Process for producing a composite adsorbent based on the ferrocyanide of a transition metal

Abstract

The present invention relates to a process for producing a composite ferrocyanide adsorbent of a transition metal in the silica gel matrix comprising the following steps: (i) the preparation of a first aqueous composition comprising at least a mixture of a sodium or potassium ferrocyanide with a sodium silicate solution so as to obtain the dissolution of salts; (ii) the preparation of a second composition comprising a 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) the mixing of the first aqueous composition with the second composition until a suspension is obtained; (iv) the addition of 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) the separation of the suspension obtained at the end of step (iv) so as to obtain a precipitate within a liquid phase; (vi) the separation of said precipitate, the rinsing and drying of 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 selected from one or more of the following compounds: aluminium, titanium, zirconium.;

Description

Title of the invention: Process for producing a composite adsorbent based on the ferrocyanide of a transition metal. Technical field of the invention

[0001] The invention relates to the processes for producing inorganic composite adsorbents based on a ferrocyanide of a transition metal, intended to extract cesium radioisotopes from solutions, such as solutions of highly alkaline saline liquid radioactive waste. State of the art

[0002] Among the sorption materials used to extract cesium ions from solutions, the poorly soluble ferrocyanides of transition metals are distinguished. These ferrocyanides have a higher selectivity towards heavy alkali metal ions, in particular towards 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 / inorganics 11030126).

[0003] The inorganic adsorbents on the basis of mixed transition metal ferrocyanides are known and are produced by the precipitation method. For example, the cobalt-potassium ferrocyanide of the brand CsTreat® is used to remove radioactive cesium from high-grade aqueous saline solutions. (Esko Tusa. Efficiency of Fortum's CsTreat® and SrTreat® in cesium and strontium removal in Fukushima Daiichi NPP, ENC2014, European Nuclear Conference, 11-14 May 2014, Marseille, France).

[0004] Despite the high sorption and selectivity capacities of such adsorbents, they generally exhibit very low mechanical resistance.

[0005] In order to improve mechanical properties, there are processes which consist of depositing the precipitate of transition metal ferrocyanides onto the surface of carriers. Examples of carriers include organic ion exchangers (EP 217143), porous carbon or cellulose (RU 2021009), or silica gels (RU 2320406).

[0006] The drawbacks of these adsorbents mentioned are: the proportion of the ferrocyanide component in the composition of the adsorbents is low and does not exceed 10%. This severely limits their cesium ion sorption capacity and provides poor chemical resistance in alkaline media with a pH greater than 10.

[0007] A process for producing the inorganic composite adsorbent based on transition metal ferrocyanide and zirconium hydroxide is known. This process includes a The process involves the successive treatment of spherical, gelatinous zirconium hydroxide granules containing 15-75% water by mass with aqueous solutions of transition metal salts such as Mn, Fe, Co, Ni, Cu, Zn, and / or Cd, followed by aqueous solutions of ferrocyanide salts of Li, Na, K, and / or ammonium, rinsing with water, and drying. According to this process, the resulting adsorbent contains up to 40% ferrocyanide by mass. It exhibits high mechanical and chemical resistance in neutral and slightly alkaline environments. (RU 2113024)

[0008] The drawbacks of this production process are the complexity of the technological process and the low sorption capacities of the adsorbent produced in highly alkaline media.

[0009] Another method for producing a composite inorganic adsorbent is known in the prior art. This method consists of mixing an aqueous solution of sodium silicate (soluble glass) containing a ferrocyanide of Na or K with an acidic solution of the transition metal salt (nickel), subsequently rinsing, and drying the resulting precipitate. This method makes it possible to produce adsorbents containing up to 29% by mass of the ferrocyanide component in the silica gel matrix. The adsorbent produced 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).

[0010] 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.

[0011] The object 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.

[0012] In particular, with regard to known processes, there is a need in the prior art to provide a new process for the production of a composite ferrocyanide adsorbent having higher cesium sorption characteristics in highly alkaline media having a pH greater than or equal to 12. Presentation of the invention

[0013] Surprisingly, the inventors have developed a process to improve the performance of the adsorbent, in particular, to increase its cesium sorption characteristics, while exhibiting optimal mechanical resistance in highly alkaline media with a pH greater than 12.

