Hydrocyclone Selection for Plant Design

Timothy J. Olson and Patrick A. Turner

ABSTRACT Hydrocyclones are used in many and various duties in mineral processing flowsheets. There is a wide range of sizes, styles and fittings to select from, however, and the focus of this paper is to provide a basis to specify a hydrocyclone for a given application. A general description of how a hydrocyclone works is included to provide background to the discussion of process and hydrocyclone geometry variables. The mechanism for selecting a hydrocyclone for an application includes the use of the corrected D50 as the key separation parameter. Important manifold design options for new projects and hydrocyclone maintenance and materials considerations are identified. Included for reference are typical mineral processing hydrocyclone applications. INTRODUCTION “It speaks highly of the versatility of the hydrocyclone that notwithstanding our lack of knowledge of its basic principles, it has proved satisfactory in so many varied applications” (Bradley, 1965). The hydrocyclone is used in various applications in many industries, from degritting sewage sludge to removing oil droplets from produced water. The governing principles are difficult to quantify because of the complexity of the fluid dynamics with multiple phases in highly swirling flows. The majority of applications are in the processing of mineral ores however, and experience has helped develop a basis for predicting the hydrocyclone classification performance in these duties. The factors that affect performance, both process and hydrocyclone design, will be covered in this paper. The focus will be on providing information that an engineer who is designing a hydrocylone system will find useful. GENERAL DESCRIPTION A cutaway of a hydrocyclone is shown in Figure 1. The slurry enters the area of the hydrocyclone called the inlet head from the inlet feed pipe. The slurry is introduced next to the wall of the

�uz�/ub � 0.74

Figure 1 Hydrocyclone Cutaway

1

Figure 2 Hydrocyclone, Axial Velocity Profile

cylindrical inlet, which induces a swirling action. This action helps develop the inertial forces that enable the classification of particles within the hydrocyclone. The slurry is further accelerated in the conical sections of the separator. The swirling action produces a low-pressure vortex in the center of the hydrocyclone where the finer, lower-mass particles migrate. The relatively light particles are removed with the overflow stream by an upward swirling flow through the vortex finder. The heavier particles are removed with an underflow stream by a downward swirling flow through the apex region of the hydrocyclone classifier.

2�p�/�ub2 � 20.3

�u��/ub � 2.4

Figure 3 Hydrocyclone, Tangential Velocity Distribution

Figure 4 Hydrocyclone, Pressure Distribution

Figures 2 and 3 show the mean axial and tangential components of the velocity at different cross-sections in the upper portion of a 250-mm diameter hydrocyclone (Petty et al., 2002). These single-phase numerical calculations were developed using the Reynolds averaged Navier-Stokes (RANS) equation, and standard transport equations for the Reynolds stress (RSM model) and the turbulence dissipation. The simulation imposes a backpressure on the overflow and underflow streams to avoid the air core. The Reynolds number based on the effective diameter of the feed entry and the volumetric flow rate of the feed stream is about 200,000. Figure 4 shows the pressure distribution predicted by the simulation. The results, which are qualitatively similar to experiments by Kelsall (1952) and to multi-phase flow calculations reported by Devulapalli and Rajamani (1994), predict a Rankine vortex flow with a maximum tangential velocity near the radius of the vortex finder (see Figure 3). This feature distinguishes hydrocyclone flows from other swirling flows encountered in centrifugal separators. As illustrated by Figure 4, the swirling action of the flow field causes a lower pressure to develop in the core of the hydrocyclone. It is noteworthy that the Computational Fluid Dynamics (CFD) simulation captures the important qualitative flow features of a hydrocyclone classifier illustrated by Figure 1. SIZING AND SELECTION OF A HYDROCYCLONE The factors involved in sizing a hydrocyclone for a specific application also provide information on the important variables related to how a hydrocyclone works. To select the appropriate hydrocyclone, the engineer must know the solids concentration and size distribution plus particle and liquid specific gravities along with the solids tonnage and slurry flowrate.

