Phase evolutions, microstructure and reaction mechanism during calcification roasting of high chromium vanadium slag

Accepted Manuscript Phase evolutions, microstructure and reaction mechanism during calcification roasting of high chromium vanadium slag Tao Jiang, Jing Wen, Mi Zhou, Xiangxin Xue PII:

S0925-8388(18)30202-0

DOI:

10.1016/j.jallcom.2018.01.201

Reference:

JALCOM 44665

To appear in:

Journal of Alloys and Compounds

Received Date: 19 June 2017 Revised Date:

11 January 2018

Accepted Date: 14 January 2018

Please cite this article as: T. Jiang, J. Wen, M. Zhou, X. Xue, Phase evolutions, microstructure and reaction mechanism during calcification roasting of high chromium vanadium slag, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.201. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Phase evolutions, microstructure and reaction mechanism during

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calcification roasting of high chromium vanadium slag

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Tao Jiang a,b,∗ , Jing Wen a, Mi Zhou a,b, Xiangxin Xue a,b

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a

School of Metallurgy, Northeastern University, Shenyang, 110819, Liaoning, China

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b

Liaoning Key Laboratory for Recycling Science of Metallurgical Resources, Shenyang 110819, Liaoning, China

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Abstract

Excess CaO was added to high chromium vanadium slag (HCVS) to systematically and

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comprehensively investigate the reaction mechanisms during calcification roasting. Samples roasted at

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specific temperatures according to thermal analysis (TG-DSC) were analyzed by XRD, FT-IR, SEM,

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XPS and leaching experiments from different perspectives. Reaction thermodynamics and kinetics were

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employed to demonstrate the mechanism theoretically. The results show that dehydration, oxidation of

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free iron and olivine, and preliminary and further oxidation and calcification of spinel were the major

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steps during calcification roasting. The formation of calcium-containing components followed the order

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of calcium vanadate > calcium chromate > calcium silicate, both practically and theoretically. Normal

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V-spinel in HCVS converted to inverse V-spinel at lower temperatures and then combined with CaO to

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generate Ca2V2O7. At higher temperatures, the final oxidation and calcification product of V-spinel with

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excessive CaO was Ca3V2O8, which was controlled by reaction kinetics rather than thermodynamics.

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Cr-spinel was more stable than V-spinel, which reacted with CaO and gradually formed CaCrO4 and

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Ca3(CrO4)2. With further temperature increases, Cr tended to conjugate to Fe to form acid-insoluble

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solid solutions (Fe0.6,Cr0.4)2O3 instead of reacting with CaO. Oxidation and calcification of spinel were

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controlled by a 2.50 order reaction. The overall apparent activation energy Ea and frequency factor A

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were 157.34 kJ•mol−1 and 4.23×107 min−1, respectively.

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Key words

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high chromium vanadium slag; calcification roasting; oxidation and calcification mechanism; phase

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evolutions; thermodynamics and kinetics

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1. Introduction

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Vanadium is one of the most important rare metals, which is growing more and more attractive to

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the modern technology and industry[1-5]. The vanadium-containing compounds such as VO, V2O5 and

∗

Corresponding author. E-mail address: [email protected] (T. Jiang).

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so on have also been studied and utilized extensively [6-9]. Vanadium slag, as the direct source for ACCEPTED MANUSCRIPT

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vanadium extraction, has been made available for the industrial production for many years[10]. It was

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produced during the vanadium-titanium bearing magnetite smelting[11, 12]. In the smelting process with

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high-chromium vanadium-titanium bearing magnetite, chromium, which has similar chemical

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characteristics with vanadium would occupy the vanadium lattices as isomorphism and form high

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chromium vanadium slag (HCVS)[13]. While HCVS has not been utilized to extract vanadium on a

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large scale until now due to the research on the transformation of valuable components containing

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vanadium and chromium is immature. Moreover, large scale high-chromium vanadium-titanium bearing

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magnetite as Hongge ore was discovered in Panzhihua in China, so it is urgent to make research on the

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reaction mechanism of HCVS systematically and comprehensively.

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As known to all, extracting vanadium from vanadium slag directly is difficult, because most of

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vanadium is concentrated on the stable spinel phase, which is dispersed in olivine phase. Roasting as the

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most commonly used method for the activation of vanadium slag can not only break down the olivine

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and spinel structure, but also promote oxidation process and form high valence vanadium. At present,

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sodium salt roasting is the most generally used technology with low cost and sample process, while the

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generation of harmful gas and materials sintering hinder its further development. Calcification roasting

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as an environmentally friendly method gradually attracting more attention, but the yield of vanadium is

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lower than sodium salt roasting. The blank roasting is limited by the components of the raw materials.

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Furthermore, several works have been undertaken to study the roasting mechanism of vanadium spinel

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and vanadium slag. Van Vuuren et al.[14, 15] revealed the oxidation kinetics of FeV2O4 in the range

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200-580 oC and the oxidation behavior of FeV2O4 by oxygen in the presence of Na2CO3. Li et al.[16]

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studied the vanadium oxidation by roasting high calcium vanadium slag with no addition. For the

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oxidation behavior of chromium in vanadium slag, Wang et al.[17] revealed that chromium (III) would

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be oxidized to the soluble sodium chromate (VI) during sodium salt roasting. Zhang et al.[12] studied

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the effects of blank roasting on oxidation behavior of chromium using vanadium slag with high

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chromium content. Besides, Zhang et al.[18] investigated the reaction mechanism on lime-free roasting

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of chromium-containing slag. While there is no work investigating the oxidation and calcification

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mechanism of HCVS during calcification roasting systematically. Furthermore, calcification roasting as

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an environmentally friendly method becomes the alternative to sodium salt roasting gradually, which is a

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valuable and potential method for the pretreatment of vanadium slag. Hence, investigating the

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calcification roasting mechanism with HCVS is essential.

