CS228533B2 - Production method of chloride dioxide - Google Patents

Abstract

Chlorine dioxide is prepd. by redn. of chlorate ions with chloride ions in an aq. reaction medium contg. above 6N sulphuric acid, with the reaction medium being held at its boiling pt. under a sub-atmospheric pressure with pptn. of an acid sulphate. Specifically, after the pptd. acid sulphate has been removed from the reaction zone, it is contacted with water in the presence of an alcohol or a water soluble ketone to form a solid neutral sulphate and an aq. phase contg. sulphuric acid which is recycled. By treating the acid sulphate prod. the render it handleable the process becomes viable unlike in prior art, and the sulphuric acid extracted from the acid sulphate is valuable when recycled. High yields of chlorine dioxide, approaching 100% theoretical are obtd.

Description

The present invention relates to a process for producing chlorine dioxide.

U.S. Pat. No. 4,081,520 discloses a highly efficient process for producing chlorine dioxide using sodium chlorate, sulfuric acid, and methanol. The mechanism by which chlorine dioxide is formed is to react chlorine together with chlorine dioxide with methanol to form chloride ions, which then reduce the chlorate ions to form chlorine dioxide and chlorine. The summary reaction can be illustrated by the following equation:

NaClO 3 + 2 H 2 SO 4 + CH 2 OH> 2 CIO 2 + + 2 NaHSO 3 + HCHO + 2 H2O

The chlorine dioxide-containing reaction medium containing sodium chlorate, methanol and sulfuric acid is maintained at a boiling point, usually at 50-85 ° C, at a pressure below atmospheric pressure. The evaporated water serves to dilute the chlorine dioxide as it exits the reaction zone. The reaction medium is highly acidic; the acid concentration exceeds 9N. The byproducts excreted from the already saturated reaction medium after the reaction is started are sodium bisulfates, sodium bisulfate (NaHSO 4) or sodium sesquisulfate (Na 3 H (SO 4) 2).

The process of said US patent is highly effective in converting chlorate ions into chlorine dioxide, as it achieves nearly 100% efficiency. The chlorine dioxide removed from the reaction zone is not contaminated with chlorine, which is advantageous in some cases for its further use.

The disadvantage of this process lies in the nature of the solid by-product; this disadvantage has so far prevented the industrial use of this method. Since the sodium sulfate precipitate is in acid form (sodium hydrogen sulphate), removal of this precipitate from the reaction zone causes a decrease in acidity in the reaction zone. In addition, sodium hydrogensulfates are difficult to handle due to their physical properties and hygroscopicity.

It is an object of the present invention to overcome the shortcomings of the prior art by exchanging solid sodium bisulfate to neutral sodium sulfate, thereby maintaining the required acidity in the reaction zone.

Although the process of the present invention is described with respect to the conversion of the hydrogen sulphate resulting from the process disclosed in the aforementioned U.S. Pat. No. 4,081,520 to neutral sodium sulfate, the process of the present invention is useful. generally to convert a solid hydrogen sulfate to sulfate, wherein said hydrogen sulfate can. be obtained by any process for producing chlorine dioxide based on sulfuric acid and a reaction medium having a high acidity which is maintained at boiling point and at a pressure below atmospheric pressure, where the hydrogen sulphate precipitates from the reaction medium in the reaction zone.

For example, the process of the invention can be used to convert sodium bisulfates forming by-products obtained from a chlorine dioxide generator, wherein the added chloride ions are used as a reducing agent in reducing chlorate in the presence of sulfuric acid in a high acid environment as described in Canadian Patent 825,084. .

In addition, sulfur dioxide can be used in this chlorine dioxide-producing process to reduce concurrently produced chlorine to chloride ions in an analogous manner to that used in the method of U.S. Pat. No. 4,081,520. The use of sulfur dioxide is described in U.S. Pat. 933 988.

Thus, the above-mentioned drawbacks have no method of producing chlorine dioxide. in the reaction zone by reducing - - - chlorate - ions - chloride-ions in an aqueous acidic reaction medium containing sulfuric acid above 6N and maintained at boiling point and below atmospheric pressure and precipitation of hydrogen sulphates in the reaction zone, According to the invention, the precipitated hydrogen sulphate is removed from the reaction zone with water in the presence of a water-soluble alcohol or ketone to form a solid neutral sulphate and an aqueous phase containing water. sulfuric acid,. wherein said aqueous phase is recirculated to the reaction zone.

