Gasification and Migration of Phosphorus from High-phosphorus Iron Ore during Carbothermal Reduction

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1 ISIJ International, Advance Publication by J-STAGE ISIJ International, ISIJ International, J-Stage Advanced Advance ISIJ Publication International, Publication, ISIJ International, by DOI: J-STAGE, Advance Vol. Publication 58 DOI: (2018), /iijinternational.ISIJINT No. by International, J-Stage 12 Vol. 58 (2018), No. 12, pp. 1 9 Gaification and Migration of Phophoru from High-phophoru Iron Ore during Carbothermal Reduction Yuanyuan ZHANG, Qingguo XUE, Guang WANG and Jingong WANG* State Key Laboratory of Advanced Metallurgy, Univerity of Science and Technology Beijing, Beijing, China. (Received on March 23, 2018; accepted on July 26, 2018; J-STAGE Advance publihed date: October 4, 2018) The effect of different gangue oxide(al 2 O 3, SiO 2 and Fe 2 O 3 ) on the gaification and migration of phophoru during the carbothermal reduction of fluorapatite ha been invetigated. The vaporization of phophoru during the carbothermal reduction of yntheized model ample demontrating a high-phophoru iron ore wa analyzed by ga ma pectrometry. Reult revealed that the dephophorization of fluorapatite wa promoted by Al 2 O 3 and SiO 2 to form CaAl 2 O 4 and CaSiO 3, repectively. The promotion effect of SiO 2 wa larger than that of Al 2 O 3. With the increae in the addition of gangue, the thermodynamic condition for the reduction of fluorapatite were continuouly optimized, thereby accelerating the dephophorization of fluorapatite. At a C/O (O originated from fluorapatite and Fe 2 O 3 ) of larger than 1 in molar ratio, P 2 wa the the dephophorization product. Wherea, at a C/O of le than 1, the dephophorization product turn to PO. With the addition of Fe 2 O 3 to fluorapatite, a large amount of phophoru wa aborbed by liquid iron, reulting in a decreae of the amount of volatilized P 2, leading to the low increae or decreae in the dephophorization ratio of the pellet. Phophoru wa aborbed by liquid iron a P 2, wherea PO ga wa completely volatilized. Gaification dephophorization mainly occured from 10 min to 25 min at C. Thee finding lead to a new idea on the dephophorization of a high -phophorou iron ore, that i, decreaing the reduction temperature to retard melting of iron and imultaneouly adding the additive to promote the dephophorization of fluorapatite. KEY WORDS: high-phophoru iron ore; fluorapatite; dephophorization; reaction mechanim. 1. Introduction High-phophoru oolitic hematite i a potential iron ore reource. China and Europe have 4 5 billion ton and more than 14 billion ton of iron reerve, repectively. 1 4) Correponding reource have alo been reported in the United State and Egypt. 5) On account of the high content of phophoru in the iron ore, which adverely affect the quality of pig iron and teel, phophoru and iron combine to form Fe 3 P, thereby forming a binary eutectic Fe 3 P Fe with iron and cauing teel to exhibit a cold and brittle phenomenon; thi phenomenon limit the ue of the Highphophoru iron ore in indutry. 6 9) The development of thi mine contitute coniderable economic importance. The high-phophoru oolitic hematite ore i embedded with extremely fine grain, with an uneven ditribution and a high content of harmful impuritie. Iron oxide are intercalated with gangue mineral, which caue iron and phophoru to be difficult to eparate. High-phophoru iron ore i a worldrecognized complex refractory iron ore. 10) On account of the above-mentioned difficultie, in recent year, experiment uing the high-phophoru iron ore have been carried out to improve the iron grade while conidering dephophoriza- * Correponding author: wangjingong@utb.edu.cn DOI: tion. Dometic and international reeracher have carried out everal experiment, among which phyical mineral eparation, leaching, magnetization roating, and magnetic eparation contitute the main proceing method ) Direct reduction i hort and imple, and the metallization rate can be controlled, with low requirement for the iron ore and reducing agent. Thi method can