FI121076B - A process for the preparation of alkali metal chlorate - Google Patents

Description

A process for the preparation of alkali metal chlorate

The invention relates to a process for the electrolytic production of alkali metal chlorate wherein the need for pH control chemicals is largely covered by the integrated production of acid 5 and alkali metal hydroxide. The method involves electrolyzing a partial stream of the prepared k1 orate electrolyte in a separator cell to produce an alkali metal hydroxide-containing catalyst and a hydrochloric acid-containing anolyte. The catalyst and anolyte are used in alkalization and acidification in the chlorate process, which greatly reduces the import of impurities with the chemicals produced outside.

Alkali metal chlorate, and especially sodium chlorate, are important chemicals in the cellulose industry, where they are used as a raw material for the production of chlorine dioxide, a major bleaching agent for cellulose fibers.

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The alkali metal chlorate is prepared by electrolysis of a chloride-enriched aqueous electrolyte according to the following formula:

MeCl + 3 H2O-> Me ClO2 + 3 H2 (1) 20 where Me = alkali metal.

The process is run in a circuit in which the chloride electrolyte is introduced into the electrolyzer to form a hypochlorite, followed by the chlorate path electrolyte to be further reacted with the reactor vessels to form the chlorate. The formed chlorate is separated by crystallization, whereas the mother liquor, on the other hand, is returned to the preparation of the chloride dielectrolyte for further electrolysis to hypochlorite.

The pH of the chlorate process cycle is adjusted at several points between 5.5 and 12 to optimize the process conditions for the corresponding unit functions. Thus, a weakly acidic or neutral pH is used in the electrolyzer and in the reaction tanks to facilitate the conversion of chlorine through hypochlorite to chlorate, while the pH is alkaline in the crystallizer to prevent gasification of hypochlorite and chlorine and to reduce the risk of corrosion.

Hydrochloric acid is usually used in acidification, but chlorine is also present. Alkaline metal hydroxide is most often used to alkalize solutions. Hydrochloric acid and alkali metal hydroxide are added as aqueous solutions. The commercially available solutions of hydrochloric acid and alkali metal hydroxide contain impurities which may originate from the chlorine / alkali plant and / or subsequent transport and storage.

5 The chloride electrolyte to be electrolyzed in the chloride cell must not contain large amounts of impurities. Ca2 +, Mg2 +, and SO42 "cause deposits on the cathodes, thereby causing higher operating voltage and energy costs, while the heavy metals decompose the formed hypochlorite into chloride and oxygen instead of the desired chlorate.

10

The alkali metal hydroxide is used to alkalize the chlorate electrolyte prior to crystallization of the chlorate, in the alkaline purification processes and in the regeneration of the ion exchanger. Alkali metal hydroxide is still used to eliminate the presence of gaseous chlorine compounds. Chlorine compounds cause odor, health and corrosion problems, and li-15 furthermore formulates hydrogen gas, which is often used as a raw material for various syntheses. Hydrochloric acid is used to acidify the chlorate electrolyte before electrolysis. In this case, the mixing must be very good, e.g. to prevent the formation of chlorine and chlorine dioxide locally in highly acidic areas, which would lead to explosion hazards and work environment problems.

20

Significant amounts of hydrochloric acid and alkali metal hydroxide are consumed in the production of chlorate, which is a significant cost. In addition, handling acid and lye is complicated due to the high safety requirements for transportation, storage and dosing. Further, the concentration of commercially available products is significantly higher than that which can be directly used in the chlorate process.

EP-A-498,484 discloses a process for the preparation of an alkali metal chlorate together with chlorine and an alkali metal hydroxide, which are used as auxiliary chemicals in the chlorate process. In the preparation of chlorine and alkali metal hydroxide according to this process, an electrolyte containing water from the condenser of a chlorate crystallization tower and purified alkali metal chloride is electrolyzed. This preparation of chlorine and alkali metal hydroxide means a chlor-alkali cell. A catalyst containing 30% by weight sodium hydroxide, corresponding to pH 15, is recycled through the cathode compartment. The anode compartment is used to recycle highly purified sodium chloride-containing anolyte-3 at pH 2 to form chlorine gas at the anode. Both the anolyte and the catalyst do not contain chlorate and they are each passed to well-purified water from the evaporation of the chlorate electrolyte 3. The chlorine gas formed at the anode leaves the anode space and must be incinerated with hydrogen gas, whereby the hydrochloric acid formed can then be introduced into the chlorate process for acidification, or dissolved in a liquid in the process. Although this process reduces the import of impurities, it requires extensive equipment, results in a risky treatment of chlorine gas, and the risk of locally strongly acidic regions in the chlorate electrolyte remains in the acidification process.

