A scalable method to rejuvenate off-grade ti64 powders using multi-component eutectic salts

Abstract

Herein disclosed is a method for removing an oxide impurity from metal particles, the method comprising: heating a mixture to a temperature of 700°C or less, wherein the mixture comprises at least one eutectic salt, a reducing agent comprising magnesium, and the metal particles, wherein the metal particles comprise the oxide impurity; cooling the mixture; heating the mixture in the absence of hydrogen, after cooling the mixture; and contacting the mixture with a mineral acid.

Description

ion No. 10202402676R, filed 29 August 2024, the content of it being hereby incorporated by reference in its entirety for all purposes.Technical Field

[0002] The present disclosure relates to a method for removing an oxide impurity from metal particles.Background

[0003] When metal or alloy powders in additive manufacturing (AM) fall out of specifications, they may undergo a costly re-melting and re-atomization process to meet standards, consuming significant energy (e.g., temperatures of more than 1800°C may be involved). Also, to mitigate oxygen contamination or surface oxygen contamination, these powders may have to be mixed with fresh ones, adding to expenses without directly tackling surface oxygen issues. Such method may be considerably ineffective in the AM industry.

[0004] Additionally, the yield of usable powders from re-atomization tends to be low. Another process that may be uneconomical, plasma spheroidization, may be used to shape powder particles but fails to address surface oxides issue.

[0005] Traditionally, a method for deoxidation process may involve using mixtures of calcium metal or calcium salts. However, based on reviews (see FIG. 1), such processes tend to require very high temperatures (more than 900°C) and may often leave residual oxygen levels above 0.2 weight percent (wt%). Sometimes, calcium impurities may remain in the deoxygenated Ti powders. Such methods may be energy-intensive and calcium may be more expensive than alternative reducing agents like magnesium. Additionally, using calcium for deoxygenation may require higher temperatures, which may lead to issues such as reactor contamination and particle sintering, which may inturn be especially problematic for producing spherical powders in additive manufacturing.

[0006] On a related note, there appears to be no effective disposal of off-grade Ti64 powders, which may then be usually left untouched in storage for long duration. A traditional practice by industry may be to add such off-grade Ti64 into virgin (i.c., unused, non-recycled) Ti64 samples and re-melt the mixture in order to reuse the off- grade Ti64 samples. Such practices do not address the accumulated surface oxide layers in the re-melted samples.

[0007] There is thus a need to provide for a solution that addresses one or more of the limitations mentioned above.Summary

[0008] In a first aspect, there is provided for a method for removing an oxide impurity from metal particles, the method comprising: heating a mixture to a temperature of 700°C or less, wherein the mixture comprises at least one eutectic salt, a reducing agent comprising magnesium, and the metal particles, wherein the metal particles comprise the oxide impurity; cooling the mixture; heating the mixture in the absence of hydrogen, after cooling the mixture; and contacting the mixture with a mineral acid.Brief Description of the Drawings

[0009] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the present disclosure. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0010] FIG. 1 is a table showing traditional deoxidation processes that involve calcium as the reducing agent in calciothermic reduction and electrochemical methods. “O” under the column “reported product chemical composition” denotes for oxygen. The term “calciothermic” herein refers to a process that involves the use of calcium (Ca), in the form of calcium metal as an example, to reduce another substance, such as a metal oxide, in a high temperature reaction.

[0011] FIG. 2A is a flow chart showing the rejuvenation process of off-grade Ti64 samples involved in the method of the present disclosure.

[0012] FIG. 2B is a table indicating the heating profile during the rejuvenation operation. In the present disclosure, the terns “h” and “hr” may be interchangeably used to denote hour. In FIG. 2B, “h” is used to denote hour.

[0013] FIG. 3A shows the reaction mechanism involved during the rejuvenation process of the present method.

[0014] FIG. 3B shows a configuration of the reactor vessel (also referred to as molten salt reactor in the present disclosure) used in the rejuvenation process (e.g., thermal reduction process) of the present method.

