applied sciences

Article High-Temperature Mineral Formation after Firing Clay Materials Associated with Mined Coal in ()

Manuel Miguel Jordán 1,* , Sergio Meseguer 2 , Francisco Pardo 3 and María Adriana Montero 1 1 Department of Agrochemistry and Environment (GEA-UMH), University Miguel Hernández, Elche. Avda. de la Universidad s/n, 03202 Elche, Alicante, Spain; [email protected] 2 Unit of Applied Mineralogy, Jaume I University, Campus de Riu Sec s/n, 1280 Castellón, Spain; [email protected] 3 Department of Education Sciences, CEU Cardenal Herrera University, Calle Grecia 31, 12006 Castellón, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-966658416

 Received: 1 April 2020; Accepted: 28 April 2020; Published: 29 April 2020 

Abstract: The production of porcelain stoneware has experienced a considerable increase. Therefore, it was necessary to undertake an investigation that would allow knowing the mineralogical evolution that porcelain stoneware undergoes during the firing process, as well as establishing the influence of the formation of mullite and other mineral or vitreous phases and their quantification. The firing transformations of mine spoils associated with mined coal in the -Escucha-Estercuel and Ariño-Andorra areas are studied in this paper. The mineralogical composition of the bulk mine spoils is kaolinite, illite, chlorite, and smectites (in traces), with quartz and feldspar, and minor hematite, calcite, and dolomite. The main objective is to understand the generation of high-temperature mineral phases after firing, and their quantification. The formation of mullite and other high-temperature phases are studied from samples that include variable proportions of illite. Samples with a high content of illite generate mullite at 995 ◦C. Cristobalite was not detected as a high-temperature phase. Mullite is the most abundant mineral. The hercynite content is higher at low temperatures (995 ◦C), and hematite content is higher at 1150 ◦C. The vitreous phase represents about 50% of fired bodies. Despite observing a porous microstructure, the non-porous areas are well sintered.

Keywords: Teruel; clay spoils; firing transformations; phase generation; vitreous phase; XRD; FTIR

1. Introduction The (NE Spain) has historically been an important coal mining area. In this area there are still abundant coal deposits, but coal mining has ceased in recent decades. In this paper, firing transformations [1] of mine spoils from the mined coal in the Utrillas-Escucha-Estercuel and Ariño-Andorra regions are studied (Figure1). The production of porcelain stoneware has experienced a considerable increase since its initial development in Italy in the late 1970s [2]. The valuation of these mine spoils can bring economic and environmental benefits [3]. However, this increase in manufacture was not accompanied by the necessary research to know the scope of the reactions that take place during the firing process of the ceramic pieces. It is worth noting the fact that the few publications that exist on the porcelain stoneware microstructure refer, above all, to issues of porosity and glass formation, without addressing the study of mullite formation and mineral quantification [1,2]. Therefore, it was considered necessary to undertake an investigation that would allow knowing the mineralogical and microstructural evolution that porcelain stoneware undergoes during its firing process, as well as establishing the influence of the mullite crystalline phase formation on some