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Corrected Recovery Curve

D50

Actual Recovery Curve

Figure 5 Recovery Curves

Figure 6 D50 For Typical Hydrocyclones

It would also be helpful to have information on the liquid and slurry viscosity and particle shape. For design of a new plant, this information is often estimated, based on experience with similar applications. In many requirements, an estimate of the feed characterization is known. In these instances the selection is completed by matching the estimated performance of a specific hydrocyclone and the required separation. Hydrocyclone performance is often evaluated using a graph of particle size versus percentage of the particles recovered to the underflow. An example of a recovery curve is shown in Figure 5. The actual recovery curve shown does not reach zero, which is typical. It has been shown that this offset is due to particle entrainment caused by the watersplit to the hydrocyclone underflow (Lynch and Rao 1975). This curve is corrected by assuming entrainment in every size in proportion to the feed concentration. After the curve has been adjusted for the water-split, this Corrected Recovery Curve can be plotted as shown in Figure 5. The characteristics of this curve are often used to describe the hydrocyclone performance, most notably the D50. The D50 is the particle diameter with a 50% recovery on the corrected recovery curve. The D50 is shown on the corrected recovery curve in Figure 5. The slope of this curve (alpha) is a measure of the sharpness of separation, which is also an important parameter. Hydrocyclones come in a variety of sizes or diameters. The larger the hydrocyclone diameter, the coarser the separation. Each size hydrocyclone has a base D50 using standard operating conditions and a “typical” geometry (Arterburn1976). The D50 (base) shown in Figure 6 is valid with the following conditions:

1. Feed Liquid – Water at 20 degrees C (viscosity 1 cp) 2. Feed Solids – 2.65 SG spheres 3. Feed Concentration - < 1% solids (wt%) 4. Pressure Drop – 10 psi 5. Hydrocyclone Geometry - Standardized hydrocyclone with; vortex finder 30% of

hydrocyclone diameter, feed orifice 7% of feed chamber area, 20-degree cone for larger hydrocyclones, cylinder section included, vertical mount.

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The actual D50 in a process is determined by adjusting the base D50 using a set of correction factors related to process and equipment variables. �

D50(actual)=D50 (base) x CP1 x CP2 x CP3 … x CD1 x CD2 x CD3 etc…. where D50 (base) is the base separation for the specified hydrocyclone diameter (Figure 6) and CP are correction factors for process variables and CD are corrections factors for hydrocyclone design variables

Process variables include feed % solids, particle specific gravity, feed pressure, slurry viscosity, among others. Hydrocyclone design variables, within a given hydrocyclone diameter, include vortex finder size, inlet area, length of cylinder, cone angle, and mounting angle. PROCESS VARIABLES Feed Concentration The most dominant process variable affecting hydrocyclone performance is the concentration of solids in the feed. This is also the most important variable because the operator can normally vary this with water addition or other means. Most of the other important variables cannot be easily changed. In grinding circuits, it is common to monitor and adjust hydrocyclone overflow size distribution with changes in the feed rate to the mill or water addition depending on system or pumping limitations. The correction for this variable is shown in Figure 7 for three separate materials with different size and viscosity characteristics. The first line represents a tailings application with a very fine feed and a high concentration of clays. The curve on the right represents a closed circuit grinding application producing a coarse product. Based on the solids concentration, the hydrocyclone feed has a high amount of very coarse, well de-slimed solids at a relatively low slurry viscosity. The curve in the middle is appropriate for most applications with broad size ranges.

