Note: Descriptions are shown in the official language in which they were submitted.
CA 02880255 2015-01-26
Process for preparing an alkali metal
Description
The invention relates to a process for preparing an alkali metal from a salt
of the alkali
metal which is soluble in a solvent.
Alkali metals, which are used as important basic inorganic chemicals, are, in
particular,
lithium, potassium and sodium. Thus, lithium is used, for example, for the
preparation
of organolithium compounds, as alloying additive to aluminum or magnesium and
for
lithium batteries. Lithium is prepared industrially by melt flux electrolysis
of a eutectic
mixture of lithium chloride and potassium chloride at from 400 to 460 C.
However, this
process has a high energy consumption. In addition, the process has the
serious dis-
advantage that only water-free lithium chloride can be used. The lithium
chloride which
is initially present as aqueous solution therefore has to be worked up to give
the water-
free solid in an energy-intensive process. Since lithium chloride is
hygroscopic, drying
and handling requires particular precautions.
When carrying out organolithium reactions, aqueous lithium salt solutions are
frequent-
ly obtained. As a result of the increasing demand for lithium batteries,
lithium-
containing waste is also obtained there. This too, can be converted into
aqueous lithi-
um salt solutions. Since lithium is also very expensive in the form of its
salts, recycling
of lithium is of interest.
Sodium is used, for example, for the preparation of sodium amide, sodium
alkoxides
and sodium borohydride. Sodium is obtained industrially by the Downs process
by
electrolysis of molten sodium chloride. This process has a high energy
consumption of
more than 10 kWh/kg of sodium. Furthermore, the process has the serious disad-
vantage that the electrolysis cells are destroyed by solidification of the
salt melt when
they are switched off. Furthermore, the sodium metal obtained by the Downs
process
has the disadvantage that it is, owing to the process, contaminated with
calcium whose
residual content can only be reduced but never completely eliminated by means
of
subsequent purification steps.
Potassium is used, for example, for the preparation of potassium alkoxides,
potassium
amides and potassium alloys. At present, potassium is obtained industrially
mainly by
reduction of potassium chloride by means of sodium. This firstly forms the
sodium-
potassium alloy NaK which is subsequently fractionally distilled. A good yield
is ob-
CA 02880255 2015-01-26
2
tamed by potassium vapor continually being taken off from the reaction zone,
as a re-
sult of which the equilibrium of the reaction is shifted to the potassium
side. However,
this process operates at high temperatures of about 870 C. In addition, the
potassium
formed comprises about 1% of sodium as impurity and therefore has to be
purified by
means of a further rectification. However, the greatest disadvantage is that
the sodium
used is expensive since it has to be obtained industrially by the Downs
process by
electrolysis of molten sodium chloride.
An alternative process for isolating an alkali metal from aqueous solution is
described
in WO 01/14616 Al. For this purpose, an aqueous solution of an alkali metal
salt is fed
to an electrolysis cell which has a cathode compartment and an anode
compartment
which are separated from one another by a solid electrolyte. The solid
electrolyte has
at least one further ion-conducting layer. The cathode compartment has a solid
cath-
ode core and is filled with a fusible alkali metal or a liquid electrolyte.
The alkali metal is
formed on the cathode and ascends in the liquid electrolyte and can then be
taken off.
Preference is given to using salt melts of the alkali metal to be isolated as
liquid elec-
trolyte. The disadvantage of the process is the increased electrical
resistance and the
unsatisfactory stability of the combination of solid electrolytes and the
further ion-
conducting layer.
A further alternative process for preparing sodium as alkali metal is
described in
DE 195 33 214 Al. Here, an electrolyte comprising essentially sodium
tetrachloroalu-
minate is electrolyzed in an anode space of an electrolysis cell, with
aluminum chloride
formed being given off as vapor and sodium being passed through a solid
electrolyte
which conducts sodium ions and taken off from the cathode space. The
disadvantage
of this process is the coupled production of aluminum chloride and sodium,
when there
is not the same demand for the products.
It is an object of the present invention to provide a process for preparing an
alkali met-
al, which firstly does not have the disadvantages known from the prior art,
especially
has a lower energy consumption and is less complicated to operate in terms of
appa-
ratus.
The object is achieved by a process for preparing an alkali metal from a salt
of the al-
kali metal which is soluble in a solvent, which comprises the following steps:
(a) carrying out of a first electrolysis in a first electrolysis cell
comprising an anode
space and a cathode space, where the anode space and the cathode space of
the first electrolysis cell are separated by a membrane which is permeable to
alkali metal cations, where the salt of the alkali metal dissolved in the
solvent is
fed to the anode space and a suspension comprising sulfur and a second sol-
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vent is fed to the cathode space and a mixture comprising the second solvent,
alkali metal cations, (poly)sulfide anions and anions of oxygen-sulfur com-
pounds is taken off from the cathode space,
(b) concentration of the mixture comprising second solvent, alkali metal
cations,
(poly)sulfide anions and anions of oxygen-sulfur compounds which is taken off
from the cathode space to give a largely solvent-free alkali metal
(poly)sulfide
melt,
(c) carrying out of a second electrolysis at a temperature above the melting
point of
the alkali metal in a second electrolysis cell comprising an anode space and a
cathode space, where the anode space and the cathode space of the second
electrolysis cell are separated by a solid electrolyte which conducts alkali
metal
cations and the alkali metal (poly)sulfide melt from step (b) is fed to the
anode
space and sulfur and unreacted alkali metal (poly)sulfide melt are taken off
from
the anode space and liquid alkali metal is taken off from the cathode space.
