Hoàn nguyên tinh quạng ilmenit Hà Tĩnh

Bài báo trình bày các kết quả nghiên cứu quá trình hoàn nguyên tinh quặng ilmenit sa khoáng Hà Tĩnh bằng than antraxit…

Solid state reduction of Ha Tinh ilmenite concentrate

THAO T. NGUYEN, THAN N. TRUONG AND BINH N. DUONG

Hanoi University of Science and Technology
Email: binh.duongngoc@hust.edu.vn
Ngày nhận bài: 27/8/2014, Ngày duyệt đăng: 24/9/2014

TÓM TẮT

Bài báo trình bày các kết quả nghiên cứu quá trình hoàn nguyên tinh quặng ilmenit sa khoáng Hà Tĩnh bằng than antraxit. Quá trình hoàn nguyên được thực hiện trong lò điện có sử dụng chất xúc tác Na2C03. Ảnh hưởng của nhiệt độ và thời gian hoàn nguyên tới quá trình hoàn nguyên được đánh giá thông qua múc độ kim loại hóa sắt. Kết quả thu được cho thấy mức độ kim loại hóa sắt tăng khi tăng nhiệt độ hoàn nguyên từ 1050 đến 1200^oC. Khi tăng nhiệt độ hoàn nguyên lên 1250°c thì mức độ kim loại hóa sẳt lại giảm đi. Tăng thời gian hoàn nguyên từ 2 đến 4h cũng làm tăng mức độ kim loại hóa sắt. Tuy nhiên, tiếp tục kéo dài thời gian hoàn nguyên trên 4h cũng lại làm giảm mức độ kim loại hóa. ỏ chế độ hoàn nguyên phù hợp (1150°c và 4h) mức độ kim loại hóa sắt đạt được khoảng 83%. Kết quả kiểm tra nhiễu xạ rơngen khẳng định sự xuất hiện của sắt kim loại trong ilmenit sau hoàn nguyên nhưng không xuất hiện các hợp chất titan có hóa trị thấp, chứng tỏ Ti⁴⁺ gần như không bị hoàn nguyên xuống các hóa tộ thấp hơn.

Từ khóa: ilmenit, hoàn nguyên ilmenit, ilmenit sa khoáng Hà Tĩnh.

ABSTRACT

In this work, solid State reduction of ilmenite concentrate has been carried out using anthracite as a reductant. The reduction was performed in an electric furnace and the catalyst used was sodium carbonate. Iron metallization (IM) in reduced ilmenite was monitored and effects of reduction temperature and time on IM were studied. Results obtained show that IM Increased when temperature rise from 1050 ^oC to 1200°C, at above 1200^o stared to decrease. Reduction time showed similar effect as the IM rose when reduction time changed from 2 to 4h, the IM value then reduced above 4h of reduction time. Approx. 83% of IM was obtained at reduction temperature and time of 1150°c and 4h, respectively. XRD analysis shows no trace of reduced titanium while confirmed the reduction of Iron ion to iron metal.

Keyword: ilmenite, ilmenlte reduction, Ha Tinh ilmenite concentrate.

1. INTRODUCTION

Ilmenite is the titanium-iron oxide mineral with the idealized formula FeTiO₃. In nature, ilmenite occurs in the form of heavy mineral sand deposit and hard rock deposit, those are main sources for the production of metallic titanium and titanium containing compound.

Throughout the years, many methods have been developed for upgrading ilmenite to rutile [1-6]. These included high temperature processing and often hot acid leaching, with associated environmental problem. Among  those, Becher process has caught general interest of researchers because of its cost-effectiveness and friendly environment. The process, which is used for upgrading ilmenite from 54-55% TiO2 to about 94% TiO₂, includes two major steps: solid state reduction of ilmenite and the removal of metallic iron in reduced ilmenite [7,8].

The reduction process of ilmenite has been extensively investigated [9,10]. At temperature below 1200 ^oC, reductive reaction occurs only on iron ion in ilmenite and thus, the process is the selective reduction of iron [11,12].

