Abstract
A weathered and a recent sample of electric arc furnace dust (EAFD), generated in a southern Brazilian steel industry, were characterized by X-ray fluorescence spectroscopy (XFA), powder X-ray diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) probe and Fourier transform infrared spectroscopy (FTIR). A quantitative phase composition model, that accounts for the observed data and for the physico-chemical conditions of formation, was postulated for each material. One sample, in the form of a wet paste, was collected from the lowest part of a landfill and corresponds to a weathered material whereas the other sample was collected from the top portion of the landfill and corresponds to a recently produced material. The dominant cations present in both samples are iron, zinc and lead with minor amounts of manganese, calcium and silicon. The dominant mineralogical phases identified in both materials are Magnetite, Franklinite and Zincite. The recent sample has Laurionite whereas the weathered sample has Hydrocerussite and Hydrozincite.
Extracts
The steel plant under study is located in Paraná State and uses EAF process in which iron scrap is the main source of iron and pig iron is a secondary source. Oxygen is injected during the melting process and act on decarburization and on the burning of natural gas and coal. Dolomitic or calcitic lime is added to make slag. An extra energy input is obtained from the combustion of natural gas and pulverized coal. The electrodes are made of carbon and are slowly consumed in the process. The fused material in the EAF reaches temperatures of 1600 °C, the molten steel is then poured in a ladle furnace for refining. Lime, to make slag, and alloying elements are added in this stage. The molten refined steel is directed to a continuous molding system where the ingots are water cooled and cut. The EAF processing generates gases, volatile organic compounds, slag and particulates known as EAF dust or simply EAFD. The EAFD is directed to a bag filter system that is periodically washed with water generating a sludge. In past plant operations this wet sludge was placed in a hazardous waste landfill. Nowadays this sludge is directed to a pelletizer that considerably reduces the humidity and material volume. No chemicals are added to make the EAFD pellets that are being landfilled on top of the sludge. The estimated generation of EAFD in this plant is 9.6 thousand tonnes/ year.
According to Brazilian standards [3], the EAFD is classified as a dangerous residue due to the potential leaching of toxic metal ions. Its disposal in controlled landfills is the main form of final destination in Brazil.
Recent work has been dedicated to the characterization of EAFD from different steel plants with focus on the solid EAFD [4,5] or on its leaching products [6,7]. Various applications for the EAFD are been suggested such as the study of its vitrification product [8], the recovery of zinc and iron [9–16], soil fertilization [17], recycling to the EAF [18–21], incorporation in cement [22,23] and incorporation in glass [24].
According to a recent study [25] the EAFD formation takes place in two steps: first, the emission of dust “precursors” (vapors, metal droplets, and solid particles) inside the furnace; second, the conversion of those precursors into dust by agglomeration and physico-chemical transformations. Out of the five possible emission mechanisms evaluated the two most important were found to be the projection of fine metal droplets by bursting of CO bubbles (coming from the decarburization of the steel bath) and volatilization at the hot spots in the arc zone, in the oxygen jet zone and in the CO bubbles. The direct fly-off of solid particles from the EAF feed, such as coal powder and lime powder depend on operational conditions and may even be absent in optimized furnaces. The projection of metal droplets at the impact points of the arc and at the impact points of the oxygen jet were found to be an unlikely mechanism because the large particles projected return to the molten bath. The so generated air-borne precursors can undergo physical transformations in their way out of the EAF and into the filtration system such as condensation of the vapors, rapid solidification of the fine metal droplets, in-flight agglomeration and coalescence of dust particles [25].
In the present work a recent and a weathered EAFD sample were collected, from a landfill of a southern Brazilian steel plant, and were characterized by means of an integrated approach, developed by our research team, that takes into account diffractometry, electron microscopy, spectroscopy, thermal analysis and the physico-chemical conditions of residue formation. This integrated approach has recently been applied to various types of industrial residues [26] including four residues of a pulp and paper plant [27]. The authors hope that this methodology can be useful to help finding industrial uses for this material and to improve EAF process control. This is the first work, to the authors’ knowledge, to characterize a recent and a weathered EAFD sample generated in the same steel mill. ...
2.1. Sampling
Sampling of the two residues was performed according to a Brazilian standard [28]. The weathered material, representative of past operations of the industry, was collected from the lowest part of a landfill, whereas the recently generated material was collected from the topmost part of the landfill.
3. Results and discussion
3.1. EAFD characterization
3.1.1. Elemental analysis
The elemental chemical composition of the weathered and recent EAFD samples can be seen in Table 1. The weathered sample and the recent sample have Fe, Zn and Pb as the major electropositive elements, intermediate amounts of Mn, Ca and Si and trace amounts of other elements. A comparison between EAFD from different origins reveals a significant variation in the elemental composition. The EAFD samples studied here have Fe, Zn, Pb, Mn, Ca, Cr and Ni percentages close to those related by Leclerc et al. [14], whereas the Fe percentage in the weathered sample is close to that reported by Sofilic et al. [4], the percentages of Zn in both samples are close to that reported by Yamada and Hara [10]. The percentage of Pb in both samples agrees with that reported by Sekula et al. [11]. These variations in compositions are mainly due to the different types of iron and steel scrap consumed in the EAF, to the type of steel being produced and to particular operations performed during the steel production. The EAFD zinc originates form the galvanized iron scrap, lead comes from the paint present in the scrap pieces, manganese, chromium, silicon, nickel, phosphorus and titanium are present in steel alloys, chromium may also come from metalized steel pieces, calcium, magnesium, barium, potassium and strontium originate from the calcitic and dolomitic lime used in the EAF, copper comes from wiring mixed with the scrap and tin comes from solder. Al is commonly present in the zinc layer of galvanized iron and steel. In the weathered EAFD Al may also come from environmental clay contamination. ...
Conclusions
A recent and a weathered sample of EAFD, generated in the same steel mill, were characterized by means of an integrated approach and a mineral phase composition model was estimated for each sample. The major chemical composition of the weathered and recent EAFD samples are similar being Fe, Zn and Pb the most dominant electropositive elements.The major phases present in both samples are also similar being spinels (Franklinite and Magnetite) and Zincite the most dominant ones. The recent sample has Laurionite as the main lead bearing phase. The weathered material has two hydroxicarbonates, Hydrozincite and Hydrocerussite, typical weathering products of zinc and lead. The pH of the samples is compatible with the presence of the assigned phases. Clay and some type of ester (or polyester) are also part of the weathered sample. Calcination of the samples, at 1000 .C for 3 h, promotes the decomposition of the hydroxicarbonates into oxides, the sublimation of lead oxide, the oxidation of iron(II) and the reaction between zincite and iron oxides to form Franklinite. The recent sample displays agglomerates of spherulitic granules being the submicrometric ones composed of Franklinite and the micrometric ones composed of Magnetite. The observed lead bearing phase in the electron micrographies of recent sample is consistent with the proposed precipitation of Laurionite. In the weathered sample the granules are completely loose and well mixed as told from the electron micrographies. During the course of the present work it was realized that the thermal treatment of EAFD may be a useful source of synthetic Franklinite, a rare mineral, whose applications in materials science are appealing. The prospect use of this EAFD as a source of synthetic Franklinite is currently being evaluated. ...
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