The process of the invention is suitable for preparing an essentially pure
alkali metal, in
particular for the preparation of sodium, potassium and lithium, very
particularly prefer-
ably for the preparation of sodium.
For the purposes of the present invention, essentially pure means that the
proportion of
foreign metal impurities in the alkali metal is not more than 30 ppm.
For the purposes of the present invention, (poly)sulfide anions are anions of
the gen-
eral formula Sx2-, where x is any integer from 1 to 6.
For the purposes of the present invention, the term alkali metal (poly)sulfide
encom-
passes all compounds of the general formula
Me2Sx
where Me is the alkali metal, for example sodium, potassium or lithium, and x
is any
integer in the range from 1 to 6.
For the purposes of the present invention, the term largely solvent-free
alkali metal
(poly)sulfide melt means that the alkali metal (poly)sulfide melt comprises
not more
than 5% by weight of solvent, preferably not more than 3% by weight of solvent
and in
particular not more than 1.5% by weight of solvent.
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To prepare the alkali metal, a first electrolysis is carried out in a first
electrolysis cell
comprising an anode space and a cathode space in the first step (a). The salt
of the
alkali metal dissolved in the solvent is fed to the anode space of the
electrolysis cell.
Alkali metal halides are particularly suitable as salt fed to the anode space
of the first
electrolysis cell. Very particular preference is given to using alkali metal
chlorides. The
solvent is for example water or an organic solvent, for example an alcohol.
The solvent
is preferably water. When the process is used for the preparation of sodium,
an aque-
ous sodium chloride solution, in particular, is fed to the anode space of the
first elec-
trolysis cell.
When using an aqueous alkali metal salt solution, for example an aqueous
sodium
chloride solution or an aqueous potassium chloride solution, preference is
given to us-
ing a solution as is also customary in chloralkali electrolysis. Before
introduction into
the anode space of the first electrolysis cell, the alkali metal chloride
solution is usually
purified in order to remove nonalkali metal ions.
When the process is used for preparing sodium and a sodium chloride solution
is fed in
as solution fed to the anode space, this solution preferably comprises not
more than
500 ppm of potassium based on the total amount of sodium and potassium
comprised
in the solution.
When the process is used for preparing potassium, preference is given to using
an
aqueous potassium chloride solution which has likewise, as is known from
chloralkali
electrolysis, been purified and is free of nonalkali metal ions. The solution
preferably
comprises not more than 0.1% by weight of sodium, based on the total amount of
po-
tassium and sodium in the solution.
The solution of the alkali metal salt fed to the anode space of the first
electrolysis cell is
preferably virtually saturated and preferably comprises, for example in the
case of so-
dium chloride, from 5 to 27% by weight, in particular from 15 to 25% by
weight, for ex-
ample 23% by weight, of sodium chloride.
A second solvent and sulfur powder are fed as a suspension to the cathode
space of
the electrolysis cell. The solution fed to the cathode space preferably
additionally corn-
prises electrolyte salts, for example alkali metal hydroxide or particularly
preferably
alkali metal (poly)sulfides, in order to increase the conductivity of the
solution. The al-
kali metal of the alkali metal hydroxide or the alkali metal (poly)sulfides is
preferably the
same as the alkali metal to be isolated. The solution fed to the cathode space
prefera-
bly comprises from 50 to 95% by weight of solvent and from 2 to 25% by weight
of el-
emental sulfur. Furthermore, from 2 to 5% by weight of alkali metal hydroxide
and from
CA 02880255 2015-01-26
0 to 48% by weight of ionic alkali metal sulfur compounds are preferably
comprised.
Particular preference is given to the solution being circulated in continuous
operation in
the cathode space. Second solvent and sulfur powder are continuously
introduced into
the circulated solution, so that the circulated solution comprises a
concentration of from
5 25 to 50% by weight of ionic sulfur compounds. This is achieved by adding
a suspen-
sion composed of from 50 to 82% by weight of water and from 18 to 50% by
weight of
sulfur powder to the circulated solution. The second solvent can be an organic
solvent,
for example an alcohol, or water. The second solvent is preferably water.
The anode space and the cathode space of the first electrolysis cell are
separated by a
membrane which is permeable to alkali metal cations and acts as a barrier to
anions.
Suitable membranes which are permeable to alkali metal cations are all cation-
selective membranes which are permeable to alkali metal cations. Suitable
cation-
permeable membranes are, for example, Nafion0 membranes, which are
commercially
available. Such a membrane usually has a framework of polytetrafluoroethylene
with
immobilized anions, generally sulfonic acid groups and/or carboxylate groups.
The anode used is, for example, an anode as is known from chloralkali
electrolysis. As
regards the electrode design, it is generally possible to use perforated
materials, for
example in the form of meshes, lamellae, oval profile struts, V-struts or
round profile
struts. The anode is preferably a dimensionally stable anode which is
generally made
up of coated titanium, with metal mixed oxides of titanium, tantalum and/or
platinum
metals such as iridium, ruthenium, platinum and rhodium being used for
coating. The
platinum metals and the proportion of the metal are selected so as to achieve
a very
low overvoltage for the formation of chlorine and a very high overvoltage for
oxygen.
