Study on gasification dephosphorization of phosphorus magnetite reduced by SiC

The mechanism of reduction and dephosphorization of phosphorus-containing magnetite was studied using SiC as reducing agent. The thermodynamic conditions for the reduction of Ca3(PO4)2 and Fe3O4 by SiC were simulated by FactSage7.1 thermodynamics software, respectively, indicating that the reduction of Ca3(PO4)2 by SiC is thermodynamically feasible, but the kinetic conditions of the system are not good when no liquid phase is formed. The Fe2SiO4 liquid phase can be formed by reducing Fe3O4 by SiC at 1200 ℃. When the SiC ratio is 10 wt.%, the liquid phase content is the largest, which can reach more than 60 wt.%. Therefore, in this paper, the Fe2SiO4 liquid phase generated by the reduction of Fe3O4 by SiC is used to improve the kinetic conditions of the reduction of Ca3(PO4)2 by SiC at high temperature, and relevant experiments were carried out. The research results show that the system generates a suitable amount of liquid phase at 1250 °C, and the gasification dephosphorization rate is the highest, which is 39.89 wt.%, at 1200 °C, the reduction reaction was not fully carried out, and the dephosphorization rate by gasification was only 23.79 wt.%. At 1300 °C, Fe3O4 was completely reduced to metallic Fe, which combined with the reduced P4 gas to form Fe2P. Gasification dephosphorization rate is only 28.25 wt.%.


Introduction
In recent years, the increase of iron and steel production capacity, the rise of iron ore import price and the release of carbon neutrality policy have restricted the sustainable and rapid development of China's iron and steel enterprises [1][2][3]. Facing the development dilemma of iron and steel enterprises, on the one hand, enterprises began to merge and unite to unify the price of imported iron ore, and strive for greater price discourse power; On the other hand, it has increased the development and utilization of domestic abundant phosphorous iron ore resources to solve the raw material price problem fundamentally. The molecular formula for phosphosiderite is FePO 4 ·2H 2 O or Fe 2 O 3 ·P 2 O 5 ·2H 2 O (P 2 O 5 =38.9%, Fe 2 O 3 =43.8%, H 2 O=17.3%). Iron black, red rhombic columnar crystal [4], Hardness 3.8, density 2.8g/cm 3 . The luster is glassy. Color light red, or light red purple. The structure is close to conglomerate, with transparent columnar protrudes, curved joints, and a kind of iron red luster, hard texture, often associated with hematite, phosphate, siderite, magnetite, specific appearance and properties close to the associated ore. Its ore grade ranges from 10 to 70% [5]. Medium and high phosphorus iron ores account for a high proportion of the total iron ore reserves in China, typical deposits are Bayan Obo medium phosphorus magnetite and western Hubei oolitic high phosphorus hematite [6]. It is of great significance to study how to remove the harmful element phosphorus from iron ores and realize the utilization of phosphorous iron ore resources and the development of iron and steel enterprises in China.
Domestic and foreign scholars have done a lot of work on iron ore dephosphorization, most of them are based on C reductant with phosphorus oxide reduction to achieve the purpose of gasification dephosphorization [7][8][9], but carbon thermal reduction, has many problems such as needing high temperature, formation stable Fe x P compounds, as a result, the gasification dephosphorization rate decreased. Some scholars have studied the reducibility of SiC [10][11][12] and confirmed that it has strong reducibility and can be used as a high-quality reducing agent in smelting, and SiC also has a certain effect on dephosphorization [13]. SiC as a reducing agent has the following characteristics: On the one hand, it has a high melting point (2700 ℃), at low temperature will not burn like coke, has a good high temperature reduction performance. [14][15][16] On the other hand, at low temperature, Si as the main reducing agent in SiC can effectively lower the sample alkalinity, which is beneficial to improve the kinetic conditions, and is conducive to the removal of phosphorus by C as the reducing agent at high temperature [17]. This paper is based on the fact that SiO 2 , the product of reduction of 3CaO·P 2 O 5 phosphate mineral in iron ore by SiC, can combine with CaO and replace P 2 O 5 , thus accelerating gasification dephosphorization [18]. Therefore, SiC is selected as the reducing agent to systematically study the mechanism of gasification dephosphorization of reduced phosphorous magnetite.