[0015] FIG. 4A is a plot of x-ray photoelectron spectroscopy (XPS) core-level analysis of (a) Ti 2p for off-grade (black lines), rejuvenated (red lines) and virgin (yellow lines) Ti64 powders.

[0016] FIG. 4B is a plot of XPS core-level analysis of O Is for off-grade (black lines), rejuvenated (red lines) and virgin (yellow lines) Ti64 powders.

[0017] FIG. 5A is a scanning electron microscopy (SEM) image of off-grade Ti64[EBMJ.

[0018] FIG. 5B is a SEM image of rejuvenated Ti64[EBM]-700°C.

[0019] FIG. 5C is a SEM image of rejuvenated Ti64[EBM]-650°C.

[0020] FIG. 5D is a SEM image of rejuvenated Ti64[EBM]-600°C.

[0021] FIG. 6A is a bar graph demonstrating for lowering of rejuvenation temperature to 55O°C in the molten salt reactor. The bar plot for rejuvenation temperature of 600°C is also shown.

[0022] FIG. 6B is a plot of XPS O Is analysis demonstrating for lowering of rejuvenation temperature to 550°C in the molten salt reactor. The XPS plot for rejuvenation temperature of 600°C is also shown.

[0023] FIG. 7 is a table indicating different types of eutectic salts based on a combination of LiCl, KC1, NaCl, CaCh, ZnCh, and / or MgCh. Two types of off-grade Ti64 powders, namely EBM and SLM types, were used.

[0024] FIG. 8 A is a bar graph demonstrating for the effects of eutectic salt composition on the rejuvenation process for off-grade Ti64[EBM].

[0025] FIG. 8B is a bar graph demonstrating for the effects of eutectic salt composition on the rejuvenation process for off-grade Ti64[SLM].

[0026] FIG. 9 demonstrates for effects of mixing methods of Ti64 powders, eutectic salt, and magnesium on the rejuvenation process. The image shown on right is a schematic diagram illustrating the two different configurations of mixing (uniform mixing and sandwich mixing). In various non-limiting examples, sandwich mixing of the mixture may be carried out to render a configuration as shown, wherein the reducing agent and the metal particles may form a mixed layer residing in the at least one eutectic salt. The left image is a bar graph plotting the results in terms of deconvoluted Ti 2p for Ti(0) metallic species (%) for the different mixing configurations.

[0027] FIG. 10 demonstrates for the promotional effects of YCh in the rejuvenation process. The right image is a plot of XPS plot O Is analysis demonstrating for the effects of a promoter (e.g., YCI3). The left image is a bar graph plotting the results in terms of deconvoluted Ti 2p for Ti(0) metallic species (%) in the presence and absence of a YCh promoter.

[0028] FIG. 11 demonstrates for scaling up of rejuvenation process from 2 g to 50 g Ti64 sample. The right image is a plot of XPS plot O ls analysis demonstrating for the scaling up. The left image is a bar graph plotting the results in terms of deconvoluted Ti 2p for Ti(0) metallic species (%) for scaling up.

[0029] FIG. 12 is a table comparing the rejuvenation temperatures when calcium reducing agents are used for the rejuvenation of pure Ti powder / ingots. The first row in the table with text in bold (where temperature ranges from 55O°C to 600°C) represents for the present method.

[0030] FIG. 13 is a table comparing the different parameters and components used in the present method (first row) against other traditional methods and components.

[0031] FIG. 14 illustrates a flow chart similar to that shown in FIG. 2A, but with additional steps included to address the loss of molten salts following each rejuvenation cycle. This is particularly relevant during the acid leaching step, where eutectic salts are often washed away. To mitigate this issue, the present method includes recovering the resulting waste solution (denoted “liquid waste”) in order to reclaim valuable salt components — specifically eutectic salt(s) (ES) and / or magnesium. These recoveredmaterials can then be recycled and reused in the next rejuvenation cycle, thereby improving material efficiency and reducing waste.

[0032] FIG. 15 demonstrates for simultaneous reduction of oxygen and nitrogen contamination via the present method. The left image is a plot of XPS plot O ls analysis demonstrating for such simultaneous reduction (-19% reduction of oxygen). The right image is a plot of XPS plot N Is analysis demonstrating for such simultaneous reduction (-19% reduction of nitrogen).Detailed Description

[0033] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the present disclosure may be practised.

