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Bubbles growth and their stability in reactive flotation process
Chemical Engineering and Processing 48 (2009) 1534–1538
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Bubbles growth and their stability in reactive flotation process A.A. El-Midany a,∗ , H.E. El-Shall b , S. Svoronos c a
Mining, Petroleum, and Metallurgical Engineering Dept, Faculty of Engineering, Cairo University, Cairo, Egypt Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States c Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, United States b
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 31 May 2009 Accepted 17 September 2009 Available online 22 September 2009 Keywords: Reactive flotation Polyvinyl alcohol Polymer coating Dynamic surface tension Bubble formation Bubble stability
a b s t r a c t Reactive flotation is a process originally used to separate dolomite from phosphate. It depends mainly on the formation of CO2 bubbles as a product of acid reaction with dolomite surface in presence of a polyvinyl alcohol (PVA) polymeric membrane. Elasticity of PVA membrane is one of the critical factors that affect the bubble stability at the interface. The dynamic surface tension (DST) at different CO2 rates was used as a measure for the membrane elasticity. The DST, at different CO2 rates, was used to simulate the bubbles formation at the interface due to the surface reaction of dolomite with acidic media. The results indicated that the high evolution of CO2 , due to vigorous reaction, negatively affects the membrane elasticity and leads to fast rupture of the formed bubbles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The presence of MgO, in a dolomitic form, increases the difficulty of phosphate processing especially when it exceeds 1%. In fact, the higher dolomitic phosphate is simply left in the mining area, discarded or blended off with lower MgO rock, as there is no economic method for removing the dolomite from the phosphate rock. Eventually, heavy media separation, the only method ever commercialized, is sitting idle due to its relatively high operating cost and poor performance. The heavy media product typically contains ≥1% MgO. Over the years the phosphate industry was looking for a number of methods for MgO removal. Grinding and screening, selective flotation, flocculation, and partial acidification were among the tested methods [1–14]. In particular, flotation methods have shown good results in the laboratory; none have been commercialized to date, generally due to a complex flow sheet, poor economics or lower P2 O5 recovery. The most promising process is the China Lianyungang Design and Research Institute (CLDRI) process [8,15,16]. However, CLDRI fine flotation process has some concerns such as: • Handling of fine concentrate (dewatering and transport), • Capital cost (grinding and multiple process steps),
∗ Corresponding author. Tel.: +20 2 35678527; fax: +20 2 35723486. E-mail address: [email protected] (A.A. El-Midany). 0255-2701/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2009.09.005
• Increase in MgO% in the phosphate rock increases the reagent consumption, and • Grinding prior to beneficiation will increase energy consumption as well as plant operating cost. On the other hand, reactive flotation (RF) process addresses the separation of dolomite from phosphate pebbles [7,17,18]. The RF idea is based on generation of carbon dioxide gas when dolomite, as a carbonate mineral, is exposed to acidic solutions. Capturing the CO2 gas at the dolomite particle surface into bubbles using a coating agent (PVA polymer) can be used to selectively separate dolomite by flotation. Thus, the presence of an acidic medium and a coating agent are basic requirements. Testing several surfactants and polymers [7] suggested that the polyvinyl alcohol (PVA) adheres to the dolomite surface, forms a stable membrane (film) that is permeable by acid and impermeable by CO2 gas, and leads to dolomite flotation. In previous work [7], the RF can float a particle size up to 5–9 mm the larger the particle size is the longer the time needed to float the particle. In addition, surface tension and contact angle measurements, calculating work of adhesion, adsorption/desorption isotherms, Zeta potential, and surface characterization using Fourier Transform InfraRed (FTIR) were used to elucidate the interaction mechanism between polyvinyl alcohol (PVA) and dolomite or apatite surfaces in the reactive flotation process.. This paper aims at studying the role of the PVA membrane elasticity in CO2 bubbles formation in terms of CO2 evolution rate and dolomite-acid reaction speed. The dynamic surface tension (DST),
A.A. El-Midany et al. / Chemical Engineering and Processing 48 (2009) 1534–1538
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Table 1 Chemical analysis of hand-picked sample. MgO%
P2 O5 %
CaO%
Al2 O3 %
Fe2 O3 %
A.I.%a
L.O.I.%b
14.8
2.6
31.52
0.99
1.42
11.1
32.12
a b
A.I.: acid insoluble. L.O.I.: loss-on-ignition.
using maximum bubble method, was used to simulate the in situ bubble formation at the dolomite particle surface due to the chemical reaction. Additionally, the DST was tested under the same conditions of PVA and H2 SO4 concentration but at different CO2 evolution rates to illustrate the role of the reaction speed on the bubble stability.
Fig. 1. Characteristic bubble pressure vs. time curve in MBP.
(MBP) technique is the most commonly used technique [19,20]. The principle of the MBP measurement is the Laplace pressure (P); the pressure inside a curved liquid interface is higher than the ambient, which can be calculated by the Laplace equation [21].
