On-line version ISSN 2411-9717
J. S. Afr. Inst. Min. Metall. vol.114 n.5 Johannesburg May. 2014
G. Chen; S. He
College of Materials Science and Engineering, Chongqing University, Chongqing, China
In this study, the effects of slag compositions, slag amount, temperature, and carbon content of steel on the manganese and phosphorus distribution ratios during converter steelmaking were analysed using the classical regular solution theory, and industrial tests were performed using two 80 t top-and-bottom combined blown converters (duplex melting process). The results indicate that the slag amount, temperature, and carbon content in steel are the main factors affecting the manganese yield when converter slag compositions remain constant. The FeO content of the slag has a strong impact on the manganese distribution ratio, while the slag basicity and MgO content have no obvious effect. The calculations and experimental results show that the phosphorus distribution ratio increases sharply with increasing slag basicity R, but then decreases with the increase of MgO and MnO contents in the slag. The final slag in converter steelmaking should have the following characteristics: 3.5 < R < 4.5, 15% < (FeO) < 20%, and 6% < (MgO) < 8%. The slag amount should be controlled appropriately at the same time. The results of this investigation would be useful in deciding on the application of manganese ore in alloying and identifying the slagging regime in converter steelmaking.
Keywords: slag compositions, distribution ratios, classical regular solution theory, slagging regime.
The increase in the international iron ore price in recent years has forced Chinese steel companies to seek domestic sources of iron ore in order to reduce costs. The availability of large amounts of phosphorus- and manganese-rich ores in the Three Gorges Reservoir region has made the development of a characteristic metallurgical technology in local steel plants possible (Wang and Dong, 2009). In the conventional steelmaking process, one converter must fulfill several functions, such as dephosphorization, decarburization, and raising temperature. It is unreasonable and uneconomical to decrease the phosphorus content using only one converter when high-phosphorus molten iron is smelted. Separation of the decarburization process from dephosphorization (using De-P and De-C converters) in the duplex melting process is advantageous, as it allows for the use of phosphorus-rich iron ore and relatively lower amounts of slag, as well as direct alloying with manganese ore in the converter. Consequently, iron procurement costs and metal losses are reduced to a considerable extent. For these advantages to be translated into commercial benefits, the dephosphorization converter should be fully exploited to produce low-phosphorus, high-carbon, high-temperature semi-steel, and the semi-steel smelting process should be further optimized by manganese ore alloying and by improving the manganese yield. A number of studies have been carried out on the improvement of the manganese yield and its impact on the converter, both in China and abroad (Suito and Inoue, 1995; Gao, Zhao, and Xing, 2011; Kaneko et al., 1993; Min and Fruehan, 1992; Lv et al., 2010; Soifer, 1958).
In this study, the main factors affecting the manganese yield are systematically investigated by thermodynamic analysis and industrial tests, and the relationship between manganese alloying and dephosphorization in the converter is discussed in detail. Finally, the final slag composition and control ranges for converter steelmaking are proposed.
The reaction of manganese ore during the alloying process in the converter is described as follows for a given slag system (Gao, Zhao, and Xing, 2011; Kaneko et al., 1993; Huang, X.H. 2008).
The equilibrium constant of the above reaction is
(Yang and Cao). The effect of [C] content of steel on the manganese distribution ratio (LMn = (%Mn)/[%Mn]) is given by Equation  at PCO = 1, f[c] = 1, and f[Mn] = 1. The activity of (MnO), a[Mn], can be obtained by using the regular ionic solution model (Huang, 2008).
where x(i) is the mole fraction of positive ion i.
The (FeO) content plays a critical role in the control of manganese ore reduction during the later stage of blowing in the converter (Suito and Inoue, 1995; Gao, Zhao, and Xing, 2011; Morales and Fruehan, 1997; Takaoka et al., 1993). This reaction is shown in Equation .
Equation  is derived from Huang (2008).
