Hydrometallurgy 152 (2015) 113–119 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet Leaching studies of alkali roasted bomarilmenite and anatase during the processing of synthetic rutile Jeya Kumari Ephraim a,⁎, Animesh Jha b a b School of Engineering and Informatics, University of Bradford, Bradford BD7 1DP, United Kingdom The Institute for Materials Research, Houldsworth Building, Clarendon Road, University of Leeds, Leeds LS2 9JT, UK a r t i c l e i n f o Article history: Received 6 March 2014 Received in revised form 2 December 2014 Accepted 5 December 2014 Available online 8 December 2014 Keywords: Sodium oxide Ilmenite Anatase Synthetic rutile a b s t r a c t The leaching of sodium (Na+) and Fe2+,3+, ions from the soda-ash roasted ilmenite (FeO–TiO2) or anatase minerals has been examined as a chemical method for beneficiating titani-ferous minerals for the production of pigment grade TiO2. In the leaching step, the roles of catalytic aeration, dilute concentrations of organic liquids (methanol) in the range of 0.5% (v/v) in 1.5% (w/v) ammonium chloride are explained. The results of leaching experiments are also compared in the presence of inorganic acid with that of ammonium salt and acetic acid for retaining the particle size. The role of pH in the removal of sodium and iron ion is also explained based on Eh–pH diagram. The paper also explains the selective separation of rare earth oxides during the processing of synthetic rutile. The data obtained at each step of processing is explained and interpreted on the basis of results obtained through XRD, SEM and EDX. Rare earth oxides are separated unattacked by the processing steps. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO2) is widely used in pigment, paper, and welding industry. It is also the main source of raw material for making titanium metal via the production of titanium tetrachloride and its reduction via liquid magnesium, the process known as the Kroll process (Kroll, 1940). More recently high-purity TiO2 is also deoxidised by direct reduction to titanium metal (N98% pure) using calcium metal as reductant (Ephraim and Patel, 2013). In all the applications of TiO2, the control of impurities is essential. For example, the rare-earth and transition metal impurities in white pigment grade TiO2 reduce the whiteness by creating colour centres, whilst their presence in titanium metal result in the embrittlement and reduced shape forming. Ilmenite (FeO·TiO2) contains 40–65% TiO2 depending on its geological environment (Barksdale, 1966). Most of the commercial processes involve a combination of thermal oxidation and reduction by roasting, leaching and physical separation steps. Iron oxide in the ore is dissolved in acid or reduced at high temperature followed by acid leaching to produce synthetic rutile (Zhang et al., 2011). The commercial technologies for the manufacture of pigment grade titanium dioxide are the sulphate process and chloride process (Langmesser et al., 1973; Chernet, 1999; Sasikumar et al., 2004; Jabłoński and Przepiera, 2001). In sulphate process ilmenite is treated with concentrated sulphuric acid to form titanylsulphate which is then hydrolysed and precipitated to form titanium dioxide (TiO2) pigment. In chloride process rutile or synthetic rutile is chlorinated in a fluidized bed reactor (FBR) to form titanium tetra ⁎ Corresponding author. E-mail address: E.J.Kumari@bradford.ac.uk (J.K. Ephraim). http://dx.doi.org/10.1016/j.hydromet.2014.12.002 0304-386X/© 2014 Elsevier B.V. All rights reserved. chloride which is then converted to TiO2 pigment. Both these processes develop huge amounts of hazardous waste leading to pollution problems (Zhang et al., 2009). In spite of the above problems there is always an increasing demand for oxide and metal products of titanium which has forced the industry