Microwave-Assisted Reduction of Electric Arc Furnace Dust with Biochar

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Microwave-assisted reduction of electric arc furnace dust with biochar: an examination of transition of heating mechanism Qing Ye, Zhiwei Peng, Guanghui Li, Joonho Lee, Yong Liu, Mudan Liu, Liancheng Wang, Mingjun Rao, Yuanbo Zhang, and Tao Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00959 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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ACS Sustainable Chemistry & Engineering

Microwave-assisted reduction of electric arc furnace dust with biochar: an examination of transition of heating mechanism Qing Ye†, Zhiwei Peng†,*, Guanghui Li†,*, Joonho Lee‡, Yong Liu§, Mudan Liu§, Liancheng Wang†, Mingjun Rao†, Yuanbo Zhang†, and Tao Jiang† †School

of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, China

‡Department

of Materials Science and Engineering, Korea University, Seoul 02841, South Korea

§Guangdong

Provincial Key Laboratory of Development and Comprehensive

Utilization of Mineral Resources, Guangdong 510650, China * Emails: [email protected] (Z. Peng); Tel.: +86-731-88877656. [email protected] (G. Li); Tel.: +86-731-88830542.

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract This present study aimed to investigate the reduction behavior of hazardous electric arc furnace (EAF) dust in the presence of biochar (reducing agent) based on self-reduction of their composites under microwave irradiation with an emphasis on the microwave heating mechanism. The experimental results showed that after microwaveassisted reduction the iron metallization degree of the product reached 94.7%, much higher than that (67.6%) by conventional heating. It was revealed that the “lens effect” promoted the directional migration of the gangue elements and the newly-generated metallic iron component in the microwave field. Further analysis of electromagnetic characteristics of the composite system demonstrated that its self-reduction relied heavily on the microwave heating mechanism which underwent multiple transitions during the reduction process. The dielectric polarization and magnetic loss dominated the initial stage of microwave heating (stage I, 1073 K), the conductive loss became more apparent because of the release of volatiles and increase of the newly-generated metallic iron phase, producing enhanced electronic conduction which was expected to speed up the reduction process.

Key words: EAF dust, biochar, permittivity, permeability, microwave heating mechanism


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Introduction With the rapid development of iron and steel industry, the global crude steel production reached about 1.69 billion tons in 2017.1 The steel plant dust is a typical byproduct, with a discharge equal to about 10 wt. % of the crude steel output.2 In a typical electric arc furnace (EAF) steelmaking process, approximately 1-2 wt. % of the charge is converted to dust.3 This dust, generally containing 30-45 wt. % iron and 3-40 wt. % zinc, was considered to be an important secondary resource.4 However, it also contains a number of heavy metal elements, including Pb and Cr, and thereby frequently categorized as a hazardous material, which demands efficient and sustainable recycling. Generally, EAF dust contains multiple mineral phases, including hematite, franklinite and ferromanganese spinel, due to the use of scrap charge.5 At present, the main methods for EAF dust processing are based on either hydrometallurgical or pyrometallurgical processes.6 For the hydrometallurgical processes, different acids were proposed for the EAF treatment. The sulfuric acid leaching was the earliest method applied for comprehensive utilization of EAF dust.7 The zinc leaching percentage achieved about 75% using diluted H2SO4 at the temperature of 353 K, with the iron extraction up to 45%.8 The shortcoming of the method was that the iron dissolved in the leachate would cause difficulties to the subsequent zinc electrodeposition process.9 Compared with acid leaching, alkaline leaching appears to be more advantageous because of the low iron dissolution in alkali solution. However, to improve its performance, pre-treatments like calcification or reductive roasting are often necessary.10 A recent study demonstrated that a significantly higher amount of zinc was extracted from the CaO-treated EAF dust compared with the raw dust without pretreatment by NaOH solution.11 It was found that with the pretreatment the majority of zinc was leached without noticeable dissolution of iron and calcium, which remained as Ca2Fe2O5 and Ca3Fe2(OH)12 in the leaching residue. However, this process consumed large amounts of calcium oxide, e.g., m(CaO)/m (Fe)>1.3, and NaOH solution, e.g. S/L ratio=1/300. In addition, the Fe-bearing leaching residue requires further recycling. Although there are many studies on developing efficient hydrometallurgical