For example, the chlorine overvoltage is from 0.1 to 0.4 volt and the oxygen
overvolt-
age is from 0.6 to 0.9 volt. Graphite is in principle also a suitable material
for the anode
but is generally not dimensionally stable under the operating conditions, so
that the
anodes made therefrom have to be adjusted and regularly replaced during
operation in
the cell, while in the case of titanium passivated with mixed oxides, the
coating has to
be replaced only after continuous operation for from 2 to 4 years.
As cathode, it is possible to use a cathode as is known from chloralkali
electrolysis, for
example a stainless steel cathode or a nickel electrode. In a preferred
embodiment, a
graphite felt is additionally introduced into the electrode gap between
stainless steel
cathode and membrane.
The first electrolysis is preferably carried out continuously, with the salt
of the alkali
metal dissolved in a solvent being fed continuously to the anode space and the
ague-
ous sulfur suspension or the (poly)sulfide/sulfur mixture recirculated from
the second
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electrolysis and second solvent being fed continuously to the cathode space.
During
the electrolysis, alkali metal cations migrate as a result of the applied
current through
the cation-selective membrane from the anode side to the cathode side.
Chlorine is
formed at the anode and is removed from the anode space. Furthermore, the
solution
comprising alkali metal salt is taken off from the anode space. The solution
of the alkali
metal salt which is taken off is, in one embodiment, dechlorinated,
concentrated to feed
concentration, purified and recirculated to the anode space. To concentrate
the solu-
tion, it is possible, for example, to introduce alkali metal salt directly
into the solution of
the alkali metal salt.
A mixture of alkali metal (poly)sulfides and ionic sulfur compounds, for
example sul-
fites, thiosulfates, is formed in the cathode space, thus giving an aqueous
solution
comprising alkali metal cations and ionic sulfur compounds. In addition, the
solution
initially comprises unreacted, undissolved elemental sulfur. The solution is
taken off
from the cathode space and preferably circulated in order to concentrate the
product,
namely the alkali metal cations and the ionic sulfur compounds. A substream is
taken
off from the mixture comprising second solvent, alkali metal cations,
(poly)sulfide ani-
ons and ionic sulfur compounds which is taken off from the cathode space and
concen-
trated in step (b).
The electrolysis in step (a) is preferably carried out at a temperature in the
range from
to 120 C, preferably in the range from 50 to 90 C and in particular in the
range from
75 to 85 C. Suitable current densities are in the range from 400 to 4000 A/m2
and suit-
able voltages are in the range from 2.5 to 6 volt.
It has been found in the electrolysis that sulfur is reduced preferentially
over cathodic
splitting of water into hydrogen and hydroxide anions, so that the mixture
leaving the
cathode space comprises alkali metal cations and essentially (poly)sulfide
anions
which on concentration and removal of the solvent form alkali metal
(poly)sulfide.
The mixture comprising second solvent, alkali metal cations and (poly)sulfide
anions
and further ionic sulfur compounds which leaves the cathode space is
concentrated by
removal of the second solvent in step (b). Preference is given here to
concentration of
the mixture comprising second solvent, alkali metal cations and (poly)sulfide
anions
and further ionic sulfur compounds which is taken off from the cathode space
being
carried out in an evaporator.
The evaporator can be operated continuously or batchwise. Here, any evaporator
known to those skilled in the art is suitable for carrying out the
concentration operation
in step (b). For example, circulation evaporators with natural convection,
circulation
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evaporators with forced circulation, falling film evaporators or thin film
evaporators are
suitable for continuous evaporation. In the case of batchwise concentration by
evapora-
tion, a stirred vessel is particularly suitable. Preference is given, both in
continuous
evaporation and in batchwise evaporation, to using an evaporator having a
condenser.
The mixture comprising alkali metal cations, (poly)sulfide anions and further
ionic sulfur
compounds and second solvent which is fed to the evaporator can be preheated
before
introduction into the evaporator. For this purpose, it is possible to use any
apparatus for
heating a liquid stream. Preference is given to using a heat exchanger.
Heating can be
carried out using a heat transfer medium or electrically. Suitable heat
transfer media
are, for example, thermooils, steam or any other heat transfer media known to
those
skilled in the art.
Concentration of the alkali metal cations and (poly)sulfide anions by
evaporation is
preferably carried out at a temperature in the range from 80 to 400 C, in
particular at a
temperature in the range from 120 to 350 C and very particularly preferably at
a tem-
perature in the range from 150 to 300 C. The pressure of the vapor in the
evaporation
is preferably in the range from 0.1 to 2 bar absolute, more preferably in the
range from
0.2 to 1 bar absolute, in particular in the range from 0.5 to 1 bar absolute.
Heating of the evaporator used can, for example up to 200 C, be carried out
using
steam. Here, it is firstly possible to convey the steam through a pipe in an
appropriate
heat exchanger or to use an apparatus having a double wall. Heating both by
means of
a pipe conducted through the apparatus and by means of a double wall is also
possi-
ble. Apart from steam, any other heat transfer medium, for example a thermooil
or a
salt melt, can also be used. Furthermore, the heat necessary for evaporation
can be
supplied by means of electric heating or direct firing.
