Centrifuge with cake churning

Abstract: A centrifuge includes a conveyor for moving cake along a cake flow path towards a cake discharge opening and further includes a baffle mounted to the conveyor and disposed along the cake flow path. The baffle is provided on an upstream side, facing substantially away from the discharge opening with a concave profile or surface. This concave profile or surface serves to direct a portion of the cake which is headed downstream along the cake flow path, into a recirculation or churning path directed partly back towards a pool. The recirculation or churning of the particulate material facilitates an enhanced washing thereof and improves the removal of solutes or undesirable impurities from the cake.

Description: The invention relates to a centrifuge comprising a bowl having an inner surface including at least one solid bowl section and at least one screen section, and conveying means to push a cake along a cake flow path to a discharge port. Such a centrifuge is disclosed in US 4 634 536 A. This centrifuge is a decanter-type centrifuge provided with a screen section for washing the cake before it is discharged. Industrial centrifugation processes for separating particulate material from various impurities include sedimentation and filtration. Generally, the particulate material is produced as a cake having different degrees of moisture depending on the type of particulate material and the particular separation process. The cake constitutes a heavy phase whereas a filtrate or centrate constitutes a light phase. In some applications, a mother liquor displaced from the dewatered cake by a washing process in a centrifuge is the valuable component while the cake is the reject. In other applications, resins or crystals in the cake are the valuable product, impurities in the cake being removed with the filtrate or centrate. A decanter-type centrifuge has a conveyor in the form of one or more helical screw wraps rotating at a slightly different angular velocity of the velocity of the bowl or outer wall. Where the bowl has a solid wall with a cylindrical shell followed by a conical shell or beach and extends from a clarifier pool at an input or feed end of the centrifuge to a cake discharge opening or openings at an output end of the centrifuge, the centrifuge is known as a decanter or a solid bowl. A sedimentation process occurs in the cylindrical portion of the centrifuge and a dewatering of the cake in the conical dry beach area. Where the bowl is provided with one or more screen sections downstream and outside of the clarifier pool, the decanter-type centrifuge is known as a screenbowl centrifuge and performs a filtration process. Another kind of filtration centrifuge is a pusher or pusher basket. Such a centrifuge includes a first cylindrical basket at an input end of the centrifuge and a second cylindrical basket of greater diameter at a cake output end of the centrifuge. The baskets rotate at a high angular speed. In addition, the baskets of this two-stage basket system are longitudinally reciprocatable relative to one another, whereby pusher plates shove the heavy phase particulate material in a layer along the first basket, from the first basket to the second basket, and along the second basket to a cake discharge port. Single-stage pushers or pushers with two-or-more stages such as quadruple-stage pushers are also available. Filtering centrifuges have been used to wash the cake to remove the impurities. There are two types of washing: a spray wash and a flood wash. In a spray wash, wash liquid is applied to a localized area on the cake surface in an attempt to displace mother liquor which contains the impurities. Spray washing is used most commonly in a screenbowl centrifuge where the cake height varies across the screen from a thin layer to a thick layer adjacent to the pressure face of the conveyor blade. Another kind of centrifuge, used particularly for the dewatering and washing of thickened slurries with particulate solids, is a conical-screen centrifuge. The centrifuge wall includes a conical screen which has an increasing diameter in the cake flow direction. The particulate solids are held by the screen as the liquid filters through. The conical screen has the advantage that the cake experiences an increasing centrifugal gravitational force as the cake travels down to the large diameter of the cone. The centrifugal gravity is proportional to the radius of the screen for a given rotational speed of the basket. Another advantage of the increasing-diameter conical screen is that, for a given cake mass, the cake height and thus the resistance to liquid drainage are reduced as the cake moves towards the large-diameter end of the cone, owing to the conservation of mass. Both of these advantages enhance the dewatering of the cake. Also, spray washing is used in conical-screen centrifuges to remove impurities dissolved in the mother liquor. In a conical-screen centrifuge a thickened or concentrated feed is introduced, after pre-acceleration to the proper tangential speed, into the centrifuge at the