Description
Hydrocyclones are cono-cylindrical in shape, with a tangential feed inlet into the cylindrical section and an outlet at each axis. The outlet at the cylindrical section is called the vortex finder and extends into the cyclone to reduce short-circuit flow directly from the inlet. At the conical end is the second outlet, the spigot. For size separation, both outlets are generally open to the atmosphere. Hydrocyclones are generally operated vertically with the spigot at the lower end, hence the coarse product is called the underflow and the fine product, leaving the vortex finder, the overflow. Figure 1 schematically shows the principal flow and design features of a typical hydrocyclone: the two vortices, the tangential feed inlet and the axial outlets. Except for the immediate region of the tangential inlet, the fluid motion within the cyclone has radial symmetry. If one or both of the outlets are open to the atmosphere, a low pressure zone causes a gas core along the vertical axis, inside the inner vortex.
Figure 1. Principal features of the hydrocyclone.
The operating principle is simple: the fluid, carrying the suspended particles, enters the cyclone tangentially, spirals downward and produces a centrifugal field in free vortex flow. Larger particles move through the fluid to the outside of the cyclone in a spiral motion, and exit through the spigot with a fraction of the liquid. Due to the limiting area of the spigot, an inner vortex, rotating in the same direction as the outer vortex but flowing upward, is established and leaves the cyclone through the vortex finder, carrying most of the liquid and finer particles with it. If the spigot capacity is exceeded, the air core is closed off and the spigot discharge changes from an umbrella-shaped spray to a ‘rope’ and a loss of coarse material to the overflow.
The diameter of the cylindrical section is the major variable affecting the size of particle that can be separated, although the outlet diameters can be changed independently to alter the separation achieved. While early workers experimented with cyclones as small as 5 mm diameter, commercial hydrocyclone diameters currently range from 10 mm to 2.5 m, with separating sizes for particles of density 2700 kg m−3 of 1.5–300 μm, decreasing with increased particle density. Operating pressure drop ranges from 10 bar for small diameters to 0.5 bar for large units. To increase capacity, multiple small hydrocyclones may be manifolded from a single feed line.
Although the principle of operation is simple, many aspects of their operation are still poorly understood, and hydrocyclone selection and prediction for industrial operation are largely empirical.
Classification
Barry A. Wills, James A. Finch FRSC, FCIM, P.Eng., in Wills' Mineral Processing Technology (Eighth Edition), 2016
9.4.3 Hydrocyclones Versus Screens
Hydrocyclones have come to dominate classification when dealing with fine particle sizes in closed grinding circuits (<200 µm). However, recent developments in screen technology (Chapter 8) have renewed interest in using screens in grinding circuits. Screens separate on the basis of size and are not directly influenced by the density spread in the feed minerals. This can be an advantage. Screens also do not have a bypass fraction, and as Example 9.2 has shown, bypass can be quite large (over 30% in that case). Figure 9.8shows an example of the difference in partition curve for cyclonesand screens. The data is from the El Brocal concentrator in Peru with evaluations before and after the hydrocyclones were replaced with a Derrick Stack Sizer® (see Chapter 8) in the grinding circuit(Dündar et al., 2014). Consistent with expectation, compared to the cyclone the screen had a sharper separation (slope of curve is higher) and little bypass. An increase in grinding circuit capacity was reported due to higher breakage rates after implementing the screen. This was attributed to the elimination of the bypass, reducing the amount of fine material sent back to the grinding millswhich tends to cushion particle–particle impacts.
Figure 9.8. Partition curves for cyclones and screens in the grinding circuit at El Brocal concentrator.
(Adapted from Dündar et al. (2014))
Changeover is not one way, however: a recent example is a switch from screen to cyclone, to take advantage of the additional size reduction of the denser payminerals (Sasseville, 2015).
Metallurgical process and design
Eoin H. Macdonald, in Handbook of Gold Exploration and Evaluation, 2007
Hydrocyclones
Hydrocyclones are preferred units for sizing or desliming large slurry volumes cheaply and because they occupy very little floor space or headroom. They operate most effectively when fed at an even flow rate and pulp density and are used individually or in clusters to obtain desired total capacities at required splits. Sizing capabilities rely on centrifugal forces generated by high tangential flow velocities through the unit. The primary vortex formed by the incoming slurry acts spirally downwards around the inner cone wall. Solids are flung outwards by centrifugal force so that as the pulp moves downwards its density increases. Vertical components of the velocity act downwards near the cone walls and upwards near the axis. The less dense centrifugally separated slime fraction is forced upwards through the vortex finder to pass out through the opening at the upper end of the cone. An intermediate zone or envelope between the two flows has zero vertical velocity and separates the coarser solids moving downwards from the finer solids moving upwards. The bulk of the flow passes upwards within the smaller inner vortex and higher centrifugal forces throw the larger of the finer particles outward thus providing a more efficient separation in the finer sizings. These particles return to the outer vortex and report once more to the jig feed.
