- Email: [email protected]
An electrostatic solvent extraction contactor for nickel-cobalt recovery
MineralsEngineering,Vol. 13, No. 12, pp. 1281-1288,2000
Pergamon 0892-6875(00)00110-2
© 2000Publishedby ElsevierScienceLtd All rightsreserved 0892-6875/00/$ - see frontmatter
AN ELECTROSTATIC SOLVENT EXTRACTION CONTACTOR FOR NICKEL--COBALT RECOVERY*
M.K. BRIGGS ~, C.Y. CHENG ~* and D.C. IBANA ~ ~[ W A School o f Mines - AJ Parker CRC for Hydrometallurgy, Curtin University of Technology, PMB 22 Kalgoorlie, W A 6430, Australia § C S I R O Minerals - AJ Parker CRC for Hydrometallurgy, PO Box 90, Bentley, W A 6982, Australia * Corresponding author. Email: [email protected]
(Received I June 2000; accepted 10 July 2000)
ABSTRACT The aim of this research is to develop an Electrostatic Solvent Extraction (ESX) contactor, to obtain fundamental data on hydrodynamics and mass transfer kinetics. ESX uses electrostatic fields to enhance either coalescence or dispersion. At low field intensities, attractive forces between the aqueous droplets predominate, favouring coalescence. At high field intensities, the aqueous drops prolate in the direction of the electrostatic field resulting in disintegration into smaller droplets. This results in large surface areas enhancing mass transfer. The application of ESX in the solvent extraction of nickel and cobalt is being studied. 7he effect of field intensity on the hold-up of the dispersed phase and its mean drop size has been investigated. An increase in the applied field intensity from 0 kV/cm to 4.6 kV/cm resulted in an increase in holdup from 0.43% to 0.94% at an electrode insulation thickness of O.5 mm. An increase in the applied field intensity from 2.5 kV/cm to 4.4 kV/cm resulted in a decrease in the mean drop size. Investigation on the separation of cobalt from nickel using Cyanex 272 in Shellsol 2046 has been carried out. Cobalt extraction doubled from 44% to 88% with an increase in applied field intensity from 3.6 kV/cm to 4.0 kV/cm. This increase in mass transfer is attributed to the smaller dispersed droplets produced and their associated vibrating and turning movements at higher field intensity. © 2000 Published by Elsevier Science Ltd. All rights reserved.
Keywords Solvent extraction; non-ferrous metallic ores
INTRODUCTION The use of an electrostatic field to enhance coalescence o f aqueous droplets in organic solvents has been used for de-watering in the petroleum industry for many years (Warren et al. 1978, Bailes 1992, Yamaguchi 1995). The use o f electrostatic fields to enhance dispersion in solvent extraction contactors has been the subject of investigation since the late 1960s. Bailes and Thornton, in 1971, investigated the mass
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
1281
1282
M.K. Briggset al.
transfer coefficients and droplet size measurements for charged furfural droplets in n-heptane. They observed that the applied voltage could control the droplet size. Also, mass transfer coefficients of the dispersed aqueous phase were significantly increased at higher applied voltages. This resulted from droplets exhibiting a high degree of mobility and internal circulation. Martin et al. (1983) reported that at low applied voltage, coalescence of the aqueous droplets was favoured. Conversely, at higher applied voltages, the disintegration of droplets was enhanced and thus favourable to mass transfer. They also showed that the applied voltages could easily control the hydrodynamic behaviour of the droplets. For instance, at a constant aqueous flowrate, the higher the electrostatic field, the smaller the mean drop size and the higher the hold-up. More recently, Usami e t a l (1992) reported that extraction efficiency depends on interfacial area as well as the renewal of the interface caused by internal droplet circulation. They also found that no electric charge exists on the aqueous droplet. The electric field was found to only act at the interface between phases. A major advantage of using electrostatic fields for mixing is that it allows easy production of small droplets. Therefore, large interfacial areas are produced at very low power input. This technique promotes turbulence and intense mixing at the droplet interface thus enhancing mass transfer (Bailes 1981). The aim of this research is to develop an ESX contactor, to obtain fundamental data on hydrodynamics and mass transfer kinetics, and to reduce the gap between laboratory testwork and industrial application.
EXPERIMENTAL
Experimental apparatus The contactor used in this study was made from transparent acrylic sheet shaped into a square crosssection. It was 850 mm high with an internal width of 80 mm and breadth of 56 mm. The two electrodes, located on opposite sides, were made from stainless steel sheets and coated with a thin insulation layer of HALAR®. The two electrodes were set approximately 50 mm apart. The length of each electrode was 600 mm. A schematic diagram of the experimental apparatus is shown in Figure 1. Aqueous Feed Distributor
H.T, Transformer
/Weir --J="-'~ LoadedOrganic
I J Positive Electrode Earthed Electrode Organic Feed ----Ib=
Bottom Section Raffinate Fig. 1 A schematic diagram of the ESX test rig.
