FI115903B - Process for the preparation of chlorine dioxide - Google Patents

Description

115903

The present invention relates to a process for the preparation of chlorine dioxide by reducing chlorate ions under acidic conditions using sulfur dioxide and methanol.

Significant amounts of chlorine dioxide are used to bleach the pulp. It is a major delignification and bleaching chemical for chemical pulp processing in an environmentally friendly ECF (Elemental Chlorine Free) bleaching process. Chlorine dioxide also has other important applications, such as waste water treatment.

A well-known process for the preparation of chlorine dioxide is the Mathieson process wherein sodium chlorate is reduced with sulfur dioxide under acidic conditions according to the following formula (1): 2NaClO3 + SO2 + H2SO4 2ClO2 + 2NaHSO4 (1)

According to Owen (Pin 1989 Bleach Plant Operations, p. 157), the stoichiometry of all commercial methods can be carried out as follows: 2HClO3 + 2HCl -? .

*. The chlorine formed in the Mathieson process reacts mainly with sulfur dioxide to provide the chloride ion required to maintain reaction (2):

t I

Cl2 + SO2 + 2H2O - »2HCl + H2SO4 (3) • * ·

Because chloride ions also disappear in the process, for example as leaving hydrogen chloride and •! 25 chlorine, it is formed by the reductive reduction of chlorate: HClO3 + 3SO2 + 3H2O HCl + 3H2SO4 (4) 2 115903

Thus, the yield based on the consumed chlorate of the chlorine dioxide is reduced. The addition of chloride ions, e.g. as sodium chloride, NaCl, is often carried out to minimize the reaction of equation (4).

The use of large amounts of SO 2 results in the removal of sulfur dioxide and its reaction with the chlorine dioxide product in the washing tower, which further reduces the yield. In order for the Mathieson generator to work, large acidifiers, typically 450-500 g / l, 9-10 N H2SO4 must be used.

As a result, the yield of a conventional Mathieson process is typically up to 87%. See. C.W. Dence, D.W. Reeve, Pulp Bleaching, Principles and Practice, 10 Tappi Press (1996), p. 880. But in practice, industrial yields have been lower than 80%, especially when generators are used at high capacity. The sulfur dioxide is consumed in excess compared to the stoichiometry of reaction equation (1). Owen (see above, p. 173) has reported consumption of 1,801 NaClCO 2 Cl 2 and 0.751 SO 2 / t ClO 2, which corresponds to a 38% excess of sulfur dioxide.

15 Reducers other than sulfur dioxide have been used to improve the Mathieson process. In Fl 20000867, sodium chlorate is incompletely reduced with sulfur dioxide in the first reactor, after which the reduction is completed with hydrogen peroxide in the second reactor. In WO 20021413, chlorate is reduced with sulfur dioxide in the presence of small amounts of formic acid 20 or a salt thereof.

: U.S. Pat. No. 4,929,434 discloses small amounts of a catalyst such as acetic acid; poo, without however touching upon the specific problem of the Mathieson process. The examples include ....; used as a reducing agent sulfur or methanol.

'' US-A-3,933,988 discloses a Mathiesson process using sulfur dioxide as a reducing agent, but mentions the use of methanol as an auxiliary reducing agent.

; _-t Even the above improved Mathieson processes have problems. First of all, the transportation, liquefaction, pressure tanks and safety aspects of gaseous SO2 constitute a significant problem. Second, the reaction of SO 2 is slow, which: '·; 30 yields poor yields. Third, S02 is an expensive reducing agent.

To solve these problems inherent in the Mathieson sulfur dioxide process, '·': the applicant has now come up with an improved method of reducing sulfur dioxide and * · methanol. The process is characterized in that the sulfur dioxide and methanol are in the form of 3,115,903 in solution, the concentration of sulfur dioxide being 5 to 70% by weight in the methanol solution.

Regardless of whether sulfur dioxide and C1-C3 alcohol are used simultaneously or sequentially to reduce the chlorate ions, it brings synergy to the process.

First, methanol itself acts as a reducing agent in the chlorine dioxide process. This is illustrated by the commercial Solvay process where the reduction is with methanol according to the following formula (5): CH3OH + 2NaCl03 + 2H2SO4 -> 2Cl2 + HCHO + 2NaHSO4 + 2H2O (5)

Reactions (2), (3) and (4) also apply to the Solvay process, but sulfuric acid is not formed in Reaction (4) when sulfur dioxide is replaced by methanol. The overall reaction is very slow, resulting in a small yield of chlorine dioxide based on sodium chlorate feed.

The formaldehyde formed could also be further reduced to formic acid and finally to carbon dioxide. According to Owen (supra, p. 173), methanol is used in an excess of 8%. This prevents formaldehyde from further acting as a reducing agent.

