DE1156054B - Process for the production of thorium oxide, uranium oxide, thorium uranium oxide or plutonium oxide sols coated with silicon dioxide - Google Patents

Description

GERMAN

PATENT OFFICE

G32243IVa / 12g

REGISTRATION DATE: MAY 10, 1961

NOTICE
THE REGISTRATION
ANDOUTPUTE
EDITORIAL: OCTOBER 24, 1963

The invention relates to the production of silicon dioxide-coated thorium oxide, uranium oxide, Thorium uranium oxide or plutonium oxide sols, which are used as fuel in liquid homogeneous nuclear reactors be used.

Some of the known aqueous homogeneous reactor systems have depended on the use of uranyl sulfate or uranyl phosphate in solution as a fuel. These reactor systems were not particularly satisfactory, because both the sulfuric acid and the phosphoric acid system have a certain tendency to Show corrosion in the system.

Because of these drawbacks, significant efforts have been made to produce fuel systems from solids such as uranium trioxide (UO ₃ ), which is added to the reactor in the form of a slurry. These slurries had some obvious drawbacks such as corrosion on the equipment and mechanical abrasion of the material itself.

It has been found that these problems can be overcome by using brines of uranium oxide, thorium oxide, Thorium dioxide-uranium oxide or plutonium oxide as fuel in aqueous homogeneous reactors can be solved. Such sols have the advantage that they are dispersions of homogeneous particles colloidal size and that the disadvantages are avoided when using thorium dioxide or uranium oxide slurries occur. For example, you don't need for a good upheaval to avoid the separation of solid parts. Because of their small size, the particles do not become mechanical abraded and the devices wear and tear is not a problem. The brines are proportionate low viscosity and can be easily circulated or pumped.

According to the present invention, a method for producing silicon dioxide-coated thorium oxide, uranium oxide, thorium-uranium oxide or plutonium oxide sols, which are suitable as nuclear reactor fuel, is proposed, which is characterized in that the non-coated sol is organic in order to achieve the desired coating silicate is added in an amount corresponding to a ratio of metal oxide to SiO ₂ is between 1: corresponds to 1, and that the mixture to 70 to 15O ⁰ C, preferably to about: 1 and 10: 1, especially between 2: 1 and 3 100 ⁰ C is heated under reflux, has been hydrolyzed to the organic silicate at a _pH value above the isoelectric point of the silicate. This speeds up the gradual ab process of manufacture
of coated with silica
Thorium oxide, uranium oxide, thorium uranium oxide or plutonium oxide sols

Applicant:

W. R. Grace & Co.,

New York, N.Y. (V. St. A.)

Representative:

Dr. rer. nat. J.-D. Mr. v. Uexküll, patent attorney,
Hamburg-Hochkamp, Königgrätzstr. 8th

Claimed priority:
V. St. v. America May 12, 1960 (No. 28 513)

Frederick T. Fitch and Jean G. Smith,

Baltimore, Md. (V. St. A.),
have been named as inventors

depositing a silicon dioxide layer on the metal oxide particles of the sol.

Since the silica in enrobing the Metalloxydteilchen gives them a negative electrostatic charge, the sol is then generally produced by adding alkali metal hydroxide such as sodium hydroxide or other alkaline solutions, stabilized, sufficient to provide the _pH value to the correct range, generally 7 to 9.5.

It has recently been proposed that a 2% silica sol, which is treated with thorium dioxide, Uranium oxide or thorium dioxide sols can be reacted to form particles coated with silicon dioxide is obtained by passing a sodium silicate solution through a cation resin. The brines obtained have an unusual hydrothermal stability and even after concentration are therefore very suitable as fuel in aqueous homogeneous reactors. The new method represents a Improvement of the previous process by greatly simplifying the process and providing a results in more uniform coating of the sol particles.

