The Role of CaCl2 in the Manufacturing of Metallic Sodium by Fused Salt Electrolysis Method (Downs Process)

The fused salt electrolysis method, also known as the Downs process, is a commonly used method for the production of metallic sodium. In this process, a small amount of CaCl2 is added as an auxiliary electrolyte. The addition of CaCl2 serves two main purposes: improving the electrical conductance and decreasing the melting point of NaCl.

Improving Electrical Conductance:
The addition of CaCl2 to the electrolyte improves the electrical conductivity of the molten salt mixture. This is because CaCl2 dissociates into Ca2+ and 2Cl- ions when dissolved in the molten salt. These additional ions increase the number of charge carriers in the electrolyte, enhancing the overall conductivity. As a result, the electrical resistance of the electrolyte is reduced, allowing for a more efficient flow of electric current during electrolysis.

Decreasing the Melting Point of NaCl:
The presence of CaCl2 also helps to decrease the melting point of NaCl. Pure sodium chloride (NaCl) has a relatively high melting point of around 801 °C. However, when CaCl2 is added to the mixture, it forms a eutectic mixture with NaCl. The eutectic mixture has a lower melting point than the pure components, allowing for the operation of the electrolytic cell at lower temperatures. This not only reduces the energy requirements for the process but also helps to minimize corrosion and extend the lifespan of the equipment.

Stabilizing the Metallic Sodium:
CaCl2 also plays a role in stabilizing the metallic sodium produced during the electrolysis process. Sodium is highly reactive and can react with moisture or air, leading to the formation of sodium hydroxide (NaOH) or sodium oxide (Na2O). These reactions can be detrimental to the quality of the sodium produced. The presence of CaCl2 in the electrolyte helps to prevent the oxidation of metallic sodium by acting as a protective layer. The Ca2+ ions present in the electrolyte form a stable compound with the oxygen, preventing its reaction with sodium.

Increase the Temperature of Electrolysis:
The statement that CaCl2 increases the temperature of electrolysis is incorrect. The addition of CaCl2 does not directly increase the temperature. However, by decreasing the melting point of NaCl, it allows for the operation of the electrolytic cell at lower temperatures, which can be advantageous in terms of energy efficiency and equipment maintenance.

In summary, the addition of CaCl2 as an auxiliary electrolyte in the fused salt electrolysis method (Downs process) for the production of metallic sodium serves to improve the electrical conductance and decrease the melting point of NaCl. These effects enhance the efficiency of the electrolysis process and help to stabilize the metallic sodium produced.