An Experimental Study on the Reduction Behavior of Dust Generated from Electric Arc Furnace

applied sciences

Article An Experimental Study on the Reduction Behavior of Dust Generated from Electric Arc Furnace

Mengxu Zhang 1, Jianli Li 1,2,3,* , Qiang Zeng 1 and Qiqiang Mou 1

1 The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 2 Hubei Provincial Key Laboratory for New Processes of Ironmaking and Steelmaking, Wuhan University of Science and Technology, Wuhan 430081, China 3 Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China * Correspondence: [email protected]

Received: 5 August 2019; Accepted: 29 August 2019; Published: 2 September 2019

Featured Application: This work expects to propose a direct-recycling process on basis of the mixture of EAFD powder and reductant (carbon or ferrosilicon) loaded into an electric arc furnace, in order to recycle Zn and Fe in the form of dust and molten steel. The results would provide the theoretical reference.

Abstract: To improve the utilization value of electric arc furnace dust (EAFD) containing zinc, the reduction behavior of non-agglomerate dust was investigated with carbon and ferrosilicon in an induction furnace. The experimental results show that when the temperature increases, the zinc evaporation rate increases. When the reducing agent is carbon, zinc evaporation mainly occurs in the range of 900–1100 ◦C. When the reducing agent is ferrosilicon, zinc begins to evaporate at 800 ◦C, but the zinc evaporation rate is 90.47% at 1200 ◦C and lower than 99.80% with carbon used as a reducing agent at 1200 ◦C. For the carbon reduction, the iron metallization rate increases with a rise in the temperature. When the reducing agent is ferrosilicon, with an increase in temperature, the metallization rate ﬁrst increases, then decreases, and ﬁnally, increases, which is mainly due to the reaction between the metallic iron and ZnO. In addition, the residual zinc in the EAFD is mainly dispersed in the form of a spinel solution near the metallic phase.

Keywords: EAFD; reduction; Zinc; FeO; evaporation; metallization

1. Introduction Electric arc furnace dust (EAFD) is one of the by-products of electric furnace steelmaking, and its production accounts for 1~2% of the charging amount of steel scrap and other metal materials [1,2]. Because heavy metal elements like lead, chromium, and cadmium are contained in EAFD, these kinds of by-products are classiﬁed as toxic waste by many countries [3]. However, dust that contains iron, lead, zinc, and chromium can be used as a secondary resource. Therefore, the recovery of these resources plays an important role in environmental protection [4–7]. However, the zinc content of EAFD is generally <25 wt%, which does not meet the raw material requirements for the non-ferrous industry. Thus, a part of the EAFD can only be stacked or landﬁlled as disposal [8]. In order to reduce pollutant emissions and promote ecological progress, the environmental protection laws of the People’s Republic of China stipulate that the taxes for steelmaking slag, steelmaking dust, coal ash, and other solid waste are 25 yuan per ton. However, if the solid waste is comprehensively utilized and meets the standards of the national and local environmental protection laws, the corresponding tax can be

Appl. Sci. 2019, 9, 3604; doi:10.3390/app9173604 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3604 2 of 11 temporarily exempted. The output of the EAFD increased rapidly with increases in the output of EAF steelmaking [9,10]. Therefore, the treatment and comprehensive utilization of EAFD is gradually focused, and a variety of studies have been carried out. At present, the Waelz kiln process, the rotary hearth furnace process (RHF), and the OxyCup process are the main processes used to treat steelmaking dust [11]. Although these processes can recycle the dust and sludge of iron and steel enterprises to some extent, they all have shortcomings. For example, the Waelz kiln process has higher requirements for raw materials, higher energy consumption, unstable production and operation, and a limited processing scale [12]. The main product of the RHF is metallized pellets, but its product has a high sulfur content, and the process has a low production efficiency and a large investment [13]. The OxyCup process’s product is molten iron, and its requirements for raw materials are relatively broad. However, its equipment operation cycle is short, and its maintenance work is difficult [11]. Furthermore, a homogenous mixture of EAF dust, reductant, and flux needs to be initially prepared in pellet form during the above processes [14]. In view of their shortcomings, it is an urgent issue to propose an eco-friendly practical process to recycle and utilize the valuable elements in the dust of iron and steel enterprises on the basis of environmental protection. Xu [15] and Zhang [16] mixed electric furnace dust with a reducing agent, without lumping or pelletizing, and then charged the mixture from the EAF directly into bags, which not only reduced the dust production by 40% and increased the zinc content in the dust to 29.7%, but also directly recycled valuable elements, such as iron, in the dust. Normal smelting is not affected by this process, but foaming slag occurs, while the zinc content of secondary dust without charging pellets is only 21% zinc. In order to further understand the behavior of zinc and iron in EAFD at different temperatures and provide theoretical reference for the recycling of EAFD containing zinc, the evaporation behavior of zinc and the metallization ratio of iron during EAFD reduction via carbon and ferrosilicon are investigated in this work.

