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Thermal Analyses of Silver-Based Sulfosalts in Air

Fiseha Tesfaye and Daniel Lindberg

Abstract The sulfosalts Ag0.93Cuj.07S (stromeyerite) and a-AgBiS2 (schapbachite) have been studied under oxidizing conditions at elevated temperatures. The sulfosalts were synthesized from the pure simple sulfides in evacuated quartz ampoules. The synthesized samples were thermally analyzed in the temperature range from 298 to jj73 K by a simultaneous DTA-TGA analyzer. Based on the DTA measurements the phase transition of stromeyerite to the solid solution (Cu, Ag)2S(hcp) in air is determined to be T = (360.9 ± 2) K. For the first time, maximum thermal stability of (Ag, Cu)2S in an oxidizing atmosphere close to the partial pressure of oxygen in air (P(O2) ^ 0.20 atm) is determined to be below T = (614 ± 2) K, above which it oxidizes to form Ag, CuO and Ag2SO4. The melting temperature of Ag2SO4 determined from the cooling DTA curve, T = (932.76 ± 2) K, is in good agreement with the literature value. Below T = 1173 K, the oxidation process for schapbachite in air has been indirectly determined to be: 2AgBiS2 + 5.5O2(g) ^ 2Ag + Bi2O3 + 4SO2(g).

Keywords Silver • Sulfosalts • Thermal stability • Oxidation • DTA-TGA analysis Pyrometallurgical roasting

Introduction

Currently, most of the available sulfide ores are low-grades and also becoming increasingly complex. Ag and Cu bearing sulfosalts are frequently encountered in the available complex ore minerals for the production of base metals and are also common sources of Ag. For instance, in the precious and base metal ores, AgBiS2 is a relatively common sulfobismuthide mineral. Several samples of hydrothermal vein-type ores including matildite (P-AgBiS2) and schapbachite (a-AgBiS2) are

F. Tesfaye (H) • D. Lindberg

Johan Gadolin Process Chemistry Centre, Abo Akademi University, Biskopsgatan 8, FI-20500 Turku, Finland e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

© The Minerals, Metals & Materials Society 2017

A. Allanore et al. (eds.), Materials Processing Fundamentals 2017,

The Minerals, Metals & Materials Series, DOI 10.1007/978-3-319-51580-9_6

reported to occur along the Northern Kinzigtal Fault (NKF) in the Schwarzwald ore district and in south western Germany [1]. Mineralogical studies of the Cu-Ag deposit in central Europe have also shown that schapbachite (a-AgBiS2) exists commonly in the mineral assemblages for the production of valuable metals [2]. Among the Ag-Cu-based sulfosalts, stromeyerite (Ag093Cu107S) is relatively common ore mineral in most hydrothermal vein and replacement deposits [3]. In order to modify operating flow sheets and strategies for processing more complex feed materials economically, valuable metal producers are in need of accurate thermal stability data of the phases, which are common in these complex ore minerals, at the pyrometallurgical processing conditions.

In recent years, the ternary chalcogenide semiconducting compound AgBiS2 has been studied intensively due to its unique electronic and magnetic properties which can be applied in linear, nonlinear, optoelectronic, and thermoelectric devices as well as optical recording media [4-12]. Recently, the high ionic conductivity and gradual disorder in the sequential phase transitions that are observed in the Ag-Cu-S ternary phases renewed the interest in the silver and copper based sulfides [13]. For instance, the ionic conductivity of superionic solid solution of Ag-Cu-sulfides at T = 573 K and composition of Ag/Cu = 1, i.e. (Cu, Ag)2S(fcc) phase, has been reported to be as high as 2.38 S cm-1 [14]. Furthermore, the solid solution (Cu, Ag)2S may have also unique optical properties [15]. There is also growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient in air. In order to optimize manufacturing of devices and accurately determine the user limits for devices incorporating the ternary phases, thermal stabilities of the phases at elevated temperatures in air is important. Consequently, data about thermal stabilities of the Ag-based sulfosalts at elevated temperatures in an oxidizing atmosphere is not only crucial for studies of ore genesis, processing of complex minerals and the optimization of the extractive metallurgy of the valuable metals, but also for improved manufacturing and user limit determination of novel electronic materials incorporating the Ag-(Bi, Cu)-S ternary phases. Therefore, the purpose of this experimental investigation was to study the thermal stabilities of the AgBiS2 and AgCuS at elevated temperatures in an oxidizing atmosphere (P (O2) ^ 0.20 atm).

 
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