PT87608B - ELECTROLYTIC PRODUCTION PROCEDURE OF CHLORO DIOXIDE - Google Patents

Abstract

Chlorine dioxide is formed by an autocatalytic electrochemical process from highly acidic chlorate solution containing a residual concentration of chlorine dioxide. Cathodic current is applied to the solution by an electrochemically-active electrode which is chemically inert and does not otherwise catalyze the chemical formation of chlorine dioxide from the acidic chlorate solution. High purity chloride dioxide is obtained while half the acid is consumed and half the water is produced, as compared with a conventional chemical chlorine dioxide generating process.

Description

the present invention relates to the production of chlorine dioxide by electrolysis of highly acidic sodium chlorate solutions.

chlorine dioxide is used as a bleach for various purposes, namely for bleaching paper pulp. Various chemical processes for producing chlorine dioxide by reducing sodium chlorate in aqueous acidic media have been described and remain in commercial operation. The chemical process can be described by equation (i):

2C1O ₃ “+ 2C1 + 4H ⁺ 2C10 ₂ + Cl ₂ + 2H ₂ 0 (I)

American patents NSs. 4,426,263 and 4,362 707 (Hardee et al) describe an electrolytic process for the production of dioxide. chlorine using an electrocatalyst comprising a metal oxide of the platinum group, as a cathode coating, in an electrolytic cell containing sodium chlorate and sul acid. furious. This patent also describes the use of platinum group metal oxides as a catalyst in the absence of applied electrical current and indicates that the latter procedure is the preferred embodiment.

As the material used as the cathode coating is itself a catalyst for the production of chlorine dioxide, the effect of an applied current cannot be easily determined, although the information presented in the patent and also in an article by Hardee describing the electrochemical process of the pateji te (see The Electrochemical Generation of Clorine Dioxide Utilizing Electrolytic Oxide Coatings, Extended Abstracts, vol. 85-1, pp 617-618, The Electrochemical Society, 1985) suggests a slight beneficial effect of the current applied in the production of chlorine dioxide .

According to the article, greater efficiencies are observed at lower current values and therefore less contributions from electrolysis to the overall process of producing di. chlorine oxide. In particular, current efficiencies as low as 20% were observed up to higher current densities

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MIS: 53-480 from. It is believed that the observed loss of efficiency stems from an additional reduction in chlorine dioxide.

The poor results obtained in electrolytic experiments conducted at higher current densities are in good correspondence with the cyclic valtamograms, mentioned in the article by Hardee mentioned above, where the maximum current density observed in the electroreduction of NaClO, 0.5M is lower 2 at 10 mA / cm which, in order of magnitude, is lower than expected for such a high concentration of reducible species.

The experimental results of Hardee's article indicate to the person skilled in the art that this prior art process is not limited by the electrochemical step involving chlorate but by a chemical step in which an electroactive species, different from chlorate, is formed, which then undergoes an electroreduction. Thus, the rate of reduction of the chlorate ion to chlorine dioxide is limited more by a chemical reaction than by an electrochemical reaction, and this chemical reaction can be accelerated by the presence of a catalyst, as described in the patents and in the article by Hardee. Although the platinum group metal oxide catalyst appears to increase the rate of chemical conversion of the chlorate ion into chlorine dioxide, its electrocatalytic properties have a detrimental effect on this electrochemical stability of the desired product, namely chlorine dioxide , when practical corrective densities are applied to electrodes having a surface of said platinum group metal oxides.

Hardee's article also states that platinum group metal oxides are the only materials that show activity in reducing chlorate, and presents results showing the alleged ineffectiveness of platinum.

According to the present invention, an electrochemical process is provided to produce chlorine dioxide, based on an autocatalytic cycle using part of the product, namely chlorine dioxide, to create the next portion of the same product.

It was surprising to see that pure chlorine dioxide,

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-4without any substantial chlorine contamination, it can be produced by passing a cathodic stream through an aqueous acidic solution of chlorate ions with a total acidity greater than that of approximately normal sulfuric acid and removing the chlorine dioxide produced from the aqueous acidic solution .

In the process of the invention, this result (a) is achieved by using a cathode constructed of an electrochemically steel material. which is also chemically inert and non-catalytic in relation to the production of chlorine dioxide from the aqueous acidic solution and, (b) maintaining a concentration of dissolved chlorine dioxide in the aqueous acidic solution throughout the process.

The mechanism for producing chlorine dioxide by the electrochemical process of the invention is believed to involve the chemical reaction between chlorate ions and short-lived chlorite ions, produced electrolytically, to form chlorine dioxide. Part of the chemically produced chlorine dioxide is reduced electrochemically to form chlorite ions, while the rest is removed from the solution as a product.

