Apparatus and method for removing strontium and/or cesium ions from an aqueous solution containing chemical hardness

Abstract

 Apparatus for removing strontium and/or cesium ions from an aqueous solution containing chemical hardness. A first column containing an organic cationic exchange resin in alkali metal form removes the chemical hardness and some of the strontium and cesium ions, and a second column containing a zeolite which can be erionite, chabazite, phillipsite, type A zeolite, or mixtures there­of, removes radioactive ions. The apparatus also includes means for passing the aqueous solution through the first column then through the second column. Also a method for removing radioactive strontium and/or cesium ions from an aqueous solution containing chemical hardness by passing the aqueous solution through a first column containing an organic cationic exchange resin in alkali metal form and then passing the aqueous solution through a second column containing a zeolite which is erionite, chabazite, phillipsite, type A zeolite, or mixtures thereof.

Description

[0001] This invention relates to an appartaus and method for removing strontium and/or cesium ions from an aqueous solution containing chemical hardness.

[0002] At the present time, aqueous solutions of low level radioactive waste are treated using a combination of scavenging, precipitation, filtration, and ion exchange to reduce the radioactivity to a safe level. This process has not proved to be entirely satisfactory because the chemical hardness in the solution interferes with the proper func­tioning of the organic ion exchange resin used to remove the radioactive ions. A high sodium concentration in the waste solution also causes functional inefficiency in the organic ion exchanger. Also, the presence of phosphate ion greater than 0.5 ppm prevents the precipitation of calcium and magnesium ions during scavenging, and the presence of phosphate and other anions, such as chloride and sulfate, reduces the efficiency of the weak acid cationic exchange resin. Finally, the removal of the strontium 90 ions in the solutions is not considered to be adequate.

[0003] Accordingly, the present invention resides in an apparatus for removing strontium and/or cesium ions from an aqueous solution containing chemical hardness characterised in that said appartaus comprises a first column containing an organic cationic exchange resin in alkali metal form; a second column containing erionite, chabazite, phillipsite, type A zeolite or mixtures of these zeolites; and means for passing said aqueous solution through said first column, then through said second column.

[0004] The invention also includes a method of removing radioactive strontium and/or cesium ions from an aqueous solution containing chemical hardness characterized by passing said aqueous solution through a first column containing an organic cationic exchange resin in alkali metal form; and passing said aqueous solution through a second column containing erionite, chabazite, phillipsite, and type A zeolite or mixtures of these zeolites.

[0005] All low radioactivity liquids, including those of high hardness concentration and those that contain anions such as phosphate, fluoride, and sulfate, can be handled regardless of sodium ion concentration.

[0006] The decontamination factor can be increased by dividing the zeolite ion exchange bed into upper and lower portions, using a different mix of zeolites in each portion.

[0007] In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accom­panying drawing which is a flow diagram of an apparatus for removing strontium and/or cesium from an aqueous solution containing chemical hardness.

[0008] In the drawing, an aqueous solution containing radioactive ions, such as cesium and strontium, as well as chemical hardness, enters filter 1 which removes any particulate matter that may be present. The solution then passes through line 2, valve 3, line 4, and valve 5 into column 6 which contains an organic cationic exchange resin in alkali metal form (Na⁺ form). The solution is atomized by atomizer 7 to help produce more surface area and thereby promote an intimate contact of the liquid and the solid resin bed. The solution, with the chemical hardness and some of the radioactive ions removed, then passes through line 8, valve 9, valve 10, and line 11 into column 12, which is packed with a specially selected zeolite, and is divided into upper portion 13 and lower portion 14, sepa­rated by liquid redistributor plate 15. The solution is atomized by atomizer 16, and leaves column 12 by line 17, passing through valve 18 into lines 19, valve 20, and line 21 for discharge. Should column 6 become exhausted or malfunction, valve 5 can be closed and valve 22 can be opened, permitting the aqueous solution to pass through line 23 into column 24 where it is atomized by atomizer 25. The aqueous solution then leaves column 24 by line 26 through valve 27 into line 28, valve 10, line 11, and into column 12. Aqueous solution leaving columns 6 and 24 can be tested for chemical hardness, and, if insufficient hardness has been removed, the solution can be recycled through valve 29 and line 30. Should column 12 become exhausted or otherwise malfunction, valve 10 can be closed and valve 31 can be opened, permitting the fluid to pass through line 32, and atomizer 33 into column 34, which is also filled with a specially selected zeolite, and is divided into upper portion 35 and lower portion 36, sepa­rated by liquid redistributor plate 37. The aqueous solution, with its radioactivity removed, leaves column 34 by line 38 and valve 39 into line 19.