[0034] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0035] The present disclosure describes for a method for removing an oxide impurity from metal particles. Advantageously, the present method offers a “gentler” approach to remove residual oxygen from powder surfaces, such as Ti64 powder surfaces, in that the present method may involve a thermal rejuvenation process that is not only scalable, but also utihzes lower temperature (550°C or less). In contrast to the present method, traditional methods tend to be energy intensive, for example, involving temperatures of 900°C or higher. It follows that the present method is sustainable in such sense.

[0036] The present method is a scalable method to rejuvenate off-grade metal powders (i.e., to remove any surface oxides, e.g., TiOx - surface titanium oxide), for example, Ti64 alloy powders, so that it can be re-used for additive manufacturing (AM) purposes. Traditional approaches do not offer effective disposal of off-grade Ti64 pow ders, which may often involve leaving the off-grade (spent) Ti64 powders untouched in storage for long duration.

[0037] In order to rejuvenate these off-grade Ti64 powders, the surface oxides impurities need to be chemically reduced sufficiently. The rejuvenation process of the present method may be straightforward. involving immersing off-grade Ti64 powders, and magnesium as a reductant, in a molten salt bath under certain temperatures and duration to remove the surface oxides. The present method docs not damage the rejuvenated Ti64 powders which retain the same surface smoothness and absent of foreign impurities.

[0038] Materials involved in the present method can be readily available, such as hydrogen gas (5% - 20% vol) and magnesium powders (5-11 wt%) used as reducing agent to deoxidize off-grade Ti64 alloy. One or more metal halides (e.g., LiCl, NaCl, KC1, MgCh, CaCh, ZnCh. or in any combination) (e.g., 15-30 wt%) may be used as eutectic salts. Optionally, rare earth halides, such as YCh (1-3 wt%), may be used as a promoter to enhance the deoxidization process.

[0039] Generally, as a non-limiting example, the method can involve titanium alloy (e.g., spherical Ti64) mixed with magnesium granules, eutectic salts and optionally promoters, in a stainless-steel holder. The mixture may be heated to 550°C to 700°C under hydrogen and / or argon flow during the deoxidation process. After completion of deoxidation, the mixture may be pickled in 0.1 M to 1 M of HC1 acid to recover the deoxidized (rejuvenated) Ti64 powders. The acid leaching waste solution potentially can be recycled to recover the eutectic salts for subsequent deoxidization process.

[0040] The present method also involves non-complex equipment (see FIG. 3B as a non-limiting example), such as a vertical tube furnace with temperature controller, reactor vessels made from stainless-steel materials, and equipped with Hs / argon flow meters. Overhead stir tank for acid leaching and filtration system to recover the rejuvenated Ti64 powders.

[0041] The present method may involve further advantageous techniques. For example, the present method may involve a promoter, such as YCh, which together with the hydrogen and magnesium, considerably and simultaneously reduce the surface oxygen and nitrogen of Ti64 powders.

[0042] The present method offers ease of quality control. For example, the baseline measurement applied in the rejuvenation process can be based on x-ray photoelectron spectroscopy (XPS) of the core-level Ti 2p, O Is and N Is. The Ti 2p, O Is and N Isof the original off-grade Ti64, rejuvenated Ti64 from the present method, and virgin Ti64 powders, were compared to quantify the rejuvenation process of the present method.

[0043] Details of various embodiments of the method, and advantages associated with the various embodiments arc now described below. Where advantages of the embodiments and features are already demonstrated in one or more examples below, they shall not be reiterated for brevity.

[0044] In the present disclosure, there is provided for a method for removing an oxide impurity from metal particles. In various embodiments, the method may comprise heating, optionally in an inert environment, a mixture (e.g., to a temperature of 700°C or less), wherein the mixture may comprise at least one eutectic salt, a reducing agent comprising magnesium, and the metal particles, wherein the metal particles may comprise the oxide impurity, cooling the mixture, heating the mixture in the absence of hydrogen, after cooling the mixture, and contacting the mixture with a mineral acid. The term “alloy”, in the present disclosure, refers to a physical mixture containing two or more distinct elements that do not form a chemical compound, wherein at least one of the elements is a metal.

