U.S. patent number 3,616,325 [Application Number 04/688,525] was granted by the patent office on 1971-10-26 for process for producing potassium peroxydiphosphate.
This patent grant is currently assigned to FMC Corporation. Invention is credited to Paul R. Mucenieks.
United States Patent |
3,616,325 |
Mucenieks |
October 26, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
PROCESS FOR PRODUCING POTASSIUM PEROXYDIPHOSPHATE
Abstract
Potassium peroxydiphosphate is produced by electrolyzing an
anolyte containing an aqueous mixture of potassium, phosphate and
fluoride ions and catholyte containing an aqueous mixture of
phosphate ions, said anolyte and catholyte being separated by
diaphragm means; peroxydiphosphate values are produced in the
anolyte at a pH of from about 9.7 to 14.5, and these values are
prevented from substantial migration to the catholyte by said
diaphragm.
Inventors: |
Mucenieks; Paul R. (Trenton,
NJ) |
Assignee: |
FMC Corporation (New York,
NY)
|
Family
ID: |
24764770 |
Appl.
No.: |
04/688,525 |
Filed: |
December 6, 1967 |
Current U.S.
Class: |
205/470 |
Current CPC
Class: |
C25B
1/28 (20130101) |
Current International
Class: |
C25B
1/00 (20060101); C25B 1/28 (20060101); C01b
015/16 () |
Field of
Search: |
;204/83,82,90 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Fichter et al., Helv. Chim. Acta 11, 323-337 (1928). Lowry,
Inorganic Chemistry, London, 1931. pp. 495-6.
|
Primary Examiner: Garvin; Patrick P.
Claims
What is claimed is:
1. Process for producing potassium peroxydiphosphate comprising
introducing aqueous mixture consisting essentially of potassium,
phosphate and fluoride ions into the anode compartment of an
electrolytic cell as the anolyte, said anolyte having a phosphate
ion concentration of from about 1 to about 4 molar, a fluoride ion
concentration of at least about 0.5 to 1.25 atoms of fluoride ion
for each phosphate ion and a potassium ion concentration sufficient
to have at least one potassium ion present for every fluoride ion
and in addition from two to three potassium ions present for each
phosphate ion in said anolyte solution, introducing an aqueous
mixture consisting essentially of phosphate ions into the cathode
compartment of said electrolytic cell as the catholyte, the
catholyte and anolyte of said electrolytic cell being contained by
means which permit ions to pass freely between the anolyte and
catholyte but which prevent any substantial amounts of
peroxydiphosphate values in the anolyte from mixing with the
catholyte, passing an electric current through said catholyte and
anolyte by means of a cathode in said catholyte and an anode in
said anolyte, converting phosphate values to peroxydiphosphate
values from said anolyte at a pH of from about 9.7 to about 14.5,
removing an anolyte enriched in peroxydiphosphate values from said
electrolytic cell, and precipitating and recovering potassium
peroxydiphosphate values from said anolyte.
2. Process of claim 1 wherein the concentration of potassium ions
in the catholyte is increased by the migration of potassium ions
from said anolyte to said catholyte, said anolyte enriched in
peroxydiphosphate values is removed from said electrolytic cell and
concentrated to precipitate potassium peroxydiphosphate values,
separating and recovering the precipitated potassium
peroxydiphosphate values from the anolyte mother liquor, recycling
said anolyte mother liquor to said anode compartment along with the
potassium-enriched catholyte solution and introducing a fresh,
aqueous mixture consisting essentially of phosphate ions into the
cathode compartment as the catholyte.
3. Process of claim 2 wherein said anolyte solution has a pH of
from 11.8 to 12.6 prior to precipitation of said potassium
peroxydiphosphate therefrom.
4. Process of claim 1 wherein said anolyte has a phosphate ion
concentration of from about 1 to about 4 molar, a fluoride
concentration of from about 0.1 to about 5 molar and a potassium
ion concentration sufficient to have one potassium ion present for
every fluoride ion and in addition from two to three potassium ions
present for each phosphate ion in said anolyte solution, and said
catholyte is an aqueous mixture consisting essentially of potassium
and phosphate values in a K/PO.sub.4 molar ratio of 0.5- 3:1.
