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Sedimentation processes are greatly affected by factors such as particle size and density, liquid density and viscosity. The relationship between these factors and sedimentation conditions is shown in Figure 1, in which three settling regimes occur; particulate, zone, and compression (Schweitzer, 1988).
Particulate settling is composed of solid particles which have little tendency to cohere to one another, such as sand grains and salt crystals, and which generally settle at a steady rate are describe as 'discrete' particles. In applications involving this type of material, clarifiers are employed; where the design is determined by particle settling rate (Schweitzer, 1988). Zone settling occurs when a rising bed of settled flocculent particles assemble until all suspended solids are in the form of a flocculent porous medium. This flocculent porous medium undergoes fast consolidation. When the initial concentration of the suspension hits the boundary between the zone settling and compression, a solid phase immediately forms; creating an interconnected coherent structure of microflocs (Somasundaran, 1979).
Thickeners may be batch or continuous units, and consist of relatively shallow tanks from which the clear liquid is taken off at the top, and the thickened suspension at the bottom. In a continuous thickener unit, flocculent suspensions of different clays or ores, as well as sludge's, form three different zones; the thickening zone at the bottom of the tank, the sedimentation zone above it, and the zone of clear liquid at the top. Incoming suspension partially disperses into water in the sedimentation zone, the balance flowing as a density current in the lower part of the sedimentation zone. The concentration of suspended solids at the top of the thickening zone is equal to the boundary concentration between zone settling and compression (See Figure 1.0), flocculent particles in the thickening zone lose their individual character. Having mutual contacts they become part of the matrix of solids which is compressed by the pressure of the overlying solids. This causes the release water to move upwards through the matrix of solids (Somasundaran, 1979).
Figure 2.0 Concentration profile in the continuous gravity thickener (Somasundaran, 1979)
The satisfactory operation of the thickener depends upon the existence of a clear-liquid overflow at the top. If the thickening zone is too shallow, some of the smaller particles may escape into the overflow. The volumetric rate of flow upwards is equal to the difference between the rate of flow feed of liquid and the rate of removal in the underflow (Wills, 1992).
In a continuous thickener, the area required for thickening must be such that the total solids flux at any level does not exceed the rate at which the solids descend to the base of the unit. If this condition does not exist, solids will accumulate and steady-state operation will not be possible. In order to prevent solids escaping in the overflow, this flux must be kept constant at all depths below the feed point. In order to calculate the required area, the concentration at which the total flux is a minimum needs to be established (Richardson, 1994).
The flux ψ is expressed as:
Eq. (1.0) (Richardson, 1994)
Where Q0 is the volumetric feed rate of suspension; A is the area of the thickener; C0 is the volumetric concentration of solids in the feed.
Figure 3.0 shows the plot of sedimentation flux ψ against volumetric concentration C. The curve passes through a maximum, and as the concentration continues to increase the curve exhibits an inflexion. At a given withdrawal rate uu the bulk flux ψu is represented by a straight line passing though the origin. At a given concentration C the flux ψ is determined by the sedimentation velocity of solids uc. The total solids flux ψT, obtained as the summation of the two curves, passes through a maximum, followed by a minimum (Richardson, 1994).
Figure 3.0 Solids fluxes as functions of concentration (Richardson, 1994)
In order to determine an appropriate area of thickener required for the limiting total flux ψTL at a specified concentration of Cu of overflow (Richardson, 1994):
Where the total flux is given at a fixed withdrawal rate of suspension (uu):
Thus if a tangent is drawn from the point on the abscissa corresponding to the required Cu, it will meet the ψ curve at a concentration value CL at which ψT has the minimum value ψTL. Hence, to determine both CL and ψTL, a tangent to the batch sedimentation (ψ) curve is drawn. The value of ψTL is determined (Richardson, 1994).
In many operations the main requirement of the thickener is to produce a thickened product of as high concentration as possible. This requires the provision of a specific depth to allow time for the desired amount of consolidation to occur. The decisive dimension is the vertical distance between the feed point and the exit. In most cases, the time of compression of the sediment will be large in comparison to the time taken for the particles to reach their terminal velocities (Richardson, 1994).
The top surface of the suspension should always be suitably below the level of the overflow weir; to provide a clear zone deep enough to allow any entrained particles to settle out. Usually such particles are present in low concentrations and so they settle at their terminal falling velocities. Provided that this requirement is satisfied, the depth of the thickener does not have any significant effect on the clarifying capability (Richardson, 1994).
In deep tanks consolidation is greatly facilitated by the use of stirring which creates channels through the sediment for the escape of water in an upwards direction. In addition the stirring eliminates any frictional forces between the solids and the walls of the vessel. The use of slow rotating rakes is generally applied in larger thickeners to direct the underflow to the central outlet at the base of the tank. The slow movement of the rakes also provides mild agitation to the sediment; facilitating its consolidation and water removal (Richardson, 1994).
Because of the nature and volume of the feed suspensions, thickeners are typically designed for heavy duty operations. The major components are; tank, feed piping and feedwell, drive support structure, drive and lifting device, rake structure, underflow solids withdrawal system, and overflow collection (Schweitzer, 1988).
