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Selective extraction of nickel from ammoniacal solutions containing nickel and cobalt by emulsion liquid membrane using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant
Minerals Engineering 22 (2009) 530–536
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Selective extraction of nickel from ammoniacal solutions containing nickel and cobalt by emulsion liquid membrane using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant R.A. Kumbasar * Department of Chemistry, Faculty of Science, Sakarya University, 54187 Adapazari, Turkey
a r t i c l e
i n f o
Article history: Received 26 April 2008 Accepted 16 July 2008 Available online 5 November 2008 Keywords: Hydrometallurgy Solvent extraction Nickel Cobalt Liquid membranes
a b s t r a c t The selective extraction and concentration of nickel from ammoniacal solutions containing nickel and cobalt by an emulsion liquid membrane (ELM) technique using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant has been presented. ELM consists of a diluent (kerosene), a surfactant (Span 80), an extractant (DBHQ), a modifier (tributyl phosphate), and a stripping solution (very dilute sulfuric acid solution containing EDTA as complexing agent, buffered at pH 4.25). Cobalt (II) in feed solution with 6 mol/L ammonia was oxidised to Cobalt (III) by H2O2 and pH of this ammoniacal solution was adjusted to 10.0 with the addition of hydrochloric acid (HCl). The important variables governing the permeation of nickel and their effect on the separation process have been studied. These variables were membrane composition, ammonia concentration in the feed solution, mixing speed, surfactant concentration, extractant concentration, pH of the feed and the stripping solutions, complexing agent concentration in the stripping solution, and phase ratio. After the optimum conditions had been determined, it was possible to selectively extract 99% of nickel from the ammoniacal solutions containing Ni and Co. The separation factors of nickel with respect to cobalt, based on initial feed concentration, have experimentally found to be of as high as 88.1 for about equimolar Co–Ni feed solutions. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The separation of cobalt from nickel in aqueous solution has always been a problem in hydrometallurgy. Their adjacent positions in the transition metal series in the periodic table results in aqueous somewhat similar chemical behavior but differences do exist. For example, although both cobalt and nickel preferentially exist as divalent hexahydrated ions in dilute aqueous solution, the rate of water exchange on the cobalt ion is very much higher than for nickel. Thus complex ion formation often proceeds much more readily with divalent cobalt than with nickel. On the other hand, the trivalent cobalt ion is much less labile and forms in preference to nickel even though the redox potentials for the Co2+/Co3+ and Ni2+/ Ni3+ couples are nearly identical. Nickel occurs within both laterite and sulphide ores and it is also associated with deep-sea nodules (Moskalyk and Alfantazi, 2002). The processing of laterites by hydrometallurgical techniques involved an ammoniacal leaching step to bring nickel into solution as its ammine complexes. The presence of other metals in the solution leads to solvent extraction as a technique to extract
* Tel.: +90 264 295 6054; fax: +90 264 295 5950. E-mail address: [email protected] 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.07.007
and separate nickel from these ammoniacal solutions (Alguacil and Cobo, 1998a, b). Alternative methods for the separation and recovery of cobalt and nickel from such solutions include solvent extraction, which is attractive for the possibilities that it offers in the separation of complex metallic solutions. There are two types of possible operation: selective extraction or co-extraction and selective stripping, and sometimes the latter processing option may be more economical as it requires fewer stages. The extraction of cobalt and nickel has attracted much interest in ammoniacal media using oximes and a-diketones (Szymanowski, 1993; Sole and Cole, 2002; Price and Reid, 1993) as well as others. To date there are numerous published works on cobalt and nickel separation by liquid–liquid systems (Preston, 1982; Dreisinger and Cooper, 1986; Komasawa et al., 1983) and supported liquid membranes (Matsuyama et al., 1987; Youn et al., 1997). However, research activities concerning application of ELMs to the separation of Co and Ni from their mixed sulphate solutions (Strzelbicki and Charewicz, 1980; Kasaini et al., 1998) and ammoniacal solutions are relatively few (Price and Reid, 1993; Kane and Cardwell, 1997). If an excess of ammonia is added to the feed solution containing Co2+ and Ni2+ ions, the complex ions called hexammine cobalt (II) 2þ and hexammine nickel (II) ions, CoðNH3 Þ2þ 6 and NiðNH3 Þ6 , respectively, are formed, as given by Eqs. (1) and (2)
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Co2þ þ 6NH3 ¢ CoðNH3 Þ2þ 6 2þ
Ni
þ
ð1Þ
6NH3 ¢ NiðNH3 Þ2þ 6
ð2Þ
In order to increase the selectivity of nickel towards cobalt, 3þ CoðNH3 Þ2þ 6 ions had to be oxidised to CoðNH3 Þ6 ions with an addition of hydrogen peroxide solution to the feed solution (Sole and ions do not give reaction with the Cole, 2002), since CoðNH3 Þ3þ 6 ions are not oxiextractant DBHQ. On the other hand, NiðNH3 Þ2þ 6 dised in the same reaction, and thus extracting nickel ammine complexes from ammoniacal solution with acidic extractant (DBHQ) proceed (Sole and Cole, 2002; Mackenzie and Virnig, 1998; Gu et al., 1992), as given by Eq. (3)
NiðNH3 Þ2þ n ðaqÞ þ 2ðDBHQ ÞðorgÞ ¢ ðDBQÞ2 NiðorgÞ þ ðn 2ÞNH3ðaqÞ þ 2NHþ4 ðaqÞ
ðn ¼ 4 or 6Þ
ð3Þ
where DBHQ represents acidic extractant 5,7-dibromo-8-hydroxyquinoline and the subscripts aq and org denote the aqueous feed and the organic membrane solutions, respectively. The reaction is an equilibrium reaction and can be driven in either direction depending on the concentrations of the species involved. Nickel can be effectively stripped by a suitable aqueous acid, for example very dilute H2SO4, as follows (Sole and Cole, 2002; Mackenzie and Virnig, 1998; Gu et al., 1992), as given by Eq. (4) 2þ
2 ðDBQÞ2 NiðorgÞ þ 2HþðaqÞ þ SO2 4 ðaqÞ ¢ NiðaqÞ þ 2ðDBHQÞðorgÞ þ SO4 ðaqÞ
ð4Þ In the present work, in order to have a better understanding of the dynamics of ELM technique, the major parameters influencing the extent of nickel extraction were studied and the selective extraction and concentration of nickel were investigated from the ammoniacal solutions, containing Ni2+ and Co2+ ions, using 5,7-dibromo-8hydroxyquinoline (DBHQ) as extractant. 2. Emulsion liquid membrane process Emulsion liquid membranes (ELMs) for metal extraction are made by forming a water-in-oil (W/O) emulsion, stabilized by a surfactant, the (W/O) emulsion contains the metal extractant in the membrane solution and the stripping reagent in the stripping solution. This emulsion is then dispersed by a relatively low agitation into the feed solution containing metal ions to be separated. After extraction, the loaded emulsion is separated from the feed solution; demulsification yields a membrane solution that can be recycled. The extraction chemistry involved in liquid membrane transport is essentially the same as that in solvent extraction, but the overall transport is governed by kinetic rather than equilibrium parameters (Pellegrino and Noble, 1990). The solvent extraction process, however, requires larger volumes of solvent inventory and equipment, and thus higher investment costs and becomes inefficient when the metal ion concentration in the effluent stream is low.