The effects of slag basicity R = (%CaO)/(%SiO2) and (FeO) and (MgO) contents of the slag on LMn are studied through the reaction in Equation  at f[Mn] = 1. Similarly, the activity coefficients of (Fe2+), γ(Fe2+) and (Mn2+), γ(Mn2+) can be obtained from Equations  and  respectively.
Moreover, the effects of temperature on LMn are calculated separately by Equations  and , and the carbon content in Equation  is set as 0.08%.
The major dephosphorization reaction between molten steel and slag in the converter is described by Equation  (Basu, 2007; Ikeda and Matsuo, 1982).
In order to calculate the phosphorus distribution ratio Lp = (%P)/[%P], the activity of the complex ion of phosphorous and oxygen can be expressed by the simplified reaction Huang (2008) :
where the KP value is 0.0234 (Huang (2008). The values of X(P5+), X(Fe2+), γ(Fe2+), and γ(P5+) can also be obtained from the regular ionic solution model (Huang (2008).
Figures 1 and 2 show that R has no obvious effect on LMn. In addition, compared with (FeO), (MgO) has a less obvious effect on the change in LMn. LMn tends to increase with an increase in (FeO) but decrease with an increase in (MgO). Therefore, LMn is more strongly affected by the (FeO) content rather than R or (MgO) content. Figure 3 shows the variation of the calculated activity coefficient of (MnO) as a function of (FeO) content. When (FeO) content in slag increases from 15% to 35%, the activity coefficient of (MnO) decreases from 1.71 to 1.29, which can also be observed in the results of Jung et al. (1993), Jung (2003), and Suito and Inoue (1984). Obviously, the results obtained in this work are in agreement with the data from the literature. In addition, as can be seen in Figure 4, the activity of FeO in slag increases strongly with an increase of (FeO) in slag over the calculated concentration range, which is in good accordance with the results of other investigators (Morales and Fruehan, 1997; Huh and Jung, 1996; Sobandi, Katayama, and Momon, 2002). The distribution ratio LMn thus increases with the increase of (FeO), resulting in a low reduction efficiency of (MnO) when (the FeO) content is increased, as shown by Equation . Hence, in order to improve the manganese yield in the converter, the (FeO) content of the final slag should be maintained at a low level in the alloying process performed using manganese ore. This has been affirmed by previous experiments (Suito and Inoue, 1995; Morales and Fruehan, 1997; Suito and Inoue, 1984; Jung, Rhee, and Min, 2002). However, the dephosphorization in the converter requires a high (FeO) content. As illustrated in Figure 5, the increase in (FeO) content initially enhances LP, but the trend is reversed beyond a certain level, and the optimal (FeO) content decreases with increasing R. Generally, LP has already reached the maximum level when the (FeO) content approaches 20% at a relatively high R value. These results are similar to those assessed thermodynamically and experimentally by previous researchers (Basu, 2007; Ikeda and Matsuo, 1982; Sobandi, Katayama, and Momon, 2002; Suito and Inoue, 1995). Therefore, the (FeO) content could be controlled between 15% and 20% to achieve a high manganese yield.
It is well known that slag with higher R and higher (FeO) content is required in the later stage of the smelting process for dephosphorization (Basu, 2007; Jeong et al., 2009; Nozaki et al., 1983). Consequently, the conditions used for converter steelmaking are suited for dephosphorization. As shown in Figures 5-7, it is obvious that the dephosphorization effect increases sharply as R is increased, which is believed to dramatically reduce the activity coefficient of (P2O5) in slag, as recognized (Basu, 2007; Sobandi, Katayama, and Momon, 2002; Suito and Inoue, 1995; Turkdogan, 2000; Suito and Inoue, 1982, 1984; Nakamura, Tsukihashi, and San, 1993). A higher R is thus indispensable for dephosphorization in the converter. Conversely, an excessively high R will worsen the kinetic conditions for manganese ore reduction and dephospho-rization. Simultaneously, taking previous studies (Lv et al., 2010; Basu, 2007; Tabata et al., 1990) into consideration, R should be high when the (FeO) content is relative low and, in general, the R of the final slag should be controlled between 3.5 and 4.5 to achieve a higher degree of dephosphorization.
