Electrochemical Performance of Ti Gr. 2 as Electrodes in Contact with Saline Suspension of Clays during the Electroflotation Process

Citation: Soliz, A.; Galleguillos- Madrid, F.M.; Cobos-Murcia, J.Á.; Angulo, S.; Salazar-Avalos, S.; Alonso-Fariñas, B.; Guzmán, A. Electrochemical Performance of Ti Gr. 2 as Electrodes in Contact with Saline Suspension of Clays during the Electroflotation Process. Appl. Sci. 2024,14, 8825. https://doi.org/ 10.3390/app14198825 Academic Editor: Angeles Sanroman Braga Received: 29 July 2024 Revised: 29 August 2024 Accepted: 6 September 2024 Published: 1 October 2024 Copyright: © 2024 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/). applied sciences Article Electrochemical Performance of Ti Gr. 2 as Electrodes in Contact with Saline Suspension of Clays during the Electroflotation Process Alvaro Soliz 1,* , Felipe M. Galleguillos-Madrid 2,* , José Ángel Cobos-Murcia 3, Sebastian Angulo 2, Sebastian Salazar-Avalos 2, BernabéAlonso-Fariñas 4and Alexis Guzmán1 1Departamento de Ingeniería en Metalurgia, Universidad de Atacama, Av. Copayapu 485, Copiapó1530000, Chile; [email protected] 2Centro de Desarrollo Energético Antofagasta, Universidad de Antofagasta, Antofagasta 1240000, Chile; [email protected] (S.A.); [email protected] (S.S.-A.) 3Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma del Estado de Hidalgo, Carr. Pachuca-Tulancingo km. 4.5, Mineral de la Reforma 42184, Hidalgo, Mexico; [email protected] 4Departamento de Ingeniería Química y Ambiental, Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain; [email protected] *Correspondence: [email protected] (A.S.); [email protected] (F.M.G.-M.) Abstract: The presence of clays in copper minerals has a significant negative impact during their processing, leading to low recoveries during the flotation process. In saline environments, the presence of these clays promotes operational problems associated with salinity, leading to decreases in the copper concentrate grade, alterations in the rheology of the mineral pulp, reduction in the selectivity of copper during the flotation process, declines in the quality of clarified water, and excessive corrosion of metallic components. This study explores the electroflotation of kaolinite and montmorillonite clays in NaCl solutions using a modified Hallimond tube coupled with Ti Gr. 2 electrodes for bubble generation via water electrolysis and the corrosion analysis of these electrodes applying the superposition model. The electroflotation results show recovery of clays close to 72.68% for kaolinite, 88.44% for montmorillonite, and 67.36% for a mixture of both clays. The presence of clays helps reduce the corrosive effects of Ti Gr. 2 from 0.069 A/m 2 in NaCl to 0.0073 A/m 2 in NaCl with montmorillonite clay. Keywords: kaolinite clay; montmorillonite clay; electroflotation; titanium Gr. 2 electrode; superposition model 1. Introduction Overexploitation of water sources in hyper-arid and arid regions, such as northern Chile, southern Peru, northern China, and parts of Africa, Asia, and Australia, has led to water scarcity, necessitating the search for new sources and better water management practices [ 1 – 3 ]. While mining globally uses a relatively small amount of water, it can be a significant consumer at the local level. In Chilean mining, approximately 3% of the total available water is utilized, drawn from three primary sources: continental water, seawater, and recirculated water. Continental water is sourced from underground aquifers, lakes, rivers, and wetlands. Seawater undergoes desalination by private entities and is then pumped to mining sites. Data from COCHILCO indicate that the use of continental water in 2022 decreased by 10.8% compared to its use in 2018 [ 4 ]. However, the use of this source has come under scrutiny due to its dwindling availability and increasing demand, prompting a shift towards seawater usage in mining operations over the years. Regarding continental water usage in mining, we can identify five primary areas: concentration processes, which emerge as the most water-intensive, consuming 74.3% of available resources; hydrometallurgy, which follows closely behind at 10.2%; the mining phase, which Appl. Sci. 2024,14, 8825. https://doi.org/10.3390/app14198825 https://www.mdpi.com/journal/applsci Appl. Sci. 2024,14, 8825 2 of 17 accounts for over 5.6% of water usage; smelting and refining operations, which utilize 3.6% of the total water volume; and other uses, including supply to third parties, accounting for 6.3% [ 4 ]. In all these cases, the reuse of water is a priority for the sustainability of the process. However, the recirculated water, used to minimize the consumption of freshwater, typically carries lower quality, containing dissolved waste and colloidal particles, such as dissolved components and especially clays, generating a negative effect on the process [ 5 ]. Clays are usually dispersed in mineral slurries as ultrafine particles, creating conditions for the slurry to behave as a non-Newtonian fluid that affects the entire mineral processing chain. In particular, the efficiency of flotation of copper sulfides is severely impacted by clay minerals. The long-term stability of clay colloidal particles in the electrolyte presents significant challenges in mineral processing due to the low shear strength of discarded solids in mineral operations, such as tailings, reduced consolidation and strength gain over time, and difficulties in water recovery during subsequent ore processing. These