Investigation on Calcination Behaviors of Coal Gangue by Fluidized Calcination in Comparison with Static Calcination

Article Investigation on Calcination Behaviors of Coal Gangue by Fluidized Calcination in Comparison with Static Calcination

Shuai Yuan 1,2,*, Yanjun Li 1,2,*, Yuexin Han 1, Peng Gao 1 and Guichen Gong 1 1 College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China; [email protected] (Y.H.); [email protected] (P.G.); [email protected] (G.G.) 2 Research Center of Coal Resources Safe Mining and Clean Utilization, Liaoning Technical University, Fuxin 123000, China * Correspondence: [email protected] (S.Y.); [email protected] (Y.L.); Tel.: +86-24-8367-6828 (S.Y.)

Academic Editor: William Skinner Received: 12 December 2016; Accepted: 10 February 2017; Published: 14 February 2017

Abstract: In order to comprehensively utilize coal gangue, we present fluidized calcination as a new thermal technology for activating coal gangue and systematical study was conducted in comparison with static calcination. The calcined products obtained by different calcination methods under various temperatures were characterized by the means of X-ray diffraction (XRD), thermal gravimetry-differential scanning calorimeter (TG-DSC), Fourier transform-infrared spectroscopy (FT-IR) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS). Chemical and physical characteristics such as aluminium leaching rate, chemical oxygen demand and whiteness of calcined products were also investigated. The results show that aluminium leaching rate could reach to the maximal value 74.42% at 500 ◦C by fluidized calcination, while the maximal value of 66.33% could be reached at 600 ◦C by static calcination. Products by fluidized calcination obtained higher whiteness and lower chemical oxygen demand (COD) under the same calcination temperature. The well-crystallized kaolinite transform to amorphous meta-kaolinite under 600 ◦C and mullite presence under 1000 ◦C according to phase transformation, chemical bond variation and microstructure evolution analysis. Fluidized calcination was more efficiently for combustion of carbon/organic matter and dehydroxylation of kaolinite, which might applied in coal gangue industry in future.

Keywords: coal gangue; ﬂuidized calcination; static calcination; thermodynamic characteristics; transformation behaviors

1. Introduction Coal gangue is an associated resource of complex industrial solid waste discharged when coal mining and washing in the production course [1,2]. The amount of coal gangue has already accumulated 3.8 billion tons and increasing at a rate of 0.2 billion tons per year in China, which has caused many serious environmental problems [3,4]. In recent years, many types of research have been conducted to develop its utilization as the abundant mineral resource for the production of paper, rubber, building materials, and chemical products, etc. [5] The main mineral composition of coal gangue is kaolinite, carbon/organic matter and iron minerals. Kaolinite (Al2O3·2SiO2·2H2O) was phyllosilicate with 1:1 type structure formed by the connection of SiO4 tetrahedral layer and AlO2(OH)4 octahedral layer [6–9], as shown in Figure1. In this crystal structure, the Si and Al elements didn‘t have the chemical activity. Thermal activation by the calcination for coal gangue at 600–900 ◦C as the most common activation method [10,11].

Minerals 2017, 7, 19; doi:10.3390/min7020019 www.mdpi.com/journal/minerals Minerals 2017, 7, 19 2 of 11 this crystal structure, the Si and Al elements didn‘t have the chemical activity. Thermal activation by Minerals 2017, 7, 19 2 of 11 the calcination for coal gangue at 600–900 °C as the most common activation method [10,11].

(a) (b)

FigureFigure 1. 1. ModelModel for for of of crystal crystal structure structure kaolinite. kaolinite. White White ball: ball: H; H; red red ball: ball: O; O; purple purple ball: ball: Al; Al; yellow yellow ball:ball: Si. Si. (a (a) )Ball-and-stick Ball-and-stick model; model; (b (b) )Polyhedron Polyhedron model. model.

