Purification of concentrated nickel sulfuric liquors via synergistic solvent extraction of calcium and magnesium using mixtures of D2EHPA and Cyanex 272

Journal Pre-proofs Purification of concentrated nickel sulfuric liquors via synergistic solvent extraction of calcium and magnesium using mixtures of D2EHPA and Cyanex 272 Alexandre Silva Guimarães, Lucas Andrade Silva, Alexandre Moni Pereira, Julio Cesar Guedes Correia, Marcelo Borges Mansur PII: DOI: Reference:

S1383-5866(19)32043-X https://doi.org/10.1016/j.seppur.2020.116570 SEPPUR 116570

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

15 May 2019 3 January 2020 13 January 2020

Please cite this article as: A. Silva Guimarães, L. Andrade Silva, A. Moni Pereira, J. Cesar Guedes Correia, M. Borges Mansur, Purification of concentrated nickel sulfuric liquors via synergistic solvent extraction of calcium and magnesium using mixtures of D2EHPA and Cyanex 272, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116570

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Purification of concentrated nickel sulfuric liquors via synergistic solvent extraction of calcium and magnesium using mixtures of D2EHPA and Cyanex 272 Alexandre Silva Guimarães ¹, Lucas Andrade Silva ², Alexandre Moni Pereira ², Julio Cesar Guedes Correia ² and Marcelo Borges Mansur ³ * ¹ Programa de Pós-graduação em Engenharia Metalúrgica, Materiais e de Minas, Universidade Federal de Minas Gerais (UFMG), Av. Antônio Carlos, 6627, Campus da Pampulha, 30160-030 Belo Horizonte, MG, Brazil ² Laboratório de Modelagem Molecular, Centro de Tecnologia Mineral (CETEM), Av. Pedro Calmon, 900, Cidade Universitária, 21941-908 Rio de Janeiro, RJ, Brazil ³ Programa de Engenharia Metalúrgica e de Materiais, Universidade Federal do Rio de Janeiro (UFRJ), Av. Horácio Macedo, 2030, Centro de Tecnologia, Cidade Universitária, 21941-598 Rio de Janeiro, RJ, Brazil * Corresponding author, E-mail: [email protected]

ABSTRACT A synergistic solvent extraction circuit consisting of D2EHPA and Cyanex 272 was applied to purify a concentrated Ni sulfuric solution ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1) similar to the MHP industrial liquors that feed the electrowinning step. The temperature was identified as a key parameter to optimize Mg extraction as well as to promote the selective recovery of coextracted Ni in the extraction stages and make the stripping of Ca feasible using HCl as a stripping agent. Ca was crowded out by Ni from the loaded D2EHPA + Cyanex 272 solution and limited the operating pH range between 2 and 4. Under optimum conditions, 98.0% Ca and 98.4% Mg were simultaneously extracted in two and three theoretical stages by D2EHPA (0.30 M) + Cyanex 272 (0.32 M) mixture (pH 4, A/O ratio = 0.7 and 50°C). Co-extracted Ni was selectively recovered (99.7%) from the loaded D2EHPA + Cyanex 272 using [H2SO4] = 0.063 M in three stages (O/A ratio = 1 and 50°C). Ca and Mg can be completely stripped out from the loaded D2EHPA + Cyanex 272 at 50°C in only 1 ([HCl] = 3 M, O/A ratio = 5) or two theoretical stages ([H2SO4] = 1 M, O/A ratio = 2). The purified liquor containing 80 g.L-1 Ni (99.9% purity) can be directed for the electrowinning step to produce Ni cathodes. Extraction and stripping kinetics are rapid (equilibrium reached within 5 minutes). Thermodynamic analysis was done to support the synergism/antagonism trends observed experimentally. Keywords: Synergistic solvent extraction, Cyanex 272, D2EHPA, calcium, magnesium, nickel purification.

1. Introduction The hydrometallurgical processing of Mixed Hydroxide Precipitates (MHP) is a typically used route to recover Ni and Co from HPAL laterite leach solutions, after the removal of Fe, Al, and Cr by precipitation [1−6]. Once transported to the refinery, the intermediate product, MHP, can be re-leached using sulfuric acid (with or without oxidant agent) to obtain a concentrated solution of Ni (70-90 g.L1),

containing Co and various impurities, such as Ca, Cu, Mn, Mg, and Zn, at

lower concentrations levels [7-9]. Before sending it to the Ni electrowinning step, this solution is normally treated by solvent extraction aiming to recover Co and remove the remaining impurities in order to obtain high purity Ni cathodes as the final product [7,9,10,11]. In the solvent extraction purification circuit, Cyanex 272 (bis(2,4,4-trimethylpenthyl) phosphinic acid) is commercially used; it selectively extracts Zn (pH 2.5, βZn/Ni = 11,865), Co, Cu, and Mn (pH 3.9, βCo/Ni = 173, βCu/Ni = 829, βMn/Ni = 455), leaving Ni in the MHP liquor [12]. However, Ca and Mg remain in the liquor in concentration levels that may severely affect the electrowinning step and the quality of the Ni products. In fact, Ca may precipitate on the diaphragms that coat the Ni cathodes, in turn disrupting the flow of MHP liquor in the electrowinning tanks [13], while Mg may cause morphological and microstructural alterations in the Ni products, thus affecting its mechanical properties. Consequently, the stages of stripping and cutting of Ni deposits can be affected [14,15]. Therefore, the Ni electrowinning liquor must be free or present a low content of impurities, such as Ca and Mg. Remarkable improvements in the efficiency of extraction and separation of various metal species Ni and Co from impurities Ca, Mn, and Mg have been successfully reported in recent decades through the use of synergistic solvent extraction [16-21]. This method consists of mixing two or more extractants. It is evaluated using the synergism enhancement factor R, which is defined by the ratio between the metal distribution for the mixture of extractants and the sum of the individual distributions obtained for each extractant separately, measured under comparable conditions. If synergism occurs (R > 1), the mixture of extractants resulted in a better separation than those using each extractant separately, otherwise antagonism is verified (R < 1) and the opposite behavior

