US3386993A - Process for drying water-wet hmx with lower alkanols - Google Patents

Description

United States Patent 3,386,993 PROCESS FOR DRYING WATER-WET HMX WHTH LOWER ALKANOLS Madhav R. Bhagwat, Indian Head, Md., assignor to the United States of America as represented by the Secretary of the Navy N0 Drawing. Filed Mar. 18, 1965, Ser. No. 440,958 4 Claims. (Cl. 260239) ABSTRACT OF THE DISCLOSURE A process for removing water from water-wet HMX which includes forming a homogeneous mixture of waterwet HMX with a lower alkanol solvent of the type in which HMX is relatively insoluble and water is infinitely soluble, and thereafter filter separating free flowing HMX.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates generally to a procedure for processing wet HMX (cyolotetramethylenetetranitramine). More specifically, this invention is directed to a new and improved method for drying water wet HMX.

HMX is shipped wet with water and isopropyl alcohol antifreeze in muslin bags and before it can be used in the production of solid propellants it must be dried to a water content that does not exceed about 5%. In the method presently employed the initial separation is effected by suction filtration. The HMX recovered from the filter has the appearance of a white plaster, the consistency of cold cream, and a water content that varies from about 25 to 40%. This consistency prevents further separation by filtration and the drying operation is completed in a dry house at about 160 F.

A disadvantage of this drying method is that it does not produce a uniform product. The outer layer dries faster than the inner parts and this results in a dry HMX, that is lumpy and uneven in its moisture content and physical makeup. Before being used in the propellant the hard chunks in the dry material must be manually broken into small pieces and softened by a soaking in acetone for twenty-four hours. In spite of this soaking, fine hard agglomerates are left which cause quite a bit of difliculty during production of the propellant.

Another disadvantage is that the drying operation is too time consuming in that it takes anywhere from three to five days to dry the wet HMX.

A further disadvantage is that this process requires too much manual handling of the friction sensitive HMX and thus exposes personnel to the hazards of an explosion.

Still another disadvantage is that drying large amounts of HMX at elevated temperatures (160 F.) creates a potential safety hazard.

Accordingly, it is an object of this invention toproduce dry HMX by a process which does not suffer from the aforementioned disadvantages.

It is a further object of this invention to dry HMX by a process which produces a uniform product.

Another object is to dry HMX by a process which is safe, economical, and more efficient.

The objects of this invention are accomplished by replacing the water in the wet HMX with a solvent in which HMX is relatively insoluble and water is infinitely soluble. It is preferred that the solvent be compatible with the propellant composition so that no further processing is necessary. As examples of solvents that are operable in this invention there may be mentioned: the lower alkanols such as methanol, ethanol and isopropanol. Of these ethanol is preferred since it is relatively inexpensive. It is to be understood, however, that the invention is not to be limited to the abovementioned solvents.

This process is based upon the unexpected finding that although HMX particles that are coated with water form a lumpy plaster-like mass that is very difficult to filter, HMX particles coated with a solvent meeting the above requirements forms a suspension that is easily filtered. Thus, it is essential to this invention that there be a uniform and intimate mixing of the solvent with the water wet HMX so that the water coating is replaced with solvent.

More particularly, the process of this invention comprises the following steps:

Step 1. Draining the wet HMX bags to remove excess water and isopropyl alcohol. This reduces the water content to an amount that is not in excess of about 40% and is usually in the range of between 25 to 40%.

Step 2.Opening the bags and feeding the wet HMX into a premix unit in which the HMX is intimately mixed with the solvent to obtain a homogeneous dispersion.

Step 3.Diluting the premix (Step 2) with additional solvent.

Step 4.Filtering the free flowing suspension (Step 3) on a filter such as a rotary vacuum filter.

Step 5.-Air drying the HMX filter cake from Step 4, if necessary, to control the water and solvent content as required in the propellant formulations.

Step 6.-Recovering the solvent from Step 4 and recycling it to the system (Step 2 or 3).

