EFFECT OF BASICITY ON BASIC OXYGEN FURNACE (BOF) SLAG SOLIDIFICATION MICROSTRUCTURE AND MINERALOGY
Chunwei Liu1, Muxing Guo1, Lieven Pandelaers1, Bart Blanpain1, Shuigen Huang1
1 KU Leuven, Department of Materials Engineering, Kasteelpark Arenberg 44 bus 2450, 3001, Belgium
Keywords: BOF slag; Valorization; Basicity; Solidification Abstract
Slag valorization in added value construction applications can contribute substantially to the sustainability of steel industry. The present work aims to investigate the crystallization behavior of a typical industrial Basic Oxygen Furnace (BOF) slag (CaO-FeOx-SiO2-based slag) by varying the basicity through hot stage engineering. A sample of industry Basic Oxygen Slag (BOF) was mixed with different quantities of silica (SiO2) to modify basicity. The effect of basicity on solidification microstructure and mineralogy was studied. The results suggest that the mineralogy of the solidified slag can be manipulated to enhance its suitability as raw material for construction applications.
1. Introduction
In steelmaking industry, steel slags are a main by-product produced during the steelmaking process, accounting for 10-15wt% of crude steel production, depending on the steel quality and the production process [1]. Recycling of slags into added value applications is therefore of great importance for the sustainability of the steel industry. In recent years, the effect of different additions and cooling methods [2-5] has been studied with respect to crystallization behavior and phase modification, aiming to use steel slags in various fields, such as landfill liner [6], hydraulic binder [7, 8] and fertilizer used in agriculture [9].
Compared to the treatment of other steelmaking slags, such as EAF (electric arc furnace) slag, AOD (argon oxygen decarburization) slag, and LM (ladle metallurgy) slag, BOF (Basic Oxygen Furnace) slag is more difficult to treat due to its higher basicity. Yet, because BOF slags make up almost half of all steelmaking slags, their valorization is key, both from an environmental and economic perspective [10]. In the past decades, utilization of BOF slag was limited due to the volume expansion occurring upon natural aging. This dimensional instability is believed to be induced by hydration and carbonation of free lime and magnesia existing inside the slag [11].
During the chemical reaction, free lime and magnesia exhibit around 10% swelling [12], leading to the disintegration of the bulk slag. Recent research stressed that 4 wt% free lime is the suggested tolerance for incorporating BOF slag in construction materials [13].
In the present work, slags with different basicities were prepared and equilibrated at 1600°C under argon atmosphere, then cooled down slowly in the furnace. X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) were used for slag sample characterization. The results indicate that it is possible to optimize the slag mineralogy for subsequent use in construction materials by controlling the basicity during hot stage engineering.
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
2. Experimental 2.1. Materials preparation
A typical BOF slag produced from a steelmaking company was used as the master slag in this work. The slag was ground and milled to powder below 200 µm. The typical range of chemical [9] and mineral compositions of the master slag is given in Table I and Table II, respectively.
The basicity R (mass ratio of CaO/SiO2) of the master slag used in our study was measured by X- ray fluorescence (XRF, Panalytical PW2400) to be 4.39.
Table I. Chemical composition of the master slag, wt%
CaO *T.Fe SiO2 MnO MgO Al2O3 P2O5
42-55 14-20 12-18 0-8 0-5 0-3 0-2
*Fe is reported as the total iron in the oxides. Measured by X-ray fluorescence (XRF, Panalytical PW2400).
Table II. Mineral composition of the master slag Mineral phase Free
lime Monoxide Aluminateferrite Beta dicalcium
silicate Magnetite Chemical
formula CaO RO (FeO-MgO-
MnO)
2CaO·(Al,
Fe)2O3 β-C2S (Ca2SiO4) Fe3O4
wt% 18.0 5.0 32.3 29.4 15.3
In order to investigate the effect of basicity on the solidification microstructure and mineralogy, sub-samples of 100g of slag was wet-mixed with 5.0 g, 10.0 g, and 15.0 g of SiO2 (>99.90%) in ethanol utilizing a multidirectional mixer (Turbula type) to prepare three adjusted slag mixtures.
After 24-hour mixing, the mixture was dried in a rotating evaporator at 65°C. The resulting mixtures have a basicity of 2.94, 2.21 and 1.77, respectively.
2.2. Experimental procedure and characterization of slag samples
10 g of sub-samples of were placed in MgO crucibles (inner diameter 15 mm, height 40 mm) and melted in a graphite heating furnace (W100/150-2200-50 LAX, FCT Systeme, Rauenstein, Germany) in argon at 1600°C for 1 h to reach equilibrium, after which each sample was cooled to room temperature in the furnace at 5°C/min.
The cooled samples were polished and characterized using electron probe micro-analysis (FE- EPMA, JXA-8530F, JEOL Ltd, Japan) that allows fully quantitative chemical analysis. During analysis, the beam current and accelerating voltage were set at 15 nA and 15 kV, respectively.
Phase identification was determined by X-ray diffraction (XRD, 3003-TT, Seifert, Ahrensburg, Germany) using Cu Kα radiation at 40 kV and 40 mA. Quantitative analysis of the XRD pattern was achieved with the aid of the Rietveld refinement method.