to explore lesser graded ores which have much higher concentrations of lanthanides and actinides (500–30,000 ppm total), calcium and aluminium (at few weight %). Consequently the lower-grade ores are difficult to beneficiate via the conventional acid leaching (Gambogi, 2009, 2010) and electric-arc smelting processes (Mukherjee et al., 2002). Soda-ash roasting of titani-ferous (e.g. ilmenite) minerals (Jeya Kumari et al., 2006) provides a much better route for selective separation of not only iron as a major element, but also the minor and trace elements. In this paper we examine the physical chemistry of leaching reaction of bomar ilmenite and anatase in particular the removal of iron and sodium from the iron–sodium titanate and sodium titanate phases, which is formed as a result of the roasting reaction between soda and ilmenite mineral at high-temperatures. These two phases appear to be insoluble in water; by contrast sodium ferrite is soluble in water. The roomtemperature aeration leaching reaction was investigated under the controlled pH and oxygen partial pressure, and the results are compared with the leaching condition when only a mineral acid was used in air. The control of pH was achieved via the addition of organic solvent methanol, acetic acid and a small fraction of ammonium chloride (Becher et al., 1965). Their roles in the generation of protons during leaching are discussed in the context of a computed Eh–pH diagram. The ion diffusion pathways in the controlled pH condition play an important role in determining the final particle size range of original 114 J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 sodium titanate as Na2TiO3, Na4TiO4 and sodium aluminate NaAlO2 shown in Fig. 1. During roasting the sodium oxide attacks the surface of the grain and creates pores on the surface of ore for the migration of sodium oxide ion into the core of the grain to react with iron and titanium to form sodium ferrite and sodium titanate respectively. When once the reaction has taken place the ilmenite crystal lattice is altered for further movement of Na+ ions into the lattice. EDX confirms the attack of sodium ions in to the core of the ilmenite lattice which is shown in Fig. 2. The ilmenite roasted with soda-ash and alumina at 900 °C was suspended in 2 M HCl acid and a magnetic stirrer was used to stir the reaction mixture. The magnetic stirrer whilst stirring, powdered the ilmenite grain. Therefore after the reaction it was found very difficult to separate the iron oxide and the beneficiated powder. Hence a new leaching vessel was designed to carry out leaching. As the leaching in HCl medium was found to be highly unsatisfactory a new fabricated leaching set-up was designed in which about 30 g of roasted ilmenite was suspended in a plastic beaker containing, 120 ml of 1.5% NH4Cl weight/weight (w/w) ratio and 0.5% volume/volume (v/ v) methanol. The pH of the aqueous medium was adjusted to 4 with the addition of acetic acid. During leaching, a continuous flow of air was maintained at the rate of 4–5 l min− 1, whilst the mixture was stirred at 1500 rotation per minute (rpm). The leaching was carried out for 5 h, after which the leached grains of roasted minerals were separated from iron oxide by repeated washing with de-ionised water. We define this leaching as the “aeration leaching” throughout the text. For detailed phase, chemical and microstructure analyses, the partially leached minerals were sampled every hour, and were washed thoroughly with water and analysed for total iron, sodium oxide and TiO2 after drying in an air oven at 100 °C. The chemical analysis for iron, sodium, minor and trace impurities and TiO2 in bomar ilmenite and Table 1 Chemical composition of roasted bomarilmenite with soda-ash at 900 °C for 4 h. Constituents Roasted ilmenite (%) Roasted anatase (%) TiO2 Fe2O3 Na2O Al2O3 Cr2O3 Mn3O4 CaO SiO2 P2O5 ThO2 CeO2 Nb2O5 ZrO2 48.3 18.69 41.8 1.36 0.13 0.17 0.01 1.52 0.2 0.24 0.97 0.63 2.41 45.2 12.32 43.1 3.25 0.08 0.33 1.35 1.38 3.62 0.07 0.82 0.76 1.13 ilmenite, which is essential for subsequent selective separation of transition and rare-earth elements. The new physical chemistry of leaching offers an excellent route for the production of high-grades of synthetic rutile for chlorination. 