2017 Volume 58 Issue 3 Pages 355-360
A two-step thermochemical reduction was developed to remove oxygen from commercial titanium dioxide, TiO2, producing metallic titanium powder. The first step was to remove >95% of the oxygen by Mg reduction in a hydrogen atmosphere, followed by a de-oxygenation step. The goal is to produce Ti powder that meets the standard specifications for titanium. Several ancillary steps including granulation and heat treatment were introduced to modify the powder morphology, particle size and powder density to make the powder suitable for a range of applications. Detailed compositional analysis indicated that the final product meets the ASTM B299 for general-purpose titanium sponge. The powder can be irregularly shaped or spherical with different granulation treatments.
Titanium is a high performance light metal, used in various areas owing to its high specific strength, exceptional corrosion resistance, and biocompatibility; however, it is too expensive for many industrial or consumer applications1–3). The high cost of titanium is partially related to attributes of the Kroll process (long processing time, batch-type processing and high-energy consumption4), as well as being associated with the machining of wrought products to their final configurations5). It has been recognized that substantial savings for titanium can be realized by the combination of improved extraction processes and powder metallurgy manufacturing techniques6). Therefore most emerging or newly developed processes, such as the FCC7–10), Armstrong (ITP)11–13), TIRO14–16), CSIR-Ti process17–19), MER process20), USTB electrolysis process21,22), and the DRTS process23,24), that are attempting to replace the Kroll process, have been seeking ways to produce powders instead of sponge for application in powder metallurgy.
Among them, TIRO, Armstrong, and the CSIR process use TiCl4 as the precursor to produce Ti metal, having the advantage that all the undesired impurities in TiCl4 can be removed by distillation, which enables the production of highly purified Ti metal. However, TiCl4 is highly corrosive, very volatile and has a boiling point of only 136.4℃, causing process complications when reacting at elevated temperatures with Mg or Na reductants to form titanium molten MgCl2 or NaCl salt, or solid salt depending on the reaction temperature17). In addition, from TiCl4 to Ti is a liquid-to-solid process, which makes it difficult to control the morphology and particle size of the final product, and fine powders with high specific surface area can be easily produced, generally leading to higher light element contamination, particularly by oxygen.
Alternatively, titanium can also be produced by using TiO2 as the precursor. Compared to using TiCl4 as a precursor, TiO2 is safe to work with and readily transported and hence it is less essential to have a titanium metal plant and a titanium precursor production plant in proximity to each other. TiO2 can alternatively be produced via sulfates or other processes instead of via TiCl4, which may be preferred in some conditions. The process of TiO2 to Ti is a solid-to-solid process that gives a greater flexibility for controlling the morphology and particle size of the final powder. Additionally, most of the other elements used in titanium alloys are commercially available as oxides, and can be blended with TiO2 and then reduced simultaneously to make titanium alloys directly. A similar approach with common metal chlorides, like AlCl3 and VCl4 would be difficult or untenable. However, using TiO2 as precursor is more difficult to reduce the oxygen content to commercially acceptable levels than to reduce chlorine levels, because in the case of oxygen, not only oxides, but also a dissolved oxygen content in the titanium of up to 14.3 mass% must be reduced. Titanium has a high chemical affinity to oxygen, which is apparent from the Ti-O bonding energy of 2.12 eV, that is comparable to the Ti-Ti bonding energy of 2.56 eV25). The oxides that are produced as by-products in metallothermic reduction processes have much higher melting points than the chlorides produced when reducing TiCl4, making the design of a suitable continuous-flow reactor for the process more complex.
Thermochemical extraction of titanium from titanium oxide has been extensively studied for several decades. Based on the Ellingham diagram, magnesium and calcium are the two possible reductants. For magnesium, there is a reducing limit (～2 mass% oxygen left in titanium at 750℃) when reduction is done in Ar. The lowest oxygen achieved in the literature has been 1.5 mass%26), which is still far from the standard specification for titanium. Therefore, most of the thermochemical reduction of TiO2 has been conducted by using calcium. The calciothermic processes reported have included the MHR process27), the O.S. process28), the EMR process29), and the PRP process30). These processes all require the excess use of calcium metal, which is quite expensive, as well as high reaction temperatures (>1123 K). The high reduction temperature leads to significant sintering between Ti powders, making the subsequent leaching extremely difficult. The purity of the final product tends to be not consistent, with the oxygen level ranging from 0.1% to 1 mass%, and the calcium level ranging from 0.1 mass% to 0.8 mass%31). In addition, powder particles made by high temperature calciothermic reduction have a spongy morphology and consist of small particles fused together during the reaction process27).
