STUDY ON SULFUR VAPORIZATION FROM COVELLITE (CUS) AND ANILITE (CU1.75S)

Covellite decomposes according to the reaction: 4.667CuS --> 2.667Cu1.75S(s) + S2(g). The sulfur vapour pressures measured in the temperature range 551.5-627 K by the torsion-effusion method are represented by the equation: log p (kPa) = (11.30 +/- 0.30) - (8290 +/- 100)/T. At high temperature, the anilite vaporizes incongruently according to the equation: 16Cu1.75S(s) --> 14Cu2S(s) + S2(g), and the sulfur pressures are well represented in the temperature range 770.5-877 K by the equation: log p (kPa) = (10.49 +/- 0.40) - (11 470 +/- 300)/T. The enthalpies associated with these reactions are, DELTAH-degrees 298 = 178 +/- 4 kJ mol-1 and 268 +/- 7 kJ mol-1 for reactions 1 and 2 respectively, obtained from second- and third-law treatment of the data. From these reactions, the heat of formation of Cu1.75S, DELTA(form)H-degrees 298 = -74 kJ mol-1, was derived.