Valorization of Bayer Red Mud in a Circular Economy Process: Valuable Metals Recovery and Further Brick Manufacture

Citation: Leiva, C.; Arroyo-Torralvo, F.; Luna-Galiano, Y.; Villegas, R.; Vilches, L.F.; Fernández Pereira, C. Valorization of Bayer Red Mud in a Circular Economy Process: Valuable Metals Recovery and Further Brick Manufacture. Processes 2022,10, 2367. https://doi.org/10.3390/pr10112367 Academic Editors: Lin He, Jun Han, Erhong Duan and Denny K. S. Ng Received: 6 September 2022 Accepted: 9 November 2022 Published: 11 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 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/). processes Article Valorization of Bayer Red Mud in a Circular Economy Process: Valuable Metals Recovery and Further Brick Manufacture Carlos Leiva , Fátima Arroyo-Torralvo * , Yolanda Luna-Galiano, Rosario Villegas, Luis Francisco Vilches and Constantino Fernández Pereira Departamento de Ingeniería Química y Ambiental, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain *Correspondence: fatimarr[email protected]; Tel.: +34-9544-82274 Abstract: In this work, the recovery of valuable metals from Bayer red mud using hydrometallurgical techniques and the subsequent use of the solid remaining after leaching as the principal component of the fired bricks were analyzed. Water, sulfuric acid, and sodium hydroxide were used as leaching agents. Different L/S ratios and contact times were also tested. According to technical, economic, and environmental considerations, the optimal conditions to recover valuable elements from red mud were 2 M H 2 SO 4 , in contact for 24 h, with an L/S ratio = 5. Under these conditions, high leaching yields of valuable elements such as La (47.6%) or V (11%) were achieved. After the leaching process, the remaining solid was mixed with clay and water to produce bricks. Two doses of red mud (50 and 80% w) and two different sintering temperatures (900 and 1100 ◦ C) were tested. When the proportion of treated RM in the mix was increased, the compressive strength of the bricks was reduced, but it was increased as the sintering temperature was increased. The environmental safety of the bricks manufactured (leaching of heavy metals and radionuclides) was also studied, and it was found that it was more favorable when red mud was treated instead of fresh red mud being used. Keywords: red mud; circular economy; hydrometallurgy; fired brick; natural radionuclides; heavy metals 1. Introduction Red mud (RM) is a hazardous waste left over from the Bayer process, in which alumina is extracted from bauxite by caustic digestion. Red mud is a mixture of the original bauxite components and other components formed or introduced by means of the Bayer process. In the process, it is necessary to use 1.9–3.6 tons of bauxite to produce 1 ton of alumina, depending on the mineral quality. The annual production of alumina in 2020 was 133 MM tons, which represents the generation of 175 MM tons of red mud [ 1 ]. As RM is considered a hazardous residue [ 2 ] due to its toxic/heavy and radioactive metal content and its high alkalinity, it is necessary to develop strategies to manage this product in a way that respects the environment. Recycling of RM has been studied previously in an attempt to find new applications for it in different sectors [ 3 ]. Red mud is also rich in different valuable metals, such as Fe, Ti, Al, Si, and Na as major elements; Mg, Ca, Mn, and V as minor elements [ 3 ]; and rare-earth elements [4]. Red mud has been widely studied for its application in building materials [ 4 ] for/in the preparation of cement and concrete [ 5 ], since red mud contains SiO 2 , Al 2 O 3 , and CaO, which could replace a portion of clay in cement and concrete. In addition, red mud has also been studied as raw material for glass in ceramic materials [6] and in the preparation of geopolymers [ 4 ]. In this last application, geopolymers could immobilize radioactive elements of red mud [7]. Metal oxides such as Al 2 O 3 , Fe 2 O 3 , and TiO 2 from RM are also used for the preparation of catalysts; many research works have studied the utilization of RM in different catalytic Processes 