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Mechanically induced reduction of ilmenite (FeTiO3) and rutile (TiO2) by magnesium
Journal of Alloys and Compounds 274 (1998) 260–265
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Mechanically induced reduction of ilmenite (FeTiO 3 ) and rutile (TiO 2 ) by magnesium N.J. Welham* Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia Received 25 February 1998
Abstract Ilmenite (FeTiO 3 ) and rutile (TiO 2 ) have been ball milled with various ratios of metallic magnesium for 100 h in a laboratory mill. The reduction of both oxides was evident directly after milling with substantial formation of MgO. Ilmenite also formed a mixed cation titanate (Mg, Fe)TiO 3 and elemental iron, no reduced phases were observed for rutile. There was no difference between the products obtained by thermal reaction and milling, although energetic reactions observed in milled powder by DTA were not seen in the milled sample and there was no obvious crystallisation of a reduced titanium phase. Solubilisation of titanium from milled powders was found to increase with magnesium content and was .90% for a starting ratio of 1: $1 ilmenite: Mg. The unleached material was composed of incompletely reduced phases such as TiO 2 , MTiO 3 and Ti 3 O 5 . 1998 Elsevier Science S.A. Keywords: Ilmenite; Reduction; Magnesium; Rutile
1. Introduction Ilmenite is the primary source of titanium compounds and several processing steps need to be performed before a suitable product is obtained. The mineral is purified by reduction to TiO 2 and elemental iron which is then leached to form an impure TiO 2 . The 88–92% TiO 2 is then chlorinated using petroleum coke as the reductant and the impurities separated by fractional distillation. For pigment TiO 2 the chloride is reoxidised and the chlorine recycled, titanium chemicals are produced from a solution of the chloride in acid. To produce metallic titanium the chloride is reacted with molten magnesium under an inert atmosphere, the MgCl 2 and remaining magnesium are leached and distilled away respectively from the titanium sponge, which is consolidated by arc melting. The use of magnesium as the reductant for rutile results in a metal containing 2.5 wt % oxygen [1] which has to be refined by vacuum melting to produce an acceptable metal. Indeed, no process for titanium metal manufacture using either rutile or more complex oxides has been commercially exploited. There is no obvious constraint on using magnesium metal as a direct reductant for ilmenite other than the *Fax: 161 2 62490511; e-mail: [email protected] 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00526-X
increase in magnesium consumption over TiO 2 . A reduction in the number of process steps may be enough to justify the use of magnesium directly. A process which converts directly from ilmenite to metal could afford potential savings in time and energy, despite the necessity to further refine the metal. Alternatively, the chemical market could be targeted by providing a high solubility of titanium without a thermal step. Mechanochemical processing has been shown to both decrease the time and temperature required for the carbothermic reduction of ilmenite [2] and induce reactions within the mill that could not be achieved without thermal processing [3]. Thus, this paper examines the feasibility of using mechanical activation to induce ilmenite and rutile reduction by magnesium.
2. Experimental The ilmenite [2] and rutile [3] were the same as that used previously. The magnesium used was nominally .99% pure but was found to contain a small fraction of Mg(OH) 2 . The ilmenite: magnesium ratios examined were 1:1, 1:2 and 1:3, these were chosen on the basis of thermodynamic predictions, summarised by reactions 1–3.
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
FeTiO 3 1 Mg ⇒ MgTiO 3 1 Fe
(1)
MgTiO 3 1 Mg ⇒ 2MgO 1 TiO
(2)
TiO 1 Mg ⇒ MgO 1 Ti
(3)
Rutile was predicted to be reduced to TiO, and then to titanium metal by reaction 3. TiO 2 1 Mg ⇒ TiO 1 MgO
(4)
Thus, two rutile: magnesium ratios were examined, 1:1 and 1:2. The 6.00 g samples were milled under vacuum for up to 100 h at room temperature in a vertical 316S stainless steel ball mill using four 10 (25.4 mm) diameter 420C stainless steel balls confined in the vertical plane. Ball motion was controlled by the positioning of magnets radially round the mill. Samples of milled powders were annealed under a controlled atmosphere to determine the effect of thermal treatment of the milled powders. Samples of as-milled powder were leached for 12 h in 3% hydrochloric acid at room temperature to determine whether the reductive process would enhance the solubility of ilmenite and rutile. The titanium in solution was measured by spectrophotometry using the standard peroxide method [4]. Ilmenite and rutile powders (pre-milled for 10 h to reduce the particle size [5]) were mixed thoroughly with the highest ratio of magnesium examined (1:3 and 1:2
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respectively) and then annealed under identical conditions to the milled powders. Selected powders were also analysed by differential thermal analysis (DTA) to detect energetic reactions (both endothermic and exothermic) during the heating process. Samples of |30 mg were heated to 10008C at 208C min 21 under argon in a Shimadzu DTA-50. The products were analysed by X-ray diffraction (XRD) ˚ with monochromatised CoKa radiation ( l 51.78896 A) using a count time of 2 s per 0.18 step size.
