U.S. patent application number 10/366107 was filed with the patent office on 2004-08-19 for process for improving the reactivity of zinc particles in producing sodium dithionite from zinc dithionite.
This patent application is currently assigned to Clariant International Ltd.. Invention is credited to Bandlish, Baldev K..
Application Number | 20040159556 10/366107 |
Document ID | / |
Family ID | 32849701 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040159556 |
Kind Code |
A1 |
Bandlish, Baldev K. |
August 19, 2004 |
Process for improving the reactivity of zinc particles in producing
sodium dithionite from zinc dithionite
Abstract
The present invention relates to a process for preparing a zinc
dithionite solution wherein the process comprises reacting an
aqueous slurry of zinc particles with sulfur dioxide in presence of
a promoter compound. The zinc particles can either be a commercial
particulate zinc material or a particulate zinc material prepared
by electrolyzing an aqueous alkaline suspension or slurry
containing zinc based species. The addition of the promoter
compound significantly increases the conversion of zinc to zinc
dithionite and significantly reduces the cycle time of the
reaction. The promoter compound is selected from the group
consisting of sodium hydroxide and metal compounds. The promoter
metal compounds are selected from the group consisting of an alkali
metal hydroxide, zinc oxide, zinc sulfite, zinc bisulfite, zinc
metabisulfite, an alkali metal zincate, an alkali metal sulfite, an
alkali metal bisulfite, and mixtures thereof.
Inventors: |
Bandlish, Baldev K.;
(Charlotte, NC) |
Correspondence
Address: |
CLARIANT CORPORATION
Industrial Property Department
4000 Monroe Road
Charlotte
NC
28205
US
|
Assignee: |
Clariant International Ltd.
|
Family ID: |
32849701 |
Appl. No.: |
10/366107 |
Filed: |
February 13, 2003 |
Current U.S.
Class: |
205/495 ;
205/477; 205/494 |
Current CPC
Class: |
C01G 9/00 20130101; C25C
5/02 20130101 |
Class at
Publication: |
205/495 ;
205/494; 205/477 |
International
Class: |
C25B 003/00 |
Claims
I claim:
1. A process for preparing a zinc dithionite solution which
comprises reacting an aqueous slurry of zinc metal with sulfur
dioxide in presence of an effective amount of a promoter
compound.
2. The process of claim 1 wherein the reacting step is maintained
at a pH above about 3.3 and a reaction temperature below about
54.degree. C.
3. The process of claim 1 wherein the promoter compound comprises
an alkali metal hydroxide and the reacting step is maintained at a
pH above about 4.0.
4. The process of claim 1 wherein the sulfur dioxide is introduced
to the aqueous slurry of zinc metal at a constant rate and the
promoter compound is admixed with the aqueous slurry of zinc metal
either before or after the sulfur dioxide is introduced.
5. The process of claim 3 wherein the constant rate of introducing
sulfur dioxide is adjusted in a series of constant rate increments
to maintain pH above about 3.2.
6. The process of claim 1 wherein the zinc metal comprises
particulate zinc material and/or zinc powder.
7. The process of claim 1 wherein the promoter compound comprises
at least one compound selected from the group consisting of an
alkali metal hydroxide, zinc oxide, zinc hydroxide, an alkali metal
zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite, an
alkali metal sulfite, an alkali metal bisulfite, and mixtures
thereof.
8. The process of claim 1 further comprising electrolyzing an
alkaline suspension or slurry of a zinc mud comprising zinc
hydroxide in an aqueous alkaline suspension to provide the aqueous
slurry of zinc metal.
9. The process of claim 8, wherein the electrolyzing step takes
place in an electrochemical cell having cathodes based on metals
selected from the group consisting of copper, magnesium, stainless
steel, and nickel.
10. The process of claim 9 wherein the electrochemical cell
comprises an undivided electrochemical cell having a copper cathode
and a nickel anode.
11. The process of claim 9 wherein the electrochemical cell
comprises an undivided electrochemical cell having a copper cathode
and a stainless steel anode.
12. The process of claim 8, wherein the electrolyzing step is
carried out at a temperature of about 20 to about 65.degree. C.
13. The process of claim 1, wherein the effective amount of the
promoter compound comprises 2% to 15% by weight of the zinc
metal.
14. The process of claim 1, wherein the effective amount of the
promoter compound comprises 3% to 9% by weight of the zinc
metal.
15. The process of claim 1, wherein the effective amount of the
promoter compound comprises 4% to 6% by weight of the zinc
metal.
16. A process for the production zinc dithionite solution, said
process comprising: a) introducing a sulfur dioxide stream to a
reaction zone containing an aqueous slurry of zinc metal in
presence of a promoter compound comprising a compound selected from
the group selected from the group consisting of an alkali metal
hydroxide, zinc oxide, zinc hydroxide, an alkali metal zincate,
zinc sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal
sulfite, an alkali metal bisulfite, and mixtures thereof; b)
reacting the zinc metal with the sulfur dioxide to produce a zinc
dithionite effluent comprising a zinc dithionite solution and
insolubles; and c) recovering the zinc dithionite solution.
17. The process of claim 16 wherein the promoter compound is
introduced to the reaction zone prior to or simultaneously with the
passing sulfur dioxide stream.
18. The process of claim 16 further comprising recovering and
returning at least a portion of the insolubles to the reaction
zone.
19. The process of claim 16 further comprising passing at least a
portion of the zinc dithionite reactor effluent at effective sodium
dithionite reaction conditions to a sodium dithionite reaction zone
and therein contacting a dilute caustic solution to provide a
sodium dithionite product solution.
20. The process of claim 16 further comprising removing at least a
portion of the insolubles from the zinc dithionite effluent and
returning at least a portion of the insolubles to an
electrochemical cell for producing the aqueous slurry of zinc
metal.
21. A process for preparing a sodium dithionite product solution
comprising: a) admixing a zinc mud stream comprising zinc oxide
with an electrolyte stream comprising water and caustic in an
undivided electrochemical reaction zone and electrolyzing said
reaction zone to provide a zinc metal suspension; b) recovering a
zinc metal stream from the zinc metal suspension; c) washing the
zinc metal stream to provide an aqueous zinc metal suspension; d)
admixing a promoter compound with water in a zinc dithionite
reaction zone to provide an aqueous zinc compound suspension; e)
admixing the aqueous zinc metal suspension with the aqueous zinc
compound suspension to provide a mixed zinc suspension; f)
contacting the mixed zinc suspension with a sulfur dioxide stream
in a zinc dithionite reaction zone and reacting the mixed zinc
suspension with the sulfur dioxide to provide a zinc dithionite
solution and insolubles; g) contacting the zinc dithionite solution
with a dilute caustic solution in a sodium dithionite reaction zone
to provide a sodium dithionite effluent stream comprising sodium
dithionite solution and a solid zinc hydroxide material; h)
separating the solid zinc hydroxide material from the sodium
dithionite effluent stream to provide a recovered zinc oxide stream
and the sodium dithionite product solution; i) washing the
recovered zinc oxide stream to provide a washed zinc oxide stream;
j) combining at least a portion of the washed zinc oxide stream
with the zinc oxide feed stream to provide the zinc mud stream; k)
combining at least a portion of the washed zinc oxide stream with
water in step (d), and l) recovering the sodium dithionite product
solution.
