FR2501061A1 - METHOD AND APPARATUS FOR GELIFYING PARTICLES - Google Patents

Abstract

THE APPARATUS ACCORDING TO THE PRESENT INVENTION INCLUDES A CENTRIFUGAL 3 ATOMIZER DRIVEN IN ROTATION BY A MOTOR 22, A DOSING PUMP 1 FOR THE STARTING SOLUTION, A THERMOSTATIC LOOP 2, AN ANNULAR DUCT 21 THROUGH WHICH THE SOLUTION IS TRANSFERRED ON THE ROTOR 3 , ON THE CIRCULAR EDGE 23 WHICH IT IS FRACTIONED INTO LIQUID PARTICLES WHICH ARE PRECIPITATED IN A GELIFICATION BATH 20 PLACED IN A GELIFICATION APPARATUS INCLUDING SEPARATE COMPARTMENTS ARRANGED CONCENTRICALLY, A DUCT 6 TO PROVIDE A GAS CURRENT, AND INERATION DUCT 4 FOR SUCTION OF AMMONIA VAPORS. APPLICATION: AMONG OTHERS, PRODUCTION OF NUCLEAR FUEL.

Description

Method and apparatus for particle zelification

  The present invention relates to a method for

  liquid particles from solutions of mietal salts and to convert said particles into

  gelled solids of controlled size, that is to say

  mined, using a centrifugal atomizer combined with a

  gelling apparatus specially designed.

  The solution of metal salts is led to ooze

  in the form of a liquid film along a touring surface

  at the end of which said solution is split

  in liquid particles distributed in an annular jet, the

  said liquid particles then being caused to fall into a gelling bath, so that one finally obtains

gelled solid particles.

  A large number of hydrometallic processes are known

  which are generally referred to in the literature as

  technical description by the expression "sol-gel process" either of the type for external gelling or of the type for gelation

  interior, said methods of transforming

  liquid particles of aqueous solutions and / or colloidal suspensions of gelled particulate inorganic compounds In processes for external gelling, the liquid particles are converted into solid particles by gelation in an ammoniacal and / or alkaline bath or by extraction using an organic solvent of the water they contain.

  In processes for internal gelation, trans-

  The formation is carried out with the aid of ammonia, which is obtained by the composition of the chemical substances, called ammonia donors, contained in the solutions forming the liquid particles, together with the compounds

concerned.

  The diameters of the starting liquid particles that are used to prepare sol-gel items, such as

  nuclear fuels, catalysts, magnesium

  special and similar ticks vary, depending on the case,

  micrometers to a few thousand micrometers and can

  be subject to strict manufacturing specifications.

  For example, in the case of mixed uranium oxide / plutonium oxide fuels for fast neutron reactors, it is considered that it is possible to prepare fuel rods by vibrationally packing two fractions of microspheres having respectively a diameter

  optimal average of 600 and 60 pim inside metal tubes.

  having an inside diameter of 6 mm, to obtain a filling density coefficient of about 80% of the

  theoretical density of the dioxide mixture.

  In the manufacture of packed fuel rods

  by vibration and intended for thermal reactors, the

  said bars having an inside diameter of about 10 mm and a fill density coefficient of about 95% of the theoretical density of the oxide, it is advisable to prepare three fractions of microspheres having average optimum diameters

  1000um, 100, um and 10Aum respectively.

  The processes indicated so far by the literature

  technical assistance and patents for the processing of

  Sol-gel solids starting solutions in solid particles by

  tion and gelling are based on the phenomenon of the stability of the liquid threads that is generated by driving said solutions through capillary tubes having a diameter of a few hundred pm, possibly in combination with the vibrations of said capillary tubes, as described by for example, British Patent Specification No. 467,281 and 1,401,962, or with the vibration of the starting solution upstream of the capillary tubes, as described by US Pat.

3,731,850.

  However, these processes are affected by

  many serious limitations as far as their application

  on an industrial scale is concerned.

  For example, a limitation that is common to all

  the embodiments of the atomization methods which use

  read capillary tubes is the minimum diameter of the

  which can be obtained with a reasonably high production rate. The fact that the maximum hourly production of liquid particles having a diameter of 46 μm,

  as stated in U.S. Patent 3,731,850, under conditions of

  also the maximum dispersion of particle diameters obtained, as low as 1.4 liters per

time is significant.

  Similarly, the hourly production of U02 particles with a diameter of 215 + 33> im, as indicated in

  examples of British Patent 1,467,284, is only 108 g.

  Another limitation of the atomization processes mentioned above is the possibility of closing the capillary tubes as a result of the premature gelation of the

  starting solution and / or the presence of foreign particles

in suspension.

