GB2053736A

GB2053736A - Trommel screen

Info

Publication number: GB2053736A
Application number: GB7925320A
Authority: GB
Original assignee: English Clays Lovering Pochin Co Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1979-07-20
Filing date: 1979-07-20
Publication date: 1981-02-11
Also published as: GB2053736B

Abstract

Wet screening apparatus comprises a trommel 1 of screening material 21 disposed upon a supporting framework which is rotated about a substantially horizontal axis, a container 26 for liquid in which the trommel is partially immersed, the container being provided with overflow means 27 to maintain a substantially constant level of liquid in the container, an apertured tube 18, closed at one end and connected at the other end to a feed supply for introducing feed suspension under pressure into the inside of the trommel, the aperture(s) being positioned so that feed suspension impinges on the screening material 21 above the level of liquid in the container, means 22 for removing a film of liquid and oversize particles from the inner surface of the screening material and means 25 for discharging oversize particles from the inside of the trommel. The means 22 may comprise a resilient flexible blade and/or an air jet or water spray. <IMAGE>

Description

SPECIFICATION Trommel Screen This invention concerns an apparatus and method for removing oversize particles from a suspension of a particulate solid material in a liquid. The invention is especially useful when a particle size separation is to be made in the size range from about 40 to about 80 microns and when the oversize particles are of a "platy" nature. By "platy" we mean that the particles have a high aspect ratio, or, in other words, the average diameter of their largest face is much greater than their thickness.

Known methods for removing oversize particles from suspensions of particulate material generally use vibrating screens which comprise flat sheets of screening material, such as woven wire mesh, which are clamped in suitable frames which may be rectangular or substantially circular and are subjected to vibration by means of an electric motor coupled to an eccentric flywheel, or similar means.

Vibrating screens are generally operated so that the part of the screen mesh nearest to the point of introduction of the feed suspension is flooded with liquid, but, at the part of the screen nearest to the point from which oversize material is discharged, liquid is allowed to drain away from the oversize material retained on the screen so that the oversize material is discharged in a condition as free as possible from suspending liquid and fine particles. When the oversize particles are of a platy nature, it is commonly found that blinding of the apertures of the screen can occur through particles adhering to the upper surface of the screen. This adherence of particles is thought to be due to a surface tension effect.As suspending liquid drains away from the screening mesh, the continuous film of liquid which covers the upper surface of the screen near the flooded part, and which provides a well lubricated surface over which oversize particles can travel under the action of the applied vibrations, breaks up into a large number of discrete drops, one in each aperture of the mesh, and each having a shape approximating to that of a concave - convex or meniscus lens with the convex surface downwards. These drops continue to lose liquid under the action of gravity and of the vibrations until a limiting state is reached in which each drop has the form of a thin biconcave lens. The thickness of this lens is so small that the effect of gravity upon the drop becomes negligible and the drop is held in place by the surface tension of the liquid.If an oversize particle rests on the upper surface of the mesh above one of these small drops, especially if the oversize particle is platy and easily wetted by the suspending liquid, the lens of liquid will change its shape so that its upper surface conforms to the approximately planar lower surface of the particle and its lower surface has a radius of curvature of the order of one half of the aperture width of the mesh. It is well known that the surface tension forces in a curved liquid film are balanced by a pressure which is directed radially outwards and is inversely proportional to the radius of curvature. The effect of the approximately plano-concave lens of liquid adhering to the underside of the particle is to exert a force on the underside of the particle acting vertically downwards and holding the particle on to the mesh.This force acting on the particle is generally of sufficient magnitude to overcome the effect of the vibrations, and the particle is therefore held permanently in place thus blinding at least one aperture of the mesh.

The result of the effect described above is to reduce progressively the number of apertures which are available for flow of fine particles and suspending liquid. It has been found in practice that a conventional vibrating screen when the screen was in a clean condition gave a good throughput rate of screened suspension. However very quickly the apertures of the screen began to blind and the throughput rate decreased until, after about 40 minutes it levelled off at a value which was about one tenth of its initial value.

The object of this invention is to provide a screening apparatus and method by which a suspension of a particulate solid material in a liquid comprising platy oversize particles can be screened continuously over a long period with substantially no reduction in the throughput rate of screened suspension due to blinding.

In accordance with the invention there is provided wet screening apparatus comprising a trommel of screening material disposed upon a supporting framework which is rotated about a substantially horizontal axis, a container for liquid in which the trommel is partially immersed, the container being provided with overflow means to maintain a substantially constant level of liquid in the container, an apertured tube closed at one end and connected at the other end to a feed supply for introducing feed suspension under pressure into the inside of the trommel, the aperture(s) being positioned so that feed suspension impinges on the inner surface of the screening material above the level of liquid in the container, means for removing a film of liquid and oversize particles from the inner surface of the screening material and means for discharging oversize particles from the inside of the trommel.

