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Chlorination of reduced ilmenite concentrates and synthetic rutile
International Journal of Mineral Processing 100 (2011) 166–171
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International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Chlorination of reduced ilmenite concentrates and synthetic rutile Andrew Adipuri 1, Yan Li, Guangqing Zhang ⁎, Oleg Ostrovski School of Materials Science and Engineering, the University of New South Wales, UNSW SYDNEY, NSW 2052, Australia
a r t i c l e
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Article history: Received 29 October 2010 Received in revised form 2 July 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Ilmenite Synthetic rutile Oxycarbide Oxycarbonitride Titanium tetrachloride Chlorination
a b s t r a c t Chlorination of reduced ilmenites of different grades (primary, secondary and HYTI 70) and synthetic rutile was investigated at 235 °C. The main phases of primary and secondary ilmenites were Fe2Ti3O9 and FeTiO3; HYTI 70 contained TiO2; synthetic rutile consisted of titania with titanium suboxides and trace amount of iron. Iron oxides were reduced to metallic iron. Titanium oxides were reduced to titanium oxycarbide or oxycarbonitride; reduced samples contained a small amount of titanium suboxides. In chlorination of reduced ilmenite concentrates and synthetic rutile, titanium oxycarbide or oxycarbonitride, metallic iron, and Ti2O3 were chlorinated. The degree of chlorination of both iron and titanium oxycarbide/oxycarbonitride was 95–98%; chlorination of iron was faster than that of titanium oxycarbonitride. The removal of iron by leaching increased the chlorination rate of titanium oxycarbide/oxycarbonitride; it was close to completion in 35 min. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium minerals are mostly processed into titanium dioxide white pigment, which is a valuable commodity used in paint, paper and plastic industries because of its exceptional scattering properties, chemical stability and lack of toxicity (Habashi, 1997). The usage of metallic titanium is limited because of its high production cost. Metallic titanium and titania white pigment are produced from titanium tetrachloride which is obtained by carbochlorination of natural or synthetic rutile at 800–1100 °C. Chlorination of titanium oxycarbide or oxycarbonitride can be implemented at 200–400 °C (Adipuri et al., 2008, 2009). Low temperature chlorination can improve efficiency of production of titanium tetrachloride. Smillie and Heydenrych (1997) investigated process developed for selective chlorination of low grade titanium-containing ores. The ore was first subjected to carbothermal reduction under nitrogen in which the titanium oxide was converted to carbonitride. Titanium carbonitride chlorinated at low temperature of 350 °C (Smillie and Heydenrych, 1997). At this temperature, other oxides were un-reactive and, therefore, a relatively pure titanium tetrachloride product can be obtained. Mostert et al. (1992) and Van Vuuren and Stone (2006) developed technologies for processing titanium minerals using reduction and
⁎ Corresponding author. E-mail address: [email protected] (G. Zhang). 1 Currently a Postdoctoral Research Fellow, Pyrometallurgy Research Centre, University of Queensland, Brisbane QLD 4072, Australia. 0301-7516/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2011.07.005
nitridation of titanium oxides. Titanium oxide in wastes or titanium ores and slags can be nitrided at temperatures of 1000–1800 °C. The optimal chlorination temperature was found between 200–500 °C. Van Vuuren and Stone (2006) indicated that at these temperatures, only the reduced metal oxides such as titanium, iron, and vanadium were selectively chlorinated, leaving residues of calcia, magnesia, silica, and alumina. In the low temperature chlorination, impurities do not chlorinate or chlorinate very slowly (Kanari and Gaballah, 1999; Hitching and Kelly, 1982; Hunter et al., 1975). This permits selective chlorination of titanium oxycarbide or oxycarbonitride, decreases the chlorine consumption and waste generation, and makes the whole technology of ilmenite processing more efficient and environmentally friendly. Carbothermal reduction of ilmenites of different grades and synthetic rutile in different gas atmospheres was studied by Dewan et al. (2010a, 2010b, 2011). Chlorination of titanium oxycarbide produced by reduction of pure rutile in argon was examined by Adipuri et al. (2008); Adipuri et al. (2009) examined chlorination of titanium oxycarbonitride produced by reduction of rutile in nitrogen. No detailed investigation of kinetics and mechanisms of chlorination of reduced ilmenites were reported in literature. The major focus of this paper is on the chlorination of titanium oxycarbide or oxycarbonitride and metallic iron produced by carbothermal reduction of ilmenite concentrates of different grades and synthetic rutile in argon or nitrogen. Chlorination of titanium oxycarbide after leaching of iron from reduced ilmenite is also examined. Investigation of the behavior of minor impurities in the ilmenite concentrates is in progress.
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2. Experimental 2.1. A. Synthesis of titanium oxycarbide or oxycarbonitride Ilmenite concentrates of different grade (primary, secondary, and HYTI 70) and synthetic rutile were supplied by Iluka Resources Ltd. Their chemical compositions, provided by the supplier, are presented in Table 1. XRD analysis of ilmenite concentrates and synthetic rutile showed that primary and secondary ilmenites consisted predominantly of pseudorutile, Fe2Ti3O9, and ilmenite, FeTiO3. HYTI 70 ilmenite concentrate had higher titanium content, which existed in the form of pseudorutile, ilmenite, and rutile. Impurities in ilmenites were present in a relatively small amount, below the detectable level by the XRD analysis. Synthetic rutile was produced from ilmenite ore by reduction and iron removal. It consisted of titania and suboxides (Ti2O3, Ti3O5, Ti4O7) according to XRD analysis. Iron oxides or metallic iron were not detected by XRD, however analysis by XRF indicated 2.75 wt.% of iron in synthetic rutile (Table 1). The particle size for all ilmenite concentrates and synthetic rutile was in the range of 50 μm to 300 μm, with an average size of 152 μm. As received ilmenite concentrates and synthetic graphite (b 20 μm) were mixed with distilled water (70 wt.% of solid mixture) and carboxymethyl cellulose (CMC, 0.3 wt.%). The carbon to TiO2 molar ratio in the mixture was 4.5. At this C/TiO2 ratio, carbon was in excess relative to the stoichiometric amount needed for reduction of titanium and iron oxides. The addition of CMC ensured uniform mixing of ilmenite and graphite. The mixture was dried at 120 °C overnight. The mixture was pressed into pellets of 2.5 g each with 15 mm in diameter and 5 mm in height by applying 40 kN pressing load for 2 min. The pellets were carbothermally reduced in a horizontal electrical furnace under flowing argon or nitrogen atmosphere (1 L∙min− 1) at 1450 °C for 180 min. The analyses of the reduced samples proved that titanium oxycarbide and oxycarbonitride prepared by this manner were of uniform and consistent compositions, which was crucial to the chlorination experiments. The variation in oxygen content of reduced samples produced in parallel reduction experiments with the same carbon to TiO2 ratio was within the error of LECO analysis. 2.2. B. Chlorination of reduced ilmenite concentrates and synthetic rutile Chlorination experiments were carried out in a transparent quartz tube in a horizontal electric furnace. The schematic of the reactor and detailed experimental procedures for sample preparation and chlorination were described elsewhere (Adipuri et al., 2008). Samples were Table 1 Chemical compositions of ilmenite concentrates and synthetic rutile, wt.%.