aminetetraacetic acid (EDTA) was used as the disodium salt and was purchased from Merck, Germany. Cobalt and nickel stock solutions were prepared from reagent grade cobalt (II) sulphate (CoSO4 7H2O) and reagent grade nickel (II) sulphate (NiSO4 6H2O). Ammonia, hydrochloric acid, sulfuric acid, acetic acid, and sodium hydroxide were of A.R. grades (Merck, Germany) that were used directly as received from the manufacturer. On the other hand, commercial kerosene is a complex mixture of aliphatic origin and also contains aromatics about 15% w/w. 3.2. Experimental method Initial ammonia concentration in the feed solution, contained cobalt and nickel ions between 100 and 500 mg/L (0.0017– 0.0085 mol/L Co or Ni), has been prepared as desired, and as a result hexammine cobalt complex, has formed, CoðNH3 Þ2þ 6 , as given by Eqs. (1) and (2). CoðNH3 Þ2þ 6 ions in the ammoniacal feed solution had to be oxidised to CoðNH3 Þ3þ 6 ions by adding a 2 mL 30% hydrogen peroxide solution (0.0194 mol/L H2O2), and thus CoðNH3 Þ3þ 6 ions would not react with the carrier DBHQ. Under these condi2þ tions, however, NiðNH3 Þ6 ions remain unoxidised, and as a result NiðNH3 Þ2þ 6 ions alone react with DBHQ. DBHQ (oxine) is a versatile chelating agent that forms a complex with many metal ions. These complexes dissolve readily in organic solvents such as kerosene, chloroform, benzene, carbon tetrachloride, and other organic solvents. The reagent has a nitrogen atom in the heterocyclic ring. A phenolic –OH group adjacent to nitrogen makes it suitable for chelate formation with the metal ion (Fig. 1). The nitrogen gets protonated at lower pH (acidic solution) and the phenolic –OH dissociates in the alkaline solution. Therefore, DBHQ is soluble in alkalis and acids. On account of this, it has low solubility in the aqueous solution around neutral pH. Although the complex is insoluble in the aqueous phase, it has sufficient solubility in kerosene so that it could be successfully used in the present study as extractant for the extraction of nickel across ELM. After oxidation process, pH of the ammoniacal feed solution was reduced the desired pH value with the addition of HCl. No detrimental effect was observed on an oxidation of DBHQ, by the addition of H2O2, since the oxidation of Co (II)–Co (III) had taken place in the feed phase before the emulsion phase was added to the feed solution and the highest concentration of H2O2 was not much higher than that of its stoichiometric ratio. Experimental conditions in emulsion liquid membrane experiments are given in Table 1. Unless otherwise stated, experimental conditions as in Table 1. Very dilute sulfuric acid solution containing 0.01mol/L EDTA as complexing agent, buffered at pH 4.25 using NaOH–acetic acid, was used as the stripping solution. The stripping solution (25 mL) was added dropwise to the membrane solution (25 mL), stirred at 1800 rpm for 20 min and passed through a burette in about 10 min. The prepared W/O emulsions were added to a 600 mL-baffled beaker which contains a 250 mL the ammoniacal feed solution to be extracted. The two-phase system was stirred by a variable speed mixer equipped with a turbine-type Teflon impeller. pH of the stripping solution was adjusted as desired using
3. Experimental
Br
3.1. Reagents The liquid membrane consists of a surfactant, an extractant, a modifier, and a diluent. The non-ionic surfactant used for stabilizing the emulsion is sorbitan monooleate which is a product of Fluka and commercially known as Span 80. The extractant is 5,7-dibromo-8-hydroxyquinoline (DBHQ) which is purchased from Merck, Germany. The modifier is tributyl phosphate (TBP) which is purchased from Daihachi Chemicals, Japan. A commercial kerosene (TUPRAS Oil Company, Turkey) were used as diluents. Ethylenedi-
N
Br OH
Fig. 1. 5,7-Dibromo-8-hydroxyquinoline.
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Table 1 Experimental conditions in ELM experiments Parameters
Optimum value
NH3 concentration in the feed solution Feed solution pH Stirring speed Surfactant (Span 80) concentration Extractant (DBHQ) concentration Modifier (TBP) concentration Diluent (kerosene) Stripping solution pH EDTA concentration in the stripping phase Phase ratio (v/v) Treatment ratio (v/v)
7M 10 400 rpm 6% w/w 4% w/w 6% w/w 84% w/w 4.25 0.01 M 5/5 50/250
NaOH–acetic acid buffer. The uptake of metal ions was monitored by removing samples of the feed solution periodically for analysis. Atomic absorption spectrophotometry (Shimadzu AA-6701F model, Tokyo, Japan) is used for metals’ (Co, Ni) determinations. Hydrogen ion concentrations were measured by pH meter with a glass electrode. To determine the important variables governing the permeation and extraction of nickel from cobalt, ammonia concentration of the feed solution, mixing speed, surfactant concentration, extractant concentration, modifier concentration, pH of the feed and the stripping solutions, complexing agent concentration in the stripping solution, and phase ratio were varied to observe their effect on nickel extraction and concentration. At the end of each run, the emulsion was recovered and subsequently broken into its constituent organic and ammoniacal solutions using a high voltage splitter with niobium electrodes. All the extraction experiments were carried out batchwise at the ambient temperature of 20 ± 1°C. All aqueous solutions were prepared using deionised water. 4. Result and discussion 4.1. Effect of ammonia concentration in the feed solution In order to investigate the effect of ammonia concentration in the feed solution, the ammonia concentrations were varied from 4 mol/L to 8 mol/L, as indicated in Fig. 2. As is observed from Fig. 2, the extent of nickel extraction slightly increases with increasing ammonia concentration from 4 mol/L to 7 mol/L. However, it slightly decreases after the 7 mol/L ammonia concentration. In fact,
the nickel ammine complex is affected by Eqs. (2)–(4). Eqs. (3) and (4) are the extraction and stripping reactions which occur simultaneously, and as ammonia concentration increases, the nickel ammine complex concentration increases and thus concentration also increases. Unlike solvent extraction, in a liquid membrane process Eq. (3) is not an equilibrium reaction and therefore the reaction shifts to the right due to the fact that extraction and stripping reactions take place simultaneously. On the other hand, an increase in ammonia concentration has also an adverse effect on reaction (3) and thus it may cause to drive the reaction (3) to the left. Eventually both effects balance one another and the extent of Ni extraction remains to be steady even though ammonia concentration increases, as shown in Fig. 2. 4.2. Effect of pH in the feed solution pH of the feed solution with 6 mol/L ammonia was adjusted to the desired pH with the addition of HCl. Fig. 3 shows the pH effect on the percentage of nickel extraction. As can be observed from this figure, nickel extraction increases up to pH 10 and then decreases. This decrease can be explained with increasing solubility of extractant DBHQ in the aqueous feed solution at these pH vales. Because, the distribution of oxine between kerosene and water is pH dependent, that is, oxine is completely soluble in kerosene at neutral pH conditions, whereas its solubility in kerosene sharply decreases both at high and low pHs. In view of this fact, it possibly causes a reduction in the extent of nickel extraction. 4.3. Effect of surfactant concentration The effect of surfactant concentration on the behavior of metal extraction by ELM is very important. The effect of surfactant concentration on nickel extraction is exhibited in Fig. 4. At the lowest concentration of surfactant, after the extraction process, both extraction efficiency and the final volume of emulsion decreased. This indicates that emulsion stability is strongly dependent on the surfactant concentration. The emulsion stability was improved with the increase in surfactant concentration. Fig. 4 exhibits that increasing concentration of surfactant from 4% to 8% (w/v) increases the stability of the liquid membrane which leads to the increase in the extraction efficiency, hence the extraction degree of nickel was also increased. However, at 10% Span 80 the extraction decreased again. Excessive surfactant tends
100
100
80
Extraction Extent (%)
Extraction Extent (%)
80
60
(Extraction time: 10 min) 40
20
60
(Extraction time: 10 min)
40
20
0
0 3
4
5
6
7
8
9
NH3 concentration, M Fig. 2. Effect of ammonia concentration in the feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
8.5
9
9.5
10
10.5
11
11.5
Feed solution pH Fig. 3. Effect of pH in the feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
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enough for forward extraction. Further increase in extractant concentration leads to the decrease in the stripping reaction rate. This is because nickel remains in nickel–DBHQ form in the membrane phase without getting stripped which in turn affected the final recovery by the emulsion liquid membrane process. Therefore, extractant concentration of 6% is optimum concentration. Thus, DBHQ merely should act as a shuttle to carry nickel ions from one side of the membrane to the other side. Since extractant is the most expensive agent among the other components of the liquid membrane system, its lower concentration is always preferred.