smaller end of the conical screen. The cake travels down the cone when the half cone angle, typically 30° to 40° with respect to the axis of the machine, is steep enough to overcome frictional forces. When the cone angle is small, typically 15° to 25°, a mechanical conveyance mechanism is used to convey the cake from the small end of the cone to the large end thereof. One mechanism is a helical screw conveyor with a single continuous lead. Another, related, mechanism is a multiple-lead screw conveyor (4 leads is common). Yet another mechanism is a set of discrete scraper blades each conforming to a helix. In any case, the conveyor rotates at a differential speed as compared to the screen, thereby conveying cake down the screen. By adjusting the differential speed, the cake movement and concomitantly the cake residence time can be adjusted. Another mechanism is a vibrator, such as rotation of eccentric weights with an axis of vibration parallel to the axis of the machine. The inertia force generated by the vibration propels the cake from the small end to the large end, the discharge end, of the centrifuge. Pusher centrifuges are excellent for washing crystals for particles having a size greater than 75-100 microns, while screen bowls provide adequate washing when the mean particle size of the processed crystals is larger than 45 microns. For chemical applications such as in fine resins separation where particles are in the 5 to 30 micron range, both types of equipment are limited by fine solids passing through the screen. Instead, batch perforate basket centrifuges are used with a filter cloth having fine openings to prevent loss of fine solids in the filtrate. Batch processes, however, require the use of surge tanks for interim storage and introduction of the feed, which may be unacceptable in certain applications. Also, with both batch and continuous centrifugal filters, the centrifugal force is limited to a maximum of 1000-2000g, which is inadequate for dewatering fine particles with low-permeability cake. Furthermore, the moisture trapped in the capillaries of the cake for the batch basket can be significant, especially for fine particles. This is compensated in part in the batch basket process by providing a long washing and dewatering time, with the result of a lower solids throughput. Solid-bowl decanters have been used for washing fine resins without the disadvantage of losing the fine particles. In one application, the resin slurry after exiting a reactor is introduced into a decanter centrifuge wherein the cake is first dewatered in a dry beach area and subsequently washed with an appropriate liquid to displace the cake mother liquor (the valuable part), which flows back to the pool. The mother liquor is then discharged with the centrate. The cake (reject part) is dewatered before discharge. In another application using a solid-bowl centrifuge, the resin or crystal solids are the valuable component. By washing, the impurities in the cake are reduced before the cake is discharged from the solid-bowl centrifuge. The impurities dissolved in the wash liquid leave the machine with the centrate. However, with the solid bowl, the washing which takes place in the dry beach after the cake has been conveyed by the screw is limited as the retention time is very short, on the order of a few seconds or less. The most important disadvantage is that the wash liquid together with the impurities or valuables in the mother liquor are conveyed out with the cake. This limits the use of solid bowls in cake washing. EP 0 787 532 A1 discloses a decanter centrifuge comprising a bowl provided with a cake discharge opening on one end and a liquid phase discharge opening at the other opposite end. The bowl has a cylindrical portion and a beach portion disposed between the cylindrical portion and the cake discharge opening. Inside the bowl a conveyor including a helical screw is disposed. For impeding the cake flow to the cake discharge opening a flow control structure in form of a weir or a baffle plate is provided proximate to the cake discharge opening. It is the object of the invention to provide a centrifuge which allows to obtain very dean cake with a highly reduced content of impurities. This object is obtained with a centrifuge having at least first and second gates, whereby the bowl is separated into at least first and second compartments and establish entrance and exit openings for the compartments between the bowl and their free edges, wherein the first compartment along the cake flow path upstream of the first gate is a reslurrying compartment including the solid bowl section and being provided with a wash liquid feed, and wherein the second compartment between the first gate and the along the cake flow path downstream second gate is a dewatering compartment by including the screen section. Preferably the solid bowl section extends past the first gate for preventing the discharge of fine particulate matter through the screen section. Advantageously the dewatering compartment defined by the screen section and the gates is provided with spray