Kinetic Mechanism and Process Optimization of Hot Metal Desulfurization Pretreatment

Abstract

A 1/5 scale water model simulation was used to study the kinetic mechanism for 300 t mechanical stirring desulfurization station, factors including rotating speed, immersion depth and size of impellers were investigated. The results show the dispersion behavior of desulfurizer can be divided into three stages, (1) the entrainment depth of desulfurizer increases with the rotating speed increasing, (2) the entrainment depth of the desulfurizer remains unchanged, (3) the desulfurizer is thrown out into the water from impeller blades. For the reason that the desulfurizer is sticky to the top surface and the middle of impeller blades, which will cause the shortening of the impeller life, the optimum rotating speed is the minimum rotating rate that the desulfurizer reaches the upper surface of impellers, which is related with immersion depth of impellers compared with the bath level and the length of impellers compared with the diameter of the ladle. After the application of a larger impeller, with 14.0% impeller life longer than before and the sulfur element is removed to 0.0006% on average.

1. Introduction

Mechanical stirring desulfurization method was invented and applied to industrial production in 1965.¹⁾ For sulfur element deteriorates the toughness and other mechanical properties of steel plates, and with the higher and higher performance requirements from downstream users, it is necessary for the converter to have low and stable sulfur content for producing high quality steel, so, hot metal desulfurization pretreatment method is now widely used in the world.

Mechanical stirring desulfurization process can be described as follows, first of all, the refractory protected impeller is inserted into the molten iron at a certain depth with rotating, and then, the desulfurizer is added into the vortex formed by the rotation, so the sulfur in the molten iron reacts with the desulfurizer during the stirring process. However, in a long period of time, for the high temperature drops, high desulfurizer consumption and other reasons, mechanical stirring desulfurization method is only applied in a small number of steel plants.^2,3)

In order to improve the desulfurization efficiency, many jobs were done, such as, Kikuchi et al.⁴⁾ found desulfurization flux efficiency increases by 1.2–1.7 times by mixing propane gas with the nitrogen carrier gas at propane gas ratio of 10–58%; Nakai et al.^5,6,7,8) did very detailed research and found that desulfurization flux consumption decreased by 19% by powder blasting with a carrier gas; Ji et al.⁹⁾ found that the stirring mode of 90–50 rpm for 8 s switch of variable-velocity stirring method is the best choice for the mixing efficiency; Deng et al.¹⁰⁾ changed the position of the stirring by increasing the impeller eccentricity; Shiba et al.¹¹⁾ investigated the operation parameters during the desulfurization process.

The desulfurization reaction is composed of three steps:¹²⁾ 1) mass transfer of the reactants [S] and [Si] in the iron; 2) desulfurization reaction at iron-slag interface; 3) diffusion of desulfurization products in slag. There are many studies on the rate-controlling step of the above steps, and the conclusions are not the same, Wang et al.¹³⁾ found the transfer of sulfur from the molten iron to the slag-iron reaction interface is the rate-controlling step through theoretical and experimental analysis; Xu et al.¹⁴⁾ argued that the main rate-controlling step is in the interface; Jiang et al.¹⁵⁾ found when the sulfur content in the melt iron is high (> 120 ppm), mass transfer in the slag side is the speed rate-controlling step, and when the sulfur content is low (< 70 ppm), mass transfer of sulfur in the iron is the rate-controlling step.

These papers have revealed the kinetic mechanism and how to improve the desulfurization efficiency, however, some of these schemes are difficult to implement in industry; at the same time, some experiments use mixing time to judge the desulfurization effect, considering the conclusion that when the KR desulfurization reaction occurs, only 10% of the desulfurizer participates in the reaction,¹⁶⁾ and most of the desulfurizer does not participate in the reaction, at this time, it is unreasonable to use mixing time to determine whether or not the molten iron is evenly mixed, and then determine whether the desulfurization reaction achieves balance. Therefore, the evaluation method needs to be optimized.

