c

3 4 4 5 6 0 0 2 3 3 0 4 3 ORN L-2894

UC-25 - Metallurgy and Ceramics

THE SOLUBILITY OF NITROGEN AND OXYGEN

IN LITHIUM AND METHODS OF

LITHIUM PUR I FlCATlON

E. E. Hoffman

'ORY

N

LEGAL NOTICE

T h i s report was prepared o s an account of Government sponsored work.

nor the Commission, nor any person oct ing on behalf of the Commission:

A. Makes any warranty or representation, expressed or implied, w i t h respect t o the accuracy,

completeness, or usefulness of the information contained i n th i s report, or that the use o f any information, apporotus, method, or process d isc losed in th i s report may not in f r inge pr ivate ly owned r ights; or

Assumes

any information, opparatus, method, or process d isc losed i n th i s report.

Neither the Uni ted States, 1

8. any l i ab i l i t i es w i th respect t o the use of, or for damages resul t ing from the use of

As used i n the above, "person act ing on behalf of the Commission" inc ludes any employee or

contractor of the Commission, or employee of such contractor, t o the extent that such employee

or controctor of the Commission, or employee of such contractor prepares, disseminutes, or

provides access to, any information pursuant t o h i s employment or contract w i t h the Commission,

or h i s employment w i t h such controctor.

Contract NO. W-7405-eng-26

METALLURGY D I V I S I O N

'PHE SOLUBILITY O F NITROGEN AJXD OXYGEN I N LI!IED.JM AND METHODS O F LITRF.JM PURIFICATION

E. E. Hoffman

DATE I S S W

R 9 1960

OAK R l D G E NATIOIVAL LABORaTORY Oak Ridge, Tennessee

Operated by UNION CARBIDE CORPOJWCION

f o r the

3 4 4 5 b 0 0 2 3 3 0 4 3

THE SOLUBIUTY OF NITROGEN,AND OXYGEN IN LITHIUM AND MF,THODS OF LITHIUM PURIFICATION

ABSTRACT

Nitrogen and oxygen are two of the most common impurities found in

commercially available lithium.

solubility of these elements in lithium in the temperature range from 250

to 450°C and to determine which of several possible purification methods was

most effective in reducing the nitrogen and oxygen content of lithium.

The solubility of nitrogen in lithium increased from 0.04 wt $ at The solubility of oxygen was found to be

Experiments were conducted to determine the

250°C to 1.31 wt % at 450°C. 0.010 wt ’$ at 250°C and increased to 0.066 wt $ at 400°C.

Vacuum distillation, low-temperature filtration, cold-trapping, and

gettering with active metals were investigated as possible purification

methods to reduce the nitrogen and oxygen content of lithium,

techniques studied, gettering with active metals was found to be the most

effective method of reducing the nitrogen concentration, while cold-trapping

and low-temperature filtration were most effective in reducing the oxygen

concentration.

Of the

I o INTRODUCTION

There is considerable interest in the application of the low-melting

alkali metals as potential coolants for nuclear reactors and other heat-

transfer applications.

attractive as a high-temperature , high-performance coolant; however, one

problem which has discouraged its use has been the severe corrosion

encountered in elevated-temperature corrosion tests

has in some cases been attributed to impurities in the lithium.

Lithium has many properties which make it most 1

The observed corrosion

The most common impurities found in commercial lithium, either

dissolved o r mechanically dispersed as lithium compounds, are nitrogen,

Hoffman, E. E., and W. D. Manly, “Comparison of Sodium, Lithium, and Iead,” Problems in Nuclear Ehgineering, 1 - (Pergamon Press, New York, 1957)

1

p 128.

- 2 -

oxygen, carbon, chlorine, hydrogen, aluminum, calcium, i ron, s i l i con ,

and sodium.'

e f f ec t of these impurities t o the corrosiveness of l i thium e x i s t s i n the

l i t e r a t u r e .

