US2749051A - Method of compacting light powders - Google Patents

Description

June 5, 1956 1.. o. SIMENSON METHOD OF COMPACTING LIGHT POWDEJRS Filed Feb. 26, 1953 P0 wder in J15 q/ven/ fi/ya rau/fc powder out hfyc/rau/lb flu/0' INVENTOR. L culls 0. 6/171 enson A 7'7'ORNEYS METHOD OF COMPACTING LIGHT POWDERS Louis 0. Simenson, Berkeley, Calif., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application February 26, 1953, Serial No. 338,941 2 4 Claims. (Cl. 2413) This invention relates to a method of compacting light and fluffy powders. It relates in particular to a method of densifying fluffy and pyrophoric alkali metal alkyl xanthates to render them safe to handle.

In the production of alkali metal alkyl xanthates, the products are first obtained as very fine, fluffy and pyrophoric powders having bulk densities near 15 to 20 pounds per cubic foot. It is known that, if these powders can be formed into pellets by compaction they are no longer pyrophoric. Most pellet presses fail to operate satisfactorily on powdered feeds with bulk densities as low as 20 pounds per cubic foot, and it is preferred to supply them with a feed with a bulk density of 40 to 60 or more pounds per cubic foot. Other materials, in the form of light, fluffy powders, present several of the same problems as the xanthates, and a method is desired which will render such powders sufliciently dense to be readily pelleted or otherwise handled safely or conveniently.

The principal object of the invention, then, is to provide a method whereby light and fluffy powders may be converted to denser powders. A particular object is to provide such a method for converting light powders of alkali metal alkyl xanthates to powders having a high enough density so that they can be pelleted satisfactorily and are no longer pyrophoric.

The present invention provides a method whereby the foregoing and related objects may be attained, and will be described with reference to the annexed drawing, the single figure of which is a diagrammatic representation of one apparatus for carrying out the new method.

According to the invention, the low density powder to be compacted is mixed with a small amount of solvent or softening agent for the powder, sufiicient to render the surface of the particles tacky and coherent, and the resulting mixture is compacted by gradually increasing pressure. The compaction step is interrupted, with the mixture held under pressure, for a short time suflicient to permit the solvent to cause the particles to cohere. The mixture then is further compressed toform a coherent mass, pressure isreleased, and the compacted mass is ground. The resulting powder has a much higher bulk density than the initial feed, and, when the feed is a pyrophoric powder, the compacted and ground product is no longer pyrophoric.

Only a small amount of solvent is required, generally from 0.5 to per cent of the weight of the flulfy feed with which it is mixed. The compaction operation is delayed for from 1 to 10 seconds, or longer, until the solvent has acted on the surface of the powder particles. The pressure required for final compaction and the pressure at which the mixture is held during the delay period vary widely, depending on the particular powder and the solvent employed, their ratio in the mass being treated, and the amount of densification desired.

In the densification of pyrophoric alkali metal alkyl xanthates, the solvent employed is most commonly methyl or ethyl alcohol in amount from 0.5 to 10 per cent of the weight of xanthate (2 per cent being a convenient nited States Patent 0 F 2,749,051 Patented June 5, 1956 amount); the time allowed for activation of the mass during compaction is from 1 to 5 seconds; the pressure during the delay period is of the order of 300 to 1000 pounds per square inch; and the final compaction pressure is-of the order of 500 to 1500 pounds per square inch. Such conditions give a compressed mass which, when ground, yields a pelletable non-pyrophoric powder having a bulk density from 40 to 60 pounds per cublic foot.

It is apparent that many forms of apparatus may be used forpracticing the method of the invention. One convenient apparatus, shown in the drawing, is an adaptation for this purpose of a Pump for Transferring Powders From One Atmosphere to'Another, which is described more fully by the present inventor in concurrently filed application Serial No. 338,942, new U. S. Patent No. 2,681,769. Operation of the apparatus will be described in conjunction with the following detailed example.

