Grinding & Classification Circuits

Grinding & Classification Circuits 2017-04-04T06:57:16+00:00
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Hydrocyclone Air Core (16 replies)

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

Is the air core formation inside a hydrocyclone is needed or not? Got confused as many research papers suggest to minimize the air core diameter by several design modifications within the hydrocyclone. On the contrary air core is necessary to carry the fine particles through the overflow.

Sugar Watkins
2 years ago
Sugar Watkins 2 years ago

Whether an air core actual exists through a cyclone is not the issue, an inner vortex is required moving upwards towards the vortex finder. Cyclone operation using flapper valves and other configurations have operated that do not have an "air" core, but do have an inner vortex. In all likelihood a small air core is probably created, but it is the inner vortex that transports the finer (or lighter) material out the vortex finder.

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

Yes this is what I have been following up. I need to know how to calibrate the flapper valves so that it opens at the exact time when it should. These arrangement is hugely used in thickening applications.

Carmen Ibanz
2 years ago
Carmen Ibanz 2 years ago

I assume you are referring to a duck bill valve fitted over the spigot. (Bottom outlet) If this is the case, the "flapper valve" is closed naturally by external air pressure as the underflow is actually below atmospheric pressure, hence why the air core is drawn in. The weight of the solids building up above the valve opens the valve naturally when there is sufficient build up. Fitting a small spigot has the same effect as you end up with the underflow roping (thickening) but this is often undesirable as you run the risk of blocking the spigot.

Ideally you should use a ceramic bottom section of the cone as the built up solids going around and around will wear a rubber / PU cone quite rapidly.

The air core has no effect of carrying the overflow solids away and as stated above is always present even when you "pot" the underflow, when the air is drawn out of the fluid itself.

Calibration is carried out by - what looks right is right! Unless you intend to build an opaque cyclone to monitor build up in the cyclone it’s down to guesswork. You will soon find out if your valve is to stiff when you come to inspect the wear within the cone. You only want a few inches of build up above the spigot.

Here I am considering a standard 10" hydrocyclone in my head hence the “few inches" but to put a buildup height to spigot diameter I'd plum for a max of 10:1.

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

We have managed to manufacture a transparent cyclone to monitor the buildup. Here is what i understand, it is actually a trial and error method as to when it should open to give my desired result. Please correct me if I am wrong.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

Role of air core in the cyclone is well described in the following:

http://www.sciencedirect.com/science/article/pii/S0892687507000271

http://www.ingentaconnect.com/content/maney/mpem/2013/00000122/00000001/art00004

http://rd.springer.com/article/10.1007%2Fs11814-010-0344-y

http://www.sciencedirect.com/science/article/pii/S0892687512003627

http://www.cfd.com.au/cfd_conf12/PDFs/040KAR.pdf

But, still it is an debatable issue in the current scenario.

Carl Jenkins
2 years ago
Carl Jenkins 2 years ago

If you are "calibrating" the flapper valve it sounds as if you are using an operatable valve (such as a pinch valve). If this is the case, it actually is a trial and error method and even this will cause variations as the feed varies. Many have had fairly good success with just a straight flapper that opens and closes based on a weight and selected by stiffness of the rubber. This should get you 80 to 90% success (unless you get large rocks). But you can often get other variations from feed and upstream operations higher than that.

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

I will follow your suggestions and get back to you with the results.

Marshal Dienes
2 years ago
Marshal Dienes 2 years ago

In my experience the flapper valve or pinch valve is usually used when you are trying to thicken the underflow of the cyclone. The overflow of the cyclone is usually designed to be under vacuum, (although it won’t be run under vacuum obviously). The overflow pipe curves up and over and down into a sump, (which must be under the underflow of the cyclone), and is better when in the overflow sump itself under the water level to prevent air intake from the pipe. The "calibration", or adjustment to this flapper valve, is usually made by having a bleed valve on top of the curve of the overflow pipe, this is usually adjustable and breaks the vacuum in a controlled manner to adjust the thickening effect of the underflow. I have fitted a few of these systems which can have their drawbacks. Usually because of the operator fiddle factor.

Ace Levy
2 years ago
Ace Levy 2 years ago

The vent pipe you have seen fitted on some hydrocyclone systems is to stop syphoning. The over flow does not have to have a negative pressure it can be at any pressure you like provided there is a pressure drop across the cyclone from inlet to overflow.