occurs. The first synergistic systems aiming at the separation of Ni from impurities consisted of the binary mixture of carboxylic acids (such as 2-methyl-2-ethylheptanoic acid (Versatic 11), naphthenic acid, 2-bromodecanoic acid) with aliphatic hydroxyoxime (5,8 diethyl-7-hydroxy-6-dodecane oxime (LIX 63)) and pyridine carboxylate esters (4-5-nonyl-pyridine); and also binary mixtures of aliphatic hydroxyoxime with organophosphorus acids (di-2-ethylhexyl phosphoric acid (D2EHPA), Cyanex 272 and 2-ethylhexylphosphonic acid, mono-2ethylhexyl ester (Ionquest 801)) and DNNSA (di-nonyl-naphthalene sulphonic acid) [22-29]. Given the number of available commercial reagents, the quantity of distinct extractive systems that could be created using mixtures of extractants is considerable [30]. The synergistic solvent extraction method was applied to treat the MHP depleted raffinate containing Zn-Co-Cu-Mn [31,32]. The study aimed to identify the best binary combination between D2EHPA (di-2-ethylhexyl phosphoric acid), Cyanex 272 and naphthenic acid in order to maximize the Ca and Mg extractions leaving Ni in the raffinate. The mixture of Cyanex 272 (0.32 M) + naphthenic acid (0.74 M) resulted in antagonism for Ca, Mg, and Ni (R < 1, with low Ca and Mg extractions at 2 ≤ pH ≤ 6), while a pronounced synergistic effect was observed for Ca (1.1 ≤ R ≤ 63), as compared to Mg (1.2 ≤ R ≤ 3.8) and Ni (1.2 ≤ R ≤ 3.2), using the mixture of D2EHPA (0.3 M) + naphthenic acid (0.74 M) at 2 ≤ pH ≤ 6. The later system is selective only for Ca (72% and βCa/Ni = 41 at pH 3.5), leaving both Mg (~77%) and Ni (~72%) in the raffinate. On the other hand, the mixture D2EHPA (0.15, 0.30 and 0.60 M) + Cyanex 272 (0.64 M) resulted in synergism for Ca (1.1 ≤ R ≤ 58) and Mg (1.1 ≤ R ≤ 5.1) extractions, as well as an antagonism for Ni (R < 0.9 at pH < 5). In fact, Ca (65-67%) and Mg (33-69%) can be simultaneously extracted from the concentrated Ni sulfuric liquor with only 2-8% Ni co-extraction at 3.5 ≤ pH ≤ 4.5. The Ca/Ni and Mg/Ni selectivity depend on the pH and the concentration of D2EHPA, reaching βCa/Ni = 18-124 and βMg/Ni = 4-28 at 3.5 ≤ pH ≤ 4.5. Based on these results, the mixture of D2EHPA + Cyanex 272 was identified as a promising system for purifying concentrated Ni sulfuric liquor. In this context, the present work aims on investigating the following operating parameters in order to optimize the selective Ca and Mg removal over Ni from

MHP liquors using the synergistic system D2EHPA + Cyanex 272: temperature, number of stages, organic to aqueous volume ratio, time, and concentrations of H2SO4, HCl, and extractants. Extraction and stripping conditions were studied. 2. Experimental 2.1 Aqueous and organic solutions The concentrated Ni sulfuric synthetic solution was prepared by dissolving analytical grade sulfates of Ca, Mg, and Ni (Synth, 98% purity) in distilled water resulting in a working solution ([Ca] = 0.5 g.L-1, [Mg] =3.1 g.L-1, [Ni] = 88 g.L-1) similar to the MHP re-leach liquor after the removal of Fe [5,33]; Cu [9]; and Co, Mn and Zn [12]. Analytical grade H2SO4 (Synth, 95% purity) and HCl (Synth, 36.5% purity) were used in the stripping tests. The pH adjustment was performed by dropping NaOH solution (1.0, 2.5, 5.0, and 10.0 M; analytical grade, Synth, 99% purity). The organic solutions were prepared by dissolving the extractants D2EHPA (Baysolvex-Lanxess, 95% purity) and/or Cyanex 272 (Cytec Canada, 85% purity) in Exxsol D80 (ExxonMobil, aliphatic kerosene, aromatic content ≤ 0.5% w/w) to achieve the required concentration. All reagents were used without further purification and no modifier agent was used. 2.2 Solvent extraction and stripping tests The extraction tests were performed using a glass reactor (1 L) immersed in a temperature-controlled water bath. When required, both aqueous and organic solutions were pre-heated separately to reach 30°C, 40°C, and 50°C. At a given A/O ratio, the solutions were mechanically stirred (Ika, model RW 20N) for 10 minutes using a marine-type impeller. The mixture was then allowed to stand for 5 min to obtain phase disengagement. The pH was measured using a pH electrode (Quimis, model 0400AS, with temperature corrector) and aliquots of both phases were withdrawn. The metal distribution isotherms were determined

