase in solution pH and reaches a minimum of -15% around pH 6. Above this pH, coal floatability increases and peaks at -pH 9 before it drops again. An increase in NaCl concentration to 0.01 mol/dm3resultain a significantdecreasein coal floatability,
Li and Somasundaran
246 Energy & Fuels, Vol. 7, No. 2, 1993 NaCl Conc.
M T
U cl
-
.-3
2
90
.c 0
i
U
I
2
I
I
I
I
I
4
6
8
10
12
concentration: 04.1 mol/dm3). 1
I
,
I
,
I
1 9 01 M
0
’
I
2
I
I
I
1
I
4
6
E
10
12
PH
PH
Figure 3. Effect of NaCl concentration on coal floatability (NaCI 100,
80
NaCl Conc. M A 0 v 1~10-~ 0 1x10:; 0 5x10
I
I
I
I
I
I
2
4
6
0
10
12
PH
Figure 4. Effect of NaCl concentration on coal floatability (NaCI concentration: 0.1-0.5 mol/dm3).
although the % Rec-pH curve is still somewhat similar in shape. The floatability of coal reaches a minimum at around pH 6, increases and peaks at pH 7 (instead of pH 9), and then decreases. Further increase in salt concentration to 0.1 mol/dm3 causes not only an additional decrease in the coal floatability but also the disappearance of the similarity in the % R e e p H curve. Above 0.1 mol/dm3 NaC1, any increase in salt concentration causes an increase of the coal floatability and thus an upward shift of the entire % Rec-pH curve (Figure 4). This observation is in agreement with the earlier findings of severalother researchersthat the floatability of naturally hydrophobic materials increases significantly upon the addition of inorganic e1ectrolytes.l4 Also, it is seen that, at a salt concentration of 0.1 mol/dm3 and above, the coal floatability is highest at the low pH end and decreases steadily with increase in pH. Role of Hydrophobicity on Coal Flotation. Hydrophobicity of fine particles is generally evaluated in terms of the induction time for the attachment of a bubble to the particle. However,bubble charge can also play a crucial role in this type of measurement and give a distorted picture of the effect of hydrophobicity. The experiments conducted in this study, as discussed earlier,were therefore designed to eliminate the electrostatic effects. Coal hydrophobicityin triply distilledwater and in NaCl solutions of various salt concentrationsis plotted in Figure 5 as a function of pH. It is seen that in triply distilled water and 10-6 mol/dm3 NaCl solution the hydrophobicity and the flotation recovery show similar trends, with both
Figure 5. Effect of NaCl concentration on the hydrophobicity of coal as determined by the film levitation technique. the responses exhibiting a minimum value at -pH 6. The hydrophobicity index of coal is -95 below pH 4.5 and reaches a minimum of 86 at around pH 6 while the floatability of coal exhibits amuch sharper decrease (from 75 to 15 wt %). Above this pH, coal hydrophobicity gradually increases with increasing pH and reaches 94 at pH 9.5, whereas the flotation recovery increases from 15 to 60%. It is thus clear that at low salt concentrations coal flotation is controlled mainly by the hydrophobicity of coal. It is also clear that the hydrophobicity of coal decreases significantlyas the salt concentration is increased from 1 X 106 to 0.1 mol/dm3, except in the pH range of 6-9 where the hydrophobicity of coal remains almost the same. This decrease in coal hydrophobicity is very likely responsible for the observed sharp decrease in the coal floatability. This result again suggests that at low salt concentrations the hydrophobicity of the coal is the controlling factor for the flotation process. Further increase in salt concentration to 0.5 mol/dm3 does not have any significant effect on the coal hydrophobicity while the flotation recovery is noticed to increase at least by a factor of 2. This means that above 0.1 mol/ dm3 NaCl the observed upward shift of the % Rec-pH curves cannot be attributed to any increase in the coal hydrophobicity. Also, it is seen that no simple correlation can be found between hydrophobicity and the flotation behavior in 0.1 and 0.5 mol/dm3 NaCl solutions. The hydrophobicity