Indian Journal of Chemical Technology Vol. II , July 2004, pp 575-58 1

Crystallization of HMX in acetone-water system

Souraseni Basu, N M Gawande, ME Apte & V L Narasimhan* Hi gh Energy Materials Research Laboratory, Sutarwadi, Pune 4 11 021 , India

Received 12 jilly 2003; revised received 26 March 2004; accepted 3 May 2004

Cyclotetraillethylene-tetranit ramine (HM X) is prepared by continuous nitrol ys is of hexamine using nitric acid and ammonium nitrate in the presence of acet ic anhydride and aceti c acid. HMX obtained alter nitrol ysis contains RDX and some nit ro compounds as impuriti es. It is purified by evaporative crystalli zation technique using acetone as the solvent. In thi s work a ternary system co nsistin g of HMX as the solute, acetone as solvent and water as an ti -so lvent was studied in order to obtai n solubilit y data and intluence of different parameters viz. percentage recovery of so lvent , stirrer speed, cooling rate, and rate of recovery of so lvent on the crys tal size dis tributi on (CSD) of HMX . The crystal morphology was also studied by usi ng scanning electron mi croscope (S EM ). The CS D of HMX obtained by using acetone-water mixlUre is co mpared with the CSD obtained by using acetone as so lvent. It has been observed that under simi lar conditions of crystall izat ion , crystals obtained from acetone-water mi xture arc smaller than those obtai ned from acetone. Also , the crystals obtained from acetone-water system arc more regular in shape and have smooth su rfaces.

IPC Code: 80 I 0 9/02 Keywords: HMX , crystalli za ti on, acetone-water system, crystal size distributi on

HMX is th e mo st powerful and thermally s tabl e

ex plosive manufactured in bulk. It is used in propellant formulati ons, plasti c bonded explosives and in different

warheads. HMX is mi xed in the propellants to increase the energy and in plastic bonded ex plos ives to attain

supe ri or performance . In the production of HMX, the ma in problem

besides its low y ie ld and high cost is the requirement of

different cry stal sizes for different end uses. For instance,

Octol needs HMX of c rysta l size 90 %< 1680 /1 , 40 %< 207 /1 and 20 % < 149 /1. RX-08 cast high ex plos ive

co mpos iti on requires HMX of c rystal s ize 100 %< 207 p and 75 % < 44~l. Different s izes of HMX cry stal s a re required to achieve desired lo adin g de nsity in

diffe rent warhead fillings. Therefore, to obta in required c rystal s ize of HMX, selec tion of proper combination of

operatin g parameters is important. He nce the present

study is taken up . Bas ic require me nts o f c rys talli z8t i o n a re to

obta in a speci fic modi ficati on of crysta l habit and spec i fi c Crysta l S ize Di stributi on (CSD). There are number of

reports on refined HMX and crystal transformati on, but

"For correspondence (E-mail: ccpp @rcmrl.org; Fax: +'.J 1-20- 586(316)

on ly a fe w about HMX c ry sta l s ize c lass ification.

Bhujbal et.al. 1 has stud ied CSD of HMX usi ng acetone as solvent. Wright and Evans2 studied growth of HMX

seed crystals using acetone as so lvent and water as non­so lvent. Svensson et af.-1 s tudied the e ffec t of cooling

rate on the CSD o f HMX from y-butyro lactone . Krober

el . ({1.4 studied the effect of coo ling program and stirre r

speed on c rystalli zati on of HMX from cyc lohexa none,

N-methylpyridine, dimethylformamide and propy lene carbonate. Jingcai et aU studied re-crystallization and

CSD of HMX using dimethyl sulfox ide as so lvent and wate r as non- so lv e nt. He ijde n a nd Duv a lo is {'

rec rys talli sed HMX using I : I mo lar mi xture of acetone

and y-butyro lac to ne. H o rs t et a l.7 s tudi ed th e re­

c rys ta lli zation and CSD for RDX w ith th e so lvents cyclohex anone, cyc lohexanone saturated w ith wa te r (3

% IV/W) and y-butyrolactone .

The a im of thi s work is to study the effec t of va ri ous pa ramete rs viz. percentage recovery, sti rre r speed , coo l ing rate and recovery rate of sol vent on CS D

of HMX using acetone as so lvent and water as ant i­solvent and its compari son with data ob ta ined us ing acetone as so lvent.

