GB2112764A

GB2112764A - Inorganic ion-exchangers based on titanium compounds

Info

Publication number: GB2112764A
Application number: GB08235238A
Authority: GB
Inventors: Panayotis Gerontopoulos; Silvio Cao; Sergio Gallone
Original assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Agip Nucleare SpA; Comitato Nazionale per la Ricerca e per lo Sviluppo dell Energia
Current assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Agip Nucleare SpA
Priority date: 1981-12-11
Filing date: 1982-12-10
Publication date: 1983-07-27
Also published as: IT1211145B; GB2112764B; IT8125516A0; DE3245811C2; FR2522533A1; DE3245811A1; FR2522533B1; BE895326A

Abstract

This invention relates to a process for preparing inorganic ion-exchangers based on titanium compounds, consisting in preparing an alcoholic solution of an organometallic compound of titanium, optionally evaporating the solution, gelling the solution into gel particles or membranes by neutralization with an alkali, and optionally calcining the particles or membranes to provide bodies having ion-exchange activity and mechanical robustness. The procedure permits one to modify the chemical composition of the exchanger by introducing chemical elements into the alcoholic solution used at the start and/or by absorption by ion-exchange on gel of titanates or hydrous titanium oxides. Exemplified is the use of such exchangers in applications of nuclear interest such as for example the separation and immobilization of toxic nuclides contained in liquid radioactive wastes.

Description

SPECIFICATION Inorganic ion-exchangers based on titanium compounds This invention relates to a process for preparing inorganic ion-exchangers based on titanium compounds, and their uses.

Processes for preparing inorganic ion-exchangers based on titanium compounds are known in the art. One of these processes is the SANDIA process, which permits one to prepare primary particles having a size of 50 to 200 Angstrom units or aggregates thereof, which particles are used for absorbing toxic radionuclides contained in highly active liquid wastes, the particles being subsequently consolidated to form a ceramic material which is adapted for the long-term storage of the readionuclides (Lynch et al, The Sandia Solidification Process - A Broad Range Aqueous Waste Solidification Method, Management of Radioactive Waste from the Nuclear Fuel Cycle, IAEA, Vienna, 1976, page 361).

Processes for separating and/or conditioning radionuclides by absorption on agglomerates of fine powders of sodium titanate have also been described in U.S. Patent No. 4156646 and in French Patent Application No. 77/32219.

It should be observed that for application in the stages of a fuel cycle, the mechanical and physicochemical properties of the exchange material are of paramount importance for a satisfactory and economically acceptable run of the process as a whole.

The conventional processes suffer from the drawback that they result in the formation of fine particles and powders. This is highly undesirable due to the hazard of rapid diffusion of radioactivity into the interior and/or towards the outside of the plants and the impossibility of carrying out the operations of absorption-elution on fixed-bed exchange columns which can be easily placed in a remote location, due to the high pressure drops involved.

According to the present invention, there is provided a process for the preparation of an inorganic ion-exchange body based on titanium and optionally one or more other elements, which process comprises (a) preparing an alcoholic solution, having a comparatively high viscosity, of an organometallic compound of titanium, which solution optionally contains, in addition to the titanium compound, at least one compound of at least one other element; (b) optionally thickening the solution by adding an organic polymer thereto; (c) gelling the solution by alkaline neutralization in an anhydrous environment or an environment having a controlled water content, thereby to obtain an ion-exchange gel based on titanium and co-precipitation of the other element(s), in any desired geometrical configuration; (d) optionally absorbing a chemical element of interest by the ion-exchanging gel, by contacting the gel with a solution which contain said element; and (e) optionally drying and/or calcining the gel.

The process of the present invention enables the above limitations to be overcome by the fact that the process gives microspheres (or membranes) for ion-exchange, having comparatively large diameters (or thicknesses) and optimum properties as to mechanical sturdiness, ability and speed of ion-exchange, these properties being retained unaltered even after stabilization heat-treatment at temperatures in the range from 400"C to 600 C.

The steps of the process according to the invention will now be described in more detail.

