These machines were first introduced to industry nearly 60 years ago and were of rather small and of a simple design. Early designs were greatly flawed. Their main limitation was at the discharge zone since the cake was contained in a fixed rim and special sealing arrangements must be provided in order to avoid the spillage of brine at the table's circumference. Another problem was that the thin heel left between the scroll and the surface of the table was dislodged by applying a back blow but not removed from the surface of the passing cell. So, as it reached the feed zone it was mixed with the incoming slurry without the cloth being washed. This can caused progressive media blinding which effects filtration rate and requires frequent stoppage of the operation for cloth washing. As the demand for more area and higher throughputs has gone up, table filters were redesigned and upgraded.
Special attention was given to the redesign of the discharge zone and a radically new engineering concept was developed to solve the circumferential sealing problem and allowing continuous cloth washing. The new table filter features a rubber belt rim that rotates together with the cells but is removed from the table just after leaving the final wash and drying zone and before reaching the cake discharge hopper. The rim then stays away from the table and returns once it passes the wash water hopper and enters the cloth drying zone. Special rollers are diverting the rim and a tension take- up roll ensures that the belt stays tight against the table and seals the circumference against leakages.
The filtration area of large table filters is more than 200 m2 and having few moving parts can rotate at a cycle time of 1.5 minutes. These machines can handle thick cakes and may be operated at high vacuum levels. The major subassemblies of the machine include:
• A series of fixed trapezoidal cells that form a rotating table and each connected to a stationary valve in the center of the filter. The cell is designed with steep sloped bottom for fast evacuation of the filtrate,
• A valve that may be raised from the top and has a bridge setting and compartments to control the various zones.
• An internal rim fixed to the table at the inner circumference and a continuous rubber belt that surrounds the table at the periphery and confine the slurry, wash liquids and the cake during the filtration cycle.
• Rollers that support the vertical loads, centering thrust and others that move the rim away from the table in the discharge zone and maintain it under tension.
• Radial rubber dams that separate between the feed, wash stages, cake discharge and cloth wash, and cloth drying zones to prevent the mixing of filtrates.
• A variable pitch screw that transports the cake radially towards the discharge point.
The cycle of a table filter that includes three countercurrent washing stages consists of the following zones:
With cells under vacuum during filtration:
• Cloudy Port Recycle or Sedimentation Pool (before applying vacuum).
• First Predrying.
• Second Predrying.
With cells purged to atmosphere:
• Cake discharged dry and conveyed by the screw to the cake hopper.
• Cloth wash and sluicing for the removal of the heel. With cells under low vacuum:
• Evacuation of cloth wash water.
Compared to tilting pan filters, these machines have fewer moving parts and hence they are easier and less costly to maintain.
Select the Table Filter
• When the process downstream requires a de-lumped cake since the screw disintegrates the solid lumps while conveying them to the periphery.
• When the solids are fast settling and cannot be kept as a homogenous slurry in bottom or side feed filters such as Drum or Disc Filters.
• When very short cycle times are required for fast dewatering cakes such as phosphate slurry.
• When a clear filtrate is required right from the start it is good practice to form a thin heel that serves as a filter medium over the exposed cloth. This is done by either a "cloudy port outlet" that is recirculated or, if solids are settling fast, by allocating a portion of the table after the cloth drying dam and prior to entering the vacuum zone to act as a "sedimentation pool".
• When intensive cake washing is required.
• When a large filtration area is required but a horizontal belt filter does not fit into the layout.
• When cakes tend to crack under vacuum measures, such as a flapper or pressure roll may assist in sealing the cracks thus avoiding loss of vacuum.
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