Ironmaking, for the steel industry, contributes ~7% of global CO2 emissions by utilising carbon fuels for process heat and the chemical reducing agent required for reduction of iron ores to metallic iron. Hydrogen (H2) ironmaking can greatly decrease these emissions when H2 is produced from renewable energy sources and used for chemical reduction. Further improvement of ironmaking sustainability and longevity relies on increased process efficiency and the origin and quality of iron ore resources used. Globally, mined iron ores are decreasing in purity, leading to the need for greater ore beneficiation or the potential valorisation of waste iron material, the latter being the focus of this research.

Iron’s high natural abundance makes it a common impurity in hydrometallurgical processes, and it requires removal through chemical precipitation, creating a voluminous, unstable, and often toxic precipitated iron residue (PIR) sludge. PIRs are generated on a large scale and often stored in waste ponds and landfills or, in countries with poor environmental regulations, disposed of without suitable environmental protection measures. Both cases create resource waste and potential environmental damage. To mitigate these impacts, there is increased motivation to utilise PIRs. PIR use in ironmaking is appealing due to ironmaking’s sufficiently large scale and growing demand for alternative high purity iron resources.

The present research evaluated the utilisation of PIRs in H2-ironmaking, specifically in the New Zealand (NZ) ironmaking context where pellets are assumed to be used in a vertical shaft furnace process. PIR was first synthesised and later pelletized alongside NZ titanomagnetite (TTM) ironsand with PIR blends of 2, 5, 10 and 20 wt.%, and individually (0 and 99 wt.% PIR). A binder was dosed into pellets at 1 wt.%. The respective physical and chemical properties of these materials and pellets where then evaluated before and during the laboratory-based ironmaking experiments, including pelleting, sintering and H2 reduction.

Results showed the 99 wt.% PIR pellets had significantly enhanced reduction kinetics compared to NZ TTM (0 wt.% PIR) after sintering at 1200°C. However, the 99 wt.% PIR pellets had decreased compressive strength and comprised considerable sulphur species, creating potential issues during industrial scale ironmaking and the subsequent steelmaking process. Therefore, it was concluded that blending of PIR with TTM was more suitable, with the addition of 5 wt.% PIR negating these issues. Pellets with 5 wt.% PIR demonstrated similar sintered compressive strength (compared with 0 wt.% PIR pellets), no increased sulphur content in DRI beyond practical levels for steelmaking, and an enhancement in H2 reduction kinetics was observed, with up to a 20% decrease in reduction time at higher temperatures. Additionally, the literature review estimated the potential global production of iron precipitates to be on the order of 100 Mtpa, sufficient for ~5 wt.% addition into existing ironmaking technologies or future H2-ironmaking. This potential blending would enhance resource valorisation and minimise harmful waste from hydrometallurgy, while providing the ironmaking industry with a viable alternative feedstock additive.