22. The process of claim 21 wherein the promoter compound is
selected from the group consisting of an alkali metal hydroxide,
zinc mud, zinc hydroxide, an alkali metal zincate, zinc oxide, zinc
sulfite, zinc bisulfite, zinc metabisulfite, an alkali metal
sulfite, an alkali metal bisulfite, and mixtures thereof.
23. The process of claim 21, further comprising recovering at least
a portion of the insolubles and returning at least a portion of
said insolubles to the undivided electrochemical reaction zone to
improve the electrolyzing step therein.
24. The process of claim 21, further comprising recovering at least
a portion of the insolubles and returning said insolubles to the
zinc dithionite reaction zone.
25. The process of claim 21, further comprising at least partially
converting the mixed zinc suspension with the sulfur dioxide in
step (f), recovering at least a portion of the zinc dithionite
solution and returning unreacted zinc suspension and insolubles to
the zinc dithionite reaction zone.
26. The process of claim 7, wherein the alkali metal comprises
sodium.
27. The process of claim 16, wherein the alkali metal comprises
sodium.
28. The process of claim 22, wherein the alkali metal comprises
sodium.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a chemical process for
preparing sodium dithionite. More particularly, the present
invention relates to a process for making zinc dithionite as an
intermediate step in the production of sodium dithionite.
BACKGROUND OF THE INVENTION
[0002] Dithionites can be manufactured by the reduction of
sulfites, bisulfites, or sulfur dioxide with reductants, e.g.,
sodium or zinc. These manufacturing processes have been reviewed by
A. Janson and coworkers in Ullmanns Encyklopadie Technischen
Chemie, 3rd edition Volume 15, published by Urban &
Schwartzenberg, of Munchen-Berlin, 1964, in pages 480-488. A
typical older process involved the reduction of liquid sulfur
dioxide with an aqueous slurry of particulate zinc material is
disclosed in British Patent 1,145,824. These references are hereby
incorporated by reference. In the chemical and metallurgical
industries, particulate zinc material is used as a reducing agent,
in the manufacture of hydrosulfite (dithionite) compounds for use
in the textile and paper industries, and to enhance the physical
properties of plastics and lubricants. Zinc dithionite is an
intermediate in the manufacturing of sodium dithionite.
[0003] Zinc metal is generally produced by pyrometallurgical or
electrochemical process. Particulate zinc material and powder are
particulate forms of zinc.
[0004] The terms dust and powder have been used indiscriminately to
designate particulate zinc materials. The term particulate zinc
material designates material produced by condensation of zinc
vapor, whereas zinc powder indicates the product obtained by
atomizing molten zinc. Particulate zinc materials 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).
[0005] In the manufacture of dithionite from zinc, the zinc
reactivity of a particulate zinc material is a critical parameter
in determining the ultimate commercial conversion of the zinc
material into the dithionite form. The zinc reactivity of
particulate zinc material is largely a function of the origin of
the zinc as well as the processing the zinc undergoes. Particulate
zinc material manufacturers use different atomization processes
such as air atomization, nitrogen atomization, etc., and various
zinc ores may have different compositions. Thus, particulate zinc
materials from different suppliers typically contain different
amounts of metallic zinc, zinc oxide, and small concentrations of
other metallic and nonmetallic impurities. For example, the
concentration of zinc oxide found in zinc material from commercial
suppliers typically is less than about 1 weight percent. Other
metallic impurities which could include compounds of lead, iron,
cadmium, indium, bismuth and copper are typically present in
amounts less than about 0.1 weight percent. Furthermore, particle
size and particle size distribution of particulate zinc materials
from different suppliers may be different. Because of these
differences, particulate zinc materials from different suppliers
can show significantly different zinc reactivity with sulfur
dioxide.
[0006] Efforts have been made to prepare zinc powder by
electrolyzing a solution of zinc based compounds in strong alkaline
solutions. I. 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, and is in the form of zinc
hydroxide. In Orszagh et al., the zinc mud was dissolved in
caustic, the resulting zinc solution is clarified and electrolyzed,
and the recovered zinc powder is used to reduce sulfur dioxide.
According to Orszag et al., the process requires a number of labor
intensive operations such as lifting of the electrodes, removal of
the zinc powder by sedimentation and filtration, drying, grinding
and sizing, etc. These labor intensive steps combine to make the
process of Orszagh et al. economically unattractive. For example,
zinc particles so produced were washed with water (20 to 60 ml
water per gram of the zinc), dried, ground, and sieved to provide
zinc particles with the desired particle size distribution.
Furthermore, the Orszagh et al. process requires a divided
electrolytical cell, which is operating and capital cost intensive.
Such electrolytic cells require high cell voltage at a low current
density which contributes to high capital and operating expenses.
In summary, this process is economically unattractive as an
alternative to the currently used particulate zinc material based
technology. Because of these disadvantages, there is a strong need
for an improved cost effective process to recycle zinc oxide
byproduct in the production of zinc dithionite.
[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. This process provides a way to
prepare an alkaline solution containing a greater concentration of
zinc based species and hence can be electrolyzed to prepare
particulate zinc material.
[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 other zinc compound
during the electrochemical process.
[0011] U.S. patent application Ser. No. 10/109,443 (filed Mar. 28,
2002) 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. Current efficiency for the
formation of zinc dithionite can 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 particulate zinc material currently used in
manufacturing zinc dithionite, conversion efficiency of zinc to
zinc dithionite is also quite high. This suggests that the current
efficiency for the zinc dithionite formation should also be quite
high. Current efficiencies for the zinc dithionite formation in an
undivided cell varied from 73% to 85% depending on several
operating variables including the nature of the electrodes, the
current density, and the concentration of sodium hydroxide.
[0012] It is highly desirable to develop a process where the use of
less reactive zinc particles will lead to a lower reaction time
and/or a higher yield of zinc dithionite. Less reactive zinc
particles can either be a commercial particulate zinc material or a
particulate zinc material prepared by electrolyzing an aqueous
alkaline solution containing zinc based species at magnesium,
stainless steel, copper, etc. based cathodes. The present invention
affords such a process.
