Hydrocyclone (Desander, Desilter) and Cut Size d50

A hydrocyclone which main part of desander and desilter is a device used to sort, classify, and separate a liquid suspension(solids) system into solids and liquid. Hydrocyclones have a wide variety of applications in many industries, particularly in oilfield application processing. The hydrocyclone is a continuously operating classification device with no moving parts. It utilizes fluid pressure to cause rotational forces that accelerate the settling of particles according to their size, shape and density.

Advantage Of Hydrocyclone

Hydrocyclones (desander and desilter) have many advantages as classifiers: they are simple and are inexpensive in terms of construction, installation, maintenance and capital cost. In closed grinding circuits in particular they require little space for installation and reduce residence time within the circuit, compared with the now obsolete rake classifiers. The apparent simplicity of operation of cyclones is misleading. They are not easily monitored and problems can often be missed by the operator.

Cut Size (d50)

The cut size (or d50) is the particle size at which there is equal chance that the particle will report to the overflow or underflow.

For example, if D50 = 74 μm, then 50% of the particles in the sample are larger than 74 μm, and 50% smaller than 74 μm. D50 is usually used to represent the particle size of group of particles. The mean of D10 is : if  D10 = 74 μm, 10% of the particles diameter smaller than 74 μm. The same as D90.

D50 is also named x50 or Dx50 particle size according to some ISO standards or user’s tradition. General relationships between cut size and hydrocyclone geometry and operating parameters have been determined, with cut size generally considered to increase with increasing hydrocyclone diameter, increasing feed solids concentration and increasing hydrocyclone inclination to the vertical. Increasing the hydrocyclone flow rate is considered to decrease the d50 whilst the d50 is increased by altering the hydrocyclone geometry using small spigots or large vortex finders.

During the classification process, some fine particles are entrained in the underflow liquid and report in the underflow product instead of the overflow. Kelsall suggested that solids of all sizes are entrained in the coarse product liquid by short circuiting in direct proportion to the fraction of feed water reporting to the underflow. The number of particles misplaced to the underflow by entrainment can be reduced by appropriate hydrocyclone operation. The water split to the underflow increases with increasing feed solids concentrations and the use of hydrocyclones with large spigot
sizes or small vortex finders. Increasing the flow rate to the hydrocyclone reduces the water split to the underflow, and therefore the number of particles misplaced due to entrainment.

Optimisation of Hydrocyclone Performance Under Cut size (d50)

Optimisation of cyclones involves achieving the d50c set by downstream processing requirements and minimising the quantity of misplaced particles in the underflow and overflow streams, i.e. maximising efficiency. The choice of d50c is based on the fineness required of the circuit product, possibly for additional grinding or in many cases for beneficiation such as flotation.