2. Experimental The EAFD was obtained from an electric arc furnace (Xingtai Iron & Steel Corp. LTD, Xingtai, China). As shown in Table1, Fe 2O3 and ZnO were the main components of the dust, together comprising 67.09 wt% of it. In the dust, there was, simultaneously, CaO, MgO, SiO2, Cr2O3, MnO, and a small amount of Al2O3. Fe and Zn mainly existed in ZnFe2O4, Fe3O4, and ZnO, which were also the main phases of EAFD (Figure1). As shown in Figure2, EAFD mainly consisted of small particles, which were less than 2 µm. Ferrosilicon and carbon powder were used as reducing agents in the experiments. The ferrosilicon contained 74.64% Si and 24.00% Fe, and the content of C in the carbon powder was >99%. Both were ground into powder for use. Fe2O3, ZnO, and MnO in the EAFD can be reduced to Fe, Zn, and Mn via the reducing agents. Based on Table1, it can be calculated that when 100 g EAFD was reduced by silicon or carbon, the required amounts of silicon and carbon were 15.63 g and 14.72 g, respectively. Since the ferrosilicon alloy was the carrier of silicon, its silicon content was 74.64%. Therefore, the actual amount of ferrosilicon alloy for 100 g dust was 20.92 g. In order to completely reduce the sample, the amount of the dispensed reducing agent should be slightly excessive. Therefore, the ﬁnal amount of the ferrosilicon alloy and carbon powder was set to 22 wt% and 16 wt%, respectively.

Table 1. The composition of electric arc furnace dust (EAFD) in wt%.

Component CaO MgO SiO2 Al2O3 Cr2O3 Fe2O3 ZnO MnO KCl CaF2 Content 6.80 5.83 5.67 0.35 5.02 40.30 26.79 2.27 0.88 1.6 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 11 Appl. Sci. 2019, 9, 3604 3 of 11

1 1 1- ZnFe2O4, Fe3O4 1- ZnFe2O4, Fe3O4 2- ZnO

2 1 2 1 1 1 1 2 2 1 2 2 1 2 2 2

20 30 40 50 60 70 2θ / ° 2θ / Figure 1. XRD diffraction diﬀraction pattern of EAFD.

Figure 2. The microstructure of EAFD.

This experiment includes two series, referred to as A and B. A total of 50 g EAFD in series A Fe2O3, ZnO, and MnO in the EAFD can be reduced to Fe, Zn, and Mn via the reducing agents. wasBased mixed on Table with 1, 8 it g can carbon be calculated powder, while that when 50 g EAFD100 g EAFD in series was B wasreduced mixed by withsilicon 11 or g ferrosiliconcarbon, the alloyrequired powder. amounts The of uniform silicon mixing and carbon samples were were 15.63 charged g and into14.72 the g, corundumrespectively. crucible Since (innerthe ferrosilicon diameter 32alloy mm, was height the carrier 65 mm, of and silicon, wall thicknessits silicon 1 mm)content with was a corundum74.64%. Therefore, cover. The the crucible actual usedamount in this of experimentferrosilicon alloy was completelyfor 100 g dust dried was in 20.92 a mu g.ffl Ine furnaceorder to under completely 200 ◦C reduce for 3 hthe in sample, order to the eliminate amount the of influencethe dispensed of water. reducing The crucible agent containingshould be the slightly materials excessive. was loaded Therefore, into an inductionthe final amount furnace (60of kW,the 3ferrosilicon kHz) and keptalloy at and diff carbonerent temperatures powder was forset 30to 22 min. wt% Six and temperatures 16 wt%, respectively. were selected: 700 ◦C, 800 ◦C, 900 ◦ThisC, 1000 experiment◦C, 1100 includes◦C, and 1200two series,◦C. The referred numbers to as of A the and experiments B. A total of are 50 showng EAFD in in Table series2. A After was keepingmixed with a constant 8 g carbon temperature powder, while for the 50 predetermined g EAFD in series time, B was the samplesmixed with were 11 naturallyg ferrosilicon cooled alloy to room-temperaturepowder. The uniform in the mixing furnace. samples were charged into the corundum crucible (inner diameter 32 mm, height 65 mm, and wall thickness 1 mm) with a corundum cover. The crucible used in this Table 2. Numbers of the experiments. experiment was completely dried in a muffle furnace under 200 °C for 3 h in order to eliminate the influence of water. The crucible containing the materials was loaded into an induction furnace (60 influenceCategory of water. 700 The◦C crucible 800containing◦C the 900 materials◦C was 1000 loaded◦C into 1100an induction◦C furnace 1200 ◦C (60 kW, 3 kHz) and kept at different temperatures for 30 min. Six temperatures were selected: 700 °C, 800 kW, 3 kHz)A and kept A1 at different temperatures A2 for A3 30 min. Six temperatures A4 were A5 selected: 700 A6 °C, 800 °C, 900B °C, 1000 °C, 1100 B1 °C, and 1200 B2 °C. The numbers B3 of the experiments B4 are shown B5 in Table B6 2. After keeping a constant temperature for the predetermined time, the samples were naturally cooled to room-temperature in the furnace. The characteristics of the samples were analyzed through X-ray fluorescence (XRF-1800, Shimadzu, Kyoto, Japan), X-ray diffraction (XRD, X Pert Pro MPD, PANalytica, Almelo, Netherlands), and Table 2. Numbers of the experiments. a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS, NanoSEM400, FEI,Category Hillsboro, 700 OR, °C USA). 800 The °C total 900 iron, °C metallic 1000 °C iron, ferrous 1100 °C iron, 1200 and °C zinc content of A A1 A2 A3 A4 A5 A6 Appl. Sci. 2019, 9, 3604 4 of 11 the samples were determined by the reduction method of titanium trichloride, the volumetric method of ferric chloride and sodium acetate, the volumetric method of potassium dichromate, and flame atomic absorption spectrometry, respectively. The evaporation rate of zinc (RZn) and the metallization rate of iron (RFe) are defined as Equations (1) and (2), respectively. It was assumed that the CaO quality in the samples remained constant during the heating process and that the quality of the samples after the reaction can be obtained according to the CaO content in the samples:

