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Desulfurizer desulphurization kinetics by the injection method
Journal of University of Science and Technology Beijing Volume 15, Number 4, August 2008, Page 375
Metallurgy
Desulfurizer desulphurization kinetics by the injection method Wei Wu, Zhijun Han, Yanbin Hu, and Wei Wu Metallurgical Technology Institute, Central Iron & Steel Research Institute, Beijing 100081, China (Received 2007-08-01)
Abstract: To obtain a better desulphurization effect in hot metal, suitable desulfurizers should be selected first according to thermodynamics. However, the effect of desulphurization is also strongly affected by kinetics. The conditions of different desulfurizers (Mg, CaC2, and lime) penetrating into hot metal, the rising up velocity in iron melt, residence time, and dissolving time are theoretically calculated and analyzed. The results are helpful to select the desulphurization process and equipment and to improve the desulphurization effect. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: hot metal; desulphurization; desulphurization kinetics; injection
1. Introduction With the promotion of the desulphurization process of hot metal in China, a lot of desulphurization methods are used individually [1-8], in which the principal method is the injection method. In this method, the desulfurizer accompanied carrier gas is injected into hot metal through a lance. Because the desulphurization reaction is an interface reaction between the desulfurizer and hot metal, when two phases contact and the desulfurizer is melted, the desulphurization reaction is generated [9]. When the desulfurizer particles immerse into the hot metal, it is necessary to overcome boundary tension, frontal resistance, and buoyancy. During the injection process, the melt, fusion, and chemical reaction of the desulfurizer partiTable 1. Desulfurizer CaO Na2O CaC2 Mg
cles are influenced by a lot of factors, such as the carrier gas pressure, flow, penetration depth, outlet diameter, outlet velocity of the lance, and the depth of hot metal [10]. Thus, it is very necessary to analyze the desulphurization kinetics of the injection method.
2. Desulfurizer selection For deep desulphurization in hot metal and to meet the requirement of sulphur content for steelmaking, the desulfurizers, which are suitable to hot metal conditions, steelmaking process, and the desulphurization target, should be selected. According to the element reaction thermodynamics, selectable desulfurizers and their characteristics are listed in Table 1 [11].
Comparison of the desulphurization effects among four kinds of desulfurizers
Desulphurization constant 6.5 5.00u104 6.9u105 2.1u104
Equilibrium [%S] 3.7u103 4.8u107 4.9u107 1.6u105
Desulphurization process Stirring Injection Injection Injection
Once a suitable desulfurizer is selected according to its characteristics listed in Table 1 and combined with practical production conditions, the next step is to add the desulfurizer into hot metal and to improve the kinetic conditions of the desulphurization process. Corresponding author: Wei Wu, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
Consumption / (kgt1) 5-8 5-7 4-6 0.4-0.6
Final [%S] <0.001 <0.003 <0.005 <0.005
Temperature drop / qC 20-30 20-30 10-20 10-15
2.1. Conditions for desulfurizer particle penetration into hot metal by the injection method When desulfurizer particles penetrate into liquid metal, it must overcome the interfacial tension, facade resistance, and buoyant force. The minimum velocity Also available online at www.sciencedirect.com
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J. Univ. Sci. Technol. Beijing, Vol.15, No.4, Aug 2008
for a particle to penetrate into metal melt is directly proportional to the square root of the surface tension of the melt but inversely proportional to the square root of the particle diameter, given by [5] v1
§ U1 · ¨ 2 0.5 ¸ Up ¹ ©
1
2
° 96V m ® ¯° 13U 1
ª § 26 U 1 · º °½ « exp ¨ ¸ 1» ¾ «¬ © 16 U p ¹ »¼ ¿°
1
2
dp
1 2
(1) where vl is the critical velocity, m/s; U1 the density of liquid metal, kg/cm3; Up the particle density, kg/cm3; and dp the particle diameter, mm.
