US3819336A - Method of making ultra-fine ammonium perchlorate particles - Google Patents

Abstract

A method of making ultra-fine ammonium perchlorate particles by spraying an aqueous ammonium perchlorate solution containing a surface active agent onto a moving film of a refrigerated organic liquid to form fine frozen droplets in said film and recovering fine ammonium perchlorate particles from the frozen droplets by freeze drying. The product particles have weight mean diameters of less than 1 micron and are especially useful in the manufacture of solid rocket propellants.

Description

[ June 25, 1974 METHOD OF MAKING ULTRA-FINE AMMONIUM PERCHLORATE PARTICLES [75] Inventors: Rudy E. Rogers; James L. Murphy,

Jr., both of Huntsville, Ala.

[73] Assignee: Thiokol Chemical Corporation,

Bristol, Pa.

[22] Filed: Dec. 20, 1972 [21] Appl. No.: 316,677

[52] US. Cl 23/302, 149/76, 264/3 C, 264/14, 62/58, 423/476 [51] Int. Cl COld 1/30 [58] Field of Search 149/76; 423/476; 264/3 C, 264/3 E, 14, 298; 23/302, 294; 62/58 3,419,899 12/1968 Tufts et a1 23/302 3,498,759 3/1970 Kralik 23/302 3,551,533 12/1970 Monforte 264/14 Primary Examiner-Stephen J. Lechert, Jr.

[5 7] ABSTRACT A method of making ultra-fine ammonium perchlorate particles by spraying an aqueous ammonium perchlorate solution containing a surface active agent onto a moving film of a refrigerated organic liquid to form fine frozen droplets in said film and recovering fine ammonium perchlorate particles from the frozen droplets by freeze drying. The product particles have weight mean diameters of less than 1 micron and are especially useful in the manufacture of solid rocket propellants.

6 Claims, 1 Drawing Figure METHOD OF MAKING ULTRA-FINE AMMONIUM PERCHLORATE PARTICLES This invention relates to the manufacture of ammonium perchlorate of a particle size adapted to be used in rocket propellants and more particularly to a novel method of making ammonium perchlorate in the form of ultra-fine particles.

Most solid rocket propellants are principally composed'of a polymeric fuel-binder having a finely divided oxidizer, usually ammonium perchlorate, dispersed therein. The oxidizer, a curing agent for the polymeric binder, and certain special purpose ingredients known in the art are dispersed in the binder while the latter is in viscous fluid form, and the resulting mixture is cast in situ in a rocket motor casing at a temperature sufficient to convert the binder to elastomeric form.

It is known that the efficiency and rate of combustion of a solid propellant can be maximized by increasing the area of contact between the oxidizer and other irihazard and numerous precautions must be taken to minimize the risk of explosion. Moreover, the risk of explosion can never be completely eliminated. Also there is a tendency for the ammonium perchlorate to become contaminated by material removed from the grinder and grinding media. In addition, grinding is a relatively expensive procedure and it has not been found possible to reduce the particle size of the oxidizer by grinding to the extent that is desirable for highly efficient propellant combustion.

Because of the foregoing disadvantages various proposals have previously been made for producing finely particulate oxidizers other than by grinding. Thus US. Pat. No. 3,452,445 discloses a process for making fine particles of ammonium perchlorate by quick freezing of aqueous ammonium perchlorate. Freezing of the aqueous solution is effected by rapidly rotating a fiask containing the solution in an acetone-dry ice bath. The ice thus formed is then sublimed under a reduced presof less than one micron. It is another object of the invention to provide a process for making ammonium perchlorate which eliminates the explosion hazard involved in the grinding operation referred to above. it is another object of the invention to provide a process for making ultra-fine ammonium perchlorate of high purity at a low unit cost. It is still another object of the invention to provide such a process than can be readily automated. It is a still further object of the invention to provide a process wherein for any given production rate the quantity of material being processed is relatively small. Other objects of the invention will be in part obvious and in part pointed out hereafter.

sure to recover ammonium perchlorate particles having an average size of 1.7 to 2.1 microns.