[0045] hr various embodiments, the mixture, throughout the method, may be absent of calcium or any calcium salt. Said differently, the method does not involve any calcium and calcium salt.

[0046] In various embodiments, the metal particles may comprise titanium or a titanium alloy. In various embodiments, the metal particles may comprise EBM Ti64 alloy or SLM Ti64 alloy. Ti64 alloy denotes titanium alloy Ti-6A1-4V, which refers to a titanium alloy containing 90% titanium (Ti), 6% aluminum (Al), and 4% vanadium (V), by weight, in the present disclosure. In the present disclosure, EBM Ti64 alloy refers to titanium alloy Ti-6A1-4V that is formed from or has been subjected to electron beam melting (EBM). In the present disclosure. SLM Ti64 alloy refers to titanium alloy Ti-6A1-4V that is formed from or has been subjected to selective laser melting (SLM).

[0047] Tn various embodiments, the oxide impurity may comprise titanium oxide.

[0048] In various embodiments, heating the mixture to the temperature of 700°C or less may comprise heating the mixture to a first temperature of 100°C to 200°C for aduration of 1 hour to 6 hours so as to remove air and moisture from the mixture, then heating the mixture to a second temperature of 550°C to 700°C for 1 hour to 12 hours.

[0049] In various embodiments, heating the mixture to the temperature of 700°C or less in the inert environment may be carried out in the presence of hydrogen, or heating the mixture to the first temperature of 100°C to 200°C may be carried out in the presence of hydrogen, or heating the mixture to the second temperature of 550°C to 700°C may be carried out in the presence of hydrogen.

[0050] In various embodiments, heating the mixture to the first temperature of 100°C to 200°C for a duration of 1 hour to 6 hours may include maintaining the mixture at the first temperature of 100°C to 200°C for 1 hour to 6 hours. In various embodiments, heating the mixture to the second temperature of 550°C to 700°C for 1 hour to 12 hours may include maintaining the mixture at the second temperature of 550°C to 700°C for 1 hour to 12 hours.

[0051] In various embodiments, the method may include heating the mixture to the temperature of 700°C or less, in the presence of hydrogen, in the inert environment. This may include, in the presence of hydrogen, heating the mixture to the first temperature of 100°C to 200°C and then heating the mixture to the second temperature of 550°C to 700°C. Said differently, heating the mixture to the first temperature and heating the mixture to the second temperature may be both carried out in the presence of hydrogen.

[0052] In various embodiments, cooling the mixture may comprise reducing temperature of the mixture to 100°C to 200°C.

[0053] In various embodiments, a metal hydride may be generated where hydrogen is involved (i) in heating the mixture to the temperature of 700°C or less, or (ii) in heating to the first temperature and / or (iii) in heating to the second temperature. In other words, a metal hydride may be generated after heating in hydrogen gas. In various embodiments, the step of heating the mixture in the absence of hydrogen, after cooling the mixture, may dehydrogenate the metal hydride.

[0054] In various embodiments, heating the mixture in the absence of hydrogen may comprise heating the mixture to a third temperature of 500°C to 700°C.

[0055] In various embodiments, (i) cooling the mixture and (ii) heating the mixture in the absence of hydrogen, may be carried out in an inert environment.

[0056] In various embodiments, the inert environment may comprise argon.

[0057] In various embodiments, the at least one eutectic salt may comprise LiCl, KC1, NaCl, CaCl2, ZnCl2, or MgCh.

[0058] In various embodiments, the mineral acid may comprise hydrochloric acid, nitric acid, sulfuric acid, or acetic acid. In various embodiments, the mineral acid may have a concentration of 1 M or less. For example, the mineral acid may be acetic acid having a concentration of 1 M or less.

