Note: Descriptions are shown in the official language in which they were submitted.
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STATE OF THE ART
Electrolysis of aqueous sodium chloride solutions to produce
chlorine and sodium hydroxide by the so-called mercury amalgam process ls
still widely used industrially as it presents several advantages over other
existing processes, for example, those utllizing diaphragm or membrane cells.
At present, in all the commercially known plants, the amalgam leaving the
electrolysis cell is decomposed in a reactor provided with 'd catalytic filling
with water, and hydrogen and caustic soda produced by the decomposition process
are recovered and mercury is recycled to the cell. ~he process is presently
very reliable and highly perfected and especially with the utilization of ~;
recently developed dimensionally stable anodes based on valve metals provided
with electrocatalytic coatings in place of the conventional graphite anodes.
One of the main factors affecting reliable operation and safety of
the mercury amalgam process is the purity of the brine introduced into the
cell as the level of impurities that can be tolerated in the process is very
low. Quantities varying from 0.3 to 0.01~ of impurities such as calcium,
magnesium and iron are usually present in salt while other heavy metals like
- Cr, V, Mo, Mn are often present in a concentration of about 0.01 ppm. These
impurities must be carefully removed from the brine since quantities higher
than 0.01 ppm in the brine can cause hydrogen to e~olve at the mercury
cathode after an extended period of time and the C12-H2 mixture formed
thereby can explode with disastrous effects.
To avoid this problem, the brine cycle used in mercury cell plan~.s
comprises the following steps: (1) dechlorination; (2) saturation of the
depleted brine with salt; (3) chemical and physical puriflcations; and (4)
ad~ustment of the pH to 4.5 to 5.5 before feeding the brine to the cell. ¦
While thls purification system permits a relatively safe operation unaffected
by sudden catastrophic phenomena, frequent periodic cleaning of the cell and
~ purificatinn of the introduced mercury by distillation are required, or
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impurities introduced in the system with the brine would accumulate in the
mercury in the long run far beyond the maximum tolerable limit.
The most critical impurities detectable in mercury after a more or
less prolonged operation in mercury cells are classified according to the
consequences they lnvolve and comprise for example: a) V, Cr, Mn, Fe, Ni,
Co, Cu, Mo, Pb, As, Sb, Se, Te, Ga and Ti as metals or oxides, hydroxides
or mixed oxides which give rise to hydrogen discharge on the amalgam and
to the formation of amalgam foam (called mercury butter) and b) Ca(O~)2,
Mg(OH)2, ~a(OH)2, Sr(OH)2, Be(OH)2 and Al(OH)3 which catalyze hydrogen dis-
charge and cause amalgam pulverization.
When impurities accumulate in the mercury circulating in the cell, '
the electrolysis process is adverserly affected by the following phenomena:
i) mercury butter formation with a consequent increase of frequency of short-
circuits in the cell and rapid inactivation of the anodes, ii) hydrogen
evolution, iii) decrease of wettability between the mercury and the cell
bottom with frequent breaking of the mercury liquid stream and consequent
corrosion of the exposed cell bottom, iv) mercury amalgam decomposition in
the cell, v) mercury oxlde formation and vi) cell voltage increase, faraday
efficiency decrease and current distribution unbalances in the various
longitudinal and transversal sections of the cell.
OBJEC~S OF THE INVENTION
It is an object of the present invention to provide a new and improved
process for maintaining the level of impurlties contained in the mercury
- circulating in the cell within limits that do not affect the electrolytic process and avoid interruptions of the cell operation.
It is a further object of the invention to provide {or a new and
improved process wherein unpurified salt is utilized as raw material; and
wherein the expensive dechlorination and brine purification plants are no
longer necessary.
It is another object of the invention to provide a process for
continuously removing impurities introduced together with unpurified salt
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from the mercury circulating in the cell whereby an equilibrium is achieved
and the level of impurities can be maintained within the admissible limits.
It is an additional object of the invention to provide a novel
denuder for decomposing amalgam and removing impurities from mercury.
These and other objects and advantages of the invention will become
obvious from the following detailed description.
THE INVENTION
The improved process of the invention for producing a halogen and
an alkali metal hydroxide solution by electrolysis of an aqueous solution of
an alkali metal halide in a mercury cathode electrolysis cell comprises
subjecting the amalgam leaving the electrolysis to decomposition to form
mercury and an alkali metal hydroxide solution and subjecting the mercury
to anodic polarization in an electrolyte with a counter-electrode maintained
at a sufficiently negative potential to remove from the mercury at least a
portion of metal impurities contained therein and recycling the purified
mercury to the electrolysis cell. The metal impurities in the mercury are
preferentially anodically dissolved in the electrolyte so that the level of
impurities in the mercury will be held below the levels which would adversely
afect the electrolytic reaction taking place in the electrolysis cell.
