Chemistry of inorganic compounds – Sulfur or compound thereof – Binary compound
Reexamination Certificate
1995-02-02
2001-10-16
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Binary compound
C423S566300, C423S443000
Reexamination Certificate
active
06303097
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for the preparation of metal sulphides.
Metal sulphides find various uses in technical and scientific applications. For instance, several rare earth and other metal sulphides are of specific interest as materials or components in optical glasses for the manufacture of, e.g., optical fibers and active fiber amplifiers. In this field of application there is an increased demand of new materials useful for fiber optics with improved optical properties. Typical of such materials are, for example, the sulphides of Lanthanum (La
2
S
3
), Praseodymium (Pr
2
S
3
), Holmium (Ho
2
S
3
), Gallium (Ga
2
S
3
) and Germanium (GeS
2
). One of the main requirements to be met for optical fiber applications, specifically for 1.3 &mgr;m fiber optic systems, is ultra high purity of the sulphides, as it is expected that an increase in purity of the sulphides could lead to a substantial increase in efficiency of the fiber optics.
Several methods for the preparation of metal sulphides are known which are based on the reaction of the metal or the metal oxide with sulphur, hydrogen sulphide or other sulphur containing reagents. For a review of methods for the preparation of rare earth metal sulphides, reference is made to Gmelin Handbook of Inorganic Chemistry, 8th Edition, Rare Earth Elements - C7, page 69-74, Springer Verlag, 1983. It is apparent that the methods reported there suffer from severe disadvantages, most of all limited applicability and ineffectiveness, and do not furnish products of high purity.
For instance, high temperature reaction of the metal with sulphur in a sealed silica tube works for a limited range of metals only, where the product sulphide is miscible with excess metal, permitting a continuous reaction of the elemental charge. The partial formation of an insoluble product sulphide, such as La
2
S
3
may cause the reaction to cease and leads to rupture of the ampule and explosion.
High temperature reaction of the metal with hydrogen sulphide is similarly limited by the formation of insoluble surface sulphide which inhibits full progress of the reaction.
High temperature reaction of the oxide with hydrogen sulphide and/or sulphur vapour is effective with elements such as Group Ia metals only, but may leave a high level of several percent of unreacted oxide impurity. It is ineffective in the case of many rare earth oxides.
Moreover, many of the rare earth oxides require temperatures above 1300° C. in this type of reaction which is inconvenient for normal furnace systems.
Carbon disulphide vapor was reported to be a more effective sulphiding agent for several rare earth metal oxides. CS
2
may be supplied into the reaction system by bubbling an inert carrier gas through a bottle of CS
2
liquid and passing the gaseous mixture into the reactor tube containing the oxide and which is heated to temperatures at about 1000° C. This is an unpleasant and dangerous operation, since CS
2
is toxic and highly flammable.
In situ generation of CS
2
from sulphur and carbon in the reaction zone was also reported. Eastman et al., J. Amer. Chem. Soc, 72, 2248 (1950), reported the preparation of cerium sulphide from the dioxide by passing a stream of hydrogen sulphide over it in a carbon furnace at elevated temperatures. An intermediate formation of CS
2
is speculated.
SUMMARY OF THE INVENTION
In-house investigations on these methods now confirmed that CS
2
is a more effective sulphiding agent. To avoid the dangerous handling of CS
2
, in situ generation from carbon and hydrogen sulfide has been studied. In the experiments the metal oxide was contained in a graphite boat which was eroded at elevated temperatures by the hydrogen sulphide in the immediate vicinity of the oxide. Analytical investigation of the products so formed showed, that the metal sulphides contain carbonaceous impurities and/or contamination with binder materials from the graphite.
It was, therefore, the task of the present invention to provide a process by which in a safe, simple and effective manner metal sulphides can be produced which are pure enough for the use in optical glass materials, specifically designated for fiber optics. It has been found out that these objects can be met by a process in which metal oxides are reacted at temperatures between 500 and 1500° C. in a stream of gaseous CS
2
, characterized in that the CS
2
is generated from elementary carbon and gaseous H
2
S by reaction at temperatures between 900 and 1500° C. upstream of the metal oxide.
The object of the invention is therefore a process for the preparation of metal sulphides by reaction of the corresponding metal oxides at temperatures between 500 and 1500° C. in a stream of gaseous CS
2
, characterized in that the CS
2
is generated from elementary carbon and gaseous H
2
S by reaction at temperatures between 900 and 1500° C. upstream of the metal oxide.
The process of the invention is based on the principle of in situ generation of CS
2
from elementary carbon in a stream of gaseous H
2
S at appropriate temperature, followed by the reaction of the so formed gaseous CS
2
with the metal oxide yielding the corresponding metal sulphide. Here, in contrast to known methods utilizing the same principle, the CS
2
is not generated in the intimate vicinity of the oxide but is formed at a separated location upstream of the oxide in a suitably designed reaction apparatus.
The main advantage of this method is that metal sulphides are obtained which are free of impurities and contaminants stemming from the carbon and which show a residual oxide content not exceeding 0.5% by weight. As a rule, the residual oxide content of sulphides obtained with the process of the invention lies within the region of 0.01-0.1% by weight. Thus, this process results in metal sulphides of extreme purity making them highly suitable as optical glass materials, specifically for optical fiber applications.
Further advantages are the easy and safe practicability and the broad applicability of the process, as practically any metal sulphide is accessible from the corresponding oxide.
The process is especially useful in cases where known processes are ineffective or even fail.
The process of the invention is preferably applicable to the preparation of the sulphides of Group IIa metals of the periodic system of the elements, especially the sulphides of Mg, Ca, Sr and Ba, of Group IIla metals, especially of Al and Ga, of Group IVa metals, specifically of Si, Ge and Sn, of transition metals and rare earth metals. Most preferred is the preparation of the sulphides of La, Pr and Ho, namely La
2
S
3
, Pr
2
S
3
and Ho
2
S
3
.
The process may be carried out in any suitably designed reaction apparatus which allows a separate placement of elementary carbon and metal oxide, an appropriate heating of both locations to reaction temperature, and a continuous feeding of gaseous H
2
S coming into contact with the carbon first.
In a preferred embodiment the reaction is carried out in a horizontally placed tube-like reactor in which in a first reaction zone a container is placed which is filled with pulverized elementary carbon and in a second reaction zone a container is placed which is filled with an appropriate amount of metal oxide, and where a stream of gaseous H
2
S is fed into the reactor, each reaction zone being heated to the necessary reaction temperature.
In the first reaction zone conversion of elementary carbon to gaseous CS
2
takes place when gaseous H
2
S is fed over the carbon charge at an appropriate temperature. The temperature should be held at least at 900° C.; 1500° C. appears to be the upper limit for reasons of practicability. Preferably, the conversion temperature is held in the region of 950-1000° C.
In the second reaction zone, the gaseous CS
2
which is generated in the first reaction zone reacts with the charge of metal oxide to form the desired metal sulphide. Suitable reaction temperatures may be chosen between 500° C. and 1500° C., depending on the nature and the specific requirements of the metal ox
Hanney Richard
Kinsman Barry Edward
Griffin Steven P.
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
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