2022,10, 2367. https://doi.org/10.3390/pr10112367 https://www.mdpi.com/journal/processes Processes 2022,10, 2367 2 of 15 processes such as hydrogenation, transesterification, pyrolysis, and oxidation [ 4 ]. Red mud could also be used in adsorption processes due to its interesting specific surface area and fine particle size [ 8 ]. Thus, some works have been carried out to investigate the potential of RM in water treatment to remove heavy metals and other inorganic, organic, and biological contaminants [ 9 , 10 ]. Other studies have described its potential in gas treatment to fix carbon dioxide, nitride, sulfur, and fluoride compounds [4,11]. Fe, Al, Si, and Ti can be recovered by pyrometallurgical methods [3]. Hydrometallurgical methods can also be used to recover valuable components [ 4 ]; acid solutions (sulfuric, hydrochloric, and nitric), organic solutions (citric, acetic), ionic liquids, carbonate reagents, and bioleaching and alkaline reagents can be used to recover Fe, Al, Ti, Zr, Si, Ga, Sc, and rare-earth elements, etc. Magnetic separation is another technology studied to separate iron from RM [ 4 ]. Combined technologies (pyrometallurgical, hydrometallurgical, and magnetic) have also been used to recover Fe, Al, Ti, and Si [ 12 ]. Biological processes using fungi and bacteria have also been evaluated [13]. The aim of this work is to turn waste management into resource management. The idea that waste can be a source of raw materials or energy is not new. In all previously published valorization routes, RM has been proposed as a construction material or as a source of valuable metals; however, both routes seem mutually exclusive. In the present work, both routes are joined and studied together: first, the potential use of RM as a source of different elements released after acid attack and leaching, and subsequently the potential valorization of the leached RM as a construction material used for brick manufacturing, analyzing the implications of the extraction process on the properties of the red mud as components of bricks. This valorization strategy would have a double profit; on the one hand, the RM disposal problem could be reduced, and on the other hand, the recovery of valuable metals and the use of leached RM in brick formulations could produce a saving of raw materials, which could involve economic and environmental benefits. 2. Materials and Methods 2.1. Characterization of Red Mud and Clay RM from a Bayer process and clay (CL) were used. The major components of RM and CL have been published elsewhere [ 14 ]. Red mud contains a lower value of SiO 2 than CL (4.87% vs. 75.66 wt%). The contents of Al 2 O 3 in RM are higher than CL (18.08 vs. 11.25 wt%). The TiO 2 content of red mud is also high (9.33 wt%), while CL has a negligible titanium content. To analyze the possible recovery of the valuable elements of RM, a complete chemical characterization was performed that included minor components (Table 1). The major and trace components were performed by X-ray fluorescence spectroscopy analysis (Bruker AXS GmbH, Karlsruhe, Germany) at the Research, Technology, and Innovation Center at the University of Seville (CITIUS). Table 1. Minor components of red mud. Components mg/kg Components mg/kg Components mg/kg As 63.9 Mo 12.3 Ta 7.0 Ba 193.7 Nb 161.2 Th 121.8 Br 4.0 Nd 84.1 U 13.0 Ce 259.0 Ni 17.3 V 1169 Cr 1818.0 P 1210 W 48.2 Cu 74.9 Pb 62.0 Y 115.8 Ga 73.4 Sb 5.6 Zn 46.0 Hf 20.2 Sc 57.9 Zr 2117 In 14.7 Se 2.0 F 2118 La 112.6 Sn 9.3 S 514 Mn 276.4 Sr 93.9 Ag, Bi, Cd, Cl, Co, Cs, Ge, Hg, Rb, Sm, and Te non-detected Processes 2022,10, 2367 3 of 15 On the one hand, RM contained some heavy metals, such as chromium, barium, and vanadium, which could cause some leaching problems, and thorium which could cause some radiological problems. On the other hand, RM contained many elements included in the fourth (last) EU list of Critical Raw Materials [ 15 ]. In some cases, the contents of Ti, Ta, Ga, V, or La were considerably high and were like those reported by other authors [ 4 ]. In Table 1the Critical Raw Materials are written in bold. 2.2. Red Mud Valorization by Leaching Leaching experiments using aqueous solutions