THE ROLE AND INFLUENCE OF IMPURITIES ON THE QUALITY OF COPPER CATHODES

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1 MultiScience - XXXI. microcad International Multidisciplinary Scientific Conference University of Miskolc, Hungary, April 2017 ISBN THE ROLE AND INFLUENCE OF IMPURITIES ON THE QUALITY OF COPPER CATHODES *Stephan Steinacker 1 and Jürgen Antrekowitsch 1 1 Christian Doppler Laboratory for Optimization and Biomass Utilization in Heavy Metal Recycling Chair of Nonferrous Metallurgy Montanuniversitaet Leoben Franz-Josef Str. 18 / 8700 Leoben / Austria (*Corresponding author: stephan.steinacker@unileoben.ac.at) ABSTRACT High-purity cathodes generally represent the main product from the copper production chain. While the share of copper itself typically reaches a value above %, there is still a broad variety of chemical elements that influences the product quality and hence the achievable price. In contrast to the production of many other metals, impurities in the copper industry cover a large section of the periodic table of elements and hence have to be carefully characterized and controlled. The present work describes possible impurities that play a role in the classical refining electrolysis and therefore influence the composition of the resulting cathode. The second part aims at the investigation of a secondary copper cathode. By applying scanning electron microscopy and different methods for an adequate surface treatment, the quality of the present materials is systematically characterized. The approach of starting macroscopically and consequently applying higher resolutions allows to draw conclusions concerning the remaining impurities in the copper cathode. INTRODUCTION The production of primary and secondary copper includes a variety of process steps. While different technologies can be applied for a primary pyrometallurgical treatment of the ore concentrate which mainly consists of chalcopyrite many types of scrap and other residues can be used for recycling. After smelting, the metal is converted, fire refined and afterwards cast to anodes. While the amount of metallic copper increases throughout this process chain, the sum of impurities decreases as the majority is removed by means of oxidation. However, a certain amount cannot be terminated from the copper product and hence accumulates in the anodes. In the last process step the refining electrolysis an additional purification takes place as the anodes dissolve in sulfuric acid and result in copper cathodes. The quality of the cathode can be defined through the share and the total amount of different impurities, as will be described in this article. [1 3] There are different factors which influence the purity of the cathodes that result from the electrolysis. These can generally be split into three fractions, the physical being the first one that aims at the prevention of turbulences and which guarantees adequate filtering steps. The chemical factors include a constant availability as well as a uniform temperature of around 65 C. Additionally, the current density represents the most important aspect of the electrical factors. While high values lead to a rapid growth of the copper plating, its negative effects include the formation of nodules and other irregularities on the cathode s surface. [4] Fig. 1 sums up the mentioned process flow of the primary copper production and results in copper cathodes with a purity of % as final product. In the following chapters, the DOI: /musci

2 impurities in this material as well as their behavior in the electrolysis form the core part of the investigation. [1,4] Fig. 1: Process steps in the primary copper production [4] IMPURITIES IN THE COPPER CATHODE Since copper anodes from the fire refining step contain a broad variety of chemical elements, these also play a role in the subsequent electrolysis and influence the quality of the resulting cathode. In general, these elements have to be prevented from entering the final product in order to meet market requirements. Table 1 states the most important impurities in the electrolytic refining step and quotes maximum values. On the one hand, the standard London Metal Exchange limits are considered, while on the other the Chilean copper producing company Codelco applies even stricter limits. [4 7] Table 1: Internationally accepted impurity contents in copper cathodes Grade A [5 7] Element Se Te Bi Sb As Sn Pb Fe Ni S Ag O Standard [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] LME Codelco The impurities behavior in the refining electrolysis is generally influenced by their position in the electrochemical series as can be seen in Table 2 and by their total share in the anode. Elements with a more positive reduction potential than copper remain solid, while others shows the tendency to dissolve anodically in the electrolyte under the applied potential.

3 The various impurities as stated before can be investigated separately in order to understand their behavior in the electrolysis and their influence on the copper cathode quality. [4] Table 2: Standard electrochemical potential of relevant elements in the copper refining electrolysis [4] Selenium and tellurium primarily occur as compounds with copper and silver in the anodes. They do not dissolve and form part of the so-called anode slime which is periodically collected and sent to a separate recovery process. Consequently, the slime contains a certain amount of Se and Te compounds which typically include Cu2(Se/Te), Ag2(Se/Te) and CuAg(Se/Te). [4] Lead shows two different modifications in the anode. While this element mainly occurs in solid solution with copper itself, it can also be found as complex oxide phase along the grain boundaries. The majority dissolves in the electrolysis and forms PbSO4 by precipitation. Together with SnO2, which displays the most important tin compound in the anode, it also shows the tendency to move to the anode slime. However, the dissolved lead sulfate can also move to other phases due to its partial solubility in the electrolyte and hence cause contamination. [4,8] According to their lower standard electrochemical potential compared to copper, bismuth, antimony, arsenic, iron, nickel and sulfur dissolve in the electrolyte during the refining electrolysis. In order to reduce the amount of dissolved impurities and hence to avoid any contamination of the cathodes, the electrolyte needs to be cleaned from time to time. The

