采用DZ988N从复杂硫酸盐溶液中高效选择性萃取铜

1 Introduction

With the rapid economic growth and social development, the total demand for copper has been rising steadily, and will exceed copper reserves by 2050 according to the current consumption of copper [1-2]. Due to the depletion of high-grade copper mineral resources, growing attention is paid to recovering copper from low-grade ores and industrial wastes [3-5]. However, during the leaching process of copper materials, impurities will be leached into solution along with the main metal. So, further treatments including precipitation [6], ion exchange [7], membrane separation [8], selective adsorption [9], and solvent extraction are required to recover valuable metals and remove impurities from the solution [10]. Among them, solvent extraction is one of the most common methods to separate and concentrate metals in an aqueous solution. SRIVASTAVA et al [11] investigated the liquid-liquid extraction of chromium from industrial effluent with tributyl phosphate as a potential extractant. ILYAS et al [12] studied the extraction of nickel and cobalt from ammonia leaching solution of Ni-laterite ore after carbon-thermal reduction roasting by 10 vol%  LIX84-I and concluded that the selective extraction rate of nickel was more than 97% over Co at the organic-to-aqueous ratio of 1. ISHFAQ et al [13] investigated the extraction of Cr(VI) and Fe(III) from electroplating waste liquid containing Cr, Fe, Zn, and free acid of chloride medium by TBP, and converted the carcinogenic metal Cr(VI) to less-toxic metal Cr(III) after treatment with ascorbic acid.

Acidic extractant and hydroxime hydroxyoxime extractant are two main types of copper extractants. If an acidic extractant is used to obtain a pure copper solution, the iron should be removed and the alkaline will be added to neutralize H+ in the solution before copper extraction at a high equilibrium pH. However, hydroxyoxime extractants, such as LIX63, LIX64, LIX65N, LIX622, LIX70, LIX860, LIX622, LIX864, LIX984, and LIX984N, were widely studied and used for copper extraction from acidic solutions due to their great extraction capacity, excellent separation efficiency from iron and lower extraction equilibrium pH [14-15]. ASGHARI et al [16] evaluated the effect of impurity ions such as Zn(II), Mn(II), Fe(III) and Fe(II) on the copper extraction by using 18% LIX®984N in kerosene, and 93.9% copper was extracted at pH of 2 and extraction time of 600 s. JUN et al [17] optimized the key experimental parameters of LIX84-I for extracting copper, such as temperature, equilibrium pH and extractant concentration, and confirmed that LIX84-I has a strong affinity with cooper due to the emergence of spontaneous inner-sphere coordination with disrupted hydration. BARIK et al [18] recovered copper from a waste heat boiler dust leaching liquor by using LIX84-I and LIX622N at a  phase volume ratio O/A (A: aqueous phase; O: organic phase) of 1.5/1 and extractant concentration of 30%, and achieved 98.64% and 99.95% efficiency with a three-stages counter-current extraction, respectively. POSPIECH [19] recovered Cu(II) from sulphate solution containing Co(II), Ni(II) and Mn(II) with a mixture of 5% Kelex 100 and 10% LIX70 at pH of 2, and 99% copper was extracted along with 10% of other metals. KUMAR et al [20] selectively extracted Cu(II) from its mixture with Ni(II) at pH of 2, and copper separation coefficient reached 6000 with 40% LIX664N. Although many studies on hydroxyoxime extractants were carried out to separate Cu(II) from other metals, DZ988N has not been applied to extract copper from the sulfuric acid leaching solution containing Cu(II), Co(II), Fe(II) and Zn(II).

This work aimed to investigate the extraction and separation of copper, cobalt, iron and zinc from the sulfuric acid leaching solution of a polymetallic residue. The reactive functional groups of DZ988N were identified, and the chelating mechanism for copper extraction was investigated. The influence factors on metal extraction efficiency (ηM) and separation coefficient (βA/B) were studied and the optimal extraction conditions were explored. Meanwhile, McCabe-Thiele isotherm diagrams were built to determine the number of stages at different phase ratios.