TZn LZn RZn = − 100% (1) TZn ×

MFe RFe = 100% (2) TFe × where TZn is the total mass of zinc in the sample before the reaction. LZn refers to the mass of the remaining zinc in the sample after reaction, TFe represents the total iron content in the sample before reaction, and MFe is the amount of metal iron in the sample after the reaction (all in grams).

3. Results The chemical compositions of the samples after treatment are summarized in Table3. When the temperature rose, the CaO, SiO2, MgO, Al2O3, Cr2O3, and metallic iron content gradually increased. The FeO content increased ﬁrst and then decreased, while the elemental zinc content gradually decreased in both series A and B. The residual zinc content was 0.07 wt% and 2.05 wt%, the metallic iron content was 37.00 wt% and 24.20 wt%, and the basicity of the sintered slags (CaO%/SiO2%) was 1.09 and 0.20 in samples A6 and B6.

Table 3. The chemical composition of the samples after the reaction (in wt%).

2+ Sample CaO SiO2 MgO Al2O3 Cr2O3 Fe MFe Zn A1 6.76 5.05 5.23 0.34 4.53 — 0.15 21.10 A2 6.80 5.40 5.37 0.34 4.61 6.75 0.65 19.10 A3 7.10 5.37 5.31 0.32 4.59 15.80 0.93 18.90 A4 8.10 6.91 7.12 0.43 6.60 11.20 23.10 14.10 A5 10.10 9.22 10.10 0.61 7.25 11.20 35.10 0.31 A6 9.90 9.10 9.99 0.59 7.37 9.10 37.00 0.07 B1 6.20 29.30 4.85 0.62 4.13 16.90 5.60 17.60 B2 6.50 30.10 5.18 0.64 4.49 14.20 16.20 12.00 B3 6.90 32.80 5.40 0.70 5.26 15.60 15.50 10.60 B4 6.90 33.70 5.41 0.72 4.63 20.30 13.80 8.75 B5 6.90 34.70 5.67 0.74 4.81 14.10 19.00 6.10 B6 7.20 35.60 5.96 0.98 4.96 16.80 24.20 2.05

3.1. Reduction and Evaporation Rates of Zinc As can be seen from Table3 and Figure3, the evaporation rates of zinc in both series increased with a rise in temperature. When the reducing agent was carbon powder, the evaporation rate of zinc was 3.30% and 99.11% at 900 ◦C and 1100 ◦C, respectively. This suggests that the evaporation of zinc mainly occurs between 900–1100 ◦C, and the evaporation process can be divided into three stages. In this way, <900 ◦C is the dormant period, 900–1100 ◦C is the evaporation period, and >1100 ◦C is the ﬁnal evaporation period. When ferrosilicon was used as the reducing agent, the evaporation of zinc increased gradually with an increase in temperature. The evaporation rate of zinc was 35.26% at 800 ◦C and only 90.47% at 1200 ◦C. Compared to the carbon powder, the temperature range of zinc evaporation is larger, and the evaporation rate is relatively smaller. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11

final evaporation period. When ferrosilicon was used as the reducing agent, the evaporation of zinc increased gradually with an increase in temperature. The evaporation rate of zinc was 35.26% at 800 °C and only 90.47% at 1200 °C. Compared to the carbon powder, the temperature range of zinc evaporation is larger, and the evaporation rate is relatively smaller. Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11

final evaporation period. When100 ferrosilicon A wasseries used as the reducing agent, the evaporation of zinc increased gradually with an increase in temperature. B series The evaporation rate of zinc was 35.26% at 800 % Appl.°C and Sci. 2019only, 9 ,90.47% 3604 at 1200 80°C. Compared to the carbon powder, the temperature range of5 zinc of 11 evaporation is larger, and the evaporation rate is relatively smaller. 60

100 40 A series B series

% 80 20 Evaporation rate of Zn / / Zn rate of Evaporation

60 0

40 700 800 900 1000 1100 1200 T / °C 20

Figure 3. The effect / Zn rate of Evaporation of the reducing agent on the reduction and evaporation of zinc.

