CA2061645A1 - Chlorine-free chlorine dioxide - Google Patents
Chlorine-free chlorine dioxideInfo
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- CA2061645A1 CA2061645A1 CA 2061645 CA2061645A CA2061645A1 CA 2061645 A1 CA2061645 A1 CA 2061645A1 CA 2061645 CA2061645 CA 2061645 CA 2061645 A CA2061645 A CA 2061645A CA 2061645 A1 CA2061645 A1 CA 2061645A1
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Abstract
Abstract of the Disclosure Process for the production of chlorine-free chlorine dioxide obtained by the reduction of sodium chlorate with acid. Chlorine gas by-product in the gaseous mixture obtained from the sodium chlorate generator is removed by selective absorption of chlorine dioxide into a selective absorbent medium, particularly sulphuric acid. The majority of the chlorine by-product remains in the gaseous mixture and collected for beneficial use, if desired. Purified chlorine dioxide is obtained from the resultant chlorine dioxide selective absorbent medium. The resultant chlorine dioxide may be further absorbed into an aqueous solvent and any residual chlorine therein removed by oxidation. The chlorine-free chlorine dioxide product is of value as a wood pulp bleaching agent providing reduced amounts of organo chloro compounds, such as dioxins.
Description
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I
Chlorine-Pree Chlorine Dio~ide Field of the Invention . . _ This invention relates to the preparation of substantially chlorine-free chlorine dioxide, apparatus therefor, and particularly to the preparation of substantially chlorine-free chlorine dioxide for use in pulp bleaching~
Background of the Invention Chlorine dioxide is extensively used in aqueous solution as a reagent for the bleaching of fibrous material, particularly wood pulp. Serious environmental concerns now exist in the bleaching of wood pulps whenever chlorinated organic compounds are produced, even in relatively small amounts, by the action of chlorine per se or chlorine when usually present in chlorine dioxide bleaching agent mixtures, upon the wood fiber. It is, therefore, becoming increasingly necessary that chlorine dioxide when used as a pulp bleaching agent be substantially chlorine contaminant free.
Chlorine dioxide is produced on a commercial scale by the acid reduction of a chlorate anion in aqueous solution according to the following general equation, viz:
C103 + 2H+ + Cl -> C102 + 1/2C12 + H20 2 2 0 ~
Typically, the reaction is achieved by the direct reduction of chlorate by hydrochloric acid as practised in, for example, the so-called "Integrated Chlorine Dioxide Process", viz:
NaC103 + 2HCl -> C102 + 1/2C12 + H20 + NaCl or by the so called R2 and related R series processes using sulphuric acid, viz:
NaC103 + NaCl + H2S04 C102 + 1/2C12 + ~2 + Na2S4 The integrated chlorine dioxide process utilizes 3 main process systems, namely, (a) a chlorate electrolysis unit, wherein sodium chloride is electrolized to produce sodium chlorate according to the following chemical reaction:
6e NaCl + 3H20 --~ NaC103 + 3~2~
(b) a chlorine dioxide generator unit, using hydrochloric acid as the chlorate-reducing agent as described hereinabove; and (c) a hydrochloric acid producing unit wherein the resultant hydrogen gas from the chlorate electrolysis unit is burned in chlorine to produce hydrochloric acid.
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The integrated process produces chlorine dioxide from chlorine, water, and electricity. Such a process, typically, produces 8-10 g/L chlorine dioxide solution strength containing 0.9-1.8 g/L chlorine.
Methods of producing chlorine dioxide from chlorate systems having minimal or no amounts of chlorine are known.
United States patent No. 2,881,052, discloses reduction of sodium chlorate by an acid in the presence of methanol according to the following equation, viz:
3 2H2S4 + CH30H 3 2C102 + 2NaHS04 + HCHO + 2H20 In an alternative process for the reduction of sodium chlorate by methanol in sulphuric acid, known in the industry as the non-integrated, methanol base chlorine dioxide process, sodium sesqui-sulfate is produced as a by-product according to the following equation:
18NaC103 + 4CH30H + 1282S04 ~18C102 + 6Na3H(S04)3 + C2 + 3HCOOH + 14H20 The gas leaving the chlorine dioxide generator is a mixture of chlorine dioxide, water vapour and trace amounts of chlorine. The gaseous mixture is, optionally, 4 2 ~ 61 ~ ~ ~
sent to a contact cooler where the water vapour is condensed and the resultant gaseous mixture passed into a packed absorption tower where chilled water is forced into intimate contact with the gas. The resultant chlorine dioxide solution is pumped to storage for subsequent use in a pulp bleaching process. Such a typical ~ethanol-reductant system provides a chlorine dioxide solution strength of 10 g/L and a chlorine concentration of 0.1 g/L
in 10 g/L chlorine dioxide solution.
British patent No. 687099 discloses reduction of sodium chlorate by nitrogen dioxide to produce chlorine dioxide according to the following equation, viz:
NaC103 + N02- ~ NaN03 + C102 U.S. Patent No. 2,866,682, discloses a related process using sodium nitrate according to the following equation, v i z :
3 NaNO2 + H2SO4 ~ 2ClO2 + Na2SO4 + NaNO3 + H2O
Alternative processes are disclosed in U.S.
Patent No. 2,866,683 wherein sodium bisulphite is used in conjunction with sulphuric acid according to the equation, viz:
2 ~
4NaC103 + 2NaHS03 + H2S04 >4C102 + 3Na2S 4 2 and the related process using sulphur dioxide, viz:
2NaC103 + H2S04 + S2 2C102 ~ 2NaH 4 2NaC103 + S02 >2Clo2 + Na2S04 French patent No. 991614 discloses the reduction of a chlorate by manganic salts and other oxidating metal salts.
It is understood by those skilled in the art that there are significant deleterious side reactions, often resulting in significant inefficiencies, in using reducing agents other than chloride for the production of chlorine dioxide from a chlorate solution for the purpose of bleaching wood pulp. However, the process of generating chlorine dioxide from chlorate using chloride as a reductant, which process is alternative to those outlined hereinabove, suffers from the increasingly disadvantageous side reaction of producing chlorine by-product.
When chloride is used as a reductant, the yield of chlorine dioxide is reduced and chlorine is produced by the following overall reaction, viz:
.. . .
0~ 3 NaC103 + 5HCl ,3C12 + 3H20 The desired overall reaction according to the equation, viz:
5NaC103 + 6HC1 6C102 + 5NaCl + 3H20 is of insignificant application.
Where a non-fully oxidised sulphur compound is used as a reductant, inefficiencies resulting in total reduction to chloride occur, for example, by the following equation:
2NaC103 + 6S02 + 6H20 - >2NaHS04 + 4H2S04 + 2HCl In this particular case the extent of the inefficiency is heightened as it reflects the effective reduction of the co-produced chlorine, which could otherwise be reduced with hydrogen and recycled in the generation of chlorine dioxide in the "Integrated Chlorine Dioxide Process". Chlorine is, however, also produced when sodium chlorate is reduced with reducing agents other than chloride; the extent of which is a function of the reaction conditions, e.g.
"` 7 2 0 ~ 5 2NaC103 + 5SO2 + 4H2O -> 2NaHSO4 + 3H2So4 + C12 In this case there is some chlorine production though reduction to chloride also occurs, viz:
C12 + S2 + 2H2 -~ 2HCl + H2SO4 Similarily, use of other reducing agents, i.e.
methanol, as disclosed in U.S. Patent No. 2,881,052, does not, in itself, prevent the production of some chlorine or reduction of chlorate to chloride.
Accordingly, while the use of chloride as a reductant is the preferred manufacturing process at present, the presence of by-product chlorine is causing reconsideration of this process as an ongoing viable commercial process for the production of chlorine dioxide for the bleaching of wood pulp.
Methods of removing chlorine from chlorine dioxide are known but, to-date, do not offer practical manufacturing processes.
