2014 Volume 54 Issue 4 Pages 766-773
With the development of multiphase slag system for hot-metal treatment to achieve better dephosphorization efficiency, it is very important to improve the distribution ratio of P2O5 between the solid solution (2CaO·SiO2–3CaO·P2O5) and liquid phase slag. This study was carried out to investigate the effects of Na2O and B2O3 on P2O5 distribution ratio and morphologies of corresponding solid solutions in CaO–SiO2–Fe2O3–P2O5 slag system. The results indicated that the distribution ratio of P2O5 would be improved with the increase of Na2O content due to the formation of (2CaO·SiO2–Na2O·2CaO·P2O5) solid solution with a similar morphology as that of reference solid solution (2CaO·SiO2–3CaO·P2O5) in the reference slag. While B2O3 plays an opposite role, it would not only reduce the phosphorus distribution ratio, but also change the morphology of its corresponding solid solution due to the formation of complex solid solution (2CaO·SiO2–Ca9.93(P5.84B0.16O24) (B0.67O1.79)). Besides, the effect of cooling rate on the size of the solid solution was also studied. It would provide an instructive way for the design of multiphase slag for hot metal treatment.
In order to meet the increasing demand for high quality grade steels such as low-phosphorus steel, it requires a high efficient hot metal dephosphorization process. The current slag system for hot metal treatment contains a large amount of solid CaO dispersed in liquid slag, which introduce significant CaO consumption due to its low solubility; especially when CaF2 as fluxing agent was restricted due to the environmental concerns. Therefore, new refining method to improve the utilization efficiency of CaO by using a solid/liquid coexisting multiphase slag was proposed, and the studies regarding to the solid solution formation mechanism, mass transfer behavior of phosphorus and the phase relationship for multi-phase slag have been conducted.1,2,3,4,5,6)
Normally, hot metal dephosphorization slag consists of CaO–SiO2–FeO–P2O5, and it has been approved that 2CaO·SiO2 forms a solid solution with 3CaO·P2O5 at the hot metal treatment temperature over a wide range of compositions.7) Kitamura et al.,8) pointed out that the solid solution plays an important role in achieving efficient dephosphorization, and (2CaO·SiO2–3CaO·P2O5) solid solution has been regarded as a absorbent for phosphorus to lower the phosphorus content in liquid phase, which makes it more capable for future dephosphorization.9)
In addition, the multiphase slag system has the potential to be recycled as a source of phosphorus, if (2CaO·SiO2–3CaO·P2O5) solid solution could contain high phosphorus content. Yokoyama et al.,10,11) found that the concentrated phosphorus phase can be separated from FetO matrix phase due to the large difference of their magnetic properties under the strong magnetic field. Therefore, it is essential to increase the distribution ratio of phosphorous between the solid solution and liquid slag as well as the phosphorus content in the solid solution.
In order to increase the efficiency of dephosphorization and to use the slag as an alternative source of phosphate ores, a series of studies regarding to the distribution ratio of phosphorous between the liquid slag and the solid solution have been carried out8,9,12,13,14). Kitamura et al.,8) studied the effect of P2O5 content and oxide additives such as MgO and MnO on the distribution ratio of phosphorus for CaO–SiO2–Fe2O3–P2O5 slag system, and the results show that the phosphorus distribution ratio would increase with the increase P2O5 content in the slag, while the influence of MgO and MnO was not significant. Shimauchi et al.,14) indicated that the similar results regarding to the influence of MgO and MnO on the distribution P2O5 ratio was obtained. Recently Pahlevani et al.,9) made a detailed study based on the previous results, it was found that, in the CaO–SiO2–P2O5–Fe2O3 system, the P2O5 distribution ratio decreased with the addition of MgO and MnO, but it did not change with the addition of Al2O3; besides, the effect of adding MgO, MnO and Al2O3 on the P2O5 distribution ratio was smaller in the case of CaO–SiO2–P2O5–FeO system than that in the case of CaO–SiO2–P2O5–Fe2O3 system. Therefore, studies regarding to the influence of oxides additives such as MgO, MnO and Al2O3 on the P2O5 distribution in the multiphase slag would provide insight for development of industrial slag.