[0014] Thus, the present invention relates to a method for producing an adsorbent a ferrocyanide composite of a transition metal in the silica gel matrix comprising the following steps:

[0015] (i) the preparation of a first aqueous composition comprising at least the mixing a sodium or potassium ferrocyanide with a sodium silicate solution so as to obtain the dissolution of salts;

[0016] (ii) the preparation of 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;

[0017] (iii) mixing the first aqueous composition with the second composition until a suspension is obtained;

[0018] (iv) the addition of an inorganic acid or base so that the suspension obtained at the end of step (iii) has a pH ranging from 4 to 9;

[0019] (v) the separation of the suspension obtained at the end of step (iv) so as to obtain a precipitate within a liquid phase;

[0020] (vi) separating said precipitate, rinsing and drying the precipitate, so as to obtain a solid precipitate,

[0021] 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 selected from one or more of the following compounds: aluminium, titanium, zirconium.

[0022] By virtue of its characteristics, the inventors have developed a process for producing a composite adsorbent based on ferrocyanide in the silicagel matrix having new chemical compositions.

[0023] Indeed, the inventors discovered that introducing specific additional compounds into the initial compositions by modifying the order of the prior art process steps unexpectedly improved the performance of the target product with respect to cesium and its mechanical strength. Specifically, the additional introduction of rubidium or cesium salt into the initial aqueous composition contributes to the formation of the less soluble phase of the ferrocyanide mixed with the transition metal and rubidium / cesium.

[0024] Also, the additional introduction in the second composition of a solution of aluminium, titanium and / or zirconium salts leads to the formation in the content of the adsorbent of the phases of aluminosilicates, titanosilicates or zirconium silicates which are more resistant in alkaline media.

[0025] Other non-limiting and advantageous features of the process according to the invention, taken individually or according to all technically possible combinations, are listed below.

[0026] Preferably, during step (i), the proportion of rubidium or cesium salt by the ratio to sodium or potassium ferrocyanide is 0.5 to 2.0 molar.

[0027] 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.

[0028] 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.

[0029] The second composition comprises a proportion of 0.1 to 1.0 molar of the soluble metal salt chosen from one or more of the following compounds: aluminium, titanium, zirconium relative to the transition metal salt chosen from one or more of the following compounds: nickel, copper, zinc, cobalt also present in the second composition.

[0030] Advantageously, the solid precipitate obtained at the end of step (vi) is ground and then sieved to recover a fraction having an average particle diameter ranging from 0.25 to 1.0 mm.

[0031] 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.: decreased efficiency).

[0032] The molar proportion of the soluble metal salt chosen from one or more of the following compounds: aluminium, titanium, zirconium relative to the transition metal is preferentially in the range 0.1-1.0. The inventors discovered that outside this range, the mechanical resistance of the resulting solid precipitate was not optimal (i.e., below 0.1, there is a decrease in the mechanical resistance of the granules formed from the solid precipitate in alkaline media, while above, there is no improvement in the stability of the granules for a higher cost price of the adsorbent).

[0033] In particular, soluble salts of rubidium, cesium, nickel, copper, zinc, cobalt, aluminum, titanium or zirconium can be used and are all accessible salts available on the market in the form of sulfates, chlorides, nitrates, acetates etc.

[0034] In the present invention, unless otherwise specified, the term "include" and its derivatives shall be understood as non-limiting and not excluding the presence of other components or steps. In certain particular embodiments, the term "include" may be understood as "consisting essentially of" or "being made up of".

[0035] Unless otherwise specified, the intervals mentioned in the present invention are understood to include the limits. Examples

[0036] The following examples allow for a better understanding of the present invention, without limiting its scope.