5. Process of claim 1 wherein said anolyte has a phosphate ion
concentration of from about 2 to about 3 molar, a fluoride
concentration of from about 1 to about 3.6 molar and a potassium
ion concentration sufficient to have one potassium ion present for
every fluoride ion and in addition from 2.5 to 2.8 potassium ions
present for each phosphate ion in said anolyte solution, and said
catholyte is an aqueous mixture consisting essentially of potassium
and phosphate values in a K/PO.sub.4 molar ratio of about 2:1.
6. Process of claim 1 wherein the ph of the anolyte is from about
11.8 to about 13.5
7. Process of claim 1 wherein the pH of the anolyte is from about
12.2 to about 12.6.
8. Process of claim 1 wherein potassium hydroxide is added to said
anolyte to maintain its pH between 9.7 and 14.5 during
electrolysis.
9. Process of claim 1 wherein the catholyte and anolyte are
separated and contained by diaphragm means.
10. Continuous process for producing potassium peroxydiphosphate
comprising introducing an aqueous mixture consisting essentially of
potassium, phosphate and fluoride ions into the anode compartment
of an electrolytic cell as the anolyte, said anolyte having a
phosphate ion concentration of from about 1 to about 4 molar, a
fluoride ion concentration of at least about 0.5 to 1.25 atoms of
fluoride ion for each phosphate ion and a potassium ion
concentration sufficient to have at least one potassium ion present
for every fluoride ion and in addition from two to three potassium
ions present for each phosphate ion in said anolyte solution,
introducing an aqueous potassium phosphate solution having a
K/PO.sub.4 molar ratio of 0.5-3:1 into the cathode compartment of
said electrolytic cell as the catholyte, the catholyte and anolyte
of said electrolytic cell being contained by means which permit at
least potassium ions to pass freely between the anolyte and
catholyte but which prevent any substantial amounts of
peroxydiphosphate values in the anolyte from mixing with the
catholyte, passing an electric current through said catholyte and
anolyte by means of a cathode in said catholyte and an anode in
said anolyte, converting phosphate values to peroxydiphosphate
values in said anolyte at a pH of from about 9.7 to about 14.5,
continuously removing an anolyte enriched in peroxydiphosphate
values from said electrolytic cell, maintaining the pH of said
anolyte solution at from 11.8 to 12.6, concentrating the removed
anolyte solution to precipitate potassium peroxydiphosphate values,
separating and recovering the precipitated potassium
peroxydiphosphate from the anolyte mother liquor, continuously
removing catholyte from said electrolytic cell, recycling a mixture
of said anolyte mother liquor and the removed catholyte to said
anode compartment and constantly introducing fresh, aqueous
potassium phosphate solution having a K/PO.sub.4 molar ratio of
0.5-3:1 into the cathode compartment as the catholyte.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of potassium
peroxydiphosphate by electrolysis of a potassium phosphate
solution.
2. Description of the Prior Art
The preparation of potassium peroxydiphosphate in the laboratory by
electrolytic methods is known. Prior workers have reported the
production of potassium peroxydiphosphate by placing a solution of
potassium phosphate, potassium fluoride, and potassium chromate
(K.sub.2 CrO.sub.6) in a platinum crucible, inserting a platinum
tube or wire into the solution and electrolyzing the solution using
the platinum crucible as the anode and the tube or wire as the
cathode. This is reported by F. Fichter and E. Gutzwiller, Helv.
Chim. Acta 11, 323-337 (1928). This process is applicable to the
formation of only laboratory amounts of potassium peroxydiphosphate
because of the inability to scale such a process up to commercial
production. The resulting product is reported as being very
hygroscopic and not subject to air drying. Moreover, prior workers
have not been able to remove chromium-containing impurities from
the finished product, and this is desirable because these
impurities act as a decomposition catalyst for the
peroxydiphosphate. In addition, only low anode current densities
can be used, otherwise the yield drops below 20 percent. Further,
temperatures below 15.degree. C. are normally required and
potassium peroxymonophosphate, which is produced as a byproduct,
must be removed periodically in order to obtain the potassium
peroxydiphosphate.
As a result, it has been impossible to produce potassium
peroxydiphosphate on a commercial basis because of the lack of an
effective process which is efficient, cheap and yields high-purity
potassium peroxydiphosphate as a product.
OBJECTS OF THE INVENTION
It is an object of the present invention to produce potassium
peroxydiphosphate in good yields and at commercially acceptable
efficiencies.