2 Materials and methods

2.1 Materials

The polymetallic residue from a wet zinc smelter in Guangdong, China, was leached with sulfuric acid, and the produced solution was used as the feed in the experiments. The concentration of the major metal components in the leaching solution were: c(Cu(II))=5.25 g/L, c(Co(II))=1.27 g/L, c(Fe(II))=2.16 g/L and c(Zn(II))=11.19 g/L.

The extractant DZ988N is a mixture of 5-nonylsalicylald oxime and 2-hydroxy-5-nonyl acetophenone oxime at a volume ratio of 1:1, and the effective components of extractant are LIX84 and LIX860N, similar to LIX984N [21]. DZ988N, a brown liquid with a density of 0.91-0.93 g/cm3, was purchased from Zhengzhou Deyuan Fine Chemicals Co., Ltd., China. Solvent oil No.260 was employed as diluent for extractant, which was provided by Shanghai Rare-Earth Chemical Co., Ltd., China.

2.2 Methods

2.2.1 Copper extraction

When the aqueous phase was mixed with the organic phase, DZ988N will chelate with copper, and will not react with other metals including cobalt, iron and zinc [22-23]. The chelate compounds formed during extraction were insoluble in water but soluble in organic, and thus copper was separated from sulfuric acid leaching solution. The above reaction process can be described as Eq. (1) [15, 24].

2HRorg+Cu2+aq⇔R2Cuorg+2H+aq

(1)

The flowsheet of copper extraction is shown in Figure 1. Aqueous and organic phase were added in a sealable test tube at a chosen phase volume ratio, and two phases completely mixed in the SHZ-82 Thermostatic water bath oscillator (Changzhou Xiangtian Experimental Instrument Factory, China) for a preset time. Then the mixed fluids were centrifuged to two individual phases in the TD6M Centrifuge (Hunan Xiangli Scientific Instrument Co., Ltd., China) and separated by a separatory funnel.

Figure 1  Flowsheet of copper extraction

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The pH value of solution was adjusted with dilute sulfuric acid, and determined with the PHS-3C pH meter (Shanghai Precision Science Instrument Co., Ltd., China). The functional groups of organic phase were detected by Nicolet 6700 Fourier transform infrared spectroscopy (FT-IR, Thrmo Fisher Scientific, USA), and the concentrations of metals in aqueous phase were determined with ICAP7400 radial inductively coupled plasma emission spectroscope (ICP, Thrmo Fisher Scientific, USA).

The metal extraction efficiency (ηM) is calculated according to Eq. (2) [25]:

ηM=c0MV0−c1MV1c0MV0×100%

(2)

where c0M and c1M are the concentrations of metal M in aqueous phase before and after extraction, respectively; V0 and V1 are the volumes of aqueous phase before and after extraction, respectively.

The metal distribution ratio (DM) was calculated according to Eq. (3) [26]:

DM=[M]org[M]aq

(3)

where [M]aq and [M]org are the concentrations of metal in aqueous phase and organic phase at extraction equilibrium, respectively.

The separation coefficient (βA/B) of two different metals (A and B) is calculated according to Eq. (4) [27]:

βA/B=DADB=[A]org/[A]aq[B]org/[B]aq

(4)

2.2.2 Counter-current extraction

To completely extract copper from the aqueous, counter-current extraction process was employed. As the operation process shown in Figure 2, the aqueous phase moved in opposite direction to the organic phase. When the aqueous phase was added in the first stage, the raffinate moved backward; when the organic phase was added in the last stage, the loading organic phase moved forward.

Figure 2  Operation process of counter-current extraction (aq: Aqueous phase; org: Organic phase; m: Extraction stage; X0: [Cu2+]aq of inlet aqueous phase; Y1: [Cu2+]org of outlet organic phase; Xm: [Cu2+]aq after the mth stage extraction; Ym+1: [Cu2+]org after the mth stage extraction)

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If the volumes of aqueous phase V(A) and organic phase V(O) remained unchanged during the counter-current extraction process, the following relationship is defined by Eq. (5) with reference of material balance principle.

