I
WILLIAM S. GINELL' Brookhaven National Laboratory, Upton, N. Y.
Oxidative Extraction of Lanthanide Metals from Molten Bismuth by Fused Salts
IN
THE
thermal breeder power reactor
(L.M.F.R.) being developed in this laboratory, the liquid metal fuel is a dilute solution of uranium-233 in bismuth (75). The breeder blanket is a suspension of the intermetallic compound thorium bismuthide in bismuth. Breeding ratio can be maximum only if neutron losses to the fission products are kept at a low level, by using a rapid, continuous chemical processing scheme (6). Bareis ( 7 ) showed the feasibility of extracting solute elements from molten bismuth by the fused salt eutectic lithium chloride-potassium chloride. Wiswall (76) and Cubicciotti (5) interpreted semiquantitatively the data based on the free energies of formation of the chlorides. This article reports small scale experiments \vhich involved determination of the equilibrium distribution of several fission product elements (lanthanide metals) betu.een bismuthmagnesium alloys and the fused salt eutectic magnesium chloride-sodium chloride-potassium chloride. Experimental All operations were carried out in vacuum or in an inert gas atmosphere. hlaterials were introduced into a stainless steel dry box through a hi5h vacuum air lock ( 2 ) , through which a constant flow of purified argon was maintained (water content 1 p.p.m.; oxidizing impurities 0.4 p.p.m.). Lanthanide metals, lanthanum, cerium, neodymium, and samarium were obtained from the Ames Labora-
' Present address, -4tomics International, Canoga Park: Calif.
tory, Iowa State College. and stored in the dry box. Prior to use, a fresh metallic surface was exposed by filing away the very thin surface oxide film. Small chips of metal, 2 to 5 mg., were cut from the larger mass with a surgical scalpel, placed in silica tubes, removed from the dry box under argon, and degassed on a vacuum system before being sealed off and neutron-irradiated in the Brookhaven reactor. After irradiation, the sealed tubes were readmitted to the dry box and broken open and master alloys prepared. The tracers xiere 'QLa, t l / z = 40 h ; I4'Ce, t , , ~= 32 d ; 147Nd, t i i f = 11.0 d ; and 153Sm,t 1 i 2 = 47 h. Additional chips were irradiated simultaneously for later use as counting standards. Magnesium was a spectrographically pure grade (Johnson, Matthey & Co.). Thta surface of the metal was filed to remove the oxide layer and pieces of requisite size cut by a small hack saw. All graphite crucibles and boats were of reactor grade (AGOT) graphite, supported on a silica triangle, and ignited in air with an oxygen torch. After cooling, crucibles were carefully wiped to remove graphite powder and outgassed by induction heating in a \'!.cor tube in vacuum. Outgassing and cleanup were considered complete when a pressure of 5 X 10-6 mm. or better was obtained at 1400' to 1600" C. Bismuth (Cerro de Pasco nominally 99.99970 pure) was ground to a coarse powder in a porcelain mortar and charged to a previously outgassed graphite casting crucible (5 X l 3 / 4 inches) contained in the Vycor assembly. A molybdenum
dip tube estended nearly to the bottom of the crucible. The assembly was brought to 500' C. under vacuum and hydrogen was bubbled into the bismuth through the dip tube while the temperature was raised to 800' (2. iifter about 30 minutes the flow \vas discontinued, the stopcocks \vex closed, and the assembly was removed from the furnace. The Lrycor container was tilted to allow the molten metal to flow into the tapered mold holes. The tube was water-quenched and, \%hencool, \vas evacuated and transferred to the dry box. This casting technique gave bismuth slugs of uniform \veight and eliminated crushing of bismuth in the dry box. Alloy Preparation. To obtain uniform alloys containing low concentratioiis of lanthanides or magnesium in bismuth, master alloys, containing about 100 p.p.m. of lanthanide metals, were prepared by charging a Iieighed chip of irradiated lanthanide metal, about 2 mg., to a graphite casting crucible \vith the required weight of purified bismuth. The crucible, smaller than that shoivn in the figure, contained in a Vycor tube under argon, was removed from the dry box and transferred to the vacuum system, where the argon \vas pumpcd out. The crucible was heated to 500" under vacuum and then to 600 " to 650 ' under 20 cm. of argon for about 30 minutes. The molten alloy was cast into approximately 0.8-gram slugs. This procedure eliminated uncertainties due to possible solute segregation during alloy cooling. Integral numbers of slugs \vere used; segregation within each slug \vas unimportant. The slugs were of uniform
