Hydrocyclones

Introduction:

Hydrocyclone is the device of separating the particles in a liquid suspension. It has two exists, the larger one at top i.e., considered as overflow and the smaller one at the bottom i.e., considered as under flow. During cyclone operation the feed slurry enters the cyclone under pressure through feed pipe into the cylindrical feed chamber. As the feed enters the chamber, it is affected by two opposing forces; these are centrifugal force and drag force. Centrifugal force acts outwards and Drag force acts inwards. The rotation of the of the slurry inside of the cyclone begins, causing centrifugal forces to accelerate the movements of the particle towards the outer wall and separates the particles according to their size, shape and specific gravity. The faster settling particles migrates downward in a spiral pattern through the cylindrical section and into the conical section, thereby it migrates to the apex opening as under flow. Due to Drag force the slower settling particles move towards the zone of low pressure along the axis and migrate toward the center, spiral upward and discharging through the vertex finder pipe as overflow. Overflow contains the finer particles and the majority of water. It should be discharged at or near atmospheric pressure. Under flow contains higher mass particles and also should be discharged at or near atmospheric pressure.

Parameters of a Hydrocyclone:

  1. Diameter
  2. Area of inlet nozzle
  3. Vertex finder
  4. Cylindrical section
  5. Conical section
  6. Apex orifice.

Diameter : It is the inside diameter of the cylindrical chamber.

Area of the inlet nozzle : It is the area of the inlet nozzle at the point of entry into the feed chamber.It approximates 0.05 times the cyclone diameter squared.

Vortex finder : It is to control both the separation and flow of the liquid. The size of the vortex finder equals 0.35 times the cyclone diameter.

Cylindrical Section : It is located between the feed chamber and conical section.

Conical section : It referred to as the cone section. The included angle of a cone section is normally between 10° and 20° and, similar to the cylindrical section, provides
retention time.

Apex orifice : It is the termination of the cone section, and its critical dimension is the inside diameter at the discharge point. Its size should be large enough to allow the
solids that have been classified to underflow to exit the cyclone without plugging.


Hydrocyclones - Mineral Process Engineering

Cyclone efficiency:

Partition curve

Cyclone efficiency determined by partition curve, which relates particle size and the weight fraction of each particle size in the feed which reports, the underflow. The cut Point or separation size, of the cyclone is defined as the size for which 50% of the particles in the feed report to the underflow, i.e. particles of this size have an equal chance of going either with the overflow or under flow. This point referred to as the D50 size.


Hydrocyclones - Mineral Process Engineering
Fig. Partition Curve for Hydrocyclone


The sharpness of the cut depends on the slope of the central section of the partition curve; the closer to vertical is the slope, the higher is the efficiency. The slope of the curve can be expressed by taking the points at which 75% and 25%.of the feed particles report to under flow. These are the d75 and d25 sizes, respectively. The efficiency of separation, or these-called imperfection I, is then given by

I = (d75-d25)/d50

Many mathematical models of hydro cyclones included the term "corrected d50" taken from the "corrected" classification curve. Kelsall (1953) 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. For example, if the feed contains 16 t/h of material of a certain size, and 12 t/h reports to the underflow, then the percentage of this size reporting to the underflow, and plotted on the normal partition curve, is 75%.However, if, say, 25% of the feed water reports to the underflow, then 25 % of the feed material will short-circuit with it; therefore, 4 t/h of the size fraction will short-circuit to the underflow, and only 8 t/h leave in the underflow due to classification. The corrected recovery of the size fraction is thus
Hydrocyclones - Mineral Process Engineering

Factors affecting cyclone performance:

Cut size :

Increases with cyclone diameter.
Increases with feed solid concentration.
Decreases with flow rate.
Increases with small apex.
Increases with small apex or large vertex finder.
Increases with cyclone inclination.

Classification efficiency:

Increases with cyclone selection.
Increases with feed solids concentration and/or viscosity.
Increased by limiting water under flow.
Increased by certain geometries.

Flow split of water to under flow:

Increases with larger apex or small vertex finder.
Decreases with flow rate.
Decreases with inclined cyclones (especially low pressure).
Increases with feed solid concentration and/or viscosity.

Flow rate:

Increases with flow rate.
Increases with cyclone diameter.
Decreases with feed solids concentration and/or viscosity.



References:

(1) Principles of Mineral Dressing by A.M.Gaudin.

(2) Mineral processing Technology by B.A.Wills.

(3) Arterbum,R.A.,”The sizing of Hydro cyclones”,Krebs Engineers, Menlo park,CA.

(4) Lynch,A.J.,Rao,T.C.and Prisbrey,”Influence of Hydrocyclone Diameter on Reduced Efficiency Curves” International Journal of Mineral processing,1, 173.


Questions and Answers:

(1) What is Hydrocyclone?
Ans: Hydrocyclone is the device of separating the particles in a liquid suspension.

(2) What are the parameters should be considered for a standard cyclone?
Ans: (a) Diameter (b) area of inlet nozzle (c) vertex finder (d) cylindrical section (e) conical section (f) apex orifice.

(3) Define Cut point or Separation size i.e., D50 of a Hydrocyclone?
Ans: It is defined as the size for which 50% of the feed reports to the under flow and 50% of the feed reports to overflow.

(4) What are the factors which affects cyclone performance?
Ans: Cut size, classification efficiency, Flow split of water to underflow, Flow rate.

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