FR2490206A1 - Chlorine di:oxide prepn. by redn. of chlorate with chloride - and treatment of the acid sulphate by=product to produce normal sulphate and sulphuric acid - 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 the production of chlorine dioxide in high yield.

In US Patent 4,081,520 in the name of the present applicant, a process for the production of chlorine dioxide with a high yield is described, using sodium chlorate, sulfuric acid and methanol. The mechanism by which chlorine dioxide is formed is that chlorine, formed at the same time as chlorine dioxide, is reacted on methanol to form chloride ions which then reduce the chlorate ions to form chlorine dioxide and chlorine. The overall reaction can be represented by the following equation 2NaC103 2H2S04 + CH30H> 2C102 + 2NaHSO4 + HCHO + 2H20
The reaction medium from which the chlorine dioxide is formed and which contains sodium chlorate, methanol and sulfuric acid is maintained at its boiling point, generally between 50 and 850C approximately, under a pressure under atmospheric. The evaporated water is used to dilute the chlorine dioxide to remove it from the reaction zone.

The reaction medium has a high total acid norm, greater than about 9N, and the by-product deposited from the reaction medium, once saturation is reached after start-up is sodium acid sulfate which can be sodium bisulfate (NaHS04) or sodium sesquisulfate (Na3 (S04) 2)
The process according to this prior patent is very effective in converting chlorate ions to chlorine dioxide.

A yield of almost 100% is obtained and the chlorine dioxide is eliminated from the reaction zone in the practically chlorine-free state, which can be advantageous in a large number of end-use cases of chlorine dioxide.

 The disadvantage of this prior process, which has hitherto prevented its industrial adoption, lies in the form and in the nature of the solid by-product. Since sodium sulphate precipitates in an acid form, there is obtained, by eliminating this body from the reaction zone, a loss of the acid contained therein. In addition, the acid sodium sulfates are physically difficult to handle and exhibit deliquescence.

 The present invention makes it possible to solve this problem of the prior art by carrying out the metathesis or conversion of sodium acid sulfate into solid phase in an original manner to form neutral sodium sulfate in solid phase and by recovering the acid content for recycling to the reaction zone.

 Although the invention is described here in particular with regard to the conversion of sodium acid sulfate formed in the process of US patent 4,081. 520 already cited in a neutral sodium sulfate, the invention is generally applicable to the conversion into neutral sulfate of a solid acid sulfate removed from any chlorine dioxide formation process based on sulfuric acid, in very acid, in which the reaction medium is kept at its boiling point under subatmospheric pressure and in which the acid sulfate precipitates from the reaction medium in the reaction zone.

 For example, the process of the invention is applicable to the conversion of sodium acid sulfate recovered as a by-product in a chlorine dioxide generator where chloride ions added are used as chlorate reducer in the presence of sulfuric acid. high acidity, as described in patent CA 825.084.

 Furthermore, the process for forming chlorine dioxide which gives the sodium acid sulfate can be a process in which sulfur dioxide is used to reduce the chlorine formed jointly into chloride ions, analogously to methanol in the process of the patent. US 4,081,520 already cited.

The use of sulfur dioxide is described in US Patent 3,933,988 granted to Hooker Chemicals & Plastics
Corporation.

 In the process of the present invention, the solid phase acid sulphate removed from the reaction zone is brought into contact with water, in the presence of a water-soluble alcohol or ketone, preferably in the presence of methanol, to form neutral sulfate in solid phase and to recover sulfuric acid.

 It has previously been suggested to use water and methanol or another water-soluble alcohol or ketone to convert sodium acid sulfate to neutral sodium sulfate.

A process of this kind is described in US Patent 4,104.

365 in the names of Howard and Lobley.

