Correlation of Nano Titanium Dioxide Synthesis and the Mineralogical Characterization of Ilmenite Ore as Raw Material

A study on mineral characterization and nano titanium dioxide synthesis from ilmenite ore of Bangka Island, Indonesia, has been carried out using a caustic fusion method and hydrochloric acid leaching. Comprehensive mineral characterization was conducted using X-ray fluorescence spectroscopy (XRF) and X-ray diffraction (XRD) to depict each fractionated particle's elemental composition and mineralogy, i.e., +80, ?80+100, ?100+150, ?150+200, -200+325, and -325 mesh. Other analyses performed are VSM to measure the magnetic properties and SEM to determine the distribution of elements at each particle size. Based on the characteristics of the ilmenite ore, magnetic separation was applied for the initial stage and analyzed gravimetrically. Later processing was the synthesis of nano titanium dioxide, conducted sequentially, including roasting and leaching. Roasting was run at 900°C with and without caustic soda, and then hydrochloric acid was applied. In reference to the elemental analysis, titanium (Ti) concentration is higher in smaller particle sizes and vice versa for iron (Fe) concentration, so the synthesis of nano titanium dioxide was carried out using a -100+150 mesh particle size. The optimum condition for nano titanium dioxide synthesis was 2:1 of NaOH and ilmenite weight ratio, 20% HCl concentration, and 4 hours of leaching time. The nano titanium dioxide (TiO₂) obtained was then characterized using XRF, XRD, particle size analyzer (PSA), and transmission electron microscopy (TEM). Roasting with caustic soda showed better nano titanium dioxide purity with 96.04% of TiO₂ with particle size in the range of 50–80 nanometers.

Caustic soda; Hydrochloric acid; Ilmenite; Leaching; Nano-TiO2¬; Roasting

One of the regions with ilmenite mineral resources in Indonesia is the Bangka Belitung Islands (Aristanti et al., 2018; Aristanti et al., 2019; Supriyatna et al., 2020). In 2017, PT Timah Tbk produced tin ore above 30,000 tons and had 129 mining business permits (IUP) covering 473,400 hectares with 796,343 tons of tin ore resources and 377,594 tons of tin ore reserves (PT Timah, 2017). In general, the minerals present in the deposits are ilmenite (FeOTiO₂), rutile (tetragonal TiO₂), anatase (tetragonal TiO₂), brookite (rhombic TiO₂) and perovskite (CaO.TiO₂) (Zhu et al., 2011; Gázquez et al., 2014; Ribeiro and De Lazaro, 2014; Liu et al., 2015; Haverkamp et al., 2016; Jabit, 2017; Jabit and Senanayake, 2018). The characteristics of minerals as materials for the synthesis of nano-TiO₂ are essential in determining the processes. Titanium dioxide in nanoparticle size can be produced using several methods, namely, sol-gel, deposition, sonochemical and microwave-assisted, hydro/solvothermal, and oxidation methods (Yuwono et al., 2010; Manhique et al., 2011; Karayan et al., 2012; Lalasari et al., 2012; Kavitha et al., 2013; Ahmad et al., 2013; Hudaya et al., 2018).

The processes commonly used to produce TiO₂ are the sulfate process and the chloride process. The sulfate process usually uses low-grade raw materials (ilmenite or titania slag), while the chloride process uses high-grade raw materials (Guo et al., 2014; Middlemas et al., 2015; U.S. Geological Survey, 2020). In the sulfate process, concentrated sulfuric acid is used to leach high-grade ilmenite or titanium slag to produce titanium sulfate, followed by the removal of iron through crystallization of titanium sulfate to precipitate titanium dioxide (Guo et al., 2014). Although the sulfate process is simple and can treat lower grade ores, the quality of the products is low, and a large amount of iron sulfate as solid waste is generated from this process (Gázquez et al., 2014). In the chloride process, titanium slag, natural rutile, or synthetic rutile is reacted with petroleum coke and chlorine gas at high temperatures to form titanium tetrachloride (TiCl₄) gas.  Then, the titanium tetrachloride is reacted with oxygen to produce pigment-grade titanium dioxide. Unfortunately, the chloride process suffers from CO₂ and other toxic emissions.