2. Experimental 2.1. Chemical analysis Dolomite particles were hand-picked from phosphate rock provided by IMC Phosphate Co., Florida, USA. The selected pieces were used to demonstrate the bubble formation at the dolomite surface. The chemical analysis of these samples are given in Table 1. 2.2. Flotation tests Dolomite particles were pre-coated by spraying 3% PVA solution (using a mist delivery nozzle) then added to the 3% H2 SO4 acidic solution. The experiment was left for 5 min. Dolomite floated to the surface, which was separated by decantation on a screen. The products (floated and unfloated fractions) were dried, weighed, digested and chemically analyzed using an Inductively Coupled Plasma (ICP) emission spectrometer (Perkin Elmer Optima 3200RL Optical Emission Spectroscopy, Norwalk, CT). The ICP was calibrated using AFPC (Association of Florida Phosphate Chemists) rock check No. 22.
P=
2 + gh r
where is the surface tension, r is the radius of curvature of the bubble, is the liquid density, g is the gravitational constant and h is the depth of the bubble in the liquid. The first term expresses the Laplace pressure due to the curved gas/liquid interface, and the second term is the hydrostatic pressure due to the liquid height above the forming bubble. The first term will vary during the life cycle of the bubble, while the second term will remain constant. Fig. 1 shows a typical cycle of a bubble and the resulting pressure within the capillary. After a bubble breaks off from the capillary tip, the pressure is the lowest. As more gas flows into the capillary, the pressure builds up as the gas is pushed out of the capillary and the radius of curvature at the tip decreases. During this expansion process, polymer is populating the new interface and acting to lower the surface tension. At the point of minimum interface radius of curvature, where a hemisphere of gas is formed at the capillary tip, the pressure reaches maximum.
2.3. PVA preparation In this study, fully hydrolyzed PVA (>99%), 150,000 average molecular weight (Celvol 165, Celanese Chemicals Co., USA), was used. PVA flakes were gradually dissolved in de-ionized water with continuous mixing by magnetic stirrer and heated at 90 ◦ C for 2 h to produce different PVA concentrations (0.5, 1, 2, 3, and 4%). 2.4. Dynamic surface tension (DST) 2.4.1. Maximum bubble pressure method The dynamic surface tension (DST) measurement is important in the dynamic interfacial processes where the equilibrium surface tension is difficult to be achieved. The maximum bubble pressure
2.4.2. Dynamic surface tension setup The maximum bubble pressure apparatus was constructed using a differential pressure transducer with a sensitivity of 0–10 in. (25 cm) H2 O (0–2500 Pa). A #23 steel needle was used as a capillary, with nominal 0.025 in. (0.64 mm) external diameter, 0.013 in. (0.33 mm) internal diameter, and a flush cut tip. The capillary diameter was chosen so that the various resistance of water to bubble growth could be ignored. Such internal and viscous effects are a potential source of error in these measurements that need to be taken into consideration [22]. All measurements were conducted with the capillary tip 1 cm beneath the liquid surface. CO2 gas was used as the bubbling gas to simulate the gas produced in the reaction of acid with dolomite and an oscilloscope connected to the
Fig. 2. The maximum bubble pressure apparatus.
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A.A. El-Midany et al. / Chemical Engineering and Processing 48 (2009) 1534–1538 Table 2 Results of −4 + 1.19 mm particle size fraction.
Fig. 3. The bubble formation mechanism.
pressure transducer was used to determine the bubble frequency and the dynamic surface tension (Fig. 2).
3. Results and discussion The reactive flotation (RF) process includes two main steps: (1) the coating of the particle with PVA, and (2) the reaction of the precoated particle with acidic media. In the second step, the sulfuric acid penetrates the polyvinyl alcohol (PVA) membrane and reacts with the dolomite surface. CO2 gas is one of the reaction products according to the following reaction: CaMg(CO3 )2 + 2H2 SO4 → CaSO4 + MgSO4 + 2CO2 + 2H2 O Because PVA is known for its very low permeability by gases [23,24], especially CO2 , the CO2 will accumulate between the particle and membrane creating a density difference between the dolomite and phosphate rocks (i.e., lowering the dolomite particle density). A schematic illustration of the mechanism of bubble formation during the reaction and how the particle density changes, is given in Fig. 3.