The decomposition of the alkali metal-mercury amalgam leaving the
electrolysis cell may be carried out in a conventional de~uder wherein the
amalgam is contacted with a catalytic material such as graphite in the presence
of water to form mercury, hydrogen and an alkali metal hydroxide solution.
The alkali metal must be substantially completely removed from the mercury
before the electrolytic purification to avoid it being anodically dissolved
before or in place of the metal impurities when mercury flows through the
electrolytic purification stage. Sodium dissolution, besides involving q
loss of caustic soda production due to soda being discharged together with
the purification electrolyte, also entails a useless consumption of electricity
which partially or completely eliminates the advantages of the present invention.
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Often this conditlon is not present in conventional pla~ts wherein mercury
leaving the decomposition stage still contains from 0.001 to 0.005~ of
sodium.
The mercury electrolytic purification process may conveniently be
carried out in the mercury inlet box of the electrolysis cell itself where
the mercury pool has a sufficiently large surface area. In this case, an
horizontal plane electrode made of iron, nickel or graphite, and preferably
foraminous, is placed at a distance of a few millimeters up to 1 or more
centimeters from the mercury surface and is cathodically polarized by a
current supply floating with respect to the mercury potential.
The electrolyte in the inlex box may be either alkaline or acidic,
but is preferably acidic. Preferably, water or a NaCl solution acidified
with hydrochloric acid is circulated through the cell inlet box and the pH
is kept between 1 and 3.5. A large amount of impurities is removed from
mercury and is together with the electrolyte removed from the inlet oox and
the electrolyte may be stripped of the metal values and recirculated.
The mercury polarization is kept between 0.1 and 1 V (NHE), pre-
ferably within 0.1 and 0.5 V(NHE), by an adequate control of the cathodic
polarization impressed on the counter-electrode depending upon the cell
parameters such as distance of mercury from the counter-electrode surface,
the electrolyte conductivity, the purity of the salt, the current density,
etc. A substantial anodic dissolution of the metal impurities contained in
the mercury is achieved by operating within the above mentioned limits. More-
over, the anodic dlssolution of the mercury itself is minimal because mercury
is much nobler than the pollutant metal impurities. Most of the mercury which
may have been anodically dissolved is cathodically reduced on the counter-
electrode and precipitates as metallic mercury in the mercury pool.
Oxidized mercury still present in the effluent electrolyte represents
only a minimum amount with respect to the mercury present in the caustic,
- 30 hydrogen and head-box washing waters effluent from the electrolysis sectlon
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of the plant and likewise is recoverable through tho available mercury
stripping systems. The decomposed metals are preferably removed frorn the
electrolyte and the purified electrolyte is recycled. It has been found
that a mercury surface area opposed to the counter-electrode in a ratio of
1/1000 with'respect to the area of the electrolysis cell mercury surface is
'sufficient although this may vary from 1/100 to 1/10,000 depending upon the
specific condition.
In a preferred embodiment of the process of the invention, the sodium
content in mercury is practically brought to zero by a complete decomposition
of the amalgam leaving the electrolysis cell, the decomposition being effected, I
at least partially, electrolytically. This treatment can be conveniently 1'
carried out in two alternative ways.
In the first alternative, the amalgam leaving the electrolysis cell is
percolated through a series of porous plates made of a conductive material, the
said plates being electrically insulated with respect to the adjacent plates
and having impressed thereon a voltage of about 0.2 - 0.4 V (lower than the
water decomposition voltage to avoid eventual oxygen evolution) between every
plate and the plates adjacent to it in the series and circulating water for
- diluting the sodium hydroxide produced counter-current to the amalgam stream.
The electrolytic denuder is electrically insulated with respect to ~he incoming
amalgam and to the exiting mercury by breaking the liquid stream during the
mercury leakage through the porous plates, preferably made of inert and non-
conductive material, placed one at the inlet and one at the outlet of the
denuder, respectively. The amalgam percolating through the denuder is
anodically polarized by contact with the porous plates connected to the positivepole of the electric current source and sodium is readily released forming
the sodium hydroxide with consequent hydrogen evolution. Therefore, the mercury
côllected at the denuder base plate is essentially free from sodium content. I
The porous plates may advantageously consist of graphite either in the solid " ~ 't
form or as a static porous bed of different grain sizes. ` '
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In the second alternative, the process can be easily integrated
into the existing commercial plants which utilize denuders provided with
graphite or other material fillings. In this a]ternative, mercury leaving
the denuder is subjected to further amalgam decomposition in order to
remove the residual sodium by subjecting an adequate portion of the mercury ¦
surface to anodic polarization with respect to a counter-electrode made from
steel, nickel, graphite or other suitable conductive materials connected to
a floating current supply with the caustic solution acting as the electrolyte.
