Effects of calcination on silica phase transition in diatomite

Accepted Manuscript Effects of calcination on silica phase transition in diatomite Renji Zheng, Zijie Ren, Huimin Gao, Anling Zhang, Zheng Bian PII:

S0925-8388(18)31688-8

DOI:

10.1016/j.jallcom.2018.05.010

Reference:

JALCOM 45983

To appear in:

Journal of Alloys and Compounds

Received Date: 6 December 2017 Revised Date:

15 April 2018

Accepted Date: 1 May 2018

Please cite this article as: R. Zheng, Z. Ren, H. Gao, A. Zhang, Z. Bian, Effects of calcination on silica phase transition in diatomite, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.05.010. 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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Effects of calcination on silica phase transition in diatomite Renji Zheng a, Zijie Ren a, b, *, Huimin Gaoa, b, Anling Zhang a, Zheng Bian a a

School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China; [email protected] (R. Zheng); [email protected] (H. Gao); [email protected] (A. Zhang); [email protected] (Z. Bian) Hubei Key Laboratory of Mineral Resources Processing & Environment, Wuhan 430070, China

* Corresponding author. E-mail address: [email protected] (Z. Ren)

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Abstract: Calcination has been a major means for the preparation of diatomite filter aids because it improves the permeability of filter aids. However, the pore structure and silica phase of diatomite could be destroyed or altered during thermal processing, which seriously restrains the properties of diatomite filter aids. In the present work, the

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calcination of diatomite was carried out to investigate its effect on silica phase transition in diatomite. Diatomite with a certain particle size distribution as raw material was sintered in a muffle furnace at a temperature from 200 to 1200

with or without flux (7 wt.% NaCO3). The phase evolution and microstructure of diatomite were

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investigated by thermal analysis (TG and DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). The results showed that the opal in diatomite began converting to cristobalite at 1000

without flux and the transformation temperature was reduced by 200

by

adding flux. In addition, there was about 64.02% content of quartz in diatomite converting to cristobalite as the calcination temperature increased from 1100

to 1200

by flux calcination. It was considered to be a universal

phenomenon that the opal in diatomite transformed into cristobalite under high-temperature calcination due to their similar microcrystalline structure. Furthermore, the quartz in raw diatomite was inclined to transform into cristobalite rather than tridymite, mainly owing to the existence of crystal nucleus of cristobalite formed from

diatomite upon calcination.

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opal-cristobalite phase transition. The findings in this paper improve understanding of silica phase transition in

Keywords: Calcined diatomite; Flux calcination; Silica phase; Phase transition.

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1. Introduction

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Diatomite is a type of porous silicate material composed of the skeletal remains of single-cell water plants (algae) with a chemical formula of SiO2·nH2O [1], the main phase of which is opal, and it belongs to amorphous silica. Diatomite has been used in a number of applications, especially in the filter aids industry, for its high permeability, high porosity, low thermal conductivity, and chemical inertness [2–4]. In addition to the application in filtration, diatomite has also excelled in dye removal due to its large specific surface area and high adsorption capacity, and it possesses good adsorption properties, especially for oils and microorganisms [5–7]. In China, there are abundant diatomite resources, but most of them only have a SiO2 content of about 60%–80%, which limits the commercial utilization of diatomite in industrial productions or applications [3,8]. Therefore, most raw diatomite needs to be treated by calcination or flux calcination for purification to remove the organic matter and carbonate compounds within [9]. In addition, calcination could further improve permeability, whiteness, and other distinctive features of diatomite filter aids [10]. However, calcination is not always a perfect process for diatomite because the desired porous structure of diatomite would be destroyed, which would seriously restrain the properties of

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diatomite filter aids. Additionally, the silica phase in raw diatomite would be transformed simultaneously upon calcination, generating the formation of crystalline silica. As has been known for many years, the industry chose to self-classify crystalline silica flours as harmful with the label Xn (Harmful) and the risk phrases R48/20 (danger of serious damage to health by prolonged exposure through inhalation) according to research from the European Industrial Minerals Association (IMA-Europe) [11]. The formation of crystalline silica could raise the content of respirable crystalline silica in airborne dust or calcined diatomite products, which would cause health effects to humans by traveling deep into the lungs (alveoli) [12]. Therefore, the formation of respirable crystalline silica was undesirable in diatomite products treated by calcination, especially for that applied in the food industry. The previous study of our tests indicated that the diatomite filter aids products with good filtration properties and whiteness could be obtained by flux calcination below 1000 , and the proportion of flux (Na2CO3) was 7 wt.% [9], but the relevant silica phase transition in diatomite has not been discussed in detail, which would provide the crucial information or mechanisms of the formation process of crystalline silica in diatomite under calcination. It is, therefore, the objective of this study to investigate the effects of calcination modification on silica phase transition in diatomite with or without flux, including the opal-cristobalite phase transition and quartz-cristobalite-tridymite phase transition processes. In addition, the transition mechanisms of those processes will be further discussed with the help of TG, DSC, XRD, FT-IR, and TEM, which can significantly improve our understanding of the transition behavior of silica in diatomite.

