Uranium Removal from Chattanooga Oil Shale by Acid Leaching

Amis Judzis

Department of Chemical EngineeringUniversity of Michigan

Ann Arbor, Michigan 48109

Brymer Williams

Department of Chemical Engineering

University of Michigan

Ann Arbor, Michigan 48109

ABSTRACT

Chattanooga oil shale which covers much of the

east-central United States contains a second energy

resource: uranium in concentrations up to 150 ppm.

Uranium is not uniformly distributed through either

the kerogen or inorganic matrix of the shale, how

ever, examination by electron mi croprobe shows the

occurrence of uraninite, multiple oxides, and uran-

iferous apatite in small grains, 5 to 20 ym in size.

Dissolution of raw ground shale by dilute sulfuric

acid in slurry reactors shows 60 to 80 percent recov

ery of uranium and solution rates as high as 1.5X10

(kg U)/(m grain surface area X sec) at room temperature.

INTRODUCTION

Devonian oil shales, covering much of east-

central United States, have been subjects of many

studies regarding its energy content. Oil and uran

ium, for example, are available in concentrations up

to 63 cm /kg (15 gal/ton) and 150 ppm, respectively

in the Chattanooga shales. This paper focuses on

the removal of uranium by acid dissolution from oil

shale samples obtained in central Tennessee and Ken

tucky. The shale's microstructure is also revealed

as a prelude to the presentation of preliminary

uranium mineral dissolution rate data.

Experimental Procedure

Chattanooga oil shales samples were obtained

from various outcrop locations in DeKalb County,

Tennessee and Russell County, Kentucky. Tennessee

State Highway 56 (Silver Point quadrangle) and Ken

tucky State Highway 379 (Creel sboro quadrangle) were

rich in accessible outcrop locations. To ascertain

oil and uranium richness, the shale samples were

assayed with the modified Fischer retort as describ

ed by Stanfield and Frost (1949) and neutron acti

vation analysis, respectively. Representative

assay results, appearing on Table 1, show the oil

and uranium contents.

Table 1.

Assays of Chattanooga Oil Shale Samples

Sample Fischer Assay Oil Yield Uranium

No. Location (cm /kg) (gal/ton) (ppm)

2 DeKalb Co 63 15.1 70

3 DeKalb Co 57 13.6 100

8 Russell Co 55 13.1 19

To study the surface microstructure of these

oil shales, an Applied Research Laboratories elec

tron mi croprobe and scanning electron microscope

were used. The electron microprobe revealed uranium-

bearing grains on highly polished shale surfaces.

The ability to identify and establish their relative

surface area is important in the subsequent analysis

of slurry reactor rate data.

3Acid dissolution runs were made in a 500 cm

glass reactor agitated at 150 rad/s with a 4 cm long

impeller. Provisions for monitoring time, reaction

temperature, oxidation-reduction potential, and agi

tation speed were included in the reactor design.

Prior to a dissolution experiment, air was bub

bled through a fritted glass disk into dilute sul

furic acid solutions, to equilibrate with ambient

atmosphere. At time, t, equal to zero, 50 g of

screened shale particles were placed in the acid

solution to react. After certain time periods,

3small (5 cm ) samples of solution were removed by

vacuum filtration through a fritted glass disk.

Solvent samples were then placed in snythetic quartz

tubing for analysis of uranium by neutron activation.

(R)A Monosortr '

surface area analyzer built byQuan-

tachrome Corporation was used to determine the sur

face area of finely divided shale. Surface area

measurements determined by nitrogen adsorption has

its theoretical basis with the B.E.T. equation.

Results and Discussion

Shale mineralogy and surface microstructure

390

basis with the B.E.T. equation.

Results and Discussion

Shale mineralogy and surface microstructure

Chattanooga shale is divided into two layers,

the Gassaway and the Dowel 1 town members. Mutschler

et:al. (1976) give average uranium contents for the

Gassaway and Dowel 1 town members of 57 and 23 ppm,

respectively. Their respective standard deviations

are 9 and 15 for a studied area of 90,000km2

in

central Tennessee and adjacent areas. High Fischer

assay oil yields for samples collected in DeKalb and

Russell Counties indicate that they are from the

Gassaway member.

