Process for separation of cesium ions from aqueous solutions

The invention relates to a process for continuous or almost continuous separation of cesium ions from aqueous solutions having high concentrations of sodium and/or potassium ions by ion exchange with ammonium molybdophosphate (AMP). A quantitative cesium separation from aqueous solutions which are high in salts, especially from highly radioactive solutions which are strongly nitric and contain nitrate, is achieved, without having to consider bleeding of the AMP and/or undesirable local overheating in the exchanger.

FIELD OF THE INVENTION 
The invention relates to a process for continuous or almost continuous 
separation of cesium ions from aqueous solutions containing high 
concentrations of sodium and/or potassium ions by ion exchange. 
TECHNOLOGY REVIEW 
Numerous ion exchange and extraction systems have been proposed before this 
time for the separation of cesium ions from aqueous reaction solutions, 
for example nitric solutions, including in the presence of high salt 
concentrations. Most of these systems suffer from an undesirable 
sensitivity to variations of the acid content or the foreign salt 
concentration, especially in the foreign nitrate concentration. Cesium 
separation is especially important in the field of the removal of 
radioactive cesium ions contained in waste water or sewage. High nitrate 
levels are encountered there particularly in the MAW evaporation 
concentrates, in which sodium nitrate represents by far the greatest 
portion of the entire salt content in the aqueous concentrate. 
An extremely high selectivity ion exchange material for cesium ions as 
compared with other alkali ions is known for the inorganic ammonium 
molybdophosphate phosphate, (NH.sub.4).sub.3 [PNO.sub.12 O.sub.40 ] ion 
exchanger, which is commercially available under the name AMP-1. However, 
this ion exchange material has not come into general use because AMP-1 is 
always in a microcrystalline state. As a result, in normal operation, 
fairly long columns filled with this material are practically impermeable. 
Another drawback of a column filled with AMP-1 is the heat rise with the 
exchange of radioactive cesium in the column because of the high cesium 
distribution coefficients under highly radioactive (hot) conditions. 
Attempts to avoid these difficulties by coating AMP on carrier substances, 
such as e.g. silica gel etc., have been unsatisfactory in the 
extractability and in the bleeding of the molybdophosphate from the 
carrier substance. Consequently the concentration of AMP in the column 
progressively decreases. Further drawbacks for example are the unsolved 
problem of the reusability of the extraction and exchange apparatus 
itself, as well as the weak exploitation of the absorbant capacity. The 
handling and treatment of such highly radioactive columns is an unsolved 
problem. 
SUMMARY OF THE INVENTION 
The invention provides a process for quantitative cesium separation from 
aqueous solutions, including high salt content aqueous solutions, 
especially from highly radioactive solutions containing nitrates and 
nitric acid, without bleeding of the ion exchange material and/or 
undesirable local overheating in the exchanger. The process of the 
invention may be performed continuously or almost continuously. It is an 
advantage of the invention that a high number of bottom layer volumes of 
solution containing cesium ions can be passed through the exchanger before 
the ion exchange material volume needs to be replaced.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the invention includes: 
(a) a starting solution with a pH.ltoreq.9.5 and containing cesium ions and 
also containing sodium and/or potassium ions is fed through 
microcrystalline ammonium molybdophospate (AMP) lying loosely on a porous 
substrate within a container or in a layer produced by deposition and 
suspended over the substrate, whereby ammonium ions are exchanged for the 
cesium ions and the less soluble cesium-molybdo phosphate is formed, 
(b) a uniform flow of the starting solution is provided with the provision 
that the AMP microcrystals not be carried out of the container with the 
decontaminated solution from which cesium ions have been removed, 
(c) the decontaminated solution from which cesium ions have been removed is 
withdrawn continuously over the microcrystalline AMP layer or over the top 
surface of a suspended volume of AMP microcrystals, 
(d) when the ion exchange material in the exchanger is exhausted, the feed 
of starting solution is halted, the exchanger is washed with water and the 
water is siphoned off and then the ion exchange material is flushed out of 
the container or is dissolved with strongly alkaline aqueous solution and 
removed from the container and 
(e) a fresh AMP layer is introduced into the container and steps (a) 
through (d) are repeated as often as desired with starting solutions 
containing cesium ions. 
It is advantageous that the uniform flow of the starting solution 
containing cesium ions be determined with the provision that the total 
suspended volume of AMP microcrystals introduced into the container does 
not exceed 7/8 of the level of the liquid column in the container. 
The porous substrate to be placed in the container, on which the loose 
layer of microcrystalline AMP is placed, may for example consist of a 
high-grade steel powder metal frit. The flowthrough velocity of the 
starting solution from which cesium ions are to be removed, which contains 
a low concentration of cesium ions and very high salt concentrations 
relative thereto, can be varied within a wide range according to 
dimensions of usable space in the AMP or the column or the container. 
