The challenge

PhD student Phillip Drain is part of a Research Team within the Steel Research Hub whose area of research focuses on understanding the effects of titanium (as [Ti] and TiO2) in the Basic Oxygen Steelmaking (BOS) charge on the removal of phosphorus (P) from the steel. The utilisation of low-cost ores, such as New Zealand ironsands, with higher Ti content has in part prompted this investigation.  As well, P in steel is traditionally controlled by selecting low P iron ores; however, P content in ores is increasing globally and low P ores are attracting premium prices.  Lower cost ores also tend to be high in other impurities.

A specific focus in this research will be to separate the effects of Ti on the physical properties of the slag and the chemistry of phosphorous transfer from steel to slag.  The reaction is difficult to control due to limited understanding of the reaction kinetics, and in particular the reaction area is difficult to define.

A key question to be addressed in this work is whether it is possible to distinguish the effects of Ti on the prevailing partial pressure of oxygen for de-phosphorisation, from the effects of Ti-based oxides on the slag properties and hence, on de-phosphorisation.

The approach

 Three approaches were used in this project:

  • Equilibrium slag/metal partition experiments to determine the thermodynamic effect of [Ti] and TiO2 on the dephosphorisation reaction.
  • Metal droplet quench experiments to determine the effect of TiO2 on the kinetics of the dephosphorisation reaction.
  • Analysis of industrial data using published LP equations to determine any observable correlation between TiO2 and other minor oxides (Al2O3 and V2O5) on the dephosphorisation reaction.

The outcome

[Ti] and TiO2 were both found to inhibit the dephosphorisation reaction at equilibrium by decreasing the slag basicity. [Ti] was also found to decrease the pO2 further inhibiting the dephosphorisation reaction.  However, the TiO2 was observed to increase the dephosphorisation rate in the metal droplet experiments. Given dephosphorisation is known to be a mixed slag/metal mass transport controlled reaction, the increased dephosphorisation rate was attributed to the formation of the Ti3+/Ti4+ pair improving the mass transport of oxygen in the slag. The analysis of plant data has allowed for the development of improved plant dephosphorisation models.

The impact

From a plant perspective, the thermodynamic and kinetic data obtained from high‐temperature experiments will be incorporated into a) an operational model that accurately predicts the steel’s final P composition enabling a reduction in overall heat‐to‐heat time and increased productivity of the existing plant, and b) a model to optimise flux usage, plant practices and incoming P loads, reducing the overall cost of converting iron ores into steel.

A new industrial P prediction model has already been implemented at Port Kembla Steelworks with an estimated saving of close to $500,000.p.a. (i.e. point a). The next step is implementation of the new flux model developed as part of this study (point b). Further development of these models to incorporate the kinetic data as well as experimental studies on the effect of TiO2 on BOS slag viscosity and confirmation of the formation of the Ti3+/Ti4+ pair in BOS slags may be avenues of future research.