Process for selective recovery of uranium from sludge

A process for recovering uranium from a waste sludge involves treating the sludge with a non-reactive, non-absorptive cellulose filtration media, adding an oxidant and heating the mixture to dissolve any uranium present. The mixture is passed through a filtration device precoated with the same filtration media, where the solids are captured while the dissolved uranium passes through as part of the filtrate, for recovery.

TECHNICAL FIELD 
This invention relates to the recovery of uranium from waste sludge. 
BACKGROUND 
During the production of componets such as fuel pellets used in fuel rods 
for nuclear reactors, there is an inevitable loss of some uranium to 
various effluent streams. These streams are typically collected with the 
uranium and various other solids settled by gravity to form a sludge. 
Even though a relatively small amount of uranium is contained in this 
effluent, its value, and concerns over disposal, makes it desirable to 
recover this uranium to the extent possible. 
While various attempts have been made to recover this uranium, these have 
had limited success. For example, the sludge typically contains other 
salts which complicate recovery schemes. The sludge will normally include 
ammonium, sodium, fluoride, and various nitrate and sulfate salts. In 
addition, the nature of the sludge is such that it is difficult to handle 
and treat, as the sludge has a slime-like consistency which typically 
coats out recovery devices. While filter aids such as diatomaceous earth, 
can be used, these typically absorb uranium and thus themselves result in 
a further loss of the material. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for selective 
recovery of the uranium from sludge. 
It is a further object to provide a process for selective recovery of 
uranium from sludge which contains high concentrations of compounds such 
as ammonium, sodium, fluoride, nitrate and sulfate salts. 
It is yet another object of the present invention to achieve an economic 
yet reliable process for the selective recovery of uranium from the waste. 
These and other objects of the present invention are achieved by a process 
comprising: 
providing sludge containing uranium in the form of a slurry; 
adding a uranium dissolving compound to the slurry; 
heating the slurry; 
providing a non-reactive cellulose filtration media; 
providing a filtration device; 
treating the filtration device with the non-reactive cellulose filtration 
media; 
passing the slurry through the filtration device and collecting a filtrate 
containing dissolved uranium. 
In a preferred embodiment, a portion of the non-reactive cellulose 
filtration media is added to the slurry to increase its body and 
filterability. Using the inventive process, recovery of uranium from 
sludge is maximized. The non-reactive cellulose filtration media, though 
considered difficult to use, has been surprisingly found to be successful 
in allowing dissolved uranium to pass through without hold-up. Such a 
process is thus effective in separating the solids components of the 
slurry from the dissolved uranium, without absorbing the uranium passing 
therethrough, and has resulted in a 200% increase in the recovery of 
uranium over conventional processing.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a schematic view illustrating the process of the 
present invention is shown. A feed tank 1 includes an agitator 2 for 
mixing a slurry 3. The slurry comprises a sludge including uranium, as 
well as liquid and solid waste containing various ingredients including 
but not limited to sand, ammonium fluoride, ammonium nitrate, ammonium 
sulfate, sodium nitrate, sodium fluoride, nitrate and sulfate salts, other 
debris, polymer slimes, phosphates, and metal compounds containing, for 
example, zirconium, aluminum, and gadolinium. Dissolved uranium may also 
be contained in the liquid portion of the slurry. Typically, this slurry 
is prepared by collecting sludge at the bottom of a lagoon which is then 
either fed directly through a line 4 to the feed tank I or the sludge may 
be collected off site, optionally dewatered, and transported to the 
location where the apparatus of the present invention is arranged for 
processing. 
In addition to the slurry 3, water 5 may be added to provide a better 
dispersion of the solids to aid in filtering. Optionally, a non-reactive 
cellulose filtration media 6 is added to the feed tank so as to improve 
the body of the material which is fed to the filter preferably provided as 
a raw cellulose slurry made up at about 0.5-1.5 lb./gal. For example, it 
is preferable to have a cellulose concentration of about 0.06 lbs. to 
0.125 cellulose per gallon of slurry added into the slurry feed tank. 