It is a further object to produce potassium peroxydiphosphate by a
process which yields a pure product and which can be carried out
commercially in batch form or in continuous production.
These and other objects will be apparent from the following
description of the invention.
SUMMARY OF THE INVENTION
I have found that potassium peroxydiphosphate can be produced in
good yields by introducing an aqueous mixture containing potassium,
phosphate and fluoride ions into the anode compartment of an
electrolytic cell as the anolyte, introducing an aqueous mixture
containing phosphate ions into the cathode compartment of said
electrolytic cell as the catholyte, the catholyte and anolyte of
said electrolytic cell being contained by means (preferably
diaphragm means) which permit potassium and/or phosphate ions to
pass freely between the anolyte and catholyte but which prevent any
substantial amounts of peroxydiphosphate values in the anolyte from
mixing with the catholyte, passing an electric current through said
catholyte and anolyte by means of a cathode in said catholyte and
an anode in said anolyte, converting phosphate values to
peroxydiphosphate values in said anolyte having a pH of about 9.7
to about 14.5, removing an anolyte enriched in peroxydiphosphate
values from said electrolytic cell, precipitating potassium
peroxydiphosphate values from the peroxydiphosphate-enriched
anolyte and separating and recovering said potassium
peroxydiphosphate values from the anolyte mother liquor.
I have found further, that the above process can be carried out on
a continuous bases by recycling said anolyte mother liquor to said
anode compartment along with the removed catholyte solution and
introducing a fresh, aqueous solution containing phosphate ions
(and preferably containing both potassium and phosphate ions in a
K/PO.sub.4 mole ratio of 0.5-3:1) into said cathode compartment as
an electrolyte, and continuing the electrolysis.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, FIG. 1 is a diagrammatical flow sheet of the
process and an electrolytic cell for carrying out the electrolysis;
FIG. 2 is a graphic illustration of the relationship between the pH
of the anolyte and current efficiencies of the electrolytic cell at
different levels of conversion.
DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS
In carrying out the present invention an aqueous anolyte solution
is prepared preferably by mixing together water, potassium
hydroxide, phosphoric acid and hydrofluoric acid. The anolyte
should have a phosphate ion concentration of from about 1 to about
4 molar and preferably from about 2 HPO.sub.to 3 molar; the
fluoride ion should be present in amounts of about 0.5 to 1.25
atoms of fluoride ion for each phosphate ion. In general, fluoride
ion concentrations of 0.1 to 5 molar (and preferably 1 to 3.6
molar) may be used. The anolyte solution should contain a potassium
ion for every fluoride ion present in the anolyte solution, and
further, additional potassium ions in the amounts of two to three
ions (preferably 2.5 to 2.8 atoms) should be present for each
phosphate ion in solution. It is preferred to make up the anolyte
solution by mixing KOH, H.sub.3 PO.sub.4 and HF in an aqueous
solution because these materials can be obtained in a pure state,
and the resulting anolyte solution contains a minimum of
impurities. However, an anolyte can also be prepared from KH.sub.2
PO.sub.4, K.sub.2 HPO.sub.4, K.sub.3 PO.sub.4, KHF.sub.2, KF, KOH
and all hydrates of the above salts, notwithstanding the generally
higher cost and higher impurity level compared with the preferred
ingredients.
The aqueous solution thus prepared is passed into the anode section
of an electrolytic cell and used as the anolyte. The cell also
contains a solution in the cathode section which serves as the
catholyte. The catholyte is an aqueous solution containing
phosphate ions obtained, e.g., by dissolving a phosphoric acid in
water, and preferably contains both potassium and phosphate ions in
a K/PO.sub.4 mole ratio of 0.5-3:1. The optimum balance of K and
PO.sub.4 values are obtained in the cell when the K/PO.sub.4 mole
ratio is 2:1. The catholyte is prepared preferably by mixing
together potassium hydroxide and phosphoric acid. The concentration
of the catholyte solution is adjusted to prescribed limits as set
forth hereinafter to permit continuous, recycle operation. For
reasons of purity and cost, it is preferred to prepare the
catholyte from potassium hydroxide and phosphoric acid although
potassium phosphate salts such as KH.sub.2 PO.sub.4, K.sub.2
HPO.sub.4 and hydrates of these salts can be used.