Chemical reactions during reduction can be separated into two states [11,12]:

a solid state reduction (typically from 860^oC to 1000^oC):

FeTiO₃  + C → Fe + TiO₂ + CO (g) (1)

and a gaseous reduction (typically above 1000^oC)

FeTiO₃  + CO (g) → Fe + TiO₂ + CO₂ (g) (2)

While the reductant is solid carbon, it has been shown that at temperature above 1000^oC the principal reaction involves carbon monoxide produced from the following reactions:

C + O₂  → CO₂ (3)
C + CO₂  → 2CO (4)

The reduction product consists of grain of metallic iron embedded in a matrix of titanium dioxide.

During reduction, titanium could also be reduced to a lower valence (typically above 1200^oC):

3TiO₂  + CO (g) → Ti₃O₅ + CO₂ (g) (5)

Sodium carbonate sometime is used as a catalyst

for iron reduction, according to Chen [11], the addition of sodium carbonate could reduce the reduction temperature by about 200^oC.

In Vietnam, ilmenite occurs mainly in the form of heavy mineral sand deposit. The ilmenite ore in Ha Tinh province is heavy mineral sand deposit and could be processed using the Becher route due to its chemical composition. The concentrate ore has about 54-55% TiO₂ and impurities such as manganese oxide, magnesia or silica.

In this work, solid state reduction of ilmenite concentrate has been carried out. Effect of important factors such as reduction temperature and time was studied. Phase transformations in the ilmenite concentrate are also observed.

2. EXPERIMENTAL

2.1. Raw materials

Ilmenite concentrate was obtained from Ha Tinh Minerais and Trading Joint Stock  Co. (Ha Tinh, Vietnam) and its chemical composition was shown in table 1.

Table1. Composition of ilmenite concentrates

Composition	Wt. (%)
TiO2	54.65
FeO	20.36
Fe2O3	21.45
SiO2	1.39
MgO	0.01
MnO	1.62
Al2O3	0.17
Others	–

Anthracite (Vang Gianh, Quang Ninh) is used as reductant, its composition was shown in table 2. Sodium carbonate was obtained from Xilong Chemical Industry Incorporated Co. Ltd. (China), the purity was of 99.8% Na₂CO₃.

2.2 Experimental

Ilmenite concentrate was supplied in the size fraction (100-200) mm. Anthracite was ground and screen to particle size fraction of < 250 mm. The phases in ilmenite samples were investigated using XRD.

Table2. Composition of anthracite

Composition	Wt. (%)
Carbon	87.15
Volatiles	7.15
Ash	5.60
Others	–

The weighted ilmenite, anthracite and sodium carbonate were throughly mixed and pelletizing into pellets of about (4-6) mm in diameter. In all experiments, the mixture was adjusted to maintain the molar ration of carbon to oxygen from iron oxide (FeO and Fe₂O₃) at 2.5. The amount of sodium carbonate used was fixed at 5%.

In a typical experiment the pellets was place in an alumina crucible and the system was place in an electric furnace chamber. The surface of the mixture was covered using alumina powder for sample protection. The crucible then heated to reduction temperature, kept for reduction time, and then cools together with furnace.

Iron metallization was determined using chemical analysis, the reduced ilmenite also subjected to XRD analysis for phase identification.

3. RESULTS AND DISCUSSION

3.1 Effect of temperature

Effect of temperature on iron metallization (IM) experiments has been carried out at the temperature range of (1050-1250)^oC and pre-fixed reduction time of 4h, results are shown in Fig 1.

As can be seen in fig 1, increasing temperature up to 1200oC has facilitated IM. The value of IM at 1200^oC was approx. 85%. Above this temperature, IM starts to decrease.

In general, temperature could influent the reductive reaction of iron in two different ways. It could directly affect the chemical reaction by shifting the activation energy of the reaction. The results obtained are  in agreement with earlier work. As calculated by Wang and Yuan [13], the activation energies of the reductive reactions were 265 kJ/mol, 164 kJ/mol and 157 kJ/mol at temperature below 1100^oC, 1100 to 1250^oC and above 1250^oC, respectively. Thus, temperature increase facilitates iron reduction.