The composition of Chattanooga shale varies

from layer to layer. A representative analysis by

Bates and Strahl (1957) of the Gassaway member fol

lows:

Clay minerals, muscovite 31%

Quartz 22

Organic matter 16-22

Pyrite, marcasite 11

Feldspar 9

Chlorite 2

Iron oxides 2

Other: tourmaline, zircon, apatite 1

The mineral association of uranium in the shale was

investigated by Judzis and Judzis (1978) in prelim

inary work using electron microprobe analysis. The

majority of the uranium appears as finely dissemin

ated uraninite ((U^i.x'^^+x^ Uranium oxide

grains up to 20 ym in size have been observed,

though hydrated forms of the mineral cannot be elim

inated. In smaller concentrations and lesser fre

quency, uraniferous apatite and multiple oxides of

titanium (bearing uranium) have been found.

The fine grained structure of Chattanooga shale

is shown in Figure 1 (Judzis and Judzis, 1978).

This photograph taken at a magnification of 1000X,

shows small grains of siliceous, carbonate, and ox

ide compounds. The larger dark particles are pyrite.

For a shale containing 70 ppm uranium, only 0.002

percent of the shale's surface has exposed uranium,

taking into consideration the specific gravity of

uraninite. Scans of various shale samples with the

electron microprobe verify this. Indeed, uranium

occurs in isolated grains, not merely distributed

uniformly within the organic matter as adsorbed

U02++

ions (Frederickson, 1948).

Figure 1. Polished Chattanooga Shale Surface

Surface area

The study of heterogeneous reactions necessi

tates that rates be expressed in terms of unit area

rather than unit volume or weight as for homogeneous

reactions. The area corresponding to the amount of

nitrogen adsorbed in a one molecular layer (Brunauer-

Emmett-Teller method) may not be the exact surface

area of ground shale particles, yet the results are

reproducible with standard procedures. Figure 2

summarizes surface area measurements for screened

DeKalb County shale particles.

Surface area is inversely proportional to the

particle diameter. Repeated surface area measure-

2ments give a standard deviation of 0.2 m /g. Sur-

2face areas of 0.64 to 3.23 m /g for the shale

particles studied are low compared to some catalysts

and the activated carbons, whose areas often exceed

21000 m /g, yet are still over 100 times that of im

permeable spheres of the same size range.

Dissolution rates of uranium-bearing grains

Variables affecting chemical kinetics and mass

transfer have frequently been ignored. Ewing et al .

(1949), for example, studied acid leaching of raw

and roasted shales. They reported on the effects of

pulp density, temperature, acid concentration, and

other variables on the percent recovery of uranium

from ground shales. Dissolution rates of shale

uranium grains, however, have remained unknown.

Assuming most shale uranium appears in an oxide

form, hexavalent uranium dissolves as the U0,

cation in the presence of sulfuric acid:

+2

391

CHATTANOOGA SHALE AREA

4 --

E

E 3

a

au

<

u

D

b

3

2 --

1 --

pulverized

200 400 600

Particle Size, /<m

FIGURE 2.

U03+

2H+

=

U02+2

+ H20

Tetravalent uranium requires oxidation prior to dis

solution. In the presence of pyrite, the mechanism

of solution appear to be:

2FeS2+7/202

+ H20 =

Fe2(S04)3+

H2S04

U02+Fe2(S04)3

=

U02S04+2FeS04

4FeS04+

2H2S04+

02=

2Fe2 (S04)3 + 2H20

A preliminary screening of important variables was

conducted with batch dissolution runs in a slurry

reactor. With short duration dissolution runs of 3

to 180 minutes, the effects of uranium assay, parti

cle size classification, sulfuric acid concentration,

and reaction temperature were considered.

TABLE 2. Dissolution Rates of Chattanooga Oil Shale Uranium

Dissolution rates of shale uranium, appearing

in Table 2, were calculated knowing the surface area

of ground shale, uranium assay, and the increase in

uranium concentration in the solution during the

first minute of reaction. The surface area of the

uranium grains were obtained from shale surface area

measurements, assuming that a known fraction of the

shale has exposed uranium. By way of comparison,

Laxen (1965) reported a dissolution rate of

-6 ?3.3X10 kg U/s-m for Witwatersrand uraninite, a

3South African ore in a 1 kg/m sulfuric acid solu

tion at15

C. For his work, agitation rates above

42 rad/s had no effect on the dissolution rate.

