ORNL-TM-11955.txt

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ORNI/TM-11955

 

ATION
APPLICABLE TO THE REACTION

OF URANIUM OXIDES WITH
CHI.ORINE TO PREPARE
URANIUM TETRACHLORIDE

P. A Haas

February 1992
 

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ORNL/TM-11955
Dist. Category UC-501
(Chemistry)

Chemical Technology Division

LITERATURE INFORMATION APPLICABLE TO THE REACTION OF
URANIUM OXIDES WITH CHLORINE TO PREPARE URANIUM TETRACHLORIDE

Paul A. Haas

Date Published: February 1992

Prepared by the
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37831
managed by
MARTIN MARIETTA ENERGY SYSTEMS, INC.
for the
U. S. DEPARTMENT OF ENERGY
under contract DE-AC05-840R21400 A TMAIE T SN 151 e

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I 4450 D35280a o
CONTENTS

ABSTRACT . . ettt e e e e e e e e e e e e e e e et et e 1
INTRODUCTION .............. B 1
LITERATURE INFORMATION ..... e PR 3
21 THERMOCHEMICAL DATA . ..ottt e e aeeeaaenns 3
2.2 PHASE DIAGRAMS ...... e 5
23 EXPERIMENTAL RESULTS FOR CHLORINATION OF URANIUM

OXIDES . . e e et et e e e e e e e e e e e e e 14
DISCUSSIONS AND CONCLUSIONS . ...\ tutitiiiietiieararaaeannns. 17
3.1 THERMOCHEMICAL CONSIDERATIONS . .. ..vutinenearanannnnn. 17
32 SELECTION OF CHLORINATION CONDITIONS ..........c.cv.n.. 22
33 SALT PROPERTIES AND CONTROL CONSIDERATIONS ........... 24
3.4 PREPARATION OF TiCl,, ZtCl,, SiCl,, and ThCl, FROM

10). 1) X 27
ACKNOWLEDGEMENT . .\ oottt e e e e e e e 28
REFERENCES . . vttt e et e e e e e e e e e e e e 29

T
LIST OF TABLES

Physical properties of U-O-Clcompounds ........... ... ... .. ... ... ... 4
Thermochemical data . ... ... ... .. . i i i e 6
Vapor pressure CQUAtioNS ... itn oo s tnnnonecrtonanneeeenannnnns 8
Heats of formation for U-O-Cl compounds at 298 K .............. ... ... ... 18
Free energies of formation for U-O-Cl compounds at 900 K (627°C) .......... 19
LIST OF FIGURES
Vapor pressures of uranium chlorides  ............ .. ... i i, 7
Phase diagram for UCI,-UQ, ... ... . . i e 9
Phasc diagrams for the compounds of uranium and chlorine ................ 10
Liquidus temperatures (°C) for UCI-MgCl,-NaCl ............... ... ... ... 12
Diagram for log P vs log PC12 at400°C . ... 13
Conversion reactions for U-O-Cl compounds and free energies at 900 K ........ 20
Solubility of UQ, in UCl,and UF, ...... ... .. 25

iv
LITERATURE INFORMATION APPLICABLE TO THE REACTION OF
URANIUM OXIDES WITH CHLORINE TO PREPARE URANIUM TETRACHLORIDE

Paul A. Haas

ABSTRACT

The reactions of uranium oxides and chlorine to prepare anhydrous
uranium tetrachloride (UCl,) are important to more economical prepara-
tion of uranium metal. The most practical reactions require carbon or
carbon monoxide (CO) to give CO or carbon dioxide (CO,) as waste gases.
The chemistry of U-O-Cl compounds is very complex with valances of 3, 4,
5, and 6 and with stable oxychlorides. Literature was reviewed to collect
thermochemical data, phase equilibrium information, and results of experi-
mental studies. Calculations using thermodynamic data can identify the
probable reactions, but the results are uncertain. All the U-O-Cl
compounds have large free energies of formation and the calculations give
uncertain small differences of large numbers. The phase diagram for UCI,-
UO, shows a reaction to form uranium oxychloride (UOCI,) that has a
good solubility in molten UCl,. This appears more favorable to good rates
of reaction than reaction of solids and gases. There is limited information
on U-O-Cl salt properties. Information on the preparation of titanium,
zirconium, silicon, and thorium tetrachlorides (TiCl,, ZrCl,, SiCl,, ThCl,)
by reaction of oxides with chlorine (Cl,) and carbon has application to the
preparation of UCI,.

 

1. INTRODUCTION

"An anhydrous UCI, salt has the properties to be an important intermediate
chemical for processing and applications of uranium compounds. This was recognized
during the World War II program to prepare nuclear weapons; preparation of UCl, was
studied at that time. These studies showed that uranium tetrafluoride (UF,) was much
easier to prepare and handle than UCl,. Also, the uranium fluorides were better than
chlorides for gaseous diffusion separation of isotopes and for batch, bomb reductions to
uranium metal. Therefore, most of the uranium that has been mined, concentrated, and

purified to give uranium ore concentrates (uranium oxides) has been converted to UF,.
The use of uranium chlorides in place of uranium fluorides would have important
economic advantages. The hydrogen fluoride (HF) and fluorine (F,) required to prepare
the uranium fluorides are expensive chemicals. Processes that use fluorine compounds end
up with toxic and troublesome wastes, such as magnesium and calcium fluoride (MgF, and
CaF,) and isotopically depleted uranium hexafluoride (UF). If uranium chlorides were
used, the recycle or reuse of the chlorides is more practical.

The proposed installation of a new industry for enrichment of uranium isotopes
could benefit from the economic advantages of using uranium chlorides. The feed to an
Atomic Vapor Laser Isotope Separation (AVLIS) process will be uranium metal.! The
principal production of uranium metal for nuclear fuel cycles has previously been by batch
metallothermic reductions of UF, using magnesium or calcium metal. For a large enrich-
ment plant (=10* ton Ufyear), the costs of the HF feed, the calcium (Ca) or magnesium
(Mg) feed, and the disposal of MgF, or CaF, waste are major parts of the total uranium
enrichment costs. Some alternate processes for preparation of uranium metal from UCl,
allow recycle of Cl, from electrolytic cells. The application of these processes requires the
reaction of uranium oxides with Cl, to prepare UCl,.