In this paper, water model simulation method and field experiment were used, factors including rotating speed, immersion depth and size of impellers which are easy to control in the industry field were investigated, to study the flux dispersion behavior during the stirring process.

2. Experiment

Water model simulation for 300 t mechanical stirring desulfurization was carried out. The geometry similarity ratio was 1:5. The experimental conditions used in the water model simulation are shown in Table 1.

Table 1. Parameters of prototype and water model.

Parameter	Prototype	Water model
Flow media	Iron	Water
Density, ρ (kg/m3)	7080	998
Viscosity, η (Pa·S)	6.4×10−3	1.019×10−3
Equivalent diameter of the ladle, dladle (m)	4.006	0.8012
Rotating speed, N (rpm)	60–150	60–150
Immersion depth, di (m)	1.6/1.7/1.8/1.9/2.0/2.1	0.32/0.34/0.36/0.38/0.40/0.42
Re=d2·N·ρ/μ	137.64~446.31×106	4.87~15.8×106

Schematic diagram of apparatus for water model simulation is shown in Fig. 1, during the water model simulation, an up and down adjustable platform was used to adjust the immersion depth of impellers in the water, and a continuous speed adjustable motor was used to control the rotating speed.

Fig. 1.

Schematic diagram of apparatus for water model simulation.

The model of ladle is made of plexiglass in order to observe the flux dispersion behavior. Expanded polystyrene plastic with a density of about 450 kg/m³ was added to the ladle to simulate the desulfurizer. The ratio between density of slag and density of iron equals to the ratio between the density of plastic and density of water, namely, ρ_slag/ρ_iron = ρ_plastic/ρ_water.

In the case of viscous incompressible fluid forced flow, the dominant role played on the flow is the viscous force rather than gravity. In this case, Fr number could be ignored and only Re number considered is appropriate. Due to the self-simulation characteristics of the forced flow of viscous incompressible fluid, Re number of the model is not guaranteed to be equal to Re number of the prototype when the Res are both in the self-simulation region. In this way, regardless of how much more or less the prototype Re than the second critical value, as long as the model Re is larger than the second threshold (Re > 10³ – 10⁴) is appropriate.¹⁷⁾

Therefore, the rotating speed of the water model simulation is set to be the same as the real rotating speed at the range of 60–150 rpm with intervals of 10 rpm. At the same time, other parameters like immersion depth, which is the distance between bath surface and blade upper surface, is set to be 0.12/0.14/0.16/0.18/0.20/0.22 m. Considering the actual molten iron loading, the bath level h_b is set to 0.780 m.

For the truncated cone shaped ladle, in the water model, the upper diameter of the ladle is 0.826 m and the lower diameter is 0.763 m, the total height is 0.909 m, as the immersion depth of impellers in the simulation is about 0.12 m to 0.22 m, an equivalent diameter d_ladle of 0.800 m is used for simplicity.

In order to investigate the impact of impeller sizes on the flux dispersion behavior, three different sizes of cross type impellers were used in the simulation, schematic diagram of impeller is shown in Fig. 2. Specific parameters like the upper length of the blade face towards the iron l_tu, the upper length of the blade back to the iron l_bu, the lower length of the blade face towards the iron l_tl, the lower length of the blade back to the iron l_bl, thickness of the blades l_th, height of the impellers h_s, total length of the upper blade (l_t = l_tu + l_th + l_bu) are shown in Table 2, and in order to evaluate the length of impellers compared with the ladle, l_t/d_ladle is calculated and shown in the last column of Table 2.

Fig. 2.

Shape of impellers.

Table 2. Parameters of impellers used in the water model.

