The separation of salt from a mixture involves leveraging the physical differences between salt and the substance it is mixed with. The goal of this process dictates the method used, whether the objective is to purify the salt itself or to reclaim the water it is dissolved in. Separation techniques rely on properties such as solubility and boiling point. Since salt is a dissolved solid in water, it forms a true solution that cannot be separated by simple straining or filtration. The two common outcomes are harvesting the solid salt or producing purified water.
Separating Salt from Water to Harvest the Salt
The most common method for obtaining solid salt from a solution involves the process of evaporation, which exploits the significant difference in boiling points between the solvent and the solute. Water has a relatively low boiling point, while salt is a non-volatile solute with an extremely high boiling point. When heat is applied, the water molecules gain enough energy to escape as vapor, leaving the salt behind to solidify through crystallization.
In a controlled environment, a saltwater solution is simply heated until all the water has boiled away, resulting in salt crystals coating the bottom of the container. On a commercial scale, the process utilizes solar evaporation, an ancient and sustainable technique for producing salt from seawater or brine. Seawater is channeled into large, shallow evaporation ponds, where the sun and wind provide the energy for the water to slowly vaporize. As the water volume decreases, the brine becomes concentrated, eventually reaching saturation where the salt precipitates out as solid crystals.
Separating Salt from Solid Impurities
When salt is mixed with insoluble solid impurities, such as sand or dirt, a multi-step process is required to isolate the salt. This separation relies on the high solubility of salt in water, a property that the solid contaminants lack. The first step involves dissolution, where the mixture is stirred vigorously in water to dissolve the salt, creating a saltwater solution, or brine. The insoluble impurities remain suspended in the water or sink to the bottom of the vessel.
Once the salt is dissolved, the next step is to separate the resulting solution from the undissolved solids using either decantation or filtration. Decantation involves carefully pouring the liquid salt solution off the top, leaving the heavier, settled solids behind. Filtration is performed by pouring the mixture through a filter paper set in a funnel, which traps the solid particles while allowing the liquid solution (the filtrate) to pass through. To harvest the pure, solid salt, this filtered solution must then be subjected to evaporation.
Industrial and Advanced Methods for Purifying Water
When the goal shifts from harvesting salt to obtaining purified water, the process is known as desalination. This is a major focus for producing fresh water globally. One thermal method used for desalination is distillation, which captures the evaporated water. Saline water is heated to its boiling point, creating steam that is free of the non-volatile dissolved salts. The pure steam is then collected and condensed back into liquid water, leaving the concentrated salt solution behind.
The current industry standard for large-scale water purification is Reverse Osmosis (RO), a membrane-based technology that is significantly more energy-efficient than thermal methods. RO uses a semi-permeable membrane with extremely small pores that allow water molecules to pass through while physically blocking larger dissolved salt ions, bacteria, and viruses. To overcome the natural osmotic pressure, high pressure is applied to the saline water. This applied force reverses the natural process, pushing the pure water through the membrane and leaving a highly concentrated brine on the pressurized side.