Gasification Behavior of Phosphorus during Biomass Sintering of High-phosphorus Iron Ore

Abstract

Developing the innovative technology for efficient utilization of high-phosphate iron ore was the inevitable choice of resource strategy. In this paper, exploring a new method of gasification removal of phosphorus by iron ore sintering technology based on biomass. Combined with theoretical analysis and experimental research, the effects of different conditions (biochar type, addition proportion, basicity) on the reduction and gasification behavior of phosphorus were studied. The results showed that the addition of biochar instead of part of the coke powder could promote the reduction of apatite and increase the dephosphorization rate compared with the full coke breeze. Among different types of biochar fuels (fruit shell charcoal, straw charcoal, bamboo charcoal), bamboo carbon had the greatest effect on the dephosphorization rate. The optimal reduction temperature was 1100°C, substitution proportion of bamboo charcoal was 20% and basicity was 0.5 for the phosphorus gasification and removal, and the corresponding dephosphorization rate was 28.77%. Selective reduction of iron and phosphorus existed in the reduction process, iron oxide took precedence over the reduction of apatite, and part of phosphorus gas entered the metal iron to form iron phosphide, resulting in the decrease of dephosphorization rate.

1. Introduction

China is rich in high phosphorus iron ore resources, accounting for more than 10% of the total iron ore resources, so the development and utilization of high-phosphorus iron ore was the inevitable choice of resource strategy.¹⁾ Phosphorus does great harm to the properties of steel, so it must be removed or reduced to a reasonable range before entering blast furnace in ironmaking process, and then completely removed in the later converter process.^2,3,4) At present, there is a lack of efficient treatment methods to realize the separation of phosphorus in iron ore, and it is difficult to recycle the resources of high-phosphorus steel slag produced in the smelting process. Metallurgical reuse will result in the enrichment of phosphorus and affect the quality of steel, while the phosphorus content is too low when directly used as the raw material of phosphate fertilizer, thus limiting the efficient use of high-phosphate ore.^5,6,7) Therefore, it is of great significance to explore the efficient utilization technology of high-phosphorus minerals and realize resource utilization.

To make efficient use of high-phosphorus iron ore, domestic and foreign researchers had carried out experimental studies on iron ore dephosphorization by using a variety of methods. Dephosphorization by beneficiation was restricted by the complex mineral structure in high phosphorus iron ore. Iron oxide grains in the ore were embedded with fine grain size, and it was mixed with apatite and ocher chlorite to form a layered oolitic structure, leading to a difficult problem for realizing the selective enrichment of iron.^8,9,10) Microbial leaching could obtain a better dephosphorization effect, but microbial self-cultivation and dephosphorization would take a long time, and the sulfur content in the ore would be increased in the production process, which increased the difficulty of subsequent use.^11,12) Chemical treatment combined with reverse flotation process to treat high phosphorus iron ore could realize the removal of phosphorus element, but the enrichment degree of the iron element was very limited, and produce a large amount of sewage, which was not conducive to environmental protection.^13,14,15) At present, due to the lack of effective iron extraction and dephosphorization process, there had been no comprehensive utilization of high phosphate iron ore in China.

The traditional sintering process proceeded in an oxidation atmosphere macroscopically, and around the carbon particles there was strong or weak reducibility. Based on the characteristics of the sintering process, there might be a possiblility to utilize the local reducibility around the carbon particles to reduce and removal phorsphous in the iron ore. Thereby, based on the sintering characteristics, it is of great significance to take measures to reduce the reduction temperature of apatite and make the reduced phosphorus gas volatilize into the flue gas under the action of sintering negative pressure.¹⁶⁾ Zhang¹⁷⁾ had carried out exploratory experiments on small pellet sintering dephosphorization and microwave sintering dephosphorization process. The results showed that the optimal dephosphorization rate was conducted with 5 mass% carbon mixing proportion, 0.75 mass% SiO₂ and 1.5% mass% CaCl₂, and the corresponding dephosphorization was 25.71%. In the early stage, we studied the gasification behavior of phosphorus in the hydrogen-rich sintering process of high-phosphorus iron ore. The result showed that compared to the carbon reduction, the dephosphorization rate in the hydrogen-rich atmosphere increased significantly in the whole temperature range, and the maximal dephosphorization rate could be improved from 15.03% to 29.51%.¹⁸⁾ To sum up, the sintering process had a certain dephosphorization ability, but it was still in the laboratory stage, and the improvement and development of system theory and basic research was an important problem to be solved.

