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A levitation technique for determining particle hydrophobicity
229
Colloids and Surfaces A: Physicochemical
and Engineering Aspects, 70 (1993)229-232 Elsevier Science Publishers B.V., Amsterdam
A levitation technique for determining particle hydrophobicity Chin Li, P. Somasundaran* Langmu~r Centerfor NY 10027, USA
and C.C. Harris
Collards and interfaces, Henry ~r~rnb School ~f~ines,
C5~~rnbia University, New York,
(Received 21 May 1992; accepted 7 November 1992) Abstract A new technique designed to evaluate the hydrophobicity of fine particles is described. Results of the hydrophobicity measurements correlate well with those of the Hallimond tube flotation tests both in triply distilled water and in I * 10e3 mol drne3 AU, solution. Experimental error analysis indicates that this technique is more sensitive for detecting small changes in the hydrophobicity than the Hallimond tube tests used for determining floatability. Keywords:
Levitation technique; particle hydrophobicity.
Introduction Hydrophobicity is an important interfacial property which controls various surface-based processes such as flotation, adhesion, emulsi~cation, flocculation and dispersion. Traditionally, contact-angle measurements have been used to assess the hydrophobicity of a substrate. This technique requires a polished surface which limits its practical applications since mineral samples rarely occur in a real system in that form. The hydrophobicity of fine particles, however, can be evaluated in terms of the induction time determined by pressing a captive bubble against a submerged particle bed. The control of the size and travelling distance of the bubble, however, is very critical especially when the induction time is very short [I]. Another technique, film flotation, assesses the hydrophobicity/hydrophili~ity of particles by sprinkling fine particles on water-methanol mixtures of varying concentrations [2-41. This technique, however, only provides information on the water-advancing
contact angle and nothing on the water-receding contact angle, which is also a critical factor for all practical processes. Also, methanol could interact with particle surfaces and alter its properties. A centrifugal immersion technique developed recently does avoid such artifacts but it also measures only the advancing contact angle [S]. Furthermore, the measured surface could conceivably be sensitive to the hydrodynamic conditions used. In this paper a hydrophobic levitation technique designed to measure the hydrophobicity of fine particles is described. This method requires no specific sample preparation so that the measured hydrophobicity is based on the surface characteristics of the particles per se. In addition, flotation experiments were carried out with a modified Hallimond tube in order to delineate the role of the hydrophobicity in the flotation process. The hydrophobi~ity of the particles, coal in this case, was altered by changing the solution pH and by adding inorganic salts.
*Corresponding author. 0927-7757/93/S~.~
0 1993 -
Elsevier Science Publishers B.V. All rights reserved.
C. Li et al./Colloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 229-232
230
Experimental Materials
Hand-picked coal samples from Bruceton mines, Pittsburgh, PA, were used in this study. This coal is a highly volatile A bituminous sample. Large pieces of coal were broken by hand into small pieces, which were subsequently crushed in a Quaker mill in open atmosphere. The crushed coal was then dry-ground in a ceramic ball mill. 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 Tyler mesh size fraction (- 425 + I80 urn). ACS certified grade sodium chloride (NaCl) and aluminum chloride (AlCl& and the 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. Methods Hydrophobicity
The hydrophobicity of the coal samples was evaluated using the hydrophobic levitation technique developed specifically for the present purpose. A modified 350 cm3 Buchner funnel provided with a coarse sintered glass frit (pore size, 40-60 urn) and an annular ring to collect the levitated product was used (Fig. 1). A coal sample (0.25 g; 35 x 80 mesh) was stirred in 25 g of the
BU~HNE~ FUNNEL RESERVOIR
Fig. 1. Schematic diagram of the device designed for measuring the particle hydrophobi~ty.
