CA1115026A - Production of chlorine dioxide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process is provided for continuously generating a mixture containing chlorine dioxide, chlorine and a neutral alkali metal salt wherein 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, hydro-chloric acid and mixtures of sulfuric acid 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 dioxide and chlorine; the temperature is maintained from about 50 to about 100°C.; the acidity of the reaction solution is maintainedwithinthe range of about 2 to about 12 normal; chlorine dioxide and chlorine produced by the reaction are withdrawn from the reactor; water is removed and neutral alkali metal salt of said mineral acid is crystallized in the form of an aqueous slurry containing minor amounts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 resides in continuously passing the slurry con-taining 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 the column at a rate sufficient to effect washing of the down-wardly flowing crystals whereby chlorate, chlorine 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.

Description

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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~

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 ~ 5 ~ ..
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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 - 5a -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 ~ 4~2~

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-- . . .

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.

-lOa-:,:, ., 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.

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 ''.' ~''' ~s~

slurry removed from the bottom of the column indicates an essentially neutral sodium chloride slurry had been produced.

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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

Claim 1 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 acid and member of the group consisting of hydro-chloric acid and phosphoric acid, are continuously reacted in a single vessel generator-evaporator-crystallizer in propor-tions to generate chlorine dioxide and chlorine;
b) the temperature is maintained from about 50 to about 100°C.
c) the acidity of the reaction solution is main-tained 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 con-tinuously upwardly through said column at a rate sufficient to effect washing of the downwardly 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.

Claim 2 The process of Claim 1 wherein the mineral acid is HCl.
Claim 3 The process of Claim 1 wherein sulfur dioxide is utilized as a reducing agent and is introduced to the process in the separatory column.
Claim 4 The process as defined by Claim 1 wherein the temperature of the wash water is maintained at from about 30 to about 70°C.
Claim 5 The process as defined by Claim 1 wherein the wash water is continuously fed into the separatory column at a rate sufficient to provide from about 0.2 to about 4 pounds of water per each pound of alkali metal salt crystals produced in said generator-evaporator-crystallizer.