DEVELOPMENT OF LEAD CRYSTAL GLASSES USING ORNAMENTAL STONE WASTE

Purpose: The objective of this work was to develop and characterize lead crystal glasses from the use of silicate wastes from ornamental stones. Theoretical framework: Glass consumption has shown considerable growth for the economy. It is predominantly used for windows and facades in modern buildings, for many practical reasons, including thermal, energy, lighting and aesthetics. The state of Espírito Santo is one of the largest producers and exporters of ornamental stones in the country. This sector is responsible for generating a large amount of waste around the world. This waste is deposited daily in ornamental stone landfills and is rich in oxides that can be used as raw materials in the manufacture of glass. Brazil generates around 2.5 million tons of fine waste per year, with the state of Espírito Santo responsible for 2 million tons. Method/design/approach: The methodology used was melting/cooling using alumina crucibles. The maximum process temperature was 1500°C with a time of 3 hours. Glass was developed using granite and quartzite residue. After melting/cooling, the glasses were cooled by the inertia of the oven and unmolded for characterization. Results and conclusion: The results showed that the glasses developed with waste showed characteristics similar to commercial glasses of the same type. X-ray diffractions (XRD) showed complete vitrification of the glasses produced. There is technical feasibility for developing glass using ornamental stone waste. Research implications: Enable initiatives that aim to develop glass using ornamental stone waste, with the aim of promoting a significant reduction in effects on the environment without compromising the sector's production pace. Originality/value: Develop lead glass, of the crystal type, and certify its technological feasibility by comparing the results obtained with commercial lead crystal glass.


INTRODUCTION
Glasses can be defined as an amorphous solid with complete absence of long-range order and periodicity, exhibiting a glass transition region.Any inorganic, organic or metal material, formed by any technique, that exhibits a glass transition phenomenon is a glass (Alves et al. 2001).
Since its discovery until the present day, there have been many advances in the glass production process, seeking quality improvements depending on the function for which it should be used.As a result, there are infinite glass formulations depending on the application, production process and availability of raw materials.However, we can divide glasses basically into fused silica or quartz, soda-lime, borosilicate, lead and alumino-borosilicate glasses (Akerman, 2000).
Lead glasses, which were developed in this work, have been known since the 17th century, when lead oxide was added to alkaline silicate to form a material called lead crystal, giving high clarity and a characteristic sound.This term is ambiguous, as it is known that glass is not a crystalline material.However, lead-enriched glass is an important material used in the luxury parts industry, which is why it is defined as crystal glass (Angeli et al., 2016;Lecanuet et al., 2022).
To attend the required weight, appearance and sound specifications, lead glass must have a lead oxide (PbO) content of 24 to 40% by weight of lead oxide (PbO).Some lead-free crystal compositions have been developed for tableware applications, but their physical and thermal characteristics cause significant changes in glass working conditions and complicate manufacturing processes (Lecanuet et al., 2022).
Regardless of the composition or technique used in the development of glass, the main raw material used in the industry is sand and, as a result, the increase in glass consumption, consequently, creates a scarcity of these natural deposits.This consumption is increasing around the world, leading to a growth in sand trade of around US$2 billion in 2019.It is further estimated that 42 to 55 billion tons of sand are extracted every year across the world.world, making sand the second most extracted resource in the world (Da and Billon, 2022).
Given these facts, attention turns to the ornamental stone industry, which is responsible for generating a large amount of waste rich in silica.This stone waste, in the form of mud, is mostly discarded in landfills and settling ponds, without any treatment process.This waste management model leads to great environmental damage due to soil and subsoil contamination, making it necessary to develop other routes for using this material, in another economic sector in which it can be used as an input for new products (Xavier et al., 2019;Almada et al., 2020).
The ornamental stone sector has been one of the fastest growing industrial activities in Brazil, placing the country among the main producers, along with China, India, Italy, Spain, Turkey, Egypt, France, Iran, Algeria and Sweden (França et al., 2018;Ashish, 2019).According to a report from the Centro Brasileiro dos Exportadores de Rochas Ornamentais, Brazilian exporting companies achieved revenues of approximately US$800 million with the export of 1.4 million tons of stones in 2023, until the month of September, showing an activity economic importance for the Brazilian gross domestic product (CENTROROCHAS, 2023).
In 2023, until September, the state of Espírito Santo alone reached a turnover of close to US$ 700 million with the export of 1,056,360 tons of stones, both in the form of blocks and slabs, ranking first among the Brazilian states with around 80% of national revenue, according to the interpretation of data provided by CENTROROCHAS until September 2023.
The high production of ornamental stones results in the large generation of waste that is divided into two main categories: coarse waste from the extraction/mining phase in quarries and fine waste from processing and transformation units.Processing waste can be divided into sawmill and polishing waste.Throughout the production chain, raw material losses of around 83% occur (Campos et al., 2014).Only 74% of a block turns into sheet metal when processed, the remainder is transformed into fine waste (Silveira, Vidal, Souza, 2014).
Adopting preventive environmental public policies means minimizing behaviors that negatively impact the environment and public health.With this in mind, on August 2, 2010, law No. 12,305 of the National Solid Waste Policy (BRASIL, 2010) was implemented, which establishes an order of priority in relation to waste generation.They are: non-generation, reduction, reuse, recycling, treatment and final disposal.Following this approach, the use of waste from the processing of ornamental stones in other industrial sectors would be included in the reuse segment, as a byproduct.Thus, it helps to reduce its final destination in landfills.According to Martínez (2022), investing in sustainability contributes to the company's growth and helps its competitiveness conditions in the long term.Socio-environmental practices can generate benefits, as they improve employee goodwill, morale and productivity (Mota and Pimentel, 2021).The scientific community began to research and analyze the possible applications of ornamental stone wastes in ceramic artifacts and other materials.Babisk et al. (2009), Babisk et al. (2010a), Babisk et al. (2010b), Bastos (2018), Babisk et al. (2019), Gomes et al., (2020) and Santos (2021) developed silicate glasses using waste from different types of stones and obtained satisfactory results regarding the physical-chemical characteristics and properties necessary for soda-lime and borosilicate glasses, comparing them commercial glass.Some of the properties verified were density, hardness, hydrolytic resistance, optical transmittance in UV-VIS-NIR and coefficient of thermal expansion.Therefore, these studies prove that there is the possibility of using ornamental stone wastes in the development of glass.

