U.S. patent number 3,905,879 [Application Number 05/411,613] was granted by the patent office on 1975-09-16 for electrolytic manufacture of dithionites.
This patent grant is currently assigned to Hooker Chemicals & Plastics Corporation. Invention is credited to Jeffrey D. Eng, Cyril J. Harke.
United States Patent |
3,905,879 |
Eng , et al. |
September 16, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electrolytic manufacture of dithionites
Abstract
Dithionites are made by a process which begins with the
production of high concentration, chloride-free sodium hydroxide
solution and chlorine at a high current efficiency from a
three-compartment electrolytic cell having membranes of a
cation-active permselective membrane material separating anode and
cathode compartments from a buffer compartment. Hydroxide ions
migrating into the buffer compartment from the cathode compartment
are converted to sulfite by reaction with sulfur dioxide, improving
the current efficiency of the three-compartment cell, and the
sulfite is removed. Subsequently, the sulfite resulting and
additional sulfur dioxide are fed to the cathode compartment of a
two-compartment electrolytic cell wherein the anode and cathode
compartments are separated by a cation-active permselective
membrane and in which chloride solution is being electrolyzed to
chlorine at the anode and sulfite solution is being electrolyzed to
dithionite at the cathode.
Inventors: |
Eng; Jeffrey D. (North
Vancouver, CA), Harke; Cyril J. (Burnaby,
CA) |
Assignee: |
Hooker Chemicals & Plastics
Corporation (Niagara Falls, NY)
|
Family
ID: |
23629632 |
Appl.
No.: |
05/411,613 |
Filed: |
November 1, 1973 |
Current U.S.
Class: |
205/345; 204/296;
205/495; 205/524 |
Current CPC
Class: |
C25B
1/14 (20130101); C25B 1/46 (20130101) |
Current International
Class: |
C25B
1/00 (20060101); C25B 1/14 (20060101); C25B
1/46 (20060101); C01b 007/06 (); C01b 017/66 ();
C01d 001/06 () |
Field of
Search: |
;204/92,98,128 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Edmundson; F. C.
Attorney, Agent or Firm: Casella; Peter F.
Claims
What is claimed is:
1. A method of electrolytically manufacturing a dithionite,
chlorine, a hydroxide and a sulfite from sulfur dioxide and a
chloride which comprises feeding chloride solution to the anode
compartment of an electrolytic cell having anode, buffer and
cathode compartments separated by cation-active permselective
membranes, an anode in the anode compartment and a cathode in the
cathode compartment, and feeding sulfur dioxide to the buffer
compartment, withdrawing chlorine from the anode compartment,
hydroxide from the cathode compartment and sulfite from the buffer
compartment, feeding such sulfite and sulfur dioxide to the cathode
compartment of a two-compartment electrolytic cell having an anode
in an anode compartment, a cathode in a cathode compartment and a
cation-active permselective membrane dividing the compartments,
maintaining the catholyte at pH 6-8, feeding chloride to the anode
compartment thereof and withdrawing chlorine from the anode
compartment and dithionite and sulfite from the cathode
compartment.
2. A method according to claim 1 wherein the material of the
cation-active permselective membranes is selected from the group
consisting of a hydrolyzed copolymer of a perfluorinated
hydrocarbon and a flurosulfonated perfluovinyl ether, and a
sulfostyrenated perflourinated ethylene propylene polymer, and the
cells employed are three- and two-compartment cells.
3. A method according to claim 2 wherein the permselective membrane
is of a hydrolyzed copolymer of tetrafluoroethylene and FSO.sub.2
CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF"CF.sub.2, which
copolymer has an equivalent weight of about 900 to 1,600.
4. A method according to claim 3 wherein the voltage drop across
the three-compartment cell is about 3 to 6 volts, that across the
two-compartment cell is about 3 to 5 volts, the current density for
the three-compartment cell is about 1 to 3 amperes/sq. in., that
for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and
the operating temperature of the three-compartment cell is about
50.degree. to 100.degree.C. and that of the two-compartment cell is
about 3.degree. to 40.degree.C.
5. A method according to claim 4 wherein the membrane walls are
from about 0.02 to about 0.5 mm. thick, the membranes are mounted
on a network screen or cloth of filaments of a material selected
from the group consisting of polytetrafluroethylene, perfluorinated
ethylene propylene polymer, polyproplyene, titanium, tantalum,
niobium and noble metals, which has an area percentage of openings
therein from about 8 to about 80% with the filaments having a
thickness of about 0.01 to about 0.5 mm.
6. A method according to claim 5 wherein the voltage drop across
the three compartment cell is from 4 to 5 volts, that across the
two-compartment cell is from 3.5 to 4.5 volts, the current density
in the three-compartment cell is from 3.5 to 4.5 volts, the current
density in the three-compartment cell is from about 1.5 to 2.5
amperes/sq. in., the current density in the two-compartments cell
is 0.2 to 1 amperes/sq. in., the current density in the
two-compartment cell is 0.2 to 1 ampere/sq. in., the operating
temperature of the three-compartment cell if 80.degree. to
100.degree.C., the operating temperature of the two-compartment
cell is 3.degree. to 25.degree.C., the feed to the anode
compartment of the three compartment cell is a chloride solution
containing 20 to 25% of chloride, the hydroxide removed from the
cathode compartment of that cell is an aqueous solution at a
concentration of 20 to 30% hydroxide, the sulfite withdrawn from
the buffer compartment of the same cell is an aqueous solution at a
concentration of 1 to 15% sulfite and accompanying it is sodium
hydroxide, at a concentration of 15 to 1% hydroxide, the chloride
feed to the anolyte compartment of the two-compartment cell is
essentially the same as that of the feed of such compartment of the
three-compartment cell, and the dithionite and sulfite removed from
the catholyte compartment of the two-compartment cell are in
aqueous solution at a concentration of 10 to 70 g./l. of the
dithionite and 100 to 250 g./l. of the sulfite.
7. A method according to claim 6 wherein the anodes are
dimensionally stable anodes of material selected from the group
consisting of noble metals, noble metal alloys, noble metal oxides,
mixtures of noble metal oxides with valve metal oxides, and
mixtures thereof, on a valve metal, and the cathode is stainless
steel.
