Regular Article
Simulation and Application of O2–N2 Mixing Top-blowing Method in MURC Steelmaking Process
2024 Volume 64 Issue 8 Pages 1251-1262
Details
2024 Volume 64 Issue 8 Pages 1251-1262
The O2 and N2 mixing top-blowing method could effectively improve the mixing degree and suppress the temperature increase rate of the molten bath in vanadium extraction converter. In this paper, four kinds of top-blowing lances designed by an extra N2 flow rate and various Mach numbers have been investigated by a series of water experiments and numerical simulations. On the basis of result, the mixing time was first increased and then decreased with the increase of lance height, and the lance height of 1400 mm obtained the longest mixing time. There were two high-velocity regions generated by impaction of top-blowing jets and stirring of bottom-blowing bubbles. Simultaneously, there were two low-velocity regions formed by the block of furnace wall, and one low-velocity region formed by the local eddy. Comparing with the current top-blowing lance, all three new kinds of top-blowing lances obviously improved the kinetic condition and impaction cavity area of molten bath, which would further be improved with a larger design Mach number. Therefore, an appropriate top-blowing lance had been selected in the industrial application research, which achieved a shorter melting time and a faster vanadium extraction rate, in contrast to the current lance.
As a high-value metallic element, the vanadium achieves low density, high strength, and stable chemical property, which is widely used in production and processing for special steel.1,2) The vanadium titano-magnetite is the main raw material for producing the vanadium-rich molten iron in BF (Blast Furnace) process, and the MURC (Multi-Refining Converter) technology is adopted for achieving the vanadium-rich liquid slag.3,4) Meanwhile, in order to decrease the operational cost of vanadium extraction, the lime is forbidden to charge in the vanadium extraction converter. For the liquid slag in the vanadium extraction converter, its main components are SiO2 and MnO, and the Si and Mn elements are mainly provided by the molten iron. That means it is hard to modulate the content of liquid slag to improve the vanadium extracting rate. Thus, for improving the V2O5 content in liquid slag, it is an effective way to strengthen the dynamic and thermal conditions of molten bath, by optimizing the oxygen lance structure, bottom-blowing nozzle arrangement, and gas injection method.
As the previous literatures reported, the water experiment and numerical simulation are the main method to investigate the effect of operational condition on the behavior of the molten bath.5,6,7) Lv et al.8) designed a series of swirl-type oxygen lances to investigate the effect of swirl angle on the mixing time, velocity and impaction cavity parameters of molten bath, and proposed an top-blowing oxygen lance structure, based on the result of industrial test. Shota et al.9) reported the behavior of molten bath formed by top-blowing gas jet and bottom-blowing nozzle, and proposed a new interference index to evaluate the spitting rate and spitting energy of molten bath. Liu et al.10) adopted the side-blow method to inject mineral powder into molten bath, and pointed out the mixing effective of molten bath increased with a bigger injection angle and a deeper installation location. Feng et al.11) discussed the application effect of O2 and CO2 mixing-blowing technology in BOF (Basic Oxygen Furnace) steelmaking process, and further proved the CO2 could considerably improve the agitation energy, comparing with the inert gas (Ar and N2), resulting in the shorter melting time and greater dephosphorization rate. Zhang et al.12) carried out the water experiment to verify the accuracy rate for various multiphase flow models in simulation process, and shown the Eulerian-multifluid VOF (Volume of Fluid) mathematical model could predict the mixing rate and impaction cavity of molten bath under a different oxygen lance heights and flow rates.
In order to control the temperature increasing rate of molten bath, the top-blowing oxygen flow rate would be suppressed in the dephosphorization and vanadium extraction converter. Meanwhile, improving bottom-blowing rate leads to faster erosion rate of bottom-blowing nozzle and furnace bottom lining. Hence, N2–O2 mixing top-blowing method has been proposed to increase the top-blowing flow rate.
With a greater top-blowing N2 flow rate, more kinetic energy of top-blowing jets is transferred into the molten bath, which improves the dynamic condition of molten bath. Simultaneously, the N2 gas flow is not reacted with the elements in molten bath. Thus, more N2 flow rate leads to more thermal energy of molten bath being removed by heat convection phenomenon, which reduces the temperature increasing rate of molten bath. That means the N2–O2 mixing top-blowing method could strengthen the dynamic and thermal conditions of molten bath, in the MURC steelmaking process.
Chengde iron and steel company adopted N2–O2 mixing top-blowing method to increase the mixing ability of oxygen jet for forming a greater dynamic and thermal conditions of molten bath. Comparing with conventional O2 top-blowing method, the result shown the [N] content in molten steel barely increased, after MURC steelmaking process. Meanwhile, the smelting time for vanadium extraction converter has been significantly decreased, at the same molten iron content and operational condition. In order to achieve a better metallurgical and economic effect, Chengde iron and steel company decided further improved the N2 flow rate in top-blowing process. Therefore, both water experiment and numerical simulation should be carried out to propose an appropriate oxygen lance parameter for industrial application test.
