Optimization of magnetite recovery circuits in a navigation coal-washing plant challenged with scaling due to gypsum formation

¹ Senior Plant Metallurgist, Khwezela Colliery, Thungela Resources, South Africa
² Metallurgical Manager, Khwezela Colliery, Thungela Resources, South Africa
³ Head of Coal Processing and Business Improvement, Thungela Resources, South Africa

Abstract

In 2023, the Navigation coal-washing plant recorded the highest magnetite consumption per tonne within the Thung ela group. With declining commodity prices, cost competitiveness remains critical for sustainability. In 2024, focus shifted to reducing magnetite consumption, improving plant efficiency, and optimising processing costs. Key interventions included optimising primary magnetic separators, improving spray-water pressure, and selecting appropriate screening media. These were supported by regular pressure-jetting, improved piping configurations, automatic flushing sprays, and maintaining consistent head-tank levels. As a result, magnetic separator efficiency increased from 95% to 99%, and magnetite adhesion losses dropped by 4.85 kg per screen tonne. Despite these gains, magnetite consumption remained 1.05 kg per feed-to-plant tonne above the 1.20 kg target. A key challenge is pipe scaling caused by gypsum formation, driven by acid mine drainage and excessive limestone dosing in water treatment. Reducing limestone use and introducing a scaling inhibitor are expected to curb sulphate-related reactions and improve recovery. Additionally, re-using water from the top of the active slimes compartment—without neutralisation—will further reduce pipe scaling. These efforts aim to enhance water quality and eliminate magnetite under-recovery, supporting long-term operational efficiency.