US2478164A - Method of recovering acid in the production of cyclonite - Google Patents

Description

Aug. 2, 1949. G. F WRIGHT El AL 2,478,164

" METHOD OF RECOVERING ACID IN THE PRODUCTION OF CYCLONITE 2 SheetsSheet 1 Filed Dec. 30, 1944 [n msniors: Gemye FPVFz'y/z? and C arlA. Wmk Z 6 r Aug. 2, 1949.

c; F. WRIGHT ET AL ,4 METHOD OF RECOVERING ACID IN THE PRODUCTION OF CYCLONITE Filed Dec. 50, 1944 2 Sheets-Sheet 2 m h m 5 a 6 04 e d W; m V e 0 0 no 5 u 2U .v b e PM 5 3 d w w 2b .W m 04 j VJ -we mmw Low W. P 4 i o a a T W a e 4 5 9 0. w w w w w 0 m m m M W N M Q fi fi b X P m R NNSQ Patented Aug. 2, 1949 UNITED STATES FATE-N T OF FlCEi M'ET-HQD 9F REGOVERING ACID PRQDUCTIQN F CYGLONITE George-F. Wright, Toronto; Ontario; and-Garl-A; Winlrler, Montreal, Quebec, Ganada; assignors to The HonoraryAdvisory Counoil f'or Scientific and-Industrial Research, Ottawa, Ontario,- Can ada, a corporationof theDominion-of Canada;

Application December 30, 1944', Serial No. 570,812

(Ch .2 Gib-541) l0-Glaimst 11 The present inventionrelates to acid recovery and 1 more. particularly to .therecovery of acetic acid from spent reaction liquors resulting from theman-ufacture' of certain explosives especially cyclonite.

In theomanufactu-re; ofpyclonite, either: by the process described in Bachman application, Ser. No; 495,078filed July- 16,- 1943, or by the: related process described in Schiessler and Ross application,- Ser. No; 444,254, filed May 23,1942, now Patent No. 2,434,230, -January-6; 1 948,-there'is produced asa' byeproducti-a. spent reaction: liquor containing .a considerablelquantity of acetic acid which may amount to as much as approximately 3.5 lbs. of acetic acid per pounduof cyclonite producedinlthe process The recovery of acetic acid from such spent reaction liquors istherefore essential if these processes .are-to ba -commercially attractive. However,- recovery operations with such spent reaction liquors are hazardous, firstly because ammonium nitrate-containing organic matter constitutes a'high: explosive and secondly because the.1iquors contain residual. amounts. of cyclonite which is knownto betanexplosive of great power and brisance.

The object of the present inventionis to provide anew, improved and safe processior the recovery 0t acetic acid from crude acidcontainingammonium nitrate and residual explosive nitramines,

A further object is to provide a method of the characterldescribed which destroys the hazardous components of spent reactionliquors resulting from the manufacture of cyclonite produced by processes involving the use of acetic anhydride and ammonium nitrate.

Another 'pbi'r-ectis an acetic acid recovery "process applicableto crude-acetic acid containing ammonium nitrate and cyclonitawhich prevents the accumulationlof explosive at any point in the systemor the retention of the explosive in the recovered acid.

Still another object is to providea process that enablesthe recovery of acetic acid without the addition of a reagent which would remain. in the recoveredacid tothe detriment of acid quality.

A further object, is the provision of an acidrecovery process which may be carried out continuously' in apparatus constructed with a minimum of strategic materials, producing recovered acetic acid-of over 99% purity;

Other objects and advantages of the process Will be apparent-as theinvention is hereinafter more fully described;

In accord-ance with the'present inventionpit has been-found that the acetic acid present in spent mother liquors of the type characterized 5 abiive maybe-recovered byfirst'fuming oii the'crude acid in the presence of a-small amount of-sulfu-ri'c acid and thereafterdistilling the fumed off *reaction liquor at a temperature sufficiently high to destroy any-organic explosivematerial contained therein. Bythe foregoing process'it'is" possible to recover acetic'acidof over99'.5% purity. Ex-

plosive hazards areminimized during therecovery operation by the'destruction" of the ammonium nitrate during the fume-off step, and of the cyclonite and"related compounds"during the distillation step;

In carrying out the processor the present invention it is desirable firstly to control the fumeofi andsecondly to'efiect 'ra'pid'and complete destructionof the organic explosive remaining in the fumed off"'liquors, without the use of a re- ,agent whichwould 'remaininthe recovered acid to the detriment of acid quality. The present inventionattains these objects by means whichw'ill hereinafter be described."