TiO2 Ti2O3 Total Fe MnO FeO SiO2 ZrO2 Al2O3 P2O5 S Nb2O3 Nb2O5 Cr2O3 CaO CeO2 MgO V2O5 Fe/Ti a b
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
53.9 – 30.5 1.63 18 0.27 0.09 0.4 0.01 0.004 – 0.1 0.047 b 0.01 – 0.18 0.17 0.79
58.20 – 25.89 1.15 11.22 0.75 0.16 0.51 0.033 0.01 – 0.14 0.05 b0.01 – 0.22 0.18 0.66
66.57 – 18.75 0.86 2.38 0.66 0.11 1.1 0.045 0.09 – 0.23 0.084 0.006 – 0.13 0.19 0.46
92.46a 12.1b 2.75 0.86 – 0.91 0.09 0.97 1.57 0.63 0.23 – 0.072 0.02 0.04 0.36 0.28 0.04
Based on total titanium content. Equivalent content of all titanium suboxides.
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heated to 235 °C; this temperature was referred to as a chlorination temperature. The sample temperature changed significantly during chlorination processes due to the exothermic nature of the chlorination reaction (Section 3B). Titanium tetrachloride vapor was absorbed in two scrubbers with 32 wt.% HCl. To prevent TiCl4 vapor from condensing between the reactor tube and scrubber, the reactor outlet tube was heated by hot air generated by a heating gun at 350 °C. Heating also avoided precipitation of ferric chloride formed in the chlorination of reduced ilmenites. Ferric chloride, FeCl3 has a boiling point of 315 °C. Without heating the reactor outlet tube, brown precipitate of FeCl3 was observed. FeCl3 vapor was absorbed by TiCl4 scrubbers. 2.3. C. Leaching of iron from reduced samples Commercially, iron is removed from roasted ilmenites by Becher process or Benelite process to produce synthetic rutile (Lasheen, 2005). The Becher process (Becher et al., 1965) uses aeration to “rust away” metallic iron, while in the Benelite process (Benelite Corp. of America, 1974), metallic iron is dissolved in a hydrochloric acid. In this investigation, removal of iron from reduced ilmenite samples was tested by acid and aeration leaching in a flask of 2.0 L volume with constant stirring. The air for aeration was supplied by a compressor. 22.5 g of the reduced ilmenite sample (Section 2A) was crushed with a ring mill into fine particles. Then it was transferred into a flask containing 1.0 L of HCl or NH4Cl solution maintained at constant temperature. In aeration experiments, air was blown into the slurry at a constant flow rate. With ammonium chloride as a catalyst, oxygen in the air oxidised metallic iron into hydroxides which precipitated outside the particles of titanium oxycarbide or oxycarbonitride. After acid or aeration leaching, the slurry was cooled and filtered. The filtrated particles were washed with distilled water. The filtered cake was then dried in an oven at 150 °C for 24 h and crushed into fine powder. The following conditions were tested to remove iron from reduced primary ilmenite concentrate: (A) Leaching at room temperature with 0.1 M hydrochloric acid for 5 h; (B) Leaching at room temperature with 3.2 M hydrochloric acid for 12 h; (C) Aeration at 40 °C with 0.1 M of NH4Cl solution and 1 L∙min − 1 of air for 3 h; (D) Aeration at 70 °C with 0.1 M of NH4Cl solution and 1 L∙min − 1 of air for 3 h; (E) Aeration at 70 °C with 0.37 M of NH4Cl solution and 2.5 L∙min− 1 of air for 5 h. The best results for iron removal were obtained under condition (E). 2.4. C. Sample characterisation Oxygen, nitrogen and carbon contents in the reduced ilmenite concentrates and synthetic rutile were determined using LECO oxygen-nitrogen analyser (TC-436) and carbon-sulphur analyser (CS-444). The extent of reduction of ilmenite concentrates was calculated from the fraction of reducible oxygen removed. The total o fraction of oxygen in raw material is defined as fO,T , while the o o “reducible” part is fO,R. The reducible oxygen (fO,R) is associated with titanium, iron, and manganese oxides. Other oxides were considered non-reducible or neglected. The fractions of carbon, oxygen, and nitrogen obtained from LECO analysis after the reduction are defined as fC, fO, and fN. Based on the oxygen removed from initial mixture, the extent of reduction in argon was calculated following Eq. (1):
XðTi;O;CÞ =
0 fO;T 0 fO;R
−
0 fO 1−fO;T 0 1−fO −fC fO;R
ð1Þ
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Table 2 Composition of ilmenites and synthetic rutile reduced in argon at 1450 °C for 3 h. Type
Carbon content fC, wt.% Oxygen content fO, wt.% Extent of reduction X(T,O,C), % Phase composition
A-1
B-1
C-1
D-1
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
12.73 4.32 83.39 C, Fe, TiCxOy, Ti3O5, Ti2O3
16.15 3.87 85.6 C, Fe, TiCxOy, Ti3O5, Ti2O3
24.85 3.56 87.45 C, Fe, TiCxOy, Ti2O3
37.51 3.27 90.20 C, Fe, TiC0.84O0.16
The extent of reduction of titania in ilmenite to titanium oxycarbonitride was calculated using the following equation:
XðTi;O;C;NÞ
0 fO 1−fO;T = 0 − fO;R f 0 1−f −f −f O C N O;R 0 fO;T
ð2Þ
When titanium sub-oxides were not detected in a reduced sample by XRD analysis, all the oxygen was assigned to TiO, which allowed calculation of fractions of TiO and TiC in the TiC–TiO solid solution or fractions of TiO, TiC and TiN in the TiC–TiO–TiN solid solution. Samples taken from the gas scrubbers at different reaction time were diluted and then analysed by Inductive Coupled Plasma-Optical Emission Spectrometer, ICP-OES (Perkin Elmer Optima 3000) for Ti and Fe contents. Based on the Ti and Fe contents in the solution, the extents of chlorination were calculated as: α=
mi × 100% moi
ð3Þ
where mi and moi are the masses of titanium or iron in the scrubber solution and in original ilmenite–graphite pellet, respectively. The accuracy of the ICP-OES analysis of titanium and iron contents in a solution (up to 100 ppm) was 1 ppm. The concentration of titanium in the solutions to analyse was targeted in the range of 50–100 ppm, what was achieved by appropriate dilution. The overall error of the ICP-OES analysis was within 2%. The overall accuracy of calculated extent of chlorination was better than 3%. The reduced pellets were characterised before and after chlorination by XRD (Siemens D5000), with Cu Kα X-ray (λ = 1.542 Å). The voltage and current used were 30 KV and 30 mA, respectively. The scanning was performed with the rate of 0.6 o/min and step of 0.01 o. 3. Results and discussion 3.1. Reduction of ilmenite concentrates and synthetic rutile Ilmenites and synthetic rutile were mixed with graphite in a C/TiO2 molar ratio of 4.5 (Section 2A). This is equivalent to a molar ratio of carbon to reducible oxygen equal to 1.47, 1.57, 1.73, and 2.02 for primary, secondary, HYTI 70 ilmenite and synthetic rutile, respectively. The compositions of samples produced by carbothermal reduction in argon and nitrogen are presented in Tables 2 and 3, respectively.
