Pyrometallurgical Refining of Copper in an Anode Furnace

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1 Title of Publication Edited by TMS (The Minerals, Metals & Materials Society), Year Pyrometallurgical Refining of Copper in an Anode Furnace H. Antrekowitsch 1, C. Wenzl 1, I. Filzwieser, D. Offenthaler 1 Christian-Doppler-Laboratory for Secondary Metallurgy of Nonferrous Metals Franz-Josef-Straße 18; Leoben, 8700, Austria Department of Nonferrous Metallurgy Franz-Josef-Straße 18, Leoben, 8700, Austria Keywords: copper, thermodynamic, anode furnace, refining, refining electrolysis Abstract The decreasing quality of the input materials in copper recycling leads to a higher content of impurities in the anode copper. Therefore an improvement of the pyrometallurgical refining process is necessary to produce high quality anodes for the copper refining electrolysis. In order to improve the metal/slag reactions as well as the volatilisation by selective oxidation in the anode furnace, the behaviour of the most important accompanying elements (e.g. nickel, tin, lead, zinc etc.) at different reaction conditions has to be investigated. This requires knowledge about thermodynamic conditions like the reaction order and the activity coefficient at the copper refining process. Additionally the interactions between different elements, but especially those for nickel, have been investigated as a function of the temperature, the content of the elements and the slag composition. These investigations were done at the Christian Doppler Laboratory for Secondary Metallurgy of Nonferrous Metals. Introduction The raw materials for copper winning contain besides copper also numerous other elements like nickel, lead, tin, zinc and iron. During the refining procedure of the copper these elements are removed by using different techniques like the selective vaporization and oxidation as well as the refining electrolysis. Nearly all copper which is won by the pyrometallurgical way (about 85 %) passes through the copper refining electrolysis, even so is most of the secondary copper. In the electrolysis the impure copper is anodically dissolved and crystallized at the cathode without impurities. The space time yield (currently about 0.03 t/m 3 ) and the specific energy consumption (about 0.4 kwh/kg Cu) represent the main key figures of the process. In order to guarantee an economical process operation it is therefore necessary to optimize those two operating figures to the highest possible extent. By increasing the current density we face the problem of anode passivation, so that the electrochemical dissolution nearly stops [1]. The consequences are a lower electricity yield as well as higher potential drops which in turn result in an increased specific energy consumption. Due to the necessity of remelting the remaining anodes a large amount of copper has to be fed again to the anode furnace. The passivation behavior of the anodes is strongly dependent on their chemical composition, in this context the contents of accompanying elements like As, Bi, Sb, Pb, O and Ni are of great importance. In many companies but especially in recycling plants the removal of those elements is very difficult since the raw material and further also the accompanying elements are more or less given. To economically process scrap it is often even necessary to feed low grade material []. Considering these aspects, there is an absolute necessity to realize further optimizations in the field of pyrometallurgical refining in the anode furnace. In this context the behavior and reactions between metal and slag as well as the conditions for a volatilization are of great

2 importance. This is because those conditions directly influence the composition of the refined copper and the anodes respectively and as a consequence also the composition of the anode slimes. During the converting period of the black copper and during the following pyrometallurgical refining all base metals as well as a part of the copper are oxidized, so that slags with high contents in different metal oxides are generated. The latter together with the oxygen potential strongly influence the liquidus area of the slag. At present these slags are recycled to the shaft furnace where the accompanying elements either accumulate in the flue dust or are transferred into the black copper. In order to break up this closed loop of several elements and to discharge them from the process it should be tried to reduce the metal oxides contained in the anode furnace- and converter slag. This reduction step is becoming of increasing importance since the quality of the scrap is continuously decreasing and therefore makes, with respect to the necessity of unloading the refining electrolysis, a further optimization of the pyrometallurgical refining absolutely necessary. Additionally it is of special interest, that the slags are very homogenous and have a low viscosity, so that a high mass transfer, ensuring high reaction rates, can be guaranteed [3]. Behavior of the accompanying elements in the anode copper If the accompanying elements aren t completely removed during the pyrometallurgical refining different compounds are formed in the anodes, which cause problems at the electrolytic refining process [4]. The anodes now represent multiphase alloys, since the various elements form solid solutions as well as intermetallic compounds. Whereas Cu-Ag, Cu-Sb, Pb-Bi and Pb-Sb are representative binary systems for the solid solutions, Cu-Sb or Cu-Se at a higher contents of Sb and Se are examples for intermetallic compounds. In figure 1 the behaviors of the various phases that occur in the anodes of a secondary copper plant are observable. The framed phases represent products, which can be found in the anode mud, in the electrolyte or in the cathodes. Figure 1: Behavior of different phases at the refining electrolysis [4]

