Grade hydrocyclones

The grade efficiency reflects the properties of the particles exploited in the separation. It is influenced by the nature of the fluid/solid system, and by the operating conditions which determine the magnitude of the separating effect, and the period during which particles are subjected to it. Such details should, therefore, accompany any experimental data on G(d). The concept is widely applied to separations using hydrocyclones as discussed in Section 1.5.4. 

In this section, the general design of the hydrocyclone and its application in the grading of solid particles, or their separation from a liquid, is considered and then the special features required in hydrocyclones required for the separation of immiscible liquids will be addressed. The use of cyclones for separating suspended particles from gases is discussed in Section 1.6.2. 

The experimental results reported in this paper demonstrate the ability of a flat-bottom hydrocyclone to separate the coarse fraction of ammonium sulfate crystals from a slurry which contains crystals of a wide size range. It appears that the grade efficiency curve, which predicts the probability of a particle reporting to the underflow of the cyclone as a function of size, can be adjusted by a change in the underflow diameter of the hydrocyclone. These two observations lead to the suggestion to use hydrocyclone separation to reduce the crystal size distribution which is produced in crystallisers, whilst using a variable underflow diameter as an additional input for process control. 

Definitions. The performance of a hydrocyclone is generally characterised by means of a grade efficiency or Tromp-curve which is the fractional mass recovery expressed as a function of particle size. 

The degree of cell separation is an important parameter to be evaluated in perfusion systems. This can be done through the use of some concepts as cell separation efficiency, grade efficiency, and cut size. These concepts are applicable to any equipment whose performance remains constant if the operational conditions do not change. They are valid, therefore, for equipment such as sedimenting centrifuges, hydrocyclones, gravitational settlers, etc. 

A hydrocyclone, 44 mm in diameter, is used to separate a suspension of limestone in water. The density of the limestone is 2885 kg/m and the gravimetric separation efficiency was 28.34% while the partition coefficient was determined as 11.62%. Derive a grade efficiency curve to find out the reduced cut size, from the data given in Table 10.2. 

Table 10.2 presents the particle size distributions for the overflow and underflow through the hydrocyclone. Therefore, Equation 10.28 would be the appropriate to derive the grade efficiency. Since the separation was carried out in a hydrocyclone, and this type of device normally presents a dead flux effect previously described. Equation 10.29 should be used to derive the reduced grade efficiency. Carrying out the proper computations using the tabulated data and the equations mentioned. Table 10.3 is obtained. 

Gravity sedimentation equipment, hydrocyclones, sedimenting centrifuges or flotation cells have also been extensively used in mineral processing for separation of minerals according to density or affinity to water. In such separations, the efficiency is expressed by the Tromp curve which, similar to the grade efficiency curve, shows probability of separation as a function of material density or mineral composition. The steepness of the curve is a measure of the sharpness of the separation. 

Another application of hydrocyclones is for solid-solid separation by particle size. As the grade efficiency of a cyclone increases with particle size, it can be used to split the feed solids into fine and coarse fractions. This may be a process requirement, by which coarse and fine solids are separated to follow different routes in the plant, such as, for example, in closed circuit wet... 

Note that the S-shaped grade efficiency curves do not necessarily start from the origin—in applications with a considerable underflow to throughput ratio (by volume) R, the grade efficiency curves tend to the value G x) = / f as X —> 0. This is a result of the splitting of the flow, or dead flux that carries even the finest solids into the underflow in proportion to the volumetric split of the feed. Section 3.4 discusses possible modifications to the efficiency definitions which account for the volumetric split and illustrate only the net separation effect. Such reduced efficiencies are widely used for hydrocyclones and nozzle-type disc centrifuges where large diluted underflows occur. 

A hydrocyclone was tested at certain operating conditions with a suspension of clay in water. Using the data given below and in Table 3.1, evaluate the grade efficiency curve for the given operating conditions, and as a function of particle size for a density difference (ps — pi) of 1000 kg m . ... 

Estimate the total efficiency that can be expected with a hydrocyclone which is identical with the one tested in section 3.3.1 and operated under the same conditions as in example 3.1, with feed material of a size distribution F(x) as given in Table 3.2. Use the average grade efficiency curve obtained from the tests in section 3.3.1, column 6 in Table 3.2. 

This reduced efficiency concept is widely used in hydrocyclones the effect of this modification on the shape of the grade efficiency curve is shown in Figure 3.9 which uses the average curve of G x) from Table 3.2 (see section 3.3.1). It should be noted that the basic relationship between the total and grade efficiencies (equation 3.32) also holds for reduced efficiencies, so that ... 

This section describes a new and simple experimental method for obtaining the reduced cut size and the rest of the reduced grade efficiency curve of an operating separator. The method relies on feeding a known and fiiUy characterized slurry to the separator under test, and on measuring only two solids concentrations (in the feed and in the overflow), one static pressure differential (or some other flow rate-dependent variable) and the slurry temperature. These measurements are best done and logged by a personal computer, and, for hydrocyclones, have to be repeated at two different pressure settings. 

For any separator with a size-dependent performance, such as a hydrocyclone, a sedimenting centrifuge or a settling vessel, the grade efficiency varies with particle size, and a graphical representation of this is called the grade efficiency curve (see section 3.2.2). As the value of the grade efficiency has the character of probability, it is sometimes referred to as the partition probability the curve then becomes the partition probability curve or Tromp curve. 

Svarovsky, L. and Svarovsky, J., A new method of testing hydrocyclone grade efficiencies , paper given at the 4th International Conference on Hydrocyclones, Southampton, 23-25 September 1992 and published in a book edited by L. Svarovsky and M. T. Thew, Hydrocyclones Analysis and Applications, Vol. 12 Fluid Mechanics and its Applications, Kluwer Academic Publishers, Dordrecht, 135-145 (1992)... 

The separation efficiency of a hydrocyclone has a character of probability. This is to do with the probability of the position of the different particles in the entrance to the cyclone, their chances of separation into the boundary layer flow and the general probability character of turbulent flow. Coarse particles are always more likely to be separated than fine particles. Effectively, the hydrocyclone processes the feed solids by an efficiency curve called grade efficiency , which is a percentage increasing with particle size (see chapter 3 for more details about grade efficiency). Figure 6.6 shows the process schematically the solids in the feed enter the cyclone and are... 

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