In the process according to the invention, sodium hydrogen sulphate is preferably used as the hydrogen sulphate, methanol is used as the water-soluble alcohol and the water is used by weight. the ratio to sodium bisulfate, considered as Na 3 H (SO 4) 2, 1.4: 1, and methanol was used in a weight ratio to sodium bisulfate, considered as Na 3 H (SO 4) 2, 0.01 to 2: 1.

Preferably, the weight ratio of water to sodium hydrogen sulphate is 0.6 to 0.8: 1 and the weight ratio of methanol to sodium hydrogen sulphate is 0.3 to 0.8: 1.

The reaction medium preferably has an acid concentration higher than 9N and the chloride ions in the reaction medium are formed by reducing the chlorine that is formed together with chlorine dioxide, methanol.

The aqueous phase obtained in the contacting step after contacting the hydrogen sulphates with water is preferably divided into two streams containing - sulfuric acid and methanol - one stream being recirculated to the reaction zone and methanol removed from the other stream to form a methanol stream from the aqueous stream, wherein the methanol stream is recirculated to the contacting stage and the aqueous stream is recirculated to the reaction zone.

An advantage of the invention is that it does not need to evaporate large volumes of methanol and concentrate large volumes of sulfuric acid, which favorably affects the economics of the chlorine dioxide production process.

A flow diagram of the method according to the invention is shown in the attached drawing.

It has previously been proposed to use water and methanol or another water-soluble alcohol or ketone to convert sodium bisulfates to neutral sodium sulfate. Such a process is described in U.S. Pat. No. 4,104,365. The process of this patent is directed to obtaining neutral sodium sulfate from the effluent liquid phase in a process for producing chlorine dioxide based on the use of high-standard sulfuric acid. Such a process is not carried out at the boiling point of the reaction medium and does not crystallize the by-product sodium sulfate from the reaction medium in the reaction vessel. The starting material in this prior art process is an aqueous solution of sodium hydrogensulfates as opposed to the solid starting material used in the process of the invention.

The earlier manufacturing process involves an initial stripping step, i.e., an operation to remove dissolved gases and residual sodium chlorate that otherwise hinders the reaction. Such an operation is not required in the process of the present invention since the starting material is solid sodium bisulfates formed in the reaction zone where chlorine dioxide is produced.

The process according to the invention differs from the former process not only because - the starting materials are in - different - physical - forms but also because - the volumes of water and methanol needed to add - to the aqueous - the effluent phase to form a solid neutral phase The sodium sulfate of the prior art are - much larger than those - which are used in the process of the invention. As a consequence of these large volumes of water and methanol, considerable evaporation is required in the earlier process, in particular to remove all the methanol used and further to concentrate the aqueous sulfuric acid solution to a concentrate suitable for reuse.

The process of the invention is described below with respect to the use of methanol, but other water-soluble alcohols or ketones could be used instead of methanol.

In the process of the invention, the weight ratio of water to sodium bisulfates (calculated as Na 3 H [SO 4) 2] is 0.4 to 1.4: 1, preferably 0.6 to 0.8: 1. The weight ratio of less than 0.4: 1 results in only limited conversion of hydrogen sulfates to neutral sulfate, while a weight ratio of greater than 1.4: 1 results in the dissolution of large amounts of sodium sulfate in the aqueous phase. Using a weight ratio within this range, the resulting aqueous phase contains sulfuric acid at a concentration sufficient to recirculate it to the chlorine dioxide production process without the need for the acid concentration.

As mentioned above, sodium hydrogen sulphates precipitated from the reaction medium may take a different form [one of which is NaH 2 SO 6] 2, depending on the overall acidity of the acid in the reaction medium. To calculate the mass of water and methanol to be used for an efficient sodium bisulfate exchange reaction, the mass of sodium bisulfate must be expressed as a mass equivalent to that of NasH 2 SO 4. When the sodium hydrogen sulphate is in a form other than NaH 2 SO 4, for example as sodium hydrogen sulphate or a mixture thereof with sodium sesquisulphate, the weight of sodium hydrogen sulphate is converted to an equivalent weight of Na 2 SO 4. If the sodium hydrogen sulphate is present in the form of Na 2 SO 4 CM 2, the weight of the exchange reaction base is the actual weight of the sodium hydrogen sulphate.