be employed a efficient pretreatment for the treatment of the highphophoru iron ore ) Several tudie have reported the mechanim for the carbothermal reduction of iron oxide. Fluorapatite i the major contituting phophoru-bearing mineral in a high-phophorou iron ore. 20) In contrat, on account of the difficult preparation of fluorapatite and the incomplete thermodynamic data, few tudie have reported the mechanim for the carbothermal reduction of fluorapatite. Only the mechanim of the carbothermal reduction of iron oxide and fluorapatite in the high-phophoru iron ore can be idefined to achieve the purpoe of improving iron grade and dephophorization. In thi tudy, according to the mineral compoition characteritic of the high-phophoru iron ore, the evolution of fluorapatite during the direct reduction of gangue wa examined. The rule for the migration of phophoru during direct reduction wa clarified, which provided the theoretical bai for the eparation of iron and phophoru from the high-phophoru iron ore. The law for the vaporization of 1

2 phophide during carbothermal reduction were invetigated a well. To permit gaification dephophorization, a concept of a new method of utilizing the high-phophoru iron ore wa propoed. 2. Experimental 2.1. Experimental Material In thi experiment, the effect of gangue on the evolution of phophoru-containing mineral in the high-phophoru iron ore during carbothermal reduction wa predominantly invetigated. Table 1 21) ummarize the chemical compoition. On account of the complex compoition and tructure of the high-phophoru iron ore, in thi tudy, pure material were ued to imulate the compoition of the high-phophoru iron ore, for highlighting the evolution of phophorou-containing mineral. Experimental material included analytical pure Fe 2 O 3, SiO 2, Al 2 O 3, and homemade high-purity fluorapatite (Ca 10 (PO 4 ) 6 F 2 ). Fluorapatite wa prepared by olid phae reaction of Ca 3 (PO 4 ) 2 with CaF 2. 22) Homemade fluorapatite wa detected by XRD and Raman analyi. The reult are hown in the Fig ) The peak poition of the homemade fluorapatite and the tandard ample correponded well, indicating that the fluorapatite ha a complete lattice and a very high purity. High-purity graphite (w% >99.9%) wa ued a the reducing agent. To decreae the effect of factor, a le than 1% ma fraction of the material (i.e., CaO and MgO) wa ignored. Ingredient were baed on the compoition of the equilibrium reaction and the raw material ratio cheme (Table 2). The ratio of each ubtance in Exp. 1 5 wa determined according to the ma ratio of each ubtance in the equilibrium equation of the reaction. For example, in Exp. 1, when the phophoru in the fluorapatite i all reduced to P 2, the ma ratio of fluorapatite to carbon i 5.5:1. In Exp. 3, the Ca 10 (PO 4 ) 6 F 2 i all reacted to form CaSiO 3 and SiF 4, the ma ratio of SiO 2 i 3.5. In Exp. 4, when the phophoru in the fluorapatite i all converted into P 2 and Fe 3 P, the ma ratio of Fe 2 O 3 i Exp. 6 and 7 were conducted to undertand the effect of carbon content on phophorou gaification. When Fe 2 O 3 and Ca 10 (PO 4 ) 6 F 2 are completely reduced, the molar ratio C/O hould be 0.7 (O from Fe 2 O 3 and Ca 10 (PO 4 ) 6 F 2 ), conidering that the reduced metallic iron i carburized, the molar ratio C/O hould be larger than 0.7 (O originated from Fe 2 O 3 and Ca 10 (PO 4 ) 6 F 2 ). So the content of carbon in Exp. 6 wa elected a the molar ratio C/O = 1 (O originated from Fe 2 O 3 and Ca 10 (PO 4 ) 6 F 2 ). In order to undertand the gaification of phophoru when carbon i inufficient, the carbon content of Exp. 7 wa elected a the molar ratio C/ O=0.4 (O originated from Fe 2 O 3 and Ca 10 (PO 4 ) 6 F 2 ). Table 1. Chemical compoition of the high-phouphoru iron ore (ma%). Compoition Fe 2O 3 Ca 10(PO 4) 6F 2 SiO 2 Al 2O 3 CaO MgO Chemical compoition of raw ore Approximate ratio Fig. 1. Homemade fluorapatite compared with the tandard material ((a) XRD pattern, (b) Raman pectra). (Online verion in color.) Experiment number Table 2. Raw material mixing ratio of fluorapatite during carbothermal reduction experiment (ma ratio and ma%). Ca 10(PO 4) 6F 2 Al 2O 3 SiO 2 Fe 2O 3 C Ca 10(PO 4) 6F 2 Ca P F O Al 2O 3 SiO 2 Fe 2O 3 C C/O ma ratio ma% ma% ma% molar ratio

