WO2004050550A1

WO2004050550A1 - Process for the production of chlorine dioxide

Info

Publication number: WO2004050550A1
Application number: PCT/FI2003/000928
Authority: WO
Inventors: Ilkka Renvall
Original assignee: Kemira Oyj
Current assignee: Kemira Oyj
Priority date: 2002-12-05
Filing date: 2003-12-04
Publication date: 2004-06-17
Anticipated expiration: 2005-06-05
Also published as: FI20022156A0; FI115903B; AU2003283468A1; FI20022156A7

Abstract

The invention relates to a process for the production of chlorine dioxide by reducing chlorate ions under acidic conditions with sulfur dioxide and a coreducing agent. The coreducing agent is methanol.

Description

Process for the production of chlorine dioxide

The present invention relates to a process for the production of chlorine dioxide by reducing chlorate ions under acidic conditions with sulfur dioxide and another compound as reducing agents.

Chlorine dioxide is used in considerable amounts for the bleaching of pulp. It is the main delignifying and bleaching chemical when treating chemical pulp in the environmently friendly ECF (Elemental Chlorine Free) bleaching process. Chlorine dioxide also has other important application areas, e.g. the treatment of waste waters.

A well-known process for the production of chlorine dioxide is the Mathieson process, in which sodium chlorate is reduced by sulfur dioxide under acidic conditions, according to the following formula (1):

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

According to Owen (Tappi 1989 Bleach Plant Operations, p. 157) the stoichiometry of all the commercial processes can be satisfied as follows:

2HC10₃ + 2HC1 → 2C10₂ + Cl₂ + 2H₂0 (2)

The HCIO₃ is fed to generator as sodium chlorate and the acid that is required to provide hydrogen ions is sulphuric acid.

The chlorine produced in the Mathieson process is largely reacting with sulphur dioxide to provide the chloride ion required for keeping the reaction (2) going:

Cl₂ + S0₂ + 2H₂0 → 2HC1 + H₂S0₄ (3)

Since chloride ions are also lost in the process, e.g. as escaped hydrogen chloride and chlorine, it is generated by an overreduction of chlorate:

HCIO₃ + 3S0₂ + 3H₂0 → HC1 + 3H₂S0₄ (4)

Thus the yield of chlorine dioxide based on consumed chlorate will decrease. An addition of chloride ions e.g. as sodium chloride, NaCl is often practised in order to minimize the reaction according to equation (4). The use of large amounts of S0₂ leads to an escape of sulfur dioxide and the reaction thereof with the chlorine dioxide product in the stripping tower, further reducing the yield. In order to operate a Mathieson generator, high acidities must be used, typically 450 to 500 g/1, from 9 to 10 N H₂S0₄.

As a consequence, the yield of a conventional Mathieson process is typically at most 87%. See C.W. Dence, D.W. Reeve, Pulp Bleaching, Principles and Practice, Tappi Press (1996), p 880. But in practice yields lower than 80% has been encountered in industry, especially when running generators at high capacity utilization. Sulfur dioxide is consumed in excess to the stoichiometry of reaction equation (1). Owen (see ibid. p. 173) has indicated a consumption of 1.80 t NaC10₃/t C10₂ and 0.75 t S0₂/t C10₂, which corresponds with an excess of 38% of sulphur dioxide.

In order to improve the Mathieson process, other reducing agents than sulfur dioxide have been used. FI 20000867 has reduced sodium chlorate uncompletely by sulfur dioxide in a first reactor, after which the reduction was completed with hydrogen peroxide in a second reactor. FI 20021413 has reduced the chlorate by means of sulfur dioxide in the presence small amounts of formic acid or a salt thereof.

Small amounts of a catalyst, such as acetic acid, have been contemplated in US 4,929,434, however without touching upon the specific problem of the Mathieson process. The examples have used sulfur or methanol as the reducing agent.