Combustion of carbon/organic matter and dehydroxylation of kaolinite are two principal Combustion of carbon/organic matter and dehydroxylation of kaolinite are two principal reactions in calcination process, after this process, performance indexes such as whiteness and reactions in calcination process, after this process, performance indexes such as whiteness and activity activity of calcined products are significantly improved for industry applications [3,12,13]. The of calcined products are significantly improved for industry applications [3,12,13]. The burnout level of burnout level of carbon/organic matter of calcined products can be measured by the chemical oxygen carbon/organic matter of calcined products can be measured by the chemical oxygen demand (COD), demand (COD), and the activity of meta-kaolinite can be evaluated by leaching rate of aluminum and the activity of meta-kaolinite can be evaluated by leaching rate of aluminum from meta-kaolinite from meta-kaolinite in hydrochloric acid solution [14–18]. in hydrochloric acid solution [14–18]. The fluidized bed technology provides the advantages of uniform temperature as well as the The fluidized bed technology provides the advantages of uniform temperature as well as the high efficiency of heat and mass transfer, which hence widely used in many industries and fields high efficiency of heat and mass transfer, which hence widely used in many industries and fields such such as alumina calcination and iron ore magnetization roasting [19,20]. In view of this, we as alumina calcination and iron ore magnetization roasting [19,20]. In view of this, we considered considered absorbing these advantages in order to form fluidized calcination technology suitable for absorbing these advantages in order to form fluidized calcination technology suitable for the coal the coal gangue utilization. gangue utilization. Prior to this, few research on fluidized calcination of coal gangue have been reported. Therefore, Prior to this, few research on fluidized calcination of coal gangue have been reported. Therefore, in order to explore the possibility of fluidized calcination for coal gangue, the calcination behaviors in order to explore the possibility of fluidized calcination for coal gangue, the calcination behaviors by fluidized calcination in comparison with static calcination should be studied comprehensively and by fluidized calcination in comparison with static calcination should be studied comprehensively systematically. In this paper, the activation behavior, the reaction in the process, performance of and systematically. In this paper, the activation behavior, the reaction in the process, performance of products and the phases and structure changes for the fluidized calcination and static calcination products and the phases and structure changes for the fluidized calcination and static calcination were were investigated in detail. investigated in detail. 2. Experimental Procedures 2. Experimental Procedures

2.1.2.1. Materials Materials TheThe coal coal gangue gangue used used in inthis this experiment experiment was was collected collected from from Shuozhou, Shuozhou, Shanxi Shanxi province. province. The mineralogicalThe mineralogical phase phaseis characterized is characterized by X-ray by diffraction X-ray diffraction (XRD) as (XRD)shown asin shownFigure 2. in Chemical Figure2. analysisChemical of analysis the sample of the was sample shown was shownin Table in Table1. It 1indicates. It indicates kaolinite kaolinite and and quartz quartz are are the the major major crystallizedcrystallized minerals minerals presented presented in in the the raw raw coal coal gangue. gangue. The The ignition ignition loss loss (LOI) (LOI) is is mainly mainly including including adsorbedadsorbed water, water, crystal crystal water, water, carbon and organic matter.matter. TheThe morphologic morphologic features features of of the the raw raw coal coal gangue gangue from from scanning scanning electron electron microscope microscope and and energy energy dispersivedispersive spectrometer spectrometer (SEM andand EDS)EDS) analysesanalyses are are shown shown in in Figure Figure3 and 3 and Table Table2. It 2. can It becan seen be seen that thatprimary primary element element of the of the particles particles are are Al, SiAl, and Si and O. TheO. The ratio ratio of Al/Si of Al/Si is between is between 0.92 0.92 and and 0.96 0.96 in thein theparticles, particles, which which is close is close to Al/Si to Al/Si of kaolinite. of kaolinite. It demonstrates It demonstrates the particles the particles are mainly are kaolinitemainly kaolinite minerals. minerals.

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FigureFigure 2. 2. X-rayX-ray diffraction (XRD) (XRD) pattern pattern of of raw raw coal coal gangue. gangue.

TableTable 1.1. ChemicalChemical compositioncomposition ofof coalcoal ganguegangue (mass(mass %).%). VM:VM: VolatileVolatile matter; matter; LOI: LOI: Loss Loss on on ignition. ignition.

Al2O3 SiO2 Fe2O3 TiO2 CaO MgO K2ONa2OP S C VM LOI Al O SiO Fe O TiO CaO MgO K O Na OPS LOI 37.062 3 46.3 2 0.262 3 0.38 2 0.055 0.046 0.0722 0.0222 0.007 0.038 C 1.63 VM 1.17 15.64 37.06 46.3 0.26 0.38 0.055 0.046 0.072 0.022 0.007 0.038 1.63 1.17 15.64

(a) (a) (b)

FigureFigure 3. Scanning 3. Scanning electron electron microscope microscope (SEM) images (SEM) imagesand energy and dispersive energy dispersive spectrometer spectrometer (EDS) analysis (EDS) of raw coal analysisgangue. ( ofa) rawSEM coal image; gangue. (b) Spot (a) SEMscanning; image; (c) ( bSurface) spot scanning;scanning of (c) Si surface element; scanning (d) Surface of Si element;scanning of O element;(d) surface (e) Surface scanning scanning of O of element; Al element. (e) surface scanning of Al element.

