2022 Volume 62 Issue 3 Pages 496-503
In order to realize the efficient utilization of high phosphorus iron ore resources, a new method of phosphorus gasification removal in hydrogen-rich sintering process was proposed. In this paper, the gasification behavior of phosphorus in hydrogen-rich sintering process of high-phosphorus iron ore was studied. Thermodynamic calculation of possible reactions is carried out by using FactSage6.1 software, and the phase transformation and distribution of phosphorus in the process of roasting reduction were analyzed by XRD, SEM-EDS and EPMA. The experimental results show that in hydrogen-rich atmosphere, the dephosphorization rate increased from 9.9% to 29.51% and then decreased to 8.62% in the temperature range of 900°C–1200°C, and the maximum value appeared at 1100°C. Compared to the carbon reduction, the dephosphorization rate in hydrogen-rich atmosphere increased significantly in the whole temperature range, and the maximal dephosphorization rate could be improved from 15.03% to 29.51%. The results of thermodynamic analysis showed that the initial temperature of direct reduction of apatite by hydrogen is higher, and adding SiO2 and Na2CO3 allowed to decrease the reduction temperature of apatite by hydrogen to about 946.50°C. With the increase of reduction temperature, the reduced phosphorus gas was absorbed by metallic iron to form stable iron phosphorus compounds, resulting in the decrease of dephosphorization rate. Therefore, in order to realize the gasification removal of phosphorus, the selective reduction of iron oxide and apatite should be realized, and the formation of liquid iron should be avoided as far as possible in the reduction process.
With the development of China’s iron and steel industry, the demand for iron ore is increasing, and the contradiction of iron ore shortage is prominent.1,2) China is rich in high-phosphorus iron ore reserves, but due to its high phosphorus content, complex mineral phase structure and small mineral size, it is difficult to use efficiently.3,4,5) In addition, in the blast furnace ironmaking process almost all the phosphorus enters into the hot metal, which will not only increase the difficulty and cost of dephosphorization in the converter steelmaking process, but also increase the phosphorus content in the steel scoria. Finally, the vicious circle of phosphorus in steel production process is formed. Thereby the huge reserves of high-phosphate iron ore have not been used on a large scale by now.6,7,8) The development and utilization of high-phosphorus iron ore is the inevitable choice of resource strategy.
The researchers have carried out a lot of theoretical and experimental research on the dephosphorization of high phosphate ore, and obtained dephosphorization methods with theoretical and application value. At present, the popular dephosphorization methods mainly include beneficiation,9,10) reduction method,11,12) chemical method,13,14) bioleaching15,16) and microwave method,17,18) etc. The above dephosphorization method has achieved acceptable dephosphorization effect, but due to the constraints of environment, production cost and efficiency, mainstream configuration of iron and steel enterprises, it is impossible to achieve the unity of high-grade, high-recovery iron concentrate and low-phosphorus products, not to mention the industrial application.19,20,21) Therefore, the investigation of industrialized dephosphorization technology with high efficiency is still a technical problem to be solved urgently.22,23)
In order to reduce CO2 emissions, JFE Steel Corporation has developed the technology of injecting hydrogen-rich gas fuel into the sintering process, which can improve the quality of sinter without increasing the ratio of coke. Injection of hydrogen-rich fuel in sintering enhances the reducing atmosphere of the bed, broadens the high temperate zone (1200°C–1400°C) of the bed. In addition, the increase in the number of pores larger than 5 mm in sinter improves the air permeability of the bed.24,25,26) Based on the hydrogen-rich sintering technology, the good reduction performance of hydrogen makes the reduction atmosphere in the sintering bed enhanced, and the combustion of hydrogen enriched gas in the sintering bed broadens the high temperature section, which provides good conditions for the reduction of apatite. Moreover, the improvement of the permeability of the sinter bed is beneficial to the discharge of phosphorus-containing gas. Therefore, it is feasible to reduce the high phosphate ore to phosphorus gas in the sintering process and discharge it with the sintering waste gas.
