Research on the process and mechanism of preparing titanium-rich materials from complex ilmenite

A new process for the development and utilization of titanium-iron sand ore from Yunnan province is proposed. We provide a detailed analysis of the chemical composition and mineral structure of the raw material, and describe a pretreatment involving low-temperature oxidation and reduction, hydrofluoric acidolysis, and hydrochloric acid leaching to remove impurities and generate titanium-rich materials. Hydrofluoric acid was used to break down the complex titanite, silica-titanium, and calcium-titanium mixture in the starting mineral into simple oxides. The results have indicated that the thermal reduction process should not be conducted at elevated temperatures. Optimal process conditions are established for the leaching process: leaching temperature of 80 °C; leaching time of 240 min; hydrofluoric acid concentration of 3%; liquid–solid ratio of 6 m l/g. Under these conditions, the extent of TFe leaching reached 96%, while the leaching of Ca and Mg reached 94%. The TiO2 grade of the final titanium-rich material was 89.95%, with a CaO content of 0.39%, a MgO content of 0.15%, and a TFe content of 7.5%. This material can serve as raw material for subsequent chlorination.


Introduction
Titanium is strategic metal that is extensively utilized in aerospace, biomedicine, marine engineering, construction, and automotive manufacturing due to its low density, high temperature stability, and exceptional resistance to acid corrosion [1].In recent decades, the global demand for titanium metal and titanium materials has significantly increased.Taking an overview of the literature, titanium dioxide, in particular, has consistently been in high demand as a result of its remarkable scattering behavior, superior optical and electronic properties, and high photocatalytic activity [2][3][4].Titanium dioxide is found in minerals such as rutile and ilmenite.Whereas the supply of natural rutile is limited, China has an abundance of ilmenite resources, notably in Sichuan, Hebei, and Yunnan.The ilmenite in Sichuan and Hebei is primarily found in rock ore, and is characterized by a low TiO 2 grade with high levels of impurities such as CaO, MgO, and SiO 2 .The ilmenite in the Yunnan region mainly consists of sand ore, the TiO 2 grade is high with significant calcium, magnesium, silicon, and other impurities.These impurities do not exist as simple oxides, but form complex substances such as titanite and olivine [5,6].
Rutile and ilmenite are the primary raw minerals used in the production of titanium dioxide.Due to the depletion of natural rutile reserves, research is now focused on the use of ilmenite in titanium dioxide production [7][8][9].Currently, the production of titanium dioxide primarily involves sulfuric acid and chlorination processes.The sulfuric acid process is time-consuming, complex, and results in low-grade titanium dioxide.Moreover, it generates significant quantities of pollutants, creating a bottleneck in the development of the titanium dioxide industry that cannot be easily overcome.In contrast, the chlorination process involves a shorter duration, large production capacity and high level of automation, resulting in high-grade titanium dioxide, and has become the principal production route.However, the chlorination process requires high-quality raw materials.The titanium slag must contain no less than 80 wt% TiO 2 , with a CaO + MgO content below 1.50 wt%.Impurities can lead to the formation of high boiling point chlorides, which may the fluidized bed chlorinator to fail [10,11].When using ilmenite for titanium dioxide production, it is necessary to first obtain titanium-rich materials by removing impurities.Given China's large-scale production of titanium-rich materials and the current gap in product quality compared with other countries, more than 90% of the raw material used in China for the chlorination process is imported.It is therefore essential to enhance the production technology for titanium-rich materials in order to address the issue of raw material imports, and achieve independent control.
