US3622655A

US3622655A - Aqueous slurry process for drying of solvent wet double base smokeless powder

Info

Publication number: US3622655A
Application number: US816022A
Authority: US
Inventors: John O Bonyata; Lynn G Rohrbaugh
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1969-04-14
Filing date: 1969-04-14
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23

Abstract

A process for rapid drying of solvent-wet double-base smokeless powder is provided in which most of the solvent entrapped in the double-base powder particles manufactured by the conventional solvent process is extracted from the powder into the aqueous phase of an aqueous slurry of the powder. The solvent-extracted smokeless powder is then dried employing rapid-drying techniques with no substantial loss of energetic plasticizer.

Description

United States Patent Inventors John O. Bonyata AQUEOUS SLURRY PROCESS FOR DRYING OF SOLVENT WET DOUBLE BASE SMOKELESS POWDER 9 Claims, No Drawings Int. Cl C06b 21/02 Field of Search 264/3;

[56] References Cited UNITED STATES PATENTS 438,816 10/1890 Lundholm et a1 149/97 2,860,042 11/1958 McDonnell et al. 149/97 3,037,891 6/1962 Maag 149/97 Primary Examiner-Benjamin R. Padgett Assistant Examiner-Stephen J. Lechert, Jr. Attorney-S. Grant Stewart ABSTRACT: A process for rapid drying of solvent-wet double-base smokeless powder is provided in which most of the solvent entrapped in the double-base powder particles manufactured by the conventional solvent process is extracted from the powder into the aqueous phase of an aqueous slurry of the powder. The solvent-extracted smokeless powder is then dried employing rapid-drying techniques with no substantial loss of energetic plasticizer.

AQUEOUS SLURRY PROCESS FOR DRYING OF SOLVENT WET DOUBLE BASE SMOKELESS POWDER This invention relates to an aqueous slurry process for highspeed drying of solvent-wet double-base smokeless powder.

The time honored conventional process for manufacture of smokeless powder involves as a first step the granulation of nitrocellulose. In this process water wet nitrocellulose is dehydrated with ethyl alcohol in a blocking press. The resulting dehydrated nitrocellulose is broken up in a block breaker and subsequently masticated in a large mixer by the addition of ethyl alcohol and acetone to form a homogeneous stiff dough. Nitroglycerin or other energetic plasticizer is admixed with the dough during the mixing process to form a double base dough. The stiff dough is blocked in large hydraulic presses and subsequently is extruded at high pressures into strands of colloided double-base powder. The strands of powder are then cut to a desired length and dried in a dry house.

In the conventional drying process the cut smokeless powder granules containing from about 8 percent to about percent solvent, based on the weight of the wet smokeless powder are taken to a dry house, placed in trays and dried in a fixed bed. Fixed bed drying of the smokeless powder granules takes place by a combination of conduction and convection and permits the processing solvents employed generally alcohol and acetone, to diffuse out of the powder granules at a slow rate carrying along only very small quantities of nitroglycerin. As the solvents reach the surface of the smokeless powder g ranules they pass from the liquid into the vapor phase. In this drying process the transfer of heat required for rapid evaporation of the solvents from the smokeless powder is restricted because of concentration gradients of solvent vapors surrounding the smokeless powder particles and because a surface film is formed on the smokeless powder particles by the presence of the solvent. From the top of the powder bed to the center of the powder bed which is formed on the drying tray there is a gradation in solvent and water vapor. For this reason smokeless powder granules at the top of the bed dry first and those granules in the center of the bed dry last.

If an attempt is made to increase the drying rate of the powder by thinning out the bed of smokeless powder being dried or by forcing air through the bed as in a fluidized bed drying system, the concentration gradient of solvent and moisture vapor through the bed is removed. When this concentration gradient is removed the solvent does not fractionate out of the powder bed and nitroglycerin which is soluble in the solvents migrates with the solvents to the surface of the smokeless powder granules and is stripped therefrom. As the concentration of nitroglycerin in smokeless powder increases, the amount of nitroglycerin that dissolves in the solvents and is stripped from the powder employing rapid-drying techniques will also increase.