SUMMARY OF THE INVENTION
[0013] According to the present invention, the zinc dithionite
solution can be prepared from either zinc particles produced as a
commercial particulate zinc material or a particulate zinc material
prepared by electrolyzing an aqueous alkaline suspension or slurry
containing zinc based species at conversions which are unexpectedly
higher than earlier processes and at overall cycle times which are
surprisingly shorter than previously known or possible before the
present invention, significantly reducing the cost and the
complexity of the overall process for the production of sodium
dithionite. We have unexpectedly learned that the addition of a
promoter compound which comprises a metal hydroxide, sulfite,
bisulfite, metabisulfite--in particular, a promoter compound
comprises at least one compound selected from the group consisting
of an alkali metal hydroxide, zinc oxide, zinc hydroxide, alkali
metal zincate, zinc sulfite, zinc bisulfite, zinc metabisulfite,
alkali metal sulfite, alkali metal bisulfite and mixtures
thereof.--to the reaction mixture before or after starting the
addition of sulfur dioxide to the zinc dithionite reaction zone
increases reactivity of the zinc particles. The term alkali metal
includes sodium, potassium, lithium, and mixtures thereof. The
preferred alkali metal is sodium. Because of the increased
reactivity of zinc particles, sulfur dioxide can be added at a
constant high rate for a longer period of time and results in a
significantly increased yield of zinc dithionite, hence sodium
dithionite, and improves the conversion of zinc particles.
[0014] In one embodiment, the present invention is a chemical
process for preparing a zinc dithionite solution which comprises
reacting an aqueous slurry of zinc metal with sulfur dioxide in
presence of an effective amount of a promoter compound selected
from the group consisting of sodium hydroxide, zinc oxide, zinc
sulfite, zinc bisulfite, zinc metabisufite and mixtures
thereof.
[0015] In another embodiment, the present invention is a process
for preparing a sodium dithionite product solution comprising:
[0016] a) admixing a zinc oxide feed stream with a water stream
comprising caustic to provide a zinc oxide slurry and passing the
zinc oxide slurry to an undivided electrochemical reaction zone and
therein electrolyzing the zinc oxide slurry to provide a zinc metal
suspension;
[0017] b) recovering a zinc metal stream from the zinc metal
suspension;
[0018] c) washing the zinc metal stream to provide an aqueous zinc
metal suspension;
[0019] d) contacting the aqueous zinc metal suspension with a
sulfur dioxide stream in the presence of an effective amount of a
promoter compound in a zinc dithionite reaction zone to provide a
zinc dithionite solution;
[0020] e) contacting the zinc dithionite solution with a caustic
solution in a sodium dithionite reaction zone to provide a sodium
dithionite effluent stream comprising sodium dithionite, water and
a solid zinc oxide material;
[0021] f) separating the solid zinc oxide material from the sodium
dithionite effluent to provide a recovered zinc oxide stream and
the sodium dithionite product stream;
[0022] g) washing the recovered zinc oxide stream to provide a
washed zinc oxide stream;
[0023] h) combining at least a portion of the washed zinc oxide
stream with the zinc oxide feed stream prior to step(a), and
optionally,
[0024] i) admixing a portion of the washed zinc oxide stream with
the aqueous zinc metal suspension prior to step(d).
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a simplified flow diagram of the process of the
present invention.
[0026] FIG. 2 is a simplified process flow diagram of the present
invention illustrating a zinc metal recovery scheme.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Reactivity of the zinc particles is of major commercial
importance because the reactivity determines the rate at which zinc
particles will react with sulfur dioxide. When reacted with sulfur
dioxide, less reactive zinc particles will lead to either an
increased reaction time and/or a lower conversion and hence a lower
yield of zinc dithionite. Increased zinc dithionite reaction time,
leads to an increase in cycle time for producing zinc dithionite,
and thereby results in significantly increased production costs. A
lower conversion will lead to the production of zinc oxide
containing a greater amount of metallic zinc particles. This is
undesirable from byproduct quality as well as safety considerations
if zinc oxide is not used as a raw material for preparing zinc
particles. For example, if commercial zinc dust is used as a raw
material and byproduct zinc oxide is sold for other uses, presence
of metallic zinc particles is highly undesirable. This is
especially true if concentration of metallic zinc particles is
relatively high. In contrast, if particulate zinc material is
prepared by electrolyzing a solution of zinc oxide in an aqueous
alkali solution, presence of metallic zinc particles in zinc oxide
byproduct is not a serious problem. In fact, presence of zinc
particles in zinc oxide will lead to an apparent increase in the
current efficiency for the zinc dithionite formation. The lower
yield of zinc dithionite leads to a higher production cost and
hence is highly undesirable. Thus, it is highly desirable to
develop a process in which the reactivity of less reactive zinc
particles is increased either to achieve a higher yield of zinc
dithionite or a lower reaction time and thereby provide a lower
cycle time for the zinc dithionite formation reaction.
[0028] Overall rate of reaction of zinc particles with sulfur
dioxide depends on several factors such as:
[0029] 1. The reactivity of metallic zinc particles.
[0030] 2. The rate at which reaction mixture is stirred and
mixed.
[0031] 3. Ratio of the amount of water to the amount of zinc
particles used.
[0032] 4. The rate at which sulfur dioxide is added to the reaction
mixture. In order to maximize the production rate, sulfur dioxide
addition rate which is generally maintained at a constant rate is
maximized. During the addition of sulfur dioxide, pH of the
reaction mixture is maintained above 3.3 and temperature is
maintained below 54.degree. C. to minimize decomposition of the
zinc dithionite solution. To increase conversion efficiency of zinc
particles, sulfur dioxide addition rate is lowered several times in
a series of constant rate increments or steps towards the end of
the reaction to maintain control of the pH of the reaction mixture
above about 3.3. This adjustment of the sulfur dioxide addition
rate during the reaction makes the process more labor intensive and
increases reaction time significantly. It is highly desirable to
achieve a higher yield of zinc dithionite and a higher conversion
of zinc particles when reactions are carried out with a constant
high addition rate of sulfur dioxide. In order to maximize sulfur
dioxide addition rate, reaction mixture is adequately stirred and
mixed. Sensitivity of the reaction rate to the rate of stirring and
mixing increases as the ratio of the amount of water to the amount
of particulate zinc material used is decreased.
[0033] Zinc particles used in the present invention can either be a
commercial particulate zinc material or can be prepared by
electrolyzing a suspension or a solution of zinc oxide in an
aqueous alkaline solution. Electrolysis can be carried out
according to the processes described U.S. patent application Ser.
