Sustainable Ceramic Materials Manufactured from Ceramic Formulations Containing Quartzite and Scheelite Tailings

In this study, we develop ceramic formulations based on quartzite and scheelite tailings collected from mining companies in the northeast of Brazil (Rio Grande do Norte State). New ceramic samples (27 wt% of kaolin, 29 wt% of plastic clay, 11 wt% of quartzite tailing, and 0–8 wt% scheelite tailing) were uniaxially pressed in two steps (20 MPa and 50 Mpa for 20 s); dried at 110 ◦C for 24 h; and sintered at 1150 ◦C, 1200 ◦C, and 1250 ◦C. The main mineralogical phases (mullite, quartz, calcite, and anorthite) of the sintered samples were identified using X-ray diffraction (XRD). After evaluation of the physical-mechanical properties (water absorption, linear shrinkage, apparent porosity, and flexural strength), it was observed that the incorporation of scheelite tailing by up to 8 wt% did not significantly alter the properties of samples sintered at all temperatures. Our results indicate that the new ceramics formulations developed have strong potentials in manufacturing sustainable materials such as ceramic tiles and porcelain stoneware.


Introduction
It is indisputable that mining activities are a significant portion of the Gross Domestic Product of several countries (such as Norway, the United States, the United Kingdom, Australia, and Brazil) and provide raw materials for industries that leverage the scientific and technological development of our society. Unfortunately, this activity can also be incredibly harmful to the environment since many tailings are generated [1,2]. Annually, it is estimated that these activities generate about 25 billion tons of solid residues, of which approximately 14 billion tons are rejects that are often improperly disposed of [3,4]. Generally, the mine tailings and water originated in the process are stored together in tanks or are improperly stored in massive residues piles near the mining companies, generating environmental impacts and damage to human health [5][6][7]. The costs associated with processing and logistics for the proper disposal of these mine tailings tend to be obstacles for mining companies. This has attracted attention from researchers interested in reusing this residue to manufacture sustainable materials [8][9][10][11]. Scheelite tailings have a composition similar to several traditional ceramic raw materials used to manufacture bricks, tiles, mortars, concrete, and porcelain materials, containing calcium oxide (CaO), silicon oxide (SiO2), aluminum oxide (Al2O3), and fluxing oxides [2,12,13]. Therefore, tailings generated from scheelite mining have strong potential to be used as an alternative raw material to substitution for the conventional one [14,15]. Also, due to the high production volume in the ceramic industry, it can absorb a large amount of mine tailings [16]. Several studies have addressed the ceramic industry's viability in absorbing different mine tailings to manufacture sustainable materials. Among these studies, it is worth mentioning that red bauxite wastes from the production of alumina have been used to produce ceramic bricks [17][18][19][20] and tiles [21,22]. The granite processing waste was used in ceramic bricks [23,24], tiles [24,25], mortars [26], and ceramic membranes [27]. Gneiss rock, slate, and limestone waste were used to produce ecological bricks [28] and tiles [29,30]. However, few studies on the development of mortars [12], concrete formulations [13], and ceramic bricks [2] addressed the recycling of scheelite waste. Studies on the use of scheelite tailing for the development of triaxial ceramic bodies are scarce in the literature, despite the high amount of scheelite waste generated each year.
Quartzite has been extensively investigated as an alternative raw material to obtain mortars, porous ceramic materials, and materials in the sanitary ware industry [31][32][33][34]. The quartzite tailing contains appreciable contents of silicon oxide (SiO2), aluminum oxide (Al2O3), calcium oxide (CaO), and alkaline oxides (K2O and Na2O), which makes it a good substitute for high-value-added raw materials that are used in the ceramic industry [34,35].
In this context, the incorporation of scheelite and quartzite tailings in the ceramic industry stands out as an alternative to minimize the environmental impact caused by its inadequate disposal since it reduces the amount of residues deposited and the extraction of conventional raw materials, which are each increasingly scarce and expensive. However, the application of this type of mine tailings to the production of ceramic tiles still need to be thoroughly investigated.