U.S. Patent No. 3,854,900, discloses a means of separating chlorine dioxide from chlorine using a twin absorption column. Unfortunately, this process leaves the production of significantly dilute chlorine dioxide solutions.
8 2 ~ 5 Japanese patent No. 4466(1956), discloses the selective removal of chlorine dioxide from chlorine contaminated chlorine dioxide gaseous mixtures by passing the mixed gases obtained from the production of sodium chlorate over metals such as iron or aluminum. The chlorine contaminant forms the respective metal chloride while the chlorine dioxide gas passes through the system.
U.S. Patent No. 2,481,241, teaches the selective reduction of chlorine by sulphur dioxide, Tappi 48:110 (1965) discloses the selective removal of chlorine by means of an alkali and Japanese patent No. 2663(1956) discloses the oxidation of chlorite with chlorine for selective removal.
All of the above chloride reductant processes cause significant inefficiencies by either loss of chlorine to the system, production of weak chlorine dioxide solutions, chlorine dioxide reduction, the formation of chlocites or for the need for additional chemical reagents.
Accordingly, it can be seen that there is an increased demand for a manufacturing process for producing chlorine-free chlorine dioxide for use as a bleaching reagent which suffers from none of the above disadvantages.
Summary of the Invention 9 2 ~
It is a primary object of the present invention to provide chlorine dioxide substantially chlorine-free and acceptable for use in a pulp bleaching process.
It is a further object of the present invention to provide a continuous substantially-closed chlorine dioxide purification process that provides for the recovery of the chlorine contaminant for subsequent use.
In a yet further object, the invention provides a pulp bleaching process which uses substantially chlorine-free chlorine dioxide wherein substantially all of the chlorine contaminant is removed by selective desorption rather than by reduction to chloride.
Applicant has surprisingly discovered that chlorine dioxide may be beneficially separated from chlorine in a gaseous mixture as is typically produced in a chlorine dioxide generator from sodium chlorate and acid, preferably hydrochloric acid, or methanol/acid, preferably sulphuric acid, mixtures as practised in the art.
Althoug~. it is known that chlorine and chlorine dioxide are soluble to some degree in suitable chlorine dioxide absorbent media, applicant has discovered that the relative degrees of solubility of these respective gases may be sufficiently different and dependent upon the nature of the absorbent medium that practical separations of chlorine from chlorine dioxide can be achieved.
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Accordingly, the invention provides a process for the production of a substantially chlorine-free aqueous solution of chlorine dioxide which process comprises generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by a chlorine dioxide generator; feeding said gaseous mixture to an absorption tower in counter-current flow to a chlorine-dioxide selective absorbent liquid to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide solution and a resultant chlorine contaminated gas stream; feeding said chlorine-dioxide solution to a stripping tower in counter current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution; feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce a chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a weak gas stream.
In yet a further aspect, the invention provides a process as hereinabove defined for use in the bleaching of a fibrous material, particularly wood pulp, in a fibrous material bleaching system, further comprising feeding said substantially chlorine-free aqueous solution of chlorine dioxide to said system and bleaching said fibrous material with said chlorine dioxide.
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Preferably, the process may be operated as a continuous process.
More preferably, the chlorine-dioxide selective absorbent liquid is selected from aqueous sulphuric acid and acetic acid.
We have surprisingly discovered that although dilute sulphuric acid having a strength of less than 15%
H2SO4 can be used in the process according to the invention to produce sufficiently low levels of chlorine in the substantially chlorine-free aqueous chlorine-dioxide, that most advantageously reduced levels of chlorine-contaminant can be obtained with stronger sulphuric acid strengths. Accordingly, acid strengths of > 15% H2SO4 is preferred, 25-50% H2S04 more preferred, and 50-95% yet more preferred. It will be appreciated by the man skilled in the art that the sulphuric acid strength may be chosen so as to not only provide the substantially chlorine-free chlorine dioxide solution by the process according to the invention, but be selected, also, as to maintain a water balance in the process and avoid the need for sulphuric acid "make-up" during the operation of the process. Water vapour enters the system in the chlorine-contaminated gaseous mixture comprising chlorine dioxide, chlorine and water vapour from the chlorine dioxide generator.
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Thus, in one embodiment, the sulphuric acid strength and operating temperature thereof in the chlorine dioxide absorption tower is selected such that the water in the form of vapour carried into the system from the chlorine dioxide generating unit balances the water carried out of the system with the resultant chlorine contaminated gas stream.
In yet a still further aspect, the invention provides apparatus for the production of substantially chlorine-free chlorine dioxide comprising: a chlorine dioxide generator for producing a chlorine-contaminated gaseous mixture comprising chlorine dioxide and said chlorine-contaminant; an absorption tower; a stripping tower; a re-absorption tower; means for feeding said gaseous mixture to said absorption tower; means for feeding a chlorine-dioxide selective absorbent liquid to said absorption tower in counter-current flow to said gaseous mixture to produce a first chlorine-dioxide solution; means for feeding said first chlorine-dioxide solution to said stripping tower; means for feeding a stripping gas to said stripping tower in counter-current flow to said first chlorine-dioxide solution to produce a stripped gaseous mixture comprising chlorine dioxide and a depleted first chlorine-dioxide solution; means for feeding said stripped gaseous mixture to said re-:
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absorption tower; means for feeding an aqueous solution tosaid re-absorption tower in counter current flow to said stripped gaseous mixture to produce an aqueous second chlorine dioxide solution and a weak gas stream; and collecting said second chlorine-dioxide solution constituting said substantially chlorine-free chlorine dioxide.
Preferably, the apparatus as hereinabove defined further comprises a fibrous material bleaching unit, particularly a wood pulp bleaching unit and means for feeding said second chlorine-dioxide solution to said bleaching unit.
In a preferred embodiment of the invention the absorbent medium is an aqueous solution of sulphuric acid.
By the term "substantially chlorine-free" is meant levels of chlorine in admixture with chlorine dioxide, either as a gaseous mixture or as a solution, lower than the ratio of chlorine:chlorine dioxide of 1:20 on a W/W basis.
By the term "chlorine dioxide selective absorbent liquid" is meant a liquid that selectively absorbs chlorine dioxide therein to a greater degree than chlorine under the absorbent conditions practised of temperature, gaseous mixture flow rates, relative concentrations of chlorine dioxide and chlorine, and the like, such that the ratio of chlorine:chlorine dioxide in the liquid is lower than in the gaseous mixture from which the chlorine dioxide is removed.
We have found that chlorine dioxide and chlorine in the ratio typically found in the gaseous mixture generated from a chlorine dioxide generator using sodium chlorate and hydrochloric acid, when fed to a selective absorbent, for example, sulphuric acid, water, or acetic acid, produces a chlorine dioxide solution containing substantially all of the chlorine dioxide originally contained in the gaseous mixture, but with significantly reduced amounts of chlorine. The remaining chlorine remains in the gaseous stream. The chlorine dioxide can be liberated from the absorbent, preferably by a suitable gas purge, such as a warm nitrogen, helium or, most preferably, an air stream; with re-absorption of the chlorine dioxide, if required, in a second liquid, preferably, water.
In those processes where further reduced levels of chlorine contaminant in the aqueous chlorine dioxide bleaching solution are desired, which chlorine levels are produced by operation of the process according to the invention at desired sulphuric acid strengths and temperatures as hereinbefore described, the chlorine contaminant level may be further reduced by treatment of 2 ~ ~ ~ & ~ ~
the chilled aqueous chlorine-dioxide solution with a reducing agent to chloride anion. Such reducing agent may be selected from the group consisting of hydrogen peroxide and a non-fully oxidised sulphur species selected from sulphur dioxide and metal salts of sulphite, bisulphite, dithionite and persulphate.