Impeller Number	ltu (m)	lbu (m)	ltl (m)	lbl (m)	lth (m)	hs (m)	lt (m)	lt/dladle
1#	0.103	0.088	0.086	0.071	0.097	0.2	0.288	0.359
2#	0.113	0.098	0.096	0.081	0.097	0.2	0.308	0.384
3#	0.123	0.108	0.106	0.091	0.097	0.2	0.328	0.409

As the iron-desulfurizer interfacial area is connected with the entrainment depth of desulfurizer, the parameter of entrainment depth of desulfurizer is used to evaluate the test results. As it is difficult to follow every desulfurizer particle in the ladle, the entrainment depth of desulfurizer is defined as the depth between the bath surface under static conditions and the height of the water where more than 5% of the section is filled with desulfurizer particles.

Finally, the simulation results are verified on 300 t desulfurization pretreatment stations, and in order to avoid the deviation of the test results caused by the difference in initial conditions, during the field test, different impellers were set up at two desulfurization stations at the same time, to ensure that the initial conditions were basically the same.

3. Experiment Results

3.1. Flow State and Distribution of the Desulfurizer

A typical flow state with an immersion depth of 0.20 m is shown in Fig. 3, the mark in the figure means “impeller number # rotating speed”, for example, 2#60 indicates that impeller 2# with a rotating speed of 60 rpm.

Fig. 3.

A typical steady flow field at different rotating speeds of impeller 2# at immersion depth of 0.20 m.

Figure 3 shows, when the impeller is rotating at a lower speed (60 rpm–90 rpm), the entrainment depth of the desulfurizer is shallow, and the area formed by desulfurizer particles in water is small. With the rotating speed increasing, the entrainment depth of desulfurizer increases, and when the entrainment depth of the desulfurizer reaches the upper surface of the impeller (100 rpm), the entrainment depth reaches the deepest.

At this time, if the rotating speed continues to increase, the desulfurizer starts to have contact with the impeller, and later the desulfurizer is entrained into the middle of the impeller blades. As the rotating speed continues to increase, the immersion depth of the desulfurizer dose not change, but the particles entrained in the middle of the impeller increase. Furthermore, if the desulfurizer between the middle of the blades accumulates to a certain amount, the desulfurizer may be thrown out into the water from impeller blades.

The flow state of other immersion depths and impeller sizes are almost the same as shown in Fig. 3. They are also divided into the above steps. Firstly, with the rotating speed increasing, the entrainment depth of desulfurizer increases, and secondly, with the rotating speed continues increasing, the immersion depth of the desulfurizer does not change. Thirdly, if the desulfurizer between the middle of the blades accumulates to a certain amount, the desulfurizer may be thrown out into the water from impeller blades. The difference in flow status brought by different parameters is mainly that, under different conditions of immersion depths and impeller sizes, the transition speeds between different stages are different.

3.2. Effect of Impeller Size on the Entrainment Depth of Desulfurizer

The relationship between the entrainment depth of desulfurizer and rotating speed/immersion depth of impellers is shown in Fig. 4. As shown in the figure, for impeller 1#, when the rotating speed is 100 rpm or less, the depth of desulfurizer entrained in water increases with rotating speed increasing. When the immersion depth of impeller is 0.12–0.18 m, the minimum speed that the desulfurizer entrained in water reaching the upper surface of impeller is 100 rpm, and 110 rpm at the immersion depth of 0.20 m, 120 rpm at the immersion depth of 0.22 m.

Fig. 4.

Relationship between entrainment depth of desulfurizer and rotating speed/immersion depth.

When the immersion depth is 0.12–0.18 m, and the rotating speed is between 100–130 rpm, the entrainment depth of desulfurizer does not increase with the rotating speed increasing. When the rotating speed is higher than 130 rpm, with the increasing rotating speed, desulfurizer entrained in water increases, and then the desulfurizer entrained in water can reach the middle of the ladle.

For impeller 2# and 3#, the results are almost the same, the differences are mainly focus on two aspects, firstly, due to the size difference of impellers, the speed of the desulfurizer that reaches the upper surface of the impeller is different under the conditions of different immersion depths; secondly, for impeller 2# and 3#, even if the rotating speed reaches the top of 150 rpm, the phenomenon of desulfurizer been thrown out into the water from impeller blades did not happen.