Sample
Wt%
Float Sink Balance Feed
14.55 85.45 100 100
Chemical analysis, %
Recovery, %
MgO
CaO
P2 O5
MgO
CaO
P2 O5
13.21 0.75 2.56 2.6
32.56 34.83 34.50 34.78
2.71 19.35 16.93 16.84
75.1 24.9 100 100
13.73 86.27 100 100
2.33 97.67 100 100
3.1. Development and application of the RF process El-Midany et al. [17,18] used statistical designs to screen the main controlling factors of the RF process. It was found that the main significant factors are particle size, polymer dosage and acid concentration. On the other hand, it was found that coating method is strongly affect the polymer consumption, therefore, the consumption of the polymer was optimized. These studies indicated that spraying the particles with 1% polymer solution can reduce the polymer dosage from 2 to 0.5 kg/t. As examples, the results of flotation of representative samples, mixture of phosphate and dolomite are given in Tables 2 and 3. Two size fractions were used in these tests, i.e., −4 + 1.19 and −2 + 1 mm. The particles were pre-coated with PVA solution then added to the acidic solution. It is clear that the smaller the particle size the lower MgO% that can be achieved. In other words, the concentrate from −2 + 1 mm fraction has 0.65% MgO while the concentrate from −4 + 1.19 mm has 0.75% MgO%. This may be due to the higher liberation at smaller size fraction. From these results, it is obvious that the RF process can produce a concentrate with MgO less than 1% according to specifications
Fig. 4. Growth of the bubbles with time on the dolomite particle surface, 50X.
A.A. El-Midany et al. / Chemical Engineering and Processing 48 (2009) 1534–1538
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Table 3 Results of −2 + 1 mm particle size fraction. Sample
Wt%
Float Sink Balance Feed
20.74 79.26 100 100
Chemical analysis, %
Recovery, %
MgO
CaO
P2 O5
MgO
CaO
P2 O5
12.3 0.65 3.06 3.04
32.82 38.33 37.19 37.45
2.9 22.3 18.27 18.13
83.37 16.63 100 100
18.30 81.70 100 100
3.29 96.71 100 100
required for phosphoric acid production even with large particle size. 3.2. Bubble formation and growth Fig. 4(a–e) shows the growth of bubbles on the dolomite surface with time. It is interesting to notice the rapid growth of bubbles within a few seconds. The reaction starts immediately after adding the particles to the acidic solution. Then, the bubbles grow as a result of the evolution of CO2 , the impermeability of the PVA membrane, and its stretching. Unless the PVA membrane is flexible enough, it will rupture and CO2 gas will be released, leaving the particle to dissolve without causing any flotation. On the other hand, Fig. 4 indicated that not all sites can be considered as active sites for reaction to form CO2 bubbles. In other words, if the entire surface is reacting, the polymer will form a uniform and homogeneous membrane around the particle.
As the reaction transpires at the dolomite particle surface, the produced CO2 gas affects the growing bubbles in terms of their stability (i.e., the stretching and elasticity of the polymer due to gas-pressure on the adsorbed polymer layer). The presence of the surfactant or polymer adsorbed at either the gas–liquid or liquid–liquid interface leads to the viscoelastic properties of that interface. They may be characterized by a surface dilatational modulus, which is defined according to Gibbs’s equation [25–27]: d d ln A
mer concentrations of up to 1%, the surface tension significantly decreases as compared to the surface tension of water. Over the concentration range from to 1% to 3%, the surface tension decreases gradually but at greater concentration levels the surface tension remains nearly constant. The decrease in surface tension can be described, according to the Ward and Tordai model [28–30], as a diffusion-controlled adsorption process. For a short time scale (from milliseconds to seconds) the equation can be written as (t) = 2co
3.3. Bubbles stability
E=
Fig. 6. PVA elasticity in terms of its concentration at different CO2 flowrate.
(1)
where is the dynamic surface tension (DST) and A is the geometric area of the surface. For that reason, the dynamic surface tension (DST) measurements using the maximum bubble pressure technique were determined at different gas flow rates: 0.23, 0.47, 0.78, and 1.55 cm3 /min. Fig. 5 shows the results of changing DST as a function of the increase in polymer concentration. Clearly, at PVA poly-
Dt 1/2
(2)
where : surface concentration at any time (g m−2 ); co : bulk concentration (g m−2 ); D: monomer diffusion coefficient (m2 s−1 ); : commonly used value; t: time (s). It is interesting to note that these decreases in surface tension are more pronounced at lower flow rates—the greater the increase in the flowrate, the smaller the variation in the DST is. This can be referred to as the time needed for the polymer to get onto the newly formed interfacial region. Therefore, at higher flow rates, the bubbles surfaces remain almost clean and the effect of the polymer as an active agent at the interfacial region will be less because the polymer molecules will not have enough time to adsorb onto it. The lower the flow rate, the higher the chance for the polymer molecules to adsorb onto the interface of the generated bubbles. Fig. 6 shows the results of using Gibbs equation to determine the elasticity in terms of PVA concentration at different CO2 gas flowrates. It is interesting to note that the elasticity increases with PVA concentration. This is due to formation of a membrane as a result of increasing the polymer concentration, which is able to resist the pressure exerted by the generated CO2 gas. However, the higher the bubble formation rate (flowrate) the lower the elasticity. This can be referred to as the lower diffusivity of the polymer to the bubble surface and, consequently, DST at high bubble formation rates. The flow rate of CO2 generation was found to be very low due to the reaction, which is closer to equilibrium conditions, and also indicates higher membrane elasticity. 4. Conclusions
Fig. 5. Dynamic surface tension of PVA concentration at different CO2 flowrate.
