Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to chlorine dioxide pro- ;
duction. More particularly, this invention relates to improve- ;
ments in a method for treating effluent slurries emanating
from single vessel process chlorine dioxide generators. -
This application is a division of Canadian Patent
Application S.N. 278,407, filed May 13, 1977.
Inasmuch as chloride dioxide is of considerab
commercial interest and importance in the areas of pulp
bleaching, water purification, fat bleaching, removal of
phenols from industrial wastes, textile bleaching and the
like, it is very desirable to provide processes by which
the chlorine dioxide can be economically produced and by
which the type of effluents produced thereby can be ~ `~
controlled.
One of the means of producing chlorine dioxide
is by the reaction of alkali metal chlorate, a chloride
and a mineral acid such as sulfuric acid, hydrochloric
acid and mixtures of sulfuric acid with phosphoric and/or
hydrochloric acids. Such reactions as occur are exemplified
by the following:
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(1) NaC103 -~ NaCl + H2S04
C102 + 0.5C12 -~ Na2S4 + ~2
(2) NaC103 + 5NaCl + H2S04 >
+ 3C12 + 3Na2S04 + 3H20
(lA) NaC103 + HCl + 0.5H2So4 > C102+0.5cl2+
0~5Na2S04+H20
(2A) NaC103 ~ 5HC1+0.5H2S04 ~ 3Cl2+o~5Na2so4+3H2o
Such reactions are employed commercially, with
the reactants continuously fed into a reaction vessel and
the chlorine and chlorine dioxide produced therein con-
tinuously removed from the reaction vessel.
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Reactions (l) and (lA), which are favored inasmuch as
they produce primarily chlorine dioxide, result from the use of .~ ~
about equimolar amounts of chlorate and chloride. -
Similarly, a methanol or sulEur dioxide reducing
agent can be used wherein it reacts with the by-product chlorine :
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to furnish the required H~SO4 and/or HCl. :~
Single vessel processes for producing Chlorine dioxide .
are set forth in U. S. Patent Number 3,563,702 and British Patent
1,347,740, wherein an alkali metal chlorate, an alkali metal .
chloride and mineral acid solutions are continuous~y fed to a
.
; single vessel generator-evaporator-crystallizer in proportions ~;
sufficient to generate chlorine dioxide and chlorine, at a tempera-
ture of from about 50 to about 100 degrees centi~rade, and an . ~
acidity of from about 2 to higher than about 12 normal, with or ~.
without a catalyst, removing water by vacuum-induced evaporation ~.
at about 100-400 millimeters of mercury absolute, with concurrent ; ~:
withdrawal of chlorine dioxide and chlorine, crystallizing the
salt of the ~ineral acid wlthin the generator and withdrawing the
. crystals from the vessel.
In those reaction systems wherein the acid normality
is maintained between~about 2 and 5, the reaction may be conducted
in the presence of a relatively small.amount of a catalyst such
as those selected from the group consisting of vanadium pentoxide, :
silver ions, manganese ions, dichromate ions and arsenic ions.
As the reaction occurs within the generator, in -
reactions where sulfuric acid is employed as a mineral acid
reactant, crystals of sodium sulfate and sodium acid sulfate in
amounts and presence dependent generally upon the acid concen-
tration used, are crystallized out and settle to the bottom
of the generator from whence they are withdrawn in the form
of a slurry.
In addition to the use of sulfuric acid, hydrochloric
acid can also be used as the mineral acid reactant, in which
instance the crystals removed from the generator are alkali -
metal chloride crystals.- However, the hydrochloric acid pro-
cess produces alkali metal chloride as a by-product, which
product is often less desirable than alkali metal sulfate.
Sodium sulfate is a valuable by-product, useful in kraft ;
pulping operations, as is the chlorine dioxide. Therefore,
systems producing chlorine dioxide and sodium sulfate are
particularly useful inasmuch as on-site co ordination can be ~ ~;
effected with pulping operations, utilizing both the primary ~ `
chlorine dioxide product and the recovered sodium sulfate in ~ ;
the pulping process, particularly in kraft mill operations.