[0059] In various embodiments, the magnesium may be absent of any molten state. For example, the magnesium may be absent of any molten state at a temperature of 650°C or less, as magnesium has a melting point of about 650°C.

[0060] In various embodiments, the method may further comprise washing the metal particles, with water or an alcohol, after contacting the mixture with the mineral acid; and / or drying the metal particles, after contacting the mixture with the mineral acid.

[0061] In various embodiments, the method may further comprise adding a promoter to the mixture, prior to heating the mixture to the temperature of 700°C or less.

[0062] In various embodiments, the promoter may comprise YCh.

[0063] In various embodiments, the method may further comprise mixing the mixture during (i) heating the mixture to the temperature of 700°C or less in the inert environment, (ii) cooling the mixture, and / or (iii) heating the mixture in the absence of hydrogen.

[0064] In various embodiments, the mixing may comprise having the reducing agent and the metal particles dispersed throughout the at least one eutectic salt so as to render uniform mixing of the mixture, or having the reducing agent and the metal particles formed a mixed layer in the at least one eutectic salt so as to render sandwich mixing of the mixture.

[0065] Advantages of aforesaid embodiments and features of the method arc already demonstrated in one or more examples below, hence not reiterated for brevity. Also, the various steps involved in the method are described in the examples and hence not reiterated for brevity.

[0066] The word “substantially” does not exclude “completely” e.g. a composition which is “substantially free” from Y may be completely free from Y. Where necessary, the word “substantially” may be omitted from the definition of the present disclosure.

[0067] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0068] In the context of various embodiments, the tilde symbolthe term “about”, and the term “approximately”, as applied to a numeric value encompasses the exact value and a reasonable variance. The variance may be ±20%, ±10%, ±5%, ±1%, ±0.5%, ±0.1%, etc.

[0069] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0070] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrccitcd elements.Examples

[0071] The method of the present disclosure, are described in further details, by way of non-limiting examples, as set forth below.

[0072] Example 1: Experimental procedures

[0073] FIG. 2A shows a flow chart of the present method (also referred to in the present disclosure as a “rejuvenation process”). The physical mixing of off-grade Ti64 samples with other components (e.g., eutectic salt, magnesium, pure Ti64) are involved. Various Ti64 samples were used, but for the purpose of demonstration and not to limit the Ti64 samples, two main samples are discussed herein, one labelled as an EBM type and the other a SLM type. Particularly, the samples tested include metal particles of EBM Ti64 alloy or SLM Ti64 alloy. Ti64 alloy denotes titanium alloy Ti-6A1-4V, which is a titanium alloy containing 90% titanium (Ti), 6% aluminum (Al), and 4% vanadium (V), by weight. EBM Ti64 alloy refers to titanium alloy Ti-6A1-4V that is formed from or has been subjected to electron beam melting (EBM). SLM Ti64 alloy refers to titanium alloy Ti-6A1-4V that is formed from or has been subjected to selective laser melting (SLM).

[0074] As shown in FIG. 2 A, off-grade Ti64 samples were mixed with an eutectic salt and magnesium to form a mixture. The mixture was then subject to thermal reduction.

[0075] FIG. 2B indicates the steps for the thermal reduction. The mixture was first heated (e.g., to a temperature of 700°C or less in an inert environment). In other words, the mixture that was heated includes at least one eutectic salt, a reducing agent that includes the magnesium, and the metal particles (e.g., Ti64), wherein the metal particles contains oxide impurity (e.g., on surface of each metal particle). After heating, the mixture was cooled. Cooling of the mixture may involve reducing temperature of the mixture to 100°C to 200°C. After cooling the mixture, the mixture was then heated in the absence of hydrogen.