The purpose and scope of this review is to collect, organize, and discuss the litera-
ture information useful to the reactions of uranium oxides and chlorine to prepare
anhydrous UCl,. The review is selective in that only one set of consistent and useful
results is presented without reference to less useful or inconsistent information. An
excellent comprehensive and critical review of the chemistry of uranium was prepared as
an account of work and information from the U. S. Manhattan Project.? Such a review for
the preparation of UCI, will not be repeated here. Well-organized and more complete
presentations of thermochemical data for uranium compounds were published by Rand
and Kubaschewski in 1963,> Fuger et al in 1983,* and Barin in 1989.> An assessment of
thermochemical data for the system uranium-oxygen-chlorine by Cordfunke and
Kubaschewski® illustrates the scatter of individual values, the limits of accuracies, and the
dependence on estimated values. Selected values will be listed and discussed in the

following sections without detailed reference to these limitations.
2. LITERATURE INFORMATION

The processes of interest for the preparation of the anhydrous UCl, are to react
the uranium oxide feeds with chlorine (ah oxidizing agent) and carbon or carbon monoxide
(reducing agents). Oxidation and reduction reactions will take place and all possible
uranium oxides, uranium chlorides, and uranium oxychlorides must be considered. Physical
properties for these compounds are tabulated (Table 1). A consistent set of thermodynam-
ic data for these uranium compounds is needed to allow calculations to identify the
probable reactions. The data for C, CO, and CO, as reactants or products and for H,O
and HCl as impurities are included for convenience. Phase diagrams are important as they
present useful equilibrium results. Finally, results are reviewed for the reported experi-
mental studies of the reactions of uranium oxides, chlorine, and a reducing agent.

The principal component of a molten salt for a chlorination will probably be UCI,.
Other physical information reported for UCI, includes:’

Heat of fusion at 863 K: 44.8 kJ/mol
Free energy of vaporization at 863 K: 218 kJ/mol
Entropy of vaporization at 1062 K: 133 J/mol K

 

Densities of molten UCl, are: Temperature Density
(°C) (g/om’)
590 3.57
600 3.55
650 3.45
700 3.36
750 3.26

The properties, preparation, and chemistry of the uranium chlorides and oxychlorides are

comprehensively reviewed by Brown.”

2.1 THERMOCHEMICAL DATA

The application for the thermodynamic data is to make calculations at the chlorina-
tion reactor conditions. The most useful values are the free energies of formation at 700

to 1100 K. A temperature of 700 K (427°C) is about the lowest temperature of interest
for both practical rates of reaction and the use of molten chloride salts. The 1100 K
Table 1. Physical properties of U-O-Cl compounds

 

 

Density Melting Boiling
Molecular  at 298 K, point point
Compound weight (g/cm?) (K) (K) Color at 298 K
U 238.03 19.05 1405 4091 Silver gray
UCl, 344.39 5.44 1114 1930 Olive-green
UuOoCdi 289.48 Dark red
Ud(l, 379.84 4.87 863 1065 Dark green
UQCl, 324.94 Green
Uo, 270.03 10.96 3110 Brown-black
(U0),Cl 685.33
U,0, 1096.12 10.9 decom.? Black
UCl 415.30 3.8 ~ 600 decom. Red-brown
UGCl, 360.39 Brown
UO,(Cl 305.48 decom. Brown-violet
U,0, 842.09 8.30 decom. Greenish-black
(UO,),Cl, 646.42 Black-brown
UCl 450.75 3.5 452 decom. Black or dark
green
UOd], 395.85
uo.Cl, 340.93 5.34 851 decom. Yellow
U0, 286.03 7.29 decom. Orange-yellow

 

*decom. = decomposes without phase change.
(823°C) is above the boiling point of UCl, and is near the highest practical temperature.
The enthalpies and entropies of formation at reference conditions are available for nearly
all the uranium chlorides and oxychlorides, but the high-temperature data is much less
complete. The enthalpies and free energies in recent (since 1975) assessments and
collections of data are mostly 20 to 30 kJ/mol smaller (less negative) than those listed
before 1970. 1t is probably inconsistent and misleading to use carly and recent data
together in one calculation.

A recommended set of data for calculations is tabulated (Table 2). This data is
from the more recent publications.**#1% Data published for uranium chlorides and
oxychlorides before 1975 is not consistent with this more recent data. The significance of
and conclusions from the thermochemical data are discussed in Sect. 3.1.

Krahe listed vapor pressure equations as shown in Table 3.' Calculated values
from these equations are shown in Fig. 1. The decomposition of UCl; (or UCly) into Cl,

and UCI, must be considered; the UCIs or UCl; are stable only when excess Cl, is present.

22 PHASE DIAGRAMS

Phase diagrams present equilibrium information in several different ways. With
only two components, a composition versus temperature type of phase diagram can show a
complete representation of the solid, liquid, and gaseous phases present. The diagram for
UQ,-UCl, gives information important to understanding the chlorination behavior. Most
of the other two-component phase diagrams for uranium oxides, oxychlorides, and chlorides
are not available in published literature. For three components, a triangular diagram can
show one variable (usually the liquidus temperature) vs all compositions. A third type of
phase diagram can be calculated from the thermodynamic data. The calculations give the
equilibrium concentration or solid phases present vs two of the concentrations as

variables.
Table 2. Thermochemical data

 

Free energy of formation (-A,G°)

 

 

-AH at 298 K Sat 298K (kI/mol)

Compound (kJ/mol U)  (J/mol-K) 700 K 900 K 1100 K
UCl, 862.1 159.0 712.2 6709 629.7
UOCI 9473 102.9 836.8 807.2 795(E)
udci, 1018.8 197.23 814.6 762.3 720(E)
UOCl, 1069.4 138.32 904.9 863(E) na®
uo, 1084.9 77.03 963.5 930.8 897.7

(U0),Cls 2197.4 326.4 na na na
U,0, 4510.8 335.93 3972.9 3827.2 3680.1
UCI, 1041.4 246.9 816.3 763(E) na
Uod], 1140.1 169.9 946.8 888.3 835(E)
UO,Cl 1169.4 112.5 na na na
U,04 3574.8 282.59 3114.0 2994.3 2874.5

(U0,),Cl,y 2404.5 276.1 na na na
UCI, 1068.2 285.8 812(E) 760(E) na
UOC], na na na na na
UO,Cl, 1145.8 150.6 1017.7 960(E) 900(E)
U0, 1223.8 96.11 1043.0 992.9 942.3