 The purpose of the latter is to recover neutral sodium sulphate in the effluent in the liquid phase originating from processes for the manufacture of chlorine dioxide based on sulfuric acid and using a high normality of total acid. These processes do not take place at the boiling point of the reaction medium and do not crystallize the sodium sulfate formed as a by-product and contained in the reaction medium of the reactor. The raw material of this prior process is an aqueous solution of sodium acid sulfate, unlike the solid phase raw material used in the present invention.

 The prior process includes an initial depletion step which is said to be necessary to remove dissolved gases and residual sodium chlorate which otherwise inhibits the reaction. Such an operation is not necessary for the process according to the invention, since the starting material is an acid sulfate of sodium in solid phase obtained in the reaction zone of formation of chlorine dioxide.

 The process of the present invention differs from the previous process, not only because the raw materials are in different physical forms, but also because the volumes of water and methanol that must be added to the aqueous effluent to form neutral sodium sulfate in the prior process are much larger than the volumes used in the process of the invention.

As a result of these large volumes of water and methanol, the previous process requires considerable evaporation, firstly to recover the methanol used and secondly to concentrate the aqueous sulfuric acid up to a concentration which is suitable for reuse in the generator.

 The invention is described below with reference to the use of methanol and, when appropriate, it should be understood that other water-soluble alcohols or ketones can also be applied.

 In the method according to the present invention, the weight ratio between water and sodium acid sulfate (calculated as Na3H (SO4) 2) is between 0.4: 1 and 1.4: 1 approximately, preferably between 0.6: 1 and 0.8: 1 approximately. Water to sodium acid sulfate ratios in the range indicated are critical to the process of the invention, since a weight ratio of less than 0.4: 1 leads only to poor sulfate conversions acid to neutral sulfate, while a weight ratio greater than 1.4: 1 leads to the dissolution of large quantities of sodium sulfate in the aqueous phase. When weight ratios included in this critical range are used, the aqueous phase obtained contains sulfuric acid having an acid normality sufficient to allow it to be recycled to the chlorine dioxide formation process without concentrating it.

 As mentioned above, the sodium acid sulfate which precipitates from the reaction medium can take different forms, one of which may be Na3H (SO4) 2, depending on the total acid norm of the reaction medium. To calculate the weight of water and methanol to be used to carry out the sodium acid sulfate metathesis, the weight of sodium acid sulfate must be expressed in equivalent weight of Na3H (SO4) 2.

In the case where it is determined that the sodium acid sulfate is in a form other than Na3H (SO4) 2, for example in the form of sodium bisulfate or a mixture of sodium bisulfate and sodium sesquisulfate, converts the weight of sodium acid sulfate to equivalent weight of Na3H (SO4) 2 and, then, the base weight for metathesis is the actual weight of sodium acid sulfate.

On the other hand, in a typical operation in accordance with the process of US Pat. No. 4,104,365 already cited, the weight ratio between water and sodium acid sulfate (considered to be
Na3H (SO4) 2 is approximately 1.83: 1 and the weight of water to be removed in order to be able to reuse the sulfuric acid solution is approximately 3.6 Kg per kg of neutral sodium sulfate recovered (i.e. approximately 4.14 kg per kg of C102 formed).

 The weight ratio between methanol and sodium acid sulfate (calculated as Na3H (SO4) 2) is less critical than the weight ratio of water and can vary from about 0.01 only to about 2: 1. The weight ratio between methanol and sodium acid sulfate is preferably between 0.3: 1 and 0.8: 1 approximately for the preferred weight ratio between water and sodium acid sulfate mentioned above.

Given its miscibility with water, the presence of methanol reduces the volume of water in which sodium sulfate can dissolve and therefore inhibits 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 decreases until a weight ratio of methanol is reached above which new quantities of methanol n do not increase the yield of neutral sodium sulfate in solid phase, the yield limit being about 80 to 85% by weight. No advantage is therefore obtained by increasing the methanol / sodium acid sulfate weight ratio beyond about 2: 1.