At present, there are more than 20 methods for preparing titanium-rich materials using ilmenite as raw material.The most widely used industrial methods include electric furnace smelting [12], the reduction-rust procedure [12], and acid leaching method [13].Electric furnace smelting makes use of an electric furnace as a high-temperature heat source to enable chemical reaction between a solid reducing agent and the iron oxides in ilmenite, producing metallic iron, and magnetic separation of iron result in the production of high titanium slag.Yu et al conducted a study of electric furnace smelting with magnetic separation to investigate the impact of carbon content, reduction time, and Na 2 CO 3 addition on process efficiency.The results demonstrated that the addition of Na 2 CO 3 was beneficial for forming a semi-molten state, which facilitated diffusion, aggregation, and growth of the metal phase.Furthermore, the use of wet milling with magnetic separation effectively separated the slag iron, a TiO 2 grade of 81.63%, an iron content of 4.53%, and a TiO 2 recovery of 93.43% [14].Nevertheless, electric furnace smelting exhibits a number of drawbacks, notably the inability to remove impurities such as calcium, magnesium, and silicon from the ferro-titanium concentrate [11,[15][16][17].The reduction rusting method involves iron oxides reduction to metallic iron using solid-phase reductants, followed by treatment with aqueous acid solution, which generates a high-grade artificial rutile product.Zheng et al have proposed the acidolysis of titanium slag using NH 4 Cl, and studied the effects of acidolysis temperature and time on the conversion of metallic iron, examining the influence of the mineralogy of the iron-containing products on titanium and iron separation.The results showed that the separation yielded a titanium-rich material with 77.81% TiO 2 , and an iron-containing product with a total iron content of 52.69% [18].Lei Zhou et al utilized the reduction-rust method to treat ilmenite, employing a novel horizontal frame capable of more efficiently removing iron when compared with traditional horizontal frames, generating a titanium-rich material with a grade as high as 86.1% and a Fe content of 4.6% [19].In common with electric furnace smelting, reduction-rust can only remove iron and other impurities are not eliminated.Acid leaching is the most widely applied procedure, and can be subdivided into sulfuric acid and hydrochloric acid methods.The sulfuric acid process has a low leaching efficiency and generates high levels of waste.It is suitable for extracting artificial rutile from high-grade, low-calcium titanium ores.In contrast, the hydrochloric acid technique does not require stringent ore selection standards and is highly effective in eliminating contaminants [13,18,20,21].Zhang et al utilized the hydrochloric acid method to synthesize artificial rutile with a TiO 2 content exceeding 90%, a total iron content below 1.37%, and a combined impurity content of CaO and MgO below 1.0 wt% from titanium ore sourced in Panxi.This level of efficiency was achieved through a sequence of oxidation, reduction, mechanical activation, and hydrochloric acid leaching [22].
In the case of ilmenite leaching using the hydrochloric acid method, a sample pretreatment is typically conducted before acid leaching.This pretreatment can involve a 'high-temperature weak reduction' or 'oxidation followed by high-temperature weak reduction', which serves to enhance the effectiveness of hydrochloric acid leaching in decontaminating rock ore.However, decontamination of Yunnan alluvial ore showed moderate efficiency where high-temperature reduction increased the levels of calcium and magnesium decontamination, but lowered the effectiveness of iron leaching [23].
In this study, the pretreatment of ilmenite from Yunnan sand ore has involved a 'low-temperature oxidation followed by low-temperature reduction' process.Before acid leaching, the ilmenite underwent decomposition with hydrofluoric acid, where hydrochloric acid treatment generates titanium-rich materials suitable for a subsequent application of the chlorination method.We have investigated the impact of carbothermal reduction on the physical phase and morphology of ilmenite.Moreover, we have considered the effects of hydrofluoric acidolysis, leaching time and temperature on the removal of iron, calcium, and magnesium impurities, and explored the mechanism of hydrochloric acid leaching.This process delivered high efficiency in removing impurities, producing a high grade titanium-rich material that improves the potential utilization of ilmenite.These advancements are of considerable significance with respect to China's titanium industrial production in lowering overall production costs.

Materials
The ilmenite obtained from Yunnan was characterized by x-ray diffraction, as shown in figure 1.The ilmenite exhibits two distinct peaks corresponding to FeTiO 3 and TiO 2 phases.Signals due to MgTiO 3 , Al 2 O 3 , Ca 2 Si 2 O 7 , SnO, and Ca 3 TiFeSi 3 O 12 are also evident.The chemical composition of the ilmenite is given in table 1.The sample was analyzed by scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS), and the results are presented in figure 2. The SEM image reveals an irregular morphology with fragments of varying size where the surface is smooth and flat.