Sedimentation tanks are usually round, and sloped to aid in the underflow removal. The rheological characteristics of the thickened slurry are used to determine the slope characteristics, which typically range from 4 to 25%. Dual-slope bottoms are used in larger thickeners to minimise center depth. Steep inner slopes encourage the courser, denser solids to settle towards the center; aiding the solid removal at the center discharge point (Schweitzer, 1988).
Feed systems consist of a feed slurry launder or pipe which terminates at the feedwell. The primary purpose of these systems is to dissipate energy and provide a transition into the settling basin with minimum turbulence. The launder or pipe is supported on a bridge; discharging into the feedwell directly. The slurry may enter either through a split outlet that disperses equal portions of the flow; dissipating the kinetic energy prior to entry into the clarification zone (Schweitzer, 1988).
Drive-support structures and drive assemblies have three basic designs; the bridge supported mechanism, the center column supported mechanism, and the traction drive mechanism. They all have a driving arm attached to a motorised carriage at the tank periphery. Bridge supported mechanisms are common on units up to 28m in diameter. Center column supported drives are used in clarifiers > 28m diameter. The center columns are usually steel or concrete and the raking arms are attached to a drive cage which rotates around the column. Traction drives are used on tanks >60m in diameter. They are more expensive than center driven units because of the heavy concrete wall required for supporting the carriage (Schweitzer, 1988).
The rake structure is usually composed of two long arms. There are three types of fixed-rake-arm designs - each adapted for specific thickening operations. The blades have sufficient area to transport around 15 to 25% of the total solids loading, as most of the solids reach the underflow by hydraulic transport. The rake blade angle is usually in the range of 20° to 45° measured with respect to the direction of rake travel (Schweitzer, 1988).
The underflow withdrawal system consists of a transitional section from the main tank body to the underflow slurry piping and the means for pumping the slurry. A steep-angled cone in the base of the tank is used with a bridge supported rake drive mechanism. Cone and trench scrapers (attached to the rake mechanism) help maintain a slurry flow to the underflow piping. From here centrifugal or positive displacement pumps transport the underflow slurry away from the sedimentation unit (Schweitzer, 1988).
Clarified overflow is removed via a peripheral launder which collects the flow and provides transport to the discharge point. The liquid flows into the launder over a V-notch or flat weir or through submerged orifices in the launder bottom. Submerged pipes are often used in applications prone to ice cap formation in the winter or when additional clarified liquor storage is to be provided at a level above the effluent pipe (Schweitzer, 1988).
Solids-recirculation clarifiers have feedwells designed to circulate previously flocculated solids into a mechanically mixed feedwell. This increases the solids concentration in the reaction well; increasing the particle contact time and hence the flocculation efficiency. A draft tube is used to lift the solids into the reaction well where the circulating solids meet the feed. Coagulants and flocculants are added into the feed line, draft tube or at the top of the reaction well (Schweitzer, 1988).
In deep-cone thickeners feed and flocculating agents are introduced near the apex of the cone, which generally has an angle of 60° to 90°. A heavy dosage of flocculent provides rapid thickening of metallurgical pulps, such as red mud or coal refuse. Relatively high underflow solids concentrations are achieved, and whilst this is partly due to compressive forces maintained by great depth of dense pulp, most of the benefit is due to the high flocculent dose which produces large, rapidly dewatering agglomerates (Schweitzer, 1988).
Suction clarifiers are employed in situations where the solids detention time within the sedimentation unit must be minimised to avoid chemical changes. To do this a means of retrieving the solids from the entire base of the clarified is required. This is achieved by using a number of suction or siphon lines connected to V- shaped blades located along the rake arms (Schweitzer, 1988).
Tray thickeners which operate with common feed and underflow systems are used for slow-settling materials which operate at an elevated temperature. A tray thickener is a series of thickeners mounted vertically above one another. They operate as separate units, where a common vertical shaft is utilised to drive the set of rakes (Wills, 1992).
Inclined-plate clarifiers, also known as lamella or tube settlers, employ a multitude of of closely spaced parallel plates inclined at an angle of between 45° and 60° from the horizontal. Each plate acts as a separate clarifier, with solids which enter in the feed suspension settling only a short distance, collecting on the plate, and sliding as a partially thickened mass into a lower compartment. Distribution of the flow is usually achieved by controlling the overflow rate of clarified liquor from each plate by orifices. These units are used for clarification of dilute suspensions. Feed suspensions that require flocculation may use external flocculators as the retention time within the clarifier is rather limited (Schweitzer, 1988).
Conventional thickeners suffer from the disadvantage that large floor areas are required, since the throughput depends primarily on the area. Therefore units known as 'high-capacity' thickeners have been introduced. Many varieties exist and are typified by a reduction in unit area requirement (Wills, 1992).
The high-capacity thickeners have specially designed feedwells and flocculation systems that maximise the effectiveness of flocculants. The high-rate feedwells are designed to transfer the flocculated slurry into the settling zone without destruction of the newly formed flocs. Dispersion and mixing of flocculent can be done hydraulically or mechanically within the high-rate feedwell. The increase in flocculation efficiency can increase the solids settling rate by up to 10 times compared to that of conventional thickeners, and thus this reduces the required unit area by a similar factor (Schweitzer, 1988).