When the particle has high kinetic energy, it can penetrate through the gas/liquid interface and enter the liquid metal. When the particle has low kinetic energy, it can not overcome the resistance of surface tension and will stick on the surface of a gas bubble and float up with the bubble. When the particle has very low kinetic energy, it will suspend in the gas bubble and finally float into air with the gas bubbles. The relation between the particle diameter and critical velocity for three frequently used agents is shown in Fig. 1.
hot metal can be calculated by the following equation [6]: § g 'U · Q s 1.74 ¨ ¸ © Ul ¹
1/ 2
(2)
d s 1/ 2
where vs is the rising up velocity, m/s; 'ȡ the density difference between the particle and liquid metal; ȡl the density of hot metal; ds the particle diameter. The calculated results are shown in Fig. 2. It can be seen from the figure that after the particle penetrates into the liquid metal, it floats up quickly. Its rising up velocity is directly proportional to the square root of the particle diameter and inversely proportional to the square root of the particle density.
Fig. 2. Relation between the diameter of solid particles (ds) and the rising up velocity (vs).
Fig. 1. Relation between the critical velocity (vl) and the diameter of solid particles (ds) for CaO, CaC2, and Mg.
Fig. 1 shows that when the diameter of the desulfurizer particle is 0.5 mm, the required velocity at the exit of the injection lance should be 45 m/s for a CaC2 particle, 65 m/s for a CaO particle, and 90 m/s for an Mg particle. For the same kind of particles, the smaller the diameter is, the higher the injection velocity is required. When the diameter of the desulfurizer particle is less than 0.2 mm the required velocity increases rapidly. When the diameter is less than 0.1 mm, the required velocity can hardly be reached, thus, a major part of the particles will be brought out from liquid metal into open air by the carrying gas. This will increase the reagent consumption significantly. 2.2. Rising up velocity of desulfurizer particles in hot metal The rising up velocity of desulfurizer particles in
When Mg is injected into the hot metal as the desulfurizer, Mg particles are evaporated to form Mg vapor bubbles quickly. Therefore, during the calculation for Fig. 2, the particle is first converted to vapor bubbles. Then, to calculate its rising up velocity, the relation between the diameter of Mg particles and the diameter of Mg vapor bubbles (dg) can be calculated by the following equation [13]: dg
(3)
1/ 3 21.2 PMg ds
The rising up velocity of Mg vapor bubbles can be calculated by the following equation [13]: 1/ 6 1/ 2 Q g 10.4 PMg ds 10.4 1 0.7 h
1/ 6
d s 1/ 2
(4)
where vg is the rising up velocity, m/s; PMg the Mg vapor pressure, Pau105; ds the particle diameter, m; h the injection depth, m. The rising up velocity of Mg vapor bubbles is directly proportional to the square root of the Mg particle diameter. Therefore, to increase the efficiency of desulphurization, the rising up velocity of Mg vapor bubbles should be decreased, i.e. the diameter of Mg particles should be decreased. However, decreasing the Mg particle size will render the particle difficult to penetrate into the hot metal.
W. Wu et al., Desulfurizer desulphurization kinetics by the injection method
This is thus a contradictory problem. A method to solve this is to increase the velocity of the carried gas at the nozzle outlet. 2.3. Resistance time of Mg, CaC2, and CaO desulfurizers in hot metal Among all desulfurizers, the Mg agent is a special one. After it is injected into hot metal, it will melt, evaporate, dissolve and float up. During these processes, Mg particles react with hot metal through gas/liquid and liquid/liquid reactions. There is no reaction between Mg and slag, thus, the residence time of Mg particles in hot metal is a predominating parameter that affects the desulphurization result. The residence time of Mg particles in hot metal includes the time for melting, evaporating, and floating up. It can be calculated by the following equation [13], and the result is shown in Fig. 3. ¦W
0.025 9.6
h(1 0.7 h) 1 / 6 u 10 2 d s1 / 2
377
According to Eq. (5), when the diameters of CaC2 and CaO particles are about 1 mm, and the injection depth is 2.5 m, the particles only stay 17.6 s in the iron melt. When the injection depth decreases, the residence time of particles in hot metal also decreases. This means the desulphurization mainly take place at the slag/iron interface. Therefore, stirring thoroughly a mixture of slag and iron is extremely important. During the injection, some accelerators such as CaCO3 and MgCO3 can be mixed with the desulfurizer. The accelerators decompose in hot metal producing many CO2 bubbles and causes strong stirring in the melt to enhance the efficiency of desulphurization. The mechanical stirring can provide a powerful stirring effect and improve the kinetic conditions for CaO injection desulphurization, thus it improves the metallurgical results. 2.4. Dissolving time of Mg particles in hot metal
(5)
The solubility of Mg in liquid iron is determined by hot metal temperature and Mg vapor pressure as follows [7]: lg[Mg] 7000/T lg PMg 5.1
(6)
where T is the absolute temperature, K; PMg the vapor pressure of the temperature.
