U.S. patent application number 10/015185 was filed with the patent office on 2003-06-12 for electrochemical process for preparation of zinc metal.
This patent application is currently assigned to Clariant International Ltd.. Invention is credited to Bandlish, Baldev K., Martin, Vincent Wise.
Application Number | 20030106806 10/015185 |
Document ID | / |
Family ID | 21769971 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030106806 |
Kind Code |
A1 |
Bandlish, Baldev K. ; et
al. |
June 12, 2003 |
Electrochemical process for preparation of zinc metal
Abstract
Disclosed is 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.
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: |
21769971 |
Appl. No.: |
10/015185 |
Filed: |
December 7, 2001 |
Current U.S.
Class: |
205/602 ;
205/603 |
Current CPC
Class: |
C25C 1/16 20130101; C25C
5/02 20130101 |
Class at
Publication: |
205/602 ;
205/603 |
International
Class: |
C25B 001/00 |
Claims
1. 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.
2. The electrochemical process of claim 1 that is a continuous
process.
3. The electrochemical process of claim 1 that utilizes an
electrochemical cell having a magnesium or copper cathode.
4. The electrochemical process of claim 1 that utilizes an
electrochemical cell having a stainless steel or nickel anode.
5. The electrochemical process of claim 1 that utilizes an
electrochemical cell having a stainless steel anode and a copper
cathode.
6. The electrochemical process of claim 1 that utilizes an
electrochemical cell having a nickel anode and a copper
cathode.
7. The electrochemical process of claim 1 that is conducted at a
temperature of from about 10.degree. C. to about 105.degree. C.
8. The electrochemical process of claim 7, wherein the temperature
ranges from about 40.degree. to about 80.degree. C.
9. The electrochemical process of claim 7, wherein the temperature
ranges from about 60.degree. to about 75.degree. C.
10. The electrochemical process of claim 1, wherein the aqueous
base comprises ions of at least one alkali or alkaline earth metal
and hydroxyl (OH.sup.-) ions.
11. The electrochemical process of claim 10, 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.
12. The electrochemical process of claim 11, wherein the compound
is selected from the group consisting of sodium hydroxide and
potassium hydroxide.
Description
FIELD OF THE INVENTION
[0001] The present invention provides an electrochemical process
for the preparation of zinc metal.
BACKGROUND OF THE INVENTION
[0002] Zinc powder is widely used in the chemical industry in
various industries. Zinc oxide containing other zinc salts, metal
impurities, etc. is produced as a byproduct. Recycling of the zinc
oxide to produce pure zinc powder is highly desirable from a cost
as well as an environmental point of view.
[0003] The electrodeposition of zinc metal is a well-known reaction
in electrochemical technology (See, for example, D. Pletcher and F.
C. Walsh, Industrial Electrochemistry, Blackie Academic, 1993). The
electrogalvanizing of steel is a process carried out on a very
large scale and aqueous acid is the normal medium. High speed, reel
to reel galvanizing of steel is carried out in sulfuric acid with
dimensionally stable anodes and uniform deposition is achieved at
high current density by inducing very efficient mass transport by
rapid movement of the steel surface. The deposition of zinc metal
is also the critical electrode reaction in the electrowinning and
electrorefining of zinc. In addition, there are a number of
technologies, which have been demonstrated for the removal of
Zn(II) from effluents. However, in these technologies,
concentration of Zn(II) is low, commonly less than 100 ppm.
Finally, the deposition of zinc has been widely investigated as the
cathodic reaction in candidate secondary batteries. In all these
applications, however, the objective is to select the conditions so
as to give an adhesive and smooth zinc coating.
[0004] Zinc metal can be produced by electrolysis either in strong
alkaline or neutral zinc containing solutions. The first patents
obtained on the alkaline electrolysis process date back to the
early thirties (German Patents, 581013, 506590, 653557). In these
methods, a low current density of 1200-1500 amperes/sq. meter
(A/m.sup.2) was used. Volume efficiency and current density of
these batch type processes are too low to be industrially
attractive. I. Orszagh and B. Vass (Hung. J. Ind. Chem., 13, (1985)
287) used these methods to recycle zinc oxide byproduct from zinc
dithionite production.
[0005] There is at present a need to carry out an electrolysis
reaction to produce zinc metal under conditions where corrosion of
the electrodes is minimized. The present invention fulfils this and
other needs. The present invention provides a low corrosion
electrochemical process for preparing zinc metal wherein air or
nitrogen is bubbled in during the electrochemical reduction process
producing the zinc metal. It has been unexpectedly found that
bubbling of air or nitrogen reduces electrode corrosion during the
electrochemical process.
[0006] U.S. Pat. No. 5,958,210 discloses a method for
electrowinning metallic zinc from zinc ion in aqueous solution,
said method comprising performing electrolysis on a mixture of
solid conductive particles and aqueous alkali solution, said
solution ranging in concentration from about 3N to about 20N alkali
and containing dissolved zinc ion at an initial concentration
ranging from about 50 to about 500 grams of zinc ion per liter of
said solution, in an electrolytic cell containing first and second
vertically arranged, parallel flat plates defined as a current
feeder and a counter electrode, respectively, said counter
electrode coated with a substance that is catalytic for oxygen
evolution, said cell further containing an ion-permeable diaphragm
parallel to each of said plates and interposed therebetween to
define a gap between said current feeder and said diaphragm, by
passing said mixture of particles and solution through said gap
such that said particles contact said current feeder and passing a
current across said gap, thereby depositing metallic zinc from said
solution onto said particles.