Given the situations described above, the quartzite and scheelite tailings were incorporated in ceramic formulations with the objective of utilizing these potential raw materials to manufacture Scheelite tailings have a composition similar to several traditional ceramic raw materials used to manufacture bricks, tiles, mortars, concrete, and porcelain materials, containing calcium oxide (CaO), silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), and fluxing oxides [2,12,13]. Therefore, tailings generated from scheelite mining have strong potential to be used as an alternative raw material to substitution for the conventional one [14,15]. Also, due to the high production volume in the ceramic industry, it can absorb a large amount of mine tailings [16]. Several studies have addressed the ceramic industry's viability in absorbing different mine tailings to manufacture sustainable materials. Among these studies, it is worth mentioning that red bauxite wastes from the production of alumina have been used to produce ceramic bricks [17][18][19][20] and tiles [21,22]. The granite processing waste was used in ceramic bricks [23,24], tiles [24,25], mortars [26], and ceramic membranes [27]. Gneiss rock, slate, and limestone waste were used to produce ecological bricks [28] and tiles [29,30]. However, few studies on the development of mortars [12], concrete formulations [13], and ceramic bricks [2] addressed the recycling of scheelite waste. Studies on the use of scheelite tailing for the development of triaxial ceramic bodies are scarce in the literature, despite the high amount of scheelite waste generated each year.
Quartzite has been extensively investigated as an alternative raw material to obtain mortars, porous ceramic materials, and materials in the sanitary ware industry [31][32][33][34]. The quartzite tailing contains appreciable contents of silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), calcium oxide (CaO), and alkaline oxides (K 2 O and Na 2 O), which makes it a good substitute for high-value-added raw materials that are used in the ceramic industry [34,35].
In this context, the incorporation of scheelite and quartzite tailings in the ceramic industry stands out as an alternative to minimize the environmental impact caused by its inadequate disposal since it reduces the amount of residues deposited and the extraction of conventional raw materials, which are each increasingly scarce and expensive. However, the application of this type of mine tailings to the production of ceramic tiles still need to be thoroughly investigated.
Given the situations described above, the quartzite and scheelite tailings were incorporated in ceramic formulations with the objective of utilizing these potential raw materials to manufacture ceramic tile and porcelain stoneware. First, the kaolin, plastic clay, and quartzite and scheelite tailings were characterized using X-ray fluorescence, XRD, particle size distributions (laser diffraction), and thermal analytical techniques (TG and DTA). Uniaxially pressed samples (50 mm × 15 mm × 5 mm) were sintered at 1150 • C, 1200 • C, and 1250 • C for 40 min. Subsequently, their main mineralogical phases were identified, and physical-mechanical properties (linear shrinkage, water absorption, bulk density, and flexural strength module) were evaluated.

Raw Materials
The scheelite tailing (SR) was collected from existing mine piles of the Mineração Tomaz Salustiano S.A. company, located in the Currais Novos City, Rio Grande do Norte State (Brazil). The quartzite tailing came from the Tecquímica company, located in the Várzea City, Paraíba State (Brazil). The Kaolin (K) was supplied from Rocha Minérios, located in Juazeirinho City, Paraíba State (Brazil); plastic clay (PC) and feldspar (F) were supplied from the Armil Mineração do Nordeste, located in Parelhas City, Rio Grande do Norte State (Brazil).

Samples Preparation and Sintering Treatments
The samples investigated in this research were prepared from 27wt% of kaolin, 29wt% of plastic clay, 11wt% of quartzite tailing, and different contents of scheelite tailing. The nominal compositions of the ceramic formulations (wt%) and their respective nomenclatures are summarized in Table 1. Before the sintering step, all raw materials were sieved (74 µm), wet-mixed (7% moisture content by wt.), and uniaxially pressed in two stages (Servitech, model CT-335, Tubarão, Brazil) in a rectangular mold (50 mm × 15 mm × 5 mm). In the first stage, called pre-pressing, a uniaxially pressing was applied at 13.5 Mpa for 10 s. The samples were then divided into two groups, where the first group was pressed at 20 Mpa and the second at 50 Mpa. In both, the pressing time was equal to 20 s. Finally, the pressed samples were dried at 110 • C for 24 h and sintered in a conventional electric oven (Flyever Equipment FE 50 RP, São Carlos, Brazil) at temperatures of 1150 • C, 1200 • C, and 1250 • C. The sintering protocol consisted of heating (30 • C·min −1 ) the samples from room temperature until the temperature of interest (1150 • C, 1200 • C, and 1250 • C) and performing an isothermal treatment there for 40 min. After sintering treatment, the oven was turned off, and the samples were cooled to room temperature.
