Reaction Mechanism and Process Control of Hydrogen Reduction of Ammonium Perrhenate

Article Reaction Mechanism and Process Control of Hydrogen Reduction of Ammonium Perrhenate

Junjie Tang 1,2, Yuan Sun 2,*, Chunwei Zhang 2,3, Long Wang 2, Yizhou Zhou 2,*, Dawei Fang 3 and Yan Liu 4

1 School of Biomedical & Chemical Engineering, Liaoning Institute of Science and Technology, Benxi 117004, China; [email protected] 2 Superalloys research department, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; [email protected] (C.Z.); [email protected] (L.W.) 3 College of chemistry, Liaoning University, Shenyang 110036, China; davidﬁ[email protected] 4 Key Laboratory for Ecological Utilization of Multimetallic Mineral, Ministry of Education, Northeastern University, Shenyang 110819, China; [email protected] * Correspondence: [email protected] (Y.S.); [email protected] (Y.Z.); Tel.: +86-024-23971767 (Y.S.); +86-024-83978068 (Y.Z.) Received: 17 April 2020; Accepted: 13 May 2020; Published: 15 May 2020

Abstract: The preparation of rhenium powder by a hydrogen reduction of ammonium perrhenate is the only industrial production method. However, due to the uneven particle size distribution and large particle size of rhenium powder, it is difficult to prepare high-density rhenium ingot. Moreover, the existing process requires a secondary high-temperature reduction and the deoxidization process is complex and requires a high-temperature resistance of the equipment. Attempting to tackle the difficulties, this paper described a novel process to improve the particle size distribution uniformity and reduce the particle size of rhenium powder, aiming to produce a high-density rhenium ingot, and ammonium perrhenate is completely reduced by hydrogen at a low temperature. When the particle size of the rhenium powder was 19.74 µm, the density of the pressed rhenium ingot was 20.106 g/cm3, which was close to the theoretical density of rhenium. In addition, the hydrogen reduction mechanism of ammonium perrhenate was investigated in this paper. The results showed that the disproportionation of ReO3 decreased the rate of the reduction reaction, and the XRD and XPS patterns showed that the increase in the reduction temperature was conducive to increasing the reduction reaction rate and reducing the influence of disproportionation on the reduction process. At the same reduction temperature, reducing the particle sizes of ammonium perrhenate was conducive to increasing the hydrogen reduction rate and reducing the influence of the disproportionation.

Keywords: ammonium perrhenate; rhenium; disproportionation reaction; hydrogen reduction

1. Introduction Rhenium as an important rare metal is widely used in metallurgy and the aerospace industry [1]. The plasma method, electrolysis method, vapor deposition method, and powder metallurgy are the main processes for the preparation of rhenium [2,3]. Jurewicz et al. [4] prepared a high-purity nanometer rhenium powder by the plasma method. Leonhardt et al. [5] used plasma spray spheroidization to control the microstructure of rhenium and obtained spherical rhenium powders. Schrebler et al. [6] also prepared spherical rhenium powder from a mixture of rhenic acid and sodium sulfate by electrolysis. Liu et al. [7] prepared small particles of superﬁne spherical rhenium powder by vapor deposition. The rhenium powders prepared by the above processes have a uniform particle size distribution and large speciﬁc surface area, and the pure rhenium materials prepared from these rhenium powders

Metals 2020, 10, 640; doi:10.3390/met10050640 www.mdpi.com/journal/metals Metals 2020, 10, x FOR PEER REVIEW 2 of 16 Metals 2020, 10, 640 2 of 15 prepared from these rhenium powders have high densities. However, due to the complexity of the abovehave high processes densities. and However, high equipment due to the requirements, complexity ofnone the aboveof the processes above processes and high equipmenthave been industrialized. The hydrogen reduction of ammonium perrhenate is a commonly used process to requirements, none of the above processes have been industrialized. The hydrogen reduction of prepare rhenium powder in industry, which has the characteristics of a simple process flow, low ammonium perrhenate is a commonly used process to prepare rhenium powder in industry, which has production cost, and low equipment requirements [8,9]. The preparation process flow chart of the characteristics of a simple process flow, low production cost, and low equipment requirements [8,9]. rhenium ingot is shown in Figure 1. However, the rhenium powders that are produced by this The preparation process flow chart of rhenium ingot is shown in Figure1. However, the rhenium preparation technology have an uneven particle size distribution, small specific surface area, and powders that are produced by this preparation technology have an uneven particle size distribution, the rhenium ingots produced have poor compactness. Moreover, due to the low efficiency of the small specific surface area, and the rhenium ingots produced have poor compactness. Moreover, due mass and heat transfer in the traditional process, the reaction with hydrogen is not complete and to the low efficiency of the mass and heat transfer in the traditional process, the reaction with hydrogen the second reduction step at a high temperature is required [10–12]. is not complete and the second reduction step at a high temperature is required [10–12].

Figure 1. Industrial preparation of rhenium ingots. Figure 1. Industrial preparation of rhenium ingots. In order to optimize the process of preparing rhenium by a high-temperature reduction, we can lookIn into order work to dealing optimize with the the process preparation of preparing of other metalrhenium powders by a high-temperature by a high-temperature reduction, reduction. we canIn recent look years,into work there havedealing been with many the studies preparatio on then preparation of other metal of metal powders powders by by a ahigh-temperature high-temperature reduction.hydrogen reductionIn recent years, [13–21 there]. For have instance, been Wang many et studies al. [22 ]on proposed the preparation a novel routeof metal to synthesize powders by Mo a powdershigh-temperature via a carbothermic hydrogen prereductionreduction [13–21]. of molybdenum For instance, oxide Wang followed et al. [22] by a proposed reduction a by novel hydrogen; route tothey synthesize removed Mo oxygen powders from via the a samplescarbothermic by a prereduction secondary reduction. of molybdenum Kanga oxide et al. followed [23] prepared by a reductionnanosized by W andhydrogen; W-Ni powdersthey removed by applying oxygen ball from milling the samples and a hydrogen by a secondary reduction reduction. of oxide Kanga powders. et al.Gua [23] et al.prepared[24] prepared nanosized Mo nanopowders W and W-Ni through powder a hydrogens by applying reduction ball of amilling combustion-synthesized and a hydrogen reduction of oxide powders. Gua et al. [24] prepared Mo nanopowders through a hydrogen foam-like MoO2 precursor. All of the above studies are based on a reducing substance pretreatment, whichreduction provides of a combustion-synthesized a certain reference experience foam-like for MoO this study.2 precursor. However, All of the the hydrogen above studies reduction are basedreaction on of a ammoniumreducing substance perrhenate pretreatment, is a complicated which process. provides This a certain process reference not only involvesexperience reduction, for this study. However, the hydrogen reduction reaction of ammonium perrhenate is a complicated but also a disproportionation reaction. Colton [25] pointed out that the disproportionation of ReO3 process. This process not only involves reduction, but also a disproportionation reaction. Colton [25] occurred above 300 ◦C to produce Re2O7 and ReO2. This disproportionation reaction is the main pointedreason why out ammoniumthat the disproportionation perrhenate cannot of ReO be completely3 occurred above reduced 300 to °C rhenium to produce at a lowRe2O temperature.7 and ReO2. ThereThis disproportionation are few reports on thereaction preparation is the ofmain a high-quality reason why Re powderammonium by a perrhenate hydrogen reduction cannot be at present.completelyBai reduced et al. [26 ]to reduced rhenium volatile at a low rhenium temperature. oxide to There prepare are Re few powder. reports However, on the preparation due to the highof a high-qualityequipment requirements Re powder by of a this hydrogen process, reduction it cannot beat present. used for Bai industrial et al. [26] production. reduced volatile rhenium oxide to prepare Re powder. However, due to the high equipment requirements of this process, it cannotChemical be Reaction used for Considerations industrial production.

