comparison of microwave and conventional heating routes

J Cent South Univ (2020) 27 2494minus2506 DOI httpsdoiorg101007s11771-020-4475-y

Comparison of microwave and conventional heating routes for kaolin thermal activation

ZHANG Liang-jing(张良静)1 2 HE Yuan(和媛)1 2 LUuml Peng(吕鹏)1 2 PENG Jin-hui(彭金辉)1 2 LI Shi-wei(李世伟)1 2 CHEN Kai-hua(陈楷华)1 2

YIN Shao-hua(尹少华)1 2 ZHANG Li-bo(张利波)1 2

1 Faculty of Metallurgical and Energy Engineering Kunming University of Science and Technology Kunming 650093 China

2 State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization Kunming University of Science and Technology Kunming 650093 China

copy Central South University Press and Springer-Verlag GmbH Germany part of Springer Nature 2020

Abstract The effect of activation properties of the precursors of zeolite directly prepared from kaolin influenced by microwave field and conventional heating was investigated XRD TG-DSC FT-IR SEM particle size analysis specific surface area (BET) pore size distribution (BJH) and N2 adsorptionminusdesorption were discussed to determine the optimal activation temperature It is concluded that the conversion of kaolin to metakaolin in the microwave field is at 500 degC holding for 30 min which is 100 degC lower than that in conventional calcination and 90 min shorter and the phase transition process of kaolin under the effect of microwave field is the same as that of conventional heating method SEM analysis indicates that the particle size is more uniform and agglomeration appears slightly in the microwave field The N2 adsorptionminusdesorption isotherm BET and BJH of kaolin indicate that the pore properties are almost invariable regardless of calcination route during the process of calcining kaolin into metakaolin It indicates that microwave calcination is superior to conventional calcination in the activation pathway of kaolin It is attributed to microwave heating relying on objects to absorb microwave energy and convert it into thermal energy which can simultaneously and uniformly heat the entire substance Key words kaolin thermal activation metakaolin microwave Cite this article as ZHANG Liang-jing HE Yuan LUuml Peng PENG Jin-hui LI Shi-wei CHEN Kai-hua YIN Shao-hua ZHANG Li-bo Comparison of microwave and conventional heating routes for kaolin thermal activation [J] Journal of Central South University 2020 27(9) 2494minus2506 DOI httpsdoiorg101007s11771-020-4475-y

1 Introduction

Kaolinite is a clay mineral of chemical formula Al2O32SiO22H2O contained abundant silicon and aluminum [1] (Figure 1) therefore it is conveniently used as a silicon source and an

aluminum source for synthetic molecular sieves Clay minerals have been used for the synthesis of zeolites as early as 1961 [2] and now are playing a pivotal role in fabricating zeolites Simultaneously the direct synthesis of zeolites from natural aluminosilicate minerals without experiencing intermediate chemicals has attracted extensive

Foundation item Projects(51604135 51504116) supported by the National Natural Science Foundational of China

Project(YNWR-QNBJ-2018-323) supported by the Yunan Ten Thousand Talents Plan Young amp Elite Talents Project China

Received date 2020-02-05 Accepted date 2020-06-03 Corresponding author YIN Shao-hua PhD Associate Professer Tel +86-871-65191046 +86-871-65174756 E-mail yinshkust

educn ORCID httpsorcidorg0000-0003-2605-2442 ZHANG Li-bo PhD Professor Tel +86-871- 65174756 E-mail zhanglibopaper126com ORCID httpsorcidorg0000-0003-3244-0142

