Deoxidation of Liquid Copper with Reducing O 2 /CH 4 Flames
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DURING liquid copper fire refining, two main steps are necessary to remove the impurities present in the blister copper before casting—namely oxidation followed by reduction. This study focuses on the latter. The fire refining process normally is carried out in batch mode in a reactor called an anode furnace; each step involved in this operation can take several hours. During this time, burners are used to maintain the temperature inside the reactor. The objective of this study was to determine the rate of reduction using reducing gases flowing above the surface of the molten copper. Industrially, at the start of reduction, the copper may have about 0.5 to 1 pct O and, at the end, have less than 0.1 pct. The gaseous reduction of copper has been investigated using carbon monoxide,[1–4] different gas mixtures including CO/CO2,[5,6] H2/N2,[7] and H2/Ar,[8] as well as reformed natural gas.[3,9,10] Using carbon monoxide, Andreini et al.[1] injected CO into about 4 kg of molten copper and studied the deoxidation kinetics by analyzing the CO2 in the off-gas. In the composition range from 0.005 to 0.1 wt pct, they concluded that liquid mass transfer was the rate-limiting step. Nanda and Geiger[2] also did a study with low oxygen concentrations in copper (below 0.05 wt pct), which confirmed that liquid mass transfer was the rate-limiting step. For higher oxygen concentrations in the melt (0.1 to 1 wt pct), Themelis and Schmidt[4] reported that the rate was independent of the oxygen concentration in the liquid and that mass transfer in the gas phase was the rate-limiting step. The same conclusion was found T. MARIN, Research Metallurgist, is with Vale-Inco, Sheridan Park, Mississauga, Ontario, Canada. T. UTIGARD, Professor, is with the University of Toronto, Materials Science and Engineering, Toronto, Ontario, Canada. Contact e-mail: [email protected]. Manuscript submitted April 25, 2006. Article published online February 5, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
by Kikuchi et al.[6] for oxygen concentrations greater than 0.25 wt pct. Although somewhat inconclusive, it can be concluded that the rates are only weakly dependent on the temperature.[1,6] With respect to the H2 reduction of oxidized copper, there is very little information available. Capocchi and De Lazzari[7] injected two different H2/N2 mixtures (15 and 33 vol pct H2) into molten copper at 1170 °C and reported that the rate of deoxidation was constant for oxygen concentrations between 0.4 and 0.15 wt pct. They concluded that gas-phase mass transfer was the rate-limiting step under their conditions. Fukunaka et al.[8] carried out a study of reducing a levitated copper droplet in an H2/Ar mixture at a very high temperature (1697 °C). From their results, they reported mixed rate-controlling steps depending on the oxygen content in the copper. They also pointed out the importance of the dissolution and accumulation of hydrogen in the copper on the reduction mechanism. This accumulation process started at the beginning of the deoxidation process. Finally, t
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