sign in sign up
<<
Home , Brookite, Ilmenite, Leucoxene, Ore

Article Extraction of from Low-Grade with Different Agents in Autoclave

Mario H. Rodriguez 1, Gustavo D. Rosales 1, Eliana G. Pinna 1 , Fernando M. Tunez 2 and Norman Toro 3,4,5,* 1 Laboratory of Extractive and Synthesis of Materials (MESiMat), ICB, UNCuyo, CONICET, FCEN, Padre Contreras 1300, General San Martín Park, CP M5502JMA Mendoza, Argentina; [email protected] (M.H.R.); [email protected] (G.D.R.); [email protected] (E.G.P.) 2 Institute of Research in Chemical Technology (INTEQUI) -UNSL-CONICET CC 290, CP 5700 San Luis, Argentina; [email protected] 3 Faculty of and Architecture, Universidad Arturo Prat, Almirante Juan José Latorre 2901, Antofagasta 1244260, Chile 4 Departamento de Ingeniería Metalúrgica y Minas, Universidad Católica del Norte, Antofagasta 1270709, Chile 5 Department of , Geological and Cartographic Department, Polytechnic University of Cartagena, 30203 Murcia, Spain * Correspondence: [email protected]; Tel.: +56-552651021

 Received: 26 March 2020; Accepted: 7 April 2020; Published: 9 April 2020 

Abstract: The progressive depletion of primary sources to obtain metals has led to the search for alternative sources for their recovery. In the particular case of titanium, titaniferous sands are a viable option for obtaining this . This paper presents the results of the dissolution of titanium from titaniferous sands of Buenos Aires province (Argentina) in a laboratory autoclave (450 mL of capacity). The operating parameters studied were as follows: different acids (HF, H2SO4 and mixtures of these acids); leaching agent concentration, 5–20% v/v; temperature, 75–150 ◦C; time, 30–180 min; solid–liquid ratio, 0.9–3.6% w/v; stirring speed, 110–550 rpm. The obtained results indicate that the increase in the leaching agent(s) concentration, temperature and time of contact with the acid mixtures have a marked effect on the dissolution reaction of titanium. Optimal conditions to achieve 89% extraction of titanium were obtained by leaching at 123 ◦C, 330 rpm, 80 min and 1.8% w/v with a mixture of 15% HF (v/v) and 10% H2SO4 (v/v).

Keywords: titanium; titaniferous sands; leaching; low-grade ore

1. Introduction Titanium is the ninth most abundant element and constitutes approximately 0.6% of the ’s crust [1]. It is resistant to corrosion, exceptionally light and extraordinarily strong, weighing 60% of by volume [2]. Due to these properties, this metal is used to manufacture parts of airplanes, spaceships, missiles and ships [3]. In addition, the biocompatible property stands out because the tissues of the human body tolerate its presence, which is why it is currently widely used in the manufacture of materials used in medicine, such as hip and knee prostheses, bone screws, antitrauma plaques, dental implants, heart valves and pacemakers. In addition, this metal is the main component of surgical instruments, such as scalpels, scissors and staples [4]. However, high production costs of metallic titanium have limited its use in some industries [5]. Nevertheless, it remains in the aerospace and biochemical industries, where the advantages of using titanium outweigh its cost [2]. Titanium, in its metallic form, cannot be easily found in nature. The main concentrated sources of titanium are the (FeOTiO2) and, with less content, the deposits of sands,

Metals 2020, 10, 497; doi:10.3390/met10040497 www.mdpi.com/journal/metals Metals 2020, 10, 497 2 of 11

such as , , and all with the TiO2 formua [3,5,6]. The importance of titaniferous sands depends on the type of occurrence, the ability to extract the elements and the exploitation of particular natural products of industrial interest (such as rutile, and ilmenite) [3]. Most of the world’s production of titanium (95%) is used to obtain as a pigment to disperse light [5]. This pigment is produced by two traditional processes: sulfate process (40% of total TiO2 production) and chloride process (60% of total TiO2 production) [5,7]. Chlorinated processes generally use the so-called Kroll method, which consists of reducing with ground in an argon atmosphere to prevent oxidation. TiCl4 is previously obtained by a chlorination process at 700–1200 ◦C, in which the formed chlorides are separated by distillation [8]. Another method used to obtain this element from ilmenite is the processing of the mineral with concentrated H2SO4 in a closed vessel at 170–200 ◦C, obtaining a precipitate of relatively pure dehydrated TiO2 [4]. Begum et al. conducted a study in which they leached in an acidic medium for the dissolution of titanium. In their tests, processes were carried out for 6 h at a temperature of 1000 ◦C by adding 0.5 g of ground anhydrous Na2CO3, after which the roasted material was contacted with H2SO4, varying the temperature, acid concentration and solid–liquid ratio. Finally, they concluded that the optimal operating conditions to achieve a 60% extraction of the titanium present in the sand were 120 min at 80 ◦C with 5 mol/L of H2SO4 [3]. On the other hand, the use of hydrochloric acid as a leaching agent, aimed at the benefit of the dissolution of titaniferous concentrates, has been the subject of numerous works considering concentrates of compositions, structures and textures that have very different reactivities and therefore diverse behaviors. Among them, we can mention those carried out by Tedesco and Rumi, who found that leaching in HCl is selective for Fe and Ti [9]. In this same medium, Tavani et al. managed to concentrate on the Ti residue, dissolving practically all the Fe present in the sands [10]. Years later, Sarno and Avanza proposed an appropriate method to leach titaniferous sands, which consists of successive leaching at low concentrations of HCl, thereby increasing the dissolution of Fe and decreasing that of Ti, achieving greater selectivity in the process [11]. These same authors also conducted kinetic studies in which they recommend that experiments must be carried out for each particular case, with the results only being appropriate to use for the design within the range of variables studied, due to the complexity of the sample [12]. In the present work, the reaction of titanium extraction from titaniferous sands in an acidic medium was comparatively studied. In addition, the effect of the operating parameters of the leaching reaction was investigated using a laboratory autoclave. In an effort to develop a novel route for titanium extraction from low-grade minerals, this will allow for a process that takes advantage of disused mining resources with low environmental impact.