US. Pat. No. 3,222,231 to Markels Jr. et. al. discloses a process wherein a saturated aqueous solution of ammonium perchlorate at a temperature of 95 C. is simultaneously agitated and subjected to ultra-sonic vibrations over a period of several hours. As the solution cools,.ammonium perchlorate crystals precipitate and are'subsequently removed from the solution, washed with acetone and ether, and dried. The product crystals had a particle size within the range 5 to 350 microns.

While such processes avoid the explosion hazards involved in mechanical grinding, the size of the ammonium perchlorate particles they produce is of the same As conducive to a clearer understanding of the present invention, it may be pointed out that in wet processes for producing ammonium perchlorate, as in most crystallization processes, the crystallization takes place in two stages, namely, crystal formation and crystal growth. In cases such as the present one where very small particle sizes are desired, it is necessary to provide either some means of inhibiting crystal growth of the particles as they are first formed or, if the particles are permitted to grow, some means of breaking them up.

In accordance with a preferred embodiment of the present process, ultra-fine particles of ammonium perchlorate are produced by preparing an aqueous solution of ammonium perchlorate containing a small amount of a crystal growth inhibitor and spraying the solution on a thin, moving film of a water-immiscible, volatile organic liquid at a temperature below the freezing point of the aqueous solution to produce a slurry of ice crystals and ultra-fine ammonium perchlorate particles. By spray-freezing the ammonium perchlorate in a moving film of a refrigerated immiscible liquid, extremely rapid freezing is achieved to yield very small crystals of ammonium perchlorate. Growth of these small crystals is inhibited by the presence of the crystal growth inhibitor which, as pointed out more fully below, is a further active agent. It has been found that the surface active agent, even when present in a relatively small amount, appears to be effective in inhibiting crystal growth in the slurry, possibly by coating the ammonium perchlorate particles, and thus stabilizes the particle size of the solid phase of the slurry produced in the spray-freezing step of the present method.

The recover the ultra-fine ammonium perchlorate particles from the slurry as thus formed, the organic liquid is vaporized from the slurry at a low enough temperature to avoid melting of the ice crystals to leave a mixture of the ice crystals and ammonium perchlorate particles. Separation of the ice crystals from the ammonium perchlorate particles is effected by freeze-drying. The mixture is maintained at a temperature of say 40 to+l0F., preferably about -20 lF., and at a reduced pressure, say 1 to micron of mercury absolute, to cause the ice crystals to sublime. The residue comprising a porous friable mass of ammonium perchlorate particle aggregates can be broken up into ultra-fine particles in any of several ways. Thus if the product ammonium perchlorate is to be used in a propellant of the type described above, a curable liquid organic fuelbinder is cured to an elastomer, the residue from the freeze-drying step can be directly incorporated in the propellant mixture prior to curing, and the usual mixing of the viscous propellant composition prior to curing will serve to disperse the ultra-fine ammonium perchloduce a dispersion of the ultra-fine ammonium perchlorate particles. In order to achieve a stable submicron particle size, a suitable coating agent, e.g., a carboxylterminated or hydroxyl-terminated liquid hydrocarbon polymer, is desirably incorporated in the organic liquid dispersion of ammonium perchlorate.

Once the ultra-fine ammonium perchlorate particles have been formed in the spray-freezing step of the present process, they should desirably be kept out of contact with atmospheric moisture, since such moisture tends to induce crystal growth, particluarly after the particles have been freeze dried.

It has been found that by carrying out the present method using a spray-freezing solution containing a surface active agent of a type described more fully below in an amount of say 0.5 to percent by weight of the ammonium perchlorate used, a product having an average particle size of 0.5 to 1.0 micron can be readily obtained, whereas a substantially larger particle size is obtained when the surface active agent is omitted from the spray-freezing solution.

The concentration of ammonium perchlorate in the aqueous solution used in the spray-freezing step of the process may vary over a relatively wide range, say 1 percent to 33 percent by weight. The preferred concentration is 5 percent to 20 percent by weight. Since the particle size produced is to some extent a function of the size of the droplets sprayed on the refrigerated liquid film, a spray capable of producing droplets of a diameter less than microns is preferably used.

The immiscible organic liquid into which the solution issprayed should desirably have a relatively low freezing point, i.e., 70F. or lower, and should be sufficiently volatile to permit it to be rapidly evaporated from the slurry formed in the spray-freezing step. The preferred organic liquids are dichlorodifluoromethane (Freon 12) and trichloroethylene. Liquid nitrogen may also be used.

