Stock material or miscellaneous articles – Web or sheet containing structurally defined element or...
Reexamination Certificate
1998-07-20
2001-04-24
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
C106S408000, C423S021100, C423S179000, C423S263000, C423S511000, C428S323000, C428S402000, C428S403000
Reexamination Certificate
active
06221473
ABSTRACT:
The present invention concerns a sulfide of a rare earth and an alkali, a process for its preparation and its use as a colored pigment.
Inorganic colored pigments are already widely used in a variety of industries, in particular paint, plastics and ceramics. In these applications, properties which are, among others, thermal and/or chemical stability, dispersibility (the ability of a product to be properly dispersed in a given medium), intrinsic color, coloring power and opacifying power, constitute a number of particularly important criteria which must be taken into consideration when selecting a suitable pigment.
Unfortunately, the majority of inorganic pigments which are suitable for applications such as those listed above and which are currently used on an industrial scale generally contain metals (in particular cadmium, lead, chromiunm or cobalt), the use of which is becoming ever more strictly regulated or even prohibited under the law of numerous countries because they are considered to be highly toxic. Particular non-limiting examples are red pigments based on cadmium selenide and/or cadmium sulphoselenide, for which we have already proposed substitutes based on rare earth sulfides. Compositions based on sesquisulfides of a rare earth and alkaline elements have also been described in European patent EP-A-0 545 746. Such compositions have proved to be particularly interesting substitutes.
However, there is a need for a wider range of pigment quality products.
The aim of the present invention is to provide such a range of products.
BRIEF SUMMARY OF THE INVENTION
The invention thus provides a sulfide of a rare earth and an alkali, characterized in that it has the following formula:
ABS
2
where A represents at least one alkali and B represents at least one rare earth, and in that it is constituted by grains with an average size of 1.5 &mgr;m at most.
In a further embodiment of the invention, the sulfide is characterized in that it has the above formula and in that it is constituted by whole grains with an average size of 1.5 &mgr;m at most.
In a third embodiment of the invention, the sulfide is characterized in that it has the above formula and in that it is constituted by agglomerates which are constituted by grains with an average size of 1.5 &mgr;m at most.
The invention also concerns a process for the preparation of a rare earth sulfide as defined above which is characterized in that at least one rare earth carbonate or hydroxycarbonate is brought into contact with at least one compound of an alkaline element and they are heated in the presence of at least one gas selected from hydrogen sulfide or carbon disulfide and the product obtained is optionally deagglomerated.
DETAILED DESCRIPTION OF THE INVENTION
Other characteristics, details and advantages of the invention will become apparent from the following description and non limiting illustrative examples.
The rare earth sulfide of the invention has the general formula given above. Throughout the present description, the term “rare earth” means elements from the group constituted by yttrium and elements from the periodic classification which have an atomic number between 57 and 71 inclusive.
In one particular embodiment of the invention, the rare earth is cerium or lanthanum.
In a further embodiment of the invention, the alkali is sodium combined with potassium or lithium.
Particular examples of products of the invention are: KCeS
2
, NaCeS
2
, and more generally products with formulae KCe
1−x
La
x
S
2
or K
x
Na
1−x
CeS
2
(0≦x≦1), the latter having a color spread from the color of KCeS
2
to that of NaCeS
2
.
An important feature of the sulfides of the invention is their granulometry. These products are constituted by grains with an average size of 1.5 &mgr;m at most, more particularly at most 1 &mgr;m. Throughout the description, the sizes given are measured using a CILAS granulometer.
In a further embodiment of the invention, the products are constituted by whole grains of the same average size. The term “whole grain” means a grain which has not been broken or crushed. Grains can be broken or crushed during grinding. Scanning electron microscope photographs of the product of the invention demonstrate that the grains forming it have not been crushed. This is due to the fact that the sulfides of the invention are deagglomeratable, i.e., if they are not directly in the form of whole grains, they can be in the form of agglomerates which are themselves constituted by grains which are agglomerated and/or slightly sintered which, on deagglomeration under mild conditions, for example air jet grinding, can produce whole grains.
The process for the preparation of the sulfides of the invention will now be described.
As seen above, the process of the invention consists of bringing at least one rare earth carbonate or hydroxycarbonate into the presence of at least one compound of an alkaline element and heating them in the presence of at least one gas selected from hydrogen sulfide and carbon disulfide and, optionally, deagglomerating the product obtained.
It is advantageous to use a carbonate or hydroxycarbonate with a fine granulometry, in particular with an average size of 1 &mgr;m at most.
Examples of alkali compounds which are suitable for use in this process. are alkali oxides, hydroxides, sulfides, polysulfides or sulphates, and oxycarbonated compounds such as alkaline oxalates, carbonates or alkali acetates.
The initial mixture can, of course, comprise several compounds of rare earths and/or alkalis.
The compounds cited above are mixed in the necessary stoichiometric proportions.
In a particular variation of the invention, the rare earth carbonate or hydroxycarbonate and the alkali element compound are mixed in water to form a solution or a suspension. The mixture obtained is then dried by spray-drying, i.e., spraying the mixture into a hot atmosphere. Any known spraying apparatus can be used for spray-drying, for example a sprinkler rose or similar spray nozzle. As an example, the gas temperature when spraying is started is usually in the range 200° C. to 300° C., and at the outlet can be between 100° C. and 200° C.
The mixture obtained after drying is then heated as described above.
The sulphiding gas used in the process of the invention may be hydrogen sulfide or carbon disulfide. In a preferred implementation of the invention, a mixture of these gases is used.
The sulphiding gas or gas mixture can be used with an inert gas such as argon or nitrogen.
Heating is generally carried out at a temperature in the range 800° C. to 1000° C., the high temperatures encouraging the production of pure product phases.
The heating period corresponds to the time required to obtain the desired sulfide; this period is shorter as the temperature increases.
The reaction is generally carried out at a partial moulding machinery of hydrogen sulfide and/or carbon disulfide which is in the range 0.1 Pa to 1×10
5
Pa.
Finally, the process can be carried out in an open reactor.
After heating, a product is obtained which has the granulometry given above or which, by simple deagglomeration or gentle grinding, can produce that granulometry.
Particular variations of the sulfides of the invention will now be described.
In a first variation, the sulfide comprises a layer based on at least one transparent oxide deposited on the surface or periphery.
The transparent oxide and its preparation process are described in our French patent application FR-A-2 703 999, which is hereby incorporated by reference.
In this variation, the product of the invention comprises a sulfide as described above which forms a nucleus, surrounded by a peripheral layer of transparent oxide.
Some variations of this structure are possible, of course. In particular, the peripheral layer surrounding the sulfide need not be perfectly continuous or homogeneous. However, the products of the invention are preferably constituted by a uniform coating layer of transparent oxide of controlled thickness, in a manner such that the original color of the sulfide befor
Aubert Maryline
Macaudiere Pierre
Le H. Thi
Rhodia Chimie
Seugnet Jean-Louis
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