types of minerals can be separated by sedimentation and/or flotation processes, but ultrafine clays pose a challenge for mineral processing due to physical properties such as particle size, shape, and density. Parameters such as those mentioned are critical for understanding the behavior of these particles in saline solutions like seawater. The presence of soluble salts can disrupt the stability of clay colloids, enhancing sedimentation and consolidation, promoting strength gain, and improving water recovery. Increased water salinity improves the volume of froth floating by diminishing settling [ 6 ]. Particle weight is the dominant force that governs the behavior of coarse clay particles in suspension, whereas inter-particle forces, due to the small particle size and high specific surface area, primarily dominate the behavior of fine-grained particles. It has been reported that the aggregation rate of nanosized clay particles present in oil sand tailings increases with rising NaCl concentration and that the initial settling rate of oil sand tailings increases with higher pH and/or the presence of divalent cations (Ca2+ and Mg2+) [7]. The presence of clay significantly hampers copper ore flotation, leading to decreased recovery rates and a subsequent decline in concentrate grades. Clays are classified as swelling and non-swelling based on their water-absorption capabilities. Kaolinite is a non-swelling clay with the general composition Al 2 Si 2 O 5 (OH) 4 , which has features that lie in the prevalence of Al 3+ in its octahedral sites, where isomorphic substitutions occur with Mg 2+ , Fe 3+ , V 3+ , and Ti 4+ [ 8 ]. Montmorillonite is a welling clay with the general composition (Na, Ca) 0.3 (Al, Mg) 2 Si 4 O 10 (OH) 2· n(H 2 O), with a nanolayered structure consisting of stacked layers about 1 nm thick [ 9 ]. In mineral processing, kaolinite presents numerous challenges, including increased fines during size reduction stages (reaching sizes < 20 µ m), interfering with the flotation process by affecting bubble stability, altering foam rheology, compromising the selectivity of valuable ore recovery, and reducing the quality of recirculated water, particularly at alkaline pH levels (pH > 8), resulting in the accumulation of suspended particles. On the other hand, due to the small particle size of montmorillonite clay and its high charge density, this clay poses challenges in mineral processing because it tends to remain suspended in solution. In the presence of montmorillonite, it is crucial to adjust the solution ionic strength to avoid significant swelling and eventual disintegration of the clay. Notably, the abundance of Cl − ions and multivalent cations (Al 3+ , Ca 2+ , Mg 2+ , K + , and Na + ) in the electrolyte notably enhances the natural sedimentation of montmorillonite compared to their performance in pure water due to an increase in the ionic strength. A technique to improve water quality in the presence of clay particles is the application of the electroflotation process. This method employs an electrochemical approach to effectively eliminate suspended particles such as colloids, microalgae, or oil from seawater, often serving as a preliminary treatment step. A widespread use of this method is in municipal water treatment and the removal of microalgae or metal ions from municipal wastewater [ 10 – 16 ]. Despite these advancements, there remains limited research in the scientific literature related to the use of electroflotation with seawater in the mining industry [ 17 , 18 ]. Specifically, there is a lack of research on its efficacy in removing ultrafine mineral particles Appl. Sci. 2024,14, 8825 3 of 17 such as clays, pyrite [ 19 ], chalcopyrite [ 20 , 21 ], hematite [ 22 ], and other minerals [ 23 – 25 ]. The liquid–solid separation by electroflotation involves lifting ultrafine particles of clay to the water surface through microbubbles of H 2 and O 2 generated by the seawater splitting process via cathodic and anodic reactions [ 17 , 26 ]. The electroflotation process offers several key advantages, notably the absence of chemical additives and the ability to regulate bubble density within the reactor by adjusting the applied potential. The literature reveals that the electroflotation process is successfully applied in the recovery of fine and ultrafine particles compared to other conventional techniques such as froth flotation, ion flotation, solvent sublation, or absorbing colloid flotation. This effectiveness is mainly attributed to the production of smaller particle sizes, ranging between 5 and 50 µ m depending on the application, uniform bubble-particle formation, and high-frequency production, which can also be manipulated by varying operational variables such as pH, current density, type of electrodes, and others [ 18 , 27 ]. The application of electroflotation for clay removal in high salinity solutions like seawater (~0.5 M NaCl) has not been deeply studied. Corrosion in saline environments poses a significant threat to the durability and effectiveness of equipment used in electroflotation processes, ultimately impacting operational costs. By understanding the corrosion mechanisms inherent in salt electroflotation, we can make informed decisions regarding material selection for equipment construction under aggressive conditions. This proactive approach not only extends the equipment’s lifespan but also reduces maintenance expenses while preserving its mechanical integrity. Moreover, corrosion directly alters the properties of