Recycling of Steel Grinding Swarf via Production of Iron Chloride Coagulants for Water Treatment

Licentiate thesis, 2024

Grinding swarf is a hazardous waste generated in the manufacturing industry when making products out of iron and steel. Annually 10-12 million tons are generated worldwide, and these numbers are projected to grow with steel production and industrialization of developing countries. Waste management of grinding swarf is challenging because of its low value, heterogeneity, flammability and small quantities produced in many workshops and as such, landfilling is today widespread. The swarf is usually contaminated by lubricant oils which classifies it as hazardous waste and requires specialized landfills for disposal. This not only makes current waste management expensive and unsustainable, but also forms an environmental threat from leached metals and lubricants ending up in soil and groundwater.

Since steel grinding swarf predominantly consists of iron, the aim of this thesis was to investigate whether it could be used as raw material in production of iron based coagulants for water treatment. Hydrometallurgy was used to extract iron from swarf by leaching with hydrochloric acid and separating the formed iron chloride solution from metal impurities by precipitation and filtration. Each step of the process was designed and optimized to find simple yet flexible solutions to make the recycling process economically feasible, and capable of handling variations in the grinding swarf.

A two-step method was proposed based on completely dissolving iron in the swarf at 60°C and pH ≤2 for 3 h, and thereafter reducing the acid concentration to pH 4 for 1 h to precipitate any leached impurities. This guarantees complete separation of Al, Cr and Mo from the solution by hydrolysis, and can to a large extent also remove Co, Cu and Ni by cementation. Lubricating oils are flocculated by solids in the slurry and can therefore also be removed by filtration.

A 3.8 M FeCl2 solution was produced which conformed to the highest purity standards for drinking water coagulants and was considered to be a good precursor for FeCl3 production. Around 30 kg hydrogen gas are also generated per ton of swarf recycled which can provide additional value in recycling if captured. Besides these products, the process also generated solid leaching residues which were determined to have potential application in high alloy steel production.