Iron and phosphorus were successfully separated from CaO-SiO2-FeO-P2O5 slag through atmospheric control, B2O3 addition and a combination of magnetic separation and flotation. For the slag with basicity (CaO/SiO2) of 2.5 and B2O3 addition of 6% (weight percentage), iron and phosphorus in the slag were enriched in the form of magnetite (Fe3O4) and calcium phosphate (Ca10P6O25) phases respectively under Ar atmosphere. Using a combination of magnetic separation and flotation, the concentrates were obtained with Fe3O4 and P2O5 content of 92.84 % and 37.66 % respectively, corresponding to the recovery ratios of 85.8 % for iron and 91.3 % for phosphorus

Guan, Dong

Li, Guoxuan

Li, Juncheng

Li, Zushu

Liang, Jinshan

Long, Jun

Sridhar, Seetharaman

Iron and phosphorus were successfully separated from CaO–SiO2–FeO–P2O5 slag through atmospheric control, B2O3 addition and a combination of magnetic separation and flotation. For the slag with basicity (CaO/SiO2) of 2.5 and B2O3 addition of 6% (weight percentage), iron and phosphorus in the slag were enriched in the form of magnetite (Fe3O4) and calcium phosphate (Ca10P6O25) phases respectively under Ar atmosphere. Using a combination of magnetic separation and flotation, the concentrates were obtained with Fe3O4 and P2O5 content of 92.84% and 37.66% respectively, corresponding to the recovery ratios of 85.8% for iron and 91.3% for phosphorus