In this paper, the gasification behavior of phosphorus in the process of hydrogen-rich sintering was studied. Phosphorus in high-phosphorus iron ore existed in gangue phase as an independent mineral. With the progress of the reaction, various mineral reactions have taken place, such as decomposition of calcium carbonate, dehydration and decomposition of chlorite (decomposes into SiO2 and Al2O3 at low temperature) and gangue transformation.3) At the same time, with the development of cracks, the inherent oolitic structure was destroyed, which made the full contact between apatite and gangue phase composition to promote the apatite reduction reaction. Therefore, SiO2 and Al2O3 in gangue phase have a significant effect on the reduction of apatite at high temperature, so pure reagent were used to simulate the chemical composition of high-phosphorus iron ore. The phase transformation and distribution of phosphorus during reduction were analyzed by XRD, SEM-EDS and EPMA. Thermodynamic calculation of possible reactions was carried out by FactSage6.1 software. The reduction mechanism and distribution law of phosphorus in the hydrogen-rich sintering process of high-phosphorus iron ore were discussed by the above methods. These work will provide theoretical basis for the reduction and dephosphorization of high-phosphorus iron ore.
The chemical composition of high phosphorus iron ore was simulated by analytical pure reagent, and the mass ratio of experimental mixture is shown in Table 1. In the course of experiment, in order to better study the law of gasification and migration of phosphorus in the reduction process, and avoid the interfering factors, the substances with mass fraction less than 1% were ignored. At the same time, in order to highlight the reduction process of apatite, the content of calcium phosphate was increased to enhance the reaction related to phosphorus. High purity graphite was selected as the carbonaceous reducing agent in this experiment to eliminate the interference of impurities. In addition, through thermodynamic calculation and previous carbothermal reduction experiments, it was found that Na2CO3 had a strong promoting effect on the reduction of apatite,27) so 1% Na2CO3 was added.
| Composition | Fe2O3 | Ca3(PO4)2 | SiO2 | MgO | Al2O3 | CaO | Na2CO3 | C |
|---|---|---|---|---|---|---|---|---|
| raw ore | 57.40 | 7.53 | 10.97 | 1.97 | 1.00 | 5.10 | – | – |
| tested sample | 58 | 11 | 11 | 2 | 1 | 5 | 1 | 5 |
In this experiment, all kinds of raw materials were crushed and ground to pass through a 0.074 mm sieve and then were mixed evenly according to the proportioning scheme described in Table 1, and the 5 g mixture was pressed under constant pressure by 5 MPa to form a cylindrical sample with a diameter of 20 mm. The cylindrical sample was dried at 100°C for 2 h until the sample no longer lost weight and the porosity of the dried sample was 27.51% by mercury intrusion porosimetry. The dried samples were reduced at different temperatures, and the reduction temperatures were 900°C, 1000°C, 1100°C and 1200°C, respectively. The reduction device is shown in Fig. 1. In order to ensure the full progress of the reduction reaction, the sample was heated from room temperature to different reduction temperature at the rate of 10°C/min, and then holding for 60 minutes. When hydrogen was used in the reduction process, the H2 and N2 flow rates were 2 L/min and 5 L/min, respectively. At the end of the reduction, the sample was cooled to room temperature with N2 protection in the furnace. When only carbon was used as reducing agent, the whole experiment process was carried out under N2 atmosphere with the flow rate of 7 L/min. The reduced sample was ground into powder (particle size<200 mesh), and a part of the reduced sample was used for chemical analysis to determine the phosphorus content.
Schematic diagram of the reduction apparatus. (Online version in color.)
The phosphorus content in roasting products was determined by chemical analysis. The reduced sample was tested by X-ray diffractometer to determine the phase composition and structure of the sample. FactSage 6.1 software was employed to calculate the thermodynamic relationship of related reactions, perfect the thermodynamic data of hydrogen reduction reaction of apatite and explain the mechanism of reduction and gasification of apatite. The element composition distribution of the sample was detected by SEM-EDS and EPMA, and the evolution law of apatite in the reduction process was clarified.
The multi-point sampling before and after roasting was used to detect the phosphorus content, and the dephosphorization rate was calculated according to the following formula.
In the formula: η represents dephosphorization rate. M0 and M1 represent the mass of tablets before and after reduction, respectively. P0 and P1 represent the content of phosphorus in the tablet before and after reduction, respectively.