0 3.2. Metallization Ratio of Iron 700 800 900 1000 1100 1200 As shown in Figure 4, the metallization rate of the iron increased somewhat in the samples of T / °C series A and B with an increase in temperature. When the carbon powder was used as the reducing agent, the metallizationFigure 3. TheThe rate effect eﬀect of of sample the reducing A3 was agent only on 3.63%,the reduction while andthat evaporation of sample ofA5 zinc. was as high as 77.12%, which is slightly lower than that of sample A6 (78.67%). Hence, the metallization percentage 3.2. Metallization Ratio of Iron of3.2. the Metallization iron-changing Ratio trend of Iron can be divided into three stages, similar to the evaporation process of zinc: As shown in Figure4, the metallization rate of the iron increased somewhat in the samples of <900 As°C shownis the dormant in Figure period, 4, the metallization900–1100 °C is rate the of metallization the iron increased occurrence somewhat period, in and the >1100samples °C ofis series A and B with an increase in temperature. When the carbon powder was used as the reducing theseries final A andmetallization B with an period.increase Figure in temperature. 4 demonstrates When that the carbonmetallization powder mainly was used occurs as thebetween reducing 900 agent, the metallization rate of sample A3 was only 3.63%, while that of sample A5 was as high as °Cagent, and the 1100 metallization °C. rate of sample A3 was only 3.63%, while that of sample A5 was as high as 77.12%, which is slightly lower than that of sample A6 (78.67%). Hence, the metallization percentage 77.12%,When which the isreductant slightly lower was ferrosilicon, than that of samplethe metalliz A6 (78.67%).ation rate Hence, of the the B seriesmetallization samples percentage increased of the iron-changing trend can be divided into three stages, similar to the evaporation process of zinc: first,of the then iron-changing decreased, trendand finally can be increased divided into with three an upturn stages, in similar temperature. to the evaporation The metallization process rate of zinc: was <900 C is the dormant period, 900–1100 C is the metallization occurrence period, and >1100 C is the as<900 high ◦°C asis 47.88%the dormant at 800 period,°C, which 900–1100 was far◦ °C higher is the than metallization the 2.58% occurrenceof A2. The metallizationperiod, and >1100 rate◦ of °C the is B4theﬁnal samples final metallization metallization reduced period. to 35.08%,period. Figure Figuresignificantly4 demonstrates 4 demonstrates lower that than metallization that the 67.12%metallization mainlyof A4. mainlyThe occurs metallization betweenoccurs between 900 rate◦C of and900 B6 1100increased°C and◦C. 1100 to 67.04%°C. again, which is lower than the 78.67% of A6. When the reductant was ferrosilicon, the metallization rate of the B series samples increased first, then decreased, and finally increased with an upturn in temperature. The metallization rate was 80 A series as high as 47.88% at 800 °C, which was Bfar series higher than the 2.58% of A2. The metallization rate of the B4 samples reduced to 35.08%, significantly lower than the 67.12% of A4. The metallization rate of B6 Fe

increased to 67.04% again,R 60 which is lower than the 78.67% of A6.

4080 A series B series Fe

R 2060 Metallic Rate of Fe / of Fe Rate Metallic

400

700 800 900 1000 1100 1200 20 T/°C Metallic Rate of Fe / of Fe Rate Metallic Figure 4. TheThe effect effect of the reducing agent on the percentage metallization of iron. 0 When the reductant was ferrosilicon, the metallization rate of the B series samples increased first, then decreased, and finally increased700 with 800 an upturn 900 in 1000 temperature. 1100 The 1200 metallization rate was as high as 47.88% at 800 C, which was far higher than the° 2.58% of A2. The metallization rate of the B4 ◦ T/ C samples reduced to 35.08%, significantly lower than the 67.12% of A4. The metallization rate of B6 Figure 4. The effect of the reducing agent on the percentage metallization of iron. increased to 67.04% again, which is lower than the 78.67% of A6. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11

3.3. Occurrence State of Zinc The microstructure and chemical components obtained by SEM-EDS are shown in Figure 5 and Table 4, individually. The mineral phases of samples A6 and B6 measured by XRD are shown in FigureAppl. Sci. 6.2019 When, 9, 3604 the reducing agent was carbon powder, the EAFD samples maintained a relatively6 of 11 intact state for their particles, but the interface between the particles became vague at 1000 °C. According to the analysis of EDS and XRD, samples in A6 were clearly divided into two phases. 3.3. Occurrence State of Zinc Phase 1 is composed of Fe and Cr metal phases, and Phase 2 comprises a CaMgSiO4 phase formed by CaO,The MgO, microstructure and SiO2. The and residual chemical zinc components in the solid samples obtained is by lower SEM-EDS than the are detection shown in limit Figure of 5SEM- and EDS.Table 4, individually. The mineral phases of samples A6 and B6 measured by XRD are shown in Figure6. WhenWhen the reducing the reducing agent wasagent carbon was ferrosilicon, powder, the EAFDthe EAFD samples samples maintained were melted, a relatively and intactthe metal state particlesfor their particles,and the silicate but the phases interface were between already the formed particles at 1000 became °C. vagueThe samples at 1000 in◦C. B6 According can be divided to the intoanalysis fourof different EDS and phases. XRD, samplesPhase 3 inis a A6 metal were phase, clearly composed divided into of chromium two phases. and Phase iron. 1 Phase is composed 4 is (Mg, of Fe,Fe andZn) CrCr2 metalO4 spinel phases, phase, and made Phase up 2 comprisesof MgO, FeO, a CaMgSiO ZnO, and4 phase Cr2O formed3. Phaseby 5 is CaO, an SiO MgO,2 phase, andSiO and2. PhaseThe residual 6 is a CaMgSi zinc in the2O6 soliddiopside samples phase, is lowercomposed than theof CaO, detection MgO, limit and ofSiO SEM-EDS.2. The residual zinc in the solid phase is mainly dissolved into a spinel crystal and diopside phase.