methods, the majority of EAF dust is still processed by pyrometallurgical processes based on carbothermic reduction of the dust.12 The Waelz process is one of the most commonly used method, accounting for more than 80% of current industrial operations.13 The main disadvantage of the process was high consumption of nonrenewable coke and coal, which serve as reducing agents, and the low metallization degree (about 75%). Furthermore, it has requirement for the zinc content of the dust that should be higher than 16% to ensure economics of the process.14-15 In fact, some recent studies demonstrated that the iron metallization degree of the dust was only 79% at 1573 K when the mass ratio of EAF dust/carbon was 6.16 The stable spinel structure of franklinite (ZnFe2O4) was the main factor that requires high energy input to decompose in the reduction process. This deficiency can only be addressed by adding excessive carbon, with which the iron metallization percentage and dezincification percentage could reach 91.35% and 99.25% after reduction at 1573 K for 30 min.17 From the above studies, it is obvious that the conventional pyrometallurgical methods suffered from high energy consumption for treatment of EAF dust. To intensify the reduction process, much attention had been devoted to the use of microwave heating for EAF dust treatment in recent years, considering its high thermal efficiency due to volumetric and selective heating features.18,19 It was reported that under microwave irradiation the reduction of metal oxides in the dust could be accomplished in 20 min with the high dezincification percentage (92.79%) using blast furnace slag as the reduction agent.20 Zhou et al. and Kim et al. also examined the reduction of zinc oxide from dust and found that the dezincification percentage reached 80%-90% rapidly under microwave irradiation.21 These studies focused on the reduction behaviors of zinc and iron components, with little attention on the other valuable components, like Mn and Cr. Meanwhile, in-depth exploration of microwave heating mechanism, which controls the reduction performance, 4 has not been carried out. Such work is really important as it would allow further enhancement of metal recovery and dust treatment efficiency via microwave processing. Obviously, despite a few reports on the reduction of EAF dust in microwave field, the heating mechanism and its transition during the whole EAF dust reduction process


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under microwave irradiation is still unclear. The present study was devoted to the systematic examination of reduction of main valuable components, including those containing Mn and Cr, in EAF dust, with an emphasis on the transition of microwave heating mechanism. It began with comparison of reduction performance of microwave and conventional heating. By examining the characteristics of microwave-materials interactions based on measurement of microwave parameters of EAF dust in a broad temperature range, the heating mechanism and its changes which accounted for the significantly higher efficiency of microwave-assisted reduction than those in conventional reduction were determined.

Experimental Materials The chemical composition of EAF dust is shown in Table 1. It contained high contents of iron (52.54 wt. %) and zinc (5.65 wt. %), in the forms of magnetite (Fe3O4) and franklinite (ZnFe2O4), respectively, as shown in Fig. 1. It also had 0.16 wt. % Cr and 1.77 wt. % Mn in the forms of chromite spinel and ferromanganese spinel (Tables 2 and 3), respectively. In this study, biochar, derived from pyrolysis of waste oak shaving, was used as reducing agent and its proximate analysis is shown in Table 4. It had fixed carbon content of 77.47 wt. % and ash content of only 5.55 wt. %. Fig. 2 shows the spherical structure of EAF dust particles and porous fiber structure of biochar. The EAF dust particles were very fine, with the size less than 1 μm and that between 1 and 20 μm accounting for 85% and 15% of the total particles, respectively. The biochar had 3 size divisions. Those with sizes less than 45 μm, between 45 and 53 μm, and larger than 53 μ m accounted for 47.5%, 35.4%, and 17.1 % of the total particles, respectively.



Table 1 Chemical composition of EAF dust

Element

Fe

Mn

Zn

Pb

Cr

F

Mg

Content (wt. %)

52.54

1.77

5.65

0.12

0.56

0.12

0.762

Element

Si

S

Ca

V

Cl

K

Na

Content (wt. %)

1.1

0.56

2.39

0.021

0.54

1.12

2.02

F

600

F- Fe3O4 Z-ZnFe2O4 C-CaMgSiO6

500

Intensity (CPS)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400 Z 300 F 200 100 0

F

10

20

C 30

C

40 50 2θ (degree)

F

F

F F

60

70

80

Fig. 1. XRD pattern of the EAF dust.

Table 2 Chemical phase analysis and distribution of Mn in the EAF dust

Ferromanganese spinel Content (wt. %)

EAF dust

1.65

Percentage (%)

Manganese oxide Content (wt. %)

Percentage (%)

0.12

6.8%

93.2%

Table 3 Chemical phase analysis and distribution of Cr in the EAF dust

Chromite spinel

EAF dust

Chromium oxide

Content (wt. %)

Percentage (%)

Content (wt. %)

Percentage (%)

0.46

82.1%

0.10

17.8%


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Table 4 Proximate analysis of biochar and its ash composition

Proximate analysis (wt. %)

Main chemical composition of ash (wt. %)

Fc a

Ad b

Vdaf c

Al2O3

SiO2

Fe2O3

TiO2

CaO

77.47

5.55

16.98

1.51

8.77

23.91

0.17

37.09

aF

c:

fixed carbon content; b Ad: ash content; c Vdaf: volatile matter content.