As indicated above, other factors being maintained constant, the use of a small amount of a surface active agent in the spray solution results in a substantial reduction in average particle size. Suitable exemplary surface active agents for use in the present process are given in Table I.

TABLE I Trade Designation Strodex PK Strodex SV 8 Surface Active Agent Potassium salt of polyphosphoric ester acid Concentrated ionized complex multi-carbon alcohol-potassium neutralized Alkyl sulfosuccinate of the formula H I cal-000m NEOaB-(B--CHr-C O NH-COONa OONa (431a) elkyl Monawet SN0 TABLE I Continued Surface Active Agent Trade Designation Na-Z-ethylhexyl sulfate Emcol DS-lO Alcohol sulfate, amine salt Duponol EP High molecular weight carbohydrate polymer composed primarily of galactomannan with borax added to produce cross-linking J B. Polymer The spray-freezing step of the present process can be carried out in any suitable apparatus capable of producing a thin film of cooled liquid in which the sprayed solution is insoluble. One suitable type of apparatus is shown in the accompanying drawing. Referring to the drawing, the freezing apparatus there shown comprises a vertically arranged tank 10 which may be of cylindrical cross section and in which a mixing weir 12 is centrally located. Mixing weir 12 may also be of generaly cylindrical configuration and coaxial with tank 10. The weir flares outward elliptically at its upper end 13 until the'top edge of its wall is essentially horizontal. This upper end 13 rests on and is attached to an upper annular flange 14 that extends inwardly from the top of an inner cylindrical wall 16. Wall 16, of a somewhat smaller diameter than tank 10, is supported at its lower end by a lower annular flange 18 that extends inwardly from the interior surface of tank 10. The top of tank 10 is enclosed by a cover 22 that is circumferentially supported by an external upper tank flange 24. A sight port 26 is provided in'tank lid 22 to permit operators of the apparatus to view the spray-freezing process.

An injector assembly 28 is positioned through the center of tank lid 22 so that a spray nozzle 30 forming part thereof extends below the top edge of weir 12. The injector assembly includes a hot water jacketed section 32 through which hot water flows to prevent precrystallization of the solution flowing to and through nozzle 30.

The ammonium perchlorate solution to be crystallized is contained in a solution storage tank 42 that is pressurized by gas'stored in a pressurization tank 44. The conduit 45, between tanks 44 and 42, contains a pressurization valve 46 that is used to control the pressurization of tank .42. A solution flow conduit 48 connects tank 42 to a pipe 31 of the injector assembly 28.

Adjustment of solution flow rate is controlled by a solu-' tion flow valve 50 that is located in conduit 48.

Located between the inner cylindrical wall 16 and the wall of tank 10 there is a heat exchanger 52 comprising a refrigeration coil 54 that is supplied with a refrigerant through a refrigerant input conduit 56. The refrigerant is cooled by a refrigeration unit 58 that supplies conduit 56. The bottom turn of coil 54 is connected to the bottom turn of a lower refrigeration coil 60 by a refrigeration coil connecting conduit 62. Coil 60 is positioned in the lower portion 64 of tank 10 and is covered by the coolant liquid that is maintained at a level 66 above the coil as shown. The uppermost turn of coil 60 is connected to a refrigerant return conduit 68 that directs the flow of the refrigerant back to refrigeration unit 58.

A coolant exitport 70 is located at the bottom of tank portion 64 and is in communication with a coolant return conduit 72. A cryogenic ball valve 74 is located in conduit 72 to provide appropriate control of the coolant flow. A second cryogenic ball valve 76 is connected to the coolant drain 78 that branches from a T- connection 79 in conduit 72. Conduit 7 is connected to the input of a coolant circulating pump 80 that directs the coolant through a coolant feed conduit 82 to the bottom of heat exchanger 52. The coolant then flows upwardly around the coil 54 which serves to further refrigerate it, and flows over the upper end 13 of weir 12, forming a thin film on the inside surface of the weir.