Practically speaking, the flowthrough velocity can be set so that the AMP 
microcrystals in the bottom part of the volume of the starting solution 
can be held in suspension, but the discharge opening of the container for 
the decontaminated solution is not reached. After the ion exchange 
material in the exchanger is loaded (i.e. converted to cesium-molybdo 
phosphate), the starting solution containing cesium is interrupted and the 
starting solution standing over the resettled AMP layer is siphoned off or 
pressed down out of the column with a tube introduced at a proper level. 
The solution from which cesium ions have been removed is replaced by 
water. The AMP is thus freed of residues of acidic solution. Then the 
remaining washing solution is removed by suction. 
The ion exchange material in the exchanger loaded with cesium ions can now 
be dissolved in ammonium hydroxide or sodium hydroxide solution and the 
waste solutions arising therefrom at the bottom end of the container are 
drawn off, without any change of the apparatus or complicated operation 
with the apparatus. In another embodiment the AMP however can also be 
flushed out of the device. The exchanger waste solution or suspension 
containing cesium can thereafter be mixed homogeneously in a simple manner 
with the matrix provided for the disposal of waste (for example for 
vitrification radioactive wastes or for the cementing of such wastes) or 
be fed to a further chemical treatment to obtain cesium commercially. 
Sodium hydroxide solution is more suitable for use as solvent in highly 
radioactive systems on account of the greater radiation resistance. 
Following the washing of the container or column fresh AMP can be fed to 
the exchanger, for instance by a pump through the decanting tube into the 
apparatus. The decanting tube which is immersed in the solution can be 
configured so that it can be raised vertically if required. In another 
embodiment it can be introduced into the column from the side. 
The device consists essentially of a container or a column, for example a 
cylindrical tube member 1, which is provided with input lines 4, 6 and 
discharge lines 5, 6, 7 at its ends and at least at the bottom end with a 
frit 2. The AMP is fed in through the delivery tube 6 as a powder before 
the beginning of the process, so that it lies loosely on the frit 2. 
Otherwise the AMP can be fed through 6 as a suspension in the medium which 
is to be decontaminated thereafter. The porosity of frit 2 plays no role, 
since it is solely to prevent the AMP falling through, and the standard 
pore size in the frit is 3 to 15 micrometers. The solution containing 
cesium is then introduced into the device through feed line 4 at a uniform 
flow velocity. This can be obtained by hydrostatic pressure or by a pump 
etc. 
Thus the AMP rises slowly upward with the flow, is distributed in the 
liquid and forms a density gradient from frit 2 upward. The flow velocity 
is determined so that the top end of the column, out of which the 
decontaminated solution flows through discharge 5, remains free of AMP 
particles. To be sure of this, a 0.5 micrometerfrit 3 can be introduced at 
the top end, which prevents AMP discharge from the column during eventual 
fluidization of the AMP, whether it is thrown up by too rapid pumping 
action or by air blasts. This filter could also be an "in-line-filter" 
built into discharge line 5 before introduction of the solution into the 
vessel (not shown in the drawing). With maintenance of certain processing 
conditions, however, frit 3 is not required. After conclusion of the 
charging of the AMP with cesium (maximum capacity about 60 g cesium/kg 
AMP), the feed of the processing solution through feed 4 is halted and the 
AMP is allowed to settle. 
The solution standing over the AMP is emptied from the column through 
delivery pipe 6 or the solution portion still remaining in the device is 
allowed to flow through discharge line 7, and additional compressed gas 
(air, N.sub.2, Ar etc.) can be fed into the device through line 5 to 
accelerate the discharge process. In case of need the column can be blown 
dry. 
The solutions required for dissolution of the AMP or flushing the device 
are fed in through inlet (the inlet feed line) 4 or in emergency through 
lines 5, 6, 7 and leave column 1 in the traditional manner through 
discharge pipe 5 during continuous operation, or delivery tube 6, after 
deposition of the AMP, or (after discharge and emptying of delivery pipe 
6) through discharge pipe 7. 
The continuous or almost continuous process according to the invention has 
shown a surprising advantage in comparison with a conventional 
discontinuous process, for example in a beaker glass (batch processing), 
which works with pure AMP, and also relative to a process in which the AMP 
was coated onto a support structure. During batch processing, a 
decontamination factor (DF) for cesium ions of on the order of 10.sup.2 
can be attained, the process according to the invention achieves a DF of 
greater than 60,000, with higher radioactive starting material a DF on the 
basis of the remaining slight residual activity of greater than 100,000 
can be achieved. The indication "greater than" used here before the 
numerical value means that the cited numerical value can be overall 
higher, but it cannot be calculated quite correctly, because the residual 
activity lies in the vicinity of the detection limit. 
SPECIFIC EXAMPLES 
In order that those skilled in the art may better understand how the 
present invention may be practiced, the following examples are given by 
way of illustration, and not by way of limitation. 
EXAMPLE 1 
1 liter of pure medium radioactive waste solution (MAW) with a dose of 8 
R/h (in contact) was pump fed through an organic absorbant material (Bio 
Bed SM7 of Bio Rad Company), which was filled into a column (.phi.=20 mm, 
H=200 mm). As a result, the organic impurities of the MAW concentrate 
solution were removed from the aqueous phase. Solid particles were 
collected in an "in-line-filter", which was mounted in front of the 
column. By this step, the radiation dose was lowered to 3.5 R/h. 