100
Extraction Extent (%)
80
Surfactant concentration
60
4 % SPAN 80 6 % SPAN 80
40
8 % SPAN 80 10 % SPAN 80 20
4.5. Effect of modifier (TBP) concentration 0 0
2
4
6
8
10
Time, minute Fig. 4. Effect of surfactant concentration on the extraction rate of Ni (DBHQ: 4%; TBP: 6%; kerosene: 80–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
to increase the resistance at the interface and this can be attributed to the increase in viscosity of the organic phase (Othman et al., 2006). Therefore, increasing surfactant concentration from 8% to 10% increased emulsion stability but the mass transfer was adversely reduced. This indicates that internal diffusion of nickel complex with DBHQ is further affected by viscosity of the membrane phase. A similar effect was observed by Reis and Carvalho (1993) and Valenzuela et al. (2005) in the recovery of zinc and copper ions. They found that the use of an unnecessarily high content of surfactant produces lower metal extraction due to the generation of higher interfacial resistance. Hence, surfactant concentration of 8% was chosen as the best concentration. 4.4. Effect of extractant concentration
4.6. Effect of mixing speed of feed solution The effect of mixing speed was studied in the range of 300– 800 rpm and is shown in Fig. 7. In ELM process, fine emulsion droplets form globules in the feed solution. An increase in mixing speed would increase interfacial area and the rate of extraction. The bigger globules may disintegrate to form smaller globules, thereby resulting in an increase in area, however further increase in speed is likely to break the emulsion droplets, thereby reducing overall enrichment and extraction (Kulkarni et al., 2000). It was observed that increasing the speed of mixing from 300 rpm to 750 rpm increased the rate of extraction. This is due to an increase in the interfacial area in the aqueous feed solution between the emulsion globules and solution, but further increase in speed of agitation from 750 rpm to 800 rpm resulted in a decrease in the extent of extraction, which was contrary to the general expectation (Itoh et al., 1990). This was attributed to the hydrodynamic instability of the emulsion at the higher speed. Thus at speed of 800 rpm the emulsion solution was broken and its final volume decreased
100
100
80
80
Extraction Extent (%)
Extraction Extent (%)
The extraction of nickel is enhanced by increasing the concentration of DBHQ from 2% to 6%, while the emulsion stability does not affect and further increase of extractant concentration decreased the extraction performance very little (Fig. 5). It is due to the access of free extractant in membrane phase. Under the present extractant concentration, the free extractant is considered to be
The effect of the modifier concentration studied was in the range of 0–8% as shown in Fig. 6. It was observed that increasing the modifier concentration from 0% to 6% increased the rate of extraction but further increase in the modifier concentration to 8% resulted in reduction in the degree of extraction from 84.8% to 84.2%. Thus, modifier concentration of 6% was maintained throughout the subsequent investigations and was chosen as optimum modifier concentration
60
Extractant concentration 40
2% 4% 6% 8%
20
DBHQ DBHQ DBHQ DBHQ
60
40
Modifier concentration 0% 2% 4% 6% 8%
20
0
TBP TBP TBP TBP TBP
0
0
2
4
6
8
10
Time, minute Fig. 5. Effect of extractant concentration on the extraction rate of Ni (SPAN 80: 6%; TBP: 6%; kerosene: 80–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
0
2
4
6
8
10
Time, minute Fig. 6. Effect of modifier concentration on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; kerosene: 82–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
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to 5.25 pH of the stripping solution decreases, and then decrease again. This decrease can be explained by the distribution of oxine between kerosene and the aqueous solution or the feed solution. Because, oxine solubility in kerosene decreases at low pHs. From Fig. 8, it was found that the stripping solution pH of 5.25 gives the highest extraction efficiency of nickel, therefore, the stripping solution pH of 5.25 is the most efficient for stripping purpose in the ELM system.
100
Extraction Extent (%)
80
60
Time, minute 2 6 10
40
4.8. Effect of EDTA concentration in stripping solution
20
0 300
400
500
600
700
800
Mixing speed, rpm Fig. 7. Effect of mixing speed of feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid).
after 10 min. This breakage can be explained as follows: as the speed of agitation increased, the co-transport of water was also increased. The emulsion solution swells, becomes highly unstable and breaks into the feed solution. The other reason is that there is an increase in shear on the emulsion solution with the increase in mixing speed. This also causes the breakage of the emulsion and finally results in the gradual reduction in extraction and enrichment efficiency as the speed is increased further (Kulkarni et al., 2000). Hence, the speed of 750 rpm was chosen as optimum mixing speed. 4.7. Effect of pH of stripping solution The stripping reaction, given by Eq. (4), plays a vital role in the extraction of metal ions from feed side to stripping side in ELM process. The effect of pH in the stripping solution on the nickel extraction was investigated in Fig. 8. The pH of stripping solution was adjusted using acetic acid–NaOH buffer. From Fig. 8, the results show that the rate of Ni extraction increases as from 5.75
ELM extraction studies reveal that the presence of EDTA as complexing agent in the stripping solution assists for stripping the extracted nickel. The stripping efficiency of EDTA in the stripping solution was studied in the range 0–0.075 mol/L, as shown in Fig. 9. The figure clearly indicates that with an increase in the concentration of EDTA, the extraction percentage almost remains constant. However, when no EDTA was added, the Ni extraction rate decreased. 4.9. Effect of phase ratio The volume ratios of the stripping solution to the membrane solution were varied between 3/5 and 6/5. The results given in Fig. 10 show the effect of variation of this ratio on the stability of the emulsion and the efficiency of the extraction. From Fig. 10, it is evident that the decrease from 5/5 to 3/5 of volume ratio of the stripping solution to the membrane solution leads to an increase in the stability of the emulsion and a decrease in the efficiency of the extraction. Nevertheless Fig. 10, the highest volume ratio of 6/5 was found to give the lowest emulsion stability leading to a decrease in the extraction efficiency. The results may be explained on the basis that increasing the stripping solution volume makes the emulsion unstable and leads to a leakage of the stripping solution into the feed solution. Hence, in order to obtain a uniform and homogeneous distribution of the membrane solution droplets in the membrane space and to avoid the influence of the stripping solution on the emulsion stability, the optimum ratio of the stripping solution to the membrane was taken to be 5/5. 4.10. Optimum conditions The optimum conditions were experimentally determined and are shown in Table 2. At the optimum conditions, the extraction
100 100
80 60
Extraction Extent (%)
Extraction Extent (%)
80
pH of stripping soultion pH = 4.25 pH = 4.75 pH = 5.25 pH = 5.50 pH = 5.75
40
s
20
60
EDTA concentration 40
0,00 M 0,01 M 0,02 M 0,03 M
20 0
0
2
4
6
8
10
Time, minute
0 0
2
4
6
8
10
Time, minute Fig. 8. Effect of pH in stripping solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
Fig. 9. Effect of phase ratio on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
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Table 3 Separation factors of Ni over Co for various feed mixtures in the optimum conditions
100
Feed mixture
Extraction into strip phase, % Ni
Co
100 mg/L Ni + 100 mg/L Co 300 mg/L Ni + 300 mg/L Co 500 mg/L Ni + 500 mg/L Co
99 99 97
2.3 1.8 1.1
Extraction Extent (%)
80
60
Phase ratio (Vs /Vm) 3/5 4/5 5/5 6/5
40
0 2
4
8
6
49.5 55.1 88.1
(99.0 ± 0.31)%, (99.0 ± 0.25)%, and (97.0 ± 0.21)%, for the three feed mixtures of 100 mg/L Co + 100 mg/L Ni, 300 mg/L Co + 300 mg/L Ni, and 500 mg/L Co + 500 mg/L Ni within 10 min, respectively. On the other hand, for the same feed mixtures the percentages of Co extraction into the stripping solution were found to be of (2.3 ± 0.54)%, (1.8 ± 0.44)%, and (1.1 ± 0.32)%, respectively.
