**1. Introduction**

Every day we come across various types of waste in our life either in houses, working areas, industries, farms, etc. These waste materials can be kitchen waste, agricultural waste, industrial waste, or poultry waste. Although there are also several other types of waste, they are outside the scope of the present study. The waste produced in our houses, such as kitchen waste and incense stick ash can be categorized as household waste [1], whereas the waste produced from the agricultural practices, such as wheat straw [2,3], wheat husks [4],

Tirth, V.; Gnanamoorthy, G.; Gupta, N.; Algahtani, A.; Islam, S.; Choudhary, N.; Modi, S.; Jeon, B.-H. Extraction of Value-Added Minerals from Various Agricultural, Industrial and Domestic Wastes. *Materials* **2021**, *14*, 6333. https://doi.org/10.3390/ ma14216333

**Citation:** Yadav, V.K.; Yadav, K.K.;

Academic Editor: Franco Medici

Received: 10 September 2021 Accepted: 16 October 2021 Published: 23 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

rice straw [5], rice husks [6], coconut shells [7], palms and dates [8], lemon peel [9], almond shells [10], etc., can be considered to be agricultural waste [11]. The waste generated by industry, such as coal fly ash (CFA) [12,13], red mud [14,15], gypsum waste [16,17], sewage sludge [18], iron tailing [19], etc., fall under the category of industrial waste [20]. Eggshells [21–23] and poultry litter [24,25] can be categorized as poultry waste; however, most investigators have considered eggshells to also belong to industrial waste. Among the above-mentioned types of waste, agricultural and industrial wastes raise major concerns for the environment, as millions of tons are produced every year around the world. Most of these waste materials are still disposed of via landfill [26,27], dumping, or disposal in water, creating pollution of the environment. The dumping of such waste may further deteriorate fertile agricultural soil [28], produce a foul odor [29] that may attract pests and mosquitos, and lead to health issues for human beings and animals [30]. Some types of industrial waste, like CFA [31] and red mud [32], are considered hazardous because of their high concentration of toxic heavy metals [33]. Heavy metals from dumping areas can leach into the surrounding soil, and when it rains, they may percolate into water bodies, thus leading to water pollution [34].

Nowadays, with the advent of new technologies, such waste can be processed into value-added minerals, especially in metallurgy. They can also be applied in the fields of agriculture [35], adsorbents [36,37], geo-polymers [38,39], ceramics [40], and environmental cleanup [41]. This article reviews and discusses a variety of waste materials in detail, including silica, alumina, calcium oxides, carbonates, etc., along with their properties, applications, and methods for their recovery.

#### **2. Industrial Waste**

Waste can be classified into several classes depending on its origin, for instance, industrial waste (CFA, paper and pulp waste), agricultural waste (coconut coir, sugarcane bagasse, and lemon peel) [42–49]. Similarly, waste materials produced in the home are referred to as domestic waste, including kitchen waste and incense stick ash, while eggshells and waste from poultry are regarded as poultry waste. The major categories of waste are shown below in Figure 1.

**Figure 1.** Classifications of the different valuable waste materials.

Industrial waste refers to byproducts, e.g., CFA, red mud, gypsum waste, and other municipal waste, generated in the industrial sector. Among the various types of waste, this is one of the major sources that requires special attention due to its hazardous nature.

#### *2.1. Fly Ash*

Coal fly ash consists of fine powders produced from the pulverized coal used in the thermal power plants during the generation of electricity. It is heterogeneous in nature, and is made up of a glassy amorphous phase and a crystalline quartz phase, such as mullite and magnetite. Fly ash is produced in large amounts in TPPs, and is mainly considered waste, although its mineral value makes it a useable material.

#### 2.1.1. Recovery of Minerals from CFA

CFA can be applied in several ways in the fields of agriculture, remediation (as an economical adsorbent), ceramics, etc. [50]. Formerly, CFA was considered a hazardous waste [51], but today, it is considered to be a useable material. CFA has high amounts of micro- and macro-nutrients that can be used as a source of nutrients for plants [52,53]. It is used in agriculture (as a fertilizer) [54], forestation [55], reclamation of wasteland [56], and for maintaining the pH of acidic soil [56]. CFA is also used as an adsorbent for the removal of pollutants—mainly dyes [57,58], pesticides [57,58], and heavy metals [59,60]—from wastewater. It is used for making nanocomposites [61] that are applicable in the defense industry [62,63] and the production of lightweight materials [64,65]. CFA can be used for making blended cement [66] tiles, bricks [67], blocks [56], RCC, kitchen panels, and geo-polymers. CFA can also be used as fillers in rubbers and tires [68], and can also be used in the mining industry [56,69] for the recovery of ferrous metals [70], cenospheres [55], mullite [71], silica [72], zeolites [73], and alumina. It can also be applied for the recovery of unburnt carbon, soot, and carbon nanomaterials like carbon nanotubes, fullerenes, and graphene. These carbonaceous materials are formed due to the burning of the organic content of the coal, while soot and unburnt carbon are due to the incomplete burning of coal. Higher grades of coal like anthracite and bituminous coal have higher carbon contents, so after the burning of such coals in thermal power plants, the resulting ash will also have a high content of carbon in the form of unburnt carbon, soot, and carbon nanomaterials like carbon nanotubes, fullerene, etc.

