US2328577A - Process and apparatus for grading and for coating with comminuted material - Google Patents

Description

Sept. 7, 1943. N. E. OGLESBY 2,328,577

PROCESS AND APPARATUS FOR GRADING AND FOR COATING WITH COMMINUTED MATERIAL Filed Jan, 12,1940 2 Sheets-Sheet 1 10L. 1 Z0 lfl/VENTGR A T TOR/V5 Y5 Sept. 7, 1943. N. E. OGLESBY 2,328,577

PROCESS AND APPARATUS FOR GRADING AND FOR COATING WITH COMMINUTED MATERIAL Filed Jan. 12, 1940 2 Sheets-Sheet 2 F X X X X X TRA/mFoRME/Q M m/aosmrr FOR m3 VA R/ABLAT mu, m as a:

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MOTOR 3 FRZQ UENC Y GENERATOR INVENTOR N/c/vo/aa E. Qg/esy ATTQRNE Y3 Patented Sept. 7, 1943 PROCESS AND APPARATUS FOR GRADING AND FOR COATING WITH COMMINUTED MATERIAL Nicholas E. Oglesby, Troy, N. Y., assignor to Behr-Manning Corporation, Troy, N. Y., a corporation of Massachusetts Application January 12, 1940, Serial No. 313,609

12 Claims.

I This invention relates to the separation or grading of comminuted material such as abrasive grains, flocks, and the like, and is particularly concerned with improved electrostatic separation or electrostatic grading methods. More speciflcally this invention also relates to coating all or part of the separated or graded comminuted material onto an adheslvely coated foundation material.

It is an object of this invention to coat the finer and less effectiveabrading particles, from a mixture of abrasive particles, on the bottom of a coated abrasive coating and to coat the coarser abrasive particles on the top 01' the coating. It is a further object of the invention to use an abrasive grain which has been less accurately graded than has heretofore been considered necessary and to coat such an abrasive in such a way that the top or effectiveabrading grains will be more uniform in grade than the average grade or size of all the grains coated. Another object.

of the novel method of coating provided in this invention is to avoid coating relatively coarse particles with which the mixture of abrasive grains delivered to the coating machine may be conterminated, or to coat a finer grade of abrasive than the average grade of abrasive delivered to the coating means. A further object of this invention is the production of a relatively deep coating of abrasive grains in oriented relation, with a non-loading or open spaced oriented grain effect at the top of the coating. Still another object of this invention is the provision of suitable apparatus for carrying out the novel processes described. It is a further object of this invention to separate abrasive grains of variable size into two or more fractions, one of which is coarser than the other. In stating the foregoing objects of the invention, abrasive grains are given as a preferred illustration of the material worked upon by the novel process and apparatus. It is to be understood that other comminuted material, such as flocks, may be coated or handled in an analogous way, and that abrasive grains are representative materials that may be processed in accordance with this invention.

The method and apparatus by which my invention is carried out will be explained by reference to the accompanying drawings, in which:

Figure 1 is a side elevation of one form of apparatus which may be used for carrying out my invention;

Filgure la is a section on the line la-ia of Figure Figure 2 is a side elevation showing a modifies tion of the apparatus used in Figure 1;

Figure 3 is a side elevation of a form of apparatus for carrying out another modification of the invention;

Figure 4 shows a wiring diagram for obtaining the variable frequency alternating current of high potential used in carrying out my invention.

Similar reference characters refer to similar parts throughout the several drawings.

Referring to Figures 1 and 2:

Numeral l is an adhesively coated web such as paper or cloth, or combinations of paper and cloth. Numerals 2 represent suitable idler rolls over which the web l is continuously drawn through the electrostatic field, by means not shown. Numeral 3 represents an electrode which may be a solid electrode or a bank of electrodes as illustrated in Figures 1 and 2. In the same way numerals 4, 6 and 7 represent electrodes which may be solid or a bank of individual electrodes, as illustrated in the figures. The electrodes may be made of suitable metal or other conducting material and they may be insulated or not insulated, but I generally prefer to use insulated electrodes, phenol formaldehyde resin or paper impregnated with such resins forming a suitable insulating covering for the electrodes or banks of electrodes. Numeral 5 represents connections between the individual electrodes forming a bank of electrodes, as for instance, electrode bank 3 may contain six electrodes connected together as indicated by numeral 5. Various sized electrodes may be used but an electrode having a dimension of about 6 inches in the direction of travel of the web and surrounded by A inch of insulation has been found generally satisfactory for my purpose. With each individual electrode having a conducting dimension of 6 inches in the direction of travel of the web and inch of insulation surrounding the conductor, the electrode bank 3 containing six such electrodes would have a total dimension of 3 feet in the direction of travel of web I. In the same way, electrode bank 4 would have a total dimension in the direction of travel of the web I of 2 feet 2 inches, but I may use greater or shorter dimensions of electrodes in the direction of travel of the web, according to the result desired. Numeral 8 denotes comminuted by conventional means, not shown. Numeral i4 is a conducting connection connecting electrode 3 or electrode bank 3 to ground. Numeral i5 is a conducting connection connecting electrode or electrode bank 4 to ground. Numeral i 6 is a conducting connection connecting electrode 6 or electrode bank 8 to a source of high potential of variable frequency and alternating polarity. Numeral I7 is a conducting lead connecting electrode 1 or electrode bank I to a source of high potential of variable frequency and alternating polarity or to a source of unidirectional high potential (direct current). The order of connections of electrodes may sometimes be reversed in that conducting lead id may be connected with a source of high potential of variable frequency, and connecting lead it may be grounded. Likewise, conducting lead it may sometimes be connected to a source of high potential of suitable characteristics, and connecting lead ll may be grounded. The abrasive grains 8 are delivered from hopper ill by feed roll iii onto the endless belt l8 which is moved continuously into the electrostatic field set up between electrodes 3 and E3. The high potential alternating current applied to electrode 6 where electrode 3 is grounded or to electrode 2 where electrode a is grounded is of a variable number of cycles, depending upon the grit sizes and variation in grit sizes being separated and coated. The number of cycles is adjusted so as to lift only the smaller grains to the adhesively coated web, the larger grains being lifted a shorter distance and dropping and/or being pulled back to the carrier belt upon the reversal of polarity. I have discovered that a superior separation'of the smaller and larger grains can be accomplished by adjusting the frequency in accordance with the size of grains that it is desired to coat in the field between electrodes 3 and 6. I generally prefer, especially in the case of fine grits, to use a voltage sumcient to lift all of the grains from the belt in, as I believe eflective separation is brought about because the smaller grains travel faster when lifted from the belt and, with the correct electrode spacing, reach the adhesively coated backing at a point of time at which the larger grains have traveled a shorter distance. By a proper selection of frequency the polarity of the field is reversed before the larger grains reach the adhesively coated backing. As stated above, the small grains are lifted to the adhesively coated web while the larger grains are lifted a shorter distance. The frequency of alterations of polarity of the field is so adjusted and controlled that the average displacement of the finer particles per electrical impulse is substantially greater than the average displacement of the larger particles. Of course, the average displacement of the larger particles is less than the free space distance between the electrodes 3 and 6, and, in fact, less than the distance between the felt I!) and the adhesively coated surface of web I. Irrespective of the mechanism it has been found possible to coat the. finer grains in oriented relationship and have the larger grains pass out of the field between electrodes 3 and 6 on th carrier belt l0. As the belt moves forward it carries the larger grains which are not coated in the field between electrodes 3 and 6 into the field between electrodes 4 and 1. Electrode l is connected to a source of high potential of variable frequency when electrode 4 is grounded or electrode 4 is connected to a source of high potential of variable frequency when electrode 7 is grounded. Iimpress upon electrode 1 or electrode 4, as the case may be,

a lower frequency alternating current whereby, for a given electrode spacing, a longer time of travel is permitted so that larger particles reach the adhesively coated backing than was the case between electrodes 3 and 6. By suitably controlling the frequency of the field between electrodes 4 and l I may control the size of particles coated in this field. Often, however, it is desired to coat all or a large portion of the particles remaining on the belt in the field between electrodes d and i. In this case I may use direct current instead of an alternating current in establishing this field or I may use an alternating current of relatively low frequency, as for instance, a frequency of 10 to 30 cycles or lower. A separation of the grain into larger and finer fractions may take place in the field between electrodes i3 and 6 or in both fields. In either case the average size of the grain coa%d depends upon the electrode spacing, the voltage, and the number of cycles.

The separation desired is not attained by the use of a simple pulsating direct current with a variable frequency of pulsation, since charges are not lost by the particles and the electrodes with sumcicnt rapidity, and a particle once charged and set in motion between the electrodes continues to travel even though the connection to the source of direct high potential is interrupted. Where alternating current is used the field is reversed, and a particle with a given charge and in motion is reversed in direction of travel with the reverse in polarity and I believe this to be the reason why better separations are attained with alternating current. Instead of using simple alternating current, a commutator may be used to reverse the polarity of the electrodes where they are energized with direct current but I prefer to use alternating current as it is simpler and very effective for my purpose.

The voltage impressed upon an electrode, the spacing between the electrodes and different banks of eectrodes, the length of the electrodes along the direction of travel of the adhesively coated web, and the frequency of the alternating current used, are varied according to the grit size and the result that it is desired to attain.

