US3238632A

US3238632A - Method and apparatus to dry powdery substances under a vacuum using electrical gas discharges

Info

Publication number: US3238632A
Application number: US220454A
Authority: US
Inventors: Voigtlaender-Tetzner Gerhard
Original assignee: Leybold Anlagen Holding AG
Current assignee: Leybold Anlagen Holding AG
Priority date: 1961-08-30
Filing date: 1962-08-30
Publication date: 1966-03-08
Anticipated expiration: 1983-03-08
Also published as: DE1140875B

Description

March 1966 G VOiGTLAENDER-TETZNER 3,238,632

METHOD AND APPARATUS TO DRY POWDERY SUBSTANCES UNDER A VACUUM USING ELECTRICAL GAS DISCHARGES Filed Aug. 30, 1962 2. Sheets-Sheet 1 K amzaomwzk. I .v 5mm...

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GERHARD VOlGTLAENDER-TETZNER ATTORNEYS GAS WITH DRAWA L March 1966 G. VOlGTLAENDER-TETZNER 3,

METHOD AND APPARATUS T0 DRY POWDERY SUBSTANCES UNDER A VACUUM USING ELECTRICAL GAS DISCHARGES Filed Aug. 30, 1962 2 Sheets-Sheet 2 Torr sec

300 min I I L BY: 40.4404 7 W ATTORNEYS United States Patent 3,238,632 METHOD AND APPARATUS T0 DRY POWDERY SUBSTANCES UNDER A VACUUM USING ELEC- TRICAL GAS DISCHARGES Gerhard Voigtlaeuder-Tetzner, Leverkuseu-Schlebusch,

Germany, assignor, by mesne assignments, to Leybold- Anlagen Holding A.G., Zug, Switzerland Filed Aug. 30, 1962, Ser. No. 220,454 Claims priority, application Germany, Aug. 30, 1961, L 39,906 12 Claims. (Cl. 34-1) The present invention relates generally to a method for vacuum drying or vacuum degassing substances in powder form, and especially substances having a small average grain size, preferably below cm.

Considerable difficulties are encountered when drying and/ or degassing substances in powder form having small grain sizes. The reason for this is that during the process there is a decrease in the size of the empty hollow spaces and cavities between powder particles and, therefore, circulation of the molecules, which is necessary for gas and/or vapor exchange, also decreases. In industrial processes, powered substances having very fine grain size including grain sizes which are less than 10"*, are used as a heat insulating material. However, in order to obtain the extremely small heat conductivity which is possible with powdered materials of the above-mentioned type, drying and degassing processes must be performed on the material. The vacuum which is used for this purpose and which, under some circumstances, is a high vacuum, can sometimes also be maintained in the container in which the body of powder is housed.

In the drying and degassing of small particles there is an undesired interaction between the individual particles which increases as the grain size decreases. When a vacuum is provided and accompanied by heating, the gaseous surface layers which surround the particles are removed and the particles can then contact each other and strong molecular surface forces become effective. By means of these forces, the particles adhere to each other and/or to the walls of the receptacle or container. Due to this interaction agglomerations of the material are often formed within the powdery contents of the container and these are of such strength or hardness that they cannot be separated even when using powerful mechanical means, such as by pounding with iron rods.

As mentioned above, the cause of this very undesirable effect is the formation of surface forces which increase to extreme extents when the particles approach intimate contact with each other. A similar effect is used in technical fields in slip gauges. In such devices the sections adhere to each other tightly when the surfaces contact one another. The reason for this is that the surfaces are very uniform or smooth and the contacting and adhering is due to molecular adhesion forces.

With these defects of the prior art in mind, it is a main object of the present invention to provide a method of drying or degassing powdery substances in a manner which is much improved over the prior art.

Another object of the present invention is to provide a method of the character described wherein agglomerations of powdery substances under the effect of surface forces are prevented during drying or degassing processes.

