Experimental Investigation and Thermodynamic Modeling of Zn +2 and Ni +2 Extraction from Zn Plant Residue using D2EHPA

Zinc plant filter cake contains valuable metals that can be reused as a source for obtaining these metals. This study describes an experimental two stage study on the extraction of zinc and nickel from waste zinc filter cake which includes acid leaching of zinc filter cake followed by organic phase aided extraction of metals from the leaching solution. To determine the optimum leaching condition a comprehensive study of the recovery of chemical elements from spent plant residues was experimentally studied at different levels of acid concentrations at different temperatures while measuring chemical elements concentration with respect to time. Experimental results showed that 99% recovery of Ni 2+ , Zn 2+ and 89% recovery of Pb 2+ can be achieved at following optimum conditions: 2M nitric acid, T= 358.15 K after 1.5 h of acid leaching at S/L=1/10. Then, the extraction of Zn 2+ , Ni 2+ , and Pb 2+ was carried out by di-(2-ethylhexyl) phosphoric acid (D2EHPA) that was diluted with kerosene in equal phase ratio and the effect of extractant concentration and pH was studied at T = 298.15 K. Results showed that an increase in pH and extractant concentration can greatly increase zinc and nickel extraction to a maximum achievable amount of 95% and 90 % for Zn 2+ and Ni 2+ , respectively by 25 (v/v%) D2EHPA at pH = 5.5 and organic to aqueous phase ratio (O/A) = 1/1. For modeling of equilibrium concentrations in organic and aqueous phases and activity coefficients calculation, Electrolyte-UNIQUAC-NRF, UNIQUAC-NRF, NRTL and NRTL-based local composition models were used. After that, adjusted parameters were successfully used for calculation of the equilibrium constant of the unknown parameters and the extraction reaction. The obtained results of thermodynamic modeling were in well agreement with the experimental data.


INTRODUCTION 1
In recent years, because of increasing the consumption of metals along with the simultaneous depletion of primary resources, more attention has been drawn to the recovery of metals from secondary resources, including waste residues from zinc plants.Recovering the valuable content of these resources can have both economic and ecological advantages.Some industrial wastes are classified as the main resources of different metals [1,2].Metals or minerals production from ores through pyro/hydro-metallurgical processes usually causes enormous amount of residues [3,4].For example, zinc industrial residue, which is produced in zinc ores processing contains some valuable metals, e.g., Ni, Pb as well as Zn that can be beneficially recovered for further processing.Thus, recovering these metals from the secondary resources as well as natural minerals for the high value products, is necessary and important for effective use of these resources [5].Nowadays, several techniques such as electrolysis, cementation, liquid-liquid extraction, precipitation, and ion exchange have been applied to separate and recover metals from zinc plant residue [6,7].The hydrometallurgical route is one of the most widely used approaches for recovering valuable metals from zinc plant residue which mainly involves leaching with an acid solution followed by the extraction of metal ions from leaching solution [8].Solvent extraction, as an efficient separation and purification technology, can achieve the selective extraction of target elements and has been widely used in metal hydrometallurgy, heavy metal wastewater treatment, and other fields [8,9].
Solvent extraction process of Ni, Zn, and Pb from aqueous solution by several commercial extractants have been studied by researchers.They were trying to understand the role and effect of influencing parameters like pH of solution and type and concentration of extractant [10,11].For example, acidic extractants (Cyanex 302 [12], D2EHPA [13], and Cyanex 272 [14]) and chelating extractants (LIX 984N [15], LIX 63 [16], and LIX 84 [17]) have been used for this purpose.Di-2ethyl hexyl phosphoric acid (D2EHPA) as a classical extractant, has been widely used in extraction and separation of the divalent transition metals such as zinc, manganese, nickel, cobalt and copper [18][19][20].According to the previous researches, for enhanced separation of Zn from acid solution by D2EHPA, another extractant along with D2EHPA must be used [21,22].For instance, Hosseini et al. [23] found the desired extraction of manganese and zinc was obtained using a mixture of Cya-nex 302 and D2EHPA.Alike results were presented by Babakhani et al. [24] for separation of Ni 2+ and Cd 2+ .Another report by Innocenzi and Veglio [25], revealed combination D2EHPA and Cya-nex 272 was more effective for separation of Mn 2+ and Zn 2+ than Cya-nex 272.
Mathematical modeling is a key step in a solvent extraction process and it is necessary for designing, controlling and optimization of the process [26][27][28][29].Thus, an appropriate model is necessary for prediction a solute distribution coefficient and thermodynamic equilibrium constant of the extraction reaction [30].Electrolyte solutions are very important in several chemical industries and biological processes.Since they are known non-ideal solutions even at low concentrations, or in some processes like gas sweetening, which electrolyte solutions have high concentration, accurate thermodynamic models are needed.Therefore, modeling and predicting the thermodynamic properties of electrolyte solutions is of great importance to predict an accurate model for a wide range of concentrations [31][32][33][34][35][36][37][38][39][40].Some models such as Bromley model, Scatchard-Hildebrand model, Pitzer virial expansion equations and Guggenheim quasilattice model have been used for activity coefficient (  ) calculation of electrolyte and nonelectrolyte solutions [37][38][39][40][41][42][43][44][45][46][47][48][49][50].Although in the mentioned studies,   was not calculated, or the non-ideality of a phase was ignored due to the complexity of the liquidliquid extraction process, several of these models might not be accurate enough to correlate the activity coefficients of high concentration of aqueous electrolyte solutions.Some significant researches have been carried out by Thomsen et al. [45], Pitzer [41,42], Cruz and Renon [43], Zhao et al. [44], Chen et al. [46], Haghtalab and Vera [47] and Sadeghi [48] that the excess Gibbs energy functions based on the concept of local composition has been developed and are among the most successful electrolyte models.Haghtalab and Peyvandi [51] developed Electrolyte-UNIQUAC-NRF model for description the behavior of binary electrolyte and multi-component solution in a concentration range and at high temperature.This model is based on localcomposition approach.Furthermore, Chen et al. [49] extended the e-NRTL model for single electrolyte solution and single solvent systems.Chen et al. [49] used Pitzer-Debye-Hückel equation [50] and local composition model based on NRTL for characterization of long and short range interactions, respectively.
In this work, we planned to examine the extraction of Zn 2+ , Ni 2+ , and Pb 2+ from zinc plant filter cake (ZPFC) combined with various metal ions.First, the influences of different processes factors on the extraction of metal ions were experimentally calculated.Next, the equilibrium extraction of Ni 2+ and Zn 2+ ions from HNO₃ solution were model based on a thermodynamic model.Hence the Electrolyte-UNIQUAC-NRF and UNIQUAC-NRF equations were used for   calculation for aqueous solution and organic phase, respectively.Finally, the result of both equations were compared with NRTL and a new NRTL-based local composition models.