[0076] The heating profiles (i.e., during thermal reduction steps) in the method (during the rejuvenation operation), are indicated in FIG. 2B. From steps 1 and 2, it can be seen that heating the mixture to the temperature of 700°C or less can include heating the mixture to a first temperature of 100°C to 200°C for a duration of 1 hour to 6 hours so as to remove air and moisture from the mixture, then heating the mixture to a second temperature of 550°C to 700°C for 1 hour to 12 hours. For steps 1 to 3 of the thermal reduction process, it can be seen that heating the mixture to the temperature of 700°C or less (e.g., in the inert environment) can be carried out in the presence of hydrogen (steps 1 and 2), or heating the mixture to the first temperature of 100°C to 200°C can be carried out in the presence of hydrogen (step 1), or heating the mixture to the second temperature of 550°C to 700°C can be carried out in the presence of hydrogen (step 2). Further, in step 3, heating the mixture in the absence of hydrogen comprises heating the mixture to a third temperature of 500°C to 700°C.

[0077] In various examples, during the thermal reduction, optionally, as shown in step 3 of FIG. 2B, (i) cooling the mixture and (ii) heating the mixture in the absence of hydrogen, can be carried out in an inert environment.

[0078] After thermal reduction, the mixture was contacted with a mineral acid (see FIG. 2A). After the completion of step 3 (sec FIG. 2B), the rejuvenated mixtures were removed and pickled in 0.1 to 1 M HC1 acid to separate the eutectic salts and magnesium from the rejuvenated Ti64 samples. The term “pickled” refers to a chemical process / step where a metal or material is treated with an acid solution to remove impurities, rust, or oxide layers from its surface. This process may also be referred to in the present disclosure as “pickling” and is used to clean and prepare metal surfaces before further processing. In general, for 2 g to 50 g of Ti64 samples, 200 mL to 1000mL of HC1 acid were used to aid in complete removal of the eutectic salts and magnesium. The rejuvenated Ti64 powders are filtered and rinsed sufficiently with of ethanol and allowed to air-dry at room temperature (e.g., 20°C to 30°C).

[0079] FIG. 3A shows a simplified schematic diagram of the rejuvenation mechanism. When oxygen is dissolved in a titanium lattice, Ti-0 can become more stable than MgO, depending on the oxygen percentage and temperature. This may limit how much oxygen can be removed from titanium by magnesium. Therefore, altering the thermodynamic stability of the Ti-0 system helps to enhance deoxygenation using magnesium as a reducing agent. Hydrogen can be used to destabilize the Ti-0 solid solution thermodynamically. In the presence of a hydrogen atmosphere, the oxygen potential of the Ti-Ox-Hy solid solution (x may range 1.5 to 2.5 and y may range from 0 to 2) can be reduced below that of MgO, allowing magnesium to remove oxygen from the Ti-0 solid solution efficiently, giving rise to Ti-Hy (y may range from 0 to 2).

[0080] Example 2: Characterisation

[0081] The baseline measurement applied in the rejuvenation process is based on the x-ray photoelectron spectroscopy (XPS) of the core-level Ti 2p and O Is. The Ti 2p and O Is of the original off-grade Ti64, rejuvenated Ti64 and virgin Ti64 powders were compared to quantify the rejuvenation process. As shown in FIG. 4A, the rejuvenated Ti64 powder showed an increased in the metallic Ti(0) species (quite similar to the virgin sample). In addition, the total surface oxygen species was analysed by the XPS O Is (see FIG. 4B), and it can be observed that there was -60% reduction in the surface oxygen species and O Is profile of the rejuvenated Ti64 samples was close to the O Is profile of the virgin samples.

[0082] Example 3: Effects of rejuvenation temperature

[0083] The rejuvenation temperatures had significant impact on the rejuvenated Ti64 samples. As shown in FIG. 5A to FIG. 5D, particularly at 700°C, there were severely damaged Ti64 as revealed in the SEM images. As the rejuvenation temperature was progressively lowered to 600°C, the surfaces of the rejuvenated Ti64 samples looked almost the same as the original off-grade Ti64 powders. Thus, it was determined that 600°C appears a suitable temperature for the rejuvenation process as it does not caused any sintering nor damages to the Ti64 surfaces.

[0084] Furthermore, the rejuvenation temperature can be further lowered to 55O°C (see FIG. 6A and FIG. 6B), which is the lowest as compared to any traditional methods. Based on the XPS O Is analysis, it can be seen that there was a reduction of oxygen species with respect to the off-grade Ti64[SLM] samples. Both 600°C and 550°C rejuvenation temperature showed almost similar -7% metallic Ti(0) species which is higher than the off-grade Ti64 samples [-2.6% Ti(0) species].

