Copyright © 2007 by Old World Industries, Inc.

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Liquid Caustic Soda is a colorless, odorless liquid. It is usually produced as part of the same process that creates chlorine. Used as a neutralizing agent, Caustic Soda is an important component in petroleum refining, pulp and paper, water treatment and aluminum industries. Caustic Soda is also used in a wide range of chemical processes. Two basic processes are used in the commercial production of chlorine. These are as follows:

• Diaphragm

• Membrane

Caustic Soda is a major base chemical manufacturing commodity. Caustic Soda is one of the workhorses of the chemical industry, sharing this position only with soda ash and sulfuric acid. There is scarcely a single chemical that does not require one or more of these three basic chemicals for its production. Furthermore, there is hardly an industrial product that does not depend upon Caustic Soda.

Caustic is also known as sodium hydroxide (NaOH), its chemical name. Used as a neutralizing agent, caustic soda is a pure alkaline, with a pH of 14. A pH of 7 is considered "neutral", because the concentration of hydrogen ions (H+) is exactly equal to the concentration of hydroxide ions (OH-) produced by dissociation of the water. 

Following are the pH levels of some common substances:

Caustic soda is used in a wide variety of industrial applications. It is used: 

• as a reactant in the manufacture of other sodium compounds, which themselves may be intermediate or end-use products: 
  Sodium hypochlorite - A household bleach and disinfectant 
  Sodium phenolate - Used in antiseptics and for the manufacture of Aspirin. 

• in the manufacturing process of soaps and surfactants. 

• in soap powders. 

• in the textile industry to remove contaminants. 

• as bleach in the treatment of scoured cloth. 

• to improve textile luster and dye absorption. 

• as an important component in the refinement of high sulfur petroleum, pulp and paper. 

• as a key component in the conversion of mined bauxite into alumina. 

Diaphragm Process

A cell with an asbestos diaphragm is the basis for all diaphragm chlor-alkali cells. The diaphragm separates the cell into two compartments. One compartment contains the cathode and one contains the anodes.

1. The sodium chloride brine solution (NaCl) enters the anode compartment and covers the anode and cathode tubes. 

2. There is continuous slow percolation of the electrolyte through the diaphragm from the anode compartment to the cathode compartment.

3. Chlorine gas rises from the surface of the anodes and is drawn off.

4. Hydrogen, released at the cathode, is collected from the top of the cathode compartment. 

5. The solution of sodium chloride and sodium hydroxide (NaOH) overflows the cell through a level control pipe on the cathode and is commonly called cell liquor. 

The cell liquor forms a 10-12 weight percent sodium hydroxide solution and also contains 14-16 weight percent unreacted sodium chloride. Modern diaphragm cell plants use triple or quadruple effect evaporators to concentrate the sodium hydroxide and to recover the sodium chloride. Caustic soda produced from diaphragm cells, after being concentrated to 50 percent, contains approximately 1 percent sodium chloride. For some applications it is desirable to remove the remaining sodium chloride and other compounds. 

Most of the Caustic Soda produced in the world is sold as a liquid containing 50 percent NaOH. Selling in liquid form provides higher transportation costs but is more economical than evaporating the remaining 50 percent water. 

Advantages of the diaphragm cell process include its ability to use lower purity brine and its lower consumption of electricity, compared to mercury cells. Disadvantages include its use of asbestos and production of lower purity caustic. It also consumes significant quantities of steam in the caustic evaporators.

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Membrane Process

In the membrane process, the anode and cathode are separated by an ion-exchange membrane that selectively transmits sodium ions, but suppresses the saturation migration of hydroxyl ions from the cathode into the anode.

1. A brine solution of NaCl is fed into the anode compartment, where chlorine gas is produced, and sodium ions migrate into the cathode through the membrane. 

2. Depleted brine is discharged from the cell. 

3. In the cathode compartment, hydrogen is produced at the cathode, leaving hydroxyl ions which, together with the sodium ions that have crossed the membrane, form caustic soda. 

4. Pure water is added to the catholyte to control the concentration of the caustic soda solution. 

Chloride ions in the anolyte are excluded by the ion exchange membrane, so the rate of diffusion of chloride from the anolyte to the catholyte is extremely low. As a result, a high purity caustic solution is produced as the catholyte effluent. Since the catholyte is strongly caustic, there is some back migration of hydroxyl ions from the catholyte into the anolyte. 

The membrane is the key component of the membrane cell. Its primary requirements are durability during electrolysis, high selectivity for sodium ion transport, low electrical resistance, and sufficient mechanical strength for practical use. The energy requirements and the quality of the solution depend on the membrane. Only ion exchange membranes made of perfluoro polymer can withstand severe conditions of exposure to chlorine and caustic soda at high temperatures. This process produces a higher purity caustic soda solution (32 to 34 percent), with very low sodium chloride content. 

Advantages of the membrane process include its energy efficiency, which allows for less costly cell operations, and its ability to produce with no asbestos or mercury environmental contaminants. Disadvantages include the high purity brine requirement and the high cost of the membrane itself. 

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Old World ships Caustic Soda from locations throughout the United States to give customers ready access to a range of Caustic Soda solutions.