1. Chemicals and Equipment
The ZPFC employed in the experiments was supplied from the zinc manufacturing plant in Zanjan (Iran) and was determined by the X-ray fluorescence (XRF) technique (ARL ADVANTX+, Switzerland).Nitric acid (purity of 65%) and the organic extractant, D2EHPA (purity of 97%) were prepared from Merck and kerosene was purchased from Esfahan Oil Refinery Co. (purity of 97%).Sodium hydroxide pellets (purity of 99%, Merck) were used to adjust pH values.The aqueous and organic phases were mixed using a mechanical stirrer.The operating temperature and pH values were monitored using a thermometer and pH meter, respectively.Moreover, metal ions concentrations in the aqueous and organic solutions were analyzed by AAS (Absorption Spectrophotometer) and mass balance, respectively.

Experimental Procedure
The experimental procedure that was used in this communication is illustrated in Figure 1.The method included two main operating units: 1. leaching process of zinc filter cake and 2. solvent extraction process; the leach solution of step 1 is utilized for the separation, according to step 1, an aqueous phase was supplied by dissolving of appropriate values of zinc filter cake and HNO3 in distilled water.During the leaching process, the influence of acid concentration, time and operating temperature were investigated.In all experiments, 30 g of zinc plant filter cake (ZPFC) was mixed with 300 mL of HNO3 (0.25-2 M), S/L=1/10, and after that stirred in the bottom three-necked flask at a speed equal to 600 rpm with a mechanical stirrer.The leaching process of all elements was investigated at time range of 30-120 min and temperature in the range of 298.15-358.15K.The concentration of each ion in the leaching solution was obtained by absorption spectrophotometer and in the second step, in all solvent extraction tests, O/A=1/1.After that, 25 mL of each phase was blended on the mechanical stirrer and the pH value was continuously controlled during the experiment.Nitric acid (2 M) and sodium hydroxide (4 M) were applied to modify the pH.The organic and aqueous phases were separated at desired pH value by a separation funnel and after 10 min the equilibrium achieved.Then, the aqueous phase was sampled for analysis of the extracted ions.The percentage of extraction of metal ions were calculated using Equation ( 1) and the distribution ratio Zn 2+ and Ni 2+ from other metal ions.The ZPFC was oven-dried for 24 h at 303.15 K and then milled into particles of less than 0.1 mm for further analysis.The XRF analysis of the ZPFC presented in Figure 2. The percentage of extraction was determined by using Equation (2): where,  is the initial metal ions concentration in the aqueous phase and  is the metal ions concentration in the aqueous phase after extraction and  is the metal ions concentration in the organic phase after extraction.
Figure1.The schematic of the experimental process