The product from Step 5 can be directly slurried into acetone for use in the propellant rnix.

Step 2 is the most crucial part of the process since a simple mixing of the solvent and water wet HMX will not produce a homogeneous dispersion but will result in either a wrapping of the plaster-like HMX around the shaft of the stirrer or in the breaking of the wet mass into agglomerates. It is only an intimate and uniform mixing that replaces water coated on the HMX with sol vent and enables an easily filtered suspension to be formed. This intimate mixing is accomplished in kneading type of equipment such as a horizontal or vertical type of dough mixer but any kind of mixer that will cause complete homogenizing is operable. The amount of solvent added in the premix step is critical since the formation of the homogeneous mixture is dependent upon the removal of water coating from the HMX particles. Thus the amount of solvent present should be sufficient to remove this water coating and this amount usually ranges from about 0.25 to about 2 parts by weight of solvent (hereinafter all parts are by weight) per part of dry HMX with about .5 to about 1 part of solvent per part of dry HMX being preferred. It is to be understood, however, that more than the critical amount of solvent could be added in the premix step. The amount of sol vent added is only critical in the initial part of the mixing operation, i.e., the solvent must be present within the above ranges only until a homogeneous mixture is formed and once formed any amount of solvent may be added. The solvent is usually added slowly so as to insure a thorough mixing of the solvent and water wet HMX. The speed of the mixer is not critical, but it is obvious to one skilled in the art that a slower rate of mixing gives a better dispersion.

Since once the homogeneous premix is formed the HMX can easily be filtered, there is nothing critical about the solvent addition or mixing in the dilution step. It is preferred, however, that there be some type of agitation in this step in order to increase the contact between the HMX and solvent but the intimate mixing employed in the previous step is not required. In this step enough solvent is added so that the total amount of solvent is sufficient to either remove all the water that is present or remove sufficient water to bring the HMX tothe desired moisture content. Since in a filtering operation a more dilute solution gives a faster filtration rate, this factor will also be taken into account in determining the amount of solvent to be added. The amount of solvent that is added is usually sufficient to bring the total amount of solvent in the mixture to about 0.5 to 4 parts of total solvent per part of dry HMX with about one to two parts of total solvent per part of dry HMX being preferred. If the amount of solvent present in the premix operation is sutficient to remove all of the water, this additional step is not necessary. However, since a two-step opera.- tion usually produces a better dispersion, this type of operation is preferred, with half of the total amount of solvent that is to be used being added in the premix step.

This process is usually run at ambient temperatures and thus reduces the hazards of handling the sensitive HMX at elevated temperatures. The process is entirely a mechanical operation and consists of a set of well regulated chemical engineering unit operations that may be adapted to a batch or continuous process. The process is illustrated by not limited by the following specific examples since various additional modifications can be made without deviating from the scope of the invention.

EXAMPLE I A one quart stainless steel beaker fitted with a bafiie was provided with an air driven shaft having a propeller with S-shaped blades that gave a kneading action. 250 grams of wet HMX (30% water) was slowly kneaded with 250 cc. of ethanol. In about eight minutes a homogeneous suspension of HMX in alcohol was formed. An additional 250 cc. of alcohol was slowly added. The diluted suspension was stirred for seven minutes and filtered on a Buchner funnel (filter paper) with partial suction. Filtration time was nine minutes. The alcohol wet HMX filter cake was about one-half inch thick. The moisture content of the cake, as determined by the Karl Fischer method, on three samples was between 1.6% and 1.3%. Total volatiles in the cake (determined on a gram sample heated at 70 C. for one hour) was 16%. Final dry HMX was a free flowing fine powder.

EXAMPLE II In this experiment the bench scale procedure was scaled up to produce about 85 pounds of dry (2.4% water) HMX.

