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Phase transformations in ilmenite induced by electric discharge assisted mechanical milling
Journal of Alloys and Compounds 469 (2009) 380–385
Phase transformations in ilmenite induced by electric discharge assisted mechanical milling D. Bishop, A. Calka ∗ Engineering Materials Institute, Faculty of Engineering, University of Wollongong, Northfields Avenue, Wollongong 2522, Australia Received 6 December 2007; accepted 24 January 2008 Available online 24 March 2008
Abstract The recently developed synthesis technique of electric discharge assisted mechanical milling (EDAMM) is extended to minerals processing. This study examines phase changes during thermal decomposition and oxidation of ilmenite concentrate induced by EDAMM in a variety of inert and oxidising atmospheres. Products were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results of this investigation demonstrate that thermal decomposition of the pseudorutile component of ilmenite concentrate occurs during EDAMM in less than 10 s and that oxidation of the ilmenite component occurs during EDAMM in less than 2 min in oxidising atmospheres. © 2008 Elsevier B.V. All rights reserved. Keywords: Oxide materials; Mechanochemical processing; Oxidation
1. Introduction Extraction of metals from naturally occurring minerals has occurred for hundreds of years. Over this time extraction processes have been continually refined, improved or replaced to increase production rates, improve efficiency and decrease costs. In modern times, society has become increasingly focused on the impact these processes have on the environment and minimising this impact has added another dimension to the drive for the development of new or improved minerals processing techniques. This includes techniques not only for the extraction step itself, but also in preparation of reagent materials such as fluxes, alloys and reductants. Two conventional processes which are used to facilitate minerals processing and extraction are reactive mechanical milling and plasma processing. Electric discharge assisted mechanical milling (EDAMM) is a relatively new process which combines these two technologies by submitting powders to a mechanical milling mode, whilst concurrently exposing the milling environment to pulsed plasma arc discharges [1]. This amalgamation of processing techniques gives rise to simultaneous reaction of
∗
Corresponding author. Tel.: +61 2 42214945; fax: +61 2 42213112. E-mail addresses: [email protected], Andrzej [email protected] (A. Calka). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.133
materials and controlled refinement of size and morphology. In addition, potential exists for the improvement in efficiency of the materials and energy consumption, whilst reducing environmental impacts. EDAMM has been demonstrated as an effective tool for inducing rapid reaction rates in solid–solid systems and solid–gas systems [2–4]. Control over the degree of reaction and particle morphology has also been demonstrated. Work conducted thus far has concentrated on synthesis reactions. Partial reduction has only been demonstrated for hematite (Fe2 O3 ) [5]. In addition, all the materials studied up to now have been high purity, laboratory grade powders. With this in mind, it is possible that phase changes induced by EDAMM in commercial grade materials could represent an intermediate step, or even the primary step in a metals extraction route. It is unknown how the presence of gangue or waste minerals present in commercial grade minerals will impact the ability of EDAMM to induce phase changes. 2. Experimental In this work natural ilmenite concentrates (NICs) were subjected to EDAMM treatment in a modified laboratory rod mill. In this mill a vibrating base produced a milling mode involving repeated impact of a hardened curved rod end on particles placed in a vibrating hemispherical container under a controlled electrical discharge in a controlled atmosphere (Fig. 1). Electric discharges occurred in the gap between the vibrating mill base, the powder particles and the rod. The
D. Bishop, A. Calka / Journal of Alloys and Compounds 469 (2009) 380–385
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Fig. 1. Schematic of electrical discharge assisted mechanical milling (EDAMM). power supply generated 1 kV, impulses within the mA and kHz range. Electric discharges resulted in hot plasma formation near ilmenite powder particles. Two sets of experiments were performed: (1) thermal decomposition of ilmenite by EDAMM in oxygen-free atmospheres (carbon dioxide, nitrogen and argon) and (2) oxidation of ilmenite in oxidising atmospheres (oxygen and air). In all experiments a milling time up to 10 min was used. X-ray diffraction (XRD) analysis was performed using a Phillips PW1730 diffractometer with a graphite monochromator and Cu K␣ radiation. Phase identification was carried out using the International Centre for Diffraction Data (JCPDS-ICDD 2000) powder diffraction files (PDF). The morphology of the powders was observed by a Leica 440 Stereoscan scanning electron microscope (SEM). This SEM is equipped with an Oxford Instruments ISIS energy dispersive spectroscopy (EDS) system which was used to examine the composition of the powders.