To realize energy conservation and emission reduction in the iron and steel industry, clean and renewable biomass energy was proposed to replace coal fossil fuels for sintering. Compared with coke breeze, the CO₂ produced by biochar combustion participated in the atmospheric circulation, coupled with the characteristics of low nitrogen and sulfur of biochar, which could reduce the production of pollutants in the sintering process from the source.^19,20) It was of great significance to realize the green and efficient utilization of high phosphate iron ore by iron ore sintering technology based on biomass. In this paper, the reduction and migration behavior of phosphorus during sintering process based on biomass were studied using a vertical tube furnace in the laboratory. The law of gasification removal and distribution of phosphorus in the reduction process was clarified.

2. Materials and Methods

The experimental batching method was consistent with the previous experiment,¹⁸⁾ and the analytical pure reagent was used to simulate the chemical composition of high phosphate iron ore. The mass proportion of the experimental mixture shown in Table 1. In order to reduce the interference factors and better study the law of gasification and migration of phosphorus in the reduction process, the substances with mass fraction less than 1% in raw iron ore were ignored. At the same time, to highlight the reduction process of apatite, the content of calcium phosphate was appropriately increased to enhance the reaction related to phosphorus.

Table 1. Mass proportion of mixture in reduction experiment (mass fraction, mass%).

Composition	Fe2O3	SiO2	CaO	MgO	Ca3(PO4)2	Al2O3	C	Na2CO3
Raw ore	57.40	10.97	5.10	1.97	7.53	1.00	–	–
Simulation proportion	58	11	5	2	11	1	5	1

Four kinds of solid fuels were used in the experiment, they were coke breeze and three kinds of biochar. The coke breeze was taken from the sintering industry site, and the three kinds of biochar were obtained through carbonization of fruit shell, straw and bamboo respectively. The industrial analysis and ash composition of fuel are shown in Table 2. Compared with coke breeze, biochar fuel had lower fixed carbon and ash, and higher volatile. Bamboo charcoal had the highest fixed carbon and volatile content among the three kinds of biochar. The ash composition analysis results showed that the ash components of biochar were mainly CaO, SiO₂, and Al₂O₃, among which the ash content of shell charcoal was alkaline, and the ash content of straw charcoal and bamboo charcoal was acidic.

Table 2. Industrial analysis and ash composition of the fuel (mass fraction, mass%).

Fuel type	Fixed carbon	Volatile matter	Ash	Chemical composition of ash
				CaO	SiO2	Al2O3
Coke	83.18	2.96	13.86	8.92	15.26	32.41
Fruit shell charcoal	79.06	13.34	7.6	33.102	32.15	11.08
Straw charcoal	74.25	13.14	12.61	15.285	57.46	9.124
Bamboo Charcoal	81.21	13.59	5.2	3.991	19.76	16.511

The specific surface area and pore structure of different fuel samples were detected by the nitrogen adsorption method. The results are shown in Table 3. The good reactivity of the fuel was beneficial to the improvement of the reduction rate. Compared with coke breeze, biochar had a smaller pore size and larger specific surface area, which determined its good reactivity. The pore structure of biochar surface was complex, a large number of micropores were formed on the surface, the number of defects sites and pores was relatively increased, and its reaction activity was further increased, which was conducive to the improvement of the reduction rate. In addition, larger pore size and good porosity were conducive to the better external diffusion of phosphorus gas produced by reduction with the flue gas. Although the pore size of biochar was small, its internal pores were connected to form a complete channel, which was conducive to gas diffusion. Bamboo charcoal in biochar had high porosity, mainly micropores, and large specific surface area, which was beneficial to reduction reaction and gas diffusion.

Table 3. Specific surface area and pore structure of fuel.