We have found that the process and apparatus according to the invention is a highly desirable addition to the typical Integrated Chlorine Dioxide Process having relatively high levels of contaminant chlorine for the production of chlorine contaminant-free chlorine dioxide solution. However, the process and apparatus according to the invention is also of value in providing an improved chlorine dioxide solution as generated from non-integrated chlorine dioxide generating systems.
Other and further objects and advantages of this invention will become apparent upon reading the following specification taken in conjunction with the accompanying drawings.
Brief Description of the Drawings In order that the invention may be better understood a preferred embodiment will now be described by way of example, only, with reference to the accompanying drawing wherein the Figure is a diagramatic sketch of a preferred process and apparatus according to the invention.
Description of a Preferred Embodiment The apparatus and process shown in the Figure generally as 10 has a chlorine dioxide generator 12, an absorption tower 14, a stripping tower 16, a re-absorption tower 18 and a heat exchanger 20.
Generator 12 is of the type well-known in the art used to produce chlorine dioxide for use as a bleaching agent of a fibrous material, such as a wood pulp by the reaction of sodium chlorate with hydrochloric acid as practised in the so-called "Integrated Chlorine Dioxide Processn, as hereinbefore described.
Connecting generator 12 to the base of absorption tower 14 is a conduit means 22. Exiting from the top of tower 14 is exit gas conduit 24. Connected from a lower part of tower 14 to heat exchanger 20 is conduit 26 and from an upper part of heat exchanger 20 to an upper part of tower 14 is conduit 28. Also exiting from an upper part of exchanger 20 to an upper part of stripping tower 16 is conduit 30 and exiting a lower part of tower 16 to a lower part of heat exchanger 20 is conduit 32. Connecting tower 16 to reabsorption tower 18 is conduit 34 and entering tower 16 at a lower part is conduit 36. Entering tower 18 at a top is conduit 36 and exiting ~ower 18 at a lower part is conduit 38 having 17 2~64~
connected thereto conduit 40. Tower 18 at an upper part ` has a return conduit 42 connected to generator 12.
In operation, circulating through absorption tower 14 and stripping tower 16, via heat exchanger 20, is a 75% aqueous sulphuric acid solution at a temperature of about 15C in tower 14 and a temperature of about 70C in tower 16.
With reference now to the gas and sulphuric acid flow systems of this embodiment, in operation a gaseous mixture of chlorine dioxide, chlorine, water vapour and air obtained from chlorine dioxide generator 12 is passed through conduit 22 to the base of absorption tower 14 where it passes in counter-current flow through tower 14 with the sulphuric acid entering the top of tower 14 from heat exchanger 20 from conduit 28. The impure chlorine dioxide gaseous feed typically comprises chlorine dioxide and chlorine in about a 4:1 ratioO Substantially all of the chlorine dioxide is absorbed in the sulphuric acid in tower 14 to provide a concentration of Ca 10-llg chlorine dioxide/litre, while the sulphuric acid absorbs chlorine to produce c~ O.lg chlorine/litre concentration at the gaseous mixture and sulphuric acid flow rates and concentrations used.
The resultant contaminant gas stream, substantially free of chlorine dioxide, exits from the top 18 2~g~
of tower 14 via conduit 24, and, optionally, sent to a hydrochloric acid synthesis unit where the chlorine gas is burned to form hydrochloric acid. The chlorine dioxide containing sulphuric acid solution exits from the base of tower 14 and is transferred via conduit 26 to heat exchanger 20, which warms the solution prior to transfer via conduit 30 to the top of stripping tower 16 wherein it is purged in counter-current flow by a stripping hot air flow having a temperature of about 70C entering tower 16 via conduit 36 to provide a stripped gaseous mixture comprising chlorine dioxide. Depleted chlorine dioxide sulphuric acid solution exits tower 16 and is recycled back to the top of absorption tower 14 through heat exchanger 20 via conduits 32 and 28.
The stripped chlorine dioxide air gaseous mixture exits stripping tower 16 via conduit 34 and enters the base of re-absorption tower 18 where it mixes in counter-current flow with chilled water having a temperature of about 8C entering the top of tower 18 via conduit 37. Chlorine dioxide dissolves in the chilled water and exits the base of tower 18 via conduit 38 for transfer directly or after storage in a storage tank (not shown) to a pulp bleaching plant (not shown). The chilled chlorine dioxide solution contains between 9 to 10 g chlorine dioxide/litre and ~ 0.1 g chlorine/litre.
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The resultant weak gas stream exiting tower 18 is recycled via conduit 42 to chlorine dioxide generator 12.
The absorption tower and re-absorption tower may be of any suitable and convenient construction to allow intimate gas-liquid contact. Most preferably, the respective gaseous mixtures rise through the respective gas/liquid contact zones under conditions of temperature, flow rates, and chlorine dioxide and chlorine concentration, as deemed sufficient to provide the selective absorption. The choice of such conditions lies well within the ability of the skilled man in the art.
Further, such parameters may be readily selected to maintain constant water balance between water entering the system as vapour via conduit 22 and that lost via conduit 24.
We have found that the lower the temperature of the circulating liquid in the absorption tower and the re-absorption tower the greater the absorption of chlorine dioxide therein, whereas the hotter the stripping tower circulating acid the geeater the amount of chlorine dioxide purged therefrom. We have found that liquid temperatures of 10 to 15C in the absorption tower, 5 to 15C in the re-absorption tower and 50 to 80 C in the stripping tower of the respective liquid absorbent 2~~
provides satisfactory absorption and de-sorption, as the case may be.
In addition to the beneficial utilisation of the heat exchanger as described in the preferred embodiment, additional heating and cooling means may be desired as determined by the man skilled in the art, either, for example, to the heat exchanger 20, re-absorption tower or stripping gas temperatures and flow rates.
Table 1 shows the experimental solubilities of chlorine dioxide and chlorine in sulphuric acid as provided by the process of the present invention in an air-diluted mixture for the same ratio of gaseous mixture feed at 15~C. The results have been corrected to a chlorine dioxide gas strength of 11 mole ~. In each case, a water blank was used to facilitate correction to some standard condition to compensate for charging off gas composition.
Table 1 gives the chlorine concentration in g/L
in approximately 10 g/L chlorine dioxide solution.
Table 1 Experimental solubilities of air-diluted chlorine dioxide and chlorine in sulphuric acid for the same ratio of feed gases at 15C.
H2S04 C102 C12 C102-Kepenski*
(%) (g/l~ (g/l) (g/l) 0 11.3 1.6 --10.6 0.7 10.7 13.3 0.7 10.3 10.1 0.4 10.0 ; 20 11.4 0.3 9.8 ~ 25 12.0 0.2 9.7 ; 30 10.5 0.2 9.7 11.8 0.1 10.8 10.7 0.1 10.4 98 10.5 0.5 --*J. Kepenski and J. Trzeszczynski, Roczniki Chemii, Ann.
Soc. Chim. Polongrum, 38, 201-211 (1964).
Table 2 shows the experimental solubilities obtained from the stripped gas purging effected in tower 18 in distilled water at 5C.
Table 2 Experimental solubilities of the stripped gas of the solutions shown in Table 1, in distilled water at 5C.
Initial circulating H2SO4 Composition of Solution Strength final solution (g/l) (wt%) ClO~ C12 0 10 1.4 9.4 0.7 9.5 0.6 9.5 0.4 10.2 0.3 Accordingly, it can be seen that the chlorine to chlorine dioxide ratio has fallen from 1:4 of the chlorine dioxide generated gaseous mixture to a 1:30 ratio in the chlorine dioxide aqueous solution product.
Optionally, but preferably, the reduced level of chlorine contaminant in the chlorine dioxide aqueous product can be readily removed by the use of a reducing 23 20~
agent, foc example, a non-fully oxidised sulphur species, such as sulphur dioxide and metal salts of sulphite, bisulphite, dithionite and persulphate, and preferably hydrogen peroxide.
