Sustainable Ilmenite Upgrading via Biochar and Sulfur Roasting

Researchers have presented a comprehensive thermodynamic and kinetic analysis of ilmenite roasting, a critical step in producing titanium-based materials. The study, published in the journal Discover Minerals, investigates the combined use of sulfurization and carbothermic reduction to upgrade low-grade ilmenite ore. By uniquely employing coconut-shell-derived biochar as a renewable carbon source, the work aims to address the growing global demand for titanium while reducing the environmental footprint of traditional processing methods. The findings provide fundamental insights into the stability regions of iron sulfides and titanium oxides, offering a theoretical framework for developing more sustainable industrial routes.

The primary objective of these high-temperature reduction techniques is to improve the efficiency of titanium recovery by facilitating the separation of titanium oxides from iron oxides and other impurities. Ilmenite, a titanium-iron oxide mineral, is a primary source of titanium, which is essential for industries ranging from paint and plastics to aerospace and green energy applications. However, the scarcity of naturally occurring high-grade rutile resources has made the processing of low-grade minerals imperative. Traditional carbothermic reduction, a solid-state process evolved since the late nineteenth century, often relies on petroleum-based carbon sources, which are less sustainable than biomass-derived alternatives.

Thermodynamic modeling using the FactSage 8.3 software identified the stability regions of various phases during the roasting process. The analysis revealed that carbothermic reactions become spontaneous above 500–600 °C, with Gibbs free energy (ΔG) values of −116.3, −52.3, and −64.1 kJ/mol at 1000 °C for specific reaction pathways. In contrast, sulfurization reduction was found to be thermodynamically feasible throughout the entire 0–1500 °C range, with a ΔG of −201.6 kJ/mol at 800 °C. This significant thermodynamic driving force suggests that sulfurization could potentially operate at lower temperatures, leading to reduced energy consumption and lower greenhouse gas emissions compared to traditional carbon-based methods.

Experimental roasting and Rietveld phase analysis were conducted to quantify the evolution of Fe- and Ti-bearing phases. The study utilized ilmenite concentrate from the Pulmoddai heavy minerals deposit in Sri Lanka, which was processed through gravity and wet-magnetic separation. The feed material was mixed with commercially available sulfur and carbonized coconut shells in an optimal mass ratio of 4:1. These mixtures were subjected to high-energy ball milling to enhance surface area and reactivity, followed by roasting in a muffle furnace at 800 °C for sulfurization and 1000 °C for carbothermic reduction. The use of coconut-shell-derived biochar was selected due to its high fixed carbon content, low ash percentage, and renewable origin, making it a cost-effective and sustainable alternative to fossil-based reductants.

Phase transformation analysis revealed distinct differences between the sulfurization and carbothermic routes. During ball milling with sulfur, new mineral phases such as brookite, pseudorutile, and pyrite emerged, accompanied by a decrease in ilmenite peak intensity. This indicates the initiation of the sulfurization reduction reaction even at ambient conditions, facilitated by the lower kinetic barrier of sulfur. In contrast, carbon-treated samples showed no formation of new crystalline phases during milling, only a slight reduction in ilmenite peak intensity, suggesting potential dissolution of carbon into the ilmenite structure. Upon roasting, the pyrite peaks in sulfur-treated samples disappeared, indicating decomposition into pyrrhotite and elemental sulfur, while carbon-treated samples formed rutile, pseudorutile, and pseudobrookite phases.

The kinetic analysis of the roasting process identified temperature-dependent rate-controlling steps. The study employed the model-fitting method to correlate experimental kinetic data with theoretical models, determining that the reactions typically follow chemical reaction-controlled kinetics governed by the shrinking-core model. However, the control mechanism can vary between interfacial chemical reaction and diffusion of reductant through micro-cracks, depending on the solid-to-reductant ratio. The activation energy for carbothermic reduction was estimated to be lower than previously reported experimental values, suggesting that the complexity of ilmenite chemistry and impurities like manganese, silica, and magnesia can hinder reaction kinetics. These impurities can concentrate around reaction interfaces, creating barriers that decrease the efficiency of iron reduction.

Despite the promising thermodynamic and kinetic insights, the study acknowledges several challenges for industrial application. Scaling up from laboratory to pilot or industrial scale may introduce variations in heat and mass transfer, particle size distributions, and reaction kinetics, which could affect process efficiency and reproducibility. Effective impurity management strategies are necessary to address the accumulation of Mn, SiO2, and MgO at reaction interfaces. Additionally, practical considerations such as feedstock availability, cost, energy consumption, and environmental compliance must be addressed to ensure the economic viability of these methods. The potential for secondary emissions of SO2 during sulfurization can be mitigated by converting them to sulfuric acid, which can be used in downstream leaching processes, aligning with circular economy principles.

The use of agricultural waste-derived biochar addresses both cost and carbon intensity concerns, offering a sustainable pathway for titanium upgrading. The sulfurization route provides opportunities for reduced CO2 emissions and improved selectivity in iron removal, which is crucial for producing high-purity titanium dioxide. As global demand for titanium continues to rise in sectors such as aerospace, pigments, and renewable energy, the insights generated by this study offer a blueprint for optimizing reduction processes for low-grade ores. The integration of thermodynamic modeling, phase transformation analysis, and kinetic evaluation provides a holistic understanding of the roasting process, guiding the development of energy-efficient and low-emission ilmenite upgrading technologies.

Future research should focus on pilot-scale validation, impurity management, and sulfur recovery and recycling strategies to fully assess the techno-economic feasibility of industrial-scale deployment. The study highlights the need for further optimization of process parameters and the development of robust impurity removal techniques to enhance the quality of the final titanium products. By leveraging renewable carbon sources and sulfurization, the industry can move towards more sustainable and cost-effective titanium production, meeting the demands of modern technology while minimizing environmental impact. This work represents a significant step forward in the metallurgical processing of titanium ores, providing a theoretical and experimental foundation for future innovations in the field.

Subject of Research: Thermodynamics, phase transformations, and theoretical perspective of roasting kinetics during sulfurization and carbothermic reduction of Ilmenite

Article Title: Thermodynamics, phase transformations, and theoretical perspective of roasting kinetics during sulfurization and carbothermic reduction of Ilmenite

Article References: Subasinghe, H. C. S., & Ratnayake, A. S. (2025). Thermodynamics, phase transformations, and theoretical perspective of roasting kinetics during sulfurization and carbothermic reduction of Ilmenite. Discover Minerals, 2(1), Article 5. https://doi.org/10.1007/s44346-025-00010-7

Image Credits: AI Generated

DOI: 10.1007/s44346-025-00010-7

Keywords: Thermodynamics, phase, transformations, theoretical, perspective, roasting, kinetics, during, sulfurization, carbothermic, reduction, Ilmenite