The trommel may be cylindrical or preferably prismatic. A prismatic shape is preferred because a cylindrical trommel must either be constructed from a single piece of screening material, in which case, if a small tear is made in the material the whole piece of material must be replaced, or from curved panels of screening material which are difficult to make with an accurately cylindrical surface because the screening material tends to assume an anticlastic surface between supports.A further advantage of a prismatic trommel is that each flat panel of screening material performs a rocking motion relative to the surface of the liquid as it passes through the container and this has the effect of applying to the panel vibrations having both vertical and horizontal components, the frequency of which increased with the speed of rotation of the trommel, and helps to separate the oversize particles from the fine particles and from the suspending liquid.

The screening material is most conveniently a woven wire mesh, which is preferably of corrosion resistant material such as brass, bronze or stainless steel. Other screening materials such as perforated sheet metal, wedge wire screens or plastics or textile material may be used under certain circumstances.

The supporting framework from the trommel is also preferably corrosion resistant and may conveniently be of aluminium or stainless steel. The trommel is conveniently rotated by means of a variable speed electric motor, such as a shunt-wound D.C. motor or by a constant speed motor driving through a variable speed gearbox.

The liquid in the container will generally be the same as the liquid in which the feed material is suspended.

Most commonly the liquid will be water.

The apertured tube may be provided with one or more slots or with a plurality of holes. Preferably the width of the aperture(s) decreases from the end of the tube which is connected to the feed supply to the further, closed end. By this means the feed suspension is substantially uniformly distributed along the length of the trommel. Conveniently the apertured tube is positioned so that the feed suspension is caused to impinge on the inner surface of the screening material between the highest point ofthetrommel and the surface of the liquid on the descending side of the trommel.Preferably the feed suspension leaves the apertured tube at an angle of elevation in the range from 10 to 60". In this way a large part of the liquid and suspended fine particles passes immediately through the screen and only a minor portion falls inside the trommel. In this way the throughput rate is increased compared with a conventional trommel screen. The liquid and suspended fine particles which pass through the screen mix with the liquid in the container and overflow into a suitable receiver.

When the aperture size of the screening material is below about 120 microns, the oversize particles are preferably removed from the inner surface of the screening material by means of a resilient flexible blade positioned along the length of the trommel, advantageously at or near its highest point. An air jet or water spray alone may be used. The resilient, flexible blade acts as a "film continuation strip". In other words it picks the film of liquid and suspended oversize particles off the inner surface of the screening material and causes itto flow down its surface which faces the rising sideofthetrommel into a suitable receiving conduit.

The blade is advantageously made of unfilled natural rubber but many other elastomeric or plastomeric materials may also be used. The unsupported width of the blade is preferably equal to from three to five times the difference between the maximum and minimum radii of the polygonal cross section of the trommel and is most preferably about four times the difference between the radii. By "maximum radius" of a regular polygon we mean the distance from the centre to a corner and by "minimum radius" the distance from the centre to the centre of one side. This width provides adequate film removal without causing the blade to flex through a very small radius, and without exerting an undesirably high pressure on the screening material.

Conveniently the end of the trommel which is connected to the driving means is closed by an imperforate plate which is preferably reinforced with radial ribs in order to transmit the torque from the driving means uniformly to the periphery of the trommel. The other end of the trommel is preferably closed by a plate having an aperture through which pass the feed tube and the conduit for discharging the oversize particles together with the support member for the flexible blade. The level of liquid inside the trommel should not be allowed to rise to the lower edge of the aperture because under these conditions unscreened feed suspension will overflow into the containerforscreened liquid.

Another aspect of the invention concerns a method for separating oversize particles from a suspension of a particulate solid material in a liquid, where the suspension is fed continuously into a rotating trommel screen of the type hereinbefore described, screened suspension being discharged over the overflow means of the liquid container and oversize particles being collected from the inner surface of the screening material and discharged from the inside of the trommel.

Preferably, in order to prevent contamination of screened suspension by unscreened feed suspension, there are provided first sensing means to generate a signal when the liquid inside the trommel rises to a level a little below that at which contamination of screened suspension with unscreened feed suspension would occur, whereby the rotational speed of the trommel is increased, and second sensing means to generate a signal whereby the rotational speed is decreased again when the liquid inside the trommel falls to a suitable operating level.

This method is most useful when the feed suspension contains a proportion of platy oversize particles.