In*order* moreclearly to describe the invention the process will be discussed with reference to the I; :THE FUME-OFF STEP In 'the manufa'ctureof cyclonite by the-Bachmann process, hexamine' (preferably dissolved in glacial acetic acid) nitric acid, ammonium nitrate and -acetic anhydride 1 are brought together in a reaction vessel at a temperature'abov'eabout 35 :C. Irr the manufacture of cyclonit'e by the Schiessler and Ross pro'cess; paraformald'ehyde {is treated with ammQnium-nitrateand acetic anhydride at' a temperature above about 35 C. The reaction mixture resulting from either of these processes'is-a slui'rWconSisting of What may convenient-ly ha -termed an-hydride cyclonite" suspendedin a mother liquor which contains among other things, ammonium nitrate and acetic acid. For purposes of convenience, the mother liquor from both of these processes may be designated as anhydride cyclonite mother liquors or reaction liquors.

The process of the present invention is based in part upon the discovery that, if the undiluted filtrate or mother liquors from the above processes are heated in the presence of a small amount of sulfuric acid to a threshold temperature of about 95 to about 100 C., a violent reaction ensues during which the color of the liquid suddenly changes from straw color to deep brown and large volumes of nitrogen oxide gases are evolved. During this fume-01f reaction, the ammonium nitrate together with a substantial portion of the nitric acid and the unstable nitramine by-products of cyclonite are decomposed. The violence of the reaction is due to the gas evolution which causes considerable foam. The foam however is unstable and disappears rapidly as soon as gas evolution ceases. The fume-off reaction is exothermic and the temperature of the reaction mixture rises to about 106 C. in the absence of added heat.

(a) Sulfuric acid concentration during fume-o1? The fume-off reaction is self-sustaining provided sufficient sulfuric acid is present in the mixture. The sulfuric acid concentration necessary to obtain a self-sustaining fume-off depends in part upon the composition of the filtrate or of the liquor. Usually, however, the minimum amount of sulfuric acid varies from about 5% to about by weight of the fume-off mixture.

(b) Nitric acid concentration during fume-0 7 If the anhydride cyclonite mother liquor or filtrate contains an appreciable amount of nitric :acid and ammonium nitrate, the fume-off is much more violent in operation. It is therefore sometimes desirable to reduce the nitric acid and ammonium nitrate content of the filtrate prior to ammonia to convert practically all of the nitric acid to ammonium nitrate which is sparingly soluble in cold (ZS-30 C.) neutralized mother liquor but fairly soluble in the mother liquor containing appreciable amounts of nitric acid. When the resulting ammonia-treated filtrate is fumed off very little nitrogen dioxide is formed but considerable nitrous oxide gas is evolved at a steady rate at a temperature of about 106 C.

In this connection it may be mentioned that if all the nitric acid is neutralized with anhydrous ammonia some difficulty may be experienced occasionally in sustaining the fume-off reaction. To avoid this difliculty it is preferable to neutralize the major proportion but not quite all of the nitric acid, leaving less than 1% of the nitric acid, for example, 0.5% unneutralized. Ammonium nitrate is only sparingly soluble in mother liquors containing this small amount of nitric acid and the mixture fumes off smoothly and continuously at about 106 C.

Either before or after the treatment with anhydrous ammonia, but in any event prior to the fume off, any large quantities of solid cyclonite present in the reaction mixture are of course removed by filtration. The ammonium nitrate may be readily removed from the separated cyclonite by passing hot water through the filter cake. In order to accelerate the rate at which the ammoand water.

nium nitrate is leached from the filter cake it is preferable to blow live steam therethrough.

