CA3209720A1 - Method and plant for production of sodium chlorite - Google Patents

Abstract

A method and plant for the production of sodium chlorite are disclosed. The method involves using an "integrated ClO2 method" to make chlorine dioxide and then reacting that chlorine dioxide with alkaline sodium hydroxide in the presence of hydrogen peroxide. The related plant comprises a subsystem for making the chlorine dioxide in this manner and a sodium chlorite subsystem for the subsequent reacting.

Description

Docket No.: Chemetics026-CA
METHOD AND PLANT FOR PRODUCTION OF SODIUM CHLORITE
Technical Field The present invention pertains to sodium chlorite production and in particular to production using chlorine dioxide obtained from a coupled chlorine dioxide subsystem.
Background Sodium chlorite is an industrial chemical commonly used as a bleaching agent for paper pulp and the textile industry, disinfection of drinking water, and sewage treatment. Sodium chlorite is produced in solid form at concentrations of approximately 80 percent by weight or aqueous solution in concentrations below 40 percent by weight.
The primary form of sodium chlorite production is through the method as described in US patent no.
6,251,357. This is done in two steps. In the first step, sodium chlorate is partially reduced to form chlorine dioxide. The second step is to react chlorine dioxide with alkaline sodium hydroxide in the presence of hydrogen peroxide to form sodium chlorite. The chlorine dioxide must be very pure with low levels of chlorine present as the chlorine will consume hydrogen peroxide and sodium hydroxide to produce undesired sodium chloride. Chlorine dioxide is also a useful chemical employed for oxidation, bleaching, and disinfection. Due to the extreme reactivity of chlorine dioxide, it is difficult to store and transport and is generally produced on site by the end user. Sodium chlorite provides a more stable solution and provides similar properties as chlorine dioxide in terms of bleaching, oxidation, and disinfection.
.. Additionally, sodium chlorite can be acidified in solution to produce chlorine dioxide virtually "on demand".
An alternative production method of sodium chlorite production is by reacting chlorine dioxide with sodium hydroxide and calcium hydroxide; however, this adds impurities such as carbonates and bicarbonates to the final product, which can result in scaling for the end user, therefore higher maintenance costs.
One of the methods to produce chlorine dioxide involves reacting a solution of inorganic chlorate with a reducing agent, which is typically an acid (sulphuric acid and methanol, hydrochloric acid, oxalic acid, hydrogen peroxide, and sodium chloride as common examples) in a generator. The method using sulphuric acid and methanol as the reducing agent produces chlorine dioxide gas along with sesquisulphate and formic acid as disclosed in Canadian patent no. 1,333,517.
The sesquisulphate and formic acid are filtered out and the reactor solution is returned to the generator. The gas leaving the Date Regue/Date Received 2023-08-21 Docket No.: Chemetics026-CA
generator is a mixture of chlorine dioxide and water that is absorbed in chilled water. An advantage of this process is virtually no chlorine is present in the final chlorine dioxide product. However, a disadvantage is the requirement of three feedstock chemicals (sulphuric acid, methanol, and sodium chlorate).
An alternative reducing agent to produce chlorine dioxide solution with an inorganic chlorate is hydrochloric acid. The method with hydrochloric acid as the reducing agent is known as the integrated chlorine dioxide method and is disclosed in US patent no. 3,607,027. In this integrated chlorine dioxide method, sodium chlorate is produced in an electrolytic chlorate cell with sodium chloride brine as the feedstock. The electrolytic reaction generates sodium chlorate and hydrogen gas. The sodium chlorate solution is then cycled through a generator with hydrochloric acid as the reducing agent to produce chlorine dioxide gas along with chlorine gas and sodium chloride salt. The chlorine and chlorine dioxide gases are separated by passing the gases through an absorption/stripping tower that can result in a chlorine dioxide solution with relatively low chlorine levels. The chloride salt can be recycled to the chlorate cell to generate more sodium chlorate. The chlorine and hydrogen gases produced in the chlorine generator, and electrolytic chlorate cell can be used in the production of hydrochloric acid required for the generation of chlorine dioxide.
In this way, the integrated chlorine dioxide method provides a low operating cost, security of supply, insulation from cyclical market prices, and the convenience and safety of not having to import, handle, and store hazardous feedstock compared to the method using sulphuric acid and methanol as the reducing agent as mentioned in "The Development of an Integrated Chlorine Dioxide Process to Produce Chlorine Dioxide Solution with Low Chlorine Content", A. Barr et al., Appita J. Vol.
59, No.6, November, 2006, Page 442-445.
Improvements to the production method of sodium chlorite remains a necessity in terms of cost savings, and efficiency. Therefore, there is a need to develop an economical process to produce sodium chlorite without the need for purchase of costly chemicals. The present invention addresses this desire and provides other benefits as disclosed below.
Summary In the present invention, sodium chlorite is produced using chlorine dioxide prepared by the integrated chlorine dioxide method. Specifically, the invention includes a method for making sodium chlorite comprising the steps of making chlorine dioxide solution using the integrated C102 method, then desorbing the C102 by into an air circuit by stripping and finally reacting that chlorine dioxide with alkaline sodium hydroxide in the presence of hydrogen peroxide. The invention also includes a related plant for making sodium chlorite according to the aforementioned method. Such a plant comprises a