The conventional fixed bed-drying process for double base smokeless powder described is a low-drying rate process because of the necessity to maintain solvent gradients through the bed of powder in order to eliminate stripping of nitroglycerin from the powder. Typical dry house conditions for drying doublebase smokeless powder granules in a fixed bed are temperatures of from about 125 F. to about 160 F., drying times from about 3 days to about 5 days, and powder bed thickness of from about 1.5 inches to about 3.0 inches.

In accordance with the method of this invention solvent-wet double-base smokeless powder is dried by a process comprising forming an aqueous slurry of solvent-wet double-base powder particles and water, the water having dissolved therein an equilibrium concentration of energetic plasticizer. The energetic plasticizer dissolved in the water is the same plasticizer employed in manufacture of the smokeless powder. The slurry which is formed has a continuous phase comprising water, and energetic plasticizer, and a dispersed phase comprising smokeless powder particles. The slurry is agitated continuously until it is comprised of a continuous phase of water,

energetic plasticizer, and solvent(s), and a dispersed phase comprising solvent-extracted smokeless powder particles. The resulting solvent-extracted smokeless powder particles are then separated from the aqueous slurry. The separated smokeless powder particles are then rapidly dried to remove residual solvent and water.

In the process described. processing solvents are rapidly extracted from the solvent-wet smokeless powder granules into the water phase of the slurry until the solvent extracted powder contains less than about 1.5 percent by weight of solvents based on the weight of the extracted powder. The water phase of the slurry also has dissolved therein an equilibrium concentration of energetic plasticizer. The term equilibrium concentration of energetic plasticizer as used herein is defined as a concentration of energetic plasticizer in the aqueous slurry at which there is substantially no tendency for the energetic plasticizer to be either absorbed into or extracted from the smokeless powder granules. The solvent extracted smokeless powder particles are separated from the slurry and rapidly dried. Due to the greatly reduced concentration of solvent in the smokeless power particles, substantially no nitroglycerin is stripped from the particles during rapid drying.

The following examples more fully illustrate this invention. All parts and percentages are by weight unless otherwise specified.

The following example illustrates the loss of nitroglycerin from smokeless powder when employing rapid drying techniques. Comparison is made with the conventional dry house" dry procedure.

EXAMPLE 1 A conventionally granulated solvent wet smokeless powder is extruded through a 0.065 inch I.D. (inside diameter) X 0.020 inch O.D. (outside diameter) die and cut at 300 cuts per lineal inch. The powder is divided into two equal portions. One-half of the smokeless powder is dried in a conventional manner in a dry house for 3 days at about l35 F. The other half of the smokeless powder is dried very rapidly in a fluidized bed by forcing air at F. through the bed of powder. The initial nitroglycerin content of the powder is 20 percent. The results of drying these powders are listed in table 1 below.

As can be seen from table I, the conventionally dried smokeless powder requires about 46 hours of drying to bring the moisture and volatile content of the powder to an acceptable level (0.50 percent or less). There is no measurable nitroglycerin loss. The powder dried by the rapid drying process requires about 5 hours of drying to bring the moisture and volatile content of the powder to the acceptable level. Over 5 percent loss of nitroglycerin from the powder results. based on the original nitroglycerin content of the powder.

Examples 2-5 which follow illustrate the rapid drying process of this invention for double base smokeless powder.

EXAMPLES 2-5 A batch of conventionally manufactured smokeless powder is extruded through 0.038 inch die and cut at 250 cuts per lineal inch. The resulting green powder containing about 12 percent by weight solvent based on the weight of the solventwet powder is divided into four lots. The first lot, example 2, is dried in a conventional manner for a period of 72 hours at a temperature of 130 F. using standard dry house facilities. The second lot, example 3, is dried over a period of 72 hours at a temperature of 130 F. in an experimental dry house wherein each drying tray is placed on a separate rack.