4 elements As, Sb and Bi play an important role concerning the passivation of the anode and therefore have to be thoroughly controlled. A minimum arsenic concentration above 300 mg/l can help to avoid this effect, where Cu 2+ ions are produced faster that they convect away. As a result, a passive CuSO4 5H2O layer precipitates on the anode and stops the progress of the electrolysis. An additional effect of elevated As levels in the electrolyte is represented by the preferred transfer of Sb and Bi to the anode slime. [4] Silver generally shows a higher standard electrochemical potential, yet a small amount also dissolves in the electrolyte due to kinetic reasons and subsequently deposits on the solid copper. Therefore, cathodes typically show a silver share of around 8-10 ppm. Compared to other impurities, however, Ag displays a component with no negative effects on the material s behavior. [4] Oxygen represents a prevalent impurity in copper anodes that can occur in different states. The monovalent Cu2O constitutes the most important compound and in contrast to the other mentioned elements dissolves chemically, not electrochemically. This effect can be explained by the strong acidity of the electrolyte. Since a certain build-up of copper in the acid results, it has to be cleaned periodically. In addition to oxygen, sulfur displays an element that strongly affects the general ductility of copper at a ppm level. [4,7] Gold and platinum-group metals (PGMs), which include Pt, Pd, Rh, Ir, Ru and Os, remain their metallic behavior and do not enter the electrolyte. Generally, a slightly different conduct compared to silver can be observed. Due to their noble nature, they also form part of the anode slime. Since they represent noble and expensive metals, their amount can play a crucial role regarding the overall profitability of the copper production process. [4,9] Small amounts of impurities can result in disastrous effects in the resulting cathodes, especially when considering the physical and most importantly the electrical properties. Considering the wiredrawing process and hence the influence on grain boundary cracks, certain elements play the most important role. These include bismuth in the first place, but also arsenic and antimony. Bi concentrations above 20 ppm cause severe brittleness and a cohesion loss in the matrix as it mainly segregates at the grain boundaries. Pb and Sb are also practically insoluble and show a similar tendency, yet noticeably alleviated. [3,7] While the content of the mentioned elements is typically controlled by the composition of the electrolyte, other impurities can also influence the overall quality of the final product. In contrast to the mentioned species, elements such as Te, Sn, Cl and Al show no considerable influence on the ductility of copper, which is more affected by oxygen and sulfur in their ppm range. The detailed investigation of an exemplary copper cathode helps to understand the presence and the effect of these elements. [3,7] INVESTIGATION OF A SECONDARY COPPER CATHODE This chapter presents an approach to determine the quality and the possible presence of impurities in a copper cathode. The investigated material represents a typical hydrometallurgically refined cathode from the electrolysis in the secondary copper industry. By applying different analytical methods, the chemical composition and the nature of the impurities can be systematically characterized. The cathodes are adequately prepared for an analysis in the scanning electron microscope by grinding and polishing the surface. First, a large scale is applied in the SEM in order to

5 determine major inclusions and impurities. Fig. 2 shows the treated cathode surface with on a millimeter scale. Fig. 2: SEM image of a secondary copper cathode This overview does not reveal any precipitations and hence leads to the first observation of a generally pure cathode. The chemical analysis also states a copper percentage of 100 % and consequently underlines this result. Sulfur inclusions which often constitute a major problem when present in the cathode could typically be monitored in the light microscope when present in the cathode and hence do not influence the present sample from a qualitative point of view. While no inclusions can be observed under the applied scale, nano precipitations may play a role for the overall quality of the cathode. These generally show the tendency to accumulate at the grain boundaries. In order to characterize these boundaries, the sample has to be prepared in an adequate way. After electropolishing, the cathode s surface is etched and hence reveals its structure more precisely, as can be observed in Fig. 3. For the etching treatment, Petzow s mixture Cu m5, which consists of ml water, ml of a 32 % hydrochloric acid and 5-10 g of iron(iii) chloride, is applied in order to obtain the desired effect. [10]