In some instances, however, the requirement for sodium
sulfate is greatly reduced or obviated. In certain types of
pulping processes, sodium sulfate is not required. In certain
kraft mill operations, the requirements for sodium sulfate may
be reduced or varied, and the disposal of excess salt produces,
problems, in view of environmental protection standaxds presently
in force. WhiLe the requirement for reduced quantities of sodium ~;
sulfate may vary, the requirement for the chlorine dioxide
remains.
In such instances where reduced quantities or no sodium
sulfate is required, the single vessel process can be converted
to utilize hydrochloric acid as the mineral acid reactant, in
which instance the by-product is sodium chloride. However, ; ~;
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such systems may not be as efficient as the systems employing
sulfuric acid. ~urther, only sodium chloride is produced and
in those instances where varying quantities of sodium sulfate
are required, to generate the required amount of sodium sul~
fate would necessitate the switching back and forth from a
catalyzed sulfuric acid system to a catalyzed hydrochloric
acid system, with all the problems attendant thereto.
It is an object of the present invention to provide
a process wherein the single vessel process may be operated
most efficiently to produce chlorine dioxide and the recovery
of by-product salt regulated to produce the salt desired in
quantities desired, without the necessity of changing conditions
in the reactor.
It is a further obiect of the present invention to
provide an improved process for treatment of the crystal slurry
produced in single vessel reactors to return chlorate and
sulfate values contained in the solids containing effluent
to the generator for further reaction, to remove sodium by
converting the sodium values to a desirable salt or acid salt,
2 as required and to add chloride and/or acid values for recycle
to the generator.
It is an object of the prese.nt invention to provide
an improved process for the treatment o the crystal slurry
produced in single vessel chlorine dioxide production, effi-
ciently separate the crystalline constituents therefrom and
to recover substantially all of the chlorate, chloride and/or
sulfuric acid values for return to the generator for further
reaction~
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In accordance with the invention there is provided
In a process for continuously generating a mixture containing
chlorine dioxide, chlorine and a neutral alkali metal salt
wherein a) an alkali metal chlorate, a reducing agent selected
from the group consisting of methanol and sulfur dioxide, and
a strong mineral acid selected from the group consisting of
sulfuric acid, hydrochloric acid and mixtures of sulfuric aci.d
and members of the group consisting of hydrochloric acid and
phosphoric acid, are continuously reacted in a single vessel
generator-evaporator-crystallizer in proportions to generate
chlorine di~xide and chlorine, b) the temperature is maintained
from about 50 to about lOO~C., c) the acidity of the reaction
solution is maintained within the range of about 2 to about 12
normal; d~ chlorine dioxide and chlorine produced by said
reaction are withdrawn from the reactor, e) water is removed
and neutral alkali metal salt of said mineral acid is
crystallized in the form of an aqueous slurry containing
minor amounts of chlorate, chloride and acid values, the
improvement which comprises: continuously passing the slurry :
containing neutral alkali metal salt crystals into the top of
a separatory column, in a downward flow, countercurrently
passing a stream of water continuously upwardly through said
column at a rate sufficient to effect washing of the down- ~.
wardly flowing crystals whereby chlorate, chloride and acid
values recovered therefrom are continuously and substantially
completely returned to the reaction of (a) aforesaid, and
continuously removing an aqueous slurry of substantially pure
neutral alkali metal salt crystals from the bottom of said
separatory column.
In the process a single vessel process slurry, ;~
suitably utilizing sulfuric acid or mixtures thereof with
hydrochloric or phosphoric acids containing alkali metal
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sulfate crystals and chlorate values, is introduced into the
top of one or more metathesis columns. The alkali metal sul-
fate may be converted to a more desirable salt by reaction
with a metathesis solution such as an acid such as ~Cl or
oxalic acid to produce a salt slurry together with sulfuric
acid or its acid sulfates. To produce the necessary salt
slurry, the acid utilized must be sufficiently dissociated in
solution that the solub1lity in that solution of the alkali
metal salt is exceeded.