No. 09/776,518 (filed Feb. 2, 2001), Ser. No. 09/776,644 (filed
Feb. 2, 2001), Ser. No. 09/965,157 (filed Sep. 27, 2001), and Ser.
No. 10/015,185 (filed Dec. 7, 2001), which are hereby incorporated
by reference. According to the process of the present invention
while converting zinc metal produced electrochemically, it was
surprisingly discovered that adding to the electrolytic cell at
least a portion of insolubles recovered from the zinc dithionite
solution, which are believed to comprise zinc sulfide, resulted in
a significant improvement in the current efficiency or yield of
both the zinc dithionite and the sodium dithionite. As shown
hereinbelow by way of example, the operation of the electrolytic
cell without the addition of the insolubles resulted in a zinc
dithionite yield of about 69 mol-%, and with the addition of the
insolubles to the electrolytic cell, the yield of zinc dithionite
rose to about 89 mol-%. When electrochemically produced zinc is the
only source of zinc included in the zinc dithionite reaction
mixture, the current efficiency is equal to the yield of zinc
dithionite.
[0034] In processing commercial zinc dusts it was unexpectedly
discovered that adding up to about 6 weight percent (weight percent
of the amount of zinc particles used) of sodium hydroxide to the
zinc dithionite reaction zone resulted in significantly increased
yield and significantly reduced cycle time. As used herein the term
cycle time refers to the time required to convert a batch of zinc
dust into zinc dithionite by reaction with sulfur dioxide in the
zinc dithionite reaction zone. A reduction in cycle time provides
the benefit of increased capacity or improved conversion. It was
unexpectedly discovered that the kinetics of the reaction was
altered by the addition of sodium hydroxide. In prior operations of
the zinc dithionite reaction zone, it had been required to lower
the sulfur dioxide addition rates as the reaction approached the
terminal pH to avoid loss in yield of zinc dithionite. The addition
of sodium hydroxide permitted the reaction to approach completion
at reduced cycle time, without decreasing the sulfur dioxide
addition rate and without significantly decreasing the yield of
sodium dithionite produced downstream in the sodium dithionite
reaction. Furthermore, the quality of the zinc oxide byproduct
produced in this manner was improved. Generally, one skilled in the
art would expect that sodium dithionite is less stable than zinc
dithionite and would expect that the addition of sodium hydroxide
to the zinc dithionite reaction zone in the presence of sulfur
dioxide would produce some sodium dithionite. The sodium dithionite
would be expected to decompose early in the reaction to produce
sodium thiosulfate. However, it was observed that the formation of
sodium thiosulfate was significantly reduced compared to production
of thiosulfate in conventional zinc dithionite reactions without
the addition of sodium hydroxide.
[0035] The addition of a promoter compound such as alkili metal
hydroxide, metal oxides (particularly, zinc oxide), sulfites,
bisulfites and metabisulfites to the zinc dithionite reaction zone
was found to significantly reduce the conventionally observed
initial sharp drop in pH which occurred as sulfur dioxide was
introduced to the zinc dithionite reaction zone. A reduction in pH
during the zinc dithionite reaction leads to the decomposition of
the desired species and results in the increased production of
impurities such as sulfur and sulfides, etc. Thus, by the addition
of a promoter to the zinc dithionite reaction zone, the rate of the
reduction of the pH is reduced and results in the following
benefits: the yield of zinc dithionite is increased, the cycle time
is reduced, the amount of impurities is reduced, and the quality of
the zinc oxide byproduct is improved. Finally, the production costs
for producing zinc dithionite are reduced by eliminating the
step-wise reduction in the rates of sulfur dioxide addition. This
unexpected result also permitted the zinc dithionite reaction to be
carried out in a series of partial conversion cycles, wherein the
zinc dithionite reaction is partially converted, the reaction is
terminated, and the zinc dithionite reaction zone effluent is
filtered to separate a zinc dithionite solution, from unreacted
zinc suspension and any insolubles. In a succeeding cycle, the
unreacted zinc suspension and any insolubles are returned to the
zinc dithionite reaction zone, and the reaction resumed.
DETAILED DESCRIPTION OF THE DRAWINGS
[0036] The process of the present invention is hereinafter
described with reference to FIGS. 1 and 2 which illustrate various
aspects of the process. It is to be understood that no limitation
to the scope of the claims which follow is intended by the
following description. Those skilled in the art will recognize that
these process flow diagrams have been simplified by the elimination
of many necessary pieces of process equipment including some heat
exchangers, process control systems, pumps, filtration systems,
etc. It may also be discerned that the process flow depicted in the
figures may be modified in many aspects without departing from the
basic overall concept of the invention.
[0037] FIG. 1 illustrates one embodiment of the present invention
which comprises the conversion of commercial zinc metal dust to
sodium dithionite employing the improved process for the
intermediate production of zinc dithionite from the zinc metal
dust. Referring to FIG. 1, a commercial metal zinc dust is
suspended in an aqueous suspension in line 10 and charged to a zinc
dithionite reaction zone 101. Typically, the commercial zinc dust
comprises fine particles of metallic zinc of a particle size less
than about 75 microns. Although the distribution of zinc particle
size will vary and may contain a small amount of particles greater
than 75 microns, preferably, less than 2 percent by weight of the
zinc dust particles have a particle size greater than 75 microns
and less than 250 microns, and more preferably, less than 1 percent
by weight of the zinc dust particles have a particle size greater
than 75 microns and less than 250 microns. Generally, the
commercial zinc dust will contain some impurities including
metallic oxides such as zinc oxide and other metallic compounds of
metals selected from the group consisting of lead, iron, cadmium,
indium, bismuth, copper, zinc, and mixtures thereof. Generally, the
zinc oxide content of commercial zinc dust will be less than about
1 weight percent.
[0038] The zinc dithionite reaction zone 101 is a stirred reactor.
