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    Topic review

    Red Mud resources for metal

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    Submitted by: Sneha Samal

    Definition

    Various scopes are suggested for the utilization of red mud to maintain a sustainable environment. The potential use of red mud covers the valuable metal recovery that could emphasize the use of red mud as a resource. Red mud could act as reduced slag in the metallurgical field for the extraction of minerals and metals for upscale application. Although many studies have revealed the potential utilization of red mud, most of them are only limited to a lab-scale basis. 

    1. Introduction

    Red mud is one of the by-products generated in the aluminum industry from the ore of bauxite during the calcination process for the extraction of aluminum dioxide. The term “red mud” is established and derived from the two words of “red”, which refers to the color, and “mud”, which refers to the waste generated after the alumina extraction from the bauxite ore, by a calcination process. Generally, 2.5–3 kg of red mud is produced in each 1 kg of Al production from the bauxite industry . As the global production of aluminum is approximately 64 million tons, this result in 160 million tons of red mud to dispose of. The current method of red mud disposal is to simply pump it into ponds or dry up the red mud with a special liner [1]. In both approaches, a large amount of land is used and ultimately the land should be maintained properly, rather than disposing of the product as waste to the surrounding area, causing serious environmental issues and health hazards. The alkaline nature of red mud and dried-up dust disposable to the environment could be minimized by spraying water on the dry red mud powders. Furthermore, the alkaline nature of red mud inhibits the vegetation growth in those areas, thus it must be corrected by adding acidic flux before its disposal into the surroundings. Given all these environmental implications, it would be appropriate to think of a new use for red mud. “Waste is a resource if we use it. Otherwise, it is waste if we waste it” [1]. Thus, the red mud residue, after the extraction of the minerals, could be considered as a potential building material for the construction of roads, landfill sites, and building materials. Recently, a combination of red mud–fly ash composite could find application in the preparation of geopolymers as an alternative material for the construction industry [2].
    The recovery of critical raw materials from red mud involves many benefits including environmental, social, financial, economic, and technological benefits [2][3]. The content of metals such as Ti, Si, Fe, Na, and Al in red mud is 2–12%, 1–9%, 14–45%, 1–6% and 5–14% respectively. Apart from representing a huge solution in the construction sector, when present in a large quantity, red mud as a resource opens up various possibilities for the extraction of minerals and ions such as the major elements Fe, Ti, Mn, Al and Ca, Na, Si, Cr, Mn, V, La, Sc, Y. Rare earth elements (REE) such as Ce (102 mg/kg), La (56 mg/kg), Sc (47 mg/kg), Nd (45 mg/kg), Sm (9 mg/kg) are also valuable elements present inside red mud. REE are the most important critical raw materials for the European Union [4]. Red mud can be also considered as sintered ceramics for electroceramic materials [5][6][7].
    In powder technology, red mud could be considered as resource for the recovery of metals such as Fe, Ti, Mn, Na, K [8][9]. Simultaneously, red mud could be used as a coating material for various composites against harsh environments and high-temperature sintering, against wear and corrosive behavior [10][11][12]. Unlike the recovery of metal ions, which will certainly not be the main business for the red mud, it could also act as the main component for construction and road fill materials [13][14]. Fly ash with acidic nature inhibits agglomeration of the volatilization of heavy metals at low temperatures within the red mud combination [15][16][17].
    In Figure 1 the combination of various technologies that could be implemented for a complete utilization strategy is shown. The comprehensive utilization of red mud as a resource opens up in various sectors such as red mud-based geopolymers in the construction and metal extraction industries. Synergetic utilization of red mud emerges as flue gas in the geopolymer industry sector for an alternative binder in cement material. The exploitation of these potential techniques, for metal extraction from red mud, is subordinate to the establishment of a small plant, near the aluminum industry, for resource utilization [18][19][20][21]. The reduction of red mud and fly ash mixtures proved the formation of reduced slag in the sintering process during lab-scale experiments. Based on the latter, it is possible to design a synergetic utilization for the red mud/fly ash mixture. It has been seen that the hazardous heavy metals could be recovered as alloy from the reduced slag [22][23]. Therefore, this environmentally friendly co-reduction process could be implemented as one sound solution for red mud and fly ash, leading to complete utilization of the resources, thus representing a zero-waste technology [24]. The optimized parameters for the reduction process were chosen as 20 wt.% fly ash with 80 wt.% red mud, at a temperature of 1100 °C for 2 h. The sintered slag contained CaO, SiO2, Al2O3, and FeO, as well as a glass phase, which is similar to ground-granulated blast-furnace slag and supports broad future applications. It has also been seen that the treatment of the red mud’s alkaline nature with an additive for surface modification will enhance the utilization on an upscale basis [25][26]. The major and minor elements of red mud are quantified in Table 1.
    Figure 1. Scheme representing the technologies implemented in red mud for its complete utilization in various sectors.
    Table 1. Quantification of major elements (wt.%) and minor elements (Conc. Mg/kg) of red mud.
    The composition shows the presence of heavy irons and minerals of Fe, Si, and Ti in the major quantity. Red mud could be considered as bricks, road surface material, and in the cement industry with potential use for building applications. However, this approach is limited due to the alkaline nature of the material. The alkaline nature of red mud is reduced by the acidic counterpart of fly ash, generating the neutral nature of the composite. The latter could be the most significant solution for the vastness of the problem, but careful consideration is required for this application. After the Al content, Fe represents the second-largest amount of metal that is separated by a magnetic separator. The nonmagnetic part of the residue can be considered as a construction material. Furthermore, some researchers searched for producing steel and cement from red mud [39]. Additionally, the recovery of Al, caustic soda, and lime could be used as catalyst for enhancing the Bayer process for increased Al production. However, despite the invaluable outcomes obtained from all techniques associated with red mud utilization, they are not practically suitable to use for recycling large amounts of red mud (currently 160 million tons annually [40]). The amount or iron in red mud is the largest, which, when disposed with red mud annually, represents a waste of metal [41]. Thus, metal recovery from red mud opens a wide field for potential utilization as resource.