J Cent South Univ (2020) 27 2494minus2506

2495

Figure 1 Kaolin structure

attention [3minus5] The synthesis of zeolite A from kaolinite comprises two steps metakaolinization and zeolitization [6] However the silicon and aluminum in kaolin are in crystal forms and crystalline kaolinite has high chemical inertness which cannot be utilized for synthesizing zeolites directly [7] Therefore kaolinite has to be calcined at high temperatures (700minus900 degC) whereby dehydroxylation takes place converting to amorphous and more reactive product (metakaolinite) prior to application for the synthesis of zeolites [8minus10] The plate-like structure will decrease the reactivity and surface area of reaction of kaolin which can be improved by appropriate treatment methods to destroy the crystal structure such as mechanical activation [11] thermal activation [12] acid-base modification activation [13 14] Due to its high passivity it is impossible to increase the reactivity of kaolin by chemical treatment even under drastic conditions [15] Heating to alter kaolin activity has become an important method which can transform the lamellar crystal structure of kaolin into disordered metakaolin structure by controlling the number of hydroxyl groups Some original groups in the inner layer of the crystal are exposed to obtain the special physical and chemical properties [16] The calcined product has the advantages of high strength good durability and corrosion resistance [17 18] However conventional calcination has the disadvantages of slow heating rate high energy consumption and uneven heating inside and outside the material Therefore it is essential to seek an alternative heating way to solve the above issues Microwave heating is a heating route that relies on objects to absorb microwave energy and convert it into heat energy so that the whole body can heat up at the same time which is completely different from other conventional heating methods

Microwave thermal [19] has been used as a new type of heating technology on account of the advantages of fast and efficient uniform interior heating relatively low reaction temperature energy saving environmental protection high product purity and no direct contact between the heating source and heated materials [20 21] ZHONG et al [22] reported a study which can reduce phase transition temperature and prepare finer and more uniform products ZHANG et al [23] recorded microwave selective heating-enhanced reaction rates for mullite preparation from kaolinite indicating that conventional heating kaolinite has to be heated at 1400 degC for 1 h to form well-ordered orthorhombic mullite (3Al2O3ꞏ2SiO2) that is accompanied by the formation of cristobalite while microwave heating at 900 degC for 5 min and 1200 degC for 1 min YOUSSEF et al [24] studied the microwave-assisted versus conventional synthesis of zeolite A from metakaolinite and came to conclusion that the rate of zeolite A formation was found to increase by 2ndash3 times in microwave treated samples with a notable enhancement in the product crystallinity and yield whether seeded or unseeded Therefore an economically viable route has been successfully applied for activating kaolin This work describes the thermal activation process of kaolin using conventional and microwave calcination The effects of the two calcinations on the temperature and time needed for the activation of kaolin were systematically studied and similarities and difference of products between the two calcination methods were comprehensively compared Analytical conclusions have been obtained by differential thermogravimetric analysis (TG-DSC) X-ray diffraction (XRD) Fourier transform infrared spectrometry (FT-IR) scanning electron microscopy (SEM) particle size analysis (PSD) specific surface area pore size distribution and N2 adsorption-desorption 2 Experimental 21 Chemical composition of kaolin Kaolin was supplied from Yunnan Xishuangbanna Wanxiang Mining Co Ltd China with the composition of elements in their oxide forms of 493 SiO2 37748 Al2O3 1271 K2O 0728 Na2O 0455 Fe2O3 0214 MgO and 0137 CaO and was used as the raw material The


2496

main impurities accorded with the standard of high quality kaolin For example the content of Fe2O3 was less than 1 [25] Its compositions are determined by X-ray florescence (XRF Instrument PANalytical Rh tube 22 kW) The results are given for elements in their oxide forms in Table 1 The XRD pattern of the kaolin sample used in this study is given in Figure 2 It is noticed that the peaks of kaolinite should be loaded at 1215deg 2019deg 2473deg 2644deg and 2802deg while that at 2667deg corresponding to quartz and muscovite and albite also is found in sample Table 1 Chemical composition of raw kaolin in mass

fraction ()

SiO2 Al2O3 K2O Na2O Fe2O3 MgO CaO

468 398 12 08 04 02 01

Figure 2 XRD pattern of kaolin samples

22 Calcination of raw kaolinite Block kaolinite was subjected to grind on a portable multifunctional powder machine After grinding 50 g kaolin was charged into crucible of d60 mmtimes75 mm to heat with microwave nor conventional field The thermal treatment of the kaolin was carried out in a laboratory muffle furnace at a heating rate of 10 degCmin in air and heated in the range of 600minus1200 degC and then remained at these temperatures for 2 h before cooling in the furnace Then sample was determined by TG-DSC analysis and the characterization was analyzed to make the change of the phase certain A certain amount of kaolin was contained in the corundum crucible and covered with a microwave absorbing medium (shown in Figure 3) The microwave frequency was 245 GHz with

