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The tipping (tilting) pan filter obviously, and the rotary table filter somewhat less obviously, are developments of the simple batch vacuum filter described in the previous chapter. The single tipping pan is a batch filter, just like the Nutsche, but other versions, including the table filter, are intended to allow more or less continuous operation.

Tipping pan and rotary filters are widely used in the mineral processing industries, and they are particularly suitable for handling rapid settling coarse crystalline solids, especially since settling aids the formation of filter cake. Very sharp separation of the mother and wash liquors is readily obtained, greatly facilitating wash liquor recirculation. By applying multi-counter current washing techniques, max imum extraction of solubles is possible, with minimum dilution.

The single tipping pan filter is a shallow rectangular pan, which is normally an open vessel, with a false bottom supporting the filter fabric. The filtrate chamber of the Nutsche filter is replaced here by a narrow space below the filter medium that is
connected to the vacuum and filtrate discharge pipe, which links the pan to the filtrate receiver. This connecting joint allows the pan to be rotated through 180° (tipping) for the discharge of accumulated solids, after they have been adequately washed.

Feed is loaded into the pan and filtration takes place either by gravity or under vacuum, depending on the design of the unit and the filterability of the solids. Cake washing can be followed by air drying, if required, prior to removal of the cake by
tipping the pan to empty out the solids. When the pans are in the inverted position, the filter fabric can then also be washed.

Tipping pan filters are also available as multi-pan units, with the pans arranged in a horizontal circle, like a wheel, with the spokes of the wheel represented by the filtrate pipe connecting each pan to a central vacuum pipe and filtrate receiver. The
pans are now trapezoidally shaped to fit together in the annular filtration zone.

A set of pans within this annulus is used to carry out the filtration cycle of feedfilter-wash-discharge and the wheel rotates after each stage of the cycle so that each pan in turn is indexed into the appropriate station. According to the number of
washing stages required, and the size of the pans, then up to four, or perhaps more, of these sets of pans may make up the complete circle.

The rotary table filter is an extension of the multi-pan filter design. The pans are replaced by segments of the annulus, which still rotates around in a circle so that each segment in turn filters and washes. The segments do not tip, however, to discharge the solid, which is instead scraped off the surface of the filter medium by a screw conveyor, over a flexible outer containing wall.

Feed to the rotary table is usually across a weir positioned over the full width of the segment. The filter medium is a single piece of material, in an annular shape, and the segments are divided underneath the medium, with appropriate filtrate discharge pipes. The action of the screw conveyor in removing the cake leaves a thin ‘heel ’ of unremoved solids (or the medium would be rapidly abraded by the screw action). If this heel would be a problem, than compressed air is injected from below the medium, just under the feed point, to free the heel and mix it with the feed slurry.

Unlike the multi-pan filter, which can have several feed and discharge points, the rotary table filter moves continuously from one feed point to one discharge zone. Both are being supplanted in industrial usage by the horizontal belt filter.

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