THE CHLOR-ALKALI INDUSTRY. Chlorine is manufactured by the electrolysis of brine. Sodium hydroxide is produced at the same time. Three different methods.

THE CHLOR-ALKALI INDUSTRY

Chlorine is manufactured by the electrolysis of brine. Sodium hydroxide is produced at the same time.

Three different methods are currently in use.

Information on each is given below. Decide which of the three is the “best” method.

NB – consider environmental aspects, energy consumption and production rate.

Mercury cell electrolysis

Mercury cell electrolysis, also known as the Castner process, in which the "primary cell", titanium anodes are placed in a sodium (or potassium) chloride solution flowing over a liquid mercury cathode. When a potential difference is applied and current flows, chlorine is released at the titanium anode and sodium (or potassium) dissolves in the mercury cathode forming an amalgam. The mercury is then recycled to the primary cell.

The mercury process is the least energy-efficient of the three main technologies and there are also concerns about mercury emissions.

Mercury cell

Diaphragm cell electrolysis

In this process, a diaphragm separates cathode and anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathode. The brine is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the sodium hydroxide is produced.

This method produces alkali that is quite dilute (about 12%) but diaphragm cells are not burdened with the problem of preventing mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings but large amounts of steam are required if the sodium hydroxide has to be evaporated to the commercial concentration of 50%.

Diaphragm cell

Membrane cell electrolysis

The electrolysis cell is divided into two "rooms" by a cation permeable membrane acting as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration. Sodium (or potassium) hydroxide solution is circulated through the cathode compartment, exiting at a higher concentration. A portion of the concentrated sodium hydroxide solution leaving the cell is diverted as product, while the remainder is diluted with deionized water and passed through the electrolyzer again.

This method is more efficient than the diaphragm cell and produces very pure sodium (or potassium) hydroxide at about 32% concentration, but requires very pure brine.

MEMBRANE CELL

SODIUM HYDROXIDE IS ESSENTIAL FOR SOAP MAKING

SAPONIFICATION

A soap molecule is a sodium or potassium salt of a long-chain fatty acid.

HOW SOAP WORKS:

A SOAP MICELLE

Non polar ends of soap or detergent molecules at the centre of the micelle are attracted to the non-polar molecules in greasy dirt. The polar ends of the molecules are attracted to the water

The formation of soap micelles allows soap to remove greasy dirt.

A detergent molecule – a sulphate group is attached to a long chain alkyl group.