Figure 3 Microwave heating device diagram

the maximum input power of 45 kW Temperature was measured by a thermocouple and the calcination conditions were selected at 500minus 1000 degC holding for 30 min 23 Characterization The composition of the starting ore was identified by XRF on an AXios max spectrometry (PANalytical) The sample phases and compositions at different temperature were identified by the X-ray diffraction analysis on a diffractometer (XRD RU-200BDMAX-RB RU-200B Japan CuKα λ=15418 Aring 40 mA and 40 kV) The thermal behavior of kaolin was investigated by thermal gravity analysis and differential scanning calorimetry (TG-DSC Netzsch STA449F3) at a ramping rate of 20 degCmin in 25minus1400 degC under argon atmosphere IR spectra (Nicolet iS50 Thermo Nicolet American) with KBr as diluent were scanned in the wavelength range of 400minus 4000 cmminus1 to analyze the functional groups The microstructures of the kaolin and fired bodies were studied with scanning electron microscope (SEM FEIQUANTA 600) with a field emission gun and without the need to conduct enhanced conductivity treatment on the sample Microparticle size and distribution of the powders were obtained by laser particle size distribution instrument (Sympatec Helos-Rodos Sichuan China) with hexametaphosphate as a dispersing solvent The specific surface area and pore size distribution were determined by N2 adsorptiondesorption isotherms at 350 degC using automated surface-area amp pore size analyzer (Quadrasorb-evo American) prior to N2


2497

physisorption samples were degassed for 4minus5 h using helium 3 Results and discussion 31 Thermal analysis The thermal behavior of the starting kaolin is very simple from a qualitative point of view (Figure 4) and Table 2 clearly clarifies the relevant chemical reactions during the activation of kaolin The DSC curves show the de-hydroxylation reaction and transformation of kaolinite which can be divided into the following stages Stage I The stage is from room temperature to 250 degC There are two endothermic peaks at 407 and 2303 degC ascribed to the removal of physical absorbed water and the removal of layer hydroxyl water resulting in the mass loss of 049 wt and 138 wt corresponding 0068 unity and 019 unity of H2O respectively Stage II In the temperature range of 250minus 800 degC there is an intensive endothermic peak occurring at 4719 degC attributed to de-hydroxylation and convert to meta-kaolin and the mass loss is about 977 wt with a loss of 137 unity of H2O Stage III The stage is from 800 to 1400 degC

Figure 4 DSC-TG curve of kaolin

The exothermic peak at 9828 degC may be attributed to the transformation of AlSi spinel to pseudomullite It can be seen that kaolin is accompanied by a large number of physical and chemical reactions in the process of calcination when it undergoes convertion from kaolin to metakaolin spinel type phase and format mullite 32 X-ray diffraction analysis The XRD of the kaolinite calcination products at different temperatures using conventional and microwave heating are shown in Figures 5(a) and (b) It is indicated that well-defined reflections at the two values of 1215deg and 2644deg (corresponding to the d values of 72243 Aring these peaks correspond to the reflections from (001)) are ascribed to kaolin At 500 degC practically all peaks corresponding to kaolinite have disappeared in the microwave field holding for 30 min generating a featureless band of X-ray amorphous metakaolin and quartz that could be identified However it requires 600 degC holding for 120 min by the conventional route suggesting that the calcined kaolin has removed a large amount of hydroxyl groups in the structure when the crystal structure is destroyed The quartz and muscovite peaks remain for the kaolin after calcination suggesting that the muscovite structure remains intact and is not dehydrated As the temperature of calcination increases the diffraction peak of mullite begins to appear in the pattern of calcined samples at 900 degC for 30 min in the microwave field and become distinct at 1000 degC The mullite diffraction peaks are visible eventually in the two calcination routes but the difference between the two is that the conventional calcination must be kept at 1100 degC for 120 min while the microwave calcination only needs to be kept at 1000 degC for 30 min The patterns exhibit no significant change at 700minus900 degC and