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subsystem for making chlorine dioxide using the integrated C102 method, and a sodium chlorite subsystem for reacting chlorine dioxide with alkaline sodium hydroxide in the presence of hydrogen peroxide. In the plant, a chlorine dioxide output from the subsystem for making chlorine dioxide is fluidly connected to a chlorine dioxide input in the sodium chlorite subsystem.
Further in the plant, the heating and cooling requirement can be reduced by a crossflow or inter heat exchanger between the C102 solution from the absorber and the absorption water from the desorber. Further still, the heat energy used to warm the C102 solution can be optionally recovered from the NaC103 electrolysis area of the plant. Yet further, the desorbing air stream in the plant can be optionally recirculated between the stripping tower and the absorber tower.
The integrated C102 subsystem can comprise further subsystems for chlorate production, hydrochloric acid production, and chlorine dioxide generation. The integrated chlorine dioxide method comprises the usual steps of:
supplying electricity and weak chlorate solution recycled from the chlorine dioxide subsystem to the circulating sodium chloride brine in the sodium chlorate production subsystem, producing sodium chlorate and hydrogen in the electrolyzer in the sodium chlorate production subsystem, supplying hydrogen and strong sodium chlorate solution produced in the sodium chlorate subsystem to the hydrochloric acid synthesis subsystem and the chlorine dioxide subsystem respectively, supplying demineralized water, imported chlorine gas, and recycled weak chlorine gas from a chlorine dioxide absorption subsystem; synthesizing hydrochloric acid solution in the hydrochloric acid subsystem, providing the hydrochloric acid solution synthesized in the hydrochloric acid synthesis subsystem to the chlorine dioxide subsystem, generating chlorine dioxide in the chlorine dioxide subsystem resulting in a production of weak chlorate from the strong chlorate solution, supplying chilled water and chlorine dioxide generated in the chlorine dioxide subsystem to the chlorine dioxide absorption subsystem, and absorbing the generated chlorine dioxide into the chilled water in the chlorine dioxide absorption subsystem, producing chlorine dioxide solution.
The sodium chlorite subsystem can comprise:
a chlorine dioxide heater comprising;
a heat exchanger;
a liquid inlet for chlorine dioxide solution;
a liquid inlet for a hot liquid;

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a liquid outlet for the heated chlorine dioxide solution produced in the absorption subsystem of the integrated chlorine dioxide subsystem;
a liquid outlet for a hot liquid; and a chlorine dioxide desorption tower comprising;
a stripping column;
a liquid inlet for chlorine dioxide solution from the chlorine dioxide heater;
a gas inlet for stripping gas;
an outlet for water partially stripped of chlorine dioxide; and an outlet for stripping gas comprising the chlorine dioxide stripped from the chlorine dioxide solution; and a sodium chlorite tower comprising;
an absorber column;
an inlet for a chlorine dioxide containing gas from the chlorine dioxide desorption tower;
an inlet for sodium chlorite solution containing hydrogen peroxide and sodium hydroxide;
an outlet for purge gas.
In the sodium chlorite subsystem, the chlorine dioxide solution from the chlorine dioxide absorption subsystem can be fluidly connected to a chlorine dioxide cross heat exchanger for heating the chlorine dioxide solution from the absorption subsystem with the desorbed chlorine dioxide solution from the chlorine dioxide desorption tower. Further, the chlorine dioxide solution outlet from the heat exchanger can be fluidly connected to the chlorine dioxide heater.
In certain embodiments, the sodium chlorite subsystem may desirably comprise a heat exchanger for cooling the desorbed chlorine dioxide solution outlet from the chlorine dioxide cross heat exchanger with chilled water. Further still, the chilled desorbed chlorine dioxide solution from the heat exchanger can be fluidly connected to the absorption subsystem and utilized as absorbing fluid.
The chlorine dioxide subsystem preferentially comprises a vacuum vertical generator, but in certain embodiments it can comprise a horizontal generator. If the generator subsystem is comprised of a vertical generator, the system consists of a vertical generator, a gas separator, a condenser, an absorber/stripper and vacuum generation system. The system operates under a deep vacuum. The gas from the generator is drawn through a condenser and into an absorber/stripper where the chlorine is air stripped out and chlorine dioxide is absorbed into chilled water resulting in a chlorine dioxide solution with low chlorine levels. The gas separator provides additional residence time to reduce acid level in the returning weak chlorate liquor

