U.S. patent application number 13/951643 was filed with the patent office on 2014-01-30 for process for preparing an alkali metal.
This patent application is currently assigned to BASF SE. Invention is credited to Anna Katharina Durr, Katrin Freitag, Karolin Geyer, Gunther Huber, Susanna Voges, Jesus Enrique Zerpa Unda.
Application Number | 20140027300 13/951643 |
Document ID | / |
Family ID | 49993812 |
Filed Date | 2014-01-30 |
United States Patent
Application |
20140027300 |
Kind Code |
A1 |
Huber; Gunther ; et
al. |
January 30, 2014 |
PROCESS FOR PREPARING AN ALKALI METAL
Abstract
Process for preparing an alkali metal from a salt of the alkali
metal which is soluble in a solvent, including a first
electrolysis, a concentration, and a second electrolysis. The first
electrolysis produces a product mixture. This product mixture is
then concentrated to give a largely solvent-free alkali metal
(poly)sulfide melt. A second electrolysis at a temperature above
the melting point of the alkali metal is then performed in a second
electrolysis cell comprising an anode space and a cathode space,
separated by a solid electrolyte which conducts alkali metal
cations. The alkali metal (poly)sulfide melt from the concentration
step is fed to the anode space. Sulfur is removed from the anode
space and liquid alkali metal is removed from the cathode
space.
Inventors: |
Huber; Gunther;
(Ludwigshafen, DE) ; Freitag; Katrin;
(Ludwigshafen, DE) ; Durr; Anna Katharina;
(Ludwigshafen, DE) ; Zerpa Unda; Jesus Enrique;
(Viernheim, DE) ; Voges; Susanna; (Shanghai,
CN) ; Geyer; Karolin; (Mannheim, DE) |
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
49993812 |
Appl. No.: |
13/951643 |
Filed: |
July 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61676360 |
Jul 27, 2012 |
|
|
|
Current U.S.
Class: |
205/345 |
Current CPC
Class: |
C25C 7/005 20130101;
C25C 3/02 20130101 |
Class at
Publication: |
205/345 |
International
Class: |
C25C 3/02 20060101
C25C003/02 |
Claims
1.-14. (canceled)
15. A process for preparing an alkali metal from a salt of the
alkali metal which is soluble in a solvent, said process comprising
(a) carrying out 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,
wherein the salt of the alkali metal dissolved in a first solvent
is fed to the anode space and a suspension comprising sulfur and a
second solvent is fed to the cathode space, and wherein a mixture
comprising the second solvent, the alkali metal cations,
(poly)sulfide anions, and further ionic sulfur compounds, is taken
off from the cathode space, (b) concentrating the mixture
comprising the second solvent, the alkali metal cations, the
(poly)sulfide anions, and the further ionic sulfur compounds, which
is taken off from the cathode space, to give a largely solvent-free
alkali metal (poly)sulfide melt, (c) carrying out 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 wherein the alkali metal
(poly)sulfide melt from step (b) is fed to the anode space and
sulfur is taken off from the anode space and liquid alkali metal is
taken off from the cathode space.
16. The process of claim 15, wherein concentration of the alkali
metal cations and the (poly)sulfide anions in the mixture
comprising the second solvent, the alkali metal cations, the
(poly)sulfide anions, and the further ionic sulfur compounds, which
is taken off from the cathode space of the first electrolysis cell,
is carried out in an evaporator.
17. The process of claim 15, wherein the concentrating step is
carried out at a temperature in the range from 80 to 400.degree. C.
and a pressure of the vapor in the range from 0.1 to 2 bar
absolute.
18. The process of claim 15, wherein the concentrated mixture
obtained in step (b) is purified before carrying out the second
electrolysis.
19. The process of claim 18, wherein the concentrated mixture
obtained in step (b) is brought into contact with a gaseous stream
comprising hydrogen sulfide to effect purification.
20. The process of claim 19, wherein the concentrated mixture
obtained in step (b) and the gaseous stream comprising hydrogen
sulfide are conveyed in counter-current.
21. The process of claim 19, wherein the purification is carried
out in a column, and wherein the concentrated mixture obtained in
step (b) is fed in at the top of the column and the gaseous stream
comprising hydrogen sulfide is fed in via a side inlet.