The evaporation can be carried out in one or more stages. In the case of a
multistage
evaporation, it is also advantageous for countercurrent vapor recirculation
with or with-
out vapor compression to be provided. The multistage evaporation is preferably
carried
out in a cascaded manner. In the case of cascaded evaporation, the same or
different
types of evaporator can be used in the individual stages of the evaporator
cascade.
The evaporation in step (b) forms an overhead stream comprising second solvent
and
possibly hydrogen sulfide.
The bottoms stream obtained in the evaporation comprises sulfur, alkali metal
(poly)sulfide and further ionic sulfur compounds and also traces of second
solvent and
possibly also sodium thiosulfate and sodium hydroxide. The evaporation residue
in the
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preparation of sodium preferably comprises, in terms of elemental analysis,
from 65 to
75% by weight of sulfur, from 20 to 25% by weight of sodium and from 4 to 10%
by
weight of oxygen, for example a proportion of 69% by weight of sulfur, 23% by
weight
of sodium and 8% by weight of oxygen.
In the preparation of potassium, the evaporation residue comprises, in terms
of ele-
mental analysis, for example from 60 to 70% by weight of sulfur, from 25 to
37% by
weight of potassium and from 4 to 10% by weight of oxygen.
After concentration of the mixture comprising second solvent, alkali metal
cations, fur-
ther ionic sulfur compounds and (poly)sulfide anions by evaporation in step
(b), the
concentrated mixture obtained as bottoms stream in the evaporation can, in a
preferred
embodiment, be purified to remove the ionic sulfur-oxygen compounds comprised
therein before carrying out the second electrolysis in step (c).
To carry out the purification, preference is given to bringing the bottoms
stream from
step (b) into contact with a gaseous stream comprising hydrogen sulfide. The
hydrogen
sulfide used for purification is preferably technical-grade hydrogen sulfide.
In addition to
the hydrogen sulfide, the gas stream fed in can also comprise gases which are
inert in
the process. Examples of gases which are inert in the process and can be
comprised
are nitrogen, hydrogen or noble gases, in particular nitrogen.
In the purification, alkali metal hydroxide, for example, still comprised in
the bottoms
stream reacts with the hydrogen sulfide to form alkali metal (poly)sulfide and
water. At
the same time, second solvent still comprised or water formed in the reaction
is re-
moved from the mixture so that essentially impurity-free alkali metal
(poly)sulfide is
formed.
To carry out the purification, the concentrated mixture from (b) and the
gaseous stream
comprising hydrogen sulfide are preferably conveyed in countercurrent. Here,
particular
preference is given to using a column, with the concentrated mixture from step
(b) be-
ing fed in at the top of the column and the gaseous stream comprising hydrogen
sulfide
being fed in via a side inlet. The hydrogen sulfide ascends in the column and
the con-
centrated mixture from step (b) runs downward in the column.
The column used is preferably a column with internals. Suitable internals are,
for ex-
ample, trays, random packing elements or structured packings.
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The apparatus in which the purification is carried out, for example the
column, is pref-
erably dimensioned so that a residence time of the concentrated mixture from
step (b)
of from at least 10 s to 30 min, preferably at least 2 min, is achieved.
In a preferred embodiment, the column in which the purification is carried out
is addi-
tionally heated below the side inlet for the gaseous stream comprising
hydrogen sul-
fide. Heating can be effected, for example, by means of a double wall or a
pipe which is
installed in the column and through which a heat transfer medium flows. As an
alterna-
tive, electric heating is also conceivable. Suitable heat transfer media are,
for example,
steam, thermooils or salt melts.
As a result of the additional heating, hydrogen sulfides formed in the mixture
are disso-
ciated into hydrogen sulfide and alkali metal (poly)sulfide. For this purpose,
a tempera-
ture in the range from 320 to 400 C, preferably in the range from 340 to 350
C, is set in
the column by means of the additional heating.
At the bottom of the apparatus for carrying out the purification, a mixture
comprising
essentially alkali metal (poly)sulfides is obtained. In addition, further
impurities in
amounts of not more than 0.5% by weight, preferably not more than 0.1% by
weight,
can be comprised. Such impurities comprise, in particular, alkali metal
hydroxide.
At the top of the apparatus for the additional purification, a gas stream
comprising sec-
ond solvent and hydrogen sulfide is obtained. The gaseous stream comprising
second
solvent and hydrogen sulfide which is taken off from the top of the
purification appa-
ratus, in particular the column, is fed to a condenser. In the condenser, the
second sol-
vent is condensed out from the stream comprising second solvent and hydrogen
sulfide
and is taken off. The second solvent condensed out is generally still
contaminated with
hydrogen sulfide and is preferably fed to the cathode space of the first
electrolysis. The
gaseous, essentially solvent-free hydrogen sulfide is recirculated to the
column.
When a multistage, cascaded evaporation is used in step (b), it is possible to
carry out
the additional purification in one of the evaporation stages, preferably in
the last evapo-
ration stage when the second solvent has been virtually completely removed.
After concentration of the mixture comprising second solvent, alkali metal
cations and
(poly)sulfide anions in step (b) or the additional purification, the resulting
stream com-
prising alkali metal (poly)sulfide is fed to a second electrolysis.
The second electrolysis is preferably carried out in a second electrolysis
cell made up
of an anode space and a cathode space which are separated by a solid
electrolyte
CA 02880255 2015-01-26
which conducts alkali metal cations. Suitable electrolysis cells for the
second electroly-
sis are, in particular, electrolysis cells whose structure corresponds to the
structure of
electrolysis cells which can be used in sodium-sulfur batteries.
