Complete Hot-recycling Technique of Dephosphorization Slag for Hot Metal Pretreatment with Convertor

A new and efficient process for hot metal pretreatment using convertor was developed in which the whole amount of dephosphorization slag at the preceding charge was left in the convertor and hot-recycled to the following charge, incorporating intermediate deslagging after desiliconization period. In this process, it is necessary to avoid rephosphorization from phosphorus-containing slag. In this study, a series of plant trial was carried out and the change in phosphorus content of hot metal was investigated. During the desiliconization period, rephosphorization or dephosphorization was observed depending on the slag basicity. Rephosphorization occurred under the condition such that the slag composition corresponds to the coexistence of CaO·SiO2 (CS) and liquid slag (L), provided that the slag is almost considered as [CaO-SiO2-FeO] ternary system. On the other hand, dephosphorization proceeded under the condition such that the slag is saturated with 3CaO·2SiO2 (C3S2) amd/or 2CaO·SiO2 (C2S) based on the ternary diagram. According to the available data, phosphorus distribution ratio in 3-phase coexistence of CS+C3S2+L and C3S2+C2S+L are in the order of 103–104, while it decreases lower than 102 in CS+L region. The range of slag basicity for such a high phosphorus distribution ratio is strongly affected by FeO content in the slag. Taking into account of kinetic advantages as well as equilibrium aspects, an optimum FeO content is discussed and proposed to be around 15 mass%, for preventing rephosphorization from the recycled slag and also for proceeding phosphorus removal sufficiently in the commercial operation. © 2024 The Iron and Steel Institute of Japan.