The separation of dolomite from phosphate is a big challenge. Reactive flotation process is used to separate dolomite from phosphate pebbles. However, it depends mainly on the in situ bubbles formed at the dolomite interface due to presence of a PVA polymer that allowed the chemical reaction of dolomite with acidic media as well as capturing the CO2 , as a reaction product, at the particle interface. Bubbles growth and their stability on the dolomite surface were tested. The results showed that the bubbles take about 10–30 s to be formed. In addition, the dynamic surface tension
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was used to simulate the CO2 evolution in the RF process using maximum bubble pressure method. It showed that the elasticity modulus increases with the PVA concentration and decreases with the flowrate. More interestingly, it was found that the rate of CO2 evolution is very low which keeps the membrane at its highest elasticity. References [1] N.A. Abdel Khalek, Evaluation of flotation strategies for sedimentary phosphate with siliceous and carbonates gangues, Journal of Minerals Engineering 13 (7) (2000) 789–793. [2] J.O. Amankonah, P. Somasundaran, Effects of dissolved mineral species on the electrokinetic of calcite and apatite, Colloids and Surfaces 15 (1985) 335–353. [3] K.P. Ananthapadmanabhan, P. Somasundaran, Surface precipitation of in organic and surfactants and its role in adsorption and flotation, Colloids and Surfaces 13 (1985) 151–167. [4] I.J. Anazia, J. Hanna, New flotation approach for carbonate phosphate separation, Minerals and Metallurgical Processing (November) (1987) 196–202. [5] B. Andersen, P. Somasundaran, The role of changing surface mineralogy on the separation of phosphatic clay waste, International Journal of Mineral Processing 38 (1993) 189–203. [6] D.A. Elgillani, A-Z.M. Abouzeid, Flotation of carbonates from phosphate ores in acidic media, International Journal of Mineral Processing 38 (1993) 235–256. [7] A. El-Midany, Separation of Dolomite from Phosphate Rock by Reactive Flotation, Ph.D. Thesis, University of Florida, 2004. [8] H. El-Shall, P. Zhang, Beneficiation technology of phosphates: challenges and solutions, Minerals and Metallurgical Processing 21 (1) (2004) 103–111. [9] H. El-Shall, P. Zhang, R. Snow, Comparative analysis of dolomite-francolite flotation techniques, Minerals and Metallurgical Processing 8 (1996) 135–140. [10] R. Houot, R. Joussement, J. Tracez, R. Brouard, Selective flotation of phosphate ores having a siliceous and/or a carbonated gangue, International Journal of Mineral Processing 14 (1985) 245–264. [11] S. Lu, K. Sun, Developments of phosphate flotation reagents in China, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, USA, 1999, pp. 21–26 (Chapter 2). [12] X. Shao, C.L. Jiang, B.K. Parekh, Enhanced flotation separation of phosphate and dolomite using a new amphoteric collector, Minerals and Metallurgical Processing 15 (2) (1998) 11–14. [13] P. Somasundaran, L. Xiao, K.V. Viswanathan, Interactions between oleate collectors and alizarin modifiers in francolite/dolomite systems, Mineral and Metallurgical Processing 6 (1989) 100. [14] P. Somasundaran, L. Zhang, Role of surface chemistry of phosphate in its beneficiation, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, Co., USA, 1999, pp. 141–1154.
[15] G. Zhengxing, G. Zhizhong, S. Zheng, Beneficiation of Florida dolomitic phosphate pebble with a fine particle flotation process, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, 1999, pp. 155–162. [16] G. Zhizhong, G. Zhengxing, Plant practices of phosphate beneficiation in China, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, USA, 1999, pp. 289–301 (Chapter 25). [17] A. El-Midany, H. El-Shall, R. Stana, Mechanisms involved in reactive flotation of dolomite, Minerals and Metallurgical Processing Journal, SME 26 (2009) 94–100. [18] A.A. El-Midany, H. El-Shall, S. Svoronos, Statistical experimental designs for optimization of dolomite separation by reactive flotation, International Mineral Processing Congress (IMPC), Turkey, 2006. [19] V. Fainerman, R. Miller, P. Joos, Colloid and Polymer Science 272 (1994) 731–740. [20] T. Tamura, Y. Kaneko, M. Ohyama, Dynamic surface tension and foaming properties of aqueous polyoxyethylene n-dodecyl ether solutions, Journal of Colloids and Interface Sciences 173 (2) (1995) 493–499. [21] A. Adamson, A. Gast, Physical Chemistry of Surfaces, 6th edition, Wiley, New York, 1997. [22] P. Garrett, D. Ward, A reexamination of the measurement of dynamic surface tensions using the maximum bubble pressure method, Journal of Colloids and Interface Sciences 132 (2) (1989) 475–490. [23] C.A. Finch, Polyvinyl Alcohol; Properties and Applications, John Wiley and Sons, New York, 1973. [24] C.A. Finch, PVA-Developments, John Wiley and Sons, New York, 1992. [25] D.O. Johnson, K.J. Stebe, An oscillating bubble technique to determine surfactant mass transfer kinetics, Colloids and Surfaces A: Physicochemical and Engineering Aspects 114 (1996) 41–51. [26] K. Lunkenheimer, K. Winsel, H. Fruhner, J. Fang, K.