Uranium Removal from Chattanooga Oil Shale by Acid Leaching Amis Judzis Department of Chemical Engineering University 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. 3 Acid 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, 3 small (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 by Quan- 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
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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