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In this work, a thermal analytical study combined with characterizations including XRD, FT-IR,

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SEM and XPS was conducted to study the phase evolutions and microstructure of HCVS during

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calcification roasting. Oxidation and calcification mechanism was acquired and confirmed by reaction

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thermodynamics and kinetics.

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2. Experimental

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2.1. Materials

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The raw material, HCVS was provided by Chengde Jianlong Iron and Steel Co., Ltd., Chengde,

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China. The chemical compositions and the X-ray diffraction patterns were shown in Table 1 and Fig. 1,

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respectively. Results show that chromium content in this slag is larger than other vanadium[19, 20].

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Vanadium and chromium are concentrated on spinel phases (Mn, Fe)(V, Cr)2O4. Silicon exists in the

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form of olivine phase (Fe, Mn)2SiO4, and some free metallic iron is also present in HCVS. Besides,

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XRD of calcium source CaO (dried at 105 °C for 24 h) was also listed in Fig. 1. Little Ca(OH)2 appears

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because CaO tends to absorb water molecules in air, which would be decomposed during roasting.

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Fig. 2 showed the bonding structures of HCVS and CaO with FT-IR spectra. The absorption bands

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of HCVS and CaO at 3430 cm-1 and 1634 cm-1 are attributed to the O-H stretching vibrations and

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H-O-H bending vibrations of water molecules absorbed on the surface. In HCVS, the strong bands at

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475 cm-1 and 595 cm-1 are the characteristic peaks of spinel, which are related to the crystal lattice

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vibration of aggregative octahedron. It is worth noting that there should theoretically be four vibration

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absorptions of normal spinel [21], while some bands with lower frequencies are in the far infrared region

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and others would be consolidated by stronger bands of other groups, which can not be shown in Fig. 2.

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Bands at 825 cm-1, 875 cm-1, 915 cm-1 and 964 cm-1 are assigned to the vibration of [SiO4] tetrahedron

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of olivine phase, and other bands existing theoretically are far infrared bands or consolidated by stronger

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bands of other groups[22]. Band at 1046 cm-1 is attributed to the Si-O-Si asymmetric stretching

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vibrations. And the specific absorption bands of CaO at 3642 cm-1 and 1414 cm-1 are narrow, which are

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consistent with previous works[23].

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Table 1

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Chemical composition of HCVS (wt.%).

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TFe

FeO

MFe

V 2O 5

Cr2O3

TiO2

SiO2

Al2O3

CaO

MnO

MgO

P2O5

33.82

23.50

4.52

13.72

9.19

10.45

14.62

1.30

1.67

6.73

1.17

0.12

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2.2. Procedure

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In order to ensure the complete reaction of CaO and HCVS, excess CaO was added after calculation

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according to the chemical composition of HCVS listed in Table 1. The vanadium (as V2O5) and

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chromium (as Cr2O3) in 10 g of HCVS would consume around 1.2 g and 0.6 g of CaO, respectively, in a

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complete reaction. To account for other secondary reactions such as the formation of calcium silicate,

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2.2 g CaO was mixed with 10 g HCVS. Dynamic heating experiments (TG-DSC) were performed at a

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heating rate of 10 °C/min from room temperature to 1000 °C. The changes in mass, along with the

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exothermic and endothermic peaks, directly reflect the specific reactions that occur during calcification

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roasting. The typical temperatures of 340 °C, 420 °C, 570 °C, 630 °C, 750 °C, 850 °C, 900 °C, and 980

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°C were chosen as the roasting temperatures according to the TG-DSC curves shown in Fig. 3. Mixtures 3

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of HCVS and CaO were put in a muffle furnace MANUSCRIPT at room temperature and heated to the above eight ACCEPTED

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predetermined temperatures at a rate of 10 °C/min. Each temperature was maintained for two hours to

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ensure thorough reactions. During roasting, the door of the muffle furnace was not fully closed so that a

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continuously oxidizing atmosphere was maintained. After roasting, samples were cooled down in the

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furnace and ground to powders (< 75 µm). Then, a series of characterizations, including XRD, FT-IR,

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SEM and XPS, along with leaching experiments, were carried out to analyze the roasted samples.

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2.3. Characterization

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HCVS chemical compositions were detected by Inductively Coupled Plasma-Atomic Emission

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Spectroscopy (ICP-AES, PerkinElmer Optima-4300DV) (except for FeO and MFe, which were

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determined by chemical analysis). Thermal gravimetry (TG) and differential scanning calorimetry

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(DSC) analysis were carried out with SDT-Q600 Simultaneous DSC-TGA. Phase compositions of

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roasted samples were identified by X-ray diffraction analysis (X’PERT PRO MPD/PW3040,

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PANalytical B.V. Corporation, Netherlands) using Cu Kα radiation from 5o to 90o. FT-IR spectra of

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samples were obtained by FT-IR spectrometer (Nicolet-380, Thermo Corporation USA). Microscopic

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observation and analysis of element distribution in samples were conducted by scanning electron

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microscopy (SEM, TESCAN VEGA III) equipped with energy disperse X-ray spectrometry (EDS,

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INCA Energy 350). X-ray photoelectron spectroscopy (XPS) experiments were carried out on Thermo

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escalab 250Xi with anode Xray sources Al Ka (hν=1486.6 eV, 150W) to characterize the valence states

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of the main elements.

