Recycle of spent acid in nitrolysis of hexamine to RDX

Recycle of spent acid in nitrolysis of hexamine to RDX is accomplished un conditions, which avoid or minimize the need to process the spent acid through conventional costly recovery operations and result in little, if any, reduction of RDX yield. The conditions include PA1 (a) effecting the simmering in spent acid containing between 0% and 2% water; PA1 (b) recycling at least part of the spent acid of 0-2% water content to dissolve the hexamine reactant; and PA1 (c) preferably also recycling a part of the spent acid, after elimination of any water contained therein with acetic anhydride, to the heel to which the reactants are added in the nitrolysis of hexamine to RDX.

BACKGROUND OF THE INVENTION 
1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) is a high explosive of great 
brisance, which is usually produced according to the Bachmann process and 
variations there of by nitrolysis of hexamethylenetretramine (hexamine) 
with a solution of ammonium nitrate in concentrated nitric acid in the 
presence of acetic anhydride according to the following overall equation: 
EQU C.sub.6 H.sub.12 N.sub.4 + 4HNO.sub.3 + 2NH.sub.4 NO.sub.3 + 6(CH.sub.3 
CO).sub.2 O.fwdarw. 2 C.sub.3 H.sub.6 O.sub.6 N.sub.6 + 12 CH.sub.3 COOH 
the reagents are preferably employed in excess over the stoichiometric 
proportions of nitric acid, ammonium nitrate and acetic anhydride, and are 
usually added incrementally to a reactor charged with a heel consisting 
essentially of acetic acid, which may contain up to a few percent of 
nitric acid, ammonium nitrate and acetic anhydride. 
The nitrolysis of hexamine to RDX according to the Bachmann process yields 
a slurry of RDX in anhydrous spent acid. Normally, the anhydrous slurry is 
diluted with water to about 40% water content and simmered to destroy 
undesirable by-products, notably linear nitramines. The slurry is then 
cooled and filtered to separate the crude RDX from the spent acid 
(filtrate). At this point the spent acid contains about 40% water, along 
with acetic acid, nitric acid, and ammonium nitrate, together with various 
small amounts of RDX and HMX (1,3,5,7-tetranitro- 
1,3,5,7-tetraazacyclooctane) and other constituents. The process, as 
industrially practiced, requires that the total volume of dilute spent 
acid be processed through the complete recovery system, including costly 
distillation and other operations, to be reconstituted to acetic acid and 
acetic anhydride prior to reuse in the manufacture of RDX. 
SUMMARY OF THE INVENTION 
Significant economic advantages might be realized in the manufacture of RDX 
by avoiding all unnecessary dilution of the spent acid, producing a more 
concentrated acid input into the acid recovery system, and by recycling 
the spent acid to the nitrolysis step without sending it through the 
complete acid recovery operations. 
The present invention is directed to a process for recycling spent acid 
obtained in the RDX nitrolysis under conditions which are novel and 
unobvious, particularly since they are critical for the achievement of 
important advantages over the prior art, as more fully described below. 
It has been unexpectedly found according to the present invention that the 
spent acid can be recycled to the nitrolysis reaction with little, if any, 
loss of RDX yield, as compared to the standard process, by a novel 
modification, which comprises: 
(1) effecting the simmering operation in spent acid from the nitrolysis 
reaction, which has been diluted with water to eliminate the acetic 
anhydride and provide a water content of between 0% and about 2%; and 
(2) recycling at least part of the spent acid of said 0-2% water content, 
after separation of the RDX product, to dissolve the hexamine reactant. 
PRIOR ART STATEMENT 
The following patents are of interest relative to the present invention: 
West German Pat. No. 1939541 
French Pat. No. 2053804 
West German Pat. No. 1939541 and French Pat. No. 2053804, relating to the 
production of HMX, disclose heating of the reaction mass, after nitrolysis 
and without dilution with water, to destroy by-products and recycling of 
the resulting spent acid. However, the French patent distills the spent 
acid to recover its content of acetic acid, which has been separated from 
HMX and RDX, while the German patent recycles the spent acid which 
apparently still contains excess nitric acid and acetic anhydride. Even 
though the HMX nitrolysis and the RDX nitrolysis employ the same reagents, 
these processes are very different, since the proportions of reagents, 
reaction temperatures, reaction mechanisms and precursors and by-product 
nitramines are significantly different.

DETAILED DESCRIPTION OF THE INVENTION 
Conventional Process 
Referring to the flow diagram of the RDX nitrolysis shown in the drawing, 
the three feedstreams which are entered into the reactor are as follows: 
(1) a solution of 100 g (0.714 moles) hexamine in 164.7 g (2.745 moles) 
acetic acid, 
(2) a solution of 166 g (2.075 moles) ammonium nitrate in 212 g (3.365 
moles) nitric acid, and 
(3) 575.1 g (5.638 moles) acetic anhydride. 