In the prior art, abrasive grains have been coated to a sheet by passing the adhesively coated backing through an electrostatic field adjacent an upper electrode with the adhesive side facing downwardly and passing abrasive grains into the electrostatic field, often by means of a belt moved adjacent to the lower electrode in its passage through the field. In this and in other methods of the prior art, spacings, voltages and frequency of the current, where alternating current has been used to energize an electrode, have been such as are conducive to coating the adhesively coated web quickly and completely with a grade of abrasive grains which at least approaches the grade of the abrasive grain delivered to the electrostatic field. In actual practice, however, with direct current, either uninterrupted or interrupted, or with alternating current of relatively low frequency and an air gap which is often relatively small, the result has been that all of the abrasive grains are propelled to the adhesively coated backing sheet. Some grains are retained by the adhesives, others immediately return to the carrier belt and are again propelled to the adhesively coated backing and this process is continued as the web travels forward through the field between the electrodes. The weight of abrasive coated gradually increases due to repeated bombardments as the web passes through the field. I! the energizing current furnished to the electrode is cut oil suddenly, the pieces of backing within and corresponding to the length of the field at the time the current is broken may be sampled and analyzed at any desired point within the coating field. It will be found that the grade of abrasive being coated as the web enters the field is usually at least substantially as coarse as the abrasive delivered to the field, and that the overall grade of abrasive decreases progressively as the weight of abrasive coated is built up from the beginning of the electrodes to the end of the electrodes in the direction of travel of the web. By computation of grades, it will be found that the last abrasive coated as the web leaves the field is very fine as compared with the grade of abrasive delivered to the field. I attribute this to the fact that as the abrasive weight coated builds up in the field the spaces between the grains become small and it is, therefore, easier to embed the fines between the grains already coated than the average size or coarser size of grains still remaining in the abrasive mixture. At any rate, in the prior art the result has been the embedding of fine grains between coarser grains toward the end of the coating operation in the electrostatic field. It should be understood that the presence of such very fine and inefiective grains between the more effective abrading grains is what we term a fine top effect in this application, a condition which is conducive to loading of the product by the work removed in abrading and which interferes with the efficiency of the product. It should be appreciated that a product built up according to the present invention avoids the difilculties of the prior art in that a substantial proportion of the fines content is coated onto the adhesiveiy coated backing at the beginning of the coating operation, thereby removing these fines from the abrasive mixture so that they will not be embedded between the coarser and more effective grains in the later stage of the Coating operation wherein I provide conditions conducive to coating coarser grains.

As an example of the use of the invention as illustrated in Figure l, I may consider a coating carried out with grit No. 280 silicon carbide abrasive. The actual grading of the abrasive of this fine number is commonly determined by sedimentation. From the sedimentation results, an accumulation curve is plotted which shows the diameter of the grain in terms of microns versus the percentage of grain. It should be appreciated that grains of this fine size and finer sizes, and sometimes somewhat coarser sizes, are graded by air or water classification methods. The grading' set forth, as determined by microns, is a statistical value, and the cleanliness of the grade can be determined by plotting a distribution curve. A clean grade is one having a high percentage at or near the nominal size and relatively free from substantially coarser or finer grades of grain. Where I speak of a flour grade being finer than another flour grade I mean that the average grain size of the finer grade is finer. Where I speak of a cleanly, well, evenly or uniformly graded material I mean that the material has a relatively steep distribution curve, shows a relatively high percentage of grain at or near the nominal size and is relatively free from grains much finer or much coarser than the nominal size. At the 50.percent point on the accumulation curve, the standard diameter for grit No. 280 silicon carbide is 44 microns. The abrasive delivered from the hopper 8 oi. Figure 1 in this coating had a diameter at the 50 percent point on the accumulation curve of 47 microns. This abrasive when coated by the regular method of upward propulsion, (i. e., electrostatic movement of particles counter-to-gravity) which does not involve the invention disclosed in this application to produce a coated abrasive article with an abrasive weight of 4.04 pounds per sandpaper ream, showed an overall grading at the 50 percent point on the accumulation curve of 44.5 microns. In the coating by the regular conventional method, the abrasive that first adhered to the sheet was coarser than that which later adhered to the sheet in the travel of the adhesively coated web between the electrodes. The abrasive which was coated onto the adhesively coated backing just before the coated web emerged from the electrodes was very fine, as was determined by stopping the machine and analyzing the grade of the abrasive at several points along the line of travel of the web through the field. For both the conventional coating and the novel method herein described, 40 pound rope paper was used as the web to be coated and the adhesive coated on the backing by conventional means to hold the grain consisted of a solution of an, oil-modified alkyd resin composition in high flash naph the, the solution containing about 65 percent of nonvolatile resinous material. In the control experiment the coating electrodes were energized with an alternating current of 25g cycles stepped up to 40,000 volts and the air gap between the belt carrying the abrasive and the upper electrodes was uniformly inch.

In coating according to the present invention the apparatus shown in Figure 1 was used. The air gap between the upper bank of electrodes 3, which was grounded, and the belt l0 carrying the abrasive, was uniformly 1% inches. The electrode bank 6 was energized by 60 cycle alternating current stepped up to 31,000 volts. It was desired to coat only a small portion, and the finer portion, of the abrasive grain of grit No. 280 on the belt ill in the field set up between. elec trodes 3 and 6. The abrasive weight attained in the field between electrodes 3 and 6 was 1.23 pounds per sandpaper ream and the grading of the abrasive coated at the b0 percent point on the accumulation curve was 37.5 microns. The adhesively coated web was 36 /2inches wide and was moved through the field, adhesive side facing downwardly, at a speed of 102 feet per minute. The feed roll H delivered to the carrier belt to, in a continuous manner, 5.4 pounds of abrasive per minute. The belt It! was moved through the field at a rate of 7 feet per minute. The layer of abrasive on belt E0 was 38 inches wide. The coarser abrasive not coated in the field between electrodes 3 and 6 was allowed to dump from the belt at the point of roll I3 and was collected for re-use. In the coating operation described the coated abrasive formed was finer in grade and more uniform in grade than the abrasive fed to the belt l0. Also, the grains were coated in oriented relation.