electrodes by modifying their surfaces and affecting other components of the electrochemical device [ 28 ]. This can lead to a decline in the flotation and recovery capacity of clays, further emphasizing the critical importance of addressing corrosion issues in saline electroflotation environments. The efficiency of ultrafine particle removal via electroflotation hinges on a series of surface electrode reactions and the inherent stability of particles, influenced by their hydrophobicity [17,25]. This work studies the electroflotation of kaolinite and montmorillonite clays, as well as their mixture, in a 0.5 M NaCl solution using Ti Gr. 2 electrodes as anode and cathode. The flotation process was conducted using a modified Hallimond tube-electroflotation device across a salinity range of 0.1 to 0.5 M of NaCl under different cell voltage and electroflotation times. Additionally, corrosion studies of the Ti Gr. 2 electrode in the clay slurries were carried out using the potentiodynamic polarization technique. This study significantly contributes to the understanding of the electroflotation process of saline clay slurries and the corrosion behavior through the use of mixed potential theory. 2. Materials and Methods 2.1. Electroflotation System The electroflotation tests were conducted using a modified Hallimond tube, specifically tailored for investigating conventional flotation processes. The Hallimond tube comprises an upper section featuring an angled tube that facilitates the ascent of the floated material to the surface, complemented by a conical receiver designed to capture the floated mass. At the base, provisions are made for air inlet and outlet for non-floated fractions. This bottom section was modified to accommodate an electrical component, housing connecting cables directly linked to the power source and electrodes, supplemented with rubber bands to insulate the electrolyte from the cables. This adaptation effectively repurposes the apparatus originally intended for air inflow, facilitating the generation of bubbles from the base. The cell used in this work had a volume of 250 cm 3 with two cylindrical electrodes of 4.4 cm 2 of Titanium Gr. 2 (Ti Gr. 2), obtained from Balance World Inc. (Putian, China), as shown in Figure 1. Appl. Sci. 2024,14, 8825 4 of 17 Appl.Sci.2024,14,xFORPEERREVIEW4of17  withtwocylindricalelectrodesof4.4cm 2 ofTitaniumGr.2(TiGr.2),obtainedfrom BalanceWorldInc.(Putian,China),asshowninFigure1.  Figure1.DiagramofthemodifiedHallimondtubeusedfortheelectroflotationtests. 2.2.MaterialsandSolutions Themontmorilloniteandkaolinitesamples,sourcedfromScienceWords(LosAngeles,CA,USA),underwentcomprehensivecharacterizationusingcutting‐edgetechniquessuchasscanningelectronmicroscopy(SEM)andenergy‐dispersiveX‐ray(EDS) analyzers(HitachiSU500,Hitachi,Japan,andZeissEVOMA10,Jena,Germany),and X‐raydiffraction(XRD)(BrukerAdvanceD8,Billerica,MA,USA).Testsolutionsofconcentrations0.1,0.3,and0.5MNaClweremeticulouslypreparedusingNaCl(analytical grade)anddeionizedwater.ThesolutionpHwasadjustedbyaddingdropsof1M NaOH.AllchemicalswerepurchasedfromMerck(Merck,Providencia,Chile). 2.3.ElectroflotationTests Fortheelectroflotationexperiments,threedifferentsyntheticsalineslurrieswere prepared:1g/Lofkaolinite,1g/Lofmontmorillonite,and1g/Lofthemixture1:1ofboth clays,consideringconcentrationsof0.1,0.3,and0.5MofNaCl.Theparticlesizesofkaoliniteandmontmorillonitewereequivalenttoa–635meshTylerwithad 80 of5µm.All electroflotationexperimentswerecarriedoutatroomtemperatureof19±0.5°Cand atmosphericpressure.Thepowersupplywasconnectedtotheanodeandcathode,operatingatfixedcellvoltages(E cell )of10,15,and20V,withelectroflotationtimesof10,15, and20min. Colloidalsuspensionpreconditioningwasperformedinanagitatortankfor30min at500rpmatafixedpHof8.Subsequently,forelectroflotation,NaClsolutionswere homogenizedfor15minat700rpmusingamagneticstirrer.Thesuspensionwas promptlytransferredtotheHallimondcelltoperformelectroflotationstudies.Upon completionoftheelectroflotationexperiment,thefloatedproductwasthoroughlyextractedusingaperistalticpumpandthencollectedinabeaker.Thecollectedproductwas thenfilteredusingavacuumpumpconnectedtoaBüchnerfunnelanddriedinauniversalovenat105°Cfor24h.Finally,themassofdriedclayparticleswasweightedonan analyticalscale.Theelectroflotationefficiency𝑅wasdeterminedusingthefollowing equation:𝑅  𝑚 𝑚∙100%(1) where𝑚istheinitialmassofclayfedtothemodifiedHallimondcell,and𝑚isthe totalmassrecoveredaftertheelectroflotationexperiments,includingTiflocsandclays. 2.4.ElectrochemicalandCorrosionStudiesforTiGr.2 Figure 1. Diagram of the modified Hallimond tube used for the electroflotation tests. 2.2. Materials and Solutions The montmorillonite and kaolinite samples, sourced from Science Words (Los Angeles, CA, USA), underwent comprehensive characterization using cutting-edge techniques such as scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analyzers (Hitachi SU 500, Hitachi, Japan, and Zeiss EVO MA 10, Jena, Germany), and X-ray diffraction (XRD) (Bruker Advance D8, Billerica, MA, USA). Test solutions of concentrations 0.1, 0.3, and 0.5 M NaCl were meticulously prepared using NaCl (analytical grade) and deionized water. The solution pH was adjusted by adding drops of 1 M NaOH. All chemicals were purchased from Merck (Merck, Providencia, Chile). 