2. Experimental materials and methods Bomar ilmenite or anatase was roasted with soda-ash and alumina (Jha et al., 2008) at around 900 °C, washed and dried prior to leaching studies. The chemical composition of the raw material used for leaching is given in Table 1. Laboratory grade and GR grade chemical reagents were used for the leaching experiments and chemical analysis, respectively. X ray diffraction studies were carried out for the bomar ilmenite and anatase that both show the presence of phases like TiO2, Fe2O3, pseudorutile, P2O5, and Al2O3. After roasting the phases present were SA AF SAF S AF SA SAF STT ST ST ST ST ST STT STT AF SA AF SA Anatase roasted SAFF ST ST ST ST STT STT SAF SA AF SAF S SAFF S SST ST ST SAFF STT SAFF SSAF Ilm meniite roasteed Fig. 1. XRD of ilmenite and anatase roasted with soda-ash + alumina at 900 °C for 4 h. ST — sodium titanate, SAF — sodium aluminium ferrite. 115 J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 Fig. 2. Bomarilmenite roasted with soda-ash + alumina for 4 h at 900 °C. anatase was carried out using the ASTM standards by adopting the wet chemical analysis, Atomic Absorption Spectroscopy (Thermo Scientific iCE 3000), XRF (Oxford ED2000 XRF), and colorimetric (Shimadzu LCMS 8050) techniques. For in detailed phase and microstructure analyses, X-ray powder diffraction (Philips PW 1825) with CuKα radiation and scanning electron microscope (SEM) (Philips XL 30 ESEM) were used. Particle size analysis was carried out using Master sizer 2000. SEM samples were prepared by dispersing the ore particles in epoxy resin and hardened. The surface was then polished by using a standard metallographic procedure. Carbon coating was given to the sample before mounting into the SEM chamber. The energy dispersive X-ray data were also obtained for relevant regions of phases during SEM analysis. Master sizer 2000, Malvern instrument was used for particle size analysis. 3. Results and discussion software called F*A*C*T* in order to find out the redox potential–pH diagram in which the iron and sodium could be preferentially dissolved in water (Factsage, 2002). In the redox potential–pH diagram, the predominance area diagram shows the thermodynamic stability limits of ionic and solid species coexisting in aqueous systems. The two variables representing the equilibrium in an aqueous system are the hydrogen ion activity measured by the pH and the oxidation potential measured by the Eh (volts) on the standard hydrogen scale. Solid iron, sodium and TiO2 are stable between a potential of −0.6 to −0.4. Above the potential of 0.5 to 1.2 solid iron oxide and TiO2 are thermodynamically stable. Above pH 5 to 7 solid ilmenite, sodium and TiO2 solid are thermodynamically stable. From the Eh–pH diagram obtained from F*A*C*T* sage software it is understood that the iron precipitation takes place at an acidic pH below 4 and a potential range of 0.01 to 0.47 where Fe2+ and Na+ separates out. During catalytic aeration pH 4 was maintained throughout the reaction by the addition of few drops of acetic acid. 3.1. Eh–pH diagram 3.2. Role of ammonium chloride It is already known that metallurgical leaching is based on Eh–pH diagram which is shown in Fig. 3. The optimum condition for selective iron removal could be established with Eh–pH diagram. A redox potential–pH (Eh–pH) diagram was constructed for the Na–Ti–Fe–H2O system at room temperature