Sodium Chlorite

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Sodium Chlorite

Background

Sodium chlorite is a compound used for water disinfection and purification. It is produced in large quantities as flakes or a solution from chlorine dioxide and sodium hydroxide. Its use as a bleach for textiles was first discovered during the 1920s. Today, sodium chlorite is an important specialty chemical with sales over $18 million annually.

In its dried state, sodium chlorite (NaClO₂) is a white or light yellow-green solid. The greenish tint comes from trace amounts of CdO2 or iron, which are production residuals. Sodium chlorite has a molecular weight of 90.44 and decomposes at about 392°F (200°C). It is generally soluble in water, but its solubility increases as the temperature of the water rises. Sodium chlorite is a powerful oxidizer that will not explode on percussion. The anhydrous salt does not absorb water and is stable for up to ten years.

Sodium chlorite is used for a variety of applications. It is used as a disinfectant and purification chemical for water. It is also employed as a textile-bleaching and water anti-fouling agent. Additionally, it is used in the paper and electronics manufacturing industries as a bleaching agent.

When put in an acid solution, sodium chlorite breaks down into chlorine dioxide. When added to a municipal water supply, chlorine dioxide helps control unwanted tastes and odors. It also aids in the removal of ions like iron and manganese. One added benefit is that it helps eliminate trihalomethanes in drinking water.

As a textile-bleaching agent, sodium chlorite is effective with various fibers. It can be used on cotton, bast fibers, and man-made fibers like nylon, Perlon, Dralon, and Rhovyl. It has an oxidizing effect on many of the natural waxes and pectins found in cellulose fibers. It helps solubilize them and makes the fiber more even and workable. It has the added benefit of destroying natural color matter without attacking the fibers themselves. This makes it useful for making permanent white fabrics without compromising tensile strength.

Sodium chlorite is also used for various industrial applications. It controls microbial contamination in industrial cooling systems and towers. It is used in place of chlorine in industrial ammonia plants because it does not react negatively with ammonia. Since it is an oxidizer, it is often a part of flue gas scrubber systems. Food-processing companies use it for washing fruits and vegetables because it is a fungicide. Meat and poultry are also washed with a solution, as is food processing equipment. Finally, it is an anti-mildew agent in detergent compositions and has been used in toothpaste and contact lens solutions.

History

The development of sodium chlorite as an industrial chemical began in 1921 when E. Schmidt found that cellulosic fibers could be purified with chlorine dioxide without being appreciably damaged. Unfortunately, chlorine dioxide gas is extremely explosive at high concentrations. These discoveries prompted researchers to look for safe and economical ways to deliver chlorine dioxide for bleaching purposes. The first company to introduce sodium chlorite for this purpose was the Mathieson Chemical Corporation.

In 1960, sodium chlorite became the standard material for continuous bleaching operations in the United States, replacing hydrogen. In subsequent years, other uses for sodium chlorite were discovered.

Raw Materials

The primary raw materials used in the production of sodium chlorite are chlorine dioxide, sodium hydroxide, and hydrogen peroxide. Chlorine dioxide is a gas at room temperature. Its color is intensely greenish-yellow. Chlorine dioxide provides the source of chlorine that is converted to sodium chlorite. In production, it is stored as a liquid solution in glass-lined steel containers.

Sodium hydroxide is a fused solid with a crystalline structure. Also known as caustic soda, it is corrosive to skin and vegetable tissue, causing severe burns. It is typically produced through the electrolysis of sodium chloride solutions. Hydrogen peroxide is a colorless liquid that is caustic and bitter to taste. Pure H2 02 is a thick, syrupy liquid that rapidly decomposes into oxygen and water. In nature, it occurs only in trace amounts in snow or rain. It is naturally generated during lightening storms. It is typically used in dilute solutions during the manufacture of sodium chlorite.

Other materials are typically added to sodium chlorite powders or solutions before they are sold. Commercial sodium chlorite bleaching solutions contain special ingredients including anticorrosive agents, buffering agents, chlorine dioxide fume controllers, and surfactants. Anticorrosive agents are used to prevent the corrosion of stainless steel bleaching equipment. Buffer salts help liberate the chlorine dioxide that is produced during the bleaching process. Surfactants help stabilize solutions and allow for cleaning and penetration effects. Stabilized sodium chlorite solution can be stored for long periods without loss of activity. When the sodium chlorite is sold as a solid, sodium chloride is often included to make it safer to handle and store.

The Manufacturing Process

While a variety of chlorites are available, sodium chlorite is the only one produced commercially. It is sold in solution or as a solid. The technical grade is made up of about 80% sodium chlorite and the rest is sodium chloride. Large scale production is based on a reaction of chlorine dioxide in a sodium hydroxide solution. Hydrogen peroxide is also present as the reducing agent. Sodium chlorite is manufactured in three phases, chlorine dioxide production, sodium chlorite generation, and recovery.

