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Gas dispersion characteristics of flotation reagents
Powder Technology 235 (2013) 329–335
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Gas dispersion characteristics of flotation reagents V. Ravichandran a, C. Eswaraiah b,⁎, R. Sakthivel b, S.K. Biswal b, P. Manisankar a a b
Department of Industrial Chemistry, Alagappa University, Tamil Nadu, 630003, India CSIR, Institute of Minerals and Materials Technology, Bhubaneswar, 751013, Orissa, India
a r t i c l e
i n f o
Article history: Received 23 January 2012 Received in revised form 5 October 2012 Accepted 19 October 2012 Available online 27 October 2012 Keywords: Frother Collector Optimization Gas hold-up Model
a b s t r a c t Gas dispersion is one of the key factors in the mineral flotation process articulated by many variables such as bubble size, gas hold-up, gas superficial velocity and bubble surface area flux (Sb). In this work, experiments were carried out on column flotation (5.08 cm in diameter and 180 cm height) with different types of reagents to evaluate the gas hold-up. The gas hold-up was measured using pressure difference technique. The parameters were optimized for enhanced gas hold-up for better flotation performance. The influence of operating parameters comprising different reagents' dosage/type and their mixture combination on gas hold-up was discussed in detail. Based on the experimental data, the dimensionless correlations were developed to predict the gas hold-up within the range of parameters studied. The correlations are found to be able to predict gas hold-up with acceptable accuracy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gas dispersion is one of the key factors in the mineral flotation process. Flotation requires the formation of a stable bubble-particle aggregate which helps the particle to be carried out from the pulp. The stability of the aggregate depends on the interfacial free energies or the corresponding interfacial tensions of the solid/liquid, solid/ vapor and liquid/vapor interfaces, which are responsible for attachment process. The tendency of replacement of solid/liquid by solid/ vapor interface depends on the hydrophobicity of the floated particles. Recent publications [1] indicate that type and dosage of frother play important roles on the applicability of the drift-flux model. Studies carried out [2] in two-phase systems (water-air) have shown that the frothers (surfactants) n-pentanol, MIBC (methyl–isobutyl carbinol), DF-250 (i.e., dowfroths-250) and F-150 (i.e., flottec-150), increase in this order, the drag coefficient of the bubble/water interface, attributed to the formation of a thin layer of water bonded to the bubble. In general, a variety of organic and inorganic reagents are used in the flotation process, such as frothers, collectors, pH regulator and depressant. Collector adsorbs on the surface of the particle to make the surface hydrophobic. The interactions between frothers and collectors have been widely reported [3]. El-Shall et al. [4] found that frother–collector interactions are dependent upon frother type; however there are few quantitative results on the effect of interaction on
⁎ Corresponding author. E-mail address: [email protected] (C. Eswaraiah). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.10.039
the concentrations of reagents remaining in solution. Measurement of the concentration of frother remaining in solution has not been widely reported. The use of a maximum bubble pressure to determine the dynamic surface tension of frother solutions is discussed by Pugh [5] and Comley et al. [6], however, at low concentrations, the trials of this method to determine the concentration of frother have proven unsuccessful. Frothers are used to decrease bubble size and promote the froth stability [7]. Frother molecules comprise a nonpolar (hydrophobic) hydrocarbon chain and a polar (hydrophilic) group, typically OH. They adsorb at the air–water interface and through forces associated with surface tension gradients and visco-elastic effects stabilize bubbles against coalescence [8], which provides both the small bubbles production and froth building functions. Two common frother classes in the mineral industry are alcohols and polyglycols. The previous communication [2] shows that the gas hold-up is strongly dependent on frother type. This permitted a simple ranking of frothers, which corresponded to those reported using other procedures. The use of gas hold-up was predicated on its relationship with bubble size through its effect on bubble velocity: as bubble size decreases bubble velocity decreases and gas hold-up increases. Three observations were made during the work that demanded the relationship be explored. One observation was the insensitivity of gas hold-up at low concentration for low ranked frothers (i.e., those where the rate of increase in gas hold-up with concentration was low); MIBC was one such frother. The second observation was the steady increase in gas hold-up at higher concentration, evident for all frothers, despite being above CCC (critical coalescence concentration) when bubble size is expected to be minimum and constant thus gas hold-up is expected to be maximum and constant [9]. The
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third observation was visual: at equal gas hold-up the various frothers appeared to yield different bubble sizes, which violates a common assumption by Azgomi et al. [10]. Previous researchers have raised the possibility that different frothers give different gas hold-up and bubble size dependence [11–14]. There is no convenient on-site method of determining frother concentration. This has arguably restricted the use of frother to achieve metallurgical targets. Approaches have included gas chromatography [15], determining the total organic carbon [16] and using calibrations of bubble size and gas hold-up vs. frother concentration [17]. Recent studies, however, have shown that shape is a key factor, that bubbles slow down the more they become spherical [18] and that the effect of frother and some salts on bubble velocity is then seen as their ability to control bubble shape. Bubbles tend to flatten (become oblate) as they rise because of the pressure drop across their dimension. In the presence of surfactants and salts, deformation is resisted. Both 1-pentanol and NaCl are less able to maintain a spherical shape compared to F150 and MIBC and this corresponds to their reduced ability to slow the bubble down [19]. Another possibility is that the water structure at the bubble surface created by H-bonding via the polar groups of the frother or hydration sheath of salt ions resists deformation and so aids in keeping the bubble spherical [20]. “It is well known fact that the energy barrier defines the particle–bubble attachment in a flotation process. The energy barrier of aggregates coal particle–bubble was determined using the extended DLVO theory.” The effect of pH and frother concentration (FC) about energy barrier using four Colombian coals was evaluated. Experiments were carried out using a column flotation in order to evaluate the bubble diameter [21]. A recent study has reported the effect of pH and frother concentration evaluated on different coals and achieved the ash, sulphur rejection, combustible recovery and ash rejection at desirable levels [22]. In the present study, an attempt is made to develop the correlations for gas hold-up in flotation column based on the reagent characteristics such as collectors like kerosene and diesel oil, frothers such as pine oil, methyl-isobutyl carbinol, Nalco-9840, eucalyptus oil and ethyl alcohol, sodium carbonate as modifier, sodium silicate as depressant and process variables such as air rate, water rate. Summary of the reagents properties used is given in Table 1.
2. Materials and methods 2.1. Materials The measurement of gas hold-up in column flotation is carried out with commercial grade frothers such as pine oil, MIBC, Nalco-9840, eucalyptus oil and ethyl alcohol and collectors namely kerosene and diesel oil are selected for the measurement. The measurement of gas hold-up is also carried out with 10% sodium carbonate as pH regulator and depressant as sodium silicate solution according to the plant conditions.
2.2. Methods The compressed air to sparger is obtained from air compressor through the air filter. The air flow rate was measured by using rotameter. The required quantity of water with and without mixing of reagent dosage was added to the flotation column. The mechanical stirring facilitates the reagents to become homogenized before sending to the column. Air was supplied at different volumetric flow rates to the sparger to generate the bubbles in the water phase.