In this research, five kinds of oxygen lances have been designed, according to different N2 flow rates. The water experiment has been carried out to measure the mixing time, impaction area and depth, at various lance heights and top-blowing flow rates. The simulation model has been built to investigate the effect oxygen lance structure on the flow field of molten bath. Then, the optimized oxygen lance would be tested in the industrial application research to evaluate the smelting time, vanadium extraction rate and T. Fe content in slag.
The flow field characteristic of molten bath is mainly affected by the gas flow generated by oxygen lance and bottom-blowing nozzle. Therefore, it is important to establish the relationship of gas flow rate between water model and industrial prototype, to satisfy standard of dynamic similarity decided by viscous force, gravity, and inertia force. Based on the similarity of dynamic and geometry, the gas flow rate in water experiment could be calculated by the modified Froude number (Fr), as follows:13)
| (1) |
| (2) |
| (3) |
where, subscript m and p present the water model and industrial prototype, respectively. ρg and ρl are the gas and liquid phase density (kg/m3), respectively. u and L are the gas velocity (m/s) and feature size (m), respectively. Q, T and H are gas flow rate (Nm3/h), temperature (K) and depth of molten bath (m), respectively. g and C are the gas flow rate (Nm3/h) and geometry ratio, respectively. The ρo is the density (kg/m3) at the standard state.
2.2. Experimental ApparatusFigure 1 showed the experimental apparatus of the water model. The vanadium extraction converter was built in plexiglass with the geometric ratio of 1:5, and the prototype and model parameters were shown in the Table 1. In the experiment, compressed air, corn oil, and water were adopted for representing gas flow, liquid slag, and molten steel, respectively. The bottom-blowing arrangement of the water model was same as that of the prototype, as shown in Fig. 2. Besides, the red hollow circles presented the bottom-blowing nozzle, and the blue filled circle depicted the top-blowing nozzle.
| Parameters | Prototype | Model |
|---|---|---|
| Molten bath depth (mm) | 1565 | 313 |
| Slag layer thickness (mm) | 250 | 50 |
| Furnace diameter (mm) | 5170 | 1034 |
| Bottom diameter (mm) | 4845 | 969 |
| Total height (mm) | 8255 | 1651 |
| Density of liquid slag (kg/Nm3) | 3000 | 900 |
| Density of molten steel (kg/Nm3) | 7200 | 1000 |
| Density of oxygen gas (kg/Nm3) | 1.43 | 1.29 |
| Density of nitrogen gas (kg/Nm3) | 1.36 | 1.29 |
| Bottom-blowing rate (Nm3/h) | 675 | 3.02 |
| Oxygen top-blowing rate (Nm3/h) | 10000 | 40.0 |
| Nitrogen top-blowing rate (Nm3/h) | 8000/9000/10000/11000 | 29.9/33.7/37.4/41.1 |
| Lance inclination angle (°) | 11.0 | 11.0 |
| Lance height (mm) | 1300/1400/1500/1600 | 260/280/300/320 |
At present, Chengde iron and steel company used the top-blowing lance, which was designed by Mach number of 1.98, O2 flow rate of 10000 Nm3/h and N2 flow rate of 8000 Nm3/h, and the lance height ranged from 1300 mm to 1700 mm in the vanadium extraction process. As mentioned, for achieving a better metallurgical and economic effect, the Chengde iron and steel company decided to inject more N2 into the molten bath, with the same O2 flow rate of 10000 Nm3/h. Comparing with the N2 flow rate of 8000 Nm3/h for the current top-blowing lance, the N2 flow rates have been selected as 10000 Nm3/h to design three kinds of new top-blowing lances. Besides, the Mach number of Laval nozzle were 1.96, 1.98, and 2.02 for the various new oxygen lances, respectively. The Table 2 shows the structure parameters of top-blowing lances.
| Label | Mach number | Diameter | Prototype | Model |
|---|---|---|---|---|
| Current top-blowing lance | 1.98 | Throat diameter (mm) | 35.3 | 7.06 |
| Exit diameter (mm) | 45.5 | 9.1 | ||
| New top-blowing lance | 1.96/1.98/2.00 | Throat diameter (mm) | 37.8/37.2/36.6 | 7.56/7.44/7.32 |
| Exit diameter (mm) | 48.3/47.9/47.6 | 9.66/9.58/9.52 |
At initial condition, both oil and water has been added into the water model, and then the compressed air would be blown into oxygen lance and bottom-blowing nozzle. After the compressed air passed to run 3 minutes, the tracer element concentration and mixing time would be measured. For calculating the mixing time, there were three conductivity electrodes have been arranged at different regions to trace the tracer concentration (KCl concentration) variation. Hence, the mixing time was defined as that when the tracer concentration at the monitor points reached 99% of the average tracer concentration in the water layer. The average mixing time (Tm) was achieved as Eq. (4), and T1, T2, and T3 were the mixing times measured by three conductivity electrodes, respectively.
| (4) |
The following assumptions have been adopted in the simulation model:
(1) The molten steel and liquid slag were assumed as the incompressible Newtonian fluid, and the foam process of the liquid slag was not considered.