3.3. Effect of Rotating Speed on the Entrainment Depth of Desulfurizer

Desulfurizer is added to the iron ladle at a very low speed after mechanical stirring started to reduce the spatter of desulfurizer. But if the rotating speed is too low, the desulfurizer spreads evenly on the ladle surface, and it is difficult for the desulfurizer to react with [S] in molten iron, resulting in a longer treatment time. Normally the desulfurizer is added to the ladle at the rotating speed of 30–60 rpm, the faster the speed without desulfurizer splashing the better, so the starting speed of water model simulation is set to be 60 rpm. The results are shown in Fig. 5.

Fig. 5.

Entrainment depth of desulfurizer at rotating speed 60 rpm.

It can be seen from Fig. 5, at the rotating speed of 60 rpm, the entrainment depth of the desulfurizer in water grows with the size of impellers, the impeller size plays a significant role in promoting for the desulfurizer rapidly dissolving at the starting of stirring. When the immersion depth is between 0.12–0.18 m, as the immersion depth of impellers increases, depth of the desulfurizer entrainment increases. And when immersion depth is between 0.18–0.22 m, the depth of the desulfurizer entrainment is almost stable, which illustrates that at the starting time of stirring, the deepest immersion depth of impellers should not exceed 0.18 m. It can be included from Fig. 5, at the rotating speed of 60 rpm, the deepest desulfurizer entrainment depths of different impeller sizes all appear at the position where the immersion depth is 0.18 m, among three impellers, the deepest entrainment depth of desulfurizer is 0.10 m brought by impeller 3#, and for impeller 2# and 1#, the numbers are 0.09 m and 0.08 m, which are both much smaller than 0.10 m, so, a larger impeller is preferred for deeper entrainment of desulfurizer at the rotating speed of 60 rpm.

As shown in Fig. 6, at the rotating speed of 100 rpm, when the immersion depth of impellers is lower than 0.18 m, the entrainment depth of the desulfurizer in water is the same as the immersion depth of impellers, at the same time, for impeller 1# and 2#, if the immersion depth is much deeper like 0.20 m or 0.22 m, the entrainment depth of the desulfurizer in water remains the same, and is smaller than the immersion depth.

Fig. 6.

Entrainment depth of desulfurizer at rotating speed 100 rpm.

As shown in Fig. 7, at the rotating speed of 150 rpm, for impeller 2# and 3#, the entrainment depth of the desulfurizer in water is the same as the immersion depth of impellers, but for impeller 1#, as shown in Fig. 4 (1#), when the rotating speed is faster than 130 rpm, the desulfurizer is thrown out into the water, and then the entrainment depth of the desulfurizer in water is much higher than the immersion depth of the impeller.

Fig. 7.

Entrainment depth of desulfurizer at rotating speed 150 rpm.

4. Discussion

4.1. The Basic Principle of Mechanical Stirring Desulfurization

Traditionally, due to the addition of CaF₂, mechanical stirring desulfurization reaction with the CaO–CaF₂ based desulfurizer   

$$4\mathrm{C}\mathrm{a}{\mathrm{O}}_{(\mathrm{s})}+[\mathrm{S}\mathrm{i}]+2[\mathrm{S}]=2\mathrm{C}\mathrm{a}{\mathrm{S}}_{(\mathrm{s})}+2\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{S}\mathrm{i}{\mathrm{O}}_{2(\mathrm{s})}$$(1)

is considered to be a liquid-liquid chemical reaction, so mixing time is used as a key index to judge the desulfurization effect, however, new findings¹⁸⁾ prove that the chemical reaction only occurs on the surface of solid CaO, it is kind of a solid-liquid chemical reaction.