Analysis and characterization
Changes to the mineral physical phases were analyzed using an x-ray diffractometer (Bruker D8 ADVANCE A25X, Germany).The elemental content was determined by energy dispersive x-ray spectroscopy (EDX-8000 ROHS-ASSY, Shimadzu, Japan) and x-ray fluorescence spectrometry.The micro-morphology of ilmenite and the leach slag was assessed using a scanning electron microscope (NOVA-NANOSEM-450, FEI, USA).The total iron and titanium dioxide contents of ilmenite and the leaching product were determined using the titration method, and the silicon contents were measured using a visible spectrophotometer (V-1100D).In addition, the calcium and magnesium contents of the leaching product were determined by plasma emission spectroscopy (ICAP7000).

Experimental procedure 2.3.1. Pretreatment experiment
A previous study has established that the ilmenite oxidation temperature should not exceed 700 °C.If the temperature is too low, sample oxidation does not reach completion, whereas the oxidation process is excessive at higher temperatures.The ilmenite was subjected to oxidation at 700 °C in air prior to titanium reduction [24].The oxidized modified ilmenite was ground and mixed with carbon powder and sodium bicarbonate in the ratio 1:0.2:0:03 in a closed environment for carbothermal reduction.

Hydrofluoric acid acidolysis
The oxidation-reduced modified ilmenite was placed in a polyethylene beaker, and acidolysis was conducted by adding a low concentration of hydrofluoric acid with stirring for 10 min, followed by immediate filtration.

Hydrochloric acid leaching
Following hydrofluoric acid treatment, the mineral was added to a 500 ml round-bottom flask, which was placed in a constant temperature water bath with a magnetic agitator.Once the preset temperature was reached, concentrated hydrochloric acid was added to the flask, which was sealed and connected to a reflux condenser to minimize evaporation of hydrochloric acid.Reaction was carried out for a predetermined time at a constant stirring speed, followed by immediate filtration.The filtrate was washed with distilled water, dried and calcined at 700 °C.Finally, the filtrate was spun on a rotary evaporator.The degree of leaching of TFe, Ca, and Mg was determined according to the equation (1).
Where η (%) represents the degree of extraction of TFe, Ca, or Mg from ilmenite, m 1 and m 2 refer to the mass of raw ilmenite before and after leaching, respectively; x 1 and x 2 represent the percentages of Fe, Ca, and Mg in ilmenite before and after leaching by oxidation-carbon thermal reduction-hydrofluoric acid treatment, respectively.The experimental procedure is illustrated in figure 3.

Results and discussion
3.1.Effect of pretreatment on ilmenite 3.1.1.Analysis of thermodynamic According to the XRD diffraction results for the original ore, it is evident that ilmenite has a complicated structure.The impurities are not present as simple oxides, but take the form of titanium and iron compounds that form complex substances.A simple oxidation treatment is not sufficient to effectively leach these impurities.A mild reduction of oxidized ilmenite is also necessary to facilitate leaching of iron impurities.
Following oxidation of ilmenite, the main products are Fe 2 O 3 and TiO 2 .In the subsequent reduction, two types of reduction reaction systems, Fe-C and Ti-C, are dominant.However, the actual reaction is primarily dominated by Fe-C reduction [24][25][26][27].The possible reactions in the thermal reduction process are shown in table 2. In order to assess the feasibility of chemical reactions (1) -(10), a plot of ΔG θ versus temperature was generated using HSC Chemistry 6.0 software (figure 4).
It can be seen that reactions (3), ( 5), ( 6), ( 7), (8), and (10) exhibit a decrease in ΔG θ as the temperature increases.The results suggest that higher temperatures are beneficial for the reduction process.The ΔG θ associated with reaction (4) is always less than 0 and largely temperature invariant.In contrast, reaction (9) shows an increase in ΔG θ with increasing temperature; ΔG θ is always less than 0, indicating that the reaction can proceed spontaneously at room temperature.The ΔG θ associated with reaction (7) also increases at higher temperatures, reflecting a decreasing tendency for spontaneous reaction.At 700 °C, the ΔG θ of reaction equation (7) exceeds 0, indicating that the reaction cannot proceed spontaneously beyond this temperature.When the thermodynamic temperature is below 700 °C, the tendency for the carbothermal reduction of Fe 2 O 3 to undergo spontaneous reaction is reduced, and some reactions do not occur spontaneously.An increase in thermodynamic temperature above 700 °C enhances spontaneous carbothermal reduction of Fe 2 O 3 .Moreover, at temperatures in excess of 900 °C, the ΔG θ of the Fe 2 O 3 carbothermal reduction reaction becomes negative, indicating an increased tendency to produce metallic Fe.Consequently, the operating carbothermal reduction temperature should be maintained between 700 °C and 900 °C.

