Impure ions removal from multicomponent leach solution of nickel sulfide concentrates by solvent extraction in impinging stream rotating packed bed

Chemical Engineering & Processing: Process Intensification 137 (2019) 54–63

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Impure ions removal from multicomponent leach solution of nickel sulfide concentrates by solvent extraction in impinging stream rotating packed bed

T

Jun Changa, , Fukang Jiaa, C. Srinivasakannanb, Kathryn A. Mumfordc, Xiaohong Yanga ⁎

a

College of Material and Chemical Engineering, Tongren University, Tongren, 554300, Guizhou, China Chemical Engineering Department, The Petroleum Institute, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates c Department of Chemical and Biomolecular Engineering, The University of Melbourne, VIC, 3010, Australia b

ARTICLE INFO

ABSTRACT

Keywords: Nickel Cobalt Impure ion IS-RPB Solvent extraction

In order to separate zinc, manganese and copper from a mixed chloride solution containing nickel and cobalt, solvent extraction with Di-2-ethylhexylphosphoric acid (D2EHPA) was investigated using an Impinging Stream–Rotating Packed Bed (IS–RPB) contactor in laboratory scale. The effects of operating parameters, such as equilibrium pH, ratio of aqueous to organic flow rates, rotation speed and concentration of D2EHPA amongst other variables were examined. A McCabe–Thiele diagram of the extraction stage predicted that at a flow rate ratio of 1:1, equilibrium pH of 3.5, D2EHPA concentration of 20% (v/v) and rotation speed of 900 rpm, 99.9% of zinc, manganese and copper would be removed within three stages. With regards to the scrubbing results, the co-extracted nickel and cobalt were scrubbed efficiently from the loaded organic solution using 0.8 M HCl, The loaded-organic phase can be reused for extraction process after stripping with 3 M HCl. IS–RPB extraction system have much higher mass transfer rates compared to conventional extractors such as mixer-settler and centrifugal extractor, etc. Finally, each of impure ions in raffinate was below 5 mg/ L, completely meeting the requirements of subsequent process of nickel and cobalt separation.

1. Introduction Cobalt and nickel are popular metals which finds very wide range of application and in formulation of many advanced materials. Cobalt is generally used in super-alloys, nuclear field, rechargeable batteries, catalysts etc. Nickel is the main raw material for the stainless steel owing to its high corrosion resistance [1]. Because of the similar physicochemical properties, the adjacent elements Co and Ni always coexist in nature. Laterite ore is an important reserve of nickel and cobalt which is also rich in iron in the form of goethite together with a small amount of aluminum, manganese, magnesium and other elements [2]. In order to improve nickel and cobalt grade, the acid leaching - sulfide precipitation is being used to deal with laterite ore to form NiS/CoS concentrates [3]. Chloride leaching is known to be an effective methods to extract nickel and cobalt from NiS/CoS concentrates. The leach liquor enrich cobalt and nickel solution together with impurities, such as iron, aluminum, copper, zinc, manganese, magnesium and calcium. In the downstream process for the recovery of nickel and cobalt, hydrolysis and fluorination precipitation are being used to remove iron, aluminum, calcium and magnesium [4]. However, the solution also contains minor residual impurities such as manganese, copper, zinc that need to be eliminated in order to obtain a high-purity product.

⁎

Therefore, removal of impure ions from cobalt and nickel solutions are mandatory. Solvent extraction is the most common and effective process for this purpose. Innocenzi and Veglio [5] have reported purification of leach liquor of NiMH spent batteries using D2EHPA in n-dodecane. More than 99% of zinc and 95% of manganese were separated in two stage crossflow liquid–liquid extraction with 20% v/v of D2EHPA at O/A ratio of 1/1. The loaded organic phase can be stripped with 4 M H2SO4 in one stage at the O/A ratio of 1/0.5. Jafari et al. have [6] reported extraction of zinc from synthetic Zn-Cd-Mn chloride solution using D2EHPA in kerosene. 97% of Zn, 14% of Mn and 3% of Cd were extracted at pH values between 2.5 and 3, a temperature of 40 °C and concentration of D2EHPA of 10 v/v%. Guimarães et al. [7] have considered the extraction of calcium and magnesium from concentrated nickel sulfate solutions (Ni 88 g/L, Mg 3.1 g/L, Ca 0.5 g/L). Ca and Mg were extracted with D2EHPA and Cyanex 272, respectively. The purified liquor containing about 80 g/L Ni was suitable for electrowinning. Cheng et al. [8] have reported the extraction of Mn, Zn, Mg and Cu from synthetic sulfate solution using D2EHPA in kerosene. The results indicated that the separation of zinc and calcium from the other elements was easy and the separation of manganese and copper from cobalt and nickel was possible. 99.9% manganese was extracted at the conditions of pH 3.5,

Corresponding author. E-mail address: [email protected] (J. Chang).

https://doi.org/10.1016/j.cep.2019.02.001 Received 23 August 2018; Received in revised form 29 January 2019; Accepted 3 February 2019 Available online 04 February 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.