The step of preparing an alcoholic solution of an organometallic compound of titanium is preferably effected by reacting a titanium halide, more particularly titanium tetrachloride, with an alcohol, preferably tetrahydrofurfuryl alcohol, the reaction, which takes place with a considerable heat build-up, resulting in the formation of a syrupy liquor which contains titanium in a form which withstands precipitation by hydrolysis in the presence of water, and presumably a polymerized ester of the general type Ti(OR)4 as the considerable increase in the viscosity of the solution, as obtained by mixing the reactants, would suggest.The viscosity of such an alcoholic solution can be further enhanced by the addition of an organic thickener consisting of an organic polymer which, preferably, is selected from cellulose derivatives and vinyl compounds and, more particularly, is hydroxypropyl cellulose or polyvinyl alcohol.

The solution prepared as above is gelled into the desired geometrical shape by alkaline hydrolysis in an anhydrous environment or an environment the water content of which is under control, by means of, for example, an alcoholic solution of an alkali. The final gel product can be produced in the form of spherical particles having a high mechanical sturdiness and a controlled diameter which can be varied, depending upon the intended uses, within a range of from a few tens of microns to 4 mm. As an alternative, the material concerned can be produced in the form of composite membranes by impregnating thin inert carriers with the solution and gelling the whole assembly in an alkaline medium.It is possible, in both instances, to give the matrix of the exchange material an evenly distributed porosity with controlled diameters, by dispersing a gas in the starting solution and subsequently effecting quick gelling of the emulsion thus obtained.

The chemical composition of the particles or membranes prepared as outlined above can be modified by the absorption of ions which are, for example, capable of modifying the exchange properties of the materials concerned, their behaviour in sintering processes and the resistance to leaching of compact ceramic bodies prepared therefrom. The chemical composition can also be varied by introducing such ions in the solution prepared in the first step and co-precipitating them together with the insoluble titanium salts in an alkaline environment. For example, the absorbed ions are selected from alkali metal ions or alkaline earth metal ions (such as Ba and/or Ca and/or Sr) in addition to B and/or Si and/or Fe. The co-precipitated ions can be selected, more particularly, from Al and/or Zr and/or Fe.

Such versatility is particularly significant for the application of the process of the invention to the synthesis of ceramic articles based on multicomponent and multiphase titanates, the chemical composition and the crystallograhic structure of which is similar to that of some naturally occurring ones which have been suggested for use in the longterm storage of toxic radionuclides as generated in the reprocessing of spent nuclear fuel elements, as an alternative to the conventional procedure of occulusion within borosilicate glass.

For example, in the synthesis of SYNROC B (a synthetic blend of pirowskite (CaTiO3), zirconolite (CaZrTi207) and hollandite (BaAl2Ti6O15)) in the form suitable for use in ceramic storage, zirconium and aluminium are introduced as salts which are soluble in the starting solution prepared as described above and co-precipitated together with titanium in an alkaline environment, barium and calcium being introduced by absorption on the gel particles so prepared by contact with aqueous solutions of calcium and barium having appropriate concentrations. The atomic ratio Ti:Al :Zr: Ba: Ca is, for example, 0.74: 0.11 :0.09:0.05:0.28.

The ion-exchange materials obtained as described above can further be stabilized by drying and, optionally, by a heat treatment at a temperature of from 1 OO"C to 5000C without any intolerable loss of their ion-exchange ability. The latter property is no doubt connected with the fact that the specific surface area of particles (or membranes) having comparatively large diameters (or thicknesses) and having a rubbery and/or glassy appearance, as measured by the BET method, is extremely high and remains so even after treatment at a comparatively high temperature. For example the BET specific surface area of particles of hydrous titanium oxide which contain sodium and have a diameter, in the form of a dry gel (xerogel), of 2 mm and which are prepared according to the procedure described in Example 2 below, is 400 m2/g.The specific surface area of the same material, when converted into particles having a glassy consistency by heating in air at 350"C, is 330 m2/g.

