Method for producing a composite adsorbent based on ferrocyanide of a transition metal

Abstract

The present invention relates to a method for producing a composite adsorbent of ferrocyanide of a transition metal in a silica gel matrix, the method comprising the following steps: (i) preparing a first aqueous composition comprising at least the mixture of a sodium or potassium ferrocyanide with a sodium silicate solution so as to dissolve the salts; (ii) preparing a second composition comprising the mixture of at least: a salt of a transition metal selected from one or more of the following compounds: nickel, copper, zinc, cobalt and an inorganic acid; (iii) mixing the first aqueous composition with the second composition until a suspension is obtained; (iv) adding an inorganic acid or base such that the suspension obtained at the end of step (iii) has a pH ranging from 4 to 9; (v) demixing the suspension obtained at the end of step (iv) so as to obtain a precipitate in a liquid phase; (vi) separating the precipitate and rinsing and drying the precipitate so as to obtain a solid precipitate, characterised in that the first aqueous composition comprises a soluble salt of rubidium or cesium, and the second composition further comprises a soluble salt of a metal selected from one or more of the following compounds: aluminium, titanium, zirconium.

Description

[0001] Description

[0002] Title of the invention: Process for producing a composite adsorbent based on the ferrocyanide of a transition metal

[0003] Technical field of the invention

[0004] The invention relates to methods for producing inorganic composite adsorbents based on a transition metal ferrocyanide for extracting cesium radioisotopes from solutions, such as high-salt and highly alkaline liquid radioactive waste solutions.

[0005] State of the art

[0006] Among the sorption materials used for the extraction of cesium ions from solutions, poorly soluble ferrocyanides of transition metals are distinguished. These ferrocyanides have a higher selectivity for heavy alkali metal ions, especially for cesium cations. (Nekrasova NA, Milyutin VV, Kaptakov VO, Kozlitin EA Inorganic sorbents for wastewater treatment from radioactive contaminants / / INORGANICS. 2023. N2 1 1; P. 126. DOL 10.3390 / inorganicsl 1030126).

[0007] Inorganic adsorbents based on mixed transition metal ferrocyanides are known and are produced by the precipitation method. For example, the cobalt-potassium ferrocyanide CsTreat® brand is used to remove radioactive cesium from high-grade saline aqueous solutions. (Esko Tusa. Efficiency of Fortum's CsTreat® and SrTreat® in cesium and strontium removal at Fukushima Daiichi NPP, ENC2014, European Nuclear Conference, 11-14 May 2014, Marseille, France).

[0008] Despite the high sorption and selectivity capacities of such adsorbents, they generally have very low mechanical resistance.

[0009] To improve mechanical properties, there are methods that consist of depositing the precipitate of transition metal ferrocyanides on the surface of carriers. Examples of carriers include organic ion exchangers (EP 217143), porous carbon or cellulose (RU 2021009), or silica gels (RU 2320406).

[0010] The disadvantages of these adsorbents are: the proportion of the ferrocyanide component in the composition of the adsorbents is low and does not exceed 10%. This severely limits their capacity to absorb cesium ions and provides poor chemical resistance in alkaline environments with a pH above 10.

[0011] A method for producing an inorganic composite adsorbent based on transition metal ferrocyanide and zirconium hydroxide is known. This method includes successive treatment of spherical gelatinous granules of zirconium hydroxide containing 15-75% by mass of water with an aqueous solution of a transition metal salt such as Mn, Fe, Co, Ni, Cu, Zn and / or Cd, with an aqueous solution of a ferrocyanide salt of Li, Na, K and / or ammonium, rinsing with water and drying. According to this method, the adsorbent produced contains up to 40% by mass of the ferrocyanide component. It has high mechanical and chemical resistance in neutral and weakly alkaline media. (RU 2113024).

[0012] The disadvantages of this production method are the complexity of the technological process and the low sorption capacities of the produced adsorbent in highly alkaline environments.

[0013] Another method for producing a composite inorganic adsorbent is known from the prior art. This method involves mixing an aqueous solution of sodium silicate (water glass) containing Na or K ferrocyanide with an acidic solution of the transition metal salt (nickel), subsequent rinsing, and drying the resulting precipitate. This method allows producing adsorbents containing up to 29% by mass of the ferrocyanide component in the silica gel matrix. The produced adsorbent based on nickel-potassium ferrocyanide in silicic acid gel has satisfactory selectivity for cesium ions and mechanical strength of the granules. (Orechovska, J.; Rajec, P. Sorption of cesium on composite sorbents based on nickel ferrocyanide. J. Radioanal. NucL Chem. 1999, 242, 387-390).