 When the preferred process of the invention is adopted, that is to say when the chlorine dioxide is produced by the process of the patent US Pat. No. 4,081,520 already cited, it is possible to directly recycle a proportion of the aqueous phase originating from the metathesis to the reaction medium for the formation of chlorine dioxide to constitute at least part and preferably all of the methanol requirement. This recycling stream also covers part of the sulfuric acid requirements of the chlorine dioxide formation process.

The methanol which is recycled in this way to the reaction medium does not need to be removed from the proportion of recycled aqueous phase. The methanol is removed from the rest of the aqueous phase to obtain a sulfuric acid solution capable of being recycled to the generator.

 The amounts of methanol used according to the present invention are in sharp contrast to those used in the prior process of US Pat. No. 4,104,365 where, in practice, a weight ratio of methanol / sodium acid sulfate (considered to be Ma3H (S04 of about 9.33: 1 and must be recovered for reuse so that the sulfuric acid solution can be concentrated to an acid normality suitable for recycling to the production process of chlorine dioxide.

 The need for water vapor of the process of the present invention, for the evaporation of the aqueous phase, is limited to the amount necessary to entrain methanol and, in a preferred and typical embodiment of the invention, it gives rise to an expenditure of approximately 16 F per tonne of chlorine dioxide produced (calculated with a cost price of 38 F per tonne of steam). This need for steam is considerably lower than that of the process of US Pat. No. 4,104,365 in which heat is required to drive out significant quantities of methanol and to concentrate the sulfuric acid solution and which, in a practical embodiment , entails an expenditure of approximately 224 F per tonne of chlorine dioxide produced, ie almost 15 times the cost of the steam necessary for the process of the present invention.

 Usually, water and methanol are added to sodium acid sulfate in solid phase in the form of a solution containing the desired proportions of water and methanol. However, since the role of methanol is exclusively to ensure a decrease in the solubility of neutral sodium sulfate in the aqueous phase, methanol can be added after initial addition of water.

The process of the invention has been described above with regard to the use of methanol, given the good accessibility of this solvent and its effectiveness in the process of the invention. However, other water-soluble alcohols and ketones can be used, if desired, for example 1
ethanol, n-propanol, isopropanol and acetone.

 The metathesis reaction used in the invention can take place over a wide range of temperatures, usually from 10 to 700C. The reaction proceeds efficiently at room temperature (about 20 to 250C), but usually elevated temperatures are preferred since the reaction rate increases as the temperature rises. Preferably, the temperature is around 20 to 500C.

 The metathesis reaction of the invention can be carried out in any suitable manner. A batch operation can be performed, but continuous operation is preferable, since the process of the invention is associated with a process for the continuous production of chlorine dioxide.

 Metathesis can be carried out in a simple reactor or in a decantation and washing column, as described in detail in DE-OS 2.856.504.

 To facilitate mixing between the aqueous methanol solution and the sodium acid sulfate for the metathesis reaction, stirring can be carried out in a reactor. Although agitation accelerates the mass transfers involved in metathesis, high shear is not necessary and it is sufficient to use gentle agitation, although this takes a longer time.

 The reaction time required to complete the metathesis can vary widely and is usually about 10 minutes with vigorous stirring to about 60 minutes in a separator.

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

 As shown in the drawing, a chlorine dioxide generator 10 produces a gaseous mixture of chlorine dioxide and water vapor passing through pipe 12 and from which chlorine dioxide is absorbed in water to obtain an aqueous solution of this body for use in bleaching wood pulp or in any desired final application.

 The generator 10 produces the chlorine dioxide according to the method of the patent US Pat. No. 4,081,520 already cited, starting from a solution of sodium chlorate supplied to the generator 10 by the pipe 14, of sulfuric acid brought to the generator 10 by the pipe 16 and methanol brought to generator 10 through pipe 18.