using a thermo chemical computational The major functions of the NH4Cl catalyst are (Farrow et al., 1987; Kumari et al., 2001) that it acts as a buffer for hydroxyl ions and prevents excessive high local pH values. The ammonia formed as a result of buffering complexes with iron ions. The chloride ions help to break down any passive film. Iron in the presence of ammonia forms a complex such as [Fe(NH3)4]2+ (Jones and Hackerman, 1968) and iron in unbuffered neutral chloride solution, formation of [Fe(OH)m(Cl)n]2-m-n takes place. It is already known (Becher et al., 1965) that in the presence of NH4Cl, the iron dissolution is prevented by the formation of some Table 2 Chemical composition of aeration leached bomarilmenite. Fig. 3. Eh–pH diagram of Na–Ti–Fe–O system calculated by using F*A*C*T*—Sage programme. Constituents Aeration leached ilmenite (%) Aeration leached anatase (%) TiO2 Fe2O3 Na2O Al2O3 Cr2O3 Mn3O4 CaO SiO2 P2O5 ThO2 CeO2 Nb2O5 ZrO2 70.89 7.7 15.62 1.15 0.13 0.11 0.01 0.35 0.02 0.12 0.33 0.18 0.72 69.58 8.8 15.89 1.16 b0.01 0.2 0.35 0.80 1.01 0.04 0.28 0.63 0.82 116 J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 Ilm+NH4Cl + CH3OH with HCl with acetic acid Ana + NH4Cl + CH3OH 60 6 pH of reaction mixture % Iron removal 50 40 30 20 4 10 0 0 2 4 6 0 2 4 Time (hrs) Fig. 4. Plot between % iron removal vs time during aeration leaching of roasted ilmenite and anatase. protective film on the iron surface. It was found (Kumari et al., 2001; Jaya et al., 2000) that certain reducing sugars or carbonyl compounds when added along with NH4Cl electrolyte increase the dissolution rate of iron and lead to the formation of iron hydroxide. In this study 0.5% of methanol has been added along with NH4Cl during aeration leaching. The detailed chemical analysis after aeration leaching is shown in Table 2. The % iron removal vs time is shown in Fig. 4. The bomar ilmenite had an iron oxide content of 21.9%. During roasting the iron oxide present on the surface of the ilmenite lattice reacts with soda-ash to form sodium ferrite which is a soluble compound. Further washing with water the soluble compound formed on the surface of the ilmenite grain dissolves out. When analysed the roasted ilmenite or anatase after washing has found that the iron oxide content has been reduced from 21.9 to 18.69 or 15.62 to 12.32 respectively. Whilst the TiO2 present in the ilmenite or anatase reacted with soda-ash to form sodium titanate which is evident from the analysis that TiO2 content decreased from 76.45% to 48.3% or 65.89 to 45.2 and sodium oxide content increased from b1% to 41.8% or 43.1 respectively. During aeration leaching iron oxide and sodium ions leached out which is evident from Figs. 4 & 5. From Fig. 4 around 60% and 30% iron removal was achieved in 5 h from roasted ilmenite and anatase respectively. From Fig. 5 it is very clear that around 65% Na2O has been removed in 5 h of leaching. 3.3. Mechanism of the aeration leaching 2Fe2 O3 þ 5H2 O þ 1=2O2 ¼ 2 FeðOHÞ2 þ 2FeðOHÞ3 Fig. 6. Plot between pH vs time during the aeration leaching of roasted ilmenite with sodaash + alumina. 2CH3 OH þ O2 ¼ 2HCHO þ 2H2 O 2þ 2FeðOHÞ2 þ HCHO ¼ 2Fe 2H2 O þ O2 þ 4e − 2þ 2Fe þ 4ðOHÞ − ðOxidationÞ þ HCOOH þ H2 O ¼ 4ðOHÞ − − ðWeathering reactionÞ ð1Þ Na2O in ilmenite Na2O in anatase 60 50 ð5Þ ¼ FeðOHÞ2 ; FeðOHÞ3 ; FeOðOHÞ þ 2NaOH  2þ 2Na2 OFe2 O3 þ 16NH4 Cl ¼ 4 FeðNH3 Þ4 þ 4NaCl þ 8H2 O 30 ð7Þ The role of pH during aeration reaction is shown in Fig. 6. It was found that when HCl was used during leaching, during the course of the reaction pH increased exponentially from 4 to 6 with time. But when acetic acid was used, pH fluctuated in between 3.8 to 4.25. This may be mainly due to the slow dissociation of acetic acid and or may be due to the conversion of methanol to formic acid during the course of the reaction. Hence