For example, aqueous hydrochloric acid having a
concentration of from about 10 to about 37 percent by weight
is added continuously or intermittently via an inlet near the
bottom of the ~etathesis column in countercurrent flow to the
downward flow of the slurry, with the crystals contained in
the slurry reacting with hydrochloric acid to produce sodium
chloride, suLfuric acid and acid sulfates, the regenerated
sulfuric acid ~tilizing the acid sulfates and the chlorate
values are washed up the column to the generator, and the
sodium chloride may be removed as an aqueous slurry via an
outlet located near the bottom of the metathesis column.
The use of such process haq many advantages. The
process permits of the employment of the more highly
efficient sulfuric acid reaction in the single vessel
generator-evaporator-crystallizer without changing to the
less efficient hydrochloric acid reaction process, in those
instances where reduced quantities of by-product sodium
sulfate are required. In
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those instances where sodium sulfate is desired in increased
or maximum quantities process permits of such increase or
maximizing by simply reducing or periodically replacing the
flow of hydrochloric acid into the bottom of the metathesis
colu~n by a flow of wash water. In such instances where
ma~imum production of sodium sulfate is to be realized, the
upward flowing wash water functions to return essentially all
chloride, chlorate and sulfuric acid values continuously to
the generator, requiring a relatively low energy input into the
L0 system. Additionally, under these conditions, and where the
! generator is operated under high acid concentrations, on the
order of about 10-11 normal, the water wash allows for the
recovery of the sodium sulfate as neutral sodium sulfate as
opposed to the undesirable acid sodium sulfates recovered by
L5 slurry filtration techniques employed in the past art.
In these instancés where HCl is the mineral acid of
the chlorine dioxide generating process, the process permits
of utiliæing an upward flow of wash water through the metathesis
column wherein the downwardly flowing slurry is continuously
wa,shed by counter-currently upwardly flowing wash water as the
alkali metal chloride crystals are conveniently separated for ;`
removal near the bottom of the column, with essentially all of
the chloride9 chlorate and acid values being returned in a
continuous flow to the generator.
The rates at which the hydrochloric acid or water are
fed into the bottom of the metathesis colu~n are, of courseJ
dependent upon the desired conversion or washing to be effected.
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In those instances where total conversion of sodium sulfate to
sodium chloride is to be effected, the amount of hydrochloric
acid fed continuously into the metathesis column will be at
least twice that of the sodium sulfate being produced in the
reactor, on a molar basis. In these instances where the
sodium sulfate to be removed is to be reduced by predetermined
amounts, the flow adjustment of hydrochloric acid into the
met~thesis column is made to effectively permit of the desired
amount of conversion, with the unconverted sodium sulfate
recovered from the bottom of the column. Similarly, oxalic
acid provides a sodium oxalate slurry which is subject to the
aforesaid description.
Alternately and concurrently, where the generating
process utilizing sulfuric acid or mixtures with hydrochloric or
phosphoric acid, chloride values may be supplied to the genera-
tor by utilizing an aqueous solution of an alkali metal or
alkali earth metal chloride in the metathesis column. For
example, potassium chloride may be utilizing wherein a potassium
sulfate slurry and sodium chloride solution is formed. The
sodium chloride will be washed to the generator and a potassium
sulfate slurry recovered. Similarly, recovery of sulfates of the
other alkali earth metals or alkali metals may be effected with
input of chloride to the generator.
It should be understood however if an acid and an
alkali metal or alkali earth metal salt or mixture thereof are
used concurrently, a mixed salt slurry will be created which may
provide further problems in separation. Such may be avoided by
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utilizing multiple metathesis and/or separatory columns on a
single generator either in parallel or cascaded to gain purer
products.
The slurry taken from the metathesis column may be
washed by elutration in a separate, separator column or
elutriated by integration in the bottom of the metathesis column
in which instance the metathesis solution intake would be
toward the middle of the column.
The size of the metathesis and separatory column may ~ ~`
be on the order of 6-24" in diameter and 10-15' or longer,
dependent primarily on the size of the generator employed.
Figures 1, 2 and 3 are illustrative of metathesis
and separatory columns which are advantageously employed in
the process of the present invention. Figures 1, 2 and 3 are
schematic vertical elevations.