Fig. 2. Microstructures of the raw materials: (a) EAF dust and (b) biochar.

Methods Thermodynamic calculation The thermodynamics of carbothermic reduction of EAF dust was studied using the software FactSage 7.0 (Thermfact/CRCT, Montreal, Quebec, Canada; GTTTechnologies, Herzogenrath, Germany). The Gibbs free energy changes of the reactions involved in the reduction of main components of EAF dust were calculated. Experimental procedure EAF dust and biochar (particle size between 45 and 53 μm) were pre-dried and then mixed at a specified mass ratio (m(C)/m(EAF dust)=0.25) using a blender. The mixture was then pressed at the pressure of 50 MPa to prepare cylindrical briquettes (EAF dust-biochar composite system) with a diameter of 20 mm and height of 10 mm. The briquettes were charged into a quartz tube in a multimode microwave tube furnace (1.5 kW, 2.45 GHz, CY-SVT1200C-SD, CHANGEMW, China) and a conventional resistance tube furnace for reduction under the same conditions, namely ramp rate of 30 K/min, reduction temperature of 1323 K, reduction time of 15 min and both in



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nitrogen atmosphere. After cooling to room temperature, the reduction products, including the reduced briquettes remained in the crucible and the volatized matter, were collected in a jar for physiochemical analysis.

Fig. 3. Schematic diagram of the microwave-assisted reduction apparatus.

The reduction efficiency of EAF dust was evaluated in terms of iron metallization degree(η), reduction degree (𝑅𝑑) and volatilization percentage (𝛾𝑖). The iron metallization degree was given as 𝛽

(1)

η = 𝛼 × 100%

where 𝛼 is the total iron content of the raw material, wt.%, and 𝛽 is the metallic iron content of the reduced briquette, wt.%. The reduction degree was expressed as 𝑅𝑑 = 1 ―

𝑚2 × 𝑤2 × 0.22 𝑚1 × 𝑤1 × 0.3

× 100%

(2)

where 𝑚1 and 𝑚2 are the masses of the raw material and the reduced briquette, respectively, g; 𝑤1 is the ferric oxide content of the raw material, wt.%, and 𝑤2 is the ferrous oxide content of the reduced briquette, wt.%. The volatilization percentage was determined as 𝑚2 × 𝛾𝑚2

𝛾𝑖 = 1 ― 𝑚1 × 𝛾𝑚1 × 100%

(3)

where 𝑖 denotes the metal volatilized (Zn or Pb), and 𝛾𝑚1 and 𝛾𝑚2 represent the contents of metal elements of the raw materials and reduced briquettes, respectively, wt.%.


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Determination of microwave absorption capability The complex relative permittivity (εr) and permeability (μr) of powdery raw materials (blended with microwave transparent paraffin with the mass ratio of 7:3) between 2-18 GHz were measured by the coaxial waveguide method using a vector network analyzer (E8362C, Agilent, Germany). To characterize the diminishing rate of waves, microwave penetration depths (Dp) of the samples, defined as the distance from surface into the material at which the traveling wave power drops to e-1 from its value at the surface, were calculated by the equation given as 1

𝐷𝑃 = 2

𝜆0

{𝜀 𝜇 ― 𝜀 𝜇 + [(𝜀 𝜇 ) + (

2𝜋

" " 𝑟 𝑟

′𝑟 ′𝑟

′𝑟 ′𝑟

2

2 𝜀"𝑟𝜇"𝑟

)

2 𝜀′𝑟𝜇"𝑟

+(

)

)]

2 𝜇′𝑟𝜀"𝑟

+(

1 2

}

―2

(4)

where 𝜆0 is the wavelength at the designated frequency in free space, m; 𝜀′𝑟 and 𝜀"𝑟 are the real and imaginary parts of complex relative permittivity of the material (relative dielectric constant and dielectric loss factor), respectively, dimensionless; 𝜇′𝑟 and 𝜇"𝑟 are the real and imaginary parts of complex magnetic permeability (relative magnetic constant and magnetic loss factor), respectively, dimensionless. For measurement of permittivity (dielectric property) of the EAF dust-biochar composite system in a broad temperature range (373-1373 K), the parallel plate capacitance method was used in the present study (HTDE1208, SANQI, China). The samples were pressed into wafers with the pressure of 50 MPa (diameter of 10 mm and thickness of 1 mm) for the measurement. The prepared samples were then heated from room temperature to 1373 K in nitrogen at the flow rate of 100 mL/min. The dielectric parameters at different temperatures were then measured. Instrumental characterization The chemical compositions and phase compositions of the reduced briquettes were examined using an X-ray fluorescence spectrometer (XRF, PANalytical, Axio Max, Netherlands) and an X-ray diffraction spectrometer (XRD, Advance D8, Bruker, Germany) under the following conditions of radiation: Cu Kα, scanning range: 1080o and scanning speed: 5 o/min, respectively. The microstructures of the reduction products were characterized using an optical microscope (Leica DM REX, Germany) and a scanning electron microscope (Quanta-200, Netherlands). The Raman spectra of