3 If the impurities are phase separated with the copper matrix or in the form of a solid solution or intermetallic compounds respectively, is of great importance for the qualitative and quantitative electrochemical dissolving. The mechanically inserted impurities are formed by suspension of impurity phases in the electrolyte. In figure the characteristics of the anodic dissolving process in the electrolyte close to the anodes are described schematically. For this special reasons an optimization of the pyrometallurgical refining has to be one of the main objectives of further investigations [5]. Figure : Schematic description of the electrochemical dissolving of the copper anode at the refining electrolysis Activity Thermodynamic fundamentals The activity of a metal oxide (a MO ) is the driving force for the dissolution of the corresponding metal in the slag [6]. The activity coefficient is indirectly proportional to the solubility. At low contents of the metal (M) in the copper and of the metal oxide (MO) in the slag Henry`s law is valid [7]. The activity coefficient of a metal oxide is a function of temperature, oxygen potential and slag composition. The influence of the temperature on the activity coefficient is shown in figure 3.

4 γ CuO0,5, γ NiO, γ SnO ,35 0,3 0,5 0, 0,15 γ ZnO, γpbo 0,1 0, T[ C] CuO0,5 NiO SnO PbO ZnO Figure 3: Activity coefficient of the metal oxide in a fayalitic slag [6] Values for the activity coefficient of several metals in liquid copper are summarized in table 1 and shown as a function of temperature in figure 4. Table I: Activity coefficients in liquid copper metal RTlnγ 0 M [cal] 0 γ M references Fe(l) T 15.95(1573K) [4] Fe 1.6 [5] Ni(l) (1573K) [4] Pb(l) T 4.37(1573K) [4] Sn(l) (1573K) [4] Zn(l) (1573K) [4] 35 0,3 30 0, 5 γ Fe, γ Ni, γ Pb , 0,1 γ Sn, γ Zn ,1 0 0, T[ C] Fe(l) Ni(l) Pb(l) Sn(l) Zn(l) Figure 4: Activity coefficient of several metals in liquid copper

5 Distribution coefficient The distribution coefficient (equation 1) describes the distribution of the accompanying elements between slag and metal and is therefore an indicator for the efficiency of the metal extraction [8]. (% M) L M = (1) % M [ ] In the slag the different metals M (e.g. Cu, Ni, Zn, Pb, Sn etc.) exist in the form of oxides, as demonstrated by the reaction in equation. Equation 3 shows the corresponding equilibrium constant to this reaction. n [ M] O ( g) ( ) + MO n () K MOn amo n = (3) a p M n O The distribution coefficient is directly proportional to p O n/ if the behavior of the metal M in the copper and of the metal oxide MO in the slag obeys Henry`s law. Accompanying elements of copper The impurities that are fed with the input materials (scrap, sludge, dust, slag etc.) have to be removed from the liquid copper during smelting by converting and refining [9]. Among the accompanying elements it has to be differentiated [10] between: Base metals with a high enthalpy of formation of the oxide, that have to be transferred into the slag in several process steps (e.g. Fe, Al, Si, P, Zn Sn and Be). Elements that are partly reduced with the copper and therefore have to be separated by accumulation in semi products or by the electrolytical refining process. Such metals are beside the noble metals elements like As, Sb, Ni and Pb that have a enthalpy of formation of the oxide which is similar to that of copper. It should be ensured that the accompanying elements are not distributed in several phases (metal, slag, dust) but accumulate in just one of those phases [11]. During the smelting process most impurities are at least partly transferred into the slag or the dust. As an exception merely Ni and the precious metals (Au, Ag, platinum group metals), with their noble character avoiding oxidation, are solved in the copper and so form the anode mud in the refining electrolysis. During converting, the base accompanying elements are either volatilized or transferred into the slag by selective oxidation. The formed converter slag is then recycled to the shaft furnace again. Due to this practice all accompanying elements are either reinserted or pass on to further process steps. Since also the off gas is cleaned there are hardly any losses of elements [10]. The accompanying elements can be removed from the copper by injection of air on the one hand or by the use of oxidizing slags on the other hand [11]. If air injection is used the oxidizing behavior of the elements depends on the gas flow rate and the temperature. After the oxidation of Zn and Sn the oxygen content of the liquid copper increases continuously until a critical value is reached, that also allows the oxidation of lead.