3. Results and discussion The two mixtures showed no reaction after mixing, as expected. DTA of the ilmenite / magnesium and rutile / magnesium mixtures are shown in Fig. 1 with both systems showing an endotherm with onset at |3208C. This was determined to be the dehydration of Mg(OH) 2 present in the magnesium, agreeing with work on pure Mg(OH) 2 [6–8]. The dehydration endotherm was followed by an exotherm which was attributed to a slow reaction between the oxide and magnesium. The ilmenite was more exothermic than the rutile suggesting the reaction was comparatively facile and may represent the partial reduction of ilmenite by reaction 1. XRD of powders annealed at 5008C for 1 h showed a slight reaction had occurred for ilmenite, with small peaks for elemental iron and mag-
Fig. 1. DTA profile of the 1:3 ilmenite:magnesium and 1:2 rutile:magnesium mixtures, argon atmosphere, 208C min 21 .
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N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
nesium oxide evident. The magnesium oxide was partially due to the dehydration of the Mg(OH) 2 but the presence of elemental iron is clear indication of a reduction reaction. Rutile also showed peaks for MgO but not MgTiO 3 so it cannot be confirmed that reaction occurred. For both mixtures there was a very large exotherm with onset at 5908C and 6108C for ilmenite and rutile respectively, these were due to one or more of the reduction reactions 1–4. The absence of further energetic events would imply that the main exotherm was due to the completion of the reduction process. The onset of reaction is below the melting point of magnesium (6508C) indicating that the initial reaction was solid state. However, because of the extremely exothermic nature of the reaction(s) the temperature in the crucible rose above that necessary to melt the magnesium for several seconds. This may have caused an acceleration of the reaction by increasing the interfacial contact area between magnesium and oxide, but conclusive evidence is lacking. The samples cooled below the melting point and on continuing heating no evidence of melting of Mg was observed confirming the complete consumption of magnesium during the exotherm. XRD of the DTA products showed MgO, Mg 2 TiO 4 and MgTiO 3 present for both powders, elemental iron was also found for ilmenite. The phases of iron and titanium with the general formula MTiO 3 and M 2 TiO 4 are very similar in their crystallographic dimensions, the magnesium end member has a slightly smaller unit cell and, hence, slightly smaller d-spacings. These phases form a complete solid solution
series between the iron and magnesium end members and mixed cation phases (i.e. Mg x Fe 12x TiO 3 and Mg x Fe 22x TiO 4 ) can be expected with peaks between those of the pure phases. Thus, it would seem likely that there will be interaction between iron and magnesium in these phases and the attribution of the magnesium end member does not exclude some substitution by iron. However, for rutile there is no iron present, other than that from abrasion of the mill and the magnesium compound is assumed. The onset temperature for the main exotherm was found to decrease by ¯208 when the ilmenite (rutile): magnesium ratio increased from 2:1 to 1:3(2) suggesting that the reaction was enhanced slightly by an increase in the interfacial area. DTA of 100 h milled powders, in the same ratio as the mixtures presented in Fig. 2, showed no energetic reactions occurred up to 10008C. Clearly, milling must perform the same function as heating the mixture up to 10008C. Comparison of the XRD of the DTA products showed no difference between the milled and mixed powders indicting they had reached the same stage of reduction by either milling or heating. The phases present in the as-milled powders changed with the ratio of ilmenite to magnesium, Fig. 2. The 1:1 ratio (2a) showed that ilmenite and MgTiO 3 could both be present (the peak positions for these phases overlap considerably), the small peaks at 2u 550 and 528 indicate magnesium oxide and elemental iron respectively. Profile fitting between 30 and 508 showed that a single peak was a better fit than two peaks for all of the peaks. The centre
Fig. 2. Intensity – 2u traces of as-milled powders. (a) 1:1, (b) 1:2 and (c) 1:3 ilmenite:magnesium ratio; (d) 1:1 and (e) 1:2 rutile:magnesium ratio. ♦ – FeTiO 3 , 앳 – MgTiO 3 , d – Fe, j – MgO, . – TiO 2 .
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
point of the fitted peak was between the accepted positions of the corresponding peaks for FeTiO 3 and MgTiO 3 [9] indicating that the ilmenite had undergone some solid-state ion-exchange. The magnesium substituting for the iron within the lattice by reduction then exchange or vice versa forming a compound of the (Fe, Mg)TiO 3 type. It is known that MgO will allow magnesium to substitute for iron in ilmenite [10] so the reaction may be reduction then ion exchange but conclusive evidence is lacking. As the ratio increased (2b and 2c), the MTiO 3 peaks weakened and peaks for iron and magnesium oxide became more intense, this is due to the increasing reduction of ilmenite to iron and an unidentified titanium product. The reduction of titanium from 41 to 31 does not allow formation of the mixed cation titanate but small amounts will almost certainly be present. The absence of an identifiable reduced titanium product maybe due to a lack of crystallinity or the presence of numerous intermediate phases e.g. Ti n O 2n21 (2#n#10) which are insufficiently crystalline or abundant to detect. After milling, neither mixture of rutile and magnesium showed many XRD peaks, Fig. 2d and e, there was only the main rutile peak and the two main peaks for MgO indicating that reduction had occurred. The MgO peaks were much more intense than any other peak implying the main phase present was MgO. After annealing under argon for 1 h at 8008C the grain size of the powders increased considerably, Fig. 3. One additional phase was revealed during annealing and the peaks were close to those of qandilite (Mg 2 TiO 4 ) but at
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slightly lower 2u, suggesting that there was some substitution of iron into this phase. Annealing at 6008C did not reveal the phase suggesting the formation of this compound was thermal rather than mechanical. The position of the peaks for the mixed MTiO 3 phase moved towards those of pure MgTiO 3 as the magnesium content increased, indicating an increasing exchange of magnesium for iron within the ilmenite lattice. Thus, it would seem that annealing involves a thermal reaction in which iron is exchanged from ilmenite forming (Fe, Mg)TiO 3 , which can then react with MgO to form Mg 2 TiO 4 which would probably contain some iron. The relative intensity of the iron peak at 2u 5528 increased with ratio compared with the peak for the titanate at 2u 5388 indicating that the reduction of titanate formed iron as a product. The main MgO peak is overlapped by both iron and the M 2 TiO 4 phase but its intensity also increased as the titanate peak decreased implying that reactions 1 and 2 were either a single direct step, 5, or, reaction 1 was rate limiting. FeTiO 3 1 2Mg ⇒ 2MgO 1 TiO 1 Fe
(5)
Annealing did not have the desired effect of crystallising the phases present after milling the rutile powders. The small rutile peak disappeared, the MgO peaks weakened and peaks for MgTiO 3 and Mg 2 TiO 4 appeared indicating a thermal reaction had occurred. A mixture of MgO and TiO 2 was annealed for 4 h at 10008C and the reaction was found to be slight, there were small amounts of MgTi 2 O 5
Fig. 3. Intensity – 2u traces of powders annealed at 8008C for 1 h. Legend as in Fig. 2, plus m – Fe 2 TiO 4 and n – Mg 2 TiO 4 .