A zinc metal stream in line 10 and an aqueous stream in line 15 are
charged to the zinc dithionite reaction zone 101 to form an aqueous
zinc suspension and a sulfur dioxide stream in line 80 is bubbled
through the zinc dithionite reaction zone 101 to chemically convert
the suspended zinc metal dust to zinc dithionite. According to the
invention, a small amount of promoter compound in line 90, such as
a zinc compound selected from the group consisting of zinc oxide,
zinc sulfite, zinc bisulfite, and mixtures thereof, is charged to
the zinc dithionite reaction zone 101 either before or at the same
time the sulfur dioxide is introduced to the zinc dithionite
reaction zone 101. Preferably, the effective amount of promoter
compound charged to the zinc dithionite reaction zone ranges from
about 2 to about 15 percent by weight of zinc metal; more
preferably, the effective amount of promoter compound charged to
the zinc dithionite reaction zone ranges from about 3 to about 9
percent by weight of zinc metal, and most preferably, the effective
amount of promoter compound charged to the zinc dithionite reaction
zone ranges from about 4 to about 6 percent by weight of zinc
metal. The sulfur dioxide chemical reaction of the zinc metal with
the sulfur dioxide is exothermic and is operated at a sulfur
dioxide reaction temperature of from about 20.degree. C. and
maintained below about 54.degree. C. As the sulfur dioxide reaction
proceeds, and as the sulfur dioxide is continuously introduced to
the zinc dithionite reaction zone 101 at a fixed rate, the pH of
the reaction mixture will gradually drop from an initial pH of
about 6-8 to about a 3.3 pH. The zinc dithionite reaction is
terminated when the pH reaches the terminal pH value of about 3.3
or the sulfur dioxide addition rate is reduced to maintain the pH
at about 3.3. According to the invention, the addition of promoter
compound in line 90 at a level of between about 2 to about 10
weight percent based on the zinc metal was found to significant
improve the cycle time over which the sulfur dioxide reaction can
be carried out, while unexpectedly improving the yield of zinc
dithionite. Without the zinc compound being present in an amount
greater than about 2 weight percent of the zinc metal, the rate at
which the sulfur dioxide in line 80 can be introduced to the zinc
dithionite reaction zone 101 had to be periodically adjusted to
progressively lower levels over long periods of time per cycle in
order to achieve the desired conversion to zinc dithionite. The
present invention not only permits fewer adjustments to the rate at
which the sulfur dioxide is introduced, but also achieves the same
or higher conversion to zinc dithionite in a substantially shorter
cycle time. At the conclusion of the zinc dithionite reaction, the
contents or effluent of the zinc dithionite reaction zone 101 are
passed to a sodium dithionite reactor 102 via line 20.
[0039] In an alternate or parallel operation, at least a portion
and up to essentially the entire contents of the zinc dithionite
reaction zone 101 after any degree of conversion may be passed via
line 22 to a zinc filtration zone 106 to recover a zinc dithionite
filtrate in line 28 and a solid zinc cake in line 24. The solid
zinc cake can optionally be caustic washed in a conventional manner
which included further washing the cake to remove residual caustic
(not shown) to remove any impurities and returned to the zinc
dithionite reaction zone 101 via line 26. A residue of sodium
hydroxide in the solid zinc cake was found to have a beneficial
effect. It was discovered that the addition of sodium hydroxide as
a promoter compound to the zinc dithionite reaction zone 101, prior
or simultaneously with the addition of sulfur dioxide, stabilized
the zinc dithionite which was formed by the reaction with the
sulfur dioxide. It is believed that the sodium hydroxide reacted
with the sulfur dioxide to form sodium sulfite, sodium bisulfite,
sodium metabisulfite, and mixtures thereof, thereby maintaining the
thiosulfate concentration at a minimum level at the start of the
reaction, while maintaining the pH of the zinc dithionite reaction
zone at a high level (preferably greater than about 4.0). At least
a portion of the solid zinc cake may be removed from the process in
line 25 to minimize the buildup of impurities in the zinc
dithionite reaction zone 101.
[0040] In the sodium dithionite reactor 102, the zinc dithionite
reactor effluent passed via line 20 and, optionally the zinc
dithionite filtrate in line 28 is contacted with a dilute caustic
solution of sodium hydroxide in an amount sufficient to convert the
zinc dithionite to sodium dithionite. Generally, 2 moles of caustic
are required for each mole of zinc dithionite to produce the sodium
dithionite. The conditions of the conversion of the zinc dithionite
to sodium dithionite are well-known to those skilled in the art. At
the conclusion of the sodium dithionite reaction, a sodium
dithionite stream in line 30 is passed to a solids separation zone
103 such as a filtration zone or a centrifuge zone to recover the
sodium dithionite solution product stream in line 40 and to remove
a crude zinc oxide solid stream in line 50. The crude zinc oxide
solid stream in line 50 is passed to a wash zone 104, wherein the
crude zinc oxide solid stream in line 50 is washed, preferably with
water, to remove any remaining sodium dithionite and to provide a
washed zinc oxide stream in line 60. Typically, the crude zinc
oxide is washed with a wash water stream. The washed zinc oxide
stream in line 60 is passed to a drying zone 105 to dry the washed
zinc oxide stream to provide a dry zinc oxide stream in line 70. In
some process arrangements, the wash zone 104 and the and the solids
separation zone 103 may be carried out in the same equipment such
as a filtration zone wherein the recovered filter cake is washed in
the same equipment. The dry zinc oxide stream in line 70 or a
portion of the washed zinc oxide stream in line 60 may be further
refined by any chemical or electrochemical means or sold.
