WO2001077012A1

WO2001077012A1 - Process for producing chlorine dioxide by the combined use of different reducing agents

Info

Publication number: WO2001077012A1
Application number: PCT/FI2001/000368
Authority: WO
Inventors: Ilkka Renvall; Anna Sundquist; Seppo Pohjanvesi; Jarmo Reunanen; Pentti ELLILÄ
Original assignee: Kemira Chemicals Oy
Priority date: 2000-04-12
Filing date: 2001-04-12
Publication date: 2001-10-18
Also published as: AU2001256376A1; FI108536B; FI20000867A0; FI20000867L

Abstract

The invention relates to a process for continuous production of chlorine dioxide by allowing sodium chlorate, sulfuric acid and a reducing agent to react in a reactor (1) at atmospheric or nearly atmospheric pressure, by continuously feeding an inert gas into the reactor, the temperature being 30 - 100 °C, from which reactor (1) the gas mixture containing mainly chlorine dioxide and inert gas is directed via a scrubber (3) to an absorption tower (4), where the chlorine dioxide is absorbed, and by feeding the reaction solution containing unreacted sodium chlorate from reactor (1) into reactor (2), into which a reducing agent and an inert gas are added, from which reactor (2) the gas mixture containing mainly chlorine dioxide and inert gas is directed via the scrubber (3) to the absorption tower (4). The reducing agent used in reactor (1) is sulfur dioxide and methanol and the reducing agent used in reactor (2) is hydrogen peroxide.

Description

Process for producing chlorine dioxide by the combined use of different reducing agents

The invention relates to a process for continuously producing chlorine dioxide from an alkali metal chlorate and sulfuric acid by using reducing agents at atmospheric or nearly atmospheric pressure. The invention specifically makes existing chloride dioxide processes more efficient without significant investment.

Cellulose mills have recently shifted to ECF bleaching, wherein the only chlorine compound that can be used is chlorine dioxide. Chlorine dioxide is thus used in considerable amounts, especially in the bleaching of cellulose. Efforts are being made to increase the capacity of cellulose mills for production cost reasons, in which case the capacity for the output of products necessary for the production, such as chlorine dioxide, may be a factor limiting the production of cellulose. Therefore it is desirable to find advantageous methods for increasing the capacity of chlorine dioxide production.

It is generally known to prepare chlorine dioxide by allowing an alkali metal chlorate to react in an acidic sulfuric acid solution with a reducing agent. The reducing agent used in the production of chlorine dioxide is hydrochloric acid, methanol, sulfur dioxide or hydrogen peroxide. In all the known processes, only one reducing agent is used. In many known processes an alkali metal chlorate and sulfuric acid are fed continuously into the reactor, into which a reducing agent is added. Gaseous chlorine dioxide is explosive at high concentrations. For this reason the gas emerging from the reactor has to be diluted. This dilution of the gas can be carried out by first directing, for example, air to the bottom of the reactor, whereby the gas leaving the reactor is diluted. This is advantageous especially when the reducing agent used is sulfur dioxide, which even otherwise has to be diluted with air before being introduced into the reactor. Other inert gases, such as nitrogen, can also be used, but in industrial processes this is often too expensive. In processes wherein the reaction is carried out at reduced pressure, it is usual to carry out the dilution by means of steam. The reaction solution from which the alkali metal chlorate has been depleted and which contains sulfuric acid and sodium salts is removed from the reactor in order to be used elsewhere in the cellulose mill, for example, for the production of tall oil by acidulation and for acidifications in different places in the cellulose mill and its bleaching plant. The amount of waste acid may, however, in certain cases be so high that all of the waste acid cannot be exploited without disturbing the sulfur-sodium balance of the mill. In such a case the waste acid has to be directed, for example, to effluent and through this into watercourses. When methanol is used as the reducing agent, the waste acid also contains organic matter formed in the reaction, which in the said case increases the biologic (BOC) and chemical oxygen consumption (COC) of the mill effluents. When the question is of processes operating at atmospheric or nearly atmospheric pressure, often at least two reactors are used, in which case the reaction solution which still contains unreacted alkali metal chlorate is directed from the first reactor to the second reactor, into which air and a reducing agent are fed in order to convert the unreacted chlorate to chlorine dioxide.

