Abstract:
A method for forming a ceramic precursor material for use in extruding ceramic honeycomb green bodies is provided. First, a plurality of dry particulate ceramic precursor ingredients are mixed to achieve an initial particulate precursor mixture. This mixture includes a percentage of particles and agglomerates with the agglomerates exhibiting a size greater than the threshold size. Following mixing, the agglomerates in the initial particulate mixture are pulverized to reduce a maximum size of at least some of the agglomerates below the threshold size to form pulverized agglomerates. Finally, a portion of the ceramic precursor ingredients are separated from the initial mixture with that portion comprising at least some of the pulverized agglomerates and at least some of the particles. The method is particularly adapted for use in the fabrication of ceramic honeycomb green bodies having thin webs between 2 and 5 mils in thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/004,678 filed on Nov. 29, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention generally relates to techniques for producing a ceramic precursor material for use in extruding ceramic honeycomb green bodies, and is specifically concerned with a system and method for producing a particulate ceramic precursor mix capable of being extruded into thin-walled ceramic honeycomb structures 
       BACKGROUND 
       [0003]    Ceramic honeycomb structures are widely used as anti-pollutant devices in the exhaust systems of automotive vehicles, both as catalytic converter substrates in automobiles, and diesel particulate filters in diesel-powered vehicles. In both applications, the ceramic honeycomb structures are formed from a matrix of thin ceramic webs which define a plurality of parallel, gas conducting channels. The web matrix is surrounded by a cylindrical or oval-shaped ceramic skin. The thickness of the ceramic webs is typically between 5.0 and 25.0 mils. 
         [0004]    Such ceramic structures are typically manufactured by first mixing together dry particulate ceramic precursor ingredients in carefully measured proportions that will form a specific ceramic material (such as cordierite or aluminum titanate) when fired in a kiln at temperatures appropriate for material consolidation. The resulting initial precursor mix is next made into a ceramic clay by mixing substances such as water and organic solvents into the dry particulate mix. The resulting ceramic clay is plasticized by an auger or a twin screw in the chamber of an extruder, and is pushed through an extrusion plate having mutually orthogonal, narrow slots. The slots form the matrix of webs of a log-shaped extrudate. The extrudate is cut into can-shaped green body ceramic honeycomb structures, which are then fired into honeycomb ceramic structures. 
         [0005]    Large particles may potentially interfere with the ability of the extruder to generate long production runs of the log-shaped extrudate. When such large particles clog the slots of an extrusion plate, the extrusion plate must be removed and cleaned or replaced to avoid formation of defects in the resulting web matrix. 
       SUMMARY 
       [0006]    One aspect of the invention described herein is a method of forming a ceramic precursor material for use in extruding ceramic honeycomb green bodies, comprising the following steps. First, a plurality of dry particulate ceramic precursor ingredients are mixed to achieve an initial particulate precursor mixture. This mixture includes a percentage of particles and agglomerates with the agglomerates exhibiting a size greater than the threshold size. Following mixing, the agglomerates in the initial particulate mixture are pulverized to reduce a maximum size of at least some of the agglomerates below the threshold size to form pulverized agglomerates. Finally, a portion of the ceramic precursor ingredients are separated from the initial mixture with that portion comprising at least some of the pulverized agglomerates and at least some of the particles. 
         [0007]    A second aspect of the invention described herein is another method of forming a ceramic precursor material for use in extruding ceramic honeycomb green bodies, comprising the following steps. (1) mixing a plurality of particulate ceramic precursor ingredients into an initial mixture, wherein the initial mixture comprises particles and agglomerates, the agglomerate exhibiting a size greater than the threshold dimension; (2) pulverizing the agglomerates in a chamber to reduce a maximum size of at least some of the agglomerates below the threshold dimension to form pulverized agglomerates; (3) removing from the chamber a portion of the ceramic precursor ingredients, the portion comprising at least some of the pulverized agglomerates and at least some of the particles. 
