Based on the measurement of viscosities and critical temperatures of CaO-MgO-SiO2- Al2O3-B2O3 slag system with various B2O3 contents, the slag with higher than 15 mas. % Al2O3 content has the lowest critical temperature and the widest solid-liquid coexisting region at about 2,0 mas. % B2O3. Furthermore, the X-ray diffractometer (XRD) result verified that bechilites whose melting point are low forms. Raman spectra revealed that the effect of network forming on viscosity is smaller than the effect of bechilites, which leads to the slag viscosity decrease with B2O3 addition. Base on the above research, so ludwigite can meet the requirements of a BF welding flux to decrease the critical temperature and improve the fluidity of the high Al2O3 slag

B. Dai

H. Long

Y. Ji

Y. Wen

English

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455METALURGIJA 59 (2020) 4, 455-458 B. DAI, H. LONG,Y. WEN, Y. JIEFFECT OF LUDWIGITE(B2O3) ON HIGH Al2O3 SLAG AND ITS MECHANISM USED AS A NEW BLAST FURNACE WELDING FLUX Received – Primljeno: 2020-03-23Accepted – Prihvaćeno: 2020-07-04Original Scientific Paper – Izvorni znanstveni radB. Dai (dbustb@163.com), H. Long, Y. Ji, Key Laboratory of Metallur-gical Emission Reduction & Resources Recycling, Anhui University of Technology, Ministry of Education, Maanshan, Anhui, China; Anhui Province Key Laboratory of Metallurgical Engineering & Resources Recycling, Anhui University of Technology, Maanshan, Anhui, ChinaB. Dai, Y. Wen, State Key Laboratory of Vanadium and Titanium Re-sources Comprehensive Utilization, Panzhihua, Sichuan, ChinaBased on the  measurement of viscosities and critical temperatures of CaO-MgO-SiO2-Al2O3-B2O3 slag system with various B2O3 contents, the slag with higher than 15 mas. % Al2O3 content has the lowest critical temperature and the widest solid-liquid coexisting region at about 2,0 mas. % B2O3. Furthermore, the X-ray diffractometer (XRD) result verified that bechilites whose melting point are low forms. Raman spectra revealed that the effect of network form-ing on viscosity is smaller than the effect of bechilites, which leads to the slag viscosity decrease with B2O3 addition. Base on the above research, so ludwigite can meet the requirements of a BF welding flux to decrease the critical temperature and improve the fluidity of the high Al2O3 slag.Key words: blast furnace; hearth activity; high Al2O3 slag; B2O3INTRODUCTIONDue to the continuous increase of steel production, Chinese iron ore external dependence was 84 % in 2015 [1].  A mass of iron ore with  high  Al2O3 have to be im-ported from foreign countries, such as Australia, India and South Africa [2], resulting in a number of blast fur-naces (BF) in China have experienced accumulation of molten slag in the BF hearth, which seriously endangers the production and safety of BFs [3]. There are great-er than 15 mas. % Al2O3 in the tapped slag due to the use of high Al2O3 ores, which has a certain influence on metallurgical properties of slag and BF operation [4]. In addition, when the hearth accumulation happens at the lower part of BF, ironmaking operators often use manganese ores for flushing to help furnace conditions [5]. But practice has proved that the environmental and energy saving [6]. Therefore, it is necessary to explore a new kind of BF welding flux. It has been reported that B2O3 could degrade the slag fluidity [7, 8], however few has explicitly investigated ludwigite as the BF welding flux and quantificationally studied the mechanism.In this work, the high Al2O3 tapped slag which result-ed in hearth accumulation on a 5 500 m3 large BF in Chi-na was as the research object, so