 The aqueous reaction medium, which has a total acid norm of more than about 9, is kept at its boiling point below a temperature above which there is a significant decomposition of the chlorine dixoyde and which is usually about 30 to 850C, under subatmospheric pressure corresponding to the boiling point and which is usually about 2.7 to 53.3 kPa and the sodium acid sulfate continuously precipitates from the reaction medium after that -ci reaches saturation after startup.

 The volume of the reaction medium in the generator 10 is kept practically constant by equilibrating the volume of aqueous phase which enters the generator with the volume of water evaporated from the reaction medium, to form the stream of gaseous product 12 and the volume is withdrawn. volume of water as a dispersion medium for sodium acid sulfate in the solid phase.

 The sodium acid sulfate precipitated from the reaction medium in the generator 10 is brought by the pipe 20 to a reactor 22. The sodium acid sulfate, usually sodium sesquisulfate, can be removed from the generator 10 in the form of a slurry with the reaction medium and it can be separated from it by filtration before bringing it to reactor 22.

In reactor 1-2, the sodium acid sulfate is brought into contact with water supplied by pipe 24 and methanol supplied by pipe 26. The weight ratio between water and sodium acid sulfate ( calculated as Na 3 H (SO 4) 2) in the reactor is between 0.4: 1 and 1.4: 1 approximately, preferably between 0.6: 1 and 0.8: 1 approximately. The weight ratio between methanol and acid sodium sulfate (calculated as
Na3H (SO4) 2) is between 0.01: 1 and 2: 1 approximately, preferably between 0.3: 1 and 0.8: 1 approximately.

 The temperature of the media which come into contact with the sodium acid sulfate in solid phase in the reactor 22 is preferably from 20 to 50 ° C. approximately and the metathesis reaction gives neutral sodium sulfate anhydrous solid. The metathesis of sodium acid sulfate gives sulfuric acid, in addition to neutral acid sulfate. The neutral sodium sulphate and the aqueous phase are separated in any suitable way, for example on a filter, and the neutral sodium sulphate is removed by the pipe 28 to use it as desired, in practice to complete the sodium and sulfur contents in a pulp mill with which the chlorine dioxide generator is associated 10.

 The aqueous phase contains sulfuric acid, methanol and a little dissolved sodium sulfate; it is withdrawn through the pipe 30 and it is divided into two streams which, in practice, represent approximately one third of the volume of the aqueous phase recycled through the pipe 32 to the methanol supply pipe 18 to the chlorine diox yde generator , so as to cover at least a part, and preferably all, of the methanol requirement of the generator 10, the possible remainder of this requirement being supplied, via the pipe 33, to the methanol supply pipe 18. The content of sulfuric acid from the aqueous phase of the pipe 32 covers part of the sulfuric acid requirement of the reaction medium of the generator 10.

 The remainder of the aqueous phase, in practice approximately 2/3 of the volume, is brought via the pipe 34 to a methanol exhauster 36, in which the methanol is removed from the aqueous phase.

The methanol vapor is sent via the pipe 38 to a condenser 40 to give liquid methanol which is brought by the pipe 42 to the methanol supply pipe 26 leading to the metathesis reactor 22, the rest of the methanol requirement of the reactor 22 being brought by the pipe 44.

 The methanol-depleted sulfuric acid solution is recycled through pipe 46 to the sulfuric acid supply stream intended for the chlorine dioxide generator 10 and passing through pipe 16. The additional need for sulfuric acid is brought by the pipe 48 to the sulfuric acid supply pipe 16.

 The process described above with reference to the drawing therefore gives chlorine dioxide practically free of chlorine, with a high yield, without it being necessary to add any catalytic species to the reaction medium. At the same time, the sodium sulphate formed as a by-product is obtained under neutral firm conditions, preferably anhydrous, so that the system does not lose sulfuric acid with the by-product.

The system uses one of the reagents, methanol, in the conversion of acid sulfate to neutral sulfate.