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This simple method is also advantageous insofar as silicate cannot be precisely determined by calculation

liable as there would be a considerable economy of having since the one needed to encase a particle

Heavy water to be used brine obtained, where- amount of silicon dioxide from the surface and

where the required amount of heavy water does not depend on the weight of the particle. That

is lower than it is when the previously used weight ratio of metal oxide to silicon dioxide is used

known process for the production of silicon is in the range of about 1: 1 to 10: 1, preferably

Dioxide coated brines was. wise between 2: 1 and 3: 1.

In order to obtain a metal oxide sol with the required thermal stability and tendency to coat, it is desirable that the initial coated sol is above the generally desired limit of sol particles spherical or mainly spherical of the specific conductivity of about 10 ^-3 mho / cm. are. Furthermore, the particles must have a limited This concentration of the anions is too high, so that size range. The particle size may require further clarification. This will be between 20 and 300 ηΐμ, but it should be best done by heating the sol under conditions that exclude particles in individual sols, preferably more uniform 15, namely between 100 and 150 πΐμ or between, in order to free anions within the micelles -250 and 300 πΐμ. The size range and the equation, whereupon the solution is cooled and with a moderation of the particles, contribute significantly to the stabilization of deionizing agents known per se, such as. B. contributed to the end product. an ion exchange resin, in connection with

One method of making this initial or 20 involves removing electrolytes. Connecting mother brine has recently been proposed. Suitable ßend the _pH value must again can be prepared for example to about 7 to 9.5 einSole by made to ensure the stability to improve the silicon While the system at an elevated temperature dioxide coated sol
The sols prepared according to the invention can be removed from dilute metal salt solutions by evaporation to a solids content up to or concentrated by controlled hydrolysis with a 50% or more. This brine is homogeneously concentrated with subsequent electrodes to a density of approximately 1.5 (e.g. lyte removal. 1.45 to 1.55), which for example leads to

The organic silicate can be added to the sol in whole or in a metal oxide content of about 30%, in small portions, whereupon the components are heated under reflux until the silicon dioxide content is less than 15% and is refluxed until the wear and the normal Ratio of metal oxide to hydrolysis is complete. Silica of about 2.2: 1 is used during the reaction. Vordes Siliciumdioxyds with the Metalloxydteilchen preferably decreases during the compression process of the p _H value. In practice, the _pH value of the fresh sol Siliciumdioxyds can be continuously added to a Niedem isoelectric point approaching, 35 Derschlag of the solid material on the surfaces to avoid the Bewas difficulties in achieving the desired boiler used. The finished sol can cause silica coating. In order difficulties by adding deionized water or culties by a fall in the p _H -value below the water with a low ion content to any isoelectric point of the Siliciumdioxyds diluted to avoid lower solids content. A to, a weak anion exchange resin 40 concentration can also by centrifuging the be added to the _pH value at the desired sol, and redispersing the solids to keep in a kleiten height. After the conversion is complete, a smaller amount of deionized water will be obtained,
the _pH value of the sol, as already described, pre- The hydrothermal stability of this brine can gezugsweise brought to 7 to 9.5, to the sol to be checked by the brine in 300 hours to stabilize. 45 a pressure vessel at temperatures of about 150

The hydrolysis proceeds satisfactorily vonstatten, heated to about 300 ⁰ C. Brine, which in aqueous

if the temperature is between 70 and 150 ° C, homogeneous reactors must be used at

however, a temperature of about 100 ° C by the operating temperatures of the system, namely about

Refluxing the sol is preferred. 300 ⁰ C, be hydrothermally stable.

