Silicate Mineral Eutectics with Special Reference to Lithium

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Article Silicate Mineral Eutectics with Special Reference to Lithium

Agata Stempkowska

Department of Environmental Engineering, Faculty of Civil Engineering and Resource Management, AGH University of Science and Technology, Mickiewicza 30 Av., 30-059 Cracow, Poland; [email protected]

Abstract: In this paper, the system of natural mineral alkali fluxes used in typical mineral industry technologies was analyzed. The main objective was to reduce the melting temperature of the flux systems. Particular attention was paid to the properties of lithium aluminium silicates in terms of simplifying and accelerating the heat treatment process. In this area, an alkaline flux system involving lithium was analyzed. A basic flux system based on sodium potassium lithium aluminosilicates was analyzed; using naturally occurring raw materials such as spodumene, albite and orthoclase, an attempt was made to obtain the eutectic with the lowest melting point. Studies have shown that there are two eutectics in these systems, with about 30% spodumene content. The active influence of sodium feldspar was found.

Keywords: Li-Na-K ﬂux system; mineral eutectic; spodumene

1. Introduction 1.1. Characteristics of Flux Minerals Lithium is a very common yet dispersed element. The largest resources are found in Citation: Stempkowska, A. Silicate seawater—billions of tons of highly diluted lithium (the range is from 0.001 to 0.020 mg/L Mineral Eutectics with Special lithium), which cannot be extracted on an industrial scale [1,2]. In dissolved form, lithium Reference to Lithium. Materials 2021, has only positive effects on supplementation of its deficiencies in humans [3,4]. Minerals 14, 4334. https://doi.org/10.3390/ containing lithium in nature are formed from the transformation of pegmatites. Although ma14154334 about 145 minerals contain this element in smaller or larger amounts, the basic industrial raw materials are spodumene, lepidolite, petalite and amblygonite [5,6]. In industry, mainly Academic Editor: Myroslav lithium-containing aluminium silicates and sometimes lithium carbonate, obtained from Sprynskyy lithium salines or from other lithium minerals mainly spodumene, are used. The use of lithium in the form of carbonate for high-temperature processes (firing) can cause a problem Received: 25 June 2021 with the outgassing of the resulting CO [7–9]. For this reason, this paper is limited to the Accepted: 29 July 2021 2 Published: 3 August 2021 characterization of natural varieties of lithium aluminum silicates—spodumene. Spodumene LiAlSi2O6 is one of three natural varieties of lithium aluminum silicate, the others being petalite and eucryptite. However, eucryptite occurs in nature only in one Publisher’s Note: MDPI stays neutral β with regard to jurisdictional claims in deposit Bikita, Southern Rhodesia. Eucryptite is usually synthesized and available in the published maps and institutional affil- variety [10,11]. Murthy and Hummel analyzed the phase system of lithium metasilicate and ◦ iations. β-eucryptite. They found that the eutectic is formed at about 1070 C[12,13]. Spodumene is a mineral of the pyroxene group. It crystallizes in a single-crystal system in the space group C2/c forming sometimes quite large crystals of whitish, gray, yellowish, lilac (kunzite) or emerald green (hiddenite) color. Spodumene melts at a temperature of about 1420 ◦C. However, in combination with quartz, feldspars, also mica, it forms low-temperature Copyright: © 2021 by the author. eutectics [14,15] At room temperature spodumene occurs in the α form. This form has a Licensee MDPI, Basel, Switzerland. 3 ◦ α This article is an open access article density of 3.2 g/cm . At a temperature of about 1080 C, an irreversible transition of the β distributed under the terms and form to occurs. This involves a rearrangement of the structure from a single chain to a 3 conditions of the Creative Commons less dense tetragonal structure. The density of β-spodumene is 2.4 g/cm . The change in Attribution (CC BY) license (https:// density represents a 30% increase in volume [6,16]. creativecommons.org/licenses/by/ 4.0/).

Materials 2021, 14, 4334. https://doi.org/10.3390/ma14154334 https://www.mdpi.com/journal/materials Materials 2021, 14, 4334 2 of 14

The formation of a vitreous phase in a mineral mass requires the addition of ﬂuxes. One of these is spodumene. With spodumene, it conducts the ﬁring so that the following reaction occurs [17]:

Li2O-Al2O3-4SiO2 + 4SiO2 → Li2O-Al2O3-8SiO2 (1)

spodumene silica β-spodumene (solid solution) The next minerals in the studied flux systems are alkali feldspars, these are raw materials rich in potassium (K2O), which are bound in them mainly in the form of aluminosilicates, i.e., potassium feldspars (orthoclase, microcline, sanidine, adular). For this ◦ reason, anorthite, Ca[Al2Si2O8], whose melting point is high (1550 C) should not be in- cluded in these raw materials although small amounts can produce eutectic. The melting point of alkali feldspars is significantly lower [18,19]. The other feldspar used as a flux is albite. Albite in nature never occurs in the pure form of sodium aluminosilicate and always contains some amount of calcium. An exception is a raw material under the name Albitte 5 (Turkey), which contains less than 1% calcium oxide and can be treated as a pure mineral [20,21]. Stechiometric albite contains 11.8 wt.% Na2O, 19.4 wt.% A12O3 and 68.8 wt.% SiO2. Its melting point is 1120–1200 ◦C and its density is 2.62 g/cm3. The characteristic of albite is lower melt viscosity compared to other feldspars. In general, it is a better flux than potassium feldspar because it melts at a lower temperature; however, it causes greater deformation in the material because of congruent melting [22]. Potassium feldspar (orthoclase) K[AlSi3O8] melts incongruently, while sodium feldspar (albite) Na[AlSi3O8] melts congruently. During the firing process, the silicate melt formed at the expense of the feldspar interacts with the solid phase and partially dissolves it. This phenomenon starts at a temperature of about 1150 ◦C. The silicate alloy leads to the expected thickening and lower porosity of the ceramic material. Feldspar raw materials characterized by the predominance of K2O to Na2O (K2O/Na2O ratio should be more than 2) are most often used in technologies. These criteria are based on theoretical premises, while there is no sufficient research data in this direction, although in the case of mineral materials it is justified by the incongruent melting of potassium feldspar [22,23]. Due to the economic and environmental aspects (fossil fuel consumption, greenhouse gas emissions), research work on lowering the temperature and reducing the firing and melting time of mineral products has been going on for many years [24,25].