In the form of apparatus shown in Figure 1, there is for a single setting, a constant air gap at all points between electrode bank 3 and the carrier belt Hi. In practicing the invention with this form of apparatus, the same or a diiferent spacing or air gap may be provided between electrodes 4 and I. In the form of apparatus shown in Figure 2 there is, as shown in the figure, a gradual decrease in the air gap from the beginning of electrodes 3 and 6 to the end of electrodes 4 and i. For certain purposes there is an advan tags in the form of apparatus shown in Figure 2, in that a wider gap, conducive to good separation, may be used for the first bank of electrodes and a narrower gap, conducive to complete coating of all the grain, may be provided in a second bank of electrodes, 4 and l. A variety of arrangemerits will occur to those skilled in the art. The distance between the electrode banks 3 and and electrode banks 4 and l is not critical so long as the two banks of electrodes are not so 'ciose to each other than there is an interference between the two fields. In general I find a spacing of about 7 inches very satisfactory.

As a further illustration oi the use of my invention I may describe the coating of grade No. hill-A waterproof sandpaper. In this instance the form of apparatus shown in Figure 2 was used. The air gap between electrode bank and the abrasive-carrying belt H0 at the point where the belt first enters the field was 1% inches. The spacing between the two pairs of electrodes was 7 inches. The electrode banks 3 and 6 were made up of six electrodes with a dimension in the direction of travel of the adhesively coated web of 6 inches each (including insulation). The second bank of electrodes was made up of only one electrode with a dimension of 6 /2 inches (including insulation) in the direction of travel of the adhesively coated web. The air gap at the end of the inch. As an adhesive, a 71 percent solution of oil-modified alkyd resin composition dissolved in high flash naphtha was coated onto the web by conventional means, not shown. The web was 40 pound rope paper previously treated by conventional means to fill the pores and waterproof the web. The web was 36 inches wide. A layer of No. 240 silicon carbide abrasive, 38 inches wide, was fed onto abrasive-carrier belt ill at the rate of 0.95 pounds per minute from hopper 9 by feed roll I l as shownin Figure 2. In the coating operation the web I was moved through the field with the adhesively coated side facing downwardly, by conventional means, not shown, at a rate of 102 feet per minute. The abrasive-carrier belt i0 moved at a speed of 7% feet per minute. The abrasive grain fed had a grading of 59.5 microns at the 50 percent point on the accumulation curve. Electrode bank 6 was energized by an alternating current of 60 cycles at 24,000 volts. Electrode 1 was energized by an alternating current of cycles at 40,000 volts. Between the electrodes 3 and 6 an abrasive weight of 2.35 pounds per sandpaper ream was coated. At the 50 percent point on the accumulation curve the abrasive so coated graded 46 microns. Between electrodes 4 and 1, 3.25 pounds per sandpaper ream of abrasive was coated, giving a total weight of coated abrasive of 5.6 pounds. The grading for the total abrasive weight applied was, at the 50 percent point on the accumulation curve, 56.5 microns. The abrasive applied in the first bank part of the abrasive, coated between electrodes 4 and 1, was much coarser than that coated between electrodes 3 and 6. In coating with both electrode banks a total of 76 percent of the abrasive delivered to the carrier belt ID was coated and 24 percent of the abrasive fed was dropped off the belt and recovered for re-use as the belt electrodes 4 and 'i where the carrier .belt it] emerges from the field was passed over roll 83. The 24 percent of grain recovered at roll l3 was suitable for use in making a coarser grade of coated abrasive. The abrasively coated web I was passed by conventional from the abrasive-applying to a conventional drying of the use of mV coating of grade is a grade controlled by screens and not by sedimentation. In this instance the form of apparatus shown in Figure 2 was used. The air gap between the electrode bank 3 and the abrasivewhere the belt first 1%; inches. of electrodes was 7 oinches. The electrode banks 3 and 0 were madeup of six electrodes each with a dimension in the direction of travel of the adhesively coated web of 6 inches (including insulation). The second bank of electrodes, Q and ii, was made up of two electrodes with a dimension of 6 inches each (including emerges from the field was inch. As an adhesive a 74 percent solution of oil-modified alkyd per minute from shown in Figure 2.

30,000 volts. Between the electrodes 3 and 6 a sand weight of pounds per sandpaper ream 1 was, therefore, decidedly coarser and freer from fines, as could be readily detected in the appearance of the coated product as compared with a product coated by conventional counter-to-gravity electrostatic means with the same abrasive to the same abrasive weight. In coating with both pairs of electrode banks, a total of 82.5 percent of the abrasive delivered to the carrier belt II) was coated and 17.5 percent of the abrasive fed was dropped oil! the belt and recovered for re-use as the belt passed over roll l3. The 17.5 percent of grain recovered was suitable for use in making a coarser grade of coated abrasive.

In a manner analogous to which No. 240-A waterproof sandpaper was made with grade No. 240 silicon carbide, I will describe a coating in which No. 320-A waterproof sandpaper was made. In this case conditions were similar to the manufacture of No. 240-A waterproof sandpaper, except that a current of 120 cycles at 29,400 volts was used to energize electrode bank 6 and a current of 25 cycles at 35,000 volts was used to energize electrode bank I. The air gap between electrode bank 3, in this case made up of three electrodes with a dimention 6 /2 inches each, in the direction of travel of web I, and carrier belt in at the point where carrier belt i enters the field was 1 /3 inches. The air gap between the carrier belt Ill and the electrode bank 4 in this case made up of two electrodes having a dimension of 6 inches each in the direction of travel of web I at the point where carrier belt l0 emerges from the field, was /2 inch. As in previou instances, the abrasive coated on the bottom between electrodes 3 and 6 was much finer than the abrasive coated between electrodes 4 and 1. and the top coating of abrasive grains was very free from fines and therefore more effective as an abrading instrument. The coated grains were well oriented.