Even the above-mentioned improved Mathieson processes have problems. First, the transportation, liquidification, pressure tanks and safety aspects of the gaseous S0₂ pose a considerable problem. Second, the reaction of S0₂ is slow, giving poor yields. Third, S0₂ is an expensive reducing agent.

In order to overcome these problems, which are specific for the Mathieson sulfur dioxide process, applicant has now invented an improved process, characterized by using, in addition to sulfur dioxide, from 46 to 97% of a C1-C₃ alcohol as reducing agent, based on the combined molar amount of the -C₃ alcohol and the sulfur dioxide.

Irrespective of whether the sulfur dioxide and the -C₃ alcohol are used simultaneously or after one another to reduce the chlorate ions, it brings synergy to the process. Firstly, the -C₃ alcohol itself acts as a reducing agent in the chlorine dioxide process. This is illustrated by the commercial Solvay process, in which the reduction takes place with methanol according to the following formula (5):

CH30H + 2NaC10₃ + 2H₂S0₄ → 2C10₂ + HCHO + 2NaHS0₄ + 2H₂0 (5)

Reactions (2), (3) and (4) are also valid for a Solvay process, but sulphuric acid is not generated in the reaction (4) when sulphur dioxide is substituted by methanol. The total reaction is very slow, which results in a low yield of chlorine dioxide calculated on the feed of sodium chlorate.

The generated formaldehyde could also be reduced further to formic acid and ultimately to carbon dioxide. According to Owen (ibid. p. 173) the methanol is used as an excess of 8%. This contradicts that formaldehyde could further act as reducing agent.

Secondly, the C1-C₃ alcohol acts as an effective solvent for the sulfur dioxide. For example, at the point of saturation, the content of sulfur dioxide in methanol is 31.3% by weight at 26°C and as much as 71.1% by weight at 0°C. The saturation point of ethanol is 25% by weight. In a process involving the use of gaseous sulfur dioxide, the C₁-C₃ alcohol absorbs the gas. In a process involving the direct use of a liquid sulfur dioxide-Cι-C₃ alcohol solution, the inconveniences of gas handling can be avoided altogether. Also, a solution of sulfur dioxide and C1-C₃ alcohol does not have the dangerous properties of pure sulfur dioxide, which is normally transported as liquefied gas.

Thirdly, by dividing the chlorate reduction between sulfur dioxide and the C₁-C₃ alcohol, a more favourable reaction system can be achieved. For, example, if a pure Solvay process is used, the methanol often reacts uncompletely forming essentially only formaldehyde. By using both sulfur dioxide and methanol, the methanol reaction goes to completion and the formation of formaldehyde can be reduced or eliminated.

Fourthly, the distribution in the reaction is even; no distributing plates, so-called spargers, for dividing the sulfur dioxide-air mixture, are needed.

Above, the claimed invention has been described as an improvement of the Mathieson process. The reason is, that it mainly solves the problem caused by using sulfur dioxide. Because in the invention, methanol is preferably used as the C₁-C₃ alcohol, the invention can also be seen as an improvement of the Solvay process. By using sulfur dioxide with methanol in the Solvay process, the yield can be improved considerably. Thus, fifthly, the total amount of reductants can be reduced compared with the Mathieson and Solvay processes.

The C₁-C₃ alcohol used as the second reducing agent is preferably methanol or ethanol, most preferably methanol.

In the claimed process, the sulfur dioxide and the C1-C₃ alcohol can be used in the form of a mixture in one step or several subsequent steps, separately in several subsequent steps, or parallel as a mixture in some steps and separately in other steps. Preferably, a process involving only one step can be used, in which sulfur dioxide and C1-C₃ alcohol are present as a mixture. The mixing can also be carried out before or immediately before contacting the reducing agents with the chlorate ions.