Furthermore, LP clearly decreases with an increase in the (MgO) and (MnO) contents of the slag at a given R (Figures 6 and 7). Also, the influences of (MgO) and (MnO) contents on LP are gradually enhanced with the increase of R from 2 to 4; the effects of R on LP become progressively weaker with increasing (MgO) and (MnO) contents. Specifically, calculations show that increasing the (MgO) and (MnO) contents of the slag can increase the activity of (P2O5), which is detrimental for LP, as confirmed by Figure 8. Separately, the dephosphorization effects weaken significantly when the slag has a high (MnO) content, as has been reported by many investigators (Suito and Inoue, 1995; Mukherjee and Chatterjee, 1996; Simeonov and Sano, 1985) and observed by previous researchers, all of whom have advised against the addition of manganese oxide to converter slag. As a result, manganese ore alloying during converter smelting would become advantageous when low-phosphorus hot metal is used as raw material. Also, the experiments of Halder and co-workers demonstrated conclusively that 2CaO∙SiO2 exists under conditions of higher slag basicity, lower steel tapping temperatures, and higher phosphorus contents of the hot metal, which comprised the majority of the solid part of the slag and also had greater solubility for phosphorus than the liquid part of the slag (Deo et al., 2004) . In addition, Suito, Inoue, and Takada (1981) also proved that slag containing 2CaO-SiO2 had a higher LP in the MgO-saturated slag of the system CaO-MgO-FeOx-SiO2, and the same result was also obtained in the CaO-CaF2-SiO2 system by Muraki, Fukushima, and Sano, (1985). However, the increase of (MgO) content in slag could result in a reduction in both the size of the 2CaO-SiO2 grains and the dissolution of phosphorus in 2CaO-SiO2. Thus, dephosphorization was greatly hindered (Deo et al., 2004). Therefore, taking only dephosphorization into consideration, the lower the MgO content the better. However, since (MgO) plays an important role in protecting the furnace lining, it is imperative that the appropriate amount of (MgO) should be present in the slag. We can manipulate this relationship by using slag-splashing protection technology for the converter. In general, the (MgO) content should be controlled between 6% and 8%.
From Equations  and , the values of LMn are calculated separately as a function of steel temperature in Figure 9. The results of the authors and of Jung et al. (1993) and Jung, Rhee, and Min (2002) are plotted for comparison. It is seen that most values of log(LMn) in this work are slightly higher compared with previous results. This may be a result of the slightly different components of the slag. In general, log(LMn) decreases linearly with increasing temperature. Equations  and  clearly indicate that the reaction in Equation  is endothermic and Equation  is exothermic. The equilibrium [Mn] content in steel is therefore expected to increase with increasing temperature. On the other hand, many studies indicate that the dissolution reaction (MnO(s) = MnO(slag) of (MnO) in slags is endothermic, so that (MnO) dissolution in slag increases with temperature (Jung et al., 1993; Suito and Inoue, 1984; Simeonov and Sano, 1985; Suito and Inoue, 1984. Accordingly, the activity of (MnO) will increase with increasing mole fraction of (MnO) in the slag, and these results are widely accepted in previous studies (Ding and Eric, 2005). Thus, the equilibrium [Mn] content increases naturally.
Figure 10 shows that the final [C] carbon content of the steel has a great influence on LMn. With a decrease in the final carbon content [C] in steel, LMn increases distinctly, and this has been experimentally confirmed by previous investigators (Kaneko et al., 1993; Yang and Cao, 2009; Tabata et al., 1990; Matsuo, Fukagawa, and Ikeda, 1990). Clearly, the [C] in the molten steel can accelerate the reduction of (MnO) and improve the manganese yield. The [C] content of steel acts as a heat source and as well as a reducing agent, and carbon is thus consumed in large amounts. At the same time, carbon reacts with oxygen to form CO gas, which can enhance the fluidity of the entire slag system, which drives the reactions towards equilibrium (Keum et al., 2007).