5 The solid electrolyte is preferably a ceramic which conducts alkali metal
cations, in par-
ticular I3-aluminum oxide, 13"-aluminum oxide or 13/13"-aluminum oxide. Alkali
metal cati-
ons of the alkali metal to be prepared are in each case bound in the ceramics.
Apart from the alkali metal 13-aluminum oxide, alkali metal 13"-aluminum oxide
or alkali
10 metal 13/13."-aluminum oxide, corresponding alkali metal analogues of
NASICONO ce-
ramics are also suitable. The alkali metal used is in each case the alkali
metal which is
to be isolated by means of the process of the invention.
When the alkali metal which is to be prepared is lithium, LISICONs and
particularly
preferably Li ion conductors having a garnet structure, for example
Li5La3Ta2012 or
Li7La3Zr2012, are also suitable.
In the second electrolysis cell, the alkali metal (poly)sulfide melt obtained
in the con-
centration operation in step (b), or the alkali metal (poly)sulfide from the
additional pun-
fication, is electrochemically separated into alkali metal and sulfur. The
electrolysis is
carried out at a temperature at which the alkali metal to be prepared is
present in mol-
ten form. The electrolysis is preferably carried out at a temperature in the
range from
290 to 330 C, in particular from 310 to 320 C, under atmospheric pressure.
On the anode side of the electrolysis cell, an electrode composed of a
stainless steel
stabilized with molybdenum, for example stainless steel having the material
number
1.4571, which can be chromium-plated, or an electrode composed of a chromium
steel,
for example steel having the material number 1.7218, is preferably used. The
cathode
is preferably an alkali metal electrode. Here, the alkali metal isolated also
serves as
electrode.
To carry out the second electrolysis, the alkali metal (poly)sulfide is fed in
liquid form to
the anode space. The alkali metal (poly)sulfide is dissociated into alkali
metal cations
and (poly)sulfide anions. The alkali metal cations are conducted through the
solid elec-
trolyte and thus go into the cathode space. In the cathode space, the alkali
metal cati-
ons take up electrons and thus form the molten alkali metal. In the anode
space, the
(poly)sulfide anions release electrons to the anode, so that reduced
(poly)sulfides are
initially formed and sulfur is ultimately formed. Owing to the temperature of
the electrol-
ysis, the sulfur is present in liquid form and can be taken off from the anode
space. The
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1 1
sulfur is usually taken off from the upper part of the anode space since
sulfur has a
lower density than alkali metal (poly)sulfide. The sulfur therefore ascends.
The sulfur obtained in the second electrolysis and unreacted ionic sulfur
compounds
are, in a particularly preferred embodiment, recirculated to the first
electrolysis. For this
purpose, the sulfur together with the unreacted ionic sulfur compounds is
preferably
sprayed in the form of a melt into the suspension fed into the cathode space
of the first
electrolysis. Here, the melt solidifies and sulfur particles finely dispersed
in the second
solvent are formed.
Examples of the invention are shown in the figures and are described in more
detail in
the following description.
In the figures:
Figure 1 shows a process flow diagram of the first electrolysis,
Figure 2 shows a process flow diagram of the concentration operation,
Figure 3 shows a process flow diagram of the additional purification,
Figure 4 shows a process flow diagram of the second electrolysis,
Figure 5 shows a process flow diagram of the overall process,
Figure 6 shows a laboratory electrolysis all for carrying out the second
electrolysis.
In figure 1, the first electrolysis is shown in the form of a process flow
diagram.
A first electrolysis cell 1 comprises an anode space 3 and a cathode space 5
which are
separated from one another by a membrane 7. An anode 9 which is preferably
made of
coated titanium, with the coating being made up of metal mixed oxides of
titanium, tan-
talum and/or platinum metals such as iridium, ruthenium, platinum and rhodium,
is pre-
sent in the anode space 3. A cathode 11 which is preferably made of stainless
steel is
accommodated in the cathode space 5.
An alkali metal salt solution is fed from a first reservoir 15 via a first
feedline 13 to the
anode space 3. The alkali metal salt solution comprised in the first reservoir
15 is pref-
erably an aqueous alkali metal halide solution, for example an aqueous alkali
metal
chloride solution. The alkali metal halide is very particularly preferably
sodium chloride.
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12
The alkali metal salt is preferably dissolved in water as solvent. However, it
is also pos-
sible to dissolve the alkali metal salt in a suitable organic solvent, for
example an alco-
hol. `a
For this purpose, the alkali metal salt is fed via an alkali metal salt line
17 into the first
reservoir 15 and the solvent, in particular water, is fed in via a solvent
line 19.
Application of an external voltage closes a current circuit and chlorine is
formed at the
anode 9 and is taken off together with circulated alkali metal salt solution
from the an-
ode space 3.
In a degassing unit 21, the chlorine is taken off from the stream taken off
from the an-
ode space and the remaining stream is recirculated to the first reservoir 15.
The chlo-
rine is taken off from the process via a chlorine offtake line 23.
In the electrolysis cell 1, alkali metal cations pass through the cation-
selective mem-
brane 17 into the cathode space 5. A suspension comprising elemental sulfur
and sec-
ond solvent, for example an organic solvent or water, preferably water, flows
via a sec-
ond feedline 25 into the cathode space.