Figure 7 Effect of Slurry

aracteristics on Feed Solids Correction Factor Ch

Correction For Feed Concentration

1

10

100

0 10 20 30 40 50 60 70Feed Concentration (%Solids, Volume)

Cor

rect

ion

Fact

or, C

P1

Standard

Coarse Closed Circuit Grinding

Tailings

Although most mining applications are generally known and the proper adjustments can be made in the selection formulas, in some cases it is recommended that a sample be tested. This is often critical for proper hydrocyclone selection. In a test loop if all the other process and hydrocyclone variables are known, the appropriate correction for solids concentration can be determined. Feed Pressure In many applications this parameter is the only other variable that can easily be adjusted. This is normally done by opening or closing feed valves to change the number of operating hydrocyclones for a given flow rate. In some cases, variable speed pumps can also effect pressure changes. Because changing the feed pressure does not make a major change in the hydrocyclone

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performance the hydrocyclone pressure is often controlled over a broad range. For about a 50% increase in pressure drop the expected change in D50 would be about 10%. However, pressure does have a large effect on component wear rates within a hydrocyclone. The effect of pressure is as follows: CP(pressure)= 3.27 x P–0.28 Where P = Pressure drop, kPa

An estimate of the pressure drop across a hydrocyclone can be determined by referring to manufacturer capacity curves which will show the hydrocyclone flow-rate as a function of inlet pressure for each vortex finder size for a selected hydrocyclone size. These capacity curves are for water, and a correction is required for slurries. Slurry Viscosity In practice the slurry viscosity is rarely known at the time of selecting the proper hydrocyclone. The slurry viscosity is directly related to the solids density by volume and to the total surface area of the solids. Thus relationships are developed to correct the D50 point for slurry viscosity by relating a specific application or size distribution of the feed solids with the slurry density. An example is shown in Figure 7. The effect of slurry viscosity is approximated by a combination of the feed solids concentration correction, discussed above, and the liquid viscosity in most hydrocyclone sizing simulations. This is because of the difficulty of measuring the slurry viscosity with coarse solids. Some work has been performed to measure this variable with special devices to keep the slurry moving (Kawatra 1996). Another approach is a methodology that allows a traditional viscosity measurement of only the fine segment of a given feed because this is the source of the material that will have a significant effect on the viscosity. In addition, with applications that have a significant amount of material that exhibits non-Newtonian flow characteristics, the apparent viscosity will vary with shear rate. Temperature also has an important but often overlooked effect on the fluid viscosity. This often can explain seasonal variations in hydrocyclone performance. For example, with a plant water temperature variation of 10 degrees from 10-20 degrees F, the fluid viscosity will change by 30% from 1.3 to 1.0. Specific Gravity Because hydrocyclones are classification devices that separate particles based on mass, the particle and liquid specific gravities are important variables. This has long been one of the important features of hydrocyclones in a grinding circuit because of the preferential way the heavier metal-bearing mineral particles report to the hydrocyclone underflow and are ground to a relatively finer size than the lighter gangue minerals. Figure 8 shows the hydrocyclone recovery curves for the lighter gangue particles and gold particles. Notice the D50 for the gold particle is 57 microns with a 160-micron D50 for the bulk solids.

Figure 8 Gold Versus Total Solids Recovery in a Grinding Circuit

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The specific gravity of the fluid also is important in applications with high concentrations of dissolved salts like potash where processing is done in a concentrated brine. Also, in some locations, the clarified water will have a build up of salts causing an increase in the liquid specific gravity as well as the viscosity. CP(specific gravity) = (1.65/(solids sg-liquid sg))0.5 DESIGN VARIABLES, HYDROCYCLONE GEOMETRY Hydrocyclone Inlet Design Hydrocyclones designed prior to 1950 featured outer wall tangential feed entry and 12-15 mm thick rubber liners. This design was not adequate for fine separations or for abrasive slurry applications. Most hydrocyclone manufacturers have redesigned their inlets to include some form of involute, ramped or scrolled feed style and all of these provide a measured advantage in hydrocyclone performance compared to earlier tangential designs. Figure 9 illustrates the various types of hydrocyclone feed entries. The inlet opening or cross-sectional area of the orifice feeding into the cylindrical section of the inlet has an effect on capacity as well as D50, and most hydrocyclone models have several options to increase or decrease this area based on the desired flowrates and cutpoint. In general, the larger inlet area, the higher the hydrocyclone capacity and the larger the predicted D50.