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3. Results and discussion

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3.1. Phase evolution

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The XRD results for products roasted at different temperatures are shown in Fig. 4(a), with Figs.

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4(b)-(d) showing local amplifications for regions (b)-(d) in Fig. 4(a). At 340 °C, almost all phases are

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consistent with those of HCVS and CaO, meaning that no chemical reaction occurs at this temperature,

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and dehydration of free water on the mixture surface causes slight weight loss. At 420 °C, peaks of

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normal spinel (Mn, Fe)(V, Cr)2O4 at 29.91°, 35.04°, 56.29° and 62.26° cannot be detected, while peaks

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at 30.05°, 35.32°, 56.96° and 62.52° appear as shown in Fig. 4(b), which proves the change of spinel

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structure. Nohair et al.[24] and Gillot et al.[25] believed that normal spinel FeV2O4 would transform to

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cation-deficient vanadium-iron spinels and produce cavity due to the oxidation of partial cation in the

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oxidation process. And Zhang et al.[12] proposed that normal spinel in vanadium slag is oxidized to

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inverse spinel at a low temperature during blank roasting. Our research also demonstrates that the

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structure of cation-deficient vanadium-iron spinel is close to that of inverse spinel. This can be attributed

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to the fact that Fe2+ ions at tetrahedral sites in normal spinel (Fe2+)[V3+2]O4 are partially oxidized to Fe3+

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ions and fill the tetrahedral sites, and other Fe2+ ions with V3+ fill the octahedral sites to form the inverse

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spinel (Fe3+)[Fe2+V3+]O4[26], Here,ACCEPTED the parentheses indicate tetrahedral sites and square brackets MANUSCRIPT

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indicate the octahedral site. Hence, the remarkable weight loss and narrow endothermic peak appearing

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before 420 °C in the TG-DSC curves in Fig. 3 are thought to be caused by the dehydration of water

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bound inside the CaO and HCVS. At 570 °C, the olivine phase (Fe, Mn)2SiO4 disappears completely, and the decomposition products

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Fe2O3 and SiO2 are found, indicating that oxidation and decomposition of olivine occur before 570 °C.

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Meanwhile, the decrease of inverse spinel peaks intensity demonstrates that Fe2O3 derives from the

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overlapping oxidation of spinel and olivine, causing the weight increase apparent in the TG-DSC curves.

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As the temperature increases further, CaO as addition participates in the reactions gradually. At 630 °C,

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there are no obvious differences compared with the peak occurring at 570 °C. Hence, the remarkable

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weight loss and the broad endothermic peak are due to the decomposition of Ca(OH)2, which

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compensates for the increased weight caused by the oxidation of spinel. At 750 °C, the spinel phase is

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almost completely decomposed, and vanadium oxide, V6O13, calcium vanadate, Ca2V2O7, and calcium

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chromate, CaCrO4, are generated according to Fig. 4(c). Moreover, a new phase (Fe0.6, Cr0.4)2O3

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resulting from the combination of FeCr2O4 or Cr2O3 with Fe2O3[27] is detected, the pattern positions of

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which have no apparent change compared with those of Fe2O3 except at about 54 °C. The peaks at

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around 24 °C, 33 °C, 54 °C and so on can be regarded as superimposed peaks of Fe2O3 and (Fe0.6,

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Cr0.4)2O3.

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At 850 °C, calcium silicate CaSiO3 appears, indicating that SiO2 reacts with CaO at higher

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temperatures, and the combination capacity between CaO and the oxides in HCVS follows the order

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V2O5 > Cr2O3 > SiO2. Furthermore, Ca2V2O7 and CaCrO4 disappear, and another calcium vanadate,

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Ca3V2O8, and calcium chromate Ca3(CrO4)2 are found, proving that the final oxidation and calcification

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product of V-spinel is Ca3V2O8 with excessive CaO, which cannot be generated directly. As the roasting

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temperature increases, another calcium chromate, Ca3(CrO4)2, forms instead of CaCrO4, which is

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consistent with previous reports[28]. Additionally, a new phase, Ca3(Fe, Ti)2[(Si, Ti)O4]3, appears.

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Further increases of temperature do not result in obvious changes in phase, as the products roasted at

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900 °C and 980 °C show, while the peak intensities of the main components change gradually according

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to Fig. 4(d). The decreases in the peak intensities of (Fe0.6, Cr0.4)2O3 and CaSiO3, along with the increase

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in the peak intensity of Ca3(Fe, Ti)2[(Si, Ti)O4]3, demonstrate that compounds containing Fe, Cr, Si and

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Ti may recombine at higher temperatures. The peak intensity of Ca3V2O8 increases, indicating that

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oxidation and calcification are more complete. In practical roasting processes, melting of components

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with low melting points leads to materials sintering when roasted at 980 °C, which is unfavorable for

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subsequent processing.

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3.2. Bonding structure changes

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The FT-IR spectra of products roasted at different temperatures are given in Figs. 5(a) and (b),

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which show the absorption bands in the fingerprint region of the infrared spectrum at 1400-400 cm-1. In 5

Fig. 5(a), broad absorption bands at ACCEPTED 3430 cm-1 andMANUSCRIPT 1634 cm-1 are assigned to water molecules attached

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to the surface of roasted products. The narrow bands at 3642 cm-1 and 1414 cm-1 are consistent with that

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of CaO in Fig. 2. When the temperature rises above 570 °C, bands of CaO gradually weaken, which is

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consistent with the XRD analysis. After 850 °C, most CaO has been consumed, while complete reaction

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is achieved after 900 °C. The roasting temperature of fully-reacted CaO according to the FT-IR spectra

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is a little higher than that estimated by XRD, which can be explained by the detection limit sensitivity of

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FT-IR spectra being higher than that of XRD[12]. Hence, structural analysis by FT-IR spectra can be

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regarded as a verification and supplement to the XRD data.