These ingredients are mixed together in the feed tank. 
The feed tank 1 preferably has means to heat the contents thereof This can 
be accomplished using internal coils, an external tank jacket or an 
external heat exchanger in a recirculating loop. However, direct steam 
injection, which is the simplest method, is preferred, and this is 
illustrated as line 7. The feed tank 1 is heated by the injection of live 
steam to a temperature of about 110-180.degree. F., preferably about 
160.degree. F. 
A precoat tank 8 is also provided. A non-reactive cellulose filtration 
media 9, usually obtained in dry form, is added to the tank 8 and mixed 
with water 10. A pump 28 is provided to pump precoat into a filter press 
14. 
A pump 11 is provided to transfer slurry to the filter press, with valves 
12 and 13 provided for selecting the flow to be delivered to the press. 
The filter press 14 is used to separate the solids from the liquid. The 
filter press is a multiple plate and frame device which include a 
plurality of alternating plates 15 and frames 16 which are pressed 
together, each frame including a filter 17 made of a material such as 
cloth. While various filters can be used, a polypropylene filter cloth 
having a pore size of about 1 to 5 microns is preferred which provides a 
filtration rate of about 1 to 3 cubic feet per minute at pressures up to 
125 psig. The solids passing through the filter press are collected on the 
cloth. After a batch of material has been transported through the filter 
press, wash water 18 is passed through the filter press to remove residual 
liquid. Then the flows are stopped, and the plates are separated so that 
solid material collected between the plates can be removed as a low water 
content cake 19. Since in this instance, the cake comprises dewater 
sludge, it is typically collected in drums 20 for disposal. 
A filtrate tank 21 is provided for capturing the liquid filtrate 22 which 
has passed through the filter press. However, it is typical to use a 
recycle line 23 to send the filtrate stream back to the feed tank until a 
steady state condition is achieved within the filter press and 
substantially all of the solids are being captured such that a clear 
filtrate is obtained. Once it has been determined that the filtrate is 
clear, the valves 24 and 25 are arranged so that the filtrate is valved to 
the filtrate tank for collection. 
In many instances, it is preferable to wash the filter cake before it is 
subject to disposal and water is typically passed through the filter press 
so as to remove any holdup of liquid which may contain, for example, the 
dissolved uranium which is recovered according to the present invention. 
The feed tank 1 is also supplied with an oxidant 26 which assures that any 
uranium within the slurry in solid form is dissolved so that it can pass 
through the filter to the filtrate tank. While various oxidants can be 
used, it is preferred to use a sodium hypochlorite solution (12% wt.) 
added at about 0.5-2% volume per volume of slurry, about 1% being 
preferred, to assure dissolution of the uranium into the liquid portion of 
the slurry. The oxidant 26 is added to the feed tank 1 selectively 
oxidizes uranium to maximize the concentration of uranium in the filtrate. 
For sodium hypochlorite, the reaction is as follows: 
EQU U.sup.4+ +NaOCl+H.sub.2 O=&gt;UO.sub.2.sup.2+ +NaCl+2H* 
Thus, the heating and oxidant assure that the uranium present in the slurry 
is dissolved into the liquid portion of the slurry. 
This material is agitated by the mixer 2 so as to prevent settling. 
As described previously, the non-reactive filtration media 9 is usually 
obtained in solid form and generally delivered in bulk. The media when 
mixed with water, forms a slurry, containing about 0.5 to about 1.5 pounds 
filtration media per gallon of water. The media is non-reactive (with 
uranium) and non-absorptive (of uranium) so that substantially all the 
uranium in the slurry is separated and collected in the filtrate. It is 
also of a type and particle size to facilitate filtration of "slimy" 
slurries, that is, those which would "blind" the filter cloth in the 
absence of the filtration medium. Generally, cotton or wood fiber 
cellulose with a particular particle size distribution is effective for 
this application. These are shown in Tables 1 and 2. While other 
filtration media are known, for example, diatomaceous earth, aluminum 
silica, ashed rice hulls, and wood fiber and cotton fiber cellulose 
materials with other particle size distributions, surprisingly only the 
particular cellulose filter media described were successful in being 
ilon-reactive with uranium and providing a dry cake with no free standing 
liquids and clear filtrate. 