2. Materials and Methods

2.1. Titanium Ore The titaniferous sands used in this work were extracted from San Blas, located in the town of Bahía Blanca in the province of Buenos Aires, Argentina. The particle size used was 297 + 105 µm; − the particle fraction was separated using sieves. The characterization of the ore by X-ray diffraction (XRD) was carried out on a Rigaku D-Max III C diffractometer operated at 35 kV and 30 mA, using Cu Kα radiation and Ni filter, λ = 0.15418 nm (Rigaku, Osaka, Japan). The experimental diffractograms were fitted with the software X’pert High Score 2.1.2, with Al2O3 (NYST) as reference. Figure1 shows the di ffractogram of the titaniferous sands, in which it is observed that the mineralogical composition was as follows: ilmenite (ICDD 01-075-1212), (ICDD 01-088-0315), (ICDD 01-089-2810), Fe2TiO5 (ICDD 01-087-1996), calcium silicate (ICDD 01-086-0401) and two phases (ICDD 01-083-0541) and (00-033-1161). Metals 20192020, 109, ,x 497FOR PEER REVIEW 3 of 12 11 Metals 2019, 9, x FOR PEER REVIEW 3 of 12

Figure 1. XX-ray-ray diffractogram diffractogram for the titaniferous sand. Figure 1. X-ray diffractogram for the titaniferous sand. The titanium content of the sample of these titaniferous sands was 3.9% 3.9%,, as dete determinedrmined by X X-ray-ray fluorescencefluorescenceThe titanium (XRF) content in a a Shimadzu Shimadzu of the sample EDX-8000 EDX of-8000 these (Shimadzu,(Shimadzu, titaniferous Kioto, Kioto,sands was Japan). Japan) 3.9%. This, as dete determinationrmined by X was-ray performedfluorescence by (XRF) the method in a Shimadzu of standard EDX addition.-8000 Using(Shimadzu, this calibration Kioto, Japan) technique . This makes determination it possible was to compensateperformed by and the solve method the ofeffects eff standardects of interference addition. Using due to this differences diff calibrationerences in technique the physical makes properties it possible of the to standardcompensate solutions and solve and the the effects sample of and interference some interference due to differences byby thethe matrix.matrix. in the physical properties of the standardThe oresolutions morphologymorphology and the waswas sample studied studied and through throughsome interference scanning scanning electron electronby the microscopymatrix. microscopy (SEM) (SEM) in ain LEO a LEO 1450 1450 VP VPmicroscope microscopeThe ore (Zeiss, morphology (Zeiss, Jena, Jena, Germany), was Germany) studied and the, throughand semiquantitative the semiquantitativescanning electron composition composition microscopy of some of particles(SEM) some in particles was a LEO obtained 1450was obtainedbyVP electron by probe electron (Zeiss, microanalysis Jena, probe Germany) microanalysis (EPMA), usingand th an (EPMA)e semiquantitative EDAX energyusing andispersive composition EDAX X-ray energy of spectrometer some dispersive particles Genesis X -wasray spectrometer2000obtained (EDAX, by Mahwah,Genesis electron 2000 NJ, probe (EDAX, USA). microanalysis Figure Mahwah2 shows, NJ (EPMA), USA) a micrograph. Figure using 2 an ofshows the EDAX untreated a micrograph energy mineral, dispersive of the previously untreated X-ray mineral,calcined.spectrometer previously It shows Genesis that calcined. 2000 the particles(EDAX, It shows Mahwah exhibit that the irregular, NJ particles, USA) edges. exhibitFigure and 2irregular shows shapes, a edgesandmicrograph some and haveshapes, of the flat anduntreated faces. some In mineral, previously calcined. It shows that the particles exhibit irregular edges and shapes, and some Tablehave 1flat, the faces. EDS In analysis Table 1, of the the EDS particles analysis marked of the in particles Figure2 markedis shown. in Figure 2 is shown. have flat faces. In Table 1, the EDS analysis of the particles marked in Figure 2 is shown.