8. A method according to claim 7 wherein the chloride is sodium
chloride, the hydroxide is sodium hydroxide, the sulfite is sodium
sulfite and the dithionite produced is sodium dithionite, the
anolytes are recirculated and the depleted anolytes are increased
in concentration to about 25% NaCl, at which concentration they are
fed to the anode compartments, by dissolving solid sodium chloride
therein.
9. A method according to claim 8 wherein the membrane copolymer
equivalent weight is from 1,100 to 1,400, the membrane wall
thickness is 0.1 to 0.3 mm., the anode is ruthenium oxide on
titanium, the pH's of the anolytes are about 2 to 4 and the
dithionite withdrawn is in an aqueous solution with sodium sulfite,
wherein the dithionite concentration is from 30 to 50 g./l.
10. A method for electrolytically maunfacturing a dithionite,
chlorine and a hydroxide from sulfur dioxide and a chloride which
comprises feeding chloride solution to the anode compartment of an
electrolytic cell having anode, buffer and cathode compartments
separated by cation-active permselective membranes, an anode in the
anode compartment and a cathode in the cathode compartment, and
feeding sulfur dioxide to the buffer compartment, withdrawing
chlorine from the anode compartment, hydroxide from the cathode
compartment and sulfite from the buffer compartment, feeding such
sulfite to the cathode compartment of a two-compartment
electrolytic cell having an anode in an anode compartment, a
cathode in a cathode compartment and a cation-active permselective
membrane dividing the compartments, maintaining the pH in the
cathode compartment of the two-compartment electrolytic cell at
about 6 to 8, feeding chloride to the anode compartment of such
cell and withdrawing chlorine from the anode compartment and
dithionite from the cathode compartment.
11. A method according to claim 10 wherein the cation-active
permselective membranes are selected from the group consisting of a
hydrolyzed copolymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluoroyinyl ether, and a sulfostyrenated
perfluorinated ethylene propylene polymer, the wall thickness of
the membranes is from about 0.02 to 0.05 mm. the hydroxide produced
in the cathode compartment of the three-compartment cell is of a
high concentration and chloride-free, the sulfite is of a
concentration of 1 to 15%, the voltage drop across the
three-compartment cell is about 3 to 6 volts, that across the
two-compartment cell is about 3 to 5 volts, the current density for
the three-compartment cell is about 1 to 3 amperes/sq. in., that
for the two-compartment cell is about 0.1 to 2 amperes/sq. in. and
the operating temperature of the three-compartment cell is about
50.degree. to 100.degree.C. and that of the two compartment cell is
about 3.degree. to 40.degree.C.
12. A method according to claim 11 wherein the permselective
membrane is of a hydrolyzed copolymer of tetrafluoroethylene and
FSO.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2,
which copolymer has an equivalent weight of of about 1,100 to
1,400, the membrane thickness is from 0.1 to 0.3 mm., the anodes
are dimensionally stable anodes of material selected from the group
consisting of noble metals, noble metal alloys, noble metal oxides,
mixtures of noble metal oxides with valve metal oxides, and
mixtures thereof, on a valve metal, the cathodes are stainless
steel, the chloride, hydroxide, sulfite and dithionite are sodium
salts, the sodium chloride is charged to the anode compartments of
the cells in an aqueous solution at a concentration of about 20 to
25% NaCl and the dithionite produced is in aqueous solution at a
concentration of 5 to 50 g./l.
Description
This invention relates to the electrolytic manufacture of
dithionites. More specifically, it is of a process for making
alkali metal dithionite from alkali metal chloride and sulfur
dioxide, utilizing a combination of electrolytic cells, one having
three compartments and the other having two compartments, the
compartments of each being separated by a cation-active
permselective membrane which, in the best embodiments of the
invention, is of a hydrolyzed polymer of a perfluorinated
hydrocarbon and a fluorosulfonated perfluorovinyl ether or is a
sulfostyrenated perfluorinated ethylene propylene polymer.
The cation-active membranes mentioned allow proportions of hydroxyl
ion generated at the cathodes of the cells to migrate to the buffer
compartments of the three-compartment cells and to the anode
compartments of the two-compartment cells. In the former case this
portion of the hydroxyl generated is reacted with sulfur dioxide to
produce sulfite and in the latter case may be converted to oxygen,
thereby interfering with the efficiency of the two-compartment cell
portion of the process. However, the proportion of hydroxide
entering the anode compartment of the two-compartment cell is very
low because it is consumed in the catholyte of that cell by
reaction with sulfur dioxide therein to form larger anions, such as
sulfite and dithionite, which do not readily penetrate the
cation-active permselective membrane. Thus, dithionite and sulfite
ions are prevented from migrating from the catholyte or buffer
solution to the buffer solution or anolyte, chloride is prevented
from migrating from the anolyte to the buffer or catholyte
compartments and hydroxyl ion is effectively prevented from passing
into the anolytes.
Dithionites and in particular, alkali metal dithionites, especially
sodium dithionite, are useful bleaching agents and have been found
to brighten or bleach wood pulps appreciably. Such a brightening or
bleaching operation is an essential portion of many papermaking
processes. Usually, the dithionite employed in the past has been
zinc dithionite but to prevent water pollution the discharging of
zinc ions into streams has been limited. Therefore, it has been
found desirable to utilize other dithionites which are less
objectionable. It has been suggested that dithionites could be made
by the electrolysis of acidic solutions of sulfer dioxide,
utilizing separating permselective membranes between anode and
cathode compartments. Such a process has been described in Pulp and
Paper Magazine of Canada, in the issue of Dec. 19, 1969, at pages
73-78. Such methods are feasible to some extent but the process of
the present invention is far superior. It electrolytically produces
hydroxide employed to make sulfite reactant, manufactures useful
chlorine simultaneously, rather than useless oxygen, and makes a
hydroxide and the bleaching product, both of which are low in
chloride content. Such low chloride contents are advantageous since
the proportion of chloride which may be discharged into streams and
ground water is also limited. Although sulfite accompanies the
dithionite, it may be usefully employed with it and is useful in
making white liquor, utilized in papermaking processes. A special
advantage of the present invention is in the utilization of the
various products of the process in industrial plants, such as
papermaking plants. The chloride-free hydroxide, dithionite,
sulfite and chlorine are all useful products for papermaking and
are produced in usable forms, without objectionable contaminants.
They are made from a limited number of starting materials,
primarily sources of chloride, e.g., salt, and sulfer dioxide,
which may be obtained from the burning of sulfur or
sulfur-containing ores.