(2) The oxygen and nitrogen were defined as a mixture phase by the species transport model, and this mixture phase was considered as a compressible ideal gas.
(3) No chemical reaction in the molten bath was taken into consideration, and the simulation model was assumed as an isothermal process, which is same as the water experiment.
3.2. Governing EquationsDue to the obviously density variation, the interface between the gas, liquid slag, and molten steel was clear under the force of gravity. Therefore, the Eulerian-multifluid VOF model was used to track the free surface zone (gas/liquid slag/molten steel interface) in computational domain, and its governing equation for the mass conservation and volume fraction of tracked phases can be also written as Eqs. (5) and (6) in the sharp interface method, respectively.14)
| (5) |
| (6) |
where, αi and ρi are the volume fraction (%) and density (kg/m3) for ith phase, respectively;
The Reynolds-Averaged Navier-Stokes (RANS) formation was used to solve the continuity, momentum and energy equations, and the SST k-ω turbulence model was used to calculate the viscous effects and vortex structure in the simulation model. Equations (7) and (8) shown the turbulence kinetic energy (k) and its dissipation rate (ε) model, referring to transport equation.15)
| (7) |
| (8) |
where, Gk and Gω are the generation of turbulence kinetic energy (m2/s2) and specific dissipation rate, respectively. Yk and Yω are the dissipation of k and ω, respectively. Γk and Γω are the effective diffusivity (m2/s) of k and ω, respectively.
As mentioned, the species transport model was selected to analyze the interaction between O2 and N2 in the numerical model, as following equation:14)
| (9) |
where, Yi, and
To investigate flow field of molten bath generated by various operational conditions, a full-scale combined-blowing converter model has been established, according to the 150 t vanadium extraction converter. The computational domain included the oxygen lance, molten bath, bottom-blowing nozzles, and the region from oxygen nozzle tip to molten bath surface. Figure 3 depicted the mesh profile of the numerical model with boundary conditions. The mesh density has been improved near the oxygen lance exit, based on the greater velocity and pressure gradients. The mass flow inlet condition (blue plane) was used at the inlet face for the Laval nozzle and bottom-blowing nozzle. Both non-slip condition and enhanced wall treatment methods were adopted for the wall boundary condition (gray plane). The pressure outlet condition (red plane) was adopted for the outlet face of the simulation model. Initially, the computational domain started with no gas passing through the top-blowing lance or bottom-blowing nozzle, and the furnace gas and molten bath remained immobile.
The flow field of molten was achieved by transient solution in the simulation process, and the PISO (pressure implicit with splitting of operators) scheme was selected to solve the pressure-velocity coupling process. The Body Force Weighted and Geo-Reconstruct models were used to calculate the pressure and volume fraction variations for the multiphase flow in gravity condition, respectively. The other equations were solved by the second-order upwind scheme. At initial condition, time step size was set as 10−5 s by the fixed type, and then the adaptive method was used to calculate the time step with the global Courant number of 1.0. Moreover, for all the numerical simulation cases, the residuals of dependent variables would be defined as convergent with the energy residual and other variables being less than 10−6 and 10−3, respectively.
3.4. Mesh Independency and Model Validation TestInitially, the mesh independency has been tested to establish an appropriate simulation model, and the vanadium extraction converter was built as following levels: coarse mesh (481, 155 cells), medium mesh (722, 721 cells), and fine mesh (953, 644 cells).
In this paper, the volume mass-average method was adopted to measure the average velocity of molten bath, during numerical simulation process. The quasi-steady-state of molten bath was addressed as the fluctuating within a stable section for the variation of average velocity. For the molten bath, its average velocity, maximum average velocity and minimum average velocity have been measured at the quasi-steady-state. Figure 4 presented the flow velocity of molten bath calculated by three kinds of mesh levels.
The result showed the variation of the average velocity of molten bath calculated by the coarse and medium mesh level was 7.0%. Comparing between the medium and fine mesh levels, their variation was just 0.7%, which could be negligible. Moreover, the computational time required for the fine mesh was approximately 1.4 times that for the medium mesh. Thus, the medium mesh level was used in the simulation model.
The behavior of top-blowing jets has a great effect on the flow field of molten bath. For testing the validation of model, the stirring ability of top-blowing jets has been calculated by the theoretical equation.16) Figure 5(a) presented the results calculated by the simulation model and theoretical Eq. (10):
| (10) |
where, Et and θ are stirring energy of per ton steel (W/t) and inclination angle of Laval nozzle (°), respectively. Q, W and H are the flow rate of top-blowing jet (Nm3/min), weight of molten bath (t) and lance height (m), respectively. de and ue are the exit diameter of Laval nozzle (m) and initial velocity of top-blowing jet at the Laval nozzle exit (m/s), respectively.
