PROPERTIES OF BAYER RED MUD BASED FLUX AND ITS APPLICATION IN THE STEELMAKING PROCESS
Yanling Zhang, Fengshan Li, Ruimin Wang
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Xueyuan Road 30, Haidian District, Beijing 100083, China
Keywords: Bayer red mud, thermodynamic properties, dephosphorization, steelmaking Abstract
Bayer red mud is characterized as highly oxidizing (high Fe2O3 content) and highly alkaline (high Na2O content), which tends to act as a flux and strong dephosphorizer in the steelmaking process. In this study, firstly, the thermodynamical properties of Bayer red mud based flux were predicted including the melting temperature and phosphorus capacity. Further, laboratory experiments on application of Bayer red mud-based flux in hot metal dephosphorization. The effects of influencing factors such as flux composition and basicity were discussed. The results gave necessary basic knowledge for promoting the application of Bayer red mud in the steelmaking process.
Introduction
With the rapid development of alumina industries in China, a large amount of red mud has been produced. Due to the high Na2O content, red mud has been classified as a hazardous waste. The red mud from Bayer process is rich in Fe2O3, Al2O3, and Na2O, during which as high as 30%-53% of Fe2O3 concentrated and much larger content could be obtained after usual magnetic or gravity separation. Accordingly, Bayer red mud tends to be a kind of ideal iron resource of ferrous metallurgy. Several previous studies [1-3]
have been carried out on extracting Fe element from red mud through direct reduction or indirectly using that as parts of raw materials in ironmaking such as sinter or blast furnace.However, due to the serious lining erosion and nodulation caused by strong vaporization of Na, the actual application of red mud in the above process is extremely limited.
As can be seen, Bayer red mud can be characterized as highly oxidizing (high content of Fe2O3) and highly alkaline (high content of Na2O), which is exactly necessary in the steelmaking process. High content of Fe2O3 in Bayer red mud can work as a good slagging flux in addition to provide plentiful iron elements, and Na2O is a well-known strong dephosphorization/desulfurization agent. Obviously, the possible corrosion of
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
Na2O on the lining furnace of BOF is an important consideration. While, due to the special shape of converter: a larger ratio of diameter to height, the erosion caused by vaporization of Na2O tends to be much less compared with that happened in Blast Furnace, which has a much smaller ratio of diameter to height and gives a much longer way to reduce vaporizing-condensing-nodules of Na2O. However, besides Fe2O3 and Na2O, some Al2O3 and a small amount of TiO2 are also brought into slag when adding Bayer red mud in steelmaking process. While the research data about the comprehensive and interacting effects of Al2O3, Na2O, and TiO2 on properties of CaO-FeO-SiO2 based steelmaking slag available is extremely limited. Therefore, in this study, in order to promote the application of Bayer red mud in steelmaking process, the thermodynamic analysis of the effects of Al2O3, Na2O, TiO2, and their interactions on the properties of CaO-FeO-SiO2-based slag was performed. Further, the laboratory experiments on application of Bayer red mud-based flux in hot metal dephosphorization were carried out.
1. Thermodynamics prediction on liquid areas of CaO-SiO2-FeO (-Al2O3-Na2O-TiO2) slag system
The phase diagram module of FactSage software was used to simulate the liquid areas of CaO-SiO2-FeO (-Al2O3-Na2O-TiO2) slag system at different temperatures. The liquid areas between 1200 °C and 1600 °C in CaO-SiO2-FeO slag are shown in Fig. 1(a).
This suggests that the melting point of the CaO-SiO2-FeO system decreases with increasing FeO content, shown by the arrows. This supports that the fluidity of the slag as well as the dissolution of lime is usually improved with increasing FeO content in slag.
The composition of initial slag is mostly around point “B” in Figure 1(a). With the progress in steelmaking, the slag composition gradually moves to near point “C”. As shown, that is in the area with melting temperature higher than 1600℃. CaF2 is often added to improve the fluidity of slag and thus obtain a good separation of steel from slag.
