稀有金属(英文版) 2019,38(03),270-276

Synergistic extraction of zinc from ammoniacal solutions using a β-diketone mixed with trialkylphosphine oxide

Ru-Long Zhu Xin-Bin Li Chang Wei Hui Huang Min-Ting Li Cun-Xiong Li Fu-Li Tang

Faculty of Metallurgy and Energy Engineering,Kunming University of Science and Technology

Yunnan Yuntong Zinc Co.,Ltd

作者简介：*Chang Wei,e-mail:weichang502@sina.cn;

收稿日期：2 March 2015

基金：financially supported by the National Basic Research Program of China (No. 2014CB643404);the National Natural Science Foundation of China (Nos.51174104, 51304093, and 51364022);

Synergistic extraction of zinc from ammoniacal solutions using a β-diketone mixed with trialkylphosphine oxide

Ru-Long Zhu Xin-Bin Li Chang Wei Hui Huang Min-Ting Li Cun-Xiong Li Fu-Li Tang

Faculty of Metallurgy and Energy Engineering,Kunming University of Science and Technology

Yunnan Yuntong Zinc Co.,Ltd

Abstract：

A simple comparative study of synergistic extraction of zinc from ammonium sulfate and ammonium chloride systems with mixtures of Mextral54-100 and tri-nalkylphosphine oxide(TRPO) dissolved in n-heptane was described. Various parameters affecting the extraction process were investigated. Experimental results show that extraction from ammonium sulfate is better than that from ammonium chloride under the same conditions owing to the additional complexation between zinc and chloride ions. Total ammonium concentration and pH have a significant effect on zinc extraction efficiency because of the formation of zinc ammine complexes. The synergistic effect is explained by the formation of adduct between zinc chelates and trialkylphosphine oxide. The thermodynamic data show that the extraction reaction is exothermic and stripping is endothermic. Infrared(IR) spectra of the zincloaded organic phases were also examined.

Keyword：

Synergistic extraction; Zinc; Ammonium sulfate; Ammonium chloride; Mextral54-100; Trialkylphosphine oxide;

Received： 2 March 2015

1 Introduction

Solvent extraction has become a key purification and separation technology for many nonferrous metals.It plays a vital role in the hydrometallurgy of zinc.A variety of reagents have been used to extract zinc from various media.Organophosphorus extractants,including di(2-ethylhexyl)phosphoric acid (D2EHPA) [ 1, 2] ,ethylhexylphosphonic acid (HEH/EHP) [ 3] ,tri-n-butylphosphate (TBP) [ 4] ,phosphinic acids (such as cyanex272,cyanex301,and cyanex302),and phosphine oxides (such as cyanex921,cyanex923,and cyanex925) [ 5, 6, 7, 8] ,have been extensively studied.Quinoline-type extractants,including LIX34 [ 9] and Kelex100 [ 10] ,pyrazolone [ 11] ,the amine extractant N1923 [ 12] ,the quaternary ammonium extractant aliquat336 [ 13] ,oxime-type extractants LIX84I [ 14] ,quaternary 3-pyridinium ketoximes [ 15] ,andβ-diketone extractants such as LIX54 [ 16] and Hostarex DK-16 [ 17] ,were also studied for extraction of zinc.With respect to ammoniacal solutions,β-diketones are regarded as the most suitable extractants for zinc owing to their strong hydrophobicity and low loading of ammonia.However,it is reported that zinc extraction is influenced by the pH of the aqueous phase as a result of the formation of zinc ammine complexes,which decrease the concentration of free zinc ions in the aqueous phase.Low zinc extraction efficiencies are therefore reported for theseβ-diketone extractants,especially under conditions of high pH and high ammonia concentrations [ 16, 18, 19] .

