Elutriation is a term for separation and is the process of detaching component particles from each other and removing them from their suspension medium. In elutriation, particles are suspended in a gas or liquid, typically air or water. Based on their size, shape and density, the particles are separated into two or more groups. The three main methods of elutriation include centrifugal processing, aspiration and bulk-tank flow. Centrifugal elutriation is used most often in medical settings, wheres aspiration is used heavily in the manufacturing and recycling of nonsoluble materials and tank flow for separating soluble substances, such as in wastewater treatment.
Centrifugal Elutriation
Centrifugal force elutriation techniques are most often used in medical settings and are designed to separate particles based on size and density at the cellular level. This is the process used to separate the components in blood and other bodily fluids, such as white and red cells, proteins and triglycerides, for medical testing and research. In used in medical settings, an elutriation rotor spins vials, causing the cells in the chamber to be acted upon by the two opposing forces, driving the particles away from the axis of rotation. This separates the particles according to size and density, causing the heavier ones to settle to the bottom.
Separation occurs according to the sedimentation rate of each particle when the forces upon them are balanced. The smaller cells with lower sedimentation rates migrate toward the axis of rotation, and the larger or heavier cells move more slowly and reach a state of equilibrium at the terminal elutriation point, meaning the spot furthest away from the axis of rotation. The centrifugal elutriation method is faster than other methods. However, it is not an ideal method for solid substances as the particles must be diluted for the process to work.
Aspirated Elutriation
Aspirated, or air-wash, elutriation is more powerful, efficient and cost-effective than other methods of elutriation. This is due to a variety of factors that include it requiring the least amount of training, lower production and long-term operational expenses, it being able to process large volumes of material as well as producing a higher quality finished product. Additionally, air wash systems have virtually no moving parts to wear out, and adjustments can be made while the equipment is in use. Aspirated elutriation is best suited to heavy industrial applications such as bulk material manufacturing and handling recyclables. Applications include textile recoveries, such as separating carpet fibers from their backing and labels from plastic bottles.
Aspirated elutriation is used in manufacturing to remove the bark from wood and sawdust from wood chips. It is used to process the rubber regrind from worn-out tires used to make new tires. Aspirated elutriation is used to separate blasting media from contaminants, so the media can be recycled, and it can also be used for dust and vapor collection.
Tank-Flow Elutriation
An elutriation tank is a simple device used to separate particles suspended in a liquid. While this method can be used on a smaller scale, tank elutriation is most commonly used for the large-scale separation of particles in industrial applications. The particles are first uniformly distributed throughout the liquid by being agitated inside the tank, and then the liquid is allowed to settle.
Depending on the density of each substance, particles will either remain suspended in the liquid or settle to the bottom. Because their terminal velocities are lower than that of the rising fluid, the lighter particles rise to the top while the heavier particles fall to the bottom due to gravitational force. Samples are then taken at different levels of the tank through some type of spigot or outlet. The samples are dried, and the various-sized particles are collected. The major downfall of tank-flow processing is the method cannot separate particles of different types having similar sedimentation values.
Conclusion
Whether using centrifugal processing, aspiration or bulk-tank flow, the method of elutriation used will depend on the material being separated. Generally speaking, the more delicate the material, the less aggressive the method must be. The type of particles, their properties, the number of different particles and the utilization of the finished product must all be considered in selecting the proper method of elutriation.