the products were examined using a laser microconfocal Raman spectrometer (Renishaw inVia, UK). The temperature-dependent magnetization (M-T) curve of the sample was measured using a vibrating sample magnetometer (D527, Quantum Design, USA) in the temperature range from 300 K to 800 K in nitrogen.

Results and discussion Thermodynamic analysis of cabothermic reduction The Gibbs free energy changes of the reactions involved in the stepwise reduction of EAF dust are shown in Table 5. Iron oxides are stepwise reduced to metallic iron in the temperature range of 906-951 K. For zinc-bearing components, zinc ferrite is decomposed to ZnO and Fe2O3 and then reduced to metallic zinc and iron at temperatures above 1200 K. Ferromanganese spinel is reduced to MnO in the initial reduction stage, as shown by Eqs. 8-10. The chromite spinel reacts with carbon to form chromium oxide and metallic iron when the temperature exceeds 1183 K (Eq. 11). The oxide, Cr2O3, can be further reduced to metallic chromium at much higher temperatures, as revealed by Eqs. 13 and 14. Fig. 4 shows the gas-phase equilibrium diagram of the reactions in which the main components of EAF dust are involved. The ferromanganese spinel remains stable in the temperature range below Ta (813 K). When the temperature increases to Tb (1183 K), the zinc-bearing component is transformed to volatilized matter and then separated from other constituents by volatilization. As the indirect reduction of iron chromite spinel cannot proceed spontaneously (Eq. 15), it is not given in the gas-phase equilibrium diagram.


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Table 5 Gibbs free energy changes of the reactions involved in stepwise reduction of EAF dust.

No.

Reaction equation

𝚫𝒓𝑮𝜽𝒎 (J/mol)

T0 (K)

1

C(s)+ CO2=2CO(g)

170700-174.5T

978

2

Fe3O4(s)+4CO(g)=3Fe(s)+4CO2(g)

-13024+16.84T

773

3

FeO(s)+CO(g)=Fe(s)+CO2(g)

-17490+21.13T

828

4

Fe3O4(s)+CO(g) = 3FeO(s)+CO2(g)

35100-41.49T

846

5

ZnO(s) +CO (g) =Zn (l) + CO2 (g)

140810-116.34T

1210

6

Zn (l) =Zn (g)

107278-91.12T

1177

7

PbO(s)+C(s)=Pb(l)+CO2(g)

-62116-143.13T

—

8

3MnFe2O4(s)+CO(g)=3MnO(s)+2Fe3O4(s)+CO2(g)

14515-44.8T

324

9

MnFe2O4(s)+CO(g)= MnO(s)+2FeO(s)+CO2(g)

22543.08-34.46T

654

10

MnFe2O4(s)+3CO(g)= MnO(s)+2Fe(s)+3CO2(g)

-13676.95+9.62T 1422

11

FeCr2O4(s)+ 4C(s)= Cr2O3(s) +Fe(s) +4CO(g)

163830-138.43T

1183

12

FeCr2O4(s)+4C(s)=2Cr(s)+Fe(s)+4CO(g)

951540-653.61T

1456

13

Cr2O3(s)+27/7C(s)=2/7Cr7C2(s)+3CO(g)

766692-546.86T

1402

14

Cr2O3(s)+ 3C(s)=2Cr (s)+3CO(g)

819936-541.2T

1515

15

FeCr2O4(s)+ CO(s)= Cr2O3(s) +Fe(s) +4CO2(g)

48103+11.44T

—



Fig. 4. Gas-phase equilibrium diagram of the main components of EAF dust.

