- Email: [email protected]
Purification of zinc sulfate solutions from chloride using extraction with mixtures of a trialkyl phosphine oxide and organophosphorus acids
Hydrometallurgy 169 (2017) 585–588
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
Purification of zinc sulfate solutions from chloride using extraction with mixtures of a trialkyl phosphine oxide and organophosphorus acids
MARK
I.Y. Fleitlikha, N.A. Grigorievaa,⁎, L.K. Nikiforovaa, O.A. Logutenkob a Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center “Krasnoyarsk Science Center SB RAS”, Academgorodok, 50/24, 660036 Krasnoyarsk, Russia b Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Kutateladze, 18, 630128 Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Zinc sulfate solutions Chloride removal Solvent extraction Trialkyl phosphine oxide CYANEX 272
Studies on the extractive purification of zinc sulfate solutions from chloride using mixtures of trialkyl phosphine oxide (TRPO, CYANEX 923) and organophosphorus acids (HR) as extractants have been carried out. The acids used were di(2-ethylhexyl)phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272). Chloride is extracted as the compound ZnCl2 ∙2TRPO. The most efficient extraction system for chloride removal was shown to be the TRPO + CYANEX 272 + tributyl phosphate mixture in kerosene. The main process parameters that provided for high chloride removal (~97%) were determined. Chloride stripping from the zinc extract is carried out with alkaline solutions (NaOH). The separation of chloride and zinc occurs at a final pH value of the strip liquors of 6–7 under which conditions chloride almost completely pass into the aqueous phase, while zinc remains in the organic phase forming zinc dialkylphosphinate. Thus the mixtures of TRPO with CYANEX 272 advantageously differ from the extraction systems containing TRPO alone wherefrom acceptable chloride stripping is not possible. Zinc stripping from the organic phase is carried out with a sulfuric acid solution.
1. Introduction High chloride concentrations in zinc electrolytes lead to significant acceleration of electrode wear, a decrease in the output current, and corrosion of equipment. Therefore, the solutions must be purified from chloride until a concentration of not > 150 mg/L is reached. Of the precipitative methods, purification from the halide by precipitation as silver chloride is most effective. However, because of its high cost, this method is not widely used. The most common purification method is based on chloride precipitation as a poorly soluble copper(I) chloride (Snurnikov, 1981). The disadvantages of this method include the length of time required for the CuCl precipitation (5–6 h) and the use of thickening and filtration operations which makes these processes inefficient. Among electrochemical methods for chloride removal, an electrodialysis method for purification of the mixed solutions ZnCl2 + ZnSO4 from chloride using membranes is of interest (Chmielarz and Gnot, 2001). It was shown that the initial chloride concentration can be lowered to 0.09 g/L using one-stage electrodialysis treatment. Commercial implementation of this method is difficult, apparently, due to the complicated hardware design and the relatively high cost of the
⁎
Corresponding author. E-mail address: [email protected] (N.A. Grigorieva).
http://dx.doi.org/10.1016/j.hydromet.2017.04.004 Received 22 February 2016; Received in revised form 13 March 2017; Accepted 2 April 2017 Available online 03 April 2017 0304-386X/ © 2017 Elsevier B.V. All rights reserved.
membranes. According to the method proposed by Liu et al. (2016) purification of zinc solutions from chloride was carried out by chloride oxidation to chlorine gas; ozone was used as an oxidizer. This method has a number of disadvantages, among which is its low productivity. The use of anion exchange resins allowed removal of chloride from zinc electrolyte reaching a concentration of ≤ 150 mg/L (Pimenov et al., 1977). The disadvantages of this method include the length of time required for sorption (1 − 2h) and, most importantly, obtaining diluted (with regard to the chloride) eluates (1–2 g/L). The best results for chloride removal from sulfate solutions were obtained when using extraction methods. Mainly, halide extraction with tertiary organic amines (R3N) has been studied (Kopanev et al., 1989; Kuhn et al., 2003). Trialkylamine of a С7–С9 fraction (Kopanev et al., 1989) or Alamine 336 (Kuhn et al., 2003) were used as extractants. In all cases extraction was satisfactory and chloride stripping occurred when using alkaline reagents NaOH (Kopanev et al., 1989) or NH4OH (Kuhn et al., 2003). The disadvantages of the use of amines include the high extraction of sulfuric acid which converts the extractant to the sulfate (R3NH)2SO4 or bisulfate (R3NH) HSO4 forms (Kuhn et al., 2003). Apart from that, an increase in the
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
method based on the degree of opalescence of silver chloride when silver reacts with the chloride.
sulfuric acid concentration in the solution resulted in a decrease in the chloride extraction, which was due to the competitive reaction proceeding between bisulfate and chloride for the extractant (Kuhn et al., 2003). In addition to organic amines, neutral organophosphorus extractants, in particular, tributyl phosphate (TBP) can be used for chloride removal (Sergievskiy and Fedyanina, 1983). A significant disadvantage of this method is the need to introduce a large amount of sulfuric acid (200–500 g/L) into the zinc solution prior extraction which is virtually impossible in the existing zinc production processes. Very good results for purification of zinc solutions from chloride have been obtained when using trialkyl phosphine oxide as the extractant (Mason et al., 2003). Together with efficient chloride removal, its extraction did not depend on pH over a wide range of H2SO4 concentrations and the degree of extraction of the acid itself was low (ε < 16%), which provides an advantageous difference for the TRPO systems compared to tertiary amine systems (Kuhn et al., 2003). However chloride stripping with water is extremely difficult (Fleitlikh et al., 2015). An attempt to strip zinc chloride (and thus chloride) with alkaline solutions (NaOH, Na2CO3) was unsuccessful due to the formation of zinc carbonate or hydroxide precipitates. At the same time, the formation of emulsions, which are almost unbreakable, took place which completely disrupts the technological process (Mason et al., 2003). The aim of this work was to search for new efficient extraction systems based on TRPO and development of a process for chloride removal from zinc sulfate solutions, comprising both its efficient extraction and its efficient stripping. Commercially available reagents, mixtures of TRPO with organophosphorus acids (HR), such as di(2ethylhexyl)phosphoric acid and bis(2,4,4-trimethylpentyl)phosphinic acid, are used as the extractants.