at changing A/O ratios by mixing the synthetic sulfuric liquor with the organic solution at a selected pH and temperature. The crowding out effect was evaluated by means of kinetic extraction tests. Based on preliminary experiments, NaOH solution (10 M) was dripped in the aqueous-organic mixture to maintain the pH approximately constant during tests. Samples of the mixing solutions were taken at given times using syringes. The stripping tests were carried out by contacting the loaded organic solution with the strip aqueous solution containing HCl or H2SO4 at a selected O/A ratio and temperature. The metal strip distribution isotherms were obtained at changing O/A ratios by mixing the loaded organic solution with the HCl and H2SO4 strip solutions at a given temperature. The concentration of metals in the aqueous solutions was determined quantitatively by atomic absorption (GBC, model XplorAA Dual). The content of the metal species in the organic solution was assessed by mass balance. 3. Results and discussion 3.1 Metals extraction A series of tests was carried out to select the best composition of the D2EHPA + Cyanex 272 mixture and the operating pH in order to purify the synthetic MHP liquor at an A/O ratio of 1:1, 50°C and 3.5 ≤ pH ≤ 4.5. The results are shown in Fig. 1. In general, the mixtures of D2EHPA + Cyanex 272 presented more affinity to extract both Ca and Mg compared to the use of D2EHPA and Cyanex 272 alone, resulting in synergism for Ca/Ni and Mg/Ni separation. A maximum of selectivity was verified at X[Cyanex 272] = 0.5. It was also observed a higher affinity of D2EHPA for Ca, mainly at lower pH levels, and a higher affinity of Cyanex 272 for Mg, mainly at higher pH levels. In fact, increasing the pH to 4.5, the βCa/Ni selectivity is undesirably reduced up to 22.4 times because Ca is crowded out by Ni from the loaded organic solution [31,32]. Although at pH 3.5 and X[Cyanex 272] = 0.5, the βCa/Ni selectivity is high (93), the βMg/Ni is relatively low (19) due to low Mg

extraction (31%). Therefore, based on these results, the organic solution containing 0.30 M D2EHPA + 0.32 M Cyanex 272 (at X[Cyanex 272] = 0.5) and pH 4 were selected for continuing tests aiming to achieve a high simultaneous removal of Ca and Mg from Ni in the same operation.

Fig. 1: Selectivities βCa/Ni and βMg/Ni at 3.5 ≤ pH ≤ 4.5 by using D2EHPA (0.60 M), Cyanex 272 (0.64 M), and mixtures of D2EHPA + Cyanex 272 ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, 50 °C, and A/O ratio = 1). The effect of temperature on the selective extraction of Ca and Mg from the Ni concentrated sulfuric liquor was evaluated using the system D2EHPA (0.30 M) + Cyanex 272 (0.32 M) and pH 4. Once the increase in Ca/Ni or Mg/Ni selectivities are associated with the respective higher extractions of Ca or Mg over Ni, the results shown in Fig. 2 reveal that Ca extraction is negatively affected when temperature was increased from 25°C to 50°C, while the opposite behavior was verified for Mg extraction. Though not very significant, the decrease in Ca extraction with temperature is possibly related to the crowding out effect of Ca by Ni, given the shift on the 𝑝𝐻𝐶𝑎 1/2 to slightly less acidic conditions. On the other hand, the increase in Mg extraction with temperature is significant, increasing βMg/Ni from 7 (25°C) to 29 (50°C) and shifting to more acidic conditions. Therefore, 50°C was chosen as the optimum temperature to purify the concentrated Ni sulfuric liquor, the same value normally used in commercial Co/Ni separation with Cyanex 272.

Fig. 2: Effect of temperature on pH1/2 and βMetal/Ni for (a) Ca and (b) Mg (organic phase: 0.30 M D2EHPA + 0.32 M Cyanex 272, diluted in Exxsol D80; aqueous phase: [Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, pH 4; A/O ratio = 1). As shown in Fig. 3, the sequence of metal extraction from the MHP synthetic liquor with pH is Ca > Mg >> Ni for pH < 4.5. The extraction of Ca increased at pH < 3, reaches a plateau of 70% at 3.2 ≤ pH ≤ 4.0, then gradually decreases until reaching 19% at pH 6. This fact occurs because the Ni extraction increases with the decrease in liquor acidity; consequently, Ca is scrubbed off from the loaded organic phase. Such behavior, named crowding out effect, is pH and time dependent, as shown in Fig. 4. For example, at pH 2.5 and 4.0, Ca was not

scrubbed off from the loaded D2EHPA + Cyanex 272, because Ni extraction over 10 minutes was only 1% (0.9 g.L-1) and 2% (1,8 g.L-1), respectively. However, at pH 6, 41% Ca were extracted in the first 2 minutes. Nevertheless, at the same time there were 9.3% (8.9 g.L-1) and 8.3% (8.1 g.L-1) of additional Ni extraction compared to pH 2.5 and 4.0. As a result of higher Ni extraction (Ni concentration is 176 times higher than that of Ca), 27% Ca were crowded out (or scrubbed off) from the loaded extract solution by Ni and returned to the liquor within 10 minutes. Contrariwise, Mg was not crowded out by Ni, since its concentration is 6.2 times higher than that of Ca. Similar results were experienced by Guimarães and Mansur [31], when Ca was extracted from concentrated Ni sulfate solution at pH > 3 using only D2EHPA as an extractant. Then, according to these results, the Ca/Ni separation should be avoided at pH > 4 because the concentration of Ca in the loaded D2EHPA + Cyanex 272 drops due to the crowding out effect of Ni (Table 1). In additon, the extractions of Ca, Mg, and Ni are satisfactorily rapid, reaching equilibrium in 2, 3, and 5 minutes, respectively, and remaining constant over 60 minutes at pH 4.