20
0
bNi/Co, 10 min
10
Time, minute Fig. 10. Effect of complexing agent (EDTA) concentration on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
4.11. Membrane selectivity The separation factors of nickel over cobalt, bNiCo/Ni, for the feed mixtures at the optimum conditions, are given by Eq. (5), based on initial feed concentration
bNi=Co ¼
Table 2 Optimum conditions Parameters
Optimum value
NH3 concentration in the feed solution Feed solution pH Stirring speed Surfactant (Span 80) concentration Extractant (DBHQ) concentration Modifier (TBP) concentration Diluent (kerosene) Stripping solution pH EDTA concentration in the stripping phase Phase ratio (v/v) Treatment ratio (v/v)
7M 10 750 rpm 8% w/w 6% w/w 6% w/w 80% w/w 5.25 0.01 M 5/5 50/250
ðCNi =CCo Þstrip ðCNi =CCo Þfeed;0
ð5Þ
where CNi and CCo are the concentrations of Ni and Co in stripping and initial feed solutions. Under the optimum conditions for the three feed mixtures of about equimolar composition, the separation factors of nickel over cobalt, bNi=Co are indicated in Table 3. After 10 min, stripping solution contains 980 mg/L Ni and 21 mg/L Co for the equimolar feed phase of 100 mg/L each, 2930 mg/L Ni and 37 mg/L Co for the equimolar feed phase of 300 mg/L each, and 4800 mg/L Ni and 52 mg/L Co for the equimolar feed phase of 500 mg/L each. Co and Ni in these stripping solutions could be further separated selectively by introducing more stages into the process using liquid membranes or solvent extraction. 5. Conclusions
100
An emulsion liquid membrane process using 5,7-dibromo-8hydroxyquinoline (DBHQ) to extract and concentrate nickel from ammoniacal solution has been investigated and the optimum conditions have been determined experimentally as given in Table 2. From this study the following conclusions can be drawn:
80
Extraction Extent (%)
Initial Ni/Co ratio of feed solution (100 mg/L Ni/100 mg/L Co) Ni (300 mg/L Ni/300 mg/L Co) Ni (500 mg/L Ni/500 mg/L Co) Ni (100 mg/L Ni/100 mg/L Co) Co (300 mg/L Ni/300 mg/L Co) Co (500 mg/L Ni/500 mg/L Co) Co
60
40
20
0 0
5
10
15
20
25
30
Time, minute Fig. 11. Effect of optimum conditions on the extraction rate of Ni (SPAN 80: 8%; DBHQ: 6%; TBP: 6%; kerosene: 80%; stripping solution: 25 mL very dilute sulfuric acid solution).
rates of nickel and cobalt for the three feed mixtures of about equimolar compositions are given in Fig. 11. As seen from Fig. 11, the percentages of Ni extraction into the stripping solution are
1. DBHQ can be successfully used as an extractant for selective extraction and concentration of nickel from cobalt in ammoniacal solutions by emulsion liquid membrane process. 2. DBHQ in combination with TBP showed a remarkable acceleration in nickel extraction by emulsion liquid membrane process. 3. The extraction of cobalt ions can be minimized provided that 3þ CoðNH3 Þ2þ 6 ions are oxidised to CoðNH3 Þ6 ions with an addition of hydrogen peroxide solution to the ammoniacal feed solution 4. The extraction was maximum when the feed solution was maintained at pH 10 and very dilute sulfuric acid solution, buffered at pH 5.25 and containing EDTA as a complexing agent, was used as a stripping solution in the receiving phase. 5. At the optimum conditions, the extraction of nickel has been achieved with an efficiency of about 97–99% from Co–Ni mixtures of about equimolar composition within 10 min. 6. At the optimum conditions, for the three feed mixtures of about equimolar composition, the separation factors of nickel over cobalt, bNi=Co based on the initial feed composition, are found to be of 49.5, 55.1, and 88.1 for the equimolar feed mixtures given, in 10 min.
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Acknowledgements The author wish to express his sincere gratitude to Sakarya University that supported this work, and Mehmet Yilmaz, coordinator of BAPK in Sakarya University. References Alguacil, F.J., Cobo, A., 1998a. Separation of copper and nickel from ammoniacal/ ammonium carbonate solutions using ACORGA PT5050. Sep. Sci. Technol. 33, 2257–2260. Alguacil, F.J., Cobo, A., 1998b. Solvent extraction equilibrium of nickel with LIX 54. Hydrometallurgy 48, 291–299. Dreisinger, D.B., Cooper, W.C., 1986. The kinetics of cobalt and nickel extraction using HEHEHP. Solv. Extr. Ion Exch. 4, 317–344. Gu, Z., Winston Ho, W.S., Li, N.N., 1992. Design considerations. In: Winston Ho, W.S., Sirkar, K.K. (Eds.), Membrane Handbook. Chapman Hall, New York, pp. 656–700. Itoh, H., Thien, M.P., Hatton, T.A., Wang, D.I.C., 1990. A liquid emulsion membrane process for the separation of amino acids. Biotech. Bioeng. 35, 853–860. Kane, W.S., Cardwell, P.H., 1997. Separation of cobalt from nickel in ammoniacal solutions using 8-hydroxyquinolines. US Patent 4 012 483. Kasaini, H., Nakashio, F., Goto, M., 1998. Application of emulsion liquid membranes to recover cobalt ions from a dual-component sulphate solution containing nickel ions. J. Membr. Sci. 146, 159–168. Komasawa, I., Otake, T., Higaki, T., 1983. Separation of cobalt and nickel with ethylhexyl phosphonic acid mono-2-ethylhexyl ester. J. Chem. Eng. Jpn. 16, 210–217. Kulkarni, P.S., Tiwari, K.K., Mahajani, V.V., 2000. Membrane stability and enrichment of nickel in the liquid emulsion membrane process. J. Chem. Technol. Biotechnol. 75, 553–560. Mackenzie, J.M.W, Virnig, M.J., 1998. Extraction of nickel from ammoniacal leach solutions: extractant and solution chemistry issues. In: Proceedings of ALTA
1998 Nickel/Cobalt Pressure Leaching and Hydrometallurgy Forum. ALTA Metallurgical Services Melbourne, Perth, Australia, May 1998. Matsuyama, H., Katayama, Y., Kojima, A., Washijima, I., Miyake, Y., Teramoto, M., 1987. Permeation rate and selectivity in the separation of cobalt and nickel by supported liquid membrane. J. Chem. Eng. Jpn. 20, 213–220. Moskalyk, R.R., Alfantazi, A.M., 2002. Nickel laterite processing and electrowinning practice. Mine. Eng. 15, 593–605. Othman, N., Mat, H., Goto, M., 2006. Separation of silver from photographic wastes by emulsion liquid membrane system. J. Membr. Sci. 282, 171–177. Pellegrino, J.J., Noble, R.D., 1990. Enhanced transport and liquid membranes in bioseparations. Trends Biotechnol. 8, 216–224. Preston, J.S., 1982. Solvent extraction of cobalt and nickel by organophosphorus acids: I. Comparison of phosphoric, phosphonic and phosphinic acid systems. Hydrometallurgy 9, 115–133. Price, M.J., Reid, J.G., 1993. Separation and recovery of nickel and cobalt in ammoniacal systems-process development. In: Logsdail, D.H., Slater, M.J. (Eds.), Solvent Extraction in the Process Industries (Proc. ISEC’93), vol. 1. SCI/Elsevier, London. p. 159. Reis, M.T.A., Carvalho, J.M.R., 1993. Recovery of zinc from an industrial effluent by emulsion liquid membranes. J. Membr. Sci. 84, 201–211. Sole, K.C., Cole, M.P., 2002. Purification of nickel by solvent extraction. In: Marcus, Y., Sengupta, A.K. (Eds.), Ion Exchange and Solvent Extraction Series Advances, vol. 15. Dekker, New York. p. 154. Strzelbicki, J., Charewicz, W., 1980. Liquid surfactant membrane separation of copper, cobalt and nickel from multicomponent aqueous solutions. Hydrometallurgy 5, 243–254. Szymanowski, J., 1993. Hydroxyoximes and Copper Hydrometallurgy. CRC Press, Boca Raton. Valenzuela, F., Fonseca, C., Basualto, C., Correa, O., Tapia, C., Sapag, J., 2005. Removal of copper ions from a waste mine water by a liquid emulsion membrane method. Mine. Eng. 18, 33–40. Youn, I.J., Lee, Y.T., Jeong, J., Lee, W.H., 1997. Analysis of Co–Ni separation by a supported liquid membrane containing HEH (EHP). J. Membr. Sci. 125, 231– 236.