3. Results and discussion 3.1. Mechanisms of extraction and stripping. General considerations As previously reported by Fleitlikh et al. (2015), zinc chloride extraction with TRPO can be described by Eq. 1:
Zn2+(a) +2Cl (a) + 2TRPO(o) ↔ ZnCl2 2TRPO(o)
(1)
where (a) and (o) denote species in the aqueous and organic phases, respectively. The introduction of organic acids (HR) into the organic phase containing TRPO will result in a decrease in the extractant activity due to the partial formation of hydrogen-bonded complexes (H-complexes) (Eq. 2) and, consequently, in a decrease in the zinc chloride distribution coefficients.
(HR)2(o) + 2TRPO(o) ↔ 2TRPO⋅HR (o)
(2)
Zinc chloride extraction with the adduct formed can be described by Eq. 3:
Zn2+(a) + 2Cl (a) + 2ТRPO·HR (о) ↔ ZnCl2·2ТRPO(о) + (НR)2(o)
(3)
To strip the chloride, the zinc extract was treated with alkaline solutions (NaOH) due to which the separation of chloride and zinc occurred. The chlorides passed into the strip liquor as NaCl while zinc remained in the organic phase according to Eq. 4.
ZnCl2⋅2TRPO(o) + (НR)2(o) + 2NaOH(a) → 2TRPO(o) + ZnR2(o) + 2NaCl(a) + 2Н2 О
(4)
The subsequent treatment of the organic phase with a sulfuric acid solution resulted in zinc stripping and, at the same time, in regeneration of the extractant which was returned to the extraction cycle (Eq. 5).
2. Experimental 2.1. Reagents
ZnR2 + 2ТRPO(о) + Н2 SO4(a) ↔ 2ТRPO⋅HR (о) + ZnSO4 (a)
Trialkyl phosphine oxide of a С6–С8 fraction (TRPO, CYANEX 923) of 93% purity was used as the extractant. D2EHPA and Cyanex 272 of 85% purity were used as additives. The reagents used as modifiers were 2-ethylhexanol (TU 26624-85) and tributyl phosphate (TBP). Petroleum paraffin of a С13 fraction of not < 98% purity and dearomatized kerosene (С10Н22, TU 38-101-454-74) were used as diluents. All reagents and diluents were used without further treatment, with the exception of TBP and D2EHPA which were preliminarily washed with a solution of NaOH (20–40 g/dm3) to remove mono- and dibutylphosphoric acids. After washing, the refined product (D2EHPA) contained 88% of D2EHPA. All reagents used were products of domestic production, with the exception of the CYANEX 923 and CYANEX 272, produced by Cytec (Canada). The mineral salts, alkalis and acids used were in chemical or analytical grade.
(5)
3.2. Chloride extraction with the TRPO and organophosphorus acid (HR) mixtures Preliminary data on chloride extraction from zinc sulfate solutions containing (g/L) 77.7 Zn, 7.0 Cl¯, 0–150 H2SO4 with 50% ТRPO solution in petroleum paraffin showed that the degree of chloride extraction was very high (> 90%) and did not depend on the acidity of the aqueous phase. The chloride and zinc concentrations in the loaded organic phase were 0.178 and 0.095 mol/L, respectively, which corresponds to the Cl to Zn molar ratio of 1.87. Since the ratio is close to 2, this indicates that zinc is extracted in the form of ZnCl2. The introduction of organophosphorus acids (HR) into the organic phase containing TRPO resulted in some decrease in the chloride extraction (Table 1). This is due to the formation of hydrogen-bonded complexes (H-complexes) between TRPO and HR, in accordance with Eq. 2, which results in a decrease in the extractant activity. The formation of such complexes between trioctyl phosphine oxide and D2EHPA previously reported by Baker and Baes (1962). It is also seen from Table 1, that chloride removal with the mixtures containing D2EHPA is always lower than with those containing CYANEX 272. The latter is due to the greater stability of the H-complexes between TRPO and D2EHPA caused by the larger acid dissociation constant of D2EHPA and lesser steric hindrance upon formation of the adduct. The pKa values for organophosphorus acids are equal to 3.01 for D2EHPA and 5.22 for CYANEX 272 (Cheng et al., 2011). As seen from Fig. 1, all extraction systems are suited for purification of the zinc solutions from chloride. Chloride removal in the system containing D2EHPA (Fig. 1, curve a) is much poorer than in the system
2.2. Procedure The liquid–liquid extraction experiments were carried out according to the published procedure (Fleitlikh et al., 2015).The pH values of the aqueous phases were determined by potentiometric titration with a glass electrode. The pH meters used were pH 673 and Akvilon pH 410. The aqueous phases were analyzed for zinc and chloride. Their concentration in the organic phases was calculated from the mass balances. In several cases, the zinc-containing organic phases were completely stripped by mixing with sulfuric acid solutions. Zinc concentration in the aqueous phase was determined by atomic absorption spectrophotometry. High chloride concentrations in the solutions were determined by potentiometric titration with silver nitrate. Low chloride concentrations were determined by a phototurbidimetric 586
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
extraction with CYANEX 272 by a cation exchange mechanism. This process is undesirable but if necessary, it can be significantly inhibited by carrying out the chloride extraction under more acidic conditions. The data obtained is clearly in favor of the TRPO and CYANEX 272 mixtures. As seen from Fig. 1, the choice of modifier in these systems is very important. Replacement of a higher alcohol (2-ethylhexanol) (curve b) by tributyl phosphate (TBP) (curve c) leads to a marked increase in the chloride extraction. The improved extraction is not due to the chloride extraction with tributyl phosphate itself as TBP does not extract zinc chloride under these conditions as previously reported by Sergievskiy and Fedyanina (1983). An increase in extraction in this case, is due to an increase in the TRPO activity. Fig. 2 shows the extraction isotherms for chloride from the zinc electrolyte with the TRPO + CYANEX 272 + TBP mixture in kerosene, together with an operating line (McCabe-Thiele diagram). Since the initial concentration of chloride in the electrolyte can vary over a wide range (from 0.5 to 5.0 g/L of Cl), the operating line was drawn from the chloride concentration of 5.0 g/L in the zinc solution. The ratio of the flows of aqueous to organic phases (λ = A:O) was taken as two. As seen from the figure, under these conditions, to achieve a residual chloride concentration in the raffinate of 0.15 g/L, 3 extraction stages are required.
