Concentration Behaviors of the Metal Ions
The plots in Fig. 2a–d display the concentration behaviors of the metal ions during evaporation. Figure 2a shows that the concentrations of Ni, Co and Mg increase exponentially as the solution becomes more concentrated, whereas, in contrast, the concentration of Ca is observed to continuously decrease, for example, the solution at XVi/Vr = 6—comprised of 89.2 g/l Ni, 4.3 g/l Co, 0.6 g/l Ca and 7.7 g/l Mg—has an ~ 80% precipitation yield of Ca. Further concentration to an XVi/Vr value of 8 results in a solution with 114.5 g/l Ni, 5.2 g/l Co, 0.6 g/l Ca and 9.0 g/l Mg and precipitation yield of Ca = 87% (as shown in Fig. 2b). These results indicate that Ca can be readily separated from the solution because of its low solubility.24 Figure 2c presents the chord length counts recorded during the evaporation period, as determined by the Particle Track system. As can be observed, the counts of chord length < 10 µm begin to increase at the XVi/Vr of ca. ~ 2 after 1.6 h, which indicates the nucleation of solid particles. With longer experimental time, the counts of < 10 µm and 10–100 µm continue to rise rapidly, demonstrating that both nucleation and solid crystal growth increase during the evaporation period.
The solid precipitate obtained at XVi/Vr = 8 was separated and washed with the deionized water three times. Analysis of the washing water showed that it contained 5.9 g/l Ni, 0.3 g/l Co, 0.2 g/l Ca and 0.6 g/l Mg, which was slightly more dilute compared with the PLS1; however, it was possible to recycle this solution back to the evaporation stage. The contents of Ni, Mg and Co in the washed solid precipitate were determined to be 3.2% (g/g), 0.3% (g/g) and 0.7% (g/g), respectively, which implied that only 0.3% Ni, 0.3% Mg and 1.6% Co were lost within the calcium precipitate. Subsequent analysis of the solid precipitate by XRD and SEM (Fig. 2d) confirmed that the main phase present in the solid precipitate was composed of CaSO4·2H2O.
Selective Extraction of Ca with Na-D2EHPA
The concentrated solution (PLS2) that originated from evaporation up to XVi/Vr = 6 (89.2 g/l Ni, 4.3 g/l Co, 0.6 g/l Ca and 7.7 g/l Mg) was used for the investigation of residual Ca removal. The effects of the extraction parameters, including equilibrium pH value (pHeq) and the concentration of the extractant Na-D2EHPA, were evaluated with an O/A ratio of 1:1 at 25°C. As shown in Fig. 3a, the extraction order of the metal ions follows Ca > Mg > Co > Ni, and these findings are consistent with results reported previously.25,26 At pHeq = 1.6, the extraction efficiency of Ca with 20 vol.% Na-D2EHPA was found to be 74%, and this was seen to increase until a maximum of 93% at pHeq = 3.0. Over this same pHeq range, the respective extraction efficiencies of Mg changed from 7% to 23% and Co from 1% to 13%, while the Ni extraction remained negligible (< 2%). When pHeq≥ 3, the extraction of Ni, Co and Mg increased, whereas the extraction of Ca decreased slightly as a consequence of the crowding out effect of Ni.27 The results in Fig. 3b reveal that after a single contact with 10 vol.% Na-D2EHPA at a pHeq = 2.5, > 89% of Ca can be extracted, while the extractions of Mg (10.5%), Co (7.2%) and Ni (0.6%) are much lower, leading to an associated SFCa/Ni of 1267. With the increase of pHeq to 3, approximately 91% Ca, 16% Mg, 8% Co and 1% Ni were extracted. Consequently, 10 vol.% Na-D2EHPA and pHeq = 2.5 were chosen as the optimum conditions. The resultant Ca-free solution (PLS 3)—with a composition of 88.7 g/l Ni, 0.06 g/l Ca, 4.0 g/l of Co and 7.0 g/l of Mg—was used in the subsequent experiments.
The concentrated solutions with different initial Ni concentrations (highlighted in Fig. 2b) were contacted with 10 vol.% Na-D2EHPA at O/A = 1:1, pHeq~ 2.5 and T = 25°C, and the corresponding extraction efficiencies of the metal ions are shown in Fig. 4. A slight decrease of the extraction efficiency of Ca from ~ 92% to ~ 88% can be observed as the initial Ni concentration increases from 16.3 g/l to 114.5 g/l. A similar outcome was obtained at XVi/Vr = 8, where 2% Ni, 8% Co and 11% Mg were co-extracted into the loaded D2EHPA. The results suggested that the residual Ca could be selectively extracted, as after extraction, the raffinate was found to have only 60 mg/l to 70 mg/l Ca over the concentration range tested.
