Separation of Fe-bearing and P-bearing Phase from the Steelmaking Slag by Super Gravity

Abstract

The Fe-bearing phase and P-bearing phase were successfully separated from a steelmaking slag by the super gravity and the separated efficiency was improved with increasing the separated time. The P-bearing phase precipitating at 1663 K was intercepted by the filter, while most residual melt went through the filter into the lower crucible to form calcium iron and aluminum and solid solution of iron, magnesium and manganese (RO phase) after centrifugal separation. Under the condition of gravity coefficient G=600 g, T=1663 K and t=15 minutes, the mass fraction of P₂O₅ in the P-bearing slag increased from 2.49 wt% before separation to 3.56 wt% and that of Fe_tO in the Fe-bearing slag from 23.99 wt% to 38.67 wt%. The recovery ratio of P₂O₅ and Fe_tO accounted for 82.2% and 68.5%, respectively.

1. Introduction

Steelmaking slag is one of the major by-products during steelmaking process. Except for limited utilization, most of the steelmaking slags are dumped into the slag pit as wastes. However, lots of Fe-bearing phases such as the calcium iron and the aluminum and RO phase exist in the steelmaking slag, which can be applied as sintering binder agent or slag dephosphorization agent back into the iron- and steelmaking processes. But, the existence of phosphorus hinders the direct recycle of these Fe-bearing phases inside metallurgy fields. Therefore, the separation of phosphorus is the key for the reuse of steelmaking slag. Fortunately, a typical technique, the selective crystallization and phase separation method, was put forward and has been extensively investigated.^1,2) According to that, the enrichment mechanism of phosphorus in steelmaking slag has been widely studied,^3,4) which indicated that phosphorus was mainly enriched in dicalcium silicate as P-bearing phase. In order to enhance the phosphorus solubility in the P-bearing phase, different additives such as CaF₂,⁵⁾ TiO₂,⁶⁾ SiO₂,⁷⁾ and Al₂O₃⁸⁾ were introduced to modify the steelmaking slag. As for the separation process, traditional methods such as magnetic field⁹⁾ flotation and reduction, have been studied,¹⁰⁾ but none of them can separate P-bearing phase from the steelmaking slag effectively. Therefore, thinking about the successful application of super gravity on other different kinds of slags^11,12,13) and effective separation of the simulated slag from previous work,¹⁴⁾ the separation of P-bearing phase from actual steelmaking slag was investigated with super gravity in this study. At the same time, the microstructure and recovery ratios of P₂O₅ and Fe_tO were analyzed and calculated, respectively.

2. Experimental

The chemical composition of steelmaking slag from Laiwu Iron and Steel Corporation is listed in Table 1. The super gravity field, as shown in Fig. 1, was generated by the centrifugal apparatus. The resistance wire heating furnace balanced with a counterweight across the rotation axis was placed into a centrifugal rotor. The temperature was controlled by a program-controlled system with an R type thermocouple, within the observed precision range of ±3 K. The super gravity coefficient, a ratio of super-gravitational acceleration to gravitational acceleration, was calculated via Eq. (1).   

$$G=\frac{\sqrt{g^2+{\left({\omega }^{2}r\right)}^2}}{g}=\frac{\sqrt{g^2+{\left(\frac{N^2{\pi }^{2}r}{900}\right)}^2}}{g}$$(1)

where N is the rotating speed of the centrifugal, r/min; ω is the angular velocity, rad/s; r is the distance from the centrifugal axis to the sample, 0.25 m; g is normal gravitational acceleration, 9.8 m/s².

Table 1. Chemical composition of the steelmaking slag from laiwu iron and steel corporation (wt%).

CaO	FetO	SiO2	MgO	MnO	P2O5	Al2O3	TiO2
48.23	24.44	14.52	5.21	2.01	2.62	1.74	1.23

Fig. 1.

Schematic diagram of centrifugal separation apparatus.1. Counterweight; 2. Centrifugal axis; 3. Base; 4. Magnesia crucible; 5. Slag melt; 6. Resistance coil; 7. Filter; 8. P-enriched phase under centrifugal separation; 9. Thermocouple; 10. Horizontal rotor; 11. Conductive slipping; 12. Temperature controller.

The recovery ratio of P₂O₅ and Fe_tO were calculated via Eqs. (2) and (3), respectively.   

$$R_P=\frac{m_p\times {\omega }_{p1}}{m_p\times {\omega }_{p1}+m_{Fe}\times {\omega }_{p2}}$$(2)

  

$$R_{Fe}=\frac{m_{Fe}\times {\omega }_{Fe2}}{m_p\times {\omega }_{Fe1}+m_{Fe}\times {\omega }_{Fe2}}$$(3)

Where R_P and R_Fe are recovery ratio of P₂O₅ and FeO in the P-enriched slag and Fe-enriched slag, pct; m_P and m_Fe are mass fractions of the P-enriched slag and Fe-enriched slag, pct; and ω_P1 and ω_P2 are mass fractions of P₂O₅ in the P-enriched slag, pct; ω_Fe1 and ω_Fe2 are mass fractions of FeO in the Fe-enriched slag, pct.