Fig. 3. Relations among the residence time of Mg particles in the melt (¦W), the diameter of Mg particles (ds), and the injection depth (h).
The total residence time of Mg particles in the hot metal, ¦W, is inversely proportional to the diameter of Mg particles, ds, and directly proportional to the injection depth (Fig. 3). If the injection depth is 2.5 m, the particle diameter 1 mm, then the total residence time of Mg particles in hot metal is 9.01 s. When a small hot metal ladle is used to desulfurize, the injection depth will be even smaller and the total residence time shorter. For example, if the injection depth is 1.5 m, the total residence time in hot metal will be only 5.15 s; thus, the desulphurization effect in a small ladle is not as good as in a big one. When CaC2 and CaO are used as desulfurizers, the desulphurization efficiency and desulfurizer consumption are influenced by the residence time of particles in hot metal. The rising up velocity of solid particles in hot metal can be calculated by Eq. (2) [13].
The solubility of Mg in liquid iron increases with the increase in pressure. At atmospheric pressure, the Mg solubilities are 0.45wt%, 0.22wt%, and 0.12wt% when the temperatures are 1200, 1300 and 1400qC respectively. The dissolving time, Wgr, of Mg particles is directly proportion to the Mg particle diameter and inversely proportion to the injection depth [14]:
W gr
0.025 1.287
ds u 10 4 (1 0.7 h) 1 / 3
(7)
The result is shown in Fig. 4. If the Mg particle diameter is 1 mm, and the injection depth is 2.5 m, then the dissolving time should be 9.21 s. It means ¦W
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J. Univ. Sci. Technol. Beijing, Vol.15, No.4, Aug 2008
(5) If the Mg particle diameter is 1 mm and the injection depth is 2.5 m, the dissolving time should be 9.21 s, but the residence time of Mg particles in hot metal is only 9.01 s. This shows that the dissolving time is longer than the residence time. The Mg particle does not have enough time to dissolve before floating up. Thus, sufficient Mg desulphurization does not occur.
References Fig. 4. Relations among the dissolving time of Mg (IJgr), particle size (ds), and injection depth (h).
3. Conclusions (1) When the particle diameter is 0.5 mm, the critical speed of the CaC2 particle from the lance nozzle should be higher than 45 m/s; for CaO particles the speed should be higher than 65 m/s, and for Mg particles 90 m/s. The smaller the particle size is, the higher the injection speed is required. When the particle diameter is less than 0.2 mm, the required speed increases rapidly. When the particle diameter is less than 0.1 mm, the required injection speed can hardly be satisfied. Under this condition, major part of the particles will be brought out of the melt by the carrying gas and therefore markedly increase its consumption. (2) To increase the desulphurization efficiency and the utilization ratio of a desulfurizer, the size of Mg particle should be decreased; however, small Mg particles can hardly penetrate into hot metal. This problem is contradictory. The best way to solve this is to increase the initial velocity of the particles from the lance nozzle. (3) When the injection depth is 2.5 m and the Mg particle diameter is 1 mm, then its residence time in hot metal is only 5.15 s. Because of this, the desulphurization effect is worse than that in a big hot metal ladle. (4) When the diameters of CaC2 and CaO particles are less than 1 mm and the injection depth is 2.5 m, their residence time in hot metal is only 17.6 s. This shows that the desulphurization reactions mainly occur at the metal/slag interface. In addition, various stirring measures are important to mix metal and slag phases.
