No.24 July 1998

Modeling of Solvent Extraction Equilibria of Cu(II) with Hydroxyoximes

Summary

Metal solvent extraction now plays a very important role in the separation and purification of many metals because of its simplicity and remarkable effectiveness for the mutual separation of metals. Knowledge of the stoichiometric and distribution relationships of metal solvent extraction is very important for predicting the separation behavior and designing the process. It is also interesting from the academic viewpoint to obtain the extraction equilibrium relationship (thermodynamic extraction equilibrium constant), because the extraction equilibrium relationship provides a knowledge of speciation and activities of the metal solutes in the aqueous and organic phases. The thermodynamic extraction equilibrium constant significantly contributes to the fundamental study and practical application of solvent extraction, because this quantity is independent of the concentration of each species in each phase. However, in industrial metal solvent extraction, the metal concentration of the aqueous solutions is usually high (generally greater than 0.01 mol·dm^-3), and the speciation of the organic solutes is unknown. As a result, the estimation of the activities of the solutes in the aqueous and organic phases is very difficult. Therefore, until now, very few thermodynamic studies on industrial metal solvent extraction have been undertaken.

Therefore, in this study, the following investigations have been undertaken to realistically discuss the thermodynamic extraction equilibria of metal solvent extraction: (i) to propose an extraction model to obtain the apparent extraction equilibrium constant, (ii) to verify this model by comparing it with the extraction experimental data, and (iii) to clarify the relationship between the apparent extraction equilibrium constant and the thermodynamic extraction equilibrium constant. The solvent extraction of Cu(II) from acid solutions with aromatic ß-hydroxyoximes is selected as a case study for this modeling. On the basis of a literature survey, the following two points have been found:

(i) A potential method to accurately estimate the activity coefficients of the solutes in the concentrated aqueous phases is to use the Pitzer's ion-interaction theory (Pitzer method).
(ii) A potential method to accurately correct the nonidealities of the organic phases is to consider the stepwise aggregation equilibria of hydroxyoxime and the equilibrium of the complex formation between hydroxyoxime and modifier.

Thus, this extraction model employs (i) the activities of cupric and hydrogen ions in the aqueous phase estimated by the Pitzer method and (ii) the molarity of the monomeric hydroxyoxime in the organic phase calculated by considering the equilibria previously mentioned. The comparison of the model with the experimental data has revealed that this extraction model provides the apparent extraction equilibrium constant independent of the concentrations of the solutes in each phase and the kinds of coexisting ions. Furthermore, the relationship between the apparent extraction equilibrium constant and the thermodynamic extraction equilibrium constant has been discussed.

This report consists of the following five chapters:

Chapter 1,

"Introduction", describes the background and the purpose of the present study. The purpose of the present study is to propose a thermodynamic model with respect to the solvent extraction equilibria of Cu(II) from acid solutions with hydroxyoxime, to verify the model by the experiment, and to discuss the results from thermodynamic viewpoint.

Chapter 2,

"Theoretical Study of the Extraction Model", details the present extraction model. In the present extraction model. the activity ratio, a(H⁺)²/a(Cu²⁺), is estimated by the Pitzer method, where a is the molality-based Henrian activities, while the molarity of the monomeric hydroxyoxime in the organic phase is estimated by considering the stepwise aggregation equilibria of hydroxyoxime and the equilibrium of the complex formation between hydroxyoxime and modifier. On the basis of these values, the apparent extraction equilibrium constant is calculated. Furthermore, by simultaneously solving (i) the equations for the apparent extraction equilibrium constant, (ii) the estimation equation for the density of the aqueous phase by the Pitzer method, (iii) material balance equations. and (iv) the electrical neutrality equation. the distribution ratio of copper is calculated on the basis of an iterative procedure. This chapter also describes a method, with respect to the system in the absence of a modifier in the organic phase, to simultaneously obtain the apparent stepwise aggregation equilibrium constant of hydroxyoxime and the apparent extraction equilibrium constant from the experimentally observed dependency of the copper distribution ratio on the concentration of hydroxyoxime