According to a preferred embodiment of the invention, the sulfur dioxide and the C₁-C₃ alcohol are used in the form of a solution. In this embodiment, the C1-C₃ alcohol not only absorbs gaseous sulfur dioxide, but also keps it in solution, thus facilitating the process radically. It is especially advantageous if the C1-C₃ alcohol solution is saturated with the sulfur dioxide at the ambient temperature. Thus, the concentration of sulfur dioxide in the solution is limited by its solubility in the - C₃ alcohol. Depending on the temperature, the pressure, and the need of the process, the concentration of sulfur dioxide in, for example, corresponding to the claimed 46 to 97 mol-% range alcohol, a solution of methanol is from 5 to 70% by weight. More preferably the S0₂ concentration is from 20 to 70% by weight, most preferably from 25 to 40% by weight.

The chlorate ion may originate from any source producing such ions under the reduction conditions of the claimed process. The source may be chloric acid HC10₃ generated e.g. electrochemically. Preferably it is an alkali metal chlorate or an alkaline earth metal chlorate and, most preferably, it is sodium chlorate, NaC10₃, or a mixture thereof with chloric acid. Sodium chlorate is typically obtained by electrolysis of an aqueous sodium chloride solution.

It has also surprisingly been found that the total amount of reduction agents can be lower than in the Mathieson and Solvay processes. Their stoichiometric exess in the invention is typically at most 10 mol-%, preferably at most 5 mol-%, most preferably at most 0 mol-%, and their stoichiometric deficit is typically at most 20 mol-%), based on the molar amount at chlorate (NaC10₃). Still, the unreacted chlorate is kept in the first vessel at an acceptable level.

When carrying out the process according to the invention, the reduction is typically carried out in an aqueous medium containing the chlorate ions. The concentration of the chlorate ion source in the aqueous medium is typically between 10 and 80%o by weight, preferably between 20 and 70% by weight, calculated as sodium chlorate. With other chlorate sources, equimolar amounts are used.

The acid conditions may be brought about via acidic chloric acid, but they are preferably accomplished with a mineral acid, most preferably sulphuric and/or hydrochloric acid. The acidity of the mineral acid is typically between 100 and 650 g/1, preferably between 400 and 500 g/1.

The other reaction parameters, such as the temperature and pressure, can be optimized normally. Typically, the temperature is from about 30°C to about 100°C. The pressure is usually atmospheric or nearly atmospheric.

The process according to the invention may be a batch process, but is typically a continuous process. It may be carried out in one or several, preferably in one or two reactors, whereby the sulfur dioxide and the methanol may be distributed between the reactors in any of the above-disclosed ways. In one embodiment, there is a first reactor containing sulfur dioxide and methanol, and a second reactor containing sulfur dioxide and methanol. In 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 presently claimed sulfur dioxide-methanol combination is not known for certain. Without limiting the scope of the patent, it is believed that both compounds selectively suppress each other's competing side reactions.

Thus, the faster reaction of sulphur dioxide with chloric acid will release chloride ions for the slower reaction of methanol with chloric acid ions, which form a necessary intermediate in the above reaction (2). Sulfur dioxide thus seems to catalyse the otherwise slow and incomplete methanol reaction, because the above reaction (5) proceeds to completion and further produces formic acid HCOOH, and not the formaldehyde intermediate HCHO, in the oxidation step, thus acting as a further reductant. Since formaldehyde has at least partly reacted into formic acid, the need in the invention of a low excess or less than stochiometric amounts of reductants can be understood.

In the following, examples will be given, the sole purpose of which are to illuminate the present invention.

Examples

Example 1 (a reference example 1)

Ten continuous reference experiments were carried out in a reactor, which had a diameter of 80 mm and a height of 4500 mm. The height of the liquor was 3500 mm. A sodium chlorate liquor containing 47 wt-% sodium chlorate at a rate of 1.26 kg/h and 93% sulphuric acid at a rate of 0.4 kg/h were fed into the reactor. A 9% sulphur dioxide gas in nitrogen was also fed into the reactor at a rate of 243 g/h of pure sulfur dioxide or as 136% of the stoichiometric amount. When the reactor reached a steady state, the content of chlorate was 30-40 g/1 and the acid content was 450-480 g/1. The temperature of the reactor was maintained at 45°C. The amount of chlorine dioxide generated was measured. The average yield of chlorine dioxide based on the consumed or reacted sodium chlorate was 86.5%.