The gels orxerogels obtained according to this invention usually have, when dried at 100C, a specific surface area in the range of from 400 to 600 m2/g and, after heating for two hours at 300"C, they still have a specific surface area above 200 m2/g.

The titanium-based ion-exchangers according to the invention can be employed for recovering uranium from sea water. Theoretical studies on this matter have shown that the economical acceptance of the extraction process, which is non-competitive at present, may be favourably influenced so as to reduce the extraction cost below the threshold value of 150 U.S. dollars per pound U308, by improving the quality of the exchange material used, such as its exchange ability and rate, its mechanical sturdiness and other properties (see F.B. Best et al, Prospects for recovery of uranium from sea water, MIT-EL 80-001, Report No. MITNE-231, January 1980, USA).

The exchangers according to the present invention can find a useful application in the separation and/or conditioning of radionuclides which are contained in liquid effluents having a high or an average activity, and generated during the reprocessing and/or remanufacture of nuclear fuels or during the use of nuclear plants for power production. It is interesting to observe that the exchangers according to this invention, once they have absorbed and/or adsorbed toxic radioactive nuclides, can be converted into ceramic bodies by, for example, either of the following two procedures. In the first procedure, the radionuclide-laden exchangers are in microsphere form.The microspheres are heated to a temperature of not less than 900 C, preferably in the range of from 1000"C to 1400 C, and, at these temperatures, sintering takes place. In the second procedure, the radionuclide-laden exchangers are also in the form of microspheres. The microspheres are calcined at a temperature of from 400"C to 800"C and thereafter are compacted under a pressure of from 1 to 6 tonne/cm2. The pellets obtained are sintered at a temperature of not less than 900"C, preferably of from 1000 C to 1400 C.

The following examples illustrate the invention.

Example 1 197.5 ml of titanium tetrachloride (TiCI4) were poured, with stirring, into 100 ml of tetrahydrofurfuryl alcohol. A reaction took place with a considerable evolution of heat and the viscosity of the resultant solution, as measured upon cooling to 24"C, was 250 centipoises. The solution was further thickened by addition of an organic polymer based on hydroxypropylmethylcellulose and manufactured under the Trade Mark METHOCEL MK4 by Dow Chemical, USA, in an amount of 18.7 g of METHOCEL MK4 per 1000 ml of the solution.The solution thus obtained was scattered by a rotary cone atomizer into a mass of liquid droplets having a diameter of from 500 microns to 750 microns, and these were immediately solidified to give gel particles having a slightly smaller diameter, a high mechanical cohesion and a clammy appearance, by contact with an alcoholic solution of an alkali (LaOH or NH40H) which is either anhydrous or has a controlled water content.

Example 2 310 ml of titanium tetrachloride (TiCI4) were poured in 1000 ml of tetrahyrdrofurfuryl alcohol and the resulting solution was converted according to the procedure set forth in Example 1 into xerogel particles having a diameter of 2.5 mm by dripping the solution through a capillary tube having an outside diameter of 1.5 mm, into an ammoniacal solution, washing in water and drying by azeotropic distillation in carbon tetrachloride, in an apparatus of the marasson type.

Example 3 2 litres of titanium tetrachloride (TiCI4) were poured with stirring into 10 litres of tetrahydrofurfuryl alcohol, and the reaciton described in Example 1 took place. To the solution there were added 500 g of a polyvinyl alcohol having a molecular weight of from 10000 to 15000 contained in 5000 ml of water. No clouding of the solution took place, and less than all any hydrolytic precipitation of the titanium content.

Closes meshed cotton gauze squares (125 mesh) having a size of 100 cm by 100 cm, held taut in specially provided frames, were impregnated by immersion in the solution and converted into ion-exchange membranes by exposure to ammonia vapour and by subsequently effecting drying in a microwave oven.

Example 4 A solution of titanium tetrachloride in tetrahydrofurfuryl alcohol, prepared according to Example 1, was poured into 1000 ml of an aqueous solution of zirconium chloride and aluminium nitrate, these compounds being present in the aqueous solution in such a proportion as to provide an atomic ratio of the metals to be contained in the combined solution, namely the ratio Ti:Al:Zr, of 0.74:0.11:0.09.