ChemicalAmmonium Reaction Considerations perrhenate decomposes into oxides with diﬀerent valence states when reduced at a high temperature, and the main oxides are Re2O7, ReO3, and ReO2 [27]. The total equation for the reductionAmmonium of ammoniumperrhenate decomposes perrhenate ininto hydrogen oxides wi isth represented different valence by Equation states when (1). Ammonium reduced at a high temperature, and the main oxides are Re2O7, ReO3, and ReO2 [27]. The total equation for the perrhenate is decomposed by heat to Re2O7, which is gradually recombined with hydrogen and ﬁnally reduction of ammonium perrhenate in hydrogen is represented by Equation (1). Ammonium perrhenate is decomposed by heat to Re2O7, which is gradually recombined with hydrogen and

Metals 2020, 10, 640 3 of 15

reduced to Re, as shown in Equations (2)–(4). ReO3 is very reactive; ReO3 is disproportionated at a high temperature to produce ReO2 and Re2O7, as shown in Equation (5).

2NH4ReO4 (s) + 7H2 (g) = 2Re (s) + 8H2O (g) + 2NH3 (g) (1)

Re2O7 (s) + H2 (g) = 2ReO3 (s) + H2O (g) (2)

ReO3 (s) + H2 (g) = ReO2 (s) + H2O (g) (3)

ReO2 (s) + 2H2 (g) = Re (s) + 2H2O (g) (4)

3ReO3 (s) = Re2O7 (s) + ReO2 (s) (5) In the present work, we determined the reduction mechanism of ammonium perrhenate through a diﬀerential thermal analysis, and innovatively proposed that the disproportionation reaction in the reduction process was the main reason aﬀecting the complete reduction of ammonium perrhenate. This work also determined an innovative process for reducing the particle size and reduction temperature of rhenium powder, aiming to produce a high-density rhenium ingot, and ammonium perrhenate is completely reduced by hydrogen at a low temperature. This is the technological innovation point of this paper. Moreover, the optimization and innovation of this process is based on the already industrialized hydrogen reduction process to produce rhenium ingot, which makes it easy to realize as an industrialized production process.

2. Materials and Methods

2.1. Instrument Instrument: The following instruments were used herein: RE-2000A rotary evaporator, Qiqiang instrument manufacturing co. LTD, Shanghai, China; SSX-550 scanning electron microscope, Shimadzu, Osaka, Japan; PW3040/60 X-ray diﬀractometer (XRD), Panalytical Company, Almelo, Netherlands; Escalab250 250 X-ray photoelectron spectroscopy (XPS), Hewlett-Packard Company, Palo alto, CA, USA; VEP223 high-temperature vacuum sintering furnace, Beizhen Vacuum Technology Co. Ltd., Shenyang, China; HYL-1076 laser particle size analyzer, Haoyu Technology Co. Ltd., Dandong, China; the self-developed recrystallization reactor, Institute of metals, Chinese academy of sciences, Shenyang, China; STA PT1600 Diﬀerential Thermal Analysis (DTA), Linseis Co. Ltd., Selb, Germany.

2.2. Materials Materials: NH ReO (99.99%, Re 69.4%) from Halin Chemical Co. LTD, Weifang, China. 4 4 ≥ 2.3. Analytical Methods XRD detection: The light tube type was a Cu target, ceramic X light tube. λ = 0.15406 nm, scan range was 10–90 degrees, the scanning speed was 2 degrees/min. DTA detection: The temperature ranged from 300 to 700 ◦C, and the gas atmosphere was nitrogen. Particle size distribution detection: the test medium was ethanol, the optical model was Mie, and the distribution type was volume distribution. The parameters of SEM: The electron acceleration voltage was 20.0 KV, the working distance was 21.8 mm, and the magniﬁcation was 15,000 times. XPS analysis parameters: The ﬁtting software was XPSPEAK (XPSPEAK.41, Hewlett-Packard Company, Palo alto, CA, USA). The read base pressure was 2.4 10 8 Pa, utilizing monochromatic Al × − Kα radiation operating at 1486.6 eV. At the pass energy of 50 eV, with a 0.1 eV step, the high-resolution scans were performed. At the pass energy of 100 eV and a step size of 1 eV, the survey spectra were acquired. The reproducible C (1 s) binding energy of all samples was 284.6 eV and the charge Metals 2020, 10, 640 4 of 15

Metals 2020, 10, x FOR PEER REVIEW 4 of 16 neutralization was achieved using low-energy electrons and argon ions. The spin–orbit splitting was Metals 2020, 10, x FOR PEER REVIEW 4 of 16 2.4.2.4 eV,Experimental and the spin Procedure orbital split intensity ratio of 4f7/2 and 4f5/2 was 4:3. 2.4. Experimental Procedure 2.4. ExperimentalAs illustrated Procedure by Figure 2 was the experimental flow chart of a hydrogen reduction of ammoniumAs illustrated perrhenate by Figurein this study2 was. Ammoniumthe experiment perrhenateal flow (99.99%,chart of Rea hydrogen≥ 69.4%) was reduction prepared of As illustrated by Figure2 was the experimental flow chart of a hydrogen reduction of ammonium withammonium deionized perrhenate water into in thisa saturated study. Ammonium solution at perrhenateroom temperature, (99.99%, theRe room≥ 69.4%) temperature was prepared was perrhenate in this study. Ammonium perrhenate (99.99%, Re 69.4%) was prepared with deionized aboutwith deionized 25 °C. Then water the saturatedinto a saturated solution solutionof ammo atnium room perrhenate temperature,≥ at room the temperatureroom temperature was placed was water into a saturated solution at room temperature, the room temperature was about 25 C. Then the inabout an RE-2000A25 °C. Then rotary the saturated evaporator, solution and partof ammo of thenium water perrhenate was evaporated at room temperatureto form a hot◦ was saturated placed saturated solution of ammonium perrhenate at room temperature was placed in an RE-2000A rotary solutionin an RE-2000A at 120 °C rotary [28]. Theevaporator, thermally and saturated part of thesolution water of was ammonium evaporated perrhenate to form wasa hot introduced saturated evaporator, and part of the water was evaporated to form a hot saturated solution at 120 C[28]. intosolution the atrecrystallization 120 °C [28]. The condensation thermally saturated reactor; solutionthe stirring of ammonium speed and perrhenatecooling temperature was introduced◦ were The thermally saturated solution of ammonium perrhenate was introduced into the recrystallization adjustedinto the recrystallizationand recrystallized condensation at 5 °C for 3 h.reactor; Finall y,the the stirring cooled speedsolid–liquid and cooling mixture temperature was filtered were and condensation reactor; the stirring speed and cooling temperature were adjusted and recrystallized at 5 C driedadjusted to obtain and recrystallized ammonium perrhenateat 5 °C for 3crystals. h. Finall Thy,e the SEM cooled diagrams solid–liquid of the recrystallized mixture was ammoniumfiltered and◦ for 3 h. Finally, the cooled solid–liquid mixture was filtered and dried to obtain ammonium perrhenate perrhenatedried to obtain are shownammonium in Figure perrhenate 3 [29], andcrystals. the median The SEM diameters diagrams (D of50 )the and recrystallized specific surface ammonium area are crystals. The SEM diagrams of the recrystallized ammonium perrhenate are shown in Figure3[ 29], shownperrhenate in Table are shown 1. The in recrystallization Figure 3 [29], and ammonium the median perrhenate diameters particles (D50) and (60 specific g) were surface reduced area with are and the median diameters (D ) and specific surface area are shown in Table1. The recrystallization theshown different in Table temperatures 1. The recrystallization (300–90050 °C) ammonium in the high-temperature perrhenate particles vacuum (60 sinteringg) were reduced furnace; with the ammonium perrhenate particles (60 g) were reduced with the different temperatures (300–900 C) in hydrogenthe different flow temperatures rate was 500 mL/min,(300–900 and°C) thein theheating high-temperature rate was 10 °C/min. vacuum After sintering 3 h of reduction, furnace;◦ the the high-temperature vacuum sintering furnace; the hydrogen flow rate was 500 mL/min, and the reductionhydrogen productflow rate of was ammonium 500 mL/min, perrhenate and the was heating obtained. rate was 10 °C/min. After 3 h of reduction, the heating rate was 10 C/min. After 3 h of reduction, the reduction product of ammonium perrhenate reduction product of◦ ammonium perrhenate was obtained. was obtained.