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Almost all characteristic absorption bands are within the fingerprint region of the infrared spectrum

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and depend on the molecular vibrational mode and frequency of the tested minerals[21]. As shown in

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Fig. 5(b), band 1 (475 cm-1) and band 2 (595 cm-1) of normal spinel in the HCVS become broader and

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weaker, and gradually shift toward lower frequencies with increases in roasting temperature. They

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become much closer to the structure of the inverse spinel[24], and then eventually disappear above 1123

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K (850 °C). Bands assigned to the vibration of the [SiO4] tetrahedron of the olivine phase at band 3 (825

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cm-1), band 4 (875 cm-1), band 5 (914 cm-1) and band 6 (962 cm-1) exist at 340 °C and 420 °C, and

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disappear fully after 630 °C. This indicates that olivine is decomposed completely before 630 °C

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(according to the higher detection-limit sensitivity of FT-IR), which corrected the decomposition

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temperature of olivine at 570 °C (by XRD). Furthermore, there are some absorption bands at bands 8

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(around 800 cm-1) above 570 °C, which are characteristic absorption bands of quartz group minerals [24]

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. Additionally, band 7 at 1046 cm-1, which is related to Si-O-Si asymmetric stretching vibrations, shifts

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to band 10 at 1062 cm-1, also demonstrating that the phase changes contain Si-O-Si. At 630 °C, the

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presence of band 11 (878 cm-1) of V2O74- groups proves the occurrence of a Ca2V2O7 phase[29].

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When roasting above 850 °C, band 8 (around 800 cm-1) belonging to SiO2 disappeared, indicating

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that SiO2 reacted with excess CaO to form calcium silicate, CaSiO3. Bands 12–15 at 600–400 cm−1 are

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related to M-O vibration, such as Fe-O from (Fe0.6, Cr0.4)2O3 and Ca3(Fe, Ti)2[(Si, Ti)O4]3. Band 17 at

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896 cm-1 and band 18 at 915 cm-1 are consistent with the asymmetric stretching vibration of [SiO4] in

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Ca3Fe2(SiO4) 3. Additionally, band 16 (680 cm-1) can be attributed to the existence of Ti-tetrahedron,

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demonstrating the isomorphic replacement of Ti with Fe and the formation of Ca3(Fe, Ti)2[(Si, Ti)O4]3.

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Band 11 (878 cm-1) of the V2O74- groups disappears, illustrating the transformation of calcium vanadate.

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Absorption bands 17–19 at 930–800 cm−1 can be assigned to the V-O stretching vibration bands of

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[VO4][30], indicating the generation of Ca3V2O8. However, it is noteworthy that many of the absorption

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bands for each group were not detected (Figs. 5(a) and (b)), which can be attributed to two reasons.

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Firstly, some bands were too weak to be detected, especially in the complicated ore. Secondly, there was

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superposition of the bands of diverse groups with close vibration frequencies.

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3.3. Macro and micro morphology Macro characteristics, along with micro analysis by SEM with EDS, were applied to demonstrate 6

morphological changes during calcification roasting and shown at Fig. 6. In the macroscopic pictures ACCEPTED MANUSCRIPT

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A-D, there is an obvious color change in the roasted samples. At 340 °C, roasted samples are gray due to

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the mass of CaO added. At 570 °C, the sample color deepens and small white particles corresponding to

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CaO become more obvious under sharp contrast, which may be attributed to the formation of iron oxide.

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At 850 °C, sample color continues to deepen and less CaO is shown, meaning that CaO has participated

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in reactions and been consumed. At 980 °C, samples show a little yellow color, and the white particles

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of CaO disappear. This illustrates that CaO reacts completely and that the main composition of roasted

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samples changes greatly at 980 °C, which is consistent with the formation of a new phase of Ca3(Fe,

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Ti)2[(Si, Ti)O4]3.

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From the microstructure of the samples shown in Fig. 6(a)-(d), along with the EDS analysis of

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feature regions in a1 to d3, component changes with increasing temperature can be determined. At 340

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°C, particles in regions a1-a3 correspond to spinel (Mn, Fe)(V, Cr)2O4, olivine (Fe, Mn)2SiO4 and CaO,

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showing the initial state of calcification roasting. At 570 °C, particles in the elliptical area are

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representative, and assigned as being at the border of spinel and olivine before roasting, and b1 and b2

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are consistent with olivine and spinel, respectively. After roasting at 570 °C, SiO2 instead of olivine is

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detected obviously in b1, proving the decomposition of olivine, while vanadium and chromate still exist

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in the form of spinel in b2. Additionally, the difference between b3 and a3 illustrate that CaO combines

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with spinel gradually as the temperature rises. At 850 °C, the particles at c1 demonstrate the formation

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of calcium vanadate or calcium chromate, and calcium silicate is also shown at c2. Particle at c3 contain

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all possible elements of the HCVS and CaO, which may result from the direct reaction of spinel and

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CaO. Hence, two possible pathways exist for the formation of calcium vanadate and calcium chromate.

