Effect of SiO2/CaO on the viscosity and structure of yellow phosphorus slag in the electric furnace

Slag within suitable viscosity range is important for well controlled slag tapping of phosphorus reduction process in electrical furnace. The suitable slag viscosity depends on the adjustment of SiO2/CaO by adding silica. However, the influence of SiO2/CaO on slag viscosity and structure at slag tapping condition is still unknown. In this work, it is found that the slag viscosity increased slightly with increasing SiO2/CaO above the liquidus temperature. This is because the reduction of free oxygen ions (O2-) leads to an increase of the polymerization degree. However, the slag viscosity decreased as the SiO2/CaO ratio increased below the liquidus temperature. The weakening of the crystallization tendency, the formation of eutectic mixtures, and the reduction of Si-O-Ca-O units with larger ionic radii improve the fluidity of the slag. In addition, the presence of phosphorus slightly promoted the polymerization of the silicate network with increasing SiO2/CaO. This is caused by the increasing proportion of modified cations released by phosphorus from the silicate network. Meanwhile, the increasing SiO2/CaO decreased the polymerization degree of the aluminate structure due to the transition of [AlO4] to [AlO5]. The optimal slag discharge temperature range for the slag with a composition of 85 wt% (SiO2 + CaO) and a SiO2/CaO ratio of 0.9 was 1252 degrees C-1285 degrees C.