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N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
but no MgTiO 3 . Whereas, the same mixture milled for 100 h then heated showed MgTiO 3 and Mg 2 TiO 4 to be the products. It was indicated that milling aided intimate mixing of the system and the solid-state diffusion of MgO into TiO 2 was the controlling step in the formation of MgTiO 3 from oxide with MgTi 2 O 5 thought to be an intermediate phase [3]. Therefore, it seems likely that the as-milled powder contained either intimately mixed MgO and TiO 2 or nanocrystalline MgTiO 3 which underwent grain growth on annealing. The leaching results for the as-milled powders, summarised in Table 1, show that the greater the fraction of magnesium the greater the solubility, both overall and of titanium. XRD of the leached powders showed that leaching removed only the dominant MgO (and Fe) peaks and enhanced smaller peaks, however, no new peaks were revealed. In all cases the solubility of the material milled with magnesium is considerably better than either ilmenite or rutile milled alone. Examination of the XRD traces showed that the ilmenite and rutile milled alone had broader, less intense peaks indicating that the crystallinity was lower when milled in the absence of magnesium. It has been shown elsewhere that ilmenite leaches most readily when the crystallite size is small [5]. After milling with magnesium the leaching is more extensive, indicating the transformation of ilmenite and rutile into a more soluble form during milling. The continued presence of titanium bearing phases in the leach residues would indicate that there was insufficient magnesium within the system for the full extent of possible reduction to occur. The rutile leaches showed a good correlation between the mass leached and titanium leached implying that the dissolution was congruent i.e. all of the elements in the powder were leaching at the same rate. This could indicate that there was a single compound containing all three elements in the original ratio, however, a mixture of TiO and MgO (by reaction 4) has the same composition as the starting material and both are known to be soluble in dilute acid. There was no apparent congruence after any ilmenite milling, with enhanced titanium solubilisation evident in all four ratios. The incongruent dissolution of ilmenite is
clear confirmation that the iron and titanium had been separated during reaction. A similar incongruence has been noted after milling with magnesium oxide [10]. The solid leach residues were annealed for 1 h at 8008C under argon, for ilmenite the positions of the peaks present did not change with increasing Mg, unlike the unleached powder. Clearly the unleached samples undergo thermal reaction during annealing to change the composition of the (Fe, Mg)TiO 3 phase towards the MgTiO 3 rich end. For rutile the 1:1 ratio showed the presence of Ti 3 O 5 as the major phase and MgTiO 3 , no other phase was identified. For the 1:2 ratio the peaks for rutile narrowed and intensified indicating crystallisation had occurred. Peaks for MgTiO 3 appeared indicating it was formed during milling (as shown previously [10]), it cannot have been formed during annealing as all magnesium and MgO had been leached away. Annealing also showed several previously unseen peaks and these were attributed to Ti 3 O 5 . The relative intensity of the main peaks of the three phases suggested that rutile was the most abundant with less Ti 3 O 5 and a much smaller fraction of MgTiO 3 . The presence of the metal titanate after all leaches and the decreasing fraction of this phase with increasing magnesium content coupled with the increasing titanium solubility confirm the insolubility of this phase. The presence of rutile would suggest that insufficient Mg was present for the reduction of this phase to soluble phases. For rutile, the presence of Ti 3 O 5 after leaching would indicate that this phase was also of limited solubility, the low solubility of this phase has been noted previously [11]. The reductive power of magnesium is sufficient to reduce ilmenite and rutile at ambient temperature to phases which are soluble. The high solubility of both systems directly after milling would seem to contradict the results obtained for the carbothermic reduction of ilmenite and rutile [11]. Reduction with carbon has been shown to form a range of titanium oxides of the general formula Tin O 2n21 n$2 depending upon the temperature. Subsequent dissolution in 3% HCl indicted .90% of iron was leached whereas, ,10% of titanium went into solution, clearly, none of the titanium oxides formed was particularly soluble. However, the dissolution of magnesium reduced ilmenite and rutile is much higher which implies that the
Table 1 Mass leached and titanium leached for as-milled powders Mixture
% mass leached
% Ti leached
Phases after leaching
Ilmenite 2:1 ilmenite:Mg 1:1 ilmenite:Mg 1:2 ilmenite:Mg 1:3 ilmenite:Mg rutile 1:1 rutile:Mg 1:2 rutile:Mg
7 42 78 86 92 8 22 80
7 50 91 94 99 7 20 78
FeTiO 3 MTiO 3 MTiO 3 , TiO 2 MTiO 3 , TiO 2 MTiO 3 , TiO 2 TiO 2 Ti 3 O 5 , MgTiO 3 TiO 2 , Ti 3 O 5 , MgTiO 3
Phases identified in the leach residue after annealing for 1 h under argon at 8008C.
