Sodium is not directly extracted from seawater. Instead, sodium chloride (common salt) is first extracted, and then sodium metal is produced from the sodium chloride. The most common method to obtain sodium chloride from seawater is through solar evaporation using solar ponds.
Solar Evaporation and Salt Production
Here's how the process typically works:
- Pond Construction: Large, shallow ponds are constructed. These ponds are designed to maximize sunlight exposure.
- Seawater Introduction: Seawater is pumped into the initial pond.
- Evaporation: Solar energy evaporates the water, increasing the concentration of salts in the remaining brine.
- Sequential Ponds: The concentrated brine is transferred to a series of subsequent ponds. In each pond, the concentration increases further.
- Salt Precipitation: As the brine becomes increasingly concentrated, different salts begin to precipitate out of solution. The order of precipitation is based on the solubility of the salts. Calcium carbonate and calcium sulfate are typically the first to precipitate.
- Sodium Chloride Crystallization: Further evaporation leads to the crystallization of sodium chloride (NaCl), which is harvested.
- Purification (Optional): The harvested sodium chloride may undergo further purification steps to remove impurities.
Production of Sodium Metal from Sodium Chloride
Once sodium chloride is obtained, the sodium metal itself is produced through electrolysis. The most common method is the Downs cell process.
- Melting the Sodium Chloride: Solid sodium chloride has a very high melting point. To make the process more energy-efficient, it's often mixed with calcium chloride to lower the melting point.
- Electrolysis: The molten sodium chloride is then electrolyzed. In the Downs cell, the sodium ions (Na+) are reduced at the cathode, forming liquid sodium metal. Chloride ions (Cl-) are oxidized at the anode, forming chlorine gas.
- Separation: The liquid sodium metal is less dense than the molten salt mixture and floats to the top of the cell, where it is collected. The chlorine gas is collected separately.
The overall reaction for the electrolysis is:
2NaCl(l) → 2Na(l) + Cl2(g)
Summary
In essence, extracting sodium from seawater is a two-step process: first, obtaining sodium chloride through solar evaporation, and second, producing sodium metal from the sodium chloride via electrolysis.
