Pre-reduction of chromite and manganese ores by green hydrogen

Abstract

The production of ferroalloys, such as ferromanganese (FeMn) and ferrochrome (FeCr), is crucial for the steelmaking industry, providing essential properties like corrosion resistance, hardness, and strength to steel used in construction, automotive, and aerospace sectors. Traditionally, FeMn and FeCr are produced through carbothermic reduction of manganese (Mn) and chromite ores, respectively, a process that is highly energy-intensive and generates significant greenhouse gas (GHG). This reliance on carbon-based reductants contributes to climate change and environmental degradation. One promising approach is the use of green hydrogen (H2), which is produced from renewable energy sources such as wind, solar, or hydropower using water. Green H2 offers an alternative to traditional methods, potentially lowering gaseous carbon emissions and reducing the overall carbon footprint of ferroalloy production. This dissertation demonstrates the effectiveness of H2 as a pre-reductant in Mn and chromite ores processing. The effects of different gas compositions and temperatures on Mn ore reduction were experimentally investigated in Chapter 3. The findings revealed that CO/CO2 and H2/H2O atmospheres reduced higher Mn- and iron (Fe) -oxides, while pure H2 effectively reduced Fe2+ to its metallic state. The main difference between these atmospheres is the oxygen partial pressure (pO2) profiles, where the CO/CO2 and H2/H2O atmospheres have similar pO2 profiles, unlike the pure H2 atmosphere, which has a significantly lower pO2 profile. Carbonates decomposed more rapidly at lower temperatures in H2-containing atmospheres compared to CO/CO2. Increasing temperature, using H2-containing atmospheres, and lowering pO2 enhanced the rate of mass loss. Although decrepitation and porosity showed minimal variation across different conditions, pure H2 significantly improved pre-reduction efficiency by accelerating carbonate decomposition and Feoxide metallisation. Considering the occurrence of the Boudouard reaction, pre-reduction using H2 not only metallised Fe-oxides to their metallic state and higher Mn-oxides to Mn2+ but also ensured that unwanted carbon dioxide (CO2) was not introduced to the smelting furnace. The effect of H2 pre-reduction on the carbothermic reducibility of chromite ore was experimentally investigated in Chapter 4. The findings revealed that H2 pre-reduction metallised 64.4 ± 6.1% of Fe-oxides, with minimal chromium (Cr) -metallisation. After H2 pre-reduction, metallic Fe was extracted from the ore through hot acid leaching, and the resultant residue was pelletised and subjected to carbon (C) -reduction. This process led to >99% Cr-metallisation, with mass loss increasing significantly at higher temperatures and longer reduction times, peaking at 27 mass% at 1300°C. The un-metallised Fe-oxides from H2 pre-reduction were also > 99% metallised during C-reduction at 1300 °C. The reduction followed a shrinking core model with an activation energy (Ea) of 126 kJ/mol. Electron microscopy revealed that reduction and leaching significantly alter particle size and structure. Thermal treatment leads to decrepitation, while leaching effectively removes the metallic shell, with Fe and Cr being leached up to the unreacted core. Mineralogical analysis confirmed the formation of metallic layers, with Fe and Cr phases removed by leaching, leaving a residual slag phase, comprised of MgAl2O4-type spinel. Four theoretical approaches using H2 and methane (CH4) as alternative reductants for chromite were reviewed in Chapter 5. Sustainability was assessed based on gaseous carbon monoxide (CO(g)) formation. The approach of pre-reducing chromite with H2 followed by primary reduction with CH4 showed the greatest drop in CO(g) formation, achieving a 75% decrease compared to the conventional method. Additionally, H2 and CH4 enable reduction at lower temperatures, reducing energy consumption. The review highlights the environmental benefits of replacing C with H2 and CH4 and discusses their potential industrial implementation. Economically, the use of so-called by-product H2 and CH4 were elaborated on, and the attractiveness of supplying them to the relevant smelters is explained.