DD239395A1 - METHOD FOR PRODUCING THORIUM YTTRIUM MIXED OXIDE POWDER - Google Patents

Abstract

Thorium-yttrium mixed oxide powder is the starting material for the production of yttrium-doped thoria. The object of the invention is to provide a pure thorium-yttrium mixed oxide powder produced from thorium oxalate and yttrium salt which is readily sinterable and has a homogeneous Th / Y distribution in the predetermined mixing ratio. The invention has for its object to provide a method by which a precipitate can be obtained from a basic Thoriumoxalato complex solution and a Yttriumsalzloesung containing the two Faellkomponenten in optimal homogeneous distribution and in the predetermined mixing ratio and after calcination a mixed oxide high Sinterfaehigkeit supplies. This object is achieved according to the invention in that the Thoriumoxalato-carbonate-complex solution is adjusted to a p H value between 7 and 9 and this solution is added while maintaining the p H value with oxalate in the amount of 1.5 to 3 times the molar amount of yttrium used corresponds to the fact that the yttrium solution is prepared separately by removing the salt in an acid and mixed with the carbonate complex solution so that the p H value of the combined solution is always kept between 1 and 3.

Description

Characteristic of the known technical solutions

For the production of thorium-yttrium mixed oxide powder, two principal process routes are known:

powder ceramic way chemical way

ThO ₂ powder

Water soluble

 Th connection

To solve

Water soluble. Y compound

To solve

Mix

Grind

seven

Mix

co-precipitation

I filtration

desiccation

calcine

Shaping sintering

The powder-ceramic process route has serious disadvantages compared to the chemical process route. These are above all the entry of attrition during grinding of the powder mixture and a generally poorer sinterability of the powder because of the low degree of dispersity. For ceramic powders based on ThO ₂ , there is therefore a tendency to produce finely dispersed powders by a chemical process.

As the water-soluble starting compound for the chemical preparation route nitrates are preferably used. For this purpose, optimum process parameters have become known, under which a co-precipitated thorium-yttrium mixed oxide powder can be obtained which, after shaping, can be sintered to dense bodies even at relatively low temperatures [DD-WP 155060].

Frequently, however, no water-soluble starting compounds of thorium are available, but insoluble oxalate.

For example, thorium oxalate is obtained in the treatment of natural materials such as monazite sand or in the isolation of technical products such as thorium-containing catalysts.

It is already known a method to obtain from Thoriumoxalat on the formation of an oxalato-carbonate complex and its decomposition in acidic solution, a pure thorium oxalate, which provides a good sinterable oxide powder with suitable process control after calcination [DD-WP G 01 F / 252852].

For the production of a homogeneous thorium-yttrium oxide, however, this method is not suitable because yttrium precipitates in the carbonate complex solution as a flaky, insoluble hydroxide precipitate. Thus, no time-constant ratio of Th to Y is present from the complex solution during decomposition, which leads to inhomogeneities of the Y distribution in the powder and ultimately in the ceramic. In addition, promotion and dosage of the hydroxide suspension are more difficult than with clear solutions. In principle, the difficulties mentioned are to be avoided if all the reactants are thorium oxalato-carbonatocomplex,

Acid to decompose the complex,

Yttrium salt solution and

Oxalic acid solution for the precipitation of the Y-oxalate

would be promoted to the reaction site in separate, proportional to the desired Th / Y ratio flow rates. However, this complex apparatus and complex control engineering solutions are required.

Object of the invention

The object of the invention is to provide a pure thorium-yttrium mixed oxide powder produced from throium oxaate and yttrium salt which is readily sinterable and has a homogeneous Th / Y distribution in the predetermined mixing ratio.

Explanation of the essence of the invention

The invention has for its object to provide a method by which a precipitate can be obtained from a basic Thoriumoxalato complex solution and an yttrium salt solution containing the two precipitation components in optimal homogeneous distribution and in the predetermined mixing ratio and after calcination a mixed oxide high sinterability supplies.

This object is achieved in that the Thoriumoxalato-carbonate complex solution is adjusted to a pH value between 7 and 9 and this solution is added while maintaining the pH with oxalate in the amount of 1.5 to 3 times the molar amount of the yttrium used corresponds to that the yttrium solution is prepared separately by dissolving the salt in an acid and mixed with the carbonate complex solution so that the pH of the combined solution is always kept between 1 and 3.

With the invention, it is possible, by combining only two single-phase solutions and the use of only the pH in the reaction vessel to control the mass flows to achieve a Oxalatgemisch with homogeneous Th / Y distribution, which after calcining a good sinterable, pure and finely dispersed Provides mixed oxide powder.

The torrium oxalato-carbonate complex solution is prepared by adding to a slurry of the torrium oxalate in water a water-soluble carbonate or CO2.

The incipient dissolution process can be accelerated by stirring or increasing the temperature. By adding ammonia, base or acid, a pH between 7 and 9 is set towards the end of the dissolution process. Conveniently, both the pH and the temperature during the dissolution process is chosen so that no gas evolution occurs, which would lead to an additional carbonate consumption.

Of course, instead of the ammonium compounds, it is also possible to use the alkali metal (hydrogen) carbonates.

Upon completion of the dissolution process, the solution is dissolved in insoluble products such as sparingly soluble carbonates, non-complexed hydroxides or water-insoluble residues, e.g. separated by filtration, resulting in addition, a cleaning effect.

The clear carbonated solution is mixed with oxalic acid, ammonium oxalate or alkali oxalate. If necessary, ammonia solution is added simultaneously to maintain the pH between 7 and 9. The amount of oxalate ions in moles (n _Ox ) depends first on the amount of thorium contained in the solution (nth) and the desired molar ratio of yttrium to thorium Χγ / m in the final product, and results

Ποχ = Χγ / Th. nTh 1 # 5 · a.