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Selective extraction of nickel from ammoniacal solutions containing nickel and cobalt by emulsion liquid membrane using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant R.A. Kumbasar * Department of Chemistry, Faculty of Science, Sakarya University, 54187 Adapazari, Turkey
a r t i c l e
i n f o
Article history: Received 26 April 2008 Accepted 16 July 2008 Available online 5 November 2008 Keywords: Hydrometallurgy Solvent extraction Nickel Cobalt Liquid membranes
a b s t r a c t The selective extraction and concentration of nickel from ammoniacal solutions containing nickel and cobalt by an emulsion liquid membrane (ELM) technique using 5,7-dibromo-8-hydroxyquinoline (DBHQ) as extractant has been presented. ELM consists of a diluent (kerosene), a surfactant (Span 80), an extractant (DBHQ), a modifier (tributyl phosphate), and a stripping solution (very dilute sulfuric acid solution containing EDTA as complexing agent, buffered at pH 4.25). Cobalt (II) in feed solution with 6 mol/L ammonia was oxidised to Cobalt (III) by H2O2 and pH of this ammoniacal solution was adjusted to 10.0 with the addition of hydrochloric acid (HCl). The important variables governing the permeation of nickel and their effect on the separation process have been studied. These variables were membrane composition, ammonia concentration in the feed solution, mixing speed, surfactant concentration, extractant concentration, pH of the feed and the stripping solutions, complexing agent concentration in the stripping solution, and phase ratio. After the optimum conditions had been determined, it was possible to selectively extract 99% of nickel from the ammoniacal solutions containing Ni and Co. The separation factors of nickel with respect to cobalt, based on initial feed concentration, have experimentally found to be of as high as 88.1 for about equimolar Co–Ni feed solutions. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The separation of cobalt from nickel in aqueous solution has always been a problem in hydrometallurgy. Their adjacent positions in the transition metal series in the periodic table results in aqueous somewhat similar chemical behavior but differences do exist. For example, although both cobalt and nickel preferentially exist as divalent hexahydrated ions in dilute aqueous solution, the rate of water exchange on the cobalt ion is very much higher than for nickel. Thus complex ion formation often proceeds much more readily with divalent cobalt than with nickel. On the other hand, the trivalent cobalt ion is much less labile and forms in preference to nickel even though the redox potentials for the Co2+/Co3+ and Ni2+/ Ni3+ couples are nearly identical. Nickel occurs within both laterite and sulphide ores and it is also associated with deep-sea nodules (Moskalyk and Alfantazi, 2002). The processing of laterites by hydrometallurgical techniques involved an ammoniacal leaching step to bring nickel into solution as its ammine complexes. The presence of other metals in the solution leads to solvent extraction as a technique to extract
* Tel.: +90 264 295 6054; fax: +90 264 295 5950. E-mail address: [email protected] 0892-6875/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2008.07.007
and separate nickel from these ammoniacal solutions (Alguacil and Cobo, 1998a, b). Alternative methods for the separation and recovery of cobalt and nickel from such solutions include solvent extraction, which is attractive for the possibilities that it offers in the separation of complex metallic solutions. There are two types of possible operation: selective extraction or co-extraction and selective stripping, and sometimes the latter processing option may be more economical as it requires fewer stages. The extraction of cobalt and nickel has attracted much interest in ammoniacal media using oximes and a-diketones (Szymanowski, 1993; Sole and Cole, 2002; Price and Reid, 1993) as well as others. To date there are numerous published works on cobalt and nickel separation by liquid–liquid systems (Preston, 1982; Dreisinger and Cooper, 1986; Komasawa et al., 1983) and supported liquid membranes (Matsuyama et al., 1987; Youn et al., 1997). However, research activities concerning application of ELMs to the separation of Co and Ni from their mixed sulphate solutions (Strzelbicki and Charewicz, 1980; Kasaini et al., 1998) and ammoniacal solutions are relatively few (Price and Reid, 1993; Kane and Cardwell, 1997). If an excess of ammonia is added to the feed solution containing Co2+ and Ni2+ ions, the complex ions called hexammine cobalt (II) 2þ and hexammine nickel (II) ions, CoðNH3 Þ2þ 6 and NiðNH3 Þ6 , respectively, are formed, as given by Eqs. (1) and (2)
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Co2þ þ 6NH3 ¢ CoðNH3 Þ2þ 6 2þ
Ni
þ
ð1Þ
6NH3 ¢ NiðNH3 Þ2þ 6
ð2Þ
In order to increase the selectivity of nickel towards cobalt, 3þ CoðNH3 Þ2þ 6 ions had to be oxidised to CoðNH3 Þ6 ions with an addition of hydrogen peroxide solution to the feed solution (Sole and ions do not give reaction with the Cole, 2002), since CoðNH3 Þ3þ 6 ions are not oxiextractant DBHQ. On the other hand, NiðNH3 Þ2þ 6 dised in the same reaction, and thus extracting nickel ammine complexes from ammoniacal solution with acidic extractant (DBHQ) proceed (Sole and Cole, 2002; Mackenzie and Virnig, 1998; Gu et al., 1992), as given by Eq. (3)
NiðNH3 Þ2þ n ðaqÞ þ 2ðDBHQ ÞðorgÞ ¢ ðDBQÞ2 NiðorgÞ þ ðn 2ÞNH3ðaqÞ þ 2NHþ4 ðaqÞ
ðn ¼ 4 or 6Þ
ð3Þ
where DBHQ represents acidic extractant 5,7-dibromo-8-hydroxyquinoline and the subscripts aq and org denote the aqueous feed and the organic membrane solutions, respectively. The reaction is an equilibrium reaction and can be driven in either direction depending on the concentrations of the species involved. Nickel can be effectively stripped by a suitable aqueous acid, for example very dilute H2SO4, as follows (Sole and Cole, 2002; Mackenzie and Virnig, 1998; Gu et al., 1992), as given by Eq. (4) 2þ
2 ðDBQÞ2 NiðorgÞ þ 2HþðaqÞ þ SO2 4 ðaqÞ ¢ NiðaqÞ þ 2ðDBHQÞðorgÞ þ SO4 ðaqÞ
ð4Þ In the present work, in order to have a better understanding of the dynamics of ELM technique, the major parameters influencing the extent of nickel extraction were studied and the selective extraction and concentration of nickel were investigated from the ammoniacal solutions, containing Ni2+ and Co2+ ions, using 5,7-dibromo-8hydroxyquinoline (DBHQ) as extractant. 2. Emulsion liquid membrane process Emulsion liquid membranes (ELMs) for metal extraction are made by forming a water-in-oil (W/O) emulsion, stabilized by a surfactant, the (W/O) emulsion contains the metal extractant in the membrane solution and the stripping reagent in the stripping solution. This emulsion is then dispersed by a relatively low agitation into the feed solution containing metal ions to be separated. After extraction, the loaded emulsion is separated from the feed solution; demulsification yields a membrane solution that can be recycled. The extraction chemistry involved in liquid membrane transport is essentially the same as that in solvent extraction, but the overall transport is governed by kinetic rather than equilibrium parameters (Pellegrino and Noble, 1990). The solvent extraction process, however, requires larger volumes of solvent inventory and equipment, and thus higher investment costs and becomes inefficient when the metal ion concentration in the effluent stream is low.