Then the aqueous solution cleaned of organic and solid impurities was 
conducted with a flow rate of 10 column volumes through a cesium 
collection column, as shown in the drawing, with 4 g of AMP-1 on the frit 
column (diameter .phi.=20 mm; height, H=200 mm). The solution was 
collected in a container and subjected to a gamma .gamma. measurement. 
Only Cs-134 and Cs-137 were removed and these with a decontamination 
factor of DF greater than 60,000, given by the detection limit of the 
gamma .gamma. spectrometer. No local overheating occurs with this 
continuous process, since the AMP-1 is cooled while moving and after use 
it is dissolved in a NaOH solution. 
Then the pump was disconnected and after rapid deposition of the AMP-1 the 
remaining, already decontaminated solution was removed through the 
delivery pipe and fed to the decontaminated solution. The entire does in 
the vessel was still 0.7 R/h. After flushing the column with water (200 
ml), the excess aqueous solution was likewise removed from the column 
through the delivery pipe (after deposition of the AMP-1); the dissolution 
of the AMP-1, charged with Cs, occurred with 20 ml M NaOH solution, fed 
into the column from below. The waste solution was then allowed to flow 
downward out of the column. After a final H.sub.2 0 washing of the column, 
the column was again fed fresh AMP-1 through the delivery pipe and the 
procedure was repeated with fresh MAW. 
EXAMPLE 2 
100 liters of simulated MAW (with traces of Cs-132) were pumped at a flow 
rate of 50 column volumes through an AMP column (.phi.=85 mm; H=500 mm), 
which was coated with 10 g AMP-1. The decontamination factor for cesium of 
the solution collected in a vessel was above 60,000. 
After loading the exchanger the pump was disconnected, and after deposition 
of the AMP-1 charged with cesium, the remaining, already decontaminated 
solution was siphoned off through the delivery pipe. 
Then the column was flushed with 5 liters of water upward from the bottom, 
and the main portion of the flushing water remaining in the column was 
also removed through the delivery pipe--after deposition of the AMP-1. The 
AMP-1 containing Cs was then dissolved in 100 ml of 1 M NaOH solution, fed 
in from below. This waste solution was allowed to flow downward. 
Then the apparatus was rinsed with H.sub.2 0 and a new charge of AMP-1 was 
fed in through the delivery pipe. This procedure was repeated 10 times and 
thus one cubic meter volume of simulated MAW was processed--corresponding 
to expectations for a regeneration treatment plant (with 4 g Cs). 
TABLE 1 
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Al 0.23 g/l 
Ca 1.5 g/l 
Cr 0.08 g/l 
Cs 0.0036 g/l 
Cu 0.15 g/l - Fe 0.38 g/l 
U 0.08 g/l 
Mg 0.75 g/l 
Mn 0.08 g/l 
K 0.08 g/l 
Mo 0.38 g/l 
Na 81.14 g/l 
Ni 0.08 g/l 
Sr 0.001 g/l 
Zn 0.15 g/l 
Zr 0.08 g/l 
HNO.sub.3 1 Mol/l. 
______________________________________ 
EXAMPLE 3 
Comparison of the decontamination factors (DF) of 
(a) 3 static tests (batches) 
(b) the corresponding dynamic tests (process of the invention in the AMP 
column) for the simulated MAW described in Example 2. 
Each 100 ml of simulated MAW with different cesium amounts was treated both 
statically (a) and dynamically (b) with 1 g AMP-1 for each treatment. 
(a) The batch experiments were carried out in 250 ml plastic flasks, and 
the solution was brought for 10 minutes into close contact with the AMP-1. 
After heating and centrifugating the Cs content in the remaining solution 
was determined. 
(b) The dynamic experiments were carried out as in Example 1. The flow rate 
was 10 column volumes per hour. The results are shown in Table 2. 
TABLE 2 
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Test AMP-1 Solutn. Cs-conc. DF 
No. (g) (ml) (mol/l) (a) batch 
(b) dyn. 
______________________________________ 
1 1 100 3.2 .times. 10.sup.-4 
450 &gt;100,000 
2 1 100 1.6 .times. 10.sup.-3 
127 &gt;100,000 
3 1 100 4.8 .times. 10.sup.-3 
90 &gt;100,000 
______________________________________ 
This table shows the unambiguous superiority of the process according to 
the invention for batch processing experiments. Even with greater loads in 
the exchanger, a DF is attained which is still greater than 60,000, which 
is limited by the detection limit of the gamma .gamma. spectrometer, while 
in the batch experiments with increasingly greater charge the DFs drop 
from 450 to 90. 
The present disclosure relates to the subject matter disclosed in European 
patent application No. 86-109194.0 filed July 5, 1986, the entire 
specification of which is incorporated herein by reference. 
It is understood that various other modifications will be apparent to and 
can readily be made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.