Electrostatic solventextractioncontactorfor nickel-cobaltrecovery
1283
Experimental procedures The aqueous test solution was pumped to the top of the distributor using a diaphragm pump where the aqueous phase dropped through a horizontal perforated plate. The organic solution was pumped from the bottom section of the contactor. The hold-up tests were carried out using a diluent (Shellsol 2046) as the organic phase. The aqueous phase was a 0.5 M H2SO4 solution. The determination of h01d-u p involved simultaneously closing the inlet and exit flows and allowing the aqueous phase to settle. The settled dispersed phase was drained from the contactor until the original interface was achieved. The hold-up tests involved using different aqueous and organic flowrates, field intensities and electrode insulation thicknesses. The behaviour of the dispersed aqueous droplets was recorded using a digital camera. The equivalent drop size for the non-spherical drops was calculated using the following equation:
de= where dl is the major axis and d2 the minor axis of the drop images (Lewis et al. 1951). For spherical drops,the drop diameter was taken as de. The mean drop size was calculated on the basis of the number of clear drop images in the photographs and their de values. The organic solution used in the cobalt extraction study was Cyanex 272 supplied by Cytec, and Shellsol 2046 supplied by Shell Chemicals. The organic reagents were used without further purification. The nickel (3.0 g/L) and cobalt (0.3 g/L) solutions were prepared by dissolving their sulphate salts in a 0.5 M acetic acid/acetate buffer solution at pH 5.5.
Sampling and assay Samples of the raffinate and loaded organic were taken at regular time intervals. Samples for the majority of the cobalt solvent extraction tests were taken at 0, 2, 5, 10, 20, 30, 60 minutes. These samples were then taken for assay using an inductively couple plasma emission spectrometer (ICP-AES).
RESULTS AND DISCUSSION
Hold-up The hold-up is the volume of the dispersed aqueous phase contained in the total phase volume, expressed as a percentage. The effect of aqueous and organic flowrates, field intensity, and the thickness of electrode insulation on the hold-up were investigated. To determine the effect of aqueous flowrate on hold-up, several tests were carried out at constant organic flowrate and constant electrode insulation thickness but with varied field intensities. The results, summarised in Figure 2, clearly show that increases in the aqueous flowrate resulted in increases in the hold-up. For example, at no field intensity (0 kV/cm), an increase in aqueous flowrate from 3.6 L/hour to 10.8 L/hour resulted in an increase in hold-up from 0.2% to 0.67%. Similar results were obtained at higher comparable field intensities (3.6, 4.0, 4.4 kV/cm) where an increase in aqueous flowrate from 3.6 L/hour to 10.8 L/hour resulted in an increase in hold-up. The effect of field intensity on hold-up was less marked. For instance, as shown in Figure 2, at 1.0 mm electrode insulation thickness, an increase in field intensity from 0 to 4.4 kV/cm only resulted in an increase in hold-up from 0.67% to 0.8% at an aqueous flowrate of 10.8 L/hour. A similar trend was observed at lower aqueous flowrates (3.6, 7.2 L/hour) over the same field intensities.
1284
M.K. Briggsetal.
The effect of the thickness of the electrode insulation on hold-up was investigated using two different electrode insulation thicknesses (0.5, 1.0 mm). The results, summarised in Figure 3, show that the thickness of electrode insulation had a much more significant effect on the hold-up than the applied field intensity. For instance, when the thickness of the electrode insulation was halved from 1.0 to 0.5 mm, the hold-up increased from 0.5% to 0.82% at a field intensity of 4.4 kV/cm. This occurred because the actual electric field intensity in the liquid is lower when thicker electrode insulation is used.
1.0 0.9 0.8 0.7 A
0.6
Aqueous 3.6 L/hr
0.5
--II--Aqueous 7.2 L/hr
0.4
~Aqueous
0.3
10.8 L/hr
- ~V-----~
0.2 0.1 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Applied Field Intensity (kV/cm)
Fig.2
The effect of aqueous flowrate on hold-up (organic flowrate of 3.6 L/hr and an electrode insulation thickness of 1.0 mm). 1.0 0.9 0.8 0.7 o.
0.6 •-e--0.5 mm --.11--1.0 mm
0.5 1
~0.4T
j
0.2 0.1 0.0 0.0
Fig.3
Mean
0.5
1.0
1.5 2.0 2.5 3.0 3.5 4.0 Applied Field Intensity (kVlcm)
4.5
5.0
The effect of the thickness of electrode insulation on hold-up (A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hour)).
d r o p size
Increases in field intensity from 0 kV/cm to 2.4 kV/cm resulted in increases in the mean drop size (Figure 4). Further increases in field intensity however, resulted in decreases in mean drop size. The initial mean drop size was about 1.6 mm. As the field intensity increased to 2.4 kV/cm the mean drop size increased to 2.2 mm. This indicated that low field intensities (<2.4 kV/cm) result in increases in polarity of the aqueous droplets, thus favouring coalescence. At higher field intensities (>2.4 kV/cm), the stresses imposed on the droplets were sufficient to break the droplets into smaller droplets. At a field intensity of 4.4 kV/cm, the mean drop size decreased to less than 1.2 mm.
