Niobium Powder Production in Molten Salt by Electrochemical Pulverization

Transcription

1 Niobium Powder Production in Molten Salt by Electrochemical Pulverization Boyan Yuan * and Toru H. Okabe ** *: Graduate Student, Department of Materials Engineering, University of Tokyo **: Associate Professor, Institute of Industrial Science, University of Tokyo Fancj May 20-28,2005 Wuhu,Anhui,China 1

2 Niobium and Tantalum Table Comparison of Nb with Ta Atomic number Nb Ta VB 41 VB 73 Crystal structure bcc bcc Melting point 2468 C 2980 C Density 8.56 g/cm g/cm 3 Dielectric constant of pentoxide Reserves 4,400,000 ton Nb 43,000 ton Ta Annual world productivity Price Major applications Commercial Aluminothermic production processreduction (ATR) Product form Development 23,000 ton Nb ~ 50 $/kg Microalloy element for steel Nb/FeNb bar Next generation capacitors Nb, a potential substitute of Ta for next generation capacitors 2,300 ton Ta ~ 700 $/kg Solid electrolytic capacitor Sodiothermic reduction (Hunter) Ta powder Higher performance capacitor 2

3 Hunter process K 2 TaF 7 (l) + 5 Na(l) Ta(s) + 5 NaF(l) + 2 KF(l) Diluent Features Well controlled powder purity and morphology Batch type process Stirrer K 2 TaF 7 and Na feeding ports Reactor Molten halides diluent Electric furnace Tantalum powder deposits Figure Schematic illustration of the Hunter process. Time and labor consuming reduction process followed by mechanical and hydrometallurgical separation operations Large amount of fluorides wastes 3

4 Direct reduction processes of oxide (a) e - A Current monitor External circuit Nb 2 O 5 powder Molten CaCl 2 Ca-Ni-Ag liquid alloy Okabe, et al Nb 2 O e - 2 Nb + O 2-5 Ca 5 Ca e - (b) Nb 2 O 5 powder Porous Ta plate Ar gas nozzle H.C.Starck, 2001 Nb 2 O 5 +5 Mg 2 Nb + 5 MgO Mg vapor reductant Mg chips (c) e - External power source Graphite anode Nb 2 O 5 pellet CaCl 2 -NaCl molten salt Fray, et al Nb 2 O e - Nb+ 5 O 2- C + x O 2- CO x + 2x e - (d) Preform (Nb 2 O 5 + flux) Stainless steel mesh Mg vapor reductant Okabe, et al Nb 2 O 5 +5 Mg 2 Nb + 5 MgO Mg (or Mg alloy) chips 4

5 Objectives of this study To develop a new, low cost, high quality niobium powder production process for capacitor or other electronic applications. Essential process features: 〇 〇 fine and homogeneous niobium powder have to be obtained. purity and morphology of the niobium powder have to be controlled. the process is required to be low cost and efficient, (semi-) continuous, environmentally sound. 5

6 Electrochemical Pulverization (EP) Anodic: Nb (bulk, s) Nb n+ (l)+ n e - Cathodic: n Dy 3+ (l) + n e - n Dy 2+ (l) Redox Nb n+ (l) + n Dy 2+ (l) Nb (powder, s) + n Dy 3+ (l) reaction in molten salt: Overall Reaction : Nb (bulk, s) Nb (powder, s) e - e - Nb n+ (l) Nb(s) Dy 2+ (l) Dy 3+ (l) Nb rod feed (anode) Mg-Ag liquid alloy (cathode) Molten salt containing Dy 2+ Nb powder Figure Schematic illustration of the configuration of EP. Features: 〇 Fine powder production by homogeneous ionic redox reaction. Purity and morphology can be controlled by: dissolution speed of Nb bulk, concentration of Dy 2+ ions reductant, temperature. Low cost: cheap ATR Nb ingot feed can be used. Environmentally sound: reductant is not consumed, molten salt can be reused. 6

7 Thermodynamic analysis (a) Nb-Dy-Cl system T = 1000 K NbCl 5 (l, in salt) NbCl 4 (l, in salt) NbCl 3 (l, in salt) NbCl 2 (l, in salt) (b) log p Cl Nb (s) β log a Dy Dy 2+ Dy 3+ e - α β Cl 2 (g) DyCl 3 (l, in salt) In molten salt Nb DyCl 2 (l, in salt) Dy (s) α log a Nb Cl- (NbCl 2 ) Nb 2+ Figure (a) Three dimensional chemical potential diagram for the Nb-Dy-Cl system at 1000 K. (b) A mechanism for niobium powder production using Dy 3+ /Dy 2+ equilibrium in molten salt. Nb ions reduction using Dy 3+ /Dy 2+ equilibrium 7 is thermodynamically feasible e- -60

