U.S. patent application number 10/109443 was filed with the patent office on 2003-10-02 for process for the preparation of zinc dithionite.
This patent application is currently assigned to Clariant International Ltd.. Invention is credited to Bandlish, Baldev K., Martin, Vincent Wise.
Application Number | 20030183535 10/109443 |
Document ID | / |
Family ID | 28453105 |
Filed Date | 2003-10-02 |
United States Patent
Application |
20030183535 |
Kind Code |
A1 |
Bandlish, Baldev K. ; et
al. |
October 2, 2003 |
Process for the preparation of zinc dithionite
Abstract
Disclosed is a process for preparing zinc dithionite comprising
reacting zinc metal with sulfur dioxide, wherein the zinc metal
used in the reaction is produced by electrochemical reduction in an
undivided electrochemical cell of an aqueous alkaline slurry or
solution of zinc oxide or any other zinc compound that reacts with
aqueous base to form zinc oxide.
Inventors: |
Bandlish, Baldev K.;
(Charlotte, NC) ; Martin, Vincent Wise; (Gastonia,
NC) |
Correspondence
Address: |
Clariant Corporation
Industrial Property Department
4331 Chesapeake Drive
Charlotte
NC
28216
US
|
Assignee: |
Clariant International Ltd.
|
Family ID: |
28453105 |
Appl. No.: |
10/109443 |
Filed: |
March 28, 2002 |
Current U.S.
Class: |
205/495 |
Current CPC
Class: |
C01G 9/00 20130101; C25C
5/02 20130101; C25C 1/16 20130101 |
Class at
Publication: |
205/495 |
International
Class: |
C25B 001/00 |
Claims
What is claimed is:
1. A process for preparing zinc dithionite comprising reacting zinc
metal with sulfur dioxide, wherein the zinc metal used in the
reaction is produced by electrochemical reduction in an undivided
electrochemical cell of an aqueous alkaline slurry or solution of
zinc oxide or any other zinc compound that reacts with aqueous base
to form zinc oxide.
2. The process of claim 1, wherein the aqueous alkaline slurry or
solution of zinc oxide or any other zinc compound that reacts with
the aqueous base to form zinc oxide, comprises solubilized zinc
based species, said species comprising at least one member selected
from the group consisting of ZnO.sub.2.sup.2-, HZnO.sub.2.sup.1-,
Zn(OH).sup.+, Zn(OH).sub.2, and Zn.sup.2+.
3. The process of claim 1, that is substantially free of
labor-intensive steps comprising removal of cathode after
completion of the electrochemical reduction of the zinc oxide.
4. The process of claim 1, that is substantially free of
labor-intensive steps comprising clarifying a slurry of zinc oxide
in an aqueous basic solution by sedimentation and filtration prior
to the electrochemical reduction of the zinc oxide.
5. The process of claim 1, that is substantially free of
labor-intensive steps comprising drying, grinding, and sizing of
zinc metal particles prior to the reaction of the zinc metal with
sulfur dioxide.
6. The process of claim 1, wherein the zinc metal used for reaction
with sulfur dioxide is in the form of wet zinc metal particles.
7. The process of claim 1, wherein the current efficiency for the
production of zinc dithionite is at least 70%, the current
efficiency for said production being obtained by multiplying the
current efficiency for zinc metal formation with the conversion
efficiency of zinc metal to zinc dithionite.
8. The process of claim 7, wherein the current efficiency for the
production of zinc dithionite is at least 75%.
9. The process of claim 1, wherein the undivided electrochemical
cell utilizes a magnesium or copper cathode.
10. The process of claim 1, wherein the undivided electrochemical
cell utilizes a stainless steel or nickel anode.
11. The process of claim 1, wherein the undivided electrochemical
cell utilizes a magnesium cathode and a stainless steel anode.
12. The process of claim 1, wherein the undivided electrochemical
cell utilizes a magnesium cathode and a nickel anode.
13. The process of claim 1, wherein the undivided electrochemical
cell utilizes a copper cathode and a nickel anode.
14. The process of claim 1, wherein the undivided electrochemical
cell utilizes a copper cathode and a stainless steel anode.
15. The process of claim 1, wherein the electrochemical reduction
is carried out at a temperature range of about 100 to about
105.degree. C.
16. The process of claim 14, wherein the aqueous base comprises
ions of at least one alkali or alkaline earth metal and hydroxyl
(OH.sup.-) ions.