Analysis of thermogravimetric
The TG-DSC measurements were used to determine the reduction characteristics of ilmenite.A standard thermogravimetric (TG) thermal decomposition experiment was performed on a mixture of ilmenite, sodium  No.
Reaction equation bicarbonate, and carbon powder (mass ratio of 1:0.03:0.2) in argon, generating the TGA-DSC curve presented in figure 5.
The curve indicates that the ilmenite weight loss process can be divided into four stages.The first stage occurred at 25 °C-200 °C, and represents sodium bicarbonate decomposition, NaHCO 3 → Na 2 CO 3 + H 2 O (g), with an associated weight loss of 6.97%.In this stage, there is an exothermic peak in the DSC curve at 120 °C associated with heat released when sodium bicarbonate decomposes into Na 2 CO 3 and water that evaporates [14,28].
The second stage ranged from 200 °C to 980 °C with a weight loss of 8.02%.This stage includes three exothermic peaks at 340 °C, 750 °C, and 930 °C, that can be attributed to the thermal decomposition of the carbon additive C (s) → CO (g) + CO 2 (g), generating carbon oxides and other volatiles [29,30].The carbon thermal reduction process is a multi-phase reaction process, involving solid phase reacting with solid phase and solid phase reacting with gas phase.The carbon thermal reduction of Fe 2 O 3 is a stepwise process: Fe

Physical and morphological analysis of ilmenite
The XRD patterns for ilmenite following carbothermal reduction at various temperatures are shown in figure 6.It can be seen that the intensity of the TiO 2 characteristic peaks increases with an increase in reduction temperature.Conversely, the characteristic peaks of Fe 2 O 3 weaken and even disappear.In addition, new peaks emerge and increase in intensity at higher temperatures.At 700 °C, the predominant physical phases of ilmenite are TiO 2 and Fe 2 O 3 , with the formation of CaTiOSiO 4 through the decomposition of Ca 3 TiFeOSi 3 O 12 that is originally present in the ore.At 800 °C, the characteristic peak intensity of TiO 2 and CaTiOSiO 4 increases, while the intensity of the peak due to Fe 2 O 3 decreases, and peaks associated with MgFeSiO 4 appear.The formation of MgFeSiO 4 , where the valence of Fe is +2, can be attributed to a temperature induced reduction of Fe (III) to Fe (II).NaHCO 3 produces Na 2 CO 3 with increasing temperature, and at high temperatures, titanium ions migrate and form NaTiO 3 with Na 2 CO 3 .The migration of titanium ions results in the formation of crevices on the surface of ilmenite, and MgO, FeO, and SiO 2 may combine through these crevices to form MgFeSiO 4 .At 900 °C, CaTiOSiO 4 decomposes to CaSiO 3 and TiO 2 , and the latter reacts with FeO to give FeTiO 3 .Furthermore, thermal reduction lowers the intensity of the TiO 2 peak.At 1000 °C, the reduction of Fe 2 O 3 results in a predominance of TiO 2 characteristic peaks in the solid product with the additional presence of titanium oxides that exhibit a lower valence state.
The results of the SEM-EDS analysis of the titanium ore at different reduction temperatures are presented in figure 7.At a reduction temperature of 700 °C (figure 7(a)), the surface of the sample consists of irregular polygonal structures tightly stacked together, with the presence of a rod-like structure embedded in the polygons.Elemental analysis has established the presence of O, Si, Ti and Fe as the principal constituent elements.The XRD results have confirmed the corresponding compounds as SiO 2 , TiO 2 and Fe 2 O 3 .As shown in figure 7(b), the rod-like structure grows with an increase in temperature (to 800 °C), precipitates and accumulates on the surface of the titanium ore.EDS analysis of the rod structure shows that the main elements of the structure are O and Ti, and combined with XRD, it can be seen that the rod structure is TiO 2 .At 900 °C (figure 7(c)), the surface appears flat with no evidence of the rod-like structures.XRD analysis has confirmed that the principal compound at this temperature is SiO 2 .At 1000 °C (figure 7(d)), the sample exhibits spherical structures embedded in the surface where EDS analysis has confirmed the presence of O and Fe.At 700 °C, sample reduction is minimal with significant separation between titanium and iron.In the temperature range 800-900 °C, SiO 2 inhibits the reduction of Fe(III), impurities such as CaO and MgO form salts with SiO 2, which impact on the degree of leaching.At 1000 °C, TiO 2 reacts with carbon, resulting in the formation of Ti that affects the grade of TiO 2 [35].Studies have shown that reduction at high temperatures is effective in removing Ga and Mg impurities but is not favorable for Fe leaching.In order to enhance the leaching of Fe, a lower temperature reduction should be chosen [36].

