Good cyclone separation depends upon control of pressure drop, pulp density and apex size. The ‘pressure drop’ may vary but should not change rapidly, and is held at safe levels by keeping an adequate sump level. A falling sump level causes cavitation in the pump and reduction in feed rate; pressure drop in the cyclone falls and solids report increasingly to the overflow until the drop approaches zero and the entire slurry stream passes into the underflow. Additionally, while maintaining the required separation parameters, the pressure drop should always be minimised to minimise energy losses, thus reducing pump and cyclone wear. The maximum ‘pulp density’ is usually about 50% solids by weight; above that level small fluctuations in density will seriously affect separation. A cone-shaped discharge of 20°–30° reduced angle usually produces optimum conditions for separation. Cyclone control is best obtained by optimising the feed density. With consistent ore types the cyclone feed density is a good indicator of cyclone overflow sizing. A ropy cyclone underflow indicates a very high-density state with a risk of plugging the ‘apex’. If control cannot be exercised either a larger apex is needed, or the addition of another cyclone.
Striking an economic balance between high D50 and low D50 separation requires:
- increasing the fine sands content of the overflow and build up of the circulating load until the cyclone feed density increases to the point of coarser separation.
- higher hydrocyclone pumping pressures with consequent higher power and maintenance costs; if the mill is unable to grind the ore at the given feed rate the final grind will not be any finer no matter what adjustments are made to the cyclone.
When processing gold, it is important to recover coarse gold as soon as possible to minimise fragmentation and smearing onto other particles during multiple passes through the grinding circuit. Unit capacity cost of an enhanced gravity concentration device is high and in order to minimise the capacity of an installed enhanced gravity concentrating device, conventional practice is to connect such a device to only a fraction of the cyclone underflow. It is considered that, statistically, any free gold escaping the grinding circuit via overflow processes will follow the laws of probability and
eventually report to the gravity concentration device after several passes through the grinding mill. It has been proposed that the incoming feed to the cyclone can be scavenged prior to cycloning. The cyclone feed pipeline is so arranged that an outlet under pressure can be installed on the bottom of a line at the end of a straight section that is either horizontal or inclined. Material taken off can be elevated to a sizing screen under its own pressure, the coarse fraction passing directly to gravity separation.
A cyclone separator imparts a rotary motion to the gases and thereby enhances the settling rate to many times that induced by gravity alone. A cyclone separator is essentially a gravitational separator that has been enhanced by a centrifugal force component. The cyclone separator grade efficiency curve, and applies to all cyclone separators, as well as to inertial and gravitational collectors.
Cyclone performance is rated in terms of particle cut diameter or cut size. The cut size, dp50, is the particle size which is captured 50%.
The relationship between particle cut diameters for this type separator, where dp is the particle diameter and the numerical subscript denotes the collection efficiency of that size particle.