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40 ℃, A/O ratio of 1:1with two theoretical extraction stages, the manganese concentration in the solution was reduced from 2.0 g/L to 3 ppm. Duan et al [9]. have studied the separation of copper, nickel and cobalt in ammoniacal solution using supported liquid membrane containing with synergistic extractant. The result showed that more than 99.5% of cobalt, 98.0% of nickel, and 98.9% of copper can be separated from a simulated feed solution with a contact time of 36 h. Mixer-settlers are widely used in industry due to their easy operation and strong adaptability for disposing the industrial leaching solution [10]. However, the requirement for long contact time/resident times, large footprint and large liquid hold-up are significant short-comings [11]. In order to improve the extraction efficiency, the continuous efforts are being made by the scientific community, to develop new technologies. In recent years, an impinging stream-rotating packed bed (IS-RPB) has emerged as a promising alternative for intensifying mass transfer for a wide variety of applications [12–14], especially for solvent extraction. Our previous publication [15] indicated that IS–RPB was effective in the separation of In(III) and Fe(III) under favorable operating conditions. The extraction and stripping efficiency as high as 99% could be achieved, at a separation factor in excess of 3000. Liu [16] has utilized LIX984 N as solvent to extract copper using IS–RPB and have reported the results covering the process variables in the following range: 1:1 phase ratio, 80 L/h flow rate, 5 vol % LIX984 N, high gravity factor of 135, extraction ratio of copper of 98.8%. Jiao et al. [17,18] have reported the extraction of nitrobenzene from aqueous solution in IS-RPB, with the % removal of nitrobenzene of 94.93%, and extraction stage efficiency of 99.99%, under the optimal experimental conditions. Modak et al. [19] have studied the extraction of methyl red using xylene from aqueous solution in rotating packed bed. 98% stage efficiency was reported with aqueous to organic flow rate ratio of 10. Karmakar et al. [20] (the latest publication of the same working group as Modak et al.) have investigated three different rotating bed contactors for liquid-liquid extraction with hexavalent chromium - Aliquat336 system. These two studies obtained the similar results that the overall volumetric mass transfer coefficient for RPB was nearly two orders of magnitude higher in comparison to mixer settler, while the volume requirement in RPB being ∼20 and ˜10 times lower compared to the countercurrent extractors and annular centrifugal extractor, respectively. The above results clearly indicate the supremacy of IS-RPB technology for its application to solvent extraction process. Although there have been many studies on the separation of impurities by solvent extraction, most of them deal with separation of impurities from synthetic solution, especially in sulfate systems. None reported utilization of industrial feed solution, especially that of chloride system. In this work, IS-RPB is utilized to identify optimal process conditions to maximize separation of contaminants like manganese, zinc, copper from an industrial chloride solution using D2EHPA– kerosene as extractant, so that nickel and cobalt could be recovered from the raffinate.

Table 1 Chemical components of the leaching solution. Element

Ni

Co

Cu

Zn

Mn

Ca

Fe

Concentration, g/L

79.08

2.60

1.12

1.04

0.63

< 0.01

< 0.01

The pH of the aqueous phase were measured by pH meter (PHS-3C, Shanghai Instrument Electric Science Instrument Ltd., China). The concentration of the metal ions except Ni in the aqueous solution before and after extraction were measured by AAS (Z-2000, HITACHI, Japan). The Ni concentration in the aqueous phase was determined by complexometric titration using 0.02 mol/L Na2EDTA standard solution with murexide as indicator agent [21]. Metal ion concentrations in the organic phase were calculated via mass balance between the aqueous and organic phase. 2.2. Apparatus and extraction procedure A schematic of the experimental setup is shown in Fig. 1. More details about the IS-PRB can be seen in our previous publication [15]. The aqueous feed solution and extractant stored in tank 1 and 5, were pumped through valves and rotameter into the IS–RPB. Then both streams impacted each other in the opposite direction via the jet nozzles. Accordingly, the impinging spray zone was formed, and the solutions were initially mixed. Following this, the impinging spray surface was sprayed onto the inner brim of RPB along the radial direction for further intensive mixing in the packing zone of the RPB under a highgravity field. In the packing zone, micro-mixing and mass transfer characteristics were greatly enhanced [22]. Afterward, the mixed phase moved outwardly and left from the outer edge of the RPB into the slit separator 7 for demixing. Finally, the raffinate and loaded organic phase were discharged into tanks 8 and 9, respectively. In the extraction process, the mixing stage was continuous, while the phase separation stage was intermittent. The initial impinging velocity can be defined as [17]:

v0 =

4Qa DH2

(1)

Where v0 is initial impinging velocity of aqueous solution, m/s; Qa is the flow rate of aqueous solution, m3/s; DH (DH = 0.15 × 10−2m) is the hole diameter of liquid distributor. The average diameter of droplets in the RPB could be calculated by following empirical equation [23]:

d = 0.7428(

r

2

)0.5

(2)

Where d is the average diameter of the droplets, m; σ is the surface tension of liquid, mN/m; r is the geometric average radius of the rotor, m; ω is the angular velocity of the rotor, rad/s; and ρ is the liquid density, kg/m3.

2. Materials and methods

2.3. Measurement of extraction efficiency and overall volumetric mass transfer coefficient

2.1. Materials and analysis The leaching solution utilized in the present work was received from a smelting process in southwest China. The main composition of the real leaching/aqueous solution are listed in Table 1. D2EHPA (Di-2-ethyl-hexyl phosphoricacid), with a purity of 97% and the molar mass 322.43 g.mol−1, was purchased from Shanghai Laiyashi Chemical Co., Ltd. It was used without any further purification. The commercial sulfonated kerosene was used as the diluent. The reagents were all of analytical grade and used without further purification. The water used in all experiments was ultra-pure water with specific resistance more than 18.2 MΩ cm (ULUP-Ⅰ, Xi 'an YouPu Instrument Equipment Co. LTD, China). The pH of the leaching/aqueous solution was modulated with 6 mol/L HCl or NaOH solution.