No specific rules can be given for electrode spacings, frequency, and voltage, for every condition that will be encountered. In general, however, electrode spacings in excess of 2 or 3 inches arenot preferred since excessive voltages would berequired and since, for some reason unknown to me, electrostatic action is more practical at smaller electrode spacings. In general with the finer grits and a well covered carrier belt 8. voltage should be used which is high enough to readily lift the abrasive grains from the carrier belt and which is not high enough to cause arcing between the electrodes. The frequency of the alternating current used to make the separation in the coating operation in the step where the finer abrasive is removed from the abrasive mixture fed to the field varies with the grit size and the electrode spacing. Generally speaking, the wider the air'gap the lower the frequency that can be used to make the separation, and the finer the grain the higher the frequency desired to make a satisfactory separation. I select a practical air gap spacing and voltage and then use that frequency most effective for obtaining the desired results of selectively removing a finer grade of abrasive from the coarser and variable grade delivered to the field. I then use conventional voltages, electrode spacings, or air gaps and a direct current, either uninterrupted or interrupted, or a suitable frequency of alternating current for coating additional abrasive as required for the results desired in the second abrasive-applying step, 'where such second step is employed, as for instance, between electrodes l and l of Figure 1 and 2.

Figure 3 illustrates another form of my invention .and differs from the forms shown in Figures 1 and 2 in that in this form of the apparatus the finer abrasive removed from the abrasive mixture fed to the device shown in Figure 3 is not permanently coated onto a reinforcing backing but is, on the contrary, removed after separation and collected for subsequent use as desired. In Figure 3, numerals 2a represent idler rolls over which an endless belt 26 is trained, belt 26 being made of paper, cloth, or other suitable material. Idler roll 2b is driven by conventional means, not shown, at any desired speed. Numerals 3, 6, 6, 6, 9, H, H and "5 denote parts which may be identical with parts denoted by similar numerals in Figures 1 and 2 and already described. Numeral Illa represents an endless belt which may be made of cloth or other suitable material. The belt Illa is trained about idler rolls l2a and a power driven roll l3a which is driven by conventional means, not shown, at any desired speed. Numeral l8 denotes a suitable receptacle in which the abrasive or other comminuted material dumped from belt Ilia at roll I30 may be recov ered. Numeral l9 represents a receptable or chamber in which the abrasive or other comminuted material removed from belt 26 may be recovered. Numeral 20 represents a heater which may be used to evaporate liquid applied to belt 26. Numeral 2| represents a rattler or beater which is driven by conventional means, not shown, and aids in the'removal of abrasive or other comminuted material from belt 26 so that it may be recovered in receptacle I8. Numeral 22 designates a rotating brush driven by conventional means. not shown, to remove the abrasive or other comminuted material, if required, and as desired, from endless belt 26, so that it may be recovered in receptacle l9. Numerals 23 denote convention-a1 coating rolls driven by conventional means, not shown, which may be used to apply a liquid coating material to the endless belt 26. Numeral 24 represents a coating trough in which the liquid to be coated to roll 26 is placed so as to contact lower coating roll 23. Numeral 25 denotes the liquid which is placed in coating trough 24. In some instances, especially in the case of very fine grits, water will be found effective. In other cases, a small quantity of a surface tension-lowering agent, such as sulphonated vegetable oil, sulphonated higher alcohol or other suitable surface tensionlowering agent is added to the water and used. In still other cases a small quantity of soluble and usually mild adhesive, such as starch in solution, is used. In lieu of water, other volatile liquids or solvents may be used. The form of the invention shown in Figure 3 is particularly useful as an abrasive-separating mechanism. Furthermore, it is particularly useful in separating the so-called flour grades or grades within the range of No. 240 and finer. The abrasive which it is desired to separate, which may be a mixture of different sizes of silicon carbide, aluminum oxide, or other abrasive, preferably containing particles of the size normally found in grades 240 and/or finer, is delivered from hopper 9 by speed roll H onto carrier belt Illa which is driven by roll l3a at any desired speed, as for instance, '7 to 10 feet per minute, through the electrostatic field set up between electrodes 3 and 6, the rate of feed being governed by the results that it is desired to attain, as previously described in instances where it was desired to permanently coat thefiner fracture of grain removed from the abrasive mixture. The endless belt 26 is coated with water or other suitable material, usually with little or mild adhesive action in the liquid state, by coating roll 23 dipping in the coating liquid 25 in .trough 214, as the endless belt 26 is moved in the direction of the arrow around idler rolls 2a by power driven roll 212 at a suitable speed, as for instance, 85 to 125 feet, although other speeds will often be found satisfactory. Electrode bank 3 or electrode bank 6 is grounded and the opposing electrode is energized by an alternating current, the frequency of which is controlled according to the results desired. I have found a frequency of 60 cycles quite effective for grit sizes of the order of No. 220, No. 240 and No. 280, provided the correct voltage is used and the correct air gap is employed. As a rule a suitable air gap for grades of this size will be about 1 to 1 /2 inches. By this I mean the air gap between belt ltd in Figure 3 and electrode bank 3. The air gap may be the same at the point where the belt Mia enters the field as where the belt leaves the field or the gap may be varied, that is, electrode banks 3 and 6 may be parallel or at a slight angle to each other in accordance with the results desired. Likewise, I may vary the length of the electrodes in the direction of travel of the endless web 26 and the endless belt lua. Generally speaking, the longer the electrodes the greater .the quantity of abrasive that will be removed from the mixture of abrasive or other comminuted material delivered to belt i012. In operating with grades of this size range, I prefer to use a voltage that will lift all or most of the grains from the abrasivecarrying belt in thefield between electrodes 3 and S.