Influence of hydrofluoric acid concentration on leaching
The impurities of calcium and silicon in ilmenite do not exist as simple oxides; rather, they combine with TiO 2 to form complex structures, such as sphene (a mixture of silicon, titanium, and calcium), silicon-titanium compounds, and perovskite.These substances have stable structures and are not easily separated or reactive to acids and alkalis.Must be a low concentration of hydrofluoric acid to prevent acidolysis, which causes the complex structure of the material to break down into simple oxides for leaching.In order to investigate the effect of hydrofluoric acid concentration on the quality of the ultimate titanium-rich material, leaching experiments were carried out under the following experimental conditions: liquid-solid ratio of 6 ml g −1 ; leaching temperature of 80 °C; leaching time of 240 min.The experiments were conducted using different mass fractions of hydrofluoric acid.The leached products were characterized by XRD analysis.The correlation of diffraction results with Ti content and the extent of TFe, Ca, and Mg leaching is shown in figure 8 as a function of hydrofluoric acid concentration.
As shown in figure 8(a), the leaching products as analyzed by XRD are primarily attributed to TiO 2 .The XRD pattern of the pretreated ilmenite without hydrofluoric acid etching exhibited diffraction peaks due to CaMg(SiO 3 ) 2 and SiO 2 .At a hydrofluoric acid mass fraction of 1%, the occurrence of SiO 2 diffraction peaks may be attributed to the low concentration of hydrofluoric acid, which is not sufficient to thoroughly corrode the complex compounds containing silicon, resulting in a significant Si residue.Applying a hydrofluoric acid mass fraction of 2% to 3%, the diffraction peaks due to SiO 2 disappeared with the appearance of diffraction peaks associated with TiO 2 .There were no detectable diffraction signals due to other impurities.At a hydrofluoric acid mass fraction of 4%, the intensity of the TiO 2 diffraction peak was reduced.As shown in figure 8(b), when the mass fraction of hydrofluoric acid was below 3%, the Ti content in the leaching product increased with increasing hydrofluoric acid concentration.The Ti content reached a maximum at a mass fraction of 3%.When the concentration of hydrofluoric acid exceeded 3%, reaction with TiO 2 resulted in a significant decrease in the From figure(c), it can be observed that the concentration of hydrofluoric acid significantly influences the leaching of TFe, Ca, and Mg, with a particularly pronounced impact on the leaching rate of Ca.When the mass fraction of hydrofluoric acid is lower than 3%, the leaching rates of TFe, Ca, and Mg increase with an increase in hydrofluoric acid concentration.The leaching rates of TFe, Ca, and Mg reach their maximum when the mass fraction of hydrofluoric acid is 3%.However, when the concentration of hydrofluoric acid exceeds 3%, the leaching rates of Ca and Mg continuously decrease, and the leaching of TFe reaches saturation.Therefore, the optimal concentration of hydrofluoric acid is 3%.