(A4) (A6)

(B4) (B6)

FigureFigure 5.5. The microstructure of the sample sample after after bein beingg reduced reduced at at 1000 1000 °C◦C and and 1200 1200 °C.◦C ( (A4A4 and and A6 A6 were reducedwere reduced by carbon by carbon at 1000 at◦C 1000 and °C 1200 and◦C, 1200 respectively. °C, respectively. B4 and B4 B6 wereand B6 reduced were reduced by ferrosilicon by at 1000 ◦C and 1200 ◦C, respectively).ferrosilicon at 1000 °C and 1200 °C, respectively. )

Table 4. Chemical components of the diﬀerent phases in Figure5 (in wt%).

Phase Ca Si Mg Al Cr Fe Zn O 1 — — — — 2.62 95.23 — — 2 36.93 18.39 7.25 — — — — 36.26 3 — — — — 0.35 98.48 — — 4 8.09 3.84 5.91 1.94 31.50 8.59 4.67 31.14 5 5.88 40.31 2.28 1.72 0.78 2.42 — 43.58 6 14.74 29.40 11.09 1.61 1.25 1.94 0.33 37.20

When the reducing agent was ferrosilicon, the EAFD samples were melted, and the metal particles and the silicate phases were already formed at 1000 ◦C. The samples in B6 can be divided into four diﬀerent phases. Phase 3 is a metal phase, composed of chromium and iron. Phase 4 is (Mg, Fe, Zn) Cr2O4 spinel phase, made up of MgO, FeO, ZnO, and Cr2O3. Phase 5 is an SiO2 phase, and Phase 6 is Appl. Sci. 2019, 9, 3604 7 of 11

a CaMgSi2O6 diopside phase, composed of CaO, MgO, and SiO2. The residual zinc in the solid phase isAppl. mainly Sci. 2019 dissolved, 9, x FOR PEER into aREVIEW spinel crystal and diopside phase. 7 of 11

3-Fe

3 4-SiO2 4 5 4 5-CaMgSi2O6 5 1 B6 5 5 3 3 6-FeCr2O4

7-CaMgSiO4 6 6 A6 6 7 7 7 3

1- ZnFe2O4, Fe3O4 2 2-ZnO 1 2 1 1 2 Original 1 2 1

20 30 40 50 60 70 80 ° 2θ /

Figure 6. The mineral components of the samples obtained at 1200 ◦°C.C. 4. Discussion Table 4. Chemical components of the different phases in Figure 5 (in wt%).

4.1. CharacteristicsPhase of Reduction Ca ProcessesSi withMg CarbonAl Cr Fe Zn O As can be seen1 from — Figure 1—, the EAFD— is composed— 2.62 of spherical95.23 particles,— — and the mixture 2 36.93 18.39 7.25 — — — — 36.26 of reducing agents and the EAFD is a dispersion system with good contact. According to the 3 — — — — 0.35 98.48 — — thermodynamics principle,4 8.09 the chemical3.84 reactions5.91 1.94 of EAFD 31.50 containing 8.59 zinc 4.67 reduced 31.14 by carbon powder are shown in Equations5 (3)–(11) 5.88 [1140.31,17 ,18],2.28 which comprise1.72 0.78 the direct2.42 reduction — of solid43.58 carbon and the indirect reduction6 of CO: 14.74 29.40 11.09 1.61 1.25 1.94 0.33 37.20

ZnO(s) + C(s) = Zn(g) + CO(g) ∆Gθ = 348480 286.1T (3) 4. Discussion 1 − 3Fe O (s) + C(s) = 2Fe O (s) + CO(g) ∆Gθ = 4536909 224.37T (4) 4.1. Characteristics of 2Reduction3 Processes with3 4 Carbon 2 − Fe O (s) + C(s) = 3FeO(s) + CO(g) ∆Gθ = 207510 217.62T (5) As can be seen from3 4 Figure 1, the EAFD is composed of3 spherical particles,− and the mixture of reducing agents and the( )EAFD+ ( )is= a dispersion( ) + ( )systemθ =with good contact. According to the FeO s C s Fe s CO g ∆G4 158970 160.25T (6) thermodynamics principle, the chemical reactions of EAFD containing− zinc reduced by carbon ZnO(s) + CO(g) = Zn(g) + CO (g) ∆Gθ = 178020 111.6T (7) powder are shown in Equations (3) – (11) [11,17,18],2 which5 comprise the− direct reduction of solid carbon and the indirect3Fe O reduction(s) + CO( gof) CO:= 2Fe O (s) + CO (g) ∆Gθ = 35380 40.16T (8) 2 3 3 4 2 6 − ZnOFe Os (+Cs) +sCO=Zn(g) =g 3FeO+CO(sg) + CO ( g ) ∆𝐺∆Gθ== 34848071940 −73.62 286.1𝑇T (3)(9) 3 4 2 7 − θ 3Fe FeOO s(s+C) +COs =2Fe(g) = FeO (ss)++COCO2g(g ) ∆ G ∆𝐺= =13160 4536909+ 17.21 − 224.37𝑇T (10)(4) 8 − θ C(s) + CO2(g) = 2CO(g) ∆G9 = 172130 177.46T (11) FeOs +Cs =3FeOs +COg ∆𝐺 = 207510− − 217.62𝑇 (5) According to the above equations, the chemical reaction of EAFD containing zinc reduced by carbon powder isFeO a complicateds +Cs =Fe heterogeneouss +COg reaction process. ∆𝐺 = 158970 As shown − 160.25𝑇 in Figure 7, when the(6) temperature is less than 900 ◦C, a direct reduction reaction is the main reaction; the reactions are of ZnOs +COg =Zng +CO g ∆𝐺 = 178020 − 111.6𝑇 (7) the solid–solid type. When the temperature is higher than 900◦C, the reaction is principally indirect, belonging to a gas–solid reaction. The reaction rate of the multiphase reaction depends predominantly 3Fe O s +COg =2Fe O s +CO g ∆𝐺 = 35380 − 40.16𝑇 (8) on the diﬀusion rate of the reaction components and the interfacial chemical reaction rate. Since the