[1] W. Wu, Y.B. Hu, and L. Liu, New technique of desulphurization by agitation injection for hot metal ladle, J. Univ. Sci. Technol. Beijing (in Chinese), 27(2005), Suppl.1, p.142. [2] Y. Shirota, Clogging-free nozzle conditions and metallurgical effects of powder injection practice in hot metal pretreatment, Trans. ISIJ, 27(1987), No.1, p.19. [3] R.P. Singh and M.J. Mcnallan, Fluid dynamics and mass transfer in submerged gas-particle jets, Metall. Mater. Trans. B, 14(1983), No.3, p.431. [4] G.A. Irons, The influence of conveying conditions on powder injection processed for desulfurization, Trans. Iron Steel Soc. AIME, 24(1984), No.5, p.41. [5] L.R. Farias and G.A. Irons, A united approach to bubbling-jetting phenomena in powder injection into iron and steel, Metall. Mater. Trans. B, 16(1985), No.2, p.212. [6] S.Y. Kitamura, T. Kitamura, and K. Shibata, Effect of stirring energy, temperature and flux composition on hot metal dephosphorization kinetics, ISIJ Int., 31(1991), No.11, p.1325. [7] H.C. Wang and S.J. Wang, Vacuum treatment for simultaneous desulphurization and dephosphorization of hot metal and molten steel, J. Iron Steel Res. (in Chinese), 16(2004), No.2, p.15. [8] N. Hao, H. Li, and H.T Wang, Application of the sulphide capacity theory on refining slags during LF treatment, J. Univ. Sci. Technol. Beijing, 13(2006), No.2, p.112. [9] Z.M. Wang, W. Sun, and X.Q. Yu, The desulphurization process of hot metal in Magang and its applied results, Res. Iron Steel (in Chinese), 32(2004), No.2, p.26. [10] S.S. Yang, Pretreatment of hot metal and pure steel smelting, China Metall. (in Chinese), 13(2003), No.9, p.16. [11] Z.W. Lu, Discussion on selection of desulphurizing agent for hot metal pretreatment, Steelmaking (in Chinese), 17(2001), No.1, p.36. [12] X.Z. Zhang, Injection Metallurgical Elements (in Chinese), Metallurgy Industry Press, Beijing, 1988, p.109. [13] G.F. Yan, On several technological and theoretical problems in desulphurization of hot metal with Mg particle injection, [in] The 13rd CSM Annual Steelmaking Conference (in Chinese), Kunming, 2004, p.71. [14] T.J. Yang, Recent progress of metal external desulphurization, Iron Steel (in Chinese), 34(1999), No.1, p.66.
Metallurgy
Desulfurizer desulphurization kinetics by the injection method Wei Wu, Zhijun Han, Yanbin Hu, and Wei Wu Metallurgical Technology Institute, Central Iron & Steel Research Institute, Beijing 100081, China (Received 2007-08-01)
Abstract: To obtain a better desulphurization effect in hot metal, suitable desulfurizers should be selected first according to thermodynamics. However, the effect of desulphurization is also strongly affected by kinetics. The conditions of different desulfurizers (Mg, CaC2, and lime) penetrating into hot metal, the rising up velocity in iron melt, residence time, and dissolving time are theoretically calculated and analyzed. The results are helpful to select the desulphurization process and equipment and to improve the desulphurization effect. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: hot metal; desulphurization; desulphurization kinetics; injection
1. Introduction With the promotion of the desulphurization process of hot metal in China, a lot of desulphurization methods are used individually [1-8], in which the principal method is the injection method. In this method, the desulfurizer accompanied carrier gas is injected into hot metal through a lance. Because the desulphurization reaction is an interface reaction between the desulfurizer and hot metal, when two phases contact and the desulfurizer is melted, the desulphurization reaction is generated [9]. When the desulfurizer particles immerse into the hot metal, it is necessary to overcome boundary tension, frontal resistance, and buoyancy. During the injection process, the melt, fusion, and chemical reaction of the desulfurizer partiTable 1. Desulfurizer CaO Na2O CaC2 Mg
cles are influenced by a lot of factors, such as the carrier gas pressure, flow, penetration depth, outlet diameter, outlet velocity of the lance, and the depth of hot metal [10]. Thus, it is very necessary to analyze the desulphurization kinetics of the injection method.
2. Desulfurizer selection For deep desulphurization in hot metal and to meet the requirement of sulphur content for steelmaking, the desulfurizers, which are suitable to hot metal conditions, steelmaking process, and the desulphurization target, should be selected. According to the element reaction thermodynamics, selectable desulfurizers and their characteristics are listed in Table 1 [11].
