A Study on the Copper Recovery from the Slag of the Outokumpu Direct-to-Copper Process
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THE Outokumpu direct-to-blister process was commercially implemented for the first time in 1978, in Głogo´w, Poland. The process requires a high oxygen potential, and therefore it is restricted to those smelting concentrates which mainly consist of minerals without iron (e.g., Cu2S, CuS, Cu9S5) or with a small iron content (Cu5FeS4). As a consequence of the high oxygen potential in the Outokumpu flash smelting furnace, the produced slag contains 12 to 16 pct Cu in the Głogo´w smelter. The copper in the slag constitutes 1/3 of the copper content in the smelting concentrate. Therefore, the recovery of the copper from this slag has a practical importance and heavily influences the operational costs. A part of the copper in the slag is in form of fine inclusions suspended in the slag; however, the majority of the copper is dissolved in it. The slag cleaning process is carried out in an electric furnace with coke and limestone additions. The limestone modifies the reduced slag in order to facilitate the process. Slags are ionic liquids,[1–3] and the copper in the slags exists mainly as Cu2O and Cu+, associated with the silicate anions, according to the reaction: MARIAN KUCHARSKI, Professor, TOMASZ SAK, PIOTR MADEJ, and MACIEJ WE ˛ DRYCHOWICZ, Ph.D. Students, are with the Department of Metallurgy of Non-Ferrous Metals, AGH University of Science and Technology, 30 Mickiewicza Avenue, 30-059 Krako´w, Poland. Contact e-mail: [email protected] WIESŁAW MRO´Z, formerly Researcher with the Department of Metallurgy of Non-Ferrous Metals, AGH University of Science and Technology, is now retired. Manuscript submitted July 25, 2012. Article published online October 8, 2013. 590—VOLUME 45B, APRIL 2014
j
!
j
O Si O Si O j
¼2
þ Cu2 O
j
j
!
O Si O Cu j
þ
:
ð1Þ
Cu2O is a basic oxide and the oxygen ion in it breaks the bridge in the Si-O network structure, which leads to the dissociation of the silicate anion complexes. During the reduction process, the situation becomes reverse, and the silicate anions undergo polymerization. This model[4] suggests that the dissolved copper is almost entirely in the form of (-Si-O-Cu+), for the slags containing about 1 wt. pct of copper. As such a slag contains a small amount of Cu2O, it causes sluggishness of the copper recovery process when it is close to completion, because the copper reduction by carbon can proceed only according to the following reactions: Cu2 O þ C ¼ 2½Cu þ CO
½2
Cu2 O þ CO ¼ 2½Cu þ CO2 :
½3
The addition of basic metal oxides (CaO, FeO, …) to the slag causes some of the copper ionic Cu+ to be replaced by Ca2+, Fe2+, …, and therefore, the concentration of Cu2O increases. There are a number of papers[5–26] devoted to the study on the kinetics of the reduction of FeO and Fe2O3 oxides from slags, but only few deal with the reduction of non-ferrous metals from slags. Therefore, an extensive study on the kinetics of copper recovery from industrial slags was undertaken to METALLURGICAL AND MATERIALS TRANSACTIONS B
improve effectiveness of the co
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