desired salt solution in a beaker, using a Tefloncoated magnetic bar, for 5 min. The pH of the slurry was then adjusted to the desired value. After an additional 5 min of conditioning, the slurry was transferred to the funnel and the excess solution was drawn out through the frit using a peristaltic pump. The pumping rate was fixed at 70 cm3 min-’ at the ~ginning and was reduced to 5 cm3 min-’ during the final stage of the process. The flow was reversed immediately after air bubbles were seen breaking through the frit to ensure that the exposure time of the coal particles was approximately the same for all the experiments. Hydrophobic particles, carried upwards by the air-water interface, were separated from the hydrophilic particles which remained submerged on the frit. These two fractions were then filtered, dried, and weighed. A hydrophobicity index, numerically equivalent to weight per cent float in these tests, is presented on a scale of 0- 100. Floatability
A coal sample (1 g; 35 x 80 mesh) was conditioned in 1OOg of the desired salt solution in a 150 cm3 beaker following the same procedure as was used in the hydrophobicity experiments. The slurry was then transferred to a modified Hallimond tube and the flotation was carried out for 10 min using nitrogen at a flow rate of 10 cm3 min-r. The float and sink fractions were filtered, dried and weighed [S]. Results and discussion The coal hydrophobicity in triply distilled water, as measured by the hydrophobic levitation technique, is plotted in Fig. 2 as a function of pH, The floatability of coal determined using the modified Hallimond tube is also presented for comparison purpose. The hydrophobicity of coal and its floatability show similar trends, with both responses exhibiting a minimum value at about pH 6. Below pH 4.5, the hydrophobicity index of coal is about 95 and reaches a minimum of 86 at around pH 6, while the ~oatability of coal exhibits a much
C. Li et al./Colloids
20
Surfaces
A: Physicochem.
Eng.
Aspects
70 (1993)
20
-
D Hydraphob;city
E) H~fophob~city 0 0
231
229-232
Floatability
‘,‘,‘,I*‘,’ 2
0 4’
8
6
10
2
12
4
8
6
PH
10
12
PH
and floatability of
Fig. 3. Comparison of the hydrophobicity and floatability of coal samples in a 1. 1C3 mol dme3 AlCl, solution.
sharper decrease (from 75 to 15 wt%). Above this pH, coal hydrophobicity increases gradually with increasing pH and reaches 94 at pH 9.5, whereas the flotation recovery increases from 15 to 60%. It has been reported that the presence of hydrolyzable metal ions (e.g. Al3 +, Ca2 + and Fe3 ‘) can depress the floatability of coal significantly. This is attributed to the formation of hydrophilic metal hydroxides at the solid/liquid interface [7]. In order to determine whether or not the observed decrease in coal floatability resulted from a decrease in the coal hydrophobicity, both the floatability and hydrophobicity of coal were measured in a 1*10S3 mol dmm3 AlC13 solution. A good correlation is obtained between the hydrophobicity and the floatability of coal below pH 9 (Fig. 3). Above pH 5, the hydrophobicity index of coal decreases drastically and becomes zero at pH 7 while the floatability of coal shows a similar decrease (from 80 to 10%). Above this pH, coal hydrophobicity starts to increase and the index reaches 90 at around pH 9, whereas flotation recovery increases from IO to 70%. Above pH 9, coal floatability was found to decrease again although the hydrophobicity index shows no change. It is clear that the observed changes in the coal floatability can be attributed to variations in its hydrophobicity. In order to explain the role of the
aluminum ion and its hydrolysis products in causing the observed changes in coal hydrophobicity, aluminum species distribution was determined as a function of pH using the data for the free energy of formation of each species (Fig. 4) [S]. It can be seen from this figure that the pH region where an AI( precipitate is expected to form (i.e. pH 5-9) corresponds well with the region where the coal hydrophobicity and floatability experience a decrease. This result suggests that AI(O a hydrophilic species, starts to precipitate on the
Fig. 2. Comparison of the hydrophobicity coal samples in triply distilled water.
-2,
,
/
/
,
,
/
(
AIf .... ..__._ .’ -4
77 -
-6
T
-8
-10
,”
,:...... ,!“ ,,I,;r ,j’.’ .,,,+y : ; ,,: , I #’ ,,: _,:’ ,’ ,’ : r’ ; ,’ : ,: ,’ ! ;’ ; ,j ; ,.: ; :’ 8 ~ 2
4
10
12
PH
Fig. 4. Aluminum species distribution diagram as a function of pH in a I *IO-’ mol drn-j AICI, solution.