[0085] Example 4: Effects of eutectic salt composition

[0086] The different types of eutectic salts based on a combination of LiCl, KC1, NaCl, CaC12, ZnCh, and / or MgCh, were investigated (see FIG. 7). Two types of off-grade Ti64 powder, namely EBM and SLM types, were used. As shown in FIG. 8A and 8B, both KCl+MgCh and LiCl+MgCh eutectic salts are demonstrated to be beneficial to the rejuvenation of off-grade Ti64 [EBM] samples. In both cases, there were an increased in the metallic Ti(0) species from 4.8% to -7.7% after the rejuvenation process. MgCh and LiCl+KCl salts also showed increase of the metallic Ti(0) species from 4.8% to -5.8%. The rejuvenation of another type of off-grade Ti64 [SLM] samples with 3 different types of eutectic salts was also studied. Likewise, KCl+MgCh showed a significant improvement of the metallic Ti(0) species from 2.6% to -6.8%, while LiCl+MgCh and NaCl+ZnCh+KCl eutectic salts showed similar rejuvenation of the metallic Ti(0) species from 2.6% to -5.5%. Nonetheless, it can be observed there is an increase in the metallic Ti(0) species after immersing different types of off-grade Ti64 samples in molten salts along with the magnesium reductants.

[0087] Example 5: Effects of mixing methods

[0088] To ensure uniformity of rejuvenation, 2 types of mixing methods were investigated: (I) uniform mixing and (II) sandwich configuration. As shown in FIG 9, the method of rejuvenation is independent of the initial mixing of the off-grade Ti64 samples and the magnesium powder, and the removal of surface oxide can still be equally efficient. For the Ti64[EBM] and Ti64[SLM] samples, no significant differences in the rejuvenated samples were observed.

[0089] Example 6: Effects of YCk promoter

[0090] The composition of the molten salt bath can be modified to include a promoter, such YCI3, which improved the efficiency of the rejuvenation process. As shown in FIG. 10, after adding small amount of YCh 0.1 wt% to 1 wt% with respect to the molten saltweight, the removal of the surface oxide of the off-grade Ti64 samples were enhanced. It was observed from the XPS O ls peaks that the freshly rejuvenated Ti64 sample using YCh promoter was almost the same as the virgin Ti64 powder, except that the virgin Ti64 had slightly higher Ti-OH species (-532 eV) comparatively.

[0091] Example 7: Scale-up results

[0092] FIG. 11 showed the results of 10 g and 50 g rejuvenation of Ti64[EBM] samples using the scale-up reactor. The rejuvenation is the same for lab-scale 2 g and scale-up 10 g samples, which is further supported by the similar XPS O1 s peaks. When linearly scaling-up from 2 g to 50 g sample size, it was observed that the rejuvenation is slightly lower than the 2 g and 10 g sample size. Based on the XPS O Is analysis, the scale-up 50 g sample rejuvenation has only marginally higher Ti-OH species. Thus, the present rejuvenation of off-grade Ti64 powders in a molten salt bath can be scaled up to remove the surface oxides.

[0093] Example 8: Further advantages

[0094] With the one or more eutectic salts mentioned above (e.g., a combination of any of LiCl / MgCh / KCl / NaCl), and optionally promoter YCh, in hydrogen atmosphere 55O°C, the present method rendered simultaneous reduction of oxygen and nitrogen contamination (-19% and -34%, respectively). In contrast, traditional approaches tend to offer only the reduction of oxygen contamination for off-grade titanium scraps (without the reduction of nitrogen).