CO 110.53 197.65 173.52 191.42 209.08
CO, 393.52 213.80 395.40 395.75 396.00
COCl, 220.08 283.80 187.05 177.84 168.66
Cd, 95.98 309.81 -1.74 -28.34 -54.56
HCI 92.31 186.90 98.75 100.15 101.43
H,0(g) 241.83 188.83 208.81 198.08 187.03

 

*(E) indicates estimated values.
*The term "na" indicates that values are not available in any of the known references.
ORNL DWG 91A-11

 

1000
800 [
600 [

400 [

20

 

A mp: 590°C

VAPOR PRESSURE (mm Hg)
o
o
i

A mp: 327°C

. L UCls o |
>800 °C

 

 

2 L | ] | L
100 200 300 400 500 600 700 800

 

TEMPERATURE ( °C)

Fig. 1. Vapor pressures of uranium chlorides.
Table 3. Vapor pressure equations®

 

 

A B C Temperature
Compound (X)
UCl; (s) 19.224 15,760 3.02 298 - 1110
UCH (1) 24.044 14,340 5.03 1110 - 1950
UCI, (s) 20.329 11,350 3.02 298 - 863
udi, () 26.079 9,950 5.53 863 - 1062
UCl; (s) 21.810 7,450 4.03 298 - 600
UCL (1) 26.027 6,210 6.29 600 - 800
UCl (s) 22.317 4,765 5.03 298 - 453
UCl, () 26.120 4,060 7.04 453 - 650

 

logP., = A-BT-ClogT.

The phase relationships between tetravalent uranium oxide and chloride are shown
in Fig. 2.1 The phasc diagram shows that there are three stable compounds over the
entire range of composition--UCI,, UOCI,, and UQ,. There is a eutectic reaction between
UCl, and the intermediate compound, UOCI, The melting point of pure UCl, is 590°C.
A minimum melting point of 545°C occurs at the eutectic composition of UCl,; + 6.9 mol
% UQ,. A maximum solubility of about 13 mol % UO, in molten UCI, is reported at
810°C. At temperatures from 810 to 855°C, UCI, vapor is in equilibrium with solid
UOCI, UQOCIL, decomposes at 855°C. At higher temperatures, vapor and solid UQ, are in
equilibrium. This phase diagram suggests practical limitations on the useful chlorination
conditions and will be discussed further in this respect (Sect. 3.2).

A phase diagram with the Cl/U atom ratio as the concentration variable shows the
melting points of the uranium chlorides and their eutectics (Fig. 3).!' This diagram is a
scries of binary diagrams for U-UCI,-UCl,-UCIs-UClg as no more than two of these

compounds can be present in equilibrium.
 

  

 

 

 

 

 

Fig. 2. Phase diagram for UCI,-UO,.”?

| wt %
99 97 95 90 85,
T TTTT T T T T
900 ; Vopor + U0, —
Vapor A___“ggs_:___ e
1 apor + UOCI, .
800 »={8I10° _|
700 —
600 -
L 3
f (6.9%) N
500H —
i
I
400H- —
|
I00H UCly ss + UOCiz —
{
| |
200 -
N
100 S -
S
oL 1 Lt 1 1 1 1t ., |
UCI42 6 10 i4 I8 50 U, —
mol %
11

For reasons discussed in Sect. 3.2, a ternary mixture of UCl,-MgCl,-NaCl might be
the preferred melt for reaction of UO,, Cl,, and C. While this ternary diagram has not
been determined, the three binary diagrams (UCl,-MgCl,, UCl,-NaCl, MgCl,-NaCl) have
been published.”® These binary diagrams give the liquidus temperatures for the three sides
of a ternary MgCl,-NaCl-UCl, phase diagram. The three binary diagrams are simple, and
simple liquidus curves for the ternary are very probable. Estimated curves were drawn
(Fig. 4) with shapes similar to those for other published ternary diagrams. These curves
are derived from the binary data and should be considered interpolations between them
instead of extrapolations. |

An important use of thermochemical data is to calculate the equilibrium composi-
tions at specified temperatures. Uranium has five major valences (0, 3, 4, 5, 6) and also
has some stable compounds of apparent intermediate valances (4.5, 5.33, 5.5). Only two of
the major valences can be in equilibrium at a specified condition. Uranium and chlorine
give a series of compounds (U, UCl;, UC,, UCl;, UCly). Each composition from Cl/U=0
to Cl/U=6 will have an equilibrium overpressure of Cl, gas. These equilibrium concentra-
tions can be calculated from thermodynamic data. They can be conveniently represented
by equations with temperature as a variable and do not require phase diagrams. The
melting points and eutectics were shown in Fig. 3. |

Thermochemical data can also be used to calculate equilibrium compositions for
the U-O-Cl system. Because of the multiple valances of uranium and the formation of
oxychlorides, over twenty U-O-Cl compounds are possible. At least seventeen of these
compounds have been reported experimentally.5 The results can be presented as diagrams
showing the composition of the solid phase with a specified temperature as a constant and
two gas activities (Cl, and O, or CO) as the variables. A result of interest for the
chlorination of UQ, to UC}, is reported by Krahe (Fig. 5)."! Chlorine pressures near 1 atm
give a melt that consists of UCl, and UCl;. UCI, is the stable composition for a wide
range of both Cl, and CO (or O,) concentrations. Krahe's result looks very good with
respect to utilization of Cl, and the formation of UCl, as the product. Cordfunke discusses
the U-O-Cl phase calculations and the limitations from the precision of the data.® He
shows that small differences in the data can cause phases to appear or disappear from the
calculated results. He estimates a need for 0.25% precision for enthalpies and 1% for

entropies. The data for uranium oxychlorides is probably not this good. The poor
TEMPERATURE (°C)

10

ORNL DWG 91A-634

 

 

 

 

 

 

 

 

 

 

 

1200 7 1
1130° |
1000
..o
8150 Ef&?
goo F— — — |-
590°
600 - 550°
400 F N\
3270 \
N\ 170°
‘\\
200
| 1700 N
U —~UCly _UCl, _UCls UClg_
0 |
0 1 3 4 5 6

ATOM RATIO OF CI/U

Fig. 3. Phase diagrams for the compounds of uranium and chlorine.’
12

   

: —PHA
IIIIIIIIIIIIIIIIIIIII

 
 
  
       
   
    

RNEX
AR
PN
50 /\ "‘“»’0“;}"@0’" INaCI. ucl,
DOPOK L PPE N
- DPOLKASXE5D
» X ‘:" ;.0:’1 f{‘:f"/':('l”"/ 700
0 0‘0 L

/AA

 
 
 
 
 
 
  
     
    

g

<O

 

     
      

N/
JOH e

Fig. 4. Liquidus tempe,raturés (°C) for UCl,-MgCl,-NaCl.
LOG P, (atm)

13

ORNL DWG 91A—-8633

 

 

 

 

 

 

 

MELT
UoCI
3 0
0 1 0
.._2 ™
..._4 -
_6 ™
_.8 e
10 | L |
~20 ~16 ~12 -8 —4 0o

LOG Pg,  (atm)

Fig. 5. Diagram for log Peg vs log Py, at 400°C.1
14

information on the composition or chemical purity of the samples that were used is an
important source of error. Cordfunke gives three phase diagrams for O, and Cl, pressures
as the concentration variables to illustrate the effects of small differences in data.® Krake
gives over twenty calculated results with gas concentrations, temperature, and the activity

of carbon or CO as variables.!