[0037] Example 1 is a comparative example illustrating the synthesis of the adsorbent according to the prior art process. Examples 5-6 confirm that the new process according to the invention is applicable for industrial use. Furthermore, the results of tests on 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 highly alkaline saline solution. The test results are shown in the table. Example 1 (according to prior art)

[0038] In a 1-liter glass beaker, 600 mL of water are poured, followed by 7.6 g of potassium ferrocyanide trihydrate and 30 g of sodium silicate solution with a concentration of 27% SiO2 by mass. 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 forms. A 10% sulfuric acid solution is added until the mixture reaches a pH of 5.0. The mixture is left undisturbed for 24 hours, and the liquid portion is discarded, while the concentrated portion 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 resulting dry product is ground using a mortar.The powder is then sieved and particles between 0.25 and 1.0 mm in size are sorted. 12.6 g of the granulated ferrocyanide composite adsorbent are produced, containing 38.8% by mass of mixed nickel-potassium ferrocyanide. The remainder is hydrated silicon dioxide and water. Example 2 (method according to the invention)

[0039] 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 (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.

[0040] In another beaker, 7.05 g of nickel sulfate heptahydrate and 0.94 g of aluminium nitrate nonahydrate are dissolved in 250 ml of 0.01 M sulfuric acid solution. The molar ratio is Al / Ni = 0.1. The two solutions are mixed and stirred for 30 minutes until a suspension is formed.

[0041] A 10% sulfuric acid solution is added until a pH of the mixture equals 5.0 is obtained.

[0042] The agitation is stopped and the suspension is kept until it is demixed.

[0043] The liquid layer is removed, the concentrated portion 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 resulting dry product is ground, and the fraction with particle sizes of 0.25–1.0 mm is sorted.

[0044] 13.2g of granulated ferrocyanide composite adsorbent are produced.

[0045] Chemical and phase analysis showed the presence of nickel-potassium and rubidium in an amount of 38.1% by mass in the mixed ferrocyanide adsorbent. The remaining adsorbent content consists of the phases hydrated silicon dioxide, hydrated aluminum oxide, and water. Example 3 (method according to the invention)

[0046] In a glass beaker with a volume of 1 liter, 600 ml of water are poured, then 8.71 g of sodium ferrocyanide decahydrate, 30 g of sodium silicate solution with a mass concentration of 27% SiO2, and 1.51 g of cesium chloride are added. The mixture is stirred until the salts are completely dissolved. The molar ratio is Cs / K4Fe(CN)6 = 0.5.

[0047] 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.

[0048] The two solutions are mixed and stirred for 30 minutes until a suspension is formed.

[0049] 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.

[0050] 14.1g of granulated ferrocyanide composite adsorbent are 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. Example 4 (method according to the invention)

[0051] In a 1-liter glass beaker, 600 ml of water are poured, followed by 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution containing 27% by mass of SiO2, and 1.09 g of rubidium chloride. The mixture is stirred until the salts are completely dissolved. The molar ratio is Rb / K4Fe(CN)6 = 0.5.

[0052] In another beaker, 7.27 g of nickel nitrate hexahydrate and 1.74 g of zirconium(IV) nitrate pentahydrate are dissolved in 250 ml of a nitric acid IM solution. The molar ratio is Zr / Ni = 0.1. The two solutions are mixed and stirred for 30 minutes until a suspension is formed.

[0053] A 10% sodium hydroxide solution is added until a pH is obtained equal to 9.0.

[0054] The remainder of the process is identical to Example 2. 13.8 g of granulated iron-rocyanide composite adsorbent are 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. Example 5 (method according to the invention)

[0055] In a glass beaker with a volume of 1 liter, 600 ml of water are poured, then 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution with a concentration of 27% by mass SiO2, and 7.02 g of cesium nitrate are added. The mixture is stirred until the salts are completely dissolved. The molar ratio is Cs / K4Fe(CN)6 = 2.0.

[0056] In another beaker, 5.95g of cobalt chloride hexahydrate and 3.33g of anhydrous aluminium chloride are dissolved in 250ml of 0.1 M hydrochloric acid solution. The molar ratio is Al / Co = 1.0.

[0057] The two solutions are mixed and stirred for 30 minutes until a suspension is formed.

[0058] A 10% hydrochloric acid solution is added until the pH of the mixture is equal to 4.0.

