Abstract:
Air is drawn upwardly through a vertical air separation chamber with an open bottom. Material to be separated is introduced into the rising stream of air. Material having a smaller ballistic cross-section rises, while heavier material falls through the open bottom. The air stream is controlled to below about 1,500 feet per minute. The dispersion of the material is accomplished with a jet of air taken from a plenum connected to an air recirculation system. The air jet is introduced immediately below the material inlet to the chamber. The jet of air breaks up and disperses the material. An air recirculation system includes a fan which draws air out of the top of the air separation chamber by way of a hydrocyclone. The air extracted from the hydrocyclone is reintroduced at the bottom of the air separation chamber from a surrounding plenum.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to apparatuses and methods for separating fractions of a particulate material. More particularly, the present invention relates to apparatuses and methods for utilizing air to separate components of a particulate material on the basis of differing attributes. 
     The separation of a particulate material into various fractions on the basis of density is performed in many industrial processes. In the mining industry, heavy minerals are concentrated from ores for extraction. In agriculture, grain is separated from chaff and leaves are separated from stalks by a current of air that lifts the lighter chaff or leaves away from the grain or stalks. In the wood pulping industry, a device known as an air density separator has been employed to separate light wood chips from chips containing knots which are more dense. 
     An air density separator uses a vertical separation chamber through which a stream of air is drawn. Wood chips to be separated are metered by an auger into the separation chamber where the high velocity air stream disperses the chips evenly over the chamber. The more dense knots fall through the uprising current of air and are rejected. The lighter chips are drawn from the separation chamber by the flow of air and separated from the air by a cyclone. 
     In the production of paper from wood fibers, the wood fibers must be freed from the raw wood. One widely used method of accomplishing this is to process the wood fibers in a cooking liquor so that the material holding the fibers together, lignin, is dissolved. To achieve rapid and uniform digestion by the cooking liquor, the wood, after it has been debarked, is passed through a chipper that reduces the raw wood to chips. 
     As a natural consequence of the harvesting and processing of pulp logs, some sand, rocks, and tramp metal find their way into the raw wood chips. Further, a certain percentage of the raw wood is comprised of knots which are in general undesired in the papermaking process because they add dark fibers that increase the bleaching requirement and because they contain resinous material. The knots, which are typically of a higher density because the wood is dense and resinous, together with tramp metal and rocks, must be separated from the raw wood chips before further processing. 
     One highly successful method of accomplishing this separation is the air density separator. In one known successful system, chips are supplied by a metering screw conveyor infeed to a separation chamber through which a stream of air is drawn. The chips are entrained in the air stream while the higher density knots, stones and tramp metal move against the current of air under the force of gravity. The acceptable chips and air then pass into a cyclone where the chips are separated from the air, the air being drawn by a vacuum into a fan and exhausted. 
     While the air density separator is the most effective and discriminating system available, it has some less desirable features. First, it requires a baghouse to remove dust from the exhaust air. The baghouse is expensive and requires labor intensive maintenance. Further, use of a baghouse results in higher energy cost because of the air pressure necessary to move the air through the filters. Conventional air density separators using air velocities of 4,000 to 5,000 feet per minute function well at dispersing and separating larger wood chips from knots, rocks, and tramp metal. However, separation of small chips from sand and dust requires a lower velocity air flow. Here the conventional method of dispersing the material to be separated in the air stream is not effective. 
     What is needed is an air density separator that eliminates the requirement for a baghouse and can process lightweight materials in a low velocity air stream. 
     SUMMARY OF THE INVENTION 
     The air density separation apparatus of the present invention draws a stream of air up through a vertical air separation chamber that has an open bottom. Material to be separated is introduced into the rising stream of air and material having a smaller ballistic cross-section rises while more dense material falls through the open bottom of the separation chamber. Because the air stream is used to separate materials of low density, the velocity of the air stream is controlled to be below about 1,500 feet per minute. The air stream, because of its low velocity, does not produce sufficient turbulence or dynamic pressure to disperse the material within the upwardly moving column of air. The dispersion of the material is accomplished with a jet of air taken from a plenum connected to an air recirculation system. The air jet is introduced immediately below the material inlet to the vertical air separation chamber. The jet of air breaks up and disperses the material so that the upwardly moving column of air can be used to separate the components of the material introduced. The air recirculation system has a fan which draws air out of the top of the air separation chamber by way of a hydrocyclone. The air extracted from the hydrocyclone is reintroduced at the bottom of the vertical air separation chamber from a plenum which surrounds the open bottom of the vertical chamber. Recirculation of air can eliminate the need to separate entrained dust with a baghouse by a process wherein, through recirculation, the dust forms larger particles which are removed by the hydrocyclone. 
     The strength of the air jet used to distribute the material introduced into the air separation chamber is adjustable by a baffle which controls the width of a slot opening which produces the air jet. Approximately ten to twenty percent of the recirculating air is used to form the jet. 
     It is a feature of the present invention to provide an air density separator that does not require a baghouse. 
     It is another feature of the present invention to provide an air density separator that can handle lightweight materials using a low velocity air stream. 
