High Temp. Mater. Proc. 2015; 34(2): 177–184

Yun Zhou, Liushun Wu*, Jue Wang, Haichuan Wang and Yuanchi Dong Separation of ZnO from the Stainless Steelmaking Dust and Graphite Mixture by Microwave Irradiation

Abstract: In this study, microwave was used to treat stain- been developed to process ZnO-bearing dust [1–5]. The less steelmaking dust containing . The effects methods can be classified into three groups: physical of heating time, content and zinc oxide content method, wet process ( process and alkaline process), on the removal efficiency of zinc oxide and the reduction pyrogenic process. Physical method, including magnetic efficiency of oxide were investigated. Experimental separation, floatation separation and gravity concentra- results show that, for the sample with 16% (mass percent, tion has been developed in early years. After the dust is the same below) graphite heated for 10 minutes by 10 kW processed by physical method, the removal efficiency of power microwave, the removal efficiency of zinc oxide is zinc oxide is between 50%–60% (mass percent, the same between 80% and 90% and the metallization ratio of iron below) [6], and part of zinc oxide still remains in the dust. oxide is between 40% and 60%; Initial zinc oxide content The removal efficiency of zinc oxide doesn’t reach expec- has a slight effect on the removal efficiency of zinc oxide. tation. Besides, lots of is required for its effective The results indicate microwave treatment is one of the fea- operation. For wet process, the essence of ZnO removal is sible ways to process metallurgical solid waste containing that zinc oxide in the dust reacts with acid or base to form the metal with low . the compounds dissolved into solution, and then the solu- tion is separated from residual by filtration. However, Keywords: microwave heating, stainless steelmaking during zinc oxide leaching, a great deal of iron oxide dust, carbon thermal reduction, zinc removal becomes soluble iron, which to low utilization of iron in dust (the part of iron can’t be recycled by PACS® (2010). 05.70.-a, 07.57.-c plant), so this technique is not suitable for the dust with low content of ZnO [7–9]. For low-grade ZnO-bearing dust DOI 10.1515/htmp-2013-0118 with high content of carbon, pyro-metallurgy technique Received November 22, 2013; accepted April 26, 2014; which is based on carbothermic reduction reaction is the published online May 28, 2014 most common way [10]. The technique can be subdivided into three groups: rotary hearth furnace, fluidization and rotary kiln which is a representative of pyro-metallurgy process. However, the methods share a disadvantage – 1 Introduction high energy consumption. To develop an environment-friendly and low energy During the smelting of stainless steel, dust generates inev- consumption technique, recently some researchers suc- itably. The dust is mainly composed of iron oxide, zinc cessfully separated zinc oxide from Zn-bearing dust using oxide, oxide, oxide, silica, and so on. the microwave processing [11–17]. Compared with other Except for zinc oxide, the other are effective consti- heating methods, microwave have better heating proper- tutes for stainless smelting. Therefore the removal of zinc ties, such as fast heating rate, selective heating. Besides, oxide from the dust makes it possible to recycle in stain- the microwave heating overcomes the barrier of tradi- less steel mill. In recent decades, some methods have tional heating model-heat transmission from external to internal. In addition, the microwave heating has two ad- vantages in the processing of ZnO-bearing stainless steel *Corresponding author: Liushun Wu: School of Metallurgical dust: rapid heating (Since the dust contains some magnet, Engineering, Anhui University of Technology, Maanshan, Anhui it has a strong ability to absorb microwave) and lower 243002, China. E-mail: [email protected] Yun Zhou, Jue Wang, Haichuan Wang, Yuanchi Dong: School reduced processing temperature (It is generally believed of Metallurgical Engineering, Anhui University of Technology, that microwave heating may lower reaction activation Maanshan, Anhui 243002, China energy [18].) 178 Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust

Table 1: Chemical compositions of EAF dusts (mass%) Table 2: Particle sizes of Zn-bearing stainless steelmaking dust (μm)

Oxides Carbon steel Stainless steel Cumulative percent Particle size dust (EAF) dust (EAF) D10 8

Fe2O3 56.83 48.69 D25 16

FeO 4.58 9.85 D50 37

MFe – – D75 68

ZnO 4.86 3.45 D90 95 CaO 14.72 12.06 Note: D10, D25, D50, D75, D90 and Dav are cumulative SiO 5.19 4.06 2 percentages of particle sizes at 10%, 25%, 50%, 75%, 90% Al O 1.27 – 2 3 and average particle size, respectively. MgO 6.46 –