According to the invention, there is provided an electrolytic process for producing alkali metal chlorate efficiently and with significantly lower risks to health and the environment, thus rendering much of the chemical introduced into conventional processes unnecessary for acidification and alkalization. The process involves electrolysis in chlorate electrolyzers containing an aqueous, purified alkali metal chloride-containing electrolyte, followed by electrolysis of a partial flow of the thus-obtained chlorate electrolyte in a separator cell to produce an alkali metal hydroxide catalyst at least each catalyst. Chlorine is formed in the anode space of the cell, which can be dissolved immediately and hydrolyzed to the alichloric acid.

The invention thus comprises a process for the preparation of an alkali metal chlorate having the characteristics set forth in the claim. According to the invention, this is an integrated process in which the major part of the process's need for protons and hydroxy-ion is covered in a cell with a separator for electrolysis of the chlorate electrolyte. Electrolysis thus yields the anolyte and the catalyst having a lower pH, respectively, higher than that of the imported chlorate electrolyte.

The process in question reduces the need for externally produced acid and alkali metal hydroxide, thereby reducing the entry of impurities into the chlorate process. Further, the risks in the transport, storage and dosing of acid and alkali metal hydroxide are reduced, so that the auxiliary chemicals are prepared directly in the process. The use of chromate buffer can also be reduced so that the production of alkali metal hydroxide can simply be increased.

The process in question can advantageously be integrated into the production of chlorine dioxide. 3 5 In certain types of integrated mills, it is not possible to introduce alkali metal ions into the process to produce chlorine dioxide. This applies primarily to processes 4 for the production of chlorine dioxide according to the total mass balance

Cl2 + 4 H2O -> 2 C102 + 4 H2 (2) 5 Here, the alkali metal ion, especially sodium, acts only as a counterion to the chlorate and chloride ions and is recycled to the chlorate electrolyte. Therefore, it is not possible to systematically introduce alkali metal hydroxide, since this would result in the accumulation of alkali metal ions. The use of a catalyst prepared according to the present process for the production of chlorine dioxide in processes in which alkali metal 10 ions cannot be systematically introduced is highly advantageous since the hydroxide ions can be prepared directly without the use of a counter ion.

In carrying out the process in question, a partial current of the chlorate electrolyte is introduced into a monopolar or bipolar cell. The partial flow may be from about 3 to about 50 m <1> / tonne of -3 to 15 alkali metal chlorate, suitably from 10 to 30 m <1> / alkali metal chloride.

The chlorate electrolyte is suitably taken from the reactor vessels or chlorate electrolyzers, preferably from the reactor vessels, since the chlorate electrolyte has reacted more fully to the chlorate ions at this point. The preparation of the chlorate electrolyte, taken from the reaction vessels or chlorate ethanol digesters and fed to a separator cell, can be carried out with a concentration of C103- of from about 100 to about 1000 g / L, suitably 300 to 650 g / L, preferably 500 to 600 g / l, calculated as sodium chlorate. The preparation of the chlorate electrolyte taken from the reaction vessels or chlorate electrolysers and introduced into a separator cell may be carried out with a CT concentration between about 30 and 200 g / L, suitably 70 to 150 g / L, and preferably 100 -130 g / l, calculated as sodium chloride.

In an anode-equipped cell with a high excess of oxygen gas, chlorine gas is evolved with 3 0 anodes, whereas hydrogen gas and hydroxide ions are obtained on the cathode side. Chlorine gas dissolves directly in the anolyte, i.e., the majority of the chlorine gas is not removed from the cell, followed by partial hydrolysis to alichloric acid as follows:

It is preferable that the pH of the anolyte does not fall below 4. Otherwise, as in the process described in EP-A-498484, chlorine gas is formed which leaves the anode space and must be burned with hydrogen to form hydrochloric acid and subsequently led to a chlorate process. Ali-chloric acid dissociates in the presence of buffer or hydroxide ions (B) to hypochloro-5 as follows: HCIO + B '-> HB + CIO- (4)

Since hypochlorite is a desirable product on chlorate electrolyzers, it is particularly preferred to use such an anolyte to oxidize the electrolyte entering the chlorate electrolyzers.