For this purpose, elemental sulfur is introduced via a sulfur feedline 27 into
a second
reservoir 31 and second solvent is fed in via a solvent feedline 29 and the
two are
mixed there. From the second reservoir 31, the mixture comprising second
solvent and
sulfur is conveyed via the second feedline 25 into the cathode space 5 of the
first elec-
trolysis cell. A small amount of alkali metal hydroxide can additionally be
added to the
mixture comprising second solvent and sulfur in the second reservoir 31 in
order to
increase the conductivity of the mixture.
As an alternative to the first reservoir 15 in which solvent and alkali metal
salt are
mixed and the second reservoir 31 in which elemental sulfur and second solvent
are
mixed, it is also possible to use any other mixing apparatus known to those
skilled in
the art. For example, it is also possible to spray the sulfur as a melt into
the second
solvent and then feed it to the cathode space 5. Furthermore, it is also
possible, for
example, to meter the alkali metal salt directly into a pipe conveying the
solvent.
A mixture comprising second solvent, alkali metal cations and (poly)sulfide
anions is
taken off from the cathode space 5 via a cathode discharge line 33. In
addition, the
mixture taken off via the cathode discharge line 33 can also comprise alkali
metal hy-
CA 02880255 2015-01-26
13
droxide. The alkali metal cations and (poly)sulfide anions comprised in the
mixture
usually form an alkali metal (poly)sulfide.
In one embodiment, the mixture taken off via the cathode discharge line 33 is
circulated
and enriched with sulfur and second solvent. For this purpose, it is possible,
for exam-
ple, to firstly recirculate the mixture taken off via the cathode discharge
line 33 to the
second reservoir 31.
When no mixture taken off via the cathode discharge line 33 is circulated, the
mixture
comprising second solvent, alkali metal cations and (poly)sulfide anions which
is taken
off via the cathode discharge line 33 is fed to a concentration operation.
When the mix-
ture taken off via the cathode discharge line 33 is circulated, a substream is
taken off
and fed to the concentration operation. Figure 2 shows by way of example a
concentra-
tion operation by means of evaporation in the form of a flow diagram.
The stream comprising second solvent, alkali metal cations and (poly)sulfide
anions
which is taken off as cathode discharge stream 33 is fed to an evaporator 41.
The
evaporator 41 is, for example, as shown in figure 2, a circulation evaporator
with natu-
ral convection. As an alternative, it is also possible to use a circulation
evaporator with
forced circulation, a falling film evaporator or a thin film evaporator. Any
other evapora-
tors known to those skilled in the art can also be used. When the evaporation
is to be
carried out batchwise, it is also possible to use, for example, a stirred
vessel in place of
the circulation evaporator with natural convection depicted here.
The evaporator 41 is preferably equipped with a liquid precipitator 43.
When using a circulation evaporator, liquid goes via a circulation line 45
into an evapo-
rator unit 47. The evaporator unit 47 can, for example, be in the form of a
shell-and-
tube heat exchanger. Here, a heat transfer medium, for example steam,
thermooil or a
salt melt, flows through the tubes of the shell-and-tube heat exchanger. In
addition or
as an alternative, the evaporator unit 47 can have a double wall for heating.
Further-
more, it is also possible for heating to be carried out electrically or by
means of direct
firing instead of heating by means of a heat transfer medium. An overhead
stream
comprising gaseous second solvent, liquid second solvent, alkali metal cations
and
(poly)sulfide anions is taken off at the top of the evaporator unit 47 and fed
to the liquid
precipitator 43. In the liquid precipitator 43, the gaseous second solvent is
separated
off and taken off from the process via a solvent offtake line 49. The mixture
comprising
second solvent, alkali metal cations and (poly)sulfide anions is circulated
until the de-
sired concentration of residual solvent is obtained. As soon as a steady state
is
reached, mixture comprising second solvent, alkali metal cations and
(poly)sulfide ani-
CA 02880255 2015-01-26
14
ons is uniformly fed in via the cathode discharge line 33 opening into the
circulation line
45 and before introduction of the mixture from the circulation line, the
concentrated
mixture comprising second solvent and alkali metal (poly)sulfide is taken off
via an
offtake line 51.
In a preferred embodiment, the mixture taken off via the offtake line 51 is
purified fur-
ther. The purification is shown schematically in figure 3 with the aid of a
flow diagram.
The concentrated alkali metal (poly)sulfide melt is optionally fed to a
preheater 53 and
heated in this. Preheating can, for example, be carried out electrically, by
means of a
heat transfer medium, for example steam, a thermooil or a salt melt. The
preheated
alkali metal (poly)sulfide melt is then preferably fed into the upper region
of a column
55. The column 55 generally comprises internals, for example trays, random
packing
elements or structured or unstructured packing.
In the lower region of the column 55, hydrogen sulfide is introduced via a
side feedline
57. The hydrogen sulfide can additionally be mixed with an inert gas, for
example nitro-
gen. In the interior of the column 55, the hydrogen sulfide and the alkali
metal
(poly)sulfide melt are preferably conveyed in countercurrent and intensively
mixed. As
a result, any alkali metal hydroxide still comprised in the alkali metal
(poly)sulfide melt
is converted into alkali metal (poly)sulfide and water.