3. Results and discussion Fig. 2. XRD patterns of ilmenite concentrate EDAMM in an inert atmosphere for 10 min: (a) ilmenite starting powder, (b) milled in carbon dioxide gas (CO2 ), (c) milled in nitrogen gas (N2 ) and (d) milled in argon gas (Ar).
3.1. Thermal decomposition of ilmenite concentrate In this work natural ilmenite concentrates were used. NICs are obtained by magnetic separation from mineral sand deposits. This concentrate is primarily an iron–titanium oxide. Two main phases are found in ilmenite concentrate and they are ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) [6]. Ilmenite was subjected to EDAMM in oxygen-free atmospheres for 10 min. These atmospheres of carbon dioxide, nitrogen and argon can be considered inert for the purposes of this work. Milling times and phases identified by XRD for the starting powder and EDAMM products are summarised in Table 1. The XRD analysis (Fig. 2) confirms the presence of both ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) in the ilmenite starting concentrate. The XRD results of EDAMM for 10 min in all three inert atmospheres show that the starting ilmenite phase has been retained. The other starting component however, pseudorutile has been eliminated and replaced with another iron–titanium oxide phase called ferric pseudobrookite (Fe2 TiO5 ). These results show that EDAMM of ilmenite concentrate in an inert atmosphere of carbon dioxide, nitrogen or argon gas
induces thermal decomposition of pseudorutile (Fe2 Ti3 O9 ). The other starting component, ilmenite (FeTiO3 ) is retained during EDAMM in the absence of oxygen. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) according to following equation: Fe2 Ti3 O9 → Fe2 TiO5 + 2TiO2 The absence of rutile from any of the XRD patterns in Fig. 2 is consistent with the work by Zhang and Ostrovski [6] who observed that rutile peaks disappeared when ilmenite concentrate was sintered in argon gas above 1673 K (1400 ◦ C). The explanation by this researcher was that at these temperatures, rutile is taken into a solid solution with ferric pseudobrookite. If this explanation is correct, then it likely means that conditions during EDAMM were equivalent to a temperature of greater than 1673 K (1400 ◦ C). During EDAMM the rutile phase was taken into solid solution with ferric pseudobrookite and subsequently cooled or quenched at a sufficient rate to maintain this solid solution at room temperature after milling ceased.
Table 1 Phases identified by XRD for ilmenite thermal decomposition Experiment type
Sample
Milling time (min)
Atmosph`ere
Identified phases
Thermal d´ecomposition
Starting powder #1 #2 #3
10 10 10
CO2 N2 Ar
Ilmenite; pseudorutile Pseudobrookite; llmenite Pseudobrookite; llmenite Pseudobrookite; llmenite
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likely they have formed by chipping from the larger, irregular particles due to the mechanical milling action. Inspection of the cross-sectioned powder products of ilmenite concentrate subjected to EDAMM in argon for 10 min (Fig. 4 inset) demonstrates that two phases are present within the material. These two phases were identified during XRD analysis as ilmenite and ferric pseudobrookite. 3.2. Oxidation of ilmenite concentrate
Fig. 3. SEM secondary electron images of ilmenite concentrate.
It can therefore be established from these experiments that EDAMM of ilmenite concentrate in an inert atmosphere induces complete thermal decomposition of the pseudorutile component in under 10 min. The ilmenite component is retained. SEM secondary electron images of the starting ilmenite sand (Fig. 3) show that this concentrate has a smooth surface, which might be expected. SEM secondary electron images of the ilmenite sample subjected to EDAMM in argon for 10 min (Fig. 4) show that the EDAMM and thermally decomposed sample has a much more irregular shape. Close inspection of this sample at higher magnifications shows what could be described as a stepped or layered edge. The top surface of the examined particle shows patches of smooth areas as well as jagged areas which appear to be an agglomeration of smaller particles or crystals. In terms of powder size it appears that marginal reduction has taken place. The starting ilmenite sand has a typical particle diameter of about 200 m whereas the sample EDAMM in argon has a typical particle diameter of about 150 m. Some much smaller particles can also be identified in the EDAMM sample which are less than 10 m. These small particles appear both on their own and adhered to the surface of the larger particles. It is
Fig. 4. SEM secondary electron image of ilmenite spark milled for 10 min in argon. Inset shows cross-sectioned particle image.