Electrostatic solventextractioncontactorfor nickel-cobaltrecovery
1285
These results are consistent with those obtained by Martin et al. (1983). They observed that, the higher the field intensity, the smaller the mean drop size at a constant aqueous flowrate. They also observed that at constant field intensity, the higher the aqueous flowrate, the larger the mean drop size. As the number of droplets per unit volume increases with the higher aqueous flowrate, the droplets are closer and thus dipole forces are more intense favouring less dispersion and larger droplets. 2,4. 2.2 2.0 1.8
1.6 j~ 1.4
~
1.2 1.0. 0.8.
0,6
0.4 0.2 0.0 0.0
Fig.4
0.5
1.0
1.5 2.0 2.5 3.0 3.5 Applied'Field Intensity (kVlcm)
4.0
4.5
5.0
The effect of field intensity on mean drop size (A/O flowrate ratio of 4:3 (aqueous 7.2 L/hour to organic 5.4 L/hour), electrode insulation thickness of 1.0 mm).
Solvent extraction of cobalt In the extraction of cobalt ions from an aqueous solution using bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) as the extractant, the overall reaction is as follows: 2R2POOH + Co 2÷ = Co (R2POOH) + 2H +
(2)
where R = 2,4,4-trimethylpentyl. Effective nickel--cobalt separation requires a pH around 5.5. If the above reaction is to proceed to the right, the H ÷ ions entering the aqueous phase must be removed. This was achieved in the testwork by buffering the aqueous phase at pH 5.5. Figure 5 shows the effect of field intensity on cobalt extraction using 5% Cyanex 272 in Shellsol 2046 at field intensities of 3.6 kV/cm and 4.0 kV/cm. The slope of the curves is a measure of the rate of cobalt extraction. The cobalt extraction at steady state increased with the increase in field intensity (with all other test conditions remaining constant). Cobalt extraction doubled from 44% to 88% as the field intensity increased from 3.6 kV/cm to 4.0 kV/cm. After 20-30 minutes into the test, the amount of cobalt extraction reached its maximum value. This remained almost constant until the end of the test. The rate of cobalt extraction at a field intensity of 4.0 kV/cm was much higher than at a field intensity of 3.6 kV/cm particularly in the first 10 minutes of the test. Many investigators including Bailes and Thornton (1971) and Yamaguchi and Kanno (1996) have studied the effects of electric fields on the rate of mass transfer in a liquid-liquid system. These investigators have suggested that an increase in field intensity results in a significant increase in mass transfer coefficients. The enhanced mass transfer is attributed to the smaller aqueous droplets produced and their vigorous vibrating and turning movement at higher field intensity. The effect of the A/O flowrate ratio on cobalt extraction was investigated. Figure 6 shows results using 8% Cyanex 272 at a field intensity of 3.6 kV/cm. At an A/O flowrate ratio of 2:1 (7.2:3.6 L/hr) cobalt extraction was almost 90%. However, when the A/O flowrate ratio was reduced to 1:1 (3.6:3.6 L/hr) cobalt extraction increased to 98%. In both tests, trace amounts (1-2%) of nickel was co-extracted with cobalt.
1286
M.K. Briggset al.
The reduction of the aqueous flowrate resulted in an increase in cobalt extraction. At a lower aqueous flowrate (3.6 L/hour), the mean drop size is smaller and the droplets are not as close to each other. Also, the number of aqueous droplets per unit volume is less and therefore there is relatively more organic solution available for extraction per unit volume.
100
90
n
m
8o 70 C
2
ttl
6o 50 40 30 20
¢
~ f - a P
"
3.6 kV/cm
- - I I - - 4.0 kV/cm
A v
0 0
10
20
30
40
50
60
70
Time (mine) Fig. 5
The effect of field intensity on cobalt extraction (5% Cyanex 272, A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hr), 23 °C, pH 5.5).
The rate of cobalt extraction for the first 5 minutes of both tests is high. However, further cobalt extraction only slightly increased with time as shown by the slope of the curves. This showed that maximum cobalt extraction could be reached in a short period of time.
100 80
80 Z r0 60
-,-tl.--A/O 1:1 (3.6:3.6)
10
0 0
10
20
30
40
Time (rains)
50
60
70
Fig. 6 The effect of A/O flowrate ratio on cobalt extraction (8% Cyanex 272, 3.6 kV/cm, 23 °C, pH 5.5). The effect of Cyanex 272 concentration on the cobalt extraction was also investigated. Figure 7 shows that when the Cyanex 272 concentration was increased from 5% to 8% at a field intensity of 3.2 kV/cm, cobalt extraction increased from 50% to 74%. This occurred using an A/O flowrate ratio of 2:1 (7,2:3.6 L/hour). A direct relationship exists between Cyanex 272 concentration and cobalt extraction under these conditions.