The feed streams were added incrementally over a 15 minute period to an 
agitated reactor containing a heel maintained at 65.degree. C. and 
consisting of 35 ml acetic anhydride, 30 ml of nitric acid/ammonium 
nitrate (56/44) solution, 20 g of ammonium nitrate and 785 ml of acetic 
acid. Following the addition of the reagents the slurry was aged for 40 
minutes at 65.degree. C. to complete the nitrolysis. 
Since the nitrolysis reaction requires anhydrous conditions, it is 
essential to avoid or minimize the presence of water in the spent acid to 
be recycled. Therefore, to determine the effect of strength of the simmer 
acid, several nitrolysis batches were carried out in the manner described 
above except that various amounts of water were added to the aged RDX 
slurry, which was then heated to 98.degree. C. for 30 minutes to effect 
the simmering operation. The results are set forth in Table I, which shows 
that there is a decrease of about 6% in yield of RDX due to solubility of 
the RDX in the stronger acid as the simmer acid concentration is varied 
from 60% (40% water content), employed in the standard process, to 100%. 
(0% water content). 
Conventional Process Modified According to the Present Invention 
The following examples illustrate specific embodiments of the method of 
carrying out the process of the present invention under conditions, which 
permit recycling of spent acid with little, if any, reduction of RDX yield 
and avoid or minimize the need to process spent acid through the 
conventional, costly recovery system. 
EXAMPLES 1-5 
These examples describe experiments, wherein spent acid was initially 
generated from a virgin batch prepared under the standard process 
conditions described above except that the simmering step was accomplished 
in spent acid containing one percent by weight of water, and the spent 
acid filtrate was recycled in four consecutive batches under the following 
condition: a portion (264.7 g) of the recycle spent acid containing 1% 
water was employed to prepare the hexamine solution. 
TABLE I 
__________________________________________________________________________ 
Standard RDX Nitrolyses - Effect of Increased Acid Strength During 
Simmer 
Assay of Spent Acid 
% Nitric 
% Acetic 
Crude RDX Pure RDX 
Batch % H.sub.2 O 
Acid Acid % HMX 
% Comp C.sup.(2) 
Yield g/g Hexamine 
Yield g/g Hexamine 
__________________________________________________________________________ 
1 (Control).sup.(1) 
40 1.88 59.04 
4.55 0.82 2.521 2.387 
2 30 2.20 63.93 
3.34 0.88 2.467 2.362 
3 20 2.48 75.90 
2.34 1.53 2.408 2.315 
4 10 2.15 83.62 
0.81 1.07 2.335 2.291 
5 1 2.95 90.00 
1.45 0.89 2.318 2.264 
6 0 2.16 91.09 
0.00 1.98 2.282 2.237 
__________________________________________________________________________ 
.sup.(1) denotes standard process 
.sup.(2) Compound C is the trivial name given to the major impurity 
present in RDX and HMX produced in the process. Compound C is presently 
unidentified but is probably a methylene nitramine polymer. 
With the above exception the nitrolysis conditions were equivalent to the 
standard process. The reclaimed spent acid from each batch was recycled to 
the next batch. 
The results set forth in Table II show that under the aforementioned 
recycle conditions, spent acid can be recycled to the process with little, 
if any reduction in yield of RDX as compared with the RDX yield obtained 
by the standard process. 
In conventional batch nitrolysis of hexamine to RDX, the hexamine is fed to 
the heel as a concentrated solution in acetic acid. Acetic acid is also 
used in the nitrolysis heel. The nitric acid-ammonium nitrate solution is 
normally fed to the reaction medium at a rate to ensure that nitric acid 
is present at all times. The free nitric acid in the reaction medium is 
one of the control parameters that can be monitored and is referred to as 
nitric acid "excess". These nitrolyses are carried out in the presence of 
acetic anhydride under anhydrous conditions so that water is not 
inherently present in the spent acid resulting from the nitrolysis. 
In carrying out the process of the present invention sufficient water is 
added to the reaction mixture after completion of the nitrolysis reaction, 
to eliminate the acetic anhydride present and to provide a slurry liquid 
of 0% to about 2% water content for the simmer purification step. The 
simmering purification step can be accomplished by heating the slurry 
liquid, adjusted to 0-2% water content, to an elevated temperature, 
preferably about 90.degree.-100.degree. C., until the by-products of the 
nitrolysis reaction, notably linear nitramines, are destroyed, after which 
the spent acid can be separated from the solid RDX, e.g. by filtration, 
and recycled. Larger amounts of water are preferably not incorporated in 
the recycled spent acid to minimize the amount of acetic anhydride 
required to make the system anhydrous for the nitrolysis reaction. 
