Using Brazilian Nepheline Syenite Waste as an Alternative Mineral Resource for Various Applications

Abstract

The high extraction of natural resources and the limited use of mining waste as alternative mineral resources are intensifying the depletion of natural reserves. The linear economic structure used by industrial sectors needs to be replaced with more sustainable models, such as the one proposed in the circular economy. This study aimed to evaluate strategies for the valorization of nepheline syenite waste (NSW) as an alternative mineral resource to natural and conventional ones. To this end, a set of criteria was adopted, consisting of a systematic approach for waste valorization, namely classification, potentiality, quantity/viability, and applicability (CPQvA). This involved investigating the properties of NSW, including its environmental, physical, chemical, morphological, and durability characteristics. The findings provide evidence of several potential applications for NSW, including the civil construction (fine aggregate and supplementary cementitious material), metallurgical (segregation of the iron fraction), and agricultural (segregation of the alkaline fraction) sectors. Methodologies for the beneficiation of NSW are suggested for each of the investigated applications. The valorization of NSW not only reduces the environmental impact of mining but also contributes to sustainable development by creating new products and economic opportunities, thereby promoting industrial symbiosis and advancement in the circular economy.

1. Introduction

The circular economy is a strategy that aims to address the issues of sustainable resource use and waste disposal associated with production systems [1,2,3,4]. It is based on a continuous loop that focuses on restoring the material cycle, maintaining functionality at various points during the useful life of materials, and transitioning to alternative sources of primary resources (raw materials). However, implementing strategies that promote the circular economy may require large investments. Less expensive actions are usually implemented first, such as using waste materials for other industries or reducing waste generation, consequently avoiding investments in landfill disposal [5].

The industrial waste valorization process provides several environmental benefits, such as making the production process cleaner and building toward a sustainable economy; technical benefits, such as maintaining or improving properties of interest for products; and financial benefits, such as the expectation of adding value and commercializing these materials [6,7,8].

Mining companies are responsible for supplying minerals for various applications, significantly contributing to national development and wealth generation [9]. However, these companies are also responsible for various environmental impacts, such as the excessive consumption of water and energy, the alteration and destruction of fauna and flora, pollution, the generation and disposal of waste, and the occurrence of large-scale disasters, such as the rupturing of dams [10,11].

The amount of waste generated during the extractive mining process is determined by the degree of weathering of the stone, the type of mineral, and its concentration. Waste generation typically ranges from 20% to 30% of the material exploited, but it can exceed 90% [12,13]. It is estimated that 100 billion tons of mining waste are generated annually worldwide [14].

In parallel, the overexploitation of natural resources is responsible for 90% of global biodiversity loss [15], and solid wastes can be converted via an appropriate flow, becoming a secondary source of valuable minerals and metals [16]. The most appropriate solution for the recycling process has been to search for new markets and applications for waste in other sectors [17,18]. Therefore, the mining sector should be integrated with other industries, promoting industrial symbiosis and contributing to the advancement of the circular economy model.

Nepheline syenite is one of the most common feldspars [19]. It is almost free of quartz and contains ferromagnesian minerals with granitic or gneissic textures. It is formed when the magma temperature falls, and there is insufficient silica to form feldspar, resulting in solutions rich in alkalis, mainly sodium and potassium [20,21]. Minerals such as nepheline, sodium and alkali feldspars, and pyroxenes represent approximately 90% of the composition of nepheline syenite [22,23,24].

Owing to its high alkali and alumina contents, nepheline syenite stone is widely used in the production of glass and ceramics [25]. However, it also has other uses, such as in fillers for paints (owing to its high whiteness, lack of reactivity, and ease of formulation); polymer manufacturing (as a filler for foams, reducing the density of the compounds compared to the use of talc) [26]; as an alternative source for the production of alumina [22], potassium [23], niobium, or rare earth elements [27], and in agriculture as a source of potassium-based fertilizers [28]. Despite the interesting mineralogical characteristics of nepheline syenite stones, few studies have reported strategies for the recovery of waste from its beneficiation process, where residual fractions are still underexplored and undervalued, leaving a scientific gap to be investigated.