1.2. Selected Thermal Parameters of Mineral Flux Systems The energy absorbed by a body during its heating or given off during its cooling is proportional to the product of the body mass m and the temperature difference of this body ∆T before and after the thermal transformation. The thermal transformation ability of a material ∆Q can be written in the form:

∆Q = cv·m·∆T (J) (2)

The heat capacity of a material is called the amount of heat required to raise the temperature of a substance by one degree. The heat capacity that accrues per unit mass of a substance is called the specific heat cv (expressed in J/kg·K). This quantity is not a constant value and depends primarily on temperature. The specific heat is an additive quantity, i.e., each degree of freedom present in a system contributes to the total heat of the system. In many amorphous, glassy and crystalline substances, the specific heat increases with increasing temperature and at high temperatures [26–28]. Another quantity that characterizes materials in terms of their thermal properties is the volumetric heat capacity. Its value b, is calculated as the product of the specific heat, cv and the material density, ρ, of which the material is made:

3 b = cv·ρ (J/(m K)) (3) Materials 2021, 14, 4334 3 of 14

Volumetric heat capacity is a measure of the amount of energy that 1 m3 of a given material will absorb while heating or give up while cooling, changing its temperature by one degree. In other words, it is the energy that raises (or lowers) the temperature of a material of unit volume by unit temperature. The materials with the highest speciﬁc density have the highest heat capacity. However, volumetric heat capacity is not sufﬁcient to describe the ability to accumulate heat. The energy that can be accumulated per unit volume of a material is an additional parameter characterizing the effectiveness of the material accumulation phenomenon. The maximum energy that can be accumulated in a unit volume of a given material bmaxv can be described by the formula [29]:

3 bmaxv = b·∆t (J/m ) (4)

The values of maximum volumetric energy for different rock types are in the range of 0.5–3.5 kJ/m3 [30,31]. The eutectic transformation during heating requires a certain heat power P, which is given by the relation [32]:

P = ∆Q/t (J/s) (5)

where: ∆Q—thermal transformation, t—heating time. The main objective of this study was to determine the image of phase transformations temperature-eutectics. The second objective was to calculate and visualize the thermal parameters of the thermal transformations of the three-component system.

2. Materials and Methods In contrast to the synthetic components of the flux system, a characterization is presented for the determination of fluxing eutectics based on naturally occurring alkalines (Table1)[ 33]. For naturally occurring lithium aluminosilicate minerals, temperatures can vary significantly. Therefore, it is advisable in each case to determine the eutectic points of mixtures of sodium potassium feldspar and naturally occurring lithium aluminosilicates for use as fluxes.

Table 1. Characteristic temperatures of the synthetic components of the system.

Spodumene Albite Microcline Melting Point (◦C) 1423 1118 1150

Naturally occurring and commercially available raw materials containing alkali minerals were used to achieve this goal. For lithium aluminium silicate, Gresflux and micronized Concentrate were used. For Na and K feldspar, Albitte 5 nd Norfloat Spar were used. All natural mineral raw materials were supplied by Otavi minerals (Neuss, Germany). The compositions of each raw material in terms of oxides are shown in Table2. Base sets differing by 20% of the individual components were prepared, then the composition of the sets was condensed into the eutectic region differing by 5 wt.% and 10 wt.%. as shown in Figure1, tests were performed for both Gresfux spodumene and Concentrate. To determine the effect of enrichment and grinding of the lithium component, enriched and micronized spodumene Concentrate with an average grain size of d50 = 3 µm and ground spodumene Gresflux with a starting grain size of d50 = 180 µm were introduced into the sets. Regardless of the degree of grinding, all sets were additionally homogenized for about 15 min in an alumina mill Fritsch pulverisette MV46 (Idar-Oberstein, Germany). The average grain size distribution of the sets did not exceed 50 µm. Measurements in high- temperature microscope Hesse-Instruments EM301 (Osterode am Harz, Germany) were performed on particular sets of specimens with the following assumptions; temperature increment from 80 ◦C/min to 650 ◦C and 10 ◦C/min in the temperature range from 650 ◦C to 1500 ◦C. On the basis of continuous observation of the specimen and recording changes Materials 2021, 14, 4334 4 of 14

in its dimensions as a function of temperature, it is possible to determine the so-called characteristic temperatures (Figure2)[14,15].