Suitably, the pH of the chlorate electrolyte entering the separator cell is between about 5.0 and 7.5, and preferably between 5.5 and 7.3. Due to this high pH, the effluent anolyte does not contain chlorine gas, but instead produces a directly acidified chlorate electrolyte which can be introduced into the chlorate process.

The pH of the prepared anolyte can be adjusted as needed by controlling the anolyte flow between about 0.5 and 5 m3 / kAh. kAh refers to the total number of Faraday charges used, i.e. the sum of the charges, which are passed through each individual cell.

Conveniently, the pH of the prepared anolyte is between about 4.5 and 6.5, preferably between 5-6. Similarly, the pH of the prepared catholyte can be adjusted as required. The pH of the prepared catholyte may be between about 9 and 13, suitably between 10 and 12 and preferably between 10 and 11.

In the preparation of the mainly chlorine-free catalyst, the catholyte stream may be significantly lower than the above anolyte stream. The catholyte flow can thus be in the range of about 0.01 to 0.1 m3 / kAh. In this case, the concentration of the alkali metal hydroxide in the catholyte may be in the range of about 13 to 130 g / l, calculated as sodium hydroxide.

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In the process in question, the temperature in the separator cell may be in the range of about 20-100 ° C, suitably 40-80 ° C.

The electrodes of the separator cells may be conventional chlorate electrodes, suitably in parallel plates, coated, for example, with RuCl2TiCl or Pt / Ir. The cathode may be made of stainless or 6 low-alloy steel or titanium, optionally activated with a platinum group metal. Suitably, a steel cathode is used.

In the electrolytic cell for the preparation of the alkali metal hydroxide containing catalyst 5, the anode and cathode spaces are separated by a separator. The separator is suitably a diaphragm or membrane, preferably diaphragm, which is mainly resistant to chlorine and alkali metal hydroxide. The diaphragm can carry charged ions in either direction. Thus, cations, preferably alkali metal ions, can pass through the diaphragm from the anolyte to the catalyst, while the anions, primarily chloride and chlorate ions, can travel in opposite directions. By contrast, the ion-selective membrane as a separator limits this transport to, for example, cations, preferably sodium.

Diaphragm means gas-separating structures which are inorganic material based on, for example, asbestos, organic materials such as fluorine-containing poly-15 polymers, polyethylene, polypropylene and polyvinyl chloride, or combinations of such materials as a fluorine-containing polymer with a for example, Polyramix (R) marketed by Oxytech, USA).

When using the membranes in the process in question, it is convenient that these are ion-20 selective. The ion-selective membranes may be cationic or anionic, preferably cationic. The use of a cation-selective membrane and an anode generating chlorine gas enables the preparation of concentrated alkali metal hydroxide having low chlorate and chloride ion contents and the preparation of a hydrochloric acid-containing anolyte having a high alichloric acid content. In this case, a partial flow of 2 to 5 chlorate electrolytes is led to the anode space, while the required amount of water is led to the cathode space.

The prepared catalyst can be taken directly from the separator cell, or recycled to the cathode space for further electrolysis until the desired concentration is reached.

Similarly, the prepared anolyte can be taken directly from the stolen cell with a separator or recycled to the anode chamber for further electrolysis until the desired concentration is reached.

The prepared catalyst containing alkali metal hydroxide can be used for alkalization of chlorate-3 electrolyte prior to crystallization of chlorate, in scrubber gas and reactor gas scrubbers, or for precipitation of impurities and regeneration of ion exchange resin 7 during technical alkali metal chloride leaching and purification. Thus, the catalyst can be used for the precipitation of alkaline earth metals, iron and aluminum hydroxides, and for the regeneration of the ion exchange resin in the first and second stages respectively to purify the fresh brine. The catalyst may also be used in cell scrubber and reactor scrubbers to remove hydrogen from the chlorate cells and to remove chlorine in the exhaust gas from the hydrogen gas burner, respectively, to absorb chlorine from the process air from the reactor vessel. The catalyst can also be used, for example, to purify chlorine compounds formed in a separator cell.