One known process for producing chlorine dioxide is the Mathieson process, wherein sulfur dioxide is used as the reducing agent in at least two reactors in series, operating at atmospheric or nearly atmospheric pressure.

The total reaction is of the form:

2NaC10₃ + S0₂ + H₂S0₄ → 2C10₂ + 2NaHS0₄ (1)

However, the total formula does not describe the intermediate stages and the partial reactions taking place in connection with chlorine dioxide production.

It is known that in all chlorine dioxide processes the reaction mediator is the chloride ion even if no chloride ions were added to the reaction system. The stoichiometry of the process is described in a publication (Dennis Owen, Operation and Maintenance of Chlorine Dioxide Generators, TAPPI 1989 Bleach Plant Operations, p. 157) with the following reaction equation:

2 HClO₃ + 2 HC1 → 2C10₂ + Cl₂ + 2H₂0 (2)

Chloric acid HC10₃ forms from sodium chlorate in acid conditions. The chlorine generated further reacts with the sulfur dioxide reducing agent to hydrochloric acid.

Cl₂ + SO₂ + 2H₂0 → 2HC1 + H₂S0₄ (3)

For this reason it is important that the portion of the chlorine formed in Reaction (2) that ends up in a gaseous state when leaving the reactor is caused to react to hydrogen chloride and then to return to the reactor, as is also the hydrochloric acid which has ended up among the gaseous chlorine dioxide.

The corresponding reactions are also valid for most other reducing agents. The reaction equations above are simplified, since in an acid solution chlorine forms hypochloric acid HOC1, via which the reactions take place.

If the amount of chloride in the reaction solution is too low, the reaction medium does not have a sufficient quantity of chloride ions to maintain the reaction, and chloride ions must be formed from chlorate via so-called overreduction, whereby the efficiency of chlorine dioxide formation is lowered. The reaction can be described as follows when sulfur dioxide is used:

2NaClO₃ + 6SO₂ + 6 H₂0 → 2HC1 + 5H₂SO₄ + Na₂SO₄ (4)

For this reason, sodium chloride is added in several chlorine dioxide processes at the time the reactor is being started. The efficiency of the process can also be raised by using in the feed solution a few per cent of sodium chloride, calculated from the weight of the sodium chlorate fed in. If the amount of the chloride fed in exceeds by approximately 6 % by weight the amount of chlorate fed in, there is the risk that the final chlorine dioxide solution will also contain chlorine, which in present-day bleaching processes is no longer desirable.

Significant loss of yield also occurs owing to the side reactions occurring at low acidity, which reactions can be described with the equation:

NaC10₃+ 3S0₂ +3H₂O → NaCl + 3H₂S0₄ (5)

To solve this problem, a high concentration of oxygen (450 - 500 g/1, 9-10 N H₂SO ) is maintained in the Mathieson process, but still the yield is typically limited to 87 % (Editors: Calton W. Dence and Douglas W. Reeve, Pulp Bleaching, Principles and Practice, Tappi Press (1996), p. 880). In practical conditions the yield of chlorine dioxide calculated of the amount of chlorate fed in may be only 80 % and even below this when reactors are loaded to produce the maximum amount of chlorine dioxide.

Another known process for producing chlorine dioxide is the Solvey process, wherein methanol is used as the reducing agent in two normal-pressured reactors in series.

The total reaction is of the form:

CH₃OH + 2NaC10₃ + H₂SO₄ → 2C10₂ + HCHO + Na₂S0₄ + 2H₂0 (6) In the Solvay process, the direct reaction between methanol and the chlorate ion is slow, and thus chloride ions are required as the actual reducing agent even in these reactions, in which case it is advantageous to add sodium chloride to the sodium chlorate fed into the reactor in order to start the reaction.

One disadvantage of the Solvay process is that a large amount of sulfuric acid is required for carrying out the reaction. In this case the amount of waste sulfuric acid is high. The composition of the waste sulfuric acid is usually indicated as sulfuric acid and sodium sulfate, although in many processes the sulfate in the waste sulfuric acid is not in the form of neutral sodium sulfate. Depending on the concentration of the sulfuric acid, the sulfate is in the form of sodium sulfate, acid sodium sesquisulfate (Na₃H(S0₄)₂) or sodium bisulfate (NaHSO₄). In the last-mentioned case, usually the salt cannot be removed, since crystallizing the compound is difficult.

