Removal of Silicon Impurity from Molten Brass Using an Innovative ZnO-containing Slag
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INTRODUCTION
LIQUID slags play an important role in protecting and refining of molten metals during processing. Numerous studies have been reported on the application of slag-metal reactions to refine molten metals.[1–12] The refining property of a slag appears when it contains oxide components with less affinity to oxygen than the dissolved impurity in molten metal. Under such circumstances, impurities convert to their oxide forms via slagmetal reactions at the interface and then join the slag phase. The presence of silicon in a-brass (containing 30 mass pct Zn and 70 mass pct Cu) decreases the formability of the alloy. Hence, elimination of silicon from brass scrap is necessary for cold forming purposes. Although several reports are available in the literature about refining of molten copper with slags and fluxes,[5–13] no information could be found concerning the refining of brass alloy. Elimination of an impurity in a liquid metal, e.g., Si in molten copper or brass, with an oxide constituent in slag (XOn) takes place according to the following reaction: n=2½Si þ ðXOn Þ ¼ n=2ðSiO2 Þ þ ½X
½1
Here, () and [ ] indicate the component dissolved in slag and metal phases, respectively. Equation [1] demonstrates that oxidation of the target impurity (silicon) coincides with the reduction of oxide component XO and with release of the element X at the interface, which would subsequently enter the metal phase. To avoid contamination of the liquid metal with a new impurity originating from the refining reaction,
M.SH. BAFGHI, Associate Professor, A.R. ASGHARI, Graduate Student, A. ZAKERI, Assistant Professor, and S.H. SEYEDEIN, Associate Professor, are with the School of Materials Engineering and Metallurgy, Iran University of Science and Technology, Tehran, I.R. Iran. Contact e-mail: [email protected]. Manuscript submitted March 27, 2008. Article published online August 20, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
the oxidizing component of the slag must be an oxide of a constituent of the metal phase. Therefore, either zinc oxide- or copper oxide-bearing slags may be selected to refine copper-zinc alloys including brass. Another criterion in selecting the oxidizing agent is thermodynamic considerations. Thermodynamic data show that silicon has higher affinity to oxygen than both copper and zinc; i.e., silicon is readily oxidized by either copper oxide or zinc oxide. However, if copper oxide is selected, it is probable that zinc atoms from molten brass are also oxidized together with silicon, because of its high concentration in comparison with minor amounts of silicon impurity. Therefore, it is reasonable to select zinc oxide as the oxidizing agent. With regard to the above argument, the refining reaction would be expected to take place according to the following: ½Si þ 2ðZnOÞ ¼ ðSiO2 Þ þ 2½Zn
½2
DG° of Eq. [2] has a large negative value indicating the feasibility of the reaction from a thermodynamic standpoint, with the net driving force being dependent on the activities of various components in the alloy and
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