nozzles. For optimizing the desired cake flow, the gates are radially adjustable. It is convenient that the entrance gate has a radial adjustment that permits only the cake adjacent to the along the cake flow path upstream solid bowl section to pass to the subsequent dewatering compartment, which cake layer is very dry. Preferably a plurality of along the cake flow path subsequent reslurrying compartments and dewatering compartments are provided. The one or more reslurrying compartments and the one or more dewatering compartments are advantageously provided along a cylindrical bowl wall. The conveying means of the centrifuge may consist of a hub and screw wraps. With this configuration the gates extend outwardly from the hub and are contiguous with and radially movably connected to the screw wraps. The gates may extend substantially perpendicularly to the screw wraps. In this case, considering the helix geometry, the gates can be located such as to form reslurrying compartments having bowl walls which are completely solid. Due to the helix geometry there may exist triangular perforate or solid wall areas in the reslurrying compartments and the dewatering compartments, respectively. In order to form reslurrying compartments having radially outer walls which are completely solid and dewatering compartments whose radially outer walls are entirely perforated, these triangular wall areas have to be replaced by corresponding triangular solid or perforate wall areas. This problem does not exist, if circumferential gates are used for producing the reslurrying and dewatering compartments. Of course, one or more reslurrying compartments and one or more dewatering compartments can be provided along a conical beach section of the bowl. Furtheron, the plurality of solid bowl sections alternating with a plurality of perforate screen sections may extend circumferentially in baskets of a pusher-type centrifuge, wherein a plurality of circumferential and radial gates are disposed at junctions between adjacent solid bowl and perforate screen sections. Preferably at least one of the gates can be provided on an along the cake flow path upstream side, facing away from the downstream discharge opening, with a concave surface. The invention will be further described referring to the accompanying drawings. Fig. 1 is a schematic longitudinal cross-sectional view of a screenbowl centrifuge in accordance with the present invention. Fig. 2 is a schematic longitudinal cross-sectional view of a screenbowl centrifuge which may be provided with a baffle or gate in accordance with the present invention. Fig. 3 is a schematic unwrapped or developed view of a screenbowl centrifuge optionally provided with baffles or gates in accordance with the present invention, showing the placement of the baffles or gates relative to alternating cylindrical solid bowl sections and screen sections. Fig. 4 is a schematic unwrapped or developed view similar to Fig. 3, showing an alternative placement of baffles or gates relative to alternating cylindrical solid bowl sections and screen sections. Fig. 5 is a schematic unwrapped or developed view similar to Fig. 3, showing modifications to the cylindrical solid bowl sections and screen sections of Fig. 3. Fig. 6 is a schematic unwrapped or developed view similar to Fig. 3, showing an alternative solid bowl sections and screen sections having a different width relative to a distance between successive conveyor screw flights. 3 Fig. 7 is a schematic unwrapped or developed view similar to Fig. 3, showing alternative gates extending circumferentially, rather than perpendicularly to conveyor flights. Fig. 8 is a schematic partial longitudinal cross-sectional view of another embodiment of a centrifuge optionally provided with baffles or gates or churning vanes in accordance with the present invention, showing reslurrying and separation in conical and cylindrical bowl sections above an annular separation pool. Fig. 9 is a schematic longitudinal cross-sectional view of a two-stage pusher-type centrifuge which may be provided with baffles in accordance with the present invention. Fig 10 is a schematic unwrapped or developed view of a conical-screen centrifuge, showing the placement of gates relative to alternating cylindrical imperforate sections and screen sections. Fig. 11 is a schematic unwrapped or developed view of another conical-screen centrifuge, showing the placement of gates relative to alternating cylindrical imperforate sections and screen sections. Fig 12 is a schematic unwrapped or developed view of yet another conical-screen showing the placement of gates relative to alternating cylindrical imperforate sections and screen sections. Fig. 13 is a schematic longitudinal cross-sectional view taken along line XVI-XVI in Fig 12. Fig. 14 shows a first embodiment of a gate in cross-section. Fig. 15 shows a second embodiment of a gate in cross-section. As diagrammatically illustrated in Fig. 1, a screenbowl centrifuge includes a bowl 66 with a cylindrical section 68 connected on one side to a transverse wall 70 and on an opposite side to a conical