FI65606C - FOERFARANDE FOER FRAMSTAELLNING AV KLORDIOXID - Google Patents

Description

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Patent · och reglsterstyralsen 'AmMomi udagd och utLtkrlfkon publlcorad /y.U^.ÖH

(32) (33) (31) fyriMty tuolk— «Bi | lf« l prlorkot 24.02.76 19.03.76, 24.08.76 England-England (GB) 7188/76, 11124/76, 35159/76 (71) Erco Industries Limited, 2 Gibbs Road, Islington, Ontario, M9B 1R1,

Canada (CA) (72) William Howard Rapson, Scarborough, Ontario,

Maurice Fredette, Mississauga, Ontario,

Norman Meyers, Islington, Ontario,

Richard Swindells, Caledon, Ontario, Canada (CA) (74) Oy Kolster Ab (54) Method for producing chlorine dioxide -Förfarande för framstäl1 Ning av klordioxid

The invention relates to the high-efficiency production of chlorine dioxide.

Canadian Patent Publication No. 913,328 discloses a process for preparing chlorine dioxide by reducing sodium chlorate with chloride ions in an acidic aqueous reaction medium whose chloride ions and acidity are due to hydrochloric acid.

This known process can be carried out continuously, keeping the reaction medium at the boiling point at the absolute pressure acting on it, while the reaction zone is at a pressure below atmospheric pressure, usually at a pressure of about 20 to about 400 mm Hg. A boiling temperature maintained below the temperature at which substantial decomposition of chlorine dioxide begins to occur causes water to evaporate from the reaction medium, with water vapor acting as a diluent gas for chlorine dioxide and chlorine. Sufficient water evaporates from the reaction medium to achieve equilibrium with the water coming through the volatile water and the reactants and the water generated in the reaction medium, so that the liquid surface in the reaction zone remains substantially constant.

Upon initiation of the process, the reaction medium becomes saturated with the by-product sodium chloride, and sodium chloride precipitates from the reaction medium. Under stable conditions of continuous use, sodium chloride continuously precipitates as the reaction proceeds and is continuously or intermittently removed from the reaction zone.

As competing reactions in the reaction medium, the reactions according to the following equations (1) and (2) take place:

NaClO3 + 2HCl - ^ C102 + 1/2 Cl2 + H2O + NaCl (1)

NaClO3 + 6HCl —3Cl2 + 3H2O + NaCl (2)

The conversion of sodium chlorate to chlorine dioxide is determined by the extent to which the reaction according to Equation (1) is predominant with respect to the reaction according to Equation (2), since no chlorine dioxide is formed in the latter. The high-efficiency production of chlorine dioxide thus depends on the extent to which the reaction of Equation (1) can be made predominant with respect to the reaction of Equation (2).

The present invention relates to the high-efficiency production of chlorine dioxide from sodium chlorate by the continuous process described above and protected by Canadian Patent 913,328 by carefully adjusting to within certain limits the parameters associated with the chlorine dioxide reaction media detailed below. The precipitation of sodium chloride from the reaction medium at the prevailing operating temperatures under constant conditions of continuous use is due to the fact that the reaction medium becomes saturated with sodium chloride. Thus, under certain operating conditions, the sodium chloride content of the reaction medium containing sodium chlorate and hydrogen chloride is determined. The solubility of sodium chloride and thus its concentration in each case depends on the temperature of the reaction medium, the concentration of sodium chlorate present in the reaction medium and the hydrogen ion concentration in the reaction medium, as defined below.

At the set temperature, the solubility of sodium chloride decreases as the sodium chlorate content increases up to the point where the reaction medium is saturated with both sodium chlorate and sodium chloride.

The point at which the solution is saturated with sodium chlorate can be called a "eutectic" point and also corresponds to the minimum concentration of dissolved sodium chloride at ambient temperature and hydrogen ion concentration and thus also the maximum molar ratio of chlorate ions and chloride ions in the reaction medium.

It has been found that when the molar ratio of chlorate ions to chloride ions changes in the reaction medium, the degree of conversion of sodium chlorate to chlorine dioxide and chlorine, respectively, changes according to reaction equation (1) and at the expense of the opposite reaction (2) to sodium chlorate. Thus, the "efficiency" of the reaction and the "gram atom atom%" of chlorine dioxide and chlorine produced in the reaction medium change with the chlorate ion-chloride ion ratio of the reaction medium, whereby the efficiency increases as the chlorate-chloride molar ratio increases.