576 INDIAN J. CHEM. TECHNOL., JULY 2004

Acetone

I ' ..... ~ !)o .. c -4()C

0 .9 \

r-7"---""*--~{---4 0 .65

0.4

0.2 0 .25 0 .3 0 .35 0 .6

0.4

Mass fraction HMX

Fig. 1 - Ternary solubility diagram of I-lM X in acetone-wa ter system

Theoretical background of crystallization The kinetic processes of nuc leat ion and crystal

g rowth require supersaturation, which can gene rall y be

obta ined by ( i) cooling in case of a pos iti ve g radient of the so lubility curve, ( ii) removi ng the so lvent (usuall y

by eva porati o n) , ( iii ) add ing drowning-out agent o r

re acti o n partn e rs. S up e rsatura te d so luti o ns are

metastab le. Several nuc le i are born in the supersaturated

solution du e to loca l co nce ntra ti o n , prese nce of

impurities and crysta ls. Nucle i thu s generated w ill grow if the ir diamete rs are g reate r than cr iti ca l diamete r required fo r g rowth . Nucl e i s m a ll e r than c riti ca l

diameter get di sso l ved because net change in free ene rgy (free ene rgy change due to c hange in surface area and vo lume) is pos iti ve. Solubility behaviour and phase

re lat ions provide useful guide lines fo r the method of operati on and choice of crysta lli ser. Systems which have la rge pos iti ve temperature so lubili ty coeffi c ient are no rmall y c rys ta llis e d by th e coo lin g m ode. Fo r substances having flat so lubility profile , c rys ta lli za ti on

is mos tly ca rri ed o ut by evaporati o n of solve nt. Solub ility behav iour of HMX in diffe rent combinati ons of acetone- wate r syste m was de te rmined in lab sca le at 40 and sODe and te rnary diagram of the sa me is shown

in Fig. I. Due to flat so lubility profil e in acetone and

acetone-wate r mi xture, the combination of evaporati ve

and cooling c rystalli zation method was adopted.

Experimental Procedure Experimental set-up

The experime nta l se t-up (as show n in F ig. 2)

consists of jacketed reactor with provision for c irculati on

of ho t/co ld water. The reac tor is prov id ed w ith a

condenser and a magneti c reflu x divide r. The re is a lso provi sion fo r the di splay and control of stirre r speed.

Method

Acetone-wate r mi xture (90: 10 V/ V) was used fo r study of c rystalli za tion by vary ing d iffe rent process paramete rs. During the ex pe riments, firs t the acetone­wate r mi xture was made saturated w ith HMX at :'iODe

and then required amount of acetone was recovered with predete rmined evaporat io n rate to ac hi eve the particula r percentage recovery. Rate of evaporat io ll can be adjusted

b y c hangin g th e he at in p u t to th e sys te m. T he supe rsaturated solution was coo led upto 30 De w ith

different cooling rates. Des ired cooling rate was attained

by adjusting the temperature of coo li ng wate r in the jacket. The product was then filtered , was hed and dried in the oven at 60 ± sDe and c harac te rised to its eSD,

BASU e/ al .: CRYSTALLIZATION OF HMX 577

FV\ME PROOF ,.CMOTOIl WIIH GEARBOX It INVERTEIl

V\l'\TER (IiOT)

JACKETED VESSEL

t:11Il .l.r:O \II! ,\TEf;. CIJ1CU IATION UNIT

Fig.2 - L"bora tory crysta lliser

shape and surface structure. The experiments were carried out in four different sets. In the first set, the recovery of solvent was varied as 40, 50 and 60% keep ing the other parameters constant. In second set the experiments were carried out by varying stirrer speed at 30, 50 and 90 rpm. In the third set, the experiments were carried out by varying cooling rate at 0.2, 0.4 and 0.6°C/min respectively. In the fourth set, the experiments were carri ed out by changi ng recovery rate to 1.8, 1.2 and 0.6 Llh respective ly.

Results and Discussion During the crystallization, there are two competing

phenomenon for new so lute as it comes out of the soluti on. First one is nuc leation i.e. birth of new crystals, and the second one is growth of these crystals to larger sizes. Kinet ics of the nucleation rate and growth rate depend on degree of supersaturat ion , temperature, solution hyd rodynamics and presence of impurities. Therefore, in order to obtain the desired crystal size,

selection of proper combination of process parameters is of prime importance.

