The extraction of zinc from chloride solutions using dibutyl butylphosphonate ( DBBP ) in Exxsol D 100 ( # )

The reaction of zinc chloride with dibutyl butylphosphonate in Exxsol DI00 has been studied. The distribution coefficient of zinc is independent of equilibrium pH, thus, suggesting a solvation extraction reaction. Experimental data indícate that this reaction is exothermic (AH°= -28.4 kJ/mol). Slope analysis for the system at various DBBP concentrations reveáis the formation in the loaded organic phases of species which probable 1:2 (Zn:DBBP) stoichiometries. This was confirmed by results obtained at full DBBP Zn-loading capacity. The stoichiometric factor of water in the extraction reaction is found to be 4, whereas experimental data also indicated that two chloride ions are involved for each metal extracted, resulting in a ZnCl2'2L-4H20 stoichiometry (L represents the extractant).


INTRODUCTION
In recent years the recovery of metáis from aqueous chloride solutions has attracted much attention and the development of solvent extraction technology has increased the possibility of metal beneficiation by hydrometallurgical processes.This technology is in widespread use for the recovery and purification
The recovery of zinc(II) from chloride solutions using solvent extraction technology is also of particular interest and various extractants had been investigated (Table I).
The aim of the present work was to study the reaction of zinc, with DBBP diluted in Exxsol D100, from aqueous chloride solutions in order to identify the stoichiometry of extracted species and to contribute to the understanding of the chemistry of such systems and their possibilities in the recovery of zinc from various chloride solutions, e. g. waste streams from hot galvanizing or zinc electroplating processes.

EXPERIMENTAL METHOD
The extractant DBBP was obtained from Albrigt and Wilson Inc., and was used without further purification.Exxsol DI00, using as diluent of the organic phase, was obtained from Exxon Chem.Iberia, its characteristics are: boiling range 234-264°C, flash point 99°C, aromatics contení 0.9, density (15°C) 0.816 g/cm 3 .All other chemicals were of AR grade.
Experiments were carried out by the next procedure.Measured amounts of the aqueous and organic phases were placed in separatory funnels provided with mechanical shaking (600 mhr 1 ) and thermostatted at the required temperature.Agitation was applied for 15 min as previous experiments showed that this time was sufficient to achieve equilibrium.All experiments were conducted at an O/A phase ratio of 1 and the organic solutions were of 50 % v/v DBBP in Exxsol D100 unless otherwise noted.After settling, the zinc content in the equilibrated aqueous solutions was analysed by A AS. Metal in the organic phase was calculated by mass balance.

RESULTS AND DISCUSSION
The distribution coefficient D is defined as the ratio of total metal concentration in the organic phase to the total metal concentration in the aqueous phase.

Influence of the pH on the extraction of zinc
Two series of experiments were performed to examine the effect of pH on zinc extraction.In the first series of experiments, the aqueous feed, with various pH valúes adjusted with HC1, contained 1.0 g/L of zinc.The results are given in figure 1.In the other case, the aqueous solutions contained 1.0 g/L of zinc and 7 mol/L of O", adding as CaCl 2 .Figure 1 also shows the effect of pH on metal extraction.In both cases, temperature was of 20 °C.
The extraction of zinc is independent of aqueous pH, being this result similar to that obtained with phosphonates but not when using TBP (11 and 13).

Influence of chloride ion on zinc extraction
From results shown above it is clear that the presence of CaCl 2 in the aqueous feed greatly improves the extraction of zinc by DBBP.To obtain further information on this behaviour, experiments were conducted with aqueous solutions of 1.0 g/L zinc and various contents of chloride ion, added as CaCl 2 .Results were presented in table II.It can be seen that the increase of the anión concentration in the aqueous feed increases the extraction of zinc.

Influence of temperature
The study of the influence of this variable on zinc extraction by DBBP was carried out using aqueous feed of 1.0 g/L zinc and 7 mol/L Cl" (added as CaCl 2 ).Temperature was varied in the range 10-50 °C.Results obtained are presented in figure 2, plotting log D Zn versus 1000/r, it can be seen that

FIG. 2.-Representación de Arrhenius para la extracción de zinc mediante DBBP
the increase of temperature decreases zinc extraction, although phase separation is good even at lower temperatures.From this figure is obtained that AH° is -28.4 kJ/mol, the reaction is exothermic.