"Grams of atomic% chlorine dioxide" (i.e., GA% Clt ^) is a quantitative expression that indicates the efficiency of the reaction according to Equation (1), i.e. the conversion of sodium chlorate to chlorine dioxide and chlorine, and indicates the number of chlorine atoms in% the total number of chlorine atoms formed in the gas mixture. It is then

Cl in C109 G.A. % C10? = ----- x 100 (3)

Cl in ClCl 2 + Cl in Cl 2

It follows from Equation (3) that the maximum gram atom atom% of the chlorine dioxide obtained is 50%, which corresponds to an efficiency of 100%, i.e. the case where the reduction of chlorate with hydrochloric acid takes place entirely according to reaction equation (1). Efficiency and G.A. The ratio of% ClCl 2 is shown in Figure 1 of the accompanying drawings.

As mentioned above, as the chlorate-chloride molar ratio in the reaction medium increases, the gram atom% of ClCl 2 formed increases. Since the molar ratio of chlorate to chloride increases as the chlorate content increases up to the eutectic point, by working so that sodium chlorate and sodium chloride are dissolved in their maximum amounts, i. the molar ratio of chlorate to chloride ions is at a maximum, the highest efficiency in the conversion of sodium chlorate to chlorine dioxide is achieved at the reaction temperature and hydrogen ion concentration in question.

In addition, as the solubility of sodium chlorate in the reaction medium increases as the temperature rises and the solubility of sodium chloride remains substantially the same or decreases, it would seem advantageous to operate 65606 at the highest possible temperature and thus achieve the highest chlorate ion-chloride ion molar ratio, and thus the highest G.A. % CIC ^ ·

However, it is known that chlorine dioxide decomposes at elevated temperatures. Thus, it could be expected that the efficiency of chlorine dioxide production could be increased by raising the temperature due to the increase of the chlorate ion to chloride ion molar ratio at the saturation point up to the decomposition temperature, where the efficiency would suddenly start to decrease. Surprisingly, however, it was found that the efficiency does not increase and decrease in this way, but that the efficiency increases as the temperature rises to a peak, and then decreases slowly as the temperature is further raised.

The reason for this unexpected finding is not entirely clear, but the theory is that there is some mechanism that causes the reaction of Equation (2) to become more active at high temperatures, negating the beneficial effects of reduced molar ratio of chlorate ions and chloride ions.

Thus, a balance must be sought between the temperature and the molar ratio of chlorate ion to chloride ion in order to achieve a high efficiency in chlorine dioxide production, which is measured by the G.A. % With C102.

Conversion of sodium chlorate to chlorine dioxide 39-50 G.A. A% ClO 2 value is considered acceptable in this invention. In order to achieve such efficiency with a chlorine dioxide-producing reaction medium saturated with sodium chloride and substantially saturated with sodium chlorate, it is essential that the temperature of the reaction medium be from about 30 ° to about 85 ° C and the chlorate ion to chloride ion molar ratio is about 5.9: 1 - 2.1: 1.

Preferably G.A. % C102 is greater than about 40, which requires temperatures of about 50 ° to about 80 ° C and chlorate ion-chloride ion molar ratios of about 5.3: 1 to about 2.7: 1. More preferably, the reaction medium has a temperature of 60-75 ° C and a molar ratio of about 4.8: 1 to about 3.3: 1. Under optimum conditions to achieve maximum efficiency, the temperature is from about 65 ° to 70 ° C and the chlorate ion to chloride ion molar ratio is from about 4.0: 1 to about 3.3: 1.

When working at the saturation point of the reaction medium for both sodium chlorate and sodium chloride, some sodium chlorate may co-precipitate with sodium chloride if the temperature varies. The presence of sodium chlorate in the solid phase may be detrimental, and it may therefore be advantageous for the reaction medium to be somewhat below the saturation limit with respect to sodium chlorate, in order to avoid precipitation. In general, the reaction medium is at least 85% saturated with sodium chlorate, preferably about 90%, and the term "substantially saturated" as used for the sodium chlorate content refers to these concentrations.

