Aqueous slurry of rare earth hydroxide particles

Details Aqueous slurry of rare earth hydroxide particles Aqueous slurry of rare earth hydroxide particles

2001-03-16

2004-01-06

Fiorilla, Christopher A. (Department: 1731)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C423S021100, C423S263000, C501S152000

Reexamination Certificate

active

06673143

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a novel aqueous slurry of rare earth hydroxide particles and to a method for the preparation thereof. More particularly, the invention relates to an aqueous slurry of rare earth hydroxide particles which can be used as a sintering aid or an additive in the preparation of a sintered ceramic body as well as to a method for the preparation of the slurry.
As is well known, certain rare earth compounds are used as a sintering aid or an additive ingredient in the preparation of various kinds of sintered ceramic bodies. When to be used in such applications, the particles of the rare earth compound are required to satisfy various requirements including not only fineness of the particle size but also good sintering behavior, high reactivity with other ingredients of the ceramic composition, excellent dispersibility in ceramic blend for sintering, and so on. In this regard of satisfying various requirements, the most widely employed rare earth compound in the ceramic preparation is a rare earth oxide in the form of a fine powder, which is prepared by several methods known in the art. For example, fine particles of an oxidatively decomposable compound of a rare earth element such as oxalates, carbonates and hydroxides are precipitated in a solution of a soluble rare earth compound and the precipitates are collected and calcined in an oxidizing atmosphere to be converted into a rare earth oxide powder. Alternatively, fine particles of a rare earth oxide can be prepared by grinding relatively coarse oxide particles in a suitable grinding machine. The rare earth oxide particles prepared by these typical prior art methods, however, are not always very satisfactory relative to the particle size because the average particle diameter of the oxide particles obtained by these methods can rarely be smaller than about 1 &mgr;m. Accordingly, it is desired in recent years to prepare particles of a rare earth compound having a finer and finer particle size in order to comply with the trend of the powdery compositions as the principal ingredient of the ceramic formulation toward a finer and finer particle size.
SUMMARY OF THE INVENTION
The present invention accordingly has an object, in view of the above described problems and disadvantages relative to powders of a rare earth compound, to provide particles of a rare earth compound or, in particular, rare earth hydroxide having an extremely small diameter of primary particles not exceeding 150 nm in the form of an aqueous slurry or dispersion in which the particles are little subject to settling with good dispersibility and stability as well as a method for the preparation thereof.
Thus, the present invention provides an aqueous slurry of rare earth hydroxide particles which comprises: a powder of a rare earth hydroxide of which the primary particles have a diameter not exceeding 150 nm; and water as the dispersion medium of the rare earth hydroxide particles, and which has an electric conductivity not exceeding 2 mS/cm.
The method of the present invention for the preparation of the above defined aqueous slurry of particles of a rare earth hydroxide comprises the steps of:
(a) mixing a first aqueous solution of a rare earth compound and a second aqueous solution of a basic compound, if necessary, in the presence of a surface active agent to precipitate rare earth hydroxide particles in an aqueous medium;
(b) collecting the rare earth hydroxide particles by separating from the aqueous medium to give a cake of the rare earth hydroxide particles;
(c) repeatedly washing the cake of the rare earth hydroxide particles with deionized water until the washing has an electric conductivity not exceeding 3 mS/cm; and
(d) dispersing the rare earth hydroxide particles in deionized water as a dispersion medium.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1
is a transmission electron microscopic photograph of the holmium hydroxide particles in the aqueous slurry prepared in Example 1.
FIG. 2
is a transmission electron microscopic photograph of holmium hydroxide particles for the same view field as in Photo 1 in a larger magnification.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The above defined inventive method for the preparation of an aqueous slurry of rare earth hydroxide particles is applicable to any rare earth element including yttrium and the elements having an atomic number in the range from 57 to 71 without particular limitations.
The rare earth compound used as the starting material in step (a) of the inventive method is not particularly limitative provided that the compound has good solubility in water including chlorides, sulfates, nitrates, aliphatic ammonium salts, salts of sulfate esters, salts of phosphate esters, sulfonates, carboxylates and ether compounds, of which salts with an inorganic acid, i.e. hydrochloric, sulfuric and nitric acids, are preferable and nitrates are more preferable.
The precipitation reaction of the rare earth hydroxide particles by mixing an aqueous solution of the rare earth compound and an aqueous solution of a basic compound is conducted optionally but preferably in the presence of a surface active agent which can be contained in either of the aqueous solutions to be mixed together although it is preferable that the surface active agent is contained in the aqueous solution of the rare earth compound. The surface active agent used here is not particularly limitative relative to the ionic nature thereof including cationic, anionic, non-ionic and amphoteric surface active agents without particular problems.
The cationic surface active agent mentioned above is exemplified by quaternary ammonium compounds such as long chain-alkyl trimethylammonium chlorides and the like. The anionic surface active agent is exemplified by sodium alkylbenzenesulfonates, sodium laurylsulfate, sodium lauryl ether sulfate and the like. The non-ionic surface active agent is exemplified by polyoxyalkylene alkyl ethers, e.g., polyoxyethylene alkyl ethers, and polyoxyalkylene alkylphenyl ethers, e.g., polyoxyethylene nonylphenyl ether. The amphoteric surface active agent is exemplified by betaine lauryl dimethylamino acetate.
Although the ionic nature of the surface active agent is not particularly limitative as is mentioned above, it is preferable to use a cationic surface active agent in respect of the high effectiveness. Various commercial products of cationic surface active agents are available and can be used in the inventive method including those sold under the trade names of Armac C (a product by Lion Akzo Co.), which is an alkyl amine salt-type surface active agent. The amount of the surface active agent contained in the aqueous solution or solutions is in the range from 0.01 to 10 g or, preferably, from 0.1 to 10 g or, more preferably, from 0.5 to 5 g per mole of the rare earth element in the rare earth compound. When the amount of the surface active agent is too small, the desired effects to be obtained thereby cannot be fully accomplished as a matter of course while no further advantages can be obtained by increasing the amount thereof to exceed the above mentioned upper limit.
The basic compound, of which an aqueous solution is mixed together with the aqueous solution of the rare earth compound in step (a) of the inventive method, is also not particularly limitative but is preferably an inorganic hydroxide such as sodium hydroxide, potassium hydroxide and ammonium hydroxide, i.e. ammonia water, of which ammonia water is preferred.
The first aqueous solution, which contains the rare earth compound, and the second aqueous solution, which contains the basic compound, can be mixed together in different ways including (1) addition of the second solution to the first solution, (2) addition of the first solution to the second solution and (3) concurrent addition of the first and second solutions to a volume of water as a receiving bath. The surface active agent can be dissolved in either one, two or all of the first solution, the second solution and the receiving bath, when used.
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