A clean production process of sodium chlorite from sodium chlorate
Introduction
Sodium chlorite is a highly efficient bleaching agent and oxidative disinfectant, widely applied in bleaching of textile, fiber, pulp and paper. It is characteristic of lower harm to fiber. It is also used to whiten sugar, starch, grease, ointment and wax. When used in purifying water, little of remained chlorine would be found. It is also used extensively in sterilization, and deodorization in sewage treatment processes.
Since reaction conditions to produce chlorine dioxide from sodium chlorite are mild, sodium chlorite is the main feedstock chemical for producing chlorine dioxide in mini-type generators. Chlorine dioxide used in aqueous solution is of considerable commercial interest in pulp bleaching, water purification, fat bleaching, medical treatment, sanitation, food processing, aquiculture, and for the removal of phenols from industrial wastewater. Chlorine dioxide is generally generated from sodium chlorite in dilute acid, which is easier than producing it from sodium chlorate. Therefore, there is a huge demand to sodium chlorite in the marketplace.
Sodium chlorite is manufactured in two ways: electrolysis and reduction. The electrolysis method is characteristic of high investment and complicated operation, while the reduction method is used in most of the plants to produce sodium chlorite. Production of sodium chlorite with a reducing agent consists of two steps: (1) preparation of chlorine dioxide by reacting sodium chlorate in sulfuric acid, and then (2) reaction of chlorine dioxide with a reducing agent in sodium hydroxide solution. The first step is the key to determine product economics and production technology. In industrial production, the chlorine dioxide generation is generally based on the reduction of sodium chlorate by reducing agents such as sulfur dioxide, hydrochloric acid, sodium chloride, methanol, and most recently, hydrogen peroxide, at high acidity [1].
There are a number of conventional chloride-based approaches to produce chlorine dioxide, named as R2, R3, R5, and R6 [2]. A typical approach is based on sodium chloride. The main stoichiometry of the reaction is [3]:2NaClO3 + 2NaCl + 2H2SO4 → 2ClO2 + 2Na2SO4 + Cl2 + 2H2O
One of the disadvantages in using chloride ions as the reducing agent is that, for each mole of chlorine dioxide formed, half a mole of chlorine gas and 1 mol of sodium sulfate are produced. In the industry, sodium chlorite has to be used to absorb chlorine for purifying the product chlorine dioxide.
Solvey and R8 processes are methanol-based production of chlorine dioxide. Stoichiometry of R8 reaction is as follows [4], [5]:30NaClO3 + 7CH3OH + 20H2SO4 → 30ClO2 + 10Na3H(SO4)2 + 3HCHO + 26H2O + 4CO2
No chlorine is produced in the methanol-based processes. However, it is still characteristic of several shortcomings:
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First, methanol is oxidized stepwise to formaldehyde, formic acid, and carbon dioxide. In commercial generator, oxidation of formic acid is slow. Formic acid is thus accumulated as a by-product. The fact that significant amounts of formic acid and methanol are found in solution in the chlorine dioxide absorber indicates that these chemicals do not react completely but instead volatize.
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Second, when ClO2 product is used in bleaching, the un-reacted methanol constitutes a pollution load to the secondary effluent treatment system of final users.
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Third, the required high acidity for operation produces waste sodium sesquisulfate, which is of little industrial use.
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Fourth, methanol as a combustible chemical constitutes a hazard to operators at the workplace.
When sulfur dioxide based process is adopted, chlorine dioxide product is vulnerable to contamination due to the presence of sulfur dioxide. Chlorine is also found in this process. Mathieson process and R1 are similar processes. The stoichiometry is:3NaClO3 + 4SO2 + 3H2O → 2ClO2 + Na2SO4 + 3H2SO4 + NaCl
Inadequacies in above-mentioned traditional processes have led to further research on the hydrogen peroxide-based process. The main stoichiometry of the reaction using hydrogen peroxide as the reducing agent is as follows:2NaClO3 + H2SO4 + H2O2 → O2 + 2ClO2 + 2H2O + Na2SO4
There are a number of advantages of a hydrogen peroxide-based process over the other approaches:
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less salt cake (Na2SO4) produced;
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elimination of an organic reducing agent;
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production of significant amounts of by-product chlorine;
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faster reaction rate.
This process can be easily used in existing facilities as a substitute and can also increase capacity. When compared with sulfur dioxide and methanol, at the same operating conditions, the hydrogen peroxide-based process has the higher yield in terms of chlorine dioxide based on the amount of chlorate reacted. Calculated as chlorate transformed, yield of chlorine dioxide in the hydrogen peroxide-based process reaches more than 95%, while yield of other processes are around 90% [4], [5], [6].
