A study on the preparation and characterization of pigment quality from mill scale steel wastes

Every day, significant quantities of solid wastes are generated in steel companies which causes environment pollution. These waste materials differ from one steel plant to other depending upon the steelmaking processes adopted and pollution control equipment installed. The most common type of solid wastes originated in steel plant are hot metal pretreatment slag, dust, GCP sludge, mill scale, scrap, etc. At present, various efforts and experiments are being carried out in order to make use of 100% solid wastes products in order to reducing cost of disposal, saving raw materials and conserving energy. Then, the purpose of our paper is the access to reuse potential of the steel abundant mill scale for sustainable industrial applications. This material is a very valuable industrial waste due to its richness in iron (about 72% Fe), chemical stability, and variable usage in multiple fields which implies that it can bring social and environmental benefits. This work aims to recover Mill scale and reuse it to synthesize three iron oxide pigments; hematite (α-Fe₂O₃, red color), magnetite (Fe₃O₄, black color), and maghemite (γ-Fe₂O₃, brown color). To achieve this objective, Mill scale needs to be refined and granted to react with sulfuric acid to obtain ferrous sulfate FeSO₄.xH₂O which is used to produce hematite by calcination between 600 and 900 °C, then, magnetite by reduction of hematite using a reducing agent at 400 °C and maghemite from a thermal treatment of magnetite at 200 °C. It was shown within the experiments that the mill scale contains between 75% and 86.66% of Fe and a uniform distribution of particles size with a low span. That gave red particles size (0.18-0.193 µm) and specific surface area SSA: 6.12 m²/g, black particles sizes (0.2-0.3 µm) and SSA 4.92 m²/g, brown particles size (0.18-0.189 µm) and SSA 6.32 m²/g. The results revealed that the mill scale was successfully converted into pigments with good qualities. So, it is recommended to work from the beginning at synthesis hematite by copperas red process and then magnetite and maghemite in order to control shape of magnetite and maghemite (spheroidal) to get the best results economically and environmentally.