Where it is desired to separate grain sizes containing a considerable proportion of grains of still smaller size, that is, sizes smaller than the range 220, 240 and 280, Iprefer to use still higher frequencies both for those forms of the invention in which the finer grains separated are permanently coated as in Figures 1 and 2 and in those cases where the finer grain is removed from the field and collected for a subsequent use as is illustrated in Figure 3. For instance, for separating the finer grains in a mixture of grains of the order of magnitude of sizes 320, 360, 400 and still finer, I may use frequencies between 60 and 180 cycles, as for instance, 120 cycles, as has already been used in illustrating the coating of grade 320 by means of the form of apparatus shown in Figure 2. Generally speaking, the conditions depend upon the nature and size of the particles in the mixture being separated, and for a given material substantially the same conditions will be found to operate where the finer grain is permanently coated as in Figures 1 and 2, and where the grain is only temporarily coated or otherwise removed from the field and collected for subsequent use as illustrated in Figure 3.

In the coarser grades, separation into different sizes is usually readily accomplished by means of screens but in some instances the present invention is useful in handling coarser comminuted material, such as the coarser grades of abrasive. Where a coarser abrasive mixture is separated by means of the present invention, a lower frequency may be used on the separating electrodes as for instance, electrodes 3 and 6 in Figures 1, 2 and 3. It should be appreciated that the controlling factor in selecting the correct frequency is that I select a frequency which permits the lifting of the finer portion of the mixture to the adhesively coated backing counter-to-gravity or in a direction with a, component counter-togravity, an angle too vertical often being conducive to ease of operation of the equipment, and a frequency which also insures a reversal of polarity before the larger grains reach the adhesively coated backing, or other means of removing the finer material from the field.-

In general the preferred operation of my process with fine grits presupposes that a definite layer of fine grits will be fed into the field on a carrier belt, as for instance, belt in, or belt 50a, and that substantially all of the grains will be lifted from the belt during the separating operation. Where relatively coarse material is being handled, it sometimes may'be preferred to coat the belt sparsely with grain and lift only the finer grains from the belt. This can be accom plished by correct adjustment of frequency, air gap and voltage. Considering solely the voltage, it should be understood that the voltage required is not always proportional to the grit sizes. For instance, there is a certain grit size, for example, of silicon carbide, that requires a higher voltage than either a smaller or larger particle to insure lifting of the particle. Considering the coarse grits, for example, of silicon carbide, that is, grits coarser than about 180, a higher voltage is required to lift the larger grains than the smaller grains in this same range. However, it frequently happens, and in fact, it is generally true, that a higher voltage is required to lift grit 280, for example, silicon carbide, than is required to lift grit or 120. In other words, there is a minimum voltage requirement in sizes in the neighborhood of grit 180 and the voltage required to lift the particles rises on the coarse side of this point as the grit size increases and rises on the finer side of this point as the grit size decreases. This condition, however, does not nullify the basis of this invention which succeeds in separating fine abrasives even though a higher voltage may be required to lift the smaller particles, since in this grit range all particles are set in motion but the finer particles once in motion travel faster after leaving the abrasive carrier belt, for example, and the coarser particles are prevented from reaching the adhesively coated web or other collecting means by a reversal in polarity at the selected frequency.

Referring to Figure 4 which illustrates diagrammatically a set-up for obtaining any desired frequency of pulsation of alternating current and any desired potential from an electrode used in my coating process, 38 is a variable speed motor connected through suitable connecting means 35 to an alternating current variable frequency generator 32. Suitable motors, connections and generators are well known in the art, The alternating current generated in through conductors 34 and 35 to the primary winding 33 of a transformer. In conductor 35 between the motor generator and the primary winding 33 of the transformer is located a rheostat 36 for varying the voltage. The transformer may be of any desired type, but for purposes of illustration, I use a core type transformer. At 3! is shown a ground for the secondary winding 38 of the transformer, the secondary winding 38 of the transformer being connected to a suitable conductor 39 which leads to a suitable terminal A: which may be one of the electrodes indicated as connected to a, high tension set in Figures 1, 2 and 3 or may be a terminal which is in turn connected to one of the aforesaid elecgenerator 32 is fed an electrostatic field of alternating Polarity. D h

trodes. By such adjustments as are known in the art, the variable speed motor 30, the generator 32, the transformer windings 33 and 33 and the rheostat 36 may be controlled to obtain any desired frequency and potential at the terminal Ai.