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to the sodium chlorate subsystem. If the generator subsystem is comprised of a horizontal generator, the system consists of a multi compartment horizontal generator operating under a slight vacuum with dilution air. The gas from the generator is drawn through an absorber by a blower where the chlorine dioxide gas is absorbed with chilled water. This embodiment results in a chlorine dioxide solution with more chlorine compared to the vertical generator. An evaporator is used to remove excess water from the sodium chlorate circuit and reduce the residual acid levels in the weak chlorate liquor returning to the sodium chlorate subsystem.
Brief Description of the Drawings Figure 1 is a schematic of a sodium chlorite plant of the invention comprising an integrated chlorine dioxide subsystem.
Figure 2 is a schematic showing greater detail of the sodium chlorate subsystem electrolysis area in Figure 1.
Figure 3 is a schematic showing greater detail of the sodium chlorate liquor circuit in Figure 1.
Figure 4 is a schematic showing greater detail of a chlorine dioxide subsystem with a vertical chlorine dioxide generator.
Figure 5 is a schematic showing greater detail of the various elements and their interconnection in the sodium chlorite subsystem of Figure 1.
Detailed Description Unless the context requires otherwise, throughout this specification and claims, the words "comprise", "comprising" and the like are to be construed in an open, inclusive sense. The words "a", "an", and the like are to be considered as meaning at least one and are not limited to just one.
Herein "hydrochloric acid" refers to hydrochloric acid with a concentration ranging from 25 to 35% by weight.

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The term "chlorine dioxide solution" refers to chlorine dioxide in water at a concentration between 3 and 15 g/L.
The term "purge gas" refers to the portion of the gas leaving the sodium chlorite tower.
The term "weak chlorate" refers to solution containing sodium chloride in concentration ranges of 90 to 180 g/L and sodium chlorate in concentrations of 300 to 450 g/L.
The term "strong chlorate" and "chlorate liquor" refers to solution containing sodium chloride in concentration ranges of 90 to 130 g/L, sodium chlorate in concentrations of 410 to 590 g/L, sodium dichromate in concentrations of 2 to 8 g/L, and sodium sulphate in concentrations of 0 to 30 g/L.
The term "sodium chlorite solution" refers to dissolved sodium chlorite at a concentration less than 50 %
by weight.
The term "solid sodium chlorite" refers to solid sodium chlorite at a concentration between 80 to 95% by weight.
The term "chilled water" refers to demineralized water or water with very low levels of impurities with a temperature in the range of 5 to 20 C
The term "integrated chlorine dioxide method" refers to a method for producing chlorine dioxide in accordance with the teachings of the aforementioned US3607027. That is, sodium chlorate is produced in an electrolytic chlorate cell using sodium chloride brine as the feedstock.
The sodium chlorate produced is reacted with hydrochloric acid as a reducing agent to produce chlorine dioxide gas (along with chlorine gas and sodium chloride salt).
Figure 1 shows a schematic of a sodium chlorite plant of the invention comprising a conventional integrated chlorine dioxide subsystem 5 for purposes of providing chlorine dioxide for sodium chlorite subsystem 4. As illustrated, the conventional integrated chlorine dioxide subsystem 5 is comprised of three plant areas: sodium chlorate production 1, hydrochloric acid synthesis 2, and chlorine dioxide production 3. The intermediate products sodium chlorate (NaC103) and hydrochloric acid (HC1) are produced in the sodium chlorate production area 1 and hydrochloric acid production area 2, respectively.
Chlorine dioxide (C102) is produced in the chlorine dioxide production area 3, which is used as the feed to