Figure 1 is illustrative of a combined metathesis ~ ~`
and elutriation column which is advantageously employed in the
process of the present invention. The apparatus is a column or -~
tower 1 made of any suitable construction materials, such as
for examplej titanium, plastics, ceramics, or the like. The
column is preferably, although not necessarily, substantially
cylindrical having an inlet means 2 at the upper portion thereof
for the introduction of crystal slurry from the single vessel
reactor ~not shown). The middle portion of the column is pro-
vided with a metathesis solution inlet 5. The lower portion of
the column is provided with water inlet 3 and a washed crystal
takeoff means 4. Column 1 is divided into multiple treatment
zones 6, 8, 10, etc. by plates incorporating at least one and
preferably multiple downwardly tapering funnel-shaped configura-
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tions 7 having openings 9 at the apex for discharging slurry
downward into turbulent metathesis zones 6 and 8 and washing
zones 10 etc. the low of slurxy directed downwardly from
plate to plate, and succeeding turbulent metathesis and
washing zones.
Each plate is provided with multiple aperatures 11
located at or near the junction of the uppermost portion of
the plate and the funnel shaped configuration. As the crystal
containing slurry moves downward onto the plate and through
the opening 9, the upcoming flow of metathesis solution is
diverted in part through openings 11, setting up a circulatory
flow around and in the funnel shaped configuration, creating
turbulence in the zones immediately below the openings 9,
causing the sodium sulfate to undergo metathesis reaction with
the metathesis solution, the similar activity of the water
in the lower section continuously freeing the salt from
chlorate, chloride and sulfuric acid values and continuously
permitting the downward pàssage of salt for removal via outlet
4 and passage to the generator. Control of the downward flow
2 of the crystals and the extent of the turbulence is conveniently
effected by adjusting the relative sizes of the openings 9 and
11 .
It should be understood that the metathesis inlet
may also be used as a wash water inlet when washing is desired
at that stage.
Zone 12 of the apparatus is a relatively non-turbulent
zone wherein the crystals settle by gravity and are not carried
by rising liquid to the washing or metathesis zone above.
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a
The column may be located immediately below the generator
in which instance the crystal slurry moves by gxavity flow from
the generator (not shown) into the column via inlet 2. Chlorate,
chloride and acid values are continuously returned directly to `
the generator in that portion of the wash water directed by up-
ward flow into the generator. In those instances where space and
other factors dictate the placement of the column at a site ad-
jacent to or remote from the generator/ the crystal slurry is
pumped by suitable pumping means ~not shown) to the column via
inlet 2.
,
The total number o~ plates situated in the column may be
varied, depending generally upon the size of the generator, amounts
of crystal slurry to be handled in a given time period, space con-
siderations. &enerally, in operations where standard generators
are used with a capacity of about 600~ gallons, columns of about
10-15 feet in length, 2-3 feed in diameter having 8-12 plates
situated approximately one foot apa~t are suitable.
Figure 2 represents a metathesls column useful in the
process of the present invention. The column, as with the column
~u of Figure 1, may be situated either adjacent to or immediately be-
low the single vessel generator. In those instances where the
column is located immediately below the single vessel generator,
crystal slurry is continuously moved from the botton of the gen-
erator (not shown) to the top of the column 1 at zone 4. Meta-
thesis solution is continuously admitted to the column via 2 and
flows upwardly through the column, continuously reacting the down
flowing crystals, and continuously returning the chlorate, chloride
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and acid values removed therefrom to the generator ~rom the
top of the column. The crystals move downward into a crystal
collection zone 5 from whence they are removed via outlet 3.
Outlet 3, shown as located at a point on the column 1 above
the metathesis solution inlet 2 may be varied in position
near the botton of the column.
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As with the column in Figure 1, this column can also
be situated adjacent the single vessel reactor, in which instance
pumping means are provided to continuously feed th~ crystal
slurry from the generator to the column, and the spent metathesis
solution containing the removed chlorate, chloride and acid ~ ;
values continuously removed from the top of the column and
returned continuously to the generator via suitable pumping
and inlet means.
