LEACHING OF ILMENITE AND PRE-OXIDIZED ILMENITE IN HYDROCHLORIC ACID TO OBTAIN HIGH GRADE TITANIUM DIOXIDE R. Vásquez, A. Molina Material Science Department. Simon Bolivar University Caracas, Venezuela 1080A. e-mail: rvasquez@usb.ve, amolina@usb.ve Abstract. In order to obtain high grade TiO 2, ilmenite from Venezuela, was oxidized and leached with a 20wt% hydrochloric acid solution at several conditions. Oxidation was performed at temperatures of 700, 800, 900 and 1050ºC while several parameters of leaching such as time, temperature, iron particles addition and agitation were evaluated. Ilmenite without oxidation was also leached for comparison. Original ilmenite and solid products of both oxidation and leaching were characterized by X-ray diffraction and Scanning electron microscopy, whereas solutions obtained after leaching were analyzed by atomic absorption and U.V. spectrophotometry for iron and titanium determination respectively. It was established that pre-oxidation contributes with iron dissolution due to the iron diffusion that occur during that process. However for iron dissolution is crucial the presence of iron particles as they reduce the ferric iron obtained after oxidation. The temperature of oxidation has an effect on type, proportion and distribution of phases. Hematite and rutile are obtained at 700 C, with an eggshell configuration, that revealed the best conditions to dissolve iron selectively. Pseudobrookite and rutile randomly distributed are obtained at higher temperatures and they showed great crystallinity and grade of product, nevertheless the presence of pseudobrookite promotes titanium dissolution. Extraction increased with leaching temperature and 105 C resulted to be the optimum value for the dissolution of species. Agitation, on the other hand, demonstrated no significant effects on dissolution of iron, but promotes titanium hydrolysis. Keywords: Rutile, Ilmenite, Leaching, Oxidation of Minerals. 1. INTRODUCTION Titanium dioxide has been widely used as pigment for the manufacture of paints and paper. Recently, the importance of this particular oxide has increased exponentially, due to its good semiconducting behavior and photosensibility, which made it adequate as solar cell material. Today, the main source for synthetic TiO 2 production is ilmenite (FeTiO 3 ), which can be found worldwide, principally in Australia, Egypt, USA, China and also in Venezuela. Usually, ilmenite is treated via sulphuric acid leaching (Chiang, B. et. al. 2005) or dry chlorination for the removal of iron (Stanaway, K. 1994). Many researches have been developed in order to improve TiO 2 extraction from ilmenite. Chun Li et al., 2007, for instance, proposed mechanical activation of ilmenite to increase dissolution rate and reduce the amounts of solution needed. Others authors (Natziger, R. et. al. 1987; Gupta, S. et. al. 1987) have suggested the use of a heat treatment followed by hydrochloric acid leaching. That
heat treatment involves mainly carbothermic reduction to reduce iron for easy removal by leaching. A stage of oxidation prior to reduction has also been proposed, because several authors have reported an increment of reduction rates (Zhang, G. Ostrovski, O. 2002). Nevertheless, the use of two heat treatments before leaching made the process more energy consumer. It is worth noting, however, that previous studies (Vásquez, R. 2005) demonstrated important changes that occur on the surface of ilmenite during high temperature oxidation, particularly the formation of hematite. This condition propitiates selective iron extraction because it is concentrated on surface. Thus, in this work it is proposed to leach preoxidized ilmenite with HCl solution. Owing to the fact that ferric iron obtained after oxidation could be difficult to dissolve, in this process it will be used iron particles, following the suggestion of Mahmoud et al., 2004, whom found that iron dissolved in HCl solution could act as reducing agent improving the dissolution. For the reasons exposed, this work presents a study of the surface modifications generated by oxidation, on the dissolution of iron from ilmenite in hydrochloric acid, in order to obtain optimum conditions for high grade TiO 2 production. 2. EXPERIMENTAL 2.1. Raw Material Preparation. Ilmenite used in the work was acquired from Bolívar State in Venezuela. It was concentrated by magnetic and electrostatic separation using optimised operation parameters previously established. The initial characterization of original Ilmenite involved X-ray diffraction (XRD) of powders and scanning electron microscopy (SEM) of transversal sections of particles. In the latter case, it was needed petrographic preparation of particles, embedded into epoxy resin. XRD patterns revealed that the principal crystalline phase corresponds to FeTiO 3, with low amounts of quartz, (figure 1). SEM image from figure 1 shows a typical ilmenite particle with its EDS spectrum. Fig. 1. XRD pattern of the concentrated ilmenite. SEM image of the transversal section of an ilmenite particle with EDS spectrum. 2.2. High Temperature Oxidation. Oxidation tests were performed in a tubular furnace Thermolyne 21100. Samples were placed inside the furnace once the temperature was reached with the help of an alumina crucible. In all cases the process was developed in open air for periods of 6 and 24 hours and at temperatures of 700, 800, 900 and 1050ºC chosen from previous results (Vásquez, R.
