Experimental Investigation on the Mass Diffusion Behaviors of Calcium Oxide and Carbon in the Solid-State Synthesis of Calcium Carbide by Microwave Heating

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Article Experimental Investigation on the Mass Diffusion Behaviors of Calcium Oxide and Carbon in the Solid-State Synthesis of Calcium Carbide by Microwave Heating

Miao Li 1,2 , Siyuan Chen 3, Huan Dai 1, Hong Zhao 1,* and Biao Jiang 1,3,*

1 Green Chemical Engineering Research Center, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China; [email protected] (M.L.); [email protected] (H.D.) 2 Green Chemical Engineering Research Center, Shanghai Advanced Research Institute, University of Chinese Academy of Sciences, Beijing 100049, China 3 Shanghai Green Chemical Engineering Research Center, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China; [email protected] * Correspondence: [email protected] (H.Z.); [email protected] (B.J.)

Abstract: Microwave (MW) heating was proven to efficiently solid-synthesize calcium carbide at 1750 ◦C, which was about 400 ◦C lower than electric heating. This study focused on the investigation of the diffusion behaviors of graphite and calcium oxide during the solid-state synthesis of calcium carbide by microwave heating and compared them with these heated by the conventional method. The phase compositions and morphologies of CaO and C pellets before and after heating were carefully characterized by inductively coupled plasma spectrograph (ICP), thermo gravimetric (TG) Citation: Li, M.; Chen, S.; Dai, H.; analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron Zhao, H.; Jiang, B. Experimental spectroscopy (XPS). The experimental results showed that in both thermal fields, Ca and C inter- Investigation on the Mass Diffusion Behaviors of Calcium Oxide and diffused at a lower temperature, but at a higher temperature, the formed calcium carbide crystals Carbon in the Solid-State Synthesis of would have a negative effect on Ca diffusion to carbon. The significant enhancement of MW heating Calcium Carbide by Microwave on carbon diffusion, thus on the more efficient synthesis of calcium carbide, manifested that MW Heating. Molecules 2021, 26, 2568. heating would be a promising way for calcium carbide production, and that a sufficient enough https://doi.org/10.3390/molecules carbon material, instead of CaO, was beneficial for calcium carbide formation in MW reactors. 26092568 Keywords: microwave heating; diffusion intensification; calcium oxide; carbon; calcium carbide Academic Editors: Farid Chemat and Ionel Mangalagiu

Received: 18 March 2021 1. Introduction Accepted: 26 April 2021 Published: 28 April 2021 Calcium carbide (CaC2), mainly used to produce acetylene and acetylene-derived products [1–10], is a vital chemical platform product. The production of CaC2 via car-

Publisher’s Note: MDPI stays neutral bon and calcium oxide to produce acetylene is an essential sustainable chemical process, with regard to jurisdictional claims in which recycles calcium and consumes a wide range of carbon, including coal [1–6,11] published maps and institutional afﬁl- and biomass [12,13]. Nowadays, the acetylene chemical industry has relatively declined, iations. mainly caused by the strong position of the ethylene industry since the 1960s, but in many countries with abundant coal and less oil, coal to carbide is one of the primary processes in the coal chemical industry. In China, the annual production of CaC2 was more than 25.88 million tons in 2019 [14]. As we know, calcium carbide production is an energy-intensive process. It is indus- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. trially produced in an electric arc furnace at a very high reaction temperature of up to ◦ · · −1 This article is an open access article 2000–2200 C, and at high energy consumption of 3300 kW h ton standard calcium distributed under the terms and carbide [3–5,15]. Partly due to the energy-intensive nature of the calcium carbide pro- conditions of the Creative Commons duction, academic research and technology improvement on coal to calcium carbide is Attribution (CC BY) license (https:// relatively limited, compared with coal to oleﬁns mainly by Fischer–Tropsch synthesis or creativecommons.org/licenses/by/ MTO processes. In the past decades, some efforts have been carried out to develop new 4.0/). CaC2 production technologies to reduce the energy consumption and production cost, such

Molecules 2021, 26, 2568. https://doi.org/10.3390/molecules26092568 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2568 2 of 14

as the rotary kiln-based process [16], the spout-fluid bed reactor with a plasma torch [17,18], and the oxygen-thermal method [4]. However, most of these technologies were abandoned at the pilot scale. The major obstacle may be attributed to the scale-up of reactors. For example, the oxygen-thermal method, the focus of numerous studies in recent years [4], has also suffered the scale-up due to the significant effect of CO pressure on the chemical reaction equilibrium. Liu et al. has studied the oxygen-thermal method with fine feeds and they found that in the small crucible of a thermogravimetric analysis (TGA) system, CaC2 ◦ was produced at temperatures of ~1750 C[3–5], but in the oxy-thermal CaC2 furnace, the appropriate reaction temperature was as high as 2200 ◦C because of the larger CO partial pressure originating from carbon combustion [4]. In recent years, MW heating has attracted intense attention owing to its advantages of overall, volumetric, instantaneous, and selective heating, high energy efficiency, cleanliness, and safety [19–23]. Especially in the fields of chemistry and chemical industry, MW heating is attractive due to its unique ability to promote mass transfer [24–26] and/or accelerate reaction kinetics [27–32]. Solid-state processing at high temperatures, such as MW synthesis of carbides, is probably the most enormous potential area for energy savings from MW processing. Recently, MW synthesis of CaC2 further evidenced the advantages of MW ◦ heating. Pillai et al. [33] reported that 71.8% CaC2 was solid-state formed at 1700 C for 30 min in a 2450 MHz MW reactor, while conventional heat treatment at 1700 ◦C for 30 min showed 14.1% CaC2; in both cases, about 10 g pressed pellets of CaO and graphite fine powder mixtures ball milled in acetone was used. The authors attributed the reaction enhancement and high CaC2 yield to the volumetric heating nature of microwaves. We have also been carrying out research on the solid-state synthesis of calcium carbide by MW heating in these years [34]. We validated the greater efficiency and productivity of the MW synthesis of calcium carbide and carried out the kilogram scale pilot using a 915 MHz microwave furnace (shown in Figure1). The acetylene gas output of produced calcium carbide reached 300 mL/g after reacting at 1720 ◦C for 1 h when the molar ratio of semi-coke to CaO was 3, suggesting an optimistic prospect of the scale-up of the MW reactor. Moreover, the mass-production of 915 MHz microwave sources with a power of 100 kilowatts has provided technical support to the promising alternative route for CaC2 industrial production, too. However, detailed information on the mass transmission associated with MW heating has not been clear, which would hamper the further development of the MW reactor. We know efficient enough diffusion of reactants is usually the prerequisite for multiphase solid-state reactions, and mass transfer is even the control-step in some cases. Binner et al. found that the diffusion was the rate-controlling step in the MW synthesis of titanium carbide [30]. There is some literature on the diffusion behaviors in the CaC2 production process by conventional heating. However, to the best of our knowledge, no unified view on it has been achieved. In earlier research, Kameyama [35] and Tagawa [36] proposed that the solid-state synthesis of CaC2 was based on the interdiffusion of C and CaO. Muller [37–39] later proposed that carbon ironically transported into the CaO lattice and enables the C3 molecules to form another “interstate” compound (CaC3O). El-Naas [18] studied the solid-phase synthesis of CaC2 in a plasma reactor and suggested that carbon diffused into the calcium oxide. Recently, Li et al. [12,13,40] studied the CaC2 formation process using high-temperature TG-MAS and reported that CaO diffused to C in the solid phase reaction stage. As for MW synthesis of CaC2, Pillai et al. [34] did not pay much attention to this point, and they suggested that MW heating promoted the formation of CaC2-CaO eutectics the same as in the conventional field, which accelerated the diffusion of CaO, hence forming more CaC2 in the MW field. In fact, due to the unique selective heating nature of MW, the reactants coupling well with MW irradiations would exhibit different diffusion behaviors from those by conventional heating, therefore causing different kinetic behaviors and operation conditions in MW reactors. We know that carbon acts as a combined reactant and susceptor via an ohmic dissipation mechanism in this system. Hence, systematic studies to Molecules 2021, 26, 2568 3 of 14