Fig. 3: Metal extraction from the synthetic MHP liquor by D2EHPA (0.30 M) + Cyanex 272 (0.32 M), diluted in Exxsol D80 ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, 50 °C, A/O ratio = 1).

Fig. 4: Kinetic behavior of metal extraction at pH 2.5 (dashed line), pH 4 (continuous line), and pH 6 (dotted line) by D2EHPA (0.30 M) + Cyanex 272 (0.32 M) ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, 50 °C, A/O ratio = 1). Table 1: Metal loading and separation factors (βCa/Ni and βMg/Ni) at changing pH by D2EHPA (0.30 M) + Cyanex 272 (0.32 M) mixture ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, 50 °C, A/O ratio = 1). Equilibrium

In loaded organic solution (g/L)

βCa/Ni

βMg/Ni

0.46

54

12

0.42

0.82

69

16

0.28

0.47

0.89

93

17

3.0

0.36

0.61

1.16

135

18

3.3

0.39

0.77

1.71

107

16

3.5

0.39

1.00

2.11

93

19

4.0

0.39

1.60

2.98

66

29

4.5

0.32

1.96

4.66

24

30

5.0

0.26

2.26

7.90

9

25

5.5

0.19

2.47

11.29

4

23

6.0

0.09

2.65

16.50

1

21

pH

[Ca]

[Mg]

[Ni]

2.0

0.13

0.18

2.5

0.22

2.7

The synergistic and antagonistic effects of the D2EHPA (0.30 M) + Cyanex 272 (0.32 M) system on the Ca, Mg, and Ni extractions were evaluated by calculating the synergistic enhancement factor (RMetal) according to Eq. 1 at 2 ≤ pH ≤ 6 [30].

𝑅𝑀𝑒𝑡𝑎𝑙 =

𝐷𝐷2𝐸𝐻𝑃𝐴 + 𝐶𝑦𝑎𝑛𝑒𝑥 272

(1)

𝐷𝐷2𝐸𝐻𝑃𝐴 + 𝐷𝐶𝑦𝑎𝑛𝑒𝑥 272

wherein DD2EHPA + Cyanex272, DD2EHPA, and DCyanex 272 are the distribution coefficient ratio of a given metal (Ca, Mg or Ni) extracted by the D2EHPA (0.30 M) + Cyanex 272 (0.32 M) mixture, D2EHPA (0.30 M) and Cyanex 272 (0.32 M), respectively. R > 1 indicates synergism, while R < 1 represents antagonism [30,34,35]. The effect of pH on the synergism enhancement factor of Ca, Mg, and Ni is shown in Fig. 5. At pH ≤ 3.5, it can be verified antagonism (logRCa ≤ 0) in the extractions + 𝐶𝑦𝑎𝑛𝑒𝑥 272 of Ca because 𝐷𝐷2𝐸𝐻𝑃𝐴 < 𝐷𝐷2𝐸𝐻𝑃𝐴 . On the other hand, at pH > 3.5 the 𝐶𝑎 𝐶𝑎

presence of Cyanex 272 (0.32 M) in the organic solution containing D2EHPA (0.30 M) may have attenuated the crowding out effect of Ni resulting in synergism + (logRCa > 0), since 𝐷𝐷2𝐸𝐻𝑃𝐴 𝐶𝑎

𝐶𝑦𝑎𝑛𝑒𝑥 272

272 > 𝐷𝐷2𝐸𝐻𝑃𝐴 while 𝐷𝐶𝑦𝑎𝑛𝑒𝑥 ≈ constant. The 𝐶𝑎 𝐶𝑎

logRCa decreased from 2.17 at pH 5 to 1.63 at pH 6 due to the higher Ni extraction, which causes a greater crowding out effect on Ca extraction (see Table 1). In relation to Mg, synergism is observed throughout the studied pH range, indicating that the D2EHPA (0.30 M) + Cyanex 272 (0.32 M) system is more efficient in extracting Mg, as compared to the sum of individual extractants D2EHPA (0.30 M) and Cyanex (0.32 M). A plateau of synergism (0.55 ≤ logRMg ≤ 0.56) is obtained at 4.5 ≤ pH ≤ 5.0. After this point, the logRMg values decrease to 0.33 at pH 6, because there was a significant increase in the extractions of Mg by Cyanex 272 (0.32 M). Regarding Ni, antagonism is obtained at 2 ≤ pH ≤ 5, favoring its separation from Ca and Mg. At pH > 5, there is a synergistic effect on Ni extractions, resulting in an undesired reduction in βCa/Ni and βMg/Ni selectivities (Table 1).