In accordance with the present invention a method for
electrolytically manufacturing a dithionite, chlorine, hydroxide
and a sulfite from sulfur dioxide and a chloride comprises feeding
chloride solution to the anode compartment of an electrolytic cell
having anode, buffer and cathode compartments separated by
cation-active permselective membranes, an anode in the anode
compartment and a cathode in the cathode compartment and feeding
sulfer dioxide to the buffer compartment, withdrawing chlorine from
the anode compartment, hydroxide from the cathode compartment and
sulfite from the buffer compartment, feeding such sulfite and
sulfer dioxide to the cathode compartment of a two-compartment
electrolytic cell having an anode in an anode compartment, a
cathode in a cathode compartment and a cation-active permselective
membrane dividing the compartments, feeding chloride to the anode
compartment thereof and withdrawing chlorine from the anode
compartment and dithionite and sulfite from the cathode
compartment. Important advantages of this process include the
manufacture of chloride-free, high concentration caustic in the
three-compartment cell at a high current efficiency, together with
useful chlorine from both cells, and the production of sodium
dithionite in the cathode compartment of the two-compartment cell
at a pH which is about neutral, preferably about 6 to 8, in which
range the dithionite is comparatively stable, so that it may be
used commercially as the aqueous solution produced, with sulfite,
for the bleaching of wood pulp and other analogous processes.
The invention will be readily understood by reference to the
following description of an embodiment thereof, taken in
conjunction with the drawing of apparatuses utilized in carrying
out the inventive process.
In the drawing:
The FIGURE is a schematic representation of a pair of electrolytic
cells and auxiliary equipment for producing dithionite by the
method of this invention.
In electrolytic cell 11, outer wall 13 and bottom 15 enclose anode
17, cathode 19 and conductive means 21 and 23, respectively, for
connecting the anode and cathode to sources of positive and
negative electrical potentials, respectively. Cation-active
permselective membranes 25 and 27 divide the cell volume into anode
or anolyte compartment 29, buffer compartment 31 and cathode or
catholyte compartment 33. An acidic aqueous solution of a halide or
brine is indicated as passing into the anode compartment through
line 35. Such brine is used for initial charging of the anolyte and
for make-up feed, although make-up may also be added before
recirculated anolyte is admitted to the resaturator, to be
described. Also, it may be desirable to dispense with brine line 35
and charge the cell initially and feed make-up through the
resaturator piping. The chloride solution for the anolyte
compartment, which may be maintained at a desired acidity by
additions of acid, e.g., HCl, by conventional means, not shown, is
circulated from the anode compartment through resaturator 37 via
line 39 and exits from the resaturator through line 41, from whence
it returns to the anode compartment. In a normal operation,
utilizing sodium chloride solution or other alkali metal chloride,
the anolyte compartment is charged with a suitable chloride, e.g.,
a 25% salt solution, and that withdrawn for resaturation is at a
lower concentration, e.g., about 22% NaCl. Chlorine, generated in
the anode compartment by electrolysis of the halide solution, is
taken off through line 43.
Water may be added to the cathode compartment 33 through piping 45
to maintain the desired level thereof and of the buffer
compartment. Hydrogen is removed from this compartment through
venting means 47. The buffer compartment has sulfur dioxide and
water added to it through lines 49 and 51, respectively, and
alkaline sodium sulfite is taken off through piping 53, through
which it is transmitted to cathode compartment 55 of
two-compartment electrolytic cell 57.
To increase circulation in the buffer compartment, effectively
increase the volume of the compartment and to allow greater
reaction times between the caustic and sulfur dioxide there may be
provided a recirculation loop, for the buffer compartment including
lines 50, 52 and 54, pump 56 and "holding tank" 58. The volume of
such system may be 10 to 100,000 times that of the buffer
compartment, preferably from 100 to 10,000 times such volume. High
strength sodium hydroxide is removed from the cell through take-off
piping 40, at a concentration of about 20 to 30% hydroxide, as
sodium hydroxide, in water, and with a low chloride content,
usually less than one gram per liter of NaCl. Some of the hydroxide
produced in the cathode compartment 33 penetrates the cation-active
permselective membrane 27 and passes into buffer compartment 31,
wherein it reacts with the sulfur dioxide to produce sodium
sulfite. The passage of the hydroxide into the buffer compartment
is represented by arrow 42. Bacause of the reaction of the
hydroxide in the buffer compartment and because the sulfite ion and
SO.sub.2 do not penetrate the membrane 25, very little hydroxide
passes into the anode compartment 29 and therefore, the chlorine
efficiency is maintained high. Also, of course, chloride ion does
not pass from the anolyte into the buffer compartment, due to the
repulsive effect of the permselective membrane. Additionally, the
membranes and buffer zone prevent hydrogen or other
cathode-produced gases from being mixed with chlorine, preventing
the production of combustible gas mixtures.
Two-compartment cell 57 has sides 59 and bottom 61 enclosing anode
63 and cathode 65, which are connected to sources of positive and
negative electrical potentials, respectively, through conductive
means 67 anad 69. Cation-active permselective membrane 71 divides
the two-compartment cell volume into anode or anolyte compartment
73 and cathode or catholyte compartment 55. Acidic aqueous halide,
e.g., chloride solution or brine passes into the anode compartment
through line 77 for initial charging of the anolyte and, if
desired, for make-up feed. The halide or chloride solution for the
anolyte compartment, also maintained at desired acidity in the same
manner described for the three-compartment cell, is taken off from
the anode compartment through line 79 and passes through
resaturator 81, exiting through line 83 and returning to the anode
compartment. Concentrations of chloride solution taken off and
returned to that compartment are about the same as with respect to
the three-compartment cell, already described. As with the
three-compartment cell operation the use of the separate brine line
may be discontinued in favor of utilization of the resaturator
elements instead, to feed brine and make-up for any losses thereof.
Also, instead of separate resaturators and attendant lines a single
resaturator and appropriate piping may be used to maintain halide
concentrations in both cell anolytes. Chlorine generated in the
anode compartment of the two-compartment cell is removed therefrom
through piping 85.