Influence of extraction time on leaching
The possible impact of leaching time on the quality of the titanium-rich material was assessed at a liquid-solid ratio of 6 ml g −1 , a leaching temperature of 80 °C, and a hydrofluoric acid concentration of 3%.The results of the XRD analysis and the correlation with Ti content and extent of TFe, Ca, and Mg leaching are given in figure 9.
As shown in figure 9(a), the material phases after 120 min and 180 min leaching consisted mainly of TiO 2 , MgSiO 3 , FeSiO 3 , and SiO 2 .Extending the duration of leaching resulted in less intense diffraction peaks due to SiO 2 and MgSiO 3 , the disappearance of the FeSiO 3 diffraction peak, and gradual increase of the TiO 2 signal.Following a 240 min leaching period, diffraction peaks for SiO 2 and MgSiO 3 were not detected, and the predominant material phase was TiO 2 .The TiO 2 XRD peak reached its maximum intensity at 240 min, and decreased slightly when the leaching period exceeded 240 min.
From a consideration of the results presented in figures 9(b) and (c), it can be seen that the leaching of TFe was essentially unaffected by the leaching time.The leaching process was incomplete for treatment times less than 240 min, and the content of Ti as well as the degree of leaching of Ca and Mg increased with longer leaching times to reach their highest levels after 240 min.At treatment times in excess of 240 min, the Ti content and leaching of Ca and Mg decreased, which may be attributed to a prolonged exposure of TiO 2 to strong acid conditions that resulted in a reduction in particle size and subsequent pulverization [37,38].Therefore, the optimal leaching time was determined to be 240 min.

Influence of liquid-solid ratio on leaching
The effect of the liquid-solid ratio on titanium content and the extent of impurity leaching was assessed at 80 °C, 240 min, and a hydrofluoric acid concentration of 3%.The tests considered various liquid-solid ratios, and the results of XRD analysis, and the correlation of titanium content and levels of impurity leaching are presented in figure 10.
As shown in figure 10(a), leaching was incomplete at a liquid-solid ratio in the range 4-5 ml g −1 , where XRD analysis is consistent with the presence of TiO 2 and SiO 2 .An increase in hydrochloric acid concentration increased the extent of leaching, with a reduction in the SiO 2 XRD peak intensity and an increase in the TiO 2 signal.At a liquid-solid ratio of 6 ml g −1 , the diffraction peak due to SiO 2 disappeared, and the product phase was primarily TiO 2 with a maximum in peak intensity.When the liquid-solid ratio exceeded 6 ml g −1 , the intensity of the TiO 2 diffraction peak gradually decreased with the appearance of the CoFe diffraction peak at a liquid-solid ratio of 8 ml g −1 .
From a consideration of figures 10(b) and (c), it can be seen that the leaching of TFe, Ca, and Mg increases with an increased liquid-solid ratio.At a liquid-solid ratio less than 6 ml g −1 , leaching was incomplete.The Ti content and the extent of Ca and Mg leaching increased at higher liquid-solid ratios, where the leaching of TFe showed a slight increase.When the liquid-solid ratio was 6 ml g −1 , the Ti content and the leaching of TFe, Ca, and Mg reached their highest values.At a liquid-solid ratio greater than 6 ml g −1 , the extent of leaching of Ti, TFe, Ca, and Mg decreased.This may be attributed to an excessively high concentration of Cl − during the hydrochloric acid leaching process.The TiO 2 that is leached by acidolysis can react with Cl − to form TiOCl 2 .When the rate of acidolysis of TiO 2 is slower than the rate of TiOCl 2 generation, Ti enters solution in the form of TiOCl 2 [35].This finding is confirmed by the XRD spectra, indicating that the optimal liquid-solid ratio is 6 ml g −1 .