The above described process provides a virtually self-contained recycling sulphuric acid system whereby the level of chlorine as a contaminant in a gaseous chlorine dioxide mixture is either adequately removed or significantly reduced to a level such that the resultant amounts of chlorine in the chlorine dioxide product can be readily removed by a reducing agent, while the majority of chlorine contaminant from the chlorine dioxide generator can be advantageously utilised in the production of valuable hydrochloric acid. Further, the above process preferably provides a weak gas recycle option to return any uncollected chlorine dioxide back to the chlorine dioxide generator for recycling.
The above embodiment of the process and apparatus according to the invention illustrates the feasibility of substantially removing chlorine contaminant from a chlorine dioxide mixture. Clearly, alternative recycling chlorine dioxide selective absorbent liquids may be used without departing from the spirit of the invention where selective absorption occurs and is of practicability.
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The dissolution of chlorine dioxide into sulphuric acid has been reported by Kepenski et al, Roczniki Chemii, Ann. Soc. Chim. Polongrum, 38, 201-211 (1964), to be dependent upon the sulphuric acid concentration whereby the chlorine dioxide solubility decreases by 13% as the sulphuric acid strength is increased from 0 to 30~, and thereafter increase over the range 30 to 60%. However, we have found that, as shown in Table l, the solubility of chlorine dioxide in aqueous sulphuric acids of various strength of 0-98% to be substantially constant. We have found, also, that the solubility of chlorine in sulphuric acid of various strength decreases significantly with increasing acid strength, as shown in Table 1. We have further found that chlorine saturation is reached in about 1/5 of the time taken for chlorine dioxide saturation in water and 20~
sulphuric acid. Most importantly, however, we have found that sulphuric acid effects a most beneficial partial separation of chlorine from admixtures of chlorine and - chlorine dioxide, and that increasing the sulphuric acid concentration to at least 20 weight ~ results in increasing separation. For typical chlorine dioxide generator gas streams, a change in molar ratio of circa 4:1 chlorine dioxide to chlorine can be increased to circa 30:1. Contrary to the findings of Kepenski et al, the `
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solubility of chlorine dioxide in sulphuric acid is essentially independent of aqueous sulphuric acid strength over the range 0 to 20% sulphuric acid, whereas the solubility of chlorine decreases as the sulphuric acid strength increases.
As hereinbefore described, with appropriate selection of sulphuric acid strength and/or operating temperature and/or pressure, there may be no net change in the water content of the sulphuric acid in a sulphuric acid absorption/de-sorption process. This latter aspect is of importance when water balance factors for the overall process are important. Thus, under the following conditions wherein: the chlorine dioxide generator air-gaseous mixture enters the sulphuric acid absorber at 70C, the sulphuric acid absorber operates at 15C, the resulting solution is stripped at 70C, constant air flow throughout the system, and near atmospheric pressure throughout, there will be a small net water loss from the sulphuric acid if the sulphuric acid strength is less than 30~ but there will be a water gain in the recycling sulphuric acid stream if the acid strength is higher than 30%.
Optionally, the gaseous mixture containing water vapour may be sent to an indirect contact cooler where the water vapour is condensed and removed to provide the 26 2 Q ~ ~ ~3~.~
resultant gaseous mixture with a relatively higher concentration of chlorine dioxide.
The process according to the invention has particular utility by removing adequate or significant amounts of chlorine in admixture with chlorine dioxide, such that the residual chlorine contained in the chlorine dioxide product may be readily removed by the addition of a reductant. We have found that hydrogen peroxide reduces chlorine essentially exclusively when the peroxide is added in an amount not greater than that required to reduce the chlorine present. We have found that the addition of sodium sulphite (and by inference, sulphur dioxide) as a reductant to chlorine containing chlorine dioxide solutions reduces both chlorine and chlorine dioxide at approximately equal rates. Thus, the additional step of removing residual chlorine by a reductant as mentioned above is now an optional, but, economically feasible step to provide a very useful pulp bleaching reagent.
In this specification, a chlorine dioxide generator means a reaction vessel having feed streams which comprise sodium chlorate feed means and acid feed means. In the non-integrated methanol based generating system a methanol feed stream means is also required. In the integrated chlorine dioxide process, the chlorine . .
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dioxide generator is provided with means for feeding hydrochloric acid and means for feeding sodium chlorate to the generator. The sodium chlorate is made from the electrolysis of sodium chloride in a standard chlorate electrolysis unit. Hydrogen gas produced as a by-product is cycled to a hydrochloric acid synthesis unit comprising chlorine feed means and hydrogen feed means, combustion reactor and hydrochloric acid product separation means.
While a preferred embodiment of the apparatus and process of the invention has been described using specific terms, it is to be understood that variations may be made without departing from or scope of the following claims. Thus, although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to these particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated herein.
I
Chlorine-Pree Chlorine Dio~ide Field of the Invention . . _ This invention relates to the preparation of substantially chlorine-free chlorine dioxide, apparatus therefor, and particularly to the preparation of substantially chlorine-free chlorine dioxide for use in pulp bleaching~
Background of the Invention Chlorine dioxide is extensively used in aqueous solution as a reagent for the bleaching of fibrous material, particularly wood pulp. Serious environmental concerns now exist in the bleaching of wood pulps whenever chlorinated organic compounds are produced, even in relatively small amounts, by the action of chlorine per se or chlorine when usually present in chlorine dioxide bleaching agent mixtures, upon the wood fiber. It is, therefore, becoming increasingly necessary that chlorine dioxide when used as a pulp bleaching agent be substantially chlorine contaminant free.
Chlorine dioxide is produced on a commercial scale by the acid reduction of a chlorate anion in aqueous solution according to the following general equation, viz:
C103 + 2H+ + Cl -> C102 + 1/2C12 + H20 2 2 0 ~
Typically, the reaction is achieved by the direct reduction of chlorate by hydrochloric acid as practised in, for example, the so-called "Integrated Chlorine Dioxide Process", viz:
NaC103 + 2HCl -> C102 + 1/2C12 + H20 + NaCl or by the so called R2 and related R series processes using sulphuric acid, viz:
NaC103 + NaCl + H2S04 C102 + 1/2C12 + ~2 + Na2S4 The integrated chlorine dioxide process utilizes 3 main process systems, namely, (a) a chlorate electrolysis unit, wherein sodium chloride is electrolized to produce sodium chlorate according to the following chemical reaction:
6e NaCl + 3H20 --~ NaC103 + 3~2~
(b) a chlorine dioxide generator unit, using hydrochloric acid as the chlorate-reducing agent as described hereinabove; and (c) a hydrochloric acid producing unit wherein the resultant hydrogen gas from the chlorate electrolysis unit is burned in chlorine to produce hydrochloric acid.
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The integrated process produces chlorine dioxide from chlorine, water, and electricity. Such a process, typically, produces 8-10 g/L chlorine dioxide solution strength containing 0.9-1.8 g/L chlorine.
Methods of producing chlorine dioxide from chlorate systems having minimal or no amounts of chlorine are known.
United States patent No. 2,881,052, discloses reduction of sodium chlorate by an acid in the presence of methanol according to the following equation, viz:
3 2H2S4 + CH30H 3 2C102 + 2NaHS04 + HCHO + 2H20 In an alternative process for the reduction of sodium chlorate by methanol in sulphuric acid, known in the industry as the non-integrated, methanol base chlorine dioxide process, sodium sesqui-sulfate is produced as a by-product according to the following equation:
18NaC103 + 4CH30H + 1282S04 ~18C102 + 6Na3H(S04)3 + C2 + 3HCOOH + 14H20 The gas leaving the chlorine dioxide generator is a mixture of chlorine dioxide, water vapour and trace amounts of chlorine. The gaseous mixture is, optionally, 4 2 ~ 61 ~ ~ ~
sent to a contact cooler where the water vapour is condensed and the resultant gaseous mixture passed into a packed absorption tower where chilled water is forced into intimate contact with the gas. The resultant chlorine dioxide solution is pumped to storage for subsequent use in a pulp bleaching process. Such a typical ~ethanol-reductant system provides a chlorine dioxide solution strength of 10 g/L and a chlorine concentration of 0.1 g/L
in 10 g/L chlorine dioxide solution.