The reactions that occur are thought to be as follows: C10 ₂ + and C10 ₂ C10 ₂ + C10 ₃ + 2H ⁺ - C10 ₂ + C10 ₂ (t) + H ₂ 0

Total: C10 ₃ + and + 2H ⁺ - * C10 ₂ ⁺

From these equations it can be seen that the process can be considered autocatalytic and that the chlorine dioxide produced is used to produce the active species for reducing chlorate ions. To maintain the autocatalytic cycle, a residual chlorine dioxide concentration must be maintained in the aqueous acid solution.

If the cell is considered in its entirety then the anodic and cathodic reactions can be described as follows:

Cathode: 2C10 ₃ “+ 2e + 4H ⁺ -¼ 2C1Q ₂ + 2H ₂ 0

Anode: H ₂ 0 - 2e - * ^{1/2 +} 0 ₂ + 2H

Cell: 2C10 2C1O ⁺ f + 2H ₂ + ^1/2 ° 2 ° ^{+ H} 2 (II)

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-5- '- ¹ ' is As you can see, by comparing equation (li) with equation (I), the electrochemical process of the invention produces the same amount of chlorine dioxide while producing half the amount of water and consumes half the amount of acid when compared to the chemical process. The chlorine dioxide produced by the process of the invention is in general substantially pure since the reactions involved do not produce chlorine.

Chlorine dioxide is not produced from the aqueous acid chlorate solution while it is in the cell in contact with the electrodes in the absence of applied cathodic current. The cathode used in the process of the invention can be constructed of any suitable electroconductive material that is chemically inert (ie does not have catalytic properties) in relation to the chemical production of chlorine dioxide by reducing chloride ions in the aqueous acid reaction medium, in contrast to the materials described in US Patent Nos.

426 263 and 4 362 707 above. The metals of the platinum group are suitable materials for the cathode and preferably, because it is cheaper and easier to use, carbon in any form, for example graphite and glassy carbon. It is also advantageous to use a carbon cathode because it stabilizes the intermediate state, that is, the chlorite ions, counteracting new electroreduction to a lower valence, such as CIO ”or Cl.

As mentioned above, the chlorine dioxide that is produced electrochemically in this invention is free of chlorine, since chlorine is not produced in the reactions described by the equations presented above. Chlorine production is only possible if chloride ions are present in the reaction medium.

There is the possibility of producing chlorine ions by acidic decomposition of chlorite ions, if the excess of chlorate ion in the acid medium is insufficient, according to the following equation:

5C10 ₂ + 4H ⁺ 4C10 ₂ + Cl ”+ 2H The chloride ion produced in this way can react chemically with the chlorate ions according to the reaction described by

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-6 (i) above, producing chlorine and chlorine dioxide. Although chlorine dioxide continues to form, co-production of chlorine represents an ineffectiveness in relation to the production of chlorine dioxide from chlorate ions, and also contributes to the current inefficiency.

electrochemical process of the invention can be conducted over a wide range of procedural conditions. It is essential for the present invention to provide a solution of aqueous acid electrolyte containing dissolved chlorate ions and having a total strength greater than that of about normal sulfuric acid. For an acidity lower than that of normal sulfuric acid, production of pure chlorine dioxide is not possible.

Acidity can very conveniently be provided by sulfuric acid although any other acid can be used. of the strong mineral other than hydrochloric acid, or a mixture of acids, such as perchloric acid (HCIO ^), orthophosphoric acid (H-PO4) or nitric acid (HNO-j). Avoid using hydrochloric acid as the introduction of chloride ions would produce an undesirable side chemical reaction with chlorate ions to produce chlorine dioxide and chlorine. It is preferred to use an aqueous acid chlorate solution with a total acidity corresponding to that of normal sulfuric acid about 9 to 11.

The chlorate ions in the electrolyte are preferably supplied by sodium chlorate, as this chemical is the most easily obtainable form of chlorate. However, other alkali metal chlorates can be used, such as potassium chlorate, lithium chlorate, rubidium chloride and cesium chlorate, as well as alkaline earth metal chlorates such as beryllium chlorate, magnesium chlorate , calcium chlorate, estrus, strontium rat, barium chlorate and radium chlorate, and mixtures of two or more of these chlorates. The concentration in chlorate ions can vary widely, from about 0.001 to about 7 molar, preferably between about 0.1 to about 2 molar.

In order to maintain the reactions believed to be involved in the electrochemical process of the invention it is essential to maintain a concentration of chlorine dioxide dissolved in the electrolyte at • Hj -H |

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-7long process. The production of chlorine dioxide ceases if all the chlorine dioxide already produced is removed. In addition, the presence of some dissolved chlorine dioxide is necessary to start the process. A concentration of dissolved chlorine dioxide, between about 0.01 and about 15 grams per liter (lpg), preferably between about 0.1 to about 8 gpl, can be used for starting and continuing the reaction .

