Extraction of Gold from Chloride Solutions Using Dibasic Esters: A Structure-Reactivity Study

ABSTRACT

In a systematic study of the extraction of gold(III) in acidic chloride solutions by a number of aliphatic esters with the focus on dibasic esters, it was found that di- or tribasic esters are more effective extractants than monobasic esters. The equilibrium for the extraction of gold(III) by a series of dibasic esters C₂H₅OCO(CH₂)_nCOOC₂H₅ (n = 0 to 8) has been studied. Diethyladipate (n = 4) was found to be the most reactive. The extraction of gold by diethyl adipate increases with increasing proton and with increasing chloride ion concentrations. The latter is due to the effect of chloride ions on the activity coefficient of the proton in concentrated HCl solutions. Gold is extracted more effectively by the use of diluents with a high dielectric constant. Gold is extracted as the tetrachloro-complex ion. Slope analysis has shown that the extracted gold complex contains three molecules of the ester and one hydrated proton. The Donor number obtained by calorimetric measurements increases with increasing number of methylene groups and is constant for greater than 3 methylene groups. The distribution coefficient correlates with the Donor number up to 4 methylene groups with steric effects resulting in a decrease in extraction for the higher chain length esters. It is proposed that gold is extracted as an ion-pair in the organic phase with a hydrated proton solvated by the esters.

KEYWORDS:

• Extraction
• gold(III)
• chloride
• esters
• structure

Acknowledgments

The authors wish to acknowledge the contribution of Dr K Sole for useful discussions and guidance of MPK.