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
titanium had been reduced beyond Ti 2 O 3 to a highly soluble form. The predicted product was titanium metal (reaction 3), however, this is not too reactive towards dilute HCl with 1% acid only attacking strongly at its boiling point and hardly at all at 258C [1]. Additionally, the presence of iron(III) in solution was shown to decrease the rate of corrosion hundredfold. Thus it seems titanium metal could not have been formed. An intermediate phase predicted was titanium monoxide, TiO, this compound is known to be soluble in dilute HCl, it is a very powerful reductant and oxidises rapidly to higher valence by reducing water to form hydrogen [12]. The presence of trivalent titanium (indicated by a violet solution) was noted for all leach solutions except those with the lowest ratio of oxide: Mg where solubility was the lowest. In the absence of crystallographic confirmation, all of the evidence points to the presence of TiO in as-milled powders. There was no evidence of TiO being present after carbothermic reduction, the sequence passed from Ti 3 O 5 (or Ti 2 O 3 ) directly to TiC, thus adding indirect evidence for TiO being the soluble phase present. Additionally, reduction of rutile by magnesium at low temperatures has been shown to form TiO as one of the reduced titanium phases [1]. The absence of titanium metal after milling under these conditions implies that there is an activation barrier present for the final reduction of TiO. The high solubility of titanium after milling ilmenite, coupled with the implied tendency of iron to remain in the solid phase may provide a method of selective solubilisation of titanium from ilmenite. The high solubility achieved with a relatively small fraction of magnesium is attractive for the production of titanium salts from solution. The standard multistep processes for titanium salt formation from ilmenite consist of reduction, leaching of the iron, chlorination, purification, dissolution and salt formation. It may be possible to form the salts directly from the liquor obtained direct after leaching the milled product with careful control of chemical conditions to prevent impurity precipitation. In the cases presented, there is a slight deficit of metallic magnesium due to the presence of Mg(OH) 2 , which does not seem to interfere significantly in the process. The highest magnesium content powders examined showed the presence of phases which were reducible indicating more magnesium was required for completion of the reduction. Indeed, the addition of quantities of Mg, in excess of the stoichiometric requirement, may lead to direct formation of Mg–Ti alloys, or the direct formation of ferro–titanium but the apparent absence of Ti metal in these experiments does not augur well.
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4. Conclusions The direct reduction of ilmenite by magnesium can be carried out within a ball mill. The reaction sequence is the concomitant formation of MgTiO 3 and reduction of iron, followed by further reduction to Ti 3 O 5 and other, soluble phases, most probably TiO. For all of the ratios examined there was no evidence of magnesium metal remaining after milling indicating that the fullest possible reduction had occurred and that the magnesium content was insufficient due to impurities. Annealing of the powders showed increased crystallinity of all phases and the appearance of a phase of general formula (Fe, Mg) 2 TiO 4 indicating that the powder underwent thermal reaction. The solubility of the powders increased with magnesium content and, hence, extent of reduction with titanium solubilities of .90% achieved from ilmenite.
Acknowledgements The author would like to thank Westralian Sands Ltd. of Capel, WA and the Australian Government for their partial financial support through a GIRD grant.Acknowledgment is also made to Tanya Hwang for performing the thermal analyses and to Michael Marsh for aiding with the milling.
References [1] A.D. McQuillan, M.K. McQuillan, Titanium, Butterworths, London, 1956, p. 439. [2] N.J. Welham, Minerals Eng. 9 (1996) 1189. [3] N.J. Welham, J. Mater. Res., 13(6) (1998) in press. [4] A.I. Vogel, Quantitative Inorganic Analysis, (Longman, London, 1961) 3rd ed., p. 788. [5] N.J. Welham, Trans. IMM, in press. [6] C. Duval, Inorganic Thermogravimetric Analysis, Elsevier, Amsterdam, 1963, 2nd ed., p. 214. [7] J. Liao, M. Senna, Mater. Res. Bull. 30 (1995) 385. [8] K. Hamada, T. Isobe, M. Senna, J. Mat. Sci. Let. 15 (1996) 603. [9] Powder Diffraction File, FeTiO 3 card no. 29-733, MgTiO 3 card no. 6-494. [10] N.J. Welham, J. Mat. Sci. 33 (1998) 1795. [11] N.J. Welham et al., The carbothermic reduction of rutile, to be submitted. [12] N.V. Sidgwick, The Chemical Elements and their Compounds vol. 1, Clarendon Press, Oxford, 1950, p. 651.