-D. Wantke, K. Siegler, Dynamic surface tension and surface area elasticity of adsorbed pulmonary surfactant layers, Colloids and Surfaces A: Physicochemical and Engineering Aspects 114 (1996) 199–210. [27] P. Pikhitsa, A. Tsargorodskaya, Possible mechanism for multistage coalescence of a floating droplet on the air/liquid interface, Colloids and Surfaces A: Physicochemical and Engineering Aspects 167 (3) (2000) 287–291. [28] E. Pefferkorn, A. Carroy, R. Varoqui, Adsorption of polyacrylamide on solid surfaces: kinetics of the establishment of adsorption equilibrium, Macromolecules 18 (1985) 2252–2258. [29] I. Nahringbauer, Dynamic surface tension of aqueous polymer solutions. I. Ethyl (hydroxyethyl) cellulose (BERMOCOLL cst-103), Journal of Colloid and Interface Science 176 (1995) 318–328. [30] V.B. Fainerman, R. Miller, Adsorption kinetics of short-chain alcohols at the water/air interface: diffusion-controlled adsorption under the conditions of a nonequilibrium surface layer, Journal of Colloid and Interface Science 178 (1) (1996) 168–175.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Bubbles growth and their stability in reactive flotation process A.A. El-Midany a,∗ , H.E. El-Shall b , S. Svoronos c a
Mining, Petroleum, and Metallurgical Engineering Dept, Faculty of Engineering, Cairo University, Cairo, Egypt Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United States c Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, United States b
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 31 May 2009 Accepted 17 September 2009 Available online 22 September 2009 Keywords: Reactive flotation Polyvinyl alcohol Polymer coating Dynamic surface tension Bubble formation Bubble stability
a b s t r a c t Reactive flotation is a process originally used to separate dolomite from phosphate. It depends mainly on the formation of CO2 bubbles as a product of acid reaction with dolomite surface in presence of a polyvinyl alcohol (PVA) polymeric membrane. Elasticity of PVA membrane is one of the critical factors that affect the bubble stability at the interface. The dynamic surface tension (DST) at different CO2 rates was used as a measure for the membrane elasticity. The DST, at different CO2 rates, was used to simulate the bubbles formation at the interface due to the surface reaction of dolomite with acidic media. The results indicated that the high evolution of CO2 , due to vigorous reaction, negatively affects the membrane elasticity and leads to fast rupture of the formed bubbles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The presence of MgO, in a dolomitic form, increases the difficulty of phosphate processing especially when it exceeds 1%. In fact, the higher dolomitic phosphate is simply left in the mining area, discarded or blended off with lower MgO rock, as there is no economic method for removing the dolomite from the phosphate rock. Eventually, heavy media separation, the only method ever commercialized, is sitting idle due to its relatively high operating cost and poor performance. The heavy media product typically contains ≥1% MgO. Over the years the phosphate industry was looking for a number of methods for MgO removal. Grinding and screening, selective flotation, flocculation, and partial acidification were among the tested methods [1–14]. In particular, flotation methods have shown good results in the laboratory; none have been commercialized to date, generally due to a complex flow sheet, poor economics or lower P2 O5 recovery. The most promising process is the China Lianyungang Design and Research Institute (CLDRI) process [8,15,16]. However, CLDRI fine flotation process has some concerns such as: • Handling of fine concentrate (dewatering and transport), • Capital cost (grinding and multiple process steps),
∗ Corresponding author. Tel.: +20 2 35678527; fax: +20 2 35723486. E-mail address: [email protected] (A.A. El-Midany). 0255-2701/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2009.09.005
• Increase in MgO% in the phosphate rock increases the reagent consumption, and • Grinding prior to beneficiation will increase energy consumption as well as plant operating cost. On the other hand, reactive flotation (RF) process addresses the separation of dolomite from phosphate pebbles [7,17,18]. The RF idea is based on generation of carbon dioxide gas when dolomite, as a carbonate mineral, is exposed to acidic solutions. Capturing the CO2 gas at the dolomite particle surface into bubbles using a coating agent (PVA polymer) can be used to selectively separate dolomite by flotation. Thus, the presence of an acidic medium and a coating agent are basic requirements. Testing several surfactants and polymers [7] suggested that the polyvinyl alcohol (PVA) adheres to the dolomite surface, forms a stable membrane (film) that is permeable by acid and impermeable by CO2 gas, and leads to dolomite flotation. In previous work [7], the RF can float a particle size up to 5–9 mm the larger the particle size is the longer the time needed to float the particle. In addition, surface tension and contact angle measurements, calculating work of adhesion, adsorption/desorption isotherms, Zeta potential, and surface characterization using Fourier Transform InfraRed (FTIR) were used to elucidate the interaction mechanism between polyvinyl alcohol (PVA) and dolomite or apatite surfaces in the reactive flotation process.. This paper aims at studying the role of the PVA membrane elasticity in CO2 bubbles formation in terms of CO2 evolution rate and dolomite-acid reaction speed. The dynamic surface tension (DST),
A.A. El-Midany et al. / Chemical Engineering and Processing 48 (2009) 1534–1538
1535
Table 1 Chemical analysis of hand-picked sample. MgO%
P2 O5 %
CaO%
Al2 O3 %
Fe2 O3 %
A.I.%a
L.O.I.%b
14.8
2.6
31.52
0.99
1.42
11.1
32.12
a b
A.I.: acid insoluble. L.O.I.: loss-on-ignition.