[0041] Referring to FIG. 2, another embodiment of the present
invention is illustrated wherein the zinc dithionite reaction zone
201 is charged with an aqueous zinc metal suspension comprising
water and a zinc metal in line 110 which is produced by the
electrolysis of a portion of a washed zinc oxide stream recovered
from a sodium dithionite reaction. According to FIG. 2, the aqueous
zinc metal suspension in line 110 is passed to the zinc dithionite
reaction zone 201 which is stirred and operates in a batch or semi
batch mode. In the zinc dithionite reaction zone 201, the zinc
metal is contacted with a sulfur dioxide stream introduced in line
125 in the presence of a zinc compound introduced in line 120. The
promoter or zinc compound is introduced at a level of about 2 to
about 15 weight percent based on the amount of zinc metal in the
zinc dithionite reaction zone 201. The sulfur dioxide stream in
line 125 is bubbled through the zinc dithionite reaction zone at a
constant rate or a series of successively reduced rates until the
contents of the zinc dithionite reaction zone 201 approaches a
terminal pH of about 3.2 at the lowest sulfur dioxide addition
rate. During the zinc dithionite reaction, the zinc dithionite
reaction temperature is maintained below about 54.degree. C., and
more preferably, during the zinc dithionite reaction, the zinc
dithionite reaction temperature is maintained from about 20 to
about 54.degree. C. A zinc dithionite solution is removed from the
zinc dithionite reaction zone 201 and passed via line 115 to a
sodium dithionite reaction zone 203 via lines 115 and 130 wherein
the zinc dithionite is contacted with a dilute caustic solution
introduced via line 145. The dilute caustic solution in line 145
typically contains sodium hydroxide, and is introduced to the
sodium dithionite reaction zone 203 at effective conditions to
convert the zinc dithionite to sodium dithionite. In the sodium
dithionite reaction zone 203, zinc oxide is produced as an
insoluble solid byproduct. Following the formation of sodium
dithionite, the effluent or in the case of a batch operation, the
contents of the sodium dithionite reaction zone 203 are passed in
line 150 to a solid separation zone 204. In the solid separation
zone 204, at least a portion of the sodium dithionite solution is
separated from any solids and a sodium dithionite product solution
is withdrawn in line 160. The solid separation zone 204 may be a
filtration zone or a centrifuge zone to separate the aqueous phase
product stream and recover a crude zinc oxide byproduct. The crude
zinc oxide byproduct is passed via line 155 to a wash zone 205
where the crude zinc oxide byproduct is washed, typically with
water, to provide a washed zinc oxide product in line 170. Spent
water is withdrawn from the wash zone 205 in line 175'. In some
process flow arrangements, the solid separation zone and the wash
zone 205 may be contained in the same apparatus. For example, when
the solid separation zone 204 is a filtration zone to separate the
crude zinc oxide byproduct from the sodium dithionite solution, the
resulting filter cake containing the crude zinc oxide can be washed
to provide the washed zinc oxide product in line 170. A portion of
the washed zinc oxide byproduct is withdrawn via line 170 for
conventional drying and sales. At least a portion of the washed
zinc oxide byproduct presscake, or zinc mud stream comprising zinc
oxide, in line 172 is passed via line 174 where is combined with a
water stream comprising caustic as an electrolyte stream in line
180 to provide a zinc oxide slurry in an electrochemical reaction
zone 206. A portion of the washed zinc oxide byproduct presscake
can be employed as a portion of all of the promoter or zinc
compound in line 120. The electrochemical reaction zone 206
comprises an undivided electrochemical reaction zone having
electrodes wherein the aqueous zinc slurry is electrolyzed to
convert at least a portion of the zinc oxide into a zinc metal
suspension. The electrodes may comprise a copper, magnesium,
nickel, or stainless steel or other suitable material as a cathode
and a nickel or stainless steel anode. A makeup zinc oxide stream
in line 185 may be added to the electrochemical reaction zone 206
to maintain the level of zinc in the process. A zinc metal
suspension in line 190 is withdrawn from the electrochemical
reaction zone 206 and passed to a zinc metal solids separation zone
207, wherein the solid zinc metal is recovered and passed via line
200 to a second wash zone 208. The zinc metal solids separation
zone 207 and the second wash zone 208 can be a single filtration
zone wherein the resulting filter cake comprising the solid zinc
metal is washed in the same filtration zone. Wash water introduced
in line 165 is employed to wash the zinc metal filter cake and
spent wash waster is withdrawn in line 175'. A liquid portion of
the effluent from the zinc metal solid separation zone 207 is
withdrawn in line 195 and reused as a water stream to form the zinc
slurry in the electrochemical reaction zone 206. In the zinc metal
wash zone 208, the recovered solid zinc metal is washed with water
to remove any residual caustic, and the washed zinc metal is
produced as an aqueous zinc metal suspension in line 110. The
aqueous zinc metal suspension in line 110 is passed to the zinc
dithionite reaction zone 201. Thus, this embodiment of the present
invention provide an environmentally safe processing scheme for the
production of sodium dithionite, with the ability to reprocess the
zinc oxide produced in the sodium dithionite reaction step. It was
unexpectedly discovered that the presence of the promoter compound
in the zinc dithionite reaction zone 201 significantly reduces the
cycle time of the zinc dithionite reaction zone while providing an
overall improved yield and recovery of zinc dithionite.
[0042] In a further embodiment of the present invention which
recognizes the problem of the build-up or accumulation of zinc
sulfide in the process, at least a portion of the zinc dithionite
solution in line 130 is optionally or periodically passed in line
130' to a zinc sulfide filtration zone 202 to remove essentially
all or at least a portion of the zinc sulfide in line 140, prior to
returning a zinc sulfide reduced stream in line 135 to the sodium
dithionite reaction zone 203. By the term zinc sulfide reduced
stream it is meant that the portion of the zinc dithionite solution
in line 135 contains a lower concentration of zinc sulfide than
unfiltered or bypassed zinc dithionite solution in line 130.
Although removal of the zinc sulfide from the process can have an
adverse effect on the operation of the electrochemical reaction
zone 206, it was discovered that high current efficiency and the
reactivity of the zinc metal particles produced in the
electrochemical reaction zone 206 are maintained by only removing a
portion of the zinc sulfide from the zinc dithionite solution in
line 115. Zinc sulfide and other insoluble materials including zinc
particles collected in the solid zinc cake in line 24 can be
employed with or without further purification as a promoter in the
zinc dithionite reaction zone 201, or in the electrochemical
reaction zone 206. The presence of such materials in the zinc
dithionite reaction zone 201, will partially limit or control the
amount of impurities generated in the zinc dithionite reaction zone
and permit the extension of the terminal pH value of the zinc
dithionite to a level higher than 3.3. Preferably, the presence of
a zinc promoter in the zinc dithionite reaction zone will permit
the extension of the terminal pH value of the zinc dithionite to a
level higher than about 4.0, and more preferably, the presence of a
zinc promoter in the zinc dithionite reaction zone will permit the
extension of the terminal pH value of the zinc dithionite reaction
to a level higher than about 4.2.
[0043] 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.
EXAMPLE
Example 1
[0044] General Procedure:
[0045] Particulate zinc material (206.3 g of commercial zinc dust
P-65, available from Noranda Zinc, Toronto, Canada) and water (700
to 1000 g) was charged into a one-liter jacketed 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. The reaction
mixture was then stirred and sulfur dioxide gas was passed through
the reaction mixture. The sulfur dioxide was added at a rate of 3.4
g/minute until pH of the reaction mixture dropped to 3.3 and
temperature was maintained at 25 to 55.degree. C. by circulating
cold water through the jacket. Sulfur dioxide addition was stopped
when pH of the reaction mixture dropped to 3.3 and 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
[0046] The results of the reaction with sulfur dioxide under
various conditions are shown in Table 1.
