The preparation of magnetite, goethite, hematite and maghemite of pigment quality from mill scale iron waste
Introduction
Stainless steel finishing operations involve several cleaning processes, which eliminate dust, scale and metallic oxides [1]. Mill scale is a steel making by-product from steel hot rolling processes and is basically composed of iron oxides and metallic iron with variable oil and grease contents [2], [3]. Its specific production is about 35–40 kg/t of hot rolled product [2]. The oil component in rolling mill scale makes the recycling difficult, and its direct re-use in sintering may lead to environmental pollution. Mill scale with high oil content is recycled after extracting the oil in a pretreatment stage. Coarse scale with a particle size of 0.5–5 mm and oil content of less than 1% can be returned to the sinter strand without any pretreatment. High oil content (>3%) results in increased emission of volatile organic compounds including dioxins and can lead to problems in waste gas purification systems, e.g. glow fires in electrostatic precipitators. Because of this mill scale needs to be pretreated before it can be re-used. Fine sludge mainly consists of very small-scale particles (0.1 mm). Since the fine particles adsorb oil to a very high degree (5–20%) this scale normally cannot be returned to the sinter strand without pretreatment [4]. The oil adsorption in the preceding line refers to the metallic mill scale and should not be confused with the oil absorption, pigment property, mentioned in the abstract and elsewhere in this paper. At MITTAL (former ISCOR), a steel manufacturing company in the Republic of South Africa, the bulk of mill scale waste is dumped in landfills. The continuous demand for more landfills and the leaching of some small percentages of heavy metals into soil and ground water, thus threatening the environment, highlight the need for more effective methods of waste disposal and productive utilisation of mill scale.
Production of iron oxide pigments is one of the possible ways of alleviating the problem facing the steel industry in RSA since mill scale has a high iron content in the form of oxides and metal. The use of iron waste in iron oxide preparations is vital because of the increasing demand for iron oxide pigments driven by the increases in construction activities, current economic needs [5] in emerging markets and growing concern over the use of heavy metal-based pigments. The increasing importance of iron oxide pigments is also based on their non-toxicity, chemical stability, durability [6], wide variety of colours and low costs.
There are many studies in the literature that deal with different methods for the preparation of magnetite [7], [8], [9], [10], [11], hematite [12], [13], [14], [15], maghemite [16], [17], [18], [19], [20] and goethite [21], [22], [23]. These are the iron oxides commonly used as pigments giving black, red, brown and yellow colours, respectively. Steel pickling chemical waste (SPW) has been thermally decomposed at various temperatures to give red iron oxide (hematite) [9], [24], [25]. The formation of mill scale (mainly FeO and Fe) can also be accompanied by the precipitation of corrosion product mixture, viz. Fe3O4, FeOOH, Fe2O3, etc. [26]. FeO is usually closest to the metal surface while Fe2O3 forms the outer layer [27]. Since corrosion products occur in a mixture and the overall mill scale is hardened and of poor colour, it is not of pigment value. Oulsnam and Erasmus [28] have succeeded in preparing magnetite from ferrous mill scale using a dry oxidation step. However, the particles of their product were too large and had to be ground (wet and dry) to <10 μm to improve the pigment qualities (colour, tinting strength, hiding power and oil absorption) [24], [25]. Hematite was prepared by calcination of the obtained magnetite and its particles also had to be ground to sizes < 10 μm.
The present study was undertaken with the aim of preparing magnetite and goethite of pigment particle size < 10 μm via water-soluble mill scale-derived precursors. Furthermore, maghemite and hematite could then be prepared by thermal treatment of the obtained magnetite and goethite, respectively.
Section snippets
Ferrous precursor
Conc. H2SO4 (analytical reagent, 300 ml) was added to 60 g of raw mill scale in a 600 ml glass beaker. The mixture when heated on a hot plate became turbid. The turbid mixture was further heated to dryness. The resulting muddy solid product was then used as the starting material for the preparation of magnetite and goethite. Preliminary investigations showed that the product was soluble in water (more readily in warm water) and a dark blue/green flaky sediment resulted when the aqueous solution
Ferrous and ferric precursor
Table 1, Table 2 give the elemental compositions of ferrous and ferric mill scale as obtained from XRF analysis. The elemental compositions show that the percentage of iron increased significantly while the amounts of other elements decreased. This means that the acid digestion of raw mill scale increases the content of iron in the products formed. The high loss on ignition (LOI) value (over 50%) may be due to the high sulphate content, which was given off as SO2 during the calcination that
Conclusion
This study has shown that it is possible to prepare magnetite (black), hematite (red), goethite (yellow) and maghemite (brown) pigments of acceptable purity and with good morphological properties (i.e. particle size, shape, colour and surface area) from mill scale iron waste through simple and cost effective methods. The formation of iron oxide precursors (sulphate-containing compounds with iron as Fe2+ in one case and Fe3+ in another) has facilitated the precipitation of both magnetite and
Acknowledgement
The financial support by the National Research Foundation in Pretoria and the University of Pretoria is gratefully acknowledged. The authors thank the MITTAL STEEL, Pretoria, for the supply of mill scale iron waste.
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