Synergism in solvent extraction describes that the effect observed when extraction of a certain metal species from an aqueous to an organic medium using a combination of two extractants is greater than the sum of the extractions by each extractant alone [ 20] .Numerous important studies were carried out on the synergistic extraction of zinc because of the abilities of such systems to improve the metal ion extraction efficiency,to enhance the stability and organic-phase solubility of the extracted complexes,and to eliminate emulsification and formation of third phases [ 21] .Hirose and Tanak [ 22] studied the synergistic extraction of zinc(Ⅱ) using capric acid mixed with pyridine and explained the synergistic effect by the formation of mixedligand complexes that replaced capric acid with pyridine in the organic-phase complex.The successful separation of Zn²⁺and Cu²⁺by mixtures of aliquat336 and tri-noctylphosphine oxide (TOPO) [ 23] and that of Zn²⁺and Cd²⁺by nonylsalicylic acid and cyanex471× [ 24] are based on synergistic extraction methods.Considerable synergistic enhancement has been observed in the extraction of Zn²⁺by mixtures of CA-100 and cyanex272 [ 25] .Mixtures of the primary amine N1923 and cyanex272exhibit evident synergistic effects on the extraction of zinc from chloride solutions [ 6] .

A typical feature of many synergistic systems is an increase in the coordination number of the central metal ion,which can exceed double its ionic charge when forming chelates with bidentate reagents [ 26] .The coordination number is therefore one of the decisive factors determining the behavior of a metal ion with respect to extraction by chelates.When the coordination is unsaturated,the addition of donor-active solvents,such as TBP,to the inert solvent leads to replacement of water molecules by the hydrophobic donor and improves the extraction of metal ions,depending on the electronic structure of metal ion and the nature of donor atoms in the ligand [ 27] .It is reported that the presence of some neutral oxygen-containing ligands can enhance extractability of metal chelates;the synergistic effect is ascribed to adduct formation between the metal chelates and the ligand [ 28] .Most attention has been focused on neutral organophosphorus oxygen-containing ligands,such as 1-phenyl-3-methyl-4-benzoyl pyrazolone-5(PMBP) with TBP or TOPO [ 26] ,thenoyltrifluoraceton (HTTA) with TBP [ 20] ,hinderedβ-diketones mixed with cyanex923 [ 29] ,4-ethyl-l-phenyl-1,3-octadione (XI-55) with TOPO [ 30] ,and benzoyl trifluoroacetone mixed with tri-n-butylphosphine oxide or TOPO [ 28] .With respect to the synergistic extraction of zinc from ammoniacal solutions,similar studies have been reported [ 29, 30] .

The addition of neutral organophosphorus oxygen-containing ligands toβ-diketone extractants has potential to improve the extraction efficiency of zinc(Ⅱ).It is therefore necessary to find a suitable synergistic system that is suitable for use with ammoniacal solutions.In the present work,Mextral54-100 and tri-n-alkylphosphine oxide(TRPO) were chosen as theβ-diketone extractant and neutral oxygen-containing ligand,respectively.The extraction behaviors of zinc from ammonium sulfate and ammonium chloride solutions using mixtures of Mextral54-100 and TRPO were studied.Effects of the aqueous pH,extractant and synergistic agent concentrations,total ammonia concentration,and temperature on zinc extraction were discussed.Infrared (IR) analysis of the extracted complexes was also examined.

2 Experimental

2.1 Reagents

Stock solutions of 6 g-L^-1 zinc(Ⅱ) were prepared by dissolving the required quantity of analytical-grade zinc oxide,ammonium sulfate or ammonium chloride,and ammonia in distilled water.Mextral54-100 was kindly supplied by Chongqing Kopper Chemical Industry Co.,Ltd,China.TRPO was obtained from Shanghai Rare Earth Chemical Co.,Ltd,China.Heptane,used as the organic-phase diluent,was supplied by Tianjin Fengchuan Chemical Reagent Science and Technology Co.,Ltd.All other chemicals were of analytical grade.