Cathode compartment 55 is charged with gaseous sulfur dioxide
through line 87 and water is added through line 89. A mixture of
dithionite and sulfite is removed via piping 91 and any hydrogen or
other gases which may be produced in the cathode compartment are
vented off via venting means 93. Analogously to the buffer solution
recirculation in the three-compartment cell, catholyte of the
two-compartment cell may also be recirculated, utilizing lines 60,
62 and 64, tank 66 and pump 68. The ratio of the total circulating
system volume to that of the cathode compartment may be from 2:1 to
100,000:1 and is preferably 100:1 to 10,000:1.
During operations of the cells high concentration, low chloride
caustic is taken off from the three-compartment cell and is ready
for use in wood pulping, bleaching or other operations and chlorine
removed from the anode compartment of the three-compartment cell is
useful in the bleaching of wood pulp or for other pulp and paper
mills' industrial purposes. The sulfite, produced in alkaline form
due to the content of hydroxide therein, is converted in the
two-compartment cell to dithionite and additional sulfite is made
by reaction of sulfur dioxide with hydroxide generated in the
cathode compartment. As is clear from the diagram, the
two-compartment cell also makes chlorine, useful in pulp bleaching.
The sulfite made by reaction of the sulfur dioxide with hydroxide
in the cathode compartment is useful in pulping operations and may
be converted to white liquor after completion of bleaching of pulp
by the accompanying dithionite. The sulfur dioxide performs the
important function of regulating the pH in the cathode compartment
of the two-compartment cell so as to maintain it in the range of 6
to 8, thereby stabilizing the dithionite produced. Although the
mechanism of the reaction has been described, applicants should not
be considered as being bound by this description, since it may also
be theorized that the sulfur dioxide charged is reduced to
dithionic acid, which is then neutralized by hydroxyl present to
form dithionite. In such case, the presence of the sulfite can help
to exert a buffering effect to maintain the desired pH.
As is illustrated schematically by arrow 95 the dithionite (and
sulfite) ions do not penetrate the permselective membrane 71 and
therefore, are held in the cathode compartment 55. Similarly,
halide ions, the path of which is indicated by an arrow identified
by numeral 97, do not pass from the anolyte to the catholyte of the
two-compartment cell. However, cations such as alkali metal ions,
e.g., Na.sup.+, indicated by M.sup.+ in the illustration, the
direction of which is represented by the arrow 99 headed toward the
right on the right side of the drawing, may pass from anolyte to
catholyte. A small proportion of hydroxyl ion may penetrate the
membrane 71 but usually the concentration of free hydroxyl is low
in the catholyte, due to reaction with sulfur dioxide and reduction
of the pH to the 6 to 8 range, so that the hydroxyl entering the
anolyte, if any, has little effect on chloride current
efficiency.
By the described process, utilizing a combination of
three-compartment and two-compartment cells, the sulfur dioxide
feed to the buffer compartment of the three-compartment cell ties
up the sodium hydroxide penetrating the membrane between the
catholyte and buffer solution and prevents it from reaching the
anode, where it could be converted to useless oxygen, thereby
decreasing current efficiency. At the same time, high strength,
chloride-free caustic is made, which is important in various
chemical operations, e.g., pulp bleaching, where chloride
discharges from industrial plants are undesirable and may be
strictly limited.
The chlorine and chloride-free caustic made are both useful
chemicals for many industrial processes, including wood pulping and
pulp bleaching. Thus, the invention has a distinct advantage over
an electrolytic method for producing dithionite by charging sulfite
or sulfur dioxide to a two-compartment cell and producing
dithionite in the cathode compartment reduction of sulfite or
reduction of sulfur dioxide, followed by neutralization to the
dithionite. That is, the sulfur dioxide which would be required to
make sulfite for the two-compartment cell electrolytic reaction,
makes the sulfite in the buffer compartment of the
three-compartment cell while chloride-free caustic is made in the
cathode compartment, and increases chlorine current efficiency of
the cell. These additional advantages improve the efficiency of the
present process and make it commerically advantageous over similar
or related processes.
Instead of adding sulfur dioxide to the cathode compartment,
wherein it acts as a source of sulfite for reduction to dithionite
and at the same time serves to help regulate the pH in the desired
6 and 8 range, sulfite may be fed to the catholyte, with other
means employed for pH regulation. By such a process, although the
results may not be as satisfactory as with that previously
described, utilizing sulfur dioxide, dithionite can be made.
However, unless the means of reducing the alkaline pH caused by the
presence of the hydroxide generated at the cathode is a chemical
which produces a useful prduct (and which is non-interfering with
the dithionite process), there will be a waste of hydroxide and
possibly, even creation of a disposal problem.
The halide solution fed to the anode compartment of both cells is
an aqueous solution of a water soluble metal chloride in the usual
case, preferably of sodium chloride. The concentration thereof is
generally in the range of 200 to 320 grams/liter for sodium
chloride and 200 to 360 g./l. for potassium chloride. preferably
such solutions contain 20 to 25% of the alkali metal halide salt,
as the solutions are charged to the cell or delivered to it from
the resaturator. Generally the chloride content will be reduced to
5 to 30% less than the original content, preferably to 10 to 20%
less and normally, as with sodium chloride, the concentration of
the halide removed from the anode compartment for resaturation and
return to such compartment is about 22 %, as NaCl, or equivalent.
Although the anolyte may be neutral, it is often acidified so as to
be of a pH in the range of about 1 to 6, preferably 2 to 4, with
acidification normally being effected with a suitable acid, such as
hydrochloric acid. Water utilized to make the initial brine charge
or added as make-up feed to the anode compartments and water added
to the other compartments of the cells will preferably be
deionized, containing less than 10 p.p.m. hardness, as CaCO.sub.3,
although tap water of comparatively low hardness, e.g., under 150
p.p.m., preferably under 50 p.p.m., can be used.
The sulfur dioxide charged to the buffer compartment of the
three-compartment cell is usually substantially pure e.g., over 90%
SO.sub.2, but lower concentrations thereof, e.g., as low as 20%,
are usable because of the desirable attributes of the membrane
material in preventing gas interchanges between cell portions.
Thus, the unreacted gas, e.g., O.sub.2, N.sub.2, may be removed
from line 53 at a suitable point, before the sulfite produced is
charged to the cathode compartment of the two-compartment cell.
In the three-compartment cell high concentration hydroxide
solution, such as alkali metal hydroxide, preferably sodium
hydroxide, is produced, normally of 20 to 30% hydroxide, although
lesser concentrations may also be made, e.g., down to as low as 5%.