beach 72. Transverse wall 70 is provided with a liquid discharge opening 74 having a radial location which defines the depth of a clarifier or separation pool 76. A conveyor 78 having one or more helical screw wraps 80 rotates at a slightly different velocity from that of bowl 66 to push particulate material or cake 82 from pool 76 and along beach 72 to a smaller-diameter cylindrical bowl section 84 disposed on a side of beach 72 opposite to pool 76 and cylindrical section 68. During the negotiation of beach 72, the particulate material or cake 82 is dewatered, with excess fluid flowing back into pool 76. In response to the differential speed of conveyor 78 relative to bowl 66, cake 82 is pushed along cylindrical bowl section 84 and over a cylindrical screen section 86 to a cake discharge port at 88. During its passage over screen section 86, cake 82 is dewatered owing to desaturation, i.e., the ejection of liquid through screen section 86, as indicated by arrows 90. Screen section 86 is flanked on an upstream side by an inlet gate 92 and on a downstream side by an outlet gate 94, as determined by the direction of cake flow. Gate 92 cofunctions with the differential speed of conveyor 78 relative to bowl 66 to control the amount of cake released to screen section 86, while gate 94 cofunctions with the differential speed of conveyor 78 relative to bowl 66 to control the retention time of the cake on the screen and the rate of cake discharge through port 88. It is to be noted here that cylindrical bowl section 84 extends past gate 92, for preventing the discharge of fine particulate matter through screen section 86. Upstream of gate 92, flood wash feed nozzles 96 are provided for reslurrying the particulate material or cake 82 after the initial dewatering and prior to further dewatering in the screen area of the bowl. This flood wash fluidizes cake 82 and also serves to enhance and displace the contaminated mother liquor, which flows back down into clarifier pool 76. Accordingly, the cake conveyed downstream past gate 92 to screen section 86 has a reduced impurities content. Gate 92 permits only the cake layer adjacent to cylindrical bowl section 84 to pass to the subsequent dewatering stage. This cake layer is the driest. Exit or outlet gate 94 controls the cake profile at the outlet end of screen section 86 and also controls the residence time of the cake on screen section 86 so as to maximize the dewatering of the cake on screen section 86. Gates 92 and 94 separate centrifuge bowl 66 into two compartments. In the first compartment, upstream of gate 92, dewatering of cake 82 occurs on beach 72 and reslurrying takes place on cylindrical bowl section 84. In the section compartment, between gates 92 and 94, dewatering is effectuated via screen section 86. It is to be noted that gates 92 and 94, as well as similar gates disclosed hereinafter are radially adjustable, as indicated by arrows 98 and 100. It is contemplated that the radial positions of gates 92 and 94 are adjustable from outside the machine without requiring a dismantling thereof. Various mechanisms for implementing such adjustability are disclosed in U.S. Patent No.5,643,169. The dewatering compartment defined by screen section 86 and gates 92 and 94 is optionally provided with spray nozzles 102. The spray wash provided by nozzles 102 is more effective than conventional spray washes insofar as the cake over screen section 86 is more uniformly distributed than in conventional centrifuges owing to the leveling effect of gate 92. Furthermore, gates 92 and 94 can assume profiles similar to those illustrated in Figs. 14 and 15 to enhance churning of the cake to set up a large recirculation flow loop to enhance cake washing. As illustrated in Fig. 2, a screenbowl-type decanter centrifuge has a bowl 104 with a first cylindrical solid bowl section 106 connected at one end to a conical beach 108 in tum connected to a second cylindrical solid bowl section 110 to be followed by alternating screen and solid wall sections downstream. Bowl section 110 is contiguous, on a downstream side as determined by a direction 112 of flow of a layer of particulate material or cake 114, with an annular or cylindrical screen section 116. On a downstream side of screen section 116 is provided another cylindrical solid bowl section 118 and then another annular or cylindrical screen section 120. A series of radially adjustable gates, 122,124, 126, 128, and 130 depending from a hub 132 of a conveyor 134 are positioned generally at the junctions between the successive cylindrical bowl sections 110, 116, 118, and 120 downstream of beach 108 to define therewith a series of compartments 136, 138, 140, and 142. Compartments 136 and 140 are provided with nozzles or passageways 144 and 146 (diagrammatically represented by arrows indicating fluid flow) extending through the conveyor hub for introducing wash liquid into those compartments for purposes of reslurrying cake 114 during its transit along a cake flow path (see direction of flow arrow 112) from a clarifier pool 148 at one end of the centrifuge to a cake