CL 4537

MINERALS AND SOLIDS PROCESSING

Demonstration: Operation of Hydrocyclone

A hydrocyclone is a tool to separate dispersed material from liquid by means of centrifugal action. It consists of a cylindrical tube or shell, closed at the top and ending at the bottom in a cone with an opening at its apex (see figure 1). Fixed in the centre of the top is a cylindrical tube (vortex finder). The suspension to be separated is injected tangentially into the upper part of the cylinder section, causing rotation of the slurry in the cyclone. In the cyclone, the central outlet on the top (called the overflow nozzle) discharges the clarified liquid (overflow), while the outlet apex discharges the concentrated slurry (underflow).

The names ‘overflow’ and “underflow” have come into use on account of the fact that the cyclone is mostly used in a vertical position, although this is not strictly necessary.

Operating a cyclone requires a certain pressure drop, which increases with the throughput and further depends on the construction of the cyclone. Inside the cyclone, there exists a pressure gradient. In nearly all practical cases a gas core develops. The absence of gas core generally results in an increase of the total pressure drop at the same throughput and a lowering of the efficiency.

The underflow discharge typically resembles an umbrella shape with centrifugal spray. However, if the total pressure drop is not high enough or if the solid concentration in the underflow is very high, this spray type discharge changes into a jet-type or drop type discharge.

The variables that affects the hydrocyclone performance can be divided into two groups: (a) those that are dependent on hydrocyclone size and proportions, (b) design variables and operating variables. The design variables effectively include feed, overflow and underflow opening sizes, the hydrocyclone size and its shape. Operating variables depend on the feed stream and include:

pressure drop (and feed flowrate)

solid concentration ( or liquid-liquid concentration)

solid size and shape

solid densities (liquid densities)

liquid medium density

liquid medium viscosity

Many of these variables interact, thus it is not possible to consider them individually. Instead, hydrocyclone efficiency is evaluated in terms of correlation using three criteria:

the performance curve

the ratio of overflow to underflow rate

pressure drop

Figure 1. Cross-Section of a hydrocyclone

Cyclone efficiency is represented by a performance or a partition curve (see figure 2). The graph relates the weight fraction, or percentage of each particle size, in the feed which reports to the apex or underflow, to the particle size. The separation size of the cyclone is often defined as that point on the partition curve for which 50% of the particles in the feed has an equal chance of reporting either with the underflow or overflow. This point is referred to as the d50 size. The sharpness of the cut depends on the slope of the central section of the partition curve; the closer to vertical the higher the efficiency.

Figure 2: Performance curve of a Hydrocyclone

Objective

To understand the operation of a hydrocyclone.

Materials

Phosphate rock slurry

Apparatus

The schematic diagram of the apparatus to be used in this exercise is illustrated in figure 3.

Figure 3. Schematic Diagram of Hydrocyclone Apparatus

Method

  • Measure the PSD and mass concentration of the feed, underflow and overflow at steady state conditions for 4 different flow rates.

Reporting

  • Note: this is a brief report – there is no size limit, but unnecessary discussion will be marked down.
  • Background:
  • Briefly discuss factors influencing hydrocyclone operation
  • Briefly discuss differences in hydrocyclone design and operation for thickening and calssifying purposes
  • Analysis
  • Calculate corrected and actual performance curves for each condition and comment on the difference between corrected and actual data and between corrected data for each test condition (e.g. sharpness of cut)
  • Calculate the corrected separation size (partition size, d50 etc.) for each condition
  • Under the conditions examined, would this cyclone be best used for thickening or classification?

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