TABLE 1 
______________________________________ 
SIZE (MICRON, um) 
WEIGHT % 
______________________________________ 
ACCEPTABLE DISTRIBUTION #1 
&lt;45 16.0 
45-100 24.3 
106-150 12.7 
150-180 8.8 
180 28.9 
&gt;250 9.3 
TOTAL 100% 
ACCEPTABLE DISTRIBUTION #2 
&lt;74 70 
74-147 22 
147-351 8 
TOTAL 100% 
______________________________________ 
Once the precoat slurry is prepared, it can be fed to the filter press, so 
as to precoat the surfaces of the filter cloth. As an example, to 
effectively precoat a filter press which has 1600 sq. feet of filter 
surface area, about 200 gallons of the raw cellulose slurry, having about 
1.25 lbs. per gallon of cellulose is further diluted with about 400 gal. 
of water to have a cellulose concentration of about 0.41 lbs. per gal. 
This is fed to the filter press where the cellulose is deposited as a 
first layer of captured solids, known as the precoat. After the filter 
cloth has received the precoat from pump 28, the pump 11 is started and 
slurry feed from tank 1 is fed to the filter. The pressure is controlled 
to prevent blinding of the filter cloth, by starting at a low pressure and 
slowly ramping up the pressure as the cake is formed on the cloth. The 
pressure is ramped up so as to overcome the increasing resistance 
encountered as the solid material thickens on the cloth of the filter 
plates. Preferably, a programmable logic controller 27 begins with an 
initial pressure of about 25 psig, with increasing pressure increments of 
about 5 psig applied at 3 minute intervals. This program has been found to 
insure that sufficient time is allowed to build up solids at the pumped 
pressure while minimizing the overall cycle time to complete formation of 
the filter cake. 
The clear filtrate 22 is collected in the filtrate tank 21, and is 
preferably sampled and analyzed to verify the uranium concentration. Since 
the uranium has been selectively removed from the slurry, the filtrate is 
readily adaptable for separation of the uranium content using known 
procedures. For example, the uranium can be recovered using conventional 
processes such as by using an ion exchange resin, by precipitation, or by 
solvent extraction. Of course, other processes may be used. 
Referring to FIG. 2, an overall process flow diagram is shown for the on 
site recovery of uranium using the process of the applicant's invention. 
Typically, waste water from a facility using uranium will accumulate within 
a vessel such as a lagoon 30. In this lagoon, a layer of solids 31 settles 
to the bottom. These solids are removed using a slurry pump 32, and are 
fed through strainers 33a and 33b and a grinder 34 to separate and/or 
break up any agglomerated material prior to entrance to a feed tank 35. 
The grinder pulverizes the large solids to produce a maximum particle size 
of less than about 1/8 inch, as larger particles could disrupt the 
filtration process. 
The feed tank 35 contains an agitator 36, and has a dedicated pump 37 for 
recirculating the material in the tank. It also has a slurry feed pump 62 
to feed the filter press. An additive tank 38 contains an oxidizing agent 
39 which is transferred by a pump 40 to the feed tank for dissolving any 
solid uranium contained in the slurry. Preferably, 12% by weight sodium 
hypochlorite solution is used as the oxidizing agent. This is added at 1% 
by volume to the volume of the slurry. For example, to treat 6,000 gals. 
of slurry would require 60 gals. of sodium hypochlorite solution. A steam 
injector 41 is located in the tank recirculating line to heat the slurry 
as it recirculates to facilitate uranium dissolution. 