Sodium chlorate and hydrochloric acid are continuously fed to the reaction medium at such a rate as to obtain substantially uniform operating conditions in the reaction medium.

Sodium chlorate is generally fed as an aqueous solution, preferably formed in situ by electrolysis of sodium chloride.

Hydrochloric acid can be of any concentration, but solutions containing from about 30 to about 37.7% HCl are preferably used to limit the amount of water introduced into the reaction medium. If desired, hydrogen chloride gas can also be fed to the reaction medium.

When a particular reaction medium is used, the temperature and the chlorate ion and chloride ion content, the total acid normality contained in the reaction medium are determined and cannot be changed without changing other parameters.

The introduction of hydrochloric acid or hydrogen chloride into the reaction medium results in a certain acidity of the reaction medium which remains substantially constant. The term "acidity" has many different meanings, but usually refers to total acidity, which is determined by titration with a predetermined strength of sodium hydroxide solution to a preselected pH endpoint. This acidity is expressed as normal, i.e. you as the number of g atoms of hydrogen ions per liter of solution, which corresponds to the titration value. The acidity thus defined is referred to herein as "total acid normality".

In aqueous systems containing anions of polybasic acids which may form other anions with hydrogen ions but which are otherwise inert to the system, and in which other anion species have too low a dissociation constant to release such hydrogen ions, the total acidity of the derivative is not the same as that of the formation of anionic species containing "bound hydrogen ions". The concentration of hydrogen ions available for the reaction is referred to herein as the "actual hydrogen ion concentration". Bound hydrogen ions, although not present in solution as dissociated, i.e. true hydrogen ions, are included in the calculated normality obtained from the total acidity titration assay.

6 65606

Thus, the total acid normality is not necessarily an actual measure of the "actual hydrogen ion concentration", i. the concentration of hydrogen ion available for the reaction. The term "actual hydrogen ion concentration" as used herein is the value obtained with a pH meter calibrated with 0.1 N hydrochloric acid, the degree of dissociation of which is assumed to be 100%. This value can be expressed as pH or normality, i.e. as the actual g-atom number of hydrogen ions corresponding to the pH determined, the solution per liter.

The rate of the sodium chlorate reactions according to Equations (1) and (2) above is determined by the actual hydrogen ion concentration and not by the total acid normality. When the chlorine dioxide system contains only chlorate, chloride and hydrogen ions, the actual hydrogen ion concentration in the reaction medium is substantially the same as the total acid normality and acid normalities in the range of about 0.05 to about 0.3 have been found to be suitable for chlorine dioxide production rates.

In the presence of polybasic acid anions of the above type, the actual hydrogen ion concentration should be in the range of about 0.05 to about 0.3 n for a satisfactory rate of chlorine dioxide production. Thus, whether or not polybasic acid anions are present, the actual hydrogen ion concentration in the reaction medium of the invention is maintained at about 0.05 to about 0.3 n.

Anions of polybasic acids that may be present and that are capable of forming other anionic species with hydrogen ions include, for example, sulfate ions, phosphate ions, and monohydrogen phosphate ions. These anions are usually added as alkali metal salts, usually sodium salts, and are used with only one species.

The effect of sulphate, phosphate and monohydrogen phosphate ions can be represented by the following equations: HSO ^ ”-— * H + + SO 4 K = 2 x 10’2 H3PO1 | H + + H2PO4 K = 7.5 x 10 “3 H2P04 ~ HP04 + H + K = 7 x 10” 8

The addition of sodium sulfate to the chlorine dioxide system in this process of the invention can be considered as illustrative of the invention.

65606

The salt addition can be made as an initial charge to the reaction liquid in an amount below the saturation limit in the reaction medium.

Once the salt has dissolved in the reaction medium, little addition, continuous or intermittent, or no addition is required to maintain its concentration because the salt does not precipitate and is not otherwise removed from the reaction medium other than in small amounts.

Sodium chloride precipitated in and removed from the reaction zone can be used to prepare a sodium chlorate solution by electrolysis in an aqueous solution, which sodium chlorate solution can again be used for the reaction.