When reduction was carried out in argon (Table 2), iron oxides were reduced to metallic iron, and most of titanium oxides were reduced to titanium oxycarbide. Small amounts of suboxides Ti3O5 and Ti2O3 were detected in reduced samples of ilmenite concentrates, which means that reduction was incomplete. No suboxides were detectable in reduced synthetic rutile. Titanium oxycarbide formed in the reduction of synthetic rutile contained 84 mol% TiC and 16 mol% TiO. Carbothermal reduction of ilmenites in nitrogen with the formation of titanium oxycarbonitride showed higher extent of reduction than in argon when titanium oxycarbide was formed. No titanium suboxides were detected in all the four samples reduced in nitrogen. According to Tables 2 and 3, samples of ilmenite of the same grade reduced in argon and nitrogen contained similar amounts of carbon, however, the main graphite peak in samples reduced in nitrogen was significantly stronger than that of samples reduced in argon. Besides, the samples reduced in argon were hard to crush, while those reduced in nitrogen were much softer. More residual (free) carbon was left in the reduced samples in nitrogen, which hindered the sintering of the reduction products. The samples reduced either in argon or nitrogen atmosphere were observed to exhibit magnetic properties as a result of formation of metallic iron. 3.2. Chlorination of ilmenite concentrates and synthetic rutile reduced in argon Chlorination reactions for titanium oxycarbide and oxycarbonitride were discussed earlier (Adipuri et al., 2008, 2009). Metallic iron formed during reduction of ilmenite concentrates was chlorinated by the following reaction: 4 =3FeðsÞ þ 2Cl2ðgÞ = 4 =3FeCl3ðgÞ
ð4Þ
ΔG = −338:947 + 0:0284T ðKJÞ
The extents of chlorination of titanium oxycarbide and iron in ilmenites of different grades and synthetic rutile reduced with the same carbon to titania molar ratio of 4.5 and the corresponding change in the samples temperature are presented in Fig. 1. Chlorine flow rate in all experiments presented in the paper was 100 mL∙min− 1. This chlorine flow rate was sufficient to avoid chlorine starvation in chlorination experiments (Adipuri et al., 2008). Chlorination of titanium oxycarbide from reduced primary and secondary ilmenites was similar, which completed in about 50 min. Reduced higher grade ilmenite HYTI 70 had a much faster chlorination rate than reduced primary and secondary ilmenites, and the rate of chlorination of reduced synthetic rutile was the highest. Difference in chlorination of iron was less significant with
Table 3 Composition of ilmenites and synthetic rutile reduced in nitrogen at 1450 °C for 3 h. Type
Carbon content fC, wt.% Oxygen content fO, wt.% Nitrogen content fN, wt.% Extent of reduction X(T,O,C,N), % Phase composition
A-2
B-2
C-2
D-2
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
12.00 0.80 10.41 97.59 C, Fe, TiC0.50O0.031N0.46
15.20 0.70 11.23 98.26 C, Fe, TiC0.45O0.029N0.53
20.70 0.70 10.88 98.89 C, Fe, TiC0.42O0.031N0.55
32.20 0.98 12.03 99.81 C, Fe, TiC0.20O0.054N0.75
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The temperature change in the process of chlorination of titanium oxycarbide obtained by reduction of rutile was measured accurately by attaching the sample pellet to the thermocouple tip (Adipuri et al., 2008). The temperature change in the chlorination process was discussed by Adipuri et al. (2008). Difference in chlorination of ilmenites of different grades can be attributed to the following factors: 1) The sample morphology. The degree of ilmenite weathering and particle porosity increased with increasing concentrate grade and was the highest for synthetic rutile (Dewan et al., 2010b). The morphology of ilmenite concentrates changed in the process of reduction; this change depended on the concentrate grade. The rate of carbothermal reduction in argon increased with increasing ilmenite grade (Dewan et al., 2010b). It can be expected that a high porosity of reduced samples had also a positive effect on the chlorination rate. 2) Different phase and chemical compositions of samples after reduction. Primary and secondary ilmenites reduced in argon contained small amounts of titanium suboxides Ti3O5 and Ti2O3 (Table 2); traces of Ti2O3 were also observed in reduced HYTI70 sample. Titanium suboxides were not detected in samples of synthetic rutile reduced in argon and ilmenites of different grades reduced in nitrogen (Tables 2 and 3). Chlorination of titanium suboxides proceeded with much slower rate than chlorination of titanium oxycarbide and oxycarbonitride (Adipuri et al., 2008). This factor can also be responsible for the observed decrease in the chlorination rate with decreasing ilmenite grade, which was calculated as the rate of conversion of titanium in the reduced sample to titanium tetrachloride. 3) Simultaneous chlorination of titanium and iron in samples with different Ti/Fe ratio. It should be noted that, although chlorination curves for iron in synthetic rutile and HYTI 70 were very close (Fig. 1), the actual amounts of chlorinated iron were very different due to significant difference in iron content. Chlorination of iron leaves less chlorine available for chlorination of titanium and vice versa. A smaller consumption of chlorine in chlorination of iron in the reduced ilmenite of a higher grade can explain the increasing rate of chlorination of titanium with increasing ilmenite grade, although the difference in iron chlorination curves in ilmenites of different grades was marginal. 4) Different amount of carbon in a reduced sample. The amount of carbon in the reduced samples increased with increasing ilmenite grade. Carbon in the reduced samples, particularly free carbon, was observed to have a retarding effect on the chlorination of titanium oxycarbide (Adipuri et al., 2009). It can be suggested that this factor was not significant in chlorination of reduced ilmenites.
Fig. 1. Extent of chlorination of titanium and iron from reduced ilmenite concentrates of different grade and synthetic rutile at 235 °C with pure chlorine at 100 mL∙min− 1. Carbon to TiO2 molar ratio was 4.5. (a) chlorination of titanium; (b) chlorination of iron; (c) change of sample temperature during chlorination.
an overall tendency of decreasing chlorination rate with decreasing grade of ilmenites. XRD analysis of the residual samples after chlorination did not show peaks of titanium oxides and iron containing phases, which is consistent with high extent of chlorination presented in Fig. 1(a), (b). As shown in Fig. 1(c), a sample temperature in chlorination of reduced synthetic rutile reached the highest peak, then decreased relatively slowly followed by a fast decrease, corresponding to completion of chlorination of titanium oxycarbide. The peak temperature in chlorination of reduced HYTI 70, secondary and primary ilmenites decreased in sequence reflecting the effect of ilmenite grade.
Combination of these factors makes the change in the titanium chlorination rate with ilmenite grade to some extent inconsistent; no difference was observed in chlorination of titanium in the reduced primary and secondary ilmenite, while the rate of chlorination of titanium in reduced HITI 70 was much higher and increased further in the chlorination of reduced synthetic rutile. 3.3. Chlorination of ilmenite concentrates and synthetic rutile reduced in nitrogen The mixtures of ilmenites and synthetic rutile with graphite were prepared with carbon to titania molar ratio of 4.5. Chlorination experiments with reduced samples were performed using an alumina boat at 235 °C for 60 min with 100 mL∙min − 1 of pure chlorine. The extents of chlorination of titanium oxycarbonitride and iron are presented in Fig. 2. Both titanium oxycarbonitride and metallic iron were rapidly chlorinated, with final extents of chlorination close to 100%. No titanium oxides or iron compounds were detected by XRD in the chlorinated residues.
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Fig. 3. X-ray diffraction patterns of ilmenite ores and synthetic rutile reduced in argon with C/TiO2 molar ratio of 4.5 and then subjected to aeration with 0.372 M of NH4Cl at 70 °C with air flow rate of 2.5 L∙min− 1 for 5 h.
Fig. 2. Extent of chlorination of titanium and iron from reduced/nitridated ilmenite concentrates of different grade and synthetic rutile at 235 °C with pure chlorine at 100 mL∙min− 1. Carbon to TiO2 molar ratio was 4.5. (a) Chlorination of titanium; (b) chlorination of iron.