Samples	BET surface area (m2/g)	Average pore diameter (nm)	Average pore volume (cm3/g)
Coke	8.2411	11.5233	0.015410
Fruit shell charcoal	16.4046	10.9191	0.011470
Straw charcoal	64.4372	9.6147	0.056901
Bamboo Charcoal	274.1646	4.5902	0.014837

The influence of reduction conditions on the reduction and migration behavior of phosphorus was studied by adjusting the proportion of biochar and basicity in the mixture, in which the proportion of biochar was adjusted according to the fixed carbon content to keep the carbon content in the mixture unchanged. The experiment of biomass iron ore sintering reduction of high-phosphate iron ore was carried out in a programmed reduction furnace as shown in Fig. 1. The equipment and methods were the same as in our previous work.¹⁸⁾ 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 105°C for 2 h to remove the moisture. The prepared tablet was heated from room temperature to 1100°C at a rate of 10°C/min in nitrogen atmosphere and then kept at that temperature for 60 minutes. At the end of the experiment, the samples were cooled to room temperature with the furnace under nitrogen protection, and then tested and analyzed.

Fig. 1.

Schematic diagram of experimental equipment for the carbon-thermic reduction process. (Online version in color.)

The phosphorus content in roasting products was determined by chemical analysis. The specific surface area and pore structure of samples under different conditions were measured by automatic surface area and porosity analyzer. The phase composition was determined by X-ray diffractometer (XRD). The element composition distribution of roasted samples was detected by scanning electron microscope and energy dispersive spectroscopy (SEM-EDS).

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.¹⁶⁾   

$$\eta =\left(1-\frac{P_1\times M_1}{P_0\times M_0}\right)\times 100\%$$(1)

Where η represents the dephosphorization rate. P₀ and P₁ represent the content of phosphorus in the tablet before and after reduction, respectively. M₀ and M₁ represent the mass of tablets before and after reduction, respectively.

3. Results and Discussion

3.1. Effect of Fuel Type on Dephosphorization

Keeping the fixed carbon content in the mixture unchanged, different kinds of biochar (fruit shell charcoal, straw charcoal, bamboo charcoal) were used instead of 40% coke breeze for reduction experiment. The effect of different biochar on the gasification dephosphorization rate in the reduction process is shown in Fig. 2. The dephosphorization rate of full coke powder as reducing agent was 14.28%, and the dephosphorization rate of fruit shell charcoal, straw charcoal, and bamboo charcoal increased to 16.83%, 19.29%, and 25.53%, respectively. Compared with full coke breeze, adding biochar could increase the gasification dephosphorization rate in the reduction process. There were differences in chemical composition and physical properties between biochar and coke breeze. The subsititution coke breeze for biochar would change the reduction behavior of apatite during sintering, and the large specific surface area of biomass determined that it had good reactivity, thus accelerating the reduction of apatite. The specific surface area of biochar was sorted as bamboo charcoal, straw charcoal and fruit shell charcoal.

Fig. 2.

The effect of fuel types on gasification dephosphorization rate. (Online version in color.)

To explore the effect of structural properties of samples with different fuel proportions on the gasification removal of phosphorus in the reduction process, the specific surface area and pore structure of different samples were analyzed. The results are shown in Table 4. This was mainly due to the selective reduction between phosphorus and iron in the carbothermic reduction process. Compared with the reduction temperature of apatite, the reduction temperature of iron oxide was lower, and the phosphorus gas volatilized in the reduction process was easy to combine with iron to generate Fe_xP, which inhibits the gasification removal of phosphorus.¹⁶⁾ The larger pore diameter and higher porosity in the reduced sample were beneficial to the volatilization of phosphorus gas into the flue gas, thus increasing the dephosphorization rate. Compared with full coke breeze, the sample with biochar had a larger pore size and specific surface area after reduction, the distribution of internal voids was numerous and uniform. and the greatest effect was obtained in the case of the addition of bamboo charcoal. This was because the biochar had high volatile matter, with the increase of temperature in the reduction process, the organic matter in the sample was decomposed and escaped by thermal decomposition, leading to the increase of pores in the sample. The content of volatile matter in carbon was ranked as bamboo charcoal, fruit shell charcoal and straw charcoal. Compared with straw charcoal, the reduced sample with fruit shell charcoal had larger pore diameter and specific surface area, but its dephosphorization rate was lower. This was mainly because the ash of fruit shell charcoal was alkaline and the ash of straw charcoal was acidic. The previous experimental results showed that the increase of SiO₂ content was beneficial to increase iron and decrease phosphorus, so the dephosphorization rate of straw charcoal was higher than that of shell charcoal. In addition, biochar had good reactivity and was easy to react with CO₂ rapidly, resulting in a large number of pores on the surface of biochar.²¹⁾ The increase of porosity and pore size of the sample was beneficial to the rapid volatilization of phosphorus gas, thus the gasification dephosphorization rate was improved. The larger pore size of the reduction sample with biochar was easy for phosphorus gas to volatilize into the flue gas, thus increasing the dephosphorization rate.

