Note: Descriptions are shown in the official language in which they were submitted.
lOS9g47
The present invention relates to a process
utilizing known electro-dialysis equipment for producing
aqueous solutions of sodium, ammonium or magnesium sulphite
or hydrogen sulphite from calcium sulphite or calcium
hydrogen sulphite. More particularly, the invention relates
to a process for electro-dialyzing an aqueous solution of
calcium sulphite or hydrogen sulphite with a second
solution, that includes a solute, preferably in the form of
a commercially available salt, corresponding to the sulphite
or hydrogen sulphite to be produced, e.g., sodium, ammonium
or magnesium chloride, or from any other available salt
solution containing the corresponding base cation.
At present, such aqueous sulphite solutions are
produced by absorption of sulphur dioxide in solutions of
the corresponding hydroxides or carbonates which known
procedures require the use of materials that are becoming
increasingly more scarce and expensive. Therefore, a
production process is needed which uses low-cost starting
materials. Whereas these currently used sodium, ammonium or
magnesium hydroxides and carbonates are relatively
expensive, their salts, e.g., sodium, magnesium and ammonium
chloride, are available in large quantities and at low-cost;
as natural raw materials and, in some industries, as waste
products.
Brief Description of Invention
The method of this invention utilizes aqueous
solutions of available, naturally occurring or otherwise
inexpensive source of salts of sodium, magnesium or ammonium
for flow through known electro-dialyzing equipment that
includes a plurality or stack of cells separated by
membranes, to react with aqueous solutions of calcium
105999~7
sulphite or calcium hydrogen sulphite to produce the desired
aqueous solution (product solution) of the sulphite of
sodium, magnesium or ammonium or their respective hydrogen
sulphites and an aqueous solution of a useful by-product.
The solutions to be elec~ro-dialyzed are made to flow
through different cells of the electro-dialyzer that are
separated on the one hand from one another by a similar cell
through which a dilute solution of the product passes and on
the other hand by a cell through which a solution of
by-product passes. The electrical energy supplied across
the stack of cells of the electro-dialyzer causes the ions
in the several solutions to migrate through the separating
membranes forming the walls of their respective cells, the
arrangement being such that certain of the ions migrate into
the product solution to produce additional sodium, ammonium
or magnesium sulphite or hydrogen sulphite as the case may
be, while others migrate into the by-product solution to
produce additional by-product.
The desired sodium, ammonium or magnesium sulphite
or hydrogen sulphite solution and the by-product solution
are then recovered. A portion of the effluent from the
respective reaction chambers may be recirculated to produce
a constant inflow concentration in the cells through which
the solution flow so that a continuous production of, for
example, an aqueous solution of a desired concentration of
NaHS03 and solution of CaC12 can be accomplished.
It is therefore an object of tbis invention to
provide a method for utilizing known electro-dialyzing
equipment for producing sodium, ammonium or magnesium
sulphite or hydrogen sulphite.
Another object of this invention is to provide a
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method for reacting readily available salts of sodium,
ammonium or magnesium in a electro-dialyzer to produce their
respective sulphites or hydrogen sulphites.
Another object of this invention is to provide a
method for using a s~ack of electrodialyzing cells to react
a readily available aqueous salt solution with calcium
sulphite or calcium hydrogen sulphite to produce sodium,
magnesium or ammonium sulphite or hydrogen sulphite.
Another object of the invention is to provide an
electrodialysis process that may be automated with known
means for the production of sodium, ammonium or magnesium
sulphite or hydrogen sulphite.
Another object of this invention is to provide a
continuously operative method for producing an aqueou~
solution of sodium, magnesium or ammonium sulphite or
hydrogen sulphite of desired concentration together with a
useful by-product.
Drawings
The single figure shows a flow sheet of the method
of this invention.
Detailed Description
The process of this invention makes use of an
electrodialysis unit which is designed as a multi-cell stack
with alternating anion-and cation-selective membranes for
receiving the feed solutions consisting of a calcium
sulphite or calcium hydrogen sulphite solution and a second
salt solution of the desired base cation that is converted
into the desired sulphite or hydrogen sulphite of the cation
by electrochemical ion exchange. A by-product of this
process is the concurrent production of an aqueous calcium
salt solution.
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In the practice of the process according to this
invention, the individual cells of the electrodialysis unit
are connected to suitable fluid circulating means, not
shown, to produce a flow of the feed solutions, a flow of
product and by-product solutions, and also a flow of one or
more electrode rinse streams. The cells are stacked to
receive the several circulating solutions such that a first
aqueous feed solution containing calcium ions and sulphite
or hydrogen sulphite ions (i.e., either calcium sulphite or
calcium hydrogen sulphite) flows through a first set of
cells in the stack and a second aqueous feed solution
containing a salt of the desired base cation to be reacted
with the sulphite ions in the first solution flows through a
second set of dif~erent cells in the stack. Dilute
solutions of the desired product and the by-product salt are
made up to flow through two additional third and fourth sets
of cells, the solutions being formed, preferably, from water
and recycled effluent.