A non-reactive cellulose filtration media 42 is received in a bulk bag 43 
which contains about 400 to 1,000 lbs. of the dry filtration media, which 
is added to a mixing tank 44 along with water 45 to provide a raw 
cellulose slurry with a solids content of about 0.5 to 1.5 lbs. per gal. 
of water about 1.25 lbs./gal. preferred. About 800 gals. of water would be 
added per 400 to 1,200 lbs. of the non-reactive cellulose filtration 
media. 
The raw cellulose slurry about 1.25 lbs. per gal., is transferred by a pump 
46 to the feed tank 35 in an amount sufficient to provide at least 0.06 
lbs. cellulose per gal. waste slurry. The cellulose in the feed tank is 
uniformly distributed in the waste slurry by the agitator 36. The purpose 
is to increase the body of the slurry to facilitate filtration. 
The pump 46 also supplies the raw cellulose slurry at about 1.25 lb./gal. 
to a precoat tank 47 where water 48 is added to provide a precoat slurry 
at about 0.16 to 0.4 lbs./gal. water. 
The precoat is fed from the tank 47 by a pump 49 to a filter press 50 of 
the plate and frame type discussed previously. However, other filtration 
devices may also be used. Water filtrate 51 leaves the filter press and 
this can be returned to the precoat tank, through line 52, returned to the 
feed tank, through line 53, or sent to a filtrate tank 54. The slurry pump 
63 pumps slurry to the filter press. As the slurry from the feed tank 
passes through the filter press, the discharge is monitored. If cloudy, 
the discharge is send back to the feed tank; if clear, it is fed to the 
filtrate tank, with a series of filters 55 preferably used to remove any 
residual solids from the filtrate. After the pressure reaches its maximum, 
and the flow through the filter drops off, water 56 is fed to the filter 
to wash the filter cake before the press is opened for cake removal. A 
pump 57 is provided for dedicated recirculation and mixing of the 
filtrate. A second pump 58 is used to transfer the filtrate to a uranium 
recovery process (not shown). 
The cellulose filtration media may vary from the distributions described 
above, with adjustments made to the process through the programmable logic 
controller, by modification to the amounts and concentrations added as 
precoat and as a body feed, etc., as would be understood by one familiar 
with filtration processes. The cellulose filtration media may have a 
particle distribution of from about 0-6% having a particle size of 351 
microns or greater, about 60-96% having a particle size of about 147 
microns and about 23-83% having a particle size of about 74 microns or 
less. Preferably, the cellulose filtration media has a particle size 
distribution in which about 24% by weight has a particle size of about 
45-100 microns and about 28% has a particle size of about 180 microns. 
Alternatively, at least 70% by weight has a particle size less than 74 
microns and 22% a particle size of 74-147 microns. 
While the term "particle size" is used, it should be understood that the 
cellulose filtration media is fibrous, and that these particle sizes 
correlate to a typical screen analysis, 351 microns being the equivalent 
of a 40 mesh screen, 147 to a 100 mesh screen and 74 to a 200 mesh screen. 
The fibers themselves have a length of about 30-200 microns and a width of 
about 10-40 microns. 
Solids from the filter press are collected in a waste drum 59 for disposal. 
Any liquid residue that accumulates during solids removal and/or cleaning 
of the press between filtration cycles is collected in a tank 60 and 
returned to the feed tank through line 61. 
Utilizing the process of the present invention, a significant increase in 
the recovery of uranium from solid waste is achieved. Prior to utilizing 
the process of the invention, recoveries in the filtrate were on the order 
of about 147 milligrams per liter. Utilizing the process of the invention, 
the amount recovered is on the order of 428 milligrams per liter, a nearly 
200% increase in uranium recovery. Further, the increased recovery 
evidences a significant decrease in the amount of uranium remaining in the 
solid material, thus reducing the amount lost through disposal. 
While preferred embodiments of the present invention have been shown and 
described, it will be understood by those skilled in the art that various 
modifications or changes can be made without varying from the scope of the 
present invention.