FeOs +COg =3FeOs +COg ∆𝐺 = 71940 − 73.62𝑇 (9)

FeOs +COg =Fes +COg ∆𝐺 = −13160 + 17.21𝑇 (10) Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11

Cs +COg =2COg ∆𝐺 = 172130 − 177.46𝑇 (11) According to the above equations, the chemical reaction of EAFD containing zinc reduced by carbon powder is a complicated heterogeneous reaction process. As shown in Figure 7, when the temperature is less than 900 °C, a direct reduction reaction is the main reaction; the reactions are of the solid–solid type. When the temperature is higher than 900 °C, the reaction is principally indirect, belonging to a gas–solid reaction. The reaction rate of the multiphase reaction depends predominantly on the diffusion rate of the reaction components and the interfacial chemical reaction Appl.rate. Sci. Since2019, 9 ,the x FOR solid PEER re REVIEWactants are in direct contact with each other before the start of the reaction,8 of 11 the reaction can be started primordially. However, a solid product layer (metal phase and slag phase) is formed betweenCs the+CO dustg =2COparticlesg and the solid ∆𝐺carbon = 172130 during − 177.46𝑇the reduction procedure.(11) Consequently, further progress of the reaction will depend on the diffusion of the reactants through According to the above equations, the chemical reaction of EAFD containing zinc reduced by the layer. carbon powder is a complicated heterogeneous reaction process. As shown in Figure 7, when the Under the influence of the Boudouard reaction, Equation (11), the carbon-reduction process is temperature is less than 900 °C, a direct reduction reaction is the main reaction; the reactions are of transformed from a solid–solid reaction to a gas–solid phase reaction. The general steps for the the solid–solid type. When the temperature is higher than 900 °C, the reaction is principally indirect, corresponding process include the external diffusion of gas reactants and the products in the belonging to a gas–solid reaction. The reaction rate of the multiphase reaction depends Appl.boundary Sci. 2019, 9 layer, 3604 on the solid surface, the internal diffusion of gas reactants, and products through8 of 11 the predominantly on the diffusion rate of the reaction components and the interfacial chemical reaction solid product layer, as well as the interfacial chemical reactions between gas and solid reactants. rate. Since the solid reactants are in direct contact with each other before the start of the reaction, the According to the microstructure of the dust samples (A5) reduced by carbon at 1100 °C, the particles reactionsolid reactants can be arestarted in direct primordially. contact with However, each other a solid before product the start layer of the(metal reaction, phase theand reaction slag phase) can beis in the sample still maintain a relatively independent shape and a certain number of pores between formedstarted primordially.between the However, dust particles a solid productand the layer solid (metal carbon phase during and slag the phase) reduction is formed procedure. between the particles, as shown in Figure 8. This provides a good condition for the diffusion of the reducing Consequently,the dust particles further and the progre solidss carbon of the duringreaction the will reduction depend procedure. on the diffusion Consequently, of the reactants further progressthrough agent, CO, and reduction product, Zn (g). theof the layer. reaction will depend on the diffusion of the reactants through the layer. Under the influence of the Boudouard reaction, Equation (11), the carbon-reduction process is transformed from a solid–solid reaction to a gas–solid phase reaction.CO The general steps for the CO2 CO CO2+Zn(g) corresponding process include the external diffusion of gas reactants and the products in the boundary layer onC the solid surface, the internal diffusion of gas reactants, and products through the solid product layer, as well as the interfacial chemical reactions +between gas and solid reactants. According to the microstructure of the dust samples (A5) reduced by carbon at 1100 °C, the particles C Dust in the sample still maintain a relatively independent shape and a certain number of pores between the particles,Figure as shown 7. Schematic in Figure diagram 8. This of the provides reaction a of good EAFD condition containing for zinc the reduced diffusion by carbon. of the reducing agent, CO, andFigure reduction 7. Schematic product, diagram Zn (g). of the reaction of EAFD containing zinc reduced by carbon. Under the influence of the Boudouard reaction, Equation (11), the carbon-reduction process is transformed from a solid–solid reaction to a gas–solid phase reaction. The general steps for the CO corresponding process include the external diffCOusion2 of gasCO reactants and the productsCO2+Zn(g) in the boundary layer on the solid surface, the internal diffusion of gas reactants, and products through the solid product layer, as well asC the interfacial chemical reactions between gas and+ solid reactants. According to the microstructure of the dust samples (A5) reduced by carbon at 1100 ◦C, the particles in the sample still maintain a relatively independent shape and a certainC number of pores betweenDust the particles, as shown in Figure8. This provides a good condition for the di ffusion of the reducing agent, CO, and reduction product, ZnFigure (g). 7. Schematic diagram of the reaction of EAFD containing zinc reduced by carbon.