In order to measure the extraction and separation efficiency, extraction efficiency (E1), scrubbing efficiency (E2), stripping efficiency (E3), distribution ratio (D), and separation factor (β) were calculated as follows. The extraction ratio (E1), defined as the metal ion concentration in the organic phase to that in the feed aqueous phase, and is generally represented as:

E1 (%) =

Ca,in

Ca, out × 100 Ca,in

(3)

Where Ca,in and Ca,out are the inlet and outlet concentration of metal ions in the aqueous solution respectively. 55

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J. Chang, et al.

Fig. 1. Schematic diagram of the experimental setup.

The scrubbing efficiency (E2), defined as the metal ion concentration in the scrubbed solution to that in the loaded organic solution, and is generally represented as:

E2 (%) =

CL,in

CL, out CL,in

× 100

packing to the outer of the packing. The boundary conditions for Eq. (10) are:

r = r1 Ca = Ca,in r = r2 Ca = Ca,out

(4)

The solution of Eq. (10) satisfying Eq. (11) is given by:

where CL,in and CL,out are the inlet and outlet concentration of metal ions in the loaded organic solution respectively. The stripping efficiency (E3), defined as the metal ion concentration in the stripped solution to that in the after scrubbing loaded organic solution, and is generally represented as:

E3 (%) =

CS,in

CS, out CS,in

× 100

Ca,in Ca, out V × a Ca,out Vo

Ca,in

A/B

KL a= (5)

Vv = Vt

KL a=

2 hr dr

(12)

C* Qa ln a h (r22 r12) Ca*

Ca,in (13)

Ca,out

(14)

Vp

Qa Ca* ln Vv Ca*

r12 )

Ca,in Ca,out

(15)

3. Results and discussion In order to optimize the solvent extraction using D2EHPA with ISRPB, the process of extraction, scrubbing and stripping important influencing factors were comprehensively investigated.

(8)

3.1. Extraction experiments

(9)

Ca )

r2 r1

) is the total Vv is the valid volume of mixing, Vt ( Vt = volume of packing bed, Vp (0.9 × 10−3 m3) is the volume of packing. So the Eq. (13) is amended to:

(6)

3.1.1. Effect of pH The percentage extraction of nickel, cobalt, manganese, zinc and copper from a chloride solution using D2EHPA 25% v/v, the same flow rate of two phases with 30 L/h, the rotor speed of 900 r/min and temperature of 30 °C with respect to pH over the range of 2 to 5 is presented in Fig. 2. According to this figure, it is seen that with increasing pH, the extraction percentages increase for all the metal ions. However, the %

where Ca is the concentration of metal ions in the aqueous phase at a radius r from the rotational axis, h is the thickness of the packed bed, and KLa is the overall volumetric mass transfer coefficient. Eq. (8) can be re-written as

Qa dCa = KL a(Ca* 2 hr dr

KL a Qa

h(r22

where the differential volume dV is

dV = 2 hr dr

=

is the equilibrium concentration of metal ions in the aqueous phase. r1 and r2 are the inner and outlet radius of packing bed. However, most of the space should be occupied by wire mesh packing in RPB, the actual mixing volume in packing area can be defined as:

(7)

Ca )dV

Ca

Ca*

The mass transfer of extraction step just discussed in view of its importance in the whole solvent extraction process. The mass balance of the solute over a differential volume in the packing assuming plug flow of the phases is expressed by [19,24,25] :

Qa dCa = KL a(Ca*

Ca*

The volumetric mass transfer coefficient was obtained by:

The separation factor (β) can be defined as the ratio of the distribution ratios of metals A and B (where DA ≥ DB):

D = A DB

dC

Ca,out

where CS,in and CS,out are the inlet and outlet concentration of metal ions in the scrubbed-loaded organic solution respectively. The distribution ratio (D), defined as the ratio of the metal ion concentration in the organic phase to that in the aqueous phase after extraction, and is expressed as:

D=

(11)

(10)

Since KLa values are obtained by the inlet and outlet liquid/liquid concentrations in the RPB, the integral path is from the inner of the 56

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exhibit significant increase in the % extraction with increase in concentration with the maximum at concentration of 20%. At concentration higher than 20%, no significant increase in the % extraction was observed. The fact that increase in the concentration of D2EHPA favored the extraction of all the metal ions can be explained by the ion exchange reaction mechanism which will be discussed in the later section. However, the viscosity of organic phase would increase with increase in the concentration that would hampering the mass transfer coefficient as result reduced % extraction. Additionally, the strengthening viscous force would increase the surface tension of the fluid molecules, causing the liquidity deterioration and the decrease of the fluid diffusion rate [26]. Fig. 3(b) shows the separation factor of 162 and 178, 3094 and 3398 being optimal at extractant concentration of 20% for Zn and Mn ions respectively with respect to both Ni and Co, But for Cu, the best separation factor of 115 and 123 with respect to Ni and Co appeared at extractant concentration of 25%. In consideration to minimize the amount of nickel and cobalt co-extraction, and to reduce the operating cost, a 20% (v/v) D2EHPA concentration is identified to be the optimal to extract manganese, zinc and copper from the chloride solution containing nickel, cobalt ion.