2.1. Materials

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2. Materials and methods

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The raw diatomite used for testing in this study was obtained from the Linjiang region of China. The flux sodium carbonate (Na2CO3, AR) powder added to the crucible with diatomite was purchased from Tianjin Bodi Chemical Co., Ltd, China. Table 1 and Fig. 1 are the chemical and phase compositions of raw material, which were determined by an X-ray fluorescence spectrometer (XRF) (Axios advanced, PAN Alytical B.V., Netherlands) and X-ray diffraction (XRD) (D/Max-IIIA, RIGAKU, Japan) with monochromatic Cu Kα radiation in 5–70° at a rate of 0.02°/s. Table 1 Chemical composition of raw diatomite. Component Content (wt.%)

SiO2 85.54

Al2O3 3.78

MgO 0.34

CaO 0.40

Fe2O3 1.81

ZnO 0.01

TiO2 0.17

Component Content (wt.%)

K2O 0.76

Na2O 0.47

P2O5 0.13

MnO 0.03

SO3 0.15

L.O.I.a 6.41

Total 100.00

a

: Loss on ignition

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Fig. 2. Standard curve of cristobalite content in mixed sample.

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Fig. 1. XRD pattern of raw diatomite.

2.2. High-temperature experiments

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For experiments in high-temperature treatment of diatomite, 25 g of diatomite dried in oven at 105 for 4 h as raw material was taken into a crucible and then calcined in a muffle furnace (SX2-8-13, Jianli, Yingshan, China) at the temperature range from 200 to 1200 for the holding time 1.5 h. Next, the calcined sample was naturally cooled to room temperature. The heating rate was controlled by a programmed system for 5 ·min-1. To investigate the effects of flux on sintering behaviors, 7 wt.% of flux powders (Na2CO3) were dried and blended with diatomite directly at room temperature for 30 min until they were adequately mixed with each other before calcination. Finally, the calcined sample was disintegrated and prepared for further testing and analysis. 2.3. Methods and characterization

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The crystal cristobalite content of calcined diatomite sample was determined by using the linear regression equation (
= −0.0362 + 1.0325 − 0.0034,  = 0.999) (Fig. 2), which was obtained by plotting a calibration curve with integral intensity of a ‘101’ plane of cristobalite (d101=4.04 Å) from its XRD pattern with a concentration from 0 wt.% to 100 wt.% mixed with diatomite. The quartz content in the calcined sample was obtained using the following equation [13]:    / = ( / )/( / ) =  ×   

(1)

where CQ (wt.%) and CC (wt.%) are the quartz content and cristobalite content of the sample, respectively. IQ (counts) and IC (counts) are the integral intensity of the ‘101’ plane of quartz and cristobalite from their XRD patterns, respectively. LC and LQ are the constants, which can be calculated from absorption coefficient and structure constant of material. In Eq. (1), LC/LQ=0.6, therefore Eq. (1) can be given as:

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(2)

3. Results and discussion 3.1. Characterization of raw diatomite

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The particle size distribution of diatomite was measured using a laser particle analyzer (BT-9300S, Dandong Bettersize Scientific Ltd., China). For characterizing the phase transition of the sample during calcination, XRD and Fourier transform infrared spectroscopy (FT-IR) (IS-10, Nicolet, US) were adopted. In addition, thermogravimetry analysis (TG) and differential scanning calorimetry (DSC) were carried out on the raw diatomite from room temperature up to 1200 with a heating rate of 10 ·min-1 by using a thermal analyzer (STA449C, NETZSCH Scientific Instruments Trading Ltd., Germany). To characterize the surface morphology and fundamental physical properties of sample, scanning electron microscopy (SEM) (JSM-5610LV, JEOL Ltd., Japan) has been a primary tool to observe the diatom frustules morphology of calcined diatomite with an accelerating voltage of 20 kV. The transmission electron microscope (TEM) (JEM-2100F, JEOL Ltd., Japan) was applied to investigate the microstructure of diatomite before and after calcination.