Materials 2021, 14, x FOR PEER REVIEW 4 of 15 Table 2. Chemical composition and melting point of raw materials used. Raw Material Norﬂoat Spar Albitte 5 Gresﬂux Concentrate Chemical Composition (wt.%) Fe2O3 0.23 0.7 0.4 0.07 SiO2 65.9 67.1 68.0 64.95 AlMnO2O3 18.5<0.05 18.8<0.05 22.0 0.2 26.80 - CaO 0.5 0.6 0.4 0.05 MgOP2O5 0.10.06 1.30.13 0.2 0.3 0.00 0.12 NaTiO22O <0.052.9 0.36 9.5 - 1.0 - 0.15 Fe2O3 0.23 0.7 0.4 0.07 K2O 12.0 0.2 1.0 0.08 MnO <0.05 <0.05 0.2 - PLi2O25O 0.06- 0.13 - 0.3 6.1 0.12 7.50 NaLOI2O 2.91.02 9.51.49 1.00.29 0.15 0.23 K2O 12.0 0.2 1.0 0.08 MeltingLi2O point - - 6.1 7.50 1434 1360 1414 1410 LOI(°C) 1.02 1.49 0.29 0.23 MeltingDominant point (◦C) 1434 1360 1414 1410 microcline albite spodumene spodumene Dominantmineral mineral microcline albite spodumene spodumene

G r e s ] ﬂ % u [ x /

r C a o p n c S e t n a t o r ﬂ a r t e o N [ % ]

FigureFigure 1. Distribution1. Distribution of samples of samples in the spodumene–albite–microcline in the spodumene–albite–microcline triangle. triangle.

To determine the effect of enrichment and grinding of the lithium component, enriched and micronized spodumene Concentrate with an average grain size of d50 = 3 µm and ground spodumene Gresflux with a starting grain size of d50 = 180 µm were introduced into the sets. Regardless of the degree of grinding, all sets were additionally homogenized for about 15 min in an alumina mill Fritsch pulverisette MV46 (Idar-Ober- stein,Germany). The average grain size distribution of the sets did not exceed 50 µm. Measurements in high-temperature microscope Hesse-Instruments EM301 (Osterode am Harz, Germany) were performed on particular sets of specimens with the following assumptions; temperature increment from 80 °C/min to 650 °C and 10 °C/min in the temperature range from 650 °C to 1500 °C. On the basis of continuous observation of the specimen and recording changes in its dimensions as a function of temperature, it is possible to determine the so-called characteristic temperatures (Figure 2) [14,15].

− shrinkage starting temperature Tg (sintering) − softening temperature Ta (corner rounding-end of sintering) − melting point Tb (hemispherical effect-formation of mineral eutectic) − spreading temperature Tc (sample base >200% or 1/3 height)

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By computer recording the change in the shape of the samples, it is also possible to determine:

− sintering interval (corner rounding temp. Ta-shrinkage temp. Tg) Materials 2021, 14, 4334 5 of 14 − melting interval (hemisphere temperature Tb-corner rounding temperature Ta) − flowing interval (spreading temp. Tc-hemisphere temp. Tb),

Figure 2. 2. MethodMethod of of determining determining characte characteristicristic temperatures temperatures and and interv intervalsals in high-temperature in high-temperature microscopy,microscopy, original original state state of ofsample sample 1 (spodumene 1 (spodumene Gresflux). Gresflux). − shrinkageInvestigations starting carried temperature out in the Thigh-tempeg (sintering)rature microscope belong to the standard − softening temperature T (corner rounding-end of sintering) investigations of thermal propertiesa of materials. Not only do they allow the determina- − melting point T (hemispherical effect-formation of mineral eutectic) tion of characteristicb temperatures, but also decomposition temperatures, sublimation − spreading temperature T (sample base >200% or 1/3 height) temperatures, phase transitionc temperatures, etc. On the basis of continuous observation of theBy sample computer and recording recording changes the change in its in dimensions the shape ofas thea function samples, of ittemperature, is also possible it is alsoto determine: possible to determine the viscosity of the alloy or wetting ability with respect to the substrate− sintering [34]. intervalData visualization (corner rounding was performed temp. Ta-shrinkage with the Surfer temp. 19 T gprogram) from Gold- enSoftware− melting delivered interval (hemisphere by Gambit (Kraków, temperature Pola Tnd).b-corner The presented rounding test temperature results are Ta the) average− flowing values intervalfrom three (spreading measurements. temp. T c-hemisphere temp. Tb), Investigations carried out in the high-temperature microscope belong to the standard 3.investigations Results and Discussion of thermal properties of materials. Not only do they allow the determi- 3.1.nation Characteristic of characteristic Temperatures temperatures, but also decomposition temperatures, sublimation temperatures,The onset phaseof shrinkage transition of individual temperatures, sets du etc.ring On heating the basis is ofthe continuous beginning observationof sintering andof the the sample extent andto which recording the corners changes round in its off dimensions defines the asextent a function of sintering. of temperature, The volume it changeis also possibleduring this to determine phenomenon the viscosityis probably of determined the alloy or by wetting three abilitymechanisms. with respect The first to isthe related substrate to the [34 thermal]. Data expansion visualization of the was grains, performed which withinduces the an Surfer approximate 19 program 1.5% from vol- umeGoldenSoftware swelling of deliveredthe samples by up Gambit to a temperat (Kraków,ure Poland). of about The 1000 presented °C. In the test case results of spodu- are the mene,average the values second from swelling three measurements. mechanism (about 30% vol.) in the temperature range 1050– 1100 °C is related to the polymorphic transformation of α to β spodumene [35,36]. In the 3. Results and Discussion next heating stage, shrinkage occurs due to sintering, melting and elimination of the gas 3.1. Characteristic Temperatures phase. The temperature of the beginning of softening, i.e., rounding of corners (end of sintering)The onset lies above of shrinkage the temp oferature individual of polymorphic sets during transformation heating is the beginning in which a of significant sintering volumeand the extentchange to occurs. which The the cornerstemperature round of off onset defines of shrinkage the extent and of sintering. rounding The of volumecorners arechange shown during in Figures this phenomenon 3 and 4. is probably determined by three mechanisms. The first is related to the thermal expansion of the grains, which induces an approximate 1.5% volume swelling of the samples up to a temperature of about 1000 ◦C. In the case of spodumene, the second swelling mechanism (about 30 vol.%) in the temperature range 1050–1100 ◦C is related to the polymorphic transformation of α to β spodumene [35,36]. In the next heating stage, shrinkage occurs due to sintering, melting and elimination of the gas phase. The temperature of the beginning of softening, i.e., rounding of corners (end of sintering) lies above the temperature of polymorphic transformation in which a significant volume change occurs. The temperature of onset of shrinkage and rounding of corners are shown in Figures3 and4.