Experimental materials
Phosphorus in most phosphorous magnetite ores in China is in the form of apatite Ca 3 (PO 4 ) 2 . In order not to be affected by impurities in iron ore, pure reagents are used as raw materials for the experiment, including Fe 3 O 4 , Ca 3 (PO 4 ) 2 and SiC. The reagent types and specifications are shown in Table 1 and the FactSage7.1 configuration conditions are shown in Table 2.

Research methods
Firstly, the reduction reaction was simulated by FactSage 7.1 thermodynamics software, to explore the thermodynamic conditions and equilibrium phase composition of the reaction of SiC reducing Ca 3 (PO 4 ) 2 and Fe 3 O 4 respectively at different temperatures, and the appropriate ratio was selected according to the stoichiometric relationship of the reaction, According to the simulation results, the simultaneous reduction of Ca 3 (PO 4 ) 2 and Fe 3 O 4 with SiC was studied experimentally. The main steps of the test are as follows: The raw material is mixed according to the stoichiometric ratio of the reaction. Press the mixed material is pressed into tablets for sample preparation; Reduction roasting of the sample. The roasted samples were tested and analyzed. Dephosphorization rate was calculated according to the phosphorus content and the quality change of samples before and after reduction roasting: where M 1 and M 2 are the sample mass before and after reduction roasting respectively; w(P)1 and w(P) 2 are the phosphorus content of samples before and after reduction roasting.
The test procedure was as follows: the raw materials were put into the mixing tank according to the ratio and mixed on the mixing machine for 3 h. After mixing, 3 g of the ingredients were weighed on a balance and put into the tablet press, 7% deionized water was added, and the  FToxide-SLAGA、FToxide-SPANA、FToxide-MeO-A、FToxide-cPyrA、FToxide-oPyr、FToxide-pPyrA、FToxide-LcPy、FToxide-WOLLA、FToxide-bC 2 S、FToxide-aC 2 S、FToxide-Mel、FToxide-OlivA samples were pressed for 2 min under 5 MPa pressure. The dried specimens were roasted in a vacuum tube furnace and cooled to room temperature after roasting, and the roasted samples were ground into powder and analyzed by means of phosphorus content, x-ray diffraction and SEM spectroscopy. According to the chemical reaction measurement relationship of SiC reducing Ca 3 (PO 4 ) 2 , pure reagents Ca 3 (PO 4 ) 2 , SiC and Fe 3 O 4 with mass fraction of 53%, 20% and 27% were added as raw materials, respectively. The sample was prepared by pressing and the temperature rising system was 5 ℃/min. The roasting time was 60min, and the pressure was 91.2 KPa (the same as the negative pressure in the sintering process). The gasification dephosphorization rate was investigated when the reduction temperature was 900 ℃, 1000 ℃, 1100 ℃, 1200 ℃ and 1300 ℃, and the optimal dephosphorization temperature was determined. In this case, The phosphorus content in the sample is measured by the mass fraction detection function of XRD diffractometer, which measures the mass fraction of the phase containing phosphorus after the reaction, and determines the phosphorus content in the sample after conversion.

Thermodynamic calculation of reduction of Ca 3 (PO 4 ) 2 by SiC
The equilibrate phase composition of SiC reduction Ca 3 (PO 4 ) 2 at 600 ∼ 1 500 ℃ was simulated and calculated using the Equilib module of Thermodynamics software FactSage7.1 and the products were P 4 gas, CaSiO 3 , Ca 3 Si 2 O 7 and C, as shown in Û. 1. The reaction equation in the standard state is: In Formula (2), When ΔG θ < 0, the reaction can occur spontaneously. It can be seen from Fig. 1(a) that the liquid phase can only be generated at 1400 ℃. According to relevant studies, the limiting condition of the reduction dephosphorization reaction of fluorapatite is the diffusion of Ca 3 (PO 4 ) 2 in the solution, and the reduction reaction of phosphate minerals is difficult to proceed without the participation of the liquid phase [19]. According to the stoichiometric relationship between Ca 3 (PO 4 ) 2 and SiC in equation (2), the mass percentage ratio of Ca 3 (PO 4 ) 2 and SiC is 75.61 wt.% : 24.39 wt.%.