22. The process of claim 21, wherein the column is heated below the
side inlet for the gaseous stream comprising hydrogen sulfide.
23. The process of claim 15, wherein the solid electrolyte which
conducts alkali metal cations of the second electrolysis cell
comprises alkali metal .beta.-aluminum oxide, alkali metal
.beta.''-aluminum oxide or alkali metal .beta./.beta.''-aluminum
oxide.
24. The process of claim 15, wherein the sulfur taken off from the
anode space of the second electrolysis cell is recirculated to the
first electrolysis in step (a).
25. The process of claim 15, wherein the alkali metal is sodium,
potassium, or lithium.
26. The process of claim 15, wherein the salt of the alkali metal
is an alkali metal halide.
27. The process of claim 15, wherein the salt of the alkali metal
is sodium chloride.
28. The process of claim 15, wherein the first solvent and/or the
second solvent is water.
Description
[0001] The invention relates to a process for preparing an alkali
metal from a salt of the alkali metal which is soluble in a
solvent.
[0002] 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.degree. C. However, this
process has a high energy consumption. In addition, the process has
the serious disadvantage 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.
[0003] When carrying out organolithium reactions, aqueous lithium
salt solutions are frequently 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
lithium salt solutions. Since lithium is also very expensive in the
form of its salts, recycling of lithium is of interest.
[0004] 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
disadvantage 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.
[0005] 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 obtained by potassium vapor continually
being taken off from the reaction zone, as a result of which the
equilibrium of the reaction is shifted to the potassium side.
However, this process operates at high temperatures of about
870.degree. 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.
[0006] An alternative process for isolating an alkali metal from
aqueous solution is described in WO 01/14616 A1. 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 cathode
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 electrolyte. 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.
[0007] A further alternative process for preparing sodium as alkali
metal is described in DE 195 33 214 A1. Here, an electrolyte
comprising essentially sodium tetrachloroaluminate 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.
[0008] It is an object of the present invention to provide a
process for preparing an alkali metal, 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
apparatus.
[0009] The object is achieved by a process for preparing an alkali
metal from a salt of the alkali metal which is soluble in a
solvent, which comprises the following steps: [0010] (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 solvent is
fed to the cathode space and a mixture comprising the second
solvent, alkali metal cations, (poly)sulfide anions and anions of
oxygen-sulfur compounds is taken off from the cathode space, [0011]
(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, [0012] (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.
[0013] 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 preferably for the
preparation of sodium.
[0014] 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.
[0015] For the purposes of the present invention, (poly)sulfide
anions are anions of the general formula S.sub.x.sup.2-, where x is
any integer from 1 to 6.
[0016] For the purposes of the present invention, the term alkali
metal (poly)sulfide encompasses all compounds of the general
formula
Me.sub.2S.sub.x
[0017] where Me is the alkali metal, for example sodium, potassium
or lithium, and x is any integer in the range from 1 to 6.
[0018] 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.
[0019] 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 aqueous sodium chloride solution,
in particular, is fed to the anode space of the first electrolysis
cell.
[0020] 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 using 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.
[0021] 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.
[0022] 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 potassium and sodium in the solution.
[0023] 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 sodium
chloride, from 5 to 27% by weight, in particular from 15 to 25% by
weight, for example 23% by weight, of sodium chloride.
[0024] 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 comprises electrolyte
salts, for example alkali metal hydroxide or particularly
preferably alkali metal (poly)sulfides, in order to increase the
conductivity of the solution. The alkali 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 preferably comprises from 50 to 95% by weight of solvent and
from 2 to 25% by weight of elemental sulfur. Furthermore, from 2 to
5% by weight of alkali metal hydroxide and from 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 25 to 50% by weight of ionic sulfur compounds. This is
achieved by adding a suspension 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.
[0025] 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,
Nation.RTM. 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.
[0026] 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 overvoltage 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.
[0027] 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.
[0028] 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 aqueous
sulfur suspension or the (poly)sulfide/sulfur mixture recirculated
from the second 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 solution, it is possible, for
example, to introduce alkali metal salt directly into the solution
of the alkali metal salt.
