Electrolytic production of praseodymium

Praseodymium oxide is electrolyzed, at temperatures at or above the melting point of praseodymium metal, in a molten electrolyte initially consisting essentially of lithium fluoride and praseodymium fluoride. Improved current efficiency is obtained when the weight ratio of lithium fluoride to praseodymium fluoride in the electrolyte is about 0.1 to about 0.4.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the electrolytic preparation of metals from fused 
baths, and more particularly to the production of praseodymium from 
praseodymium oxide, in a molten lithium fluoride-praseodymium fluoride 
electrolyte. 
2. Description of the Art 
Rare earth metals, once only scientific curiosities, are finding 
ever-increasing industrial utility. In particular, recently developed rare 
earth alloy high-strength permanent magnets have greatly increased the 
demand for certain rare earths having lower atomic numbers, most notably 
samarium and neodymium. Samarium-cobalt magnets have become particularly 
important, due to their very high strengths. 
The rare earth praseodymium also forms high-strength magnets, when alloyed 
with cobalt. In addition, praseodymium is useful in magnets as a 
replacement for some of the samarium in a samarium-cobalt alloy, due to 
the relatively higher cost of samarium. 
Praseodymium metal is frequently prepared by the metallothermic reduction 
of a praseodymium halide (such as the fluoride), wherein the halide is 
loaded into a corrosion-resistant container with an active metal, such as 
calcium metal, then is heated in an inert atmosphere to temperatures above 
the melting point of praseodymium (about 935.degree. C.), and held at such 
temperatures until the praseodymium is reduced to the metal. After cooling 
the reaction mixture to room temperature, the product is separated from 
active metal halide slag and the container. This procedure has certain 
disadvantages, including the limited quantity of metal which can be 
prepared in a batch and the numerous steps which involve personal 
attention from an operator. 
Rare earth metals have been prepared for quite some time by fused salt 
electrolysis techniques, several such techniques being reviewed by E. 
Morrice and M. M. Wong, "Fused-Salt Electrowinning and Electrorefining of 
Rare-Earth and Yttrium Metals," Minerals Science Engineering, Vol. 11, 
July 1979 (pages 125-135). Early techniques involved electrolysis of rare 
earth chlorides, using electrolytes of molten sodium and potassium 
chlorides; some investigators avoided undesired reactions of the metal 
products by conducting the electrolysis below the metal melting point, 
thus producing rare earth metal sponges or nodules. 
Other investigators have used molten fluoride electrolytes to produce rare 
earth metal directly from rare earth oxides. Results for praseodymium are 
reported by E. Morrice and T. A. Henrie, Electrowinning High-Purity 
Neodymium, Praseodymium, and Didymium Metals from Their Oxides, U.S. 
Department of the Interior, Bureau of Mines Report of Investigations 6957, 
May, 1967. Those workers employed a "thermal gradient" electrolysis cell 
for the production of praseodymium, in which average electrolyte 
temperature was 1030.degree. C., but metal product was collected in a 
cooled area, at an average temperature of 800.degree. C., slightly above 
the solidus point of the electrolyte. By not maintaining product metal at 
the high formation temperature, better yields and purity were obtained. 
The electrolyte used was a mixture of 60 percent by weight PrF.sub.3 and 
40 percent by weight LiF. 
Further information on electrolyzing praseodymium oxide was reported by E. 
Morrice, E. S. Shedd, and T. A. Henrie, Direct Electrolysis of Rare-Earth 
Oxides to Metals and Alloys in Fluoride Melts, U.S. Department of the 
Interior, Bureau of Mines Report of Investigations 7146, June, 1968. A 
similar "thermal gradient" technique was described, and this report 
appears to restate the data for praseodymium production obtained by 
Morrice and Henrie, supra. 
Both of the described praseodymium oxide-to-praseodymium electrolysis 
reports were confined to small laboratory-scale, batch procedures. To 
recover product, it was necessary to completely cool the electrolyte, 
crush it, and separate the product metal nodules. Such procedures 
generally are not suitable for large-scale, commercial production 
undertakings, which normally require more continuous, less labor-intensive 
production methods. 
In order to make a more or less continuous process, however, it is 
necessary to maintain product metal in a molten state, so that the metal 
can be withdrawn without otherwise affecting the ongoing electrolysis cell 
operation. Problems observed with such higher-temperature operation 
involve both higher levels of corrosivity to cell construction materials 
and product losses, due to competing reactions in the molten electrolyte. 
One such competing reaction is reaction of praseodymium metal product 
and/or oxide feed with the electrolyte to form an oxyfluoride-containing 
sludge. A symptom of this problem is a reduced current efficiency, since 
produced metal is subsequently being reacted. 