#### Silica

Silica accounts for up to 40–60% of CFA [74], depending on the source of the CFA, the type of coal, the operating parameters of the thermal power plant, the furnace temperature, etc. Silica is present in CFA in either crystalline or amorphous form [75]. The crystalline form of silica is present mainly as quartz, sillimanite, and mullite [76], whereas the glassy amorphous form is the only form of amorphous silica. Crystalline silica is mainly inert, and does not easily react with acids or bases. Therefore, silica can only be extracted from the amorphous form, as it reacts easily with strong bases. In addition, silica can easily be extracted from CFA through treatment with strong bases like NaOH, KOH, and sodium bicarbonate [77]. Silica is mainly extracted using either the alkali treatment [78] method or the alkali fusion method [79]. The complete extraction of silica using the alkali treatment method is depicted in Figure 2, while in alkali fusion method, the silica source is calcinated with NaOH at high temperature in order to form a new mineral (such as nephaline), from which it is easier and more efficient to extract the silica. Yadav et al. reported the extraction of nanosilica from fly ash tiles using the alkali fusion method [80]. Silica can be used in the production of glass and ceramics [80], in drug delivery [81], and used in medicine, foundries, adsorbents, catalytic processes [82], and molecular sieves [83]. Silica is nontoxic and mesoporous, meaning it can be readily used in various industries. The silica extracted from the CFA is nanosized (20–60 nm), but aggregates together to form lumps.

**Figure 2.** Schematic diagram of the synthesis of silica from silica-rich waste materials.

#### Alumina

Alumina, which is amphoteric in its nature, can be obtained from CFA or other alumina-rich materials using both acids and bases. Depending on the CFA type and the source of the coal, the alumina content in CFA can range between 20 and 40% [84]. In CFA, alumina can mostly be found in the form of crystalline aluminosilicates [85] such as mullite. These aluminates are highly inert, and rarely react with acids; thus, only low yields can be obtained. They can be treated with strong mineral acids that can be directly converted to powder by thermal decomposition. They can also be extracted using alkali like NaOH at high temperatures, i.e., 125–1100 ◦C. Alumina forms in numerous meta-stable phases, including gamma [γ]-, epsilon [η]-, delta [d]-, theta [θ]-, kappa [κ]- and *χ*-alumina. While the most stable phase of alumina is [*a*] alpha. Of all of the phases, the gamma-phase of alumina is the most important and widely used nanosized material. Gamma alumina is widely used in the petroleum and automobile industries as a catalyst and catalyst substrate, to produce ceramics [86] and glass, adsorbents, spacecraft materials, microelectronics, thermal-resistant materials, and as a coating material for thermal wear and abrasives [87], optoelectronics, and metallurgy. Alumina nanoparticles have high compression strength, high chemical resistance, a high degree of refractoriness, high thermal shock resistance, high abrasion strength, and high dielectric strength. Alumina is also applied in ceramics, adsorbents, fire-retardant materials, acid, and alkali-resistant materials [88].

#### Cenospheres

Cenospheres are aluminosilicate spheres [89,90], the size of which varies by microns. They are lightweight nanostructured materials that are created when coal is combusted within a furnace. These are used as lightweight materials for aircraft, in the field of defense, in fire-proof materials [55], and for acid and alkali wear-resistant materials [91]. Today,

cenospheres that are tiny hollow spheres with diameter of approximately 10–1000 μm are among the most desirable byproducts that can be obtained from coal combustion processes. They normally comprise 1–2% of CFA obtained during the process of coal combustion. Cenospheres possess properties such as very high mechanical strength and low density; thus, they are regarded as a highly significant issue in coal-fired power plants. A number of parameters affect cenosphere properties, including the grinding operations, the nature of the coal used, the combustion parameters, and withdrawal when generating electricity. These materials are mainly possess a glassy surface and a crystalline matrix such as mullite with a nano-film covering with a thickness of 30–50 nm. Their form is similar to that of a shell, with a thickness varying between 2 and 30 microns. They are extracted using the density-based centrifugation method, during which finer lightweight cenospheres float at the top of the slurry, while the particles of heavier ferrous metals settle at the bottom [55]. The cenospheres are collected from the top and dried before use. The formation mechanism of cenospheres during pulverized coal combustion is complex and is highly dependent on fuel properties and combustion parameters. The formation mechanism of cenospheres is very similar to the procedure of glass blowing [92]. Therefore, it would be beneficial to take a closer look into glass formation principles.