e PYREX. KOVAR-GLISS SEAL-
VICOR-
All weighing and filling operations were carried out in a stainless steel, argon atmosphere dry box
Bismuth-lanthanide metal alloys were cast in a graphite crucible VOL. 51, NO. 2
0
FEBRUARY 1959
185
Crucible parts were made from 3-mil tantalum foil
concentration (within countifig error) and usually within 10% of the calculated composition as determined from the measured specific activity of the pure irradiated lanthanide metal. Salts. Sodium and potassium chlorides (c.P.)were recrystallized twice from concentrated hydrochloric acid, washed with water and sublimed in vacuum from graphite crucibles. The water-cooled cold-finger condenser was a platinumplated copper tube. Temperatures of 700 'to 800 ' C. were required to sublime small batches of salt at convenient rates. The sublimed salts were removed from the cold finger in the dry box and stored in glass-stoppered weighing bottles. Comparatively anhydrous magnesium chloride was obtained from the Carborundum Co., contaminated with small amounts of zirconium, titanium, iron, and copper. The powdered salt was put into a graphite boat, placed in a horizontal Vycor tube in a tube furnace, and dehydrated in a stream of dry hydrogen chloride between 240' and 280' C. Accurate temperature control was necessary, because below 240 O incompletedehydration resulted and above 280 " small amounts of MgOHCl were formed by hydrolysis ( 7 7). Following complete removal of water, the temperature was raised to volatilize chlorides of zirconium, iron, and other impurities. Finally, the salt was melted to a transparent liquid. The boat was cooled in an atmosphere of hydrogen chloride and transferred to the vacuum sublimation apparatus. The salt was pumped a t 550" to allow any MgOHCl to decompose to magnesium oxide and hydrogen chloride. When the pressure had dropped to 0.1 micron, the temperature was raised to 680' to 700 " C. at which point sublimation occurred. In the dry box, the sublimed magnesium chloride was easily removed from the cold finger by gentle chipping. The eutectic mixture, 50 mole yo magnesium chloride, 30% sodium chloride, and 20% potassium chloride, was weighed out and intimately ground in a glass mortar (melting point 396" C.). Tantalum was selected as the equilibration crucible material because of its negligible solubility in bismuth and fused salts. I t was easily outgassed by induction heating in vacuum. Crucible com-
1 86
ponents were formed from 3-mil tantalum foil in a hydraulically operated die (70). A ring of crucibles was assembled by spot-welding the flanges of six crucibles together, outgassed, and annealed by induction heating to 1500" C. at 10-6 mm. Imperfect flange welds were easily detected by filling each crucible with acetone. Welds impermeable to acetone were satisfactory for containing both molten bismuth and the fused ternary eutectic. The crucibles were once again outgassed, this time in the flanged L'ycor tube, 64 mm. in outside diameter and 250 cm. long, to be used in the equilibration experiment. When cooled, the evacuated tube was transferred to the dry box. Experimental Procedure. The crucibles were loaded in the dry box. In a typical experiment, 1 gram of the ternary salt was added to each crucible, with one or more slugs of lanthanide master alloy, magnesium master alloy, and pure bismuth to bring the total weight of metal to 5 grams. All weighings were performed on an analytical balance to the nearest 0.2 mg. A small polonium alpha source aided in