2.3. Electroflotation Tests For the electroflotation experiments, three different synthetic saline slurries were prepared: 1 g/L of kaolinite, 1 g/L of montmorillonite, and 1 g/L of the mixture 1:1 of both clays, considering concentrations of 0.1, 0.3, and 0.5 M of NaCl. The particle sizes of kaolinite and montmorillonite were equivalent to a − 635 mesh Tyler with a d 80 of 5 µ m. All electroflotation experiments were carried out at room temperature of 19 ± 0.5 ◦ C and atmospheric pressure. The power supply was connected to the anode and cathode, operating at fixed cell voltages (E cell ) of 10, 15, and 20 V, with electroflotation times of 10, 15, and 20 min. Colloidal suspension preconditioning was performed in an agitator tank for 30 min at 500 rpm at a fixed pH of 8. Subsequently, for electroflotation, NaCl solutions were homogenized for 15 min at 700 rpm using a magnetic stirrer. The suspension was promptly transferred to the Hallimond cell to perform electroflotation studies. Upon completion of the electroflotation experiment, the floated product was thoroughly extracted using a peristaltic pump and then collected in a beaker. The collected product was then filtered using a vacuum pump connected to a Büchner funnel and dried in a universal oven at 105 ◦ C for 24 h. Finally, the mass of dried clay particles was weighted on an analytical scale. The electroflotation efficiency RMF was determined using the following equation: RMF =mf mi ·100% (1) where mi is the initial mass of clay fed to the modified Hallimond cell, and mf is the total mass recovered after the electroflotation experiments, including Ti flocs and clays. 2.4. Electrochemical and Corrosion Studies for Ti Gr. 2 The electrochemical and corrosion behavior of Ti Gr. 2 in 0.5 M NaCl solutions, both with and without clays, were studied with a focus on the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and titanium oxidation reaction (TOR) as the main electrochemical partial reactions [ 29 – 34 ]. Potentiodynamic polarization measurements were conducted using Ti Gr. 2 working electrodes connected to a rotating disk electrode interface. These working electrodes were small cylindrical bars, with 4 mm in diameter Appl. Sci. 2024,14, 8825 5 of 17 and 10 mm in length, which were embedded concentrically into a PTFE tube with an 8 mm diameter and sealed with epoxy adhesive resin. Before potentiodynamic measurements, the electrodes were abraded and polished successively with silicon carbide sandpaper from 600 to 1200 mesh. The experiments were carried out in triplicate at a temperature of 20 ±0.5 ◦C by circulating hot water through a jacket cell. All potentials reported are referred to the standard hydrogen electrode (SHE). A BASI/RDE-2 rotating electrode interface connected to an Epsilon potentiostat/ galvanostat (Basi, IN, USA) was used for electrochemical measurements. Platinum wire as a counter electrode and Ag/AgCl (4 M KCl Sat.) as a reference electrode were used. Potentiodynamic polarization measurements were performed in a conventional 3-electrode cell following the methodology reported in previous works [ 32 , 35 ] and covering a potential range between − 1100 and 200 mV/SHE at a scan rate of 2 mV/s. The rotation electrode for the Ti Gr. 2 was 1200 rpm. 2.5. Kinetic Corrosion Analysis Corrosion of pure titanium can produce several anodic and cathodic reactions, including Ti species that interact with hydrogen protons and hydroxide ions depending on the solution pH [ 29 , 31 , 32 , 36 ]. In this work, the corrosion kinetic investigation was conducted utilizing non-linear fitting applied to experimental potentiodynamic curves, employing the superposition model based on mixed potential theory. This approach aligns with the methodology outlined in our prior research, where the total current density can be expressed as follows [32,35,37]: i=iO2+iH2+iTi (2) where irepresents the total current density; iO2 and iH2 denote the partial reduction current densities for ORR and HER, respectively; and iTi denotes the partial oxidation current density for TOR. Although direct experimental measurements of iO2 , iH2 , and iTi are not feasible, their values can be deduced using a fitting methodology to determine the kinetic parameters of each reaction. To achieve this, the electrochemical reactions of TOR and HER are considered to be controlled by charge transfer kinetics, while the ORR is controlled by a mixed-charge diffusion mechanism, according to the following expressions [35]: iH2=i0,H2×exp−2.303 ×ηH2 tH2(3) iO2=i0,O2×exp−2.303 ×ηO2 tO2×  1+ i0,O2×exp−2.303×ηO2 tO2 il,O2    −1 (4) iTi =i0,Ti ×exp2.303 ×ηTi tTi (5) where the ηO2=E−Eeq,O2 , ηH2=E−Eeq,H2 , and ηTi =E−Eeq,Ti are the ORR, HER, and TOR overpotentials, respectively; i0,O2 , i0,H2 , and i0,Ti are the exchange current densities for ORR, HER, and TOR, respectively; il,O2 is the limiting current density for ORR; tO2 , tH2 , and tTi are the Tafel slopes for ORR, HER, and TOR, respectively; and Eeq,O2 , Eeq,H2 , and Eeq,Ti are the equilibrium potentials which are equal to 752 mV, − 479 mV, and − 1804 mV for ORR, HER, and TOR, respectively, assuming that the main anodic reaction is Ti →Ti2++2e− [ 34 ]. The root mean square error (RMSE) was used to evaluate the quality of curve fitting. 3. Results 3.1. Mineralogical Characterization of Clays Figure 2shows morphological patterns of kaolinite and montmorillonite clays as received. For montmorillonite samples (Figure 2a,c), the morphology consists of grains Appl. Sci. 2024,14, 8825 6 of 17 with a flat shape and irregular edges, with particle sizes of 5 µ m. Conversely, for kaolinite clay (Figure 2b,d), the morphology consists of a structure of silicate sheets joined together with layers of irregular edges, approximately 1 µ m in length. Kaolinite presents particles of uniform composition and size, suggesting a more homogeneous structure compared to montmorillonite. Appl.Sci.2024,14,xFORPEERREVIEW6of17  mV,and–1804mVforORR,HER,andTOR,respectively,assumingthatthemainanodic reactionisTi →Ti2e[34].Therootmeansquareerror(RMSE)wasusedtoevaluatethequalityofcurvefitting. 