3 2.2. Experimental Method According to the mixing ratio hown in Table 2, raw material were uniformly mixed, and 50 mg of the mixture wa preed into cylindrical pellet and ubequently dried for ue in experiment. The quadrupole ma pectrometry (QMS) wa ued to detect the ga pecie during the reduction proce. When the furnace temperature wa contant at C, Ar ga wa introduced at a rate of 100 ml/min. After five minute, the ample wa placed in a contant temperature zone of the quartz reactor, and the reactor wa rapidly connected to the quadrupole ma pectrometer. The vacuum condition of the QMS ytem were maintained at torr. The ga pecie were determined by detecting the ionic mae, and Table 3 how the molecular ion and their mae ionic analyzed in thi experiment. Figure 2 how the chematic of the experimental apparatu. SEM-EDS wa employed to invetigate the change occurring during the reduction of fluorapatite for the purpoe of determining the type of product obtained. Thermogravimetric analyi (TGA) and chemical analyi were employed to determine whether dephophorization occur, and the dephophorization ratio wa calculated by the following formula. To better undertand the evolution of phophoru-containing mineral in the high-phophoru iron ore during direct reduction, the mechanim for the reduction and gaification of fluorapatite wa dicued. P30min m30min 1 P0min m0min In the formula, η repreent the dephophorization rate. P 0min and P 30min repreent the content of P in the pellet before and after the reaction, repectively. m 0min and m 30min repreent the ma of the pellet before and after the reaction, repectively. Table 3. Detected the molecular ion and their mae. 31 P 47 PO 62 P P P 2O P 4O PO 2 84 AlF 3 78 P 2O 104 SiF 4 3. Reult and Dicuion 3.1. Macrocopic Analyi for the Carbothermal Reduction of Fluorapatite and Gangue Figure 3 how the thermogravimetric analyi reult obtained for the carbothermal reduction of fluorapatite and gangue. Firt, fluorapatite and graphite were mixed and calcined at C (Experiment 1): The ample barely lot weight. However, ample exhibited different degre of weight lo after the mixing and calcination of fluorapatite and gangue. Without iron oxide, gentle weight lo curve attaining 5 12% weight lo at 30 min were oberved (Experiment 2, 3). With the addition of Fe 2 O 3, the ample rapidly lot weight in the firt 10 min, and the weight lo curve gradually became flat, indicating a more rapid reduction of iron oxide compared to fluorapatite (Experiment 4). With the addition of only Fe 2 O 3, the reduction of iron oxide followed a Fe 2 O 3 Fe 3 O 4 FeO Fe, wherea with the addition of SiO 2 and Al 2 O 3 (Experiment 5), the iron oxide reduction pathway wa changed. Some FeO motly reacted with gangue mineral toform hard-to-reduce intermediate, Fe 2 SiO 4 and FeAl 2 O 4, 21) leading to lower weight lo. To determine the relationhip between weight lo and dephophorization, the dephophorization of different ample wa ummarized (Fig. 4). With the addition of only carbon, the content of phophoru in the pellet wa almot the ame a the theoretical phophoru content, indicating that the fluorapatite i not reduced. With the addition of gangue, fluorapatite wa dephophorylated to varying degree. With the addition of Al 2 O 3 or Fe 2 O 3, the dephophorization ratio of the ample reached 20%, and the dephophorization ratio of SiO 2 in the fluorapatite-containing carbon pellet reached 30%. SiO 2 exhibited the tronget effect on the dephophorization of fluorapatite. With the imultaneou addition of SiO 2, Al 2 O 3, and Fe 2 O 3, the dephophorization ratio did not correpond to thoe of the three gangue becaue during carbothermal reduction, a large amount of intermediate product were generated by three gangue. Comparing the reult obtained from Experiment 6 and 7, the dephophorization ratio decreaed with increaing carbon content (7.1 to 16.7 ma%) becaue reduced iron underwent carburization to increae the amount of liquid iron, and a large amount of Fig. 2. Schematic of the experimental apparatu. (Online verion in color.) Fig. 3. TGA curve of the reaction between fluorapatite and gangue with the addition of carbon. (Online verion in color.) 3