The anolyte and catholyte solutions of the electrolytic cell must
be contained in a manner to prevent any substantial amounts of
peroxydiphosphate values which are formed in the anolyte from
mixing with the catholyte, but without preventing the flow of an
electric current between the solutions. It is preferred that the
anolyte and catholyte be separated by a diaphragm which permits the
flow of an electric current between the anolyte and catholyte but
which prevents substantial amounts of peroxydiphosphate values in
the anolyte from diffusing into the catholyte. The flow of current
between the anolyte and catholyte takes place by passing certain
ions, principally potassium and/or phosphate ions, through the
diaphragm and is a necessary part of the electrolytic circuit of
the cell. The diaphragm in the electrolytic cell can be any porous
membrance such as a porous, porcelain sheet, asbestos membrance,
plastic membrane, or a cation exchange membrane, etc. In the case
of a cation exchange membrane, this will permit the passage of
potassium ions from the catholyte to the anolyte selectively
without permitting anions, such as peroxydiphosphate or phosphate
anions, from passing through the cation exchange membrane.
An electrode then is immersed in each catholyte and anolyte
solution. These electrodes can be any material which can conduct an
electric current and which does not react with the solutions of the
cell during electrolysis. Generally, noble metals such as platinum
or gold or tantalum-reinforced platinum are preferred as the
anodes. The cathodes can be made of lead, graphite, stainless steel
or of noble metals such as platinum or gold. The cathodes can be in
the form of plates, wire screens or in the form of spirals or other
configurations made of a hollow tubing. If the cathode is in the
form of a hollow tubing, a cooling solution can be passed through
the tube and serve to cool the catholyte. An electric potential is
placed across the electrodes (the anode and cathode) by means of a
battery, rectifier, or other source of direct current to complete
the electrolytic cell.
The electric potential which is applied across the electrodes must
be sufficient to cause a positive electric current to flow outside
the cell, from cathode to anode, so that hydrogen gas is generated
at the cathode, and phosphate values are converted to
peroxydiphosphate values at the anode. Normally an EMF of at least
about 2.0 volts has been found operable with from about 3 to 8
volts being preferred. The electrolytic conversion of phosphates to
peroxydiphosphates may be conducted at any temperature of from
10.degree. to 90.degree. C. with temperatures of from about
10.degree. to about 70.degree. C. being preferred. The preferred
anode current density which is used in operating the cell may range
from 0.1 to 1 a./cm..sup.2 of anode surface, with 0.3 a./cm..sup.2
of anode surface being optimum.
In carrying out the present process it is necessary to control the
pH of the anolyte to obtain maximum conversion of phosphate ions to
peroxydiphosphate ions at high current efficiencies. The current
efficiency is determined by comparing the amount of
peroxydiphosphate values formed by a unit quantity of electricity
with the theoretical amount of peroxydiphosphate which that amount
of electrical energy can produce. The current efficiency is a
separate and distinct measurement from the degree of conversion, in
that the latter expresses only the percent of phosphate ions
converted to peroxydiphosphate ions, regardless of the quantity of
electricity used to effect the conversion.
In general, the anolyte should be maintained at a pH of from about
9.7 to about 14.5 to obtain conversions (phosphate ions to
peroxydiphosphate ions) greater than 20 percent at current
efficiencies of at least 50 percent. Highest conversions are
obtained when the pH of the anolyte is from about 11.8 to about
13.5 with the optimum being obtained at a pH of about 12.2 to 12.6.
The pH of the anolyte can be controlled most readily by regulating
the concentration of potassium hydroxide in the anolyte
solution.
During electrolysis, the pH of the anolyte changes and becomes
progressively less alkaline. Accordingly, optimum conditions for
obtaining maximum conversion can be obtained by constantly
adjusting the pH of the anolyte in the electrolytic cell by the
addition of KOH or by commencing operation on the alkaline side of
the preferred range and continuing electrolysis until the anolyte
has reached the lowest pH at which operation is desired.