3. Experimental setup and procedure The schematic of the laboratory column flotation experimental setup was used for the present work and shown in Fig. 1. The volumetric flow rate was measured by using rotameter (Blacka Instrument, Chennai, India). U tube manometer was used to know the volume of air flow passing to the column. Air was introduced through a ceramic sparger (S) at the bottom of the column and its flow rate was observed using air rotameter (AR). The flotation column is made of acrylic tube of 180 cm height above the base with 5.08 cm diameter. The column has been graduated for determining the volume, and the maximum volume of the column is 3650 ml. The sparger is a porous element made of ceramic material with 2.5 cm diameter and 7.5 cm height. The pore diameter of the porous element is 5 μm. Internal sparger is used for all the experimental runs and is fitted at the bottom of the column. Initially, the column is filled with the water varied from 1 to 3 liters, and the volume was noted down. Air was supplied through the sparger and is connected with non returning valve so that the water does not come through the sparger to the compressor. Internal sparger is used in the present work, because with internal sparger material has a modest effect on bubble size. Flexible sparger is preferred over rigid sparger as they are less prone to the plugging by particles. The metallurgical performance in ceramic type diffuser is better in comparison with filter cloth. It has been noted that irrespective of sparger type, as the superficial sparger area decreases, the bubble size increases and vice-versa. In external sparger, the air and the liquid are contacted outside the column, and the mixture is carried through a pipe into the column from which it is expelled through holes about 1 mm in diameter. The experiments were carried out with water at different air flow rates with different reagents by varying their dosage. The column was thoroughly washed after each experimental run. All the experiments were carried out at ambient temperature. The mechanical stirring facilitates the reagents to become homogenized before sending to the column. The requisite volume of water without/with mixing of required reagent dosage was added to the flotation column through funnel fitted on the column top. Air at pressure 2 kg/cm 2 was supplied at different volumetric flow rates to the sparger to generate the bubble in the water phase. Air bubbles so generated will occupy
Table 1 Summary of reagents properties.a Reagent
Molecular weight, g/mole
Color
Flash point °F
Boiling/melting point °C
Density g/cc
Pine oil MIBC Nalco-9840 Eucalyptus oil Ethyl alcohol Kerosene Diesel oil Sodium carbonate Sodium silicate
154 102 NA 154 46 170 (approx.) 233 106 122
Colourless to pale yellow Matches with APHA Dark yellow Colorless Colorless Saybolt 10 Pale yellow Colorless Colorless
b141 41 96 Non-flammable 12 35 35 Non-flammable Non-flammable
200–220 132 180–350 176–177 78 20–300 85–95 1633 1088
0.90 0.89 0.84 0.90 0.79 0.79–0.82 0.82–0.60 1.09 2.40
a
Physical properties taken from supplier MSD sheets.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
331
Fig. 1. Schematic diagram of column flotation.
some volume in the column. The overall volume of two phases (water and air) in the column will increase. The change of volume in each case was recorded. The effect of reagents on gas hold-up with respect to hydrostatic head of water height and air rate was studied in batch process. Initially the gas hold-up data was collected in the air–water system using sparger. The data was then collected in air–water with different flotation reagents. The experiments were also carried out without adding reagents. The water volume was varied from 1 to 3 l with 0.5 l increment, whereas air rate was varied from 1 to 10 l per min with an increment of 1 l/min. The change of total volume of water and gas phases in each case was recorded for the evaluation of the gas hold-up. Similarly, experiments were also conducted by introducing reagents with various concentrations ranging from 0.02 to 0.1 ml/l with an increment of 0.02 ml/l.
4. Results and discussion 4.1. Effect of different parameters on gas hold-up 4.1.1. Different water volumes The initial height of water at different volumes without passing air is shown in Fig. 2. It is observed that the height of water with respect to volume is following a linear relationship. 4.1.2. Effect of water volume on gas hold-up The gas is introduced into the column, a liquid or slurry is displaced. The volumetric fraction displaced is called the gas hold-up, εg. The optimal conditions were selected based on the detailed gas hold-up studies and their data interpretation. The variations of gas hold-up with the change of water volume at optimized conditions are shown in Fig. 3 (i.e., 2.5 l of water, 5 l/min of air, 0.04 ml/l of MIBC and ethyl alcohol at 90:10 ratios). The figure shows the effect of volume of raw water and water mixed with frothers on the gas hold-up. The results indicated
160
Raw Water
12
140 10 120
Gas hold-up, %
Height of water in column, cm
180
100 80 60
Frother
9.1
10.6
10.5
10.0
9.5
8 6
4.6
4.3
5.0
4.8
5.1
4 2
40 20
0 0.5
1.0
1.5
2.0
2.5
3.0
Volume of water, litre Fig. 2. Variation of water height with water volume in the column.
3.5
1.0
1.5
2.0
2.5
3.0
Volume of water, litre Fig. 3. Variation of gas hold-up with volume of water at the optimized conditions.
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20 Frother
15
13.0 9.4
10
10.5
13.8
6.3
5 2.8
3.3
5.0
4.5
3.8
5.9
5.3
6.3
6.8
7.2
0 1
2
3
4
5
6
7
8
Water volume: 2.5 liter Frother conc.:0.04 ml/lit Air flow rate: 5 liter/min
10.5
10
11.5
8.0 5.1
15.0
12
16.2
Gas hold-up, %
Gas hold-up, %
Raw Water
9
7.3 7.0 7.3 6.9 6.7 7 .1 7 .1 6.8 7.1
8 6
5.0
4 2
10
Air rate, lit/min
0 A
Fig. 4. Variation of gas hold-up with air rate at the optimized conditions.
B
C
D
E
F
G
H
I
J
K
Mixture of frother type that the volume of gas hold-up is increasing steadily with an increase in volume of water. It was also observed that the gas hold-up volume increases significantly with the addition of dipolar surfactants like frothers. The increase in gas hold-up is due to a decrease in surface tension thereby the generation of micro bubbles. 4.1.3. Effect of air flow rate on gas hold-up The air flow rate increased in arithmetic progression from 1 to 10l/min with a step interval of 1 l/min. The results are shown in Fig. 4. It is observed that the volume of gas hold-up significantly decreases at below 5 l/min air flow rate. Hence, a minimum of 5 l/min air flow rate is required to maintain the minimum bubble size. Unstable bubble formation occurs at high air flow rate. In this work, an optimum air flow rate of 5 /min is chosen to obtain the smaller size and stable bubbles. Previous studies [23] carried out on similar size column and recommended the optimum air flow rate of 5–6 l/min. Experiments were also carried out more than 10 l/min of air rate; however, the gas hold-up is increasing but the bubble collision takes place eventually when bubble breakage occurs. Similar observations were found in this work. An optimum air flow rate of 5 l/min is chosen to obtain the smaller size and stable bubbles. 4.1.4. Selection of frother for gas hold-up The full factorial designs of experiments were conducted before the selection of reagents, reagent mixture and their dosages. The results of individual frothers and mixture of frothers are shown in Figs. 5 and 6 respectively. From the figures, it was observed that the individual MIBC at a concentration of 0.04 ml/l shows the maximum
10
Water volume: 2.5 liter Frother conc.:0.04 ml/lit MIBC+EA mixture (90:10 ratios) Air flow rate: 5 liter/min
Fig. 6. Selection of frother combination for the gas hold-up at the optimized conditions. A–raw water, B-MIBC+ Ethyl alcohol (90:10), C-Pine oil+MIBC(75:25), D-Pine oil+MIBC (50:50), E- Pine oil+MIBC(25:75), F-MIBC+Nalco(75:25), G- MIBC+Nalco(50:50), H- MIBC+Nalco(25:75), I-MIBC+ Eucalyptus oil (75:25),J- MIBC+ Eucalyptus oil (50:50), K– MIBC+ Eucalyptus oil(25:75).
