A rotating screen

Abstract: A method of and apparatus for continuously separating relatively coarse particles from a suspension of mixture of particles in a liquid by impinging the suspension upon the surface of a rotating screen (12) so that liquid containing the relatively fine particles passes through the screen (12) and the relatively coarse particles are flung radially outwardly to the periphery of the screen. The preferred screen is a woven wire mesh with aperture sizes conveniently in the size range from about 0.020 mm to about 2.00 mm.

Description: This invention concerns an improved screening process and apparatus for separating coarse particles from a suspension of a particulate solid material in a liquid. Coarse particles are conveniently separated from a suspension of a particulate solid material in a liquid by means of a screen or sieve which generally consists of a perforated plate, a grid or grating or a mesh material. The mesh may be of metallic or plastics material. When the coarse particles to be separated are relatively small, i.e. in the size range from about 0.040mm to 1mm, a wire mesh material is the preferred separating medium. Especially when the suspension contains a relatively large proportion of solid particles of relatively coarse size it is generally necessary to agitate the screen, for example by applying vibration thereto by means of an electric motor connected to the screen through a suitable reciprocating linkage. Such means increase the cost of the screening operation and lead to more rapid wear and consequent tearing of the screen or sieve. The screening apparatus is also noisy in operation. Screening processes generally also suffer from the disadvantage that constant supervision is necessary because there is a tendency for the screens to blind or become blocked with solid particles which in turn causes unscreened suspension to overflow from the screens by the route intended for the oversize particles with consequent waste of fine particles. It is known to provide screening apparatus in the form of a trommel of screening material rotating about a substantially horizontal axis, which was intended to overcome the disadvantages associated with a vibrating screening device. Such an apparatus is described in British Patent No. 2053736. However the apparatus, although quieter and less susceptible to wear and fracture of the separating medium, suffered from blinding of the screen by coarse particles. A screening apparatus having coaxial conical or frusto-conical screens which are rotatable at different angular velocities and in different directions about a vertical axis is known from European Patent Specification No. 0278124. According to one embodiment described in that Patent specification a mixture of fine and coarse particles is dumped near the apex of an upper conical screen rotating with its apex upward. Particles retained on the screen move to the periphery of this screen and are transferred to a lower frusto-conical screen rotating with its imaginary apex downwards and in the opposite direction. Fine particles pass through the two rotating screening surfaces while the coarse particles are eventually discharged over the peripheral rim of the lower frusto-conical screen. The apparatus is designed for dry separations, for example for separating dust or ultrafine particles from cattle feed pellets [column 1, lines 46-49]. A screening apparatus having at least one circular screening member rotating in its own plane about a central axis is known from British Patent specification No. 2088750. However in that case the screening member consists of an array of radially extending bars or rods which together make up a grating and the apparatus is for dry separation of relatively coarse solid material such as coal. An apparatus and process for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid is disclosed in DE-A-2018677 which is considered to be the most relevant disclosure prior to the present invention; in this prior disclosure a planar screen presents an upwardly directed concave frusto conical surface and is mounted on and over the open top of a hollow drum so that the screen and drum are rotatable about a vertical axis. The suspension is deposited on the central region of the rotating screen for coarse particles in the suspension to move radially over the screen towards the periphery thereof while liquid containing relatively fine particles passes through the screen into the drum and therefrom through an opening in the bottom of the drum for collection. It is an object of the present invention to provide an apparatus and process for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid by subjecting the suspension to a screening medium and by which efficient separation may be effected as an improvement over the separation provided by the prior proposals, especially the disclosure in DE-A-2018677. According to the present invention there is provided a process for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid wherein the suspension is caused to impinge upon the surface of a substantially planar screening medium which is rotating in its own plane about a substantially vertical axis so that the relatively coarse particles are retained on the surface of the screening medium and are caused to move radially outwardly