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Figure 3. Shrinkage onset temperature Tg of the flux system: Greflux–Albitte 5–Norfloat Spar (a), Figure 3. Shrinkage onset temperature Tg of the flux system: Greflux–Albitte 5–Norfloat Spar (a), andFigure Concentrate–Albitte 3. 3. Shrinkage Shrinkage onset onset temperature 5–Norfloat temperature T(bg ).of T theg of flux the system:flux system: Greflu Greflux–Albittex–Albitte 5–Norfloat 5–Norfloat Spar (a), Spar (a), andand Concentrate–Albitte Concentrate–Albitte 5–Norfloat 5–Norfloat 5–Norfloat ( b).). (b).

Figure 4. Rounding temperature Ta of the corners of the flux system Greflux–Albitte 5–Norfloat Spar Figure 4. Rounding temperature T of the corners of the flux system Greflux–Albitte 5–Norfloat Spar Figure(a), and 4.Concentrate–Albitte Rounding temperature 5–Norfloata Ta of Spar the corners(b). of the flux system Greflux–Albitte 5–Norfloat Spar (Figure(aa),), and 4. Concentrate–AlbitteConcentrate–Albitte Rounding temperature 5–Norfloat 5–Norfloat Ta of Spar the Spar corners (b). (b). of the flux system Greflux–Albitte 5–Norfloat Spar (a), andThe Concentrate–Albitte stage of eutectic formation 5–Norfloat of the Spar stud (bied). system, in general, was defined as the The stage of eutectic formation of the studied system, in general, was defined as the change in the shape of the samples from the temperature of rounding of the corners, i.e., changeThe in stage the shape of eutectic of the samples formation from theof the temperature studied ofsystem, rounding in general, of the corners, was i.e.,defined the as the the beginningThe stage of oftheir eutectic softening, formation to the form of theation stud of ieda hemisphere system, in (Figure general, 2). Thewas visuali- defined as the changebeginning in ofthe their shape softening, of the tosamples the formation from ofthe a hemispheretemperature (Figure of rounding2). The visualization of the corners, i.e., zationchange of in these the measurementsshape of the samplesis shown fromin Figure the 5.temperature of rounding of the corners, i.e., theof these beginning measurements of their issoftening, shown in Figureto the5 form. ation of a hemisphere (Figure 2). The visuali- zationthe beginning of these ofmeasurements their softening, is shownto the formin Figureation 5.of a hemisphere (Figure 2). The visualization of these measurements is shown in Figure 5.

FigureFigure 5. 5. MeltingMelting temperature temperature (hemisphere) (hemisphere) T Tbbofof the the flux flux system system Greflux–Albitte Greflux–Albitte 5–Norfloat 5–Norfloat Spar ((aa),), and and Conce Concentrate–Albittentrate–Albitte 5–Norfloat 5–Norfloat Spar ( b).

Figure 5. Melting temperature (hemisphere) Tb of the flux system Greflux–Albitte 5–Norfloat Spar (Figurea), and 5. Conce Meltingntrate–Albitte temperature 5–Norfloat (hemisphere) Spar T(bb).of the flux system Greflux–Albitte 5–Norfloat Spar (a), and Concentrate–Albitte 5–Norfloat Spar (b). Materials 2021, 14, x FOR PEER REVIEW 7 of 15 Materials 2021, 14, 4334 7 of 14

The measurements showed the first eutectic point, i.e., 1263 °C (Concentrate) and ◦ 1273 °CThe (Gresflux) measurements at compositions showed theof 50 first wt.% eutectic potassium point, feldspar i.e., 1263 andC 45 (Concentrate) wt.% spodumene and ◦ and1273 30C wt.%(Gresflux) potassium at compositions feldspar, 30 of wt.% 50 wt.% sodium potassium feldspar feldspar and 40 and wt.% 45 wt.% spodumene, spodumene re- and 30 wt.% potassium feldspar, 30 wt.% sodium feldspar and 40 wt.% spodumene, respec- spectively. This applies to both micronized (d50 = 3 µm) and ground spodumene (d50-180 tively. This applies to both micronized (d50 = 3 µm) and ground spodumene (d50-180 µm). µm). A second eutectic with a higher melting point of ◦1382 °C (Gresflux) and ◦1376 °C (Concentrate)A second eutectic exists with at the a higherpoint with melting proportions point of of 1382 50/45/5C (Gresflux) wt.% (B-eutectic). and 1376 TheC study (Con- showedcentrate) that exists the atmelting the point point with of the proportions different sets, of 50/45/5 measured wt.% as the (B-eutectic). point of hemisphere The study showed that the melting point of the different sets, measured as the point of hemisphere formation, did not differ significantly depending on the fineness and enrichment of the formation, did not differ significantly depending on the fineness and enrichment of the spodumene. Using micronized spodumene (Concentrate), the melting point of the system spodumene. Using micronized spodumene (Concentrate), the melting point of the system can be lowered, at most, by about 10 °C relative to ground spodumene (Gresflux). This can be lowered, at most, by about 10 ◦C relative to ground spodumene (Gresflux). This fact can probably be explained by the influence of the spodumene surface development fact can probably be explained by the influence of the spodumene surface development and the increased amount of lithium by about 1.4 wt.% (Table 2) compared to the Gresflux and the increased amount of lithium by about 1.4 wt.% (Table2) compared to the Gresflux spodumene. An active effect of sodium feldspar on the melting point of the system can be spodumene. An active effect of sodium feldspar on the melting point of the system can be observed as well as a shift of the eutectic towards higher spodumene contents. The study observed as well as a shift of the eutectic towards higher spodumene contents. The study shows that spodumene decreases the melting temperature of the system more than other shows that spodumene decreases the melting temperature of the system more than other feldspars. The characteristic spreading temperatures are shown in Figure 6. feldspars. The characteristic spreading temperatures are shown in Figure6.