17. The process of claim 15, wherein the alkali and alkaline earth
metal ions are selected from sodium, potassium, and mixtures
thereof and are provided in the form of a compound selected from
hydroxides, and oxides.
18. The process of claim 15, wherein the compound is selected from
the group consisting of sodium hydroxide and potassium
hydroxide.
19. The process of claim 1, wherein the electrochemical reduction
is carried out at a current density of about 500 to about 30,000
Amps/m.sup.2.
20. The process of claim 1, wherein the reaction of the zinc metal
produced electrochemically in an undivided cell with sulfur dioxide
is carried out at a temperature of about 20 to about 60.degree.
C.
21. The process of claim 1, wherein the reaction of zinc metal with
sulfur dioxide to produce zinc dithionite is conducted in the
presence of cadmium sulfate.
22. The process of claim 1, wherein the reaction of zinc metal with
sulfur dioxide to produce zinc dithionite is conducted in the
substantial absence of cadmium sulfate.
Description
FIELD OF THE INVENTION
[0001] The present invention provides a process for the preparation
of zinc dithionite that utilizes electrochemically prepared zinc
metal as a starting material.
BACKGROUND OF THE INVENTION
[0002] Zinc metal is generally produced by pyrometallurgical or
electrochemical process. Zinc dust and powder are particulate forms
of zinc. The terms dust and powder have been used indiscriminately
to designate particulate zinc materials. The term zinc dust
designates material produced by condensation of zinc vapor, whereas
zinc powder indicates the product obtained by atomizing molten
zinc. Zinc dusts are manufactured in various size ranges and a
typical commercial dust has an average particle diameter between 4
and 8 .mu.m. Usually, dusts are screened to be essentially free of
particles coarser than 75 .mu.m (200 mesh). The particle size
distribution for commercial zinc powders typically range from 44 to
841 .mu.m. (325-20 mesh).
[0003] In the chemical and metallurgical industries, zinc dust is
used as the reducing agent, in the manufacture of hydrosulfite
compounds for the textile and paper industries, and to enhance the
physical properties of plastics and lubricants. Efforts have been
made to prepare zinc powder by electrolyzing a solution of zinc
based compounds in strong alkaline solutions.
[0004] Orszagh and V. Vass (Hungarian Journal of Industrial
Chemistry, Vol. 13, pp. 287-299 (1985)) disclose an electrochemical
method to regenerate zinc mud obtained during sodium dithionite
production. The zinc mud obtained in sodium dithionite production,
contains 50-60% water and 30-35% zinc, mist in the form of zinc
hydroxide. The zinc mud was dissolved, the solution clarified, and
electrolyzed, and the zinc powder obtained was used to reduce
sulfur dioxide. According to Orszag and Vaas, their process
requires a number of labor intensive operations such as lifting of
the electrodes, removal of the zinc powder, drying, grinding and
sizing, etc., that make this process economically unattractive.
Removal of all insoluble material was carried out by using
sedimentation and filtration. Solution of the zinc oxide in an
aqueous NaOH solution was electrolyzed to prepare zinc particles.
Zinc particles so produced were washed with water (20 to 60 ml
water per gram of the zinc), dried, ground and sieved to give zinc
particles with the desired particle size distribution
[0005] Furthermore, their process requires a divided electrolytical
cell. This leads to a significant increase of operating,
maintenance and capital expenses. In order to reduce the required
operating, maintenance and capital expenses, it is highly desirable
to develop a process which uses an undivided cell. The present
invention affords such a process.
[0006] The process of Orsagh and Vass further requires high cell
voltage at a lower current density. This increases capital as well
as operational expenses. In summary, this process is economically
unattractive as an alternate to the currently used zinc dust based
technology. Because of these disadvantages, there is a strong need
for an improved cost effective process to recycle zinc oxide
byproduct to zinc dithionite. The present invention provides such a
process.
[0007] U.S. patent application Ser. No. 09/776,518 (filed Feb. 2,
2001) discloses an electrochemical process for preparing zinc
powder which involves: a) providing to an electrochemical cell a
basic solution of zinc oxide or any other zinc compound that reacts
with an aqueous base to produce zinc oxide, the basic solution
prepared by dissolving the zinc oxide or the other zinc compound in
an aqueous 2.5 to 10.0 M base solution; and b) passing current to
the cell at a current density of about 10,000 to about 40,000
A/m.sup.2 for a time period sufficient to electrochemically reduce
the zinc oxide to zinc powder, wherein the electrochemical process
has a current efficiency of at least 70% and is substantially free
from electrode corrosion.