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Firstly, CaO reacts with the decomposition products of spinel, as shown in c1. On the other hand, some

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spinel in direct contact with CaO or not decomposing completely before calcification may react directly

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with CaO and form complex compounds, as shown in c3. At 980 °C, Cr and Fe are concentrated on d1

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and d2, which is consistent with the formation of (Fe0.6, Cr0.4)2O3. Furthermore, wrapping is remarkable

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in d3, which makes it unfavorable for further processing.

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3.4. Valence states shift

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The X-ray photoelectron spectroscopy (XPS) technique was employed to analyze the valence states

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of constituent elements in samples roasted at 340 °C, 570 °C, 750 °C and 900 °C, with results shown in

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Fig. 7. The global XPS survey in Fig. 7(a) proves the existence of Fe, V, Cr, Si, Ca and O in roasted

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samples according to the specific binding energy of different orbits. Moreover, detailed XPS spectra of

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the Fe2p, V2p, Cr2p and O1s present in samples are shown in Fig. 7(b) to illustrate the valence state

34

changes of Fe, V and Cr during calcification roasting. For Fe, the peaks at around 709 eV and 723 eV

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correspond to the Fe 2p3/2 and Fe 2p1/2 of Fe2+, and the peaks of Fe3+ are at approximately 711 eV and

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724.5 eV. Meanwhile, the peak area of 2p3/2 is twice as that of 2p1/2 according to spin-orbit splitting[31].

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At 340 °C, Fe exists mainly in the form of Fe2+, and the peak at around 708 eV is the Fe0 present in 7

1

HCVS. Satellite peaks of Fe 2p3/2 are also consistent with those observed in previous work[32]. All of ACCEPTED MANUSCRIPT

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these results illustrate that most Fe exists in the form of Fe2+ in spinel and olivine in the initial state. As

3

the temperature rises, more Fe3+ is observed, meaning that the Fe2+ in olivine and spinel is gradually

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oxidized to Fe3+ at 570 °C and 750 °C. At 900 °C, Fe can barely be detected, which may due to the fact

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that Fe tends to conjugate to Cr to form (Fe0.6, Cr0.4)2O3 solid solutions, which cannot be detected with

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XPS as it is a surface analysis technique. For Cr, there are no obvious peaks at 340 °C corresponding to Cr 2p, which is because Cr-spinels

8

and V-spinels are distributed in a layered core/shell configuration according to the nucleation and growth

9

kinetics of spinel crystals in vanadium slag[33], This causes the Cr element, which is centralized in the

10

central core, to not be detected by XPS. With temperature increases, the distribution of Cr valence states

11

becomes considerably complex. Most peaks at 570 °C are concentrated around 577 eV, which is

12

assigned to Cr 2p3/2 of Cr3+, while further Cr 2p3/2 peaks at around 579 eV describe the formation of Cr6+

13

at 750 °C. The changes of binding energy of Cr 2p3/2 demonstrate that Cr is oxidized gradually and

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shows higher valence states as the roasting temperature rises. At 900 °C, Cr can barely be detected,

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which can also be attributed to the formation of (Fe0.6, Cr0.4)2O3.

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For V, peaks with binding energies at around 515.5 eV and 517 eV correspond to the V 2p3/2 of V3+

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and V5+, respectively. At 340 °C, most V appears in the form of V3+, and peaks at 513 eV also

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demonstrate that there is some V2+ in HCVS[34]. As the temperature rises, the binding energy of the

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main peak increases gradually, demonstrating the oxidation of spinel. At 900 °C, V exists mainly in the

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form of V5+ because of the formation of calcium vanadate. For O, peaks at around 530.5 eV are assigned

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to lattice oxygen[35] of M-O (M: V, Cr, Fe) of spinel and olivine, and peaks at around 533 eV indicate

22

the presence of adsorbed H2O. As the roasting temperature increases further, more and more peaks with

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different binding energies appear because of the formation of different compounds such as SiO2, calcium

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vanadate, calcium chromate and Ca3(Fe, Ti)2[(Si, Ti)O4]3.

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3.5. Leaching experiments

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To evaluate the effects of roasting activation on the leaching stage, leaching experiments were

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performed under the certain sulfuric acid concentration (20% VH2SO4/V), S/L (1:10), leaching

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temperature (80 oC) and time (2h). Leaching rates of V, Cr and Fe during calcification roasting were

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calculated according to the previous work[36]. Fig. 8 showed the variation trends of leaching rates of V,

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Cr and Fe. At low temperature, leaching rate of Fe reaches around 80%, while that of V and Cr are less

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than 20% and 10%, respectively, meaning that free metallic iron and iron oxide existing in spinel and

33

olivine can be leached with acid, and transfer to liquid phase, while V and Cr in the stable spinel are

34

difficult to be leached. Moreover, sample at 420 oC has lower V and Cr leaching rates than in 340 oC,

35

indicating that structure changes of spinel may have remarkable effects on its solubility, and normal

36

spinel is more likely to be dissolved. As temperature goes up, opposite variation trends of leaching rates

37

among V and Fe are presented. During 570 oC to 750 oC, Fe leaching rate decreases dramatically and 8

1

o reaches a minimum of 20.07% at 750ACCEPTED C, while V leaching rate increases obviously and reaches 67.95%, MANUSCRIPT

2

and Cr leaching rate increases slightly. These are due to the formation of acid-soluble calcium vanadate

3

and calcium chromate, along with the generation of acid-insoluble (Fe0.6, Cr0.4)2O3. When temperature

4

continues to rise, leaching rates of V, Cr and Fe increase simultaneously, and reach 80.18%, 17.58 and

5

30.83% at 980 °C respectively, illustrating the thorough oxidation and calcification of HCVS.