Effect of percentage recovery of solvent

The effect of percentage recovery of so lvent on CSD for acetone-water system is compared with the data reported for acetone I and is shown in Fig. 3. For acetone­water system 19% crystals are >250 fl , II % crysta ls are >500 fl and 6% crystals are >850 fl fo r 60% recovery of ~: olvent. In case of 50% recovery of solvent, 14% crystals are >250 fl; 3% crystal s are >500 fl and 0.4% crys tals are >850 fl. For 40% recovery, 9% c rystal s are >250 fl; 1.5% c rystals are >500 fl and a ll crystals have sizes less than 850 fl . The resul ts show that c rystal size obtained in acetone is larger as compared to crystal size obtained in acetone-water mixture as so lvent. It is observed that as the percentage recovery increases, percentage of large c rystals increases in both cases. This is due to increase in net driving force for crystal growth. Al so, crystals remain in sol ut ion for a longer time as pe rcentage

80

70

60 u

'" . ~ 50 W a: 40

~ 30

20

10

0

0 . 1 0 .3 0 .5 0.7 0 .9

P arti c le s ize (mm)

h g.3-Ettect o t percent recovery on particle s ize dis tribution

~60% reco very in acetone-water

----6---- 40 % recovery in acetone- water

____ 50% recovery in acetone

Table I - Erfec t or % recovery or solvent on the y ield of crystalline HMX

Recovery Yi eld (Acetone) Yield (Acetone-water)

% % %

40 49 45 .8

50 57 65.4

(iO 65.6 77.5

recovery increases and recovery rate remains constant. Prod uct y ie ld inc reases w:th increase in percentage recovery which is shown in Table I.

Effed (If stilTer speed

Ge ne rall y c rys ta l size in batch crystall izer dec reases with increase in stirre r speed. For acetone wate r sys te m, it is observed that crystal size decreases as stir,·e r speed increases from 30 to 50 rpm. However, crys tal s ize increases for further increase in stirre r speed from 50 to 90 rpm. Th is can be explained by "diffusion layer mode l" of c rystal growth and " terminal settling ve loc ity" of crysta ls. Acco rdin g to this mode l the di ffusion of solute through boundary layer pl ays a s igni ficant role in c rystal growth . As the stirrer speed increases, the boundary layer thickness decreases and in turn increases c rystal growth rate . Also, as ag itation speed increases from 50 to 90 rpm ve locity of crystal is marc than te rminal settling ve locity. As a result, c rystals reillain in suspens ion ror a longer time and prov ide a 1 ~ II·gcr surface area to g row. T he e ffects of st irrer speed

-e- 50% recovery in acetone -water

____ 60% recovery in acetone

--6-- 40% recovery in acetone

on CSD for acetone-water system is compared with the data reported for acetone I and are shown in Fig. 4 . For

acetone-water sys te m 38% crystals are >250 11 ; 14% crystals are >500 11 and 2 .5% crystals are >850 p for stirrer speed of 30 rpm . In case of 50 rpm, 14% crystals are >250 fJ.; 3% crystals are >500 fJ. and 0 .3% crystals are >850 fJ. . For 90 rpm, 22% crystals a re >250 1-' , 4% crysta ls are >500 I-' and all the c rysta ls have sizes less than 850 11 . It is observed that perce ntage of crystal s with crystal size >500 11 is higher at 30 and 90 rpm and lower at a stirrer speed of 50 rpm in acetone water system but this phenomenon is not observed for acetone as solvent. The results show that the crystal size is larger in acetone than that in acetone-water mixture .