of H 2 SO 4 and NaOH were tested. The leaching tests were performed in two stages: first, a screening step was carried out to determine the extraction capacity of each leaching agent, and in a second step the leaching conditions were optimized. Table 2summarizes the test conditions carried out in the first and second stages. The influence of leaching time and the L/S ratio on the metal recovery from RM was evaluated using an experimental design. For this purpose, a two-factor factorial design was used that included 4 ×3 experiments. Table 2. Leaching tests carried out in stages 1 and 2. Screening Tests (Contact Time = 24 h and L/S = 10) Leaching agents Concentration Water - Sulfuric Acid 0.5 M, 1 M, 2 M Sodium hydroxide 0.5 M, 1 M, 2 M Optimization of leaching operating conditions (2 M H2SO4) L/S ratio 2, 5, 10 L/kg Contact time 1, 2, 6, 24 h The use of H 2 SO 4 as a leaching agent to recover different elements of RM has been previously described [ 16 , 17 ]. Sulfuric acid concentrations were selected based on the results of Cui et al. [ 18 ], which showed a high extraction yield of REE at room temperature when the H 2 SO 4 concentration was greater than 1 N (0.5 M). The use of NaOH as a leaching solution to valorize RM has also been previously described [18,19]. NaOH concentrations were based on a study performed by Borra et al. [ 16 ], which showed that at alkaline pH, the minimum leaching yield of some toxic elements was achieved at a pH around 13. The experimental extraction yields were fitted by a mathematical model using Stat- Ease Design-Expert software (Version 10, Minneapolis, MN, USA), and the analysis of variance was used for statistical analysis. The quality of fit of the polynomial model was expressed by R 2 , and its statistical significance was examined using the F test. The experiments were carried out in duplicate employing 100 g of RM. After contact, the liquid and solid phases were separated for chemical analyses. 2.3. Brick Manufacture The brick raw materials (red mud-treated (RM-T), clay (CL), and water) were first mixed for 4 min. Subsequently, cylindrical molds were filled with wet pastes and compressed at 5 MPa for five minutes. The characteristics of molding spectrum pressures were between 4 and 50 MPa [ 20 ]. Finally, the specimens were cured at 25 ◦ C for 48 h, followed by drying at 100 ◦ C for two days. Table 3shows the proportions of RM-T and CL, and the water/solid ratio. Two firing temperatures (900 and 1100 ◦ C) were tested. As can be seen in Table 3, the water/solid ratio was higher as the RM-T content increased to obtain a fluid mixture. Processes 2022,10, 2367 4 of 15 Table 3. Brick compositions (wt%). CL RM-T Water/Solid Ratio CL-900 100 0 0.12 RM-T-50-900 50 50 0.30 RM-T-80-900 20 80 0.35 CL-1100 100 0 0.12 RM-T-50-1100 50 50 0.30 RM-T-80-1100 20 80 0.35 2.4. Heating Method for Firing Bricks Different heating phases (Figure 1) were used: (A) from 25 ◦ C to 500 ◦ C at 100 ◦ C/h, (B) from 500 ◦ C to the final temperature (900 ◦ C or 1100 ◦ C) at 50 ◦ C/h, and (C) keeping the final temperature constant for 8 h and then cooling to room temperature. The two heating methods were selected because they were used in a previous work, with red mud without a previous leaching process, in order to analyze the difference between the recycling of both red muds [14]. Processes 2022, 10, x FOR PEER REVIEW 4 of 16 water/solid ratio. Two firing temperatures (900 and 1100 °C) were tested. As can be seen in Table 3, the water/solid ratio was higher as the RM-T content increased to obtain a fluid mixture. Table 3. Brick compositions (wt%). CL RM-T Water/Solid Ratio CL-900 100 0 0.12 RM-T-50-900 50 50 0.30 RM-T-80-900 20 80 0.35 CL-1100 100 0 0.12 RM-T-50-1100 50 50 0.30 RM-T-80-1100 20 80 0.35 2.4. Heating Method for Firing Bricks Different heating phases (Figure 1) were used: A) from 25 °C to 500 °C at 100 °C/h, B) from 500 °C to the final temperature (900 °C or 1100 °C) at 50 °C/h, and C) keeping the final temperature constant for 8 h and then cooling to room temperature. The two heating methods were selected because they were used in a previous work, with red mud without a previous leaching process, in order to analyze the difference between the recycling of both red muds [14]. Figure 1. Brick heating program. 