reveal the diffusion behaviors of calcium oxide and carbon in the MW field are crucial for MW reactor design and process optimization. In this study, the diffusion behaviors of C and CaO were experimentally investigated in the MW field and compared with those in the conventional field. A suite of diagnostic techniques including ICP, TG, XRD, XPS, and SEM-EDS were used to monitor the morphol- Molecules 2021, 26, x FOR PEER REVIEWogy, composition, and chemical state of graphite and calcium oxide pellets before and3 afterof 14 heating. After that, the influence of microwaves on the diffusion process was elucidated.

Figure 1. The MW heating equipment for CaC 22 synthesis.synthesis.

2. MaterialsThere is andsome Methods literature on the diffusion behaviors in the CaC2 production process by 2.1.conventional Materials heating. However, to the best of our knowledge, no unified view on it has beenAnalytical-grade achieved. In earlier calcium research, oxide Kameyama (CaO > 99.8%, [35] from and Greagent) Tagawa [36] was proposed calcined at that 1000 the◦C forsolid-state 6 h followed synthesis by of crushing CaC2 was and based grinding on the to interdiffusion obtain CaO withof C and a grain CaO. size Muller of about [37– 50039] later mesh. proposed Analytical-grade that carbon graphite ironically (graphite, transported >99.99%, into fromthe CaO Greagent, lattice particleand enables size wasthe ◦ aboutC3 molecules 200 mesh) to form was another treated at“interstate” 300 C for compound 6 h under (CaC N2 flow.3O). El-Naas About 8[18] g treatedstudied CaOthe powdersolid-phase or graphite synthesis powder of CaC2 wasin a carefullyplasma reactor pressed and into suggested pellets (40.5that carbon mm in diffused diameter) into at 60the MPa calcium and oxide. precisely Recently, weighed. Li et In al. this [12,13,40] way, we studied could assumethe CaC that2 formation the pellets process were using thick enoughhigh-temperature for the diffusion TG-MAS while and the reported diffusion that elements CaO diffused were concentrated to C in the solid in a thinphase layer. reaction stage. As for MW synthesis of CaC2, Pillai et al. [34] did not pay much attention to 2.2.this Diffusionpoint, and Experiments they suggested that MW heating promoted the formation of CaC2-CaO eutecticsThe diffusionthe same as experiments in the conventional of graphite field, and which CaO wereaccelerated carried the out diffusion in a multi-mode of CaO, high-temperaturehence forming more vacuum CaC2 microwavein the MW ovenfield. withIn fact, a maximum due to the output unique power selective of 4.5 heating kW at 2.45nature GHz of (ChangshaMW, the reactants Longtai coupling Microwave well Thermal with MW Engineering irradiations Co., would Ltd., Changsha, exhibit different China), asdiffusion shown behaviors in Figure1 from. In order those to by obtain convention a convincingal heating, measurement therefore causing on temperature, different ki- an infrarednetic behaviors thermometer and operation was used conditions to detect in the MW temperature reactors. We of know materials. that carbon Simultaneously, acts as a threecombined temperature reactant measuring and susceptor rings via wrapped an ohmic by graphitedissipation paper mechanism were respectively in this system. placed atHence, the upper, systematic middle, studies and bottomto reveal of the the diff heatedusion material behaviors to of correct calcium the oxide infrared and tempera- carbon turein the measurement. MW field are crucial The comparative for MW reactor tests weredesign carried and process out in optimization. a corundum tube furnace (ShanghaiIn this Chenhua study, the Electric diffusion Furnace behaviors Co., of Ltd., C and Shanghai, CaO were China, experimentally the maximum investigated service temperaturein the MW field was and 1650 compared◦C). In diffusion with those experiments, in the conventional four pieces field. of A graphite suite of pelletsdiagnostic and threetechniques pieces including of calcium ICP, oxide TG, pellets XRD, XPS, with and smooth SEM-EDS surfaces were were used interlaced to monitor like the a burger mor- forphology, heating, composition, as shown inand Figure chemical2. Noticeably, state of graphite in the MWand calcium field, the oxide burger pellets was before placed and in aafter boron heating. nitride After crucible, that, the which influence was infilled of microwaves with a layer on the of 1 diffusion cm thick process graphite was powders eluci- beforehanddated. to avoid spark generation. The crucible containing samples was placed in the corundum tube or in the microwave transparent insulation barrel (an infrared temperature 2. Materials and Methods 2.1. Materials Analytical-grade calcium oxide (CaO > 99.8%, from Greagent) was calcined at 1000 °C for 6 h followed by crushing and grinding to obtain CaO with a grain size of about 500 mesh. Analytical-grade graphite (graphite, >99.99%, from Greagent, particle size was about 200 mesh) was treated at 300 °C for 6 h under N2 flow. About 8 g treated CaO powder or graphite powder was carefully pressed into pellets (40.5 mm in diameter) at 60 MPa

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and precisely weighed. In this way, we could assume that the pellets were thick enough for the diffusion while the diffusion elements were concentrated in a thin layer.