Since the methanol used as a reducing agent does not completely break up to carbon dioxide and water, in the waste solution there is left organic matter which, when directed to effluent, increases the biologic and chemical oxygen consumption of the effluent.

The use of hydrogen peroxide as the only reducing agent in the production of chlorine dioxide is known from US patent 2 332 181. A corresponding reducing agent is also used in US patent 5 380 517. This patent describes a process wherein chlorine dioxide is formed in at least one reaction step, preferably at a pressure of approximately 400 - 900 mmHg (53 - 120 kPa), but also at atmospheric pressure, and at a temperature of approximately 35 - 100 °C within an acidity range of 4 - 14 N. The reaction according to the patent is carried out as substantially chloride free.

The total reaction is of the form:

2NaC10₃ + H₂0₂ + 2H₂S0₄ → 2C10₂ + 0₂ + 2NaHS0₄ + 2H₂0 (7)

EP application 473 559 describes a process wherein chlorine dioxide is produced, by using one reaction vessel, from an alkali metal chlorate with a reaction of a mineral acid and a hydrogen peroxide reducing agent at a temperature of 50 - 100 °C, at reduced pressure and within an acidity range of 2 - 5 N. EP application 473 560 describes a process of a corresponding type, wherein the acidity range is 5 - 11 N. In these processes, however, it is necessary to remove the precipitated alkali metal sulfate from the crystallization zone of the reaction vessel. In Rapson's R2 process an alkali metal chlorate is reduced with hydrochloric acid, which is generated from sodium chloride and sulfuric acid. When the reaction is performed at atmospheric or nearly atmospheric pressure, by using air in the reactor to carry and dilute the chlorine dioxide, it is necessary to use highly acid conditions, whereby a very large amount of waste acid is formed.

NaC10₃ + NaCl + H₂SO₄ → C10₂ + 1/2 Cl₂ + H₂0 + Na₂SO₄ (8)

As a byproduct there is obtained chlorine, which partly ends up among the chlorine dioxide when the chlorine dioxide is absorbed into water, and partly it has to be absorbed into water or sodium hydroxide, whereby sodium hypochlorite is formed. It is characteristic of the R2 process, as is also of the Mathieson and Solvay processes, that there is formed in the process a large amount of waste sulfuric acid that contains sodium bisulfate. The R2 reaction is carried out in one reaction vessel, since the reaction is considerably more rapid than the Mathieson and Solvay reactions. In the more advanced version R3 of the process, the reaction is carried out in a vacuum and the carrier gas is water vapor, whereby it has been possible to decrease the amount of sulfuric acid. There has also been developed a partial hydrochloric acid process, the so-called R3H process, wherein it has been possible to halve the amount of the sodium sulfate formed. Even in this case a large amount of gaseous chlorine ends up among the chlorine dioxide.

Since shifting to vacuum processes would require considerable investment costs, cellulose mills continue to have interest in increasing the production capacities of reactors being conventionally in two series, operating at atmospheric or nearly atmospheric pressure and using continuous-working processes, such as the Mathieson and Solvay processes and in reducing the adverse effects present therein and thereby in increasing the efficiency.

Thus the object of the present invention is to increase the efficiency and production capacity of conventional processes operating at atmospheric or nearly atmospheric pressure. In particular, processes using the Mathieson and Solvay methods are concerned. The aim is a process wherein, in addition to achieving a high efficiency, the production may be varied within a wide range.

Below, mainly the special characteristics of only the Mathieson process are described in greater detail, but the principle can also be applied to other processes operating at atmospheric or nearly atmospheric pressure, such as the Solvay process. In the Mathieson process, the reducing agent used is sulfur dioxide, which is a gas, and so the reaction occurs between two phases. For this reason the reaction volume must be sufficiently large in order to achieve a high efficiency and a high conversion. If the amount of sulfur dioxide is too low, the reaction medium does not have a sufficient quantity of chloride ions for carrying out the reaction and chloride must be formed from chlorate by overreduction, which lowers the efficiency. If sulfur dioxide is used in excess, there is the risk that the gas scrubber tower will have an excess of sulfur dioxide relative to the chlorine to be reduced. If this sulfur dioxide passes with chlorine dioxide to the absorption step, it reacts with the chlorine dioxide, thereby lowering the yield.