  Finally, an additional difficulty inherent in the use of atomizers comprising capillary tubes is the deformation that the liquid particles, springing from the capillary orifice at a speed of a few meters per second, swell when they strike the free surface of the

  gel bath. For example, according to the British patent

  1 401 962, a device used to cushion this shock is to cover the free surface of the gelling bath with

  a layer of foam that must be renewed continuously.

  These limitations are completely eliminated at the same time as a considerable economic and qualitative advantage is obtained with regard to the products obtained, thanks to the atomization and gelling process according to the present invention which can be applied both to the sol-gel process

  for internal gelation and the sol-gel process for gelation

  external process, said method according to the present invention being described hereinafter with reference to the accompanying drawings, in which: Figure 1 is a diagram of an apparatus for carrying out the method according to the present invention; and FIG. 2 is a diagram of a variant of the apparatus of FIG. The method according to the present invention for producing particles comprises the steps of causing the metal salt solution to ooze in the form of a liquid film along a surface which is rotatable and at the end of wherein said solution is dispersed in liquid particles distributed in an annular jet, after which said liquid particles drop by drop

  in a gelling bath and one thus obtains parti-

gelled solids.

  Particles of metal salt solutions can be obtained in accordance with the present invention by dropping the particles into different annular baths, and these baths can be, in particular, ammonia baths or an alkaline bath. In addition, according to one aspect of the present invention, the starting solution can

  contain chemical compounds that may give rise to

  moniac, while gelling baths are made up

  by chemically heated liquids inert.

  According to an alternative embodiment of the invention,

  The annular baths are arranged concentrically and are separated from one another and placed in an appropriate manner.

  as compared with the atomized liquid jet generated in the ato-

  centrifugal misery that we get a dimensional ranking

gelled products.

  According to other variants of the invention, the

  liquids that do not correspond dimensionally to the specifications are directly returned

to the starting solution ..

  This phase is performed in one of the following ways:

  (1) Collector compartments are filled

  of the gelling machine with an inert organic liquid having a lower specific gravity than that of the gelling product or, alternatively, (2) cooling the corresponding collecting compartments to temperatures below the ammonia from ammonia donors

  contained in the starting solution.

  In a preferred embodiment of the invention, the ammoniacal vapors released from the ammonia bath are sucked into the space covering the gelling bath and the outer surfaces of the centrifugal atomizer are

swept by gas currents.

  The spherical gel particles which are obtained by the process according to the present invention have been

  a diameter of between 10 and 3000 μm.

  In the process illustrated in FIG. 1, the starting solution of the treatment is fed into the apparatus by a metering pump 1, is heated or cooled, depending on the case, to the desired temperature by flow through the thermostatic loop 2, is transferred to the walls of the rotor 3 through the intermediate of the annular conduit 21, seeps along the walls of said rotor in the form of a liquid film adhering to said walls and, finally, when this film reaches the edge

  circular 23 of the rotor 3, is dispersed in parti-

  fluid layers distributed to form an annular jet 24, in accordance with the principles of operation of

centrifugal atomizers.

  The liquid particles thus formed are collected in the gelling bath 20 and converted into solid particles; this bath, in the case of sol-gel processes for external lyeing, consists of an aqueous solution of ammonium hydroxide, or of another alkaline product, or of a mixture of organic solvent containing an extracting solvent] water, as an alcohol and, optionally, a basic compound, -6

  for example a high molecular aliphatic amine.

  On the other hand, in the case of sol-gel processes for internal gelling, the gelling bath consists of an inert organic solvent which is heated to a temperature above the decomposition temperature of the ammonia donor contained in the starting solution. For example, in the case where hexamethylene tetramine is used as an ammonia donor, the organic solvent is heated.

  inert to a temperature of about 900C.

  The premature gelation of the solution is avoided

  feeding on the inner surface 10 of the rotor 3, gelling

  cation that is caused by the ammonia vapors are

  gelling baths in gelling processes

  external cation using ammonium hydroxide and / or in processes for internal gelling, by sending a gas stream introduced into the duct 6 and deflected by the action of the baffle 7 in the slot 26 which is formed between the walls

of the rotor 3 and the stator 11.

  In addition, the premature gelation of the starting solution is prevented on the circular edge 23 of the rotor 3, or in the immediate vicinity of this rotor, by bringing a stream of shielding gas through the conduit 27 and sucking controlled via conduit 4

  the ammonia vapors with the aid of the fan-vacuum 5.