Fig. 2. Effect of equilibrium pH on extraction rate of five ions.

increase in extraction for Co and Ni up to a pH of 3.5 is insignificant while that of the other ions (Mn, Zn, Cu) were in excess of 90%. The % extraction of Zn was near 100%, while that of Mn and Cu were in excess of 90%. It should also be noted that the % extraction of Co and Ni was significant with the maximum % extraction of near 20% at a pH of 5. The distribution ratio of metal ions will be increased with the increase of solution pH until hydrolysis. The distribution ratio of five ions at each pH and the separation factor were calculated are presented in Table 2. Based on the data presented in Table 2, the distribution ratio for Mn, Zn and Cu shows a sharp increase with increase in pH from 3 to 3.5 as compared to Ni and Co, recommending the pH to be optimal for separation. The separation factor for Zn was maximum with reference to Ni and Co at pH of 3.5 with the values being 2633 and 2800. And the separation factor for Cu with reference to Ni and Co being 115 and 122, at the pH of 3.5. Both clearly indicate the favorable pH to be 3.5. However, for manganese the optimal pH appears to be 4.5 with the specific values being 299 and 247 with reference to Ni and Co. In order to reduce co-extraction of nickel and cobalt, the optimal pH is determined as 3.5. As shown in Fig. 2, the zinc separation is easier than that of manganese and copper which was also confirmed by the higher separation factor Zn as compared to manganese and copper. This indicates that the easiness in extraction of impure ions is in order of zinc > manganese > copper in chloride system. These results are found to be in consistent with Cheng et al. [8].

3.1.3. Effect of flow ratio (A/O) In the extraction process, the flow ratio (flow rate of aqueous phase /organic phase) is a very important parameter. The parameter not only affected the % removal of impure ions, but also affected the regeneration cost on stripping. Fig. 4 presents the effect of (A/O) ratio on the % extraction of all the five ions at a total flow rate of 60 L/h, pH of 3.5, temperature of 30 °C and the rotor speed of 900 r/min. As shown in Fig. 4, % extraction for all the ions decrease with increase in the A/O ratio. Except Zn, for all other ions progressive decrease was observed with increase in A/O ratio, while for Zn, it was found to be stable until an A/O ratio of 6/5, beyond which it was found to decrease. For Ni and Co, the rate of decrease was higher until an A/O ratio of 5/5, beyond which it was marginal. With the separation factor being the target to maximize the optimal A/O ratio could be identified to be 1. The nickel and cobalt co-extraction decreased from 20.03 and 24.78% at A/O of 1/5 to 6.67 and 5.83% at A/O of 5/5, respectively. It has been stated by Zhang et al. and Gao et al. [27,28] that a change in A/O was achieved by adjusting the flow ratio of the two phases, which potentially alter the initial velocity of the liquid phase in the IS-RPB. In addition, the impact surface formed by the two fluid phase collision will be shifted to one side with a smaller momentum at the unequal volumetric flow ratio, resulting in "nozzle blockage", thus affecting the micro-mixing effect. But sufficient extractant are provided at A/O < 1, “nozzle blockage" effect is not dominant, so the extraction rate stays pretty good. When the A/O is equal to 1, “nozzle blockage" effect is minimized, due to minimization of impact velocity of two fluids. The larger the dispersion degree of the liquid after the collision, the more likely the liquid is to disperse and form the liquid microelement (such as drops, wires or films), enhancing the mass transfer area and the mixing efficiency. At A/O > 1, “nozzle blockage" effect plays a significant role, reducing the extraction efficiency of impurities gradually. Thus, in order to reduce the dosage of extractant (D2EHPA) and to minimize the operational cost, the flow ratio (A/O) of 1 was identified to be most suitable.

3.1.2. Effect of extractant concentration The effect of D2EHPA concentration on the % extraction and separation factor of nickel, cobalt, zinc, manganese and copper at an A/O ratio of 1:1, flow rate of 30 L/h, rotor speed of 900 r/min, temperature of 30 °C and pH 3.5 is presented in Fig. 3. Fig. 3(a) shows that the extraction of all the five ions increase with increase in extractant concentration. However, the increase was marginal with the maximum being at concentration of 30% for Ni and Co. On the other hand Mn, Zn and Cu

Table 2 The distribution ratio and separation factor of Mn, Zn, Cu from a Ni/Co containing chloride solution. pH

2.0 2.5 3.0 3.5 4.0 4.5 5.0

Distribution ratio

Separation factor (β = DM/DNi)

Separation factor (β = DM/DCo)

Ni

Co

Mn

Zn

Cu

Mn

Zn

Cu

Mn

Zn

Cu

0.013 0.041 0.059 0.084 0.13 0.19 0.31

0.012 0.028 0.048 0.079 0.15 0.23 0.33

0.11 0.35 1.14 10.88 21.03 56.80 75.92

2.52 9.33 23.94 221.22 226.27 231.56 226.27

0.13 0.61 2.21 9.71 10.85 11.85 12.64

8.46 8.54 19.32 129.52 161.77 298.95 244.90

193.85 227.56 405.76 2633.57 1740.54 1218.74 729.90

10.00 14.88 37.46 115.59 83.46 62.37 40.77

9.17 12.50 23.75 137.72 140.20 246.96 230.06

210.00 333.21 498.75 2800.25 1508.47 1006.78 685.67

10.83 21.78 46.04 122.91 72.33 51.52 38.30

57

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Fig. 3. Effect of extractant concentration on the extraction efficiency: (a) % extraction, and (b) separation factor between Zn, Mn, Cu and Ni, Co.

Fig. 5. Effects of rotation speed on extraction rate of five ions.