Chlorination of titanium oxycarbonitride from reduced primary and secondary ilmenites was close to completion after 50 min, while its chlorination from reduced HYTI 70 and synthetic rutile occurred at much faster rate and was close to completion after 30 min. The rate of chlorination of iron from nitrided ilmenite concentrates and synthetic rutile was significantly higher than that of titanium: it was close to completion after 30 min for all grades of ilmenite concentrates and synthetic rutile. Chlorination of titanium oxycarbonitride was slower than chlorination of oxycarbide formed from the same ilmenite. Titanium oxycarbonitride is thermodynamically more stable than titanium oxycarbide, what explains the difference in their chlorination rate (Adipuri et al., 2009). Relatively slow chlorination of titanium oxycarbonitride left a higher amount of chlorine for chlorination of iron; the chlorination of iron in ilmenites reduced in nitrogen was faster in comparison with chlorination of iron in ilmenites reduced in argon. The chlorination rate of titanium in oxycarbonitride in reduced ilmenite also had a tendency to increase with increasing ilmenite grade (Fig. 2), although it was not consistent; the chlorination rate of titanium in the reduced primary ilmenite was the same as in the reduced secondary ilmenite, and no difference was observed in the rate of chlorination of HITI70 and synthetic rutile. This tendency can be attributed to the higher porosities of higher grade ilmenites.
oxycarbonitride, respectively, together with metallic iron. Chlorination was not selective, and metallic iron was chlorinated together with titanium oxycarbide and oxycarbonitride. To study the effect of iron on chlorination of titanium oxycarbide and oxycarbonitride, iron was leached out from the reduced samples; afterwards samples were subjected to chlorination under the same conditions as samples containing iron. The effectiveness of leaching was qualitatively assessed by the relative strength of iron peak in the XRD spectra of the leached samples. Leaching with higher concentration of hydrochloric acid and longer time was more effective (condition B vs A); increasing aeration temperature had a similar effect (condition D vs C). However, removal of iron by leaching or aeration under conditions A through D was not complete. After treatment under condition E which used higher concentration of NH4Cl, higher temperature and air flow rate, iron in the sample was undetected by the XRD analysis. As a result of iron removal, samples lost its magnetic property. Fig. 3 presents the XRD spectra of primary, secondary, HYTI 70 ilmenites and synthetic rutile reduced in argon and then subjected to iron removal by aeration under condition E. Although only titanium
3.4. Chlorination of titanium oxycarbide and oxycarbonitride after removal of iron by leaching As described in Section 3A, the reduction of ilmenite concentrates in argon and nitrogen at 1450 °C produced titanium oxycarbide and
Fig. 4. Extent of chlorination of titanium from titanium oxycarbide or oxycarbonitride from reduced and leached ilmenite concentrates of different grade at 235 °C with pure chlorine at 100 mL∙min− 1.
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oxycarbide and residual graphite were detected by XRD, XRF analysis detected iron content in the range of 0.88 to 1.71 wt.% after aeration. Leaching of iron from ilmenites reduced in nitrogen was tested only under condition E. XRD analysis of samples after leaching detected only titanium oxycarbonitride and graphite. The XRD spectra of these samples were similar to the leached samples of ilmenites reduced in argon. The carbon to titania molar ratio of the ilmenite–graphite mixture was kept at 4.5; reduced samples were chlorinated at 235 °C with pure chlorine flow rate of 100 mL∙min− 1. The extent of chlorination of titanium in reduced ilmenite concentrates after iron removal is presented in Fig. 4. Chlorination of titanium oxycarbide and oxycarbonitride was faster than direct chlorination of reduced ilmenites without removal of iron. Chlorination of titanium from titanium oxycarbide prepared from HYTI 70 became comparable to that from synthetic rutile (Fig. 4 vs Fig. 1a). Investigation on the status of impurities in ilmenites and their behavior during carbothermal reduction and chlorination is in progress. 4. Conclusions The main phases of primary and secondary ilmenites were Fe2Ti3O9 and FeTiO3; HYTI 70 also contained TiO2; synthetic rutile consisted of titania with traces of metallic iron and suboxides (Ti2O3, Ti3O5, Ti4O7). Carbothermal reduction of ilmenites at 1450 °C in argon produced titanium oxycarbide, while in reduction in nitrogen oxycarbonitride was formed. Iron oxides were reduced to metallic iron. Low temperature chlorination of titanium oxycarbide obtained by carbothermal reduction of ilmenite concentrates is feasible at such low temperature as 235 °C. However, the chlorination is not selective with regards to iron. In chlorination, titanium oxycarbide or oxycarbonitride, metallic iron, and Ti2O3 were chlorinated. The rate of chlorination of titanium and iron increased with increasing ilmenite grade. The effect of the ilmenite grade was more significant on the chlorination of titanium than that of iron. Chlorination of iron in samples reduced in nitrogen when titanium oxycarbonitride was formed was faster than chlorination of iron in samples reduced in argon, when titanium oxycarbide was formed. The aeration of reduced samples in 0.372 M NH4Cl solution with 2.5 L∙min − 1 of air at 70 °C was efficient in removing iron from reduced ilmenites. After leaching, no iron peaks were detected by XRD; iron
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content was decreased to 0.88–1.71 wt.% based on XRF analysis. Higher rate and extent of chlorination of titanium oxycarbide and oxycarbonitride were observed after iron removal. Chlorination was completed after 30 min of reaction. Acknowledgements The technical assistance of Rabeya Akter and Dorothy Yu in ICPOES analysis is gratefully acknowledged. This research was supported under Australian Research Council's Discovery Projects funding scheme (project number DP0771059). Professor Ostrovski is the recipient of an Australian Research Council Professorial Fellowship. References Adipuri, A., Zhang, G., Ostrovski, O., 2008. Chlorination of titanium oxycarbide produced by carbothermal reduction of rutile. Metall. Trans. B 39B, 23–34. Adipuri, A., Zhang, G., Ostrovski, O., 2009. Chlorination of titanium oxycarbonitride produced by carbothermal reduction of rutile. Ind. Eng. Chem. Res. 48, 779–787. Becher, R.G., Canning, R.G., Goodheart, B.A., Uusna, S., 1965. A new process for upgrading ilmenitic mineral sands. Australasian Inst. Min. Met. Proc. No. 214, 21–44. Benelite Corp. of America, 1974. US Patent No. 3825419. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2010a. Carbothermal reduction of a primary ilmenite concentrate in different gas atmospheres. Metall. Mater. Trans. B 41B, 182–192. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2010b. Phase development in carbothermal reduction of ilmenite concentrates and synthetic rutile. ISIJ Int. 50, 647–657. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2011. Carbothermal reduction of ilmenite concentrates and synthetic rutile in different gas atmospheres. Miner. Process. Extr. Metall. 120, 111–117. Habashi, F., 1997. Handbook of Extractive Metallurgy, Vol. 2. Wiley-VCH, Weinheimm, Germany, p. 1135. Hitching, K.D., Kelly, E.G., 1982. Carburization/chlorination process for production of titanium tetrachloride from titaniferous slag. Trans. Inst. Min. Metall, Sect. C 91, C97–C99. Hunter, W.L., White, J.C., Stickney, W.A., 1975. Highly pure titanium tetrachloride from Ilmenite slag. US Patent 3899569, 7pp. Kanari, N., Gaballah, I., 1999. Chlorination and carbochlorination of magnesium oxide. Metall. Trans. B 30B, 383–391. Lasheen, T.A.I., 2005. Chemical benefication of Rosetta ilmenite by direct reduction leaching. Hydrometall. 76, 123–129. Mostert, G.J., Rohrmann, B.R., Wedlake, R.J., Baxter, R.C., 1992. Titanium recovery from ores and slags by nitriding and chlorinating. Brit. UK Pat. Appl. GB 2246345, 12 pp. Smillie, L.D., Heydenrych, M., 1997. Low temperature TiO2 production process, Titanium Extraction and Processing. In: Mishra, B., Kipouros, G.J. (Eds.), Titanium Extraction and Processing. TMS, Indiana, USA, p. 129. Van Vuuren, D.S., Stone, A.K., 2006. Selective recovery of titanium dioxide from low grade sources. http://researchspace.csir.co.za/dspace/bitstream/10204/937/1/van %20Vuuren_2006_D1.pdf.