The variations of stirring energy between theoretical and simulation data at H of 0.50, 0.75, 1.00 and 1.25 m were 8.2, 9.2, 10.6 and 11.7%, respectively. Based on the result, the simulation model proposed in this article can be used to describe a supersonic jet flow field. In the Fig. 5(a), the axis line of Laval nozzle and top-blowing jet trajectory have been addressed as solid line and dot line, respectively. The top-blowing jets showed a coalescence phenomenon due to static pressure gradient, which made their trajectory as a curve line. And Fig. 5(b) showed the trajectory of multi-jets, which proved the interference phenomenon occured in the simulation model. For the theoretical formula proposed by Nakanishi et al., it did not take the coalescence phenomenon into the consideration. Therefore, the variation of stirring energy between theoretical and simulation data was increased, with a higher lance height.
The stirring effect of molten bath could be represented by the mixing time in the water experiment, and the stirring effect of molten bath improves with a shorter mixing time. Figure 6 depicts the mixing time profiles in water experiment generated by various operational conditions. In this paper, the different top-blowing lance kinds are defined by their Mach number and design flow rate. For instance, the 1.98–18000 lance presents that the top-blowing lance designed by Mach number of 1.98 and design flow rate of 18000 Nm3/h.
The result shows the effect of top-blowing lance structure on the mixing time is obviously different, at the same bottom-blowing rate. Increasing the top-blowing flow rate leads to greater initial total kinetic energy of top-blowing jets. More kinetic energy of top-blowing jets could be transferred into the molten bath, which improves the mixing effect of molten bath. Hence, the mixing time reduces with the increase of top-blowing flow rate, as represented in Fig. 6(a).
With a higher lance height, the distance from Laval nozzle tip to the molten bath surface increases, which improves the entrainment phenomenon between top-blowing jets and ambient gas. During this process, the kinetic energy of top-blowing jets would be absorbed by ambient gas, and the stirring ability of top-blowing jets is suppressed. Thus, when the lance height increases from 1300 mm to 1400 mm, the mixing time is improved.
In the molten bath, both top-blowing jets and bottom-blowing bubbles generate a serious local eddies. When these eddies collide with each other, the total kinetic energy of eddies is reduced, resulting in a lower stirring effect of molten bath and a longer mixing time. At some certain conditions, a higher lance height could suppress the collision phenomenon among the eddies, resulting in a shorter mixing time, as reported in literatures 17, 18 and 19. Hence, when the lance height increases from 1400 mm to 1600 mm, the mixing time keeps reducing, as shown in Fig. 6(b). The results present the longest and shortest average mixing time are generated by the lance height of 1400 and 1600 mm, respectively.
The average mixing time generated by 1.98–18000, 1.96–20000, 1.98–20000, and 2.00–20000 lance are 94.5, 90.7, 85.6, and 81.1 s, respectively. Hence, the 2.00–20000 lance obtains the shortest average mixing time, which proves the 2.00–20000 lance achieves the best stirring ability at the test condition. Moreover, the 1.98–18000 lance obtains the longest average mixing time. That means all three kinds of new top-blowing lances achieve greater stirring ability than the current top-blowing lance.
With a larger Mach number, the initial gas velocity and kinetic energy at the Laval nozzle exit would be increased under the same operational condition, which improves the stirring ability of top-blowing multi-jets. Therefore, for three new kinds of top-blowing lances, the mixing time reduces with a larger Mach number. Meanwhile, the 2.00–20000 lance achieves the shortest average mixing time.
Based on the result, the mixing time formed by 1.96–20000 lance is smaller than that of 1.98-18000 lance. Meanwhile, the Mach number of 1.98–18000 lance is larger than that of 1.96–20000 lance, but the design flow rate of 1.98–18000 lance is smaller than that of 1.96–20000 lance. Thus, in this research, the design flow rate of top-blowing lance has a greater influence on reducing mixing time, comparing with the Mach number of top-blowing lance. The result shows the mixing time of molten bath further reduces, with a bigger design flow rate.
Figure 7 shows the impacting diameters profiles in water experiment at a series of flow rates and lance heights. In this paper, the impacting diameter is defined as the maximum diameter for the projection plane of impaction cavity along the axial direction of furnace. A bigger impaction diameter leads to a bigger impaction area, which improves the contacting degree of top-blowing multi-jets and molten bath. As increasing the flow rate, the total kinetic energy and stirring ability of top-blow multi-jets improves, resulting in a bigger impaction diameter. With a higher lance height, the top-blow multi-jets could contact more surface area of molten bath at the same flow rate, which increases the impaction diameter, as previous literatures reported.20,21)
The average impaction diameter formed by 1.98–18000, 1.96–20000, 1.98–20000, and 2.00–20000 lance are 207.7, 214.8, 221.7, and 233.0 mm, respectively. As a larger Mach number and a greater design flow rate, the 2.00–20000 lance achieves the biggest impaction diameter, and that means the 2.00–20000 lance could improve the contact area and mixing degree of top-blowing multi-jets and molten bath. Whereas, the 1.98–18000 lance achieves the smallest impaction diameter at all the operational conditions, which further represents the three new kinds of top-blowing lances achieve greater metallurgical effect than the current top-blowing lance.
