ESTIMATION OF EXTRACTION BEHAVIOR OF Co(II) AND Ni(II) BY McCABE-THIELE ANALYSIS

Journal of Engineering Science and Technology Special Issue on SOMCHE 214 & RSCE 214 Conference, January (215) 7-77 School of Engineering, Taylor s University ESTIMATION OF EXTRACTION BEHAVIOR OF Co(II) AND Ni(II) BY McCABE-THIELE ANALYSIS K. MORIYAMA, N. MURAYAMA, J. SHIBATA * Department of Chemical, Energy and Environmental Engineering, Faculty of Engineering, Kansai University, Japan *Corresponding Author: shibata@kansai-u.ac.jp Abstract Industrial solvent extraction for the purpose of separation and recovery of the metal ions has been carried out in the countercurrent multistage process. The determination of the operating conditions is performed by McCabe-Thiele analysis for the equilibrium extraction isotherm obtained in extraction experiments. In such an analysis, it is possible to analyze the condition of the extraction of metal ions. For example, however, in the case of mixed solution of Co(II) and Ni(II), the analysis is very complicated. This is because under the conditions where the extraction of Co(II) takes place, the stripping of Ni(II) happens at the same time. In considering the separation of the metal ions in the multicomponent system, it is better to establish the analytical method to provide an important guideline for actual operation. In this study, the conditions where Co(II) is extracted and Ni(II) is stripped are considered. The extraction behavior of Co(II) and stripping behavior of Ni(II) could be estimated from the extraction isotherms. From the estimated results of each metal ion, the separation performance of each metal ion is evaluated by using purity as an indicator. By varying the phase ratio and stage number, the extraction and stripping behavior of the respective metal ions could be reasonably estimated Keywords: Solvent extraction, McCabe-Thiele analysis, Separation behavior, Acidic extractant, Cobalt and nickel 1. Introduction Solvent extraction is one of the methods to separate and purify inorganic compounds and metal ions [1, 2]. Solvent extraction in an industrial operation is mainly carried out by countercurrent multistage process from the viewpoint of separability and efficiency. It is possible to determine the operating conditions of 7

Estimation of Extraction Behavior of Co(II) and Ni(II) by McCabe-Thiele Analysis 71 the single component system by McCabe-Thiele analysis using the equilibrium extraction isotherm obtained in extraction experiments [3-5]. However, the separation of multi-component system has not been discussed. This is because the stripping of Ni(II) happens under the conditions where the extraction of Co(II) takes place. Since industrial operations are the multicomponent system, this type of study provides an important guideline for the separation of metal ions [6, 7]. In this study, the conditions where Co(II) is extracted and Ni(II) is stripped are considered. The separation performance between Co 2+ and Ni 2+ is examined by using McCabe-Thiele analysis. 2. Experiment 2.1. Preparation of aqueous phase and organic phase A stock solution of Co(II) or Ni(II) was prepared by dissolving CoSO 4 7H 2 O or NiSO 4 6H 2 O in pure water respectively. The concentration of both metal ions was.5-.5mol/dm 3, and the ph was adjusted by using NaOH and H 2 SO 4. In the condition shown above, the obtained solutions were used as a feed aqueous solution. PC-88A (2-Ethylhexyl phosphonic acid mono-2-ethylhexyl) diluted to 1mol/dm 3 with kerosene was used as a feed organic solution. 2.2. Solvent extraction The both feed solutions of 15cm 3 were put in a centrifuge tube with a glass stopper, and it was kept in a constant temperature bath at 25 for 1 minutes. The tube was shaken for 2 minutes at 3 spm by a vertical shaker, and then it was centrifuged. After keeping it in the constant temperature bath at 25 for 1 minutes, equilibrium ph of the aqueous phase was measured. The residual metal ion concentration in the aqueous phase was determined by an atomic absorption spectrophotometer (AA-7, Shimadzu Co., Ltd. Japan). The equilibrium metal concentration in the organic phase was determined by the difference between the equilibrium metal concentration in the aqueous phase and the metal concentration in the initial aqueous solution. 2.3. Estimation of extraction behavior of Co(II) and Ni(II) The extraction isotherms of Co 2+ and Ni 2+ were prepared by using extraction equilibrium data. A schematic diagram of countercurrent multistage extraction is shown in Fig. 1. Mass balance equation for the extraction system is given by the following equation [3-5]. Ax 1 Oy Ax (1) n 1 Oy n where A and O are the flow rate of aqueous phase and organic phase, x and y indicate the metal concentrations in the aqueous phase and organic phase, respectively. The following equation is obtained from Eq. (1), n 1 x1 y y A / O x (2) n Journal of Engineering Science and Technology Special Issue 1 1/215