towards the periphery of the screening medium while the liquid containing relatively fine particles passes through the screening medium into an open top hollow drum which is beneath the screening medium and rotates about a substantially vertical axis, characterised in that said liquid passing through the screening medium is deposited on a closed lower end of the drum and said liquid in the drum is displaced from the drum during rotation thereof in a direction radially outwardly of its rotational axis through an array of holes provided in the periphery of the drum. Further according to the present invention there is provided apparatus for continuously separating relatively coarse particles from a suspension of a mixture of particules in a liquid by the process as specified in the immediately preceding paragraph and comprising a substantially planar screening medium mounted to be rotatable in its own plane about a substantially vertical axis; means for depositing said suspension onto said screening medium at a central region thereof; an open top hollow drum located beneath said screening medium and mounted to be rotatable about a substantially vertical axis and drive means for rotating said screening medium and drum whereby during said rotation relatively coarse particles in the solution deposited on the screening medium are caused to move over the screening medium radially outwardly of its axis of rotation to the periphery of the screening medium while liquid containing relatively fine particles passes through the screening medium into the drum characterised in that the drum has a closed lower end and is provided at its periphery with holes through which, during rotation of the drum, said liquid is displaced from the drum in a direction radially outwardly of the axis of rotation of the drum. In a preferred form the screening medium is rotated at an angular velocity such as to give a peripheral velocity of at least 2 ms⁻¹. The screening medium may be a perforated plate, a grating or a woven or welded mesh, made from metallic, ceramic or plastics filaments, but a woven wire mesh is especially preferred. The aperture size of the screening medium is conveniently in the size range from about 0.020mm to about 2.00mm but the invention is especially advantageous in cases in which a screening medium having an aperture size in the range from 0.020mm to 0.100mm is required. The screening medium is most suitably of circular planar shape. The peripheral margin of the screening medium may be impervious. For example a film of latex, or a thin plate adhered or welded to the surface of the screen may be provided, or, in the case of a perforated screen, a suitable peripheral band of the screen may be left unperforated. If desired two or more similar screening media in accordance with the invention may be provided in series and may, in particular, be disposed one above the other. The angular velocity of the screening medium is preferably such as to provide a peripheral speed of at least 7.5ms⁻¹. However, higher or lower peripheral speeds may be advantageous for certain applications. The screening medium or media may be rotated at changing speed cycles during the screening operation and the direction of rotation may be reversed at intervals. The suspension containing the mixture of particles is preferably caused to impinge on the surface of the rotating screening medium at or near the centre of rotation of the screening medium. The particles in suspension may be prevented from passing through the screen adjacent the periphery of the screen by the provision of an annular band of imperforate or impervious material. The process in accordance with the invention is preferably performed so that the coarse particles are allowed to overflow continuously from the periphery of the screening medium into suitable collecting and discharge means. However the process may also be conducted on a batchwise basis and, in this case, a suitable dam may be provided around the periphery of the screening medium against which the coarse particles build up. The screen may be vibrated in addition to being rotated, and the vibration may be effected ultrasonically. The efficiency of the screening device according to the invention depends on the peripheral speed of the screen. When the screen is stationary it rapidly blinds with coarse particles and the percentage by volume of the feed suspension which passes through the screen rapidly decreases. When the screen is rotated at relatively low speeds, i.e. with a peripheral speed of up to about 2ms⁻¹, most of the feed suspension is flung off the screen by centrifugal action, due to the at least partial blinding of the screen. Surprisingly, when the rotational speed of the screen is within a critical range from around 2ms⁻¹ to 10ms⁻¹ for many typical feed suspensions, there is a dramatic decrease in the amount of suspension which is flung off, with the result that most of the feed suspension then flows through the screen. In experiments it was found that, at an optimum peripheral speed for a particular feed suspension, up to 95% by volume of the feed suspension passed through the screen. In view of the relatively high viscosity of the feed suspension this is a most surprising result. In comparison with the conventional type of vibratory screen, the rotary screen