I

1

,..I

. 1 * 1 ' .

AEC RESEARCH AND DEVELOPMENT REPORT Chemistry-Separat ORN L-2239 ?/ on Processes for Plutonium an(

l l l l l Ill I \ Ill l l 3 4 9 5 6 0 3 6 3 0 4 5 7

LABORATORY DEVELOPMENT- OF THE

DAREX PROCESS. I . DISSOLUTION OF

A NICKEL-CHROMIUM ALLOY

M. Le Hyman

lranium

This document cantoins Secret Restricted Data relating to Civilign Applications of Atomic Energy

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' 1

This document consists of 44 pages. copy 4 of ST6 copies. Series A,

Contract NO. W-7405-eng-26

CHEMICAL TECHNOLOGY DIVISION

Chemical Development Section B

LABORATORY DEVELOPMENT OF TRE DARM PROCESS. I. DISSOLUTION OF A NICKEL-CHROMIUM ALLOY

Covering work done July 1955-November 1956

M. L. Hyman

DATE ISSUED

SEP i 8 i957

OAK RIDGE NATIONAL LABORATORY Operated by

UNION CARBIDE NUCLEAR COMPANY A Division of Union Carbide and Carbon Corporation

Post Office Box X

30. v

40. 41. 42.

-ii- ORNL - 22 3 9 Chemistry-SeBaration

fo r Plutonium and

1. C. E. Cente 2. Biology L i b

15. J. P. Mwray

1

45 46. 47 9

48.

24.*J. H. Frye, Jr. 25. S. C. Lind 26. A , H. Snell 27. A. Hollaender 28. K. Z. Morgan 29. M. T. Kelley 30. T. A . Lincoln 31. R. S. Linvingston 32. A. S. Householder 33. C. S. Harrill 34. C . E. Winters 35. D. Phi l l ips 36. D. W. Cardwell

67 68.

69-74. 75 9

76 0

77 -80. 81-82.

83 a4.

85-86.

49. 50 51 52 53 54 55 56 57 9

58 59- 60. 61. 62. 63 64. 65

EXTERNAL DISTRIBUTION

k r i Teciwcal Intelligence center Alco "p$oducts, Inc . Argonne National Laborato Armed Forces Special Weeipps Project, Sandia Armed Forces Special We: s Project, Washington Atomic Energy Commislp Atomics International . Battel le Memorial In s t i t u t e B e t t i s Plant (WAPD) Brookhaven National Laboratory

+ .

W o c es ses Uranium

M-3679 (20th ad. )

W. K. Eister F. R. Bruce D. E. Ferguson R. B. Lindauer H. E. Goeller R. A. Charpi%' J. A. .Lane M. J. Skinner, R. E. Blanco G . E. Boyd - W. E. Unger R. R. Dickison A. T. Gresky E. D. Arnold C. E. Guthrie J. W. Ullmann K. B. Brown K. 0. Johnsson H. M. McLeod J. C. Bresee M.' L. m a n W.' W. Weinqich (consa tan t ) M.' D. Petedson (consqtant) D. L, Katz (consultant) G. T. Seaborg (consultant) M. Benedict ( consultant ) C. E. h r s o h (consultant) L. Squires (consultant) ORNL - Y - 1 2 Technical Library, Document Reference Section

7

.

I .

1

D 87. 88 0

89 0

900 91. Dow Chemical Company (Rocky Flats)

96 e dupont Company, Wilmington 92-95. duPont Company, Aiken

97-108. General Electric Company, Richland log e . . 110. 111 112 0

113 -114. 115 ., 116. 117. National Advisory Committee fo r Aeronautics, Cleveland 118. National Lead Company of Ohio llg. Naval Research Laboratory 120. New York Operations Office 121. Pat 122. Pat

123-126. Phi 127. 'rile 128. Uni

129-131. UniuwGa, 132-133. University of' California Radiation Laboratory, Berkeley 134-135. University of California Radiation Laboratory, Livermore

137-176. Technical Information Service Extension (FOP Official AEC Use) 136. Vitro Engineering Division

CONTENTS

,

0.0 ABSTRACT

1.0 INTRODUCTION

2.0 FLOWSBEET FOR AQUA REGIA DISSOLUTION OF NICHROME V--CONTAINING FUEL ELEMENTS

3.0 GENERAL CONSIDERATIONS

3 .1 Theories of the Behavior of Aqua Regia 3.2 Previous Open-literature Studies

3.3 Previous Project Studies

3.4 Composition of Nichrome V Metal 3.5 Corrosion Resistance of NichromgV

4.0 DISSOLUTION EXPERlMENTS

4.1 Dissolution in Hot Dilute Aqua Regia

4.2 Dissolution in Cold Concentrated Aqua Regia 4.3 Reaction Stoichiometry

4.4 Chloride Economy 4.5 Alteration of Flowsheet Conditions

4.6 Analysis of Solutions

5.0 REFERENCES

4

6

6 7 8 9 10

18

12

27 29 31 35 35

37

- 1 -

The laboratory developaent of the Darex process9 a proposed method For the dissolution of oxidation- resistant fuel elemn%zt incorporating dwoaaim- containing aUoys, 1s presented, The fuels are dis- solved in dilute HCl-Hm03, the residua& chloride is moved, and the solution is adjuste8 Lo s W e x ty-pe feed for conventional solvent extraction of the uranium.

This report covers studies made on the dissolution of a nickel-chromium alloy,

1 0 INTRODUCTION

This report presents the extensive laboratory-scale experiments

on dissolution of Nichrome V that were carried out in developnent of the Darex process.

dissolution of fuel elements incorporating chromium-containing alloys

such as Nichrome V or stainless steel, The diasolvent is a dilute mixture of nitric and hydrochloric acidso

schemes for heterogeneous power reactor fuel e&,ments, uranium and/or thorium are recovered by solvent ex%raelion of a nitrate solution

1 The Darex process is a proposed method for the

In current processing

resulting from their dissolution, There are two general categories of power reactor fuel elements-those eonhining chromium alloys such

as stainless steel or Nichrome V and those containing zirconium. These metals do not dissolve in most aqueous reagents, but both stainless steel and Nichrome V dissolve readily in dilute aqua regia,

In the Darex flowsheet,2 fuel elements are dissolved in dilute aqua regia, residual chloride ion is removed by distillation or oxi -

dation with nitric acid, and the chloricte-Free solution is adjusted

to concentrations similar to those of the Brex process feed,

uranium is subsequently extracted w%th tributyl phosphate in stainless

steel equipment. Although processing of both Nichponte V and stainlees

The

- 2 -

steel-conlaining &el elements by this method is visualized, Nickvme V was selected for the first study because it is more d i f f i cu l t t o dissolve*ehan s ta inless steel, i n general, applicable t o stainless steel, but the converse is not true.

