3.1. Reconstruction
Lime and bauxite were selected as tempering components to reconstruct the BOF slag at a high temperature of 1290 °C according to previous study [
20], in order to increase the content of C
2S and C
4AF as the main mineral components of BOF slag and reduce the content of the RO phase. The specific mixing ratio is shown in
Table 2.
The X-ray diffraction of reconstructed BOF slag (RS) and original BOF slag (OS) is shown in
Figure 1. The main mineral phases of BOF slag are α-dicalcium silicate (α-C
2S), β-dicalcium silicate (β-C
2S), dicalcium ferrite (C
2F), tetracalcium ferroaluminate (C
4AF), and other cementitious mineral phases, as well as non-gelling magnetite (Fe
3O
4) and the RO phase. It can be seen that the characteristic peaks of C
2S and C
4AF in the reconstructed BOF slag increase with the addition of tempering components, indicating that the addition of tempering components promotes the increase of the C
2S and C
4AF content. C
2S and C
4AF exhibit good hydration activity and grindability in cement clinker, and the properties of BOF slag will be improved with the increase of their contents. At the same time, the characteristic peak of the RO phase disappeared, and the characteristic C
4AF peak appeared. It shows that with the addition of tempering and tempering components, the RO phase in the steel slag is precipitated and decomposed; partly converted into MgFe
2O
4, and partly converted into iron phases.
Unmodified BOF slag and reconstructed BOF slag were ground for 10 min by vibrating mill, respectively, the SSA and particle distribution of BOF slag are tested, and the results are as follows. It can be seen from
Figure 2. that the small-size particles in the BOF slag increase, while the large-size particles are less after high-temperature reconstruction. The average particle size of the BOF slag is reduced from 51.75 μm to 39.96 μm as plotted in
Figure 2. Meanwhile, the SSA of the fine powders prepared by the reconstructed slag and the unmodified steel slag under the same grinding time was studied. It is found that the SSA of the reconstructed steel slag powder increased from 303.1 m
2/kg of the unmodified steel slag to 354.5 m
2/kg as shown in
Table 3, which fully shows that the grindability of the reconstructed steel slag modified by the calcium-aluminum composition is better than that of the unmodified converter steel slag.
As the XRD analysis mentions above, a mass of C
2S and other silicate phases with good grindability are newly formed, with the addition of the tempering components. Fe also transforms from a metal oxide solid solution-RO phase, and combines with calcium ferrite into a combined state of C
4AF. The content of calcium ferrite and the RO phase in BOF slag is reduced in the process of high-temperature reconstruction. The content of the calcium ferrite phase and the RO phase in BOF slag decrease in the process of high temperature modification, and these two phases are also the most difficult to grind from the perspective of mineral hardness [
12]. Therefore, the grindability of steel slag is improved after high-temperature reconstruction treatment due to the changes of these two factors.
3.2. SiC Foaming Modification
In the research of the previous chapter of this article, it can be found that the grindability of BOF slag is improved to a certain extent by high-temperature reconstruction, but it is still not ideal. This chapter describes a method by which a high-temperature foaming agent, SiC, is added to a high-temperature slag reconstruction process for further improvement of the grindability of slag from the structure. SiC begins to react at about 1000 °C, which is mainly the oxidation reaction as Equation (1), and the reaction continues with the increase in temperature. This reaction will result in a weight gain of the solid phase, and result in a weight increase of 50 wt % theoretically, when the SiC is completely converted to SiO
2. In practice, however, SiC begins to gain weight at around 1000 °C, but it increases slowly, and its weight gain is only 2.2 wt % at 1300 °C, which means that only 4.4% of SiC has undergone oxidation. Studies have found [
21] that SiO
2 formed by the oxidation of SiC will form a dense oxide layer on the surface of SiC, blocking the continued reaction of SiC.
In this chapter, 0.4%, 0.8%, 1.2%, 1.6%, and 2% of the SiC foaming agent micro powder is added to the basic ratio of BOF slag reconstruction to further improve the grindability of the BOF slag. The sintering temperature is set to 1290 °C, the holding time is set to 90 min, and the cooling system adopts air-cooling and rapid quenching. The specific configuration is shown in the
Table 4 below.
