US2270341A

US2270341A - Method of coating granular materials

Info

Publication number: US2270341A
Application number: US307312A
Authority: US
Inventors: Harold P Ransburg
Original assignee: Harper J Ransburg Co Inc
Current assignee: Harper J Ransburg Co Inc
Priority date: 1939-12-02
Filing date: 1939-12-02
Publication date: 1942-01-20
Anticipated expiration: 1959-01-20

Description

Ja'n 2o, 1942.

H. P. RANSBURG METHODL 0F COATING GRANULAR MATERIALS 2 sheets-sheet 1 Filed Dec. 2, 1959- Jan. 20,Y 1942. H. P. RANsBuRG METHD OF COAIING GRANULAR MATERIALSl Filed Dec. 2, 1939 2 Sheets-Sheet 2 "RNEYs,

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\ Patented Jan. 20, 1942 ,y

UNITED STATE Harold P. Ransburg, Indianapolis, Ind., assignor to Harper J. Ransburg, doing business as Harper J. Ransburg Company, Indianapolis, Ind.

Application December z, 1939, serial riogso'rznzV realms.

This invention relates to a method of coat-I ing granular materials. The method is particularly adapted to the coating of ground cork with adhesive material for the manufacture of molded cork articles such as the cork seals used in crown bottle caps. the coating of ground cork or other similar material with paint or coloring matter in the manu-A facture of press board or insulating roofing material. Many other similar uses of the invention are possible. u

In the past it has been the practice to coat such granular materials by mixing the same with the coating material in a pug mill or other form of apparatus by which they are mechanically mixed. This process is not entirely satisfactory vfor giving a thorough and uniform coating to all particles of the granular material unless Iexcessively long periods are allowed in the pug mill. Even withA a lengthy pugging operation there is a tendency for the smaller particles to accumulate in masses coated only on theoutside,V the individual particles within the mass having allor a large portion of their surface uncoated. The prior process, moreover, is restricted .in use since someV materials can not be handled at all in that manner.

One object of the present invention is to provide a simple and efcient method of coating granular materials of the type described which insures a thorough coating of the surface of all particles. i

Another object of the invention is to provide` a method for l,coating such materials with coatings which have not been possible by the prior methods.

In its simplest form the invention provides for intermingling dispersed streams of the granular material and coatingy material in an electrostatic field in such manner that the particles of the coating material are drected to the particles of granular material by electrostatic attraction. In one form of the invention the electrostatic attraction is produced by impressing static surface charges of opposite potential upon the particles of the two streams. tion like charges impressed on the particles of granular material serve to scatter the particles by electrostatic repulsion and so facilitate access of the coat'ng material to individual particles. This in itself is of great importance even when the coating material is not electrically charged. In another form of the invention a stream of the granular material is passed over the surface' of an electrode which is used to attract electro- Another use of the invention is in In this form of the invenstatically a spray of coating material toward the stream of granular material.

Preferably vthe two streams of material are intermingled in midair while unsupported, as distinguished from spraying coating material on a stream ofmaterial travelling on'a conveyor. In this way the' greatest dispersion of the granular particles is obtained and all parts of each particle are accessible '"to the coating material.

Other ,objects and features of the invention will be understood from the accompanying drawings and the following description and claims:

Figs. 1, 2, 3 and 4 are vertical sectional views partly diagrammatic of several forms of apparatus by means of which the method of the invention may be carried out. Fig. 5 is an elevational view of a portion of the apparatus shown in Fig. 1. Fig. 6 is a similar view of a portion' of the apparatus shown in Fig. 3.