(0) Temperature during fume-017 Temperature is an important factor in controlling the violence of the fume-off reaction. If the temperature is high enough to initiate rapid fume-off (about to about C.) the heat of the reaction is sufiicient to raise the liquid temperature to about 106 C. This temperature can be maintained without further outside heat provided fresh mother liquor-sulfuric acid mixture is added at an appropriate rate. If outside heat is supplied during fume-off until the temperature of the liquid rises to 112 C., the evolution of nitrogen dioxide may cease for reasons at present unknown and the evolution may or may not be restored by lowering the liquid temperature to the proper range between about 95 C. and about 106 C. However, Once the fume-off reaction has ceased due to overheating, the restoration of the fume-off is sometimes very difficult to brin about. It is therefore important to control the temperature of the fume-off reaction so that it takes place below about 112 C., and preferably so that it takes place in the neighborhood of 106 C.

Inasmuch as the constant addition of fresh mother liquor-sulfuric acid mixture assists in maintaining the fume-off at an easily controlled constant rate, continuous operation during fumeoff is preferred although batch fume-off may be carried out where desired. However, in batch fume-off of even a small volume of mother liquor the reaction is sometimes quite violent and for this additional reason continuous fume-01f is much to be preferred.

A convenient method of carrying out the fumeoff is to start with a small volume of mother liquor maintained at fume-off temperature in a suitable reaction vessel and then to add sulfuric acid or oleum to the mother liquor until fumeoff begins to take place. Thereafter additional mother liquor containing from about 5% to about 10% by weight sulfuric acid may be constantly added to the reaction vessel, the feed being preheated at a temperature (about 50 C.) sufficient to maintain the fume-off reaction mixture at a temperature in the neighborhood of 106 C. By providing the reaction vessel with a draw-01f tap, the fumed off liquor may be withdrawn from the reaction vessel at a rate sufficient to maintain a fairly constant level in the reactor.

(d) Impurities in the famed 017 liquor The composition of the fumed 01f mother liquor depends of course upon the composition of the original mother liquor. The ammonium nitrate and hydrolyzable organic impurities, such as N- acetylpolymethylene nitramines, are removed during the fume-01f but approximately the original amounts of residual cyclonite and homocyclonite remain in the fumed-off mother liquor as were originally present in the mother liquor prior to the fume-01f reaction. Traces of nitric acid may also be present in the fumed-off mother liquor, together with larger amounts of sulfuric acid Generally, the water content will be considerably higher in the fumed-off mother liquor than in the original mother liquor; the average increase in Water content being about 1.6% regardless of the original water content of the mother liquor prior to fume-off. Apparently the water formed during fume-off results almost ennitrate in the mother liquor.

t. i =5 1, .2. L12 t .w wflrflzt "dehydration" reaction involving mq i r eaud nit oapiqerror: or THE" FUMED- OFF Mo 1 r IQQUQR F, V ('a) Destruction of leyclom'teond homocyclomte pointed "out above," the iiuii i'e ofi" reaction notdestroy the residual cyclonit'e and homokfy ion'ite' containedih the mother liquor. 'In acooifdahe with a further aspectof the present in vn'tion'it' has beenfound that this residualcy clon'ite and homocyclonite maybe completely def strbyed during distillation of the acetic'acidfpro: 'vided the fumed-off mother liquor is distilled from a sulfuric acid-acetic acid mixturecontain ing a relatively high-percentage of sulfuric acid, .maint'ained at a temperature within the range of about 125 to about 150 C.

The rate of destruction of cycloni'te in mix- THER tures of acetic acid and sulfuric acid is dependent partly-upon the temperature of the mixture and partly upon the sulfuric acid concentration. Fig. 2 is a graphical representation of the rate of destruction of 'e ucmte in fumed-off mother liquorsulfuric acid iniX-tures at the boiling point of the mixtures, From this graph it will be apparent that 100% of the cyclonite may be destroyed in less than- 2 minutes provided the sulfuric acid content of the boiling'mixture is above about by weight; Fig. 3 is a graphical representation sulfuric acid by volume will be above about 130 C Accordingly in effecting the distillation of the acetic" acid in accordance with the present invention,' complete and rapid destruction of the residual cyclonite and homocyclonite is insured by introducing the'fumed-ofi mother liquor into a mixture of sulfuric acid and acetic acid of 'at least 20% by weight sulfuric acid strength main tained ata temperature above about 120 110140" C In order to ensurevery rapid and'complete destruction of the residual organic explosive, the fumed-oft mother-liquor is distilled through a mixture consisting of about 40% sulfuric acid and 60% glacial acetic acid. Furthermore, inasmuch as the 'fume-off reaction is preferably carried out continuously it is desirable to effect the distillation in an apparatus adapted for continuous operation, 1 Y I do) Compositiomofihe distillate... it 1,