gas hold-up. The results indicate that the MIBC frother has a significant effect on the gas hold-up compared to other frothers. This phenomenon may be attributed to the fact that a greater surface activity would result in the presence of a frother. This in turn would reduce coalescence and lead to higher gas hold-up, whereas, the mixture of MIBC and ethyl alcohol ratio of 90:10 shows the synergistic variation in gas hold-up. This may be the effect due to the drastic surface tension gradient to generate relatively small bubbles. From the figure, it is observed that the individual MIBC frother is more effective than other individual frothers at a concentration of 0.04 ml/l. However, a mixture of MIBC and ethyl alcohol confirmed synergistic effect than the individual frother. Hence, it was decided to use the mixed frother such as 90:10 ratios of MIBC and ethyl alcohol instead of individual frothers. Similar observations [23] were found in some studies with coal and graphite flotation and suggested that the mixture of alcohol and MIBC has a synergistic effect to generate the small bubbles. Combination of frother mix is attributed to the synergistic effect due to the drastic surface tension gradient to generate relatively small bubbles. From the above experiment, the individual frother and mixture of frothers at optimized conditions of frother concentration at 0.04 ml/l and the air rate of 5 l/min at 2.5 l water volume were selected. 4.1.5. Effect of frother dosage The effect of frother dosage on gas hold-up is shown in Fig. 7. It is observed that as the frother concentration increases, the gas hold-up Water volume: 2.5 liter Frother: MIBC+EA mixture (90:10 ratios) Air flow rate: 5 liter/min
7.3
8 6.3
6
6.9
6.4
5.0
4 2 0 RW
12
Gas hold-up, %
Gas hold-up, %
8.2
PO
MIBC
NA
EU
EA
Frother type Fig. 5. Selection of frother reagent for the gas hold-up at the optimized conditions. RW – Raw Water, PO – Pine oil, MIBC-Methyl iso butyl carbinol, NA–Nalco, EU – Eucalyptus oil, EA- Ethyl alcohol.
10.5
10.9
11.0
11.2
10.1
0.02
0.04
0.06
0.08
0.1
10 8 6
5.0
4 2 0 0
Frother concentration, ml/lit Fig. 7. Effect of frother dosage on the gas hold-up.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
Water volume: 2.5 liter Frother conc.: 0.04 ml/lit MIBC+EA mixture at 90:10 ratios Air flow rate: 5 liter/min
Kerosene
Diesel Oil 5.8
5.8 5.7
5.7 5.6
5.6
5.8 5.8
5.8 5.8
5.7 5.6
5.5
5.5 5.4 5.3 0.02
0.04
0.06
0.08
0.1
Collector concentration, ml/lit Fig. 8. Effect of collector concentration on the gas hold-up at optimized conditions.
increases gradually. It is also noticed that the optimized gas hold-up occurs at 0.04 ml/l concentration of MIBC and ethyl alcohol of 90:10 ratio at an air rate of 5.0 l/min and 2.5 l of water volume. 4.1.6. Effect of collector dosage The experiments were carried out with two different collectors such as kerosene and diesel oil individually. The results are shown in Fig. 8. The results indicated that the gas hold-up is marginally increased with kerosene with better selectivity compared to the diesel oil over the concentration ranges being studied. The dosage variation indicated a minimum concentration of 0.06 ml of collector. The previous laboratory flotation test work also arrived at a similar conclusion [23]. 4.1.7. Selection of collector and frother mixture With the same conditions, the full factorial designs of experiments were conducted to assess the effect of different frothers and collectors and their combinations at various proportions. The effect of collectors such as kerosene and diesel oil with the selected frother of MIBC and ethyl alcohol of 90:10 ratios on gas hold-up was studied. It is observed that the mixture of frother MIBC and ethyl alcohol of 90:10 with kerosene (ratio at 10:90) as collector gives better results compared to the diesel oil and the comparative results are shown in Fig. 9. Further, for the increase in ethyl alcohol concentration, the
Gas hold-up, %
7
7.3
7.5 6.8
6.3
4.1.8. Effect of modifiers Experiments were carried out with sodium carbonate modifier and sodium silicate at varying dosages according to the plant conditions to know the gas hold-up effect. The modifier and depressant did not improve the volume of gas hold-up due to the increase in surface tension. It is evidenced from the results that the percentage of gas hold-up is very minimal compared to the water. This may be due to the increase in surface tension that leads to the reduction of the froth formation. Sodium silicate is used as a depressant and measured the gas hold-up. 10% sodium silicate solution was prepared from its pure compound as mentioned in Table 1. In the experiment, modifier and the depressant concentration varied from 0.02 to 0.1 ml (10% sodium silicate solution) with 0.02 ml increment. The effect of sodium carbonate and sodium silicate dosage on gas hold-up is shown in Fig. 10. From the figure, it is observed that the gas hold-up variation is very minimal. In summary, the results indicated that the volume of gas hold-up is increasing steadily with an increase in the volume of water. It is also observed that the gas hold-up volume increases significantly with the addition of dipolar surfactants like frothers. The increase in gas hold-up is due to the decrease in surface tension thereby the generation of micro bubbles in column. 5. Correlation for gas hold-up The gas hold-up was determined from the data obtained from all the experiments. The data generated were used to develop a correlation in terms of process parameters, such as volume of water, air rate and concentration of different reagents. It was found that data with different reagents could be combined using regression analysis. The dimensionless groups involved in the correlation are the ratio of volume of water to the optimum volume of water, air rate to the optimum air rate and frother concentration to the optimum frother concentration. Similarly, the data from different particle size groups could be brought together by including individual effects as well as combined effect in the correlation. 5.1. Correlation for gas hold-up with individual MIBC frother The regression analysis of the present data consisting of 250 data points of individual MIBC resulted in the following dimensionless correlation:
Water volume: 2.5 liter Air flow rate: 5 liter/min
8
marginal increase in gas hold-up is observed, but it is expensive. After analyzing the results carefully, we have selected the frothers, collectors and their combination and the same is evidenced from the figures.