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The amount of alkali metal hydroxide produced in the separator cell can be about 50 kg, calculated as sodium hydroxide per ton of dry sodium chlorate. Suitably, the amount is between 10 and 25 kg, calculated as sodium hydroxide per ton of dry sodium chlorate.

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The chlorine or alichloric acid to be prepared can be used as an acidifier in the preparation of alkali metal chlorate. In particular, the electrolyte entering the chlorate electrolyzers can be acidified with the removed acidic anolyte. The acidic anolyte may be added to an electrolyte stream entering a chlorate electrolysis preparation, a precursor to circulating mother liquor from chlorate crystallization, and to a branched electrolyte from reactor reservoirs. Suitably, an addition occurs between the electrolyte cooling heat exchangers and the electrolyzers by adding the electrolyte to the main stream.

The process in question is conveniently used for the preparation of sodium or potassium chlorate, preferably sodium chlorate, but other alkali metal chlorates may also be prepared. The potassium chlorate may be prepared by adding a purified potassium chloride solution to an alkalized, electrolytically prepared sodium chlorate partial stream, followed by precipitation of the crystals by cooling and evaporation. Chlorate is preferably prepared by a continuous process, but a batch process is also useful.

The invention will now be described with reference to Figure 1, which shows a schematic description of an apparatus for preparing sodium chlorate according to the invention. Further, the preparation of anolyte and catalyst in a diaphragm cell is described, but a cell with a membrane is also useful.

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Sodium chloride in the form of a technical salt and raw water are used for the preparation of a salt slurry (1). Such a preparation is described e.g. EP-A-0 498 484. The salt slurry thus purified is used to prepare the electrolyte (2) for the preparation of the chlorate, together with the chlorate electrolyte from the reaction vessels (5) and the mother liquor for the crystallization of chlorate-5. The electrolyte thus concentrated contains from 100 to 140 g of sodium chloride per liter and from 500 to 650 g of sodium chloride per liter, preferably from 110 to 125 g of sodium chloride per liter and 550 to 580 g of sodium chloride per liter. The electrolyte is cooled and the pH (3) adjusted to 5.5 to 6.5 by addition of an acidic anolyte from the diaphragm cell (12) and possibly fresh hydrochloric acid, followed by the electrolyte being introduced into the chlorate electrolyzer cells (4). The total flow to the chlorate cells is usually 75 to 200 m3 of electrolyte per ton of chlorate produced. Each chlorate cell operates at 50-100 ° C, suitably 60-80 ° C, and has a current density of about 10-100 A / liter electrolyte circulating. The chlorate electrode 1 core is led from (4) to the reactor (5), where the reaction to form chlorate continues. Part of the chlorate electrolyte 15 is recycled from (5) to (2), part is led to anode and cathode spaces (12), and part to alkali and electrolyte filtration (6) and final pH adjustment (7) before (8) . In this way, the alkaloid electrolyte (7) is evaporated in (8), whereupon the sodium chlorate crystallizes and is removed by means of a filter or centrifuge, while the removed water is condensed. The mother liquor, which is saturated with chlorate and contains high concentrations of sodium chloro-20, is returned directly to (2) and through the cell scrubbers (9) and the reaction gas scrubbers (10) to (2) and / or (5). .

The alkaline catalyst prepared in the cathode space of the diaphragm cell is used to alkalize the chlorate electrolyte in (6) and (7) to prevent the removal of chlorine compounds in 2 (8), respectively, to precipitate metal impurities as metal hydroxides. The alkaline catalyst is further used in the scrubbing system of the carrier gas, reactor gas and dialysis gas (9, 10, respectively 11) to absorb the acidic components from the gas streams from (4), (5), respectively (12).

3 0 The anode forms chlorine, which is directly soluble in the electrolyte and hydrolysed to the alichloric acid. After removal from the cell, the pH of the anolyte is about 5.5. The removed anoyl is subjected to a pH adjustment (3) before the chlorate electrolyzers (4).