These objects and others ancillary thereto are accomplished according to preferred embodiments of the invention wherein a body of powdery substance is provided and is subjected to the influence of a vacuum. During the period it is subjected to the vacuum, an electrical gas discharge is created i the body of powdery material. As used in the following specification and claims, the term gas discharge has the conventional meaning of a discharge 3,238,632 Patented Mar. 8, 1 966 formed under reduced pressure by the application of a break-down voltage between a pair of electrodes. The discharge paths thereby formed create channels in the powder layers and thus loosen the powder particles. By properly arranging the discharge electrodes, the electrode discharge processes can be caused to occur to the greatest degree in certain preferred portions of the body of powdery material. However, in many instances a uniform effect is desired, i.e., an effect of the electrical discharge upon the entire powdery body is desired and may be uniform throughout.

The gas discharge may be caused by a high frequency electric alternating field which may preferably be generated with the aid of a known Tesla transformer or coil. This type of discharge causes only a little heat to be generated in the powdery substance and this heat also contributes to the acceleration of the drying and degassing processes.

Instead of using the irregular spark discharges, glow discharges of low luminous flow may be used within the powder layer. However, the greater mechanical forces provided by the spark discharge are generally more advantageous. Since, in the instant invention, an electrical gas discharge is to be maintained in the body of powdery material, it should be noted that at least during a portion of the drying and degassing process the pressure within the receiving container must not be lowered to the extent that the electrical discharge will be extinguished.

Additional objects and advantages of the present invention will become apparent upon consideration of the following description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic view of an arrangement for degassing fine grain powders.

FIGURE 2 is a graphical view illustrating the relative gas withdrawal of fine grain powders according to the present invention and according to prior art processes.

FIGURE 1 illustrates a vessel or receiving container 1 in which a body of fine grain powdery material 2 is disposed. This powdery material can be formed of Aerosil powder having a medium grain size of 10- cm. A heating coil 3 is disposed around the outer wall of the container 1 and this coil may be activated or heated by connected it with a current supply.

A discharge probe 4 extends into the powdery body of material 2 and is connected at one end to a Tesla transformer 6 by means of a connection line 5. The other pole of the Tesla transformer 6 is grounded by means of connection line 7. The inner wall of the container 1 acts as a counter electrode for the discharge probe 4, and is also grounded by means of a line 8.

The receiving container 1 is sealed by closure element 11 and is connected to a vacuum pump 10 by means of an evacuating line or conduit 9. A pressure measuring device 13 of a known construction, for example, an ionization manometer, is connected to the evacuating line 9. Also a ventilating valve 14 is connected to the evacuating line 9 and both of these connections are provided by means of a cross piece 12 formed of conduit material. A filter 15 is disposed in the evacuating line 9 and in the flow path and upstream of, or before, the cross piece 12 and prevents the fine grained powder from entering the measuring device 13 and the pump 1t). Downstream or behind the cross piece 12 and in the evacuating line 9 is a sealing or blocking valve 16.

During operation of the device the interior of container 1 is evacuated by means of line 9 and the vacuum pump 10, with the valve 16 being open. The amount of pressure which is provided can be determined by resorting to the pressure measuring device 13. At this point the ventilating valve 14 remains closed. Also, the body of powdery material is heated by means of coil 3 and elec- Torr liters as the abscissa seconds where 1 TO1T=1 mm. of Hg, and pumping time in min utes as the ordinate. Curve a illustrates the nature of the gas withdrawal without the benefit of the elfect of an additional electrical discharge. However, curve b illustrates the more advantageous conditions which are provided when such a discharge is simultaneously provided during the vacuum operation.

In this example the Aerosil powder had a medium or average grain size of 10' cm. and an initial moisture content of 1.5%. The graph clearly shows that a desired high value of gas withdrawal of the powder can be obtained with the method according to the present inven tion. In other words, a more extensive degassing of the body of powdery material is provided than is possible with evacuation or operation without the benefit of this discharge effect. From practical experiments it is seen that the vacuum treatment of the body of powdery material, when gas discharge is carried out simultaneously therewith, is degassed about ten times as much as is provided without this auxiliary measure. It is essential that such a strong degassing can be accomplished with minimal heating of the powdery body.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. A method for vacuum treatment of powdery substances having small average grain size of below 10- cm. comprising the steps of depositing a body of powdery material in an evacuable chamber, subjecting the body of powdery material to a vacuum, and, at least during a portion of the time that it is subjected to the vacuum, creating an electrical gas discharge through the interior portions of the body of powdery material so as to cause agitation thereof.