In practice it has been possible to increase the capacity of Mathieson reactors by increasing the height of the primary reactor. This method can be used to a certain limit, whereafter the use of the system becomes difficult, since, for example, the hydrostatic pressure of the liquid column in the reactor causes too high a counterpressure to the feed of gaseous sulfur dioxide and air in processes operating at atmospheric or nearly atmospheric pressure.

The problem with a secondary reactor is that the primary reactor does not contain a sufficient quantity of chloride ions for performing the reaction. For this reason a portion of the chlorate has to be left unreacted or, if the reaction is carried out too far, chloride ions are formed by overreduction, which causes a drop in the efficiency. Overreduction is observable as an increase in the sulfuric acid concentration even if sulfuric acid is not separately added into the secondary reactor. Both cases lead to the situation that the efficiency ratio of chlorine dioxide production decreases and causes a difficult optimization and operating problem in the process, i.e. how much chlorate is it profitable to leave unreacted.

The idea of the present invention is that in a process operating at normal atmospheric pressure or at a nearly atmospheric pressure, such as the Mathieson process, the reaction of the chlorate coming from the primary reaction vessel to the secondary reactor can be promoted, whereby a higher efficiency is achieved as calculated from the chlorate. On the other hand, the concentration of chlorate can be raised in the primary reactor from the normal level 15 - 25 g/1, and the rest of the reaction can be allowed to take place in the secondary reactor in conditions where chlorine dioxide forms more rapidly than normally in a secondary reactor when hydrogen peroxide is used as the reducing agent. Many advantages can be gained through this process. First, the conditions in the primary reactor need not be maintained extreme in order to achieve maximum production. Thus, if it is desired to maintain the production at the same level, the process makes it possible to decrease the production in the first reactor by rendering the conditions milder, and to compensate for this decrease by more efficient production in the secondary reactor. On the other hand, the production of chlorine dioxide can be increased and varied within a wide range while achieving a high, higher than previously, overall efficiency. Production can thus be increased by maintaining the production capacity of the first reactor unchanged and by gaining additional production through more efficient production in the second reactor.

The objects of the invention can be attained by using the conventional Mathieson process taking place in at least two reactors and at atmospheric or nearly atmospheric pressure so that in the first reactor (primary reactor) the reduction is carried out using sulfur dioxide and in the second reactor (secondary reactor) hydrogen peroxide is used as the reducing agent.

According to the invention, the reaction can be carried out in favorable conditions in the first reactor, and the reaction of chlorate can be carried out efficiently to completion in the second reactor.

It has been shown in the present invention that by running the first reactor to a lower than normal conversion and by using hydrogen peroxide for reduction in the second reactor, a financially good production capacity is achieved.

According to the invention, there is thus provided a process for continuous production of chlorine dioxide by feeding an alkali metal chlorate, sulfuric acid, and sulfur dioxide or methanol as the reducing agent into the first reactor and by allowing them to react therein at atmospheric or nearly atmospheric pressure, the temperature being 30 - 100 °C, from which reactor the gas mixture containing chlorine dioxide is directed via a scrubber to an absorption tower, where the chlorine dioxide is absorbed, and by directing the reaction solution containing unreacted alkali metal chlorate from the first reactor to the second reactor, into which hydrogen peroxide is fed as a reducing agent, from which second reactor the gas mixture containing chlorine dioxide is directed via the scrubber to the absorption tower.

According to the invention there is additionally provided a process for increasing the production capacity of an existing or known apparatus for continuous production of chlorine dioxide, which apparatus has a first reactor into which an alkali metal chlorate, sulfuric acid, and sulfur dioxide or methanol as the reducing agent are fed at atmospheric or nearly atmospheric pressure, the temperature being 30-100 °C, and from which reactor the gas mixture containing chlorine dioxide is directed via a scrubber to an absorption tower, where the chlorine dioxide is absorbed, and a second reactor into which the unreacted reaction solution containing alkali metal chlorate from the first reactor and additionally hydrogen peroxide as a reducing agent are fed, and from which second reactor the gas mixture containing chlorine dioxide is directed via the scrubber to the absorption tower.