Thermodynamic calculation of reduction of Fe 3 O 4 by SiC
The phase equilibrium of SiC reduction Fe 3 O 4 was calculated using Factsage7.1. according to different products, standard state reaction equation is obtained by reaction module mainly including: (2) 2Ca3(PO4)2 + 5SiC = 3CaSiO3 + P4 + Ca3Si2O7 + 5C According to the thermodynamics conditions, the ΔG θ < 0 of the reactions (3) ~ (6) can be spontaneously carried out in the range of 700~1700 ℃. The relationship between gibbs free energy and temperature in the standard state of reactions (3) ~ (6) is shown in Fig. 2.
The thermodynamic simulation of the reduction of Ca 3 (PO 4 ) 2 by SiC shows that liquid phase can be formed only when the temperature is above 1400 ℃. Although the reaction can be carried out in thermodynamics, the kinetic conditions of the reduction of Ca 3 (PO 4 ) 2 are very poor without the participation of liquid phase, and the reaction is difficult to occur. Since the melting point of Fe 2 SiO 4 , the product of SiC reduction of Fe 3 O 4 , is about 1200 ℃, the kinetic conditions of dephosphorization reaction can be improved by using the liquid phase of Fe 2 SiO 4 generated by SiC reduction of Fe 3 O 4 At high temperature, and the product gas CO or CO 2 is also conducive to the gasification and removal of phosphorus [20]. Therefore, thermodynamic simulation was carried out for the reaction of SiC reduction of Fe 3 O 4 at 1200 ℃, and the equilibrium phase composition and liquid phase amount were calculated when SiC content were 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% respectively, as shown in Fig. 3.
According to Fig. 3, as the SiC content gradually increased from 5 to 20 wt.% at 1200 ℃, the Fe 2 SiO 4 liquid phase increased first and then decreased, reaching the maximum at 10wt.% SiC, which was above 60%. The content of Fe increases first and then decreases, and reaches the maximum at 15wt.%SiC, which is over 40%. The content of FeO decreases until it disappears. SiO 2 (6) 3Fe3O4 + SiC = 7FeO + Fe2SiO4 + CO and Fe 3 C appear at about 15wt.% SiC and have a tendency to increase gradually.

Experimental results and analysis
Combined with thermodynamic simulation results of SiC reduction of Ca 3 (PO 4 ) 2 and Fe 3 O 4 , in order to further clarify the mechanism of phosphorus-containing magnetite gasification dephosphorization reaction, and make the system produce enough liquid volume to promote the smooth reaction, appropriate ratio of Fe 3 O 4 , Ca 3 (PO 4 ) 2 and SiC were selected for experimental study. According to the stoichiometric relationship of reaction (2), the ratio of Ca 3 (PO 4 ) 2 and its reducing agent SiC remained at 75.61 wt.% : And the sum of the mass of the two accounts for 70 wt.% of the total mass fraction of the system. 30 wt.% of the total mass fraction of the system is replaced by Fe 3 O 4 and its reducing agent SiC, where the mass ratio of Fe 3 O 4 to SiC is 90 wt.%:10 wt.% (SiC content is 10 wt.%), the most Fe 2 SiO 4 liquid phase can be generated. Ensure that the dephosphorization reaction is fully carried out under sufficient liquid volume. The ratio of raw materials was shown in Table 3. The Fe 2 SiO 4 liquid phase will be produced at 1200 ℃, and the amount of liquid phase may increase with the increase of temperature, which is conducive to the dephosphorization reaction.Therefore, the reduction roasting temperatures were selected as 1200 ℃, 1250 ℃ and 1300 ℃, respectively.