Table 4. Specific surface area and pore structure of samples with different fuel type after reduction.

Samples	BET surface area (m2/g)	Average pore diameter (nm)	Average pore volume (cm3/g)
Coke breeze	0.1425	0.000226	31.4614
Fruit shell charcoal	0.2377	0.000456	12.4933
Straw charcoal	0.1763	0.000276	16.1315
Bamboo Charcoal	0.4488	0.000820	14.4009

To better analyze the phase transition of phosphorus in the reduction process with different fuel proportions, the reduced samples were analyzed by XRD, and the results are shown in Fig. 3. In all samples, the diffraction peak of metallic iron could be detected and with high intensity, meaning that the iron oxides were reduced to generate a large amount of metallic iron. In addition, the existence of FeO peak was also detected, which indicated that the iron oxide was not completely reduced at this time. Compared with full coke breeze, after adding biochar, the reduced sample had a stronger metallic iron diffraction peak and weaker FeO diffraction peak. This was mainly because the large specific surface area of biochar improved the contact area with iron oxide, accelerating the reduction reaction. Bamboo charcoal had the highest specific surface area, correspondingly the diffraction peak of reduced metallic iron was the strongest. Fayalite and spinel were detected in the reduction products of all samples, which was mainly due to the step-by-step reduction of iron oxide. Part of the FeO generated in the reduction process reacted with SiO₂ and Al₂O₃ to form fayalite and spinel which were hard to reduce, resulting in the Fe²⁺ retaining in the gangue phase. Accordingly, the further reduction of iron oxides was hindered, which was beneficial to the gasification and removal of phosphorus due to the less combination of phosphorus and iron. Metallic iron reacted with carbon to form Fe₃C and the carburization of iron led to the increase of molten iron, which provided favorable conditions for phosphorus entering the metallic iron.²²⁾ In addition, the existence of Fe₂P diffraction peak was detected, which indicated that part of phosphorus gas generated by apatite reduction entered the metallic iron, thus inhibiting the gasification and removal of phosphorus.

Fig. 3.

The phase composition of reduced samples from XRD analysis (A-Fe, B-Fe₂SiO₄, C-CaSiO₃, D-Fe₃C, E-Ca₂SiO₄, F-Na₂CaSi₅O₁₂, H-NaAlSiO₄, M-FeO, O-Fe₂P, X-CaMgSiO₄). (Online version in color.)

The diffraction peak of Na₂CaSi₅O₁₂ was detected in the products of different samples. In the case of only carbon and apatite, the high start reduction temperature of apatite made the reduction reaction difficult to occur. When adding SiO₂ and Na₂CO₃, the reaction temperature of apatite decreased, which promoted the reduction reaction to generate phosphorus-containing gas.¹⁶⁾ In addition, CaO, SiO₂, Al₂O₃, and Na₂CO₃ interacted to form Na₂CaSi₅O₁₂ and NaAlSiO₄ phases with a low melting point. The formation of low melting point compounds improved the fluidity of the system and destroyed the inclusion structure of gangue relative to apatite in the ore, therefore the reduction reaction of apatite proceeded better. However, excessive Na₂CO₃ would further reacted with apatite to form CaNaPO₄, hardly to be reduced. In addition, the research showed that alkali metals could cause lattice deformation of iron oxide, and lattice transformation was beneficial to the formation of porous iron, promoting phosphorus to enter the iron phase and thus inhibiting the gasification and removal of phosphorus.²³⁾