8 Experimental procedure (a) Flowchart of Experimental procedure NaCl KCl MgCl 2 Dy, Ag Nb rod Molten salt preparation NaCl-KCl-MgCl 2 molten salt Dy 2+ addition into molten salt NaCl-KCl-MgCl 2 -DyCl 2 molten salt Producing Nb n+ ions by anodic dissolution Reduction of Nb n+ by Dy 2+ in molten salt Nb powder with salt Leaching CV analysis Dy + Ag + MgCl 2 DyCl 2 + Ag-Mg CV analysis Galvanostatic technique CV analysis Nb powder (b) Experimental conditions for EP of ATR-Nb XRD, SEM Particle size XRF, ICP-AES analysis Exp. # A B Dy add. w Dy /g w ms /g Molten salt Composition (mol%) NaCl-36KCl-9MgCl 2-1DyCl 2 NaCl-36KCl-8MgCl 2-2DyCl 2 Temp. Current T / K i / A 2 2 8

9 Cyclic voltammogram of Nb (a) Experimental setup Ar gas inlet/outlet Thermocouple Heating element Nb wire working electrode Ni quasi-reference electrode Graphite counter electrode Silver shots Ceramics insulator (a) Cyclic voltammogram of Nb in NaCl-36 mol%kcl-10 mol%mgcl 2 molten salt Mg 2+ /Mg -1.1 Nb/Nb 2+ Nb 2+ /Nb Current density, j / A cm A A' Potential, E / V vs. Ni quasi-ref B C 9

10 CV before and after Dy 2+ addition Current density, j / A cm -2 Current density, j / A cm A' Dy lump Ag shot Stainless steel holder Mg-Ag liquid alloy RE Graphite WE Mg 2+ / Mg RE Graphite CE -1 Mg 2+ /Mg A NaCl-36KCl-10MgCl 2-2 Mg 2+ /MgDy 3+ /Dy A' A E' E Potential, E / V vs. Mg-Ag liquid alloy D D Cl - /Cl g Dy add g Dy add. (a) Exp. apparatus (b) CV before Dy addition (c) CV after Dy addition 10

11 EP of Nb in Dy 2+ containing molten salt (a) Experimental apparatus for EP of Nb rod anode Electrochemical interface Thermocouple Molten salt containing Dy 2+ Nb rod anode Mg-Ag liquid alloy cathode Stainless steel Nb powder collecting dish (b) Chronopotentiomatric curve of Nb anode (i = 2 A) Potential, E / V vs. Mg-Ag liq. alloy E = 1.9 V t / ks Anodic current efficiency (Nb Nb n+ + n e - ) n=2, 58% n=3, 87% E Nb 3+ /Nb 2+ E Nb 2+ /Nb E Dy 3+ /Dy 2+ E Mg 2+ /Mg 11

12 Appearances before and after EP (a) Nb anode before and after EP Before After EP Initial Nb rod dissolved Nb rod Nb rod was dissolved Immersed part of Nb rod 10 mm 10 mm 10 mm (b) Stainless steel holder of liquid alloy cathode after EP Cathode current lead Supporting rod for cathode Stainless steel holder 10 mm No Nb deposition on cathode was observed (c) Nb deposits in powder collecting dish after EP Supporting rod for collecting dish Nb powder Nb powder with salt was obtained in 12 collecting dish Collecting dish 10 mm

13 XRD and XRF analysis (a) XRD pattern of the Nb powder obtained by EP. Intensity, I (a.u.) : Nb JCPDS # Angle, 2θ (degree) (b) XRF results of the Nb powder obtained by EP. Exp. # A B Concentration of element i, C i (mass%) Nb Fe Cr Ni Ag Mg W Ta < < <0.01 Yield 92% 98% Presently, niobium powder with purity of 98 mass% was obtained. 13

14 SEM and particle size analysis 2 µm Frequency, F (%) (a) Exp. A: in NaCl-36 mol%kcl-9 mol% MgCl2-1 mol% DyCl D10 = 1.0 µm D50 = 1.8 µm D90 = 3.0 µm Particle size, d (µm) 2 µm Frequency, F (%) (b) Exp. B: in NaCl-36 mol%kcl-8 mol% MgCl2-2 mol% DyCl D10 = 0.2 µm D50 = 0.5 µm D90 = 0.9 µm Particle size, d (µm) Figure SEM image and particle size distribution profile of the Nb powder obtained by EP technique. Fine and homogeneous Nb powder was obtained 14

15 Summary and future work Summary: The electrochemical pulverization technique of bulk niobium in molten NaCl-KCl-MgCl 2 salt containing Dy 2+ ions was demonstrated to be effective in producing fine and homogeneous niobium powder. Nb 2+ Nb Dy 2+ Dy 3+ Future work: Mg Mg 2+ Development of powder purity and morphology controlling techniques. Improvement of current efficiency. Development of the electrochemical pulverization technique to be applied to the production technology of niobium powder for next generation high performance capacitors. 15