Effect of temperature on gasification dephosphorization rate
Five samples at each temperature were selected as a group for parallel reduction roasting experiments. The weight loss of samples after roasting and cooling at 1200 ℃, 1250 ℃ and 1300 ℃ were recorded. The phosphorus content of products after roasting at different temperatures was detected, and the gasification dephosphorization rate was calculated according to Eq. (1) Fig. 4 shows the phosphorus content and dephosphorization rate of roasting products at different temperatures. As can be seen from Fig. 4, as the temperature increases from 1200 to 1300 ℃, gasification dephosphorization first increases and then decreases. At 1200 ℃, the gasification dephosphorization rate is 23.79 wt.%, and at 1250 ℃, the highest dephosphorization rate is 39.89 wt.%. When the temperature rises to 1300 ℃, the gasification dephosphorization rate decreases to 28.25 wt.%. Due to the increased gradually with the increase of experimental temperature liquid content in the system, so the sample after 1200 ℃ calcination cooling for molten state at first, has less liquid, gasification dephosphorization rate is low, at 1250 ℃ cooling samples show semi-molten state, gasification dephosphorization rate is higher, up to 1300 ℃ when sample complete melting, product structure is compact, porosity decreases, The escape resistance of phosphorous gas increases, which is not conducive to gasification dephosphorization. The highest dephosphorization rate was achieved at the temperature of 1250 °C, and phosphorus was not combined with Fe to form Fe 2 P; on the one hand, Fe 3 O 4 was mostly reduced to Fe 2 SiO 4 at this temperature. In the presence of C reductant, the binding ability of Fe to C was better than that of Fe to P, and Fe 3 C was formed, consuming most of the Fe. On the other hand, the insufficient amount of liquid phase at this temperature leads to insufficient kinetic conditions for the formation of Fe 2 P [21]. It can be seen that appropriate reduction temperature and liquid volume are the key to improve the effect of gasification dephosphorization. The experimental results are basically consistent with the thermodynamic simulation, but in Fig. 1 (b), the dephosphorization rate decreases when the temperature is higher than 1300°C, while this temperature is 1250°C in the experiment. The analysis suggests that, on the one hand, the thermodynamic calculation belongs to the equilibrium state reaction with the same relative concentration at each place, while the raw material used in the experiment, with a high local SiC content, undergoes local dephosphorization at a relatively low temperature. On the other hand, the local overheating in the experiment led to the early generation of the liquid phase, which promoted the diffusion of fluorapatite in the liquid phase and facilitated the dephosphorization, while the local overheating did not exist in the thermodynamic calculation. Therefore, the optimal dephosphorization temperature in the experiment is lower than the thermodynamic simulation calculation temperature.