The production of chlorate by electrolysis is usually enhanced by sodium dichromate. The sodium chlorate solution recirculated to the generator contains sodium dichromate dissolved under these conditions, so that the dichromate is also fed to the generator. Feeding the dichromate results in the accumulation of sodium dichromate in the reaction medium until it becomes saturated with sodium dichromate, and sodium dichromate precipitates from the reaction medium together with sodium chloride.

When the sodium chloride thus precipitated is fed to the chlorate cell, the aqueous solution therein will also contain precipitated dichromate. Thus, with the process running smoothly, the liquid in the chlorate cell being fed to the generator and the sodium dichromate being used in the chlorate cell, the reaction medium is saturated with sodium dichromate, and the sodium dichromate required by the chlorate cell is fed to the chlorate cell together with the sodium precipitated in the generator.

Sodium chloride can also be used in a bleaching plant in the form of sodium hydroxide and chlorine by electrolysis of its aqueous solution.

If neither of these electrolysis processes is desired, sodium chloride can be discarded.

As in the process of Canadian Patent 913,328, in the process of the invention, the volume of the reaction medium is kept substantially constant by equilibrating the water removed as steam with the sodium chlorate solution and hydrochloric acid and the water formed in the reaction.

Some water may be removed with the sodium chloride precipitate, but it is then swept away again.

8 65606

The mixture of chlorine dioxide and chlorine diluted in the reaction with water vapor is removed from the reaction zone. Since there is no complete vacuum in the reaction zone, air is allowed to flow there, so that the gas mixture removed from the reaction zone contains some air in addition to water vapor, chlorine dioxide and chlorine.

The volume ratio of water vapor to chlorine dioxide in the gas mixture obtained from the reaction zone can vary widely, although in the present invention it is always considered to be greater than a value at which decomposition of chlorine dioxide is essential.

Chlorine dioxide is recovered from the gas mixture as an aqueous solution, which can be used, for example, for bleaching wood pulp.

It is known from Canadian Patent Publication 969,735 to improve the reduction of sodium chlorate with hydrochloric acid to form chlorine dioxide using various metal catalysts such as silver ions, manganese ions, arsenic ions, vanadium pentoxide and dichromate ions.

As mentioned above, the chlorine dioxide production efficiency of the present invention can be improved by controlling the operating parameters of the reaction medium. If desired, the efficiency of chlorine dioxide production can be further improved, typically up to 50 G.A. Up to% ClO 2 using one or more such catalysts.

However, in some cases it may be undesirable to include such catalysts in the reaction medium. For example, when silver ions are used and sodium chloride precipitates in the reaction zone, which is removed therefrom and used as an aqueous solution for electrolysis to form sodium chlorate, which is recycled to the chlorine dioxide zone, the silver contained in sodium chloride catalyzes the decomposition of electrolysis products to oxygen.

The invention is illustrated by the following examples.

Example 1

A single-chamber chlorine dioxide generator fed with sodium chlorate solution and hydrochloric acid was continuously operated at atmospheric pressure at the boiling point of the reaction medium at about 63 ° C. The reaction medium was kept saturated with sodium chloride at all times, and the chloride ion-chlorate ion molar ratio was varied by controlling the sodium chlorate concentration in the reaction medium with a suitable feed. Chlorate conversion efficiency to chlorine dioxide expressed as G.A. % Clt 2 values, were determined for each molar ratio, and the results are shown graphically in Figure 2.

'65606 i t

As seen in Figure 2, G. A.% p109 is higher with increasing chlorate ion-chloride ion molar ratio. From these values and the known aqueous solubilities of sodium chlorate and sodium chloride at different temperatures, the chlorate ion-chloride ion molar ratio can be extrapolated to give an approximate theoretical G.A. Maximum of the% ClCl 2 value.

The following table shows the solubilities and extrapolation:

Table

Temperature ° C NaCl 10 NaCl C 10 ~ / Cl "G.A.% CIO

xxmm 40 5.54 2.26 2.4 39.7 50 6.24 2.14 2.9 40.5 60 6.95 1.98 3.6 41.3 70 7.63 1.71 4.5 42.2 80 8.28 1.44 5.9 43.1 90 9.03 1.55 7.7 44.0 100 9.82 0.72 14.3 45.5 x These values are solubilities in water. Hydrochloric acid, chlorine dioxide and chlorine in the reaction medium reduce these values somewhat in the case of the reaction medium used to produce chlorine dioxide.