Cr2O3 – 16.66 NiO – 2.96 Others 6.09 2.27 Table 3: The ratio of the dust to graphite in mixture (mass%)

EAF dust Graphite

92% 8% 88% 12% The above mentioned researches focused on the con- 84% 16% ventional Electric Arc Furnace (EAF) steelmaking dust. Compared with the conventional EAF steelmaking dust, the stainless steelmaking dust has a higher content of 2+ 3+ Cr2O3 and a better ratio of Fe and Fe which is closer to 76 μm. Besides, the screen underflow was also analyzed 2+ 3+ the ratio of Fe and Fe in Fe3O4 (see Table 1). The iron by a Laser Granulometer (S3500, American Microtrac oxide combines with ZnO or Cr2O3 to form spinel (the Company). Table 2 shows the size distribution of screen has a high ), and the difference in underflow. The sizes of the particles are between 37 μm to the ratio of Fe2+ and Fe3+ may means the difference in the 68 μm. content of Fe3O4 (Fe3O4 shows excellent heating behavior To study the effect of graphite content on the removal than FeO or Fe2O3, as seen in Table 5). This may to efficiency of ZnO, graphite was added to the dust to obtain the difference in heating and reduction behavior of the different graphite-dust mixtures. To study the effect of stainless steelmaking dust from that of the conventional zinc oxide content on the removal efficiency of ZnO, the EAF steelmaking dust. Therefore, the heating and reduc- different ZnO-graphite-dust mixtures were prepared by tion behavior of the stainless steelmaking under micro- mixing the graphite-dust mixture and different amount of wave irradiation will be investigated in this study. The zinc oxide. study will give guidance for effective utilization of stain- In a general run, a predetermined amount of graphite less steelmaking dust containing zinc oxide. (<76 μm) was added into the stainless steelmaking dust (the ratios of the dust to graphite are shown in Table 3) or a predetermined amount of zinc oxide was added into the 2 Experimental graphite-dust mixture. The mixture (50 g) was well mixed in a pot mill. After dried for 4 hours, the mixture in a The chemical composition of the stainless steelmaking quartz crucible (inner diameter: 50 mm, height: 60 mm) dust and carbon steel dust from Bao Steel Co. Ltd. in China was heated in a microwave furnace (maximum power: was analyzed by an ISP Emission Spectrum (type: Thermo 10 kW, frequency: 2450 MHz). Figure 1 shows a schematic Elemental-IRIS Intrepid) and titration (Analysis result is diagram of the microwave furnace. gas was shown in Table 1). The morphology and mineral structure used as protection gas to enhance reduction atmosphere of the dust were analyzed by a Scanning Electron Micro- (0.1 L/min). A double platinum thermocouple scope (JSM-6490LV) equipped with an Energy Dispersive (Pt – 6% Rh / Pt – 30% Rh) was inserted into the sample to X-ray Spectroscopy (EDX). measure temperature. The thermocouple was protected To determine the distribution of particle size, the by two layer pipes: inner layer of alumina pipe and outer dust was screened by a sieve with 200 meshes, and 85% layer of stainless steel (Type: 316L) pipe. Since stainless of samples passed through the mesh. The result indi- steel had a strong ability to reflect microwave, the protec- cates that 85% of particles in the dust are smaller than tive pipe could effectively protect the interruption on the Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust 179

Fig. 1: Schematic diagram of microwave furnace

temperature measurements by the microwave. After pro- 3 Results and discussion cessing, shutting off the furnace and increasing the flow rate of nitrogen gas (5 L/min), and then moving the sample Figure 2 presents SEM image of the dust and Table 4 shows along with the crucible out of the furnace to quench. the chemical compositions of the particles in the dust. As Usually, the sample could be cooled to 800 K in 2 minutes. shown in Fig. 2 and Table 4, iron oxide exists in most of The quenched sample was collected for analysis. the particles in the dust and its content is above 30%. Zinc The phases in the sample were analyzed by a Scanning oxide only exists in the big particles in the dust. The differ- Electron Microscope. The amount of metallic iron in the ence in the distribution of the oxides may be caused by the sample was analyzed by dichromate titration difference in the source of the oxides. During EAF smelt-

(Metallic iron was leached using CuSO4 solution or FeCl3 ing, due to the splashing of liquid iron, a few liquid iron solution, and leach solution and residual were separated is oxidized on the surface of EAF to form iron oxide. Then by filtration. The leach solution was used to determine the the iron oxide goes into dust removal device. Zinc turns content of metallic iron in the dust. The residual was dis- solved by solution. The dissolved solu- tion was used to analyze the content of FeO and Fe2O3), and the content of ZnO in the sample was analyzed by ISP Emission Spectrum (type: Thermo Elemental-IRIS In- trepid). The reduction efficiencies of zinc oxide and iron oxide were calculated by Eq. (1) and Eq. (2).