     It is a further feature of the present invention to provide an air density separator which provides clumping of fines so they can more easily be removed from the air stream by a cyclone. 
     It is yet another feature of the present invention to provide an air density separator feed system which distributes lightweight materials into the air stream of the air chamber of an air density separator. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view, partially cut-away in section and somewhat schematic view of the air density separator of this invention. 
     FIG. 2 is an isometric view, partially cut-away in section, of the separation chamber and infeed mechanism of the air density separator of FIG.  1 . 
     FIG. 3 is a schematic view of the air and particle paths within the lower portion of the separation chamber of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring more particularly to FIGS. 1-3, wherein like numbers refer to similar parts, an air density separator  20  is shown in FIG.  1 . The air density separator  20  has a vertically disposed chamber  22  with walls  23  which define a vertical air separation chamber  24 . As shown in FIG. 3, mixed particulate material  26  to be separated is introduced into the separation chamber  24  from a material hopper  28  through a material inlet  35 . An auger  30  is provided to distribute the particulate material  26  across the hopper  28 . However, depending on the feed system and the natural angle of repose of the material  26 , baffles alone may be substituted for the auger  30 . 
     In the air density separator  20  dispersion of the material  26  is accomplished by a jet or curtain of air formed by an adjustable slot  32  in the wall  33  directly below the material inlet  35 . The slot  32  allows air from a plenum  34  to enter the separation chamber  24 . Air in the plenum is at a higher pressure than air in the chamber  24 , so the pressure drop as the air passes through the slot  32  accelerates the air passing through the slot to form the jet indicated by arrows  36 . The size and velocity of the jet is controlled by a movable damper  38  which is held in place by screws  40 . As material  26  flows through an opening  35  into the separation chamber  24 , it falls through the jet of air flowing from the slot  32 . The effect of the jet is to disperse the material  26  and accelerate the material towards the opposite side  42  of the chamber  24  opposite the slot  32 . 
     A flow of air, indicated by arrows  44 , is introduced at the base of the recirculation chamber, and flows upwardly. Where the upwardly flowing air meets the air from the jet exiting the slot  32 , a turbulent recirculation zone is formed, indicated by arrows  46 . Material  26  caught in the recirculation zone, if it is lightweight, travels upwardly with the upwardly moving air indicated by arrows  48 . If heavy material is caught in the recirculation zone, it falls downwardly where it is accelerated by the air jet from the slot  32 . Arrows  50  in FIG. 3 show the trajectory of that material which is caught by the air jet and accelerated. Such material entrained in the air jet moves out across the duct until air resistance slows the individual particles&#39; lateral velocity and the particles are either drawn upwardly, as shown by arrows  48 , or fall downward, as indicated by arrows  52 , through the uprising air. The jet of higher velocity air formed by the slot  32  breaks up and disperses the material  26  to be separated. In a chamber having a rectangular cross-section with dimensions of approximately eight by two feet, the air curtain would be about one to two inches wide and extend across the width of the longer eighth foot chamber wall  33  beneath the material inlet  35 . 
     The air density separator  20  is configured to recirculate the air and entrained fines. The entrained fines conglomerate and are removed by a cyclone  56  which eliminates the need for a baghouse in many circumstances and hence minimizes emissions without the cost associated with a baghouse to remove fines. 
     As shown in FIG. 1, the air separation chamber  24  is connected by a first duct  54  to the cyclone  56 . A fan  58  is positioned adjacent the lower end  60  of the air separation chamber  24 , and draws air through a second duct  62  out of the cyclone  56  for reintroduction into the air chamber  24 . The fan  58  thus draws air through the first duct  54  from the air separation chamber  24 . The fan  58  exhausts into the vertical air separation chamber  24  adjacent to the bottom  63  of the chamber  24  through a plenum  64  by way of a duct  65 . A third duct  82  conducts ten to twenty percent of the total air moving through the fans  58  to the plenum  34  which supplies air to the slot  32  which forms the jet of air used to disperse the material  26  added to the separation chamber  24 . 
     When the material  26  is introduced into the upwardly moving air stream within the air separation chamber  24 , heavy particles fall down past the plenum  64  at the bottom  63  of the chamber  24 . A stream of air, indicated by arrows  66 , enters the chamber  24  from the plenum  64 , and is drawn upward through the first duct  54  into the cyclone  56 , where denser particles are thrown outwardly to the walls of the cyclone. Most of the air and the less dense particles such as fines is drawn out of the cyclone  56  through the second duct  62  for reintroduction into the air separation chamber  24  at the plenum  64 . 
     Materials having a lower ballistic coefficient, that is those which are lighter in proportion to their area, will be entrained in the upwardly moving air and will leave the separation chamber through the first duct  54 . The remaining particulate material which is not entrained will exit the separation chamber  24  through the bottom  63  of the chamber  24 . Material exiting the bottom of the chamber  24  may be collected on a conveyor or the like. Very lightweight dust and particles are too light to be removed by the cyclone  56  and thus recirculate with the air. Over time the fine particles conglomerate into larger clumps which the cyclone can remove. The precise mechanism for agglomeration is not fully understood but may include the dust grains developing an electrical charge which causes them to attract each other. 