In these experiments ilmenite concentrate was subjected to EDAMM under oxidising atmospheres (oxygen and air) for 10 min. Phases identified by XRD for the starting powder and EDAMM products are summarised in Table 2. Labelled XRD patterns for ilmenite concentrate subjected to EDAMM in oxidising atmospheres are shown in Fig. 5. The XRD results confirm the presence of both ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) in the ilmenite starting concentrate. Samples after EDAMM in an oxidising atmosphere of oxygen or air developed the iron–titanium oxide, ferric pseudobrookite (Fe2 TiO5 ). The ilmenite and pseudorutile starting phases were both eliminated during EDAMM. The XRD results show that EDAMM of ilmenite concentrate in an oxidising atmosphere of oxygen or air induces thermal decomposition of the pseudorutile (Fe2 Ti3 O9 ) component and complete oxidation of the ilmenite (FeTiO3 ) component in less than 10 min. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) according to the equation: Fe2 Ti3 O9 → Fe2 TiO5 + 2TiO2 Likewise, the expected products from oxidation of ilmenite are also ferric pseudobrookite and rutile. The overall reaction is
Fig. 5. XRD patterns of ilmenite concentrate EDAMM in oxidising atmospheres for 10 min: (a) ilmenite starting powder, (b) milled in oxygen gas (O2 ) and (c) milled in air.
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Table 2 Phases identified by XRD for ilmenite oxidation Experiment type
Sample
Milling time (min)
Atmosph`ere
Identified phases
Oxidation
Starting powder #4 #5
10 10
O2 Air
Ilmenite; pseudorutile Pseudobrookite Pseudobrookite
shown in the below equation: 2FeTiO3 + (1/2O2 ) → Fe2 TiO5 + TiO2 The absence of rutile from any of the XRD patterns in Fig. 5 is consistent with the work by Zhang and Ostrovski [6] who observed that rutile peaks disappeared when ilmenite concentrate was sintered in argon gas above 1673 K (1400 ◦ C). The absence of these rutile peaks was also noted during EDAMM in an inert atmosphere. The explanation by Zhang and Ostrovski [6] was that at these temperatures, rutile is taken into a solid solution with ferric pseudobrookite. Again, if this explanation is correct, then it likely means that conditions during EDAMM were equivalent to a temperature of greater than 1673 K (1400 ◦ C). During EDAMM the rutile phase was taken into solid solution with ferric pseudobrookite and subsequently cooled or quenched at a sufficient rate to maintain this solid solution at room temperature after milling ceased. SEM secondary electron images of the ilmenite sample subjected to EDAMM in air for 10 min (Fig. 6) show that the EDAMM and oxidised sample has a much more irregular shape than the starting NIC. Close inspection of this sample at 2500× shows that the morphology of the oxidised product consists of acicular or needle-like crystals. This is consistent with Amethyst Galleries (2007) description of the crystal habit of ferric pseudobrookite as “including small acicular or thin prismatic crystals aggregated together as sprays of only a few individuals”. In terms of powder size it appears that some size reduction has taken place. The starting ilmenite sand has a typical particle diameter of about 200 m whereas the sample EDAMM in air has a typical particle diameter of about 100 m. Some much smaller particles can also be identified in the EDAMM sample
which are less than 10 m. These small particles appear both on their own and adhered to the surface of the larger particles. This bimodal size distribution was also identified when ilmenite was subjected to EDAMM in an inert argon. It is likely that they have formed by chipping from the larger, irregular particles. Inspection of the SEM images of cross-sectioned powders (Fig. 6 inset) does not reveal any atomic number contrast which indicates that these are the single phase particles with little or no segregation of elements. From above results it can be concluded that EDAMM of ilmenite concentrate in an oxidising atmosphere, induces complete thermal decomposition of the pseudorutile component and complete oxidation of the ilmenite component in under 10 min. The complete reaction sequence was unable to be identified from the XRD patterns as only the final oxidation products were present. In the complete reaction sequence [6] hematite (Fe2 O3 ) was identified as a product of an intermediate reaction in the oxidation of ilmenite. This phase was not identified from XRD patterns. Therefore in this study EDAMM experiments under the same conditions for a range of shorter milling times were subsequently performed to establish the reaction sequence as well as the time to complete the reactions. It was thought that intermediate reaction products might be able to be identified from this work. This experiment is described in the following section. 3.3. Oxidation of ilmenite concentrate over a range of durations Ilmenite concentrate was subjected to EDAMM in air over a range of durations up to 10 min. Milling times and phases identified by XRD for the starting powder and EDAMM products are summarised in Table 3. Labeled XRD patterns for ilmenite concentrate EDAMM in oxidising atmospheres are shown in Fig. 7.