The invention is further illustrated by the following examples
EXAMPLE 1
This example illustrates the preparation of sodium acid sulfate according to the method of US patent 4,081,520 already cited.

A chlorine dioxide generator was operated to form chlorine dioxide from sodium chlorate, sulfuric acid and methanol. The reaction medium was kept at its boiling point under atmospheric pressure, and the reaction medium allowed to deposit sodium sesquisulfate. The operating parameters are indicated in Table 1 below.
Working conditions
Temperature 740C
Pressure 18 kPa
Concentration of reagents and flow rate
CH3OH 33%, 3.4 ml / min
H2S04 9 M, 3.6 ml / min
Na103 6.74 M, 10.5 ml / min
Generator liquor concentrations
H2SO4 9.3 N
1.1 M NaC103
Solid phase crystals Na3H (S04) 2
Chlorine dioxide production
flow rate 0.48 g / l / min
relatively yield
with chlorate gas analysis C102) 99% C12> 1%
EXAMPLE II
This example illustrates the metathesis of sodium sesquisulAte under various conditions.

 A series of experiments was conducted in which solid sodium sesquisulfate was contacted with water and / or methanol at room temperature (200C), while stirring the slurry slowly for about 15 minutes.

The series of experiments was carried out to establish the critical limits of the metathesis process and also the optimal operating conditions. In each case, the weight of sodium sulfate recovered and the proportion of sulfuric acid remaining in the sodium sulfate were determined. Considering the efficiency of the process, a residual concentration of sulfuric acid of less than about 1% indicated practically 100% conversion of sodium acid sulfate to neutral sodium sulfate.

(a) Critical lower limit of the weight ratio between water
and sodium acid sulfate.

 Table II below indicates the results of the experiments carried out to show the critical nature of a lower limit of the weight ratio water / Na3H (SO4) 2 of 0.4: 1.

Table II
Exp- Weight ratio yield, H2SO4 in H20 / Na3H (S04) 2 CH30H / Na3H (S04) 2% by weight solid n0 (q)
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: (1) The yield is meaningless given the high acid content of these products.

 The results in Table II above clearly demonstrate that it is essential to provide a water / sodium acid sulfate weight ratio of at least 0.4: 1 to ensure complete conversion to neutral salt by metathesis.

(b) critical upper limit of the water / water weight ratio
sodium acid sulfate.

 The results of the experiments carried out to show the critical nature of an upper limit of the water / Na 3 H (SO 4) 2 weight ratio of 1.4: 1 are indicated in Table III below.

Table III
Exp- Weight ratio H 2 SO 4 in H 2 O / Na 3 H (S 4) 2 CH 3 OH / Na 3 H (S 4) 2% by weight solid No. 4 (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 precipitate
The results of Table III above clearly demonstrate that it is essential to provide a water / sodium acid sulphate weight ratio not exceeding 1.4: 1 in order to obtain a precipitate following the reaction.

(c) Criticality of the presence of methanol during the
metathesis reaction
Table IV below reproduces the results of experiments carried out to show the critical nature of the presence of methanol during the reaction.

Table IV
Exp- Weight ratio yield H 2 SO 4 in rience H 2 O / Na 3 H (S04) 2 CH 30 H / Na 3 H (S0 4) 2% by weight the solid nO (s)
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 in Table IV demonstrate a considerable increase in the yield of neutral sodium sulfate with a methanol / sodium acid sulfate weight ratio of only 0.1: 1.

(d) Criticality of the upper limit of the ratio in
weight methanol / acid sulfate.

The results of the experiments conducted to demonstrate the critical nature of the upper limit of the methanol / sodium acid sulfate weight ratio are indicated in the table.
V below.

