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Summary and Conclusions

Thermal stabilities of Ag093Cu107S and AgBiS2 in an oxidizing atmosphere have been studied by applying the simultaneous DTA-TGA analyses. 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, which is in agreement with the value T = (361.2 ± 1) K reported after calorimetric measurements in an argon atmosphere [30]. For the first time, maximum thermal stability of (Ag, Cu)2S in an oxidizing atmosphere close to 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 in air determined from the cooling DTA curve in Fig. 1, T = (932.76 ± 2) K, is in good agreement with the melting temperature reported by Singh et al. [34], T =931 K.

An attempt to study the oxidation processes of AgBiS2 failed due to the reaction of the products with Al2O3 crucible. However, based on the SEM-EDS analysis of the oxidized sample, it’s evident that Bi2O3 has been produced, which implies the oxidation process for AgBiS2 may have occurred according to reaction (7). It has been found that for measurements of Bi containing samples at elevated temperatures and in an oxidizing atmosphere Al2O3 crucibles are not recommended.

Acknowledgements The authors are grateful to the Academy of Finland for financial support. This work was made under the project “Chemistry of biomass impurities at reducing conditions in future thermal conversion concepts” as part of the activities of the Johan Gadolin Process Chemistry Center at Abo Akademi University.

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