Any process developed for Nichrome is,

If it should be reqwlred tbat a reprocessing plant handle, for example, both N i c h r a m e and stainless steel fuel elements, it i s obvious that there is an advantage i n employing one process u t i l i z ing the mxue head-end apparatus and flowsheet for both kinds of fuel elanents, appears thz t the Darex process may be generally applicable t o the growing class of oxidation-resistant fuel element alloys whose common constituent is chromium. Iron, nickel, aluminum, cobalt, molybdenum, and other metals may o r may not be present also, but it i s e w c t e d that these alloys w i l l be dissolvable i n di lute aqua regia. advantage seen f o r Darex is i t s application t o continuous dissolution of a wide range of fuel elements without the necessity of mechanical

It

The major

handling e i Various schemes have been proposed and studied for the dissolution

of s ta inless steel and Nichrome V fuels preceding solvent extraction (Table 1). In the case of the stainless s t e e l fuels, dissolution of the s ta inless r.’+el jacket i n sulkuric acid followed by n i t r i c acid dissolution of the U02 fuel was proposed by KAPL This process i s now being tested at the Idaho Chemical Processing Plant, with n i t r i c acid i s applicable, perhaps, t o certain specific fue l ele- ment configuratcons. The Darex process’lias been selected f o r intensive study at ORNL.

3 for the SIR fuels.

Mechanical dejacketing and subsequent leaching of the fuel

(The APPR fuel element i s a notable exception,)

For Nichrome V fuels, only dissolution i n aqua regia and elec- These two techniques are used t ro ly t i c dissolution are applicable,

a t Y-12 fo r recovering highly enriched uranium from nonirradiated prototype Nichrome V-containing fie1 elements i n glass apparatus, Aqua regia is used a t Y - 1 2 for dissolution of both whole fuel element

- 3 -

Table 1. Objectionable Aspects of Proposed Dissolution Processes for Fuel Elements,Containing Nichrcane V and Stainless Steel

Disadvantages* for Disadvantage# for Process Ref,. Stainless Steel Nichr- -v

Dissolution in HC1; con- version to nitrate solution 4 Dissolution in HBr; con- version to nitrate solution 4 Dissolution in 3 (dis-

Dissolution in chromic acid

Dissolution in molten metal

solve fie1 in 3i;:j 3

catalyzed HN03 5

followed by HNO dis- solution

Electrolytic dissolution into HN03 6 Carburization and HNO leach Mechanical dejacketing and HNO leach 2

Darex 2

1 9 3 3

798 3

3

3,496

3,496

3,697

Ineffective

899 6

Ineffective

Ineffective

Ineffective

Ineffective

2,8,10

Ineffective

' 6

*Key to disadvantages:

1. 2. 3. 4, Slow dissolution rate. 5. 6. 7. 8. 9. 10.

Incmplete dissolution and subsequent loss of uranium. ~ i g h processing temperature (> 2000~). Hydrogen in dissolution off-gas (safety hazard).

Unreliability of process with various alloys. Premium materials of construction required. Lowered solvent extraction plant capacity. Lack of suitable mechanical equipment. Process not applicable to all fuel configurations. Insufficient laboratory data for evaluation.

Q

- 4 - t

sections and of "tails" remaining in the electrode clamps after eles- trolytic dissolution. The necessity for processing "Lails" and other

mechanical difficulties militate against use of electrolytic disso-

lution for irradiated fuel elements.

Application of the Darex process to zirconium- or aluminum-

containing Fuel elements is not recommended.

are essentially inert to aqua regia. Aluminum dissolves readily in

nitric acid containing mercuric nitrate as a catalyst, and the more

drastic aqua regia treatment is therefore not required unless the

rapid dissolution rates that may be achieved in aqua regia are of

particular interest.

Zirconium and its alloys

Grateful acknowledgment to J. E. Savolainen of this Division is made for his helpf'ul suggestions, and to W. R. Laing and his associates of the Analytical Chemistry Division for their analytical services.

2.0 FLOWSHEET FOR AQUA REGIA DISSOLUTION OF NICKROME V-CONTAINING FUEL EIZMEUTS

The flowsheet for batch dissolution of Nichrome V-containing

fuel elements (Mg, 1) in aqua regia uses 6.0 M HNO and 1-2 M - HC1 as the dissolvent. Based on maximum dissolution rates and minimum solu-

tion volumes to be processed, 6.0 M HNO However, the corrosive action of chloride ion on stainless steel makes

removal of the chloride necessary before further processing to protect

both the stainless steel solvent extraction apparatus and aqueous

waste tanks, Chloride removal by distillation with nitric acid vapor

stripping is now visualized. Preliminary data have shown that the

chloride concentration in the feed to the distillation column as w e l l

as the concentration of the nitric acid in the stripping vapor have a

critical effect on chloride oxidation. That is, in order to recover

the chloride values as HC1 rather than lose them in the off-gas as C$

- 3

and 2 M - HC1 are optimum, - 3

2

- 5 -

i

FUEL EJBENTS 1 OFF-GAS

Fuel element

1.0 kg (assumed)

Dissolvent

HCL, M

Volume, liters m03 ,-E

Dissolver solution

Ni, g/liter Cr, g/liter

U, g/liter H+, - N NO3-, M C1- M- v o l k ~ , liters

DISSOLVER I- DISSOLVER SOLUTION

Dissolution tempemture, O C

Average dissolution rate, mg/cm2 emin

Chloride econmy (see t ex t )

850 g Nichrame V, 150 g U02 -(assumed)

1.0

18.4 6.0

10 5 2.0 6,O 6,o 9.7 ,705

37 70 910 5 9-3 17.6 22.5

7.2 13.7 17.7

0.6 0.9 1.2 18.1 8 0 9 6e9

4.0 2.4 1.4 5.4 5.2 5.1

- 103 - 103 -103

13 30 37

4.2 6.1 6.0

Fig. 1. Tentative Schematic Flowsheet for Batch Dissolution of Nichrome V-containing Fuel Elements in Aqua Regia

- 6 -

and NOC1, it may be necessary to adjust the HC1 concentration in ?2ae

dissolvent to a lower concentration than would be desirable from consideration of dissolution rates alone.

between dissolution and distillation conditions may be required.

