This invention relates to liquid/solid separation systems and, more particularly, to an improved belt filter press for removing excess liquid from a partially dewatered liquid/solid suspension.
In the process of making many solid waste by-products, and in the field of industrial and municipal water and wastewater treatment, a number of devices are utilized in the general category of liquid/solid separation. These devices include centrifuges, screw presses, vacuum filters, belt filter presses, pressure filters and others. Although they all operate on different principles, the basic purpose of each is to receive a pumpable liquid/solid suspension, to accomplish as complete a separation as possible of the solid suspended matter from the liquid, and to discharge the suspended matter in a semi-solid non-pumpable form, referred to as "cake".
The principle of operation of the device for which patent application is being made is a modification of the concept used in the devices generally categorized as "belt filter presses". Reference to FIG. 1 and 2, showing a typical prior art belt filter press will illustrate the principle of operation of belt filter presses.
A slurry 10 comprised of liquid and solids is pumped onto a horizontally positioned portion 13 of a belt 11. Belt 11 is of a liquid-permeable material, generally a polyester fabric. In most cases some type of organic compound has been added to the slurry to make it more amenable to filtration. A second similar belt 12 then converges on top of belt 11, trapping slurry 10 between the two belts. The sandwiched material then passes over any number of rollers, illustratively rollers 7, 8 and 9, which rotate around shafts 3, 4 and 5, respectively, in FIGS. 1 and 2. The liquid is driven from the solids by the tension created in the device or by direct application of pressure. The vertical arrows in FIG. 1 indicate the direction of flow of expressed liquid. A semi-solid cake 15 is discharged at the point where belts 11 and 12 diverge.
In all of the various designs of belt filter presses, provisions are made for cleaning both belts with high-pressure wash-water before the belts make contact with the incoming slurry. In addition, provisions are generally made to ensure that the belts remain centered on the rollers, since they are driven toward the direction of discharge and have a tendency to wander to one side of the machine or the other. However, the number of rollers, the relative diameters of the rollers with respect to one another, and the relative positions of the rollers with respect to one another differ greatly from one design to another.
One of the major deficiencies in all belt filter press designs is that the moisture removal efficiency is not as high as should be theoretically attainable. The primary cause for this inability to achieve optimum performance is that the moisture which is expressed through the top belt 12 has no means of being removed and is eventually reabsorbed by the filter cake.
All existing belt filter press designs incorporate rollers which are on a horizontal plane as illustrated in FIG. 3 which shows the orientation of roller 7, as viewed from the slurry feed end of the belt filter press shown in FIGS. 1 and 2. Roller 7 rotates about shaft 3. Left end 30 of shaft 3 is the same distance x from a horizontal reference plane 35 as is right end 31. A horizontal reference plane is defined as any plane parallel to horizontal portion 13 of the path traveled by belt 11. Because of this orientation, the force of gravity acts upon the moisture expressed through the top belt 12 with the solids captured between the two belts as an intermediary only in a downward direction whereby the gravity force vector is perpendicular to the plane of the trapped solids causing liquid expressed through the top belt to be reabsorbed through the fabric back into the trapped slurry.