using maximum bubble method, was used to simulate the in situ bubble formation at the dolomite particle surface due to the chemical reaction. Additionally, the DST was tested under the same conditions of PVA and H2 SO4 concentration but at different CO2 evolution rates to illustrate the role of the reaction speed on the bubble stability.
Fig. 1. Characteristic bubble pressure vs. time curve in MBP.
(MBP) technique is the most commonly used technique [19,20]. The principle of the MBP measurement is the Laplace pressure (P); the pressure inside a curved liquid interface is higher than the ambient, which can be calculated by the Laplace equation [21].
2. Experimental 2.1. Chemical analysis Dolomite particles were hand-picked from phosphate rock provided by IMC Phosphate Co., Florida, USA. The selected pieces were used to demonstrate the bubble formation at the dolomite surface. The chemical analysis of these samples are given in Table 1. 2.2. Flotation tests Dolomite particles were pre-coated by spraying 3% PVA solution (using a mist delivery nozzle) then added to the 3% H2 SO4 acidic solution. The experiment was left for 5 min. Dolomite floated to the surface, which was separated by decantation on a screen. The products (floated and unfloated fractions) were dried, weighed, digested and chemically analyzed using an Inductively Coupled Plasma (ICP) emission spectrometer (Perkin Elmer Optima 3200RL Optical Emission Spectroscopy, Norwalk, CT). The ICP was calibrated using AFPC (Association of Florida Phosphate Chemists) rock check No. 22.
P=
2 + gh r
where is the surface tension, r is the radius of curvature of the bubble, is the liquid density, g is the gravitational constant and h is the depth of the bubble in the liquid. The first term expresses the Laplace pressure due to the curved gas/liquid interface, and the second term is the hydrostatic pressure due to the liquid height above the forming bubble. The first term will vary during the life cycle of the bubble, while the second term will remain constant. Fig. 1 shows a typical cycle of a bubble and the resulting pressure within the capillary. After a bubble breaks off from the capillary tip, the pressure is the lowest. As more gas flows into the capillary, the pressure builds up as the gas is pushed out of the capillary and the radius of curvature at the tip decreases. During this expansion process, polymer is populating the new interface and acting to lower the surface tension. At the point of minimum interface radius of curvature, where a hemisphere of gas is formed at the capillary tip, the pressure reaches maximum.
2.3. PVA preparation In this study, fully hydrolyzed PVA (>99%), 150,000 average molecular weight (Celvol 165, Celanese Chemicals Co., USA), was used. PVA flakes were gradually dissolved in de-ionized water with continuous mixing by magnetic stirrer and heated at 90 ◦ C for 2 h to produce different PVA concentrations (0.5, 1, 2, 3, and 4%). 2.4. Dynamic surface tension (DST) 2.4.1. Maximum bubble pressure method The dynamic surface tension (DST) measurement is important in the dynamic interfacial processes where the equilibrium surface tension is difficult to be achieved. The maximum bubble pressure
2.4.2. Dynamic surface tension setup The maximum bubble pressure apparatus was constructed using a differential pressure transducer with a sensitivity of 0–10 in. (25 cm) H2 O (0–2500 Pa). A #23 steel needle was used as a capillary, with nominal 0.025 in. (0.64 mm) external diameter, 0.013 in. (0.33 mm) internal diameter, and a flush cut tip. The capillary diameter was chosen so that the various resistance of water to bubble growth could be ignored. Such internal and viscous effects are a potential source of error in these measurements that need to be taken into consideration [22]. All measurements were conducted with the capillary tip 1 cm beneath the liquid surface. CO2 gas was used as the bubbling gas to simulate the gas produced in the reaction of acid with dolomite and an oscilloscope connected to the
Fig. 2. The maximum bubble pressure apparatus.
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A.A. El-Midany et al. / Chemical Engineering and Processing 48 (2009) 1534–1538 Table 2 Results of −4 + 1.19 mm particle size fraction.
Fig. 3. The bubble formation mechanism.
pressure transducer was used to determine the bubble frequency and the dynamic surface tension (Fig. 2).