the effect of B2O3 on the high Al2O3 slag and its mechanism were researched, in-cluding the measurement of viscosity and critical tem-perature by rotating cylinder method, confirming the ISSN 0543-5846METABK 59(4) 455-458 (2020)UDC – UDK 669.162.2:536.777/624.027:546.621/541.451:546.431/541.451=111 mineral phase and structure by XRD and Raman spectra. Based on the research,  the ludwigite which is regarded as a new choice of BF welding flux is put forward for BF operators to restore the hearth activity. EXPERIMENTALThe experimental chemical composition of the slag is shown in Table 1.Table 1 Chemical composition of the slags / mas. %No. B2O3 MgO SiO2 Al2O3 CaO CaO/SiO21 0 7,35 34,72 16,96 40,97 1,182 0,2 7,35 34,63 16,96 40,86 1,183 0,5 7,35 34,49 16,96 40,70 1,184 0,8 7,35 34,35 16,96 40,54 1,185 1,0 7,35 34,26 16,96 40,43 1,186 2,0 7,35 33,80 16,96 39,89 1,187 3,0 7,35 33,34 16,96 39,35 1,18According to the composition of the tapped slag sample, slags are prepared using reagent-grade chemi-cals of CaO, SiO2, MgO, Al2O3 and B2O3. According to the desirable proportions in Table 1, the reagents are weighed 140g precisely and mixed with an agate mortar thoroughly. The molybdenum crucible (inner diameter 39 mm, high 60 mm) is used to hold the slag sample.Based on the rotating cylinder method, the viscosity is measured by a comprehensive  analyzer for physical properties of melts (RTW-10 Type). The comprehensive analyzer mainly consists of a computer, viscometer, and high temperature tubular furnace with U-shape MoSi2 heating elements. After the viscosity measurements, the slag samples were reheated to 1 500 °C and quenched by water rapidly to obtain amorphous slag samples for XRD and Raman spectra.456 B. DAI et al.: EFFECT OF LUDWIGITE (B2O3) ON HIGH Al2O3 SLAG AND ITS MECHANISM USED AS A NEW BLAST... METALURGIJA 59 (2020) 4, 455-458RESULTS AND DISCUSSIONFigure 1 shows the effect of B2O3 on the viscosity, the viscosities of the slag samples with various B2O3 contents at 0 mas. %, 0,2 mas. %, 0,5 mas. %, 0,8 mas. %, 1,0 mas. %, 2,0 mas. % are almost coincident before the inflection point, which indicates that the viscosities of high Al2O3 slag samples are not significantly changed during the process of increasing B2O3 content from 0 to 2,0 mas. % when the temperature is higher than the crit-ical temperature, although the slag composition is somewhat different. But as soon as B2O3 content comes up to 3,0 mas. %, the viscosity of the slag is signifi-cantly reduced and almost unchanged in high tempera-ture region from 1 400 °C to 1 480 °C, which indicates that the fluidity and stability of the slag with 3,0 mas. % B2O3 is better than the others. As shown in Figure 2, the addition of B2O3 significantly reduces the critical tem-perature of the high Al2O3 slags. Specifically, when B2O3 content was increasing from 0 mas. % to 0,8 mas. %, the rate of decrease is basically the same. During the process of increasing B2O3 content from 0,8 mas. % to 1,0 mas. %, the rate of decrease is accelerated. As soon as B2O3 content is increasing from 1,0 mas. % to 2,0 mas. %, the rate of decrease is slowing down again. When B2O3 content comes up to 3,0 mas. %, there is no obvious viscosity inflection point, and the fluidity of the slag is different from the general BF slag, so there is no critical temperature of the slag.F igure 3 shows the XRD patterns of CaO-MgO-SiO2-Al2O3-B2O3 system with various B2O3 contents. It can be seen that when there was no B2O3 addition, the mineral phase of the slag is just gehlenite. With the ad-dition of 1,0 mas. % B2O3, the mineral phases of bechi-lite is appearing, indicating