Table V
Exp- Ratio by weight H2SO yield in H20 / Na3H (S04) 2 CH3OH / Na3H (S04) 2 x by weight solid
n0 (g) 13 0.75 1.0 64.5 77 0.43 14 0.75 2.0 67.5 81 0.27 15 0.75 2.5 66.8 80 0.47
As can be seen from the results in the table
V above, when the methanol / sodium acid sulfate molar ratio exceeds 2: 1, no further increase in yield is obtained.

(e) Results in the preferred ranges.

 The results of the experiments carried out between the critical limits of ratio H20 / Na3H (S04) 2 and CH 30H / Na3H (S04) 2 to show the optimal results are shown in Table VI below.

Table VI
Exp- Ratio by weight yield H SO in H20 / Na3H (S04) 2 CH30H / Na3H (504) 2% by weight the solid n (q) 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 0.75 0.8 70.7 85 0.13
As can be seen from the results in the table
VI, when the H2O / Na3H (S04) 2 weight ratio is within the preferred range, that is to say between 0.6: 1 and 0.8: 1, and when the CH30H / Na3H (SQ4) 2 molar ratio is included in the preferred range, that is to say between 0.3: 1 and 0.8: 1, yields of product greater than 75% are obtained at an acid level less than 0.6% of H2SO4.

EXAMPLE III
This example illustrates the use of alcohols and ketones other than methanol.

 The process of Example II was repeated, except that the methanol was replaced by acetone and methanol, in two operations carried out at 200C for 10 minutes with slow stirring. The results are shown in Table VII below.

Table VII
Solvent Weight ratio Weight of H2S04 Normality Na2SO4
H20Na3H (SO4) 2 solvent: recovered in
Na3H (SO4) 2 Na2SO4 NA2SO4 aqueous phase (g)
Acetone 0.75: 1 0.4: 1 62.4 1.6 5.1
Ethanol 0.75: 1 0.4: 1 75.9 0.13 5.1
The results of Table VII above illustrate the utility of acetone and ethanol in the metathesis reaction.

 In summary, the present invention provides a very efficient process for the production of chlorine dioxide, in which chlorine dioxide is obtained which does not contain chlorine and as a by-product, neutral sodium sulfate. Modifications are possible within the scope of the invention.

Claims (5)

 1. Process for obtaining chlorine dioxide by reduction of chlorate ions by means of chloride ions in a reaction zone, in an aqueous acid reaction medium containing sulfuric acid in an amount greater than 6N, the medium of reaction being maintained at its boiling point under atmospheric pressure, with precipitation of an acid sulfate in the reaction zone, characterized in that after removing the precipitated acid sulfate from the reaction zone, it is puts in contact with water in the presence of a water-soluble alcohol or ketone to form a neutral sulfate in solid phase and an aqueous phase containing sulfuric acid and that the sulfuric acid is recycled to the reaction medium.  2. Method according to claim 1, characterized in that the acid sulphate is an acid sodium sulphate, the alcohol or the water-soluble ketone is methanol, while a water / sodium acid sulphate weight ratio is used ( calculated as Na3H (SO4) 25 comprised between 0.4: 1 and 1.4: 1 and a methanol / sodium acid sulfate weight ratio (calculated as Na3H (SO4) 2) of between 0.01 is used: 1 and 2: 1.  3. Method according to claim 2, characterized in that the weight ratio water / sodium acid sulfate is between 0.6: 1 and 0.8: 1 and the weight ratio methanol / sodium acid sulfate is between 0.3: 1 and 0.8: 1.  4. Method according to any one of claims 1 to 3, characterized in that the reaction medium has an acid norm greater than 9N and the chloride ions are formed in the reaction medium by reducing with methanol chlorine which forms at the same time as chlorine dioxide.  5. Method according to claim 4, characterized in that the aqueous phase is separated resulting from the step of bringing into contact into two streams containing sulfuric acid and methanol, one of the streams is recycled to the reaction medium, methanol is removed from the other stream to form a methanol stream and an aqueous stream, the methanol stream is recycled to the contacting step and the stream is recycled aqueous to the reaction medium.