Figure 2. The experimental working procedure diagram. Figure 2. The experimental working procedure diagram. Figure 2. The experimental working procedure diagram.

Figure 3. SEM images of recrystallized ammonium perrhenate particles at different agitation intensities Figure 3. SEM images of recrystallized ammonium perrhenate particles at different agitation and ammonium perrhenate raw material particles (a) ammonium perrhenate raw material particles, intensitiesFigure 3. SEMand ammoniumimages of recrystallizedperrhenate raw ammonium material perrhenateparticles ( aparticles) ammonium at different perrhenate agitation raw (b) 100 rpm stirring strength recrystallized particles, and (c) 200 rpm stirring strength recrystallized material particles, (b) 100 rpm stirring strength recrystallized particles, and (c) 200 rpm stirring particles)intensities [ 29and]. ammonium perrhenate raw material particles (a) ammonium perrhenate raw strengthmaterial recrystallizedparticles, (b) particles)100 rpm [29].stirring strength recrystallized particles, and (c) 200 rpm stirring strength recrystallizedTable 1. Specific particles) surfaces [29]. and median diameters (D50s) of the NH4ReO4 particles. Table 1. Specific surfaces and median diameters (D50s) of the NH4ReO4 particles. 2 Ammonium Perrhenate Specific Surface (m /kg) D50 (µm) Table 1. Specific surfaces and median diameters (D50s) of the NH4ReO4 particles. Ammonium Perrhenate Specific Surface（m2/kg） D50 (µm) 100 rpm stirring strength 21.72 81.05 100 rpmAmmonium stirringrecrystallized strength Perrhenate recrystallized Specific Surface21.72 （m2/kg） D81.0550 (µm) 200 rpm stirring200 rpm strength stirring strength recrystallized 26.93 71.17 100 rpm stirring strength recrystallized 26.9321.72 71.17 81.05 200 rpmRaw stirring materialrecrystallized strength particles recrystallized 14.7926.93 123.9071.17 Raw material particles 14.79 123.90 Raw material particles 14.79 123.90 Preparation of rhenium ingots: 20 g rhenium powder was put into the powder press mold (the heightPreparation was 32.00 ofmm rhenium and the ingots: inner diameter20 g rhenium of the powder mould waswas put16.60 into mm) the to powder press. Thepress pressure mold (the of height was 32.00 mm and the inner diameter of the mould was 16.60 mm) to press. The pressure of

Metals 2020, 10, 640 5 of 15 Metals 2020, 10, x FOR PEER REVIEW 5 of 16 the powderPreparation press ofwas rhenium 7,4000 ingots:N, the sintering 20 g rhenium furnace powder temperature was put was into 2300 the °C, powder and the press sintering mold time(the heightwas 3 h. was The 32.00 theoretical mm and density the inner of rhenium diameter is of 21.04 the mould g/cm3. was 16.60 mm) to press. The pressure of the powder press was 74,000 N, the sintering furnace temperature was 2300 ◦C, and the sintering 3.time Results was 3and h. The Discussion theoretical density of rhenium is 21.04 g/cm3.

3.1.3. Results Reduction and Mechanism Discussion Analysis 3.1. ReductionIn order to Mechanism research the Analysis reaction mechanism of ammonium perrhenate in the reduction process, the decomposition products of ammonium perrhenate (Re, ReO2, and ReO3) were analyzed by a In order to research the reaction mechanism of ammonium perrhenate in the reduction process, differential thermal analysis (DTA), and the differential thermal analysis curve is shown in Figure 4. the decomposition products of ammonium perrhenate (Re, ReO , and ReO ) were analyzed by a It can be seen that an obvious endothermic peak appeared at 3502 to 400 °C.3 Re is stable at a high differential thermal analysis (DTA), and the differential thermal analysis curve is shown in Figure4. temperature, and the decomposition temperature of ReO2 is 700 °C. Therefore, the generation of this It can be seen that an obvious endothermic peak appeared at 350 to 400 ◦C. Re is stable at a high endothermic peak can only be due to the disproportionation of ReO3. The disproportionation temperature, and the decomposition temperature of ReO2 is 700 ◦C. Therefore, the generation of this reaction products of ReO3 are ReO2 and Re2O7, and the reaction equation is shown in Equation (5). endothermic peak can only be due to the disproportionation of ReO3. The disproportionation reaction In the hydrogen reduction process, the ammonium perrhenate is firstly decomposed into Re2O7, the products of ReO3 are ReO2 and Re2O7, and the reaction equation is shown in Equation (5). In the Re2O7 reacts with hydrogen to form ReO3, and ReO3 reacts with hydrogen to form ReO2 until they hydrogen reduction process, the ammonium perrhenate is firstly decomposed into Re O , the Re O are reduced to Re. In the process of a hydrogen reduction of ammonium perrhenate,2 7 if 2the7 reacts with hydrogen to form ReO , and ReO reacts with hydrogen to form ReO until they are reduced disproportionation reaction and3 reduction3 reaction exist simultaneously, the2 disproportionation to Re. In the process of a hydrogen reduction of ammonium perrhenate, if the disproportionation reaction will be the main reason affecting the complete reduction of ammonium perrhenate at a low reaction and reduction reaction exist simultaneously, the disproportionation reaction will be the main temperature. reason affecting the complete reduction of ammonium perrhenate at a low temperature.

