materials Review Utilization of Red Mud as a Source for Metal Ions—A Review Sneha Samal   Citation: Samal, S. Utilization of Red Mud as a Source for Metal Ions—A Review. Materials 2021, 14, 2211. https://doi.org/10.3390/ma14092211 Academic Editor: F. Pacheco Torgal Received: 9 April 2021 Accepted: 22 April 2021 Published: 25 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the author. 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/). FZU-Institute of Physics of Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Prague, Czech Republic; samal@fzu.cz; Tel.: +42-266-05-2361 Abstract: An overview is presented on the prospective use of red mud as a resource in this review. 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. Therefore, a large-scale investigation on recycling of red mud for the extraction in the area of the metal recovery section will draw attention to the extensive use of red mud. Metal ions of major elements Fe (44 wt.%), Al (18.2 wt.%), Si (14.3 wt.%), Ti (9.3 wt.%), Na (6.2 wt.%), Ca (4.4 wt.%) as major elements and of Mg, V, Mn, Cr, K as minor elements and rare earth elements such as Ce (102 mg/kg), La (56 mg/kg), Sc (47 mg/kg), Nd (45 mg/kg), Sm (9 mg/kg). Moreover, an appropriate in-house metal recovery facility with the alumina industry will come out as a cost–benefit analysis. Keywords: red mud; resources; fly ash; reduced slag; metal ion recovery; mineralogical 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 [1]. 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 [2]. 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” [3]. 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 [4,5]. The recovery of critical raw materials from red mud involves many benefits including environmental, social, financial, economic, and technological benefits [6,7]. 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 Materials 2021, 14, 2211. https://doi.org/10.3390/ma14092211 https://www.mdpi.com/journal/materials