2.5. Testing Methods 2.5.1. Materials Characterization A D8 Advance A25 equipment from BRUKER (Bruker AXS GmbH, Karlsruhe, Germany) was utilized for the X-ray diffraction of RM (before and after treatment), and DIFFRAC-EVA software (Bruker AXS GmbH, Karlsruhe, Germany) was used for phase identification. Phase identification and accurate quantitative phase analysis (amorphous and crystalline contents) are based on the RIR (reference intensity ratio) method. The software works with multiple reference databases (ICDD PDF2/PDF4+/PDF4 Minerals/PDF4 Organics databases) . Changes in mass versus temperature were carried out with duplicate thermogravimetric analysis. A TA Instrument analyzer (Q600, TA Instruments, Barcelona, Spain) using a heating rate of 20 °C/min from 20 °C to 1100 °C in an air atmosphere [21] was used. Figure 1. Brick heating program. 2.5. Testing Methods 2.5.1. Materials Characterization A D8 Advance A25 equipment from BRUKER (Bruker AXS GmbH, Karlsruhe, Germany) was utilized for the X-ray diffraction of RM (before and after treatment), and DIFFRAC-EVA software (Bruker AXS GmbH, Karlsruhe, Germany) was used for phase identification. Phase identification and accurate quantitative phase analysis (amorphous and crystalline contents) are based on the RIR (reference intensity ratio) method. The software works with multiple reference databases (ICDD PDF2/PDF4+/PDF4 Minerals/PDF4 Organics databases). Changes in mass versus temperature were carried out with duplicate thermogravimetric analysis. A TA Instrument analyzer (Q600, TA Instruments, Barcelona, Spain) using a heating rate of 20 ◦C/min from 20 ◦C to 1100 ◦C in an air atmosphere [21] was used. 2.5.2. Compressive Strength and Physical Tests According to EN 772-13 [ 22 ], the weight and volume of four samples were used to calculate the material’s bulk densities. According to EN 772-21 [ 23 ], the water absorption capacity was assessed in triplicate. A Tinius–Olsen machine (model TO 317, Tinius Processes 2022,10, 2367 5 of 15 Olsen Ltd., Surrey, England) was used to evaluate three samples of each composition’s compressive strength according to EN 772-1 [24]. 2.5.3. Environmental Assessment: Leaching Experiments An environmental evaluation must be carried out when a secondary raw material containing heavy metals is fired to manufacture bricks. The mass loss that occurs during the heating process causes an increment of the proportion of heavy metals (which are not volatilized) in the finished brick. However, the brick’s matrix is altered during the sintering process, and some heavy metals may end up stabilizing in this matrix. Since red mud retains some metals, two types of leaching tests (batch and monolithic) were tested on raw materials and final bricks, respectively, to achieve an extensive environmental characterization. EN 12457-4 [ 25 ] is a batch static extraction test for granular materials that agitates the sample for one day at a liquid/solid ratio of 10 L/kg. This test was conducted on RM and CL. The bricks underwent a monolithic test, as specified in NEN 7375 [26], which was the other leaching test employed. A water leaching solution with a pH of 7 must be changed eight times in accordance with NEN 7375. This test simulates the effect of rain, the main leaching agent outdoors. An ICP spectrometer (Agilent Technologies, Madrid, Spain) from the Research, Technology, and Innovation Center at the University of Seville (CITIUS) was used. Two samples of each composition were used for each of the two leaching tests. 