Chapter 3,

Experimental Study of the Extraction'', describes the purpose of each series of experiments. the experimental method, and the results of the analyses. The extraction experiment was carried out using the batch shaking method at a temperature of 298 K and at the molarities of Cu(II) in the initial aqueous phase of 0.025 - 0.5 mol.dm^-3. First, the extraction with P-5100, a typical industrial hydroxyoxime extractant, was carried out to investigate the effects of (i) various mineral acids (nitric acid, hydrochloric acid. and sulfuric acid: molarities of 0.0001-2 mol·dm^-3) and (ii) coexisting alkali-metal chlorides (lithium chloride, sodium chloride, and potassium chloride: molarities of 0.5 - 3 mol·dm^-3) on the distribution ratio. The present extraction model was applied to analyze the experimental data to obtain the apparent extraction equilibrium constant. As a result, the apparent extraction equilibrium constant is found to be constant, independent of the solute concentrations of each phase and the kinds of coexisting ions in the aqueous phase. Next. the extraction with LIX65N and SME529, which are typical industrial hydroxyoxime extractants, from hydrochloric acid solutions (molarities of 0.01-1 mol·dm^-3) was carried out. The experimental data were analyzed by the present extraction model to simultaneously obtain the apparent aggregation equilibrium constant and the apparent extraction equilibrium constant. As a result, the present extraction model is found to be capable of simultaneously evaluating, from the one series of experiment, the aggregation behavior of hydroxyoxime and the extraction behavior for a given organic phase.

Chapter 4,

"General Discussion", considers the relationship between the apparent extraction equilibrium constant and the thermodynamic extraction equilibrium constant. As a result, the apparent extraction equilibrium constant in the present extraction model has been found to be expressed as the product of the thermodynamic extraction equilibrium constant (molarity-based Henrian activities for the organic species and molality-based Henrian activities for the aqueous species are employed) and the ratio of the activity coefficients, y*()²/y*(). Here, y*() and y*() are the molarity-based Henrian activity coefficients, respectively, of hydroxyoxime (HR) and Cu(II)- hydroxyoxime complex (CuR₂) at infinite dilution in the organic phase of unit Raoultian water activity, where the bars represent the species in the organic phase. This relationship suggests that the activity coefficients of HR and CuR₂ in the organic phase containing water are constant when the Raoultian activity of water in the organic phase is near the value realized by the solvent extraction equilibria (usually greater than 0.9). This chapter also mentions that the model presented in this report is applicable not only to the extraction of Cu(II) with hydroxyoxime but also to other extraction systems if one can estimate the concentrations of the organic species by considering the aggregation equilibria of the extractant and the equilibrium of the complex formation between the extractant and modifier.

Chapter 5,

"Summary", abstracts this report and states the conclusions.

As discussed so far, this report describes the equilibrium model for the extraction of Cu(II) with hydroxyoxime from acid solutions in order to thermodynamically analyze the metal solvent extraction equilibria. This model defines the apparent extraction equilibrium constant where the Pitzer's ion-interaction theory is applied to the aqueous phase and the stepwise aggregation equilibria of hydroxyoxime and the equilibrium of the complex formation between hydroxyoxime and modifier are considered in the organic phase. Subsequently, the model is experimentally verified, and the comparison between the apparent extraction equilibrium constant and the thermodynamic extraction equilibrium constant is carried out.

Principal new findings shown in this report are as follows:

(i) The activities of the solutes in the concentrated aqueous solutions are adequately estimated by the Pitzer method.
(ii) The model provides information on the aggregation in the organic phase.
(iii) The model allows the quantitative calculation of the distribution ratio.
(iv) The apparent extraction equilibrium constant is related to the thermodynamic extraction equilibrium constant by a simple equation.

The results obtained by this report present a new method for analyzing the distribution in metal solvent extraction.