In order to analyze the desulfurization behavior of CaO–CaF₂ based desulfurizer, slag phase of the desulfurization reaction is observed using SEM, the slag was taken from the KR desulfurization station of Shougang Jingtang Company, and the desulfurizer is CaO–CaF₂ with the ratio of 9/1. The SEM micrograph is shown in Fig. 8, and the chemical composition of points 1–9# in the figure is shown in Table 3.

Fig. 8.

SEM micrograph of desulfurization slag with CaO–CaF₂ desulfurizer.

Table 3. Chemical composition of points 1#–9 # in Fig. 8.

Composition, %	1#	2#	3#	4#	5#	6#	7#	8#	9#
CaO	100	100	100	69.4	69.1	72.8	–	–	–
SiO2	–	–	–	17.3	19.5	12.3	–	–	–
CaF2	–	–	–	10.3	9.6	9.8	–	–	–
CaS	–	–	–	–	–	–	100	100	100
Al2O3	–	–	–	3.0	1.7	5.0	–	–	–
Description	Lime phase			CaO–SiO2–CaF2 phase			CaS phase

It can be seen from Fig. 8, the chemical composition of points 1#–3# is pure CaO, which is the lime in desulfurizer, indicating that majority of the sulfur element in the iron did not enter into the internal of the solid CaO and reacts with it. The chemical composition of points 4#–6# is CaO–SiO₂–CaF₂ phase (containing a small amount of Al₂O₃), which indicates the chemical reaction occurs on the surface of solid CaO, at the same time, CaF₂ in desulfurizer reacts with the high melting point 2CaO·SiO₂ and 3CaO·SiO₂ and a liquid phase is formed. The liquid CaO–SiO₂–CaF₂ phase provides a diffusion channel for the sulfur element, and the desulfurization product CaS is formed on the solid phase of CaO, which is shown as point 7#–9# in Fig. 8.

The SEM micrograph shows the mechanism of desulfurization: when solid CaO is added to the molten iron, the sulfur element in the molten iron passes through the liquid CaO–SiO₂–CaF₂ phase which is a diffusion channel on the surface of CaO, and forms the desulfurization product CaS on the surface layer of the CaO solid phase. Therefore, providing a large surface area for the chemical reaction, and promoting a successive contact between the desulfurizer and the sulfur in molten iron is the key to achieving high desulfurization efficiency.

4.2. Dispersion Behavior of Desulfurizer

It can be summarized from Figs. 3 and 4, during the rotating period, the dispersion behavior of desulfurizer can be divided into three stages as shown in Fig. 9, stage 1, with the rotating speed of impellers increasing, the entrainment depth of desulfurizer increases, until to a certain speed the desulfurizer reaches the upper surface of the impeller; stage 2, when the desulfurizer reaches the upper surface of impeller, the impeller prevents the desulfurizer from further being entrained, under this condition, even if the rotating speed of the impeller increases, the curve of entrainment depth changing with rotating speed appears a plateau and therefore the entrainment depth remains unchanged, in this stage, with the rotating speed continues increasing, the desulfurizer is entrained into the middle of the impeller blades; stage 3, with the desulfurizer entrained in the middle of the impeller blades accumulating to a certain extent by the rotating speed increasing, the desulfurizer is thrown out into the water around the impeller blades.

Fig. 9.

Flow field schematic diagram at different rotating speeds.

To be more precise, as shown in Fig. 10, at lower rotating speed, the molten iron in the horizontal position of the impeller rotates horizontally under the driving force of the impeller, it is not a V-shape with sharp corners, but a U-shape with a flat bottom. At this time, as the rotating speed increases, the depth of the vortex formed by stirring increases. Because the desulfurizer in the vortex floats on the surface of the vortex, the contact area of the desulfurizer with the molten iron increases accordingly.

Fig. 10.

Movement path of desulfurizer.