Table 2 Related chemical reactions of kaolin transformation

Temperature rangedegC Thermal change Reaction equation Mass loss Heat effect

Room temperatureminus70 Removal of physical

absorbed water Al2O3∙2SiO2∙(156H2O+0068H2O)rarr Al2O3∙2SiO2∙156 H2O+0068H2Ouarr

049 Endothermic

70minus250 Removal of layer hydroxyl water

Al2O3∙2SiO2∙156H2Orarr Al2O3∙2SiO2∙137H2O+019H2Ouarr

138 Endothermic

250minus800 Removal of dehydroxylation

to meta-kaolin Al2O3∙2SiO2∙137H2Orarr Al2O3∙2SiO2+137H2Ouarr

977 Endothermic

800minus1400 Transformation of AlSi spinel

to pseudo-mullite 2Al2O3∙3SiO2rarr3Al2O3∙2SiO2+SiO2 0 Exothermic


2498

Figure 5 XRD pattern of kaolin at different temperatures

(a) Conventional heating (b) Microwave heating

600minus800 degC and characteristics of a quasi- amorphous material in conventional and microwave heating systems The above analysis shows that well-ordered kaolinite is transformed to less reactive metakaolinite by conventional calcination at 600 degC for 120 min as well as by microwave at 500 degC for 30 min Thus the microwave calcination time is significantly shortened which is sufficient to show the superiority of microwave heating energy saving and high efficiency 33 Fourier infrared spectroscopy (FT-IR) Distinctly different from the starting ore the FT-IR bands of calcination products show extreme distinctions in Figures 6 and 7 The FT-IR spectra of the crude clay show peaks at 3695 3619 1634 1030 911 789 753 693 536 468 and 429 cmminus1

Figure 6 Infrared spectrum of original kaolin

Figure 7 Infrared spectra of kaolin samples after

calcination at different calcination temperatures

(a) Conventional calcination (b) Microwave field

characteristic of kaolinite [23] The 3619 cmminus1 peak has been appointed to the internal hydroxyl group the 3695 cmminus1 peak corresponds to the internal surface OminusH group the 1098 and 1030 cmminus1 peaks are attributed to SiminusO stretching The AlminusOH is assigned to 911 cmminus1 the peaks at 789 and 753 cmminus1 are assigned to vs (SiminusOminusSi) SiminusOminusAl vibration bands lies in 536 cmminus1 and finally the peaks at 468


2499

and 429 cmminus1 are adapted to the deformation vibration of SiminusO [26 27] As the temperature increases the absorption peaks of 3695 3619 and 911 cmminus1 disappeared indicating that the internal structure water of kaolin is lost and the kaolin structure has been destroyed and transformed into metakaolin A low broad peak at ~3440 cmminus1 is attributed to the absorption of associated hydroxyls formed by the coupling agent molecules on the surface of the mineral [28] The peak near 1637 cmminus1 ascribed to the bending vibration mode of physisorbed water on the surface of free silica produced is quite intense [29] The presence of the vibration band at 1000minus1100 cmminus1 for metakaolinite is assigned to the stretching SiminusO bonds in amorphous silica [30] The broad band of metakaolinite located at 791 cmminus1 assigned to the AlminusO bonds in Al2O3 is observed The broad AlminusO octahedral stretching band at 555minus558 cmminus1 can be clearly identified The results are probably associated with the formation of four-coordinated Al species [31] The framework of mullite phase formed completely when the thermal treatment temperature was increased to near 1000 degC which is in good agreement with the XRD characterization results These bands all prove the conversion of kaolin to metakaolin which needs to be calcined to 600 degC for 120 min in the conventional route while that only needs to be kept at 500 degC for 30 min in the microwave field In comparison the peak at 555minus558 cmminus1 occurred in conventional as well as microwave field at different temperatures indicating that there is a heat gap between microwave heating and conventional calcaination where microwave heating efficiency is higher and the required temperature is lower 34 SEM analysis The SEM images of the crude ore at different magnifications are shown in Figure 8 The surface morphology of crude ore is a mixture of flakes and rods wherein the flaky crystal forms are pseudo hexagons exhibiting an irregular shape and relatively poor crystallinity [32] As shown in Figures 9 and 10 with the increase of temperature the surface morphology of kaolin changed significantly For example the rod structure decreased the flaky structure increased the slab-like fragments increased and varying degrees of stacking and agglomeration appeared The kaolin