Fig. 4. Effect of flow ratio (A/O) on extraction rate.

theoretical extraction stages are needed to extract almost all the manganese and copper. Table 3 shows the metal concentration left in the raffinate after three stages extraction. Indicating that three ideal extraction stages is sufficient to remove Zn, Mn and Cu from nickel and cobalt in a chloride solution system. With respect to this result, the concentrations of impurities are acceptable for the separation of nickel and cobalt process.

3.1.4. Effect of rotating speed The variation of all five metal ions extraction efficiency along with the rotating speed is shown in Fig. 5, while other parameters were held at 30 °C, pH of 3.5 and at a total flow rate at 60 L/h with extractant concentration of 20%. The % extraction of zinc, manganese and copper increased obviously with increase in rotation speed from 500 to 900 rpm beyond which almost constant. At a rotation speed of 900 r/min, the % extraction of zinc, manganese and copper were 99.53%, 91.09% and 86.74%, respectively, with the coextraction of Ni and Co being low at 6.31 and 5.91% respectively. In the rotated packing bed, the liquid mainly exists in two forms, either as liquid film on the filler wire or as a small droplets in the voidage [29]. When the speed increases, the movement of liquid in the packing gets accelerated, increasing the shear force, as a result of which reduced droplet size. The smaller droplets would increasing the probability of coagulation and dispersion, thus strengthening the mixing and mass transfer process [30]. However, when the rotation speed was further increased, the contact time for these two liquids was shortened, as a result of which the extraction efficiency was reduced [31]. Additionally, the two phases stratified due to the violent mixing at high rotational speed. And hence the optimal rotating speed for IS-RPB was chosen to be 900 r/min.

3.1.6. Extraction mechanism Acidic organophosphorus extractants of D2EHPA as cation exchange type (HA) extractant usually affected by equilibrium pH of the aqueous phase, extractant and metal concentrations, solvent phase properties which determines the formation of dimers or polymers in the organic phase, polymerization of extracted complex in the solvent phase. The interweaving of these factors make the extraction mechanism become complicated [32]. The reaction processes of D2EHPA generally include the steps of distribution of extractant from organic to aqueous phase, dissociation of extractant as cationic H+ and anionic A− ions. complex formation of A− ion of the extractant combine with metal ions in the aqueous phase and enter organic phase [32,33]. The extraction reaction for a divalent cation is written as: + 2HA (org) + M2(aq)

3.1.5. Extraction equilibrium stages In order to investigate the extraction behavior of D2EHPA and determine the number of extraction stages, a series of experiments were conducted to simulate the first stages of counter-current extraction(the same organic was contacted with fresh aqueous solution six times) with A/O ratio of 1:1, extractant concentraction of 20%(v/v), rotor speed of 900 r/min, pH of 3.5 at 30℃. Fig. 6 shows the zinc(6a), manganese(6b) and copper(6c) extraction distribution isotherm and the McCabe–Thiele diagram constructed at 30℃ from the chloride solution containing nickel and cobalt system. As seen from this figure, only one theoretical extraction stage is needed to extract almost all the zinc while three

MAn(org) + 2H+(aq)

(16) +

It is known from Eq. (16) that H are continuously released when D2EHPA extracts metal ions, and the acidity of the aqueous phase increases, which cannot maintain the optimal pH range for metal extraction. To have a reasonably good extraction the extractant is saponified to a desired extent by addition of NaOH solution. 65% D2EHPA pre-neutralization for the following experiments. The neutralization reaction can be written as [34]:

NaOH (aq) +

58

1 (HA)2(org) 2

NaA (org) + H2 O(aq)

(17)

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Fig. 6. McCabe Thiele diagram for zinc, manganese and copper removal with IS-RPB.

exchanged by every mole of each extracted ion, the convincing explanation for this observation is that a part of D2EHPA in the acidic form was involved in the reaction. The plot of logD versus the varied concentration of extractant at a fixed equilibrium pH also gave the linear with a slope of about 2 for each extracted ion (Fig. 8), which suggests 2 moles of D2EHPA are involved in metal complex formation. The above analysis clearly certify that both neutralized and acid form of the D2EHPA take part in metal complexation and ultimate metal transfer from aqueous to organic phase. In addition, from the slope of logD versus log[extractant], the value 3 of y can be calculated easily, the value of m can be estimated to 1 based on principle of electrical neutrality of complex molecules. In conclusion, the mechanism of Zn, Cu and Mn with extractant is as follows.

Table 3 Metal extractions and concentrations with three stages at an A/O ratio of 1:1. Element

Ni

Co

Cu

Zn

Mn

Raffinate (g/L)

71.14

2.43

0.004

0.001

0.003

Then, the sodium salt of extractant, NaAorg monomer can be replaced by the metal ion M according to the following reaction [35]: + 2NaA (org) + M2(aq)

MA2(org) + 2Na+(aq)

(18)

In order to find out a possible mechanism in this study, we suppose that the extraction of divalent metal ion in present work is as follows: + M2(aq) + mNaA (org) +

1 (x+ y)(HA)2(org) 2 + mNa+aq

MA (m+x) (HA) y(org) + xH+aq

M2aq+ + NaA (org) + 2(HA) 2(org)

(19)

where m and (x + y)/2 represent the coefficients of NaA and (HA)2, respectively, participating in the reaction. Based on this reaction, equilibrium constant, Kex, is written by

K ex =

x+y 2

(20)

where

D=

[MAm+x (HA) y ] (21)

[M2 +]

As a result:

log(DM) = logK ex + mlog[NaA] +

x+ y log[(HA)2] 2

mlog[Na+]

(23)

3.1.7. Mass transfer To clearly explain the intensification of the extraction process in ISRPB, volumetric mass transfer coefficient is employed to characterize the performance of a contactor. In this study KLa based on the aqueous phase is evaluated using Eq. (15) [36].