The solution was thickened by adding thereto 18.4 g of METHOCEL MK4, and was then formed, by a rotary cone atomizer, into liquid droplets having a diameter of from 500 to 750 microns. The liquid droplets were converted into gelled particles by allowing them to fall into an alkaline bath consisting of a 15% solution of NaOH methanol.

The gelled particles obtained in this manner were washed with water and subsequently adjusted with a 0.1 M solution of CaCI2 and a 0.01 M solution of BaCI2, the ratios between the quantities of the particles and the volume of the alkaline earth metals being so adjusted that, as a result of the absorption of Ca and Ba by the gelled particles, a final overall atomic ratio Ti:Al:Zr:Ba:Ca of 0.74:0.11 :0.09:0.05:0.28 was obtained.

The particles so obtained were characterized by satisfactory ion-exchange properties, and could be used as "precursors" for conditioning radioactive wastes according to the SYNROC procedure.

Example 5 0.4 g of particles prepared according to Example 1 were contacted with 50 ml of a solution having a concentration of 0.09 M of Sr(NO3)2 and of 0.5 M of NH40H. The quantity of strontium absorbed by the gelled particles, after a contact time of 30 minutes, corresponded to 5 milliequivalents of Sr per gram of TiO2.

Example 6 The procedure described in Example 5 was repeated, employing 0.4g of the particles obtained as described in Example 1, calcined in air at 300"C. The quantity of strontium absorbed by the particles after a contact time of 30 minutes corresponed to 3 millequivalents of Sr per gram of TiO2.

Example 7 0.1 g of particles prepared as in Example 1 and dried in air at 100 C, was contacted with a solution of Am having a concentration of 0.07 mg/litre and a pH of 2.68. After two minutes from the start of the contact, the americium fraction bonded by the titanate particles corresponded to 98% of the total amount. After 30 minutes from the start of the contact time, the absorbed fraction exceeded 99.9% of the total quantity.

Example 8 The procedure of Example 7 was repeated, employing 0.1 g of particles prepared according to Example 1 and calcined in air at 3000C. After two minutes from the start of the contact, the Am fraction fixed by the titanate particles corresponded to 90% of the quantity which was present in total, and, 30 minutes from the start of the contact time, the absorbed fraction exceeded 99.5% of the total.

Example 9 Particles prepared according to Example 4 were calcined in air at 250"C and contacted with a solution of liquid radioactive wastes having the following composition and pH: Rare earths . 94.3 mol % U + Th 5.0 mol % Am + Cm + Pu + Np 0.7 mol% pH 2.5.

The particles, which contained fission products and transuranic elements in a quantity of 10% of the total weight of the metals which were present, were calcined in air at a temperature of 700"C and compacted into pellets having a size of 30mm by 30 mm, using a double-acting hydraulic press and a pressure of 4 tonne/cm2. The "green" pellets were heated to a temperature of 1200 C, the rate of temperature increase being 100"C per hour. After 3 hours at the maximum temperature (1200"C), there were obtained ceramic bodies which have a high specific gravity and a high resistance to leaching, and which were adapted to long-term storage of the toxic nuclides embedded therein.

Example 10 Particles prepared according to Example 2 and calcined at 250"C, were contacted with the solution of fission products plus actinides described in Example 9, until they attained a content of 10 grams of radioactive nuclides per 100 g of TiO2. The nuclide-laden particles were treated at 1050'Cfortwo hours to obtain microspheres having a specific gravity above 4.1 g/cm3, which were suitable for the permanent storage of the toxic nuclides embedded therein by occlusion in a glass or a metal block.