The hydrogen gas formed in (4) is conducted to (9), and the gases leaving (5) 35 are directed to (10), while the gases coming from the cathode space (12) are conducted to (9) (10), or separate dialysis washers (11). Purified hydrogen gas 9 can be used, for example, for various syntheses or as a fuel.

The invention and its advantages will be further described by the following example, which is intended only to illustrate the invention without limiting it. The percentages and percentages used in the specification, claims 5, and example are by weight, and by weight unless otherwise indicated.

Example 1 A chlorate electrolyte containing 110 g of NaCl and 600 g of NaClCE / L and minor amounts of Na2Cr2C7, NaC104 and Na2SO4 were pumped through the anode space of the electrolytic cell at a rate of 10-20 rpm. Prior to the electrolysis, the chlorate electrolyte had a pH of about 6 and a temperature of about 50 ° C.

15 The cell was a laboratory cell equipped with a diaphragm made of sintered polyethylene. The electrodes had a surface area of 1 dm2. A dimensionally stable titanium chlorine anode (DSA) and an uncoated titanium cathode were used. The distance between the diaphragm and each electrode was 7 mm. The cell volume was 2 L.

The chlorate electrolyte was pumped through the anode space at a flow rate of 10-20 rpm. Through the cathode chamber and the 2 L reactor with overflow discharge, the catholyte was circulated at a flow rate of 4 L / min. The catholyte was introduced into the same chlorate electrolyte at a flow rate of 0.141 / min.

Electrolysis was carried out at an electrolyte temperature of 50-70 ° C, a current density of 1-3 kA / m 2 and at pH 11.8 for the catalyst and at pH 5-6 for the anolyte. The current ranged between 10 and 30 A.

Under all operating conditions, the current efficiency was over 90% and in most cases over 95%. The current efficiency is then calculated between the actual and theoretical maximum production of sodium hydroxide. The production of hydroxide ions was determined by analyzing the concentration of hydroxide ions in the catalyst and multiplying by the collected flow. Relative to the production of protons and hydroxide ions, expressed as sodium hydroxide and hydrochloric acid, the energy consumption was between 2000 and 2300 kWh / tonne sodium hydroxide and an equivalent amount of hydrochloric acid. Equivalent amount of hydrochloric acid 3 5 refers to the total amount of hydrochloric acid and alichloric acid formed by chlorohydrolysis, calculated as pure hydrochloric acid.

Claims (11)

A process for the production of alkali metal chlorate by electrolysis of an aqueous electrolyte containing purified alkali metal chloride, characterized in that a sub-stream of the obtained chlorine electrolyte is electrolysed in a cell provided with separator to produce a catholyte metal hydroxide containing catholyte metal hydroxide containing Tone is partially used in the production of alkali metal chlorate, and chlorine is produced in the anode compartment of the cell and dissolved in the prepared anolyte and hydrolyzed to subchloric acid in the anode compartment. Process according to claim 1, characterized in that the concentration of chlorate in the electrolyte supplied to the cell with separator is in the range 300 - 650 g / l calculated as sodium chlorate. 15 Method according to claim 1 or 2, characterized in that the separator in the cell is a diaphragm. Method according to claim 1 or 2, characterized in that the separator in the cell 20 is a non-selective membrane. 5. A process according to claim 4, characterized in that the separator is cation-selective. 2 5 Process according to any of the preceding claims, characterized in that the prepared catholyte is used for the alkalization of the chlorate electrolyte before crystallization of chlorate, in cell gas and reactor gas scrubbers or in the precipitation of impurities and the regeneration of ion exchange resin in connection with dissolution and purification of the alkali and purification technology. 30 Process according to any of the preceding claims, characterized in that the chlorine or subchloric acid produced is used in the production of alkali metal chlorate. 35 Method according to any of the preceding claims, characterized in that the chlorate electrolyte is supplied to the anode compartment of the cell. Method according to any of the preceding claims, characterized in that the chlorate electrolyte is supplied to the anode compartment and cathode compartment of the cell. Process according to any of the preceding claims, characterized in that the prepared catholyte is used for the production of chlorine dioxide, in integrated processes, where a unit for the production of chlorate is combined with a unit for the production of chlorine dioxide, and where alkali metal ions cannot be added systematically. 10 Process according to any of the preceding claims, characterized in that the pH of the produced anolyte is in the range of about 4.5 - about 6.5.