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As shown in Table 1, the raw diatomite contained a high content of SiO2 (85.54%). In addition, the impurities such as iron, calcium, potassium, and sodium within were low, indicating that the raw diatomite possessed good quality for practical application. The X-ray diffraction of the raw diatomite powder before calcination is given in Fig. 1. There were a small number of crystalline phases such as quartz, illite, kaolinite, and plagioclase existing in the sample. Because of the presence of the amorphous phase, it was obvious to see a broad diffraction peak between 15° and 30° 2θ in the XRD pattern of raw diatomite. It could be concluded that the appearance of the amorphous phase mainly belonged to opal (SiO2·nH2O) in combination with the results of the chemical analysis above. The particle size of raw diatomite was considered one of the important factors in the preparation of filter aids. A previous study revealed that the formation of cristobalite was always favored by a small grain size of the starting powder during the high-temperature process, and the smaller the particles, the easier was the phase transformation [14]. Fig. 3 shows the particle size distribution of raw diatomite used in the test, with most of the diatomite powder distributed in the interval from 20 µm to 30 µm, and the median particle diameter (D50) of 21.69 µm, which is suitable for the preparation of diatomite filter aids [9].

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Fig. 3. Size distribution curves of raw diatomite.

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Fig. 4 presents the SEM images of diatomite samples prepared in experiments. There were many porous structures in the diatomite shell and many diatom fragments and impurities on the surface of the diatomite from Fig. 4a. After calcination, the diatom fragments and impurities on the diatomite were obviously fused compared to the non-fired sample (Fig 4b). In addition, the porous structure of diatomite became more advanced and complete, which could enhance the permeability of diatomite filter aids. However, the porous structure of diatomite was fused and destroyed by adding too much flux (Fig. 4c), and the undamaged diatomite shell broke and collapsed simultaneously (Fig. 4d) under excessive high-temperature calcination. Hence, the calcination of diatomite should be carried out under an appropriate temperature with suitable addition of flux.

Fig. 4. SEM images of diatomite sample; (a) raw diatomite, (b) ideal calcined diatomite, (c)

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Powder X-ray diffraction (XRD) is one of the primary techniques used for tracking changes in the layer spacing and new phase formation of crystalline phase [15]. Fig 5 shows the XRD patterns of diatomite calcined at different temperatures without or with flux. At low temperature below 1000 without flux (Fig. 5a), there was a broad diffraction peak existing between 15° and 30°, which meant the amorphous opal was still the same as the main phase composition of sintered sample, with a small quantity of quartz within. A new crystalline phase cristobalite began emerging at 1000 for the appearance of the ‘101’, ‘200’, and ‘102’ characteristic planes (d101=4.073 Å, d200=2.497 Å, d102=2.863 Å) of crystalline cristobalite (JCPDS#39-1425), and then the diffraction intensity of the characteristic reflection peak (‘101’) was enhanced with increase of sintering temperature. Meanwhile, diffracted intensity of the ‘101’ plane of quartz obviously reduced as the temperature increased from 1100 to 1200 , indicating that the content of quartz in the calcined diatomite decreased. Compared with non-flux calcination, the phase transition temperature from opal to cristobalite decreased by about 200 (Fig. 5b) under flux calcination, which meant the flux (Na2CO3) could facilitate the transformation of opal to cristobalite. Similarly, the diffracted intensity of the ‘101’ plane of quartz also reduced continuously as the heating temperature exceeded 800 , especially when the temperature increased from 1100 to 1200 .

Fig. 5. XRD patterns of diatomite calcined at different temperature; (a) without flux, (b) with flux. 3.2.2. FT-IR analysis of the sintered samples The FT-IR spectra of calcined diatomite samples at different temperatures are given in Fig. 6, the absorption bands at about 1100 cm-1, 795 cm-1, and 471 cm-1 represented the asymmetric stretching, symmetric stretching, and bending vibrations of Si-O-Si bond, respectively [16]. After calcination or flux calcination, the absorption band at 795 cm-1 became narrow and unabridged, revealing the enhancement of crystallinity of the silica phase in the fired samples. In addition, a without flux and 800 with flux, new absorption band at 616 cm-1 turned up at 1100

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respectively, which indicated the vibration characteristics of the silica tetrahedron in cristobalite, demonstrating the formation of the crystalline phase cristobalite [16–18]. The results of the FT-IR analysis were in agreement with the results of the XRD analysis above.