discharge opening or openings 150 at an opposite end of the centrifuge. Conveyor 134 includes one or more screw wraps 152 for pushing cake 114 along the cake flow path identified by cake flow direction 112. As described in detail hereinafter, gates 122, 124,126,128, and 130 are contiguous with and movably connected to conveyor wraps 152. Gates 122,124, 126,128, and 130 establish entrance and exits openings for the various interleaved compartments 136, 138, 140, and 142 and control cake thickness at the entrance sides of the compartments. As discussed above with reference to Fig. 1, low-flow-rate spray nozzles (not shown) may be provided in dewatering compartments 138 and 142. Gates 124 and 128 force the cake in compartments 138 and 142 to have a uniform thickness, thereby facilitating or enhancing the removal of residual mother liquor through drainage in those compartments. On the other hand, gates 122 and 126 force the cake in compartments 136 and 140 to have a uniformity, thereby facilitating reslurrying of the particulate matter by the wash liquid. The uniform cake thickness allows a better reslurrying as channeling through thinner cake with reduced flow resistance is not possible. As cake 114 is conveyed along cake flow path 112, it is dewatered first in beach section IO8, then reslurried in compartment 136, dewatered or desaturated in compartment 138, reslurried again in compartment 140, and finally dewatered or desaturated again in compartment 142. As indicated by phantom line 154, liquid extracted from cake 114 via screen section 120 maybe returned as wash liquid to compartment 136 via nozzle 144 to wash the cake upstream, i.e., in a countercurrent washing with the wash liquid becoming increasingly saturated with impurities as the wash travels upstream while the cake becomes increasingly pure after each wash in traveling downstream towards the cake exit. It is to be understood that solid bowl sections 110 and 118 may be formed as blinds which are inserted into bowl 104 to overlie spaced cylindrical portions of a single screen at the output end of the machine. This manner of assembly is especially appropriate in retrofits. Of course, solid bowl sections 110 and 118 may be solid cylindrical plates like bowl section 106. It is accordingly clear that the term "solid bowl section" as used herein is meant to denote plate sections of a centrifuge bowl and sections of a screen bowl covered with blinds to render those sections effectively solid for purposes of permitting reslurrying of materials. In using such a counter-current reslurry/wash, incoming fresh wash liquid is used to flood wash the exiting cake in compartment 140 and, optionally, to spray wash the exiting cake in compartment 142. Filtrate is collected and used to reslurry/wash the cake further upstream. Filtrate obtained from upstream dewatering compartment 138 via screen section 116 is concentrated in impurities and is discarded. Fig 3 depicts an unwrapped or developed bowl of a screenbowl centrifuge wherein the bowl is provided in a conical beach and/or a cylindrical bowl wall downstream thereof with a plurality of annular solid bands or circumferentially extending solid bowl sections 156 interleaved in the axial direction with a plurality of annular screen sections or circumferentially extending perforate screen sections 158. A conveyor 160 has a plurality of interleaved screw wraps 162 and 164 extending at an angle a1 relative to the solid bowl sections 156 and the perforate screen sections 158. Screw wraps 162 and 164 define a plurality of helical channels 166 and 168 along which cake flows from a clarifier pool and beach (neither shown) to a cake discharge port (also not shown). Along each channel, plural reslurrying compartments 170 and multiple dewatering compartments 172 alternating with one another in a cake flow direction are defined in part by radially adjustable entrance and exit gates 174 which are contiguous with and extend substantially perpendicularly to wraps 162 and 164. Gates 174, formed as baffle plates, extend outwardly from a centrifuge hub (not shown) and are connected to wraps 162 and 164. Cake flows through the opening formed between the bowl wall and the free edges of the gates. As shown in Fig 3, due to the helix geometry, end effects arise which are associated with the entrance and exit of each compartment 170 and 172. These end effects include the disposition of either a triangular perforate wall area 176 in a reslurrying compartment 170 or a solid wall area 178 in a dewatering compartment 172. Figs 4 to 7 depict different techniques for eliminating these end effects. In Figs. 4 to 7, reference numerals from Fig. 3 are used to designate the same structural elements as in Fig. 3. As illustrated in Fig. 4 gates 180 are installed which are so located as to form reslurrying compartments 182 having bowl walls which are completely solid. Thus, gates 180 are shifted relative to gates 174 to eliminate triangular perforate areas 176. Reslurrying compartments 182 in the embodiment of Fig.4 are shorter than reslurrying compartments 170 in Fig 3. Concomitantly, dewatering