British patent No. 687099 discloses reduction of sodium chlorate by nitrogen dioxide to produce chlorine dioxide according to the following equation, viz:
NaC103 + N02- ~ NaN03 + C102 U.S. Patent No. 2,866,682, discloses a related process using sodium nitrate according to the following equation, v i z :
3 NaNO2 + H2SO4 ~ 2ClO2 + Na2SO4 + NaNO3 + H2O
Alternative processes are disclosed in U.S.
Patent No. 2,866,683 wherein sodium bisulphite is used in conjunction with sulphuric acid according to the equation, viz:
2 ~
4NaC103 + 2NaHS03 + H2S04 >4C102 + 3Na2S 4 2 and the related process using sulphur dioxide, viz:
2NaC103 + H2S04 + S2 2C102 ~ 2NaH 4 2NaC103 + S02 >2Clo2 + Na2S04 French patent No. 991614 discloses the reduction of a chlorate by manganic salts and other oxidating metal salts.
It is understood by those skilled in the art that there are significant deleterious side reactions, often resulting in significant inefficiencies, in using reducing agents other than chloride for the production of chlorine dioxide from a chlorate solution for the purpose of bleaching wood pulp. However, the process of generating chlorine dioxide from chlorate using chloride as a reductant, which process is alternative to those outlined hereinabove, suffers from the increasingly disadvantageous side reaction of producing chlorine by-product.
When chloride is used as a reductant, the yield of chlorine dioxide is reduced and chlorine is produced by the following overall reaction, viz:
.. . .
0~ 3 NaC103 + 5HCl ,3C12 + 3H20 The desired overall reaction according to the equation, viz:
5NaC103 + 6HC1 6C102 + 5NaCl + 3H20 is of insignificant application.
Where a non-fully oxidised sulphur compound is used as a reductant, inefficiencies resulting in total reduction to chloride occur, for example, by the following equation:
2NaC103 + 6S02 + 6H20 - >2NaHS04 + 4H2S04 + 2HCl In this particular case the extent of the inefficiency is heightened as it reflects the effective reduction of the co-produced chlorine, which could otherwise be reduced with hydrogen and recycled in the generation of chlorine dioxide in the "Integrated Chlorine Dioxide Process". Chlorine is, however, also produced when sodium chlorate is reduced with reducing agents other than chloride; the extent of which is a function of the reaction conditions, e.g.
"` 7 2 0 ~ 5 2NaC103 + 5SO2 + 4H2O -> 2NaHSO4 + 3H2So4 + C12 In this case there is some chlorine production though reduction to chloride also occurs, viz:
C12 + S2 + 2H2 -~ 2HCl + H2SO4 Similarily, use of other reducing agents, i.e.
methanol, as disclosed in U.S. Patent No. 2,881,052, does not, in itself, prevent the production of some chlorine or reduction of chlorate to chloride.
Accordingly, while the use of chloride as a reductant is the preferred manufacturing process at present, the presence of by-product chlorine is causing reconsideration of this process as an ongoing viable commercial process for the production of chlorine dioxide for the bleaching of wood pulp.
Methods of removing chlorine from chlorine dioxide are known but, to-date, do not offer practical manufacturing processes.
U.S. Patent No. 3,854,900, discloses a means of separating chlorine dioxide from chlorine using a twin absorption column. Unfortunately, this process leaves the production of significantly dilute chlorine dioxide solutions.
8 2 ~ 5 Japanese patent No. 4466(1956), discloses the selective removal of chlorine dioxide from chlorine contaminated chlorine dioxide gaseous mixtures by passing the mixed gases obtained from the production of sodium chlorate over metals such as iron or aluminum. The chlorine contaminant forms the respective metal chloride while the chlorine dioxide gas passes through the system.
U.S. Patent No. 2,481,241, teaches the selective reduction of chlorine by sulphur dioxide, Tappi 48:110 (1965) discloses the selective removal of chlorine by means of an alkali and Japanese patent No. 2663(1956) discloses the oxidation of chlorite with chlorine for selective removal.
All of the above chloride reductant processes cause significant inefficiencies by either loss of chlorine to the system, production of weak chlorine dioxide solutions, chlorine dioxide reduction, the formation of chlocites or for the need for additional chemical reagents.
Accordingly, it can be seen that there is an increased demand for a manufacturing process for producing chlorine-free chlorine dioxide for use as a bleaching reagent which suffers from none of the above disadvantages.
Summary of the Invention 9 2 ~
It is a primary object of the present invention to provide chlorine dioxide substantially chlorine-free and acceptable for use in a pulp bleaching process.
It is a further object of the present invention to provide a continuous substantially-closed chlorine dioxide purification process that provides for the recovery of the chlorine contaminant for subsequent use.
In a yet further object, the invention provides a pulp bleaching process which uses substantially chlorine-free chlorine dioxide wherein substantially all of the chlorine contaminant is removed by selective desorption rather than by reduction to chloride.
Applicant has surprisingly discovered that chlorine dioxide may be beneficially separated from chlorine in a gaseous mixture as is typically produced in a chlorine dioxide generator from sodium chlorate and acid, preferably hydrochloric acid, or methanol/acid, preferably sulphuric acid, mixtures as practised in the art.
Althoug~. it is known that chlorine and chlorine dioxide are soluble to some degree in suitable chlorine dioxide absorbent media, applicant has discovered that the relative degrees of solubility of these respective gases may be sufficiently different and dependent upon the nature of the absorbent medium that practical separations of chlorine from chlorine dioxide can be achieved.
.. . :
~ o 2 ~
Accordingly, the invention provides a process for the production of a substantially chlorine-free aqueous solution of chlorine dioxide which process comprises generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by a chlorine dioxide generator; feeding said gaseous mixture to an absorption tower in counter-current flow to a chlorine-dioxide selective absorbent liquid to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide solution and a resultant chlorine contaminated gas stream; feeding said chlorine-dioxide solution to a stripping tower in counter current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution; feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce a chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a weak gas stream.
In yet a further aspect, the invention provides a process as hereinabove defined for use in the bleaching of a fibrous material, particularly wood pulp, in a fibrous material bleaching system, further comprising feeding said substantially chlorine-free aqueous solution of chlorine dioxide to said system and bleaching said fibrous material with said chlorine dioxide.
11 2~&~
Preferably, the process may be operated as a continuous process.
More preferably, the chlorine-dioxide selective absorbent liquid is selected from aqueous sulphuric acid and acetic acid.
We have surprisingly discovered that although dilute sulphuric acid having a strength of less than 15%
H2SO4 can be used in the process according to the invention to produce sufficiently low levels of chlorine in the substantially chlorine-free aqueous chlorine-dioxide, that most advantageously reduced levels of chlorine-contaminant can be obtained with stronger sulphuric acid strengths. Accordingly, acid strengths of > 15% H2SO4 is preferred, 25-50% H2S04 more preferred, and 50-95% yet more preferred. It will be appreciated by the man skilled in the art that the sulphuric acid strength may be chosen so as to not only provide the substantially chlorine-free chlorine dioxide solution by the process according to the invention, but be selected, also, as to maintain a water balance in the process and avoid the need for sulphuric acid "make-up" during the operation of the process. Water vapour enters the system in the chlorine-contaminated gaseous mixture comprising chlorine dioxide, chlorine and water vapour from the chlorine dioxide generator.
12 2~
Thus, in one embodiment, the sulphuric acid strength and operating temperature thereof in the chlorine dioxide absorption tower is selected such that the water in the form of vapour carried into the system from the chlorine dioxide generating unit balances the water carried out of the system with the resultant chlorine contaminated gas stream.