2005). Oxidation products were characterized by X-ray diffraction and scanning electron microscopy. As before, preparation of transversal sections by petrographic techniques was required. 2.3. Leaching Leaching was carried out with hydrochloric acid at a concentration of 20wt %. In all cases 2 grams of ilmenite were mixed with 100ml of acid solution. Also, 0.5 grams of metallic iron particles were added to the reactor, 20 minutes later of the beginning of the test. The tests were performed at different temperatures specifically at 25, 84 and 105ºC, using hotplates with magnetic agitation. Vessels were adapted with condensers to maintain the acid concentration stable during the test. Also, each reactor was provided with apertures to allow the egress of intermediate samples for kinetic studies. The products were separated by filtration. Solids were washed and introduced in an oven at 90 C for SEM and XRD characterization. Liquids were analyzed by U.V. and atomic absorption spectrophotometry in order to determine iron and titanium dissolved. 3. RESULTS AND DISCUSSION 3.1. Effects of preoxidation Figure 2 shows the kinetic curve of iron extraction from original and 700 C preoxidized ilmenite, both leached in HCl 20%wt solutions at 105 C, with iron particles addition as described above. Fig. 2. Iron removal (%) from leaching of standard and 700 C preoxidized ilmenite. Titanium removal (%) from leaching of standard and 700 C preoxidized ilmenite. Leaching conditions are: 6 hours, 20wt% HCl, 105ºC with 0.5 grams of iron particles. It is clear the improvement of iron removal when the mineral is oxidized, which reach almost 88% extraction, in comparison with original ilmenite, whose maximum removal oscillated around 58%. This result could be explained in terms of the events that occur during ilmenite oxidation. Pre-oxidation of ilmenite produce surface modifications as a result of diffusional processes that take place at high temperature on air. In this particular case, iron moves faster than titanium and they tend to migrate towards the high oxygen potential region (Martin, M. 2003). Once the iron cations arrive to the surface of the particle, they are oxidized, and a hematite eggshell is formed. The oxidation process is given by (Zhang, G. Ostrovski, O. 2002): 2FeTiO 3 + ½ O 2 = Fe 2 O 3 + 2TiO 2 (1)
This phenomenon occurs at 700ºC of oxidation at times as shorter as 6 hours as observed in figure 3, which shows the two phases: external hematite and internal iron doped rutile. This concentration of iron on the surface made the process more selective respect to iron, which increases efficiency of leaching process under the conditions used. As a result, the dissolution of ilmenite is represented by (Mahmoud et al., 2004): Fe 2 O 3 + 6HCl = 2FeCl 3 + 3H 2 O (2) In the case of original ilmenite, although the main iron cations are in the ferrous form, which are easier to dissolve, they have to migrate to the surface to be leached. Thus, the process even faster at the beginning becomes slower with time. In addition, it has been reported that dissolution of ilmenite follows the reaction specified by (Mahmoud et al., 2004): FeTiO 3 + 4HCl = FeCl 2 + TiOCl 2 + 2H 2 O (3) This made it clear that HCl not only promotes iron removal but also titanium could be dissolved. Figure 2 illustrates titanium extraction from both samples. This reveals the high removal of titanium from non preoxidized samples. Even though it has been demonstrated that titanium oxide could precipitate later from solutions, it is a reagent consumer during the process. Fig. 3. SEM images of the transversal section of ilmenite oxidized 6 hours at 700ºC indicating the two phases (1 and 2) obtained after oxidation. EDS of both phases are included. In both cases a steady state is reached with time due to dissolution is a diffusion dependent process. Even though preoxidized sample has iron concentrated on its surface, part of it is still dissolved on rutile and at the end it has to diffuse to be removed. In original ilmenite this behavior is reached earlier because iron is distribute homogeneously in ilmenite. Thus, as pre-oxidation plays an important roll on iron dissolution from ilmenite, the following leaching tests were performed using preoxidized samples. 3.2. Effects of temperature of preoxidation It was demonstrated before, the improvement of iron removal when using preoxidized ilmenite during leaching, as a result of iron diffusion towards the surface. It is also well known the effect of temperature on diffusion. Thus the next series of tests involved the leaching of samples preoxidized at temperatures of 800, 900 and 1050 C in order to check the effect of that parameter on acid leaching process. Figure 4 shows the resulting kinetic curves of iron extraction under those conditions. Leaching of ilmenite preoxidized at 700 C was included for comparison. It is observed that in all cases occurs the dissolution of iron, because leaching is carried out at conditions where ferric ion obtained during oxidation is