The Archimedes method was used to measure water absorption (WA), apparent porosity (AP), and an apparent density (AD). Linear shrinkage (LS) and flexural tensile strength measurements were carried out using three points bending test. Each experiment was repeated 10 times for different samples. The procedures for carrying out these tests are described in other works [38,44,45].  Table 2). For the quartzite tailing (Figure 2b), the mineralogical phases present were mica (JCPDS 83-1808), feldspar (JCPDS 84-0710), and quartz (JCPDS 46-1045). As the quartzite tailing has quartz and feldspar, it can have a physical characteristic similar to the non-plastic raw materials used in the traditional ceramic industries.

Characterization of Raw Materials
Sustainability 2020, 12, x FOR PEER REVIEW 4 of 14 model TA 60H, Kyoto, Japan) and differential thermal analysis (DTA) (Instrumentec BP, model RB-3000, Campinas, Brazil), with a heating rate of 12.5 °C·7min −1 , under the air atmosphere and using calcined aluminum oxide (Al2O3) as standard [36,37,43]. The Archimedes method was used to measure water absorption (WA), apparent porosity (AP), and an apparent density (AD). Linear shrinkage (LS) and flexural tensile strength measurements were carried out using three points bending test. Each experiment was repeated 10 times for different samples. The procedures for carrying out these tests are described in other works [38,44,45].   Table 2. Chemical compositions of the raw materials (kaolin, plastic clay, and feldspar) and the mine tailings (quartzite and scheelite) used in the new ceramic formulation studied.  Table 2 summarizes the chemical compositions of raw materials (kaolin, plastic clay, and feldspar) and the mine tailings (quartzite and scheelite). SiO2 (45.7% and 54.5%) and Al2O3 (39.5% and 27.5%) were the main oxides identified in the kaolin and plastic clay, respectively. These oxides commonly have origin from the structure of clay minerals and the presence of free silica [46,47]. Also,  Table 2. Chemical compositions of the raw materials (kaolin, plastic clay, and feldspar) and the mine tailings (quartzite and scheelite) used in the new ceramic formulation studied.  Table 2 summarizes the chemical compositions of raw materials (kaolin, plastic clay, and feldspar) and the mine tailings (quartzite and scheelite). SiO 2 (45.7% and 54.5%) and Al 2 O 3 (39.5% and 27.5%) were the main oxides identified in the kaolin and plastic clay, respectively. These oxides commonly have origin from the structure of clay minerals and the presence of free silica [46,47]. Also, the high content of K 2 O (3.9%) was detected in plastic clay. The presence of K 2 O is important because it is a known flux and acts by lowering the sintering temperature, bringing economic gains to industrial processes. The high content levels of CaO (44.7%) and MgO (2.7%) identified in the scheelite tailing are associated with the presence of calcite, dolomite and SiO 2 (20.8%); in the form of quartz as shown in Figure 2a. The presence of calcium oxide (CaO) and magnesium oxide (MgO) is essential because they reduce the refractoriness. The high fire loss measured in the scheelite tailing (15.8%) can be attributed to the thermal decomposition of calcium carbonate and the release of gases. The higher amount of K 2 O (12.1%) present in feldspar contributes to the formation of the vitreous phase during sintering [46]. The quartzite tailing contained SiO 2 (76.5%) and Al 2 O 3 (12.1%) as major constituents, and in smaller proportions, CaO (0.7%), MgO (1.1%) and Fe 2 O 3 (1.5%). For the quartzite tailing, it was also observed that this residue has a silico-aluminous composition, i.e., the sum of the oxides contents SiO 2 and Al 2 O 3 is greater than 50%, (SiO 2 + Al 2 O 3 > 50%), indicating its potential as a flux agent and to lower the temperature maturation of the ceramic bodies, enabling a reduction in energy consumption [48]. Table 3 shows the particle size distribution of the kaolin, plastic clay, feldspar, and scheelite and quartzite tailings. The scheelite and quartzite tailings presented the most considerable fraction of particles with diameters above 20 µm (70.0% and 50.2% of the accumulated volume, respectively). For the other raw materials, the most considerable fraction of particles has diameters between 2 µm and 20 µm, with 69.0%, 79.9%, and 60.9% of accumulated volume for kaolin, plastic clay, and feldspar, respectively. It is known that the coarser particle size can interfere with sintering reactions kinetics, dramatically influencing the packaging of green samples and the product sintering step. The smaller particle sizes provide greater surface areas and reactivities between the particles, favoring the reactions kinetics and the diffusion process during the phase transformations [37,38,49]. Table 3. Summary of the particle size distribution of the kaolin, plastic clay, feldspar, and mine tailings (quartzite and scheelite).