the concentration of acid concentration was estimated during the course of the reaction. Table 3 Variations in free acid concentration during the aeration leaching of ilmenite roasted with soda-ash + alumina with NH4Cl + CH3OH + acetic acid. 20 10 0 2 ð6Þ The Fe2O3 present after roasting and washing would be totally distorted because of the partial removal of Fe2O3 as sodium ferrite which created vacancies in the roasted ilmenite lattice. Therefore when subjected for aeration leaching, chemical reactions are possible as explained above which selectively removes major quantities of sodium oxide and iron oxide. 40 0 ð3Þ ð4Þ ¼ 2FeðOHÞ2 Na2 OFe2 O3 þ 4ðOHÞ ð2Þ 3.4. Effect of pH on aeration leaching 70 % Na2O removal 6 Time (hrs) 4 6 Time (hrs) Fig. 5. Plot between % Na2O removal with time from ilmenite and anatase during aeration leaching. Time (hrs) Acid concentration (%) 0 1 2 3 4 5 0.029 0.028 0.033 0.017 0.021 0.022 J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 117 a b Fig. 7. a. XRD of processed ilmenite after aeration leaching. Phases such as sodium titanate, rutile, anatase, pseudorutile and pseudobrookite are formed after aeration leaching. b. XRD of synthetic rutile after acid wash. Phases such as anatase, pseudobrookite and rutile are present after acid wash. A—anatase, ST—sodium titanate, R—Rutile, PR—Psedorutile, PB— Pseudobrookite. 3.5. Acid concentration During the aeration leaching a small amount of leachant was withdrawn at regular intervals of time to determine the acid concentration present in the mixture. The acid concentration is given in Table 3. From the acid concentration it is confirmed that when organic compounds were added during leaching the pH could be maintained since free acids are being generated during the course of the reaction which reflects in the rate of the iron removal also. When HCl was used for leaching after 5 h it was found that around 14.2% total iron oxide was left behind. But when NH4Cl + CH3OH + acetic acid was used after 5 h of leaching the total iron oxide left behind was 7.7% in roasted ilmenite. The better iron removal could be accounted for the stability of pH during the reaction. The major phases present at XRD of the aeration leached sample were sodium titanate as Na4TiO4, Na2TiO3, iron hydroxide FeOOH and iron oxide Fe2O3. The presence of 7.7% iron after aeration leaching analysed through wet chemical analysis is further confirmed by XRD shown in Fig. 7. SEM of beneficiated bomar ilmenite through aeration leaching is shown in Fig. 8 which shows that a lot of cracks and pores have been developed on particle. The formation of cracks and voids may be due to the removal of sodium ferrite and iron hydroxide during leaching. The EDX (Fig. 8) also show the disappearance of Kβ and Kα phase of iron which is an evident of removal of iron. The presence of iron around 8.8% in anatase even after leaching may be due to the locking of iron oxide in to TiO2 matrix. This confirms the theoretical findings of Eh–pH diagram that better iron removal could be obtained below pH 4. Fig. 8. Roasted bomarilmenite obtained after aeration leaching with NH4Cl + CH3OH + acetic acid. 118 J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 Table 4 Chemical analysis of the acid washed synthetic rutile. Constituents Acid washed ilmenite with 4 M HCl (%) Acid washed anatase with 4 M HCl (%) TiO2 Fe2O3 Na2O Al2O3 Cr2O3 Mn3O4 CaO SiO2 P2O5 ThO2 CeO2 Nb2O5 ZrO2 90.1 4.8 5.68 1.15 0.13 0.11 0.01 0.25 0.02 0.12 0.33 0.18 0.72 88.2 5.09 5.25 0.9 0.05 0.03 – 0.11 0.01 0.02 0.12 0.015 0.21 3.6. Acid wash aluminium phosphate or as zirconium silicate hosts. Rare earth oxides are separated as undisturbed aluminium phosphate or zirconium silicate grains. The separated rare earth oxide can be removed by subjecting the processed material for a physical separation means such as para magnetic separation or magnetic separation. The schematic SEM and EDX of rare earth oxide are shown in Fig. 10. 