Zninitial− Zn final η=Zn ×100% (1) Znfinal

MFe η=Fe ×100% (2) TFe

where Zninitial is the initial zinc content before processing,

Znfinal is the zinc content after processing, TFe is the total iron content before processing, and MFe is the metallic iron content after processing. Fig. 2: SEM image of ZnO-bearing stainless steelmaking dust 180 Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust

Table 4: Chemical compositions of each particle shown in Fig. 2 (EDX analysis, mass%)

Phase O Ca Cr Ni Fe Zn Mn Mg Si

A 31.22 10.85 9.55 – 40.97 5.82 1.60 – – B 39.09 3.30 7.18 – 34.46 8.25 1.23 3.28 1.47 C 45.42 21.34 5.23 1.65 10.93 2.76 – 9.11 3.56 D 32.89 22.08 1.76 4.55 33.50 2.32 – – 1.97 E 23.69 0.67 7.72 5.77 56.45 5.69 – – – F 61.76 18.29 0.91 – 3.13 – – 12.70 1.76 G 23.84 – 1.74 4.70 69.71 – – – – H 21.88 0.59 8.84 – 68.69 – – – – I 38.38 0.49 34.67 – 26.46 – – – – J 34.24 – 8.57 6.26 50.93 – – – – K 47.44 4.31 6.28 1.80 31.97 1.89 1.35 3.68 1.29

into vapor (since boiling point of zinc is about 1180 K, zinc slowly first and then rapidly rises up to a high tempera- easily becomes vapor at steelmaking temperature), which ture, thereafter slowly goes up. rises along with iron oxide. During zinc vapor rising, most According to Fig. 3, the heating process can be divided of zinc is oxidized by , and water into three stages. The relation between the heating rate vapor in the upper part of EAF or inside flue to form zinc and the heating time can be described by the mathematic oxide. The zinc oxide is adsorbed on the surface of the iron model shown in Fig. 4, which is similar to the model of oxide particles. This is the reason for zinc oxide existing in carbon removal during converter steelmaking. bigger particles. Other oxides, such as , silica and oxide come from slag or refractory. In dT The first stage: = kt (3) short, the oxides in the dust come from different sources. dt 1 dT The second stage: = k 2 (4) 3.1 The relation between heating rate dt dT with heating time The third stage: = −kt (5) dt 3 Fig. 3 presents the relation between the temperatures of the mixture with heating time. As shown in Fig. 3, for the where k1, k2 and k3 are the coefficients, T is the tempera- mixture heated by microwave, the temperature rises ture, and t is the heating time.

Fig. 3: The relation between heating rate with heating time Fig. 4: Schematic diagram of the model for the heating process Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust 181