The distillate'hroducedin accordance with the present invention consists of acetic acid or above 95% urityfthe precise composition of'the distilla'te'being tat-item ionof the residue'm'aintained in the stillduring"distillation." The distillate contains afvarylng but usually very small amount of water whrenus derived from' three possible sources, namaym the Water in the fumed' off mother dependent partly upon'the sulfuric acid K liquor resulting from the decomposition of amgreatly reduces'the solub'ilityof the ammonium duceci bythe econ'd sourceis residue in the stil-liattains a composition of about 14.2% sulfuricacid; 2.6% water and 83.-2-%acetic acid, it -appears -thatsulfuric acid exercises no appreciabledehydrating action on the acetic' acid. From about 15 to about 35 sulfuric acid content thedehydrating efiiciency orthe sulfuric a-cidincreases to a -maximum giving a distillate of "progressively higher acetic acid strength.= After a maximal dehydrating efficiency at about 35 sulfuric acid in the stillresidue,'- the efficiency decreases sharply. Accordingly the highest qual--- ity aceticacid distillateis obtained at a maximum dehydrating concentration of'about 35% sulfuricacid in the still 'mixture. However, thetotal recoVe-ry of" acetic acid is notmaXimal under these;

' to about 40% sulfuric acid in the still residues;

represent convenient operative ranges.

*As an illustration of the composition ofthe distillate obtainable in accordance with the proc-,- ess of the present invention, an experiment was. carried out with 7. l. of fumed oil mother liquor: containing about 5.4% sulfuric acid. Thisfumedi oiT mother liquor was distilled through 130 cc; of'a mixture consisting of 40% sulfuric racidandl 60% glacial acetic acid using a reflux ratioof 0:1,. The distillate consist-odor aceti acid of over 99 purity. In some instances very smallamounts of aldehydes maybe present.v Where it is desired to reduce the aldehyde contentof the recovered acid, the acid may be digested .witha small amount of 'aniline and then distilled. This practice, however, is usually not necessary.

III. 'ILLusrRA mvE EXAMPLE In order to disclose the nature of thepresent invention still more clearly a typical example will hereinafter be described with reference to Fig. I or" the drawing. It-sliould be clearly understood that this is done solely by way of illustration-and is notintended to delineate the invention in its broadest aspects.

Inthis example-it will be assumed that the cyclonite was produced in accordancewith the Bachmann process by reacting hexamine dise solved in acetic acid with ammonium nitrate-and nitric acid-inthe presence of acetic anhydride at a temperature Off about 35 to about C; After-the reactantsxare brought together in the reaction vessel, the resulting slurry is withdrawn from the reactor. and cooled to about 25 to about 30" .C; This undiluted but cooled slurry from -the Bac'hmann process is then treated with nearlystoichio-metric amounts of anhydrous ammonia in.

an amount suificient to reduce the nitric acid.

content to about 0.5%, thereby reducing the: solubility of the ammonium nitrate in the reaction liquor. a After this ammonia pretreatment, the slurry i filtered,'the cyclonite and other solid materials: remaining in the filter bed being worked up by hot water wash and by passing live steam through the-filter Ibed, prior to the purification procedure "described in the above identifiedBa-chmann application. y

Afterthe'removal' of the solid cyclonite' thefiltrate is then ready for the recovery process ractically': inala terable i n quantity; -The water produced-from the" third source varies with the isulfuric acid concentration of -the -sti1l-;resiclues;- Until-the:

7 which may conveniently be carried out in the apparatus diagrammatically illustrated in Fig. 1. The apparatus as shown includes a fume-off chamber Ii) (preferably constructed of Duriron) a series of boiling tubes l I, an entrainment separator l2 and an acetic acid condenser [3.