         [0008]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description, serve to explain the principles and operation of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram illustrating the system of the invention wherein a powderizer separates the particles of the initial particulate precursor mix prior to the formation of a ceramic precursor clay from the dry precursor mix; 
           [0011]      FIG. 2  shows comparative graphs illustrating the effect of the powderizer on the average diameter of the dry precursor mix particles; the left side show 10%, 50% and 90% of the particles (solid line) vs. the average diameters for 10%, 50% and 90% of the particles without the powderizer (dashed line); the right side shows the effect of the powderizer on the average diameter of the dry precursor mix particles for the largest 20% of the particles (solid line with squares) vs. without the powderizer (dashed line with circles), and 
           [0012]      FIG. 3  is a table illustrating how different setting of the controls of the powderizer effect average particle distribution of the final dry precursor mix. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    While not intending to limited by theory, applicants believe that premature plugging of protective screens positioned before an extruder die is due to a combination of particle agglomerates caused by van der Waal forces and static electricity, and of micro-debris such as micro-fibers of binder materials and metal particles that were inherently present in the particulate ceramic ingredients as a result of the manufacturing techniques used, or were later introduced into the particulate ceramic ingredients from the shipping containers or packaging. 
         [0014]    Applicants found that premature plugging can be reduced by processing the initial dry precursor mix through a powderizer (sometimes also referred to in this application as an impact and classifying mill) prior to forming the precursor clay that is ultimately extruded into the green body ceramic honeycomb structures. Hence the system of the invention includes a mixer that mixes a plurality of dry particulate ceramic precursor ingredients into an initial particulate precursor mix, and a powderizer that both pulverizes and separates the smaller diameter particles to form a final dry precursor mix. Despite the fact that such powderizers are designed to process the particles of a single ingredient, the applicants found that such a powderizer worked well to reduce or eliminate oversized particles when multiple ceramic ingredients were processed through the powderizer, and that such a powderizer outputted the processed particulates in almost exactly the same proportion as inputted, despite differences in the densities of the various ingredients. The system not only overcomes the aforementioned screen clogging problems, but also allows larger mesh, lower pressure protective screens to be used in the extruder and allows long runs of continuous extruding, thereby expediting the manufacturing process of the resulting green body structures. 
         [0015]    A portion of the particles in the initial precursor mix have diameters that are greater than a threshold dimension. However, the pulverization of the particles of the different ingredients forming the initial precursor as well as the agglomerates helps to reduce the portion of particles and agglomerates having dimension above the threshold dimension and helps to reduce the diameter of any trace amounts of contaminating debris in the mix. The pulverization also lowers the average particulate diameter, which helps to lower the pressure applied to the protective screen during the extrusion process. In some batches, applicants have found about 90% of said particles and agglomerated in an initial precursor mix have a diameter of about 19 microns or less. By contrast, about 90% of the particles and agglomerates separated by said powderizer have a diameter of about 14 microns or less. The separation of the particles and agglomerates having dimension below the threshold dimension in the precursor mixture via cyclonic forces generated by a blower in the powderizer further reduces (if not entirely eliminates) the portion of particles having diameters that are greater than threshold dimension 
         [0016]    The system may also include a metal particle separator that detects and separates metal particle from said initial precursor mix prior to the introduction of said mix into the powderizer. A vibratory screen may be disposed between the mixer and powderizer to separate particles, agglomerates, debris and fibers having an average diameter above a threshold dimension from said initial particulate precursor mix. 
