Process for separating radioactive and hazardous metal contaminants from soils

A process for treating radioactive contaminated soils to remove radioactive metal oxide contaminants therefrom comprises creating a suspension of particles of the soil in a column of water, alternately forcing fresh water in the column upwardly to force ligher soil particles upwardly in the column and allowing heavier particles to gravitationally settle in the bottom of the water column. The heavy particles comprising radioactive metal oxides are collected and handled for radioactive waste material storage. The aqueous slurry of lighter soil particles is directed to a separator for removing substantial amounts of water after which the particles are directed to a conveyor and spread out to a substantially uniform thickness and detected for any radioactivity. Portions of material in which radioactive particles are detected ar diverted and the uncontaminated soil material is recovered.

BACKGROUND OF THE INVENTION 
A rather large amount of radioactive soil material has accumulated in areas 
where nuclear device testing has been carried out over the years. Of 
special significance is the amount of radioactive soil on a number of 
South Pacific Islands, the radioactivity being generally present in coral 
soils, often referred to as transuranic material, containing usually 
radioactive plutonium and/or americium. Other nuclear testing areas also 
contain significant amounts of these and similar radioactive materials in 
the soil. Because of the great volume of such radioactive soils, handling, 
transportation and storage problems and costs are substantial. In other 
locations large amounts of soils containing hazardous metals such as lead, 
mercury, silver, arsenic, cadmium and chromium often result in unusable 
land masses of such great volumes that the cost of disposal is 
economically impractical, if not prohibitive. 
SUMMARY OF THE INVENTION 
It has been found, according to the present invention, that the hazardous 
or radioactive components of contaminated soils can be removed by creating 
a suspension of the soil particles in a column of water, alternately 
forcing water in the column upwardly to float away the lighter 
uncontaminated particles while allowing the heavier contaminated particles 
to settle to the bottom of the water column. The portion of the soil 
containing the lighter and generally uncontaminated material is then 
inspected by radioactive detection means and remaining contaminated soil 
portions are separated and the uncontaminated soil is recovered.