In starting the unit in operation the crude acid (i.e., the mother liquor from which the cyclonite has been filtered) together with sulfuric acid, is introduced into the fume-off chamber l through the charge lines 2!) and 2|, the proportions being so adjusted that the resulting mixture contains from about 5% to about sulfuric acid, the higher concentration being generally more satisfactory in starting up the continuous system. A sufiicient amount of this mixture is introduced through the charge lines to fill the hydraulic legs 22 and 23 and partially fill the pot Ill. The mixture in the hydraulic leg 22 is then heated by passing steam through the steam jacket 24'. After fumes begin to evolve in the fume-off pot In the crude acid and sulfuric acid (preferably preheated to about 50 C.) are constantly fed through the charge lines 20 and 2i in a ratio of roughly 95 parts by weight of crude acid to 5 parts by weight of sulfuric acid, the feed rate being adjusted to maintain a substantiall constant liquid level in the fume-off chamber 80. During the fume-off operation the temperature of the liquid in the fume-off pot I0 is maintained as closely as possible at about 106 C. The evolved gases and vapors pass upwardly through the conduit 25 into a condenser 26, any condensate being returned to the fume-ofi pot through the trapped line 21; uncondensed vapors and gases pass through the vapor outline 29 to the fume stack (not shown).

The fumed-off reaction mixture overflows through the hydraulic legs 22 and 23 into the upper boiling tube II. The boiling tubes consist of a number of steam jacketed horizontaltubes disposed above one another and so connected as to form a continuous flow path. Each of the horizontal tubes is provided with suitable dams 28 so that each horizontal tube is maintained partially full of liquid. The fumed-01f mother liquor flows into one end of the upper tube and thence in sequence through the lower horizontal boiling tubes. The lowermost boiling tube is provided with a take-off tap 30 through which residual sulfuric acid-acetic acid may be withdrawn. The temperature of the liquid in the uppermost boiling tube is maintained at about 125 C. while that of the lowermost boiling tube is held at about 140 C.

In order to minimize back pressure effects, a vapor line 3i connects the vapor space of the horizontal boiling tubes to the sulfuric acid entrainment separator l2.

As the vapors from the boiling tubes ascend through the entrainment separator [2 (which is preferably packed with Raschig rings or other suitable surface material) the entrained sulfuric acid or other liquid in the vapors is separated, the vapors ascending to the condenser i3. The condensed acetic acid thence flows into the recovered acetic acid storage tank 32.

Inasmuch as the residual acid flowing from the lowermost boiling tube may contain 60% acetic acid or more, it is preferable to recirculate this acid to the fume-off vessel, this being accomplished by the return line 33 which connects with the recycle storage tank 34. The residual acid in the storage tank 34 is forced by the pump 35 through the return line 36 to the overhead 8 storage tank 3'! whence it flows through the valved line 38 to the fume-off pot ID.

The boiling tubes II are preferably steamjacketed (not shown) in order to supply heat to the liquid undergoing distillation.

By way of recapitulation, in using the apparatus described with reference to Fig. 1, assuming that the process is .to be used in connection with the manufacture of anhydride cyclonite, the cycle of operations is preferably as follows:

(1) Treat the undiluted reaction mixture (consisting of a slurry of cyclonite in the undiluted reaction liquor) with anhydrous ammonia until nearlyv all of the nitric acid has been neutralized,

leaving approximately 0.5% of the nitric acid unneutralized. The resulting slurry will then consist of cyclonite, acetic acid, ammonium nitrate, about 0.5% unneutralized nitric acid, and other reaction products, impurities, etc.

(2) Filter the cyclonite and other solid products.

(3) Mix the filtrate with sulfuric acid, forming a solution preferably containing about 9 to 10% sulfuric acid in starting up the acid recovery system, the filtrate and the sulfuric acid entering the fume-off vessel preferably being prewarmed to about 50' C.

(4) Heat the liquid in the hydraulic leg 22 until fume-off is initiated and thereafter maintain the temperature of the liquid in the fumeoff pot at about 106 C.

(5) After the fume-off has started, continue charging to the fume-01f vessel It], prewarmed (50 C.) crude acid and sulfuric acid in the proportions of about by weight crude acetic acid and 5% by weight sulfuric acid.

(6) Permit the fumed-01f liquor to flow into the boiling tube evaporator, maintaining the temperature in the uppermost tubes of the evaporator at a temperature slightly above the boiling point of acetic acid (for example at C.) and the lowermost tubes of the evaporator at a temperature not exceeding about C.