Although some inefficiency can be tolerated, in order to minimize the side reactions that produce chlorine and maximize the total chemical efficiency of producing one mole of chlorine dioxide for each mole of chlorate ion consumed, the concentration of chlorate ion in the electrolyte should be in substantial excess of the concentration of dissolved chlorine dioxide, in g. a molar excess of at least about 2: 1, preferably at least about 10: 1, usually up to about 1000: 1.

In general, the concentration of dissolved chlorine dioxide is maintained at a substantially constant level during the process, by removing chlorine dioxide at the rate of its formation. The chlorine dioxide decomposes spontaneously at high partial pressures and it is necessary to dilute the chlorine dioxide well below the partial decomposition pressure, usually below about 100 mmHg. Any convenient diluent gas, usually air, can be used to remove the chlorine dioxide produced from the electrolytic cell and to give the necessary dilution. 0 di. chlorine oxide can be recovered from the gas outlet stream by dissolving it in water.

electrical potential applied to the cathode during the electrochemical reaction depends on the construction material of the electrode and usually varies between about +1.0 and about -0.5 volts, when compared to a saturated calomel electrode (SCE). For a carbon electrode, the preferred potential is approx. at +0.4 Volts while for a platinum electrode, the preferred potential is approximately +0.7 Volts. The process is usually operated under conditions of constant potential difference, with the current also preferably constant.

The operating temperature of the cell affects the purity of the obtained chlorine dioxide gas. Higher temperatures fa67 707

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-8the formation of chloride ions by decomposition of chlorite ions, as described above, according to the equation:

5C10 ₂ + 4H ⁺ -> 4C1O ₂ + Cl ”+ 2H ₂ 0

As mentioned above, the formation of chloride ions in this way results in the formation of chlorine, with the consequent loss of efficiency and purity of chlorine dioxide. Thus, it is preferred to operate at temperatures below about 40 ° C, preferably at ambient temperatures from about 209 to about 25 ° C.

The process of the present invention can be carried out in any convenient cell arrangement in which the anode and cathode electrodes are placed and between which current can flow. The cell can be physically divided into anolyte and catholyte chambers by any convenient cation exchange membrane. With a split cell arrangement, the aqueous acid chlorate solution is fed to the cathode compartment while the anode compartment is fed with water, the latter compartment containing an electrolyte as a solution. acidic reaction.

However, it is also possible to operate in a non-dividing cell or in a cell with a simple separator, other than a membrane. As mentioned above, the electrochemical reaction on the cathode surface is believed to be that of the formation of chlorite ions from chlorine dioxide. In an undivided cell, these chlorite ions will try to migrate to the anode but are consumed by the chlorate ions present in large excess in the electrolyte, forming chlorine dioxide, so that the chlorite ions never reach the anode and, in addition, its lifetime in an acid medium is very short.

The production of chlorine dioxide by the process of the invention is accompanied by the formation of by-products. As already noted, the anode reaction in the cell produces gaseous oxygen that can be released in any convenient way. Other by-products are water produced by the electrochemical reaction and a salt of the chlorate cation and the acid anion consumed in the process. These can be removed by any convenient procedure, e.g. by evaporation and crystallization67 707

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-9 action outside the cell.

The present invention provides for the first time an electrochemical process for producing chlorine dioxide from chlorate, which is not based on chemically catalytic electrode materials. Chlorine dioxide is produced in pure form from an aqueous acidic chlorate solution, passing a cathodic current through the solution, from a cathode constructed of material chemically inert to the formation of chlorine dioxide from the solution. This result is achieved by maintaining a concentration of chlorine dioxide dissolved in the solution.

In the following Examples, reference is made to the attached drawing, in which:

Fig. 1 contains a series of three voltammetric curves obtained in the experiments described in these Examples.

invention is illustrated by the following Examples.

Example 1

Cyclic voltammetric studies were performed in an aqueous solution containing about 0.1 g / l of chlorine dioxide, NaClO ^

M and 10 N, using, in one case, a glassy carbon electrode and, in another case, a platinum disc electrode, each having a surface area of 0.196 cm. The current was plotted as a function of the applied potential difference and the results are reproduced as curves a (glassy carbon) and .b (platinum) in Figure 1. The initial potential applied was +1.0 V vs. SCE, and a scan rate of 0.1 Vs ” ^{1 was used} . Another operation was carried out using glassy carbon in a solution from which sodium chlorate was absent. These results are reproduced as curve C in Figure 1.