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FigureFigure 1. 1. LocationLocation of of the the studied sampling areas.areas. Legend:Legend: OE:OE: -Estercuel; Oliete-Estercuel; AA: AA: Ariño-Andorra Ariño-Andorra [3]. [3]. Mine spoils from the lignite mines associated with the Lignitos de Escucha (Escucha Lignites) andMine Arenas spoils de Utrillasfrom the (Utrillas lignite kaoliniticmines associated sands) formationswith the Lignitos belong de to Escucha the upper (Escucha part of theLignites) Lower andCretaceous Arenas de [3]. Utrillas These mine(Utrillas spoils kaolinitic have a greatsands) importance formations in belong the ceramic to the cluster upper ofpart Castell of theón Lower (Spain), Cretaceousdue to their [3]. possibilities These mine for spoils being have used a as great ceramic impo rawrtance materials in the Theseceramic clayed cluster mine of spoilsCastellón for industrial(Spain), dueuses, to ontheir the basispossibilities on their for mineralogy being used and as chemical ceramic composition, raw materials can These be classified clayed asmine ball spoils clays [1for–3 ]. industrialSome of these uses, spoils on the are basis mined on nowadaystheir mineralogy [4]. High-temperature and chemical composition, mineral phases can are be generatedclassified inas firingball claysprocesses. [1–3]. MineralSome of phase these transformations spoils are mined in a nowa ceramicdays firing [4]. processHigh-temperature have previously mineral been phases described are in generatedliterature [in3,7 ].firing Previously, processes. a powerful Mineral study phase was tr carriedansformations out on the in determination a ceramic firing of high-temperature process have previouslymineral phases been described using these in mineliterature spoils [3,7]. as ceramic Previously, raw materiala powerful [3]. study The formationwas carried of mulliteout on the and determinationother high-temperature of high-temperature phases were mineral studied phases in 2015 using [3]. However,these mine it spoils was not as possibleceramic toraw quantify material the [3].mineral The formation phases and of themullite vitreous and phaseother high-temperature present for each firing phases temperature. were studied There in 2015 is no [3]. information However, in itthe was literature not possible about to the quantify quantification the mineral of high-temperature phases and the phasevitreous generation phase present and amorphous for each firing phase temperature.formation from There these is no mine information spoils for in di thefferent litera firingture about temperatures. the quantification Knowledge of high-temperature and quantification phaseof high-temperature generation and minerals amorphous generated phase usingformation these from mine these spoils mine as raw spoils materials for different is interesting firing in temperatures.industrial processes Knowledge for manufacturing and quantification stoneware of hi andgh-temperature ceramic tiles, asminerals well as refractorygenerated ceramics using these [5–9 ]. mineIn this spoils work, as raw for thematerials first time, is interesting a quantification in industrial of mineral processes phase for inmanufacturing the thermal transformationsstoneware and ceramicexperienced tiles, as by well kaolinitic as refractory mine spoils ceramics has been [5–9]. carried In this out. work, The for ceramic the first industry time, a hasquantification been shown of to mineralhave great phase potential in the thermal to absorb transformations industrial waste experienced [8,9]. However, by kaolinitic the commercial mine spoils production has been carried of tiles out.from The waste ceramic materials industry is very has been limited shown and, to in have some great countries, potential non-existent. to absorb industrial Some reasons waste include [8,9]. However,possible contaminationthe commercial by production the waste used,of tiles the from absence waste of relevantmaterials standards, is very limited and slow and, acceptance in some countries,by industry non-existent. and consumers. Some reasons The main include objectives possible of this contamination paper are:(i) by to the understand waste used, the the generation absence ofof relevant high-temperature standards, mineraland slow phases acceptance after firing;by industry (ii) to addressand consumers. a quantitative The main approach objectives to minerals of this paperand amorphousare: (i) to understand phases present the ge inneration ceramic tileof high-temperature bodies; (iii) to study mine theral microstructure phases after firing; and porosity (ii) to addressof ceramic a quantitative tile bodies; approach and (iv) to to contribute minerals and to a amorphous better knowledge phases of present firing behaviorin ceramic of tile mine bodies; spoils, (iii)in orderto study to promotethe microstructure their use in and the porosity Spanish of ceramic ceramic industry tile bodies; as raw and material (iv) to contribute in the manufacture to a better of knowledgeporcelain stoneware. of firing behavior of mine spoils, in order to promote their use in the Spanish ceramic industry as raw material in the manufacture of porcelain stoneware. Appl. Sci. 2020, 10, 3114 3 of 9