DETAILED DESCRIPTION OF THE INVENTION 
In treating soils according to the process of the invention, an apparatus 
shown generally in FIG. 1 is useful. The radioactive materials are usually 
present as metal oxides, whereas the aforesaid hazardous metals may be 
free metals or present as compounds of the metals. Soil containing the 
contaminants is placed in a hopper 12 or similar chute using a loader or 
other convenient means. The material is directed to a sizing apparatus 
such as a vibrating screen 14 which separates smaller particles from 
larger particles, the latter passing to a crusher 18. Any other means for 
separating out particles too large to be conveniently handled in the 
subsequent process may be used, and that shown is by way of illustration 
only. The sizing apparatus conveniently passes soil particles of 4 mm 
while larger particles or "overs" are directed to the crusher 18 for 
crushing the particles to a nominal size not larger than about 4 mm 
diameter. Conveyer 20 then moves the material to an apparatus for 
gravitationally separating heavier and lighter particles. Optionally, at a 
convenient location, prior to being introduced into the separation phase 
of the system, radioactively contaminated soil may be inspected with a 
radiation detector to provide an indication of the level of contamination. 
Such detection may also be useful in rating the effectiveness of the 
process by comparing the radioactive level obtained subsequently in the 
process. For this purpose detector 15 is shown above conveyor 20, although 
such detection equipment may be placed at any location upstream of jig 22. 
In FIG. 1, a gravitational separator in the form of a mineral jig 22 is 
used for creating a slurry or suspension of soil particles in a column of 
water and separating heavier particles from lighter particles. This 
separation is further explained by reference to FIGS. 2 and 3 which 
describe the apparatus in more detail. In FIG. 2 the mineral jig apparatus 
is shown in which soil particles 60 are suspended in a column of water in 
screen compartment 54. The column of water is subjected to alternate 
upward force of water created by a reciprocating diaphragm 53 and rotating 
water valve 57 which alternately feeds fresh water via pipe 41 to hutch 
52. A motor (not shown) drives pulley 51 and rotates drive shaft 58 for 
alternately opening and closing water valve 57. Thus, fresh water is 
alternately pumped into the hutch 52 which creates the alternate upward 
movement of water in the water column present in screen compartment 54. 
Rod 55 is simultaneously driven reciprocally causing reciprocal movement 
of diaphragm 53. In the position shown in FIG. 2, fresh water is entering 
the hutch via inlet pipe 41 and through valve 57 while the downward stroke 
of diaphragm 53 forces water in the water column in screen compartment 54 
to rise. In the alternate position (not shown), valve 57 is closed while 
diaphragm 53 is in an upward or elevated position. In this condition, the 
heavier particles settle to the bottom of the column through bar screen 56 
and pass into the bottom of hutch 52 for collection and removal. 
Observing also FIG. 3, as the mixture of solid light and heavy particles 60 
passes into screen compartment 54, large and small heavier particles 62 
and 64 fall gravitationally through the water column to the bottom of 
screen compartment 54 while lighter particles 65 float near the top of the 
water in the compartment. A trash screen 76, may be present to filter off 
or direct any floating materials at the top of the water column and 
prevent them from otherwise accumulating in the screen compartment. As the 
water moves from left to right in FIG. 3, smaller heavier contaminated 
particles filter through bedding material 66 which may be of any desired 
thickness and selected opening sizes. These contaminated particles then 
pass through screen 56 which supports the bedding material 66. The larger 
heavier particles 62 are forced along with the moving water stream beneath 
diverter 72 and through port 70 into draw-off pipe 74. 
The heavier sinking particles small enough to pass through the bedding 
material 66 will be collected at the narrow base of hutch 52 while the 
larger heavier particles 62 drawn off through port 70 and pipe 74 are 
collected in a radioactive waste container. The smaller particles may be 
drawn off through a discharge fitting 36 at the bottom of hutch 52 as 
desired. The apparatus may be provided with handle means 68 for raising or 
lowering a diverter 72 for occluding and/or opening port 70 as desired. 
The soil radioactive contaminants are normally heavy metal oxides occurring 
as tiny little spheres, or are attached to fragments of metal, usually 
steel and the like. Radioactive metal oxides mostly consist of plutonium 
oxide having a specific gravity of 11.46 or americium oxide, having a 
specific gravity of 11.68. Iron, the principal element of steel has a 
specific gravity of 7.86. On the other hand, uncontaminated soil 
components have substantially lower specific gravities. For example, pure 
calcium carbonate has a specific gravity ranging between about 2.7 and 
about 2.9 while silica, another major soil ingredient has specific 
gravities ranging from about 2.2 to about 2.6. Coral samples which have a 
relatively high porosity have specific gravities ranging between about 1.5 
and about 2.0. It is because of these significant differences in specific 
gravities of radioactive contaminated materials and uncontaminated soil 
particles that provides for separation according to the process of the 
invention. Generally, for successful separation, the radioactive 
contaminants must have a specific gravity of at least about twice that of 
the uncontaminated particles. 
The lighter soil particles in the slurry or suspension float in the water 
column and are directed into an apparatus 26 for separating substantial 
amounts of water from the mixture. Preferably at least about 70% of the 
water is removed from the suspension in this step of the process. Suitable 
water separating apparatus may include centrifuges, filters or other 
equivalent devices. A particularly useful apparatus is a spiral classifier 
as shown in FIG. 1 which allows for continuous removal of the water as the 
process is carried out. A hose 32 directs water and any remaining 
suspended soil particles from the classifier to a tank 30. A stirrer 34 
agitates the aqueous mixture while a pump 25 directs the material back 
into mineral jig 22 via pipe 28 and valve 57 This return provides further 
separation of light or heavy particles that may have inadvertently passed 
through the system. 
Once the material has passed through classifier 26, there will still be 
normally about 12-20% of water present although the material can be 
handled substantially as a particulate solid. It is then directed onto 
conveyer 38 where a gate or bar 46 is maintained at a predetermined 
elevation above the conveyer and spreads the solid material to a 
substantially uniform thickness for being exposed to radioactive detection 
means. Preferably the thickness of the layer of material spread by the 
gate is no greater than about 4 inches. 
Any radioactive contaminants remaining in the soil material are detected on 
conveyer 38 by scintillation detectors as the soil is passed along on the 
conveyer belt. As shown in FIG. 1, a detector housing 40 is provided with 
radiation detectors 48. Preferably a plurality of such devices are 
stationed side-by-side so that the detection field extends entirely across 
the space in which the layer of soil travels on the conveyer belt. 
Suitable detectors are preferably low energy radiation probes operating in 
an energy range of between about 13 and about 24 Kev. These detectors will 
continually scan the moving layer of soil passing beneath the detector 
housing and will detect any radioactive decontaminants present. As the 
soil layer continually passes beneath the detectors 48, when radioactive 
material is detected, a signal is passed to a controller 50 which will 
operate a suitable drive mechanism for lowering knife gate 42. When the 
knife gate is lowered, it passes through the layer of soil on the conveyer 
belt and deflects the soil off of the belt into a suitable container 44 
for radioactive material. 0f course, any kind of container may be used for 
receiving the radioactive material as may any other equivalent or suitable 
means for removing detected radioactive soil material from the conveyer 
belt and for directing it to such a suitable container. Although an 
automatic knife gate means as shown in the drawing may be used for 
directing contaminated soil from the conveyer belt once radiation 
contamination is detected, this may also be done manually, for example, 
with the radioactive detector being connected to an alarm. Once the alarm 
is given, an operator may drop a knife blade or other means for directing 
the radioactive contaminated soil portion off the conveyer belt to a 
suitable collection or storage container. Any other suitable means for 
removing the soil from the conveyor may be substituted for the knife gate. 
After radioactive material is no longer sensed by detector 48, knife blade 
42 may again be raised and the clean soil material passed on to the end of 
conveyer 38 where it may be suitably recovered. The recovered clean soil 
material may be treated further for detecting any remaining residual 
radioactivity that may have passed undetected. For example, the material 
may be taken to a suitable area and laid down to a uniform thickness and 
inspected further with a radioactive detection means. Once radioactivity 
is no longer detected, the soil material may be used in an unrestricted 
manner. Moreover, the contaminated soil recovered in containers 24 and 44 
may be further treated or processed to concentrate the heavy metal 
contaminants. For example, the contaminated material may be subjected to 
an electrostatic field in which lighter particles are deflected more than 
heavier uncontaminated particles. Advantage may be taken of such 
deflection differences in separating the materials. Alternatively, the 
radioactive components may be preferentially dissolved by selective 
solvent means and the solution subjected to chemical or electrochemical 
precipitation or deposition. Other components for accomplishing the 
process steps described above may be used or substituted as may additional 
treatment steps, all within the purview of the invention. 
Although the process of the invention has been particularly described for 
use in separating radioactive materials, it may be used to separate 
hazardous metals or metal compounds. The only substantial difference in 
treating the hazardous metal soils is that the radioactive detection steps 
and equipment are eliminated. Instead, the soil composition recovered from 
the classifier is tested by chemical and/or physical analysis to determine 
that it meets or passes requirements and standards for safe materials.