However, utilization of CaF2 has been extremely limited due to its potential environment pollution, and alternative fluxes have to be pursued. The effects of Al2O3 and Na2O on the liquid areas of CaO-SiO2-FeO system at 1400 °C are shown in Fig. 1(b) and Fig. 1(c), respectively. Fig. 1(d) shows the effect of the mixture of Al2O3, Na2O, and TiO2 (denoted as “A” in the figure) with a mass ratio of Al2O3:Na2O:TiO2 = 1:0.25:0.15 (which is roughly estimated based on the typical composition of Bayer red mud in China after accounting for the possible evaporation loss of Na2O) on the 1400 °C liquid areas of the CaO-SiO2-FeO system. As shown, with the addition of Al2O3, Na2O, and A, the liquid areas of the CaO-SiO2-FeO system at 1400 °C is obviously enlarged. Upon adding 10%
Al2O3 (Fig. 1(b)), 5% Na2O (Fig. 1(c)), and 10% A (Fig. 1(d)), the liquid areas below 1400 C (over the range of %CaO/%SiO2 = 0.8–1.5) is nearly independent of the FeO content. This tends to indicate that it is possible to acquire good fluidity of slag with much lower FeO content, which is of great importance for decreasing iron loss.
Furthermore, Fig. 1 shows that with increasing Al2O3 (Fig. 1(b)) or A content (Fig. 1(d)),
the 1400 °C liquid areas of the CaO-SiO2-FeO system obviously moves towards the lower left corner, in the direction of increasing basicity. This indicates that the presence of Al2O3 or A can greatly decrease the melting temperature of the CaO-SiO2-FeO slag with higher %CaO/%SiO2 value, or it can promote the dissolution of lime in the slag. As known, the removal of phosphorus from steel to slag is an important operation in steelmaking, which thermodynamically needs a slag with higher basicity but kinetically needs a slag with lower melting temperature (good fluidity). This means that the addition of Al2O3 as well as Bayer red mud benefits the dephosporization performance, and it can be expected to effectively prohibit rephosphorization because of the excellent slag situation (higher basicity and good fluidity). Diao’s experiments suggested that dephosphorization ratio increased with the addition of some amount of Al2O3 and Na2O into CaO-FeO-SiO2 slag, and their kinetic models showed that both Al2O3 and Na2O obviously promoted the growth of overall mass transfer coefficient [4]. It agrees reasonably well with this research data.
Fig.1. Phase diagram and liquid areas of the CaO-SiO2-FeO.slag system: (a) phase diagram of CaO-SiO2-FeO, (b) effect of Al2O3 on liquid areas at 1400 °C , (c) effect of
Na2O on the 1400 °C liquid areas, (d) effect of A on the 1400 °C liquid areas.
2. Experiments on hot metal dephosphorization by using Bayer red mud based flux
2.1 Materials
The high-iron red mud used in this study was sampled from Shandong Weiqiao aluminum plant, and its chemical composition determined by XRF is shown in Table 1.
Table1. Main compositions of Bayer red mud, wt%
Compositions Fe2O3 Al2O3 SiO2 Na2O TiO2 CaO Others
% 59.09 16.18 10.59 7.59 4.13 0.67 1.75
Some of the experiments were performed in a silicon molybdenum furnace. In this case the main materials used were chemically pure reagents. Saturated molten iron samples with saturated carbon and different P/Si contents were prepared by using reduced iron (AR, Fe≧99%), graphite powder (CP, C≧99.95%), ferrophosphorus (P=27.50%) and ferrosilicon (Si=98.95%). The dephosphorizer in experiments was mainly made up of Bayer red mud and CaO. Another two runs of experiments were carried out in an induction furnace, during which the industrial pig iron was chosen as main iron material, whose components are listed in Table 2.