[0008] U.S. patent application Ser. No. 09/776,644 (filed Feb. 2,
2001) discloses a continuous electrochemical process for preparing
zinc powder which involves: providing to an electrochemical cell a
solution or suspension in an aqueous 1.25 Molar to 10.0 Molar base
solution of zinc oxide or any other zinc compound that reacts with
an aqueous base to produce zinc oxide, the solution or suspension
containing at least 2 millimoles of solubilized zinc based species
per 100 grams of electrolyte; and b) passing current to the cell at
a current density of about 500 to 40,000 A/m.sup.2, for a time
period sufficient to electrochemically reduce the solubilized zinc
based species to zinc powder, while continuously or intermittently
adding a sufficient amount of the zinc oxide or the other zinc
compound to the cell to maintain the concentration of the
solubilized zinc based species at a level of at least 2 millimoles
per 100 grams of electrolyte and continuously or intermittently
removing at least a portion of the zinc powder formed; wherein the
electrolyte includes the aqueous base solution and the zinc oxide
or the other zinc compound.
[0009] U.S. patent application Ser. No. 09/965,157 (filed Sep. 27,
2001), provides a process for preparing a solution of zinc oxide in
an aqueous base, said process comprising diluting a more
concentrated solution of zinc oxide in aqueous sodium or potassium
hydroxide to produce a resulting dilute solution of zinc oxide
having a concentration of zinc oxide that is higher than that
obtained by dissolving solid zinc oxide in aqueous sodium or
potassium hydroxide, wherein the concentration of the aqueous
sodium or potassium hydroxide used for dissolving the solid zinc
oxide is substantially the same as the concentration of the aqueous
sodium or potassium hydroxide in the resulting dilute solution of
zinc oxide, and wherein the concentration of the aqueous sodium
hydroxide in the resulting dilute solution ranges from 5 wt % NaOH
to about 35 wt % NaOH, and the concentration of the aqueous
potassium hydroxide in the resulting dilute solution ranges from 10
wt % KOH to about 55 wt % KOH.
[0010] U.S. patent application Ser. No. 10/015,185 (filed Dec. 7,
2001) provides a low corrosion electrochemical process for
preparing zinc metal which comprises electrochemically reducing an
aqueous basic solution or slurry of zinc oxide or any other zinc
compound that reacts with an aqueous base to produce zinc oxide,
wherein the electrochemical process is carried out in an undivided
electrochemical cell, and wherein air or nitrogen is bubbled in
through the solution or slurry of zinc oxide or said other zinc
compound during said electrochemical process.
SUMMARY OF THE INVENTION
[0011] The present invention provides a process for preparing zinc
dithionite comprising reacting zinc metal with sulfur dioxide,
wherein the zinc metal used in the reaction is produced by
electrochemical reduction in an undivided electrochemical cell of
an aqueous alkaline slurry or solution of zinc oxide or any other
zinc compound that reacts with aqueous base to form zinc oxide.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] Careful washing and drying of the zinc powder produced by
electrolysis of zinc based species is of major importance because
the wet powder, with its large specific surface area, is quite
susceptible to oxidation.
[0013] Even water oxidizes this high surface area zinc. For this
reason, wet zinc powder produced electrochemically loses some
reactivity during storage. For this reason, wet zinc should be used
immediately. This high surface area zinc is also expected to be
oxidized with oxygen and thereby lowers the current efficiency for
zinc formation. It is probably for these reasons, that previous
work (e.g., Orszag and coworkers, (Hungarian Journal of Industrial
Chemistry, Vol. 13, pp. 287-299 (1985) and references cited
therein) aimed at the development of an industrial process for the
electrochemical regeneration of zinc from the zinc oxide byproduct
generated during the sodium dithionite production used a divided
cell to avoid the contact of the zinc produced with the oxygen
generated at the anode. If an undivided cell is used, oxygen
evolution from the anode introduces the possibility that it will
react with the high surface area zinc and thereby will reduce the
current efficiency.
[0014] The following are the highlights of the process used by I.
Orszagh and V. Vass (Hungarian Journal of Industrial Chemistry,
Vol. 13, pp. 287-299 (1985)) to recycle zinc oxide byproduct to
zinc dithionite:
[0015] 1. Zinc oxide byproduct was dried to a zinc content of 60 to
65%.
[0016] 2. Dried zinc oxide byproduct was dissolved in an aqueous
sodium hydroxide solution to give a slurry.