Comparison of the desulphurization effects among four kinds of desulfurizers
Desulphurization constant 6.5 5.00u104 6.9u105 2.1u104
Equilibrium [%S] 3.7u103 4.8u107 4.9u107 1.6u105
Desulphurization process Stirring Injection Injection Injection
Once a suitable desulfurizer is selected according to its characteristics listed in Table 1 and combined with practical production conditions, the next step is to add the desulfurizer into hot metal and to improve the kinetic conditions of the desulphurization process. Corresponding author: Wei Wu, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.
Consumption / (kgt1) 5-8 5-7 4-6 0.4-0.6
Final [%S] <0.001 <0.003 <0.005 <0.005
Temperature drop / qC 20-30 20-30 10-20 10-15
2.1. Conditions for desulfurizer particle penetration into hot metal by the injection method When desulfurizer particles penetrate into liquid metal, it must overcome the interfacial tension, facade resistance, and buoyant force. The minimum velocity Also available online at www.sciencedirect.com
376
J. Univ. Sci. Technol. Beijing, Vol.15, No.4, Aug 2008
for a particle to penetrate into metal melt is directly proportional to the square root of the surface tension of the melt but inversely proportional to the square root of the particle diameter, given by [5] v1
§ U1 · ¨ 2 0.5 ¸ Up ¹ ©
1
2
° 96V m ® ¯° 13U 1
ª § 26 U 1 · º °½ « exp ¨ ¸ 1» ¾ «¬ © 16 U p ¹ »¼ ¿°
1
2
dp
1 2
(1) where vl is the critical velocity, m/s; U1 the density of liquid metal, kg/cm3; Up the particle density, kg/cm3; and dp the particle diameter, mm.
When the particle has high kinetic energy, it can penetrate through the gas/liquid interface and enter the liquid metal. When the particle has low kinetic energy, it can not overcome the resistance of surface tension and will stick on the surface of a gas bubble and float up with the bubble. When the particle has very low kinetic energy, it will suspend in the gas bubble and finally float into air with the gas bubbles. The relation between the particle diameter and critical velocity for three frequently used agents is shown in Fig. 1.
hot metal can be calculated by the following equation [6]: § g 'U · Q s 1.74 ¨ ¸ © Ul ¹
1/ 2
(2)
d s 1/ 2
where vs is the rising up velocity, m/s; 'ȡ the density difference between the particle and liquid metal; ȡl the density of hot metal; ds the particle diameter. The calculated results are shown in Fig. 2. It can be seen from the figure that after the particle penetrates into the liquid metal, it floats up quickly. Its rising up velocity is directly proportional to the square root of the particle diameter and inversely proportional to the square root of the particle density.
Fig. 2. Relation between the diameter of solid particles (ds) and the rising up velocity (vs).
Fig. 1. Relation between the critical velocity (vl) and the diameter of solid particles (ds) for CaO, CaC2, and Mg.
Fig. 1 shows that when the diameter of the desulfurizer particle is 0.5 mm, the required velocity at the exit of the injection lance should be 45 m/s for a CaC2 particle, 65 m/s for a CaO particle, and 90 m/s for an Mg particle. For the same kind of particles, the smaller the diameter is, the higher the injection velocity is required. When the diameter of the desulfurizer particle is less than 0.2 mm the required velocity increases rapidly. When the diameter is less than 0.1 mm, the required velocity can hardly be reached, thus, a major part of the particles will be brought out from liquid metal into open air by the carrying gas. This will increase the reagent consumption significantly. 2.2. Rising up velocity of desulfurizer particles in hot metal The rising up velocity of desulfurizer particles in
When Mg is injected into the hot metal as the desulfurizer, Mg particles are evaporated to form Mg vapor bubbles quickly. Therefore, during the calculation for Fig. 2, the particle is first converted to vapor bubbles. Then, to calculate its rising up velocity, the relation between the diameter of Mg particles and the diameter of Mg vapor bubbles (dg) can be calculated by the following equation [13]: dg
(3)
1/ 3 21.2 PMg ds
The rising up velocity of Mg vapor bubbles can be calculated by the following equation [13]: 1/ 6 1/ 2 Q g 10.4 PMg ds 10.4 1 0.7 h
1/ 6
d s 1/ 2
(4)
where vg is the rising up velocity, m/s; PMg the Mg vapor pressure, Pau105; ds the particle diameter, m; h the injection depth, m. The rising up velocity of Mg vapor bubbles is directly proportional to the square root of the Mg particle diameter. Therefore, to increase the efficiency of desulphurization, the rising up velocity of Mg vapor bubbles should be decreased, i.e. the diameter of Mg particles should be decreased. However, decreasing the Mg particle size will render the particle difficult to penetrate into the hot metal.
