Mineralogical and Leaching Characteristics of Altered Ilmenite Beach Placer Sands

Article Mineralogical and Leaching Characteristics of Altered Ilmenite Beach Placer Sands

Munyadziwa Mercy Ramakokovhu 1,* , Peter Apata Olubambi 2, Richard Kady Kadiambuji Mbaya 1, Tajudeen Mojisola 2 and Moipone Linda Teﬀo 1

1 Institute for NanoEngineering Research, Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria 0001, South Africa; [email protected] (R.K.K.M.); teﬀ[email protected] (M.L.T.) 2 Centre for NanoEngineering and Tribocorrosion, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, Johannesburg 2000, South Africa; [email protected] (P.A.O.); [email protected] (T.M.) * Correspondence: [email protected]; Tel.: +27-12-382-4858

Received: 7 July 2020; Accepted: 7 August 2020; Published: 17 November 2020

Abstract: In order to have a good understanding of the treatment process and improvement on the market value of ilmenite beach placer sands, knowledge of its mineral composition and phase distribution is fundamental. In this study, a combination of characterization techniques including high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron (HR-TEM) microscope, and X-ray diffraction (XRD) techniques was used to understand the mineralogical characteristics of ilmenite beach placer sands obtained from Richards Bay, South Africa. The mineral phase constituents of the ilmenite beach placer sands were studied before pre-oxidation and additive leaching in a chloride environment. During observations using XRD and HR-SEM, the ilmenite beach placer sands exhibited signature rhombohedral crystal form and crescentic pits with evidence of alteration phases. The characterized pre-oxidized ilmenite showed the presence of a ferric oxide film deposit of the particles. The leaching characteristics of both raw and pre-oxidized ilmenite was studied in the presence of additives. The leaching efficiency of the pre-oxidized ilmenite in the presence of additives increased by 20% at atmospheric conditions. The characterized residues show the improved amenability of pre-oxidized leach in chloride media. The formation of new phases containing pseudo-rutile indicated crystallographic disintegration by the movement of atoms during dissolution. Some particles retained the crescentic pit and the subangular grain structure; however, the phase changes were observed at the grain boundaries and grain edges. The leached residue’s EDS results still indicated the presence of pseudo-rutile and some minor unreacted oxides such as SiO2, Al2O3, and other trace metals. The trace metals impurities present in the as-received ilmenite were reduced by 80% in the final residue after the leaching.

Keywords: ilmenite; characterization; pseudo-rutile; leaching; mineralogy

1. Introduction Synthetic rutile production from ilmenite has gained momentum with increasing demand in pigment production, ceramics, coverings, and pharmaceutical applications [1,2]. With the scarcity of natural rutile, ilmenite and its alteration products have become a signiﬁcant source of synthetic rutile and the titanium ﬁnal product. Given the lucrative properties of titanium and titanium-based products, the rise in exploiting the heavy mineral sands is high. More than half of the global reserves of ilmenite are situated in both South Africa and Australia [3,4]. Ilmenite is mined from altered igneous rocks. These beach placer sands are extracted from the nearby dunes by dredging and gravity concentration

Minerals 2020, 10, 1022; doi:10.3390/min10111022 www.mdpi.com/journal/minerals Minerals 2020, 10, 1022 2 of 18 methods. The recovery of these reserves requires a closer look at the structural orientation of each respective deposit. The complexity and variance of ilmenite phase distribution create processing challenges [5,6]. Several studies [7–9], have dealt extensively with the natural weathering process of ilmenite. However, its susceptibility to this process poses that Mg2+, Mn2+, and Fe3+ can replace a challenge for process design as a substitution of the principal phase constitution Fe2+ apart from forming a large variety of intergrowth with Iron titanium oxides. The presence of Cr, Al, Si, and Mn as impurities complicates the deposits’ characteristics even further and decreases the market value [10,11]. Many processes commercially used or proposed to upgrade ilmenite to synthetic rutile involve pyrometallurgy and hydrometallurgy and are generally expensive [12–14]. Over the years direct hydrometallurgical leach processes were found to be advantageous in processing abundant ilmenite ores and low energy consumption and producing sufficiently high-quality synthetic rutile [15]. The complexity in using hydrometallurgical processes to convert ilmenite economically to synthetic rutile lies in its mineralogical composition and properties [16]. In aqueous solution, the titanium 2 trioxide ions are still bound together as single (TiO3) − ions, moving independently through the solution, with iron ions, represented as Fe2+. When ilmenite crystals are subjected to acidic media, the ionic bonds are broken first because they are non-directional and allow the charged species to move freely [17]. Secondly, when salts dissolve into water, the ionic bonds are typically broken by the interaction with water, but the covalent bonds continue to hold. Fe2+ dissolves, and oxygen from titanium trioxide gets attracted to hydrogen ions. As a result of the dissolution challenges, research efforts have focused on adding additives to enhance the solubility of iron from the ilmenite compound [18]. Ilmenite reactivity in chloride media can be significantly enhanced by creating a reducing environment using metallic iron [19]. The inclusion of sulfate of divalent metal (Ca and Mg) as a catalyst has proved to increase the ilmenite leaching efficiency. The catalyst’s sulfate ions in solution lower the energy gap of the chemical reaction while decreasing the free acid content and fine generation in the spent solution, enhancing the leaching efficiency [20]. Chloride leaching processes have evolved and developed in techniques and strategies, but still face challenges in waste management, cost, and lengthy residence time [21]. This study seeks to determine the mineralogical structural changes of raw and pre-oxidized ilmenite during leaching, the formation of different structures, and the possible influence these structural changes have on the leaching characteristics of ilmenite. The focus will be directed on the characteristics of the deposit and the impact of changing mineralogy by pre-oxidation aid amenability of altered ilmenite beach placer sands to chloride additive leaching.

2. Materials and Methods

2.1. Mineralogical Studies on the Ilmenite Beach Placer Sands The ilmenite samples used in this study were ilmenite beach placer sands collected from Richards Bay Minerals in South Africa. The particle size distribution of the sands was determined using the Malvern particle size analyzer and confirmed by the standard laboratory Tyler series sieves as both methods determine the cut size. The X-ray fluorescence spectrometry technique was used to determine the ilmenite sands’ chemical composition, while the phases within the sand were identified using the X-Ray diffractometer technique using the PANalytical Empyrean Powder X-ray diffraction. The morphologies and the backscattered electrons of the sand were observed using a Jeol HR-SEM (model JSM 7600F, JEOL, Akishima, Japan), and the elemental compositions were mapped using energy dispersive spectrometry (EDS) techniques.