Electrostatic solvent extraction contactor for nickel-cobalt recovery
1287
100 90 ---.. 80 v 70 .o 60
50
$
8% Cyanex 272
40
"-
5% Cyanex 272
8 20 10
0 0
Fig.7
I
i
10
20
I
I
30 40 ~me(mins)
I
I
50
60
70
The effect of Cyanex 272 concentration on cobalt extraction (3.2 kV/cm, 23 °C, pH 5.5, A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hour).
CONCLUSIONS The fundamentals of ESX were investigated using a laboratory scale contactor. The effect of field intensity on the hold-up of the dispersed aqueous phase and its drop size has been studied. Investigation of the separation of cobalt from nickel using Cyanex 272 in Shellsol 2046 has commenced. The main results are as follows: 1.
An increase in the aqueous flowrate results in an increase in hold-up. As the aqueous flowrate increased from 3.6 L/hour to 10.8 L/hour, the hold-up increased from 0.21% to 0.72%, at a field intensity of 3.6 kV/cm.
2.
The mean drop size initially increased and then decreased with increasing field intensity. At an A/O flowrate ratio of 4:3 (7.2 to 5.4 L/hour) and an electrode insulation thickness of 1.0 mm, the initial mean drop size was around 1.6 mm. As the field intensity increased to 2.4 kV/cm the mean drop size increased to 2.2 mm. As the field intensity increased further to 4.4 kV/cm the mean drop size decreased to less than 1.2 mm.
3.
Cobalt extraction increased with an increase in field intensity. Using 5% Cyanex 272, cobalt extraction doubled from 44% to 88% when the field intensity was increased from 3.6 kV/cm to 4.0 kV/cm.
.
Cobalt extraction increased with an increase in Cyanex 272 concentration. When Cyanex 272 concentration was increased from 5% to 8% at a field intensity of 3.2 kV/cm, cobalt extraction increased from 50% to 74%.
.
Cobalt extraction increa~d with a decrease in aqueous flowrate. At an A/O flowrate ratio of 2:1, cobalt extraction was almost 90% after a period of 60 minutes. However, when the A/O ftowrate ratio was reduced to 1: 1, cobalt extraction increased to 98%.
1288
M.K. Briggs et al.
REFERENCES
Bailes, P.J., Electrostatic Extraction for Metals and Non-metals. In Proc. International Solvent Extraction Conference, ISEC 77. CIM Special Volume 21, 1979, pp. 233-241 Bailes, P.J., Solvent Extraction in an Electrostatic Field. In Industrial & Engineering Chemistry -- Process Design & Development. 1981, 20, 564-570. Bailes, P.J., Electrically augmented settlers and coalescers for solvent extraction. Hydrometallurgy, 1992, (30), 417 - 430. Bailes, P.J., and Larkai, S.K.L., Liquid Phase Separation in Pulsed D.C Fields. Transactions Institution oJ Chemical Engineers, 1982, Vol. 60, pp. 115-121. Bailes, P.J., and Larkai, S.K.L., The application of electric fields to phase separation in solvent extraction system. In Proceedings International Solvent Extraction Conference, ISEC 83, 1983, pp. 180-181 Bailes, P.J., and Thornton, J.D., Electrically augmented liquid-liquid extraction studies in a two-component system I - Single droplet studies. In Solvent Extraction -- Proceedings of the International Solvent Extraction Conference, ISEC 71 Vol. 2, Society of Chemical Industry London, 1971, pp. 1431-1439. Gu, Z.M., Reagent Advances in Solvent Extraction, Advances in Chemical Engineering (in Chinese), 1988, (Vol.2), 22-28. Lewis, J.B., Jones, I. and Pratt, H.R.C., Transactions Institution of Chemical Engineers, 1951, 29, 126. Martin, L., Vignet, P., Fombarlet, C., and Lancelot, F., Electrical Field Contactor for Solvent Extraction. Separation Science and Technology, 1983, 18 (14&15), 1455-1471. Prestridge, F.L., Johnson, B.C. and Sublette, K.L., Electrostatic Coalescence in a Solvent Extraction Process. Society of Mining Engineers of AIME Transactions, 1984, Vol. 274, pp. 1959-1962. Suyama, T., Awakura, Y., Hirato, T., Konto, M. and Majima, H., Extraction and Stripping Characteristics of Ni (II) with Di(2-ethylhexyl) Phosphoric Acid under a High Electrostatic Field. Materials Transactions, JIM, 1993, Vol. 34, No.l, pp. 37--42. Warren, K.W., Prestridge, F.R., and Sinclair, B.A., Electrostatic Separators May Supplant Mixer-Settlers. Society of Mining Engineers, 1978, Vol. 30 (4), 355-357. Yamaguchi, M., Application of Electric Fields to Solvent Extraction. In Electric Field Applications in Chromatography, Industrial and Chemical Processes, ed. T. Tsuda. VCH, Germany, 1995, pp. 185-203. Yamaguchi, M. and Kanno, M., Hydrodynamics in an Electrostatic Liquid-Liquid Contactor - Holdup, Drop Size and Drop Velocity, ISEC '96 International Solvent Extraction Conference, Melbourne, Australia, 1996, pp. 1191-1196.