Fig. 5: Effect of pH on the logRMetal values of Ca, Mg, and Ni by the D2EHPA (0.30 M) + Cyanex 272 (0.32 M) system, diluted in Exxsol D80 ([Ni] = 88 g.L-1, [Mg] = 3.1 g.L-1, [Ca] = 0.5 g.L-1, 50 °C, A/O ratio = 1). The isotherms of Ca and Mg distributions and their respective McCabe-Thiele diagrams were obtained at changing A/O ratios (1:5, 1:3, 1:1, 2:1, 3:1, 5:1, and 10:1), pH 4 and 50°C. It can be seen from Fig. 6 that the Ca isotherm is steeper than those of Mg due to the higher extractions of Ca. However, the loading capacity of Ca is limited up to 0.65 g.L-1, because at high A/O ratios (5:1 and 10:1) the competition between Ca and Ni by D2EHPA and Cyanex 272 increases; consequently, Ni crowds Ca out of the organic solution. Regarding Mg, it could be verified that, although it was not possible to determine its maximum loading capacity under such experimental conditions, only 4.4 g.L-1 were obtained in the organic solution along the isothermal distribution, thus indicating that its loading capacity is low. According to the McCable-Thiele diagrams, only 2 and 3 theoretical stages are required to extract Ca and Mg from the MHP synthetic liquor within an operation A/O ratio of 1:1.4, producing a raffinate solution containing only 0.01 g.L-1 Ca and 0.05 g.L-1 Mg.

Fig. 6: McCabe-Thiele diagrams for Ca and Mg extractions from concentrated Ni sulfuric liquor by D2EHPA (0.30 M) + Cyanex 272 (0.32 M), diluted in Exxsol D80 ([Ca] = 0.5 g.L-1, [Mg] = 3.1 g.L-1, [Ni] = 88 g.L-1, pH 4, 50 °C). 3.2 Thermodynamic analysis of metal extraction The data fitting procedure described by Innocenzi and Veglio [36] was modified to incorporate the findings of Darvishi et al. [37], in which mixtures of organophosphorus extractants are used to determine the real moles of the D2EHPA + Cyanex 272 mixture involved in the extraction of Ca, Mg, and Ni from the MHP sulfuric synthetic liquor. The model, given by the following equilibrium reaction, assumes: + 𝑀(𝑎𝑞)+ 𝑛(𝑅/𝑅′)𝐻(𝑜𝑟𝑔) 𝑀(𝑅/𝑅′)𝑛(𝑜𝑟𝑔) + 𝑛𝐻(𝑎𝑞)

(2)



real concentrations of Ca, Mg, and Ni in the synthetic sulfuric liquor;



simultaneous co-extraction of Ca, Mg, and Ni, and the formation of nonextractable intermediate complexes, such as metal ion-other metal ion, by a mixture of extractants where R and R’ denote D2EHPA and Cyanex 272 molecules, respectively;



formation of adducts and ionization of some of extractant and diluent molecules;



Eq. (2) is valid at high metal loadings by single organophophorous extractants [30], since it incorporates the simultaneous homogeneous reaction ocurring in the organic solution producing extractant molecules in the form of dimers [38,39].

where M represents all metal species in the aqueous solution (Ca, Mg, and Ni), (R/R’)H is the mixture of extractants, n is the stoichiometric number of extractant moles involved in the reaction obtained by data fitting and M(R/R’)n represents all organometallic complex species. The overall apparent equilibrium constant, K, can be defined as:

𝐾 = ∏𝐾𝑖 =

(

𝑛 [𝑀 (𝑅/𝑅′) ] [𝐻 + ] 𝑖 𝑛 ∏ [𝑀 ] [(𝑅/𝑅′)𝐻] 𝑖

)

=𝐷

(

𝑛 [𝐻 + ] [(𝑅/𝑅′)𝐻]

)

(3)

where i = Ca, Mg, and Ni and D = DCaDMgDNi. Taking logarithm of Eq. (3): log(D) = log(K) + n pH + n log[(R/R’)H]

(4)

The equilibrium concentrations of the extractants can be calculated by: [(𝑅/𝑅′)𝐻](𝑜𝑟𝑔) = [(𝑅/𝑅′)𝐻]𝑖𝑛𝑖𝑡𝑖𝑎𝑙 - ∑𝑖[𝑀𝑖(𝑅/𝑅′)𝑛](𝑜𝑟𝑔)

(5)

[𝑀𝑖(𝑅/𝑅′)𝑛](𝑜𝑟𝑔) =([𝑀𝑖]𝑖𝑛𝑖𝑡𝑖𝑎𝑙(𝑎𝑞) - [𝑀𝑖]𝑓𝑖𝑛𝑎𝑙(𝑎𝑞))Vaq/Vorg

(6)

where Vaq and Vorg are the volume of aqueous and organic phases, respectively. The data fitting procedure by slope analysis at pH range 2-4 (because Ca is crowded out by Ni at pH > 4) is shown in Fig. 7. The experimental data were fitted on straight lines with R2 > 0.86. The sum of the real moles of D2EHPA + Cyanex 272 mixture involved in the extraction reactions of 1 mol of Ca, Mg, and Ni are 0.4869, 0.5849, and 0.4211 moles, respectively, obtaining ntotal = 1.4929 moles. It can be inferred from these tests that the simultaneous extraction of 0.5 g.L-1 Ca (0.012 M), 3.1 g.L-1 Mg (0.125 M), and 88 g.L-1 Ni (1.5 M) from the synthetic liquor

requires 0.0061, 0.073, and 0.63 moles of D2EHPA + Cyanex 272 mixture resulting in ntotal = nCa + nMg + nNi ≈ 0.7091 moles. The apparent equilibrium constants were calculated by substituting the values of distribution coefficient, equilibrium concentrations of D2EHPA + Cyanex 272, and equilibrium pH for each data into Eq. (3). The activity coefficients are assumed to be 1. The values of K for each metal was found to be: KCa = 0.077 > KMg = 0.0087 > KNi = 0.0013, while the overall apparent equilibrium constant K = KCaKMgKNi = 1.17x10-6 (Table 2).