Example 2 (reference example 2)

Two continuous experiments were carried out in the same reactor as in example 1 and with the same sodium chlorate feed, but using only methanol as the reduction agent. The feed of methanol was 110 g/h or 123% of the stochiometric amount. The reaction temperature was 50°C.

The acidity was about 470 g/1. The chlorate content after the first reaction vessel was 20.5 resp. 20.4 g/1. The yields obtained were 75.0 and 77.4% calculated from the sodium chlorate feed.

Example 3

Three continuous experiments were carried out with 36% sulphur dioxide dissolved in methanol at the same conditions than in example 1. The feed was 90 g/h and the reaction temperature was 50°C. The acidities varied between 470 and 534 g/1. The chlorate in the spent chlorate varied between 24 and 35 g/1. The average yield obtained was 96.7%. The total amount of reductants was 83% of the stochiometric amount calculated based on that sulfur dioxide would convert into sulphuric acid and methanol into formaldehyde. No formaldehyde could be found and more than 70% of the methanol had converted into formic acid.

The examples show that a much improved yield of chlorine dioxide can be achieved and that the total amount of reductants can be reduced when using the mixture of sulphur dioxide and methanol than using either sulphur dioxide or methanol alone.

Claims

1. A process for the production of chlorine dioxide by reducing chlorate ions under acidic conditions with sulfur dioxide and a C1-C₃ alcohol, characterized in that the amount of C₁-C₃ alcohol is from 46 to 97%, based on the combined molar amount of the C₁-C₃ alcohol and sulfur dioxide.

2. A process according to claim 1, characterized in that the sulfur dioxide and the C₁-C₃ alcohol are in the form of a solution.

3. A process according to claim 2, characterized in that the solution is a C₁-C₃ alcohol solution saturated with sulfur dioxide.

4. A process according to claim 1, 2 or 3, characterized in that the C₁-C₃ alcohol is methanol or ethanol, preferably methanol.

5. A process according to claim 4, characterized in that in the methanol solution, the concentration of sulfur dioxide is from 5 to 70% by weight, preferably from 20 to 70% by weight, most preferably from 25 to 40% by weight.

6. A process according to any one of the preceding claims, characterized in that the chlorate ion source is chloric acid and/or an alkali metal chlorate or an alkaline earth metal chlorate, preferably sodium chlorate or a mixture thereof with chloric acid.

7. A process according to any one of the preceding claims, characterized in that the stoichiometric excess of the total amount of reducing agents is at most 10 mole-

%, preferably at most 5 mole-%, most preferably at most 0 mol-%, and the stoichiometric deficit thereof is at most 20 mole-%, based on the molar amount of chlorate (NaC10₃).

8. A process according to any one of the preceding claims, characterized in that the reduction is carried out in an aqueous medium containing the chlorate ions.

9. A process according to claim 8, characterized in that the chlorate ion concentration calculated as sodium chlorate is between 10 and 80% by weight, preferably between 20 and 70% by weight.

10. A process according to any one of the preceding claims, characterized in that the acid conditions are brought about with a mineral acid, preferably hydrochloric and/or sulfuric acid.

11. A process according to claim 10, characterized in that the acidity of the mineral acid is between 100-650 g/1, preferably 400-500 g/1.

12. A process according to any one of the preceding claims, characterized in that it is continuous.

13. A process according to any one of the preceding claims, characterized in that it is carried out in two successive reactors.

14. A process according to claim 13, characterized in that both reactors contain sulfur dioxide and methanol.

15. A process according to claim 13, characterized in that the first reactor contains sulfur dioxide and methanol, and the second reactor contains hydrogen peroxide.

16. A process according to any preceding claim, characterized in that the sulfur dioxide and the C₁-C₃ alcohol are mixed before, preferably immediately before, contacting them with the chlorate ions.