According to the invention, an inert gas, such as air or nitrogen or a mixture of these, is fed into the first reactor and the second reactor, whereupon a gas mixture containing mainly chlorine dioxide and the inert gas leaves the first reactor, and a gas mixture likewise containing mainly chlorine dioxide and the inert gas leaves the second reactor.

The alkali metal chlorate fed into the first reactor is preferably sodium chlorate.

When hydrogen peroxide is used according to the invention as a reducing agent in the second reactor, the only byproduct is oxygen, whereby the overall amount of detrimental byproducts is further decreased.

The process according to the invention can be carried out in all types of known continuous-working reaction apparatuses wherein atmospheric or nearly atmospheric pressure is used. The reactors used in the Mathieson and Solvay processes are especially suitable. In the preferred embodiment of the invention the process is carried out in two reactor steps by using two reactors. The process can also be implemented by using three reactors if there is a need for further increase of the capacity. In the third reactor it is preferable to use hydrogen peroxide as the reducing agent. Preferably the second reactor is as large as or smaller than the first reactor.

The invention is described below in greater detail with reference to accompanying Figure 1, which shows one process diagram of the principle of the Mathieson process, equipped with a gas scrubber.

In one embodiment of the invention, the generation of chlorine dioxide takes place in reactors 1 and 2. Sodium chlorate and sulfuric acid, as well as sulfur dioxide used for reduction, are fed into reactor 1. An inert gas is fed into reactors 1 and 2. The gaseous chlorine dioxide generated in reactor 1 travels via a scrubber 3 to an absorption tower 4, where the waste gases leave and an aqueous solution of chlorine dioxide is collected into a storage container. The reaction solution is directed from reactor 1 continuously to reactor 2, where reduction is carried out with hydrogen peroxide. From reactor 2 the gaseous chlorine dioxide generated is directed via the scrubber 3 to the absorption tower 4. The residue from reactor 2 is directed to a stripper 5.

Usually air is used as the inert gas in the process according to the invention. When emphasis is given to safety it is good to use nitrogen or a mixture of nitrogen and air. It is especially safe to use nitrogen when the process is being started. Carbon dioxide and process exit gases are also possible.

It is preferable to maintain in reactor 1 a chlorate concentration higher than 15 g/1. In reactor 1 the concentration of sulfuric acid is preferably within a range of 100 -

650 g/1. The sulfur dioxide used for reduction is preferably added in an amount of

50 - 110 % of the stoichiometric total amount required for the reduction and hydrogen peroxide in an amount of 10 - 70 %. Usually air is fed into both reactors. nitrogen can also be used as the inert gas, for mixing the reactor medium and for diluting the generated chlorine dioxide. In reactors 1 and 2 a temperature of 30 -

100 °C is usually maintained, and atmospheric or nearly atmospheric pressure is used.

When methanol is used in accordance with the invention for reduction in the first reactor, it is added preferably in an amount of 50 - 100 % of the stoichiometric total amount required for the reduction, and respectively hydrogen peroxide is added into the second reactor in an amount of 10 - 70 %.

According to the invention it is advantageous to return the acid gases to the reactors in order to achieve a high efficiency. In the following examples, return of acid components was not used, owing to practical difficulties. Thus the results obtained are not necessarily the best possible, but they well illustrate the advantages to be gained through the invention.

Example 1 (reference)

Experiments were performed with an apparatus of the type of Figure 1 , made up of two reactors in series. The inner diameter of each reactor was 90 mm and height 4500 mm. The liquid level in the reactors was 2250 mm. Sodium chlorate, sulfuric acid and water, in total 2.90 kg/h, were fed into the first reactor. In the feed, sodium chlorate 6.25 mol h and sulfuric acid 7.40 mol h entered the reactor. Into each reactor there was fed 500 1/h (NTP) of a mixture of nitrogen and sulfur dioxide, of which the amount of sulfur dioxide used as a reducing agent was in each reactor 2.23 mol/h. After the system had reached an equilibrium, the following analyses were obtained for the concentration of sodium chlorate in the solution: feed 227 g/kg, output from the first reactor 153 g/kg, and output from the second reactor 54 g/kg. Thus the total conversion of sodium chlorate was 76 %. The yield of chlorine dioxide was 266 g/h, i.e. 63 % of the sodium chlorate fed in. Of the reacted sodium chlorate, 82 % was recovered as chlorine dioxide.

