*Corresponding author. Tel.: #1-423-574-0437; fax: #1-423-574-1900.

E-mail address: m54@ornl.gov (C. Marcus)1Managed by Lockheed Martin Energy Research Corp. un-

der contract DE-AC05-96OR22464 for the US Department ofEnergy.

Nuclear Instruments and Methods in Physics Research A 438 (1999) 30}35

The reduction and distillation of isotopically enriched zincoxides under high vacuum conditions

C. Marcus*, L.A. Zevenbergen

Oak Ridge National Laboratory1, Isotope Enrichment Program, Chemical Technology Division, P.O. Box 2009,Bldg 9204-3, Oak Ridge, TN 37831-8044, USA

Abstract

Historically, enriched zinc (Zn) metal was produced at the Oak Ridge National Laboratory's Isotope EnrichmentFacility (IEF) by either electrodeposition, followed by melting to produce a metal ingot, or puri"ed by hydrogenreduction and distillation at atmospheric pressure in a tube furnace as a prelude to electroplating. Electroplated materialwas generally poor in quality, and losses were high during subsequent melting. Adapting the distillation purifyingtechnique as an ultimate means of recovery of Zn metal proved to be di$cult and ine$cient. To resolve these problems,the well-established vacuum reduction/distillation process was adapted for the conversion of Zn oxide to metal. ( 1999Elsevier Science B.V. All rights reserved.

Keywords: Induction heating; Reduction of oxide; Vacuum deposition; Zinc

1. Introduction

The reduction of gram quantities of ZnO tometal and the consolidation of the metal intoa bead for further processing, in a manner thatyields product of both an acceptable quality ande$ciency, can be a formidable task. Tests using theconventional methods, electroplating and hydro-gen reduction, did not provide a reasonable meansfor satisfying both objectives. Therefore, experi-ments were conducted by adapting the well-estab-

lished vacuum reduction/distillation method [1,2]to obtain Zn metal. The development of the reduc-tion/distillation process for Zn and meltingtechniques to form ingots for further processing isdescribed, along with the successful processing ofenriched Zn isotopes.

2. Background

Preliminary work demonstrated that the elec-troplating of acid-dissolved Zn in aqueous solu-tions with varying normalities of NH

4Cl yielded an

e$cient quantity of metal. However, the qualitywas unacceptable due to the complex compoundsformed between the bath residuals and the platedmetal. Adjustments in the plating current density(resulting in layered "lms or dendrite formations),plating bath temperature, and solvent washes of

0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 6 4 4 - 0

Fig. 1. Reduction/distillation components showing stainless steel still and thermal di!user.

plated Zn did not remedy the poor quality anddegradation of material resulting during beadformation } even when the beading was performedunder hydrogen. Final process e$ciencies were lessthan 70%. To complicate matters, the formation offree radicals from the plating solution createda hydrogenation reaction at the cathode whichcaused it to become embrittled and fracture atthicknesses of less than 0.5 mm. Nevertheless, elec-trodeposition is e!ective under some circumstances.Milligram quantities of Zn have been electroplatedfrom an acetate solution to produce pure depositsof metal [3].

As an alternative, Zn distillation tests were per-formed building upon previous work at the IEF[4]. Carbon was used as a reductant within a tubefurnace heated under hydrogen at atmosphericconditions. The result was a pure product,but metal recovery was impractical because of thecharacteristic thin-"lm metal deposition onthe surface of the inner tube. Unfortunately,attempts to remedy this condensing pattern withthe setup of cold zones within the tube proved to beunsuccessful.

3. Equipment

A vacuum system with a glass bell jar that was43 cm high and had an approximately 31-cm-diam-eter base was used to conduct the Zn distillations.The vacuum system consisted of an approximately8-l/s mechanical pump for roughing and backing,as well as an oil di!usion pump with a 15-cmdiameter. A radio-frequency induction power sup-ply with a 7.5 kW capacity and adjustable controlsto vary the grid and plate currents was used forheating. An optical pyrometer was used to measureapproximate temperatures of the still.

Examples of the experimental components andassembly are shown in Figs. 1 and 2. Possible reduc-tion reactions occurring during the equipment runscan be expressed by the following equations:

(a) ZnO#CH%!5P Zn#CO

(b) 2 ZnO#CH%!5P 2 Zn#CO

2

(c) ZnO#COH%!5P Zn#CO

2.