compartments 184 in Fig. 4 are longer than dewatering compartments 172 in Fig 3. In this embodiment of a screenbowl centrifuge, the outer wail of each dewatering compartment 184 has a screen or perforate portion and two triangular solid portions. As shown in Fig 5, triangular perforate wall areas 172 (Fig. 3) may be replaced with respective triangular solid sections or plates 186, thereby forming reslurrying compartments 188 which have radially outer walls which are completely solid. Similarly, triangular solid areas 178 (Fig. 3) are replaced with triangular perforate sections or screens 190, thereby forming dewatering compartments 192 whose radially outer walls are entirely perforated. Compartments 188 and 192 may have the same length as compartments 170 and 172, respectively. As depicted in Fig 6, the centrifuge bowl wall can be formed with alternating cylindrical solid sections 194 and cylindrical screen sections 196 which are wider relative to the width of channels 166 and 168, i.e., relative to the separation of adjacent conveyor wraps 162 and 164. The increase in bowl section width together with the placement of gates or baffles 198 produces longer reslurrying compartments 200 and longer dewatering compartments 202,relative to compartments 170 and 172 in Fig 3. Gates 198 maybe placed, as discussed above with reference to Fig. 4, to eliminate vestigial perforate areas in reslurrying compartments 200. Another arrangement which eliminates the end effects is the use of circumferential gates or weirs 204 to produce reslurrying and dewatering compartments 206 and 208, as shown in Fig. 7. Unlike with perpendicular gates 174,180, and 198, the cake in the embodiment of Fig. 7 sees non-uniform resistance as it meets circumferential weirs 204, producing the undesirable consequence of cake jamming in corners 210. As illustrated in Fig. 8, a bowl 212 of a screenbowl centrifuge may be provided along a beach 214 with one or more reslurrying compartments 216 and 218 and one or more dewatering compartments 220 interleaved or alternating with the reslurrying compartments. Flood wash liquid is fed to reslurrying compartments 216 and 218 via nozzles at 222 and 224. Wash liquid and mother liquor, schematically represented by an arrow 226, exits cake 228 via a conical screen section 230 along an outer side of dewatering compartment 220. Screen section 230 is flanked on opposite sides by conical solid bowl sections 232 and 234 defining outer walls of reslurrying compartments 216 and 218. Compartments 216,218 and 220 are defined in part by gate or baffle elements 236. Bowl 212 may include an output cylinder 238 optionally provided with one or more additional dewatering compartments 240 and 242 and one or more additional reslurrying compartments 244. Fig 8 diagrammatically represents flood wash reslurrying by showing slurry levels 246, 248, and 250 in reslurrying compartments 216,218, and 244 Fig. 9 depicts a pusher-type centrifuge including a cylindrical first basket 252 and a cylindrical second basket 254 disposed downstream of the first basket along a cake flow path 256. Basket 252 has a smaller diameter than basket 254. The centrifuge also includes pushers 258 and 260 for pushing a particulate cake layer 262 along baskets 252 and 254, respectively. Baskets 252 and 254 have a plurality of circumferentially extending solid bowl sections 264 alternating with circumferentially extending perforate screen sections 266, while a plurality of circumferential and radial gates 268 are disposed at junctions between adjacent solid bowl and perforate screen sections 264 and 266. Flood wash liquid is supplied to reslurrying compartments 269 by nozzles, as indicated by arrows 270, while spray nozzles 272 may be provided in dewatering compartments 274 for spraying cake 262 during dewatering or desaturation thereof. Tests conducted on an 18-in diameter screenbowl show promise of the above-described reslurrying-and-separation design. This is especially suitable for screenbowl operating at high flow rate and low centrifugal gravity . As much as 4-6% washing efficiency improvement can be obtained with the reslurrying-and-separation arrangement as compared to a conventional washing arrangement with a screenbowl despite the open screen area is much reduced. Multiple reslurrying and separation stages in a single pusher centrifuge may be carried out for single-stage pushers as well as multi-stage pushers, each with a large diameter basket with alternating stages reciprocating, and all the stages also rotating concurrently. Fig. 10 is an unwrapped or developed view of a conical-screen centrifuge having a single-lead conveyor 280. A series of baffles or gates 282 are provided substantially perpendicular to the conveyor lead or blade for separating the screen area or conical wall of the centrifuge into reslurrying compartments 284 and dewatering compartments 286 alternating therewith. The conical wall of the centrifuge is formed with circumferentially extending solid or imperforate sections or bands 288 interleaved with circumferentially extending perforated screen