6 T /μV

V 3 μ

T, T, Endothermic peak  2

Exo 0 300 400 500 600 700

Temperature, °C

Figure 4. The differential thermal analysis curve of Re, ReO2, and ReO3. Figure 4. The differential thermal analysis curve of Re, ReO2, and ReO3. In order to research the effect of disproportionation on the hydrogen reduction of ammonium perrhenate,In order the to hydrogen research reductionthe effect experimentsof disproportio of ammoniumnation on the perrhenate hydrogen were reduction carried outof ammonium at the same perrhenate,reduction time the andhydrogen at diff erentreduction reduction experiments temperatures. of ammonium The reducing perrhenate substance were wascarried recrystallized out at the same reduction time and at different reduction temperatures. The reducing substance was ammonium perrhenate at a 200 rpm stirring strength (D50 was 71.17 µm, specific surface was recrystallized26.93 m2/kg). ammonium The XRD patterns perrhenate of the at reductiona 200 rpm products stirring ofstrength ammonium (D50 was perrhenate 71.17 µm, at dispecificfferent 2 surfacetemperatures was 26.93 are shownm /kg). in The Figure XRD5. Thepatterns characteristic of the reductio peaksn of products the reduction of ammonium products wereperrhenate complex at different temperatures are shown in Figure 5. The characteristic peaks of the reduction products at lower temperatures (300–600 ◦C), and the diffraction peaks of the reduction products indicated Re were complex at lower temperatures (300–600 °C), and the diffraction peaks of the reduction and ReO2, and ReO3. The characteristic peaks of Re did not change obviously from 400 to 600 ◦C. products indicated Re and ReO2, and ReO3. The characteristic peaks of Re did not change obviously However, in the range of 300 to 400 ◦C, the characteristic peaks of ReO3 were enhanced. In the range of from 400 to 600 °C. However, in the range of 300 to 400 °C, the characteristic peaks of ReO3 were 400 to 600 ◦C, the characteristic peaks of ReO2 were enhanced, while that of ReO3 were weakened. enhanced. In the range of 400 to 600 °C, the characteristic peaks of ReO2 were enhanced, while that When the temperature reached 700 ◦C, the characteristic peaks of the reduction products were Re, and of ReO3 were weakened. When the temperature reached 700 °C, the characteristic peaks of the other crystal peaks were not observed. This result suggested that the contents of ReO3 in the reduction reduction products were Re, and other crystal peaks were not observed. This result suggested that products increased in the range of 300 to 400 ◦C. However, within the temperature range of 400 to the contents of ReO3 in the reduction products increased in the range of 300 to 400 °C. However,

Metals 2020, 10, 640 6 of 15 Metals 2020, 10, x FOR PEER REVIEW 6 of 16

within the temperature range of 400 to 600 °C, the content of Re in the reduction products did not 600 ◦C, the content of Re in the reduction products did not increase signiﬁcantly, while the content of increase significantly, while the content of ReO2 increased significantly. ReO2 increased signiﬁcantly.

• ⎯Re  ⎯ReO  2 • ⎯Re  ⎯ReO ♦ ⎯ReO Figure (a) Figure (b) 2 3 ♦ ⎯ReO  ⎯Re O • 3 2 7

    °    • • 700 C   400°C • •  • • •  •

  Intensity/a.u. •   

• Intensity/a.u. °   ° • 600 C   350 C • • •     •  • • 

 •    ° • 500°C  300 C  •          

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 2θ/degree 2θ/degree

FigureFigure 5. XRD 5. XRD patterns patterns of the of reductionthe reduction products products of ammonium of ammonium perrhenate perrhenate (D 50(Dwas50 was 71.17 71.17µm, µm, specific surfacespecific was 26.93surface m was2/kg) 26.93 at di mfferent2/kg) at temperatures. different temperatures. Figure (a) Figure was the (a) XRD was ditheffraction XRD diffraction pattern of the

reducedpattern product of the at reduced 300 to 400product◦C; Figureat 300 to (b )400 was °C; the Figure XRD (b di) ﬀwasraction the XRD pattern diffraction of the reduced pattern of product the at reduced product at 500 to 700 °C. 500 to 700 ◦C.

In orderIn order to further to further clarify clarify the influences the influences of reduction of reduction temperatures temperatures on an ammoniumon an ammonium perrhenate hydrogenperrhenate reduction, hydrogen X-ray reduction, photoelectron X-ray photoelectro spectroscopyn spectroscopy was used forwas the used rhenium for the rhenium atomic quantitativeatomic quantitative analysis in different valence states. According to the references [30–32], a rhenium analysis in different valence states. According to the references [30–32], a rhenium atom has a split atom has a split energy level (f), where the spin–orbit splitting was 2.4 eV, and the spin orbital split energy level (f), where the spin–orbit splitting was 2.4 eV, and the spin orbital split intensity ratio of intensity ratio of Re 4f7/2 and Re 4f5/2 was 4:3. The background was a mixed Shirley background Re 4f(Shirley7/2 and Re+ straight 4f5/2 was line) 4:3. [33], The and background the slope of was the a mixedline was Shirley eight. backgroundThe peak positions (Shirley and+ straightfit line)paramters [33], and for the all slope samples of the are linegiven was in Table eight. 2. TheThe X-ray peak photoelectron positions and spectroscopy fit paramters of the for reduced all samples are givenproducts in Table at the2 different. The X-ray reduction photoelectron temperatures spectroscopy are shown in of Figure the reduced A1. products at the di fferent reduction temperatures are shown in Figure A1. Table 2. The peak positions and fit paramters for Re 4f7/2 and Re 4f5/2. Table 2. The peak positions and fit paramters for Re 4f and Re 4f . Fitting Parameters Re Re4+ Re76+/2 Re5/27+ B.E (4f7/2)Fitting Parameters 40.31 Re 42.7Re 4+ Re6 45.2+ Re7+ 46.1 B.E (4f5/2) 42.71 45.1 47.6 48.5 FWHM B.E (4f7/2)1.3 40.311.7 42.7 45.21.75 46.1 1.8 B.E (4f5/2) 42.71 45.1 47.6 48.5 FWHM 1.3 1.7 1.75 1.8 The X-ray photoelectron spectroscopy of the reduced products at different reduction temperatures are shown in Figure A1. According to the peak areas of the different valence states of rhenium,The X-ray the photoelectron content of each spectroscopy valence state of thewas reduced calculated, products as shown at di infferent Table reduction 2. The percent temperatures of are shownrhenium in atom Figure in each A1 .valence According state was to thecalculated, peak areasas shown of thein Table different 3. valence states of rhenium, the content of each valence state was calculated, as shown in Table2. The percent of rhenium atom in each valence state was calculated, as shown in Table3.

Table 3. The percent of rhenium atom in each valence state of the reduction products of ammonium 2 perrhenate (D50 was 71.17 µm, speciﬁc surface was 26.93 m /kg) at diﬀerent temperatures.

4+ 6+ 7+ Reduction Temperature (◦C) Re (%) Re (%) Re (%) Re (%) 300 75.9 20.0 3.0 1.1 350 69.6 16.4 24.0 0.0 400 59.3 14.1 26.6 0.0

500 65.9 16.2 17.9 0.0 600 76.1 18.1 7.8 0.0 700 100 0.0 0.0 0.0 Metals 2020, 10, x FOR PEER REVIEW 7 of 16

Table 3. The percent of rhenium atom in each valence state of the reduction products of ammonium

perrhenate (D50 was 71.17 µm, specific surface was 26.93 m2/kg) at different temperatures.