(X0−Xm)⋅V(A)=(Y1−Ym+1)⋅V(O)

(5)

To determine the needed number of stages during counter-current extraction process, the extraction isotherm diagram (McCabe-Thiele diagram) including equilibrium isotherm and operation line, was built at a chosen phase volume ratio O/A [28]. The equilibrium isotherm described the copper distribution in two phases, which was obtained under the chosen extraction conditions, continuously contacting fresh aqueous phase with loading organic phase until the extractant saturation. In addition, according to Eq. (5), the operation line was defined by Eq. (6), passing through two points (X0, Y1) and (Xm, Ym+1) with a slope 1/R. R is the phase volume ratio of organic to aqeous phase.

Y1=V(O)V(A)(X0−Xm)+Ym+1=R(X0−Xm)+Ym+1

(6)

3 Results and discussion

3.1 Chelating mechanism of extraction process

The FT-IR pattern of fresh and loading organic phases both containing 20% DZ988N are shown in Figure 3. The bands at 2850, 2920 and 2955 cm-1 corresponded to the alkane C—H stretching vibrations of —CH2 and —CH3, and their bending vibrations were associated to the bands at 1460 and 1375 cm-1, respectively. The bands at 721 and 829 cm-1 were assigned to —CH external bending vibrations on the aromatic ring [29]. The bands at 1020 and 1640 cm-1 were assigned to the N—O stretching vibration and the C=N stretching vibration, in the oxime group (—C=NOH), respectively [30, 31]. The band at 1543 cm-1 attributed to the N—H bending vibration with hydrogen bonding (N…H—O). The N—H stretching vibration is associated to the band at       3421 cm-1, which is overlapped with a wide absorption peak of O—H stretching vibration in the hydroxyl group (—OH) [32, 33]. The bands of —C=NOH and R—OH groups appeared in Figure 3, indicating that effective components of extractant DZ988N exist in the organic phase.

Figure 3  FT-IR pattern of fresh organic phase and loading organic phase

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Comparing FT-IR curves of fresh and loading organic phases in Figure 3, there are significant changes of several absorption peaks. The band of N—H bending vibration with hydrogen bonding (N…H—O) at 1543 cm-1 disappears after extraction, while the band of —OH stretching vibration at 3421 cm-1 is weakened. The two donor atoms of nitrogen in the —C=NOH group and oxygen in the R—OH group are simultaneously coordinated to the Cu(II) to form a redundant bond, and the hydrogen of the —C=NOH group forms an internal hydrogen bond with the oxygen of the           R—OH group, thus forming the complex shown in Figure 4 [34-35].

Figure 4  Complex structure of organic phase and Cu(II)

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3.2 Extraction efficiency and separation coefficient

3.2.1 Effect of DZ988N concentration

The effect of DZ988N concentration on the extraction efficiency and separation coefficient was studied under the following conditions: pH of aqueous phase of 2.0, extraction temperature of 25 °C, extraction time of 5 min, phase ratio O/A of 1/1. The DZ988N concentration increased from 5% to 35%, with the results shown in Figure 5.

Figure 5  Effect of DZ988N concentration (v/v) on (a) extraction efficiency and (b) separation factor

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Figure 5(a) illustrated that the Cu(II) extraction efficiency increased from 65.50% to 97.06% with DZ988N concentration increased from 5% to 25%. No further obvious raise was detected when DZ988N concentration exceeded 25%. The extraction efficiencies of Co(II), Fe(II) and Zn(II) had a slightly increase by about 1.7%, 2.2% and 1.1%, respectively. In Figure 5(b), the separation coefficients, βCu/Zn, βCu/Fe and βCu/Co showed a growth trend similar with Cu(II) extraction efficiency, and reached 2112, 1595 and 2998, respectively, at DZ988N concentration of 25%. Therefore, to ensure a higher copper extraction efficiency with a lower consumption of extractant, DZ988N concentration of 25% was appropriate to extract copper from the sulfuric acid leaching solution.

