Method for obtaining cesium from aqueous starting solutions

Abstract

The disclosed subject matter relates to a method for obtaining cesium from aqueous solutions.

Claims

1. A method for obtaining cesium from aqueous starting solutions with cesium ion contents in the range of 50 ppm to 5000 ppm, characterized in that the cesium ions in the aqueous solution are, in a first step, precipitated as a double salt having divalent cations with an at least 1.1 times overstoichiometric amount of solution containing prussiate selected from the group consisting of K.sub.4[Fe(CN).sub.6], Na.sub.4[Fe(CN).sub.6], Ca.sub.2[Fe(CN).sub.6] and mixtures thereof, in the pH range of 2 to 12 and in the temperature range of 10 to 80° C., wherein the divalent cations are either already present in the starting solutions in an amount at least equimolar to the cesium ion contents or added as a water-soluble salt, and, in a second step, the cesium ions are converted back into a water-soluble form by thermal decomposition carried out in a calcining step under oxidative conditions at temperatures of 400° C. to 800° C. and, in a third step, the cesium ions are separated from insoluble residues.

2. The method according to claim 1, characterized in that aqueous starting solutions with cesium ion contents in the range of 100 ppm to 1000 ppm are used.

3. The method according to claim 1, characterized in that an overstoichiometric amount of solution containing prussiate in the range of 1.15 to 1.5 times the stoichiometric amount is used.

4. The method according to claim 1, characterized in that, as divalent cations, calcium and/or magnesium ions are obtained in at least equimolar amount or added at least until the equimolar amount is reached.

5. The method according to claim 1, characterized in that the precipitation of the double salt is carried out in a first step in the pH range of 4 to 11.

6. The method according to claim 1, characterized in that the precipitation of the double salt is carried out with the addition of inorganic filtering aids.

7. The method according to claim 1, characterized in that the overstoichiometric amount of prussiate remaining in the starting solution is precipitated by the addition of a water-soluble iron(III) salt in the pH range of 4 to 7 to the already formed double salt.

8. The method according to claim 7, characterized in that iron(III) sulfate is used in an excess of up to 100% by weight with respect to the amount of prussiate remaining in the solution.

9. The method according to claim 1, characterized in that a calcining residue formed in the calcining step is introduced into demineralized water and soluble components are separated from insoluble components.

10. The method according to claim 9, characterized in that the cesium salts contained in the solution are further purified by recrystallization.

11. The method according to claim 2, characterized in that an overstoichiometric amount of solutions containing prussiate in the range of 1.15 to 1.5 times the stoichiometric amount is used.

12. The method according to claim 11, characterized in that, as divalent cations, calcium and/or magnesium ions are obtained in at least equimolar amount or added at least until the equimolar amount is reached.

13. The method according to claim 12 characterized in that, as divalent cations, calcium and/or magnesium ions are obtained in at least equimolar amount or added at least until the equimolar amount is reached.

14. The method according to claim 13, characterized in that the precipitation of the double salt is carried out in a first step in the pH range of 4 to 11.

15. The method according to claim 14, characterized in that the precipitation of the double salt is carried out with the addition of inorganic filtering aids.

16. The method according to claim 15, characterized in that the overstoichiometric amount of prussiate remaining in the starting solution is precipitated by the addition of a water-soluble iron(III) salt in the pH range of 4 to 7 to the already formed double salt.

17. The method according to claim 16, characterized in that iron(III) sulfate is used in an excess of up to 100% by weight with respect to the amount of prussiate remaining in the solution.

18. The method according to claim 17, characterized in that a calcining residue formed in the calcining step is introduced into demineralized water and the soluble components are separated from the insoluble components.

Description

DESCRIPTION

(1) According to the invention, the aim is achieved by a method for extracting cesium from aqueous starting solutions with cesium ion contents in the range of 50 ppm to 5000 ppm, in which method, in a first step, the cesium ions contained in the aqueous solutions are precipitated as a double salt having divalent cations with the aid of an at least 1.1-times overstoichiometric amount of solutions containing prussiate of potash, selected from the group consisting of K.sub.4[Fe(CN).sub.6], Na.sub.4[Fe(CN).sub.6], Ca.sub.2[Fe(CN).sub.6] or mixtures thereof, in a pH range of 2 to 12 and a temperature range of 10 to 80° C., wherein the divalent cations are either already present in the starting solutions in an amount at least equimolar to the cesium content or added as a water-soluble salt at least until reaching the equimolar amount, and, in a second step, they are converted back into a water-soluble form by thermal decomposition and, in a third step, separated from the insoluble residues. The invention is characterized by the use of typical “contaminants” in aqueous solutions such as, for example, magnesium and calcium, in order to precipitate the cesium present, by the addition of yellow prussiate of potash, as a mixture of different sparsely soluble double salts having the exemplary composition Cs.sub.2Mg[Fe(CN).sub.6] and Cs.sub.2Ca[Fe(CN).sub.6], and to remove it by filtration.