6.9
εg ¼ 2:5
6.1
0:14 0:49 0:12 Vw Qa Fc Vo Qo Fo
ð1Þ
6 5
Water volume: 2.5 liter Frother conc.: 0.04 ml/lit Air flow rate: 5 liter/min
4 3
5
2 1 0 A
B
C
D
E
F
Mixture of frother and collector combination Fig. 9. Selection of collector and frother mix dosage on the gas hold-up. A-MIBC+ Ethyl alcohol (90:10) with kerosene (5:95), B- MIBC+ Ethyl alcohol (90:10) with kerosene (10:90), C- MIBC+ Ethyl alcohol (90:10) with kerosene (15:85), D-MIBC+ Ethyl alcohol (90:10) with diesel oil (5:95), E- MIBC+ Ethyl alcohol (90:10) with diesel oil (10:90), F- MIBC+ Ethyl alcohol (90:10) with diesel oil (15:85).
Gas hold-up, %
Gas hold-up, %
5.9
333
Sodium carbonate
Sodium silicate 4.8
4.8
4.7 4.6
4.7 4.6
4.6
4.6
4.5
4.5
4.5
4.4
4.4 4.2 0.02
0.04
0.06
0.08
0.1
Concentration, ml/lit Fig. 10. Effect of modifiers on the gas hold-up at the optimized conditions.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
% gas holdup, predicted
334
13
can be seen that a reasonably good agreement, with a RMS error of 10.2% is obtained.
11
5.3. Correlation for gas hold-up with a mixture of frother and collector The regression analysis of the present data consisting of 250 data points of mixture of frother and collector at optimum conditions resulted in the following dimensionless correlation:
9 7
ε g ¼ 2:0
5 3 3
8
13
% gas holdup, observed Fig. 11. Comparison of predicted values estimated from Eq. (1) and observed values for individual MIBC frother.
where, Vw is the volume of water in liters, Qa is the air rate in liter/minute and Fc is the frother concentration in ml. Vo, Qo and Fo are the optimum values of volume of water, air rate and frother concentration respectively. A comparison between the predicted and the observed values is given in Fig. 11. It can be seen that a reasonably good agreement, with a mean relative error of 6.1% is obtained. 5.2. Correlation for gas hold-up with a mixture of frothers The regression analysis of the present data consisting of 1500 data points of mixture of frothers resulted in the following dimensionless correlation:
ε g ¼ 1:97
0:22 0:42 0:11 Vw Qa Fc : Vo Qo Fo
ð2Þ
In the correlation, it is not shown as a separate entity, however, the effect of proportion of the mixture of frother data points has been considered while developing correlation (2). A comparison between the predicted and the observed values is given in Fig. 12. It
0:2 0:38 0:13 Vw Qa Fc : Vo Qo Fo
ð3Þ
The effect of the proportion of mixture of frother and in the presence of collector was studied and the corresponding data points have been considered while developing the above correlation. In all the correlations, the values of Vo, Qo and Fo are the optimum values 2.5, 5.0, and 0.04 of the volume of water, air rate and frother concentration respectively which are considered to develop correlations. A comparison between the predicted and the observed values is given in Fig. 13. It can be seen that a reasonably good agreement, with a RMS error of 6.6% is obtained. However, the above correlations (1), (2) and (3) have restrictions on water volume, air flow and frother concentration. These correlations are effectively applicable in the range of water volume 1–3 l, air flow in the range of 1–10 l/min and frother concentration range of 0.01 to 0.1 ml/l. 6. Conclusions In this work, the volume of water, air flow rate, reagent concentration and reagent dosage were optimized for the better gas hold-up. The mixture of MIBC and ethyl alcohol with 90:10 ratio frothers was found to have better performance compared to the individual performance of MIBC and ethyl alcohol. However, MIBC yields better efficiency compared to individual ethyl alcohol. A dimensionless model has been developed to predict the gas hold-up using the experimental data. The correlations can be used to predict the gas hold-up for a given set of operating conditions within the parameters range studied. The model can predict the gas hold-up with reasonable accuracy. Nomenclature MIBC methyl-isobutyl carbinol F-150 flottec-150 DF-250 dowfroths-250 CCC critical coalescence concentration
14 12
% Gas holdup, Predicted
% gas holdup, predicted
12 10
8
6
4
10 8 6 4 2
2 2
4
6
8
10
12
% gas holdup, observed Fig. 12. Comparison of predicted values estimated from Eq. (2) and observed values for the mixture of frothers.
2
4
6
8
10
12
14
% Gas holdup, Observed Fig. 13. Comparison of predicted values estimated from Eq. (3) and observed values for mixture of frother and collector at a ratio of 90:10.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
S AR εg Vw Vo Qa Qo Fc Fo RMS
sparger air rotameter gas hold-up volume of water, liter optimum volume of water, liter air rate, l/min optimum air rate, l/min frother concentration, ml optimum frother concentration, ml root means square error
Acknowledgments The authors express their thanks to the Chairman and Managing Director, TAMIN, Chennai and Scientists, CSIR-IMMT, Bhubaneswar for providing relevant support and encouragement in preparing the manuscript. References [1] M. Massinaei, M. Kolahdoozan, M. Noaparast, M. Oliazadeh, J. Yianatos, R. Shamsadini, M. Yarahmadi, Hydrodynamic and metallurgical characteristics of industrial and pilot columns in rougher circuit, Mineral Engineering 22 (2009). [2] F. Azgomi, C.O. Gomez, J.A. Finch, Correspondence of gas hold-up and bubble size in presence of different frothers, International Journal of Mineral Processing 83 (2007). [3] J. Leja, Surface Chemistry of Froth Flotation, Plenum Press, 1982. [4] H. El-Shall, N.A. Abdel-Khalek, S. Svoronos, Collector–frother interaction in column flotation of Florida phosphate, International Journal of Mineral Processing 58 (1–4) (2000). [5] R.J. Pugh, Dynamic surface tension measurements in mineral flotation and deinking flotation systems and the development of the on line dynamic surface tension detector (DSTD), Minerals Engineering 14 (9) (2001). [6] B.A. Comley, P.J. Harris, D.J. Bradshaw, M.C. Harris, Frother characterisation using dynamic surface tension measurements, International Journal of Mineral Processing 64 (2–3) (2002). [7] S.R. Rao, J. Leja, Surface Chemistry of Froth Flotation, second ed. Kluwer Academic/Plenum Publishers, New York, London, 2004.