C o r r e s p o n d e n c e on papers published in Minerals Engineering is invited b y e-mail to bwills @ m i n - e n g . c o m
Pergamon 0892-6875(00)00110-2
© 2000Publishedby ElsevierScienceLtd All rightsreserved 0892-6875/00/$ - see frontmatter
AN ELECTROSTATIC SOLVENT EXTRACTION CONTACTOR FOR NICKEL--COBALT RECOVERY*
M.K. BRIGGS ~, C.Y. CHENG ~* and D.C. IBANA ~ ~[ W A School o f Mines - AJ Parker CRC for Hydrometallurgy, Curtin University of Technology, PMB 22 Kalgoorlie, W A 6430, Australia § C S I R O Minerals - AJ Parker CRC for Hydrometallurgy, PO Box 90, Bentley, W A 6982, Australia * Corresponding author. Email: [email protected]
(Received I June 2000; accepted 10 July 2000)
ABSTRACT The aim of this research is to develop an Electrostatic Solvent Extraction (ESX) contactor, to obtain fundamental data on hydrodynamics and mass transfer kinetics. ESX uses electrostatic fields to enhance either coalescence or dispersion. At low field intensities, attractive forces between the aqueous droplets predominate, favouring coalescence. At high field intensities, the aqueous drops prolate in the direction of the electrostatic field resulting in disintegration into smaller droplets. This results in large surface areas enhancing mass transfer. The application of ESX in the solvent extraction of nickel and cobalt is being studied. 7he effect of field intensity on the hold-up of the dispersed phase and its mean drop size has been investigated. An increase in the applied field intensity from 0 kV/cm to 4.6 kV/cm resulted in an increase in holdup from 0.43% to 0.94% at an electrode insulation thickness of O.5 mm. An increase in the applied field intensity from 2.5 kV/cm to 4.4 kV/cm resulted in a decrease in the mean drop size. Investigation on the separation of cobalt from nickel using Cyanex 272 in Shellsol 2046 has been carried out. Cobalt extraction doubled from 44% to 88% with an increase in applied field intensity from 3.6 kV/cm to 4.0 kV/cm. This increase in mass transfer is attributed to the smaller dispersed droplets produced and their associated vibrating and turning movements at higher field intensity. © 2000 Published by Elsevier Science Ltd. All rights reserved.
Keywords Solvent extraction; non-ferrous metallic ores
INTRODUCTION The use of an electrostatic field to enhance coalescence o f aqueous droplets in organic solvents has been used for de-watering in the petroleum industry for many years (Warren et al. 1978, Bailes 1992, Yamaguchi 1995). The use o f electrostatic fields to enhance dispersion in solvent extraction contactors has been the subject of investigation since the late 1960s. Bailes and Thornton, in 1971, investigated the mass
* Presented at Hydromet 2000, Adelaide, Australia, April 2000
1281
1282
M.K. Briggset al.
transfer coefficients and droplet size measurements for charged furfural droplets in n-heptane. They observed that the applied voltage could control the droplet size. Also, mass transfer coefficients of the dispersed aqueous phase were significantly increased at higher applied voltages. This resulted from droplets exhibiting a high degree of mobility and internal circulation. Martin et al. (1983) reported that at low applied voltage, coalescence of the aqueous droplets was favoured. Conversely, at higher applied voltages, the disintegration of droplets was enhanced and thus favourable to mass transfer. They also showed that the applied voltages could easily control the hydrodynamic behaviour of the droplets. For instance, at a constant aqueous flowrate, the higher the electrostatic field, the smaller the mean drop size and the higher the hold-up. More recently, Usami e t a l (1992) reported that extraction efficiency depends on interfacial area as well as the renewal of the interface caused by internal droplet circulation. They also found that no electric charge exists on the aqueous droplet. The electric field was found to only act at the interface between phases. A major advantage of using electrostatic fields for mixing is that it allows easy production of small droplets. Therefore, large interfacial areas are produced at very low power input. This technique promotes turbulence and intense mixing at the droplet interface thus enhancing mass transfer (Bailes 1981). The aim of this research is to develop an ESX contactor, to obtain fundamental data on hydrodynamics and mass transfer kinetics, and to reduce the gap between laboratory testwork and industrial application.
EXPERIMENTAL
Experimental apparatus The contactor used in this study was made from transparent acrylic sheet shaped into a square crosssection. It was 850 mm high with an internal width of 80 mm and breadth of 56 mm. The two electrodes, located on opposite sides, were made from stainless steel sheets and coated with a thin insulation layer of HALAR®. The two electrodes were set approximately 50 mm apart. The length of each electrode was 600 mm. A schematic diagram of the experimental apparatus is shown in Figure 1. Aqueous Feed Distributor
H.T, Transformer
/Weir --J="-'~ LoadedOrganic
I J Positive Electrode Earthed Electrode Organic Feed ----Ib=
Bottom Section Raffinate Fig. 1 A schematic diagram of the ESX test rig.