Figure 10 Hydrocyclone Cylinder Length

Figure 9 Hydrocyclone Inlet Styles

Cylinder Section Typically hydrocyclones have a cylinder section length equal to the hydrocyclone diameter. This can be a separate section or integral to the inlet head. Figure 10 illustrates a hydrocyclone without a cylinder section plus hydrocyclones with a single and double cylinder. While the longer cylinder section provided greater residence time and thus more capacity, it also reduces the tangential velocity. The added cylinder length results in minimal improvement in hydrocyclone separation and will increase hydrocyclone capacity at the same pressure by 8-10%. Larger 660-840mm diameter hydrocyclones typically have shorter cylinder sections. Cone Section Figure 11 illustrates the different hydrocyclone cone angles that are used in different applications. The 20-degree cones have been a standard in the minerals industry. The flat bottom hydrocyclone has been used to make very coarse separations with characteristic D50’s 2-3 times that of the standard hydrocyclone. The longer 10-degree cone will produce a finer and sharper separation at a higher unit capacity compared with the 20-degree hydrocyclone. Use of this longer cone angle can

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change the predicted D50 by 15-20%. A hydrocyclone that has multiple cone angles is also shown in Figure 11.

Figure 11 Various Hydrocyclone Cone Angles

Figure 12 Apex Capacity

Vortex Finder A range of vortex finder (diameter) sizes are normally available for each model hydrocyclone. The vortex finder sizes can range in sizes from 20-45% of the hydrocyclone diameter. Larger vortex finders will increase the hydrocyclone capacity but provide a relatively coarser separation. Smaller vortex finders will have the reverse affect. The correction for vortex finder size is:

CD(vortex finder) = (Dv/(0.3 x Dc)0.6 Apex Design The apex angle and design also have a large effect on performance. Proper selection of both the apex opening and the angle will allow a high underflow % solids as well as maintain the intended hydrocyclone separation. This design incorporates an optimal apex angle in combination with a straight section to maintain the finest possible separation with maximum underflow solids concentration. The proper apex size must be selected to insure the maximum underflow density and limit the fines that the additional water will entrain and carry to the underflow. Some hydrocyclone sizing methodologies have used apex opening as a factor in determining the expected hydrocyclone performance. Others maintain that apex size is a matter of determining the correct apex size to handle the estimated solids flow at the highest density. However, closer inspection shows that changing apex opening, in most cases, also means changing the apex angle and, in that context, the apex change does have an effect on the expected D50 produced. Figure 12 illustrates the capacity of different size apexes. The selection of apex or spigot size is performed after the basic hydrocyclone for the application and the expected material balance has been performed. The apex is then selected by knowing the expected tonnage and flow that will report to the underflow and selecting the correct opening from the attached chart. For existing installations, the discharge pattern of an apex can provide information about the required size of apex opening. A wide spray pattern is indicative of a dilute underflow and an apex that is too large for the application. If an apex is too small, the underflow will “rope” with a very tight narrow discharge. Sampling the underflow to determine % solids for different openings is recommended. Underflow solids concentrations of 50% by volume are normally a good target.

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Mounting Angle Hydrocyclones can be mounted at angles ranging from vertical to nearly horizontal. The effect of lowering the mounting angle will increase the expected D50 for a given hydrocyclone by 20-40% depending on the angle. This has been a popular tool to increase concentrator tonnage by effectively producing a coarser product. However, mounting at angles less than 45 degrees from horizontal have resulted in some maintenance problems, most notably with inlet head wear life.