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Table 2. EDS analysis of raw coal gangue. Table 2. EDS analysis of raw coal gangue. Content (wt %) Elements Content (wt %) ElementsPoint-1 Point-2 Point-3 O 52.98 Point-1 Point-262.90 Point-3 49.29 Al O22.77 52.98 62.9018.20 49.29 24.24 Si Al24.25 22.77 18.2018.90 24.24 26.47 Al/Si Si0.94 24.25 18.900.96 26.47 0.92 Al/Si 0.94 0.96 0.92 2.2. Calcination Methods 2.2. Calcination Methods The raw coal gangue was dry grinding for −45 μm, occupying 85% of the specific surface area of 1030 Them2/kg raw with coal minimum gangue was fluidization dry grinding velocity for − of45 0.08µm, occupyingm/s. The sample 85% of was the specificcalcined surface at various area 2 temperaturesof 1030 m /kg (via with 400 minimum to 1000 fluidization °C) for 180 velocity min by of 0.08fluidized m/s. Thecalcination sample wasand calcinedstatic calcination, at various ◦ respectively.temperatures The (via fluidized 400 to 1000 calcinationC) for 180 experiments min by fluidized were calcination performed and in a static customized calcination, vertical respectively. tubular furnaceThe fluidized system calcination as shown experimentsin Figure 4, the were internal performed diameter in a customizedof the tubular vertical furnace tubular was 25 furnace mm. During system theas shown experiments, in Figure the4, the15 g internal sample diameter was put ofon the the tubular gas distributor, furnace was and 25 the mm. furnace During was the heated experiments, at the ◦ ratethe 15of g10 sample °C/min, was the put air was on the supplied gas distributor, as the iner andt gas the through furnace the was bottom heated of at the the tubular rate of furnace 10 C/min, at a constantthe air was flow supplied rate of as 800 the mL/min. inert gas The through static the calcination bottom of was the tubularused closed furnace programmable at a constant box-type flow rate resistanceof 800 mL/min. furnace The with static the calcination temperature was controlled used closed (KSL-1400X; programmable HF-Kejing box-type Materials resistance Technology furnace withCo., Ltd.,the temperature Hefei, China), controlled and the (KSL-1400X;15 g sample was HF-Kejing put in the Materials crucible, Technology and the resistance Co., Ltd., furnace Hefei, China),was heated and ◦ atthe the 15 rate g sample of 10 was°C/min. put in the crucible, and the resistance furnace was heated at the rate of 10 C/min.

Figure 4. Schematic of fluidized calcination experimental apparatus. Figure 4. Schematic of fluidized calcination experimental apparatus. 2.3. Testing Methods 2.3. Testing Methods TG-DSC (thermal gravimetry-differential scanning calorimeter) technique was the method TG-DSC (thermal gravimetry-differential scanning calorimeter) technique was the method widely used to characterize the combustion behavior of the coal gangue powder during calcination. widely used to characterize the combustion behavior of the coal gangue powder during calcination. Experiments were carried out in a simultaneous thermal analyzer (Netzsch Scientific Instruments Experiments were carried out in a simultaneous thermal analyzer (Netzsch Scientific Instruments Trading Ltd., Selb, Germany), and the experiment was carried out under air atmosphere from room Trading Ltd., Selb, Germany), and the experiment was carried out under air atmosphere from room temperature to 1200 °C at a heating rate of 10 °C/min [13]. temperature to 1200 ◦C at a heating rate of 10 ◦C/min [13]. The content of Al2O3 of calcined coal gangue was analyzed to determine by the chemical titration. The content of Al2O3 of calcined coal gangue was analyzed to determine by the chemical titration. The conventional dichromate method (K2Cr2O7 method) was used to measure chemical oxygen The conventional dichromate method (K Cr O method) was used to measure chemical oxygen demand (COD) value according to the National2 2 7 Standard of China (JC/T 2156-2012) [21]. The demand (COD) value according to the National Standard of China (JC/T 2156-2012) [21]. The whiteness whiteness of calcined coal gangue was measured with the whiteness meter WSB-2 [17]. of calcined coal gangue was measured with the whiteness meter WSB-2 [17]. The leaching processes as follows: 2 g of calcined coal gangue and 50 mL 20 wt % hydrochloric The leaching processes as follows: 2 g of calcined coal gangue and 50 mL 20 wt % hydrochloric acid solutions were added in a 3-mouth flash with solid–liquid ratio of 1:25 g/mL. The flask was acid solutions were added in a 3-mouth flash with solid–liquid ratio of 1:25 g/mL. The flask was heated to 90 °C in magnetic stirrers and stirred for 60 min. Then the mixture was filtered and washed heated to 90 ◦C in magnetic stirrers and stirred for 60 min. Then the mixture was filtered and washed with deionized water for many times, the filtrate was analyzed to determine aluminum by inductively coupled plasma atomic emission spectrometer (ICP-AES), and the dissolution