Table 1 Effect of organophosphorus acids on chloride extraction with TRPO. Aqueous phase, g/L: 97.7 Zn (ZnSO4), 50 H2SO4, 8.0 Cl (NaCl). Organic phase: TRPO + HR + 2-ethylhexanol in kerosene. O:A = 1:1. No.
1 2
Organic phase composition, M
1.1 M TRPO 1.1 M TRPO + 0.6 M CYANEX 272 + 0.64 M 2-ethylhexanol 1.1 M TRPO + 0.6 M D2EHPA + 0.64 M 2ethylhexanol 0.73 M TRPO + 0.6 M CYANEX 272 + 0.64 M 2ethylhexanol 0.73 M TRPO + 0.6 M D2EHPA + 0.64 M 2ethylhexanol
3
4
5
20
Chloride concentration in the phases, g/L
DCl
ε, %
Aqueous
Organic
0.5 0.58
7.5 7.42
15 12.8
93.75 92.75
0.76
7.24
9.5
90.50
1.03
6.97
6.8
87.13
1.5
6.5
4.23
81.25
CCl(org), g/L b
16
c
3.3. Chloride stripping from the TRPO and CYANEX 272 mixtures
a
Earlier, it was noted that stripping of the zinc chloride is extremely difficult in the systems with TRPO. It will be shown below that the introduction of an organophosphorus acid into the organic phase containing TRPO, in particular CYANEX 272, solves this problem quite easily. To strip the chloride, the zinc extract was treated with alkaline solutions (NaOH) which resulted in the separation of the chloride and zinc. The chlorides passed into a strip liquor as NaCl while zinc remained in the organic phase forming zinc dialkyldithiophosphinate (see Eq. 4). The data on chloride stripping with NaOH are shown in Tables 2 and 3. As seen from the tables, at a final pH value of 6–7, chloride practically completely passes into the aqueous phase while zinc remains in the organic phase. Based on the data obtained, it was proposed to carry out chloride stripping with a stoichiometric amount of sodium hydroxide with a concentration of ~3.5 mol/L to a final pH value of 6.0–7.0 in 2 stages. This gives sufficiently concentrated chloride strip liquors (~ 70 g/L).
12
8
4
CCl(aq), g/L 0 0
1
2
3
4
5
Fig. 1. Isotherms for chloride extraction from zinc sulfate solutions with the mixtures of TRPO and HR in kerosene. Organic phases: a – 0.73 M TRPO + 0.6 М D2EHPA + 0.64 М 2-ethylhexanol in kerosene; b – 0.73 M TRPO +0.6 M CYANEX 272 + 0.64 M 2ethylhexanol in kerosene; c – 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP in kerosene. Aqueous phases, g/L: a, b – 97.7 Zn, 50 H2SO4, 8.0 Cl; c – 78.9 Zn, 50 H2SO4, 0.0–16.3 Cl. a, b – O:A ≠ const; c – O:A = 1:1.
3.4. Zinc stripping from the TRPO and CYANEX 272 mixtures
CCl(org), g/L
Zinc stripping is easily carried out with sulfuric acid (according to Eq. 5). At a concentration of sulfuric acid in the stripping solution of 50–100 g/L, zinc was stripped almost completely. The results on sulfuric acid extraction with a 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP solution in kerosene showed that the degree of acid extraction was not high and did not exceed 12.1%.
λ=A:O =2:1
3.5. Schematic flowsheet for chloride extraction from zinc solutions with the TRPO and CYANEX 272 mixtures
CCl(aq), g/L The present study has been focused only on the zinc and chloride distribution in extraction systems with TRPO. As previously reported by Fleitlikh et al. (2015), during extraction with TRPO from zinc chloride solutions, such impurities as Cu, Co, Pb, and Mn are almost not extracted while Fe(III) is extracted only to a small extent. The results of the experiments allow one to recommend the following sequence of operations for chloride removal from the zinc sulfate solutions:
Fig. 2. Chloride distribution isotherm and McCabe-Thiele diagram for chloride extraction from zinc sulfate solutions with the TRPO + CYANEX 272 + TBP mixture in kerosene. Organic phase: 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP in kerosene. Aqueous phase, g/L: 78.9 Zn, 50 H2SO4, 0.0–16.3 Cl.
with CYANEX 272 (Fig. 1, curve b). For the TRPO with CYANEX 272 mixtures, when extractant is in excess, a low molar ratio of chloride to zinc in the extracts takes place; the Cl(о):Zn(о) molar ratio is < 2 (1.5–1.7). In excess water phase, this ratio is close to 2 (1.9–2.0). The increased zinc content in the extract relative to chloride is due to zinc
• Using the TRPO + CYANEX 272 + TBP extractant mixtures in a diluent; chloride extraction is carried out in 3 stages. • Washing of the extract from impurities (mainly iron) is carried out with dilute sulfuric acid solutions (~ 40–50 g/L) in 1–2 stages.
587
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
Table 2 Dependence of chloride stripping upon pH for the mixtures of TRPO and CYANEX 272 Organic phase: 0.73 M TRPO + 0.6 М CYANEX 272 + 0.6 M TBP in kerosene; CCl(o) = 7.5 g/L, СZn (o) = 7.5 g/L. Stripping solution: aqueous solutions of different alkali (NaOH) content. εCl – degree of chloride stripping, εZn – degree of zinc extraction, O:A = 1:1. No.
1 2 3 4 5
NaOH Concentration in the stripping solution (mol/ L)
Water 0.1 M NaOH 0.2 M NaOH 0.3 M NaOH 0.4 M NaOH
рН (equilibrium)
1.3 2.5 4.6 7.0 7.5
Chloride concentration in the phases, g/L Aq.