    2. Sources and Utilization of Red Mud

    2.1. Relevant Sources for Literature Review

    A broad range of literature sources, dating from 1991 to 2021, in the areas of red mud and red mud composites were reviewed for this article. The databases searched for this literature survey include various sources such as MDPI, Scopus, Science Direct, Google Scholar, and Springer. Articles, conference proceedings, data, reviews, chapters, and books of similar topics were filtered using search terms such as “red mud”, “composite”, “mineral”, “microstructure evolution”, “metal ion recovery”, “mineralogical characteristics of the materials”. Section 1 in the introduction includes all the potential previous studies in this area. Section 2 includes various types of red mud and composites with potential applications. The basic and advanced application of red mud and composite is followed in Section 3 with emphasis on some recent literature surveys. Section 4 compares the data with the present scenario through an exhaustive literature survey. Figure 2 displays the total publication from 2010–2021 in the area of red mud that consider it as a source of metal and ions.
    Figure 2. Total publications as function of year for red mud considered as source of ions (data collected from Web of Science).

    2.2. Utilization of Red Mud as Metal Resource

    Although researchers highlight that red mud is a large contributor to the construction sector, it is generally recognized as waste material. The term “waste” creates, both psychologically and from a media viewpoint, an obstacle in the application areas. Thus, replacing the term “waste” with “resource” could add significant interest in the extraction of minerals and their use. In this work, an investigation was carried out for review in the area of utilization of red mud as a source of metallic ions and resource material. Red mud, added with various weight percentages of fly ash to neutralize the acidity, undergoes the sintering process for the conversion into a reduced slag material. This sintered product could act as a basic resource for the extraction of metal ions and as a major by-product for the mineral industry. Figure 3 illustrates the red mud utilization from the as-received stage towards the final stage for industrial utilization.
    Figure 3. Schematic flow sheet on the iron, alumina, and slag for recovery of various metals.
    The dry red mud undergoes magnetic and non-magnetic separation that follows up the smelting process for iron recovery. Non-magnetic parts undergo the leaching process for Al recovery. Figure 3 portrays the flow sheet of various types of red mud utilization. Preliminary treatment involves the magnetic separations of bulk iron parts from the red mud. Accordingly, the magnetic and non-magnetic parts undergo different treatment in the further steps for iron recovery on the acceptable norms. If the magnetic parts are non-acceptable, they undergo smelting for iron recovery. The non-magnetic parts undergo leaching for alumina recovery and the residue undergoes slag recovery as the utilization of the major parts. A key point to benefit in terms of human resources and the economy could be the establishment of a plant for the beneficiation of red mud as resource alongside the bauxite industry. Particularly, to avoid transportation costs, the waste utilization facility processes and tools such as electric arc furnace, sintering of red mud, and leaching facility should be present in the proximal areas of the aluminum industry. One of the innovative processes in the production of pig iron is a by-product from reduced red mud by the carbothermal reduction process. The various process and active areas in which red mud can be treated can be divided into major and minor activities (Figure 4). Red mud can be used as a primary resource in the construction industry, for example, as bricks and other suitable materials for making houses, or as material for pavement. Red mud could be used in the industrial sector of iron recovery or metal extraction and smelting for the by-product of pig iron and calcium titanium-rich compounds for recovery of titanium. Finally, it can be used in the carbothermal reduction process for iron recovery, which could be a possible step for steel making. The rest of the residual red mud could be considered as the reductant for alumina recovery. Major use in the areas of construction and landfill opens the application of red mud in combination with metal recoveries such as Al, Fe, and its integrated combination towards the reduction process for the steelmaking. Integrating the red mud with other materials could improve its use in synergetic utilization [41].
    Figure 4. The various areas of red mud utilizations.
    Foaming ceramics are emerging as a new group of materials that could improve performance that could act as energy-saving materials [42]. Sintering and thermal plasma open the possibility of the synthesis of energy-saving materials by generating porosity in the sintered material [43][44]. In these cases, sintering is one of the effective processes of using carbo-thermal reduction inside the furnace that facilitates the formation of sintered slag. The quantity of fly ash content (wt.%) reduces the mixture of red mud and fly ash that undergoes chemical and physical reduction processes as a function of sintering temperature. The mineralogical evolution in the sintered product and the end-product was examined to confirm the presence of minerals and ions at the end of the process.