Figure 13 Mounting Angle

Most large mill circuits have 660 – 840 mm diameter hydrocyclones. These large diameter hydrocyclones are normally 2,500 to 3,000 mm tall. These tall hydrocyclones provide a substantial head on the underflow discharge of the hydrocyclone. In order to maintain a high underflow density, the apex size must be closely monitored and maintained. Installing the hydrocyclones at 45 degrees from horizontal greatly reduces the head on the underflow discharge. A consistently high underflow density can be achieved because apex diameter is not as critical as in vertical installations. In addition, the lower head results in reduced velocity of the slurry spraying out of the hydrocyclone. This increases the component life of the apex by about 100% compared to vertically mounted hydrocyclones. Approximately 50% of the hydrocyclones installed in the past 10 years on large SAG circuits have been installed at 45 degrees from horizontal. The reason for this trend is to maintain a higher average underflow density compared to vertically mounted hydrocyclones. This also allows the operator greater flexibility to change tonnage or density because the oversized apex will not easily plug and misplace coarse material to the hydrocyclone overflow. Drawbacks include longer overflow pipes, shorter inlet headliner life, and difficulty accessing the lower part of the hydrocyclone. For hydrocyclones installed at 45 degrees, most operations must completely remove the hydrocyclone in order to work on the apex or lower cone. SAMPLE CALCULATION The discussion above is intended to serve as a guide to the relative importance of hydrocyclone variables on performance. To select a hydrocyclone for a specific application there are several simulation programs that use similar methods to calculate the expected performance of a given size hydrocyclone in a selected application. To complete our general example: Where:

D50(actual)=D50 (base) x CP1 x CP2 x CP3 … x CD1 x CD2 x CD3 etc… If we start with the following hydrocyclone: 250mm hydrocyclone with Standard Configuration: 20 degree cone angle, 250 mm cylinder,

75mm vortex finder, vertical mount, 123 mm2 inlet area, (CD1, CD2, CD3, etc., all 1.0 for standard hydrocyclone)

D50 (base) = 24 microns

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At the following feed conditions for a silica sand classification process: 20% Feed Concentration, Solids Liquid SG 2.65 and 1.0, 69 KPA Pressure, 20 Degrees C The CP for feed solids is 2.0 from the middle curve and the others calculate to 1 for the standard specific gravity water viscosity and pressure. D50(actual) = 24 microns x 1.0 (all CD factors) x 2.0 (solids factor) x 1.0 (all other CP factors) = 48 microns

TYPICAL MINERAL PROCESSING HYDROCYCLONE APPLICATIONS Closed Circuit Grinding One of the most prevalent hydrocyclone applications in a concentrator is to classify grinding mill discharge. This can be from a SAG, primary, secondary, regrind, or other auxiliary mill. Depending on the application and mineral liberation of the ore, the hydrocyclone will typically achieve an overflow product size ranging from a P80 of 300 micron to a P95 of 25 microns in closed circuit grinding duties. Typical hydrocyclone performance is shown for the large 840mm hydrocyclones in a coarse copper circuit and 510mm hydrocyclone data from a gold circuit producing a P80 of about 75 microns in the following table. Table 1 also shows hydrocyclone data from primary grinding circuits in two different iron ore installations. The first example is from a concentrator producing a very fine product and, for contrast, the second is a much coarser grind in another location.

In most plant design situations where a grinding circuit is involved, the hydrocyclone feed conditions are not known, and the selection is based only on specified conditions for the overflow product. Hydrocyclone selection requires an experienced-based correlation that links the required overflow size with the separation size required In grinding circuit applications, the separation achieved by the hydrocyclones is normally defined by a percent passing point in the hydrocyclone overflow rather than the D50 point or "mesh of separation". The most common definition of the separation is the "P80" point or particle size that is 80% finer in the overflow. The P80 is different from D50 because the P80 point is dependent upon both the D50 separations achieved by the hydrocyclone and the size distribution of the hydrocyclone feed. For example, if the hydrocyclone feed size distribution is 50% +1000 microns and 50% -1 micron, the P80 achieved by the hydrocyclone will be less than 1 micron regardless of the size of hydrocyclone installed. Thus, it is not a simple matter to go from D50 separation to the P80 of the hydrocyclone overflow. In some applications, this relationship is one to one, but it varies dramatically from application to application. The D50 separation is normally not an important data point for the hydrocyclone operator. They normally will want to know the "mesh of separation" or the particle size that has a 95-99% (D95 - D99) chance of reporting to the underflow. This is often called the cut or separation achieved by the hydrocyclone and is typically two times the D50 point. The recovery curve shown in Figure 5 illustrates the relationship of the various recovery points versus the D50 point. In some instances, the hydrocyclone and the mill are modeled as a circuit and the complete system performance is evaluated as a function of hydrocyclone selection variables. This method is often helpful in an existing circuit where most parameters are known. Conditions in a grinding circuit are not constant, and the hydrocyclone feed solids concentration and particle size distribution change continuously. The changing parameters and the self-regulating nature of the hydrocyclone in a grinding circuit tend to mask many of the effects of hydrocyclone changes.