sections or bands 290. Wash liquid is introduced into the reslurrying compartments, as indicated by arrows 292. Cake moves along a helical path, as indicated by cake flow arrows 294, as the conveyor 280 and the conical centrifuge wall rotate at differential speeds about an axis 296 as indicated by arrows 298 and 300. The arrangement of Fig. 10 also applies to conical-screen centrifuges with multiple-lead conveyors. Fig 11 shows a conveyor with 4 discrete leads or blades 302 which do not wrap 360° about the conveyor hub (not shown). Gates 304 and interleaved cylindrical solid wall and perforate sections 306 and 308 are provided to form alternating reslurrying and dewatering compartments 310 and 312. Cake flows in an approximately longitudinal direction as indicated by an arrow 313. As illustrated in Figs. 12 and 13, a vibrating conical-screen conveyor with a plurality of annular or circumferentially oriented rotating gates or baffles 314. Together with interleaved cylindrical solid wall sections 316 and cylindrical perforate or screen wall sections 318, baffles 314 define a series of alternating reslurrying compartments 320 and dewatering compartments 322. A feed slurry 324 is delivered to an input compartment 326 of the machine, where bulk filtration and cake formation occurs. A wash liquid is provided to reslurrying compartments 320, as indicated by arrows 328. Filtrate 330 exits the centrifuge through screen sections 318. In the embodiments of Figs. 10 to 13, the last gate towards the cake discharge end of the machine, at the large diameter end of the conical screen area, provides a means for controlling the retention/residence time of final cake dewatering at maximum centrifugal gravity before the cake is discharged from the machine. Thus, reslurrying and dewatering as described herein is effective in enhancing the removal of impurities in all types of conical-screen centrifuges, whether including large cone angles with centrifugal gravity driving the cake down the cone or shallow cone angles with a conveyor or vibration driving the cake Reslurrying and separation insitu in a single centrifuge provides an important technology allowing substantial purification of difficult-to-wash cake which otherwise could not have achieved with conventional spray or flood wash. Any gate or baffle which partitions a screening-type centrifuge into reslurrying and dewatering compartments, as discussed hereinabove with reference to Figs. 1 to 13, may be formed with a concave profile on an upstream side (as defined by cake flow), as discussed below. More particularly, any such gate or baffle which is oriented substantially perpendicularly to the direction of cake flow, for example, a gate 268 in Fig. 9, may be formed on an upstream side with a substantially cylindrical concave surface 268a extending about an axis 268b which is parallel to the cake layer and perpendicular to the cake flow direction, facilitating a turning back of the wetter cake particles in an upper (radially inner) portion of the moving cake layer towards the interior of the compartment on the upstream side of the respective gate. The gate 42 are provided on respective upstream side, facing substantially away from discharge opening, with a substantially cylindrical concave profile or surface 46 (Fig.14). This concave profile or surface 46 serves to direct a portion of cake, which is headed downstream along cake flow path, into a respective looped recirculation or churning path, directed partly back towards a slurry pool. The recirculation or churning of the particulate material induces additional shear and restructuring of the cake matrix. This facilitates an As depicted in Fig. 15, the gate 42' may be provided on a downstream side with a concave profile or surface 420. This additional concave surface 420 enhances the cake churning action by facilitating a redirection, back in a downstream direction, of relatively wet cake traveling towards slurry pool In other words, concave surfaces 420 facilitate or enhance the establishment of recirculation paths.

Claims: Centrifuge comprising a bowl (66, 104, 212) having an inner surface including at least one solid bowl section (84, 110, 118, 156, 186, 194, 232, 234, 264, 306, 316) and at least one screen section (86, 116, 120, 158, 190, 196, 230, 266, 308, 318), and conveying means (78, 134, 160, 258, 260, 280) to transport a cake (82, 114, 228, 262, 313) along a cake flow path (112, 166, 168, 256, 294) to a discharge port (88, 150), at least first and second gates (92, 122, 126, 130; 94, 124, 128; 174, 180, 198, 204, 236, 268, 282, 304, 314) which separate the bowl (66, 104, 212) into at least first and second compartments (84, 136, 140, 170, 182, 188, 200, 206, 216, 218, 244, 269, 284, 310, 320; 86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322) and establish entrance and exit openings for the compartments (84, 136, 140, 170, 182, 188, 200, 206, 216, 218, 244, 269, 284, 310, 320; 86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322) between the bowl (66, 104, 212) and the free edges of the gates (92, 122, 126, 130; 94, 124, 128; 