3.Results 3.1.MineralogicalCharacterizationofClays Figure2showsmorphologicalpatternsofkaoliniteandmontmorilloniteclaysas received.Formontmorillonitesamples(Figure2a,c),themorphologyconsistsofgrains withaflatshapeandirregularedges,withparticlesizesof5µm.Conversely,forkaoliniteclay(Figure2b,d),themorphologyconsistsofastructureofsilicatesheetsjoinedtogetherwithlayersofirregularedges,approximately1µminlength.Kaolinitepresents particlesofuniformcompositionandsize,suggestingamorehomogeneousstructure comparedtomontmorillonite.  (a)(b)  (c)(d) Figure2.SEMimagesof(a,c)montmorilloniteand(b,d)kaoliniteclays. Figure3showsimagesoftheelementalanalysisforbothkaoliniteandmontmorilloniteclays,andTable1presentstheirchemicalcompositionexpressedinweightpercent (%).Forkaoliniteclay(Figure3a),siliconisthepredominantelementconstitutingit,followedbyaluminum,withaminorpresenceofironandtitanium.Ontheotherhand, montmorilloniteclay(Figure3b)exhibitsahigherpercentageofsiliconcomparedto aluminum,wherethesamplealsocontainssignificantamountsofcalcium,magnesium, andminoramountsofsodium.Thesecompositionsreflectthedistinctmineralogical characteristicsofkaoliniteandmontmorillonite,providinginsightsintotheirpotential effectsandbehaviorsinmineralprocessingapplications. Figure 2. SEM images of (a,c) montmorillonite and (b,d) kaolinite clays. Figure 3shows images of the elemental analysis for both kaolinite and montmorillonite clays, and Table 1presents their chemical composition expressed in weight percent (%). For kaolinite clay (Figure 3a), silicon is the predominant element constituting it, followed by aluminum, with a minor presence of iron and titanium. On the other hand, montmorillonite clay (Figure 3b) exhibits a higher percentage of silicon compared to aluminum, where the sample also contains significant amounts of calcium, magnesium, and minor amounts of sodium. These compositions reflect the distinct mineralogical characteristics of kaolinite and montmorillonite, providing insights into their potential effects and behaviors in mineral processing applications. Table 1. Chemical composition of kaolinite and montmorillonite clays. Elements Kaolinite, wt% Montmorillonite, wt% Fe 2.47 - Al 44.53 21.71 Si 48.57 67.48 Ti 4.44 - Na - 0.83 Mg - 1.81 S - 1.91 Ca - 6.25 Appl. Sci. 2024,14, 8825 7 of 17 Appl.Sci.2024,14,xFORPEERREVIEW7of17   (a)  (b) Figure3.ElementalmappingbyEDSanalysisof(a)kaoliniteand(b)montmorilloniteclays. Table1.Chemicalcompositionofkaoliniteandmontmorilloniteclays. ElementsKaolinite,wt%Montmorillonite,wt% Fe2.47‐ Al44.5321.71 Si48.5767.48 Ti4.44‐ Na‐ 0.83 Mg‐ 1.81 S‐ 1.91 Ca‐ 6.25 Figure 3. Elemental mapping by EDS analysis of (a) kaolinite and (b) montmorillonite clays. XRD analysis was performed to obtain comprehensive characterization of kaolinite and montmorillonite samples, providing valuable insights into their mineralogical composition as shown in Figure 4. The patterns have been analyzed based on Powder Diffraction File (PDF) cards. For the montmorillonite sample (Figure 4a), the results exhibited signals associated with quartz (SiO 2 ) (PDF: 05-0490) and three forms of montmorillonite: montmorillonite with a phase composed by MgOAl 2 O 3 5SiO 2· xH 2 O (PDF: 03-0014), montmorillonite with sodium content composed by Na x (Al, Mg) 2 Si 4 O 10 (OH) 2· zH 2 O (PDF: 12-0204), and montmorillonite with calcium content composed by Ca 0.2 (Al, Mg) 2 Si 4 O 10 (OH) 2· 4H 2 O (PDF: 13-0135). Conversely, XRD analysis for the kaolinite sample (Figure 4b) displayed patterns and signals corresponding specifically to kaolinite (PDF: 89-6538) and other minor phases such as quartz (SiO 2 ), magnetite (Fe 3 O 4 ), anatase (TiO 2 ), and rutile (TiO 2 ). Kaolinite and montmorillonite, despite their similar phyllosilicate structures, exhibit distinct compositions and morphologies that can significantly impact processing efficiency. In Appl. Sci. 2024,14, 8825 8 of 17 mineral processing operations, the presence of these clays plays a significant role where several challenges such as increased fines, compromised flotation selectivity, and water quality degradation are observed. Kaolinite’s ability to reach ultrafine sizes and interfere with bubble movement exacerbates these issues, while its chemical composition, mainly aluminum and silicon, influences its interactions with other minerals and chemicals. Furthermore, montmorillonite, with its diverse elemental composition including calcium, magnesium, and sodium, introduces additional hurdles due to its complex interactions with processing elements, suggesting potential interactions that may influence flotation behavior, rheological properties, and water quality. Understanding the mineralogical composition and elemental distribution of these clays is crucial for developing tailored strategies to optimize processing efficiency and minimize environmental impacts. Further research into clay–mineral interactions promises innovative solutions to address these challenges effectively. Appl.Sci.2024,14,xFORPEERREVIEW8of17  XRDanalysiswasperformedtoobtaincomprehensivecharacterizationofkaolinite andmontmorillonitesamples,providingvaluableinsightsintotheirmineralogical compositionasshowninFigure4.ThepatternshavebeenanalyzedbasedonPowder