Zeolites Synthesis from CFA

Zeolites are another class of materials that are widely used in industry for petroleum cracking, cation exchange resins, water softening processes, etc. These zeolites are also hydrates of alumina and silica, mainly containing Al, Si, cations from gp II (Ca, Mg, Na) and water molecules. Due to the presence of 40–60% silica and 20–40% alumina, the CFA serves as an economical resource for zeolite material synthesis. To date, zeolites have been synthesized from CFA by using 4–8 M, NaOH treatment along with continuous heating at 90–105 ◦C, for 1 h to 24 h with rigorous stirring. The pH, temperature, heating and stirring time decides the morphology and class of zeolites to be synthesized. There are several reports in the literature in which CFA from different parts of the globe has been used for the synthesis of zeolites. Yadav et al., 2019 and 2021, reported the synthesis of zeolites from CFA collected from the Gandhinagar and Gujarat TPPs in India. The size of the synthesized zeolites varied between 80 nm and 180 nm in width, while their length varied from 120 nm to 300 nm [99].

#### *2.2. Red Mud*

Red mud is a hazardous byproduct of the bauxite industry [101], and is produced at the time of extraction of alumina from bauxite using Bayer's method [102]. It is considered hazardous because of its high alkalinity and the presence of various toxic heavy metals; however, it is also a rich source of titanium, iron, and aluminum (composition is shown in Table 1). In addition, red mud is recognized as a bauxite residue [103], as it is what is left after the extraction of all the extractable alumina from bauxite. The resulting mud is a mixture of the insoluble fraction of solid and metallic oxides, and ore, which remains after the extraction of the aluminum-containing components [104]. It is typically disposed of as a slurry with a solid concentration ranging between 10 and 30%, pH in the range of 13, and high ionic strength [14]. This disposal problem is compounded by the fact that typical bauxite processing produces up to three times as much toxic red mud as aluminum. Figure 4 shows a typical SEM micrograph of red mud. The particles of red mud aggregate together to form lumps.


**Table 1.** Elemental composition of red mud.

Approximately 44 million tons [MTs] of primary aluminum are produced annually around the world [105]; by that count, roughly 132 MTs of red mud enter retention ponds and some dry stack tailing areas annually [106]. The alumina plants in the Indian context have an annual capacity of 1.692 MTs; they produce 0.6 MTs of metal, and approximately 2 MTs of red mud per year [107]. The chemical analysis of RM reveals the presence of silica, aluminum, iron, calcium, and titanium, as well as an array of minor constituents, namely Na, K, Cr, V, Ni, Ba, Cu, Mn, Pb, Zn, etc. [15,108]. There is a high variance in chemical composition among various red muds generated around the world [14].

**Figure 4.** SEM micrograph of red mud, adopted from Zhang et al., open access journal Applied Sciences, 2018 [109].

Currently, in the alumina industry, alumina is obtained by carrying out Bayer's process on bauxite. The byproduct of such processes is red mud, which is generally regarded as dangerous waste, because it contains high concentrations of some heavy metals, iron oxide, and other metal oxides. Industry does not recycle it, and there is no available disposal method for it. Currently, companies collect this waste in their vicinity after the extraction of alumina. This continuous piling up of red mud constitutes a major and global threat to the environment [110,111]. This red mud contains high amounts of iron oxides and heavy metals, which can leach out into water bodies and cause damage to aquatic fauna. In India, two major alumina-producing companies are NALCO and HINDALCO, who generate a huge amount of waste every year in the form of red mud. This causes water pollution through its entry into water streams, leaching toxic heavy metals into water bodies. This also affects the aquatic flora and fauna. In addition, it has adverse effects on the biological oxygen demand (BOD) and chemical oxygen demand (COD) of the water bodies, resulting in a lowering of these parameters. This waste changes the community structure of water bodies. The alkalinity of water bodies is increased as a result of releasing NaOH into the water. In any alumina refinery, large amounts of land are required for the handling of this waste.

#### 2.2.1. Recovery of Alumina from Red Mud

The red mud-based recovery of alumina involves several steps in series—recovery of ferrous particles, leaching of dried red mud with strong mineral acids, filtration to obtain the filtrate, crystallization of the leachate, the recovery of acids, and finally calcination—in order to obtain the alumina powder. A complete flow chart depicting the extraction of alumina from red mud is given below in Figure 5.