2. Materials and Methods

2.1. Mine Spoils The geological background is described in [8]. The samples analyzed in this study were taken from a lignite mining area in the Iberian Range (Teruel, Spain). Sixty bulk samples of mine spoils were mineralogically and chemically analyzed, 40 of which came from the Oliete-Estercuel (OE) area, and 20 from the Ariño-Andorra (AA) mine area (Teruel, Spain). Ten representative samples from each area were selected, in order to separate them into three grain size fractions and, subsequently, carry out their mineralogical analysis. These fractions were separated from the bulk mine spoils following the standard protocols from ceramic industries. These mine spoils were classified as ball clays, and are mined today [1,3,4]. A representative sample of each deposit was selected for firing tests. The mineralogical analysis was carried out using X-ray diffraction (XRD). The three fractions analyzed were: F1 (particle size <2 µm); F2 (particle size between 2 and 50 µm); and F3 (particle size >50 µm). In the case of F1 and F2, oriented aggregates were prepared for the characterization of phyllosilicates, treated with ethylene glycol, and calcined at 550 ◦C for 2 h. XRD mineralogical quantitative approach was carried out (DIFFRACT.EVA and EDQ software v. 6.1.2, Bruker, Mannheim, Germany). The X-ray diffraction experiments were conducted in a Siemens D5000D powder diffractometer (Berlin, Germany) with the Bragg–Bentano θ:2θ configuration, using Cu Kα radiation (40 kV, 30 mA), aperture slit = 2 mm, divergence slit = 2 mm, detector slit = 0.2 mm, anti-scatter slit = 0.6 mm, and a graphite secondary monochromator. Samples were analyzed within the range of 3–65◦. Measurements were taken at every 0.04◦ in a time step of 3.2 s. The X-ray diffraction plots were analyzed using Siemens automatic software for peak recognition, mineral identification, and peak intensity calculations. The chemical analysis of the main elements was carried out by X-ray fluorescence (XRF) using conventional techniques, using a PHILIPS PW2400 X-ray Spectrometer (Mahwah, NJ, USA). Statistical correlations and cluster analysis were established, following the methodology described in [9].

2.2. Fired Samples Four representative mine spoil samples from the OE area and two representative samples from the AA area were selected. From each selected sample, three replicas were prepared, following the protocol explained below. The selected clayed mine spoils (18 replicas) were moistened, mixed, and sieved (1 mm) until homogeneous agglomerates (6.5% of water) were made. They were pressed (300 kg/cm2, 80 40 5 mm) using a laboratory press (NANNETI mod. Mini-P, Faenza, Italy). The × × samples were fired to 995, 1050, 1100, and 1150 ◦C, keeping them for 4 h at the maximum firing temperature using a NANNETI kiln Mod. CVKN-CVKN/S, Faenza, Italy. A firing cycle similar to the one used in the local ceramic industry was simulated. A mineralogical analysis of the ceramic tile bodies (18 replicas 4 temperatures) was carried out by XRD following the methodology described × in [7–10], and by FTIR following the methodology described in [1]. In order to quantify mineral phase, ceramic pieces were ground in an agate hammer, homogeneously mixed with 10 wt% corundum, and analyzed by XRD, following the conventional RIRcor method [11]. Powder-fired samples were characterized by XRD techniques using a Siemens D5000D powder diffractometer with Cu-Kα radiation (λ = 1.5406 A), 2θ in the interval between 5 to 65◦, and a scanning rate of 2◦/min. A method for crystalline and amorphous phase quantification was used. Once the majority of pure phases in the fired samples were identified, the corresponding calibration curves were built using 676-NIST (National Institute of Standards and Technology, Gaithersburg, MD, USA) as a reference. The RIRcor values of mullite, hematite, quartz, and hercynite were taken from [4]. For the FTIR analysis, a fine powder of the different ceramic tile bodies was placed on the glass window of the ATR-FTIR instrument spectrometer (BRUKER IFS 66/S, Mannheim, Germany). This device has a medium IR source, a KBr beamsplitter, and two detectors, including a DLaTGS detector (Mannheim, Germany) at room temperature for routine 1 measurements and obtaining spectra between 7000 and 400 cm− . This method relies on the use of Appl. Sci. 2020, 10, 3114 4 of 9 the main band of a pure mineral as a reference for the normalization of the IR spectrum of a mineral sample. In this way, the molar absorptivity coefficient in the Lambert–Beer law and the components of a mixture, in mol percentage, can be calculated. GAMS equation modeling environment and the NLP solver CONOPT (©ARKI Consulting and Development, Bagsvaerd, Denmark) were used to correlate the experimental data in the samples considered. MCT (Mannheim, Germany) detector was cooled by high liquid nitrogen sensitivity and speed of measurement. Spectra were also compared to the RRUFF database (2237 Public Mineral Species with Samples, 3710 total Minerals Species with Samples): https://rruff.info/. This methodology allows the use of spectra libraries and linear regression algorithms for quantification purposes. A mercury penetrometer (POROSIZER 9329, Faenza, Italy) was used to determine some parameters that describe the porous texture of the studied ceramic tile bodies. Total volume of intrusion, the total pore area, the average pore radius, and apparent density of the test bodies were calculated from the intrusion curves for each of the fired tile bodies. Ceramic tile bodies were studied by back-scattering scanning electron microscopy (SEM) with energy dispersion microanalysis (EDS), using a PHILIPS XL-30 microscope (Mahwah, NJ, USA) with EDAX DX-4 microanalysis v.3.6 (0.3 mbar and 20 kV).

