Kinetics study on the H2 reduction of Nchwaning manganese ore at elevated temperatures

Abstract

Replacing solid carbon with hydrogen gas in ferromanganese production presents a forward-thinking, sustainable solution to reducing the ferro-alloy industry’s carbon emissions. The HAlMan process, a groundbreaking and eco-friendly method, has been meticulously researched and scaled up from laboratory experiments to pilot tests, aiming to drastically cut CO2 emissions associated with ferromanganese production. This innovative process could potentially reduce CO2 emissions by about 1.5 tonnes for every tonne of ferromanganese produced. In this study, a lab-scale vertical thermogravimetric furnace was used to carry out the pre-reduction of Nchwaning manganese ore, where direct reduction occurred with H2 gas under controlled isothermal conditions at 700, 800, and 900°C. The results indicated that higher pre-reduction temperatures (800 and 900°C) effectively converted Fe2O3 to metallic iron and Mn2O3 to MnO. By continuously monitoring the mass changes during the reduction, both the rate and extent of reduction were assessed. A second-order reaction model was applied to validate the experimental outcomes of H2 reduction at various temperatures, showing apparent activation energies of 29.79 kJ/mol for dried ore and 61.71 kJ/mol for pre-calcined ore. The reduction kinetics displayed a strong dependence on temperature, with higher temperatures leading to quicker and more complete reductions. The kinetics analysis suggested that the chemical reaction at the gas–solid interface between hydrogen and the manganese ore is likely the rate-limiting step in this process.

1. Introduction

Manganese ferro-alloy is primarily produced by reducing manganese ores with carbon in a submerged arc furnace (SAF). In this procedure, metallurgical coke plays a dual role, acting as both the main source of energy and the reducing agent, with approximately 0.3 to 0.5 tonnes of coke required per tonne of the final alloy. This approach is highly demanding in terms of energy, requiring between 2000 and 3000 kWh for each tonne of metal, and results in the emission of 1 to 1.4 tonnes of CO2 for every tonne of metal produced [1–2]. Aligned with the Paris Agreement, there is a goal to decrease CO2 emissions by 45% by 2030, with a vision of achieving net-zero emissions by 2050 [3]. The Norwegian industrial sector have mapped out a plan, targeting the “integration of economic growth with attaining zero emissions by 2050” as a strategic pathway to fulfill their environmental objectives [4]. As a result, the development of novel sustainable manganese production processes is critical for mitigating future greenhouse gas emissions.

Studies have shown that manganese ores can be transformed into MnO using just H2 and CO gases. Research has explored how factors such as temperature, gas mixture, mineral properties, and particle size influence the reduction method. Additionally, it has been observed that adding H2 to the CO gas mixture improves the effectiveness of the reduction [5]. In the investigation conducted by Bruijn and his team (1980), particle sizes in the range of 0.0675 and 0.1275 mm were investigated. The team utilised both a shrinking-core model and a cracking-core model to study the two-step reduction of MnO2 to MnO using H2, with the intermediate formation of Mn3O4. The cracking-core model matched the data well, though it did not fully explain equilibrium conversions less than one [6]. Despite the variability in manganese ore compositions, research consistently reveals two to three separate phases in the reduction process across a range of ore samples [7]. For instance, when Mamatwan ore was reduced with graphite at temperatures between 1523 and 1623 K (1250 to 1350°C), three distinct reaction stages were noted for particles smaller than 300 µm. In contrast, at lower temperatures of 1373 to 1473 K (1100 to 1200°C), only two reaction stages were observed [7–8]. Ngoy et al. (2020) utilised a thermogravimetric (TG) furnace for non-isothermal testing to explore how hydrogen impacts the pre-reduction of Nchwaning and Comilog ores when combined with CO gas. The study involved treating the ores with CO/CO2 gas mixtures that included varying amounts of hydrogen. The results revealed that incorporating hydrogen into the CO/CO2 mix enhanced the reduction rate by 20 to 30 percent [5]. Barner and Mantell (1968) examined the reduction of synthetic MnO2 under varying partial pressures of H2 at temperatures between 200 and 500°C. Their research covered particles...