Hydrocyclones

The fluid flow in a hydrocyclone has been described in section 17.2.3. Hydrocyclones are used for separating particles that are denser than the fluid or for separating lighter elements (oil droplets, for example). In the first case, the particles are thrown onto the conical sidewall of the apparatus and then extracted through the spigot. In the second, the droplets migrate toward the Oz axis of the apparatus and are extracted via the overflow. 

A hydrocyclone is a device employing centrifugal separation without the need for mechanically moving parts, other than a punq). They are cheap, conq)act and versatile as a means of solid-liquid separation. It is similar in operation to a centrifuge, but with much larger values of g -force (ranging from 800 g in a 300 mm diameter cyclone to 50 000 g in a 10 mm diameter cyclone). This force is, however, applied over a much 

The most noticeable flows in the hydrocyclone are the primary and secondary vortices. The primary vortex lies outside the secondary one, and carries suspended material down the axis of the hydrocyclone. 

Settlers Floe or particulates do not settle size different from design/density different from design/unanticipated turbulence or back eddies/liquid residence time too short/feed flowrate design/incorrect design of inlet/faulty design of solids removal. Supernatant not clear [floe or particulates do not settle] /faulty design of overflow weirs/short circuiting. 

Particle diameter 4 to 400 pm feed solids concentration 4 to 30 % solids v/v but usual application is separating particles 50 pm and concentrations 10% v/v with pressure loss of 20 to 100 kPa. 

Regular hydrocyclones particle settling velocity 2 pm/s to 5 mm/s and clarified liquid product of 0.1 to 70 L/s. 

Determine diameter of hydrocyclone from D [cm] = 6 X 10 Feed flowrate [L/s]/ target diameter [ptn ] Ap [kPa] (pl, density liquid [Mg/m j, X liquid viscosity [mPa s]/Aq, density difference, [Mg/m ].  

Typical uses Suspension thickening, clarification and particle classification. 

Cyclones designed for use with liquids are referred to as hydrocyclones, hydraulic cyclones or hydroclones. The basic principle employed to effect either concentration or classification of the solids is centrifugal sedimentation, caused by introducing the feed suspension tangentially into the unit they are particularly attractive for many applications because they have no moving parts. Specific aspects of design are provided by Svarovsky (1984). 

Cyclones are basically aerodynamic or hydrodynamic separators, in which the fluid being handled is caused to develop a vortex flow, as a result of which the more dense particles are thrown out of the main fluid stream. Since cyclones work according to the mass of the particles involved, they may also be applicable to the separation of light and dense solids in suspension from one another, or of mixtures of liquids. The same principle is also utilized in pipeline air filters for separating out water and oil droplets, and heavier solids, before the contaminated air actually reaches a filter. 

Cyclones may be dry or wet in operation, with dry cyclones providing soM-gas or liquid droplet-gas separation as in air or gas cleaners. Wet cyclones provide solid-liquid or liquid-liquid separation, and are specifically desalbed as hydrocyclones. 

Hydrocyclones operate under pressurized conditions and, where sufficient pressure is not already available in the feed, pumping is required. Capital and operating costs are generally low and hydrocyclones are particularly suitable for classification as well as basic separation duties. 

Individual cyclones can handle exceptionally large flow volumes. For the most part assemblies of hydrocyclones are less costly than other types of separator. Also, 

Cyclones are also used as de-aeration devices with centrifuges. Because of air movement in the casing of a centrifuge, the centrate often leaves the centrifuge mixed with a large volume of air. A liquid-gas cyclone directly attached to the centrate outlet provides effective de-aeration. 

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Earlier on when we described the cutting action of the drill bit we learned about the drilling fluid or mud. The mud cools the bit and also removes the cuttings by carrying them up the hole outside the drill pipe. At the surface the mud runs over a number of moving screens, the shale shakers (Fig. 3.11) which remove the cutting for disposal. The fine particles which pass through the screens are then removed by desanders and desilters, usually hydrocyclones. 

To reduce oil content to levels which meet disposal standards it is often necessary to employ rather more sophisticated methods. Two such techniques which can reduce oil in water to less than 40 ppm use gas flotation and hydrocyclone processes. 

A promising technique currently under development is downhole separation whereby a device similar to a hydrocyclone separates oil and water in the well bore. The water is subsequently compressed into a zone beneath the producing interval and only the oil is produced to surface. 