. Result of Acid Concentration
The influence of some important factors, containing temperature, operation time and acid concentration are studied on the leaching process.ZPFC is leached with nitric acid solution followed by dissolution and solvent extraction of objective ions from the leached solutions [52].Nevertheless, hydrometallurgical behavior creates a vast acidic wastes value that if disposed in environment reasons dangerous health problems and ecological imbalance.Among a huge number of acids, HNO3 is most accomplished leaching agent as high oxidation potential of HNO3 and less non-soluble residue evolution.Despite high value of HNO3, it can be recovered again with hydrometallurgical behavior.HNO3 can be easily recycled via external NOx oxidation [53][54][55][56].Generally, concentration of acid has pronounced influence on the extraction of metal and it is one of the main factors to be investigated for optimization condition for dissolution of metal ions, range of which was studied with conducting a number of experiments at various acid concentration using ZPFC.Figure 3

2. Solvent Extraction
After investigating leaching process, the extraction of Zn Aghazadeh et al. [56] studied zinc extraction from synthetic sulfate solution using D2EHPA diluted with kerosene.In their study tributyl-phosphate (TBP) showed positive synergism at concentration of 5% (v/v) and negative synergism effect at concentrations of 2% and 10%.Zinc extraction efficiency increased from 90 to 98% for experiments with 5%, 15%, and 20% D2EHPA concentrations when TBP concentration was 5%.However, their study was limited to zinc extraction from synthetic sulfate solution.Aghdam et al. [57] also, studied the possibility of separation of Zn 2+ and Cd 2+ metal ions from chloride solutions by (D2EHPA) in kerosene as a diluent.In fact, the aqueous phase was obtained by brine leaching of zinc leaching filter cakes.They concluded D2EHPA is capable of extracting and separating zinc from cadmium in chloride solutions at pH = 3 with approximately 99% yield a negligible coextraction of cadmium.

Thermodynamic Modeling
The   in organic and aqueous phases are needed for calculation The superscripts "cal"of and "exp" are the calculated and the experimental values, respectively and N is the number of experimental points.The thermodynamic modeling will be performed based on following algorithm: and the distribution coefficient, D, can be expressed as Equation ( 6): By taking the logarithm of Equation ( 5) and by Equation (7): The equilibrium concentration of H2A2 in Equation ( 4) was considered by following (Equation ( 8)): The plot of   −  vs.  [( 2  2 )] is linear if the ideality is assumed for the system, i.e.   = 1.The line slope (n) characterizes H2A2 stoichiometric coefficient whereas its intercept denotes Kex without considering the non-ideality of the substances.
In ionic strength calculation, it is required to consider H + , Na + , NO -3 , Zn 2+ , and Ni 2+ ions in the aqueous solution.According to Equation (4), the metal ions during the extraction reaction are substituted with H + .The organic solution contains of three components: Zn 2+ and Ni 2+ , free extractant dimers (H2A2), and kerosene as diluent.Therefore, the system contains five unknown concentrations and five equations were needed.The equations were obtained by charge and mass balances in the aqueous solution as following Equations (9-14):

2. Determination of Stoichiometric Coefficient
The obtained experimental data that are used to obtained stoichiometric coefficient are shown in Table 1.The experimental tests were performed by D2EHPA (2-25 (v/v%)), at 298.15 K, 3.8 ˂ pH ˂ 4.5.In Equation ( 2), the slope analysis procedure is applied to calculate the organic complex of Zn 2+ and Ni 2+ and unknown n (stoichiometric coefficient) of D2EHPA dimer.Thus, the behavior of   −  vs. log(( 2  2 )) must be plotted.Figure 5 demonstrates this behavior.Based on Figure 5, accuracy  of both correlations are satisfactory (R 2 ≥ 0.98, R 2 is Rsquared).According to curve fitting results, extraction reaction between D2EHPA, Zn 2+ and Ni 2+ dimers is as following Equation ( 15): Using UNIQUAC- UNIQUAC-NRF and Electrolyte-UNIQUAC. Next, the values of equilibrium concentration were calculated by using the trust region dogleg iteration method.As can be shown in Figure 6, all obtained and calculated experimental data of Ni 2+ and Zn 2+ in the aqueous solution and organic media the at equilibrium condition.