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produce sodium chlorite (NaC102) in the sodium chlorite subsystem 4. Further, the area for chlorine dioxide production 3 comprises distinct areas for chlorine dioxide generation 3a and for chlorine dioxide absorption 3b. Further still, the subsystem for sodium chlorite production comprises distinct areas for chlorine dioxide desorption 4a and sodium chlorite production 4b.
Figure 2 shows in more detail a sodium chlorate electrolytic system in sodium chlorate production area 1.
The sodium chlorate electrolytic system is comprised of a reaction vessel 101, heat exchanger 102, supply header 103, electrolyzer cells arranged in parallel 104, which are connected to pipes 105 that allow the liquid to rise into the degasifier 106 to allow for separation of gas and solution. The function of the sodium chlorate electrolytic system is to produce sodium chlorate liquor and hydrogen gas from the electrolysis of a sodium chloride solution. In the electrolyzer cells 104, gaseous chlorine, hydrogen, and sodium hydroxide are formed. The reaction above may be represented by the following overall equation:
(1) NaCl + 3 H20 ¨> NaC103+ 3H2 The electrolyzer cells 104 do not separate the anodic and cathodic products but allow them to react further to produce sodium chlorate. Sodium hydroxide reacts with the chlorine to form sodium hypochlorite, which further reacts to form sodium chlorate. When electric current is applied in series to the electrolyzer cells 104, the hydrogen generated creates a natural circulation that causes the chlorate liquor to rise in parallel up the connected pipes 105 to the degasifier 106 to separate hydrogen gas 13 from the chlorate liquor 12. The hydrogen gas 13 passes through the headspace in the reaction vessel 101 before being sent to the hydrochloric acid synthesis area 2. The strong chlorate 12 flows into the reaction vessel 101 and overflows via circuit 15 to tank 107 as shown in Figure 3. The chlorate liquor flows from the reaction vessel 101 through the heat exchanger 102 via circuit 10 to remove the heat generated in the electrolyzers 104. To maintain the concentration of sodium chloride in the chlorate liquor, weak chlorate 32 from the chlorine dioxide generation area 3a is recycled to the reaction vessel 101.
The sodium chlorate electrolytic system 1 can alternatively be comprised of various electrolysis cells connected in parallel to a reaction tank such as the one described in US
patent no. 4,414,088.
In the hydrochloric acid synthesis area 2, hydrochloric acid 21 is produced by burning chlorine and hydrogen gas. The reaction above may be represented by the following overall equation:
(2) H2 C12 -> 2HC1 The hydrogen gas 14 is supplied by the sodium chlorate production area 1. Weak chlorine gas 38 is obtained as a by-product from the chlorine dioxide generation system 3.
Additional chlorine 22, and

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demineralized water 23 are supplied to the hydrochloric acid area 2 as necessary to meet hydrochloric acid 21 requirements for chlorine dioxide generation 3a.
Figure 3 shows in more detail the circulation of chlorate liquor produced in the sodium chlorate production area 1. Prior to chlorine dioxide generation 3, the sodium chlorate is stored in tank 107, cooled in heat exchanger 108, and passed through filter 109 to remove solid particles. Strong sodium chlorate 17 is reacted with hydrochloric acid to form chlorine dioxide in the chlorine dioxide generator 301. Weak chlorate is a by-product of the generation of chlorine dioxide, which is recovered from the chlorine dioxide generator 301. The recovered weak chlorate is heated up in heat exchanger 305 to form chlorine, and the chlorine is allowed to disengage in the gas separator 306. The resulting weak chlorate solution 32 in vessel 306 is recycled to the reaction vessel 101 for strong chlorate production.
Figure 4 shows in more detail the chlorine dioxide generation system 3 with a vertical generator. The system is comprised of a vertical generator vessel 301 operating under an absolute pressure ranging from 190 to 300 mmHg where strong chlorate 17 and hydrochloric acid 21 are circulated around the generator 301 to provide residence time to react sodium chlorate and hydrochloric acid to produce chlorine, water, sodium chloride, and preferentially chlorine dioxide. The reaction above may be represented by the following overall equation:
(3) NaC103 + 2HC1¨> NaCl + C102+ V2 C12+ H20 The strong chlorate 31 in generator 301 is passed through heater 302 to evaporate the water in the generator 301 to ensure the correct water balance in the strong chlorate circuit. The generator 301 off-gases 33 that contain chlorine, water, and chlorine dioxide passes through condenser 305 to condense a portion of the water in gas 33. To ensure the correct liquid level in the generator 301 and sodium chlorate concentration in the generator liquor, some of the weak chlorate is removed from the circuit and pumped through the chlorate heater 303 to increase the temperature of the weak chlorate to react excess hydrochloric acid and sodium chlorate to form chlorine. The weak chlorate from the chlorate heater 303 is sent to the gas separator 304 to allow for the disengagement of chlorine formed. The weak chlorate 32 from the gas separator 304 is recycled to the sodium chlorate system 1. The chlorine gas produced in the gas separator 304 is sent to condenser 305 and sent through separate tubes than the generator 301 gases 33 to condense water vapour in gas 34. The vapour mixture 35 from condenser 305 is pulled through the absorber/stripper 307 by the vacuum jet 308. The absorber/stripper contains two stages; the top stage serves as an absorption unit for chlorine and chlorine dioxide with chilled water or chilled desorbed chlorine dioxide solution 36. The bottom section acts as a preferential chlorine stripper. Chlorine that already has been absorbed is subsequently stripped out by small amount of air 37 introduced at the bottom