is found to have about five times the capacity on the basis of flow rate per unit area. Also the wear and tear and the noise of conventional vibratory screens as well as the tendency for the screen cloth to rupture through fatigue are unacceptable and the use of a rotary screen overcomes these further disadvantages. One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is a diagrammatic view in section of a screening apparatus according to the invention. As seen in the drawing, a hollow drum 1, open at the top, is mounted on a boss 2 which is keyed and bolted on to the upper end of a shaft 3 which is supported in bearings 4 and 5 and rotated by means of an electric motor 6, the drive being transmitted by a belt 7 passing over pulleys 8 and 9. The upper rim of the drum 1 is provided with a flange 10 to which is bolted an annular member 11 to which is clamped a circular piece of woven wire mesh screen cloth 12 having a nominal aperture size of 0.053mm. Also clamped to the upper rim of the drum 1 is an annular skirt member 13 of L-shaped cross section over which coarse particles which are retained on the screen cloth and which are caused to move to the periphery of the screen cloth under centrifugal action pass into a collecting launder 14 having an outlet 15 is provided below and adjacent to the skirt member. The drum 1 is provided around the lower part of its periphery with a number of holes 25. The drum is also closed at its lower end and has a lower skirt member 26 beneath which is located a collecting launder 27 having an outlet 28. An inlet pipe 17, is provided at its lower end with a flange 18 and a hollow cylindrical shell 20 is suspended therefrom by bolts passing through spacing members 19. The shell is open at its upper end and its lower end is formed by a foraminous plate 21 in order to minimize the danger of damage which could occur to the screen cloth 12 by any large size particles falling directly on to it. The plate 21 is provided with holes to from 5mm. to 10mm. in diameter which is sufficient to retard the mixture on its passage to the rotating screen cloth. The mixture is generally screened through a sieve of approximately 2mm. before being introduced into the apparatus of the invention but the foraminous plate is nevertheless found to be advantageous and prolongs the life of the screen cloth. To further prolong the life of the screen cloth 12, one or more additional foraminous plates, of course screens, may be provided between the plate 21 and the screen cloth. In use, a mixture of coarse and fine particles, in suspension, is introduced through the pipe 17 and passes through the foraminous plate onto the rotating screen cloth 12 where the relatively fine particles pass through the screen into the interior of the drum 1 to pass through the holes 25 and thence via the launder 27 to the outlet 28. The relatively coarse particles, which do not pass through the screen cloth are urged, under centrifugal action, towards the periphery of the drum from where they pass over the annular skirt member into the launder 14 and thence to the outlet 15. EXAMPLE 1 An experimental screening apparatus of the type shown in the drawing was provided with a circular piece of woven wire mesh screen cloth of diameter 440mm and aperture size 0.053mm [No. 300 mesh British Standard sieve]. The feed was a flocculated kaolin suspension containing 20% by weight of dry kaolin in water, and this suspension was fed to the screening apparatus at two different flow rates of 0.114 m³.min⁻¹ [25gpm.] and 0.250 m³.min⁻¹ [55gpm.] respectively. The speed of rotation of the screen was 300 r.p.m. giving a peripheral speed of 6.9ms⁻¹. The screening efficiency or percentage by weight of particles larger than 0.053mm contained in the feed which are removed by the screening device, was determined for each flow rate and the results are set forth in Table I below. Flow Rate [m³.min⁻¹] % by wt. of + 0.053mm particles removed 0.114 92 0.250 92 These results show that the efficiency of the screening device remains substantially constant over a wide range of feed rates. EXAMPLE 2 The screening device used in Example 1 was fed with a similar feed suspension to that used in Example 1 at a flow rate of 0.182 m³.min⁻¹ [40gpm.], but five different experiments were performed at five different speeds of rotation of the screen. In each case the underflow rate, or the rate of flow of suspension passing through the screen was measured and the percentage by volume of the feed suspension which appeared in the underflow was determined. The results are set forth in Table II below:- Rotational Speed [rpm] Peripheral Speed [ms⁻¹] Underflow Rate [m³.min⁻¹] % by volume of underflow 300 6.9 0.134 73.5 350 8.1 0.152 84.0 400 9.2 0.164 90.0 450 10.35 0.162 88.7 485 11.2 0.158 86.7 These results show that for this particular feed suspension the optimum rotational speed of the screen is about 400 r.p.m. giving an optimum peripheral speed of about 9ms⁻¹. EXAMPLE 3 The screening device used in Examples 1 and 2 was fed with a flocculated kaolin suspension containing 17% by weight of dry kaolin in water. The rotational speed of the screen was maintained constant at 400 rpm giving a peripheral speed of 9.2ms⁻¹ and the suspension was fed to the screen at three different flow rates. For each flow rate the percentages