From the data presented in curves a and b in Figure 1, it can be seen that both the reduction current from Cl0 ₂ to Cl0 ₂ ”, which is proportional to the concentration of C10 ₂ , and the corresponding reoxidation current from C10 ₂ ” back to C10 ₂ , recorded during consecutive explorations of potential between +1.0 t / and +0.6 1 / verses SCE, increase substantially with the duration of the experiments

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-10 multicyclics. This result indicates a progressive accumulation or self-perpetuating multiplication of chlorine dioxide in the vicinity of the electrode, both for the case of the vitreous carbon electrode and for the platinum electrode. By comparison, curve C in Figure 1 shows that there is no accumulation of chlorine dioxide in the absence of chlorate ions.

Example 2

Electrolytic studies were conducted in a split H cell using crosslinked glassy carbon foam as the cathode material and platinum sheet as the anode material. A potentiostatic mode of operation was adopted at a cathodic potential of +0.2 Volt vs. Hg / Hg ₂ S0 ^ as the reference cathode.

catholyte with a volume of approximately 100 ml contains H ₂ SO ₂ about 10 N, NaClO 1 about 1 M and a variable initial concentration of dissolved C ₁ O ₂ . During electrolysis, the gaseous products, C1O ₂ and Cl ₂ , were extracted into a potassium iodide (Kl) mousetrap by bubbling nitrogen and applying a small level of vacuum. Both the nitrogen flow and the vacuum have been adjusted to maintain a subs electrolyte level. substantially constant in the compartments and, at the same time, in order to maintain a practically constant concentration of chlorine dioxide dissolved in the catholyte, resulting from which it is seen. Chlorine dioxide extraction rate was approximately equal to the chlorine dioxide production rate.

electrolyte was analyzed for C1O ₂ , Cl ₂ , C10 ^ ", Cl", C1O ₂ "and acidity both before and after electrolysis, while the KI trap was analyzed and changed every 30 to 60 minutes.

The results obtained are shown in Table I below

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-12 As can be seen in Table I, the chemical and current efficiencies obtained were very high, especially at room temperature. The current required depended both on the concentration of the dissolved chlorine dioxide and on the temperature, the effect of temperature being much more marked. As you can see from experience N9. 4, a significant increase in the concentration of sodium chlorate did not significantly affect the process. only.

The molar ratio CIC ^ / CIC ^, and therefore the purity of chlorine dioxide, although good in all experiments, was much better at room temperature and is in good correspondence with the fact that the current efficiency depends on the temperature.

Chlorite ions, postulated as short-lived intermediates, in the autocatalytic process were not detected in either the catholyte or anolyte, before and after electrolysis.

Maintaining a residual concentration of chlorine dioxide was a critical condition for the operation of the process. In an experiment in which all the chlorine dioxide was extracted from the solution. tion, chlorine dioxide electroproduction was no longer observed and the current measured under potentiostatic conditions was up to zero.

Summarizing this description, the present invention presents a new process of production of chlorine dioxide by an autocatalytic electrochemical reduction of chlorate ions. Modifications are possible within the scope of this invention.

Claims (9)

1 - Electrochemical process for the production of chlorine dioxide by passing a cathodic electrical current through an aqueous acid solution of chlorate ions with a total acidity greater than that of a 7 N sulfuric acid solution and removal of chlorine dioxide formed from aqueous acid solution, characterized by: (a) using a cathode constructed of an electrochemically active material that is also chemically inert and non-catalytic in relation to the production of chlorine dioxide by said aqueous acid solution, and (b) maintaining a concentration of chlorine dioxide dissolved in said aqueous acid solution during said process. Process according to claim 1, characterized in that an excess molar concentration of chlorate ions is maintained, relative to the concentration of chlorine dioxide dissolved in said aqueous acid solution, of at least about 2: 1. Process according to claim 2, characterized in that the excess molar concentration is up to about 1000: 1. Process according to claim 2 or 3, characterized in that the excess molar concentration is at least about 10: 1, because the aqueous acid solution has a chlorate concentration of about 0.1 to about of 2 molar and because said aqueous acid solution contains sulfuric acid from about 9 N to about 11 N. Process according to one of Claims 1 to 4, characterized in that the concentration of chlorine dioxide dissolved in said aqueous acid medium is in the range of about 0.01 to about 15 g / l. Process according to claim 5, characterized in that the concentration of dissolved chlorine dioxide is in the range of about 0.1 to about 8 g / l. 67 707 MIS: at 53-480 Process according to one of Claims 1 to 6, characterized in that said concentration of dissolved chlorine dioxide remains substantially uniform in said aqueous acid solution during said process, by removing the chlorine dioxide generated from the aqueous acid solution, at a ratio corresponding to the reason for its formation. Process according to one of Claims 1 to 7, characterized in that said cathode is made of carbon. Process according to one of Claims 1 to 8, characterized in that an electrode potential of about +1.0 to about -0.5 Volt is applied to the cathode when compared to a saturated calomel electrode. Process according to one of Claims 1 to 9, characterized in that it is carried out at a temperature below about 402 ° C.