Electrodeposition of rare earth elements

Abstract

A method of obtaining a purified rare earth metal deposit from a rare earth metal halide salt, by: (i) producing a rare earth metal borohydride salt by the following reaction:n·ABH4+m·(RE)X3→RE(BH4)3+n·AXwherein the variables are defined as follows: A is an alkali metal cation; RE represents one or more rare earth metal cations; X represents a halide selected from chloride, bromide, and iodide; n is the molar amount of ABH4; and m is the molar amount of (RE)X3; wherein the ratio of n:m is 3:1 to 5:1; (ii) mixing RE(BH4)3 and AX with a halogen-free organic solvent to form a solution containing the RE(BH4)3 salt dissolved therein and with AX undissolved and precipitated; and (iii) contacting the solution from step (ii) with an anode and cathode in electrical communication, and electrically charging the anode and cathode to electroplate the rare earth metal onto the cathode.

What is claimed is: 
     
         1 . A method of obtaining a purified rare earth metal deposit from a rare earth metal halide salt, the method comprising:
 (i) producing a rare earth metal borohydride salt from the rare earth metal halide salt by the following reaction:
     n ·ABH 4   +m ·(RE)X 3   RE(BH 4 ) 3   +n ·AX
 
   wherein the above reaction is conducted as a solid state reaction in the absence of a solvent, and wherein the variables are defined as follows:   A is an alkali metal cation which counterbalances the borohydride (BH 4   − ) anion;   RE represents one or more rare earth metal cations selected from scandium, yttrium, and lanthanide elements;   X represents a halide selected from chloride, bromide, and iodide;   n is the molar amount of ABH 4 ; and   m is the molar amount of (RE)X 3 ; wherein the ratio of n:m is 3:1 to 5:1;   (ii) mixing RE(BH 4 ) 3  and AX with a halogen-free organic solvent in which RE(BH 4 ) 3  is fully dissolved and in which AX is highly insoluble and precipitates, thereby forming a solution containing the RE(BH 4 ) 3  salt dissolved therein in the substantial absence of AX; and   (iii) contacting the solution from step (ii) with an anode and cathode in electrical communication, and electrically charging the anode and cathode to electroplate the rare earth metal onto the cathode.   
     
     
         2 . The method of  claim 1 , wherein A is lithium. 
     
     
         3 . The method of  claim 1 , wherein RE represents one or more lanthanide elements. 
     
     
         4 . The method of  claim 3 , wherein RE comprises Nd or Dy. 
     
     
         5 . The method of  claim 1 , wherein the ratio of n:m is about 4:1. 
     
     
         6 . The method of  claim 1 , wherein the RE metal deposit does not contain a fluorine-containing passivation layer. 
     
     
         7 . The method of  claim 1 , wherein the halogen-free organic solvent is selected from ethers, dialkyl sulfides, nitriles, and carbonates. 
     
     
         8 . The method of  claim 1 , wherein the halogen-free organic solvent is selected from ether solvents. 
     
     
         9 . The method of  claim 8 , wherein the ether solvent is selected from the group consisting of diethyl ether, ethyl propyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, methyl t-butyl ether, dimethoxy ethane (monoglyme), diethylene glycol dimethyl ether (diglyme), di triethylene glycol dimethyl ether (triglyme), tetrahydrofuran, and dioxolane. 
     
     
         10 . The method of  claim 1 , wherein the halogen-free organic solvent is selected from dialkyl sulfide solvents. 
     
     
         11 . The method of  claim 10 , wherein the dialkyl sulfide solvent is selected from the group consisting of dimethyl sulfide, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, and ethyl propyl sulfide. 
     
     
         12 . The method of  claim 1 , wherein the electroplating step is conducted directly on the solution from step (ii) in which AX salt has precipitated. 
     
     
         13 . The method of  claim 1 , wherein the electroplating step is conducted by first removing AX salt that precipitated from the solution from step (ii) to form an AX-free solution, followed by contacting the AX-free solution with an anode and cathode in electrical communication, and electrically charging the anode and cathode to result in rare earth metal deposition on the cathode. 
     
     
         14 . The method of  claim 1 , wherein said solid state reaction is achieved by ball milling of ABH 4  and (RE)X 3  in the absence of a solvent. 
     
     
         15 . The method of  claim 1 , wherein steps (i) and (iii) are conducted under an inert atmosphere. 
     
     
         16 . The method of  claim 1 , wherein RE in the RE(BH 4 ) 3  salt is a single RE metal and the purified rare earth metal deposited on the cathode is a single metal. 
     
     
         17 . The method of  claim 1 , wherein RE in the RE(BH 4 ) 3  salt contains at least two RE metals and the purified rare earth metal deposited on the cathode is an alloy. 
     
     
         18 . The method of  claim 1 , wherein the solution containing the RE(BH 4 ) 3  salt in step (ii) is fluorine-free. 
     
     
         19 . The method of  claim 1 , wherein the halogen-free organic solvent is non-ionic. 
     
     
         20 . The method of  claim 1 , wherein the solution containing the RE(BH 4 ) 3  salt in step (ii) has a concentration of at least 0.5 M of RE(BH 4 ) 3  salt. 
     
     
         21 . The method of  claim 1 , wherein ABH 4  is in admixture with the RE(BH 4 ) 3  salt in the solution containing the RE(BH 4 ) 3  salt in step (ii). 
     
     
         22 . The method of  claim 21 , wherein the ABH 4  in the solution is present in an amount of at least 20% of the amount of RE(BH 4 ) 3  salt.