[MAm+x (HA) y ][H+]x [Na+]m [M2 +][NaA]m [(HA)2]

MA2 3HA org + H+aq + Na+aq

xpH (22)

The coefficients x and y were determined by slope analysis. The extraction data of Zn, Mn and Cu presented in Fig. 2 was further analysed as a function of equilibrium pH variations at fixed concentrations of D2EHPA. The plots of logD versus equilibrium pH (from 2.0 to 3.5) of Zn, Mn and Cu are linear with slopes of ∼1, indicating x = 1, and only one mole of H+ ions from the extractant during the complex formation and ultimate metal transfer to the organic phase (Fig. 7). However, according to the Eq.(16), 2 moles of H+ ions from the extractant should be

Fig. 7. Relationship between lgD and the equilibrium pH. 59

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Fig. 10. Effect of rotating speed on the volumetric overall mass transfer coefficients.

Fig. 8. Relationship between lgD and the concentration of extractant.

As for IS-RPB, rotating speed and initial impinging velocity are the most important factors for liquid-liquid mass transfer process. In the following section, the effects of rotating speed and initial impinging velocity on KLa are discussed. As shown in Fig. 9, an increase in the flow rate is attributed to improving KLa for a constant rotating speed of 900 rpm, and at equal flow rate ratio. According to Eq. (1), when the flow rate increases from 10 to 50 L/h, the fluid initial velocity increased from 1.57 to 7.85 m/s simultaneously. In other words, a higher liquid flow rate provided a higher initial velocity. The increase of initial velocity increased the relative velocity of collision between the fluids and the rotating packing, the impact strength and the kinetic energy increased, and the interface area of liquid -liquid was enlarged [17]. For example for Zn, KLa increased from 19 × 10−3 to 160 × 10−3 s-1 (The KLa comparisons with other extraction equipment will be discussed in detail later in this paper). Although the KLa of Mn and Cu were not as large as the Zn, they also increased distinctly with the increase of flow rate. However, not the larger flow rate, the higher overall volumetric mass transfer coefficient, the better consequence to the solvent extraction in ISRPB, because more vigorous collision between liquids at larger flow rate caused serious emulsification. These results indicated that impurities of Zn, Mn, Cu can be perfectly separated with higher extraction efficiency under suitable liquid velocity. The liquid flow rate ratio was 1. The aqueous phase flow rate of the blade was 30 L/h. Fig. 10 plots the effect of the rotor speed on overall volumetric mass transfer coefficient. It can be seen that volumetric overall mass transfer coefficient increases with an increase in rotor speed from 500 to 900 rpm, and then decreased when the rotor speed exceeds 900 rpm. This variation trend applies to each extracted ion. The reason for this behavior could be due to decreasing of drop size and increasing uniform distribution of two liquid phase with the enhancement of rotor speed. Consequently, this results in enhancement of

interfacial area [37]. However, the higher rotor speeds will lead to the tremendous increase of angular velocity, which will reduce the size of droplets sharply according to Eq. (2), the production of very fine droplets would be causing a terrible phases separation. Moreover, the reduction of contact time of two liquid phases at higher speed is another reason for the decrease of volumetric overall mass transfer coefficient. 3.2. Scrubbing of Co(II) and Ni(II) from loaded organic phase The objective of scrubbing stage is to maximize removal of the coextracted nickel and cobalt while minimizing other metals. Scrubbing tests were carried with the extracted organic phase determined at the optimum conditions, identified in the earlier section. The nickel, cobalt, zinc, manganese and copper concentrations in the loaded D2EHPA phase were 5.674, 0.168, 1.438, 0.837, 1.447 g/L respectively. 3.2.1. Effect of acid concentration on scrubbing efficiency Fig. 11 shows the scrubbing behavior of all five ions from the loaded D2EHPA by hydrochloric acid solutions with the condition of rotational speed of 900 rpm, A:O ratio of 1:5 at ambient temperature. As can be seen from Fig. 11, Co(II) and Ni(II) were efficiently scrubbed from the loaded D2EHPA while that of Cu(II), Mn(II), Zn(II) was negligible in the range of HCl concentration 0.2 to 0.8 M. Both cobalt and nickel removal increased with increase in HCl concentration due to the increased rate of reaction/solubility at higher concentration. At concentration of hydrochloric acid higher than 0.8 M, the aqueous and organic phase reacted violently and exothermically, enhancing the removal of Mn(II), Zn(II) and Cu(II) as well. To keep the Mn(II), Zn(II) and Cu(II) in the loaded organic phase, the optimal HCl concentration was identified to be 0.8 M. 3.2.2. Scrubbing equilibrium stages To know the number of stages required for scrubbing of nickel and cobalt, the loaded organic phase was contacted with 0.8 M HCl at A:O ratios within 1:1 to 1:10 and 1:1 to 1:15, respectively. The nickel and cobalt equilibrium distribution and their McCabe– Thiele diagrams with the D2EHPA system are shown in Fig. 12. As shown in Fig.12, only one theoretical scrubbing stage is needed to extract almost all the cobalt while two theoretical scrubbing stages are needed to extract almost all the nickel. After a single contact at an A/O ratio of 1:8, the scrubbing efficiency of cobalt and nickel were 99% and 95%, respectively, while that of manganese, zinc and copper, were negligible, thereby avoiding the loss of nickel and cobalt. 3.3. Metal stripping from the loaded organic solution The objectives of the stripping is to generate stripped organic solution containing low residual metals suitable for recycling to the extraction stages.