Generally, Na2O and B2O3 have been chosen as the effective additives for various slags to enhance the dissolution rate of CaO and to increase the phosphorus distribution between lime-based slag and liquid iron. Pak and Fruehan15) observed a fivefold increase in the phosphorus partition coefficient between lime-based slag and liquid iron, for a 5-wt% addition of Na2O to a CaO-saturated CaO–FeOt–SiO2 slag at 1873 K. Hamano et al.,16,17) reported that the phosphorous partition ratio between lime-based slag and liquid iron didn’t change, when SiO2 was replaced by B2O3 for the FetO–CaO–MgOsatd.–SiO2 slag at 1873 K. Above results have shown that Na2O and B2O3 would tend to influence the phosphorus partition between lime-based slag and liquid iron; however, the effect of the addition of Na2O and B2O3 on the distribution ratio of P2O5 between solid solution and liquid slag in multi phase slag system hasn’t been conducted, which may have the potential to further remove phosphorus from liquid iron to liquid slag and then into solid solution in a multiphase slag system.
This paper would first investigate the effect of different oxides, i.e. Na2O and B2O3 contents on the distribution ratio of P2O5 between the solid solution and liquid slag in the CaO–SiO2–Fe2O3 slag system, followed by the study of the effect of cooling rate on the distribution ratio of P2O5. Finally the morphology and size of the solid solution under the different conditions was also studied.
A schematic of the experimental apparatus was shown in Fig. 1, where the electric resistance furnace with MoSi2 heater was used, the accuracy of temperature control was about ± 5 K, and it could reach up to 1923 K due to its good insulation properties. The heating profile could be designed according to the experimental requirement. Besides, a peephole was mounted on the top of the heating furnace tube for the in-situ observation to check whether the mixed slag samples in resistance furnace has been melted or not.
Schematic drawing of experimental apparatus.
The design of samples was based on the CaO–SiO2–Fe2O3 system, the isothermal CaO–SiO2–Fe2O3 ternary phase diagram at 1623 K was calculated by FactSage software as shown in Fig. 2, where the composition of the reference slag (except P2O5) was represented by a red circle, The chemical compositions of all slag samples used in this study are listed in Table 1, where Sample O was designed as the reference sample, and others were prepared by adjusting the proportion of Na2O and B2O3. All slag samples were prepared by using regent-grade SiO2, Fe2O3, CaO, 3CaO·P2O5, Na2CO3 and B2O3. After weighing, the reagents were fully mixed in various ratios to produce CaO–SiO2–Fe2O3–P2O5–(Na2O or B2O3) slag system. The desired criterion for the slag systems in the study is that it should be in a homogenous liquid state at 1873 K and in a semisolid state at 1623 K.
The isothermal CaO–SiO2–Fe2O3 ternary phase diagram at 1623 K.
The mixed regents were first placed in an corundum crucible as the effect of Al2O3 on distribution ratio of P2O5 is slight in the CaO–SiO2–P2O5–Fe2O3 system,5,9) then the crucible was heated up to 1873 K in the electric resistance furnace, and was held for 60 min to eliminate bubbles and homogenize chemical composition for producing a homogenous slag. Then, the homogenous slag was cooled to 1623 K at a cooling rate of 5.0 K/min. After that, it was held at 1623 K for 60 min to produce a semisolid slag and ensure that the equilibrium between the solid phase and the liquid phase would be obtained.9,14) Finally, the slag sample was quenched through a rotating water bath for further analysis. The thermal profile for the experiments was shown in Fig. 3. It should be mentioned that Fe2O3 was used in the present work, even most of the iron oxide would be in the form of FeO in practical steelmaking; however, it has been indicated that the trend regarding to the effect of oxide additives on the distribution behavior of P2O5 between solid solution and liquid phase was consistent when the iron oxide was changed from FeO to Fe2O3.9,12)
Thermal profile for slag melting, cooling and holding process.
The samples selected from the core of each quenched slag were embedded in the polyester resin; then, they were polished following the standard metallographic procedure. Finally the polished samples were coated by Au evaporation for SEM observation. The composition of each phase in different regions was analyzed by SEM/EDS, as described in previous papers.9,18) Through the element analysis, the amounts of oxides, such as Fe2O3, CaO, SiO2, P2O5, Na2O and Al2O3 in different phases can be obtained. Then, the distribution ratio of P2O5 (LP) between the solid solution and the liquid phase was calculated by Eq. (1). Meanwhile, some extra slag samples was crushed and ground into powders for XRD tests. XRD data were collected by using Cu Ka radiation (1.54184 Å), in a range of 2θ = 10 to 80 deg with a step size of 4 deg/s.