Org.
2.2 3.2 6.2 7.5 7.5
5.2 4.3 1.25 0.02 –
εCl, %
29.33 42.66 82.66 99.73 100.0
Zinc concentration in the phases, g/L Aq.
Org.
2.15 0.88 0.11 – –
6.4 6.7 7.45 7.5 7.5
εZn, %
85.33 89.33 99.33 100.0 100.0
Table 3 Chloride stripping under different A/O ratios with NaOH solutions from the mixtures of TRPO and CYANEX 272 Organic phase: 0.73 M TRPO + 0.6 М CYANEX 272 + 0.6 M TBP in kerosene; CCl(o) = 7.0 g/L, СZn(o) = 7.0 g/L. Stripping solution: aqueous solutions of different alkali (NaOH) content. A:O
1:1 1:2.5 1:5 1:10
NaOH concentration in the stripping solution (mol/ L)
0.35 0.875 1.75 3.5
рН (equilibrium)
7.2 7.05 6.6 6.2
Chloride concentration in the phases, g/L Aqueous
Org.
7.0 18.4 36.4 72.0
0.2 0.42 – –
• Chloride stripping is carried out with a stoichiometric amount of •
Org.
0.03 0.04 0.05 0.05
7.0 7.0 7.0 7.0
99.57 99.43 99.29 99.29
References Baker, H.T., Baes Jr., C.F., 1962. An infra-red and isopiestic investigation of the interaction between di(2-ethylhexyl)phosphoric acid and tri-n-octylphosphine oxide in octane. J. Inorg. Nucl. Chem. 24 (10), 1277–1286. Cheng, С.Y., Barnard, K.R., Zhang, W., Robinson, D.J., 2011. Synergistic solvent extraction of nickel and cobalt: a review of recent developments. Solvent Extr. Ion Exch. 29 (5–6), 719–754. Chmielarz, A., Gnot, W., 2001. Conversion of zinc chloride to zinc sulphate by electrodialysis – a new concept for solving the chloride ion problem in zinc hydrometallurgy. Hydrometallurgy 61 (1), 21–43. Fleitlikh, I.Yu., Pashkov, G.L., Nikiforova, L.K., Grigorieva, N.A., 2015. Zinc extraction from chloride solutions with mixtures of trialkyl phosphine oxide and p-tertbutylphenol. Chemistry for Sustainable Development (Russ. J.) 23, 279–284. Kopanev, A.M., Skvortsov, A.Yu., Laskorin, B.N., Vodolazov, L.I., Zhukova, N.G., 1989. Sorption-extraction technology of chlorine utilization from the solutions of zinc production. Non-ferrous Metals (Russ. J.) 9, 33–34. Kuhn, J.M., Mason, C.R.S., Harlamovs, J.R., Bell, M.W., Buchalter, E.M., 2003. Piloting of Halogon™ process with mixer-settlers and Bateman pulsed columns. In: Proceedings of the Fifth International Conference in Honor of Professor I. Ritchie, Hydrometallurgy – 2003, 24–27 August 2003, Vancouver, Canada. 1. pp. 777–786. Liu, W., Zhang, R., Liu, Z., 2016. Removal of chloride from simulated zinc sulfate electrolyte by ozone oxidation. Hydrometallurgy 160, 147–151. Mason, C.R.S., Grinbaum, B., Harlamovs, J.R., Dreisinger, G.B., 2003. Solvent extraction of halides from metallurgical solutions. In: Proceedings of the Fifth International Conference in Honor of Professor I. Ritchie, Hydrometallurgy – 2003, 24–27 August 2003, Vancouver, Canada. 1. pp. 765–776. Pimenov, V.B., Startsev, V.N., Pavlov, Yu.I., Malkhov, G.V., Kulenov, A.S., Pashkov, G.L., Yatsuk, V.V., Korotin, A.D., 1977. Method for purification of metal sulfate solutions from chlorine. Certificate of Authorship, SU No. 552 987. Sergievskiy, V.V., Fedyanina, L.B., 1983. Method for chloride ions recovery from zinc sulfate solutions. Certificate of Authorship, SU No. 994 410. Snurnikov, A.P., 1981. Zinc hydrometallurgy. Elsevier “Metallurgiya”, Russia, Moscow 384 рр.
• efficient chloride removal (is recovered as ZnCl ) takes place over a 2
•
Aqueous
εZn, %
Our special thanks go to CYTEC INDUSTRIES INC. Canada for free samples of the CYANEX extractants. Appreciation for valuable remarks is extended to Dr. D.S. Flett.
In this work, it has been shown that:
• •
97.14 94.0 100.0 100.0
Zinc concentration in the phases, g/L
Acknowledgments
sodium hydroxide to a final pH value of 6.0–7.0 in 2 stages. These solutions can be directed to dumping or to the production of sodium hypochlorite. Zinc stripping is carried out with a stoichiometric amount of sulfuric acid having a concentration of 50–100 g/L in 2 stages. The extractant solution is returned to the extraction cycle.
4. Conclusions
•
εCl, %
wide range of sulfuric acid concentrations. The degree of extraction of the acid itself is not high and does not exceed 12.1%; the introduction of organophosphorus acids D2EHPA or CYANEX 272 into the organic phase containing TRPO results in some decrease in the chloride extraction which is due to the formation of H-complexes between TRPO and HR. In the mixtures containing CYANEX 272, chloride removal is always higher than in the presence of D2EHPA; the TRPO + CYANEX 272 + TBP mixture in kerosene is the optimum extraction system; to strip the chloride, the zinc extract is treated with alkaline solutions (NaOH). In this way, the separation of chloride ions and zinc occurs; chloride almost completely passes into the aqueous phase, while zinc remains in the organic phase as zinc dialkylphosphinate; zinc stripping from the organic phase is easily carried out with sulfuric acid solutions.
Based on the data obtained, a new method for chloride extraction from zinc sulfate solutions with the mixtures of TRPO and CYANEX 272 has been developed.