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Mechanically induced reduction of ilmenite (FeTiO 3 ) and rutile (TiO 2 ) by magnesium N.J. Welham* Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia Received 25 February 1998
Abstract Ilmenite (FeTiO 3 ) and rutile (TiO 2 ) have been ball milled with various ratios of metallic magnesium for 100 h in a laboratory mill. The reduction of both oxides was evident directly after milling with substantial formation of MgO. Ilmenite also formed a mixed cation titanate (Mg, Fe)TiO 3 and elemental iron, no reduced phases were observed for rutile. There was no difference between the products obtained by thermal reaction and milling, although energetic reactions observed in milled powder by DTA were not seen in the milled sample and there was no obvious crystallisation of a reduced titanium phase. Solubilisation of titanium from milled powders was found to increase with magnesium content and was .90% for a starting ratio of 1: $1 ilmenite: Mg. The unleached material was composed of incompletely reduced phases such as TiO 2 , MTiO 3 and Ti 3 O 5 . 1998 Elsevier Science S.A. Keywords: Ilmenite; Reduction; Magnesium; Rutile
1. Introduction Ilmenite is the primary source of titanium compounds and several processing steps need to be performed before a suitable product is obtained. The mineral is purified by reduction to TiO 2 and elemental iron which is then leached to form an impure TiO 2 . The 88–92% TiO 2 is then chlorinated using petroleum coke as the reductant and the impurities separated by fractional distillation. For pigment TiO 2 the chloride is reoxidised and the chlorine recycled, titanium chemicals are produced from a solution of the chloride in acid. To produce metallic titanium the chloride is reacted with molten magnesium under an inert atmosphere, the MgCl 2 and remaining magnesium are leached and distilled away respectively from the titanium sponge, which is consolidated by arc melting. The use of magnesium as the reductant for rutile results in a metal containing 2.5 wt % oxygen [1] which has to be refined by vacuum melting to produce an acceptable metal. Indeed, no process for titanium metal manufacture using either rutile or more complex oxides has been commercially exploited. There is no obvious constraint on using magnesium metal as a direct reductant for ilmenite other than the *Fax: 161 2 62490511; e-mail: [email protected] 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00526-X
increase in magnesium consumption over TiO 2 . A reduction in the number of process steps may be enough to justify the use of magnesium directly. A process which converts directly from ilmenite to metal could afford potential savings in time and energy, despite the necessity to further refine the metal. Alternatively, the chemical market could be targeted by providing a high solubility of titanium without a thermal step. Mechanochemical processing has been shown to both decrease the time and temperature required for the carbothermic reduction of ilmenite [2] and induce reactions within the mill that could not be achieved without thermal processing [3]. Thus, this paper examines the feasibility of using mechanical activation to induce ilmenite and rutile reduction by magnesium.
2. Experimental The ilmenite [2] and rutile [3] were the same as that used previously. The magnesium used was nominally .99% pure but was found to contain a small fraction of Mg(OH) 2 . The ilmenite: magnesium ratios examined were 1:1, 1:2 and 1:3, these were chosen on the basis of thermodynamic predictions, summarised by reactions 1–3.
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
FeTiO 3 1 Mg ⇒ MgTiO 3 1 Fe
(1)
MgTiO 3 1 Mg ⇒ 2MgO 1 TiO
(2)
TiO 1 Mg ⇒ MgO 1 Ti
(3)
Rutile was predicted to be reduced to TiO, and then to titanium metal by reaction 3. TiO 2 1 Mg ⇒ TiO 1 MgO
(4)
Thus, two rutile: magnesium ratios were examined, 1:1 and 1:2. The 6.00 g samples were milled under vacuum for up to 100 h at room temperature in a vertical 316S stainless steel ball mill using four 10 (25.4 mm) diameter 420C stainless steel balls confined in the vertical plane. Ball motion was controlled by the positioning of magnets radially round the mill. Samples of milled powders were annealed under a controlled atmosphere to determine the effect of thermal treatment of the milled powders. Samples of as-milled powder were leached for 12 h in 3% hydrochloric acid at room temperature to determine whether the reductive process would enhance the solubility of ilmenite and rutile. The titanium in solution was measured by spectrophotometry using the standard peroxide method [4]. Ilmenite and rutile powders (pre-milled for 10 h to reduce the particle size [5]) were mixed thoroughly with the highest ratio of magnesium examined (1:3 and 1:2
261
respectively) and then annealed under identical conditions to the milled powders. Selected powders were also analysed by differential thermal analysis (DTA) to detect energetic reactions (both endothermic and exothermic) during the heating process. Samples of |30 mg were heated to 10008C at 208C min 21 under argon in a Shimadzu DTA-50. The products were analysed by X-ray diffraction (XRD) ˚ with monochromatised CoKa radiation ( l 51.78896 A) using a count time of 2 s per 0.18 step size.
3. Results and discussion The two mixtures showed no reaction after mixing, as expected. DTA of the ilmenite / magnesium and rutile / magnesium mixtures are shown in Fig. 1 with both systems showing an endotherm with onset at |3208C. This was determined to be the dehydration of Mg(OH) 2 present in the magnesium, agreeing with work on pure Mg(OH) 2 [6–8]. The dehydration endotherm was followed by an exotherm which was attributed to a slow reaction between the oxide and magnesium. The ilmenite was more exothermic than the rutile suggesting the reaction was comparatively facile and may represent the partial reduction of ilmenite by reaction 1. XRD of powders annealed at 5008C for 1 h showed a slight reaction had occurred for ilmenite, with small peaks for elemental iron and mag-
Fig. 1. DTA profile of the 1:3 ilmenite:magnesium and 1:2 rutile:magnesium mixtures, argon atmosphere, 208C min 21 .