The final decomposition stage can be easily realized at the bottom of a con-
ventional denuder by inserting a counter-electrode placed at a distance
varying from some millimeters to 1 or 2 cm from the surface of the mercury
pool which collects on the denuder bottom with the electrode being cathodically
polarized with respect to the mercury. `~
Therefore, according to a preferred embodiment of the invention, mercury
is continuously subjected to two anodic polarization stages, a first stage
carried out in an alkaline environment to remove completely the sodium content
and to partially remove metal impurities such as potassium, lithium, barium,
aluminum, etc., which can be easily anodically dissolved in an alkaline
environment, and a second stage carried out preferably in an acid environment
for removing impurities such as oxides, hydroxides and heavy metal oxysalts.
One of the advantages of the invention is the elimination of the
dechlorination treatment of the brine which can be sent to the cell without
being sub;ected to any purification treatment. The diluted chlorine, which
poses a difficult problem for its disposal, is no longer produced. According
to the present inventLon, every cell may be provided with an autonomous system
of saturation and feeding of the brine. The system is very easy to realize.
In this way, the entire centralized system for brine treating,distributing and ¦ .
recycling is no longer necessary, resulting in a considerable saving.
According to another embodiment of the invention, it is also possible
to feed the salt directly to the cell onto the mesh anodes. The turbulence
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formed by the gaseous ehlorine evolution i9 utilized to effect salt dis-
solution and to avoid channeling phenomena.
The process of the invention has been mainly described by referring
to sodium chloride electrolysis due to its great industrial importance but
it is obvious that other alkali metal halides such as potassium chloride may
be considered as well.
Referring now to the drawings:-
Figs. 1 to 3 schematically illustrate the flow of mercury in threedifferent embodiments of the invention.
Fig. 4 is a schematic view of the electrolytic mercury purification
cell of Figs. 1 to 3 indicated therein as 4.
Fig. 5 is a schematic partial cross-sectional view of the bottom of
a denuder provided with an electrolytic final decomposition stage of Fig. 2.
Fig. 6 is a schematic cross-sectional view of an electrolytic amalgam
denuder of the invention to eompletely remove sodium from the amalgam.
Fig. l illustrates the mereury eireuit in a ehlorine plant wherein
brine is electrolyzed in mereury eleetrolysis cell 1. The amalgam leaving
the eell 1 is introdueed at the upper por~ion of denuder 2 whieh is filled
with a static porous bed of catalytic material such as graphite granules.
Water is introdueed by line 11 into the lower portion of denuder 2 and flows
counter current to the amalgam durlng which sodium is stripped from the amalgam
to form sodium hydroxide and hydrogen is evolved. The hydrogen i5 removed
through outlet 13 and the sodium hydroxide solution is removed through outlet
12. ~he mercury from the bottom of denuder 2 is eondueted by pump 3 to the
eleetrolytie purifieation eell 4 and then baek to eleetrolysis cell 1 which is
provided also with brine inlet 16, brine diseharge 17 and chlorine outlet 18.
Electrolyte is added to purifieation eell 4 by line 14 and is diseharged
through outlet 15.
Flg. 2 illustrates a preferred embodiment of the process of the
0 invention wherein the mercury flow is the same as in Fis. l with the addition
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of an electrolytic decomposition stage 5 provided at the bottom of denuder 2
to eliminate any residual sodium in the mercury before the electrolytic
purification step of cell 4. ~he stage 5 is illustrated further in Fiq. 5
which is described infra.
Fig. 3 illustrates another embodiment of the process of the invention
wherein the mercury flow is as in Fig. 1 but the denuder 2 is replaced with
an electrolytic amalgam denuder 6 which is illustrated in greater detail in
Fig. 6 to remove the sodium from the amalgam.
In the electrolytic purification cell illustrated in Fig. 4, the
cell consists of a container 19 provided with a cover 20, both made of a l;
corrosion resistant material such as rubber-lined steel and as noted above,
the electrolyte is introduced through inlet 14 and hydrogen is removed by
outlet lS. Mercury is introduced at the bottom through inlet 21 to maintain ,~
a layer 22 of mercury on the cell bottom. Counter electrode 23 made of steel,
nickel, graphite or other suitable material is placed at a certain distance
from the mercury and a direct current by means not shown is placed on the
mercury-counter electrode with the counter-electrode being negatively polarized .
with respect to the mercury by a floating electric current supply whose positivepole is preferably connected to the bottom of container 19. Any mercury
deposited on counter-electrode 23 will fall back to the pool of mercury 22 on
the container bottom.