Waste valorization is a strategic process aimed at identifying alternative sources of materials, thereby extending their life cycle, minimizing the volume of waste destined for landfills, and consequently reducing the extraction of virgin raw materials from the environment [29,30].

The use of mining waste in civil construction is a promising strategy for reducing the sector’s carbon footprint. When applied as aggregates or supplementary cementitious materials (SCMs), these residues help conserve natural resources and reduce CO₂ emissions, particularly through the partial replacement of sand or Portland cement. Additionally, their use as a source of industrial minerals broadens reuse opportunities in other production chains, reinforcing process circularity and sustainability [31,32,33].

Thus, this study aims to investigate the waste from nepheline syenite stone processing to promote its valorization as a mineral alternative, thereby contributing to the formation of industrial symbiosis and a strategy for progress toward the circular economy. The classification, potentiality, quantity/viability, and applicability (CPQvA) criteria proposed by Raupp-Pereira [34] are being applied to this type of waste for the first time. The criteria adopted by this system can be considered for assessing the potential for reuse of waste or by-products.

2. Materials and Methods

The nepheline syenite waste (NSW) investigated in this study was obtained from a mining waste pile of a company located in Santa Catarina, Brazil. This study used the CPQvA criteria for NSW valorization, as shown in Figure 1. This systematic method is guided by five decision-making criteria, as follows: classification (C), potentiality (P), quantity (Q)/viability (v), and applicability (A), which are related to environmental regulations and standards, physico-chemical characteristics, production data, recycling challenges, and possible applications, respectively. In previous studies, the authors have applied this systematic guideline to other wastes [10,35,36,37].

These criteria help determine whether waste material can be transformed into a useful raw material, thereby promoting sustainability and resource-use efficiency. The application of the CPQvA framework is essential for strategic decision-making regarding the valorization of discarded materials.

Using these criteria, a qualitative evaluation of NSW as an alternative mineral resource was conducted. The material was properly prepared for analysis by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Tests were also carried out in accordance with the Brazilian Standards (NBR) established by the Brazilian Association of Technical Standards (ABNT). The criteria, standards, and analysis adopted are presented in Table 1.

Microstructural analysis was performed using a scanning electron microscope (SEM, EVO 10, Zeiss, Hong Kong, China). Gold coating was performed using the Q156R-ES equipment (Quorum, Singapore). Chemical analysis was carried out using an X-ray fluorescence spectrometer (EDX 7000, Shimadzu, Kyoto, Japan). Mineralogical compositions were obtained using an X-ray diffractometer (LabX XRD-6000, Shimadzu), equipped with a copper X-ray tube (CuKα), operating in the 2θ range of 10–80°, with a scanning rate of 0.05°/s.

To determine the presence of pozzolanic activity, the NSW was preliminarily comminuted by a porcelain centrifugal grinding mill until less than 20% of the material was retained on a 45 μm sieve. The resulting material, powder nepheline syenite waste (PNSW), was subjected to pozzolanicity tests using lime and Portland cement.

The pozzolanic activity of the PNSW was evaluated using the following two methods: one with hydrated lime and another with Portland cement. In the former, mortars composed of PNSW, lime, and sand were tested to assess the formation of cementitious compounds; the material was considered pozzolanic if it reached a compressive strength ≥ 6 MPa. In the second approach, reference mortars and mortars with 25% cement replacement by PNSW were compared; the material was classified as pozzolanic if the performance index was ≥90% [38,39].

The durability of cementitious matrices in terms of dimensional variations was assessed using prismatic specimens (2.5 × 2.5 × 28.5 cm) containing NSW, subjected to three curing conditions—air, saturated lime solution (reference), and sodium sulfate solution (aggressive environment). Twelve specimens were cast, four for each condition. After 48 h in molds, the specimens were exposed to the respective curing environments. Mortars were prepared at a 1:3.2 (cement/sand) ratio with a water/cement ratio of 0.60, following NBR 13583 [40]. Dimensional changes were measured using a dial gauge (0.001 mm resolution) at 3, 7, and 14 days, and every 14 days thereafter to 182 days, using the 3-day reading as a reference.