Based on average values of different top-blowing lances depicted in Fig. 7, the slopes of the linear regression are 0.0059, 0.0063 and 0.0064 when flow rates are in the range of 18000–19000 19000–20000 and 20000–21000 Nm3/h, respectively. That means the uptrend of impaction diameter would be slightly accelerated with a greater flow rate at the tested condition. Contrary to the lance height, the slopes of the linear regression are 0.12, 0.08 and 0.03 when lance heights are in the range of 1300–1400 1400–1500 and 1500–1600 Nm3/h, respectively. That means the uptrend of impaction diameter would be decelerated with a greater lance height.
The impaction depth of molten bath indicates the penetration ability of top-blowing multi-jets. The deeper impaction depth is, the greater penetration ability is. With a deeper impaction depth, there are more top-blowing multi-jets would be contact with molten steel, which improves the reaction rates among the oxygen gas and elements of molten bath. Figure 8 shows the contrast of impacting depth of oxygen lance with four kinds of top-blowing lances at varying flow rates and lance heights.
As mentioned, a bigger flow rate of multi-jets improves its initial kinetic energy, which also increases its penetration ability. As the supersonic jet is blowing though the ambient gas, the low-velocity ambient gas keeps absorbing the kinetic energy from the supersonic jet, which suppresses impaction depth of top-blowing multi-jets. When the lance height increases, the distance, between top-blowing lance tip to the surface of molten bath, has been prolonged. Consequently, there is more kinetic energy would be removed from top-blowing multi-jets by the ambient gas, resulting in a smaller impaction depth.
The slopes of the linear regression are 0.008, 0.0010 and 0.004 when flow rates are in the range of 18000–19000 19000–20000 and 20000–21000 Nm3/h, respectively. In this research, with a greater flow rate, the increasing rate of impaction depth first improves, and then decreases obviously. Meanwhile, the slopes of the linear regression are −0.04, −0.06 and −0.15 when flow rates are in the range of 1300–1400 1400–1500 and 1500–1600 Nm3/h, respectively. That proves the reducing rate of impaction depth keeps accelerating with a higher lance height.
As represented in Fig. 8, the average impaction depth generated by 1.98–18000, 1.96–20000, 1.98–20000, and 2.00–20000 lance are 81.1, 85.6, 90.7, and 94.5 mm, respectively. That means the 2.00–20000 lance obtains the biggest impaction diameter and depth, which effectively improves the contact area among top-blowing multi-jets, liquid slag and molten bath. At the same time, the shortest mixing time also formed by the 2.00–20000 lance, resulting in a greater mixing effect of molten bath.
With increasing the design top-blowing N2 flow rate, the mixing degree of oxygen gas and elements in molten bath would be obviously improves, making a better kinetic condition. And with more nitrogen gas, the temperature increasing rate of molten bath has been suppressed, which is conducive to vanadium extraction process, resulting in a greater thermal condition of molten bath.
Comparing with the new three kinds of top-blowing lances, the current top-blowing lance achieves the smallest impaction area and depth, and the longest mixing time. Thus, it further proves the increasing N2 flow rate for the N2–O2 mixing top-blowing method is an efficient way to improve the vanadium extraction rate in MURC steelmaking process.
4.2. Numerical Simulation AnalysisFor the all simulation models, their lance height, top-blowing and bottom-blowing flow rate are 1700 mm, 20000 Nm3/h and 675 Nm3/h, respectively. That means the operational condition are same in various simulation model, and the top-blowing lance structure is the only variable. Figure 9 is the flow velocity profile on the longitudinal section using various top-blowing lance. Moreover, there are five typical flow regions, which have been addressed as A, B, C, D and E, respectively, as presented in Fig. 9.
The A region nears the impaction cavity of molten bath. At the impaction cavity, the kinetic energy of top-blowing jets would be transferred into the molten bath, which obviously improves the velocity of molten bath streams.
The B region distributes in the area, where the bottom-blowing bubbles flow through. During the bottom-blowing bubbles rising to the surface of molten bath, the molten bath is kept stirring due to buoyancy force, and the velocity of molten bath is increased.
Therefore, there are two obviously high-velocity regions in molten bath formed by the impaction of top-blowing jets and stirring of bottom-blowing bubbles, which are depicted in A region and B region, respectively, as shown in Fig. 9(a).