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Mineralswith deionized 2017, 7, 19 water for many times, the filtrate was analyzed to determine aluminum by inductively5 of 11 coupled plasma atomic emission spectrometer (ICP-AES), and the dissolution percentages of Al2O3 percentageswere calculated of Al [218O3, 22were]. The calculated dissolution [18,22]. of AlThe3+ dissolutionin hydrochloric of Al3+ acid in hydrochloric solution was acid the solution indicator was to theevaluate indicator the activationto evaluate efficiency the activation of calcined efficiency coal of gangue. calcined Leaching coal gangue. of calcined Leaching coal of gangue calcined in coal HCl ganguesolution in with HCl the solution main reaction:with the main reaction: +→ + Al23 O 6HCl 2AlCl 3 3H 2 O (1) Al2O3 + 6HCl → 2AlCl3 + 3H2O (1) The phase transformation of calcination products was investigated by PANalytical X’pert The phase transformation of calcination products was investigated by PANalytical X’pert PW3040 PW3040 (PANalytical B.V. Ltd., Almelo, The Netherlands). The operating voltage and current were (PANalytical B.V. Ltd., Almelo, The Netherlands). The operating voltage and current were 40 kV and 40 kV and 40 mA, respectively. The diffraction angle was scanned from 5° to 90°. 40 mA, respectively. The diffraction angle was scanned from 5◦ to 90◦. The chemical structures of calcined coal gangue under different calcination temperatures were The chemical structures of calcined coal gangue under different calcination temperatures were analyzed by Fourier transform infrared spectra using Nicolet 380 FT-IR spectrometer (Thermo Fisher analyzed by Fourier transform infrared spectra using Nicolet 380 FT-IR spectrometer (Thermo Fisher Scientific Ltd., Waltham, MA, USA), and 1 mg sample accompanied with 100 mg KBr usually [23]. Scientific Ltd., Waltham, MA, USA), and 1 mg sample accompanied with 100 mg KBr usually [23]. The spectra were recorded in the range of 4000–500 with 4 cm−1 resolution. The spectra were recorded in the range of 4000–500 with 4 cm−1 resolution. The morphology and microstructure of calcined products were observed by scanning electron The morphology and microstructure of calcined products were observed by scanning electron microscopy (SEM; S-3400N; Hitachi, Ltd., Tokyo, Japan) and composition analysis was carried out by microscopy (SEM; S-3400N; Hitachi, Ltd., Tokyo, Japan) and composition analysis was carried out by energy-dispersive spectrometry (EDS) combined with SEM. At the same time, samples were coated energy-dispersive spectrometry (EDS) combined with SEM. At the same time, samples were coated with gold using a sputter coater to increase conductivity prior to SEM characterization [24]. with gold using a sputter coater to increase conductivity prior to SEM characterization [24].

3.3. Results Results and and Discussion Discussion

3.1.3.1. Thermal Thermal Analysis Analysis TheThe result result of of TG-DSC TG-DSC analysis analysis of of the the sample sample is is presented presented in in Figure Figure 5.5 .The The sample sample undergoes undergoes a seriesa series of ofphysical physical and and chemical chemical variations variations with with th thee increasing increasing temperature, temperature, and and these these variations variations are: are: TheThe weight weight loss loss of of 1.89% 1.89% is is attributed attributed to to the the absorb absorbeded water water is is liberated liberated and and the the ignitable ignitable substances substances decomposedecompose and and burn burn before before 455.6 455.6 °C.◦C. Structural Structural hy hydroxyldroxyl groups groups were were removed removed from from kaolinite kaolinite by by a 2 3 2 reorganizationa reorganization of ofthe the octahedral octahedral laye layerr of of kaolinite kaolinite to to meta-kaolinite (Al 2OO3∙2SiO·2SiO)2 )as as described described by by EquationEquation (2), (2), when when temperature temperature raised raised up up to to a range of 455.6~691.1 °C,◦C, resulting resulting in in weight weight loss loss of of aboutabout 13.62%. 13.62%.

Figure 5. TG and DSC analysis of raw coal gangue under air flow. Figure 5. TG and DSC analysis of raw coal gangue under air ﬂow.