The chloride content thereof is low, usually being less than 5
g./l. and often less than 1 g./l. The concentration of the
hydroxide may be regulated by controls of the rate of feed of water
to the catholyte, flow of electric current and, in some cases,
nature of the feed to the cathode compartment (dilute caustic may
sometimes be fed in at least partial replacement of water).
The sulfite produced by reaction of the sodium hydroxide and sulfur
dioxide in the buffer compartment may be of any of various
concentrations. These are controllable by regulating the feed of
sulfur dioxide to the buffer compartment. The more sulfur dioxide
charged, the greater the quantity of sulfite in the buffer
effluent, in comparison to that of the hydroxide. Generally, the
sulfite will be an aqueous solution of 1 to 15% strength and the
hydroxide removed fro the buffer compartment will also be a
corresponding 15 to 1% solution, with more sulfite than hydroxide
in the buffer compartment. Preferably the sulfite and hydroxide
concentrations total about 10 to 20%, e.g., about 15%, and in more
preferred embodiments of the invention the concentration of
hydroxide is maintained at less than 5% while that of the sulfite
is up to about 10%.
In the two-compartment cell the feeds to the catholyte of
sulfite-hydroxide solution from the buffer compartment and SO.sub.2
are so regulated as to maintain the desired pH for the formation of
a stable dithionite. Such a pH should be in the range of about 6 to
8, preferably 6 to 8 and most preferably about 7. It may be
regulated by controlling the feed of sulfur dioxide, which has the
additional beneficial effect of diminishing the hydroxide
concentration to a very small proportion, preventing all but a very
minor proportion of the hydroxide generated at the cathode from
migrating through the membrane to the anolyte, where it could have
been converted to oxygen, with a loss of electrical efficiency. The
effluent from the cathode compartment is a mixture of dithionite
and sulfite and the concentrations of these components are usually
in the ranges of 0.5 to 30% sulfite and 0.5 to 10% dithionite.
Within such ranges the normal ranges are from 10 to 20% of sulfite
and 1 and 5% of dithionite. The conversion of sulfite or sulfur
dioxide to dithionite will usually be at a current efficiency of
from about 40 to 80%, normally within the 60 to 75% range. The
dithionite removed from the cathode compartment of the
two-compartment cell will generally have a concentration of 10 to
70 g./l., within which range 30 to 50 g./l. is usual. From 100 to
250 g./l. will be the concentration of the sulfite drawn off with
it.
To obtain the desired operation of these cells, as described, the
voltage drop across the three-compartment cell is maintained at
about 3 to 6 volts, preferably 4 to 5 volts and that across the
two-compartment cell is about 3 to 5 volts, preferably 3.5 to 4.5
volts. The current density for the three-compartment cell is about
1 to 3 amperes/sq. in., preferably 1.5 to 2.5 a.s.i., and that of
the two-compartment cell is 0.1 to 2 a.s.i., preferably 0.2 to 1
a.s.i. The operating temperature of the three-compartment cell is
about 50.degree. to 100.degree.C., preferably 80.degree. to
100.degree.C., whereas that of the two-compartment cell is
3.degree. to 40.degree.C., preferably 3.degree. to 25.degree.C. A
low temperature is desirable for operation of the two-compartment
cell because of the greater stability of the dithionite at such low
temperatures.
The anodes employed are preferably dimensionally stable anodes of a
material selected from the group consisting of noble metals, noble
metal alloys, noble metal oxides, mixtures of noble metal oxides
with valve metal oxides and mixtures thereof, on a valve metal,
whereas the cathodes are preferably of stainless steel. Instead of
the dimensionally stable anodes, anodes of noble metals or oxides
thereof may also be employed, e.g., platinum, iridium, ruthenium or
rhodium. Alternatively, other anodes resistant to the anolytes can
be used, although they are not usually preferred. The anodes and
cathodes may be connected to sources of electrical potential by
conductive metals, such as copper, silver, aluminum, steel and iron
but these materials are normally shielded from contact with the
electrolytes. Preferable dimensionally stable anode surfaces, all
on titanium or tantalum substrates, are ruthenium oxide-titanium
oxide mixtures, platinum, ruthenium, platinum oxide and mixtures of
ruthenium and platinum and mixtures of their oxides. A preferred
dimensionally stable anode is a ruthenium oxide-titanium dioxide
mixture on a titanium substrate, connected to a source of positive
electrical potential by a titanium-clad copper conductor.
The cathodes employed should be resistant to the corrosive
catholyte and therefore it had been found that noble metal, noble
metal oxide and stainless steel cathodes are preferred. Ordinary
iron or steel cathodes soon become deteriorated in use, although
they may be employed for short term operations. Graphite cathodes
are not preferred because of their poorer conductivity and other
physical properties. Of the noble metals, those previously
described are satisfactory and of the stainless steels those
containing small proportions of molybdenum, in addition to
chromium, nickel and iron, are preferred. These include Stainless
Steel Types Nos. 316 to 317. However, other stainless steels of
high resistances to corrosion by the catholyte environments may
also be employed, many of which may contain about 18% of chromium
and 8% of nickel. The various stainless steels from which
corrosion-resistant anodes may be made are described in Section 24
of the Steel Products Manual, issed by the American Iron and Steel
Institute in February, 1949, under the heading "Stainless and
Heat-Resisting Steels". A summary of such steel formulations and
corresponding type numbers is found in the Handbook of Engineering
Fundamentals by Eshback, Second Edition, published in 1952 by John
Wiley & Sons, Inc., New York, page 12-40 and discussions of
such steels and their corrosion resistances is at page 12-39. In
addition to the stainless steels, other corrosion resistant steels
such as silicon steels, nickel steels, and other conductivve
materials resistant to corrosion may also be employed as cathode
materials or surfaces.
The presently preferred cation-permselective membrane is of a
hydrolyzed copolymer of perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether. The perfluorinated
hydrocarbon is preferably tetrafluoroethylene, although other
perfluorinated anad saturated and unsaturated hydrocarbons of 2 to
5 carbon atoms may also be utilized, of which the monoolefinic
hydrocarbons are preferred, especially those of 2 to 4 carbon atoms
and most especially those of 2 to 3 carbon atoms, e.g.,
tetrafluoroethylene, hexafluoropropylene. The sulfonated
perfluorovinyl ether which is most useful is that of the formula
FS0.sub.2 CF.sub.2 CF.sub.2 OCF(CF.sub.3)CF.sub.2 OCF=CF.sub.2.
