PROCESS FOR THE PREPARATION OF ANHYDROUS SODIUM SULFIDE

PROCESSUS DE PREPARATION DE SULFURE DE SODIUM ANHYDRE

English Abstract

The invention provides a process for the preparation of
anhydrous sodium sulfide by heating sodium sulfide having a
water content of about 35 to 45 % in a drier under reduced
pressure, the temperature of the water-containing sodium
sulfide to be dried being increased from approximately 20°C
at the drier inlet to at least about 180°C at its outlet.
Preferably, the apparatus is a contact-type drier, and the
sodium sulfide is in a comminuted form, more particularly
in the form of scales, lamellae or flakes having an edge
length of from 5 to 8 mm. The method can produce anhydrous
sodium sulfide having a content of at least about 98% Na2S.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A method for the preparation of anhydrous sodium
sulfide by heating water-containing sodium sulfide under
reduced pressure, in which sodium sulfide having a water
content of from 35 to 45 % is conveyed, with mixing,
through an apparatus in which the temperature of the sodium
sulfide is raised continuously from approximately 20°C at
the point where the solid sodium sulfide is fed into the
apparatus to ~ 180°C at the apparatus outlet, while
maintaining a low pressure of < 20 torr.
2. A method according to claim 1, wherein a pressure of
< 11 torr is maintained during conveyance of the sodium
sulfide through the apparatus.
3. A method according to claim 1 or 2, in which the
water-containing sodium sulfide is in comminuted form.
4. A method according to claim 3, wherein the
water-containing sodium sulfide is in the form of scales,
lamellae or flakes having an edge length of from 5 to 8 mm.
5. A method according to any one of claims 1 to 4, in
which the water-containing sodium sulfide is resident in
the apparatus for a time sufficient to provide at the
outlet an anhydrous sodium sulfide having a content of at
least 98 wt.% Na2S.

6. A method according to claim 5, in which the anhydrous
sodium sulfide is about 99 wt.% Na2S.
7. A method according to any one of claims 1 to 6, in
which a contact drier is used as the apparatus.
8. A method according to claim 7, in which the apparatus
is a paddle drier.
9. A method according to claim 7, in which the apparatus
is a plate drier.
10. A method according to claim 9, in which the plates of
the plate drier are combined into sets which provide three
or more temperature zones such that the temperature of the
sodium sulfide in the first zone after feeding is from 20
to 70°C, and its temperature just before the apparatus
outlet is greater than about 180°C.
11. A method according to claim 9 or 10, carried out with
a continuous conveyance of sodium sulfide through the
apparatus.
12. A method according to claim 8, in which the sodium
sulfide dehydration is carried out discontinuously.

Description

CA 022~6134 1998-12-1~
Process For The Preparation Of Anhydrous Sodium Sulfide
The invention relates to the preparation of anhydrous
sodium sulfide by heating water-containing sodium sulfide
under reduced pressure.
Methods for preparing an anhydrous sodium sulfide from
Na2S 9H2O, Na2S 6H2O or Na2S 5H2O are already known from the
prior art.
According to E.P.O. 345,136, the monohydrate is prepared
in the first dehydration step. The elimination of water
requires a time of 2 hours or more. The remaining water
contained in the monohydrate is then removed at a
temperature of from 90 to 200~C and under a reduced
pressure of approximately from 2.66 kPa (20 torr) to 66.5
kPa (500 torr).
A similar, two-step method for the preparation of
crystalline anhydrous sodium sulfide is described in
E.P.O. 361,998.
The disadvantage of these known methods is that it is
necessary to wait for the reaction in the first step, for
example to the monohydrate, to be complete (E.P.O.

CA 022~6134 1998-12-1~
345,136) before this compound can be conveyed to the
second temperature step.
A product that is available commercially is a water-
containing sodium sulfide having an Na2S content of from 60
to 62 %. According to stoichiometric calculation, that
corresponds approximately to Na2S 3H2O.
The introduction of hydrogen sulfide into concentrated
lo sodium hydroxide solution yields, at temperatures above
100~C, a sodium sulfide hydrate melt having a content of
approximately from 60 % to 62 % Na2S. The melt is placed
on a cooling conveyor, on which it solidifies (T < 90~C).
Comminution generally yields a flaky product, which is
then available commercially.
An object of this invention is to provide a method by
means of which there is obtained a sodium sulfide that is
anhydrous to a greater possible extent.
According to this invention, there is provided a process
for the preparation of anhydrous sodium sulfide by heating
water-containing sodium sulfide under reduced pressure, in
which sodium sulfide having a water content of from 35 to
45 %, is conveyed, with mixing, through an apparatus in
which the temperature of the Na2S is raised continuously

CA 022~6134 1998-12-1~
from approximately 20~C at the point where the solid
sodium sulfide is fed into the apparatus to 2 180~C at the
apparatus outlet, while maintaining a low pressure of < 20
torr.
In this manner, undesired melting of the material that is
to be dehydrated is minimized.
A pressure of < 11 torr is preferably maintained during
lo the dehydration process.
For the process according to the invention contact driers
are particularly useful. Contact driers are to be
understood as being apparatuses in which the heat from a
heating surface is transferred to the solid that is in
contact with this surface. Such apparatuses include, for
example, paddle driers and plate driers, the former
generally being operated discontinuously and the latter
continuously.
Plate driers that are installed vertically have proved to
be especially suitable. In these apparatuses, the plates,
which are arranged from top to bottom, are combined into
three or more sets of plates of different temperature
zones. The solid temperature in the first set after the
point at which the solid is fed in is from 20 to 70~C, and

CA 022~6134 1998-12-1~
the solid temperature (product temperature) in front of
the solid outlet is greater than 180~C.
The water-containing Na2S is preferably used in the form of
scales, lamellae or flakes having an edge length of from 5
to 8 mm.
According to the invention it is possible markedly to
reduce the dwell time of the sodium sulfide that is to be
lo dehydrated in the drying process as compared with the
prior art, and to make available a dehydration process
which may be carried out discontinuously and, preferably,
continuously.
Where a vacuum plate drier is used in continuous
operation, the process can yield an anhydrous sodium
sulfide having a content of at least 98 wt.%, and
preferably 99 wt.% Na2S, even after dwell times in the
drier of 2 90 minutes.
Example
A mass flow of from 300 to 400 kg/h of Na2S having a water
content of from 38 to 40 wt.% is fed in the form of flakes
having an edge length of from 5 to 8 mm into a vacuum
plate drier in which a vacuum of < 15 mbar is maintained.

CA 022~6134 1998-12-1~
The crude product passes through the drier in a meandering
manner from top to bottom, during which it is heated and
gives up the water bound in the form of the hydrateO
The individual heating plates are combined to form three
sets of plates, the solid temperature in the uppermost set
of plates rising from approximately 20~C to approximately
70~C and the temperature of the solid as it emerges at the
solid outlet at the end of the third set being 2 180~C.
o The passage time through the drier is 2 90 minutes.
The starting compounds, or the products that are drying,
are regularly rearranged on the plates.
In this manner, a product having a content of more than
99 wt.% Na2S is obtained.