6 7

3.6. Reaction mechanism Based on the analyses from characterizations and leaching experiments, we can safely draw the

9

calcification roasting mechanism with HCVS. The entire reaction procedure can be divided into several

10

stages, and the schematic representation was shown in Fig. 9. At I (room temperature-340 oC), free water

11

on the surface of HCVS and CaO dehydrated. At II (340 oC-420 oC), normal spinel converts to the

12

inverse spinel because of the oxidation of some Fe2+. Besides, the narrow endothermic peak at 400 oC

13

and the weight decreases of 1.7% is corresponding to the dehydration of bound water inside CaO and

14

HCVS. At III (420 oC-630 oC), the decomposition and oxidation of olivine, along with the free metallic

15

iron oxidation leads to sample weight increases. While the decomposition of Ca(OH)2 causes the weight

16

decrease during (570 oC-630 oC). At IV (630 oC-750 oC), spinel was oxidized and calcified initially to

17

form calcium vanadate, Ca2V2O7 and calcium chromate, CaCrO4 indirectly or directly, and some

18

vanadium oxide such as V6O13 is also generated. At V (750 oC-980 oC), spinel was calcified further by

19

excess CaO and form calcium vanadate Ca3V2O8, and another calcium chromate Ca3(CrO4)2 is generated

20

instead of CaCrO4. Besides, CaO also combines with SiO2 from the decomposition of olivine and form

21

calcium silicate CaSiO3. New phases (Fe0.6, Cr0.4)2O3 and Ca3(Fe, Ti)2[(Si, Ti)O4]3 appears. The main

22

chemical reactions of above reactions during calcification roasting are summarized in Table 2.

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Table 2

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Main chemical reactions in calcification roasting using CaO of HCVS. Stage

Reaction

No.

Dehydration of free water

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∗

(RT -340 oC)

Dehydration of bound water

II

(340 oC-420 oC)

III (420 oC-630 oC)

FeV2O4 + O2 → Fe2VO4

(1)

(4FeO + O2 = 2Fe2O3)

(2)

2Fe2SiO4 + O2 = 2Fe2O3 + 2SiO2

(3)

2Fe + O2 = 2FeO

(4)

Ca(OH)2 = CaO + H2O ↑ IV (630 oC-750 oC)

∗

4FeV2O4 + O2 = 2Fe2O3 + 4V2O3

(5)

V2O3 + O2(g) = V2O5

(6)

RT: room temperature 9

V2O5 +ACCEPTED 2CaO = Ca2V2O7MANUSCRIPT (Indirect oxidation)

(7)

FeV2O4 + O2 + CaO → Ca2V2O7 + Fe2O3

(8)

V2O3 + 2/3O2(g) = 1/3V6O13

(9)

4FeCr2O4 + O2(g) = 2Fe2O3 + 4Cr2O3

(10)

Cr2O3+1.5O2(g)+2CaO=2CaCrO4

(13)

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V2O5 + 3CaO = Ca3V2O8

(14) (15)

Fe2O3 + Cr2O3 → (Fe0.6, Cr0.4)2O3 FeCr2O4 + Fe2O3→ (Fe0.6, Cr0.4)2O3

(16) (17)

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Fe2O3 + SiO2 + TiO2 + CaO→ Ca3(Fe, Ti)2[(Si, Ti)O4]3 1

3.7. Thermodynamic analysis

(18)

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(11) (12)

CaO + SiO2 = CaSiO3

(750 oC -980 oC)

(Indirect oxidation)

FeCr2O4 + O2(g) + CaO →CaCrO4+ Fe2O3 (Direct oxidation)

Cr2O3+O2(g)+3CaO=Ca3(CrO4)2 V

(Direct oxidation)

Relationships between the standard Gibbs free energy changes (∆rGo) and temperature of the main

4

reactions shown in Table 2 were given in Fig. 10 using HSC Chemistry version 6.0 software. ∆rGo are

5

always negative over the temperature range from 200-1000 oC (except for reactions (9) and (11)),

6

indicating that every stage during calcification roasting is thermodynamically feasible. Comparative

7

analyses of reaction (3), (5) and (10) illustrate that oxidation and decomposition of olivine, V-spinel and

8

Cr-spinel follow the order of V-spinel > olivine > Cr-spinel (> means priority). However in actively,

9

olivine is prior to being decomposed to form SiO2, that can be attributed that spinel phase is dispersed

10

by olivine phase, and only when the wrapping olivine is decomposed can spinel be exposed to the

11

oxidizing atmosphere and be oxidized. Comparing ∆rGo of the formation of calcium vanadate, calcium

12

chromate and calcium silicate according to reaction (7), (11), (13) and (15), the order is in accordance

13

with Ca2V2O7 > Ca3V2O8 > CaCrO4 > CaSiO3 after 500 oC, meaning that CaO is more likely to combine

14

with vanadium and form calcium vanadate. And the formation of calcium vanadate is controlled by the

15

reaction kinetics rather than thermodynamics. Besides, Cr3+ is oxidized to form CaCrO4 before 800 °C,

16

while with temperature increases further, other chromate Ca3(CrO4)2 may form. All of above is

17

consistent with the analyses from characterization.