Scrubbing of the Loaded D2EHPA
The scrubbing/stripping behavior of the metal ions from the loaded D2EHPA—which contained 1.2 g/l Ni, 0.3 g/l Co, 0.6 g/l Ca and 0.7 g/l Mg—was studied by varying the H2SO4 concentration from 0 to 1 M at O/A = 1:1 and T = 25°C. The scrubbing efficiencies of both Ni and Co were enhanced as the concentration of H2SO4 was increased from 0.01 M to 0.05 M (49% to 95% for Ni and 48% to 84% for Co). These findings show that Ni and Co can be selectively scrubbed from the loaded D2EHPA with low H2SO4 concentrations (Fig. 5a), i.e., using 0.025 M H2SO4, results in a solution with 1.0 g/l Ni, 0.25 g/l Co and 0.3 g/l Mg (PLS4). With higher concentrations of H2SO4, Ca can be also stripped, for example, 1.0 M H2SO4 is shown to increase Ca yield to ~ 90% (Fig. 5a). Use of more concentrated H2SO4 enhances Ca precipitation as gypsum, especially with higher O/A ratios (> 2:1). Therefore, based on the literature, the HCl solution can be suggested as an optional reagent for Ca stripping from the loaded organic phase and to regenerate the D2EHPA.21
Selective Extraction of Co and Mg with Na-Cyanex 272
The scrubbing solution (PLS4) from the loaded D2EHPA was integrated into the Ca-free solution (PLS3) for the recovery of Ni and Co.21 Nevertheless, the volume of the scrubbing solution that can be added is dependent on the Ni concentration of the feeding solutions used in the second extraction step. In the case detailed here, a combination of the PLS3 (~ 80 vol.%) and the PLS4 (~ 20 vol.%) resulted in a feedstock (PLS5) that contained approximately 67.5 g/l Ni, 3.8 g/l Co and 6.3 g/l Mg. The pH of PLS5 was adjusted to around 5.5 with 10 M NaOH before being treated with Na-Cyanex 272 to extract Co and Mg.26 The effects of pHeq and Na-Cyanex 272 concentration on the extraction efficiencies of Co, Mg and Ni were examined as detailed in Fig. 6. From Fig. 6a, the extraction efficiencies of Co and Mg gradually increase with the increase of pHeq. When the pHeq value is < 5.2, the extraction efficiencies of Co and Mg are < 95% and 44%, respectively, whereas the Ni extraction is < 2%. Mg appears to be difficult to extract compared with Co, which is in accordance with the relative order of extraction selectivity previously determined for Cyanex 272.28 A further increase in pHeq to 5.6 gives rise to extraction efficiencies of ~ 97% Co and ~ 51% Mg; however, crud begins to be observed when the pHeq is > 6.3. The formation of crud within the solution at elevated pHeq is believed to result from the formation of insoluble Ni(OH)2, as proposed by both Cheng et al.29 and Guimarães et al.21,30 In Fig. 6b, the increase in the concentration of Na-Cyanex 272 from 10 vol.% to 30 vol.% at pHeq ~ 5.5 results in an increase in the extraction of all metal ions, especially Mg. Nevertheless, the extraction efficiency of Ni is relatively high at ~ 5% with 30 vol.% Na-Cyanex 272. Consequently, 20 vol.% Na-Cyanex 272 was determined to be the optimum level required to selectively extract Co and Mg.
The isotherms of Co and Mg extractions were evaluated at varying A/O ratios (2:1, 1:1, 1:2, 1:3, 1:4 and 1:5) by exposing the mixed solution (PLS5) to a single contact with 20 vol.% Na-Cyanex 272. The extraction distribution isotherms and their McCabe-Thiele diagrams are displayed in Fig. 7a and b. The results in Fig. 7a show that at the A/O ratio of 1:1, two theoretical stages are required to extract ~ 100% Co, whereas Fig. 7b highlights that two theoretical extraction stages at the A/O ratio of 1:2 are needed to extract ~ 95% Mg. It is worth noting that with the increase in the O/A ratio, the co-extraction of Ni becomes significantly higher as more extraction sites are freely available. Therefore, for the successful separation of Co and Mg vs. Ni, the O/A ratio plays a crucial role.32 A lower O/A ratio (1:1) with an additional extraction stage can be used to suppress Ni co-extraction and ensure efficient Mg separation. Use of an O/A ratio of 2:1 in the first stage, followed by two subsequent stages at an O/A ratio of 1:1, resulted in a raffinate (PLS6) that contained 61.7 g/l Ni, 0.007 g/l Co and 0.3 g/l Mg, which is of an acceptable purity and concentration to be used as a feedstock for state-of-the-art hydrometallurgical high-purity Ni recovery processes.31,32
Scrubbing of the Loaded Cyanex 272
The scrubbing behavior of Ni, Co and Mg from the loaded Cyanex 272—which contained 1.4 g/l Ni, 3.5 g/l Co and 2.8 g/l Mg—were studied with different concentrations (0–0.1 M) of H2SO4 solution. As shown in Fig. 8, with a 0.025 M H2SO4 solution, approximately 91% Ni and 13% Mg can be selectively scrubbed, while only 1% Co is removed from the loaded Cyanex 272. When 0.05 M H2SO4 is used, the scrubbing efficiency of Mg increases significantly to around 50%, whereas the associated efficiencies of Ni and Co increase to 97% and 3%, respectively. Accordingly, the H2SO4 concentration for the selective scrubbing Ni from the loaded Cyanex 272 was chosen as 0.025 M to minimize impurities in the resultant Ni solution. After the selective scrubbing of Ni, the loaded Cyanex 272 can be further scrubbed to produce a relatively Co-rich strip solution that can then be treated via sulfide precipitation to recover the Co.33,34