2.5.4. Radionuclide Activity Particle sizes of bricks were reduced using a mill, then dust was introduced into an 80 cm3volume polystyrene Petri dish. To prevent the escape of 222-Rn, the dish was later vacuum-sealed in a plastic bag. The gamma emissions of 214-Pb can be used to estimate the activity of 226-Ra. The gamma emissions of 40 K at 1460 keV were used to directly assess its activity, and the activity concentration of 232-Th activity was obtained from the emissions of 228-Ac. A Canberra low-background high-purity germanium GR-6022 reverse electrode coaxial detector (Mirion Technologies (Canberra) GmbH, Hamburg, Germany) was employed as the main gamma-ray detector. It displays a 60% relative efficiency while being shielded by a 10 cm layer of high-purity lead. There were two composition measurements made. The minimum detectable activity and the decision level for 40-K, 214-Pb, and 228-Ac were calculated according to the ISO 11929-4 standard [ 27 ] at the Research, Technology, and Innovation Center at the University of Seville (CITIUS). 3. Results 3.1. Leaching Extraction Table 4shows the composition of the leachates obtained when the leaching agents were used in the screening stage. The pH values of the leachate varied between 0.3 and 0.8 in the case of sulfuric acid and between 13.2 and 13.3 in the case of NaOH. When only water was used, the pH was slightly acidic (5.4). The leaching yields when water was used as leaching agent were very low for almost all elements. Only 12% of Se was leached with water. When NaOH was used, a certain amount of V (5–7%) and Ga (3–4%) could be recovered. Se (31–72%) was also leached. In the H 2 SO 4 leaching tests, higher extraction yields were obtained. Ce and La were recovered from RM, achieving a yield greater than 60%. When the rest of the elements were classified according to their recovery percentage, four groups could be defined: •Less than 10%: As, Ba, Ga, Mo, Mn, Pb, Se; •10–20%: Cr, Cu, Ni, Ti, and V; •20–50%: Sb and Zn; •More than 50%: Ce, La, Sr, and Th. In pH-dependent leaching behavior, elements have been classified in cationic, anionic, or amphoteric patterns [ 18 ]. The leaching behavior of the studied elements suggested an anionic pattern for Se and a cationic pattern for Ce, La, Sr, Th, Sb, and Zn. The mechanism Processes 2022,10, 2367 6 of 15 of leaching for most elements from RM was dissolution [ 18 ]. In the case of the element with low recovery percentage, leaching was probably controlled by the dissolution of oxides, since in the case of elements showing high recovery yields, leaching probably occurred due to the dissolution of hydroxides. Table 4. Leachate composition in screening experiments. Water Sulfuric Acid Sodium Hydroxide 0.5 M 1 M 2 M 0.5 M 1 M 2 M pH 5.4 0.8 0.4 0.3 13.1 13.2 13.3 Component Metal content (ppb) As <20 230 290 550 200 340 310 Ba 9 67 80 175 <5 <5 12 Cr 2575 5400 13,890 19,000 2324 2200 1980 Cu 14 850 960 1030 <10 <10 <10 Mn <2 750 1347 1580 <5 <5 <5 Mo 27 <30 30 76 <100 <100 <100 Ni <5 66 110 187 <5 <5 <5 Pb <5 98 800 560 <25 <25 <25 Se 36 38 <30 <10 90 210 120 Sn <25 <50 <50 <50 <50 <50 <50 Sr 7 5945 5679 4710 <5 15 <5 Ti <5 63,000 452,000 612,000 <25 <25 <25 V 133 8260 16,800 17,900 7800 8340 5470 Zn 111 560 580 1500 170 140 490 Sb 31.5 <25 114 139 <25 <25 <25 Ga 18.3 360 522 700 288 290 225 Ce <25 12,000 16,700 17,100 <50 <50 <50 La <5 5690 7220 7437 <5 <5 237 Th <10 1860 5170 6270 <25 <25 <25 From the point of view of the valorization of RM and the recovery of valuable metals, H 2 SO 4 seems to be the best leaching option, which is consistent with other studies [ 28 , 29 ]. The most valuable metals potentially recovered by H 2 SO 4 leaching were Ni ( € 43/kg), Sn ( € 38/kg), Ga ( € 338/kg), V ( € 21/kg of V 2 O 5 ), La ( € 4/kg), and Th ( € 160/kg), according to the London Metal Exchange [30] and ISE [31]. According to Borra et al. [ 32 ], the main disadvantages of RM acid leaching are the acid consumption, the low-pH effluents handling, and the difficulty in using the residual RM after leaching. However, regarding the problem of acid effluents, it should be noted that the acid leachate would be sent to a metal recovery process, which in most cases will include one or more stages of precipitation at an elevated pH, thus implying the neutralization of the effluent [33]. To reduce the negative environmental impacts, residue reutilization is analyzed in another section. In relation to the volume of effluents generated during the valorization process, in the second phase of the leaching experiments, L/S was varied from 10 to 2 to reduce effluents. Furthermore, the contact time between the RM particles and H 2 SO 4 in the leaching stage was also studied. The results obtained using 2 M H 2 SO 4 as the leaching agent are shown in Table 4. 