The previously described criteria (classification, potentiality, and quantity/viability) were used in conjunction with an analysis of the information collected and a discussion of the results to identify potential applications for using the waste as an alternative mineral raw material. These applications aim to support the circular economy and strengthen industrial symbiosis. The main identified applications are presented and discussed.

Table 1. Criteria and procedures for analyzing NSW.

Criteria	Standard	Analysis
C—Classification	NBR 10004 [41]	Environmental classification
P—Potentiality	SEM XRD XRF NBR NM 248 [42] NBR NM 46 [43] NBR NM 52 [44] NBR NM 45 [45] NBR NM 30 [46] NBR 15261 [47] NBR 13583 [40] NBR 5751 [48] NBR 5752 [49]	Morphological aspects Mineralogical identification Chemical composition Particle size distribution Powder materials Specific gravity Bulk density Water absorption Dimensional variation in air Dimensional variation in sodium sulfate Pozzolanic test using lime Pozzolanic test using cement Portland
Q/v—Quantity/viability	Mining reports and scientific articles	Data on mineral reserves, processing and waste generated

Table 1. Criteria and procedures for analyzing NSW.

Criteria	Standard	Analysis
C—Classification	NBR 10004 [41]	Environmental classification
P—Potentiality	SEM XRD XRF NBR NM 248 [42] NBR NM 46 [43] NBR NM 52 [44] NBR NM 45 [45] NBR NM 30 [46] NBR 15261 [47] NBR 13583 [40] NBR 5751 [48] NBR 5752 [49]	Morphological aspects Mineralogical identification Chemical composition Particle size distribution Powder materials Specific gravity Bulk density Water absorption Dimensional variation in air Dimensional variation in sodium sulfate Pozzolanic test using lime Pozzolanic test using cement Portland
Q/v—Quantity/viability	Mining reports and scientific articles	Data on mineral reserves, processing and waste generated

3. Results and Discussion

3.1. Classification (C) and Mineral Processing

The environmental classification of the waste began with the identification of the process and activity that produced it, detailing how its segregation occurred at the source. Nepheline syenite is commonly processed via a dry route, and the stone blocks are then transported to an industrial plant and dumped into a feeder, where primary crushing is performed using a jaw crusher, and secondary crushing is performed using a cone crusher. The product can be reprocessed to adjust its size. Coarser materials are directed to a tertiary crusher (cone or impact), the intermediate fraction is sent to a primary mil, and the finer fraction is advanced to the next stage. This cycle is repeated until all materials are less than 1.18 mm in size. The appropriately sized mineral fraction is directed to a low-intensity magnetic separator (magnetic or electromagnetic drum) capable of removing any strongly magnetic material, and the resulting magnetic material is identified as a by-product of the process. The weakly magnetic or non-magnetic fractions fall freely to a discharge point, allowing the beneficiation process to continue in the inertial air gravitational classifier (air classifier), which has an exhaust system that pushes the finest particles. After leaving the air classifier, the materials undergo a new magnetic separation process using high-intensity magnetic or electromagnetic rollers capable of removing even weakly magnetic materials.

The material not retained by the magnetic rollers is the commercially valuable fraction of nepheline syenite, which can be comminuted repeatedly until the required fineness is achieved. The retained fraction is the primary waste in the process and is typically deposited in the form of piles in landfill areas.

Thus, after successive beneficiation processes, as illustrated in a simplified way in Figure 2, a portion of the stone is marketed as iron ore (magnetite), the most considerable non-magnetic fraction is marketed as nepheline syenite, which is extremely fine, with low iron oxide content and a high standard of whiteness. The final portion comprises the NSW, which is the subject of this study.

Thus, the beneficiation process consists of dry physical processing for material comminution, size separation, and magnetic separation. NSW has a known origin, does not belong to any specific hazardous source listed in the standard NBR 10004, and lacks characteristics such as biodegradability or combustibility. As a result, NSW is classified as a Class II waste (non-hazardous), and therefore it presents the possibility of being reused and valorized as a mineral resource. This classification is consistent with the literature on stone waste with similar processing and also aligns with the classification by the European standard [50,51,52].