Consequently, a comgromise

The presence of uranium in an actual fuel element, in contrast to the pure Nicbrome V used in this s t u d y , should have essentiaUy

no effect on the dissolution conditions, In the flowsheet sham, only about 5$ of the nitric acid specified is required for oxidation of the U02 in the fuel element to U02++* composition was calculated from the Nichrome V dissolution data with allowance for the assumed U02 content of the fuel element.

The anticipated final dissolver

The flowsheet d a t a are based on batch dissolution to a s m a l l metal heel. The average dissolution rate for complete metal disso- lution would, of course, be lower.

Vigorous reaction can be expected to start in the neighborhood of 90°C and to continue readily at about lO3OC,

the various dissolvents given will never exceed 115OC at 1 a b pressure.

Foaming is not serious, but a batch dissolver should be provided with

at least 1009 freebard to provide for too rapid reaction.

The boiling points d 4

Materials for construction of the head-end system have been suggested and are being tested,'

and glass-lined steel.

for use in the process.

These include tantalum, titanium,

Crystal-bar zirconium may also be suitable

3.0 GENERAL CONSIDERATIONS

3*l Theories of the Behavior of Aqua Regia

Despite the fact that aqua+regia has been known since the time of the alchemists, when it was used for dissolving the noble metals, there have been no attempts to study at length the unique behavior

i

- a -

of this combination of hydrochloric and n i t r i c acids. derance of literature is concerned With descr ibbg optimum proportions of the two acids for rapid dissolution 02 the noble metals or, more recently, some of the base metals such as iron and nickel,

Ml lo r l0 discusses the theories of the nineteenth century. reviews Gay-Lussac's inference %ha% the peculiar action 09 aqua regia on metals is due t o the chlorine liberated by its decomposition, the ni t rosyl chlorides passing off as they do when the l iquid alone i s heated. regia, questioned whether the efficacy of aqua regia is due t o chlorine liberated v ia the now accepted reaction

The prepon-

He

However, Moore,ll writing i n a preliminary pager on aqua

3HCa + HNO = NWL + C12 + 2%0 3 He suggested the possibi l i ty t h a t the chloride ion acts as a c a t d y t i c agent i n oxidation reactions. precluded any definit ive description of the reaction mechanism.

The preliminary nature of the work

Briner12 confined the reactants t o a sealed tube and showed that the above equation is reversible. regia is formed, consisting of the aqueous acid phase and a red l iquid phase containing liquid. MOCl and cuor ine ,

A t equilibrium, a stabil ized aqua

A current view on the action of aqua regia on noble metals has been

increases the value of the reduction potential of the metal. been sham, for example, by comparison 02 the Eo values for the re- duction of gold and s i lver w i t h and -t&t&out chloride present. t h i s theory may be applicable a l so Lo %be action of aqua regia on base metals and alloys, it i s probably only one of many reaction mechanisms involved i n a dissolution.

by the explanation. that the presence of chloride ion This has

Although

3.2 Previous Open-literature Studies

Most investigators 14,15 sugges% %hat fo r dissolving %he noble

- 8 -

metals, i .e . , gold and platinum alone or alloyed, a volume r a t i o of about 4 parts of concentrated (38%) hydrochloric acid t o 1 part of concentrated ( 7 6 ) n i t r i c acid is best. w i t h as l i t t l e as 14$ t o as much as an equal volume of water has

been advised.

Dilution of t h i s mixture

Bonilla16 recognized i n 1932 that no work had been done w i t h aqua regia of low hydrochloric acid content and that a study of the dissolving of base metals i n this reagent had not been made. determined that fo r nickel the m a a m * um dissolution rate occurred with aqua regia consisting of 1 m l of 38% H C l and 13 ml of 70% HN03 (0.88 M HC1 and 14.7 M €IN0 ) , 15-2OoC during dissolution. The maximum dissolution rate of 7.0 mg/ cm2.rnin was obtained i n an immersion time of about 30 sec. extremely short immersion time, which measures only the i n i t i a l dis-

solution rate, makes the data valueless fo r comparison t o the work reported here. t o 0.35 C ) , Bonilla found a maximum dissolution rate of 117 mg/cm2.min w i t h aqua regia made from 7 m l of 38$ H C 1 and 20 ml of 7 6 HNO (3.2 - M HC1 and ll. 7 - M HN03). The acid w a s aged 10 min, which was the optimum time for this aqua regia composition to achieve maximum corrosiveness,

and allowed t o b o i l during i ts short action on the metal. action on iron, Bonilla found tha t , up t o about 9.4 M H C 1 i n concen- trated aqua regia, the corrosiveness increased w i t h aging time after the two component acids had been mixed. composition 4.5 M H C 1 and 10.2 M HNO 20 min, while for acid 9.4 M HC1 and 3.8 M HNO the optimum aging t i m e was about 90 min.

He

The acid was aged 30 min and maintained at - 3 - This

Using a similar procedure for low carbon steel (0.2

3

For i t s

- For example, for an acid

the optimum aging time was about - 3' - - 3 -

The acids were aged a t room temperature.

3.3 Previous Project Studies

A study of dissolution of Nichrome V--containing f'uels was made a t the Idaho Chemical Processing Plant i n late 1954.l~ Using th in Nichrome V w i r e , preliminary experiments were performed on the dis-

solution of the metal i n various acids. Dissolution rates a t

- 9 -

boiling temperatures in various sulfuric acid concentrations and in

nitric acid were t o o low t o Be of use.

metal was penetrated at a rate of" about 3 mi1s/hre of acid was used,

mixtures made up with concentrated acids for the range 5 to 75 vol $ hydrochloric acid, the balance being nitric acid,

solution rate of 261 mils/& (9 5 mg/cm min) was reported to occur with boiling 60 vol $ HC1--40 vol$ HNO HN03). steel dissolutions ,4 that chloride removal is Imperative if the

feed solution is to be extracted in stainless steel squipent; eva-

poration of the dissolvent with excess nitric acid gave a chloride- free solution.

Later,18 a Nichrome V-uranium fuel alloy was dissolved in both

En 85$ phosphoric acid %he A large excess

Dissolution rates were reported for aqua regia

The masrjbmum &s- 2

(7.38 - M,HC1--6.33 - H 3 It was recognized, as it halt been earlier with stainless

boiling concentrated phosphoric acid and aqua regia.

the dissolving time was much longer and a 1arg;e fraction of the

uranium remained as an insoluble residue. A residue also resulted from the aqua regia dissolution:

much as 13 wt $ for oxidized metale 1.5 w t $ of the initial specimen weigbt. Little, id" any, uranium remained in the insoluble fraction, which, except for the highly hydrated silica, was reduced by prolonged digestion in the dissolvent.