         [0017]    The mixer may include a mixing bin having walls formed at least in part from a porous material, a source of pressurized gas connected to an outside surface of said walls such that a flow of said initial particulate precursor mix is enhanced without the need for static-inducing vibrators that might create unwanted particle agglomerates, and a metering device that determines a feed rate of the initial mix into the powderizer. 
         [0018]    The system may also include a digital processor that is connected to the metering device in order to control a rate of flow from the mixer to the powderizer. The powderizer may have a blower damper control and a classifier wheel speed control, both of which are also connected to the digital processor. Finally, the system may also include a particle diameter monitor connected to an outlet of the powderizer that monitors the average diameter of particles separated by the powderizer that communicates with said digital processor, and the digital processor may operate to adjust the metering device, blower damper control, and the classifier wheel speed control in response to an output of the particle/agglomerate diameter monitor to minimize the portion of the particle/agglomerate diameters that are greater than a threshold dimension 
         [0019]    The invention further includes a method which is implemented by the system of the invention. 
         [0020]    With reference now to  FIG. 1 , the system  1  of the invention includes a precursor mixer  2  for mixing the various ceramic precursor ingredients  3   a ,  3   b  of the final ceramic composition desired. Examples of such final ceramic compositions include cordierite and aluminum titanate. While only two ingredients  3   a ,  3   b  are shown in this example of the system, it should be noted that the number of ingredients required to form final compositions is often substantially greater. In the case of cordierite, three major ingredients are required to form the precursor mix (i.e. primarily SiO 2 , Al 2 O 3 , MgO) along with a smaller percentage of one or more other compounds to improve, for example, thermal expansion characteristics. The particulate average diameter of the raw ingredients is selected to be about one-tenth of the slot width used in the extrusion plate of the extruder. Consequently, for a slot width of 2.5 mils, the average diameter of the particles of raw material should be 0.25 mils, or 6.35 microns, and no particle should have a diameter greater than 63.5 microns, or the slot could become clogged. The mixer  2  is lined with porous metal walls  4   a  which communicate with a source of compressed air  4   b  to promote the flow of the particulate precursor ingredients  3   a ,  3   b  down the funnel-shaped walls without the need for vibratory devices which might induce agglomerate-promoting static electricity in the ingredients. 
         [0021]    A metering device  5   a  regulates the flow of initial precursor mix through the outlet of the mixer  2 . Metering device  5   a  includes a variable speed electric motor (not shown) connected to a rotary airlock valve via an appropriate drive train (also not shown), and flow of the mix can be increased or decreased in accordance with increasing or decreasing the rpm of the variable speed motor. Upon leaving the outlet of the mixer  2 , the particulate ingredients are sifted through a vibratory screen  5   b  in order to remove at least some of the agglomerates and debris particles or fibers which may be present in the precursor mix. Because the screen is not the primary separator of oversized particles, the screen may have a mesh size (for example, between about 6 and 12 when the ingredient particles are sized for a 2.50 mil slot) which is fine enough to remove some oversized particles but not so fine as to result in frequent cloggings and the discarding of an overly large percentage of the precursor mix. After being sifted through the vibratory screen  5   b , the precursor mix is directed through a metal particle remover  6  that determines the presence of contaminating metal particles, and directs any portion of the precursor mix so contaminated to an outlet  7 . To this end, the metal particle remover  6  includes an eddy current detecting circuit that detects the presence of metals via fluctuations in an induction field, and the diversion of and contaminated portion of the stream of precursor mix is accomplished via solenoid valves. 