Referring now to Fig. 1, there is provided a chamber I having sidewalls II, top I2 and hopper bottom I3. The walls, top and bottom of the chamber are preferably constructed of an electrically non-conducting material such as plywood, breboard or the like. A supply hopper Il provided with a control gate I communicates with the chamber III by means of an inlet opening I6. The supply hopper I4 is adapted to contain a supply of material to be coated which may be fed into the chamber I0 through the opening I6 and the flow of which may bev controlled by the gate I5. In the upper portion of each side of the chamber I0 there is provided Ian opening I1 y through which a coating material, such as paint, glue, varnish or other similar substances, may be sprayed, as by pneumatic spray guns I8. The

spray guns I8 may be of any conventional form well known in the art and suitable for the particular coating material to be supplied. For certain materials the rapid cooling effect of expanding compressed air associated with a pneumatic spray gunis detrimental, Insuch cases, a wellf known type of centrifugal spray device may be provided instead.

Associated with each of the openings I1 and with the openingI I6 there'is provided a grid construction shown particularly in Fig. 5. Said construction consists of a pair of supports I9 mounted on the wall of the chamber and carrying an 60 electrical conductor 20 which is repeatedly passed from one to the other of 'the supports I9'to form a grid between the strands of which the materials must pass in entering the chamber I0. A

generator 2l adapted to supply direct current at a relatively high voltage may be located at any l rn.

generator preferably supplies a potential of the order of 100,000 volts and lseveral types well known vin the art are available for the purpose. One terminal of the generator is connected by a conductor 22 to the rid 20 arranged beneath the opening I6. The o posite terminal of the generator is connected by a conductor 23 to both of the grids 20 associated with the coating material openings I1. By means of this construction a high potential electrostatic field is established within the chamber l0, the portion thereof adjacent the\'granular material supply opening I6 being charged with one potential and that adjacent the coating material openings l1 being into the Ichamber 30. The-.stream of granular material and the stream of coating material are an even more eiiicient mixing is produced ,by the provision of grids 31 similar to the grids 20 and arranged adjacent the openings 3| and 32. Said grids are'electrically ycharged in opposite senses by a generator 38. The operation of apparatus of this form is similar to that of the apparatus shown in Fig. l. The streamy of incoming granular material is dispersed by electrostatic repulsion anid lthe oppositelyl charged particles of the charged in the oppositesense. Preferably the spacing of the ,grids and the voltage applied thereto is properly chosen so that a corona discharge takes place since the voltages associated with such dischargarefound to have the necessary effectin charging the atmosphere adjacent the grids and the particles of material passing through the grids.'

In the operation of this form of the apparatus,

the gate I5 is adjusted to permit a stream of the granular material 4to fall freely into the chamber l0. Even thoughV the material is not such as would ordinarily be classed as a conductor of electricity still the particles thereof may pick up surface electric charges in passing through the grid and the highly charged atmosphere surrounding the same. Since like charges are picked up by the'particles, they tend to repel each other two streams are electrostatically attracted to each other in much the same manner as previously described. l v I Referring now to Fig. 3, there' is shown therein a construction similar in many respects to that of Fig. 2 and in which parts numbered in the one hundred series have the same construction and function as correspondingly numbered parts and the stream of material is thereby scattered.

At the same time, the particles of coating material sprayed into the chamber l0 pick up electrical charges opposite to those on the particles of the granular material'. The sprays of coating material are-directed as best seen in 1 to converge upon the stream of granular material and since the latter stream has become dispersed by electrostatic repulsion of its particles, each particle has va much greater chanceto receive its full complement of coating material than it would have if the stream lwere not so dispersed. The opposite electrical charges of the particles of granular material and coating material cause the' same to be attracted toward each other and to the grids 3l are n'ot used and are replaced by a construction shown in detail in Fig. .6. In said construction a plurality of downwardly extending electrical conductors 39 are mounted on a support 40 and are electrically connected to each other. Beneath each of the openings y|3I and |32 there is mounted a conductor 4| in opposed relation to the conductors 39. The conductors 39 associated with the granular material supply opening |3| are connected to one terminal of the generator |38 by a conductor 42 and the conductors 39 associatedI with the coating material supply are connected to the opposite terminal of the generator by a. conductor 43. the conductors. are grounded.

lBy means of this construction separate electrostatic fields are set up between each of the conductors 39 and its associated conductor 4|. Since the gap between said conductors is much less than half ofthe gap between the grids'31 of neutralize each other. An intimate mixing of the maintaining a relatively high temperature therei 'I'his temperature may be maintained in many cases fsufiiciently 4high to evaporate the solvent used in the coating material. The coated particles fall to the bottom of the chamber l0 and may be discharged through an .opening 25 therein forlsuch further steps in the process of manufacture as may be necessary.

