Dense Medium Cyclone Principles
The cyclone is a piece of equipment which utilizes fluid pressure energy to create rotational fluid motion. This rotational motion causes relative movement of the materials suspended in the fluid, thus permitting separation of these materials from one another. The rotational motion of the fluid is produced by tangential injection of the fluid into the cyclone.
The raw coal to be treated is suspended in a very fine medium, normally finely ground magnetite, and this pulp is fed tangentially into the cyclone through the inlet to a short cylindrical section, which also carries what is termed the vortex finder. Separation is made in the cone-shaped part of the cyclone or frustum. The discard portion of the raw coal (sinks) leaves the cyclone at the spigot, and the clean coal (floats) via the vortex finder. The cylindrical section of the cyclone can be extended by the introduction of a barrel section which effectively increases the residence time within the cyclone and can improve the sharpness of separation between the clean coal and discard.
The separation process that occurs inside a cyclone is thought to be driven by the centrifugal force acting radially outward and an inwardly acting drag force. The centrifugal force developed inside the cyclone accelerates the settling rate of the particles, thereby separating them according to specific gravity in the medium. Thus the more dense material is flung to the outer wall of the cyclone where the settling velocity is at its lowest and progresses downwards along the cyclone wall in a spiral flow pattern until it exits at the spigot in an umbrella shaped spray. At the spigot, a reverse vortex begins to form creating a low pressure zone (generally referred to as the air core) flowing upwards along the axis of the cyclone, through the vortex finder and exits at the overflow. The less dense material, due to the action of the drag force, settles more slowly. This material is captured in the upward flow of the reverse vortex and exits through the overflow. The medium density at which the separation between more dense and less dense material occurs is called the cut point density. Invariably, there often is a percentage of the feed coal distribution which gets trapped in an envelope of zero velocity inside the cyclone where the centrifugal force equals the drag force. Such material has an equal chance of reporting either to the overflow or the underflow and is often termed near density material.