Example 2 (invention)

The following experiment was performed with the apparatus according to Example 1. Sodium chlorate, sulfuric acid and water, in total 2.88 kg/h, were fed into reactor 1. In the feed, sodium chlorate 6.21 mol h and sulfuric acid 7.49 mol/h entered the reactor. Into the first reactor there was fed 500 1/h (NTP) of a mixture of nitrogen and sulfur dioxide, of which the amount of sulfur dioxide used as a reducing agent was 2.23 mol/h. Into the second reactor, 500 1/h (NTP) of pure nitrogen was fed. As a reducing agent, 1.68 mol/h of hydrogen peroxide was fed into this reactor. After the system had reached an equilibrium, the following analyses were obtained for the concentration of sodium chlorate in the solution: feed 226 g/kg, output from the first reactor 148 g/kg, and output from the second reactor 57 g/kg. Thus the total conversion of sodium chlorate was 75 %. The recovery of chlorine dioxide was 276 g/h. i.e. 66 % of the sodium chlorate fed in. Of the reacted sodium chlorate, 87 % was recovered as chlorine dioxide.

Example 3 (invention)

The following experiment was performed with the apparatus according to Example 1. Sodium chlorate, sulfuric acid and water, in total 2.28 kg/h, were fed into reactor 1. In the feed, sodium chlorate 6.21 mol/h and sulfuric acid 7.49 mol/h entered the reactor. Into the first reactor there was fed 500 1/h (NTP) of a mixture of nitrogen and sulfur dioxide, of which the amount of sulfur dioxide used as a reducing agent was 2.23 mol/h. Into the second reactor, 500 1/h (NTP) of pure nitrogen was fed. As a reducing agent, 2.19 mol/h of hydrogen peroxide was fed into this reactor. After the system had reached an equilibrium, the following analyses were obtained for the concentration of sodium chlorate in the solution: feed 226 g/kg, output from the first reactor 151 g/kg, and output from the second reactor 33 g/kg. Thus the total conversion of sodium chlorate was 85 %. The recovery of chlorine dioxide was 327 g/h. i.e. 78 % of the sodium chlorate fed in. Of the reacted sodium chlorate, 91 % was recovered as chlorine dioxide. Example 4 (reference)

The apparatus of Example 1 was altered so that the liquid level in the 1^st reactor was 3750 mm and in the 2^nd reactor 1500 mm. Sodium chlorate, sulfuric acid and water, in total 2.225 kg/h, were fed into reactor 1. In the feed, sodium chlorate 6.09 mol/h and sulfuric acid 7.34 mol/h entered the reactor. 500 1/h (NTP) of nitrogen was fed into reactor 1 and 300 1/h (NTP) into reactor 2. Sulfur dioxide in total 4.46 mol/h was fed as a reducing agent into both reactors so that 85 % was fed into reactor 1 and 15 % into reactor 2. After the system had reached an equilibrium, the following analyses were obtained for the concentration of sodium chlorate in the solution: feed 292 g/kg, output from the first reactor 54 g/kg, and output from the second reactor 3 g/kg. Thus the total conversion of sodium chlorate was 99 %. The recovery of chlorine dioxide was 392 g/h, i.e. 95 % of the chlorate fed in. Of the reacted sodium chlorate, 96 % was recovered as chlorine dioxide. The yield in the first reactor was 97 % and in the second reactor 89 %.

Example 5 (invention)

The following experiment was performed with the apparatus according to Example 4. Sodium chlorate, sulfuric acid and water, in total 2.679 kg/h, were fed into reactor 1. In the feed, sodium chlorate 7.11 mol/h and sulfuric acid 8.87 mol/h entered the reactor. 500 1/h (NTP) of nitrogen was fed into reactor 1 and 300 1/h (NTP) into reactor 2. As a reducing agent, sulfur dioxide 3.79 mol/h was fed into reactor 1 and hydrogen peroxide 0.96 mol/h into reactor 2. After the system had reached an equilibrium, the following analyses were obtained for the concentration of sodium chlorate in the solution: feed 283 g/kg, output from the first reactor 82 g/kg, and output from the second reactor 9 g/kg. Thus the total conversion of sodium chlorate was 97 %. The recovery of chlorine dioxide was 445 g/h, i.e. 93 % of the chlorate fed in. Of the reacted sodium chlorate, 96 % was recovered as chlorine dioxide. The yield in both reactors was 96 %.