Electrostatic solventextractioncontactorfor nickel-cobaltrecovery
1283
Experimental procedures The aqueous test solution was pumped to the top of the distributor using a diaphragm pump where the aqueous phase dropped through a horizontal perforated plate. The organic solution was pumped from the bottom section of the contactor. The hold-up tests were carried out using a diluent (Shellsol 2046) as the organic phase. The aqueous phase was a 0.5 M H2SO4 solution. The determination of h01d-u p involved simultaneously closing the inlet and exit flows and allowing the aqueous phase to settle. The settled dispersed phase was drained from the contactor until the original interface was achieved. The hold-up tests involved using different aqueous and organic flowrates, field intensities and electrode insulation thicknesses. The behaviour of the dispersed aqueous droplets was recorded using a digital camera. The equivalent drop size for the non-spherical drops was calculated using the following equation:
de= where dl is the major axis and d2 the minor axis of the drop images (Lewis et al. 1951). For spherical drops,the drop diameter was taken as de. The mean drop size was calculated on the basis of the number of clear drop images in the photographs and their de values. The organic solution used in the cobalt extraction study was Cyanex 272 supplied by Cytec, and Shellsol 2046 supplied by Shell Chemicals. The organic reagents were used without further purification. The nickel (3.0 g/L) and cobalt (0.3 g/L) solutions were prepared by dissolving their sulphate salts in a 0.5 M acetic acid/acetate buffer solution at pH 5.5.
Sampling and assay Samples of the raffinate and loaded organic were taken at regular time intervals. Samples for the majority of the cobalt solvent extraction tests were taken at 0, 2, 5, 10, 20, 30, 60 minutes. These samples were then taken for assay using an inductively couple plasma emission spectrometer (ICP-AES).
RESULTS AND DISCUSSION
Hold-up The hold-up is the volume of the dispersed aqueous phase contained in the total phase volume, expressed as a percentage. The effect of aqueous and organic flowrates, field intensity, and the thickness of electrode insulation on the hold-up were investigated. To determine the effect of aqueous flowrate on hold-up, several tests were carried out at constant organic flowrate and constant electrode insulation thickness but with varied field intensities. The results, summarised in Figure 2, clearly show that increases in the aqueous flowrate resulted in increases in the hold-up. For example, at no field intensity (0 kV/cm), an increase in aqueous flowrate from 3.6 L/hour to 10.8 L/hour resulted in an increase in hold-up from 0.2% to 0.67%. Similar results were obtained at higher comparable field intensities (3.6, 4.0, 4.4 kV/cm) where an increase in aqueous flowrate from 3.6 L/hour to 10.8 L/hour resulted in an increase in hold-up. The effect of field intensity on hold-up was less marked. For instance, as shown in Figure 2, at 1.0 mm electrode insulation thickness, an increase in field intensity from 0 to 4.4 kV/cm only resulted in an increase in hold-up from 0.67% to 0.8% at an aqueous flowrate of 10.8 L/hour. A similar trend was observed at lower aqueous flowrates (3.6, 7.2 L/hour) over the same field intensities.
1284
M.K. Briggsetal.
The effect of the thickness of the electrode insulation on hold-up was investigated using two different electrode insulation thicknesses (0.5, 1.0 mm). The results, summarised in Figure 3, show that the thickness of electrode insulation had a much more significant effect on the hold-up than the applied field intensity. For instance, when the thickness of the electrode insulation was halved from 1.0 to 0.5 mm, the hold-up increased from 0.5% to 0.82% at a field intensity of 4.4 kV/cm. This occurred because the actual electric field intensity in the liquid is lower when thicker electrode insulation is used.
1.0 0.9 0.8 0.7 A
0.6
Aqueous 3.6 L/hr
0.5
--II--Aqueous 7.2 L/hr
0.4
~Aqueous
0.3
10.8 L/hr
- ~V-----~
0.2 0.1 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Applied Field Intensity (kV/cm)
Fig.2
The effect of aqueous flowrate on hold-up (organic flowrate of 3.6 L/hr and an electrode insulation thickness of 1.0 mm). 1.0 0.9 0.8 0.7 o.
0.6 •-e--0.5 mm --.11--1.0 mm
0.5 1
~0.4T
j
0.2 0.1 0.0 0.0
Fig.3
Mean
0.5
1.0
1.5 2.0 2.5 3.0 3.5 4.0 Applied Field Intensity (kVlcm)
4.5
5.0
The effect of the thickness of electrode insulation on hold-up (A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hour)).
d r o p size
Increases in field intensity from 0 kV/cm to 2.4 kV/cm resulted in increases in the mean drop size (Figure 4). Further increases in field intensity however, resulted in decreases in mean drop size. The initial mean drop size was about 1.6 mm. As the field intensity increased to 2.4 kV/cm the mean drop size increased to 2.2 mm. This indicated that low field intensities (<2.4 kV/cm) result in increases in polarity of the aqueous droplets, thus favouring coalescence. At higher field intensities (>2.4 kV/cm), the stresses imposed on the droplets were sufficient to break the droplets into smaller droplets. At a field intensity of 4.4 kV/cm, the mean drop size decreased to less than 1.2 mm.