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International Journal of Mineral Processing j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j m i n p r o
Chlorination of reduced ilmenite concentrates and synthetic rutile Andrew Adipuri 1, Yan Li, Guangqing Zhang ⁎, Oleg Ostrovski School of Materials Science and Engineering, the University of New South Wales, UNSW SYDNEY, NSW 2052, Australia
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Article history: Received 29 October 2010 Received in revised form 2 July 2011 Accepted 10 July 2011 Available online 20 July 2011 Keywords: Ilmenite Synthetic rutile Oxycarbide Oxycarbonitride Titanium tetrachloride Chlorination
a b s t r a c t Chlorination of reduced ilmenites of different grades (primary, secondary and HYTI 70) and synthetic rutile was investigated at 235 °C. The main phases of primary and secondary ilmenites were Fe2Ti3O9 and FeTiO3; HYTI 70 contained TiO2; synthetic rutile consisted of titania with titanium suboxides and trace amount of iron. Iron oxides were reduced to metallic iron. Titanium oxides were reduced to titanium oxycarbide or oxycarbonitride; reduced samples contained a small amount of titanium suboxides. In chlorination of reduced ilmenite concentrates and synthetic rutile, titanium oxycarbide or oxycarbonitride, metallic iron, and Ti2O3 were chlorinated. The degree of chlorination of both iron and titanium oxycarbide/oxycarbonitride was 95–98%; chlorination of iron was faster than that of titanium oxycarbonitride. The removal of iron by leaching increased the chlorination rate of titanium oxycarbide/oxycarbonitride; it was close to completion in 35 min. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium minerals are mostly processed into titanium dioxide white pigment, which is a valuable commodity used in paint, paper and plastic industries because of its exceptional scattering properties, chemical stability and lack of toxicity (Habashi, 1997). The usage of metallic titanium is limited because of its high production cost. Metallic titanium and titania white pigment are produced from titanium tetrachloride which is obtained by carbochlorination of natural or synthetic rutile at 800–1100 °C. Chlorination of titanium oxycarbide or oxycarbonitride can be implemented at 200–400 °C (Adipuri et al., 2008, 2009). Low temperature chlorination can improve efficiency of production of titanium tetrachloride. Smillie and Heydenrych (1997) investigated process developed for selective chlorination of low grade titanium-containing ores. The ore was first subjected to carbothermal reduction under nitrogen in which the titanium oxide was converted to carbonitride. Titanium carbonitride chlorinated at low temperature of 350 °C (Smillie and Heydenrych, 1997). At this temperature, other oxides were un-reactive and, therefore, a relatively pure titanium tetrachloride product can be obtained. Mostert et al. (1992) and Van Vuuren and Stone (2006) developed technologies for processing titanium minerals using reduction and
⁎ Corresponding author. E-mail address: [email protected] (G. Zhang). 1 Currently a Postdoctoral Research Fellow, Pyrometallurgy Research Centre, University of Queensland, Brisbane QLD 4072, Australia. 0301-7516/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2011.07.005
nitridation of titanium oxides. Titanium oxide in wastes or titanium ores and slags can be nitrided at temperatures of 1000–1800 °C. The optimal chlorination temperature was found between 200–500 °C. Van Vuuren and Stone (2006) indicated that at these temperatures, only the reduced metal oxides such as titanium, iron, and vanadium were selectively chlorinated, leaving residues of calcia, magnesia, silica, and alumina. In the low temperature chlorination, impurities do not chlorinate or chlorinate very slowly (Kanari and Gaballah, 1999; Hitching and Kelly, 1982; Hunter et al., 1975). This permits selective chlorination of titanium oxycarbide or oxycarbonitride, decreases the chlorine consumption and waste generation, and makes the whole technology of ilmenite processing more efficient and environmentally friendly. Carbothermal reduction of ilmenites of different grades and synthetic rutile in different gas atmospheres was studied by Dewan et al. (2010a, 2010b, 2011). Chlorination of titanium oxycarbide produced by reduction of pure rutile in argon was examined by Adipuri et al. (2008); Adipuri et al. (2009) examined chlorination of titanium oxycarbonitride produced by reduction of rutile in nitrogen. No detailed investigation of kinetics and mechanisms of chlorination of reduced ilmenites were reported in literature. The major focus of this paper is on the chlorination of titanium oxycarbide or oxycarbonitride and metallic iron produced by carbothermal reduction of ilmenite concentrates of different grades and synthetic rutile in argon or nitrogen. Chlorination of titanium oxycarbide after leaching of iron from reduced ilmenite is also examined. Investigation of the behavior of minor impurities in the ilmenite concentrates is in progress.
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2. Experimental 2.1. A. Synthesis of titanium oxycarbide or oxycarbonitride Ilmenite concentrates of different grade (primary, secondary, and HYTI 70) and synthetic rutile were supplied by Iluka Resources Ltd. Their chemical compositions, provided by the supplier, are presented in Table 1. XRD analysis of ilmenite concentrates and synthetic rutile showed that primary and secondary ilmenites consisted predominantly of pseudorutile, Fe2Ti3O9, and ilmenite, FeTiO3. HYTI 70 ilmenite concentrate had higher titanium content, which existed in the form of pseudorutile, ilmenite, and rutile. Impurities in ilmenites were present in a relatively small amount, below the detectable level by the XRD analysis. Synthetic rutile was produced from ilmenite ore by reduction and iron removal. It consisted of titania and suboxides (Ti2O3, Ti3O5, Ti4O7) according to XRD analysis. Iron oxides or metallic iron were not detected by XRD, however analysis by XRF indicated 2.75 wt.% of iron in synthetic rutile (Table 1). The particle size for all ilmenite concentrates and synthetic rutile was in the range of 50 μm to 300 μm, with an average size of 152 μm. As received ilmenite concentrates and synthetic graphite (b 20 μm) were mixed with distilled water (70 wt.% of solid mixture) and carboxymethyl cellulose (CMC, 0.3 wt.%). The carbon to TiO2 molar ratio in the mixture was 4.5. At this C/TiO2 ratio, carbon was in excess relative to the stoichiometric amount needed for reduction of titanium and iron oxides. The addition of CMC ensured uniform mixing of ilmenite and graphite. The mixture was dried at 120 °C overnight. The mixture was pressed into pellets of 2.5 g each with 15 mm in diameter and 5 mm in height by applying 40 kN pressing load for 2 min. The pellets were carbothermally reduced in a horizontal electrical furnace under flowing argon or nitrogen atmosphere (1 L∙min− 1) at 1450 °C for 180 min. The analyses of the reduced samples proved that titanium oxycarbide and oxycarbonitride prepared by this manner were of uniform and consistent compositions, which was crucial to the chlorination experiments. The variation in oxygen content of reduced samples produced in parallel reduction experiments with the same carbon to TiO2 ratio was within the error of LECO analysis. 2.2. B. Chlorination of reduced ilmenite concentrates and synthetic rutile Chlorination experiments were carried out in a transparent quartz tube in a horizontal electric furnace. The schematic of the reactor and detailed experimental procedures for sample preparation and chlorination were described elsewhere (Adipuri et al., 2008). Samples were Table 1 Chemical compositions of ilmenite concentrates and synthetic rutile, wt.%.
TiO2 Ti2O3 Total Fe MnO FeO SiO2 ZrO2 Al2O3 P2O5 S Nb2O3 Nb2O5 Cr2O3 CaO CeO2 MgO V2O5 Fe/Ti a b
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
53.9 – 30.5 1.63 18 0.27 0.09 0.4 0.01 0.004 – 0.1 0.047 b 0.01 – 0.18 0.17 0.79
58.20 – 25.89 1.15 11.22 0.75 0.16 0.51 0.033 0.01 – 0.14 0.05 b0.01 – 0.22 0.18 0.66
66.57 – 18.75 0.86 2.38 0.66 0.11 1.1 0.045 0.09 – 0.23 0.084 0.006 – 0.13 0.19 0.46
92.46a 12.1b 2.75 0.86 – 0.91 0.09 0.97 1.57 0.63 0.23 – 0.072 0.02 0.04 0.36 0.28 0.04
Based on total titanium content. Equivalent content of all titanium suboxides.