Figure 4 schematically shows the typical four stages during the experimental process of sample powders. Stage I is a period in which the different particle reacts with each other when the slag is heated. Stage II is the 60 minutes period of melting for bubbles elimination and chemical composition homogenization at 1873 K. Stage III is the time of the solid solution crystallization due to the drop of temperature with the rate of 5.0 K/min. Stage IV is the time of the growth of the solid solution when the temperature at the 1623 K holds for 60 min. The results regarding to the average compositions of the precipitated solid phases and matrix liquid phases at 1623 K are summarized in Table 2. In this table, the distribution ratio of P2O5 between the solid and liquid phase for each sample is also shown. It should be mentioned that the element of Boron is not measured because its atomic number is too small to be analyzed by SEM/EDS, which might cause the slight increase of the other compounds content. Besides, the results of Sample O and Sample B at the different cooling rate of 2.5K/min and 10.0 K/min are also shown in the Table 2.
Illustration of the melting, crystallization and growth processes of sample slag.
Note: (XX) represents different cooling rate
(1) Effect of Na2O on the Distribution Ratio of P2O5
Figure 5 shows the relationship between distribution ratio of P2O5 and varied Na2O content. It can be seen that the distribution ratio of P2O5 between the solid solution and liquid slag increases with the addition of Na2O. As for the reference slag Sample O, the distribution ratio of P2O5 is 6.39; and it goes up to 6.69 and 8.16, when the Na2O content increases from 1% to 5%.
The distribution ratio of P2O5 versus mass fraction of Na2O.
(2) Effect of B2O3 on the Distribution Ratio of P2O5
Figure 6 shows the relationship between the distribution ratio of P2O5 and B2O3 content for CaO–SiO2–Fe2O3–P2O5 slag, and it is clear that the distribution ratio of P2O5 decreases from 6.39 to 3.19 with the addition of B2O3 content. Compared with the effect of Na2O, the results indicates that when dephosphorization is conducted by using multiphase slag, the addition of B2O3 might not be a good option for the enrichment of phosphorus from the liquid phase to solid solution.
The distribution ratio of P2O5 versus mass fraction of B2O3.
(3) Effect of Cooling Rate on the Distribution Ratio of P2O5
In order to understand the effect of the crystallization behavior of the solid solution on the distribution ratio of P2O5, the effect of cooling rate on crystallization of the precipitated solid solution was investigated. Figure 7 shows the phosphorus distribution ratio of both Sample O and Sample B at the different cooling rate of 2.5 K/min, 5.0 K/min and 10.0 K/min under the same holding time of 60 min. It could be observed that the cooling rate has very trivial effect on P2O5 distribution ratio, as it is firstly reduced and then getting increased slightly for both samples. Meanwhile, the phosphorus distribution ratio of Sample B is always higher than that of Sample O regardless of cooling rate, which is consistent with the results in Fig. 5.
Effects of cooling rate on the phosphorus distribution ratio.
In order to make best use of the solid solution as an alternative source for phosphate ore, it is important not only to increase phosphorus content in the solid solution, but also to understand the characteristics of the solid solution such as the morphology and size. Therefore, the morphology and size of the solid solution under the different conditions was investigated.
(1) Effects of Na2O on Morphology of the Solid Solution
Figure 8 shows the SEM images for the reference Sample O along with other three Sample A, B and C. The morphology of the solid solution is appearing three kinds of structures: dendrite, strip and nummular for each sample. Compared all fours samples, it was found that addition of Na2O would not change the morphology of the solid solution.
SEM images of samples with different Na2O content.
(2) Effects of B2O3 on Morphology of the Solid Solution
The SEM images of the reference Sample O and other three B2O3 containing Sample D, E and F are shown in Fig. 9. It is clear shown that the morphology of the solid solution in Samples D, E and F is different from that of Sample O, where the irregular polygonal structure was formed in solid solution, which indicated that the morphology of the solid solution was changed with the addition of B2O3.
SEM images of samples with different B2O3 content.
(3) Effects of Cooling Rate on Size of the Solid Solution
The effect of cooling rate on the crystal size of the solid solution was investigated by comparing their SEM images with a low magnification of 100 as shown in Fig. 10. It is found that the size of the solid solution at the cooling rate of 2.5 K/min is larger than that of 10 K/min. This may because the molten slag viscosity becomes larger with the addition of cooling rate,19,20) which makes it more difficult for the transportation of molten species. Thus it requires a larger driving force to initiate the crystallization, and results in a larger undercooling. As the size of the crystal is mainly determined by the growth rate of the crystals, and the growth rate in a larger cooling system is limited due to the increased difficulty of the transportation of molten species, the crystal size tends to become smaller with the increase of cooling rate.
SEM images of Sample O and Sample B at the different cooling rate.