[0014] Although satisfactory, this technical solution is not optimal and has limitations, including a low sorption capacity in very alkaline media (pH>10), which is linked to the low chemical resistance of the ferrocyanide component and the carrier in alkaline media.

[0015] The aim of the present invention is to provide a new method for manufacturing inorganic composite adsorbents based on a ferrocyanide of a transition metal which avoids all, or part, of the disadvantages described above. In particular, with regard to known methods, there is a need in the state of the art to provide a new method for producing a ferrocyanide composite adsorbent having higher cesium sorption characteristics in highly alkaline media having a pH greater than or equal to 12.

[0016] Presentation of the invention

[0017] Surprisingly, the inventors have developed a process for improving the performance of the adsorbent, in particular, for increasing its cesium sorption characteristics, while presenting optimal mechanical resistance in very alkaline environments with a pH greater than 12.

[0018] Thus, the present invention relates to a method for producing a composite adsorbent of ferrocyanide of a transition metal in the matrix of silica gel comprising the following steps:

[0019] (i) the preparation of a first aqueous composition comprising at least the mixture of a sodium or potassium ferrocyanide with a sodium silicate solution so as to obtain the dissolution of salts;

[0020] (ii) preparing a second composition comprising the mixture of at least: a transition metal salt selected from one or more of the following compounds: nickel, copper, zinc, cobalt, and an inorganic acid;

[0021] (iii) mixing the first aqueous composition with the second composition until a suspension is obtained;

[0022] (iv) adding an inorganic acid or base so that the suspension obtained at the end of step (iii) has a pH ranging from 4 to 9;

[0023] (v) demixing the suspension obtained at the end of step (iv) so as to obtain a precipitate within a liquid phase;

[0024] (vi) separating said precipitate, rinsing and drying the precipitate, so as to obtain a solid precipitate, characterized in that the first aqueous composition comprises a soluble salt of rubidium or cesium and the second composition further comprises a soluble salt of metal chosen from one or more of the following compounds: aluminum, titanium, zirconium.

[0025] Due to its characteristics, the inventors have developed a process for producing a composite adsorbent based on ferrocyanide in the silica gel matrix having new chemical compositions. Indeed, the inventors discovered that the introduction of specific additional compounds into the initial compositions by modifying the order of the steps of the process according to the prior art unexpectedly allowed to improve the performance of the targeted product with respect to cesium and with respect to its mechanical resistance. Indeed, the additional introduction of the rubidium or cesium salt into the first aqueous composition contributes to the formation of the less soluble phase of the ferrocyanide mixed with the transition metal and rubidium / cesium.

[0026] Also, the additional introduction into the second composition of a solution of aluminum, titanium and / or zirconium salts leads to the formation in the content of the adsorbent of phases of aluminosilicates, titanosilicates or zirconium silicates more resistant in alkaline media.

[0027] Other non-limiting and advantageous characteristics of the method according to the invention, taken individually or in all technically possible combinations, are listed below.

[0028] Preferably, in step (i), the proportion of rubidium or cesium salt relative to sodium or potassium ferrocyanide is from 0.5 to 2.0 molar. According to the invention, a value "from 0.5 to 2.0" includes the following values ​​and all intervals between these values: 0.5; 0.6; 0.7; 0.8; 0.9; 1.0; 1.1; 1.2; 1.3; 1.4; 1.5; 1.6; 1.7; 1.8; 1.9; 2.0.

[0029] In particular, during step (ii) and / or step (iv), the inorganic acid may be chosen from one or more of the following acids: sulfuric acid, hydrochloric acid, nitric acid; the inorganic base may be sodium hydroxide.

[0030] The second composition comprises a proportion of 0.1 to 1.0 molar of the soluble salt of metal chosen from one or more of the following compounds: aluminum, titanium, zirconium relative to the salt of the transition metal chosen from one or more of the following compounds: nickel, copper, zinc, cobalt also present in the second composition.