Carbothermic reduction of EAF dust According to the above thermodynamic analysis, the metal components of EAF dust can be separated above 1183 K (Tb). The ferromanganese spinel and chromite spinel can be reduced to MnO and Cr2O3, respectively. Further reduction of these oxides requires higher temperature. For this reason, the iron metallization degree and reduction degree were used as main indexes to evaluate the reduction efficiency of EAF dust. Based on the previous study, 22 the reduction behaviors of EAF dust under microwave irradiation and conventional condition were investigated by fixing the ramp rate of 30 K/min, reduction temperature of 1323 K, and reduction time of 15 min. For comparison, the results of microwave-assisted and conventional reductions are illustrated in Fig. 5. The results showed that the former one promoted the reduction efficiency. The iron metallization degree reached 94.7% under microwave irradiation while it was only 67.6% using the conventional method. In addition, the volatilization percentages of zinc and lead were 99.6% and 92.9%, respectively, under microwave irradiation. The XRD patterns of the reduced briquettes and volatized matters obtained after conventional and microwave-assisted reduction are presented in Fig. 6. It was shown that there was insufficient reduction of the iron-bearing components in the conventional reduction, as demonstrated by the presence of FeO in the XRD pattern


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(Fig. 6 (a)). On the contrary, all the iron-bearing components of EAF dust were reduced to metallic iron under microwave irradiation. For the volatized matters collected in both cases, metallic zinc was the only phase component (Fig. 6 (b)).

100

Index (%)

80 60 40 20 0 Microwave-assisted reduction

Conventional reduction

Iron metallization degree

Reduction degree

Volatilization percentage of zinc

Volatilization percentage of lead

Fig. 5. Comparison of reduction indexes of the products obtained by microwave-assisted reduction and conventional reduction. F-Fe O-FeO C-CaMgSiO6


C

O

O

O

F

F

20

30

Z Z Z

40

50

60

70

80

ZZ

Z

Microwave-assisted reduction

F

Z-Zn


Z

Microwave-assisted reduction C 10

Z

(b)

Intensity (A. U.)

F

(a)

Intensity (A. U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

20

2θ (degree)

30

Z

40

Z

50

2θ (degree)

Z

60

Z

70

80

Fig. 6. XRD patterns of (a) reduced briquettes and (b) volatized matter.

In order to further investigate the reduction behavior of the EAF dust-biochar composite system, the Raman spectra of reduced briquettes and off-gas analysis results during the reduction process were obtained, as shown in Fig. 7. Compared with biochar, the intensity ratios of D band to G band, denoted by R (ID/IG), of the reduced briquettes were decreased after reduction (Fig. 7 (a)), indicating that the ordered structure of



biochar was destroyed in the process. In view of the higher R value of conventionally reduced sample than that of the briquettes collected in microwave field, there existed a lower graphitization degree of the reduced briquettes, thereby a higher carbon utilization efficiency in microwave-assisted reduction. Fig. 7 (b) shows that the CO concentration increased and then remained stable as the reduction time was prolonged under microwave irradiation. In contrast, it kept increasing up to 800 K under the conventional condition. The CO concentration was sharply decreased as the temperature increased. When the temperature continually increased to 1000 K, the CO concentration of off-gas increased and then remained stable. According to the thermodynamic analysis, the reduction of iron oxides to metallic iron in 800-1000 K were exothermal while the Boudouard reaction was highly endothermic. The poor efficiency of heat transfer in conventional reduction cannot sustain the consumption of the Boudouard reaction, producing a “cool center” inside the briquettes. Due to this phenomenon, the CO concentration and reduction reaction rate were decreased. On the other hand, the reduction rate of the Boudouard reaction increased with temperature, leading to increase of CO concentration before it remained stable. From these analyses, the conventional reduction was less sufficient than microwave-assisted reduction, which agreed well with the experimental findings (Fig. 5).

G

R=1.09 R=1.0

(a) 500

1000

100

Biochar Microwave-assisted reduction Conventional reduction

Microwave-assisted reduction Conventional reduction

80

CO/(CO+CO2) (%)

D Intensity (A. U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R=0.89

1500

2000

60

40

20

2500

3000

3500

0

(b) 500

600

700

-1

Raman shift (cm )

800

900 1000 1100 1200 1300

Temperature (K)

Fig. 7. (a) Raman spectra of biochar and reduced briquettes and (b) variation of CO concentration of off-gas.

