Modelling copper solvent extraction from acidic sulphate solutions using MOC 45

A mathematical model to predict the extraction has been developed for the Cu-MOC 45 system. The model consists of sets of nonlinear mass action and mass balance equations where the dimerization of the oxime was also considered. The predictive model calculated the equilibrium concentradons from the total oxime concentration, total copper concentration, initial pH valué and the O/A volume phase ratio. The model suggests that the extraction of copper can be defined by the existence of two species into the organic solution: CuR2 (Kext = 4.2) and Cu(HR2)2 (^ext = 10.000)The initial oxime concentration defines the predominancy in the organic solution of both species. The model can be used to predict copper extraction isotherm as well as copper stripping with sulphuric acid.


INTRODUCTION
Copper production technology saw a drastical change in the late 1960s with the implementation of the solvent extraction-electrowinning circuit as a copper obtention method.The advent of this technology produced the most economical copper in the world, increasing the copper production by this method over the two last decades.Oximes are used for copper extraction in acidic aqueous solutions whereas (3-diketones seem to be favoured in ammoniacal media (1 and 2).
The design of new solvent extraction-electrowinning plants or even the modification of existing ones, usuallys needs of an extensive number of ex- periments until acceptable and efficient operating parameters are obtained.At this point is where modelling enters in scene.A predictive model can take a number of examples, choose starting conditions and predict with an acceptable degree of accuracy the equilibrium valúes and thus decreasing the number of preliminary experimentes which otherwise would be required.
Many types of prediction models have been suggested for copper extraction (3)(4)(5)(6)(7)(8), at the same time that new oxime extractants are being introduced into the market with, at the present time, few data available on the literature about their performance on copper solvent extraction.
Here a model is presented from which the extraction of copper by the new oxime MOC 45 can be predicted using the starting feed (organic and aqueous streams) conditions.

EXPERIMENTAL
The oxime MOC 45 was obtained from the Chilean branch of Allco Chem.and was used as obtained from the manufacturer.The active species of this reagent is the 2-hydroxy-5-nonylacetophenone oxime.As a diluent for the organic phase, kerosene from Campsa (10 % aromatic content) was used.
Stock solutions of copper were prepared by dissolving copper sulphate (AR grade) in water, the pH valué was adj usted to the desired valué using H 2 S0 4 (AR grade).
Copper extraction-stripping experiments were carried out by shaking equal volumes (unless otherwise stated) of the organic and aqueous solutions in separatory funnels, for 20 min at 20 °C.After phase disengagement, copper was analyzed in the raffinates by A AS, whereas the corresponding acidity was determined by titration with standard NaOH solution or measuring the equilibrium pH.Copper in the loaded organic solutions was calculated by mass balance.Some copper extraction equilibrium results were taken from the literature (9).

Copper loading by MOC 45
The loading capacity of MOC 45 was determined contacting 25 cm 3 of the organic solution containing 20 % MOC 45 with 25 cm 3 of fresh aqueous phase containing 5 g/L Cu at pH 2.2.A máximum loading of 10.2 g/L Cu was reached after six extraction stages (Fig. 1).

The predictive model
The assumed reactions in building the model are represented by the following equations: Cu 2+ aq + (HR) 2org ^ CuR 2org + 2 H+ aq K 2 [2] 2 HR org ¿3 (HR) 2org K 3 [3] where HR represents the oxime molecule.The corresponding mass-action equilibrium expressions for the reactions listed above are: [6] In addition to the above mass-action equations, the model also includes the following mass-balance equations: aq where [Cu] t is the total copper in the system, [HR] t is the total oxime in the system, [H + ]j is the hydrogen ion concentration of the aqueous feed solution and V is the organic:aqueous volume ratio.Then all the equilibrium concentrations can be calculated from these parameters.Furthermore, the experimental data were treated numerically using the program LETAGROP-DISTR (10) in order to obtain the valúes of the extraction constant for the different extracted species.The numerical program minimizes the error-square sum function, Í7, defined as in eq.[10], where D cal and D exp are the calculated and experimental valúes of the distribution coefficient assuming a given chemical model.The valué of K 3 for the present model was taken from the literature considering the apparent similarity of MOC 45 and SME 529 extractant (11).Results of the numerical calculation is given in table I.
Figure 2 shows how well the model fits the experimental data o ver the full range of conditions: MOC 45 (0.08 mol/L to 0.40 mol/L), aqueous pH (0.5 to 3) and copper concentration (0.60 g/L to 15 g/L).
Figure 3 shows the distribution of the Cu-MOC 45 species as a function of the total extractant concentration.The dimer complex is predominant at high oxime concentrations, whereas the monomer complex is predominant at low MOC 45 concentrations.
Additional data were obtained for the present extraction system determining two copper extraction isotherms using A:0 ratios ranging from 1:10  FIG.3.-Diagrama de distribución del cobre en función de la concentración total de oxima.pH de equilibrio, 2,0.
to 10:1 to evalúate the influence of these relationships on the model.Figure 4 shows the equilibrium curve for the system at various MOC 45 concentrations, whereas in figure 5 the calculated extracted data are compared with the experimental copper extraction valúes.This last figure shows that the calculated valúes match the experimental results reasonably well.FIG.6:-Comparación de la concentración de cobre en la fase orgánica predicha y experimental en la reextracción de este metal.
The model was also used to predict the results of stripping copper-containing organic solutions with different concentrations of sulphuric acid.The organic phase was of MOC 45 (0.18 mol/L) loaded with 3 g/L Cu, the sulphuric acid concentration was varied from 10 to 150 g/L.The model fits the data well as shown in figure 6.

CONCLUSIONS
The extraction of copper from acidic sulphate solutions by the commercially available MOC 45 oxime can be predicted by the use of a mathematical model which uses as a starting input the total oxime concentration, the total copper concentration, the initial hydrogen concentration and the OrA phase ratio.The model also predicted reasonably well the extraction of copper at various O:A phase ratios, and henee the obtention of copper extraction isotherms, and the strip of copper using H 2 S0 4 solutions.
g/L org.ph2.-Comparison of the copper concentrationpredicted with the experimental results.

Fig. 5 .
Fig. 5.-Comparison of predicted and experimental copper extraction using MOC 45 at different phase ratios.
FIG.6.-Comparison of the predicted copper concentration in the stripped organic phase with experimental results.

TABLE . L
-Equilibrium constants for the extraction of copper from acidic solutions by MOC 45