2.2. Diffusion Experiments The diffusion experiments of graphite and CaO were carried out in a multi-mode high-temperature vacuum microwave oven with a maximum output power of 4.5 kW at 2.45 GHz (Changsha Longtai Microwave Thermal Engineering Co., Ltd., Changsha, China), as shown in Figure 1. In order to obtain a convincing measurement on temperature, an infrared thermometer was used to detect the temperature of materials. Simulta- neously, three temperature measuring rings wrapped by graphite paper were respectively placed at the upper, middle, and bottom of the heated material to correct the infrared temperature measurement. The comparative tests were carried out in a corundum tube furnace (Shanghai Chenhua Electric Furnace Co., Ltd., Shanghai, China, the maximum service temperature was 1650 °C). In diffusion experiments, four pieces of graphite pellets and three pieces of calcium oxide pellets with smooth surfaces were interlaced like a burger for heating, as shown in Figure 2. Noticeably, in the MW field, the burger was Molecules 2021, 26placed, 2568 in a boron nitride crucible, which was infilled with a layer of 1 cm thick graphite 4 of 14 powders beforehand to avoid spark generation. The crucible containing samples was placed in the corundum tube or in the microwave transparent insulation barrel (an infrared temperaturemeasuring measuring hole holein in thethe centercenter of its lid). In In order order to to improve improve the the uniformity uniformity of microwave of microwave irradiation,irradiation, the the storage storage chassis, chassis, on on which which the the microwave transparent insulationinsu- barrel was lation barrel wasput, put, rotated rotated at aat speed a speed of 20of revolutions20 revolutions per per minute. minute. The The samples samples were were heated to the target heated to the targettemperature temperature (1320–1620 (1320–1620◦C) in°C) 2 hin and2 h and held held for 1 for h by1 h adjusting by adjusting the MWthe output power MW output powerunder under the flowthe flow of Ar of (80 Ar mL.min (80 mL.min−1). The−1). The relatively relatively low heatinglow heating rate inrate the in MW field would the MW field woulddiminish diminish the possible the possible temperature temperature heterogeneity heterogeneity between between CaO and CaO graphite and pellets, which graphite pellets,was which possibly was possibly caused caused by their by different their different microwave microwave absorbing absorbing properties. prop- As for electric erties. As for electricheating, heating, considering considering the poor th resistancee poor resistance of the corundum of the corundum tube to rapid tube cooling to and heating, rapid cooling andthe heating, temperature the temperature of the electric of tubethe electric furnace tube was furnace firstly raised was firstly to 1000 raised◦C, then the crucible to 1000 °C, thencontaining the crucible samples containing was carefullysamples putwas incarefully and heated put toin theand target heated temperature to the at a rate of target temperature2 ◦C.min at a −rate1. Thus,of 2 °C.min the whole−1. Thus, heating the upwhole time heating by electric up time heating by waselectric much longer than heating was muchthat longer by MW than heating, that by meaning MW heating, a longer meaning diffusion a longer time in diffusion the conventional time in thermal field. the conventionalAfter thermal cooling field. to After 300 ◦ C,cooling the treated to 300 samples°C, the treated werequickly samples transferred were quickly to a glove box for transferred to a XRD,glove TG,box andfor XRD, ICP test TG, preparation, and ICP test and preparation, the remaining and samplesthe remaining were sealed sam- and stored in a ples were sealeddry and box stored filled in with a dry nitrogen box filled for with other nitrogen analyses. for other analyses.

Figure 2. SchematicFigure 2. Schematic set-up for set-up microwave for microwave heating. 1:heating. microwave 1: microwave source, 2:source, microwave 2: microwave cavity,3: cavity, infrared 3: thermometer, 4: insulationinfrared barrel, thermometer, 5: crucible. 4: insulation barrel, 5: crucible.

2.3. Sample Analysis2.3. Sample Analysis For TG, XRD, andFor TG,microstructure XRD, and microstructure observation analysis observation by analysisSEM, sample by SEM, powders sample powders were were prepared preparedby cutting by a cuttingquarter aof quarter a target of pellet a target and pellet then andgrounding then grounding evenly in evenlyan in an agate agate mortar operatedmortar in operated a glove inbox. a glove For XP box.S analysis, For XPS the analysis, sample thepowders sample were powders care- were carefully fully scraped downscraped from down the contacted from the surface contacted of another surface quarter of another of the quarter target of sample. the target As sample. As for the contacted surfaces analysis by SEM and EDS, another quarter of the target sample was used for analysis directly. The diffused Ca amount was examined by ICP on Thermo iCAP7600, while the diffused C amount was determined by TG analysis performed on a TA SDT Q600 in He/O2 flow, in which the value was the weight loss in O2 flow subtracted by the weight loss in He flow so that the possible interference of calcium carbonate could be avoided. XRD was recorded with a Rigaku MiniFlex 600 X-ray diffractometer, using filtered Cu Kα (λ = 0.15406 nm) as the radiation source operating at 40 kV and 15 mA and operating with the step size of 0.02◦. SEM and EDS mapping were observed using a JEOL JSM-7800F Prime, and XPS was carried out on a Kratos Axis Ultra DLD operating at 30 eV. The dielectric permittivity test used a 2018D1F5 network analyzer for measurement.

3. Results and Discussion 3.1. Morphology Observations In order to investigate the changes of the phase composition and texture morphologies before and after MW heating, we carried out the characterization of the samples. Figure3a shows the photographs of graphite and CaO pellets before and after the typical MW heating process at 1520 ◦C. Clearly, the heated CaO pellet turned from white to gray, while the heated graphite pellet kept the dark black color. Beyond that, the heated CaO pellet exhibited an obvious contracted outsize from 40.5 × 6.3 mm to 34.5 × 4.6 mm, but the Molecules 2021, 26, x FOR PEER REVIEW 5 of 14

for the contacted surfaces analysis by SEM and EDS, another quarter of the target sample was used for analysis directly. The diffused Ca amount was examined by ICP on Thermo iCAP7600, while the diffused C amount was determined by TG analysis performed on a TA SDT Q600 in He/O2 flow, in which the value was the weight loss in O2 flow subtracted by the weight loss in He flow so that the possible interference of calcium carbonate could be avoided. XRD was recorded with a Rigaku MiniFlex 600 X-ray diffractometer, using filtered Cu Kα (λ = 0.15406 nm) as the radiation source operating at 40 kV and 15 mA and operating with the step size of 0.02°. SEM and EDS mapping were observed using a JEOL JSM-7800F Prime, and XPS was carried out on a Kratos Axis Ultra DLD operating at 30 eV. The dielectric permittivity test used a 2018D1F5 network analyzer for measurement.

3. Results and Discussion 3.1. Morphology Observations In order to investigate the changes of the phase composition and texture morphologies before and after MW heating, we carried out the characterization of the samples. Fig- Molecules 2021, 26, 2568 ure 3a shows the photographs of graphite and CaO pellets before and after5 of 14the typical MW heating process at 1520 °C. Clearly, the heated CaO pellet turned from white to gray, while the heated graphite pellet kept the dark black color. Beyond that, the heated CaO pellet exhibited an obvious contracted outsize from 40.5 × 6.3 mm to 34.5 × 4.6 mm, but heated graphite pellet expanded from 40.5 × 3.8 mm to 42.0 × 4.3 mm. This was because the heated graphite pellet expanded from 40.5 × 3.8 mm to 42.0 × 4.3 mm. This was because of their different thermal expansion properties for graphite and calcium oxide. Figure3b–e of their different thermal expansion properties for graphite and calcium oxide. Figure 3b– illustrates the SEM images of graphite and CaO particles in the heated and unheated pellets. e illustrates the SEM images of graphite and CaO particles in the heated and unheated The results demonstrated the remarkable sintering and agglutination of CaO particles, pellets. The results demonstrated the remarkable sintering and agglutination of CaO par- while no pronounced morphology change was observed for graphite particles. ticles, while no pronounced morphology change was observed for graphite particles.

Figure 3. The appearanceFigure 3. ofThe CaO appearance and graphite of CaO tables and before graphite and tables after beforeMW heating and after (a) MWand the heating SEM ( aimages) and the of SEMCaO (b,d) and graphite (c,e); before (b,c) and after (d,e) MW heating at 1520 °C. images of CaO (b,d) and graphite (c,e); before (b,c) and after (d,e) MW heating at 1520 ◦C.