Reduction Temperature (°C) Re (%) Re4+ (%) Re6+ (%) Re7+ (%) 300 75.9 20.0 3.0 1.1 350 69.6 16.4 24.0 0.0 400 59.3 14.1 26.6 0.0 500 65.9 16.2 17.9 0.0 600 76.1 18.1 7.8 0.0 700 100 0.0 0.0 0.0 Metals 2020, 10, 640 7 of 15

According to the results in Table 3, the percents of rhenium atom in each valence state of the reducedAccording products to theat diff resultserent in temperatures Table3, the percentswere plotted, of rhenium as shown atom in in Figure each valence 6. Combined state of with the reducedTable 2 productsand Figure at di 6,ff erentit can temperatures be concluded were that plotted, the aspercent shown of in Re Figure6+ in6 .the Combined reduction with products Table2 andincreased Figure in6, itthe can range be concluded of 300 to that400 the°C, percentand with ofin Re the6+ intemperature the reduction range products of 400 increased to 600 °C, in the 4+ rangepercent of of 300 Re to in 400 the◦C, reduction and within products the temperature did not increase range of significantly, 400 to 600 ◦ C,while the percentthe content of Re of in Re the reductionincreased productssignificantly. did notThese increase results significantly, showed that while the theReO content3 content of Rein4 +theincreased reduction significantly. products Theseincreased results in showedthe temperature that the ReO range3 content of 300–400 in the °C reduction and decreased products with increased the increase in the temperature in the reduction range oftemperature; 300–400 ◦C the and Re decreased content withdecreased the increase in the intemperature the reduction range temperature; of 300–400 the °C Re and content increased decreased with inthe the increase temperature in the rangereduction of 300–400 temperature.◦C and increasedIn order to with explain the increase this rule, in thethe reductionfollowing temperature.conclusions Inwere order obtained to explain by combining this rule, the the XRD following and DTA conclusions detection were results: obtained the disproportionation by combining the reaction XRD and of DTAReO3 detectionoccurred results:between the 350 disproportionation and 400 °C, ReO3 reactiondecomposed of ReO into3 occurred ReO2 and between Re2O7, 350and and Re2O 4007 was◦C, ReOconverted3 decomposed to ReO3 intoby a ReO hydrogen2 and Re reduction.2O7, and ReTherefore,2O7 was convertedin the temperature to ReO3 by range a hydrogen of 350 to reduction. 400 °C, Therefore,the effect of in disproportionation the temperature range hindered of 350 the to norm 400 ◦alC, reduction the effect process, of disproportionation and this is why hindered ammonium the normalperrhenate reduction cannot process, be reduced and this to more is why rhenium ammonium at 350 perrhenate to 400 °C cannot in the be same reduced reduction to more time. rhenium This atphenomenon 350 to 400 ◦ Cshows in the that same the reduction disproportionation time. This of phenomenon ReO3 hindered shows the that normal the disproportionationreduction reaction ofin ReOa certain3 hindered temperature the normal range, reduction but with reaction the increase in a certain in the temperature temperature, range, the butreduction with the reaction increase rate in thegradually temperature, increased, the reductionand the content reaction of rate rhenium gradually in the increased, reduction and products the content also ofincreased. rhenium At in the reduction productstemperature also increased.of 300 °C, At the the disproportionation reduction temperature of ReO of 3003 did◦C, not the disproportionationoccur, and only the of ReOreduction3 did notreaction occur, was and onlycarried the ou reductiont. Therefore, reaction the wascontent carried of Re out. in Therefore, the reduction the content products of Re was in thehigher. reduction As discussed products above, was higher.in the process As discussed of a hydrogen above, in reduction the process of ammonium of a hydrogen perrhenate, reduction the of ammoniumdisproportionation perrhenate, of ReO the3 disproportionationdecreased the rate ofof ReO the3 reductiodecreasedn reaction, the rate ofand the the reduction increase reaction, in the andtemperature the increase can inincrease the temperature the reduction can reaction increase rate the reductionand reduce reaction the effect rate of and disproportionation reduce the effect on of disproportionationthe reduction process. on theIncreasing reduction the process. reduction Increasing time and the lowering reduction the time reduction and lowering temperature the reduction below 3 temperaturethe temperature below at thewhich temperature the disproportionation at which the disproportionation of ReO occurs can of also ReO increase3 occurs the can content also increase of Re thein the content reduction of Re products. in the reduction products.

100

） Re

% 60 4+ Re （ Re6+ 40 Re7+ Percent 20

300 400 500 600 700 Temperature (℃)

Figure 6.6. OxidationOxidation state state atomic atomic content content changes changes of ammonium of ammonium perrhenate perrhenate (D50 was (D50 71.17 wasµ m,71.17 speciﬁc µm, surfacespecific wassurface 26.93 was m2 26.93/kg)hydrogen m2/kg) hydrogen reduction reduction reaction reaction at diﬀerent at different temperatures. temperatures.

3.2. Influence of Particle Size on Reduction In the solid-state reaction system, the reaction rate is related not only to the temperature but also to the diffusion rate of the reactant [34]. The particle size of the reactant decreases, which is conducive to increasing the diffusion rate [35]. In order to research the effects of particle sizes of ammonium perrhenate on the reduction effect, the recrystallized ammonium perrhenate (D50 81.05 µm) was used for the reduction experiments under the same operating conditions. The XRD patterns of the reduction products of ammonium perrhenate at different temperatures are shown in Figure7. The characteristic peaks of the reduction products were complex at lower temperatures (300–700 ◦C), and the diffraction peaks of the reduction products indicated Re and ReO2, and ReO3. The characteristic peaks of Re did Metals 2020, 10, x FOR PEER REVIEW 8 of 16

3.2. Influence of Particle Size on Reduction In the solid-state reaction system, the reaction rate is related not only to the temperature but also to the diffusion rate of the reactant [34]. The particle size of the reactant decreases, which is conducive to increasing the diffusion rate [35]. In order to research the effects of particle sizes of ammonium perrhenate on the reduction effect, the recrystallized ammonium perrhenate (D50 81.05 µm) was used for the reduction experiments under the same operating conditions. The XRD patterns of the reduction products of ammonium perrhenate at different temperatures are shown in FigureMetals 2020 7. ,The10, 640 characteristic peaks of the reduction products were complex at lower temperatures8 of 15 (300–700 °C), and the diffraction peaks of the reduction products indicated Re and ReO2, and ReO3. The characteristic peaks of Re did not change obviously from 500 to 800 °C. However, in the range not change obviously from 500 to 800 ◦C. However, in the range of 300 to 400 ◦C, the characteristic peaks of 300 to 400 °C, the characteristic peaks of ReO3 were enhanced. In the range of 500 to 800 °C, the of ReO3 were enhanced. In the range of 500 to 800 ◦C, the characteristic peaks of ReO2 were enhanced, characteristic peaks of ReO2 were enhanced, while that of ReO3 were weakened. When the while that of ReO3 were weakened. When the temperature reached 900 ◦C, the characteristic peaks temperature reached 900 °C, the characteristic peaks of the reduction products were Re and ReO2, of the reduction products were Re and ReO2, and other crystal peaks were not observed. This result and other crystal peaks were not observed. This result suggested that the contents of ReO3 in the suggested that the contents of ReO in the reduction products increased in the range of 300 to 400 C. reduction products increased in the3 range of 300 to 400 °C. However, within the temperature range◦ However, within the temperature range of 500 to 800 C, the content of Re in the reduction products of 500 to 800 °C, the content of Re in the reduction products◦ did not increase significantly, while the did not increase signiﬁcantly, while the content of ReO2 increased signiﬁcantly. content of ReO2 increased significantly.

