ISIJ International, Vol. 49 (2009), No. 6, pp. 771–776 Review Thermodynamic and Kinetic Analysis of Nitrogenization in Desulfurization of Hot Metal by Magnesium Injection Haiping SUN,1,2) Yung-Chang LIU1) and Muh-Jung LU1) 1) Steel & Aluminum Research & Development Department, China Steel Corporation, Kaohsiung 81233, Taiwan. E-mail: [email protected] 2) School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia. E-mail: [email protected] (Received on November 25, 2008; accepted on February 18, 2009) Literature study and thermodynamic/kinetic analysis have been carried out on the effect of carrier gas on hot metal desulfurization by magnesium injection. The literature study shows that the magnesium efficiency of the process could be deteriorated by using nitrogen as carrier gas, but the difference in magnesium effi- ciency for the process using argon and that using nitrogen was not clearly identified. Thermodynamic/ki- netic analysis shows that when nitrogen is used as carrier gas for introducing magnesium into hot metal, the formation of magnesium nitride is possible in the regions close to the lance tip. The nitride formed at lance tip may cause lance clogging. Magnesium nitride is unstable in hot metal or in gas at high tempera- tures; and after leaving lance tip regions, magnesium nitride will undergo decomposition. Magnesium loss in process off gas will be increased by the decomposition of magnesium nitride that occurs too closely to bath surface or by any un-decomposed magnesium nitride at bath surface. The magnesium loss by nitroge- nization and the clogging problem could be minimized by optimizing injection conditions. KEY WORDS: steelmaking; desulfurization; injection; magnesium; nitrogenization; carrier gas. stand the magnesium nitrogenization and the decomposi- 1. Introduction tion of the nitride in magnesium desulfurization process Magnesium injection has been proven to be one of most when nitrogen is used as carrier gas. efficient, economical, fast and environmental friendly processes for hot metal desulfurization in integrated steel- 2. Literature Study making plant. However, the effect of carrier gas on the magnesium efficiency in the magnesium injection processes The nitrogenization of solid, liquid or gaseous magne- is still remained as a puzzling issue in open publications.1–7) sium may occur during magnesium injection process. These Some investigations suggested that the magnesium effi- reactions not only consume magnesium but also cause ciency is lowered by using nitrogen, while many others sug- lance clogging problem during magnesium desulfurization gested the effect of nitrogen is small. There was no signifi- process. A statistic analysis of operational data by Chen cant difference in magnesium efficiency can be confirmed et al.1) shows that, there was about 16 to 20% of magne- between the process using nitrogen and that using argon in sium is nitrogenized, which heavily reduces the magnesium experimental or industrial trials. In this study, thermody- desulfurization efficiency of the injection process. The ni- namics and kinetics of nitrogenization in magnesium desul- trogenization loss could be avoided if nitrogen is replaced furization process was analyzed. Although magnesium ni- by argon as carrier gas for the injection. By carefully as- tride was found to be unstable in the high temperature sys- sessing both aspects of magnesium desulfurization effi- tem formed in magnesium injection process, the nitride is ciency and carrier gas cost, they selected nitrogen for the possible to generate at localized regions close to the lance injection process. tip. Once magnesium nitride is generated, it may be present The performances of the magnesium injection process in gas bubbles as suspended solid particles and rise to the of a Chinese steelmaker2) using nitrogen and that of an bath surface without fully decomposing, or the nitride may Ukrainian steelmaker2) using argon as carrier gas for intro- attach to the lance tip to cause nozzle clogging. ducing magnesium granules into hot metal are compared in The analysis shows that the magnesium loss by nitroge- Table 1. In the injection process using argon gas, magne- nization may not be significant and could be minimized by sium consumed for removing sulfur in hot metal is 1.22 kg optimizing injection conditions. Replacing argon by nitro- Mg/kg S, which is 3% less than that in the process using ni- gen as carrier gas will provide significant economic advan- trogen gas where 1.26 kg Mg/kg S was consumed. It is not tage for the process because nitrogen is about ten times less clear that the slight higher magnesium efficiency in the expensive than argon. Therefore, it is important to under- process using argon is contributed by carrier gas or not. 771 © 2009 ISIJ ISIJ International, Vol. 49 (2009), No. 6 Table 1. Comparison of performance of Mg injection desul- Table 2. Experimental results of desulphurization of hot metal phurization processes using nitrogen or argon gases by magnesium