Valorization of mill scale as a sustainable fuel source

Details

Supervisors

Jacques, Pascal ; Choisez, Laurine

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

This study explores the potential of mill scale, an industrial waste product, as a viable source of iron. By investigating the recycling and reduction processes of mill scale, the research aims to contribute to more sustainable energy production. Recently, iron powder has emerged as a promising sustainable fuel, as its combustion releases large amounts of heat without CO2 emissions. The resulting iron oxides can be reduced back to iron, forming a metal fuel cycle. Direct hydrogen reduction is studied for this second step and is also increasingly explored in the steel industry to lower CO2 emissions and energy consumption. The experimental analysis focused on reducing iron powder (mill scale and combusted pure iron powder) at different temperatures (400°C, 450°C, 500°C, and 660°C-700°C) with and without pre-oxidation (at 700°C, 800°C and 900°C) using a thermogravimetric analyzer and a fluidized bed reactor. Changes in microstructure, composition, and morphology were characterized using granulometry, inductively coupled plasma (ICP) analysis, x-ray diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopy (EDX) at different reduction stages. Key findings indicate that pre-oxidation significantly enhances the reduction rate of mill scale, with the fastest reduction achieved with pre-oxidation at 800°C. Below 570°C, the analyses highlighted the importance of wüstite decomposition into magnetite and ferrite. Kinetic analyses indicate an activation energy (Ea) of 106.79 kJ/mol with pre-oxidation (reduction of hematite to ferrite) and 74.81 kJ/mol without pre-oxidation (reduction of magnetite to ferrite). These reactions are assumed to occur in two steps. For pre-oxidized mill scale, hematite is reduced to magnetite (Ea=118.91kJ/mol) according to a phase boundary model, and magnetite is simultaneously reduced to ferrite (Ea=104.59 kJ/mol) following a nucleation and growth model. Without pre-oxidation, wüstite decomposes into magnetite and ferrite (Ea=65.84 kJ/mol) according to a phase boundary mechanism, while magnetite is simultaneously reduced to ferrite (Ea=71.32 kJ/mol) following a nucleation and growth model. Experiments above 570°C showed slower reduction kinetics at 700°C due to the formation of dense iron around unreduced wüstite islands; no kinetic analyses were performed at this temperature. Comparative assessments in the fluidized bed reactor demonstrated high reduction efficiencies for both mill scale and combusted iron powder, with near-complete reduction at 500°C. Nevertheless, combusted iron exhibited greater sintering.