GB2278790A - Hydrocyclone - Google Patents

Abstract

A hydrocyclone uses an auxiliary fluid, such as water/magnetite slurry, fed into the vessel 1 through tangential inlet 12, to create a vortex into which a mixture of particles to be separated, such as coal and shale, in slurry form is fed through axial inlet 4. Heavy particles are discharged with one fluid fraction through outlet 13 at the narrow end of the vessel. The rest of the fluid carries the light particles through annular gap between inlet 4 and vortex finder 6 to outlet 10. <IMAGE>

Description

SEPARATOR AND PROCESS This invention relates to coal or mineral oreprocessing technology and, more particularly, it relates to a separator for separating particulate materials such as coal or mineral ores from unwanted materials, such as shale or gangue. It also relates to a process for effecting such separation.

In the processing of coal and minerals it is in many instances necessary to separate the wanted material, such as coal or a desired ore, from unwanted materials, such as shale or gangue. This can often be achieved by utilising a difference in density or specific gravity between the wanted and unwanted materials. Cyclones are a common form of device used for such separations.

Shale is often a constituent of raw coal and is denser than coal. To separate coal from shale a fluid medium is typically used that has a density intermediate between those of coal and of shale, for example a slurry of magnetite in water that has a density of about 1.5. Such a medium is often termed a dense medium. Coal is lighter than shale and, by passing the raw coal with the magnetite/water slurry through a cyclone, coal can be separated in an acceptable manner from shale.

Mineral ores are frequently associated with unwanted materials, such as silica. Such unwanted materials or gangue can also be separated from the desired ore with the aid of a cyclone.

In one conventional design of cyclone, originally developed in about 1945 by Dutch States Mines separation takes place in an elongate vessel of generally circular cross section which has an upper generally cylindrical part and a lower frusto-conical part which tempers towards its open lower end. The axis of the device is typically inclined to the horizontal plane at an angle approximating to the half cone angle of the lower frusto-conical part.

The upper cylindrical part of the vessel has a tangential inlet and it is also open at its upper end. A mixture of particulate material of different densities, such as crushed raw coal, and a dense medium, for example a magnetite/water slurry, is pumped into the separator via the tangential inlet so as to provide a helical flow of the pumped dense medium travelling around the wall of the separator towards the lower open end thereof. Alternatively the mixture is fed under gravity from an elevated feed tank, which is typically mounted about 10 metres above the cyclone so as to provide a sufficient hydraulic pressure at the inlet to the cyclone.A first vortex is created adjacent the vessel wall which exerts a centrifugal force on the coal and such particles, under the influence of which particles with density greater than that of the dense medium, which are sometimes termed "sinks", are carried towards the open lower end of the vessel. As the particles move down the vessel, the radius of the vessel wall decreases thus increasing the peripheral velocity of the vortex and increasing the centrifugal force exerted on the particles in the medium.

These "sinks" flow out from the separator through its open lower end along with a portion of the dense medium, the flow of which was responsible for the creation of the first vortex, at the upper end of the separating vessel. If a sufficiently high feed rate of the mixture is used not all of the dense medium can escape through the open lower end and the surplus forms a second vortex whose direction of flow is opposite to that of the first vortex and is created within the region defined by the first vortex. Under the influence of the second, inner vortex, particles with density less than that of the dense medium are carried towards the first end region of the separator. These "floats are recovered from the separator along with the dense medium through the first end.

If a pump is used to feed the mixture of particulate material and dense medium, a major disadvantage is the excessive wear on the internal parts of the pump and on the inner wall of the cyclone at the feed point caused by the abrasive particulate materials, and the technological problems involved with pumping larger particles of feed material. If an elevated feed tank is used a disadvantage is that a tall building is required to house the cyclone.

In another conventional design of cyclone, sold under the trade mark Dynawhirlpool, the raw material to be separated is fed, at a minimum head, optionally in admixture with some dense medium, through an axial feed tube, into the upper end of an inclined cylindrical vessel into whose lower opposed end region the dense medium is pumped. The dense medium is pumped tangentially into the lower end region of the cylindrical vessel so as to create a helical flow of dense medium travelling from the lower end toward the upper end of the vessel. Particles with density greater than that of the dense medium are carried under the influence of this vortex towards the upper end region of the vessel and then exit from the separating vessel.Particles with density less than that of the dense medium are not constrained by the action of centrifugal force to follow the dense medium flow around the wall of the separating vessel, but drift downwards through the central region of the separating vessel, in a direction opposite to the overall direction of flow of the vortex and exit the separating vessel at its lower end through an axial pipe or vortex finder which projects somewhat into the vessel.