DiffractionFile(PDF)cards.Forthemontmorillonitesample(Figure4a),theresultsexhibitedsignalsassociatedwithquartz(SiO2)(PDF:05‐0490)andthreeformsofmontmorillonite:montmorillonitewithaphasecomposedbyMgOAl2O35SiO2·xH2O(PDF: 03‐0014),montmorillonitewithsodiumcontentcomposedbyNax(Al, Mg)2Si4O10(OH)2·zH2O(PDF:12‐0204),andmontmorillonitewithcalciumcontentcomposedbyCa0.2(Al,Mg)2Si4O10(OH)2·4H2O(PDF:13‐0135).Conversely,XRDanalysisfor thekaolinitesample(Figure4b)displayedpatternsandsignalscorrespondingspecificallytokaolinite(PDF:89‐6538)andotherminorphasessuchasquartz(SiO2),magnetite (Fe3O4),anatase(TiO2),andrutile(TiO2).Kaoliniteandmontmorillonite,despitetheir similarphyllosilicatestructures,exhibitdistinctcompositionsandmorphologiesthatcan significantlyimpactprocessingefficiency.Inmineralprocessingoperations,thepresence oftheseclaysplaysasignificantrolewhereseveralchallengessuchasincreasedfines, compromisedflotationselectivity,andwaterqualitydegradationareobserved.Kaolinite’sabilitytoreachultrafinesizesandinterferewithbubblemovementexacerbatesthese issues,whileitschemicalcomposition,mainlyaluminumandsilicon,influencesitsinteractionswithothermineralsandchemicals.Furthermore,montmorillonite,withits diverseelementalcompositionincludingcalcium,magnesium,andsodium,introduces additionalhurdlesduetoitscomplexinteractionswithprocessingelements,suggesting potentialinteractionsthatmayinfluenceflotationbehavior,rheologicalproperties,and waterquality.Understandingthemineralogicalcompositionandelementaldistribution oftheseclaysiscrucialfordevelopingtailoredstrategiestooptimizeprocessingefficiencyandminimizeenvironmentalimpacts.Furtherresearchintoclay–mineralinteractionspromisesinnovativesolutionstoaddressthesechallengeseffectively.  (a)(b) Figure4.XRDpatternsof(a)montmorilloniteand(b)kaoliniteclays. 3.2.ElectroflotationofClaysinNaClSolutions TheelectroflotationprocesswasperformedusingamodifiedHallimondtubewitha pairofcylindricalTiGr.2electrodes,whichwereconnectedfromthebottomofthedevicetoavariablepowersupplyasshowninFigure1.Theparameteremployedtoassess thewaterqualitytreatedbytheseelectrochemicalmethodsistheelectroflotationefficiency(Equation(1)).Figure5showsimagesoftheslurriesbeforeandaftertheelectroflotationprocess.Inthefigure,itispossibletoobservethechangesinthesolutionappearance,wherealargepartoftheclayswasseparated. 10 20 30 40 50 60 70            Quartz (PDF: 05-0490)  Montmorillonite (Mg, Al, Si) (PDF: 03-0014)  Montmorillonite (Na, Mg, Al, Si) (PDF: 12-0204)  Montmorillonite (Ca, Mg, Al, Si) (PDF: 13-0135)     Atomic unit (AU) 2   10 20 30 40 50 60 70                        Quartz (PDF: 05-0490)  Anatase (PDF: 89-4921)  Rutile (PDF: 21-1276)  Kaolinite (PDF: 89-6538)  Magnetite (PDF: 19-0629) Atomic unit (AU) 2  Figure 4. XRD patterns of (a) montmorillonite and (b) kaolinite clays. 3.2. Electroflotation of Clays in NaCl Solutions The electroflotation process was performed using a modified Hallimond tube with a pair of cylindrical Ti Gr. 2 electrodes, which were connected from the bottom of the device to a variable power supply as shown in Figure 1. The parameter employed to assess the water quality treated by these electrochemical methods is the electroflotation efficiency (Equation (1)). Figure 5shows images of the slurries before and after the electroflotation process. In the figure, it is possible to observe the changes in the solution appearance, where a large part of the clays was separated. During the electroflotation process, the produced foam revealed a sticky consistency, which is a characteristic inherent to clays when they are in contact with aqueous mediums. Through experiments employing applied cell potentials of 10, 15, and 20 V over durations of 10, 15, and 20 min, it was observed that the current intensity varies significantly, as does the amount of clay recovered over time, reaching a steady-state value close to 0.04 A at the end of the electroflotation process, as is tabulated in Table 2. Under these electroflotation conditions, no notable difference was observed in the cell current at the end of the process. However, changes in this variable could be attributed to several factors, including variations in the solution chemistry due to the presence of multiple ions such as Al +3 , Fe +2 , Fe +3 , Mg +2 , and Na+1, which come from the dissolution of kaolinite and montmorillonite clays due to their solubility [ 38 ]. Additionally, the cell current intensity can be affected by the reduction in the number of active sites on the anodic electrode surface and near the solid–liquid interface, caused by the oxidation of the anodic electrode and further formation of a passive Appl. Sci. 2024,14, 8825 9 of 17 film composed mainly of TiO 2 , following the sequence of electrochemical and chemical reactions [39]: Ti −2e− →Ti2+−e− →Ti3+−e− →Ti4+(6) Ti4++4OH−→TiO2+2H2O (7) Appl.Sci.2024,14,xFORPEERREVIEW9of17   (a)(b) Figure5.ModifiedHallimondtube,(a)beforeand(b)aftertheelectroflotationprocessofclays. Duringtheelectroflotationprocess,theproducedfoamrevealedastickyconsistency,whichisacharacteristicinherenttoclayswhentheyareincontactwithaqueousmediums.Throughexperimentsemployingappliedcellpotentialsof10,15,and20Vover durationsof10,15,and20min,itwasobservedthatthecurrentintensityvariessignificantly,asdoestheamountofclayrecoveredovertime,reachingasteady‐statevalue closeto0.04Aattheendoftheelectroflotationprocess,asistabulatedinTable2.Under theseelectroflotationconditions,nonotabledifferencewasobservedinthecellcurrentat