As the coolant film flows downwardly on the inner surface of weir 12, it is struck by the spray emitted from nozzle 30. This contact with the coolant immediately freezes the spray droplets, thereby forming a slurry of the frozen droplets in the coolant. This slurry then continues to flow down the inner surface of weir 12 and drops into a basket 86 positioned in a basket support 88. Basket 86 is formed from a tightly woven screen, for example, 325 mesh, which holds the frozen droplets but permits passage of the coolant to the lower portion 64 of tank 10. The basket support 88 extends through the wall of tank and an insulation and vapor barrier 90, which encloses the entire tank.

in operation, a constant, predetermined flow of the coolant liquid over the upper circumferential edge of weir 12 is established. The sight port 26 can be used to view this flow to ensure that the coolant only forms a thin film on the inner surface of the weir. As previously described, the droplets of solution discharged by spray head 30 impinge upon the flowing refrigerated liquid and are immediately frozen to form a fine particle size slurry that flows from the bottom of weir 12 into the basket 86. Most of the coolant liquid drains through the basket into the lower portion 64 of tank 10 and is cooled and recirculated as described above.

When a desired amount of thickened slurry has collected in basket 86, the accumulated slurry is removed from the spray-freezer and residual coolant liquid is separated from the frozen particles by evaporation. This step of the process can be carried out in any suitable apparatus. It is only necessary that the coolant be evaporated in a substantially moisture-free environment and at a temperature low enough to prevent melting of the frozen particles of ammonium perchlorate solution.

When the frozen solution particles are essentially free from residual coolant, they are transferred to a freeze drier, which may be a known, commercially available type of equipment, for example, a model 41 RePP sublimator manufactured by The Virtis Co., Inc. Such freeze driers commonly comprise an evacuated chamber having shelves adapted to support trays containing the material to be freeze dried and a condenser for condensing the water vapor formed by sublimation of the ice crystals. Desirably the shelves are heated to increase the sublimation rate.

Upon completion of the freeze drying step, the solid residue comprises essentially a friable aggregate of very fine ammonium perchlorate particles which, if they are to be stored, are packaged in hermetically sealed containers. As indicated above, if the ammonium perchlorate product is to be used in solid propellant manufacture of the type in which an inorganic oxidizer is mixed with a liquid curable organic polymeric fuelbinder, the product may be added directly to a conventional propellant mix, and the normal mixing operation will disperse the fine particles of the product. Alternatively, the product may be dispersed in a liquid non-solvent with vigorous mixing and/or ultra-sonic treatment to produce a dispersion of the fine ammonium perchlorate particles.

In order to point out more fully the nature of the present invention, the following specific examples are given of illustrative embodiments of the process of the invention. Example 1 A solution of 392 grams of ammonium perchlorate in 3,600 ml. of water was prepared and 3.93 grams of a potassium salt of polyphosphoric ester'acid (Strodex PK- was added thereto as a surface active agent. Thus the solution contained about 10 percent by weight of ammonium perchlorate and the surface active agent constituted about i percent by weight of the ammonium perchlorate.

The prepared solution was pressurized in a closed stainless steel vessel to 900 p.s.i.g. and sprayed from the vessel through a 1212 (Spray Engineering Co.) hydraulic nozzle having an orifice of 0.012 inch diameter. The nozzle was wrapped with copper heating coils to prevent freezing of the solution before spraying. The temperature of the solution at the outlet of the spray was 55F.

The sprayed solution was frozen in apparatus of the type described above and shown in the accompanying drawing using a refrigerated film of Freon l2 chilled to 62F. by circulation over refrigerated coils. The stream of Freon 12 with entrained droplets of frozen solution was fed to a 325-mesh screen which separated the frozen particles.

The separated slush of frozen particles was stored in a low temperature chamber at about 30F. until the Freon 12 had evaporated, after which the frozen material was transferred to pre-cooled trays-of a freeze dryer. A vacuum of 50 microns was established in the freeze drying chamber and a shelf temperature of F. was established on the shelves supporting the trays containing the frozen material to vacuum sublime the ice. After 24 hours, the material was sufficiently dry and the dry ammonium perchlorate powder was packaged and sealed. The resulting particles had a weight means diameter of 0.47 microns. The total residual moisture content of the ammonium perchlorate was 0.061 percent by weight.