Figure 2. MicrographMicrograph of titaniferous sands prior to heat treatment. Figure 2. Micrograph of titaniferous sands prior to heat treatment. Table 1. EDS analysis, in atomic percentage. Table 1. EDS analysis, in atomic percentage. Region AnalyzedTable %Fe 1. ED %OS analysis, %Ti in atomic %Sipercentage. %Ca %Mg %Al Region Analyzed %Fe %O %Ti %Si %Ca %Mg %Al RegionPointPoint Analyzed 1 1 61.461.4%Fe 29.729.7%O 3.9%Ti3.9 2.6%Si2.6 0.5%Ca0.5 -%Mg- 1.9%1.9Al PointPointPoint 21 2 49.7449.7461.4 20.7820.7829.7 1.791.793.9 20.6220.622.6 2.912.910.5 4.164.16- - 1.9- Point 2 49.74 20.78 1.79 20.62 2.91 4.16 - PointPoint 3 3 44.0444.04 18.118.1 2.032.03 26.2526.25 3.463.46 5.095.09 1.031.03 Point 3 44.04 18.1 2.03 26.25 3.46 5.09 1.03 The results of Table 1 state that these arenas have high Fe and O contents and small amounts of The results of Table1 state that these arenas have high Fe and O contents and small amounts of Si, Si, Ti Theand resultsCa. The of high Table Fe 1 content state that is duethese to arenas the presence have high of hematite Fe and Oand contents ilmenite, and while small the amounts presence of TiSi, and Ti and Ca. Ca. The The high high Fe contentFe content is due is due to the to presencethe presence of hematite of hematite and ilmenite,and ilmenite, while while the presence the presence of Si of Si is due to the occurrence of the quartz and CaSiO4 (calcium silicate) phases. Ti is present in the of Si is due to the occurrence of the quartz and CaSiO4 (calcium silicate) phases. Ti is present in the

Metals 2020, 10, 497 4 of 11

is due to the occurrence of the quartz and CaSiO4 (calcium silicate) phases. Ti is present in the form of ilmenite and titanium (pseudo brookite). Finally, Mg could come from the structure MgSiO3 (enstatite). In addition, enstatite was not identified by XDR. These results are coincident with those obtained by XRD.

2.2. Reactant and Leaching Tests The experimental tests were carried out in a Parr reactor, built in Monel , with a capacity of 450 mL and equipped with electromagnetic agitation, a heating mantle and a control unit. To this device, 5 g of the sample and a volume of 275 mL of leaching solution were added. Then, N2 (gas) was bubbled in order to remove air and thus reduce the corrosive effects on the reactor of O2 dissolved in the hot HF medium. The mixture was subsequently heated with stirring and a heating program between 5 and 10 ◦C/min, depending on the working temperature. The reaction time was measured from the moment the set temperature for each test was reached. After the leaching tests, the reactor was allowed to cool for approximately 25 min without stirring. The reactor contents were subsequently filtered through blue band filter paper. Then, the paper with the unreacted solid was dried and calcined, and the resulting residue was stored for subsequent characterization by XRD, SEM and EDS. Titanium was analyzed by XRF, using the calibration curve method to calculate the extraction percentage in all the experiments by the following equation:

Ti f %X = 100 Tim where Tim is the initial amount of Ti in the ore, and Tif is the quantity of Ti from leach liquors. The operating parameters studied were as follows: different acids (HF, H2C2O4 and H2SO4) and mixtures of these acids (HF–H2C2O4 and HF–H2SO4); leaching agent concentration, 5–20% v/v; temperature, 75–150 ◦C; time, 30–180 min; solid–liquid ratio, 0.9–3.6% w/v; stirring speed, 110–550 rpm. The experimental study of this work was performed by univariate analysis. In order to evaluate the experimental error, each test was replicated three times. The average extraction efficiency and standard deviation were calculated for each parameter studied.

3. Results

3.1. Type of Leaching Agent

The leaching agents studied were HF, H2C2O4,H2SO4, HF–H2C2O4 and HF–H2SO4. In all tests, the other operating parameters were kept constant at the following values: temperature, 123 ◦C; reaction time, 80 min; solid–liquid ratio, 1.82% w/v; stirring speed, 330 rpm; concentration of the leaching agent(s) between 10% and 15% v/v. The obtained results are shown in Figure3. As seen in Figure3, the best titanium extraction was achieved using the mixture of sulfuric acid–hydrofluoric acid and hydrofluoric acid at 123 ◦C. These results can be explained taking into 4+ account the high affinity of the Ti cation with the F− anion to form stable aqueous complexes to 2 satisfy its coordination sphere, forming [TiF6]− complex, which in an acid medium forms H2[TiF6] (Reactions 1 and 2) [8]. In the case of the HF/H2SO4 mixture, the effect is greater due to a synergistic 2 effect, where firstly the HF acts according to Reactions 1 and 2, and then the [TiF6]− complex reacts with H2SO4 to give Ti(SO4)2 and HF (aq) which can react again with the titanium minerals in the titaniferous sands (Reaction 3). According to these results, these leaching agents were chosen to carry out the study of the other operative variables. From these results, the dissolution reactions of the Metals 2020, 10, 497 5 of 11

titanium ore with HF and H2SO4 were proposed, which would be the leaching agents that extract the titanium from both minerals:

FeTiO + 7 HF H TiF + FeOF + 2 H O + 1/2H (1) 3(s) (aq) → 2 6(aq) (s) 2 2(g) Fe TiO + 8 HF H TiF + 2 FeOF + 3 H O (2) 2 5(s) (aq) → 2 6(aq) (s) 2 Metals 2019, 9, x FOR PEER REVIEW 5 of 12 H TiF + 3 H SO Ti(SO ) + 6HF + 3 H O (3) 2 6(aq) 2 4(aq) → 4 2(aq) (aq) 2