FigureFigure 6. 6. TheThe beginning beginning of of the the flux flux system spreading T c,, Greflux–Albitte Greflux–Albitte 5–Norfloat 5–Norfloat Spar Spar ( (aa),), and and Materials 2021, 14, x FOR PEER REVIEWConcentrate–AlbitteConcentrate–Albitte 5–Norfloat 5–Norfloat Spar Spar ( (bb).). 8 of 15

ItIt can can also also be be observed observed that that spodumene spodumene (b (bothoth Gresflux Gresﬂux and and Concentrate) Concentrate) in in amounts amounts aboveabove 60 60 wt.% wt.% causes causes significant signiﬁcant swelling swelling of of samples up to 35%35 vol.% vol. (Figure 77).). In the case of mineral materials, high swelling can lead to the deformation of the products. In the case of an alloy, there is no risk and the so-called softening stage is usually defined as the difference between the temperature of the onset of shrinkage and the temperature of the hemisphere. In the case of alloys, this is justified by the usually narrow transition interval between solid and liquid. From the functional point of view in terms of meltability, the reactivity of spodumene at small amounts in the set, up to 5 wt.%, is interesting. This fact is not known so far in the literature.

Figure 7. Swelling phenomenon of ﬂux systems containing increased amounts of spodumene, Figure(diameter 7. of Swelling the pastille phenomenon was 5 cm). of flux systems containing increased amounts of spodumene, (diameter of the pastille was 5 cm).

3.2. Sintering, Melting and Flowing Intervals The sintering interval of the sets, calculated from the temperature of the beginning of contraction to the temperature of the rounding of the corners, is of less importance with respect to flux systems. However, the smaller the interval is, the faster the system goes to the melting phase (faster time of eutectic transformation). Studies have shown that the sintering interval is shorter the more the composition of the sets approaches the eutectic point. One can also notice a greater discrepancy in the measurement results when Gresflux is used especially at a higher proportion. Probably the swelling effect caused by the polymorphic change of spodumene has a significant influence in this case. It is worth noting the two increased sintering intervals at 20 wt.% spodumene and 40 and 70 wt.% albite, respectively (Figure 8).

Figure 8. Sintering interval of the flux system Greflux–Albitte 5–Norfloat Spar (a), and Concentrate– Albitte 5–Norfloat Spar (b).

Melting of the system begins when the corners of the samples are rounded. The melting interval measured by the difference between the corner rounding temperature (end of specimen shrinkage and corner rounding) and the melting temperature (hemisphere) is shown in Figure 9. Materials 2021, 14, x FOR PEER REVIEW 8 of 15

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In the case of mineral materials, high swelling can lead to the deformation of the products. In the case of an alloy, there is no risk and the so-called softening stage is Figureusually 7. defined Swelling as thephenomenon difference betweenof flux systems the temperature containing of increased the onset ofamounts shrinkage of andspodumene, (di- ameterthe temperature of the pastille of the was hemisphere. 5 cm). In the case of alloys, this is justified by the usually narrow transition interval between solid and liquid. From the functional point of view in 3.2.terms Sintering, of meltability, Melting the reactivity and Flowing of spodumene Intervals at small amounts in the set, up to 5 wt.%, is interesting. This fact is not known so far in the literature. The sintering interval of the sets, calculated from the temperature of the beginning of3.2. contraction Sintering, Melting to the and temperature Flowing Intervals of the rounding of the corners, is of less importance with respectThe to sintering flux systems. interval However, of the sets, calculatedthe smaller from the the interval temperature is, the of faster the beginning the system goes to theof contraction melting phase to the temperature(faster time ofof the eutectic rounding transformation). of the corners,is Studies of less importancehave shown that the sinteringwith respect interval to flux systems.is shorter However, the more the the smaller composition the interval of is,the the sets faster approaches the system the eutectic goes to the melting phase (faster time of eutectic transformation). Studies have shown point.that the One sintering can intervalalso notice is shorter a greater the more disc the compositionrepancy in of the the setsmeasurement approaches theresults when Gresfluxeutectic point. is used One especially can also notice at a a higher greater discrepancyproportion. in Probably the measurement the swelling results wheneffect caused by theGresflux polymorphic is used especially change at of ahigher spodumene proportion. has a Probably significant the swelling influence effect in causedthis case. by It is worth notingthe polymorphic the two changeincreased of spodumene sintering hasintervals a significant at 20 influence wt.% spodumene in this case. Itand is worth 40 and 70 wt.% albite,noting therespectively two increased (Figure sintering 8). intervals at 20 wt.% spodumene and 40 and 70 wt.% albite, respectively (Figure8).

Figure 8.8.Sintering Sintering interval interval of the of fluxthe flux system syst Greflux–Albitteem Greflux–Albitte 5–Norfloat 5–Norfloat Spar (a), and Spar Concentrate– (a), and Concentrate– Albitte 5–Norfloat 5–Norfloat Spar Spar (b). (b). Melting of the system begins when the corners of the samples are rounded. The meltingMelting interval of measured the system by the begins difference when between the corn theers corner of the rounding samples temperature are rounded. (end The melt- ingof specimen interval shrinkage measured and by corner the difference rounding) andbetween the melting the corner temperature rounding (hemisphere) temperature is (end of specimenshown in Figure shrinkage9. and corner rounding) and the melting temperature (hemisphere) is shownThe in total Figure softening 9. interval of the system increases slightly in favor of additional milling and enrichment. This phenomenon is related to the surface development of the micronized lithium aluminum silicate and faster reaction in the melting range. The flowing interval determines the difference between the spreading temperature and the melting temperature (hemisphere temperature). The results are visualized in Figure 10. Materials 2021, 14, x FOR PEER REVIEW 9 of 15

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Figure 9. Melting interval of flux system Greflux–Albitte 5–Norfloat Spar (a), and Concentrate–Al- bitte 5–Norfloat Spar (b).