3.2. Mathematical Leaching Models Description and Statistical Evaluation From the data in Table 5, the extraction yields were calculated, and the results were analyzed using Design Expert. A statistical analysis was then performed to check the variation in performance of each element extraction yield with operating variables. As a result, a mathematical model was obtained that relates the leaching performance with the contact time (t) and the L/S ratio (L/S) for each element. All the models obtained were represented and can be seen in the Supplementary Materials. Processes 2022,10, 2367 7 of 15 Table 5. Leachates composition in optimization experiments. Contact Time 1 h 2 h 6 h 24 h L/S ratio 2 5 10 2 5 10 2 5 10 2 5 10 As (ppb) 700 680 500 756 570 500 785 507 500 1460 500 550 B (ppb) 100 100 100 100 100 100 100 100 100 200 100 175 Cr (ppb) 12,400 9920 5490 14,050 10,950 6650 29,128 12,400 7930 36,560 20,200 19,000 Cu (ppb) 1910 1330 670 2100 1290 690 2940 1320 740 4300 1540 1030 Mn (ppb) 1386 1020 530 1660 1130 650 4070 1390 860 4010 2220 1580 Mo (ppb) 100 100 100 100 100 100 100 100 100 100 100 76 Pb (ppb) 290 860 530 260 970 620 550 770 700 1700 1010 560 Sr (ppb) 9090 8410 5200 9790 7740 5340 12,109 7740 5350 18,530 8260 4710 Ti (ppb) 183 337 204 232 355 253 500 413 315 612 632 612 V (ppb) 32,160 27,570 14,860 32,710 24,920 15,330 39,100 25,010 16,160 60,800 27,110 17,900 Zn (ppb) 2840 810 1370 1780 710 410 2710 1470 985 3580 1750 1500 Ce (ppb) 19,150 17,470 10,330 20,520 16,890 11,100 30,800 17,290 11,570 46,090 20,420 17,100 La (ppb) 11,420 9340 5400 11,980 9090 5690 16,970 9180 5880 32,030 10,720 7437 Th (ppb) 6210 5580 3060 6620 5630 3430 11,290 5960 3760 15,370 7360 6270 Except in the case of the model of Ce, the mathematical models obtained were representative, with the p-value < 0.0001 in most cases, and < 0.001 in the models of Mn, Mo, Ti, and Th. Therefore, these models allowed for navigation in the study space. In the Supplementary Materials, the terms that should not be included in the models due to their low representativeness (p-value > 0.05) are marked in gray. The R 2 of the mathematical models that modeled the leaching of Ce, La, Pb, Th, and V were >0.75, and the rest were greater than 0.9. In the models obtained for all elements except Mn, the L/S ratio showed less influence on the extraction performance than the contact time. The maximum leaching yields of each element were reached for different combinations of operational factors (L/S ratio and leaching time) (see Supplementary Materials). The software Design Expert allows for the determination of the optimal combination that simultaneously satisfies the criteria placed on a group of responses (leaching yields). In this case, five objectives were imposed: maximize the recovery of elements such as La (1) or Ce (2), and minimize the presence of potentially hazardous elements such as As (3), Cr (4), and Th (5). The software determined that the optimal operating conditions to satisfy the objectives must be L/S = 10 and time = 24 h (Table 6). However, although the L/S ratio increased the overall extraction yield when a larger amount of leaching agent was used, some disadvantages arose because the metal concentration in the leachates was lower. This will probably penalize the subsequent recovery stage because the consumption of reagents in the next effluent management steps increases, and this entails an economic and environmental penalty. However, although the L/S = 2 ratio achieved a leachate with a very high concentration of metals, the recovery yields were lower than those achieved when higher L/S ratios were used. Considering the reasons mentioned above and the manageability of the mixture, the L/S = 5 ratio was finally chosen. Therefore, 2 M H 2 SO 4 was selected as the leaching solution, with a contact time of 24 h and an L/S ratio = 5 as the leaching conditions. Table 5displays the recovery yields. As can be observed, the yields obtained experimentally and those predicted by the mathematical model were very similar. Although in this work the