Figure4 further exhibits the SEM images of their contacted surfaces after MW heating at 1520 ◦C, on which elemental mapping analysis was induced. Clearly, in Figure4a the more compact microstructure was observed on the CaO pellet surface, compared with that on the graphite pellet surface shown in Figure4a 0, suggesting the concentration and aggregation of CaO particles after MW heating. Figure4b,c showed the corresponding EDS mapping diagrams on the heated CaO pellet surface, suggesting the distributions of Ca and C on the CaO pellet surface. The results revealed that a large amount of C was uniformly distributed on the contacted surface of calcium oxide pellets, indicating that C easily diffused to the calcium oxide pellet under microwave heating. The EDS mapping diagram in Figure4b 0,c0 showed C and Ca distribution on the contacted surface of the graphite pellet. The larger spots in Figure4c 0 should be the calcium oxide particles which adhered on the graphite pellet surface. The compositional distribution suggested a smaller diffused amount of Ca atoms into the graphite pellet. The mapping results suggested that CaO and C inter-diffused, but C diffused faster. Molecules 2021, 26, x FOR PEER REVIEW 6 of 14

Figure 4 further exhibits the SEM images of their contacted surfaces after MW heating at 1520 °C, on which elemental mapping analysis was induced. Clearly, in Figure 4a the more compact microstructure was observed on the CaO pellet surface, compared with that on the graphite pellet surface shown in Figure 4a′, suggesting the concentration and aggregation of CaO particles after MW heating. Figure 4b,c showed the corresponding EDS mapping diagrams on the heated CaO pellet surface, suggesting the distributions of Ca and C on the CaO pellet surface. The results revealed that a large amount of C was uniformly distributed on the contacted surface of calcium oxide pellets, indicating that C easily diffused to the calcium oxide pellet under microwave heating. The EDS mapping diagram in Figure 4b′,c′ showed C and Ca distribution on the contacted surface of the graphite pellet. The larger spots in Figure 4c′ should be the calcium oxide particles which Molecules 2021, 26, 2568 adhered on the graphite pellet surface. The compositional distribution suggested a smaller6 of 14 diffused amount of Ca atoms into the graphite pellet. The mapping results suggested that CaO and C inter-diffused, but C diffused faster.

Figure 4. SEM images of the contacted surface of MW heated CaO pellet (a) and graphite pellet (a’), and the corresponding Figure 4. SEM images of the contacted surface of MW heated CaO pellet (a) and graphite pellet (a’), and the corresponding EDS mapping images of C (b,b′) and Ca (c,c′) at 1520 °C. EDS mapping images of C (b,b0) and Ca (c,c0) at 1520 ◦C. 3.2. Phase Structure Analysis 3.2. Phase Structure Analysis XRD analysis was performed to investigate the MW heating temperature on the XRD analysis was performed to investigate the MW heating temperature on the phase phase composition of the heated pellets. The obtained results are shown in Figure 5. For composition of the heated pellets. The obtained results are shown in Figure5. For the the unheated calcium oxide sample shown in Figure 5a, the diffraction peaks at 32.5°, unheated calcium oxide sample shown in Figure5a, the diffraction peaks at 32.5 ◦, 37.7◦, 37.7°, 54.1°, 64.4°, and 67.6° could be noted, which corresponded to (111), (200), (220), 54.1◦, 64.4◦, and 67.6◦ could be noted, which corresponded to (111), (200), (220), (311), and (222)(311), ofand calcium (222) of oxide, calcium respectively. oxide, respectively. After MW After heating, MW aheating, new peak a new of graphitepeak of graphite (002) at 26.6(002)◦ appeared,at 26.6° appeared, and its intensity and its increasedintensity increased with an increase with an in increase the temperature. in the temperature. The results conﬁrmedThe results the confirmed diffusion the of Cdiffusion to calcium of oxide,C to calcium and the oxide, diffused and amount the diffused increasing amount with in- an increasecreasing inwith temperature. an increase Meanwhile, in temperature. the diffraction Meanwhile, peak the intensities diffraction of CaOpeak also intensities increased of Molecules 2021, 26, x FOR PEER REVIEW withCaO thealso increasing increased temperature,with the increasing suggesting temper an increasingature, suggesting grain size, an whichincreasing7 of 14 coincided grain withsize,

thewhich microstructure coincided with showed the microstructure by SEM. showed by SEM.

Figure 5. XRDFigure pattern 5. XRD of pattern CaO and of CaO graphite and graphite samples samples subjected subjected to microwave to microwave treatment treatment in a temperaturein a tem- range of 1320–1620 ◦C:perature (a) CaO; range (b) graphite.of 1320–1620 °C: (a) CaO; (b) graphite.

Figure 5b exhibitedFigure the5b exhibitedXRD patterns the XRDof the patterns corresponding of the correspondinggraphite pellets. graphite It can be pellets. It can seen that, exceptbe seenfor the that, diffraction except for peaks the at diffraction 26.5° and peaks 54.5° attributing at 26.5◦ and to 54.5 (002)◦ attributing and (004) to (002) and diffraction peaks(004) of diffractiongraphite, respectively, peaks of graphite, there respectively,was no other there apparent was nocharacteristic other apparent dif- characteristic fraction peaks to calcium compounds, such as CaO and CaC2. This should be because the peak strength of graphite was much higher than that of calcium oxide, which weakened the signals of calcium oxide. As shown in Figure 5b, the mixture of calcium oxide and graphite with a molar ratio of 1:1 exhibited very strong graphite characteristic peaks but extremely weak CaO signals.

3.3. Chemical State Analysis In order to further explore the chemical states of Ca and C on the contacted surfaces of MW heated samples, the microwave heated samples heated at 1470 °C were investigated by XPS analysis. Figure 6a shows the survey spectrum of their contacted surfaces. Both the calcium oxide and graphite pellets contained the three elements of C, O, and Ca. Their molar percentages of C, O, and Ca on the surface of the calcium oxide plate were 40.2%, 44.1%, and 15.7%, respectively, while on the corresponding graphite plate surface molar percentages of them were 89.1%, 8.2%, and 2.7%, respectively. The above result further indicated the faster diffusion of C in the MW heating field. Figure 6b–d showed the high-resolution Ca 2p, O 1s, and C 1s spectra on the contact faces of the calcium oxide and graphite pellets. As can be seen from Figure 6b, both the graphite and calcium oxide pellets showed two characteristic peaks. The peak at 347.3 eV was ascribed to Ca 2p3/2 of CaCO3, and the other one at 350.8 eV was ascribed to Ca 2p1/2 of CaCO3, suggesting calcium compounds changed to CaCO3 in the XPS analysis process [34]. Two small but obvious signals of Ca 2p3/2 and Ca 2p1/2 over the graphite pellet were observed, manifesting Ca diffusion from the CaO pellet to the graphite pellet. Figure 6c exhibited O1s signals for both calcium oxide and graphite pellets. The calcium oxide pellet gave a high-intensity peak near 531.9 eV, which was the characteristic peak of O1s for CaCO3, but the graphite pellet offered a much weaker peak here. The results in Figure 6b,c indicated that a small amount of Ca diffused to the graphite pellet, which also agreed with the described SEM and XRD results. Figure 6d displayed C 1s signals on the contacted surfaces of the CaO and graphite pellet. The high-strength peak at 284.5 eV was the characteristic peak of graphite carbon, and the weak peak near 289.8 eV corresponded to C1s of CaCO3. Clearly, the graphite pellet showed the very strong C1s signal for graphite carbon at 284.5 eV and a hardly visible C1s signal for CaCO3 at 289.8 eV, but the CaO pellet exhibited a middle-intensity peak at 284.5 eV and a weaker signal at 289.8 eV. The result