Zander Barcalow
2 years ago
Zander Barcalow 2 years ago

To control the "flapper" valve, which I assume is being used for thickening the underflow, you establish a siphon in the design stage, then use the syphon break pipe, with a valve to control the air intake and hence control how the "flapper" valve works. More siphoning/ vacuum will try to try to close the valve allowing for a build-up of coarse or heavies which then relives on weight; but by using a siphon break valve or vacuum control valve you can adjust the effect and control the amount it thickens up to.

See web links for what I mean. https://www.911metallurgist.com/C/GEO/Basic%20Hydrocyclone%20Operation.pdf

http://www.weirminerals.com/products__services/hydrocyclones/hydrocyclones_-_dewatering/cavex_dewatering_hydrocyclones.aspx

Bill Rico
2 years ago
Bill Rico 2 years ago

We have a number of the flapper type "dewatering cyclones" installed to dewater our tails, before discharging on to dewatering screen for further dewatering. Operator adjust small vacuum valve, attached to overflow pipe to control density to underflow - fine balance - drive to maximise % solids in underflow vs solids in overflow water which is again used in process.

Marshal Meru
2 years ago
Marshal Meru 2 years ago

It is a natural phenomenon in cyclones. The more the diameter of apex the more is the air core.

When apex diameter is small in cyclones of one inch air core is too small. For very thick slurries > 40% solids air core may be blocked. When you run with only water (zero solids) not even a single drop of water falls below apex. 100% water goes to over flow. This is a natural phenomenon. In any cyclone you will never see underflow % solids is less than over flow. It is this principle we have taken advantage to separate particles in size, in a cyclone.

Ace Levy
2 years ago
Ace Levy 2 years ago

Has any comparison been done between the Partition value curves (Tromp curves) of conventional cyclones with air cores and duck bill valve cyclones with little or no air core for the separation sizes and densities?

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

You are right not even a single drop of water comes through the apex. Myself here at Tega and as well as in RRL we found the same phenomenon with a 1 inch cyclone. But as we go with higher size of cyclones this phenomenon does not replicate. So that means no separation occurs if there is no air core. But as we go up with the cyclone size, air core starts forming and eventually separation starts. But now if i start using a duckbill valve at the apex and restrict the air core formation then it would definitely affect the classification efficiency.

So far I have not done the experiments to get an idea of the effect with the two designs. Now if such comparisons has already been done then it will help me in getting a fair idea as to what should be done and what not to.

Carl Jenkins
2 years ago
Carl Jenkins 2 years ago

Regarding air core you will get many references in minerals engineering as well as powder technology. The very purpose of air core generation is explained in all the literature.

You can carry out simulation studies and can easily assess the variation of different parameters on air core. The difference in the pressure drop creates the air core.

Carmen Ibanz
2 years ago
Carmen Ibanz 2 years ago

Some few fundamentals all know about cyclone.

Air core:

This will become Zerto in ropy underflow. In Ropy under flow few coarse particles which are supposed to come out from under flow, will try to escape form top. Hence underflow should be provided with air core. Increase water addition in feed. More water will create more air core causing fine particles to escape from under flow. This stage is called umbrella shaped under flow. Angle of spray should be in-between both. Ideal condition. We don't have today commercial application of controlling air core.

Cyclone overflow:

Over flow discharge point should be just above feed point. If it is below, then siphoning effect will be created. To avoid this you provide a small vent pipe just above vortex finder, in over flow bend.

Cyclone pressure:

In general it is good to operate between 16 to 20 psi. High or low -- pressures will give more fines in under flow.

Cyclone Underflow fines:

We cannot avoid fines in underflow. Minimum of 10% report in underflow. Water coming down will carry such fines.

Control of cut size:

After cyclone designing for the stipulated parameters size is controlled by water addition or also called as Wt% solids in cyclone feed. In commercial operation it plays a major role.

For variations in specific gravity of solids cyclone over flow contain coarse particles. For such type of problems you need to have long cone or increased length of cone or reduced cone angle.

Apex is the heart of cyclone performance. As apex starts wearing and diameter becomes more air core diameter becomes more and cause more recirculation of fines. Best option to overcome this problem is to have Silicon Carbide apex. It is next to diamond in hardness.

You should never change apex diameter design, without consulting the manufacturer.

Always have pressure gauge and record its variations. More variations cause cyclone to surging.

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