Very l i t t l e quant i ta t ive information which r e l a t e s the

It has been shown t h a t nitrogen additions t o l i thium increase i t s corrosiveness t o s t a in l e s s s t e e l , resu l t ing i n grain-boundary a t t ack of the

al loy. Recent corrosion t e s t s have been conducted on t e n s i l e - t e s t

specimens of type 316 s t a in l e s s s t e e l exposed t o pure lithium,5 t o l i thium

contaminated with 2 w t $ l i thium oxide, and t o l i t h i u m contaminated with

2 w t $ l i thium n i t r ide .

resulted i n grain-boundary at tack of the s t a in l e s s s t e e l with corresponding

decreases i n the room-temperature t ens i l e strength and d u c t i l i t y of the

specimens following t e s t . The specimens exposed t o pure l i thium were

e s sen t i a l ly unaffected by the t e s t , while the specimens tes ted i n lithium

contaminated with l i thium oxide were attacked, but t o a very s l i g h t degree

compared t o specimens tes ted i n the nitrogen-contaminated lithium.

4

6 The addition of l i t h i u m n i t r i d e t o l i thium

Since both ni%ride and oxide a,dditions t o l i thium were found t o a f f e c t

the corrosiveness of lithium, a program was i n i t i a t e d t o determine the

so lub i l i t y of nitrogen and oxygen i n lithium, and a l so t o determine which of

several possible pur i f ica t ion methods was most e f fec t ive i n removing these

impurities.

'Lyon, R. N., (ed. ) Liquid Metals Handbook (rev. ed. ), Office of Naval Research, NAVEXOS F-733 , p T ( June 1 9 r -

- 'Hoffman, E. E., "Liquid Metal Corrosion, E' Corrosion Fundamentals

4 (Univ. of Tenn. Press, 1956) pp 79, 80.

Corrosion t e s t conditions: S t a t i c t e s t s a t 816"~ f o r 100 hr.

5Lithium contained 25 ppm N2 and 135 ppm 02. 6 Hoffman, E. E., unpublished work.

- 3 -

11. REXIEN OF PAST WORK

Solubi l i ty of Nitrogen and Oxygen i n Lithium

No information could be found i n the l i t e r a t u r e r e l a t ive t o the

so lub i l i t y of nitrogen and oxygen i n lithium.

Purif icat ion of Lithium

V i e n ~ ~ ' ~ refined l i thium by vaporization a t low pressure, but only the

lowering of sodium and potassium contents was considered. @stein and Howland purif ied a small quantity of l i thium by d i s t i l l a t i o n of the metal onto a "cold f inger , " but no quantitative determinations of the oxygen and

nitrogen contents of the pcr i f ied metal were given.

prepared "high-purity" l i thium by vacuum d i s t i l l a t i o n , but again no

Numerous investigators have attempted t o purify lithium. Rogers and

9

Baker -- e t al.," a l so

determinations of the oxygen and nitrogen contents were included i n the

study. Horsleyll made an extensive study of the theore t ica l fac tors which

should influence the production of high-purity l i thium and although both

nitrogen and oxygen removal were discussed, no experimental work was done

t o substant ia te the proposed purif icat ion procedures.

A n attempt was made t o reduce the nitrogen and oxygen contents of

large quant i t ies of l i thium by low-temperature f i l t r a t i o n but the r e su l t s were inconclusive. l2

was unknown since no so lub i l i t y data f o r nitrogen o r oxygen i n l i thium as a The poten t ia l usefulness of t h i s method of pur i f ica t ion

function of temperature were available i n the l i t e r a t u r e .

7Rogers, R. R., and G. E. Viens, '*Refining L i t h i u m by Vaporation a t Low Pressure , I' Can. Mining and Met

8 of Lithium by Di s t i l l a t i on , " - J. Electrochem. - - SOC., - 98(12), 483-487 (1951).

'Epstein, L. F., and W. KO Eowland, "The D i s t i l l a t i o n of Lithium Metal, " Science, - 114, 443-444 { 1951).

Pur i ty Lithium Metal by Vacuum Dis t i l l a t i on , " Science, - 118, 778-780 (1953).

Bull., p 1 5 4 0 (Jan. 1951). 3- -- _I

Rogers, Ro R., and C-. E. Viens, "Effect of Pressure on the Refining

_c

"Bker, P. S o , F. R. Duncan, and H. 3. Gree~e , "Preparation of High-

- _ - II Horsley, G o , - The Purif icat ion of Commercial Li th ium, Atomic Energy

Research Establishment, Harwell, EngEnd, Report No. AERX-M/R-1251 (Septa 26, 1953).