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[8] H. Fruhner, K.D. Wantke, K. Lunkenheimer, Relationship between surface dilational properties and foam stability, Colloids and Surfaces A: Physicochemical and Engineering Aspects 162 (1999). [9] Y.S. Cho, J.S. Laskowski, Effect of flotation frothers on bubble size and foam stability, International Journal of Mineral Processing 64 (2002). [10] F. Azgomi, C.O. Gomez, J.A. Finch, Characterizing frothers using gas hold-up, in: Z. Xu, Q. Liu (Eds.), Interfacial Phenomena in Fine Particle Technology, 6th UBC-McGill-UA International Symposium on Fundamentals of Mineral Processing, COM 2006, Montreal, Oct 1–4 2006, pp. 429–442. [11] Z.A. Zhou, N.O. Egiebor, L.R. Plitt, On bubble size determination in a flotation column, in: G.E. Agar, B.J. Huls, D.B. Hyma (Eds.), Proceedings of International Conference on Column Flotation, vol. 1, 1991, pp. 249–262. [12] Z.A. Zhou, N.O. Egiebor, L.R. Plitt, Frother effect on bubble motion in a water column, Canadian Metallurgical Quarterly 31 (1992). [13] Z.A. Zhou, N.O. Egiebor, L.R. Plitt, Frother effects on bubble motion in a swarm, Canadian Metallurgical Quarterly 32 (1993). [14] Z.A. Zhou, N.O. Egiebor, L.R. Plitt, Frother effects on bubble size estimation in a flotation column, Minerals Engineering 6 (1993). [15] G. Tsatouhas, S.R. Grano, M. Vera, Case studies on the performance and characterization of the froth phase in industrial flotation circuits, in: G. Jameson, R.-H. Yoon (Eds.), Centenary of Flotation Symposium, Australasian Institute of Mining and Metallurgy (AusIMM) Publication, Brisbane, 2005, pp. 377–384. [16] K. Hadler, A.Z. Aktas, B.J.J. Cilliers, The effects of frother and collector distribution on flotation performance, Minerals Engineering 18 (2005). [17] T. Weber, C.O. Gomez, J.A. Finch, A frother concentration meter, in: Proceedings of the 35th Annual Meeting of the Canadian Mineral Processors (Division of CIM), 2003, pp. 639–652. [18] M. Wu, M. Gharib, Experimental studies on the shape and path of small air bubbles rising in clean water, Physics of Fluids 14 (7) (2002). [19] W. Kracht, J.A. Finch, Effect of frother on initial bubble shape and velocity, International Journal of Mineral Processing 94 (2010). [20] S.H. Kuan, J.A. Finch, Impact of talc on pulp and froth properties in F150 and 1-pentanol frother systems, Minerals Engineering 23 (2010). [21] J. Pineres, J. Barraza, Energy barrier of aggregates coal particle–bubble through the extended DLVO theory, International Journal of Mineral Processing 100 (1–2) (2011). [22] J. Pineres, J. Barraza, Effect of pH, air velocity and frother concentration on combustible recovery, ash and sulphur rejection using column flotation, Fuel Processing Technology 97 (2012). [23] S.K. Biswal, P.S.R. Reddy, S.K. Bhaumik, Bubble size distribution in a flotation column, Canadian Journal of Chemical Engineering 72 (1) (1994).
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Gas dispersion characteristics of flotation reagents V. Ravichandran a, C. Eswaraiah b,⁎, R. Sakthivel b, S.K. Biswal b, P. Manisankar a a b
Department of Industrial Chemistry, Alagappa University, Tamil Nadu, 630003, India CSIR, Institute of Minerals and Materials Technology, Bhubaneswar, 751013, Orissa, India
a r t i c l e
i n f o
Article history: Received 23 January 2012 Received in revised form 5 October 2012 Accepted 19 October 2012 Available online 27 October 2012 Keywords: Frother Collector Optimization Gas hold-up Model
a b s t r a c t Gas dispersion is one of the key factors in the mineral flotation process articulated by many variables such as bubble size, gas hold-up, gas superficial velocity and bubble surface area flux (Sb). In this work, experiments were carried out on column flotation (5.08 cm in diameter and 180 cm height) with different types of reagents to evaluate the gas hold-up. The gas hold-up was measured using pressure difference technique. The parameters were optimized for enhanced gas hold-up for better flotation performance. The influence of operating parameters comprising different reagents' dosage/type and their mixture combination on gas hold-up was discussed in detail. Based on the experimental data, the dimensionless correlations were developed to predict the gas hold-up within the range of parameters studied. The correlations are found to be able to predict gas hold-up with acceptable accuracy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gas dispersion is one of the key factors in the mineral flotation process. Flotation requires the formation of a stable bubble-particle aggregate which helps the particle to be carried out from the pulp. The stability of the aggregate depends on the interfacial free energies or the corresponding interfacial tensions of the solid/liquid, solid/ vapor and liquid/vapor interfaces, which are responsible for attachment process. The tendency of replacement of solid/liquid by solid/ vapor interface depends on the hydrophobicity of the floated particles. Recent publications [1] indicate that type and dosage of frother play important roles on the applicability of the drift-flux model. Studies carried out [2] in two-phase systems (water-air) have shown that the frothers (surfactants) n-pentanol, MIBC (methyl–isobutyl carbinol), DF-250 (i.e., dowfroths-250) and F-150 (i.e., flottec-150), increase in this order, the drag coefficient of the bubble/water interface, attributed to the formation of a thin layer of water bonded to the bubble. In general, a variety of organic and inorganic reagents are used in the flotation process, such as frothers, collectors, pH regulator and depressant. Collector adsorbs on the surface of the particle to make the surface hydrophobic. The interactions between frothers and collectors have been widely reported [3]. El-Shall et al. [4] found that frother–collector interactions are dependent upon frother type; however there are few quantitative results on the effect of interaction on
⁎ Corresponding author. E-mail address: [email protected] (C. Eswaraiah). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2012.10.039
the concentrations of reagents remaining in solution. Measurement of the concentration of frother remaining in solution has not been widely reported. The use of a maximum bubble pressure to determine the dynamic surface tension of frother solutions is discussed by Pugh [5] and Comley et al. [6], however, at low concentrations, the trials of this method to determine the concentration of frother have proven unsuccessful. Frothers are used to decrease bubble size and promote the froth stability [7]. Frother molecules comprise a nonpolar (hydrophobic) hydrocarbon chain and a polar (hydrophilic) group, typically OH. They adsorb at the air–water interface and through forces associated with surface tension gradients and visco-elastic effects stabilize bubbles against coalescence [8], which provides both the small bubbles production and froth building functions. Two common frother classes in the mineral industry are alcohols and polyglycols. The previous communication [2] shows that the gas hold-up is strongly dependent on frother type. This permitted a simple ranking of frothers, which corresponded to those reported using other procedures. The use of gas hold-up was predicated on its relationship with bubble size through its effect on bubble velocity: as bubble size decreases bubble velocity decreases and gas hold-up increases. Three observations were made during the work that demanded the relationship be explored. One observation was the insensitivity of gas hold-up at low concentration for low ranked frothers (i.e., those where the rate of increase in gas hold-up with concentration was low); MIBC was one such frother. The second observation was the steady increase in gas hold-up at higher concentration, evident for all frothers, despite being above CCC (critical coalescence concentration) when bubble size is expected to be minimum and constant thus gas hold-up is expected to be maximum and constant [9]. The
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third observation was visual: at equal gas hold-up the various frothers appeared to yield different bubble sizes, which violates a common assumption by Azgomi et al. [10]. Previous researchers have raised the possibility that different frothers give different gas hold-up and bubble size dependence [11–14]. There is no convenient on-site method of determining frother concentration. This has arguably restricted the use of frother to achieve metallurgical targets. Approaches have included gas chromatography [15], determining the total organic carbon [16] and using calibrations of bubble size and gas hold-up vs. frother concentration [17]. Recent studies, however, have shown that shape is a key factor, that bubbles slow down the more they become spherical [18] and that the effect of frother and some salts on bubble velocity is then seen as their ability to control bubble shape. Bubbles tend to flatten (become oblate) as they rise because of the pressure drop across their dimension. In the presence of surfactants and salts, deformation is resisted. Both 1-pentanol and NaCl are less able to maintain a spherical shape compared to F150 and MIBC and this corresponds to their reduced ability to slow the bubble down [19]. Another possibility is that the water structure at the bubble surface created by H-bonding via the polar groups of the frother or hydration sheath of salt ions resists deformation and so aids in keeping the bubble spherical [20]. “It is well known fact that the energy barrier defines the particle–bubble attachment in a flotation process. The energy barrier of aggregates coal particle–bubble was determined using the extended DLVO theory.” The effect of pH and frother concentration (FC) about energy barrier using four Colombian coals was evaluated. Experiments were carried out using a column flotation in order to evaluate the bubble diameter [21]. A recent study has reported the effect of pH and frother concentration evaluated on different coals and achieved the ash, sulphur rejection, combustible recovery and ash rejection at desirable levels [22]. In the present study, an attempt is made to develop the correlations for gas hold-up in flotation column based on the reagent characteristics such as collectors like kerosene and diesel oil, frothers such as pine oil, methyl-isobutyl carbinol, Nalco-9840, eucalyptus oil and ethyl alcohol, sodium carbonate as modifier, sodium silicate as depressant and process variables such as air rate, water rate. Summary of the reagents properties used is given in Table 1.