The first stage (0–1.5 minutes) is the incubation period It is generally believed that conductivity has a posi- to mobilize the sample. The heating rate increases linearly tive effect on heating rate, and heat capacity and with the heating time. For the mixture of 92% the dust have negative effects on heating rate [20]. However, for and 8% graphite, the heating rate increases from 0 to high conductivity material, for example metal, microwave 510 K/min. For the mixture containing 12% graphite, the cannot go deep into the material because microwave is heating rate increases from 0 to 428 K/min. For the mixture ­reflected easily by the surface of the metal, which results containing 16% graphite, the heating rate increases from in a great loss in energy. Low conductivity material has a 0 to 382 K/min. weak capacity to absorb microwave. The two kind of mate- The second stage (1.5–3 minutes) is the rapid heating rial cannot be heated to a higher temperature at higher period. The temperature of the mixture rises linearly with heating rate under microwave irradiation. The conductiv- heating time. The mixture of 92% the dust and 8% graph- ity of graphite is between that of the two kinds of the ma- ite was heated up to 1233 K at the rate of 510 K/min, the terials above mentioned. It has a better capacity to absorb mixture containing 12% graphite was heated to 1100 K at microwave, so the temperature rises rapidly under micro- the rate of 428 K/min, and the mixture containing 16% wave irradiation. Besides, graphite has a smaller thermal graphite was heated to 955 K at the rate of 382 K/min. The capacity (710 J/(kg·K)) than iron oxide and calcium oxide results also indicate that graphite content has a positive (about 1700–2100 J/(kg·K)) which are the main compo- effect on the heating rate. nents in the dust [21]. In addition, graphite possesses a The third stage (3 minutes) is the slow heating period. performance density (ρ), which is beneficial to the rapid Heating rate decreases with heating time. For the mixture heating. In the case, the conductivity of the mixture in- containing 8% graphite, the heating rate decreased from creases, and performance density and thermal capacity 72.9 to 0 K/min in 10 minutes. For the mixture containing decreases with the content of graphite, which improve the 12% graphite, the heating rate decreased from 97.2 to ability to absorb microwave. In short, the heating rate 0 K/min in 10 minutes. For the mixture containing 16% ­increases with graphite content. graphite, the heating rate decreased from 120.6 to 0 K/min in 10 minutes. In a word, the temperature rises up slowly with heating time. 3.2 Reduction of ZnO-bearing stainless It is generally believed that the heating rate is deter- making dust mined by the absorption microwave capacity of material. Table 5 [19] shows the heating rates of some minerals The ZnO-bearing stainless steelmaking dust contains many under the same power microwave irradiation. As seen in metallic oxides, such as iron oxide, zinc oxide and chro- Table 5, compared with the other oxides in the dust, graph- mium oxide above mentioned. They were all involved in ite has a biggest heating rate under microwave ­irradiation, reduction reaction during microwave heating. The equa- i.e., it has the strongest capacity to absorb ­microwave. tions on zinc oxide reduction are shown below (some

Table 5: Heating effect of MW of minerals and compound [19]

Mineral and compound Chemical composition Temperature (K) Time (s) ΔT/Δt (K/s)

Graphite (<200 mesh) C 1053 360 2.10 hematite Fe2O3 455 420 0.37 magnetite Fe3O4 1026 150 4.85 ferric oxide Fe2O3 407 270 0.40 calcium oxide CaO 449 150 1.01 chromic oxide Cr2O3 403 420 0.25 aluminum oxide Al2O3 430 150 0.88 zinc sulphide ZnS 340 180 0.23 manganous oxide MnO 386 360 0.24 MgO 362 150 0.43 quartz SiO2 346 150 0.32 cassiterite SnO2 1480 60 19.7 stannic oxide SnO2 1010 150 4.75 182 Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust equations were cited from literature [22] and some based on thermodynamics calculation).

3ZnFe2O4(s) + 4C(s) → ZnO(s) + 2Fe3O4(s) + 4CO(g) (7) ZnO(s) + C(s) → Zn(g) + CO(g) (8)

ZnO(s) + CO(g) → Zn(g) + CO2(g) (9)

C(s) + CO2(g) → 2CO(g) (10)

The reduction reaction of iron oxide and chromium oxide are similar to the reactions involving ZnO, shown as follows.

Fe3−xCrxO4(s) + (1 − x/2)C(s)

→ (x/2)Cr2O3(s) + (3 − x)FeO(s) + (1 − x/2)CO (11) Fig. 5: The relation between the weight loss ratio with heating time

FeOx(s) + xC(s) → Fe + xCO(g) (12)

FeOx(s) + xCO(g) → Fe + xCO2(g) (13)

Cr2O3(s) + 3C(s) → 2Cr + 3CO(g) (14)

Cr2O3(s) + 3CO(g) → 2Cr + CO2(g) (15)