(2) Preferably, aqueous starting solutions with cesium ion contents in the range of 100 ppm to 1000 ppm are used.

(3) Particularly preferable is a method in which an overstoichiometric amount of solutions containing prussiate of potash in the range of 1.15- to 1.5-times the stoichiometric amount, which shifts the precipitation equilibrium far toward the product side.

(4) Also preferable is a method in which, as divalent cations, calcium and/or magnesium ions are contained in at least equimolar amount or added at least until the equimolar amount is reached.

(5) In the method, it is particularly preferable that the precipitation of the double salt is carried out in a first step in a pH range of 4 to 11.

(6) The method can advantageously be designed in that the precipitation of the double salt is carried out with addition of inorganic filtering aids such as kieselguhr or diatomaceous earth.

(7) A particularly advantageous variant of the method consists in that the overstoichiometric amount of alkali prussiate of potash salt remaining in the starting solution is precipitated by the addition of a water-soluble iron(III) salt in the pH range of 4 to 7 to the already formed double salt. The applied excess of prussiate of potash is precipitated by addition of iron(III) salts and separated. The Cs.sub.2Mg[Fe(CN).sub.6] crystals already present act as “seed crystals” for the Prussian blue, which as a result can be removed more simply by filtration. Surprisingly, the Prussian blue binds on its surface additional cesium from the solution by adsorption, so that the residual solubility of Cs in the 20 ppm solution (only by precipitation Cs.sub.2Mg[Fe(CN).sub.6] and Cs.sub.2Ca[Fe(CN).sub.6]) can be reduced to approximately 10 ppm. Advantageously, with this step, not only is the necessary excess of yellow prussiate of potash removed from the solution, but at the same time a further and improved Cs enrichment is achieved. This increases the Cs yield in the case of optimal consumption of the precipitation reagent used and thus also makes it possible to economically use water sources with low cesium contents.

(8) The method can be further improved in that iron(III) sulfate is used in an excess of up to 100% by weight with respect to the amount of alkali prussiate of potash remaining in the starting solution.

(9) The method is particularly advantageous since the thermal decomposition in the second step is carried out in a calcining step under oxidative conditions at temperatures of 400° C. to 800° C.

(10) Advantageously, in the method, the calcining residue is introduced into demineralized water, in accordance with the DIN specification, standard DIN 55997 (2006-12), and the soluble components are separated from the insoluble components.

(11) In an advantageous design of the method, the cesium salts contained in the solution are further purified by recrystallization.

(12) The precipitation is advantageously carried out in a reaction vessel without intermediate filtration at room temperature. The reaction is rapid, with a reaction time of approximately 1 hour, and tolerant with respect to other contaminants. The filter residue consists of a mixture of different sparsely soluble Cs salts which contain, with respect to the weight after separation of the mother liquor, approximately 40 to 50% by weight of cesium.

(13) The moist precipitation salt mixture is converted in a calcining step in air at 600° C. into insoluble oxides and soluble Cs compounds. Except for the cesium components and Na/K, all the other elements form water-insoluble hydroxides, oxides or carbonates. The calcining residue is leached with water, and a Cs salt solution is obtained, from which the insoluble components are removed by filtration. By washing or resuspension of the residue in water, the Cs yield can be increased to approximately 90%.

(14) In summary, the present invention has the following advantages:

(15) a) economic recovery of Cs compounds,

(16) b) compliance with environmental waste water limit values by Cs removal for the discharge of waste water into bodies of water,

(17) c) utilization for removing radioactive .sup.137Cs from wastewater and thus reduction of the radiation amount,

(18) d) use of cost-effective precipitants such as K.sub.4[Fe(CN).sub.6], Na.sub.4[Fe(CN).sub.6], Ca.sub.2[Fe(CN).sub.6] or mixtures thereof,

(19) e) very reliable reaction running independently of numerous interfering ions and contaminants, wherein the precipitation occurs rapidly,

(20) f) good filtration properties of the Prussian blue which is otherwise difficult to filter, by epitaxial growth on the Cs.sub.2Ca[Fe(CN).sub.6] crystals already present,

(21) g) simple procedural steps in the form of stirring, precipitation, filtration,

(22) h) optimal use of the precipitation reagent

(23) The invention is explained further below in reference to an embodiment example.