injection after Nakanishi et al. as carrier gas. This is because many process parameters in Ukrainian steelmaker are better than those in Chinese steelmaker for achieving higher magnesium efficiency. For example, the injection is deeper, the size of ladle or the mass of hot metal is larger and the temperature is lower in Ukrainian steel- maker than those in Chinese steelmaker. Therefore, the ef- fect of replacing argon with nitrogen as carrier gas on the magnesium desulfurization efficiency may not be signifi- cant. Fig. 1. Change of sulfur during the injection of magnesium by In the earlier study by Voronova et al., factors that influ- nitrogen and argon gases after Nakanishi et al. encing the magnesium desulfurization efficiency were sys- tematically investigated.3) When compared with nitrogen or Table 3. Summary of literature study on magnesium desul- air, the higher magnesium desulfurization efficiency was phurization process using different carrier gases. achieved when natural gas was used as carrier gas. Accord- ing to this result, natural gas is suggested by Sun et al.4) for improving magnesium desulfurization efficiency of the in- jection process because magnesium loss by nitrogenization as well as by oxidation will be prevented by natural gas. Furthermore, the carbon and hydrogen decomposed from natural gas at high temperatures will facilitate further desulfurization. Base on thermodynamic analysis, Li5) suggested that the nitrogenization of magnesium under nitrogen atmosphere is possible at temperature between 573–1 713 K. The magne- sium nitride is unstable in air; it will react with moisture in air to produce MgO and NH3. To investigate the effect of carrier gas, Nakanishi et al.6) carried out lab scale experiments where 15 kg blast furnace hot metal was injected by magnesium granulates using argon and nitrogen at 1 450°C. The experimental results are shown in Table 2 and the changes of sulfur during the ex- servation by authors, lance clogging was confirmed when periments are shown in Fig. 1. In Table 2, the Mg/metal nitrogen was used as carrier gas for injecting magnesium. ratio is the amount of magnesium in gram per kg of hot The lance clogging materials was confirmed to be mixture metal. The desulfurization ratio is the fraction of sulfur re- of Mg3N2, MgO and Mg by X-ray diffraction analysis. The moved by the magnesium injection, and the Mg utilization clogging phenomenon should bring our attention when ni- ratio is the fraction of injected magnesium that reacted with trogen is used as carrier gas in the practice. sulfur in hot metal. As seen in Fig. 1, the desulfurization The experimental results shown in Table 2 and Fig. 1 is rate was essentially same in two runs of different carrier supported by the thermodynamic analysis by Sun et al.7) gases. This indicates the desulfurization performance is not Magnesium nitride (Mg3N2) is found to be unstable in hot affected by replacing argon with nitrogen. The desulfuriza- metal at higher temperatures and therefore the effect of ni- tion ratio and the Mg utilization ratio are compared in Table trogen as carrier was suggested to be insignificant because 2 for 2 runs. This comparison clearly shows the similar Mg3N2 (if generated) will eventually decompose in the hot desulfurization performance in the run with nitrogen and metal and Mg(g) generated from the decomposition will that with argon. Therefore, the effect of nitrogenization on contribute to desulfurization. the magnesium desulphurization efficiency is considered Table 3 shows the summary of findings in the past stud- unimportant when temperature is as high as 1 450°C or ies related to the effect of carrier gas on the performance of when the injection is shallow. According to the visual ob- magnesium injection process. Some authors suggested se- © 2009 ISIJ 772 ISIJ International, Vol. 49 (2009), No. 6 vere magnesium loss by nitrogenization in the process when nitrogen is used as carrier gas for injecting magne- sium granules, while many others suggested the loss is unimportant. No experimental or industrial trial evidence can be found to show the magnesium desulfurization effi- ciency is deteriorated by nitrogen as compared with argon under otherwise similar injecting conditions. The issue is unfortunately a controversial one in open publications and is needed to be clarified. 3. Thermodynamic and Kinetic Analysis 3.1. Nitrogenization of Magnesium in Injection Process Figure 2 shows the schematic diagram of hot metal desulfurization process by magnesium injection. Hot metal contained in a ladle is injected with gas–solid mixture Fig. 2. Nitrogenization reactions in Mg injection process. through an immersed lance. Magnesium granules are car- ried by nitrogen gas and injected deeply into metal bath. Table 4. Equilibrium values for nitrogenization. Magnesium granules are heated up in the lance where a portion of magnesium granules may melt, vaporize or be nitrogenized. The gas forms numerous bubbles after leaving the lance tip.