TABLE II 
__________________________________________________________________________ 
RDX Nitrations Using Acid Recycle and 1% Water in Hexamine Solution 
Assay of Spent Acid 
% Nitric 
% Acetic Crude RDX Yield 
Pure RDX Yield 
Example 
Acid Acid % HMX 
% Comp C 
g/g Hexamine 
g/g Hexamine 
__________________________________________________________________________ 
1 (virgin 
3.41 91.61 
5.44 2.79 2.52 2.31 
batch) 
2 3.53 91.36 
6.30 4.87 2.57 2.28 
3 3.43 91.56 
2.47 1.46 2.46 2.36 
4 3.52 90.73 
2.70 1.33 2.43 2.33 
5.sup.(a) 
1.55 93.09 
6.84 2.52 2.42 2.19 
__________________________________________________________________________ 
.sup.(a) Nitric Acid - ammonium nitrate was reduced 20% 
The amount of recycle spent acid of 0-2% water content employed to dissolve 
the hexamine reactant is ordinarily restricted to that which is sufficient 
to dissolve the hexamine. About 15-20% of the spent acid generated per 
batch is required to dissolve the hexamine feedstock for a subsequent 
nitrolysis batch. This amount of spent acid is fixed by requirements of 
solution composition and is near the minimum amount required to dissolve 
the hexamine. The use of smaller amounts of spent acid is less preferred, 
since it would result in incomplete dissolution of the hexamine; while the 
use of larger amounts of spent acid to dissolve the hexamine is also less 
preferred, since it would have the adverse effect of increased volume 
throughout with no RDX yield increase. 
When the spent acid from the simmer purification step is recycled to the 
hexamine feed solution and the nitrolysis heel, the excess nitric acid 
usually present therein is not neutralized with sodium acetate, or 
equivalent, to avoid the loss of RDX yield at the expense of increased HMX 
yield. Spent acid recycled to the nitrolysis heel is mixed with acetic 
anhydride if necessary, to eliminate any water contained therein so as to 
provide the anhydride conditions required in the nitrolysis reaction. 
Optimum results are obtained when a portion of the recycled spent acid, 
obtained by simmering at high acid strength (98-99% corresponding to 1-2% 
water content), is utilized without modification to prepare the hexamine 
solution (i.e. neither the excess nitric acid nor water content is 
eliminated), and the remainder of the recycle spent acid is made anhydrous 
with acetic anhydride and then utilized as the heel, as illustrated in the 
examples. 
The conditions under which the recycling of spent acid is feasible 
according to the present invention are based on several important 
discoveries discussed below. 
(1) As shown in Table I, it has been found that RDX can be simmered in 
strong acid in concentations up to 100 % (0% water content) to destroy 
by-products e.g. linear nitramines, with little or no loss in yield except 
that due to the greater solubility of the RDX in the more concentrated 
acid. 
(2) It has also been found that the spent acid from the simmering step can 
be recycled repeatedly without progressive buildup of byproduct 
nitramines. Table III shows the results of experiments, wherein spent acid 
was generated initially from a virgin batch prepared under standard 
conditions and the simmer step was carried out in spent acid containing 1% 
and 2% water. The spent acid filtrate was used in four consecutive 
recycles under the following conditions: 
a. The recycle acid was used to dissolve hexamine (264.7 g acid per 100 g 
hexamine). Acetic anhydride was added to the recycle acid sufficient to 
restore anhydrous conditions i.e. no excess anhydride was used, prior to 
hexamine dissolution. 
b. The heel was standard in composition and volume. Spent acid was not 
recycled to the heel. 
c. With the above exceptions, the nitrolysis conditions were equivalent to 
the standard process conditions described above. Reclaimed spent acid from 
each batch was recycled to the next batch. 
(3) It has also been found that the reaction product formed beween hexamine 
and the recycle spent acid employed to prepare the hexamine feedstock 
solution is quite dependent on the water content of the spent acid and in 
turn has an important effect on the yield of RDX produced. Thus, for 
example, 
a. When the hexamine feedstock solutions were prepared with recycled spent 
acid containing 1% water and normal (about 2%) nitric acid excess, using 
264.7 g spent acid per 100 g hexamine, and the remainder of the spent acid 
was recycled to the heel after first removing its water content by 
addition of acetic anhydride, as described in Examples 1-5, the following 
results were obtained: 
(1) The nitric acid in the recycled spent acid reacted with hexamine to 
form hexamine mononitrate, leaving no free nitric acid in the solution. 
About 12% of the hexamine reacted in the mixing step. 
