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Chlorinated Volatilization Roasting: Decomposition of Calcium Chloride. Chlorination of Gold
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The temperature-composition diagram of the system gold-chlorine has been determined by differential thermal analysis and high-temperature X-ray diffraction. Two compounds exist: AuCl and Au2Cl6. AuCl2 is not a compound, but a eutectic mixture of Au2Cl6 and AuCl; the eutectic temperature is 260°C. Au2Cl6 melts at 304°C, and AuCl(s) decomposes peritectically at 342°C into Au and a liquid of composition AuCl1.45. The heat of fusion of Au2Cl6 is estimated to be 29 ± 5 kcal/mole.
Large single crystals of AuCl were prepared by a vapour-transport method. AuCl is tetragonal, a = 6.734, , space group 141/amd, Z = 8. The structure consists of zig-zag chains of Au and Cl; Au is linearly coordinated by Cl at distances of 2.36 Å, the bond angles Au-Cl-Au are 92 °.
Processes that depend upon volatility are widespread throughout extractive metallurgy and maybe found represented in any of the stages in the sequence from raw material to finished metal. The removal of arsenic as the volatile AS406 by roasting gold-bearing concentrates and the removal by volatilisation under vacuum of impurities from nickel alloy melts, represent the extremes. In general, despite the example of zinc retorting, the field of application of volatilisation processes lies in achieving a separation, either of valuable material from gangue, or of unwanted or deleterious material at any stage in the processing. Unless the volatile substance is produced by an exothermic chemical reaction at a temperature where its vapour pressure is appreciable, the volatilisation will be endothermic and hence energy be supplied to the system. Consequently, the deliberate vaporising of substances during processing will introduce a high cost element. In the past, volatilisation techniques may not have been deliberately sought because of this factor, but have appeared adventitiously during the evolution of classical extractive metallurgy.
The kinetics and vapor-solid equilibria of the gold-chlorine system have been studied between 70 ° and 8oo °C at chlorine pressures ranging from 76 to 760 torr. Four kinetic regions, differentiated by separate controlling mechanisms, were found. Kinetic factors were correlated with equilibria involving (1) the vapor pressure of Au2Cl6, (2) chlorine and Au2Cl6 vapor in equilibrium with Au2Cl2, (3) chlorine and Au2Cl6 vapor over gold, and (4) chlorine and Au2Cl2 vapor over gold.
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Gold Transport by Complex Metal Chloride Vapors
- J Eisele
- D Fischer
- H Heinen
- D Kester-Ke
J. EISELE, D. FISCHER, H. HEINEN, D. KESTER-KE: "Gold Transport by Complex Metal Chloride
Vapors", BUMines RI 7489, 1971.
High Temperature Vaporiza-OPYKTor nAOYTOrtMINERAlWEALTH 69
- S James
- J Hager
S. JAMES, J. HAGER: "High Temperature Vaporiza-OPYKTor nAOYTOrtMINERAlWEALTH 69/1990
tion Chemistry in the Gold -Chlorine System Including Formation of Vapor Complex Species of Gold and Silver with Copper and Iron
tion Chemistry in the Gold -Chlorine System
Including Formation of Vapor Complex Species of
Gold and Silver with Copper and Iron ", Metallurgical Transactions B., Vol. 9B, December 1978, p.p
501-508.