DOI 10.1515/htmp-2013-0097 High Temp. Mater. Proc. 2014; 33(5): 469 – 476 Pekka Taskinen*, Sonja Patana, Petri Kobylin and Petri Latostenmaa Oxidation Mechanism of Copper Selenide Abstract: The oxidation mechanism of copper selenide atmosphere [2]. The roasting option is industrially attrac- was investigated at deselenization temperatures of copper tive as it leads to essentially 100% separation of selenium refining anode slimes. The isothermal roasting of syn- from the other components of anode slime and produces thetic, massive copper selenide in flowing oxygen and relatively pure crude selenium in a single step. oxygen – 20% sulfur dioxide mixtures at 450–550 °C indi- The industrial copper anode slimes are complex mix- cate that in both atmospheres the mass of Cu2Se increases tures of the insoluble substances in the electrolyte and a as a function of time, due to formation of copper selenite significant fraction of it comes from residues of the mold as an intermediate product. Copper selenide oxidises to paint in the anode casting, typically barium sulfate. Sele- copper oxides without formation of thick copper selenite nium is present in the slimes fed to the deselenization scales, and a significant fraction of selenium is vaporized process mostly as copper and silver selenides as well as as SeO2(g). The oxidation product scales on Cu2Se are elementary selenium [3], depending on the processing porous which allows transport of atmospheric oxygen to steps prior to the actual selenium roasting. Therefore, the reaction zone and selenium dioxide vapor to the their detailed mineralogical analysis is not straightfor- surrounding gas. Predominance area diagrams of the ward on the microscopic scale. copper-selenium system, constructed for selenium roast- Decomposition of the selenides present in the anode ing conditions, indicate that the stable phase of copper in slime is a necessary step before the oxidation of selenium a selenium roaster gas with SO2 is the sulfate CuSO4. The to gaseous SeO2(g) can take place. The vapour pressure of cuprous oxide formed in decomposition of Cu2Se is further elementary selenium or that in its intermetallic com- sulfated to CuSO4. pounds is much lower than that of SeO2(g) [4]. The thermal stability of Cu2Se is, however, high and it melts congru- Keywords: precious metals, copper refining, tank house, ently before the decomposition. Thus, the oxidation of electrolysis, anode slime selenium must occur on the surfaces of the intermetallic compound where formation of ternary oxides with copper PACS® (2010). 64.75.Lm, 81.05.Bx, 82.30.Lp is possible. An extensive review of the selenium roasting studies was written by Barbante et al. [5]. In addition, Ishi- hara [6], Gospodinov and Bogdanov [7] and Segarra et al. *Corresponding author: Pekka Taskinen: Department of Materials Science and Engineering, Aalto University CHEM, Aalto FI-00076, [8] have studied the phenomena involved in the thermal Finland. E-mail: [email protected] and oxidation behaviour of copper selenide and selenite. Sonja Patana: National Board of Patents and Registration of Finland, Copper selenide leaching in aqueous media with ferric ion Helsinki, Finland as oxidizing agent has been studied by Dutrizac and Chen Petri Kobylin: Outotec (Finland), Pori FI-28101, Finland [9]. Petri Latostenmaa: Boliden Harjavalta, Pori FI-28101, Finland The phase relations of copper selenides in oxidizing atmospheres are not well known. Properties of the binary metallic system have been compiled by Chakrabarti and Laughlin [10]. Gospodinov [11] determined the decomposi- tion products of copper selenite. Copper selenite seems to 1 Introduction be solid up to its decomposition temperature [11, 12]. The crystal structure above 170 °C is of cubic fluorite type and Copper anode slimes are globally an important source for it is metallic conductor at room temperature due to forma- selenium and tellurium. The slimes in copper refinery tion of Frenkel defects [13]. tank houses also contain precious metals, and their recov- The mechanism and role of sulfur dioxide in the in- ery is crucial to copper smelting and its economy. For their dustrial selenium roasting is still unclear. The aim of this extraction, a number of technologies have been devel- study is to explore the oxidation mechanism of roasting oped [1]. Many process chains include a roasting step for pure copper selenide at typical deselenization tempera- vaporizing selenium as a gaseous oxide SeO2(g), using tures 450–550 °C and the impact of SO2(g) to the roasting oxygen or oxygen-sulfur dioxide mixtures as the roasting rate and its products. 