2. Apparatus for vacuum treatment of powdery substances, comprising, in combination:

(a) a vacuum tight chamber for receiving powdery substances;

(b) vacuum pump means for producing a vacuum connected to the interior of said chamber; and

() gas discharge producing means comprising an electrode disposed within said chamber and a power supply for applying a break-down voltage to said electrode, said gas discharge producing means adapted to produce an agitating gas discharge through the interior portions of a body of powdery substance in said chamber while a vaccum is produced therein.

3. The apparatus according to claim 2 wherein said electrode is disposed within saidchamberso as to project into the interior portion of the body of powdery substance.

4. The apparatus according to claim 3 wherein said vacuum tight chamber is a grounded electrically conductive container and provides a discharge path for the gas discharge produced by said gas discharge producing means.

5. The apparatus according to claim 2 wherein said gas discharge producing means is adapted to produce a spark discharge at said electrode disposed within said chamber.

6. The apparatus according to claim 5 wherein said electrode is disposed within said chamber so as to project into the interior portions of the body of powdery substance.

7. The apparatus according to claim 5 wherein said gas discharge producing means includes a Tesla transformer device adapted to produce a spark discharge.

8. The apparatus according to claim 2 including, in addition to said gas discharge producing means, heating means adapted to produce heating of the body of powdery substance.

9. The apparatus according to claim 8 wherein said electrode is disposed within said chamber so as to project into the interior portions of the body of powdery substance.

10. A method for vacuum treating an average powdery substance having a grain size below 10 cm., comprising the steps of depositing the powdery substance in a vacuum chamber so as to at least partially bury an electrode disposed within said vacuum chamber, evacuating said vacuum chamber, and producing a gas discharge at said electrode so as to cause agitation of the powdery substance during said evacuation step.

11. The method defined in claim 10 wherein the gas discharge produced is a spark discharge.

12. The method defined in claim 10 wherein the gas discharge produced is a glow discharge.

References Cited by the Examiner UNITED STATES PATENTS 773,875 11/1904 Lorillard 34l5 773,876 11/1904 Lorillard 34-15 X 2,038,251 4/ 1936 Vogt. 2,442,114 6/1948 Brown 34-1 2,585,825 2/1952 Nyrop 341 2,605,554 8/1952 Flosdorf 34-1 3,011,543 12/1961 McCormick 159-4 3,060,297 10/ 1962 Sargent.

FOREIGN PATENTS 898,989 5/ 1945 France.

704,491 2/1954 Great Britain.

911,885 11/1962 Great Britain.

WILLIAM F. ODEA, Primary Examiner.

GEORGE D. MITCHELL, ROBERT A. OLEARY,

MEYER PERLIN, Examiners.

J. SOFER, W. E. WAYNER, Assistant Examiners.

Claims

1. A METHOD FOR VACUUM TREATMENT OF POWDERY SUBSTANCES HAVING SMALL AVERAGE GRAIN SIZE OF BELOW 10**-4 CM. COMPRISING THE STEPS OF DEPOSITING A BODY OF POWDERY MATERIAL IN AN EVACUABLE CHAMBER, SUBJECTING THE BODY OF POWDERY MATERIAL TO A VACUUM, AND, AT LEAST DURING A PORTION OF THE TIME THAT IT IS SUBJECTED TO THE VACUUM, CREATING AN ELECTRICAL GAS DISCHARGE THROUGH THE INTERIOR PORTIONS OF THE BODY OF POWDERY MATERIAL SO AS TO CAUSE AGITATION THEREOF.