Figure 8. Microstructure of the reduced samples (A5) at 1100 °C.

Since the boiling point of zinc is 908.47 °C, the zinc vapor could be separated from EAFD and discharged as a gas. The evaporation rate of zinc is governed by an interfacial chemical reaction, the diffusion of the zinc vapor, and the temperature. In the low temperature stage (900 °C), the evaporation rate of zinc in the sample is very low because of the boiling point of zinc. With the mechanism transformation of the carbon-thermal reduction, the interface chemical reaction and the

Figure 8. MicrostructureMicrostructure of the reduced samples (A5) at 1100 ◦°C.C.

Since the boiling point of zinc is 908.47 ◦°C,C, th thee zinc vapor could be separated from EAFD and discharged as a gas. The evaporation rate of zinc is governed by an interfacial chemical reaction, the diffusiondiffusion ofof the the zinc zinc vapor, vapor, and theand temperature. the temperatur In thee. lowIn the temperature low temperature stage (900 stage◦C), the (900 evaporation °C), the evaporationrate of zinc inrate the of sample zinc in isthe very sample low becauseis very low of the because boiling of pointthe boiling of zinc. point With of thezinc. mechanism With the mechanismtransformation transformation of the carbon-thermal of the carbon-thermal reduction, the reduction, interface chemical the interface reaction chemical and the reaction diffusion and of COthe and zinc (g) are improved when the temperature is higher than 908.47 ◦C. Therefore, the evaporation rate of zinc increased quickly in the temperature range of 900–1100 ◦C. The alteration of the metallization rate can be explained in the following ways. In the low temperature stage (below 900 ◦C), the reduction of Fe2O3 mainly occurred in the sample because it was restricted by a solid–solid reaction, as in Equations (4) and (5). According to Table3, it can be 2+ inferred that when the temperature increased from 700 ◦C to 900 ◦C, the Fe content in the samples increased from 0 to 15.80%, but the content of MFe increased from 0.15 to 0.93%. When the temperature increased to above 900 ◦C, the interfacial chemical reaction transformed from a solid–solid reaction to Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 11 diffusion of CO and zinc (g) are improved when the temperature is higher than 908.47 °C. Therefore, the evaporation rate of zinc increased quickly in the temperature range of 900–1100 °C. The alteration of the metallization rate can be explained in the following ways. In the low temperature stage (below 900 °C), the reduction of Fe2O3 mainly occurred in the sample because it Appl.was Sci.restricted2019, 9, 3604by a solid–solid reaction, as in Equations (4) and (5). According to Table 3, it can9 of be 11 inferred that when the temperature increased from 700 °C to 900 °C, the Fe2+ content in the samples increaseda gas–solid from reaction, 0 to 15.80%, and good but conditions the content were of M providedFe increased for from the di 0.15ffusion to 0.93%. of CO When in the the samples. temperature In this increased to above 900 °C, the interfacial chemical reaction transformed from a solid–solid reaction way, the metallization rate of iron increased rapidly in the range of 900–1100 ◦C. to a gas–solid reaction, and good conditions were provided for the diffusion of CO in the samples. In this4.2. Characteristicsway, the metallization of Silicon rate Reduction of iron increased rapidly in the range of 900–1100 °C. The reduction process accomplished by the ferrosilicon alloy is a solid–solid reaction, and the 4.2. Characteristics of Silicon Reduction rate-controlling step of this reaction potentially includes the following possible situations: an interfacial chemicalThe reduction reaction, theprocess diffusion accomplished of reactants by throughthe ferrosilicon the product alloy layer,is a solid–solid and an interfacial reaction, chemical and the rate-controllingreaction and diff usion.step of The this ferrosilicon reaction potentially reduction isincludes a strong the exothermic following process, possible which situations: is different an interfacialfrom the carbon-thermal chemical reaction, reduction the diffusion reaction. of Thereac heattants releasedthrough bythe this product reaction layer, locally and promotesan interfacial the chemicalfusion of thereaction sample. and According diffusion. toTh Figuree ferrosilicon9, the disappearance reduction is a of strong the interface exothermic between process, the particleswhich is differentin the sample from starts the carbon-thermal at 800 ◦C, and the reduction sintering reacti phenomenonon. The heat occurs released in the sameby this neighborhood. reaction locally As promotesshown in Figurethe fusion5B4, of the the whole sample. sample According melted to after Figure further 9, the increases disappearance in temperature. of the interface between the particles in the sample starts at 800 °C, and the sintering phenomenon occurs in the same neighborhood. As ZnOshown(s) +inFe Figu(s) =re Zn5B4,(g) +theFeO whole(s)∆ Gsampleθ = 201066.8 melted 138.33after Tfurther increases(12) in 10 − temperature.