Molecules 2021, 26, 2568 7 of 14

diffraction peaks to calcium compounds, such as CaO and CaC2. This should be because the peak strength of graphite was much higher than that of calcium oxide, which weakened the signals of calcium oxide. As shown in Figure5b, the mixture of calcium oxide and graphite with a molar ratio of 1:1 exhibited very strong graphite characteristic peaks but extremely weak CaO signals.

molar percentages of C, O, and Ca on the surface of the calcium oxide plate were 40.2%, 44.1%, and 15.7%, respectively, while on the corresponding graphite plate surface molar percentagesfurther suggested of them that were a large 89.1%, amount 8.2%, of and grap 2.7%,hite respectively.C diffused from The the above graphite result pellet further to indicated the faster diffusion of C in the MW heating ﬁeld. CaO pellet, but a smaller amount of Ca diffused from the CaO pellet to the graphite pellet.

FigureFigure 6.6. XPSXPS spectraspectra of of the the microwave-heated microwave-heated CaO CaO and and graphite graphite pellets pellets at 1470at 1470◦C. °C. (a) ( Surveya) Survey spectrum, spectrum, (b) ( high-resolutionb) high-resolu- Cation 2p Ca spectrum, 2p spectrum, (c) high-resolution (c) high-resolution O 1s spectrum,O 1s spectrum, (d) high-resolution (d) high-resolution C 1s spectrum.C 1s spectrum.

3.4. TheFigure Diffusion6b–d showedAmount theof C high-resolution and Ca Ca 2p, O 1s, and C 1s spectra on the contact facesIn of order the calcium to know oxide better and about graphite the diffusio pellets.n behaviors As can beof C seen and from CaO Figureby MW6b, heating, both thewe graphitequantitatively and calciumanalyzed oxide the diffused pellets showed C amount two and characteristic Ca amount peaks. at different The peakheating at 347.3temperatures eV was ascribed and compared to Ca 2p them3/2 of with CaCO those3, and by the electric other oneheating. at 350.8 Figure eV was 7a showed ascribed the to Caresults 2p1/2 byof electric CaCO3 heating., suggesting C was calcium checked compounds out in the changed heated toCaO CaCO pellet,3 in theand XPS Ca was analysis also processchecked [ out34]. in Two the smallheated but carbon obvious pellet, signals suggesti of Cang that 2p3/2 C andand Ca inter-diffused 2p1/2 over the by graphite electric pelletheating. were In detail, observed, the diffusion manifesting amount Ca diffusion of C and from Ca increased the CaO with pellet temperature to the graphite increasing pellet. Figureat lower6c exhibitedtemperature. O1s When signals the for temperatur both calciume was oxide higher and graphitethan 1520 pellets. °C, the The diffused calcium Ca oxideamount pellet unexpectedly gave a high-intensity decreased and peak showed near 531.9 a di eV,stinct which volcano was shape the characteristic with the increasing peak of temperature, while the diffused C amount continued to increase slowly, and became higher than the corresponding Ca amounts. According to DD e, the diffusion rate should increase exponentially with the increase of temperature. The results suggested a different kinetic behavior at a higher heating temperature. The diffusion of C and Ca, especially Ca, were hindered at a higher temperature, causing the diffused C amount to become relatively greater than that of Ca. The results agreed with the report that C and CaO inter-diffused by Yamanaka [35] and Tagawa [36], but far away from the proposal that CaO should diffuse to C reported by Li et al. [12,13]. The great influence of temperature on the diffusion behaviors may be partially caused by these different reports.

Molecules 2021, 26, 2568 8 of 14

O1s for CaCO3, but the graphite pellet offered a much weaker peak here. The results in Figure6b,c indicated that a small amount of Ca diffused to the graphite pellet, which also agreed with the described SEM and XRD results. Figure6d displayed C 1s signals on the contacted surfaces of the CaO and graphite pellet. The high-strength peak at 284.5 eV was the characteristic peak of graphite carbon, and the weak peak near 289.8 eV corresponded to C1s of CaCO3. Clearly, the graphite pellet showed the very strong C1s signal for graphite carbon at 284.5 eV and a hardly visible C1s signal for CaCO3 at 289.8 eV, but the CaO pellet exhibited a middle-intensity peak at 284.5 eV and a weaker signal at 289.8 eV. The result further suggested that a large amount of graphite C diffused from the graphite pellet to CaO pellet, but a smaller amount of Ca diffused from the CaO pellet to the graphite pellet.

3.4. The Diffusion Amount of C and Ca In order to know better about the diffusion behaviors of C and CaO by MW heating, we quantitatively analyzed the diffused C amount and Ca amount at different heating temperatures and compared them with those by electric heating. Figure7a showed the results by electric heating. C was checked out in the heated CaO pellet, and Ca was also checked out in the heated carbon pellet, suggesting that C and Ca inter-diffused by electric heating. In detail, the diffusion amount of C and Ca increased with temperature increasing at lower temperature. When the temperature was higher than 1520 ◦C, the diffused Ca amount unexpectedly decreased and showed a distinct volcano shape with the increasing temperature, while the diffused C amount continued to increase slowly, and became higher −Q than the corresponding Ca amounts. According to D = D0 × e RT , the diffusion rate should increase exponentially with the increase of temperature. The results suggested a different kinetic behavior at a higher heating temperature. The diffusion of C and Ca, especially Ca, were hindered at a higher temperature, causing the diffused C amount to become relatively greater than that of Ca. The results agreed with the report that C and CaO inter-diffused by Yamanaka [35] and Tagawa [36], but far away from the proposal that CaO should diffuse to Molecules 2021, 26, x FOR PEER REVIEW 9 of 14 C reported by Li et al. [12,13]. The great inﬂuence of temperature on the diffusion behaviors may be partially caused by these different reports.

Figure 7. FigureThe diffused 7. The diffused C amounts C amounts in CaO pelletsin CaO andpellets the and diffused the diffused CaO amounts CaO amounts in graphite in graphite pellets pellets heated by electric heating (aheated) and MW by electric heating heating (b) under (a) Arand atmosphere. MW heating (b) under Ar atmosphere.