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Fig. 6. FT-IR spectra of diatomite calcined at different temperature; (a) without flux, (b) with flux. 3.2.3. Thermal analysis of raw diatomite

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The thermal behavior of diatomite in terms of TG-DSC analysis is given in Fig. 7. From the DSC curve, several endothermic and exothermic peaks of the sample can be seen clearly during the heating process. The first endothermic peak at about 91.0 mainly belongs to desorption of absorbed water from the surface of sample, accompanied by the weight loss of 2.52%. Another endothermic peak occurs at about 696.9 , which is mainly assigned to the decomposition of the carbonate impurities in the samples, e.g., the flux (Na2CO3) added in the sample. At 335.7 , there is a strong exothermic peak due to the oxidation reactions of organic matter in the diatomite. There is an exothermic peak existing at 930.7 without weight loss, which occurs mainly due to the phase transition from opal to cristobalite in diatomite. The weight of the sample did not decrease with further heating as the temperature exceeded 900 , signifying that the oxygenolysis process or decomposition reaction had been terminated.

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3.3. Opal-cristobalite phase transition

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Fig. 7. Thermal analysis of raw diatomite under calcination.

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The previous studies demonstrated that opal (SiO2·nH2O) was the main phase composition of diatomite, and it belongs to opal-A, but neither opal-CT nor opal-C [19,20]. Opal-A could convert into crystalline silica during high temperature process [9]. The XRD results of the fired sample above clarified that the scope of broad diffraction peak between 15° and 30° 2θ in XRD pattern, the characteristic diffraction peaks of opal, became narrow and weak continuously with temperature increase. The broad diffraction peak almost disappeared after the formation of crystalline silica cristobalite. It was interesting that the formation of cristobalite at comparatively low temperature of 1000–1200 , where the thermodynamic stable region of tridymite [14]. The phenomenon presented in the present work seemed anomalous because cristobalite was unstable at that temperature range according to thermodynamic phase equilibria, while the tridymite should be formed. Moreover, the considered abnormal formation of cristobalite suggested that the cristobalite formed from amorphous to crystalline transformation. Actually, opal phase composed of Si-O tetrahedral mesh structure is not entirely amorphous. It has been verified that opal is the aggregation of crystallite which is made up of cristobalite microcrystalline, and the ordered size of opal microcrystal is about 1–2 nm in statistically, called medium-range order hovering between short-range order and long-range order in crystallology, reflecting the spatial orientation of silicon-oxygen tetrahedron in opal [21–24]. In addition to the thermodynamic process, the kinetics of cristobalite crystallization is also an important factor to consider, and it plays a crucial role during the formation process [22]. The nonequilibrium thermodynamics behavior where cristobalite formed in a metastable state could be explained according to the Ostwald’s rule [25]: It is unnecessary for an unstable phase to transform into its most stable phase immediately, and it only needs lesser free energy loss of system to convert into its transient or intermediate phase. For the cristobalite crystallization from the opal phase, due to their similar microcrystalline structure, the opal and cristobalite could be regarded as an unstable phase and transient phase, respectively [24]. In addition, tridymite has been demonstrated to have the stable phase at 1000–1200 [14]. Therefore, it was interpreted

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that the opal in diatomite was inclined to transform into cristobalite rather than tridymite following the Ostwald’s rule, even though the latter was thermodynamically stable phase. The phase transition temperature from opal to cristobalite reduced about 200 when the flux Na2CO3 was added because the alkali ions (Na+) would partly broke down the Si-O-Si chains of opal and occupy interspaces of the mesh structure, finally facilitating the formation of crystalline silica phase with high temperature process [9,26]. The cristobalite crystallization from the amorphous opal phase in diatomite could also be verified through the TEM micrographs of diatomite before and after calcination (Fig. 8). The crystalline phase was found on the surface of diatomite after calcination, from which it could be deduced the crystalline phase was cristobalite as well as from XRD and thermal analyses above.

Fig. 8. TEM micrographs of diatomite before (a) and after (b) calcination. 3.4. Quartz-cristobalite-tridymite phase transition

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The theoretical sequence of the silica transformation involves three periods upon heating [14]. As shown in Fig. 9, in the first period, where the heating temperature is below 870 , the α-quartz would turn to β-quartz at about 573 , and this transformation is reversible because of no phase transition happening. In the second period, the β-quartz can be converted to high tridymite (β-tridymite) when the heating temperature is below 1470 . A phase transition occurred during this period for the different crystal structure of quartz and tridymite, which means the happening of breakage and recombination of Si-O-Si bond. A previous study showed that the formation of β-tridymite required particular impurities to be present [18]. In the third period, where the heating temperature is above 1470 and below the melting temperature of β-cristobalite (1720 ), the β-tridymite would continue to transform into β-cristobalite at 1470 . Both high tridymite and β-cristobalite are unstable at room temperature. Upon cooling, high tridymite transforms into medium tridymite at 163 and low tridymite (α-tridymite) at 117 , and β-cristobalite turns into its tetragonal α phase at 270 , as it was unable to overcome the activation energy barrier to change into the stable silica phase [14].