Dissolution Rates, kgU/sm grain

Sulfuric Acid

Concentration,

kg/m3

DeKalb

27C

County

71C

Russell

27C

County

71C

4.9

25.5

49.0

3.00 XIO'6

3.33

5.67

5.50 XIO'6

15.33

1.00 XIO'6

1.67

2.17 X10"f

3.17

392

Activation energies of 1.2 XIO4

and 1.5 XIO4

J/mol were estimated for the DeKalb and Russell Coun

ty shale samples, respectively in the 4.9kg/m3

sul

furic acid solution. Normally, activation energies

below 2.1 X 10 J/mol indicate dissolution rates are

controlled by a diffusion process.

Consider the reaction of liquid with a porous

solid of unchanging size. A spherical particle will

first see a reaction near the outer surface, followed

by diffusion through the pores to the unreacted

"core". Levenspiel (1972) develops in terms of

complete conversion time, t, an expression relating

reaction time, t, with fractional shale uranium con

version, Xy. For complete conversion of shale uran

ium, the unreacted core diameter equals zero, and

x=puR2/6bDeC

where, p= density uranium grain

R = particle radius

b = moles fluid reactant per mole solid

reactant

D = effective diffusion coefficient

C = fluid reactant concentration at surface.

Expressed in terms of fractional conversion, the

ratio of t/x becomes:

? = t/t = 1 - 3(1 -

Xu)2/3

+ 2(1 - Xy) where

(1 - X ) = (volume unreacted U grains/total volume U

grains)

Figure 3 summarizes the results of a 24 hour

dissolution run with coarsely ground DeKalb County

shale. The reaction time constant, t, is 2 X 10 s,

and the diffusion coefficient is estimated to be

-9 22 X 10 cm /s, reasonable for the liquid phase

present. A dissolution run with finely ground shale

particles (75 to 150 ym in diameter) at the same re

actor conditions led to a diffusion coefficient of

1.3 X10"7

cm2/s.

Data on Table 2 indicates that uranium dissolu

tion rates for Russell County shale are lower than

for samples from DeKalb County. Unless the uranium

mineral surface areas are calculated in error, uran

ium assay alone cannot explain the difference. Fis

cher assay oil yields for the shale samples do not

3differ greatly, 55 and 57 cm oil/kg shale. Organic

contents are thus similar, assuming the carbon to

Diffusion Controlling Dissolution

O

4 --

3 -

2 -

1 --

27C

4.9 kg/m*

H2S04

+ +

8 16

+

24

TIME, hr

FIGURE 3.

393

hydrogen ratio varies little from county to county.

Grandstaff (1976) has, however, suggested that

organic molecules bond with and block surface oxida

tion sites. Kerogen in shale is made of organic

macromolecules, and its effect on uranium dissolution

mechanisms is unknown.

Laxen (1965) noted large differences in dissol

ution rates for South African uranium ores. He

suggested one cause to be thorium. With increasing

Th02 content, uranothorianite has been found to be

less soluble than uraninite in acidic solutions.

A complete solid solution series exists between U0o

and Th02. Chattanooga shale samples from DeKalb

and Russell Counties have thorium concentrations of

3 and 9 ppm, respectively though thorium's location

within the shale, matrix has not been identified

with the electron microprobe.

Refractory (tetravalent) uranium requires the

maintenance of proper oxidizing conditions during

dissolution in a slurry reactor with an oxidation-

reduction potential (emf) at least in the range of

-400 to -500 mV (Merritt, 1971). The oxidation-

reduction potential in the shale slurry reactor was

monitored with platinum and saturated calomel elec

trodes. In the presence of pyritic shales, most of

the iron is present in the ferric state in the emf

range of -400 to -500 mV. Potentiometric titrations

on uranium leach liquors containing iron by Toohey

and Kaufman (1954) showed the oxidation of iron from

Fe to Fe is complete at a potential of-0.61*

0.04 V. Batch slurry dissolution runs with DeKalb

and Russell County shales had oxidation-reduction

potentials averaging -630 and -275 mV, respectively.

High ratios of ferric to ferrous iron, therefore,

decreased the dissolution of refractory uranium in

TABLE 3. Shale Uranium

(DeKalb Co. Shale - 75 to

U Assay = 70 ppm

Slurry Temperature = 27C

4..9 kg/nr H2S04

Time, s Conversion (Xu)

0 0

120 0.10

300 0.12

900 0.14

3600 0.20

10800 0.43

Russell County shale.

The effects of major reactor variables have

been examined with short duration dissolution runs.

High acid concentrations and high temperatures

insure higher reaction rates. With pore diffusion

influencing dissolution, particle size itself

affects the recovery potential of shale uranium.