232
C. Li et al./Colloids
coal surface at about pH 5.0, causing a reduction in hydrophobicity. An increase in hydrophobicity was observed above pH 7 due to dissolution of the precipitated AI( species from the surface. The reproducibility of the levitation tests was determined by repeating the experiment 5-10 times under selected conditions, and the experimental error was calculated to be around 2.5%. The estimated error based on mass balances was only about 1%. This difference can be attributed largely to the small sample size (0.25 g) used in this experiment. However, the experimental error for the Hallimond tube flotation tests was found to be at least three times that of the hydrophobicity experiments, although l-g samples were used in the flotation tests. The higher error obtained in the flotation tests is possibly due to the turbulent flow conditions in the Hallimond tube compared with those in the hydrophobic levitation technique. It is thus clear that this technique is much more sensitive and reliable in detecting small variations in the hydrophobicity than the Hallimond tube tests, which strictly should be used for determining floatability [9]. This study demonstrates that the hydrophobic levitation technique is a simple and effective tool for determining the hydrophobicity of fine particles. This technique requires no special sample preparation (as in the contact-angle measurement), bubble size and motion control (as in the induction-time
Surfaces
A: Physicochem.
Eng. Aspects 70 (1993) 229-232
measurement), use of non-aqueous solvents (as in the film flotation), and thus provides an accurate assessment of hydrophobicity as encountered in practicat situations. Under the conditions tested, the results of the hydrophobicity measurements correspond well with those obtained from the floatability measurements. Acknowledgments The authors wish to acknowledge the financial support of NSF(INT-87-04303) and the New York Mining and Mineral Resources Research Institute. References Y. Ye, SM. Khandrika and J.D. Miller, Int. J. Miner. Process., 25 (1989) 221. P.L. Walker, E.E. Petersen and CC. Wright, Ind. Eng. Chem., 44 (1952) 2389. MC. Williams and D.W. Fuerstenau, Int. J. Miner. Process., 20 (1987) 153. D.W. Fuerstenau, MC. Williams, KS. Narayanan, J.L. Diao and R.N. Urbina, Energy Fuels, 2 (1988) 237. R. Ramesh and P. Somasundaran, J. Colloid Interface Sci., 139 (1990) 291. P. Somasundaran and K.P. Ananthapadmanabhan, in N.L. Weiss (Ed.), S.M.E. Mineral Processing Handbook, Society of Mining Engineers, New York, 1985, pp. 30-87. M.S. Celik and P. Somasundaran, Sep. Sci. Technol., 21 (1986) 393. D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nuttall, J. Phys. Chem. Ref. Data, 1I (1982) 2-127. C.C. Harris, Min. Mag., 126 (1972) 439.
Colloids and Surfaces A: Physicochemical
and Engineering Aspects, 70 (1993)229-232 Elsevier Science Publishers B.V., Amsterdam
A levitation technique for determining particle hydrophobicity Chin Li, P. Somasundaran* Langmu~r Centerfor NY 10027, USA
and C.C. Harris
Collards and interfaces, Henry ~r~rnb School ~f~ines,
C5~~rnbia University, New York,
(Received 21 May 1992; accepted 7 November 1992) Abstract A new technique designed to evaluate the hydrophobicity of fine particles is described. Results of the hydrophobicity measurements correlate well with those of the Hallimond tube flotation tests both in triply distilled water and in I * 10e3 mol drne3 AU, solution. Experimental error analysis indicates that this technique is more sensitive for detecting small changes in the hydrophobicity than the Hallimond tube tests used for determining floatability. Keywords:
Levitation technique; particle hydrophobicity.
Introduction Hydrophobicity is an important interfacial property which controls various surface-based processes such as flotation, adhesion, emulsi~cation, flocculation and dispersion. Traditionally, contact-angle measurements have been used to assess the hydrophobicity of a substrate. This technique requires a polished surface which limits its practical applications since mineral samples rarely occur in a real system in that form. The hydrophobicity of fine particles, however, can be evaluated in terms of the induction time determined by pressing a captive bubble against a submerged particle bed. The control of the size and travelling distance of the bubble, however, is very critical especially when the induction time is very short [I]. Another technique, film flotation, assesses the hydrophobicity/hydrophili~ity of particles by sprinkling fine particles on water-methanol mixtures of varying concentrations [2-41. This technique, however, only provides information on the water-advancing
contact angle and nothing on the water-receding contact angle, which is also a critical factor for all practical processes. Also, methanol could interact with particle surfaces and alter its properties. A centrifugal immersion technique developed recently does avoid such artifacts but it also measures only the advancing contact angle [S]. Furthermore, the measured surface could conceivably be sensitive to the hydrodynamic conditions used. In this paper a hydrophobic levitation technique designed to measure the hydrophobicity of fine particles is described. This method requires no specific sample preparation so that the measured hydrophobicity is based on the surface characteristics of the particles per se. In addition, flotation experiments were carried out with a modified Hallimond tube in order to delineate the role of the hydrophobicity in the flotation process. The hydrophobi~ity of the particles, coal in this case, was altered by changing the solution pH and by adding inorganic salts.