Example 2 A solution of 529.4 grams of ammonium perchlorate in 3.000 ml. of water was prepared and 5.3 grams of Strodex PK-90 was dissolved therein as a surface active agent. The solution was spray frozen and freeze dried as in Example I.

The weight mean diameter of the freeze-dried particles was 0.535 microns and their water content was 0.087 percent.

Example 3 A solution to be freeze-dried was prepared to contain 10 percent by weight of ammonium perchlorate in 90 percent water. To the solution there was added as a surfactant 1 percent by weight of the ammonium perchlorate of alkylamine sulfosuccinate (Emcol 4,200).

The prepared solution was pressurized in a closed stainless steel vessel to 800-1000 p.s.i.g. and sprayed into a flowing film of refrigerated liquid as in Example 1. However, trichloro-ethylene at a temperature of 55 to 75F. was used in place of the Freon 112 of Example l. The flow rate of the trichloroethylene over the weir plate was about 6 gal/min. The stream of trichloro-ethylerle was entrained frozen droplets of the solution was fed to a 325-mesh screen to separate the frozen particles.

The frozen material was transferred to trays on the shelves of a freeze dryer, and vacuum established in the drying chamber. The trichloroethylene from the frozen slush collected in the bottom of the drying chamber where it was drained from the system periodically.

After the trichloroethylene was removed, heat was supwas used as a surfactant in place of the Emcol 4,200.

The weight means diameter of the ammonium perchlorate particles was 0.88 microns and their water content was 0.13 percent. Example The procedure of Example 3 was followed except that a sulfosuccinate surface active agent (Monawet SNO) was used in place of the Emcol 4,200.

The weight mean diameter of the ammonium perchlorate particles was 0.84 microns and their water content was 0.15 percent.

From the foregoing description it should be apparent that the present invention provides a process capable of achieving the objectives set forth at the beginning of the. specification. By using an aqueous solution of ammoniurn perchlorate containing the disclosed surface active agents, product particles having weight mean diameters of less than one micron can be consistently obtained. The surface active agents inhibit crystal growth not only in the spray freezing step of the process, but also in the subsequent process steps such as the coolant evaporation and freeze drying steps, as well as in the subsequent storage of the material.

It is, of course, to be understood that the foregoing examples are intended to be illustrative and that numerous changes can be made in the ingredients, proportions and conditions disclosed without departing from the spirit of the invention as defined in the appended claims.

We claim:

1. The method of making ultra-fine ammonium perchlorate particles which comprises preparing an aqueous solution of ammonium perchlorate containing a surface active agent, spraying said solution on a thin, moving film of a water-immiscible, volatile, organic liquid at a temperature below the freezing point of said solution to form a slurry'of ice crystals and ultra-fine ammonium perchlorate particles, separating said organic liquid from said slurry, heating the resulting mixture at a reduced pressure to cause said ice crystals to sublime and recovering said ultra-fine ammonium perchlorate particles.

2. A method according to claim 1 wherein said aqueous solution contains from 1 percent to 33 percent by weight of ammonium perchlorate.

3. A method according to claim 1 wherein said aqueous solution contains from 5 percent to 20 percent by weight of ammonium perchlorate.

4. A method according to claim 1 wherein said aqueous solution contains from 0.5 to 5 percent of said surface active agent based on the weight of ammonium perchlorate therein.

5. A method according to claim 1 wherein said organic liquid is selected from dichlorodifluoromethane and trichloro-ethylene.

6. A method according to claim 1 wherein said organic liquid is separated from said slurry by evaporation.

Claims (5)

1. 2. A method according to claim 1 wherein said aqueous solution contains from 1 percent to 33 percent by weight of ammonium perchlorate.
2. 3. A method according to claim 1 wherein said aqueous solution contains from 5 percent to 20 percent by weight of ammonium perchlorate.
3. 4. A method according to claim 1 wherein said aqueous solution contains from 0.5 to 5 percent of said surface active agent based on the weight of ammonium perchlorate therein.
4. 5. A method according to claim 1 wherein said organic liquid is selected from dichlorodifluoromethane and trichloro-ethylene.
5. 6. A method according to claim 1 wherein said organic liquid is separated from said slurry by evaporation.

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