2. Materials and methods 2.1. Materials The measurement of gas hold-up in column flotation is carried out with commercial grade frothers such as pine oil, MIBC, Nalco-9840, eucalyptus oil and ethyl alcohol and collectors namely kerosene and diesel oil are selected for the measurement. The measurement of gas hold-up is also carried out with 10% sodium carbonate as pH regulator and depressant as sodium silicate solution according to the plant conditions.
2.2. Methods The compressed air to sparger is obtained from air compressor through the air filter. The air flow rate was measured by using rotameter. The required quantity of water with and without mixing of reagent dosage was added to the flotation column. The mechanical stirring facilitates the reagents to become homogenized before sending to the column. Air was supplied at different volumetric flow rates to the sparger to generate the bubbles in the water phase.
3. Experimental setup and procedure The schematic of the laboratory column flotation experimental setup was used for the present work and shown in Fig. 1. The volumetric flow rate was measured by using rotameter (Blacka Instrument, Chennai, India). U tube manometer was used to know the volume of air flow passing to the column. Air was introduced through a ceramic sparger (S) at the bottom of the column and its flow rate was observed using air rotameter (AR). The flotation column is made of acrylic tube of 180 cm height above the base with 5.08 cm diameter. The column has been graduated for determining the volume, and the maximum volume of the column is 3650 ml. The sparger is a porous element made of ceramic material with 2.5 cm diameter and 7.5 cm height. The pore diameter of the porous element is 5 μm. Internal sparger is used for all the experimental runs and is fitted at the bottom of the column. Initially, the column is filled with the water varied from 1 to 3 liters, and the volume was noted down. Air was supplied through the sparger and is connected with non returning valve so that the water does not come through the sparger to the compressor. Internal sparger is used in the present work, because with internal sparger material has a modest effect on bubble size. Flexible sparger is preferred over rigid sparger as they are less prone to the plugging by particles. The metallurgical performance in ceramic type diffuser is better in comparison with filter cloth. It has been noted that irrespective of sparger type, as the superficial sparger area decreases, the bubble size increases and vice-versa. In external sparger, the air and the liquid are contacted outside the column, and the mixture is carried through a pipe into the column from which it is expelled through holes about 1 mm in diameter. The experiments were carried out with water at different air flow rates with different reagents by varying their dosage. The column was thoroughly washed after each experimental run. All the experiments were carried out at ambient temperature. The mechanical stirring facilitates the reagents to become homogenized before sending to the column. The requisite volume of water without/with mixing of required reagent dosage was added to the flotation column through funnel fitted on the column top. Air at pressure 2 kg/cm 2 was supplied at different volumetric flow rates to the sparger to generate the bubble in the water phase. Air bubbles so generated will occupy
Table 1 Summary of reagents properties.a Reagent
Molecular weight, g/mole
Color
Flash point °F
Boiling/melting point °C
Density g/cc
Pine oil MIBC Nalco-9840 Eucalyptus oil Ethyl alcohol Kerosene Diesel oil Sodium carbonate Sodium silicate
154 102 NA 154 46 170 (approx.) 233 106 122
Colourless to pale yellow Matches with APHA Dark yellow Colorless Colorless Saybolt 10 Pale yellow Colorless Colorless
b141 41 96 Non-flammable 12 35 35 Non-flammable Non-flammable
200–220 132 180–350 176–177 78 20–300 85–95 1633 1088
0.90 0.89 0.84 0.90 0.79 0.79–0.82 0.82–0.60 1.09 2.40
a
Physical properties taken from supplier MSD sheets.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
331
Fig. 1. Schematic diagram of column flotation.
some volume in the column. The overall volume of two phases (water and air) in the column will increase. The change of volume in each case was recorded. The effect of reagents on gas hold-up with respect to hydrostatic head of water height and air rate was studied in batch process. Initially the gas hold-up data was collected in the air–water system using sparger. The data was then collected in air–water with different flotation reagents. The experiments were also carried out without adding reagents. The water volume was varied from 1 to 3 l with 0.5 l increment, whereas air rate was varied from 1 to 10 l per min with an increment of 1 l/min. The change of total volume of water and gas phases in each case was recorded for the evaluation of the gas hold-up. Similarly, experiments were also conducted by introducing reagents with various concentrations ranging from 0.02 to 0.1 ml/l with an increment of 0.02 ml/l.
4. Results and discussion 4.1. Effect of different parameters on gas hold-up 4.1.1. Different water volumes The initial height of water at different volumes without passing air is shown in Fig. 2. It is observed that the height of water with respect to volume is following a linear relationship. 4.1.2. Effect of water volume on gas hold-up The gas is introduced into the column, a liquid or slurry is displaced. The volumetric fraction displaced is called the gas hold-up, εg. The optimal conditions were selected based on the detailed gas hold-up studies and their data interpretation. The variations of gas hold-up with the change of water volume at optimized conditions are shown in Fig. 3 (i.e., 2.5 l of water, 5 l/min of air, 0.04 ml/l of MIBC and ethyl alcohol at 90:10 ratios). The figure shows the effect of volume of raw water and water mixed with frothers on the gas hold-up. The results indicated
160
Raw Water
12
140 10 120
Gas hold-up, %
Height of water in column, cm
180
100 80 60
Frother
9.1
10.6
10.5
10.0
9.5
8 6
4.6
4.3
5.0
4.8
5.1
4 2
40 20
0 0.5
1.0
1.5
2.0
2.5
3.0
Volume of water, litre Fig. 2. Variation of water height with water volume in the column.