Crocker, A.. R., R. L. Pot ter , R. J. Spera, T . D. McLay, and S o H. 12

Esleeck, Design and Operation of a Sodium-to-Lithium-to-Air --- Heat Transfer System, Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C. , APEX-327 {Dee. 1954) p 117.

I---- -

- 4 -

111. sA94pLIWG METHODJ AWlTICA,L PROCEDURES, AND PURITY OF COMMERCIAL LITHIUM

Inconsistencies in analytical results on duplicate samples of lithium

taken from purification systems were observed throughout the early portions of this study. given lithium samples often were found to be inconsistent with the past

history of the lithium.

be responsible f o r some of the inconsistencies observed, it was recognized that inadequate sampling procedures also contributed to the experimental

errors.

In addition, the concentrations of nitrogen and oxygen in

Although analytical difficulties were thought to

The sampling procedure developed to overcome these problems is

illustrated in Fig. 1.

it was necessary to take samples in inert containers at the temperatures of

interest, quench them rapidly from the test temperature to avoid segregation

of the impurity elements, and protect them from atmospheric contamination

at all times.

1.

In order to obtain meaningful analytical results,

The procedure consisted of the following steps:

Pressurization of the container to be sarqpled with argon, forcing

lithium into a heated sampling tube.

Removal of the furnace from around the sampling tube and rapid

quenching of the sample by means of a chilled-oil bath. Cutting of the sampling tube13 and transference to the inert-

atmosphere chamber for sectioning and loading of samples into

the transfer capsules which were submitted f o r chemical analysis.

2.

3 .

The analytical method used to determine the nitrogen content of lithium

samples was that reported by Gilbert and co-workersllc and was found to be

quite satisfactory.

determine the oxygen content of lithium samples.

and Steinmetz15 utilizing methanol and Karl Fischer reagent was investigated

Two analytical procedures were used initially to

A method described by Sax

13Air-contaminated ends of the sampling tubes were discarded. 14 Gilbert, T. W., Jr., A. S. Meyer, Jr., and J. C. White, "Spectro-

photometric Determination of Lithium Carbide in Metallic Lithium as the Acetylene-Silver Perchlorate Complex," Anal. Chem., 29, 1627-1630 (1957). - - - -

15saxJ w., and H. Steinmetz, Determination of Oxygen in I;ithium *tal - - -' Om-2570 (Oct. 15, 1958).

UNC?.ASSIFIED ORNL-LR-DWG 33124

THIS LINE IS NOT BROKEN UNTIL OIL QUENCH IS FINISHED I TMODIFIED HOKE 1172 VALVE

P R E S S U R E 7 \ TO VACUI

I FURNACES-

INERT ATMOSPHERE CHAMBER

/

GROUND JOINT

TITANIUM TUBE

QUARTZ TRANSFER CAPSULE

XJND JOINT j STEEL)

ESBURY - VALVE I

u3 -+ TO NITROGEN DETERM- INATION BY STEAM DISTIL- t LATION FOLLOWED BY COLORIMETRIC ANALYSIS

TO OXYGEN DETERM-

ANALYSIS ' INATION BY ACTIVATION -

STEP 3 STEP 4 v

STEP 2 v

STEP 1

Fig. 1. L i t h i u m Sampling Procedure.

- 6 -

but did not yield oxygen r e su l t s which were as reproducible as those which

were obtained by the act ivat ion analysis technique reported by Leddicotte.

The reproducibi l i ty and sens i t i v i ty par t icu lar ly a t low oxygen concentrations,

which were obtained by act ivat ion analysis, a re i l l u s t r a t e d by the r e su l t s

of the analyses of samples taken during the determination of oxygen

so lub i l i t y i n l i t h i u m .

16

The li thium used i n these studies was obtained from the producer i n

gas-tight s t a in l e s s s t e e l containers packed under an argon atmosphere. The

as-received pur i ty of several typ ica l shipments of metal i s given below:

Shipment Nitrogen Oxygen Quantity (ppm) (PPm) (W

A.