It has been found by experiment that the rate of production of screened suspension rises with the speed of rotation of the trommel, but the percentage by weight of oversize particles in the suspension discharged by the oversize conduit is decreased. This latter effect is probably due to the fact that oversize particles have less time to drain when the trommel is rotating faster. Preferably the feed suspension is deflocculated with a suitable dispersing agent if the percentage by weight of solids exceeds about 20%.

The suspension is preferably fed into the rotating trommel screen at a positive pressure of at least 30 kilonewtons per square metre.

For a better understanding of the invention, and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 is an isometric drawing showing the general arrangement of a trommel screen in accordance with the invention.

Figure2 is a cross section on the line II - II of Figure 3 Figure 3 is a cross section on the line Ill - Ill of Figure 2 Atrommel screen 1 comprises a framework of octagonal cross section comprising lateral members 2 of aluminium angle, longitudinal members 3 of aluminium strip bent longitudinally to an angle of 135", an imperforate first end plate 4 to which is bolted a hub plate 5 with reinforcing radial ribs 6, and a second end plate 7 which is provided with a circular aperture 8 of radius approximately one half of the maximum radius of the octagonal cross section.The hub plate 5 is keyed to a stub shaft 9 which is journalled in two plummer block bearing 10 and driven by a synchronous A.C. motor 11 through a variable speed gearbox 12, a pulley 13 on the drive shaft of the gearbox, driving belts 14 and a pulley 15 keyed to the stub shaft 9. The other end of the trommel is supported by an idler roller 16 which co-operates with the periphery of the circular aperture 8 at its highest point and is rotatably mounted on an upright steel beam 17.

Feed suspension is introduced into the inside ofthetrommel under pressure through a perforated pipe 18.

The diameters of the holes decrease progressively from the end which is connected to the source of supply to the further, closed end. The actual sizes of the holes depend upon the volumetric feed rate which is required. For example, if the feed rate required is in the range from about 500 to about 1200 litres per min. a tube having holes of the following diameters in millimetres, from the input end, is found to be suitable.

24,23,22, 21,20,19,18,17,16,15, 14,13,12 If the flow rate is in the range from about 150 to about 700 litres per min., a tube having holes of the following diameters in millimetres, again from the input end, should be used: 18,17,16,15,14,13,12,10,10,10,8.8 The feed tube is positioned so that the jets of feed suspension are projected towards the screening surface at an angle of elevation of 30 on the descending side of the trommel.

The screening surface is made up of eight rectangular panels 19 each of which comprises a framework 20 of aluminium angle and a rectangular piece of stainless steel mesh 21 which is secured to the framework by means of an epoxy resin. The panels are bolted to the trommel framework and are replaceable as separate units. A rubber seal 28 between each panel and the trommel framework ensure a watertight joint. The mesh is generally fine, i.e. having an aperture size between about 40 and 80 microns.

Oversize particles are removed from the insidde of the trommel by means of a blade 22 of natural rubber of thickness 5 - 6mm which is bonded to a support 23 of sheet aluminium which has an upper, planar portion 24 and a lower portion 25 which is bent to form a U-shaped trough for the collection and removal of the suspension of oversize particles. The trough is inclined so that the suspension runs down to and through the aperture 8. The unsupported width of the blade is four times the difference between the maximum and minimum radii of the octagon. A blade of this width found to remain in contact with the whole surface of each panel and to ride over the longitudinal members 3 without being flexed through an undesirably small radius of curvature and without exerting undue pressure on the screening material.This arrangement is effective in picking the film of oversize particles and suspending liquid off the inner surface of the screening material and causing it to flow down the planar portion of the support into the collection trough.

The lower part of the trommel rotates in a tank 26 provided with an overflow weir 27 which maintains a level of liquid in the tank which is approximately 1/ 2 of the maximum radius of the octagon below the axis of the trommel, so that at all times at least two panels of the trommel are submerged.

In operation a suspension of the feed material is delivered by means of a centrifugal pump (not shown) to the feed pipe 18 at a pressure of 85 kilonewtons per square metre and the trommel is rotated at a speed in the range from 10 to 25 rpm. Above 70% by volume of the feed suspension passes straight through the mesh of the trommel panels and falls into the tank 26. The remaining feed suspension which contains all the oversize particles runs down the inside surface of the trommel and some drainage of liquid and fine particles to the outside of each panel occurs as the panel descends towards the liquid level in the tank.

As each panel emerges from the far side of the pool of liquid, drainage occurs first from inside to outside, but as each panel passes the level of the axis of the trommel, the direction of drainage is reversed and a little liquid flows back through the mesh thus holding the oversize particles away from the surface of the mesh.