, Electricity of characteristics suitable for carrying out my invention is readily supplied by apparatus of the type illustrated in Figure 4. This type of apparatus supplies electricity or characteristics well adapted for use in energizing electrode Ii of Figure 1, electrode 8 of Figure 2 and electrode 6 of Figure 3. The same type of apparatus may be used to supply alternating current, usually of lower frequency, to electrode I of Figure 1 or electrode 1 of Figure 2, where alternating current is used to energize electrode 1 of Figure 1 or of Figure 2. It is to be understood that the electrostatic separations carried out as illustrated in Figure 1 between electrodes.

3 and 6, in Figure 2 between electrodes 3 and B and in Figure 3 between electrodes 3 and i are analogous. The fundamental difference is that where the apparatus is used for permanently coating to a reinforcing backing the finer portion of comminuted material, separated from the comminuted material of variable size, a relatively strong adhesive is applied to the reinforcing backing 1 of Figures 1 and 2, whereas, when it is desired to recover for subsequent use as such, the finer portion of comminuted material separated from the comminuted material of variable size, as between electrodes 3 and 6, in Figure 3, a liquid such as water or a mild adhesive is used to temporarily anchor the finer portion of comminuted material to the endless belt or carrier 26 so that the finer portion of material may b collected as illustrated in Figure 3 in receptacle I! for further use as such. Recovery of the finer portion of material removed-from the field by carrier 26 is facilitated by the heater of Figure 3 which evaporates some or all of the liquid used to wet and adhere the grains temporarily to the carrier 26 or evaporates the volatile portion of the mild adhesive coated to carrier 26, so that the temporarily adhered grains may be more readily removed from the web by beater or rattler 2| and/ or revolving brush 22.

It will thus be seen that there has been provided by this invention, methods and apparatus suitable for coating a controlled grade of comminuted matter such as abrasive grains to an adhesively coated backing, for separating com minuted material such as abrasive grain into two or more portions, one of which is of finer grade than the other portion, and for combining the steps of separating or grading comminuted ma terial, such as abrasives, with the coating of the same to an adhesively coated backing, in a single operation. There has thus been provided by this invention, methods and apparatus in which the various objects hereinbefore set forth, together with many thorough practical advantages, are successfully achieved.- As various possible embodiments might be made of the mechanical features of the above invention, and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A process of separating finer sized particles from comminuted material having particles of different sizes therein, comprising establishin ing said comminuted material through said field, adjusting the frequency of alternations of the polarity of said field to separate from the comminuted material in the field only the finer sized particles of comminuted material, removing the finer sized particles so separated from said field, and collecting separately from the finer sized particles so removed, the coarser particles of comminuted material.

2. A process of separating finer sized particles and coarser sized particles from comminuted material having particles of diiferent sizes therein,

comprising establishing an electrostatic field of alternating polarity, simultaneously passing through said field separate means for remov ing therefrom the finer sized particles and the coarser sized particles, adjusting the frequency of alternationsof the polarity of said field to separate from the comminuted material introduced into the field and to deposit upon the firstmentioned particle removing means only the finer sized particles, thereby removing the finer sized particles from the field, and separately removing on the second-mentioned particle removing means the coarser portion of said comminuted material.

3. In a process of coating a reinforcing backing with an abrasive material, the steps comprising passing an adhesively coated web through an electrostatic field of alternating polarity of controlled frequency adjacent an upper electrode of said field and with the adhesively coated side facing downwardly, passing abrasive grains of variable size through said electrostatic field adjacent the lower electrode, adjusting the frequency of alternation of the polarity of said field so that a finer portion only of said abrasive ma terial is propelled to and embedded in said adhesive in oriented relation whereby the abrasive coating applied to said adhesively coated web is of a substantially finer grade than the abrasive grain delivered to said field and whereby a coarser abrasive grain with less variation in size is prepared for other abrasive use.

4. A process of coating a reinforcing backing with an abrasive material comprising passing an adhesively coated web through an electrostatic field of alternating polarity of controlled frequency adjacent an upper electrode of said field and with the adhesively coated side facing downwardly, passing abrasive grains of variabl size through said electrostatic field adjacent a lower electrode, adjusting the frequency of alternation of the polarity of said field so that a partial coating of a liner portion only of said abrasive material is propelled to and embedded in said adhesive in oriented relation, passing said web so coated through a second electrostatic field adjacent an upper electrode of said second field, passing the abrasive grains not coated in the first electrostatic field into the second electrostatic field adjacent the lower electrode, and creating electrostatic forces in said second elec trostatic field to cause a coarser portion of abrasive to be propelled to and embedded in said adhesive whereby the final composite abrasive coating applied to said adhesively coated web is relatively free from fine grains that have been em bedded between the exposed portions of the coarser grains, and drying said coated web to set the adhesive and fix the abrasive grains to the backing.

5. A process of coating a reinforcing backing with abrasive material comprising passing an adhesively coated web through an electrostatic field with the adhesively coated side facing abrasive supplied to said field, passing abrasive grains containing a substantial proportion of grains finer than the coarser portion of said grains through said electrostatic field and electrically propelling a partial coating of only a finer por- 2 tion of said abrasive onto said adhesively coated backing in said field, passing said adhesively coated backing so partially coated with abrasive grains, while the adhesive is still in a condition to receive additional grains, through a second electrostatic field, supplying to said second electrostatic field coarser abrasive grains relatively free from grains of the size of grains coated in the first electrostatic field, and electrically applying an additional portion of a coarser size of abrasive to said previously partially coated backing, in said second electrostatic field, and setting the adhesive on the web so coated to hold the abrasive grains in place.