[0029] A mixture of alkali metal (poly)sulfides and ionic sulfur
compounds, for example sulfites, 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 anions and ionic sulfur compounds which is
taken off from the cathode space and concentrated in step (b).
[0030] The electrolysis in step (a) is preferably carried out at a
temperature in the range from 25 to 120.degree. C., preferably in
the range from 50 to 90.degree. C. and in particular in the range
from 75 to 85.degree. C. Suitable current densities are in the
range from 400 to 4000 A/m.sup.2 and suitable voltages are in the
range from 2.5 to 6 volt.
[0031] 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.
[0032] 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.
[0033] 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 evaporators with forced circulation, falling film
evaporators or thin film evaporators are suitable for continuous
evaporation. In the case of batchwise concentration by evaporation,
a stirred vessel is particularly suitable. Preference is given,
both in continuous evaporation and in batchwise evaporation, to
using an evaporator having a condenser.
[0034] 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.
[0035] 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.degree. C., in particular at a temperature
in the range from 120 to 350.degree. C. and very particularly
preferably at a temperature in the range from 150 to 300.degree. 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.
[0036] Heating of the evaporator used can, for example up to
200.degree. 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 possible. 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.
[0037] 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 without 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.
[0038] The evaporation in step (b) forms an overhead stream
comprising second solvent and possibly hydrogen sulfide.
[0039] 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 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.
[0040] In the preparation of potassium, the evaporation residue
comprises, in terms of elemental 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.
[0041] After concentration of the mixture comprising second
solvent, alkali metal cations, further 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).
[0042] 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.
[0043] 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 removed from the mixture so that essentially
impurity-free alkali metal (poly)sulfide is formed.
[0044] 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) being 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 concentrated mixture
from step (b) runs downward in the column.
[0045] The column used is preferably a column with internals.
Suitable internals are, for example, trays, random packing elements
or structured packings.
[0046] The apparatus in which the purification is carried out, for
example the column, is preferably 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.
[0047] In a preferred embodiment, the column in which the
purification is carried out is additionally heated below the side
inlet for the gaseous stream comprising hydrogen sulfide. 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 alternative, electric heating is also
conceivable. Suitable heat transfer media are, for example, steam,
thermooils or salt melts.
[0048] As a result of the additional heating, hydrogen sulfides
formed in the mixture are dissociated into hydrogen sulfide and
alkali metal (poly)sulfide. For this purpose, a temperature in the
range from 320 to 400.degree. C., preferably in the range from 340
to 350.degree. C., is set in the column by means of the additional
heating.
[0049] 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.
[0050] At the top of the apparatus for the additional purification,
a gas stream comprising second 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
apparatus, in particular the column, is fed to a condenser. In the
condenser, the second solvent 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.
[0051] 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 evaporation stage
when the second solvent has been virtually completely removed.
[0052] 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 comprising
alkali metal (poly)sulfide is fed to a second electrolysis.
[0053] 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 which conducts
alkali metal cations. Suitable electrolysis cells for the second
electrolysis are, in particular, electrolysis cells whose structure
corresponds to the structure of electrolysis cells which can be
used in sodium-sulfur batteries.
[0054] The solid electrolyte is preferably a ceramic which conducts
alkali metal cations, in particular .beta.-aluminum oxide,
.beta.''-aluminum oxide or .beta./.beta.''-aluminum oxide. Alkali
metal cations of the alkali metal to be prepared are in each case
bound in the ceramics.
[0055] Apart from the alkali metal .beta.-aluminum oxide, alkali
metal .beta.''-aluminum oxide or alkali metal
.beta./.beta.''-aluminum oxide, corresponding alkali metal
analogues of NASICON.RTM. ceramics 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.
[0056] When the alkali metal which is to be prepared is lithium,
LISICONs and particularly preferably Li ion conductors having a
garnet structure, for example Li.sub.5La.sub.3Ta.sub.2O.sub.12 or
Li.sub.7La.sub.3Zr.sub.2O.sub.12, are also suitable.
[0057] In the second electrolysis cell, the alkali metal
(poly)sulfide melt obtained in the concentration operation in step
(b), or the alkali metal (poly)sulfide from the additional
purification, 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 molten form. The
electrolysis is preferably carried out at a temperature in the
range from 290 to 330.degree. C., in particular from 310 to
320.degree. C., under atmospheric pressure.