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Table 1 Primary Grinding Circuit, Typical Hydrocyclone Performance

Copper GoldIron Ore Fine

Iron Ore Coarse

Cyclone Product Feed

Over-flow

Under-flow Feed

Over-flow

Under-flow Feed

Over-flow

Under-flow Feed

Over-flow

Under-flow

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Cum Wt% Finer

Mesh Microns 4 4800 93 100 90 100 100 100 100 100 100 99 100 98

10 1700 75 100 73 97 100 97 99 100 99 97 100 9420 850 65 99 59 97 100 96 94 100 8928 600 54 98 46 88 100 86 95 100 92 92 100 8635 417 43 93 35 82 100 78 92 100 89 87 100 8048 300 32 82 23 75 100 68 90 100 85 81 99 7465 212 24 70 15 53 100 35 87 100 82 71 96 58

100 150 17 57 10 50 99 32 84 100 78 56 86 40150 106 13 46 7 34 93 17 81 100 74 44 72 27200 75 10 37 5 24 81 12 78 100 69 34 59 18270 53 8 30 4 18 68 8 72 100 61 28 49 12325 45 16 63 7 68 99 55 24 41 11400 37 7 25 3 15 60 6 62 97 47 500 25 51 91 35

% Solids 65 36 81 43 21 67 14 5 78 66 54 80Circulating Load 430 305 240 130Pressure, PSI 11 16 30 11Cyclone Diameter 510mm (20") 840mm (33") 380mm (15") 660mm (26")

It should be noted that the hydrocyclone is often limited in closed circuit applications by the capacity of the grinding mill. Changes made to affect the hydrocyclone classification will result in a change in the circulating load and the amount of material returning to the mill feed. A finer separation will result in a higher circulating load and a higher tonnage feeding the mill at a constant new feed rate. This will increase the feed % solids feeding the hydrocyclone and this will coarsen the separation. Therefore changes to the hydrocyclone must be evaluated in the context of the overall grinding system. Tailings Hydrocyclones Construction of tailings dams is a very important consideration in a hard rock mining project. Hydrocyclones are often used to produce a low cost source of sand from mill tailings. The required characteristics of this dam building material are that it must:

1. Provide suitable drainage characteristics to allow materials to consolidate adequately and display full-strength.

2. Minimize the risk of failure by liquefaction under dynamic loading conditions. 3. Provide good percolation rates of water through the coarser solids.

A different but very closely related application is for the production of paste back-fill in

underground mines. The amount of available sand will depend on the grind, and as a result, tailings applications are much more typical in copper operations which have a comparatively coarse tailings. The traditional “Rule of Thumb” for high quality sand is a particle size less than 20% finer than 74 microns (Turner 1997). Recent experience has shown that this is not a reliable measurement in that it does not quantify the amount of silts and clays. The amount of minus 20 micron or the amount of 5 micron or finer material is a much better measure of the sand quality. Hydrocyclone size (diameter) can vary from 250-660mm for tailings applications. Hydrocyclones can either be mounted along the crest of the dam with the underflow discharging directly or on a hydrocyclone manifold at a central station where the underflow is pumped to the dam location. The hydrocyclones are often mounted horizontally in this application to decrease the