The geometry and operating conditions within the spiral flow pattern of a typical hydrocyclone are described in Fig. 8.13. Operational variables are pulp density, feed flow rate, solids characteristics, feed inlet pressure and pressure drop through the cyclone. Cyclone variables are area of feed inlet, vortex finder diameter and length, and spigot discharge diameter. The value of the drag coefficient is also affected by shape; the more a particle varies from sphericity the smaller is its shape factor and the greater its settling resistance. The critical stress zone may extend to some gold particles as large as 200 mm in size and careful monitoring of the classification process is thus essential to reduce excessive recycling and the resulting build up of slimes. Historically, when little attention was given to the recovery of 150 μm gold grains, carry-over of gold in the slime fractions appears to have been largely responsible for gold losses that were recorded to be as high as 40–60% in many gold placer operations.
8.13. Normal geometry and operating conditions of a hydrocyclone.
Figure 8.14 (Warman Selection Chart) is a preliminary selection of cyclones for separating at various D50 sizings from 9–18 microns up to 33–76 microns. This chart, as with other such charts of cyclone performance, is based upon a carefully controlled feed of a specific type. It assumes a solids content of 2,700 kg/m3 in water as a first guide to selection. The larger diameter cyclones are used to produce coarse separations but require high feed volumes for proper function. Fine separations at high feed volumes require clusters of small diameter cyclones operating in parallel. The final designparameters for close sizing must be determined experimentally, and it is important to select a cyclone around the middle of the range so that any minor adjustments that may be required can be made at the start of operations.
8.14. Warman preliminary selection chart.
The CBC (circulating bed) cyclone is claimed to classify alluvial gold feed materials up to 5 mm diameter and obtain a consistently high jig feed from the underflow. Separation takes place at approximately D50/150 microns based upon silica of density 2.65. The CBC cyclone underflow is claimed to be particularly amenable to jig separation because of its relatively smooth size distribution curve and almost complete removal of fine waste particles. However, although this system is claimed to produce a high-grade primary concentrate of equant heavy minerals in one pass from a relatively long size range feed (e.g. mineral sands), no such performance figures are available for alluvial feed material containing fine and flaky gold. Table 8.5gives the technical data for AKW hydrocyclones for cut-off points between 30 and 100 microns.
Table 8.5. Technical data for AKW hydrocyclones
| Type (KRS) | Diameter (mm) | Pressure drop | Capacity | Cut point (microns) | |
|---|---|---|---|---|---|
| Slurry (m3/hr) | Solids (t/h max). | ||||
| 2118 | 100 | 1–2.5 | 9.27 | 5 | 30–50 |
| 2515 | 125 | 1–2.5 | 11–30 | 6 | 25–45 |
| 4118 | 200 | 0.7–2.0 | 18–60 | 15 | 40–60 |
| (RWN)6118 | 300 | 0.5–1.5 | 40–140 | 40 | 50–100 |
Developments in iron ore comminution and classification technologies
A. Jankovic, in Iron Ore, 2015
8.3.3.1 Hydrocyclone separators
The hydrocyclone, also referred to as cyclone, is a classifying device that utilizes centrifugal force to accelerate the settling rate of slurryparticles and separate particles according to size, shape, and specific gravity. It is widely used in the minerals industry, with its main use in mineral processing being as a classifier, which has proved extremely efficient at fine separation sizes. It is extensively used in closed-circuit grinding operations but has found many other uses, such as desliming, degritting, and thickening.