by weight of particles larger than 0.053 mm in the feed and in the underflow respectively and the percentage by volume of the feed suspension which appeated in the underflow were determined and the results are set forth in Table III below:- Flow rate [m³.min⁻¹] % by volume of underflow wt. % + 0.053mm particles in feed wt. % + 0.053mm particles in underflow 0.091 90.5 0.0138 0.0002 0.136 89.0 0.0138 0.0003 0.182 87.5 0.0138 0.0002 These results show that the quality of the underflow is not affected by changes in feed rate over the range covered by this experiment and the percentage by volume passing through the screen is only very slightly affected. EXAMPLE 4 The screening device used in the foregoing Examples was installed in a factory and run continuously over a period of several weeks. The rotational speed was maintained constant at 365 rpm giving a peripheral speed of 8.4ms⁻¹ and the feed rate was maintained constant at 0.182 m³.min⁻¹ [40 gpm]. Each working day several samples were taken from the feed, bulked together and tested for percentage by weight of particles larger than 0.053mm. The underflow was sampled and tested in the same way, and the average percentage by volume of underflow for each day was determined. The results are set forth in Table IV below:- Day % by wt. of kaolin in feed suspension % by volume of underflow wt. % + 0.053mm in feed underflow 1 16 96 0.0130 0.0005 2 17 96.5 0.0081 0.0006 3 16 94.5 0.0028 0.0005 4 16 93.7 0.0036 0.0007 5 16 95.3 0.0032 0.0006 6 17 95.0 0.0044 0.0005 7 15 90.1 0.0048 0.0009 8 17 93.6 0.0041 0.0004 9 18 93.7 0.0061 0.0004 10 17 93.5 0.0051 0.0001 11 17.5 93.4 0.0043 0.0004 12 19 92.3 0.0451 0.0006 13 18 90.9 0.0043 0.0006 14 20 92.1 0.0132 0.0006 These results show that the screening device in accordance with the invention reduces the percentage by weight of particles larger than 0.053mm to very low levels when operated under factory conditions for long periods of time. It was observed that the screen showed no sign of blinding and at no stage needed to be brushed by hand to remove accumulated coarse material. It is well known that conventional vibrating screens blind rapidly and may require frequent brushing of the surface. EXAMPLE 5 The throughput of the rotating screen was compared with the throughput of two commercially available vibrating screens. The first of these was of an old design, and had a rectangular screen cloth of area 0.4m². When fed with a suspension of flocculated kaolin at 15wt% solids content, the screen had a maximum capacity of 4.8m³h⁻¹. Therefore the screening capacity is 12m³h⁻¹ of slurry per square metre of screen area. The second vibrating screen was of modern design with a circular screen cloth 1.17 metres in diameter. It was fed with a very similar kaolin suspension to the first vibrating screen. The maximum flow rate that could be sustained without slurry overflowing the screen was 16.4m³h⁻¹. Therefore the screening capacity is 15.2m³h⁻¹ of slurry per square metre of screen area. The rotating screen was fed with a very similar kaolin suspension to the two vibrating screens described above. The maximum flow rate that could be reliably sustained for long periods of operation was 13.6m³h⁻¹. Since the screen area was 0.15m², the screening capacity of the rotating screen is 80m³h⁻¹ of slurry per square metre of screen area. Therefore it is concluded that the rotating screen has at least 5 times the screening capacity of a vibrating screen, on an equal area basis. EXAMPLE 6 The screening device used in the foregoing Examples was fed with a flocculated suspension of marble powder which had been ground to a particle size distribution such that 75% by weight consisted of particles having an equivalent spherical diameter smaller than 2 microns. The suspension contained 30% by weight of dry marble in water. Portions of the marble suspension were fed to the screening device at two different feed rates for each of three different speeds of rotation of the circular screening surface. For each feed rate and rotational speed, the percentages by weight of particles larger than 0.053mm in the feed and underflow, respectively, and the percentage by volume of the feed suspension which appeared in the underflow were determined and the results are set forth in Table V below:- For example, because the linear velocity of the central portion of the screen is very small, it may be advantageous to provide an inverted conical member between the foraminous plate and the screen in order to ensure that the mixture passing down the feed-in pipe is initially spread some distance from the vertical axis of rotation of the screen. Although the mesh screen shown in the drawing is a flat screen , it may in fact be conical in form or dished, and the apex or convex side may face upwardly or downwardly. In a further modification, the screen 12, as well as rotating, may be vibrated, for example, by ultrasonics. It is sometimes found to be advantageous to rotate the screen at varying speed cycles, and a control system may be provided to that end. Thus, the power input to the electric drive motors may be provided with an overriding or manual control. The programme may be arranged to vary the peripheral speed from zero to a maximum of, say 15ms⁻¹. It may also be arranged to vary the direction of rotation if so desired.