C. Marcus, L.A. Zevenbergen / Nuclear Instruments and Methods in Physics Research A 438 (1999) 30}35 31

I. ISOTOPE ENRICHMENT

Fig. 2. A mock-up of the typical setup used for reduction/distil-lation runs inside the vacuum chamber.

The still, which contained the ZnO/carbon mix-ture in a loose-powder or pressed-pellet form, wassupported on an insulating pedestal. Zinc was con-densed on the conical tip of a copper rod (`cold"ngera) that was mounted in a brass chill block.This assembly was adjustable to ensure that anadequate "t was formed in the top of the still. Thechill block was mounted on an insulating support.Occasionally, a stainless steel washer was installedon the top of the still to improve the "t between thecopper rod and the still so as to minimize theescape of Zn vapor.

Several still materials were evaluated for theirperformance when subjected to the chemical andphysical conditions of the experiment * graphite,304 stainless steel, and tantalum (Ta). Typically,approximate still dimensions consisted of a mater-ial thickness of 0.5 and 2.5 mm for metal and graph-ite, respectively, with an inside diameter of 1.5 cm

and a length of 10.5 cm. In addition, a thermaldi!user, comprised of tiered Ta spindles mountedon a tungsten rod, was designed and inserted in thestill for runs using loose-powder feed material. Thepurpose of the di!user was to attempt to createa minimal temperature gradient in the still, therebyensuring that the ZnO/carbon reductant mixturewould be heated in a more uniform manner.

4. Experimental procedures

Zinc oxide was mixed and crushed with a slightexcess (&10%) of the stoichiometric amount ofultrahigh-purity carbon powder, using a mortarand pestle. The quantity of carbon added was basedupon the `worst-casea assumption that reaction (a)was dominant. If the other reactions occurred, aneven greater excess of carbon would be present. Themixture was either loaded into the still as a loosepowder or pressed (in an approximate range of68}103 MPa) into pellets prior to placement in thestill.

The preliminary experimental runs with naturalmaterial were made with quantities having 1, 2.5,and 5 g of Zn, with the latter being the ultimate goal.To complete a typical 5-g run, two to four iterationswere performed, with a duration of approximately8 h each, until it was apparent that no additionalmetal had collected on the cold "nger. A vacuumlevel between 1.3 and 0.1 mPa was maintained dur-ing the outgassing from the reduction/distillationreactions, while the typical temperature range was1170}1470 K during heating. Prior to restartinga run with powdered feed, the residual carbon andZnO were remixed to ensure homogeneous contactbetween the two constituents. As Fig. 3 indicates,the condensed Zn vapor would accumulate on thecold "nger as crystal growth formations.

Two types of graphite stills were evaluated dur-ing the experimental runs. One was a single-piececomponent, while the other was a two-piece assem-bly. A thin-walled Ta liner was inserted in thegraphite stills in later runs. Its purpose was to serveas a barrier to prevent the di!usion of Zn vaporthrough the graphite. Because of poor performanceusing the graphite still, further e!orts with it werediscontinued.

32 C. Marcus, L.A. Zevenbergen / Nuclear Instruments and Methods in Physics Research A 438 (1999) 30}35

Fig. 3. Zinc metal collected on the `cold "ngera as mounted onthe brass holder.

Fig. 4. Samples of isotopic Zn formed from the bead fabricationof collected metal.

Stills made of stainless steel, which was a cost-e!ective choice for fabrication, were constructedand evaluated in runs using natural Zn and isotopi-cally enriched 66Zn. A Ta still was also fabricatedand used for runs using isotopically enriched 67Znand 68Zn.

Although the Zn was collected as a crystalgrowth, it was fabricated into a metal bead forinventory purposes. The collected metal waspressed (in an approximate range of 103}138 MPa)into two &2.5-g pellets or less, depending on thequantity of Zn in the feed. Each pellet was separate-ly placed inside a quartz tube back"lled with argon,together with an adequate amount of #uxing agent,NH

4Cl, to promote fusion of the Zn metal. An

oxygen-natural gas torch was used as the heatsource to provide rapid heating in order to minim-ize product loss during the pellet melting. The "nalappearance of the beads for isotopically enriched66Zn, 67Zn and 68Zn is shown in Fig. 4.