The effect of roasting temperature on the gasification dephosphorization rate of the sample in different reduction atmosphere is shown in Fig. 2. It could be seen from the figure that temperature had a significant effect on the gasification process of phosphorus. In the case of only carbon as reductant, improving the reduction temperature from 900°C to 1200°C, the dephosphorization rate first increased from 3.2% to 15.03%, then decreased to 8.62%, and the maximum value appeared at 1100°C. In hydrogen-rich atmosphere, the dephosphorization rate first increased from 9.90% to 29.51%, and then decreased to 21.37%, the peak value was at 1100°C. Compared to carbon reductant, the dephosphorization rate under hydrogen-rich conditions increased significantly during the whole temperatures range, and the maximum dephosphorization rate increased from 15.03% to 29.51%. In addition, compared with the previous pre-reduction sintering experimental results, when the carbon content reaches 20%, the dephosphorization rate reaches 31%.27) Therefore, the reduction atmosphere had a significant effect on the gasification dephosphorization rate in the sintering process. This was mainly because the starting-reduction temperature of iron oxide was lower than that of apatite. When the reducing atmosphere is weak, the reducing agent was mainly used the reduction of iron oxide, and only a small amount of apatite were reduced, so the gasification dephosphorization rate was low. When the reducing atmosphere was gradually strengthened, the reduction of iron oxide was completed, and the residual reducing agent promoted the reduction process of apatite, which made the gasification dephosphorization rate increase rapidly.
Effect of temperature on dephosphorization rate of samples in different reducing atmospheres. (Online version in color.)
In order to better understand the phase transformation of phosphorus in the process of hydrogen-rich reduction, the phase of the reduced sample was analyzed by XRD. The results are shown in Fig. 3 and Table 2.
Phase composition of reduced samples from XRD analysis (A-Fe, B-Fe2SiO4, C-CaSiO3, D-Fe3C, F-Na2CaSi5O12, H-NaAlSiO4, I-FeAl2O4, O-Fe2P, V-Ca3(PO4)2). (Online version in color.)
| Samples | Products after reduction roasting from XRD patterns | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Temperature /°C | A | B | C | D | F | H | I | O | V | |
| 900 | √ | √ | √ | √ | √ | √ | √ | |||
| 1000 | √ | √ | √ | √ | √ | √ | √ | √ | ||
| 1100 | √ | √ | √ | √ | √ | √ | √ | √ | ||
| 1200 | √ | √ | √ | √ | √ | √ | √ | √ | ||
The existence of Fe, Fe2SiO4, and FeAl2O4 has been observed at 900°C, indicating that the iron oxide has been reduced. At the same time, the Fe3C was detected, which proved the carburizing reaction happened. Because the reduction of iron oxides was carried out step by step, when iron oxides were reduced to FeO, part of FeO reacted with SiO2 and Al2O3 to form fayalite and hercynite, which were difficult to be reduced further, resulting in Fe2+ remained in gangue phase and hindering the further reduction of iron. In addition, the presence of Ca3(PO4)2 was detected, meaning that most of Ca3(PO4)2 was not reduced at this time.
With a gradual rise of reduction temperature, the existence of Fe2P was detected at 1000°C, indicating that apatite began to be reduced, and part of the phosphorus gas entered the metal iron. When the reduction temperature up to 1100°C, the diffraction peak intensity of Fe2P increased obviously, indicating that a large amount of apatite was reduced to get phosphorus gas, and part of them was absorbed by metal iron. Consequently it is necessary to restrain the phosphorus absorption behavior of metal iron in the reduction process of high phosphorus iron ore.