2.2. Pre-Oxidation and Leaching Studies

The ilmenite was pre-oxidized by heating the material to 817 ◦C in a muﬄe furnace in the presence of oxygen. The ramping furnace rate was 100 ◦C/h. The pre-oxidized ilmenite sands were air-cooled to Minerals 2020, 10, x FOR PEER REVIEW 3 of 20

SEM (high-resolution scanning microscope) (model JSM 7600F). The Jeol HR-TEM (high resolution- transmission microscope) was used to determine the crystallographic structure of the material. MineralsFigure2020 1 ,shows10, 1022 a schematic overview of the leaching process. Ilmenite was leached using a 5003 of 18 mL reactor placed on a heated plate. A mechanical stirrer with a maximum speed of 450 rpm was used to keep the slurry suspended during the experiment. De-ionized (DI) water was used to make room temperature. The PANalytical Empyrean powder X-ray diﬀraction determined the phase analyses, up the slurry, and anhydrous hydrochloric acid was dosed to regulate the pH of the solution. The and the morphology of the pre-oxidized ilmenite was observed using a Jeol HR-SEM (high-resolution leaching was carried out at atmospheric pressure varying parameters such as temperature, acid scanning microscope) (model JSM 7600F). The Jeol HR-TEM (high resolution-transmission microscope) concentration, agitation, solid to liquid ratio, and the residence time. The pH meter with a was used to determine the crystallographic structure of the material. temperature gauge was used to measure both temperature and the pH. Magnesium sulfate was Figure1 shows a schematic overview of the leaching process. Ilmenite was leached using a 500 mL introduced at varying volumes and concentrations as a catalyst. Metallic iron was also introduced to reactor placed on a heated plate. A mechanical stirrer with a maximum speed of 450 rpm was used to the solution as a reducing agent. The pregnant leach solution was sampled intermittently at 30 min keep the slurry suspended during the experiment. De-ionized (DI) water was used to make up the intervals for the duration of the leaching cycle. For statistical viability, the tests were repeated three slurry, and anhydrous hydrochloric acid was dosed to regulate the pH of the solution. The leaching times. Ammonium difluoride (NH4HF4) was used as an additive because of its ability to attack the was carried out at atmospheric pressure varying parameters such as temperature, acid concentration, glassy silica component that forms during leaching. agitation, solid to liquid ratio, and the residence time. The pH meter with a temperature gauge was The leaching efficiency is calculated based on the amount of iron and impurities removed from used to measure both temperature and the pH. Magnesium sulfate was introduced at varying volumes ilmenite. The pregnant leach solution was characterized using ICP-OES (inductively coupled plasma- opticaland concentrationsemission spectrometry). as a catalyst. The Metallic residue iron was was characterized also introduced using to thea Jeol solution HR-SEM as a reducing(model JSM agent. 7600F)The and pregnant Jeol HR-TEM. leach solution Equation was (1) sampled shows the intermittently reaction involved at 30 min during intervals leaching. forthe duration of the leaching cycle. For statistical viability, the tests were repeated three times. Ammonium diﬂuoride 3 2 2 2 (NH4HF4) was used as an additiveFeTiO + because4HCl of FeCl its ability+ TiOCl to + attack 2H O the glassy silica component(1) that forms during leaching.

Figure 1. Leaching process schematic overview. Figure 1. Leaching process schematic overview. The leaching eﬃciency is calculated based on the amount of iron and impurities removed 3. Results from ilmenite. The pregnant leach solution was characterized using ICP-OES (inductively coupled 3.1.plasma-optical Mineralogy of As-Received emission spectrometry).Ilmenite The residue was characterized using a Jeol HR-SEM (model JSM 7600F) and Jeol HR-TEM. Equation (1) shows the reaction involved during leaching. In Figure 2, the particle size distribution means that a particle size (d50) of 136 µm was observed. The diffraction patterns of eachFeTiO particle+ 4HCl size range ofFeCl the as-received+ TiOCl + ilmenite2H O are shown in Figure 3. (1) 3 −→ 2 2 2 The background peaks and peak position in the diffractogram showed ilmenite (FeTiO3), rutile (TiO2),

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3. Results

3.1. Mineralogy of As-Received Ilmenite In Figure2, the particle size distribution means that a particle size (d50) of 136 µm was observed. Minerals 2020, 10, x FOR PEER REVIEW 4 of 20 The diffraction patterns of each particle size range of the as-received ilmenite are shown in Figure3. The background peaks and peak position in the diffractogram showed ilmenite (FeTiO3), rutile (TiO2), and silica (SiO2) as the major mineral phases in both finer and coarse fractions. The lower-sized and silica (SiO2) as the major mineral phases in both finer and coarse fractions. The lower-sized fractions fractions (−106, −75-micrometer size) showed the (Fe2)SiO4 iron-silicate phase. The silicate phase in ( 106, 75-micrometer size) showed the (Fe )SiO iron-silicate phase. The silicate phase in ilmenite −ilmenite− deposits hosts impurity inclusions2 as it is4 the major gangue mineral phase. In line with XRD deposits hosts impurity inclusions as it is the major gangue mineral phase. In line with XRD results, results, the chemical composition of as-received ilmenite material shown in Table 1 comprises 48.23% the chemical composition of as-received ilmenite material shown in Table1 comprises 48.23% Fe 2O3 Fe2O3 and 42.93% TiO2. The results showed that SiO2 (2.94%), Al2O3 (1.71%), and MnO (1.11%) and 42.93% TiO2. The results showed that SiO2 (2.94%), Al2O3 (1.71%), and MnO (1.11%) represent represent the gangue minerals; however, the crystalline structure of MnO and Al2O3 could not be the gangue minerals; however, the crystalline structure of MnO and Al O could not be detected by detected by XRD as they were below the detection limit. Other associated2 3 gangue materials were XRD as they were below the detection limit. Other associated gangue materials were observed as observed as traces <1%. The presence of Al2O3 and SiO2 further suggested that there was an alteration traces <1%. The presence of Al O and SiO further suggested that there was an alteration of the of the ilmenite mineralogical 2structure.3 The2 alumina and silica enrichment facilitates impurity ilmeniteinclusions mineralogical within the structure structure. post-deposition. The alumina and silica enrichment facilitates impurity inclusions within the structure post-deposition.

Figure 2. Particle size distribution curve. Figure 2. Particle size distribution curve. Table 1. Chemical compositions of the ilmenite (weight fraction, %). Table 1. Chemical compositions of the ilmenite (weight fraction, %). Title Fe2O3 TiO2 SiO2 AL2O3 MnO MgO V2O5 Cr2O3 ZrO2 Na2O CaO Na2O

TitleIlmenite Fe48.232O3 TiO 42.932 SiO 2.942 AL 1.712O3 1.11MnO 0.80MgO 0.39V2O5 0.17Cr2O3 0.16 ZrO 0.132 Na2O 0.13 CaO 0.13 Na2O 75 µm 46.49 41.47 4.63 2.17 2.17 1.08 0.40 0.35 0.34 0.15 0.43 0.15 Ilmenite− 48.23 42.93 2.94 1.71 1.11 0.80 0.39 0.17 0.16 0.13 0.13 0.13 +75 µm 48.10 43.55 2.49 1.63 1.63 0.84 0.42 0.24 0.22 0.12 0.12 0.11 −75 µm 46.49 41.47 4.63 2.17 2.17 1.08 0.40 0.35 0.34 0.15 0.43 0.15 +106 µm 48.39 43.34 2.51 1.65 1.65 0.81 0.40 0.14 0.12 0.11 0.11 0.18 +75+ 150µmµ m 48.1047.20 43.55 39.99 2.49 5.61 2.211.63 2.21 1.63 0.90 0.84 0.330.42 0.40 0.24 0.090.22 0.09 0.12 0.24 0.12 0.29 0.11 +106 µm 48.39 43.34 2.51 1.65 1.65 0.81 0.40 0.14 0.12 0.11 0.11 0.18 +150 µm 47.20 39.99 5.61 2.21 2.21 0.90 0.33 0.40 0.09 0.09 0.24 0.29