3.5
1.0
1.5
2.0
2.5
3.0
Volume of water, litre Fig. 3. Variation of gas hold-up with volume of water at the optimized conditions.
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20 Frother
15
13.0 9.4
10
10.5
13.8
6.3
5 2.8
3.3
5.0
4.5
3.8
5.9
5.3
6.3
6.8
7.2
0 1
2
3
4
5
6
7
8
Water volume: 2.5 liter Frother conc.:0.04 ml/lit Air flow rate: 5 liter/min
10.5
10
11.5
8.0 5.1
15.0
12
16.2
Gas hold-up, %
Gas hold-up, %
Raw Water
9
7.3 7.0 7.3 6.9 6.7 7 .1 7 .1 6.8 7.1
8 6
5.0
4 2
10
Air rate, lit/min
0 A
Fig. 4. Variation of gas hold-up with air rate at the optimized conditions.
B
C
D
E
F
G
H
I
J
K
Mixture of frother type that the volume of gas hold-up is increasing steadily with an increase in volume of water. It was also observed that the gas hold-up volume increases significantly with the addition of dipolar surfactants like frothers. The increase in gas hold-up is due to a decrease in surface tension thereby the generation of micro bubbles. 4.1.3. Effect of air flow rate on gas hold-up The air flow rate increased in arithmetic progression from 1 to 10l/min with a step interval of 1 l/min. The results are shown in Fig. 4. It is observed that the volume of gas hold-up significantly decreases at below 5 l/min air flow rate. Hence, a minimum of 5 l/min air flow rate is required to maintain the minimum bubble size. Unstable bubble formation occurs at high air flow rate. In this work, an optimum air flow rate of 5 /min is chosen to obtain the smaller size and stable bubbles. Previous studies [23] carried out on similar size column and recommended the optimum air flow rate of 5–6 l/min. Experiments were also carried out more than 10 l/min of air rate; however, the gas hold-up is increasing but the bubble collision takes place eventually when bubble breakage occurs. Similar observations were found in this work. An optimum air flow rate of 5 l/min is chosen to obtain the smaller size and stable bubbles. 4.1.4. Selection of frother for gas hold-up The full factorial designs of experiments were conducted before the selection of reagents, reagent mixture and their dosages. The results of individual frothers and mixture of frothers are shown in Figs. 5 and 6 respectively. From the figures, it was observed that the individual MIBC at a concentration of 0.04 ml/l shows the maximum
10
Water volume: 2.5 liter Frother conc.:0.04 ml/lit MIBC+EA mixture (90:10 ratios) Air flow rate: 5 liter/min
Fig. 6. Selection of frother combination for the gas hold-up at the optimized conditions. A–raw water, B-MIBC+ Ethyl alcohol (90:10), C-Pine oil+MIBC(75:25), D-Pine oil+MIBC (50:50), E- Pine oil+MIBC(25:75), F-MIBC+Nalco(75:25), G- MIBC+Nalco(50:50), H- MIBC+Nalco(25:75), I-MIBC+ Eucalyptus oil (75:25),J- MIBC+ Eucalyptus oil (50:50), K– MIBC+ Eucalyptus oil(25:75).
gas hold-up. The results indicate that the MIBC frother has a significant effect on the gas hold-up compared to other frothers. This phenomenon may be attributed to the fact that a greater surface activity would result in the presence of a frother. This in turn would reduce coalescence and lead to higher gas hold-up, whereas, the mixture of MIBC and ethyl alcohol ratio of 90:10 shows the synergistic variation in gas hold-up. This may be the effect due to the drastic surface tension gradient to generate relatively small bubbles. From the figure, it is observed that the individual MIBC frother is more effective than other individual frothers at a concentration of 0.04 ml/l. However, a mixture of MIBC and ethyl alcohol confirmed synergistic effect than the individual frother. Hence, it was decided to use the mixed frother such as 90:10 ratios of MIBC and ethyl alcohol instead of individual frothers. Similar observations [23] were found in some studies with coal and graphite flotation and suggested that the mixture of alcohol and MIBC has a synergistic effect to generate the small bubbles. Combination of frother mix is attributed to the synergistic effect due to the drastic surface tension gradient to generate relatively small bubbles. From the above experiment, the individual frother and mixture of frothers at optimized conditions of frother concentration at 0.04 ml/l and the air rate of 5 l/min at 2.5 l water volume were selected. 4.1.5. Effect of frother dosage The effect of frother dosage on gas hold-up is shown in Fig. 7. It is observed that as the frother concentration increases, the gas hold-up Water volume: 2.5 liter Frother: MIBC+EA mixture (90:10 ratios) Air flow rate: 5 liter/min
7.3
8 6.3
6
6.9
6.4
5.0
4 2 0 RW
12
Gas hold-up, %
Gas hold-up, %
8.2
PO
MIBC
NA
EU
EA
Frother type Fig. 5. Selection of frother reagent for the gas hold-up at the optimized conditions. RW – Raw Water, PO – Pine oil, MIBC-Methyl iso butyl carbinol, NA–Nalco, EU – Eucalyptus oil, EA- Ethyl alcohol.
10.5
10.9
11.0
11.2
10.1
0.02
0.04
0.06
0.08
0.1
10 8 6
5.0
4 2 0 0
Frother concentration, ml/lit Fig. 7. Effect of frother dosage on the gas hold-up.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
Water volume: 2.5 liter Frother conc.: 0.04 ml/lit MIBC+EA mixture at 90:10 ratios Air flow rate: 5 liter/min
Kerosene
Diesel Oil 5.8
5.8 5.7
5.7 5.6
5.6
5.8 5.8
5.8 5.8
5.7 5.6
5.5
5.5 5.4 5.3 0.02
0.04
0.06
0.08
0.1
Collector concentration, ml/lit Fig. 8. Effect of collector concentration on the gas hold-up at optimized conditions.