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heated to 235 °C; this temperature was referred to as a chlorination temperature. The sample temperature changed significantly during chlorination processes due to the exothermic nature of the chlorination reaction (Section 3B). Titanium tetrachloride vapor was absorbed in two scrubbers with 32 wt.% HCl. To prevent TiCl4 vapor from condensing between the reactor tube and scrubber, the reactor outlet tube was heated by hot air generated by a heating gun at 350 °C. Heating also avoided precipitation of ferric chloride formed in the chlorination of reduced ilmenites. Ferric chloride, FeCl3 has a boiling point of 315 °C. Without heating the reactor outlet tube, brown precipitate of FeCl3 was observed. FeCl3 vapor was absorbed by TiCl4 scrubbers. 2.3. C. Leaching of iron from reduced samples Commercially, iron is removed from roasted ilmenites by Becher process or Benelite process to produce synthetic rutile (Lasheen, 2005). The Becher process (Becher et al., 1965) uses aeration to “rust away” metallic iron, while in the Benelite process (Benelite Corp. of America, 1974), metallic iron is dissolved in a hydrochloric acid. In this investigation, removal of iron from reduced ilmenite samples was tested by acid and aeration leaching in a flask of 2.0 L volume with constant stirring. The air for aeration was supplied by a compressor. 22.5 g of the reduced ilmenite sample (Section 2A) was crushed with a ring mill into fine particles. Then it was transferred into a flask containing 1.0 L of HCl or NH4Cl solution maintained at constant temperature. In aeration experiments, air was blown into the slurry at a constant flow rate. With ammonium chloride as a catalyst, oxygen in the air oxidised metallic iron into hydroxides which precipitated outside the particles of titanium oxycarbide or oxycarbonitride. After acid or aeration leaching, the slurry was cooled and filtered. The filtrated particles were washed with distilled water. The filtered cake was then dried in an oven at 150 °C for 24 h and crushed into fine powder. The following conditions were tested to remove iron from reduced primary ilmenite concentrate: (A) Leaching at room temperature with 0.1 M hydrochloric acid for 5 h; (B) Leaching at room temperature with 3.2 M hydrochloric acid for 12 h; (C) Aeration at 40 °C with 0.1 M of NH4Cl solution and 1 L∙min − 1 of air for 3 h; (D) Aeration at 70 °C with 0.1 M of NH4Cl solution and 1 L∙min − 1 of air for 3 h; (E) Aeration at 70 °C with 0.37 M of NH4Cl solution and 2.5 L∙min− 1 of air for 5 h. The best results for iron removal were obtained under condition (E). 2.4. C. Sample characterisation Oxygen, nitrogen and carbon contents in the reduced ilmenite concentrates and synthetic rutile were determined using LECO oxygen-nitrogen analyser (TC-436) and carbon-sulphur analyser (CS-444). The extent of reduction of ilmenite concentrates was calculated from the fraction of reducible oxygen removed. The total o fraction of oxygen in raw material is defined as fO,T , while the o o “reducible” part is fO,R. The reducible oxygen (fO,R) is associated with titanium, iron, and manganese oxides. Other oxides were considered non-reducible or neglected. The fractions of carbon, oxygen, and nitrogen obtained from LECO analysis after the reduction are defined as fC, fO, and fN. Based on the oxygen removed from initial mixture, the extent of reduction in argon was calculated following Eq. (1):
XðTi;O;CÞ =
0 fO;T 0 fO;R
−
0 fO 1−fO;T 0 1−fO −fC fO;R
ð1Þ
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Table 2 Composition of ilmenites and synthetic rutile reduced in argon at 1450 °C for 3 h. Type
Carbon content fC, wt.% Oxygen content fO, wt.% Extent of reduction X(T,O,C), % Phase composition
A-1
B-1
C-1
D-1
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
12.73 4.32 83.39 C, Fe, TiCxOy, Ti3O5, Ti2O3
16.15 3.87 85.6 C, Fe, TiCxOy, Ti3O5, Ti2O3
24.85 3.56 87.45 C, Fe, TiCxOy, Ti2O3
37.51 3.27 90.20 C, Fe, TiC0.84O0.16
The extent of reduction of titania in ilmenite to titanium oxycarbonitride was calculated using the following equation:
XðTi;O;C;NÞ
0 fO 1−fO;T = 0 − fO;R f 0 1−f −f −f O C N O;R 0 fO;T
ð2Þ
When titanium sub-oxides were not detected in a reduced sample by XRD analysis, all the oxygen was assigned to TiO, which allowed calculation of fractions of TiO and TiC in the TiC–TiO solid solution or fractions of TiO, TiC and TiN in the TiC–TiO–TiN solid solution. Samples taken from the gas scrubbers at different reaction time were diluted and then analysed by Inductive Coupled Plasma-Optical Emission Spectrometer, ICP-OES (Perkin Elmer Optima 3000) for Ti and Fe contents. Based on the Ti and Fe contents in the solution, the extents of chlorination were calculated as: α=
mi × 100% moi
ð3Þ
where mi and moi are the masses of titanium or iron in the scrubber solution and in original ilmenite–graphite pellet, respectively. The accuracy of the ICP-OES analysis of titanium and iron contents in a solution (up to 100 ppm) was 1 ppm. The concentration of titanium in the solutions to analyse was targeted in the range of 50–100 ppm, what was achieved by appropriate dilution. The overall error of the ICP-OES analysis was within 2%. The overall accuracy of calculated extent of chlorination was better than 3%. The reduced pellets were characterised before and after chlorination by XRD (Siemens D5000), with Cu Kα X-ray (λ = 1.542 Å). The voltage and current used were 30 KV and 30 mA, respectively. The scanning was performed with the rate of 0.6 o/min and step of 0.01 o. 3. Results and discussion 3.1. Reduction of ilmenite concentrates and synthetic rutile Ilmenites and synthetic rutile were mixed with graphite in a C/TiO2 molar ratio of 4.5 (Section 2A). This is equivalent to a molar ratio of carbon to reducible oxygen equal to 1.47, 1.57, 1.73, and 2.02 for primary, secondary, HYTI 70 ilmenite and synthetic rutile, respectively. The compositions of samples produced by carbothermal reduction in argon and nitrogen are presented in Tables 2 and 3, respectively.