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Figure 3. Diffractometer patterns of as-received ilmenite samples at different particle size (µm) ranges with black ( 75 µm) red (+75 µm) blue (+106 µm) green (+150 µm) pink (before sieving). Figure 3. Diffractometer− patterns of as-received ilmenite samples at different particle size (µm) Theranges particle with black size and(−75 shapeµm) red(+75 identification µm) blue by (+106 high-resolution µm) green (+150 scanning µm) pink electron (before microscope sieving). (HR-SEM) analysis is shown in Figure4. At lower magnification ( 100), the observed embedded The particle size and shape identification by high-resolution scanning× electron microscope (HR- micrographsSEM) analysis show is an shown elongated in shapeFigure feature 4. At of lo ilmenite.wer magnification Ilmenite crystalline (×100), structure the observed is rhombohedral embedded asmicrographs per the observation; show an the elongated particle size shape ranges feature from 50–200of ilmenite.µm. The Ilmenite tiny grooves crystalline (pits) structure and veins is observed at higher magnification ( 500, 1000) suggest the migration of atoms from the crystalline rhombohedral as per the observation;× th×e particle size ranges from 50–200 µm. The tiny grooves (pits) structuraland veins lattice observed further at confirmshigher magnification mineralogical (×500, alteration ×1000) of suggest the material. the migration The abrasion of atoms and from cracks the observedcrystalline are structural a result of lattice the particle further to confirms particle impactmineralogical and collision alteration during of the transportation material. The from abrasion the seaand bed. cracks observed are a result of the particle to particle impact and collision during transportation fromThe the cross-sectioned sea bed. particle micrograph (Figure5) shows the pits protruding through the minerals structure. The suggested atoms migration is evident as there are spots that show darker colors than the original structure. The migration leaves pits allowing impurity atoms to deposit on the empty sites. The backscattering electron micrographs (Figure6) shows the di fferent minerals’ phases within the ilmenite structure. Using the elemental mapping technique (Figure7), the ilmenite phase is concentrated on the grey colored phase, silica (dark black) and alumina (light grey) are inter-deposited within the ilmenite grain on the pits. Similar to the XRD and XRF results, the EDS elemental mapping micrographs (Figure7) show the presence of impurity inclusion within the ilmenite structure, confirming the assertion of the material mineralogical orientation has been altered. According to Buick and Dunlop [22], ilmenites have corundum structures with oxygen anions in the hexagonal tightly packed lattice and Fe and Ti cations occupying two-thirds of the interstices. Typically the ilmenite alterations and deposits in the Richards Bay area have chemical compositions containing a wide range of elements (Ca2+, Mn2+, Fe3+), which can substitute the Fe2+ in the lattice structure [23]. The manganese substitutions in the weathering transformation phase (pseudorutile) occur in the range of 0.6–1%, which is similar to what is observed in the chemical composition of this material [24].

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Figure 4. As-received ilmenite micrographs under different magnification. (a) Cracks, (b) Furrows, (c) Particle shape.

The cross-sectioned particle micrograph (Figure 5) shows the pits protruding through the minerals structure. The suggested atoms migration is evident as there are spots that show darker colors than the original structure. The migration leaves pits allowing impurity atoms to deposit on the empty sites. The backscattering electron micrographs (Figure 6) shows the different minerals’ phasesFigureFigure within 4. As-received the ilmenite ilmenite ilmenite struct micrographsure. micrographs Using underthe under elemental different diﬀerent magnification.mapping magniﬁcation. technique (a) Cracks, (a) Cracks, (Figure (b) Furrows, (b 7),) Furrows, the ilmenite phase((cc )is) ParticleParticle concentrated shape. on the grey colored phase, silica (dark black) and alumina (light grey) are inter- deposited within the ilmenite grain on the pits. The cross-sectioned particle micrograph (Figure 5) shows the pits protruding through the minerals structure. The suggested atoms migration is evident as there are spots that show darker colors than the original structure. The migration leaves pits allowing impurity atoms to deposit on the empty sites. The backscattering electron micrographs (Figure 6) shows the different minerals’ phases within the ilmenite structure. Using the elemental mapping technique (Figure 7), the ilmenite phase is concentrated on the grey colored phase, silica (dark black) and alumina (light grey) are inter- deposited within the ilmenite grain on the pits.

Figure 5.5. Cross-sectional image of the as-received ilmeniteilmenite particle.particle.

Figure 5. Cross-sectional image of the as-received ilmenite particle.

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(a) (b)

FigureFigure 6. 6. BackscatteredBackscattered electron electron micrograph micrograph of as-received of as-received ilmenite (a ilmenite) charged (particlea) charged (b) cross- particle sectioned particle. (Mineralsb) cross-sectioned 2020, 10, x FOR particle. PEER REVIEW 8 of 20 Similar to the XRD and XRF results, the EDS elemental mapping micrographs (Figure 7) show the presence of impurity inclusion within the ilmenite structure, confirming the assertion of the material mineralogical orientation has been altered. According to Buick and Dunlop [22], ilmenites have corundum structures with oxygen anions in the hexagonal tightly packed lattice and Fe and Ti cations occupying two-thirds of the interstices. Typically the ilmenite alterations and deposits in the Richards Bay area have chemical compositions containing a wide range of elements (Ca2+, Mn2+, Fe3+), which can substitute the Fe2+ in the lattice structure [23]. The manganese substitutions in the weathering transformation phase (pseudorutile) occur in the range of 0.6–1%, which is similar to what is observed in the chemical composition of this material [24].

FigureFigure 7. EDS 7. EDS elemental elemental mapping mapping micrographs micrographs ofof the as-received as-received ilmenite ilmenite particle. particle. (a) Cross (a) Cross sectioned sectioned

area,area, (b) EDX (b) EDX spectra, spectra, (c) ( Silica,c) Silica, (d ()d Aluminum,) Aluminum, ((ee)) Titanium,Titanium, ( (ff) )Managanese. Managanese.

Changes in the mineralogical orientation of deposits create new dynamics in the structure’s amenability to leaching. The impurity amounts shown in Figures 6 and 7 determine the weathering and process’s extent, which affects the quality and recoverability of ilmenite. The TEM micrographs (Figure 8) of the as-received ilmenite indicated the ilmenite deposit’s polycrystalline nature. The latticed-spacing of the ilmenite shown in Figure 8 is 0.3740 nm. The mineral is in plate form with spiky and cracked morphology. The cracks observed penetrate deeper in the particle to the 250 nm size range. Due to the visible cracks, the particle has been altered, alteration of individual particles occurs along the rims and cracks as iron is readily displaced from the lattice.

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Changes in the mineralogical orientation of deposits create new dynamics in the structure’s amenability to leaching. The impurity amounts shown in Figures6 and7 determine the weathering and process’s extent, which aﬀects the quality and recoverability of ilmenite. The TEM micrographs (Figure8) of the as-received ilmenite indicated the ilmenite deposit’s polycrystalline nature. The latticed-spacing of the ilmenite shown in Figure8 is 0.3740 nm. The mineral is in plate form with spiky and cracked morphology. The cracks observed penetrate deeper in the particle to the 250 nm size range. Due to the visible cracks, the particle has been altered, alteration of individual particles occurs along the rims and cracks as iron is readily displaced from the lattice. Minerals 2020, 10, x FOR PEER REVIEW 9 of 20

FigureFigure 8. 8.TEM TEM micrographmicrograph of an as-received ilmenite ilmenite particle. particle.

3.2.3.2. Pre-Oxidized Pre-Oxidized Ilmenite Ilmenite Mineralogy Mineralogy The pre-oxidized ilmenite micrographs (Figure9) show the ferric oxide phase’s presence emanating 2FeTiO3 + ½O2 → Fe2O3·2TiO2 (2) from the oxidation process. The gradual formation of the Fe O 2TiO intermediate phase (Equation (2)) 2 3· 2 can be observedThe pre-oxidized at a temperature ilmenite of micrographs 800 ◦C. The relatively (Figure smooth9) show and the homogeneous ferric oxide surfacephase’s morphologypresence emanating from the oxidation process. The gradual formation of the Fe2O3·2TiO2 intermediate phase with eccentric pits (Figure9) allows the Fe 2O3 content of the intermediate phase to further break down(Equation to ferric (2)) oxide, can be which observed is amenable at a temperature to leaching. of 800 McConville °C. The relatively and Lee smooth [25] reported and homogeneous the breaking surface morphology with eccentric pits (Figure 9) allows the Fe2O3 content of the intermediate phase down of the pseudo rutile at temperatures between 600 ◦C and 900 ◦C to form hematite and rutile dueto tofurther diﬀusion break during down prolongedto ferric oxide, heating which promoting is amenable the breakdownto leaching. fromMcConville the primary and Lee ilmenite [25] reported the breaking down of the pseudo rutile at temperatures between 600 °C and 900 °C to form particles changing the surface morphology of the particles creating a porous structure on the surfaces. hematite and rutile due to diffusion during prolonged heating promoting the breakdown from the The formation of the porous structure would favor the diﬀusion of the lixiviant during leaching. primary ilmenite particles changing the surface morphology of the particles creating a porous structure on the surfaces. The formation of the1 porous structure would favor the diffusion of the 2FeTiO + O Fe O 2TiO (2) lixiviant during leaching. 3 2 2 → 2 3· 2

3.3. Leaching Characteristics It is well established that the additives and reductants significantly affect the leaching efficiency of ilmenite from the electrochemical results. The response of as-received and pre-oxidized ilmenite to additive and reductive leaching was studied in a series of test works performed at atmospheric conditions. Figure 10 shows the leaching efficiency of ilmenite over time. It can be observed that when only metallic iron is added to the solution, the leaching efficiency of ilmenite is the lowest for both as-received and pre-oxidized samples. The NH4HF4 substantially increased the as-received ilmenite leaching efficiency. The active HF component etches the passivation layer formed during leaching allowing interface reaction to continue increasing the dissolution rate of ilmenite. It can be noted that the pre-oxidized ilmenite response to leaching supersedes that of the as-received ilmenite at atmospheric conditions. In the same Figure 10, the leaching efficiency of the pre-oxidized ilmenite for all additives increased compared to the as-received ilmenite. The porous structure attained during the pre-oxidation step favors the solid–liquid interaction process during the ilmenite leaching process. Figure 9. Pre-oxidized ilmenite backscattered electron micrographs at different magnification.

3.3. Leaching Characteristics It is well established that the additives and reductants significantly affect the leaching efficiency of ilmenite from the electrochemical results. The response of as-received and pre-oxidized ilmenite to additive and reductive leaching was studied in a series of test works performed at atmospheric conditions. Figure 10 shows the leaching efficiency of ilmenite over time. It can be observed that when only metallic iron is added to the solution, the leaching efficiency of ilmenite is the lowest for both

Minerals 2020, 10, x FOR PEER REVIEW 9 of 20

Figure 8. TEM micrograph of an as-received ilmenite particle.

3.2. Pre-Oxidized Ilmenite Mineralogy

2FeTiO3 + ½O2 → Fe2O3·2TiO2 (2) The pre-oxidized ilmenite micrographs (Figure 9) show the ferric oxide phase’s presence emanating from the oxidation process. The gradual formation of the Fe2O3·2TiO2 intermediate phase Minerals(Equation2020, 10 (2)), 1022 can be observed at a temperature of 800 °C. The relatively smooth and homogeneous9 of 18 surface morphology with eccentric pits (Figure 9) allows the Fe2O3 content of the intermediate phase to further break down to ferric oxide, which is amenable to leaching. McConville and Lee [25] The micropores enhance the mass transfer rate of leaching reagents through the ferric oxide layer reported the breaking down of the pseudo rutile at temperatures between 600 °C and 900 °C to form promoting the dissolution reaction. Therefore, this validates the assertion by Liu and Xiang [26] that hematite and rutile due to diffusion during prolonged heating promoting the breakdown from the the exposure of hematite through pre-oxidation promotes dissolution and untreated ilmenite has low primary ilmenite particles changing the surface morphology of the particles creating a porous reactivity to HCl. Mahamoud et al. [19] reported that the addition of metallic iron reduces the dissolved structure on the surfaces. The formation of the porous structure would favor the diffusion of the 3+ 2+ Felixiviantto Fe duringand signiﬁcantly leaching. improves the iron dissolution rate.

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as-received and pre-oxidized samples. The NH4HF4 substantially increased the as-received ilmenite leaching efficiency. The active HF component etches the passivation layer formed during leaching allowing interface reaction to continue increasing the dissolution rate of ilmenite. It can be noted that the pre-oxidized ilmenite response to leaching supersedes that of the as-received ilmenite at atmospheric conditions. In the same Figure 10, the leaching efficiency of the pre-oxidized ilmenite for all additives increased compared to the as-received ilmenite. The porous structure attained during the pre-oxidation step favors the solid–liquid interaction process during the ilmenite leaching process. The micropores enhance the mass transfer rate of leaching reagents through the ferric oxide layer promoting the dissolution reaction. Therefore, this validates the assertion by Liu and Xiang [26] that the exposure of hematite through pre-oxidation promotes dissolution and untreated ilmenite has low reactivity to HCl. Mahamoud et al. [19] reported that the addition of metallic iron reduces the dissolved Fe3+ to Fe2+ and significantly improves the iron dissolution rate. FigureFigure 9. 9. Pre-oxidizedPre-oxidized ilmenite backscatteredbackscattered electron electron micrographs micrographs at diatﬀ differenterent magniﬁcation. magnification.

Figure 10. The leaching characteristics of as-received and pre-oxidized ilmenite in the presence of Figure 10. The leaching characteristics of as-received and pre-oxidized ilmenite in the presence of reductants and additives at atmospheric conditions. reductants and additives at atmospheric conditions. 3.3.1. Effect of Temperature on the Leaching Efficiency of Ilmenite 3.3.1. Effect of Temperature on the Leaching Efficiency of Ilmenite The effect of temperature on ilmenite’s leaching efficiency was investigated at 9.6 M, HCL in the The effect of temperature on ilmenite’s leaching efficiency was investigated at 9.6 M, HCL in the presence of NH4HF4, and metallic iron. From the results presented in Figure 11, it can be noted that thepresence increase of in NH temperature4HF4, and metallic increased iron. the From leaching the eresultsfficiency presented of ilmenite. in Figure Moreover, 11, it thecan pre-oxidizedbe noted that the increase in temperature increased the leaching efficiency of ilmenite. Moreover, the pre-oxidized sample had the highest leaching efficiency at all temperatures. The operational temperature was kept at 90 °C to avoid the formation of HF and HCL vapors at higher temperatures.

Minerals 2020, 10, 1022 10 of 18 sample had the highest leaching eﬃciency at all temperatures. The operational temperature was kept Minerals 2020, 10, x FOR PEER REVIEW 11 of 20 at 90 ◦C to avoid the formation of HF and HCL vapors at higher temperatures.

Figure 11.11. TheThe e ffeffectect of of temperature temperature on theon leachingthe leaching efficiency efficiency of as-received of as-received and pre-oxidized and pre-oxidized ilmenite, when using NH HF and Fe as additives in chloride media. ilmenite, when using4 4 NH4HF4 and Fe as additives in chloride media. 3.3.2. The Behavior of Impurities during Leaching 3.3.2. The Behavior of Impurities during Leaching Manganese, alumina, and silica form part of the crystalline structure of ilmenite coexisting as Manganese, alumina, and silica form part of the crystalline structure of ilmenite coexisting as intra and inter impurities. In application, these impurities affect the quality of the final product. intra and inter impurities. In application, these impurities affect the quality of the final product. While While maintaining the same leaching conditions, the behavior of the impurities was observed. maintaining the same leaching conditions, the behavior of the impurities was observed. Figure 12 Figure 12 presents the leaching characteristics of impurities associated with ilmenite. It can be observed presents the leaching characteristics of impurities associated with ilmenite. It can be observed that that alumina had the highest leaching efficiency, which can be attributed to alumina’s deposition alumina had the highest leaching efficiency, which can be attributed to alumina’s deposition characteristics. When observing the ilmenite crystal structure (Figures5–7), the alumina deposited on characteristics. When observing the ilmenite crystal structure (Figures 5–7), the alumina deposited the intra part of the crystallographic structure, making it easy to be leached out. on the intra part of the crystallographic structure, making it easy to be leached out. In the case of manganese, it was optimally leached to lower levels. The manganese grains are embedded within the ilmenite crystalline structure during alteration hence limiting complete extraction. Silica has the lowest extraction rate as it has larger grains embedded within the structure.

3.3.3. Mineral Phase Constitution of Ilmenite Leaching Residues Figure 13 shows the mineral phase constitution of the as-received ilmenite leached at the optimum condition at different time intervals from 45 min to 1 h 30 min, at 90 ◦C. Compared with Figure3 (as-received ilmenite XRD diffractogram), the x-ray diffractograms of the residue samples were studied after each leaching time interval. New diffraction peaks appeared after a 45-min leach indicating surfacing rutile peaks as ilmenite dissolution occurs. Intermediate phases transformed into rutile; however, most unreacted ilmenite peak intensity is visible in the material.

MineralsMinerals2020 2020, 10, ,10 1022, x FOR PEER REVIEW 12 of11 20 of 18

Minerals 2020, 10, x FORFigureFigure PEER 12. 12. REVIEWThe The characteristic characteristic behaviorbehavior of impurities during during leaching. leaching. 13 of 20

In the case of manganese, it was optimally leached to lower levels. The manganese grains are embedded within the ilmenite crystalline structure during alteration hence limiting complete extraction. Silica has the lowest extraction rate as it has larger grains embedded within the structure.

3.3.3. Mineral Phase Constitution of Ilmenite Leaching Residues Figure 13 shows the mineral phase constitution of the as-received ilmenite leached at the optimum condition at different time intervals from 45 min to 1 h 30 min, at 90 °C. Compared with Figure 3 (as-received ilmenite XRD diffractogram), the x-ray diffractograms of the residue samples were studied after each leaching time interval. New diffraction peaks appeared after a 45-min leach indicating surfacing rutile peaks as ilmenite dissolution occurs. Intermediate phases transformed into rutile; however, most unreacted ilmenite peak intensity is visible in the material.

Figure 13. XRD mineral phase constitution during leaching of the as-received ilmenite, Black (45 min) RedFigure (60 min) 13. BlueXRD (1 mineral h 15 min) phase Green constitution (1 h 30 min). during leaching of the as-received ilmenite, Black (45 min) Red (60 min) Blue (1 h 15 min) Green(1 h 30 min). Figure 14 shows the phase constitution of the pre-oxidized ilmenite during leaching at diﬀerent temperatures.Figure It14 can shows be observed the phase that constitution ilmenite peaked of the pre- at highoxidized intensity ilmenite at lower during temperatures leaching at (25 different and temperatures. It can be observed that ilmenite peaked at high intensity at lower temperatures (25 and 45 ◦C). It can be attributed to the low iron dissolution rates at lower temperatures. At 90 ◦C, there are high-intensity45 °C). It can peaks be attributed of the rutile to the phase, low conﬁrmingiron dissolu thattion iron rates dissolution at lower temperatures. increases with At an 90 increase °C, there in are temperature.high-intensity When peaks comparing of the rutile the pre-oxidized phase, confirming ilmenite that with iron the as-receiveddissolution material, increases the with rutile an peaksincrease arein more temperature. intense in theWhen pre-oxidized comparing sample the pre-oxidized than the as-received ilmenite material.with the as-received The rutile phase material, dominance the rutile peaks are more intense in the pre-oxidized sample than the as-received material. The rutile phase dominance in pre-oxidized material is attributed to the ferric oxide layer, which leaches faster, leaving the rutile phase exposed.

Figure 14. XRD mineral phase constitution during leaching of the pre-oxidized ilmenite at different temperatures, Black (25 °C) Red (45 °C) Blue (90 °C).

Minerals 2020, 10, x FOR PEER REVIEW 13 of 20

Figure 13. XRD mineral phase constitution during leaching of the as-received ilmenite, Black (45 min) Red (60 min) Blue (1 h 15 min) Green(1 h 30 min).

Figure 14 shows the phase constitution of the pre-oxidized ilmenite during leaching at different temperatures. It can be observed that ilmenite peaked at high intensity at lower temperatures (25 and 45 °C). It can be attributed to the low iron dissolution rates at lower temperatures. At 90 °C, there are Mineralshigh-intensity2020, 10, 1022 peaks of the rutile phase, confirming that iron dissolution increases with an increase12 of 18 in temperature. When comparing the pre-oxidized ilmenite with the as-received material, the rutile peaks are more intense in the pre-oxidized sample than the as-received material. The rutile phase in pre-oxidizeddominance in material pre-oxidized is attributed material to is the attributed ferric oxide to the layer, ferric which oxide leaches layer, faster,which leaving leaches the faster, rutile phaseleaving exposed. the rutile phase exposed.

Figure 14. XRD mineral phase constitution during leaching of the pre-oxidized ilmenite at different Figure 14. XRD mineral phase constitution during leaching of the pre-oxidized ilmenite at different temperatures, Black (25 C) Red (45 C) Blue (90 C). temperatures, Black (25◦ °C) Red (45◦ °C) Blue (90 ◦°C). Figure 15 shows the phase pattern of the pre-oxidized ilmenite leached in the presence of ammonium additives. It can be observed that at 25 ◦C there is a higher peak ilmenite peak intensity with few picks of TiO2 and nitrogen. The peak intensity for ilmenite reduces as temperature increases; there are phase changes as the overlap between sulfur, TiO2, and ilmenite observed when leaching was conducted at 45 ◦C. At 90 ◦C, a precise observation of the dominant TiO2 phase constitution confirms the assertion that the pre-oxidized ilmenite leached successfully in the presence of ammonium. When contrasting with the XRD pattern for the leached as-received ilmenite, pre-oxidized leached without additives, the additive leach shows a significant increase in the TiO2 as temperature increases. To have a fully comprehensive understanding of how the crystallographic orientation was affected by leaching, a further mineralogical investigation was required.

3.3.4. Mineralogical Characterization of Leaching Residue Using SEM Therefore, the residue was analyzed using a high-resolution scanning electron microscope (SEM) to determine the surface morphology and mineralogical orientation of the residue at each interval for both standard and additive leaching. Figure 16 shows the electron micrographs of the optimally leached ilmenite at different temperature intervals. The morphology of the residues after leaching at 25 ◦C observed (Figure 16a) showed no deposition or surface film build up during the leaching test, indicating that there was minimal dissolution that occurred. When observing Figure 16b,c, precipitation or a surface film can be observed, with a thicker layer observed on Figure 16c, it confirms that there is indeed a higher dissolution rate that occurs at high temperatures. The film on the particles shows distinct character, but there is no crystal disorientation; however, the fine particle cluster formation or deposition observed as the mineral build-up is targeted explicitly at the rough edges of the mineral surface. The surface layer is also thicker at the grooves, attributed to deposition and polymerization of titanium on the ridges during the leaching of iron from the crystal lattice. Minerals 2020, 10, x FOR PEER REVIEW 14 of 20

Figure 15 shows the phase pattern of the pre-oxidized ilmenite leached in the presence of ammonium additives. It can be observed that at 25 °C there is a higher peak ilmenite peak intensity with few picks of TiO2 and nitrogen. The peak intensity for ilmenite reduces as temperature increases; there are phase changes as the overlap between sulfur, TiO2, and ilmenite observed when leaching was conducted at 45 °C. At 90 °C, a precise observation of the dominant TiO2 phase constitution confirms the assertion that the pre-oxidized ilmenite leached successfully in the presence of ammonium. When contrasting with the XRD pattern for the leached as-received ilmenite, pre- oxidized leached without additives, the additive leach shows a significant increase in the TiO2 as temperature increases. To have a fully comprehensive understanding of how the crystallographic Mineralsorientation2020, 10, was 1022 affected by leaching, a further mineralogical investigation was required. 13 of 18

Figure 15. XRD mineral phase constitution during leaching of the pre-oxidized ilmenite at diﬀerent Figure 15. XRD mineral phase constitution during leaching of the pre-oxidized ilmenite at different temperaturesMinerals 2020, 10, in x FOR the presencePEER REVIEW of ammonium additive, Black (25 ◦C) Red (45 ◦C) Blue (90 ◦C). 15 of 20 temperatures in the presence of ammonium additive, Black (25 °C) Red (45 °C) Blue (90 °C).

3.3.4. Mineralogical Characterization of Leaching Residue Using SEM Therefore, the residue was analyzed using a high-resolution scanning electron microscope (SEM) to determine the surface morphology and mineralogical orientation of the residue at each interval for both standard and additive leaching. Figure 16 shows the electron micrographs of the optimally leached ilmenite at different temperature intervals. The morphology of the residues after leaching at 25 °C observed (Figure 16a) showed no deposition or surface film build up during the leaching test, indicating that there was minimal dissolution that occurred. When observing Figure 16b,c, precipitation or a surface film can be observed, with a thicker layer observed on Figure 16c, it confirms that there is indeed a higher dissolution rate that occurs at high temperatures. The film on the particles shows distinct character, but there is no crystal disorientation; however, the fine particle cluster formation or deposition observed as the mineral build-up is targeted explicitly at the rough edges of the mineral surface. The surface layer is also thicker at the grooves, attributed to deposition and polymerization of titanium on the ridges during the leaching of iron from the crystal lattice.

FigureFigure 16. Electron 16. Electron micrographs micrographs of of the the as-received as-received ilmenite ilmenite leaching leaching resi residuesdues at different at diﬀerent temperature temperature intervals (a) 25 °C, (b) 45 °C, and (c) 90 °C. intervals (a) 25 ◦C, (b) 45 ◦C, and (c) 90 ◦C.

FigureFigure 17 shows17 shows the the electron electron micrographs of of the the pre-oxidized pre-oxidized ilmenite ilmenite leaching leaching residues residues in the in the presencepresence of of reducing reducing agent agent and andammonium ammonium additives. additives. When observing When observingthe microstructures, the microstructures, it can be it candeduced be deduced that the that ilmenite the ilmenite crystallographic crystallographic structure structurewas broken was down broken during down leaching. during When leaching. comparing the micrographs in Figures 16 and 17, it is evident that the dissolution kinetics of the iron When comparing the micrographs in Figures 16 and 17, it is evident that the dissolution kinetics were altered by the presence of a reducing agent, in this case, metallic iron. Sasikumar, Srikanth [27] also observed that iron alters the iron titania bond’s chemical composition, making it unstable and amenable to dissolution by hydrochloric acid. Figure 17a,b the particles show partial dissolution under microscopy at similar magnification and lower temperatures. It can be observed that the particle disintegration started at the edges of the particle, which is the area where the ferric oxide layer was deposited. The removal of iron and other base metal from the lattice also creates gaps in the crystal lattice resulting in line grooves. At higher temperatures (Figure 17c), the particle shows evident disintegration of the crystalline structure because of the high dissolution rate of iron and associated impurities. In comparison with the leached as-received ilmenite sample in Figure 16, it can be deduced that the pre-oxidation enhanced the leaching efficiency of ilmenite. In Figure 16, there is no evident sign of particle breakdown, as in contrast to the evident disintegration of the euclic bond shown in Figure 17. To further understand the nature of the crystallographic disorientation, a closer look at the single leached particle through TEM was instigated.

Minerals 2020, 10, 1022 14 of 18 of the iron were altered by the presence of a reducing agent, in this case, metallic iron. Sasikumar, Srikanth [27] also observed that iron alters the iron titania bond’s chemical composition, making it unstableMinerals 2020 and, 10 amenable, x FOR PEER to REVIEW dissolution by hydrochloric acid. 16 of 20

FigureFigure 17. 17.Electron Electron micrographs micrographs of the of pre-oxidized the pre-oxidized ilmenite leachingilmenite residuesleaching at residues diﬀerent temperatureat different intervalstemperature (a) 25 intervals◦C, (b) 45(a)◦ C,25 and°C, ( (bc)) 45 90 °C,◦C. and (c) 90 °C.

Figure 17a,b the particles show partial dissolution under microscopy at similar magnification Figure 18 shows the TEM leached residue’s TEM micrographs for the pre-oxidized ilmenite and lower temperatures. It can be observed that the particle disintegration started at the edges of the leached in the presence of reductant and ammonium salt. The micrograph (Figure 18) observed particle, which is the area where the ferric oxide layer was deposited. The removal of iron and other showed the amorphous and partly polycrystalline nature of the residue. As shown in Figure 18, the base metal from the lattice also creates gaps in the crystal lattice resulting in line grooves. At higher latticed-spacing difference is due to the mineral phase transformation to titania. The crystallographic temperatures (Figure 17c), the particle shows evident disintegration of the crystalline structure because disintegration of ilmenite particles is due to the migration of iron and impurity mineral atoms to the of the high dissolution rate of iron and associated impurities. solution. As per the observation, it can be deduced that the amorphous part in the ilmenite crystal In comparison with the leached as-received ilmenite sample in Figure 16, it can be deduced that represents the disintegrated area shown in Figure 17. the pre-oxidation enhanced the leaching efficiency of ilmenite. In Figure 16, there is no evident sign of particle breakdown, as in contrast to the evident disintegration of the euclic bond shown in Figure 17. To further understand the nature of the crystallographic disorientation, a closer look at the single leached particle through TEM was instigated. Figure 18 shows the TEM leached residue’s TEM micrographs for the pre-oxidized ilmenite leached in the presence of reductant and ammonium salt. The micrograph (Figure 18) observed showed the amorphous and partly polycrystalline nature of the residue. As shown in Figure 18, the latticed-spacing difference is due to the mineral phase transformation to titania. The crystallographic disintegration of ilmenite particles is due to the migration of iron and impurity mineral atoms to the solution. As per the observation, it can be deduced that the amorphous part in the ilmenite crystal represents the disintegrated area shown in Figure 17.

Figure 18. TEM micrograph of the leach residue d-spacing of the crystal.

The polycrystalline area represents the unreacted part of the ilmenite particle. It can be observed that the amorphous area is predominantly on the edges of the crystal, which confirms the assertion that during pre-oxidation, the ferric oxide, which is amenable to leaching, is moved to the periphery

Minerals 2020, 10, x FOR PEER REVIEW 16 of 20

Figure 17. Electron micrographs of the pre-oxidized ilmenite leaching residues at different temperature intervals (a) 25 °C, (b) 45 °C, and (c) 90 °C.

Figure 18 shows the TEM leached residue’s TEM micrographs for the pre-oxidized ilmenite leached in the presence of reductant and ammonium salt. The micrograph (Figure 18) observed showed the amorphous and partly polycrystalline nature of the residue. As shown in Figure 18, the latticed-spacing difference is due to the mineral phase transformation to titania. The crystallographic disintegration of ilmenite particles is due to the migration of iron and impurity mineral atoms to the Mineralssolution.2020 As, 10 per, 1022 the observation, it can be deduced that the amorphous part in the ilmenite crystal15 of 18 represents the disintegrated area shown in Figure 17.

Figure 18. TEM micrograph of the leach resi residuedue d-spacing of the crystal.

The polycrystalline area represents the unreacted part of the ilmenite particle. It can be observed MineralsThe 2020 polycrystalline, 10, x FOR PEER areaREVIEW represents the unreacted part of the ilmenite particle. It can be observed17 of 20 that thethe amorphousamorphous area area is is predominantly predominantly on on the the edges edges of theof the crystal, crystal, which which confirms confirms the assertionthe assertion that duringthatof the during pre-oxidation,ilmenite pre-oxidation, particle. the ferric theTherefore, ferric oxide, oxide, whichduring which is amenableleaching, is amenable tothe leaching, edges to leaching, isare moved disintegrated is moved to the to periphery theto peripheryform of the ilmenite amorphous particle. phase. Therefore, during leaching, the edges are disintegrated to form the amorphous phase. Figure 19 shows the micrograph ofof thethe leachleach residueresidue crystals.crystals. It can be observed that the various ilmeniteilmenite residueresidue crystalscrystals areare inin thethe plate-likeplate-like structure.structure. The crystalcrystal orientation exhibits a similarsimilar orientation of the flatflat plate-likeplate-like structure,structure, whichwhich hashas aa spikyspiky orientation.orientation. The flatflat plate structure is attributed to the rhombohedral nature of the ilmeniteilmenite crystals.crystals. It can be deduced from the originorigin ofof thethe deposit wherewhere duringduring transportationtransportation ofof thethe ilmeniteilmenite minerals,minerals, thethe collisioncollision ofof thethe particles caused cracks hence the spikyspiky orientation.orientation.

Figure 19. TEM micrograph of the leach residue crystal orientation.

The leaching residue at didifferentfferent leachingleaching timetime intervalsintervals waswas mappedmapped using TEM, and thethe micrographs and EDS results are presentedpresented in FiguresFigures 2020–22.–22. The observed micrograph (Figure 20),20), after 30 min of leaching, showsshows thethe presencepresence ofof allall thethe impuritiesimpurities inin thethe crystallinecrystalline structure.structure. In Figure 21, after 20 min, the manganese peaks were eliminated with low iron intensity peaks and chloride peaks from the leach solution. In comparison with the mapping in Figure 20, the leach residue decreases iron peaks as the leaching retention time increases. The final residues (Figure 22) confirm the assertion that there is a decrease in the intensity of the iron peaks; similarly, there is also the elimination of manganese, aluminum in the crystalline structure. The magnesium and silica impurity

Figure 20. TEM micrograph of the mapped leach residue crystal orientation and EDS after 30 min of leaching.

In Figure 21, after 20 min, the manganese peaks were eliminated with low iron intensity peaks and chloride peaks from the leach solution. In comparison with the mapping in Figure 20, the leach residue decreases iron peaks as the leaching retention time increases. The final residues (Figure 22) confirm the assertion that there is a decrease in the intensity of the iron peaks; similarly, there is also the elimination of manganese, aluminum in the crystalline structure. The magnesium and silica impurity peaks are very low in intensity, suggesting that the ICP-OES results confirm a significant reduction in their concentration.

Minerals 2020, 10, x FOR PEER REVIEW 17 of 20

of the ilmenite particle. Therefore, during leaching, the edges are disintegrated to form the amorphous phase. Figure 19 shows the micrograph of the leach residue crystals. It can be observed that the various ilmenite residue crystals are in the plate-like structure. The crystal orientation exhibits a similar orientation of the flat plate-like structure, which has a spiky orientation. The flat plate structure is attributed to the rhombohedral nature of the ilmenite crystals. It can be deduced from the origin of the deposit where during transportation of the ilmenite minerals, the collision of the particles caused cracks hence the spiky orientation.

Minerals 2020, 10, 1022 Figure 19. TEM micrograph of the leach residue crystal orientation. 16 of 18

The leaching residue at different leaching time intervals was mapped using TEM, and the peaksmicrographs are very lowand inEDS intensity, results suggestingare presented that in theFigu ICP-OESres 20–22. results The observed conﬁrm micrograph a signiﬁcant (Figure reduction 20), in theirafter concentration. 30 min of leaching, shows the presence of all the impurities in the crystalline structure.

FigureFigure 20. 20.TEM TEM micrograph micrograph ofof thethe mapped leach leach residue residue crystal crystal orientation orientation and and EDS EDS after after 30 min 30 of min Minerals 2020, 10, x FOR PEER REVIEW 18 of 20 Mineralsof leaching. 2020, 10 , x FOR PEER REVIEW 18 of 20

FigureFigure 21. 21.TEM TEM micrograph micrograph of the mapped mapped leach leach residue residue crystal crystal orientation orientation andand EDS EDS afterafter 60 min 60 of min Figure 21. TEM micrograph of the mapped leach residue crystal orientation and EDS after 60 min of ofleaching. leaching.

FigureFigure 22. 22.TEM TEM micrograph micrograph ofof thethe mappedmapped final ﬁnal leac leachh residue residue crystal crystal orientation orientation and and EDS. EDS. Figure 22. TEM micrograph of the mapped final leach residue crystal orientation and EDS. 4. Conclusions 4. Conclusions 4. Conclusions TheThe pre-oxidation pre-oxidation of of the the altered altered ilmeniteilmenite beach placer sands sands provides provides a aunique unique opportunity opportunity to to The pre-oxidation of the altered ilmenite beach placer sands provides a unique opportunity to transformtransform the the sands’ sands’ mineralogical mineralogical characteristics. characteristics. By By comparing comparing thethe initialinitial impurityimpurity characteristicscharacteristics of transform the sands’ mineralogical characteristics. By comparing the initial impurity characteristics theof as-received the as-received ilmenite ilmenite beach beach sands, sands, it it is is evident evident that that thethe pre-oxidation increased increased the the amenability amenability of the as-received ilmenite beach sands, it is evident that the pre-oxidation increased the amenability ofof the the sands sands to to chloride chloride leach. leach. TheThe shreddedshredded and disarranged surface surface orientation orientation of ofthe the ilmenite ilmenite of the sands to chloride leach. The shredded and disarranged surface orientation of the ilmenite increasedincreased the the iron iron dissolution dissolution rate. rate. TheThe ilmeniteilmenite mineral phase phase was was converted converted to to pseudo-rutile pseudo-rutile at at increased the iron dissolution rate. The ilmenite mineral phase was converted to pseudo-rutile at intermediateintermediate time time intervals, intervals, asas the retention retention time time increases increases the the final ﬁnal mineral mineral phase phase transformation transformation to intermediate time intervals, as the retention time increases the final mineral phase transformation to rutile. During the dissolution of iron, there is a change in d-spacing, the polycrystalline structure to rutile. During the dissolution of iron, there is a change in d-spacing, the polycrystalline structure to amorphous. amorphous. Author Contributions: Conceptualization and methodology, M.M.R., T.M., and P.A.O.; formal analysis, AuthorR.K.K.M.; Contributions: investigation, M.M.R.;Conceptualization data curation, and M.L.T.; methodology, writing—original M.M.R., draft T.M., prepar and ation,P.A.O.; M.M.R. formal and analysis, P.A.O.; R.K.K.M.;writing—review investigation, and editing, M.M.R.; R.K.K.M. data curation, All authors M.L.T.; have writing—original read and agreed draft toprepar the ation,published M.M.R. version and P.A.O.;of the writing—reviewmanuscript. and editing, R.K.K.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Research Foundation (NRF) Thuthuka grant. Funding: This research was funded by the National Research Foundation (NRF) Thuthuka grant. Acknowledgments: The authors gratefully acknowledge the financial support received from the National Acknowledgments:Research Foundation (NRF) The authors and the gratefully Tshwane Universityacknowledge of Technology,the financial South support Africa. received from the National Research Foundation (NRF) and the Tshwane University of Technology, South Africa. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the Conflictsstudy; in the of Interest:collection, The analyses, authors or declare interpretation no conflict of data of interest.; in the writing The funders of the manuscript,had no role inor thein the design decision of the to study;publish in the the results. collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Minerals 2020, 10, 1022 17 of 18 to rutile. During the dissolution of iron, there is a change in d-spacing, the polycrystalline structure to amorphous.

Author Contributions: Conceptualization and methodology, M.M.R., T.M., and P.A.O.; formal analysis, R.K.K.M.; investigation, M.M.R.; data curation, M.L.T.; writing—original draft preparation, M.M.R. and P.A.O.; writing—review and editing, R.K.K.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Research Foundation (NRF) Thuthuka grant. Acknowledgments: The authors gratefully acknowledge the financial support received from the National Research Foundation (NRF) and the Tshwane University of Technology, South Africa. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. 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]

2. El-Sherbiny, S.; Morsy, F.; Samir, M.; Fouad, O.A. Synthesis, characterization and application of TiO2 nanopowders as special paper coating pigment. Appl. Nanosci. 2014, 4, 305–313. [CrossRef] 3. Amer, A. Alkaline pressure leaching of mechanically activated Rosetta ilmenite concentrate. Hydrometallurgy 2002, 67, 125–133. [CrossRef] 4. Moila, A.; Chetty, D.; Ndlovu, S. The application of process mineralogy on a tailings sample from a beach placer deposit containing rare earth elements. J. South Afr. Inst. Min. Metall. 2017, 117, 615–621. [CrossRef] 5. Ahmad, S.; Rhamdhani, M.A.; Pownceby, M.I.; Bruckard, W.J. Analysis of sulﬁdation routes for processing weathered ilmenite concentrates containing impurities. In 6th International Symposium on High-Temperature Metallurgical Processing; Springer: Berlin/Heidelberg, Germany, 2015. 6. Pownceby, M.I.; Sparrow, G.J.; Fisher-White, M.J. Mineralogical characterisation of Eucla Basin ilmenite concentrates–First results from a new global resource. Miner. Eng. 2008, 21, 587–597. [CrossRef] 7. Bruckard, W.J.; Pownceby, M.I.; Smith, L.K.; Sparrow, G.J. Review of processing conditions for Murray Basin ilmenite concentrates. Miner. Process. Extr. Metall. 2015, 124, 47–63. [CrossRef] 8. Panigrahi, M.; Shibata, E.; Iizuka, A.; Nakamura, T. Production of Fe–Ti alloy from mixed ilmenite and titanium dioxide by direct electrochemical reduction in molten calcium chloride. Electrochim. Acta 2013, 93, 143–151. [CrossRef] 9. Pownceby, M.; Fisher-White, M. Chemical variability in chrome spinel grains from magnetically fractionated ilmenite concentrates: Implications for processing. Miner. Process. Extr. Metall. 2006, 115, 213–223. [CrossRef] 10. Wang, Y.; Yuan, Z.; Matsuura, H.; Tsukihashi, F. Reduction extraction kinetics of titania and iron from an

ilmenite by H2–Ar gas mixtures. ISIJ Int. 2009, 49, 164–170. [CrossRef] 11. Grey, I.; MacRae, C.; Silvester, E.; Susini, J. Behaviour of impurity elements during the weathering of Ilmenite. Miner. Mag. 2005, 69, 437–446. [CrossRef] 12. Wei, L.; Hu, H.; Chen, Q.; Tan, J. Eﬀects of mechanical activation on the HCl leaching behavior of plagioclase, ilmenite and their mixtures. Hydrometallurgy 2009, 99, 39–44. [CrossRef]

13. Wu, F.; Li, X.; Wang, Z.; Xu, C.; He, H.; Qi, A.; Yin, X.; Guo, H. Preparation of high-value TiO2 nanowires by leaching of hydrolyzed titania residue from natural ilmenite. Hydrometallurgy 2013, 140, 82–88. [CrossRef] 14. Zhang, L.; Hu, H.; Liao, Z.; Chen, Q.; Tan, J. Hydrochloric acid leaching behavior of diﬀerent treated Panxi ilmenite concentrations. Hydrometallurgy 2011, 107, 40–47. [CrossRef] 15. Vasquez, R.; Molina, A. Leaching of ilmenite and pre-oxidized ilmenite in hydrochloric acid to obtain high grade titanium dioxide. In Proceedings of the 17th International Metallurgical & Materials Conference Proceedings: METAL, Hradec and Moravici, Czech Republic, 13–15 May 2008.

16. El-Hazek, N.; Lasheen, T.; El-Sheikh, R.; Zaki, S.A. Hydrometallurgical criteria for TiO2 leaching from Rosetta ilmenite by hydrochloric acid. Hydrometallurgy 2007, 87, 45–50. [CrossRef] 17. Welham, N.; Llewellyn, D. Mechanical enhancement of the dissolution of ilmenite. Miner. Eng. 1998, 11, 827–841. [CrossRef] 18. Li, C.; Liang, B.; Guo, L.-H.; Wu, Z.-B. Eﬀect of mechanical activation on the dissolution of Panzhihua ilmenite. Miner. Eng. 2006, 19, 1430–1438. [CrossRef] Minerals 2020, 10, 1022 18 of 18

19. Mahmoud, M.; Aﬁﬁ, A.; Ibrahim, I. Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile. Hydrometallurgy 2004, 73, 99–109. [CrossRef] 20. Gireesh, V.; Vinod, V.; Nair, S.K.; Ninan, G. Catalytic leaching of ilmenite using hydrochloric acid: A kinetic approach. Int. J. Miner. Process. 2015, 134, 36–40. [CrossRef]

21. Mukherjee, A.; Raichur, A.M.; Modak, J.M. Dissolution studies on TiO2 with organics. Chemosphere 2005, 61, 585–588. [CrossRef] 22. Buick, R.; Dunlop, J. Evaporitic sediments of early Archaean age from the Warrawoona Group, North Pole, Western Australia. Sedimentology 1990, 37, 247–277. [CrossRef] 23. Deysel, K. Leucoxene study: A mineral liberation analysis (MLA) investigation. In Proceedings of the 6th International Heavy Minerals Conference ‘Back to Basics’, The Southern African Institute of Mining and Metallurgy, Richards Bay, South Africa, 9–14 September 2007. 24. Das, G.; Pranolo, Y.; Zhu, Z.; Cheng, C. Leaching of ilmenite ores by acidic chloride solutions. Hydrometallurgy 2013, 133, 94–99. [CrossRef] 25. McConville, C.J.; Lee, W.E. Microstructural development on ﬁring illite and smectite clays compared with that in kaolinite. J. Am. Ceram. Soc. 2005, 88, 2267–2276. [CrossRef] 26. Liu, S.-L.; Xiang, J.-Y. The eﬀects of thermal pretreatment on leaching of yunnan ilmenite with hydrochloric acid. Metall. Mater. Trans. B 2016, 47, 1334–1339. [CrossRef] 27. Sasikumar, C.; Srikanth, S.; Mukhopadhyay, N.; Mehrotra, S. Energetics of mechanical activation–Application to ilmenite. Miner. Eng. 2009, 22, 572–574. [CrossRef]

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