6. A combined process of grading abrasives and coating abrasives onto a reinforcing backing comprising passing an adhesively coated web through an electrostatic field with the adhesively coated side facing downwardly and adjacent an upper electrode, passing abrasive grains of suitable but variable size through said field, adjusting the electrode spacing, the voltage and the frequency of the alternating current used to energize an electrode of said field in accordance with the grade and quantity of abrasive that it is desired to coat on the backing in said electrostatic field and thereby propelling a controlled coating of abrasive grains into said adhesive on said web, removing the web so coated with adhesive and abrasive from said field and setting the adhesive, and collecting the uncoated abrasive grains, which pass through said field and are of a different grade from those coated for subsequent abrasive use.

7. A process of coating abrasives onto a reinforcing backing, comprising providing at least one electrostatic field adapted to coat a finer grade of abrasive and at least one electrostatic field adapted to coat a coarser grade of abrasive, passing an adhesively coated backing through the first-mentioned electrostatic field and then through the second-mentioned electrostatic field, passing abrasive grains having finer and coarser grades therein first through the first-mentioned field and then through the second-mentioned field and coating on the adhesively coated side of the backing by electrostatic means, thereby, at least one finer portion of abrasive grains and thereafter at" least one coarser portion of abrasive grains whereby a composite abrasive coating is built up on the backing which is relatively free from finer grains that have been embedded between the exposed portion of the coarser grains, and then setting the adhesive to anchor the grains to the backing.

8. A process of grading comminuted material such as abrasive grains having finer and coarser portions intermixed therein, comprising establishing between spaced electrodes, an electrostatic field of alternating polarity of controlled frequency with the direction of force of said field having a component parallel to the force of gravity, conducting through said field separate means for removing therefrom, at least one fiiner portion of comminuted material introinto the field and at least one coarser graded portion of such comminuted material, adjusting the voltage, the electrode spacing and the frequency of alternations of the polari y 01' said field in accordance with the grade and quantity of material to be separated as the finer graded portion of the material, and thereby mov ing a'finer graded portion of the material to the first-mentioned material-removing means for removal therewith from said field, and removing on the second-mentioned material-removing means the coarser graded portion of the material not so separated by the electrostatic field, whereby the comminuted material fed to the electrostatic field is separated into at least one finer graded portion and at least one coarser graded portion.

9. A process of manufacturing coated abrasives,

controlled frequency, said field having a component force of gravity, foundation material through the said electrostatic field adjacent an upper electrode of said field with the adhesively coated side facing downwardly, feeding abrasive grains of variable size through said field below said adhesively coated foundation material and adjacent a lower electrode, adjusting the voltage, the electrode spacing and the frequency of the alternation of the polarity of said field in accordance with the grade and quantity of material that it is desired to deposit electrostatically upon said adhesive, and depositing thereby a controlled weight of a controlled grade of abrasive onto said adhesively coated side of said foundation material, removing the residual uncoated abrasive from said field and setting the adhesive to anchor the coated abrasive grains to the said foundation material.

parallel to the coated backing, electrodes, each has a component of force parallel to the force of gravity, means for passing an adhesively coated backing in sequence through the first and second pairs of electrodes adjacent an upper electrode in. each means for establishing an electrostatic field of alternating polarity between electrodes, means-for controlling the frequency of alternation of said field of alternating polarity and whereby a predominantly finer grade of comminuted material may be propelled to the adhesively coated side of said adhesively coated backing between the first coarser grade of comminuted material may be coated electrically, and means for feeding comminuted material of variable size in sequence through the first pair of electrodes and then through the second pair of electrodes.

11. Apparatus for coating comminuted material, such as abrasive grains onto an adhesively pairs of tlierebetween which has a component of force force of gravit the electrode spacings gradually decreasing from a maximum of electrodes to passing an adhesively coated trode in each pair of electrodes, means for establishing an electrostatic field of alternating p0- larity between said first pair of electrodes, means for controlling the frequency of alternation of said field of alternating polarity and whereby a predominantly finer grade of comminuted material may be propelled to the adhesively coated side of said adhesively coated backing between the first pair of electrodes, means for establishing an electrostatic field between the second pair of electrodes such that a coarser grade of comminuted material may be electrically coated, and means for feeding comminuted material of variable size in sequence through the first pair of electrodes and then through the second pair of electrodes.

12. A process of separating comminutedmaterial having particles of difierent sizes therein into finer and coarser portions comprising establishing an electrostatic field of alternating polarity between spaced electrodes having free space for motion of particles between the electrodes, passing said comminuted material into said field, controlling the frequency of alternations of polarity of said field so that the average displacement of the finer particles per electrical impulse is substantially greater than the average displacement of the larger particles per electrical impulse and so that the average displacement of the larger particles per electrical impulse is substantially less than the free space distance between the electrodes, and whereby there is created in the field and adjacent one electrode, a zone containing a finer portion of comminuted material and. relatively free from the coarser portions of comminuted material, removing the finer portion of comminuted material from this zone and collecting it for a-subsequent use, and separately removing the coarser portion of comminuted material and collecting it for a separate subsequent use.

NICHOLAS E. OGLESBY.

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