6 In a liquid Cu-Pb-O metal phase the oxidation rate of lead is lower than in a Cu-Zn-Sn-Pb-O phase, indicating that the activity coefficient of lead oxide (γ PbO ) has a lower value in slags where Cu O coexists with ZnO and SnO [11]. Even though by increasing the flow rate higher oxidation rates of the accompanying elements can be realized, this practice generates larger amounts of slag and therefore causes higher copper losses [11]. The oxygen content of the liquid copper increases rapidly with temperature. Whereas the oxidation reactions of Zn and Sn are exothermic and therefore an increased temperature decreases the oxidation rates of those elements, the oxidation rate of Pb is hardly dependent on temperature [11]. In this context the most important aim of the pyrometallurgical refining process is to produce a slag with a low copper content and a high absorption potential for the accompanying element oxides. In order to be able to describe the behavior of each element it is necessary to determine the activity and the distribution coefficient of the corresponding elements in thermodynamic experiments and calculations. Copper losses in the slag In the slag copper exists in form of enclosed metal drops, Cu 0, as well as in the solved form of Cu +. Enclosed metallic copper It are physical slag properties like density, surface tension and viscosity that determine the amount of enclosed copper. These copper losses can either be decreased, by giving the metallic drops enough time for sedimentation or by lowering the slag-viscosity by reducing the content of magnetite in the slag. At higher temperatures the influence of the slag melting point and the slag-viscosity vanishes certainly, but fuel consumption and as a consequence also process costs increase. From this point of view the overall objective of the investigations should be to guarantee a low slag-viscosity at lower process temperatures. The sedimentation rate of the enclosed metal particles can be estimated by Stoke`s law [1]. v = 9 g ( ρ ρ ) D µ S S r D (4) v sedimentation velocity [m/s] g acceleration of gravity [m/s²] ρ D, ρ S density of the enclosed particles and of the slag [kg/m³] r D radius of the enclosed particles [m] µ S viscosity of the slag [kg/m. s] According to Stoke s law small metal particles will settle rather slow. By the injection of gas into the slag/metal system more metal is transported into the slag by the gas bubbles. The rising gas bubbles are covered with a layer of liquid metal that bursts when the bubbles are entering the slag. That s why the amount of enclosed metal increases with the flow rate. By the higher turbulences the sedimentation of bigger particles is retarded too[1]. Solved oxidic copper The content of copper oxide in the slag depends first of all on the p O but also on the temperature and the slag composition [13]. Oxidic copper dissolves in the slag according to the following reaction (5).

7 1 Cu() l + O ( g) CuO0, 5 ( l ) (5) 4 acuo 0,5 () l K = (6) 1 4 acu() l p The temperature dependence of the equilibrium constant is determined by equation (7) and graphically shown in figure 5 [14]. O 7361 lnk =,639 (7) T 5 0 K(CuO0,5) T[ C] Figure 5: Equilibrium constant for the oxidic dissolution of copper as a function of temperature. Figure 6 shows the correlation between the copper content and the oxygen potential. For the dissolution of copper in siliceous slags Henry`s law can be applied, whereby the activity coefficient of CuO 0,5 shows marginal dependence of the slag composition. The solubility decreases with increasing content of SiO and CaO. Furthermore also the addition of CaO, MgO and Al O 3 to SiO saturated fayalitic slags decreases the copper solubility[8].

