## 1 Introduction  
Red muds, or red sludges, are the most concerning and produced industrial waste from primary aluminum production, i.e., the Bayer process. Depending on the quality of the raw material processed, 1-2.5 tons of red mud is generated per ton of alumina produced [1; 2; 3]. From 2000 to 2018 alumina production increased from approximately 55 Mt to 130 Mt, mainly due to a fast growth in Chinese production capacity [4]. This means about 130-325 Mt of red mud produced in only 2018. This amount of hazardous waste still continues to be one of the most important disposal problems in the mining industry [5]. Red mud is composed of a mixture of metal oxides. The red color arises from iron oxides, which comprise up to 60% of the mass of the red mud. In addition, the other components include silica, unleached residual aluminum, titanium oxide [5; 6], and rare earth traces [7; 8]. Red muds are classified by the EEA (European Environment Agency) as special non-hazardous waste (EWC 01 03 09), however, a mirror entry exists in the European waste list (EWC 01 03 10 *) [9]. This means that the red muds can be classified as absolutely hazardous or not as a function of their chemical composition, further complicating their management. Due to their high alkalinity, with a pH ranging from 10 to 13 [2; 10; 11], red mud cannot be disposed easily and in most countries where red muds are produced, the cheapest method is to pump them into holding ponds or lagoons [10; 11]. Unfortunately, only 2-3% of plants dispose of the muds in the sea [12], posing risks to the marine ecosystem. An alternative and more expensive method consists of repeatedly rinsing the red mud. This red mud suspension, with a density between 1.2 and 1.3 g/cm\({}^{3}\) and a dry matter concentration from 250 to 350 g/L, is then dried and stacked (dry stacking or dry cake). However, this technique still leads to serious problems, as the cakes take up a large land area that can neither be built on nor farmed, even when the mud is completely dry. The liquid phase of the suspension contains about 7 g/L of Na\({}_{2}\)O, a serious pollutant due to its high alkalinity, which is dumped in selected and prepared areas, usually open fields, and convoyed to water treatment plants [3; 12; 13]. The recent ecological disaster occurred in October 2010 at Ajka (Hungary) has drawn attention to the worrying situation of the red mud basin owned by Euralumina in Portovesme (Italy). There, 20 Mm\({}^{3}\) of bauxite residues are lagooned on an area of 120 ha. Even if the Italian red mud is naturally inertized by the sea water (thanks to the MgO content that neutralizes the caustic soda through the precipitation of \"Bayer\" hydrotalcite Mg:A\({}_{2}\)OH\({}_{18}\)(CO\({}_{32}\)-, SO\({}_{4}\)\({}^{2-}\))\(\times\)H\({}_{2}\)O [14; 15]) its polluting footprint must be considered, especially related to toxic metal leaching. In fact, in Hungary, non-negligible concentrations of Cr\({}^{\text{VI}}\) into water supplies and surface waters were detected in a large contaminated area [16] from the Marcal and Danube rivers [17], as well as high concentrations of biologically available Al [18]. Many studies focusing on the red mud are mainly oriented to the recovery of alumina and soda [19; 20; 21] or iron, titanium dioxide, and other minor elements, such as vanadium and rare earths [7; 22; 23; 24; 25; 26; 27; 28; 29]. However, these processes could not be economically justified, leading to economic loss. In general, chemical processes aimed at recovering a constituent of red mud have suffered from inadequate revenues and the creation of further disposal problems. In particular, attempts for a utilization of red mud for iron production have a long history [30]. For instance, before and during the Second World War, the red mud was dried by the German iron and steel industry on deck ovens and sold as a catalyst for the liquid phase of the coal hydrogenation. The active component thereby formed finely divided iron hydroxide.