Prethickening of filter feeds can be done with a variety of equipment such as gravity thickeners, hydrocyclones, or sedimenting centrifuges. Even cake filters can be designed to limit or completely eliminate cake formation and therefore act as thickening filters and be used in this thickening duty. 

The hydrocyclone, commonly referred to simply as cyclone, is a nonmechanical sedimentation-type classifier (2,6,10,27) (Fig. 7). It has no moving parts or power attachments directly connected to it. The hydrocyclone has become the workhorse of most mineral processing operations because of its simplicity, short residence time, compactness, and low cost of operation. It is, however, characterized by lack of sharpness of separation. Equipment... 

Time is a critical variable because the magnetite settles if the mixture is held too long in a static condition. Use of hydrocyclones makes possible the separation in a fraction of a minute (21). These devices impart centrifugal force to the system, thereby permitting a separation to be made at a specific gravity less than that required in static, heavy-Hquid separations. 

Mechanical Gleaning. A cleaner is a hydrocyclone device utilizing fluid pressure to create rotational fluid motion (20). Pulp is introduced tangentially near the top of the cleaner. Contaminants denser than water such as chemically treated toner inks and sand migrate toward the outer wall of the cleaner and exit in a separate (reject) stream. For most forward cleaners, optimal ink removal efficiency is obtained at a pulp consistency of 0.2—0.3%. Most forward cleaners deinking efficiency declines at pulp feed consistencies greater than 0.4%. However, a cleaner said to be efficient at 1.2% pulp consistency has been reported (39). 

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. 

Centrifuges. Sohd-bowl centrifuges have been proposed as an alternative classifying device to hydrocyclones for cut sizes below 10 pm. The results appear to be mixed (21). In one apphcation, where the cut size was 6.5 pm and the sharpness index 0.7, there was essentially no apparent bypass. However, in other apphcations operating at higher feed concentrations, the cut size ranged from 5—8 pm, but the sharpness index was between 0.3—0.5 and the apparent bypass between 10—30% or higher (22). Smaller cut sizes have also been reported (23). 

Xanthates are used in a froth flotation process of soils contaminated with mercury. The soil to be treated is mn through hydrocyclones, and the slurries are flocculated, dewatered, and removed to a secure landfill. The effluent water is recycled. The process is suitable for treating industrial land sites contaminated with mercury droplets (115). 

In 1981, a novel flotation device known as the air-sparged hydrocyclone, shown in Figure 3, was developed (16). In this equipment, a thin film and swid flotation is accompHshed in a centrifugal field, where air sparges through a porous wall. Because of the enhanced hydrodynamic condition, separation of fine hydrophobic particles can be readily accompHshed. Also, retention times can be reduced to a matter of seconds. Thus, this device provides up to 200 times the throughput of conventional flotation cells at similar yields and product quaHties. 

Fig. 3. Air-sparged hydrocyclone, where A represents the tangential feed that estabHshes swid flow B, the area of small bubbles formed by high shear at the porous wall and C, the outlet for the (D) hydrophilic particles rejected by the swid flow. The (B) hydrophobic particles are in the axial froth flow.

Classified removal of course material also can be used, as shown in Figure 16. In a crystallizer equipped with idealized classified-product removal, crystals above some size ate removed at a rate Z times the removal rate expected for a perfecdy mixed crystallizer, and crystals smaller than are not removed at all. Larger crystals can be removed selectively through the use of an elutriation leg, hydrocyclones, or screens. Using the analysis of classified-fines removal systems as a guide, it can be shown that the crystal population density within the crystallizer magma is given by the equations... 

L., and Thew M. T., (eds.). Hydrocyclones Analysis and Applications, p. 95, Kliiwer Academic Publishers, Dordrecht, The Netherlands, 1992. Dahlstrom D. A., Fundamental of Solid-Liquid Separation, Mnlar A. L., and Anderson,... 

Hydrocyclones are available in numerous sizes and types ranging from pencil-sized 10-mm diameters of plastic to the 1.2-m (48-in) diameter of rubber-protected mild or stainless steel. Porcelain units 25 to 100 mm (1 to 4 in) in diameter are becoming popular, and in the 150-mm (6-in) size the starch industry has standardized on special molded nylon types. Small units for fine-size separations are usually manifolded in multiple units in parallel with up to 480 ten-mm... 

FIG. 19-79 Variations in the basic column design a) packed column, (h) Jameson cell, and (c) air-sparged hydrocyclone. 

Arterbum, R.A.,"The Sizing of Hydrocyclones, Krebs Engineers, Menlo Park, CA. (1976)... 

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