The total softening interval of the system increases slightly in favor of additional milling and enrichment. This phenomenon is related to the surface development of the micronized lithium aluminum silicate and faster reaction in the melting range. The flowing interval determines the difference between the spreading temperature and the melting FigureFigure 9. 9. MeltingMelting interval of fluxflux systemsystem Greflux–Albitte Greflux–Albitte 5–Norfloat 5–Norfloat Spar Spar (a ),(a and), and Concentrate–Albitte Concentrate–Al- temperature (hemisphere temperature). The results are visualized in Figure 10. bitte5–Norfloat 5–Norfloat Spar Spar (b). (b).

FigureFigure 10. 10.FlowingFlowing interval interval of of a a flux flux system system Greflux–Albitte Greflux–Albitte 5–Norfloat 5–Norfloat Spar Spar ( (aa),), and and Concentrate– Concentrate– AlbitteAlbitte 5–Norfloat 5–Norfloat Spar ( b).).

Again,Again, the the greater greater fineness fineness and and enrichment enrichment of ofspodumene spodumene reduces reduces the themelt melt flow flow in- ◦ tervalinterval of ofthe the system system by by about about 10 10 °C.C. The The effe effectct is is particularly particularly pronounced pronounced with with increased increased spodumenespodumene content. content. Technological Technological experience experience in indicatesdicates that that a a smaller smaller meltflow flow interval (lower(lower melt melt viscosity) viscosity) is is more more advantageous advantageous fo forr the the preparation preparation of of vitreous vitreous coatings as it Figure 10. Flowing interval of a flux system Greflux–Albitte 5–Norfloat Spar (a), and Concentrate– allowsallows better better gas-phase gas-phase evacuation evacuation and and thus thus a a reduction reduction in the number of defects. In this Albitte 5–Norfloat Spar (b). respect,respect, for for the the production production of of amorphous amorphous coat coatings,ings, a a larger larger spodumene grain grain size size is is not advantageous. advantageous.Again, the greater fineness and enrichment of spodumene reduces the melt flow in- terval3.3. Thermal of the system Parameters by about of a Three-Component 10 °C. The effe Fluxct is particularly System pronounced with increased 3.3. Thermal Parameters of a Three-Component Flux System spodumeneOn the content. basis of Technological the percentage experience of particular indicates oxides that contained a smaller in melt the raw flow materials interval (lowerOn melt the viscosity)basis of the is morepercentage advantageous of particul forar the oxides preparation contained of vitreous in the raw coatings materials as it (Table2) and their tabulated c v values (Table3), the resultant specific heat of the materials (Table 2) and their tabulated cv values (Table 3), the resultant specific heat of the materials allowsstudied better was gas-phase calculated. evacuation The highest and value thus ofa reduction resultant specificin the number heat is of characterized defects. In this by studied was calculated. The highest value of resultant specific heat is characterized by respect,spodumene for the micronized production Concentrate, of amorphous it is 828.95 coatings, J/kgK, a larger (Table spodumene4). Specificheat grain is size a quantity is not spodumene micronized Concentrate, it is 828.95 J/kgK, (Table 4). Specific heat is a quantity advantageous.that is particularly sensitive to phase transformations of all types, i.e., a quantity whose that is particularly sensitive to phase transformations of all types, i.e., a quantity whose value changes significantly near phase transformations. It carries valuable information 3.3.about Thermal the nature, Parameters latent of a heat Three-Component or critical exponent Flux System of the transformation. The incidental specificOn heatthe basis depends of the closely percentage on the of chemical particul compositionar oxides contained of the raw in the material, raw materials which is (Tablewhy quality 2) and controltheir tabulated is so extremely cv values important. (Table 3), Inthe the resultant calculations, specific reference heat of the was materials made to studiedthe oxides was of calculated. the individual The elements, highest value which of build resultant natural specific minerals, heat for is the characterized reason that theby spodumenespatial structure micronized of silicates Concentrate, consists of it istetrahedral 828.95 J/kgK, metal–oxygen (Table 4). Specific bonding heat [22 is]. a quantity that is particularly sensitive to phase transformations of all types, i.e., a quantity whose Materials 2021, 14, 4334 10 of 14

Table 3. Tabular-speciﬁc heat.

c Oxide v (J/kg·K)

SiO2 742 Al2O3 775 Fe2O3 655 MgO 924 CaO 750 Na2O 1115 K2O 764 Li2O 1811

Table 4. Calculated speciﬁc heat and density of raw materials.

Raw Material Norﬂoat Spar Albitte 5 Gresﬂux Concentrate c v 762.63 772.14 811.79 828.95 (J/kg K) density 2.58 2.61 3.1 3.1 (g/cm3)

The materials with the highest speciﬁc density are characterized by the highest thermal storage capacity b. Volumetric heat capacity of metals with density 7000–9000 kg/m3 is 1.5–3.5 MJ/(m3 K). Natural rock formations have lower volumetric heat capacity than metals, e.g., granite—about 1.8 MJ/(m3 K) gabbro about 2.2 MJ/(m3 K), granodiorite about 2.3 MJ/(m3 K). Even smaller volumetric heat capacity is characterized by brick and sand— about 1.2 MJ/(m3 K) [29,31,37]. Mineral eutectics are characterized by similar values of parameter b to natural rocks. Analyzing the thermal accumulation capacity b contained in Tables5 and6, it was found that the highest value is characterized by sample 18 with a composition of 20 wt.% albite and 80 wt.% spodumene, and this applies to both types of spodumene Gresﬂux and Concentrate. This seems to be due to the synergistic effect that sodium and lithium have on each other [14].