[0017] 3. This slurry was clarified by sedimentation and
filtration. This step was necessary because these impurities are
reported to contaminate the zinc powder and foul the diaphragm
separating the anode from the cathode. This further suggests that
the use of the slurry of zinc oxide byproduct from sodium
dithionite production in an aqueous solution of sodium hydroxide
may give a contaminated zinc powder and may be less reactive than
the zinc dust used in manufacturing sodium dithionite.
Contamination may arise because of the deposition of these
impurities on the zinc produced. For example, zinc produced by
electrolyzing the alkaline slurry of zinc salts in an undivided
cell contains metal ion impurities generated from the anode
corrosion.
[0018] 4. The zinc oxide solution was electrolyzed in a diaphragm
type electrolysis unit (a divided cell). Electrolysis is carried
out at a constant current density of 1000-3000 Amps/M.sup.2 and
this resulted in a cell voltage of 4 or more volts.
[0019] 5. Once the electrolysis was completed, the cathode was
removed along with the diaphragm. The zinc powder was removed by
the metal plate. The wet powder was filtered, washed, dried, milled
and screened to achieve the desired particle size distribution.
[0020] 6. Zinc powder was then used in the synthesis of zinc
dithionite.
[0021] As already mentioned, the present invention does not suffer
from the disadvantages resulting from the method of Orszagh and
Vass.
[0022] The present invention provides a process for preparing zinc
dithionite comprising reacting zinc metal with sulfur dioxide,
wherein the zinc metal used in the reaction is produced by
electrochemical reduction in an undivided electrochemical cell of
an aqueous alkaline slurry or solution of zinc oxide or other zinc
compound (such as zinc sulfate, zinc acetate, and zinc carbonate)
that reacts with aqueous base (alkali) to form zinc oxide.
[0023] The use of an undivided cell in the electrochemical
production of zinc metal in the present invention requires lower
capital. Furthermore, operational and maintenance costs are also
lower when an undivided cell is used. The design of the undivided
cell is simpler and the cell voltage required to achieve the
desired current density is lower because of the lower ohmic
resistance. This means that the electrical cost is generally lower
where an undivided cell is used. Furthermore, capital cost required
with the undivided cell is significantly lower than the divided
cell. Details for the electrochemical production of zinc metal
utilizing solutions or slurries of zinc based species in aqueous
base (alkali) have been previously disclosed in U.S. patent
application Ser. No. 09/776,518, 09/776,644, 09/965,157 and and
10/015,185.
[0024] In one embodiment, the aqueous alkaline slurry or solution
of zinc oxide or any other zinc compound or compounds that react
with the aqueous base to form zinc oxide comprises solubilized zinc
based species, the species comprising at least one member selected
from the group consisting of ZnO.sub.2.sup.2-, HZnO.sub.2.sup.1-,
Zn(OH).sup.+, Zn(OH).sub.2, and Zn.sup.2+. Zinc oxide is known to
dissolve by reacting with water to form a variety of species (which
includes ionic and neutral species) depending upon pH. Thus a
solution of zinc oxide in alkaline solution may contain species
such as ZnO.sub.2.sup.2-, HZnO.sub.2.sup.1-, Zn (OH).sub.2,
Zn(OH).sup.+, and Zn.sup.2+. Therefore, solubilized zinc based
species may comprise one or more of these species in the
solution.
[0025] The aqueous base solutions employed in the process of the
invention are prepared by combining water with a source of alkali
metal or alkaline earth metal ions, such as lithium sodium and
potassium, and a source of hydroxyl (OH.sup.- ions). A single
source may of course provide both types of ions. The various alkali
or alkaline earth metal ions are preferably supplied from various
compounds such as hydroxides and oxides. Preferred base solutions
are sodium and potassium hydroxide solutions.
[0026] The anode of the undivided electrochemical cell of the
present invention may be made from any conventional suitable
material such as platinum, or iridium, either of which may be
coated over an inert support such as niobium or titanium. The anode
may also be made of nickel, or from conventional materials having
good alkali corrosion resistance, e.g., lead or stainless steel.
The cathode may be made from any conventional suitable materials
having good alkali corrosion resistance, such as magnesium,
magnesium alloy, copper, lead, nickel and stainless steel.
Preferably, the anode in the present invention is formed of
stainless steel or nickel and the cathode is formed of stainless
steel, magnesium, magnesium alloy or copper. In one embodiment, the
cathode and the anode are magenesium and stainless steel
respectively, and in one embodiment, magnesium and nickel
respectively, and in one embodiment, copper and nickel
respectively, and in one embodiment, copper and stainless steel
respectively.
