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 Water Only
Water-Only Hydrocyclones
Water-Only Cyclones are also used to "clean" (or "wash") raw coal. This hydrocyclone design relies on the fact that the mineral matter is denser than the coal. In this case the truncated cone bottom allows refuse bed to form which rejects lighter coal particles, while the relatively long vortex finder "vacuums" up the light particles.
- A problem exists with this design in that some coal losses take place through the apex because larger coal particles are classified. This problem can be mitigated by using a 2-stage water-only hydrocyclone circuitry where primary losses are re-cleaned in secondary hydrocyclones. A much more efficient system results.
- Finer coals must be cleaned in smaller diameter hydrocyclones, otherwise the drag forces in larger diameter hydrocyclones cause all the fine particles to report to "clean coal".
- Pressure and feed solids affect the separating gravity.
- Can adjust separating density with apex size.
  
Materials
Liner Material for Coal and Other Ceramic Lined Hydrocyclones
It is FLSmidth Krebs’ policy to offer the customer the most cost-effective materials available for each application. For this reason, we have several elastomer and ceramic options for the liners of each hydrocyclone. Because the coarse, heavier materials migrate to the bottom of a hydrocyclone and because the velocities in this region are the highest, it stands to reason this is where would expect accelerated wear to take place. In some relatively easy applications, a nitride bonded silicon carbide ceramic apex insert along with upper liners of urethane or other elastomer is adequate to combat the wear. It’s important to understand that the wearing away of upper liners before lower liners creates a ledge at the liner interface. This can cause particles that normally are spiraling downward toward the apex to be "kicked" inward toward the center of the hydrocyclone and forced out the overflow by drag forces; this misplacement of coarse or heavy material is usually unacceptable in all applications.
Most coal applications require all wetted hydrocyclone surfaces to be ceramic lined especially Heavy Media and Water-Only Cyclones where dense magnetite and raw coal (containing sand and rocks) impinges upon the liners. For severe applications, we may recommend that upper portions of the hydrocyclone be protected with nitride bonded silicon carbide ceramic liners, while the lower sections and apex be protected with more abrasive resistant, high purity alumina liners. In extremely severe applications a reaction bonded silicon carbide apex insert might be recommended. We suggest you discuss liner options with a Krebs Sales Engineer or your local Representative.
Maintenance of Ceramic Lined Hydrocyclones
With the aforementioned ceramic liner options, frequent maintenance is usually not a requirement. Preventative maintenance, as with other pieces of process equipment, is important. However, for Classifying, Water-Only and Heavy Media Cyclones, apex wear is probably the most important aspect and the easiest to monitor. With all pump motor electrical switches properly secured, a flashlight and measuring device are usually the only items required for this practice.
To inspect the other liners, usually an apex assembly or overflow pipe and vortex finder can be removed. If this still doesn’t allow adequate view of all liners, additional disassembly will be required. Because the liners are somewhat brittle, all ceramic lined hydrocyclones should be handled carefully; hammering on any surface of the hydrocyclone is not recommended. To install new liners please click here for instructions.
Because ceramic lined hydrocyclones will last such a long period of time, some maintenance personnel may forget certain aspects of repairing them. In this case, please contact Krebs or your local Representative for assistance; some of the representatives may also be able to provide you with a rebuild or exchange service if you’re interested. Q: Why can’t I use larger diameter hydrocyclones to classify so I won’t need so many?
A: Larger diameter hydrocyclones will always make a coarser separation than smaller diameter hydrocyclones because the internal velocities and resultant g-forces are always of a lower value…with less force pushing the particles toward the hydrocyclone wall, the particles will report out the vortex finder.
Q: What is the best solids content for the feed to a classifying hydrocyclone.
A: This somewhat depends on the size of the feed particles, but usually we recommend that 10% solids is not exceeded. Otherwise, the fines in the feed will create a slurry viscosity that will force light, relatively coarse particles out the overflow like a heavy media hydrocyclone.
Q: Why does Krebs offer so many apex sizes for their classifying hydrocyclones?
A: The smaller the apex, the less liquid is allowed through the orifice. If the apex is too small, a very heavy, dense discharge will be observed; this is called "roping" due to the narrow coiling discharge. On the other hand, an apex that is too large allows excessive amounts of liquid to pass. This allows slimes that are carried by the water to also pass into the stream where only the large particles are desired. A roping discharge will lead to coal being discharged to the overflow. In addition, if the apex angle is not properly designed coal will be lost to overflow.
Q: I need a part badly…how soon can you ship it?
A: We can usually ship the part within a 24 hour period.
Q: Why are heavy media hydrocyclones mounted nearly horizontal?
A: To allow for a large apex that will pass relatively large amounts of coal refuse. The angle, however should always be more than half the cone angle to allow the magnetite to freely drain after shutting the circuit off.
Q: Why am I seeing material misplaced only lately by the water-only hydrocyclones?
A: Assuming no other changes have taken place, it’s quite likely that the truncated cone liner is starting to wear unevenly often causing high ash particles to be "kicked" into the upward moving current adjacent to the vortex finder. New liners will remedy the problem.
Q: What pressure should I be reading on my heavy media hydrocyclone gauge?
A: This is not easily answered unless the density of the circulating media is known. As the media density is increased, the gauge reading is correspondingly higher. Each Krebs Heavy Media Cyclone is designed to operate within a certain pressure range. Confirm this range with a Krebs sales engineer or your local representative.
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