588
Contents lists available at ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Technical note
Purification of zinc sulfate solutions from chloride using extraction with mixtures of a trialkyl phosphine oxide and organophosphorus acids
MARK
I.Y. Fleitlikha, N.A. Grigorievaa,⁎, L.K. Nikiforovaa, O.A. Logutenkob a Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center “Krasnoyarsk Science Center SB RAS”, Academgorodok, 50/24, 660036 Krasnoyarsk, Russia b Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, Kutateladze, 18, 630128 Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Zinc sulfate solutions Chloride removal Solvent extraction Trialkyl phosphine oxide CYANEX 272
Studies on the extractive purification of zinc sulfate solutions from chloride using mixtures of trialkyl phosphine oxide (TRPO, CYANEX 923) and organophosphorus acids (HR) as extractants have been carried out. The acids used were di(2-ethylhexyl)phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272). Chloride is extracted as the compound ZnCl2 ∙2TRPO. The most efficient extraction system for chloride removal was shown to be the TRPO + CYANEX 272 + tributyl phosphate mixture in kerosene. The main process parameters that provided for high chloride removal (~97%) were determined. Chloride stripping from the zinc extract is carried out with alkaline solutions (NaOH). The separation of chloride and zinc occurs at a final pH value of the strip liquors of 6–7 under which conditions chloride almost completely pass into the aqueous phase, while zinc remains in the organic phase forming zinc dialkylphosphinate. Thus the mixtures of TRPO with CYANEX 272 advantageously differ from the extraction systems containing TRPO alone wherefrom acceptable chloride stripping is not possible. Zinc stripping from the organic phase is carried out with a sulfuric acid solution.
1. Introduction High chloride concentrations in zinc electrolytes lead to significant acceleration of electrode wear, a decrease in the output current, and corrosion of equipment. Therefore, the solutions must be purified from chloride until a concentration of not > 150 mg/L is reached. Of the precipitative methods, purification from the halide by precipitation as silver chloride is most effective. However, because of its high cost, this method is not widely used. The most common purification method is based on chloride precipitation as a poorly soluble copper(I) chloride (Snurnikov, 1981). The disadvantages of this method include the length of time required for the CuCl precipitation (5–6 h) and the use of thickening and filtration operations which makes these processes inefficient. Among electrochemical methods for chloride removal, an electrodialysis method for purification of the mixed solutions ZnCl2 + ZnSO4 from chloride using membranes is of interest (Chmielarz and Gnot, 2001). It was shown that the initial chloride concentration can be lowered to 0.09 g/L using one-stage electrodialysis treatment. Commercial implementation of this method is difficult, apparently, due to the complicated hardware design and the relatively high cost of the
⁎
Corresponding author. E-mail address: [email protected] (N.A. Grigorieva).
http://dx.doi.org/10.1016/j.hydromet.2017.04.004 Received 22 February 2016; Received in revised form 13 March 2017; Accepted 2 April 2017 Available online 03 April 2017 0304-386X/ © 2017 Elsevier B.V. All rights reserved.
membranes. According to the method proposed by Liu et al. (2016) purification of zinc solutions from chloride was carried out by chloride oxidation to chlorine gas; ozone was used as an oxidizer. This method has a number of disadvantages, among which is its low productivity. The use of anion exchange resins allowed removal of chloride from zinc electrolyte reaching a concentration of ≤ 150 mg/L (Pimenov et al., 1977). The disadvantages of this method include the length of time required for sorption (1 − 2h) and, most importantly, obtaining diluted (with regard to the chloride) eluates (1–2 g/L). The best results for chloride removal from sulfate solutions were obtained when using extraction methods. Mainly, halide extraction with tertiary organic amines (R3N) has been studied (Kopanev et al., 1989; Kuhn et al., 2003). Trialkylamine of a С7–С9 fraction (Kopanev et al., 1989) or Alamine 336 (Kuhn et al., 2003) were used as extractants. In all cases extraction was satisfactory and chloride stripping occurred when using alkaline reagents NaOH (Kopanev et al., 1989) or NH4OH (Kuhn et al., 2003). The disadvantages of the use of amines include the high extraction of sulfuric acid which converts the extractant to the sulfate (R3NH)2SO4 or bisulfate (R3NH) HSO4 forms (Kuhn et al., 2003). Apart from that, an increase in the
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
method based on the degree of opalescence of silver chloride when silver reacts with the chloride.
sulfuric acid concentration in the solution resulted in a decrease in the chloride extraction, which was due to the competitive reaction proceeding between bisulfate and chloride for the extractant (Kuhn et al., 2003). In addition to organic amines, neutral organophosphorus extractants, in particular, tributyl phosphate (TBP) can be used for chloride removal (Sergievskiy and Fedyanina, 1983). A significant disadvantage of this method is the need to introduce a large amount of sulfuric acid (200–500 g/L) into the zinc solution prior extraction which is virtually impossible in the existing zinc production processes. Very good results for purification of zinc solutions from chloride have been obtained when using trialkyl phosphine oxide as the extractant (Mason et al., 2003). Together with efficient chloride removal, its extraction did not depend on pH over a wide range of H2SO4 concentrations and the degree of extraction of the acid itself was low (ε < 16%), which provides an advantageous difference for the TRPO systems compared to tertiary amine systems (Kuhn et al., 2003). However chloride stripping with water is extremely difficult (Fleitlikh et al., 2015). An attempt to strip zinc chloride (and thus chloride) with alkaline solutions (NaOH, Na2CO3) was unsuccessful due to the formation of zinc carbonate or hydroxide precipitates. At the same time, the formation of emulsions, which are almost unbreakable, took place which completely disrupts the technological process (Mason et al., 2003). The aim of this work was to search for new efficient extraction systems based on TRPO and development of a process for chloride removal from zinc sulfate solutions, comprising both its efficient extraction and its efficient stripping. Commercially available reagents, mixtures of TRPO with organophosphorus acids (HR), such as di(2ethylhexyl)phosphoric acid and bis(2,4,4-trimethylpentyl)phosphinic acid, are used as the extractants.