A new process, the CTL process, is currently being commercialized to produce pigment grade. The process involves leaching ilmenite ore in a mixed chloride lixiviant, i.e., HCl and MgCl₂, followed by solid-liquid separation and successive solvent extraction stages to produce high-purity iron and titanium pregnant strip liquors (Lakshmanan et al., 2012). Solid-state reduction involves several processes, such as the Becher (iron oxidized to Fe₂O₃ and reduced to metallic Fe by coal at 1200°C, followed by leaching using NH₄Cl and H₂SO₄), the Benilite (carbon-thermo reduction and leaching using 18–20% HCl), the Murso (oxidation using hydrogen-rich reductant and leaching using 20% HCl), the Dunn (selective chlorination of iron in ilmenite with Cl₂), the Kataoka (conversion to ferrous form and leaching using H₂SO₄) the Austpac (magnetization at 800–1000°C, followed by leaching using 25% HCl), and the Laporte (lower temperature for iron conversion to FeO with controlled CO₂ pressure) processes in which iron is converted to soluble ferrous or elemental forms by reduction at high temperature followed by acid leaching to obtain synthetic rutile (Zhang et al., 2011).

Several researchers have carried out TiO₂ extraction research from Bangka ilmenite. For example, some studies consist of decomposition using KOH followed by leaching in sulfuric acid (Subagja, 2016) and decomposition using KOH or NaOH and leaching in sulfuric acid with dextrin and Fe (Lalasari, 2014). The other studies were leaching using HCl with NaCl addition (Setiawan, 2012) and acid leaching at high pressure (Lalasari, 2014). However, the only study that successfully produced nano-TiO₂ was the one conducted by Lalasari (2014). Lalasari conducted acid leaching at high pressure. This process requires special attention in terms of equipment, which must be resistant to high pressure and acid so that it is more costly than atmospheric leaching (Zhang and Nicol, 2010; Zhang et al., 2011; Middlemas et al., 2013; Jia et al., 2014; Yousef, 2015).

This work aims to investigate the characterization of ilmenite obtained from PT Timah Tbk to more deeply determine the characteristics of the Bangka ilmenite and study the nano-TiO₂ synthesis using the roasting with caustic soda addition, followed by atmospheric leaching using hydrochloric acid, which is a new method in this study. The mineral characterization and study of nano-TiO₂ synthesis are expected to illustrate the possibility of alkaline fusion processes and leaching using hydrochloric acid in synthesizing nano-TiO₂ from the Bangka ilmenite mineral, which is cheaper and more accessible.

      The mineral characterization results showed that the smaller the particle size, the more the contents of Fe decreased and vice versa for Ti. Mineralogical analysis using XRD shows that the dominant minerals contained in Bangka ilmenite are, namely, ilmenite, rutile, cassiterite, rhodonite, and quartz. VSM analysis indicates that the Fe element content influences the magnetic properties of Bangka ilmenite samples. Titanium dioxide products produced by caustic fusion have higher Ti = 94.84% (96.04% TiO₂) than TiO₂ products without caustic fusion. The PSA analysis results show that the size distribution of the TiO₂ particles produced is two. The first size is 51.97 nm, with a volume percentage of 84.8%, and the second is 243.7 nm, with a volume percentage of 15.2%. The homogeneity of the nano-TiO₂ particles obtained was relatively high: more than 75% had a size of 51.97 nm. TEM analysis results show TiO₂ products with the fusion method and hydrochloric acid leaching larger than 20 nanometers. Further research is needed to optimize the fusion and leaching processes to determine the optimum operating conditions for nano-TiO₂ synthesis.

        We would like to thank the Research Unit for Mineral Technology-Indonesian Institute of Sciences (BPTM-LIPI), Department of Chemical Engineering (Sustainable Mineral Processing Research Group), Faculty of Engineering, Gadjah Mada University, and Deputy for Research and Development Strengthening, Ministry of Research and Technology/National Innovation Agency for the facility and the financial support to complete this study.

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