Bismuth and antimony removal in electrolytic copper refining
The electrolytic refining of raw copper allows large-scale access to high-purity copper, which is primarily required in the electrical and electronics industry. During this process, both noble metals and non-precious elements are removed from the raw copper. Bismuth and antimony dissolve in the electrolyte. To prevent them from contaminating the cathode again during copper deposition, they must be selectively removed from the electrolyte. Lewatit MDS TP 260, an ion exchange resin from the German specialty chemicals group Lanxess, is used for this purpose in an Australian plant.
Copper electrorefining – A complex purification process
The copper concentrate extracted from the copper ore contains 60 – 70 % copper. The rest is mainly iron and sulphur. Anode plates are produced from the up to 99 % blister copper obtained after separation, which are then further purified electrolytically. The main impurities are lead and nickel, precious and platinum metals such as gold, silver, palladium and platinum, as well as arsenic, tin, selenium, bismuth and antimony. While the precious metals precipitate elementarily in the anode sludge during electrolysis, the non-precious impurities either dissolve or form insoluble oxides (e.g. tin oxide SnO2) or sulphates (e.g. lead sulphate PbSO4). Antimony and bismuth mainly remain in the electrolyte and must be removed from it to prevent them from being deposited again at the cathode together with the copper. High-purity copper (> 99.99 % Cu) can thus be obtained.
Ion exchange for efficient bismuth
and antimony removal
Bismuth (Bi) and antimony (Sb) are frequently separated from increasingly copper-poor electrolytes (down to and below 1 g/l Cu) in a multi-stage electrolytic process (electrowinning). Such a process was also used in an Australian copper extraction plant in electrolytic refining until 2023 in order to purify a bleed stream of the electrolyte so to reduce or control the concentration of impurities by feeding it back to the mainstream. Alternatively, solvent extraction or adsorption, e.g. on activated carbon, are also used on a technical scale for this purpose. The Australian operators of the electrolysis plant decided at the time to use another alternative process in future. The choice fell on the use of ion exchange (IEX) resins, namely the macroporous selective IEX resin Lewatit MDS TP 260 from Lanxess. It contains chelating aminomethylphosphonic acid (AMPA) groups which are able to bind Bi and Sb ions with high selectivity. According to the operators, it was the resin’s high capacity for Bi and Sb and its low affinity for arsenic and lead that were mainly responsible for this choice. In addition, the selective resin does not extract copper from the electrolyte, so that no copper losses occur during the cleaning process.
The so-called MDS resin (Mono Disperse Small) used consists of significantly smaller polymer beads with a diameter of approx. 0.4 mm compared to conventional AMPA resins with a typical bead diameter of approx. 0.63 mm. “Tests in our laboratories have shown that these smaller beads enable better exchange kinetics, resulting in a higher total capacity. In addition, the small beads are mechanically more stable. All these properties make the MDS resin particularly suitable for the application described here,” explains Dr. Dirk Steinhilber, Application Technology Manager in Business Development, Applications & Innovation at LANXESS Deutschland GmbH.
Electrolyte purification in detail
Table 1 provides an overview of the process parameters of this electrolyte purification and the concentration of ions dissolved in the feed of the IEX column. The column is filled with 6 m³ of resin and equipped with a bypass to divert the electrolyte past the column during the regeneration phase. Fig. 1 schematically shows the cleaning and regeneration step. Up to almost 2200 m³ of electrolyte flow through the IEX column every day. It is particularly important that this electrolyte contains as few suspended solids as possible, as these could otherwise clog the resin bed. “The lowest possible concentration of iron(III) ions is also crucial for the efficient removal of bismuth and antimony, as these would otherwise be preferentially and very firmly bound by the resin, causing its capacity for bismuth and antimony to decrease rapidly and permanently,” adds Steinhilber. This is guaranteed in the present case because both the total iron concentration and the iron(III) content are significantly lower than the concentrations of Bi and Sb.
The selectivity of the resin for Bi and Sb can best be read from a c/c0 diagram (Fig. 2), in which the concentration of the relevant ion in the effluent of the IEX column is plotted relative to the concentration in the feed. Values close to zero indicate extensive adsorption of the ion in question, whereas a value of 1 would indicate no binding at all. Fig. 2 shows that up to a flow volume of approx. 129 bed volumes (BV), more than 60 % of the Bi and Sb ions contained in the electrolyte are adsorbed, whereby Bi is initially bound more efficiently than Sb. This relationship is reversed beyond 129 BV. However, the absorption capacity for both types of ions then also decreases continuously. Overall, the selectivity also decreases significantly and irreversibly with the number of cycles run, especially in the area of high bed volumes (BV > 130), so that the resin should be renewed after approx. 150 cycles.
The measurement of the Bi and Sb ion concentration in the effluent of the IEX columns (Fig. 3) shows that the Sb content of the electrolyte in particular remains below 70 mg/l over several hundred BV, whereas the Bi concentration increases significantly at high BV and almost reaches the concentration in the feed for BV greater than 260. This is consistent with the findings from the c/c0 diagram (Fig. 2).
As expected, an opposite behavior can be observed during regeneration with hydrochloric acid/thiourea. As shown in Fig. 4, the elution of Bi reaches a clear maximum after only two BV, while the elution of Sb is much slower and shows a less pronounced maximum. According to findings from the literature (Riveros, P.A., The removal of antimony from copper electrolytes using amino-phosphonic resins: improving the elution of pentavalent antimony. Hydrometallurgy 105 (2010) 110-114. https://doi.org/10.1016/j.hydromet.2010.08.008), the addition of small quantities of thiourea causes reduction and/or complexation of Sb(V) ions which are very tightly bound to the IEX resin. They would otherwise remain on the resin and gradually poison it. Several hundred kilograms of bismuth and antimony are removed from the electrolyte in this way every day and transferred to tailing ponds. The main amount of hydrochloric acid is recovered beforehand, for which membrane processes or distillation are suitable. This acid can be used again as an eluent, which significantly reduces the regeneration costs.
Conclusion
“Our ion exchange resin has proven itself in this application on an industrial scale, not only in Australia, but also already in Japan and Spain. Even in the presence of more than a hundredfold excess copper and in a strongly sulphuric acid solution, large quantities of bismuth and antimony can be reliably separated from the electrolyte,” summarizes Steinhilber. After around one and a half years of operation, the first resin change is currently due in the Australian electrolysis plant. The operator is also satisfied with the results and will continue to use the IEX process with the selective Lanxess resin also in the future.