3.2.2 Effect of pH value of aqueous phase

The influence of pH value of aqueous phase on the extraction efficiency and separation coefficient was investigated under a condition with DZ988N concentration of 20%, extraction temperature of 25 °C, extraction time of 5 min, phase ratio O/A of 1/1. The pH value of aqueous phase increased from 0.4 to 2.8, with the results presented in Figure 6.

Figure 6  Effect of pH value of aqueous phase on (a) extraction efficiency and (b) separation factor

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Figure 6(a) indicates that the Cu(II) extraction efficiency first increased from 49.8% to 95.55% with pH value increased from 0.4 to 2.0, and then remained unchanged when pH value has continuously risen to 2.8. The pH value had a limited effect on the extraction efficiency of Zn(II) and Co(II). However, when pH reached 2.8, a significant increase was detected in iron extraction efficiency. A small amount of Fe(II) was oxidized to Fe(III), then hydrolyzed and precipitated at pH 2.8, so that the concentration of iron in the raffinate decreased and the iron extraction efficiency calculated by Eq. (2) was higher than the actual one. Figure 6(b) further illustrates that the separation coefficients βCu/Co, βCu/Fe and βCu/Zn firstly increased from 67, 53 and 106 to 1351, 1037 and 1904, respectively, with the pH value increased from 0.4 to 2.0, and then decreased when pH value exceeded 2.0. Considering both copper extraction efficiency and separation coefficient, 2.0 was selected as the preferred pH value of aqueous phase.

3.2.3 Effect of extraction temperature

Figure 7 illustrates the effect of extraction temperature on the extraction efficiency and separation coefficient with DZ988N concentration     of 20%,initial pH of aqueous phase of 2.0, extraction time of 5 min, phase ratio O/A of 1/1, and extraction temperature in the range of 25 ℃ to 75 ℃.

Figure 7  Effect of extraction temperature on (a) extraction efficiency and (b) separation factor

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The extraction process was performed in a sealed test tube. As the results shown in Figure 7, both the copper extraction efficiency and separation coefficient increased along with temperature from 25 ℃ to 75 ℃. The high temperature can facilitate the mass transfer between the two phases; therefore, the early increase in temperature will promote copper extraction efficiency [36]. However, excessive temperature rise will hinder the extraction reaction to some extent due to the exothermic reaction of copper extraction, which would play a more dominant role than mass transfer [37]. The aqueous phase and organic phase will volatile at high temperature in an open extraction environment. After 5 min extraction, the loss of two phases was 4% at 25 ℃, and 46% at 75 ℃, respectively. Considering both the copper extraction efficiency and the volatilization loss, 25 ℃ was suggested as the optimal extraction temperature where the copper extraction efficiency can reach 95.59% with almost no volatilization loss in two phases, and βCu/Co, βCu/Fe, βCu/Zn were 1385, 1088, 1941, respectively.

3.2.4 Effect of extraction time

The extraction time of two phases in the range of 1 to 10 min was selected as the factor to investigate its effect on the extraction efficiency and separation coefficient (βA/B) under fixed conditions of DZ988N concentration of 20%, pH of aqueous phase of 2.0, extraction temperature of 25 ℃, and phase ratio O/A of 1/1. The results are shown in Figure 8.

Figure 8  Effect of extraction time on (a) extraction efficiency and (b) separation factor

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It was observed in Figure 8(a) that the metals’ extraction efficiency increased with extraction time increasing from 1 to 6 min, and kept constant when extraction time continuously rose. After 6 min extraction, the extraction capacity of DZ988N almost achieved equilibrium, and the extraction efficiencies of Co(II), Fe(II), Cu(II) and Zn(II) were 1.70%, 2.12%, 97.50% and 1.11%, respectively. In Figure 8(b), the separation coefficients βCu/Co, βCu/Fe, and βCu/Zn kept growth with an increase in extraction time, and reached 2255, 1797 and 3468 at the extraction time of 6 min, respectively, while βCu/Zn decreased at 3 min due to the growth of DZn greater than that of DCu. Therefore, 6 min was selected as the suitable extraction time to achieve an acceptable copper extraction.