[0009] Some low level radioactive waste solutions consist of evaporator condensates or overshoots (resulting from higher activity streams or evaporator malfunction) that contain very low or no chemical hardness. These condensates can enter the system by line 40. If chemical hardness is present, they can pass through line 41 and valve 42 into line 30 for treatment in column 6 or column 24. If chemical hardness is not present, valve 42 is closed and valve 43 is opened, and the condensate passes through line 44 and valve 45 through atomizer 46 into column 47, which is also packed with a specially selected zeolite, and is divided into an upper portion 48 and a lower portion 49, separated by liquid redistributor plate 50. Condensate leaving column 47 by line 51 through valve 52 can be sent through lines 19 and 21 to discharge if the radioactivity is sufficiently low to meet the discharge limits. However, if any of the fluids leaving columns 47, 12, or 34 do not meet the discharge limits, valve 20 can be closed and valve 53 can be opened so that the fluids pass through line 54, which recycles them through columns 47, 12, or 34, until the radioactivity is reduced to the discharge limits. Should column 47 become exhausted or malfunction, valve 45 can be closed and valve 55 can be opened, and the condensate will pass through line 56 into column 34.

[0010] The waste water treated according to this inven­tion contains cesium and strontium ions and also contains chemical hardness such as calcium, magnesium, and/or iron ions. The aqueous solution contains no particulate matter as it is removed by a filter that precedes treatment in the ion exchange columns. The aqueous liquid also should not contain organic liquids as they interfere with the removal of radioactive ions from the solution. While any amount of cesium and strontium ions may be present, a typical low level waste solution will contain about 10⁻³ to about 10 microcuries per cubic centimeter and about 5 to about 1000 parts per million (ppm) of chemical hardness as calcium, magnesium, and iron.

[0011] The organic cationic exchange columns remove chemical hardness from the solution and reduce the solu­tion's conductivity. In addition, they also remove some of the strontium. The organic cationic exchange resin must be in alkali metal from, preferably the sodium form, rather than the acid form, as the acid form is not effective in removing calcium. It is preferable to use a strong acid form rather than a weak acid form because a strong acid form is more durable and lasts longer. A sulfonic acid based cationic exchange resin is preferable as it is readily available and works well. While either the gel or macroreticular type of exchange resin can be used, it is preferable to use a macroreticular exchange resin as that type of resin does not expand, is easier to work with, and is easier to dispose of. If phosphate ion is present in the aqueous solution, it is preferable to include about one to about 20 percent (all percentages herein are of weight unless otherwise specified) of an organic anionic ion exchange resin in with the organic cationic exchange resin in order to remove the phosphate ion. Also, as is shown in the drawing, it is preferable to use two columns so the process can continue by switching to a second column should one column become exhausted, plugged, or otherwise break down. The fluid leaving the organic cationic exchange column should be tested for calcium to determine when the column has become exhausted.

[0012] In the second stage of the process of this invention the aqueous solution is passed through a second type of column containing particular zeolites. The zeolites that are used in this invention are erionite, chabazite, type A zeolite (a synthetic zeolite), and phil­lipsite. We have discovered that these particular zeolites have a higher capacity for removing cesium and strontium than do other ion exchange materials, such as organic cationic exchange resins. The zeolites preferably have a pore size of about 4 to about 5 angstroms.