When the panel reaches the highest point of the circuit the thin film of liquid and suspended oversize particles is removed by the blade 22 and runs down the surface of the planar position of the support 23 on the side facing the rising side of the trommel and thence flows down the trough 25 and out through the aperture 8,An upper level probe (not shown) provides a signal if the level inside the trommel rises to just below the lowest point of the aperture 8, which signal increases the speed of rotation of the trommel, and a lower level probe (not shown) reduces the speed when the level inside the trommel falls to that of the liquid outside.

A typical trommel screen according to the invention has a length of 1 metre and a maximum radius of 500mm.

The invention is further illustrated by the following Examples.

Example 1 A deflocculated aqueous suspension contained 10.5% by weight of an English china clay having a particle size distribution such that 45% by weight consisted of particles having an equivalent spherical diameter smaller than 2 microns, 12% by weight of particles having an equivalent spherical diameter larger than 10 microns and only a trace of particles having a diameter larger than 53 microns. As the dispersing agent there had been used 0.3% by weight of sodium hexametaphosphate based on the weight of dry clay. To this suspension there was added 2% by weight, based on the weight of dry clay, of a flaky muscovite mica substantially all of which consisted of particles having a diameter larger than 53 microns.Samples of the resultant suspension were fed to a trommel screen of the type hereinbefore described having a length of 200mm and a maximum radius of 100mm and the speed of rotation of the trommel was varied in the range from 15 to 50 rpm. The trommel was fitted with stainless steel wire mesh of aperture size 53 microns. At each speed the maximum fed rate per unit area of screening surface (i.e. when the level of water inside the trommel just rose to the lower edge of the aperture 8), the flow rate of the aqueous suspension of oversize particles flowing down the trough 25 and the percentage by weight in the oversize suspension of particles having a diameter larger than 53 microns were measured.The results are set forth in Table 1 below: TABLE I Trommel Speed (r.p.m.) 15 20 25 30 35 40 Maximum Feed Rate (litres/Min/m2) 95 132 164 182 213 228 Oversize discharge rate (litres/min/m2) 3.4 5.1 7.0 9.1 11.2 13.1 %byweight+531lminoversize 10.7 10.2 9.6 9.2 8.8 8.4 suspension These results demonstrate that this maximum throughput rate can be increased, or the level of liquid inside the trommel lowered, by increasing the speed of rotation of the trommel. As the speed of the trommel is increased the proportion by weight of oversize particles in the oversize discharge suspension is decreased, presumably because the oversize particles have less time to drain before being removed from the screening surface.

Example 2 Aflocculated aqueous suspension contained 17.5% by weight of an English china clay of the same type as was used in Example 1. The percentage by weight of particles having a diameter larger than 53 microns was found to be 0.041%. This suspension was fed to the same trommel screen as was used in Example 1, rotating at15rpm.

The maximum feed rate was found to be 276 litres/min/m2, the corresponding oversize discharge rate was 5.0 litres/min/m2 and the percentage by weight in the oversize suspension of particles having a diameter largerthan 53 microns was 0.76%.

A portion of the same aqueous suspension was then deflocculated with 0.3% by weight, based on the weight of dry clay, of sodium hexametaphosphate and the resultant deflocculated suspension was passed through the trommel again rotating at 15 rpm. The maximum feed rate was now found to be 281/litres/min/m2, the corresponding oversize discharge rate 3.1 litres/min/m2 and the percentage by weight in the oversize suspension of particles having a diameter larger than 53 microns was 1.45%.

These results show that deflocculating the suspension give a small increase in throughput rate, reduces the amount of fine material lost in the oversize fraction and increases the concentration of oversize particles in this fraction.

Example 3 A flocculated aqueous suspension contained 18% by weight of an English china clay having a particle size distribution such that 25% by weight consisted of particles having an equivalent spherical diameter smaller than 2 microns, 25% by weight of particles having an equivalent spherical diameter larger than 10 microns and not more than 0.05% by weight of particles having a diameter larger than 53 microns. To this suspension there was added 2% by weight, based on the weight of dry clay, of a flaky muscovite mica substantially all of which consisted of particles having a diameter larger than 53 microns.Samples of the resultant suspension were fed at a pressure of 85 kilonewtons per square metre to a trommel screen of the type hereinbefore described having a length of 1 metre and a maximum radius of 500mm and the speed of rotation of the trommel was varied in the range from 14 to 24 rpm. The trommel was fitted with stainless steel wire mesh of aperture size 53 microns. At each speed the maximum feed rate per unit area of screening surface and the flow rate of the aqueous suspension of oversize particles flowing down the trough 25 were measured and the results are set forth in Table II below: TABLE II Trommel speed (r.p.m.) 14 17.2 20 22.5 24 Maximum Feed rate (litres/min/m2) 635 728 980 1520 1980 Oversize discharge rate (litres/min/m2) 7.1 9.7 12.7 15.4 18.6 By comparison the maximum throughput rate of conventional vibrating screens covered with the same screening material and handling the same type of feed suspension was found to lie in the range from 98 to 245 Litres/min/m2 after the screens had settled down to a steady state with a proportion of the apertures of the screening material blinded by oversize particles.