Na2CaSi5O12 and NaAlSiO4 were observed to exist in the samples at different reduction temperatures. Because their melting points were lower, the liquid phase amount of the system increased with the increase of temperature, and the kinetic conditions of mass transfer in the system were improved, which was consistent with the thermodynamic calculation results. In addition, the additive Na2CO3 reacted with gangue in iron ore, which destroyed the package structure and improved the contact condition between apatite and reducing agent, as a result the reduction reaction could proceed better.28,29) At the same time, SiO2 reacted with FeO to form fayalite and it was difficult to be reduced, which inhibited the further reduction of FeO and hindered the movement of phosphorus to the iron phase. However, due to the low melting point of Na2CO3, Na2O dissociated from excess Na2CO3 will appear in the liquid phase, which will hinder the internal diffusion of gas, and facilitate the diffusion and aggregation of iron, so promoting the combination of iron and phosphorus.30) In addition, excessive Na2CO3 will further react with apatite to form CaNaPO4.31) Therefore, adding excessive Na2CO3 is not conducive to the dephosphorization reaction.
According to the phase transition analysis of roasting products, SiO2, Al2O3 and Na2CO3 have significant effects on the reduction of apatite. The thermodynamic calculation of possible reduction reactions under different composition conditions was carried out by FactSage 6.1 software, and the thermodynamic relationship of each reaction was obtained as shown in Eqs. (1), (2), (3). When taking SiO2 and Na2CO3 as additives, the starting reduction temperature of apatite decreased significantly, and the reduction reaction could occur at about 946.50°C, which made the reaction easily proceed during sintering process. When SiO2, Na2CO3 and Al2O3 existed at the same time, the starting reduction temperature of apatite was 1609.3°C, which was higher than that of adding SiO2 and Na2CO3. From the calculation results, it could be seen that the starting temperature of hydrogen reduction apatite could be significantly reduced by adding appropriate SiO2 and Na2CO3 in the hydrogen-rich sintering process, making it meet the reduction temperature conditions of apatite in the sintering process. Therefore, the gasification and removal of apatite are possible under the combined action of hydrogen and coke in the process of hydrogen-rich sintering.
| (1) |
| (2) |
| (3) |
The mineral reaction in the reduction process can be preliminarily understood by XRD detection, but the change process of apatite cannot be explained clearly. Therefore, the evolution of high phosphate iron ore and the process of phosphorus entering iron were explained by the SEM-EDS. The SEM-EDS images of the roasted products at different reduction temperatures are shown in Fig. 4.
Representative SEM-EDS analysis of reduced samples: (a) 1000°C (b) 1100°C (c) 1200°C. (Online version in color.)
As can be seen from Fig. 4, when the reduction temperature was 1000°C, the reduced metal iron particles were fine and dispersed. The distribution of P was almost coincident with Ca, the distribution area of Ca and Si had a clear boundary, and there was almost no overlap between Fe and P, indicating that there was a small amount of apatite was reduced at this time.
When improving the reduction temperature to 1100°C, the metallic iron increased gradually, and the accumulation of metallic iron could be observed. The overlapping area of P and Ca decreased, while Ca and Si overlapped increasingly, indicating that the reduction reaction of apatite increased. We also noticed that there was no obvious overlapping area between Fe and P distribution, only a few P elements entered into the interior of the Fe phase by diffusion, which also meant the diffusion of phosphorus to iron phase started.
Improving the temperature further, the reduction reaction of apatite and iron oxide enhanced accordingly. The accumulated metallic iron particles appeared at 1200°C and the condition of phosphorus entering the metallic iron became more favorable. The phosphorus entering the iron phase gradually increased and extended from the edge to the interior, gradually the phosphorus tended to be uniformly distributed in the metallic iron.
The element distribution of roasted products with different reduction temperature was analyzed by EPMA, as shown in Fig. 5, Tables 3 and 4. The results showed that upon allowing the reaction to proceed further, the distribution region of the P element in metallic iron enlarged gradually, and the content of phosphorus increased accordingly. The increasing trend of carbon content showed good consistency with the phosphorus element. When the reduction temperature reached 1200°C, the carbon content in high phosphorus content area could be high up to 3.64%. In addition, only Fe, P and C were detected in the iron phase, indicating that phosphorus did not diffuse into metallic iron in the form of Ca3(PO4)2, but migrated into metallic iron in the form of phosphorus gas.