The result of TG-DSC analysis of the sample is presented in Figure 5. The sample undergoes a The result of TG-DSC analysis of the sample is presented in Figure5. The sample undergoes series of physical and chemical variations with the increasing temperature, and these variations are: a series of physical and chemical variations with the increasing temperature, and these variations are: The weight loss of 1.89% is attributed to the absorbed water is liberated and the ignitable substances The weight loss of 1.89% is attributed to the absorbed water is liberated and the ignitable substances decompose and burn before 455.6 °C. Structural hydroxyl groups were removed from kaolinite by a decompose and burn before 455.6 ◦C. Structural hydroxyl groups were removed from kaolinite by reorganization of the octahedral layer of kaolinite to meta-kaolinite (Al2O3∙2SiO2) as described by a reorganization of the octahedral layer of kaolinite to meta-kaolinite (Al O ·2SiO ) as described by Equation (2), when temperature raised up to a range of 455.6~691.1 °C, 2resulting3 2in weight loss of about 13.62%. The endothermic peak presented at 532.3 °C in Figure 5 might be contributed by the dehydroxylation of kaolinite and formation of meta-kaolinite; the exothermic peak at 1009.7 °C is due

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Equation (2), when temperature raised up to a range of 455.6~691.1 ◦C, resulting in weight loss of aboutMinerals 13.62%. 2017, 7, 19 6 of 11 The endothermic peak presented at 532.3 ◦C in Figure5 might be contributed by the ◦ dehydroxylationto the transformation of kaolinite of meta-kaolinite and formation into of mullite meta-kaolinite; (3Al2O3∙2SiO the exothermic2) as presented peak by at 1009.7EquationC is(3) due [25– to the27]. transformationIt deduced that of kaolinite meta-kaolinite in coal intogangue mullite begins (3Al to2O decompose3·2SiO2) as into presented amorphous by Equation meta-kaolinite (3) [25–27 at]. ◦ It450 deduced °C and thattransforms kaolinite to in the coal crystallized gangue begins mullite to decompose when the temperature into amorphous reaches meta-kaolinite up to 1000 at °C. 450 TheC ◦ andtwo thermal transforms induced to the processes crystallized can be mullite described when by the the temperature following reactions reaches [5,28]: up to 1000 C. The two thermal induced processes can be described by the following reactions [5,28]: ⋅⋅⎯⎯⎯450°C→⋅+ Al23 O 2SiO 2 2H 2 O Al 23 O 2SiO 2 2H 2 O()g (2) 450◦C Al2O3 · 2SiO2 · 2H2O −−−→ Al2O3 · 2SiO2 + 2H2O(g) (2) ()⋅⎯⎯⎯1000°C→⋅+ (3) 3 Al23 O 2SiO 2 3Al 23 O 2SiO 2 4SiO 2 1000◦C 3(Al2O3 · 2SiO2) −−−−→ 3Al2O3 · 2SiO2 + 4SiO2 (3)

3.2. Chemical and Physical Characteristics

It can can be be seen seen from from Figure Figure 6, the6, thecontent content of Al of2O3 Al in 2theO3 fluidizedin the fluidized calcination calcination products increased products increasedfaster and fasterhigher and than higher the static than calcination the static calcination products before products 600 before °C, indicating 600 ◦C,indicating that the fluidized that the fluidizedcalcination calcination was better was efficiency better efficiency to obtain to obtain high highquality quality products products by bycombustion combustion of of carbon, dehydroxylation ofof kaolinitekaolinite and and removal removal of of other other impurities impurities at at low low temperatures. temperatures. The The Al 2AlO23Ocontent3 content of productsof products by twoby differenttwo different calcination calcination methods method tendeds totended be stable to afterbe stable 700 ◦C, after which 700 was °C, corresponded which was wellcorresponded with the thermalwell with analysis. the thermal The contentanalysis. of The Al 2contentO3 increased of Al2O to3 increased the maximum to the value maximum 44.99% value and 44.95%,44.99% and respectively 44.95%, respectively at 1000 ◦C. at 1000 °C. The hydrochloric acid leaching calcination product presents a different behavior by fluidizedfluidized calcination and static calcination showed in Figure6 6.. AluminiumAluminium leachingleaching raterate ofof calcinedcalcined productsproducts increases withwith calcination calcination temperature temperature and and reaches reaches to the to maximal the maximal value at value 500–600 at ◦500–600C then decreased °C then dramaticallydecreased dramatically when temperature when temperature rise to 1000 rise◦C. to This 1000 was °C. mainly This attributedwas mainly to attributed the formation to the of meta-kaoliniteformation of meta-kaolinite derived from derived the dehydration from the dehydration of kaolinitestart of kaolinite at 450 ◦ C,start which at 450 would °C, which increase would the reactivityincrease the of andreactivity the aluminium of and the extraction aluminium rate extraction of calcined rate coal of gangue calcined [29 coal]. It isgangue noted that[29]. aluminiumIt is noted leachingthat aluminium rate from leaching calcined rate products from calcined by fluidized products calcination by fluidized is 25% calcination higher thanis 25% the higher leaching than rate the ofleaching static calcinationrate of static at calcination 500 ◦C. It impliedat 500 °C. that It im theplied fluidized that the calcination fluidized wascalcination more efficient was more to increaseefficient theto increase reactivity the of reactivity calcined of products. calcined Aluminum products. Aluminum leaching rate leaching was reduced rate was to reduced a minimum to a atminimum 1000 ◦C, whichat 1000 was °C, duewhich to was the transformationdue to the transformation of meta-kaolinite of meta-kaolinite into mullite. into mullite.