3.2.5 Effect of phase volume ratio O/A

The phase volume ratio O/A also has a significant influence on the copper extraction process. The experiments were carried out under the conditions of DZ988N concentration of 20%, pH of aqueous phase of 2.0, extraction temperature of 25 ℃, extraction time of 5 min. The phase volume ratio O/A decreased from 3.0/1 to 1/4.0, and the results were presented in Figure 9.

Figure 9  Effect of phase ratio O/A on (a) extraction efficiency and (b) separation volume factor

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Figure 9 shows that both the extraction efficiency and separation factor generally presented a downward trend with the phase ratio O/A rising. When O/A decreased from 3.0/1 to 1/1.5, Cu(II) extraction efficiency decreased slowly from 98.44% to 95.06%. However, a significant drop in separation coefficients βCu/Co, βCu/Fe and βCu/Zn, decreasing from 2477, 2091 and 3030 to 1450, 1056 and 1867, respectively, were detected. When O/A decreased to 1/4.0, copper extraction decreased obviously to 57.94%, because of the limited extraction capacity of DZ988N for copper. Extraction at a low O/A will lead to a high depletion of the extractant, therefore, 1/1.5 was considered as a suitable phase volume ratio O/A.

A one-stage extraction was carried out under the optimal conditions achieved from above experiments, and the extraction efficiencies of Co(II), Fe(II), Cu(II) and Zn(II) were 1.56%, 2.03%, 97.53% and 1.08%, respectively; the separation coefficients βCu/Zn, βCu/Fe and βCu/Co, were 2492, 1905 and 3306, respectively, indicateing that DZ988N had a high extraction for copper in sulfuric acid solution, while cobalt, iron and zinc were hardly extracted and can be separated well with copper.

3.3 Counter-current extraction

The McCabe-Thiele diagrams including equilibrium isotherm and operation line, were built to determine the stages of counter-current extraction. The extractions were performed under the conditions of DZ988N concentration of 25%, pH of aqueous phase of 2, extraction temperature of 25 °C, extraction time of 6 min. The phase volume ratios O/A of 1/1.0, 1/1.5 and 1/2.0 were investigated, respectively. The copper equilibrium concentrations in the two phases are presented in Table 1, and the parameters of operation line defined by Eq. (6) with an aim of [Cu2+]aq less than 0.005 g/L, are presented in Table 2.

Table 1  Copper equilibrium concentrations in two phases

m V(O)/V(A)=1/1.0 V(O)/V(A)=1/1.5 V(O)/V(A)=1/2.0

Xm/

(g·L-1)

Ym+1/

(g·L-1)

Xm/

(g·L-1)

Ym+1/

(g·L-1)

Xm/

(g·L-1)

Ym+1/

(g·L-1)

1 0.1114 5.1391 0.2147 7.5538 0.4667 9.5677

2 1.3450 9.0446 1.8385 12.6718 3.0231 14.0226

3 2.4175 11.8776 4.4225 13.9138 4.9195 14.6846

4 4.3605 12.7676 5.1095 14.1253 5.1610 14.8636

5 5.1305 12.8876 5.2230 14.1665 5.2245 14.9156

6 5.1965 12.9416 5.2505 14.1665 5.2505 14.9156

7 5.2505 12.9416 — — — —

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Table 2  Parameters of operation lines at different O/A

V(O)/V(A) X0/(g·L-1) Xm/(g·L-1) Y1/(g·L-1) Ym+1/(g·L-1)

1/1 5.2505 0.005 5.2455 0

1/1.5 5.2505 0.005 7.8683 0

1/2 5.2505 0.005 10.491 0

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According to the data in Tables 1 and 2, the equilibrium isotherm and the operation line at each phase volume ratio O/A are graphically presented in Figure 10. The number of steps drawn between equilibrium isotherm and operation line were defined as the needed stages of counter-current extraction. Figure 10 illustrates that when the O/A was 1/1.0, 1/1.5 and 1/2.0, the required stages were 2, 2 and 3, respectively. According to Figure 9(a), copper extraction efficiency decreased with increasing O/A. Therefore, more stages of counter-current extraction were suggested to achieve the acceptable copper concentration in raffinate.