3. Results and Discussion

3.1. Mine Spoils The mineralogical composition of bulk materials was: quartz, feldspars, kaolinite, illite, chlorite and smectites (in traces), and minor hematite, calcite, and dolomite. Quantification of main minerals is shown in Table1. The minerals detected in F1 were kaolinite and illite, sometimes they were also accompanied by chlorite.

Table 1. Mineralogical composition of mine spoils (bulk mine spoil). Mean value of the 60 samples analyzed, and range for each series (40 Oliete-Estercuel (OE) samples and 20 Ariño-Andorra (AA) samples).

Series Quartz Kaolinite Illite/Muscovite Feldspars Calcite/Dolomite (%) Mean Range Mean Range Mean Range Mean Range Mean Range OE 19.8 48–5 53.6 90–20 20.5 40–5 1.4 3–0 2.2 7–0 AA 40.3 59–17 35.8 49–17 19.6 50–8 0.9 6–0 -

The relative percentage of the main components for F1, illite and kaolinite, is recorded in Table2. F2 has a similar composition to F1, but the kaolinite decreases and the illite content increases substantially (Figure2). In F3, quartz is the main component and is always present, and illite /muscovite, dolomite, calcite, feldspars, and hematite can be present (Figure2).

Table 2. Mineralogical composition of clay fractions (%kaolinite, %illite/muscovite). Mean value of the 60 samples analyzed, and ranges for each series (40 OE samples and 20 AA samples).

Series Kaolinite Illite (%) Mean Range Mean Range OE 80.5 96–45 19.5 60–5 AA 65.3 80–39 34.7 65–21

Table3 shows the chemical composition of the main components (mean and range) for each sampling area. Appl. Sci. 2020, 10, 3114 5 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 10

Figure 2. XRD diagrams for each particle size fraction: F1, F2, and F3 (from top to bottom, respectively. Legend: Q: quartz; I: Illite; K: kaolinite; Cl: chlorite; Fd: feldspar; Cc: calcite. Figure 2. XRD diagrams for each particle size fraction: F1, F2, and F3 (from top to bottom, respectively. Legend: Q: quartz; I: Illite;Table K: kaolinite; 3. Main elements Cl: chlorite; (wt%). Fd: LOI:feldspar; Loss Cc: on ignition.calcite.

SiO Al O CaO MgO K O Fe O LOI SeriesTable 2. Mineralogical2 composition2 3 of clay fractions (%kaolinite, %illite/muscovite).2 2 3 Mean value of the 60Mean samples Range analyzed, Mean and Range ranges Mean for each Range series Mean (40 RangeOE samples Mean and Range 20 AA Mean samples). Range Mean Range OE 55.5 64–41 25.7 33–19 0.72 3.0–0.2 0.70 1.8–0.4 1.92 3.5–0.6 4.38 8.2–1.3 15.8 17.2–6.2 Series Kaolinite Illite AA 64.6 70–58 20.71 24–13 0.41 0.6–0.2 0.41 0.6–0.2 2.09 3.8–0.1 3.12 4.5–1.3 8.59 10–4.8 (%) Mean Range Mean Range OE 80.5 96–45 19.5 60–5 3.2. Fired Bodies AA 65.3 80–39 34.7 65–21 Table4 3 show shows the the mineral chemical quantification composition ofof analyzedthe main firedcomponents bodies for(mean each and temperature. range) for each All analyzedsampling samplesarea. contain a high percentage of kaolinite and illite. The presence of micaceous minerals along with kaolinite is quite common in many ceramic clays [12]. This is the case for all the analyzed samples that contain illite. Therefore,Table 3. Main in thiselements section, (wt%). we considerLOI: Loss theon ignition. firing transformations experienced by clayed mine spoils. In other words, the formation of mullite and other high-temperature phases SiO2 Al2O3 CaO MgO K2O Fe2O3 LOI Series wereMean studied Range from Mean mine spoils Range that Mean have variableRange Mean proportions Range ofMean illite (betweenRange 4%Mean and Range 50%) andMean/or Range chlorite (<5%). Cristobalite was not detected as a high-temperature phase in either of the series (OE 17.2– OE 55.5 64–41 25.7 33–19 0.72 3.0–0.2 0.70 1.8–0.4 1.92 3.5–0.6 4.38 8.2–1.3 15.8 or AA). Table4 summarizes the aforementioned results regarding the observations made with FTIR 6.2 AA (Figure 64.6 3) and 70–58 XRD 20.71 (Figure 4 24–13). 0.41 0.6–0.2 0.41 0.6–0.2 2.09 3.8–0.1 3.12 4.5–1.3 8.59 10–4.8