Figure 3. Effect of the leaching agent. Figure 3. Effect of the leaching agent. These reactions were obtained using HSC 5.1 software (,Espoo, ), based on theAs construction seen in Figure of the 3, the potential best titanium versus pHextraction diagrams. was achieved using the mixture of sulfuric acid– hydrofluoric acid and hydrofluoric acid at 123 °C. These results can be explained taking into account 3.2.the Temperaturehigh affinity E ffofect the Ti4+ cation with the F− anion to form stable aqueous complexes to satisfy its −2 coordinationThe results sphere, of the forming influence [TiF of6] temperature complex, which on thein an extraction acid medium reaction form ofs H titanium2[TiF6] (Reaction from thes 1 titaniferousand 2) [8]. In sands the case are of shown the HF/H in Figure2SO4 mixture,4, working the ineffect the followingis greater due conditions: to a synergistic solid–liquid effect, ratio,where firstly the HF acts according to Reactions 1 and 2, and then the [TiF6]−2 complex reacts with H2SO4 to 1.82% w/v; stirring speed, 330 rpm; reaction time, 80 min; leaching agents HF (15% v/v) and H2SO4 Metals 2019, 9, x FOR PEER REVIEW 6 of 12 (10%give vTi(SO/v)–HF4)2 (15% and HFv/v). (aq) which can react again with the titanium minerals in the titaniferous sands (Reaction 3). According to these results, these leaching agents were chosen to carry out the study of the other operative variables. From these results, the dissolution reactions of the titanium ore with HF and H2SO4 were proposed, which would be the leaching agents that extract the titanium from both minerals:

FeTiO3(s) + 7 HF(aq)  H2TiF6(aq) + FeOF(s) + 2 H2O + 1/2H2(g) (1)

(2) Fe2TiO5(s) + 8 HF(aq)  H2TiF6(aq) + 2 FeOF(s) + 3 H2O

(3) H2TiF6(aq) + 3 H2SO4(aq)  Ti(SO4)2(aq) + 6HF(aq) + 3 H2O

These reactions were obtained using HSC Chemistry 5.1 software (Outotec,Espoo, Finland), based on the construction of the potential versus pH diagrams.

3.2. Temperature Effect Figure 4. Effect of reaction temperature. The results of the influenceFigure of temperature 4. Effect of reaction on the temperature. extraction reaction of titanium from the titaniferous sands are shown in Figure 4, working in the following conditions: solid–liquid ratio, As seen in Figure 4, the increase in temperature to an increase in the dissolution of 1.82% w/v; stirring speed, 330 rpm; reaction time, 80 min; leaching agents HF (15% v/v) and H2SO4 titanium. The highest extraction values are achieved by leaching with the mixture of sulfuric acid and (10% v/v)–HF (15% v/v). hydrofluoric acid at 150 °C (92.0% and 70.5% respectively). However, important dissolution values are obtained at 123 °C for both leaching media. These results are as expected for a solid–liquid solution reaction since the magnitude of the dissolution of the metals present in the solid depends on the temperature (the general rule is that the increase of the temperature also increases the extent of the dissolution). In addition, it should be taken into account that the increase in temperature also increases the reactivity of solids [13–16]. Thus, 123 °C was selected to continue the study of the other operating parameters.

3.3. Reaction Time Effect

The results of the effect of time on the dissolution reaction of titanium are shown in Figure 5. The other operating parameters were kept constant at the following values: solid–liquid ratio, 1.82% w/v; stirring speed, 330 rpm; temperature, 123 °C ; leaching agents HF (15% v/v) and H2SO4 (10% v/v)–HF (15% v/v).

Figure 5. Effect of leaching time.

The results shown in Figure 5 show that the dissolution of titanium increases with the increase in reaction time, for the three leaching media studied. In addition, above 120 min of reaction, the

Metals 2019, 9, x FOR PEER REVIEW 6 of 12

Figure 4. Effect of reaction temperature. Metals 2020, 10, 497 6 of 11 As seen in Figure 4, the increase in temperature leads to an increase in the dissolution of titanium.As seen The in highest Figure4 extraction, the increase values in temperature are achieved leads by leaching to an increase with the in mixture the dissolution of sulfuric of titanium. acid and Thehydrofluoric highest extraction acid at 150 values °C are(92.0 achieved% and 70.5% by leaching respectively). with the mixtureHowever, of sulfuricimportant acid dissolution and hydrofluoric values are obtained at 123 °C for both leaching media. These results are as expected for a solid–liquid acid at 150 ◦C (92.0% and 70.5% respectively). However, important dissolution values are obtained solution reaction since the magnitude of the dissolution of the metals present in the solid depends on at 123 ◦C for both leaching media. These results are as expected for a solid–liquid solution reaction sincethe temperature the magnitude (the of general the dissolution rule is that of thethe metalsincrease present of the intemperature the solid depends also increases on the temperaturethe extent of (thethe generaldissolution). rule isIn that addition, the increase it should of the be temperaturetaken into account also increases that the the increase extent in of temperature the dissolution). also Inincreases addition, the it shouldreactivity be takenof solids into [13 account–16]. Thus, that the123 increase °C was inselected temperature to continue also increases the study the of reactivity the other operating parameters. of solids [13–16]. Thus, 123 ◦C was selected to continue the study of the other operating parameters.