Figure 10  McCabe-Thiele diagrams for copper extraction under different phase volume ratios: (a) V(O)/V(A)=1/1.0; (b) V(O)/V(A)=1/1.5; (c) V(O)/V(A)=1/2.0

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Under the optimal extraction conditions achieved from the single factor experiments, a two-stage counter-current extraction process was carried out. The copper extraction efficiency was 99.92%, and [Cu2+]aq=0.0044 g/L≤0.005 g/L. Obviously, the counter-current extraction improved copper extraction compared with the single-stage extraction.

3.4 Extraction-stripping

Using acidic water of pH=3.0 can wash away most impurity ions carried in the extraction process, and the removal rate of each metal ion was: Cu 0.23%, Co 93.24%, Fe 94.267% and Zn 92.384%. Then, for efficient stripping of copper along with loaded organic, the concentration of H2SO4 varied in the range of 0.5 to 3.0 mol/L under conditions         of phase ratio O/A 1:1.0, temperature 25 °C and contact time 5 min.

The results presented in Figure 11 show that only 68.21% Cu was stripped with 0.5 mol/L H2SO4. Further increase in the acid concentration to 1.5 mol/L yielded 99.92% Cu and about 0.7% other metals stripping, indicating the complete recovery of copper.

Figure 11  Effect of H2SO4 concentration on stripping rate

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According to the previous experimental results, the distribution of various metals in organic and aqueous phases during the extraction recovery of copper was calculated, as shown in Table 3. In the whole recovery process, 99.61% Cu was finally enriched in the stripped solution, and a total of 0.39% copper was lost, and 98.34 % Co, 97.80% Fe and 98.66% Zn remained in the raffinate, indicating a pure Cu2SO4 solution with a concentration of 7.844 g/L.

Table 3  Distribution of metals in process of copper recovery by DZ988N

Metal ion Raffinate Washing water Stripped solution Stripped solvent

[Cu2+]/(g·L-1) Proportion/% [Cu2+]/(g·L-1) Proportion/% [Cu2+]/(g·L-1) Proportion/% [Cu2+]/(g·L-1) [Cu2+]/%

Cu2+ 0.004 0.08 0.018 0.23 7.844 99.61 0.006 0.08

Co2+ 1.249 98.34 0.030 1.55 0 0 0.002 0.11

Fe2+ 2.113 97.80 0.067 2.07 0 0 0.004 0.13

Zn2+ 11.040 98.66 0.208 1.24 0 0 0.017 0.10

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4 Conclusions

1) The chelating mechanism of DZ988N was investigated. When Cu(II)-contained aqueous phase was mixed with DZ988N, copper will substitute H in H…N—O group, and the hydrogen bonds disappeared. As a result, Cu(II) was chelated and extracted by DZ988N.

2) The optimal extraction conditions were determined as: DZ988N concentration 25%, pH value of aqueous phase 2.0, extraction temperature 25 °C, time 6 min, and phase ratio O/A of 1/1.5. Under the optimal conditions, extraction efficiencies of Co(II), Fe(II), Cu(II) and Zn(II) reached 1.56%, 2.03%, 97.53% and 1.08%, respectively, with a single-stage extraction; the separation coefficients βCu/Zn, βCu/Fe and βCu/Co, were 2492, 1905 and 3306, respectively.

3) The number of stages for counter-current extraction was determined by the McCabe-Thiele diagrams. When the phase ratios O/A were 1/1.0,

1/1.5 and 1/2.0, the needed extraction stages were 2 , 2 and 3, respectively. Under the previously obtained optimal extraction conditions, extraction efficiency of copper reached 99.92% and [Cu2+]aq=0.0044 g/L when applying a two-stage counter-current extraction process.

4) In the whole copper recovery process, 99.61% Cu was enriched into the reverse extraction solution, and 0.39% was lost. Finally, a pure Cu2SO4 solution with a concentration of 7.844 g/L was obtained.