FI87935B - ELEKTROLYTISK PRODUKTION AV KLORDIOXID - Google Patents

Description

87935 • | Electrolytic production of chlorine dioxide

Elektrolytlsk Produktion av klordioxid g The present invention relates to the production of chlorine dioxide by electrolysis of highly acidic sodium chlorate solutions.

Chlorine dioxide is used as a bleaching agent in a number of different environments, especially for bleaching wood pulp. Many different chemical methods for producing chlorine dioxide by reducing sodium chlorate in aqueous acidic media have been described in the art in the past, and these methods are now in commercial use. This chemical method can be represented by Equation (I): 2C103 * + 2Cl '+ 4H + - * 2C102 + Cl2 + 2H2O (I). . U.S. Patent Nos. 4,426,263 and 4,362,707 to Hardee et al. Describe an electrolytic process for producing chlorine dioxide using an electrocatalyst comprising an oxide of a platinum group metal 20 as a cathode coating in an electrolyte cell containing sodium chlorate and sulfuric acid. This patent also describes the use of platinum group metal oxides as a catalyst without the use of electric current, and according to these publications this latter procedure is the most preferred embodiment.

25

Since the cathode coating material itself acts as a catalyst in the production of chlorine dioxide, the effect of the electric current used cannot be easily determined, but in this patent and in Hardee's article describing the electrochemical process of this patent, (see "The Electrochemical Generation of Chlorine Dioxide Utilizing Elect Coatings ", Extended Abstracts, vol. 85-1, pages 617-618, The Electrochemical Society, 1985), the electric current used has only a small beneficial effect on the evolution of chlorine dioxide.

According to this article, efficiencies are found to be better at lower current values and thus the contribution of electrolysis to the overall process leading to the evolution of chlorine dioxide 35 87935 2 1 is lower. In particular, efficiencies as low as 20% were observed at higher current densities. The observed decrease in efficiency was assumed to be due to further reduction of chlorine dioxide.

5

The poor results obtained in electrolytic experiments with high current densities correspond well to the periodic voltammograms presented in the above-mentioned Hardee article, in which the maximum observed current density jQ in the electro-reduction of 0.5 molar NaClOj is less than 10 mA / cm 2, which is one order of magnitude lower than the reduction in the case of such a high concentration, the expected value.

The experimental data presented in Hardee's article show one skilled in the art that the electrochemical step comprising chlorate does not limit this prior art method, but rather it. . limited by the chemical phase in which non-chlorate-forming electroactive species are formed, which species are then electrically reduced. Thus, the rate at which chlorate ion is reduced to chlorine dioxide is limited by a chemical rather than an electrochemical reaction, and this chemical reaction can be accelerated by the presence of a catalyst, as described in Hardee's patents and article. Although the platinum group metal oxide acting as a catalyst appears to improve the rate at which the chlorate ion is chemically converted to chlorine dioxide, its electrocatalytic properties adversely affect the electrochemical stability of the desired product, chlorine dioxide, when electrodes having such platinum group metal oxide are applied. densities.

30

Hardee's article further mentions that platinum group metal oxides are the only materials that are active for chlorate reduction. The article presents data showing the presumed inefficiency of platinum.

According to the present invention, there is provided an electrochemical process for producing a chloroloxlide based on an autocatalytic cycle utilizing a portion of a product, i.e., 35 3,879,35 l of chlorine dioxide, to develop the next portion of the same product.

Surprisingly, it has been found that pure chlorine dioxide which is not substantially contaminated with chlorine is produced by passing a cathode stream through an acidic aqueous solution of chlorate ions having a total acidity greater than about 7% normal sulfuric acid and removing the chlorine dioxide evolved therefrom.

10

In the process according to the invention, this result is obtained by (a) using a cathode made of an electrochemically active material which is also chemically inert and has no catalytic effect on the formation of chlorine dioxide in this acidic aqueous solution, and (b) maintaining a dissolved concentration of chlorine dioxide in said acidic aqueous solution throughout the process.

The mechanism by which chlorine dioxide is evolved in the electrochemical process of the invention is believed to involve a chemical reaction between chlorate ions and electrolytically generated short-lived chlorite ions to form chlorine dioxide. Some of the chemically formed chlorine dioxide is electrochemically reduced to produce chlorite ions, while the remainder 25 is removed as a product from solution.

