Original Research PaperSynthesis of rutile TiO2 powder by microwave-enhanced roasting followed by hydrochloric acid leaching
Graphical abstract
Introduction
Rutile TiO2 is one of the important raw materials for the production of titanium pigment and titanium sponge [1]. With the gradual depletion of natural rutile, a low-cost, environmentally friendly production technique for the rutile TiO2 is in urgent demand [2], [3], [4], [5]. Ilmenite and titanium slags are, therefore, becoming main raw materials for rutile TiO2 in titanium industry [6]. The methods for preparation of rutile TiO2 from ilmenite or titanium slag including electrothermal reduction, molten salt roasting and acid leaching [7], [8], [9], [10]. However, the quality of rutile TiO2 prepared by electrothermal reduction is challenged by the stubborn impurities such as Fe and Mg [7]. The NaOH or KOH applied in the molten salt roasting is highly corrosive [11], [12]. Thus, acid leaching, namely sulfuric acid leaching process and hydrochloric acid leaching process, are the most widely used method among the technologies reported for producing rutile TiO2 [9], [10].
However, the application of sulfuric acid leaching process is limited by the complicated by-products including intractable waste [13], [14]. Conversely, hydrochloric acid leaching process achieves more attention since it has the advantages of high leaching efficiency, strong impurity removal ability, high product grade and recycling of hydrochloric acid [10], [15]. High purity synthetic rutile was successfully obtained by mechanical activated hydrochloric acid leaching and solution reduction method (adding iron powder as reducing agent in hydrochloric acid leaching process), respectively [16], [17]. It was reported that rutile TiO2 with 92.65% grade of purity was synthetized by pressure leaching at 140 °C using 20% hydrochloric acid with ilmenite (liquid-solid mass ratio 8:1) [18]. The purpose for the application of pressure leaching is to reduce the leaching time from 6 h to 4 h.
Recently, Chen et al. introduced a novel process with five steps including hydrochloric acid leaching for the synthesis of rutile TiO2 from titanium slags, which utilized a microwave sodium salt (Na2CO3) roasting to improve the leaching efficiency [19]. Microwave irradiation technology is known to be superior to the use of conventional heating on accelerating reaction rates, improving yields, and selectively activating or suppressing reaction pathways [20], [21], [22], [23], [24]. Microwaves are in the radio frequency portion of the electromagnetic spectrum between 0.3 and 300 GHz with corresponding wavelengths ranging from 1 m to 1 mm [25], [26], [27], [28], [29], [30]. Microwave energy passing through a sample, and resulting internal friction produces heat. Thus, the sample is heated from within, and heating time is greatly reduced compared with heating by external application of heat [31]. Nowadays, microwave irradiation technology is being widely used in laboratory scale as well as for commercial industrial manufacturing processes [32], [33], [34], [35], [36].
The role of sodium salt during the preparation of rutile TiO2 from titanium slags was to decompose the anosovite into alkali titanate [37]. It was also noticed that the sodium salt acted as a flux during the roasting process. During the leaching process, the alkali titanate dissociates into insoluble rutile (TiO2) in aqueous media, and separate with other soluble component [38]. Laxmi et al. recommended an additive (Na2CO3) to ilmenite ratio of 1:1 for the soda ash roasting of red sediment ilmenite (47.03% TiO2) [39]. The molar ratio of Na2CO3/titanium dioxide for the roasting of ilmenite (42.00% TiO2) recommended by El-Tawil et al. was 0.5 [40]. Mass ratio of the additive Na2CO3 was proved to be a key factor for the reaction rates during the roasting process.
In this study, rutile TiO2 powder was prepared from titanium slag using a novel microwave associated method with steps as follows: microwave roasting with Na2CO3, hydrochloric acid leaching and microwave calcination. Compared with similar conventional roasting-leaching combining methods for the preparation of rutile TiO2, the proposed method is simpler and more effective. With the application of microwave roasting associated with an optimized mass ratio of Na2CO3, the leaching efficiency was largely improved without using pressure leaching. To find out the appropriate mass ratio of Na2CO3, the effect of the mass ratio of Na2CO3 on the synthesis of rutile TiO2 was analyzed by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopic analysis, Raman spectroscopy techniques and scanning electron microscopy (SEM).
Section snippets
Materials
The titanium slag used in the present work was received from Panzhihua (Sichuan province, China). All the other chemical reagents utilized in this study were of analytical grade and used without further purification. The main chemical composition of titanium slag were analyzed by X-ray Fluorescence (Model: EDX-8100, Shimadzu, Japan). The composition of the as-received slag is presented in Table 1, revealing 75.34% of TiO2, 9.72% of total Fe, 5.87% of Al2O3, 5.23% of SiO2, 1.23% of MgO, 1.81% of
Results and discussion
The chemical reactions during the sodium salt roasting can be represented by the following equations:Al2TiO5 + 2Na2CO3 = Na2TiO3 + 2NaAlO2 + 2CO2FeTi2O5 + 2Na2CO3 = 2Na2TiO3 + FeO + 2CO2CaTi2O5 + 2Na2CO3 = 2Na2TiO3 + CaO + 2CO2MgTi2O3 + 2Na2CO3 = 2Na2TiO3 + MgO + 2CO2SiO2 + Na2CO3 = Na2SiO3 + CO2TiO2 + Na2CO3 = Na2TiO3 + CO2
The relation between the standard Gibbs free energy and temperature for the above reactions is shown in Fig. 2(a). Apparently, the standard Gibbs free energy for the Eqs. (1)
Conclusion
In this paper, the synthesis of rutile TiO2 powder from titanium slag by microwave-assisted activation roasting followed by hydrochloric acid leaching was investigated. The main findings of the study are summarized as follows:
- (1)
The results of XRD, Raman spectra and FT-IR spectra indicating that rutile TiO2 powder was successfully prepared with the designed experimental process. Since the microwave heating is superior to the use of conventional heating on accelerating reaction rates and
Acknowledgments
The authors acknowledge the financial support from the National Natural Science Foundation of China (No: U1802255, 51504110 and 51424114), Yunnan Applied Basic Research Project of China (No: 2017FD117) and Innovative Research Team (in Science and Technology) in University of Yunnan Province were sincerely acknowledged.
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