     In a conventional air density separator, air is drawn up through the separation chamber at four to five thousand feet per minute while the granular material to be separated such as wood chips is dispensed into the air chamber either by a chute with an air lock or by an auger which distributes the material across the separation chamber. In a conventional air density separator the high velocity air stream moving up through the separation chamber is usually effective to disperse the granular material being separated in the air stream. Materials which are sufficiently dense fall down through the separation chamber whereas lighter materials become entrained in the air and are drawn into a cyclone where they are separated. The recirculating air density separator  20  shown in FIG. 1 may be used with any suitable air velocity for a particular application. However the use of an air curtain or jet is particularly advantageous where lightweight materials are being dispersed into a low velocity stream of air. 
     An air density separator separates a particulate material depending on what is known in the aerodynamic field as ballistic coefficient. Ballistic coefficient is a function of the density of the object, the area of the object presented to the air stream, and a shape-dependent coefficient. Thus, the ballistic coefficient of an object increases with its density, decreases with increasing area and decreases with increasing bluntness of the object facing the air stream. Ballistic coefficient controls the maximum rate at which an object will fall through a still column of air. Because resistance to motion of an object through the air increases with velocity, an object which is accelerated by the earth&#39;s gravitational force eventually reaches an equilibrium velocity where the acceleration force of gravity is balanced by the drag force produced by the air through which the object is moving. 
     This principal is used to separate the granular material into two or more components based on the ballistic coefficient of the granules. By introducing the granules into an upwardly moving stream of air which has a velocity which is greater than the terminal velocity of some of the particles and less than the terminal velocity of other particles, the granular material will be separated into two fractions. Thus, for separating wood chips from wood knots, an air velocity in the range of four to five thousand feet per minute is chosen which exceeds the terminal velocity of the wood chips, thereby causing them to rise to the top of the air chamber and be transported through a duct to a cyclone. On the other hand, the knots, which have a terminal velocity greater than four to five thousand feet per minute, fall through the air to exit the bottom of the separation chamber. 
     An exemplary problem addressed by the low velocity air density separator  20  is separating small wood chips and sawdust from sand and dirt. The high cost of wood fiber combined with a desire to minimize waste has produced a demand for the capability to recover wood fiber from material which may have been discarded in the past. Because wood chips, sawdust fines and needles of wood are of lower density than the sand and dust with which they are mixed, they have a higher ballistic co-efficient and can be separated in theory in an air density separator. However, all small particles have relatively low ballistic coefficients because the area of the particle dominates as particles become smaller. To separate particles with low ballistic coefficients the velocity of the air in the air density separator must be lower, preferably in the range of five hundred to a thousand feet per minute. 
     The problem with using these low velocities in an air density separator can be readily demonstrated by taking a handful of paper confetti such as the punchings from a paper punch and dropping them into the air. Some of the paper punchings will become dispersed and rapidly reach their terminal velocity and slowly settle to the floor. Others, however, will clump together and fall as a unit reaching the floor before the dispersed punchings. Thus, with lightweight materials, they must be adequately dispersed in the column of air moving up through the vertical air separation chamber  24  if it is desired to reliably separate them on the basis of their ballistic coefficients. The relatively slow upward moving stream of air in the air separation chamber  24  is insufficient to reliably disperse the lightweight material. 
     The cyclone  56  uses centrifugal forces to separate the majority of the particulate material from the air stream. The cyclone has an air lock  68  which allows the lighter fraction to be removed from the cyclone. The air that is withdrawn from the cyclone passes through the fan  58  and is then reinjected into the bottom  63  of the of the air separation chamber  24  through the plenum  64 . The plenum  64  is a rectangular box  70  which is fed tangentially with air from the fan  58 . Portions  72  of the walls  74  of the air separation chamber  24  adjacent to the plenum  64  are angled into the plenum  64 . The gap  76  between the angled portions  72  and the wall  74  of the plenum  64  is closed with a grid of metal  78  with ½ inch holes  80 . The gap  76  forms a continuous opening about the circumference of the chamber  24 . The grid  78  produces a pressure drop as air moves from the plenum  64  into the separation chamber  24 . The pressure drop helps to equalize the air flow into the chamber  24 . 
     It should be understood that the low velocity air density separator of this invention may be used to separate shredded post-consumer plastic containers. The recycling of post-consumer plastic bottles results in a feed stock formed by the shredding of plastic milk bottles or plastic pop bottles. The feed stock contains both plastic from the bottles and paper from the labels associated with the bottles. Because the plastic shards are of a thicker gauge of material than the paper or light grade plastic labels, they can be separated in an air density separator. The velocity of the air in the air density separator will be preferably in the range of seven to eight hundred feet per minute. 
     It should also be understood that the precise amount of air injected into the separation chamber will depend on the size of the air separator and the material being separated. However, the amount of air will generally be about ten to twenty percent, if the air injected through the slot is too great, the injection of air will result in too great a difference in air velocity above and below the air injection point. Control of the air injected can be used as an additional variable which can be controlled to adjust the separation conditions within the air density separator  20 . U.S. Pat. No. 5,829,597 is incorporated herein by reference. 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.