The mineral reaction process was preliminarily understood by XRD, but the migration law of phosphorus in the reduction process was not clearly explained. Therefore, the SEM-EDS method was used to explain the mineral evolution of high phosphate iron ore and the process of phosphorus entering iron through the analysis of microscopic mineral change law. The results are shown in Fig. 4. Seeing from the element area distribution of phosphorus, iron and calcium, there was no obvious overlap between P and Ca in the reduced products, and most of the phosphorus was uniformly distributed in the metallic iron, indicating that part of the reduced phosphorus gas was absorbed by liquid iron and aggregated in the iron phase, which was consistent with the above XRD analysis results. In addition, the overlapping distribution of iron and calcium was not observed, indicating that the phosphorus in the product did not diffuse into the metallic iron in the form of Ca₃(PO₄)₂, but migrated into the metallic iron in the form of phosphorus gas. In addition, due to the better reducibility of biomass charcoal, the overlapping distribution of phosphorus and iron was more aggregated, and the reduction of bamboo charcoal was the most obvious.

Fig. 4.

Representative SEM-EDS analysis of reduced samples: (a) Coke (b) Fruit shell charcoal (c) Straw charcoal (d) Bamboo Charcoal. (Online version in color.)

3.2. Effect of Bamboo Charcoal Proportion on Dephosphorization

The research results of the influence of different fuels on dephosphorization showed that adding bamboo charcoal had the highest gasification removal rate. Therefore, according to the principle of constant carbon content in the mixture, the influence of the proportion of bamboo charcoal on phosphorus removal in the reduction process was studied. The reduction experiment was carried out by mixing different quantity of bamboo charcoal (0 mass%, 20 mass%, 40 mass%, 60 mass%, 80 mass%) with reagents. The gasification dephosphorization rate at different bamboo charcoal addition proportions is shown in Fig. 5. The results showed that with the increase of bamboo charcoal, the dephosphorization firstly increased and then decreased, and the dephosphorization rate reached the maximum of 28.77% when 20% bamboo charcoal was added. Compared with coke breeze, bamboo charcoal had good reactivity, so adding appropriate amount of bamboo charcoal was beneficial to improve the dephosphorization rate. Thermodynamically, the reduction temperature of iron oxide was lower than that of apatite. With the addition of appropriate amount of bamboo charcoal, its good reactivity was conducive to the improvement of the reduction rate, and the increase of sample pores was conducive to the external diffusion of phosphorus gas. However, with the addition of excessive bamboo charcoal, the reductant was more involved in the reduction of iron oxide, and the reduction of iron oxide is relatively complete. The residual reducing agent promotes the reduction of apatite in the ore, and there was a large amount of metal iron before the formation of phosphorus gas. Because the phosphorus gas was easy to combine with iron, the gasification dephosphorization rate was reduced.

Fig. 5.

The effect of bamboo charcoal proportion on gasification dephosphorization rate. (Online version in color.)

To explore the effect of structural properties of samples with different bamboo charcoal proportions on the gasification removal of phosphorus in the reduction process, the specific surface area and pore structure of different samples were analyzed. The results are listed in Table 5. The dephosphorization rate was restricted by reduction reaction rate and phosphorus gas diffusion. Compared with coke breeze, bamboo charcoal had good reactivity and was easy to react with calcium phosphate quickly. The results showed that the specific surface area and pore size of the sample increased with the increase of bamboo charcoal proportion. The bamboo charcoal had higher volatile than coke breeze, improving the proportion of bamboo charcoal would bring a large amount of organic matter decomposed and escaped when they were heated meanwhile the charcoal reacted with CO₂ at a specific temperature, as a result a large number of pores formed on the surface of biomass. The increase of porosity and pore size promoted the increase of the reduction reaction rate. At the same time, a large number of holes and channels made the phosphorus gas generated by reduction enter the sintering flue gas. The comprehensive effect of these two factors made bamboo charcoal beneficial to the gasification and removal of phosphorus when it replaces part of coke breeze.

Table 5. Specific surface area and pore structure of samples with different bamboo charcoal proportion after reduction.