Raw Materials
Fine Figure 3a-e shows the TG/DTG data for the kaolin, plastic clay, feldspar, and scheelite and quartzite tailings. Kaolin (Figure 3a) showed a significant loss of mass (13.6%) between 352 • C and 754 • C, probably associated with the dihydroxylation of kaolinite and mica. The TG/DTG curve obtained from the plastic clay (Figure 3b) shows that mass loss occurred in two stages. In the first stage, the mass loss was equal to 3.4% and was observed in the temperature range of 22 • C to 300 • C. The origin of the first thermal event is associated with the loss of free and adsorbed water. A mass loss of 7.3% was observed during the second stage (300-788 • C). This second stage can be attributed to the loss of organic matter and hydroxyl groups present in the clay minerals. The total mass loss for kaolin and plastic clay was 16.1% and 10.9%, respectively. Feldspar (Figure 3c) and quartzite tailing (Figure 3d) showed shallow mass loss (1.6% and 1.5%, respectively), corroborating with the fire loss analysis (see Table 2). In addition, no carbonates, sulfates, organic matter, and clay minerals were found in these materials. The TG/DTG curve of the scheelite tailing ( Figure 3e) shows a significant loss of mass (13.9%) between 602 • C and 901 • C. The mass loss of the scheelite tailing is associated with the Figure 4a-e shows the differential thermal analysis (DTA) data for kaolin, plastic clay, feldspar, and tailings (scheelite and quartzite). In the DTA curve of kaolin ( Figure 4a) and plastic clay ( Figure  4b), two endothermic and one exothermic event were identified. The endothermic events are related to loss of free water (~92 °C for kaolin and ~108 °C for plastic clay) and loss of hydroxyls (~601 °C for kaolin and ~586 °C for plastic clay) of the octahedral layer present in its structure. The exothermic peak with a maximum of ~975 °C and 987 °C for kaolin and clay, respectively, corresponds to the mullite nucleation with β-quartz release from the amorphous structure created previously [50]. The DTA curve acquired from the quartzite tailing (Figure 4c) showed an endothermic peak at 51 °C and is related to the free water. The exothermic peak at 579 °C is associated with the polymorphic transformation of α-quartz to β-quartz. Still in the quartzite DTA curve, the exothermic peak at 949 °C is related to the structural and characteristic of the crystalline lattice's destruction. The DTA curve of the scheelite tailing (Figure 4e) shows an exothermic peak at ~465 °C and two endothermic peaks (866 °C and 906 °C) that correspond to the carbonate decomposition.  4a-e shows the differential thermal analysis (DTA) data for kaolin, plastic clay, feldspar, and tailings (scheelite and quartzite). In the DTA curve of kaolin ( Figure 4a) and plastic clay (Figure 4b), two endothermic and one exothermic event were identified. The endothermic events are related to loss of free water (~92 • C for kaolin and~108 • C for plastic clay) and loss of hydroxyls (~601 • C for kaolin and~586 • C for plastic clay) of the octahedral layer present in its structure. The exothermic peak with a maximum of~975 • C and 987 • C for kaolin and clay, respectively, corresponds to the mullite nucleation with β-quartz release from the amorphous structure created previously [50]. The DTA curve acquired from the quartzite tailing (Figure 4c) showed an endothermic peak at 51 • C and is related to the free water. The exothermic peak at 579 • C is associated with the polymorphic transformation of α-quartz to β-quartz. Still in the quartzite DTA curve, the exothermic peak at 949 • C is related to the structural and characteristic of the crystalline lattice's destruction. The DTA curve of the scheelite tailing (Figure 4e) shows an exothermic peak at~465 • C and two endothermic peaks (866 • C and 906 • C) that correspond to the carbonate decomposition.  