To cut down the intensive \(\text {CO}_2\) emissions of the steel industry, hydrogen plasma smelting reduction (HPSR) has been suggested as a new method for ore-based steelmaking. Previous research has shown the usability of optical emission spectroscopy (OES) to monitor the process in real time. This study focuses on the connection between the chemical composition of the HPSR-processed hematite leach residue—a sidestream material—and the OES data. A theoretical metallization degree of 87 pct was achieved after 10 minutes of processing with Ar-10-pct \(\text {H}_2\) plasma, but because of evaporation, the actual amount of Fe obtained is lower. A wüstite phase and two spinel phases resembling hercynite ( \(\text {FeAl}_2\text {O}_4\) ) and chromite ( \(\text {FeCr}_2\text {O}_4\) ) were distinguished from the slag. The emission line intensities and, therefore, the evaporation of the elements from the slag mostly increased with time, showing the enrichment of these elements as Fe reduced. Aluminum evaporation increased most notably, indicating its potential in estimating the state of the reduction process. The results support the suitability of hematite leach residue for HPSR and show how the optical intensities are connected to changes in chemical composition, which can potentially be utilized when scaling up the process.