Referring now to Fig. 2, there is shown therein a second form of apparatus for carrying out the invention. In that form a chamber y3|l which may be of substantially similar construction to the chamber I0 is provided with side openings 3| and 32. Adjacent the. opening 3| there is a pneumatic blow gun 33 by means of which a supply of granular material 34 may be blown into the chamber 30. Adjacent the opening 32 there is a spray gun 35v which may be similar to the guns I8 and which sprays a coating material-36 Fig. 2 the voltage required to establish corona discharge is much less. In addition, the ends of the conductors 39 constitute discharge points which intensify corona discharge. TheV two elds,

since. they are connected to opposite terminalsv of the generator, charge the atmosphere in their `immediate neighborhoods in opposite senses 1so-that particles of the granular material entering the. opening |3| are charged in opposite sense to particles of coating material entering the' opening |32. The resulting interaction of the charged particles is similar to that previ- Aousl'y described in connection with Figs. 1 and 2.

Referring now to Fig. `4, there' is illustrated an apparatus suited for the coating `of materials which are not so well adapted-tol the reception of electrostatic surface charges. In the use of this construction, less reliancev is placed upon the electrostatic attraction between the particles of the two materials and additional means are provided forI directing the spraylof coatingy material upon the stream of granular material. In this form of apparatus there is provided a casing 50 of electrical non-conducting material deilning a chamber 5|. Adjacent opposite sides of thechamber 50 there 4are provided

electrodes

52 and 53 connected t'o opposite terminals of a generator 54 adapted to supply high voltage direct current. The electrode 52 is preferably in the form of a vertical wire having very small surface area. Electrode 53 is a plate of conducting material having many times the surface area of the Both or'` electrode 52. Above the chamber 5I there is provided a storage hopper 55 adapted to contain a supply 56 of the granular material to be coated. Said material is supplied from a suitable opening in the bottom of the hopper 55 to a rotating gate roller 51 which is adapted to regulate by its speed of rotation the flow of material from the hopper. Said material is permitted to fall freely across the face of the electrode 53 and thus passes through the electrostatic field established between the

electrodes

52 and 53. A spray gun 58 is mounted to supply a spray of coating material 59 into the upper part of the chamber 5I between the ystream of granular material and the electrode 52. As discussed in more detail in co-pending application of Harold P. Ransburg and Harry J. Green, Serial No. 281,766, illed June 29, 1939, coating material sprayed into an electrostatic eld is attracted to the electrode having the largest surface area in much greater degree than toward the electrod of small surface area. 'Ihus the spray of coating' material is attracted to the electrode 53 but is intercepted by'the stream of granular material dispersed over the surface of the electrode4 and is intimately mixed therewith.

Several forms of apparatus by means of which f the method of the invention as defined in the appended claims may be carried out have been described. Other forms will be apparent to those of said points, introducing a stream of liquid coating material in the neighborhood of the second of said points whereby said streams are electrically charged in opposite sense, and directing said streams to convergence within said ileld whereby particles of one stream are attracted to particlesfof the other.

2. A method of coating granular material comprising establishing a corona electric discharge between a pair of oppositely charged electrodes, and introducing converging streams of said material and a coating material into the space between said electodes, one of said streams passing suiilciently close to one of said electrodes to receive electrical chargesA therefrom and the other similarly recelvingvcharges from the other of said electrodes, whereby particles of one stream j prising establishing an electrostatic eld of high potential between two electrodes one of which has a surface area many times greater than the other, passing a freely falling stream of said material between said electrodes and close to the larger electrode, and spraying coating material into the space between said stream and the smaller electrode, said coating material being electrostatically attracted toward said larger electrode and thereby mingling with the stream of material to be coated.

HAROLD P. RANSBURG.