theendoftheprocess.However,changesinthisvariablecouldbeattributedtoseveral factors,includingvariationsinthesolutionchemistryduetothepresenceofmultiple ionssuchasAl +3 ,Fe +2 ,Fe +3 ,Mg +2 ,andNa +1 ,whichcomefromthedissolutionofkaolinite andmontmorilloniteclaysduetotheirsolubility[38].Additionally,thecellcurrentintensitycanbeaffectedbythereductioninthenumberofactivesitesontheanodicelectrodesurfaceandnearthesolid–liquidinterface,causedbytheoxidationoftheanodic electrodeandfurtherformationofapassivefilmcomposedmainlyofTiO 2 ,followingthe sequenceofelectrochemicalandchemicalreactions[39]: Ti  󰇒 ⎯ ⎯ 󰇏 Ti  󰇒 ⎯ 󰇏 Ti  󰇒 ⎯ 󰇏 Ti(6) Ti4OH→TiO2HO(7) Althoughthecellcurrentvaluesdecreasedinmagnitude(rangingbetween0.03and 0.05A,equivalentto68and114A/m 2 )comparedtothebeginningoftheelectroflotation process(t=0min),underthestudiedconditions(10,15,and20V),waterelectrolysiswas notunfavorable,andoxygenandhydrogenbubblegenerationwasmaintainedovertime. Thisconfirmsthatcellcurrentdensitiesbetween68and114A/m 2 aresufficientforthis process.Thisobservationcouldberelatedtotheincreasedpassivefilmgrowthwithhigh anodicoxygenbubblegenerationatthebeginningoftheprocess,wheretransitionsinthe crystallinephaseofTiO 2 haveoccurred,whicharecausedbyhigherappliedcellcurrent densities[40,41].However,thegoodcorrosionresistanceofTiO 2 passivefilminsaline environmentscanbecompromisedbythehighcellvoltageandcurrentintensity,promotingtheformationoftitaniumoxychloridesoverthepassivefilmbytheactionof chlorideions,accordingtothechemicalreaction[29]: TiO2Cl→TiOCl2e(8) Table2summarizestheelectroflotationparametersforbothkaoliniteandmontmorilloniteandtheirmixturesundersimilarelectroflotationtimes,cellvoltages,andNaCl concentrations. Figure 5. Modified Hallimond tube, (a) before and (b) after the electroflotation process of clays. Table 2. Operational and response parameters obtained for the electroflotation of clays using a modified Hallimond tube with an anode and cathode made of Ti Gr. 2. Test Clay Type NaCl, M tEF, min Ecell, V Icell, A (t = 0 min) Icell, A (t = tEF)%RMF 1 Montmorillonite 0.1 10 10 0.14 0.05 1.49 2 0.3 15 15 0.17 0.05 43.52 3 0.5 20 20 0.40 0.03 72.68 4 Kaolinite 0.1 10 10 0.15 0.03 2.37 5 0.3 15 15 0.23 0.04 45.72 6 0.5 20 20 0.80 0.03 88.44 7 Mix (1:1 wt%) Kao./Mont. 0.1 10 10 0.40 0.04 2.33 8 0.3 15 15 0.56 0.04 31.36 9 0.5 20 20 0.71 0.05 67.36 Although the cell current values decreased in magnitude (ranging between 0.03 and 0.05 A, equivalent to 68 and 114 A/m 2 ) compared to the beginning of the electroflotation process (t = 0 min), under the studied conditions (10, 15, and 20 V), water electrolysis was not unfavorable, and oxygen and hydrogen bubble generation was maintained over time. This confirms that cell current densities between 68 and 114 A/m 2 are sufficient for this process. This observation could be related to the increased passive film growth with high anodic oxygen bubble generation at the beginning of the process, where transitions in the crystalline phase of TiO 2 have occurred, which are caused by higher applied cell current densities [ 40 , 41 ]. However, the good corrosion resistance of TiO 2 passive film in saline environments can be compromised by the high cell voltage and current intensity, promoting the formation of titanium oxychlorides over the passive film by the action of chloride ions, according to the chemical reaction [29]: TixOy+2Cl−→TixOyCl2+2e−(8) Appl. Sci. 2024,14, 8825 16 of 17 16. Zouboulis, A.I.; Matis, K.A. Cadmium Ion Removal by Electroflotation onto Sewage Sludge Biomass. Int. J. Environ. Waste Manag. 2012,9, 245–256. [CrossRef] 17. Madrid, F.M.G.; Arancibia-Bravo, M.P.; Sepúlveda, F.D.; Lucay, F.A.; Soliz, A.; Cáceres, L. Ultrafine Kaolinite Removal in Recycled Water from the Overflow of Thickener Using Electroflotation: A Novel Application of Saline Water Splitting in Mineral Processing. Molecules 2023,28, 3954. [CrossRef] 18. Kyzas, G.Z.; Matis, K.A. Electroflotation Process: A Review. J. Mol. Liq. 2016,220, 657–664. [CrossRef] 19. Kydros, K.A.; Gallios, G.P.; Matis, K.A. Electrolytic Flotation of Pyrite. J. Chem. Technol. Biotechnol. 1994,59, 223–232. [CrossRef] 20. Bhaskar Raju, G.; Khangaonkar, P.R. Electro-Flotation of Chalcopyrite Fines. Int. J. Miner. Process. 1982,9, 133–143. [CrossRef] 21. Makuei, F.; Tadesse, B.; Albijanic, B.; Browner, R. Electroflotation of Ultrafine Chalcopyrite Particles with Sodium Oleate Collector. Miner. Eng. 2018,120, 44–46. [CrossRef] 22. Hacha, R.R.; LeonardoTorem, M.; Gutiérrez Merma, A.; da Silva Coelho, V.F. Electroflotation of Fine Hematite Particles with Rhodococcus Opacus as a Biocollector in a Modified Partridge–Smith Cell. Miner. Eng. 2018,126, 105–115. [CrossRef] 23. Llerena, C.; Ho, J.C.K.; Piron, D.L. Effects of pH on Electroflotation of Sphalerite. Chem. Eng. Commun. 1996,155, 217–228. [CrossRef] 24. Liu, A.; Fan, P.; Han, F.; Han, H.; Li, Z.; Wang, H.; Fan, M. Effect of Electroflotation on Quartz and Magnetite and Its Utilization on the Reverse Flotation of Magnetic Separation Concentrate. Miner. Eng. 2022,175, 107292. [CrossRef] 25. Tadesse, B.; Albijanic, B.; Makuei, F.; Browner, R. Recovery of Fine and Ultrafine Mineral Particles by Electroflotation—A Review. Miner. Process. Extr. Metall. Rev. 2019,40, 108–122. [CrossRef] 26. Hacha, R.R.; Merma, A.G.; Couto, H.J.B.; Torem, M.L. Measurement and analysis of H 2 and O 2 bubbles diameter produced by electroflotation processes in a modified Partridge-Smith cell. Powder Technol. 2019,342, 308–320. [CrossRef] 27. Alam, R.; Shang, J.Q.; Khan, A.H. Bubble size distribution in a laboratory-scale electroflotation study. Environ. Monit. Assess. 