Rock salt, primarily sodium chloride (NaCl), contains various impurities. Purification involves removing these contaminants to yield pure table salt. Two primary methods achieve this: filtration and evaporation.
Purification Techniques
Filtration
This initial step removes larger insoluble impurities like sand and other solid matter. Dissolving the rock salt in water creates a solution. This solution is then passed through a filter (e.g., filter paper or cloth) to separate the solid impurities from the dissolved salt.
Evaporation
After filtration, the remaining salt solution contains dissolved impurities. Evaporation removes the water, leaving behind purified salt crystals. This can be done through various methods including:
- Solar evaporation: Utilizing sunlight to evaporate water from large shallow ponds, a cost-effective technique frequently used for large-scale salt production.
- Thermal evaporation: Applying heat to boil off the water more quickly. This method offers faster production than solar evaporation but requires more energy.
- Vacuum evaporation: Lowering the pressure to decrease the boiling point of water, allowing for evaporation at lower temperatures and saving energy.
Industrial Purification Processes
Industrial salt purification goes beyond simple filtration and evaporation. Processes may involve:
- Clarification and De-aeration: Removing suspended solids and dissolved gases from the brine solution.
- Precipitation: Utilizing chemicals like lime and sodium carbonate to precipitate out specific impurities (e.g., calcium and magnesium ions).
- Crystallization: Controlling the crystallization process to enhance salt purity and crystal size.
Examples of Impurity Removal
The references illustrate various impurity removal techniques depending on the level of purity desired and the scale of operation. For instance, some methods focus on removing sand by dissolving the salt in water and filtering out the insoluble sand. Other techniques, commonly used on an industrial scale, involve complex steps of precipitation and crystallization to remove a range of impurities to achieve higher purity salt, suitable for chemical or human consumption.