Effect of cooling rate

My e rson X has d esc rib e d th e batch cooling crystallization . For a batch coo ling crys ta llization , if a co nstant cooling rate is maintained throu g hout th e c rystallization, a significantly high supersaturati on is generated at the beg inning o f the run , which results in high nucleation and low growth ratc. So there is an opt imum cooling rate which can ,dance both the phenomenon . The e ffects of cooling rate on CSD for acetone-water system is compared with the data repo rted fo r acetone l and are shown in Fig. 5. For acetone-water system coo ling rate of 0 .6°Clmin g i ve~; 30% crysta ls

> 125 p; 6% crystal s >300 I-' and all the c rysta ls have the s izes smaller than 850 11 . For coo ling rate of OAoC/min , 59% crystals are> 125 1-' , 10% crysta ls are >300 I-' and 0 .3% crystal s are >850 11 and for coo ling rate 0.2°CI

BASU el al.: CRYSTALLIZATION OF HMX 579

90

80

70

60

50

40

'" 30

20

10

0. 1 0.3 0.5 0.7 0 .9

Particle s ize (mm )

Fig.4- Effect of s tirrer speed on particle s ize distribution

-<>-30 rpm (acelan<>-waler) ---6- 50 '1:1T1 (acelone-waler) --s-- 90 rfnl (acelone-waler)

--+- 30 rpm ( acelare) -.- 50 rfnl (acelmo) ____ 90 rfnl (acelone)

70

60

u 50 Q)

.~ 40 W a: 30 ~ 0

20

10

0 0. 1 0. 3 0 .5 0.7 0. 9 1.1

Panlcles!7.e (mm)

Fig.6-Effect of recove ry ra te on par ticl e s ize di str ib ution

-e-- O.6Uhr (acelane-wale r) -<>- 1.2Uhr (acetone-water)

---6- 1.8lJhr (acelone-wale r) --.-o.6Uhr (acelone) -.-1 .2Uhr (acelone) -.- 1.BUhr (acetone)

min , 42% crysta ls are > 125 p, 8% crystals are >300 p and all crystals have size less than 850 p. These results show that as the cooling rate decreases from 0.6 to OAoC/ min , the formati on of larger crystal increases but further decrease in the coo ling rate from OA to O,2DChnin the fo rmati on of smaller crys tal increases. For both systems coo ling rate of OA DC/min is optimum for reali sing larger crys tal s. These results show that crystal size is larger in acetone than in acetone-water mi xture.

Effect of sol vent l'ccovery rate

The effec ts of recovery rate of solvent on CS D for acetone-waleI' system is compared with the data reported fo r acetone I and are shown in Fig. 6. For acetone-water system recovery rate of 0.6 Llh gives 3 1 % crysta ls >200 p, 2% crys tals >600 p. and 0.9% crystals >850 ~l.ln the case of 1.2 Llh recovery rate, 27 % crysta ls are >200 p ; 1.7% crysta ls are >600 ~L and 0.8% crys tals are >850 ~l. For 1.8 Llh recovery rate, 25% crystals are >200~; 1% crystals are >600 ~ and 0.3% crystal s are >850 ~, The resu 1 ts show that as the rate of recovery decreases, crystal size increases , This is due to the dependence of

80

70

60

-g 50 c

~ 40 a: "if!. 30

20

0. 1 0.3 0.5 0.7 0 .9 1.1

Particle size (mm)

Fig.5- Effect of cooling rate on particle s ize distribution

---Q-O. 2"C I min (acetone-water)

--6--0.6-C/min (acetone-water)

_O.4·C/m in (acetone)

-e- 0.4 "C/ min (acetone-w ater)

______.O.2 "C/ min (acetone)

-r- O.G·C/ min (aceto n e)

Table 2 - Effect of sol vent recovery rate on the yield of crys talline HMX

Rate of Yield Yi eld recovery (Llh) (Acetone) % (Acetone-water) %

0.65 54. 1 65.4

1.2 48 .3 54. 1

1.8 57 65 .4

nucleat ion process on the rate of super saturati on. Also at a low rate of recovery, crystals get more ti me to grow than at higher rate of recovery as the percentage recovery is constant. It is observed that crys ta l size obtained in acetone is larger as compared to the crystal size obtai ned in acetone-water mi xture as solvent. It is observed that crys tal yie ld of HMX dec reases as recove ry rate increases from 0.6 to 1.2 Llh but increases with further increase in recove ry rate fro m .Ii,l fo I .8 Llh. It is shown ill Table 2.

Crystal morphology

HMX ex ists in four p<i> lymorphic forms known as ex, ~ , y and 8. The rol e of so lvent on the nature of " .' polYlllorphs and its stability is not wellullclerstoocl . The room temperature stable 13 form is used for mi I itary applications due to its hi gh density and low sensiti vity. The infrared differences are so striking for different polymorphs of HMX that they serve as well as the X­ray diffraction patterns for identificat ion'} . In general the spectrum of I3-HMX differ largely from ex, alld 8 HMX both in respect of several methylene modes and al so of the strong NN0

1 absorption in the 1]00 CI11,I

range.