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For more information, please contact the WRAP Team at: wrap@warwick.ac.uk.  1  A Novel Process for Separation of Magnetite and Phosphorous Phases 1 from a CaO-SiO2-FeO-P2O5 slag 2 Guoxuan Li,1) Jinshan Liang,1) Jun Long,1) Dong Guan,1) Zushu Li,2) Seetharaman Sridhar3) and 3 Juncheng Li1) * 4 1)School of materials science and Engineering, Jiangsu University, Zhenjiang 212013, China 5 2)WMG, The University of Warwick, Coventry, CV4 7AL UK  6 3)School for Engineering of Matter, Transport and Energy, Arizona State University Tempe, AZ 7 85287, USA 8   9 2  Synopsis: 10 Iron and phosphorus were successfully separated from CaO-SiO2-FeO-P2O5 slag through 11 atmospheric control, B2O3 addition and a combination of magnetic separation and flotation. For the 12 slag with basicity (CaO/SiO2) of 2.5 and B2O3 addition of 6% (weight percentage), iron and 13 phosphorus in the slag were enriched in the form of magnetite (Fe3O4) and calcium phosphate 14 (Ca10P6O25) phases respectively under Ar atmosphere. Using a combination of magnetic separation 15 and flotation, the concentrates were obtained with Fe3O4 and P2O5 content of 92.84 % and 37.66 % 16 respectively, corresponding to the recovery ratios of 85.8 % for iron and 91.3 % for phosphorus. 17 Keywords: Steelmaking slag; B2O3 addition; Atmospheric control; Magnetite; Calcium phosphate 18 19 3    20 1. Introduction 21 Converter slag, a significant by-product during the steelmaking process, primarily contains CaO, 22 SiO2, FetO and P2O5, which are considered to be potential material sources for road construction,1) 23 cement,1) fertilizer,2) civil engineering projects3) as well as feedstock for re-use in the steelmaking 24 process. Re-use in the steel plant is limited by the enriched phosphate in steelmaking slags, since 25 phosphorus may reverse to the hot metal. The FetO, in the steelmaking slag is furthermore, a potential 26 source for ferrous feedstock if it could be seperated. Various techniques such as flotation,4) magnetic 27 separation,5) super gravity separation,6-8) carbothermal reduction,9) and selective leaching10-12) have 28 been employed to extract phosphorus from steelmaking slags. However, none of these approaches 29 have achieved simultaneous recovery of both iron and phosphorus. A novel technology based on the 30 principle of selective enrichment-selective growth-selective separation has been successfully applied 31 to extract boron,13) vanadium,14) titanium from metallugical slags,15) and many trials16) have been 32 carried out to selectively separate phosphorus from steelmaking slag. However, solid solution 33 2CaO∙SiO2-3CaO∙P2O5 (C2S-C3P) was the main phosphorus-enriched phase in steelmaking slag and 34 the maximum content of P2O5 in C2S-C3P was reported no more than 20%, particularly for the slag 35 with basicity above 2.0.17,18) Moreover, the abovementioned approaches primarily focused on 36 extracting phosphorus from steelmaking slag, rather than simultaneously recycling iron and 37 phosphorus as well as reusing tailings. Here, we present a successful method, based on the 38 combination of magnetic separation and flotation methods, to selectively separate iron and 39 phosphorus from CaO-SiO2-FeO-P2O5 slag through atmospheric control and B2O3 addition. 40 2. Materials and methods 41 Based on the dephosphorization slag composition of the two converter steelmaking process (i.e. 42 the first converter for dephosphorization and the second for decarburization), we synthesized the slag 43 required for this study. The chemical composition of synthetic slags was presented in Table 1. As 44 seen in the Table 1, the P2O5 content in all the slags were adjusted to 10 % and basicity (CaO/SiO2) 45 of the slags were kept at 2.5.  46 The powders of dry chemical agents were well mixed and then placed into a platinum crucible with 47 diameter of 30 mm and height of 40 mm. The mixed powder (40 g for each slag) was heated in a 48 4  vertical tube furnace that was controlled by a program controller with U-type (Pt–30%Rh/Pt–6%Rh) 49 thermocouple, within the observed precision range of ±3 oC. The experimental material was melted 50 in the vertical tube furnace under air/argon gas atmosphere (with flow rate 0.5 L/min, 99.999%) at 51 1600 oC for 120 minutes to ensure it fully melted and then rapidly cooled to 900 oC at a cooling rate 52 of 10 oC/min. Then the slags were water quenched and divided into two parts. One part was crushed 53 by ball mill, and characterized by X-ray diffraction (TTRIII from Rigaku Corporation) and XRF (ARL 54 Advant’X IntelliPower-4200) to obtain the respective mineral component and chemical composition, 55 while the other part was characterized on the scanning electron microscopy (SEM), energy disperse 56 spectrum (EDS; ZEISS EVO 18) and electron probe microanalysis (EPMA SHIMADZU 8050G) to 57 obtain micromorphology and element distribution of the products. In order to separate phosphorus 58 and iron phases from the quenched slags, a wet magnetic separator (XCGQ-500 from Hengcheng 59 equipment Co., Ltd) with the magnetic field intensity of 3.0 KOe was firstly employed to separate 60 magnetite phase from the quenched slags. The magnetic separation tube was located between the two 61 ends of the C-type electromagnet and is equipped with a glass tube for reciprocating movement and 62 swing. When the sorted sample passes through the magnetic field area, the magnetic part is attached 63 near the tube wall, while the non-magnetic part is washed out by water during mechanical movement. 