4 Fig. 4. Standardized phophorou content and dephophorization ratio of carbon-bearing pellet. (Online verion in color.) Fig. 5. SEM-EDS analyi of the reduced pellet (a-experiment 2, b-experiment 3, c-experiment 4, d-experiment 5). (Online verion in color.) phophoru wa aborbed. From Fig. 3 and 4, with the imultaneou addition of three gangue oxide, the ample underwent a violent reaction, but the dephophorization ratio of carbon-containing pellet wa low poibly becaue of the following reaon. Firt, reduced phophoru wa aborbed by the iron phae; Second, fluorapatite wa reduced and decompoed, generating intermediate phophorou product that were retained in the pellet, leading to a low dephophorization ratio Effect of Gangue Oxide on the Evolution of Phophoru-containing Mineral To reaonably explain the above experimental phenomena, SEM-EDS analyi wa employed to microcopic ally analyze the carbothermal reduction of fluorapatite with each of the gangue oxide. Figure 5 how the microanalyi of the reduced pellet under different condition, and Table 4 ummarize the reult obtained from EDS point canning. Ca wa eparated from P and ditributed with Al in overlapped (1, 4), indicating that fluorapatite wa decompoed and reduced to form a new gangue phae, and EDS analyi revealed that the new gangue product wa determined to be CaAl 2 O 4 (Fig. 5(a)). However, P and F were not detected from the product obtained by the decompoition and reduction of fluorapatite, uggeting that P and F were volatilized a ga. Some new gangue wa formed in the contact part between fluorapatite and SiO 2 (Fig. 5(b)). The reaction wa a typical oli-olid reaction, and SiO 2 wa gradually conumed. Product were wrapped around the SiO 2 urface to hinder the progre of the reaction, leading to the incomplete reaction of fluorapatite. Reult of EDS analyi revealed that 4

5 Table 4. Quantitative EDS Analyi of Element in Reduced Pellet (number are correponding to number in Fig. 5). Ele Ca (at.%) Al (at.%) Si (at.%) O (at.%) CaSiO 3 i generated by the reaction. Similarly, phophoru wa not obtained a a reaction product from fluorapatite, indicating that the phophoru produced by the reduction of fluorapatite i releaed a ga. A new gangue phae wa formed at the contact interface between fluorapatite and metallic iron (Fig. 5(c)). EDS quantitative analyi revealed the preence of Ca and O in the new gangue phae, indicating that the dephophorization of fluorapatite lead to the formation of CaO, and a majority of P i volatilized a ga. However, with the progre of the reaction, a part of the phophoru wa aborbed by the iron phae becaue metal iron undergoe carburization to form liquid iron, which exhibit a trong ability to aborb phophoru. P 2 ga wa aborbed to form an Fe P alloy in the iron phae a indicated by the overlapping area of Fe and P hown in Fig. 5(c). CaO wrapped around the unreacted fluorapatite urface hindered the further reaction of fluorapatite. With the imultaneou addition of three gangue oxide, the reult obtained from the carbothermic reduction of fluorapatite i hown in Fig. 5(d). Fluorapatite particle underwent complete reaction. In the gangue phae, blacktriped Ca Si Al and gray Ca Fe Si Al P gangue phae were oberved. Some of the unreduced phophoru and iron to form Fe P O phae exit in the gray phae. 23) EDS analyi revealed that Ca Si Al gangue i determined a CaAl 2 Si 2 O 8. The metling point of the newly generated gangue phae wa low; hence, a liquid phae i formed, which improve the ma tranfer condition of the ytem, and the unreacted graphite particle traction to the fluorapatite urface to increae the contact area of graphite and fluorapatite to facilitate eay reduction Effect of Gangue Oxide on the Gaification of Phophoru During carbothermal reduction, fluorapatite wa reduced, phophoru wa not oberved in the reduction product by EM-EDS analyi, and reduced phophoru wa preumed to