Patent GB-791520-A (GB)
Title: Improvements in and relating to the wet screening of solid particles and apparatus therefor
IPC: B07B1/00
Filed: 1954-12-24
Abstract: 791,520. Filters. STAMICARBON N.V. Dec. 24, 1954 [Dec. 24, 1953 ; May 4, 1954 ; May 5, 1954 ; June 22, 1954 ; June 22, 1954; July 15, 1954; Sept. 7, 1954], No. 37323/54. Class 46. In a method of wet screening a mass of solid particles in which fractions of particles of different sizes are separated, or in which all the particles are separated from the liquid, the particles are entrained in a liquid medium flowing as a layer along the concave side of a screening deck which is curved along the line of flow and is formed by assembled spaced bars arranged perpendicularly to the line of flow, the curvature of the deck, the flow velocity or velocities and the spacing and shape of the bars being such that liquid substrata and particles (or liquid substrata only) are deflected by the bars and flow through the slots between the bars wholly or mainly by reason of their kinetic energy and are not deflected, or only negligibly so, by reason of gravitational attraction. The bars 3, Fig. 5, forming the screening deck may be supported at one or more positions between their ends by being looped around rods 13. The particles passing through the discharge end portion of the deck may become coarser than those passing through the feed end of the deck on account of the wear of the bars (by impingement of the suspension) being less towards the discharge end. To rectify this the particles collected in hopper 20, Fig. 6, from suspension supplied via reservoir 14, weir 16 and wall 18, are separated from those collected in hopper 19 by a partition 24, and recirculated. The proportion recirculated may be varied by pivoting the partition about axis 25. In Fig. 7 (not shown) the oversize particles from one screening deck orientated as in Fig. 6 are entrained in liquid fed to a second deck similarly orientated and located at the end of the first. In Fig. 8 (not shown) the second deck is orientated in the same way as 2a in Fig. 9. In Fig. 9, liquid is supplied at v and solid at u, the oversize particles being entrained by liquid sprayed at 35 onto the second deck 2a. For operation with a low velocity feed, e.g., 50 cm. /sec, the suspension may be supplied to a reservoir at the top of a single deck, the upper edge of the deck constituting the overflow weir, Fig. 10 (not shown). Filtration may take place under gas pressure in a closed casing. In Fig. 11 compressed gas may be fed in at 54 and air vented at 52. Pressures on both sides of the deck are maintained equal by pipes 46, 47, where the feeding means 50 comprises a number of contiguous pipes extending across the deck. Alternatively clearances may be left between the pipes themselves and the side walls. The direction of flow along the screening deck may be reversed periodically to render wear of the bars more uniform by mounting the deck in a box 62, Fig. 12, pivoted at 63 and capable of being rocked manually. Undersize particles are discharged through a side pipe 70. Alternatively, Fig. 15 (not shown) they may be discharged by a spout into a collecting tank located beside a trough for receiving oversize particles. In Fig. 16 periodic reversal of the flow direction is effected by movement of the suspension feeding-means 80. The feed velocity must be high enough to overcome the difference in level between the feed end and the centre of the deck and to prevent liquid and undersize particles which impinge on the underside of a hood 85 from passing back again through the deck. For a deck radius of 50 cm. a head of 2 metres of suspension is sufficient. The collecting tank 76 may be divided by partitioning into two parts for separating the undersize particles into two fractions as in Fig. 6. Optionally, to assist this division, a partition extending downwards to the deck may be provided in the middle of the underside of the hood 85. The first-mentioned partition may be constituted by flaps coupled to the feeding device 80 to be changed in position with the latter. In Fig. 20 raw coal containing fine particles is supplied to a separator 99 and magnetite suspension to effect the separation at 100. The shale fraction is discharged at 101 and the coal fraction to a curved screen deck 2 having slots, e.g., 1À5 mm. wide and bars, e.g., 3 mm. wide. The coarse particles from 2 are drained at the feed end of a vibrating screen 103, the drainings, and undersize particles from 2, being recirculated. The coal is washed on the remainder of the vibrating screen and discharged at 110. The dilute suspension from the reservoir 105 is supplied to a further curved screen deck 2 with slots, e.g., 0À3 mm. wide and bars, e.g., 1À2 mm. wide. The oversize particles are treated by a magnetic separator 112 to yield coal at 114 and magnetite, which, together with undersize particles from 2c, is passed to a thickener 115. Clarified liquid is discharged at 119 and thickened magnetite is returned to the separator 99. The embodiment of Fig. 21 (not shown) differs in detail from that of Fig. 20. In Fig. 22 fine coal is supplied at 130 and magnetite suspension at 131 to a reservoir 129 and thence to a hydrocyclone 133. The resulting fractions are drained and sprayed on vibrating screens 136 and 140. Shale particles of 1 to 8 mm. are discharged at 144 and coal particles of 1 to 8 mm. at 145. Suspension containing particles less than 1 mm. is discharged at 148 onto a curved screening deck 2f whence separation-suspension at 149 may be returned to the reservoir 129. Oversize particles from the screen 2f and suspension supplied at 150 are fed to a hydrocyclone 153 which discharges to two curved screen decks 2g and 2h supplied with clarified liquid at 157 and 161 respectively. The fine fractions consist of impure magnetite suspension withdrawn at 156. The coarse fractions are magnetically separated at 159 and 163 respectively, yielding fine shale at 164, fine coal at 160 and magnetic particles in suspension at 165. In all cases the material may be fed to the screen through a flat tube, e.g., of rubber ; the thickness of the layer flowing tangentially onto the screen may be regulated by a compressing device which controls the flattening of the tube. By appropriate selection of slot width in relation to a given size range of particles, and of the flow velocity, a suspension of solid particles may be wet screened so that a liquid fraction substantially free of solid particles is obtained through the slots.