Removal of Metallic Iron from Reduced Ilmenite by Aeration Leaching

: Aeration leaching was used to obtain synthetic rutile from a reduced ilmenite. The reduced ilmenite, obtained from the carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1100 ◦ C, contained 62.88% TiO 2 and 28.93% Metallic iron. The particle size was about 200 µ m and the size distribution was uniform. The e ﬀ ects of NH 4 Cl and HCl concentrations, stirring speed, and aeration leaching time on the extent of removal of metallic iron from the reduced ilmenite were studied at room temperature. The results revealed that aeration leaching is feasible at room temperature. When using the NH 4 Cl system, the metallic iron content was reduced to 1.98% in synthetic rutile, but the TiO 2 content only reached 69.16%. Higher NH 4 Cl concentration did not improve the leaching. Using 2% NH 4 Cl with 3% HCl, we were able to upgrade the synthetic rutile to 75%, with a metallic iron content as low as 0.14% and a total iron content of about 4%. Synthetic rutile could be upgraded to about 90% using HCl solution alone. HCl and NH 4 Cl are both e ﬀ ective on the aeration leaching process. However, within the scope of this experiment, hydrochloric acid is more e ﬃ cient in aeration leaching. reduced ilmenite before iron removal were Fe, TiO 2 , and FeTi 2 O 5 . The di ﬀ raction peaks of FeTi 2 O 5 and TiO 2 were strong. Peaks for the metallic iron phase were not to be found in the sample after aeration leaching, but di ﬀ raction peaks of FeO(OH) were detected. The di ﬀ raction peaks of FeO(OH) and TiO 2 had the same intensity, which indicated that metallic iron transformed into FeO(OH). There were just two main phases in the sample after the aeration leaching process. These results indicate that the transformation of reduced ilmenite into rutile was achieved under these experimental conditions.


Introduction
Titanium dioxide (TiO 2 ) is the most widely used titanium product, being employed as pigment, as filler in the paper, plastic, and rubber industries, and as flux in glass manufacture. Synthetic rutile (SR) is one of the major sources of TiO 2 [1][2][3]. Industrial processes usually involve the initial preparation of titanium dioxide, followed by titanium metal production [4,5]. Several commercial or proposed processes are available to produce SR or high-grade titanium slag from ilmenite which is mainly composed of FeTiO 3 . These involve a combination of thermal oxidation and reduction by roasting, leaching, and physical separation steps. Iron is converted to soluble ferrous or elemental forms by reduction at a high temperature, followed by acid leaching to obtain a SR product.
Ilmenite generally contains impurities such as iron, which leads to its low grade and cannot be directly used. Synthetic rutile is a kind of titanium rich raw material with the same composition and structural properties as natural rutile by separating most iron components from ilmenite. An industrial process for upgrading ilmenite to SR is typically represented by the Becher process [6][7][8]. Ilmenite contains 40-65% titanium as TiO 2 , with the rest being iron oxide. The Becher process removes the iron oxide, leaving a residue of SR that contains more than 90% TiO 2 . The Becher process comprises four major steps: oxidation, reduction, aeration, and acid leaching [9,10]. Oxidation involves heating the ilmenite in a rotary kiln with air to convert the contained iron to iron oxide: 4FeTiO 3(s) +O 2(g) → 2Fe 2 O 3 · TiO 2(s) +TiO 2(s) (1) This allows for the use of a wide range of ilmenite materials with various Fe(II) and Fe(III) contents for the subsequent step. Reduction is performed in a rotary kiln with a mixture of pseudobrookite (Fe 2 O 3 ·TiO 2 ) and coal at about 1200 • C to reduce iron oxide to metallic iron: Fe 2 O 3 · TiO 2(s) + 3CO → 2Fe (s) + 2TiO 2(s) + 3CO 2(g) (2) Metallic iron is then oxidized and precipitated from the solution as a slime in an aeration or 'rusting' step in large tanks using 1% ammonium chloride solution at 80 • C: The finer iron oxide is then separated from the larger SR particles. When most of the iron oxide is removed, the residual portion is leached using 0.5 M sulfuric acid and then separated from the SR. In the aeration leaching step, the removal of metallic iron from the reduced ilmenite (RI) grains is essentially a redox reaction, which can be represented by the following half-cell reactions: The oxidation of ferrous ions is then given by: In current industrial practice, the aeration step of the Becher process can take as long as 22 h to complete [11]. Some reports show that the rusting process can be accelerated by improving aeration [12] or by adding a component such as acetic, tartaric, or citric acid [13,14]; a ligand, such as ethylenediammonium dichloride; various phenolic and aldehyde compounds, such as pyrogallol, saccharin, starch, and formaldehyde; sugars, such as glucose and sucrose; and water-soluble redox catalysts, namely, methyl viologen dichloride and diquat dibromide [11,[15][16][17][18]. These additives differ in effectiveness and cost. Most prior research was carried out at relatively high temperature (70 • C). Other related hydrometallurgical processes include, for example, ultrasonic-assisted acid leaching for iron removal from quartz sand [19][20][21] and the goethite process for iron removal from hydrochloric acid leaching solution of reduced laterite [22].
In the present work, we report a study of aeration leaching of reduced ilmenite at room temperature. Aeration leaching experiments using the hydrochloric acid system with oxygen injection at room temperature are rarely studied. The effects of hydrochloric acid and ammonia chloride in improving the aeration efficiency were evaluated. The effects of leaching parameters, including stirring speed and NH 4 Cl and hydrochloric acid concentrations, were investigated. Through the above research, the method of strengthening the aeration process at room temperature is explored to provide a new way to obtain high-grade SR.

Materials
A Chinese source of reduced ilmenite, produced by carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1100 • C, was used. The chemical composition and particle size is reported in Table 1 and Figure 1, respectively. MFe stands for metal iron and TFe stands for all iron in Table 1.
The composition of reduced ilmenite and SR obtained by XRF analysis and MFe was determined by chemical titration. Figure 1 shows that almost 80% of the particles were distributed between 90 and 400 µm, with a mode value of about 200 µm and a uniform distribution.

Aeration Conditions
The aeration leaching experiments were performed in a 1 L stirred reactor. Details of the experimental apparatus are illustrated in Figure 2. The inner diameter of the stirred reactor was 80 mm and the agitator was a four-blade propeller. The blade length as 30 mm. The initial reaction mixture comprised 640 mL solution and 320 g reduced ilmenite, which were added to the stirred reactor. The solution contained different concentrations of ammonium chloride and/or hydrochloric acid. The pulp was stirred by a four-blade agitator. Aeration gas was then introduced and passed through the pulp for the entire duration of the experiment. After 4 h, fine iron oxides were separated from the SR by wet screening. Particles of iron oxides and SR were washed and dried for analysis. We used the content of metallic iron (MFe) remaining in the SR to measure the efficiency of aeration leaching: it was found that the lower the residual iron content, the better the effect.
Aeration leaching is a process of oxygen absorption corrosion of metal iron. Three kinds of corrosion systems are generally selected: ammonium chloride, ammonium chloride plus hydrochloric acid, and hydrochloric acid. It is recognized that the anion provided by hydrochloric

Aeration Conditions
The aeration leaching experiments were performed in a 1 L stirred reactor. Details of the experimental apparatus are illustrated in Figure 2. The inner diameter of the stirred reactor was 80 mm and the agitator was a four-blade propeller. The blade length as 30 mm.

Aeration Conditions
The aeration leaching experiments were performed in a 1 L stirred reactor. Details of the experimental apparatus are illustrated in Figure 2. The inner diameter of the stirred reactor was 80 mm and the agitator was a four-blade propeller. The blade length as 30 mm. The initial reaction mixture comprised 640 mL solution and 320 g reduced ilmenite, which were added to the stirred reactor. The solution contained different concentrations of ammonium chloride and/or hydrochloric acid. The pulp was stirred by a four-blade agitator. Aeration gas was then introduced and passed through the pulp for the entire duration of the experiment. After 4 h, fine iron oxides were separated from the SR by wet screening. Particles of iron oxides and SR were washed and dried for analysis. We used the content of metallic iron (MFe) remaining in the SR to measure the efficiency of aeration leaching: it was found that the lower the residual iron content, the better the effect.
Aeration leaching is a process of oxygen absorption corrosion of metal iron. Three kinds of corrosion systems are generally selected: ammonium chloride, ammonium chloride plus hydrochloric acid, and hydrochloric acid. It is recognized that the anion provided by hydrochloric The initial reaction mixture comprised 640 mL solution and 320 g reduced ilmenite, which were added to the stirred reactor. The solution contained different concentrations of ammonium chloride and/or hydrochloric acid. The pulp was stirred by a four-blade agitator. Aeration gas was then introduced and passed through the pulp for the entire duration of the experiment. After 4 h, fine iron oxides were separated from the SR by wet screening. Particles of iron oxides and SR were washed and dried for analysis. We used the content of metallic iron (MFe) remaining in the SR to measure the efficiency of aeration leaching: it was found that the lower the residual iron content, the better the effect.
Aeration leaching is a process of oxygen absorption corrosion of metal iron. Three kinds of corrosion systems are generally selected: ammonium chloride, ammonium chloride plus hydrochloric acid, and hydrochloric acid. It is recognized that the anion provided by hydrochloric acid can destroy the passivation film on the surface of metallic iron in the aeration process [23]. The role of NH 4 + is to combine with ferrous ions in the ore particles to form a complex that cannot be separated, so as to prevent oxidation and hydrolysis in the ore particles. The complex immediately decomposes when encountering water, and so acts as a carrier. The effect of ammonium chloride was examined using concentrations of 2%, 4%, 6%, and 8% (m/v) NH 4 Cl. The stirring speed was 800 rpm. An ambient temperature was employed. Wet separation of the fine iron oxide from the coarse titanium mineral particles was done by using hydrocyclones and spiral classifiers. We measured the MFe and TiO 2 contents in the SR after aeration leaching for 4 h. The particle size of the samples was analyzed by a laser diffraction particle size analyzer (Bettersize V8.0, Dandong Baite Instrument Co., Ltd., Dandong city, China). The structure and morphology of the reduced ilmenite samples and the product after aeration leaching were characterized by an X-ray diffractometer (BRUKER Inc., Karlsruhe, Germany), applying Cu Kα radiation at 40 kV and 40 mA, with 2θ recording from 10 • to 80 • with a step size of 0.02 • and a counting time of 0.1 s per step. The metallic iron content of the solids was determined by potassium dichromate titration in FeCl 3 solution. Other elements were determined by a ZSX PrimusIV X-ray fluorescence spectrum (Japan Neo Confucianism Co., Ltd., Tokyo, Japan).

Effect of Solution Composition
The results are shown in Figure 3. With increasing NH 4 Cl concentration from 2% to 8%, the metallic iron content in the SR increased from 1.85% to 6.75%, the total iron content decreased from 19.63% to about 16%, and the TiO 2 content increased from 64.97% to about 70%. It can be deduced that an increase in NH 4 Cl concentration is not conducive to the aeration leaching process. When the concentration of 2% ammonium chloride was the same as that in Reference [21] and the reaction time was reduced by one hour, the removal rate of MFe in this paper was as high as 98.15%, while the removal rate of iron in Reference [21] was less than 50%. The TFe content was close to 20% and this is not sufficient for the aeration leaching products.
Metals 2020, 10, x FOR PEER REVIEW 4 of 9 role of NH4 + is to combine with ferrous ions in the ore particles to form a complex that cannot be separated, so as to prevent oxidation and hydrolysis in the ore particles. The complex immediately decomposes when encountering water, and so acts as a carrier. The effect of ammonium chloride was examined using concentrations of 2%, 4%, 6%, and 8% (m/v) NH4Cl. The stirring speed was 800 rpm. An ambient temperature was employed. Wet separation of the fine iron oxide from the coarse titanium mineral particles was done by using hydrocyclones and spiral classifiers. We measured the MFe and TiO2 contents in the SR after aeration leaching for 4 h. The particle size of the samples was analyzed by a laser diffraction particle size analyzer (Bettersize V8.0, Dandong Baite Instrument Co., Ltd., Dandong city, China). The structure and morphology of the reduced ilmenite samples and the product after aeration leaching were characterized by an X-ray diffractometer (BRUKER Inc., Karlsruhe, Germany), applying Cu Kα radiation at 40 kV and 40 mA, with 2θ recording from 10° to 80° with a step size of 0.02° and a counting time of 0.1 s per step. The metallic iron content of the solids was determined by potassium dichromate titration in FeCl3 solution. Other elements were determined by a ZSX PrimusIV X-ray fluorescence spectrum (Japan Neo Confucianism Co., Ltd., Tokyo, Japan )

Effect of Solution Composition
The results are shown in Figure 3. With increasing NH4Cl concentration from 2% to 8%, the metallic iron content in the SR increased from 1.85% to 6.75%, the total iron content decreased from 19.63% to about 16%, and the TiO2 content increased from 64.97% to about 70%. It can be deduced that an increase in NH4Cl concentration is not conducive to the aeration leaching process. When the concentration of 2% ammonium chloride was the same as that in Reference [21] and the reaction time was reduced by one hour, the removal rate of MFe in this paper was as high as 98.15%, while the removal rate of iron in Reference [21] was less than 50%. The TFe content was close to 20% and this is not sufficient for the aeration leaching products. Aeration leaching tests using 2% NH4Cl with 0% to 3% hydrochloric acid were then carried out. The results are shown in Figure 4. For the same reaction time and other conditions, the contents of TFe and MFe in the SR monotonically decreased with an increase in hydrochloric acid concentration from 1% to 3%, while the TiO2 content increased. The addition of hydrochloric acid helped to improve Aeration leaching tests using 2% NH 4 Cl with 0% to 3% hydrochloric acid were then carried out. The results are shown in Figure 4. For the same reaction time and other conditions, the contents of TFe and MFe in the SR monotonically decreased with an increase in hydrochloric acid concentration from 1% to 3%, while the TiO 2 content increased. The addition of hydrochloric acid helped to improve the aeration leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. Similar to Figure 3, the reaction rate of MFe was high but there was over 10% content of TFe in the aeration leaching products. The aeration leaching was better with the addition of hydrochloric acid than with the NH4Cl alone, so pure hydrochloric acid was considered for comparative analysis. The hydrochloric acid concentration was selected as 1.5% (m/v). The resulting MFe and TiO2 contents in the SR are shown in Figure 5.
In the hydrochloric acid system, the TFe content in the SR was about 4%, compared with over 10%, and even up to 18%, in the NH4Cl system. This proved that hydrochloric acid is better for aeration leaching than ammonium chloride. However, it is more difficult to store and transport hydrochloric acid, and the accumulation of chloride ion is not conducive to recycling of the corrosion solution. Therefore, comprehensive consideration is needed to select the best aeration leaching solution.

Effect of Stirring Speed
Stirring is one of the most important factors in mixing processes in the chemical industry and metallurgy. The purpose is to mix evenly, accelerate the dissolution, or accelerate the reaction process. Generally, too slow a stirring speed will lead to uneven mixing and too fast a stirring speed The aeration leaching was better with the addition of hydrochloric acid than with the NH 4 Cl alone, so pure hydrochloric acid was considered for comparative analysis. The hydrochloric acid concentration was selected as 1.5% (m/v). The resulting MFe and TiO 2 contents in the SR are shown in Figure 5.
In the hydrochloric acid system, the TFe content in the SR was about 4%, compared with over 10%, and even up to 18%, in the NH 4 Cl system. This proved that hydrochloric acid is better for aeration leaching than ammonium chloride. However, it is more difficult to store and transport hydrochloric acid, and the accumulation of chloride ion is not conducive to recycling of the corrosion solution. Therefore, comprehensive consideration is needed to select the best aeration leaching solution. The aeration leaching was better with the addition of hydrochloric acid than with the NH4Cl alone, so pure hydrochloric acid was considered for comparative analysis. The hydrochloric acid concentration was selected as 1.5% (m/v). The resulting MFe and TiO2 contents in the SR are shown in Figure 5.
In the hydrochloric acid system, the TFe content in the SR was about 4%, compared with over 10%, and even up to 18%, in the NH4Cl system. This proved that hydrochloric acid is better for aeration leaching than ammonium chloride. However, it is more difficult to store and transport hydrochloric acid, and the accumulation of chloride ion is not conducive to recycling of the corrosion solution. Therefore, comprehensive consideration is needed to select the best aeration leaching solution.

Effect of Stirring Speed
Stirring is one of the most important factors in mixing processes in the chemical industry and metallurgy. The purpose is to mix evenly, accelerate the dissolution, or accelerate the reaction process. Generally, too slow a stirring speed will lead to uneven mixing and too fast a stirring speed can damage the product. High-speed mixing consumes more electric energy, which results in an

Effect of Stirring Speed
Stirring is one of the most important factors in mixing processes in the chemical industry and metallurgy. The purpose is to mix evenly, accelerate the dissolution, or accelerate the reaction process. Generally, too slow a stirring speed will lead to uneven mixing and too fast a stirring speed can damage the product. High-speed mixing consumes more electric energy, which results in an increase in production cost. Selection of an appropriate mixing speed is, therefore, essential [23]. The effect of stirring speed on the removal of metallic iron from the reduced ilmenite was investigated by varying the impeller speed in the range of 400 to 1000 rpm. The concentration of hydrochloric acid was 1.5% m/v, the reaction time was 4 h, and the aeration gas was present in excess.
As shown in Figure 6, the metallic iron remaining in the SR decreased from 6.95% at 400 rpm to 0.49% at 800 rpm, corresponding to a reduction in metallic iron content of 6.46% points. The metallic iron content increased to 2.85% at 1000 rpm and the iron content increased by 2.36% compared with the value of 2.85% at 800 rpm.
Metals 2020, 10, x FOR PEER REVIEW 6 of 9 varying the impeller speed in the range of 400 to 1000 rpm. The concentration of hydrochloric acid was 1.5% m/v, the reaction time was 4 h, and the aeration gas was present in excess. As shown in Figure 6, the metallic iron remaining in the SR decreased from 6.95% at 400 rpm to 0.49% at 800 rpm, corresponding to a reduction in metallic iron content of 6.46% points. The metallic iron content increased to 2.85% at 1000 rpm and the iron content increased by 2.36% compared with the value of 2.85% at 800 rpm. The presence of agitation can break up bubbles, increase the specific surface area of bubbles, and accelerate mass transfer from the gas phase to the liquid phase. Agitation can also promote uniform suspension of reduced ilmenite particles, increase the liquid-solid contact area, accelerate the internal diffusion process, and prevent corroded iron ions from reducing ilmenite particles in an in situ reaction. The vortex will be formed at high speed, which will lead to uneven mixing of gas, liquid, and solid. Figure 7 presents XRD spectra of reduced ilmenite before and after iron removal by aeration under the conditions: room temperature, t = 4 h, excess oxygen, 1.5% hydrochloric acid. The major phases in the reduced ilmenite before iron removal were Fe, TiO2, and FeTi2O5. The diffraction peaks of FeTi2O5 and TiO2 were strong. Peaks for the metallic iron phase were not to be found in the sample after aeration leaching, but diffraction peaks of FeO(OH) were detected. The diffraction peaks of FeO(OH) and TiO2 had the same intensity, which indicated that metallic iron transformed into FeO(OH). There were just two main phases in the sample after the aeration leaching process. These results indicate that the transformation of reduced ilmenite into rutile was achieved under these experimental conditions. The presence of agitation can break up bubbles, increase the specific surface area of bubbles, and accelerate mass transfer from the gas phase to the liquid phase. Agitation can also promote uniform suspension of reduced ilmenite particles, increase the liquid-solid contact area, accelerate the internal diffusion process, and prevent corroded iron ions from reducing ilmenite particles in an in situ reaction. The vortex will be formed at high speed, which will lead to uneven mixing of gas, liquid, and solid. Figure 7 presents XRD spectra of reduced ilmenite before and after iron removal by aeration under the conditions: room temperature, t = 4 h, excess oxygen, 1.5% hydrochloric acid. The major phases in the reduced ilmenite before iron removal were Fe, TiO 2 , and FeTi 2 O 5 . The diffraction peaks of FeTi 2 O 5 and TiO 2 were strong. Peaks for the metallic iron phase were not to be found in the sample after aeration leaching, but diffraction peaks of FeO(OH) were detected. The diffraction peaks of FeO(OH) and TiO 2 had the same intensity, which indicated that metallic iron transformed into FeO(OH). There were just two main phases in the sample after the aeration leaching process. These results indicate that the transformation of reduced ilmenite into rutile was achieved under these experimental conditions. Philips ssx-550 scanning electron microscope (SEM) images of the sample before and after aeration leaching are shown in Figure 8. There are obvious differences between the raw material and the product of the aeration leaching process: the sample before aeration leaching was compact and we could not find holes in the surface; after aeration leaching, the interior was full of holes, giving rise to a network structure, which maintained the sample integrity. The holes are attributed to the transformation of metallic iron into iron oxide by the aeration leaching reaction and its removal from the interior of sample.  Philips ssx-550 scanning electron microscope (SEM) images of the sample before and after aeration leaching are shown in Figure 8. There are obvious differences between the raw material and the product of the aeration leaching process: the sample before aeration leaching was compact and we could not find holes in the surface; after aeration leaching, the interior was full of holes, giving rise to a network structure, which maintained the sample integrity. The holes are attributed to the transformation of metallic iron into iron oxide by the aeration leaching reaction and its removal from the interior of sample. Philips ssx-550 scanning electron microscope (SEM) images of the sample before and after aeration leaching are shown in Figure 8. There are obvious differences between the raw material and the product of the aeration leaching process: the sample before aeration leaching was compact and we could not find holes in the surface; after aeration leaching, the interior was full of holes, giving rise to a network structure, which maintained the sample integrity. The holes are attributed to the transformation of metallic iron into iron oxide by the aeration leaching reaction and its removal from the interior of sample.

