Pekka Taskinen*,1, Sonja Patana*,2, Petri Kobylin*,3 & Petri Latostenmaa** *Aalto University CHEM, Department of Materials Science and Engineering, P.O. Box 16200, FI-00076 Aalto (Finland) **Boliden Harjavalta Oy, P.O. Box 60, FI-28101 Pori (Finland), process engineer 1

Professor, 2 research associate, 3 doctoral candidate

ROASTING SELENIUM FROM COPPER REFINERY ANODE SLIMES

ABSTRACT Selenium is present in the slimes fed to roasting process mostly as copper and silver selenides as well as elementary selenium, depending on the processing steps prior to the deselenisation. The decomposition of silver (Ag2Se), copper (Cu2Se) and mixed (AgCuSe) selenides is a necessary step in the roasting, before the oxidation of selenium to SeO2 can take place. Vapour pressure of elementary selenium or that in its intermetallic compounds is much lower than that of pure SeO2(g). Thus, the oxidation of selenium bound in selenides must occur on the surfaces of the intermetallic compounds, where formation of complex selenium oxides with silver or copper is possible. The selenium roasting of anode slimes is a complex process where the deselenisation rate is linked with the chemical form of selenium present in the slime. Elemental selenium is removed from the slime easily at low temperatures, around 200 °C, where its oxidation starts. Silver selenide Ag2Se is oxidized to selenite Ag2SeO3 almost quantitatively, and no gaseous selenium dioxide is released from slime to the roaster gas. The reaction rate is low, due to the small decomposition pressure of silver selenite. Copper selenides if present in the anode slime react much in the same way in oxygen as silver selenides. Due to the higher decomposition pressure only a fraction is oxidized directly to Cu2SeO4 or CuSeO3, and selenium is released as SeO2(g) to the gas phase. In SO2-bearing roaster atmospheres, the stable forms of copper and silver are their sulphates. Thus the gravimetric data of slime roasting is a combination of oxidation, deselenisation and sulphation reactions. The reaction rates in O2+SO2 atmospheres are enhanced by porosity and cracks formed in reaction scales which allow the gas transport between the reaction atmosphere and selenite. This feature of the roasting products explains the beneficial properties of sulphur dioxide in the anode slime roasting compared to simple roasting in oxygen or air.

Chapter: Introduction

Key words: copper refining, tank house, anode slime, selenium, roasting, precious metals

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INTRODUCTION Anode slimes in copper refinery tankhouses contain precious and platinum group metals, and their recovery is crucial to copper smelting and its economy. Copper refinery anode slimes are also globally an important source for selenium and tellurium. For their extraction, a number of technologies have been developed [1, 2] and many industrial process chains include a roasting step for vaporising selenium as a gaseous oxide SeO2, using oxygen or oxygen-sulphur dioxide mixtures as the roasting atmosphere. Post roasting the recovery of selenium is straightforward and uses typically SO2 reduction in an aqueous medium to grude metal [2]. Selenium is present in the slimes fed to roasting process mostly as copper and silver selenides as well as elementary selenium [3], depending on the processing steps prior to the deselenisation. In decopperising by pressure leaching with gaseous oxygen, elementary selenium is an oxidation product of selenides [4]. Thus decomposition of silver (Ag2Se), copper (Cu2Se) and mixed AgCu selenides (AgCuSe) is necessary step in the roasting, before the oxidation of selenium to gaseous SeO2 can take place. The thermal stabilities of Ag2Se and Cu2Se are high and they melt congruently before decomposition. Thus, the oxidation of selenium bound in selenides occurs on the surfaces of the intermetallic compounds, where formation of complex selenium oxides with silver or copper is possible [5, 6]. The mineralogy of anode slimes has been under intensive scrutiny by various groups in the past [3, 7-9]. An extensive review of the selenium roasting studies was written by Barbante et al. [10]. Very few experimental data on roasting anode slimes are available in isothermal roasting conditions [11]. Some authors have used thermoanalytical methods [12-14] and a few selenium oxide pressure measurements on silver selenite for finding out suitable temperature ranges of the deselenisation have been made [15-16].

Chapter: Introduction

The use of oxygen-sulphur dioxide mixtures as roasting atmosphere in the selenium removal [7] replaces the old sulphuric acid or sulphation roasting, where a mixture of the refinery slime and sulphuric acid was used as feedstock of the roasting furnace [17]. The aim of this study is to examine the role of silver to selenium ratio to the roasting rate and the final selenium concentration achieved post roasting. This study deals with industrial anode slimes and in previous papers [5-6] the authors investigated the mechanisms of roasting of synthetic copper and silver selenides in oxygen and oxygen-sulphur dioxide mixtures.

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EXPERIMENTAL MATERIALS The anode slimes were obtained from Boliden Harjavalta Oy Copper refinery (Pori, Finland) after decopperising. They were washed with pure water for removing any soluble material and sulphuric acid, and dried in an oven at 60 °C over night. The dry materials were homogenized in a mortar by hand. Two slime types were studied, with different Ag/Se ratios. The slime analyses are given in Table I. Tellurium concentrations in the slimes were low, 1.4 ±0.2% Te. Table I. Concentrations of selected elements in the anode slimes used.

Ag Bi Cu Pb Se

(wt%) (wt%) (wt%) (wt%) (wt%)

High-Ag Low-Ag slime slime 21.5 10.6 6.7 7.9 0.6 1.7 13.2 12.8 9.5 10.5

Most chemical analyses were made using inductively coupled plasma spectroscopy (ICP). Total carbon was analysed with an Eltra C/S analyser and the residual organic substances were low (