[0031] Advantageously, the solid precipitate obtained at the end of step (vi) is ground, then sieved in order to recover a fraction having an average particle diameter ranging from 0.25 to 1.0 mm.

[0032] The molar proportions used of the rubidium or cesium salt relative to the sodium or potassium ferrocyanide of the first composition are preferably equal to 0.5-2.0. The inventors have discovered that outside this range, the efficiency of cesium sorption in alkaline media was not optimal (i.e.: decrease in efficiency).

[0033] The molar proportion of the soluble metal salt chosen from one or more of the following compounds: aluminum, titanium, zirconium relative to the transition metal is preferably in the range 0.1 -1.0. The inventors have discovered that outside this range, the mechanical resistance of the solid precipitate obtained is not optimal (i.e.: below 0.1, a decrease in the mechanical resistance of the granules formed from the solid precipitate in alkaline media is observed, while above, no improvement in the stability of the granules is observed for a higher cost price of the adsorbent).

[0034] In particular, soluble salts of rubidium, cesium, nickel, copper, zinc, cobalt, aluminium, titanium or zirconium can be used and are all salts accessible and present on the market in the form of sulphates, chlorides, nitrates, acetates etc.

[0035] In the present invention, unless otherwise specified, the term "comprising" and its derivatives should be understood as non-limiting and not excluding the presence of other components or steps. In certain particular embodiments, the term "comprising" may be understood as "consisting essentially of" or "consisting of".

[0036] Unless otherwise specified, the intervals mentioned in the present invention are understood to be inclusive.

[0037] Examples

[0038] The following examples provide a better understanding of the present invention, without limiting its scope.

[0039] Example 1 is a comparative example which illustrates the synthesis of the adsorbent according to the prior art method. Examples 5-6 confirm that the new method according to the invention is applicable for industrial use. In addition, the test results of the targeted products produced according to Examples 1-6 are mentioned, these results were obtained when using the targeted products for the sorption of cesium radionuclides from the highly alkaline saline solution. The test results are mentioned in the table.

[0040] Example 1 (according to the prior art)

[0041] In a glass beaker of volume 1 liter are poured 600 ml of water, then are added 7.6 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution with the concentration 27% by mass of SiO2. The mixture is stirred until the salt is completely dissolved. In another beaker, 7.05 g of nickel sulfate heptahydrate are dissolved in 250 ml of a 0.01 M sulfuric acid solution. The two solutions are mixed and stirred for 30 minutes until a suspension is formed. A 10% sulfuric acid solution is added until the pH of the mixture is equal to 5.0. The mixture is kept without stirring for 24 h and the liquid part is removed, while the concentrated part is filtered through a paper filter under vacuum. The precipitate on the filter is rinsed with distilled water, then returned to the oven and dried for 12 hours at 80°C. The dry product produced is ground using the mortar.The powder is then sieved and the particles of size between 0.25-1.0mm are sorted. 12.6g of the granulated ferrocyanide composite adsorbent is produced, containing 38.8% by mass of nickel-potassium mixed ferrocyanide. The remainder is hydrated silicon dioxide and water.

[0042] Example 2 (method according to the invention)

[0043] In a 1 liter glass beaker, 600 ml of water are poured, then 7.6 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution at 27% by mass of SiO2 and 5.31 g of rubidium nitrate are added. The mixture is stirred until the salts are completely dissolved. The molar ratio is Rb / K4Fe(CN)6 = 2.0.

[0044] In another beaker 7.05g of nickel sulfate heptahydrate and 0.94g of aluminum nitrate nonahydrate are dissolved in 250ml of 0.01M sulfuric acid solution. The molar ratio is Al / Ni = 0.1. The two solutions are mixed and stirred for 30 minutes until a suspension forms.

[0045] A 10% sulfuric acid solution is added until the mixture reaches a pH of 5.0.

[0046] Stirring is stopped and the suspension is kept until it is demixed.

[0047] The liquid layer is removed, the concentrated part is filtered through a paper filter under vacuum and then the precipitate on the filter is rinsed with distilled water. The wet precipitate on the filter is returned to the oven and dried for 12 hours at 80°C. The dry product produced is ground, and the fraction with particle sizes 0.25-1.0 mm is sorted.