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N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
nesium oxide evident. The magnesium oxide was partially due to the dehydration of the Mg(OH) 2 but the presence of elemental iron is clear indication of a reduction reaction. Rutile also showed peaks for MgO but not MgTiO 3 so it cannot be confirmed that reaction occurred. For both mixtures there was a very large exotherm with onset at 5908C and 6108C for ilmenite and rutile respectively, these were due to one or more of the reduction reactions 1–4. The absence of further energetic events would imply that the main exotherm was due to the completion of the reduction process. The onset of reaction is below the melting point of magnesium (6508C) indicating that the initial reaction was solid state. However, because of the extremely exothermic nature of the reaction(s) the temperature in the crucible rose above that necessary to melt the magnesium for several seconds. This may have caused an acceleration of the reaction by increasing the interfacial contact area between magnesium and oxide, but conclusive evidence is lacking. The samples cooled below the melting point and on continuing heating no evidence of melting of Mg was observed confirming the complete consumption of magnesium during the exotherm. XRD of the DTA products showed MgO, Mg 2 TiO 4 and MgTiO 3 present for both powders, elemental iron was also found for ilmenite. The phases of iron and titanium with the general formula MTiO 3 and M 2 TiO 4 are very similar in their crystallographic dimensions, the magnesium end member has a slightly smaller unit cell and, hence, slightly smaller d-spacings. These phases form a complete solid solution
series between the iron and magnesium end members and mixed cation phases (i.e. Mg x Fe 12x TiO 3 and Mg x Fe 22x TiO 4 ) can be expected with peaks between those of the pure phases. Thus, it would seem likely that there will be interaction between iron and magnesium in these phases and the attribution of the magnesium end member does not exclude some substitution by iron. However, for rutile there is no iron present, other than that from abrasion of the mill and the magnesium compound is assumed. The onset temperature for the main exotherm was found to decrease by ¯208 when the ilmenite (rutile): magnesium ratio increased from 2:1 to 1:3(2) suggesting that the reaction was enhanced slightly by an increase in the interfacial area. DTA of 100 h milled powders, in the same ratio as the mixtures presented in Fig. 2, showed no energetic reactions occurred up to 10008C. Clearly, milling must perform the same function as heating the mixture up to 10008C. Comparison of the XRD of the DTA products showed no difference between the milled and mixed powders indicting they had reached the same stage of reduction by either milling or heating. The phases present in the as-milled powders changed with the ratio of ilmenite to magnesium, Fig. 2. The 1:1 ratio (2a) showed that ilmenite and MgTiO 3 could both be present (the peak positions for these phases overlap considerably), the small peaks at 2u 550 and 528 indicate magnesium oxide and elemental iron respectively. Profile fitting between 30 and 508 showed that a single peak was a better fit than two peaks for all of the peaks. The centre
Fig. 2. Intensity – 2u traces of as-milled powders. (a) 1:1, (b) 1:2 and (c) 1:3 ilmenite:magnesium ratio; (d) 1:1 and (e) 1:2 rutile:magnesium ratio. ♦ – FeTiO 3 , 앳 – MgTiO 3 , d – Fe, j – MgO, . – TiO 2 .
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
point of the fitted peak was between the accepted positions of the corresponding peaks for FeTiO 3 and MgTiO 3 [9] indicating that the ilmenite had undergone some solid-state ion-exchange. The magnesium substituting for the iron within the lattice by reduction then exchange or vice versa forming a compound of the (Fe, Mg)TiO 3 type. It is known that MgO will allow magnesium to substitute for iron in ilmenite [10] so the reaction may be reduction then ion exchange but conclusive evidence is lacking. As the ratio increased (2b and 2c), the MTiO 3 peaks weakened and peaks for iron and magnesium oxide became more intense, this is due to the increasing reduction of ilmenite to iron and an unidentified titanium product. The reduction of titanium from 41 to 31 does not allow formation of the mixed cation titanate but small amounts will almost certainly be present. The absence of an identifiable reduced titanium product maybe due to a lack of crystallinity or the presence of numerous intermediate phases e.g. Ti n O 2n21 (2#n#10) which are insufficiently crystalline or abundant to detect. After milling, neither mixture of rutile and magnesium showed many XRD peaks, Fig. 2d and e, there was only the main rutile peak and the two main peaks for MgO indicating that reduction had occurred. The MgO peaks were much more intense than any other peak implying the main phase present was MgO. After annealing under argon for 1 h at 8008C the grain size of the powders increased considerably, Fig. 3. One additional phase was revealed during annealing and the peaks were close to those of qandilite (Mg 2 TiO 4 ) but at
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slightly lower 2u, suggesting that there was some substitution of iron into this phase. Annealing at 6008C did not reveal the phase suggesting the formation of this compound was thermal rather than mechanical. The position of the peaks for the mixed MTiO 3 phase moved towards those of pure MgTiO 3 as the magnesium content increased, indicating an increasing exchange of magnesium for iron within the ilmenite lattice. Thus, it would seem that annealing involves a thermal reaction in which iron is exchanged from ilmenite forming (Fe, Mg)TiO 3 , which can then react with MgO to form Mg 2 TiO 4 which would probably contain some iron. The relative intensity of the iron peak at 2u 5528 increased with ratio compared with the peak for the titanate at 2u 5388 indicating that the reduction of titanate formed iron as a product. The main MgO peak is overlapped by both iron and the M 2 TiO 4 phase but its intensity also increased as the titanate peak decreased implying that reactions 1 and 2 were either a single direct step, 5, or, reaction 1 was rate limiting. FeTiO 3 1 2Mg ⇒ 2MgO 1 TiO 1 Fe
(5)
Annealing did not have the desired effect of crystallising the phases present after milling the rutile powders. The small rutile peak disappeared, the MgO peaks weakened and peaks for MgTiO 3 and Mg 2 TiO 4 appeared indicating a thermal reaction had occurred. A mixture of MgO and TiO 2 was annealed for 4 h at 10008C and the reaction was found to be slight, there were small amounts of MgTi 2 O 5
Fig. 3. Intensity – 2u traces of powders annealed at 8008C for 1 h. Legend as in Fig. 2, plus m – Fe 2 TiO 4 and n – Mg 2 TiO 4 .
