Nghiên cứu quá trình thủy phân chế tạo sắc tố TiO2 theo phương pháp kiềm nóng chảy
NGUYỄN THỊ THẢO, DƯƠNG NGỌC BÌNH, TRẦN VŨ DIỄM NGỌC*
Viện Khoa học và Kỹ thuật vật liệu, Trường Đại học Bách khoa Hà Nội, Số 1 Đại Cồ Việt, Hà Nội Email: firstname.lastname@example.org
Ngày nhận bài: 11/5/2020, Ngày duyệt đăng: 26/6/2020
Upgrading ilmenite, a product of the Becher process for ilmenite processing, was purified using the molten NaOH method. The process includes alkaline roasting of upgrading ilmenite in molten NaOH, followed by washing and acid leaching, solution hydrolysis, and calcination of the hydrolysis product. The upgrading ilmenite (81.89%TiO2) was first roasted in molten NaOH at 500 oC for 1 h. The roasted product is then washed using water and leached with 20 % HCl at 50 oC for 2 hours. The TiOCl2 solution was hydrolyzed with 20-50 g/l TiO2 and hydrolysis time from 30 to 90 min at 95 oC. The hydrolysis efficiency is obtained up to 96 % with 40 g/l TiO2 for 90 min. The precipitation of the hydrolysis process was then calcinated at 650 oC for 1 h. The final product contained up to 98.47%TiO2. This product can be used as pigment after further particle refinement and surface treatment.
Keywords: Ilmenite, NaOH, TiO2, hydrolysis, pigment
Phương pháp kiềm nóng chảy được sử dụng để chế tạo sắc tố TiO2 từ ilmenit nâng cấp chứa 81,89 %TiO2, là sản phẩm trung gian trong quá trình chế biến rutin tổng hợp từ công nghệ Becher. Nguyên liệu được nung phân hủy trong kiềm nóng chảy ở 500 oC. Sản phẩm sau nung được rửa bằng nước rồi hòa tách trong dung dịch axit 20 %HCl thu được dung dịch chứa TiOCl2. Dung dịch sau hòa tách được thủy phân với nồng độ TiO2 từ 20-50 g/l và thời gian từ 30-90 phút ở nhiệt độ 95 oC. Kết quả nghiên cứu cho thấy, nồng độ TiO2 dưới 40 g/l và thời gian 90 phút cho hiệu quả thủy phân cao nhất, khoảng 96 %. Kết tủa sau quá trình thủy phân đem nung ở 650 oC, thời gian 1 h thu được sản phẩm chứa 98,47 %TiO2, sau khi xử lý cấp hạt và bề mặt, sản phẩm này có thể được sử dụng để làm sắc tố TiO2.
Từ khóa: Ilmenit, NaOH, TiO2, thủy phân, sắc tố
Titanium dioxide (TiO2) pigment is the most widely used pigment in various applications such as in coatings, fillers for plastic and paper, adsor- bents, cosmetics, catalysts, gas sensors, etc. The TiO2 pigment has high chemical stability, heat sta- bility, and exceptional optical, mechanical and electronic properties [1-3].
Millions ton of TiO2 pigment are being used annually all over the world. It could be directly produced from titanium ores such as ilmenite or rutile. Nevertheless, synthetic rutile and titanium slag are the most common raw materials used due to their high TiO2 content . In order to be used as pig- ment, those raw materials are purified, ground to appropriate particle size and subjected to surface treatment.
There are two process to produce TiO2 pigment used in industry; the sulphate process and the chloride one [4-7]. In the sulfate process, the purification step begins with the leaching of raw material in hot concentrated H2SO4 followed by dilution, solution purification, hydrolysis and calci- nation. The process is responsible for nearly haft of the world current TiO2 pigment production. However, sulfate process generates a huge amount of solid waste, larger than the amount of TiO2 product. The chloride process used for the other haft of the world current TiO2 pigment pro- duction is considered more advanced than the sul- fate process. Nevertheless, this process still cre- ates a considerable amount of carbon emission and consumes a large amount of energy.
Recently, a new process called “molten NaOH process” is developed for the purification of TiO2 [8-17]. The process includes alkaline roasting of titanium slag [8-15] or ilmenite [16,17] followed by washing, acid leaching, hydrolysis and calcination. The process has the advantages of high reaction activity, high conversion of Ti, and less waste emission to the environment. Effect of alkaline roasting, water treatment and leaching were inves- tigated in [9-11,14,18], where the parameters of these stages were determined. However, hydroly- sis of Ti solution is unclear.
In this work, upgrading ilmenite (approx. 82 %TiO2) was used as raw material for the molten NaOH process. The experiments are focused on the hydrolysis stage of the process, several factor such as the concentration of Ti in solution and the duration the hydrolysis stage are investigated.
2. EXPERIMENTAL PROCEDURE
Upgrading ilmenite used in the present study was prepared from ilmenite . The chemical composition of upgrading ilmenite was analyzed by X-ray Fluorescence (XRF-Viet Space 5008P) as shown in table 1. The X-ray diffraction (XRD- Bruker D8-Advance) analysis of the upgrading ilmenite (Fig. 1) indicates that the main titanium compositions are TiO2 and Fe3Ti3O10. The sodium hydroxide (NaOH) and hydrochloric acid (HCl) were supplied by Xilong Scientific Co., Ltd (China).