The reactions that are expected to occur can be represented as follows: CIO2 + e - ♦ CIO2 30

C102 '+ C103 + 2H + - C102 + C102 (t) + H2O of which: CIO3 · + e + 2H + - »CIO2 + HjO

It can be seen from these equations that the process can be considered autocatalytic in that the chlorine dioxide formed is used to produce active species for the reduction of chlorate ions. The residual concentration of chlorine dioxide in the acidic aqueous solution must be maintained in order to maintain the autocatalytic cycle.

g Looking at the whole cell, the anodic and cathodic reactions can be represented as follows:

Cathode: 2CIO3 · + 2e + 4H + -► 2C102 + 2H2O Anode: H2O - 2e - hO2 + 2H + ^ 10 -

Cell: 2CIO3 · + 2H + + 2C102 + hO 2 + H 2 O (II) • As can be seen from Equation (II) compared to Equation (I), the electrochemical process according to the invention produces the same amount of chlorine dioxide-15 dia, while half less water is generated and half acid is consumed. less compared to the chemical process. According to the invention. . the chlorine dioxide produced in the process is generally substantially pure; because no chlorine is generated in the reactions involved in the process.

No chlorine dioxide is formed in the acidic aqueous solution of chlorate ions when it is in contact with the electrodes in the absence of a cathode current. The cathode used in the process of the invention may be made of any suitable electrically conductive material that is chemically inert (i.e., without catalytic properties) for the chemical formation of chlorine dioxide due to reduction of chlorate ions in an acidic aqueous reaction medium, contrary to the above U.S. Patents. 426 263 and 4 362 707. Suitable cathode materials are, for example, platinum group metals and preferably carbon in any form, for example graphite and vitreous carbon, due to its cheapness and ease of use. The use of a carbon cathode is also advantageous because it stabilizes the intermediate state, i.e. the chlorite ions, so that it is not further electrically reduced to a lower valence state, such as CIO 'or Cl "ions.

As noted above, in the present invention, the electrochemically produced 5,879,35 L of chlorine dioxide is generally obtained without chlorine because no chlorine is generated in the reactions according to the above equations.

Chlorine formation is only possible if chloride ions are present in the reaction medium.

5

There is a formation of chloride ions according to the following equation ... possibility of acid decomposition of chlorite ions if the excess of chlorate ions in the acidic medium is insufficient: 1Q 5C102 '+ 4H + -► 4C102 + Cl * + 2H20 The chloride ion thus formed can then react chemically with the chlorate ions above according to the reaction shown in Equation (I) to give chlorine and chlorine dioxide. Although J5 chlorine dioxide is formed, the formation of chlorine at the same time results in inefficient formation of chlorine dioxide. . ions as well as current inefficiency.

The electrochemical process of the invention can be carried out under many different process conditions. It is essential in the present invention to provide an aqueous acidic electrolyte solution containing dissolved chlorate ions and having a total acidity greater than about 7 normal sulfuric acid. At acids lower than the acidity of about 7-normal sulfuric acid, the formation of pure chlorine dioxide is not possible.

The acidity is most conveniently obtained with sulfuric acid, although any other strong mineral acid, with the exception of hydrochloric acid, or mixtures of these acids may be used, for example perchloric acid (HClO 4), orthophosphoric acid (H 3 PO 4) or nitric acid (HNO 3). Hydrochloric acid is avoided because the incorporation of chloride ions would produce an undesired chemical side reaction with chlorate ions to form chlorine dioxide and chlorine. An acidic aqueous solution of chlorate ions having a total acidity equivalent to about 9-11 normal sulfuric acid is preferred.

The chlorate ions of the electrolyte are preferably obtained from sodium chlorate, 6,879,35 l because this chemical is the most readily available form of chlorate. However, other alkali metal chlorates such as potassium chlorate, lithium chlorate, rubidium chlorate and cesium chlorate, as well as alkaline earth metal chlorates such as beryllium chlorate, magnesium chlorate, calcium chlorate, strontium chlorate, barium chlorate and radium chlorate and radium chlorate can also be used in the process. The concentration of chlorate ions in the electrolyte can vary over a wide range, being about 0.001 to 7 molar, preferably about 0.1 to 2 molar.

10

For the continuous implementation of the reactions which are presumed to be involved in the electrochemical process according to the invention, it is essential: to maintain the dissolved concentration of chlorine dioxide in the electrolyte throughout the process. The formation of chlorine dioxide ceases if all the chlorine dioxide formed is removed. In addition, a small amount of dissolved chlorine dioxide is necessary during the start-up phase. At the first start-up and during the reaction, the concentration of dissolved chlorine dioxide may be kept in the range of about 0.01 to 15 grams / liter (g / l), preferably in the range of about 0.1 to 8 g / l.