The reactions that presumably occur at each of the half-cells are
as follows:
At the cathode:
2H.sup.++2 electrons H.sub.2
At the anode:
2PO.sub.4 - 2 electrons P.sub.2 O.sub.8
The desired overall reaction of the electrolytic cell is:
2PO.sub.4 +2H.sup.+ H.sub.2 +P.sub.2 O.sub.8
In the above description of the electrolytic cell, phosphate ions
are converted to peroxydiphosphate ions at the anode, while H.sup.+
ions are converted to hydrogen gas at the cathode. The electronic
flow necessary to carry out these conversions within the cell is
obtained by the transfer of ions through the diaphragm. For
example, potassium ions may flow from the anode compartment through
the diaphragm into the cathode compartment, and/or PO.sub.4 ,
HPO.sub.4 or H.sub.2 PO.sub.4 .sup.- ions may flow from the cathode
compartment through the diaphragm to the anode compartment. When
substantial potassium ion transfer occurs, the anolyte, depleted of
its K.sup.+ values, becomes progressively less alkaline;
simultaneously the catholyte becomes more alkaline as its K.sup.+
concentration increases. In addition to the transfer of ions
through the diaphragm, some catholyte solution may be permitted to
flow through the diaphragm into the anolyte. However, it is not
desirable to have any substantial flow of anolyte through the
diaphragm into the catholyte because of the possible transfer and
loss of peroxydiphosphate values.
The electrolysis is continued until the desired conversion of
phosphate to peroxydiphosphate ions has been obtained. The exact
amount of conversion desired will depend upon the minimum current
efficiencies which can be tolerated, regardless of whether the
system is operated as a batch process or a continuous process;
current efficiencies normally decrease as higher conversions are
obtained. When the desired peroxydiphosphate concentration has been
obtained, the anolyte is removed from the anode compartment of the
cell and placed in an evaporator-crystallizer unit. In this unit,
water is evaporated from the solution, and potassium
peroxydiphosphate is crystallized and separated from the mother
liquor.
When the anolyte is removed from the electrolytic cell and passed
into the evaporator-crystallizer, the pH of the anolyte should be
adjusted to between 11.8 and 12.6, and preferably 12 to 12.6. This
can be done by the addition of KOH if the solution has a pH below
11.8, or by the addition of phosphoric acid, if the solution has a
pH above 12.6. Obviously, other acids or bases can be used to
achieve the same results, but the above additives are desired since
they do not introduce foreign ions into the system. The pH of the
anolyte is adjusted between 11.8 and 12.6 before evaporation of
water and crystallization of the potassium peroxydiphosphate
product in order to facilitate separation of the potassium
peroxydiphosphate product from the mother liquor. If the anolyte
has a pH outside of this range, the resulting potassium
peroxydiphosphate crystals retain excess amounts of water and
become extremely difficult to filter or otherwise separate from the
resulting mother liquor. When the anolyte is maintained at optimum
pH conditions for conversion of the phosphate to peroxydiphosphate,
namely a pH of from about 12 to 12.6 by the constant addition of
KOH into the anolyte during the electrolysis, the anolyte which is
removed from another compartment of the electrolytic cell will be
at the proper pH for immediate evaporation and concentration to
recover the potassium peroxydiphosphate product.
The mother liquor containing some unconverted phosphate values,
potassium values and fluoride values is then mixed with the exit
catholyte solution, and the mixture is recycled to the anode
compartment of the electrolytic cell. In this way, succeeding
anolytes are made up from the mother liquor obtained after
crystallizing potassium peroxydiphosphate from the anolyte and the
exit catholyte solution. A fresh catolyte solution is then made up,
preferably in the form of an aqueous, K.sub.2 HPO.sub.4 solution.
The fresh catholyte is made up preferably by mixing together
potassium hydroxide and phosphoric acid. In general, the
concentration of the fresh catholyte is adjusted so that the
outgoing catholyte from the electrolytic cell is at the proper
concentration, when mixed with the mother liquor from the anolyte,
to provide a suitable anolyte as previously defined. Providing a
phosphate ion containing solution as the catholyte serves several
purposes. Initially, it serves as a method of collecting and
recycling any and all potassium values which migrate into the
catholyte from the anolyte during electrolysis. Additionally, the
presence of phosphate ions in the cathode compartment provides the
necessary conductivity of the catholyte. Finally, the use of
phosphate ions (preferably with potassium ions in a K/PO.sub.4 mole
ratio of 0.5- 3:1) in the catholyte prevents any foreign ions from
diffusing into the anolyte solution and contaminating or destroying
peroxydiphosphate values.