Based on thermodynamics, the formation of the spinel phases Fe3−xCrxO4 and ZnFe2O4 leads to the decrease in the activities of ZnO and FeOx, which in turn influences the equilibrium yield of Zn and Fe. Relatively, spinel phase has a less impact on the reduction of ZnO as Zn go away along with other gases. Based on kinetics, the existing of the spinel phases result in the increase in the melting ­temperature of dust and the viscosity of liquid phase (the spinel phases have high melting point), which hinders mass transport, decreasing the rate of carbothermal re- Fig. 6: The relation between the reduction efficiencies of ZnO and duction. FeOx with heating time Fig. 5 shows the relationship between the weight loss ratio and the heating time during the reduction of ZnO- bearing stainless steelmaking dust heated by microwave. As shown in Fig. 5, the weight loss curve is similar to the heating curve shown in Fig. 4, only having a time lag. As seen in Fig. 4, under the irradiation of 10 kW power micro- wave, the temperature of the mixture rises up quickly to a comparatively high value in about 3 minutes. However, the weight loss ratio reaches a similar stage in 8 to 10 minutes. The variation of weight loss ratio lags behind that of the temperature of the mixture about 5 to 7 minutes, which implies that the rate-determining step is not the chemical reaction step but the spread step of the reducing agent during the reduction. Fig. 6 shows the relation between the reduction effi- ciencies of zinc oxide and iron oxide with heating time. Fig. 7 shows the image of the collected Zn-product. Table 6 lists the chemical composition of the collected Zn-product. Fig. 7: The collected Zn-product Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust 183

Table 6: Chemical composition of the collected Zn-product

Components Content (mass%)

ZnO 99.13 PbO 0.34 Others 0.53

As seen in Fig. 6 (Note: Reduction reaction takes place at the temperature of less than 800 K which is lower than conventional reduction temperature of iron oxide. The possible reason for it is that microwave heating lowers the reaction activation energy or microwave heating leads to the uneven temperature), the reduction process of zinc oxide could be divided into two stages. At the first stage Fig. 8: SEM image of the reduced dust (3–7 minutes), the reduction efficiency rapidly increases to a high value of 83.7% at the rate of 14.2% per minute. At the second stage, the reduction efficiency increases slowly with heating time at the rate of 1.2% per minute. The quick Table 7: The composition of minerals in the dust heated for 10 min reduction stage corresponds to the rapid heating stage (EDX) shown in Fig. 5. The reason for the quick reduction of zinc oxide is that, zinc oxide is reduced to gaseous zinc, break- Components A (mass%) B (mass%) C (mass%) ing away quickly from reaction zone, which leads to the Fe 98.67 45.39 12.67 decrease in the partial pressure of zinc vapor. This in turn Zn – – 1.33 promotes the reduction reaction of zinc oxide based on O – 53.55 56.38 the theory of Le Chatelier. Si 0.62 – 13.12 Ca – 1.06 11.01 The relationship between equilibrium Al 0.53 – 3.07 of zinc and temperature is expressed by formula (16) [23]. C 0.18 – –

6697 lg p =−−1.21 lg T + 12.247 (16) Zn T

Calculation results by Eq. (12) show that the vapor 3.3 Influence of the initial zinc content pressure of zinc is 7.69 × 103 Pa at 1180 K and equilibrium on reduction vapor pressure is 2.31 × 105 Pa at 1273 K. This means that liquid zinc easily turns into vapor at high temperature Fig. 9 presents the relation between the removal efficiency

[24]. Besides, zinc vapor readily escapes from reaction of ZnO and the reduction efficiency of FeOx with the initial zone with CO and CO2 together, which leads to the de- ZnO content in the mixture. As shown in Fig. 9, with the crease in the partial pressure of zinc vapor. The decrease initial content of zinc oxide in the mixture increasing, the of zinc vapor pressure enhances the driving force for re- removal efficiency of zinc oxide decreases from 86.7% to duction reaction of zinc oxide. 81.4%, and the reduction efficiency of iron oxide decreases After reducing, part of iron oxide turns into iron parti- from 53.8% to 34.1%. The results imply that zinc oxide in cles (see Fig. 8). The composition of the iron particles is the mixture could restrain the reduction of iron oxide. The listed in Table 7. The relation between the reduction effi- possible reason for this is that, under microwave irradia- ciency of iron oxide and heating time is also shown in tion, zinc oxide preferentially reacts with carbon monox- Fig. 6. As seen in Fig. 6, after heating for 20 minutes, the ide to form zinc vapor, which escapes from the surface of reduction efficiency is about 80%, which implies the re- the particles in the dust. The reduction reaction of zinc duction reaction of iron oxide is controlled by the spread oxide consumes much , which results in of reduction agent-carbon. With much carbon monox- the decrease in partial pressure of carbon monoxide. This ide forming, indirect-reduction of iron oxide accelerated in turn leads to the decrease in the reduction efficiency of greatly. iron oxide. 184 Y. Zhou et al., Separation of ZnO from the Stainless Steelmaking Dust


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