An overhead stream 59 comprising water and hydrogen sulfide is taken off at
the top of
the column 55. The overhead stream 59 is introduced into a condenser 61 in
which the
water is condensed out. The remaining hydrogen sulfide present in gaseous form
is
conveyed via a circulation line 63 back to the column 55. The water, which may
still
comprise residues of hydrogen sulfide, is taken off from the condenser 61 and,
if water
is used as second solvent, recirculated via an offtake line 65 to the cathode
space of
the first electrolysis.
A stream 67 which comprises essentially solvent-free alkali metal
(poly)sulfide is taken
off at the bottom of the column 55.
The alkali metal (poly)sulfide melt obtained in the evaporation or the stream
67 corn-
prising alkali metal (poly)sulfide which is obtained when carrying out the
work-up as
shown in figure 3 is fed to a second electrolysis. This is shown by way of
example in
figure 4.
The second electrolysis can be carried out in a plurality of stages. For this
purpose, a
plurality of electrolysis cells 71 are connected in parallel.
CA 02880255 2015-01-26
The electrolysis cells 71 each have an anode space 73 in which a plurality of
electrode
units 75 are installed in the embodiment depicted here. The electrode units 75
each
comprise a cylindrical body composed of a solid electrolyte and thus separate
a cath-
5 ode space located in the interior of the solid electrolyte from the anode
space 73. The
alkali metal (poly)sulfide melt from the evaporation shown in figure 2 or,
when a further
purification is carried out, the alkali metal (poly)sulfide from the
purification shown in
figure 3 is fed via a feedline 79 to the anode space 73 of the respective
electrolysis
cells.
During operation of the electrolysis cells 71, the alkali metal (poly)sulfide
is dissociated
electrolytically into alkali metal and sulfur. Here, alkali metal cations pass
through the
solid electrolyte which conducts alkali metal cations into the cathode space
in which
alkali metal is formed. The alkali metal is taken off from the cathode space
and dis-
charged via a product line 77. At the same time, sulfur is formed from the
polysulfide at
the anode. The electrolysis is operated at a temperature at which the alkali
metal is
present in liquid form.
For this purpose, a stainless steel electrode is preferably accommodated in
the anode
space. The sulfur formed rises since it has a lower density than the alkali
metal
(poly)sulfide. The sulfur can then be taken off via a sulfur offtake line 81
at the upper
part of the anode space 73. The sulfur taken off via the sulfur offtake line
81 is prefera-
bly recirculated to the first electrolysis shown in figure 1. For this
purpose, the sulfur is,
for example, conveyed via the sulfur feedline 27 to the second reservoir 31.
As an al-
ternative, it is also possible, as described above, to spray the sulfur taken
off as sulfur
melt from the second electrolysis into the second solvent and then feed it to
the first
electrolysis cell 1.
The overall process without the additional purification shown in figure 3 is
shown by
way of example in figure 5.
When sodium is to be prepared by the process of the invention, sodium chloride
is in-
troduced via the alkali metal salt feedline 17 and preferably water is
introduced via the
solvent feedline 19, the sodium chloride is dissolved in the water and
introduced via the
first feedline 13 into the electrolysis cell. In the first electrolysis cell
1, the sodium chlo-
ride is separated into sodium ions and chlorine. The chlorine is taken off
together with
circulating sodium chloride solution from the anode space of the first
electrolysis cell 1.
The chlorine is separated off and removed from the process via the chlorine
offtake line
23. The remaining sodium chloride solution is concentrated by addition of
additional
sodium chloride and conveyed back into the anode space of the first
electrolysis cell 1.
CA 02880255 2015-01-26
16
The sodium ions pass through the cation-permeable membrane 7 and go into the
cath-
ode space 5. A mixture comprising solvent, preferably water, and sulfur flows
through
the cathode space 5. Since sulfur is reduced preferentially over hydrogen,
sodium
(poly)sulfide is formed in the cathode space and the sodium (poly)sulfide is
dissociated
into sodium cations and (poly)sulfide anions. The solution comprising sodium
(poly)sulfide is fed from the cathode space to the evaporator 41. In the
evaporator 41,
the sodium (poly)sulfide is concentrated by evaporation of the water. The
concentrated
sodium (poly)sulfide is subsequently fed to the second electrolysis cells 71
in which the
sodium (poly)sulfide is electrolytically dissociated into sodium and sulfur.
The sodium
ions pass through the solid electrolyte which conducts sodium ions and go into
the
cathode space from which the sodium formed there is taken off in molten form.
Sulfur is
taken off from the anode space and recirculated to the first electrolysis.
Example:
First electrolysis stage:
The electrolysis of the aqueous sodium chloride solution was carried out in
the electrol-
ysis cell shown in figure 1. The electrolysis cell was divided by means of a
cation-
exchanging membrane (Nafion0 324) into an anode space and a cathode space. As
anode, use was made of an Ru/lr-titanium mixed oxide-coated titanium anode in
the
form of expanded metal. The cathode was stainless steel expanded metal having
the
material number 1.4571.
The electrolysis was carried out batchwise with stepwise introduction of
further sodium
chloride. The anolyte was circulated by pumping from the first reservoir 15
through the
anode space 3 of the electrolysis cell by means of a laboratory centrifugal
pump. At the
beginning, 1566 g of a 23% strength aqueous sodium chloride solution were
placed in
the cell as anolyte.