Samples/%	BET surface area (m2/g)	Average pore diameter (nm)	Average pore volume (cm3/g)
0	0.1425	0.000226	31.4614
40	0.4488	0.000820	14.4009
80	2.3891	0.004004	12.0659

The phase changes of the reduced samples were analyzed by XRD, and the results are shown in Fig. 6. The results showed that with improving the proportion of bamboo charcoal, the intensity of Fe diffraction peak increased and the intensity of FeO peak decreased. At a small proportion of bamboo charcoal, the less contact area between reactants led to a slow reduction rate. The improvement of the bamboo charcoal proportion enhanced the contact between bamboo charcoal and iron ore due to its large specific surface area, thus promoting the reduction of iron oxide and apatite. In thermodynamics, the reduction of iron oxides was prior to the reduction of phosphorus-containing minerals, therefore a large amount of metallic iron existed before the formation of phosphorus gas. In addition, the more sufficient contact between iron and carbon promoted the carburization, resulting in an increase in the amount of liquid iron at high temperature, further inhibiting the gasification and removal of phosphorus.²⁴⁾ Moreover, the increase of bamboo charcoal proportion would make the fayalite and spinel to be reduced more quickly, which increased the amount of SiO₂ involved in the apatite reduction reaction, and promoted the apatite reduction. At the same time, the increase of the amount of metallic iron provided more contact between phosphorus and iron, accelerating the migration of phosphorus to metallic iron, and reducing the gasification dephosphorization rate.

Fig. 6.

The phase composition of reduced samples from XRD analysis (A-Fe, B-Fe₂SiO₄, C-CaSiO₃, D-Fe₃C, E-Ca₂SiO₄, F-Na₂CaSi₅O₁₂, H-NaAlSiO₄, M-FeO, O-Fe₂P, X-CaMgSiO₄). (Online version in color.)

The migration process of phosphorus in the reduction process was analyzed by SEM-EDS, and the results are shown in Fig. 7. The overlapping distribution of phosphorus and iron in the reduction products of different samples, and there was no obvious overlap area between phosphorus and calcium, which indicated that part of the reduced phosphorus gas was absorbed by iron. With the increase of the proportion of bamboo charcoal, the distribution of phosphorus became more aggregated, which was mainly because the carburizing reaction was enhanced due to the more contact between iron and carbon. Because Fe₂P and Fe₃C had a similar crystal structure, the phosphorus gas generated during reduction entered the Fe₃C lattice to form Fe₂P, consequently the phosphorus content in the iron phase was improved as a result of the increase of carbon content.¹⁶⁾

Fig. 7.

Representative SEM-EDS analysis of reduced samples: (a) 20% (b) 40% (c) 60%. (Online version in color.)

3.3. Effect of Basicity on Dephosphorization

The bamboo charcoal proportion was fixed at 20%, and the reduction experiment was carried out by adding CaO to adjust the basicity of samples at 0.5, 1.0, 1.5, 2.0 and 2.5. The effect of basicity on the dephosphorization rate is shown in Fig. 8. The results showed that with the increase of basicity, the dephosphorization rate decreased from 28.77% to 14.28%.

Fig. 8.

The effect of basicity on gasification dephosphorization rate. (Online version in color.)

The specific surface area and pore structure of different samples were analyzed as listed in Table 6. The results showed that with the increase of basicity, the specific surface area and pore size of the sample increased, which could be attributed to the difficulty of aggregation of iron and other products due to the increased slag content and high melting point substances in the reduction products. In the process of sintering, with the downward movement of the high-temperature layer, the apatite in the burden layer was reduced, and the resulting phosphorus gas was concentrated in the sintering suction and discharged into the flue gas. With the increase of basicity, the increase of porosity and pore diameter was beneficial to the gasification and discharge of phosphorus gas. However, thermodynamics shows that the increase of SiO₂ content was beneficial to reduce the reduction temperature of apatite, and the increase of alkalinity was not conducive to the reduction of apatite.

Table 6. Specific surface area and pore structure of samples with different basicity after reduction.

Samples	BET surface area (m2/g)	Average pore diameter (nm)	Average pore volume (cm3/g)
R = 0.5	0.2841	0.000525	17.8126
R = 1.5	3.1804	0.004986	10.2717
R = 2.5	4.5287	0.008225	7.2649

The phase changes of the reduced samples with different basicity were analyzed by XRD, as shown in Fig. 9. The results showed that with the increase of basicity, the diffraction peak intensity of Fe decreased and the diffraction peak of Fe₃C increased. Meanwhile the diffraction peaks of CaSiO₃, Ca₂SiO₄, and other substances with a high melting point in the slag phase increased either. SiO₂ could reduce the reduction temperature of apatite and promoted the gasification removal of phosphorus in the reduction process. With the increase of basicity, SiO₂ would give priority to react with CaO, thus inhibiting the reduction of apatite. The reduction of apatite was more difficult than iron oxide, so most of the carbon consumed in the reduction of iron oxide, and the residual carbon entered metallic iron, enhancing the diffraction peak of Fe₃C. In addition, although the formation of high melting point compounds increased the porosity of the products, it worsened the kinetic conditions of the reduction, thus inhibiting the reduction reaction of the apatite and reducing the dephosphorization rate.