Extraction mechanism
DBBP is a solvating extractant (14) and considering the results obtained above, e.g. the extraction is independent of aqueous pH and the positive influence of chloride ion on zinc extraction, the extraction reaction can be generalized as: Znt: + 2Cl aq + qL org «=> ZnCl 2 • qL org [1] where L represents the extractant molecule and aq and org the respective aqueous and organic phases.On the other hand, and considering the solvating characteristics of DBBP it can be assumed that water molecules also enter the metal-DBBP extracted species.The equilibrium constant for the above reaction can be expressed, in terms of activities, as: and for zinc speciation in the aqueous solution: The formation constant for the above complexes is defined as: where n varíes from 1 to 4. By substituting, eq.[3] becomes: The substitution of eq.[5] into eq.[2] and considering the definition of D Za gives: ¿ext[cn z (i + Xp n [cr] n ) -iH\_A L^Jorg [6] Results of extraction experiments at 20 °C are presented in table III.In all the cases the total chloride ion concentration in the aqueous solution was maintained at 7 mol/L, and considering that the amounts of zinc and chloride extracted are much smaller than the amounts of water and Cl", the activities of both water and chloride ion can thus be considered as constant.
Equation ( 6) can be rewritten as:  where ^Text is an effective extraction constant defined as: ^ext = ^ext[Cn 2 (l+2P n [ClT) [8] A plot of log D Zn versus log [DBBP] org will give a straight line of slope q.The equilibrium zinc concentration in the equilibrated organic solutions is much smaller than the initial extractant concentration, thus, it can be assumed that the equilibrium DBBP concentration in the organic phase is equal to the initial reagent concentration and at lower extractant concentrations the activities can be considered equal to the concentrations.By plotting log D Zn against log [DBBP] org , as shown in figure 3, a straight line of slope 2 is obtained, thus the coefficient q of eq.[1] can be considered 2 and the valué of log ÁT ext being 0.19.
The possibility of extractant dimers formation at higher DBBP concentrations should result in a slope different than 2 either if the extraction constant remained unaletered or changed with the formation of dimers.On the other hand, it was found that the viscosity of the loaded DBBP organic phases increases with the loading of zinc into the organic solutions.
The formation of zinc chlorocomplexes can be written as: x n-l [9] where the overall constant, K w is defined as: 0.0--1.5 [ZnCl^n] [ZnCl^nCF] [10] where n changes from 1 to 4. The reported valúes of the respective constants (K x through K 4 ) varied.In any case, ZnCl 2 should not be the predominant species in the solution at certain chloride concentrations.On the other hand, the 1:2 (Zn:Cl~) is the major species that DBBP can extracts.It is a fact that independent of which species of zinc is extracted, at chloride ion activities constant, the slope analysis can be carried out because the kinetics of eq.[9] are fast and, thus, there is a redistribution of the zinc species in favour of the 1:2 species.
A number of experiments was carried out to satúrate the organic solution with zinc.The initial concentration of DBBP in the organic solution was 50 % v/v, while the zinc concentration in the initial aqueous solution varied from 0.5 to 180 g/L.In all the cases the total Cl" concentration was maintained constant at 7 mol/L by addition of CaCl 2 .The results are shown in table IV, from which molar ratios concentrations of DBBP to zinc in the organic phase were calculated and plotted against the zinc equilibrium concentration in the raffinate (Fig. 4).
The molar ratio DBBP/[Zn] in the organic solution decreases to near 2 when the metal concentration in the aqueous feed increases to near  The corresponding zinc equilibrium loading isotherm is shown in figure 5.The isotherm can be mathematically described by the equation:  The stoichiometric factor of chloride ion in this system was estimated by varying the chloride ion concentration from 1 to 3.5 mol/L by CaCl 2 addition.As preliminary IR studies had shown, the possibility of finding water associated with the extracted species can not be neglected, thus a water stoichiometric factor was also estimated considering that the activity of water varíes with the chloride ion concentration.
The activity coefficient of chloride ion was obtained from reported data (15 y 16), whereas the activity of water was determined accordingly with the literature (17 y 18).The results are given in table V.
Plots of log D Zn versus (2 log a cl + b log a H20 ) were made using different b valúes.With b = 4, the plot can be represented by a straight line with slope near to 1, as shown in figure 6.Thus, the stoichiometry of the extracted species can be represented by ZnCl 2 -2L-4H 2 0. It is estimated that the hydration number of zinc chloride is 10-15 (19), during the extraction of zinc by DBBP (20), a number of water molecules in the primary coordination layer of zinc chloride are replaced by DBBP molecules, remaining only 4 water molecules coordinated with zinc.

CONCLUSIONS
The extraction of zinc from chloride solutions using DBBP dissolved in Exxsol DI00 has been studied.The extraction is independent of aqueous pH but dependent on the presence of chloride ions in the aqueous media, a solvation reaction is suggested.This reaction is exothermic (AH°= -28.4 kJ/mol).Slope analysis indicated that the stoichiometric factor between Zn and the extractant is 1:2 (ZmDBBP), this has been confirmed at máximum zinc load of the DBBP organic solution.The equilibrium loading isotherm has been obtained at 20 °C.The experimental data indícate that the stoichiometric factor for chloride and water in the FIG. 1.-Influence of pH on the extraction of zinc by DBBP.FIG.1.-Influencia del pH en la extracción de zinc mediante DBBP.

Temperature: 20
FIG. 4.-Influence of zinc concentration in the aqueous feed on the molar ratio [DBBP]/[Zn] in the organic phase.FIG.4.-Influencia de la concentración de zinc en la fase acuosa en la relación molar [DBBP]/[Zn]en la fase orgánica.100g/L.Further increases of zinc concentration in the aqueous solution do not decrease this ratio, because DBBP has reached its máximum loading capacity.The corresponding zinc equilibrium loading isotherm is shown in figure5.The isotherm can be mathematically described by the equation: ] org and[Zn]  aq represented the corresponding zinc concentrations at equilibrium in the respective organic and aqueous phases.

TABLE I .
-Solvent extraction systems for zinc extraction from chloride systems

TABLE II .
-The effect of Cl" on zinc extraction by DBBP TABLA II.-El efecto de la presencia de Cl sobre la extracción de zinc mediante DBBP

TABLE IIL -
The influence of DBBP on zinc extraction

TABLE V .
-Activities of chloride ion and water at different chloride ion concentrations FIG.6.-Plot of log D Zn vs 2 log a cl + 4 log a mo .FiG. 6.-Representación de log D Zn frente a 2 log a cl + 4 log a H20 .organic phase are 2 and 4, respectively; thus, the final stoichiometry of the extracted species can be represented by ZnCl 2 -2L-4H 2 0, being L the extractant.