         [0022]    The resulting stream of sifted and de-metallized precursor mix is then directed into the inlet  9  of an impact and classifying mill or powderizer  10 . While the vibratory screen  5   b  has eliminated a substantial portion of the oversized particles, the mix entering the inlet  9  still has an unacceptable amount of oversize particles  8 , a large portion of which are agglomerates created by van der Waals forces and static electricity during the packaging of the raw ingredients  3   a ,  3   b , and the mixing and conveying of these ingredients  3   a ,  3   b  through the mixer. The powderizer  10  substantially removes all of these oversize particles. To this end, the powderizer  10  includes a vacuum damper  11 , a high speed rotor disc  12  to which a plurality of impactor hammers  14  are connected, a motor  15  for rotating the disc  12 , and a classifying wheel  16  rotated by a motor  17   a  whose rotational speed is regulated by a motor controller  17   b  and the powderizer  10  also includes a blower  20   a  connected to an outlet of the powderizer  10 . A damper control  20   b  controls the output of the blower  20   a . The classifier wheel is circumscribed with blades  22  that generate cyclonic forces within the housing of the powderizer  10  which lift and expel particles above a certain size through outlet  23 . To prevent premature wear, the impact hammers  14  of the mill  10  are faced with tungsten carbide, and various portions of the interior of the powderizer are reinforced with ceramic armor or tungsten carbide. The metering device  5   a , classifier motor control  17   b , and damper control  20   b  are preferably connected to the output of a digital processor  21  which coordinates these controls  5   b ,  17   b , and  20   b  in a manner to be described hereinafter. 
         [0023]    In operation, dry precursor mix flows out of the mixer  2  through the metering device  5   a , vibratory screen  5   b  and metal particle remover  6  and in to the inlet  9  of the powderizer  10  as shown at a controlled rate of flow. Air currents generated by the blower  20   a  and regulated by the blower damper  20   b  pull the flow of precursor mix to the impact hammers  14  on the rotor disc  12 . The impact hammers  14  proceed to pulverize the precursor mix, which breaks up oversized particles caused by agglomerates, and further lowers the average particle diameter of the mix. The pulverized precursor mix generated by the action of the impact hammers  14  is subjected to cyclonic wind forces generated by the rotation of the blades  22  of the classifying wheel  16  interacting with the air stream generated by the blower  20   a  and regulated by the blower damper  20   b . The lighter, smaller diameter particles are conveyed by the cyclonic wind forces to the outlet  23 . The heavier, larger diameter particles and agglomerates  8  are continuously recycled through the impact hammers  14  until they are broken up into particles small enough to be carried to the outlet  23  via the cyclonic wind forces within the mill  10 . The system further includes a particle diameter monitor  24  located on the outlet  23  for periodically or continuously monitoring the average diameter of the particles of the final precursor mix in route to the inlet  31  of the extruder  33 . In the preferred embodiment, the particle diameter monitor  24  may be a laser diffraction-type diameter monitor such as a Malvern Insitec monitor manufactured by Malvern Instruments of Southborough, Mass. Preferably, the output of the monitor  24  is connected to an input of an additional digital processor (not shown) connected to processor  22  so that the processor  22  can manipulate the controls  5   a ,  17   b , and  20   b  to minimize the amount of oversize particles as well as the wear on the powderizer  10 . 
         [0024]    The applicants have observed that the powderizer  10  is able to quickly remove oversize particles from the initial precursor mix and to generate a final precursor mix out of the outlet  23  having the same proportions of ceramic ingredients  3   a ,  3   b  as was introduced in to the mixer  2 . This is surprising in view of the fact that the different ceramic ingredients  3   a ,  3   b  have different densities and different hardnesses, both of which would indicate a different rate of separation by the classifying wheel  16 . While applicants do not understand exactly why such serendipitous results occur, applicants believe it is because the most problematical oversize particles were the agglomerates that formed in the initial precursor mix as a result of van der Waals forces and static electricity, and that only a relatively brief amount of pulverizing and separation is necessary for these agglomerates to be effectively eliminated from the precursor mix. 
         [0025]    The final, dry precursor mix flows into a precursor paste mixer  25 , where it is mixed with substances such as water and organic solvents from source  27  to form a precursor paste or clay  29 . The resulting clay  27  is introduced into the inlet  31  of an extruder  33 . While the extruder  33  is indicated in  FIG. 1  as being screw-type extruder, ram-type extruders may also be used in the system  1  of the invention. The extruder forces the clay  29  through an assembly  35  having a protective screen  37  that screens out just about all of the last remaining oversize particles. The screened clay is then squeezed through an extrusion plate  40  to form an extruded green body log  42  having in its interior a matrix of web walls the same thickness as the spacing between the slots in the extrusion plate  40 . The extruded green body log  42  is carried by an air bearing table  44  to a cutting station (not shown) to ultimately create green body honeycomb structures that are fired into a final ceramic product. 
