Underflow, hydrocyclones

A basic appreciation of slurry rheology, or flow behaviour, is in oitant in many solid-liquid separations, e.g. when feeding pressure filters, punq)ing thickener underflow, hydrocyclone feed and exit streams and during cross-flow filtration. This Appendix is designed to introduce some of the terminology and basic concepts. A more thorough text such as Wilkinson [1960] should be referred to for further details, if necessary. 

The vessel design features a Chinese hat-like conical core stopper above the underflow sump, which is there to prevent the vortex from reaching the latter and reentraining the settled soHds. The core stopper is also beheved to stabilize and locate the vortex flow in the vessel. Overflow from the vessel is through a wide cylindrical insert through the Hd, similar to a vortex finder in a hydrocyclone (16), and an optional provision can be made for collecting any floatables in a float trap. 

The diameter of a hydrocyclone can range from 10 mm to 2.5 m, cut sizes from 2 to 250 /am, and flow rate (capacities) from 0.1 to 7200 m3/hr. Pressure drop can range from 0.3 to 6 atm (Svarovsky, 1984). For aerocy-clones, very little fluid leaves with the solids underflow, although for hydrocyclones the underflow solids content is typically 45-50% by volume. Aerocyclones can achieve effective separation for particles as small as 2-5 pm. 

An emulsion separation method using hydrocyclones. The emulsion comprises a continuous phase, a discontinuous phase and fine solid particles. In the first step, the original emulsion is separated into an overflow emulsion and an underflow emulsion, in a first hydrocyclone. The overflow emulsion comprises portions of the continuous phase, the discontinuous phase and the fine solid particles. The overflow emulsion is inverted in which the continuous phase of the overflow emulsion is now a second discontinuous phase and the original discontinuous phase becomes a second continuous phase. Then, the inverted emulsion is directed to one or more subsequent hydrocyclones and the second continuous and discontinuous phases are collected. The fine solid particles remain in the second discontinuous phase. 

The energetics slurry is transferred from the energetics slurry feed tank into the anolyte feed tank through a hydrocyclone. The concentration of the energetics in the hydrocyclone underflow is nominally 40 weight percent. This underflow drains into the anolyte tank. The overflow, depleted in energetics, goes to a second hydrocyclone. The underflow from the second... 

Solids. Treatment of mustard agents by the SILVER II process results in the precipitation of silver chloride, which, if not removed, could cause cell plugging. This precipitate is removed from the main circuit in a slurry underflow from hydrocyclones in the anolyte feed stream circuit. Subsequently, this slurry is dewatered in a centrifuge to approximately 50 weight percent AgCl, with the remainder of the slurry consisting of water, nitric acid, silver nitrate, and trace amounts of other materials in the anolyte loop. 

The flow rate to the hydrocyclones for the full-scale SILVER II unit, based on a 14-mm vortex finder and a 6.4-mm underflow spigot, would need to be approximately 3.2 m3/hr at a pressure of around 3.5 bar. This flow rate should be sufficient to achieve the overflow flow rate of 1.8 m3/hr required to feed the SILVER II cells. As noted previously, the hydrocyclones used in the 12-kW energetics and agent simulant trials for EDS II handled solids at the planned design loading (AEA, 2001a). 

As flow patterns are influenced only slightly by gravitational forces, hydrocyclones may be operated with their axes inclined at any angle, including the horizontal, although the removal of the underflow is facilitated, with the axis vertical. 

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. 

Fig. 4. Influence of pressure drop on ceU viability in the underflow and overflow of the following hydrocyclones Dorr-Oliver (a),Mozley (b) and Bradley (c) with diameters of 10 mm, 10 mm and 7 mm, respectively [66]...

An essential factor for good table operation is that the rate of feed must be uniform, both as to tonnage and as to physical properties. No one factor will cause more trouble to the table operator than to have a surging feed. The feed to tables may be unsized, or it may be either screened or hydraulically classified. For treating fine coals a common procedure is to use hydrocyclones both to deslime the material and to give a cyclone underflow of about 40 percent solids, which constitutes the table feed. 

Design nsing the same principles as liquid-solid hydrocyclone. Section 16.11.5.8. For flooded underflow, the pressure drop is about two to seven times greater than for air core operation. 

Typical target diameter is 5 to 100 pm and is the diameter that 50% reports to the overflow and 50% reports to the underflow. Typically three times target diameter is the diameter below which all particles in disttibution are removed. The standard hydrocyclone has an inlet diameter of 0.28 D the overflow exit diameter = 0.34 D the vortex finder length is 0.4 D cylindrical body of height of 0.4 D, vertical length of cone = 5 D or cone angle about 10°. Underflow diameter adjustable to adjust the volume split between the overflow and underflow. 

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