3. Results and discussion 3.1. Mechanisms of extraction and stripping. General considerations As previously reported by Fleitlikh et al. (2015), zinc chloride extraction with TRPO can be described by Eq. 1:
Zn2+(a) +2Cl (a) + 2TRPO(o) ↔ ZnCl2 2TRPO(o)
(1)
where (a) and (o) denote species in the aqueous and organic phases, respectively. The introduction of organic acids (HR) into the organic phase containing TRPO will result in a decrease in the extractant activity due to the partial formation of hydrogen-bonded complexes (H-complexes) (Eq. 2) and, consequently, in a decrease in the zinc chloride distribution coefficients.
(HR)2(o) + 2TRPO(o) ↔ 2TRPO⋅HR (o)
(2)
Zinc chloride extraction with the adduct formed can be described by Eq. 3:
Zn2+(a) + 2Cl (a) + 2ТRPO·HR (о) ↔ ZnCl2·2ТRPO(о) + (НR)2(o)
(3)
To strip the chloride, the zinc extract was treated with alkaline solutions (NaOH) due to which the separation of chloride and zinc occurred. The chlorides passed into the strip liquor as NaCl while zinc remained in the organic phase according to Eq. 4.
ZnCl2⋅2TRPO(o) + (НR)2(o) + 2NaOH(a) → 2TRPO(o) + ZnR2(o) + 2NaCl(a) + 2Н2 О
(4)
The subsequent treatment of the organic phase with a sulfuric acid solution resulted in zinc stripping and, at the same time, in regeneration of the extractant which was returned to the extraction cycle (Eq. 5).
2. Experimental 2.1. Reagents
ZnR2 + 2ТRPO(о) + Н2 SO4(a) ↔ 2ТRPO⋅HR (о) + ZnSO4 (a)
Trialkyl phosphine oxide of a С6–С8 fraction (TRPO, CYANEX 923) of 93% purity was used as the extractant. D2EHPA and Cyanex 272 of 85% purity were used as additives. The reagents used as modifiers were 2-ethylhexanol (TU 26624-85) and tributyl phosphate (TBP). Petroleum paraffin of a С13 fraction of not < 98% purity and dearomatized kerosene (С10Н22, TU 38-101-454-74) were used as diluents. All reagents and diluents were used without further treatment, with the exception of TBP and D2EHPA which were preliminarily washed with a solution of NaOH (20–40 g/dm3) to remove mono- and dibutylphosphoric acids. After washing, the refined product (D2EHPA) contained 88% of D2EHPA. All reagents used were products of domestic production, with the exception of the CYANEX 923 and CYANEX 272, produced by Cytec (Canada). The mineral salts, alkalis and acids used were in chemical or analytical grade.
(5)
3.2. Chloride extraction with the TRPO and organophosphorus acid (HR) mixtures Preliminary data on chloride extraction from zinc sulfate solutions containing (g/L) 77.7 Zn, 7.0 Cl¯, 0–150 H2SO4 with 50% ТRPO solution in petroleum paraffin showed that the degree of chloride extraction was very high (> 90%) and did not depend on the acidity of the aqueous phase. The chloride and zinc concentrations in the loaded organic phase were 0.178 and 0.095 mol/L, respectively, which corresponds to the Cl to Zn molar ratio of 1.87. Since the ratio is close to 2, this indicates that zinc is extracted in the form of ZnCl2. The introduction of organophosphorus acids (HR) into the organic phase containing TRPO resulted in some decrease in the chloride extraction (Table 1). This is due to the formation of hydrogen-bonded complexes (H-complexes) between TRPO and HR, in accordance with Eq. 2, which results in a decrease in the extractant activity. The formation of such complexes between trioctyl phosphine oxide and D2EHPA previously reported by Baker and Baes (1962). It is also seen from Table 1, that chloride removal with the mixtures containing D2EHPA is always lower than with those containing CYANEX 272. The latter is due to the greater stability of the H-complexes between TRPO and D2EHPA caused by the larger acid dissociation constant of D2EHPA and lesser steric hindrance upon formation of the adduct. The pKa values for organophosphorus acids are equal to 3.01 for D2EHPA and 5.22 for CYANEX 272 (Cheng et al., 2011). As seen from Fig. 1, all extraction systems are suited for purification of the zinc solutions from chloride. Chloride removal in the system containing D2EHPA (Fig. 1, curve a) is much poorer than in the system
2.2. Procedure The liquid–liquid extraction experiments were carried out according to the published procedure (Fleitlikh et al., 2015).The pH values of the aqueous phases were determined by potentiometric titration with a glass electrode. The pH meters used were pH 673 and Akvilon pH 410. The aqueous phases were analyzed for zinc and chloride. Their concentration in the organic phases was calculated from the mass balances. In several cases, the zinc-containing organic phases were completely stripped by mixing with sulfuric acid solutions. Zinc concentration in the aqueous phase was determined by atomic absorption spectrophotometry. High chloride concentrations in the solutions were determined by potentiometric titration with silver nitrate. Low chloride concentrations were determined by a phototurbidimetric 586
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
extraction with CYANEX 272 by a cation exchange mechanism. This process is undesirable but if necessary, it can be significantly inhibited by carrying out the chloride extraction under more acidic conditions. The data obtained is clearly in favor of the TRPO and CYANEX 272 mixtures. As seen from Fig. 1, the choice of modifier in these systems is very important. Replacement of a higher alcohol (2-ethylhexanol) (curve b) by tributyl phosphate (TBP) (curve c) leads to a marked increase in the chloride extraction. The improved extraction is not due to the chloride extraction with tributyl phosphate itself as TBP does not extract zinc chloride under these conditions as previously reported by Sergievskiy and Fedyanina (1983). An increase in extraction in this case, is due to an increase in the TRPO activity. Fig. 2 shows the extraction isotherms for chloride from the zinc electrolyte with the TRPO + CYANEX 272 + TBP mixture in kerosene, together with an operating line (McCabe-Thiele diagram). Since the initial concentration of chloride in the electrolyte can vary over a wide range (from 0.5 to 5.0 g/L of Cl), the operating line was drawn from the chloride concentration of 5.0 g/L in the zinc solution. The ratio of the flows of aqueous to organic phases (λ = A:O) was taken as two. As seen from the figure, under these conditions, to achieve a residual chloride concentration in the raffinate of 0.15 g/L, 3 extraction stages are required.
