FILTER PRESS BELT

Abstract: A filter press belt comprises a non-woven mesh membrane (10) moulded from thermoplastics material so as to provide a planar load transfer surface.

Description: FILTER PRESS BELT The present invention relates to a filter press belt. Belt filter presses are used in a wide number of industries to separate solids from fluids. A common application for belt filter presses is in removing water from sludge in, for example, processes used in paper mills, sewage works, water treatment plants or mines. One application for belt filter presses involves dewatering mineral waste products such as gypsum produced as a result of lime solution being sprayed into coal-fired electricity generating station flue gases to remove sulphur. Belt filter presses are also used for dewatering raw materials such as coal, buaxite, iron ore, potash or phosphates, or for screening materials. They are also used in chemical production processes, for example, in the manufacture of ascorbic acid, citric acid, pectin or phosphoric acid. During the production of phosphoric acid the filter press is used to remove the gypsum waste product from the acid. In the above processes the water content is reduced so that when the sludge if removed for transportation the water is not being transported. This decreases transportation costs. Water removal is also beneficial if the sludge is to be incinerated. A particular application of belt filter presses is in pulp dewatering. Pulping mills are normally located near the wood source, i.e. forested areas and the pulp is then transported to paper mills, usually in non-forested regions. If the transportation distance is small the pulp is converted into a high consistency (i.e. high solids content) "crumb". More commonly transportation distances are great and in this case the pulp is converted into a high consistency "wetlap" form where a double-wire press initially dewaters the stock down to about 40% wt. dryness, followed by a heavy-duty press where dryness is increased to 48- 50% wt.. The pulp web is then cut into sheets by a rotating knife drum and stacked onto pallets. Belt filter presses generally comprise two endless belts, the lower belt being arranged generally horizontally. Water is drained from the initial section of the lower belt under gravity. The upper belt runs over the end section of the lower belt in a so- called "wedge zone" in which the upper belt is set at an angle to the lower belt such that the two belts merge at the end of the horizontal section of the lower belt so as to apply increasingly greater pressure to the pulp provided therebetween. The two belts then pass through a series of roller arrangements which in the most modern machines includes one or more nip rolls. The condensed pulp is collected therefrom. Heavy duty presses again comprise two moving endless belts which form a nip therebetween. Conventional fabrics used in a pulp dewatering presses are simple single-layer woven structures with a plain or twill weave, comprising coarse, such as 1.2 mm diameter monofilaments of, for example, polyester or nylon. These woven structures transfer the press load to the sludge. This pressure transfer is concentrated at the yarn knuckles or floats and therefore the transfer of press load is not completely even or efficient. Furthermore the woven structures are complicated in nature and therefore costly to produce. According to the present invention there is provided a filter press belt comprising a non-woven mesh membrane moulded from thermoplastics material. The moulding process which is used to make the membrane is quicker, less complicated and less critical than the prior art weaving processes. Ideally the filter press belt solely consists of one or more of said mesh membranes, and provides a planar load transfer surface. The planar nature of the outer membrane surface, which contacts the material to be filtered, provides much greater transfer and more even transfer of press load compared with woven structures where pressure transfer is concentrated at the yarn knuckles or floats, thereby improving dewatering efficiency. Furthermore the manufacture of membrane belts does not involve the complicated weaving, heat-setting and seaming techniques associated with making woven dewatering press belts. The membrane is preferably resilient, as these belts operate at high press loads. Modern machines work at a press nip load of up to 100 kN/m. This resilience may be achieved by using an elastomeric material for forming the mesh membrane. Preferred elastomeric materials include any of the following either alone or in combination with other materials:- polyurethane, polyester, polyamide or silicone polymers. Further materials may be used to optimise belt properties. For example, an abrasion-resistant nylon or polyurethane matric may be reinforced by dimensionally stable PET or PEN mono or multifilament machine direction yarns. The membrane may conveniently be manufactured in accordance with the process described in US 4,740,409. This allows a full range of thermoplastic or thermoset polymers to be used as matrix material, enabling abrasion resistance, resilience and pH resistance to be optimised. It is noted that the membrane may be formed as an endless tube as is described in GB 2254287 or it may include a seamed arrangement of the type described in US 5,169,670 or EP 0518494. Multi-layered membrane structures may be used to create a permeability gradient. Here at least two membranes would be used having different hardness values, different open areas, different mesh profiles and/or different mesh hole distributions. One or more yarn members may be incorporated within at least one of the machine direction lands of the membrane, or at least one of the membranes for multi- layered structures, so as to provide adequate strength in the running direction of the belt. Cross-direction rigidity can be improved by incorporating one or more ya members within the cross-machine direction yams of the membrane, or at least one of the membranes for multi-layered structures. Yams in the different membrane layers may extend parallel or at right angles to each other. The membrane may be laminated with woven fabric, knitted fabric and/or non- woven material on one or both of its sides. Such structures would be for use as filter belt applications for recovery fine particulate matter. Here the fabric and particularly the non-woven material performs a filtration function. Non-woven knitted or woven fabric can also be laminated for improving cross direction rigidity. For such purposes the knitted or woven fabric may be very open. Such fabrics are ideally located on the side of the membrane that is remote from the side of the membrane that, when in use, initially contacts the material to be filtered. The membrane may comprise channels for draining liquid, the channels ideally running in the machine direction of the belt. These channels are formed on the side of the membrane that is remote from the side of the fabric that initially contacts the material to be filtered so as not to affect the planar nature of that side of the fabric. The channels may be formed by utilising membranes which have lands of differing depths. A machine direction drainage channel may be formed by moulding a membrane having at least some and ideally all cross machine direction lands which are not as deep as the machine direction lands. In order that the present invention may be more readily understood specific embodiments thereof will now be described by way of example with reference to the accompanying drawings in which: - Fig.1 is a perspective view of part of one filter press belt in accordance with the invention; Fig.2 is a cross section in the cross machine direction of a second belt in accordance with the present invention; Fig.3 is a cross section in the cross machine direction of a third belt in accordance with the present invention; and Fig.4 is a cross section in the cross machine direction of a fourth belt in accordance with the present invention. Referring to fig.l a filter press belt 10, only part of which is shown comprises a mesh membrane of elastomeric material. The membrane comprises machine direction lands 11 and cross-machine direction lands 12. Rectangular apertures 13 are defined between the lands. A ya member is encapsulated in each machine direction land 11. Yam members may optionally be encapsulated in the cross-machine direction lands 12. In use the membrane is used in a double wire press or heavy duty press of a pulp dewatering apparatus. Referring to fig.2 a filter press belt 15 comprises a mesh membrane of the type shown in fig.1. Similar reference numerals are used to indicate like parts. The mesh membrane is laminated to a layer of non- woven material 16 comprising a batt of staple fibres. The belt has a similar use to the belt described with reference to fig.1. Referring to fig.3 a batt 17 is illustrated which is similar to that illustrated with reference to fig.2 and again similar reference numerals are used to indicate like parts. Here a needlefelt 18; i.e. a composite woven and non-woven material comprising a batt of fibres needled to a woven web is used in place of the non- woven batt 16 illustrated in fig.2. Again the belt has a similar use to that described with reference to figs.1 and 2. Referring to fig.4 there is shown a belt 20 similar to that described with reference to fig.2 except in that the cross machine direction 12 lands of the machine are not as deep as the machine direction lands 11. This creates machine-direction channels 19 which aid water drainage. It is noted that all embodiments of the invention as described above provide a planar upper land surface. It is to be understood that the above described embodiments are by way of illustration only. Many modifications and variations are possible.