B

C

600 390 1940 415 331.0 1980

20

46 46

I V . S O L U B I L I T Y OF NITROGEN AND OXYGEN I N LITHIUM

No information was found i n the l i t e r a t u r e r e l a t ive t o the equilibrium

Such information was s o l u b i l i t i e s of nitrogen or oxygen i n molten l i t h i u m .

needed i n order t o determine the poten t ia l usefulness of low-temperature f i l t r a t i o n or cold-trapping'' as methods of purifying lithium.

The so lub i l i t y experiments were conducted i n the sampling apparatus i l l u s t r a t e d i n Fig. 2. For eacn so lub i l i t y determination, su f f i c i en t l i thium oxide or l i thium n i t r i d e was added t o bring the nltrogen o r oxygen content of the bath t o 5 w t $I. l i thium through a 20-p. s t a in l e s s s t e e l f i l t e r a f t e r allowing the system t o equi l ibra te a t the sampling temperature f o r 24 hr . and prepared for analysis by the sampling technique previously described.

Samples of the l i thium were taken by forcing the

The samples were quenched

'%ddicotte, G. W., and L. C. Bate, "Determination of Oxygen by ---- - -'

"Cold-trapping i s a method of liquid-metal pur i f ica t ion which i s based Activation Analysis," Anal. Chem. Prog. Rep., E. 31, 1957

upon the temperature dependence of so lub i l i t y of impurities i n the l iqu id metal.

ORNL-2453.

- 7 -

UNCLASSIFIED ORNL-LR-DWG 38974

TO STAINLESS STEEL AND TITANIUM SAMPLING TUBE ASSEMBLY AND VACUUM SYSTEM

20-p STAINLESS STEEL FILTER THIS SECTION REPLACED

SWAGELOK CONNECTION

LITHIUM; SATURATED WITH EITHER Li20 OR Li3N ----.,

1 I 1 I I I I I I 1 I I I I 1 '\ 1

i OR Li3N

....:- / 4.5-

Fig. 2. Solubi l i ty Sampling Rig.

- 8 -

18 The li thium n i t r i d e and l i thium oxidelg used i n these experiments

were qui te pure.

over l i thium which was held i n a tantalum boat inside a gas-tight s t a in l e s s

s t e e l container a t 800°c. Lithium oxide was prepared by vacuum treatment

of a pure grade of l i thium hydroxide monohydrate i n a nickel container a t

Lithium n i t r i d e was prepared by passing nitrogen gas

60ooc.

The r e su l t s of experiments t o determine the s o l u b i l i t y of nitrogen i n l i thium are:

Sampling Nitrogen Temperature

("c.1 250

300

3 50 400 450

Content * .LE& 0.04 0.16 1.19 1.21

1.31

* Nitrogen content i s average of four determinations a t each temperature.

It may be noted t h a t the s o l u b i l i t y i s appreciable (- 400 ppm) a t 25OoC

(482'~) and increases qui te rapidly with increasing temperature.

The s o l u b i l i t y of oxygen was determined a t four temperatures with a minimum of seven samples taken a t each temperature. l i s t e d and plot ted i n Fig. 3, show tha t the s o l u b i l i t y of oxygen a t tempera- tu res s l i g h t l y above the melting point of l i thium i s l e s s than 1QO ppm.

The re su l t s , which are

V. EXF'ERIMEDPUL METHODS OF PURIFICATION AND RESUJiTS

Typical batches of high-purity commercial l i thium were found t o vary i n nitrogen and oxygen content from several hundred t o several thousand pa r t s

per mill ion.

included vacuum d i s t i l l a t i o n , low-temperature f i l t r a t i o n , cold-trapping and The methods investigated t o pur i fy l i thium of these impurit ies

"Lithium n i t r i d e analysis: 99.9 w t p L ~ ~ N

lgUthium oxide analysis: 99.8 w t $ Li20

- 9 -

800

700

600

- 500

Q Y

z l- 0 a a 2 400 0 z 0 0 z w c3

0 300

200

400

0

UNCLASSIFIED ORNL-LA-DWG 36034

OXYGEN CONCENTRATION ( p p m ) 58, 74, 85, 92,96,101,109,121 176,190,193,205, 212,214, 214,237, 253 348,368,374,383, 392,396

2 50 3 00 3 50 400 4 50 500 200

TEMPERATURE ("GI

Fig. 3. Solubility of Lithium Oxide in Lithium.