Figure 3 is an embodiment of a metathesis column alone
of Figure 1, wherein elutriation is achieved by a separate
vessel.
Though the process of the present invention is parti-
cularly suitable to a single vessel chlorine dioxide generating
process but is not limited to such and may be readily adapted to `
lS a chlorine dioxide generating system wherein the spent generator
slurry 7 S crystallized in a separate vessel by providing trans- -
port of the recovered chlorate, chloride and acid values to the
generator.
Utilizing the process of the present invention, it was
found that the amount of chlorate, chloride and acid values
remaining in the recovered salt were approximately 0.2 to about
0.25 that remained in the salt when attempting standard separatory
techniques with filters. Additionally, utilizing the process
of the present invention, where the single vessel generator is
operated utilizing sulfuric acid and mixtures thereof at high
acid normalities, the undesirable acid sulfates produced are
converted into neutral sodium sulfate during the washing, not
possible when utilizing filter or centrifuge-type separatory
procedures.
~ hen utilizing sulfuric acid and mixtures thereof
in the generator, the rates of flow of the sulfate slurry
downward and the flow of washing and/or metathesis solution up- -
ward are adjusted so as to provide maximum washing and/or
conversion efficiency without substantially increasing the
steam requirements for the vacuum evaporation in the generator.
Generally, the washing and conversion reaction requires
the adjustments of flow rates as to provide for a retention time
of from about 10 to about 60 minutes, preferably from about
15 to about 40 minutes.
The following examples serve to illustrate the present
invention.
EXAMPLE 1
15Utilizing the metathesis column shown in Figure 1
having 11 ledges, the dilute hydrochloric acid inlet was located
between the fifth and sixth ledges from the bottom with a water
inlet located at the bottom of the column. The reaction pro-
ducing chlorine dioxide in the single vessel generator utilized
2~ as a mineral acid, sulfuric acid was adjusted to produce sodium
sulfate at the rate of 57.6 pounds per hour. Aqueous hydro-
chloric acid, as 36 percent acid, was fed into the metathesiscolumn at the rate of 28 pounds per hour with 50 pounds per hour
. of hot water added to the bottom of the column to wash the
sodium chloride produced in the metathesis column. Sulfuric acid,
in the amount of 23.5 pounds per hour was recovered from the top
of the column. Analysis of the salt slurry recovered from the
bottom of the column indicatel 28.06 pounds per hour recovery of
sodium chloride, 43.82 pounds per hour takeoff of water 0.08 pound
per hour hydrochloric acid and 0.02 pounds per hour sodium sulfate,
indicative of essentially complete conversion of the sodium
sulfate to sodium chloride in the column.
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EXAMPLE II
In a manner similar to Example 1, aqueous potassium
chloride is introduced into a metathesis column at an inlet
located between ledges in the appropriate middle o~ the column.
The reaction producing chlorine dioxide is adjusted to produce
~ sodium sulfate at the rate of 71 pounds per hour. Aqueous
; potassium chloride, a 30% solution, is fed into the metathesis
column at the rate of 250 pounds per hour with 200 pounds per
hour of hot water added to the bottom of the column to wash
the potassium sulfate produced in the metathesis column.
Sodium chloride, in the amount of 58 pounds per hour is recovered
for the top of the column.
EXAMPLE III
Utilizing the column and generating process of
Example 1, the generator was adjusted to produce sodium sulfate
at the rate of 57.6 pounds per hour. Water was added to the
bottom of the column at the rate of 50 pounds per hour
counter-currently upwardly washing the downwardly flowing sodium
sulfate. Analysis of the salt slurry recovered from the bottom
of the column indicates an essentially neutral sulfate slurry
has been produced.
EXAMPLE IV !l
The process of Example III was operated utilizing
hydrochloric acid as the mineral acid of the generator, wherein
approximately 50 pounds per hour of sodium chloride salt slurry
was produced. Water was added to the bottom of the column at
the rate of 45 pounds per hour counter-currently upwardly washing ~-
the downwardly flowing sodium chloride. Analysis of the salt
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slurry removed from the bottom of the column indicates an
essentially neutral sodium chloride slurry had been produced.
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