Soon, the entrainment depth of the desulfurizer reaches the top surface of the impeller, due to the blocking of the impeller, it is difficult for the desulfurizer in the bottom of the u-shaped vortex to go further lower. As the speed increases, the entrainment depth of the desulfurizer will not change within a certain speed range, especially when the size of the impeller is relatively large. At this time, if the entrainment depth of the desulfurizer does not change and the stirring speed keeps increasing, the desulfurizer will adhere to the top surface of the impeller.

Furthermore, the desulfurizer will be drawn between the impeller blades. Within a certain range of stirring speed, part of the desulfurizer will always stay between the blades and adhere to the blades with the stirring. When the speed further increases and the desulfurizer accumulated between the blades reaches a certain amount, the desulfurizer will be thrown out into the molten iron by the impeller blades, but no matter what the situation, it is inevitable that a large amount of desulfurizer particles will adhere between the blades.

4.3. Influence of Rotating Speed on Desulfurization Effect

As shown in Fig. 9, when the rotating speed continues to increase, the slag/desulfurizer starts contact with the impeller, and then the desulfurizer get entrained in the middle of the impeller blades (in stage 2), with parts disperses into molten iron (in stage 3), and parts gets adhered between impeller blades.

The adhesion of the desulfurizer to impeller causes two bad effects, firstly, the slag/desulfurizer is attached to the impeller refractory, and then the reaction between the two happens and induces the fusion of each other, eventually, as the sticky slag is difficult to clean up, and in order to maintain a good shape of the impeller, mechanical cleaning is always used to clean the impeller, and then the life of the refractory is also affected, which will also adversely affect the production organization, the related research is reported in another article by the author;¹⁹⁾ secondly, as the slag is always sticky to the blade face towards the iron, the effective area of the blade face towards the iron will become smaller, as shown in Fig. 11, and then the kinetic conditions are deteriorated and the desulfurization effect becomes worse.

Fig. 11.

Slag/desulfurizer gets adhered to impeller blades.

At the same time, as reported in the article,¹⁹⁾ as the stirring speed increases, the shear stress on the impeller increases, which leads to cracks in the refractory material, and the wear of the refractory material caused by the washing of molten iron increases, high speed is not conducive to the control of the life of the impeller, so higher rotating speed should not be used for the good of the refractory life.

When the desulfurizer is entrained between the blades of the impeller, it is very easy to cause the desulfurizer to adhere to the blades, resulting in the reduction of the effective stirring area of blades face towards the iron, resulting in a decrease in the desulfurization efficiency of subsequent heats, based on comprehensive consideration, the operation is unreasonable. Therefore, the optimum rotating speed is the minimum rotating rate that the desulfurizer reaches the upper surface of impellers. According to Figs. 9 and 10, the lowest speeds that the desulfurizer reaches the upper surface of impellers under a certain immersion depth are shown in Table 4.

Table 4. Lowest speed the desulfurizer reaches the upper surface of impellers.

Immersion depth of impellers (di, m)	1# (rpm) lt/dladle =0.359	2# (rpm) lt/dladle =0.384	3# (rpm) lt/dladle =0.409
0.12	100	90	80
0.14	100	90	90
0.16	100	100	90
0.18	100	100	90
0.20	110	100	100
0.22	120	110	110

In order to find the relationship among the lowest speed that the desulfurizer reaches the upper surface of the impeller (N_u), immersion depth of impellers (d_i) compared with the bath level h_b and the length of impellers compared with the diameter of the ladle (l_t/d_ladle), non-linear fitting method is used to do the analyze job, the best option with the correlation coefficient (R) 0.95 is as follows:   

$$N_u=25\mathrm{\hspace{0.17em} }432\times {(d_i/h_b)}^{5.56}+35.12\times {(l_t/d_{ladle})}^{-0.99}$$(2)

N_u is the best rotating speed for the impellers to rotate, for a certain d_i/h_b and l_t/d_ladle, when the real rotating speed is lower than N_u, with the rotating speed increasing, the entrainment depth of desulfurizer increases. When the rotating speed comes to N_u, that means the desulfurizer reaches the upper surface of the impeller, the entrainment of the desulfurizer is the deepest, at this time, the contact area of the desulfurizer with the iron is maximum.