• • • ⎯Re  ⎯ReO 2 • ⎯Re  ⎯ReO ♦ ⎯ReO Figure (a) Figure (b) 2 3 ♦ ⎯ReO  ⎯Re O  3 2 7    • •   • • • • 900°C • • •  ° •  • 400 C • •    • • •   • 800°C • • • •   • •  ° • 350 C  •  Intensity/a.u. • •    Intensity/a.u. • •   °  • • •  700 C • • • •    •  •  ° •  • 600 C • • • • °  300 C • •  •     500°C • • • • • •

10 20 30 40 50 60 70 80 90 0 102030405060708090 2θ/degree 2θ/degree

Figure 7.7. XRDXRD patternspatterns of of the the reduction reduction products products of ammonium of ammonium perrhenate perrhenate (D50 was (D50 81.05 wasµ m,81.05 specific µm, 2 specificsurface wassurface 21.72 was m /21.72kg) at m di2ff/kg)erent at temperatures. different temperatures. Figure (a) wasFigure the XRD(a) was diff ractionthe XRD pattern diffraction of the patternreduced of product the reduced at 300 toproduct 400 ◦C; at Figure 300 to (b 400) was °C; the Figure XRD di(bff) ractionwas the pattern XRD diffraction of the reduced pattern product of the at reduced500 to 900 product◦C. at 500 to 900 °C.

InIn order to further clarify the influence influence of part particleicle sizes on an ammonium perrhenate hydrogen reduction, X-rayX-ray photoelectronphotoelectron spectroscopy spectroscopy was was used used to quantitativelyto quantitatively analyze analyze this feature.this feature. The X-ray The X-rayphotoelectron photoelectron spectroscopy spectroscopy of the reducedof the reduced products pr atoducts different at different reduction reduction temperatures temperatures are shown are in shownFigure inA2 Figure. The percentA2. The of percent rhenium of rhenium atom in eachatom valence in each statevalence was state calculated, was calculated, as shown as inshown Table in4. TableAccording 4. According to the results to the in results Table 4in, theTable percents 4, the ofpercents rhenium of rhenium atom in each atom valence in each state valence of the state reduced of the reducedproducts products at different at temperaturesdifferent temperatures were plotted, were as plotted, shown inas Figureshown8 .in Combined Figure 8. withCombined Table4 andwith TableFigure 48 ,and it can Figure be concluded 8, it can thatbe concluded the content that of Re the in content the reduction of Re productsin the reduction increased products with the increased decrease of ReO between 350 and 800 C. At the reduction temperature of 300 C, the disproportionation of with the3 decrease of ReO3 between◦ 350 and 800 °C. At the reduction◦ temperature of 300 °C, the disproportionationrhenium trioxide did of notrhenium occur trioxide and the contentdid not ofoccur rhenium and the was content higher. of The rhenium composition was higher. law of The the compositionreduction products law of wasthe thereduction same as products that of previous was the reductionsame as that experiments, of previous which reduction proved experiments, the accuracy whichof the reductionproved the mechanism accuracy of analysis. the reduction When mech theanism reduction analysis. temperature When the reached reduction 800 ◦temperatureC, rhenium reachedtrioxide still800 existd.°C, rhenium It can betrioxide concluded still thatexistd. the disproportionationIt can be concluded of ReOthat 3theexisted disproportionation in the range of 350 of to 800 C. Compared with the previous reduction experiment, the temperature range at which the ReO3 existed◦ in the range of 350 to 800 °C. Compared with the previous reduction experiment, the temperaturedisproportionation range at reaction which the occurs disproportionation had increased. Thisreaction was occurs because had the increased. particle size This of was ammonium because perrhenate increased, the solid reaction diffusion rate decreased, and the contact between the reactant and hydrogen was not ideal. Therefore, under the same operating conditions, reducing the particle size of ammonium perrhenate can increase the hydrogen reduction diffusion rate and reduce the influence of the disproportionation on the reduction process. Metals 2020, 10, x FOR PEER REVIEW 9 of 16

the particle size of ammonium perrhenate increased, the solid reaction diffusion rate decreased, and the contact between the reactant and hydrogen was not ideal. Therefore, under the same operating conditions, reducing the particle size of ammonium perrhenate can increase the hydrogen reduction diffusion rate and reduce the influence of the disproportionation on the reduction Metalsprocess.2020, 10, 640 9 of 15

Table 4. The percents of rhenium atom in each valence state of the reduction products of Tableammonium 4. The percents perrhenate of rhenium (D50 was atom 81.05 in each µm, valence specific state surface of the was reduction 21.72 productsm2/kg) at ofdifferent ammonium 2 perrhenatetemperatures. (D50 was 81.05 µm, speciﬁc surface was 21.72 m /kg) at diﬀerent temperatures.

Reduction Temperature (°C) Re (%) Re4+4+ (%) Re6+ (%)6+ Re7+ (%)7 + Reduction Temperature (◦C) Re (%) Re (%) Re (%) Re (%) 300 65.7 20.0 10.8 3.5 300350 65.758.9 21.8 20.0 19.1 10.8 0.2 3.5 350 58.9 21.8 19.1 0.2 400 36.3 10.7 45.9 7.1 400 36.3 10.7 45.9 7.1 500 68.1 15.7 16.2 0.0 500 68.1 15.7 16.2 0.0 600600 72.272.2 18.2 18.2 9.6 9.60.0 0.0 700700 72.772.7 22.0 22.0 5.3 5.30.0 0.0 800800 79.079.0 17.2 17.2 3.8 3.80.0 0.0 900900 80.180.1 19.9 19.9 0.0 0.00.0 0.0

100

） 60 Re 4+

% Re 6+ （ Re 40 Re7+

Pencert 20

300 400 500 600 700 800 900 Temperature (℃)

FigureFigure 8. Oxidation 8. Oxidation state state atomic atomic content content changes changes of ammonium of ammonium perrhenate perrhenate (D50 (Dwas50 was 81.05 81.05µm, µm, speciﬁc 2 surfacespecific was 21.72surface m was/kg) 21.72 hydrogen m2/kg) hydrogen reduction reduction reaction reaction at diﬀerent at different temperatures. temperatures.

3.3. Analysis3.3. Analysis of the of Productsthe Products of theof the Rhenium Rhenium Ingots Ingots The ReThe powders Re powders were were prepared prepared by by unrecrystallized unrecrystallized ammonium ammonium perrhenate perrhenate and and recrystallized recrystallized ammoniumammonium perrhenate perrhenate with with a di ﬀaerent different D50, D and50, and the rheniumthe rhenium ingots ingots were were prepared prepared from from these these rhenium powders.rhenium The powders. relevant The physical relevant parameters physical paramete of the productsrs of the areproducts shown are in shown Table 5in. TheTable particle 5. The size distributionparticle size of the distribution rhenium of powder the rhenium is shown powder in Figureis shown9. Itin canFigure be 9. seen It can from be seen Table from5 and Table Figure 5 9 that theand rheniumFigure 9 that powder the rhenium prepared powder by ammonium prepared by perrhenate ammonium withperrhenate a small with particle a small size particle is also of relativelysize is small also particleof relatively size; small the particle particle size size; distribution the particle uniformity size distribution of rhenium uniformity powder of rhenium prepared by powder prepared by recrystallized ammonium perrhenate was improved obviously, and the recrystallized ammonium perrhenate was improved obviously, and the rhenium ingot prepared has a rhenium ingot prepared has a higher density. The theoretical density of rhenium is 21.04 g/cm3, and higher density. The theoretical density of rhenium is 21.04 g/cm3, and the density of the rhenium ingot the density of the rhenium ingot prepared by D50 71.17 µm ammonium perrhenate reached 20.106 3 preparedg/cm by3. The D50 rhenium71.17 µ mingot ammonium prepared under perrhenate this conditio reachedn was 20.106 close g/cm to the. The theoretical rhenium density. ingot preparedThe underSEM this images condition of the was surface close defects to the of theoreticalthe rhenium density. ingots are The shown SEM in images Figure 10. of theIt can surface be seen defects that of the rheniumthe rhenium ingots ingot are prepared shown by in the Figure small 10 particle. It can size be rhenium seen that powder the rhenium not only ingothas a preparedhigh density by the small particle size rhenium powder not only has a high density but also has a small surface hole defect. Therefore, reducing the particle size of the rhenium powder is a key factor in preparing a high-density rhenium ingot.