Fig. 9. Effect of flow rate on the volumetric overall mass transfer coefficients. 60

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Fig. 13. Effect of flow rate ratio on stripping.

Fig. 11. Scrubbing of all five ions from the loaded D2EHPA by dilute HCl solutions.

two streams of fluid at the inner edge of the packing. With the increase of rotating speed, the large liquid droplets sheared into liquid microelement, the water-in-oil emulsion has break and the interphase surface was greatly increased. However, the contact time of two phases is greatly shortened under higher rotating speed, it is not benefit to thorough reaction. Hence, the rotating speed of 1200 rmp was selected as the preferred parameter for the strip stage from loaded organic solution.

3.3.1. Effect of A/O ratio on metal stripping Metal stripping was performed for the loaded organic solution containing 0.829 g/L Mn, 1.434 g/L Zn, 1.436 g/L Cu using HCl at various A/O ratios (Fig. 13). The experimental conditions were set with the following parameters: at ambient temperature, flow rate of organic phase 30 L/h, rotational speed 1000 rpm, 3 M HCl. It is shown that copper and manganese could be readily stripped at relative ease up to 100% between 10:1 to 1:2 of A/O. While at A/O ratio in excess of 1:2, the stripping efficiency was found to decrease. However, zinc stripping decreased rapidly with decreasing A/O ratio lower than 2:1. This can be explained with a fact that the packing has small opening size and the liquid can easily flow through the small opening size at low total flow rate. However, when the A/O ratio lower than 1:5, the total liquid flow rate increased greatly, the liquid couldn’t through the packing timely and the droplet may incorporate severely, thereby decreasing the micromixing efficiency [26]. Of course, the outstanding results can be obtained with increasing A/O ratio more than 5:1 because of rich strip reactant play the major role compared with micromixing effect. In order to reduce the amount of hydrochloric acid usage for industrial production, the A/O ratio of 1:2 was adopted for the following experiments.

3.3.3. Effect of number of stripping stages In the first stripping 3 M HCl solution was used, O/A volume ratio was 2 and two stages of stripping were used, whereas in the second test 4 M HCl solution was used, O/A volume ratio 2 and single step of stripping. Both of two stripping operation the organic phase containing metals was mixed with fresh hydrochloric acid solution with the condition of rotational speed of 1200 rpm, flow rate of organic phase 30 L/ h at ambient temperature. The stripping efficiency of Zn, Mn and Cu were invested in a single and two stage stripper and the results are presented in Table 4. Table 4 lists typical stripping efficiency of single and two stages. With a single stage stripper with 3 M HCl over 96% of zinc and 99% of manganese and copper were stripped. However, the residual zinc concentration in the stripped organic phase was still up to 37 mg/L. The stripping efficiency was near 100% after a two stage stripper for each loaded ion. The similar results also belong to a single stage stripper with 4 M HCl. Summarizing the above results, the use of higher concentrations of hydrochloric acid solution can reduce the number of stages, but the corrosion on the equipment will be more serious. Therefore, two stages with 3 M HCl is more suitable for the process.

3.3.2. Effect of rotating speed on metal stripping The effect of rotating speed on the stripping of manganese, zinc and copper, which was used to determine the optimal operating conditions in the IS-RPB, is shown in Fig. 14. The experimental conditions were set with the following parameters: at ambient temperature, A/O = 1:2, flow rate of organic phase 30 L/h, 3 M HCl. The stripping efficiency of Zn, Cu and Mn ion increased with an increase of the rotating speed from 900 to 1200 rpm, and then decreased with the increasing of rotating speed. The reason for this phenomenon was possible that due to the high viscosity of the loaded organic phase, poor dispersibility with low rotating speed in multi-layer teflon mesh packing. Inaddition, the large liquid droplets of organic phase will wrap up the aqueous droplets, so it is not conducive to the mixing of

3.4. Performance comparison 3.4.1. Comparison with conventional solvent extraction study To evaluate the merits of IS-RPB contactor, 2 L of the constant solution was treated with a laboratory scale mixer-settler system at the similar experimental conditions. The results are shown in Table 5.

Fig. 12. Nickel and cobalt scrubbing distribution isotherm with 0.8 M HCl and its McCabe–Thiele diagram. 61

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for circulating extractants and formation of emulsion are much more serious as compared to IS-RPB, which leads to the extension of phase separation time and the increase of entrainment amount [38]. 3.4.2. Comparison mass transfer Though there are several studies on removal of impure ions by liquid-liquid extraction, none of these studies uses IS-RPB technology. Furthermore, studies reporting overall volumetric mass transfer coefficients for strictly using identical phase system in other liquid-liquid equipment such as mixer settlers, centrifugal extractors, pulsed packed extraction column and microchannel etc. has not be found. Despite lack of such data it is still worth to compare the KLa values reported in different liquid system and types of extractors to have a deep understanding of typical KLa values reported in publications. Table 6 shows this comparison. It is seen that the range of overall volumetric mass transfer coefficient obtained in IS-RPB is nearly two orders of magnitude higher in comparison to traditional extractor like mixer settler, and also in good agreement with the finding by Modak’s [19]. The maximum value of the KLa observed in the present study is higher than in other process intensifying contactor such as pulsed packed extraction column, annular centrifugal extractor, hollow fibre membrane contactor and serpentine microchannel. In addition, the total volume of packing bed had 1.32 × 10−3 m3 and was able to operate at flow rates up to 8.35 × 103 m3/s. The volume requirement in IS-RPB was tens even hundreds times lower compared to other extractors. In other words, the contactor volume could be greatly decreased by carrying out the solvent extraction process with IS-RPB contactor.

