The invention concerns filter cloths, and has more particular reference to filter cloths of the kind used in filter presses for the separation of the solid/liquid phases of slurries, and especially those used in the winning of precious metals in an ore extraction process.

It is known in the art to provide a seal at the peripheral region of a press-type filter cloth by the application of a sealant to the filter cloth in that region thereof which is to be clamped between successive press-plates. Conventionally the sealant comprises a water - or solvent - based neoprene latex material, and such material is applied by superimposing a template on the cloth and pouring latex into the uncovered areas, a reduced pressure being provided at the underside of the cloth to draw the latex into the cloth. An alternative method involves what is an essentially "hand painting" process, but both methods are such that accuracy and uniformity of impregnation are difficult to achieve. Furthermore, proper location of the template relative to the cloth and the subsequent creation of other impermeable regions therein for eventual accurate registration with feed ports in the press-plates leaves much to be desired.

The object of the present invention is to provide an alternative to the conventional method of providing a seal in the requisite regions of a filter cloth intended for. use in a filter press which facilitates the production of a

filter cloth having a sealing characteristic in selected, accurately located regions thereof, and avoids the highly labour intensive methods hithereto practised.

According . to the present invention there is proposed a method for the production of a filter cloth having selected regions thereof rendered impermeable and capable of providing a seal when located in position between the successive plates of a filter press, the method including the steps of silk screen printing the cloth in selected areas with a suitable sealant, the screen having permeable and impermeable regions respectively corresponding to regions of intended impermeability and permeability of the filter cloth, and effecting a cure of the sealant material.

According to a preferred feature, those regions of intended impermeability which are to be placed in registration with and about feed or delivery ports in the press plates are apertured to provide flow passages through the cloth.

The invention also includes a filter cloth for disposition between successive plates of a filter press, the cloth including a sealing region defining a closed loop for location in clamped relationship between the flanges of said plates to provide a seal thereat, the closed loop conforming to the shape of the flange of the press plates to which the cloth is to be applied, and at least one further printed region remote from said closed loop for registration with a feed aperture in said plates, the said at least one further

printed region being apertured to provide a flow passage through the cloth.

According to a further preferred feature, the sealing region defining the closed loop is of a width in excess of the corresponding width of the plate flange, to provide an impermeable peripheral region inwardly of the flange in the use condition of the filter cloth.

The invention will now be described further, by way of example only, with reference to the accompanying diagrammatic drawings illustrating one embodiment thereof and in which : -

Fig. 1 is a section taken longitudinally of a filter press of the kind to which the filter cloth of the invention is to be applied;

Fig. 2 is an enlarged view of a part of Fig. 1;

Fig. 3 is a view on line III-III of Fig. 1;

Fig. 4 is a plan view of the filter cloth of the invention; and

Fig. 5 shows the successive stages in the method of the invention.

Referring now to Figs. 1 and 2 of the drawings, a filter press 11 comprises a multiplicity of recessed press plates 12 arranged in face-to-face disposition, there being a filter cloth 13, 14 at either side of each press chamber 15 and the peripheries of the cloths 13, 14 being clamped between the flanges 16 of the press plates 12 and being secured in relation to centre feed apertures 17 of the

respective said plates, all in conventional manner. The recesses in the filter plates will ordinarily include stay bosses 18 to maintain flow passages between the filter cloths 13, 14 and the floor 19 to the recesses, thereby to facilitate drainage of liquids passing through such cloths. Drainage conduits 21 are provided in the plate flanges 16 in fluid flow connection with a drainage passage 22 in the flange, through which liquids are removed from the press.

In order to ensure an adequate seal at the flanges 16, that region of the filter cloths 13, 14 existing between such flanges is impregnated with a suitable sealing material. The present invention is concerned with filter cloths for use as aforesaid, and relates particularly to a method of imparting a sealing characteristic to the edge region, and indeed to other relevant regions of the filter cloth, and proposes the use of a silk screen printing technique as a means whereby an appropriate sealing material might accurately be applied to selected regions of the filter cloth. The invention also proposes a modification to the extent to which the edge region of the filter cloths are rendered impermeable.

-Thus referring now to Fig. 5 of the drawings, a suitable sealing material 51 is applied in a predetermined pattern to the surface 52 of a filter cloth 53 through a silk screen 54, the filter cloth 53 being supported on a support surface 55, the silk screen 54 being disposed in overlying relationship thereto, and the cloth being printed with a

pattern corresponding to that appearing on the screen by drawing a damming edge 56 across the screen thereby to force the sealing material 51 through the screen 54 and onto the filter cloth 53. After application of the sealing material 51, the printed filter cloth 53 is removed from the support surface 55 and is placed in an oven 57 to effect cure thereof.

The silk screen bears a pattern of permeable and impermeable areas according to specific requirements, and in the present instance, see now Fig. 4, is such as to print a rectangular frame-like outer seal region 41 in closely spaced disposition relative to the edges of the filter cloth 42, the inner corners 43 of the outer seal region being truncated, as at 44, an annular seal region 45 centrally of the filter cloth, and a multiplicity of eyelet regions 46 outwardly of the outer seal region 41. In addition, an array of stay boss seal regions 47 is provided in positions to correspond with the stay bosses present on the filter plates. A non-printed region 48 is provided within each corner 43 of the frame-like outer seal region 41.

After screen printing and curing the edges of the cloth are trimmed in any convenient manner, and apertures are punched in the filter cloth 42 in register with the annular seal region 45, to provide a centre feed aperture, and with the non-prin ed regions 48 in the corners 43 of the frame-like outer seal region 41, to connect the aligned drain passages in the successive press plates with which such

regions are aligned when applied to the filter press.

The individual eyelet regions 46 are also punched out, to receive eyelets thereat.

In the case of the outer seal region 41 and the annular seal region 45 _r the material applied to the filter cloth 42 serves as a sealant to prevent leakage of liquid through the fabric in the plane thereof- In the case of the stay boss seal regions 47 and the eyelet regions 46, the added material serves to protect the filter cloth against wear and damage.

It is to be observed from Fig. 3 that the width of the outer seal region b_ is materially in excess of the flange width c_ of the press plates, such not being the case with conventional filter cloths wherein the sealing material is provided to a width equal to or less than that of the flange width. • It has been found that by increasing the width such that the seal region extends to a position within the chamber, and indeed at least partially into the non-rectilinear peripheral regions thereof, a differential cake wash water flow is created within the chamber such as promotes uniformity of washing throughout the full volume of the chamber and thus,. for example, maximises the metal-winning capability of the press.

The sealing material will preferably be a water based neoprene latex, in view of the facility whereby such material may be cured, but other materials, such as u.v. curable polymers, for example acrylated silicones, may be

preferred in some instances.

The fabric will ordinarily be of a kind conventionally used in the art, that is to say a structure woven from synthetic yarns, although alternative structures, such as for example, needle felts may be used.

It is to be observed that with closely woven structures or needle felts, the provision of a reduced pressure at the underside of the felt will improve sealant take-up, and encourages the production of an in-depth rubbery gasket.

We have found that by means of our invention we are able to provide a readily reproducible press felt at a higher rate of production than has hitherto been achieved without the need for, and the problems inherent in, separately marking out eyelet fixing holes to support the cloth and the site of the barrel neck component. The normally labour intensive separate marking operation, which is prone to error and is irksome is avoided.

The invention is not limited to the detail of the embodiment herein set forth, since alternatives will readily present themselves to one skilled in the art. Thus, for example, whilst the provision of reinforcement in those regions of the cloth which will overlie the stay bosses, if the same are present, is convenient, such reinforcement need not be provided.