These insoluble nickel and chromium oxides were easily filterable.

For the former,

3 wt $ for unoxidized fuel and as In both cases silica was about

3.4 Camposition of Nichrome V Metal

Nichrome V is a proprietary aaLoy whose noainal composition is

80% nickel and 20% chromium. Company, Harrison, New Jersey. (The &asigna%ion 'V distinguishes this particular alloy from earlier, an8 now discontinued, composi-

tions, although NichPome 1x1, 85$ nickeP--l5$ chromium, is still available.)

It is manufactured by Driver-Harris

The metal finds wide use in electrical heating elements

- 10 -

because of its resistance to oxidation at elevated

The metal used in these dissolution studies was obtained from the stock of the ORNL Metallurgy Division (Table 2). was analyzed by ORNL Special Analysis Laboratory. 2 for comparison are the compositions of two samples of Nichrume V submitted by General Electric C0mpa.n~ to the U, S, Bureau of Standards for its work in developing coatings for this alloy.

analyses were performed by Driver-Harris Company; the metal is

probably representative of material that would be used in certain

The metal

Included in Table

These

fuel elements

3.5 Corrosion Resistance of Nichrome V

In general, Nichrome V shows fair to good corrosion resistance 20 to all aqueous environments except aqua regia,

the metal has demonstrated fair to excellent resistance $0 sulfuric, sulfhrous, and organic acids, alkalies, salt solutions, and dry

gases,

but at 96'~ the dissolution rate was 0,53 mg/cm emin, hydrofluoric acid at room temperature was negligible, but at elevated temperatures attack was severe, Phosphoric acid had no effect except

at elevated temperatures, In terms of dissolution rates, nitric acid of all concentrations and at all reasonable temperatures had little

effect on Nichrome V.

occurred with red f'uming nitric acid containing 13 to 18s dLssolved

In corrosion studies,

At 25OC 37 w t $ hydrochloric acid barely attacked Nichrome V, 2 The effect of

The most rapid dissolution with this acid

NO2 for which a corrosion rate of 35.4 in,/year (1.46 mg/cm2emin) at 108 to 170°C was reported for a 6-hr test,

4 0 DISSOLUTION EXPEWMTS

The laboratory-scale dissolution experiments were designed to

\

*

1

Cons-&tuent

Ni CP

c Mn Si cu Fe Zr A1

Table 2, Composition 02 Nichrome \$.

P 77.634 19 e 18 0 ~ 8 0,31

.long 0 0 021

0 ~ 3 2 O*l?

0 .I%c

78 J*69 ~ 9 ~ 8 0 0 2 8 0,05

0,52

0.098

0 ~ 2 5

0,04

0,Ol

a6 089 19 0 13 0~03

~ 8 0

1,28

0003

0.61

0 -09 0,10

Sample sources: O.O$h-in,-thiek sheet used in t h i s stu 19; (bv) sample B of reference 19,

(1) 5/n6-inO hexagonal rod used in this study; (1x1 a

j (111) smple A of reference

'Analyzed by O m . c Analyzed by Driver-Harris Company ,

- 12 -

determine optimum dissolvent compositions and temperatures of cbisso-

lution, dissolution rates possible under these conditions, the metal

concentrations of the final solutions, changes in dissolvent compo-

sition as the reaction proceeded, and the efficiency of utilization

of the reagents.

the chloride before the solvent extraction step, minimization of

chloride in the dissolvent is desirable. For this reason, dissolution

in hot dilute aqua regia was most extensively studied, although some

experiments were made with cold concentrated aqua regia. Dissolution

was satisfactory in both reagents.

concentrated aqua regia was not practicable. Above P5OC the reaction

is vigorous, with considerable foaming and gas evolution.

Since the present flowsheet calls for removal of

Dissolution of Nichrame V in hot

4.1 Dissolution in Hot Dilute Aqua Regia

With total acid concentrations of about 10 N and lower the dis- - solution reaction is relatively quiet. There is little initial

foaming and any foam that does form can be readily dispersed by cold

water on the top of the flask. the acid can be gently refluxed with mild applications of heat to increase the reaction rate.

Once the initial reaction is over,

a. Dissolution Rate

Optimum Acid Concentration. At all constant HC1 concentrations

except 0.5 M - HC1, the maximum dissolution rate of Nichrome V in boiling dilute aqua regia occurs at 6 M HNO

HNO

increasing H C 1 concentrations up to a maximum value of 2-4 - M HC1 depending on the HNO

(Fig. 2). At constant - 3 concentration the dissolution rate increases rapidly with 3

concentration (Fig. 3 ) . 3 Variation with Time. The reaction rates decreased exponentially

with time (Fig. h), but were erratic.

is not surprising since the reaction rate depends on factors other The irregularity of the data

1 . 4c

t .- E

E 3c nl

0 \

E ,. W l- E Z 0 t- 3 -1 0

a

-

2 - 20 a

a

a

W c3

E w >

10

0 0

- 13 - UNCLASS I FI ED ORNL-LR-DWG I9 31 5

TE M PE R ATU RE : IOO- i I O°C ( boi I i ng 1 TIME: ~ 1 5 min ACID CONCENTRATIONS ARE p\- 4.OM HCI

- IN1 T lAL

4 8 42 IN ITlAL HN03, M

Fig. 2. Effect of Ni t r ic Acid Concentration on Rate o f Dissolution o f a Chromium- containing A l l oy in Hot Dilute Aqua Regia.

0

- 14 - UNCLASS I FI ED ORNL-LR- DWG 19316

TEMPERATURE: boiling TIME: ~ 1 5 min ACID CONCENTRATIONS ARE INITIAL /

c

2 4 INITIAL HCI, M

6

Fig. 3 . E f fec t of Hydrochloric Acid Concen.tration on Rate of Dissolution of a Chromium-containing Alloy in Hot Dilute Aqua Regia.

- 15 -

I I I I I l l 1

UNCLASS IF1 E D OR N L- L R-DWG 4 934 7

I I I l l l

60 TEMPERATURE: 80- 100°C

\

Fig. 4 . Progressive Dissolution Rate of a Chromium-containing Al loy in Hot Dilute Aqua Regia. Data obtained in 40-min dissolutions o f specimens o f N 8 c m 2 area in 6 0 m l o f acid.