Table 1 Effect of organophosphorus acids on chloride extraction with TRPO. Aqueous phase, g/L: 97.7 Zn (ZnSO4), 50 H2SO4, 8.0 Cl (NaCl). Organic phase: TRPO + HR + 2-ethylhexanol in kerosene. O:A = 1:1. No.
1 2
Organic phase composition, M
1.1 M TRPO 1.1 M TRPO + 0.6 M CYANEX 272 + 0.64 M 2-ethylhexanol 1.1 M TRPO + 0.6 M D2EHPA + 0.64 M 2ethylhexanol 0.73 M TRPO + 0.6 M CYANEX 272 + 0.64 M 2ethylhexanol 0.73 M TRPO + 0.6 M D2EHPA + 0.64 M 2ethylhexanol
3
4
5
20
Chloride concentration in the phases, g/L
DCl
ε, %
Aqueous
Organic
0.5 0.58
7.5 7.42
15 12.8
93.75 92.75
0.76
7.24
9.5
90.50
1.03
6.97
6.8
87.13
1.5
6.5
4.23
81.25
CCl(org), g/L b
16
c
3.3. Chloride stripping from the TRPO and CYANEX 272 mixtures
a
Earlier, it was noted that stripping of the zinc chloride is extremely difficult in the systems with TRPO. It will be shown below that the introduction of an organophosphorus acid into the organic phase containing TRPO, in particular CYANEX 272, solves this problem quite easily. To strip the chloride, the zinc extract was treated with alkaline solutions (NaOH) which resulted in the separation of the chloride and zinc. The chlorides passed into a strip liquor as NaCl while zinc remained in the organic phase forming zinc dialkyldithiophosphinate (see Eq. 4). The data on chloride stripping with NaOH are shown in Tables 2 and 3. As seen from the tables, at a final pH value of 6–7, chloride practically completely passes into the aqueous phase while zinc remains in the organic phase. Based on the data obtained, it was proposed to carry out chloride stripping with a stoichiometric amount of sodium hydroxide with a concentration of ~3.5 mol/L to a final pH value of 6.0–7.0 in 2 stages. This gives sufficiently concentrated chloride strip liquors (~ 70 g/L).
12
8
4
CCl(aq), g/L 0 0
1
2
3
4
5
Fig. 1. Isotherms for chloride extraction from zinc sulfate solutions with the mixtures of TRPO and HR in kerosene. Organic phases: a – 0.73 M TRPO + 0.6 М D2EHPA + 0.64 М 2-ethylhexanol in kerosene; b – 0.73 M TRPO +0.6 M CYANEX 272 + 0.64 M 2ethylhexanol in kerosene; c – 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP in kerosene. Aqueous phases, g/L: a, b – 97.7 Zn, 50 H2SO4, 8.0 Cl; c – 78.9 Zn, 50 H2SO4, 0.0–16.3 Cl. a, b – O:A ≠ const; c – O:A = 1:1.
3.4. Zinc stripping from the TRPO and CYANEX 272 mixtures
CCl(org), g/L
Zinc stripping is easily carried out with sulfuric acid (according to Eq. 5). At a concentration of sulfuric acid in the stripping solution of 50–100 g/L, zinc was stripped almost completely. The results on sulfuric acid extraction with a 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP solution in kerosene showed that the degree of acid extraction was not high and did not exceed 12.1%.
λ=A:O =2:1
3.5. Schematic flowsheet for chloride extraction from zinc solutions with the TRPO and CYANEX 272 mixtures
CCl(aq), g/L The present study has been focused only on the zinc and chloride distribution in extraction systems with TRPO. As previously reported by Fleitlikh et al. (2015), during extraction with TRPO from zinc chloride solutions, such impurities as Cu, Co, Pb, and Mn are almost not extracted while Fe(III) is extracted only to a small extent. The results of the experiments allow one to recommend the following sequence of operations for chloride removal from the zinc sulfate solutions:
Fig. 2. Chloride distribution isotherm and McCabe-Thiele diagram for chloride extraction from zinc sulfate solutions with the TRPO + CYANEX 272 + TBP mixture in kerosene. Organic phase: 0.73 M TRPO + 0.6 M CYANEX 272 + 0.6 M TBP in kerosene. Aqueous phase, g/L: 78.9 Zn, 50 H2SO4, 0.0–16.3 Cl.
with CYANEX 272 (Fig. 1, curve b). For the TRPO with CYANEX 272 mixtures, when extractant is in excess, a low molar ratio of chloride to zinc in the extracts takes place; the Cl(о):Zn(о) molar ratio is < 2 (1.5–1.7). In excess water phase, this ratio is close to 2 (1.9–2.0). The increased zinc content in the extract relative to chloride is due to zinc
• Using the TRPO + CYANEX 272 + TBP extractant mixtures in a diluent; chloride extraction is carried out in 3 stages. • Washing of the extract from impurities (mainly iron) is carried out with dilute sulfuric acid solutions (~ 40–50 g/L) in 1–2 stages.
587
Hydrometallurgy 169 (2017) 585–588
I.Y. Fleitlikh et al.
Table 2 Dependence of chloride stripping upon pH for the mixtures of TRPO and CYANEX 272 Organic phase: 0.73 M TRPO + 0.6 М CYANEX 272 + 0.6 M TBP in kerosene; CCl(o) = 7.5 g/L, СZn (o) = 7.5 g/L. Stripping solution: aqueous solutions of different alkali (NaOH) content. εCl – degree of chloride stripping, εZn – degree of zinc extraction, O:A = 1:1. No.
1 2 3 4 5
NaOH Concentration in the stripping solution (mol/ L)
Water 0.1 M NaOH 0.2 M NaOH 0.3 M NaOH 0.4 M NaOH
рН (equilibrium)
1.3 2.5 4.6 7.0 7.5
Chloride concentration in the phases, g/L Aq.