174, 180, 198, 204, 236, 268, 282, 304, 314), characterized by a first compartment (84, 136, 140, 170, 182, 188, 200, 206, 216, 218, 244, 269, 284, 310, 320) along the cake flow path (112, 166, 168, 256, 294) upstream of the first gate (92, 122, 126, 130) is a reslurrying compartment which includes the solid bowl section (84, 110, 118, 156, 186, 194, 232, 234, 264, 306, 316) and is provided with a wash liquid feed (96, 144, 146, 222, 224, 270, 328), and wherein the second compartment (86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322) along the cake flow path (112, 166, 168, 256, 294) between the first gate (92, 122, 126, 130) and the second gate (94, 124, 128) downstream thereof is a dewatering compartment which includes the screen section (86, 116, 120, 158, 190, 196, 230, 266, 308, 318). Centrifuge according to claim 1, characterized in that the solid bowl section (84) extends past the first gate (92) for preventing the discharge of fine particles matter through the screen section (86). Centrifuge according to claim 1 or 2, characterized in that the dewatering compartment defined by the screen section (86) and the gates (92, 94) is provided with spray nozzles (102). Centrifuge according to one of the preceding claims, characterized in that the gates (92, 122, 126, 130; 94, 124, 128; 174, 180, 198, 204, 236, 268, 282, 304, 314) are radially adjustable. Centrifuge according to claim 4, characterized in that the entrance gate (92, 122, 126, 130) for each dewatering compartment (86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322) has a radial adjustment that permits only the cake (82, 114, 228, 262, 313) adjacent to the upstream solid bowl section (84, 110, 118, 156, 186, 194, 232, 234, 264, 306, 316) along the cake flow path (112, 166, 168, 256, 294) to pass to the subsequent dewatering compartment (86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322). Centrifuge according to one of the preceding claims, wherein a plurality of additional reslurrying compartments (84, 136, 140, 170, 182, 188, 200, 206, 216, 218, 244, 269, 284, 310, 320) and dewatering compartments (86, 138, 142, 172, 184, 192, 202, 208, 220, 240, 242, 274, 286, 312, 322) are provided along the cake flow path (112, 166, 168, 256, 294). Centrifuge according to one of the preceding claims, characterized in that the one or more reslurrying compartments (84, 136, 140, 170, 182, 188, 200, 206, 244, 269, 284, 310, 320) and the one or more dewatering compartments (86, 138, 142, 172, 184, 192, 202, 208, 240, 242, 274, 286, 312, 322) are provided along a cylindrical bowl wall. Centrifuge according to one of the preceding claims, characterized in that the conveying means (134, 160) has a hub (132) and screw wraps (152, 162, 164) extending at an angle (α 1 ) relative to the solid bowl sections (156) and perforate screen sections (158), and that the gates (122, 124, 126, 128, 130, 174) extend outwardly from the hub (132) and are contiguous with and radially movably connected to the screw wraps (152, 162, 164). Centrifuge according to claim 8, characterized in that the gates (174, 180, 198) extend substantially perpendicularly to the screw wraps (162, 164). Centrifuge according to claim 9, characterized in that considering the helix geometry the gates (180) are so located as to form reslurrying compartments (182) having bowl walls which are completely solid. Centrifuge according to claim 9, characterized in that the, due to helix geometry, triangular perforate or solid wall areas in the reslurrying compartments and the dewatering compartments, respectively, are replaced by corresponding triangular solid or perforate wall areas (186, 190) thereby forming reslurrying compartments (188) having radially outer walls which are completely solid, and dewatering compartments (192) whose radially outer walls are entirely perforated. Centrifuge according to claim 8, characterized in that circumferential gates (204) are used to produce reslurrying compartments (206) and dewatering compartments (208). Centrifuge according to one of the claims 1 to 8, characterized in that one or more reslurrying compartments (216, 218) and one or more dewatering compartments (220) are provided along a conical beach section (214) of the bowl (212). Centrifuge according to one of the claims 1 to 7, characterized in that the plurality of solid bowl sections (264) alternating with a plurality of perforate screen sections (266) extend circumferentially in baskets (252, 254) of a pusher-type centrifuge, and that a plurality of circumferential and radial gates (268) are disposed at junctions between adjacent solid bowl and perforate screen sections (264, 266). Centrifuge according to one of the preceding claims, characterized in that at least one of the gates (92, 122, 126, 130) is provided with a concave surface on an upstream side, facing along the cake flow path (112) away from the downstream discharge port (88, 150).

Inventors: LEUNG WALLACE WOON-FONG, LEUNG, WALLACE WOON-FONG
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1998
Main classification: B04B1
IPC codes: B04B15, B04B3, B04B1