Influence of temperature on phase composition of roasted products
The product after reduction roasting was broken into powder by crusher for X-ray diffraction analysis, to determine the phase composition of the product at different temperatures, and analyze the reduction law of SiC to iron oxide and phosphorus oxide. Fig. 5 shows the XRD patterns of the products at 1200 ℃, 1250 ℃ and 1300 ℃.
As can be seen from Fig. 5, the main phases at 1200 ℃ are CaSiO 3 , Fe 3 O 4 , SiO 2 , Ca 3 (PO 4 ) 2 , Fe 2 SiO 4 and SiC. The diffraction peak intensity of Ca 3 (PO 4 ) 2 is the highest. In addition, the diffraction peak of SiC is also strong, and the diffraction peak of reactant Fe 3 O 4 also exists. It indicates  Research Article SN Applied Sciences (2023) 5:56 | https://doi.org/10.1007/s42452-022-05264-w that the reduction reaction at 1200 ℃ is incomplete and the reaction degree is low. At 1250 ℃, the diffraction peak of CaSiO 3 is obviously enhanced, the diffraction peak of Ca 3 (PO 4 ) 2 is weakened, and the diffraction peak of FeO is detected. The reduction reaction is better than that at 1200 ℃. When the temperature reached 1300 ℃, FeO and Fe 2 SiO 4 diffraction peak disappear, and SiC could not be detected, and there were more diffraction peaks of Fe 2 P. This was because there was a lot of liquid phase at 1 300 ℃, and the reduction reaction of Fe 3 O 4 was quite thorough. Most of Fe 3 O 4 was reduced to Fe, which combined with the reduced phosphorus gas to form Fe 2 P. The XRD analysis shows that the reduction reaction at 1250 ℃ is suitable and the rate of gasification dephosphorization is high. The phosphorus gas at 1250 ℃ is more conducive to phosphorus removal due to the change of kinetic conditions, which is consistent with the conclusion in Fig. 1 that the dephosphorization rate is higher under 1300 ℃.
In order to further clarify the gasification dephosphorization mechanism of phosphorus-bearing magnetite by SiC, SEM energy spectrum analysis was carried out on roasting products at different temperatures, and element distribution was observed by plane scanning, as shown in Fig. 6.
By comparing the SEM analysis images at different temperatures, it can be seen that, at 1200 ℃, there are more overlap areas between Ca and P, while there are also more overlap areas between Fe and O. The distribution area of Fe element is uneven and lump-shaped. It is speculated that the overlap area between Ca and P is Ca 3 (PO 4 ) 2 , and the overlap area between Fe and O is Fe 3 O 4 . The results showed that when the liquid phase was small at 1200 ℃, the reactants Ca 3 (PO 4 ) 2 and Fe 3 O 4 remained more and the reaction effect was not good. At 1250 ℃, the overlap area of P and Ca element decreases, the distribution area of Ca, Si and O element is the same, and Fe element is evenly distributed. It is speculated that the overlap area of Ca, Si and O element is CaSiO 3 , and the distribution area of uniformly dispersed Fe element is mostly Fe 2 SiO 4 phase. The results showed that the reduction reaction was better at 1250 ℃. There were a large number of CaSiO 3 and Fe 2 SiO 4 in the product, and there were still some overlap areas between Ca and P. Ca 3 (PO 4 ) 2 was not completely reduced, while Fe was evenly distributed and Fe 3 O 4 was completely reduced. At 1300 ℃, it can be clearly seen from the plane scanning diagram that Ca and P are almost completely separated, and the distribution region of Fe and P has a high degree of overlap. It is speculated that the phase in the overlap region of Ca, Si and O is CaSiO 3 , and the phase in the overlap region of Fe and P at point B is Fe 2 P. It indicates that Ca 3 (PO 4 ) 2 and Fe 3 O 4 are completely reduced when the liquid phase is larger at 1300 ℃, and the product Fe generated by them combines with phosphorus gas to form Fe 2 P.
In summary, the temperature increases from 1200 to 1300 ℃ as the amount of liquid phase increases the extent to which the reduction reaction proceeds. At 1200 ℃, due to poor kinetic conditions, the reduction degree of Ca 3 (PO 4 ) 2 and Fe 3 O 4 is low, most of Ca 3 (PO 4 ) 2 is not reduced, and the gasification dephosphorization rate is low. At 1250 ℃, the liquid phase increased, and the kinetic conditions improved at high temperature so that Ca 3 (PO 4 ) 2 was mostly reduced, and Fe 3 O 4 was mostly reduced to Fe 2 SiO 4 , which increased the gasification dephosphorization rate. At 1300 ℃, Ca 3 (PO 4 ) 2 is basically completely reduced, but the reduction degree of Fe 3 O 4 is further increased with the improvement of kinetic conditions, and Fe 2 SiO 4 is reduced to Fe. Although more phosphorus gas is produced than 1250 ℃, more phosphorus gas is combined with Fe to form Fe 2 P, and the gasification dephosphorization rate decreases. This is essentially the same trend as in Fig. 1(b), where the Ca 3 (PO 4 ) 2 dissolution temperature in the thermodynamic calculations, can be used as a supplement to the experimental data. The dephosphorization rate decreases at a temperature slightly higher than the experimental temperature. When the temperature was higher than 1300 ℃, the dephosphorization rate began to decrease, and the temperature was higher than 1400 ℃, which prompted most of Ca 3 (PO 4 ) 2 to enter the liquid phase to be reduced, and the P 2 O 5 content in the liquid phase increased sharply, and the dephosphorization rate decreased significantly. Author contributions GPL: The theory of the paper is guided, and the relevant content of the paper has been modified. SH: The experimental results are analyzed and a paper is written. TH: Provide experimental data and assist in analysis. JZ: Computational aid is provided and experimental work is carried out. XHJ: Provide thermodynamic calculation data. All authors have read and agreed to the published version of the manuscript.

Conflict of interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.
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