23  EXPERIMENTAL RESULTS FOR REACTIONS OF CHLORINE WITH

URANIUM OXIDES

A search of technical literature did not reveal any significant reports for the
preparation of pure UCI, from reaction of uranium oxides with Ci, and C or CO. There is
extensive literature on the production of UC], by reaction of carbon tetrachloride (CCl,)
with uranium oxides.>!* One such process was used at Oak Ridge for producing calutron
feed for isotope separations. The CCl, cannot be produced by a simple reaction of C with
Cl,. Therefore, the reactions of CCl, with uranium oxides do not provide efficient overall
reactions of Ci, to prepare UCI,.

There are many literature descriptions concerning the reactions of chlorine with
uranium oxides. Some of these were intended to produce uranium chlorides with some
results for carbon as a reactant and CO, or CO as producis. Experiments using chlorine
gas feed to graphite distributors immersed in molten salts showed good rates of reaction of
Cl,. These graphite distributors provide a carbon source of relatively low-surface area and
reactivity, and the rates of formation of CO and CO, are much better than might be
expected. Canning demonstrated nearly complete utilization of Cl, for up to 90%
chlorination of uranium oxides in NaCl-MgCl, at 700 and 800°C."> Analyses indicate 80 to
85% UC], and 15 to 20% UCI; at the end of chlorination. Gibson claims a similar result
with all the uranium soluble in the NaCl-KC! melt at the end of chlorination.'® Gens
studied the volatilization of uranium chlorides from nuclear fuels and appeared to find the

formation of some non-volatile UO,Cl,.!” Lyon reported rapid reactions of uranium oxides
in molten NaCI-KClI at 850°C with Cl, to give UO,CL,.'*

The teactions of uranium oxides with CCl, have been more carefully studied than
the reaction with Cl, and C or CO. Since the free energy of formation of CCl, is positive

above 415°C, the use of CCl, above this temperature is somewhat thermodynamically
15

equivalent to use of C and Cl,. Budayev provides good thermodynamic analyses and
experimental results of reactions with CCl,.'* Reaction products at 200 and 300°C were
UQO, and UCl,. Reaction products at 400 to 700°C included UO,, UCl,, UOCl,, UOCI,,
and U,0,Cl;. The experimental results showed stepwise reaction with many intermediate
products, including CO, COCl,, Cl,, and all the uranium oxychlorides.

Gens reported that the UyO;4 treated with CCl,-Cl, is first converted to UO,Cl, and
is then further reacted to give UCl,, UCl;, and UCI."® Jangg found high conversions to
volatile UCI, and UCI; at 700 to 900°C using CCl, while Cl, gave mostly UO,CL,.”

Katz and Kabonowitch published an excellent review of the literature on the
chemistry of uranium up through 1946.> The overall results for preparation of uranium
chlorides indicate the following conclusions:

1. There were no complete and practical conversions of UO, to pure UCl, by reactions
with chlorine and C or CO.

2. UO, was clearly the preferred uranium oxide feed. Uranyl compounds were much less
reactive. Higher oxides, such as U,Oz and UQ,, gave larger amounts of UCl as
compared to the UC yield from UO, at similar conditions.

3. The conversion of UO, to UOC!, appears to liberate more energy than the conversion
of UQCI, to UCl,. Therefore, an incomplete conversion is likely to leave large
amounts of UOCI, instead of unreacted UO, and UCl,.

4. The first reported preparation of UC], was by the reaction of a UO,-C mixture with Cl,
gas. The major disadvantages were the major yield of UCI,, the high reaction tempera-
ture, and the phosgene in the waste gas.

5. The reaction of UO, with CCl, proceeds at lower temperature and gives less UCl; and
phosgene than UO, with C and Cl,, Common conditions were 350 to 450°C in CCl,
vapor or 150 to 250°C in liquid CCl, under pressure.

6. Phosgene (COCl,) was an effective reagent at temperatures of 450°C or higher.

7. The higher chlorides (UCl;, UCL) are formed when the higher oxides (U;0,, UQ;) are
converted to chlorides and are also formed by reaction of UCl, with Cl,.
17

3. DISCUSSIONS AND CONCLUSIONS

The application of literature information to plan a development program for UCI,
preparation is given here. The use of thermodynamic data is the logical first step, but has
important limitations. The experimental results reported in the literature show reactions of
chiorine and uranium oxides with little information on what reactions are occurring. The
chemistry and preparation of TiCl,, ZrCl,, ThCl,, and SiCl, have important similarities to
thase of UCl,. Therefore, references for preparation of these compounds are reviewed as

sources of information applicable to UCl,.

3.1 THERMOCHEMICAL CONSIDERATIONS

Uranium dioxide is one of the most stable metal oxides and has a larger free energy of
formation than UCIl,. This means that many of the reactions that might convert UO, to
UC], are thermodynamically unfavorable. The displacement of the oxygen in UO, by
reaction with Cl, is not practical. While the rate of reaction of UCl, with air is low at
room temperature, the thérmodynamic eqdilibrium is a high ratio of Cl,/O, in the gas. At
225°C or higher, UC], reacts with air to release Cl,.

The practical conversion of UO, to UCI, by a chemical reaction requires a reducing
agent which yields an oxide product that is more stable than the chloride (i.e., it does not
react with UCl,). Carbon and carbon monoxide meet this criteria. Both have large free
energies of formation while CCl, has a zero value at about 415°C and decomposes
thermally at higher temperatures. Hydrogen oxide and hydrogen chloride are about equally
stable, and the oxide (water) reacts with uranium chlorides to form HCl and UQCI, or
UQO,. The equilibrium pressure of H,O over UQ, in HC! gas is very small, and conversion
to UCl, by countercurrent treatment of UO, with HC] (as used to prepare UF, using HF)
is completely impractical. For the same reasons, UCl, prepared in aqueous solutions
cannot be dehydrated to anhydrous UCl,. Thermal or other treatments give removal of
HCI leaving UO, or UOCI, as the product.