1TABLE 1 Zinc Agitation Moles Conv. Reaction Exp. Zinc Dust Rate Of
Of zinc Yield of time, No. dust g Water g moles ZnO g rpm
ZnS.sub.2O.sub.4 dust.sup.3 % ZnS.sub.2O.sub.4 % Minutes 7I-16
206.3 714 3.15 0 560 2.1 -- 66.7 83 7I-15 206.3 714 3.15 19.7.sup.1
560 2.49 82.7% 79.0 97 7I-19 206.3 906 3.15 20.sup.1 560 2.42 84.2
76.8 97 7I-18 206.3 906 3.15 0 560 2.33 78.1 74.0 92 7I-39.sup.4
206.3 906 3.15 0 560 2.30 79.1 73.0 92 7I-54.sup.3 206.3 906 3.15
22.sup.1 560 2.53.sup.1 86.3 80.3 133 7I-40.sup.4 206.3 906 3.15
22.sup.1 560 2.52 85.4 80.3 106 7I-49.sup.4 206.3 906 3.15 14.sup.1
560 2.56 84.7 81.3 102 7I-50.sup.3 206.3 906 3.15 10.sup.1 560 2.50
86.1 79.4 104 7I-53 206.3 906 3.15 4.sup.1 560 2.43 81.9 77.1 92
7I-51.sup.3 206.3 906 3.15 6.sup.1 560 2.48 84.8 78.7 93
7I-46.sup.3 206.3 906 3.15 30.sup.1 560 2.60 85.4 82.5 106
7I-44.sup.3 206.3 906 3.15 0 560 2.60.sup.2 87.9 82.5 146
7I-41.sup.3 206.3 906 3.15 22.sup.1 560 2.72.sup.2 94.6 86.4 133
7I-53 206.3 906 3.15 4.sup.1 560 2.43 81.9 77.1 92 7I-45.sup.3
206.3 906 3.15 14.sup.1 560 2.71.sup.2 92.2 86.0 128 7H-25 206.3
906 3.15 0 560 2.32 79.2 73.6 86 7H-19 206.3 906 3.15 22.sup.4 560
2.68 93.6 85.1 115 Notes: .sup.1ZnO was 68% dry byproduct from zinc
dithionite production. This byproduct was obtained from Clariant
Chemical Corporation. .sup.2Sulfur Dioxide is added at a constant
rate of 3.4 g/minute. Until pH dropped to 3.3. Sulfur dioxide
addition rate was then lowered several times to maintain pH above
3.3. Lowest sulfur dioxide addition rate was 0.5 g/minute.
.sup.3Pitched blade stirrer was used. .sup.4ZnO was 61% dry
byproduct from zinc dithionite production. This byproduct was
obtained from Clariant Chemical Corporation. 8 ml of cadmium
sulfate solution was also added to the reaction mixture.
Example 3
[0047] General Procedure:
[0048] 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. Zinc oxide was also added during the experiment
to maintain a maximum concentration of solubilized zinc oxide. A
thermometer, desired cathodes and anodes were positioned in the
cell using laboratory clamps. Mixing was achieved by bubbling
nitrogen and by using a mechanical stirrer. 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
Amps/m.sup.2. A portion of the zinc deposited on the cathode was
removed periodically. 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 reaction mixture was then heated
to 43.degree. C. 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 3.3 and temperature
was maintained at 25 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 4
[0049] The results of electrolysis of zinc oxide followed by the
reaction with sulfur dioxide under various conditions are shown in
Table 2.
2TABLE 2 ZnO.sup.1 Reaction Moles Exp. NaOH Source/ X ZnO.sup.1,2
Cathode/ Time of e.sup.-1 Moles C.E. No. (wt %) wt % A g ml
Source/g Anode Min. passed ZnS.sub.2O.sub.4 (%) 7F-36.sup.3,4 25
W/6.0 1015 8 0 Cu/Ni 133 6.32 2.20 70 7F-37.sup.3,4 25 W/6.0 1121 8
F/10 Cu/Ni 124 6.32 2.84 90 7G-1.sup.4 25 F/6.0 926 8 F/10 Cu/Ni
159 6.32 2.57 81 7G-3.sup.4 25 F/6.0 946 8 0 Cu/Ni 197 6.32 2.28 72
Notes: .sup.1F: Zinc oxide from Fisher Chemical Inc., was used W:
Zinc oxide byproduct from sodium dithionite production was used.
.sup.2Amount of ZnO added to the reactor before passing sulfur
dioxide. .sup.3Zinc produced electrochemically was first washed
with 25% sodium hydroxide solution and then washed with water.
.sup.4Sulfur dioxide addition rate was lowered several times to
maintain the reaction mixture's pH above 3.3.
Example 5
[0050] Particulate zinc material (206.3 g), zinc sulfite dihydrate
(25.0 g), water (906 g) were charged into a one-liter jacketed
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. The
reaction mixture was then stirred and sulfur dioxide gas was added
at a rate of 3.4 g/minute for 100 minutes. During this period, pH
of the reaction mixture dropped to 3.3. Temperature of the reaction
mixture was maintained at 25 to 43.degree. C. by circulating cold
water through the jacket. The resulting reaction mixture was
decanted to give a translucent solution of the zinc dithionite. It
was then titrated to determine the concentration of zinc
dithionite. Yield of zinc dithionite was 83.2% and conversion of
zinc particles was 86.4.
Example 6--Comparative Example Cycle Time
[0051] With reference to Table 1, Experiment 71-44 illustrated the
operation of a zinc dithionite reaction without the presence of any
promoter compound in the zinc dithionite reaction zone and
Experiment 7H-19 illustrated the operation of the zinc dithionite
reaction in the presence of about 6.5 weight percent zinc oxide.
The presence of the zinc oxide resulted in a decrease in the cycle
time, or reaction time for the zinc dithionite reaction by about 31
minutes, by surprisingly reducing the required reaction time from
146 minutes to 115 minutes, or about a 21 percent reduction in
cycle time, accompanied by an unexpected 5.7% increase in the
conversion of the zinc dust to 93.6 percent, and bringing the
overall yield of zinc dithionite to 85.1 percent.
Example 7
[0052] Production of Sodium Dithionite with Removal of Insolubles
from Zinc Dithionite Solution
[0053] A resin kettle (5 inch (12.7 cm) in diameter and 18 inch
(45.7 cm) high) was used as an electrolytic cell. A solution of
zinc oxide (KADOX-920P, Zinc Corporation of America) (299 g), and
sodium hydroxide (1248 g) in water (3444 g) was charged to the
resin kettle. Zinc oxide (189 g) was also added in installments
during the experiment to maintain a maximum concentration of
solubilized zinc oxide, according to the instant invention. A
thermometer, 2 nickel based anodes and one magnesium based cathode,
were positioned in the cell using laboratory clamps. Mixing was
achieved by using a mechanical stirrer. Parts of the cathode and
anode surfaces were covered with Teflon and electrical tape to
achieve the desired active cathode and anode surface areas.
