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BACKGROUND OF THE INVENTION 
     The invention relates to manufacture of mineral wool-based acoustical ceiling tile. 
     PRIOR ART 
     Mineral wool-based acoustical ceiling tile is conventionally made in a wet felting process using Oliver or Fourdrinier machines, well known in the art. Mineral wool and binder have long been used to form acoustical ceiling tile. Mineral wool is used because it is relatively inexpensive and inert. Mineral wool is typically shipped from a site where it is manufactured to another site where it is used to manufacture acoustical ceiling tiles. Typically, the mineral wool is compressed in bales to reduce its volume to facilitate handling and shipment. A bale of mineral wool may be compressed to a density of at least 8 lbs. per cubic foot. 
     When compressed and bound into bales, individual mineral fibers become entangled with one another. Long established practice has been to open a bale and break it up with power rotated metal tines or fingers and to deliver the coarsely separated mass of fibers to a mixing tank. The density of the mineral fibers while quite variable due to the coarseness of the bale opening devices can be, by way of example, 5 to 6 lbs. per cubic foot. In the mixing tank, a high speed rotary impeller is used to open and separate the fibers while simultaneously mixing other constituents of the tile basemat to be formed. The conventional practice is to rely on the mixing paddle blades of the mixing tank impeller to detangle the mineral fibers since the fibers were to be received in the tank and the mixing blade was to be employed to disperse the other constituents through all of the water in the tank. 
     This procedure has been found, however, to have deleterious effects on the mineral wool. The circular mixing action has a tendency to roll the fibers into little balls or nodules that are of greater density than desired. Fiber breakage results in reduced strength in the finished tile and nodulation can reduce potential NRC (noise reduction coefficient) values due to a loss of porosity. The nodules do not interlock or co-mingle with the other constituents. 
     SUMMARY OF THE INVENTION 
     The invention is a process for making mineral wool-based acoustical ceiling tile with improved acoustical performance. The inventive process takes the traditional task of fully separating the mineral fibers in a water filled mixing tank and moves the task upstream where dispersion of the mineral fibers is accomplished mechanically in an air environment. It has been discovered that the mineral fibers can be successfully and sufficiently separated, as measured by bulk density of the fibers with mechanical devices operated in air and, optionally, with an air stream so that a ceiling tile with improved properties results. Less fiber bundling occurs when they are dispersed without significant agitation in a water mixing tank allowing the fiber content in a ceiling tile to be reduced without a loss of strength. The reduced fiber content, for a given caliper of a tile, results in greater porosity and, consequently, a higher NRC value. Also adding to the ability to decrease overall fiber content is a reduction or elimination of fiber nodules. Nodules are otherwise created by extended fiber mixing by a high speed water immersed impeller. The nodules are relatively dense and, therefore, do not contribute proportionately to sound reduction. Fiber separation in air upstream of the mixing tank can have the beneficial effect of reducing the content of shot, i.e. the un-fiberized portion, in a given volume of mineral wool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation of a prior art process for preparing a dilute slurry of mineral fiber and binder for a wet felted acoustical ceiling tile; 
         FIG. 2  is a diagrammatic representation of an exemplary process for preparing a dilute slurry of mineral fiber in accordance with the invention; and 
         FIG. 3  illustrates a multi-station system for refined control of mineral fiber separation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Conventionally, in the manufacture of wet felted mineral wool-based acoustical tile, a dilute water slurry of mineral fiber and binder of starch and/or latex, and other minor amounts of solid components is used. The slurry is distributed from a mixing tank onto a travelling wire screen, sometimes simply called a wire. A mixing tank for the slurry is shown in  FIG. 1  at  10 . It has been the practice to receive large bales  11  of mineral wool held in a compressed state by bands or otherwise from a manufacturer or other source. By way of example, a bale  11  may weigh 1000-1300 lbs. and the mineral wool in the bale can be compressed to a density, for example, of between about 60 and about 65 lbs. per cubic foot. Upon release of the constraining bands, a bale  11  will expand somewhat on its own. The loose bales  11  are conveyed to a bale breaking station  12  where spiked rolls grab and separate clumps  13  of mineral wool from the loose bale  11 . The clumps, separated from a bale  11 , are directed by the spiked rolls and or chutes (not shown) into the mixing tank  10 . The ordinary practice is to separate the clumps  13  of wool into loose fibers with a high speed impeller  14  that simultaneously serves to mix and suspend other constituents in the tank water. 
     The bulk density of the clumps  13  is approximately 5½ lbs. per cubic foot. In order to disperse the fibers of the mineral wool clumps  13 , a typical mixing time can range between about 12 and 15 minutes. 
     The impeller  14  tends to tumble the mineral fibers and induces the fibers to form tight balls or nodules. The fiber nodulation limits the strength of the tiles in which the fibers are incorporated. The fiber nodules diminish the potential sound absorption ability of the tiles since they decrease homogeneity. 