In order to further explain the flow characteristics at the C, D and E regions, the Fig. 10 represents the velocity vector distributions of the molten bath at the C, D and E regions using 2.00–20000 lance. The C region is near to the side-wall of furnace. Because of the obstruction effect by the side-wall, when the molten bath stream flows to the side-wall, the trajectory of molten bath stream keeps changing, as shown in Fig. 10(a). During this process, parts of molten bath stream kinetic energy would be transmitted into the internal energy of furnace lining, which reduces velocity of molten bath stream and generates low-velocity C region.
The D region distributes in the bottom center of furnace. The Fig. 10(b) represents the flow directions of molten bath streams have been changed due to the collision among the eddies at the bottom of molten bath, which suppresses kinetic energy of molten bath and forms a low-velocity region.
The E region has been defined as the area at the center area of the eddy. When the molten bath stream rotates around its center of eddy, its velocity keeps reducing, as the molten bath stream becoming closer to its center of the eddy. Hence, the average velocity at the center area of eddy is smaller than that at the peripheral area of eddy, resulting in the low-velocity E region, as shown in the Fig. 10(c).
In order to further investigate the velocity distribution of molten bath, there are three flow zones have been addressed by the bottom-blowing nozzle arrangement, as depicted in Fig. 11. The four red points represent the Laval nozzle arrangement in the steelmaking process. The flow field of molten bath in the centre and middle flow zones are mainly influenced by the top-blowing jets and bottom-blowing bubbles, respectively. The flow field of molten bath in the outer flow zone is decided by both bottom-blowing bubbles and backflow of top-blowing jet. Table 3 presents the average velocities of flow zone and molten bath.
| Label | Outer flow zone | Middle flow zone | Centre flow zone | Molten bath |
|---|---|---|---|---|
| 1.98–18000 | 0.069 m/s | 0.119 m/s | 0.115 m/s | 0.086 m/s |
| 1.96–20000 | 0.072 m/s | 0.124 m/s | 0.119 m/s | 0.089 m/s |
| 1.98–20000 | 0.074 m/s | 0.125 m/s | 0.122 m/s | 0.091 m/s |
| 2.00–20000 | 0.076 m/s | 0.125 m/s | 0.124 m/s | 0.093 m/s |
| Average | 0.073 m/s | 0.123 m/s | 0.120 m/s | 0.090 m/s |
The result shows the average velocities of molten bath in various flow zones generated by all three new kinds of top-blowing lance are bigger than that by the current lance (1.98–20000 lance), which further proves the extra N2 flow rate for O2+N2 mixing top-blowing method could improve the kinetic condition of molten bath. Moreover, a larger designed Mach number will further enhance the average velocities in different flow zones of molten bath, and the 2.00–20000 achieves the biggest average velocity value.
Table 3 shows the ascending order of the average velocity in three flow zones, that is, outer flow zone, centre flow zone and middle flow zone. The outer flow zone is far away from the top-blowing lance and bottom-blowing bubbles, resulting in the lowest velocity value among three flow zones. Although the molten bath absorbs amount of kinetic energy from the high-velocity top-blowing jets at the impaction cavity, the average velocity in centre flow zone would be suppressed by the low-velocity molten steel stream, as shown in Fig. 9(b). Meanwhile, the 8 bottom-blowing nozzles could effectively stir the molten steel in the middle flow zone, which generates the B high-velocity region, as shown in Fig. 9(a). As a result, the average velocity of middle flow zone is bigger than that of centre flow zone, at the test condition.
As mentioned, for the molten steel stream near the surface of furnace wall and in the center of eddy, its velocity is relatively lower than others region in molten bath, which generates the velocity dead-zone. In this paper, the 0.0045 m/s, which is the 5 percent of the average of molten bath, has been selected to calculate the volume of velocity dead-zone. The Fig. 12 is the distribution of the relationship between the average velocity of molten bath and volume of velocity dead-zone in the molten bath.
The result shows the volume of velocity dead-zone reduces with a bigger average velocity. Moreover, as the average velocity increasing by 0.002 m/s, the average volume of velocity dead-zone reduces by 0.006 m3, at the test condition. The volume of velocity dead-zone formed by all three new kinds of top-blowing lance are all smaller than that by the current lance (1.98–20000 lance). The 2.00–20000 lance achieves the smallest volume of velocity dead-zone of 0.015 m3, which means it could evidently improve the mixing degree and obtain a great kinetic condition in vanadium extraction process.
The mass transform rate between liquid slag and molten steel is an important index to evaluate the vanadium extraction rate, and it is improved with a bigger velocity at the interface between liquid slag and molten bath, as depicted in Fig. 13.
The average velocity at the interface formed by 1.98–18000, 1.96–20000, 1.98–20000, and 2.00–20000 lance are 0.077, 0.081, 0.085, and 0.088 m/s, respectively. Thus, the 2.00–20000 lance could effectively improve the mass transform rate between liquid slag and molten steel, resulting in a biggest vanadium extraction rate. However, the FeO in the liquid slag would be also quickly reacted with the [C], [Si] and [Mn], with a greater mass transform rate. As a result, the viscosity of liquid slag increases, which suppresses the vanadium extraction rate at a certain level. For this simulation model, no chemical reaction in the molten bath was taken into consideration, and it is difficult to make accurate predictions about effect of viscosity of liquid slag on the vanadium extraction rate.