METHODOLOGY
Two glassy mixtures were prepared, one with white Fortaleza granite and the other with Naica quartzite.P.A. reagents were used to adjust the typical compositions of the oxides necessary for the manufacture of lead crystal glasses.The source of silica was waste collected from an ornamental stone processing industry in the municipality of Cachoeiro de Itapemirim-ES.

Characterization of Raw Materials
Chemical composition analyzes were performed by X-ray fluorescence (XRF) and were expressed in % weight.They are averages of 3 readings and were determined by semiquantitative analysis (standardless) on an x-ray fluorescence spectrometer -(WDS-1), model AxiosMax (Panalytical).Loss on ignition (LOI) of the samples was carried out in a Muffle furnace.Aliquots of each sample were separated, placed in a muffle furnace at 1000°C for 16 hours and after cooling, they were weighed to check loss on ignition.
For mineralogical analysis, the prepared wastes were subjected to crystalline phase analysis using the X-ray diffraction (XRD) method and determined by the powder method, using Bruker-D4 Endeavor equipment, under the following operating conditions: Co Kα radiation (35 kV/40 mA); goniometer speed of 0.02º 2θ per step with a counting time of 1 second per step and collected from 5 to 80º 2θ.Qualitative spectrum interpretations were carried out by comparison with standards contained in the ICDD-PDF02 database (ICDD, 2006) in Bruker AXS Diffrac.Plus software.
For physical analysis, the size and particle size distribution of the stone wastes used in the Malvern Mastersizer equipment (model 2000) were verified, using the low-angle laser light scattering technique, known generically as light scattering.
To evaluate the morphology of the ornamental stone waste, optical microscopy (OM) and scanning electron microscopy (SEM) were used.Optical microscopy was performed using a 1600x Zoom HD USB Digital Microscope.Scanning electron microscopy used the FEI Quanta 400 equipment, in high vacuum, electron acceleration voltage of 20 kV.The samples were adhered in natura to double-sided adhesive tape, and covered with silver on a BalTech SCD 050 sputter (using a vacuum with argon and applying a current of 30 mA for 300 seconds).

Glass Manufacturing
The proportions of P.A. reagents used to adjust the typical compositions of the oxides necessary for the manufacture of lead crystal glasses were calculated based on Table 1, a reference for this type of glass.The mixtures were homogenized, placed in alumina crucibles and placed in a DuraCEr oven, model 1700/12L BM for melting.The melting process took place in 16 hours, with different heating rates up to a temperature of 1500 °C, for 3 hours.After this period, the oven cooling process began, reducing the temperature every 10 °C, until reaching 30 °C.