In yet a still further aspect, the invention provides apparatus for the production of substantially chlorine-free chlorine dioxide comprising: a chlorine dioxide generator for producing a chlorine-contaminated gaseous mixture comprising chlorine dioxide and said chlorine-contaminant; an absorption tower; a stripping tower; a re-absorption tower; means for feeding said gaseous mixture to said absorption tower; means for feeding a chlorine-dioxide selective absorbent liquid to said absorption tower in counter-current flow to said gaseous mixture to produce a first chlorine-dioxide solution; means for feeding said first chlorine-dioxide solution to said stripping tower; means for feeding a stripping gas to said stripping tower in counter-current flow to said first chlorine-dioxide solution to produce a stripped gaseous mixture comprising chlorine dioxide and a depleted first chlorine-dioxide solution; means for feeding said stripped gaseous mixture to said re-:
.~, .
13 2 B ~
absorption tower; means for feeding an aqueous solution tosaid re-absorption tower in counter current flow to said stripped gaseous mixture to produce an aqueous second chlorine dioxide solution and a weak gas stream; and collecting said second chlorine-dioxide solution constituting said substantially chlorine-free chlorine dioxide.
Preferably, the apparatus as hereinabove defined further comprises a fibrous material bleaching unit, particularly a wood pulp bleaching unit and means for feeding said second chlorine-dioxide solution to said bleaching unit.
In a preferred embodiment of the invention the absorbent medium is an aqueous solution of sulphuric acid.
By the term "substantially chlorine-free" is meant levels of chlorine in admixture with chlorine dioxide, either as a gaseous mixture or as a solution, lower than the ratio of chlorine:chlorine dioxide of 1:20 on a W/W basis.
By the term "chlorine dioxide selective absorbent liquid" is meant a liquid that selectively absorbs chlorine dioxide therein to a greater degree than chlorine under the absorbent conditions practised of temperature, gaseous mixture flow rates, relative concentrations of chlorine dioxide and chlorine, and the like, such that the ratio of chlorine:chlorine dioxide in the liquid is lower than in the gaseous mixture from which the chlorine dioxide is removed.
We have found that chlorine dioxide and chlorine in the ratio typically found in the gaseous mixture generated from a chlorine dioxide generator using sodium chlorate and hydrochloric acid, when fed to a selective absorbent, for example, sulphuric acid, water, or acetic acid, produces a chlorine dioxide solution containing substantially all of the chlorine dioxide originally contained in the gaseous mixture, but with significantly reduced amounts of chlorine. The remaining chlorine remains in the gaseous stream. The chlorine dioxide can be liberated from the absorbent, preferably by a suitable gas purge, such as a warm nitrogen, helium or, most preferably, an air stream; with re-absorption of the chlorine dioxide, if required, in a second liquid, preferably, water.
In those processes where further reduced levels of chlorine contaminant in the aqueous chlorine dioxide bleaching solution are desired, which chlorine levels are produced by operation of the process according to the invention at desired sulphuric acid strengths and temperatures as hereinbefore described, the chlorine contaminant level may be further reduced by treatment of 2 ~ ~ ~ & ~ ~
the chilled aqueous chlorine-dioxide solution with a reducing agent to chloride anion. Such reducing agent may be selected from the group consisting of hydrogen peroxide and a non-fully oxidised sulphur species selected from sulphur dioxide and metal salts of sulphite, bisulphite, dithionite and persulphate.
We have found that the process and apparatus according to the invention is a highly desirable addition to the typical Integrated Chlorine Dioxide Process having relatively high levels of contaminant chlorine for the production of chlorine contaminant-free chlorine dioxide solution. However, the process and apparatus according to the invention is also of value in providing an improved chlorine dioxide solution as generated from non-integrated chlorine dioxide generating systems.
Other and further objects and advantages of this invention will become apparent upon reading the following specification taken in conjunction with the accompanying drawings.
Brief Description of the Drawings In order that the invention may be better understood a preferred embodiment will now be described by way of example, only, with reference to the accompanying drawing wherein the Figure is a diagramatic sketch of a preferred process and apparatus according to the invention.
Description of a Preferred Embodiment The apparatus and process shown in the Figure generally as 10 has a chlorine dioxide generator 12, an absorption tower 14, a stripping tower 16, a re-absorption tower 18 and a heat exchanger 20.
Generator 12 is of the type well-known in the art used to produce chlorine dioxide for use as a bleaching agent of a fibrous material, such as a wood pulp by the reaction of sodium chlorate with hydrochloric acid as practised in the so-called "Integrated Chlorine Dioxide Processn, as hereinbefore described.
Connecting generator 12 to the base of absorption tower 14 is a conduit means 22. Exiting from the top of tower 14 is exit gas conduit 24. Connected from a lower part of tower 14 to heat exchanger 20 is conduit 26 and from an upper part of heat exchanger 20 to an upper part of tower 14 is conduit 28. Also exiting from an upper part of exchanger 20 to an upper part of stripping tower 16 is conduit 30 and exiting a lower part of tower 16 to a lower part of heat exchanger 20 is conduit 32. Connecting tower 16 to reabsorption tower 18 is conduit 34 and entering tower 16 at a lower part is conduit 36. Entering tower 18 at a top is conduit 36 and exiting ~ower 18 at a lower part is conduit 38 having 17 2~64~
connected thereto conduit 40. Tower 18 at an upper part ` has a return conduit 42 connected to generator 12.
In operation, circulating through absorption tower 14 and stripping tower 16, via heat exchanger 20, is a 75% aqueous sulphuric acid solution at a temperature of about 15C in tower 14 and a temperature of about 70C in tower 16.
With reference now to the gas and sulphuric acid flow systems of this embodiment, in operation a gaseous mixture of chlorine dioxide, chlorine, water vapour and air obtained from chlorine dioxide generator 12 is passed through conduit 22 to the base of absorption tower 14 where it passes in counter-current flow through tower 14 with the sulphuric acid entering the top of tower 14 from heat exchanger 20 from conduit 28. The impure chlorine dioxide gaseous feed typically comprises chlorine dioxide and chlorine in about a 4:1 ratioO Substantially all of the chlorine dioxide is absorbed in the sulphuric acid in tower 14 to provide a concentration of Ca 10-llg chlorine dioxide/litre, while the sulphuric acid absorbs chlorine to produce c~ O.lg chlorine/litre concentration at the gaseous mixture and sulphuric acid flow rates and concentrations used.
The resultant contaminant gas stream, substantially free of chlorine dioxide, exits from the top 18 2~g~
of tower 14 via conduit 24, and, optionally, sent to a hydrochloric acid synthesis unit where the chlorine gas is burned to form hydrochloric acid. The chlorine dioxide containing sulphuric acid solution exits from the base of tower 14 and is transferred via conduit 26 to heat exchanger 20, which warms the solution prior to transfer via conduit 30 to the top of stripping tower 16 wherein it is purged in counter-current flow by a stripping hot air flow having a temperature of about 70C entering tower 16 via conduit 36 to provide a stripped gaseous mixture comprising chlorine dioxide. Depleted chlorine dioxide sulphuric acid solution exits tower 16 and is recycled back to the top of absorption tower 14 through heat exchanger 20 via conduits 32 and 28.
The stripped chlorine dioxide air gaseous mixture exits stripping tower 16 via conduit 34 and enters the base of re-absorption tower 18 where it mixes in counter-current flow with chilled water having a temperature of about 8C entering the top of tower 18 via conduit 37. Chlorine dioxide dissolves in the chilled water and exits the base of tower 18 via conduit 38 for transfer directly or after storage in a storage tank (not shown) to a pulp bleaching plant (not shown). The chilled chlorine dioxide solution contains between 9 to 10 g chlorine dioxide/litre and ~ 0.1 g chlorine/litre.
19 20Sl~
The resultant weak gas stream exiting tower 18 is recycled via conduit 42 to chlorine dioxide generator 12.
