Photo induced reductive leaching of iron from ilmenite in hydrochloric acid solutions

Introduction

Titanium dioxide (TiO₂) is one of the extensively used materials in a number of industrial applications such as paints, paper, semiconducting material, etc. The main source of TiO₂ in the global market originates from the naturally occurring ilmenite. Pyrometallurgical methods (Mohanty and Smith, 1993; Mackey, 1994) and hydrometallurgical methods (Lanyon et al., 1999; Ogasawara and de Araujo, 2000; Lasheen, 2005) are the two widely used industrial methods to extract titanium dioxide from ilmenite minerals. Conversion of ilmenite into synthetic rutile has become an important research area due to the shortage of natural rutile. Mahmoud et al. (2004) have reported five types for the production of synthetic rutile from ilmenite, where all the processes mainly depend on the reductive and/or oxidative pretreatment of ilmenite. Synthetic rutile production from ilmenite through the Becher process (Becher, 1963; Becher et al., 1965) is the major feedstock for the production of titania pigments and it is widely used in Australia over 30 years. Removal of iron and other impurities such as Mn₃O₄, Al₂O₃, CaO, Cr₂O₃ and MgO is essential for the production of pigment grade TiO₂. The soda ash roasting method has been reported (Lahiri and Jha, 2001, 2009) for the production of synthetic rutile, where rare earth oxides and impurities from lower grade titaniferous become water soluble in Fe–Ti–O–Na system and can be selectively separated out. The enhanced separation was attributed to the lattice strains and subsequent fracturing developed by ions such as Na⁺, K⁺ and Al³⁺, which facilitates the reaction between entrapped iron oxide inside the complex structure with alkali and alumina. Various physical, inorganic and thermodynamic aspects of titaniferous ores were also investigated by Jha et al. (2008), and they have reported that the efficiency of selective separation of impurities such as lanthanides and actinides is reported based on chemical analysis of the materials derived after leaching. van Dyk et al. (2002) have extensively studied the kinetics of ilmenite dissolution in hydrochloric acid (HCl) solutions. Their work reports the investigation of the particle size, acid concentration, temperature, stirring speed, acid to ilmenite molar ratio and additives used on the dissolution. Their proposed mechanism suggests under what conditions the rate determining step for the ilmenite leaching in HCl will operate. In this study, we have found that the rate of dissolution of ilmenite is greatly enhanced by subjecting the reaction medium to the ultraviolet (UV) light and a possible mechanism is discussed.

Experimental conditions

Ilmenite samples were obtained from Lanka Mineral and Sands Ltd in Sri Lanka and used as it is. X-ray diffraction (XRD) analyses were performed using a Stoe STADI/P powder diffractometer and SEM images were obtained using a Jeol JSM-5600 instrument. Phase purity and identity were confirmed by X-ray powder diffraction using a STOE Stadi P transmission diffractometer (Cu K_α1 = 1·5406 Å) and an EDX Inca Energy System. The SEM images were obtained using a Jeol JSM-5600 instrument. Particle sizes were analysed from SEM images using ImageJ software. The UV irradiation was carried out using a Philips medium pressure Hg arc 250 W lamp. The lamp and reaction vessel were kept at constant position during this study to avoid any variations. In a typical experiment, 5·0 g of ilmenite was mixed with 50·00 mL HCl solution. In the case of nitrogen purged experiments, commercially available gas was used with approximate nitrogen content of ∼98%. Total dissolved iron was determined using atomic absorption spectrometry in experiments involving H₂O₂ (model GBC 932 plus) and standard colorimetric 1, 10-phenanthroline test, which has been reported in a previous study (Priyadarshana and Jayaweera, 2009). Mixture was allowed to sit in the dark for 30 min before ultraviolet–visible (UV–Vis) spectroscopy analysis, during which time characteristic reddish orange colour developed. The UV–Vis absorption measurements were carried out using a ThermoSpectronic Heλios α and JASCO V-570 UV–Vis–near infrared spectrophotometers.

Results and Discussion

The XRD pattern of the ilmenite sample used in this work closely matches with the previously published XRD pattern (Gao et al., 2008). The absence of peaks corresponding to other materials such as hematite suggests that the sample is fairly pure and has the rhombohedral FeTiO₃ structure. The SEM images show that the average particle size is about 100±25 μm.