Fig. 7: Real moles of D2EHPA + Cyanex 272 mixture to extract Ca, Mg, Ni, and (d) Ca+Mg+Ni from the sulfuric synthetic solution ([Ca] = 0.5 g.L-1, [Mg] = 3.1 g.L1,

[Ni] = 88 g.L-1, 50 °C, A/O ratio = 1).

Table 2: Thermodynamic parameters and number of moles of D2EHPA + Cyanex 272 to extract Ca, Mg, and Ni from the synthetic sulfuric liquor at pH 4. Ca

Mg

Ni

Moles of metal in the aqueous solution

0.012

0.125

1.500

Moles of D2EHPA + Cyanex 272 in the organic solution

0.0061

0.0730

0.6300

Apparent equilibrium constants (Ki)

0.0770

0.0087

0.0013

∆H (KJ.mol-1)

11.55

36.08

20.24

∆G (KJ.mol-1)

-2.78

-0.42

1.41

∆S (J.mol-1.K-1)

48.07

122.44

63.17

The change of enthalpy (∆H) for Ca, Mg, and Ni extraction reaction by D2EHPA + Cyanex 272 system at pH 4 and A/O ratio = 1 was calculated according to Eq. (7) [40,41]. The plots of log DMetal versus 1000/T are shown in Fig. 8. ∆log 𝐷𝑖 1

∆𝑇

=

―∆𝐻 2.303𝑅

(7)

wherein R is 8.314 J.mol-1.K-1. The change of Gibbs free energy (∆G) was determined as follows: ∆Gi = ∆Gi° + RTlnQ

(8)

wherein Q can be obtained according to Eq. 2:

Q=

[𝑀𝑖(𝑅/𝑅′)𝑛] [𝑀𝑖]

(

𝑛 [𝐻 + ] [(𝑅/𝑅′)𝐻]

)

(9)

At the equilibrium ∆G = 0, then: ∆Gi° = ― RTln𝐾𝑖

(10)

wherein Ki corresponds to the apparent equilibrium constant of each metal (Ca, Mg or Ni). The change of entropy (∆S) can be determined by Eq. (11):

∆S =

∆𝐻 ― ∆𝐺 𝑇

(11) The values of ∆H, ∆G, and ∆S (pH 4 and A/O ratio = 1) are shown at Table 2. It can be seen that the enthalpy variation of Ca, Mg, and Ni by the D2EHPA + Cyanex 272 system is positive, thus indicating that the extraction reaction of these metals (Eq. 2) is endothermically driven. The negative value of the Gibbs free energy for Ca and Mg reveals naturally spontaneous synergistic reactions,

while opposite behavior for Ni was identified resulting on antagonism character corroborating the experimental observations.

Fig. 8: Relationship between distribution coefficient of Ca, Mg, and Ni by D2EHPA (0.30 M) + Cyanex 272 (0.32 M) system and temperature at pH 4 ([Ca] = 0.5 g.L-1, [Mg] = 3.1 g.L-1, [Ni] = 88 g.L-1, A/O ratio = 1). 3.3 Metal stripping 3.3.1 Selective recovery of Ni Despite the low coextraction of Ni (3.3%), as compared to Ca (70%) and Mg (50%), at pH 4 (Fig. 3), the Ni concentration in the organic phase is significant; therefore, its recovery from the loaded organic solution at pH < 𝑝𝐻𝐶𝑎 1/2 = 2.8 is feasible [30]. Stripping tests were then performed to selectively recover Ni from the loaded D2EHPA + Cyanex 272 at T = 25°C and 50°C (O/A ratio = 1) using H2SO4 solution aiming to reuse the loaded strip liquor containing Ni in the MHP leaching step, thus avoiding both Ni losses and contamination of the circuit with chlorides. Tests at 0.01 M < [H2SO4] < 0.13 M shown in Fig. 9 revealed that the highest βNi/Ca and βNi/Mg selectivity values were obtained using 0.063 M H2SO4 (pH 0.9) at 50ºC as stripping solution. Temperature is a key factor on the Ni selective recovery. It can been seen also from Fig. 10 that decreasing the

temperature from 50°C to 25°C increases the number of successive stages from 3 to 7 and reduces the Ni recovery from 99.7% to 86%, respectively. In addition, the temperature reduction to 25°C also resulted in non-selective recovery of Ni, because Ca and Mg were stripped out together with Ni from the loaded D2EHPA + Cyanex 272, obtaining unsatisfactory values of βNi/Ca and βNi/Mg. Contrary, when T = 50°C, the recovery of Ni is very selective, obtaining in each stage increase in the values of βNi/Ca (22, 142, 256) and βNi/Mg (37, 270, 484), indicating greater efficiency in the separation of Ni/Metal from the loaded D2EHPA + Cyanex 272. From these tests, it can be concluded that 50°C is the optimum temperature to recover Ni from the loaded D2EHPA + Cyanex 272. The kinetics of selective Ni stripping at T = 50°C is very fast reaching an average recovery at an equilibrium of 51% in only 30 s (Fig. 11).