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of the column. The gas from the absorber/stripper 38 containing chlorine is sent to the hydrochloric acid synthesis area 3.
In certain embodiments, the chlorine dioxide generation system 3 with a vertical generator can comprise a vacuum pump 309 as shown in Option # lb in Figure 4 compared to Option # la with the vacuum jet and condenser 308. Further still, the condenser can be comprised of condenser 305 fluidly connected to the gases from the gas separator 34 and vertical generator 33 as shown in Option #
2a, or condenser 305 and condenser 306: one that is fluidly attached to the gas separator outlet gas stream 34 and one that is fluidly connected to the outlet gases from the vertical generator as shown by Option #
2b in Figure 4. For option 2b, the gas outlet from condenser 306 is directly sent to the weak chlorine stream 38.
The chlorine dioxide generation system 3 can alternatively be comprised of a horizontal generator as described above.
Figure 5 is a schematic showing greater detail of the sodium chlorite subsystem 4. The sodium chlorite subsystem 4 comprises a heat exchanger 401, stripping tower 402, and absorber 403. Chlorine dioxide solution 39 produced in the chlorine dioxide generation system 3 passes through heat exchanger 401 to heat the chlorine dioxide solution such that the solubility of chlorine dioxide is reduced. In stripping tower 402 the chlorine dioxide gas is stripped out from the solution 41 by a counter-current air stream 42. The depleted chlorine dioxide solution at the bottom stripping tower 402 is called absorption water 44. The chlorine dioxide rich gas stream 43 from the stripper 402 is then sent to absorber 403. In absorber 403, the air flows counter-current to the sodium chlorite solution 47 passed through the top. Sodium hydroxide solution 46 and hydrogen peroxide solution 45 are added to the sodium chlorite solution 47 to absorb the chlorine dioxide and generate sodium chlorite, water, and oxygen. The solution at the bottom of the tower is rich in sodium chlorite and consumed of hydrogen peroxide and sodium hydroxide. The solution is cooled in heat exchanger 406 to remove the heat of reaction prior to entering absorber 403 and being sent as product 49. The vent gas 48 from the absorber 403 can be recycled to fan 407 and be used as stripping air for stripping tower 402. The reaction above may be represented by the following overall equation:
(4) 2C102 + H202+ 2NaOH ¨> 2NaC102 +02+ 2H20 The sodium chlorite product 49 can comprise further processing in the form of evaporation or crystallization to form a solid sodium chlorite product.

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If desired to recover heat from the sodium chlorite subsystem 4, the subsystem can comprise heat exchanger 404 prior to heat exchanger 401. The absorption water 44 can be passed through the heat exchanger 404 to heat the chlorine dioxide solution 39 and cool the absorption water 44. Additionally, the condensate produced in the chlorine dioxide generation system 3 can be combined with the absorption water 44 prior to heat exchanger 404. The cooled absorption water from heat exchanger 404 can be sent through heat exchanger 405 to chill the liquid and be used as absorbing fluid 36 for absorber 307 in the chlorine dioxide generation system 3.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.
Date Regue/Date Received 2023-08-21

Claims (2)

Claims

1. A method for making sodium chlorite comprising the steps of:
making chlorine dioxide solution using the integrated ClO2 method; desorbing the ClO2 gas from the ClO2 solution with air and finally reacting the chlorine dioxide into a solution with alkaline sodium hydroxide in the presence of hydrogen peroxide.

2. A plant for making sodium chlorite according to the method of claim 1 comprising:
a subsystem for making chlorine dioxide using the integrated ClO2 method; and a sodium chlorite subsystem for reacting chlorine dioxide with alkaline sodium hydroxide in the presence of hydrogen peroxide;
wherein a chlorine dioxide output from the subsystem for making chlorine dioxide is fluidly connected to a chlorine dioxide input in the sodium chlorite subsystem.