be volatilized a ga. To determine the vaporization of phophoru, the ga phae volatile during reduction were detected by quadrupole ma pectrometry. P 2, PO, SiF 4 and AlF 3 were oberved. Figure 6 how the reult. Fluorapatite wa not coniderably reduced in the abence of gangue oxide. With the addition of gangue oxide, fluorapatite wa reduced to varying degree. Reult revealed that a horter time i required to generate P 2, and the formation of P 2 ga from pellet with SiO 2 wa more rapid than that from pellet with Al 2 O 3 and Fe 2 O 3. The promotion of SiO 2 on the gaification and dephophorization of fluorapatite i tronger than that of Al 2 O 3 and Fe 2 O 3. In the preence of Fe 2 O 3 in the ample, the P 2 volatilized during the later period of the Fig. 6. Analyi of ga from the volatile obtained during direct reduction. (Online verion in color.) reaction decreaed poibly becaue the reduced phophoru i aborbed by the pellet, leading to the decreae in the amount of the volatilized P 2 ga. With the coexitence of Al 2 O 3, SiO 2, and Fe 2 O 3, the reaction of fluorapatite wa more evere; the time required for the generation of P 2 wa the hortet; and the increae of P 2 wa more rapid. At thi 5

6 time, fluorapatite wa eaily dephophorized reductively. PO ga wa generated in the later tage of reduction (Fig. 6(b)). With the progre of the reaction, a large amount of the reducing agent wa conumed, leading to the partial reduction of fluorapatite. Some of the phophoru wa preent a oxide, PO. In the preence of Fe 2 O 3, the generated PO curve did not exhibit a decreaing trend a that oberved for the P 2 ga curve. Thi obervation i poibly related to the fact that phophoru in fluorapatite entered into iron a elemental phophoru, and PO wa not aborbed by iron. The generated PO ga wa completely dicharged from the ample, which wa advantageou for dephophorization. The generation of phophoru-containing ga wa accompanied by the formation of a fluorine-containing ga (Fig. 6(c)). The defluorination of fluorapatite wa promoted by Al 2 O 3 and SiO 2, generating more eaily reduced Ca 3 (PO 4 ) 3 and promoting the gaification of phophoru. At a C/O (O originated from fluorapatite and Fe 2 O 3 ) of greater than 1 in molar ratio, fluorapatite wa completely reduced, and P 2 wa the dephophorization product (Fig. 7). At 25 min, the P 2 ga curve tended to decreae. Thi reult could be explained a follow. Metal iron began to melt a a reult of carburization, leading to higher aborption of phophorou due to a higher olubility compared with it to olid iron. At a C/O of le than 1, phophoru wa not completely reduced. PO wa the dephophorization product, and the PO curve did not exhibit a decreaing trend, indicating that liquid iron can only aborb elemental phophoru and cannot aborb phophoru oxide Change in the Dephophorization Ratio and Migration of Phophoru According to the variation in the curve of P 2 in Fig. 7, phophoru wa aborbed by iron after a reduction time of 25 min. To further verify thi reult, the dephophorization of carbon-containing pellet (Experiment 6) wa invetigated by chemical analyi during reduction. Figure 8 how the reult. Dephophorization lowly increaed in the firt 10 min, indicating that only a mall amount of fluorapatite i reduced during thi time, and the reduced P i volatilized a P 2 ga. The dephophorization ratio rapidly increaed from 10 min to 25 min, indicating that the dephophorization of fluorapatite mainly occur during thi period, and a large amount of phophoru-containing ga generated by reduction i volatilized, leading to the increae in the dephophorization ratio of pellet. However, the dephophorization ratio decreaed at 30 min poibly becaue reduced P 2 wa aborbed by the iron phae. To further verify thi reult, the ditribution of P and Fe wa invetigated by SEM. Figure 9 how the ditribution of P and Fe at different time. At 15 min, the metallic iron particle were maller, with a dipere ditribution; a large amount of phophoru wa till retained in fluorapatite; the ditribution of P and Fe did not overlap; and the reduced phophoru-containing ga wa completely volatilized. At 