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N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
but no MgTiO 3 . Whereas, the same mixture milled for 100 h then heated showed MgTiO 3 and Mg 2 TiO 4 to be the products. It was indicated that milling aided intimate mixing of the system and the solid-state diffusion of MgO into TiO 2 was the controlling step in the formation of MgTiO 3 from oxide with MgTi 2 O 5 thought to be an intermediate phase [3]. Therefore, it seems likely that the as-milled powder contained either intimately mixed MgO and TiO 2 or nanocrystalline MgTiO 3 which underwent grain growth on annealing. The leaching results for the as-milled powders, summarised in Table 1, show that the greater the fraction of magnesium the greater the solubility, both overall and of titanium. XRD of the leached powders showed that leaching removed only the dominant MgO (and Fe) peaks and enhanced smaller peaks, however, no new peaks were revealed. In all cases the solubility of the material milled with magnesium is considerably better than either ilmenite or rutile milled alone. Examination of the XRD traces showed that the ilmenite and rutile milled alone had broader, less intense peaks indicating that the crystallinity was lower when milled in the absence of magnesium. It has been shown elsewhere that ilmenite leaches most readily when the crystallite size is small [5]. After milling with magnesium the leaching is more extensive, indicating the transformation of ilmenite and rutile into a more soluble form during milling. The continued presence of titanium bearing phases in the leach residues would indicate that there was insufficient magnesium within the system for the full extent of possible reduction to occur. The rutile leaches showed a good correlation between the mass leached and titanium leached implying that the dissolution was congruent i.e. all of the elements in the powder were leaching at the same rate. This could indicate that there was a single compound containing all three elements in the original ratio, however, a mixture of TiO and MgO (by reaction 4) has the same composition as the starting material and both are known to be soluble in dilute acid. There was no apparent congruence after any ilmenite milling, with enhanced titanium solubilisation evident in all four ratios. The incongruent dissolution of ilmenite is
clear confirmation that the iron and titanium had been separated during reaction. A similar incongruence has been noted after milling with magnesium oxide [10]. The solid leach residues were annealed for 1 h at 8008C under argon, for ilmenite the positions of the peaks present did not change with increasing Mg, unlike the unleached powder. Clearly the unleached samples undergo thermal reaction during annealing to change the composition of the (Fe, Mg)TiO 3 phase towards the MgTiO 3 rich end. For rutile the 1:1 ratio showed the presence of Ti 3 O 5 as the major phase and MgTiO 3 , no other phase was identified. For the 1:2 ratio the peaks for rutile narrowed and intensified indicating crystallisation had occurred. Peaks for MgTiO 3 appeared indicating it was formed during milling (as shown previously [10]), it cannot have been formed during annealing as all magnesium and MgO had been leached away. Annealing also showed several previously unseen peaks and these were attributed to Ti 3 O 5 . The relative intensity of the main peaks of the three phases suggested that rutile was the most abundant with less Ti 3 O 5 and a much smaller fraction of MgTiO 3 . The presence of the metal titanate after all leaches and the decreasing fraction of this phase with increasing magnesium content coupled with the increasing titanium solubility confirm the insolubility of this phase. The presence of rutile would suggest that insufficient Mg was present for the reduction of this phase to soluble phases. For rutile, the presence of Ti 3 O 5 after leaching would indicate that this phase was also of limited solubility, the low solubility of this phase has been noted previously [11]. The reductive power of magnesium is sufficient to reduce ilmenite and rutile at ambient temperature to phases which are soluble. The high solubility of both systems directly after milling would seem to contradict the results obtained for the carbothermic reduction of ilmenite and rutile [11]. Reduction with carbon has been shown to form a range of titanium oxides of the general formula Tin O 2n21 n$2 depending upon the temperature. Subsequent dissolution in 3% HCl indicted .90% of iron was leached whereas, ,10% of titanium went into solution, clearly, none of the titanium oxides formed was particularly soluble. However, the dissolution of magnesium reduced ilmenite and rutile is much higher which implies that the
Table 1 Mass leached and titanium leached for as-milled powders Mixture
% mass leached
% Ti leached
Phases after leaching
Ilmenite 2:1 ilmenite:Mg 1:1 ilmenite:Mg 1:2 ilmenite:Mg 1:3 ilmenite:Mg rutile 1:1 rutile:Mg 1:2 rutile:Mg
7 42 78 86 92 8 22 80
7 50 91 94 99 7 20 78
FeTiO 3 MTiO 3 MTiO 3 , TiO 2 MTiO 3 , TiO 2 MTiO 3 , TiO 2 TiO 2 Ti 3 O 5 , MgTiO 3 TiO 2 , Ti 3 O 5 , MgTiO 3
Phases identified in the leach residue after annealing for 1 h under argon at 8008C.