Table 5. Volumetric heat capacity b, and eutectic formation time, of three-component system: Gresﬂux spodumene, Albitte 5, Norﬂoat Spar, according to the compositions shown in Figure1.

b t b t Set Number Set Number (MJ/(m3K)) (s) (MJ/(m3K)) (s) 1 1.976 7794 A 2.268 7002 2 1.986 7656 B 2.213 7182 3 1.996 7620 C 2.159 6912 4 2.006 7542 D 2.196 6882 5 2.073 7098 E 2.111 6936 6 2.083 7224 F 2.092 6828 7 2.093 7074 G 2.141 6858 8 2.103 7056 H 2.169 6882 9 2.112 7044 I 2.143 6816 10 2.180 7146 J 2.147 6864 11 2.190 7044 K 2.042 7110 12 2.201 7050 L 2.067 6900 13 2.211 7062 M 2.093 6894 14 2.290 7560 N 2.049 7134 15 2.301 7410 O 2.027 7272 16 2.311 7428 P 2.054 7092 17 2.402 7650 R 2.223 7123 18 2.413 7998 Materials 2021, 14, x FOR PEER REVIEW 11 of 15

Materials 2021, 14, 4334 10 2.180 7146 J 2.147 686411 of 14 11 2.190 7044 K 2.042 7110 12 2.201 7050 L 2.067 6900 13 2.211 7062 M 2.093 6894 Table14 6. Volumetric heat2.290 capacity b,7560 and eutectic formation N time, of 2.049three-component 7134 system: Concentrate spodumene, Albitte 5, Norfloat Spar, according to the compositions shown in Figure1. 15 2.301 7410 O 2.027 7272 16 2.311b 7428t P 2.054b 7092t Set Number Set Number 17 (MJ/(m2.4023K)) 7650(s) R (MJ/(m 2.2233K)) (s)7123 181 2.413 1.976 75787998 A 2.295 7746 2 1.986 7566 B 2.235 7632 Table 6. 3Volumetric heat 1.995 capacity b, and 7602 eutectic formation C time, of three-component 2.175 system: 7722 Con- centrate 4spodumene, Albitte 2.005 5, Norfloat 7596 Spar, according to D the compositions 2.215 shown in Figure 7662 1. 5 2.082 7044 E 2.121 7938 6b 2.091 t 6858 Fb 2.102 7998t Set Number Set Number 7MJ/(m 2.1023K)] (sec) 6936 GM J/(m 2.1563K)) (sec) 7800 8 2.112 6864 H 2.185 7716 1 1.976 7578 A 2.295 7746 9 2.122 6942 I 2.158 7782 210 1.986 2.200 7566 6954 B J2.235 2.161 7632 7824 311 1.995 2.200 7602 6882 C K2.175 2.047 7722 8220 412 2.005 2.220 7596 6942 D L2.215 2.074 7662 8100 513 2.082 2.230 7044 7056 E M2.121 2.102 7938 7992 14 2.320 7410 N 2.053 8226 615 2.091 2.331 6858 7422 F O2.102 2.029 7998 8286 716 2.102 2.341 6936 7392 G P2.156 2.058 7800 8184 817 2.112 2.444 6864 7902 H R2.185 2.255 7716 8246 918 2.122 2.454 6942 7830 I 2.158 7782 10 2.200 6954 J 2.161 7824 11The next step2.200 was to visualize6882 selected thermalK parameters2.047 of the spodumene–albite– 8220 microcline12 (Li-Na-K)2.220 three-component 6942 system.L These data are2.074 presented in Figures 8100 11–13. 13The transformation2.230 energy7056∆Q of a three-componentM mixture2.102 depends on 7992 the mass m, the14 resultant specific2.320 heat cv,7410 and the temperatureN difference2.053 during thermal 8226 exposure. The temperature15 2.331 difference is the7422 beginning ofO heating of the2.029 system and reaching 8286 its melting temperature, that is phase transformation (hemisphere point of Figure2). From 16 2.341 7392 P 2.058 8184 a technological point of view, the lowest energy value is the most desirable—a mineral 17 2.444 7902 R 2.255 8246 melt is obtained with the minimum possible heat input. Figure 11 shows the thermal 18 2.454 7830 transformation ∆Q map of the system. It can be seen the fields with the lowest values at a spodumene content of about 30 wt.%. When using micronized spodumene, the minimum energyThe value next isstep about was 20 to kJvisualize lower (Gresflux, selected th lowestermal energyparameters fields of 890 the kJ, spodumene–albite– while Concentrate microcline870 kJ). (Li-Na-K) three-component system. These data are presented in Figures 11–13.