Claims

1. A process for the preparation of an inorganic ion-exchange body based on titanium and optionally one or more other elements, which process comprises (a) preparing an alcoholic solution, having a comparatively high viscosity, of an organometallic compound of titanium, which solution optionally contains, in addition to the titanium compound, at least one compound of at least one other element; (b) optionally thickening the solution by adding an organic polymer thereto; (c) gelling the solution by alkaline neutralization in an anhydrous environment or an environment having a controlled water content, thereby to obtain an ion-exchange gel based on titanium and co-precipitation of the other element(s), in any desired geometrical configuration; (d) optionally absorbing a chemical element of interest by the ion-exchanging gel, by contacting the gel with a solution which contain said element; and (e) optionally drying and/or calcining the gel.

2. A process according to claim 1, werein the organometallic compound of titanium is prepared by reacting a titanium halide and an alcohol.

3. A process according to claim 2, wherein the titanium halide is TiCI4.

4. A process according to any of claims 1 to 3, wherein the alcohol is tetrahydrofurfuryl alcohol.

5. A process according to any of claims 1 to 4, wherein the organic polymer is selected from cellulose derivatives and vinyl compounds.

6. A process according to claim 5, wherein the cellulose derivative is hydroxypropylmethylcellulose.

7. A process according to claim 5, wherein the vinyl compound is polvyinyl alcohol.

8. A process according to any of claims 1 to 7, wherein gas bubbles are introduced into the alcoholic solution in order to produce an inorganic ion exchanger having a controlled porosity.

9. A process according to claim 1, substantially as described in any of the foregoing Examples.

10. An inorganic ion-exchange body prepared buy a process according to any of claims 1 to 9.

11. Ion-exchange gel or xerogel as claimed in claim 10, having, when dried at 100 C, a specific surface area of from 400 to 600 m2/g, and having, after heating for two hours at 300 C, a specific surface area greater than 200 m2/g.

12. Gel or xerogel as claimed in claim 10 or 11, wherein the ions absorbed are ions of alkali metal or of alkaline earth metal.

13. Gel or xerogel as claimed in claim 12, wherein the ions are Ba ions and/or Ca ions and/or Sr ions.

14. Gel or xerogel as claimed in claim 10 or 11, wherein the absorbed ions are B ions and/or Si ions and/or Fe ions.

15. Gel or xerogel as claimed in any of claims 10 to 14, wherein, in addition to titanium and the absorbed ions, it contains elements selected form Al and/or Zr and/or Fe.

16. Gel or xerogel as claimed in claim 13 or 15, wherein the ratio Ti:Al:Zr:Ba:Ca is 0.74:0.11 :0.09:0.05:0.28.

17. Gel or xerogel as claimed in any of claims 10 to 16, the geometrical configuration thereof being microspherical.

18. Gel or xerogel as claimed in any of claims 10 to 16, the geometrical configuration thereof being that of a membrane borne by an appropriate carrier.

19. The use of an inorganic ion-exchanger as claimed in any of claims 10 to 18 for extracting uranium from sea water.

20. The use of an inorganic ion exchanger as claimed in any of claims 10 to 18 for immobilizing toxic nuclide contained in liquid radioactive waste produced in the reprocessing and/or remanufacture of nuclear fuel elements, or in the cooling or during the operation of a nuclear power station.

21. A procedure for the conversion into a ceramic body, of a gel orxerogel as claimed in any of claims 10 to 18, on which has been absorbed and/or adsorbed radioactive toxic nuclide, the gel or xerogel being in the form of microspherical particles, which procedure comprises heating the microspheres at a temperature of not less than 900'Cso as to sinter the microspheres.

22. A procedure for the conversion, into a ceramic body, of a gel or xerogel as claimed in any of claims 10 to 18, on which has been absorbed and/or adsorbed radioactive toxic nuclide, the gel or xerogel being in the form of microspherical particles, which procedure comprises calcining the microspheres at a temperature of from 400 to 800"C, compacting the micropsheres at a pressure of from 1 to 6 tonne/cm2, and sintering the microspheres at a temperature of not less than 900"C.

23. A procedure according to claim 21 or 22, wherein the microspheres are sintered at a temperature of from 1000 to 1400"C.

24. A procedure according to claim 21 or 22, substantially as described herein.