3.8. Effect of particle size during various processing steps The synthetic rutile obtained after processing steps such as roasting, aeration leaching and acid wash was being subjected for particle size analyses. The mid-point range of the particle is represented as d10, d50 and d90. Around 10% of the grains were of 72.35 μm, 50% of 141.34 μm and 90% of 276.68 μm. The particle size analysed after the processing steps is shown in Table 5. From the particle size analysis report it was found that the particle size obtained for synthetic rutile is well suited to be used in a Fluidised Bed Chlorination Reactor (FBCR) for chlorination to convert to TiO2 pigment grade. By controlling the roasting time, temperature and under optimised conditions of leaching the particle size of the grains were retained. The observations from Table 2 reveal that a total iron oxide and sodium oxide content of 7.7%, 8.8%, 15.62% and 15.89% still exist in the roasted ore minerals bomar ilmenite and anatase even after aeration leaching. Therefore acid wash was carried out by suspending the leached ore minerals in 4 M HCl in a leaching vessel which had a fitted condenser and a Teflon stirrer. The acid wash was carried out for 2 h at 50 °C. After acid wash the synthetic rutile was separated and washed well with water, dried and analysed. The chemical analysis report is given in Table 4. From the chemical analysis it is found that around 90% and 88% synthetic rutile has been beneficiated from bomar ilmenite and anatase ore respectively. The presence of around 4% iron and 5% sodium oxide is also revealed through XRD as Fig. 7b has the phases like pseudorutile, pseudobrookite and sodium titanate respectively. The acid washed synthetic rutile micrograph shown in Fig. 9 reveals the removal of sodium oxide considerably as the peak height has reduced significantly. However still the presence of 5% sodium oxide observed through AAS is being proved by EDX (Fig. 9). A high temperature alkali roasting followed by aeration leaching and acid wash of lean ores such as bomar ilmenite and anatase is explained. The effect of several reaction conditions on the extraction of synthetic rutile is studied. From the results obtained from wet chemical, XRF, AAS, XRD, SEM and EDX the following conclusions were obtained. Soda-ash roasting followed by leaching is an alternative route for the processing of lean quality ores with rare earths dispersed throughout the titaniferous grains. Better iron and sodium oxide removal was observed when NH4Cl + CH3OH + acetic acid was used as the leaching medium compared to HCl acid. Optimum pH required for the iron removal was 4. Unaltered rare earth oxides could be selectively separated after the processing steps. The particle size of the processed material was comparable to the size required for further chlorination. 3.7. Selective separation of rare earth oxides Acknowledgements As explained earlier bomar ilmenite is a lean ore with the distribution of rare earth oxides throughout the mineral lattice. During sodaash roasting and post-leaching steps the rare earth oxides removed from the mineral lattice have been found to be unattacked by soda. The rare earth oxides present in the mineral is associated either as We are grateful to EPSRC, Millennium Inorganic Chemicals, and DTI (RGMATS 448696) for the financial support. Also the authors acknowledge the technical support of Mr. Simon Lloyd. The first author is grateful for the facilities provided by the Institute for Materials Research, University of Leeds, UK to carry out this work. 4. Conclusion Fig. 9. Synthetic rutile obtained after acid wash with 4 M HCl at 50 °C. Most of the impurities have removed leaving behind high grade