The above electrolysis reaction can be conducted batchwise or
continuously. In a continuous operation, electrolytic cells should
be used in which the separate electrolytes are introduced at one
end of the cell and flow along opposite sides of the membranes
until they are removed from the cell at the opposite end. During
this flow the concentration of the peroxydiphosphate ion in the
anolyte increases. Thereafter the peroxydiphosphate values are
recovered and the remainder of the anolyte and the exit catholyte
are recycled to the anode compartment along with any makeup
chemicals required to obtain a desired anolyte composition. The
solutions can be introduced in other such cells in cascade, if
desired, so that instead of a single cell, a plurality of
electrolytic cells can be used.
The present invention will now be described by reference to the
drawings in which FIG. 1 is a diagrammatical flow sheet of one
embodiment of the process, and FIG. 2 shows, in graphic form, the
current efficiencies as a function of the pH of the anolyte at
different levels of conversion.
In FIG. 1 of the drawings, the anolyte solution enters the
electrolytic cell 2 through line 4. The electrolytic cell 2 is made
up of anode compartment 6 and a cathode compartment 8, separated by
diaphragm 10. In the anode compartment 6 an anode 12 is connected
by electric battery 14 to the cathode 16 in the cathode compartment
8. The catholyte is made up preferably by mixing potassium
hydroxide, phosphoric acid and water in catholyte makeup tank 18,
and the resulting aqueous, potassium phosphate solution is passed
through line 20 into the cathode compartment 8. The electrolytic
cell 2 is then energized by connecting anode 12 and cathode 16 to
battery 14. In the anode compartment 6 phosphate ions are converted
to peroxydiphosphate ions at the anode 12. The electric current is
carried between the anode and cathode compartments principally by
the transfer of potassium ions along with some hydrogen ions
through the diaphragm 10 from anode compartment 6 to cathode
compartment 8. As this occurs the anolyte becomes less basic, and
the pH decreases. If the anolyte is at the optimum pH of 12 to 13.3
when electrolysis commences, the pH of the anolyte can be
maintained within this range by the addition of potassium hydroxide
through line 22 and line 24 directly into the anode compartment 6
of the electrolytic cell 2. If direct introduction of KOH into the
anode compartment is not desired, the pH of the anolyte which
enters the anode compartment 6 can be maintained at initially high
alkaline pH, e.g., about 13 to 14, and the pH allowed to drop, with
continued electrolysis to a pH of not lower than about 9.7. In this
way, electrolysis of the anolyte through the optimum pH range will
take place during a large portion of the electrolysis.
In the cathode compartment 8 hydrogen ions are converted at the
cathode 16 to hydrogen gas which is evolved from the cathode
compartment 8. The potassium ions (and some hydrogen ions) which
carry the electric current in the electrolytic cell and pass from
the anode compartment 6 through the diaphragm 10 into the cathode
compartment 8 enrich the potassium content of the catholyte. At the
same time the catholyte increases in pH because of the loss of
H.sup..sup.+ions. Electrolysis is continued until the desired
conversion of phosphate ions to peroxydiphosphate ions in anode
chamber 6 has been obtained commensurate with permissible current
efficiencies. When this occurs the anolyte is removed from the
anode compartment 6 through line 26 and is treated to adjust the pH
between 11.8 and 12.6 and preferably between 12 and 12.6. This is
most conveniently done by adding KOH from line 22 and line 22
directly into the withdrawn anolyte solution. The withdrawn anolyte
solution from line 26, whose pH has been adjusted between 11.8 and
12.6, is then placed in the evaporator-crystallizer 30 where a
portion of the water in the anolyte is evaporated. When sufficient
amounts of the water have been evaporated, potassium
peroxydiphosphate crystallizes from the evaporated solution and is
separated from the mother liquor. The potassium peroxydiphophate is
removed from the evaporator-crystallizer 30 through line 32 and
recovered as product, while the mother liquid is separated through
line 34. The catholyte is also removed from cathode compartment 8
after the electrolysis has been completed through line 36. The exit
catholyte can be mixed with the anolyte through line 38 prior to
evaporation and crystallization of the potassium peroxydiphosphate
product, if desired. Alternately, the exit catholyte can be mixed
with the mother liquor from the evaporator-crystallizer 30 by
passing the catholyte through lines 36 and 40 until it mixes with
the mother liquor in line 34. The mixture of catholyte and mother
liquor is sent through line 4 back to the anode compartment 6 as
makeup anolyte. The pH of the anolyte in line 4 can be controlled
by adding KOH through lines 22 and lines 44 to give the desired pH
range, normally between 9.7 and 14.3.Also, provision is made for
the addition of HF through line 46 into the anolyte makeup solution
in line 4 to replace any fluoride ion lost in processing. Fresh
catholyte solution from makeup tank 18 is then passed into the
cathode compartment 8 through line 20 to complete the electrolytic
cell for continued production of potassium peroxydiphosphate.
