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Abstract The roasting of enargite (Cu 3 AsS 4) in the temperature range 673 K (400 °C) to 900 K (673 °C) has been considered in the light of three-dimensional predominance volume diagrams (PVDs). Condensed phases considered were Cu, Cu 3 As, Cu 2 O, Cu 2 S, CuO, CuS, CuSO 4 , CuO·CuSO 4 , Cu 3 AsO 4 , Cu 3 As 2 O 8 , Cu 3 AsS 4 , Cu 6 As 4 S 9 , and Cu 12 As 4 S 13. Gases considered were As 4 , As 4 O 6 , AsS, As 4 S 4 , O 2 , S 2 , SO 2 , and SO 3. There are four line segments associated with each of the nine invariant points while each corner has three. The PVD at 900 K shows the gas composition during vacuum decomposition of tennantite with selected O 2 infiltration. Calculations show that increasing the O/Tennantite reacted ratio in the system leads to the formation of Cu 3 As 2 O 8 at 900 K and CuSO 4 at 673 K. Roasting experiments at 673 K showed that enargite transformed into tennantite (Cu 12 As 4 S 13) in the absence of oxygen; whereas, in the presence of oxygen it transformed into CuSO 4 and CuO·CuSO 4. Enargite does not appear to transform directly into CuSO 4 , but follows a path that includes Cu 2 S or CuS. Sulfate formation is favored over oxide at lower roasting temperatures. Graphical abstract Image 1 Highlights • Quaternary Cu-As-S-O system studied at 673 K and 900 K. • Three-dimensional predominance volume diagrams (PVDs) constructed. • Total pressure plane (0.25 atm) superimposed. • Experimental roasting results compared with equilibrium thermodynamics. • Volatilization of arsenic determined. [ABSTRACT FROM AUTHOR]

While predominance area diagrams (PADs) are useful for the determination of the stable phases in the metal-sulfur-oxygen system, they are constructed at constant temperature. Varying the temperature results in shifting of the equilibrium lines and the stability areas thereof. Since the available tools in hands of an extractive metallurgist are temperature and oxygen potential, a more useful diagram would be one that shows the stability areas of different phases as a function of temperature and percent of oxygen in the roast gas. Such diagrams, called ‘roaster’ diagrams, derive directly from PADs. Although such diagrams are used by extractive practitioners, this type of diagram is unavailable in the open literature. In this paper, the relationship between the PADs and roaster diagrams is discussed and PADs and roaster diagrams for the Zn-S-O and Pb-S-O ternary systems are constructed. This information will serve as the foundation for more complex four-component systems to follow in subsequent publications. [ABSTRACT FROM AUTHOR]

Although several hydrometallurgical treatment options for dirty copper sulfide concentrates have been developed in recent years, these processes have not been implemented by industry. Attention has therefore shifted to pyroprocessing such concentrates in order to provide clean feed for the well-established sulfide smelting process. In this paper, recent publications on pyrometallurgical processing of enargite and gold concentrates have been reviewed, which indicate that little has been added to already existing knowledge of the thermal behavior of enargite. The arsenic fix-roast approach using Na 2 CO 3 and Ca(OH) 2 to capture the arsenic during roasting does not seem to be a practical approach due to insufficient capture and also the complexity of the subsequent phase separations. A more pragmatic approach would be further development of neutral/reductive roast of enargite concentrates in order to produce clean feed for the existing smelters and to convert arsenic into its sulfides which are more stable and compact than its oxides. Potentially high temperatures (> 650 °C) required for effective arsenic removal from enargite concentrates has represented a significant barrier for the development of roasting equipment. This calls for further development of pyroprocessing strategies through a better understanding of the thermodynamics of the Cu-As-S-(O) system. The results of recent research indicate that the mysterious role of pyrite, always present in enargite concentrates in thermal transformation of enargite, has been largely underestimated and not well understood. This necessitates a thorough understanding of the even more complicated Cu-Fe-As-S-(O) system. [ABSTRACT FROM AUTHOR]

While the growing demand for copper has compelled the industry to adapt new technologies for the treatment of copper-arsenic (enargite) concentrates, the refractory nature of such concentrates combined with the troublesome presence of arsenic has created a major metallurgical and environmental challenge. Preliminary results of the acid bake-leach process at the University of Utah have shown some potential advantages for the treatment of enargite concentrates. While the transformation of enargite to copper sulfate, arsenolite, and elemental sulfur has already been established experimentally, thermodynamic evaluation of the sulfuric acid baking process provides further understanding which should be useful. In this article, the available thermodynamic data for the species involved in the Cu-As-S-O system are compiled. These data were used to calculate the phase stability (Kellogg) diagrams as well as equilibrium compositions at 473 K (200 °C) using the STABCAL and HSC Chemistry 5.1 software packages. The equilibrium composition calculations indicate that enargite can transform to copper sulfate either directly or through chalcocite and/or covellite. The major gaseous species during baking were found to be SO and HO. The results of the thermodynamic calculations were further compared with two confirmatory baking experiments involving a high-quality enargite sample. The condensed reaction products from sulfuric acid baking based on XRD results include CuSO, AsO, CuO·CuSO, and S under both neutral and oxidative conditions. While all these compounds were predicted through equilibrium calculations, some of the predicted compounds were not detected in the sulfuric acid-baked enargite. None of the calculations indicated any appreciable amounts of arsenic-bearing gases at the baking temperature of 473 K (200 °C). Consistent with thermodynamic predictions, no HS gas was detected during the sulfuric acid baking experiment. Approximately, 80 pct of the baked enargite samples were leached in water. [ABSTRACT FROM AUTHOR]

Abstract: Thiocyanate has been identified and studied as a promising alternative lixiviant for gold in acidic solutions. Eh–pH and ion species distribution diagrams for SCN–H2O, Au–SCN–H2O, Ag–SCN–H2O, Cu–SCN–H2O, and Fe–SCN–H2O systems were constructed to predict the behavior of each metal ion in the thiocyanate system and also to explain the experimental results. Thermodynamic analyses suggest that gold can be leached by thiocyanate under appropriate leaching potentials, forming aurous or auric complexes with thiocyanate, depending on the thiocyanate concentration and leaching potential. According to species distribution diagrams, silver (I) and copper (I) form insoluble salts at moderate thiocyanate concentrations and are soluble at low and high thiocyanate concentrations. Ferric ion forms a series of complexes with thiocyanate. The study of the ferric ion effect indicates that gold can be leached in acid thiocyanate solution with ferric sulfate as the oxidant. Also the presence of excess ferric ion reduces the apparent thiocyanate activity for copper (I) and silver (I) dissolution. The findings of this thermodynamic assessment are useful in the analysis of some of the phenomena encountered in the leaching and recovery of gold from thiocyanate solutions as discussed in subsequent papers. [Copyright &y& Elsevier]