Such a material, named as perfluoro
[2-(2-fluorosulfonylethoxy)-propyl vinyl ether], referred to
henceforth as PSEPVE, may be modified to equivalent monomers, as by
modifying the internal perfluorosulfonylethoxy component to the
corresponding propoxy component and by altering the propyl to ethyl
or butyl, plus rearranging positions of substitution of the
sulfonyl thereon and utilizing isomers of the perfluorolower alkyl
groups, respectively. However, it is most preferred to employ
PSEPVE.
The method of manufacture of the hydrolyzed copolymer is described
in Example XVII of U.S. Pat. No. 3,282,875 and an alternative
method is mentioned in Canadian Pat. No. 849,670, which also
discloses the use of the finished membrane in fuel cells,
characterized therein as electrochemical cells. The disclosures of
such patents are hereby incorporated herein by reference. In short,
the copolymer may be made by reacting PSEPVE or equivalent with
tetrafluoroethylene or equivalent in desired proportions in water
at elevated temperature and pressure for over an hour, after which
time the mix is cooled. It separates into a lower perfluoroether
layer and an upper layer of aqueous medium with dispersed desired
polymer. The molecular weight is indeterminate but the equivalent
weight is about 900 to 1,600 preferably 1,100 to 1,400 and the
percentage of PSEPVE or corresponding compound is about 10 to 30%
preferably 15 to 20% and most preferably about 17%. The
unhydrolyzed copolymer may be compression molded at high
temperature and pressure to produce sheets or membranes, which may
vary in thickness from 0.02 to 0.5 mm. These are then further
treated to hydrolyze pendant --SO.sub.2 F groups to --S0.sub.3 H
groups, as by treating with 10% sulfuric acid or by the methods of
the patents previously mentioned. The presence of the --SO.sub.3 H
groups may be verified by titration, as described in the Canadian
patent. Additional details of various processing steps are
described in Canadian Pat. No. 752,427 and U.S. Pat. No. 3,041,317,
also hereby incorporated by reference.
Because it has been found that some expansion accompanies
hydrolysis of the copolymer it is preferred to position the
copolymer membrane after hydrolysis onto a frame or other support
which will hold it in place in the electrolytic cell. Then it may
be clamped or cemented in place and will be true, without sags. The
membrane is preferably joined to the backing tetrafluoroethylene or
other suitable filaments prior to hydrolysis, when it is still
thermoplastic, and the film of copolymer covers each filament,
penetrating into the spaces between them and even around behind
them, thinning the films slightly in the process, where they cover
the filaments.
The membrane described is far superior in the present processes to
all other previously suggested membrane materials. It is more
stable at elevated temperatures, e.g., above 75.degree.C. It lasts
for much longer time periods in the medium of the electrolyte and
the caustic product and does not become brittle when subjected to
chlorine at high cell temperatures. Considering the savings in time
and fabrication costs, the present membranes are more economical.
The voltage drop through the membranes is acceptable and does not
become inordinately high, as it does with many other membrane
materials, when the caustic concentration in the cathode
compartment increases to above about 200 g./l. of caustic. The
selectivity of the membrane and its compatibility with the
electrolyte do not decrease detrimentally as the hydroxyl
concentration in the catholyte liquor increases, as has been noted
with other membrane materials. Furthermore, the caustic efficiency
of the electrolysis does not diminish as significantly as it does
with other membranes when the hydroxyl ion concentration in the
catholyte increases. Thus, these differences in the present process
make it practicable, whereas previously described processes have
not attained commerical acceptance. While the more preferred
copolymers are those having equivalent weights of 900 to 1,600,
with 1,100 to 1,400 being most preferred, some useful resinous
membranes produced by the present method may be of equivalent
weights from 500 to 4,000. The medium equivalent weight polymers
are preferred because they are of satisfactory strength and
stability, enable better selective ion exchange to take place and
are of lower internal resistances, all of which are important to
the present electrochemical cell operations.
Improved versions of the above-described copolymers may be made by
chemical treatment of surfaces thereof, as by treatments to modify
the --SO.sub.3 H group thereon. For example, the sulfonic group may
be altered or may be replaced in part with other moieties. Such
changes may be made in the manufacturing process or after
production of the membrane. When effected as a subsequent surface
treatment of a membrane the depth of treatment will usually be from
0.001 to 0.01 mm. Caustic efficiencies of the invented processes,
using such modified versions of the present improved membranes can
increase about 3 to 20%, often about 5 to 15%. Exemplary of such
treatments is that described in French patent publication No.
2,152,194, in which one side of the membrane is treated with
NH.sub.3 to form S0.sub.2 NH.sub.2 groups.
In addition to the copolymers previously discussed, including
modifications thereof, it has been found that another type of
membrane material is also superior to prior art films for
applications in the present processes. Although it appears that
tetrafluoroethylene (TFE) polymers which are sequentially
styrenated and sulfonated are not useful for making satisfactory
cation-active permselective membranes for use in the present
electrolytic processes it has been established that perfluorinated
ethylene propylene polymer (FEP) which is styrenated and sulfonated
makes a useful membrane. Whereas useful lives of as much as three
years or more (that of the preferred copolymers) may not be
obtained, the sulfostyrenated FEP's are surprisingly resistant to
hardening and otherwise failing in use under the present process
conditions.
To manufacture the sulfostyrenated FEP membranes a standard FEP,
such as manufactured by E. I. DuPont de Nemours & Co. Inc., is
styrenated and the styrenated polymer is then sulfonated. A
solution of styrene in methylene chloride or benzene at a suitable
concentration in the range of about 10 to 20% is prepared and a
sheet of FEP polymer having a thickness of about 0.02 to 0.5 mm.,
preferably 0.05 to 0.15 mm., is dipped into the solution. After
removal it is subjected to radiation treatment, using a
cobalt.sup.60 radiation source. The rate of application may be in
the range of about 8,000 rads/hr. and a total radiation application
is about 0.9 megarad. After rinsing with water the phenyl rings of
the styrene portion of the polymer are monosulfonated, preferably
in the para position, by treatment with chlorosulfonic acid, fuming
sulfuric acid or S0.sub.3. preferably chlorosulfonic acid in
chloroform is utilized and the sulfonation is completed in about
one-half hour.