[0058] 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.
[0059] 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 electrolyte and thus go into the cathode space.
In the cathode space, the alkali metal cations 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 electrolysis,
the sulfur is present in liquid form and can be taken off from the
anode space. The 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.
[0060] 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.
[0061] Examples of the invention are shown in the figures and are
described in more detail in the following description.
[0062] In the figures:
[0063] FIG. 1 shows a process flow diagram of the first
electrolysis,
[0064] FIG. 2 shows a process flow diagram of the concentration
operation,
[0065] FIG. 3 shows a process flow diagram of the additional
purification,
[0066] FIG. 4 shows a process flow diagram of the second
electrolysis,
[0067] FIG. 5 shows a process flow diagram of the overall
process,
[0068] FIG. 6 shows a laboratory electrolysis cell for carrying out
the second electrolysis.
[0069] In FIG. 1, the first electrolysis is shown in the form of a
process flow diagram.
[0070] 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, tantalum
and/or platinum metals such as iridium, ruthenium, platinum and
rhodium, is present in the anode space 3. A cathode 11 which is
preferably made of stainless steel is accommodated in the cathode
space 5.
[0071] 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 preferably an
aqueous alkali metal halide solution, for example an aqueous alkali
metal chloride solution. The alkali metal halide is very
particularly preferably sodium chloride.
[0072] The alkali metal salt is preferably dissolved in water as
solvent. However, it is also possible to dissolve the alkali metal
salt in a suitable organic solvent, for example an alcohol.
[0073] 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.
[0074] 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 anode space
3.
[0075] In a degassing unit 21, the chlorine is taken off from the
stream taken off from the anode space and the remaining stream is
recirculated to the first reservoir 15. The chlorine is taken off
from the process via a chlorine offtake line 23.
[0076] In the electrolysis cell 1, alkali metal cations pass
through the cation-selective membrane 17 into the cathode space 5.
A suspension comprising elemental sulfur and second solvent, for
example an organic solvent or water, preferably water, flows via a
second feedline 25 into the cathode space.
[0077] 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 electrolysis 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.
[0078] 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.
[0079] 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
hydroxide. The alkali metal cations and (poly)sulfide anions
comprised in the mixture usually form an alkali metal
(poly)sulfide.
[0080] 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 example, to firstly
recirculate the mixture taken off via the cathode discharge line 33
to the second reservoir 31.
[0081] 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
mixture taken off via the cathode discharge line 33 is circulated,
a substream is taken off and fed to the concentration operation.
FIG. 2 shows by way of example a concentration operation by means
of evaporation in the form of a flow diagram.
[0082] 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 FIG. 2, a circulation evaporator with natural
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 evaporators 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.
[0083] The evaporator 41 is preferably equipped with a liquid
precipitator 43.
[0084] When using a circulation evaporator, liquid goes via a
circulation line 45 into an evaporator 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.
Furthermore, 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 desired concentration
of residual solvent is obtained. As soon as a steady state is
reached, mixture comprising second solvent, alkali metal cations
and (poly)sulfide anions 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.
[0085] In a preferred embodiment, the mixture taken off via the
offtake line 51 is purified further. The purification is shown
schematically in FIG. 3 with the aid of a flow diagram.
[0086] 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.
[0087] 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 nitrogen. 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.
[0088] 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.
[0089] A stream 67 which comprises essentially solvent-free alkali
metal (poly)sulfide is taken off at the bottom of the column
55.
[0090] The alkali metal (poly)sulfide melt obtained in the
evaporation or the stream 67 comprising alkali metal (poly)sulfide
which is obtained when carrying out the work-up as shown in FIG. 3
is fed to a second electrolysis. This is shown by way of example in
FIG. 4.
[0091] 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.
[0092] 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 cathode 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 FIG. 2 or, when a further purification is
carried out, the alkali metal (poly)sulfide from the purification
shown in FIG. 3 is fed via a feedline 79 to the anode space 73 of
the respective electrolysis cells.
[0093] 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 discharged 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.
[0094] 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 preferably recirculated to the first
electrolysis shown in FIG. 1. For this purpose, the sulfur is, for
example, conveyed via the sulfur feedline 27 to the second
reservoir 31. As an alternative, 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.
