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An Introduction to a Dense Medium Cyclone Circuit

For processing ores and coal a circuit that is seeing greater interest and use is the dense medium cyclone circuit.  It is seeing use in diamond operations and heavy mineral processing.  Its use in coal goes back to the middle of the last century (circa 1945).

 TYPICAL CIRCUIT

A typical dense medium cyclone circuit is shown in the attached sketch.

This circuit consist of an ore feed, a set of desliming screens to pre-size the feed to the cyclones, a cyclone feed sump, dense medium cyclone(s), media drain and rinse screens, and a media control circuit.  It is often combined with a fines circuit to process material of a finer size.

 DENSE MEDIUM CYCLONE

Dense medium (sometimes called heavy media) cyclones were developed during World War II by the Dutch State Mines.  The original application was for coal cleaning.  A cyclone pilot plant was successfully operated in Europe during 1945, and observed by H. F. Yancey of the US Bureau of Mines (USBM, now part of the Department of Energy (DOE)).  Mr. Yancey started a similar program for the USBM, with a published report in 1948.   Cyclone plants quickly became widely used in Europe, but the first United States plant was not built until 1961.

 Several different configurations are available, the standard cyclone is available from several vendors, either as individual cyclones or as complete separating systems.  In addition variations on the basic cyclone are available such as the Dyna-Whirlpool and the "tri-clone".  Large diameter cyclones (30”, 33, and larger) are also available, with capacities approaching vessels also.

Today dense medium cyclones are the most common coal cleaning device.  For hard to clean coal (+10% near gravity material) in the size range of 50mm to 0.5 mm (28 mesh), the dense medium cyclone is widely used.  In its operation, a slurry of ore or coal and media (magnetite dispersed in water) is admitted at a tangent near the top of a cylindrical section that is affixed to a cone shaped lower sec­tion.  The slurry forms a strong vertical flow.  Under the force of gravity, the higher specific gravity particles, move along the wall of the cone and are discharged at the apex.  The particles having a lower specific gravity move toward the center of the cyclone. In the center a counter rotating vortex moves the light fraction upwards. The light fraction is discharged through the vortex finder.  The dense medium cyclone functions efficiently even with large amounts of near gravity material in the feed.  

 CYCLONE SIZING

The size of a dense medium cyclone is usually expressed in gallons/minute of pulp capacity to the inlet.   Alternatively as the tons/hour of raw coal feed to cyclone.  Dense medium cyclone sizing depends on how much pulp you can feed through the inlet.   Once in the cyclone you have to make sure that the overflow and underflow can handle the amount of material reporting to each.  All of this while maintaining the needed efficiencies to make the separation desired.

Dense medium cyclones behave similar to classifying cyclones in volumetric handling capacity.  And any general capacity information for cyclones will apply.  A latter section of this article describes general cyclone sizing.  Specifically for dense medium cyclones the attached capacity table (Table 1) should be used.  This chart is based upon the information over collected from many projects. 

 

EXAMPLE

The following is an example of sizing and selecting a dense medium cyclone.  It is included for reference only.   In actual practice many different factors can cause the specific selection to change.

 Conditions:

200 T/hr of raw coal

1 " x 1/4" (25 mm x 6.5 mm)

1.50 separating gravity

80% (at 1.50 Sp.Gr.) reporting to clean coal.

 From Table 1, to handle a 1" particle a 24" diameter (600 mm) cyclone is needed.  Physical check should be made of the feed to determine the amount of material at or near 1", if it is relatively small (<10%) a smaller cyclone might work.

 With 80 % reporting to clean coal, this gives 20% reporting to refuse or 40 T/hr.  Again from Table 1, two 24" diameter cyclones are needed.  For 160 T/hr of clean coal, three 26" diameter or four 24" diameter cyclones are needed.  From Table 1 both sizes can handle the 200 T/hr (3 @ 26" = 3 * 82.5 = 247 T/hr; 4 @ 24" = 4 * 70 = 280 T/hr).

 Laying out an even number of cyclones is easier then an odd number.  This has to do with the number of drain and rinse screens, and distributing the clean coal and refuse to them and collecting the products from them.

 For flowsheet calculations, from Figure 3, 1.5 Separating Gravity requires a media gravity of 1.45 using a media that has 95% - 50 micron magnetite.

 From Figure 4 a 1.45 media gravity and 80% to the overflow gives a clean coal media of 1.3 Sp.Gr. units, and 20% to the underflow gives a refuse media of 1.9 Sp.Gr. units

 MEDIA CIRCUIT CALCULATION

Calculation of a dense medium circuit is the determination of the amount of media circulating in that circuit and the amount of media (magnetite) lost to dilute media. 

 The amount of magnetite lost to the dilute media circuit is directly dependent on the average grain size and specific gravity of the material being separated (i.e. coal or refuse). 

 DILUTE MEDIA:  The total amount of media carry-over to the dilute media circuit is calculated as follows: 

 Dense medium Cyclone

           

Where D_n is the average grain size of the solids (coal or refuse) and SG is the specific gravity of the solids.  This is the solids in the feed stream to the drain and rinse screens or sieve bend.  T/H is the solids flow rate in tons per hour to the D&R device.  G/M is the gallons per minute of pulp.

 SEPARATING GRAVITY:  From figure 4 determine the desired separating gravity and the desired media gravity.  From this information refer to pulp conversion data to determine the volume of magnetite in the media. 

 DILUTION WATER:  From the screen sizing sections determine the amount of rinse water to be used on the D&R Screens.  In addition, determine the amount of surface mois­ture on the final products (this is often a given and the circuit must be designed to meet it). 

 LEVEL CONTROL:  To balance the circuit an amount of media must be bled to the dilute media circuit or water and media added.  In actual practice the amount of media lost on final product and in the dilute media circuit will require a positive addition of water and magnetite.  In balancing a plant the calculations may indicate a build-up which will require a circuit bleed.  To prevent fines build-up a bleed is often desirable. 

 TOTAL DILUTE MEDIA:  The total dilute media will be

C/C adhering media + Refuse adhering media + Level Control Volume + Rinse Water + Products surface moisture = Total dilution