In FIG. 2, there is shown, in graphic form, the relationship
between current efficiencies and the pH of the anolyte at different
levels of conversion. The uppermost curve illustrates that to
obtain current efficiencies of about 50 percent or above, when the
degree of conversion of phosphate ions to peroxydiphosphate ions is
no more than 20 percent, the pH of the anolyte must be between the
range of about 9.7 to about 14.3. The next curve adjacent the
uppermost curve illustrates that to obtain current efficiencies of
50 percent or above, when the degree of conversion is from 20 to 40
percent, the pH of the anolyte must be between about 10.7 and about
14.2. The remaining curves show the relationship between anolyte pH
and current efficiencies at levels up to about 80 percent
conversion of phosphate ions to peroxydiphosphate ions. As will be
observed from FIG. 2, the maximum current efficiencies, at any
conversion level is obtained when the pH of the anolyte is about
12.5.
FIG. 2 also illustrates that current efficiencies decrease with
increased conversion of the phosphate ions to peroxydiphosphate
ions, regardless of the pH value of the anolyte. This decrease in
current efficiency is due to the use of increasingly larger
portions of the electronic current to electrolyze undersired side
reactions rather than the principal conversion of phosphate ions to
peroxydiphosphate ions.
The following examples are given to illustrate the present
invention and are not deemed to be limited thereof.
EXAMPLE 1
A two-compartment electrolytic cell was made up of whose dimensions
were 10 .times. 12 .times. 20 cm. and which was equipped with a
platinum mesh anode having a surface area of 14 cm..sup.2 and a
coiled, stainless steel (SS304 ), tubular cathode having a surface
area of 340 cm..sup.2 A porous, ceramic plate served as the
diaphragm between the cathode compartment and anode compartment and
had a surface area of 57 cm..sup.2 A tubular glass coil was placed
in the anode compartment and was connected by a hollow rubber
tubing to the stainless steel tube serving as the cathode. Water
was passed through the glass coil in the anode compartment, through
the rubber tubing connection and finally through the steel tubing
cathode in the cathode compartment to maintain the temperature of
the cell constant during electrolysis.
One liter of an anolyte solution was made up containing 2 moles of
phosphoric acid, 2.4 moles of HF and 8.0 moles KOH. The catholyte
solution had a volume of 0.5 liters and contained 0.96 mole
phosphoric acid and 1.19 moles KOH. The catholyte and anolyte were
then added to their respective compartments in the cell and the
solutions electrolyzed with a current of 4.2 amperes. The current
density of 0.3 a./cm..sup.2 at the anode, 0.012 a./cm..sup.2 at the
cathode and 0.075 a./cm..sup.2 at the diaphragm. Electrolysis was
continued until 60 percent of the phosphate present in the anolyte
was converted to peroxydiphosphate. This took 9 hours and 34
minutes. The original anolyte solution which had a pH of about 12
was maintained at this pH by the addition of 0.54 mole of KOH
during the electrolysis. After the electrolysis was complete, the
anolyte was removed from the electrolytic cell and evaporated in a
vacuum-type crystallizer until 0.48 mole (166 g.) of K.sub.4
P.sub.2 O.sub.8 was precipitated and recovered. The mother liquor
recovered after evaporation of the anolyte solution was mixed with
the exit catholyte and sufficient water to make 1 liter; this
mixture was placed in the anode compartment of the electrolytic
cell. Five hundred milliliters of fresh catholyte solution was made
up containing 0.96 mole H.sub.3 PO.sub.4 and 1.49 moles of KOH.
