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Home , Copper selenide, Selenium dioxide

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

Abstract: The oxidation mechanism of atmosphere [2]. The roasting option is industrially attrac- was investigated at deselenization temperatures of copper tive as it to essentially 100% separation of refining anode slimes. The isothermal roasting of syn- from the other components of anode slime and produces thetic, massive copper selenide in flowing and relatively pure crude selenium in a single step. oxygen – 20% 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 sulfate. Sele- copper 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 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 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 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 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 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 . The slimes in copper refinery tion of Frenkel defects [13]. tank houses also contain precious metals, and their recov- The mechanism and role of 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 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.

Fig. 2: Copper oxide scales on Cu2Se generated by a flowing dry O2 roasting atmosphere at 450 °C within 4 h reaction time; the outer layer against atmosphere is CuO (points 1–5) and inner predominantly of Cu2O (points 6–8) with traces of selenium-copper-oxides (dark grey, points 9, 11–13) next to Cu2Se (points 10, 14).

Cu2Se + 2O2(g) = Cu2SeO4. (3) mass change suggests that more than 50% of selenium is converted into selenite in the oxygen roasting. The or ­copper-selenium oxides identified in the scale were mix-

Cu2Se + 2O2(g) = CuO + CuSeO3 (4) tures of Cu2O and CuSeO3 as their analysed Cu/Se-ratios

were much higher than that of CuSeO3. This explains the mass increase in pure oxygen roasting Copper selenite is obviously stable in the roasting and the observed TG curve in oxygen given in Figure 1 is a conditions used, but has such a high selenium dioxide composite of two parallel reactions (1–2) and (3–4). A pressure and decomposition rate that only some thin sele- short mass balance calculation ending up with a positive nite layers can be found on the oxide-selenide interfaces. 472 P. Taskinen et al., Oxidation Mechanism of Copper Selenide

Fig. 3: Copper oxide scales on Cu2Se generated by a flowing dry O2 roasting atmosphere at 550 °C within 4 h reaction time; the outer layer against the atmosphere is CuO (points 1, 4) and the inner predominantly of Cu2O (points 2–3, 5) with scales of selenium-copper-oxides

(points 6–7), the non-reacted base material (area 8) is of Cu2Se.

Fig. 4: Oxidation scales formed on Cu2Se at 550 °C in an O2–20% SO2 atmosphere: black outer layer of CuSO4 (point 1) on Cu2O (point 2, 8) and a Cu-Se-oxide (points 9–11); CuO is also visible in the fine grained surface scale and points (3–7) give the analysis of non-reacted Cu2Se with about 60% Se.

Significant porosity is also formed on the Cu2Se-Cu2O-CuO phases containing oxygen. The outer surfaces of copper interface. oxides are generating copper sulfate layer densely on CuO. The surface scales on copper selenide roasted in The oxidation sequence can thus be written as: flowing O2-20 vol% SO2 atmosphere at 550 °C are shown in 1 Figure 4. The reaction products are essentially separated Cu2Se + 1 2 O2(g) = Cu2O + SeO2(g) (5) by a large porosity from the primary Cu2Se, in a similar 1 Cu2O + 2SO2(g) + 1 2 O2(g) = 2CuSO4 (6) way as in the case of Ag2Se [15]. The EDS analyses indicate that the scale also contains traces of selenium-bearing Cu2Se + 2O2(g) = Cu2SeO4. (7) P. Taskinen et al., Oxidation Mechanism of Copper Selenide 473

Cu2SeO4 = CuO + CuSeO3 = 2CuO + SeO2(g) (8) The phase relations of oxidizing copper selenide were estimated using the copper selenite data from the data- Cu O + 1O (g) = 2CuO. (9) 2 2 2 base of HSC [18]. The selenium dioxide partial pressures generated by the oxidizing processes of copper selenides at 450 °C are shown in Figure 6. The graphs include the 4 Discussion boundary condition in industrial selenium roasting of 1 atm total pressure, the gas composition line with Ptot =

The observations on the reaction products when oxidizing P[O2(g)] + P[SeO2(g)] = 1 atm. synthetic, dense copper selenide indicate clearly the As indicated by Figure 6, Cu2Se can also oxidize di- phases forming in the roasting process, whereas the fine rectly to Cu2O and CuO. At 450 °C the selenium dioxide particle size, partially amorphous structure and complex pressure at selenide-copper oxide-copper selenite equilib- mineralogy of the industrial anode slimes prevent any rium is slightly above 0.02 atm, and at 550 °C it is close to direct observation [3]. The results obtained will be anal- 1 atm. The prevailing selenium dioxide pressure in the ysed based on thermodynamic properties of the oxidation Cu-O-Se system generated by oxygen of the anode slime products. roasting atmosphere is higher than in the silver-selenium- Predominance area diagrams of copper in oxygen and oxygen system [15]. sulfur dioxide bearing conditions at 450 and 550 °C are Copper selenide is oxidised to copper oxides without shown in Figure 5. The stable phases of copper in condi- formation of thick copper selenite scales, and a great deal tions of a selenium roaster gas with SO2 is its sulfate of selenium is obviously vaporized directly as SeO2(g)

