US 3451789 A
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June 24, 1969 M. J. MCINTOSH,
OXIDIZER RECOVERY PROCESS Filed April 21, 1965 mwka Aw NO 09 mm V 5.5 023000 g mm woiipzwu mw-j 5 mu me 10:24 P4- .POI
HEVGQB B wnewmm in Nn Zmmmom Jwsiomk mwhis Immmm INVENTOR. MELDON J. MCINTOSH ATTORNEY 3,451,789 OXIDIZER RECOVERY PROCESS Meldon J. McIntosh, North Ogden, Utah, assignor to Thiokol Chemical Corporation, Bristol, Pa., a corporation of Delaware Filed Apr. 21, 1965, Ser. No. 449,721 Int. Cl. C01b 11/18 US. Cl. 23-302 15 Claims ABSTRACT OF THE DISCLOSURE A process for recovering oxidizers from solid propellant, safely and inexpensively. The propellant is reduced in size to expose crystals of oxidizer dispersed throughout said propellant, contacted with leach water to leach out oxidizer therefrom, and the oxidizer recovered from the leach water.
This invention relates to a process for recovering oxidizers from waste solid propellants and more particularly to a process for recovering oxidizers from waste solid propellants wherein such oxidizers are interspersed in water-insoluble matrices.
In the manufacture of solid propellant rocket motors, uncured propellants in liquid form are poured into motor cases and thereafter cured to the solid state in situ. To compensate for the propellant volume loss which occurs during curing, an amount of uncured propellant greater than that which will be required for the grain, or charge, is generally placed in the motor case, the excess being removed after the propellant has been cured. This solid propellant which is removed from rocket motor cases, as Well as overage propellant and propellant scrapped for other reasons, has heretofore been discarded as waste because its physical characteristics, particularly its extremely high flammability and rubbery consistency, have been regarded as prohibitive of any recovery of the propellant components. Since solid propellant rocket motors are produced in large numbers, a considerable amount of waste propellant is accumulated, with the resultant loss of large quantities of expensive propellant components, such as oxidizers, that could be reused if they were recoverable. Furthermore, because of their highly combustible and sometimes even explosive nature, waste solid propellants cannot be handled so readily as most waste materials but ordinarily must be carefully burned, a disposal method attended with considerable hazard, expense, and inconvenience. Therefore, an inexpensive process for recovering oxidizers waste solid propellants, such as is hereinafter disclosed, constitutes a significant advance in the art of solid propellant manufacture.
It is, therefore, an object of this invention to provide an inexpensive process for recovering oxidizers from waste solid propellants.
Another object of this invention is to provide a process by means of which oxidizers can safely be recovered from waste solid propellants.
An additional object of this invention is the elimination of the hazards heretofore associated with the disposal of Waste solid propellants.
Further objects and advantages of this invention will become apparent to persons skilled in the art of solid propellant manufacture as they read the following description of a process which constitutes a preferred embodiment of the concepts of the invention, the description being considered in conjunction with the accompanying schematic representation of the process.
As is well known, many solid propellants comprise metallic particles such as aluminum powder, and an oxidizer, such as ammonium perchlorate, which are in- 3,451,789 Patented June 24, 1969 terspersed in a combustible binder material, or matrix, the latter generally being in the liquid form when the metal and oxidizer are mixed therewith and being subsequently cured to the solid state in a motor case. One of the most widely used oxidizer materials is the aforementioned ammonium perchlorate, and this oxidizer is recovered in the preferred process of the invention that is described hereinafter. It shouldbe understood, however, that the invention is not limited to the recovery of ammonium perchlorate from solid propellants, the principles of the invenion being applicable to the recovery of other oxidizers that are commonly employed in solid propellants, e.g., potassium perchlorate, sodium perchlorate, and many other oxidizers.