Examples of useful membranes made by the described process are
products of RAI Research Corporation, Hauppauge, New York,
identified as 18ST12S and 16ST12S, the former being 18% styrenated
and having two-thirds of the phenyl groups monosulfonated and the
latter being 16% styrenated and having thirteen-sixteenths of the
phenyl groups monosulfonated. To obtain 18% styrenation a solution
of 17-1/2% of styrene in methylene chloride is utilized and to
obtain the 16% styrenation a solution of 16% of styrene in
methylene chloride is employed.
The products resulting compare favorably with the preferred
copolymers previously described, giving voltage drops of about 0.2
volt each in the present cells at a current density of 2
amperes/sq. in., the same as is obtained from the copolymer.
The membrane walls will normally be from 0.02 to 0.5 mm. thick,
preferably from 0.1 to 0.5 mm. and most preferably 0.1 to 0.3 mm.
When mounted on a polytetrafluoroethylene, asbestos, titanium or
other suitable network, for support, the network filaments or
fibers will usually have a thickness of 0.01 to 0.5 mm., preferably
0.05 to 0.15 mm., corresponding to up to the thickness of the
membrane. Often it will be preferable for the fibers to be less
than half the film thickness but filament thickness greater than
that of the film may also be successfully employed, e.g., 1.1 to
five times the film thickness. The networks, screens or cloths have
an area percentage of openings therein from about 8 to 80%,
preferably 10 to 70% and most preferably 30 to 70%. Generally the
cross-sections of the filaments will be circular but other shapes,
such as ellipses, squares and rectangles, are also useful. The
supporting network is preferably a screen or cloth and although it
may be cemented to the membrane it is preferred that it be fused to
it by high temperature, high pressure compression before hydrolysis
of the copolymer. then, the membrane-network composite can be
clamped or otherwise fastened in place in a holder or support. It
is preferred to employ the described backed membranes as walls of
the cell between the anolyte and catholyte compartments and the
buffer compartment(s) but if desired, that separating the anolyte
and buffer compartments may be of conventional diaphragm material,
e.g., deposited asbestos fibers or synthetic polymeric fibrous
material (polytetrafluoroethylene, polypropylene). Also, treated
asbestos fibers may be utilized and such fibers mixed with
synthetic organic polymeric fibers may be employed. However, when
such diaphragms are used efforts should be made to remove hardness
ions and other impurities from the feed to the cell so as to
prevent these from prematurely depositing on and blocking the
diaphragms.
The material of construction of the cell body may be conventional,
including concrete or stressed concrete lined with mastics,
rubbers, e.g., neoprene, polyvinylidene chloride, FEP, chlorendic
acid based polyester, polypropylene, polyvinyl chloride, TFE or
other suitable plastic or may be similarly lined boxes of other
structural materials. Substantially self-supporting structures,
such as rigid polyvinyl chloride, polyvinylidene chloride,
polypropylene or phenol formaldehyde resins may be employed,
preferably reinforced with molded-in fibers, cloths or webs.
The processes of this invention obtain good current efficiencies
for the manufacture of chlorine and acceptable current efficiencies
for producing hydroxide, sulfite and dithionite. In preferred
embodiments of the invention, when sodium chloride is utilized and
sodium sulfite and sodium dithionite are made, the current
efficiencies for the productions of chlorine in both cells are from
90 to 99%, usually being 94 to 97%, e.g., 96%. The production of
caustic in the three-compartment cell, including caustic produced
in the cathode compartment, whether removed therefrom the buffer
compartment and whether removed from the buffer compartment as
caustic or sulfite, is at a current efficiency or sodium ion
efficiency of about 70 or 75 or 90%. Approximately 5 to 50% of the
hydroxide produced in the cathode compartment migrates to the
buffer compartment and usually this will be from 5 to 25%. In the
two-compartment cell the current efficiency for the production of
the dithionite will normally be from 40 to 80%, usually 60 to 75%,
with the conversion of sulfur dioxide or sulfite to dithionite
being about 20 to 50%. such efficiencies are acceptable and
although the efficiency for the manufacture of dithionite might
appear low, considering that useful sulfite is also made, it is
satisfactory.
The present cells may be incorporated in large or small
electrochemical plants, those producing bleaching dithionite and
accompanying sulfite while also making from 20 to 1,000 tons per
day of chlorine or equivalent derivative. In all cases the
efficiencies obtainable are such as to make the processes
economically desirable. It is highly preferred, however, that the
installation should be located near to and should be used in
conjunction with a groundwood or woodpulp bleaching plant so that
the dithionite produced can be employed promptly as a bleach and
the other chemicals may also be used for pulping or bleaching
purposes without the need to ship them long distances to ultimate
consumers. Of course, if desired, the chlorine and caustic may be
so shipped or may be chemically converted to other materials. In
some instances the chlorine may be liquefied and the caustic may be
evaporated to a higher concentration so as to facilitate shipment
or transfer.
The following examples illustrate but do not limit the invention.
Unless otherwise indicated, all parts are by weight and all
temperatures are in .degree.C.
EXAMPLE 1
Utilizing the apparatus illustrated in the FIGURE, useful sodium
dithionite in aqueous solution, accompanied by sodium sulfite, is
produced and is successfully employed in the bleaching of
groundwood pulp.
The materials of construction of the three-compartment and
two-compartment cells include as a preferred material, asbestos
filled polypropylene. I anodes are dimensionally stable anodes of
titanium having ruthenium-titanium oxide coatings. The titanium
mesh-based anodes are connected to sources of electricity by
titanium-clad copper rods. The cathodes are of Type 316 stainless
steel. In other experiments, yielding essentially the same results,
the internal cell walls are of such materials as chlorinated
polyethylene or chlorinated polypropylene, the anodes are of
platinum or platinum-iridium alloy and the cathodes are of Type 317
stainless steel.
The cation-active permselective membranes employed have a wall
thickness of 7 mils (about 0.2 mm.) and the membrane portion
thereof is joined to a backing or supporting network of
polytetrafluoroethylene (Teflon) filaments having a diameter of
about 0.1 mm. and woven into cloth form such that the area
percentage of openings therein is of about 25%. The cross-sectional
shape of the filaments is substantially circular and the membranes
mounted on them are originally flat and are fused onto the screen
or cloth by high temperature, high compression pressing, with
portions of the membranes actually flowing around the filaments
during the fusion processes to lock onto the cloth. The described
permselective membranes are obtainable from E. I. Du Pont de
Nemours and Company, Inc., Plastics Department, Wilmington, Del.