- g

- 16 -

than dissolvent eomposi-tion, e o g o z tempra twe, metal concentration, acid aging time, heat flu, acid-metab r a t i o , specimen condition, etc. For a given n i t r i c acid concentration, varying the BC1 concentration i n the range 2-6 M - did not significantly affect the rate of change of the reaction. rate. The empirical i n Fig, 4 is of %he form

w where R is the dissolution rate,* n and k are empirical consd;ttnts.

equation fo r the dissolutions shown

= k%n

mg/cm o m i n ; t is the t i m e , min; and Thrt curves i n ~ i g , 4 are specific

2

f o r the experimental conditions used, longest series studied, 4 M HNO

The form of the curve for the is mcer%ain beyond about 200 min, - 3$

although the depazture from a %&a&&%

~ t a are less rel iable for the 2 and 6 l ine a t 2.000 mfn is not great. M HNO series because of scatter, - 3

For a t l ea s t the first 100 m i n of reaction increase i n the order 2, 4, 6

the dissolution the rates of Y M HN03. The rates of decrease

of the reaction rate fo r 6 and 4 M wN@, apgear t o be of the same

tions are augmented by a plot (Fig. 5 ) of the average dissolution r a t e calculated from %he weight Loss and avemge surface mea after l hr dissolution, The m.te increases markedly as the HNQ coacentration approaches 4 - M, but the change i n rate 18 small beyond this eoncen- t ra t ion, there i s l i t t l e difference between tihe rates for these two acid con- centmtions,

3 - magnitude, but for 2 M HNQ the r a t e i s much steadier, The observa- 0 - 3

3

The closeness of the C ~ C B %or 4 and 6 - M H C 1 confirms that

There is a mria,tPon of 30-158, d even sOS$ in one case, for dissolution rates de%emined from data. obtained by direct dissolution of a specimen fo r 15 min (Fig, 2) and those de%emnined from data obtained i n PO-anin dissolution incrementa (Fgg, 4), fo r the diwerepancy l ies i n the temperatwe of the acid, which was

5% main reason

.

m e penetration rate2 mi~s/b , f o r Nichrome v of specific gravity 8.4 i s found by multiplying the dissoLu$fon rate i n mg/an2*min by 2.82,

- 17 -

OO

.

2c

10

UNCLASSIFIED ORNL-LR-DWG 19318

4M HCI 0

6 M HCI

\

TEMPERATURE: boiling ACID CONCENTRATIONS ARE IN I TI A L

4 IN IT IAL HN03, M

8

Fig. 5. Average I - h r Dissolution Rate o f a Chromiurn- containing A l l oy in Hot Di lute Aqua Regia.

generally 10-20°@ lower i n the 15-min dissolution, is the diff icul ty i n reproducing dissolution data. w i l l naturally occur when s l igh t ly varying techniques are used. duplication of experimental conditions is d i f f i cu l t without t i m e - consuming ef for t .

A second reason Discrepancies

kract

c

b. Metal Concentrations

The f i n a l m e t a l concentrations ob%ained by "saturating" the dis- solvent, i .e., by allowing the reaction t o continue u n t i l no further reaction w a s observed, foblowed vexy closely the pattern set by the metal dissolution rates. A t a l l constant He1 eorseentrations except 0.5 M, the ma.ximum metal concentration was obtained with 6 M €IN0

the aqua regia (Fig. 6). W%th 6 M HNO t ra t ion increased rapis$9y and l inear ly with increasing hydrochloric acid concentration up t o about 2 - M HCl md then leveled off (Fig. 7). A metal concentration of about 138 g/l i ter cannot be exceeded under the dissoluzion conditions described here.

i n i n i t i a l l y , the metal. concen-

- - 3 - 3

c. Changes i n Disso1iren-k ComDosition

Chloride Ion. A t all eonstant i n i t i a l HC% concentrations, the fract ional decrease of i n i t i a l chloride ion that remained i n the dissolver solution became greater 8s the HNO creased (Fig. 8). (and, similarly, of Figs

concentration w a s in- 3 The values on the axis of' ordinates of Fig. 8

9 and l o ) were calculated: as fobpows : aoo(c,- C f >

Percent decrease of C1" concentpation = c i

where Ci i s the number of moles of CP" i n i t i a l l y i n the solution and Cf is the f ina l number found by analysis. fo r the reduction i n dissolvent volme where t h h occurred. Chloride leaves the system i n the vapor phase as C 1 2 and NQCb, which are gener- ated by the n i t r i c acid--hydrochlor%c acid reaction,

This computation allows

The substantial decrease i n chloride concentration a t high n i t r i c acid concentrations explains the a2mos-b negligible effect of the more concentrated aqua regia, when hot, on NLchrome V, The excess nitrate

- 19 - UNCL ASS I FI E D ORNL-LR-DWG t 9 31 9

TEMPERATURE: boi l ing

AC I D CONCENTRATIONS ARE INITIAL

A--- 2.OM HCI

4 8 I N I T I A L HNO3, M

Fig, 6 . Ef fect of Ni t r ic Acid Concentration on Metal Concentration of Solution in Dissolution of a Chromium-containing Alloy in Aqua Regia to Point of No Reaction,

- 20 - UNC L ASS I F I E D O R N L - L R - D W G 19320

TEMPERATURE: boil ing ACID CONCENTRATIONS ARE I N I T I A L

I I I I I I

INITIAL HCI , M

Fig. 7 . Ef fec t of Hydrochloric Acid Concentration on Meta l Concentration of Solution in Dissolution o f a Chromium-containing Al loy in Aqua Regia to Point o f No Reaction.

8

0 a

.. z

t- LT I- z W 0 z 0 0 1 0 2

u v, W LT 0 W

- -

a

n

I oc

8C

60

40

20

OO

- 21 - UNCLASS IF1 ED

ORNL-LR-DWG 49324

ACID

4 8 I N I T I A L H N O , , M

Fig. 8 . Decrease in Chloride Concentration during Dissolution o f a Chromium- containing AI l oy in Aqua Regia.