At stoichiometric conversion the factor a would have the value a = 1. Although it is well known that yttrium oxalate is also practically insoluble in acidic solution [e.g. B. Remy, Textbook of Inorganic Chemistry, Akademische Verlagsgesellschaft Geest & Portig K.-G. Leipzig, 1954.7. In the process according to the invention, complete precipitation of the yttrium oxalate only occurs at values a greater than 1 in the process according to the invention. Conveniently, a value of 1.4 <a <2 has been found.

The acid solution is prepared by dissolving an yttrium salt in mineral acid, preferably nitric acid. The amount of acid results from the amount of carbonate contained in the carbonate solution (bound in the complex and free in solution) and from the pH of the complex solution. A calculation of the required amount of acid for the decomposition of the carbonate complex and adjustment of a specific pH in the reaction space is possible. Easier is the titrimetric determination of the amount of acid. For this purpose, a small sample of complex solution with acid is titrated to the desired pH value. This immediately results in the amount of acid required for the entire complex solution.

If yttrium oxide is dissolved in the mineral acid instead of the yttrium salt, this must be taken into account when determining the necessary amount of acid.

The thus prepared yttrium-containing acid solution is now in a reaction vessel with the oxalic-containing ThoriumoxalatOrKarbonatokomplexlösung with vigorous mixing z. B. so combined by means of agitator that the pH is always between 1 and 3. In the process, the carbonate complex decomposes with the deposition of thorium oxalate. At the same time yttrium reacts to poorly soluble yttrium oxalate. The recommended for the production of pure thorium oxalate on the thorium oxalato carbonatoin pH below 1 [DD-WP G 01 F / 252852] leads in the precipitation of the Th-Y-mixed oxalate surprisingly to products that are poorly sinterable after calcination.

On the other hand, the best sinterability of the resulting oxide powder mixture is obtained at pH values around 2. The concentration ratios of protons and yttrium set in the acid solution ensure that the deposition of thorium and yttrium oxalate always takes place in the same molar ratio.

The precipitation process can be carried out both continuously and discontinuously.

The temperature in the reaction chamber strongly influences the grain spectrum of both the oxalate and the oxide powder producible therefrom. For the production of a well sinterable mixed oxide powder, the reaction temperature is below 5O ⁰ C to obtain.

After completion of the precipitation, the oxalates ζ. B. separated by filtration from the solution, washed and dried. The calcination is conveniently carried out at temperatures between 600 and 1200 ⁰ C in an air atmosphere.

embodiment

In an exemplary embodiment for the preparation of a mixed oxide powder with 7.5% by weight of Y.sub.2 O.sub.3, the general procedure is to be explained further:

1000 g thorium oxalate (impurity content 2%) are slurried in 21 water. To this suspension are added 460 g of ammonium bicarbonate and 450 ml of concentrated ammonia solution (25%). Promoted by repeated stirring, the dissolution process is completed after 5 hours. The solution is adjusted to a pH of 8.3 with a little ammonia solution and sucked through a medium-hard filter. 144g of ammonium oxalate dissolved in water are added to the clear filtrate, the total amount of solution is increased to 51 by the addition of water and the pH is readjusted to the value of 8.3.

25 ml of the total solution are now titrated with concentrated nitric acid to a pH of 2.0. This requires 3.6 ml of concentrated acid. Therefore, in 720 ml of concentrated nitric acid, yttrium nitrate is dissolved corresponding to an amount of 39.3 g of Y ₂ O ₃ .

Both solutions are simultaneously added to the reaction vessel at room temperature in such a way that hourly

21 complex solutions are implemented. The reaction vessel is equipped with stirrer and glass electrode. The amount of acid containing yttrium is metered so that the glass electrode always displays values around pH-2.

After about 2 h, the precipitation process is completed, both solutions are completely combined, the pH of the solution is 2.0. After settling the precipitate and decanting the supernatant clear solution is filtered through a medium-hard filter. The precipitate is washed with 21 distilled water and finally with 200 ml of alcohol. After drying (12 h at 80 ° C, then 8 h at 200 ⁰ C) and calcination (2 h in air at 800 ⁰ C in sintered shells) a pure finely dispersed mixed oxide powder is obtained, which without addition of auxiliaries at a pressure of 150MPa by Cold compacting can be compressed and sintered in hydrogen at 1700 ⁰ C and 2 h temperature hold time provides a shaped body of more than 95% of the theoretical density.

Claims (1)

Invention claim: Process for the preparation of thorium-yttrium mixed oxide powder by co-precipitation of an oxalate mixture of triumchlorium and yttrium compounds in solution and subsequent calcination, starting from a triumoxalato-carbonate complex solution and an yttrium salt solution, characterized in that the thorium oxalato-carbonate complex solution is adjusted to a pH value between 7 and 9 and this solution is added while maintaining the pH with oxalate ions in the amount corresponding to 1.5 to 3 times the molar amount of yttrium used that the yttrium solution by dissolving the salt in a Acid is prepared separately and mixed with the carbonate complex solution so that the pH of the combined solution is always kept between 1 and 3. Field of application of the invention The invention relates to a process for the preparation of thorium-yttrium mixed oxide powder. Thorium-yttrium mixed oxide powder is the starting material for the production of yttrium-doped thorium oxide ceramics. Because of its special electrical properties, this ceramic is mainly used as a solid electrolyte material in measuring probes for determining the oxygen potential under strongly reducing conditions. In addition, applications in laser technology and high-temperature technology are emerging.