[0013] The second stage of the process is itself prefer­ably divided into two portions within the column. The column is preferably divided to achieve a more selective Sr-90 partition/decontamination in the upper portion and more selective Cs-137 partition/decontamination in the lower portion. The minimum packing height of the upper stage is preferably five feet. The steam exiting the upper stage drop into a liquid redistributor plate, then trickles into the lower stage. The upper portion contains a higher proportion of type A zeolite, which removes strontium better, and the lower portion contains a higher proportion of chabazite and/or erionite, which removes cesium better. By dividing the second stage into upper and lower stages, a higher decontamination factor (DF, equal to radioactivity before treatment divided by radioactivity after treatment) is obtained. The upper stage preferably contains from 55 to 60% by volume type A zeolite and from 40 to 45% by volume chabazite and/or erionite. The lower stage preferably contains from 25 to 30% by volume type A zeolite and from 70 to 75% by volume chabazite and/or erionite.

[0014] When the ion exchange columns are exhausted, they can be sluiced to a cement mixture where they are solidi­fied in cement, or otherwise disposed of.

[0015] The invention will now be illustrated with reference to the following Example.

EXAMPLE

[0016] This example shows the dynamic column test data obtained by passing a radioactive solution containing chemical hardness through an organic ion exchange prefilter and then through a zeolite bed. The organic ion exchange bed removes chemical hardness (Ca⁺⁺, Mg⁺⁺, etc.) very effectively and at the same time decontaminates a very substantial quantity of Sr-90 and Cs-137. The effluent from the organic ion-filter bed goes to the zeolite bed where the remaining traces of radioactivity is so effec­tively removed that the Cs-137 and Sr-90 levels of the effluent zeolite bed are undetectable. (<10⁻⁸uCi/c.c). The organic ion exchange prefilter was strong acid cation exchange resin in Na⁺ form sold by Rohm & Hass under the trade designation IR-122. The zeolite was erionite sold by Phelps dodge zeolites under the trade designation PDZ-50. The following tables give the conditions and results of this experiment.

Claims

1. Apparatus for removing strontium and/or cesium ions from an aqueous solution containing chemical hardness characterized in that said appartaus comprises a first column containing an organic cationic exchange resin in alkali metal form; a second column containing erionite, chabazite, phillipsite, type A zeolite or mixtures of these zeolites; and means for passing said aqueous solution through said first column, then through said second column.

2. Apparatus according to claim 1 characterized in that the aqueous solution is radioactive to the extent of from 10⁻³ to 10 microcuries/cc, and contains from 5 to 1000 ppm of calcium, magnesium or iron ion.

3. Apparatus according to claim 1 or 2, charac­terized in that the alkali metal is sodium.

4. Apparatus according to claim 1, 2 or 3, characterized in that the organic cationic exchange resin is a strong acid type.

5. Apparatus according to claim 4, characterized in that the organic cationic exchange resin is sulfonic acid based.

6. Apparatus according to any of claims 1 to 5, characterized in that the organic cationic exchange resin is a macroreticular type.

7. Apparatus according to any of claims 1 to 6, characterized in that the aqueous solution includes phos­phate ion and the organic cationic exchange resin is mixed with from 1 to 20% organic anionic exchange resin.

8. Apparatus according to any of claims 1 to 7, characterized in that said apparatus includes means for testing said aqueous solution for calcium ions after it has passed through said first column, and for recycling it through said organic cationic exchange resin bed if signif­icant concentrations of calcium ions are present.

9. Apparatus according to any of claims 1 to 8, characterized in that said apparatus includes means for atomizing the aqueous solution as it enters the first and second columns.

10. Apparatus according to any of claims 1 to 9, characterized in that said apparatus includes means for filtering the aqueous solution before it enters said first column.

11. Apparatus according to any of claims 1 to 10, characterized in that the second column is divided into an upper portion and a lower portion, and in said upper portion are from 55 to 60% by volume type A, zeolite and from 40 to 45% by volume chabazite, erionite, phillipsite or mixtures thereof, and in said lower portion are from 25 to 30% by volume type A zeolite and from 70 to 75% by volume chabazite, erionite, phillispsite or mixtures thereof.

12. Apparatus according to any of claims 1 to 11, characterized in that the zeolite has pore size of from 4 to 5 angstroms.

13. A method of removing radioactive strontium and/or cesium ions from an aqueous solution containing chemical hardness characterized by passing said aqueous solution through a first column containing an organic cationic exchange resin in alkali metal form; and passing said aqueous solution through a second column containing erionite, chabazite, phillipsite, and type A zeolite or mixtures of these zeolites.

Drawing