Recently, a new approach was reported by Fang et al for the direct reduction of Ti-slag using Mg23). The Mg reduction is carried out in a hydrogen atmosphere to deliberately form titanium hydride (TiH2). Mg was chosen as the reducing agent, the same as the Kroll process, in part because it is a less expensive reducing agent than calcium. The impurities in the Ti-slag and the by-product of the reduction (MgO) are removed by leaching the reduction product, which consists of TiH2, MgO, and the impurities. However, a challenge for this approach is to prevent TiH2 from being dissolved and lost during the leaching process. Therefore, the alternative is to use purified TiO2 as the precursor as reported by Zhang et al24,32). Furthermore, the Mg reduction is followed by a deoxygenation step to ensure the oxygen content in the final product is sufficiently low, less than 0.15% as required by ASTM B299 for general purpose Ti. The deoxygenation can be carried out using either Ca33) or Mg as the reducing agent34). When Mg is used, the use of hydrogen atmosphere is necessary because hydrogen destabilizes Ti-O solid solutions, making it possible to reduce oxygen content in Ti to very low levels (<0.1%) by reacting it with Mg34). The combination of the hydrogen assisted magnesium reduction and deoxygenation (HAMR) process, is an integrated approach that made it possible to produce Ti powders that meet the ASTM specification for Ti.
To produce such powders for a range of applications, particle size and morphology are also very critical in additional to chemical compositions. These properties affect bulk density, flow-ability and specific surface area, as well as powder processing properties like packing density, compact green density, and sintering behavior. The particle size and morphology depends on many factors including the preparation of the precursor material, the exact processing routes, and the process parameters for the reduction and deoxygenaiton. However, the effects of these process details are yet to be studied and reported for the HAMR process. In this study, the focus is to investigate the dependence of particle size and morphology on the processing routes and parameters. Commercial TiO2 was used as raw materials. Several steps including granulation and heat treatment were introduced to modify the powder morphology, and particle size. The chemical composition, particle size and morphology were characterized in detail. The results demonstrate that HAMR process is a flexible method for producing Ti powders with controlled particle size and morphologies.
Raw materials used in this study included: TiO2 powder (0.5–2 μm, >99.5% purity, provided by Kronos), MgCl2 (>99% purity, purchased from Sigma Aldrich), and Mg powder (<250 μm, >99.5 mass% purity, purchased from Sigma Aldrich). The amount of Mg used was calculated according to the oxygen content in stoichiometric MgO, plus an excess to push a chemical equilibrium forward to create more desired products and compensate for possible losses at high temperatures. The amount of MgCl2 can be in the range of 30% to 100% the mass of the powder to be treated, and 60% was used in this study.
2.2 Processing routesDifferent processing routes were investigated, as shown in Fig. 1. In route I, TiO2 was directly reduced without any pre-treatment. After reduction and leaching, the fine powder underwent a sintering process at >1000℃ to make a dense and brittle chunk of material, which was then crushed into small particles. The crushed powder was then deoxygenated to further reduce and insure the oxygen fell into the range of standard specification. In route II, a special granulation and sintering process was introduced before reduction, fine TiO2 was first granulated and sintered to make coarse TiO2 powder with a desired particle size and size distributions. Granulation in this route was can be done by mixing TiO2 with a binder and then broken into granules. Sintering of the granulated TiO2 can be done at 1400℃ for a number of hours under air. Heat treatment of the reduced and leached powder can be done at temperatures greater than 900℃ to reduce specific surface areas of the powder and control the particle size. A deoxygenation step was followed to further remove the oxygen to meet the standard specification for titanium. In route III, the sequence of reduction and granulation was reversed. Granulation was done using spray drying to form spherical granulaes. Details of the processes for spray drying, debinding and sintering can be seen in a previously published work35). The other steps are similar to route II.
Processing routes investigated for making Ti powder.
TiO2, Mg powder and the anhydrous MgCl2 salt were mixed at particular ratios in a tubular mixer for 5 min, put into a molybdenum lined stainless steel crucible, transferred to a tube furnace and then heated at 10℃/min to 750℃ in a flowing H2 atmosphere (1 L/min), isothermally held for 12 h, and then cooled to room temperature at 10℃/min. The reduced product was leached with dilute hydrochloric acid solution to remove the water-soluble salt, by product MgO and excess Mg, and further washed with high-purity water. Finally, the remaining wet solids were dried in a fume hood in air.
