This invention relates to a filter cloth securing device which attaches the filter cloth to the filter plates of a plate filter press in the zone of the plate opening (port) provided for the passage of the slurry. The securing device has two flanged sleeves inserted into the slurry port from opposite sides of the filter plate and a threaded tensioning sleeve which also passes through the slurry port and which axially clamps the flanged sleeves to one another. The flanges of the flanged sleeves extend essentially parallel to the plane of the filter plate.
In chamber-type or diaphragm chamber-type filter presses where the slurry inlet is arranged in the zone of the filtering face of the filter plates, the filter cloth must be sealed with respect to the edge zones of the slurry ports. Such seal is of importance in case of individual or continuous filter cloths as well as in case of tubular or pass-through filter cloths in order to prevent a filtering effect and thus a filter cake formation within the slurry channel constituted by the series of axially aligned slurry ports. As securing and sealing means generally the earlier-noted flanged sleeves are used whose flanges press the filter cloth against the respective filter plate side in the wall zone of the slurry port.
In order to ensure an optimal utilization of a filter press, it is desired that the filter cakes in all filter chambers be identically thick. This is of particular importance in case the filter cakes have to be washed or dried in the filter press, because only a uniform filter cake thickness ensures a uniform washing and drying effect. A uniform cake thickness, however, preconditions identical chamber depths which in turn require an accurately central positioning of the filtering face in the individual filter plates. Particularly in filter plates made of synthetic materials, deformations of the filtering face from the central plate plane may occur because of stresses applied during manufacture, shipping or storage. Furthermore, in case of hot filtering, the filtering face may undergo bulging deformations even in case of very small pressure differences resulting, for example, from non-uniform filter cloth soiling. Such deformations may lead to a cracking or breakage of the filter plate.
In diaphragm-type filter presses a bending of the filtering face and thus an increased filter chamber depth on one filter plate side has the additional disadvantage that the diaphragm may press annularly into the slurry channel on that side so that the diaphragm may be torn away from its securing device or may be torn open.
In order to prevent a bending deformation of the filtering face, U.S. Pat. No. 3,503,326 discloses the use of tubular screw-in flanges which are threaded into corresponding passages of the filter plate from both sides and wherein the tubular part of the components constitutes the slurry port and the flanges sealingly clamp the filter cloth against a cooperating counterface of the filter plate. The flanges have such a thickness dimension that in the closed (pushed-together) state of the filter plate stack they lie on one another and thus are in a mutually supportive relationship. The slurry inflow in the individual filter chambers is effected by radial grooves provided in the outer surfaces of the flanges. In such a structure the grooves of two superposed flanges of adjoining filter plates complement each other to form a closed channel. It is a disadvantage of this arrangement that the screw flanges must have an accurate angular (screwed-in) orientation in order to position the grooves such that in the closed state of the filter plate stack the grooves of adjoining flanges complement each other to form a slurry channel. Considering the permissible tolerance deviations, this requirement cannot be satisfied with the desired accuracy.
It is a further disadvantage of the above-outlined prior art arrangement that upon tightening the screw flanges, as a result of friction that part of the filter cloth which lies underneath the flange faces is dragged by the flange As a result, folds may appear which can cause leaks between the slurry channel and the filtering spaces. If the threaded flanges are not tightened sufficiently, the individual filter cloth securements may abut one another in an axial direction even before the filter plates are tightly pressed to one another at their outer sealing edges. As a result, the filtering chambers will not be separated from the environment in a fluidtight manner. It is still another disadvantage of this prior art arrangement that during the emptying process solid deposit residues may clog the channels formed by the grooves.
A similar construction is known in the field of chamber-type filter plates which are utilized as a combination with diaphragm-type filter plates known from German Patent Nos. 2,322,044 and 2,324,876. In this arrangement too, screw-in flanges are provided which are tightened against one another by a threaded bushing and thus clamp the filter cloth. In this arrangement too, the slurry is admitted into the filter chamber from a slurry port formed by the threaded sleeve and through radial grooves provided in the outer faces of the flanges. The flanges are configured such that the diaphragms of the adjoining diaphragm filter plates annularly lie against the flanges upon inflation and cover the mouth of the radial grooves in the filter chamber. This prior art arrangement too, has the disadvantage that the radial grooves are clogged by the filter cake and also, during the pressing step, the diaphragms additionally press filter cake portions from the filter chamber into the radially extending grooves.
In all known structures of filter cloth securing devices solid deposits do not detach by themselves from the radial grooves during the opening of the filter press. Further, as the filter cake falls down in the zone of the slurry intake, solid parts are sheared off the filter cake and remain suspended in the slurry ports, particularly because of the recessed end faces of the threaded sleeves. Risks are high that these residues may block the radial grooves in the outer faces of the flanges during the subsequent operational cycle. Consequently, the grooves as well as the slurry ports of the individual filter plates have to be carefully cleaned manually in each filter chamber after opening the filter press in order to avoid any damage during the subsequent filtering cycle. A cleaning of the slurry ports and the radial groove is, however, not readily feasible because of the particular configuration of the filter cloth securing devices. Furthermore, because of a need for the manual cleaning, the advantage of a mechanized plate shifting apparatus which should make possible an automatic filtering operation is partially lost, and thus the time needed for the manual cleaning work adversely affects the output of the filter press.