  In addition to preventing the premature gelation of the starting solution, it is also possible to use the stream of gas sent by the conduits 27 against the outer surface of the rotor 3 to modify the geometrical shape of the jet 24 of particles and, more particularly, to reduce its width, for example, whenever fissile nuclear material is processed for which the geometric shape of the safety devices

  is subject to limitations dictated by the need to

  to avoid criticality accidents. The same effect of reducing the width of the atomizing jet can be obtained by charging the particles electrostatically and creating a directional electrostatic field in the free space of the machine.

  which is above the surface of the gelling bath.

  Referring again to the scheme of FIG. 1, it can be seen that all the particles which are generated by the centrifugal atomizer come into contact with the free surface 25 of the gelling bath 20 and are transformed into solid particles which are automatically sorted into three fractions of different particle size to be collected in the three coaxial collecting compartments formed by the walls 19. The particles having the largest diameter are collected in the outer compartment which communicates, through the valve

  28, with the collecting receptacle 13, the parts

  cules having an intermediate diameter are collected in the same manner in the receptacle 14 and those having the smallest diameter are collected in the receptacle 15. It is obviously possible to increase the number of compartments and adjust by trial and error the distance between the partitions 19 various compartments in accordance with the practical needs of

classification of final products.

  As an alternative to what has been described above, only a fraction of the liquid particles having a predetermined size are converted into solid particles, while the fractions which do not do not meet the specifications

regarding their size.

  In the case of sol-gel processes for external gelling which uses an alkaline precipitation bath, the process will be better understood with reference to FIG. 20 The sequence of operations until the formation of the jet 24 of liquid particles in the The arrangement of FIG. 2 is in all respects similar to that described with reference to FIG.

  the only difference being the location of the gelling bath.

  Indeed, in the embodiment that will now be described, the alkaline gelation bath is contained in the middle compartment and up to the level shown in phantom in 31. The remaining space of the middle compartment and the remaining space of the receptacle 19 are filled with an inert organic solvent having a lighter specific weight than that of the gelling bath. The liquid particles which are collected in the middle compartment 29 are transformed into gelled solid particles immediately as they pass through the interface 31. The particles which are collected in the outer compartment and the central compartment again form a continuous liquid medium which is separated from the inert organic solvent by

  interfaces 30, and are recycled via the receivers

  tackles 13 and 15, respectively. We will describe below what

  typical working examples which are not to be considered as limiting the invention, nor as regards the chemistry of the test examples, nor with regard to details of the design and / or operation of the apparatus

  used for gelation and atomization.

Example 1

  An aqueous solution containing 98 grams per liter (g / l) of uranium, in the form of a solution of uranyl nitrate, 2 g / l of plutonium present in the form of a solution of tetravalent plutonium nitrate and 400 g / l of tetrahydrofurfuryl alcohol is thickened by the addition of an organic polymer, such as hydroxypropyl methylcellulose or polyvinyl alcohol until a viscosity of cP. reached, said viscosity being measured at room temperature. The surface tension, measured at the same temperature as above, is 55 dynes / cm. The solution is heated by the thermostatic loop 2 of the machine

  shown in the drawing of Figure 1 to a temperature of

  40 C, and atomized by the atomizer 3, the latter being

  rotated at 1400 rpm. by a motor 22.

  The diameter of the circular edge 23 of the rotor 3, used in this particular test, is 3 cm. The flow rate of the starting solution is 25 liters per hour. The elution bath is composed of 11 molar XH 4 OH. The flow rate of the air stream sent into the jacket formed between the rotor 3 and the stator 11, as well as the flow rate of the current sent through the conduits 27 are maintained at 10 liters per hour. Liquid particles are obtained which, for 95 g of the total product, have a diameter of between 500 and 750, one. These values correspond, respectively, to the final diameters of 100-150 μm (approximately) during the transformation of the gel particles I0 obtained as described above into particles of (10:98; -PuO: 2?) By a treatment. under high temperature in a reducing atmosphere. Such gel particles

  are characterized by highly satisfactory sphericity properties

  and are collected in the median compartment of

  the apparatus shown in FIG. 1 o, in the particular case

  considered here, the maximum distance between the walls

  intermediate in their upper sections is 7 cm.

  3.5%) of the total product, from the largest diameter (750 μm) liquid particles, and the remaining 1.5% of the total product from the lowest diameter liquid particles (500 μm). and less) are collected in the outer compartment and in the central compartment

The device, respectively.

Example 2

  Example 1 is repeated with the arrangement shown

  in Figure 2, using silene as the organic solvent.

  In this case, fractions of liquid particles having diameters greater than or equal to 500-750 μm are collected with a continuous liquid medium in the receptacles 13 and 15, respectively, and are returned to the main stream of the starting solution of the apparatus. In this case, the particles are accelerated through the silene / ammonium hydroxide interface 31, by modifying the surface tension (ie the aqueous phase by adding an agent to the latter).

water soluble wetting.