aminetetraacetic acid (EDTA) was used as the disodium salt and was purchased from Merck, Germany. Cobalt and nickel stock solutions were prepared from reagent grade cobalt (II) sulphate (CoSO4 7H2O) and reagent grade nickel (II) sulphate (NiSO4 6H2O). Ammonia, hydrochloric acid, sulfuric acid, acetic acid, and sodium hydroxide were of A.R. grades (Merck, Germany) that were used directly as received from the manufacturer. On the other hand, commercial kerosene is a complex mixture of aliphatic origin and also contains aromatics about 15% w/w. 3.2. Experimental method Initial ammonia concentration in the feed solution, contained cobalt and nickel ions between 100 and 500 mg/L (0.0017– 0.0085 mol/L Co or Ni), has been prepared as desired, and as a result hexammine cobalt complex, has formed, CoðNH3 Þ2þ 6 , as given by Eqs. (1) and (2). CoðNH3 Þ2þ 6 ions in the ammoniacal feed solution had to be oxidised to CoðNH3 Þ3þ 6 ions by adding a 2 mL 30% hydrogen peroxide solution (0.0194 mol/L H2O2), and thus CoðNH3 Þ3þ 6 ions would not react with the carrier DBHQ. Under these condi2þ tions, however, NiðNH3 Þ6 ions remain unoxidised, and as a result NiðNH3 Þ2þ 6 ions alone react with DBHQ. DBHQ (oxine) is a versatile chelating agent that forms a complex with many metal ions. These complexes dissolve readily in organic solvents such as kerosene, chloroform, benzene, carbon tetrachloride, and other organic solvents. The reagent has a nitrogen atom in the heterocyclic ring. A phenolic –OH group adjacent to nitrogen makes it suitable for chelate formation with the metal ion (Fig. 1). The nitrogen gets protonated at lower pH (acidic solution) and the phenolic –OH dissociates in the alkaline solution. Therefore, DBHQ is soluble in alkalis and acids. On account of this, it has low solubility in the aqueous solution around neutral pH. Although the complex is insoluble in the aqueous phase, it has sufficient solubility in kerosene so that it could be successfully used in the present study as extractant for the extraction of nickel across ELM. After oxidation process, pH of the ammoniacal feed solution was reduced the desired pH value with the addition of HCl. No detrimental effect was observed on an oxidation of DBHQ, by the addition of H2O2, since the oxidation of Co (II)–Co (III) had taken place in the feed phase before the emulsion phase was added to the feed solution and the highest concentration of H2O2 was not much higher than that of its stoichiometric ratio. Experimental conditions in emulsion liquid membrane experiments are given in Table 1. Unless otherwise stated, experimental conditions as in Table 1. Very dilute sulfuric acid solution containing 0.01mol/L EDTA as complexing agent, buffered at pH 4.25 using NaOH–acetic acid, was used as the stripping solution. The stripping solution (25 mL) was added dropwise to the membrane solution (25 mL), stirred at 1800 rpm for 20 min and passed through a burette in about 10 min. The prepared W/O emulsions were added to a 600 mL-baffled beaker which contains a 250 mL the ammoniacal feed solution to be extracted. The two-phase system was stirred by a variable speed mixer equipped with a turbine-type Teflon impeller. pH of the stripping solution was adjusted as desired using
3. Experimental
Br
3.1. Reagents The liquid membrane consists of a surfactant, an extractant, a modifier, and a diluent. The non-ionic surfactant used for stabilizing the emulsion is sorbitan monooleate which is a product of Fluka and commercially known as Span 80. The extractant is 5,7-dibromo-8-hydroxyquinoline (DBHQ) which is purchased from Merck, Germany. The modifier is tributyl phosphate (TBP) which is purchased from Daihachi Chemicals, Japan. A commercial kerosene (TUPRAS Oil Company, Turkey) were used as diluents. Ethylenedi-
N
Br OH
Fig. 1. 5,7-Dibromo-8-hydroxyquinoline.