One hundred and thirty-five pounds of wet HMX (30% water) was slowly kneaded in a vertical dough mixer into 96 pounds of ethyl alcohol to secure a free flowing premix suspension of HMX particles. The premix was diluted and stirred with 145 pounds of additional alcohol. The alcohol was added gradually to maintain a uniform suspension. The suspension was stirred for about twenty minutes and filtered. A stainless steel Nutsche filter of six feet diameter (muslin cloth) with steam jet suction was used. 95.3 pounds of alcohol wet cake of thickness varying from A" to /2" was produced. Total volatiles in the filter cake-l3%, water content of cake-2.4%.

Using the procedure of Example II between 900 to 1000 pounds of dry HMX, about 2 to 3% water was prepared. This HMX gave satisfactory results in propellant formulations.

EXAMPLE III Equipment similar to Example I was used. 500 cc. of alcohol was placed in the beaker and 1200 grams of wet HMX (20% water) was slowly added while the stirrer was on. In addition 700 cc. of alcohol was added and the kneading action was continued until a homogeneous system was formed.

The premix was transferred to another beaker that has no baflles and an ordinary stirrer. The premix was slowly diluted with an additional 1200 cc. of alcohol while stirring. The stirring was continued for thirty minutes and then the mixture was filtered on a 10" diameter Buchner (filter paper) with about 30 (Hg) vacuum. The filtration was completed in fifteen minutes with the filter cake having an average thickness of of an inch. The HMX contained two to three percent water.

EXAMPLE IV Example III was repeated but in the filtration coarse muslin was used instead of filter paper. There was very littie change in the filtration. The cake contained 1.9% water.

EXAMPLE V An operation similar to Example III was employed on 1200 grams of wet HMX (23.7% water). In this operation 1050 cc. of ethanol (95%) was used in the premix operation and 1950 cc. of ethanol in the dilution step.

The total volatiles in the filter cake was 19.1% and the water content was 2%.

Net weight of alcohol recovered from the filtrate was 1950.4 grams.

EXAMPLE VI 600 grams of wet HMX (23.7%) was added to cc. of ethanol (95%) in a vertical dough type of mixer. An additional 750 cc. of ethanol was added and the mixing was continued for twenty minutes. A homogeneous mixture was formed. The diluent step was carried out in the same mixer with an increased speed of agitation. 750 cc. of ethanol was slowly added and the agitation continued for fifteen minutes. The mixture was filtered similarly to Example 111. The total volatiles in the cake was 22.4% and the water content 2.8%.

The process of this invention has the following major advantages:

1. It can be set up as well controlled chemical engineering unit operations, thus producing a uniform prodnet.

(2) It does not affect the physical properties of the HMX.

(3) It can be run at ambient temperatures and thus it eliminates the hazards involved in handling HMX at ele vated temperatures.

(4) It reduces the hazards of an explosion caused by friction since the HMX is handled wet with either water or solvent throughout the operation.

(5) It is more efficient since the overall time for drying the HMX is less than in prior processes.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A process for drying water wet HMX which comprises,

slowly adding from about 0.25 to 2 parts of a solvent per part of dry HMX to the water wet HMX, said solvent being a lower alkanol solvent in which HMX is relatively insoluble and Water is infinitely soluble, kneading the solvent and HMX during the addition, said kneading being effected until a homogeneous mixture is formed, diluting the homogeneous mix- 5 6 ture with additional solvent, and filter separating the References Cited HMX from the mixture. 2. The process of claim 1 wherein the water wet HMX UNITED STATES PATENTS contains initially from about 25 to 40% water. 1,398,911 11/1921 5661 260-223 3. The process of claim 2 in which the additional sol- 5 2,102,799 12/ 1937 l y et 6 3 vent is added in an amount such that the total solvent 3,297,631 1/ 1967 Wright et 260239 present is in an amount from about 0.5 to 4 parts per part of dry HMX. FOREIGN PATENTS 4. The process of claim 3 in which the initial solvent 590,851 1/1960 Canada.

is present in an amount from about 0.5 to 1 part per 10 part of dry HMX. ALTON D. ROLLINS, Primary Examiner.