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Fig. 9. Schematic diagram of phase transition during the heating and cooling process of quartz.

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In addition to the existence of the main composition opal, there was also a small amount of quartz in raw diatomite. It has been analyzed that the diffracted intensity of the ‘101’ plane of quartz decreased continuously when the heating temperature exceeded 800 (Fig. 5b). There was about 64.02% content of quartz in raw diatomite turning to cristobalite by flux calcination from 1100 to 1200 using Eq. (2), where cristobalite could only exist as a metastable phase out of its stable field. The most possible reason could be predicted the quartz in raw diatomite was inclined to transform into cristobalite rather than tridymite because of the existence of crystal nucleus of cristobalite formed from opal in calcined diatomite. With the purpose of verifying the prediction proposed above, the following test was designed and carried out. A certain amount of quartz and cristobalite powder were fired with flux (7 wt.% Na2CO3) adding, simulating a similar condition where raw diatomite was calcined. As showed in Fig. 10, it was interesting to discover that quartz transformed into cristobalite firstly through flux calcination at 1200 , and then was converted to tridymite with an increase in holding time (Fig. 10a). The diffraction intensity of characteristic reflection peak (‘101’) of cristobalite decreased progressively when the holding time was extended from 0.5 h to 6.0 h; meanwhile, the diffraction intensity of characteristic reflection peak (‘220’ and ‘004’) of tridymite enhanced gradually, indicating the cristobalite phase had turned into tridymite. Through the calcination of thee cristobalite powder, more and more cristobalite transformed into tridymite with an increase in holding time (Fig. 10b). The phenomenon cristobalite born as a metastable phase at 1200 , where the thermodynamic stability region of tridymite, was in agreement with the results of the fired diatomite presented above. Finally, the sintered sample transformed into tridymite phase because longer holding time could provide enough power for overcoming high activation energy barrier of cristobalite-tridymite transformation.

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Fig. 10. XRD patterns of (a) quartz and (b) cristobalite calcined at 1200

4. Conclusions

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The present study was undertaken to investigate the silica phase transition in diatomite samples under high-temperature modification with or without flux, including an opal-cristobalite and quartz-cristobalite-tridymite phase transition. On the basis of the study results, it could be concluded that: 1. A new crystalline phase cristobalite formed at 1000 ℃ without flux and at 800 ℃with flux in calcined diatomite. The phase transition temperature was reduced by about 200 ℃ when the flux Na2CO3 was added mainly because the alkali ions (Na+) would partly broke down the Si-O-Si chains, facilitating the phase transition of opal to cristobalite. 2. Amorphous opal, the main phase composition of diatomite, transformed into cristobalite rather than other silica phases mainly because opal was the aggregation of cristobalite microcrystalline. 3. There was about 64.02% content of quartz in the diatomite turning to cristobalite during high temperature process from 1100 ℃ to 1200 ℃ by flux calcination. The quartz in raw diatomite was inclined to transform into cristobalite upon calcination rather than tridymite mainly due to the existence of crystal nucleus of cristobalite formed from opal-cristobalite phase transition, which could reduce the activation energy barrier from quartz to cristobalite. The findings in this paper improve understanding of silica phase transition and the relevant phase transformation mechanism in diatomite upon calcination, providing theoretical guidance for the effective control of inhalable crystalline silica in calcined diatomite in future research work.

Acknowledgements

The authors acknowledge the financial support of this study from the Key Science and Technology Support Programs (2011BAB03B07) of the Ministry of Science and Technology of China and the Independent Innovation Research Fund of Wuhan University of Technology (2017-ZH-B1-09).

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ACCEPTED MANUSCRIPT Highlights The thermal processing of diatomite by calcination or flux calcination was carried out in detail. The amorphous opal in diatomite transforms into cristobalite at 1000

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The flux can facilitate the transformation of opal in diatomite to cristobalite by calcination.

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The quartz in diatomite is inclined to transform into cristobalite rather than tridymite upon calcination.