470 P. Taskinen et al., Oxidation Mechanism of Copper Selenide 2 Experimental The sample temperature in the furnace was measured with a Pt/Pt10Rh thermocouple, delivered by Johnson- The synthetic intermetallic compound Cu2Se was pre- Matthey, in a 3 mm fo alumina sheath located next to the pared using a vacuum ampoule technique at elevated selenide sample. The measured inaccuracy of the thermo- temperatures. Pure selenium (99.99% Se from Cerac) and couple below 1100 °C was ±1 °C and the observed stability electronic grade OF copper (99.99% Cu from Outokumpu) of furnace temperature over the period of 4–8 hours was were used as starting materials. Carefully weighted typically ±3 °C. amounts of copper and selenium were sealed in a fused quartz ampoule in vacuum. It was heat treated at 650 °C for 100 hours and subsequently melted at 1200 °C. After two hours melting and homogenization period, the 3 Results ampoule was cooled down to room temperature along The isothermal roasting experiments of synthetic copper with the furnace. selenide in flowing oxygen and oxygen – 20% sulfur The synthetic selenide obtained was characterised by dioxide mixtures, with duration of four hours at 450–550 X-ray diffraction using Cu Ka radiation for confirming its °C are shown in Figure 1. Due to the tiny surface area of the phase assemblage. Copper selenide contained orthorhom- material, the reaction rates are at all temperatures as well bic, monoclinic and tetragonal Cu Se as well as Cu Se , 2 2 x as in every atmosphere very low. The sample mass in- which are formed at low temperatures due to slow cooling creases as a function of time in both atmospheres. The [10]. Copper selenide was analysed by EDS to contain small initial weight loss at 450 °C within the first 5–10 40.15 mass% Se and 59.85 mass% Cu, which is slightly minutes in both atmospheres is obviously due to loss of superstoichiometric in respect with Cu Se, but it is in the 2 selenium from the synthetic selenide surface. The samples homogeneous region of copper selenide at experimental do not show any indication of the formation of liquid temperatures [14]. Cu Se-Se phase which is stable at 523 °C and above [10]. The roasting studies were carried out isothermally in 2 This indicates that the synthetic Cu Se was single-phase a thermogravimetric furnace equipped with a Mettler 2 material at the experimental temperatures. Toledo AB 104-S semi-microbalance. It was calibrated The reaction rates indicate that the oxidation process prior to the experiments with Mettler-Toledo AG standard of copper selenide proceeds over the whole period of four weights from 1 mg to 200 g. The thermobalance furnace or eight hours. Assuming that copper oxidizes in oxygen was of split-type and it was moved around and from the immediately after selenium has vaporized as SeO (g), the furnace tube before and after experiment, thus allowing 2 observed mass change of Cu Se should be negative. fast heating up and cooling down the sample in the exper- 2 The reaction zones on the selenide surfaces were imental roasting atmosphere. The furnace tube had a examined with SEM and EDS from polished sections. The diameter of f = 30 mm. Mass of the sample was 1.3 ± 0.5 g. i condensed roasting products of pure copper selenide at No effort was made for getting identical samples for each 450 and 550 °C in 1 atm oxygen are shown as SEM micro- run as the reaction was predominantly a surface process graphs in Figures 2 and 3, respectively. with a relatively small degree of advancement. The sample The layers forming on reacting Cu Se were analysed was suspended in a platinum wire basket. The roasting 2 by EDS to be selenium-free oxides of copper, cuprous time was 4 or 8 hours. oxide Cu O and on cuprous oxide a layer of CuO. The The roasting gas was regulated with Aalborg rota- 2 oxidation products of copper selenide are thus solid at all meters calibrated for oxygen and sulfur dioxide. 99.5% selenium roasting temperatures used in this work. The oxygen and 99.98% sulfur dioxide, supplied by AGA oxidation sequence of copper selenide roasting in pure Linde, were used throughout this work. The gas flow rate oxygen atmosphere can be written as: in the experiments was 0.217 mLN/s O2 and 0.043 mLN/s SO2. For safety reasons, the sulfur dioxide containing 1 off-gas was scrubbed with dilute hydrogen peroxide solu- Cu2Se + 1 2 O2(g) = Cu2O + SeO2(g) (1) tion before discharging into atmosphere. Cu O + 1O (g) = 2CuO. (2) The microstructures and phase analyses were carried 2 2 2 out with a Jeol 6490 LV scanning electron microscope (SEM) equipped with an Oxford Inca EDS analyser. The Only traces of copper-selenium oxides at 450–550 °C can sample preparation was carried out using normal wet be found in the SEM micrographs, scattered in the copper methods. oxide scales, see Figures 3 and 4, as a result of reactions P. Taskinen et al., Oxidation Mechanism of Copper Selenide 471 Fig. 1: Roasting kinetics of synthetic, dense Cu2Se in dry oxygen and oxygen – 20% SO2 mixtures at 450–550 °C; note the slow but continuous rate of oxidation at all temperatures and atmospheres.