Figure 9. The microstructure of samples in B2 reduced at 800 ◦°C.C.

In this way, ZnO should be successfully reduced to elemental zinc, and good conditions for the ZnOs +Fes =Zng +FeOs ∆𝐺 = 201066.8 − 138.33𝑇 (12) internal structure of the sample are required for the evaporation of zinc in EAFD. In the initial reaction, the contactIn this isway, good ZnO between should the be EAFDsuccessfully and ferrosilicon reduced to alloy, elemental and a zinc, large and reaction good interfaceconditions exists. for the In internalthis way, structure the interfacial of the reductionsample are reaction required can for be carriedthe evaporation out smoothly. of zinc When in EAFD. the temperature In the initial is reaction,lower than the 800 contact◦C, the is reactiongood between rate of the ZnO EAFD in the and EAFD ferrosilicon reduced alloy, by silicon and a is large slow, reaction and the interface reaction exists.is carried In outthis relatively way, the slowly. interfacial However, reduction the instantaneous reaction can heatbe carried released out by thesmoothly. reduction When reaction the temperatureof ferrosilicon is changeslower than the 800 internal °C, the structure reactionof rate the of sample ZnO in with the EAFD an increase reduced in temperature.by silicon is slow, The andreduction the reaction reaction is cancarried occur out smoothly, relatively and slowly. a good However, condition the for instantaneous the escape of heat Zn (g)released still remains. by the Therefore,reduction thereaction evaporation of ferrosilicon rate of zinc changes is 35.26% the intern at 800 al◦C, structure far higher of than the thesample 0.50% with of the an reductionincrease in of temperature.EAFD by carbon The powder. reduction However, reaction SiOcan2 ,occur the oxidation smoothly, product and a good of silicon, condition gradually for the increased escape andof Zn a (g)silicate-phase still remains. layer Therefore, formed duringthe evaporation the preceding rate of reaction, zinc is 35.26% as shown at 800 in Figure °C, far5 .higher The ferrosilicon than the 0.50% and ofEAFD the particlesreduction are of separated EAFD by by carbon this product powder. layer, However, so further SiO reduction2, the oxidation is limited. product At the sameof silicon, time, graduallythe samples increased started toand be a sintered silicate-phase and melted, layer andform theed porosityduring the gradually preceding decreased, reaction, which as shown was not in Figureconducive 5. The to theferrosilicon diﬀusion and and EAFD the evaporation particles are of sepa the reductionrated by this product product (zinc). laye Ther, so evaporation further reduction rate of iszinc limited. in sample At the B6 same is 90.47%, time, whichthe samples is less started than that to be of samplesintered A6, and 99.80%. melted, According and the porosity to the test gradually results, even though the reactants diﬀused very slowly through the solid product layer, the reduction and evaporation of zinc still occurred slowly and mainly depended on the reduction of the iron particles, as shown in Equation (12) [19,20]. On the basis of Figure4 and Table3, the relationship between the metal iron and FeO content suggests that ZnO is reduced by part of the metallic iron that is reduced via silicon during the heating process. Appl. Sci. 2019, 9, 3604 10 of 11

5. Conclusions (1) When carbon is used as the reducing agent, the evaporation of zinc mainly occurs in the temperature range of 900–1100 ◦C. For the ferrosilicon reduction, the evaporation rate of zinc increased from 700 to 1200 ◦C and is only 90.47% at 1200 ◦C. The residual zinc in the samples mainly exist in the form of a spinel solution at 1200 ◦C. (2) The beginning temperature of the metallization of the iron is 900 ◦C, and the percentage of the metallization is 78.67% at 1200 ◦C for carbon reduction. When ferrosilicon is used as the reducing agent, the variation of metallic iron content in the sample is complicated with the temperature rising due to the reaction between zinc oxide and metallic iron at a high temperature. (3) The microstructure of the samples changed with an increase in temperature. When the carbon is the reducing agent, the relatively independent state among the particles is still preserved in the sample at 1100 ◦C, which provides a good condition for the diﬀusion of CO and zinc (g). The sinter of the samples starts locally at a relatively low temperature, and the diﬀusion of Zn (g) is hindered during the ferrosilicon-reduction process.

Author Contributions: M.Z. drafted the manuscript and conducted the experiments, Q.M. and Q.Z. helped to carry out the experimental work, and J.L. analyzed the experimental data and modified and polished the draft. Funding: The work was supported by the National Natural Science Foundation of China (No. 51974210), the Hubei Provincial Natural Science Foundation (No. 2019CFB697), the China Postdoctoral Science Foundation (No. 2014M562073), and the State Key Laboratory of Refractories and Metallurgy. Acknowledgments: The authors would like to thank the anonymous reviewers for their constructive comments and suggestions. Conflicts of Interest: The authors declare no conflicts of interest.

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