Claims: A process for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid wherein the suspension is caused to impinge upon the surface of a substantially planar screening medium (12) which is rotating in its own plane about a substantially vertical axis so that the relatively coarse particles are retained on the surface of the screening medium (12) and are caused to move radially outwardly towards the periphery of the screening medium while the liquid containing relatively fine particles passes through the screening medium into an open top hollow drum (1) which is beneath the screening medium (12) and rotates about a substantially vertical axis, CHARACTERISED IN THAT said liquid passing through the screening medium is deposited on a closed lower end of the drum (1) and said liquid in the drum (1) is displaced from the drum (1) during rotation thereof in a direction radially outwardly of its rotational axis through an array of holes (25) provided in the periphery of the drum (1). A process according to claim 1 wherein the screening medium (12) is rotated at changing speed cycles during the screening operation. A process according to either claim 1 or claim 2 wherein the suspension containing the mixture of particles is prevented from passing through part of the screening medium (12) by reason of an impervious annular band (11) adjacent the periphery of the screen. A process according to any of the preceding claims conducted on a batch-wise basis and a suitable dam is provided around the periphery of the screening medium (12) against which the coarse particles build up. A process according to any of the preceding claims wherein the suspension containing the mixture of the particles is assisted in its passage through the screening medium (12) by vibration of said screening medium in addition to its being rotated. Apparatus for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid by the process as claimed in claim 1 and comprising a substantially planar screening medium (12) mounted to be rotatable in its own plane about a substantially vertical axis; means (17) for depositing said suspension onto said screening medium (12) at a central region thereof; an open top hollow drum (1) located beneath said screening medium (12) and mounted to be rotatable about a substantially vertical axis and drive means (6, 7) for rotating said screening medium (12) and drum (1) whereby during said rotation relatively coarse particles in the solution deposited on the screening medium (12) are caused to move over the screening medium radially outwardly of its axis of rotation to the periphery of the screening medium (12) while liquid containing relatively fine particles passes through the screening medium into the drum (1) CHARACTERISED IN THAT the drum (1) has a closed lower end and is provided at its periphery with holes (25) through which, during rotation of the drum (1), said liquid is displaced from the drum in a direction radially outwardly of the axis of rotation of the drum. Apparatus as claimed in claim 6 in which the screening medium (12) is substantially flat. Apparatus as claimed in either claim 6 or claim 7 in which the screening medium (12) is carried on an upper rim (10) of the open top hollow drum (1). Apparatus according to any one of claims 6 to 8 and comprising a foraminous plate (21) through which said suspension is deposited on the screening medium (12) at or near to the centre of rotation of the screening medium. Apparatus according to any one of claims 6 to 9 and comprising means for vibrating the screening medium (12) in addition to rotation thereof. Apparatus according to any one of claims 6 to 10 wherein there is provided a suitable dam around the periphery of the screening medium (12) against which coarse particles may build up. Apparatus according to any of claims 6 to 11 wherein the screening medium (12) is provided with an impervious or imperforate annular band (11) adjacent its periphery. Apparatus according to any one of claims 6 to 12 in which the lower end of the drum (1) has a lower skirt member (26) over which liquid displaced through the holes (25) passes to enter a lower collecting launder (27) having an outlet (28). Apparatus according to any one of claims 6 to 13 in which the upper end of the drum (1) carries an annular skirt member (23) over which relatively coarse particles in the solution displaced from the periphery of the screening medium (12) pass to enter an upper collecting launder (14) having an outlet (15).

Inventors: JONES THOMAS RICHARD, PHILLIPS REGINALD LEYLAND, JONES, THOMAS RICHARD, PHILLIPS, REGINALD LEYLAND
Applicants: ECC INT LTD, ECC INTERNATIONAL LIMITED
Application year: 1990
Main classification: B07B1
IPC codes: B07B1