This design overcomes the problems associated with excessive wear on the machinery by not requiring the abrasive raw material to be pumped into the separating vessel. It also avoids the necessity of having to erect a tall building to house an elevated feed tank. However, the separation of the dense particulate material achieved is generally inferior to that achieved with the Dutch States Mines design of cyclone.

In a development of this design introduced in approximately 1965 a longer cylindrical vessel is used with a transverse annular partition in which is mounted an axial vortex finder. This partition divides the vessel into upper and lower sections, each with an involute dense medium inlet at its lower end and an involute exit pipe for dense particles and dense medium at its upper end. Raw material, optionally in admixture with dense medium, is fed axially into the upper end of the upper section. "Floats" and dense medium from the upper section pass through the vortex finder into the lower section, while "sinks" and dense medium exit through the exit pipe at the upper end of the upper section.

Further separation occurs in the lower section with a particulate fraction of intermediate density exiting the upper end of the lower section through the involute exit pipe, whilst "floats" from the process exit the lower end of the lower section through the vortex finder.

A further design of cylcone was introduced in about 1981 under the trade mark Tri-Flo. This is generally similar in concept to that of the Dynawhirlpool design, except that there is no vortex finder projecting into the vessel from the central aperture therein.

It is an object of the present invention to provide an efficient means of separating dense particulate materials, wherein the abrasion on the machinery used by the abrasive raw material is minimised and the costs associated with the construction of buildings designed to house such separators are also minimised.

According to the present invention there is provided a separator for separating relatively dense particles from relatively light particles present in a particulate mixture comprising relatively light particles and relatively dense particles by use of a dense medium whose density is intermediate the densities of the relatively dense particles and of the relatively light particles respectively, said separator comprising an elongate vessel whose interior wall is substantially symmetrical about a longitudinal axis extending between first and second open ends, said vessel including a tapered vessel portion which tapers towards said second open end, particle feed means for supplying a particulate material to be separated axially into the vessel through said open first end, fluid inlet means adjacent said first open end for introduction of dense medium to the vessel with a tangential component of flow so as to induce vortex formation within the vessel, and collection means for collecting a mixture of dense medium and relatively light particles that exits the vessel through said open first end, the arrangement being such that incoming dense medium generates a first vortex substantially coaxial with the longitudinal axis of the vessel and with an overall direction of flow which serves to carry relatively dense particles towards said second open end, that relatively dense particles are separated under the action of the resulting centrifugal force from relatively light particles within said first vortex and exit the vessel together with a portion of the dense medium via said second open end, and that the remainder of the dense medium in excess of said portion forms a second vortex within the vessel substantially coaxial with the longitudinal axis and with an overall direction of flow which serves to carry relatively light particles towards said first open end and said collection means.

The invention further provides a method for separating relatively dense particles from relatively light particles present in a particulate mixture comprising relatively dense particles and relatively light particles by use of a dense medium whose density is intermediate the densities of the relatively dense particles and of the relatively light particles respectively which comprises feeding the particulate mixture into an elongate vessel whose interior wall is substantially symmetrical about a longitudinal axis extending between first and second open ends and includes a tapered vessel portion which tapers towards said second open end, said feeding step being effected axially through said first open end and into said vessel, supplying dense medium to said vessel at a predetermined rate adjacent said first open end with a tangential component of flow so as to induce vortex formation within said vessel, allowing a first vortex to be formed adjacent the interior wall of the vessel and coaxial with the vessel with an overall direction of flow towards said second open end, thereby to impose a centrifugal force upon particulate material in the dense medium, allowing dense medium in admixture with relatively dense particles to exit said vessel through said second open end at an exit rate of flow of dense medium less than said predetermined rate, allowing dense medium in excess of said exit rate of flow to form a second vortex within the first vortex with an overall direction of flow towards the first open end of said vessel and coaxial with the vessel, and recovering from the first open end of said vessel a mixture comprising relatively light particles and dense medium.

By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawing which is a vertical section through one embodiment of a dense medium separator in accordance with the present invention.