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3.8. Kinetics analysis

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The oxidation and calcification of HCVS is a non-isothermal heterogeneous gas–solid reaction. In a

21

gas–solid reaction aA(s)+bB(g)=cC(s), when the reaction rate is controlled by substance A, the reaction

22

rate follows Equation (19): dα / dt = kf (α) = k (1 − α) n

23

(19)

where n is the order of reaction, α is the fraction of reacted reactant at time t in equation (20), and k 10

1

is the reaction rate constant expressed by Arrhenius MANUSCRIPT equation (21) respectively ： ACCEPTED α = (m t − m 0 ) /(m ∞ − m 0 )

(20)

k = A ⋅ exp(− E a / RT)

(21)

where mt, m0, and m∞ are the sample weight at time t, starting state and completion state of

3

reaction, T is the Kelvin temperature, Ea is apparent activation energy, R is the gas constant, and A is the

4

pre-exponential factor. For a linear heating, heating rate β is usually a function of time according to

5

equation (22) :

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dT / dt = β

dα / dT = A / β exp(−E a / RT)f (α)

Separation of variables and integration of equation (24) :

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α0

T0

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∫ dα / f (α) = A / β ∫ exp(−E a / RT) = g(T)

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Then the equation would be combined to equation (23) :

6

(22)

(23)

(24)

According to the Coats-Redfern method, integration of Equation (25) yield:

8

[1 − (1 − α)1− n ] /(1 − n ) = ART 2 / αE a [1 − 2RT / E a ]exp(−E a / RT ) Taking natural logarithm yields:

9

[

]

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When n=1

10

ln − ln(1− α) / T2 = ln[AR/ βEa (1− 2RT/ Ea )] − Ea / RT≈ lnAR/ βEa − Ea / RT

When n≠1

11

(26)

(27)

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ln[1 − (1 − α)1−n ] /[T 2 (1 − n )] = ln[AR(1 − 2RT / E a ) / βE a ] − E a / RT ≈ ln AR / βE a − E a / RT

(25)

The plot of ln[-ln(1-α)/T2] or ln[1-(1-α)(1-n)/T2(1-n)] should present a straight line with the slope of

13

–Ea/R, and intercept of ln[1-(1-α)(1-n)/T2(1-n)] to determine the suitable value of n. And then the

14

activation energy Ea and preexponential factor A can be calculated.

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The TG data are fitted to the kinetic equations with different n, and results were shown in Fig. 11.

16

With n increasing from 1.5 to 4.5, the correlation coefficient (R2) has proven to increase firstly and

17

decrease later. However, an opposite variation trend of the residual sum of squares (RSS) is obtained

18

with the n increasing from 2.3 to 2.7. According to the comprehensive consideration of the R2 and RSS,

19

the suitable value of n should be 2.50. Hence, oxidation and calcification of spinel occurs from 630

20

o

21

from Fig. 11, the overall apparent activation energy Ea and the frequency factor A were were 157.34

22

kJ•mol−1 and 4.23×107 min−1. Moreover, comparisons of the main kinetic parameters using different

23

roasting methods and roasting parameters of vanadium slag were shown at Table 3. The main

24

discrepancies may come from the differences of chromium content and the heating rate.

C-980 oC is controlled by a 2.50 order reaction. According to the slope and intercept of fitting curve

11

ACCEPTED MANUSCRIPT

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Table 3 Comparisons of the main kinetic parameters of vanadium slag roasting. Apparent activation Roasting

Temperature

V content

Cr content

Heating

Reaction

method

range (oC)

(wt.%)

(wt.%)

rate(oC/min)

order

650-950

15.32

12.54

10

energy Ea (kJ•mol−1)

Blank

Zhang et

608-959

14.30

4.40

3

roasting Calcification 657-914

14.30

4.40

5

roasting Calcification 630-980

13.72

9.19

10

1.50

3.00

2.50

al.[12] Zhang et

140.20

al.[37] Zhang et

247.50

157.34

al.[37] This work

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Calcification

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roasting

4 5

Ref.

4. Conclusions

In this work, the effects of calcification roasting on HCVS were systematically investigated by

7

XRD, FT-IR, SEM, XPS and leaching experiments. Thermodynamics and kinetics were used to explain

8

the mechanism further. Conclusions can be drawn as follows:

9

(1) The entire procedure of calcification roasting of HCVS contains many steps: dehydration of free and

10

bound water inside HCVS and CaO, transformation of normal spinel into inverse spinel, oxidation of

11

free iron and olivine, preliminary and further oxidation and calcification of spinel, and the combination

12

of excess CaO with silicon, titanium and iron in HCVS.

13

(2) Complete decomposition and oxidation of olivine occur at 570–630 °C and forms SiO2 and Fe2O3. At

14

higher temperatures above 850 °C, excessive CaO would react with SiO2 to form calcium silicate

15

CaSiO3.

16

(3) Normal V-spinel in HCVS converts to inverse V-spinel at lower roasting temperatures. Above 630 °C,

17

V-spinel or its decomposition product, vanadium oxide, combines with CaO initially and generates

18

Ca2V2O7. Meanwhile, the final oxidation and calcification product of V-spinel with mass CaO is

19

Ca3V2O8, which appears at higher temperatures above 750 °C.

20

(4) According to thermodynamic analyses, Cr-spinel is more stable than V-spinel, which reacts with CaO

21

and forms CaCrO4 at 630 °C. With further temperature increases, Ca3(CrO4)2 is generated. Meanwhile,

22

Cr is likely to conjugate to iron to form acid-insoluble solid solutions (Fe0.6,Cr0.4)2O3 rather than reacting

23

with CaO, causing lower leaching rates of Fe and Cr at high temperatures.