Figure 7b showedFigure the 7effectb showed of heating the effect temperature of heating on temperature the diffusion on the amounts diffusion of amountsC of C and and Ca by MW heating.Ca by MW Clearly, heating. the Clearly, carbon theamount carbon in amountheated CaO in heated pellets CaO constantly pellets constantly in- increased creased with temperature,with temperature, and the andobstruction the obstruction to C diffusion to C diffusion in a high in temperature a high temperature range range became became indistinct.indistinct. Moreover, Moreover, the carbon the carbonamount amount in the inheated the heated CaO CaOpellet pellet was wasmuch much higher than higher than that thatheated heated by the by thetraditional traditional elec electrictric heating. heating. In our In ourexperiments, experiments, the thewhole whole heating time heating time in thein the MW MW field field was was much much shorter shorter than than that that by by traditional traditional heating, heating, so that the significantly the significantly improvedimproved carboncarbon diffusiondiffusion amount indicated indicated the the intensification intensification of of MW MW irradiation on irradiation on C Cdiffusion. diffusion. As for the diffused Ca amount, a more obvious volcano shape was observed over the MW heated samples. The calcium amount at first increased with temperature, and was clearly higher than those by electric heating but lower than the corresponding C amount. That is to say, by MW heating, Ca diffusion was slower than the corresponding C diffusion, but faster than that heated by the electric method. It indicated that MW heating showed a positive effect on Ca diffusion, but the intensification effect was not as great as that on graphite. Moreover, by MW heating the maximum diffusion amount of Ca appeared at 1420 °C, which was 100 °C lower than that by traditional heating. Then, the Ca diffusion amount quickly decreased with the increase of temperature until 1570 °C. The result suggested that Ca diffusion to C was blocked at a lower temperature by MW irradiation. Additionally, by continuing to increase the heating temperature over 1570 °C, the Ca diffusion amount increased but was still maintained at a low level. The results indicated that the obstruction effect on Ca diffusion did not disappear or get weaker at a higher temperature and ran through the whole high temperature range. From the above experimental results, it was clear that there was a “microwave effect” on the mass transfer of calcium oxide and carbon in the solid-state synthesis of calcium carbide. MW irradiation enhanced the diffusion of C and Ca, especially C diffusion to Ca. Whittaker [26] investigated the effects of the electric field produced by MW heating upon the mass transport during the sintering of ceramic materials. He found that the intense electric field MW irradiation produced at the lattice defects and interparticle boundaries would enhance ion mobility, and that the mass transport rate at a given temperature would be enhanced by the presence of MW. This “nonthermal effect” of MW irradiations, resulting from the ponderomotive driving force of the electric field, was a function of both the sample permittivity and the strengths of the high-frequency fluctuating electric field. As shown in Figure 8, the dielectronic constant of graphite at 2450 MHz was more than 20, and its tanδ was over 0.15 in all temperature ranges, suggesting that graphite was an excellent MW absorber. The excellent coupling of graphite with MW enhanced its diffusion to CaO and changed the diffusion trend at high temperatures exhibited in the traditional thermal field. As for CaO, its dielectric constant was about 5.0 and tanδ was about 0.08, much lower than graphite, suggesting its weaker coupling with MW and then weaker MW enhancement on its diffusion behavior.

Molecules 2021, 26, 2568 9 of 14

As for the diffused Ca amount, a more obvious volcano shape was observed over the MW heated samples. The calcium amount at first increased with temperature, and was clearly higher than those by electric heating but lower than the corresponding C amount. That is to say, by MW heating, Ca diffusion was slower than the corresponding C diffusion, but faster than that heated by the electric method. It indicated that MW heating showed a positive effect on Ca diffusion, but the intensification effect was not as great as that on graphite. Moreover, by MW heating the maximum diffusion amount of Ca appeared at 1420 ◦C, which was 100 ◦C lower than that by traditional heating. Then, the Ca diffusion amount quickly decreased with the increase of temperature until 1570 ◦C. The result suggested that Ca diffusion to C was blocked at a lower temperature by MW irradiation. Additionally, by continuing to increase the heating temperature over 1570 ◦C, the Ca diffusion amount increased but was still maintained at a low level. The results indicated that the obstruction effect on Ca diffusion did not disappear or get weaker at a higher temperature and ran through the whole high temperature range. From the above experimental results, it was clear that there was a “microwave effect” on the mass transfer of calcium oxide and carbon in the solid-state synthesis of calcium carbide. MW irradiation enhanced the diffusion of C and Ca, especially C diffusion to Ca. Whittaker [26] investigated the effects of the electric field produced by MW heating upon the mass transport during the sintering of ceramic materials. He found that the intense electric field MW irradiation produced at the lattice defects and interparticle boundaries would enhance ion mobility, and that the mass transport rate at a given temperature would be enhanced by the presence of MW. This “nonthermal effect” of MW irradiations, resulting from the ponderomotive driving force of the electric field, was a function of both the sample permittivity and the strengths of the high-frequency fluctuating electric field. As shown in Figure8, the dielectronic constant of graphite at 2450 MHz was more than 20, and its tan δ was over 0.15 in all temperature ranges, suggesting that graphite was an excellent MW absorber. The excellent coupling of graphite with MW enhanced its diffusion to CaO and changed the diffusion trend at high temperatures exhibited in the traditional thermal field. As for CaO, its dielectric constant was about 5.0 and tanδ was about 0.08, much lower than Molecules 2021, 26, x FOR PEER REVIEW graphite, suggesting its weaker coupling with MW and then weaker MW10 of 14 enhancement on

its diffusion behavior.

Figure 8. TheFigure dielectric 8. The constant dielectric (a constant) and tan (aδ) ( andb) of tan graphiteδ (b) of and graphite calcium and oxide. calcium oxide.

It should be statedIt should that, MW be stated heating that, enhancement MW heating on enhancement the mass transport on the masshere, transportwhich here, which was obviously wasnot originated obviously from not originated the “thermal from effect” the “thermal of MW irradiations, effect” of MW would irradiations, have a would have great benefit toa the great solid beneﬁt synthesis to the of solid CaC2 synthesis. This conclusion of CaC 2was. This different conclusion from wasthe pro- different from the posal by Pillai proposalet al. [33], by that Pillai the etfast al. heating [33], that rate the and fast higher heating temperature rate and higher than temperaturethose ac- than those tually observedactually by MW observed heating promoted by MW heating the diffusion promoted of theCaO diffusion to carbon, of CaOhence to forming carbon, hence forming more CaC2. more CaC2.