reduced. Nevertheless, the removal is highest when using ilmenite oxidized at the lowest temperature tested, i.e., 700 C, which reached about 88% extraction, while 60.5, 73.8 and 67.9 % of extraction were attained for samples oxidized at 800, 900 and 1050 C respectively. Fig. 4.. Iron removal (%) and Titanium removal from leaching ilmenite preoxidized at 700, 800, 900, 1050 C. Leaching conditions are: 6 hours, 20wt% HCl, 105ºC with 0.5 grams of iron particles. To explain the differences, it is considered necessary to recall what takes place during oxidation of ilmenite at those temperatures. Previous results (Vásquez, R. 2005) demonstrated that temperature of oxidation affects the type, proportion and distribution of phases produced. Figure 5, for example, shows XRD patterns of ilmenite oxidized at different temperatures. Fig. 5. XRD patterns of products obtained at different temperatures of oxidation. XRD patterns of Products of obtained after leaching of ilmenite preoxidized at different temperatures. Leaching conditions are: 6 hours, 20wt% HCl, 105ºC with 0.5 grams of iron particles. It is evident that at 700 C, the products of oxidation are hematite and rutile, as mentioned before. At higher temperatures, however, a new phase: ferric pseudobrookite is formed as a result of the following reaction: Fe 2 O 3 + TiO 2 = Fe 2 TiO 5 (4)
The proportion and distribution of pseudobrookite changes with temperature as observed in figures 6 and for ilmenite preoxidized at 800 C and 1050 C respectively. It is clear that at lower temperature pseudobrookite is localized between Fe 2 O 3 and TiO 2 whereas at 1050 C, a matrix of Fe 2 TiO 5 with TiO 2 distributed randomly are the only detectable products. Large amounts of porosities are also observed. Then, the microestructural variation seems to affect iron dissolution. In the first place, at 700 C, iron in the form of hematite is localized at the surface of particles. Thus iron removal at the beginning of process is easier. Due to the formation of pseudobrookite at higher temperatures, iron is involved in another more complex phase which made it slightly difficult to extract. At the same time as the new phase contains titanium, during iron dissolution also that element is dissolved, as occurred with original ilmenite. Hematite pseudobrookite rutile Pseudobrookite (clear) Rutile (gray) Fig. 6. SEM images from preoxidized sample. 800ºC of treatment. Morphology of 1050ºC sample. As an evidence for that statement figure 4 illustrates titanium extraction from samples of ilmenite oxidized at different temperatures and leached as described. It made clear that titanium extraction increases with temperature. Thus, leaching of samples preoxidized at higher temperatures involves pseudobrookite dissolution. Figure 5 shows the XRD patterns of products obtained after leaching samples of ilmenite preoxidized at different temperatures. It is evident that pseudobrookite peak intensity diminishes with treatment while original peak of rutile increases. The differences observed in each case indicate high purity and proportion of the desired phase (TiO 2 ). This is the result of the increment of oxidation temperature that procures extensive iron diffusion from rutile previously formed. Another evidence of the mechanism involves during leaching of preoxidized ilmenite could be seen in figure 7, which shows SEM images of some particles of ilmenite preoxidized at different temperatures and leached at the conditions mentioned. The figure 7, for instance, is a representative particle, enriched on titanium as EDS indicates. This was obtained after leaching of ilmenite preoxidized at 700 C. The surface irregularities suggest that iron dissolution takes place on that region as a result of the hematite localized originally there. Similar results were observed after leaching particles of ilmenite preoxidized at 800 C (figure 7). However, different features are observed after leaching of ilmenite particles preoxidized at 900 and 1050 C, observed on figures 7(c) and (d) respectively. In these cases, dissolution takes place massively on each particle because of the random distribution of pseudobrookite on them. Due to the high proportion of pseudobrookite still is reported high proportion of iron in the products. In addition, there is a strong evidence of the effect of porosities on the microstructure which facilitate solution penetration through the particle assisting on iron removal; otherwise dissolution could be probably more difficult. Thus, the microstructural changes produced when preoxidation temperature increase has an important
