THE CHLORALKALI AND SOLVAY PROCESSES

Contents for this page	Related topics
Introduction Electrolysis of sodium chloride Manufacture of soap The Solvay process Additional questions	The petroleum industry The polymer industry (1) The polymer industry (2) The fertiliser industry Ammonia - Nitric acid - Sulphuric acid Electrochemical cells	Data Glossary
Learning Outcomes
After studying this section, you will be familiar with the chemical principles underlying the chloralkali process, the manufacture of soap, and the Solvay process.

Introduction

Sodium hydroxide, sodium carbonate, and chlorine are substances produced on a large scale by the chemical industry. The term CHLORALKALI PROCESS refers to the industrial production of the alkali sodium hydroxide, NaOH, and chlorine, Cl₂ from common salt, sodium chloride, NaCl. Sodium carbonate, known in the industry as SODA ASH is produced by the Solvay process. The importance of these chemicals, produced in the millions of tons world-wide annually, is illustrated by the table below.

Electrolysis of sodium chloride:

Sodium hydroxide and chlorine are produced industrially by the electrolysis of brine, which is a near-saturated solution of sodium chloride, NaCl. The reactions involved are

When a current is passed through brine (an aqueous solution of sodium chloride), hydrogen gas is produced at the cathode, since H⁺ ions are more easily discharged than Na⁺ ions (except under the conditions used in the mercury process - see below). The depletion of H⁺ ions near the cathode means that hydroxide(OH^-) ions, as sodium hydroxide, accumulate in the cathode compartment. Chlorine gas is produced at the anode.

Three processes are used, the MEMBRANE CELL, the NELSON DIAPHRAGM CELL, and the MERCURY PROCESS.

The membrane cell:

In the membrane process, the anode and cathode compartments are separated by an ION EXCHANGE MEMBRANE that allows water and sodium ions to diffuse through, but not chloride ions. In the anode compartment, chloride ions migrate to the titanium anode, where they are discharged to form chlorine gas. In the cathode compartment, hydrogen ions migrate to a nickel cathode, where hydrogen is produced. Electrical neutrality is maintained by sodium ions moving through the membrane from the anode compartment to the cathode compartment. This is the preferred industrial method for the preparation of chlorine and sodium hydroxide. Very pure product NaOH is obtained, with minimal effects on the environment. The energy requirements of this cell are said to be more advantageous than with the diaphragm or mercury cells.

The Nelson diaphragm cell:

In the Nelson cell, saturated sodium chloride solution is fed continuously into a steel-mesh container lined with an asbestos diaphragm. Hydrogen is produced on the outside of the asbestos, and escapes out of the cell, where it is collected and used (for example, for making hydrogen chloride). The steam creates a heated environment, and, through condensation on the steel mesh, washes out the sodium hydroxide, which collects at the bottom of the cell, from where it is drawn off. Chlorine gas is produced at the carbon anode, and escapes out of the cell for further treatment.

This process is being phased out, as asbestos is harmful to the workers, causing fatal lung diseases generally called ASBESTOSIS.

The mercury (Castner-Kellner) process:

Saturated sodium chloride solution is fed continuously into a container fitted with titanium anodes. The bottom of the cell consists of a layer of mercury, circulated by means of a pump. This layer serves as cathode. Under the conditions that prevail, sodium ions are preferentially discharged. The sodium metal dissolves in the mercury to form an amalgam. This amalgam is reacted with water in a "decomposer", producing hydrogen gas and sodium hydroxide solution:

The mercury is cooled and recycled. Chlorine gas is formed at the anodes, and is collected. A disadvantage of this process is the use of mercury, which is poisonous and which may have harmful effects, not only for the workers, but also for the environment.

Manufacture of soap:

Soaps are sodium or potassium salts of long-chain carboxylic acids, such as stearic, palmitic and oleic acids. The production of soaps dates back to antiquity, when humans discovered that wood ash and fats produced substances having a cleansing effect. Soaps are produced by the action of strong alkalis, such as potassium or sodium hydroxides on fats and oils (). The reaction, called SAPONIFICATION, is the hydrolysis of esters of the trihydric alcohol (), glycerol, and long-chain fatty acids. These esters collectively form a group of organic compounds called TRIGLYCERIDES.

When a triglyceride is treated with sodium or potassium hydroxides, hydrolysis takes place forming glycerol and the sodium or potassium salts of the acids, that is, soap. The glycerol is refined, and sold as GLYCERINE, which is used, amongst other things, for the manufacture of nitroglycerine, the explosive in dynamite.

The Solvay process:

Note that the Solvay process is not part of the South African Grade 12 syllabus.

The Solvay process (also known as the ammonia-soda process) was invented by the Belgian chemical engineer Ernest Solvay, and is used for the manufacture of sodium carbonate (known as soda ash and/or washing soda) and sodium hydrogen carbonate (sodium bicarbonate). Calcium chloride, CaCl₂, is a by-product. The starting materials are cheap and common substances, namely limestone, CaCO₃, and sodium chloride, NaCl.

Step 1: A saturated solution of sodium chloride is treated with ammonia, and then with carbon dioxide. This causes sodium hydrogen carbonate, NaHCO₃ to be formed and to precipitate out of the solution. Ammonium chloride is also formed, and this becomes a source of ammonia, as shown in Step 3.

Step 2: The sodium hydrogen carbonate is washed, dried, and heated to form sodium carbonate, water and carbon dioxide (which is recuperated and used in the reaction above).

Step 3: The ammonium chloride formed in Step 1 is treated with quicklime (calcium oxide, CaO), forming ammonia and calcium chloride (equation on the left below). The quicklime is itself obtained by heating limestone, producing carbon dioxide in the process (equation on the right, below), which is used in Step 1:

The Solvay process is very efficient in terms of materials used. Ammonia is only needed to make up for losses in the process. The calcium chloride finds numerous uses, a major one being the de-icing of roads and airport runways.

Additional questions