Influence of extraction temperature ratio on leaching
The effects of leaching temperature on titanium content and impurity leaching were examined at a hydrofluoric acid concentration of 3%, a leaching time of 240 min, and a liquid-solid ratio of 6 ml g −1 .The results of XRD analysis are presented in figure 11 with a correlation of titanium content and impurity leaching as a function of temperature.
Temperature had a significant impact on the leaching process.The XRD patterns (figure 11(a)) have revealed that the material phases are mainly TiO 2 , SiO 2 , and MgSiO 3 at a leaching temperature of 50 °C.When the temperature was increased to 60 °C, the diffraction peak due to SiO 2 disappeared, the intensity of the diffraction peak for MgSiO 3 increased with the appearance of a signal associated with CaSiO 3 .With a further increase in temperature to 70 °C, the intensity of the MgSiO 3 and TiO 2 diffraction peaks reached the highest values, the diffraction peak for CaSiO 3 disappeared, and a peak due to FeSiO 3 appeared.When the leaching temperature exceeded 80 °C, the MgSiO 3 and FeSiO 3 diffraction peaks disappeared, and the intensity of TiO 2 diffraction peaks decreased slightly.
The results presented in figures 11(b) and (c) indicate that the leaching of TFe is not influenced by temperature.However, the leaching of Ca is sensitive to temperature changes, while the leaching of Mg is moderately affected by the leaching temperature.When the temperature was increased from 50 °C to 80 °C, the Ti content and the degree of leaching of Ca and Mg increased.However, the leaching of TFe only showed a slight increase.At a leaching temperature of 80 °C, the Ti content and leaching of Ca and Mg reached maximum levels.When the operating temperature exceeded 90 °C, the Ti content decreased, and the leaching of Ca and Mg showed a slight decrease.Higher temperatures should increase the activity of hydrochloric acid, which favors the leaching reaction.Therefore, the leaching rate of Ca and Mg should increase with increasing temperature.However, in the case of Ti, elevated temperature and hydrochloric acid volatilization facilitate hydrolysis of TiOCl 2 , resulting in the dissolution of TiOCl 2 into fine TiO 2 particles, which then precipitate.The interplay between these two processes causes the degree of leaching of Ti to initially increase and then decrease with increasing temperature.Consequently, the intensity of TiO 2 XRD peaks in the leaching products showed a slight decrease at 80 °C.
3.6.Characterization and analysis of leached products 3.6.1.Chemical composition The chemical composition of the leaching product under optimal experimental processing conditions is given in table 3.These conditions involve a hydrofluoric acid concentration of 3%, a leaching temperature of 80 °C, a liquid-solid ratio of 6 ml g −1 , and a leaching time of 240 min.Under these process conditions, the leaching product contains 89.95% TiO 2 , 5.44% TFe, 3.72% SiO 2 , 0.39% CaO, and 0.15% MgO.

SEM-mapping analysis
The leaching products were subjected to SEM analysis to determine morphology under the following experimental conditions: hydrofluoric acid concentration of 3%; leaching temperature of 80 °C; liquid-solid ratio of 6 ml g −1 ; leaching time of 240 min.The results are presented in figure 12 where it can be seen that the product morphology is characterized by irregular shapes and different sizes (figure 12-a).The magnified image (figures 12(a-1)) shows an uneven surface.The SEM images presented in figures 12(a-2) and (a-3) reveal a range of distinct morphologies, including surface granular and rod-like structures.The cross-section is relatively flat  with evidence of an irregular porous structure, suggesting that the micro-interior of the leached product is not a solid but hollow.The granular and rod-like TiO 2 morphologies may be attributed to the high local temperature resulting from uneven heating during oxidation.In addition, the TiO 2 crystal size can decrease at a higher operating temperature.An EDS mapping of the leaching product has revealed the presence of Ti, O, Fe, and Si elements, as illustrated in figure 13.The analysis presented establishes Ti and O as the predominant elements, accounting for Ca.70% of the composition.The mapping confirms an even distribution of Ti and O over the entire sample, where the content of the other elements is relatively low.The distribution of Fe appears uniform, whereas Si largely occurs along the fragment edges.The incomplete leaching of Si can be attributed to the low concentration of hydrofluoric acid.The initial decomposition involving CaTiOSiO 4 , followed by reaction with decomposed SiO 2 that resulted in an uneven distribution and higher concentration of Si in the sample.

Hydrochloric acid leaching reaction mechanism
Samal et al [39] have demonstrated that the leaching of ilmenite may be operated under chemical control.This control can involve the diffusion of reacting species and resultant products to or from the reaction interface, or it can be controlled by the diffusion of these substances through the product layer on the particle surface.It has been shown that the formation of Ti(IV) during hydrochloric acid leaching depends on the concentrations of Ti ions and Cl − in solution.The concentration of Cl − is dependent on the starting acid concentration, where the concentration of Ti ions increases as the dissolution process proceeds.When the concentration of Ti ions in solution exceeds 10 −3 M, a polymerization, or hydrolysis, occurs.
The leaching process of Ti in ilmenite can be divided into three stages.The first stage involves the acidolysis of ilmenite.In the second stage, titanium oxychloride is generated, which undergoes hydrolysis and polymerization in the third stage.Within a specific acidolysis timeframe, the concentration of TiO 2 reaches saturation, leading to phase nucleation and the generation of colloidal crystal species that subsequently undergo hydrolysis.The dissolution process is controlled by reaction (11), whereas the hydrolysis process is controlled by reactions (12) and (13).The leaching mechanism is illustrated in figure 14.In the leaching process, the rate of TiO 2 acidolysis in ilmenite and hydrolytic polymerization of titanium oxychloride affects the grade of TiO 2 in the leaching product.If the rate of acid digestion is greater than the rate of hydrolysis polymerization, the quality of TiO 2 in the leaching product decreases.Conversely, if the rate of acid digestion is lower than the rate of hydrolysis polymerization, the ultimate grade of TiO 2 will increase.As shown in figure 10(b), the Ti content with respect to the liquid-solid ratio first increases and then decreases.A higher liquid-solid ratio is accompanied by an increase in Cl − , which serves to increase the rate of hydrolysis with a resultant dissolution of TiO 2 .Consequently, the rate of acid digestion and hydrolysis polymerization is crucial in controlling the concentration of Cl − .If the Cl − concentration is too high, this promotes the hydrolysis reaction but hinders TiO 2 leaching.