Second, methanol acts as an effective solvent for sulfur dioxide. For example, at the saturation point, the content of sulfur dioxide in methanol is 31.3% by weight at 26 ° C and up to 71.1% by weight at 0 ° C. In a process using gaseous sulfur dioxide, methanol absorbs the gas. In a process where: a liquid solution of sulfur dioxide and methanol is used directly, the concept of gas can be completely avoided. Also, the sulfur dioxide and methanol solution does not have the hazardous properties of pure sulfur dioxide, which is usually carried as liquid gas.

“· 25 Third, a more favorable reaction system can be achieved by dividing the reduction of chlorate between sulfur dioxide and methanol. For example, if a pure Solvay process is used, the methanol often reacts incompletely to form mainly formaldehyde. Using both sulfur dioxide and methanol, the methanol reaction is complete and the formation of formaldehyde can be avoided. 30 can be prevented or prevented.

Fourth, the distribution is uniform in the reaction, the equalizing plates, so-called,; '' Spargers '', no need for splitting the sulfur dioxide-air mixture.

4, 115903

The above claimed invention has been described as an improvement on the Mathiesson process. This is because it mainly solves the problem of sulfur dioxide use. Because the invention uses methanol, the invention can also be seen as an improvement on the Solvay process. By using sulfur dioxide with methanol in the Solvay-5 process, the yield can be significantly improved. Thus, fifth, the total number of reducing agents can be reduced compared to the Mathieson and Solvay processes.

In the claimed process, sulfur dioxide and methanol can be used as a mixture in one step or in several successive steps, to Erik 10 in several successive steps or in parallel as a mixture in some steps and separately in other steps. Preferably, only a one step process can be used in which the sulfur dioxide and methanol are in a mixture. The mixing may also be carried out before or shortly before contacting the reducing agents with the chlorate ions.

According to a preferred embodiment of the invention, sulfur dioxide and methanol are used as a solution. According to this embodiment, methanol not only absorbs gaseous sulfur dioxide but also holds it in solution, thereby greatly facilitating the process. It is particularly advantageous if the methanolic solution is saturated with sulfur dioxide at ambient temperature. Thus, the concentration of sulfur dioxide in solution 20 is limited by its solubility in methanol. Depending on the temperature, pressure and need of the process, the concentration of sulfur dioxide in, for example, methanol solution is 5 to 70% by weight. ... percent, more preferably 20-70% by weight, most preferably 25-40% by weight.

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The chlorate ion may be derived from any source forming such ions under the reduction conditions of the process of claim 25. The source may be hydrochloric acid HClO3, which is formed, e.g., electrochemically. Preferably. ** ·. it is an alkali metal chlorate or an alkaline earth metal chlorate, and most preferably it is sodium chlorate, NaCl 3, or a mixture thereof with chloric acid. Sodium-. . Chlorate is typically obtained by electrolysis of an aqueous solution of sodium chloride.

; It has also surprisingly been found that the total amount of reducing agents may be less than in the Mathieson process and the Solvay process. Their stoichiometric excess is typically no more than 10 mol%, preferably no more than 5 mol%, most preferably no more than 0 mol%, and they are stoichiometric. The deficiency is typically up to 20 mol% based on the molar amount of chlorate (NaCl 3) 5,115,903. Still, the amount of unreacted chlorate in the first container remains at an acceptable level.

When the process of the invention is carried out, the reduction is typically carried out in an aqueous medium containing chlorate ions. The content of chlorine ion source p-5 in the aqueous medium is typically 10-80% by weight, preferably 20-70% by weight, based on sodium chlorate. Other chlorate ions are used in equimolar amounts.

The acidic conditions may be obtained by the acidic hydrochloric acid, but preferably by the inorganic acid, preferably by sulfuric acid and / or hydrochloric acid. The acidity of the inorganic acid is typically 100-650 g / l, preferably 400-500 g / l.

Other reaction parameters such as temperature and pressure can be optimized in the usual way. Typically, the temperature is from about 30 ° C to about 100 ° C. The pressure is usually at or near atmospheric pressure.

The process of the invention may be a batch process, but is typically a continuous process. It may be carried out in one or more reactors, preferably in one or two reactors, whereby sulfur dioxide and methanol may be distributed into the reactors by any of the above methods. According to one embodiment, it has a first reactor containing sulfur dioxide and methanol, and a second reactor containing sulfur dioxide and methanol. According to another embodiment: the first reactor contains sulfur dioxide and methanol and the second '' · '* reactor contains a third reducing agent, preferably hydrogen peroxide.

The chemical reason for the good performance of the sulfur dioxide-methano * combination as claimed now is uncertain. Without limiting the scope of the patent, it is believed that both compounds selectively suppress competing side reactions of one another.