Electrostatic solventextractioncontactorfor nickel-cobaltrecovery
1285
These results are consistent with those obtained by Martin et al. (1983). They observed that, the higher the field intensity, the smaller the mean drop size at a constant aqueous flowrate. They also observed that at constant field intensity, the higher the aqueous flowrate, the larger the mean drop size. As the number of droplets per unit volume increases with the higher aqueous flowrate, the droplets are closer and thus dipole forces are more intense favouring less dispersion and larger droplets. 2,4. 2.2 2.0 1.8
1.6 j~ 1.4
~
1.2 1.0. 0.8.
0,6
0.4 0.2 0.0 0.0
Fig.4
0.5
1.0
1.5 2.0 2.5 3.0 3.5 Applied'Field Intensity (kVlcm)
4.0
4.5
5.0
The effect of field intensity on mean drop size (A/O flowrate ratio of 4:3 (aqueous 7.2 L/hour to organic 5.4 L/hour), electrode insulation thickness of 1.0 mm).
Solvent extraction of cobalt In the extraction of cobalt ions from an aqueous solution using bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) as the extractant, the overall reaction is as follows: 2R2POOH + Co 2÷ = Co (R2POOH) + 2H +
(2)
where R = 2,4,4-trimethylpentyl. Effective nickel--cobalt separation requires a pH around 5.5. If the above reaction is to proceed to the right, the H ÷ ions entering the aqueous phase must be removed. This was achieved in the testwork by buffering the aqueous phase at pH 5.5. Figure 5 shows the effect of field intensity on cobalt extraction using 5% Cyanex 272 in Shellsol 2046 at field intensities of 3.6 kV/cm and 4.0 kV/cm. The slope of the curves is a measure of the rate of cobalt extraction. The cobalt extraction at steady state increased with the increase in field intensity (with all other test conditions remaining constant). Cobalt extraction doubled from 44% to 88% as the field intensity increased from 3.6 kV/cm to 4.0 kV/cm. After 20-30 minutes into the test, the amount of cobalt extraction reached its maximum value. This remained almost constant until the end of the test. The rate of cobalt extraction at a field intensity of 4.0 kV/cm was much higher than at a field intensity of 3.6 kV/cm particularly in the first 10 minutes of the test. Many investigators including Bailes and Thornton (1971) and Yamaguchi and Kanno (1996) have studied the effects of electric fields on the rate of mass transfer in a liquid-liquid system. These investigators have suggested that an increase in field intensity results in a significant increase in mass transfer coefficients. The enhanced mass transfer is attributed to the smaller aqueous droplets produced and their vigorous vibrating and turning movement at higher field intensity. The effect of the A/O flowrate ratio on cobalt extraction was investigated. Figure 6 shows results using 8% Cyanex 272 at a field intensity of 3.6 kV/cm. At an A/O flowrate ratio of 2:1 (7.2:3.6 L/hr) cobalt extraction was almost 90%. However, when the A/O flowrate ratio was reduced to 1:1 (3.6:3.6 L/hr) cobalt extraction increased to 98%. In both tests, trace amounts (1-2%) of nickel was co-extracted with cobalt.
1286
M.K. Briggset al.
The reduction of the aqueous flowrate resulted in an increase in cobalt extraction. At a lower aqueous flowrate (3.6 L/hour), the mean drop size is smaller and the droplets are not as close to each other. Also, the number of aqueous droplets per unit volume is less and therefore there is relatively more organic solution available for extraction per unit volume.
100
90
n
m
8o 70 C
2
ttl
6o 50 40 30 20
¢
~ f - a P
"
3.6 kV/cm
- - I I - - 4.0 kV/cm
A v
0 0
10
20
30
40
50
60
70
Time (mine) Fig. 5
The effect of field intensity on cobalt extraction (5% Cyanex 272, A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hr), 23 °C, pH 5.5).
The rate of cobalt extraction for the first 5 minutes of both tests is high. However, further cobalt extraction only slightly increased with time as shown by the slope of the curves. This showed that maximum cobalt extraction could be reached in a short period of time.
100 80
80 Z r0 60
-,-tl.--A/O 1:1 (3.6:3.6)
10
0 0
10
20
30
40
Time (rains)
50
60
70
Fig. 6 The effect of A/O flowrate ratio on cobalt extraction (8% Cyanex 272, 3.6 kV/cm, 23 °C, pH 5.5). The effect of Cyanex 272 concentration on the cobalt extraction was also investigated. Figure 7 shows that when the Cyanex 272 concentration was increased from 5% to 8% at a field intensity of 3.2 kV/cm, cobalt extraction increased from 50% to 74%. This occurred using an A/O flowrate ratio of 2:1 (7,2:3.6 L/hour). A direct relationship exists between Cyanex 272 concentration and cobalt extraction under these conditions.