3.2. Fired Bodies Table 4. Mineral quantification of fired tile bodies (%). Table 4 show the mineral quantification of analyzed fired bodies for each temperature. All T(◦C)/Mineral Phase (%) Mullite Quartz Hematite Spinel/Hercynite Vitreous Phase analyzed samples contain a high percentage of kaolinite and illite. The presence of micaceous Serie OE minerals along with kaolinite is quite common in many ceramic clays [12]. This is the case for all the 995 18 15 5 5 57 analyzed samples that contain illite. Therefore, in this section, we consider the firing transformations 1050 23 10 7 4 56 experienced by1100 clayed mine spoils. 28 In other 6words, the 9 formation 3of mullite and 54other high- temperature phases1150 were studied fr 39om mine spoils 2 that have 12 variable proportions 1 of illite 46 (between 4% and 50%) and/or chlorite (<5%). CristobaliteSerie was AA not detected as a high-temperature phase in either of the series995 (OE or AA). Table 8 4 summa 21rizes the 9 aforementioned 7 results regarding 55 the observations made1050 with FTIR (Figure 11 3) and XRD 11 (Figure 4). 7 5 66 1100 17 8 12 3 60 1150 28 4 19 2 47 S.D 1. ± Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 10

Table 4. Mineral quantification of fired tile bodies (%).

T (°C)/ Mineral Phase (%) Mullite Quartz Hematite Spinel/Hercynite Vitreous Phase Serie OE 995 18 15 5 5 57 1050 23 10 7 4 56

Appl. Sci. 20201100, 10, x 3114 FOR PEER REVIEW28 6 9 3 54 6 6of of 10 9 1150 39 2 12 1 46 Serie AA

995 8 21 9 experimental7 55 1050 11 11 7 5 66 1100 17 8 12 3 60 995 °C 1150 28 4 19 2 47 intensity 2 S.D ± 1 R = 0.9898

−1 At 1050 °C, the 55 °C rise in firing temperaturewavenumber causes (cm an) increase in the percentage of mullite in both samples, and the formation of mullite is much more intense. These samples show the most intense and closest reflections. At 1100 °C, we obtained(a) diffractograms with very intense and well- defined lines of mullite (Figure 4). At 1150 °C, the development of mullite is clear in all samples, and with high crystallinity. experimental According to the quantification XRD analyses (Table 4), mullite is the most abundant mineral. The hercynite content is higher at low temperatures (9951150 °C), °C and hematite content is higher at 1150 °C. The highest quartz content is slightly over 20% in samples from the AA mine area. Compared to 2 intensity R = 0.9978 the other crystalline phases present in the fired samples, an intermediate phase between spinel (MgO·Al2O3) and hercynite (FeO·Al2O3) is originated from the destruction of illite [10]. In the samples richer in illite, hercynite forms directly [10]. Thiswavenumber phase (cmis −1formed) at 995 °C and remains up to a temperature of 1150 °C. Firstly, at 995–1100 °C, a great change in the structure takes place and a spinel phase is formed. The crystallinity of spinel and glassy phases are evolved in a wide temperature range between 995 and 1150 °C. (b) Figure 3. FTIRFTIR experimental experimental spectra spectra for for fired fired sample sample bodies bodies and and their their calculated calculated FTIR FTIR spectra.

Temperatures: 995 ◦°CC( (a) and 1150 ◦°CC( (b).

