Temperature at the reaction interfaces

A thermal gradient can exist inside a BOF converter due to the formation of localized 630

reaction zones. Numerous researchers attempted to measure the temperature at different 631

zones of the converter.[63, 64] Chiba et al.[63] reported that the temperature of the hot spot 632

jumps suddenly to 2273 K (2000 °C) at the beginning stage of the blowing, then fluctuates 633

35

between 2373 K (2100 °C) to 2773 K (2500 °C) during the main blow period and finally 634

equals to the hot metal temperature. Rote and Flinn observed that the temperature difference 635

between the top surface and bottom of the vessel varies between 200 and 400 K depending on 636

the blowing type (soft or hard blowing).[64] Since the temperature is an essential factor in the 637

equilibrium partitioning of refining elements, the model calculations for interfacial 638

temperature in different reaction zones were developed. 639

In common with Chiba et al., it was assumed the temperature in the hot spot increases 640

linearly from 2273 K (2000 °C) to 2573 K (2300 °C) during the first 25% of the blow. During 641

the main blow, between 25 to 80 % of the blow, the temperature was maintained at a constant 642

value of 2573 K (2300 °C). Finally, the temperature gradient between the hot spot and the 643

liquid bath begins to disappear and hot spot temperature gradually decreases after 80% of the 644

blow.[65] Industrial measurements indicated that the temperature of slag is generally 20 to 100 645

K hotter than the hot metal during the blowing period.[66] The temperature difference between 646

the metal and slag was reported to be high during the initial period and the gradient becomes 647

smaller towards the end blow period. In the present work, for the sake of simplicity, the 648

average temperature of the slag was assumed to be 100 K higher than the hot metal 649

temperature. 650

36 651

Figure 9: Temperature change across various reaction interfaces inside the furnace during the 652

blowing time 653

The surface temperature of the moving droplets in the slag-metal emulsion was calculated by 654

applying Eq. 33. The variation of temperature in different zones of the converter used in the 655

model is shown in Fig. 9. It was observed that the surface temperature of the droplets is 90 to 656

200 K higher than the metal bath temperature. The temperature profile of droplet surface 657

varies linearly with the blowing time during almost all the part of the blow. Toward the end 658

of the blow, there is a decreasing trend observed which is due to a reduction in the hot spot 659

temperature as a result of slowing down of the decarburization reaction. It should be 660

acknowledged that the current procedure for estimation of interfacial temperature is based on 661

several simple assumptions and no rigorous heat balance model was applied in the 662

calculation. A dynamic heat balance model focusing the micro and macrokinetics of heat 663

transfer in the recirculated metal drops in the emulsion, coupled with the present multi-zone 664

model can provide a clear insight of reactions in a BOF process. 665

37 3.2.Kinetics of refining of droplet in the emulsion 666

The model predictions of the compositional change of two classes of droplets having average 667

diameter 6×10-4 m (0.6 mm) and 9×10-4 m (0.9 mm) at 2.5th minutes of blowing time are 668

shown in Fig. 10. The reaction rate of Si, Mn and P in the droplet are found to be rapid and 669

reaches the state of equilibrium within a few seconds in the emulsion. In the 0.6 mm droplets, 670

the concentration of Si, Mn and P approaches its equilibrium value within 2 seconds. In 671

contrast, the refining of C continues during the entire 27 seconds of residence in the emulsion 672

phase. It was also observed that the refining rate of droplets, particularly decarburisation is a 673

function of droplet size. The droplets in the lower region of size spectrum exhibit high 674

efficiency of refining and make a greater contribution to the conversion process of Si, C, Mn 675

and Pduring the reaction in the emulsion. About ~60% of decarburization was observed for 676

0.6 mm droplet in contrast to ~18% when the droplet size was increased by 0.3 mm. The may 677

be due to a shorter reaction time (~10 s) of 0.9 mm diameter droplet as compared to 0.6 mm 678

droplets (~27 s). Here the extend of decarburisation reaction is limited by the time of reside 679

of droplet in emulsion. The model prediction of the droplet refining kinetics has been found 680

to be consistent with the observed refining of metal drops reported by IMPHOS pilot plant 681

experiments.[18] The measurements of droplet composition collected from the emulsion 682

sample shows a high depletion of Mn, P and Si but the C concentration is more than 1 wt pct 683

during the initial blowing period. The rapid removal rate of Si, Mn, and P during the opening 684

stage of oxygen blow is thought to be the result of high thermodynamic driving force and 685

large surface area created by small size metal drops in the emulsion. 686

38 687

Figure 10: Removal kinetics of C, Si, Mn and P of a single metal drop in emulsion (a) Initial 688

droplet diameter = 6x10-4 m (b) Initial droplet diameter = 9x10-4 m 689

3.3.Validation