- 22 -

ion oxidizes the chloride ion t o C 5 and NOCl rapidly, and these a re driven into the vapor phase so quickly by the mass action effect that

there i s no t i m e for the gross reaction between these products and the metal. Morgan and decomposition of n i t r i c and hydrochloric acids that chloride ion de-

composition is very rapid a t first; it can be deduced from the i r data that the chloride decomposition rate is markedly affected by n i t r a t e ion concentration. They found such a rapid decomposition rate a t the highest n i t ra te concentration studied, 6.27 - M, that it is eviheat that a t 8-12 M - NO3' the decomposition must be immediate and almost quantitative.

shared i n the i r br ief work on the mutual

It is noteworthy that th i s mutual acid decomposition takes place even i n the presence of Nichrome V metal. regia, which has a characteristic orange-brown color, turns darker when heated, evolves copious amounts of brown NO2 f'umes, and continues t o do so u n t i l the chloride is depleted. almost unaffected e

The concentrated aqua

The metal specimen remains

Hydrogen Ion. Hydrogen ion consumption follows closely the m e t a l concentration curves, A t constant i n i t i a l H C l concentration, the fractional decrease of i n i t i a l hydrogen ion that remained i n the dissolver solution was a maximum w i t h 6 M HNO i n the aqua regia

(Fig. 9 ) and i s relat ively unaffected by i t s awn high concentrat&on i n regard t o the HC1-HNO reaction.

- 3 It is apparent that H" enters the metal dissolution reaction

3 Nitrate Ion. The role of the. n i t ra te ion i n the reaction is t o

be the sole oxidizing agent for a l l the oxidizable species i n the reaction. The net oxidation reactions, ignoring any internal reaction such as €$ --+ 2 H+ + 2e, can be m i t t e n

c l - - 1/2 CI; + e

N i o - N i + + + 2e

C r o - Cr + 3e +++

- 23 -

8C

8 6C z 0 I-

LT I- z W 0 z 0 0

- a

+ r z 40

a

- W m W LT 0 w n

20

1 (

0 2

ACID CONCENTRATIONS ARE INITIAL UNCLASS IF1 ED

ORN L-LR -DWG I 9 3 2 2

' \ \ y 4 . O M HCI

\-- 2.OM HCI L l f \

I I I I 6 INITIAL HNO3, M

40

Fig. 9. Decrease in Hydrogen Ion Concentration during Dissolution of a Chromium-containing A l l oy in Aqua Regia.

The possible reduction equations* are

A high consumption where considerable sumption where 61-

NOa - e N03

ao3

N03

N03

- - NO - 3e

* __c l /2 N20 - k 0

0 s_+. 112 N~ - 5e

of NO metal was dissolved, and also some moderate con-

would therefore be expected i n the runs 3

is oxidized, as i n those runs with high n i t r i c acid concentrations. This i s found t o be the ease (mg. 10). The NO3' concentration of the dissolver solution was calculated from the analysis of the solution. were reduced. The various errors accumulate i n t h i s calculation and consequently only the m s with 1.5 and 2,O - M H e 1 were plotted,

In most rum only small amounts of NO - 3

d. Experimental Procedure

T e s t specimens 1 in. long were cut from 5/16-ine hexagonal Nichrome V rod.

g/cm . cut from bars, were more convenient t o work w i t h than th in f la t metal sections w i t h their low mass/area rat io

The specimens had a mass/area r a t i o of about 1,4 2 Preliminary runs bad indicated that massive pieces of Nbchrome,

The specimens were washed w i t h carbon tetrachloride and acetone and allowed t o dry i n air , the nearest decigram. sisted of a 300-ml round-bottomed flask f i t t ed w i t h a water-cooled reflux condenser and thermometer. a small bunsen burner, which is more convenient than an e lec t r i c mantle when rapid heating and cooling are necessary,

They were then measured and weighed t o The apparatus h;et up %or the dissolution con-

The flask was heated d i rec t ly w i t h

*Hydrogen is not evolved during dissolution of a metal i n aqua regia because of the highly oxidizing nature of the acid, confirmed2 for the dissolution of stainless steel , and i s certainly true f o r Nichrome V.

This has been

- 25 -

UNCLASSIFIED ORN L-LR- DWG 4 9 3 2 3

Fig. IO. Decrease in N i t r a t e Ion Concentration dur ing Dissolution o f a Chromium- containing Al loy in Aqua Regia.

- 26 -

The specimens were treated with 60 ml of aqua regia which had been prepared by volume measurement of reagent-grade 38qd HCb and

TO$ HN03, t o the acid t o effect dissolut2on of the Nichrame V specimen never exceeded 2 min. pre-mixed aqua regia was poured in to the flask.

using the more concentrated acid, there was l i t t l e o r no reaction a t this point between the acid and m e t a l (acid temperature about 3OoC) Heat was immediately applied t o the flask, and the acid was brought t o i t s boiling point as rapidly as possible,

The time between mixing the acids and application of heat

With the metal specimen i n place i n the flask, the Except for the runs

The in i t ia t ion of the acid-metal reaction was evidenced by vigorous gas evolution at the metal surface accompanied by copious amounts of red-brown nitrogen dioxide f'umes i n the f lask and condenser, In addition, a reaction yielding as l i t t l e as 1 g of metal per l i t e r i n the acid imparts a definite l ight bluish-green color t o the usual yellow or yellow-brown aqua regia, This makes visual recognition of a reaction easy. The reaction was allowed t o proceed, i n general, 1

for 15 min except i n some runs where it was obviously complete much sooner. A t the desired time the reaction was quenched by immersion of the f lask i n cold water and, after cooling, the specimen was removed, washed i n water and acetone, dried, measured, and weighed. The average dissolution rate was calcul.a.ted&om the measured w e i g h t loss of metal, dissolution time, and average surface area of the specimen.

For those acid compositions where the reaction was incomplete a f t e r 15 min, the m e t a l specimen, after being weighed, was returned t o the acid i n the f lask and the solution was again heated t o resume the dissolution. completion-sometimes as much as 1.5 hr-and then the heat source was removed and the solution allowed t o cool. weighed as before and the metal-saturated solution was analyzed. dist inction between "reaction" and "no reaction" when dealing with

In these runs the action was allowed t o go t o

The samples were The

I'

. metals dissolving i n aqua regia is, 0f eolarses ofim a subtle oneo E;Pnperieno-"a has indicated, Bweww, %ha% unless the reaction between the metal and acid is visually observable, then the reaction rate (of the order of 1 mg/m omLn> is of l i t t l e practical use for this acid- metal system,

