Using centrifugal pumps in the foam process of producing non-woven webs

#The foam-laid process for forming non-woven fibrous webs is basically disclosed in U.S. Pat. Nos. 3,716,449, 3,871,952, and 3,938,782 (the disclosures of which are incorporated by reference herein). The foam-laid process has a number of advantages over the water-laid process that is most conventionally used for making synthetic or cellulose fiber webs. The invention relates to a method and assembly for implementing the foam-laid process so as to improve aspects thereof.

#While the foam process has a number of advantageous over the water-laid process in the production of fibrous non-woven webs, one of the practical disadvantageous thereof that has limited its commercialization to particular types of foams, is the relatively narrow width of the webs that have been produced from the foam-laid process in the past (e.g. typically in the range of 1-1.5 meters), compared with the width of the web in typical paper machines using the water-laid process which can be more than ten meters. Also, the production speed of the foam process has in the past been typically significantly under 100 meters per minute.

#The main limitation in the width of the web and speed of operation in prior art systems implementing the foam process has been the pumps used to implement the process. The pumps are positive displacement pumps, such as screw pumps, twin screw pumps, twin rotor pumps, or the like, which have limited pumping capacity. Some of these positive displacement pumps are relatively insensitive to the material being pumped and, therefore, operate well in the production of fiber and gas containing fluids, which are, of course, characteristic of the foam process and it is for that reason that they are used. However, some of these pumps are easily worn out, are expensive, and easily damaged. Consequently, if the production is to be increased (as by increasing the size of the web produced by increasing the width of the wire or other foraminous element to more than 1.5 meters) several pumps must be used in parallel. This increases the expense of the assembly dramatically, and also introduces the risk of one of the pumps becoming damaged and thereby shutting down the entire process to repair or replace the damaged pump.

#FIG. 1 is a general schematic illustration of a foam laid process system in which the method of the invention may be practiced and the apparatus of the invention utilized;

#FIG. 2 is a detail schematic view, partly in cross-section and partly in elevation, showing the feed of a foam/fiber slurry from the mixer to the pump feeding the manifold and headbox of the system of FIG. 1; and

#FIG. 3 is a side view, mostly in cross section, but partly in elevation, of an exemplary degassing centrifugal pump that is utilized in the practice of the present invention.

#An exemplary foam-laid process system for practicing a foam laid process with which the invention is desirably utilized is illustrated schematically at 10 in FIG. 1. The system includes a mixing tank or pulper 11 having a fiber input 12, a surfactant input 13, and an input 14 for other additives, such as pH adjustment chemicals like calcium carbonate or acids, stabilizers, etc. The particular nature of the fibers (whether glass, synthetic, and/or cellulosic or natural), surfactant, and additives is not critical and they may be varied widely depending upon the exact details of the product being produced (including its basis weight). It is desirable to use a surfactant that can be fairly readily washed out since a surfactant reduces the surface tension of the final web if it is still present, and that is an undesirable feature for some products. The exact surfactant used, from the thousands that are commercially available, such as described generally in U.S. Pat. Nos. 3,716,449, 3,871,952, and 4,856,456, is not part of the present invention.

#The tank 11 is per se entirely conventional, being the same type of tank that is used as a pulper in conventional paper making systems using the water-laid process. The only differences are that the side walls of the mixer/pulper 11 are extended upwardly about three times the height in the water-laid process since the foam has a density about a third that of water. The rpm and blade configuration of the conventional mechanical mixer in the tank 11 is varied depending upon the particular properties of the product being produced, but is not particularly critical, and a wide variety of different components and variables may be employed. Brakers may also be provided on the walls. There is a vortex at the bottom of the tank 11 from which the foam drains, but the vortex is not visible once start up occurs because the tank 11 is filled with foam and fiber.

#The tank 11 also preferably includes therein a large number of pH meters 15 for measuring the pH at a number of different points. pH affects surface tension, and thus desirably is accurately determined. The pH meters are calibrated daily.