Extractive Metallurgy Division - Sulfating of Cuprous Sulfide and Cuprous Oxide

The oxidation of Cu2S in oxygen and the sulfating of Cu2O in oxygen-sulfur dioxide atmospheres was carried out under a variety of conditions. The oxidation of Cu2S was found to be retarded by entrapment of SO, and O2, which stabilized internal sulfates for long periods of time. The course of the reaction was followed by measuring weight changes and also by SO, evolution. Sulfating of Cu2O was a maximum at ratios of SO, to O2 approximating maximum SO, production. At elevated temperatures SO, was found to increase the rate of oxidation of Cu2 O to CuO even though sulfates did not form. All sulfating reactions followed the parabolic rate law indicating diffusion. MANY studies of the roasting of copper sulfides have been reported in recent years. Diev et al.1 investigated the roasting of chalcocite (Cu2S) in air, and oxygen enriched air. Lewis et al.2 also studied the oxidation of natural and synthetic chalcocite in air and oxygen atmospheres and their studies indicated that the maximum formation of water soluble sulfates occurred at approximately 450oC. Ashcroft3 reported that oxide production during the roasting of chalcocite resulted only from secondary decomposition of sulfates which were formed as primary products. peretti4 refuted this claim by showing that a layer of Cu2O appeared directly adjacent to the Cu2S during roasting of cylindrical briquettes of cupric sulfide, CuS. The linear advance of the Cu2S-Cu2O interface was used as a measure of the kinetics of the roasting reaction. The reactions proposed were: 2 CuS—Cu2S + 1/2 S2 [4-1] 1/2 S2 + O24 SO2 [4-2] cuzs +3/2 O2 4 Cu2O + SO2 [4-3] cu2o + 1/2 O2—2 cuo [4-4] At temperatures above 663oC, CuO was the only final solid phase reported. Below 663" C increasing amounts of sulfate were found mixed with the CuO. McCabe and Morgan5 investigated the roasting of discs of synthetic chalcocite and reported the following sequence of products beginning at the sulfide surface: Cu2O, a mixture of Cu2O and CuSO4, Cum,, CuO . CuSO4, and CuO. The principal reactions were reported to be: Cu2S + 3/2 O2-Cu2O + SO2 [5-1] CU2O + 2 SO2 + 3/2 O2—2 CUSO4 [5-21 2 CUSO4— CUO . cum, + SO3 [ 5-31 cuo . cuso4—-2 cuo + SO, [ 5-41 Eq.15-11 supports the claim of Peretti, Eq. [4-31, that CuzO is formed directly from Cu2S rather than as a secondary product from a sulfate as suggested by Ashcroft. On the other hand CuO was found to form as a secondary product from the decomposition of copper sulfate and basic copper sulfate, Eqs. [5-31 and [5-41. The formation of sulfates was explained by McCabe and morgan5 to be a direct reaction of Cu2O with 0, and SO, or SO, at distinct regions in which the partial pressures of each were such as to form the sulfate. Thornhill and pidgeon6 roasted both natural and synthetic chalcocite grains in air at temperatures between 420" and 550° C. They found a dense primary oxidation layer in contact with the sulfide. A secondary layer of porous oxidation products was found to expand with roasting time. The oxide products were leached away and the remaining core was studied by X-ray diffraction. The X-ray patterns showed an increased conversion of chalcocite to digenite with time. Digenite,7 a defect structure of cuprous sulfide, occurs naturally as Cu,-,S where x = 0.12 to 0.45, with an average analysis of Cu, ,S. The mechanism of digenite formation was proposed as: Cu2S + oxygen—Cu1-8S + 0.1 Cu2O [6-1] Cuj.eS + oxygen—0.9 Cu2S + SO2 [6-2] It is apparent from the above studies that the oxidation of Cu2S, ultimately ending in CuO, may be divided into ihree general stages (all of which may occur simultaneously): 1) primary oxidation to Cu2O; 2) secondary sulfate formation; and 3) sulfate decomposition. Consequently reactions of O2 and SO, with Cu20 constitute important aspects of the roasting of chalcocite. Virtually no studies have been made regarding sulfating reactions involving Cu,O. Mills and Evans8 noted the effect of sulfur dioxide on the oxidation of copper at low temperatures and low SO, partial pressures. They reported a measurable increase in the oxidation rate of copper when SO2, was present. Interest in the Cu2O-CuO-0, system has been limited predominantly to misciblllty studies and determinations of heats of formation by