Effect of Borax-, KOH-, and NaOH-Treated Coal on Reducing Carbon Waste and Activated Carbon Production in Synthetic Rutile Production from Ilmenite

Coal is commonly used as both fuel and reducing agent in producing synthetic rutile from ilmenite (FeTiO3) via the Becher process, which upgrades ilmenite to high-purity TiO2 (>88%). However, coal-based reduction generates significant carbon waste. This study investigated the effect of adding 1–5% w/w potassium hydroxide (KOH), sodium hydroxide (NaOH), and sodium tetraborate (borax) to coal during ilmenite reduction to improve metallisation and reduce carbon burn-off. Results showed that 1% w/w additives significantly increased metallisation to 96% (KOH), 95% (NaOH), and 93% (borax), compared to 80% without additives, while higher concentrations (3–5% w/w) decreased metallisation. Scanning electron microscopy (SEM)analysis showed cleaner particle surfaces and optimal metallisation at 1% w/w, whereas higher additive levels caused agglomeration or sintering due to elevated silica and alumina activity. Additive type also influenced TiO2 quality, with KOH enhancing TiO2 at low concentrations but causing negative effects at higher levels, while NaOH and borax reduced TiO2 quality via sodium-based compound formation. All additives reduced carbon burn-off, with KOH producing the greatest reduction. The iodine number of the carbon residue increased with higher additive concentrations, with KOH achieving 710 mg/g at 1% w/w and 900 mg/g at 5% w/w, making the residue suitable for water treatment. Overall, KOH is the most effective additive for producing high-quality synthetic rutile while minimising carbon waste.