3.3.3.3. ReactionReaction TimeTime EEffectffect TheThe resultsresults of of the the e ffeffectect of of time time on on the the dissolution dissolution reaction reaction of titanium of titanium are shownare shown in Figure in Figure5. The 5. otherThe other operating operating parameters parameters were were kept kept constant constant at the at following the following values: values: solid–liquid solid–liquid ratio, ratio 1.82%, 1.82%w/v; stirringw/v; stirring speed, speed, 330 rpm; 330 temperature, rpm; temperature, 123 ◦C; 123 leaching °C ; leaching agents HFagents (15% HFv/v ()15% and v/v H2SO) and4 (10% H2SOv/v4 )–HF(10% (15%v/v)–vHF/v). (15% v/v).

FigureFigure 5.5.E Effectffect ofof leachingleaching time.time.

The results shown in Figure5 show that the dissolution of titanium increases with the increase in The results shown in Figure 5 show that the dissolution of titanium increases with the increase reaction time, for the three leaching media studied. In addition, above 120 min of reaction, the plateau in reaction time, for the three leaching media studied. In addition, above 120 min of reaction, the of the curve is reached. The synergistic effect can also be observed using the HF/H2SO4 mixture, since the extraction values are considerably improved if they are compared with the experiments that work with pure acids. The optimal extraction of titanium, 89%, was achieved at 80 min for the lixiviant H2SO4–HF medium (10% v/v and 15% v/v, respectively). These results are expected since fluid–solid reactive reactions are generally slow [13–16]. Similar results to ours were obtained by Das et al., Agatzini et al., Begum et al., Habashi, Nguyen and Lee and Haverkamp et al. [1–6], who conducted operating parameter studies on sands, mud and minerals with sulfuric and hydrochloric media, finding in all cases that the increase in reaction time increases the dissolution of these materials.

3.4. Concentration Effect of Leaching Agents The ore leaching tests with different concentrations of acids and mixtures were performed under the following conditions: solid–liquid ratio, 1.82% w/v; stirring speed, 330 rpm; reaction time, 80 min; temperatures, 75 and 123 ◦C. The results obtained are shown in Figure6. Metals 2019, 9, x FOR PEER REVIEW 7 of 12

plateau of the curve is reached. The synergistic effect can also be observed using the HF/H2SO4 mixture, since the extraction values are considerably improved if they are compared with the experiments that work with pure acids. The optimal extraction of titanium, 89%, was achieved at 80 min for the lixiviant H2SO4–HF medium (10% v/v and 15% v/v, respectively). These results are expected since fluid–solid reactive reactions are generally slow [13–16]. Similar results to ours were obtained by Das et al., Agatzini et al., Begum et al., Habashi, Nguyen and Lee and Haverkamp et al. [1–6], who conducted operating parameter studies on sands, mud and minerals with sulfuric and hydrochloric media, finding in all cases that the increase in reaction time increases the dissolution of these materials.

3.4. Concentration Effect of Leaching Agents The ore leaching tests with different concentrations of acids and mixtures were performed under the following conditions: solid–liquid ratio, 1.82% w/v; stirring speed, 330 rpm; reaction time, 80 min; Metalstemperatures,2020, 10, 497 75 and 123 °C. The results obtained are shown in Figure 6. 7 of 11

. Figure 6. Effect of the concentration of the leaching medium at 75 and 123 ◦C: (a) HF; (b)H2SO4 (10%

v/Figurev)–HF. 6. Effect of the concentration of the leaching medium at 75 and 123 °C: (a) HF; (b) H2SO4 (10% v/v)–HF. Figure6 shows that the dissolution of titanium increases with the increase in the concentration of leachingFigure agent. 6 shows This that is becausethe dissolution the extent of titanium of solid–fluid increases reactions, with the in increase which soluble in the productsconcentration are formed,of leaching depends agent on. This the is concentration(s) because the extent of the of leachingsolid–fluid agent(s). reactions, In addition, in which thissoluble phenomenon products are is favoredformed, by depends the increase on the in temperature,concentration which(s) of notthe onlyleaching increases agent the(s) activity. In addition, of the ionsthis responsiblephenomenon for is the dissolution of the solid but also increases the solubility of the compounds formed. These results are similar to those found by Das et al., Agatzini et al., Begum et al., Habashi, Nguyen and Lee and Haverkamp et al. [1–6]. They reported that the increase in the concentration of acid leads to high extraction percentages of Ti in the leaching tests of minerals, or black sands with HCl or H2SO4. On the other hand, comparing Figure6a,b, it is observed that when using the mixture of HF and H2SO4, better titanium extraction values are obtained for both temperatures (89% and 71% for 123 and 75 ◦C, respectively) with H2SO4 (10% v/v)–HF (15% v/v) than with HF (15% v/v) (Figure6b).