Fig. 8 compares the microstructures of reduced briquettes obtained in conventional


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and microwave-assisted reduction. The microstructures of the briquettes indicated that the metallic iron particles remained fine and dispersive in the briquettes after conventional reduction (Fig. 8 (a)). A part of carbon was remained in the reduced briquettes. In contrast, the metallic iron particles with larger sizes were obtained under microwave irradiation (Fig. 8 (b)), which was favorable for the particle liberation by subsequent grinding and magnetic separation. The SEM-EDS analysis results of the reduced briquettes are shown in Fig. 9. The gangue elements like Si, Ca and Al were interweaved with the newly generated metallic iron phase based on solid phase reaction under the conventional condition, as shown in spectrum A. The newly generated metallic iron phase was only associated with the elements Mn and Cr in microwave field with the contents of 2.58 at. % and 0.31 at. %, respectively, as revealed in spectrum B in Fig. 9. This observation indicated that the main components of EAF dust had directional migration in the reduction process under microwave irradiation. As the penetration depths of the microwave in gangue components were larger than those of spinel structured phases (e.g., Fe3O4, FeMn2O4 and FeCr2O4), more microwave was guided into the composite system. Due to the microwave “lens effect”, the metallic particles in the core of the briquettes would be heated faster in the microwave field. 23 Hence, the difference of microwave absorption capabilities of the main components in EAF dust led to the directional migration of the valuable components under microwave irradiation. Most of the gangue components were excluded from the newly-generated metallic phase. The siderophile elements, e.g., Cr and Mn, were believed to be present in the forms of MnO and Cr2O3 after reduction of ferromanganese spinel and chromite spinel. Further separation of these two elements can only be achieved by elevating temperature.



Fig. 8. Microstructures of the reduced briquettes obtained by (a) conventional reduction and (b) microwave-assisted reduction.

Fig. 9. SEM-EDS analysis of the reduced briquettes obtained by (a) conventional reduction and (b) microwave-assisted reduction.

Evidently, microwave-assisted reduction promoted the reduction efficiency in comparison with the conventional method. The volumetric heating feature of microwave energy prevented the formation of a “cold center” that is commonly found in the conventional reduction, enhancing the reduction performance. In addition, the migration of the valuable components of EAF dust was directed under microwave


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irradiation and the resulting components could be separated by proper subsequent operation. Without doubt, these findings were dependent on the microwave-material interaction during the reduction. In other words, the microwave heating mechanism during EAF dust reduction should be clarified for optimal reduction.

Microwave heating mechanism Microwave absorption capabilities According to the above analysis, the microwave-assisted reduction had advantages on the reduction efficiency which relies closely on the microwave absorption capabilities of EAF dust and biochar. Fig. 10 (a) and (b) show the values of complex relative permittivity and permeability of EAF dust and biochar in the frequency range of 2-4 GHz. The biochar had relative dielectric constant (𝜀′𝑟) much higher than EAF dust and its magnetic constant (𝜇′𝑟) remained around 1 with slight fluctuations in the examined frequency range. The dust presented a larger relative magnetic loss factor (𝜇"𝑟 ) than the corresponding dielectric loss factor (𝜀″𝑟) at 2.45 GHz. In contrast, the biochar was featured by a higher dielectric loss factor. Fig. 10 (c) and (d) show the effect of mass ratio (m(C)/m(EAF dust)) on microwave penetration depth of the composite system. When the examined frequency was 2.45 GHz, the value of Dp decreased firstly and then increased. At the mass ratio of 0.25, the microwave penetration depth of the composite system was 13.8 mm, indicating its good microwave absorption at room temperature. 4.4

13.6

4.3

13.2

ε'r

12.8

μ

4.2 4.1 4.0

Relative permittivity and permeability

Relative permittivity and pemeabillity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ε'r ε"r μ'r μ"r

1.5 1.0 0.5 0.0 2.0

(a) 2.4

2.8 3.2 Frequency (GHz)

3.6

4.0

ε"r

' r

μ"r

12.4 3.4 3.2 3.0 1.16 1.12 1.08 0.01 0.00 2.0

(b) 2.4


2.8 3.2 Frequency (GHz)

3.6

4.0


1.0 0.00 15.9 31.9 47.8 63.8 79.7 95.6 112 128

0.8

m(C)/m(EAF dust)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

(mm)

0.4

(d)

0.2

0.0

2

4

6

8 10 12 Frequency (GHz)

14

16

Fig.10. Relative permittivities and permeabilities of (a) EAF dust and (b) biochar; (c) microwave penetration depths and (d) their two-dimensional projection of the EAF dust-biochar composite system.