increases gradually. It is also noticed that the optimized gas hold-up occurs at 0.04 ml/l concentration of MIBC and ethyl alcohol of 90:10 ratio at an air rate of 5.0 l/min and 2.5 l of water volume. 4.1.6. Effect of collector dosage The experiments were carried out with two different collectors such as kerosene and diesel oil individually. The results are shown in Fig. 8. The results indicated that the gas hold-up is marginally increased with kerosene with better selectivity compared to the diesel oil over the concentration ranges being studied. The dosage variation indicated a minimum concentration of 0.06 ml of collector. The previous laboratory flotation test work also arrived at a similar conclusion [23]. 4.1.7. Selection of collector and frother mixture With the same conditions, the full factorial designs of experiments were conducted to assess the effect of different frothers and collectors and their combinations at various proportions. The effect of collectors such as kerosene and diesel oil with the selected frother of MIBC and ethyl alcohol of 90:10 ratios on gas hold-up was studied. It is observed that the mixture of frother MIBC and ethyl alcohol of 90:10 with kerosene (ratio at 10:90) as collector gives better results compared to the diesel oil and the comparative results are shown in Fig. 9. Further, for the increase in ethyl alcohol concentration, the
Gas hold-up, %
7
7.3
7.5 6.8
6.3
4.1.8. Effect of modifiers Experiments were carried out with sodium carbonate modifier and sodium silicate at varying dosages according to the plant conditions to know the gas hold-up effect. The modifier and depressant did not improve the volume of gas hold-up due to the increase in surface tension. It is evidenced from the results that the percentage of gas hold-up is very minimal compared to the water. This may be due to the increase in surface tension that leads to the reduction of the froth formation. Sodium silicate is used as a depressant and measured the gas hold-up. 10% sodium silicate solution was prepared from its pure compound as mentioned in Table 1. In the experiment, modifier and the depressant concentration varied from 0.02 to 0.1 ml (10% sodium silicate solution) with 0.02 ml increment. The effect of sodium carbonate and sodium silicate dosage on gas hold-up is shown in Fig. 10. From the figure, it is observed that the gas hold-up variation is very minimal. In summary, the results indicated that the volume of gas hold-up is increasing steadily with an increase in the volume of water. It is also observed that the gas hold-up volume increases significantly with the addition of dipolar surfactants like frothers. The increase in gas hold-up is due to the decrease in surface tension thereby the generation of micro bubbles in column. 5. Correlation for gas hold-up The gas hold-up was determined from the data obtained from all the experiments. The data generated were used to develop a correlation in terms of process parameters, such as volume of water, air rate and concentration of different reagents. It was found that data with different reagents could be combined using regression analysis. The dimensionless groups involved in the correlation are the ratio of volume of water to the optimum volume of water, air rate to the optimum air rate and frother concentration to the optimum frother concentration. Similarly, the data from different particle size groups could be brought together by including individual effects as well as combined effect in the correlation. 5.1. Correlation for gas hold-up with individual MIBC frother The regression analysis of the present data consisting of 250 data points of individual MIBC resulted in the following dimensionless correlation:
Water volume: 2.5 liter Air flow rate: 5 liter/min
8
marginal increase in gas hold-up is observed, but it is expensive. After analyzing the results carefully, we have selected the frothers, collectors and their combination and the same is evidenced from the figures.
6.9
εg ¼ 2:5
6.1
0:14 0:49 0:12 Vw Qa Fc Vo Qo Fo
ð1Þ
6 5
Water volume: 2.5 liter Frother conc.: 0.04 ml/lit Air flow rate: 5 liter/min
4 3
5
2 1 0 A
B
C
D
E
F
Mixture of frother and collector combination Fig. 9. Selection of collector and frother mix dosage on the gas hold-up. A-MIBC+ Ethyl alcohol (90:10) with kerosene (5:95), B- MIBC+ Ethyl alcohol (90:10) with kerosene (10:90), C- MIBC+ Ethyl alcohol (90:10) with kerosene (15:85), D-MIBC+ Ethyl alcohol (90:10) with diesel oil (5:95), E- MIBC+ Ethyl alcohol (90:10) with diesel oil (10:90), F- MIBC+ Ethyl alcohol (90:10) with diesel oil (15:85).
Gas hold-up, %
Gas hold-up, %
5.9
333
Sodium carbonate
Sodium silicate 4.8
4.8
4.7 4.6
4.7 4.6
4.6
4.6
4.5
4.5
4.5
4.4
4.4 4.2 0.02
0.04
0.06
0.08
0.1
Concentration, ml/lit Fig. 10. Effect of modifiers on the gas hold-up at the optimized conditions.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
% gas holdup, predicted
334
13
can be seen that a reasonably good agreement, with a RMS error of 10.2% is obtained.
11
5.3. Correlation for gas hold-up with a mixture of frother and collector The regression analysis of the present data consisting of 250 data points of mixture of frother and collector at optimum conditions resulted in the following dimensionless correlation:
9 7
ε g ¼ 2:0
5 3 3
8
13
% gas holdup, observed Fig. 11. Comparison of predicted values estimated from Eq. (1) and observed values for individual MIBC frother.
where, Vw is the volume of water in liters, Qa is the air rate in liter/minute and Fc is the frother concentration in ml. Vo, Qo and Fo are the optimum values of volume of water, air rate and frother concentration respectively. A comparison between the predicted and the observed values is given in Fig. 11. It can be seen that a reasonably good agreement, with a mean relative error of 6.1% is obtained. 5.2. Correlation for gas hold-up with a mixture of frothers The regression analysis of the present data consisting of 1500 data points of mixture of frothers resulted in the following dimensionless correlation:
ε g ¼ 1:97
0:22 0:42 0:11 Vw Qa Fc : Vo Qo Fo
ð2Þ
In the correlation, it is not shown as a separate entity, however, the effect of proportion of the mixture of frother data points has been considered while developing correlation (2). A comparison between the predicted and the observed values is given in Fig. 12. It
0:2 0:38 0:13 Vw Qa Fc : Vo Qo Fo
ð3Þ
The effect of the proportion of mixture of frother and in the presence of collector was studied and the corresponding data points have been considered while developing the above correlation. In all the correlations, the values of Vo, Qo and Fo are the optimum values 2.5, 5.0, and 0.04 of the volume of water, air rate and frother concentration respectively which are considered to develop correlations. A comparison between the predicted and the observed values is given in Fig. 13. It can be seen that a reasonably good agreement, with a RMS error of 6.6% is obtained. However, the above correlations (1), (2) and (3) have restrictions on water volume, air flow and frother concentration. These correlations are effectively applicable in the range of water volume 1–3 l, air flow in the range of 1–10 l/min and frother concentration range of 0.01 to 0.1 ml/l. 6. Conclusions In this work, the volume of water, air flow rate, reagent concentration and reagent dosage were optimized for the better gas hold-up. The mixture of MIBC and ethyl alcohol with 90:10 ratio frothers was found to have better performance compared to the individual performance of MIBC and ethyl alcohol. However, MIBC yields better efficiency compared to individual ethyl alcohol. A dimensionless model has been developed to predict the gas hold-up using the experimental data. The correlations can be used to predict the gas hold-up for a given set of operating conditions within the parameters range studied. The model can predict the gas hold-up with reasonable accuracy. Nomenclature MIBC methyl-isobutyl carbinol F-150 flottec-150 DF-250 dowfroths-250 CCC critical coalescence concentration
14 12
% Gas holdup, Predicted
% gas holdup, predicted
12 10
8
6
4
10 8 6 4 2
2 2
4
6
8
10
12
% gas holdup, observed Fig. 12. Comparison of predicted values estimated from Eq. (2) and observed values for the mixture of frothers.
2
4
6
8
10
12
14
% Gas holdup, Observed Fig. 13. Comparison of predicted values estimated from Eq. (3) and observed values for mixture of frother and collector at a ratio of 90:10.
V. Ravichandran et al. / Powder Technology 235 (2013) 329–335
S AR εg Vw Vo Qa Qo Fc Fo RMS
sparger air rotameter gas hold-up volume of water, liter optimum volume of water, liter air rate, l/min optimum air rate, l/min frother concentration, ml optimum frother concentration, ml root means square error
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