Fig. 9.

The phase composition of reduced samples from XRD analysis (A-Fe, B-Fe₂SiO₄, C-CaSiO₃, D-Fe₃C, E-Ca₂SiO₄, F-Na₂CaSi₅O₁₂, H-NaAlSiO₄, M-FeO, O-Fe₂P, X-CaMgSiO₄). (Online version in color.)

The migration process of phosphorus in the reduction process was further analyzed by SEM-EDS, and the results are shown in Fig. 10. The results showed that, the metallic iron and phosphorus aggregated in the reduction products when the basicity was low, most of the phosphorus and iron presented an overlapped distribution, and the phosphorus had diffused into the interior of the metallic iron. Improving the basicity, the slag phase in the product increased, and the metallic iron was widely distributed, companied by the phosphorus element widely distributing in the metallic iron and slag phase, which indicated that the phosphorus element had not been completely reduced at this time. This validated the results of the XRD analysis, indicating that the increase of basicity inhibited the reduction of apatite.

Fig. 10.

Representative SEM-EDS analysis of reduced samples: (a) R = 0.5 (b) R = 1.5 (c) R = 2.5. (Online version in color.)

To sum up, based on the characteristics of sintering based on biomass, there was a problem of selective reduction of iron and phosphorus in the reduction process. The reduction of phosphorus and iron could be realized by adjusting the fuel type, proportion, and basicity of the mixture. Part of the reduced phosphorus gas was volatilized into the flue gas and discharged with it, and part reduced phosphorus entered the metallic iron to form Fe_XP. Therefore, the gasification of phosphorus was realized by adjusting the sintering process parameters, which was of great significance to reduce the dephosphorization pressure in the steelmaking process and reduce the amount of high phosphorus steel slag.

4. Practical Application Design of Dephosphorization in the Biomass Sintering Reduction Process

Based on the study of the gasification behavior of phosphorus in the sintering process of high-phosphate iron ore, a new method for the phosphorus gasification and recovery in sintering process was proposed. The technological process is shown in Fig. 11. In the sinter process, add the appropriate amount of carbonaceous reducing agent and dephosphorization agent. After mixing and pelletizing sufficiently, uniformly distribute them onto the sintering trolley for ignition and sintering. By controlling the proper basicity of the mixture, strengthening the reducing atmosphere, and decreasing the oxygen content of the sintered layer, iron oxide and apatite would be reduced quickly in the sintering process. The phosphorous minerals would be reduced to gaseous phosphorus above 1100°C. The phosphorus gas downward movement under the action of sintering negative pressure. The phosphorus gas in the flue was recovered by stages to produce yellow phosphorus. At present, a preliminary reduction experiment had been carried out in the laboratory and preliminarily proved that it was feasible to gasification and recovery of phosphorus based on the sintering process.

Fig. 11.

Schematic diagram of gasification dephosphorization and recovery system for biomass sintering. (Online version in color.)

In the traditional sintering process, there was a strong oxidizing atmosphere and a weak reducing atmosphere, and part of the P₂ gas produced during the sintering process was oxidized to form P₂O₅, which would inhibit the reduction and collection of phosphorus. At present, we developed the cloud explosion sintering technology in cooperation with a domestic technology company and have conducted preliminary industrial experiments. The cloud explosion sintering technology was to inject cloud explosion agent (gas-liquid mixture) into the ignited sinter layer, and a certain amount of combustion-supporting air was ignited in the upper part of the sinter surface, which not only triggered the secondary combustion of the layer surface, but also produced certain amount of reducing gases (CO and H₂) in the sintered layer, as a result the oxygen content in the sintered layer was reduced from 21% to about 5.0%, and the sintering time was significantly prolonged. Based on the cloud explosion sintering technology, the carbon content in the sintered layer was properly increased to further enhance the reducing atmosphere, and the iron oxide and apatite in the mixture were quickly reduced.