20 g steelmaking slag from the Laiwu Iron and Steel Corporation was ground and put into the upper magnesia crucible (I.D. 18 mm and H. 60 mm) with four parallel slits (W. 0.4 mm) across the bottom. The graphite sheet was placed on the crucible cover to ensure that the sample was in the protection atmosphere of CO₂ and CO. And a carbon fiber felt was put inside and right above the bottom of the crucible as filter, whose thickness is 4 mm and density is 0.16 g/cm³. Another lower magnesia crucible (I.D. 18 mm and H. 20 mm) was used to hold the slag that went through the filter. The slag was heated to the desired temperature T=1663 K (1663 K) at heating rate of 5°C/min and kept for 10 minutes to ensure the slag being in the state of solid-liquid mixture, after which the centrifugal apparatus was started. Referring to the gravity coefficient in the study by Li et al.,¹²⁾ the specified angular velocity was adjusted to an appropriate value of 1465 r/min, namely, G=600 and kept at the constant temperature T=1663 K for different time. After that, the centrifugal apparatus was turned off and the sample was taken out to cool in air. The samples on and through the filter were cut into halves along the central axis. One part was polished and investigated on the scanning electron microscope, and the other was characterized by X-ray fluorescence (XRF-1800X from Shimadzu Corporation) and X-ray diffraction (TTRIII from Rigaku Corporation) to determine the chemical components and mineral compositions. Comparatively, the parallel sample was prepared at 1663 K for 15 minutes without centrifugal separation.

3. Results and Discussion

Figure 2 shows the cross section of the samples obtained by super gravity under the conditions of different separating times t=1, 5, 10, 15 minutes, gravity coefficient G=600 and temperature T=1663 K, compared with the parallel sample under the conditions of gravity coefficient G=1, temperature T=1663 K and time t=15 minutes. It is noted from Fig. 2(a) that there was no stratification phenomenon presenting in the parallel sample under normal-gravity field. In comparison, after centrifugal separation, part of slag went through the filter into the lower crucible and the significant stratification was presented in the samples, as shown in Figs. 2(b), 2(c), 2(d) and 2(e), and as the separating time increased, the mass of the slag into the lower crucible augmented. Figure 3 shows the microstructure of the stratified sample, which indicates that there are a large number of elliptical particles existing in the upper part, while the phases in the lower part is significantly different and has no fixed shape. With the help of X-ray distraction analysis result as shown in Fig. 4, it can be seen that the slag in the upper part was mainly composed of a large quantity of solid solution of tricalcium phosphate and dicalcium or tricalcium silicate and a few Fe-bearing phase, which was defined as P-bearing slag; and that in the low part was mainly composed of a large quantity of calcium iron and aluminum, solid solution of iron, magnesium and manganese and a few P-bearing phase, which was defined as Fe-bearing slag.

Fig. 2.

Macrographs of the samples obtained by centrifugal separation with different times at G=600, T=1390°C: (a) t=0 min, (b) t=1 min, (c) t=5 min, (d) t=10 min, (e) t=15 min.

Fig. 3.

Microstructure of the Stratified Sample Obtained by Super Gravity ((a) and (b) refer to A and B marked in Fig. 2(e), respectively).

Fig. 4.

X-ray diffraction of the sample after centrifugal separation and the parallel sample.

Table 2 shows the X-ray fluorescence result of the stratified sample with the G=600, T=1663 K and t=15 min. The mass fraction of P₂O₅ in the P-enriched slag was up to 3.56%, while that in the Fe-enriched slag was just 1.04%, which presented a clear differentiation phenomenon. The opposite result appeared on the distribution of Fe_tO, and the mass fraction of Fe_tO in the Fe-enriched slag was up to 38.67%, while that in the P-enriched slag was just 13.18%. The recovery ratio of P₂O₅ in the P-enriched slag was 82.2%; meanwhile, that of Fe_tO in the Fe-enriched slag was up to 68.5%, as shown in Table 3.

Table 2. X-ray fluorescence result of the sample after centrifugal separation (wt%).

Sample	CaO	FetO	SiO2	MgO	P2O5	MnO	Al2O3	TiO2
P-bearing slag	54.20	13.14	20.78	4.59	3.56	1.85	1.08	0.76
Fe-bearing slag	39.97	38.67	6.35	6.47	1.04	2.77	2.94	1.79
Parallel slag	48.13	23.99	14.67	5.39	2.49	2.24	1.87	1.20

Table 3. Recovery ratio of P₂O₅ and Fe_tO in the stratified samples after centrifugal separation (Pct).

Sample	Mass(g)	Mass Fraction	Recovery Ratio of P2O5	Recovery Ratio of FetO
P-enriched slag	11.5	57.5	82.2	–
Fe-enriched slag	8.5	42.5	–	68.5

It is well known that the dicalcium silicate firstly precipitates with the tricalcium phosphate from the steelmaking slag during the solidification process to form a P-bearing phase. When the temperature reached 1663 K, the steelmaking slag was in a solid-liquid coexistence state, namely the solid P-bearing phase and residual melt. Due to the high viscosity, the P-bearing phase cannot be separated from residual melt under the normal gravity. However, the super gravity has the characterization of reinforced separation of different phases in fluid. So the residual melt went through the filter into the lower crucible, while the solid P-bearing phase was blocked by the filter and remained in the upper crucible.

4. Conclusions

It was experimentally confirmed that the super gravity was a novel effective method to successfully separate the P-bearing phase and Fe-bearing phase from the steelmaking slag. The solid P-bearing phase precipitating at 1663 K was intercepted by the filter, while the residual melt went through the filter into the lower crucible to form the Fe-bearing phase. After centrifugal separation at the parameter of G=600, T=1663 K and t=15 minutes, the mass fraction of P₂O₅ in the P-bearing slag increased from 2.49 wt% before separation to 3.56 wt% and that of Fe_tO in the Fe-bearing slag from 23.99 wt% to 38.67 wt%; the recovery ratio of P₂O₅ and Fe_tO were 82.2% and 68.5%, respectively.

Acknowledgement

This work is supported by the National Natural Science Foundations of China (No. 51234001 and No. 51404025) and the Fundamental Research Funds for the Central Universities (FRF-TP-15-009A2), which is acknowledged with gratitude.

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