Upgrading copper concentrate by hydrothermally converting chalcopyrite to digenite
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I.
INTRODUCTION
C O P P E R smelter capacity may be constrained physically or by environmental limitations. Smelter expansion is expensive and may not be permitted at specific smelter sites. Increasing the concentration of copper in the concentrate feeding a smelter is an alternative for increasing smelter capacity. SOx recovery and fugitive SOx environmental constraints are relieved by lower sulfur grades in the concentrate, and higher grade mattes can be produced. Reduced iron concentrations lower slag volumes and slag copper losses. The reduction in mass associated with concentrate upgrading decreases concentrate shipping costs when smelters are distant from the ore concentration plant. For primary ores, chalcopyrite is the predominant copper mineral, and this mineral accounts for most of the world's copper extraction. This treatise is concerned with the hydrothermal chemical upgrading of copper concentrates that are predominantly chalcopyrite. Secondary ore deposits are formed when chalcopyrite is slowly enriched by reaction with cupric sulfate solutions that are formed by oxidation of copper minerals above the water table and percolate down to the zone of enrichment. [1,2] Rapid enrichment of chalcopyrite with cupric sulfate, using elevated temperatures to accelerate the reaction kinetics, has been studied as a metallurgical process in recent decades. [3-~~ The metathesis reaction is 3CuFeS2 + 6CuSO4 + 4H20 = 5CUl.8S + 3FeSO4 + 4H2SO4
[1]
Other metallurgical process approaches to chalcopyrite upgrading have included reduction by 5 0 2 U'SAll and reaction with metallic copper. [u~ Research on the cupric sulfate metathesis reaction t61 led to a pilot plant using reground Butte concentrates that
ROBERT W. BARTLETT, Professor of Metallurgical Engineering and Dean, College of Mines and Earth Resources, is with the University of Idaho, Moscow, ID 83843. Manuscript submitted March 22, 1991. METALLURGICAL TRANSACTIONS B
involved continuous feed to two sequential autoclave reactors. Pressure oxidation occurred in the first autoclave to produce cupric sulfate, CuFeS2 + 402
= C u S O 4 "~
FeSO4
[2]
and the CuSO4 was reacted with additional concentrate, without oxygen, in the second autoclave according to Eq. [1]. In the course of this pilot plant study, tl~ direct chalcopyrite conversion to digenite, Cul.sS, without a cupric sulfate reaction was discovered to occur at elevated temperatures when oxygen transfer to the mineral slurry was not fast enough to maintain a high oxidation potential in the slurry solution. Some of the ensuing pilot plant experiments used only one autoclave, while simultaneously injecting oxygen at limiting rates stoichiometrically related to the concentrate composition and feed rate. This process, operating at an Eh of about 500 mV, S.H.E., selectively oxidizes iron and part of the sulfur and converts chalcopyrite to digenite by the following net reaction: 1.8CuFeS2 + 0.8H20 + 4.802 = Cul8S + 1.8FeSO4 + 0.8H2SO4
[3]
Metallographic inspection of partially reacted grains shows that digen
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