A typical hydrocyclone (Figure 8.12a) consists of a conically shaped vessel, open at its apex, or underflow, joined to a cylindrical section, which has a tangential feed inlet. The top of the cylindrical section is closed with a plate through which passes an axially mounted overflow pipe. The pipe is extended into the body of the cyclone by a short, removable section known as the vortex finder, which prevents short-circuiting of feed directly into the overflow. The feed is introduced under pressure through the tangential entry, which imparts a swirling motion to the pulp. This generates a vortex in the cyclone, with a low-pressure zone along the vertical axis, as shown in Figure 8.12b. An air-core develops along the axis, normally connected to the atmosphere through the apex opening, but in part created by dissolved air coming out of solution in the zone of low pressure. The centrifugal force accelerates the settling rate of the particles, thereby separating particles according to size, shape, and specific gravity. Faster settling particles move to the wall of the cyclone, where the velocity is lowest, and migrate to the apex opening (underflow). Due to the action of the drag force, the slower-settling particles move toward the zone of low pressure along the axis and are carried upward through the vortex finder to the overflow.
Figure 8.12. Hydrocyclone (https://www.aeroprobe.com/applications/examples/australian-mining-industry-uses-aeroprobe-equipment-to-study-hydro-cyclone) and hydrocyclone battery. Cavex hydrocyclone overvew brochure, https://www.weirminerals.com/products_services/cavex.aspx.
Hydrocyclones are almost universally used in grinding circuits because of their high capacity and relative efficiency. They can also classify over a very wide range of particle sizes (typically 5–500 μm), smaller diameter units being used for finer classification. However, cyclone application in magnetite grinding circuits can cause inefficient operation due to the density difference between magnetite and waste minerals (silica). Magnetite has a specific density of about 5.15, while silica has a specific density of about 2.7. In hydrocyclones, dense minerals separate at a finer cut size than lighter minerals. Therefore, liberated magnetite is being concentrated in the cyclone underflow, with consequent overgrinding of the magnetite. Napier-Munn et al. (2005) noted that the relationship between the corrected cut size (d50c) and particle density follows an expression of the following form depending on flow conditions and other factors:
where ρs is the solids density, ρl is the liquid density, and n is between 0.5 and 1.0. This means that the effect of mineral density on cyclone performance can be quite significant. For example, if the d50c of the magnetite is 25 μm, then the d50c of silica particles will be 40–65 μm. Figure 8.13 shows the cyclone classification efficiency curves for magnetite (Fe3O4) and silica (SiO2) obtained from the survey of an industrial ball mill magnetite grinding circuit. The size separation for silica is much coarser, with a d50c for Fe3O4 of 29 μm, while that for SiO2 is 68 μm. Due to this phenomenon, the magnetite grinding mills in closed circuits with hydrocyclones are less efficient and have lower capacity compared to other base metalore grinding circuits.
Figure 8.13. Cyclone efficiency for magnetite Fe3O4 and silica SiO2—industrial survey.
High Pressure Process Technology: Fundamentals and Applications
M.J. Cocero PhD, in Industrial Chemistry Library, 2001
Solids-separation devices
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Hydrocyclone
This is one of the simplest types of solids separators. It is a high-efficiency separation device and can be used to effectively remove solids at high temperatures and pressures. It is economical because it has no moving parts and requires little maintenance.
The separation efficiency for solids is a strong function of the particle-size and temperature. Gross separation efficiencies near 80% are achievable for silica and temperatures above 300°C, while in the same temperature range, gross separation efficiencies for denser zircon particles are greater than 99% [29].
The main handicap of hydrocyclone operation is the tendency of some salts to adhere to the cyclone walls.
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Cross micro-filtration
Cross-flow filters behave in a way similar to that normally observed in crossflow filtration under ambient conditions: increased shear-rates and reduced fluid-viscosity result in an increased filtrate number. Cross-microfiltration has been applied to the separation of precipitated salts as solids, giving particle-separation efficiencies typically exceeding 99.9%. Goemans et al.[30] studied sodium nitrate separation from supercritical water. Under the conditions of the study, sodium nitrate was present as the molten salt and was capable of crossing the filter. Separation efficiencies were obtained that varied with temperature, since the solubility decreases as the temperature increases, ranging between 40% and 85%, for 400 °C and 470°C, respectively. These workers explained the separation mechanism as a consequence of a distinct permeability of the filtering medium towards the supercritical solution, as opposed to the molten salt, based on their clearly distinct viscosities. Therefore, it would be possible not only to filter precipitated salts merely as solids but also to filter those low-melting-point salts that are in a molten state.
The operating troubles were mainly due to filter-corrosion by the salts.