In contrast, in a typical process of U.S. Pat. No. 4,104,365, the weight ratio of water to sodium bisulfates [considered as NaH 2 SO 4] is about 1.83: 1 and the weight of water to be evaporated to allow reuse of the sulfuric acid solution is about 3.61 kg per kg of neutral sodium sulfate obtained (or about 4.14 kg per kg of chlorine dioxide formed).

The weight ratio of methanol to sodium bisulfates [calculated as Na 2 SO 4] 2 is less critical than the weight ratio of water and may vary from as low as 0.01 to 2: 1. The weight ratio of methanol to sodium bisulfates is preferably 0.3 up to 0.8: 1 for the above-mentioned preferred ratio of water to sodium bisulfates. Due to the miscibility of methanol with water, the presence of methanol reduces the volume of water in which sodium sulfate can dissolve, and therefore prevents the dissolution of neutral sodium sulfate in the aqueous phase.

As the weight ratio of methanol increases, the proportion of neutral sodium sulfate dissolved in the aqueous phase also decreases until a weight ratio of methanol is reached above which the addition of additional methanol no longer yields a solid neutral sodium sulfate yield, limited to about 80-85%. weight. Therefore, there is no benefit in increasing the weight ratio of methanol to sodium bisulfates above 2: 1.

If preferred forms of the process according to the invention are used, namely when chlorine dioxide is produced by the process according to the above-mentioned U.S. Pat. No. 4,081,520 adopted by the measures of the invention, then a proportion of the water phase obtained from the exchange reaction can be recirculated directly to the reaction zone chlorine dioxide is formed to obtain at least a portion, preferably all methanol. The recirculated stream will also cover part of the sulfuric acid needed to produce chlorine dioxide. The methanol recirculated to the reaction zone in this manner need not be recovered from the recirculated fraction of the aqueous phase. Methanol is removed from the remainder of the aqueous phase to obtain a sulfuric acid solution suitable for recirculation to the reaction zone.

The amount of methanol used according to the invention differs markedly from that used in the prior art process described in U.S. Pat. No. 4,104,365, using a typical weight ratio of methanol to sodium bisulfates [considered as NaH 2 SO 6] of about 9.33: 1 and methanol must be used. recovered for reuse, the sulfuric acid solution being concentrated to a normality suitable for recirculating the solution to the chlorine dioxide production process.

The steam consumption of the process according to the invention for evaporating the aqueous phase is limited to stripping off methanol. Thus, the amount of steam required is considerably less than the process of U.S. Pat. No. 4,104,365, where heat is required to drive off substantially all of the methanol and to thicken the sulfuric acid solution, which is nearly 15 times the steam cost of the process .

Typically, water and methanol are added to the solid sodium hydrogensulfate phase as a solution containing the desired ratio of water to methanol. However, since methanol has an exclusive role in decreasing the solubility of neutral sodium sulfate in the aqueous phase, methanol may be added after the initial addition of water.

The above-described process according to the invention can be carried out using methanol because of its easy availability and its effectiveness in the process according to the invention. However, other water-soluble alcohols and ketones such as methanol, n-propanol, isopropyl alcohol and acetone may also be used if desired.

The exchange reaction used in the process of the invention can be carried out over a wide range of temperatures, usually from 10 to 70 ° C. The reaction proceeds efficiently at room temperature (about 20-25 ° C), although elevated temperatures are preferred, as the rate of reaction increases with increasing temperature. Preferably, a temperature of 20 to 50 ° C is used.

The exchange reaction of the process of the invention may be carried out by any of the usual methods. Although the process can be carried out in batches, a continuous process is preferred because the process of the invention follows the continuous process for producing chlorine dioxide.

The exchange reaction can be carried out in a simple reaction vessel or in a decantation wash column, for example as described in detail in German Patent Application No. 28 56 504.

Mixing the aqueous methanol solution with sodium hydrogen sulphates to achieve said exchange reaction may be accomplished by stirring in the reaction vessel. The mass supplied to the exchange reaction does not require intensive mixing; it is sufficient to use only gentle stirring, even if it takes longer to mix.