W. Wu et al., Desulfurizer desulphurization kinetics by the injection method
This is thus a contradictory problem. A method to solve this is to increase the velocity of the carried gas at the nozzle outlet. 2.3. Resistance time of Mg, CaC2, and CaO desulfurizers in hot metal Among all desulfurizers, the Mg agent is a special one. After it is injected into hot metal, it will melt, evaporate, dissolve and float up. During these processes, Mg particles react with hot metal through gas/liquid and liquid/liquid reactions. There is no reaction between Mg and slag, thus, the residence time of Mg particles in hot metal is a predominating parameter that affects the desulphurization result. The residence time of Mg particles in hot metal includes the time for melting, evaporating, and floating up. It can be calculated by the following equation [13], and the result is shown in Fig. 3. ¦W
0.025 9.6
h(1 0.7 h) 1 / 6 u 10 2 d s1 / 2
377
According to Eq. (5), when the diameters of CaC2 and CaO particles are about 1 mm, and the injection depth is 2.5 m, the particles only stay 17.6 s in the iron melt. When the injection depth decreases, the residence time of particles in hot metal also decreases. This means the desulphurization mainly take place at the slag/iron interface. Therefore, stirring thoroughly a mixture of slag and iron is extremely important. During the injection, some accelerators such as CaCO3 and MgCO3 can be mixed with the desulfurizer. The accelerators decompose in hot metal producing many CO2 bubbles and causes strong stirring in the melt to enhance the efficiency of desulphurization. The mechanical stirring can provide a powerful stirring effect and improve the kinetic conditions for CaO injection desulphurization, thus it improves the metallurgical results. 2.4. Dissolving time of Mg particles in hot metal
(5)
The solubility of Mg in liquid iron is determined by hot metal temperature and Mg vapor pressure as follows [7]: lg[Mg] 7000/T lg PMg 5.1
(6)
where T is the absolute temperature, K; PMg the vapor pressure of the temperature.
Fig. 3. Relations among the residence time of Mg particles in the melt (¦W), the diameter of Mg particles (ds), and the injection depth (h).
The total residence time of Mg particles in the hot metal, ¦W, is inversely proportional to the diameter of Mg particles, ds, and directly proportional to the injection depth (Fig. 3). If the injection depth is 2.5 m, the particle diameter 1 mm, then the total residence time of Mg particles in hot metal is 9.01 s. When a small hot metal ladle is used to desulfurize, the injection depth will be even smaller and the total residence time shorter. For example, if the injection depth is 1.5 m, the total residence time in hot metal will be only 5.15 s; thus, the desulphurization effect in a small ladle is not as good as in a big one. When CaC2 and CaO are used as desulfurizers, the desulphurization efficiency and desulfurizer consumption are influenced by the residence time of particles in hot metal. The rising up velocity of solid particles in hot metal can be calculated by Eq. (2) [13].
The solubility of Mg in liquid iron increases with the increase in pressure. At atmospheric pressure, the Mg solubilities are 0.45wt%, 0.22wt%, and 0.12wt% when the temperatures are 1200, 1300 and 1400qC respectively. The dissolving time, Wgr, of Mg particles is directly proportion to the Mg particle diameter and inversely proportion to the injection depth [14]:
W gr
0.025 1.287
ds u 10 4 (1 0.7 h) 1 / 3
(7)
The result is shown in Fig. 4. If the Mg particle diameter is 1 mm, and the injection depth is 2.5 m, then the dissolving time should be 9.21 s. It means ¦W
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J. Univ. Sci. Technol. Beijing, Vol.15, No.4, Aug 2008
(5) If the Mg particle diameter is 1 mm and the injection depth is 2.5 m, the dissolving time should be 9.21 s, but the residence time of Mg particles in hot metal is only 9.01 s. This shows that the dissolving time is longer than the residence time. The Mg particle does not have enough time to dissolve before floating up. Thus, sufficient Mg desulphurization does not occur.