72 K. Moriyama et al. Equilibrium ph and the metal concentration in the inlet aqueous phase X in (x n+1 ) and organic phase Y in (y ) are given as an initial condition. The metal concentrations in the outlet aqueous phase X out (x 1 ) and organic phase Y out (y n ) are determined by providing the setting value of recovery of Co 2+ and Ni 2+. The extraction behavior of Co 2+ and stripping behavior of Ni 2+ could be estimated from the extraction isotherm and the operating line expressed by Eq. (2). Purity is applied as an indicator of the separation of Co 2+ and Ni 2+, and it is calculated from the following equations: Purity Co Purity 2 2 Y Co 2 2 YoutCo YoutNi 2 2 X Ni Ni 2 2 X Co X Ni out out (3) out (4) out y (Y in ) y 1 y 2 1 2 Organic(O) y 3 y n-1 y n (Y out ) Aqueous(A) x 2 x 1 (X out ) x 3 x 4 x n x n+1 (Y in ) Fig. 1. Countercurrent Multistage Extraction. 2.4. Extraction with acidic extractant Extraction reaction of divalent metal ions with acidic extractant is represented by the following equation [8], 2 aq. M 2HR org., org. Haq. MR2 2 (5) where HR is the extractant molecule, and MR 2 means the extracted species, the subscripts of aq. and org. indicate species in the organic phase and aqueous phase. Extraction equilibrium constant K ex is defined as, K ex D MR2 org. H 2 M HR 2 aq. 2 aq. (6) org. Distribution ratio, D, of the metal ions is expressed as follows by definition: MR 2 org. 2 M aq. By combining the Eqs. (6) and (7), the next equation can be obtained,. log D 2pH log K 2log (8) ex HR org When the metal ion concentration in the initial aqueous phase is extremely low compared with the extractant concentration, HR org. can be regarded as a constant value. This means that the relationship between equilibrium ph and logd shows a straight line with slope of two [9, 1]. (7) Journal of Engineering Science and Technology Special Issue 1 1/215

logd Conc. in org. phase [mol/dm 3 ] Estimation of Extraction Behavior of Co(II) and Ni(II) by McCabe-Thiele Analysis 73 3. Results and Discussion 3.1. Extraction isotherms of Co(II) and Ni(II) Figure 2 shows the relationship between equilibrium ph and logd. In any initial metal ion concentration, the linear relationship is obtained. The slope is close to valence of Co 2+ in the area of low metal ion concentrations. As the metal ion concentration is increased, the slope becomes smaller. This is because the extractant concentration before and after extraction is not regarded to be a constant when the concentration of metal ions in the initial aqueous phase increases. From the distribution ratio at each ph, the equilibrium concentration of metal ions can be determined. The extraction isotherms of Co 2+ and Ni 2+ are shown in Fig. 3. The metal concentration in organic phase increases with increasing the equilibrium ph. The isotherm of Co 2+ was higher than that of Ni 2+ at each equilibrium ph. As shown in Eq. (5), the extraction reaction of the metal ions with an acidic extractant, such as PC-88A, is based on a cation-exchange reaction between the metal ions in aqueous phase and protons in the extractant. The ratio of dissociated extractant is increased with an increasse in ph, and extraction of metal ions proceeds. By comparing the extraction behavior of the two metal ions, it is understood that Co 2+ is more extracted than Ni 2+. 2 1.5.4 Co 2+ ph=4.5 ph=4. ph=3.5 Ni 2+ ph=4.5 ph=4. ph=3.5-1 C M,initial [mol/dm 3 ] :.5 :.25 :.45.3.2.1-2 2 3 4 5 6 Equilibrium ph Fig.2 Relationship between equilibrium Fig. 2. Relationship between Equilibrium ph and logd for Cobalt Extraction..1.2.3.4.5 Conc. in aq. phase [mol/dm 3 ] Fig.3 Extraction isotherm of Co 2+ and Ni 2+ Fig. 3. Extraction Isotherm of Co 2+ and Ni 2+. 3.2. Extraction behavior of Co(II) and Ni(II) The conditions where Co 2+ is extracted and Ni 2+ is stripped are considered. The extraction behavior of Co 2+ and stripping behavior of Ni 2+ could be estimated from the extraction isotherms. Figure 4 shows the extraction isotherm and the behavior of Co 2+ in countercurrent multistage extraction. The operation line is constructed according to slope of unity, 1% recovery of Co 2+ and the stage number of three. The stripping behavior of Ni 2+ in a countercurrent multistage operation is shown in Fig. 5. The recovery of Ni 2+ is 66.1% by three stage stripping. In order to obtain a high recovery, it is necessary to lower the metal concentration in the outlet organic phase (Y out ) and increase the metal ion concentration in the outlet aqueous phase (X out ). At this time, since the operating line approaches the isotherm, stage number is increased. Journal of Engineering Science and Technology Special Issue 1 1/215