2.2 Extraction procedure

The aqueous pH of the solutions was adjusted prior to each test by adding NaOH or H₂SO₄ to achieve the required value.Extraction tests were carried out in a thermostated vessel (298.0 K±0.5 K) by shaking equal volumes(20 ml) of aqueous and organic phases in separatory funnels for 10 min which was sufficient to reach equilibrium.Once equilibrium was obtained,the phases were separated.The concentration of zinc(Ⅱ) in aqueous phase was determined by ethylene diamine tetraacetic acid (EDTA) volumetric titration [ 31] .The concentration of zinc(Ⅱ) in the organic phase was obtained by mass balance.A PHSJ-5digital pH meter (Shanghai INESA Scientific Instrument Co,Ltd,Shanghai,China) was used to measure the pH of the aqueous phases.IR measurements were obtained over the wavenumber range of 4000-400 cm^-1 using a Nicolet6700 spectrophotometer with KBr windows.Stripping tests were performed by shaking the loaded organic phases and H₂SO₄ solutions for 10 min at an organic-to-aqueous (O/A)volumetric ratio of unity.The distribution ratio (D) of zinc and extraction and stripping efficiencies (measured as percentages) were calculated from the relationships:

where C₀,C_org,and C_strip refer to the initial zinc concentration in aqueous phase,the equilibrium concentration of zinc in the loaded organic phase,and the zinc concentration of the spent stripping solution,respectively.

3 Results and discussion

3.1 Extraction of zinc(Ⅱ)

The extraction of palent zinc by aβ-diketone extractant can be represented by the following reaction [ 16] :

When a neutral oxygen-containing ligand is added to the organic phase,the extraction equilibrium can be given by:

where HA and L denote Mextral54-100 and TRPO,respectively,and the subscripts‘aq’and‘org’refer to the aqueous and organic phases,respectively.

In ammonium sulfate and ammonium chloride solutions,the complexation equilibria of Zn(Ⅱ) with ammonia and chloride can be described as follows:

3.1.1 Examining synergistic effect

Extraction studies were performed to examine the synergistic effect of mixtures of HA and TRPO on zinc extraction from ammoniacal solutions ([NH₃]_total=3 mol·L^-1) at an initial pH of 7.25.The sum of the concentrations of HA and TRPOwas kept constant at 0.5 mol·L^-1.A plot of zinc extraction efficiency as a function of the content of TRPO in organic phase (χTRPo) is shown in Fig.1.The synergistic behavior of the ammonium sulfate system is similar to but stronger than that of the ammonium chloride system.The lower zinc extraction efficiency in NH₄Cl-NH₃ system is attributed to the stronger complexation between zinc and chloride ion.For both systems,the extraction efficiency reaches a maximum when XTRPO is about 30 mol%,which indicates that the maximum molar ratio of HA to TRPO is about 2,i.e.,zinc is extracted as the ZnA₂L synergistic adduct.The maximum extraction efficiency is 95.6%for the ammonium sulfate system and 84.0%for the ammonium chloride system.

Fig.1 Relationship between extraction efficiency for zinc(Ⅱ) and TRPO content (χTRPo) in organic phase

3.1.2 Effect of pH

The initial solution pH was varied from 6.00 to 9.50,while keeping the concentrations of HA and TRPO at 0.45 and0.05 mol.L^-1,respectively,and[NH₃]_total=3 mol.L^-1.For comparison,the extraction of zinc by TRPO alone as a function of pH for the two systems was also presented.The results are illustrated in Fig.2.Increasing the initial pH increases the zinc extraction efficiency up to pH 7.25 for the ammonium sulfate system and up to pH 7.02 for the ammonium chloride system;above these pH values,the extraction efficiency decreases.This is ascribed to the formation of zinc chloride complexes, (n=1,2,3,4),in ammonium chloride.At sufficiently high initial pH,ammonia is produced in both systems,and zinc ammine complexes, (m=1,2,3,4),form which cannot be extracted into the organic phase bec ause of zinc c ation strong complex with ammonia.Organic extractants are not able to compete with the ammonium ions complexing the zinc ion [ 16] .Greater zinc extraction efficiency is measured from the ammonium sulfate system than from ammonium chloride.It is worth noting that,although neutral organophosphorus extractants can extract zinc chloride complexes at low pH,only negligible extraction of zinc by TRPO occurs from ammonium sulfate solutions.This explains the greater extraction efficiency of zinc from the ammonia chloride system below pH 7.02.