3.5. Verification of the Impediment by CaC2 On the other hand, it was necessary for us to explore the resistance effect on the diffusions occurring at a high temperature range. In our tested system, there were three chemical elements including C, Ca, and O, so there should be some new chemical material produced in the heating process that hindered the diffusion of Ca. It was reported that the critical temperature for calcium carbide formation could be as low as about 1460 °C [3,40] and 1440 °C [41]. Moreover, Ca2+ with a larger ion radius was reported to possess high barrier energy in the calcium carbide crystal [42]. In this study, the inflection point of the Ca diffusion amount was 1470 °C for the MW heating and 1570 °C for the electrical heating. Thus, it might be reasonable for us to propose that CaC2 crystals would be formed at the interfaces in both thermal fields, which would obstruct the diffusion of C and Ca, especially the diffusion of Ca. The thermodynamic analysis proposed that the higher CO partial pressure would inhibit the formation of calcium carbide. Under the CO atmosphere, the critical formation temperature of calcium carbide was above 1780 °C [41]. Thus, the diffusion amount of both calcium and carbon would increase steadily with the increase of temperature under a CO atmosphere in the investigated temperature range. Therefore, in order to confirm the formation of CaC2 in the heating processes, we carried out a verification test, in which 250 g of mixed raw materials was MW heated at 1570 °C for 2 h to synthesize CaC2. In detail, 560 g of CaO, 360 g of graphite, and 300 g of water were mixed together. The well mixed slurry was then kneaded into spherical balls with a diameter of about 1 cm. After being dried at 120 °C for 24 h, the obtained balls were used as a reactant. In the experiment, the system pressure was kept at about 1000 kPa using a vacuum pump to imitate the low CO partial pressure in the diffusion test. After reaction, the products were analyzed by XRD. Hydrolysis testing was also performed to quantify the synthesized CaC2 in the C2H2 gas generation and collection apparatus. Figure 9 demonstrated the XRD pattern of the samples. We know four temperature- induced CaC2 modifications were experimentally reported, including the common form of CaC2-I (I4/mmm, Z = 2, 139), two metastable low-temperature monoclinic modification CaC2-Ⅱ (C2/c, Z = 4, 15), and CaC2-Ⅲ (C2/m, Z = 4, 12), and cubic high-temperature modification CaC2-Ⅳ (Fm-3m, Z = 4, 225) [43]. From Figure 9 we can see, besides the characteristic peaks of CaO and graphite, there were two small but distinct new peaks at 30.4° and 31.1°, which respectively corresponded to (210) and (004) diffraction peaks of CaC2-Ⅲ.

Molecules 2021, 26, 2568 10 of 14

3.5. Verification of the Impediment by CaC2 On the other hand, it was necessary for us to explore the resistance effect on the diffusions occurring at a high temperature range. In our tested system, there were three chemical elements including C, Ca, and O, so there should be some new chemical material produced in the heating process that hindered the diffusion of Ca. It was reported that the critical temperature for calcium carbide formation could be as low as about 1460 ◦C[3,40] and 1440 ◦C[41]. Moreover, Ca2+ with a larger ion radius was reported to possess high barrier energy in the calcium carbide crystal [42]. In this study, the inflection point of the Ca diffusion amount was 1470 ◦C for the MW heating and 1570 ◦C for the electrical heating. Thus, it might be reasonable for us to propose that CaC2 crystals would be formed at the interfaces in both thermal fields, which would obstruct the diffusion of C and Ca, especially the diffusion of Ca. The thermodynamic analysis proposed that the higher CO partial pressure would inhibit the formation of calcium carbide. Under the CO atmosphere, the critical formation temperature of calcium carbide was above 1780 ◦C[41]. Thus, the diffusion amount of both calcium and carbon would increase steadily with the increase of temperature under a CO atmosphere in the investigated temperature range. Therefore, in order to confirm the formation of CaC2 in the heating processes, we carried out a verification test, in which ◦ 250 g of mixed raw materials was MW heated at 1570 C for 2 h to synthesize CaC2. In detail, 560 g of CaO, 360 g of graphite, and 300 g of water were mixed together. The well mixed slurry was then kneaded into spherical balls with a diameter of about 1 cm. After being dried at 120 ◦C for 24 h, the obtained balls were used as a reactant. In the experiment, the system pressure was kept at about 1000 kPa using a vacuum pump to imitate the low CO partial pressure in the diffusion test. After reaction, the products were analyzed by XRD. Hydrolysis testing was also performed to quantify the synthesized CaC2 in the C2H2 gas generation and collection apparatus. Figure9 demonstrated the XRD pattern of the samples. We know four temperature- induced CaC2 modifications were experimentally reported, including the common form of CaC2-I (I4/mmm, Z = 2, 139), two metastable low-temperature monoclinic modification CaC2-II (C2/c, Z = 4, 15), and CaC2-III (C2/m, Z = 4, 12), and cubic high-temperature modification CaC2-IV (Fm-3m, Z = 4, 225) [43]. From Figure9 we can see, besides the characteristic peaks of CaO and graphite, there were two small but distinct new peaks at 30.4◦ and 31.1◦, which respectively corresponded to (210) and (004) diffraction peaks of CaC2-III. Here, no CaC2-I was observed, manifesting a low generation temperature of calcium oxide by MW heating. The further hydrolysis testing showed the product with 91 mL/g C2H2 gas generation, equaling 26 wt % of CaC2 in the product, confirming that CaC2 was formed at a lower heating temperature. Figure 10 exhibits the diffusion amounts of Ca and C heated under a CO atmosphere at different temperatures. Unsurprisingly, under a CO atmosphere there was no inflection point observed, and the diffusion amounts of Ca and C constantly increased with the heating temperature. When the temperature was over 1520 ◦C, the Ca diffusion amount was lower than the C diffusion amount. The results convinced the suppression of the formed calcium carbides on the diffusion of reactants, especially on the diffusion of Ca. The lower inflection temperature of calcium diffusion in the MW field indicated a lower generation temperature of CaC2, suggesting that MW heating only accelerated the mass diffusion but also enhanced the chemical reaction. Molecules 2021, 26, x FOR PEER REVIEW 11 of 14

Here, no CaC2-I was observed, manifesting a low generation temperature of calcium oxide by MW heating. The further hydrolysis testing showed the product with 91 mL/g C2H2 gas generation, equaling 26 wt % of CaC2 in the product, confirming that CaC2 was formed at a lower heating temperature.

Molecules 2021, 26, x FOR PEER REVIEW 11 of 14

Here, no CaC2-I was observed, manifesting a low generation temperature of calcium oxide Molecules 2021, 26, 2568 11 of 14 by MW heating. The further hydrolysis testing showed the product with 91 mL/g C2H2 gas generation, equaling 26 wt % of CaC2 in the product, confirming that CaC2 was formed at a lower heating temperature.

Figure 9. XRD pattern of MW heated product at 1570 °C.

Figure 10 exhibits the diffusion amounts of Ca and C heated under a CO atmosphere at different temperatures. Unsurprisingly, under a CO atmosphere there was no inflection point observed, and the diffusion amounts of Ca and C constantly increased with the heating temperature. When the temperature was over 1520 °C, the Ca diffusion amount was lower than the C diffusion amount. The results convinced the suppression of the formed calcium carbides on the diffusion of reactants, especially on the diffusion of Ca. The lower inflection temperature of calcium diffusion in the MW field indicated a lower generation temperature of CaC2, suggesting that MW heating only accelerated the mass diffusion but FigureFigure 9.9. XRDXRD patternpattern ofof MWMW heatedheated productproduct atat 15701570 ◦°C.C. also enhanced the chemical reaction. Figure 10 exhibits the diffusion amounts of Ca and C heated under a CO atmosphere at different temperatures. Unsurprisingly, under a CO atmosphere there was no inflection point observed, and the diffusion amounts of Ca and C constantly increased with the heating temperature. When the temperature was over 1520 °C, the Ca diffusion amount was lower than the C diffusion amount. The results convinced the suppression of the formed calcium carbides on the diffusion of reactants, especially on the diffusion of Ca. The lower inflection temperature of calcium diffusion in the MW field indicated a lower generation temperature of CaC2, suggesting that MW heating only accelerated the mass diffusion but also enhanced the chemical reaction.

