Sustainable Recovery of Fe(II) Oxalate from Steel Industry Waste Using Leaching, Hydrothermal, and Photo-Reduction Routes

The iron and steel industries have continuously expanded in recent years. These manufacturing companies are one of the main contributors to the environmental problems due to their by-products such as slag and mill scale. These solid wastes from iron and steel industries consist of a high iron element. Hence, the recovery of iron from these steel wastes can reduce secondary pollution and can have an economic benefit. Fe(II) oxalate is a critical building block material for several applications such as lithium-ion batteries, photocatalysts, pigment, etc. This work proposes a green synthesis technology with near-zero waste for the preparation of Fe(II) oxalate from steel waste in Indonesia. The following steps of leaching, photochemical reduction, and hydrothermal methods were used to achieve the purpose. The leaching process of steel waste using oxalic acid was conducted to produce a yellow precipitate of Fe(II) oxalate and the greenish filtrate (Fe(III) oxalate aqueous solution) as a by-product. The optimum dose of steel waste in oxalic acid solution was found at 50 mg/mL. Subsequently, photochemical reduction and hydrothermal were utilized to produce another Fe(II) oxalate from the greenish filtrate with high purity of Fe(II) oxalate in the form of α -FeC 2 O 4 ⋅2H 2 O and β -FeC 2 O 4 ⋅2H 2 O. A rod-like microstructure of Fe(II) oxalate was found in various sizes, depending on the synthesis route. A major particle length in the range of 4–10 μm was found for the leaching and photoreduction routes, while a larger size with a length of 150–350 μm was found for the hydrothermal one. Our results indicate that different methods influenced α or β crystalline structure as well as the particle size of Fe(II) oxalate. Graphical Abstract