Accordingly, it is an object of the present invention to provide an 
improved electrolytic method for producing praseodymium metal from 
praseodymium oxide. 
An additional object is to provide such an improved method wherein metal is 
collected and removed from an electrolysis cell, in a molten state. 
A further object is to provide such a method wherein the increased cell 
temperatures, needed to produce molten metal, do not result in uneconomic 
current efficiency. 
These and other important objects of the invention will more clearly appear 
from consideration of the following disclosure. 
SUMMARY OF THE INVENTION 
Praseodymium oxide is introduced into a molten lithium 
fluoride-praseodymium fluoride salt bath, maintained at temperatures at 
least about the melting point of praseodymium, and is electrolyzed to form 
praseodymium metal. By utilizing weight ratios of lithium fluoride to 
praseodymium fluoride about 0.1 to about 0.4 in the salt bath, improved 
current efficiency is obtained for the continuous production of 
praseodymium metal. 
DESCRIPTION OF THE INVENTION 
The invention pertains to the continuous-flow production of praseodymium 
metal, by electrolytic reduction of praseodymium oxide in a molten lithium 
fluoride-praseodymium fluoride electrolyte. 
The term "continuous-flow" is used herein to describe methods wherein metal 
collection is conducted above the melting point of the metal product. It 
is not necessary for a method to be actually run in a continuous manner to 
qualify as "continuous-flow," since the capability for periodic or 
intermittent withdrawals of molten metal is considered to be equivalent to 
continuous operation. 
Current efficiency is determined by calculating the amount of product 
expected to be formed (1.753 grams praseodymium from Pr.sub.2 O.sub.3 per 
ampere-hour) in the cell, and comparing the actual metal product recovery. 
Electrolysis cells which are suitable for the practice of the present 
invention include those which are known in the art, such as the cells 
described in reports by Morrice et al., supra. Numerous cell 
configurations are considered useful, utilizing various materials of 
construction; such matters are not considered to be critical for the 
successful conduction of the claimed method. 
Physical properties of lithium fluoride-praseodymium fluoride mixtures have 
previously been studied. The melting points of these mixtures were 
reported by R. E. Thoma, by means of a phase diagram showing the effect of 
increasing praseodymium fluoride concentration, in L. Eyring (ed.), 
Progress in the Science and Technology of the Rare Earths, Vol. 2, 
Pergamon Press, New York, 1966, page 110. The diagram expresses 
concentration in terms of mole percentages; restatement in terms of weight 
ratios gives the data in Table 1. 
TABLE 1 
______________________________________ 
Weight LiF Melting Point 
Weight PrF.sub.3 
.degree.C. 
______________________________________ 
Pure LiF 850 
1.180 810 
0.524 750 
0.439 720 
0.400 750 
0.306 830 
0.197 970 
0.131 1070 
0.100 1130 
______________________________________ 
As can be seen in the table, electrolyte compositions of the invention, 
which have weight ratios of lithium fluoride to praseodymium fluoride 
between about 0.1 and about 0.4, have melting points between about 
750.degree. C. and about 1130.degree. C., the temperature increasing as 
the proportion of lithium fluoride decreases. Since some of the weight 
ratios have melting points higher than that of praseodymium metal, it is 
clear that electrolyte melting point must be considered when choosing 
operating conditions for practice of the method. 
The term "lithium fluoride and praseodymium fluoride" is used herein to 
describe an electrolyte which initially consists essentially of those 
compounds. Thus, the term encompasses the relatively pure starting 
electrolyte, as well as an impure electrolyte as it exists after use to 
produce praseodymium, it being recognized that electrolyte composition can 
be significantly altered during use. 
Most of the electrolytic cells which have been used to produce lanthanide 
metals (i.e., elements having the atomic numbers 39 and 59 through 71) 
utilize carbon anodes, at which the following reactions (1) through (3) 
are believed to occur during the production of praseodymium from Pr.sub.2 
O.sub.3 or from Pr.sub.6 O.sub.11 (Pr.sub.2 O.sub.3 .multidot.4PrO.sub.2): 
EQU Pr.sub.2 O.sub.3 +3/2 C.fwdarw.2Pr+3/2 CO.sub.2 ( 1) 
EQU PrO.sub.2 +2C.fwdarw.Pr+2CO (2) 
EQU PrF.sub.3 +3/4 C.fwdarw.Pr+3/4 CF.sub.4 ( 3) 
with reaction (3) occurring primarily when the praseodymium oxide 
concentration near the anode becomes very low. Reaction (3) clearly has an 
effect upon electrolyte composition, by removing fluoride. 