[0048] 13.2 g of granulated ferrocyanide composite adsorbent were produced. The chemical and phase analysis carried out showed the presence of nickel-potassium and rubidium in the amount of 38.1% by mass in the mixed ferrocyanide adsorbent. The remaining adsorbent content consists of the phases in the form of hydrated silicon dioxide, hydrated aluminum oxide and water. Example 3 (method according to the invention)

[0049] In a 1 liter glass beaker, 600 ml of water are poured, then 8.71 g of sodium ferrocyanide decahydrate, 30 g of sodium silicate solution at 27% by mass of SiO2 and 1.51 g of cesium chloride are added. The mixture is stirred until the salts are completely dissolved. The molar proportion is Cs / K4Fe(CN)e = 0.5.

[0050] In another beaker, 3.41 g of zinc sulfate heptahydrate and 4.74 g of titanium (IV) chloride are dissolved in 250 ml of a 6 M hydrochloric acid solution. The molar ratio is Ti / Zn = 1.0.

[0051] The two solutions are mixed and stirred for 30 minutes until a suspension forms.

[0052] A 15% sodium hydroxide solution is added until a pH of 4.0 is obtained. The rest of the process is identical to Example 2.

[0053] 14.1g of granulated ferrocyanide composite adsorbent is produced, containing 33.7% by mass of mixed zinc-sodium and cesium ferrocyanide. The remainder consists of hydrated silicon dioxide, hydrated titanium dioxide and water.

[0054] Example 4 (method according to the invention)

[0055] In a glass beaker with a volume of 1 liter are poured 600 ml of water, then are added 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution at 27% by mass of SiO2 and 1 .09 g of rubidium chloride. The mixture is stirred until the salts are completely dissolved. The molar proportion is Rb / K4Fe(CN)6 = 0.5.

[0056] In another beaker, 7.27g of nickel nitrate hexahydrate and 1.74g of zirconium (IV) nitrate pentahydrate are dissolved in 250ml of a 1M nitric acid solution. The molar ratio is Zr / Ni = 0.1. The two solutions are mixed and stirred for 30 minutes until a suspension forms.

[0057] A 10% sodium hydroxide solution is added until a pH of 9.0 is obtained. The rest of the process is identical to Example 2. 13.8 g of granulated ferrocyanide composite adsorbent is produced containing 36.1% by mass of mixed nickel-potassium and rubidium ferrocyanide. The remainder consists of hydrated silicon dioxide, hydrated zirconium dioxide and water.

[0058] Example 5 (method according to the invention)

[0059] In a glass beaker of volume 1 liter are poured 600 ml of water, then are added 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution with the concentration 27% by mass SiO2 and 7.02 g of cesium nitrate. The mixture is stirred until the salts are completely dissolved. The molar ratio is Cs / K4Fe(CN)6 = 2.0.

[0060] In another beaker are dissolved 5.95g of cobalt chloride hexahydrate and 3.33g of anhydrous aluminum chloride in 250ml of 0.1M hydrochloric acid solution. The molar ratio is Al / Co = 1.0.

[0061] The two solutions are mixed and stirred for 30 minutes until a suspension forms.

[0062] A 10% hydrochloric acid solution is added until the mixture pH is 4.0.

[0063] The rest of the process is identical to Example 2. 13.6 g of granulated ferrocyanide composite adsorbent is produced, containing 33.4% by mass of mixed cobalt-potassium and cesium ferrocyanide. The remainder is hydrated silicon dioxide, hydrated aluminum oxide, and water.

[0064] Example 6 (method according to the invention)

[0065] In a glass beaker of volume 1 liter are poured 600 ml of water, then are added 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution with the concentration of 27% by mass of SiO2 and 1 .09 g of rubidium chloride. The mixture is stirred until the salts are completely dissolved. The molar proportion is Rb / K4Fe(CN)6 = 0.5.

[0066] In another beaker are dissolved 3.00 g of cobalt chloride hexahydrate, 2.95 g of nickel chloride hexahydrate and 7.50 g of aluminum nitrate nonahydrate and 2.14 g of zirconium (IV) nitrate pentahydrate in 250 ml of a 0.5 M nitric acid solution. The molar ratio is AI / (Co + Ni) = 1.0.

[0067] The two solutions are mixed and stirred for 30 minutes until a suspension is formed. A 10% nitric acid solution is added until the mixture reaches a pH of 4.0.

