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Gravity separation separates suspensions by density and size. It is used in medicine and metallurgy. The most common methods are static separation, centrifugation, and jigging. Particle size and water balance are important for efficient separation. Centrifugation is commonly used in medicine, while jigging is an older but effective technique.
Gravity separation is a method of separating a suspension into its individual components. It is commonly used in the medical field to separate red blood cells from plasma and in the metallurgical industries to separate particles of different metals. Effective separation relies on the different densities and sizes of particulate matter and works by combining the forces of gravity with resistance to motion. The finite concentration is a ratio of the difference between the density of the heavier particle and the fluid to the difference between the density of the lighter particle and the fluid:
Concentration = Dh – Df / Dl – Df
There are many ways to achieve gravity separation, the most common being static separation, centrifugation and jigging. For all methods, a correct water balance in the suspension is important to ensure efficient separation and the fluid must be free of sludge, which increases viscosity and hinders particle movement. For the most effective gravity separation, particle sizes should be relatively close together, with very small or very large particles being filtered out.
The static method is gravity separation in its purest form and relies solely on gravity to achieve separation. The suspension is stirred, then left completely undisturbed. As the mixture settles, the larger, denser particles will stop first and settle to the bottom of the container. Subsequent larger and denser particles will settle on top, forming a distinct layer. The process continues until all particles settle into their respective layers, with the smallest and lightest particles on top. All gravity separation methods work in this way, but differ in the mechanisms applied to facilitate the separation and speed up the process.
Centrifugation is the most frequently used separation method in medical settings, but it is also occasionally used in more specialized metallurgical operations where small quantities and larger particles are involved. In the case of blood, the sample is injected into a rubber stoppered glass test tube and, once clotted, it is placed in a counterbalanced centrifuge. The centrifuge can rotate at very high speeds, which produces centrifugal forces that push the particles down through the fluid until they collect into a compact mass at the bottom of the tube. Some tubes contain a gel separator that is heavier than plasma but lighter than blood cells and forms a barrier to keep the two from mixing after centrifugation.
Jigging is an older, but still effective gravity separation technique that achieves separation down to 150 micrometres. The most basic jigging mechanism involves placing the substance to be separated on a bed of “rags”, such as ball bearings, in a water chamber. The rag layer rests on a perforated plate above another chamber, which sends alternating suction and impulses upwards through the rag. The pulses vibrate the rag, creating spaces for the heavier particles to fall, and the suction collects them in the lower chamber. The lightest particles are washed from the surface by a constant stream of water over the substance in question and collected in a separate container.
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