N. J. Welham / Journal of Alloys and Compounds 274 (1998) 260 – 265
titanium had been reduced beyond Ti 2 O 3 to a highly soluble form. The predicted product was titanium metal (reaction 3), however, this is not too reactive towards dilute HCl with 1% acid only attacking strongly at its boiling point and hardly at all at 258C [1]. Additionally, the presence of iron(III) in solution was shown to decrease the rate of corrosion hundredfold. Thus it seems titanium metal could not have been formed. An intermediate phase predicted was titanium monoxide, TiO, this compound is known to be soluble in dilute HCl, it is a very powerful reductant and oxidises rapidly to higher valence by reducing water to form hydrogen [12]. The presence of trivalent titanium (indicated by a violet solution) was noted for all leach solutions except those with the lowest ratio of oxide: Mg where solubility was the lowest. In the absence of crystallographic confirmation, all of the evidence points to the presence of TiO in as-milled powders. There was no evidence of TiO being present after carbothermic reduction, the sequence passed from Ti 3 O 5 (or Ti 2 O 3 ) directly to TiC, thus adding indirect evidence for TiO being the soluble phase present. Additionally, reduction of rutile by magnesium at low temperatures has been shown to form TiO as one of the reduced titanium phases [1]. The absence of titanium metal after milling under these conditions implies that there is an activation barrier present for the final reduction of TiO. The high solubility of titanium after milling ilmenite, coupled with the implied tendency of iron to remain in the solid phase may provide a method of selective solubilisation of titanium from ilmenite. The high solubility achieved with a relatively small fraction of magnesium is attractive for the production of titanium salts from solution. The standard multistep processes for titanium salt formation from ilmenite consist of reduction, leaching of the iron, chlorination, purification, dissolution and salt formation. It may be possible to form the salts directly from the liquor obtained direct after leaching the milled product with careful control of chemical conditions to prevent impurity precipitation. In the cases presented, there is a slight deficit of metallic magnesium due to the presence of Mg(OH) 2 , which does not seem to interfere significantly in the process. The highest magnesium content powders examined showed the presence of phases which were reducible indicating more magnesium was required for completion of the reduction. Indeed, the addition of quantities of Mg, in excess of the stoichiometric requirement, may lead to direct formation of Mg–Ti alloys, or the direct formation of ferro–titanium but the apparent absence of Ti metal in these experiments does not augur well.
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4. Conclusions The direct reduction of ilmenite by magnesium can be carried out within a ball mill. The reaction sequence is the concomitant formation of MgTiO 3 and reduction of iron, followed by further reduction to Ti 3 O 5 and other, soluble phases, most probably TiO. For all of the ratios examined there was no evidence of magnesium metal remaining after milling indicating that the fullest possible reduction had occurred and that the magnesium content was insufficient due to impurities. Annealing of the powders showed increased crystallinity of all phases and the appearance of a phase of general formula (Fe, Mg) 2 TiO 4 indicating that the powder underwent thermal reaction. The solubility of the powders increased with magnesium content and, hence, extent of reduction with titanium solubilities of .90% achieved from ilmenite.
Acknowledgements The author would like to thank Westralian Sands Ltd. of Capel, WA and the Australian Government for their partial financial support through a GIRD grant.Acknowledgment is also made to Tanya Hwang for performing the thermal analyses and to Michael Marsh for aiding with the milling.
References [1] A.D. McQuillan, M.K. McQuillan, Titanium, Butterworths, London, 1956, p. 439. [2] N.J. Welham, Minerals Eng. 9 (1996) 1189. [3] N.J. Welham, J. Mater. Res., 13(6) (1998) in press. [4] A.I. Vogel, Quantitative Inorganic Analysis, (Longman, London, 1961) 3rd ed., p. 788. [5] N.J. Welham, Trans. IMM, in press. [6] C. Duval, Inorganic Thermogravimetric Analysis, Elsevier, Amsterdam, 1963, 2nd ed., p. 214. [7] J. Liao, M. Senna, Mater. Res. Bull. 30 (1995) 385. [8] K. Hamada, T. Isobe, M. Senna, J. Mat. Sci. Let. 15 (1996) 603. [9] Powder Diffraction File, FeTiO 3 card no. 29-733, MgTiO 3 card no. 6-494. [10] N.J. Welham, J. Mat. Sci. 33 (1998) 1795. [11] N.J. Welham et al., The carbothermic reduction of rutile, to be submitted. [12] N.V. Sidgwick, The Chemical Elements and their Compounds vol. 1, Clarendon Press, Oxford, 1950, p. 651.