**Figure 5.** Flow chart for the recovery of alumina from red mud/CFA.

The recovery of alumina and ferric oxides from iron-rich red mud has been reported using the reduction sintering technique [112]. Different experiments have shown that up to 89.71% of alumina can be extracted, with a Fe recovery rate of 60.67%, under optimal conditions [113]. Zhang et al. [114] investigated andradite-grossular hydrogarnet formation in the hydrothermal process with the aim of examining its effect on alumina and alkali recovery from Bayer red mud. For the evaluation of the parameters with the highest impact on the recovery process, they took into consideration the batch experiments and parameters such as caustic ratio (molar ratio of Na2O to Al2O3 in sodium solution), reaction temperature, residence time, and sodium concentration. Zhu et al. [115] reported the recovery of alumina and alkali from red mud using a novel calcification–carbonation method under mild reaction conditions. Batch experiments were performed, and the effects of temperature, pressure, and additive addition on the extraction efficiency of alumina were examined, and the extraction efficiency of alumina was 95.2%. In another study, Meher [116] reported the extraction of alumina from red mud using a calcium carbonate and sodium carbonate sintering process. They studied the impacts of Na2CO3 and CaCO3 additives, sintering time and temperature, and leaching time on the effectiveness of alumina extracted from red mud. The alumina extraction was up to 97.64% at a sintering temperature of 1100 ◦C for 4 h with red mud.

#### 2.2.2. Applications of Red Mud

Many studies have confirmed the benefits of red mud in the process of treating water and removing inorganic anions (e.g., fluoride, phosphate, and nitrate), toxic heavy metals and metalloid ions, as well as organic substances (e.g., phenolic compounds, dyes, and bacteria) [117,118]. Moreover, red mud can be employed as an effective catalyst in processes such as hydrocarbon oxidation, hydrodechlorination, and hydrogenation [119]. The broader areas of application of red mud are given in Figure 6.

**Figure 6.** Diagrammatic representation of red mud in their broader applications.

#### *2.3. Iron Slags/Scraps*

There are several iron-based industries, like the steel industry, where iron particles are generated as waste materials [120]. These iron materials can be processed for the recovery of highly pure iron, zero-valent iron, or iron oxides. These can be used directly as filings or in coke industries [121]; thus, they have drawn much attention. These iron particles can be recovered using strong magnets in either wet or slurry form. These iron particles can further be treated with acids to obtain iron-rich leachates that can be used as precursor materials for the synthesis of different types of iron oxide particles. The synthesized iron oxide particles can be recovered by precipitation, chemical precipitation, and calcination. These iron oxides are so highly pure that they can act as an adsorbent in processes like the treatment of wastewater, environmental cleanup, in ceramics, and in the steel industry [122,123]. The major advantages of such iron oxide particles include their ability to be easily recovered from the reaction site, their recyclability, and their easy external manipulation using strong magnets [124]. The synthesis of iron oxide particles from waste is cost-effective and environmentally friendly.

Tang et al. [125] attempted a coal-based smelting reduction method for the recovery of Fe, Ni, and Cr from pickling sludge waste. Their findings showed that the Fe recovery was 98.1% under optimized conditions. Tang et al. [126] reported the recovery of iron from iron ore tailings. They used it to develop a concrete composite mixture. They investigated the impacts of different parameters upon the extraction of ferrous particles. Up to 83.86 wt% of iron was recovered from a feed iron grade of 12.61 wt%. Zhang et al. [127] attempted to recover iron from the waste slag of pyrite processing using a reduction roasting magnetic separation method. The iron content of the concentrate was initially 57%, of which 87% was extracted using the aforementioned method. It should also be noted that, through further treatment using chlorinated segregation–magnetic separation, the iron content in the slag was increased to 83%.

In the study carried out by Wang Yu et al. [128], the co-precipitation and magnetic separation methods were adopted with the aim of recovering iron from waste ferrous sulphate. They investigated the impacts of various reaction parameters on the iron recovery, and also examined the impacts of milling time and magnetic induction intensity on the separation of magnetic particles. The mixed magnetic particles were wet-milled for 20 min before magnetic separation. The grade and recovery rate of iron in the magnetic concentrate drastically increased from 51.41% to 62.05%, and from 84.15% to 85.35%, respectively.