The main drawbacks of the above-mentioned processes are the formation of sodium sulfate as a by-product, which has to be removed from the reactor, either in the form of a solid salt cake or as a waste acid. It is hard to find any use of the salt cake currently and it is normally regarded as an unwanted by-product. In order to avoid formation of sulfate by-product, it has been found to be possible to provide all the acid needed for the chlorine dioxide generation from chloric acid, which can be prepared electrochemically from sodium chlorate. Such methods are described in, for example, U.S. Patents [7], [8], [9]. However, it has been found to be difficult to achieve satisfactory efficiency in production of strong chloric acid, which is desirable in order to provide efficient chorine dioxide generation. Twardowski discloses a process in which chlorine dioxide is generated from sodium chlorate and hydrochloric acid, in which process the generator liquor is acidified electrochemically and recycled back to the reactor [10]. However, this process necessarily results in co-formation of chlorine, which is not welcome in modern environmentally friendly bleaching processes.
There are many U.S. Patents [11], [12], [13], [14] disclosing the electrolytic conversion of sodium sulfate into sulfuric acid and sodium hydroxide. Most of them make use of electrolytic cells employing diaphragms or ion permeable membranes to separate the product solutions from the feed solution, thus avoiding contamination of the products by the feedstock material.
The objective of the proposed cleaner production process presented in this paper is to make rational use of resources, to minimize harm to humankind and the environment, and for best social benefits in the product manufacture and during consumption. The advantages of hydrogen peroxide-based chlorine dioxide production can be a solid foundation of cleaner production of sodium chlorite.
The optimal operating conditions and how to make use of waste acid in reaction system were investigated and are presented in this paper. In the end of the paper, a novel cleaner production process for producing sodium chlorite, with minimal resources used and wasted, and maximal economic potential, is put forward.
Section snippets
Reaction mechanism
The following is the basic reaction equation reducing sodium chlorate with hydrogen peroxide as the reducing agent in the presence of sulfuric acid [15]:H2O2 + 2ClO3− + 2H+ → O2 + 2ClO2 + 2H2O
When the concentration of sulfuric acid is higher than 5.5 mol L−1, the produced salt is bisulfate. The reaction stoichiometry is:2NaClO3 + 2H2SO4 + H2O2 → O2 + 2ClO2 + 2H2O + 2NaHSO4
When the concentration of sulfuric acid is between 2.5 mol L−1 and 5.5 mol L−1, little chlorine is found in the by-products. Reaction rate and yield are
Experimentals and chemical analysis
Experimental facilities were set up to investigate the new process and conditions of chlorine dioxide generation and sodium chlorite production. It is shown in Fig. 1. The experimental installation consists of a chlorine dioxide generator, several chemical containers in series of absorbers, buffer, and a vacuum pump. A four-neck round bottle reaction flask, submerged in a bath of constant temperature, was used as the reactor to generate chlorine dioxide. Sodium chlorate solution is pre-mixed
Results and discussions
To determine the optimal reaction conditions and optimal reaction yield in the chlorine dioxide production process, the orthogonal design of experiments with factorial analysis was employed. Five factors and four levels were adopted in the experiments. The five factors investigated were:
- A:
reaction temperature, which is also the bath temperature and kept constant during the reaction;
- B:
concentration of sodium chlorate, calculated as in the pre-mixed feed solution of sodium chlorate and hydrogen
A clean production process of sodium chlorite
As the price of hydrogen peroxide decreases sharply in the marketplace, the hydrogen peroxide-based approach becomes the most promising approach to realize the cleaner production process of sodium chlorite.
Firstly, efficiency of the hydrogen peroxide-based process is high. The conversion ratio of the key component of materials sodium chlorate is high. Compared with other process in which chlorine gas might be produced [18], [19], [20], [21], there are few exhaust gases found in this hydrogen
Conclusions
Laboratory experiments were performed to investigate a new cleaner production process for producing sodium chlorite. Operating conditions were explored. Feasible and optimal operating conditions investigated included: reaction temperatures, concentrations of sulfuric acid, molar ratio of hydrogen peroxide to sodium chlorate, concentration of sodium chlorite solution, and molar ratio of sulfuric acid to sodium chlorate. Sodium chlorate conversion to sodium chlorite of 95–100% was achieved. The
Acknowledgments
Financial supports to this work from the Excellent Young Scientist Fund of China (20225620), China Natural Science Fund (20376025, 20536020), and the Outstanding Young Professor Award from Guangdong Province Bureau of Education are gratefully acknowledged.
Yu Qian is a professor in the School of Chemical Engineering at South China University of Technology, Guangzhou, China. He is engaged in recent years on a number of chemicals and chemical process development projects, including clean fuels, clean oxidation agents. He holds a PhD degree in chemical engineering from Tsinghua University, in Beijing, China. He was then a postdoctoral fellow at the Norwegian Institute of Technology, in Trondheim, Norway; Research associate at the University of
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Yu Qian is a professor in the School of Chemical Engineering at South China University of Technology, Guangzhou, China. He is engaged in recent years on a number of chemicals and chemical process development projects, including clean fuels, clean oxidation agents. He holds a PhD degree in chemical engineering from Tsinghua University, in Beijing, China. He was then a postdoctoral fellow at the Norwegian Institute of Technology, in Trondheim, Norway; Research associate at the University of British Columbia, in Vancouver, Canada, before joining the South China University of Technology.