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An Introduction to Water-only Cyclones

Water-only or Compound-water cyclones (WOC) are commonly used in a fines cleaning circuit, either as a first stage ahead of tables or spirals or as a two stage circuit of their own.  The separating efficiency of WOC’s has limited their application, but used properly they are easy to set up and operate.  They work best in two stage circuits (but then most jigs have a primary and a secondary compartment so are two stage also(.

TYPICAL CIRCUIT

Water-only Cyclones are commonly used in a fines cleaning circuit, either as a first stage ahead of tables or spirals or as a two stage circuit of their own.  In the coal industry a very common circuit is a two stage sink re-cleaning.   Figure 1 shows a typical circuit

Figure 1 – Typical WOC Circuit

 EQUIPMENT DESCRIPTION

In the early 1960's research on cyclones led to the development of the water-only cyclone which performs a specific gravity separation using only water and inertia.  Water-only or Compound-water cyclones (WOC) separate based on differential specific gravity, with the lighter particles going to overflow.   The design feature which permits the use of "water-only" operation is the wide angle or angles in the conical bottom.  This promotes the formation of hindered settling bed, as the dense particles move down the side wall.   Less dense particles cannot penetrate this heavy bed and move back into the main hydraulic current to be discharged out of the top of the unit through the vortex finder.  Applied in easier cleaning operations than heavy media cyclones, water-only cyclones have been used to wash coals with a top size of 50 mm to 28 mesh.  The most common application is on coals with a size range of 28 mesh x 0.   Figure 2 shows typical features of a water-only cyclone.

Figure 2 – Water-only Cyclone

 EQUIPMENT SIZING

The size of a WOC is usually expressed in gallons/minute of pulp capacity to the inlet, alternatively as the tons/hour of solids fed to cyclone.  WOC 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.

 WOC behave similar to classifying cyclones in volumetric handling capacity.  And any general capacity information for cyclones will apply.  For more information see General Cyclone Sizing.   A latter section of this article describes general cyclone sizing.  For rough sizing and preliminary approximations the information in Table 1 can be used as a guide.    This chart is based upon the information over collected from many projects. 

Table 1 – Water-only Cyclone Capacity

	Water-only Cyclone Capacity
		Feed		G/M		% Wt.			% Wt.
Size		Range (t/hr)		Average	Feed			Underflow
3"	10	-	40	30	10	-	20	35	-	50
4"	30	-	70	50	10	-	20	35	-	50
6"	75	-	150	100	10	-	20	35	-	50
8"	125	-	250	175	10	-	20	35	-	50
12"	200	-	400	300	10	-	20	35	-	50
14"	350	-	600	450	10	-	20	35	-	50
18"	600	-	800	700	10	-	20	35	-	50
20"	700	-	1000	850	10	-	20	35	-	50
24"	800	-	1200	1000	10	-	20	35	-	50
26"	1000	-	1600	1200	10	-	20	35	-	50
30"	1500	-	2500	2000	10	-	20	35	-	50

 EXAMPLE

The following is an example of sizing and selecting a water-only cyclone.  It is in­cluded as reference, many different factors can cause the specific selection to change.

Conditions: 

            50 ton/hour of raw coal

            28 mesh x 0 mesh size distribution

            1.70 separating gravity

            80% (at 1.70 Sp.Gr.) reporting to light fraction (cyclone overflow).

 From Table 1, normal feed concentration would be about 15 % by weight.  For 50 T/hr this would give:    50/(15/100) = 335 T/hr of pulp or 335 *4 = 1350 G/m of pulp

Using one (1) cyclone this would be a 26” cyclone.  For water-only cyclones, the larger the size the less efficient they are.  While large diameter cyclones are seeing more usage, common practice has been to use several smaller cyclones.   It would be more common to use three (3) 14” or four (4) 12” cyclones.  For maintenance, an additional (installed spare) would be used, calling for a cluster of four (4) 14” or five (5) 12” cyclones.

For refuse handling the 80% to clean coal gives 20% or 10 tons/hour reporting to the cyclone underflow, and from Table 1 this would be at approximately 40% solids or 10/(45/100) = 22 T/hr of pulp or 22 *4 = 90 G/m of pulp.

For the second stage this would be diluted back to 15% solids or 10/(15/100) = 67 T/hr of pulp or 67 *4 = 270 G/m of pulp.  From this Table 1 shows that one (1) 12” cyclone would work, but a more common way would be to dilute the feed to allow two (2) cyclone operation with an installed spare.

The overflow would go back to the primary feed and the underflow go to dewatering.

The overflow would be approximately 8 t/hr of solids and 250 g/min of pulp, which added in a circulating load to the primary feed makes the feed 58 t/hr and 1550 g/min of pulp, selection of  five (5) 12” cyclones operating (six (6) installed) a good match.