4.4. Industrial Application Results

As shown in the previous chapters, the deeper the immersion depth, the deeper the desulfurizer entrainment depth, but due to the mechanical equipment restrictions, the maximum immersion depth of Shougang Jingtang Company mechanical stirring station can only be set to 0.22 m (the equivalent of 1.1 m at the mechanical stirring station). The entrainment depth of desulfurizer with rotating speed of different impellers at the immersion depth of 0.22 m is shown in Fig. 12, it can be concluded from the figure, in the range of 60–100 rpm, the entrainment depth of the desulfurizer increases as the size of the impeller increases. At the speed of 80 rpm, for example, the entrainment depth of impeller 3# increased by 30% compared with impeller 1#.

Fig. 12.

The entrainment depth of desulfurizer at immersion depth of 0.22 m.

Taking into account the desulfurizer entrainment depth, platform appeared at speed of 100–130 rpm range, and the reduction of impeller refectory life with rotating speed above the high speed of 130 rpm, impeller 3# with deeper entrainment depth of the desulfurizer is best for Shougang Jingtang mechanical stirring desulfurization, and then the rotating speed is set to be 100 rpm for a new impeller, and the rotating speed changes with the impeller life due to the reduction of the effective stirring area. This basic law ensures that the desulfurizer is deep enough to ensure the desulfurization efficiency, but also to avoid the shortening of impeller life.

On-site application effect of Shougang Jingtang Company mechanical stirring station is as follows, with the operating conditions that, the initial molten iron sulfur content is about 0.06%, the target end-point sulfur content is divided into three grades, respectively ≤ 0.0010%, 0.0011–0.0020% and > 0.0020, the stirring time is about 8–10 minutes, and the desulfurizer is CaO–CaF₂ with the ratio of 9/1 , the amount of desulfurizer added is 8–10 kg/t iron depending on the target end-point sulfur content, and as for the impeller life, the maximum value, minimum value and average value of impeller 1# and impeller 3# are as shown in Fig. 13. Due to the larger size and lower rotating speed, the average life expectancy for impeller 3# is 318 heats, up to 355 heats, which are 14.0% and 9.2% higher than impeller 1# with an average of 279 heats and a maximum of 325 heats, as shown in Fig. 13.

Fig. 13.

Comparison of impeller life.

Industrial application results are shown in Fig. 14, it can be seen that before and after the application of impeller 3#, with the same stirring time of 8–10 minutes, the sulfur element is removed to 0.0006% on average, and at the end of pretreatment, the sulfur content of all heats are below 0.0025%, both the maximum sulfur content and average sulfur content of the desulfurization endpoint are significantly lower than before the application.

Fig. 14.

Industrial application results.

5. Conclusions

Water model simulation was used to improve the desulfurization efficiency, the conclusions are:

(1) The dispersion behavior of desulfurizer can be divided into three stages during the stirring process, firstly, the entrainment depth of desulfurizer increases with the rotating speed increasing, secondly, a platform occurs, and the entrainment depth of the desulfurizer remains unchanged; thirdly, with the rotating speed continues increasing, the desulfurizer is thrown out into the water from impeller blades.

(2) Considering the bad effect of the desulfurizer entrained into the middle of impeller blades, the optimum rotating speed is the minimum rotating rate that the desulfurizer reaches the upper surface of impellers (N_u), which is related to the immersion depth of impellers (d_i) compared with the bath level h_b and the length of impellers (l_t) compared with the diameter of the ladle (d_ladle), N_u=25432*(d_i/h_b)^5.56+35.12×(l_t/d_ladle)^−0.99.

(3) After the application of a larger impeller, with 14.0% impeller life longer than before and the sulfur element is removed to 0.0006% on average.

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