Figure 6. AluminiumAluminium leaching leaching rate rate of of calcined calcined products products by by fluidized ﬂuidized calcination ( a)) and static calcination (b) at various temperatures.

Figure7 7 shows shows thatthat thethe CODCOD valuevalue ofof ﬂuidizedfluidized calcination calcination products products reduce reduce to to 428 428 µμg/gg/g and the whiteness increases to 81.8 as the temperature increasedincreased to 1000 ◦°C.C. Meanwhile, the COD of static calcination product reduces to 820 µμg/g and and the the whiteness whiteness increases increases to to 75.3 75.3 at at the the same same temperature. temperature. The COD value and the whiteness in coal-bearing kao kaolinlin are mainly influenced inﬂuenced by carbon and organic matter, and they will be reduced or eliminated by combustion as the temperature increased in harmony with thermal analysis. It was found that calcined products could obtain lower COD value and higher whiteness by the fluidized calcination, which suggest that fluidized calcination was more efficient than static calcination for removal of impurities such as carbon and organic matter.

Minerals 2017, 7, 19 7 of 11 matter, and they will be reduced or eliminated by combustion as the temperature increased in harmony with thermal analysis. It was found that calcined products could obtain lower COD value and higher Minerals 2017, 7, 19 7 of 11 whitenessMinerals 2017 by, 7, the19 fluidized calcination, which suggest that fluidized calcination was more efficient7 thanof 11 static calcination for removal of impurities such as carbon and organic matter.

Figure 7. Chemical oxygen demand (COD) and whiteness of calcined products by fluidized Figurecalcination 7.7.Chemical Chemical (a) and oxygenstatic oxygen calcination demand demand (COD) (b ) (atCOD)and various whiteness and temperatures. whiteness of calcined of productscalcined by products fluidized calcinationby fluidized (a) calcinationand static calcination (a) and static (b) atcalcination various temperatures. (b) at various temperatures. 3.3. X-ray Diffraction Analysis 3.3. X-ray Diffraction Analysis XRD patterns of calcined coal gangue by different calcination methods at various temperatures are shownXRD patterns in Figure ofof calcinedcalcined8. The calcination coal coal gangue gangue level by by different differentcan be calcinationevaluated calcination by methods methodsthe crystallinity at variousat various temperaturesof temperatureskaolinite and are areshownquartz, shown inthe Figure in two Figure8 .primary The 8. calcinationThe diffraction calcination level peaks canlevel be ofcan evaluated kaolinite be evaluated byare the at by crystallinity 12.4° the crystallinityand of24.9°, kaolinite and of kaolinite the and primary quartz, and ◦ ◦ quartz,thediffraction two primarythe peaktwo diffractionofprimary quartz diffraction is peaks at 21°. of kaolinite Accompaniedpeaks of are kaolinite at 12.4temperature andare 24.9at rise,12.4°, and theand the intense primary24.9°, peaksand diffraction the of kaoliniteprimary peak ◦ diffractionofgradually quartz is disappear,peak at 21 of. Accompanied quartz which is causedat 21°. temperature byAccompanied the breakages rise, the temperature intenseof Al–OH peaks rise,bonds of the kaolinite and intense the gradually dehydroxylation peaks of disappear, kaolinite of whichgraduallykaolinite caused [30].disappear, by The the intense breakages which peak caused of Al–OHof bykaolinite the bonds breakages at and 12.4° the ofcompletely dehydroxylation Al–OH bonds disappear and of kaolinite theat 600dehydroxylation [°C30]. by The fluidized intense of ◦ ◦ ◦ kaolinitepeakcalcination of kaolinite [30]. but The remain at 12.4intense atcompletely 600 peak °C ofby kaolinite disappearstatic calcination, at at 12.4° 600 Ccompletelywhich by fluidized means disappear dehydroxylation calcination at but600 remain°C of by kaolinite fluidized at 600 byC calcinationbyfluidized staticcalcination, calcination but remain whichwas at 600 more means °C effectiveby dehydroxylationstatic thancalcination, static calcination. ofwhich kaolinite means The by dehydroxylation fluidized kaolinitecalcination becomes of kaoliniteamorphous was more by ◦ fluidizedeffectivefrom 450 than°Ccalcination and static transforms calcination. was more to mullite effective The kaoliniteat 1000 than °C, stat becomes andic calcination.the amorphousfluidized The calcination fromkaolinite 450 produced becomesC and transforms more amorphous mullite to ◦ ◦ frommulliteat 1000 450 °C at °C 1000than and thetransformsC, andstatic the calcination to fluidized mullite asat calcination 1000shown °C, in and producedFigure the fluidized 8. It more is noted calcination mullite that atmullite 1000produced hasC than low more activities, the mullite static atcalcinationwhich 1000 was°C than asalso shown theconsistent static in Figure calcination with8. the It is analysisas noted shown that results in mullite Figure of the has 8. thermalIt low is noted activities, analysis that whichmullite and wasthe has leaching also low consistent activities, rate of withwhichaluminum. the was analysis alsoTherefore, consistent results the of temperaturewith the thermalthe analysis should analysis results be andcont of the rolledthe leaching thermal accurately rateanalysis ofin aluminum.order and tothe get leaching Therefore,better activityrate the of aluminum.temperatureproperties. Therefore, should be the controlled temperature accurately should in be order cont torolled get better accurately activity in properties.order to get better activity properties.