3.2. Potentiality (P)

Figure 3 shows an SEM image of the NSW, revealing its morphological characteristics, as well as the diffractogram of NSW, which indicates the type of microstructure and mineralogical composition of the material. The NSW exhibits irregular particles with a predominance of elongated, flattened, and angular shapes, with a rough texture. These forms are typical of the crushing comminution process and are frequently found in artificial aggregates. Irregular and rough particles not only increase friction but also reduce fluidity. They are more likely to form regions with water accumulation due to the exudation effect, which result in a weaker interfacial transition zone (ITZ), thereby reducing performance, hindering workability, and necessitating greater paste consumption [53,54].

Figure 3 also shows that the NSW is a crystalline material containing felsic silicates such as nepheline and albite, which typically contain silica, alumina, and alkalis. The same minerals were reported in the nepheline syenite stone [22,26,27].

According to the XRF results (

Table 2. Chemical characterization of NSW by XRF (wt%).

SiO2	Al2O3	K2O	Na2O	Fe2O3	CaO	TiO2	MnO	MgO	Others	Loss on Ignition
56.16	20.46	4.68	10.35	5.48	1.15	0.46	0.53	0.07	0.03	0.63

), the NSW is mostly composed of silica (SiO₂), alumina (Al₂O₃), and alkalis (K₂O and Na₂O). The concentrations of these elements in the NSW are a strong indication of its potential for use in the steel and ceramics sectors because the alkalis act as melters, reducing the required temperatures and fuel consumption, and alumina provides the products with increased chemical resistance and durability [25]. High silica and alumina contents are also desirable for construction because they provide hard and inert particles that can improve the mechanical performance of cementitious matrices [55,56].

The results of the physical properties are presented in Figure 4. It is observed that the NSW exhibits a uniform curve, with particle sizes distributed mostly within the usable zone but with a 10% fraction outside the normative limits. Its fineness modulus of 2.23 confirms that the NSW has a fraction within the optimal range for use (between 2.20 and 2.90), but with the absence of larger particles than 1.18 mm due to the beneficiation process, where the comminution and screening cycle ensures the maximum particle size admitted.

The 2.75% powdery material content indicates that the NSW is adequate because amounts greater than 3% (normative limit in NBR 7211 [57]) can increase the surface area, requiring more water to wet the particles and contributing to the shrinkage and cracking of cementitious matrices [55]. The NSW has a specific gravity of 2.49 g/cm³ and a bulk density of 1.34 g/cm³, with a water absorption of 0.70%. The results show that the waste has low porosity, normal density, and a compact structure, similar to the results reported in the literature for natural mineral inputs such as igneous stones often use in civil construction [58,59].

The swelling coefficient of fine aggregates is a decimal value representing the volumetric variation caused when free water on the particle surface promotes surface tension, causing the aggregate particles to separate and occupy a larger volume [55,60]. This effect is strongest for the critical moisture content and, if ignored, can significantly increase dosage errors. Thus, the NSW values of 7.28% for critical moisture and 1.66 for the swelling coefficient do not preclude its use but do necessitate caution from users during cementitious matrix dosage procedures.

Mining waste can cause adverse reactions when used as aggregates owing to the dimensional variation of cementitious matrices. Alkali aggregate reactions, for example, occur when amorphous or deformed silica is present in high concentrations in combination with alkalis, such as sodium and potassium [13,61]. Adverse reactions can also occur due to the exposure of sulfates to the hydration products of cement, where the aggregates used may intensify these reactions by altering the microstructure, leading to increased porosity and permeability of the matrix [62]. As a result, when using solid waste as aggregates for cementitious matrices, it is critical to conduct durability analyses, such as those using accelerated methods in aggressive environments or those evaluating long-duration behavior, in environments with air curing. Figure 5 illustrates the dimensional variations in the mortars containing NSW subjected to curing in both air and Na₂SO₄ environments.

The results show that the mortar using NSW as the fine aggregate exhibits a maximum matrix shrinkage of approximately 1% after six months of air curing. This dimensional variation is caused by drying shrinkage, which occurs as a result of the volume change in the cementitious matrix caused by the loss of internal water absorbed by evaporation [63,64]. It is also observed that there is a stabilization of shrinkage after three months of curing, which is consistent with the results obtained by He et al. [65], who identified stability for the cementitious matrix at 80 days. This dimensional variation does not promote pathology in the produced specimens, and similar results are expected under real-world conditions.