Once isotopically enriched material was beingprocessed, a routine procedure was established toprevent the cross-contamination of Zn isotopes.First, the still was placed in a dilute HNO

3solution

and su$ciently heated on a steam bath to dissolveany residual Zn. Then it was air-dried at ambientconditions and, "nally, was "red under vacuum

conditions, slightly above the run temperaturerange, to remove any residuals remaining in thestill. The chill block was also cleaned with diluteHNO

3to remove any residual metal `weldeda to

the copper, and dried in a similar manner.

5. Process results

The preliminary process e$ciencies were evalu-ated relative to the types of stills used. To summar-ize, the e$ciencies for the earlier runs using thefour-turn coil with the graphite stills were approx-imately 40%. With the insertion of the Ta liner, thee$ciencies signi"cantly improved to approximately70%. They increased further with the stainless steeland Ta stills to approximately 90% and 80%, re-spectively. The latter runs were performed with thesix-turn induction heating coil and with the addi-tion of a thermal di!user to provide a wider, moreuniform heat distribution, as well as with improve-ments from modi"cations in the cold "nger design.Relative to isotopically enriched material, the pre-liminary process e$ciencies for 66Zn, 67Zn and68Zn were 91%, 78%, and 80%, respectively.

Material balance calculations were also per-formed to account for and to quantify the losses ofZn in the isotopic runs. After incorporating re-coverable losses, the xnal process e$ciency for the66Zn run was '95%; for the 67Zn and 68Zn runs,it was 88% and 85%, respectively.

C. Marcus, L.A. Zevenbergen / Nuclear Instruments and Methods in Physics Research A 438 (1999) 30}35 33

I. ISOTOPE ENRICHMENT

6. Discussion

During the early experiments, it was observedthat the pressed-feed mixture would segregate dur-ing distillation. This inhibited the carbon reductionof the ZnO because of the diminished surface con-tact between the reactants. Therefore, for sub-sequent runs, it was decided to remix the feedmaterial prior to performing the next run iteration.In addition, it was observed that carbon in anexcess of '10% of the stoichiometric amountwould result in too much unreacted carbon in thecondensed Zn metal. Ultimately, this excess of un-reacted carbon would interfere with the subsequentbead melting. The problem was further com-pounded in runs where the still was overheated.Overheating had to be avoided because it causedthe underlying layers of crystal growth to `welda tothe copper cold "nger. The carbon content alsoembrittled the components of the metal still, andtherefore, excessive amounts would accelerate thedeterioration of the still. It was evident that thethermal di!user would have to be replaced aftercompleting a set of reduction iterations, while thestill would withstand three to four sets of reduc-tions before the structural integrity would becomesuspect.

During experimental runs with both types ofgraphite stills, it was evident that the porosity of thegraphite was signi"cant enough to allow the Znvapor to readily permeate through the still (as in-dicated by the formation of a dark "lm on the belljar, still pedestal, and chill-block assembly). Thisphenomenon was further substantiated when usingthe two-piece assembly by the observation of a "lmdeposition on the bell jar in proximity to the seamof the still. An attempt was made to prevent thedi!usion of Zn vapor by inserting a Ta liner insidethe graphite still. Although the permeation of Znvapor appeared to have been e!ectively inhibited inruns using the graphite still with the Ta liner, it wasdecided to consider metal as an alternative materialfor still fabrication due to the resulting deteriora-tion of the liner, limited availability of existingliners, and the need to obtain higher process ef-"ciencies.

Stainless steel (type 304) and Ta were selected asmaterials for fabrication of the still because each

metal was perceived to be an acceptable candidatefor withstanding the thermal conditions of the pro-cess. As a result of the associated experimental runs,several changes were made to obtain a more uni-form temperature in the still. First, the four-turncoil was replaced with a six-turn coil to provide formore uniform heating. In conjunction with the coilchange, a thermal di!user was fabricated and in-serted in the still to transfer heat from the hotterbase region to the higher regions of the still and,thus, more e!ectively maintain the Zn as vaporuntil it reached the cold "nger. In addition, a cold"nger with a longer taper was used to minimize thecondensation of Zn vapor along the cooler regionsof the still. Furthermore, the feed material wasloaded as a loose material to accommodate theshape of the thermal di!user.