A faster reaction of sulfur dioxide with chloric acid thus releases the chloride ions to a slower reaction of methanol with the chloric acid ions which form * ;;; an essential intermediate in the above reaction (2). Sulfur dioxide thus appears to catalyze the otherwise slow and incomplete reaction of methanol, since •; ': the above reaction (5) is completed and forms in the oxidation step ...: additionally formic acid HCOOH, no intermediate formaldehyde HCHO, i.e. acting as further reducing agent . Since formaldehyde has reacted, at least in part, to formic acid, the need of the invention for small excess or stoichiometric amounts of reducing agents can be understood.

The following are examples which are intended solely to illustrate the present invention.

5 Examples

Example 1 (Comparative Example 1)

Ten continuous comparative experiments were performed in a reactor having a diameter of 80 mm and a height of 4,500 mm. The height of the liquid was 3 500 mm. Sodium chlorate solution containing 47% by weight of sodium chlorate was introduced into the reactor at a rate of 1.26 kg / h and 93% sulfuric acid at a rate of 0.4 kg / h. The reactor was also fed with 9% sulfur dioxide in nitrogen at a rate of 243 g / h of pure sulfur dioxide or 136% of the stoichiometric amount. When the reactor reached equilibrium, the chlorate concentration was 30-40 g / l and the acid content was 450-480 g / l. The reactor temperature was maintained at 45 ° C. The amount of chlorine dioxide formed was measured. The average yield of chlorine dioxide based on consumed or reacted sodium chlorate was 86.5%.

Example 2 (Comparative Example 2)

Two continuous experiments were conducted in the same reactor as in Example 1 and using the same sodium chlorate feed, but using only ** as a reducing agent; 20 methanol. The methanol feed was 110 g / h or 123% of the stoichiometric amount. The reaction temperature was 50 ° C.

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The acidity was about 470 g / l. The chlorate concentration was in the first reaction vessel. after 20.5 vs. 20.4 g / L. The yields obtained were 75.0 and 77.4% based on sodium chlorate feed.

Example 3 ':' Three continuous experiments were conducted using 36% sulfur dioxide in methanol under the conditions described in Example 1. The feed was 90 g / h and the reaction temperature was 50 ° C. Acids ranged from 470-534 g / l. The chlorate concentration of chlorate consumed ranged from 24 to 35 g / l. The average yield ...: 30 was 96.7%. The total amount of reducing agents was 83% of the stoichiometric amount based on the conversion of sulfur dioxide into sulfuric acid and methanol to form maldehyde. No formaldehyde was detected and more than 70% of the methanol had been converted to formic acid.

The examples show that a significantly improved yield of chlorine dioxide can be achieved and that the total amount of reducing agents can be reduced when using a mixture of sulfur dioxide and methanol as compared to using either sulfur dioxide or methanol alone.

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Claims (14)

115903 A process for the preparation of chlorine dioxide by reduction of chlorate ions under acidic conditions using sulfur dioxide and methanol, characterized in that the sulfur dioxide and methanol are present in a solution, wherein the concentration of sulfur dioxide in methanol solution is 5 to 70% by weight. Process according to claim 1, characterized in that the solution is a methanolic solution saturated with sulfur dioxide. Process according to Claim 1 or 2, characterized in that the concentration of sulfur dioxide in the methanol solution is 20 to 70% by weight, preferably 25 to 40% by weight. Process according to any one of the preceding claims, characterized in that the chlorate ion source is chloric acid and / or alkali metal chlorate or alkaline earth metal chlorate, preferably sodium chlorate or a mixture thereof with chloric acid. Process according to any one of the preceding claims, characterized in that the total amount of stoichiometric excess of the reducing agent is not more than 10 mol%, preferably not more than 5 mol%, most preferably not more than 0 mol%, and its stoichiometric deficiency is not more than 20 mol% calculated. «T» Method according to one of the preceding claims, characterized in that ''; 20 that the reduction is carried out in an aqueous medium containing chlorate ions. • » Process according to Claim 6, characterized in that the chlorate ion content, calculated as sodium chlorate, is 10 to 80% by weight, preferably; * '*; 20-70% by weight. Process according to one of the preceding claims, characterized in that the acidic conditions are obtained by means of an inorganic acid, preferably: hydrochloric acid and / or sulfuric acid. Process according to Claim 8, characterized in that the acidity of the inorganic acid is 100-650 g / l, preferably 400-500 g / l. ,! '; A method according to any one of the preceding claims, characterized in that it is continuous. 115903 Process according to one of the preceding claims, characterized in that it is carried out in two successive reactors. Process according to Claim 11, characterized in that both reactors contain sulfur dioxide and methanol. Process according to claim 12, characterized in that the first reactor contains sulfur dioxide and methanol and the second reactor contains hydrogen peroxide. Process according to any one of the preceding claims, characterized in that the sulfur dioxide and methanol are mixed before, preferably just before, their contact with the chlorate ions.