20

Although a slight inefficiency is acceptable, in order to minimize side reactions leading to chlorine formation and to maximize the overall efficiency of the chemical reaction of one mole of chlorine dioxide per mole of chlorate ion consumed, chlorine ions should be present in the electrolyte in a substantial excess. this molar excess generally corresponding to a ratio of at least about 2: 1, preferably at least about 10: 1, usually at most about 100: 1.

30

The concentration of dissolved chlorine dioxide is generally maintained at substantially the same level during the process by removing chlorine dioxide at its rate of formation. Chlorine dioxide decomposes spontaneously at high partial pressures, and thus chlorine dioxide must be diluted to a pressure well below this decomposing partial pressure, usually about 100 mm. Any suitable diluent gas, usually air, can be used to remove the 7,879,35 L of chlorine dioxide formed from the electrolyte cell and provide the necessary dilution. Chlorine dioxide can be recovered from the exhaust stream by dissolving in water.

g The amount of electrical voltage applied to the cathode during the electrochemical reaction depends on the electrode material and usually ranges from about +1.0 to -0.5 volts compared to a saturated calomel electrode (KKE). In the case of a carbon electrode, the preferred voltage is approximately +0.4 volts, while in the case of a platinum electrode, the preferred voltage is approximately +0.7 volts. The method is usually carried out under conditions in which the voltage remains constant, the current being likewise preferably constant.

The operating temperature of the cell affects the purity of the chlorine dioxide gas jg obtained. Higher temperatures favor the formation of chloride ions upon decomposition of chlorite ions, as shown above, according to the following equation: 5C102 '+ 4H + - ♦ 4C102 + Cl * + 2H2O 20

As shown above, the formation of chloride ions in this way leads to the formation of chlorine, which adversely affects the efficiency and purity of chlorine dioxide. Thus, it is preferred to operate at temperatures less than about 40 ° C, more preferably at an ambient temperature of about 20-25 ° C.

The method according to the present invention can be carried out using any cell arrangement comprising anode and cathode electrodes, between which a current can be conducted. Cell 30 may be physically divided into anolyte and catholyte chambers by any suitable cation exchange membrane. In a shared cell arrangement, an acidic aqueous solution of chlorate ions is fed to the cathode compartment, while water is fed to the anode compartment, the latter compartment containing an electrolyte such as an acid solution.

However, it is also possible to use an undivided cell 8 87935 Ί or a cell comprising a simple separator which is not a membrane in the method. As mentioned above, the electrochemical reaction at the cathode surface is believed to be the formation of chlorite ion from chlorine dioxide. In an undivided cell, such chlorite-5 ti ions tend to migrate to the anode, but the chlorate ions present in large excess in the electrolyte consume them to form chlorine dioxide so that the chlorite ions never reach the anode, and in addition their lifespan in an acidic medium is very short.

io

In addition to the evolution of chlorine dioxide, the present invention involves the formation of • by-products. As previously stated, the anode reaction of the cell produces gaseous oxygen that can be led out of the cell by any suitable means. Other by-products are the water formed in the electrochemical reaction and the salt between the cationic part of the chlorate and the anionic part of the acid consumed in the process. They can be removed, respectively, by any appropriate:: - operation, such as evaporation or crystallization on the outside of the cell::.

20

The present invention provides, for the first time, an electrochemical process for producing chlorine dioxide from chlorate, which process is not based on chemically catalytic electrode materials. Chlorine dioxide is formed pure from an acidic aqueous solution of chlorate ions by passing a cathode stream through the solution from a cathode whose material is chemically inert to the formation of chlorine dioxide from this solution. Such a result is obtained by maintaining the dissolved concentration of chlorine dioxide in solution.

30

The following examples refer to the accompanying drawing, in which:

Figure 1 shows a series of three buttermetric curves obtained in the experiments described in these examples.