The effect of the amount of slag on the manganese yield ((Ws * ([Mn]f - [Mn]s))/(0.3 * WMn), where [Mn]f and [Mn]s are the [Mn] content in the final steel and semi-steel respectively; Ws and WMn are the yield of final steel (tons) and manganese ore charged (tons), respectively) is shown in Figure 11. While the slag amount affects the manganese yield, it does not influence ZMn. The manganese ore yield decreases sharply with an increase in the slag amount, due to the fact that the (MnO) content in the slag will decrease and the balanced [Mn] content in steel also will decline. As can be seen, since the manganese yield is less than 35% when the slag amount exceeds 60 kg/t, the slag amount must be maintained at a value less than 40 kg/t to ensure a high Mn yield (>45%). Similar results have been reported by other researchers (Kaneko et al., 1993; Tabata et al., 1990; Mukherjee and Chatterjee, 1996).
In conclusion, steelmaking by a process that involves manganese ore alloying and the use of low quantities of slag is an effective measure for lowering the consumption of raw materials and raising the manganese yield, which are the typical advantages of the De-P/De-C steelmaking process.
Industrial process description
To verify the results of the thermodynamic calculations, we carried out industrial tests using two 80 t converters at a steel plant in China. The blast furnace, De-P converter, De-C converter, refining, and continuous casting route has been been adopted. The experimental conditions used for the industrial-scale tests are shown in Table I. Tables II and III show the compositions of the molten iron and manganese ore used in the tests, respectively; the additions of manganese ore and compositions of semi-steel, final steel, and final slag are shown in Table IV. In this exploratory study, only a small amount of manganese ore (0~810 kg) was added to ascertain the factors affecting alloying, which can provide reference data for further industrial-scale production.
Results and analysis
The values of LMn obtained from the results are shown against (FeO) contents in Figure 12, where the R values range from 3.0 to 5.5. The values of LMn change irregularly with the increase of R at a given (FeO) content, and LMn increases with an increase in (FeO) content at a given R, which shows a relatively consistent tendency with thermodynamics (Figure 1). This is also confirmed in Figures 13 and 14. LMn increases as the FeO content increases when [C] content and temperature remain constant. In addition, LMn shows a rapid increase as the [C] in the steel decreases at the same FeO content (Figure 13), and the temperature has a similar impact on LMn, as illustrated in Figure 14. In Figure 13, the experimental values of LMn locate between 25 and 45, indicating that these values agree well with the calculated values (which are between 20 and 47) according to Equation  (Figure 10), under the condition of 0.04-0.09% [C] carbon content. However, these experimental values turn out to be somewhat less than the calculated values according to Equation  (Figure 10), probably because the reduction of MnO in slag by carbon [C] in steel leads to a significant increase of the [Mn] content of the steel, and in turn, the oxidization of [Mn] by FeO in the slag may not reach a thermodynamic equilibrium. As shown in Figures 12-14, as expected, low LMn values result when the (FeO) content is less than 25%, the [C] content exceeds 0.06%, and the temperature is higher than 1920K. As is well known, the increase in temperature depends on the oxidization of carbon during blowing in the BOF, and there is a strong negative correlation between [C] carbon content and temperature. On the other hand, the oxidation of [C] carbon will be depressed while the oxidation of [Fe] will be facilitated when the [C] content is less than about 0.05% (Huang, 2008), which will cause excessive (FeO) content in slag. As demonstrated in Figure 15, when the final carbon content is less than 0.06%, the majority of the corresponding (FeO) content in slag exceeds 28%. However, a high [C] carbon content, high temperature, and low (FeO) content are required in order to attain a low LMn. More intensive research is required to determine the appropriate amount of carbon powder that should be added to the slag when the [C] carbon content of the semi-steel is insufficient to provide adequate heat and reduce the manganese ore to a large extent.