6 Fig. 3: SEM image of an electropolished and etched copper cathode surface The chosen scale in the µm range leads to a better resolution of the present sample. The observation of twinnings does not correlate to the presence of electrolytic copper and rather indicates a mechanically formed structure. However, this effect can be explained by the preparation of the sample which uses strong mechanical stress and hence leads to the formation of twinnings. The comparison of the present cathode with other samples confirms this observation. Beside the mentioned effect, the presence of the black spots at the grain boundaries constitute another factor that needs to be investigated. Fig. 4 highlights a section of the preceding SEM image with an increased micrometer scale. Fig. 4: Detail from the SEM image in Fig. 3

7 The black holes that can be observed at the grain boundaries represent pores that result from the electropolishing step. During the electrochemical treatment defects which primarily occur at the boundaries are especially susceptible to this attack. Therefore, the black holes display the result of electrochemical corrosion. Since no precipitations can be found in the present sample, the electropolished and etched copper cathode does not allow to draw conclusions concerning any impurities. The so-called focused ion beam FIB treatment represents an additional method that allows an even more precise investigation of the present copper cathode. This method offers an option for a sophisticated microscale treatment which leads to a planar and amorphous surface. Fig. 5 displays the SEM image of an FIB treated copper cathode surface. [11,12] Fig. 5: SEM image of an FIB treated cathode surface The FIB treated cathode surface generally shows a very homogeneous build-up. The small bright zones in the copper matrix prove the presence of nano precipitations. The yellow markers show that their size lies between 50 and 200 nm and hence do not allow a further chemical characterization in the scanning electron microscope. Their bright appearance, however, indicates that these particles display a higher density than copper. On the one hand, lead can be made responsible for these segregations as it represents a common by-element in the secondary copper industry, while on the other side noble metals could also be relevant for these impurities. For instance, gold or silver may not experience optimum conditions for a separation in the electrolysis and hence remain with the copper matrix instead of moving to the anode slime. To sum up, the present cathode is characterized by a very high purity and only possesses a minimum amount of undesired by-elements which accumulate as nano precipitations. CONCLUSION The presented chapters lead to the conclusion that a broad variety of chemical elements influences the quality of the copper cathode. While impurities such as silver, tin or aluminum only play a minor role, the presence of bismuth, arsenic and antimony can cause disturbing effects on the material properties. As a result, their behavior in the refining electrolysis and their resulting distribution has to be controlled severely.

8 The second part of this work investigates the quality of a secondary copper cathode. Since no impurities can be detected on a micrometer scale, the focus is set on a more detailed resolution of the material. After applying different treatments including an electropolishing step combined with etching as well as an ICP treatment, nano precipitations can be observed. Their analysis reveals certain concentrations of dense elements which either consist of gold, silver or lead. Since the total amount only reaches an insignificant concentration, their influence on the material properties results in minor importance. Hence, the process parameters of the investigated cathode production fully confirm a perfectly functioning refining electrolysis. ACKNOWLEDGMENTS The authors want to acknowledge their thanks as the present work was written with the financial help of the Christian Doppler Forschungsgesellschaft, the Montanuniversitaet Leoben as well as the Austrian government. REFERENCES [1] Hoffmann, J.E.: The Purification of Copper Refinery Electrolyte, Journal of Metallurgy, 2004, p [2] Wang, S.: Copper Leaching from Chalcopyrite Concentrates, Journal of Metallurgy, 2005, p [3] Wang, S.: Impurity Control and Removal in Copper Tankhouse Operations, Journal of Metallurgy, 2004, p [4] Schlesinger, M.E., M.J. King, K.C. Sole and W.G. Davenport: Extractive Metallurgy of Copper, 5 th Edition, Elsevier, Amsterdam, The Netherlands, [5] W.M. Tuddenham and R.J. Hibbein: Sampling and analysis of copper cathodes, American Society for Testing and Materials, Philadelphia, Pa, [6] London Metal Exchange, LME Molybdenum, Date: [7] Camurri, C., C. Carrasco and S. Albretch: Impurities on Cathodic Copper: Study of their Influence on the Ductility of Copper Wires and Development of Mechanical Tests Sensible to such Impurities, Metal 2010, p [8] Hyvärinen, O.: Process for selective removal of bismuth and antimony from an electrolyte, especially in electrolytic refining of copper, Patent US A, [9] Jermann, B. and J. Augustynski, Long-term Activation of the Copper Cathode in the Course of CO2 Reduction, Electrochimica Acta, 1994, p [10] Petzow, G.: Metallographisches, plastographisches, keramographisches Ätzen, 6 th Edition, Borntraeger, Berlin, Stuttgart, [11] Motz, C., T. Schöberl and R. Pippan: Mechanical properties of micro-sized copper bending beams maching by the focused ion beam technique, Acta Materialia, 2005, p [12] Melngailis, J.: Focused ion beam technology and applications, Journal of Vaccum Science & Technology B: Microelectronics Processing and Phenomena, 1986.