that B2O3 have changed the basic mineral phase in the slag after the doping process. With the increasing of B2O3, the diffraction peak inten-sity of gehlenite is weakened, on the other hand, the diffraction peak intensity of bechilite is strengthened. The XRD results indicate that the bechilite whose melt-ing point is low really forms in the slag. With the bechi-lite continuously and massively forming, the critical temperature of the slag decreases as well, which is also consistent with the previous viscosity and critical tem-perature experiment.In order to further explain the reason of viscosity variation, the room-temperature Raman spectra were investigated to obtain the structure information of molten slags with different B2O3 contents. The back-ground of the measured Raman spectra was subtracted B2O3 / mas.%      1  0       2  0,2       3  0,5       4  0,8       5  1,0       6  2,0       7  3,0Viscosity / Pa.sTemperature / °C12345670,00,51,01,52,02,53,03,54,01 200 1 300 1 400 1 500Figure 1 Effect of B2O3 on viscosity Critical temperature / °  B2O3 / mas. %0,0 0,5 1,0 1,5 2,01 3001 3101 3201 3301 3401 3501 3601 370Figure 2 Effect of B2O3 on critical temperature10 20 30 40 50 60 702  / degree B 2O 3 = 3,0 mas. % B 2O 3 = 2,0 mas. % B 2O 3 = 1,0 mas. %01 7003 4005 100I n t e n s i t y01 7003 2004 800 B 2O 3 = 0 mas. %01 5003 0004 500GehleniteBechilite02 1004 2006 300Figure 3  XRD patterns of the high Al2-O3 slags with various B2O3Table 2 Assignments of Raman bands for slagsBridging oxygenRaman shift / cm-1Raman assignmentsQ0 840 - 870 Symmetric stretching vibrations of silicate tetrahedral with zero bridging oxygenQ1 900 - 930 Symmetric stretching vibrations of silicate tetrahedral with one bridging oxygenQ2 950 - 980 Symmetric stretching vibrations of silicate tetrahedral with two bridging oxygenQ3 - 1060 Symmetric stretching vibrations of silicate tetrahedral with three bridging oxygen457 B. DAI et al.: EFFECT OF LUDWIGITE (B2O3) ON HIGH Al2O3 SLAG AND ITS MECHANISM USED AS A NEW BLAST...METALURGIJA 59 (2020) 4, 455-458and the raw Ram an spectra were deconvoluted by Gaussian-Deconvolution me-thod with the minimum correlation coefficient r2 ≥ 0,99. The assignments of Ra-man bands are provided in Table 2. The different Ra-man spectra with different B2O3 are showed as Figure 4.From Fig ure 4, it can be seen that the band areas of Q0 units decrease with the increase of B2O3 content, which is consistent with the studies of Kim et al. [10] and Wang et al. [11]. This indicates the network struc-ture of the slags may be polymerized. On the basis of previous studies [12], a certain amount of Si- O-B bonds exist in borosilicate glasses by NMR spectroscopy anal-ysis. Therefore, the forming of Si-O-B bonds may lead to the average number of bridging oxygen increase. Based on our structural research by Raman spectra, the degree of polymerization of the present slags are also improved by B2O3 that behaves as a network former to polymerize the network structure. That means the real reasons for the decrease of viscosity could not be the lower degree of polymerization. Based on this, we spec-ulate that in spite of the new forming of B-O, but the bond energy of B-O is weaker than the bond energy of Si-O because of low melting point of B2O3 and B-O containing flow units are easily broken or slided [13]. The other reason is due to the formation of low-melting bechilites with B2O3 addition, which is advantageous to viscous flow. The effect of network forming on viscos-ity is smaller than the effect of low melting point com-pound formations, as a result, the slag viscosity is de-creased with B2O3 addition.  