Inaccessibility of uranium minerals when surface

area per unit weight is small prevents large con

versions, Kerogen is inert to dilute acids and

will not increase in porosity through time.

Dissolution runs with shale particles 75 to

150 ym in size show that uranium recoveries up

to 60 to 80 percent are possible in dilute sulfuric

acid. Table 3 shows the results of two selected

dissolution runs wit DeKalb County shale. Extended

runs past 10,800 seconds will increase recovery,

though at much lower solution rates.

CONCLUSIONS

Shale mineralogy

1. Uranium-bearing minerals exist in the

Gassaway member of Chattanooga shale.

Finely disseminated uraninite,uran-

iferous apatite, and multiple oxides

of titanium have been observed

2< Shales containing 100 ppm uranium do

not expose more than 0.003 percent

of its surface area with uranium-bearing

grains.

Surface area

3. The surface area of finely ground

shale particles varies inversely

with the diameter.

Recovery (Conversions)

150 ym particle sizes)

U Assay = 100 ppm

Slurry Temperature = 71C

49kg/m"3

H2S04

Time, s

0

60

120

180

Conversion (Xu)

0

0.56

0.60

0.70

394

4. Porosity and surface irregularity is suffic

ient to expose surface areas over 100 times

that of assumed impermeable spheres.

Dissolution rates of uranium-bearing grains

5. Uranium dissolution rates of 3 to 15 X10"6

2kg U/s*m grain have been observed for

Chattanooga oil shale particles in sulfuric

acid.

6. The dissolution of refractory uranium

grains requires oxidation through the

ferric-ferrous mechanism.

7. The application of an unreacted core model

with unchanging particle sizes and low

activation energies (1.2 - 1.5 XIO4

J/mol)

suggests that diffusion through pores or

irregular surfaces controls the dissolution

rates of shale uranium.

8. Diffusion coefficients, D , of 2 X 10

-7 2e

and 1 X 10 cm/s have been estimated for

slurry reactions at 71C, typical values

for the liquid phase.

Recovery of shale uranium

9. Preliminary dissolution runs indicate that

over 70 percent of shale's uranium may be

recovered. Dissolution rates proceed,

controlled by a diffusion process, until

all exposed uranium grains have reacted.

Grinding finer particles increases the

percentage of uranium which becomes exposed.

ACKNOWLEDGEMENT

The assistance provided to one of the authors

by the Michigan Memorial -Phoenix Project and a Ford

Fellowship (Ford Motor Company) is gratefully

acknowledged. Assistance provided by S. A. Wilson

of the Department of Chemical Engineering, Univer

sity of Michigan, in measuring shale surface areas

is also appreciated.

Frederickson, A.F., "Some Mechanisms for the Fixa

tion of Uranium in CertainSediments,"

Science,

108, 184-185 (1948).

Grandstaff, D. E., "A Kinetic Study of the Dissolu

tion ofUraninite,"

Econ. Geol., 71, 1493-

1506 (1976).

Judzis, A., and A. Judzis, Jr., "Uranium Minerals

in ChattanoogaShale,"

11th Oil Shale Symposium

Proceedings, Colorado School of Mines, 343-349

(1978).

Laxen, P. A., "The Dissolution of Uranium Minerals

from South African Ores in AcidSolutions,"

Report PEL-121, Atomic Energy Board, Pelindaba,

Pretoria, South Africa, 434-475 (1965).

Levenspiel, 0., Chemical Reaction Engineering, 2nd

Edition, John Wiley and Sons, Inc., New York

(1972).

Merritt, R. C, The Extractive Metallurgy of Uranium,

Colorado School of Mines Research Institute,

Golden, CO (1971).

Mutschler, P. H., J. J. Hill, and B. B. Williams,

Uranium from Chattanooga Shale - Some Problems

Involved in Development: IC 8700, U.S. Bur.

Mines (1976).

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ing Oil Shale by a Modified Fischer Retort:

RI 4477, U.S. Bur. Mines (1949).

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Valence State of Iron, Vanadium and Uranium in

Sulfuric Acid Leach Liquors: ACC0-60, U.S.

Atomic Energy Commission (1954).

REFERENCES

Bates, T. F., and E. 0. Strahl , "Mineralogy, Petro

graphy, and Radioactivity of Representative

Samples of ChattanoogaShale,"

Bulletin of the

Geological Society of America, 88, 1305-1313

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395