[0068] The rest of the process is identical to the example 213.8g of granulated ferrocyanide composite adsorbent is produced, containing 34.3% by mass of mixed cobalt-nickel-potassium and rubidium ferrocyanide. The remainder is hydrated silicon dioxide, hydrated aluminum oxide, hydrated zirconium dioxide and water.

[0069] Examples 2-6 are presented to illustrate the best variants of the embodiment of the present invention, without volume limit. The tests confirmed the obtaining of improved cesium sorption rates in alkaline medium compared to Example 1 obtained according to the method of the prior art (see the test below).

[0070] Example 7: Testing of adsorbents

[0071] Sorption of microquantities of the radionuclide 137Cs from the solution with the composition 0.5 mole / liter NaOH +3.9 mole / liter NaNO3, pH=13.7 is carried out with the samples of the adsorbents produced according to the present invention (examples 2-6) and according to the literature method (example 1). The mark 137 Cs is introduced into the initial solution until the specific activity of 3.2x105 Bq / liter is obtained. To carry out the sorption, 0.1000g of dry adsorbent is mixed with 20 cm3 of solution for 24 hours. Then the mixture is filtered through a paper filter, and the specific activity of 137 Cs in the filtered product is estimated. According to the results of the analyses, the value of the distribution coefficient (Kd) 137 Cs is calculated according to the formula: where, Ao, Ap are the specific activity of the 137 Cs respectively in the initial solution and in the filtered product, expressed in Bq / litre;

[0072] Vp is the volume of the liquid phase expressed in ml; mc is the mass of adsorbent expressed in g;

[0073] The produced values ​​of Kd from the 137 These are mentioned in Table 1.

[0074] Table 1 As seen from the tests carried out, the adsorbents produced according to the method of the present invention have much higher sorption characteristics compared to the comparative method (example 1) when sorbing cesium in alkaline medium. Furthermore, it is established that upon contact with the alkaline solution, with the pH in the range of 12 to 14, for 24 hours, the granules of the adsorbent produced according to the method of the present invention completely retain their shape and mechanical properties, while the granules produced according to the literature method are almost completely destroyed. The technical feasibility of the method according to this invention has been well validated. Thus the reproducibility of the production process.

Claims

Claims

1. A method for producing a composite adsorbent of transition metal ferrocyanide in silica gel matrix comprising the following steps: (i) the preparation of a first aqueous composition comprising at least the mixture of a sodium or potassium ferrocyanide with a sodium silicate solution so as to obtain the dissolution of salts; (ii) preparing a second composition comprising the mixture of at least: a transition metal salt selected from one or more of the following compounds: nickel, copper, zinc, cobalt, and an inorganic acid; (iii) mixing the first aqueous composition with the second composition until a suspension is obtained; (iv) adding an inorganic acid or base so that the suspension obtained at the end of step (iii) has a pH ranging from 4 to 9; (v) demixing the suspension obtained at the end of step (iv) so as to obtain a precipitate within a liquid phase; (vi) separating said precipitate, rinsing and drying the precipitate, so as to obtain a solid precipitate, characterized in that the first aqueous composition comprises a soluble salt of rubidium or cesium, and the second composition further comprises a soluble salt of metal chosen from one or more of the following compounds: aluminum, titanium, zirconium.

2. A method according to claim 1, wherein the solid precipitate obtained at the end of step (vi) is ground, then sieved in order to recover a fraction having an average particle diameter ranging from 0.25 to 1.0 mm.

3. A method according to any one of claims 1 or 2, wherein in step (i) the proportion of rubidium or cesium salt relative to sodium or potassium ferrocyanide ranges from 0.5 to 2.0 molar.

4. A method according to any one of claims 1 to 3 wherein in step (ii), in the second composition, the molar proportion of the soluble metal salt chosen from one or more of the following compounds: aluminum, titanium, zirconium, relative to the transition metal salt selected from one or more of the following compounds: nickel, copper, zinc, cobalt, is in the range 0.1 -1.0 molar.

5. Process according to any one of claims 1 to 4, wherein during step (ii) and / or step (iv), the inorganic acid or base is chosen from one or more of the following products: sulfuric acid, hydrochloric acid, nitric acid, sodium hydroxide.

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