More or less significant amounts of amorphous material and glassy phase were detected in all fired samples. According to the literature [13,14], there is no interaction between the kaolinite and illite particles of clays fired under 1000 °C. Mullite formation after high-temperature illite breakdown has been studied in phyllosilicate-rich bricks [15]. At T ≥ 900 °C, illite dehydroxylation is followed by partial melting that triggers the nucleation and growth of mullite [15]. This absence of interaction would be the cause behind the temperature of formation of the spinel phase being separate from the illite content. Therefore, the mullitization process can be explained independently. The creation of mullite from illite at a lower temperature [15] than the one that leads to the transformation of kaolinite can be attributed to the effect of potassium and other alkaline or alkaline-earth cations present in (or near) the porous hexagonal network of the tetrahedral sheets (containing Si) in 2:1 layers of illite. The effect of K in these positions is similar to the role conducted by this element in glass formation. As the Si–O bonds break, the potassium ions move, keeping the structure open and not allowing atoms to bond back together. As a result, the stability of the tetrahedral layer is significantly reduced compared to its normal stability, which facilitates the structural transformation into mullite at a low temperature. Potassium, therefore, decreases the activation energy required to create mullite. The relative amount of alumina present in illite that is available for the formation of mullite at high temperatures is lower than in kaolinite. As a result, in mine spoils with a high proportion of illite/kaolinite,Figure 4. XRD the diagramsamount of testmullite bodies that heated can atbe di createdfferenttemperatures. in the fired tile Legend: bodies Q: is quartz; considerably H: hematite; smaller than M:in pure mullite. kaolinite. The non-existence of cristobalite in clays that are rich in illite can be explained by Figuretaking 4.into XRD consideration diagrams of thetest chemical bodies heated composition at different of illite. temperatures. Legend: Q: quartz; H: More or less significant amounts of amorphous material and glassy phase were detected in all hematite;By increasing M: mullite. the content in illite, when going from pure kaolinites to mixtures and pure illite, firedthe amount samples. of silica According available to the for literaturethe formation [13,14 of], cristobalite there is no also interaction increases between slightly, the but kaolinite the content and illiteof alkaline particles metals of clays that fired create under glass 1000 increases◦C. Mullite much formation more. As after a result, high-temperature the absence illiteof cristobalite breakdown is has been studied in phyllosilicate-rich bricks [15]. At T 900 C, illite dehydroxylation is followed attributed to the presence of glass-forming cations supplied≥ by◦ the illite. by partialAt 995 melting °C, the that formation triggers of the a noticeable nucleation amount and growth of mullite of mullite can be [15 attributed]. This absence to the ofhigh interaction content wouldof illite. be Even the causeso, lengthy behind firing the temperature times are needed of formation to be able of theto observe spinel phase clearly-defined being separate reflections from the of illitemullite content. in the diffractogram. Therefore, the mullitization process can be explained independently. The creation of