Org.
2.2 3.2 6.2 7.5 7.5
5.2 4.3 1.25 0.02 –
εCl, %
29.33 42.66 82.66 99.73 100.0
Zinc concentration in the phases, g/L Aq.
Org.
2.15 0.88 0.11 – –
6.4 6.7 7.45 7.5 7.5
εZn, %
85.33 89.33 99.33 100.0 100.0
Table 3 Chloride stripping under different A/O ratios with NaOH solutions from the mixtures of TRPO and CYANEX 272 Organic phase: 0.73 M TRPO + 0.6 М CYANEX 272 + 0.6 M TBP in kerosene; CCl(o) = 7.0 g/L, СZn(o) = 7.0 g/L. Stripping solution: aqueous solutions of different alkali (NaOH) content. A:O
1:1 1:2.5 1:5 1:10
NaOH concentration in the stripping solution (mol/ L)
0.35 0.875 1.75 3.5
рН (equilibrium)
7.2 7.05 6.6 6.2
Chloride concentration in the phases, g/L Aqueous
Org.
7.0 18.4 36.4 72.0
0.2 0.42 – –
• Chloride stripping is carried out with a stoichiometric amount of •
Org.
0.03 0.04 0.05 0.05
7.0 7.0 7.0 7.0
99.57 99.43 99.29 99.29
References Baker, H.T., Baes Jr., C.F., 1962. An infra-red and isopiestic investigation of the interaction between di(2-ethylhexyl)phosphoric acid and tri-n-octylphosphine oxide in octane. J. Inorg. Nucl. Chem. 24 (10), 1277–1286. Cheng, С.Y., Barnard, K.R., Zhang, W., Robinson, D.J., 2011. Synergistic solvent extraction of nickel and cobalt: a review of recent developments. Solvent Extr. Ion Exch. 29 (5–6), 719–754. Chmielarz, A., Gnot, W., 2001. Conversion of zinc chloride to zinc sulphate by electrodialysis – a new concept for solving the chloride ion problem in zinc hydrometallurgy. Hydrometallurgy 61 (1), 21–43. Fleitlikh, I.Yu., Pashkov, G.L., Nikiforova, L.K., Grigorieva, N.A., 2015. Zinc extraction from chloride solutions with mixtures of trialkyl phosphine oxide and p-tertbutylphenol. Chemistry for Sustainable Development (Russ. J.) 23, 279–284. Kopanev, A.M., Skvortsov, A.Yu., Laskorin, B.N., Vodolazov, L.I., Zhukova, N.G., 1989. Sorption-extraction technology of chlorine utilization from the solutions of zinc production. Non-ferrous Metals (Russ. J.) 9, 33–34. Kuhn, J.M., Mason, C.R.S., Harlamovs, J.R., Bell, M.W., Buchalter, E.M., 2003. Piloting of Halogon™ process with mixer-settlers and Bateman pulsed columns. In: Proceedings of the Fifth International Conference in Honor of Professor I. Ritchie, Hydrometallurgy – 2003, 24–27 August 2003, Vancouver, Canada. 1. pp. 777–786. Liu, W., Zhang, R., Liu, Z., 2016. Removal of chloride from simulated zinc sulfate electrolyte by ozone oxidation. Hydrometallurgy 160, 147–151. Mason, C.R.S., Grinbaum, B., Harlamovs, J.R., Dreisinger, G.B., 2003. Solvent extraction of halides from metallurgical solutions. In: Proceedings of the Fifth International Conference in Honor of Professor I. Ritchie, Hydrometallurgy – 2003, 24–27 August 2003, Vancouver, Canada. 1. pp. 765–776. Pimenov, V.B., Startsev, V.N., Pavlov, Yu.I., Malkhov, G.V., Kulenov, A.S., Pashkov, G.L., Yatsuk, V.V., Korotin, A.D., 1977. Method for purification of metal sulfate solutions from chlorine. Certificate of Authorship, SU No. 552 987. Sergievskiy, V.V., Fedyanina, L.B., 1983. Method for chloride ions recovery from zinc sulfate solutions. Certificate of Authorship, SU No. 994 410. Snurnikov, A.P., 1981. Zinc hydrometallurgy. Elsevier “Metallurgiya”, Russia, Moscow 384 рр.
• efficient chloride removal (is recovered as ZnCl ) takes place over a 2
•
Aqueous
εZn, %
Our special thanks go to CYTEC INDUSTRIES INC. Canada for free samples of the CYANEX extractants. Appreciation for valuable remarks is extended to Dr. D.S. Flett.
In this work, it has been shown that:
• •
97.14 94.0 100.0 100.0
Zinc concentration in the phases, g/L
Acknowledgments
sodium hydroxide to a final pH value of 6.0–7.0 in 2 stages. These solutions can be directed to dumping or to the production of sodium hypochlorite. Zinc stripping is carried out with a stoichiometric amount of sulfuric acid having a concentration of 50–100 g/L in 2 stages. The extractant solution is returned to the extraction cycle.
4. Conclusions
•
εCl, %
wide range of sulfuric acid concentrations. The degree of extraction of the acid itself is not high and does not exceed 12.1%; the introduction of organophosphorus acids D2EHPA or CYANEX 272 into the organic phase containing TRPO results in some decrease in the chloride extraction which is due to the formation of H-complexes between TRPO and HR. In the mixtures containing CYANEX 272, chloride removal is always higher than in the presence of D2EHPA; the TRPO + CYANEX 272 + TBP mixture in kerosene is the optimum extraction system; to strip the chloride, the zinc extract is treated with alkaline solutions (NaOH). In this way, the separation of chloride ions and zinc occurs; chloride almost completely passes into the aqueous phase, while zinc remains in the organic phase as zinc dialkylphosphinate; zinc stripping from the organic phase is easily carried out with sulfuric acid solutions.
Based on the data obtained, a new method for chloride extraction from zinc sulfate solutions with the mixtures of TRPO and CYANEX 272 has been developed.
588