effect on the mechanism involved in the removal of iron during acid leaching and also in the proportion of the desired phase obtained. Conversely the iron extraction is only slightly affected. (c) (c) (d) (d) Fig. 7. SEM images of the transversal section and powder morphology of samples of ilmenite with different treatments. Pre-oxidized at 700ºC. 800ºC. (c) 900ºC. (d) 1050ºC. All the samples were leached 24hr at 105ºC with 0.5gr Fe powder. EDS analysis are also presented. 3.3 Effects of temperature of leaching Leaching temperature has demonstrated to have an important effect on dissolution kinetic and extraction due to the fact that leaching of ilmenite is a diffusion dependent process, thus this parameter was evaluated. Figure 8 illustrated the iron extraction obtained after leaching at 25, 84 and 105 C for 24 hours, ilmenite preoxidized at 1050 C. It is clear the increment of iron extraction with temperature, which suggest the increase of pseudobrookite dissolution. This is evident from figure 9 which present the XRD patterns of the solid product obtained after leaching at the conditions mentioned above, which confirm the pseudobrookite consumption and the increase of rutile peak intensities. Also, it is worth noting that after steady state is achieved, there are not significant changes on iron extraction. Fig. 8. Iron removal (%) of 1050ºC preoxidized ilmenites by atomic absorption analysis at tree different leaching temperatures. Fig. 9. XRD patterns of the 1050ºC preoxidized samples leached at tree different temperatures.
3.4 Effects of iron powder presence and agitation Some tests without addition of iron particles and without agitation were performed for leaching ilmenite preoxidized. The results are present on figure 10. It is clear that addition of iron particles play an important effect because of the enhancement on the dissolution of Fe 3+ cations as proposed by Mamoud et. al. 2007. Regards to agitation effect, figure 10 reveal that iron removal is slightly lower in comparison to stirring conditions. This can be a consequence of the reduction of contact between acid solution and solid particles. Conversely, titanium dissolution is higher probably because at static conditions it is formed a particle bed which generates a reaction front, that promote the formation of Ti enrichment layers that tends to dissolve. Fig. 10. Iron removal and titanium removal in samples leached at 105ºC with 20%wtHCl, without Fe and without agitation. Samples were leached for 24 hours in 20%wtHCl at 105ºC with 0.5gr of iron particles. 4. CONCLUSIONS. Pre-oxidation, contributes with iron removal thanks to diffusion processes that occurs during it, creating more selective conditions for hydrochloric acid leaching. Temperature of oxidation affects the type, proportion and distribution of phases produced. Oxidation at 700 C generates an eggshell configuration which is the best iron removal conditions. Pre-oxidation at 1050 C propitiates a random two phase distribution, whose leaching generates the high grade of the desired product. The mechanism of iron removal from pre-oxidized ilmenite involves dissolution of hematite when oxidation is performed at 700 or 800 C and dissolution of pseudo-brookite on samples oxidized at higher temperatures. The optimal leaching temperature is 105ºC, and the iron removal is greatly enhanced with addition of metallic iron particles. Agitation demonstrated no significant effects on dissolution of iron, but promotes dissolution of titanium. REFERENCES CHUN LI, BIN LIANG, LING-HONG GUO., 2007. Dissolution of mechanically activated Panzhihua ilmenites in dilute solutions of sulphuric acid. Hydrometallurgy 89, 1 10. GUPTA, S., RAJAKUMAR, V., GRIEVESON P., 1987, Kinetics of reduction of ilmenite with graphite at 1000 to 1100ºC. Metallurgical Transactions B. Volume 18B December. 713-717. MARTIN, M. Pure Appl. Chem. Vol. 75 No. 7, (2003), pp. 889-903. M.H.H. MAHMOUD, A.A.I. AFIFI, I.A.IBRAHIM., 2004. Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile. Hydrometallurgy 73, 99 109. NATZIGER, R.H., ELGER, G.W., 1987, Preparation of titanium feedstock from Minnesota ilmenite by smelting and sulfation leaching. U.S. Bureau of Mines. Report of Invest. 9065. STANAWAY, K.J., 1994. Overview of titanium dioxide feedstock. Min. Eng. 46(12). 1367-1370. VÁSQUEZ, R. MOLINA, A., 2005. Mechanism of transformation of ilmenite during air oxidation. Simon Bolivar University. Grade Project. ZHANG, G. OSTROVSKI, O., 2002, Effect of preoxidation and sintering on properties of ilmenites concentrates, Miner. Process. 64, 201-218.