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amount of water and fines that are carried out the apex. In some cases, a two-stage hydrocyclone system is required to recover an adequate amount of sand at an acceptable quality. This involves another set of hydrocyclones to process the first stage underflow. A low cost alternative to the two-stage process is to introduce an attachment to the lower portion of the hydrocyclone where fresh water is introduced to wash entrained slimes to the overflow. Because of the importance of achieving a good quality dam building material, hydrocyclone testing is recommended. The test-work can be performed on actual tailings samples collected at the concentrator and sent to a pilot plant. On large operations, predicting the amount sand that can be recovered at an acceptable quality often will determine the viability of a project. Dewatering and Desliming Dewatering and desliming applications are very common in non-metallic operations where the amount of very fine material, including clays, has a large impact on reagent usage as well as product grade and recoveries in the subsequent process. In some phosphate operations, for example, the ore is deslimed with hydrocyclones prior to feeding a hydrosizer. The products from the hydrosizer are also dewatered in hydrocyclones to increase the feed solid concentration for adequate conditioning prior to flotation. The rougher flotation products are dewatered again in hydrocyclones prior to conditioning for cleaner flotation. Similar applications are found in coal and industrial sand processing. In many dewatering applications, where hydrocyclone underflow density is important, special attachments can be provided to the hydrocyclone that control or restrict the flow of material out of the apex. This is done with a duck bill shaped attachment added below the apex and an elongated pipe on the hydrocyclone overflow, specially designed to create a siphon. A valve regulates this siphon, and the amount of the siphon effects the force required to open the duckbill valve. This design is used in low solids applications with variable feed conditions, which makes proper apex sizing impossible. This is shown in Figure 14.

Figure 14 Apex Attachment, Controlled with Siphon

Ultra-fine Particle Separations In some applications, very fine particle separations are required. It is often necessary to remove ultra fines to enable the separation and recovery of minerals like cassiterite that occur in these very fine sizes. Typically 50mm (2”) diameter hydrocyclones are the preferred size for these fine high capacity separations. The design of the 50mm hydrocyclones has been refined to allow separations in the 5-10 micron D50 range. Smaller diameter 13-25mm hydrocyclones are available but because of the inherent lower capacities and smaller orifices of these very small hydrocyclones the operational problems are not insignificant with any amount of tramp oversize. Small diameter hydrocyclones are used in high flow-rate applications by incorporating them in a pod or group of hydrocyclones, with a common feed and overflow compartment and accessible individual underflows where any plugging of the apex can be seen and addressed. Many pods can be fed though a radial distributor as if they were individual hydrocyclones, to process these larger flows. An example of the installation of these is shown in Figure 15.

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Figure 16 Radial Manifold or Cyclopac, Large Hydrocyclones

Figure 15 Canister or Pod Design, Small Hydrocyclones

Figure 17 Spider Manifold

MANIFOLD SYSTEMS The specified design for hydrocyclone manifold systems are also an important consideration. In most mineral processing plants, this consists of a radial feed distributor with isolation valves on the lines feeding the individual hydrocyclones (figure 16). The valves are normally a heavy-duty knife gate style that is either actuated manually with a hand wheel or automatically with a pneumatic cylinder. Most modern plants will have the capability to remotely open and close hydrocyclones based on changes in hydrocyclone feed pressures. Pressure transducers are also included on the feed header so that the hydrocyclone feed pressure can be monitored remotely. A diaphragm is recommended to keep the pressure gauge or transducer from plugging with solids. In coarse grinding circuits the feed distributor and the launder collecting the hydrocyclone underflow are lined with at least 12mm (1/2”) rubber. The wear is not as concentrated in the hydrocyclone overflow launder and the pipes feeding each hydrocyclone, and, as a result, these will be lined with 6mm (1/4”) rubber in most cases. In lighter duty applications, the rubber lining is reduced in the feed and underflow launders and eliminated in the feed pipes and overflow launder. An alternate material that has been used recently in hydrocyclone overflow piping is HDPE. This is more wear resistant than unlined steel pipe and also fairly inert to chemical attack.

