Characterization and Reduction Behavior of Mill Scale

Abstract

This paper presents an initial part of a project devoted to the recycling of mill scale in the form of selfreducing briquettes. First chemical and morphological characteristics of mill scale were investigated and next its gaseous reduction behavior was studied by thermogravimetry. The chemical characterization showed that wustite is the major constituent of this waste matter, with small amounts of magnetite, hematite and metallic iron. The microscopic examination of the scale revealed its complex and layered microstructure with three distinct zones. The outer layer is relatively thin and porous. It is mainly composed of hematite and magnetite. The intermediate layer is made of the dense, columnar grains of wustite. The inner layer is a very porous wustite. The gaseous reduction by carbon monoxide has a topochemical character regardless of initial morphology of scale and, depending on temperature and reducing gas composition it produces a porous iron or the iron whiskers. The unreacted shrinking core model with one interface fits quite well the kinetic data and the activation energy of reduction is about 80 kJ/mol.

Carbon Gasification in Self-reducing Mixtures

“…The activation energy of the fine iron oxide reduction by CO reported by Moon and Sahajwalla varie from 36 to 75 kJ/mol. 14) Fortini and Fruehan 17) estimate the activation energy of wustite reduction by CO as high as 168 kJ/mol which is a bit higher then upper limit of the 60-150 kJ/mol range given by Coetze et al 19) In our previous work 3) on the reduction of the scale by CO, the activation energy of reaction rate constant was found to be about 80 kJ/mol. It was also observed that the gaseous reduction of scale occurs according to the shrinking core model under chemical control, i.e.…”

“…In fact, in our earlier study 3) on gaseous reduction of scale, two different morphologies of iron were observed: the porous iron, which prevailed in the samples reduced at higher temperatures (1 100°C and 1 200°C), and the iron whiskers formed in the scale samples reduced at lower temperatures (900°C and 1 000°C). It was thus interesting to examine and compare the morphology of iron produced with different solid carbonaceous materials, in order to identify the conditions in which the self-reducing reactions occur.…”

“…The content of reducible oxygen in the scale, calculated from both analysis and confirmed by gaseous reduction of scale, is 22.3 mass%. 3) As received, the scale is a mixture of coarse particles, mainly having a shape of platelets (20 mass% above 2 mm) and fines (20 mass% under 0.3 mm). As it is shown in Fig.…”

“…3) In the integrated steel plants, the mill scale is habitually used as raw material at the sintering plants but the recycling of this waste has also been reported in the form of briquettes used in BOF steelmaking. [4][5][6] In the case of mini-mills which operate using electric arc furnaces (EAF) and where there are no reduction reactors, such as a blast furnace, the scale recycling is more difficult and this waste is often sent to landfills or used in the cement industry.…”

“…Bearing this in mind, the scale used in this work was first characterized and its reduction behaviour in pure CO and CO-CO2 mixtures was investigated. 3) In the next step, which was the goal of this work, four carbonaceous materials was evaluated as the potential scale reducing agents for the self-reducing briquettes to use in the EAFs. For that gasification kinetics of each reductant by pure carbon dioxide was first measured and next the selfreduction tests were performed in order to determine the rate and controlling steps of process.…”