When the mortar is subjected to more severe curing conditions in an aggressive sodium sulfate environment, the results show an increased expansibility with exposure time. The standard recommends that the test be performed for eight weeks, during which time the NSW demonstrates an expansibility of 0.30%, and no pathological manifestations are observed. However, longer exposure causes mapped cracks in the specimens, followed by longitudinal and transverse cracks, with a progressive increase until collapse at twenty weeks (140 days). The SO₄²⁻ ions penetrate the paste and react with monosulfate crystals (which may be incorporated into the C-S-H), causing the formation of late ettringite, as well as with portlandite, forming calcium sulfates (gypsum). The ettringite and gypsum, due to confinement and a larger volume than the previous phases, stress the matrix, generating significant expansion, which causes cracking in the matrix [62]. These results indicate that when there is continuous exposure to aggressive environments, cementitious matrices using NSW as fine aggregate require solutions to be developed to mitigate deleterious effects. Some possibilities are related to reducing porosity and permeability of the matrix [62], as well as the use of supplementary cementitious materials [66].

When pozzolanic material is used to make mortar, it is expected that the material will chemically react at room temperature after being finely ground in the presence of water and calcium hydroxide, forming compounds with cementitious properties. The results for the pozzolanicity analysis of PNSW, using lime and Portland cement, are presented in

Table 3. Analysis of pozzolanic activity of PNSW using lime and Portland cement.

Criteria	Lime (NBR 5751) [48]	Cement Portland (NBR 5752) [49]
% retained (45 μm sieve)	17.40 (<20.00)	17.40 (<20.00)
Average size (μm)	22.75 (<45.00)	22.75 (<45.00)
Lime (g)	208.00	0.00	0.00
Portland cement (g)	0.00	624.00	468.00
PNSW (g)	530.00	0.00	156.00
Standardized sand (g)	1872.00	1872.00	1872.00
Water (g)	460.00	300.00	300.00
Compressive strength (MPa)	3.10 (>6.00)	24.49	21.64
Performance index (%)	51.67 (>100)	88.36 (>90)
Classification	Non-pozzolanic	Non-pozolanic

. It is observed that the results are similar, indicating that PNSW is a non-pozzolanic material. However, the method using Portland cement yielded a result slightly below the normative limit.

These results suggest that PNSW can act as a filler, where despite not having predominantly cementitious or pozzolanic properties, it can contribute to the physical and chemical effects, modifying the particle distribution and pore size, decreasing the total porosity, increasing the packing density, acting as a heterogeneous nucleation point, and reducing C3A hydration [67]. This analysis suggests that PNSW can be used in construction at concentrations as low as 25% to replace Portland cement.

3.3. Quantity/Viabibility (Qv)

In the Brazilian state of Santa Catarina (Figure 6), only two companies beneficiate nepheline syenite, which is used as the primary substitute for feldspar in the ceramics market of the region, one of the strongest in the country. In 2018, the generated nepheline syenite waste comprise approximately 23% when waste generation is calculated as the difference between the gross quantity extracted and the quantity commercialized [68]. This value is similar to that of a previous study reporting that mining companies discarded one quarter of the nepheline syenite crude stone in tailing piles [69].

Regarding the deposit size, a mineral reserve of nepheline syenite stone of approximately 523 million tons was estimated in 2018 in Santa Catarina, the leading state for exploration activity in Brazil. Smaller deposits of nepheline syenite have also been identified in the Brazilian states of Rio de Janeiro, São Paulo, Minas Gerais, and Bahia, increasing the environmental impacts associated with its extraction and beneficiation [68]. The most significant reserves worldwide are reported in Russia, Canada, and Norway [24]. Figure 6 shows the locations of major nepheline syenite stone deposits. However, there is a need for more up-to-date data from management agencies on the quantities of reserves, fractions processed, and generation of solid waste related to the processing of nepheline syenite, both nationally and globally. However, the available information strongly indicates its viability and justifies the need to develop alternatives for applications of NSW.