Table 3 EDAMM variables and phases identified by XRD for ilmenite oxidation in air Experiment type
Oxidation in air Fig. 6. SEM secondary electron images of ilmenite EDAMM for 10 min in air. Inset shows cross-sectioned particle image.
Sample Starting powder #6 #7 #8 #9 #10 #11
Milling time
Identified phases Ilmenite; pseudorutile
10 s 30 s 1 min 2 min 5 min 10 min
Ilmenite; pseudobrookite; rutile Ilmenite; pseudobrookite; rutile Ilmenite; pseudobrookite; rutile Pseudobrookite; rutile Pseudobrookite Pseudobrookite
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The XRD results (Fig. 7) show that subjecting ilmenite concentrate to EDAMM in air induces thermal decomposition of the pseudorutile (Fe2 Ti3 O9 ) component and complete oxidation of the ilmenite (FeTiO3 ) component in less than 2 min. The pseudorutile component was not able to be identified after just 10 s of EDAMM time which indicates that its thermal decomposition was very rapid. Some ilmenite was retained up to 1 min of EDAMM duration which indicates that its oxidation is not as rapid. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) as was discussed in Section 3.1. Likewise, the expected products from oxidation of ilmenite are also ferric pseudobrookite and rutile. Both of these product phases can be seen from 10 s of EDAMM duration, however the rutile phase disappears from the XRD patterns after 2 min of EDAMM. This was also discussed in Section 3.1 and is consistent with the work of Zhang and Ostrovski [6] who suggested that at temperature greater than 1673 K (1400 ◦ C), rutile is taken into solid solution with ferric pseudobrookite and therefore cannot be identified by XRD. It was thought that hematite (Fe2 O3 ), as a product of an intermediate step in the reaction sequence for ilmenite oxidation might be identified in the shorter milling durations. This was not the case and indicates that EDAMM conditions provide an environment of sufficient temperature for subsequent intermediate steps to occur as soon as the reagents become available. It can therefore be established from these experiments that subjecting ilmenite concentrate to EDAMM in an oxidising atmosphere induces complete thermal decomposition of the pseudorutile component in less that 10 s. In addition, complete oxidation of the ilmenite component occurs in 1–2 min. 4. Conclusions
Fig. 7. XRD patterns of ilmenite concentrate EDAMM in air over a range of durations.
The XRD results shown in Fig. 7 demonstrate that after just 10 s of milling time the pseudorutile (Fe2 Ti3 O9 ) starting component has transformed. The ilmenite (FeTiO3 ) starting component is still present after 1 min of EDAMM but has been transformed between 1 min and 2 min. After 10 s, the product phases ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) have begun to appear. Ferric pseudobrookite is retained right through the entire EDAMM time series however rutile disappears between 2 min and 5 min of EDAMM duration.
Natural ilmenite concentrate has been subjected to EDAMM in both inert and oxidising atmospheres in this study. It has been shown that in inert atmospheres such as carbon dioxide, argon and nitrogen, the pseudorutile component is thermally decomposed to ferric pseudobrookite whilst the ilmenite component remains unreacted. It has also been shown that in oxidising atmospheres, such as air and oxygen, the pseudorutile component is again thermally decomposed to ferric pseudobrookite, whilst the ilmenite component is oxidised to ferric pseudobrookite. Conducting EDAMM experiments on NIC over a range of durations demonstrated that during EDAMM, the pseudorutile component is thermally decomposed in less than 10 s, whilst the ilmenite component is completely oxidised in less than 2 min. XRD results indicate that rutile, which is a product of these reactions is taken into solid solution with ferric pseudobrookite during EDAMM, indicating that conditions in the reaction zone are equivalent to a temperature of at least 1673 K (1400 ◦ C). Knowing that high temperature reactions can be induced during EDAMM indicates that a range of other mineral processing reactions are likely to be also possible using this technique.
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References [1] A. Calka, D. Wexler, Nature 419 (2002) 147–151. [2] S.A. Needham, A. Calka, G.X. Wang, P. Germanas, H.K. Liu, J. Mater. Chem. 16 (46) (2006) 4488–4493. [3] A. Calka, D. Oleszak, J. Met. Compd. 440 (1/2) (2007) 346–348.
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[4] S.A. Needham, A. Calka, G.X. Wang, A. Mosbah, H.K. Liu, Electrochem. Commun. 8 (2006) 434–438. [5] D. Wexler, A. Calka, D.P. Dunne, J. Metastable Nanocryst. Mater. 26 (2005) 16–21. [6] G. Zhang, O. Ostrovski, Int. J. Miner. Process. 424 (2002) 201–218.