The remaining two lots of conventionally granulated powder are placed in a vessel and jointly slurried with water to form a slurry comprising a dispersed phase of smokeless powder particles and a continuous phase of water. The resultant slurry contains 12 percent by weight of smokeless powder and 88 percent by weight of water based on the weight of the slurry. The water has dissolved therein 0. l percent by weight of nitroglycerin which is an equilibrium concentration of nitroglycerin. The slurry is agitated to maintain the dispersion of the smokeless powder in the slurry and temperature of the slurry is increased from ambient temperature 68 F. to 175 F. This temperature is maintained for about 5 minutes. Colloidal graphite (type GPW, an aqueous paste containing 22 percent solids, sold by Graphite Products Corporation) is.

added to the agitated slurry and the slurry maintained at the 175 F. temperature for an additional ten minutes. The slurry is then cooled and centrifuged.

The resulting glazed smokeless powder contains approximately 15 percent by weight of moisture and processing solvents. The processing solvents are about 1 percent by weight based on the weight of the wet powder. This powder is divided into two lots, 3 and 4. Lot 3 (example 4) is dried in a dry house under the same conditions as example 3. From lot 4(example 5) two one-half pound samples are taken and dried in a small fluid bed type dryer operating at 150' 5 F. The drying time, percent moisture, and percent nitroglycerin, in the dried smokeless powder particles prepared in the foregoing examples is set forth in table 2 below.

TABLE 2.MOISTURE LEVEL AND NITROGLYCERIN LOSS ON FOUR DRYING TECHNIQUES minutes 0. 50 38:84

The foregoing examples illustrate the reduction in time in drying solvent-wet smokeless powder in accordance with the process of this invention (example 5). Substantially no nitroglycerin loss results from the process of this invention. Consequently, the process is considered as safe as the conventional dry house process for drying smokeless powder.

in the drying process heretofore described it is generally preferred to prepare an aqueous slurry comprised of from about'70 percent to about 92 percent by weight of water, and

from about 30 percent to about 8 percent by weight of smokeless powder particles. lf more than 92 percent water is employed, equipment size and processing time increase to make processing unwieldy. If less than about 70 percent water is employed the extraction efficiency is reduced.

The water employed in preparation of the aqueous slurry has dissolved therein an equilibrium concentration of the same energetic plasticizer employed in manufacture of the solvent wet smokeless powder. Energetic plasticizer is readily dissolved in water to form the equilibrium concentration. Recycle water from previous cycles can be advantageously employed. The presence of the plasticizer in the water is necessary to maintain a plasticizer-water-nitrocelIulose equilibrium. When operating within the water-smokeless powder slurry concentration range heretofore described, this equilibrium concentration of energetic plasticizer is from about 0.1 to about 0.3 percent by weight based on the weight of the water. If the energetic plasticizer content in the water is in excess of about 0.3 percent, the equilibrium is shifted in favor of driving the energetic plasticizer into the smokeless powder particle. if the energetic plasticizer concentration falls below about 0.1. percent then migration of energetic plasticizer into the water is favored. Neither condition is desirable since it is important to closely control the composition of the smokeless powder which in turn controls burning rate characteristics of the powder.

After forming the aqueous slurry of this invention, extrac tion of processing solvents from the solvent-wet smokeless powder into the continuous water phase of the slurry begins. The rate of extraction of solvents from the powder is increased with increasing slurry temperature. Preferred temperature ranges for extraction are from about to about 190 F. When operated in the preferred extraction temperature ranges, the time required for solvent extraction will vary from about 5 minutes to about 15 minutes. The resultant smokeless powder particles are separated from the slurry by a suitable means such as filtration or decantation and contain from about 0.5 to about l.5 percent by weight of residual solvent based on the weight of the extracted powder. This low amount of solvent can be safely removed in a high-speed forced air fluid bed dryer without causing any substantial loss of nitroglycerin from the powder during drying as illustrated in example 5.