PROPOSAL OF LUDWIGITE TO RESTORE HEARTH ACTIVITY IN BF  One proposal is to restore the hearth activity by the feeding of ludwigite (Mg,Fe)2Fe(BO3)O2) as a source of B2O3 which is a magnetite stabilizing element(Table 3). For the BF production, the  critical temperature of the tapped slag generally does not exceed 1 350 °C. Therefore, based on the experimental results, it can be known that slag with higher than 15 mas. % Al2O 3 con-tent has the lowest critical temperature and the widest solid-liquid coexisting region at about 2,0 mas. % B2O3. It is also observed that the viscosity of slag with 2,0 mas. % B2O3 exhibits a small enough value at higher temperature than 1 450 °C, however it does not signifi-cantly changes with varying B2O3 content. It is indicated that the appropriate content of B2O3 in the high Al2O3 tapped slag is 1,0 - 2,0 mas. % by the feeding of lud-wigite, aiming to decrease the critical temperature and improve the fluidity of the high Al2O3 tapped slag, which can meet the requirements of a BF welding flux.Figure 4 Raman spe ctra of slags with different B2O3 100200300400500600IntensityRaman Shift / cm -1Q2Q1Q0Original spectrumB2O3 = 0 mas. %800 850 900 950 1 000 1 050 1 100150200250300350IntensityB2O3 = 1,0 mas. %Original spectrumQ1Q2Q0Raman Shift / cm -1800 850 900 950 1 000 1 050 1 100150200250300350400IntensityOriginal spectrumQ1Q2B2O3 = 2,0 mas. %Raman Shift / cm -1800 850 900 950 1 000 1 050 1 100150200250300350400IntensityB2O3 = 3,0 mas. %Original spectrumQ1Q2800 850 900 950 1 000 1 050 1 100Raman Shift / cm-1458 B. DAI et al.: EFFECT OF LUDWIGITE (B2O3) ON HIGH Al2O3 SLAG AND ITS MECHANISM USED AS A NEW BLAST... METALURGIJA 59 (2020) 4, 455-458Table 3 Chemical compositions of lud-wigite / mas. %Composition TFe SiO2 MgO PMas. 53,61 3,81 10,22 0,02Composition S Al2O3 B2O3Mas. 1,0 0,27 4,53CONCLUSIONThe slag with higher than 15 mas. % Al2O3 content has the lowest critical temperature and the widest solid-liquid coexisting region at about 2,0 mas. % B2O3. The viscosity of tapped slag with 2,0 mas. % B2O3 exhibits a small enough value at higher temperature than 1 450 °C, however it does not significantly changes with varying B2O3 content. With the addition of B2O3, bechilite was appearing, indicating that B2O3 have changed the basic mineral phase in the slag after the doping process. One proposal is to restore the hearth activity by the feeding of ludwigite as the source of B2O3 which is a magnetite stabilizing element. The appropriate content of B2O3 in the high Al2O3 tapped slag is 1,0 - 2,0 mas. %, aiming to decrease the critical temperature and improve the fluid-ity of the high Al2O3 tapped slag, which can meet the requirements of a BF welding flux.REFERENCES[1] W. Chen, Y. Lei, Y. Jiang, et al. Influencing factors analysis of China’s iron import price: based on quantile regression model[J], Resources Policy 48 (2016), 68-76.[2] X. Zhang, T. Jiang, X. Xue, et al. Influence of MgO/Al2O3 ratio on viscosity of blast furnace slag with high Al2O3 content[J], Steel Research International 87 (2016), 87-94.[3] Z. Liu, J. Zhang, H. Zuo, et al. Recent progress on long service life design of Chinese blast furnace hearth[J], ISIJ International 52 (2012), 1713-1723.[4] M. Nakamoto, T. Tanaka, J. Lee, et al. Evaluation of visco-sity of molten SiO2-CaO-MgO-Al2O3 slags in blast furnace operation[J], ISIJ International 44 (2004), 2115-2119.[5] H. Zuo, C. Wang, C. Xu, et al. 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Effect of ludwigite (B2O3) on high Al2O3 slag and its mechanism used as a new blast furnace welding flux

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