Technical Article Series

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1 ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz _+=- hi {} []\; :,./ Ω π~<>? èùìòàéù`íóá ëÿüïöä êûîôâõãñ ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz _+[]{} \;: Ω π~,.<>?/ èùìòàéù`íóá ëÿüïöä êûîôâõãñ Technical Article Series The Evolution of Modern High Capacity Recycle Clarifiers KASON CORPORATION East Willow St. Millburn, NJ USA Tel: Fax: KASON CORPORATION, EUROPE Units 12 & 13 Park Hall Business Village Park Hall Road Longton Stoke-on-Trent ST3 5XA UNITED KINGDOM Tel: (+44) Fax: (+44) SEPARATOR ENGINEERING LTD Midland Ave., #85 Scarborough, Ontario M1P 3E6 CANADA Tel: Fax:

2 The Evolution of Modern High Capacity Recycle Clarifiers After the construction an operation of circular screen separators are discussed, the article covers the significant changes that were made to provide twice the screening area of a single-deck unit with a capital cost substantially less than two machines. By L. H. Stone, President, Kason Corporation Introduction For many years the designers of Kason circular separators have searched for new design configurations that would enhance the performance of these units when performing liquid/solids separations. In addition to a basic desire to improve the performance of new machinery being installed for the first time, Kason designers were sensitive to the need to increase the capacity of equipment currently operating in the field but being subject to higher processing loads than their original requirements called for. How The Machine Operates The basic assembly consists of a motor plus a number of interchangeable frames that contain screen-cloth decks and discharge outlets. Mounted rigidly to the main screen assembly, the motor has a double-extension shaft, which is fitted at its ends with variable eccentric weights. The screen assembly is supported on the circular base by springs that allow the assembly to vibrate freely, while preventing transmission of vibration to the floor. In properly designed units, the wire screen-cloth does not flex; thus, its life is prolonged. All parts above the spring level, including the motor, are integrated into a rigid structure. The spring's isolation from the support base makes the system self-balancing, requiring minimum power and avoiding mechanical stress. Typical power requirements are 1/3 hp ( kw) for 2-ft (610 mm) diameter machine, 1 hp ( kw) for a 4-ft (1220 mm) machine, and 2 hp ( kw) for a 6-ft (1830 mm) unit. These requirements are a function of size and construction of the separator, and not of material throughput. The motor must have sufficient low-speed torque to accelerate the eccentric weights to a speed just above the resonant frequency of the system, as well as overcome the hysteresis loss in the springs, and losses due to the slight flexing of the frames. Material to be separated is fed to the center of the top screen. Oversized particles move to the screen periphery where they are discharged, while undersize particles or liquids pass rapidly through it. Units contain up to five frames. In multiple-frame units, each lower screen is preceded by a Flow patterns on a screen are changed by varying the angle between top and bottom weights. feed tray that redirects undersize material to the center of the screen beneath. This increases efficiency by forcing each particle to travel the full distance from the center to the periphery. Multiple-deck construction also saves operating space. Vibratory Motion The motion of these separators is three-dimensional. The top eccentric weight of the double-extension shaft motor is in a plane close to the center of the mass of the assembly, imparting a horizontal throw to it. The bottom weight is below the center of the mass, giving the assembly a high-frequency tilt. The third dimension of motion (tangential) results from the vector combination of the horizontal and vertical components. The tangential component helps move oversize material laterally across the screen in a spiral path while undersize particles flow down through the openings. On circular-screen separators, top and bottom weights are independently adjustable. The mass and angle between the weights can both be varied, offering a great deal of control over the three vibrating components, and thus permitting optimization of equipment for varying conditions and materials. Varying The Flow Patterns