64 Subsequently, the flotation machine (XFD-0.75L from Weiming equipment Co., Ltd) was used to 65 separate phosphorus-enriched phase from the non-magnetic part of magnetic separation. The flotation 66 machine is driven by the trigonometric generation of the motor to drive the impeller to rotate, resulting 67 in centrifugal action to form negative pressure. On the one hand, the sufficient air is mixed with the 68 slurry, and the slurry is mixed with the flotation reagent, including inhibitors, collectors and pH 69 regulators. At the same time, the foam is refined, so that the mineral adheres to the foam and floats to 70 the slurry surface to form a mineralized foam. Through adjusting the height of the gate and controlling 71 the liquid level, the useful foam is scraped out by the scraper. During the flotation process, the pH 72 value was controlled at 8.5-9 by adding sodium carbonate, the samples were roughened once and 73 selected four times with water glass as inhibitor, oxidized paraffin soap and tall oil as mixed capture 74 agent. In the process of roughing, sodium carbonate, water glass, oxidized paraffin soap and tall oil 75 were 1500 g/t, 3000 g/t, 700 g/t and 500 g/t respectively. While for cleaning, sodium carbonate, water 76 glass, oxidized paraffin soap and tall oil were 300 g/t, 500 g/t, 500 g/t and 300 g/t respectively (mixing 77 for 3 min). 78 5  3. Results and discussion 79 The phases presented in the synthetic slags were characterized by XRD and the results are shown 80 in Fig. 1. It can be seen that the main phases presented in the slag 1# were calcium ferrite (Ca2Fe2O5), 81 calcium silicate (Ca2SiO4) and phosphorus solid solution (Ca7Si2P2O16). This is in agreement with the 82 experimental results19,20) that wustite is inclined to oxidize to hematite under an air atmosphere and 83 then stabilizes the free lime by forming brownmillerite (Ca2(Al, Fe)2O5). However, with an 84 atmosphere change from air to high purity argon, the main iron-containing phase (Ca2Fe2O5) were 85 found to change to the magnetite phase (Fe3O4), while other phases remain the same as the original 86 slag. Furthermore, with the introduction of 6% B2O3 (slag 3#), the dicalcium silicate (2CaO·SiO2) 87 and solid solution phase of Ca7Si2P2O16 were changed into CaB2O4, CaFeSi2O6 and phosphorus rich 88 phase of calcium phosphate (Ca10P6O25), while the magnetite phase remained the same. In summary, 89 with the simultaneous atmospheric control and B2O3 addition into the simulated steelmaking slag, 90 iron and phosphorus containing phases precipitated in the form of Fe3O4 and Ca10P6O25 respectively 91 during the cooling process from 1600 oC to 900 oC. 92 From the results of SEM-EDS analysis (Fig. 2 and Table 1), it is clear that the iron phase in the 93 slags changed from irregular calcium ferrite (Ca2Fe2O5) to regular spinel (Fe3O4), while the 94 phosphorus phase in the slags transformed from slag matrix (Ca7Si2P2O16) to lath-shaped phase 95 (Ca10P6O26). In addition, the round granular dicalcium silicate phase (2CaO·SiO2) turned into slag 96 matrix. The measured chemistry of the white spinel phase was close to Fe3O4.3, while the lath-shaped 97 phase’s chemistry was closest to Ca2P0.83O3.51, corresponding to the Fe3O4 and Ca10P6O25 phases in 98 Fig. 1.   99 In order to quantitatively analyze the composition of various phases, slag 3# was analyzed using 100 EPMA. The P2O5 content in the phosphorus enriched phase was detected to be 34.1%, which is almost 101 twice as high as previous studies.17,18) Moreover, the dominant composition of the spinel phase was 102 Fe3O4 even through it was presented as Fe2O3 in Table 3. Therefore, in the whole paper, Fe2O3 content 103 is converted to Fe3O4 based on the stoichiometry of 3Fe2O3 = 2Fe3O4 + O.  104 In order to separate the abovementioned spinel (Fe3O4) and calcium phosphate (Ca10P6O25) phases 105 from modified slags, a combination of magnetic separation and flotation separation were carried out. 106 The XRD results of the concentrates and tailings obtained by magnetic separation and flotation 107 6  separation are shown in Fig. 4. It can be seen that magnetic separation can separate the treated slag 108 into two parts: the concentrates of magnetite Fe3O4 and the mixture (remaining slag) of CaB2O4, 109 CaFeSi2O6 and Ca10P6O25. After applying the flotation separation, the remaining slag from magnetic 110 separation can be split into concentrates Ca10P6O25 and tailings (primarily CaB2O4 and CaFeSi2O6).  111 The products obtained in different stages were tested by XRF, the results were shown in Table 4. 112 Fe3O4 in the concentrate of magnetic separation was 92.84%, P2O5 content in the concentrate after 113 floatation separation was 37.66%, and P2O5 contents in the tailing was 1.95%. 114 The recovery ratios of Fe and P in the concentrates and tailings can be calculated by Eqs. (1) and 115 (2): 116 100pct××× × ×=+ +1 11 1 2 2 3c FeFec Fe c Fe r Fem ωm ω m ω m ωε                                           (1) 117 100pct××× × ×=+ +2 21 1 2 2 3c PPc P c P r Pm ωm ω m ω m ωε                                             (2) 118 where  Fe  and P  are the recovery ratios of Fe and P in the slag, pct; 1cm , 2cm  and rm are the 119 mass of magnetic separation concentrate, flotation concentrate and flotation tailings respectively, pct; 120 Fe1 , Fe2 , Fe3  and P1 , P 2 , P3  are the mass fraction of Fe and P in magnetic separation 121 concentrate, flotation concentrate and flotation tailings, pct. As shown in Table 4, the recovery ratio 122 of iron and phosphorus were 85.8 % and 91.3 %. 