    2.3. Sources of Metal Ions

    In this article, an effort was made to create a review in the area of utilization of red mud as a source of metal ions. Various steps and process related to the mineralogical evolution of various metal and rare earth ions in red mud are covered and discussed. Simultaneously, the application of red mud in various fields is covered, where red mud could be given importance as a resource rather than waste.
    Table 2 shows the red mud generated from various plants with different chemical compositions.
    Table 2. Major elemental composition of red mud from various locations in the countries.
    Composition wt.%
    Location Al2O3 Fe2O3 SiO2 TiO2 CaO Na2O Mn P2O5 V2O5 Gd2O3 MgO K2O LOI REFs
    Ajka Aluminum Industry, Hungary 16–18 33–48 9–15 4–6 0.5–3.5 8–12 - - 0.2–0.3 - 0.3–1 - - [31]
    Aluminium Pechiney, Gardanne, France 15.00 26.62 4.98 15.76 22.21 1.02 - -- - - 0.95 - 12.10 [33]
    Bauxite ore refinery, Guinea 26.60 48.40 5.50 - 1.30 - - - - - 0.90 - 14.60 [32]
    ALCOA factory, San Cibrao (Northwest of Spain) 12.00 37.00 9.00 20.00 6.00 5.00 - - - - - - - [35]
    Korea Chemical Co. 23.70 16.60 22.90 6.70 6.70 11.60 - - - - - - - [36]
    Shandong Aluminium Factory, China 7.96 6.57 21.90 - 38.84 2.32 - - - - 1.60 0.41 17.42 [37]
    Greek red mud, Greece 15.60 42.50 9.20 5.90 19.70 2.40 - - - - - - - [38]
    Slurry pond from Worsley Alumina, Australia 15.00 60.00 5.00 5.00 - 16.00 - - - - - - - [39]
    Alpart factory and the Alcan Ewartonred mud pond, Jamaica 18.80 45.30 4.30 6.40 3.10 1.50 - - - - - -   [40]
    Shandong Aluminium Corporation, Shandong, China 6.93 12.76 19.14 3.43 46.02 2.37 - - - - 1.15 1.20 5.73 [41]
    Alumina-aluminio of San Ciprian, Lugo, Spain 20.10 31.80 6.10 22.60 4.78 4.70 - - - - 0.20 0.03   [42]
    Etibank Seydiehir Aluminium Plant, Konya, Turkey 20.39 36.94 15.74 4.98 2.23 10.10 - - 0.05 - -   8.19 [43]
    Aluminium of Greece S.A. 15.65 45.58 6.96 7.07 14.84 3.26 - - - - - 0.07 - [44]
    Eurallumina alumina plant, Italy 17.19 30.45 9.58 8.61 7.77 12.06 - - - - 0.86 0.30 12.38 [45]
    Queensland Alumina Ltd. refinery, Gladstone, Australia 25.45 34.05 17.06 4.90 3.69 2.74 - - - - 1.86 0.20 - [46]
    Seydiehir Aluminium Plant, Konya, Turkey 14.10 38.30 2.50 - 4.10 - - - - - - -   [47]
    HINDALCO Renukoot, India 21.9 28.1 7.5 15.6 10.2 4.5 - - 12.2 [48]
    IND ALMuri, India 24.3 24.5 6.2 18.0 5.3 - - [48]
    BALCO Kobra, India 19.4 27.9 7.3 16.4 11.8 3.3 - - 12.6 [48]
    NALCo Damanjodi, India 14.8 54.8 6.4 3.7 2.5 4.8 1.1 0.67 0.38 0.01 - - 9.5 [48]
    INDALBelgam, India 19.2 44.5 7.0 13.5 0.8 4.0 -- - 10.0 [48]
    MALCO Mettur Dam, India 14.0 18.0 56.0 50.0 2.0–4.0 6.0–9.0 1.0–2.0 - - 12.60 [48]

    The entry is from 10.3390/ma14092211

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