3.5. Solid–Liquid Ratio Effect Figure7 shows the e ffect of the solid–liquid ratio on the extraction of titanium. In all the tests, the other operating parameters were kept constant at the following values: temperature, 75 and 123 ◦C; reaction time, 80 min; stirring speed, 330 rpm; H2SO4 (10% v/v)–HF (15% v/v) or HF (15% v/v) leaching agents. Metals 2019, 9, x FOR PEER REVIEW 8 of 12

favored by the increase in temperature, which not only increases the activity of the ions responsible for the dissolution of the solid but also increases the solubility of the compounds formed. These results are similar to those found by Das et al., Agatzini et al., Begum et al., Habashi, Nguyen and Lee and Haverkamp et al. [1–6]. They reported that the increase in the concentration of acid leads to high extraction percentages of Ti in the leaching tests of minerals, red mud or black sands with HCl or H2SO4. On the other hand, comparing Figure 6a,b, it is observed that when using the mixture of HF and H2SO4, better titanium extraction values are obtained for both temperatures (89% and 71% for 123 and 75 °C, respectively) with H2SO4 (10% v/v)–HF (15% v/v) than with HF (15% v/v) (Figure 6b).

3.5. Solid–Liquid Ratio Effect Figure 7 shows the effect of the solid–liquid ratio on the extraction of titanium. In all the tests, the other operating parameters were kept constant at the following values: temperature, 75 and 123 °C; reaction time, 80 min; stirring speed, 330 rpm; H2SO4 (10% v/v)–HF (15% v/v) or HF (15% v/v) Metals 2020, 10, 497 8 of 11 leaching agents.

FigureFigure 7. 7.E Effectffect of of solid–liquidsolid–liquid ratio. ratio. The results of Figure7 show that increasing the solid–liquid ratio decreases the dissolution of The results of Figure 7 show that increasing the solid–liquid ratio decreases the dissolution of titanium, which is due to the leachate becoming saturated as the content of dissolved metals increases titanium, which is due to the leachate becoming saturated as the content of dissolved metals increases owing to the increase in the solid–liquid ratio, for a fixed volume of liquid. This hinders the interaction owing to the increase in the solid–liquid ratio, for a fixed volume of liquid. This hinders the between the leaching agent and the sample, thus decreasing the leaching reaction. interaction between the leaching agent and the sample, thus decreasing the leaching reaction. 3.6. Stirring Speed Effect 3.6. Stirring Speed Effect Figure8 shows the e ffect of the stirring speed on the extraction of titanium. The other parameters Figure 8 shows the effect of the stirring speed on the extraction of titanium. The other parameters were kept constant at the following values: temperatures, 75 and 123 ◦C; reaction time, 80 min; Metalswere 2019kept, 9 constant, x FOR PEER at theREVIEW following values: temperatures, 75 and 123 °C; reaction time, 80 min; solid9 of 12– solid–liquid ratio, 1.82%; H2SO4 (10% v/v)–HF (15% v/v) or HF (15% v/v) leaching agents. liquid ratio, 1.82%; H2SO4 (10% v/v)–HF (15% v/v) or HF (15% v/v) leaching agents.

Figure 8. Effect of stirring speed. Figure 8. Effect of stirring speed. The results of Figure8 indicate that the increase in the stirring speed slightly a ffects the titanium The results of Figure 8 indicate that the increase in the stirring speed slightly affects the titanium dissolution, which would allow us to say that the transfer of mass (or leaching agent) through the dissolution, which would allow us to say that the transfer of mass (or leaching agent) through the boundary layer (Nernst layer) is maximal [13–16]. boundary layer (Nernst layer) is maximal [13–16]. 3.7. Residues Characterization 3.7. Residues Characterization The characterization of the residues was performed by XRD, SEM and EDS. Figure9 shows the XRDThe of the characterization waste from diff oferent the leachingresidues conditions.was performed by XRD, SEM and EDS. Figure 9 shows the XRD of the waste from different leaching conditions.

Figure 9. XRD of the residues at 123 °C : (a) HF; (b) HF (15% v/v)-H2SO4 (10% v/v).

In the diffractograms of Figure 9, there are no diffraction lines that fit the structures of some of the titanium compounds, which is consistent with the high titanium extraction values achieved in the leaching test. In the diffractogram of the residue from dissolution in HF medium (Figure 9a), the presence of fluorite and magnetite was observed. In the XRD obtained after treating the mineral with

Metals 2019, 9, x FOR PEER REVIEW 9 of 12

Figure 8. Effect of stirring speed.

The results of Figure 8 indicate that the increase in the stirring speed slightly affects the titanium dissolution, which would allow us to say that the transfer of mass (or leaching agent) through the boundary layer (Nernst layer) is maximal [13–16].

3.7. Residues Characterization The characterization of the residues was performed by XRD, SEM and EDS. Figure 9 shows the XRDMetals of2020 the, 10 waste, 497 from different leaching conditions. 9 of 11