Phase transformations in ilmenite induced by electric discharge assisted mechanical milling D. Bishop, A. Calka ∗ Engineering Materials Institute, Faculty of Engineering, University of Wollongong, Northfields Avenue, Wollongong 2522, Australia Received 6 December 2007; accepted 24 January 2008 Available online 24 March 2008
Abstract The recently developed synthesis technique of electric discharge assisted mechanical milling (EDAMM) is extended to minerals processing. This study examines phase changes during thermal decomposition and oxidation of ilmenite concentrate induced by EDAMM in a variety of inert and oxidising atmospheres. Products were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The results of this investigation demonstrate that thermal decomposition of the pseudorutile component of ilmenite concentrate occurs during EDAMM in less than 10 s and that oxidation of the ilmenite component occurs during EDAMM in less than 2 min in oxidising atmospheres. © 2008 Elsevier B.V. All rights reserved. Keywords: Oxide materials; Mechanochemical processing; Oxidation
1. Introduction Extraction of metals from naturally occurring minerals has occurred for hundreds of years. Over this time extraction processes have been continually refined, improved or replaced to increase production rates, improve efficiency and decrease costs. In modern times, society has become increasingly focused on the impact these processes have on the environment and minimising this impact has added another dimension to the drive for the development of new or improved minerals processing techniques. This includes techniques not only for the extraction step itself, but also in preparation of reagent materials such as fluxes, alloys and reductants. Two conventional processes which are used to facilitate minerals processing and extraction are reactive mechanical milling and plasma processing. Electric discharge assisted mechanical milling (EDAMM) is a relatively new process which combines these two technologies by submitting powders to a mechanical milling mode, whilst concurrently exposing the milling environment to pulsed plasma arc discharges [1]. This amalgamation of processing techniques gives rise to simultaneous reaction of
∗
Corresponding author. Tel.: +61 2 42214945; fax: +61 2 42213112. E-mail addresses: [email protected], Andrzej [email protected] (A. Calka). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.133
materials and controlled refinement of size and morphology. In addition, potential exists for the improvement in efficiency of the materials and energy consumption, whilst reducing environmental impacts. EDAMM has been demonstrated as an effective tool for inducing rapid reaction rates in solid–solid systems and solid–gas systems [2–4]. Control over the degree of reaction and particle morphology has also been demonstrated. Work conducted thus far has concentrated on synthesis reactions. Partial reduction has only been demonstrated for hematite (Fe2 O3 ) [5]. In addition, all the materials studied up to now have been high purity, laboratory grade powders. With this in mind, it is possible that phase changes induced by EDAMM in commercial grade materials could represent an intermediate step, or even the primary step in a metals extraction route. It is unknown how the presence of gangue or waste minerals present in commercial grade minerals will impact the ability of EDAMM to induce phase changes. 2. Experimental In this work natural ilmenite concentrates (NICs) were subjected to EDAMM treatment in a modified laboratory rod mill. In this mill a vibrating base produced a milling mode involving repeated impact of a hardened curved rod end on particles placed in a vibrating hemispherical container under a controlled electrical discharge in a controlled atmosphere (Fig. 1). Electric discharges occurred in the gap between the vibrating mill base, the powder particles and the rod. The
D. Bishop, A. Calka / Journal of Alloys and Compounds 469 (2009) 380–385
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Fig. 1. Schematic of electrical discharge assisted mechanical milling (EDAMM). power supply generated 1 kV, impulses within the mA and kHz range. Electric discharges resulted in hot plasma formation near ilmenite powder particles. Two sets of experiments were performed: (1) thermal decomposition of ilmenite by EDAMM in oxygen-free atmospheres (carbon dioxide, nitrogen and argon) and (2) oxidation of ilmenite in oxidising atmospheres (oxygen and air). In all experiments a milling time up to 10 min was used. X-ray diffraction (XRD) analysis was performed using a Phillips PW1730 diffractometer with a graphite monochromator and Cu K␣ radiation. Phase identification was carried out using the International Centre for Diffraction Data (JCPDS-ICDD 2000) powder diffraction files (PDF). The morphology of the powders was observed by a Leica 440 Stereoscan scanning electron microscope (SEM). This SEM is equipped with an Oxford Instruments ISIS energy dispersive spectroscopy (EDS) system which was used to examine the composition of the powders.
