Obtaining of hematite from industrial steel waste using dry-milling and high temperature

Abstract

In this work, the mill scale (MS) of hot rolling, a waste of steel processing, composed by a complex mixture of iron oxides, is transformed in a nanocrystalline powder containing only the hematite (α-Fe 2 O 3 ) phase. For that, the MS was dry-milled using a high-energy ball mill for 30 min and heat-treated up to 1200 °C for 25 min. The samples of this study were characterized by X-ray diffraction, Thermogravimetric Analysis and Vibrating Sample Magnetometry. The milling process decreases the amount of FeO phase content (FeO:Fe 3 O 4 phase ratio from 3:1 to 1:1), reduces the average crystallite size and increases the micro-strain of the iron oxides. The high-temperature experiments showed the complete transformation of the Fe 3 O 4 and FeO nanophases to the nanocrystalline α-Fe 2 O 3 with average crystalline domains greater than 180 nm after few hours of heat treatment. The thermal expansion coefficient of the α-Fe 2 O 3 nanophase was obtained considering anisotropic effects, mainly for the a -axis. Thermogravimetric analysis shows the oxidation of Fe 3 O 4 to α-Fe 2 O 3 . The milled MS sample shows a mixed magnetism that transforms to a weak ferromagnetism (or antiferromagntism) after thermal treatment. The temperature of the Morin transition of the α-Fe 2 O 3 nanophase at 110 K and a second magnetic transition at 50 K, related to a spin-glass state, seems to be strongly affected by the nanometric size of crystalline domains (about 180 nm).