Conclusions
This study has adopted the 'low-temperature oxidation and low-temperature reduction -hydrofluoric acid acidolysis -hydrochloric acid leaching' process to remove impurities from ilmenite and prepare titanium-rich materials that are suitable for chlorination processing.The study has demonstrated the following: 1.The oxidation-reduction pretreatment generates cracks in the ilmenite morphology and disrupts the mineral phase structure.
2. Optimization of process conditions in the 'hydrofluoric acid treatment -hydrochloric acid leaching' of ilmenite to prepare titanium-rich materials.Under the optimum conditions generate a TiO 2 grade of 89.95%, CaO content of 0.39%, and MgO content of 0.15%.The titanium-rich material meets the requirements of the chlorination method for the production of applicable titanium dioxide raw materials.
3. The mechanistic analysis of ilmenite leaching by hydrochloric acid has revealed three stages in the process: acidolysis, generation of titanium oxychloride, and hydrolysis polymerization.The key to controlling the rate of acid digestion and hydrolysis polymerization lies in managing the concentration of Cl − .

Figure 3 .
Figure 3. Oxidation-carbon thermal reduction-hydrofluoric acid acidolysis-hydrochloric acid leaching procedure in the preparation of titanium-rich material.

2 O 3 →
Fe 3 O 4 → FeO → Fe.When the temperature exceeds 750 °C, Fe 2 O 3 reacts with carbon powder, resulting in the reduction of Fe 2 O 3 to Fe 3 O 4 with gaseous product release, accompanied by the exothermic peaks at 340 °C, 750 °C, and 930 °C corresponding to the generation of CO 2 , Fe 3 O 4 , and CO, respectively [31, 32].The third stage occurred at 980 °C with a significant downward trend in the TG curve at higher temperatures, and a weight loss of 13.97%.The exothermic peak at 1080 °C can be ascribed to the reaction between Fe 2 O 3 and carbon

Figure 4 .
Figure 4. Shows the relationship between ΔG θ and temperature in the ilmenite reduction process.

Figure 5 .
Figure 5. TG-DTG curves for the ilmenite reduction process.

Figure 6 .
Figure 6.The XRD patterns of reduced ilmenite at different temperatures.

Figure 8 .
Figure 8.Effect of hydrofluoric acid concentration on leaching: (a)The XRD patterns of leaching products; (b) Plots of titanium content in leaching products; (c) Plots of impurity leaching rate.

Figure 9 .
Figure 9.Effect of leaching time on leaching: (a) The XRD patterns of leaching products; (b) Plots of titanium content of leaching products; (c) Plots of impurity leaching rate.

Figure 10 .
Figure 10.Effect of liquid-solid ratio on leaching: (a) The XRD patterns of leaching products; (b) Plots of titanium content of leaching products; (c) Plots of impurity leaching rate.

Figure 11 .
Figure 11.Effect of temperature on leaching :(a)The XRD patterns of leaching products; (b) Plots of titanium content of leaching products; (c) Plots of impurity leaching rate.

Figure 12 .Figure 13 .
Figure 12.The SEM images of leached products under optimal conditions.

Table 2 .
Possible reactions involved in the reduction process.

Table 3 .
Chemical composition of leaching product under optimal process conditions (wt%).