- 10 -

gettering with active metals.

which were found to be most efficient and practical in reducing the nitrogen

and oxygen contents of large quantities of lithium to levels of 100 ppm or less.

Bnphasis will be placed on those methods

Vacuum Distillation

Vacuum distillation of lithium at 650"c in the apparatus illustrated

in Fig. 4 was found to be ineffective in lowering the nitrogen and oxygen concentrations of the lithium. The results of these experiments are listed

in Table I. The failure of this method of purification is not fully under-

stood but may indicate the need for operating such a system at a lower pressure

than the 10 Distillation at lower

temperatures might have resulted in a purer product; however, the distillation

rate was only 30 g/hr or 2.5 g/hr/in.2 of liquid-vapor interface area when the

distillation temperature was 650"~.

-4 rum Hg that was used in these experiments.

Recent experiments by Berkowitz -- et al. regarding the sublimation of

lithium oxide, indicate that effective separation of lithium oxide from the

metal by vacuum distillation should be possible.

could be found regarding the volatility of lithium nitride; however,

estimates based on the free-energy relationships suggest that there can be

no effective separation of the nitride from lithium at distillation tempera-

tures of 600"~ and above.

No definite information

21

Low -Temperature Filtration

If it is assumed that the solubilities of nitrogen and oxygen in lithium

are quite low at temperatures slightly above the melting point of lithium,

it should be possible to purify lithium by forcing the metal through a filter.

This method of purification is effective in decreasing the oxygen content

of sodium. 22

The effectiveness of low-temperature filtration in reducing the oxygen

and nitrogen content of lithium is illustrated by the solubility results

which have been presented (Section IV).

contaminated with 5 wt $ oxygen reduced the oxygen concentration to less than 100 ppm.

Filtration at 250°C of lithium

Since the solubility of nitrogen is quite high (a 400 ppm)

2%erkowitz, J., W. A. Chupka, G. D. Blue, and J. L. Margrave, "Mass Spectrometric Study of the Sublimation of Lithium Oxide," - - - Jo Phy. Chem,, - 63(5), 644 (my 1959). -

*%orsley, G. , - The Purification of Commercial Lit,kium, Atomic Energy Research Establishment, Harwell, EhglGd, Report No. AERE-M/R-1251, p 9 (Septa 26, 1953).

Ak0mi.C Energy Comission, Department of Navy, Washington, Do C. (1955) p 146. 22Jackson, C. B. (ed), Liquid Metals Handbook, -- Sodium-NaK Supplement,

- 11 - UNCLASSIFIED

ORNL-LR-DWG. 36139

UNCLASSIFIED ORNL-LR-DWG 22264

FULTON

M LINE (94-In TUBE) ItOv TO AUTOMATIC SAFETY SOLENOID VALVE (NORMALLY CLOSED)

-in FLANGE AND .O'RING

VACUUM LINES ARE Iqn COPPER TUBING; ALL OTHER MATERIAL TYPE 347 STAINLESS STEEL BELLOWS-TYPE VEECO.

VACUUM SYSTEM VALVES ARE 1Jb-1..

3-m BOILER P VAPOR BAFFLE

4 STRIP HEATERS (250 WATTS EACH)

DIFFUSION PUMP DRAIN LINE m STORAGE TANKS

ROUGHING PUMP

Fig. 4. L i t h i u m Distillation System.

TA.BLF: I

EFFECT OF VA,CUUM DISTILLA.TIONa A.T 1202OF (650°C) ON TH3 NITROGEN AND OXYGEN CONTENT OF LITHIUM

Pressure a t cold t r a p with system cold: 2 x lo-4 5 mm Hg Pressure a t cold t r a p with system a t temperature: D i s t i l l a t i on ra te :

1 x 10- m ~g 30 g/hr

Nitrogen (ppm) Oxygen b ( P P )

Separate Separate b Dis t i l l a t ion No. Analyses A.ve rage Analys e s A.verage Remarks

Charge t o s t i l l 1170, 1250 1210 226 (226) As-received and f i l t e r e d a t (1100 g ) 300°C through a 5-micron

f i l t e r

1400, 1300 1350 370, ,285 330 1

1080 700, 389, 500 2 1260, 1150, 885, ioio 3-76? 533

2450 375 (375) Condenser leaked near end of 3 3000, 1900 run

4 4200, 5300 4750 1120, 1210 1170 Some contamination of system occurred during r epa i r of leak following D i s t i l l a t i o n No. 3

5 1600, 1600, 1730 203, 198 200 2000

1800, 2100 24 30 701, 758 690 6 2700, 3100 688, 630

‘80% of each charge was d i s t i l l e d and the remaining 20% was discharged through the dra in l i n e .