Table2. Compositions of industrial pig iron, wt%
C Si Mn Al P S Ca Cr
4.4 0.12 0.27 0.07 0.03 0.03 0.02 0.02
2.2 Apparatus and experimental procedure
The structure diagram of silicon molybdenum furnace used for dephosphorization experiments of hot metal pretreatment was shown in Figure 2. Raw materials were firstly thoroughly mixed in a mortar according to the requirements of the initial iron sample (C=4.5%, P=0.12%, Si=0~0.5%). 250 g of the iron sample was placed in an alumina or magnesia crucible (Φ50×140) and the amount of dephosphorization reagent (mixture of Bayer red mud and CaO) was set as 50 g or 25 g. The experiment was started when the iron sample melted completely. The initial iron sample was taken in a Φ6 mm tube and the dephosphorizer was added into the sample crucible in three batches. The iron and slag samples were taken out in the end of the experiment and were analyzed after crushing.
Another two runs of experiments were performed in a 10 kg induction furnace. The amount of pig iron was about 5 kg each time, and the mass ratio of dephosphorizer (mixture of Bayer red mud and CaO) to iron was about 10%. Firstly, the pig iron was pre-melted in the crucible, after pig iron melted completely the initial iron sample was taken out from the crucible with a Φ6mm tube. All of the slag was added into the crucible. 25 min later, the iron and slag samples were taken out and analyzed after crushing.
Table 3 shows the specific experimental arrangements. It should be noted that the dephosphorizers of No.1 and No.2 were prepared by using pure oxides while the other ones consisted of Bayer red mud and CaO. The experiments of Nos.1-8 were conducted
in a silicon molybdenum furnace, and those of Nos.9-10 were performed in an induction furnace.
Fig. 2 The diagram of silicon molybdenum furnace Table 3 The experimental scheme of dephosphorization Experiments Group Red mud:
CaO(:Fe2O3) [%P]0 [%Si]0 Slag ratio
Temperature
/ ˚C [%P]f 𝜂𝑃
Hot metal pretreatment
1 2:1 0.12 0 0.2 1350 0.075 37.5%
2 1:1 0.12 0.5 0.2 1400 0.099 17.5%
3 2:1 0.12 0 0.1/0.2 1350 0.043
/0.022 64.0%
/81.93%
4 1:1 0.12 0.5 0.2 1400 0.098 18.33%
5 2:1 0.12 0.2 0.1/0.2 1350 0.060
/0.028 49.0%
/76.5%
6 2:1 0.12 0.5 0.1/0.2 1350 0.109
/0.074 8.71%
/38.06%
7 1:1 0.12 0 0.2 1350 0.018 85.0%
8 4:1 0.12 0 0.2 1350 0.070 41.2%
9 1:1:0.7 0.099 0.12 0.2 1350 0.011 90.8%
10 2:1 0.11 0.088 0.1 1350 0.070 36.4%
2.3 Analysis method
The contents of C, Si and P elements were obtained by chemical analysis method.
The slag compositions of the samples were obtained by XRF analysis method. The
dephosphorization efficiency was mainly described by the dephosphorization rate, which can be calculated by Eq. (1).
𝜂𝑃=[%𝑃][%𝑃]0−[%𝑃]𝑓
0 (1) Here, [%𝑃]0 is the percentage of initial phosphorus; [%𝑃]𝑓 is the percentage of final phosphorus in steel; 𝜂𝑃 is the dephosphorization rate.
2.4 Results and discussion
2.4.1 Separating conditions of slag and metal phase
A complete separation between slag and metal phase is necessary for promoting the dephosphorization reaction. In the dephosphorization process, CaO-FeO-SiO2 is generally the basic slag system. In order to investigate the fluxing action of Bayer red mud, the following comparative experiments were carried out. Based on the same mass ratio of CaO:SiO2:Fe2O3, one type of dephosphorizer was prepared using pure oxides of CaO, SiO2, and Fe2O3 (No. 1 and
No. 2 in Table 3); and the other ones consisted of Bayer red mud and CaO (No. 3 and No. 4 in Table 3).
After experiments, the separating conditions of iron and slag under different situations were shown as the follows. Fig. 3(a) and (c) showed those of using Bayer red mud-based dephosphorizer (corresponding to No. 4 and No. 3 in Table 3, respectively), and Fig.3(b) and (d) showed the separating conditions of iron and slag when using pure compounds (corresponding to No. 2 and No. 1
in Table 3, respectively). Obviously, a much better separation between the slag and the iron phases was obtained when Bayer red mud-based fluxes were used (Fig. 3(a) and (c)).