Thermodynamic calculations are a logical first step to identifying the probable chemical
reactions for conversion of UO, to UCl,. The use of thermochemical data does not

identify the probable reactions with any certainty or degree of confidence. There are
18

several major causes of uncertainty. Uranium chemistry is complex with stable valences of
3, 4, 5, and 6, and with stable oxychlorides. UQ,Cl, and UOCI, are well known compounds
and others are possible. All of the possible products must be considered. The uranium
chlorides are more volatile with increasing valence, but UCl and UCl; are less stable with
increasing temperature and decreasing Cl, partial pressure. All of the uranium compounds
have large heats of formation, and calculating the free energy of these reactions usually
results in a small difference from two large numbers. Small percentage uncertainties for
the large numbers give large uncertainties for the differences.

The relationships between the U-O-Cl compounds can be illustrated by a matrix listing.
Data are available for heats and entropics of formation at 298 K for nearly all of these
compounds (Tables 2 and 4). The free energies of formation at 900 K would allow more
realistic and useful calculations. However, because the free energy of formation data at
high temperatures is much less complete, this matrix listing contains less certain or

estimated values (Table 5).

Table 4. Hceats of formation for U-O-Cl compounds at 298 K

 

- {Heats of formation), kJ/mol U

 

Valence All dl Onec O Two O >Two O
0 u? U U U
3 UCl, UOoCl
862.1 9473
4 ucCl, Uodi, uo,
1018.8 1069.4 1084.9
4.5 (U0),Clg U,0,
1098.7 1127.7
5 UCl UOd, Uuo,Cl
1041.4 1140.1 1169.4
5.33 (UO,).Cl, U,;04
or 5.5 1202.3 1191.6
o UCI, uQod, UQO,Cl, U0,
1068.2 ~ 1140 1145.8 1223.8

 

aAfI’IO = {),
19

Table 5. Free energies of formation for U-O-Cl compounds at 900 K (627°C)

 

: - (Free energy of formation), kJ/mol U
Valence All Cl One O Two O >Two O

 

0 U U - U U
ucl, Uocl
670.9 807.2
4 ucl, Uoc, U0,
762.3 863 930.8
4.5 - (U0),Clq U,0,
880 (Est.) 956.8
5 UCl uoC, UO,Cl
763 888.3 960 (Est.)
5.33 (U0,),Cl, U0,
or 5.5 990 (Est.) 998.1
6 UCy, uoci, Uo,ClL, U0,
760 900 (Est.) 960 992.9

 

The complexity of the uranium conversion chemistry is partly shown by a diagram
giving the free energies of the simple oxidation and reduction reactions (Fig. 6). This
diagram was simplified by omitting the compounds of U(4.5) and U(5.5) vailences. It also
does not show the reactions of two U-O-Cl compounds to give a third U-O-Cl compound.
The reductions are shown for 4C to 4CO,. Similar conclusions would apply for C to CO
or CO to CO, as the three free energics are 197.9, 191.4, and 204.3 kJ at 900 K. The
three free energies are equal at 973 K (700°C), and the probable products from carbon are
CO, below 700 K and CO at higher temperatures.

Some general conclusions from examinations of Tables 4 and 5 and Fig. 6 are:
® At a given valence state, the uranium oxychlorides are more stable than the chlorides.

® At a given valence state, the uranium oxides are more stable than the chlorides.
20

ORNL DWG 91A-632

 

 

 
 
 
   

  
  
  

  
  

C 0/U, ATOM/ATOM
URANIUM
VALENCE 0 ‘ 2 >2
U(0) U U U U
* : : :
-224 ~538 | | ,
s : ' '
-136 —-465 1 I
U(III) UCly UocCl 8 ; :
= ' |
-91 g —-56 g I ~374 :
i i ¢ !
u(Iv) uc uocl, uo, |
. - . - l
: 25 § -29 } 90 |
~1 4 : : {
i § i UsOsg
u(v) uc UOCIy Uo,Cl (VALENCE—5.33)
. » ° l
: -12 2 0 3
: : s |
3 3 : $ +16
3 : $ !
¥ { ¥ '
—-14 . -B0 —~33
U(VI) UCI6 Ww UQCI, seesrtsssmsssinsioe UO2Clo  sesersrsrrerenssm  UO3

LEGEND: esasesaessap CHLORINATIONS; FREE ENERGY IN kJ/ATOM CI

————s OXIDATIONS; FREE ENERGY IN kJ/(0 + Cl,)
sessssssrenssn DISPLACEMENTS; FREE ENERGY IN kd/[0 — Clg]

e REDUCTIONS; FREE ENERGY IN kJ/(C-CO)

Fig. 6. Conversion reactions for U-O-Cl compounds and free energies at 900 K.
21

e All additions of chlorine to oxides or oxychlorides of lower valence {less than U(VI)]
are favorable to yield oxychlorides of higher valence.

® The oxychlorides can be formed by both direct reaction of chlorine and by reaction of a
uranium chloride with an oxide or oxychloride.

Since oxychlorides are the intermediate compounds for conversion of UO, to UCI,,
their stability can be of critical importance to complete conversions. A conversion may
appear favorable overall, but one of the steps for the intermediate may be much less

favorable. For example, consider the following overall reaction:

U0, + 2C + 2Cl, - UC, + 2CO.

At 900 K, AG = -214.3 k], but individual steps show:
UO, + Cl, - UO,(Cl, AG = -29.2;
UO(l, + C - UOCI, + CO, AG = -94.4;
uod, + Cl, » U0, AG = -37.0; and
UoCl, + C-»UCl, + CO, AG = -54.

These numbers indicate that the reactions should take place, but older data indicate they
could stop at UO,Cl, or UOCI,. If using CO to give CO, as the product is considered,

then the two reduction reactions change to:

UO,CL, + CO - UOCI, + CO,, AG = -107.3 and
UOC], + CO - UC), + CO,, AG = -66.6.

The uncertainties for A(G® values of UO,Cl, and UOCI, may be larger than the above AG
values, so it is difficult to be certain that the reactions are thermodynamically favorable.
The more favorable calculation for CO as compared to C may also be misleading. The C
would be present as a solid with a thermodynamic activity of 1, while CO would be mixed

with other gases and would have a lower activity for 1 atm total pressure.