The sinter layer of cloud explosion sintering technology was in a low oxygen and strong reduction atmosphere. Under the strong reducing atmospheres, iron oxide was reduced systematically. Then, the metal phase aggregated and grew during the mineral reaction process. Finally, only the metal and slag phases with uniform texture existed in the sinter, and the boundary between them was obvious. At the same time, Fe₃C is formed due to the Carburizing reaction of metallic iron. With the progress of the reaction, apatite was reduced to P₂ gas, as shown in Eqs. (2), (3), (4), (5). In the process of cloud explosion sintering, the heat storage capacity of the sintering bed was enhanced, and the extension of the high temperature section was beneficial to the occurrence of apatite reduction reaction, at the same time, the increase of the amount of macroporous sinter formed by biomass reduction in the mixture improves the air permeability of the sintering bed and was beneficial to the removal of phosphorus gas. However, the reduction temperature of iron oxide was lower than that of apatite, and a large amount of metallic iron was produced before apatite was reduced to P₂. Therefore, P₂ gas was partially absorbed by metallic iron to form iron–phosphorus compounds (Fe₂P), and the diffusion of phosphorus gas was the limiting link of its volatilization process.²⁵⁾   

$$\begin{array}{r}\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2+6\mathrm{C}+6\mathrm{S}\mathrm{i}{\mathrm{O}}_2+\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\\ =\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_6{\mathrm{O}}_{16}+{\mathrm{P}}_2+7\mathrm{C}\mathrm{O}\end{array}$$(2)

  

$$\begin{array}{r}\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2+11\mathrm{C}+9\mathrm{S}\mathrm{i}{\mathrm{O}}_2+6\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\\ =3\mathrm{N}{\mathrm{a}}_4\mathrm{C}{\mathrm{a}}_3\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_9+{\mathrm{P}}_2+17\mathrm{C}\mathrm{O}\end{array}$$(3)

  

$$\begin{array}{r}\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2+13\mathrm{C}+9\mathrm{S}\mathrm{i}{\mathrm{O}}_2+3\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\\ =3\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{S}{\mathrm{i}}_3{\mathrm{O}}_9+2{\mathrm{P}}_2+16\mathrm{C}\mathrm{O}\end{array}$$(4)

  

$$\begin{array}{r}\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{P}{\mathrm{O}}_4\right)}_2+8\mathrm{C}+15\mathrm{S}\mathrm{i}{\mathrm{O}}_2+3\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\\ =3\mathrm{N}{\mathrm{a}}_2\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{i}}_5{\mathrm{O}}_{12}+{\mathrm{P}}_2+11\mathrm{C}\mathrm{O}\end{array}$$(5)

  

$$4\mathrm{F}\mathrm{e}+{\mathrm{P}}_2\left(\mathrm{g}\right)=2\mathrm{F}{\mathrm{e}}_2\mathrm{P}$$(6)

5. Conclusions

In this study, the pure reagents were taken to simulate the high phosphate iron ore, through the laboratory experiments the reduction behavior of apatite during the reduction process was studied by adjusting the type and proportion of biomass fuel, and the basicity of mixture. The migration law of phosphorus in the reduction process was analyzed by XRD and SEM-EDS. The following conclusions were drawn.

(1) Compared with full coke breeze sintering, the addition of biochar could promote the reduction of phosphorus in the reduction process, and the addition of bamboo charcoal significantly improved the reduction of apatite.

(2) When the reduction temperature was 1100°C and the basicity was 0.5, the gasification dephosphorization firstly increased and then decreased with the addition of bamboo charcoal and the maximum dephosphorization rate was 28.77% when 20% was added. The reduced phosphorus gas was easy to enter the iron phase to generate Fe₂P.

(3) When the reduction temperature was 1100°C and the proportion of bamboo charcoal was 20%, with the basicity of the sample increasing from 0.5 to 2.5, change of slag phase and liquid phase in reduction products, thus inhibiting the reduction of apatite.

(4) The cloud explosion sintering technology was put forward, it could help to adjust the sintering process parameters to enhance the reducing atmosphere and reduce the oxygen content of the sintered layer. Using this method accelerated the reduction of phosphorus.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (U1960205) and Qinhuangdao Xinte Technology Co., Ltd.

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