[0059] The remainder of the process is identical to Example 2. 13.6 g of granulated ferrocyanide composite adsorbent are produced, containing 33.4% by mass of mixed cobalt-potassium and cesium ferrocyanide. The remainder is hydrated silicon dioxide, hydrated aluminum oxide, and water. Example 6 (method according to the invention)

[0060] In a glass beaker with a volume of 1 liter, 600 ml of water are poured, then 7.20 g of potassium ferrocyanide trihydrate, 30 g of sodium silicate solution with a concentration of 27% by mass of SiO2, and 1.09 g of rubidium chloride are added. The mixture is stirred until the salts are completely dissolved. The molar ratio is Rb / K4Fe(CN)6 = 0.5.

[0061] In another beaker, 3.00 g of cobalt chloride hexahydrate, 2.95 g of nickel chloride hexahydrate, 7.50 g of aluminium nitrate nonahydrate, and 2.14 g of zirconium(IV) nitrate pentahydrate are dissolved in 250 ml of a 0.5 M nitric acid solution. The molar ratio is Al / (Co+Ni) = 1.0.

[0062] The two solutions are mixed and stirred for 30 minutes until the suspension is formed.

[0063] A 10% nitric acid solution is added until a pH of the mixture equals 4.0 is obtained.

[0064] The remainder of the process is identical to the example 213.8g of iron- composite adsorbent Granulated rocyanide 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.

[0065] Examples 2-6 are presented to illustrate the best embodiments of the present invention, without volume limitation. The tests confirmed the attainment of improved cesium sorption rates in an alkaline medium compared to Example 1 obtained according to the prior art process (see the test below). Example 7: Adsorbent Tests

[0066] The sorption of microquantities of the radionuclide 137Cs from a solution with the composition 0.5 mol / L NaOH + 3.9 mol / L NaNO3, pH=13.7, is carried out using samples of adsorbents produced according to the present invention (Examples 2-6) and according to the method described in the literature (Example 1). The 137Cs marker is introduced into the initial solution until a mass activity of 3.2 x 10⁵ Bq / L is obtained. To perform the sorption, 0.1000 g of dry adsorbent is mixed with 20 cm³ of solution for 24 hours. The mixture is then filtered through a paper filter, and the mass activity of 137Cs in the filtered product is estimated. Based on the results of the analyses, the value of the distribution coefficient (Kd) of 137Cs is calculated according to the formula:

[0067] where, Ao, Ap are the mass activity of 137Cs respectively in the initial solution and in the filtered product, expressed in Bq / litre;

[0068] Vp is the volume of the liquid phase expressed in ml;

[0069] mc is the mass of adsorbent expressed in g;

[0070] The Kd values ​​produced for 137Cs are mentioned in Table 1.

[0071] [Tables 1] Example No. (Previous Art) 2 3 4 5 6 Kd137Cs, ml / g 80 4240 5120 5370 3460 3150

[0072] As can be seen from the tests carried out, the adsorbents produced according to the process of the present invention have much higher sorption characteristics compared to the comparative process (example 1) when sorbing cesium in an alkaline medium.

[0073] Furthermore, it is established that upon contact with an alkaline solution, with a pH in the range of 12 to 14, for 24 hours, the adsorbent granules produced according to the process of the present invention completely retain their shape and properties mechanical, while the granules produced according to the process-literature are almost completely destroyed.

[0074] The technical feasibility of the process according to this invention has been validated. Thus, the reproducibility of the production process has also been confirmed.

Claims

Demands

1. A process for producing a composite ferrocyanide adsorbent of a transition metal in the silica gel matrix comprising the following steps: (i) preparing a first aqueous composition comprising at least a mixture of a sodium or potassium ferrocyanide with a sodium silicate solution to obtain the dissolution of salts; (ii) preparing a second composition comprising a 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) to obtain a precipitate within a liquid phase;(vi) the separation of said precipitate, rinsing and drying of 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 selected from one or more of the following compounds: aluminium, titanium, zirconium.;

2. A process according to claim 1, wherein the solid precipitate obtained at the end of step (vi) is ground and then sieved to recover a fraction preferably having an average particle diameter of 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 varies 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 selected from one or more of the following compounds: aluminum, titanium, zirconium, relative to the metal salt of the chosen transition from one or more of the following compounds: nickel, copper, zinc, cobalt, lies in the 0.1-1.0 molar range.

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