The catholyte was circulated by pumping from the second reservoir 31 through
the
cathode space 5 of the electrolysis cell by means of a laboratory centrifugal
pump. At
the beginning, 1700 g of a 2.5% strength aqueous sodium tetrasulfide solution
were
placed in the cell as catholyte. 80 g of sulfur powder were added to this
solution.
The electrolysis was carried out at a temperature in the range from 75 C to 80
C, a
current density i of 2000 A/m2 and a cell voltage in the range from 3.5 to 5
volt.
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17
The electrolysis was carried out batchwise in 4 stages of 40 Ah each, so that
a total of
160 Ah were introduced into the cell. After the first electrolysis stage with
40 Ah, 85 g of
sodium chloride were added to the anolyte and 80 g of sulfur were added to the
catho-
lyte. This was carried out a total of 3 times, so that a total of 320 g of
sulfur and 225 g
of sodium chloride were added.
During the electrolysis, the anode side was flushed with nitrogen. The anode-
side off-
gas went through two scrubbers which were operated using 10% strength aqueous
NaOH and were connected in series.
The cathode side was likewise flushed with nitrogen. The cathode-side offgas
was
passed through a gas analysis instrument which determined the hydrogen
content.
The solutions were discharged after the electrolysis and subjected to
elemental analy-
sis.
Analytical results:
Anolyte discharged: 864 g
Chloride 13.9% by weight
Sulfur 0.01% by weight
Sodium ion 9.2% by weight
Catholyte discharged: 2294 g
Chloride 0.06% by weight
Sulfur 15.2% by weight
Sodium ion 6.5% by weight
175 g of chloride were found in the two scrubbers for the anode offgas.
Concentration:
The cathode output was evaporated batchwise in an electrically heated
distillation flask
at increasing temperature and while stirring. The boiling temperature
increased from
102 C to 200 C during the concentration operation. The evaporation system was
lim-
ited to 200 C. The contents of the distillation flask remained liquid over the
course of
the concentration operation. The distillation was stopped when no more
distillate went
over.
1684 g of vapor condensate were obtained. The contents of the flask were then
cooled
to room temperature, resulting in the contents solidifying. The solidified
sodium
(poly)sulfide melt was crushed in a glove box made inert by means of nitrogen,
giving a
CA 02880255 2015-01-26
18
sodium (poly)sulfide powder. A partial amount of this sodium (poly)sulfide
powder was
subjected to quantitative elemental analysis.
Analytical results:
Catholyte concentrate discharged: 494.4 g
Oxygen 10.4% by weight
Sulfur 59.0% by weight
Sodium 28.4% by weight
Second electrolysis stage:
The electrolysis of the sodium (poly)sulfide melt was carried out in the
laboratory appa-
ratus shown in figure 6, which was provided with electric heating 101 and a
steel hous-
ing 100. The electrolysis cell 90 was a U-tube made of borosilicate glass,
with the two
electrodes together with the ceramic membrane being arranged in an
electrolysis leg
91 while the second leg 92 remained without internals. The membrane 93 was a
beta"-
A1203 ceramic which conducted sodium ions. The membrane 93 had the form of a
tube
closed at one end, with sodium 94 being kept within the tube and the sodium
(poly)sulfide melt 95 being kept outside the tube. The useable surface of the
tubular
membrane 93 was 14 cm2. As anode 96, use was made of a graphite felt type
GFD5EA
(from SGL) which was connected electrically to the plus side of the power
supply via 4
contact plates 97 made of chromium-plated steel having the material number
1.4404.
The molten sodium 94 which was electrically connected via a stainless steel
rod 98 to
the minus side of the power supply served as cathode. Both electrolysis
chambers
were made inert by means of nitrogen.
The electrolysis was carried out batchwise. Before commencement of the
electrolysis,
40 g of the sodium (poly)sulfide powder obtained in the glove box after
concentration
were introduced into the free leg 92 of the U-tube. The filling opening 99 was
then
closed. The electrolysis apparatus was then heated from room temperature to
300 C
over a period of 10 hours. This resulted in the sodium (poly)sulfide powder
melting.
This melt was transferred to the electrolysis zone by means of application of
slightly
superatmospheric pressure to the free leg.
The electrolysis was carried out at a temperature in the range from 290 C to
310 C, a
current of 1.4 A and a cell voltage in the range from 2.5 to 3 volt over an
electrolysis
time of 7 h.
After the electrolysis, 8 g of sodium metal were discharged.
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19
List of reference numerals
1 first electrolysis cell
3 anode space
5 cathode space
7 membrane
9 anode
11 cathode
13 first feedstream
15 first reservoir
17 alkali metal salt line
19 solvent line
21 degassing unit
23 chlorine offtake line
25 second feedstream
27 sulfur feedline
29 solvent feedline
31 second reservoir
33 cathode discharge stream
41 evaporator
43 liquid precipitator
45 circulation line
47 evaporator unit
49 solvent off-take line
51 offtake line
53 preheater
55 column
57 side inlet
59 overhead stream
61 condenser
63 circulation line
65 offlake line
67 stream comprising essentially alkali metal (poly)sulfide
71 second electrolysis cell
73 anode space
75 electrode unit
77 product line
79 feedline
81 sulfur discharge line
90 electrolysis cell
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91 electrolysis leg
92 free leg
93 membrane
94 sodium
5 95 sodium (poly)sulfide melt
96 anode
97 contact plate
98 stainless steel rod
99 filling opening
10 100 steel housing
101 electric heating