[0095] Example 9: Summary and commercial applications

[0096] Singapore can be a regional hub to rejuvenate and reuse the off-grade Ti64 powders. A technical feature of the present method includes the involvement of eutectic salt(s) (e.g., multi-component eutectic salts), which can be composed of a combination of, for example, LiCl-KCl-MgCk-YCh halide salts to modify for the rejuvenation of different types of off-grade Ti64 samples. Furthermore, the present rejuvenation method does not require the magnesium reducing agent to be in the molten state (melting point of magnesium is about 650°C). As shown in the various results, the surface oxides of different types (SLM and EBM) off-grade Ti64 powders were capable of being reduced in a molten salt reactor at mild temperatures of 55O°C to 600°C, which are below the melting point of magnesium. As compared to reported data (see FIG. 12),traditional methods utilize very high rejuvenation temperatures (at least 700°C or more) when calcium reducing agents are used for the rejuvenation of pure Ti powders / ingots.

[0097] Economically, the present method affords USD$228.36 / Kg of rejuvenated Ti64 powder while the cost of virgin Ti64 is ~USD$223.23 / Kg, and the price of rejuvenated Ti64 powder from the present method can be reduced as the method scales up.

[0098] While the present disclosure has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present disclosure as defined by the appended claims. The scope of the present disclosure is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS1. A method for removing an oxide impurity from metal particles, the method comprising: heating a mixture to a temperature of 700°C or less, wherein the mixture comprises at least one eutectic salt, a reducing agent comprising magnesium, and the metal particles, wherein the metal particles comprise the oxide impurity; cooling the mixture; heating the mixture in the absence of hydrogen, after cooling the mixture; and contacting the mixture with a mineral acid.

2. The method of claim 1, wherein the metal particles comprise titanium or a titanium alloy.

3. The method of claim 1 or 2, wherein the oxide impurity comprises titanium oxide.

4. The method of any one of claims 1 to 3, wherein heating the mixture to the temperature of 700°C or less comprises: heating the mixture to a first temperature of 100°C to 200°C for a duration of 1 hour to 6 hours so as to remove air and moisture from the mixture, then heating the mixture to a second temperature of 55O°C to 700°C for 1 hour to 12 hours.

5. The method of claim 4, wherein: heating the mixture to the temperature of 700°C or less in the inert environment is carried out in the presence of hydrogen; or heating the mixture to the first temperature of 100°C to 200°C is carried out in the presence of hydrogen; or heating the mixture to the second temperature of 550°C to 700°C is carried out in the presence of hydrogen.

6. The method of any one of claims 1 to 5, wherein cooling the mixture comprises reducing temperature of the mixture to 100°C to 200°C.

7. The method of any one of claims 1 to 6, wherein heating the mixture in the absence of hydrogen comprises heating the mixture to a third temperature of 500°C to 700°C.

8. The method of any one of claims 1 to 7, wherein (i) cooling the mixture and (ii) heating the mixture in the absence of hydrogen, are carried out in an inert environment.

9. The method of any one of claims 5 to 8, wherein the inert environment comprises argon.

10. The method of any one of claims 1 to 9, wherein the at least one eutectic salt comprises LiCl, KC1, NaCl, CaCl2, ZnCl2, or MgCl2.

11. The method of any one of claims 1 to 10, wherein the mineral acid comprises hydrochloric acid, nitric acid, sulfuric acid, or acetic acid.

12. The method of any one of claims 1 to 11, wherein the magnesium is absent of any molten state.

13. The method of any one of claims 1 to 12, further comprising: washing the metal particles, with water or an alcohol, after contacting the mixture with the mineral acid; and / or drying the metal particles, after contacting the mixture with the mineral acid.

14. The method of any one of claims 1 to 13, further comprising: adding a promoter to the mixture, prior to heating the mixture to the temperature of 700°C or less.

15. The method of claim 14, wherein the promoter comprises YCh-16. The method of any one of claims 1 to 15, further comprising: mixing the mixture during (i) heating the mixture to the temperature of 700°C or less in the inert environment, (ii) cooling the mixture, and / or (iii) heating the mixture in the absence of hydrogen.

17. The method of claim 16, wherein the mixing comprises: having the reducing agent and the metal particles dispersed throughout the at least one eutectic salt so as to render uniform mixing of the mixture; or having the reducing agent and the metal particles formed a mixed layer in the at least one eutectic salt so as to render sandwich mixing of the mixture.

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