CuSO4. Copper oxidizes first into Cu2O and in low SO2 pres- when selenite is formed. This is in good agreement with sures to CuO, before forming sulfate. the chemical analyses of Ishihara [6]. The reaction product

The thermodynamic stabilities of copper selenites are scales on Cu2Se are also porous which allows transport of available in the compilation by Barin [16]. According to oxygen to the reaction zone and selenium dioxide vapour the assessment of Olin [4], copper selenite CuSeO3 decom- to the surrounding atmosphere. poses to CuO and SeO2(g) below 427 °C, whereas at higher Formation of the porosity in solid Cu2Se is evidently temperatures CuO·CuSeO3 is formed as an intermediate linked with clustering of copper vacancies in the diffusion product prior to the final decomposition to solid CuO and processes where copper have much higher migration gaseous SeO2(g). No accurate data on this equilibrium are, rate than selenium, due to the superionic conduction however, available [4, 17]. mechanism [19]. There are also indications that selenium

Fig. 5: The stable phases of copper at 450 and 550 °C (···) with oxygen and sulfur dioxide pressures as variables, calculated using the standard database of HSC Chemistry (vers. 6.07) [18]. 474 P. Taskinen et al., Oxidation Mechanism of Copper Selenide

Fig. 6: The stable phases in the system Cu-Se-O at a typical selenium roasting temperature of 450 °C, as a function of the prevailing oxygen partial pressure of the system; the solid curve shows the atmospheric composition of 1 atm total pressure expressed as P + P = 1, O2(g) SeO2(g) forming a boundary condition of the industrial selenium roasting.

can diffuse through Cu2O with a reasonable rate at low Table 1: (r), molar volume (v) and molecular weight (M) of temperatures [20]. The observations are in agreement selected substances at room temperature [25, 26]. with the previous X-ray data on the oxidation of copper r 3 3 selenide [8], silver-copper selenide [6, 20] and decomposi- Substance (g/cm ) v (cm /mol) M (g/mol) tion of selenite in air [7, 12, 21]. Cu 8.92 7.1 63.55

The oxidation of copper on copper selenide seems to Cu2Se 6.65–7.7 28.7 206.05 proceed in parallel by two mechanisms: a coupled oxida- Cu2SeO4 5.01 53.9 270.05 tion with selenium to copper selenite and direct oxidation CuSeO3 4.73 40.3 190.50 CuSO 3.61 44.2 159.60 to cuprous oxide. Copper oxide grows outside from metal 4 Cu2O 6.0 23.8 143.09 surface by copper transport through neutral copper va- CuO 6.45 12.3 79.55 cancies in Cu2O [22]. Its further oxidation to cuprous oxide CuO takes place through a mechanism of copper intersti- tials or copper vacancies, enabling outward diffusion of Cu2Se → Cu2SeO4 → Cu2O → 2CuO copper in the growing copper(II) oxide scale [23, 24]. The v(system): 28.7 53.9 23.8 24.6 cm3/mol diffusion of selenium through cuprous oxide may be pos- PBR: 1.88 0.44 1.03 sible through oxygen vacancies which have relatively low energy of formation [27]. Thus the release of gaseous The Pilling-Bedworth ratios (PBR) shown after each oxida-

SeO2(g) from Cu2Se takes place through porosity and tion step can be used for estimating the stress formation in cracks in the copper oxide scales. the reaction product scale and thus also its porosity gener-

This general oxidation pattern of Cu2Se is not essen- ated in an oxidation process [28], depending on the direc- tially different in SO2 bearing roaster atmospheres. The tion of mass transfer in the product scale [29]. The volu- cuprous oxide formed in selenium oxidation is further metric increase in the formation of selenites is large and it

­sulfated to CuSO4, according to the equilibria presented in may to cracking and peeling off the solid product the predominance area diagram of Figure 5. layer from the copper selenide surface at small scale thick- The formation of roasting products of copper selenide nesses. The formation of the final oxidation products of also involve large volumetric contributions, in particular Cu2Se, copper oxides and copper sulfate, takes place as when selenite or sulfate is formed, see Table 1. The volu- separate islands on selenite, and they tend to grow to- metric differences as molar volumes (v) can be written gether as well as cover the selenite surface if the conver- based on the data in Table 1 as: sion to sulfate is complete. P. Taskinen et al., Oxidation Mechanism of Copper Selenide 475

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