     Referring to  FIG. 2 , there is depicted an example of the inventive process for effectively dispersing the fibers of mineral wool from one another prior to delivery into a conventional mixing tank  10  so as to eliminate the need for further fiber dispersing action in the mixing tank. Bales  11  of the character described above are released from their binding and carried along a path that ultimately delivers adequately dispersed mineral fibers to the conventional mixing tank  10 . The bales  11  are received at a preliminary opening station  15  where meshed spikes or tines  16  on counter-rotating rolls  17  separate the bale  11  into wool clumps or tufts  18  that are subsequently fed into a fiber separator  19 . The fiber separator  19 , by way of example, but not limitation, may be of the type disclosed in U.S. Pat. Nos. 4,111,493 and 4,978,252. The relatively dense small pieces or clumps  18  of the bales  11  are directed to a hopper  21  of the fiber separator  19  where they are circulated over and caused to fall through a hopper outlet  22 . The clumps  18  are received on a grid  23  (extending perpendicular to the plane of  FIG. 2 ) of parallel bars. The mineral wool clumps  18  are converted into separated fibers by tines or spikes  24  on a rotating shaft  26 . The tines  24  pass through the plane of the grid  23  enabling them to operate on the clumps  18 . A second set of tines  27  on a shaft  28  meshes between the first set of tines  24  to further separate the fibers. The fibers are propelled downwardly by the tangential motion of the tines  27  and by gravity into a rotary pocket feeder  29 . Fibers delivered to a lower part of the feeder  29  are forcibly discharged pneumatically from the fiber separator  19  by compressed air at above atmospheric pressure to a conduit  31 . 
     The conduit  31  discharges into the top opening of the mixing tank  10 . The mineral fibers are dispersed to the degree that they have a bulk density of preferably about 2 lbs. per cubic foot, and most preferably between 1.2 and 1.0 lbs. per cubic foot. 
     Preferably, the tank  10  is filled with water and tile components including a binder of starch and/or latex and optional constituents such as expanded perlite, paper fiber, a filler such as clay, and glass fiber. Preferably, these constituents are premixed before the mechanically air dispersed mineral fibers are delivered into the mixing tank  10 . This premixing can minimize the exposure of the dispersed mineral fibers to the breaking and modulating effects of the mixer rotor or impeller  14 . Ideally, the mineral fiber slurry is discharged from the tank immediately upon a desired consistency being reached. 
     The slurry is discharged from the tank  10  onto a moving screen of a wet felting machine such as an Oliver or Fourdrinier machine. 
     The ability to adequately disperse the mineral wool fibers from the compacted wool bales  11  in an air environment such as by mechanical fingers and with an air stream to transport the fibers pneumatically to a state where they are sufficiently disentangled and dispersed has significant advantage in the manufacture of acoustical ceiling tile. The uniformity and separation of the mineral fibers by dispersion in air can result in a more open mat in a finished ceiling tile than has been practical to obtain by the water dispersion of the prior art. 
     Variations in the manner of dispersing mineral fibers in air for direct use in a water slurry without significant mixing in the water slurry are contemplated by the invention. Various arrangements using rotating, reciprocating and/or oscillating mechanical fingers or tines and/or air jets and/or air stream can be implemented in a path from a bale receiving station to a slurry mixing tank. These devices and expedients should reduce the density of the fibers to at least 2 lbs. per cubic foot before they are introduced to the mixing tank. An air stream can be provided by an air source operating above atmospheric pressure or a suction device operating below atmospheric pressure. While the relatively simple fiber separation system disclosed above has proven to obtain improvements in the performance of acoustical tile, it is expected that more elaborate air dispersion processes can be used with even greater success and higher throughput. 
       FIG. 3  illustrates a more extensive fiber separating system which can afford more control over mineral fiber separation than that afforded by the system of  FIG. 2 . The system  36  comprises a number of stations that employ bale and fiber handling units that are commercially available from one or more manufacturers. A first station  37  provides a tipper unit  38  for an unbound bale  39  of compacted mineral fiber. The tipper unit  38  deposits the unbound bale  39  of mineral fiber onto a conveyor  41  of a bale opener  42  comprising the second unit of the system  36 . The bale opener  42  includes spiked rolls  43  that break the bale  39  down into uncompacted fibers. From the bale opener  42  the fibers are delivered to an auxiliary fiber separator unit  46  representing the third station of the system  36 . The auxiliary fiber separating unit  46  can reduce the shot content of the mineral fiber stream. 
     Fibers are delivered from a rotating paddle wheel  47  of the auxiliary unit  46  to a final or fourth station  51 . Fibers received in the fourth station are conveyed and elevated to a weighing hopper  52  at which well separated mineral fibers are collected until a predetermined weight or mass of such fibers is gathered. When the fiber weight reaches the predetermined level, the hopper  52  is opened to release the fibers into a mixing tank  53  which serves the same function as the tank  10  described in connection with  FIGS. 1 and 2  above. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
A method of forming a dilute water slurry for water felting a basemat for an acoustical ceiling tile comprising delivering a bale of compressed mineral wool with a density of at least 8 lbs. per cubic foot at an unbaling station, releasing a binding holding the bale in compression, mechanically separating the fibers of a mineral wool bale with mechanical instrumentalities arranged to disperse the fibers to a generally uniform density of less than 2 lbs. per cubic foot prior to passage of the fibers through the tank inlet, and causing the separated mineral fibers to pass into the mixing tank for contact with water suspended binder.