The system is heated long enough to ensure that the full 50 Special care must be taken to ensure constant hydrolysis of the organic silicate to a minimum. Obviously, the amount of time required depends, as described above, on the type and concentration of the silicate and on the concentration of undesirable ionic other considerations. For example, in all material, the hydrolysis of ethyl orthosilicate is conveniently measured by determining the specific conductivity. The specific 100 ⁰ C ended in 10 to 30 hours. Any conductivity of the sols according to the invention can be used in any organic silicate, generally kept below 10 ^-3 mho / cm. The hydrochloric silicon dioxide released during hydrolysis, the thermal stability of a given sol, is left through. Suitable organic silicates are alkyl- reducing its ion content improved; made of silicates, preferably with alkyl groups, which are smaller than butyl.
is easily hydrolyzable. The preferred organic For the present description, the specific silicate compound is that fish conductivity at 25 ⁰ C and at a frequency is easily obtained tetraethyl orthosilicate and hydrolyzed at an acceptable overall of 1000 cycles using a Standardschwindigkeit. 65 conductivity bridge with one in one junction

The metal oxide sol should be measured when the cell is relatively thin. The cell constant will to ensure good and rapid mixing with a 0.01 normal KCl solution according to the following. The ratio of the metal sol to the organic equation is determined (where the conductivity of the

KCl solution taken from the conductivity tables will):

K = L _Ka R,

in which K is the cell constant in cm " ¹ , R is the bridge resistance in ohms and L is the conductivity in mho / cm of the standard KCl solution.

The specific conductivity L of the sol in question is determined by measuring its resistance in the same cell and using the following equation:

_R ,

in which K is the cell constant in cm " ¹ and R is the resistance in ohms.

The electron micrographs were produced according to the usual methods.

Although only the use of alkali hydroxide and especially sodium hydroxide for setting of the pn value of the brine has been mentioned, other alkalis can also be used. The only However, a limitation in the selection of these alkalis is the fact that they consist of elements with a low thermal neutron cross-section and must be stable under nuclear reactor conditions.

The reference to dispersions of silica-coated metal oxide particles in water includes both heavy water and natural water.

The following examples serve to further illustrate the invention.

example 1

1760 g thorium nitrate solution with a content of 5% thorium dioxide in deionized water was placed in a heated compression vessel and heated to 97 to 98.degree. The solution was circulated in an amount of about 60 ml / min through a water cooler to reduce the temperature to 30 to 4O ⁰ C and passed through the cathode compartment of a cell, which was divided by an anion permeable membrane. Each electrode compartment had a capacity of 350 ml and was provided with a stirrer. The platinum electrodes attached to each side of the membrane were about 3 mm apart. The solution leaving the cathode cell was passed through a heat exchanger, where it was ^{heated to 97 to 98 °} C. and then passed back into the compression vessel. Evaporation losses were avoided by equipping the cell with a condenser and periodically adding deionized water to make up for the inevitable losses. The solution was circulated for a total of 10.5 hours with an interruption at night after 8 hours. During this time the solution was allowed to cool to room temperature. During the electrodialysis, the current strength fell from about 10 to about 0.4 amperes, while the _pH value rose from 2.7 to 4.6. The sol had a density of 1.04 g / ml and was concentrated to a density of 1.13 by partially removing water.

250 g of this sol having a content of about 12% Thoriumdioxyd, which had a _pH value of 4.6 and a density of 1.13 and made of a relatively spherical particles with a diameter of

ίο was about 70 πΐμ, was placed in a three-necked flask equipped with a stirrer, reflux condenser and thermometer. A total of 250 grams of deionized water and 39.9 grams of ethyl orthosilicate were added to the flask and the mixture was stirred at reflux for about 20 hours. At the end of this period, the _pH value was the solution to 2.2 liked. The _pH value was increased by the addition of 38 ml lnormalen Natriumhydroxyds on. 9 The product was concentrated by evaporation ml to about 130, and the resulting sol was treated for 24 hours at 15O ⁰ C in an autoclave; then passing it through a mixed bed of cation-anion exchange resin, provided the _pH value to 8 and then concentrated, the sol to a density of 1.5. The final _pH value of the sol was 7.9 and the conductivity was 6.14 · IO ⁴ mho / cm.