2

The effect OB" time on $he CkLssolutfon rates was d.etemxi,n& i n a separate stuw by dissolving Nickorme V specimens in aqua regia pre- pared from 2 , 4, and 6 M RETO and 2, 4, and 6 M - El. Except %or the series using 4 M BNO intervals and a final 3O-min period, reacted fo r three LQ-win, two 3O-min, one ?Q-m%n, a d one 15-hr period, metal described previowly except that a l/16-ino hole was drilled i n one end of the sample, w i r e i n a 100-m$ three-neeked round-bottomed flask, The procedure

was essentially the 8- as that &xaTbedb prsviously except that the specimens were measured a d weighed a t the end of each reaction period and the new surface area was eah.a%Bated, I n general, the dissolutions were carried out a% gently refluxing temperatures, 80- 106OC; these varied w i t h acid concentration a d time,

were reac%ed for the &s$red time, appUcation of hest $0 the flask

was stopped, and the specimen was removed from the flask by means of the tungsten wise, was replaced in the f lask m d the next dissolution

- 3 specimens w e r e reacted for t h e e lQ-Bh$Q

- 3 - 3'

The! series with 4 M HNO was

The specimens were l,-ino pieces of rod %Ipm the same 10% of

Each spcs%m,en was supported w i t h t ugs t en

The specimens

After being weighed and measmed, the epcimen r$od was begun,

4.2

a, Disaolutdon Rate

Optimum Acid Concentration, - The dis;solutiorn rate 02 N'%@hasewe V

i n concentrate& aqua regia a% roan temperature w a ~ a maxfnrum when the acid @omposition was 3.8 M aCl.--lO.g M HX8 (Fig, lb), w%th a sharp

PI - 3 drop i n rate on either side of this composition, This is quite a

- 28 -

4

.5 3 E

E

E W t- LT z 0 t- 3 2 -J 0 CI) cn (3

W c3

LT w

(u

0 \ 0

I

a

-

-

a

z 4

0

0 I I I

/ /

/ /

/ /

/ /

/ / I

UNCLASS IF1 ED OR N L- L R - D WG 4 9 3 2 4

r

4

I I HCI 0 2 4 6 8 10 12.3 HNO3 15.8 43.3 10.7 8.1 5.5 2.9 0

INITIAL ACID CONCENTRATION,

Fig. 1 1 . Rate o f Dissolut ion o f a Chromium-containing Alloy in Cold Concentrated Aqua Regia.

- 29 -

different value from the 8*2 M HC~--2.6 M HNO (HCl/HN03/I$0 volume r a t i o of 4/1/1) often recommended (Sec, 3.2) for the most rapid dis -

solution of the noble metals. The discrepancy between the O R m work and Bonilb8s,16 who found even less HC1 i n concentrated aqua regia t o be efficacious i n dissolving nickel, undoubtedly lies i n the extremely short dissolving time tha t he used and i n the 20$ & d u m content of Nichrome V. s tantiated by the work of Schmitz,22 who concluded that the protective agent i n alloy steel i n the presence of oxidizing agents i s chrcanium while i n di lute and concentrated hydrochloric acid it is nickel,

- 3 -

The protective action of chromium is sub-

Variation w i t h Time. The dissolution rate for the opt.lmum acid concentration decreased with time (Fig, 12). Tbe slope of the fine i n Fig. 12 i s greater than the steepest l ines (4 M HNO ) in Fig. 4. This indicates tha t the reaction rate i n cold concentrated aqua regia decreases more rapidly wi th time than i n hot di lute acid and, consequently, the reaction is l e s s uniform.

- 3

b . Experimental Procedure

Nichrome V sheet rolled from thin-walled tubing was cut into specimens approximately 1,25 cm wide by 4.0 cm long by 0.058 cm thick, The specimens had a mass/area r a t i o of 0.2 g/cm . various compositions, prepared from l2,3 M HC1 and 15.8 M HNO was aged f o r 10 min. The specimens were submerged i n the aged acid for 10 min, and were then removed, rinsed i n water, dried, and weighed. In studying the variation of the ra te w i t h time, t h i s procedure was followed repeatedly on the same sample. near 24OC by immersing the reaction f lask i n a cold water bath,

2 Aqua regia of

- 3' -

The temperature was kept

4.3 Reaction Stoichiometry

There are too few data t o define even approximately the stoichio- metry of the Nichrome V--aqua regia reaction. The fac t tha t the

- 30 -

UNCLASS I FI ED ORNL-LR-DWG 49325

TEMPERATURE: 24OC INITIAL ACID: 40.9M HN03--3.8MHC

e

e \

.

Fig. 42. Progressive Dissolution Rate of a Chromium-containing Alloy in Cold Concentrated Aqua Regia.

- 31 -

.

reaction is extremely complex is obvious.

that even the HNO -HC1 reaction in some of the dilute solutions used here cannot be &fined stoichiometricably,

known that the extent of reduction of nitric acid depends on the concentration of the nitric acid as well as on the nature and strength of the reducing agent (here HC1). reactant, the metal to be dissolved, complicates the reaction further.

Morgan and Bond2' found

3 Furthermore, it is well

%e presence of a heterogeneous

Since in an oxidation-reduction reaction the change in normality of the reductants must equal the change in normality of the W d a n t s , an attempt was made to determine the average valence change which the nitrogen in the nitrate ion undergoes during the dissolution reaction

(Table 3) e

formed was added to the change in chloride normality, and this sum was divided by the change in nitrate normality. average valence change for nitrogen in the reaction.

with 2 M HC1 could f'urnish reliable data for this calculation, and even here the results are erratic,

change for nitrogen in the reaction appears to be greater than 3 e

The number of equivalents of metal ions (Ni* and Cr+*)

The quotient is the

Only the runs

- Q It can be seen that the valence

Confirmation of this is f'wther suggested by examining the off-

gas analysis 'for type 304L stainless steel in 4 M HC1-3 M FINO aqua regia.2

- 3 - The nitrogen valence change calculated from these data

appears to be about 3.6 (Table 4) e

4.4 Chloride Economy

"Chloride economy" may be defined as the ratio, in equivalents, of dissolved metal (nickel and chromium) to chloride consumed, chloride being consumed when it leaves the dissolution system as a

nonrecoverable or nonusable molecular form off chlorine, such as C12 or NOCl.

chloride involved in some way with the oxidation of the nickel and

The term is introduced here to distinguish between that

chromium, and that chloride evolved from the acid-metal system

- 32 -

Normality Change Reductants I Nitrate Ion

Table 3. Valence Change of Nitrogen in Dissolution of Nichrcone V by Aqua Regia

Valence Change of Nitrogen

Initial Concentration M , - 7 Z q 7 z + 2 4 6 8 10 12

2 2 2 2 2 2

Metal in Dissolvent, @;/liter

10.9

21.8

32.4 53.4

0.11 0.85 4.34 2.72 0.32 0.06

Nitrogen Valence Change

3.5 3 *6 3.5 3.7

0.03

1.29

0.54' 0.89

0.35

1.54

.