Table 5. The rhenium materials prepared from ammonium perrhenate of diﬀerent particle sizes.

D of Ammonium Speciﬁc Surface Area of Density of Rhenium 50 D of Re Powders (µm) Perrhenate (µm) 50 Re (m2/kg) Ingots (g/cm3) 81.05 34.04 97.54 18.779 71.17 19.74 163.70 20.106 123.90 52.15 72.73 17.726 Metals 2020, 10, x FOR PEER REVIEW 10 of 16

but also has a small surface hole defect. Therefore, reducing the particle size of the rhenium powder is a key factor in preparing a high-density rhenium ingot.

Table 5. The rhenium materials prepared from ammonium perrhenate of different particle sizes.

Density of D50 of Ammonium D50 of Re Specific Surface Area of Rhenium Ingots Perrhenate (µm) Powders (µm) Re (m2/kg) (g/cm3) 81.05 34.04 97.54 18.779 71.17 19.74 163.70 20.106 Metals 2020, 10, 640123.90 52.15 72.73 17.72610 of 15

Figure 9. The particle size distribution of rhenium powder prepared by ammonium perrhenate with Figure 9. The particle size distribution of rhenium powder prepared by ammonium perrhenate with diﬀerent particle sizes ((a) prepared by 81.05 µm D50 ammonium perrhenate, (b) prepared by 71.17 µm Metals 2020different, 10, x FOR particle PEER REVIEW sizes (( a) prepared by 81.05 µm D50 ammonium perrhenate, (b) prepared11 by of 71.1716 D50 ammonium perrhenate, (c) prepared by 123.90 µm D50 ammonium perrhenate). µm D50 ammonium perrhenate, (c) prepared by 123.90 µm D50 ammonium perrhenate).

FigureFigure 10. 10.SEM SEM images images of of the the surface surface defectsdefects ofof the rhenium ingots ingots (( ((aa) )prepared prepared by by 52.15 52.15 µmµm D50 D 50ReRe powders,powders, (b )(b prepared) prepared by by 34.04 34.04µ µmm D D5050 ReRe powders,powders, ( c)) prepared prepared by by 19.74 19.74 µmµm D D5050 ReRe powders). powders).

3.4. Proposed Flow Sheet Based on the experimental results, the flow sheet for the processing of rhenium ingots by ammonium perrhenate was tentatively suggested, which is shown in Figure 11. The ammonium perrhenate particles were refined by homogeneous recrystallization, and the D50 of the ammonium perrhenate particles was refined from 123.90 to 71.17 µm. The recrystallized ammonium perrhenate was completely reduced by hydrogen at 700 °C for 3 h, and the D50 of 19.74 µm rhenium powder was obtained. The density of the rhenium ingots pressed by these rhenium powders was 20.106 g/cm3. The theoretical density of rhenium is 21.04 g/cm3, and a rhenium ingot that reaches the theoretical density of more than 90% is a high-quality product. In this study, the rhenium ingot density was 95.56% of the theoretical density, which reached the high-quality product standard. The optimized production process of rhenium ingot not only realized a low-temperature reduction, but also increased the density of the rhenium ingot, which can provide a theoretical basis and practical experience for industrial production.

Figure 11. Proposed flow sheet for the preparation of high-density rhenium ingot by homogeneous recrystallization of ammonium perrhenate by a hydrogen reduction.

4. Conclusions In this study, the influences of disproportionation on the hydrogen reduction of ammonium perrhenate were investigated and the following conclusions were drawn: (1) In the process of a hydrogen reduction of ammonium perrhenate, the disproportionation of ReO3 decreased the rate of the reduction reaction, and the increase in the reduction temperature was conducive to increasing the reduction reaction rate and reducing the influence of disproportionation on the reduction process. (2) At the same reduction temperature, reducing the particle sizes of ammonium perrhenate was conducive to increasing the hydrogen reduction rate and reducing the influence of the disproportionation on the reduction process.

Metals 2020, 10, x FOR PEER REVIEW 11 of 16

Figure 10. SEM images of the surface defects of the rhenium ingots ((a) prepared by 52.15 µm D50 Re Metals 2020, 10, 640 11 of 15 powders, (b) prepared by 34.04 µm D50 Re powders, (c) prepared by 19.74 µm D50 Re powders).

3.4. Proposed Flow Sheet Based on the experimental results, the flow flow sheetsheet for the processing of rhenium ingots by ammonium perrhenate was tentatively suggested, which is shownshown inin FigureFigure 1111.. The ammoniumammonium perrhenate particles were refinedrefined byby homogeneoushomogeneous recrystallization,recrystallization, andand thethe DD5050 of the ammoniumammonium perrhenate particles was refinedrefined fromfrom 123.90123.90 toto 71.1771.17 µµm.m. TheThe recrystallizedrecrystallized ammonium perrhenate was completelycompletely reducedreduced by by hydrogen hydrogen at at 700 700◦C °C for for 3 h,3 andh, and the the D50 Dof50 19.74of 19.74µm µm rhenium rhenium powder powder was obtained.was obtained. The densityThe density of the of rhenium the rhenium ingots ingots pressed pressed by these by rheniumthese rhenium powders powders was 20.106 was g20.106/cm3. Theg/cm theoretical3. The theoretical density density of rhenium of rhenium is 21.04 gis/cm 21.043, and g/cm a rhenium3, and a ingotrhenium that reachesingot that the reaches theoretical the densitytheoretical of moredensity than of 90%more is than a high-quality 90% is a high-quality product. In thisproduct. study, Inthe this rhenium study, the ingot rhenium density ingot was 95.56%density ofwas the 95.56% theoretical of the density, theoretical which density, reached which the high-quality reached the product high-quality standard. product The optimizedstandard. productionThe optimized process production of rhenium process ingot of not rhenium only realized ingot not a low-temperature only realized a reduction,low-temperature but also reduction, increased thebut densityalso increased of the rhenium the density ingot, of which the rhenium can provide ingot, a theoreticalwhich can basis provide and practicala theoretical experience basis and for industrialpractical experience production. for industrial production.