When reduction was carried out in argon (Table 2), iron oxides were reduced to metallic iron, and most of titanium oxides were reduced to titanium oxycarbide. Small amounts of suboxides Ti3O5 and Ti2O3 were detected in reduced samples of ilmenite concentrates, which means that reduction was incomplete. No suboxides were detectable in reduced synthetic rutile. Titanium oxycarbide formed in the reduction of synthetic rutile contained 84 mol% TiC and 16 mol% TiO. Carbothermal reduction of ilmenites in nitrogen with the formation of titanium oxycarbonitride showed higher extent of reduction than in argon when titanium oxycarbide was formed. No titanium suboxides were detected in all the four samples reduced in nitrogen. According to Tables 2 and 3, samples of ilmenite of the same grade reduced in argon and nitrogen contained similar amounts of carbon, however, the main graphite peak in samples reduced in nitrogen was significantly stronger than that of samples reduced in argon. Besides, the samples reduced in argon were hard to crush, while those reduced in nitrogen were much softer. More residual (free) carbon was left in the reduced samples in nitrogen, which hindered the sintering of the reduction products. The samples reduced either in argon or nitrogen atmosphere were observed to exhibit magnetic properties as a result of formation of metallic iron. 3.2. Chlorination of ilmenite concentrates and synthetic rutile reduced in argon Chlorination reactions for titanium oxycarbide and oxycarbonitride were discussed earlier (Adipuri et al., 2008, 2009). Metallic iron formed during reduction of ilmenite concentrates was chlorinated by the following reaction: 4 =3FeðsÞ þ 2Cl2ðgÞ = 4 =3FeCl3ðgÞ
ð4Þ
ΔG = −338:947 + 0:0284T ðKJÞ
The extents of chlorination of titanium oxycarbide and iron in ilmenites of different grades and synthetic rutile reduced with the same carbon to titania molar ratio of 4.5 and the corresponding change in the samples temperature are presented in Fig. 1. Chlorine flow rate in all experiments presented in the paper was 100 mL∙min− 1. This chlorine flow rate was sufficient to avoid chlorine starvation in chlorination experiments (Adipuri et al., 2008). Chlorination of titanium oxycarbide from reduced primary and secondary ilmenites was similar, which completed in about 50 min. Reduced higher grade ilmenite HYTI 70 had a much faster chlorination rate than reduced primary and secondary ilmenites, and the rate of chlorination of reduced synthetic rutile was the highest. Difference in chlorination of iron was less significant with
Table 3 Composition of ilmenites and synthetic rutile reduced in nitrogen at 1450 °C for 3 h. Type
Carbon content fC, wt.% Oxygen content fO, wt.% Nitrogen content fN, wt.% Extent of reduction X(T,O,C,N), % Phase composition
A-2
B-2
C-2
D-2
Primary ilmenite
Secondary ilmenite
HYTI 70
Synthetic rutile
12.00 0.80 10.41 97.59 C, Fe, TiC0.50O0.031N0.46
15.20 0.70 11.23 98.26 C, Fe, TiC0.45O0.029N0.53
20.70 0.70 10.88 98.89 C, Fe, TiC0.42O0.031N0.55
32.20 0.98 12.03 99.81 C, Fe, TiC0.20O0.054N0.75
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The temperature change in the process of chlorination of titanium oxycarbide obtained by reduction of rutile was measured accurately by attaching the sample pellet to the thermocouple tip (Adipuri et al., 2008). The temperature change in the chlorination process was discussed by Adipuri et al. (2008). Difference in chlorination of ilmenites of different grades can be attributed to the following factors: 1) The sample morphology. The degree of ilmenite weathering and particle porosity increased with increasing concentrate grade and was the highest for synthetic rutile (Dewan et al., 2010b). The morphology of ilmenite concentrates changed in the process of reduction; this change depended on the concentrate grade. The rate of carbothermal reduction in argon increased with increasing ilmenite grade (Dewan et al., 2010b). It can be expected that a high porosity of reduced samples had also a positive effect on the chlorination rate. 2) Different phase and chemical compositions of samples after reduction. Primary and secondary ilmenites reduced in argon contained small amounts of titanium suboxides Ti3O5 and Ti2O3 (Table 2); traces of Ti2O3 were also observed in reduced HYTI70 sample. Titanium suboxides were not detected in samples of synthetic rutile reduced in argon and ilmenites of different grades reduced in nitrogen (Tables 2 and 3). Chlorination of titanium suboxides proceeded with much slower rate than chlorination of titanium oxycarbide and oxycarbonitride (Adipuri et al., 2008). This factor can also be responsible for the observed decrease in the chlorination rate with decreasing ilmenite grade, which was calculated as the rate of conversion of titanium in the reduced sample to titanium tetrachloride. 3) Simultaneous chlorination of titanium and iron in samples with different Ti/Fe ratio. It should be noted that, although chlorination curves for iron in synthetic rutile and HYTI 70 were very close (Fig. 1), the actual amounts of chlorinated iron were very different due to significant difference in iron content. Chlorination of iron leaves less chlorine available for chlorination of titanium and vice versa. A smaller consumption of chlorine in chlorination of iron in the reduced ilmenite of a higher grade can explain the increasing rate of chlorination of titanium with increasing ilmenite grade, although the difference in iron chlorination curves in ilmenites of different grades was marginal. 4) Different amount of carbon in a reduced sample. The amount of carbon in the reduced samples increased with increasing ilmenite grade. Carbon in the reduced samples, particularly free carbon, was observed to have a retarding effect on the chlorination of titanium oxycarbide (Adipuri et al., 2009). It can be suggested that this factor was not significant in chlorination of reduced ilmenites.
Fig. 1. Extent of chlorination of titanium and iron from reduced ilmenite concentrates of different grade and synthetic rutile at 235 °C with pure chlorine at 100 mL∙min− 1. Carbon to TiO2 molar ratio was 4.5. (a) chlorination of titanium; (b) chlorination of iron; (c) change of sample temperature during chlorination.
an overall tendency of decreasing chlorination rate with decreasing grade of ilmenites. XRD analysis of the residual samples after chlorination did not show peaks of titanium oxides and iron containing phases, which is consistent with high extent of chlorination presented in Fig. 1(a), (b). As shown in Fig. 1(c), a sample temperature in chlorination of reduced synthetic rutile reached the highest peak, then decreased relatively slowly followed by a fast decrease, corresponding to completion of chlorination of titanium oxycarbide. The peak temperature in chlorination of reduced HYTI 70, secondary and primary ilmenites decreased in sequence reflecting the effect of ilmenite grade.
Combination of these factors makes the change in the titanium chlorination rate with ilmenite grade to some extent inconsistent; no difference was observed in chlorination of titanium in the reduced primary and secondary ilmenite, while the rate of chlorination of titanium in reduced HITI 70 was much higher and increased further in the chlorination of reduced synthetic rutile. 3.3. Chlorination of ilmenite concentrates and synthetic rutile reduced in nitrogen The mixtures of ilmenites and synthetic rutile with graphite were prepared with carbon to titania molar ratio of 4.5. Chlorination experiments with reduced samples were performed using an alumina boat at 235 °C for 60 min with 100 mL∙min − 1 of pure chlorine. The extents of chlorination of titanium oxycarbonitride and iron are presented in Fig. 2. Both titanium oxycarbonitride and metallic iron were rapidly chlorinated, with final extents of chlorination close to 100%. No titanium oxides or iron compounds were detected by XRD in the chlorinated residues.
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Fig. 3. X-ray diffraction patterns of ilmenite ores and synthetic rutile reduced in argon with C/TiO2 molar ratio of 4.5 and then subjected to aeration with 0.372 M of NH4Cl at 70 °C with air flow rate of 2.5 L∙min− 1 for 5 h.
Fig. 2. Extent of chlorination of titanium and iron from reduced/nitridated ilmenite concentrates of different grade and synthetic rutile at 235 °C with pure chlorine at 100 mL∙min− 1. Carbon to TiO2 molar ratio was 4.5. (a) Chlorination of titanium; (b) chlorination of iron.