Testes comparativos
Comparative tests were carried out between the developed glasses and a commercial lead crystal glass (Figure 1).XRD confirmation of the vitrification of the mixtures was carried out, if the glasses were completely amorphous, and, therefore, without the presence of crystalline phases.The chemical compositions of manufactured glass and commercial glass were also analyzed using the XRF method.Both tests used the same equipment described in the waste characterization.Density measurements were carried out using the pycnometry method.
The glass hydrolytic resistance tests were carried out in accordance with ISO 719 (1985) standard Glass -Hydrolytic resistance of glass at 98°C -Method of test and classification and classification).The results were analyzed in accordance with what was established by the standard.

RESULTS AND DISCUSSION
The following topics shows the results of waste characterization and comparative tests of glasses made from ornamental stone waste and commercial glass.

Mineralogical Characterization of Waste
The mineralogical compositions of the quartzite waste can be seen in Figure 2.This waste is composed entirely of the mineral quartz (ICDD-PDF2: 00-046-1045), which is the   7 results corroborate those showed in the literature, as the minerals identified are similar to those of research that used these types of waste to produce glass, some of which have already been mentioned previously.

Chemical characterization of waste
Table 2 shows the chemical composition of the waste.The majority presence of silica (SiO2) can be observed in quartzite (95.2%) and granite (76.6%), which proves the efficiency of these wastes as raw material for the production of silicate glasses.Silica is the main oxide that forms the glass network also in lead crystal glasses, elaborated in this dissertation.The concentration of alumina (Al2O3), 3.02 and 11.5% in quartzite and granite, respectively, also favored the formation of the glass network, which despite being an intermediate oxide, behaves in the silica network as an oxide forming network.
Other alkaline and alkaline earth oxides, such as potassium (K2O), sodium (Na2O), calcium (CaO) and magnesium (MgO), which act as network modifiers in the structure of glasses, are found in higher percentages in granite waste, totaling 8.72%.Titanium (TiO2) and iron (Fe2O3) oxides are show in the waste, in smaller proportions.These are compounds of 3d transition metals, used to impart color to glasses (Alves et al. 2001).

Morphological analysis
In general, stone waste has irregular and angular shaped particles.This particle morphology comes from the processing process in which the stones are ground, promoting the cutting or polishing of the pieces (Almada et al., 2020).
Figure 4 illustrates the optical microscopy analysis of the wastes.It can be seen that the quartzite waste is much more homogeneous than the granite waste.Furthermore, the other tests carried out in this work showed that the granite waste is composed of a mixture of oxides, which corroborates the images presented by optical microscopy.8 Scanning electron microscopy of the granite waste can be seen in Figure 5, in which the micrographs at magnifications of 300x and 1200x allow us to verify that the waste particles have angular grains, which favors the fusion that starts at the edges and, consequently, energy savings in the glass manufacturing process.In Figure 6 we can see the micrograph of quartzite, in which, like granite, the particles also have angular grains, which favor fusion.Furthermore, the comparison between Figures 5  and 6 allows us to observe that, despite the heterogeneity of the granite waste, it has smaller particles.While quartzite waste, as it contains more silica, has larger particles, this is because silica particles are more resistant and, consequently, more difficult to crush, in addition to being larger than other minerals.

Particle size and distribution analysis
The results obtained for the size and distribution of the particles corroborate the results obtained in the morphological analyses, since it is possible to confirm that the quartzite grains are larger than the granite grains.This occurs because the quartzite particles contain more quartz, which is a more resistant mineral and consequently more difficult to saw, making the particles less fine, as can be seen in Figure 7. Granite, on the other hand, as it has a mixture of various feldspars, mica, muscovite, among other minerals, makes it more heterogeneous and, consequently, is an easier stone to sawdust, resulting in finer particles, as illustrated in Figure 8.In Table 3 we can more easily visualize the numerical values of particle sizes for 10, 50 and 90% of each waste.

Developed glasses
The glass produced with quartzite waste showed a bluish color (Figure 9a), possibly due to the presence of titanium in the composition of the waste used, which, even at a low concentration of 0.11%, is the only coloring metallic oxide present in the matrix of the glass produced.According to Alves et al. (2001), colorants are composed of 3d transition metals or the 4f lanthanide series, in which the final color obtained will depend on the oxidation state of the metal.
As for the glass produced with granite waste, a greenish coloration was observed, very characteristic of this type of glass due to the presence of iron oxide in the composition of the waste (Figure 9b).