[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.
SUMMARY OF THE INVENTION
[0009] The present invention 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.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] The present invention 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 (such as zinc sulfate) 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.
[0011] The anode 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 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 stainless steel and copper respectively, and in one
embodiment nickel and copper respectively.
[0012] 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.
[0013] For electrolysis, temperatures higher than ambient are
generally desired because of the beneficial effects on the kinetics
of all steps in an electrode process. At higher temperatures, the
diffusion coefficient, the exchange current density and the rates
of chemical reactions generally are increased. The decrease in
viscosity and increase in diffusion coefficient leads to the
increased mass transport rates. This increased mass transport of
zinc ions from the bulk of the solution to the cathodic region is
highly desirable. However, increase in the rate of chemical
reaction such as the oxidation of zinc produced with oxygen and
mass transport of the byproduct oxygen to the bulk of the solution
may not be desirable. In the present invention, higher than ambient
temperatures are found to be favorable for the electrolytic
reduction of zinc oxide to zinc, and are preferred.
[0014] In one embodiment, the presently claimed electrochemical
reduction process is conducted at a temperature of from 110.degree.
C. to 105.degree. C., preferably from 40.degree. to 80.degree. C.,
and more preferably from 60.degree. to 75.degree. C.
[0015] In one embodiment, the electrochemical process of the
present invention is a continuous process. The meaning of
"continuous" in the present context is well understood by one of
ordinary skill in the art. As used herein, it relates a process
wherein zinc oxide or the other zinc compound can be added
continuously to the electrochemical cell and at least a portion of
the zinc metal formed is removed continuously or intermittently
during the electrochemical process. A continuous electrochemical
process has been disclosed in U.S. application Ser. No. 09/776,664
(filed Feb. 2, 2001).
[0016] 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
[0017] General Procedure Used for Electrolytic Reduction of Zinc
Oxide to Zinc Metal:
[0018] In these experiments, a resin Kettle (5 inch in diameter and
18 inch high) is 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. is charged into the resin kettle. A
thermometer, desired cathodes and anodes are positioned in the cell
using laboratory clamps. Mixing is achieved by bubbling air or
nitrogen through the solution or slurry of ZnO. In some experiments
mechanical stirring in addition to bubbling of air or nitrogen is
used. Control experiments use only mechanical stirring for mixing
(no air or nitrogen bubbling). Parts of the cathode and anode
surfaces are covered with Teflon tape to achieve the desired active
cathode and anode surface areas. Electrolysis is carried out at a
current density of about 5000 Amps/m.sup.2. A portion of the zinc
deposited on the cathode is removed periodically. At the end of the
experiment, zinc particles are separated from the electrolyte by
decantation, washed with water and then dried. Dried zinc particles
were analyzed to determine the zinc content.
Example 2
[0019] The results of electrolysis of zinc oxide under various
conditions are shown below in Table 1.
1 TABLE 1 Zinc Current Anode Cathode Current NaOH Soln/ Cathode/
Moles formed Efficiency Corrosion Corrosion Density (Wt%) Slurry
Anode4 Mixing electrons (moles) (%) g/45.5 kg Zn g/45.5 kg Zn
(Amps/m.sup.2) 1 25 Soln Mg/SS Mech 3.66 1.63 89 6.8 0.3 5000 2 25
Soln Mg/SS N.sub.2 5.39 2.43 90 1.0 0.3 5000 3 26 Soln Mg/SS
N.sub.2 4.56 2.19 96 0.0 0.0 5050 4 26 Soln Mg/SS Air 4.56 2.23 98
3.2 0.0 5050 5 25 Soln Cu/SS N.sub.2 4.56 2.10 92 3.8 0.0 5000 6 24
Slurry Cu/SS N.sub.2 7.26 3.58 99 4.8 1.0 5000 7 25 Slurry Cu/SS
N.sub.2/mech 5.61 2.62 94 4.7 0.2 5053 8 29 Slurry Cu/Ni mech 7.26
3.59 99 5.8 2.7 5000 9 29 Slurry Cu/Ni N.sub.2/mech 7.26 3.50 96
0.0 0.2 5000 10 26 Slurry Cu/Ni mech 7.26 3.49 96 1.4 0.1 5000 11
25 Slurry Cu/Ni N.sub.2/mech 5.82 2.74 94 0.3 0.1 5053 Notes: SS =
stainless steel "Mech" refers to mechanical stirring only;
"N.sub.2/mech" refers to refers to use of mechanical stirring in
addition to nitrogen bubbling. All solution based electrolysis used
saturated solutions of zinc oxide in NaOH solution. All slurry
based electrolysis contained undissolved ZnO in the electrolyte
during the entire electrolysis. Zn particles deposited on the
copper cathode were easily removable. The precision of the
corrosion results is about .+-.1 g/45.5 kg Zn (the values expressed
in g/45.5 kg Zn are normalized values).
[0020] The data in Table 1 clearly indicate that electrode
corrosion is diminished when nitrogen or air is bubbled through the
reactor during the electrochemical process (compared to the control
process when only mechanical stirring is used). Furthermore, the
use of mechanical stirring in conjunction with nitrogen or air
bubbling does not adversely affect electrode corrosion
[0021] 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), and the like are to be understood to be modified
by the word "about".
[0022] 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.
* * * * *