         [0026]    The left side of  FIG. 2  is a graph illustrating the effect of the powderizer  10  on the average diameter of the dry precursor mix particles for 10%, 50% and 90% (d 10 , d 50  and d 90  respectively) of the particles. Specifically, the solid line graph illustrates the average particle diameter in such a mix processed through a mill  10 , while the dashed line graph illustrates the average particle diameter in such a mix that has not been processed through such a mill  10 . As is evident from these graphs, the powderizer  10  has the effect of lowering the average diameter of the precursor mix such that 90% of the particles have a diameter of 14.43 microns or less. By contrast, without the powderizer  10 , 90% of the particles have a diameter of 18.83 microns or less. Such lowering of the average diameter of the particles not only has the effect of reducing the number of agglomerates  8  and oversize particles, but further helps reduce the amount of pressure needed to squeeze the resulting precursor paste  29  through the protective screen  37  of the extruder  33 . 
         [0027]    The values d 10  and d 50  are defined as the diameters at 10% and 50% of the cumulative particle size distribution, with d 10 &lt;d 50 . Thus, d 50  is the median particle/agglomerate diameter, and d 10  is the particle/agglomerate diameter at which 10% of the particle/agglomerates are finer. The value of d 90  is the particle/agglomerate diameter for which 90% of the particles/agglomerates are finer in diameter; thus d 10 &lt;d 50 &lt;d 90 . For example mixing a plurality of particulate ceramic precursor ingredients into an initial mixture, wherein the initial mixture comprises fine particles and coarse particles is interpreted to mean that the mixture exhibits an initial d 90  of some initial value; for instance if the d 90  was 18 microns, that would imply that 90% of the particles are 18 microns or smaller.; 
         [0028]    The right side of  FIG. 2  compares how the diameter distribution of the precursor particles is changed by the powderizer for the largest 20% of the particles. Specifically, the solid line graph marked with squares illustrates the particulate diameter distribution with the powderizer  10 , while the dashed line graph marked with circles illustrates the particle distribution without the powderizer  10 . Note that when the powderizer  10  is used, 99.40% of the particles have an average diameter of 60 microns or less, and hence are unlikely to clog an extrusion plate having 2.50 mil wide slots (which corresponds to 63.5 microns). By contrast, when the powderizer  10  is not used, 98.81% of the particles have an average diameter of 60 microns or less, which amounts to twice as many particles having average diameters that can potentially clog the slots of an extrusion plate  40 . 
         [0029]    Table 3 illustrates how different setting of the controls of the powderizer effect average particle distribution of the final dry precursor mix, and in particular illustrates how the digital processor  21  can adjust the settings of the metering device  5   a , classifier motor control  17   b , and blower damper  20   b  to reduce the percentage of oversized particles that must be removed by the protective screen  37  even further. The particular powderizer  10  that was used to compile the information in the Table 3 was a Sturtevant Model NSP1 available from Sturtevant, Inc. located in Hanover, Mass. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 POWDERIZER 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 RUN 
                 SETTINGS 
                 DV10 
                 DV50 
                 DV90 
                 2μ 
                 5μ 
                 10μ 
                 20μ 
                 30μ 
                 40μ 
                 50μ 
                 60μ 
                 70μ 
                 80μ 
                 90μ 
                 100μ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 1565 classifer rpm, at 
                 0.68 
                 2.90 
                 11.6 
                 37.9 
                 71.2 
                 87.5 
                 95.9 
                 98.1 
                 99.0 
                 99.5 
                 99.7 
                 99.9 
                 99.9 
                 100.0 
                 100.0 
               