conditions described above were selected, using this mathematical model it is possible to estimate the leaching yields reached in any operating conditions within the study range. Under the selected conditions (L/S = 5 and 24 h), acid leaching with 2 M H 2 SO 4 was carried out at 5 L/1 kg RM, so that there was enough solid waste to perform all the tests described below. Processes 2022,10, 2367 8 of 15 Table 6. Experimental and predicted leaching yields (%). As Ba Cr Cu Mn Mo Pb Ti V Zn La Ce Th L/S = 10 and 24 h of leaching time Experimental 2.5 0.5 32.0 7.9 9.7 1.6 1.8 2.6 15.3 32.6 66.0 56.0 51.5 Predicted 2.5 - 30.4 9.3 9.5 1.8 2.5 2.2 14.6 27.8 61.2 57.6 51.0 L/S = 5 and 2 h of leaching time Experimental 3.9 0.3 55.6 10.2 14.1 4.1 8.1 8.3 11.5 19.0 47.6 39.4 30.2 Predicted 5.5 - 58.6 11.2 15.2 3.5 7.2 10.2 10.6 20.4 47.7 41.7 31.3 3.3. Evaluation of the Solid Waste Remaining after Leaching The solid waste remaining after the acid leaching attack of RM (RM-T) was dried at 105 ◦ C for 24 h, and then its use for brick fabrication was evaluated. Figure 2shows the mineralogical composition of the RM before and after leaching extraction and the clay. XRD data of the as-received RM were previously described by Arroyo et al. [ 14 ]. A large peak in practically all of the two ranges was visible in the XRD pattern, which is indicative of an amorphous material (82.6%). Gibbsite, hematite, and titanium–nickel oxide were the main mineralogical phases observed in RM. Compounds of iron and molybdenum were also observed. The solid obtained after acid attack (RM-T) showed a similar pattern, with practically the same mineralogical phases (iron and molybdenum compounds practically disappeared). The amorphous content of RM-T was 73.4%, resulting in a reduction of 11.1%. This was the main effect of the attack on RM according to this technique. On the contrary, clay is a less amorphous material (its amorphous content measured by XRD reached 23.8%), with αquartz, muscovite, and nontronite as the main crystalline phases [14]. Processes 2022, 10, x FOR PEER REVIEW 8 of 16 achieved when higher L/S ratios were used. Considering the reasons mentioned above and the manageability of the mixture, the L/S = 5 ratio was finally chosen. Therefore, 2 M H 2 SO 4 was selected as the leaching solution, with a contact time of 24 h and an L/S ratio = 5 as the leaching conditions. Table 5 displays the recovery yields. As can be observed, the yields obtained experimentally and those predicted by the mathematical model were very similar. Although in this work the conditions described above were selected, using this mathematical model it is possible to estimate the leaching yields reached in any operating conditions within the study range. Under the selected conditions (L/S = 5 and 24 h), acid leaching with 2 M H 2 SO 4 was carried out at 5 L/1 kg RM, so that there was enough solid waste to perform all the tests described below. Table 6. Experimental and predicted leaching yields (%). As Ba Cr Cu Mn Mo Pb Ti V Zn La Ce Th L/S = 10 and 24 h of leaching time Experimental 2.5 0.5 32.0 7.9 9.7 1.6 1.8 2.6 15.3 32.6 66.0 56.0 51.5 Predicted 2.5 - 30.4 9.3 9.5 1.8 2.5 2.2 14.6 27.8 61.2 57.6 51.0 L/S = 5 and 2 h of leaching time Experimental 3.9 0.3 55.6 10.2 14.1 4.1 8.1 8.3 11.5 19.0 47.6 39.4 30.2 Predicted 5.5 - 58.6 11.2 15.2 3.5 7.2 10.2 10.6 20.4 47.7 41.7 31.3 3.3. Evaluation of the Solid Waste Remaining after Leaching The solid waste remaining after the acid leaching attack of RM (RM-T) was dried at 105 °C for 24 h, and then its use for brick fabrication was evaluated. Figure 2 shows the mineralogical composition of the RM before and after leaching extraction and the clay. XRD data of the as-received RM were previously described by Arroyo et al. [14]. A large peak in practically all of the two ranges was visible in the XRD pattern, which is indicative of an amorphous material (82.6%). Gibbsite, hematite, and titanium–nickel oxide were the main mineralogical phases observed in RM. Compounds of iron and molybdenum were also observed. The solid obtained after acid attack (RM-T) showed a similar pattern, with practically the same mineralogical phases (iron