Many of the reactions to change between the U-O-Cl compounds are shown in Fig. 6.
Large values for the negatives of the free energies of reaction (kJ/equiv at 900 K) show

reactions that are thermodynamically favored. Positive values in Fig. 6 indicate that the
22

reactions are not thermodynamically favorable. The data for the oxychlorides are uncer-
tain, and values ranging from -20 to +20 kJ do not justify predictions. The formation of
UCI; and UCI appears to require excesses of Cl, and will not be complete. Otherwise, all
additions of Cl, are favorable, and oxychlorides should add chlorine to give UO,CI, or.
UOC],. The reductions of UO; or U;0,4 to UQO, are highly favorable. The reduction of
UQCI,; to give UCI, is unfavorable. Some of the other reactions give free energies of
rcaction that are too near zero to justify predictions. A practical preparation of UCI,
probably requires that the chlorinations to U(VI) and the reductions of UO,Cl, and UOCI,
be possible since the intermediates would otherwise accumulate as stable products.

The equilibrium mixture among the uranium compounds for normal operation of a
chlorination reactor will always be UCl, and uranium oxychlorides containing one oxygen.
The feed of UQO, will be limited to prevent excessive solids; therefore, O/U ratios will be
much less than one. Because of the lower stability of the chlorides as compared to the
oxychlorides, any uranium compounds containing two oxygens will react with a uranium
chloride to form two oxychlorides with one oxygen in each. Any U(VI) or U(V) will be
much more stable as oxychlorides than as UClg or UCl;. Therefore, the amounts of UC];
and UCl, will be small unless the moles of U(VI) and U(V) exceed the moles of oxy-

chlorides. Some of the reactions and their free energies at 900 K are:

UCl, +UO, » 2UOCl,, AG = -32.9 kJ;
UCl; + UO,Cl - 2UOCH, AG = -53.9 kJ;
UCl, + UO,CL, - 2UOC],, AG = -80 kJ;
UC!, + UO,CL, - 2U0C, AG = -54.3 kI;
UCl, + UO, » UO,ClL, + UCl,, AG = -31.5 kJ; and
UCI, + UOCL, - UOC, + UCl, AG = -39.3 k.

The first five reactions indicate that the uranium chlorides will react to give oxychlorides.
The first four show that an oxychloride containing two oxygens will react with uranium
chlorides to give two oxychlorides. The last two show that UCl, will react with a U(IV)

oxychloride to give UCl, and an U(VI) oxychloride.

3.2 SELECTION OF CHLORINATION CONDITIONS

One feed material must be the uranium oxides from ore refineries. Recycling consider-

ations to eliminate large amounts of waste require the use of Cl, gas from electrolytic cells.
23

Thermochemical data show larger free energies of formation for uranium oxides than for
chlorides of the same valence. Therefore, the overall reaction must include a reducing
agent that has a much more stable oxide than chloride. Carbon or carbon monoxide are
the mostk practical reducing agents that meet this requirement. Hydrogen (as H, or
hydrocarbons) does not meet this requirement since H,O will react with uranium chlorides
to form HCl. Sulfur and phosphorus (and some of their compounds) can meet this
thermochemical requirement, but they are expensive feeds and give troublesome waste
oxides in‘comparison {0 carbon. _

The overall reaction from the above considerations shows one or two solids (uranium
oxides, C) and gaseous feed (Cl,, perhaps CO) and a gaseous product (CO, or CO). Even
with stepwise reactions, the reactions between two phases with one of them a solid tend to
be slow or incomplete. The phase diagram for UO, and UCI4 (Fig. 2) shows several
indications toward practical conditions foriconversion of U'O2 into UCl,. The use of a
liquid melt with reaction of UQ, and UCl, to give UOC, in solution appezirs favorable to
high rates of reaction. Then the Cl, and carbon can react with the UOCI, or other
oxychlorides in solution. The Cl, can also react with UCl, to give UCl; or UCl6 as soluble
chlorinating agents in the melt. |

Both the temperature and the fraction of UQO, in the charge must be limited to
maintain the desirable liquid melt condition. All of the UQ, reacts with UCl, to form
UOCl, and any UOC], above the solubility is present as solids. An equimolar mixture of
UG, and UCl, gives all UOCI, solids without any melt. Since C will also be a solid, the
preferred concentration to ensure a fluid melt will be less UO, than the solubility limit
(from 6.9 mol % at 545°C to about 13 mol % at 810°C). The preferred temperatures will
be intermediate between the melting point and boiling point of UCI, (590 to 792°C). The
vapor pressure of UC, is also an important consideration.

The addition of a diluent salt to the melt would relax some of the composition and
temperature limits indicated by the UCl,-UO, phase diagram. The diluent salt should be
unreactive with the carbon, Cl,, UO,, and UCL,, should have a low volatility at 600 to
800°C, and should melt below 600°C. An equimolar mixture of MgCl,-NaCl meets these
requirements better than any single salt. Mixtures of MgCl,-NaCl are commonly used for
electrolytic production of Mg and Cl, and have good properties.

There are no published phase diagrams for UC],-UO,-MgCl,-NaCl or for any ternary

mixtures of these components. However, reasonable liquidus temperatures and UO,
24

solubilities can be estimated from four binary phase diagrams. Binary diagrams for MgCl,-
NaCl, MgCl,-UCl,, and NaCl-UCI," can be used to give liquidus temperatures for the
three sides of a ternary MgCl,-NaCl-UCl, phase diagram. The three known binary
diagrams are all simple and simple liquidus curves for the ternary are very probable.
Estimated curves were drawn (Fig. 4) with shapes similar to those for other published
ternary diagrams. The UCI,-UO, binary phase diagram gives the data needed for a UQO,
solubility versus temperature in units of moles UQ,/UCI, (Fig. 7). Experimental data for
fluoride salt mixtures show that the UQ, solubilities are proportional to the UF, concentra-
tion; that is, moles UO,/moles UF, is dependent on the temperature and the other salt
compositions but independent of the UF, concentration. Figure 7 shows the data for the
solubility of UO, in UF, also. The data for UCl, and UF, could be represented by a single
curve. Considering the MgCl,-NaCl-UCJ, ternary together with the UO,-UCI, binary, it is
likely that replacement of part of the UCl, by UQ, up to the solubility limits shown by Fig.
7 would have only small effects on the liquidus temperatures of Fig. 4. As an example of
the results of these assumptions, a charge of 20 mol % MgCl,-20 mol % NaCl-60 mol %
(UC1,+U0O,) would be expected to be all liquid above 500°C with UQ, solubilities from 4
mol % at 500°C to 8 mol % at 800°C.