Temperature variation also shifts the CO/CO₂ ratio, which has significant effect on the reduction. At higher temperature, equilibrium of the Boudouard reaction (reaction 4) shifts to the right, results in more CO in the environment. At temperature above 1100^oC, the environment consists of more than 95% CO and therefore, facilitates reduction.

According to Williams and Douglas [14], when titanium-iron oxide mineral is heated to 1130^oC in the presence of magnesia and alumina, a compound called anosovite is formed typical formula is [(Mg,Fe,Ti)O.2TiO₂].n[(Fe,Al,Ti)₂O₃.TiO₂]. The anosovite formation increased when temperature increased.

The formation of anosovite in the reduction zone might limit the diffusion of reductant and reaction product, therefore, decrease the value of iron metallization.

As depicted in fig 1, the values of IM at 1150^oC and 1200^oC are almost identical. 1150^oC could be picked as suitable reduction temperature for Ha Tinh ilmenite concentrate.

3.2 Effect of time

Effect of time on iron metallization experiments has been carried out at 1150^oC, results are shown in Fig 2.

At 1150^oC, iron metallization increased as reduction time increased. The maximum value of IM obtained was 83% after 4 hours of reduction. With the reduction time prolonged, the value of IM reduced.

Generally, longer reduction time is expected to give higher IM value. However, as reduction time passed 4h, opposite results were observed. As discussed above, the formation of anosovite, which increases over time, might result in negative impact to the reduction of iron. As the reduction of iron halted, re-oxidation of metallic iron might occur and thus, reduces IM.

The reduction of iron was also influenced by the impurities. It was known that manganese oxide and magnesium oxide have negative effect on the reduction of iron in ilmenite [15,16]. During the reduction, Mn and Mg diffused ahead of the advancing reaction interface to form a narrow enrichment zone in which the Fe²⁺ in the ilmenite was replaced by Mn²⁺ or Mg²⁺. At last, the manganese (magnesium) concentration became so high that the reduction of Fe²⁺ to metallic iron was impossible [17].

It is reported that magnesium oxide has somewhat larger effect on the reduction kinetics than manganese oxide [13]. This may be due to the fact that magnesium oxide forms a more stable solid solution with titanium and iron oxides than manganese oxide. The silica oxide also inhibited reduction due to the formation of fayalite (Fe₂SiO₄) during the reducing process which decreased the reactivity of the iron oxide [13].

3.3. Phase transformation

XDR analysis was used to determine the phase in reduced ilmenite. Results are shown in fig 3.

As can be seen in fig 3, the largest difference is the appearance of metallic iron after reduction. Others phases show different behaviors, rutile intensity was vastly increased while intensity of ilmenite reduced. Pseudoutlle intensity is almost the same as before reduction.

The results indicated that during reduction, part of the ilmenite was decomposed and the iron ions were reduced whist the titanium ions were left as rutile. The  results are in agreement with Wang and Yuan [13] research, the ilmenite intensity reduced when temperature rises and the ilmenite phase disappeared at above 1300^oC.

The XRD pattern also showed no trace of  reduced titanium; prove that titanium could not be reduced by carbon at this temperature.

4. CONCLUSION

In this work, solid state reduction of ilmenite concentrate has been carried out using anthracite as a reductant. The reduction was performed in an electric furnace and the  catalyst used was sodium carbonate. Iron metallization (IM) in reduced ilmenite was monitored and effects of reduction temperature and time on IM were studied.

Results obtained show that IM increased when temperature rise from 1050 to 1200^oC, at above 1200^oC, IM started to decrease. Reduction time show similar effect as the IM rose when reduction time changes from 2 to 4h, the IM value then reduced above 4h of reduction time.

For further study, 1150^oC and 4h could be chosen as suitable reduction temperature  and reduction time, respectively.

XRD analysis shows no trace of reduced titanium while confirmed the reduction of iron ion to iron metal.

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