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The Table Filters belong to the top feed group, introduced in the early 40's and were rather small and of a simple design. Their main limitation was at the discharge zone since the cake was contained in a fixed rim and special sealing arrangements had to 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 has caused progressive media blinding which effected filtration rate and required frequent stoppage of the operation for cloth washing.
As the demand for more area and higher throughputs has gone up in the 60's the Table Filters and Tilting Pan Filters were redesigned and upgraded since the rubber technology was unable to catch-up with the filtration area requirements of Horizontal Belt Filters.
A typical flowscheme for a counter-current washing may be seen on the left. The flowscheme consists of three washing stages and is a conventional design for phosphoric acid plants. The filter in the flowscheme is a table filter however the same diagram applies also to Tilting Pan Filters and Horizontal Belt Filters.
In the old design the rim that contained the cake was fixed to the main structure and had an opening just at the discharge point to allow cake removal. In this design a thin heel was left continuously on the filtering surface, could not be removed and caused loss of filtration rate due to blinding. Likewise owing to the friction between the moving cake and the stationary rim the drive was subjected to high power consumption.
To solve the problems encountered with fixed rim a radically new engineering concept was developed in the late 60's. 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 moves away from the table, the cake discharges and the heel is removed by high impact jets to a cloth wash water to a separate chute. It then continuous to a cloth drying zone for such applications that the dilution of mother liquor is undesired and returns to the slurry feed section.
Special rollers are diverting the rim at the cake discharge zone 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 since there are few moving parts, contrary to Pan Filters, the table can rotate at a cycle time of 1½ minutes.
These filters can handle thick cakes and may be operated at high vacuum levels. The major subassemblies of the Table Filter are:
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 point of discharge.
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The criteria for selecting a Table Filter are:
When the process downstream requires a de-lumped cake since the screw disintegrates the solid lumps while conveying them to the periphery.
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.
The cycle of a Table Filter that includes three counter-current washing stages consists of the following zones:
When compared to Tilting Pan Filters the Table Filter has less moving parts so the major components to be checked as part of the preventive maintenance schedule are:
The clearance between the cake discharge screw and the cloth surface. If, for some reason, this clearance becomes smaller the friction and shear forces during the radial transport of the cake towards the discharge hopper may cause cloth wear.
The condition of the rubber belt that surrounds the table at its periphery and serves to contain the feed slurry, wash liquid and cake.
The table support rollers, centering thrust rollers and the rollers in the discharge zone.
The valve and wear plate faces for tight sealing against vacuum leaks.