In cured solid propellants, individual crystals of oxidizer are surrounded by the matrix material, which is usually insoluble in water. Consequently, although many oxidizers utilized in solid propellants are soluble in water, the oxidizer contained in fragments of solid propellant discarded as waste cannot be dissolved out of the propellant merely by placing said fragments in water. Therefore, in accord with the principles of the present invention, as schematically represented in the accompanying drawing, Waste solid propellant is conveyed by means of a conveyor belt 10 from a water flooded waste propellant storage tank 12 to a shredder 14 in which the solid propellant particles are reduced in size so that they will pass through a 20 mesh screen. For the solid propellants most commonly employed in rocket motors (namely, those in which the oxidizers thereof have a crystal size within the range of about 40 to about 600 microns and in which the oxidizer-matrix weight ratio is within the range of about 2.0 to about 6.0), it has been found the aforementioned size reduction permits almost full recovery of the oxidizer from the waste propellant, more specifically, about percent of the oxidizer can be removed from propellant matrix, by means of the process steps described hereinafter.
In the preferred process described herein, the shredder 14 is a wet rotating knife type machine. However, other types of size-reducing may also be employed to divide the waste propellant into particles that will pass through a 20 mesh screen. For example, the shredder 14 may be replaced with a ball mill, or with various other types of pulverizing or grinding machines, the operation of which permits wet size reduction of the solid propellant particles.
As illustrated in the drawing, water is introduced into the shredder 14 in a continuous flow from a spray head 16 as the waste propellant is divided therein. This water serves several purposes: namely, to render the propellant feed insusceptible to ignition while it is in the shredder 14, to fl-ush the rubbery propellant particles from the shredder into a leach tank 18 where oxidizer is dissolved out of said particles, and to provide make-up water for the leach tank 18. Transfer of oxidizer from the propellant particles to the water introduced into the shredder 14 is initiated in that unit, but most of the oxidizer is removed from the propellant particles in the leach tank 18, in which the retention period of the feed can be regulated. With a propellant feed rate of about 30 pounds per minute (this propellant feed including oxidizer in the crystal size range of about 40 to about 600 microns and Within the oxidizer-matrix weight ratio of about 2.0 to about 6.0) and a leach tank temperature of at least 195 F., it has been found preferable to provide a propellant retention time in the leach tank 18 of about minutes, during which period the propellant particles are continuously mixed by the agitator 20. It will be obvious that the amount of water that is introduced into the shredder 14 must be sufiicient to dissolve the oxidizer that can be recovered from the waste propellant (as hereinbefore mentioned, about 85 percent of the oxidizer in propellants most commonly employed) at the temperature of the leach tank 18, taking into consideration any cooling of the water therein that results from the dissolving of the oxidizer. Since a portion of the water introduced into the shredder 14 is recycle-water that contains dissolved oxidizer, the sources of this recycle-water being described hereinafter, the amount of make-up water supplied to the leach tank 18 must also be calculated with this additional weight of oxidizer in mind. In the preferred process herein described, about 1.5 pounds of water (including recycle-water) is introduced into the shredder 14 per pound of prepellant. This water is stored in a process water tank 22 which is provided with three inputs, namely, a fresh water input 24 and two recycle-water inputs 26 and 28, the sources of which are disclosed hereinafter. The flow rate of water to the shredder 14 can be adjusted by means of a valve 30 in the feed pipe 32 interconnected to the process water tank 22 and the spray head 16. The temperature of water introduced into the shredder is maintained above 195 F. by a heat transfer means 34 located in the process water tank 22. A heat transfer means 36 also maintains the temperture of the leach tank 18 above 195 F.
The slurry formed in the leach tank 18, comprising propellant residue in the solid form and an oxidizer-water solution, overflows at an outlet pipe 38 connected to said tank and discharges upon a trommel screen, generally designated by the numeral 40, which comprises two sections 40a, 40b. The bulk of the insoluble propellant residue is separated from the oxidizer-water solution at the first section 40a of the trommel screen, the oxidizer-water solution, together with a small amount of finer solids, then passing through an outlet pipe 42 to a clarifier 44 for further separation. -At the second section 40b of the trommel screen 40 fresh water, maintained at a temperature of at least 195 F., is distributed upon the coarse propellant residue by means of a plurality of spray heads 46 connected to a supply pipe 48. This water washes entrapped oxidizer-water solution out of the coarse propellant residue and subsequently is recycled with this oxidizer-water solution through the pipe 26 to the process water tank 22, a pump 50 being connected in said pipe to effect this flow. Recycle-water, the source of which will be disclosed hereinafter, is also distributed upon the coarse propellant residue at the second section 40b of the trommel screen 40, a plurality of spray heads 52, each connected to a supply pipe 54, being provided to effect this distribution. The recycle-water from the spray heads 52 also remove entrapped oxidizer-water solution from the coarse propellant residue and then becomes part of the recycle-water passing through the pipe 26 to the process water tank 22. As previously mentioned, about 85 percent of the oxidizer in the most commonly used propellant is removed from the propellant matrix in the process herein described, this removal being effected mainly in the leach tank 18. The coarse propellant residue that leaves the trommel screen 40 at its discharge pipe 56 contains only about 3 percent oxidizer, and can be safely handled by means of conventional disposal methods.