EXAMPLES

Example 1

(24) Precipitation of Cs ferrocyanide from concentrated, natural, chloride containing salt solution pH 4 to 10 (brine with 14% by weight NaCl, 7% by weight CaCl.sub.2), 1% by weight MgCl.sub.2, <1% by weight KCl, <1% by weight SrCl.sub.2)

(25) $\underset{484.07}{2{\mathrm{Na}}_4\left[\mathrm{Fe}\left(\mathrm{CN}\right)6\right]\times 10H_{2}O}+2\mathrm{CaCl2}+4\mathrm{CsCl}.fwdarw.{\mathrm{Cs}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]\underset{1035.69}{{\mathrm{Ca}}_2}[\mathrm{Fe}\left(\mathrm{CN}\right)\alpha +8\mathrm{NaCl}3{\left({\mathrm{Fe}\left(\mathrm{CN}\right)}_6\right)}^{4-}+4{\mathrm{Fe}}^{3+}\stackrel{}{.fwdarw.}{\mathrm{Fe}\left[{\mathrm{FeFe}\left(\mathrm{CN}\right)}_6\right]}_{3}14\text{-}16H_{2}O\downarrow$

(26) TABLE-US-00001 TABLE 1 Precipitation of Cs ferrocyanide and subsequent precipitation of Prussian blue Molecular weight Molarity Weight g/mol mmol g Remarks Salt solution amount — — 15 000   with content of Cs 470 ppm 132.91 53.0    7.05 Ca 2.6% by weight 40.08 9730 390 Mg 0.27% by weight 24.31 1666  40 Addition Na.sub.4[Fe(CN).sub.6] × 10 H.sub.2O 484.07 36.7   17.8 Excess: +38% by weight +10.2 mmol Fe.sub.2(SO.sub.4).sub.3 399.88 11.2 6.0 g (75% Excess: (21% by weight Fe) 22.4 mmol Fe by weight) +120% by weight +12 mmol

(27) Na.sub.4[Fe(CN).sub.6]×10 H.sub.2O is added at room temperature in the form of an aqueous solution or a solid and stirred for 30 minutes. The precipitation occurs spontaneously. Subsequently, Fe.sub.2(SO.sub.4).sub.3 is added in the form of an aqueous solution or a solid and stirred for 30 minutes.

(28) The further precipitation also occurs spontaneously. Subsequently, filtration through a folded paper filter is carried out, and the unwashed residue is dried at 100° C.

(29) Starting solution 15 0008 g with 470 ppm Cs (7.1 g Cs)

(30) Filtrate: 15 0008 g with 20 ppm Cs (0.3 g Cs)

(31) Residue: 25.8 g with 26% by weight Cs (6.78 g Cs, 98% of the theory)

(32) TABLE-US-00002 TABLE 2 Analysis of the filtered leaching solution Cs Fe Ca Mg Na Sr K % by % by % by % by % by % by % by weight weight weight weight weight weight weight Starting solution 0.047 <0.0001 2.6 0.27 5.5 0.15 0.14 Solution after precipitation 0.003 0.0014 2.6 0.27 5.5 0.15 0.13 of Cs ferrocyanide Solution after precipitation 0.002 0.0041 2.6 0.27 5.4 0.15 0.13 of excess ferrocyanide Residue of the two precipitations 26 10.1 5.3 1.9 3.9 0.15 0.25 (unwashed, dried) Final solution of the residue 4.5 <0.0001 0.001 0.0001 0.9 0.005 0.04 of the thermal decomposition

(33) 5.0 g of the residue are heated in a crucible made of Al.sub.2O.sub.3 in the tube furnace at 600 CC, the temperature is maintained for 3 h, and 50 l.sub.n/h of air is passed over it. The waste gas is introduced into a solution of H.sub.2O.sub.2 and NaOH, in order to oxidize poisonous waste gases such as CO, (CN).sub.2 and HCN. Residue: 4.0 g (weight loss: 20% by weight)

(34) Leaching residue: oxides/hydroxides/carbonates of Fe, Ca, Mg, Sr and K.

(35) Table 3 shows the composition of the Cs solution obtained by thermal decomposition of the precipitation residue and leaching of the decomposition residue with at least the amount of demineralized water necessary for complete dissolution.

(36) TABLE-US-00003 TABLE 3 Analysis of the product solution % by weight meq/g Cs.sup.+ 4.5 +/− 0.2 0.34 Na.sup.+ 0.92 0.40 Ca.sup.2+ 0.0013 0.0003 K.sup.+ 0.04 0.01 Total 0.75 OH.sup.− 0.10 CO.sub.3.sup.2− 0 0 Cl.sup.− 0.67 SO.sub.4.sup.2− 0.03 0.006 NO.sub.3.sup.− 0.12 0.02 Total 0.79

(37) The residue of the thermal decomposition is leached here with at least the amount of demineralized water necessary for complete dissolution and is separated by filtration from insoluble components. The aqueous solution contains 1.4 g Cs (100% of the theory).

(38) Composition of the solution: 3.8% by weight CsCl/1.7% by weight CsOH/2.3% by weight NaCl/<0.1% by weight KCl.