Referring to the drawing a dense medium separator comprises an elongate vessel 1 of circular cross section whose axis is inclined at an angle to the horizontal.

Vessel 1 has a cylindrical upper end portion 2 and a tapered lower end portion 3. An axial feed tube 4 projects through an opening 5 in the upper end of cylindrical portion 2 which is surrounded by a vortex finder 6 that projects somewhat into cylindrical portion 2. Preferably feed tube 4 projects into vessel 1 approximately as far as the junction between cylindrical vessel portion 2 and tapered vessel portion 3.

Elbow piece 7 connects the upper end of feed tube 4 to a chute 8 into which is fed a mixture of particulate material to be separated, e.g. a crushed raw coal comprising a mixture of coal pieces up to 10 cm across and of particles of shale. If desired, a proportion of the dense medium fluid can also be supplied to feed chute 8. Conveniently feed chute 8 is arranged to receive the particulate material from a desliming screen (not shown).

A collection device in the form of an overflow cap 9 is attached around the upper end of the vortex finder 6 and the feed tube 4 and is furnished with an outlet pipe 10.

Reference numeral 11 indicates support stays within overflow cap 9.

At its upper end cylindrical section 2 has an inlet pipe 12 for dense medium fluid, such as a water/magnetite slurry. This is connected either in an involute or tangential fashion to the vessel 1 so that the dense liquid medium enters vessel 1 tangentially so as to encourage vortex formation.

At its lower end vessel 1 has an opening 13. It also has an external flange 14.

In use dense particulate material, such as crushed raw coal or a crushed mineral ore, is fed, at minimum head, into vessel 1 through feed chute 8, optionally in admixture with some dense medium fluid. It is carried into vessel 1 via feed tube 4. A dense medium such as a water/magnetite slurry having a density of approximately 1.5, is pumped into vessel 1 via inlet pipe 12 to allow cyclonic flow of fluid medium into vessel 1. A first vortex of dense medium flowing helically from the upper cylindrical end portion 2 towards the tapered lower frusto-conical end portion of the separating vessel 1 is thus created. This vortex rotates about the axis of vessel 1 and exerts a centrifugal force on the particulate material.The relatively dense particles, e.g. shale, which have a density greater than that of the water/magnetite slurry or other dense medium fluid migrate to the vessel wall while the relatively light particles, whose density is less than that of the water/magnetite slurry or other dense medium fluid, migrate towards the axis of the vessel 1. In this way the relatively light particles are separated from the relatively dense particles in the dense medium fluid. As the mixture of dense medium and particles in the first vortex moves towards opening 13 it passes into tapered end portion 3 and the peripheral speed of the first vortex increases due to the reduction in diameter as the mixture passes towards opening 13. Because of the restricted size of opening 13 only a portion of the dense medium can exit vessel 1 through opening 13 if the feed rate of the dense medium exceeds a threshold value.

Relatively dense particles exit vessel 1 as "sinks" through opening 13 together with a proportion of the dense medium.

The remainder of the dense medium forms a second vortex within the first vortex and also coaxial with the vessel 1. This second vortex moves in the opposite overall direction to that of the first vortex, i.e. towards the upper end of vessel 1, carrying with it relatively light particles. This mixture of dense medium and relatively light particles, or "floats", exits vessel 1 through the annular passage 5 between vortex finder 6 and feed tube 4 and passes into overflow cap 9 from which it exits via exit pipe 10. In operation an air core extends from opening 13 to and around feed tube 4. Air can escape from the eye of the inner second vortex via annular opening 5. In this way the vortices are stabilised.

As illustrated the lower end of feed tube 4 is open. If desired the lower end of feed tube 4 can be fitted with a deflector (not shown) to cause the incoming particulate feed material to be deflected outwardly into the inner second vortex. In this way the risk of particulate material passing through vessel 1 along the air core and out of the opening 13 without being subjected to centrifugal action in the vortices is obviated. This modification may be particularly desirable if, for example, the axis of the vessel is inclined at a greater angle to the horizontal than is illustrated, for example at an angle of 300 to 400 to the horizontal.

Because the particulate feed material is dry or contains only a limited amount of liquid, it is not necessary to pass it to an elevated feed tank so as to develop a hydraulic head. Hence it is not necessary to construct a tall building to house an elevated feed tank.