and molybdenum compounds practically disappeared). The amorphous content of RM-T was 73.4%, resulting in a reduction of 11.1%. This was the main effect of the attack on RM according to this technique. On the contrary, clay is a less amorphous material (its amorphous content measured by XRD reached 23.8%), with αquartz, muscovite, and nontronite as the main crystalline phases [14]. Figure 2. Mineralogical composition of red mud before and after the leaching process. Figure 2. Mineralogical composition of red mud before and after the leaching process. The particle size distribution of RM before and after the leaching attack is depicted in Figure 3. As can be seen, after leaching, particle size increased due to the agglomeration process, possibly produced during the leaching process. Clay (CL) was previously sieved prior to use, and its particle size was less than 150 µm. Thermogravimetric analysis of RM, RM-T, and CL was carried out, and the results are shown in Figure 4. Mass loss due to moisture was observed in the range of 20–200 ◦ C, which was higher in RM-T than in RM due to the leaching process. It is possible to see many endothermic peaks between 200 and 400 ◦ C because of the dehydration of gibbsite to produce boehmite and alumina [ 2 ]. Between 600 and 800 ◦ C, the decomposition of calcium carbonate is shown. Processes 2022,10, 2367 9 of 15 Processes 2022, 10, x FOR PEER REVIEW 9 of 16 The particle size distribution of RM before and after the leaching attack is depicted in Figure 3. As can be seen, after leaching, particle size increased due to the agglomeration process, possibly produced during the leaching process. Clay (CL) was previously sieved prior to use, and its particle size was less than 150 µm. Figure 3. Particle size distribution of red mud before and after leaching extraction. Thermogravimetric analysis of RM, RM-T, and CL was carried out, and the results are shown in Figure 4. Mass loss due to moisture was observed in the range of 20–200 °C, which was higher in RM-T than in RM due to the leaching process. It is possible to see many endothermic peaks between 200 and 400 °C because of the dehydration of gibbsite to produce boehmite and alumina [2]. Between 600 and 800 °C, the decomposition of calcium carbonate is shown. Figure 4. Thermogravimetric analysis of RM, RM-T, and CL. Figure 3. Particle size distribution of red mud before and after leaching extraction. Processes 2022, 10, x FOR PEER REVIEW 9 of 16 The particle size distribution of RM before and after the leaching attack is depicted in Figure 3. As can be seen, after leaching, particle size increased due to the agglomeration process, possibly produced during the leaching process. Clay (CL) was previously sieved prior to use, and its particle size was less than 150 µm. Figure 3. Particle size distribution of red mud before and after leaching extraction. Thermogravimetric analysis of RM, RM-T, and CL was carried out, and the results are shown in Figure 4. Mass loss due to moisture was observed in the range of 20–200 °C, which was higher in RM-T than in RM due to the leaching process. It is possible to see many endothermic peaks between 200 and 400 °C because of the dehydration of gibbsite to produce boehmite and alumina [2]. Between 600 and 800 °C, the decomposition of calcium carbonate is shown. Figure 4. Thermogravimetric analysis of RM, RM-T, and CL. Figure 4. Thermogravimetric analysis of RM, RM-T, and CL. In the case of CL, from 20 to 200 ◦ C, a mass loss of 4% was produced, which could be attributed to the loss of moisture and physically adsorbed water. Between 60 and 160 ◦ C, clay dehydration may be considered [ 34 ]. In the range of 200–400 ◦ C, the weight remained almost constant. Between 400 and 800 ◦ C, the combustion of some organic compounds [ 16 ] and the dehydroxylation of muscovite [ 35 ] could be responsible for the mass loss (3%) observed. Above 900 ◦C, no substantial weight changes were observed. 3.4. Physical Properties and Compressive Strength of Bricks Figure 5shows that a higher firing temperature produced a higher brick density with the same RM-T content. The flow of a viscous amorphous phase into the internal pores