123 Based on the investigation results in this study, it is possible to separate iron and phosphorus 124 enriched phases from steelmaking slag as well as comprehensively utilize the steelmaking slag as 125 illustrated in Fig. 5, and further investigation will focus on the enriching mechanism of iron and 126 phosphorus phases in the slags with atmospheric control and B2O3 addition.  127 4. Conclusion 128 In summary, it was confirmed by the experimental result that iron and phosphorus could be 129 effectively recoved from CaO-SiO2-FeO-P2O5 through atmospheric control and B2O3 addition. Using 130 a combination of magnetic separation and flotation separation, the concentrates were obtained with 131 the Fe3O4 and P2O5 content of 92.84 % and 37.66 % respectively, corresponding to the recovery ratios 132 of 85.8 % for iron and 91.3 % for phosphorus. The results suggest the current approach to be a 133 potential method for the comprehensive utilization of basic oxygen steelmaking slag. 134 7  Acknowledgement 135 This work was supported by Jiangsu University (19JDG011). J LI would like to acknowledge the 136 support from innovation training program for undergraduate (202010299076 and 202110299463X) 137 and scientific research projects (18A001, 18A003,18A014 and 20A007).  138 8  Reference 139 1) J. Li, C. Luo, M. Sun, W. Shen, B. Cao and X. Li: Key Eng. Mater., 3019(2014), 9. http://doi.org/10.4028/www.scien140 tific.net/KEM.599.98. 141 2) L. Lin, Y. Liu, J. Zhi, S. He, X. Li, Z. Hou and L. Zhang: Ironmak. Steelmak., 48(2021), 334. http://doi.org/10.1080/0142 3019233.2020.1780366. 143 3) S. Israel, T. Carlos, A. P. Juan, S. Jesus and T. Pablo: Appl. Sci., 10(2020), 773. http://doi.org/10.3390/app10030773. 144 4) Z. Wang, B. Liang and J. Zhang: Appl. Mech. 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B., 47(2016), 3237. http://doi.org/1171 0.1007/s11663-016-0809-4. 172   173 9  Table captions: 174 Table 1 Composition and oxygen partial pressure of simulated steel slag； 175 Table 2 EDS analysis data of the composition of the samples before and after modification； 176 Table 3 EPMA analysis data of the composition of slag 3#； 177 Table 4 XRF results of products and recovery ratio of P and Fe in the slag   178 10  Table 1 Composition and oxygen partial pressure of simulated steel slag 179 Slag No Composition(mass%) Atmosphere  CaO SiO2 FeO P2O5 B2O3 1 38.57 15.34 36 10 0 Air 2 38.57 15.34 36 10 0 Ar 3 34.29 13.71 36 10 6 Ar  180   181 11  Table 2 EDS analysis data of the composition of the samples before and after modification 182 Slag No. Position Phases Ca(at.%) Si(at.%) O(at.%) P(at.%) Fe(at.%) Potential Formula 1 Pt1 Fe-rich 20.61 5 20.66 - 53.73 Ca2Fe3.7O4.8 Pt2 Matrix 50.9 16.32 31.67 - 1.02 Ca2Si0.91O3.1 Pt3 P-rich 43.26 20.06 22.33 14.35 - Ca7Si4.67 P2.98O9.1 2 Pt4 Fe-rich 3.32 - 26.35 - 70.33 Fe3O3.93 Pt5 Matrix 50.04 15.9 33.61 - 0.45 Ca2Si0.91O3.4 Pt6 P-rich 44.82 19.77 23.06 12.35 - Ca7Si4.44P2.15O11 3 Pt7 Fe-rich - - 29.11 - 70.89 Fe3O4.3 Pt8 Matrix 41.78 16.98 27.5 1.34 12.4 CaFe0.21Si0.58O1.64 Pt9 P-rich 49.37 0.06 34.55 16.02 - Ca2P0.83O3.51 -The amount of the phase is below the lower detection limit. The X-ray energy of element B is very low which is hard to detect, so B is not listed in Table 2.  183   184 12  Table 3 EPMA analysis data of the composition of slag 3# 185 Spots Phases Composition(mass%) CaO SiO2 “Fe2O3” P2O5 B2O3 Total 1 P-rich  51.79 5.407 0.97 34.1 3.66 95.95 2 Matrix 40.21 25.37 13.38 1.612 11.9 92.48 3 Fe-rich 0.5 0.02 99.19 0 0 99.7 The measured composition of iron is reported as Fe2O3 for presentation purposes.  186   187 13  Table 4 XRF results of products and recovery ratio of P and Fe in the slag  188 Products Mass(g) Composition(mass%) Recovery ratio(Pct) CaO SiO2 P2O5 Fe2O3 Corresponding Fe3O4 P Fe Concentrate(magnetic) 14.2 3.2 1.3 0.46 96.04 92.84 1.55 85.8 Concentrate(flotation) 10.2 59.8 1.74 37.66 0.8 0.72 91.3 0.475 Tailing(flotation) 15.6 48.26 35.78 1.95 14.01 13.54 7.13 13.73  189   190 14  Figure captions： 191 Figure 1 XRD analysis of samples； 192 Figure 2 SEM morphology of samples: (a) 1#; (b) 2#; (c) 3#； 193 Figure 3 EPMA image characterization of slag 3#； 194 Figure 4 XRD patterns of the phase after separation； 195 Figure 5 Flow sheet of comprehensively utilization of steelmaking slag 196  197  198  199  200  201  202  203  204  205  206  207  208  209  210  211  212  213  214  215  216  217  218  219 15  Figure 1 220  221  222  223  224  225  226  227  228  229  230  231  232  233  234  235  236 16  Figure 2 237  238  239  240  241  242  243  244  245  246  247  248  249  250  251  252  253  254  255  256  257  258 17  Figure 3 259  260  261  262  263  264  265  266  267  268  269  270  271  272  273  274 18  Figure 4 275  276  277  278  279  280  281  282  283  284  285  286  287  288  289  290  291  292  293  294  295  296  297  298  299  300 19  Figure 5 301  302 

A novel process for separation of magnetite and phosphorous phases from a CaO-SiO2-FeO-P2O5 slag

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A novel process for separation of magnetite and phosphorous phases from a CaO-SiO2-FeO-P2O5 slag

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