3. Results and discussion Fig. 2. XRD patterns of ilmenite concentrate EDAMM in an inert atmosphere for 10 min: (a) ilmenite starting powder, (b) milled in carbon dioxide gas (CO2 ), (c) milled in nitrogen gas (N2 ) and (d) milled in argon gas (Ar).
3.1. Thermal decomposition of ilmenite concentrate In this work natural ilmenite concentrates were used. NICs are obtained by magnetic separation from mineral sand deposits. This concentrate is primarily an iron–titanium oxide. Two main phases are found in ilmenite concentrate and they are ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) [6]. Ilmenite was subjected to EDAMM in oxygen-free atmospheres for 10 min. These atmospheres of carbon dioxide, nitrogen and argon can be considered inert for the purposes of this work. Milling times and phases identified by XRD for the starting powder and EDAMM products are summarised in Table 1. The XRD analysis (Fig. 2) confirms the presence of both ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) in the ilmenite starting concentrate. The XRD results of EDAMM for 10 min in all three inert atmospheres show that the starting ilmenite phase has been retained. The other starting component however, pseudorutile has been eliminated and replaced with another iron–titanium oxide phase called ferric pseudobrookite (Fe2 TiO5 ). These results show that EDAMM of ilmenite concentrate in an inert atmosphere of carbon dioxide, nitrogen or argon gas
induces thermal decomposition of pseudorutile (Fe2 Ti3 O9 ). The other starting component, ilmenite (FeTiO3 ) is retained during EDAMM in the absence of oxygen. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) according to following equation: Fe2 Ti3 O9 → Fe2 TiO5 + 2TiO2 The absence of rutile from any of the XRD patterns in Fig. 2 is consistent with the work by Zhang and Ostrovski [6] who observed that rutile peaks disappeared when ilmenite concentrate was sintered in argon gas above 1673 K (1400 ◦ C). The explanation by this researcher was that at these temperatures, rutile is taken into a solid solution with ferric pseudobrookite. If this explanation is correct, then it likely means that conditions during EDAMM were equivalent to a temperature of greater than 1673 K (1400 ◦ C). During EDAMM the rutile phase was taken into solid solution with ferric pseudobrookite and subsequently cooled or quenched at a sufficient rate to maintain this solid solution at room temperature after milling ceased.
Table 1 Phases identified by XRD for ilmenite thermal decomposition Experiment type
Sample
Milling time (min)
Atmosph`ere
Identified phases
Thermal d´ecomposition
Starting powder #1 #2 #3
10 10 10
CO2 N2 Ar
Ilmenite; pseudorutile Pseudobrookite; llmenite Pseudobrookite; llmenite Pseudobrookite; llmenite
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likely they have formed by chipping from the larger, irregular particles due to the mechanical milling action. Inspection of the cross-sectioned powder products of ilmenite concentrate subjected to EDAMM in argon for 10 min (Fig. 4 inset) demonstrates that two phases are present within the material. These two phases were identified during XRD analysis as ilmenite and ferric pseudobrookite. 3.2. Oxidation of ilmenite concentrate
Fig. 3. SEM secondary electron images of ilmenite concentrate.
It can therefore be established from these experiments that EDAMM of ilmenite concentrate in an inert atmosphere induces complete thermal decomposition of the pseudorutile component in under 10 min. The ilmenite component is retained. SEM secondary electron images of the starting ilmenite sand (Fig. 3) show that this concentrate has a smooth surface, which might be expected. SEM secondary electron images of the ilmenite sample subjected to EDAMM in argon for 10 min (Fig. 4) show that the EDAMM and thermally decomposed sample has a much more irregular shape. Close inspection of this sample at higher magnifications shows what could be described as a stepped or layered edge. The top surface of the examined particle shows patches of smooth areas as well as jagged areas which appear to be an agglomeration of smaller particles or crystals. In terms of powder size it appears that marginal reduction has taken place. The starting ilmenite sand has a typical particle diameter of about 200 m whereas the sample EDAMM in argon has a typical particle diameter of about 150 m. Some much smaller particles can also be identified in the EDAMM sample which are less than 10 m. These small particles appear both on their own and adhered to the surface of the larger particles. It is
Fig. 4. SEM secondary electron image of ilmenite spark milled for 10 min in argon. Inset shows cross-sectioned particle image.