The invention is illustrated by the following examples: 9 37935 1 Example 1

Periodic voltammetric studies were performed with an aqueous solution containing about 0.1 g / l chlorine dioxide, 1 M compound ΝβΟΙΟβ, and g 10 N sulfuric acid H2SO4, using in one case a glassy carbon electrode and in another case a platinum disc electrode with a surface area of 0.196 cm 2 each. The current is plotted as a function of voltage applied, and the results are plotted as curves a (glassy carbon) and b (platinum) in Figure 1. The voltage initially applied was +1.0 V compared to the saturated calomel electrode, and the sweep rate used was: 0.1 Vs "1. the rest of the run was performed using glassy carbon as well as a solution lacking sodium chlorate These results are shown as curve c in Figure 1.

jtj From the data shown in curves a and b in Figure 1, it can be seen that both the stream which reduces chlorine dioxide CIO2 to CIO2 ', which is proportional to the C102 content, and the corresponding stream which oxidizes the C102' ion back to chlorine dioxide C102, which streams were measured · _ · During successive voltage sweeps in the range of +1.0 to +0.6 V (compared to the saturated calomel electrode 20) increases substantially with the duration of the multivariate experiments. This result indicates that chlorine dioxide gradually accumulates in the vicinity of the electrode or that it increases continuously by itself in the vicinity of the electrode, as well as in the case of a glassy carbon or platinum electrode. For comparison, curve c in Figure 1 shows no accumulation of chlorine dioxide in the absence of chlorate ions.

Electrolytic studies were performed in a split H-cell using cross-linked glassy carbon foam as the cathode material and platinum foil as the anode material. A potentiostatic mode of operation was used in the experiment with a cathode voltage of +0.2 volts compared to the reference Hg / 1 2 SO 4 electrode.

35

The catholyte, which had an estimated volume of 100 ml, contained about 1N sulfuric acid, about 1M NaClO 2, and the initial concentration of CIO 2 dissolved in chlorine dioxide varied. During the electrolysis, the gaseous products, CIO2 and CI21, were removed with a separator containing potassium iodide (KI) by bubbling nitrogen through the solution and using a small vacuum. Both the nitrogen flow and the vacuum were adjusted so that the amount of electrolyte in the compartment remained substantially constant and at the same time the concentration of dissolved chlorine dioxide in the catholyte remained substantially constant, thus the rate of chlorine dioxide removal was approximately equal to the rate of chlorine dioxide formation.

The electrolyte was analyzed for CIO2, Cl2, CIO3 ', Cl *, CIO2 * and acidity both before and after electrolysis, while the KI separator was analyzed and replaced after 30-60 minutes.

The results obtained are shown in the following table: 15: *! ' 20 25 30 35 11 87935 I-1-1