At the clarifier 44 further separation of insoluble propellant residue from the oxidizer-water solution is accomplished, most of the residue that reaches this unit passing, together with a small volume of the oxidizer-water solution, through an outlet pipe 58 to a pump 60 and thence through a pipe 61 to a wash tank 62. The wash tank 62 is provided with a supply pipe 64, which is connected to the pipe 48 and which therefore supplies fresh Wash water at a temperature of at least 195 F., and with an agitator 66 by means of which the fresh wash water is mixed with the oxidizer-water solution introduced into said tank. Three decant taps 68a, 68b, 68c are vertically spaced on the side of the wash tank 62, each of said taps being connected to the pipe 54. Thus the clear upper portion of the xidizer-water solution in the wash tank 62 can selectively be recycled to the second section 40b of the trommel screen 40 where, as previously mentioned, it is used to wash entrapped, higher-concentration oxidizer-water solution from the coarse propellant residue. The fine propellant residue that reaches the wash tank 62 settles to the bottom thereof and is drawn off through an outlet pipe 72, provided with a flow control valve 74, to a residue disposal bin (not shown).
The oxidizer-water solution that overflows from the clarifier 44 passes through a pipe 76 to a Surge tank 78, in which the temperature of the oxidizer-water solution is again raised to at least 195 F. by means of a heat transfer means 80. The hot oxidizer-water solution is then pumped through a pipe 82 by means of a pump 84 to a pressure filter 86 Where the last traces of propellant residue are removed. Until a cake which provides effective filtering has been built up in the pressure filter 86, the oxidizer-water solution from said filter is recycled to the surge tank 78 through the pipe 88. The addition of asbestos fibers or other filter aids can be used to develop the desired precoat cake. After a suitable cake has been formed in the pressure filter, the control valve 90 is closed and the clear oxidizer-water solution fiows through a pipe 92 to a crystallizer 94 which is provided with an agitator 96 and a heat transfer means 98. In the crystallizer the temperature of the oxidizer-water solution is carefully lowered to about 80 F. at which temperature crystals of the ammonium perchlorate oxidizer are formed. When crystal formation in the crystallizer 94 has reached a maximum, the slurry in the crystallizer, which comprises oxidizer crystals and a water solution including a small amount of oxidizer together with a small amount of other water soluble material present in the waste propellant feed, is fed through a pipe 100 to a centrifuge 102 where the oxidizer crystals are separated from the water solution. The oxidizer crystals removed at the centrifuge 102 are subsequently dried in a dryer 104 and stored in a storage bin 106, and the low concentration oxidizer-water solution removed from the oxidizer crystals at the centrifuge is recycled through the pipe 28 to the process water storage tank 22. Since the solution drawn off at the decant taps 68a, 68b, 68c of the wash tank 62 and the fresh wash water entering the second section 40b of the trommel screen 38 from the pipe 48 are also returned to process water storage tank 22 through the pipe 26, there is very little loss, from the units of the process, of the oxidizer that is dissolved in water at the leach tank 18.
The process described herein is operated in continuous flow through the pressure filter 86, with the crystallizer being operated on a batch basis. It Will be readily appreciated, however, that all of the process units can be operated as batch processes. Furthermore, although in the described process all steps thereof are carried out at atmospheric pressure, it will be recognized that certain of the process units, e.g., the crystallizer 94 and dryer 104, may also be operated under pressures other than atmospheric to accelerate the operation conducted therein. The amount of waste propellant fed into the shredder 14, as well as the amount of water introduced therewith, can of course be varied from the amounts stated for the described process, which has been described in detail for the purpose of example only.