2.4 DeoxygenationTi powder obtained in the previous step were then subjected to deoxygenation. Mg powder and the anhydrous MgCl2 salt were mixed at particular ratios, put into a crucible, transferred to a tube furnace and then heated to over 700℃ in a flowing H2 atmosphere (1 L/min), isothermally held for 4 h, and then cooled to room temperature. The deoxygenated powder was leached with dilute hydrochloric acid solution to remove the water-soluble salt, by product MgO and excess Mg, and further washed with high-purity water. Finally, the remaining wet solids were dried in a fume hood in air.
After de-oxygenation with Mg in hydrogen atmosphere, titanium powder contains a certain amount of hydrogen. To obtain commercial pure titanium powder, the hydrogen containing titanium powder was dehydrogenated at 600℃ for 12 h.
2.5 CharacterizationThe oxygen/nitrogen/hydrogen contents in the powder were determined by using a LECO TCH 600, and the carbon content was analyzed by using a LECO C/S 230. The concentrations of metallic elements in the powder were analyzed by using an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The composition (except C, N, O) of the final powder from route II was analyzed by an independent certified analytical testing lab in Luvak Inc. The morphology and phase composition of the powders were analyzed using scanning electron microscopy and X-ray diffraction analysis. Particle size distribution of the powder was measured by laser diffraction analyzer.
Figure 2 shows the morphology of the product at each major step of the process. The particle size of the raw TiO2 was less than 1 μm. After reduction, the morphology and particle size did not significantly change, as shown in Fig. 2(b). Figure 2(c) shows the crushed and sieved powder with particle size ranging from 106–212 μm. Table 1 lists the composition of C, N, and O in the product at each step. After reduction, oxygen was lowered to 1.68 mass% from the original 40 mass%. The de-oxygenation step further decreased the oxygen to 0.063 mass%. All of the lighter elements, including C, N, O fell within the prescribed range for ASTM B299. Figure 3 shows the XRD patterns of the product at each step in the process. The as-received titanium oxide was a pure single rutile phase. After reduction with Mg in a hydrogen atmosphere, the reduced and leached powder indicated a pure titanium hydride phase, as shown in Fig. 3(a). Dehydrogenation of titanium hydride occurred during sintering, and the pure α-titanium phase was observed, rather than TiH2 after the sinteing. After de-oxygenation, the detected phases did not change, while the peaks of the patterns shifted slightly to the right.
SEM images of the product during each major step of the process: (a) raw TiO2;(b) reduced TiH2; (c) Sintered and crushed powder; (d) final powder.
| Samples | O | C | N |
|---|---|---|---|
| TiO2 | ～40 | <0.001 | -- |
| TiH2 | 1.68 | 0.009 | -- |
| Sintered powder | 1.72 | 0.01 | 0.08 |
| Deoxygenated powder | 0.063 ± 0.007 | 0.02 ± 0.009 | 0.012 ± 0.003 |
| ASTM B299 | 0.15 | 0.03 | 0.02 |
XRD patterns of the product from route I at each major process step.
Similar to the HDH process7,36), the particle size and morphology of the powder from route I was controlled during the crushing process.
3.2 Making powder via route IIFigure 4 shows the morphology of the product at each step. After granulation, irregular TiO2 granulate formed. A sintering step was followed to obtain coarse TiO2 powders. The product after granulation and sintering was shown in Fig. 4(b). After reduction, the powder shape was maintained, and, no obvious broken powder was observed as shown in Fig. 4(c). The higher magnification image in Fig. 4(d) shows that the reduced powder has a porous structure. Figure 4(e) and (f) indicate that the powder becomes denser after the heat treatment step. The images of the final powder and cross-section of the sintered powder are shown in Fig. 5. The sintered powder showed a dense structure. Not much difference was observed in micrographs taken before (see Fig. 4(e) and Fig. 4(f)) and after deoxygenation (see Fig. 5(a)).
SEM images of the product from route 2 at each step: (a) raw TiO2; (b) granulated and sintered TiO2; (c) Higher magnification image in (b); (d) Reduced TiH2; (e) Heat treated powder (f) Higher magnification image in (e).
SEM images of (a) the final powder and (b) cross-section of the final powder from route II.