Figure 5 shows the effect of the sintering temperature on the color of the samples. It was observed that the tones varied between light (1150 °C), yellow (1200 °C), and gray (1250 °C). The color variation was verified throughout all ceramic formulations. It was also found that ceramic formulations with a higher percentage of scheelite tailing presented colors with gray tones at higher temperatures. This color variation is observed because, at temperatures above 1000 °C, oxidation occurs from divalent iron to trivalent iron, responsible for accentuating the samples color for yellow tons. At temperatures above 1100 °C, the percentage of trivalent iron decreases, generating a dark reddish-brown to black color. This explains why there was a decrease in the yellow tone giving rise to a gray color with increased sintering temperature. The samples color is also dependent on the  Figure 5 shows the effect of the sintering temperature on the color of the samples. It was observed that the tones varied between light (1150 • C), yellow (1200 • C), and gray (1250 • C). The color variation was verified throughout all ceramic formulations. It was also found that ceramic formulations with a higher percentage of scheelite tailing presented colors with gray tones at higher temperatures. This color variation is observed because, at temperatures above 1000 • C, oxidation occurs from divalent iron to trivalent iron, responsible for accentuating the samples color for yellow tons. At temperatures above 1100 • C, the percentage of trivalent iron decreases, generating a dark reddish-brown to black color. This explains why there was a decrease in the yellow tone giving rise to a gray color with increased sintering temperature. The samples color is also dependent on the contents coloring oxides (such as Fe 2 O 3 , MgO, NaO, etc.). As the ceramic formulation increases, there will be a more significant variation in the sample tone. contents coloring oxides (such as Fe2O3, MgO, NaO, etc.). As the ceramic formulation increases, there will be a more significant variation in the sample tone.     and mullite (Al 6 Si 2 O 13 ) are desirable phases in ceramic materials due to their excellent mechanical properties. The increase in temperature from 1150 • C to 1250 • C led to a decrease in the quartz peaks' intensity, indicating their partial dissolution and the disappearance of the anorthite peak. contents coloring oxides (such as Fe2O3, MgO, NaO, etc.). As the ceramic formulation increases, there will be a more significant variation in the sample tone.      (Figures 7a and 8a) increased when sintering temperature increases from 1150 • C to 1200 • C. This behavior is probably related to the higher degree of sintering and densification due to the higher production of liquid phase with increasing temperature [51]. However, when the temperature increases from 1200 • C to 1250 • C, the linear shrinkage tends to decrease due to the smaller glass phase expansion. The lowest LS values were observed for ceramic formulations with higher contents of scheelite tailing, i.e., above 2wt% (F4, F5, F6, and F7 samples).

Mineralogical Phases and Physical-Mechanical Properties of the Sintered Samples
production of liquid phase with increasing temperature [51]. However, when the temperature increases from 1200 °C to 1250 °C, the linear shrinkage tends to decrease due to the smaller glass phase expansion. The lowest LS values were observed for ceramic formulations with higher contents of scheelite tailing, i.e., above 2wt% (F4, F5, F6, and F7 samples).