synthetic rutile. J.K. Ephraim, A. Jha / Hydrometallurgy 152 (2015) 113–119 119 Fig. 10. Rare earth particle separated un-attacked by the chemical processing steps. Table 5 Particle size analysis report (d is the midpoint range of the particle sizes). Materials Particle size (microns) d10 d50 Sodium titanate obtained by roasting ilmenite with soda-ash + alumina and aerated Synthetic rutile obtained after roasting ilmenite with soda-ash + alumina, aerated and acid washed Synthetic anatase obtained by roasting anatase with soda-ash + alumina, aerated and acid washed 144.4 244.03 294.49 d90 72.35 141.34 276.68 164.75 409.13 755.43 References Barksdale, J., 1966. Titanium: Its Occurrence, Chemistry and Technology. Second ed. The Roland Press Company, New York. Becher, R.G., Canning, R.G., Good Heart, B.A., Ulsra, S., 1965. A new process for upgrading ilmenite mineral sand. Aust. Inst. Min. Met. Proc. 214, 21–44. Chernet, T., 1999. Applied mineralogical studies on Australian sand ilmenite concentrate with special reference to its behavior in the sulphate process. Miner. Eng. 12 (5), 485–495. Ephraim, Jeya, Patel, Raj, 2013. Improved Metal Production, PCT/GB/052719. Factsage, 2002. FACT-Sage Ver. 5,2002. Thermodynamic Software Developed by Bale, C. W., et al. at ÉcolePolytechnique CRCT, Montréal, Québec, Canada. Farrow, J.B., Ritchie, I.M., Mangano, P., 1987. The reaction between reduced ilmenite and oxygen in ammonium chloride solutions. Hydrometallurgy 18, 21–38. Gambogi, J., 2009. Titanium mineral concentrates. US Geol. Surv. 172–173. Gambogi, J., 2010. Titanium and titanium dioxide, mineral commodity summaries. US Geol. Surv. 176–178. Jabłoński, M., Przepiera, A., 2001. Kinetic model for the reaction of ilmenite with sulphuric acid. J. Therm. Anal. Calorim. 65 (2), 583–590. Jaya, Kumari E., Koshy, Peter, Mohan Das, P.N., 2000. Investigations on the effect of certain carbonyl compounds on the removal of iron during the rusting of reduced ilmenite. Trans. Indian Inst. Met. 53 (6), 573–579. Jeya Kumari, E., Lahiri, Abishek, Jha, Animesh, 2006. Selective separation of rare earths from titaniferous ores during the production of high-grade synthetic rutile. Proc. Sohn International Symposium on Advanced Processing of Metals and Materials: Principles, Technologies and Industrial Practice, August 27–31, San Diego, California, USA. Jha, A., Lahiri, Kumari, E.J., 2008. Beneficiation of titaniferous ores by selective separation of iron oxide impurities and rare earth oxides for the production of high-grade synthetic rutile. Miner. Process. Extract. Metall. 117 (3), 157–165. Jones, D., Hackerman, N., 1968. The corrosion of Fe in the NH4NO3–NH3–H2O system. Corros. Sci. 8, 565–572. Kroll, J.W., 1940. The production of ductile titanium. Trans. Am. Electrochem. Soc. 78, 35–47. Kumari, E.J., Bhat, K.H., Sasibhushanan, S., Mohan Das, P.N., 2001. Catalytic removal of iron from reduced ilmenite. 14 (3), 365–368. Langmesser et al., 1973, P.W. Langmesser, H.G. Volz, G. Kienast, 1973. Process leading to the production oftitanium dioxide pigment with a high degree of whiteness. U.S. Patent 3,760,058. Mukherjee, P.S., Choudury, S.K., Misra, V.N., Mohan Das, P.N., 2002. Melt separation of titania rich slag and pig iron from pre-reduced ilmenite using thermal plasma. Trans. Indian Inst. Met. 55 (6), 543–550. Sasikumar, C., Rao, D.S., Srikanth, S., Ravikumar, B., Mukhopadhyay, N.K., Mehrotra, S.P., 2004. Effect of mechanical activation on the kinetics of sulfuric acid leaching of beach sand ilmenite from Orissa, India. Hydrometallurgy 75 (1–4), 189–204. Zhang, Yongjie, Qi, Tao, Zhang, Yi, 2009. A novel preparation of titanium dioxide from titanium slag. Hydrometallurgy 96, 52–56. Zhang, Wensheng, Zhu, Zhawu, Yong Cheng, Chu, 2011. A literature review of titanium metallurgical processes. Hydrometallurgy 108, 177–188.