Fig. 14. Effects of rotating speed on stripping efficiency. Table 4 Zinc, manganese and copper stripping with single and two stages. Concentration of HCl

3M 4M

No. stages

1 2 1

Stripping (%)

Stripped org. phase (mg/L)

Zn

Mn

Cu

Zn

Mn

Cu

96.40 99.95 99.94

99.95 99.99 99.99

99.92 99.99 99.99

37 5 5

2 <1 <1

<1 <1 <1

Compared to mixer-settler reactor, the single stage extraction efficiency was higher and the co-extraction rate was lower with IS-RPB contactor. The experiment was also found that six extraction stages, three scrubbing stages and three stripping stages would be adopted with mixer-settler for obtaining satisfactory separation effect, and the zinc, manganese and copper concentration was 8, 23 and 17 ppm in the raffinate, respectively. Due to the pretty performance of IS-RPB, the process of separation impure ions from chloride system is shorter than mixer-settler. The entrainment loss rate of solvent for mixer-settler and IS-RPB was calculated from the difference between the initial and final volume after collecting the solvent carefully and was found to be 1.4 and 0.9%, respectively. Some possible reasons can be explained above mentioned phenomenon. For mixer-settler extraction equipment, the ideal mass transfer process depends on a large interfacial mass transfer area and high intensity turbulence by using external force. However, it is impossible to make both effects occur simultaneously under conventional agitator with general operating speed of 100˜150 rpm, so it is not easy to make high-speed motion in small droplets. Compared to mixer-settler, the frequency of the vigorous impingement and coalescence dispersion of the liquid is much faster in IS-RPB, which would lead to smaller droplet size and better dispersion quality contributing simultaneous increase in both the mass transfer coefficient and interfacial surface area, the extraction efficiency and selectivity are increased. Secondly, the adherence of solvent with mixer-settler walls, tubes used

4. Conclusion A novel concept of a liquid-liquid extraction for separation of zinc, manganese and copper from the mixed chloride solution was investigated in IS-RPB using D2EHPA as the extractant. The initial pH, concentration of extractant, flow rate ratio and rotating speed were found have significant influence on the extraction efficiency. In excess of 99.9% of the zinc, manganese and copper could be extracted, with the raffinate concentration of Zn, Cu and Mn less than 5 mg/L in a three stage extractor. The co-extracted nickel and cobalt in the organic phase were scrubbed thoroughly in two stages with 0.8 M HCl. A two-stage stripping at an O/A ratio of 2:1 using 3 M HCl could strip Zn, Cu and Mn in excess of 99.9%. The slope values of the plots of log D vs. log [extractant] and log D vs. equilibrium pH revealed that association of 2 moles of the extractant could release a mole of H+ at the extraction process. KLa values obtained in this contactor vary between 0.008 and 0.16 s−1 are much higher than other intensifying contactor which discussed at present work. In IS-RPB the efficiency of mass transfer and two phases splitting were greatly improved and the process flow is obviously shortened as compared to mixer settler extractor. Additionally the application of IS-RPB could reduce the formation of interfacial crud and extractant consumption. In short, IS-RPB is a new approach for strengthen micromixing efficiency, and has a brighter prospect for industrial application in solvent extraction process.

Table 5 The comparison between mixer-settler and IS-RPB. Style

Mixer-settler IS-RPB

Single stage extraction/%

Zn Mn Cu Zn Mn Cu

92.46 83.18 77.23 99.52 91.47 86.66

Stage Number

Time of phase separation/s

Entrainment %

Coextraction %

Extraction

Scrubbing

Stripping

6

3

3

120

1.4

Ni Co

10.29 10.34

3

2

2

100

0.9

Ni Co

6.82 5.91

62

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Table 6 Comparison of overall volumetric mass transfer coefficients reported for different types of reactors. References

Contactor

System

Qo(m3/s)×106; Qa(m3/s)×106

KLa×103

Present work

Mixer–settler IS-RPB rotating packed bed rotor-stator spinning disc extractor pulsed packed extraction column Annular centrifugal extractor Hollow fibre membrane contactor serpentine microchannel

Leaching solution-D2EHPA-Kerosene

0.33; 0.33 2.78-13.89;2.78-13.89 0.83–2.1; 4.16–20.83 0.017-0.085; 0.017-0.085 0.69-1.04;0.56-0.97 3.33-11.67;3.33-11.67 2.7–7.1; 3.6–7.9 0.0026-0.05; 0.0026-0.05

2.5-10 8-160 15–205 0.39–14.9 1.02-4.43 15.5-46.5 4.5-42 0.5–30

[19] [24] [39] [40] [41] [42]

Water–Methyl red–Xylene phosphoric acid-TBP- kerosene Toluene-acetone-water Water (succinic acid) n-butanol Water–Ibuprofen–Octanol zinc sulfate solution- D2EHPA- dodecane

Acknowledgements

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