3.4. The Reaction Mechanism of High-Phosphate Iron Ore in Vacuum Reduction
Therefore, in order to further investigate the reasons for the low dephosphorization efficiency of fluoroapatite during VCTR, the reduction mechanism of fluoroapatite in the vacuum reduction process of high-phosphorus iron ore was studied. As shown in
Table 3, Samples were prepared according to the optimal conditions (
T = 1100 °C,
t = 30 min). The samples were, respectively, reduced in vacuum and N
2 atmosphere for a different reduction time (2.5–30 min) under the same conditions.
The macroscopic morphology changes of the reduced pellets at different times are, respectively, shown in
Figure 8 and
Figure 9 under vacuum and nitrogen conditions. It could be found that the reduced pellets in the vacuum condition reacted more uniformly and tended to consolidate; while the reduced pellets under nitrogen condition expressed more cracks and non-uniformity. The results indicated that the reduction reaction of pellets in vacuum conditions was excellent. In general, the control of pellets in the reduction process indicated that reduction temperature was reasonable. Chemical analysis was performed on the reduced pellets after 30 min reaction under the two conditions, and the metallization ratio and dephosphorization ratio were calculated, as presented in
Table 4.
Figure 10 and
Figure 11 show the XRD spectra of the reduced pellets as a function of reduction time under two conditions, respectively. Comparative analysis of the XRD patterns under the two conditions clearly indicates that the reduction reaction of Fe
2O
3 to FeO was basically completed in the first 5 min. Higher peak intensities of Fe and Fe
3C obtained in the vacuum condition indicated that the reduction of FeO to Fe was more thorough. However, the peak of FeO could be found in the entire reduction process under the nitrogen condition, which illustrated that the metallization rate in nitrogen condition was lower.
Figure 10 and
Figure 11 exhibit the appearance of the phosphorus-containing phase (Fe
2P) in the spectrum; however, the peak intensity does not change significantly due to its low content. No other iron-phosphorus phase is observed in the two Figures, thus it was necessary to conduct further analysis on the phosphorus-containing phase. Furthermore, the spectra of the two cases always exhibited the peak of graphite, which indicated that the carbon content in the reduction reaction was sufficient.
The results summarized in
Table 4 indicate a remarkable increase in the metallization ratio of vacuum-reduced pellets to 94.7%, while an undesirable decrease in the dephosphorization ratio from 13.5 to 6.2% compared to that for the nitrogen-reduced pellets. The reasons might be attributed to the following two aspects: (1) with the aggravation of the solution loss reaction of carbon (C + CO
2 = 2CO) in the vacuum condition, a great deal of fixed carbon was consumed to generate CO. In terms of iron oxide, the ability of CO to reduce Fe
2O
3 was stronger than that of carbon, so the reduction reaction of iron oxide greatly intensified with the increase of CO content. In contrast, the ability of CO to reduce fluorapatite was weaker than that of carbon; therefore, the reduction reaction of fluorapatite was indirectly inhibited due to the large consumption of fixed carbon. (2) The influences of iron oxides and reduced iron on the reduction process of fluorapatite were not considered. The reduction reaction of fluorapatite with fixed carbon and gangue might no longer generate gaseous phosphide, but iron-phosphorus compounds, such as Fe
3P, Fe
2P, and FeP.
For the second reason, the microscopic behavior of pellets in the process was observed by SEM-EDS.
Figure 12 shows no change in the distribution of iron oxide before and after the vacuum reduction reaction. The reduced metallic iron, with small size between 50–80 µm was uniformly distributed in the ore phase. Nonetheless, under the nitrogen condition, the accumulation and growth of metallic iron was found and it eventually formed large clumps with particle sizes between 200 and 300 μm. This was mainly ascribed to the fact that metallic iron grew too late to agglomerate based on the faster reduction rate in vacuum condition. Under vacuum conditions, the fluoroapatite began to be reduced at the 10th minute, and as the reaction progressed, the phosphorus content in the reduced iron also gradually increased. When phosphorus migrated into the iron phase, the high-phosphorus iron phase combined with the low-phosphorus or non-phosphorus iron phase from one side and gradually diffused into the iron phase. The distribution of phosphorus in the iron phase was uniform until the reduction reaction reached 30 min. The fluoroapatite did not start to react until 12.5 min under the nitrogen condition and the migration form of phosphorus entering the iron was the same as under the vacuum condition. This indicated that not all of the reduced phosphorus formed gaseous phosphides (P
2, P
4, PO and PO
2) during the reduction of fluoroapatite. In contrast, most of the reduced phosphorus entered the metallic iron. The migrated form of phosphorus might diffuse from high-phosphorus iron phase, produced by the reduction process of fluoroapatite, into low-phosphorus iron phase.
In order to determine the products in the reaction process, the local micro-areas of the iron-phosphorus phase under the vacuum condition and the nitrogen condition were respectively analyzed as presented in
Figure 13 and
Table 5.
Table 5 demonstrates an interesting result that the atomic ratio of iron and phosphorus was about 6–8:1. Therefore, this confirmed that iron phosphorus compounds existed in the form of Fe
3P, Fe
2P, and FeP.
Based on the aforementioned research results of the reduction process under different conditions, the reaction mechanism of high-phosphorus iron ore was discussed. Under vacuum condition, Fe
2O
3 was almost completely reduced to FeO after 5 min of reduction, and Fe began to be formed. Then, the stage of rapid formation of Fe occurred in 5–10 min, after which most of the element Fe transformed into the metal Fe. After 10 min, the fluorapatite began to be reduced. At this time, it was easy to ignore the effect of the large amount of reduced metallic iron on the reduction reaction. Some research [
26,
27,
28] considered that iron did not participate in the reaction, and the fluoroapatite reacted only with the fixed carbon and gangue. The reduced phosphorus was volatilized in the form of gas phase such as P
2 or PO. However, the test results had shown that the reduced phosphorus was not completely volatilized in the vapor phase, and most of the iron phosphorus compounds were formed. This indicated that a large amount of metal iron formed in 5–10 min, might have participated in the reduction reaction of the fluoroapatite, and the iron phosphorus compounds were directly formed. Therefore, a new reduction mechanism of fluoroapatite was proposed in this study, as shown in Equations (3)–(5) [
29,
30], and thermodynamic analysis was carried out:
In these equations, SiO2 was represented as gangue mineral. According to the type and content of gangue minerals, the products were also different.
Table 6 and
Figure 14 show that all reactions could be performed, except for the reaction (5) at 10
5 Pa, in the pressure range of 10
0–10
5 Pa. This indicated that metallic iron could promote the reduction of fluorapatite, and the effect of metallic iron on the reaction could not be ignored. In order to intuitively show the reduction process of high-phosphorus iron ore, the reduction route is represented in
Figure 15 and the schematic illustration of the fluorapatite reduction process is shown in
Figure 16.
Figure 16 demonstrates the completion of the reduction process of iron oxides before the beginning of the reduction of fluorapatite. Fluorapatite, reduced iron, and gangue contacted and reacted with each other with the diffusion of solid phase. Compared to that under nitrogen condition, the particle size of reduced iron in a vacuum was smaller and well distributed. The contact area with fluoroapatite was much larger. Therefore, the vacuum condition with a bad effect on generating gaseous phosphide was more favorable for the formation of Fe
xP. The generated Fe
xP phase attached to the edge of the iron phase and gradually gathered with the aggregation of the iron phase. Moreover, the phosphorus diffused from the high-phosphorus region to the low-phosphorus region during the process of accumulation, and the higher the reduction temperature, the more uniform the distribution of phosphorus.