The reaction time required to complete the exchange reaction can vary within wide limits, and is typically about 10 minutes with very vigorous stirring to about 60 minutes while the reaction is in a decanting scrubber.

The invention is described in more detail with reference to the accompanying drawing, which shows a flow diagram of a preferred embodiment of the method according to the invention.

It can be seen from the figure that the chlorine dioxide generator 10 produces a gaseous mixture of chlorine dioxide and steam and is discharged through line 12 from which the chlorine dioxide is absorbed into water to form an aqueous solution thereof suitable for use in pulp bleaching or for other purposes .

Said generator 10 produces chlorine dioxide as described in U.S. Pat. No. 4,081,520 from a sodium chlorate solution fed to generator 10 via line 16. Methanol is supplied to generator 10 via line 18,

The aqueous reaction medium with a total acidity greater than 9 N is maintained at boiling point at which no temperature is observed. substantial decomposition of chlorine dioxide, usually at a temperature of 30 to 85 ° C and at a pressure below atmospheric pressure,. taking. as a rule, it uses a pressure of 2 to 50 psi. . Sodium hydrogen sulphate is continuously precipitated from the reaction medium when the reaction medium reaches a saturated state after the reaction is started.

The volume of the reaction medium in the generator 10 'is maintained substantially constant by equalizing the volume of the aqueous phase entering the generator 10 with the volume of water vaporized from the reaction medium in the form of vapor flowing through line 12 and the volume of water removed as a suspension medium for solid sodium hydrogen sulphate.

Sodium hydrogen sulphate precipitated from the reaction medium in the generator 10 is passed through a transfer line 20 to the reactor 22. Sodium hydrogen sulphate, typically sodium sesquisulphate, can be removed from the generator 10 as a suspension in the reaction medium and separated from this suspension by filtration prior to its introduction into the reactor. Reactor 22.

Sodium bisulfate is introduced in reactor 22. The weight ratio of water to sodium hydrogen sulphate [calculated as Na 3 H (SO 4) 2] in reactor 22 is 0.4 to 1.4: 1, preferably 0.6 to 0.8: 1. The weight ratio of methanol to sodium bisulfate [calculated as Na2SO4] 2 is 0.01 to 2: 1, preferably 0.3 to 0.8: 1.

The temperature of the medium containing solid sodium bisulfate in reactor 22 is preferably 20 to 50 ° C. The exchange reaction produces solid anhydrous neutral sodium sulfate. The sodium bisulfate exchange reaction takes place to form sulfuric acid and the aforementioned neutral sodium sulfate. The neutral sodium sulfate is separated from the aqueous phase by any known method, for example by filtration. The neutral sodium sulphate is discharged to the desired use, usually for the preparation of sodium and sulfur compounds useful in pulp production; the chlorine dioxide generator 10 is technologically linked to this production.

The aqueous phase containing sulfuric acid, methanol and a certain amount of dissolved sodium sulphate is discharged via line 30 and divided into two streams, typically one third of the volume of the aqueous phase being recirculated through line 32 to line 18 to obtain at least a portion, preferably all % of the methanol required in the chlorine dioxide generator 10. Optionally, the remainder of the methanol required is metered from line 33 to line 18. The sulfuric acid content of the aqueous phase in line 32 provides some of the sulfuric acid required in the reaction medium of the chlorine dioxide generator 10.

The remaining volume of the aqueous phase (typically two-thirds of this volume) is passed through a conduit 34 to a methanol stripper 36 where methanol is separated from the aqueous phase. Methanol vapors pass through a conduit 38 to a condenser 40 where the vapors condense to form liquid methanol. is fed via line 42 to the methanol feed line 26 to the exchange reactor 22. The methanol required to equalize the required amount of methanol in the reactor 22 is metered in via feed line 44.

The methanol-free sulfuric acid solution is recirculated through line 46 to the sulfuric acid stream fed to the chlorine dioxide generator 10 via line 16. The additionally needed sulfuric acid is metered in by the inlet 48 of sulfuric acid flowing into line 16.

The process according to the invention therefore produces chlorine dioxide which is substantially free of chlorine, with high efficiency and without the need for

The addition of catalyst additives to the reaction medium. At the same time as. by-product obtains. sodium sulfate, . preferably in anhydrous form; Consequently . is not lost by the formation of the by-product. ze. system no sulfuric acid.