Chlorine dioxide production

1 While there are five principal methods for generating chlorine dioxide, the most common is the Hooker R-2 process, which generates chlorine dioxide from sodium chlorate. During production, solutions of both sodium chlorate and sodium chloride are pumped into a reaction vessel in approximately equal ratios. Concentrated sulfuric acid is also added to the reaction. Next, air is bubbled into the bottom of the container to create rapid agitation and dilution of the chlorine dioxide that is produced. During this process, both chlorine dioxide and chlorine gas are created.
2 These gases are separated out from the reaction vessel. The chlorine dioxide is separated by being absorbed in a conventional, water chilled tower. The chlorine gas is passed through separation towers and is picked up as sodium or calcium hypochlorite. This process produces about a 95% yield of chlorine dioxide.

Sodium chlorite generation

3 The chlorine dioxide gas is pumped into a vessel containing a cooled, circulating solution of sodium hydroxide. These compounds react to form sodium chlorite and sodium chlorate in approximately equal amounts. Water and oxygen are also generated. To minimize sodium chlorate production, a reducing agent is added. Typically, hydrogen peroxide is used, although sodium peroxide and sodium amalgam may also be employed. This step is closely monitored because sodium chlorate is highly undesirable in the final product.

Isolation and purification

4 Even though steps are taken to minimize its production, sodium chlorate must still be reduced before the sodium chlorite can be isolated. This is accomplished by adding extra hydrogen peroxide.
5 The spent reactive solution is then pumped through a fractional crystallization tower to purify the sodium chlorite. This method takes advantage of the large solubility differences between the chlorite and other related salts that can be formed. After purification, the sodium chlorite solution is evaporated and tumble dried. If an anhydrous (devoid of water) product is desired, the evaporated powder is mixed with water at lOO°F (38°C). The solution is saturated and cooled to 77°F (25°C). When this happens, the anhydrous salt spontaneously crystallizes out of the solution. A rotary drum, steam heated dryer is used to isolated the crystals, resulting in flakes or a fine powder. Occasionally, multiple drying steps are required.
6 The anhydrous salt can then be converted into powder, granules, or a solution. Granules are used more often because they are safer, with lower toxic risks and fire hazards, and a homogeneous composition can be created. Using typical methods, the particle size of the granules can be tightly controlled. Prior to packaging, solid sodium chlorite is mixed with sodium chloride to make it safer to handle.
7 Solutions are prepared by mixing powdered sodium chlorite with various anticorrosive agents, buffering agents, and surfactants in a mixing vessel. These solutions are used for commercial bleaching processes and can be formulated to be extremely stable.
8 Depending on the final use, sodium chlorite solution is packaged in plastic containers, drums, tote tanks, and tanker trucks. In the United States, powdered or flaked sodium chlorite is shipped in lined drums. Bulk transportation of the solid is not allowed because of safety concerns.

Quality Control

To ensure the quality of the sodium chlorite that is produced, the production process is monitored at each stage. The starting raw materials and the final product are all subjected to a variety of chemical and physical tests to determine that they meet the required specifications. Some of the commonly tested characteristics include appearance, odor, pH, density, specific gravity, and melting point. If the final product is a solution, its chemical activity is tested to make sure it has the correct concentration. For solid granules, particle size is determined and modified if necessary.

Byproducts/Waste

Manufacturing sodium chlorite produces some undesirable byproducts, such as chlorine dioxide, that cannot be released into the immediate environment. Concentrated fumes of chlorine dioxide are toxic, and cause sickness, appetite loss, and nausea in line operators. In the production plant, circulation of fresh air is essential. The chlorine dioxide gas is also highly corrosive. For this reason, sodium chlorite solutions must be stored in specially coated containers. Materials such as glass, porcelain, some plastics, or earthenware are typically used. Titanium is the most resistant metal used today. In the textile industry, molybdenum alloy stainless steels are used to store the sodium chlorite bleach solutions.

The Future

With increased applications for chlorine dioxide, improvements in sodium chlorite production are currently being studied. Sodium chlorite research is focused on reducing the environmental impact of bleaching systems and finding quicker, less expensive production methods. New bleaching formulations are constantly being developed by formulating chemists.

Where to Learn More

Books

Kirk-Othmer. "Carbon & Graphite Fibers to Chlorocarbons & Chlorohydrocarbons." In Encyclopedia of Chemical Technology.Vol. 5, edited by Jacqueline I. Kroschwitz and Mary Howe-Grant. New York: John Wiley & Sons, Inc., 1993.