*Corresponding author. 0927-7757/93/S~.~
0 1993 -
Elsevier Science Publishers B.V. All rights reserved.
C. Li et al./Colloids Surfaces A: Physicochem. Eng. Aspects 70 (1993) 229-232
230
Experimental Materials
Hand-picked coal samples from Bruceton mines, Pittsburgh, PA, were used in this study. This coal is a highly volatile A bituminous sample. Large pieces of coal were broken by hand into small pieces, which were subsequently crushed in a Quaker mill in open atmosphere. The crushed coal was then dry-ground in a ceramic ball mill. 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 Tyler mesh size fraction (- 425 + I80 urn). ACS certified grade sodium chloride (NaCl) and aluminum chloride (AlCl& and the 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. Methods Hydrophobicity
The hydrophobicity of the coal samples was evaluated using the hydrophobic levitation technique developed specifically for the present purpose. A modified 350 cm3 Buchner funnel provided with a coarse sintered glass frit (pore size, 40-60 urn) and an annular ring to collect the levitated product was used (Fig. 1). A coal sample (0.25 g; 35 x 80 mesh) was stirred in 25 g of the
BU~HNE~ FUNNEL RESERVOIR
Fig. 1. Schematic diagram of the device designed for measuring the particle hydrophobi~ty.
desired salt solution in a beaker, using a Tefloncoated magnetic bar, for 5 min. The pH of the slurry was then adjusted to the desired value. After an additional 5 min of conditioning, the slurry was transferred to the funnel and the excess solution was drawn out through the frit using a peristaltic pump. The pumping rate was fixed at 70 cm3 min-’ at the ~ginning and was reduced to 5 cm3 min-’ during the final stage of the process. The flow was reversed immediately after air bubbles were seen breaking through the frit to ensure that the exposure time of the coal particles was approximately the same for all the experiments. Hydrophobic particles, carried upwards by the air-water interface, were separated from the hydrophilic particles which remained submerged on the frit. These two fractions were then filtered, dried, and weighed. A hydrophobicity index, numerically equivalent to weight per cent float in these tests, is presented on a scale of 0- 100. Floatability
A coal sample (1 g; 35 x 80 mesh) was conditioned in 1OOg of the desired salt solution in a 150 cm3 beaker following the same procedure as was used in the hydrophobicity experiments. The slurry was then transferred to a modified Hallimond tube and the flotation was carried out for 10 min using nitrogen at a flow rate of 10 cm3 min-r. The float and sink fractions were filtered, dried and weighed [S]. Results and discussion The coal hydrophobicity in triply distilled water, as measured by the hydrophobic levitation technique, is plotted in Fig. 2 as a function of pH, The floatability of coal determined using the modified Hallimond tube is also presented for comparison purpose. The hydrophobicity of coal and its floatability show similar trends, with both responses exhibiting a minimum value at about pH 6. Below pH 4.5, the hydrophobicity index of coal is about 95 and reaches a minimum of 86 at around pH 6, while the ~oatability of coal exhibits a much
C. Li et al./Colloids
20
Surfaces
A: Physicochem.
Eng.
Aspects
70 (1993)
20
-
D Hydraphob;city
E) H~fophob~city 0 0
231
229-232
Floatability
‘,‘,‘,I*‘,’ 2
0 4’
8
6
10
2
12
4
8
6
PH
10
12
PH
and floatability of
Fig. 3. Comparison of the hydrophobicity and floatability of coal samples in a 1. 1C3 mol dme3 AlCl, solution.