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Table 1 Experimental conditions in ELM experiments Parameters
Optimum value
NH3 concentration in the feed solution Feed solution pH Stirring speed Surfactant (Span 80) concentration Extractant (DBHQ) concentration Modifier (TBP) concentration Diluent (kerosene) Stripping solution pH EDTA concentration in the stripping phase Phase ratio (v/v) Treatment ratio (v/v)
7M 10 400 rpm 6% w/w 4% w/w 6% w/w 84% w/w 4.25 0.01 M 5/5 50/250
NaOH–acetic acid buffer. The uptake of metal ions was monitored by removing samples of the feed solution periodically for analysis. Atomic absorption spectrophotometry (Shimadzu AA-6701F model, Tokyo, Japan) is used for metals’ (Co, Ni) determinations. Hydrogen ion concentrations were measured by pH meter with a glass electrode. To determine the important variables governing the permeation and extraction of nickel from cobalt, ammonia concentration of the feed solution, mixing speed, surfactant concentration, extractant concentration, modifier concentration, pH of the feed and the stripping solutions, complexing agent concentration in the stripping solution, and phase ratio were varied to observe their effect on nickel extraction and concentration. At the end of each run, the emulsion was recovered and subsequently broken into its constituent organic and ammoniacal solutions using a high voltage splitter with niobium electrodes. All the extraction experiments were carried out batchwise at the ambient temperature of 20 ± 1°C. All aqueous solutions were prepared using deionised water. 4. Result and discussion 4.1. Effect of ammonia concentration in the feed solution In order to investigate the effect of ammonia concentration in the feed solution, the ammonia concentrations were varied from 4 mol/L to 8 mol/L, as indicated in Fig. 2. As is observed from Fig. 2, the extent of nickel extraction slightly increases with increasing ammonia concentration from 4 mol/L to 7 mol/L. However, it slightly decreases after the 7 mol/L ammonia concentration. In fact,
the nickel ammine complex is affected by Eqs. (2)–(4). Eqs. (3) and (4) are the extraction and stripping reactions which occur simultaneously, and as ammonia concentration increases, the nickel ammine complex concentration increases and thus concentration also increases. Unlike solvent extraction, in a liquid membrane process Eq. (3) is not an equilibrium reaction and therefore the reaction shifts to the right due to the fact that extraction and stripping reactions take place simultaneously. On the other hand, an increase in ammonia concentration has also an adverse effect on reaction (3) and thus it may cause to drive the reaction (3) to the left. Eventually both effects balance one another and the extent of Ni extraction remains to be steady even though ammonia concentration increases, as shown in Fig. 2. 4.2. Effect of pH in the feed solution pH of the feed solution with 6 mol/L ammonia was adjusted to the desired pH with the addition of HCl. Fig. 3 shows the pH effect on the percentage of nickel extraction. As can be observed from this figure, nickel extraction increases up to pH 10 and then decreases. This decrease can be explained with increasing solubility of extractant DBHQ in the aqueous feed solution at these pH vales. Because, the distribution of oxine between kerosene and water is pH dependent, that is, oxine is completely soluble in kerosene at neutral pH conditions, whereas its solubility in kerosene sharply decreases both at high and low pHs. In view of this fact, it possibly causes a reduction in the extent of nickel extraction. 4.3. Effect of surfactant concentration The effect of surfactant concentration on the behavior of metal extraction by ELM is very important. The effect of surfactant concentration on nickel extraction is exhibited in Fig. 4. At the lowest concentration of surfactant, after the extraction process, both extraction efficiency and the final volume of emulsion decreased. This indicates that emulsion stability is strongly dependent on the surfactant concentration. The emulsion stability was improved with the increase in surfactant concentration. Fig. 4 exhibits that increasing concentration of surfactant from 4% to 8% (w/v) increases the stability of the liquid membrane which leads to the increase in the extraction efficiency, hence the extraction degree of nickel was also increased. However, at 10% Span 80 the extraction decreased again. Excessive surfactant tends
100
100
80
Extraction Extent (%)
Extraction Extent (%)
80
60
(Extraction time: 10 min) 40
20
60
(Extraction time: 10 min)
40
20
0
0 3
4
5
6
7
8
9
NH3 concentration, M Fig. 2. Effect of ammonia concentration in the feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
8.5
9
9.5
10
10.5
11
11.5
Feed solution pH Fig. 3. Effect of pH in the feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
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enough for forward extraction. Further increase in extractant concentration leads to the decrease in the stripping reaction rate. This is because nickel remains in nickel–DBHQ form in the membrane phase without getting stripped which in turn affected the final recovery by the emulsion liquid membrane process. Therefore, extractant concentration of 6% is optimum concentration. Thus, DBHQ merely should act as a shuttle to carry nickel ions from one side of the membrane to the other side. Since extractant is the most expensive agent among the other components of the liquid membrane system, its lower concentration is always preferred.
100
Extraction Extent (%)
80
Surfactant concentration
60
4 % SPAN 80 6 % SPAN 80
40
8 % SPAN 80 10 % SPAN 80 20
4.5. Effect of modifier (TBP) concentration 0 0
2
4
6
8
10
Time, minute Fig. 4. Effect of surfactant concentration on the extraction rate of Ni (DBHQ: 4%; TBP: 6%; kerosene: 80–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
to increase the resistance at the interface and this can be attributed to the increase in viscosity of the organic phase (Othman et al., 2006). Therefore, increasing surfactant concentration from 8% to 10% increased emulsion stability but the mass transfer was adversely reduced. This indicates that internal diffusion of nickel complex with DBHQ is further affected by viscosity of the membrane phase. A similar effect was observed by Reis and Carvalho (1993) and Valenzuela et al. (2005) in the recovery of zinc and copper ions. They found that the use of an unnecessarily high content of surfactant produces lower metal extraction due to the generation of higher interfacial resistance. Hence, surfactant concentration of 8% was chosen as the best concentration. 4.4. Effect of extractant concentration
4.6. Effect of mixing speed of feed solution The effect of mixing speed was studied in the range of 300– 800 rpm and is shown in Fig. 7. In ELM process, fine emulsion droplets form globules in the feed solution. An increase in mixing speed would increase interfacial area and the rate of extraction. The bigger globules may disintegrate to form smaller globules, thereby resulting in an increase in area, however further increase in speed is likely to break the emulsion droplets, thereby reducing overall enrichment and extraction (Kulkarni et al., 2000). It was observed that increasing the speed of mixing from 300 rpm to 750 rpm increased the rate of extraction. This is due to an increase in the interfacial area in the aqueous feed solution between the emulsion globules and solution, but further increase in speed of agitation from 750 rpm to 800 rpm resulted in a decrease in the extent of extraction, which was contrary to the general expectation (Itoh et al., 1990). This was attributed to the hydrodynamic instability of the emulsion at the higher speed. Thus at speed of 800 rpm the emulsion solution was broken and its final volume decreased
100
100
80
80
Extraction Extent (%)
Extraction Extent (%)
The extraction of nickel is enhanced by increasing the concentration of DBHQ from 2% to 6%, while the emulsion stability does not affect and further increase of extractant concentration decreased the extraction performance very little (Fig. 5). It is due to the access of free extractant in membrane phase. Under the present extractant concentration, the free extractant is considered to be
The effect of the modifier concentration studied was in the range of 0–8% as shown in Fig. 6. It was observed that increasing the modifier concentration from 0% to 6% increased the rate of extraction but further increase in the modifier concentration to 8% resulted in reduction in the degree of extraction from 84.8% to 84.2%. Thus, modifier concentration of 6% was maintained throughout the subsequent investigations and was chosen as optimum modifier concentration
60
Extractant concentration 40
2% 4% 6% 8%
20
DBHQ DBHQ DBHQ DBHQ
60
40
Modifier concentration 0% 2% 4% 6% 8%
20
0
TBP TBP TBP TBP TBP
0
0
2
4
6
8
10
Time, minute Fig. 5. Effect of extractant concentration on the extraction rate of Ni (SPAN 80: 6%; TBP: 6%; kerosene: 80–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
0
2
4
6
8
10
Time, minute Fig. 6. Effect of modifier concentration on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; kerosene: 82–86%; stripping solution: 25 mL very dilute sulfuric acid solution).
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R.A. Kumbasar / Minerals Engineering 22 (2009) 530–536
to 5.25 pH of the stripping solution decreases, and then decrease again. This decrease can be explained by the distribution of oxine between kerosene and the aqueous solution or the feed solution. Because, oxine solubility in kerosene decreases at low pHs. From Fig. 8, it was found that the stripping solution pH of 5.25 gives the highest extraction efficiency of nickel, therefore, the stripping solution pH of 5.25 is the most efficient for stripping purpose in the ELM system.