It is also not necessary to pump the particulate feed material as a slurry. Hence larger particles can be included in the feed material having dimensions up to 10 cm or more. Since the particulate feed material is supplied to the eye of the inner second vortex it is not necessary to provide a hydraulic head in a feed slurry. Hence a relatively low inexpensive building can be used to house the illustrated separator.

It will be appreciated by those skilled in the art that in contrast to a conventional cyclone of the type originally developed by Dutch State Mines the raw coal or other particulate material to be separated is fed first not into the outer first vortex but into the air core of the second inner vortex. The cylindrical vessel portion provides a zone in which the particulate material can spend a significant residence time. Because the two vortices move in opposite directions axially of the vessel a circulation pattern can be set up whereby particles can be carried down the vessel wall adjacent the interior wall towards the opening 13 by the first vortex and then get carried back up again by the second vortex before diffusing out into the first vortex again. In this way efficient separation of light and dense particles can be achieved.

Claims (9)

1. A separator for separating relatively dense particles from relatively light particles present in a particulate mixture comprising relatively light particles and relatively dense particles by use of a dense medium whose density is intermediate the densities of the relatively dense particles and of the relatively light particles respectively, said separator comprising an elongate vessel whose interior wall is substantially symmetrical about a longitudinal axis extending between first and second open ends, said vessel including a tapered vessel portion which tapers towards said second open end, particle feed means for supplying a particulate material to be separated axially into the vessel through said open first end, fluid inlet means adjacent said first open end for introduction of dense medium to the vessel with a tangential component of flow so as to induce vortex formation within the vessel, and collection means for collecting a mixture of dense medium and relatively light particles that exits the vessel through said open first end, the arrangement being such that incoming dense medium generates a first vortex substantially coaxial with the longitudinal axis of the vessel and with an overall direction of flow which serves to carry relatively dense particles towards said second open end, that relatively dense particles are separated under the action of the resulting centrifugal force from relatively light particles within said first vortex and exit the vessel together with a portion of the dense medium via said second open end, and that the remainder of the dense medium in excess of said portion forms a second vortex within the vessel substantially coaxial with the longitudinal axis and with an overall direction of flow which serves to carry relatively light particles towards said first open end and said collection means.

2. A separator according to claim 1, in which said vessel includes a substantially cylindrical vessel portion adjacent said first open end and in which said tapered vessel portion is attached to the end of the cylindrical vessel portion opposite said first open end of said vessel.

3. A separator according to claim 2, in which said fluid inlet means is connected to said cylindrical vessel portion.

4. A separator according to any one of claims 1 to 3, in which a vortex finder tube extends through said first open end into said vessel.

5. A separator according to any one of claims 1 to 4, in which said particle feed means comprises a feed tube extending through said first open end and in which an annular opening surrounds said feed tube for exit of dense medium and relatively light particles.

6. A separator according to any one of claims 1 to 5, in which said particle feed means includes a feed chute connected thereto into which can be supplied particulate material to be separated, optionally together with a minor stream of dense medium.

7. A separator according to any one of claims 1 to 6, in which said collection means comprises an overflow cap into which dense medium and relatively light particles from said second vortex may overflow.

8. A separator according to any one of claims 1 to 7, in which said fluid inlet means comprises an inlet pipe connected in involute or tangential fashion to said vessel.

9. A method for separating relatively dense particles from relatively light particles present in a particulate mixture comprising relatively dense particles and relatively light particles by use of a dense medium whose density is intermediate the densities of the relatively dense particles and of the relatively light particles respectively which comprises feeding the particulate mixture into an elongate vessel whose interior wall is substantially symmetrical about a longitudinal axis extending between first and second open ends and includes a tapered vessel portion which tapers towards said second open end, said feeding step being effected axially through said first open end and into said vessel, supplying dense medium to said vessel at a predetermined rate adjacent said first open end with a tangential component of flow so as to induce vortex formation within said vessel, allowing a first vortex to be formed adjacent the interior wall of the vessel and coaxial with the vessel with an overall direction of flow towards said second open end, thereby to impose a centrifugal force upon particulate material in the dense medium, allowing fluid medium in admixture with relatively dense particles to exit said vessel through said second open end at an exit rate of flow of dense medium less than said predetermined rate, allowing dense medium in excess of said exit rate of flow to form a second vortex within the first vortex with an overall direction of flow towards the first open end of said vessel and coaxial with the vessel, and recovering from the first open end of said vessel a mixture comprising relatively light particles and dense medium.