FigureFigure 10. 10.The The diffusiondiffusion amounts amounts of of Ca Ca ( a(a)) and and C C ( b(b)) under under Ar Ar and and CO CO atmosphere. atmosphere.

3.6.3.6. TheThe SchematicSchematic DiagramDiagram ofof thethe DiffusionDiffusion Model Model in in the the MW MW Field Field BasedBased onon ourour experimentalexperimental results,results, it it was was clear clear that that the the diffusion diffusion behavior behavior of of carbon carbon andand calcium calcium oxide oxide should should be be divided divided into into two two different different stages stages within within the the examined examined temper- tem- atureperature range range in the in MWthe MW ﬁeld. field. The The diffusion diffusion schematic schematic diagram diagram was was shown shown in Figure in Figure 11. At11. aAt lower a lower temperature, temperature, carbon carbon and and calcium calcium would would directly directly inter-diffuse inter-diffuse on the on interfaces, the interfaces, and theand diffusion the diffusion rate increasedrate increased with with temperature. temperature. When When the temperaturethe temperature was was higher, higher, carbon car-

andbon calcium and calcium would would partly diffusepartly throughdiffuse thro the producedugh the produced calcium carbide calcium crystals. carbide However, crystals. becauseFigureHowever, 10. of The betterbecause diffusion microwave of amountsbetter absorptionmicrowave of Ca (a) and andabsorpti C ( lessb) under atomicon and Ar and radiusless CO atomic ofatmosphere carbon, radius C. diffusionof carbon, in C the MW field consistently increased with the increasing temperature and was much faster than3.6. The that Schematic of Ca. Thus, Diagram at a of lower the Diffusion temperature, Model in the the interdiffusion MW Field of C and Ca occurred, but atBased higher on temperatures, our experimental the diffusion results, it of was carbon clear was that prominent the diffusion in the behavior MW field. of carbon In the industrialand calcium production oxide should of CaC be2 dividedusing an into arc-furnace, two different excess stages CaO iswithin necessarily the examined used to formtem- CaCperature2-CaO range liquid in the eutectics, MW field. which The promote diffusion CaO schematic diffusion diagram to carbon was shown to accelerate in Figure CaC 11.2 generation.At a lower temperature, However, when carbon C was and insufficient calcium would in the directly reaction inter-diffuse system, the on produced the interfaces, CaC2 wasand easythe diffusion to decompose rate increased into Ca andwith C. temperature. When the temperature was higher, carbon and calcium would partly diffuse through the produced calcium carbide crystals. (CaC2 + 2CaO = 3Ca + 2CO). However, because of better microwave absorption and less atomic radius of carbon, C Thus, excess CaO would accelerate the decomposition of CaC2, add the heat load, and reduce carbide content of the product.

Molecules 2021, 26, x FOR PEER REVIEW 12 of 14

diffusion in the MW field consistently increased with the increasing temperature and was much faster than that of Ca. Thus, at a lower temperature, the interdiffusion of C and Ca occurred, but at higher temperatures, the diffusion of carbon was prominent in the MW field. In the industrial production of CaC2 using an arc-furnace, excess CaO is necessarily used to form CaC2-CaO liquid eutectics, which promote CaO diffusion to carbon to accelerate CaC2 generation. However, when C was insufficient in the reaction system, the produced CaC2 was easy to decompose into Ca and C.

(CaC2 + 2CaO = 3Ca + 2CO). Molecules 2021, 26, 2568 12 of 14 Thus, excess CaO would accelerate the decomposition of CaC2, add the heat load, and reduce carbide content of the product.

FigureFigure 11. 11.The The schematic schematic diagram diagram of of the the diffusion diffusion of of C C and and CaO CaO in in the the MW MW field. field. 4. Conclusions 4. Conclusions In this study, the diffusion behaviors of C and CaO were experimentally investigated in In this study, the diffusion behaviors of C and CaO were experimentally investigated the MW field and compared with those in the conventional field. The obtained experimental resultsin the manifestedMW field and the compared high-efficient with and those dominant in the conventional diffusion of C field. toward The CaO obtained in the experi- solid- mental results manifested the high-efficient and dominant diffusion of C toward CaO in synthesis of CaC2 by MW heating. The effective mass transfer of reactants in the MW thermalthe solid-synthesis field was no of longer CaC2 by dependent MW heating. on CaO The diffusion effective tomass carbon, transfer and of excess reactants CaO in was the MW thermal field was no longer dependent on CaO diffusion to carbon, and excess CaO not needed to promote the formation of CaO-CaC2 eutectics. In essence, carbon reactant playedwas not a keyneeded role into bothpromote mass the and formation heat transfer of CaO-CaC in the MW2 eutectics. reactor. AnIn essence, appropriate carbon C/CaO reac- ratiotant inplayed the MW a key reactor role in was both close mass to and the heat stoichiometric transfer in ratiothe MW 3, suggesting reactor. An that appropriate in MW reactors,C/CaO ratio higher in puritythe MW calcium reactor carbide was close would to the be morestoichiometric easily and ratio efficiently 3, suggesting obtained. that in MW reactors, higher purity calcium carbide would be more easily and efficiently ob- Authortained. Contributions: Conceptualization, H.Z. and B.J.; research ideas, B.J., H.Z. and S.C.; method- ology, M.L., H.D. and S.C.; software, M.L. and H.D.; validation, H.Z. and B.J.; formal analysis, M.L., H.D.Author and Contributions: S.C.; investigation, Conceptualization, M.L. and S.C.; H.Z. data and curation, B.J.; resear M.L.ch and ideas, S.C.; B.J., writing—original H.Z., and S.C.; draftmeth- preparation,odology, M.L., M.L. H.D., and H.D.;and S.C.; writing—review software, M.L. and and editing, H.D.; H.Z. validation, and B.J.; H.Z. visualization, and B.J.; formal M.L. and analysis, H.D.; supervision,M.L., H.D., S.C.;and projectS.C.; investigation, administration, M.L. H.Z. and and S.C. B.J.;; data funding curation, acquisition, M.L. an H.Z.d S.C.; and writing—original B.J. All authors havedraft read preparation, and agreed M.L. to theand published H.D.; writing—review version of the manuscript.and editing, H.Z. and B.J.; visualization, M.L. and H.D.; supervision, S.C.; project administration, H.Z. and B.J.; funding acquisition, H.Z. and B.J. Funding:All authorsThis have work read was and supported agreed byto thethe Shanghaipublished Science version and of Technology the manuscript. Committee (15DZ1206800). InstitutionalFunding: This Review work Board was Statement:supportedNot by applicable.the Shanghai Science and Technology Committee Informed(15DZ1206800). Consent Statement: Not applicable. DataInstitutional Availability Review Statement: Board Statement:Not available. Not applicable. Acknowledgments:Informed Consent Statement:This work Not was applicable. supported by Shanghai Science and Technology Committee. TheData authors Availability would Statement: like to express Not available. their sincere thanks to the Science and Technology Planning Project of Shanghai, China (15DZ1206800) and Shanxi Lu’an Mining (Group) Co., Ltd. for financially supporting this study. Conflicts of Interest: The authors declare no conflict of interest.

Sample Availability: Not available.

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