Figure 8 SEM images of crude ore (a) 5000 times

(b) 20000 times (c) 50000 times

desorbs the internal and external hydroxyl groups at the same time the structure of the aluminoxy octahedron is destroyed during the calcination process but the silicon tetrahedron still maintains a layered structure resulting in the crystal lattice of the kaolin change However agglomeration appeared slightly in the microwave field compared to the conventional route It is attributed to the difference of the mechanism of conventional heating and microwave heating where microwave heating makes the material be more evenly heated resulting in agglomeration less


2500

Figure 9 SEM images of calcined kaolin at different temperatures by conventional route (a) 600 degC (b) 700 degC (c) 800 degC (d) 900 degC (e) 1000 degC (f) 1100 degC (g) 1200 degC


2501

Figure 10 SEM images of calcined kaolin at different temperatures in microwave field (a) 500 degC (b) 600 degC

(c) 700 degC (d) 800 degC (e) 900 degC (f) 1000 degC

35 Particle size distribution analysis The particle size distributions of kaolin at 500 degC in the microwave field and conventional calcination at 600 degC are shown in Figures 11(a) and (b) It can be seen that particle size distributions are (D90minusD10)(2timesD50)=1095 and 1351 in the microwave field and conventional calcination respectively the value is closer to 1 showing that the particle size distribution is narrower The particle size distribution of the calcination kaolin in the microwave field is mostly distributed in the range of 10minus50 μm and the particle size

distribution range is narrow while it is relatively divergent for that obtained by conventional calcination at 600 degC It is attributed to the fact that specific surface area of the microwave treated product is relatively stable as the temperature increases while the conventional calcination specific surface area fluctuates greatly resulting in uneven particle growth 36 Pore size analysis The N2 adsorptionminusdesorption isotherms of the kaolin raw material used in the experiment are


2502

Figure 11 Particle size distribution (a) Conventional

calcination (b) Microwave calcination

shown in Figure 12 At low pressure (PP0lt08) the adsorption amount (W) slowly increases however a sharp increase occurs posterior to PP0gt08 with capillary condensation due to pore size limitation In addition the adsorption curve and desorption curve in Figure 12 do not completely coincide and the existence of a hysteresis loop can be observed The adsorption and desorption isotherm of the kaolin belongs to the type IV adsorption and desorption isotherm and the hysteresis ring belongs to the type A hysteresis loop It is known from calculation that the specific surface area of kaolin is 20065 m2g (Table 3) It can be seen from Figure 12 that the pore size distribution of raw kaolin is relatively wide and there are abundant of mesopores and a handful of macropores Figures 13 and 14 show the N2 adsorptionminus desorption isotherm BET specific surface area curve and BJH pore size distribution curve of kaolin under the conventional condition (600 degC) and microwave field (500 degC) respectively It can be seen that the adsorptionminusdesorption isotherms the BET specific surface area curve and the BJH

Figure 12 N2 adsorption and desorption isotherm (a)

BET specific surface area curve (b) and BJH pore size

distribution chart (c) of raw kaolin

Table 3 Pore property analysis of kaolin raw material

Specific surface area(m2∙gminus1)

Pore volume (mL∙gminus1)

Average pore sizenm

20065 0062 12669

pore size distribution curve of kaolin are not significantly different from that prior to calcination After calcination the adsorptionminus


2503



distribution chart (c) of conventional calcined kaolin

(600 degC)

desorption isotherm is still considered to be the type IV of adsorptionminusdesorption isotherm and the hysteresis ring belongs to the type A hysteresis ring indicating that the pore structure is a cylindrical capillary with open ends In addition the pore size distribution of both routes after calcination is relatively extensive In the conventional calcination



distribution chart (c) of microwave calcined kaolin

(500 degC)

the distribution is concentrated at about 3835 nm while that is more concentrated near 383 nm in the microwave field After 15 nm the curve gradually flattens indicating that there are still substantial of mesopores and a small fraction of macropores after calcination By the calculation of the specific surface area graph drawn according to the BET


