Design of optimal electrolyte circulation based on the kinetic modelling of copper dissolution in silver electrorefining
Introduction
Copper (Cu) is a common base metal impurity in the anodes of the silver (Ag) electrorefining process, either from primary sources [Habashi, 1997; Marsden and House, 2006] or various secondary sources [Balde et al., 2017]. Classic silver electrorefining uses a Moebius cell in AgNO3 electrolyte with a silver concentration, [Ag+], of 30 to 200 g/dm3, free acid concentration, [HNO3], of 0 to 10 g/dm3 and operating temperature of 25–55 °C [Mantell, 1940; Leigh, 1973; Cornelius, 1982; Pletcher and Walsh, 1990; Renner et al., 1993; Jaskula and Kammel, 1997; Pophanken and Friedrich, 2017]. The main silver anode compositions consist of silver, gold and copper with typical contents as shown in Table 1 [Mantell, 1940; Leigh, 1973; Cornelius, 1982; Pletcher and Walsh, 1990; Jaskula and Kammel, 1997; Pophanken and Friedrich, 2017].
Copper dissolution in a low concentration of nitric acid solution follows the reactions outlined in Eqs. (1), (2), (3), (4), (5), (6) [Martinez et al. 1993; Ozmetin et al. 1998]. The overall reaction presented in Eq. (7) is based on a revised version published previously [Demir et al. 2004] and subsequently recalculated with HSC Chem 8.0 from Outotec.
Kinetic models of copper dissolution related to the effect of [HNO3] and temperature proposed by Berg [Travnicek and Weber, 1961] are shown in Eq. (8), (9) and a more recent model is detailed in Eq. (10) as derived from the results of [Khodari et al. (2001).
The increase of [Cu2+] provides a benefit in the form of increased conductivity of the silver electrolyte [Aji et al., 2016a, Aji et al., 2016b]. Furthermore, with the lower standard potential of reduction, theoretically the presence of Cu should not affect the deposition process. Nevertheless, studies have found that Cu is co-deposited - which results in impurities within the Ag crystal product - at a certain level of [Cu2+] in the electrolyte, and therefore the level of [Cu2+] in the electrolyte needs to be limited [Leigh, 1973; Hunter, 1975; Pophanken and Friedrich, 2017; Pophanken and Friedrich, 2015]. While there are suggestions that co-deposition is either caused by the electrochemical deposition or mechanical inclusion of Cu salt into the Ag crystal product [Eger, 1924; Hunter, 1975; Prior, 1986; Renner et al., 1993; Haranczyk and Sedzimir, 1995;], several studies have concluded that to avoid Cu co-deposition, both the [Cu2+] should be lower than that of [Ag+] in the electrolyte (see Table 2) [Johnson, 1967; Leigh, 1973; Hunter, 1975; Pophanken and Friedrich, 2017;].
The established concentration limitation of [Cu2+] ˂ [Ag+] results in the critical [Cu2+]/[Ag+] ratios primarily within the range of 0.8–1. In this paper, we have studied the dissolution kinetics of pure copper in silver electrolyte by electrochemical methods. With the development knowledge related to Cu dissolution within the more complex parameters of silver electrorefining, [Cu2+] accumulation in the electrolyte and the quality of the product can be maintained even at higher current density operation. The objectives of this study are to develop a Cu dissolution kinetic model and design the optimal electrolyte circulation required to control the accumulation of [Cu2+] in the electrolyte.
Section snippets
Materials and methods
Synthetic silver electrolyte parameters were selected in such a way as to simulate industrial silver electrorefining process conditions: [Ag+] range of 40 to 100 g/dm3, [HNO3] between 5 and 15 g/dm3 and a temperature range of 25–45 °C (Table 3). The chemicals used were silver nitrate (AgNO3, 99.8%, VWR Chemicals), copper nitrate (Cu(NO3)2·3H2O, 99%, VWR) and nitric acid (HNO3, 65%, Merck).
Copper electrodes of 99.99% grade were prepared by coating with an inert resin (Struers Aps, Denmark) and
Theoretical background
According to the phase diagram of AgCu shown in Fig. 1, while silver and copper form a solid solution at high temperature, both metals tend to segregate at approximately 300 °C in a slow cooling process. This segregation of Ag and Cu can result in the formation of copper or a copper-rich zones within silver anodes, which is the basic assumption of this paper.
Results and discussion
The current density operation of silver electrorefining can be divided into a moderate current density of 200–600 A/m2 and high current density of 600–1000 A/m2 [Aji et al., 2018]. High current density operation is ideally at 200–300 mV vs. Ecorr,Ag or in the range of 625–725 mV vs. Ag/AgCl [Aji et al., 2018]. In addition, the presence of gold suggests that the optimal potential range is between 700 and 720 mV vs. Ag/AgCl [Aji et al., 2019]. Currently, high current density silver
Conclusions
The presence of copper in silver anodes as the major dissolved impurity in silver electrorefining poses a challenge due to the co-deposition of Cu in the silver crystal product, thus reducing the product quality. While copper is easily and rapidly dissolved, the large difference in reduction potential between copper and silver allows operation even at a [Cu2+]/[Ag+] ratio as high as 0.8 without the co-deposition of copper on the cathode. Furthermore, [Cu2+] improves electrolyte conductivity
Credit author statement
Arif. T. Aji was responsible of the conceptualization, design of experiments and performed the experimental work, Joseph Hamuyuni supported on the thermodynamics analysis and design of experiments, B.P·W provided critical and constructive comments during the manuscript writing and reviewing, all the works was conducted under supervision of Jari Aromaa and Mari Lundström.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
This research was carried out using facilities provided by the Academy of Finland's RawMatTERS Finland Infrastructure (RAMI-FIRI) at Aalto University. In addition, Arif T. Aji would like to acknowledge the funding support from LPDP (Indonesian Endowment Fund for Education), and Business Finland funded projects Circular Metal Ecosystem (CMEco) and Symmet.
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