FigureFigure 8.8. XRDXRD patternspatterns ofof calcinedcalcined productsproducts byby ﬂuidizedfluidized calcinationcalcination (a)) andand staticstatic calcinationcalcination ((b)) atat Figurevariousvarious 8. temperatures.temperatures. XRD patterns of calcined products by fluidized calcination (a) and static calcination (b) at various temperatures. 3.4. Fourier Transform Infrared Spectroscopy Analysis 3.4. Fourier Transform Infrared Spectroscopy Analysis A comparison of the functional groups for calcined products by different calcination methods is determinedA comparison by FTIR of thespectra functional in the groupsregion for4000–500 calcined cm products−1 and described by different in Figurecalcination 9. FTIR methods spectra is determinedresults are important by FTIR spectrain investigating in the region combustion 4000–500 of carbon/organiccm−1 and described matter in andFigure dehydroxylation 9. FTIR spectra of results are important in investigating combustion of carbon/organic matter and dehydroxylation of

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3.4. Fourier Transform Infrared Spectroscopy Analysis

MineralsA 2017 comparison, 7, 19 of the functional groups for calcined products by different calcination methods8 of 11 is determined by FTIR spectra in the region 4000–500 cm−1 and described in Figure9. FTIR spectra kaolinite.results are We important can see that in investigating calcined products combustion have similar of carbon/organic development matter rules andby fluidized dehydroxylation calcination of andkaolinite. static Wecalcination. can see that According calcined to products Hongfei have Cheng similar [23], development bands at 3695 rules and by 3619 ﬂuidized cm−1 calcinationwere O–H −1 stretchingand static calcination.vibrations, which According are the to Hongfeihydroxyl Cheng stretching [23], of bands inner at surface 3695 and hydroxyl 3619 cm andwere the inner O–H hydroxylstretching of vibrations, kaolinite respec whichtively. arethe The hydroxyl band at 1632 stretching cm−1 is attributed of inner surface to O–H hydroxyl stretching and and the bending inner −1 vibrationshydroxyl of of kaolinite absorbed respectively. water, indicates The band that absorb at 1632ed cm wateris attributedpresent on to the O–H raw stretching coal gangue. and None bending of thevibrations preceding of absorbed bands was water, present indicates on thatthe absorbedspectra of water calcined present product on the after rawcoal 600 gangue.°C, which None suggest of the ◦ suggestspreceding that bands the was coal present gangue on theunderwent spectra ofdehydroxylation calcined product and after produced 600 C, which meta- suggest kaolinite. suggests The disappearancethat the coal gangue of bands underwent at 914 and dehydroxylation 539 cm−1 signifies and the produced breakages meta- of Al–OH kaolinite. and Si–O–Al The disappearance functional −1 groupsof bands in at kaolinite. 914 and 539The cm absencesigniﬁes of 803 the and breakages 697 cm of−1 Al–OH and appearance and Si–O–Al of functional1100 and groups810 cm in−1 −1 −1 demonstrateskaolinite. The that absence the depolymerization of 803 and 697 cm of silicanda tetrahedrons. appearance ofAromatic 1100 and CH 810 bond cm arounddemonstrates 756 cm−1 − arethat intense the depolymerization in uncalcined ofcoal silica gangue tetrahedrons. and gradually Aromatic eliminated CH bond with around the temperature 756 cm 1 are increased, intense in whichuncalcined attributed coal gangue to combustion and gradually of carbonaceous eliminated withor organic the temperature matter and increased, combustion which in attributedcoal gangue to [31].combustion of carbonaceous or organic matter and combustion in coal gangue [31].

Figure 9.9.Fourier Fourier transform transform infrared infrared spectroscopy spectroscopy spectra sp ofectra calcined of productscalcined by products fluidized calcinationby fluidized (a) calcinationand static calcination (a) and static (b) atcalcination different temperatures.(b) at different temperatures.