It was reported that the rapid realignment of dipole polarization caused heat to be generated within the entire volume of materials under microwave field.24 The dielectric molecules would be restored to a state of disordered orientation with an average dipole moment of zero. The dielectric relaxation was a response process due to inertia of the molecule itself and the viscosity of the medium. The Debye dipolar relaxation, which can be described by the Cole–Cole semicircle, is commonly used to express the dielectric relaxation process.25 Regarding the Debye dipolar relaxation, the relative permittivity (𝜀𝑟) can be expressed as follows.26 𝜀𝑠 ― 𝜀∞

(5)

𝜀′𝑟 = 𝜀∞ + 1 + 𝜔2𝜏2 𝜀"𝑟 =

(𝜀𝑠 ― 𝜀∞)𝜏𝜔

(6)

1 + 𝜔2𝜏2

where 𝜔 is angular frequency; 𝜏 is the polarization relaxation time, and 𝜀𝑠 and 𝜀∞ are the stationary permittivity and optical dielectric constant at the high-frequency limit, respectively. According to Eqs. (5) and (6), the relationship between 𝜀′𝑟 and 𝜀"𝑟 can be expressed as: 1

1

[𝜀′𝑟 ― 2(𝜀𝑠 + 𝜀∞)]2 +(𝜀"𝑟)2 = 4(𝜀𝑠 ― 𝜀∞)2

(7)

Based on Eq. (7), the curve of 𝜀′𝑟 versus 𝜀"𝑟 would form a semicircle, which is usually denoted as a Cole-Cole semicircle, as shown in Fig. 11. It was shown that the EAF dust-biochar composite system presented more than one semicircle, which


Page 19 of 28

suggested the existence of multiple relaxation effects in room temperature. 2.4

2.0

Dielectric loss factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1.2

0.8

0.4

0.0

5.4

5.6

5.8

6.0

6.2

6.4

6.6

6.8

7.0

7.2

7.4

Dielectric constant

Fig. 11. Cole-Cole semicircles of the EAF dust-biochar composite system.

Microwave heating mechanism of the EAF dust-biochar composite system in the high temperature range The heating mechanism of EAF dust during microwave carbothermic reduction can be revealed by determining the variations of high-temperature microwave absorption capabilities through permittivity and permeability measurements. According to the above results (Fig. 10), the magnetic loss appeared to be an important factor that contributed to the microwave heating of the composite system. Fig. 12 shows the temperature-dependent magnetization (M-T) curve of the EAF dust-biochar composite system. The temperature dependent magnetization data was collected in the presence of 1000 Oe external magnetic field strength in the temperature range of 300800 K. The magnetic moment/mass decreased as the temperature increased. The results of the first derivative of the plot of moment/mass (ΔM/ΔT) versus temperatures indicated that 777 K was the Curie temperature, at which the spontaneous magnetization of ferromagnetic components, including Fe3O4 and ferromanganese spinel in the EAF dust-biochar composite system, dropped to unity. In accordance with the above analysis (Fig. 10), the dielectric polarization and magnetic loss played a major role in the microwave heating below the Curie temperature and the contribution from



magnetic loss decreased to zero when the reduction temperature was further increased. 45 40

Moment/mass (emu/g)

35 30 25

0.0 -0.1

20 15 10 5 0

-0.2

ΔM / ΔT

Moment/mass (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.3 -0.4 -0.5 -0.6 -0.7 -0.8 730

777 K 740

750

760

770

780

790

800

Temperature (K)

300

400

500 600 Temperature (K)

700

800

Fig. 12. Temperature-dependent magnetization (M-T) curve of the EAF dust-biochar composite system.

Fig. 13 shows the temperature dependence of relative permittivity of the EAF dustbiochar composite system. The variation of the dielectric parameters with temperature was consisted of three stages. In stage I, the relative dielectric constant and loss factor remained stable up to about 873 K. The ferromanganese spinel was reduced to MnO and FeO in this stage. There were no strong reduction reactions of Fe3O4 and ZnFe2O4. Thereafter, in stage II there existed an obvious increase in the relative dielectric constant and loss factor in the temperature range up to 1073 K at 2.45 GHz, as a result of newly generated metallic iron and decomposition of ZnFe2O4.22 A small amount of lead associated with zinc ferrite spinel may also be reduced in this stage. Both changes contributed to the important role of dielectric polarization in the heating. In stage III, the relative dielectric constant and loss factor increased substantially. The remarkable change of the dielectric loss factor at about 1073 K was associated with the release of volatile matters from biochar and newly generated metallic iron phase, resulting in the increased electronic conduction and higher conductive loss.27 The dielectric constant


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kept increasing, but its increasing rate declined. Such phenomenon occurred above 1183 K was probably attributed to the phase transformations, such as reduction of chromite spinel and volatilization of metallic zinc generated during the process. The temperature dependence of dielectric loss tangent at 2.45 GHz is further

illustrated in Fig. 13 (b). The value of dielectric loss tangent gradually decreased and then remained stable at low temperatures, showing the contribution of adsorbed moisture evaporation and dipole polarization of stable spinel structure. When the temperature was elevated, new ferromagnetism substances (e. g., FeO and MnO) were generated due to reduction of ferromanganese spinel. The loss caused by polarization increased, generating a loss tangent (tan δ) peak at 873 K, as denoted by Tm. Actually, the maximum dielectric loss tangent relies solely on the applied frequency. When the temperature exceeded Tm, the dielectric loss tangent declined, in association with the relaxation time of polarization.27-28 When the temperature exceeded 1073 K, metallic iron was formed due to further reduction of iron oxide. A large amount of volatile zinc was produced as the temperature exceeded 1183 K. More free charges became available and they contributed to increased response of the composite system to microwave field, demonstrating the major role of electronic conduction in microwave heating during the microwave-assisted reduction of EAF dust at high temperatures. Under this condition, the reduction process is expected to speed up due to higher microwave absorption in association with enhanced electronic conduction.24