index remains constant at a value of 88 below pH 6. Above this pH, the hydrophobicity index starta increasing and reaches a value of 93 at -pH 9.5 before it decreases again, whereas the floatability testa show a continuous decrease in flotation with increasing solution pH. These results suggest that above 0.1 mol/ dm3NaCl the coal hydrophobicity is no longer controlling the flotation process. Role of Electrostatic Interaction on Coal Flotation. As mentioned earlier, coal particles are positively charged below pH 6 and negatively charged above this pH while bubbles are negatively charged in the complete pH range of 2-12. It is then clear that below pH 6 an attractive force will exist between bubbles and particles since they are oppositely charged. This attractive force decreases with increase in pH and vanishes at pH 6 where coal particles are neutral. Above pH 6, the force between bubbles and coal particles becomes repulsive and increases in magnitude with increase in solution pH. The effect of electrostaticinteraction can be demonstrated more clearly in terms of an “attraction index”. According to Healy and
Coal Flotation in NaC1 Solutions
Energy & Fuels, Vol. 7, No. 2, 1993 247
Table 11. Estimated Values of Zeta Potential (in mV) for Bubbles and Particles in Various NaCl Solutions
PH [NaCl], mol/dm3
3
4
5
6
7
8
9
10
0.5 M bubble particle 0.3M bubble particle 0.2 M bubble particle
-6
-12 3 -14
-14 0 -17 -20
7
0
-5
-21 -15 -26 -18 -30 -22
-23 -23 -27 -28 -32
13
4 -17 4
-17 -4 -21 -4 -24
-19 -8 -23
-8
-8 5 -10 6 -12
9 -7
11
c ~ - w o r k e r s , ~the ~ Jdouble ~ layer interaction between two spherical particles is a function of the product of their Stern potentials 41 and 42. If the bubbles are considered to be spherical particles, and the zeta potential assumed to be the same as the Stern potential, the double layer interaction between the bubbles and the coal particles will be a function of the product of their zeta potentials, Therefore, the attraction index is defined here as the product of the zeta potential of the bubble and that of the particle with a negative sign. Negative term is introduced to show attraction as a positive value and repulsion as negative. Due to the limitations of the Lazer Zee Meter, zeta potential values for bubbles and coal particles cannot be obtained experimentally in NaCl solutions above 0.1 mol/ dm3 and have to be estimated. For a charged surface of potential (60, the potential cp at a distance x from the surface is = ‘Po exp(-rx) (1) where K is the reciprocal of the electrical double layer thickness and can be expressed as follows cp
where e is the elementary electric charge, ni is the number of ions per unit volume of type i, Zi is the valency of ion i , E is the permittivity, k is the Boltzmann constant, and T is the absolute temperature. If x in eq 1 is the distance of the shear plane from the particle surface, cp can be replaced by 5; and eq 1 then becomes
r = ro exP(-Kx)
(3) Since for the unknown zeta potential, t1 = fa exp(-qx) and for the known experimental value si = t o exp(-rzx), thus
= f2 exp[(KZ - K1)Xl (4) For the present calculations, 52 was assumed to be that obtained in the 0.1 mol/dm3solution and x to be 3.72 Azo for both the bubbles and coal particles (assuming the location of the shear plane is the same as the Stern plane), ~1were estimated to be 1.47 X lo9, 1.80 X lo9, and 2.32 X lo9 m-l for 0.2, 0.3, and 0.5 mol/dm3 NaCl solutions, respectively, and K Z 1.04 X lo9 m-l. The results obtained are tabulated in Table 11. In Figure 6, the floatability of coal is plotted as a function of the attraction index, -S;Ji, for various NaCl solutions. In 10-5 and mol/dm3 NaCl solutions, it is found that the floatability of coal first decreases with decreasing (18)HOUU, R.;Healy, T. W.; Fuerstenau, D. W. Tram. Faraday sot. 1966,62,1638-1651. (19)Wiese, G.R.;Healy,T. W. Trans.Faraday SOC.1970,66,490-499. (20) Devanathan, M.A. V.: Tilak. B. V. K. S.R.A. Chem. Rev. 1965, 65,635-684.