4.3. Industrial Application ResearchThe results of the water experiment and the numerical simulation proves that the 2.00–20000 lance achieves the greatest mixing and reaction degrees, at the test condition. In order to further investigate the metallurgical effects of O2+N2 mixing top-blowing method for the MURC steelmaking process, both 1.98–18000 and 2.00–20000 lances were used in a 150 t vanadium extraction converter. The mass balance of oxygen has been calculated before the industrial application research, to achieve the theoretical melting time for the vanadium extraction process. And the validity of the industrial application research has been confirmed. There were 180 heats collected in the industrial application research, including 90 heats using 1.98–18000 lance and 90 heats using 2.00–20000 lance, and the same operation mode has been adopted for two kinds of top-blowing lances.
In this paper, the liquid iron represents the initial conditions before blowing and vanadium extraction process, and the semi-steel represents the final condition after blowing and vanadium extraction process. Table 4 represents the detail of the average components and temperatures for semi-steels. Moreover, the initial conditions of liquid iron are stable, which means the liquid iron condition have little influence on the vanadium extraction process. The results show the [N] contents in the semi-steel generated by 1.98–18000 and 2.00–20000 are same. That proves the O2+N2 mixing top-blowing method could be adopted in the MURC steelmaking process, and has little influence on the [N] content in the end-point semi-steel.
| Top-blowing Lance | 1.98–18000 | 2.00–20000 | |
|---|---|---|---|
| Liquid iron | C (mass%) | 4.15 | 4.16 |
| V (mass%) | 0.205 | 0.204 | |
| Si (mass%) | 0.153 | 0.154 | |
| Mn (mass%) | 0.181 | 0.180 | |
| Temperature (K) | 1319 | 1318 | |
| Semi-steel | C (mass%) | 3.45 | 3.46 |
| V (mass%) | 0.022 | 0.012 | |
| N (mass%) | 3.5×10−5 | 3.5×10−5 | |
| Temperature (K) | 1347 | 1346 | |
| Melting time (min) | 5.2 | 5.0 | |
The Sherwood number (Sh) is a dimensionless number used in mass-transfer operation, which represents the ratio of convective to diffusive mass transport. That means the mass transfer rate of molten bath is increased by a larger Sherwood number. The Sherwood number can be calculated by Reynolds number (Re) and Schmidt number (Sc), as following equation:
| (11) |
where, ρ and μ are the density (kg/m3) and viscosity (Pa·s) of molten bath, respectively. D and d are diffusion coefficient and characteristic length of molten bath, respectively. v is the velocity (m/s) of molten bath steam.
As mentioned, the initial condition of the liquid iron is the same for different top-blowing method. Thus, the initial parameters of molten bath, such as density, viscosity, diffusion coefficient and characteristic length, have also little influence on the vanadium extraction process. The increasing in velocity of molten would lead to a raise in larger mass transfer rate of molten bath, due to a larger Sh, as shown in Eq. (11).
Based on the water experiment and simulation result, the kinetic energy of 2.0–20000 lance is greater than that of 1.8–18000 lance, due to a larger flow rate and a larger Mach number, which indicates the velocity of molten bath using 2.0–20000 lance is larger than using 1.8–18000 lance. Therefore, both Sh and mass transfer rate of the molten bath would be increased by 2.0–20000 lance, comparing with the 1.8–18000 lance.
Besides, for the vanadium extraction process in BOF steelmaking process, its main rate-determining step is considered to be the mass transfer of [V] in the molten metal and (V2O5) in the liquid slag. That indicates the vanadium extraction rate using 2.0–20000 lance is faster than that using the 1.8–18000 lance, based on the Sh. Thus, a shorter melting time is achieved by a faster vanadium extraction rate, when the 2.0–20000 lance is adopted. As a result, the melting time using 2.00–20000 lance is reduced by 0.2 min from 4.5 to 4.3 min, comparing with the 1.98–18000 lance.
The decrease in melting time leads to a decrease in the thermal energy of the melting bath, which is removed by thermal convection and radiation phenomena. Hence, the end-point temperature of semi-steel B increases when the melting time is decreased. Based on the optimal condition of O2+N2 mixing top-blowing method, more N2 flow gas would be injected into the molten bath by the 2.00–20000 lance, comparing with the 1.98–18000 lance. Meanwhile, increasing N2 flow rate leads to more thermal energy of molten bath being removed by heat convection phenomenon, due to the temperature variation between low-temperature N2 and high-temperature molten bath. Thus, the end-point temperature of semi-steel B reduces when the N2 flow rate is improved. As a result, the end-point temperature of semi-steel B remains basically unchanged, as shown in Table 4.