Example 2

Several attempts were made in which ethyl orthosilicate was hydrolyzed to one To achieve silicon dioxide coating on thorium dioxide particles.

In each of these experiments, sufficient ethyl orthosilicate was added to the thorium dioxide sols obtained by electrodialysis according to Example 1 to obtain a ratio of thorium dioxide to silicon dioxide of 2.2: 1. The ethyl orthosilicate was slowly added to the thorium dioxide sol using a three-necked flask equipped with a stirrer and condenser as a reaction vessel. To keep the _pH value as near as possible at 3, small quantities of anion exchange resin were added during the entire clogging of Äthylorthosilikat without, however, excess resin was added. The mixture was heated to 8O ⁰ C and the resin removed by being filtered through glass wool. The product was treated at about 100 ⁰ C to reflux, cooled, and the _pH value using sodium hydroxide lnormalem adjusted to 9. Electron microscopic examinations of the sol treated in the autoclave showed that uniform silicon dioxide coatings had formed on the surface of the thorium dioxide. Information on the duration of the reflux treatment in the presence of ethyl orthosilicate, the duration of the autoclave treatment, density according to the concentration and stability of the sols produced in two of the experiments are shown in the table.

Ethyl orthosilicate addition
hours

Reflux

End _pH value

Autoclave treatment

at 150 ° C
Hours I density density
after concentration

Hydrothermal stability,

stable for the long term

of at least

24
16

2.6
2.2

17th
17th

1.51
1.50

205 hours at 250 ° C
393 hours at 275 ° C

These data indicate that silica coated and suitable for use in nuclear reactors Thorium dioxide brine with a high degree of hydrothermal stability through ethyl orthosilicate hydrolysis can be produced.

Example 3

The sols were deionized and concentrated by the following procedure to improve their stability.

Each of the brine shown in the table was adjusted to a _pH value of 9 and treated under reflux overnight. The sol was then allowed to cool and passed through a mixed bed ion exchange resin to remove electrolytes. This treatment resulted in a sol having a specific conductivity in the range of about 10 ^-3 to 10 ^-6 mho / cm. These sols were treated with! .Normalem sodium hydroxide so that the _pH value increased to about 8, and were evaporated under reflux and stirring, rise to the density to 1.3. The _pH value was adjusted with! .Normalem sodium hydroxide back to about 8, and the sol was treated for a further 16 hours at a temperature of 150 ° C in an autoclave. The product was cooled, anion exchange cations passed through a mixed, adjusted to a _pH value of about 8 and evaporates at constant volume with continuous addition of fresh sol rise to the density to 1.5; then the sol was filtered through glass wool.

Electron micrographs of the particles before and after concentration showed that the Autoclaving most of the silica into an even surface layer converted the particle.

Claims (4)

PATENT CLAIMS: 1. A process for the production of silicon dioxide-coated thorium oxide, uranium oxide, thorium-uranium oxide or plutonium oxide sols, which are suitable as nuclear reactor fuel, characterized in that the uncoated sol organic silicate is added in an amount that corresponds to a ratio of metal oxide to silicon dioxide is between 1: 1 and 10: 1, preferably between 2: 1 and 3: 1, corresponding to the mixture at 70 to 15O ⁰ C, is preferably heated to about 100 ° C, under reflux, hydrolyzed to the organic silicate has been, wherein the _pH value is maintained above the isoelectric point of Siliciumdioxyds and the sol is then freed in manner known per se from contaminating ions. 2. The method according to claim 1, characterized in that the _pH value is brought of the obtained coated sol to 7 to 9.5. 3. The method according to claim 1 and 2, characterized in that an alkyl silicate in which the alkyl groups contain 1 to 3 carbon atoms, in particular tetraethyl orthosilicate, are used will. 4. The method according to claim 1 to 3, characterized in that the sol obtained by evaporation is concentrated. © 309 729/294 10.63