Since SiC is surrounded by silicate forming components or liquid phases under high-temperature alkaline molten salt conditions, the silica protective layer reacts with the alkaline molten salt to form a silicate liquid phase, causing corrosion or cracking of the protective layer. The rapid diffusion of oxygen through the protective layer causes the chemical reaction between SiC and oxygen to increase greatly, and a large amount of CO
2 and CO gas is generated [
20]. The oxidation was schematic as shown in
Figure 3. The generated gas was not discharged to the outside in time, remained in the test cake, and caused closed cells. In turn, the test cake was deformed, the pressure of the blocked pore gas was higher than the capillary pressure, and the test cake swelled as a result. The sintering process of this material is expressed by the following equation [
22], in which ε is porosity, η
s is the effective viscosity of the system, P
c is the capillary pressure produced in the fine pores of the piece, and P
g is the pressure that the gas inside the pores exerts.
The SEM micrographs of the reconstructed steel slag and the porous restructured steel slag containing 1.6% SiC produced by sintering at 1290 °C are shown in
Figure 4. The reconstructed steel slag not doped with SiC presents a relatively dense microstructure with fewer pores, while those structure with the addition of SiC as a high-temperature foaming agent become looser, and the pore size increases. The sample contains many isolated spherical closed pores with an average pore diameter of about 10 μm, and there are also long and narrow open pores, when the SiC mass fraction is 1.6%, as shown in
Figure 4b.
In fact, SiC particles are easily oxidized at high temperatures when they are exposed to O
2 [
23]. Nevertheless, the formation of a dense SiO
2 protective film on the surface of SiC particles can effectively prevent their further oxidation [
24]. Flux oxides (ie, K
2O, Na
2O, and CaO, etc.) have been found that have a significant corrosive effect on the SiO
2 protective film. The addition of quenching and tempering components such as CaO and Al
2O
3, meanwhile, reduces the viscosity of the system and makes the BOF slag in the molten liquid phase form at a lower temperature. The formation of more liquid phases reduces the expansion resistance of the pores, and accelerates the chemical reaction between SiC and O
2, thereby increasing the level of SiC oxidation [
24,
25].
The reconstructed steel slag and SiC foamed porous reconstructed steel slag are tested through MIP. The relationship between the amount of non-wetting liquid entering pores and the continuously increasing pressure are tracked basing on the Washburn equation [
26], and the results are shown in
Table 4. P is the pressure of the liquid level, γ is Surface tension, θ is contact angle, and r is radius of curvature.
The pore size distribution diagram and the cumulative mercury intrusion curve of BOF slag are shown in
Figure 5a,b respectively. The total area of the mercury intrusion curve equals the total volume of the unit mass pores according to the graph. The pore size distribution presents multiple peaks, and the coarse pores increase slightly after SiC foaming, which may be because some small pores combine to form larger pores due to the large dosage of SiC. The total pore volume per unit mass of steel slag increases from 0.1228 mL/g to 0.1872 mL/g, the porosity increases from 21.48% to 31.79%, the total pore area increases from 14.554 m
2/g to 26.724 m
2/g, the medium pore diameter increases from 48.4 nm to 52.4 nm, and the average pore size is reduced from 33.7 nm to 28 nm, as plotted in
Table 5. The reconstructed steel slag after SiC foaming has a porous structure, which is consistent with the results of the microscopic morphology analysis.
The FBT-5 SSA diameter is used to test the ground steel slag powder separately, and the results obtained are as shown in
Table 6: The SSA of the reconstructed BOF slag powder increases with the increase of the SiC content, when the SiC content is less than 1.6%. The SSA of the PRS4 group BOF slag powder with a SiC content of 1.6 and a porosity of 31.79% reached 424.4 m
2/kg. The pore structure of the porous reconstituted steel slag deteriorates and the SSA begins to decrease when the SiC content continues to increase, but it is still higher than that of the unfoamed reconstructed steel slag. The porosity of the reconstructed steel slag after SiC high-temperature foaming increases, its mechanical strength decreases, and the grinding efficiency is improved. The grindability of the reconstructed steel slag is improved to varying degrees.
The average particle size of the fine powder ground from the PRS4 porous reconstructed steel slag is reduced to 24.36 μm compared to 39.96 μm of the reconstructed steel slag as shown in
Figure 6, which is basically consistent with the SSA test result, and further confirms that the grindability of reconstituted steel slag is improved after high-temperature foaming of SiC. This method, which can improve the grindability of BOF slag, is carried out industrially using the residue heat of high temperature molten steel slag. Despite the fact that the foaming process of SiC releases CO
2, it is still relatively environmentally friendly compared to the manufacturing process of conventional cement materials.