The ilmenite leaching reaction in HCl solutions can be described by the following chemical equation (1) The rate of leaching will depend on many parameters and extensive work on these effects was reported in the literature (van Dyk et al., 2002). In this study, the dissolution of ilmenite was investigated under HCl concentrations of 1·0, 1·5, 2·0 and 7·0M and at temperatures of 30±1 and 70±3°C, with and without UV irradiation. The results obtained for the dissolution of ilmenite under four different conditions in 1·0M HCl solutions are shown in Fig. 1a. The four different conditions are: air purged under dark, air purged under UV irradiation, N₂ purged under dark and N₂ purged under UV irradiation. The results clearly show that in the presence of UV radiation with air purging, the rate of dissolution is enhanced approximately by twofold when compared with N₂ purged experiment under dark condition. In all cases, UV light enhances the rate of dissolution of ilmenite. Figure 1b shows the experiments performed in purging air in 1·5M HCl solutions with and without UV light and it clearly shows again that iron dissolution rate is enhanced in the presence of UV radiation. An experiment performed at an elevated temperature of 70°C in 1·0M HCl solution, in the presence and absence of UV radiation also confirms the enhanced rate of dissolution of ilmenite (see Fig. 1c). Furthermore, experiments performed with sunlight also show enhanced leaching rate. However, leaching rate is lower than the direct UV exposure experiments. Line (v) in Fig. 1d shows an air purged experiment performed under afternoon sunlight in 1·0M HCl solution.

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Figure 1

Leaching of iron from ilmenite in a 1·0M, b 1·5M HCl solutions at 30°C, c 1·0M HCl solution at 70°C and d 1·0M HCl, 30°C sunlight experiment: (i) air purged and under dark; (ii) air purged and under UV irradiation; (iii) N₂ purged and under dark; (iv) N₂ purged and under UV irradiation; (v) air purged and under sunlight; inserted: rates of dissolutions (5·0 g of FeTiO₃ in 50·00 mL of HCl was used)

In order to understand the effect of UV radiation on the rate of dissolution of ilmenite, UV–Vis absorption spectroscopic analysis was performed as a function of time, in the presence and absence of UV light. Figure 2a shows the UV–Vis absorption spectra accumulated for the supernatant solution as a function of dissolution time in the absence of UV light. The supernatant solution produces two well defined absorption peaks at 272 and 334 nm and increases their intensities with the dissolution time. Figure 2b shows the absorption spectra accumulated for the supernatant solution as a function of dissolution time in the presence of UV light. During spectral accumulation, UV light was blocked. What is interesting is that absorbance values are somewhat lower in the case of UV irradiated dissolution when compared with an experiment without UV. This effect is more visible, when the intensities of 272 and 334 nm peaks are plotted with dissolution time. It clearly shows that both peaks have lower absorbance values, indicating a steady state photo induced dissociation of either a Ti or Fe species formed during dissolution. The stability of both Ti and Fe species generated during dissolution was investigated by separating the supernatant solution from ilmenite particles and subjecting it to UV irradiation and keeping a reference solution in the dark. Figure 3a shows the UV–Vis absorption spectra accumulated, subjecting the separated supernatant solution to the UV radiation. It clearly indicates a drop of intensities of both peaks at 272 and 334 nm. This drop halts at a certain level and further irradiation does not decrease the absorbance. The controlled experiment (Fig. 3b) shows no change in the absorbance, suggesting that dissolved species are stable under dark conditions. Therefore, findings suggest that a component in the solution undergoes UV induced degradation. The absorption spectrum of TiCl₄ in concentrated HCl solution, in the presence of very small amount of water, shows a blue shift for the λ_max of 276–228 nm. This is due to the formation of titanium oxychloride (TiOCl₂) species and further addition of water causes to form colloidal TiO₂ particles as shown in reaction (2) (2) Exposure of these solutions, i.e. TiCl₄, TiOCl₂ and FeCl₃ in HCl solutions for prolonged, ∼60 min UV irradiation, does not reduce their absorption values, indicating that species are photostable.

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Figure 2

Ultraviolet–visible absorption spectra accumulated for supernatant solution during dissolution of 5·0 g of FeTiO₃ in 50·00 mL of 1·0M HCl, as function of time:

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Figure 3

Ultraviolet–visible absorption spectra of supernatant solution, separated out from ilmenite particles after 50 min dissolution of 5·0 g of FeTiO₃ in 50·00 mL of 1·0M HCl, as function of time:

van Dyk et al. (2002) have reported that, once the concentration of Ti(IV) exceeds a value of 10⁻³M and at [H⁺] >0·5M, Ti(IV) polymerises in HCl solutions. Therefore, the diffusion of the Ti(IV) polymer chains away from the ilmenite particles acts as the rate determining step in the leaching process. In a recent study, Cottineau et al. (2008) have reported the synthesis and characterisation of photosensitive titanium oxopolymers. It has been found that TiOCl₂ in HCl is hydrolysed into polymeric form of titanium oxides as evident from X-ray absorption spectroscopic experiments. Fowles et al. (1968) have reported the most likely structures for the complexes TiOCl₂, 2L type as oxygen bridged dimers or polymers. Also they have not ruled out the presence of Ti-Cl→Ti bridged structures. What is clear from our finding is that the enhanced rate for the iron leaching from ilmenite arises as a result of UV induced degradation of Ti polymer species or precursor(s) formed during dissolution. This facilitates the diffusion of degraded products away from the ilmenite HCl interface and minimises the deposition of Ti(IV) polymeric species in the pores of ilmenite particles. As a result, rate determining step becomes the chemical reaction between ilmenite and HCl, enhancing the rate of leaching. Furthermore, leaching studies under air purged conditions have shown the highest rate in the presence of UV light. This enhancement could be explained by the highly reactive oxidising radical species such as OH, HO₂ and H₂O₂ formed through Fe²⁺/Fe³⁺/H₂O Fenton type reactions (Bauer et al., 1999; Kassinos et al., 2009) as shown below (3) (4) (5) (6) It has been reported (Cottineau et al., 2008) that excited Fe(III) complexes can also undergo photo induced reduction to generate reactive radical species, which in turn could act as potential titanium oxy polymer destructive species (reaction (7)) (7) where L = OH⁻, H₂O and .

The effect of radical species and hydrogen peroxide (H₂O₂) on the rate of leaching was investigated by adding 10·00 mL of 6% (w/v) H₂O₂ into the leaching medium maintaining the HCl concentration at 1·0M. Results obtained in the presence and absence of UV light are shown in Fig. 4. These experiments clearly confirm the role of Fe²⁺/Fe³⁺ driven Fenton process and the reactions of OH and HO₂ species on the enhancement of rate of leaching of iron from ilmenite. In the presence of UV light, photo Fenton reactions generate more hydroxyl radical species, further enhancing the rate of dissolution (reactions (8) and (9)) (8) (9) The appearance of white cloudiness in the solution suggests that TiO₂ particles are formed during leaching via reaction (2). Furthermore, there is a possibility for the TiOCl₂ species to undergo TiO₂ formation through proposed reactions (10) and (11) (10) (11) Peroxy radicals are considered as the intermediates between the hydroxyl (OH) radical and ozone formation or destruction (Fleming et al., 2006; Ginovska et al., 2008). Mehandjiev et al. (2001) have investigated the heterogeneous catalytic activity of mixed metal systems having ilmenite structure. They have reported that surface oxygen is highly active at room temperature when the two metal cations are in octahedral coordination. This results an enhanced heterogeneous catalytic decomposition of ozone and ozone-induced oxidations. The reactions can be summarised as follows, where Z and R are mixed metal system having ilmenite structure and organic molecule respectively (12) (13) (14) Therefore, one cannot rule out the formation (Caridade et al., 2001; Kanno et al., 2005) of O₃, further enhancing the rate of destruction of polymeric titanium oxy species and consequently enhanced rate of dissolution. Furthermore, lattice defects in oxide structures play a major role in a number of properties such as diffusion of ions and conductivity. It has been reported (Camargo et al., 2008) that oxygen vacancies affect the mobility of iron and titanium cations on a crystal lattice, such as FeTiO₃. Therefore, it is clear that titanium and iron show different mobilities in areas with different concentrations of defects. At elevated temperatures, more point defects, i.e. oxygen vacancies are expected to generate in the lattice. Therefore, preheated ilmenite may show different photo induced rates of leaching.

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Figure 4

Leaching of iron from ilmenite in 1·0M HCl in presence of H₂O₂ at 30°C under conditions of a air purged and under dark and b air purged and under UV irradiation (5·0 g of FeTiO₃ in 1·0M HCl and 10·00 mL of 6% (w/v) H₂O₂; total volume 50·00 mL)

Experiments performed allowing only the HCl solution to expose to the UV light and blocking UV light falling to the ilmenite particles, showed similar rates for the iron dissolution. This confirms that there is no influence arising from ilmenite band gap excitation and the rate of dissolution enhancement is due to the degradation of polymeric species in the solution phase.

Summary

The rate of leaching of iron from ilmenite in HCl solutions is greatly enhanced when exposed to UV radiation. The UV light enhanced leaching was attributed to the photo induced decomposition of titanium oxy polymers formed and non-deposition in pores of ilmenite particles. The enhanced dissolution takes place both at elevated temperatures and at higher HCl concentrations. Air purged dissolution in the presence of UV radiation shows a higher rate. This enhancement arises from the oxidising radical species such as OH and HO₂ formed through Fe³⁺/Fe²⁺/H₂O photo reactions. The addition of H₂O₂ into the reaction medium further enhances the rate, generating more radical species.