19898, as XR Perfluorosulfonic Acid Membranes. The material thereof
is a hydrolyzed copolymer of a perfluorinated hydrocarbon and a
fluorosulfonated perfluorovinyl ether. The hydrolyzed copolymer is
of tetrafluoroethylene and FS0.sub.2 CF.sub.2 CF.sub.2 OCF(CF 3)
CF.sub.2 OCF CF.sub.2 and has an equivalent weight in the 1,100 to
1,400 range, about 1,250.
Although in the FIGURE, for clarity of presentation, sides of the
membranes the electrodes are apart from the membranes, in the
practice of the present process the electrodes are in contact with
the membranes in the three-compartment cell, with the
"flatter"sides of the membranes facing the contacting electrodes.
In the three-compartment cell the buffer compartment volume is
about 10% of the total of the anode and cathode compartment
volumes, which are of about the same volume. In the two-compartment
cell, cell volumes are about equal and the electrodes are about
one-forth inch or 6.3 mm. apart.
The feeds to the anode compartments of both cells are 25% sodium
chloride solutions in water and the depleted anolytes in both cases
are at 22% sodium chloride contents with circulations of the
depleted anolytes through the resaturators (or a single
resaturator) being controlled by sensors, valves and pumps to
maintain this desired difference in concentration between feed and
take-off solutions to/from the anode compartment.
In the case of the three-compartment cell the feed of sulfur
dioxide to the buffer compartment is regulated so as to produce an
effluent from that compartment comprising about 10% of sodium
sulfite and 10 % of sodium hydroxide in water. Water feed to the
buffer compartment and water feed and caustic producing conditions
in the cathode compartment may also be regulated to adjust the
proportion of sulfite to hydroxide leaving the buffer compartment.
The pH of such solution is that the caustic, 14. Under best
operating conditions of the three-compartment cell the proportion
of hydroxide passing from the cathode compartment to the buffer
compartment is or averages about 25% of that produced at the
cathode and this ratio is in the range of 5 to 50%. The high
concentration, low chloride content hydroxide taken off from the
cathode compartment is a 25% hydroxide and has a chloride content
of about 0.05%. The temperature of the electrolyte is maintained at
about 90.degree.C. during the process, with 4.5 volts impressed
across the electrodes and a current density of 2 a.s.i., the
current flow being 90 kiloamperes.
In the two-compartment cell the feed to the catholyte is the
effluent from the buffer compartment of a three-compartment cell
and preferably it is cooled en route by cooling means, not
illustrated in the drawing, so as to enter the cathode compartment
of the two-compartment cell at the desired cell temperature, about
20.degree.C. (within a range of 15.degree. to 35.degree.C.). sulfur
dioxide is added to the cathode compartment at such a rate as to
maintain the pH of the catholyte at 7, although it may vary between
6 and 8. Under flow rates described, about 60% of the cathodic
current is utilized in the production of dithionite and about 40%
to make sulfite from hydroxide and sulfur dioxide. The effluent
from the cathode compartment is an aqueous solution containing 16%
of sodium sulfite and 3.7% of sodium dithionite.
The installation described produces 0.36 ton per day of sodium
dithionite, in a 3.7% concentration aqueous solution, with 33%
conversion of sulfur dioxide to dithionite and with the dithionite
obtained at 75 percent current efficiency, calculated on the basis
of useful products obtained. The chlorine produced from the
two-compartment cell is at the rate of 0.3 ton per day and the
current efficiency is 95%. With respect with the three-compartment
cell, the chlorine production is at the rate of 3 tons per day,
also with a 95% current efficiency. The sodium hydroxide taken of
the cathode compartment of the three-compartment cell is produced
at the rate of 2.28 tons per day and is in 25% aqueous solution.
The sulfur dioxide feed to the buffer compartment cell is 0.49 ton
per day with production of sodium sulfite from that compartment
being at 0.97 ton per day and with 0.39 ton per day of sodium
hydroxide accompanying it. Current efficiency for the production of
sulfite and hydroxide in the three-compartment cell, or sodium ion
efficiency, is about 90%.
The solution of dithionite and sodium sulfite from the cathode
compartment of the two-compartment cell is continuously employed to
bleach groundwood pulp, after dilution to a 1% dithionite solution.
The groundwood charge is an 85:15 mixture of West Coast hemlock and
balsam, the rate of application is 1.1 percent of sodium
dithionite, on a dry pulp basis and the pulp is in a 3 percent
aqueous slurry buffered to a pH of about 6.5 with potassium
hydrogen phosphate before addition of the dithionite. A brightness
increase of about 10 units is obtained at a brightening temperature
of 60.degree.-70.degree.C. after about 30 minutes treatment.
Reversion in such cases is about 2 units.
The bleach liquor is recovered and mixed with black liquor which is
subsequently converted to white liquor used in pulping.
EXAMPLE 2
The procedure of Example 1 is followed except for the addition of
sulfur dioxide to the catholyte of the two-compartment cell.
Instead of the sulfur dioxide, additional sulfite is added and the
desired pH of 7 is maintained in the cathode compartment by
continuous addition of sulfuric acid, sodium bisulfate, sodium
bisulfite or any other suitable acidic or alkaline neutralizing
agent or buffer. Although the current efficiency is not as good as
in the processes utilizing a sulfur dioxide feed to the catholyte
of the two-compartment cell, the process is operative and
production of dithionite and other product is at essentially the
same rate as previously described. The dithionite solution obtained
is effective for groundwood bleaching, as described in Example 1,
and is useful for other bleaching purposes, too.
In variations of this process and that of Example 1 the sulfur
dioxide is fed to the buffer compartment of the three-compartment
cell and to the cathode compartment of the two-compartment cell as
aqueous solutions containing about 8% of sulfur dioxide. Utilizing
the solutions fewer problems of gas bubbling and interference with
electrode reactions are experienced but weaker product is obtained.
In other modifications of the experiments, batch and continuous
processes are employed. The continuous processes, sometimes with
recycles of each of the compartment contents, are generally
superior, yielding a more consistent product and readily lending
themselves to automatic control.
The invention has been described with respect to working examples
and illustrative embodiments but is not to be limited to these
because it is evident that one of ordinary skill in the art will be
able to utilize substitutes and equivalents without departing from
the spirit of the invention or the scope of the claims.
* * * * *