The Castner-Kellner process, developed in the late 19th century, is one of the methods used to obtain sodium in its pure form. Here's an overview of the process:

1. Preparation of Sodium Amalgam:

- Brine (a concentrated solution of sodium chloride) is electrolyzed using a mercury cathode and a carbon anode.

- Metallic sodium is produced at the cathode, but instead of directly collecting it, the sodium atoms dissolve in the mercury to form sodium amalgam.

2. Decomposition of Sodium Amalgam:

- The sodium amalgam formed in the previous step is pumped into a separate vessel called a decomposer.

- Here, the amalgam is heated in an iron retort. As the temperature increases, the mercury vaporizes and is condensed, while the sodium remains behind.

3. Purification of Sodium:

- The molten sodium, obtained from the decomposer, may contain impurities. To purify it, the molten sodium is filtered through a layer of steel wool or a similar filter to remove solid impurities.

4. Casting into Ingots:

- The purified molten sodium is poured into molds and allowed to solidify, forming sodium ingots. These ingots can be further processed as needed.

In summary, the Castner-Kellner process involves electrolysis of brine to form sodium amalgam, followed by the decomposition of the amalgam to obtain pure sodium. This process enables the production of high-purity sodium metal.