[0095] The overall process without the additional purification
shown in FIG. 3 is shown by way of example in FIG. 5.
[0096] When sodium is to be prepared by the process of the
invention, sodium chloride is introduced 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 chloride 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.
[0097] The sodium ions pass through the cation-permeable membrane 7
and go into the cathode 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
[0098] First Electrolysis Stage:
[0099] The electrolysis of the aqueous sodium chloride solution was
carried out in the electrolysis cell shown in FIG. 1. The
electrolysis cell was divided by means of a cation-exchanging
membrane (Nafion.RTM. 324) into an anode space and a cathode space.
As anode, use was made of an Ru/Ir-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.
[0100] 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.
[0101] 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.
[0102] The electrolysis was carried out at a temperature in the
range from 75.degree. C. to 80.degree. C., a current density i of
2000 A/m.sup.2 and a cell voltage in the range from 3.5 to 5
volt.
[0103] 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 catholyte. 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.
[0104] During the electrolysis, the anode side was flushed with
nitrogen. The anode-side offgas went through two scrubbers which
were operated using 10% strength aqueous NaOH and were connected in
series.
[0105] 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
analysis.
[0106] Analytical Results:
TABLE-US-00001 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
[0107] 175 g of chloride were found in the two scrubbers for the
anode offgas.
[0108] Concentration:
[0109] The cathode output was evaporated batchwise in an
electrically heated distillation flask at increasing temperature
and while stirring. The boiling temperature increased from
102.degree. C. to 200.degree. C. during the concentration
operation. The evaporation system was limited to 200.degree. 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.
[0110] 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
sodium (poly)sulfide powder. A partial amount of this sodium
(poly)sulfide powder was subjected to quantitative elemental
analysis.
[0111] Analytical Results:
TABLE-US-00002 Catholyte concentrate discharged: 494.4 g Oxygen
10.4% by weight Sulfur 59.0% by weight Sodium 28.4% by weight
[0112] Second Electrolysis Stage:
[0113] The electrolysis of the sodium (poly)sulfide melt was
carried out in the laboratory apparatus shown in FIG. 6, which was
provided with electric heating 101 and a steel housing 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''-Al.sub.2O.sub.3
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 usable surface of the tubular membrane 93 was 14
cm.sup.2. 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.
[0114] 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.degree. 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.
[0115] The electrolysis was carried out at a temperature in the
range from 290.degree. C. to 310.degree. 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.
[0116] After the electrolysis, 8 g of sodium metal were
discharged.
LIST OF REFERENCE NUMERALS
[0117] 1 first electrolysis cell
[0118] 3 anode space
[0119] 5 cathode space
[0120] 7 membrane
[0121] 9 anode
[0122] 11 cathode
[0123] 13 first feedstream
[0124] 15 first reservoir
[0125] 17 alkali metal salt line
[0126] 19 solvent line
[0127] 21 degassing unit
[0128] 23 chlorine offtake line
[0129] 25 second feedstream
[0130] 27 sulfur feedline
[0131] 29 solvent feedline
[0132] 31 second reservoir
[0133] 33 cathode discharge stream
[0134] 41 evaporator
[0135] 43 liquid precipitator
[0136] 45 circulation line
[0137] 47 evaporator unit
[0138] 49 solvent offtake line
[0139] 51 offtake line
[0140] 53 preheater
[0141] 55 column
[0142] 57 side inlet
[0143] 59 overhead stream
[0144] 61 condenser
[0145] 63 circulation line
[0146] 65 offtake line
[0147] 67 stream comprising essentially alkali metal
(poly)sulfide
[0148] 71 second electrolysis cell
[0149] 73 anode space
[0150] 75 electrode unit
[0151] 77 product line
[0152] 79 feedline
[0153] 81 sulfur discharge line
[0154] 90 electrolysis cell
[0155] 91 electrolysis leg
[0156] 92 free leg
[0157] 93 membrane
[0158] 94 sodium
[0159] 95 sodium (poly)sulfide melt
[0160] 96 anode
[0161] 97 contact plate
[0162] 98 stainless steel rod
[0163] 99 filling opening
[0164] 100 steel housing
[0165] 101 electric heating
* * * * *