Claims

1. Wet screening apparatus comprising a trommel of screening material disposed upon a supporting framework which is rotated about a substantially horizontal axis, a container for liquid in which the trommel is partially immersed. The container being provided with overflow means to maintain a substantially constant level of liquid in the container, an apertured tube closed at one end and connected at the other end to a feed supply for introducing feed suspension under pressure into the inside of the trommel, the aperture(s) being positioned so that feed suspension impinges on the inner surface of the screening material above the level of liquid in the container, means for removing a film of liquid and oversize particles from the inner surface of the screening material and means for discharging oversize particles from the inside of the trommel.

2. Wet screening apparatus according to claim 1, wherein the apertured tube is provided with one or more slots.

3. Wet screening apparatus according to claim 1, wherein the apertured tube is provided with a plurality of holes.

4. Wet screening apparatus according to claim 2 or 3, wherein the width of the aperture(s) decreases from the end of the apertured tube which is connected to the feed supply to the further closed end.

5. Wet screening apparatus according to any one of claims 1 to 4, wherein the apertured tube is positioned so that the feed suspension is caused to impinge on the inner surface of the screening material between the highest point of the trommel and the surface of the liquid on the descending side of the trommel.

6. Wet screening apparatus according to claim 5, wherein the feed suspension leaves the apertured tube at an angle of elevation in the range from 10 to 609.

7. Wet screening apparatus according to any one of claims 1 to 6, wherein the means for removing a film of liquid and oversize particles from the inner surface of the screening material is an air jet or a water spray.

8. Wet screening apparatus according to any one of claims 1 to 6, wherein the means for removing a film of liquid and oversize particles from the inner surface of the screening material is a resilient, flexible blade used alone or in conjunction with an air jet or water spray.

9. Wet screening apparatus according to any one of claims 1 to 8, wherein the trommel is of polygonal cross section.

10. Wet screening apparatus according to claim 8 and claim 9 wherein the width of the resilient, flexible blade is equal to from three to five times the difference between the maximum and minimum radii of the trommel.

11. Wet screening apparatus according to claim 10, wherein the width of the resilient, flexible blade is about four times the difference between the maximum and minimum radii of the trommel.

12. Wet screening apparatus according to claim 8, 10 or 11, wherein the resilient flexible blade is of unfilled natural rubber.

13. Wet screening apparatus according to any one of claims 1 to 12, wherein the screen material is a corrosion-resistant woven wire mesh.

14. Wet screening apparatus according to claim 13, wherein the aperture size of the wire mesh is in the range from 40 to 80 micrometres.

15. A method for separating oversize particles from a suspension of a particulate solid material in a liquid, wherein the suspension is feed continuously into a rotating trommel screen of the type hereinbefore described, screened suspension being discharged over the overflow means of the liquid container and oversize particles being collected from the inner surface of the screening material and discharged from the inside of the trommel.

16. A method according to claim 15, wherein there are provided first sensing means to generate a signal when the liquid inside the trommel rises to a level a little below that at which contamination of screened suspension with unscreened feed suspension would occur whereby the rotational speed of the trommel is increased, and second sensing means to generate a signal whereby the rotational speed is decreased again when the liquid insided the trommel falls to a suitable operating level.

17. A method according to claim 15 or 16, wherein, if the solids content of the suspension of the particulate solid material in the liquid exceeds 20% by weight, the suspension is deflocculated with a suitable dispersing agent.

18. A method according to claim 15, 16 or 17, wherein the suspension is fed into the rotating trommel screen at a positive pressure of at least 30 kilonewtons per square metre.

19. A method according to any one of claims 15 to 18, wherein the particulate solid material contains a proportion of platy oversize particles as hereinbefore defined.

20. Wet screening apparatus as hereinbefore described with reference to the accompanying drawings.

21. A method for separating oversize particles from a suspension of a particulate solid material in a liquid, which method is as claimed in claim 15 and substantially as described in any one of the foregoing Examples.