3 Flow patterns are varied by changing either the amount of the weights or their relative position. Increasing the mass of the top eccentric weight increases horizontal throw of the screen, speeding up the rate of discharge of oversize material. This is especially useful for applications that contain large amounts of oversize solids, such as dewatering. Adding to the bottom eccentric weight will result f1 a larger vertical component of motion, promoting turnover (tumbling of the material at the screen surface). This maximizes the quantity of undersize material passing through the screen. Increased vertical motion also inhibits blinding of the screen by near-size particles (those slightly larger than the mesh openings). When processing light or fragile solids, it is sometimes desirable to reduce the bottom weight to minimize vertical motion, thereby avoiding particle breakage and attrition. Tangential motion of the screen is controlled by the relative angular positions of the top and bottom weights. The screening pattern, therefore, is changed by altering the relative angle of these weights. Typical flow patterns generated by various angles are shown in Fig. I. When the weights are aligned and move in phase, the tangential component is at a minimum, and there is no tendency for the material to spiral; travel is radially from the center. When the bottom eccentric leads the top eccentric, a spiral motion is induced. Under some conditions, it is possible to prevent discharge of oversize particles from the screen surface. Such a flow pattern is used when there is a small amount of oversized material. Adjustment of the weights can be made in a few minutes by the operator. Capacity of circular-screen separators may be limited either by a particular unit's design or by what happens at the screen surface. The Recycle Deck Concept Design engineers recognized that if decks were configured in series, a slurry fed to the center of the machine would be concentrated on the first deck (top deck) that it came in contact with and would achieve formal dewatering of the solid~ on the lower deck. Machines configured in this manner would provide twice the screening area of a single deck unit with a capital cost significantly less than two machines; thereby attaining higher production rates per dollar of installed cost. These early recycle deck configurations are shown in Figures 2 and 3. In actual practice these early external recycle decks exceeded the original expectations in two areas but were not considered adequate in one area. These are summarized below: 1. Feed rates realized were significantly higher than twice a single deck unit because the upper deck is operated under a flooded condition, thereby developing a hydraulic head which radically increases the throughput rate on the upper deck about two to four times the gallons per minute per square foot (liters per minute per square meter) of screening area of a single deck machine. 2. When handling slurries with air bubbles in the feed, the upper deck Schematic Diagram of External Recycle Deck would separate the solids, air bubbles and some liquid, and discharge showing liquid/solids separation. this frothy mixture to the lower deck where the bubbles would coalesce and drain through the screen allowing the discharge of damp solids. On single deck units processing feeds with air bubbles, capacities were lower and solids discharge wetter because of insufficient screen area for coalescence to occur. 3. Results were less impressive with feeds having a high volumetric percentage of solids or where the solids were very fine and would not discharge well, thus causing solids to back up onto the upper screen deck, ultimately reducing throughput or, in some cases, actually blinding the upper screen completely. Internal Recycle/Peripheral Discharge² Necessity being the mother of invention, Kason's success over the years with 360 Kascade decks has

4 indicated these designs would be ideal to overcome the problems outlined in Item 3 above. Previous experience with 360 discharge decks had demonstrated that even the most difficult materials such as fine mica, which is a wafer-like solid, could be easily discharged with a 360 peripheral discharge. Moreover, material handling problems of high solids loading were also eliminated since the length of discharge (7tD) is so large in relation to the loading. Furthermore, the 360 peripheral discharge provided the option of installing a peripheral dam to control the depth of liquid on the upper screen deck. Adjusting the dam height allowed the separator to operate at the best balance between solids or thickened slurry discharge over the dam and clarified effluent rates through the screen. The higher the dam, the greater the throughput rates, as long as the solids or thickened slurry would discharge over the dam to feed the lower deck. In practice these dams range from 1/4" (63 mm) to 1/2" (127 mm) in height. Illustration of the addition of a Recycle Deck to an existing circular screen separator. Clarified effluent from the upper screen is collected in a cone attached to the upper screen and directed through a central opening in the lower screen to the discharge frame below where it combines with clarified effluent draining through the lower deck and is discharged via the clarified effluent spout (see Figures 4,5, and 6). Recycle deck arrangements are of modular construction allowing them to be incorporated into a wide variety of circular screen configurations to meet a broad range of processing requirements, either as new equipment going into an initial installation or as retrofits to existing machinery. Schematic diagram of an Internal Recycle Clarifier showing liquid-solids separation. Internal Recycle Deck mounted on a spacing frame showing annular slots to convey thickened slurry to lower deck for further drainage.

5 Internal Recycle Deck looking down through the screen mesh at the clarified liquid discharge cone.