The several cells of all of the sets are arranged
side by side in the electrodialyzer so that the first and
second solutions are alternately disposed on the opposite
sides of one each of the cells forming the third and fourth
sets of cells.
The calcium sulphite and calcium hydrogen sulphite
used in this process can be readily produced at low cost and
by known procedures from sulphur dioxide and lime. Thus,
with the process according to the invention, the desired
sulphite or hydrogen sulphite solutions can be produced
using, besides sulphur dioxide, only the low-cost starting
materials of lime and salts of the bases corresponding to
the sulphite or hydrogen sulphites ts be produced, e.g.,
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sodium, ammonium or magnesium chloride.
The electrodialysis technology on which the
process according to the invention is based is already being
used on an industrial scale for the desalination of seawater
and brackish water and for the desalination or concentration
of aqueous solutions of electrolytes in waste water
technology and in the food processing industry. The present
process can thus be carried out using only slightly modified
existing technologies and apparatus, which is highly
advantageous for the implementation of the process.
Contrary to the known applications of
electrodialysis for desalination or concentration, the
special arrangement of the cells of the electrodialysis unit
as taught herein, makes use of the selective ion exchange
membranes and the passage of the various streams of
electrolytes in the sequence described above. In following
this invention, a continuous electrochem~cal ion exchange is
effected i~ the desired way to produce the sodium, magnesium
or ammonium sulphite or hydrogen sulphite and a useful
by-product or aqueous solution of a separable salt. By
using the multi-cell electrodialysis unit according to the
present invention, the process may be carried out in a
relatively compact area and can easily be automated by means
of conventional devices, not shown herein, for the easy and
exact control of throughput and electric current.
The multi-cell electrodialysis unit - generally
known as a membrane stack - which is to be used for carrying
out the process according to the invention, consists of
alternatingly arranged anion- and cation-selective ion
exchange membranes that are commercially available. Between
the individual membranes, cell frames are arranged which
105994~
hold spacers (mostly consisting of extruded plastic mesh~
that serve to prevent adjacent membranes from coming into
contact with each other and to provide a uniform turbulent
flow across the entire surface of the membranes. The
resulting membrane stack is arranged between end plates
which hold the electrodes and have boreholes for the
required circulation of the liquid streams. The stack is
usually compressed by tie rods so that the multi-cell stack
is sealed towards the outside by rubber-covered rims on the
cell frames or spacers.
The required liquid streams are ~ed into and
discharged from the stack through the boreholes in the end
plates with liquid distribution within the stack being
effected through suitable conduits and appropriately
arranged boreholes in the separators and membranes.
Conventional electrodialysis units generally comprise
electrode rinse conduits and two electrolyte conduits for
the dialysate and the concentrate. For the process
according to the invention, however, besides the two
electrode rinse conduitQ, four electrolyte conduits are
required, which are passed by the aforementioned solutions
in the described order.
As can be seen from the attached process flow
sheet, the electrodialysis unit used for the present
process, makes use of a multi-cell stack formed by anion
selective membranes A alternating with cation-selective
membranes C positioned between electrodes 7,8. As
schematically shown, electrode rinse conduits 1 and 6 for
electrodes 7 and 8 are connected to a recirculating flow of
rinse streams. In a typical example, an aqueous sodium salt
solution is circulated through the rinse conduits when
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either sodium sulphite or sodium hydrogen sulphite is to be
produced.
In the cathode rinse cell 1, an alkaline medium is
formed in accordance with the reaction:
2 ~2 + 2e --~ H2 + 2 OH-,
while hydrogen is set free. In the anode rinse cell 6, free
oxygen and acid is produced according to the reaction:
2 H2O ~~~ 2 ~ 4 H+ + 4e~,
and the use of rinse streams containing chloride results in
the additional formation of chlorine according to:
2 Cl~ C12 + 2e~,
In many cases it is advantageous to let the two streams of
acidic and alkaline rinse streams follow each other to
achieve mutual neutralization.