Fig. 9: Effect of temperature and H2SO4 concentration as stripping agent on [Ni]/[Metal]aqueous ratio (0.30 M D2EHPA + 0.32 M Cyanex 272 in Exxsol D80 containing [Ca] = 0,5 g/L; [Mg] = 3,03 g/L; [Ni] = 6,05 g/L; A/O ratio = 1; dotted curves: T = 25°C; continuous curves: T = 50°C).

Fig. 10: Effect of temperature and number of crosscurrent stages on the Ni recovery from the loaded D2EHPA + Cyanex 272 at [H2SO4] = 0.063 M and O/A ratio = 1 ([Ni] = 6.05 g.L-1, [Ca] = 0.50 g.L-1, [Mg] = 3.03 g.L-1) (Dotted curves: T = 25°C; continuous curves: T = 50°C; green curves: Ni stripping; red curves: βNi/Ca; blue curves: βNi/Mg).

Fig. 11: Metal stripping kinetics from the loaded D2EHPA (0.30 M) + Cyanex 272 (0.32 M) ([Ni] = 5.2 g.L-1, [Ca] = 0.50 g.L-1, [Mg] = 3.1 g.L-1, [H2SO4] = 0.063 M, 50 °C, O/A ratio = 1).

3.3.2 Stripping of Ca and Mg from the loaded D2EHPA + Cyanex 272 The stripping of Ca (~0.44 g.L-1) and Mg (~2.78 g.L-1) from the scrubedd D2EHPA + Cyanex 272 solution was evaluated using H2SO4 as a stripping agent. It was verified that the effect of decreasing the temperature from 50°C to 25°C (1 M and O/A ratio = 2) did not affect the stripping of Ca and Mg (variation < 1.2%). However, at O/A ratio ≥ 2 and [H2SO4] = 2 M, precipitation of Ca was observed in the loaded aqueous solution. This effect is undesirable because the precipitate may accumulate in pipes, pumps, and control valves causing hydraulic restrictions in continuous operation using mixer-setler device [42]. In order to avoid the formation of precipitates, the stripping distribution isotherms of Ca and Mg (O/A ratio = 1:2, 1:1, 3:2 and 2:1) was obtained using a strip solution containing 1 M H2SO4 at 50°C (temperature selected for comparison purposes). From Fig. 12, 2 and 1 theoretical stripping stages are required to strip 99.9% of Ca and Mg, respectively (O/A ratio = 2). Only 1 ppm of both metals remained in the organic solution and the final loaded aqueous solution contained approximately 0.5 g.L-1 Ca and 1.4 g.L-1 Mg. The stripping kinetic of Ca from the loaded D2EHPA + Cyanex 272 is rapid, reaching equilibrium in only 1 minute, with an efficiency of greater than 99%, as shown in Fig. 15, while those for Mg are a bit slower, reaching 99% in 5 minutes.

Fig. 12: Stripping distribution isotherms and McCabe-Thiele diagrams for Ca (0.44 g.L-1) and Mg (2.78 g.L-1) from the scrubbed loaded D2EHPA (0.30 M) + Cyanex 272 (0.32 M) by H2SO4 solution (1 M) and at 50°C. Based on previous study [32], the stripping operation was then alternativelly evaluated with HCl solution because chlorides of Ca, Mg and Ni are more soluble than their respective sulfates. Initially, the effect of the temperature (25°C and 50°C) on the simultaneous stripping of Ca (~0.44 g.L-1) and Mg (~2.78 g.L-1) from the scrubedd D2EHPA + Cyanex 272 solution was evaluated at 1 M ≤ [HCl] ≤ 3 M and O/A ratio = 10. As verified in Fig. 13, the temperature affected metal stripping, being more significant to Ca than to Mg. In fact, decreasing the temperature from 50°C to 25°C, Ca stripping dropped from 58.6% to 14.2% while Mg stripping dropped from 99.9% to 77.8% at [HCl] = 3 M. Consequently Ca and Mg stripping at low temperatures and high O/A ratios may require a greater number of stages for their complete removal from the organic solution. This may be disadvantageous for commercial applications. Based on these tests, 50°C and [HCl] = 3 M were selected to obtain the stripping distribution isotherms (O/A ratio = 1:1, 3:1; 5:1, 8:1, 10:1, and 12:1) and the McCable-Thiele diagrams.

Fig. 13: Effect of temperature and HCl concentration on Ca (0.44 g.L-1) and Mg (2.78 g.L-1), stripping from the scrubbed loaded D2EHPA (0.30 M) + Cyanex 272 (0.32 M) at O/A ratio = 10. As can be seen from Fig. 14, one theoretical stripping stage is required to strip 99.9% of Ca and Mg from the loaded D2EHPA + Cyanex 272 using O/A ratio = 5. Less than 2 ppm of both metals remained in the organic solution. The final loaded aqueous solution contains approximately 2.2 g.L-1 Ca and 13.9 g.L-1 Mg. The stripping kinetics of Ca and Mg is fast, reaching an equilibrium in 1 and 3 minutes, respectively, obtaining a recovery of higher than 99% (Fig. 15). This difference in Ca and Mg stripping kinetics may be related to the sum of the specific interfacial effects and the possibility of D2EHPA-Mg-Cyanex272 complexes being more hydrophobic, which would result in a slightly longer diffusivity time of Mg [43,44].