FigureFigure 11. 11. TransformationTransformation energy energy ∆∆QQ ofof the the three-component three-component system, system, Greslux Greslux (a ()/Concentratea)/Concentrate (b) ( b–) Albitte–Albitte 5–Norfloat 5–Norﬂoat Spar Spar.. Materials 2021, 14, x FOR PEER REVIEW 12 of 15

The transformation energy ∆Q of a three-component mixture depends on the mass m, the resultant specific heat cv, and the temperature difference during thermal exposure. The temperature difference is the beginning of heating of the system and reaching its melting temperature, that is phase transformation (hemisphere point of Figure 2). From a tech- Materials 2021, 14, x FOR PEER REVIEWnological point of view, the lowest energy value is the most desirable—a mineral 12melt of 15 is obtained with the minimum possible heat input. Figure 11 shows the thermal transformation ∆Q map of the system. It can be seen the fields with the lowest values at a spodu- meneThe content transfor ofmation about 30energy wt.%. ∆ QWhen of a usinthree-componentg micronized mixturespodumene, depends the minimumon the mass en- mergy, the valueresultant is about specific 20 kJheat lower cv, and (Gresflux, the temperature lowest energy difference fields during 890 kJ, thermal while Concentrate exposure. The870 temperature kJ). difference is the beginning of heating of the system and reaching its melting temperature, that is phase transformation (hemisphere point of Figure 2). From a technological point of view, the lowest energy value is the most desirable—a mineral melt is obtained with the minimum possible heat input. Figure 11 shows the thermal transformation ∆Q map of the system. It can be seen the fields with the lowest values at a spodu- Materials 2021, 14, 4334 mene content of about 30 wt.%. When using micronized spodumene, the minimum12 ofen- 14 ergy value is about 20 kJ lower (Gresflux, lowest energy fields 890 kJ, while Concentrate 870 kJ).

Figure 12. Maximum volumetric energy bmax of the three-component system Greslux (a) /Concen- trate (b) –Albitte 5–Norfloat Spar.

The maximum thermal energy that the system is able to accumulate depends directly proportional to the density of the components, and to the calculated specific heat (Table 4). The highest value of the maximum volumetric energy amounting to over 440 MJ/m3 exactly (447 MJ/m3) was obtained in the system with a component proportion of 20 wt.% FigureFiguremicronized 12. 12. MaximumMaximum spodumene, volumetric volumetric 20 wt.% energy energy microcline bmax bmax of of th theande three-component three-component 60 wt.% albite (pointsystem 6Greslux in Figure ( a) /Concen-1), where tratetratethe (effect (bb))–Albitte –Albitte of grain 5–Norﬂoat5–Norfloat size is Spar.Spar.clearly visible (Figure 12).

FigureFigure 13.13. ThermalThermal power power P P of of the the Greslux Greslux (a)/Concentrate (a)/Concentrate (b) (–Albitteb) –Albitte 5–Norfloat 5–Norﬂoat Spar Spar three-com- three- componentponent mineral mineral system. system.

TheThe maximumheat power thermal of the energymineral that set theis linea systemr and is increasing able to accumulate with the dependsspodumene directly con- proportionaltent. The thermal to the power density strictly of the depends components, on the and amount to thecalculated of energy speciﬁcconsumed heat by (Table the sys-4 ). 3 Thetem highest∆Q in a valuegiven time of the unit. maximum The time volumetric of energy accumulation energy amounting t was taken to over as 440the moment MJ/m exactly (447 MJ/m3) was obtained in the system with a component proportion of 20 wt.% micronized spodumene, 20 wt.% microcline and 60 wt.% albite (point 6 in Figure1), where Figurethe effect 13. Thermal of grain power size is P clearly of the Greslux visible ( (Figurea)/Concentrate 12). (b) –Albitte 5–Norfloat Spar three-com- ponentThe mineral heat powersystem.of the mineral set is linear and increasing with the spodumene content. The thermal power strictly depends on the amount of energy consumed by the system The heat power of the mineral set is linear and increasing with the spodumene con- ∆Q in a given time unit. The time of energy accumulation t was taken as the moment tent. The thermal power strictly depends on the amount of energy consumed by the sys- of eutectic formation (Table5), and the whole system was heated at 10 ◦C/min in the temtemperature ∆Q in a given range time 650–1500 unit. The◦C. time For of the energy micronized accumulation Concentrate t was taken spodumene, as the moment the ∆Q values were several watts higher. This is due to the higher calculated speciﬁc heat (Table4).

4. Conclusions The degree of milling of spodumene concentrate lowers the eutectic temperatures of the system of spodumene–albite–microcline, but this change is marginal. In the case of the characteristic hemisphere temperature, the change is about 12 ◦C and the softening temperature (rounding of corners) is about 10 ◦C. It was observed that micronized spodumene concrete undergoes a faster polymorphic transformation. This phenomenon causes less swelling and thus prevents the cracking of Materials 2021, 14, 4334 13 of 14

the samples at spodumene contents >60%. Furthermore, a significant effect of spodumene grain size on all thermal parameters of the three-component system was observed. The characteristic temperatures of the applied natural flux raw materials are different from those of pure synthetically derived minerals (literature data). In particular, the characteristic melting temperature (hemisphere) of spodumene as a synthetic component is lower by about 7–15 ◦C, while albite and microcline are lower by about 242 ◦C and 184 ◦C, respectively. The first eutectic melting points of the mineral system spodumene (Gresflux/Concentrate)– potassium feldspar (Norfloat Spar)–sodium feldspar (Albitte 5) occurred at 1275 ◦C (Gresflux) and 1263 ◦C (Concentrate) with proportions of 40/30/30 wt.% (A-eutectic). A second eutectic with a higher melting point of 1382 ◦C (Gresflux) and 1376 ◦C (Concentrate) exists at the point with proportions of 50/45/5 wt.% (B-eutectic). The fields of the lowest values of the energy of eutectic transformation are observed when the spodumene content (of both types) is about 30 wt.%. However, the maximum thermal energy accumulated by the system is 447 MJ/m3, which refers to the system with micronized spodumene with the composition of 20 wt.% Concentrate, 20 wt.% Norfloat and 60 wt.% Albitte 5. Visualizations of selected thermal parameters of three-component mixtures clearly show that the formation of eutectic is the result of many parameters, however, strictly depends on the components’ proportions and their chemical composition. The next stage of work will be the study of high-temperature viscosity of mineral eutectics, and the behavior of Li-Na-K system in the presence of raw materials containing Ca and Mg.

Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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