After a few runs using the aforementioned modi-"cations, it became apparent that the maximumtemperature level for experiments with the stainlesssteel still was near the `operating limita for stainlesssteels under vacuum conditions [5]. Althoughbased on limited sampling using collected 66Zn,contamination from stainless steel constituents didnot prove signi"cant. Spectrographic analysis,using the spark-source method, showed concentra-tions for elemental impurities at less than 20 ppm.Nevertheless, it was decided to select a Ta still forother reduction and distillation runs using isotopi-cally enriched Zn because it had a much highertemperature limit and any potential for majorcontamination of Zn with other collectedimpurities could be avoided. Both stainless steeland Ta appeared to serve as su$cient barriers inpreventing the di!usion of Zn vapor through thestill walls.

Measurement of the temperature pro"le withinthe heating range of the still veri"ed that the ther-mal di!user made a signi"cant improvement inreducing the temperature gradient. During runswithout the di!user, the temperature gradient wasas high as 160 K, with the hottest temperatureoccurring at the base. However, when it was used,the range was reduced to as low as 60 K whilechanging the temperature gradient to a more nor-mal, or `bell-curvea, distribution. In addition, itwas apparent that the thermal di!user increasedthe reduction rate, as indicated by the vessel pres-

34 C. Marcus, L.A. Zevenbergen / Nuclear Instruments and Methods in Physics Research A 438 (1999) 30}35

sure levels showing accelerated #uctuations in therate of rise from normal vacuum conditions, incomparison with earlier runs.

7. Conclusions

Carbon reduction and distillation under a vac-uum condition provides an e$cient means to pro-duce quality Zn. By controlling the incrementalheating of the still and, thus, satisfactorily predic-ting the e!ect on the still pressure, Zn vapor lossresulting from gas evolution from feed and byprod-uct reactants can be minimized.

Experimental results demonstrated that the dis-tilled metal is of a higher quality than that of theplated metal when comparing the relative ease withwhich they bead during melting.

The carbon reduction and distillation reactionsfor ZnO under vacuum conditions can be main-tained and controlled such that the resulting prod-uct losses are minimized. Using these processconditions, xnal process e$ciencies (which accountfor recoverable losses) between 85% and 95% havebeen achieved for isotopically enriched Zn metal byusing metal stills.

Since carbon has a tendency to embrittle orreact with metal at the operating temper-atures used, caution must be exercised in determin-ing the condition of the still if it is used formultiple runs.

Acknowledgements

Gratitude is expressed to J.R. Parks for his mech-anical craft services, which produced some of theessential process components. Also, appreciation isextended to Curtis Boles Jr., for accommodatingthe photographic needs in an expedient manner.Finally, special thanks go to Scott Aaron, whoseongoing mentoring and guidance ensured a suc-cessful outcome for this project.

References

[1] E.H. Kobisk, H.L. Adair, Conversion of isotope compoundsto metals by reduction-distillation methods, Proceedings ofthe Eighth World Conference of INTDS, November 1979,Nucl. Instr. and Meth. A 167 (1979) 153}160.

[2] J.M. Heagney, J.S. Heagney, Evaporation of high vaporpressure materials * some problems and some solutions,Proceedings of the Fourth World Conference of INTDS,September 30}October 2, 1975, Argonne National Labora-tory, document no. ANL/PHY/MSD-76-1, pp. 41}43.

[3] J.M. Heagney, J.S. Heagney, A simple and e$cient methodfor reducing small quantities of zinc oxide, Proceedings ofthe Sixth World Conference of INTDS, October 19}20,1977, Lawrence Berkeley Laboratory, LBL-7950, pp. 35}36.

[4] H.H. Caudill et al., Concurrent reduction and distillation} an improved technique for the recovery and chemicalre"nement of the isotopes of cadmium and zinc, Proceed-ings of the 11th World Conference of INTDS, October 6}8,1982, Seattle, WA, University of Washington Nuclear Phys-ics Laboratory, pp. 132}147.

[5] G.J. Shubat et al., Metals Handbook}8th Edition, Vol. 1:Properties and Selections of Metals, 1973, American Societyfor Metals, Materials Park, OH, pp. 622, 624.

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