The absorption tower and re-absorption tower may be of any suitable and convenient construction to allow intimate gas-liquid contact. Most preferably, the respective gaseous mixtures rise through the respective gas/liquid contact zones under conditions of temperature, flow rates, and chlorine dioxide and chlorine concentration, as deemed sufficient to provide the selective absorption. The choice of such conditions lies well within the ability of the skilled man in the art.
Further, such parameters may be readily selected to maintain constant water balance between water entering the system as vapour via conduit 22 and that lost via conduit 24.
We have found that the lower the temperature of the circulating liquid in the absorption tower and the re-absorption tower the greater the absorption of chlorine dioxide therein, whereas the hotter the stripping tower circulating acid the geeater the amount of chlorine dioxide purged therefrom. We have found that liquid temperatures of 10 to 15C in the absorption tower, 5 to 15C in the re-absorption tower and 50 to 80 C in the stripping tower of the respective liquid absorbent 2~~
provides satisfactory absorption and de-sorption, as the case may be.
In addition to the beneficial utilisation of the heat exchanger as described in the preferred embodiment, additional heating and cooling means may be desired as determined by the man skilled in the art, either, for example, to the heat exchanger 20, re-absorption tower or stripping gas temperatures and flow rates.
Table 1 shows the experimental solubilities of chlorine dioxide and chlorine in sulphuric acid as provided by the process of the present invention in an air-diluted mixture for the same ratio of gaseous mixture feed at 15~C. The results have been corrected to a chlorine dioxide gas strength of 11 mole ~. In each case, a water blank was used to facilitate correction to some standard condition to compensate for charging off gas composition.
Table 1 gives the chlorine concentration in g/L
in approximately 10 g/L chlorine dioxide solution.
Table 1 Experimental solubilities of air-diluted chlorine dioxide and chlorine in sulphuric acid for the same ratio of feed gases at 15C.
H2S04 C102 C12 C102-Kepenski*
(%) (g/l~ (g/l) (g/l) 0 11.3 1.6 --10.6 0.7 10.7 13.3 0.7 10.3 10.1 0.4 10.0 ; 20 11.4 0.3 9.8 ~ 25 12.0 0.2 9.7 ; 30 10.5 0.2 9.7 11.8 0.1 10.8 10.7 0.1 10.4 98 10.5 0.5 --*J. Kepenski and J. Trzeszczynski, Roczniki Chemii, Ann.
Soc. Chim. Polongrum, 38, 201-211 (1964).
Table 2 shows the experimental solubilities obtained from the stripped gas purging effected in tower 18 in distilled water at 5C.
Table 2 Experimental solubilities of the stripped gas of the solutions shown in Table 1, in distilled water at 5C.
Initial circulating H2SO4 Composition of Solution Strength final solution (g/l) (wt%) ClO~ C12 0 10 1.4 9.4 0.7 9.5 0.6 9.5 0.4 10.2 0.3 Accordingly, it can be seen that the chlorine to chlorine dioxide ratio has fallen from 1:4 of the chlorine dioxide generated gaseous mixture to a 1:30 ratio in the chlorine dioxide aqueous solution product.
Optionally, but preferably, the reduced level of chlorine contaminant in the chlorine dioxide aqueous product can be readily removed by the use of a reducing 23 20~
agent, foc example, a non-fully oxidised sulphur species, such as sulphur dioxide and metal salts of sulphite, bisulphite, dithionite and persulphate, and preferably hydrogen peroxide.
The above described process provides a virtually self-contained recycling sulphuric acid system whereby the level of chlorine as a contaminant in a gaseous chlorine dioxide mixture is either adequately removed or significantly reduced to a level such that the resultant amounts of chlorine in the chlorine dioxide product can be readily removed by a reducing agent, while the majority of chlorine contaminant from the chlorine dioxide generator can be advantageously utilised in the production of valuable hydrochloric acid. Further, the above process preferably provides a weak gas recycle option to return any uncollected chlorine dioxide back to the chlorine dioxide generator for recycling.
The above embodiment of the process and apparatus according to the invention illustrates the feasibility of substantially removing chlorine contaminant from a chlorine dioxide mixture. Clearly, alternative recycling chlorine dioxide selective absorbent liquids may be used without departing from the spirit of the invention where selective absorption occurs and is of practicability.
24 20~ S
The dissolution of chlorine dioxide into sulphuric acid has been reported by Kepenski et al, Roczniki Chemii, Ann. Soc. Chim. Polongrum, 38, 201-211 (1964), to be dependent upon the sulphuric acid concentration whereby the chlorine dioxide solubility decreases by 13% as the sulphuric acid strength is increased from 0 to 30~, and thereafter increase over the range 30 to 60%. However, we have found that, as shown in Table l, the solubility of chlorine dioxide in aqueous sulphuric acids of various strength of 0-98% to be substantially constant. We have found, also, that the solubility of chlorine in sulphuric acid of various strength decreases significantly with increasing acid strength, as shown in Table 1. We have further found that chlorine saturation is reached in about 1/5 of the time taken for chlorine dioxide saturation in water and 20~
sulphuric acid. Most importantly, however, we have found that sulphuric acid effects a most beneficial partial separation of chlorine from admixtures of chlorine and - chlorine dioxide, and that increasing the sulphuric acid concentration to at least 20 weight ~ results in increasing separation. For typical chlorine dioxide generator gas streams, a change in molar ratio of circa 4:1 chlorine dioxide to chlorine can be increased to circa 30:1. Contrary to the findings of Kepenski et al, the `
.
2s 2~
solubility of chlorine dioxide in sulphuric acid is essentially independent of aqueous sulphuric acid strength over the range 0 to 20% sulphuric acid, whereas the solubility of chlorine decreases as the sulphuric acid strength increases.
As hereinbefore described, with appropriate selection of sulphuric acid strength and/or operating temperature and/or pressure, there may be no net change in the water content of the sulphuric acid in a sulphuric acid absorption/de-sorption process. This latter aspect is of importance when water balance factors for the overall process are important. Thus, under the following conditions wherein: the chlorine dioxide generator air-gaseous mixture enters the sulphuric acid absorber at 70C, the sulphuric acid absorber operates at 15C, the resulting solution is stripped at 70C, constant air flow throughout the system, and near atmospheric pressure throughout, there will be a small net water loss from the sulphuric acid if the sulphuric acid strength is less than 30~ but there will be a water gain in the recycling sulphuric acid stream if the acid strength is higher than 30%.
Optionally, the gaseous mixture containing water vapour may be sent to an indirect contact cooler where the water vapour is condensed and removed to provide the 26 2 Q ~ ~ ~3~.~
resultant gaseous mixture with a relatively higher concentration of chlorine dioxide.
The process according to the invention has particular utility by removing adequate or significant amounts of chlorine in admixture with chlorine dioxide, such that the residual chlorine contained in the chlorine dioxide product may be readily removed by the addition of a reductant. We have found that hydrogen peroxide reduces chlorine essentially exclusively when the peroxide is added in an amount not greater than that required to reduce the chlorine present. We have found that the addition of sodium sulphite (and by inference, sulphur dioxide) as a reductant to chlorine containing chlorine dioxide solutions reduces both chlorine and chlorine dioxide at approximately equal rates. Thus, the additional step of removing residual chlorine by a reductant as mentioned above is now an optional, but, economically feasible step to provide a very useful pulp bleaching reagent.
In this specification, a chlorine dioxide generator means a reaction vessel having feed streams which comprise sodium chlorate feed means and acid feed means. In the non-integrated methanol based generating system a methanol feed stream means is also required. In the integrated chlorine dioxide process, the chlorine . .
27 20~
dioxide generator is provided with means for feeding hydrochloric acid and means for feeding sodium chlorate to the generator. The sodium chlorate is made from the electrolysis of sodium chloride in a standard chlorate electrolysis unit. Hydrogen gas produced as a by-product is cycled to a hydrochloric acid synthesis unit comprising chlorine feed means and hydrogen feed means, combustion reactor and hydrochloric acid product separation means.