100
Extraction Extent (%)
80
60
Time, minute 2 6 10
40
4.8. Effect of EDTA concentration in stripping solution
20
0 300
400
500
600
700
800
Mixing speed, rpm Fig. 7. Effect of mixing speed of feed solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid).
after 10 min. This breakage can be explained as follows: as the speed of agitation increased, the co-transport of water was also increased. The emulsion solution swells, becomes highly unstable and breaks into the feed solution. The other reason is that there is an increase in shear on the emulsion solution with the increase in mixing speed. This also causes the breakage of the emulsion and finally results in the gradual reduction in extraction and enrichment efficiency as the speed is increased further (Kulkarni et al., 2000). Hence, the speed of 750 rpm was chosen as optimum mixing speed. 4.7. Effect of pH of stripping solution The stripping reaction, given by Eq. (4), plays a vital role in the extraction of metal ions from feed side to stripping side in ELM process. The effect of pH in the stripping solution on the nickel extraction was investigated in Fig. 8. The pH of stripping solution was adjusted using acetic acid–NaOH buffer. From Fig. 8, the results show that the rate of Ni extraction increases as from 5.75
ELM extraction studies reveal that the presence of EDTA as complexing agent in the stripping solution assists for stripping the extracted nickel. The stripping efficiency of EDTA in the stripping solution was studied in the range 0–0.075 mol/L, as shown in Fig. 9. The figure clearly indicates that with an increase in the concentration of EDTA, the extraction percentage almost remains constant. However, when no EDTA was added, the Ni extraction rate decreased. 4.9. Effect of phase ratio The volume ratios of the stripping solution to the membrane solution were varied between 3/5 and 6/5. The results given in Fig. 10 show the effect of variation of this ratio on the stability of the emulsion and the efficiency of the extraction. From Fig. 10, it is evident that the decrease from 5/5 to 3/5 of volume ratio of the stripping solution to the membrane solution leads to an increase in the stability of the emulsion and a decrease in the efficiency of the extraction. Nevertheless Fig. 10, the highest volume ratio of 6/5 was found to give the lowest emulsion stability leading to a decrease in the extraction efficiency. The results may be explained on the basis that increasing the stripping solution volume makes the emulsion unstable and leads to a leakage of the stripping solution into the feed solution. Hence, in order to obtain a uniform and homogeneous distribution of the membrane solution droplets in the membrane space and to avoid the influence of the stripping solution on the emulsion stability, the optimum ratio of the stripping solution to the membrane was taken to be 5/5. 4.10. Optimum conditions The optimum conditions were experimentally determined and are shown in Table 2. At the optimum conditions, the extraction
100 100
80 60
Extraction Extent (%)
Extraction Extent (%)
80
pH of stripping soultion pH = 4.25 pH = 4.75 pH = 5.25 pH = 5.50 pH = 5.75
40
s
20
60
EDTA concentration 40
0,00 M 0,01 M 0,02 M 0,03 M
20 0
0
2
4
6
8
10
Time, minute
0 0
2
4
6
8
10
Time, minute Fig. 8. Effect of pH in stripping solution on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
Fig. 9. Effect of phase ratio on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
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R.A. Kumbasar / Minerals Engineering 22 (2009) 530–536
Table 3 Separation factors of Ni over Co for various feed mixtures in the optimum conditions
100
Feed mixture
Extraction into strip phase, % Ni
Co
100 mg/L Ni + 100 mg/L Co 300 mg/L Ni + 300 mg/L Co 500 mg/L Ni + 500 mg/L Co
99 99 97
2.3 1.8 1.1
Extraction Extent (%)
80
60
Phase ratio (Vs /Vm) 3/5 4/5 5/5 6/5
40
0 2
4
8
6
49.5 55.1 88.1
(99.0 ± 0.31)%, (99.0 ± 0.25)%, and (97.0 ± 0.21)%, for the three feed mixtures of 100 mg/L Co + 100 mg/L Ni, 300 mg/L Co + 300 mg/L Ni, and 500 mg/L Co + 500 mg/L Ni within 10 min, respectively. On the other hand, for the same feed mixtures the percentages of Co extraction into the stripping solution were found to be of (2.3 ± 0.54)%, (1.8 ± 0.44)%, and (1.1 ± 0.32)%, respectively.
20
0
bNi/Co, 10 min
10
Time, minute Fig. 10. Effect of complexing agent (EDTA) concentration on the extraction rate of Ni (SPAN 80: 6%; DBHQ: 4%; TBP: 6%; kerosene: 84%; stripping solution: 25 mL very dilute sulfuric acid solution).
4.11. Membrane selectivity The separation factors of nickel over cobalt, bNiCo/Ni, for the feed mixtures at the optimum conditions, are given by Eq. (5), based on initial feed concentration
bNi=Co ¼
Table 2 Optimum conditions Parameters
Optimum value
NH3 concentration in the feed solution Feed solution pH Stirring speed Surfactant (Span 80) concentration Extractant (DBHQ) concentration Modifier (TBP) concentration Diluent (kerosene) Stripping solution pH EDTA concentration in the stripping phase Phase ratio (v/v) Treatment ratio (v/v)
7M 10 750 rpm 8% w/w 6% w/w 6% w/w 80% w/w 5.25 0.01 M 5/5 50/250
ðCNi =CCo Þstrip ðCNi =CCo Þfeed;0
ð5Þ
where CNi and CCo are the concentrations of Ni and Co in stripping and initial feed solutions. Under the optimum conditions for the three feed mixtures of about equimolar composition, the separation factors of nickel over cobalt, bNi=Co are indicated in Table 3. After 10 min, stripping solution contains 980 mg/L Ni and 21 mg/L Co for the equimolar feed phase of 100 mg/L each, 2930 mg/L Ni and 37 mg/L Co for the equimolar feed phase of 300 mg/L each, and 4800 mg/L Ni and 52 mg/L Co for the equimolar feed phase of 500 mg/L each. Co and Ni in these stripping solutions could be further separated selectively by introducing more stages into the process using liquid membranes or solvent extraction. 5. Conclusions
100
An emulsion liquid membrane process using 5,7-dibromo-8hydroxyquinoline (DBHQ) to extract and concentrate nickel from ammoniacal solution has been investigated and the optimum conditions have been determined experimentally as given in Table 2. From this study the following conclusions can be drawn:
80
Extraction Extent (%)
Initial Ni/Co ratio of feed solution (100 mg/L Ni/100 mg/L Co) Ni (300 mg/L Ni/300 mg/L Co) Ni (500 mg/L Ni/500 mg/L Co) Ni (100 mg/L Ni/100 mg/L Co) Co (300 mg/L Ni/300 mg/L Co) Co (500 mg/L Ni/500 mg/L Co) Co
60
40
20
0 0
5
10
15
20
25
30
Time, minute Fig. 11. Effect of optimum conditions on the extraction rate of Ni (SPAN 80: 8%; DBHQ: 6%; TBP: 6%; kerosene: 80%; stripping solution: 25 mL very dilute sulfuric acid solution).
rates of nickel and cobalt for the three feed mixtures of about equimolar compositions are given in Fig. 11. As seen from Fig. 11, the percentages of Ni extraction into the stripping solution are
1. DBHQ can be successfully used as an extractant for selective extraction and concentration of nickel from cobalt in ammoniacal solutions by emulsion liquid membrane process. 2. DBHQ in combination with TBP showed a remarkable acceleration in nickel extraction by emulsion liquid membrane process. 3. The extraction of cobalt ions can be minimized provided that 3þ CoðNH3 Þ2þ 6 ions are oxidised to CoðNH3 Þ6 ions with an addition of hydrogen peroxide solution to the ammoniacal feed solution 4. The extraction was maximum when the feed solution was maintained at pH 10 and very dilute sulfuric acid solution, buffered at pH 5.25 and containing EDTA as a complexing agent, was used as a stripping solution in the receiving phase. 5. At the optimum conditions, the extraction of nickel has been achieved with an efficiency of about 97–99% from Co–Ni mixtures of about equimolar composition within 10 min. 6. At the optimum conditions, for the three feed mixtures of about equimolar composition, the separation factors of nickel over cobalt, bNi=Co based on the initial feed composition, are found to be of 49.5, 55.1, and 88.1 for the equimolar feed mixtures given, in 10 min.
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