3. Results and discussion 3.1 Microstructures of slags with different basicities
Figure 1 shows typical microstructures of slags with different basicities produced through furnace cooling under Ar atmosphere. The microstructure of the master slag, without additional SiO2 addition, is shown in Figure 1(a). This master slag contains approximately 60 µm sized free lime. With SiO2 addition and therefore decreasing basicity, the amount of free lime is decreased.
In all cases, dicalcium silicate (C2S) and aluminateferrite (C2AF) are observed as the major phases except for the sample with a basicity of 1.77. Chemical analysis indicates that C2S typically contains P2O5 (>2 wt%), which agrees with Rubio’s study [14]. According to previous studies, 0.5 wt% P2O5 is enough to stabilize β-C2S which plays an important role for cement production [15]. Another major phase is monoxide solid solution (RO). The RO phase is a FeO- based monoxide containing MgO, MnO and CaO. There are two typical morphologies of RO:
one has tiny crystals (<10 µm), dispersed in C2S and free lime; the other one has larger (>20 µm) separate grains. At a basicity of 1.77, C2AF phase disappears and also the C2S phase can be hardly found. Instead, a substantial amount of bredigite is formed. The chemical composition of bredigite (Ca1.75Mg0.25SiO4) is very close to C2S (Ca2SiO4) in which some CaO is substituted by MgO.
Figure 1. Microstructures of slags with different basicities R produced through furnace cooling in Ar. (a) R=4.39 (master slag); (b) R=2.94; (c) R=2.21; (d) R=1.77. Scale bar is 10 µm.
The elemental distribution of the slag at a basicity of 4.39 and 1.77 is shown in Figure 2. In the case of R=4.39, Ca exists in free lime and C2S,while for R=1.77, Ca is present in both C2S and bredigite, with the concentration of Ca being slightly higher in the former, as expected from the stoichiometric formulas for C2S (Ca2SiO4) and bredigite (Ca1.75Mg0.25SiO4). In terms of Mg distribution, in the case of R=4.39, it is present in RO and C2S. In the case of R=1.77, most Mg combines with Fe and Mn and forms the monoxide (RO) phase, whereas some Mg is present in bredigite and the rest stays in the C2S phase. It is interesting that no free MgO exists in the slag.
WDS analysis reveals that in the RO phase the content of MgO in the case of R=4.39 is 26.91 wt%, but decreases to 21.28 wt% for R=1.77, because bredigite also combines MgO in the latter case. Whatever the basicity, most P dissolves in C2S. Some P is in bredigite when R=1.77. For PO bearing slag, [PO]3- substitutes for [SiO]4− during solidification, yielding a modified
formula Ca2SiO4·Ca3PiO4 [16, 17], in brief C2S·C3P. According to the composition, it can be concluded that only CaO contributes to the consumption of P. The lower concentration of P in bredigite compared to C2S may be due to the lower CaO/SiO2 ratio in bredigite. Because in bredigite it is possible that the CaO content is insufficient to consume P after it combines with SiO2. Therefore, the phosphorus is less concentrated in bredigite than that in C2S phase.
Figure 2. Elemental distribution in the slag (a) without SiO2 addition (R=4.39); with 15 wt%
SiO2 addition (R=1.77) 3.2 Quantitative estimation of minerals content in the slag
The results of the quantitative analysis of the XRD patterns are presented in Figure 3. The high basicity industrial slag has 20 wt% free lime. With the aid of SiO2 addition, the free lime content can be significantly decreased, as indicated by the solid line. When 10 and 15 wt% SiO2 were added to the slag (corresponding to R=2.21 and R=1.77 respectively), free lime is completely eliminated. At the same time, the amount of RO and β-C2S is increased. In contrast, C2AF is decreased by increasing SiO2, probably because SiO2 combines with more CaO forming more β- C2S. For the case of 15 wt% SiO2 addition (R=1.77), bredigite becomes a major phase and accordingly the amount of β-C2S is significantly decreased. This trend shows good agreement with our microstructural analysis. In the current study, the amount of β-C2S increases to a maximum of 44.4 wt% for 10 wt% SiO2 addition (R=2.21). At this basicity there is also 15.7 wt%
of C2AF. The stabilized β-C2S and C2AF phases play an important role in preparing cement products, implying a potential valorization direction [18]. The bredigite phase, however, makes little contribution for the cement properties due to its poor hydration behavior and should be avoided for preparing cement [2].
Figure 3. Quantitative XRD analysis of slags with different basicities (R), synthesized through furnace cooling in Ar. The solid line indicates the free lime content.
4. Conclusions
The effect of basicity on BOF slag solidification was studied by lab scale experiments.
Stabilization of free lime was demonstrated by lowering the slag basicity with SiO2 additions to high basicity BOF slag (R > 4). Meanwhile, the content of the β-C2S was increased by decreasing the basicity to an appropriate range. When the basicity is too low (R=1.77 for current case), a large amount of bredigite phase is formed, incorporating some MgO. In the present study, the total amount of β-C2S and C2AF phases reaches 60.1 wt% when reducing the basicity to 2.21, showing a potential application in cement production.
Acknowledgement
The authors acknowledge the support from IWT grant 140514. Chunwei Liu acknowledges the support of China Scholarship Council (CSC).
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