8 Figure 6: Solubility of copper in FCS slags as function of p O [1] (R and Q represent the slag composition and the basicity respectively) Equation (8) results from the validity of Henry`s law and the limited solubility of copper: (% Cu) k acuo 0, 5 = (8) The activity coefficient of copper oxide can be calculated according to equation (9): K MCu acu() l ( ) = nt po γ (9) CuO0,5 (% Cu) In the system of CaO-FeO x -SiO high values of γ CuO 0, (maximum 13) are achieved at values of 5 Q between 0.45 to 0.55 and of R of about 0. [1] Total copper losses The total copper loss depends not only on the copper solubility of the slag but also on the total slag amount, that is indirect proportional to the iron content of the slag [13]. If copper exists in oxide form, the copper content in the slag and the total amount of in fayalitic slag dissolved copper can be reduced by decreasing the content of SiO. During melting and refining copper losses can be minimized by: Reducing conditions in the melting aggregates (but the formation of solid metallic iron has to be absolutely avoided) An oxidic atmosphere at the refining process, that is as low as possible but high enough to remove the impurities. A slag composition that guarantees a low copper solubility as well as a small amount of copper inclusions. 1 4

9 Nickel losses in the slag As an important alloying element for copper nickel is introduced by feeding secondary material. Although nickel is oxidized easier than copper it is removed as nickel sulphate in the refining electrolysis by precipitation from the electrolyte. The nickel containing slags from the converter and the anode furnace are fed to the shaft furnace. The losses of nickel through the shaft furnace slag are with 0.5% rather low. The highest nickel contents can be found in the black copper and the converter slag [10]. The oxidic dissolution of Ni in the slag takes place according to the following reaction: anio() l K = (10) 1 ani () l po 1 Ni() l + O ( g) NiO( l) (11) Experimental The number of experiments were calculated by the software MODDE 7.0. Furthermore a stoichiometric amount of reducing agent was used for the reactions. For the calculation of this amount it was assumed that all non ferrous metal oxides (CuO 0.5, NiO, PbO, SnO, ZnO) in the slag are reduced to metal. Although iron should remain in the slag, it should be reduced from FeO 1.5 to FeO. The addition of iron was necessary for the exact ratio of Fe/SiO. Additionally iron served as reducing agent for the oxides of Zn, Pb, Sn, Ni and Cu whereas the iron itself is oxidized to FeO, which is produced also by the reduction of FeO 1.5. The ratio of Fe/SiO and of CaO/SiO were the investigated parameters in these experiments. The composition of the slag for the investigations is given in table II. Table II: Composition of the anode furnace slag [%] [%] Cu As Fe 6.9 Ag Pb 4.9 SiO 11.0 Sn 3.1 Al O Ni.6 MgO 1.8 Sb 0. CaO.0 The investigations were carried out in an induction furnace. For the experiments a sintered alumina crucible with a height of 38 mm, a diameter of 3 mm and a wall thickness of 1 mm was used. The crucible was fed with the anode furnace slag (table II) and several additions (SiO, CaO, Al O 3, MgO, Graphite). The reaction time for each experiment was 4 hours. After 4 hours the thermocouple was removed and the crucible was cooled down in the furnace. To investigate the viscosity of the slag the temperature was kept constant at 1300 C during the experiment. It turned out that a higher basicity increases the viscosity of the slag at this temperature, which in turn leads to a higher metal content in the slag. The experiments mainly focused on the behavior of Ni, which can strongly influence the electrolysis, and Cu. Since the Ni distribution in the anodes depends on the solidification conditions, the Ni content fairly varies at the cross section of the anodes, which then causes different conditions during the electrolysis. The distribution of the element Ni along the cross section of an anode as a function of the solidification conditions is shown in figure 7. As it can

be seen it is very important, if the form of the primary copper crystals is

1,13 1,11 1,09 nickel in % 1,07 1,05 1,03 spheriodal dendritic 1,01 0,99 0,97

the cross-section of the anode as a function of the solidification conditions

10 be seen it is very important, if the form of the primary copper crystals is either dendritic (figure 8) or globulitic (figure 9). 1,13 1,11 1,09 nickel in % 1,07 1,05 1,03 spheriodal dendritic 1,01 0,99 0,97 0 0,5 1 1,5 cross section of the anode in cm Figure 7: Ni distribution along the cross-section of the anode as a function of the solidification conditions Figure 8: Dendritic microstructure of anode copper Figure 9: globulitic microstructure of anode copper