3. Results and discussion The reactive flotation (RF) process includes two main steps: (1) the coating of the particle with PVA, and (2) the reaction of the precoated particle with acidic media. In the second step, the sulfuric acid penetrates the polyvinyl alcohol (PVA) membrane and reacts with the dolomite surface. CO2 gas is one of the reaction products according to the following reaction: CaMg(CO3 )2 + 2H2 SO4 → CaSO4 + MgSO4 + 2CO2 + 2H2 O Because PVA is known for its very low permeability by gases [23,24], especially CO2 , the CO2 will accumulate between the particle and membrane creating a density difference between the dolomite and phosphate rocks (i.e., lowering the dolomite particle density). A schematic illustration of the mechanism of bubble formation during the reaction and how the particle density changes, is given in Fig. 3.
Sample
Wt%
Float Sink Balance Feed
14.55 85.45 100 100
Chemical analysis, %
Recovery, %
MgO
CaO
P2 O5
MgO
CaO
P2 O5
13.21 0.75 2.56 2.6
32.56 34.83 34.50 34.78
2.71 19.35 16.93 16.84
75.1 24.9 100 100
13.73 86.27 100 100
2.33 97.67 100 100
3.1. Development and application of the RF process El-Midany et al. [17,18] used statistical designs to screen the main controlling factors of the RF process. It was found that the main significant factors are particle size, polymer dosage and acid concentration. On the other hand, it was found that coating method is strongly affect the polymer consumption, therefore, the consumption of the polymer was optimized. These studies indicated that spraying the particles with 1% polymer solution can reduce the polymer dosage from 2 to 0.5 kg/t. As examples, the results of flotation of representative samples, mixture of phosphate and dolomite are given in Tables 2 and 3. Two size fractions were used in these tests, i.e., −4 + 1.19 and −2 + 1 mm. The particles were pre-coated with PVA solution then added to the acidic solution. It is clear that the smaller the particle size the lower MgO% that can be achieved. In other words, the concentrate from −2 + 1 mm fraction has 0.65% MgO while the concentrate from −4 + 1.19 mm has 0.75% MgO%. This may be due to the higher liberation at smaller size fraction. From these results, it is obvious that the RF process can produce a concentrate with MgO less than 1% according to specifications
Fig. 4. Growth of the bubbles with time on the dolomite particle surface, 50X.
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Table 3 Results of −2 + 1 mm particle size fraction. Sample
Wt%
Float Sink Balance Feed
20.74 79.26 100 100
Chemical analysis, %
Recovery, %
MgO
CaO
P2 O5
MgO
CaO
P2 O5
12.3 0.65 3.06 3.04
32.82 38.33 37.19 37.45
2.9 22.3 18.27 18.13
83.37 16.63 100 100
18.30 81.70 100 100
3.29 96.71 100 100
required for phosphoric acid production even with large particle size. 3.2. Bubble formation and growth Fig. 4(a–e) shows the growth of bubbles on the dolomite surface with time. It is interesting to notice the rapid growth of bubbles within a few seconds. The reaction starts immediately after adding the particles to the acidic solution. Then, the bubbles grow as a result of the evolution of CO2 , the impermeability of the PVA membrane, and its stretching. Unless the PVA membrane is flexible enough, it will rupture and CO2 gas will be released, leaving the particle to dissolve without causing any flotation. On the other hand, Fig. 4 indicated that not all sites can be considered as active sites for reaction to form CO2 bubbles. In other words, if the entire surface is reacting, the polymer will form a uniform and homogeneous membrane around the particle.
As the reaction transpires at the dolomite particle surface, the produced CO2 gas affects the growing bubbles in terms of their stability (i.e., the stretching and elasticity of the polymer due to gas-pressure on the adsorbed polymer layer). The presence of the surfactant or polymer adsorbed at either the gas–liquid or liquid–liquid interface leads to the viscoelastic properties of that interface. They may be characterized by a surface dilatational modulus, which is defined according to Gibbs’s equation [25–27]: d d ln A
mer concentrations of up to 1%, the surface tension significantly decreases as compared to the surface tension of water. Over the concentration range from to 1% to 3%, the surface tension decreases gradually but at greater concentration levels the surface tension remains nearly constant. The decrease in surface tension can be described, according to the Ward and Tordai model [28–30], as a diffusion-controlled adsorption process. For a short time scale (from milliseconds to seconds) the equation can be written as (t) = 2co
3.3. Bubbles stability
E=
Fig. 6. PVA elasticity in terms of its concentration at different CO2 flowrate.