(2) The RDX yields were only about 1% lower than control batches due to 
greater volume and solubility. 
b. When the hexamine solutions were prepared with recycled spent acid 
containing 1% acetic anhydride and normal (about 2%) nitric acid excesses 
and using standard heel conditions (composition and volume), the following 
results were obtained: 
(1) 1-acetamidomethylhexamine1-nitrate was formed and remained in solution, 
leaving no free nitric acid in the solution. About 20% of the hexamine 
reacted during the mixing step. 
(2) The RDX yields obtained in the RDX nitrolysis batches were about 23% 
lower than control batches due to the formation of larger amounts of HMX. 
Since 1-acetamidomethylhexamine-1-nitrate has been shown to nitrolyze to a 
product rich in HMX, its presence may possibly explain the smaller RDX 
yields and greater amounts of HMX obtained. 
c. When the hexamine solutions were prepared with recycled spent acid, 
wherein the normal nitric acid excess was removed by neutralization with 
sodium acetate, two conditions were investigated: 
(a) The neutralized recycle acid was used only for preparing the hexamine 
solution and standard heel conditions (composition and volume) were 
employed, i.e. the neutralized acid was not recycled to the heel. 
(b) The neutralized recycle acid was used for preparing the hexamine 
solution and also as a nitrolysis heel. 
As shown in Table IV, recycle of the neutralized spent acid to both the 
hexamine feed and the heel resulted in a large increase in yield of HMX 
produced at the expense of RDX yield, but not when it was recycled only to 
the hexamine feed. Thus, the average HMX content of the crude product 
progressed from 2.28% (for spent acid-hexamine solution) to 5.06% 
(standard) and the 23.86% (recycle to both hexamine feed and nitrolysis 
heel). This result is of potential interest for simultaneous production of 
RDX and HMX from a single nitrolysis reaction and at RDX economics. The 
economics involve a trade-off between RDX losses, HMX gains and the use of 
a single nitrolysis system rather than separate facilities, as in current 
practice. 
The foregoing disclosure and drawings are merely illustrative of the 
principles of this invention and are not to be interpreted in a limiting 
sense. We wish it to be understood that we do not desire to be limited to 
the exact details of construction shown and described, because obvious 
modifications will occur to a person skilled in the art. 
TABLE III 
__________________________________________________________________________ 
RDX Nitrolyses - Strong Acid Simmer and Recycle 
Assay of Spent Acid 
% Nitric % Acetic 
Crude RDX Pure RDX Batch from which 
Batch 
Acid Acid % HMX 
% Comp C 
Yield g/g Hexamine 
Yield g/g Hexamine 
Acid was Recycled 
__________________________________________________________________________ 
1.sup.(a) 
2.99 91.26 
0.89 1.55 2.50 2.44 Virgin batch 
2.sup.(a) 
3.96 91.21 
0.00 0.34 1.91 1.90 1 
3.sup.(a) 
3.57 87.81 
3.10 0.40 1.98 1.91 2 
4.sup.(b) 
3.59 90.89 
1.20 1.40 1.93 1.88 3 
5.sup.(b) 
1.87 91.90 
4.90 1.25 1.91 1.79 4 
__________________________________________________________________________ 
.sup.(a) Simmered in spent acid adjusted to 1% water content by addition 
of water. 
.sup.(b) Simmered in spent acid adjusted to 2% water content by addition 
of water. 
Byproduct linear nitramines were not detected in any of the recycled spen 
acids by thin layer chromatography (TLC) and gas liquid chromatography 
(GLC) techniques. 
TABLE IV 
__________________________________________________________________________ 
RDX Nitrolyses Using Acid Recycle With Nitric Acid Content Neutralized 
Assay of Spent Acid 
Crude RDX Pure RDX 
% Nitric 
% Acetic Yield g/g 
Yield g/g 
Batch from Which 
Batch 
Acid Acid % HMX 
% Comp C 
Hexamine 
Hexamine 
Acid was Recycled 
__________________________________________________________________________ 
1 1.53 93.53 
3.92 2.19 2.49 2.34 Virgin Batch 
2.sup.(a) 
3.29 87.23 
22.42 
1.92 2.00 1.51 1 
3.sup.(a) 
3.05 86.67 
25.30 
2.16 2.05 1.49 2 
4 3.26 92.28 
6.20 1.44 2.54 2.35 Virgin Batch 
5.sup.(b) 
3.73 87.98 
2.25 1.52 2.03 1.95 4 
6.sup.(b) 
3.93 90.92 
2.30 1.68 2.04 1.96 5 
__________________________________________________________________________ 
.sup.(a) Acid was recycled to both the heel and the hexamine 
.sup.(b) Acid was recycled only to the hexamine solution