Appl. Sci. 2020, 10, 3114 7 of 9 mullite from illite at a lower temperature [15] than the one that leads to the transformation of kaolinite can be attributed to the effect of potassium and other alkaline or alkaline-earth cations present in (or near) the porous hexagonal network of the tetrahedral sheets (containing Si) in 2:1 layers of illite. The effect of K in these positions is similar to the role conducted by this element in glass formation. As the Si–O bonds break, the potassium ions move, keeping the structure open and not allowing atoms to bond back together. As a result, the stability of the tetrahedral layer is significantly reduced compared to its normal stability, which facilitates the structural transformation into mullite at a low temperature. Potassium, therefore, decreases the activation energy required to create mullite. The relative amount of alumina present in illite that is available for the formation of mullite at high temperatures is lower than in kaolinite. As a result, in mine spoils with a high proportion of illite/kaolinite, the amount of mullite that can be created in the fired tile bodies is considerably smaller than in pure kaolinite. The non-existence of cristobalite in clays that are rich in illite can be explained by taking into consideration the chemical composition of illite. By increasing the content in illite, when going from pure kaolinites to mixtures and pure illite, the amount of silica available for the formation of cristobalite also increases slightly, but the content of alkaline metals that create glass increases much more. As a result, the absence of cristobalite is attributed to the presence of glass-forming cations supplied by the illite. At 995 ◦C, the formation of a noticeable amount of mullite can be attributed to the high content of illite. Even so, lengthy firing times are needed to be able to observe clearly-defined reflections of mullite in the diffractogram. At 1050 ◦C, the 55 ◦C rise in firing temperature causes an increase in the percentage of mullite in both samples, and the formation of mullite is much more intense. These samples show the most intense and closest reflections. At 1100 ◦C, we obtained diffractograms with very intense and well-defined lines of mullite (Figure4). At 1150 ◦C, the development of mullite is clear in all samples, and with high crystallinity. According to the quantification XRD analyses (Table4), mullite is the most abundant mineral. The hercynite content is higher at low temperatures (995 ◦C), and hematite content is higher at 1150 ◦C. The highest quartz content is slightly over 20% in samples from the AA mine area. Compared to the other crystalline phases present in the fired samples, an intermediate phase between spinel (MgO Al O ) · 2 3 and hercynite (FeO Al O ) is originated from the destruction of illite [10]. In the samples richer in · 2 3 illite, hercynite forms directly [10]. This phase is formed at 995 ◦C and remains up to a temperature of 1150 ◦C. Firstly, at 995–1100 ◦C, a great change in the structure takes place and a spinel phase is formed. The crystallinity of spinel and glassy phases are evolved in a wide temperature range between 995 and 1150 ◦C. Along with the incongruent fusion of the feldspars, the formation of the vitreous phase and the final mullitization were observed (Figure5). Between 1050 and 1100 ◦C there is an important decrease in the open porosity in AA fired samples, coinciding with the first phase of vitrification (Table5). This behavior is not observed in the OE fired samples, where the open porosity increases between 1050 and 1100 ◦C. This behavior is probably due to the higher mullite content of the OE fired samples at these temperatures. At 1150 ◦C, both fired samples OE and AA close their pores, and the porosity decreases significantly [7,10]. An Al- and Si-rich vitreous phase is formed by the decomposition of the initial minerals due to the increase in temperature, and it represents about 50% of the fired bodies. The higher percentages in mullite and vitreous phases at 995 ◦C is explained by the presence of an abundance of illite in all samples [15]. Appl. Appl.Sci. 2020 Sci.,2020 10, x, 10 FOR, 3114 PEER REVIEW 88 of 910

Figure 5. Growth of minerals, and presence of abundant vitreous phase and low porosity in tile bodies Figurefired 5. Growth at 1150 ◦ C.of (minerals,A) The presence and presence of quartz of andabundant amorphous vitreous phase phase were and observed low porosity together in with tile bodies small scale-shaped crystals of type I primary mullite (<0.2 µm in size). (B) Crystallizations on the vitreous fired at 1150 °C. (A) The presence of quartz and amorphous phase were observed together with small phase on the fracture surface. Hematite coatings are observed forming crusts. scale-shaped crystals of type I primary mullite (<0.2 μm in size). (B) Crystallizations on the vitreous phase on the fractureTable 5. surface.Parameters Hematite that describe coatings the are porous observed texture forming of the ceramiccrusts. tile bodies.