The extraction rate of the solvents from the smokeless powder into the continuous aqueous phase of the slurry can be increased by continuously separating extracted solvents from the slurry. A simple and effective manner of continuously separating these solvents from the slurry is by passing an inert gas or air through the agitated and heated slurry. The gas passing through the slurry will aid vaporization of the solvents from the slurry. The resulting solvent rich gaseous mixture can then be passed through a condenser to recover the solvents. Other methods of separating solvent from the slurry such as continuously withdrawing a portion of the slurry, stripping solvent therefrom, and recycling of the water-plasticizer phase to the slurry are acceptable.

In all of the processing steps heretofore described, the agitation must be sufficient to maintain the smokeless powder par ticles dispersed throughout the water phase of the slurry. This type of agitation is readily supplied by simple paddle or propeller type mixers.

Prior to separating the extracted smokeless powder particles from the aqueous slurry heretofore described, colloidal graphite can be admixed with the slurry. The function of the colloidal graphite is to provide an electrically conductive coating on the particles to reduce electrostatic potential on the particle permitting removal of residual solvent in safety and to prevent agglomeration of the smokeless powder particles. When drying large smokeless powder particles such as cannon powder in which the powder is about 0.15 inches in diameter or greater, the colloidal graphite can be omitted since no agglomeration problem exists. Colloidal graphite applied in the slurry coats the surface of the smokeless powder particles and firmly adheres thereto when the smokeless powder particles are separated from the aqueous slurry. The amount of colloidal graphite employed on the powder is generally from about 0.1 to about 2.0 percent by weight based on the weight of the smokeless powder particles (dry).

The temperature of the slurry required for graphite glazing is 150 F. or higher. Preferred glazing temperatures are from about l70-l90 F. The time required to graphite glaze the smokeless powder particles when operating in the preferred temperature range is from about 5 minutes to about 30 minutes.

Following the graphite glazing step the heating of the slurry is stopped and the solvent-extracted and graphite-coated smokeless powder particles are separated from the aqueous slurry and are dried.

Rapid removal of residual solvent from the extracted double base powder can be conducted in a fluidized-bed-type dryer. Operating conditions for fluidized bed drying will vary, depending upon powder bulk density, and degree of dryness desired. These conditions include gas flow rate, gas temperature, depth of smokeless powder, and residence time. Typical gas temperatures are from about 130 to about 160 F. Gas flow rate must be controlled to avoid excessive turbulence of particles which can cause degradation of the particle geometry and result in an unsafe operating condition. It is well within those skilled in the art of drying to adjust gas flow rates and temperature 5 to rapidly dry the solvent extracted smokeless powder of this invention.

While the foregoing process description has made specific reference to preventing loss of nitroglycerin as the energetic plasticizer in the aqueous slurry drying process of this invention it is not so limited. Other energetic plasticizers which can be employed in the double-base powder and dried by the process of this invention include the nitrate esters of polyhydric alcohols such as diethylene glycol dinitrate, ethylene glycol dinitrate and the like. Double-base powders often contain mixtures of energetic plasticizers such as those described and nonenergetic plasticizers such as triacetin, dibutyl phthalate, tricresyl phosphate, and the like. When a mixture of plasticizers has been prepared, the water portion of the slurry can contain in addition to the equilibrium concentration of energetic plasticizer, an equilibrium concentration of nonenergetic plasticizers. These concentrations can be readily determined by simple mixing of a smokeless powder aqueous slurry, and sampling the water phase of the slurry. When the plasticizer level remains constant, the equilibrium concentration for the smokeless powder employed at the particular ratio of smokeless powder to water in the slurry, is determined. When a recycle system is employed, the equilibrium concentration of energetic plasticizer in the continuous aqueous phase is rapidly attained, and this concentration will remain constant in subsequent cycles in the process.

