A centrifuge cannot separate salt from water. Centrifuges use the force of gravity to affect the separation of solids from liquids. Salt dissolves in water to form a solution. The centrifugal force exerted by a centrifuge separates immiscible phases such as sand from water but not miscible phases such as salt in water.
A centrifuge cannot separate salt from water because salt dissolves in water. The dissolution of salt in water is a chemical process wherein the salt molecules form a chemical bond with the water molecules forming a cohesive liquid.
A centrifuge works by acting on the different specific gravities of fluids in a mixture. Since a solution such as salt and water does not have phases with different specific gravities, a centrifuge cannot separate them.
Yes, under certain conditions, a centrifuge cannot separate immiscible solids from liquids. Since the centrifuge uses differential sedimentation to separate solids from liquids, it requires the solids to be denser or heavier than the liquid.
When the solid particles are light and minuscule, the centrifugal force is insufficient to push the particles out of the liquid. In such cases, the solid particles remain suspended in the liquid without separating.
An example of this failure of centrifugal separation is the carbon particles in used engine oil. These particles are sub-micron level and are predominantly combustion byproducts such as ash. The relatively low specific gravity combined with the tiny particle size makes it impossible for a centrifuge to separate these particles.
Also, these particles cause the black color of used engine oil. And therefore, processing used engine oil through a centrifuge does not change the color of the oil, and the oil remains black post centrifugation.
There are multiple examples of solids solutions with liquids and liquids with liquids that one cannot separate with a centrifuge. Some examples of such solutions are listed below.
The following is a list of standard liquid/liquid and liquid/solid mixes that a centrifuge can separate.
As explained above, a centrifuge cannot separate dissolved solids from liquids. So the obvious question is - how to separate dissolved solids from liquids?
The most common method of separating dissolved solids from liquids is evaporation or distillation. The evaporation process involves transforming a liquid into its vapor form through higher temperatures (boiling) or reducing the pressure below vapor pressure (distillation).
In either case, the carrier liquid changes state to a gaseous form while the dissolved solids remain. The most common example of this process is salt production from seawater.
Salt production involves the evaporation of seawater in large lagoons. The water evaporates due to the heat from sunlight leaving behind the dissolved salt in sea salt crystals.
In some instances, chemicals in the form of polymers or precipitants aid the separation of dissolved solids from liquids using mechanical separation devices such as centrifuges.
For example, heavy metal compounds dissolved in water precipitate into solid particles in the presence of catalysts or through a chemical reaction with precipitation agents.
It is clear from the above article that centrifuges cannot separate salt from water or any solid that is dissolved in a liquid.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge