Cyclone

Details Cyclone Cyclone

2001-06-15

2003-07-22

Reifsnyder, David A. (Department: 1723)

Liquid purification or separation

Tangential flow or centrifugal fluid action

C055S459100, C209S719000, C209S720000, C209S721000, C209S732000, C209S733000

Reexamination Certificate

active

06596169

ABSTRACT:

This invention relates to a cyclone.
This invention relates particularly, but not exclusively, to dense medium cyclones for treating fine particles and classification cyclones and it will be convenient hereinafter to describe the invention with reference to these example applications. However, it is to be clearly understood that the invention is capable of application to other cyclones for example, water washing cyclones and dense medium cyclones for treating coarse particles.
In the specification the term “cyclone” is to be interpreted broadly and specifically to include hydrocyclones. Hydrocyclones which treat liquids containing entrained particles are thus a subset of the term “cyclones”.
Broadly, a cyclone comprises a body defining an interior space having an upper cylindrical portion and a lower frusto conical portion. Fluid having entrained particles enters via a tangential or involute inlet towards the upper end of the body and passes out through an axial underflow outlet towards the bottom end of the body. Fluid and particles which do not pass out through the underflow outlet travel upwardly in an air core through a central region of the interior space of the cyclone and out through the overflow outlet. The overflow outlet is formed by a vortex finder which projects in through the top of the body of the cyclone into the interior space.
The shape of the body of the cyclone induces a helical spiral flow of fluid in the body in a radially outer region of the interior space. Then the flow changes direction and an air core spirals upwardly through a radially inner region of the interior space of the cyclone. The spiralling flow of fluid applies a centrifugal force to particles entrained within the fluid and exerts differing forces on the particles depending on their size and/or specific gravity. Heavier and/or larger particles are radially displaced towards the radially outer region of the interior space from where they pass out through the underflow outlet. Lighter and/or smaller particles tend to gravitate towards a radially inner region of the interior space and are carried upwardly with the air core which flows out through the overflow outlet. This thereby effects a separation of particles on the basis of specific gravity or size which is the key function of the apparatus.
Dense media cyclones are used for beneficiation of mineral ores, e.g. separation of heavy minerals or coal from unwanted gangue or tailings on the basis of difference in specific gravity. One application of dense media cyclones is to separate coal from non coal material in the ore which is mined out of the ground. In dense media cyclones, the relatively light coal particles are predominantly carried with the dense medium or liquid through the overflow outlet while the relatively heavier non coal particles are predominantly passed out through the underflow outlet. The efficiency of the cyclone is measured by its ability to provide a relatively sharp separation of coal and non coal particles, e.g. to reduce contamination of waste materials in the product and to reduce loss of valuable product with the waste materials.
A known prior art dense media cyclone is the cyclone illustrated in FIG.
1
. This cyclone has a side wall comprising an upper wall portion of circular cylindrical configuration and a lower wall portion defining an interior space. One part of the lower wall portion has a frusto-conical configuration and the other part is in the form of a spigot projecting outwardly away from the end of the frusto-conical part. The cyclone has a vortex finder which has a relatively thin wall and does not occupy more than one third of the cross sectional area of the interior space of the body of the cyclone adjacent the inlet. Thus, the annular cross sectional area for fluid flow defined between the vortex finder and the upper wall portion is fairly large and the velocity of the fluid drops when it enters the cyclone. Another feature of the prior art dense media cyclone is that the internal junction between the frusto-conical part and the spigot part of the lower wall portion is smooth and without any interruption.
One limitation of these prior art cyclones is that there is a tendency for fluid to short circuit from the inlet to the overflow outlet without being subjected to the vigorous centrifugal forces developed in the interior space. There is also a tendency for fluid to short circuit along the side wall of the body or to be positioned in a boundary layer adjacent the side wall. Further there is a tendency for heavier particles to be entrained in an axial air core flowing up from the underflow outlet to the overflow outlet if the air core is too close to the side wall of the cyclone. The air core is very unstable. These occurrences detract from the efficiency of these cyclones.
Another limitation of the cyclone is its ability to separate out fine particles, eg 0.5 mm to 0.1 mm. This is because of the limited amount of centrifugal force generated in the cyclone. Currently, separation of very fine heavy minerals (2 mm to 0.1 mm) is undertaken using gravity concentrators, such as shaking tables, spirals and diaphragm jigs. However, the separation efficiency of this equipment is typically quite low. The concentrate from the separation often contains a considerable amount of waste material and a further separation, e.g. a downstream heavy liquids process, is often required.
Clearly therefore it would be advantageous if a dense media cyclone having increased efficiency and reduced contamination of waste material and product could be devised. It would also be advantageous if a dense media cyclone could be devised with an increased ability to separate out fine particles and thereby obviate the need for a separate step.
Classification cyclones are widely used in many industries for a variety of tasks in liquid-solid separation, including classifying, thickening, clarification, and desliming. Conventional classification cyclones have similar structural features to the dense media cyclone described above, but with different dimensions.
FIG. 2
is a schematic illustration of a conventional classification cyclone.
During operation of the cyclone, the spiralling flow and resultant centrifugal force, causes solids to be flung to the lower wall portion and spiral down to the underflow outlet. The bulk of the fluid e.g. liquid and very fine particles spiral upwards and leave the cyclone through the overflow outlet.
For any inlet pressure and rotational speed there is a theoretical “cut” size at which the centrifugal and drag forces are in balance. Theoretically, particles finer than the cut size are dragged with the bulk of the liquid through the vortex finder, and particles coarser than the cut size report to the underflow outlet.
Currently, the performance of conventional classification cyclones in industry is most unsatisfactory and contamination of particle sizes occurs in both the underflow and overflow material.
The reasons for this are varied and include the reasons articulated above in relation to the dense media separation cyclones. Some efforts have been directed towards the following areas in an effort to reduce these problems: change of feed parameters; modification of underflow and overflow outlets; introduction of additional components or formations to alter fluid-flow patterns; modification of the shape of the lower wall portion; and multi-stage cyclones. However, none have been commercially successful, mainly due to cost.
It would therefore be advantageous if a classification cyclone, e.g. a hydrocyclone, could be devised which at least partially overcame these problems.
According to one aspect of this invention there is provided a cyclone for effecting a separation on a fluid stream containing entrained particles, the cyclone including:
a body having a circumferential side wall extending between upper and lower ends and defining an interior space, the side wall comprising an upper wall portion and adjacent lower wall portion tapering inwardly in a direction away from the upper wall portion, the upper w

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