3.1.3 Effect of HA concentration

The effect of HA concentration on zinc extraction efficiency from the ammoniacal solutions ([NH₃]_total=3 mol·L^-1,pH=7.25) was examined.The TRPO concentration was kept constant at 0.05 mol·L^-1.As shown in Fig.3,an increase in HA concentration causes an increase in the extraction effciency of zinc in both systems,although the performance of the sulfate system is better than that of the chloride system.When the HA concentration is fixed at0.6 mol·L^-1,the extraction efficiencies are 87.2%and67.5%,respectively.Finally,HA concentration of0.45 mol·L^-1 was chosen for zinc extraction.

Fig.2 Relationship between zinc extraction efficiency and initial pH of aqueous phase

3.1.4 Effect of TRPO concentration

The TRPO concentration was varied from 0.01 to0.10 mol·L^-1 while keeping the concentration of HA constant at 0.45 mol·L^-1,[NH₃]_total=3 mol-L^-1,and pH=7.25.As shown in Fig.4,the extraction efficiency for zinc increases with TRPO concentration increasing in both systems.The effect is greater in the ammonium sulfate system.At a TRPO concentration of 0.1 mol·L^-1,the extraction efficiencies are 92.5%and 81.1%for the sulfate and chloride systems,respectively.When metal ions form chelates with bidentate reagents,their coordination numbers can exceed double their charge in synergistic extraction.In the absence of the donor-active solvent TRPO,the coordination of the zinc ion cannot be saturated by two HA molecules,which will occupy its four coordination sites in the form of bidentate ligands.The remaining one or two sites will be occupied by water and/or ammonia for coordinatively saturated complexes.TRPO has stronger donor ability than water and ammonia ligands;therefore,the addition of the hydrophobic donor TRPO can completely replace water,chloride,or ammonia molecules coordinated with the zinc complexes [ 32] .TRPO is associated with the zinc through an oxygen atom of the phosphoryl group,resulting in the formation of a synergistic adduct [ 26] ,which improves the extraction efficiency of zinc and the stability of the zinc chelate compound.The synergistic effect accounts for the formation of adduct between zinc chelate and TRPO.

Fig.3 Relationship between zinc extraction efficiency and HA concentration

Fig.4 Relationship between zinc extraction and TRPO concentration

3.1.5 Effect of total ammonia concentration

The extraction behaviors of zinc with a mixture of0.45 mol.L^-1 HA and 0.05 mol·L^-1 TRPO as a function of total ammonia concentration,ranging from 2 to 6 mol.L^-1,at pH 7.25 are shown in Fig.5.Increasing the total ammonia concentration leads to a decrease in zinc extraction efficiency in both systems.This is attributed to the formation of zinc ammine complexes with total ammonia concentration increasing,which decreases the concentration of free zinc ions.When the total ammonia concentration increases from 2 to 6 mol·L^-1,the extraction efficiency decreases from 85.8%to 58.3%for the ammonium sulfate system and from 71.6%to 48.5%for the ammonium chloride system.The negative influence of total ammonia concentration is more evident in the ammonium chloride system.This is explained by the additional formation of zinc complexes, ,which cannot be extracted into the organic phase.

Fig.5 Relationship between zinc extraction efficiency and total ammonia concentration

3.1.6 Effect of temperature

Temperature plays an important role in extraction processes.The extraction of zinc from ammoniacal solution([NH₃]_total=3 mol·L^-1,pH=7.25) by a mixture of0.45 mol·L^-1 HA and 0.05 mol·L^-1 TRPO was studied at temperatures from 20 to 45℃.As shown in Fig.6,zinc extraction efficiency decreases with temperature increasing in both systems.Extraction is obviously superior for the ammonium sulfate system.Based on the relationship between distribution ratio and temperature,the change of enthalpy (ΔH) of the reaction was calculated according to the Van’t Hoff equation:

where R is gas constant and T is temperature.The change of enthalpy (ΔH) is calculated as-16.8 kJ·mol^-1 for the ammonium sulfate system and-7.3 kJ.mol^-1 for the ammonium chloride system.These results indicate that the synergistic extraction is exothermic in both systems.