20 min, the overlap area of Ca and P wa extremely mall, indicating that a large amount of fluorapatite i reduced. The ize of the iron particle increaed, P wa gathered on the iron particle urface, and a mall amount of P wa aborbed by iron. At 25 min, P wa diffued into the interior of the iron phae, and the content of phophoru in the iron phae increaed. At 30 min, the content of phophoru in the iron phae increaed further, and a large amount of phophoru wa aborbed by iron. SEM analyi wa conitent with the above experimental reult. Before 20 min, the content of carbon in the metal wa low, the metal wa not melted, and the olubility of phophoru in the olid-phae iron wa particularly mall; hence, phophoru i completely volatilized during thi period. At 20 min, metallic iron wa ubjected to carburization, leading to the appearance of liquid iron. On account of the higher olubility of phophoru in liquid iron, phophoru wa aborbed by the iron phae. With the progre of the reaction, the content of iron gradually increaed promote liquid iron formation; hence, the content of phophoru in the iron phae increae. The mot effective time range wa min. After 25 min, a large amount of reduced P 2 wa aborbed by the iron phae, leading to the increaed content of P in metallic iron Phophoru Gaification Mechanim Phophoru mainly exit a fluorapatite in highphophoru iron ore. In addition, iron oxide and variou gangue (uch a Al 2 O 3, SiO 2, etc.) are alo preent in the high-phophoru iron ore. Thee iron oxide and gangue have an impact on the migration and gaification of pho- Fig. 7. Effect of Carbon Content on Phophoru Gaification Product. (Online verion in color.) Fig. 8. Dephophorization ratio of carbon-containing pellet (with Al 2O 3, SiO 2, and Fe 2O 3). (Online verion in color.) 6

7 Fig. 9. SEM-EDS urface can of the reduced iron phae (A-15min, B-20min, C-25min, D-30min). (Online verion in color.) phoru. 24) Therefore, the carbothermal reduction of fluorapatite, which i extremely complicated, comprie different tage. TGA and QMS reult only indicated whether the fluorapatite i reduced and the pecie of ga-phae product in the preence of different component; however, the reaction mechanim of fluorapatite ha not been etablihed thu far. To clarify the carbothermal reduction of fluorapatite, the thermodynamic of thi reaction wa dicued in detail. (1) Carbothermal reduction reaction of Ca 10 (PO 4 ) 6 F 2. The equation for the poible dephophorization i expreed a follow: Ca10( PO4) 6F2() 15C() CaF2() 15CO(g)... (1) 3P ( g) 9CaO() 2Ca10( PO4) 6F2() 30C() 2CaF2() 30CO(g)... (2) 3P ( g) 18CaO()p 4 2 Fig. 10. Relationhip between ΔG θ and temperature. (Online verion in color.) Ca10( PO4) 6F2() 9C() CaF2() 9CO(g)... (3) 6PO(g) 9CaO() Ca10( PO4) 6F2() 3C() CaF2() 3CO(g)... (4) 6PO ( g) 9CaO() Figure 10 how the relationhip between the Gibb free energy and temperature for the above four equation. Thermodynamic calculation revealed that in the cae of only carbon, fluorapatite i reduced to temperature greater than C. According to the calculation reult, it i eaier for fluorapatite to be reduced to generate P 2 and P 4 than to generate the phophoru-containing oxideat temperature above C. At high temperature without oxygen, P 4 i untable and prone to homologou tranformation into P 2 2 and P 2 wa a mall molecule; hence, it can be eaily volatilized. Therefore, the following thermodynamic calculation wa baed on P 2 a the phophoru gaification product. (2) Carbothermal reduction of Ca 10 (PO 4 ) 6 F 2 with Al 2 O 3 and SiO 2. Al 2 O 3 and SiO 2 in gangue can react with Ca 10 (PO 4 ) 6 F 2 to form CaAl 2 O 4 and CaSiO 3, repectively. The independent reaction between Ca 10 (PO4) 6 F 2 and gangue i expreed a follow: Ca10( PO4) 6F2() 15C() 9Al2O3()... (5) CaF2() 15CO(g) 3P2( g) 9CaAl2O4() 3Ca10( PO4) 6F2() 45C() 31Al2O3()... (6) 2AlF3( g) 45CO(g) 9P2( g) 30CaAl2O4() 7