Figure 9. XRD of the residues at 123 ◦C: (a) HF; (b) HF (15% v/v)-H2SO4 (10% v/v). Figure 9. XRD of the residues at 123 °C : (a) HF; (b) HF (15% v/v)-H2SO4 (10% v/v). In the diffractograms of Figure9, there are no di ffraction lines that fit the structures of some of the titaniumIn the diffractograms compounds, whichof Figure is consistent9, there are with no diffraction the high titanium lines that extraction fit the structures values achieved of some of in theMetals titanium leaching 2019, 9, x compounds, test.FOR PEER In the REVIEW diwhichffractogram is consistent of the with residue the high from titanium dissolution extraction in HF values medium achieved (Figure 10in 9 ofthea), 12 leachingthe presence test. ofIn fluorite the diffractogram and magnetite of the was residue observed. from In d theissolution XRD obtained in HF aftermedium treating (Figure the mineral9a), the presencewithHF–H HF–H2SO of4 2( fluoriteFigureSO4 (Figure 9b and), diffraction 9magnetiteb), di ffraction lineswas observed. linescorresponding corresponding In the to XRD magnetite, to obtained magnetite, a aftermixed a mixedtreating oxide oxide of the Fe, mineral of Al Fe, and Al with Mg, and Mg,and calcium and calcium sulfate sulfate were were detected. detected. The presence of the calcium compounds in the diffractograms diffractograms of the residues can be explained, taking into account that, in addition to the dissolution of ilmenite and the the titanium oxide oxide and and iron, iron, leachingleaching ofof the accessory minerals occurs,occurs, includingincluding thethe metalmetal contained.contained. This leads, according to the leachingleaching media,media, toto thethe formationformation ofof CaSOCaSO44 and CaF22. In In addition, addition, the the formation formation of is due to thethe reactionreaction ofof ironiron oxyfluorideoxyfluoride (Reaction (Reaction 2) 2) with with magnesium magnesium and and aluminum aluminum from from the the dissolution dissolution of anorthiteof anorthite and and enstatite enstatite (MgSiO (MgSiO3). 3). Figures 10 and 11 show the SEM micrographs of two residues residues obtained under different leaching conditions.

FigureFigure 10. MicrographMicrograph of of the the leaching leaching residue residue at 123 °C◦C with with 15% 15% v/vv/v HF.HF.

Figure 10 shows the micrograph of the residue, where it can be seen that the ore particles are selectively attacked over an area; around the particle, small fragments of the disintegrated mineral are seen due to the progress of the reaction.

Figure 11. SEM of the leaching residue at 123 °C, with HF (15% v/v)–H2SO4 (10% v/v).

Figure 11 shows the micrograph of the leaching residue at 123 °C, with H2SO4 (10% v/v)–HF (15% v/v), where the formation of a rhombic crystalline structure can be seen. An EDS analysis was performed on different points of this particle. The results are presented in Table 2.

Table 2. EDS analysis, in atomic percentage.

Sector Analyzed %Fe %O %Mg %Al %Ca

Metals 2019, 9, x FOR PEER REVIEW 10 of 12

HF–H2SO4 (Figure 9b), diffraction lines corresponding to magnetite, a mixed oxide of Fe, Al and Mg, and calcium sulfate were detected. The presence of the calcium compounds in the diffractograms of the residues can be explained, taking into account that, in addition to the dissolution of ilmenite and the titanium oxide and iron, leaching of the accessory minerals occurs, including the metal contained. This leads, according to the leaching media, to the formation of CaSO4 and CaF2. In addition, the formation of mixed oxide is due to the reaction of iron oxyfluoride (Reaction 2) with magnesium and aluminum from the dissolution of anorthite and enstatite (MgSiO3). Figures 10 and 11 show the SEM micrographs of two residues obtained under different leaching conditions.

Figure 10. Micrograph of the leaching residue at 123 °C with 15% v/v HF.

Figure 10 shows the micrograph of the residue, where it can be seen that the ore particles are

Metalsselectively2020, 10 attacked, 497 over an area; around the particle, small fragments of the disintegrated mineral10 of 11 are seen due to the progress of the reaction.

Figure 11. SEMSEM of of the the leaching leaching residue residue at 1 12323 °C,◦C, with with HF ( (15%15% v/vv/v)–H)–H22SO4 ((10%10% v/vv/v).).

Figure 10 shows the micrograph of the residue, where it can be seen that the ore particles are Figure 11 shows the micrograph of the leaching residue at 123 °C, with H2SO4 (10% v/v)–HF selectively(15% v/v), attackedwhere the over formation an area; of around a rhombic the particle, crystalline small structure fragments can of be the seen. disintegrated An EDS analysis mineral was are seenperformed due to on the different progress po ofints the of reaction. this particle. The results are presented in Table 2. Figure 11 shows the micrograph of the leaching residue at 123 ◦C, with H2SO4 (10% v/v)–HF (15% v/v), where the formationTable of a rhombic2. EDS analysis, crystalline in atomic structure percentage. can be seen. An EDS analysis was performed on different points of this particle. The results are presented in Table2. Sector Analyzed %Fe %O %Mg %Al %Ca Table 2. EDS analysis, in atomic percentage.

Sector Analyzed %Fe %O %Mg %Al %Ca Point 1 61.4 29.7 8.3 0.30 0.5 Point 2 55.3 30.5 7.5 0.28 0.3

The EDS analysis of the particles corresponding to this phase (marked particles) indicates that they have a high content of , aluminum, iron and magnesium, while in smaller quantity we also find the presence of aluminum and calcium. These results indicate that these particles would correspond to the structure of the mixed oxide Fe0.7Al0.23Mg1.97O4 that has been identified by XRD. The formation of iron compounds during leaching can benefit the subsequent recovery of titanium in a next stage of the process.