In the cases where pure oxides were used, a clear interface between the slag and the iron phases could not be obtained, suggesting that because of its low fluidity, the CaO-FeO-SiO2 based slag cannot be completely separated from the iron phase. These observations clearly show that the presence of components such as Al2O3 and Na2O in Bayer red mud can act as a kind of flux to decrease the melting point of the CaO-FeO-SiO2 based slag.
2.4.2 Effect of components in dephosphorizer and initial silicon content of hot metal on dephosphorization ratio
Fig. 3 Separating conditions between slag and metal phase ((a)(c) red mud based dephosphorizer were
utilized, (b)(d)pure oxide were utilized).
The effect of composition of Bayer red mud based dephosphorizer and the initial silicon content in hot metal on the dephosphorization rate was shown in Fig. 4.
Fig. 4 The influence of different ratio of Red mud:CaO, slag ratio and initial silicon contenton dephosphorization rate.
As shown in Fig. 4(a), the dephosphorization ratio decreased with increasing the mass ratio of red mud to CaO at 1350°C. When the mass ratio of red mudto CaO was changed from 1:1 to 2:1, the dephosphorization ratio changed a little, but remained > 80%
and the final [P] was less than 0.018%. However, it decreased sharply to be less than 40%
for red mud:CaO = 4:1. This is because the addition of excessive amounts of red mud reduced the basicity of the slag and weakened the phosphorus capacity of the dephosphorizer. As shown in Fig. 4(b), the initial [Si] in the hot metal also had a big influence on the dephosphorization rate. The higher the initial [Si], the lower was the dephosphorization rate; but for initial [Si] values less than 0.2%, this parameter had little effect on the dephosphorization rate especially when slag ratio was high (20%). This result demonstrates that the Bayer red mud-based flux can remove phosphorus and silica simultaneously if the initial [Si] is less than 0.2%. The dephosphorization ratio decreased sharply when the initial [Si] was as high as 0.5% and under these conditions, dephosphorization cannot effectively be carried out.
2.4.3 Effect of final slag basicity on dephosphorization ratio
Fig. 5(a) and 5(b) show the relation of the dephosphorization rate to the binary basicity and the optical basicity of the final slag, respectively. As can be seen, the dephosphorization rate increased with increasing basicity. Especially, when the binary basicity was as high as 12, the dephosphorization rate was 91% and the final [P] was 0.011% (see Table 3). In addition, good slag fluidity and an excellent separation between the slag and iron phase was observed even under strong alkaline conditions. This fact further proved that Al2O3 and Na2O in the Bayer red mud can act as flux materials to decrease the melting point of CaO-FeO-SiO2 slag system. And there was a more clearly linear relationship between dephosphorization rate and optical basicity (Fig. 5(b)), which illustrated that it is more accurate to use the optical basicity to describe the alkaline nature of the multiple dephosphorizer slag system used in this study.
Fig. 5 The influence of (a) (%CaO/%SiO2), and (b) optical basicity on dephosphorization 3. Conclusions
1. The liquid areas of CaO-SiO2-FeO system are obviously enlarged with the addition of Al2O3, Na2O, and mixture A; In the cases of adding 10% Al2O3, or 5% Na2O, or 10% A, the liquid areas below 1400℃are nearly independent of FeO content.
2. During the hot metal dephosphorization experiments, a much better slag-iron separating result was observed in the case of that Bayer red mud was used. That suggested that Al2O3 and Na2O in Bayer red mud can improve the fluidity of CaO-FeO-SiO2 basedslag.
3. In this study, when the mass ratio of Bayer red mud : CaO was between 1:1 to 2:1, a good dephosphorization effect was obtained, whose dephosphorization ratio can be reach more than 80%, and the final [P] was lower than 0.018%.
Acknowledgement
The authors are grateful to the financial support from the National Natural Science Foundation of China (No. 51474021).
References
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