[0027] In one embodiment, the electrochemical reduction of the
presently claimed process is conducted at a temperature of from
10.degree. C. to 105.degree. C., more preferably from 20.degree. C.
to 80.degree. C.
[0028] The zinc metal prepared electrochemically is used to prepare
zinc dithionite by reaction of the zinc metal with sulfur dioxide
by conventional processes well known to those of ordinary skill in
the art. In one embodiment, the zinc method used in the present
invention is the form of wet zinc metal particles. By electrolyzing
a slurry of zinc oxide by-product (from the reaction of zinc
dithionite and sodium hydroxide to form sodium dithionite), wet
zinc particles can be prepared (particle size distribution
depending on the cathode and anode used), which can be washed and
used without further processing to prepare zinc dithionite.
[0029] The process of the present invention is substantially free
of labor-intensive steps such as removal of cathode after
completion of the electrochemical reduction. Labor intensive and
time consuming steps such as the removal of cathodes after complete
electrolysis are replaced by mechanical or manual scraping of
cathodes during electrolysis. The process of the present invention
is also substantially free of other labor-intensive steps such as
clarifying a slurry of zinc oxide in an aqueous basic solution
sedimentation and filtration prior to the electrochemical
reduction. Such labor intensive steps are set forth in the Orszag
and Vass reference. Also, as set forth above, the present invention
employs less costly undivided cell for preparing zinc metal and it
is found that such zinc metal is adequately suitable for the
preparation of zinc dithionite. Furthermore, it has been
unexpectedly found that wet zinc can be used without the
labor-intensive steps of drying, grinding and sizing of zinc metal
particles prior to the reaction of zinc metal with sulfur dioxide
to prepare zinc dithionite.
[0030] Current efficiency (C.E.) for the presently claimed process
for the formation of zinc dithionite can be defined as follows:
C.E. for the formation of ZnS.sub.2O.sub.4=[(Moles of
ZnS.sub.2O.sub.4)/(Moles of electrons passed/2)].times.100.
[0031] Current efficiency for the formation of zinc dithionite can
also be obtained by multiplying current efficiency for the zinc
formation with conversion efficiency of zinc to zinc dithionite.
Current efficiency of zinc formation is quite high (such
as>90%). If reactivity of the zinc formed is similar to the zinc
dust currently used in manufacturing zinc dithionite, conversion
efficiency of zinc to zinc dithionite is also quite high. This then
suggests that the current efficiency for the zinc dithionite
formation should also be quite high. In one embodiment, the current
efficiency for the formation of zinc dithionite is at least 70%,
and in one embodiment, at least 75%. Experimental current
efficiencies for the zinc dithionite formation in an undivided cell
varied from 73% to 85% depending on several variables such as the
electrodes used, current density used (such as 500 to 20,000
Amps/m.sup.2, preferably 2500 to 7500 Amps/m.sup.2, and more
preferably 4000 to 6000 Amps/m.sup.2), concentration of sodium
hydroxide used to dissolve zinc oxide byproduct, etc. This also
depends on the amount and nature of impurities in the zinc oxide
byproduct. These results clearly suggest that the presently claimed
process (using an undivided cell) is highly effective and
advantageous for the preparation of zinc dithionite from the zinc
oxide by-product from sodium dithionite production.
[0032] In one embodiment of the present invention, the reaction of
zinc metal with sulfur dioxide to produce zinc dithionite is
conducted in the presence of cadmium sulfate, and in one embodiment
in the substantial absence of cadmium sulfate. While not wishing to
be bound by theory, it is believed that the addition of cadmium
sulfate has a positive effect on the production of zinc dithionite,
in that the yield of zinc dithionite is slightly improved. However,
the reaction can also be carried out in the substantial absence of
cadmium sulfate (e.g., where cadmium sulfate is not added to the
reaction mixture), with good results.
[0033] The following specific examples will provide detailed
illustrations of the methods of producing and utilizing
compositions of the present invention. These examples are not
intended, however, to limit or restrict the scope of the invention
in any way and should not be construed as providing conditions,
parameters or values which must be utilized exclusively in order to
practice the present invention. Unless otherwise specified, all
parts and percents are by weight.