A detailed ICP analysis and Leco combustion analysis, shown in Table 2, indicate that the composition of all elements met the standard specification for general purpose titanium sponge.
| mass% | O | C | N | Fe | Si | Al | Ca | Cl | Mg | Y | V | Cr |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| TiO2 | ～40 | <0.001 | -- | 0.024 | 0.015 | 0.036 | 0.06 | -- | 0.04 | -- | 0.02 | 0.02 |
| TiH2 | 0.78 | 0.009 | -- | 0.03 | 0.024 | 0.036 | 0.05 | -- | 0.4 | -- | 0.06 | 0.044 |
| Heat treated powder | 0.83 | 0.01 | 0.01 | 0.03 | 0.028 | 0.034 | 0.07 | -- | 0.2 | -- | 0.06 | 0.039 |
| Final powder | 0.11 ± 0.09 | 0.008 ± 0.003 | 0.005 ± 0.002 | 0.032 | 0.012 | 0.024 | 0.002 | <0.001 | 0.064 | <0.0005 | 0.005 | 0.021 |
| ASTM B299 | 0.15 | 0.03 | 0.02 | 0.15 | 0.04 | 0.05 | -- | -- | 0.5 | -- | -- |
In route II, the particle size was mainly controlled by the granulation process, making nano-sized particle into micro-sized particle. Through the reduction process, the particle becomes porous, as >95% oxygen is removed, leaving void/pores in the final structure. Figure 6 shows the particle size distribution of the product at each step during processing. As can be seen, the characteristics of particle size and particle size distribution were maintained with little change during processing.
shows the particle size distribution of the product at each step during processing;(a) after granulation and sintering, (b) after reduction, (c) after heat treatment, (d) after deoxygenation.
To avoid significant sintering during deoxygenation, a low temperature deoxygenation method33) was used to remove the residual oxygen. The low temperature deoxygenation method can lower the oxygen level to the range of the ASTM specification. Here the temperature used was lower than 750℃, and it was possible to retain the original size and morphology of the particles, with limited sinter-bonding between the particles.
3.3 Making powder from route IIIFigure 7 shows the morphology of the product at each step. The raw TiO2 had a particle size of < 1 um. After reduction, the morphology did not significantly change. The spray-dried powder had a spherical morphology, and after debinding and sintering, the individual powder sintered very well with little attachment observed between the powders. Figure 7(e) shows the image of the final powder. The cross-section of the powder indicates the powder is 100% dense, consistent with the density measurement of 99.8% relative density. Table 3 indicates all of the lighter elements of the final powder meet the ASTM standard specification for general-purpose Ti sponge.
SEM images of the product from route III at each step: (a) raw TiO2; (b) reduced TiH2; (c) granulated TiH2; (d) sintered Ti; (e) final powder. (f) cross-section of final powder.
| mass% | O | C | N |
|---|---|---|---|
| TiO2 | ～40 | <0.001 | -- |
| TiH2 | 1.53 | 0.009 | -- |
| Sintered powder | 1.61 | 0.059 | 0.01 |
| De-oxygenated powder | 0.079 ± 0.008 | 0.061 ± 0.005 | 0.015 ± 0.003 |
| ASTM B299 | 0.15 | 0.03 | 0.02 |
It is also worth mentioning that to have good control of the morphology, route III is a better option, as reduction is done before granulation. This process route is ideal for making spherical powder. Approximately 95% of the oxygen is removed after reduction. During de-oxygenation, there is no significant volume change, as only ～1–2 mass% oxygen in the solid solution needs to be removed, which does not affect the crystal structure.
Commercial TiO2 was reduced by Mg in a hydrogen atmosphere. A further deoxygenation process was carried out to ensure the purity met the standard specification for titanium. The combination of the hydrogen assisted magnesium reduction and deoxygenation (HAMR) process, is an integrated approach that made it possible to produce Ti powders that meet the ASTM specification for Ti based on a simple process of thermal reduction using Mg. In this study, particle sizes and morphology of Ti powder produced through three optional routes are presented. From route I and route II, irregular shaped powders were produced, while from route III, a spherical powder was produced. They all met the ASTM B299 titanium sponge standard for general purpose applications.
This research was supported by the METALS program from the Advanced Research Projects Agency (ARPA), U.S. Department of Energy (DOE) under contract number DE-AR0000420. We thank Dr. Mark Koopman for proofreading the manuscript and constructive suggestions.