References Fig. 4. Relations among the dissolving time of Mg (IJgr), particle size (ds), and injection depth (h).
3. Conclusions (1) When the particle diameter is 0.5 mm, the critical speed of the CaC2 particle from the lance nozzle should be higher than 45 m/s; for CaO particles the speed should be higher than 65 m/s, and for Mg particles 90 m/s. The smaller the particle size is, the higher the injection speed is required. When the particle diameter is less than 0.2 mm, the required speed increases rapidly. When the particle diameter is less than 0.1 mm, the required injection speed can hardly be satisfied. Under this condition, major part of the particles will be brought out of the melt by the carrying gas and therefore markedly increase its consumption. (2) To increase the desulphurization efficiency and the utilization ratio of a desulfurizer, the size of Mg particle should be decreased; however, small Mg particles can hardly penetrate into hot metal. This problem is contradictory. The best way to solve this is to increase the initial velocity of the particles from the lance nozzle. (3) When the injection depth is 2.5 m and the Mg particle diameter is 1 mm, then its residence time in hot metal is only 5.15 s. Because of this, the desulphurization effect is worse than that in a big hot metal ladle. (4) When the diameters of CaC2 and CaO particles are less than 1 mm and the injection depth is 2.5 m, their residence time in hot metal is only 17.6 s. This shows that the desulphurization reactions mainly occur at the metal/slag interface. In addition, various stirring measures are important to mix metal and slag phases.
[1] W. Wu, Y.B. Hu, and L. Liu, New technique of desulphurization by agitation injection for hot metal ladle, J. Univ. Sci. Technol. Beijing (in Chinese), 27(2005), Suppl.1, p.142. [2] Y. Shirota, Clogging-free nozzle conditions and metallurgical effects of powder injection practice in hot metal pretreatment, Trans. ISIJ, 27(1987), No.1, p.19. [3] R.P. Singh and M.J. Mcnallan, Fluid dynamics and mass transfer in submerged gas-particle jets, Metall. Mater. Trans. B, 14(1983), No.3, p.431. [4] G.A. Irons, The influence of conveying conditions on powder injection processed for desulfurization, Trans. Iron Steel Soc. AIME, 24(1984), No.5, p.41. [5] L.R. Farias and G.A. Irons, A united approach to bubbling-jetting phenomena in powder injection into iron and steel, Metall. Mater. Trans. B, 16(1985), No.2, p.212. [6] S.Y. Kitamura, T. Kitamura, and K. Shibata, Effect of stirring energy, temperature and flux composition on hot metal dephosphorization kinetics, ISIJ Int., 31(1991), No.11, p.1325. [7] H.C. Wang and S.J. Wang, Vacuum treatment for simultaneous desulphurization and dephosphorization of hot metal and molten steel, J. Iron Steel Res. (in Chinese), 16(2004), No.2, p.15. [8] N. Hao, H. Li, and H.T Wang, Application of the sulphide capacity theory on refining slags during LF treatment, J. Univ. Sci. Technol. Beijing, 13(2006), No.2, p.112. [9] Z.M. Wang, W. Sun, and X.Q. Yu, The desulphurization process of hot metal in Magang and its applied results, Res. Iron Steel (in Chinese), 32(2004), No.2, p.26. [10] S.S. Yang, Pretreatment of hot metal and pure steel smelting, China Metall. (in Chinese), 13(2003), No.9, p.16. [11] Z.W. Lu, Discussion on selection of desulphurizing agent for hot metal pretreatment, Steelmaking (in Chinese), 17(2001), No.1, p.36. [12] X.Z. Zhang, Injection Metallurgical Elements (in Chinese), Metallurgy Industry Press, Beijing, 1988, p.109. [13] G.F. Yan, On several technological and theoretical problems in desulphurization of hot metal with Mg particle injection, [in] The 13rd CSM Annual Steelmaking Conference (in Chinese), Kunming, 2004, p.71. [14] T.J. Yang, Recent progress of metal external desulphurization, Iron Steel (in Chinese), 34(1999), No.1, p.66.