The vanadium extraction rate can be calculated from the difference of vanadium content in liquid iron and semi-steel. The results show the vanadium extraction rate using 1.98–18000 and 2.00–20000 lance are 86.5% and 92.6%, respectively. That proves the extra N2 flow rate for O2+N2 mixing top-blowing method could significantly improve the vanadium extraction rate. Figure 14 illustrates the vanadium distribution in semi-steel using two kinds of top-blowing lances.
Based on the Table 4 and Fig. 14, when the 1.98-18000 lance is used for vanadium extraction, the average content of vanadium in semi-steel fluctuates from 0.016 to 0.028 mass%, with a fluctuation range of 0.012%. Similarly, the average content of vanadium in semi-steel using 2.00–20000 lance fluctuates between 0.009 and 0.015 mass%, with a fluctuation range of only 0.006 mass%. In this research, the 2.00–20000 lance decreases the content and fluctuation range of vanadium content in semi-steel, which further proves the extra N2 flow rate for O2+N2 mixing top-blowing method could improve the stability of vanadium extraction process.
Figure 15 depicts the main content of endpoint slag. In order to improve the level of vanadium content in slag, only rich iron ore is used for controlling the slag fluidity in slag forming process. None other slag-forming material would be allowed to add into the molten bath, during the vanadium extraction process. Therefore, the main components of slag are FeO, V2O5, SiO2 and MnO, and the Fe, V, Si and Mn elements are mainly provided by the molten iron. The Table 4 presents the liquid iron conditions are basically same for both the 1.98–18000 and 2.00–20000 top-blowing lances. That means the quantities of slag are also basically same, when the 1.98–18000 and 2.00–20000 top-blowing lances are used, and quantity of slag has a limited influence on the vanadium extraction process.
Based on the results of water experiment and numerical simulation, the kinetic and thermal conditions in molten bath have been enhanced by the 2.00–20000 lance. As a result, the V2O5 content in the endpoint slag is increased by 0.9 mass%, and the FeO content in the endpoint slag is decreased by 3.6 mass%, when the 2.00–20000 lance has been used in vanadium extraction converter. Meanwhile, the fluidity of liquid slag decreases with a greater V2O5 content and lower FeO content. Therefore, the sticky slag formed by the 2.00–20000 lance suppresses the M. Fe transport efficiency from liquid slag into molten bath, resulting in the M. Fe in endpoint slag being increased by 0.5 mass%. For the 2.00–20000 lance, the reduction mass of FeO is bigger than the increase mass of M. Fe, which makes the T. Fe content decreased by 2.3 mass%.
In this paper, the extra N2 flow rate for O2+N2 mixing top-blowing method was investigated, and three kinds of top-blowing lances were analyzed in the water experiment and numerical simulation. Based on industrial application research, the metallurgical effects of optimized top-blowing lance in vanadium reaction process also were studied. The main conclusions can be described as follows:
(1) Combing with the mixing time, impaction depth and area generated by the four top-blowing lance, the 2.00–20000 lance was considered as the best lance structure to improve the stirring ability and oxygen contact interface of molten bath. For all kinds of top-blowing lances, the mixing time first increases and then reduces with a higher lance height, and the lance height of 1400 mm obtained the longest mixing time.
(2) The top-blowing jets and bottom-blowing bubbles are the main stirring source for the molten bath, which distinctly improves the velocity of molten bath in the A and B regions. The furnace wall would change the flow trajectory of molten steel streams, which generates C and E regions with a low-velocity value. Meanwhile, when the collision phenomenon formed among the various velocity vector of flow stream, the velocity of molten bath has been suppressed, resulting in a low-velocity region addressed as D region.
(3) Comparing with the 1.98–18000 lance, the average velocity of molten bath is increased by the other three new kinds of top-blowing lance. With the same top-blowing rate of 20000 Nm3/h, a larger designed Mach number would further enhance the average velocity of molten bath, which is opposite to the volume of velocity dead-zone.
(4) Although the molten bath absorbs amount of kinetic energy from the high-velocity top-blowing jets at the impaction cavity, the average velocity in centre flow zone would be suppressed by the low-velocity molten steel stream at the bottom of the furnace. Simultaneously, the 8 bottom-blowing nozzles could effectively stir the molten steel in the middle flow zone, which generates the B high-velocity region. Thus, the average velocity of middle flow zone is bigger than that of centre flow zone, at the test condition.
(5) From the result of the industrial plan test, melting time is reduced by 0.2 min, the vanadium extraction rate is increased by 6.1 mass%, and the T. Fe is decreased by 2.3 mass% compared with the 1.98–18000 lance, when the 2.00–20000 lance has been adopted.
The authors would like to express their thanks for the support by National Nature Science Foundation of China (NSFC 52074024) and the National Nature Science Foundation of China (NSFC 51974024).