The influence of the slag amount on the manganese yield is shown in Figure 16. The yield of the manganese ore decreases sharply with an increasing slag amount at similar values of LMn, and it falls below 25% when the slag amount is 60 kg/t. Generally, the manganese yields in the industrial tests are fairly low (usually below 20%); this is the inevitable consequence of the presence of a large amount of slag. The main reason is that the iron ore used by the aforementioned steel plant is rich in phosphorus, and the [P] content of the semi-steel charged in the De-C furnace is still high and a large amount of time and energy will lost in dephosphorization. Consequently, a large amount of slag is generated. At the same time, a large amount of slag is required for deep dephosphorization, which in turn has an enormous negative influence on manganese yield.
The influences of R and the (FeO), (MnO), and (MgO) contents of the slag on LP are shown in Figures 17-19. From the experimental results, it is obvious that LP increases with increasing R from 3.0 to 5.5, regardless of whether the (FeO), (MnO), or (MgO) contents are held constant. It is also found that the higher the (FeO) content of slag, the more the LP increase at a given R. However, the optimum (FeO) content is not observed. This is corroborated by the theoretical calculations and has been reported in previous investigations (Basu, 2007; Ikeda and Matsuo, 1982; Sobandi, Katayama, and Momon, 2002; Suito and Inoue, 1995). It is considered that this phenomenon is due to the increase in the amount of slag, even if the LP is decreased slightly with an (FeO) content higher than the optimum. As shown in Figure 15, the results confirm a strong positive correlation between (FeO) content and slag amount. In addition, as expected, the dephosphorization effect weakens markedly with increasing (MgO) and (MnO) contents in slag when R is greater than 4. However, when R is less than 4, no regular relationship has been found. Furthermore, the impact of R on LP is weakened when the (MgO) and (MnO) contents are high, respectively.
From the data pertaining to the smelting test conducted in 27 heats, we can conclude that the average final [P] content of the steel and the average degree of dephosphorization are 0.016% (0.008-0.023%), and 87.4%, respectively, when the average [P] content of the semi-steel charged in the De-C furnace is 0.126% (0.05-0.22%). Since a larger amount of slag (on average, about 68.3 kg/t) with high (FeO) content (mean, 25.6%) is used in the converter to decrease the [P] content to the required steel grade, a relatively good degree of dephosphorization is achieved, but the manganese yield is only 17.2% on average. Hence, for manganese ore alloying to be beneficial, the process in the De-P furnace should be optimized to decrease the [P] content of the semi-steel further.
Obviously, the data obtained in the test work is basically in good agreement with the results of the thermodynamic calculations. This, in turn, shows that the choice of the method of calculation is reasonable.
On the basis of the thermodynamic calculations and the industrial test results, it is concluded that coordinated control between the dephosphorization ability and manganese ore alloying technology in the De-C converter should be considered carefully. The characteristics of the final slag for converter steelmaking should be controlled in the following ranges: 3.5 < R < 4.5, 15% < (FeO) <20%, and 6% < (MgO) < 8%.
A thermodynamic analysis and industrial tests of manganese ore alloying and dephosphorization in converter steelmaking were carried out. The conclusions can be drawn as follows.
(1) The main factors affecting the alloying process performed using manganese ore in the converter are the slag amount, temperature, and the [C] content of the steel with a given slag system
(2) The (FeO) content of the slag has an enormous impact on LMn but shows no clear relationship with the slag basicity or the (MgO) content of the slag
(3) The LP increases sharply with increasing slag basicity, but weakens with increasing (MgO) and (MnO) contents in the slag
(4) The characteristics of the final slag for converter steelmaking should be controlled in the following ranges: 3.5 < R < 4.5, 15% < (FeO) < 20%, and 6% < (MgO) < 8%. The slag amount should be controlled appropriately at the same time.
The authors greatly appreciate the funding support from Chongqing Science and Technology Key Project (CSTC2008AB4018)
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