3.3 SALT PROPERTIES AND PROCESS CONTROL CONSIDERATIONS

The practical chlorination of uranium ore concentrates to UCI, on a large scale would
require continuous processes with controlled inventories of the process materials. The
literature data indicate that a molten salt reaction with feeds of UQ,, Cl,, and C or CO
might be most practical. The UCI, product must have low concentrations of unreacted
carbon or oxygen. Large amounts of unteacted chlorine in the product gas are also very
undesirable. Countercurrent tlows with high conversion to UCI, in a melt (low oxychlo-
rides, UCI and UCIy) would be extremely difficult to accomplish. A more practical
concept is to remove UCI, as a vapor from a melt with compositions favorable to high
utilizations of the Cl, feed. The chlorination reactor will have three reactant feed streams

and two products leaving the reactor.

The most likely overall reaction will be one of:

U0, + C + 2CL, -» UC], + CO,
U0, + 2CO + 2Cl, - UC], + 2C0, .
U0, SOLUBILITY (mol UG, /mol UX,)

0.40

0.30

G.20

ORNL DWG 91A4-631

 

 

 

 

1 r 1 ! &’
/'
a "”’, -
UOCI, DECOMPOSES ’/,/'
AT »855° C .
— ... —
0‘. - )
. ® FROM UCIl, —UO, PHASE DIAGRAM;'?
",,‘ DISSOLVED U0, IS UoCl,
e ® DATA FOR UF, (GREENFIELD
~ PR AND HYDE IN AERE-R 6463) B
e |
! 1 | 1 i
400 600 800 1000 1200 1400 1600

TEMPERATURE (°C)

Fig. 7. Solubility of UQ, in UCl, and UF,.

¢c
26

Reactions with either CO or carbon and with other uranium oxides as feed will also have
five primary flows of feeds and products. Small amounts of unreacted Cl, or of CO in
addition to CO, and large amounts of a nonreactive diluent gas (N, or CO,) do not change
the need to control five process inventories or process flows.

All gases including the gaseous products, unreacted Cl, or CO, and nonreactive diluent
gas leave by displacement without need for a control measurement. This leaves four other
flows requiring control.

One flow can be set to cstablish the system capacity. This is most logically the Cl, feed
rate. Since there is little or no chlorine inventory in the melt, a change in the chlorine
feed rate can give an immediate change in the rate of Cl, reactions. It would normally be
desirable to have a good inventory of UO,, C, or CO and the molten chloride salt to allow
high utilizations of Cl,. Depleting one of these charge components to provide control of
the reaction rate contributes to high, undesitable losses of unreacted Cl,.

The condensed phase in the chlorination reactor is likely to contain more UCI, than
any other component. Therefore, it is logical to measure the condensed phase level or
volume and usc this measurement to control the UC], exit rate. The gas leaving the
charge should be at a vapor pressure equilibrium; that is, saturated with UCI, vapor. The
UCI, vapor rate could be changed by changing the charge temperature (to increase the
vapor pressure of UCI,) or by changing a diluent gas rate. Liquid or solid UCl, could be
recycled to the charge to control the net outflow of UCI,. Use of nonvolatile diluent salts
such as CaCl,, MgCl,, or NaCl would reduce the vapor pressure of UCI, by reducing the
UCI, concentration. The diluent salts could provide some automatic changes in the rate of
UCI], vapor. When the UCI, is depleted, the lower concentration of UCl, would result in a
lower rate of vaporization. Excess UCl, would result in a higher rate of UCI, vaporization.
For purc UCI,, concentration in the melt and the vapor pressure do not change as the
amount in the charge varies.

After the rates of gas exit flow, Cl, feed, and UC], vapor are determined as described,
the feed rates of UO, and C or CO remain to be controlled. While the rate of Cl,
losses to the exit gas can depend on the charge inventories, measurement of Cl, losses is a
poor control criteria. A high utilization of Cl, is desirable and conditions that allow
increascs in Cl, losses are undesirable. Also, increases in Cl, losses could result from

deficiencies in either UQ,, or C-CO and could also result from excessive UOCI, or C solids
27

with poor gas-charge contact. Since a high Cl, loss might have several causes, it would not
be a dependable control measurement for one feed. The UO, and C feed rates should be

controlled on a long-term basis by some direct measurement on the reactor charge—either
measurements of salt properties or analyses of samples for C and oxygen. A review of the

limited literature on uranium salt properties did not indicate any promising possibilities for

use of measurements of the salt properties at 600 to 800°C.
Based on the lack of information for measurement and use of salt properties for

control, the UQ, and C feed rates will probably be sct to agree with the Cl, feed rate with
periodic adjustments from analyses of charge samples. Good inventories of UO, and C
appear desirable both to ensure good utilization of Cl, and to make control from periodic

samples more practical.

3.4 PREPARATION OF TiCl,, ZrCl,, SiCl,, AND ThCl, FROM OXIDES

The chemistry and preparation of these metal chlorides have important similarities to
those of UCI,. For all of them, the tetrachlorides have been produced from oxides using

Cl, and carbon as follows:

MO, + C + 2C], - COy,y + MCly,, or
MO, + 2C + 2Cl, » 2C0O, + MCl,,,.

The ThCl, preparations were small scale. The other three conversions have been produc-
tion processes and should provide practical information that applies to production of UCl,.
For silicon, the free energy of reaction is positive, but the reactions can be completed by
removal of the SiCl, gas.

This conversion of TiO, to TiCl, is used on a large scale (10° tonsfyear) to prepare
titanium dioxide pigments. The TiCl, is a more volatile product (mp 248 K and bp 409 K)
than UCl,, and can be handled as a liquid at room temperature. While the process
concepts for preparation of TiCl, are old and well known, the details of plant design and
operation are proprietary with little publication as technical literature. The descriptions

published in the 1950s*! and 1980s* are very similar.
28

One description of a plant operation is as follows: A mixture of 20 to 30 wt %
calcined coke and 70 to 80 wt % TiO, ore is fluidized at 900 to 1000°C using Cl, gas. The
reactor is lined with SiO, brick and some oxygen feed may be used to preheat the reactor
or replace heat losses (the chlorination reactions are exothermic). The exit gases contain
the TiCl, product as vapor, some excess Cl,, CO,, and CO (ratios near 2 mol/mol), and
volatile chlorides of impurity metals such as Fe, Mn, Cr, and Al. The chlorides are
separated using the differences in volatility and melting points to give a purified TiCl,
liquid as the product. An alternate to the fluidized bed is to flow the chlorine up through
a fixed bed of TiO,-carbon briquettes. Electrical resistance heating may be used to
generate supplementary heat in the fixed bed. The use of SiO, brick linings without
excessive attack is possible as a result of the low surface area of the brick as compared to
the mixture of fine SiO, and carbon that is used for preparation of SiCl,.