Table 4. Valence Change of Nitrogen in Dissolution of Stainless Steel by 4 y ~ c 1 - 3 M mo3a

From Table 7.5 of reference a 2.

-

c

-c

-. 0

3 -

-

0

Y

0

0

lu

P

EQ

UIV

ALE

NTS

OF

ME

TA

L D

ISS

OLV

ED

E

QU

IVA

LEN

T

OF

CH

LOR

IDE

CO

NSU

MED

P

I W

-r

I

Iu

cn

, 1

- 35 -

4 .. 5 Alteration of Flowsheet Conditions Considering the various process conditions that &$eane the

flowsheet (Fig, 11, the optimum aqua regia composi$%on for &.sjs;olvilag Nichrome V is 6 M HNO -2 M HC1. removal shows that a concentration of only 1,O OP lo5 E4 HCX is desirable in order to 1-t chloride oxidstion, the flowsheet would

be altered substantially. For example, with 6 M HNO -1 M El, instead of 6 M HN03-2 ,M HC1, the Nichrame V dhsolution pate and metal concentrations are reduced abmt 6O$ to 14 mg/m omJgn, and

49 g/liter respectively. chloride from the dissolver solution by a distillation stripping

method, the dissolvent composition cannot be ehjtablished,

continuous f'1omheete3 was used for stainless steel processing, the actual dissolvent composition at steady state would abslost certainly differ from that indicated here as optinnun,

the overall behavior of an integrated dissolver-ehloricle strip

column-rectifier column system when operated continuously,

3owever, if later work on chloride - 3 - -

- 3 - - 2

Until more work is done on the remo-tml. of

%f a

This would foUow from

c

4.6 Analysis of Solutions

Tbe difficulties encountered in accurately analyzing the dis- solver solutions make it worthwhile to outline briefly %he 8uecessful

techniques.

Samples of dissolver solution were submitted to %he Special

Analysis Laboratory"lp for detemination of ebomium, nickel, acidi%y, and chloride.

phenylcarbazide method, but results were erratic,

was persulfate oxidation followed by titration of the dichromate with ferrous ammonium sulfate,

the colorimetric dimethylglyoxime method, but results were high, Volumetric cyanide titration was finally used, and with this method

*ORNL Analytical Chemistry Division.

Chromium was analyzed first by the co%orP%metrie di- A better method

Nickel was originally determined by

- 36 -

the amount of nickel and c-um in the dissolver solutions agreed

within less than lo$ with the weight loss of the neW specimens. A few samples were analyzed for Cr(VI), but none was found. exists in the dissolver solution entirely in the trivalent state,

Chrox~Lum

The acid concentration of samples was determined by campleXing the cations with neutral potassium omlate, followed by potentio- metric titration of the free acid,

potentiometric silver nitrate titration; sodium pyrophosphate was Chloride was detemined by a

added first to complex the chromium and release the chloride for titration.

Whenever substantial amounts of Nichrome V were dissolved, gelatinous silica was found in the dissolver solution. showed that it absorbed negligibly small amounts of nickel and chromium salts.

Spot checks

.

- 37 -

5.0 REF'ERENCES

1. "Chemical Technology Division Semiannual Progress Report for Period Ending September 30, 1955, Om-2000

2. Did. , August 31, 1956, 0 ~ ~ ~ - 2 1 6 9 .

3.

4.

"Fuel Recovery Process f o r the SI3 Mark A," fcApz-933, July 17, 1953.

"Separations Processes, Progress Report, November, December 1950, January 1951," -461, March 19, 1951.

5 . "Chemical Technology Division Semiannual Progress Report for Period Ending March 31, 1956," ORNL,-2079.

6. E o C. Pitzer, '$lectrolytic Dissolution of Stainless Steel--clad Fuel Assemblies, KAPL-653, December 21, 195L

7. R. E , Adams, i n 'lkletallurgy Division Semiannual Progress Report," ORNL-2078, October 10, 1955, pp. 81-82.

8. R. E , AaamS, i n "Metallurgy Division Semiannual Progress Report," ORNL-2217, October 10, 1956, ppe 185-186.

9. R. E o Blanco, W. K. Eis ter , D. E. Ferguson, "Power Reactor Fuel Processing Status Report for August 1956, om-CF-56-8-101, pp. 11-12.

10. J. W. Mellor, "A Comprehensive Treatise on Inorganic and Theore- t icab Chemistry," vol, VIP%, pp. 617-619, L o n m s (1928).

U.

12.

W. C, Moore, J. Am. Chem. SOC., 33: lO9l (1911)-

E. Briner, Compt. rend., 162: 387-389 (191.6)~

13. "Encyclopedia of Chemical Technology,'' R. E , Kirk and D. F. Other (editors), vole 2, p. 109, The Interscience Encyclopedia (N.Y,) (19W 0

14, E, Priwoznik, Oesterr. Z, Berghcttenw., 58: 549-550 (1910); Chem. Abs., 5 : 41 ( l g l l ) .

15.

16.

C, M, Hoke, R. He Moore, Metal Ind. (N. Y . ) , 14: 296-298 (1916).

C F. Bonilla, Ind, Eng, Chem., Anal. Ed,, 4: 128-130 (1932)

17. C. E , Stevenson, "Progress Report for October through December 1954, Technical Branch, Idaho Cheahical Processing Plant," D O - 14350, pp. 46-47.

:: I . ;.'

18. F. P. Vance, "a6daho Chemical. Processing Bbat Progress Report, B y , 1955," IDO-14342, p. 14,

19. "Investigation to Develop Heat-resisting Coatings t o Protect Metal Alloys Against Corrosion, Quarterly Progress Report No, 9," NBS-k-115, January 1, 1955.

20. H. H. Uhlig, "The Corrosion Handbook," John Wibey and Sons, (N.Y.) (1948), p. 28 Tf,

D. T. Morgan, Go W. Bond, Jr,, "Liquid-Vapor Equilibria for Nitric Acid, Hydrochloric Acid, and Aqua Regia Solutions," M.I.T. Eng. Practice School Memorandum EPS-X-258, April 6, 1956

21.

c

23. C, E. Guthrie, "Darex: HEP Design and Recommenda%ions , 'I ORNL-CF- . 56-8-167, August 23, 1956,

c

.