dispelling static charges during weighing. The initial lanthanide concentration in the bismuth was 10 to 30 p.p.m. and magnesium concentration varied from 10 to 400 p.p.m. After loading, the flanged Vycor tube was sealed by borosilicate glass cap which carried a stopcock and a ball joint. The flanges of the tubes and caps were ground and lapped flat using 600-mesh Carborundum and a glass plate. A vacuum-tight seal was effected with a grease composed of a 1 to 2 mixture of Apiezon N and Apiezon T. The high melting point of T grease, and the superior lubricating properties of N grease, produced a lubricant' and seal satisfactory a t the elevated temperatures to which the flanges were subjected. Temperature was measured by a platinum-platinum, 13% rhodium thermocouple, introduced through a Kovarglass seal in the ball joint attached to the cap. Potentials were read on a Rubicon microvolt potentiometer. The sealed tube was brought out of the dry box and evacuated to 5 X 10-6 mm. A regulated constant temperature resist-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ance furnace was then raised around the Vycor tube. The crucibles were brought to 500' C. under vacuum and 20 to 30 cm. of argon admitted to minimize salt and bishuth volatilization. Temperatures were maintained constant to f2' C. Equilibration times ranged from 2 to 16 hours. Previous experiments had indicated that equilibrium was established within 1 hour under quiescent conditions. A kinetic study of the rate of approach to equilibrium showed that equilibrium was reached within minutes ( 3 ) . At the end of the experiment, the crucibles were cooled rapidly by quenching in water. The salt solidified within 30 seconds. The salt and metal phases were separated by cutting away the welded seams of the tantalum crucibles with scissors, and peeling the two halves of the crucible from the salt-bismuth ingot. Each crucible was extracted with 1 F hydrochloric acid, which dissolved the salt and repressed hydrolysis of lanthanide chloride. The salt solution was filtered through a fritted-glass filter, the filtrate and residue being analyzed separately for the lanthanides by counting in a welltype scintillation counter. The metallic phase was dissolved in concentrated nitric acid and an aliquot was counted in the same counter. Specific activities were of the order of 2 X lo3 counts per minute per microgram of lanthanide, and material balances were within 5%. Magnesium was determined in the bismuth spectrographically to &lo%.
Results and Discussion The oxidation reaction may be represented as:
Lanthanum is chosen as a representative lanthanide metal in the +3 oxidation state and magnesium chloride is the most active oxidizing agent in the ternary salt. The stability of both sodium and potassium chlorides is so much greater than that of magnesium chloride that their contribution to the oxidizing potential of the salt may be neglected, to a first approximation. In effect, the fused salt is a solution of magnesium chloride in a low melting solvent. The equilibrium constant, K,,, for Reaction 1 is:
Expressed in terms of activity coefficient and mole fractions, Equation 2 becomes:
where a = thermodynamic activity, X = mole fraction, f = activity coefficient, and f" = limiting activity coefficient at infinite dilution. As the mole fractions
NUCLEAR TECHNOLOGY
w-
-
-
I3 $1
' + Cs+MgCIZ
-
CeC13tMg La+MqCl2 SLOPE
-5
IO
Figure 1. Distribution of cerium, lanthanum, neodymium, and samarium between molien ternary eutectic sah I O O L magnesium chloridesodium chloride-potassium chloride and mol; ten magnesium-bismuth alloys, and of neodymium between mol- p ten binary eutectic salt MgCI,-KCL.NoC magnesium chloride- 5 5 i 0 Nd t M g sodium chloride and f 1 C N d + Mg+U Nd + M ~ i U + Z r molten magnesium-bis- ; + + S m i Mg muth alloys, is a funcMpCI,-NoCl Nd+Mg tion of magnesium conNdiMgrU centration in fuel
\-
-
-3
-4
"1
;
IO2
IO
10
'up.~
l
21 i
,
-
-1
lob
I
1
.
r
\
I
MOLE FRACTION MAGNESIUM IN BISMUTH PHASE
of lanthanide metals and magnesium in bismuth and of lanthanide chlorides in salt were very low (
