Abstract
Impurity elements in copper electrorefining (ER) electrolyte have been becoming a crucial issue on purity of copper cathode. The objective of this doctoral research is to develop a speciation model to understand impurity behavior in aqueous ER solutions.
A thermodynamic model of Fe(II)-Fe(III)-Cu(II)-H2SO4-H2O system is developed and shown to reliably simulate the species distribution in industrial copper electrorefining electrolyte from 25°C to 70°C. The previously developed model of Fe(II)-Fe(III)-H2SO4-H2O system under leaching conditions was first evaluated. It has proved that its applicability can be extended to much higher acid concentration (185 g/L) and high amount of copper (40-50 g/L). Cu(II) species were then identified from literature, and their thermodynamic data were collected and assessed for modeling calculation. Results reveal that after addition of high amount of copper, Fe(II) still distributes as free Fe2+, FeHSO4+ and FeSO4°; Fe(III) distributes as free Fe3+, FeSO4+, FeHSO42+ and Fe(SO4)2-. Cu(II) dissolves as Cu2+, CuSO4° and CuHSO4+. The proposed model was validated by reliable and accurate prediction of measured oxidation and reduction potential (ORP) throughout all solution conditions. Analysis indicates that Fe(II)-Fe(III)-H2SO4-H2O solutions with a high H2SO4 and Cu(II) concentration can still be solely determined by Fe3+/Fe2+ couple. The results also prove the suitability of B-dot equation at high solution ionic strength up to 3M and a previously developed expression for ORP prediction.
The extension of above Fe(II)-Fe(III)-Cu(II)-H2SO4-H2O model was made in solutions, containing additional As(III), As(V), Sb(III), Sb(V) and Bi(III) impurity elements with concentrations based on industrial condition. Identified species including aqueous H3AsO3o, AsO+, H2AsO4-, H3AsO4o, SbO+, H3SbO3o, Sb(OH)6-, HSb(OH)6o, BiO+ and Bi3+ were collected after critical review from literature as well as their thermodynamic data. Experimental ORP measurements were also performed for validation of the proposed model. The distribution percentage of each species was quantitatively illustrated in detail from 25°C to 70°C. The results show that redox couple of free Fe3+/Fe2+ from Fe(III)/Fe(II) plays a predominant role on ORP determination after addition of As(III, V) and Sb(III, V). Furthermore, it has been verified that the previously developed expression with only two variables (nominal Fe3+/Fe2+ ratio and temperature) is capable of predicting ORPs in acidic sulfate solutions with high accuracy.
The findings of this work can provide a deeper understanding on speciation in copper electrorefining solutions and further facilitate the industrial process improvement.