The invention is further illustrated by the following examples.

PíkUdl

This example illustrates a process for producing sodium bisulfate according to the procedure of US Patent No. 4,081,520.

The chlorine dioxide generator is operated to produce chlorine dioxide from sodium chlorate, sulfuric acid, and methanol. The reaction medium is maintained at boiling temperature below atmospheric pressure and sodium sesquisulphate is excluded from the reaction medium.

Manufacturing. the parameters are given in the following Table I.

Table I

Production conditions:

Chlorine dioxide produced:

speed. 0.48 g. i-1 min1 efficiency related. to chlorate. .......> 99% gas analysis CIO2> 99%

C12 <1%.

Example 2

This one. the example illustrates the sodium sesquisulfate exchange reaction under various conditions.

A number of experiments are carried out in which solid sodium sesqususulate is contacted with water and / or methanol at room temperature (20 ° C), the suspension is stirred slowly for about 15 minutes. In each case, the weight of the sodium sulphate obtained and the proportion of sulfuric acid remaining in sodium sulphate are determined, taking into account the efficiency of the process, a residual sulfuric acid concentration below about 1% demonstrates essentially 100% conversion of the hydrogen sulphate. sodium to neutral sodium sulfate.

temperature pressure ° C

17,956 kPa

(a) Critical lower limit of the weight ratio of water to sodium hydrogen sulphate

Reaction conditions and dosing rate:

methanol

H 2 SO 4 NaClO 5%, 3.4 mL / min

9M, 3.6 mL. mini 6.74 M, 10.5 mn m

The results of the experiments carried out are of decisive importance. the lower limit of the water to Na2SO4 ratio of 0.4: 1 is reproduced in Table II below.

showing massConcentrator fluid concentration:

Table II

H2SO4

NaClOs

Crystals, solid phase

9.3 N

1,1M

NasH (SO 4) 2

Attempt number Weight ratio Yield H2SO4 in solids (%) H2O: : NasHfSOíjz CHsOH: : Na 3 H (SO 4) 2 mass (.%) (G) 1 0 1.1 99 (1) 15.2 2 0.125 0.7 93.3 (1) 18.4 3 0.25 0.6 84.5 (1) 10.4 4 0.4 0.6 64.2 77 0.92 5 0.5 0.4 67.3 81 0.84 Note: 0.4: 1 to achieve complete replaceable re-

action on neutral salt.

(1) The yield value is insignificant due to the high acid content of the product.

(b) Critical upper limit of the weight ratio of water to sodium hydrogen sulphate

The results from Table II clearly demonstrate that it is essential to achieve a water to sodium sesquisulfate weight ratio of at least

The results of the experiments carried out showing the decisive importance of the upper limit of the ratio of water to NasH 2 SO 4 in the 1.4: 1 ratio are reproduced in Table III below.

Table III

Attempt number Weight ratio Yield H2SO4 in solids (%) HzO: : Na 3 H (SO 4) 2 CH 3 OH: Na'sH (SC4) 2 weight (g) (%) 6 1,2 0.6 47.9 56 0.86 7 1.4 0.6 39.8 48 0.97 8 1.6 0.6 No clots -

The results from Table III clearly demonstrate that it is essential to achieve a water to sodium sesquisulfate weight ratio not greater than 1.4: 1 to obtain a precipitate from the reaction medium.

c) The crucial importance of the presence of methanol during the exchange reaction

The results of the experiments showing the decisive importance of the presence of methanol during the reaction are reproduced. in Table IV below.

Table IV

Experiment Mass ratio Yield H2SO4 in

number H2O: : Na 3 H (SO 4) 2 CH3OH: : Na 3 H (SC 4) 2 weight (g) (%) fixed share (%) 9 0.75 0 16.8 20.2 1.0 10 0.75 0.1 57.3 69 1.1 11 0.875 0.1 37.5 45 0.73 12 0.75 0.2 52.3 63 0.94

The results of Table IV show a significant increase in the yield of neutral sodium sulfate at a weight ratio of methanol to sodium sesquisulfate of 0.1: 1.

(d) Critical upper limit of the ratio by weight of methanol to sodium sesquisulphate

The results of the experiments showing the decisive importance of the upper limit of the weight ratio of methanol to sodium sesquisulfate are reproduced in Table V below.

