Thomas Ottink,

Almost 2 billion tons of steel are produced annually, accounting for 7.2% of global greenhouse gas emissions, and recycling plays a crucial role in reducing this impact by conserving energy and materials. Grinding sludge is a hazardous metal waste generated during machining processes throughout the steel value chain. Due to limited recycling options, it is widely incinerated and landfilled today, representing a significant environmental and economic burden for its producers, with disposal fees up to 1000 €/ton. New waste management strategies are needed, and given that the sludges contain 50-80 wt% iron, an open-loop hydrometallurgical recycling process to produce iron-based chemicals was developed in this work. It was shown that >95% of the iron could be recovered as ferrous (FeCl₂) or ferric (FeCl₃) chloride solutions, which are vital in water treatment and other sustainability-related applications. This was achieved by first washing the sludges with a solvent to recover lubricant oils and removing abrasive wheel materials by magnetic separation. Cleaned swarf was then leached with hydrochloric acid (HCl) or FeCl₃ to convert iron into soluble FeCl₂. Codissolved impurities, including chromium, copper, nickel and residual lubricant oils, were removed directly from the slurry with grinding swarf as a precipitation agent, and made separable from the solution by filtration. Purified FeCl₂ solutions were converted to FeCl₃ via pressure oxidation with oxygen or electrolysis, with the coproduction of hydrogen gas (H₂), yielding a solution of 36 wt% FeCl₃ that met drinking water coagulant purity standards. Further assessments showed that the recycled product performed comparably to a commercial 40 wt% FeCl₃ solution in water treatment and synthesis of iron phosphate for lithium-ion batteries. A net profit of 520 €/ton sludge recycled was estimated, with FeCl₃ and H₂ representing 71% and 8% of the process revenue streams. This can generate significant income, serving as a stable foundation for grinding sludge recycling.