8 Ca10( PO4) 6F2() 15C() 9SiO2()... (7) CaF2() 15CO(g) 3P2( g) 9CaSiO3( ) 2Ca10( PO4) 6F2() 30C() 21SiO2()... (8) SiF4( g) 30CO(g) 6P2( g) 20CaSiO3() (3) Carbothermal reduction of Ca 10 (PO 4 ) 6 F 2 with Fe 2 O 3. The mechanim for the reduction of Ca 10 (PO 4 ) 6 F 2 by Fe 2 O 3 wa different from that of Al 2 O 3 and SiO 2, and the reduced liquid iron exhibited a trong aborption capacity for P 2 and promoted the dephophorization of Ca 10 (PO 4 ) 6 F 2. The equation for the poible reaction between Ca 10 (PO 4 ) 6 F 2 and Fe 2 O 3 i expreed a follow: Ca10( PO4) 6F2() 18C() Fe2O3()... (9) CaF () 18CO(g) 3P ( g) 9CaO() 2Fe() 2 2 Ca10( PO4) 6F2() 18C() Fe2O3() CaF () 45CO(g) 2P ( g) 9CaO()... (10) 2 2 2Fe P() 14Fe() 3 Ca10( PO4) 6F2() 45C() Fe2O3()... (11) CaF () 45CO(g) 9CaO() 6FeP() Fe() (4) Carbothermal reduction of Ca 10 (PO 4 ) 6 F 2 with three gangue. The dephophorization of Ca 10 (PO 4 ) 6 F 2 i expreed a follow: 2Ca10( PO4) 6F2() 36C() 41SiO2() 20Al2O3() 2Fe2O3() 20CaAl2SiO 2 8() 4Fe() 36CO(g) 6P2( g) SiF4 ( g)... (12) Table 5 ummarize the main thermodynamic data obtained from reaction (1) (12). 25) The Gibb free energy for the reduction of Ca 10 (PO 4 ) 6 F 2 wa decreaed by the Table 5. Thermodynamic data obtained from reaction (1) (12). 25) Reaction formula ΔG θ ((kj mol 1 )) t b ( C) (1) E T (2) E T (3) E T (4) E T (5) E T (6) E T (7) E T (8) E T (9) E T (10) E T 936 (11) E T 873 (12) E T 871 Note: ΔG θ tandard Gibb free-energy variable; T thermodynamic temperature; t b reaction tart temperature in the tandard tate addition of gangue oxide, promoting the gaification dephophorization of fluorapatite (Table 5). The initial reaction temperature (temperature at ΔG θ = 0) for reaction (6), (8), and (10) (12) wa le than C, but the initial reaction temperature of reaction (1) (5), (7), and (9) wa greater than C. At thi point, the dephophorization of fluorapatite wa hindered. Firt, from thermodynamic calculation, at a C/O (O originated from fluorapatite) of greater than 1, the initial temperature (temperature at ΔG θ = 0) for the dephophorization of fluorapatite wa lower; fluorapatite wa completely reduced; and phophoru wa mainly volatilized a P 2. At a C/O of le than 1, the initial temperature (temperature at ΔG θ = 0) of fluorapatite wa relatively high, and fluorapatite wa not completely reduced. The reduced phophoru wa predominantly volatilized a oxide. With the increae in the amount of C, the contact area of fluorapatite with C and the poroity of pellet increaed, and the thermodynamic and kinetic condition favorable for the reductive dephophorization of fluorapatite were obtained. Second, the addition of Al 2 O 3 and SiO 2 wa known to improve the thermodynamic condition of the ytem, and the temperature at which fluorapatite tarted to undergo dephophorization wa reduced. In particular, at a high content of Al 2 O 3 and SiO 2, adequate Al 2 O 3 or SiO 2 allowed for more cloe contact with fluorapatite to improve the diffuion condition. Fluorapatite wa defluorinated to generate fluorine-containing ga; the partial preure of phophoru-containing ga decreaed; the volatilization of phophoru wa promoted; a low-melting point lag phae wa formed; the reaction kinetic wa improved; the dephophorization of fluorapatite wa promoted; and the reduction temperature i reduced, which wa favorable for the dephophorization of fluorapatite. The promotion effect of SiO 2 i tronger than that of Al 2 O 3. Secondly, the mechanim for the reduction of fluorapatite by Fe 2 O 3 wa different from that of Al 2 O 3 and SiO 2, and liquid iron exhibited a trong ability to aborb phophoru to form Fe 3 P, thereby promoting the dephophorization of Ca 10 (PO 4 ) 6 F 2. In the reduction of a high-phophoru iron ore, the phophoru aborption of metal iron hould be avoided a much a poible; hence, focu on method to effectively uppre thi behavior i crucial for future reearch. With the imultaneou preence of three gangue oxide, the reaction of fluorapatite wa extremely complicated; hence, only the total reaction equation i given. The dephophorization of fluorapatite wa eaier at thi time becaue the gangue phae wa tranformed to form low-melting FeAl 2 O 4 and Fe 2 SiO 4 during carbothermic reduction. Thermodynamic and kinetic condition for the dephophorization of fluorapatite were improved for the promotion of the dephophorization of fluorapatite. In ummary, during the carbothermal reduction of a highphophoru iron ore, the migration and gaification of phophoru i gradually clarified A new idea wa propoed for the dephophorization of the high-phophoru iron ore. That i, reducing the reduction temperature led to the retardation of the melting of the iron, and the reduction of fluorapatite wa promoted by the addition of the dephophorization additive via the vaporization of phophoru. 8