2504

equation a conclusion is drawn that the specific surface area of kaolin is 22383 m2g after conventional calcination while that is 23486 m2g in the microwave field Tables 4 and 5 show the pore performance analysis of calcined kaolin The specific surface area pore volume average pore size and external surface area of the calcined kaolin and the raw materials are not significantly different which indicates that calcination will not significantly affect the pore performance of kaolin Table 4 Pore property analysis of calcined kaolin



Average pore sizenm

22383 0068 12325

Table 5 Pore property analysis of kaolin raw material Specific

surface area(m2∙gminus1) Pore

volume(mL∙gminus1) Average

pore sizenm

23486 0065 11433

4 Conclusions A comparison of the properties of the calcination kaolin samples subjected to conventional thermal treatment and microwave heated is studied and the conclusions are drawn as follows 1) The phase transition process of kaolin under the effect of microwave field is the same as that of conventional heating method from crystal phase (kaolin) to amorphous phase (metakaolin) and finally to crystal phase (mullite) 2) Compared with conventional calcination the time required for kaolin to transform into amorphous metakaolin under microwave field is reduced by 90 min the optimum temperature of kaolin transformed to metakaolin in the microwave field is 500 degC which is correspondingly reduced by 100 degC 3) Through SEM and laser particle size analysis the products obtained by microwave heating are uniformly distributed of the overall particle sizes and agglomeration appears less in the microwave field compared to the conventional thermal treatment 4) The N2 adsorptionminusdesorption isotherm BET specific surface area curve and BJH pore size distribution curve of kaolin indicate that the pore properties are almost invariable regardless of

calcination route during the process of calcining kaolin into metakaolin

Contributors ZHANG Liang-jing LUuml Peng and HE Yuan performed the experiments LI Shi-wei and YIN Shao-hua conceived and designed the study YIN Shao-hua and CHEN Kai-hua reviewed the manuscript ZHANG Liang-jing edited and reviewed the manuscript Peng Jin-hui and Zhang Li-bo reviewed the whole manuscript all the authors have read and reviewed this manuscript Conflict of interest ZHANG Liang-jing HE Yuan LUuml Peng PENG Jin-hui LI Shi-wei CHEN Kai-hua YIN Shao-hua ZHANG Li-bo declare that they have no conflict of interest

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TG-DSCFT-IRSEM粒度分析比表面积(BET)孔径分布(BJH)和 N2吸附minus脱附等温线以确定最

佳热活化温度结果表明微波场中高岭土向偏高岭土的转化在 500 degC 下保温 30 min 就能实现这

比常规煅烧温度低 100 degC时间缩短 90 min在微波与常规加热方法中高岭土相变过程相同SEM分析表明在微波场中产物粒度更均匀略有团聚高岭土的 N2 吸附minus解吸等温线BET 和 BJH分析表明在高岭土煅烧为偏高岭土的过程中无论以何种方式煅烧其孔隙性质几乎不变以上结

论表明在微波场中活化高岭土优于常规活化这主要是因为微波依靠物体吸收微波能量并将其转换

成热能来加热从而可以均匀地加热整个物质关键词高岭土热活化偏高岭土微波


2495

Figure 1 Kaolin structure

attention [3minus5] The synthesis of zeolite A from kaolinite comprises two steps metakaolinization and zeolitization [6] However the silicon and aluminum in kaolin are in crystal forms and crystalline kaolinite has high chemical inertness which cannot be utilized for synthesizing zeolites directly [7] Therefore kaolinite has to be calcined at high temperatures (700minus900 degC) whereby dehydroxylation takes place converting to amorphous and more reactive product (metakaolinite) prior to application for the synthesis of zeolites [8minus10] The plate-like structure will decrease the reactivity and surface area of reaction of kaolin which can be improved by appropriate treatment methods to destroy the crystal structure such as mechanical activation [11] thermal activation [12] acid-base modification activation [13 14] Due to its high passivity it is impossible to increase the reactivity of kaolin by chemical treatment even under drastic conditions [15] Heating to alter kaolin activity has become an important method which can transform the lamellar crystal structure of kaolin into disordered metakaolin structure by controlling the number of hydroxyl groups Some original groups in the inner layer of the crystal are exposed to obtain the special physical and chemical properties [16] The calcined product has the advantages of high strength good durability and corrosion resistance [17 18] However conventional calcination has the disadvantages of slow heating rate high energy consumption and uneven heating inside and outside the material Therefore it is essential to seek an alternative heating way to solve the above issues Microwave heating is a heating route that relies on objects to absorb microwave energy and convert it into heat energy so that the whole body can heat up at the same time which is completely different from other conventional heating methods