Stage number [-] Purity of metal ion [%] Conc. in org. phase [mol/dm 3 ] Conc. in org. phase [mol/dm 3 ] 74 K. Moriyama et al..25.2.15 X in ;.1mol/dm 3, Y in ; mol/dm 3 A/O=1, Eq. ph; 4,.3.2 X in ; mol/dm 3, Y in ;.2mol/dm 3 A/O=1, Eq. ph; 4 (X out, Y in ).1.5 (X in, Y out ) Stage number; 3 Recovery; 1% (X out, Y in ).5.1.15.2.25 Conc. in aq. phase [mol/dm 3 ] Fig.4 McCabe-Thiele analysis for Fig. 4. McCabe-Thiele analysis for Countercurrent Extraction of Co 2+..1 (X in, Y out ) Stage number;3 Recovery; 66.1%.1.2.3 Conc. in aq. phase [mol/dm 3 ] Fig. 5. McCabe-Thiele Analysis for Countercurrent Stripping of Ni 2+. 3.3. Extraction behavior of Co(II) and Ni(II) in the various conditions The extraction behavior of Co 2+ and stripping behavior of Ni 2+ are estimated by changing the stage numbers. Figure 6 shows the relationship between recovery of metal ions and stage number. In order to recover 1% of Co 2+, three stages are required. Even if 1 stage operation is applied, the recovery of Ni 2+ increases only up to 81%. When the stage number is increased to infinity, the recovery of Ni 2+ reaches asymptotically to 87%. As shown in Fig. 5, when varying the metal ion concentration in the outlet organic phase (Y out ) and the metal ion concentration in the outlet aqueous phase (X out ), the operating line is in contact with the isotherm. The recovery obtained at this time becomes the maximum in the phase ratio of 1. The relationship between stage number and purity of metal ions is shown in Fig. 7. With increasing the stage number, the purity of both metal ions is increased. The purity of Co 2+ and Ni 2+ are 93.6% and 1% by three stage operation, respectively. This is because even if the stage number is increased, all Ni 2+ ions are not stripped in the phase ratio of 1 as shown in Fig. 5. 1 8 6 4 Co 2+ [mol/dm 3 ]; X in =.1,Y in = Ni 2+ [mol/dm 3 ]; X in =,Y in =.2 A/O; 1, Eq. ph; 4 ; Co 2+ in org. phase ; Ni 2+ in aq. phase 1 8 6 4 Co 2+ [mol/dm 3 ]; X in =.1,Y in = Ni 2+ [mol/dm 3 ]; X in =,Y in =.2 A/O; 1, Eq. ph; 4 ; Co 2+ in org. phase ; Ni 2+ in aq. phase 2 2 2 4 6 8 1 Recovery [%] Fig.6 Relationship beween recovery of Fig. 6. Relationship between Recovery of Metal Ions and Stage Number. 2 4 6 8 1 Stage number [-] Fig.7 Relationship beween stage number Fig. 7. Relationship between Stage Number and Purity of Metal Ions. Journal of Engineering Science and Technology Special Issue 1 1/215