Titanium dioxide (TiO2) has been fabricated from upgrading ilmenite by the NaOH molten salt method, including alkaline roasting of upgrading ilmenite followed by water treatment, acid leach- ing, hydrolysis, and calcination as show in Fig.2. The upgrading ilmenite (81.89 % TiO2) is decom- posited at 500 °C for 1 h by NaOH molten to obtain sodium titanate (Na2TiO3) which is then converted into hydrate titanium dioxide (H2TiO3) through water treatment with a liquid-to-solid mass ratio of 10:1 at 50 oC for 10 minutes. The solid intermedi- ate was dissolved in 20 %HCl solution at 50 °C for 2 h, and the titanium chloride (TiOCl2) solution was obtained. Then, the TiOCl2 solution was subse- quently hydrolyzed under conditions of variation of TiO2 concentration (20 – 50 g/l) and time (30 – 180 mins) at temperature 95 oC to yield precipitates of hydrous titanium oxides and calcinated at 650 oC to prepare high pure TiO2. The crystal structure of final TiO2 sample was analyzed by XRD and chemical composition by XRF.
Table 1. Chemical composition of upgrading ilmenite
3. RESULTS AND DISCUSSIONS
Decomposition of upgrading ilmenite at 500 oC for 1 h in molten NaOH is the first step in the production of TiO2 pigment by molten NaOH. The role of this step is to make compound of titanium diox- ide and other impurities with sodium.
The TiO2, Fe3Ti3O10 and some impurities in upgrading ilmenite are reacted with NaOH in the following main reactions:
TiO2 + 2NaOH = Na2TiO3 + H2O (1)
2Fe3Ti3O10+12NaOH+1/2O2 = 6Na2TiO3 + Fe2O3 + 6H2O (2)
The process of washing the products after decomposition in molten NaOH is essentially the separation process in aqueous solvent, where the Na2TiO3 reacts with H2O to form H2TiO3 (solid). The main reaction occuring when washed in aque- ous solvents is:
Na2TiO3 + 2H2O = H2TiO3 + 2NaOH (3)
The chemical composition of solid intermediate after water washing treatment is listed in table 2. The TiO2 content is up to 84.47. The impurities of Al, Si dissolved into water are removed after sep- aration. The Al2O3 and SiO2 contents are reduced 4.21 and 56.10 %, respectively.
Table 2. Chemical composition of solid intermediate after washing treatment
In order to obtain metatitanic acid with high Ti content the solid intermediate was dissolved in the 20 %HCl solution at 50 oC, with solid/acid mass ratio of 1/10 for 2 h, and the titanium oxychloride (TiOCl2) solution was obtained. The impurities of the solid intermediate were also dissolved into the acid solution . The main reaction oCcurring when the leaching is:
H2TiO3 + 2HCl = TiOCl2 + 2H2O (4)
The titanium oxychloride was hydrolyzed to precipitate hydrous titanium oxides separated from impurities by filtration. There are some reports on the hydrolysis of TiOCl2 solutions [9, 20, 21]. The TiOCl2 solution was obtained from hydrometallur- gical process using HCl acid leaching [20, 21]. The thermal hydrolysis of TiOCl2 solution in the NaOH molten method is mentioned . An approach to clarify the mechanism of TiOCl2 solution hydroly- sis could be carried out with experimental parameters such as: TiO2 concentration in hydrolysis solu- tion (g/l), hydrolysis time.
The effect of TiO2 concentration on hydrolysis efficiency of TiOCl2 solution was shown in Fig. 3. The hydrolysis efficiency of the diluted solutions (with 20 to 45 g/l TiO2 content) was significantly higher than that of the original solution (the non- diluted solution containing 50 g/l of TiO2). The TiO2 concentration was 45 g/l with a dilution factor of 1.11 times gives a hydrolysis yield of only 84.88 %. When the TiO2 concentration was reduced to 40 g/l, the hydrolysis efficiency increased to 95.85 % with a dilution ratio of 1.25. However, when the concentration of TiO2 was reduced to 20 g/l, the hydrolysis efficiency was not changed significantly from 95.98 % to 96.76 %. When the solution is diluted, the concentration of OH– in the solution or the pH increases, increasing the rate of hydrolysis reaction [9, 22].
According to Scott C. Middlenmas  when hydrolysis at 95 oC, the hydrolysis product is formed by reaction:
Ti4+ +2H2O = TiO(OH)2 + 2H+ (5)
The hydrolysis efficiency is inversely propor- tional to the concentration of TiO2 in the solution. However, if the TiO2 concentration is too low or the solution is too many times diluted, it will directly affect the device yield and indirectly add further impurities to the solution. Therefore, the appropriate TiO2 content is chosen here as 40 g/l. Effect of hydrolysis time on the hydrolysis efficiency of TiOCl2 solution was shown in Fig. 4. The hydroly- sis efficiency increased from 79.95 to 97.00 % after 30 to 90 minutes and increased slightly to 97.17 % after 120 minutes. When hydrolysis time increased from 90 to 180 minutes, the hydrolysis efficiency increased not significantly, because the first 60 minutes is time for nuclei formed and grown and after 60 minutes, almost Ti in solution had been hydrolysed, Therefore, the hydrolysis time chosen here is 90 minutes.
After hydrolysis process, the H2TiO3 precipitate is heated at 650 oC to obtain TiO2 by the following reaction:
H2TiO3 →to TiO2 + H2O (6)
The reaction product has a concentration of 98.47 wt. %TiO2, as shown in Table 3. The XRD analysis of the final product indicates that only rutile TiO2 is present as shown in Fig. 5.
Table 3. Chemical composition of final product
Titanium oxide was purified from the upgrading ilmenite (TiO2 81.89 wt.%) by the molten NaOH method. In the hydrolysis process, the TiOCl2 solution containing 40 g/l TiO2 was subsequently hydrolyzed at temperature 95 oC for 90 minutes to precipitate hydrous titanium oxides. The rutile TiO2 was obtained by calcination at 650 oC for 1 hour.
The TiO2 content of final product was 98.47 %, which can be used as pigments after further treatment of coating and crushing.
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