sharper decrease (from 75 to 15 wt%). Above this pH, coal hydrophobicity increases gradually with increasing pH and reaches 94 at pH 9.5, whereas the flotation recovery increases from 15 to 60%. It has been reported that the presence of hydrolyzable metal ions (e.g. Al3 +, Ca2 + and Fe3 ‘) can depress the floatability of coal significantly. This is attributed to the formation of hydrophilic metal hydroxides at the solid/liquid interface [7]. In order to determine whether or not the observed decrease in coal floatability resulted from a decrease in the coal hydrophobicity, both the floatability and hydrophobicity of coal were measured in a 1*10S3 mol dmm3 AlC13 solution. A good correlation is obtained between the hydrophobicity and the floatability of coal below pH 9 (Fig. 3). Above pH 5, the hydrophobicity index of coal decreases drastically and becomes zero at pH 7 while the floatability of coal shows a similar decrease (from 80 to 10%). Above this pH, coal hydrophobicity starts to increase and the index reaches 90 at around pH 9, whereas flotation recovery increases from IO to 70%. Above pH 9, coal floatability was found to decrease again although the hydrophobicity index shows no change. It is clear that the observed changes in the coal floatability can be attributed to variations in its hydrophobicity. In order to explain the role of the
aluminum ion and its hydrolysis products in causing the observed changes in coal hydrophobicity, aluminum species distribution was determined as a function of pH using the data for the free energy of formation of each species (Fig. 4) [S]. It can be seen from this figure that the pH region where an AI( precipitate is expected to form (i.e. pH 5-9) corresponds well with the region where the coal hydrophobicity and floatability experience a decrease. This result suggests that AI(O a hydrophilic species, starts to precipitate on the
Fig. 2. Comparison of the hydrophobicity coal samples in triply distilled water.
-2,
,
/
/
,
,
/
(
AIf .... ..__._ .’ -4
77 -
-6
T
-8
-10
,”
,:...... ,!“ ,,I,;r ,j’.’ .,,,+y : ; ,,: , I #’ ,,: _,:’ ,’ ,’ : r’ ; ,’ : ,: ,’ ! ;’ ; ,j ; ,.: ; :’ 8 ~ 2
4
10
12
PH
Fig. 4. Aluminum species distribution diagram as a function of pH in a I *IO-’ mol drn-j AICI, solution.
232
C. Li et al./Colloids
coal surface at about pH 5.0, causing a reduction in hydrophobicity. An increase in hydrophobicity was observed above pH 7 due to dissolution of the precipitated AI( species from the surface. The reproducibility of the levitation tests was determined by repeating the experiment 5-10 times under selected conditions, and the experimental error was calculated to be around 2.5%. The estimated error based on mass balances was only about 1%. This difference can be attributed largely to the small sample size (0.25 g) used in this experiment. However, the experimental error for the Hallimond tube flotation tests was found to be at least three times that of the hydrophobicity experiments, although l-g samples were used in the flotation tests. The higher error obtained in the flotation tests is possibly due to the turbulent flow conditions in the Hallimond tube compared with those in the hydrophobic levitation technique. It is thus clear that this technique is much more sensitive and reliable in detecting small variations in the hydrophobicity than the Hallimond tube tests, which strictly should be used for determining floatability [9]. This study demonstrates that the hydrophobic levitation technique is a simple and effective tool for determining the hydrophobicity of fine particles. This technique requires no special sample preparation (as in the contact-angle measurement), bubble size and motion control (as in the induction-time
Surfaces
A: Physicochem.
Eng. Aspects 70 (1993) 229-232
measurement), use of non-aqueous solvents (as in the film flotation), and thus provides an accurate assessment of hydrophobicity as encountered in practicat situations. Under the conditions tested, the results of the hydrophobicity measurements correspond well with those obtained from the floatability measurements. Acknowledgments The authors wish to acknowledge the financial support of NSF(INT-87-04303) and the New York Mining and Mineral Resources Research Institute. References Y. Ye, SM. Khandrika and J.D. Miller, Int. J. Miner. Process., 25 (1989) 221. P.L. Walker, E.E. Petersen and CC. Wright, Ind. Eng. Chem., 44 (1952) 2389. MC. Williams and D.W. Fuerstenau, Int. J. Miner. Process., 20 (1987) 153. D.W. Fuerstenau, MC. Williams, KS. Narayanan, J.L. Diao and R.N. Urbina, Energy Fuels, 2 (1988) 237. R. Ramesh and P. Somasundaran, J. Colloid Interface Sci., 139 (1990) 291. P. Somasundaran and K.P. Ananthapadmanabhan, in N.L. Weiss (Ed.), S.M.E. Mineral Processing Handbook, Society of Mining Engineers, New York, 1985, pp. 30-87. M.S. Celik and P. Somasundaran, Sep. Sci. Technol., 21 (1986) 393. D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney and R.L. Nuttall, J. Phys. Chem. Ref. Data, 1I (1982) 2-127. C.C. Harris, Min. Mag., 126 (1972) 439.