Study of Cadmium Extraction with Aliquat 336 from Highly Saline Solutions

Abstract

A large number of model solutions with high ionic strength were synthesised to mimic industrial conditions and were used as a first approach to study Cd extraction in the presence of chloride at high salinity, as experienced in real industrial solutions. The extractant used throughout in this work was Aliquat 336, a quaternary ammonium salt well known to the hydrometallurgical industry. The effects of some selected anions in addition to chloride (i.e., perchlorate, nitrate, and sulfate) were studied. The distribution of cadmium was measured using ¹⁰⁹Cd as a tracer. Liquid-scintillation spectroscopy quantified the concentration of ¹⁰⁹Cd in both phases. Raman and NMR spectroscopy were employed to gain further insight into the extraction chemistry. A careful analysis of all Cd extraction data showed that within specific windows of the reactant concentrations the chemical reactions could be represented by simplified equations, as discussed thoroughly in the text. Equilibrium constants for the extraction of ${\text{CdCl}}_{3}^{ - }$ from chloride and chloride/sulfate media were determined to be log₁₀K_ext = 4.9 ± 0.8 and log₁₀K_ext = 5.7 ± 0.5, respectively. For the nitrate environment, an exchange reaction involving a LiNO₃ ion pair is proposed and agrees with the experimental data, but was not proven. ¹⁴N-NMR and Raman spectroscopy confirmed that the relative affinity of Aliquat 336 for the relevant anions followed the order: perchlorate > nitrate > chloride > sulfate. Finally, ¹⁴N-NMR enabled the equilibrium constant of the exchange reaction between nitrate and chloride for Aliquat 336 to be determined.

Appendices

Appendix A: Distribution Ratios

See Tables 6, 7, 8 and 9.

Table 6 D values of 0.1 mmol·kg⁻¹ Cd at 21 ± 2 °C from aqueous chloride, using A336 in toluene as the extractant

Table 7 D values of 0.1 mmol·kg⁻¹ Cd at 21 ± 2 °C with LiCl and LiNO₃ present and the variation of them

Table 8 D values of 0.1 mmol·kg⁻¹ Cd at 21 ± 2 °C with both LiCl and Li₂SO₄ present the variation of them

Table 9 D values for 0.1 mmol·kg⁻¹ Cd at 21 ± 2 °C with perchlorate present the variation of them

Appendix B: Speciation Estimation

Figure 1 shows the speciation of Cd as functions of sulfate and chloride concentrations. The speciation diagrams were made by calculating the fraction of free Cd at a given chloride or sulfate concentration:

$$f({\text{Cd}}^{2 + } ) = \frac{{[{\text{Cd}}^{2 + } ] }}{{\left[ {{\text{Cd}}^{2 + } } \right] + \left[ {{\text{CdCl}}^{ + } } \right] + [{\text{CdCl}}_{2} ] \ldots }}$$

(B1)

By using, the product of Cd speciation constants, and some rearranging, the fraction can be calculated as:

$$f({\text{Cd}}^{2 + } ) = \frac{1 }{{1 + \mathop \sum \nolimits_{i = 1 }^{4} \beta_{i} \left[ {{\text{Cl}}^{ - } } \right]^{i} }}$$

(B2)

Here, [Cl⁻] is the concentration of chloride in the solution, $\beta_{i}$ is the stability constant of a Cd species with i ligands, where the stability constant is defined as the product of the equilibrium constants.

The same systematic approach as described above was applied for sulfate:

$$f({\text{Cd}}^{2 + } ) = \frac{1 }{{1 + \mathop \sum \nolimits_{i = 1 }^{2} \beta_{i} \left[ {{\text{SO}}_{4}^{2 - } } \right]^{i} }}$$

(B3)

The fraction of individual species can then be calculated by:

$$f({\text{CdCl}} _{i} ) = f\left( {{\text{Cd}}^{2 + } } \right)\beta_{i} [\text{Cl}^{ - } ]^{i}$$

(B4)

and for sulfate:

$$f({\text{CdSO}}_{4} ) = f\left( {{\text{Cd}}^{2 + } } \right)\beta_{i} \left[ {{\text{SO}}_{4}^{2 - } } \right]^{i}$$

(B5)

If chloride and sulfate is present in the aqueous solution the equation for the fraction of free Cd then becomes:

$$f({\text{Cd}}^{2 + } ) = \frac{1 }{{1 + \mathop \sum \nolimits_{i = 1 }^{4} \beta_{i} \left[ {{\text{Cl}}^{ - } } \right]^{i} + \mathop \sum \nolimits_{i = 1 }^{2} \beta_{i} \left[ {{\text{SO}}_{4}^{2 - } } \right]^{i} }}$$

(B6)

and then Eqs. 4 and B5 can be used to calculate the individual fractions of the species.