4. Conclusions In this study, the results of the extraction reaction of titanium from titaniferous sands with different leaching agents are presented. The main findings of this study are the following: the leaching media composed of HF or a mixture of H2SO4–HF produces important extractions of titanium in the working conditions investigated. The leaching temperature, time, concentration of the fluid phase and the solid–liquid ratio have a marked influence on the extraction reaction of titanium contained in these titaniferous sands. The stirring speed, between 110 and 550 rpm, has a slight effect on the solid extraction reaction. The optimal conditions to achieve of 89% extraction of titanium were obtained to be as follows: HF (15% v/v)–H2SO4 (10% v/v) as leaching agent; temperature of 123 ◦C; reaction time of 80 min; solid–liquid ratio of 1.82% w/v; stirring speed of 330 rpm. The analysis of the leaching residues corroborated that no titanium precipitate is formed during the dissolution of the ore. In addition, precipitation of iron compounds occurs, which can then favor the recovery process of titanium from leach liquor in a next stage of the process.

Author Contributions: The following statements should be used “Conceptualization, G.D.R. and M.H.R.; methodology, E.G.P., N.T., G.D.R., F.M.T. and M.H.R.; validation, G.D.R., E.G.P., F.M.T. and M.H.R.; formal analysis, Metals 2020, 10, 497 11 of 11

G.D.R., N.T., F.M.T. and M.H.R.; investigation, G.D.R., F.M.T. and M.H.R.; resources, M.H.R.; data curation, E.G.P. and G.D.R.; writing—original draft preparation, M.H.R. and G.D.R.; writing—review and editing, E.G.P., N.T., F.M.T., G.D.R. and M.H.R.; visualization, G.D.R. and M.H.R.; supervision, M.H.R.; project administration, M.H.R.; funding acquisition, N.T. and M.H.R. All authors have read and agreed to the published version of the manuscript”. Funding: This research received no external funding. Acknowledgments: The authors thank the Universidad Nacional de Cuyo, Secretaria de Investigación, Internacionales y Posgrado (SIIP) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), for the. financial support. Conflicts of Interest: The authors declare they have no conflict of interest.

References

1. Das, G.K.; Pranolo, Y.; Zhu, Z.; Cheng, C.Y. Leaching of ilmenite by acidic chloride solutions. 2013, 133, 94–99. [CrossRef] 2. Agatzini-Leonardou, S.; Oustadakis, P.; Tsakiridis, P.E.; Markopouloset, C.H. Titanium leaching from red mud by diluted sulfuric acid at atmospheric pressure. J. Hazard. Mater 2008, 157, 579–586. [CrossRef] [PubMed] 3. Begum, N.; Maisyarah, A.; Bari, F.; Rafezi Ahmad, K.; Hidayah, N. Leaching behaviour of Langkawi black sand for the recovery of titanium. APCBEE Procedia. 2012, 3, 1–5. [CrossRef] 4. Habashi, F. Handbook of ; Wiley-VCH: Hoboken, NJ, USA, 1997; Volume II. 5. Nguyen, T.H.; Lee, M.S. A review on the recovery of titanium dioxide from ilmenite ores by direct leaching technologies. . Process. Extr. Metall. Rev. 2018, 231–247. [CrossRef] 6. Haverkamp, R.G.; Kruger, D.; Rajashekar, R. The digestion of New Zealand ilmenite by hydrochloric acid. Hydrometallurgy 2016, 163, 198–203. [CrossRef] 7. Gázquez, M.J.; Bolívar, J.P.; Garcia-Tenorio, R.; Vaca, F. A review of the production cycle of titanium dioxide pigment. Mater. Sci. Appl. 2014, 5, 441–458. [CrossRef] 8. Housecroft, C.; Sharpe, A. Química Inorgánica, 2nd ed.; Editorial Pearson Prentice Hall: Madrid, España, 2006. (In Spanish) 9. Tedesco, P.H.; Rumi, V.B. Estudio de la factibilidad de la recuperación de titanio a partir de arenas titaníferas mediante extracción con fosfato de tributilo. Ind. y Qca. 1978, 247, 44. (In Spanish) 10. Tavani, E.L.; Tedesco, P.H.; Krenkel, T.G. Beneficio de las arenas titaníferas de Bahía San Blas por vía hidrometalúrgica. Rev. Latin. Amer. Ing. Qca. Qca. Aplic. 1981, 77, 13–24. (In Spanish) 11. Sarno, M.C.; Avanza, J.R. Análisis cinético de la lixiviación de un mineral concentrado. Facena 1986, 6, 259–270. (In Spanish) 12. Sarno, M.C.; Avanza, J.R. Lixiviación de arenas titaníferas con ácido clorhídrico. Facena 1987, 7, 265–270. (In Spanish) 13. Rosales, G.D.; Pinna, E.G.; Suarez, D.S.; Rodriguez, M.H. Recovery process of Li, Al and Si from lepidolite by leaching with HF. Minerals 2017, 7, 36. [CrossRef] 14. Suarez, D.S.; Pinna, E.G.; Rosales, G.D.; Rodriguez, M.H. Synthesis of fluoride from spent lithium-ion batteries. Minerals 2017, 7, 81. [CrossRef] 15. Habashi, F. Principles of Extractive Metallurgy; Gordon and Breach Science Publishers, Inc.: New York, NY, USA, 1980; Volume I. 16. Quiroga, O.D.; Avanza, J.R.; Fusco, A.J. Modelado Cinético de las Transformaciones Fluido-Sólido Reactivo; Editorial Universitaria de la Universidad del Nordeste (EUDENE): Corrientes, Argentina, 1996. (In Spanish)

© 2020 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 (http://creativecommons.org/licenses/by/4.0/).