Similar trends are observed for Sample B at the different cooling rate. Furthermore, comparing the results of Sample O and Sample B at the same cooling rate, it is found that the crystal size of the solid solution in Sample O is smaller than that in Sample B, it indicates that adding Na2O oxides is favorable for the crystallization of the solid solution, which is again in good agreement with the results in Fig. 5, the possible reason would be analyzed in Section 4.1.
There is a parcel phenomenon observed in many cases. Take Sample A with 1% Na2O for an example, it can be observed from Fig. 11 that several gray phases are embedded in the black solid phase, and its color is almost as same as the liquid phase. In order to determine the composition of the gray phase, EDS line analysis was made from point (M) to point (N). The obtained results are shown in Fig. 12. According to the line scanning results, the average content of elements in gray phase is almost equal to that in the liquid phase. Therefore, the gray phase was confirmed as the liquid phase. A possible explanation was found in another Sample B with 3% Na2O, as shown in Fig. 13. The formation process could be considered as follows: (1) several adjacent solid phases gradually precipitate and grow up (Fig.13(A)); (2) the adjacent solid phases continue to grow and merge with each other (Fig.13(B)); (3) when the equilibrium between the liquid and the solid phase is obtained, there will be no more phase transformation and the retaining liquid in the solid during the crystals merge would be reserved in the solid phase during quenching (Fig.13(C)). In order to elaborate this phenomenon, further studies are needed in the future.
SEM image of Sample A with 1% Na2O.
Results of line analysis from M to N by EDS.
SEM image of Sample B with 3% Na2O.
In the present work, the influence of Na2O and B2O3 on both the distribution ratio of P2O5 and the corresponding solid solution morphology for CaO–SiO2–Fe2O3–P2O5 system was investigated. The results show that Na2O and B2O3 play different roles for above system. In order to further investigate above oxides introduced variations, Sample O, Sample B with 3% Na2O and Sample E with 3% B2O3 were chosen as representative examples for XRD testing, respectively. The XRD results are shown in Fig. 14 through Fig. 16, the intensity of detected peaks for each sample is not very strong due to the presence of glass phase obtained through quenching of liquid slag. According to the XRD analysis, it is found in Fig.14 that the solid solution in the reference slag sample O is (2CaO·SiO2–3CaO·P2O5), which has been confirmed by previous paper,14) Figure 15 shows that the major phase of solid solution in Sample B is the (2CaO·SiO2–Na2O.2CaO·P2O5), and (2CaO·SiO2–Ca9.93(P5.84B0.16O24) (B0.67O1.79)) was formed in Sample E, B2O3-containg slag, as shown in Fig. 16.
X-ray diffraction of Sample O without oxide additive.
X-ray diffraction of Sample B with 3% Na2O.
X-ray diffraction of Sample E with 3% B2O3.
It is necessary to understand the formation mechanism of (2CaO·SiO2–3CaO·P2O5) solid solution before the discussion of phosphorus partition. According to the ionic solution model, Silicon and Phosphorus are existed in the form of [SiO4]4– tetrahedral unit and [PO4]3– tetrahedral unit in the molten slag, respectively. As the silicon ionic (Si4+) radius is 0.039 nm that is close to that of phosphorus ionic (P5+) radius which is 0.035 nm, the [SiO4]4– tetrahedral unit and [PO4]3– tetrahedral unit could be easily replaced with each other in a crystal lattice. Therefore, the molten slag with complex silicate-phosphate structure tends to form solid solution (2CaO·SiO2–3CaO·P2O5) during cooling.21)
When adding Na2O into CaO–SiO2–Fe2O3–P2O5 (12%) slag system, Sodium and Calcium will present in the form of ions in the molten slag as described above. As pointed by Hume-Rothery rule,22) when (where r1 and r2 stand for the radius of two different ions in the solute, respectively), both ions in the molten system could be replaced with each other and form the solid solution. Therefore, Calcium ion (Ca2+) and Sodium ion (Na+) could be replaced with each other in the molten multi phase slag system, as the ionic radius of Na+ (0.095 nm) is very close to that of Ca2+ (0.106 nm), such that the value of = 10.38%, is less than 15%.
For the molten slag, each ion has its electrostatic field, and the cation prefers to be together with the anion, which has the large electrostatic field.23) However, for the Na2O-containing slag, there are sufficient calcium ions (Ca2+) in the molten slag as the composition of slag is located in the C2S saturation zone. Therefore, the Sodium ion (Na+) would be mainly together with [PO4],3– because the electrostatic field of [PO4]3– is larger than that of [SiO4]4–.23) Thus, Sodium ions (Na+) would mainly replace the Calcium ions (Ca2+) that bonded with [PO4]3– during the cooling and a new solid solution of (2CaO·SiO2–Na2O·2CaO·P2O5) was obtained. There would be more (2CaO·SiO2–Na2O·2CaO·P2O5) solid solution precipitated with the increase of Na2O content. Besides, a better kinetic condition would be obtained, as Na2O could lower the viscosity of slag that is favorable for the nucleation and growth of the crystals. Combing above two issues, the phosphorus partition between the solid solution and liquid phase would increase with the increase of Na2O content.