I ro i I

I u - «I I

I v. Ό | Ο ΙΛ fO O 'O CM CO i I (M O £ I ^ M M N M r · »- 1

1–031 I

I u E <o I I

I I I

I-T "-1

I I I

I · (0 I I

I · - 3 I I

1-31 I

I - * I I

1 «JtfX I in K) O 'K K» in Ό I

I · - o | O 'O' CO CO O O CO CO I

- I E C .C I I

. * ö) I 9t if V I I

• - - * I ^ c «» I I

:. - I -I-1

. . Ό I I I

., —Le · i I

'' - tt I ID. * I I

. * I t- O tft I I

o | t-x3X) »nmeooK >> and ^ l

- I · - Φ 3 I o O 'co eo co Ό> o I

"* U I> * * I I

; *. till I

. Olmi I

0 I 0 »I I

•. - - I m o I θ 'Ο · Ό ^ ro ο Ό I

Jd | 4- * CK I O 'O' O 'O' o O O 'I

. '. . e I - m I I

• - - φ I o a I I

"* C | _L_J

Eitti I

> * 13 I O 00 Kl O O O 1

♦ J I <0 l-ι | O * - -4 * * “O 'O O I

(0 jto j Ο Ο Ό O O O O I

· - 1 «· - · I fO ^ Κί Ο Ό *" o I

Jd I> I T- I

- I_ I_ J

4> f - I I

a I · I I

I g I I

:; ; e I · - ο ιοκ> οοβο «-» /> ΐΛΐ

Φ j. * + J r-ι | -41 '' ί ΙΛ W "t Ό« 0 O I

- e I CO t <I ** · * «** I

- | Φ - - | O O O O O O * “l

0: 0 I W> I I

m Jd μ -—- 1-- "Ή

XL I I I

... 3: <0 I 1-. I I

- * - »W I <J | ΙΛ (ft ID ΙΛ ΙΛ ΙΛ IS) I

• 3 I t- O I (NJ C> J ίΜ CNJ ΙΛ Ό Ό I

• * “Φ C I * - · I I

· - 2 1 -! - i

. 4U j · <- j I

“‘ + * I C - · I I

---->. I. · M I Ό Ό Ό co rvj co I

x I ^ a j k k go m ό o o I

..... - I O E tt | »·« »» «» I

<0 1 (/) 1.3 I o o o o o o o I

4- * I rg o 3 I I

a I x e «o I I

Jrf I -, --- 1 ----- 1

oil I

4-4 I I I

3 I - »I I

(D I o I Ό Ό Ό ^ CO> ί Ό I

I (0 | »·« »» »· I

C I z loooooool c! · -l -1

O I K> I I

- I O I CO 00 O '<NJ O' ID O 'I

1 lv O I O O N * fNJ o o · - I

m I: (0 (D I O O O 00 o o o I

O I t z I «r-« - I

- * I: (0 «_u _—- —I

O I: (0 - * I I

IEO I 00 ¢ 0 O '* - CO © O' I

I: 0 fi Λ4 | 00 »- * - 0t - * - |

I * E - I ....... I

I «C u IOOOOOOOI

I - 1 - j

I I I

I <M I Ό Ό - * - ΗΊ CO Κ) I

I - | ID ID O 'ID ID ID ΙΌ I

I U | I

I I I · f W M r * r r | 1 ------- · l -i

I O I I

1C | * - r \ i m v »in Ό h- I

I I I

I_I_I

i2 87935 j As can be seen from Table I, the achieved chemical efficiency and current efficiency were very high, especially at ambient temperature. The current required depended on both the concentration of dissolved chlorine dioxide and the temperature, with the effect of temperature being much more significant. Like driving no. 4 can be seen, the clear increase in sodium chlorate concentration did not significantly affect the process.

: The molar ratio CIO2 / Cl2 and thus the purity of chlorine dioxide, which was jq good in all experiments, was much better at ambient temperature, and corresponded well to the temperature dependence of the current efficiency.

- - Chlorite ions, which are claimed to be short-lived intermediates in the autocatalytic process, could not be detected in the catholyte or in the anolyte before or after electrolysis.

Maintaining a residual chlorine dioxide concentration was critical to carrying out the process. In an experiment in which all chlorine dioxide was removed from solution, chlorine dioxide was no longer found to be electrically generated ·· 20, and the current measured under potentiostatic conditions ... decreased to zero.

To summarize this publication, the present invention provides a novel process for the production of chlorine dioxide by autocatalytic cathodic electrochemical reduction of chlorate ions. Modifications are possible within the scope of this invention.

30 35

Claims (10)

An electrochemical process for producing chlorine dioxide by passing a cathodic electric current through an acidic aqueous solution of chlorate ions having a total acidity greater than about 7-normal sulfuric acid and removing the chlorine dioxide formed from this acidic aqueous solution, characterized in that the cathode used in process (a) is an electrochemically active material which is also chemically inert and has no catalytic effect in producing chlorine dioxide from said acidic aqueous solution, and (b) the concentration of dissolved chlorine dioxide in the acidic aqueous solution is maintained throughout the process. Process according to Claim 1, characterized in that a molar excess of chlorate ions relative to the dissolved chlorine dioxide content is maintained in the acidic aqueous solution in a ratio of at least about 2: 1. ! 1.1 Process according to Claim 2, characterized in that the molar excess is at most about 1000: 1. _ · _ · 1; 25 Process according to claim 2 or 3, characterized in that the molar excess is at least about 10: 1, the chlorate content in the acidic aqueous solution is about 0.1-2 molar, and in that the acidic aqueous solution contains about 9-11 normal sulfuric acid. • 30 Process according to one of Claims 1 to 4, characterized in that the concentration of dissolved chlorine dioxide in the acidic aqueous solution is approximately in the range from 0.01 to 15 g / l. Process according to Claim 5, characterized in that the concentration of dissolved chlorine dioxide is approximately in the range from 0.1 to 8 g / l. U 37935 Process according to one of Claims 1 to 6, characterized in that the concentration of dissolved chlorine dioxide is kept substantially constant in the acidic aqueous solution throughout the process by removing the chlorine dioxide formed from the acidic aqueous solution at a rate corresponding to its rate of formation. Method according to one of Claims 1 to 7, characterized in that the cathode is made of carbon. Method according to one of Claims 1 to 8, characterized in that the electrode potential applied to the cathode is approximately +1.0 to -0.5 volts compared with a saturated calomel electrode. Process according to one of Claims 1 to 9, characterized in that it is carried out at a temperature of less than about 40 ° C. 1 · · is 87935