Using the example given above of the process
according to the invention, four additional and separate
streams are provided for feeding water and the desired
aqueous solutions into adjacent cells of the electrodialysis
unit. For example, an aqueous NaCl solution is passed
through a ~irst cell 2 adjacent cathode cell 1 which is
separated therefrom by a cation selective membrane C. A
solution of-water and the by-product CaC12 is pas~ed through
the next cell 3 adjacent cell 2 and separated therefrom by a
anion-selective membrane A. An aqueous solution of
Ca(HSO3)2 is then passed through cell 4, which is separated
from cell 3 by a cation-selective membrane and a dilute
solution of water and the desired product NaH~O3 is
circulated through cell 5 which is separated from cell 4 by
an anion selective membrane A. If this were one unit, cell
5 would then be followed by NaCl chamber 2", anode rinse
cell 6 and anode 8 to complete the unit. This combination
of cells 2-5 can then be considered as the basic unit which
can be duplicated up to 100 units between the two electrodes
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7 and 8. In ~he drawing, two basic cell structures are
shown, the second unit being represented by the numerals
2' - 5'.
Cells of the same type can be connected in series,
to achieve a higher concentration of the electrolyte or in
parallel to increase the throughput. The attached flow
sheet shows an embodiment of the invention with parallel
connection. When 3pplying d.c. voltage of adequate
polarization to the electrodes, the cations flow to cathode
7 and the anions to anode 8.
With the arrangement of anion and cation exchange
membranes and the liquid streams of the example shown in the
attached flow sheet, the selectivity of the membranes A and
C results in increasing the concentration of the CaC12
solution (the by-product stream) and the NaHSO3 solution
(the product stream) in cells 3 and 5, respectively, while
decreasing the concentration of the NaCl and Ca(HSO3)2
solutions (the feed streams) in cells 2 and 4, respectively.
In this process, and with reference to the basic unit of
cells 2 - 5, Na+ ions from cell 2' in the next adjacent unit
(or cell 2 " if the cell consisted of only one unit) and
HSO3- ions from cell 4 migrate into cell 5, while Cl- ions
from cell 2 and Ca++ ions from cell 4 migrate into cell 3.
The process can be described by the equation:
2 NaCl + Ca(HSO3)2 ~~energy~~> 2 NaHSO3 + CaC12,
and thus constitutes a continuous electrochemical ion
exchange process.
The described multi-cell stack has substantial
advantages over three - or five - cell stacks with a
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different arrangement of the ion exchange membranes in that
the CaC12 and NaHSO3 solutions obtained with the embodiment
of the invention, whose flnal concentration is higher than
their initial concentration, contaih no impurities from the
electrolytes of the starting solutions and for one ~araday
equivalent, ~n~ equivalents of salt are transferred, where
~n" designates the number of basic units of cells 2 to 5
between the two electrodes.
When carrying out the process according to the
invention, it is of utmost importance that electrodialysis
takes place without concentration polarization.
Concentration polarization occurs when the ion flow due to
the electric field equals or exceeds the diffusion-induced
ion flow at the electrolyte/membrance interface. This
results in hydrogen or hydroxyl ions participating in the
current transport 80 that a shift in the pH value is
._, .. .. .. ..
encountered. This not only leads to a decrease in the
current efficiency but there is also the risk of slightly
soluble salts being deposited, especially on the
anion-selective membranes, which deposition produces scaling
that increases the membrane resistance and impedes ion
transport through the membrane. Since calcium forms several
slightly soluble salts (e.g. calcium carbonate by reaction
with atmospheric carbon diozide), it is preferred that the
electrolyte of the by-product be acidified (in this case the
calcium chloride solution) with hydrochloric acid which can
simply be added to the water in-flow in accordance with
known procedures to prevent salt deposition on the
membranes.
Industrial electrodialysis apparatus of this type
can be constructed according to conventional design
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1059947
principles for electrodialysis units as described, for
example, by M.S. Mintz in Ind. Engineering Chem. 55 ~1963)
6, 18-28.
The attached flow sheet is based on a process in
which constant input concentration is achieved by replacing
appropriate proportions of the CaC12 and NaHSO3 solutions
with water; the converted NaCl and Ca(HSO3)2 being also
replaced continuously. This bleed-and-feed process thus
permits the continuous production of NaHSO3 solution of
constant concentration, the by-product being CaC12 solution.
The above description covers the preferred mode of
operation of our method. An example of the continuous
formation of sodium hydrogen sulphite in an aqueous solution
has been set forth. It is apparent that if calcium ~ulphite
is substituted for calcium hydrogen sulphite on the infeed
that the same procedure can be used for the production of
sodium sulphite. Similarly, ammonium and magnesium chloride
salts may be substituted for the sodium chloride salt
indicated on the drawing to produce aqueous solutions of
ammonium or magnesium sulphite or hydrogen sulphite.
Further, other salts of ammonium, sodium and magnesium may
be used as a starting material such as fluorides, bromides,
and also hydroxides.
These and other modifications of this method may
occur to those skilled in the art that will fall within the
scope of the following claims.
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