Speciation study of the aqueous Fe-Cu-As-Sb-Bi-H2SO4 system and prediction of redox potential in copper electrorefining from 25 °C to 70 °C

Abstract

• A thermodynamic model for solutions in copper electrorefining was developed. • Species of Fe, Cu, As, Sb and Bi were identified and thermodynamic data was collected and assessed. • Distribution of each species was obtained and discussed up to 70 °C. • The model was validated by reliable and accurate prediction of measured ORPs. • Fe 3+ /Fe 2+ couple dominates in ORPs with presence of Fe(II, III), As(III, V) and Sb(III, V). The control of impurities is crucial for production of high-purity cathode copper by electrorefining, especially due to its surging demand stimulated by the promotion of electric vehicles. In the present study, a thermodynamic model has been developed to reliably simulate the distribution of impurity species such as Fe(II, III), As(III, V), Sb(III, V) and Bi(III) in industrial copper electrorefining solutions up to 70 °C. Through extensive literature review, relevant aqueous species were identified and thermodynamic data were collected and critically assessed, including free Fe 2+ , FeHSO 4 + , FeSO 4 °, free Fe 3+ , FeSO 4 + , FeHSO 4 2+ , Fe(SO 4 ) 2 - , Cu 2+ , CuSO 4 °, CuHSO 4 + , H 3 AsO 3 o , AsO + , H 2 AsO 4 - , H 3 AsO 4 o , SbO + , H 3 SbO 3 o , Sb(OH) 6 - , HSb(OH) 6 o , BiO + and Bi 3+ . The previously developed model of Fe(II, III)-Cu(II)-H 2 SO 4 -H 2 O system was firstly evaluated. Then, additional impurities including As(III, V), Sb(III, V) and Bi(III) were gradually added for model simulation. The distribution of each species was obtained and discussed in terms of solution composition and temperature. Species concentrations were thus quantified and used for analysis of oxidation and reduction potentials (ORPs). The reliable and accurate prediction of measured ORPs confirms that the proposed model was quantitatively validated. Analysis of measured and model-calculated ORPs strongly support that the redox couple of free Fe 3+ /Fe 2+ from Fe(III)/Fe(II) plays a predominant role in ORPs determination, even after addition of As(III, V) and Sb(III, V). The expression developed previously to predict ORPs can still be applied in this work. The pH and ionic strength were also obtained and discussed. These findings can provide a deeper understanding on speciation in copper electrorefining solutions.