Another important consideration is the design of the feed distributor. It is important to have the feed evenly distributed to each of the operating hydrocyclones. It is in this area that system designers can be tempted into considering a less expensive in-line feed distribution design. This always leads to an unequal distribution in both the feed solids concentration as well as size distribution. This in turn leads to different hydrocyclone performance within the same bank, often requiring different apex sizes. The metallurgist’ task in hydrocyclone performance evaluation or optimization is made much more difficult as a result. A radial design feed distributor, with the feed coming from either the top or bottom, and into a header with multiple nozzles allowing the feed slurry to be distributed to each hydrocyclone feed line, is the preferred design. This is shown in figures 15-17 for different style manifolds. The header should have a removable top for maintenance. It is also a good idea to allow extra nozzles for possible future capacity or as a location to sample hydrocyclone feed. MATERIALS OF CONSTRUCTION AND MAINTENANCE CONSIDERATIONS Hydrocyclones used in the mining industry normally have steel or fiberglass housings with replaceable liners. In some less wear sensitive applications, molded urethane hydrocyclones are used. In metallurgical hydrocyclones the most prevalent lining is gum rubber. These liners are

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normally available in 12mm (1/2”) or 25mm (1”) thickness. Because of the amount of coarse solids feeding a hydrocyclone in many of the mining applications the more wear resistant hydrocyclone design will include the thicker rubber liners in the upper portion of the hydrocyclone and utilize a combination of ceramic liners for the lower cones and apex. In large hydrocyclones (660 and 840 mm), the wear is highest in the apex followed by the lower cone and then the middle cone. The next highest wearing part is the inlet headliner. Thus, in order to keep all of the parts on the same cycle, premium ceramics are required in the apex and lower cone to bring the life of these parts up to the inlet headliner life. It is common commercial practice to use one type of ceramic in the apex, another type in the lower cone liner, and a third type in the middle cone liner (figure 17). The objective is to increase the life of the lower liners to be consistent with the inlet head liner life. The ideal situation is to have all the liners wear out at the same time. Hydrocyclone maintenance practice can also affect performance. In addition to worn apexes causing low underflow solids, worn cone liners can have grooves or edges that will cause misplaced coarse solids to the overflow. If a worn lower cone is replaced but the cone right above is also worn and not replaced, a negative edge may result which will adversely affect performance. In most cases at the plant design stage, because the actual hydrocyclone feed conditions and wear requirements are unknown, the hydrocyclones are provided with all rubber liners and rubber apexes with alternate sizes. After the optimum apex size has been determined at the commissioning stage a proper size ceramic apex is provided.

Figure 18 Hydrocyclone Cross-section

Figure 19 Various Hydrocyclone Liners and Materials

While the rubber and silicon carbide liners are the primary wear material used in closed circuit grinding applications many mining hydrocyclone installations use different wear materials. For example, many phosphate and iron ore installations utilize polyurethane liners in the hydrocyclones. In coal, hydrocyclones are either lined with an alumina or silicon carbide ceramic or polyurethane. Different hydrocyclone liner materials are illustrated in Figure 19.

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ACKNOWLEDGMENTS We are grateful to Dr. Charles Petty, Professor Department of Chemical Engineering, Michigan State University and Dr. Steve Parks also of Michigan State University for their assistance and review of this work. REFERENCES Agar, G.E., and J. A. Herbst “The Effect of Fluid Viscosity on Cyclone Classification”,

Transactions SME, December 1966. Arterburn, R.A., “The Sizing of Hydrocyclones”, Krebs Engineers, 1976. Arterburn, R.A., “The Sizing and Selection of Hydrocyclones”, Design and Installation of

Comminution Circuits, Volume 1, Chapter 32, 597-607. Bradley,D., “The Hydrocyclone” , Pergamon Press, 1965. Devulapalli,B. and R. K. Rajamani., “Hydrocyclone Modeling of Swirling Flow and Particle

Classification in Large Scale Hydrocyclones,” KONA Powder and Particle Journal, No. 12, pp. 95-104,1994.

Dorr, J.V.N., and A. Anable, “Fine Grinding and Classification”, Trans. AIME, Vol. 112, pp. 161-177, 1934.

Hill, L.N., “Installation of 0.84 M (33 IN) Cyclones on the Primary Grinding Circuit at Cyprus Sierrita Corporation”, SME Annual Meeting, February 14-17, 1994.

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