0
0
-10 -27 -12
-33
1
-6000
-4000
-2000
0
2000
A t t r a c t i o n Index
Figure 6. Flotation recovery as a function of the attraction index.
attraction indexuntila minimum is reached at 0 attraction index. The floatabilitystarta increasingonce the attraction index becomes negative. This indicates that the electrostatic interaction is not controlling the flotation process at least when both the bubbles and the particles are negatively charged. This result further supports the suggestion made in the previous section that at low salt concentration the flotation process is controlled mainly by the coal hydrophobicity. It is to be noted that all the flotation recovery data at 0.1 mol/dm3NaCl solution and those with positive attractive indices at and mol/ dm3solutions can be represented by a single curve, whereas above 0.1 mol/dm3, the results obtained at different concentrations are widely separated from each other. In each case, the floatability of coal decreases steadily with decreasing attraction index suggesting that the coal flotation under these conditions is controlled mainly by the electrostatic interaction between the bubbles and the particles. Thus, at high salt concentrations (0.1 mol/dm3 and higher) the electrostatic interaction between bubbles and particles becomes the major factor controlling the flotation process. It can be concluded at this point that, below 0.1 mol/ dm3 NaCI, the flotation is mainly controlled by the hydrophobicity of the coal, whereas above this salt concentration it is controlled by the electrostatic interaction. This is possibly due to the long-range nature of the electrical double layer forces at low salt concentrations in comparison to the short-range hydrophobic interactions. Kinetic energies of the bubbles and the particles are sufficient to overcome the energy barrier resulting from the electrostatic interaction enabling them to approach each other within a distance at which the hydrophobic interaction can take place and cause the three-phase contact. As the salt concentration is increased, the effective distance for the double layer interaction also decreases due to the compression of the double layers. The electrostatic interaction becomes significant and controlling once the effective distance of the electrostatic
248 Energy & Fuels, Vol. 7, No. 2, 1993
Li and Somasundaran
Table 111. Mean Bubble Diameter in NaCl Solutions at Different pH
Kharlamovl has shown that no significant change in bubble size could be observed for NaCl solutions below 0.25 N.
mean bubble diameter (pm) [NaClI, mol/dm3
10-1 10-2 10-5
3 -
-
102
PH 4
6
10
11
110 102 110
109 106 104
110 103 107
110
-
109
interaction between bubbles and particles is comparable to that of the hydrophobic interaction. Above 0.1 mol/dm3, any increase in salt concentration will result ina significant increase in coal floatability which is not explained by the changes in either the hydrophobicity or the electrostatic interaction. The reason for this is not yet clear; one possible explanation is that an increase in salt concentration can destabilize the hydration layer around coal particles and thus facilitate the drainage of the water film between the bubble and the particle making the coal particles more floatable. It may be suggested that the observed changes in the coal flotation behavior be attributed to the variations in the bubble size. Results of the bubble size measurement given in Table I11 clearly shows that there is no significant change in bubble size a t least below 0.1 mol/dm3NaCl in a pH range of 3-11 while the floatability of coal does change markedly under these conditions. Laskowskiand Iskraghave also suggested that the frothing effect in the salt flotation of naturally hydrophobic materials can be neglected. Moreover,
Conclusions Analysis of the effect of salt addition on the zeta potential of bubbles and coal particles showed the sodium chloride to behave as an indifferent electrolyte in both cases. The isoelectric point of coal was pH 6.0 and that of the bubble, obtained by extrapolation, was pH 1.5. The floatability behavior of coal, obtained using a modified Hallimond tube, was found to be complex with respect to variations in both the pH and the salt concentration. At low salt concentrations (below 0.1 mol/dm3) flotation of coal decreases with increase in salt exhibiting a minimum at around pH 6 and a maximum between pH 7 and 9. Above 0.1 mol/dm3,however, flotation exhibitaa marked increase in flotation with salt concentration without any such maximum or minimum. Hydrophobicity of coal particles is measured by a new hydrophobic levitation technique and it is shown that the floatability of coal depends predominantly on the hydrophobicity of coal at low salt concentrations. Above 0.1 mol/dm3, on the other hand, the flotation is governed mainly by electrostatic interactions represented here as “attraction index”.
Acknowledgment. The authors acknowledge the financial support of NSF(1NT-87-04303)and the New York Mining and Mineral Resources Research Institute.