EXAMPLES
Example 1
[0034] General procedure:
[0035] In these experiments, a resin Kettle (5 inch (12.7 cm) in
diameter and 18 inch (45.7 cm) high) was used as the cell. A
solution or slurry of zinc oxide in the aqueous sodium hydroxide
solution (3 to 3.5 liters) at 20 to 80.degree. C. was charged into
the resin kettle. A thermometer, desired cathodes and anodes were
positioned in the cell using laboratory clamps. Mixing was achieved
by using a mechanical stirrer. In some experiments bubbling
nitrogen in addition to mechanical stirring was used. Parts of the
cathode and anode surfaces were covered with Teflon or electrical
tape to achieve the desired active cathode and anode surface areas.
Electrolysis was carried out at a current density of about 5000 to
20,000 Amps/m.sup.2. A portion of the zinc deposited on the cathode
was mechanically removed periodically and collected on the bottom
of the kettle. At the end of the experiment, zinc particles were
separated from the electrolyte by decantation, and washed with
water. Wet zinc (A grams) was charged into a one-liter resin kettle
equipped with a thermometer, nitrogen inlet and outlet adapter, a
gas disparager, mass flow meter to measure the amount of sulfur
dioxide passed, a pH probe, and a mechanical stirrer. An amount (X
ml of 0.18%) of an aqueous solution of cadmium sulfate was then
charged into the resin kettle. The step of adding the cadmium
sulfate can, however, be eliminated. The reaction mixture was then
heated to 430C and sulfur dioxide gas was passed through the
reaction mixture. The sulfur dioxide addition rate was controlled
to maintain pH of the reaction mixture above 3.7 and temperature
was maintained at 43 to 55.degree. C. Initial addition rate of
sulfur dioxide was 3.4 g/minute. Zinc dithionite solution was
decanted to give a translucent solution of the zinc dithionite. It
was then titrated to determine the concentration of zinc
dithionite.
Example 2
[0036] The results of electrolysis of zinc oxide followed by the
reaction with sulfur dioxide under various conditions are shown
below in Table 1.
1TABLE 1 ZnO.sup.1 Mechanical Zn.sub.2S.sub.2O.sub.4 Exp. NaOH
Source Stirring/N.sub.2 A X Soln/ Cathode/ Moles e.sup.-1 Moles
C.E. conc. C.D..sup.5 No. (wt %) (wt %) passed g ml Slurry Anode
passed Zn.sub.2S.sub.2O.sub.4 (%) (g/l) (A/m.sup.2) 1 15 F/7.25
Yes/No 920. 0 Soln.sup.2 Mg/SS 6.02 2.37 79 459 20,000 2.sup.3 25
W/6.0 Yes/yes 926 8 slurry Mg/Ni 6.32 2.62 83 550 5000 3 25 W/6.0
Yes/Yes 926 8 slurry Cu/Ni 6.32 2.4 76 500 5000 4 25 W/6.0 Yes/Yes
926 8 soln Cu/Ni 6.32 2.4 76 503 5000 5.sup.4 -- -- -- 926 8 -- --
-- 2.68 85 605 -- 6.sup.4 -- -- -- 926 8 -- -- -- 2.63 83 553 --
Notes: 1. F: Zinc oxide from Fisher Chemical Inc., was used W: Zinc
oxide by-product from sodium dithionite production was used. 2.
Zinc oxide was dissolved in a 50% aq. sodium hydroxide solution and
then diluted to give a solution containing 15% sodium hydroxide and
7.25% zinc oxide. 3. Zinc was stored under water for 48 hours. 4.
Zinc dust (3.15 moles) available commercially from Umicore was
used. In Exp. Nos. 5 and 6, approximately 83-85% of the zinc dust
is converted into the zinc dithionite. 5. Current density in
Amps/m.sup.2
[0037] Each of the documents referred to above is incorporated
herein by reference in its entirety, for all purposes. Except in
the Examples, or where otherwise explicitly indicated, all
numerical quantities in this description specifying amounts and
concentrations of materials, reaction and process conditions (such
as temperature, current density, current efficiency), and the like
are to be understood to be modified by the word "about".
[0038] It must be noted that as used in this specification and the
appended claims, the singular forms "a," "an," and "the" include
plural reference unless the context clearly dictates otherwise.
Unless defined otherwise all technical and scientific terms used
herein have the same meaning as commonly understood to one of
ordinary skill in the art to which this invention belongs.
[0039] While the invention has been explained in relation to its
preferred embodiments, it is to be understood that various
modifications thereof will become apparent to those skilled in the
art upon reading the specification. Therefore, it is to be
understood that the invention disclosed herein is intended to cover
such modifications as fall within the scope of the appended
claims.
* * * * *