Chlorination of titanium oxycarbonitride from reduced primary and secondary ilmenites was close to completion after 50 min, while its chlorination from reduced HYTI 70 and synthetic rutile occurred at much faster rate and was close to completion after 30 min. The rate of chlorination of iron from nitrided ilmenite concentrates and synthetic rutile was significantly higher than that of titanium: it was close to completion after 30 min for all grades of ilmenite concentrates and synthetic rutile. Chlorination of titanium oxycarbonitride was slower than chlorination of oxycarbide formed from the same ilmenite. Titanium oxycarbonitride is thermodynamically more stable than titanium oxycarbide, what explains the difference in their chlorination rate (Adipuri et al., 2009). Relatively slow chlorination of titanium oxycarbonitride left a higher amount of chlorine for chlorination of iron; the chlorination of iron in ilmenites reduced in nitrogen was faster in comparison with chlorination of iron in ilmenites reduced in argon. The chlorination rate of titanium in oxycarbonitride in reduced ilmenite also had a tendency to increase with increasing ilmenite grade (Fig. 2), although it was not consistent; the chlorination rate of titanium in the reduced primary ilmenite was the same as in the reduced secondary ilmenite, and no difference was observed in the rate of chlorination of HITI70 and synthetic rutile. This tendency can be attributed to the higher porosities of higher grade ilmenites.
oxycarbonitride, respectively, together with metallic iron. Chlorination was not selective, and metallic iron was chlorinated together with titanium oxycarbide and oxycarbonitride. To study the effect of iron on chlorination of titanium oxycarbide and oxycarbonitride, iron was leached out from the reduced samples; afterwards samples were subjected to chlorination under the same conditions as samples containing iron. The effectiveness of leaching was qualitatively assessed by the relative strength of iron peak in the XRD spectra of the leached samples. Leaching with higher concentration of hydrochloric acid and longer time was more effective (condition B vs A); increasing aeration temperature had a similar effect (condition D vs C). However, removal of iron by leaching or aeration under conditions A through D was not complete. After treatment under condition E which used higher concentration of NH4Cl, higher temperature and air flow rate, iron in the sample was undetected by the XRD analysis. As a result of iron removal, samples lost its magnetic property. Fig. 3 presents the XRD spectra of primary, secondary, HYTI 70 ilmenites and synthetic rutile reduced in argon and then subjected to iron removal by aeration under condition E. Although only titanium
3.4. Chlorination of titanium oxycarbide and oxycarbonitride after removal of iron by leaching As described in Section 3A, the reduction of ilmenite concentrates in argon and nitrogen at 1450 °C produced titanium oxycarbide and
Fig. 4. Extent of chlorination of titanium from titanium oxycarbide or oxycarbonitride from reduced and leached ilmenite concentrates of different grade at 235 °C with pure chlorine at 100 mL∙min− 1.
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oxycarbide and residual graphite were detected by XRD, XRF analysis detected iron content in the range of 0.88 to 1.71 wt.% after aeration. Leaching of iron from ilmenites reduced in nitrogen was tested only under condition E. XRD analysis of samples after leaching detected only titanium oxycarbonitride and graphite. The XRD spectra of these samples were similar to the leached samples of ilmenites reduced in argon. The carbon to titania molar ratio of the ilmenite–graphite mixture was kept at 4.5; reduced samples were chlorinated at 235 °C with pure chlorine flow rate of 100 mL∙min− 1. The extent of chlorination of titanium in reduced ilmenite concentrates after iron removal is presented in Fig. 4. Chlorination of titanium oxycarbide and oxycarbonitride was faster than direct chlorination of reduced ilmenites without removal of iron. Chlorination of titanium from titanium oxycarbide prepared from HYTI 70 became comparable to that from synthetic rutile (Fig. 4 vs Fig. 1a). Investigation on the status of impurities in ilmenites and their behavior during carbothermal reduction and chlorination is in progress. 4. Conclusions The main phases of primary and secondary ilmenites were Fe2Ti3O9 and FeTiO3; HYTI 70 also contained TiO2; synthetic rutile consisted of titania with traces of metallic iron and suboxides (Ti2O3, Ti3O5, Ti4O7). Carbothermal reduction of ilmenites at 1450 °C in argon produced titanium oxycarbide, while in reduction in nitrogen oxycarbonitride was formed. Iron oxides were reduced to metallic iron. Low temperature chlorination of titanium oxycarbide obtained by carbothermal reduction of ilmenite concentrates is feasible at such low temperature as 235 °C. However, the chlorination is not selective with regards to iron. In chlorination, titanium oxycarbide or oxycarbonitride, metallic iron, and Ti2O3 were chlorinated. The rate of chlorination of titanium and iron increased with increasing ilmenite grade. The effect of the ilmenite grade was more significant on the chlorination of titanium than that of iron. Chlorination of iron in samples reduced in nitrogen when titanium oxycarbonitride was formed was faster than chlorination of iron in samples reduced in argon, when titanium oxycarbide was formed. The aeration of reduced samples in 0.372 M NH4Cl solution with 2.5 L∙min − 1 of air at 70 °C was efficient in removing iron from reduced ilmenites. After leaching, no iron peaks were detected by XRD; iron
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content was decreased to 0.88–1.71 wt.% based on XRF analysis. Higher rate and extent of chlorination of titanium oxycarbide and oxycarbonitride were observed after iron removal. Chlorination was completed after 30 min of reaction. Acknowledgements The technical assistance of Rabeya Akter and Dorothy Yu in ICPOES analysis is gratefully acknowledged. This research was supported under Australian Research Council's Discovery Projects funding scheme (project number DP0771059). Professor Ostrovski is the recipient of an Australian Research Council Professorial Fellowship. References Adipuri, A., Zhang, G., Ostrovski, O., 2008. Chlorination of titanium oxycarbide produced by carbothermal reduction of rutile. Metall. Trans. B 39B, 23–34. Adipuri, A., Zhang, G., Ostrovski, O., 2009. Chlorination of titanium oxycarbonitride produced by carbothermal reduction of rutile. Ind. Eng. Chem. Res. 48, 779–787. Becher, R.G., Canning, R.G., Goodheart, B.A., Uusna, S., 1965. A new process for upgrading ilmenitic mineral sands. Australasian Inst. Min. Met. Proc. No. 214, 21–44. Benelite Corp. of America, 1974. US Patent No. 3825419. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2010a. Carbothermal reduction of a primary ilmenite concentrate in different gas atmospheres. Metall. Mater. Trans. B 41B, 182–192. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2010b. Phase development in carbothermal reduction of ilmenite concentrates and synthetic rutile. ISIJ Int. 50, 647–657. Dewan, M.A.R., Zhang, G., Ostrovski, O., 2011. Carbothermal reduction of ilmenite concentrates and synthetic rutile in different gas atmospheres. Miner. Process. Extr. Metall. 120, 111–117. Habashi, F., 1997. Handbook of Extractive Metallurgy, Vol. 2. Wiley-VCH, Weinheimm, Germany, p. 1135. Hitching, K.D., Kelly, E.G., 1982. Carburization/chlorination process for production of titanium tetrachloride from titaniferous slag. Trans. Inst. Min. Metall, Sect. C 91, C97–C99. Hunter, W.L., White, J.C., Stickney, W.A., 1975. Highly pure titanium tetrachloride from Ilmenite slag. US Patent 3899569, 7pp. Kanari, N., Gaballah, I., 1999. Chlorination and carbochlorination of magnesium oxide. Metall. Trans. B 30B, 383–391. Lasheen, T.A.I., 2005. Chemical benefication of Rosetta ilmenite by direct reduction leaching. Hydrometall. 76, 123–129. Mostert, G.J., Rohrmann, B.R., Wedlake, R.J., Baxter, R.C., 1992. Titanium recovery from ores and slags by nitriding and chlorinating. Brit. UK Pat. Appl. GB 2246345, 12 pp. Smillie, L.D., Heydenrych, M., 1997. Low temperature TiO2 production process, Titanium Extraction and Processing. In: Mishra, B., Kipouros, G.J. (Eds.), Titanium Extraction and Processing. TMS, Indiana, USA, p. 129. Van Vuuren, D.S., Stone, A.K., 2006. Selective recovery of titanium dioxide from low grade sources. http://researchspace.csir.co.za/dspace/bitstream/10204/937/1/van %20Vuuren_2006_D1.pdf.