I

I-‘ p\)

I

b A l l samples obtained a t end of 80% d i s t i l l a t i o n .

- 13 - a t 25OoC, f i l t r a t i o n a t t h i s temperature i s not an ef fec t ive method of

reducing the nitrogen concentration t o less than LOO p p .

Cold-Trapping

The effectiveness of cold-happing a s a method of purif icat ion i s a l so dependent on the so lub i l i t y of the impurit ies a t the cold-trap temperature.

The procedure used i n these experiments consisted of holding a relative1.y

large volume (500 g ) of l i thium at 400°C i n a container of the type

i l l u s t r a t e d i n Fig. 5 . maintained i n the lower section of t h i s system.

the impurities, nitrogen

l i thium (179°C ) , then continual purif icat ion should r e s u l t when the system

i s held under these conditions. The r e su l t s of two cold-trap pur i f ica t ion

experiments are l i s t e d i n Table 11. in te rva ls .

A temperature gradient; from 400 t o 150°C was If the soPubi l i t ies of

and oxygen, are low near the melting point of

Uthfum samples were taken a t 24-hr

Despite several anomalous resu l t s , a de f in i t e decrease i n oxygen

content was apparent as a r e su l t of cold-trapping.

decreased during cold-trapping, but pur i f ica t ion t o nitrogen leve ls of less than 100 ppm appears t o be d i f f i c u l t , i f not impossible, by t h i s procedure.

These r e su l t s were consistent with the oxygen and nityogen so lub i l i t y values

reported above.

The nitrogen content

Gettering of Oxygen and Nitrogen with Active Metals

The addition t o l i thium of metals whose oxides and n i t r ides a re mre s tab le than those of l i thium was found t o be the mst p rac t i ca l and ef fec t ive purif icat ion method investigated. various metals with l i thium n i t r i d e and l i thium oxide calculated from standard f r ee energies of formation were used as guides i n the select ion of

metals f o r the get ter ing s tudies . On the basis of the possible reduction

reactions f o r l i thium ni t r ide23 l i s t e d i n Table 111, titanium, zirconium,

and yttrium were selected f o r study as poten t ia l purifying agents f o r nitroger,.

Both titanium and zirconium were found t o be very e f fec t ive get ter ing agents i n removing nitrogen from li thium a t 816"c, but were not effect ive i n

Standard f r ee energies of reaction of

2 3 Q u i l l , L. L., The Chemistry and Metallurgy - of Miscellaneous Materials (McGraw-Hill, New YorkxgSO),p42.-

- li: -

L I T H I U M ___

F I L L

w

UNCLASSIFIED O R N L - L R - D W G 40850R

DISCHARGE THROUGH FILTER (20 micron) AND VALVE TO SAMPLER

347 STAINLESS STEEL TUBE (4- in . DIA X 0 . 0 6 5 - i n . W A L L )

316 STAINLESS STEEL TUB (l'/z-in. DIA x 0 0 3 5 - i n WA

COPPER COOLING COIL (AIR COOLANT)

,

Fig. 5. Cold-Trap Purification System.

- 15 -

RESULTS OF EXPERHmS TO PURIFY IJTHIm OF LITHIUM OXIDE ANTI LITHIuplI NITRIDE BY COI;D-WPIMG

Container Temperature : 400°C (752OF) Minimum Cold-Trap Temperature: 150°C (302OF)

a b Sampling Time Oxygen Content Nitrogen Content ( h r ) (PPm) ( PPm 1

Test No. 1 0

24 48 96

Test No. 2 0

24 48

72 96

810 10

270 162

965 710 680 675

a Oxygen contents are average of three determinations.