Electrolysis of the cell was then resumed and was carried out as
set forth previously until 60 percent of the phosphate ions present
were converted to peroxydiphosphate ions. This took 7 and 40
minutes. During the electrolysis the pH of the anolyte which was at
about 12 during the commencement of the electrolysis was maintained
at about 12 by the addition of 0.43 mole of KOH into the anolyte as
electrolysis progressed. The anolyte solution, having a pH of from
12 to 12.6, was removed from the cell, partially evaporated, and
166 g. of potassium peroxydiphosphate was precipitated and
recovered. The resulting product contained 93 percent potassium
peroxydiphosphate and 1 percent KF. The product was further
purified by recyrstallization from water, and the resulting
potassium peroxydiphosphate was found, on analysis, to be 99+
percent pure, with no detectable fluoride or chromium content. It
was a dry, white, free-flowing, stable, crystalline product and was
not hygroscopic.
EXAMPLE 2
Four electrolytic cells, each having substantially the same
structure described in example 1, were connected together in series
so that the catholyte and anolyte of each cell flowed into the
cathode and anode chamber of the succeeding electrolytic cells
continuously. An anolyte solution was then made up containing 2.4
moles HF, 2.0 moles H.sub.3 PO.sub.4 and 8.0 moles KOH per liter of
solution. The solution was pumped into the anode chambers of the
four cells in series at a rate of 418 ml./hr. A catholyte was
formulated and passed through the cathode compartments of the cells
in series so that the equivalent of 0.4 mole/hour of H.sub.3
PO.sub.4 and 0.50 mole/hour of KOH were passed through the cells.
The water content of the catholyte was controlled so that the
catholyte solution removed from the cells and the mother liquor
recovered from the evaporator-crystallizer totaled about 418
ml./hr. The anolyte entering the first cell had a pH of about 12.
Potassium hydroxide was added to the third cell of the series at a
rate of about 0.23 mole/hour in order to keep the pH at about 12.
After leaving the anode compartment, the anolyte was placed in an
evaporator-crystallizer and water was evaporated until 80 percent
of the potassium peroxydiphosphate in solution was precipitated and
recovered. The mother liquor recovered from the
evaporator-crystallizer and the catholyte issuing from the last
cell were mixed together to give a total solution of 418 ml./hr.
This was recycled to the anode compartment as recycle anolyte,
continuously. The fluoride content of the recycle anolyte stream
was continuously monitored and adjusted to 2.4 molar by adding HF.
The current flow during electrolysis was 4.2 a. The production rate
of potassium peroxydiphosphate during this portion of the run was
69.5 g./hour.
As the above system was operating in continuous cycle, conditions,
particularly flow rates, were changed until the system reached
equilibrium. At that point the recycle anolyte solution, which is a
mixture of motor liquor from the crystallizer and catholyte from
the cathode chamber, contained the equivalent of 0.12 mole K.sub.4
P.sub. 2 O.sub.8 1.76 moles H.sub.3 PO.sub.4, 2.4 moles KF and 4.93
moles KOH per liter and was recycled at a rate of 522 ml./hour. The
cathode solution circulated the equivalent of 0.50 mole of H.sub.3
PO.sub.4 per hour and 0.78 mole/hour of KOH passed through the
cathode compartments of the cells. Sufficient water was added to
the catholyte so that after mixing the cathode effluent with the
mother liquor coming from the evaporator-crystallizer, the total
flow rate of the resulting recycle anolyte stream was 522 ml./hour.
Under these equilibrium conditions, 80 percent of the
peroxydiphosphate present in the anolyte solution was precipitated
in the evaporator-crystallizer, yielding 86.8 g./hour of K.sub.4
P.sub.2 O.sub.8. When equilibrium was reached, the current
efficiency in the above example was 80 percent at an anode current
density of 0.3 a./cm..sup.2 The conversion of phosphate to
peroxydiphosphate per pass through the anode cells was 70 percent,
and the potassium peroxydiphosphate removal in the crystallizer per
pass was 80 percent.
Pursuant to the requirements of the patent statutes, the principle
of this invention has been explained and exemplified in a manner so
that it can be readily practiced by those skilled in the art, such
exemplification including what is considered to represent the best
embodiment of the invention. However, it should be clearly
understood that, within the scope of the appended claims, the
invention may be practiced by those skilled in the art, and having
the benefit of this disclosure otherwise than as specifically
described and exemplified herein.
* * * * *