9 4. Concluion (1) With the addition of Al 2 O 3 and SiO 2, CaAl 2 O 4 and CaSiO 3 were generated by the reaction between gangue and fluorapatite, repectively, which promoted the dephophorization of fluorapatite. The promotion effect of SiO 2 wa larger than that of Al 2 O 3. With the increae in the addition amount of gangue, the thermodynamic condition for the reduction of fluorapatite were continuouly optimized, thereby accelerating the dephophorization of fluorapatite. (2) During the initial tage of reduction, P 2 wa obtained from the gaification and dephophorization of fluorapatite becaue the urrounding carbon of fluorapatite wa relatively abundant, fluorapatite wa completely reduced, and the Gibb free energy required to generate P 2 wa lower. With the progre of the reaction, carbon wa continuouly conumed; carbon around fluorapatite wa not ufficient; fluorapatite wa not completely reduced; and PO ga wa generated. (3) With the addition of Fe 2 O 3, after the metal iron wa melted, a large amount of phophoru wa aborbed by the iron, and volatilized P 2 wa reduced, leading to the low increae or even decreae in the dephophorization ratio of the pellet. Phophoru wa aborbed by iron a P 2, and PO ga wa completely volatilized. (4) Gaification dephophorization mainly occurred from 10 min to 25 min at C. A large amount of phophoru wa volatilized a ga. After 25 min, P 2 wa aborbed by the liquid phae, leading to the decreae in the dephophorization ratio. Acknowledgment Project wa funded by the National Natural Science Foundation of China ( ), Fundamental Reearch Fund for the Central Univeritie (FRF-TP A1) and China Potdoctoral Science Foundation (2016M600919). State Key Laboratory of Advanced Metallurgy, Univerity of Science and Technology Beijing (No ). REFERENCES 1) L. Li-qun and Z. Han-quan: J. Cent. South Univ., 24 (2017), ) C. Cheng, Q. G. Xue, Y. Y. Zhang, F. Han and J. S. Wang: ISIJ Int., 55 (2015), ) H. M. Baioumy: Chermiie der Erde, 67 (2007), ) G. S. Liu, V. Strezov, J. A. Luca and L. J. Wibberley: Thermochim. Acta, 410 (2004), ) E. Donkoi, R. I. Olivare, D. L. S. Mcelwain and L. J. Wibberley: Ironmaking Steelmaking, 33 (2006), 24. 6) H. H. Wang, G. Q. Li, J. H. Ma and D. Zhao: RSC Adv., 7 (2017), ) H. H. Wang, G. Q. Li, D. Zhao, J. H. Ma and J. Yang: Hydrometallurgy, 171 (2017), 61. 8) R. J. Fruehan: Metall. Mater. Tran. B, 8 (1977), ) T. Coetee, P. C. Pitoriu and E. E. D. Villiere: Miner. Eng., 15 (2002), ) H. H. Wang, G. Q. Li, J. H. Ma and D. Zhao: RSC Adv., 7 (2017), ) Y. C. Liu, Q. X. Li and Y. C. Liu: Adv. Mater. Re., (2012), ) Y. Y. Zhang, Q. G. Xue, G. Wang and J. S. Wang: ISIJ Int., 57 (2017), ) M. Miyata, T. Tamura and Y. Higuchi: ISIJ Int., 57 (2017), ) M. S. Jena, H. K. Tripathy, J. N. Mohanty, S. K. Da and P. S. Reddy: Sep. Sci. Technol., 50 (2015), ) E. Matinde and M. Hino: ISIJ Int., 51 (2011), ) I. D. Santo and J. F. Oliveira: Miner. Eng., 20 (2007), ) Y. S. Sun, Q. Zhang, Y. X. Han, P. Gao and G. F. Li: JOM, 70 (2018), ) Y. Y. Cao, T. C. Sun, J. Kou, C. Y. Xu and E. X. Gao: J. Wuhan Univ. Technol. Mater. Sci. Ed., 32 (2017), ) C. C. Yang, D. Zhu, J. Pan and L. M. Lu: JOM, 69 (2017), ) W. Yu, Q. Y. Tang, J. A. Chen and T. C. Sun: Int. J. Miner. Metall. Mater., 23 (2016), ) C. Cheng, Q. G. Xue, G. Wang, Y. Y. Zhang and J. S. Wang: Metall. Mater. Tran. B, 47B (2016), ) R. Fábián, I. Koti, P. Zimany and P. Halmo: Talanta, 46 (1998), ) D. Zhao, G. Q. Li, H. H. Wang and J. H. Ma: ISIJ Int., 57 (2017), ) H. Wang, G. Q. Li, J. Yang, J. H. Ma and B. S. Khan: Metall. Mater. Tran. B, 47B (2016), ) Y. Y. Zhang, Q. G. Xue, G. Wang and J. S. Wang: Metal, 8 (2018),