Electrolysis was carried out at a current density of about 5000
Amps/m.sup.2. A portion of the zinc deposited on the cathode was
removed periodically. At the end of the experiment, zinc metal
particles were separated from the electrolyte by decanting, and
were washed with water.
[0054] A zinc dithionite reaction mixture of wet zinc metal (223.1
grams), zinc oxide (KADOX-920P) (10 g), and water (882 g) were
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. Sulfur dioxide gas was then passed
through the zinc dithionite reaction mixture at a rate of 3.4
g/minute to produce a zinc dithionite solution. The sulfur dioxide
addition rate was lowered several times to maintain pH of the
reaction mixture above 3.3 and temperature was maintained between
25 and 55.degree. C. until the reaction was terminated at a pH of
about 3.3. The zinc dithionite solution was mixed with filter cell
(20 g), filtered, and washed with water to provide a filtered
solution of zinc dithionite (2.19 moles; current efficiency of
69.6%) and a wet cake of insolubles (42 g).
[0055] The zinc dithionite solution was further reacted with sodium
hydroxide to produce sodium dithionite. A solution of sodium
hydroxide (235 g) in 1209 g water was charged into a four neck
round bottom flask equipped with a mechanical stirrer, nitrogen
inlet and outlet adapters, and an addition funnel. The filtered
zinc dithionite solution was then added slowly into the sodium
hydroxide solution while controlling temperature below 45.degree.
C. Reaction mixture was stirred for 10 minutes, filtered, and
washed with water to provide a solution of sodium dithionite and a
wet cake of zinc oxide (440.1 g).
Example 8
[0056] A Portion of Zinc Dithionite Insolubles Added to
Electrolyte).
[0057] The procedure of Example 7 was repeated with the addition of
a portion of the insolubles recovered from the zinc dithionite
solution in Example 7 being added to the electrolytic cell. A resin
Kettle (5 inch (12.7 cm) in diameter and 18 inch (45.7 cm) high)
was used as the cell. Electrolyte--prepared by mixing the decanted
electrolyte from the control run, a zinc oxide wet cake (440 g)
produced during the hydrolysis of zinc dithionite in the control
run, a wet cake of insolubles (38 g) produced in the control run,
and 100 g of sodium hydroxide--was charged into the resin kettle. A
thermometer, 2 nickel based anodes and one magnesium based cathode,
were positioned in the cell using laboratory clamps. Mixing was
achieved by using a mechanical stirrer. Parts of the cathode and
anode surfaces were covered with Teflon and electrical tape to
achieve the desired active cathode and anode surface areas.
Electrolysis was carried out at a current density of about 5000
Amps/m2. A portion of the zinc deposited on the cathode was removed
periodically. At the termination of the electrolysis step, zinc
metal particles were separated from the electrolyte by decanting,
and were washed with water.
[0058] The zinc metal was converted to a zinc dithionite solution
as follows. Wet zinc (353 grams), zinc oxide (KADOX-920P) (10 g)
and water (747 g) were 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. Sulfur
dioxide gas was then passed through the reaction mixture at a rate
of 3.4 g/minute. The sulfur dioxide addition rate was lowered
several times to maintain pH of the reaction mixture above 3.3 and
temperature was maintained between 25 to 55.degree. C., until the
zinc dithionite reaction was terminated at a pH of 3.3. Recovery of
the zinc dithionite solution comprised mixing the zinc dithionite
solution with filter cell (20 g), filtering the zinc dithionite
solution and washing the filter with water to provide a zinc
dithionite solution (2.81 moles; current efficiency of 89.2%) and a
wet cake of insolubles (45 g).
[0059] The zinc dithionite solution was converted to a sodium
dithionite solution as follows. A solution of sodium hydroxide (257
g) in 1000 g water was charged into a four neck round bottom flask
equipped with a mechanical stirrer, nitrogen inlet and outlet
adapters, and an addition funnel. The above produced filtered zinc
dithionite solution was then added slowly to the sodium hydroxide
solution while maintaining the temperature in the flask below
45.degree. C. In order to maintain pH above 12.0, 70 g of a 16%
solution of sodium hydroxide in water was added during the
hydrolysis. Reaction mixture was then stirred for 10 minutes,
filtered, and washed with water to provide a solution of sodium
dithionite and a wet cake of zinc oxide (498.6 g). Thus, the
addition to the electrolytic cell of at least a portion of the
insolubles recovered from the zinc dithionite solution, which are
believed to comprise zinc sulfide, resulted in a significant
improvement in the current efficiency or yield of both the zinc
dithionite and the sodium dithionite. When electrochemically
produced zinc is the only source of zinc included in the zinc
dithionite reaction mixture, the current efficiency is equal to the
yield of zinc dithionite.
Example 9
[0060] Sodium Hydroxide Promoter
[0061] Particulate zinc material (206.3 g), water (906 g) were
charged into a one-liter jacketed 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. Prior to the introduction of
the particulate zinc, sodium hydroxide was added to the water to
bring the water to the indicated sodium hydroxide level, expressed
as a weight percent of the zinc. The reaction mixture was then
stirred and sulfur dioxide gas was added at a constant rate of 3.4
g/minute for about 100 minutes. The resulting yield of zinc
dithionite at various levels of sodium hydroxide addition between 0
and about 9 weight percent is shown in Table 3. A significant
increase in the yield of zinc dithionite resulted for those runs
when sodium hydroxide was present in an amount up to about 9 weight
percent.
3TABLE 3 EFFECT OF THE ADDITION OF SODIUM HYDROXIDE Gassing Time,
ZnS.sub.2O.sub.5 ZnS2O3 Yield of Exp. No. NaOH % min moles moles
ZnS.sub.2O.sub.4 % 7H-BKB-18 0 100 0.085 0.016 74 1-BAC-25 2 95
0.123 0.022 77 1-BAC-28 5 98 0.162 0.029 83 7I-BKB-47 6 107 0.171
0.01 84 1-BAC-31 9 96 0.212 0.024 79
Example 10
[0062] Sodium Hydroxide with Sulfur Dioxide at a Variable Rate
[0063] The procedure of Example 9 was repeated for a variable
sulfur dioxide addition rate until the reaction vessel reached a
terminal pH value of about 3.3, where upon the sulfur dioxide
addition rate was reduced in a step-wise manner to maintain the pH
about 3.3 for a total period of about 150 minutes. Runs were made
using 0 and 6 grams of sodium hydroxide in the 906 grams of water.
The resulting yield of zinc dithionite when no sodium hydroxide was
present was 83 percent, compared to a yield of 89 percent zinc
dithionite when the sodium hydroxide was present at a level of
about 3 weight percent of the zinc.
[0064] 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.
[0065] 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.
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