What I claim and desire to protect by Letters patent is:

1. An aqueous slurry process for rapid drying of solvent-wet double-base smokeless powder particles said aqueous slurry drying process comprising:

a. forming an aqueous slurry of solvent-wet doublebase smokeless powder particles and water, said water having dissolved therein an equilibrium concentration of energetic plasticizer, said energetic plasticizer dissolved in the water being a plasticizer of the same composition employed in manufacture of the solvent-wet double-base smokeless powder particles, said aqueous slurry initially having a continuous phase comprising water and dissolved energetic plasticizer, and a dispersed phase comprising solvent-wet smokeless powder particles, said slurry being agitated to maintain the smokeless powder particles in the dispersed phase until the slurry is comprised of a continuous phase comprising water, energetic plasticizer and solvent, and a dispersed phase comprising solvent-extracted smokeless powder particles,

b. separating the resulting solvent-extracted smokeless powder articles from the aqueous slurry, and c. drying t e resultant powder to remove residual solvent and water.

2. The process of claim 1 in which the energetic plasticizer is nitroglycerin.

3. The process of claim 2 in which the aqueous slurry of step (a) is heated to a temperature of from about to about 190 F., said aqueous slurry comprising from about 70 to about 92 percent by weight of water and from about 30 to about 8 percent by weight of solvent-wet double base smokeless powder particles, said water containing from about 0.1 to about 0.3 percent of nitroglycerin based on the weight of the water.

4. The process of claim 3 wherein the aqueous slurry is agitated until the solvent extracted smokeless powder contains less than about 1.5 percent solvent based on the weight of the solvent extracted smokeless powder.

5. The process of claim 4 in which the solvents extracted from the solvent-wet double-base smokeless powder particles into the continuous phase of the aqueous slurry are continuously separated therefrom.

6. The process of claim 5 in which the solvents extracted from the solvent-wet double-base smokeless powder particles into the continuous phase of the aqueous slurry are separated from the aqueous slurry by blowing inert gas or air through the slurry.

7. The process of claim 3 in which the aqueous slurry of step (a) is heated to a temperature of at least F. and colloidal graphite is dispersed in the heated aqueous slurry, the temperature of the heated slurry being maintained at at least l50 F. until substantially all of the colloidal graphite adheres to the surfaces of the smokeless powder particles.

8. The process of claim 7 wherein from about 0.1 to about 2 percent colloidal graphite is dispersed in the slurry based on the weight of the smokeless powder.

9. The process of claim 7 in which the separated, solvent-ex' tracted and graphite glazed smokeless powder particles are dried in a fluidized bed drier.

Claims

2. The process of claim 1 in which the energetic plasticizer is nitroglycerin.
3. The process of claim 2 in which the aqueous slurry of step (a) is heated to a temperature of from about 80* to about 190* F., said aqueous slurry comprising from about 70 to about 92 percent by weight of water and from about 30 to about 8 percent by weight of solvent-wet double base smokeless powder particles, said water containing from about 0.1 to about 0.3 percent of nitroglycerin based on the weight of the water.
4. The process of claim 3 wherein the aqueous slurry is agitated until the solvent extracted smokeless powder contains less than about 1.5 percent solvent based on the weight of the solvent extracted smokeless powder.
5. The process of claim 4 in which the solvents extracted from the solvent-wet double-base smokeless powder particles into the continuous phase of the aqueous slurry are continuously separated therefrom.
6. The process of claim 5 in which the solvents extracted from the solvent-wet double-base smokeless powder particles into the continuous phase of the aqueous slurry are separated from the aqueous slurry by blowing inert gas or air through the slurry.
7. The process of claim 3 in which the aqueous slurry of step (a) is heated to a temperature of at least 150* F. and colloidal graphite is dispersed in the heated aqueous slurry, the temperature of the heated slurry being maintained at at least 150* F. until substantially all of the colloidal graphite adheres to the surfaces of the smokeless powder particles.
8. The process of claim 7 wherein from about 0.1 to about 2 percent colloidal graphite is dispersed in the slurry based on the weight of the smokeless powder.
9. The process of claim 7 in which the separated, solvent-extracted and graphite glazed smokeless powder particles are dried in a fluidized bed drier.