bNitrogen contents are average of two determinations e

- 16 -

STANDARD FREF: ENERGY CHA.NGES FOR REACTIONS BETWEEN

L i N AND VARIOUS METAL9 -3

L i N + - M = - M N + 3 J X 3 Y Y X Y

X 1

*&8 Kcal/g-atom of N M N

X Y

ZrN - 38.0 UN

TiN

m3N4 YN

3N2 CbN

- 37-7 - 36.2 - 33.6 - 26.7 - 23.4 - 15.7

TaN - 14.9 Mg3N2 - 10.8

ca3N2 Bra 3 2

- 9.3 + oo6

- 17 -

reducing the oxygen content.

data shown i n Fig. 6 . Typical t e s t r e su l t s a re shown i n Table I V which

gives the data obtained f o r titanium. I n these tests, f l a t titanium specimens

having measurable surface areas were used so t h a t the get ter ing r a t e as a function of the surface area could be determined. The metallographic

appearance of the titanium before and a f t e r the get ter ing experiment i s i l l u s t r a t e d i n Fig. 7.

This was expected from the oxide f r e e energy 24

Titanium sponge was used i n subsequent pur i f ica t ion experiments i n

order t o obtain the benefi t of a very high surface area-to-weight r a t io .

These t e s t s were conducted a t 8 1 6 " ~ f o r 24 hr on 40-lb batches of commercial

l i thium t h a t were held i n s t a in l e s s s t e e l containers t o which 5 l b of t i tanium

sponge had been added. This procedure reduced the nitrogen content of lithium t o l e s s than 10 ppm and was subsequently used routinely t o produce low-nitrogen lithium.

The r e su l t s of experiments i n which zirconium sheet specimens were used

indicated t h a t zirconium was s l i g h t l y l e s s e f fec t ive than titanium as a

nitrogen ge t t e r under s imilar t e s t conditions. Since it was c l ea r t h a t no

s ignif icant improvement could be achieved using zirconium, which i s more

expensive than titanium, no fur ther t e s t ing on zirconium was undertaken. The r e su l t s of preliminary studies using yttrium indicated t h a t t h i s

metal i s more ef fec t ive than e i t h e r titanium o r zirconium i n reducing the

oxygen content of molten lithium. The l i thium which was purif ied by exposure

t o y t t r i u m turnings a t 816"~ f o r 100 h r contained less than LOO ppm each of

oxygen and nitrogen. Chemical analyses obtained on y t t r i u m metal specimens exposed t o impure l i t h i u m f o r various time periods a re l i s t e d i n Table V and are indicat ive of the r e l a t ive effectiveness of yttrium as a get ter ing agent f o r these impurities. A.dditiona1 t e s t s w i l l be required t o confirm the r e su l t s of these preliminary s tudies .

- 18 -

UNCLASSIFIED ORN L-LR- DWG 33580

25

- 20 z u (3 > X

LL 0 E

0 15

2 10

E 2

0 I

\ " 5 - 0 0 Y v

0 z 0 I- o

E L L 0 > LT W z W

- a w -5

(3 40

; -15

% -20

LT LL

0 -

TEMPERATURE ("C) 0 500 1000

2 L i 2 0 t T i +- 4Li + TiO,

- 25

Fig. 6.

2Li2C t Zr + 4 Li t Z r 0 2 2Li20 + U -4L i t UO, L i 2 0 t B a + 2 L i + B a O L i 2 0 + M g - 2 L i + M g 0

Li20 t Be -2 Li + B e 0 3Li20 +2Y-+6Li+Y203

3 L i 2 0 + 2 L a -6Li t La203 2 L i 2 0 t Th - 4Li +Tho2

Li,O + Ca+ZLi +CaO

300 500 4000 TEMPERATURE ( O K )

Standard Free Energy Chznges for

1500

L i t h i ~ u i i Oxide Reduction Reactions.

- 19 -

TABLE IV

FBSULTS OF LITHIUM PURIFICATION EXpERIMEXt' UTILIZING

TITANIUM SREET AS THE GETI'ERING METAL

Test Conditions : Temperature : 1 5 0 0 ~ ~ (816"~ ) Container: ~ y p e 347 Stainless Steel Weight of Lithium: 730 g Volume of Lithium: 94.8 in. Dimensions of Titanium Sheets: Number of Titanium Sheets: 10 Total Weight of Titanium Sheets:* Ratio of Total Surface Area of

8.0 x 1.75 x 0.012 in.