2017,189, 193. [CrossRef] [PubMed] 28. Mraz, R.; Krysa, J. Dimensionally Stables Anodes with a Long Lifetime for Electroflotation. In Precision Process Technology; Springer: Berlin/Heidelberg, Germany, 1993; pp. 681–688. 29. El-Ghenymy, A.; Alsheyab, M.; Khodary, A.; Sirés, I.; Abdel-Wahab, A. Corrosion Behavior of Pure Titanium Anodes in Saline Medium and Their Performance for Humic Acid Removal by Electrocoagulation. Chemosphere 2020,246, 125674. [CrossRef] [PubMed] 30. Wang, Z.B.; Hu, H.X.; Liu, C.B.; Zheng, Y.G. The effect of fluoride ions on the corrosion behavior of pure titanium in 0.05 M sulfuric acid. Electrochim. Acta 2014,135, 526–535. [CrossRef] 31. Pei, D.-N.; Gong, L.; Zhang, A.-Y.; Zhang, X.; Chen, J.-J.; Mu, Y.; Yu, H.-Q. Defective titanium dioxide single crystals exposed by high-energy {001} facets for efficient oxygen reduction. Nat. Commun. 2015,6, 8696. [CrossRef] 32. Madrid, F.M.G.; Arancibia-Bravo, M.; Cisterna, J.; Soliz, Á.; Salazar-Avalos, S.; Guevara, B.; Sepúlveda, F.; Cáceres, L. Corrosion of Titanium Electrode Used for Solar Saline Electroflotation. Materials 2023,16, 3514. [CrossRef] 33. Kelly, E.J.; Bronstein, H.R. Kinetics and Mechanism of the Hydrogen Evolution Reaction on Titanium in Acidic Media. J. Electrochem. Soc. 1984,131, 2232–2238. [CrossRef] 34. Chen, C.C.; Chen, J.H.; Chao, C.G.; Say, W.C. Electrochemical characteristics of surface of titanium formed by electrolytic polishing and anodizing. J. Mater. Sci. 2005,40, 4053–4059. [CrossRef] 35. Soliz, A.; Cáceres, L. Corrosion behavior of carbon steel in LiBr in comparison to NaCl solutions under controlled hydrodynamic conditions. Int. J. Electrochem. Sci. 2015,10, 5673–5693. [CrossRef] 36. Wang, Y.; Fugetsu, B.; Sakata, I.; Fujisue, C.; Kabayama, S.; Tahara, N.; Morisawa, S. Monolayered Platinum Nanoparticles as Efficient Electrocatalysts for the Mass Production of Electrolyzed Hydrogen Water. Sci. Rep. 2020,10, 10126. [CrossRef] [PubMed] 37. Soliz, A.; Cáceres, L.; Pineda, F.; Galleguillos, F. Erosion–Corrosion of AISI 304L Stainless Steel Affected by Industrial Copper Tailings. Metals 2020,10, 1005. [CrossRef] 38. Werling, N.; Kaltenbach, J.; Weidler, P.G.; Schuhmann, R.; Dehn, F.; Emmerich, K. Solubility of calcined kaolinite, montmorillonite, and illite in high molar NaOH and suitability as precursors for geopolymers. Clays Clay Miner. 2022,70, 270–289. [CrossRef] 39. Zhong, Y.; Yang, Q.; Li, X.; Yao, F.; Xie, L.; Zhao, J.; Chen, F.; Xie, T.; Zeng, G. Electrochemically Induced Pitting Corrosion of Ti Anode: Application to the Indirect Reduction of Bromate. Chem. Eng. J. 2016,289, 114–122. [CrossRef] 40. Xing, J.H.; Xia, Z.B.; Hu, J.F.; Zhang, Y.H.; Zhong, L. Time dependence of growth and crystallization of anodic titanium oxide films in potentiostatic mode. Corros. Sci. 2013,75, 212–219. [CrossRef] 41. Liu, Z.; Liu, X.; Donatus, U.; Thompson, G.E.; Skeldon, P. Corrosion behaviour of the anodic oxide film on commercially pure titanium in NaCl environment. Int. J. Electrochem. Sci. 2014,9, 3558–3573. [CrossRef] 42. Hatch, C.D.; Wiese, J.S.; Crane, C.C.; Harris, K.J.; Kloss, H.G.; Baltrusaitis, J. Water Adsorption on Clay Minerals as a Function of Relative Humidity: Application of BET and Freundlich Adsorption Models. Langmuir 2012,28, 1790–1803. [CrossRef] 43. Chen, L.; Zhao, Y.; Bai, H.; Ai, Z.; Chen, P.; Hu, Y.; Song, S.; Komarneni, S. Role of Montmorillonite, Kaolinite, or Illite in Pyrite Flotation: Differences in Clay Behavior Based on Their Structures. Langmuir 2020,36, 10860–10867. [CrossRef] 44. Qiao, Y.; Qin, Y.; Zhou, H.; Yang, L.; Wang, X.; Wang, Z.; Liu, Z.; Zou, J. Electrochemical hydrogen charging on corrosion behavior of Ti-6Al-4V alloy in artificial seawater. Chin. J. Mech. Eng. 2024,37, 2. [CrossRef] 45. Xue, C.; Zhang, P.; Wei, D.; Hu, H.; Li, F.; Yang, K. Corrosion and Tribocorrosion Behaviors for TA3 in Ringer’s Solution after Implantation of Nb Ions. Appl. Sci. 2020,10, 8329. [CrossRef] Appl. Sci. 2024,14, 8825 17 of 17 46. Talat, R.; Asghar, M.A.; Liaqat, F.; Satti, M.; Fatima, S.; Haider, A.; Ali, S. Adsorption and anticorrosion studies of newly designed Schiff bases for the protection of EN3B mild steel in 3.5% NaCl solution: A combined experimental and theoretical approach. J. Mol. Liq. 2023,388, 122776. [CrossRef] 47. Lazi´c, M.M.; Miti´c, D.; Radovi´c, K.; Ðor ¯ devi´c, I.; Majeriˇc, P.; Rudolf, R.; Grgur, B.N. Corrosion Behavior of Nickel–Titanium Continuous-Casted Alloys. Metals 2024,14, 88. [CrossRef] 48. Kostelac, L.; Pezzato, L.; Colusso, E.; Natile, M.M.; Brunelli, K.; Dabalà, M. Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities. Appl. Sci. 2023,13, 12280. [CrossRef] 49. Tiwari, A.K.; Singh, R.K.; Ji, G. Investigation on corrosion behaviour of electrical discharge machined surfaces of Titanium-C2 in NaCl solution. Mater. Today Proc. 2023, in press. [CrossRef] 50. Fekry, A.M. The Influence of Chloride and Sulphate Ions on the Corrosion Behavior of Ti and Ti-6Al-4V Alloy in Oxalic Acid. Electrochim. Acta 2009,54, 3480–3489. [CrossRef] 51. Ganor, J.; Mogollón, J.L.; Lasaga, A.C. The effect of pH on kaolinite dissolution rates and on activation energy. Geochim. Et. Cosmochim. Acta 1995,59, 1037–1052. [CrossRef] 52. Rozalén, M.L.; Huertas, F.J.; Brady, P.V.; Cama, J.; García-Palma, S.; Linares, J. Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25 ◦C. Geochim. Cosmochim. Acta 2008,72, 4224–4253. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.