1800 1600

1.2

ε'r

1.1

ε"r

1.0

Stage Ⅰ

1400

Stage Ⅰ

Stage Ⅰ

0.9

1200

0.8 0.7

1000

tanδ

Relative permittivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

800

0.6 0.5

600

0.4 0.3

400

0.2 200 0 300

(a) 450

600

750

900

1050

1200

1350

1500

0.1

Tm

(b)

0.0 200 400 600 800 100012001400

Temperature (K)

Temperature (K)

Fig. 13. Temperature dependence of (a) relative permittivity and (b) dielectric loss tangent of the EAF dust-biochar composite system at 2.45 GHz.

Conclusion The microwave-assisted reduction of the EAF dust-biochar composite system was investigated based on thermodynamic and experimental analyses, with a focus on the transition of microwave heating mechanism. The thermodynamic analysis demonstrated that the metal components of EAF dust can be separated above 1183 K. The experimental results showed that after microwave-assisted reduction of the EAF dust-biochar composite system at 1323 K for 15 min with the mass ratio of the materials (m(C)/m(EAF dust)) of 0.25, the iron metallization degree of the product (reduced briquettes) reached 94.7%, which was much higher than that (67.6%) by conventional reduction. In addition, the reduction led to higher volatilization percentages of zinc and lead (99.6% and 92.9%, respectively). Further analysis of the reduced briquettes demonstrated the metal elements in the briquettes had directional migration under microwave field, resulting in exclusion of gangue components from newly generated metallic iron phase. Due to the “lens effect” of metallic iron particles in the microwave field, the reduction reactions and the growth of new phases were promoted. Based on the electromagnetic analysis, the transition of microwave heating mechanism during the reduction process consisted of three stages. In stage I, the dielectric polarization and


Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


magnetic loss played a major role in the initial microwave heating, promoting reduction reaction volumetrically. The spontaneous magnetization in the EAF dust-biochar composite system dropped to unity until the temperature went up to 777 K. Both of dielectric constant and dielectric loss factor remained stable. In stage II, the dielectric constant kept increasing due to the strong reduction reactions of Fe3O4 and ZnFe2O4 up to 1070 K. The remarkable change of the dielectric loss factor in this stage was associated with the release of volatilized zinc and generation of metallic iron phase. In addition, there existed a dielectric loss tangent peak, which verified the critical role of dielectric polarization in the heating. In stage III, more free charges became available and they improved the composite system’s response to microwave field, showing the main contribution of electronic conduction (i.e., conductive loss) in microwave-assisted reduction at high temperatures which was expected to speed up the reduction process under microwave irradiation.

Notes The authors declare no competing financial interest.

Acknowledgements This work was partially supported by the National Natural Science Foundation of China under Grants 51774337,51504297, 51811530108, and 51881340420, the Natural Science Foundation of Hunan Province, China, under Grant 2017JJ3383, the Innovation-Driven Program of Central South University under Grant 2016CXS021, the Hunan Provincial Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources under Grant 2014-405, the Research Fund Program of Guangdong Provincial Key Laboratory of Development and Comprehensive Utilization of Mineral Resources under Grant SK-201801, the Project for Guangdong Public Welfare Research and Capacity Building under Grant 2017A070702011, the Project for Innovative Capacity Building of Guangdong Academy of Sciences under Grant 2017GDASCX-0109, the Project for Guangdong Collaborative Innovation and Platform Environment Building under Grant 2017B090904035, the Fundamental



Page 24 of 28

Research Funds for the Central Universities of Central South University under Grants 2018zzts222, 2018zzts798, and 2018dcyj056, and the Open-End Fund for the Valuable and Precision Instruments of Central South University under Grant CSUZC201706.

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Table of Content (TOC) graphic

Synopsis Microwave energy was used for efficient treatment of EAF dust in the presence of biochar based on exploration of the microwave heating mechanism.


Page 28 of 28

Microwave-Assisted Reduction of Electric Arc Furnace Dust with Biochar

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