Comparing (2CaO·SiO2–3CaO·P2O5) and (2CaO·SiO2–Na2O·2CaO·P2O5) solid solution’s structure, the change is that part of the Calcium ions (Ca2+) in the lattice was replaced by the Sodium ions (Na+) that is with almost the same radius, and this replacement does not affect the main structure of the (2CaO·SiO2–3CaO·P2O5) solid solution. Thereby, the new solid solution of (2CaO·SiO2–Na2O·2CaO·P2O5) remains the similar structure as that of (2CaO·SiO2–3CaO·P2O5).
Comparing the XRD results of Figs.14 and 16, the solid solution in multiphase slag was changed to (2CaO·SiO2–Ca9.93(P5.84B0.16O24) (B0.67O1.79)) when B2O3 was added. For the B2O3-containg slag, Boron could form two types of structure unit as [BO3]3– triangular and [BO4]4– tetrahedral, while there would be only one type of structure formed by Phosphorus and Silicon. The structure units such as [BO4]4–, [PO4]3– and [SiO4]4– are 3D, except for [BO3]3– which is a planar shape but it could link with other tetrahedral units.24,25) [BO4]4– and [PO4]3– tetrahedral units prefer to be together in the molten slag to form complex B–O–P bond based network during the cooling process,24) and a minor portion of [BO3]3– triangular units may connect to [BO4]4– or [PO4]3– tetrahedral units; thus, it would result in the formation of (2CaO·SiO2–Ca9.93 (P5.84B0.16O24) (B0.67O1.79)) solid solution during molten slag solidification. Therefore, the addition of B2O3 is favorable for improving the glass-forming ability and weakening the crystallization ability.26) So the crystallization of the solid solution is restrained with the increase of B2O3, and the phosphorus partition between the solid solution and liquid phase would decrease with the increase of B2O3 content.
Based on the above analysis, it was indicated that the crystal structure of the (2CaO·SiO2–Ca9.93(P5.84B0.16O24)(B0.67O1.79)) solid solution is different from that of the (2CaO·SiO2–3CaO·P2O5). Therefore, the crystal structure of the new solid solution was eventually changed to irregular polygonal structure with the addition of B2O3.
In order to increase the efficiency of dephosphorization and to use the slag as an alternative source of phosphate ores, the influences of Na2O and B2O3 on the distribution ratio of P2O5 between the solid solution and the liquid phase and the morphology of the solid solution for the CaO–SiO2–Fe2O3–P2O5 system, were investigated by using the melt-quenching technique. The conclusions obtained in the present study are summarized as follows:
(1) The distribution ratio of P2O5 in the solid solution was enhanced with the increase of Na2O content, as the new (2CaO·SiO2–Na2O·2CaO·P2O5) solid solution was formed due to the replacement of Na+ with Ca2+. Besides, the addition of Na2O would tend to provide a better kinetic condition for the crystal nucleation and growth, which in turn to improve the distribution ratio. However, the morphology of the solid solution does not change significantly, as the lattice replacement of Sodium ions by Calcium ions in the phosphate tetrahedral structure is very small.
(2) The addition of B2O3 study showed that the distribution ratio of P2O5 and the P2O5 concentration in the solid solution would reduce with the increase of B2O3 content, as the new formed complex (2CaO·SiO2–Ca9.93(P5.84B0.16O24) (B0.67O1.79)) solid solution is hard to precipitate from the liquid slag due to the complexity of the new formed network structure. The morphology of the new-formed solid solution also changed to irregular polygon with change of complex networks.
(3) The parcel phenomenon was observed during the formation of solid solution, which may be due to the growth of the crystals and liquid-solid equilibrium between the solid solution and liquid slag. The slow cooling rate would favorable for the precipitation and growth of the solid solution, and it leads to a larger crystal size than the one caused by a faster cooling rate. Thus, it is important not only to slow the cooling rate, but also to avoid the parcel phenomenon for the utilization of multiphase dephosphorization slag.
The financial support from NSFC (51274244, 51322405, 513111020) and the Fundamental Research Funds for the Central Universities (2011JQ010) is greatly acknowledged.