3.4. Application (A)

After analyzing the collected information and results, the following possible applications are proposed for the nepheline syenite beneficiation waste: in civil construction as a fine aggregate or supplementary cementitious material for the development of cementitious matrices, and as a source of oxides in the steel industry (iron oxide and alkali) and agriculture (potassium oxide). These applications are discussed in the following sections.

3.4.1. Use as Small Aggregates

Aggregates primarily used in the construction industry represent the largest volume of solid material extracted globally, with approximately 50 billion tons extracted per year, which is still insufficient to meet the entire demand [70]. However, the annual amount of mining waste generated, exceeds 100 billion tons [15]. This waste can be viewed as a source of valuable minerals and as an alternative aggregate source, providing a counterpoint to the volumes generated by mining companies and those demanded by the construction industry.

NSW exhibits physico-chemical properties that characterize the material as suitable for use as a fine aggregate in cement matrices, with results comparable to those of igneous stones. The NSW has a fineness modulus of 2.23 and a uniform particle size distribution curve; a fine material content of 2.75%; specific gravity of 2.49 g/cm³; bulk density of 1.34 g/cm³; and water absorption of 0.70%. Chemical and morphological analyses revealed that the NSW is mainly composed of silica (56%), alumina (20%), and total alkalis (15%), with a crystalline structure and the main minerals being nepheline and albite. As a result, there is sufficient evidence for its potential use in the construction industry. It is suggested that tests of mortar and concrete specimens be performed to verify the properties of interest and develop appropriate dosages for use. Figure 7 presents a suggested methodology for using NSW as fine aggregate for cementitious matrices.

3.4.2. Use as Supplementary Cementitious Material (SCM)

SCMs are alternative materials composed of fine particles and soluble siliceous or aluminosilicate substances that can potentially replace Portland cement and provide several benefits [71]. Some of the most significant SCMs are derived from solid waste, such as coal fly ash (thermal power plants), granulated blast furnace slag (iron ore processing in blast furnaces), silica fume or micro-silica (ferrosilicon production), and metakaolin (paper production). Other wastes such as ground glass, sewage sludge, and diverse fly ash (rice husks, sugar cane bagasse, biomass combustion, palm oil combustion, wood, bamboo, and oyster shells) can also be used as alternatives [66].

Mechanical activation has received considerable attention as a method to increase the pozzolanicity of materials because it promotes a physico-chemical change through comminution processes that reduce the particle size and trigger transformations in the crystalline microstructure, producing defects and distortions in the network and thus making the structure metastable and more reactive [72].

PNSW is a crystalline material with a predominantly aluminosilicate chemical composition (76.62%), and the results classify it as non-pozzolanic. However, its use at a concentration of 25% yielded results close to the reference material. Thus, its use as an SCM is still possible, but investigation into activation mechanisms is recommended, indicating analysis through mechanical activation, assessing parameters such as processing time and rotation speeds in high-energy mills. It is worth noting that the inclusion of a new waste comminution process, even if it consumes significant energy resources, is still well below the energy consumption required for clinker production [66]. Figure 8 presents a suggested methodology for using NSW as supplementary cementitious material for cementitious matrices.

3.4.3. Iron Oxide

Magnetic or electromagnetic equipment can be used to separate ferromagnetic elements such as iron oxide. The effectiveness of recovery, on the other hand, is determined by the magnetic field generated by the equipment, as well as the minerals present and their quantity. Comminution of the material sometimes facilitates or allows the extraction of iron-bearing minerals. Magnetic separators used in industrial plants range in intensity from 8000 to 15,000 Gauss [23,69].

The ferrous fraction in nepheline syenite stone was studied post-grinding. Leaching with sulfuric and hydrochloric acids showed recoveries of 19% and 51%, respectively. However, a 15,000 Gauss magnetic field produced the best results, achieving a 70% recovery rate [23].

Given that the NSW has an iron oxide content of 5.48% (Table 2), an investigation into separating the ferrous fraction present in the residue using high-intensity magnets in both dry and wet processes is suggested. It is worth noting that part of the equipment is already present in the mining company’s industrial plant, which already has a market for the by-product (iron ore), facilitating the valorization of the NSW. Figure 9 presents a suggested methodology for separating the ferrous fraction present in the NSW.