In Fig. 5, the lower portion of denuder 5 is provided with an
electrolytic decomposition zone below divider plate 24 in which a pool 26 of
mercury collects in the denuder bottom. A counter electrode 25 made of
graphite, steel, nickel or other suitable, electrically conductive material
is placed a certain distance from mercury pool 26 and the electrode 25 is
cathodically polarized with respect to pool 26 by a floating direct electric
current supply ~not shown) whose positive pole is directly connected to pool
26. The electrolyte for the decomposition stage is the water introduced by
line 11 to form sodium hydroxide solution during its passage through the
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denuder.
In Fig. 6, the amalgam electrolytic denuder consists of a container
27 provided with a cover 28, both preferably made of an inert, electrically
non-conductive material or steel coated on its interior surfaces with an
inert, electrically non-conductive material. The container 27 is provided
with a series of hori~ontal porous plates with each plate being electrically
insulated from the two adjacent plates. Plates 29, 31 and 33 made of
electrically conductive, amalgam resistant material such as graphite are
connected to the negative pole of a floating direct current electrical supply
means (not shown) and plates 30, 32 and 34, also made of electrically conductive,
amalgam resistant material such as graphite are connected to the positive pole
of said electrical supply means.
Top plate 35 and bottom plate 36 are made of graphite or other porous
material which need not be electrically conductive and the plates break the
liquid stream of incoming amalgam and exiting mercury, respectively, to effect
electrical insulation of the denuder from the mercury potential in the
electrolysis cell 1. The amalgam from the cell 1 is introduced by line 37 into
the top of the denuder and percolates down through the series of porous plates
which interrupt the stream at every pass from one plate to the lower plate. As
the amalgam contacts the positively polarized plates, the sodium is readily
released for anodic dissolution and gives rise to hydrogen evolution and
sodium hydroxide eormation.
- Each of the porous plates is provided with a hole 41, preferably co-
axial, to form a type of chimney for hydrogen passage and a suitable weir is
provided about the upper edge of each hole 41 to prevent amalgam from falling
through the holes. Water is introduced at the bottom of the denuder through
line 38 and flows counter-current to the mercury and is discharged through
outlet 39 while hydrogen is removed by outlet 40. The mercury collects on the
denuder bottom wherein it is sent by outlet 42 to the electrolytic purification .
stage 4 of Fig. 3.
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In the following examples there are described several preferred ,
embodiments to illustrate the invention. ~owever, it is to be understood
that the invention is not intended to be limited to the specific embodiments.
_XAMPLE 1
Reduced side tests were conducted using the mercury flow scheme of
Figs. 1 and 4 wherein the ratio of the area of the mercury surface in
electrolysis cell 1 to surface in electrolytic purification cell 4 was 1,000:1
and the ratio of electrolysis current density between the said cells was
10,000:1. The electrolyte circulated in electrolytic purification cell 4
was aqueous hydrochloric acid with a constant pH of 3. The brine fed to the
cell 1 through inlet 16 was not purified in any manner and contained as
impurities: 0.5 to 0.01% of Fe, 0.1 to 0.05~ of Ca, 0.1 to 0.15~ of Mg and
0.01 to 0.005 ppm of chromium. The cell 1 was operated continuously for 6
days and the amount of impurities determined is reported in Table I. No
operating deterioration in the electrolysis cell was observed and the hydrogen
content in the chlorine was constant within 0.5~ and the faraday efficiency
varied from 96 to 97~.
The electrolysis cell 1 was then shut down and graphite counter-
electrode 23 was removed from electrolytic purification cell. The cell 1
was then operated for 8 hours after which the impurities in the brine were
determlned. The results are reported in Table I. At the end of the 8 hours
of operation, the faraday efficiency had fallen to 91~ and the hydrogen content
in the chlorine had increased rapidly to 5~.
TAEJLE I
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PPM ¦
Wlth electro- Without electro-
Impurity lytic purification lytic purification
Fe 2 to 20 100 to 700
Ca 0.1 to 2 10 to 200 .
Mg 0.05 to 1.5 5 to 00
Cr 0.001 to 0.01 0.01 to 0.02
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The said test clearly shows that the process of the invention may
be operated without salt purification for prolonged periods of time while
the impurity level without the electrolytic purification quickly rises to
undesirable levels resulting in increased hydrogen generation and a sharp
drop in faraday efficiency.
EXAMPLE 2
The test of Example 1 was repeated except the salt was added
directly to the electrolysis cell 1 onto the mesh anodes above the mercury
surface and the salt slowly dissolved in the circulating electrolyte. After
10 days of operation with electrolytic purification, the cell was still
operating satisfactorily.
Various modirications of the process and apparatus of the invention ;
may be made without departing from the spirit or scope thereof and it is
intended to be limited only as defined in the appended claims.
This application is a division of copending Canadian application
Serial No. 307,898 filed July 21, 1978.
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