As the sintering temperature increased, the water absorption (WA) and apparent porosity (AP) decreased (Figures 7c and 8c). This may be related to the pores' filling due to the melting of the fluxing oxides (Fe2O3, K2O, and Na2O) present in the ceramic masses, which leads to greater packaging of the piece due to the formation of a liquid phase [37,38]. In summary, an increase in flexural strength (FS) is observed with an increase in temperature from 1150 °C to 1200 °C (Figures 7d and 8d). This behavior is related to the reduction of porosity in the parts. This effect is desirable since the pores act as stress concentrators. With the temperature increases to 1250 °C, there is a considerable change in the samples' mechanical behavior. The formulations that present a higher percentage of scheelite residue (above 2wt%) have their flexural strength modules reduced due to the excessive increase in the hardness of the ceramic body, increasing the fragility of the samples. In general, it is observed that the increase in the pressing pressure from 20 MPa to 50 MPa promoted a reduction in the values of linear shrinkage, water absorption, and apparent porosity.  As the sintering temperature increased, the water absorption (WA) and apparent porosity (AP) decreased (Figures 7c and 8c). This may be related to the pores' filling due to the melting of the fluxing oxides (Fe 2 O 3 , K 2 O, and Na 2 O) present in the ceramic masses, which leads to greater packaging of the piece due to the formation of a liquid phase [37,38]. In summary, an increase in flexural strength (FS) is observed with an increase in temperature from 1150 • C to 1200 • C (Figures 7d and 8d). This behavior is related to the reduction of porosity in the parts. This effect is desirable since the pores act as stress concentrators. With the temperature increases to 1250 • C, there is a considerable change in the samples' mechanical behavior. The formulations that present a higher percentage of scheelite residue (above 2wt%) have their flexural strength modules reduced due to the excessive increase in the hardness of the ceramic body, increasing the fragility of the samples. In general, it is observed that the increase in the pressing pressure from 20 MPa to 50 MPa promoted a reduction in the values of linear shrinkage, water absorption, and apparent porosity.   Table 4 summarizes the physical-mechanical properties (water absorption, linear shrinkage, apparent porosity, and flexural strength) measured from F0, F1, F2, F3, F4, F5, F6, and F7 samples pressed at different conditions (20 MPa and 50 MPa) and sintered at 1150 °C, 1200 °C, and 1250 °C. The ceramic formulation without scheelite tailing (F0 samples, see Table 1) was defined as the control sample. At 1150 °C, the scheelite-tailing-containing ceramics formulations presented better performance than the F0 sample, and the F5, F6, and F7 samples are potential candidates for the production of ceramic tiles. At 1200 °C, the ceramic formulations with scheelite tailing showed physico-mechanical properties had been performance statistically similar to the F0 sample, except the F3 sample that shown a behavior inferior. At 1250 °C, of the ceramic formulations with a higher content of scheelite tailing (i.e., a higher amount of fluxes), only the F1 and F2 samples showed performance similar to that of the F0 sample, and these samples present potential to be used as porcelain stoneware tile.  Table 4 summarizes the physical-mechanical properties (water absorption, linear shrinkage, apparent porosity, and flexural strength) measured from F0, F1, F2, F3, F4, F5, F6, and F7 samples pressed at different conditions (20 MPa and 50 MPa) and sintered at 1150 • C, 1200 • C, and 1250 • C. The ceramic formulation without scheelite tailing (F0 samples, see Table 1) was defined as the control sample. At 1150 • C, the scheelite-tailing-containing ceramics formulations presented better performance than the F0 sample, and the F5, F6, and F7 samples are potential candidates for the production of ceramic tiles. At 1200 • C, the ceramic formulations with scheelite tailing showed physico-mechanical properties had been performance statistically similar to the F0 sample, except the F3 sample that shown a behavior inferior. At 1250 • C, of the ceramic formulations with a higher content of scheelite tailing (i.e., a higher amount of fluxes), only the F1 and F2 samples showed performance similar to that of the F0 sample, and these samples present potential to be used as porcelain stoneware tile.

Conclusions
The incorporation of quartzite and scheelite tailings in ceramic masses proves to be a sustainable and viable alternative, making it possible to obtain ceramic pieces with low water absorption and low porosity, in addition to flexural strength in ranges of values that allow their application for the sustainable production of ceramic tiles and porcelain stoneware.
The incorporation of scheelite tailing by up to 8 wt% did not significantly alter the physical-mechanical properties for samples sintered at 1150 • C. However, the effects were more evident in samples sintered at 1200 • C and 1250 • C. Overall, the results showed that quartzite and scheelite tailings could be considered to be an alternative raw material for use in the ceramic tile and porcelain stoneware industry.