3.1.7 IR studies

The IR spectra of the zinc-loaded organic phases obtained from the extraction of zinc from the ammoniacal solutions([NH₃]_total=3 mol.L^-1,pH=7.25) using a mixture of0.45 mol·L^-1 HA and 0.05 mol·L^-1 TRPO are shown in Fig.7.For comparison,the spectrum of the fresh organic phase is also shown.The presence of ammonia in the extracted compounds results in absorption peaks at 645,1145,1596,3220,and 3350 cm^-1;such bands are absent in the spectra of the loaded zinc organic phases,thereby providing confirmatory evidence of the nonextraction of zinc ammine complexes.In addition,almost no differences exist between the spectra of the loaded organic phases generated from both systems,indicating that the structure of the extracted compounds is not affected by the nature of the anion,i.e.,these anions are not being extracted into organic phases.

Fig.6 Relationship between zinc extraction efficiency (D=C_org/C₀) and 1000/T

Fig.7 IR spectra of fresh and loaded organic phases.(1) Fresh HA+TRPO;(2) zinc-loaded organic phase for (NH₄)₂SO₄-NH₃system;(3) zinc-loaded organic phase for NH₄Cl-NH₃ system

3.2 Stripping of zinc(Ⅱ)

3.2.1 Effect of sulfuric acid concentration

The stripping of zinc is essentially inverse process of extraction.The sulfuric acid concentration of the stripping liquor was varied from 0.01 to 0.10 mol·L^-1 to investigate its effect on zinc stripping.The results are shown in Table 1.Stripping efficiency increases with acid concentration increasing,and good zinc recovery can be achieved with low acidic concentrations.Stripping is slightly easier in the ammonium chloride system,which is mainly attributed to the lower loading of zinc (3.7 g·L^-1) than that in the ammonia sulfate system (4.6 g·L^-1).During the stripping process,hydrogen ion displaces zinc ion from the organic phase.Complete zinc stripping is obtained for a sulfuric acid concentration of 0.1 mol·L^-1.

3.2.2 Effect of temperature

Figure 8 indicates that increasing the temperature from 20to 45℃causes a slight increase in zinc stripping efficiency using 0.05 mol·L^-1 acid.The zinc-loaded capacity is 3.7and 4.6 g·L^-1 in ammonium chloride and ammonium sulfate systems,respectively.Based on Eq.(9),the change of enthalpy of the reaction can be calculated as3.4 kJ·mol^-1 for ammonium chloride system and4.6 kJ-mol^-1 for ammonium sulfate system;thus,the reaction has a endothermic character.

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Table 1 Effect of sulfuric acid concentration on zinc stripping

Fig.8 Relationship between zinc stripping efficiency (D=C_strip/C₀)and 1000/T

4 Conclusion

The synergistic extraction of zinc from ammonium sulfate and ammonium chloride solutions using mixtures of Mextral54-100 and TRPO dissolved in n-heptane was investigated.The extraction efficiency of zinc increases with pH increasing and reaches a maximum at pH 7.25 for ammonium sulfate system and pH 7.02 for ammonium chloride system and then decreases,respectively.But zinc extraction efficiency from ammonium sulfate is greater than that from ammonia chloride.This is attributed to the additional complexation between Zn(Ⅱ) and chloride ions.Increasing total ammonia concentration decreases the zinc extraction efficiency in both systems.The formation of zinc ammine and zinc chloride complexes in the aqueous phase decreases the concentration of free zinc ions and suppresses the extraction of zinc.The formation of an adduct gives rise to the synergistic effect.The change of enthalpy in extraction process is-16.8 and-7.3 kJ.mol^-1,and that in stripping process is 3.4 and 4.6 kJ·mol^-1,respectively.The thermodynamic data suggest that the extraction process is exothermic and stripping process is endothermic.IR spectra of the zinc-loaded organic phases support the evidence that zinc ammonia and zinc chloride complexes cannot be extracted into the organic phase.

Acknowledgments This study was financially supported by the National Basic Research Program of China (No.2014CB643404) and the National Natural Science Foundation of China (Nos.51174104,51304093,and 51364022).

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