It has been found that the process described herein produced ammonium perchlorate crystals (at the crystallizer 94) having a purity of about 99.6 percent. The process thus produces oxidizer that can be reused in the manufacture of solid propellant for rocket motors. Furthermore, as has been mentioned hereinbefore, after the oxidizer has been dissolved from the waste propellant entering the process, the propellant residue can be handled without the hazard, inconvenience and expense heretofore associated with waste solid propellant disposal. An important advantage of the invention is that the size-reduction of the waste propellant to the desired particle size is accomplished safely through the wetting of the surfaces of said propellant with water in the shredder 14.
Therefore, although the invention has been described in detail with reference to a process that constitutes a preferred embodiment thereof, it is to be clearly under stood that the same is by way of example only and is not to be taken by way of limitation, the scope of the invention being limited only by the terms of the appended claims.
What is claimed is:
1. A process for recovering water soluble oxidizers from a solid propellant, said solid propellant comprising a rubbery water-insoluble matrix in which said oxidizer is dispersed as discrete crystalline particles, comprising the steps of:
reducing said propellant to particulate form to expose a substantial number of oxidizer crystals on the surfaces thereof;
contacting said reduced size propellant particles with water to leach out said oxidizer crystals from said particles until an aqueous solution of oxidizer and a slurry of water insoluble matrix particles is formed; separating said aqueous solution from said slurry; and cooling said aqueous solution until crystallization of said oxidizers occurs.
2. The process of claim 1 wherein the step of reducing said propellant to particulate form is carried out while said propellant is wetted with water.
3. The process of claim 1 wherein said leaching step is carried out with water at a temperature of at least 195 F.
4. The process of claim 1 wherein the leaching step is carried out for at least 150 minutes.
5. The process of claim 1 wherein the reducing step is carried out until the particles formed will pass through a 20 mesh screen.
6. The process of claim 1 wherein the reducing step is carried out by grinding.
7. The process of claim 1 wherein the reducing step is carried out in a rotating knife machine.
8. The process of claim 1 wherein the reducing step is carried out in a ball mill.
9. The process of claim 1 wherein said water soluble oxidizers in said propellant are perchlorates.
10. The process of claim 9 wherein the perchlorates are selected from the group consisting of ammonium perchlo rate, potassium perchlorate and sodium perchlorate.
11. A process of recovering water soluble oxidizers from a solid propellant, said solid propellant comprising a rubbery binder of water insoluble material in which said oxidizer is dispersed as discrete crystalline particles, comprising the steps of dividing said solid propellant into particles in a shredder while wetting said propellant with water;
contacting said divided propellant particles with water to leach out said oxidizer crystals therefrom until an aqueous solution of oxidizer is formed;
separating said propellant particles remaining after said aqueous solution is formed from said aqueous solution; washing said separated propellant particles with water to remove any residual aqueous solution of said oxidizer entrapped therein; recycling said wash Water to said shredder;
filtering said aqueous solution after separation thereof from said propellant particles to remove residual propellant material therefrom;
cooling said filtered aqueous solution until crystallization of said oxidizer occurs; and
drying said crystallized solution to recover said oxidizer.
12. The process of claim 11 further including a centrifuging step after said cooling of said aqueous solution step for separating said oxidizer crystals from said aqueous solution.
13. The process of claim 12 wherein the liquid remaining after separation of said crystals is recycled to said shredder.
14. The process of claim 11 wherein said water soluble oxidizers in said propellant are perchlorates.
15. The process of claim 14 wherein said perchlorates are selected from the group consisting of ammonium perchlorate, sodium perchlorate and potassium perchlorate.
References Cited UNITED STATES PATENTS 1,894,514 1/ 1933 Hechenbleikner 233 12 2,892,697 6/1959 Davies et al 241--17 2,929,697 3/1960 Perry et a1. 149-19 3,191,535 6/1965 Mulloy 149-2 3,215,492 11/1965 Hartley 241-16 3,215,509 11/1965 Adams 23312 3,252,842 5/1966 Williams 241-16 WILBUR L. BASCOMB, JR., Primary Examiner.
U.S. Cl. X.R.