(7) Condense the acetic acid vapors passing through the entrainment separator I2, in the condenser 13.

(8) Continuall withdraw residual acid (about 40% sulfuric, 60% acetic acid) from the tap-off line 30, recycling either all or a portion thereof where desired to the fume-off vessel It.

It will be apparent to those skilled in the art that many variations may be made in the foregoing detailed procedure without departing from the spirit and scope of the invention. We therefore intend to be limited only in accordance with the following patent claims.

We claim:

1. A method of recovering acetic acid from mixtures containing ammonium nitrate, nitric acid, acetic acid and residual nitramine explosives, which comprises heating the reaction mixtures in the presence of sulphuric acid to effect fume-off, and thereafter distilling the fumed-off reaction, mixture from a residual acid containing about 15 to about 40% sulfuric acid.

2. A method of recovering acetic acid from mixtures containing ammonium nitrate, nitric acid, acetic acid and residual nitramine explosives, which comprises heating the reaction mixture at a temperature from about 100 to about 112 C. in the presence of sufficient sulfuric acid to effect fume-off, and thereafter distilling the fumed-off reaction mixture from a residual acid comprising about 15 to about 40% sulfuric acid and about 60 to about 85% acetic acid, said re- 9 sidual acid being maintained at a temperature of about 120 to about 140 C.

3. The method of claim 2 wherein said sulfuric acid is present to the extent of about 5% to about by weight.

4. The method of claim 2 wherein said fumedoff reaction mixture is distilled from a mixture of sulfuric acid and acetic acid of about 35 to about 40 sulfuric acid strength.

5. In the recovery of acetic acid from residual reactions mixture liquors obtained in the manufacture of nitramine explosives, the method of working up the reaction mixture which comprises treating the undiluted reaction product with anhydrous ammonia to reduce the nitric acid content thereof to below about 1%, separating the solid reaction products from the mother liquor, heating the mother liquor from which the solid has been removed to a temperature between about 95 to about 112 C. in the presence of sufficient sulfuric acid to effect a fume-off, distilling the fumed-off mother liquor from a residual acid comprising about to about 40% sulfuric acid and about 85% to about 60% acetic acid, maintained at about 120 to about 140 C., and condensing the acetic acid distilled from said mixture.

6. The method of claim 5 wherein the separated solid reaction products are treated with hot water to leach out the water soluble and water hydrolyzable impurities therein, whereby to recover cyclonite.

7. The method of claim 5 wherein said sulfuric acid is present to the extent of about 5 to about 10% by Weight.

8. The method of claim 5 wherein said residual acid is utilized in the fume-off of further quantitles of said crude acid.

9. A method of recovering acetic acid from a crude acid containing ammonium nitrate, nitric acid, acetic acid, and a nitramine explosive, which comprises mixing said crude acid with sulfuric acid to form a mixture containing about 5 to about 10% sulfuric acid, heating said mixture at a temperature of about -112 C. to effect a fume-off of said mixture Withdrawing a portion of the fumed-off mixture and distilling the same fro-m a residual acetic acid-sulfuric acid mixture having a boiling point between about and about 140 C.

10. A method for the recovery of acetic acid from residual reaction liquor obtained in the manufacture of cyclonite, which comprises: treating the undiluted reaction mixture with sufficient anhydrous ammonia to reduce the nitric acid content thereof to below 1% by weight, separating the solid reaction products including solid cyclonite from the reaction mixture: heating the mother liquor remaining after separation of said cyclonite in the presence of about 5% to about 10% by weight sulfuric acid, at a temperature sufiicient to produce a fume-01f, whereby to destroy the ammonium nitrate and nitric acid content of said mother liquor; distilling the acetic acid from the fumed-off mother liquor from a body of about 15 to about 40% sulfuric acid and about 85% to about 60% acetic acid, maintained at a temperature of about C. to about 0., whereby to destroy any residual cyclonite contained in said mother liquor, condensing the acetic acid distilled from said mixture; and mixing the residual undistilled sulfuric acid-acetic acid mixture with further quantitles of said mother liquor prior to fume-off and distillation thereof.

GEORGE F. WRIGHT. CARL A. WINKLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,609,393 Bader Dec. 7, 1926 2,355,770 Wyler Aug. 15, 1944

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