3.5. Scanning Scanning Electron Microscope Analysis Morphology and microstructure evolution analysis of calcined products by different different calcination methods are are collected collected in in the the Figure Figure 10. 10 It. can It can be known be known from from SEM SEM images images (a-600 (a-600 and b-600) and b-600)that meta- that kaolinitemeta-kaolinite had the had irregular the irregular characteristics characteristics of lamellar of lamellar structures structures under under 600 600 °C,◦C, and and more more scale-shaped scale-shaped lamellar structures dissociated fromfrom thethe groupgroup particleparticle byby thethe fluidizedfluidized calcination.calcination. ThisThis might be due to the dehydroxylation of kaolinite and phase tr transformansform to meta-kaolinite. The structure of the aluminum oxide oxide octahedral octahedral layer layer was was destroyed, destroyed, while while the the silicic silicic oxygen oxygen tetrahedron tetrahedron preserved preserved the originalthe original lamellar lamellar structure, structure, which which made made the the meta-kaolinite meta-kaolinite maintain maintain the the majority majority of of lamellar structures. As mentioned in the previous Section 3.2,3.2, we could confirm confirm that the leaching rate and react activity of calcined calcined mainly mainly depend depend on on the the non-crystallizing non-crystallizing degree degree of of kaolinite kaolinite after after calcination. calcination. The meta-kaolinite lamellar structure produced th thee disordered structure at 800 ◦°C,C, and fluidized fluidized calcination products appeared to have more obvious disordered structure. The lamellar structure disappeared by fluidized fluidized calcination and a small amount of lamellar structure is still maintained by static calcination at 1000 °C,◦C, which coincides well with thermal analysis, meta-kaolinite transformed to mullite mullite when when temperature temperature reaches reaches up up to to1000 1000 °C.◦ TheC. The microstructural microstructural evolution evolution demonstrated demonstrated that activationthat activation of coal of coalgangue gangue using using calcination calcination with with fluidized fluidized calcination calcination has has the the good good effect effect for accelerating crystal transformation and reactivity.

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Figure 10. Scanning Electron Microscope images of calcined products by fluidized ﬂuidized calcination ( a) and static calcination (b) at different temperatures.

4. Conclusions Thermal activation ofof coalcoal ganguegangue by by fluidized fluidized calcination calcination and and static static calcination calcination was was discussed discussed in inthis this work. work. The The major major findings findings can can be be concluded concluded as as below: below: (1) Kaolinite, quartz, unburned carbon and organic matter were the main mineral compositions (1)in theKaolinite, raw coal quartz, gangue. unburned TG-DSC carbon analysis and organic indicates matter that were kaolinite the main begins mineral to compositions decomposein into the amorphousraw coal meta-kaolinite gangue. TG-DSC at 450 analysis °C and indicatestransforms that to kaolinite mullite as begins temperature to decompose reaches into up amorphousto 1000 °C. ◦ ◦ (2)meta-kaolinite According to at chemical 450 C and and transforms physical characteri to mullitestics as temperatureof calcined products, reaches up fluidized to 1000 calcinationC. (2)productsAccording could obtain to chemical higher and activity, physical higher characteristics whiteness of and calcined lower products,COD under fluidized same calcination temperatureproducts compared could obtain with higher static calcination. activity, higher whiteness and lower COD under same calcination (3)temperature XRD analysis compared illustrated with that static kaolinite calcination. peaks at 12.4° disappeared completely at 600 °C and (3)more XRDmullite analysis peaks illustrated presence that at kaolinite1000 °C peaks by fluidized at 12.4◦ disappeared calcination, completely which was at 600proved◦C and by more the disappearancemullite peaks of O–H presence stretching at 1000 vibration◦C by fluidized in infrared calcination, spectra results. which was Morphology proved by evolution the disappearance analysis

Minerals 2017, 7, 19 10 of 11

of O–H stretching vibration in infrared spectra results. Morphology evolution analysis showed that fluidized calcination could accelerate crystal phase transformation and improve the activity of calcined products. (4) Combustion of carbon/organic matter and dehydroxylation were more quickly and effectively by ﬂuidized calcination, due to the high efﬁciency of heat and mass transfer, which could produce product with excellent performance and stable quality.

Acknowledgments: This work was funded by Open Projects of Resarch Research Center of Coal Resources Safe Mining and Clean Utilization, Liaoning (LNTU16KF14) and Natural Science Foundation of China (51674065). Author Contributions: Shuai Yuan and Yuexin Han conceived and designed the experiments; Shuai Yuan and Guichen Gong performed the experiments; Shuai Yuan and Yanjun Li analyzed the data; Peng Gao contributed reagents/materials/analysis tools; Shuai Yuan wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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