Figure 11.11. Proposed flowflow sheet for thethe preparationpreparation ofof high-densityhigh-density rheniumrhenium ingotingot byby homogeneoushomogeneous recrystallization ofof ammoniumammonium perrhenateperrhenate byby aa hydrogenhydrogen reduction.reduction. 4. Conclusions 4. Conclusions In this study, the influences of disproportionation on the hydrogen reduction of ammonium In this study, the influences of disproportionation on the hydrogen reduction of ammonium perrhenate were investigated and the following conclusions were drawn: perrhenate were investigated and the following conclusions were drawn: (1) In the process of a hydrogen reduction of ammonium perrhenate, the disproportionation of (1) In the process of a hydrogen reduction of ammonium perrhenate, the disproportionation of ReO3 decreased the rate of the reduction reaction, and the increase in the reduction temperature was ReO3 decreased the rate of the reduction reaction, and the increase in the reduction temperature was conducive to increasing the reduction reaction rate and reducing the influence of disproportionation conducive to increasing the reduction reaction rate and reducing the influence of disproportionation on the reduction process. on the reduction process. (2) At the same reduction temperature, reducing the particle sizes of ammonium perrhenate (2) At the same reduction temperature, reducing the particle sizes of ammonium perrhenate was conducive to increasing the hydrogen reduction rate and reducing the influence of the was conducive to increasing the hydrogen reduction rate and reducing the influence of the disproportionation on the reduction process. disproportionation on the reduction process. (3) The rhenium ingot prepared by the small particle size rhenium powder not only has a high density but also has a small surface hole defect. Therefore, reducing the particle size of the rhenium powder is a key factor in preparing high-density rhenium ingot. (4) It is feasible to increase the density of rhenium ingot by reducing the particle size of the rhenium powder. The particle size of ammonium perrhenate was reduced to a rhenium powder with a 2 D50 of 19.74 µm and a specific surface area of 163.70 m /kg, which was pressed into a rhenium ingot with a density of 20.106 g/cm3, close to the theoretical density of rhenium.

Author Contributions: J.T.: Writing—original draft; Data curation; Formal analysis; Investigation; Funding acquisition. Y.S.: Data curation; Formal analysis; Funding acquisition; Resources. C.Z.: Data curation; Software. L.W.: Resources; Writing—review and editing. Y.Z.: Investigation; Supervision; Funding acquisition; Resources. D.F.: Writing—review and editing; Resources. Y.L.: Writing—review and editing; Funding acquisition. All authors have read and agreed to the published version of the manuscript. Metals 2020, 10, 640 12 of 15

Funding: Liaoning provincial department of science and technology doctoral research initiation fund, grant number 2019-BS-130; Open project fund of key laboratory of ecological metallurgy of polymetallic symbiosis in ministry of education, Northeastern University, grant number NEMM2019003. Acknowledgments: School of Biomedical & Chemical Engineering, Liaoning Institute of Science and Technology and the analysis and test center of institute of metals, Chinese academy of sciences undertook the sample test for this study. The authors are grateful for these supports. ConﬂictsMetals 2020 of, 10 Interest:, x FOR PEERThe REVIEW authors declare no conﬂict of interest. 13 of 16

Appendix A Appendix A

6 8 a-300℃ Re 4f 7/2 b-350℃ 5 Re 4f7/2 Re 4f5/2 Re 4f5/2 6 Re(Ⅳ)4f7/2 4 Re(Ⅳ)4f Re(Ⅳ)4f 7/2 ） 5/2 s

） Re(Ⅳ)4f Re(Ⅵ)4f s 5/2

（ 7/2

（ 3 4 Re(Ⅵ)4f7/2 Re(Ⅵ)4f5/2 Re(Ⅵ)4f Re(Ⅶ)4f 5/2 7/2 2 Counts Counts 4 Re(Ⅶ)4f5/2 4 10

2 10 1

0 0 52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

3 5 c-400℃ Re 4f7/2 d-500℃ Re 4f Re 4f5/2 7/2 4 Re 4f Re(Ⅳ)4f7/2 5/2

Re(Ⅳ)4f Re(Ⅳ)4f7/2 ） 5/2

s 2 3 Re(Ⅳ)4f Re(Ⅵ)4f7/2 ） 5/2 （ s Re(Ⅵ)4f Re(Ⅵ)4f 5/2 （ 7/2 2 Re(Ⅵ)4f5/2 Counts 4 Counts

10 1 1 4 10 0

52 50 48 46 44 42 40 38 36 34 32 0 Binding Energy (eV) 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV)

8 12 e-600℃ f-700℃ Re 4f7/2 Re 4f7/2

Re 4f5/2 10 Re 4f5/2 6 Re(Ⅳ)4f7/2 ）

Re(Ⅳ)4f ） 8 s 5/2 s （ Re(Ⅵ)4f7/2 （ 4 6 Re(Ⅵ)4f5/2 Counts Counts

4 4 4 10

10 2 2

0 0 52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

Figure A1. X-ray photoelectron spectroscopy of the reduction products of ammonium perrhenate Figure A1. X-ray photoelectron spectroscopy of2 the reduction products of ammonium perrhenate (D50 was 71.17 µm, speciﬁc surface was 26.93 m /kg) at diﬀerent temperatures. (D50 was 71.17 µm, specific surface was 26.93 m2/kg) at different temperatures.

Metals 2020, 10, 640 13 of 15 Metals 2020, 10, x FOR PEER REVIEW 14 of 16

a-300℃ Re 4f7/2 b-350℃ 6 Re 4f7/2 Re 4f5/2 4 Re 4f5/2 Re(Ⅳ)4f7/2 Re(Ⅳ)4f7/2 Re(Ⅳ)4f5/2 Re(Ⅳ)4f ）

） 5/2 s

s 4 Re(Ⅵ)4f7/2 Re(Ⅵ)4f （（ 7/2

Re(Ⅵ)4f5/2 Re(Ⅵ)4f5/2 2 Re(Ⅶ)4f7/2 Re(Ⅶ)4f7/2 Counts Counts 4 2 Re(Ⅶ)4f5/2 4 Re(Ⅶ)4f5/2 10 10

0 0 52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

10 c-400℃ Re 4f 7/2 d-500℃ Re 4f Re 4f 7/2 5/2 8 Re 4f 2 Re(Ⅳ)4f 5/2 7/2 Re(Ⅳ)4f Re(Ⅳ)4f 7/2 ）

5/2 ）

s Re(Ⅳ)4f s 6 Re(Ⅵ)4f 5/2 （ 7/2 （ Re(Ⅵ)4f Re(Ⅵ)4f 7/2 5/2 Re(Ⅵ)4f 4 5/2 Re(Ⅶ)4f7/2 Counts

4 1 Counts Re(Ⅶ)4f5/2 4 10 10 2

0 0 52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

8 e-600℃ Re 4f f-700℃ 7/2 Re 4f 8 7/2 Re 4f5/2 Re 4f5/2 Re(Ⅳ)4f7/2 6 Re(Ⅳ)4f7/2 ） Re(Ⅳ)4f ）

s 6

5/2 s Re(Ⅳ)4f5/2 （ Re(Ⅵ)4f7/2 （ Re(Ⅵ)4f7/2 4 Re(Ⅵ)4f5/2 4 Re(Ⅵ)4f5/2 Counts Counts 4 4 10 2 10 2

0 0 52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

12 10 h-900℃ g-800℃ Re 4f 10 Re 4f7/2 7/2 Re 4f Re 4f 5/2 8 5/2 Re(Ⅳ)4f 8 Re(Ⅳ)4f7/2 7/2 ）） s Re(Ⅳ)4f s Re(Ⅳ)4f 5/2 6 5/2 （（ 6 Re(Ⅵ)4f7/2 Re(Ⅵ)4f 5/2 4

4 Counts 4 Counts 4 10 10 2 2

0 0

52 50 48 46 44 42 40 38 36 34 32 52 50 48 46 44 42 40 38 36 34 32 Binding Energy (eV) Binding Energy (eV)

Figure A2. X-ray photoelectron spectroscopy of the reduction products of ammonium perrhenate 2 Figure(D50 was A2. 81.05 X-rayµm, photoelectron speciﬁc surface spectroscopy was 21.72 m of/ kg)the atre diductionﬀerent products temperatures. of ammonium perrhenate (D50 was 81.05 µm, specific surface was 21.72 m2/kg) at different temperatures.

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