Representative EPMA analysis of reduced samples: (a) 1000°C (b) 1100°C (c) 1200°C. (Online version in color.)
| Element | Fe | P | C |
|---|---|---|---|
| 1 | 98.7 | 0.27 | 1.03 |
| 2 | 97.66 | 0.39 | 1.95 |
| 3 | 96.75 | 0.85 | 2.40 |
| 4 | 94.89 | 1.47 | 3.64 |
| Element | Fe | P | C | Mg | Al | Si | Ca | O | Na |
|---|---|---|---|---|---|---|---|---|---|
| 5 | 18.24 | 1.30 | 1.05 | 1.31 | 0.12 | 19.76 | 14.92 | 42.34 | 0.96 |
| 6 | 16.53 | 1.10 | 1.74 | 1.35 | 0.13 | 22.42 | 17.09 | 38.51 | 1.13 |
| 7 | 16.89 | 1.23 | 1.87 | 1.30 | 0.13 | 23.75 | 18.44 | 35.27 | 1.12 |
Under the condition that the gasification dephosphorization rate at different temperatures was known, the distribution ratio of unremoved phosphorus in iron phase and slag phase was estimated approximately according to the phosphorus content in iron phase and slag phase obtained by EPMA quantitative analysis, as shown in Fig. 6. When the reduction temperature was 1000°C, nearly 69.37% of the phosphorus still existed in the gangue phase, only about 30.63% of the apatite was reduced, of which about 16.23% of the phosphorus was removed in the form of gas phase, and about 14.40% of the phosphorus was absorbed by iron phase. Rising the reduction temperature to 1100°C, about 47.80% of the phosphorus could be reduced, of which about 29.51% of the phosphorus entered into the gas phase, and about 18.29% of the phosphorus was absorbed by the iron phase. However, when the reduction temperature was set at 1200°C, about 53.49% of the phosphorus was reduced, but the proportion of phosphorus removed by gas-phase did not increased correspondingly, a mount of the phosphorus was absorbed by metallic iron phase, resulting in a significant increase in phosphorus content of iron, which finally arrived at 32.12%. To sum up, with the increase of reduction temperature, the reduction reaction of apatite further increased, and the trend of phosphorus into the iron phase also gradually increased. The absorption of phosphorus by iron becomes a critical reason to limit the gasification removal of phosphorus.
Distribution ratio of phosphorus in gas-iron-gangue phase at different reduction temperatures. (Online version in color.)
The results of EPMA quantitative analysis showed that the phosphorus content in iron phase was closely related to the carbon content of metallic iron, in another word, the area with high phosphorus content was also with high carbon content. The main reason for this was the diffusion rate of carbon atoms in metal iron was ranked as surface > grain boundary > grain. When the temperature was low, only the surface of metal iron particles melted, and the phosphorus was absorbed by liquid iron and accumulated on the surface of iron phase. With the increase of reduction temperature, the existing form of reduced iron changed from ferrite to austenite, and the ability to dissolve carbon increased greatly. The carburization of metal iron led to the increase of liquid metal iron.32) The grain boundary began to melt, phosphorus diffused into the metal along the grain boundary, and the solubility of phosphorus in hot metal increased. When the temperature reached 1200°C, the interior of the grain began to melt, phosphorus diffused into the metal along the grain boundary, and a large amount of reduced phosphorus was absorbed by liquid iron, resulting in an increase in the content of phosphorus in iron and the uniform distribution of phosphorus in the iron phase.
To sum up, in the process of hydrogen-rich sintering reduction of high-phosphate iron ore, the liquid iron phase has a strong ability to absorb phosphorus. Therefore, in the process of carbothermal reduction of high phosphate iron ore, the ratio of hydrogen-rich in sintering can be increased, accordingly reducing the coke breeze ratio, thereby the carburization can be alleviated and the formation of liquid iron is inhibited. As a result, the carbon content of metal iron can be limited in the solid phase area of Fe–C phase diagram, and to ensure that the iron phase does not melt at the reduction temperature. At the same time, additives are added to promote the rapid gasification and removal of apatite at low temperature. In addition, through the sintering sucking pressure regulation, more phosphorus gas can be quickly discharged from the sintering layer, so as to inhibit the rapid absorption of phosphorus by iron in the subsequent reduction process.