Example 2

A second set of experiments was performed at temperatures of 70 ° C and above, keeping the reaction medium saturated in each case with sodium chlorate and sodium chloride. At each temperature, G.A. The% ClOj value and results are shown graphically in Figure 3.

It can be seen from Figure 3 that the obtained G.A. The% ClO 2 decreases rapidly as the temperature rises above about 85 ° C, although the chlorate ion-chloride ion molar ratio increases.

Interdependence and effect of two variables, temperature and molar ratio G.A. The% ClO 2 value is shown in Figure 4, which is obtained by combining the results of Figures 2 and 3. As can be seen from Figure 4, a very clear G.A. % ClCl 3 peak 41.7% using a chlorate ion-chloride ion molar ratio of 4.0: 1 at 65-60 ° C to 67-68 ° C and both NaClO 3 and NaCl saturated in solution.

It can also be seen from the curve in Figure 4 that the temperature must be selected from a narrow range and also the molar ratio in order for the chlorine dioxide production to be high efficient.

To prevent precipitation of sodium chlorate from the reaction medium, the reaction medium can be kept saturated with 90% sodium chlorate, resulting in a corresponding reduction in molar ratio of 12% and G.A. % Decrease in C102 by about 0.6% CIO2. The maximum molar ratio G.A. achieved under these conditions. % CIO2 is 41.1% CIO2 at 67 ° C and a chlorate ion-chloride ion molar ratio of 3.6: 1.

Example 3

A chlorine dioxide generating system in which a sodium chlorate solution and hydrochloric acid were fed to a single-chamber chlorine dioxide generator was operated continuously at subatmospheric pressure at the boiling point of the reaction medium. No other anion species were present in the reaction medium, and the actual hydrogen ion concentration was about 0.1 n.

Sodium sulfate was then added to the reaction medium to a concentration of 0.34 m, whereupon the reaction medium became colorless, indicating that chlorine dioxide production had ceased.

The actual hydrogen ion concentration was calculated to be about 0.007 n.

Hydrochloric acid was added to increase the actual hydrogen ion concentration to 0.053, at which point the color of the solution returned to indicate that the reaction had begun. The total acid normality after this addition of chlorohydric acid was Q, 3-n.

Example 4

A similar chlorine dioxide production system as in Example 3 was used without added anions at a hydrogen ion concentration of 0.13-n. Sodium sulfate was added to a concentration of 0.1 m. As in Example 3, the reaction medium became colorless, indicating that the formation of chlorine dioxide had ceased. The actual hydrogen ion concentration was then calculated to be 0.002.

Sufficient hydrochloric acid was added to bring the actual hydrogen ion concentration to its original value of 0.13-n. The color returned, indicating that the reaction had begun again. The total acid normality after this addition of hydrochloric acid was about 1.0 n.

Claims (7)

A process for producing chlorine dioxide by reducing sodium chlorate with hydrochloric acid in a one-piece generator containing an aqueous acidic reaction medium maintained at a boiling point of 30 to 85 ° C by a vacuum of 20 to 400 mmHg, characterized in that the reaction medium is substantially saturated with sodium and sodium. 0.05 - 0.3 N. Process according to Claim 1, characterized in that the reaction medium is also substantially saturated with sodium dichromate. Process according to Claim 1 or 2, characterized in that at least one polybasic acid anion is present in the reaction medium, which is able to form others with anions and hydrogen ions, but is otherwise inert to the system, the other anions having a low dissociation constant to release these hydrogen ions. . Process according to Claim 3, characterized in that the at least one anion is sulphate, phosphate or monohydrogen phosphate. Process according to Claim 3 or 4, characterized in that the anion (s) are present in the reaction medium in the form of a salt, which is added to the reaction medium at the beginning of the reaction in a concentration up to the saturation concentration of the reaction medium under the reaction conditions. Process according to one of the preceding claims, characterized in that the molar ratio of chlorate ions to chloride ions is 5.9: 1 to 2.1: 1. Method according to one of the preceding claims, characterized in that sodium chloride precipitates in the generator and is removed therefrom.