Recovery [%] Purity of metal ion [%] Estimation of Extraction Behavior of Co(II) and Ni(II) by McCabe-Thiele Analysis 75 In order to improve the recovery of Ni 2+, extraction and stripping behavior of Co 2+ and Ni 2+ are evaluated by varying the phase ratio (A/O). Figure 8 shows the relationship between phase ratio and recovery of metal ions. The recovery of Co 2+ in the organic phase is reduced with an increase in the phase ratio. This is why the reduction of the volume of organic phase leads to the insufficient extent amount required for Co 2+ extraction. In the case of the phase ratio of unity, the recovery of Co 2+ is 1%. On the other hand, the recovery of Ni 2+ increases with an increase in the phase ratio. The number of protons needed for stripping increases with increasing the volume of the aqueous phase. When phase ratio is 1, the recovery of Ni 2+ becomes 99.6%. Figure 9 shows the relationship between phase ratio and purity of metal ions. The purity of Co 2+ is increased with an increase in phase ratio since the stripping reaction of Ni 2+ in the organic phase proceeds. On the other hand, the purity of Ni 2+ is reduced with an increase in the phase ratio. 1 1 8 Co 2+ [mol/dm 3 ]; X in =.1,Y in = Ni 2+ [mol/dm 3 ]; X in =,Y in =.2 Eq. ph; 4 Stage number; 3 8 ; Co 2+ in org. phase ; Ni 2+ in aq. phase 6 6 4 4 2 ; Co 2+ in org. phase ; Ni 2+ in aq. phase 2 2 4 6 8 1 Phase ratio (A/O) [-] Fig.8 Relationship beween phase ratio Fig. 8. Relationship between Phase Ratio and Recovery of Metal Ions. 2 4 6 8 1 Phase ratio (A/O) [-] Fig.9 Relationship beween phase ratio Fig. 9. Relationship between Phase Ratio and Purity of Metal Ions. 3.4. Separation condition and separation performance Table 1 shows the separation performance (stage number 3) of Co 2+ and Ni 2+ estimated from Figs. 6-9. The target value of purity of both metal ions is set to be more than 95.% in stage number of three. When the phase ratio is 1.6, the purity of Ni 2+ and Co 2+ is 95.4% and 97.6%. Table 2 shows the separation performance of Co 2+ and Ni 2+ when the target value of recovery and purity of both metal ions is set to be more than 99.9%. When the phase ratio is less than 2., the recovery of Co 2+ is increased and on the other hand, the recovery of Ni 2+ is decreased. The purity of Co 2+ is decreased because the amount of Ni 2+ contained in the organic phase increases. When the phase ratio is more than 2., the recovery of Co 2+ is decreased, while the recovery of Ni 2+ is increased. The purity of Co 2+ is increased because the amount of Ni 2+ contained in the organic phase decreases. Thus, the behavior of extraction and stripping of both metal ions affect the purity of each metal ion. When the stage number is three, the recovery of Co 2+ and Ni 2+ is 95% and 87% at phase ratio of A/O=2.. These recoveries are increased with increasing the stage number and 24 stage operations give 99.9% recovery and purity for both metal ions. Journal of Engineering Science and Technology Special Issue 1 1/215

76 K. Moriyama et al. Table 1. Separation Performance of Co 2+ and Ni 2+. Co 2+ [mol/dm 3 ] Ni 2+ [mol/dm 3 ] Recovery [%] Purity [%] Aq. phase (X out ) 5.2 1-4 1.1 1-2 Ni 2+ =8.5 Ni 2+ =95.4 Org. phase (Y out ) 1.59 1-1 3.94 1-3 Co 2+ =99.2 Co 2+ =97.6 <Extraction of Co 2+ > [mol/dm 3 ] Stage number; 3, Phase ratio; 1.6 Y (Y in )= Y 1 =9.19 1-3 Y 2 =5.99 1-2 Org. 1 2 X 1 (X out )=5.2 1-4 X X 3 =3.82 1-2 2 =6.52 1-3 Y 3 (Y out )=1.59 1-1 Aq. X 4 (X in )=1. 1-1 Table 2. Separation Performance of Co 2+ and Ni 2+. Co 2+ [mol/dm 3 ] Ni 2+ [mol/dm 3 ] Recovery [%] Purity [%] Aq. phase (X out ) 2.51 1-18 9.99 1-3 Ni 2+ =99.9 Ni 2+ =1 Org. phase (Y out ) 2. 1-1 2. 1-5 Co 2+ =99.9 Co 2+ =99.9 Stage number; 24, Phase ratio; 2. <Extraction of Co 2+ > Y 1 =3.7 1-17 Y 23 =.195 Y (Y in )= Y 2 =1.88 1-16 Y 24 (Y out )=.2 Org. 1 2 24 Aq. X 1 (X out )=2.51 1-18 X 3 =9.39 1-17 X 25 (X in )=.1 X 2 =1.54 1-17 Y 24 =.974 <Stripping of Ni 2+ > Y 1 =.14 Y 23 =4.36 1-5 Y (Y in )=.2 Y 2 =.958 Y 24 (Y out )=2 1-5 Org. 1 2 24 Aq. X 1 (X out )=.999 X 3 =.478 X 25 (X in )= X 2 =.699 Y 24 =1.18 1-5 4. Conclusion The separation performance of Co 2+ and Ni 2+ was estimated by McCabe-Thiele analysis using extraction isotherms. The separation process of Co 2+ and Ni 2+ could be evaluated by changing the operating conditions such as stage number and phase ratio in countercurrent multi-stage extraction. The information for the separation and purification of the two metal components was obtained with a smaller amount of experiments. References 1. Nishimura, S. (1989). Solvent extraction technology-recent applications and features. Journal of MMIJ, 15(2), 142-146. 2. Shibata, J. (22). Solvent extraction of precious metals. Journal of the Surface Finishing Society of Japan, 53(1), 641-646. Journal of Engineering Science and Technology Special Issue 1 1/215

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