Figure 7 shows the schematic diagram of phosphorus gasification and recovery in hydrogen-rich sintering process. In the sintering process, the mixture of air, oxygen, hydrogen and circulating flue gas is injected into the sinter bed through the nozzle. Under the reduction atmosphere of coke and hydrogen, apatite began to be reduced. The phosphorus gas produced by reduction moves downward into the flue under the action of suction pressure of sintering.
Schematic diagram of hydrogen-rich sintering gasification dephosphorization and recovery system (1-Distributor, 2-Sintering machine, 3-Flue, 4-Precipitator, 5-Fan, 6-Igniter, 7-Gas hood, 8-Jet nozzle, 9-Flame nozzles, 10-Windbox, 11-Condensing heat exchangers, 12-Condensing ball, 13-Spray nozzle, 14-Settling tank, 15- Phosphorus storage tank, 16-Flue gas purifier). (Online version in color.)
The phosphorus-containing gas will be condensed and adsorbed on the surface of the material bed when it passes through the over-wet zone. In addition, due to the flue gas of the upper material bed contains a lot of water vapor, it is cooled and condensed when it passes through the low temperature material bed, which greatly reduces the air permeability of the material bed and is not conducive to the discharge of phosphorus-containing gas. Therefore, the reduction and recovery of phosphorus are carried out in stages. Taking the vanishing place of the over-wet zone (No. H bellows) as the boundary, the gas hood is divided into two parts, the reduction section (A-H) and the recovery section (I-X). There is a flame nozzles every two air jet in the reduction section, and the continuous combustion of flame nozzles during sintering provides heat for the material bed, which is beneficial to the reduction of apatite in high phosphate ore. The over-wet zone of the recovery section disappears, the resistance of phosphorus gas disappears, and a large amount of phosphorus gas is discharged with the sintering flue gas, so the flue gas in this stage is treated to recover phosphorus. After the mixed flue gas of the recovery section is purified by the precipitator, two condensing heat exchangers with unique design in series are connected. The gaseous phosphorus is condensed and gathered in the phosphorus collector, and the product yellow phosphorus is obtained after treatment. In addition, the condensing heat exchanger is connected with the hydrogen storage tank through the pipeline to realize the condensation recovery of phosphorus gas while complete the preheating of injecting hydrogen at the same time.
In the process of hydrogen-rich sintering, phosphorus in iron ore can be efficiently removed. The dephosphorized sintered finished ore can be used as blast furnace burden for iron making, which can not only significantly reduce the dephosphorization burden of converter, but also promote the recycling of steel slag resources and environmental protection and energy saving in steel production. The collected yellow phosphorus is an important industrial raw material, which plays an important role in many fields such as pesticides and chemical industry.
The reduction behavior of apatite and the distribution characteristics of phosphorus during hydrogen-rich sintering were studied in the temperature range of 900°C–1200°C, the conclusions were drawn:
(1) The addition of Na2CO3 or SiO2 during hydrogen-rich sintering can significantly decrease the hydrogen reduction temperature of apatite. When the reduction temperature is 1100°C, the maximum gasification dephosphorization rate is 29.51%. At this time, there is more phosphorus removed by gas phase and lower phosphorus content in metallic iron, which is beneficial to the dephosphorization of high phosphorus iron ore.
(2) In the process of sintering, part of the phosphorus gas produced by reduction enters the metal iron to form iron-phosphorus compounds, which leads to the decrease of dephosphorization rate. In order to achieve the purpose of phosphorus removal, it is necessary to realize the selective reduction of iron oxide and apatite.
(3) In the process of sintering, the absorption of phosphorus by metallic iron is related to the carburizing reaction. The amount of carbon can be reduced by increasing the injection ratio of hydrogen-rich fuel in the sintering process, so as to reduce carburizing and inhibit the absorption of phosphorus by metallic iron.
(4) The research shows that hydrogen has a positive effect on phosphorus removal by gasification in sintering process, but the reaction rate of apatite reduction by hydrogen is unknown. In addition, the influence of pore structure change of ore caused by hydrogen enrichment in sintering process on phosphorus gas removal efficiency is not clear. Therefore, the following work needs to further study the influence of apatite reaction rate and structure change on dephosphorization rate in reduction process.
The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (U1960205).
