The Rate of the Phosphorous Reaction Between Liquid Iron and Slag
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IN recent years, electric furnace production of higher quality steels has increased. Many of these steels require lower residual elements such as copper, nickel, tin, and phosphorous. Scrap alternatives such as pig iron, direct reduced iron (DRI), and hot briquetted iron (HBI) are frequently used to dilute noble impurities in scrap, such as copper, and often represent more than 30 pct of charge. These scrap alternatives frequently contain higher phosphorous contents than scrap, making phosphorus control as an issue of special concern. The observed phosphorous distribution between slag and metal (Lp) is generally lower (10 to 50) for EAF steelmaking than for oxygen steelmaking (50 to 200) Lp ¼
ðpct PÞ ½pct P
½1
where ( ) and [ ] indicate that the specie is in the slag and metal, respectively. Electric arc furnaces also have lower slag volumes than typical oxygen steelmaking furnaces, further reducing the potential for phosphorus removal. The analysis of plant data indicates that the phosphorous reaction does not reach equilibrium in either of the steelmaking processes. It is reasonable to assume that mass transfer controls the rate. However, the analysis of plant data is complicated by slag formation: specifically, the formation and reduction of iron oxide, and because the rate of dissolution of CaO greatly influences dephosphorization. In addition, the temperatures of CHRISTOPHER P. MANNING, President, is with Materials Processing Solutions, Inc., PO Box 1203, Easton, MA 02334. RICHARD J. FRUEHAN, Professor, is with Carnegie Mellon University, 5000 Forbes Avenue, Wean Hall 4319, Pittsburgh, PA 15213-3890. Contact e-mail: [email protected] Manuscript submitted August 14, 2012. Article published online November 29, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
the slag and metal can differ greatly and are continuously changing during the melting cycle. As a result, the thermodynamic driving force for phosphorus transfer between the metal and slag is a continuously moving target under actual steelmaking conditions. The behavior of phosphorous in DRI and HBI has been investigated by the current authors.[1] The phosphorous in DRI and HBI is initially contained in the oxide gauge phase of the material. The oxygen/carbon balance and gangue chemistry of most of the commercial DRI and HBI materials will result in a very low intrinsic phosphorus partition ratio between the melt-in gangue and metal phases. During the melting process, the phosphorus is rapidly transferred from the gangue phase to the metal phase of the individual DRI pellets or HBI briquettes across the large interfacial area between the microscopic particles of gangue and the surrounding metal phase. Further phosphorus transfer occurs as the melting DRI/HBI metal and oxide gangue phases report to the bulk metal and slag phases within the furnace.
II.
LITERATURE REVIEW
The phosphorous equilibrium has been extensively studied[2–11] and been recently reviewed by Reference12. Many of these studies have developed correlation to predict Lp as a fu
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