130 g

2,98 in. /in. 2 3 Titanium to Lithium Volume:

-purity Content (pp)

Material Analyzed M2 O2 Remarks

Li thiwn

Before test 2450 370 After 24 hr 740 910

After 48 hr 380 730

Titanium Sheet

Before test 74 1500

After 24 hr 1600 1000

After 48 hr 3200 1400

Rate of M2 loss (0-24 hr),

Rate of N2 loss (2448 hr), 1 5 ppm/hr.

71 pIrm/hr.

X-ray examination of surface showed TIN; metallographic examination showed 0.0004-in. layer of TiN.

X-ray examination of surface showed TIN; metallographic examination showed 0.0006-in. layer of TiN.

%eight change of titanium specimens during 48-hr test: 1.0 g (3.55 mg/inO2) a

-20 -

UNCLASSIFIED

ORNL-LR-DWG 35962

Fig. 7 Surface of Titanium Gettering Specimen:

Following Exposure t o Lithium. Gettering Test Conditions: 81.6"~ (1500°F)

100 hr; Ratio of Titanium Surface Area t o Lithium Volume - 2.98 i n . /in.3;

Specimen Thickness - 0.012 i n .

2450 ppm. After Test - 380 ppm. Etchant:

(a) Specimen Before and (b) Specimen

2

Nitrogen Content of Lithium: Before Test - HF, HNO 3' Glyceria (25-25-50 vo l '$).

w m V COMPOSITIONS~ OF YTTRIUM GETPER SPECIMENS~ BEFORE AND m m

EXPOSURE TO LITHIUM CONTAINING NITROGEN AND OXYGEN

Test Temperature: 1500°F (816"~) Total Time of Test: 72 hr

Y t t r i u m Time Period Oxygen Content of Y t t r i u m (ppm) Nitrogen Content of Y t t r i u m (ppm) Specimen of Exposure, Before After Before After

Test Test Change Test Test Change Number ( h r )

1 0-72 1300 3800 + 2500 450 1100 + 650 2 24-72 1400 3900 + 2500 570 850 + 280 3 48-72 940 1800 + 860 110 170 + 60

a

bDimensions of cylindrical specimens:

As determined by vacuum-fusion analysis.

Diameter, 0.3 in . - Length, 1.0 in .

I

Iu P

I

- 22 -

VI' CONCLUSIONS

The results of determinations of the solubilities of nitrogen and

oxygen in lithium indicate that nitrogen has an appreciably higher

solubility in the temperature range studied.

Vacuum distillation was fouad to be ineffective in reducing the nitrogen

and oxygen content of lithium.

Low-temperature filtration was effective in reducing the oxygen

concentration of contaminated lithium to levels of approximately 100 ppm

but did not reduce the nitrogen concentration to this level.

The results of cold-trapping experiments indicate that this technique

of purification reduces the oxygen concentration to acceptable levels but

does not reduce the nitrogen concentration to the desired level.

Gettering with an active metal, such as titanium, is the most practical

and efficient, as well as the cheapest method of decreasing the nitrogen

concentration of lithium to less than 50 ppm. Reduction of the oxygen

concentration of lithium by gettering was found to be quite difficult

compared t o the relative ease with which the nitrogen content can be lowered. This is attributed to the thermodynamic stability of lithium oxide.

The purification procedure recommended as a result of these experiments

is as follows: 1. Filtration of the gas-packed, commercial lithium at 250°C to remove

gross amounts of nitrogen and oxygen contamination generally found

in commercial-grade lithium.

Gettering by an active material, such as titanium sponge, at 800"t: 2 .

for 24 hr or more to reduce the nitrogen concentration to less than 100 ppm.

Cold-trapping for approximately 100 hr to reduce the oxygen concentra-

tion to less than 100 ppm.

3.

The author wishes to acknowledge the assistance of J. L. Scott and J. R. DiStefano in reviewing this paper and the contributions of J. W. Hendricks and L. R. Trotter in conducting the various solubility and purification experi-

ments. Appreciation is extended to those members of the Oak Ridge National

Laboratory Analytical Chemistry Divisio2 who performed the many analyses reported.

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