Electrostatic solvent extraction contactor for nickel-cobalt recovery
1287
100 90 ---.. 80 v 70 .o 60
50
$
8% Cyanex 272
40
"-
5% Cyanex 272
8 20 10
0 0
Fig.7
I
i
10
20
I
I
30 40 ~me(mins)
I
I
50
60
70
The effect of Cyanex 272 concentration on cobalt extraction (3.2 kV/cm, 23 °C, pH 5.5, A/O flowrate ratio of 2:1 (aqueous 7.2 L/hour to organic 3.6 L/hour).
CONCLUSIONS The fundamentals of ESX were investigated using a laboratory scale contactor. The effect of field intensity on the hold-up of the dispersed aqueous phase and its drop size has been studied. Investigation of the separation of cobalt from nickel using Cyanex 272 in Shellsol 2046 has commenced. The main results are as follows: 1.
An increase in the aqueous flowrate results in an increase in hold-up. As the aqueous flowrate increased from 3.6 L/hour to 10.8 L/hour, the hold-up increased from 0.21% to 0.72%, at a field intensity of 3.6 kV/cm.
2.
The mean drop size initially increased and then decreased with increasing field intensity. At an A/O flowrate ratio of 4:3 (7.2 to 5.4 L/hour) and an electrode insulation thickness of 1.0 mm, the initial mean drop size was around 1.6 mm. As the field intensity increased to 2.4 kV/cm the mean drop size increased to 2.2 mm. As the field intensity increased further to 4.4 kV/cm the mean drop size decreased to less than 1.2 mm.
3.
Cobalt extraction increased with an increase in field intensity. Using 5% Cyanex 272, cobalt extraction doubled from 44% to 88% when the field intensity was increased from 3.6 kV/cm to 4.0 kV/cm.
.
Cobalt extraction increased with an increase in Cyanex 272 concentration. When Cyanex 272 concentration was increased from 5% to 8% at a field intensity of 3.2 kV/cm, cobalt extraction increased from 50% to 74%.
.
Cobalt extraction increa~d with a decrease in aqueous flowrate. At an A/O flowrate ratio of 2:1, cobalt extraction was almost 90% after a period of 60 minutes. However, when the A/O ftowrate ratio was reduced to 1: 1, cobalt extraction increased to 98%.
1288
M.K. Briggs et al.
REFERENCES
Bailes, P.J., Electrostatic Extraction for Metals and Non-metals. In Proc. International Solvent Extraction Conference, ISEC 77. CIM Special Volume 21, 1979, pp. 233-241 Bailes, P.J., Solvent Extraction in an Electrostatic Field. In Industrial & Engineering Chemistry -- Process Design & Development. 1981, 20, 564-570. Bailes, P.J., Electrically augmented settlers and coalescers for solvent extraction. Hydrometallurgy, 1992, (30), 417 - 430. Bailes, P.J., and Larkai, S.K.L., Liquid Phase Separation in Pulsed D.C Fields. Transactions Institution oJ Chemical Engineers, 1982, Vol. 60, pp. 115-121. Bailes, P.J., and Larkai, S.K.L., The application of electric fields to phase separation in solvent extraction system. In Proceedings International Solvent Extraction Conference, ISEC 83, 1983, pp. 180-181 Bailes, P.J., and Thornton, J.D., Electrically augmented liquid-liquid extraction studies in a two-component system I - Single droplet studies. In Solvent Extraction -- Proceedings of the International Solvent Extraction Conference, ISEC 71 Vol. 2, Society of Chemical Industry London, 1971, pp. 1431-1439. Gu, Z.M., Reagent Advances in Solvent Extraction, Advances in Chemical Engineering (in Chinese), 1988, (Vol.2), 22-28. Lewis, J.B., Jones, I. and Pratt, H.R.C., Transactions Institution of Chemical Engineers, 1951, 29, 126. Martin, L., Vignet, P., Fombarlet, C., and Lancelot, F., Electrical Field Contactor for Solvent Extraction. Separation Science and Technology, 1983, 18 (14&15), 1455-1471. Prestridge, F.L., Johnson, B.C. and Sublette, K.L., Electrostatic Coalescence in a Solvent Extraction Process. Society of Mining Engineers of AIME Transactions, 1984, Vol. 274, pp. 1959-1962. Suyama, T., Awakura, Y., Hirato, T., Konto, M. and Majima, H., Extraction and Stripping Characteristics of Ni (II) with Di(2-ethylhexyl) Phosphoric Acid under a High Electrostatic Field. Materials Transactions, JIM, 1993, Vol. 34, No.l, pp. 37--42. Warren, K.W., Prestridge, F.R., and Sinclair, B.A., Electrostatic Separators May Supplant Mixer-Settlers. Society of Mining Engineers, 1978, Vol. 30 (4), 355-357. Yamaguchi, M., Application of Electric Fields to Solvent Extraction. In Electric Field Applications in Chromatography, Industrial and Chemical Processes, ed. T. Tsuda. VCH, Germany, 1995, pp. 185-203. Yamaguchi, M. and Kanno, M., Hydrodynamics in an Electrostatic Liquid-Liquid Contactor - Holdup, Drop Size and Drop Velocity, ISEC '96 International Solvent Extraction Conference, Melbourne, Australia, 1996, pp. 1191-1196.
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