The separation of iron from the waste is limited by technical and economic factors, as the segregation process is costly, yields low recovery rates, and predominantly results in a non-commercial mineral fraction composed of aegirine and augite.

3.4.4. Potassium Oxide

The primary potassium oxide source minerals and their respective concentrations are sylvite (63%), sylvinite (36%), langbeinite (23%), kainite (19%), carnallite (17%) and polyhalite (16%) [73,74]. However, most of these reserves are concentrated in the Northern Hemisphere, and alternative minerals with lower potassium oxide contents (ultrapotassic syenite, glauconitic sandstone, and nepheline syenite) have been investigated [74,75].

Different beneficiation techniques can be used to recover K₂O from nepheline syenite stone. However, physical techniques such as magnetic separation and flotation are expensive and inefficient. The recovery obtained after grinding and application of a magnetic field of 15,000 Gauss is only 21%. The flotation recovery using oleic acid as a collector and sodium silicate as a dispersant is 27%. However, potassium oxide extraction can be improved by using the leaching method with either sulfuric or hydrochloric acid, achieving identical recoveries of approximately 50% [23]. Another method, roasting nepheline syenite with calcium chloride, a potassium oxide recovery of more than 90% according to the literature [23,75]. As a possibility for K₂O recovery, the same technique can be applied to NSW.

Potassium chloride (KCl) is the most commonly used form for marketing K₂O (90%), followed by potassium sulfate (5%), and double potassium magnesium sulfate (5%). Brazil is the world’s second-largest consumer of potash in the form of KCl, consuming approximately 4.6 million tons per year. In 2020, national production met less than 20% of this demand [76]. In this scenario, the calcium chloride roasting technique for recovering the K₂O present in NSW may be effective and has a promising market, making NSW a source of mineral input, promoting its valorization, and introducing the agricultural sector as a potential application field. Figure 10 presents a suggested methodology for separating the potassium fraction present in the NSW.

To make the process of segregating the potassium fraction through thermal treatment more economically attractive, the resulting non-alkaline fraction can be utilized as a supplementary cementitious material. This fraction contains elements such as silicon, aluminum, and calcium, and exhibits high fineness and possibly amorphous structure, which enhances its potential and applicability.

4. Conclusions

Waste from nepheline syenite stone beneficiation processing was investigated to obtain sufficient evidence to consider it as a mineral alternative for different industrial sectors. As a result, through valorization, this waste presents the potential for the formation of industrial symbioses and the advancement of the circular economy.

NSW can be classified as non-hazardous; it is composed mainly of silica, alumina, and alkalis, with nepheline and albite as its main mineral constituents. NSW exhibits irregular particles with elongated, angular, and cubic forms, typical of the crushing comminution process.

The analyzed physical properties showed that NSW is similar to natural mineral inputs, such as igneous stones, and meets standard requirements, suggesting that NSW can replace fine natural aggregates in cement materials. There is evidence that NSW, as a supplementary cementitious material, can be used to replace Portland cement at proportions of up to 25%. The results for the dimensional variation of cementitious matrices using NSW indicate that it has the potential to be used in recurring applications; however, it should be further investigated when subjected to prolonged aggressive environments. Consequently, NSW has the potential for use in the construction industry in the development of mortars and concretes.

Other potential applications of NSW include the metallurgical sector, following the recovery of the metallic fraction containing iron oxide as well as sodium and potassium alkaline oxides. Although NSW contains important oxides such as silica, alumina, and alkalis, its lack of whiteness prevents its use in the ceramic and glass industries.

The authors believe that these preliminary results provide evidence for various potential applications for nepheline syenite waste and suggest conducting further tests to verify the properties of interest and the suitability of NSW for the proposed uses.

For each of the investigated applications, methodologies for the beneficiation of NSW are suggested, aiming to facilitate its development and promote its valorization as a mineral alternative, thereby contributing to the formation of industrial symbiosis and a progressive strategy towards a circular economy.

In conclusion, the CPQvA framework proved to be a valuable and effective tool for supporting decision-making processes related to waste valorization. In this study, its application demonstrated the potential of nepheline syenite waste (NSW) for use in various industrial alternatives, reinforcing the relevance of systematic evaluation in identifying viable reuse pathways.