11 The copper and nickel content of the slag as a function of the CaO/SiO und Fe/SiO ratio is given in figure 10. It has to be considered, that these investigations have been carried out at a temperature of 1300 C. Therefore the viscosity of the slag influence the reaction and the sedimentation of the liquid metal. In industrial operation these circumstance have to be considered at higher basicities to get a slag with a low viscosity. Figure 10: Cu and Ni content as a function of the CaO/SiO and Fe/SiO ratio at a temperature of 1300 C and a stoichiometric amount of the reduction agent. The copper and nickel content of the slag under the investigated conditions can be calculated according to equations (1) and (13): (% Cu) (% Ni) Fe CaO Fe CaO = SiO SiO SiO SiO (1) Fe CaO Fe CaO = SiO SiO SiO SiO (13) Further experiments at higher temperatures (1400 C) have shown, that the content of the elements but especially that of copper decreases significantly. Due to this fact in future investigations also the temperature will be a parameter that has to be varied. Although this practice will increase energy costs, it is justified by an increased process yield and an improved slagging of the accompanying elements. Conclusion Due to the lower quality of the input materials and the necessity for a continuous increase of the space time yield the pyrometallurgical refining step in the primary and secondary copper industry has to be optimized. The changed process conditions also change the behavior of the accompanying elements at selective oxidation and evaporation reactions. For the description of the process conditions and for further investigations knowledge about parameters like viscosity, temperature and basicity of the slag is essential. The investigations showed, that the temperature is one of the most important parameters for influencing the slag viscosity.

12 The continuous improvement of the pyrometallurgical refining process in copper secondary metallurgy is of great importance for an unproblematic operation of the final refining electrolysis. The different elements strongly influence the refining electrolysis where they cause e.g. passivation of the anodes which results in a lower yield of the whole process. Further investigations should enable the production of anode copper, which can be inserted into the refining electrolysis without any restrictions, although low quality scrap is used. References 1. Jin, Sh. E. Ghali and A. Adnot: XAES study on the passivation of copper anodes in H SO 4 -CuSO 4 solution. Proc. Emerging Separation Technologies for Metals and Fuels, TMS, 1993, Gumowska, W. and I. Sedzimir: Influence of the lead and oxygen content on the passivation of anodes in the process of copper electro-refining. Hydrometallurgy 8 (199), Bach, X., A.C. Feneau und B. Gongarinoff: Anodenprobleme bei der elektrolytischen Kupferraffination. Erzmetall (1969), Beiheft, B10 - B17 4. Buhrig, E., K. Hein und H. Baum: Verteilung von Fremdelementen bei der Kristallisation von Kupfer. Metall 33 (1979), Lange, H.-J., K. Hein und D. Schab: Anodenprozesse bei der Raffinationselektrolyse. Freiberger Forschungshefte (1977), Degterov S. A. und A. D. Pelton: A thermodynamic database for copper smelting and converting. Metallurgical and Materials Transactions B, Vol.30B, August 1999, Krajewski W. und Krüger J.: Schlacken beim Schmelzen von Kupfer und Kupferstein. Schlacken in der Metallurgie, Hrsg. Koch K., Düsseldorf, Verlag Stahleisen, Kim H. G. und H. Y. Sohn: Effects of CaO, Al O 3 and MgO additions on the copper solubility, ferric/ferrous ratio and minor-element behavior of iron-silicate slags. Metallurgical and Materials Transactions B, Vol.9B, June 1998, Biswas A. K. und W. G. Davenport: Extractive Metallurgy of Copper, 3 rd edition. 1994, Pergamon Press, ISBN Hanusch K. und H. Bussmann: Behavior and removal of associated metals in the secondary metallurgy of copper. Third International Symposium on Recycling of Materials and Engineered Materials, The Minerals, Metals and Materials Society, 1995, Nakashima K., K. Yamamoto, E. Shibata, H. Tahori und K. Mori: Oxidation rate of impurities in liquid copper by gas and slags. Second International Conference on Processing Materials for Properties, San Francisco, November Poggi D., R. Minto und W. G. Davenport: Mechanisms of metal entrapment in slags. JOM, Vol.1, November 1969, Reddy R.G. und C.C. Acholonu: Activity coefficient of CuO 0,5 in alumina saturated iron silicate slags. Metallurgical and Materials Transactions B, Vol.15B, December 1984, Yazawa A., Y. Takeda und S. Nakazawa: Ferrous calcium silicate slag to be used for copper smelting and converting. Copper 99, Vol.VI., The Minerals, Metals and Materials Society, 1999, Vartiainen A. und M. Kytö: Olivine slags - the ultimate solution to low copper slags?. Scandinavian Journal of Metallurgy, Vol.31, Issue 5, October 00

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