T VT ST ϕ ϕap Kp.10–16 r.10–8 Sample Table 5. Parameters that describe2 the porous texture of the ceramicε tile bodies. (◦C) (mL/g) (m /g) (g/mL) (g/mL) (m) (m) T 995VT 0.142 ST 2.09 1.895𝝋 2.603𝝋𝒂𝒑 0.375 8.72Kp.10–16 13.44 r.10–8 Sample ε 1050 0.0742 1.78 2.186 2.577 0.180 1.59 8.22 OE (°C) (ml/g) (m /g) (g/ml) (g/ml) (m) (m) 1100 0.135 2.29 1.944 2.601 0.344 6.01 11.97 995 0.142 2.09 1.895 2.603 0.375 8.72 13.44 1150 0.030 1.684 2.243 2.480 0.083 0.06 5.98 1050 0.074 1.78 2.186 2.577 0.180 1.59 8.22 OE 995 0.143 3.505 1.919 2.668 0.285 2.43 8.40 1100 10500.135 0.100 2.29 1.917 1.944 2.125 2.6962.601 0.2220.344 3.25 6.01 10.40 11.97 AA 1150 11000.030 0.061 1.684 1.345 2.243 2.286 2.6202.480 0.1420.083 1.32 0.06 8.86 5.98 995 1150 0.143 0.038 3.505 1.745 1.919 2.352 2.5812.668 0.0920.285 0.25 2.43 4.33 8.40 Legend:1050 T (temperature);0.100 VT (total Hg1.917 volume that penetrates2.125 into the2.696 pores); ST (specific0.222 surface); ϕ 3.25(density); ϕap 10.40 AA (apparent density); ε (open porosity); Kp (coefficient of permeability); r (average pore radius). 1100 0.061 1.345 2.286 2.620 0.142 1.32 8.86 1150 0.038 1.745 2.352 2.581 0.092 0.25 4.33 The presence of quartz and amorphous phase were observed by SEM, together with small Legend: T (temperature); VT (total Hg volume that penetrates into the pores); ST (specific surface); 𝜑 scale-shaped crystals of type I primary mullite. Also, hematite coatings were observed forming crusts. (density); 𝜑 (apparent density); ε (open porosity); Kp (coefficient of permeability); r (average pore Most of the small quartz (<25 µm) is included in the vitreous matrix. Despite observing a porous microstructure,radius). the non-porous areas are well sintered.

4.Along Conclusions with the incongruent fusion of the feldspars, the formation of the vitreous phase and the final mullitization were observed (Figure 5). Between 1050 and 1100 °C there is an important decrease Mine spoils with a high amount of illite create mullite at 995 C. Cristobalite was not detected as a in the open porosity in AA fired samples, coinciding with the first◦ phase of vitrification (Table 5). high-temperature phase. More or less significant amounts of amorphous material and glassy phase Thiswere behavior detected is not in allobserved fired samples in the (OE>45%). fired The samples, higher where percentages the open of mullite porosity (18–8%) increases and vitreousbetween 1050 and 1100 °C. This behavior is probably due to the higher mullite content of the OE fired samples phase (>55%) at 995 ◦C were due to the presence of an abundant amount of illite in all samples. At at these temperatures. At 1150 °C, both fired samples OE and AA close their pores, and the porosity 1150 ◦C, the content of mullite in the fired OE selected samples (39%) was higher than the ones from the decreasesAA series significantly (28%). Between [7,10]. 1050 An andAl- 1100and ◦Si-richC, there vi istreous an important phase is decrease formed in by the the open decomposition porosity in AA of the initialfired samples. minerals However, due to the this increase behavior in is temperatur not observede, and in the it OErepresents fired samples, about where50% of the the open fired porosity bodies. The increasedhigher percentages between 1050 in andmullite 1100 and◦C. Thevitreous role of phases the mullite at 995 in this°C is behavior explained could by be the an areapresence for future of an abundancenew research. of illite Despite in all samples observing [15]. a porous microstructure, the non-porous areas are well sintered. The presenceThe presence of quartz of quartz and amorphous and amorphous phase were phase observed were observed together by with SEM, small together scale-shaped with small crystals scale- of shapedmullite crystals and hematiteof type I primary coatings formingmullite. crusts.Also, hematite coatings were observed forming crusts. Most of the small quartz (<25 μm) is included in the vitreous matrix. Despite observing a porous microstructure, the non-porous areas are well sintered.

4. Conclusions Mine spoils with a high amount of illite create mullite at 995 °C. Cristobalite was not detected as a high-temperature phase. More or less significant amounts of amorphous material and glassy phase Appl. Sci. 2020, 10, 3114 9 of 9

Author Contributions: Conceptualization, M.M.J. and S.M.; methodology, M.M.J. and S.M.; software, S.M.; validation, S.M., F.P. and M.A.M.; formal analysis, S.M. and M.M.J.; investigation, M.M.J., S.M., M.A.M. and F.P.; writing—original draft preparation, M.M.J.; writing—review and editing, M.M.J.; supervision, M.M.J.; project administration, M.M.J. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to express their gratitude to Emilio Galán for his help in the discussion and comments; and dedicate this manuscript to the memory of T. Sanfeliu, for his hard work in the study of Spanish ceramic clays. Conflicts of Interest: The authors declare no conflict of interest.

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