Other reactions which can affect the electrolyte composition and/or the 
current efficiency are those involving the product metal, such as 
reactions (4) and (5): 
EQU 2 Pr+3 CO.sub.2 .fwdarw.Pr.sub.2 O.sub.3 +3 CO (4) 
EQU 2 Pr+3 CO.fwdarw.Pr.sub.2 O.sub.3 +3 C (5) 
In addition, praseodymium can form the oxide Pr.sub.6 O.sub.11 and can 
react in the molten electrolyte to form oxyfluorides such as PrOF. The 
oxyfluorides contribute to the formation of insoluble sludge in the cell, 
as well as altering electrolyte composition by removing fluoride ions. 
The method of the invention utilizes weight ratios of lithium fluoride to 
praseodymium fluoride in the initial electrolyte composition about 0.1 to 
about 0.4. These values correspond to molar ratios of lithium fluoride to 
praseodymium fluoride about 0.76 (melting point about 1130.degree. C.) to 
about 3.05 (melting point about 750.degree. C.). Preferred weight ratios 
are in the range about 0.2 to about 0.3. Such ratios provide an 
electrolysis of improved current efficiency, possibly due to the decreased 
solubility of molten praseodymium metal in the molten electrolyte and 
improved metal coalescence. 
A general procedure for use of the present invention comprises the steps 
of: 
(a) preparing a molten electrolyte which comprises lithium fluoride and 
praseodymium fluoride, wherein the weight ratio of lithium fluoride to 
praseodymium fluoride is about 0.1 to about 0.4; 
(b) passing a direct current through the electrolyte, while introducing 
praseodymium oxide into the electrolyte; and 
(c) collecting praseodymium metal, at or above the melting point of the 
metal. 
The invention is further illustrated by the following examples, which are 
illustrative of various aspects of the invention and are not intended as 
limiting the scope of the invention as defined by the appended claims.

EXAMPLE I 
The solubility of praseodymium in molten mixtures of lithium fluoride and 
praseodymium fluoride is determined by placing a piece of the metal, 
weighing 1 gram, in a molybdenum foil-lined graphite boat with 50 grams of 
the salt mixture, heating the boat in a tube furnace for 1 hour, under an 
argon atmosphere, cooling the boat, and weighing the recovered metal 
piece. 
Results obtained at 950.degree. C. are summarized in Table 2, showing that 
praseodymium solubility increases as lithium fluoride concentration 
increases. 
TABLE 2 
______________________________________ 
Weight LiF Number Grams Pr Dissolved 
Weight PrF.sub.3 
of Tests Median Mean 
______________________________________ 
0.667 5 0.71 0.75 
0.250 4 0.44 0.39 
______________________________________ 
EXAMPLE II 
The effect of electrolyte composition upon current efficiency is measured, 
using a small cell having a diameter of three inches, a graphite anode and 
a molybdenum or tungsten cathode. 
Electrolysis is conducted at temperatures about 920.degree. C. to about 
980.degree. C., with two different electrolyte compositions and for a 
period of about one hour. A total of 30.0 grams of praseodymium 
sesquioxide (Pr.sub.2 O.sub.3) is fed to the cell during the run. Results 
are summarized in Table 3, showing an increase in cathode current 
efficiency when the electrolyte lithium fluoride concentration is lowered. 
This increase, however, is accompanied by a higher voltage drop in the 
cell. 
TABLE 3 
__________________________________________________________________________ 
##STR1## 0.25 
0.25 0.25 
0.67 0.67 
Total Current (ampere-hours) 
13.5 
15.0 15.0 
15.0 15.0 
Grams Metal collected 
18.47 
18.70 
20.87 
5.24 9.8 
Cell Voltage (volts) 
4.2-4.9 
4.4-4.7 
4.5-4.8 
3.4-3.6 
3.7-4.1 
Cathode Current Efficiency (%) 
78.1 
71.2 79.4 
19.9 37.3 
__________________________________________________________________________ 
EXAMPLE III 
A series of experiments, similar to that of the preceding example, shows 
cathode current efficiency as a function of electrolyte composition. 
Results, summarized in Table 4, are obtained at cell temperatures about 
950.degree. C. and total currents of 15 ampere-hours. This demonstrates 
the higher efficiency obtained when the electrolyte has a lower 
concentration of lithium fluoride. 
TABLE 4 
______________________________________ 
Weight LiF Cathode Current 
Weight PrF.sub.3 
Efficiency, percent 
______________________________________ 
0.389 43.0, 61.2 
0.316 65.3, 72.3, 80.5 
0.250 83.0 
0.176 78.2, 83.6 
______________________________________ 
Various embodiments and modifications of this invention have been described 
in the foregoing description and examples, and further modifications will 
be apparent to those skilled in the art. Such modifications are included 
within the scope of the invention as defined by the following claims.