The results obtained in the examples presented above are compiled in the following Table 1. Table 1

ExReducing Feeds Results ClO₂ yield ample agent NaClO₃ SO₂ H₂0₂ C10₂ NaC10₃ Of Of converchlorate reacted sion fed in chlorate

Rl R2 mol/h mol/h mol/h mol/h % % %

1 S0₂ so₂ 6.25 4.46 3.94 76 63 82

2 S0₂ H₂0₂ 6.21 2.23 1.68 4.09 75 66 87

3 so₂ H₂0₂ 6.21 2.23 2.19 4.84 85 78 91

4 so₂ so₂ 6.09 4.46 5.82 99 95 96

5 so₂ H₂0₂ 7.11 3.79 0.96 6.59 97 93 96

From the results in the table, a clear improvement can be observed when the process with two reducing agents according to the invention is used as compared with the conventional Mathieson process in which sulfur dioxide alone is used as the reducing agent. Thus Example 3 illustrating the invention leads to a considerably higher yield of chlorine dioxide than does Example 1 (reference), and Example 5 illustrating the invention shows that the production quantity, i.e. the production capacity of the reactors, can be improved considerably relative to Example 4 (reference). Since the parameters used in the examples were not optimized, the yields and efficiencies obtained are only by way of reference and do not precisely describe an optimized process.

Claims

1. A process for continuous production of chlorine dioxide by feeding an alkali metal chlorate, sulfuric acid and a reducing agent into a first reactor (1) and by allowing them to react therein at atmospheric or nearly atmospheric pressure, the temperature being 30 - 100 °C, from which reactor (1) the gas mixture containing chlorine dioxide is directed via a scrubber (3) to an absorption tower (4), where the chlorine dioxide is absorbed, and by feeding the reaction solution containing unreacted alkali metal chlorate from the first reactor (1) into a second reactor (2), into which a reducing agent is added, from which second reactor (2) the gas mixture containing chlorine dioxide is directed via the scrubber (3) to the absorption tower (4), characterized in that the reducing agent used in the first reactor (1) is sulfur dioxide or methanol and the reducing agent used in the second reactor (2) is hydrogen peroxide.

2. The process according to Claim 1, characterized in that an inert gas, such as air, nitrogen or a mixture thereof, is fed into the first reactor (1) and the second reactor (2).

3. The process according to Claim 1 or 2, characterized in that the concentration of chlorate in the first reactor (1) is higher than 15 g/1.

4. The process according to any of the preceding claims, characterized in that the concentration of sulfuric acid in the first reactor (1) is 100 - 650 g/1.

5. The process according to any of the preceding claims, characterized in that chloride ions are added into the first reactor (1).

6. The process according to any of the preceding claims, characterized in that sulfur dioxide is used for the reduction in an amount of 50 - 110 % of the total stoichiometric amount required for the reduction.

7. The process according to any of Claims 1 - 5, characterized in that methanol is used for the reduction in an amount of 50 - 100 % of the total stoichiometric amount required for the reduction.

8. The process according to any of the preceding claims, characterized in that hydrogen peroxide is used for the reduction in an amount of 10 - 70 % of the total stoichiometric amount required for the reduction.

9. A process for raising the production capacity of an existing or known apparatus used for continuous production of chlorine dioxide, having a first reactor (1) into which an alkali metal chlorate, sulfuric acid and a reducing agent are fed continuously at atmospheric or nearly atmospheric pressure, the temperature being 30 - 100 °C, and from which reactor (1) the gas mixture containing chlorine dioxide is directed via a scrubber (3) to an absorption tower (4), where the chlorine dioxide is absorbed, and a second reactor (2), into which there are fed the reaction solution containing unreacted alkali metal chlorate from the first reactor (1 ) and additionally a reducing agent, and from which second reactor (2) the gas mixture containing chlorine dioxide is directed via the scrubber (3) to the absorption tower (4), characterized in that the reducing agent used in the first reactor (1) is sulfur dioxide or methanol and the reducing agent used in the second reactor (2) is hydrogen peroxide.