In these experiments ilmenite concentrate was subjected to EDAMM under oxidising atmospheres (oxygen and air) for 10 min. Phases identified by XRD for the starting powder and EDAMM products are summarised in Table 2. Labelled XRD patterns for ilmenite concentrate subjected to EDAMM in oxidising atmospheres are shown in Fig. 5. The XRD results confirm the presence of both ilmenite (FeTiO3 ) and pseudorutile (Fe2 Ti3 O9 ) in the ilmenite starting concentrate. Samples after EDAMM in an oxidising atmosphere of oxygen or air developed the iron–titanium oxide, ferric pseudobrookite (Fe2 TiO5 ). The ilmenite and pseudorutile starting phases were both eliminated during EDAMM. The XRD results show that EDAMM of ilmenite concentrate in an oxidising atmosphere of oxygen or air induces thermal decomposition of the pseudorutile (Fe2 Ti3 O9 ) component and complete oxidation of the ilmenite (FeTiO3 ) component in less than 10 min. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) according to the equation: Fe2 Ti3 O9 → Fe2 TiO5 + 2TiO2 Likewise, the expected products from oxidation of ilmenite are also ferric pseudobrookite and rutile. The overall reaction is
Fig. 5. XRD patterns of ilmenite concentrate EDAMM in oxidising atmospheres for 10 min: (a) ilmenite starting powder, (b) milled in oxygen gas (O2 ) and (c) milled in air.
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Table 2 Phases identified by XRD for ilmenite oxidation Experiment type
Sample
Milling time (min)
Atmosph`ere
Identified phases
Oxidation
Starting powder #4 #5
10 10
O2 Air
Ilmenite; pseudorutile Pseudobrookite Pseudobrookite
shown in the below equation: 2FeTiO3 + (1/2O2 ) → Fe2 TiO5 + TiO2 The absence of rutile from any of the XRD patterns in Fig. 5 is consistent with the work by Zhang and Ostrovski [6] who observed that rutile peaks disappeared when ilmenite concentrate was sintered in argon gas above 1673 K (1400 ◦ C). The absence of these rutile peaks was also noted during EDAMM in an inert atmosphere. The explanation by Zhang and Ostrovski [6] was that at these temperatures, rutile is taken into a solid solution with ferric pseudobrookite. Again, if this explanation is correct, then it likely means that conditions during EDAMM were equivalent to a temperature of greater than 1673 K (1400 ◦ C). During EDAMM the rutile phase was taken into solid solution with ferric pseudobrookite and subsequently cooled or quenched at a sufficient rate to maintain this solid solution at room temperature after milling ceased. SEM secondary electron images of the ilmenite sample subjected to EDAMM in air for 10 min (Fig. 6) show that the EDAMM and oxidised sample has a much more irregular shape than the starting NIC. Close inspection of this sample at 2500× shows that the morphology of the oxidised product consists of acicular or needle-like crystals. This is consistent with Amethyst Galleries (2007) description of the crystal habit of ferric pseudobrookite as “including small acicular or thin prismatic crystals aggregated together as sprays of only a few individuals”. In terms of powder size it appears that some size reduction has taken place. The starting ilmenite sand has a typical particle diameter of about 200 m whereas the sample EDAMM in air has a typical particle diameter of about 100 m. Some much smaller particles can also be identified in the EDAMM sample
which are less than 10 m. These small particles appear both on their own and adhered to the surface of the larger particles. This bimodal size distribution was also identified when ilmenite was subjected to EDAMM in an inert argon. It is likely that they have formed by chipping from the larger, irregular particles. Inspection of the SEM images of cross-sectioned powders (Fig. 6 inset) does not reveal any atomic number contrast which indicates that these are the single phase particles with little or no segregation of elements. From above results it can be concluded that EDAMM of ilmenite concentrate in an oxidising atmosphere, induces complete thermal decomposition of the pseudorutile component and complete oxidation of the ilmenite component in under 10 min. The complete reaction sequence was unable to be identified from the XRD patterns as only the final oxidation products were present. In the complete reaction sequence [6] hematite (Fe2 O3 ) was identified as a product of an intermediate reaction in the oxidation of ilmenite. This phase was not identified from XRD patterns. Therefore in this study EDAMM experiments under the same conditions for a range of shorter milling times were subsequently performed to establish the reaction sequence as well as the time to complete the reactions. It was thought that intermediate reaction products might be able to be identified from this work. This experiment is described in the following section. 3.3. Oxidation of ilmenite concentrate over a range of durations Ilmenite concentrate was subjected to EDAMM in air over a range of durations up to 10 min. Milling times and phases identified by XRD for the starting powder and EDAMM products are summarised in Table 3. Labeled XRD patterns for ilmenite concentrate EDAMM in oxidising atmospheres are shown in Fig. 7.