A similar chlorination of ZrG,-C briquettes at 600-800°C was developed for prepara-
tion of ZrCl,” Fixed beds of 66-cm diam were operated using ZrO,-C briquettes to
produce 100 mol ZrCl/h. The ZrCl, leaves the reactor as vapor and is collected as con-
densed solids.

ThCl, has been prepared using by reacting crushed ThO,-C compacts with Cl, in a
KCIi-NaCl melt at 900°C.?* Careful operation was required to give complete conversions to

ThCl, without residues of ThOCI,. The containers were silica (quariz).

4. ACKNOWLEDGEMENT

This review resulted from a development program for preparation of uranium metal
feed for the AVLIS (Atomic Vapor Laser Isotope Separation) program. Discussions with
and suggestions from H. W. Hayden, J. C. Mailen, and M. J. Stephenson contributed to the
evolution of the review. Some references were supplied by H. W. Hayden and W. D.
Bond. Any oversights or errors in the selection of data and the discussion and conclusions
arc the responsibility of the author. The information in this review was used to plan and
correlate results for a laboratory study. The experimental results show high rates of
chemical reaction, good utilizations of chlorine, and collection of condensed UCI,

product.®
10Q.

11.

12.

13.

14.

15.

29

5. REFERENCES

Phillip G. Sewell and Norman Haberman, "AVLIS Program Powers Ahead in the
United States," Nucl. Eng. Int., pp. 17-19 (October 1988).

Joseph J. Katz and Eugene Rabinowitch, The Chemistry of Uranium, Part 1. The
Element, Iis Binary and Related Compounds, McGraw Hill (1951).

M. H. Rand and O. Kubaschewski, The Thermochemical Properties of Uranium
Compounds, Interscience Publishers (1963).

J. Fuger et al,, The Chemical Thermodynamics of Actinide Elements and Compounds-
Part 8: The Actinide Halides, International Atomic Energy Agency, Vienna (1983).

Thsan Barin, Thermochemical Data of Pure Substances, Cambridge, New York (1989).

E. H. P. Cordfunke, "Basic Thermodynamic Data in Nuclear Technoldgy: New
Developments, Old Problems," J. of Nucl Mater., 130, 82-93 (1985).

David Brown, "Compounds of Uranium with Chlorine, Bromine, Iodiné," Gmelin
Handbuch der Anorganischen Chemie, Vol. C9 (1979).

E. H. P. Cordfunke et al., Thermochemical Data for Reactor Materials an Fission
Products, Eur.-Contract no. ETSN-0005-NL (1988).

E. H. P. Cordfunke and O. Kubaschewski, "The Thermochemical Properties of the
System Uranium-Oxygen-Chlorine," Thermochim. Acta, 74, 235-245 (1984).

M. W. Chase et al., JANAF Thermochemical Tables, Third Edition, Amer. Chem. Soc.,
Am. Inst. Physics, and National Bureau of Standards (1986).

Josef Krahe and Franz Miller, "Zur Thyermochemie der Stoffsysteme U, Th, Pa, C,
O,, Cl,," Institute fir Chemisehe Technologie, Jul 565-CT (December 1968).

Y. M. Sterlin and V. V. Artamonov, cited by E. M. Levin and H. ¥. McMurdie, Phase
Diagrams for Ceramists: 1975 Supplement, The American Chemical Society, Colum-
bus, OH, 1975, p. 396.

H. F. McMurdie, Editor, Phase Diagrams for Ceramists, Vol I to VII, American
Ceramic Society (1964 to 1989).

I. V. Budayev and A. N. Volsky, "The Chiorination of Uranium Dioxide and Plutoni-
um Dioxide by Carbon Tetrachloride," Proc. of Second United Nations International
Conf. on the Peaceful Uses of Atomic Energy, p.2195, pp. 316-330, {1959).

R. G. Canning, "The Production of Uranium Metal Powders by Electrolysis in Molten
Chlorides,” Australian Atomic Energy Symposium-1958, pp. 115-122 (June 1958).
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17.

18.

19.

20.

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25.

30

A. R. Gibson et al., "Processes for the Production of Uranium Oxide," U. S. Patent
3,117,386 (January 14, 1964).

T. A. Gens, Laboratory Development of Chloride Volatility Processes for the Recovery of
Uranmium Directly from Spent Rover Fuel or from its Combustion Ash, ORNL-3376
(June 1963).

W. L. Lyon and E. E. Voiland, The Preparation of Uranium Dioxide from a Molten
Salt Solution of Uranyl Chloride, HW-62431 (October 1959).

T. A. Gens, Chloride Volatility Experimental Studies: The Reaction of U;04 with
Carbon Tetrachloride and Mixtures of Carbon Tetrachlonde and Chlorine, ORNL/TM-
1258 (August 1965).

G. Jangg et al., "Chlorination of Uranium Oxides," Afompraxis, 7, 332-336 (1961).
A. D. Mcquillan and M. K. McQuillan, Titanium, Academic Press (1956).

Minoru Qgawa et al., "A Study of Titanium Resources and Its Chlorination Process,"
Titanium *80 Science and Technology, pp. 1936-1945, Kyoto, Japan (May 19-22, 1980).

S. M. Skelton et al., "Zirconium Metal Production,” Proc. of the International Conf. on
the Peaceful Uses of Atomic Energy, 8, 505 (1956).

A. R. Gibson et al., "Thorium Metal Production by a Chlorination Process," Proc. of
Second United Nations International Conf. on the Peaceful Uses of Atomic Energy, 4,
237-242 (1959).

P. A. Haas et al., Reaction of Uranium Oxides with Chlorine and Carbon or Carbon
Monoxide to Prepare Uranium Chlorides, ORNL/TM-11792, Oak Ridge National
Laboratory (November 1991).
ORNL/TM-11955

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