A light (18 pounds per cubic foot) and pyrophoric powder of potassium amyl xanthate was mixed with 2 per cent of its weight of methanol in a mixer 10, of the Banbury type, and the mixture was fed, through chute 11, to compaction cylinder 12 when piston 13 rose above the egress of chute 11 on its up-stroke. At the top of the up-stroke, arm 26 contacted switch 2'7 which closed valve 15 and opened valve 16, admitting hydraulic fluid under pressure of nearly 1500 pounds per square inch through line 17 to the upper portion 18 of hydraulic cylinder 19. Piston 13, in compaction cylinder 12, was forced downward closing chute 11 and compacting the solvent-powder mixture. As the mixture was compacted the pressure registered by the gage on line 17 increased. When it reached 700 pounds per square inch, pressuresensitive time-delay switch 14 closed valve 16 providing an unvarying pressure on the mixture for about 4 seconds after which switch 14 reopened valve 16, forcing piston 13 downward again and compacting the solventactivated powder into a dense plug 20 against disk 21. The said disk 21 was mounted for rotation on shaft 22 which passed downward through hydraulic cylinder 23. Hydraulic fluid was maintained in cylinder 23 under suffi' cient pressure to hold disk 21 firmly over the bottom of compaction cylinder 12 until the plug 20 was well compacted. The pressure in cylinder 23 was finally overcome by that in chamber 18 allowing shaft 22 and disk 21 to drop slightly. At that time, hydraulic fluid was circulated through a positive displacement gear motor 24, at a rate to turn shaft 22 and disk 21 at about revolutions per minute. The rotation of disk 21 against the lower end of plug 20 resulted in abrasion of the plug, forming a powdered product which was discharged through hopper 25. This powder had a bulk density of about 55 pounds per cubic foot, and was nonpyrophoric. It was freed from final traces of alcohol and supplied to a pelleting machine and behaved normally therein, forming coherent pellets Without difiiculty. Piston 13 was raised to the starting position, disk 21 was pressed against cylinder 12, and the cycle was repeated.

The drawing shows, in outline, one suitable arrangement of pipes, valves, control switches, pressure accumulator, and related items whereby repetitive cyclic operation of the apparatus has been attained. These features form no part of the present invention, however, and it is understood that any type of control system which will provide the necessary sequence of steps and pressure conditions may be used.

It is not essential that the compacted plug 20 be reconverted to powder form by an abrader disk 21, as numerous other means may be used for comminuting the coherent cake. For example, when plug 20 has been compacted, it may be extruded from cylinder 12 substantially intact, and may be broken up in a hammer mill or other grinder.

The invention has been illustrated with respect to the compaction or densiiication of an alkali metal alkyl xanthate. It has been used with numerous such compounds, including sodium and potassium salts of ethyl, isopropyl, sec-butyl, amyl and hexyl Xanthates, and is applicable as well to diverse other powders or dusts. Thus, extremely fine dusts of sodium hydroxide can be compacted using small amounts of water .as the sovent or cohesive agent, and polymeric powders too line to be used in a screw extruder may be densified with the aid of small amounts of organic solvents which swell the polymer particles. Thus, emulsion-polymerized vinylidene chloride copolymers maybe obtained in particle sizes down to a fraction of a micron in diameter and, in such sizes, cannot be extruded readily. Such powders may be mixed with small amounts (0.5 to per cent) Of tetrahydrofuran or other swelling agent, compacted and ground to form a denser powder which canibe extruded easily.

I claim:

,1. The method which comprises mixing a finely divided flutfy powder of an alkali ,meta'l'a'lkyl xanthate having a bulk density near to pounds per cubic foot with a liquid capable of softening the powder, in amount sufficient to effect only superficial softening, partially compacting the mixture under a pressure of the order of 300 to 1000 pounds per square inch, letting the partially compacted mixture stand at constant pressure in said range for from 1 to 5 seconds, further compacting the mixture under a higher pressure of the order of 500 to 1500 pounds per square inch to form a coherent mass, and comminuting the said mass, thereby to form a powder of considerably higher density than that of the initial powder.

2. The method which comprises mixing a pyrophoric alkali metal alkyl xanthate powder with from 0.5 to 10 per cent of its Weight of a volatile organic liquid capabio of softening the xauthatc, partially compacting the mixture under pressure of the order of 300 to 1.000 pounds per square inch, letting the partially compacted mixturestandat constant pressure in said range for from 1 to 5 seconds, further compacting the mixture under a higher pressure of the order of 500 to 1500 pounds per square inch to form a coherent mass, and comminuting the said mass, thereby to form a non-pyrophoric powder whose density is near to pounds per cubic foot.

3. The method claimed in claim 2, wherein the volatile organic softening agent is methanol.

4. The method claimed in claim 2, wherein the volatile organic softening agent is ethanol.

References Cited in the file of this patent UNITED STATES PATENTS 1,098,651 'Washburn June 2, 1914 1,580,787 Lanhoffer et al Apr. 13, 1926 1,590,795 Asplundh June 29, .1926 2,617,599 Dammers Nov. 11, 1952

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