The literature presents two approaches for the extraction of cenospheres from fly ash, namely dry separation and wet separation. The conventional cenosphere extraction method is primarily performed using wet processes, namely simple sedimentation and flotation [93]. There are two methods for estimating the degree of separation when recovering cenospheres from coal CFA, namely the float method and the sink method. To separate cenospheres from CFA, various liquids, viz., water (1 g/cc) and acetone (0.789 g/cc), can be used. Fly ash is kept in a vessel and water is added to it. The complete mass is stirred for four hours; afterwards, it is allowed to settle for ten hours. Then, all cenospheres with densities lower than 1 g/cc will float up and can be separated [94].

#### Mullite

Mullite, consisting of micron-sized particles (1–1.5 microns in length and 0.3–0.5 microns in width), is a rarely observed crystalline mineral that contains aluminum silicate (3Al2O3 2SiO2) [95] and is mainly made up of Al, Si, and O; however, its composition can be quite variable. In the process of combusting aluminosilicate raw materials, mullite is created. This material is a key component in porcelains, ceramic whiteware, high temperature insulation, refractory materials and traditional ceramics [96]. Mullite is a compositional orthorhombic aluminosilicate, and generally possesses the composition Al2 (Al2+2*x*Si2−2*x*)O10−*x*. Mullites are non-stoichiometric compounds whose structure is similar to magnetite containing impurities. It is rarely formed in nature, because of its formation conditions, which require high temperature in combination with low pressure. Synthesis of mullites is possible from high-silico-aluminous CFA only, as they are rich in silica and alumina, with lower contents of iron oxides. Iron oxides have a negative effect on mullite. Generally, for the purpose of synthesizing mullites from CFA, cenospheres are the most effective materials, since they have lower contents of iron oxides.

Mullites are formed in CFA from the organic and inorganic materials present in coal as a result of different melting and firing processes. After the extraction of mullites using a hydrofluoric process, it can be characterized by XRD and SEM-EDS in CFA. A clearer picture of mullite can be obtained using XRD and NMR. Both of these techniques can be used to efficiently investigate the value of x and the oxygen hole rate in the general formula, thus obtaining the mullite composition. Mullites can be used as refractory materials because of their high melting point (1840 ◦C).

Mullites are aluminosilicate minerals with the general formula Al4+2x Si 2+2x O10−<sup>x</sup> (with the value of x varying between 0.17 and 0.59). Mullites are capable of forming two stoichiometric forms, namely 3Al2O32SiO2 and 2Al2O3 SiO2. Mullite is known to be the only stable binary phase of the A12O3-SiO2 system that exists under ambient conditions. From an empirical perspective, its chemical compositions include 71.8 wt% A12O3 and 28.2 wt% SiO2, designated as 3/2-mullite (3A12Oy2SiO2). Mullite has two common morphologies: a platelet shape and a needle shape. In the platelet shape, it has a low aspect ratio, whereas in the needle shape, it possesses a high aspect ratio. In addition, mullite has low thermal conductivity, low thermal expansion, high thermal stability, high corrosion stability, high strength, and high fracture toughness. It has excellent creep resistance, acceptable thermal shock and stress resistance, and acceptable strength wear resistance, and it can be used at high temperatures. Mullites can be applied as an effective replacement for platinum in diesel engines, furnace liners, electrical insulators, protection tubes, kiln furniture, rollers, heat exchanger components, heat insulation parts, pressed parts, and isostatically pressed parts [97–100].

Yadav et al., 2021 reported the recovery of needle-shaped mullite, 90–300 nm in size, extracted from CFA using 16 M HF acid. An optimum ratio of CFA and HF was mixed and kept for interaction in an incubator shaker. The CFA was collected from the Gandhinagar and Gujarat thermal power plants. The source of the coal was anthracite/bituminous coal, i.e., higher grades of coal. The detailed mechanism for recovery of mullites from CFA is given below in Figure 3 [94].

**Figure 3.** Schematic diagram for the recovery of mullite from CFA adopted from Yadav et al., open access journal Crystals, 2021 [94].
