Process Overview of Caustic Soda

By: doyouknow | Views: 3121 | Date: 28-Nov-2011

Over 95% of the capacity to produce chlorine and essentially 100% of the capacity to produce caustic soda (sodium hydroxide, NaOH) are based on the electrolysis of brine. In this process a sodium chloride (NaCl, salt) solution (brine) is electrolytically decomposed to elemental chlorine (in the anode compartment), and sodium hydroxide solution and elemental hydrogen (in the cathode compartment).

Process Overview

Over 95% of the capacity to produce chlorine and essentially 100% of the capacity to produce caustic soda (sodium hydroxide, NaOH) are based on the electrolysis of brine. In this process a sodium chloride (NaCl, salt) solution (brine) is electrolytically decomposed to elemental chlorine (in the anode compartment), and sodium hydroxide solution and elemental hydrogen (in the cathode compartment). A chemical facility which co-produces caustic soda and chlorine is typically referred to as a chlor-alkali (C/A) facility.

The overall reaction for the electrolytic production of caustic soda and chlorine is:

The electrolytic route to making caustic soda, by using sodium chloride as a raw material, is a co-product process, i.e., for each 1.1 pounds of sodium hydroxide produced, one pound of chlorine is also produced. This ratio is referred to as an electrochemical unit (ECU).

Dow utilizes two electrolytic processes to produce caustic soda solution and chlorine. These processes are:
• Diaphragm cell
• Membrane cell

Diaphragm Cell

Using diaphragm cell technology, chlorine, caustic soda and hydrogen are produced simultaneously. Saturated brine enters the anode compartment of the cell, where chlorine gas is liberated. The function of the diaphragm is to separate the brine from the caustic solution (called cell effluent) at the cathode side, which is also where hydrogen gas is released.


Membrane Cell

Membrane cell technology is a relatively recent development. It differs from diaphragm cell technology in that the solutions surrounding each electrode are separated by a membrane rather than a diaphragm. The membrane is very selective and primarily allows the migration of sodium ions from the anode chamber to the cathode chamber. Saturated brine enters the anode compartment of the cell where chlorine gas is liberated. Since only sodium ions can pass through the membrane to the cathode (brine cannot pass through the membrane), the caustic soda (cell effluent) contains substantially less sodium chloride. No salt removal capabilities are required as in the diaphragm cell process.


1. DOW Caustic Soda Solution plays an important role in many industries such as: pulp and paper, textiles, soaps and detergents, bleach manufacturing, petroleum products, aluminum, and the Chemical Processing Industry (CPI).

2. Pulp and Paper Industry

The pulp and paper industry is the largest consumer of caustic soda worldwide. Uses include the de-inking of waste paper and water treatment, as well as being a raw material in the pulping and bleaching processes.

3. Textile Industry

Caustic soda is used in the chemical processing of cotton, including desizing, scouring, mercerization, and dying synthetic fibers, such as nylon and polyester.

4. Soaps and Detergents Industry

Caustic soda is the alkali material frequently used in the saponification or conversion of fat, tallow and vegetable oils in the soap manufacturing process. The largest use of caustic soda in detergents is in the manufacture of anionic surfactants.

5. Bleach Manufacturing Industry

Caustic soda is used to make bleach (sodium or calcium hypochlorite).

6. Petroleum Products Industry

Caustic soda is used in the exploration, production and processing of petroleum and natural gas. A major use of caustic soda is in the removal of objectionable acidic materials, such as H2S and mercaptans, from hydrocarbons and off-gases generated in processing.

7. Aluminum Industry

The production of alumina from bauxite is a major end-use application for caustic soda. It is used to dissolve bauxite as a first step in the production of aluminum.

8. Chemical Processing Industry (CPI)

Caustic soda is a basic feedstock for a wide range of downstream products in the chemical processing industry. It is used as an intermediate and as a reactant in processes that produce solvents, plastics, synthetic fabrics, adhesives, coatings, herbicides, dyes, inks, pharmaceuticals, and many more industrial products. It is also used for the neutralization of acidic waste streams and scrubbing of acidic components from off-gases.


Physical Properties

Caustic soda is also known as sodium hydroxide, caustic, and lye. Anhydrous (100%, solid) sodium hydroxide has a chemical formula of NaOH and a molecular weight of 40.00.

Basic Properties of Caustic Soda 50%

Vapor Pressure- 1.5 mmHg @ 68º F (0.20 kPa @20º)

Boiling Point- Approx 293º F (145ºC)

Freezing Point- Approx 58º F (14º C)

pH- 14

Specific Gravity- 1.52 g/ml @ 68º F (20º C)

Caustic soda, as a 50% solution, is an odorless and colorless liquid. In all forms, caustic soda is highly corrosive and reactive. Caustic soda solution reacts readily with metals such as aluminum, magnesium, zinc, tin, chromium, bronze, brass, copper, and tantalum. Galvanized (zinc coated) materials should be avoided. Contact with acids, halogenated organics, organic nitro compounds, and glycol should be avoided. It reacts with most animal tissue, including leather, human skin, and eyes. It also reacts readily with various reducing sugars (i.e., fructose, galactose, maltose, dry whey solids) to produce carbon monoxide.

Upon cooling, the viscosity of the solution increases rapidly as the temperature falls below 65°F (18°C).

Stability and Storage Life

Caustic soda solution is a stable product but its storage life is dependent upon the storage conditions. If the caustic is exposed to air, a change in the product quality will be seen over time, since the caustic soda solution will pick up carbon dioxide to form sodium carbonate (Na2CO3) solids. In addition, iron pick up is common in carbon steel storage vessels or in lined carbon steel storage vessels where the liner has been damaged. Therefore, minimizing its exposure to air and its direct contact with iron containing metals will extend the storage life of caustic soda solution.


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