               
                   
                 damper 80% open, motor is 
               
               
                   
                 at 50 hertz metering set 
               
               
                   
                 automatically from particle 
               
               
                   
                 size analyzer feedback 
               
               
                 2 
                 2500 classifier rpm; damper 
                 0.65 
                 2.69 
                 10.8 
                 40.6 
                 74.1 
                 88.9 
                 96.3 
                 98.3 
                 99.2 
                 99.6 
                 99.8 
                 99.9 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 Hertz); automatic 
               
               
                   
                 ametering 
               
               
                 3 
                 2500 classifier rpm; damper 
                 0.60 
                 2.33 
                 8.0 
                 42.8 
                 77.2 
                 93.2 
                 98.3 
                 99.4 
                 99.8 
                 99.9 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz); metering 
               
               
                   
                 device @ 15 rpm 
               
               
                 4 
                 2500 classifier rpm; damper 
                 0.66 
                 2.72 
                 10.2 
                 41.0 
                 75.3 
                 89.6 
                 96.9 
                 98.7 
                 99.4 
                 99.7 
                 99.8 
                 99.9 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz); metering 
               
               
                   
                 device @ 10 rpm 
               
               
                 5 
                 2000 classifier rpm; damper 
                 0.67 
                 2.77 
                 10.2 
                 39.9 
                 74.2 
                 89.5 
                 96.9 
                 98.7 
                 99.4 
                 99.7 
                 99.9 
                 99.9 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz) metering 
               
               
                   
                 device @ 10 rpm 
               
               
                 6 
                 2000 classifier rpm; damper 
                 0.69 
                 2.91 
                 11.4 
                 38.8 
                 72.4 
                 87.8 
                 96.1 
                 98.2 
                 99.1 
                 99.5 
                 99.7 
                 99.9 
                 99.9 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz) metering 
               
               
                   
                 device @ 18 rpm 
               
               
                 7 
                 2000 classifier rpm; damper 
                 0.71 
                 3.06 
                 12.2 
                 37.0 
                 69.8 
                 86.5 
                 95.5 
                 97.9 
                 98.9 
                 99.4 
                 99.6 
                 99.8 
                 99.9 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz); metering 
               
               
                   
                 device @ 19 rpm 
               
               
                 8 
                 1565 classifier rpm; damper 
                 0.70 
                 3.00 
                 12.6 
                 36.5 
                 69.1 
                 86.2 
                 95.1 
                 97.6 
                 98.7 
                 99.2 
                 99.5 
                 99.7 
                 99.9 
                 100.0 
                 100.0 
               
               
                   
                 70% (60 hertz) automatic 
               
               
                   
                 metering 
               
               
                 9 
                 330 classifier rpm; damper 
                 0.67 
                 2.82 
                 11.3 
                 37.4 
                 70.4 
                 87.9 
                 96.0 
                 98.2 
                 99.1 
                 99.5 
                 99.8 
                 99.9 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
                 80% (50 hertz); automatic 
               
               
                   
                 metering 
               
               
                 10 
                 1565 classifier rpm, damper 
                 0.69 
                 2.92 
                 11.7 
                 37.4 
                 70.7 
                 87.3 
                 95.9 
                 98.1 
                 99.0 
                 99.5 
                 99.7 
                 99.9 
                 99.9 
                 100.0 
                 100.0 
               
               
                   
                 80% (50 hertz), automatic 
               
               
                   
                 metering 
               
               
                   
               
             
          
         
       
     
         [0030]    The first settings in Run # 1  were the ones initially set by the powderizer  10  automatically from feedback from the particle diameter monitor  24 . Run # 1  of this table indicates that the average diameter of 99.7% of the particles in the final precursor mix may be reduced to 60 microns or less when the metering device  5   a  is set automatically from feedback from the particle diameter monitor  24 , the classifier wheel motor control  17   b  is set to 1565 rpm, the blower damper control  20  is set to 80% open. The blower motor is operated at a current frequency of 50 Hz. The resulting 99.7% compares favorably to only 98.81% of the particles having an average diameter of 60 microns or less when the powderizer  10  is not used (from the table in  FIG. 2A ) and indicates that the powderizer, at the settings of Run # 1 , reduces oversize particles and agglomerates by 75% (i.e., 1.19% being oversized without the powderizer vs. only 0.3% being oversized with the powderizer). The best results, however, were achieved with the settings of Run # 3 . Here, setting the metering device  5   a  to 15 rpm, the classifier wheel motor control to  17   b  to 2500 rpm, the blower damper control  20   b  to 70%, and operating the blower motor at a current frequency of 60 Hz, resulted in 100% of the particles having an average diameter of 60 microns or less. 
         [0031]    Different modifications, additions, and variations of this invention may become evident to the persons in the art. All such variations, additions, and modifications are encompassed within the scope of this invention, which is limited only by the appended claims, and the equivalents thereto.