(1)
where is the dynamic surface tension (DST) and A is the geometric area of the surface. For that reason, the dynamic surface tension (DST) measurements using the maximum bubble pressure technique were determined at different gas flow rates: 0.23, 0.47, 0.78, and 1.55 cm3 /min. Fig. 5 shows the results of changing DST as a function of the increase in polymer concentration. Clearly, at PVA poly-
Dt 1/2
(2)
where : surface concentration at any time (g m−2 ); co : bulk concentration (g m−2 ); D: monomer diffusion coefficient (m2 s−1 ); : commonly used value; t: time (s). It is interesting to note that these decreases in surface tension are more pronounced at lower flow rates—the greater the increase in the flowrate, the smaller the variation in the DST is. This can be referred to as the time needed for the polymer to get onto the newly formed interfacial region. Therefore, at higher flow rates, the bubbles surfaces remain almost clean and the effect of the polymer as an active agent at the interfacial region will be less because the polymer molecules will not have enough time to adsorb onto it. The lower the flow rate, the higher the chance for the polymer molecules to adsorb onto the interface of the generated bubbles. Fig. 6 shows the results of using Gibbs equation to determine the elasticity in terms of PVA concentration at different CO2 gas flowrates. It is interesting to note that the elasticity increases with PVA concentration. This is due to formation of a membrane as a result of increasing the polymer concentration, which is able to resist the pressure exerted by the generated CO2 gas. However, the higher the bubble formation rate (flowrate) the lower the elasticity. This can be referred to as the lower diffusivity of the polymer to the bubble surface and, consequently, DST at high bubble formation rates. The flow rate of CO2 generation was found to be very low due to the reaction, which is closer to equilibrium conditions, and also indicates higher membrane elasticity. 4. Conclusions
Fig. 5. Dynamic surface tension of PVA concentration at different CO2 flowrate.
The separation of dolomite from phosphate is a big challenge. Reactive flotation process is used to separate dolomite from phosphate pebbles. However, it depends mainly on the in situ bubbles formed at the dolomite interface due to presence of a PVA polymer that allowed the chemical reaction of dolomite with acidic media as well as capturing the CO2 , as a reaction product, at the particle interface. Bubbles growth and their stability on the dolomite surface were tested. The results showed that the bubbles take about 10–30 s to be formed. In addition, the dynamic surface tension
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was used to simulate the CO2 evolution in the RF process using maximum bubble pressure method. It showed that the elasticity modulus increases with the PVA concentration and decreases with the flowrate. More interestingly, it was found that the rate of CO2 evolution is very low which keeps the membrane at its highest elasticity. References [1] N.A. Abdel Khalek, Evaluation of flotation strategies for sedimentary phosphate with siliceous and carbonates gangues, Journal of Minerals Engineering 13 (7) (2000) 789–793. [2] J.O. Amankonah, P. Somasundaran, Effects of dissolved mineral species on the electrokinetic of calcite and apatite, Colloids and Surfaces 15 (1985) 335–353. [3] K.P. Ananthapadmanabhan, P. Somasundaran, Surface precipitation of in organic and surfactants and its role in adsorption and flotation, Colloids and Surfaces 13 (1985) 151–167. [4] I.J. Anazia, J. Hanna, New flotation approach for carbonate phosphate separation, Minerals and Metallurgical Processing (November) (1987) 196–202. [5] B. Andersen, P. Somasundaran, The role of changing surface mineralogy on the separation of phosphatic clay waste, International Journal of Mineral Processing 38 (1993) 189–203. [6] D.A. Elgillani, A-Z.M. Abouzeid, Flotation of carbonates from phosphate ores in acidic media, International Journal of Mineral Processing 38 (1993) 235–256. [7] A. El-Midany, Separation of Dolomite from Phosphate Rock by Reactive Flotation, Ph.D. Thesis, University of Florida, 2004. [8] H. El-Shall, P. Zhang, Beneficiation technology of phosphates: challenges and solutions, Minerals and Metallurgical Processing 21 (1) (2004) 103–111. [9] H. El-Shall, P. Zhang, R. Snow, Comparative analysis of dolomite-francolite flotation techniques, Minerals and Metallurgical Processing 8 (1996) 135–140. [10] R. Houot, R. Joussement, J. Tracez, R. Brouard, Selective flotation of phosphate ores having a siliceous and/or a carbonated gangue, International Journal of Mineral Processing 14 (1985) 245–264. [11] S. Lu, K. Sun, Developments of phosphate flotation reagents in China, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, USA, 1999, pp. 21–26 (Chapter 2). [12] X. Shao, C.L. Jiang, B.K. Parekh, Enhanced flotation separation of phosphate and dolomite using a new amphoteric collector, Minerals and Metallurgical Processing 15 (2) (1998) 11–14. [13] P. Somasundaran, L. Xiao, K.V. Viswanathan, Interactions between oleate collectors and alizarin modifiers in francolite/dolomite systems, Mineral and Metallurgical Processing 6 (1989) 100. [14] P. Somasundaran, L. Zhang, Role of surface chemistry of phosphate in its beneficiation, in: P. Zhang, H. El-Shall, R. Wiegel (Eds.), Beneficiation of Phosphates: Advances in Research and Practice, SME, Co., USA, 1999, pp. 141–1154.
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