Fig. 14: Stripping distribution isotherms and McCabe-Thiele diagrams for Ca (0.44 g.L-1) and Mg (2.78 g.L-1) from the scrubbed loaded D2EHPA (0.30 M) + Cyanex 272 (0.32 M) by HCl solution (3 M) and at 50°C.

Fig. 15: Kinetic stripping of Ca (0.50 g.L-1) and Mg (3.1 g.L-1) from the loaded D2EHPA (0.30 M) + Cyanex 272 (0.32 M) by using HCl (1 M, dotted curves) and H2SO4 solutions (0.25 M, continuous curves) at O/A ratio 1:1. It can be concluded from the Ca and Mg stripping tests that using HCl would require less stages and water, the volume of which can be reduced by half. This may be advantageous for commercial applications, but washing of the purified organic stream may be required before feeding it in the Ni purification step to avoid any contamination with chlorides [18]. Future studies in semi and fully countercurrent continuous runs using mixer settlers are suggested to further optimize the operating conditions obtained in this work and collect data for process designs. 4. Conclusions The synergistic system D2EHPA + Cyanex 272 was successfully applied to selectively remove Ca and Mg aiming to purify concentrated Ni sulfuric liquors. This result could be used to treat MHP liquors before the Ni electrowinning step. Under optimal operating conditions, 98% Ca and 98.3% Mg were selectively removed from the synthetic liquor, using an organic phase containing D2EHPA (0.30 M) + Cyanex 272 (0.32 M) dissolved in Exxsol D80 in 3 theoretical stages at pH 4, 50°C, and A/O ratio = 0.7. Because of the high metal concentration

difference in the feed phase, Ni crowded Ca out of the loaded D2EHPA + Cyanex 272, thus limiting the extraction operating pH to be ≤ 4. The slope analysis method was used to calculate the apparent equilibrium constants for Ca, Mg, and Ni extractions by the synergistic system as well as thermodynamic parameters to support the synergistic/antagonism metal reaction trends experimentally observed. The increase in temperature is a decisive factor to (i) maximize Mg extraction; (ii) allow the selective recovery of co-extracted Ni in the extraction step by using diluted H2SO4 solution, and (iii) facilitate the Ca stripping by using concentrated HCl solution. At 50°C, the loaded D2EHPA + Cyanex 272 could be selectively stripped in 3 stages to obtain 99.7% Ni recovery (O/A ratio = 1). The extraction and stripping kinetics of Ca, Mg, and Ni are fast, reaching equilibrium within 5 minutes. Ca and Mg can be stripped out from the scrubbed loaded D2EHPA + Cyanex 272 at T = 50°C by both HCl (3 M, 1 theoretical stage and O/A ratio = 5) and H2SO4 (1 M, 2 theoretical stages, and O/A ratio = 2). A flowsheet of the proposed process including metal composition of the main aqueous and organic streams is shown in Fig. 16.

D2EHPA (0.3 M) + Cyanex 272 (0.32 M)

MHP liquor (pH 4)

[Ni] = 88 g.L-1 [Ca] = 0.5 g.L-1 [Mg] = 3.1 g.L-1

H2SO4 solution (0.063 M)

Ni electrowinning

Ni PURIFICATION T = 50°C, A/O ratio = 1 and 3 stages [Ni] = 5.5 g.L-1 [Ca] = 0.49 g.L-1 [Mg] = 3.05 g.L-1

[Ni] = 82.5 g.L-1 [Ca] = 0.01 g.L-1 [Mg] = 0.05 g.L-1

Ni RECOVERY

MHP leaching

T = 50°C, O/A ratio = 1 and 3 stages [Ni] = 0.02 g.L-1 [Ca] = 0.44 g.L-1 [Mg] = 2.78 g.L-1

HCl solution (3 M)

[Ni] ≈ 0 g.L-1 [Ca] ≈ 0 g.L-1 [Mg] ≈ 0 g.L-1

[Ni] = 5.48 g.L-1 [Ca] = 0.05 g.L-1 [Mg] = 0.27 g.L-1

Ca and Mg REMOVAL T = 50°C, O/A ratio = 5 and 1 stage

Tailing [Ni] = 0.1 g.L-1 [Ca] = 2.2 g.L-1 [Mg] = 13.9 g.L-1

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Rio de Janeiro, 03/01/2020.

Dear Dr. Bart Van der Bruggen and Dr. Andrá de Haan, Editor-in-Chief and Editor, Separation and Purification Technology

Authors state there is no conflict of interest on this research work.

Prof. Marcelo Borges Mansur Program/Department of Metallurgical and Materials Engineering UFRJ, Brazil

Highlights:

    

Concentrate Ni liquors were successfully purified by solvent extraction Synergistic D2EHPA + Cyanex 272 selectively extracted Ca and Mg simultaneously Temperature is a key factor to improve separation efficiency Thermodynamic analysis for Ca, Mg and Ni extractions was performed Purification of MHP liquors feeding Ni electrowinning step was highlighted