Table 3 EDAMM variables and phases identified by XRD for ilmenite oxidation in air Experiment type
Oxidation in air Fig. 6. SEM secondary electron images of ilmenite EDAMM for 10 min in air. Inset shows cross-sectioned particle image.
Sample Starting powder #6 #7 #8 #9 #10 #11
Milling time
Identified phases Ilmenite; pseudorutile
10 s 30 s 1 min 2 min 5 min 10 min
Ilmenite; pseudobrookite; rutile Ilmenite; pseudobrookite; rutile Ilmenite; pseudobrookite; rutile Pseudobrookite; rutile Pseudobrookite Pseudobrookite
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The XRD results (Fig. 7) show that subjecting ilmenite concentrate to EDAMM in air induces thermal decomposition of the pseudorutile (Fe2 Ti3 O9 ) component and complete oxidation of the ilmenite (FeTiO3 ) component in less than 2 min. The pseudorutile component was not able to be identified after just 10 s of EDAMM time which indicates that its thermal decomposition was very rapid. Some ilmenite was retained up to 1 min of EDAMM duration which indicates that its oxidation is not as rapid. The expected products of thermal decomposition of pseudorutile are ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) as was discussed in Section 3.1. Likewise, the expected products from oxidation of ilmenite are also ferric pseudobrookite and rutile. Both of these product phases can be seen from 10 s of EDAMM duration, however the rutile phase disappears from the XRD patterns after 2 min of EDAMM. This was also discussed in Section 3.1 and is consistent with the work of Zhang and Ostrovski [6] who suggested that at temperature greater than 1673 K (1400 ◦ C), rutile is taken into solid solution with ferric pseudobrookite and therefore cannot be identified by XRD. It was thought that hematite (Fe2 O3 ), as a product of an intermediate step in the reaction sequence for ilmenite oxidation might be identified in the shorter milling durations. This was not the case and indicates that EDAMM conditions provide an environment of sufficient temperature for subsequent intermediate steps to occur as soon as the reagents become available. It can therefore be established from these experiments that subjecting ilmenite concentrate to EDAMM in an oxidising atmosphere induces complete thermal decomposition of the pseudorutile component in less that 10 s. In addition, complete oxidation of the ilmenite component occurs in 1–2 min. 4. Conclusions
Fig. 7. XRD patterns of ilmenite concentrate EDAMM in air over a range of durations.
The XRD results shown in Fig. 7 demonstrate that after just 10 s of milling time the pseudorutile (Fe2 Ti3 O9 ) starting component has transformed. The ilmenite (FeTiO3 ) starting component is still present after 1 min of EDAMM but has been transformed between 1 min and 2 min. After 10 s, the product phases ferric pseudobrookite (Fe2 TiO5 ) and rutile (TiO2 ) have begun to appear. Ferric pseudobrookite is retained right through the entire EDAMM time series however rutile disappears between 2 min and 5 min of EDAMM duration.
Natural ilmenite concentrate has been subjected to EDAMM in both inert and oxidising atmospheres in this study. It has been shown that in inert atmospheres such as carbon dioxide, argon and nitrogen, the pseudorutile component is thermally decomposed to ferric pseudobrookite whilst the ilmenite component remains unreacted. It has also been shown that in oxidising atmospheres, such as air and oxygen, the pseudorutile component is again thermally decomposed to ferric pseudobrookite, whilst the ilmenite component is oxidised to ferric pseudobrookite. Conducting EDAMM experiments on NIC over a range of durations demonstrated that during EDAMM, the pseudorutile component is thermally decomposed in less than 10 s, whilst the ilmenite component is completely oxidised in less than 2 min. XRD results indicate that rutile, which is a product of these reactions is taken into solid solution with ferric pseudobrookite during EDAMM, indicating that conditions in the reaction zone are equivalent to a temperature of at least 1673 K (1400 ◦ C). Knowing that high temperature reactions can be induced during EDAMM indicates that a range of other mineral processing reactions are likely to be also possible using this technique.
D. Bishop, A. Calka / Journal of Alloys and Compounds 469 (2009) 380–385
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[4] S.A. Needham, A. Calka, G.X. Wang, A. Mosbah, H.K. Liu, Electrochem. Commun. 8 (2006) 434–438. [5] D. Wexler, A. Calka, D.P. Dunne, J. Metastable Nanocryst. Mater. 26 (2005) 16–21. [6] G. Zhang, O. Ostrovski, Int. J. Miner. Process. 424 (2002) 201–218.