3. 1 . 2 .
Result of Temperature and TimeFigure 3 (b-d) illustrates the influence of both time reaction and operating temperature on dissolution of metal ions with 2 M HNO3.As can be seen, both temperature and

Figure 3 .
Figure 3. Factors affecting metal ions dissolution from ZPFC, S/L=1/10, (a) Influence of amount of acid on Zn 2+ , Ni 2+ , and Pb 2+ dissolution at T = 358.15K, and t = 1.5 h, Effect of temperature and time on (b) Ni 2+ , (c) Zn 2+ , and (d) Pb 2+ dissolution in concentration of 2 M acid, S/L=1/10, speed of agitation: 600 rpm 2+ , Ni 2+ , and Pb 2+ were studied.All experimental tests were performed at the pH range 2-6, O/A=1/1, and T = 298.15K. Referring to Figure 4(a), complete extraction of Ni 2+ and Zn 2+ ions, was occurred at higher value of pH.According to Equation (4) di-(2-ethylhexyl) phosphoric acid release hydrogen ion (H + ) during extraction process.Thus, increasing pH leads to increase extraction of metal from aqueous media.The extraction of Ni 2+ and Zn 2+ is remarkably being depending on an acidic extractant like D2EHPA and pH.There is significant difference in the treatment of each metal.The roughly a half of Zn 2+ and almost all of Ni 2+ ions were extracted by D2EHPA whereas Pb 2+ remained in the aqueous solution.Furthermore, the percentage of the extraction of Pb 2+ was 12.8 % and the pH variations had negligible effect on it.The influence of concentration of D2EHPA on the Zn 2+ , Ni 2+ and Pb 2+ extraction using 2-25 percent volume/volume of D2EHPA in kerosene at pH = 5.5, and T = 298.15K was studied.Referring to Figure 4(b), increasing D2EHPA concentration have no remarkable effect on Pb 2+ extraction, while the extraction percentage of Zn 2+ and Ni 2+ had different behavior.The obtained experimental results showed rising in D2EHPA concentration leads to remarkable extraction of Ni 2+ (95.5%) and Zn 2+ (96.1%).Consequently, the high values of D2EHPA concentration had effective influence on Zn 2+ and Ni 2+ extraction.

2 : 3 . 3 . 1 .
Calculate the concentration of each species [i] / from the equation system Initial Calculate the concentration of each species [i] from the equation system at γi =1 7: If OF ˃ ε [i]=[i] / and return to step 4 If OF ˂ ε Calculate γi from models using [i] 8: If OF ˃ ε Calculate the concentration of each species [i] / from the equation system and return to step 3 If OF ˂ ε Print parameters of models and Kx Extraction Mechanism of Ni and Zn by D2EHPA In this study, D2EHPA was used as a solvent extraction, which extracts Zn 2+ and Ni 2+ ions.The divalent extraction metals such as Ni 2+ and Zn 2+ using di-(2-ethylhexyl) phosphoric acid is shown using the equilibrium reaction (Equation (4)):  () 2+ + ( 2  2 ) () ↔ ( 2 () 2−2 ) () extractant in dimeric form and the stoichiometric coefficient of H2A2 displayed by n.The D2EHPA molecules is well known being generally as dimers in the non-polar organic diluents.H2A2 shows the dimer of di-(2-ethylhexyl) phosphoric acid and the over -bar symbol illustrates the substances in the organic phase.Kex parameter of the extraction reaction of Ni 2+ , Zn 2+ and D2EHPA can be given as Equation (5):

Figure 6 .
Figure 6.The Ni 2+ and Zn 2+ concentration in organic phase at 298.15 K: The experimental results vs. thermodynamic modeling.

TABLE 1 .
The extraction experimental data of zinc and nickel (O/A=1/1 and T = 298.15K)