While a preferred embodiment of the apparatus and process of the invention has been described using specific terms, it is to be understood that variations may be made without departing from or scope of the following claims. Thus, although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to these particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated herein.
Claims (16)
1. A process for the production of a substantially chlorine-free aqueous solution of chlorine dioxide for use in the bleaching of a fibrous material which process comprises:
generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by a chlorine dioxide generator;
feeding said gaseous mixture to an absorption tower in counter-current flow to an aqueous sulphuric acid solution to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide solution and a resultant chlorine contaminated gas stream;
feeding said chlorine-dioxide solution to a stripping tower in counter-current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution;
feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a depleted gas stream.
generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by a chlorine dioxide generator;
feeding said gaseous mixture to an absorption tower in counter-current flow to an aqueous sulphuric acid solution to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide solution and a resultant chlorine contaminated gas stream;
feeding said chlorine-dioxide solution to a stripping tower in counter-current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution;
feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a depleted gas stream.
2. In a process for the bleaching of a fibrous material of the type wherein the fibrous material is bleached in a bleaching unit with a chlorine dioxide solution obtained from a chlorine dioxide generator, the improvement comprising:
generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by said chlorine dioxide generator;
feeding said gaseous mixture to an absorption tower in counter-current flow to an aqueous sulphuric acid solution to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide containing sulphuric acid solution and a resultant chlorine contaminated gas stream;
feeding said chlorine-dioxide containing sulphuric acid solution to a stripping tower in counter-current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution;
feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a weak gas stream;
feeding said substantially chlorine-free aqueous chlorine-dioxide solution to said bleaching unit and bleaching said fibrous material.
generating a chlorine-contaminated gaseous mixture comprising chlorine dioxide and chlorine by said chlorine dioxide generator;
feeding said gaseous mixture to an absorption tower in counter-current flow to an aqueous sulphuric acid solution to substantially selectively absorb said chlorine-dioxide therein to produce a chlorine-dioxide containing sulphuric acid solution and a resultant chlorine contaminated gas stream;
feeding said chlorine-dioxide containing sulphuric acid solution to a stripping tower in counter-current flow to a stripping gas to produce a stripped gaseous mixture comprising chlorine-dioxide and a depleted chlorine-dioxide solution;
feeding said stripped gaseous mixture to a re-absorption tower in counter-current flow to a chilled aqueous solution to produce chilled said substantially chlorine-free aqueous chlorine-dioxide solution and a weak gas stream;
feeding said substantially chlorine-free aqueous chlorine-dioxide solution to said bleaching unit and bleaching said fibrous material.
3. A process as claimed in claim 2 wherein said sulphuric acid has a concentration of greater than 15 H2S04.
4. A process as claimed in claim 3 wherein said sulphuric acid is selected from the range 25% - 95% H2S04.
5. A process as claimed in claim 3 wherein said sulphuric acid is selected from the range 50% - 95% H2S04.
6. A process as claimed in claim 4 wherein said gaseous mixture is treated in said absorption towers with said sulphuric acid having a temperature selected from the-range 0°C to 30°C and said chlorine-dioxide solution in said stripping tower has a temperature of greater than 40°C.
7. A process as claimed in any one of claims 1 to 6 which is continuous.
8. A process as claimed in claim 2 further comprising treating said chilled aqueous chlorine-dioxide solution with a reducing agent to reduce any chlorine-contaminant present.
9. A process as claimed in claim 8 wherein said reducing agent is selected from the group consisting of hydrogen peroxide and a non-fully oxidised sulphur species selected from sulphur dioxide, and metal salts of sulphite, bisulphite, dithionite and persulphate.
10. A process as claimed in claim 2 further comprising recycling said depleted chlorine dioxide solution to said absorption tower and said weak gas stream to said chlorine-dioxide generator.
11. A process as claimed in any one of claims 2 to 10 wherein said fibrous material is wood pulp.
12. A process as claimed in claim 2 wherein said sulphuric acid in said absorption tower has a strength and temperature such that the amount of water entering the absorption tower and contained in the chlorine-contaminated gaseous mixture from the chlorine dioxide generator as water vapour substantially balances the amount of water leaving said absorption tower in said resultant chlorine contaminated gas stream.
13. An improved apparatus for the production of chlorine dioxide for use as a bleaching agent for fibrous material of the type having a chlorine dioxide generator for the production of a chlorine-contaminated gaseous mixture comprising chlorine dioxide and said chlorine-contaminant;
wherein the improvement comprises;
an absorption tower;
a stripping tower;
a re-absorption tower;
means for feeding said gaseous mixture to said absorption tower;
means for feeding an aqueous sulphuric acid to said absorption tower in counter-current flow to said gaseous mixture to produce a first chlorine-dioxide solution;
means for feeding said first chlorine-dioxide solution to said stripping tower;
means for feeding a stripping gas to said stripping tower in counter-current flow to said first chlorine-dioxide solution to produce a stripped gaseous mixture comprising chlorine dioxide and a depleted first chlorine-dioxide solution;
means for feeding said stripped gaseous mixture to said re-absorption tower;
means for feeding an aqueous solution to said re-absorption tower in counter-current flow to said stripped gaseous mixture to produce an aqueous second chlorine dioxide solution and a weak gas stream; and means for collecting said second chlorine-dioxide solution constituting said substantially chlorine-free chlorine dioxide.
wherein the improvement comprises;
an absorption tower;
a stripping tower;
a re-absorption tower;
means for feeding said gaseous mixture to said absorption tower;
means for feeding an aqueous sulphuric acid to said absorption tower in counter-current flow to said gaseous mixture to produce a first chlorine-dioxide solution;
means for feeding said first chlorine-dioxide solution to said stripping tower;
means for feeding a stripping gas to said stripping tower in counter-current flow to said first chlorine-dioxide solution to produce a stripped gaseous mixture comprising chlorine dioxide and a depleted first chlorine-dioxide solution;
means for feeding said stripped gaseous mixture to said re-absorption tower;
means for feeding an aqueous solution to said re-absorption tower in counter-current flow to said stripped gaseous mixture to produce an aqueous second chlorine dioxide solution and a weak gas stream; and means for collecting said second chlorine-dioxide solution constituting said substantially chlorine-free chlorine dioxide.
14. Apparatus as claimed in claim 13 further comprising heat exchanger means between said absorption tower and said stripping tower to effect heat transfer between said first chlorine-dioxide solution and said depleted first chlorine-dioxide solution.
15. Apparatus as claimed in claim 13 or claim 14 further comprising:
means for recycling said depleted first chlorine-dioxide solution to said absorption tower and means for recycling said weak gas stream to said chlorine dioxide generator.
means for recycling said depleted first chlorine-dioxide solution to said absorption tower and means for recycling said weak gas stream to said chlorine dioxide generator.
16. Apparatus as claimed in claim 15 further comprising means for feeding said substantially chlorine-free chlorine dioxide to a fibrous material pulp bleaching unit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2061645 CA2061645A1 (en) | 1992-02-21 | 1992-02-21 | Chlorine-free chlorine dioxide |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2061645 CA2061645A1 (en) | 1992-02-21 | 1992-02-21 | Chlorine-free chlorine dioxide |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2061645A1 true CA2061645A1 (en) | 1993-08-22 |
Family
ID=4149311
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2061645 Abandoned CA2061645A1 (en) | 1992-02-21 | 1992-02-21 | Chlorine-free chlorine dioxide |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2061645A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3992144A1 (en) * | 2020-10-29 | 2022-05-04 | Bertram Schmid | Device and method for the production of chlorine dioxide |
-
1992
- 1992-02-21 CA CA 2061645 patent/CA2061645A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3992144A1 (en) * | 2020-10-29 | 2022-05-04 | Bertram Schmid | Device and method for the production of chlorine dioxide |
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