Claims: CLAIMS 1. A filter press belt comprising a non-woven mesh membrane moulded from thermoplastics material. 2. A filter press belt comprises a non- woven mesh membrane as claimed in claim 1, wherein said membrane provides a planar load transfer surface. 3. A filter press belt as claimed in claim 1 or claim 2, wherein the filter press belt comprises a single said membrane. 4. A filter press belt as claimed in claim 1 or claim 2, wherein the filter press belt comprises at least two of said membranes. 5. A filter press belt as claimed in claim 4, wherein the said two membranes have different hardness values, different open areas, different mesh profiles and/or different mesh hole distributions. 6. A filter press belt as claimed in claim 3, wherein at least one yam member is provided within at least one of the machine direction lands of the said membrane. 7. A filter press belt as claimed in claim 4, wherein at least one yam member is provided within at least one of the machine direction lands of at least one of said membranes. 8. A filter press belt as claimed in claim 3, wherein the belt comprises at least one yam member in at least one of the cross-machine direction lands of the said membrane. 9. A filter press belt as claimed in claim 4, wherein the belt comprises at least one yam member in at least one of the cross machine direction lands of at least one of the said membranes. 10. A filter press belt as claimed in any preceding claim, wherein the membrane is laminated with woven cloth, knitted cloth and/or non-woven material. 11. A filter press belt as claimed in any preceding claim, wherein the membrane comprises lands having differing depths. 12. A filter press belt as claimed in any preceding claim, wherein the membrane has at least some cross-machine lands which are not as deep as the machine direction lands.

Inventors: HAASMA HANS VELLO WILHELM, HAASMA, HANS, VELLO, WILHELM
Applicants: HAASMA HANS VELLO WILHELM, SCAPA GROUP PLC, HAASMA, HANS, VELLO, WILHELM
Application year: 1998
Main classification: B01D33
IPC codes: B01D33