24

(5) The formation of calcium vanadate with CaO is controlled by reaction kinetics rather than

25

thermodynamics. Oxidation calcification of spinel is controlled by a 2.50 order reaction. The overall

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apparent activation energy Ea and ACCEPTED frequency factor A were 157.34 kJ•mol−1 and 4.23×107 min−1, MANUSCRIPT

2

respectively.

3

Acknowledgments This research was financially supported by the Programs of the National Natural Science

5

Foundation of China (Nos. 51604065, 51574082 and 51374052), the National Basic Research Program

6

of China (973 Program) (No.2013CB632603), the Fundamental Funds for the central universities

7

(Nos.150203003, 150202001).

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References [1] X. Liang, G. H. Gao, Y. D. Liu, T. Q. Zhang,G. M. Wu, J. Alloy. Comp. 715 (2017) 374-383. [2] P. Kumar,L. H. Hu, J. Alloy. Comp. 655 (2016) 79-85. [3] M. Karthikeyan,S. Um, J. Alloy. Comp. 695 (2017) 1770-1777.

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[16] X. S. Li,B. Xie, Int. J. Miner. Metall. Mater. 19 (2012) 595-601. [17] H. G. Wang, M. Y. Wang,X. W. Wang, Miner. Process. Extr. Metall. Rev. 124 (2015) 127-131. [18] K. P. Yu, H. L. Zhang, B. Chen, H. B. Xu,Y. Zhang, Metall. Mater. Trans. B. 46 (2015) 2553-2563.

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[19] P. Cao, Iron Steel VanadiumTitanium. 33 (2012) 30-34. [20] X. S. Li, B. Xie, G. E. Wang,X. J. Li, Trans. Nonferrous Met. Soc. China. 21 (2011) 1860-1867. [21] V. C. Farmer, The Infrared spectra of minerals, (1982) Science Press, Beijing. [22] R. Jeanloz, Phys. Chem. Minerals. 5 (1980) 327-341. [23] N. R. Yang,W. H. Yue, The Handdook of Inorganic Matalloid Materials Atlas (2000) Wuhan Industrial University Press, Wuhan. [24] M. Nohair, D. Aymes, P. Perriat,B. Gillot, Vib. Spectrosc. 9 (1995) 181-190. [25] B. Gillot,V. Nivoix, Mater. Res. Bull. 34 (1999) 1735-1747. [26] P. Chen, Crystallization mineralogy, (2006) Chemical industry Press, Beijing. [27] W. Li, G. Q. Fu, M. S. Chu,M. Y. Zhu, Ironmaking Ironmaking. 44 (2016) 294-303. [28] H. Y. Hu, Z. Xu, H. Liu, D. K. Chen, A. J. Li, H. Yao,I. Naruse, Proc. Combust. Inst. 35 (2015) 2397-2403. [29] I. O. Mazali,O. L. Alves, J. Braz. Chem. Soc. 15 (2004) 464-467. [30] V. B. Taxak, Sheetal, Dayawati,S. P. Khatkar, Curr. Appl. Phys. 13 (2013) 594-598. [31] M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau, A. R. Gerson,R. S. C. Smart, Appl. Surf. Sci. 257 (2011) 2717-2730. [32] T. Yamashita,P. Hayes, Appl. Surf. Sci. 254 (2008) 2441-2449.

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[34] X.M. Qing, B. Xie , D. K Li,Q. Y. Huang, The Chinese Journal of Process Engineering. 9 (2009) 122-126. [35] A.P. Grosvenor, B.A. Kobe,N. S. McIntyre, Surf. Sci. 572 (2004) 217–227. [36] H. Y. Li, H. X. Fang, K. Wang, W. Zhou, Z. Yang, X. M. Yan, W. S. Ge, Q. W. Li,B. Xie, Hydrometallurgy. 156 (2015) 124-135. [37] J. H. Zhang, W. Zhang, L. Zhang,S. Q. Gu, Int. J. Miner. Process. 138 (2015) 20-29.

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Figures Captions List

2

Fig. 1. XRD pattern of HCVS and CaO.

3

Fig. 2. FT-IR spectra of HCVS and CaO.

4

Fig. 3. TG-DSC of the mixture of HCVS and CaO.

5

Fig. 4. XRD of products roasted at different temperatures (a) 340-980 oC; (b) 340-570 oC; (c) 630-850 oC; (d) changes

6

of peaks height at 850-980 oC. Fig. 5. FT-IR of products roasted at different temperatures (a) 4000-400 cm-1; (b) 1400-400 cm-1.

8

Fig. 6. Macro and micro morphology of roasted samples at different temperatures (A)-(D) Images of samples; (a)-(d)

9

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Backscattered electron image of sample.

Fig. 7. XPS of roasted samples at different temperatures (a) Global XPS; (b) valence states change of Fe, Cr, O and V.

11

Fig. 8. Effects of calcification roasting on the change of mass weight and leaching rates of V, Cr and Fe.

12

Fig. 9. Schematic representation of calcification roasting.

13

Fig. 10. Relationships between standard Gibbs free energy changes and temperatures.

14

Fig. 11. Values of R2 and RSS for TG data fitting with kinetic equations (0.25
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linearization for TG data.

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ACCEPTED MANUSCRIPT Highlights: 1. Oxidation and calcification mechanism of high chromium vanadium slag were analyzed. 2. Final reaction product of V-spinel with excessive CaO was Ca3V2O8. 3. Cr tended to form acid-insoluble solid solutions (Fe0.6,Cr0.4)2O3 during calcification roasting.

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4. The remarkable microstructure evolutions and surface morphology prove the reaction

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mechanism of calcification roasting.