Microwave thermal [19] has been used as a new type of heating technology on account of the advantages of fast and efficient uniform interior heating relatively low reaction temperature energy saving environmental protection high product purity and no direct contact between the heating source and heated materials [20 21] ZHONG et al [22] reported a study which can reduce phase transition temperature and prepare finer and more uniform products ZHANG et al [23] recorded microwave selective heating-enhanced reaction rates for mullite preparation from kaolinite indicating that conventional heating kaolinite has to be heated at 1400 degC for 1 h to form well-ordered orthorhombic mullite (3Al2O3ꞏ2SiO2) that is accompanied by the formation of cristobalite while microwave heating at 900 degC for 5 min and 1200 degC for 1 min YOUSSEF et al [24] studied the microwave-assisted versus conventional synthesis of zeolite A from metakaolinite and came to conclusion that the rate of zeolite A formation was found to increase by 2ndash3 times in microwave treated samples with a notable enhancement in the product crystallinity and yield whether seeded or unseeded Therefore an economically viable route has been successfully applied for activating kaolin This work describes the thermal activation process of kaolin using conventional and microwave calcination The effects of the two calcinations on the temperature and time needed for the activation of kaolin were systematically studied and similarities and difference of products between the two calcination methods were comprehensively compared Analytical conclusions have been obtained by differential thermogravimetric analysis (TG-DSC) X-ray diffraction (XRD) Fourier transform infrared spectrometry (FT-IR) scanning electron microscopy (SEM) particle size analysis (PSD) specific surface area pore size distribution and N2 adsorption-desorption 2 Experimental 21 Chemical composition of kaolin Kaolin was supplied from Yunnan Xishuangbanna Wanxiang Mining Co Ltd China with the composition of elements in their oxide forms of 493 SiO2 37748 Al2O3 1271 K2O 0728 Na2O 0455 Fe2O3 0214 MgO and 0137 CaO and was used as the raw material The


2496


fraction ()


468 398 12 08 04 02 01






2497








049 Endothermic



138 Endothermic



977 Endothermic




2498










2499



(b) 20000 times (c) 50000 times



2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2496


fraction ()


468 398 12 08 04 02 01






2497








049 Endothermic



138 Endothermic



977 Endothermic




2498










2499



(b) 20000 times (c) 50000 times



2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2497








049 Endothermic



138 Endothermic



977 Endothermic




2498










2499



(b) 20000 times (c) 50000 times



2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2498










2499



(b) 20000 times (c) 50000 times



2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2499



(b) 20000 times (c) 50000 times



2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2500



2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2501






2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2502










Average pore sizenm

20065 0062 12669



2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2503




(600 degC)





(500 degC)



2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2504




Average pore sizenm

22383 0068 12325




pore sizenm

23486 0065 11433









2020 297 110046 DOI 101016jmicromeso2020110046





19820300301





006









2020 293 109772 DOI 101016jmicromeso2019109772





030





2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2505

001





010




107639 DOI 101016jinoche2019107639






2020 257 120399 DOI 101016jjclepro2020120399






144






2019117092






105187 DOI 101016jclay2019105187





23285 DOI 101016jmicromeso201606010




101016jceramint201610115





jclay201005012





jclay201706029




2018 171 558minus565






101016jceramint201904035





2017 77 12minus27 DOI 101016jrser201703135







003







101039C3RA43767A





030



(in Chinese)





S0022-2860(03)00249-7





101021jp001075s







1996031211





184 DOI 101016jmicromeso200507008




2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读








2506


jclay200901014



DOI 101016S0169-1317(01)00057-6


中文导读







comparison of microwave and conventional heating routes

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