Conclusions
The following conclusions were drawn on the basis of the results obtained in this work. Aeration leaching is feasible at room temperature. For a reaction time of 4 h and adequate stirring speed of 800 rpm, the effectiveness of metallic iron removal differed for different solution systems. When using NH4Cl, the MFe content could be reduced to 1.98% in SR, but the TiO2 content only reached 69.16%. A higher NH4Cl concentation did not improve aertion leaching. Using 2% NH4Cl with hydrochloric acid, the presence of the acid helped to improve the leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. In the hydrochloric acid system, the MFe content was as low as 0.14% and TFe content was about 4%, indicating that the SR could be upgraded to about 90%.

Conclusions
The following conclusions were drawn on the basis of the results obtained in this work. Aeration leaching is feasible at room temperature. For a reaction time of 4 h and adequate stirring speed of 800 rpm, the effectiveness of metallic iron removal differed for different solution systems. When using NH 4 Cl, the MFe content could be reduced to 1.98% in SR, but the TiO 2 content only reached 69.16%. A higher NH 4 Cl concentation did not improve aertion leaching. Using 2% NH 4 Cl with hydrochloric acid, the presence of the acid helped to improve the leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. In the hydrochloric acid system, the MFe content was as low as 0.14% and TFe content was about 4%, indicating that the SR could be upgraded to about 90%.