250106 1

Example 3

  An aqueous solution containing 180 g / l of thorium, present in the form of thorium nitrate, was thickened by adding to this solution hydroxymethylpropylcellulose until a viscosity of 80 cP was obtained. This solution is atomized at room temperature using an atomizer having a diameter of 15 mm and rotated at a speed of 550 rpm. Liquid particles with an average diameter of 2500 μm are obtained which, when gelled and heat-treated at 1300 C in air, become ThO 2 particles having

diameter of 750 am.

  EXAMPLE 4 A solution containing 2.9 mol / l of uranyl nitrate which was partially denitrated until a ratio of NO 3: 0 = 1.5 was obtained and which additionally contained 2.5 moles / l of urea, 3.5 moles / l of hexamethylene tetramine and the equivalent of 108 g / l of carbon, present in the form of a suspension of carbon black, is atomized and converted into gelled particles of diuranate ammonium + carbon by gelling in a bath of paraffin oil maintained at a temperature

  of 94 C according to the operating phases of Example 3. When

  they are heat-treated at a high temperature under vacuum,

  the gelled particles become particles of mono-

  uranium carbide having a diameter of 450 + 25 nm.

  In FIGS. 1 and 2, the reference 8 designates partitions, the reference 9 a liquid level control device, the reference 11 a protective cap to protect the film of liquid between the ammoniacal vapors, the reference 12 a device for adjusting screw to adjust the height of the cone 11, the reference 16 a command for the discharge valve 28, the reference 17 a circulating pump for the liquid contained in the apparatus, and the reference 18 a space in

which the liquid is sprayed.

R1EVNDICATIONS

  1. Process for producing liquid particles

  solutions of metal salts and to transform the

  particles of gelled solid particles of a determined size, characterized by the fact that the

  solution in the form of a liquid film along an over-

  a rotor face at the end of which said solution is fractionated into liquid particles distributed in an annular jet, said liquid particles then falling into a gelling bath so as to obtain

gelled solid particles.

  2. Process for producing liquid particles

  solutions of metal salts and to transform the

  so-called particles in gelled solid particles according to the

  claim 1, characterized in that said parties

  They fall into several annular baths.

  3. Process for producing next particles

  claims 1 or 2, characterized by the fact that

  uses an ammoniac bath or an alkaline bath as a bath of

gelation.

  4. Process for producing next particles

  claims 1 or 2, characterized by the fact that

  uses starting solutions which contain chemical compounds capable of releasing ammonia when heated and gelling baths consisting of

heated inert liquids.

  5. Process according to claims 1 or 2,

  characterized in that said annular baths are arranged concentrically and are separated from each other and appropriately positioned with respect to the atomized liquid jet generated by the centrifugal atomizer in such a way that a sizing is obtained. of particles

gelled products.

  Process according to any one of the claims

  1, 2, 3 and 5, characterized in that the liquid particles which do not correspond to their size specifications are returned directly to the starting solution by filling the resecating collecting compartments of the gelling apparatus with a liquid. inert organic material having a lighter density than

gelation medium.

  7. Process according to any one of the claims

  1, 3 and 5, characterized in that the liquid particles which do not

  do not correspond to the specifications cc-, er "r: t r - ..

  size by cooling the collecting receptacles correspon-

  at temperatures below the

  Ammonia solution of the ammonia donors contained in the solution of

  8. Process according to any one of the claims

  previous ones, characterized by the fact that

  bilges released by the ammonia bath are sent by vacuum

  ration in the space covering the gelation bath and that

  the outer surfaces of the centrifugal atomizer are

washed by gas currents.

  9. Process for the production of particles according to claim 1, characterized in that it reduces the

  the width of the jet of liquid particles generated by atomic

  centrifuge using an incident gas stream or

an electrostatic effect.

  A process for producing spherical particles of a mineral compound gel according to any one of

  preceding claims, characterized in that the

  particles have a determined diameter between 10 and 3000 pmo 11. Apparatus for the manufacture of particles following

  one of the preceding claims, characterized

  in that it comprises a centrifugal atomizer (3)

  in rotation by a motor (22), a metering pump (1) for the starting solution, a thermostatic loop (2), an annular duct (21) through which the solution is conveyed onto the rotor (3) on the circular edge (23) from which it is fractionated into liquid particles which are precipitated in a gelling bath (20) placed in a purification apparatus comprising separate compartments arranged concentrically, a conduit (6) for supplying a current inert gas protection and a conduit (4) for

aspirate the ammonia vapors.