Paper: Recycling and Recycled Materials
M.R. Doshi, J.M. Dyer, in Reference Module in Materials Science and Materials Engineering, 2016
3.3 Cleaning
Cleaners or hydrocyclones remove contaminants from pulp based on the density difference between the contaminant and water. These devices consist of conical or cylindrical-conical pressure vessel into which pulp is fed tangentially at the large diameter end (Figure 6). During passage through the cleaner the pulp develops a vortex flow pattern, similar to that of a cyclone. The flow rotates around the central axis as it passes away from the inlet and toward the apex, or underflow opening, along the inside of the cleaner wall. The rotational flow velocity accelerates as the diameter of the cone decreases. Near the apex end the small diameter opening prevents the discharge of most of the flow which instead rotates in an inner vortex at the core of the cleaner. The flow at the inner core flowsaway from the apex opening until it discharges through the vortex finder, located at the large diameter end in the center of the cleaner. The higher density material, having been concentrated at the wall of the cleaner due to centrifugal force, is discharged at the apex of the cone (Bliss, 1994, 1997).
Figure 6. Parts of a hydrocyclone, major flow patterns and separation trends.
Cleaners are classified as high, medium, or low density depending upon the density and size of the contaminants being removed. A high density cleaner, with diameter ranging from 15 to 50 cm (6–20 in) is used to remove tramp metal, paper clips, and staples and is usually positioned immediately following the pulper. As the cleaner diameter decreases, its efficiency in removing small sized contaminants increases. For practical and economic reasons, the 75-mm (3 in) diameter cyclone is generally the smallest cleaner used in the paper industry.
Reverse cleaners and throughflow cleaners are designed to remove low density contaminants such as wax, polystyrene, and stickies. Reverse cleaners are so named because the accepts stream is collected at the cleaner apex while the rejects exit at the overflow. In the throughflow cleaner, accepts and rejects exit at the same end of the cleaner, with accepts near the cleaner wall separated from the rejects by a central tube near the core of the cleaner, as shown in Figure 7.
Figure 7. Schematics of a throughflow cleaner.
Continuous centrifuges used in the 1920s and 1930s to remove sand from pulp were discontinued after the development of hydrocyclones. The Gyroclean, developed at Centre Technique du Papier, Grenoble, France, consists of a cylinder that rotates at 1200–1500 rpm (Bliss, 1997; Julien Saint Amand, 1998, 2002). The combination of relatively long residence time and high centrifugal force allows low density contaminants sufficient time to migrate to the core of the cleaner where they are rejected through the center vortex discharge.
M.T. Thew, in Encyclopedia of Separation Science, 2000
Synopsis
Though the solid–liquid hydrocyclone has been established for most of the 20th century, satisfactory liquid–liquid separation performance did not arrive until the 1980s. The offshore oil industry had a need for compact, robust and reliable equipment for removing finely divided contaminant oil from water. This need was satisfied by a significantly different type of hydrocyclone, which of course had no moving parts.
After explaining this need more fully and comparing it with solid–liquid cyclonic separation in mineral processing, the advantages that the hydrocyclone conferred over types of equipment installed earlier to meet the duty are given.
Separation performance assessment criteria are listed prior to discussing performance in terms of feed constitution, operator control and the energy required, i.e. the product of pressure drop and flowrate.
The environment for petroleum production sets some constraints for materials and this includes the problem of particulate erosion. Typical materials used are mentioned. Relative cost data for types of oil separation plant, both capital and recurrent, is outlined, though sources are sparse. Finally, some pointers to further development are described, as the oil industry looks to equipment installed on the sea bed or even at the bottom of the wellbore.
Sampling, Control, and Mass Balancing
Barry A. Wills, James A. Finch FRSC, FCIM, P.Eng., in Wills' Mineral Processing Technology (Eighth Edition), 2016
3.7.1 Use of Particle Size
Many units, such as hydrocyclones and gravity separators, produce a degree of size separation and the particle size data can be used for mass balancing (Example 3.15).
Example 3.15 is an example of node imbalance minimization; it provides, for example, the initial value for the generalized least squares minimization. This graphical approach can be used whenever there is “excess” component data; in Example 3.9 it could have been used.
Example 3.15 uses the cyclone as the node. A second node is the sump: this is an example of 2 inputs (fresh feed and ball milldischarge) and one output (cyclone feed). This gives another mass balance (Example 3.16).
In Chapter 9 we return to this grinding circuit example using adjusted data to determine the cyclone partition curve.
Post time: May-07-2019