T table V

Attempt number Weight ratio Yield H2SO4 in solids (%) H2O: NasH (SO4) 2: CHsOH: Na3H (SO2O2 weight (g) (0 / (1) 13 0.75 1.0 64.5 77 0.43 14 0.75 2,0 67.5 81 0.27 15 Dec 0.75 2.5 66.8 80 0.47 As is clear from the results shown in Results of experiments under condition when the molar ratio of methanol to among critical ones by H2O boundaries: sodium sesquisulphate exceeds 2: 1; ; NasH (SO4) 2 and CH3OH: Na3H (SO4) 2, a further increase in yield is obtained. optimal results, are listed below Table VI. (e) Results within the preferred range Table VI Try Weight ratio Yield H2SO4 v number H2O: CH3OH: mass (%) solids : Na3H (SO4) 2 NasH (SO 4) 2 (G) (%) 16 0,645 0.34 63.3 76 0.53 17 0.75 0.4 64.5 77 0.56 18 0.75 0.6 67.3 81 0.34 19 Dec 0.75 0.8 70.7 85 0.13

As can be seen from the results of Table VI, with a weight ratio of H 2 O: Na 5 H (SO 4) 2 in the preferred range of 0.6 to 0.8: 1 and a molar ratio of Na 2 N 2: NaH (SO 4) 2i in the preferred range of 0.3 to 0.8: 1 the product yield is greater than 75%, with an acid content of less than 0.6% H2SO4.

Example 3

This example illustrates the use of alcohols or ketones other than methanol.

The process of Example 2 was repeated except that methanol was replaced with acetone and ethanol in two experiments carried out at 20 ° C for 10 minutes with slow stirring. The results are reproduced in Table VII below.

Table VII

Solvent Weight ratio H2O: : Na 5 H (SO 4) 2 Solvent: : Na 3 H (SO 4) 2 Acetone 0.75: 1 0.4: 1 Ethanol 0.75: 1 0.4: 1

Mass of Na 2 SO 4 recovered (G) % H2SO4 in NazSCH Normality of the aqueous phase 62.4 1.6 5.1 75.9 0.13 5.1

The results of Table VII above illustrate the use of acetone and ethanol in the exchange reaction.

In summary, the present invention provides a highly efficient process for the production of chlorine dioxide, wherein chlorine dioxide is formed without chlorine and by-product neutral sodium sulfate is obtained.

Claims (5)

A process for producing chlorine dioxide in a reaction zone by reducing chlorate ions with chloride ions in an aqueous acidic reaction medium containing sulfuric acid at a concentration greater than 6N and maintained at boiling point below atmospheric pressure and precipitating hydrogen sulphate in the reaction zone, by contacting the precipitated hydrogen sulphate after removal from the reaction zone with water in the presence of a water-soluble alcohol or ketone to form a solid neutral sulfate and an aqueous phase containing sulfuric acid, said aqueous phase being recirculated to the reaction zone. 2. A process according to claim 1, characterized in that sodium hydrogen sulphate is used as the hydrogen sulphate, methanol being used as alcohol and water is used in a weight ratio to sodium hydrogen sulphate, considered as Na3H (SO4) 2, 0.4 to 1.4. And methanol is used in a weight ratio to sodium bisulfate, considered as Na 3 H (SO 4) 2, 0.01 to 2: 1. vynalezu 3. The process of claim 2 wherein the weight ratio of water to sodium hydrogen sulfate is 0.6 to 0.8: 1 and the weight ratio of methanol to sodium hydrogen sulfate is 0.3 to 0.8: 1. 4. A process according to any one of claims 1 to 3 wherein the reaction medium contains sulfuric acid in a concentration greater than 9N and the chloride ions in the reaction medium are produced by reducing the chlorine formed together with chlorine dioxide by methanol. 5. A process according to claim 4, wherein the aqueous phase produced in the contacting step after contacting the hydrogen sulphate with water is separated into two streams containing sulfuric acid and methanol, wherein one stream is recirculated to the reaction zone and methanol is removed from the other stream. forming a methanol stream and an aqueous stream, wherein the methanol stream is recirculated to the contacting stage and the aqueous stream is recirculated to the reaction zone.