X-ray Diffraction of Glasses
The X-ray diffraction analysis was carried out both on glasses developed with granite and quartzite wastes, and on commercial crystal glass, in order to compare the results obtained.The diffractograms of the glasses can be seen in Figure 10.The absence of crystallinity peaks in the diffractograms is observed, as no crystalline phase was identified.The amorphous band around 27°, the peak of greatest intensity of silica (SiO2), is typical in silicate glasses due to the majority presence of this mineral in the glass matrix.With this we can consider that all glasses developed, as well as commercial glass, are amorphous, that all prepared mixtures vitrified completely, and that the cooling rate was sufficient, not providing any crystallization.
These results are favorable to the colors showed by the developed glasses.The granite glass, which presented a greenish color, is due to the presence of iron oxide (Fe2O3).These results were also obtained in other studies that used granite waste to produce glass.
As for the quartzite glass, a blue-colored glass was obtained.This possibly occurred due to the presence of titanium ion, the only metallic oxide from the 3d transition metal family present in the composition of the glass matrix.Previous research that produced other types of silicate glasses with quartzite wastes obtained different shades, such as colorless glasses by Babisk et al. (2018) and green by Babisk & Vidal, (2010).
Commercial glass had a chemical composition similar to that of produced glasses, in terms of the amounts of silica (SiO2) and lead oxide (PbO).As expected, because it is colorless, commercial glass did not present any coloring oxides in its chemical composition.

Glass Density
The densities were determined by the pycnometry method for both the developed glasses and the commercial glass, allowing a comparative analysis.The values found are described in Table 5.
The 3 glasses presented very close density values, which confirms that the glasses developed with waste are similar to commercial lead crystal glass.

Hydrolytic Resistance of Glasses
The hydrolytic resistance will determine how likely the glass is to resist water attack and, consequently, how much it will be able to react with the product that you want to store in the glass container.Therefore, resistance to hydrolytic attack will depend on the composition of the glass, as the more alkaline it contains, the more soluble it will be and the less resistant to chemical attack (AKERMAN, 2013).
The hydrolytic resistance test, showed in Table 6, shows that the glasses developed with ornamental stone waste were more resistant to hydrolytic attack than commercial glass, presenting HGB3 for both granite and quartzite glasses, while lead crystal glass commercial showed a hydrolytic resistance classified as HGB4, which is a low resistance, in accordance with the ISO 719 standard.

CONCLUSION
The characterization of the ornamental stone wastes used proved their potential for use as a source of silica (SiO2), the main raw material in the development of silicate glasses, therefore, in lead crystal glass.Quartzite waste is composed entirely of the mineral quartz, which is the crystalline form of silica.The granite waste is mainly composed of quartz and silicate minerals.
Morphological and granulometric analyzes of the wastes indicated appropriate size and morphology for the development of silicate glasses.Both wastes have angular grains, which favors the fusion of the raw material.
The chemical compositions of glasses developed with waste were close to the chemical composition of commercial glass.Oxides such as titanium oxide, in quartzite, and iron, in granite, which contributed to the coloring of the glasses.The diffractograms proved that the glasses developed with the waste are completely amorphous, just like commercial glass.Glass with waste was more resistant than commercial glass and had similar densities.
Wastes of silicate ornamental stones are technologically viable for use in the development of glasses.The use of waste to manufacture new products contributes to reducing waste disposal, which amounts to around millions of tons, and becomes a possibility to collaborate with the circular economy, since this waste returns to the production cycle.The new product created with waste becomes a sustainable and environmentally friendly alternative.

Figure 1 .
Figure 1.Commercial lead crystal bowl Source: Prepared by the authors.

Figure 2 .
Figure 2. X-ray diffraction diagram of the quartzite waste.Source: Prepared by the authors.

Figure 3 .
Figure 3. X-ray diffraction diagram of the granite waste.Source: Prepared by the authors.

Figure 4 .
Figure 4. Optical microscopy of the wastes.(a) Quartzite (b) Granite.Source: Prepared by the authors.

Figure 7 .
Figure 7. Particle size distribution of Quartzite waste Source: Prepared by the authors.

Figure 8 .
Figure 8. Granite waste particle size distribution Source: Prepared by the authors.

Figure 9 .
Figure 9. Developed glasses: a) quartzite waste and b) granite waste Source: Prepared by the authors.

Figure 10 .
Figure 10.X-ray diffraction diagram of the produced glasses and commercial crystal.Source: Prepared by the authors.

Table 1 .
Typical commercial composition for lead glass (% by weight)

Table 2 .
Chemical composition of the waste used (% weight).

Table 3 .
Average particle size

:
Prepared by the authors.

Table 4 .
Chemical composition of produced and commercial glasses (%).

Table 5 .
Densities of developed and commercial glasses.

Table 6 .
Hydrolytic resistance of developed and commercial glasses.