580 IND IAN J . C HEM. TECHNOL. , JULY 2004

%T

112.9 -r--~---

11 0 '

100

70

<to'--

I !

\; V ?9." ~--hl ----.--- .---,---,---J I --r------,- -,---- --,·- - --,,----,---r------T··········---·, -

16CO 1500 1400 1300 "tZOO 1100 1000 900 800 700 600 50 0

CM-l

Fig.7- IR Spec tra or HMX crys tals rro m acetone-Willer mi xture

Th e c rys tals obta in ed from ace tone-water mi xture were co nfirmed by infrared spec tra in KBr matri x as shown in Fig. 7 . The results were compared with the spectra obta ined by Bedard (' I of.') and Achuthan (' I af. JO It is confirmed that the crystal obtai ned from the acetone-water mixture is on Iy in f3 polymorphic form. Fig. 8 and Fi g. 9 (a,b,c) show the scanning electron mi crosco pe (SEM) photog raphs of HMX crystals in acetone-water mixture and in acetone_ It is observed that all the crysta ls do not have the same shape, though they have the same internal structure_ The relative areas of the faces present arc different. Thi s is due to different faces of crystals grown at different rates, at different mi cro mi xing conditi ons as ex ternal habits of the crysta ls arc controll ed by mi xing co nditions. In both the cases th e crys tal s obtain ed are monoc lini c. Th e crys tal s obtained from acetone have rough surfaces, cracks and inclusions. whereas crys tals from acetone-water mixture have comparati vely smooth surfaces and no cracks and inclusions. The crys tal s observed by both sys tems have terraces. steps and kinks. Crystalli zati on in both acetone­wa te r mixture and ace to ne show s th e c rys tal agg lomerati on but crys tal size is sma ll er in acetone­wa ter than in acetone.

Conclusion To obtain the desired crys tal size di ;; tribution ,

one ha s to co ntrol th e competi ng phenomenon of nu cleati on and crystal growth. For crysta lli zation of

b) Crystal habit

c) Agglomeration

Fig.8- SEM images or HMX crys tals rrom acetone-water mi xture

BASU et al .: CRYSTALLIZATION OF HMX 58 1

a) Different shapes

b) Crystal habit

c) Crystal defect

Fig.9- SEM images of H MX crys ta ls from acetone

HMX fro m ace tone-wate r mixture, the paramete rs affec ting thi s phenomenon are percentage recovery of solvent , st irrer speed, cooling rate and rate of recovery of so lvent. By optimis ing these parameters requi red crysta l s ize di stributi ons can be rea li zed . Optimized parameters for getting large crystal size in acetone-water mi xture are so lvent recovery 50%, stirrer speed 30 rpm, recovery rate 0.6 Llh and cooling rate O.4°C/min . Under

these conditions CSD obtained are 64% crys tals> 200

Jl, 72% crys tals> 150 Jl, 3% crystals> 1700 ~L which can be used in preparati o n of a hi g h ex pl osive compos iti on octo l. Th e optimum paramete rs to get smaller crystal size are so lvent recovery 50%, sti rrer speed 90 rpm, recovery rate 1.8 Llh and cooling rate 0 .6°Chnin . Under these conditions CS D obta ined are

15% crysta ls> 200 Jl , 32% crystals> 150 ~L and a ll the crys tals have sizes lass than 850 Jl . Hence, by selecti ng prope r combination of these operating parameters and by controlling them, it is poss ible to get th e des ired crystal s ize di stributi on for diffe rent e nd uses.

Acknowledgement The auth ors are grateful to Dr. Hari dwar S ingh,

Direc to r HEMRL fo r hi s e ncourage me nt and ki nd permi ss ion to pu bli sh the paper.

References

Bhujbal G J, Gawande N M & Syal R K, Tailorill g of' HMX

Crystallizat io ll . 54'h Annua l Session ur In dia n Institut e ur Chem ica l Engineers and Indian Chemi ca l Engineer in g Congress, Chennai, 19-22 Dec 200 1.

2 Wri ght Sam B & Evans Albert D, u.s. Pat 3.297.68 / . 1%7; Chelll Astr, 66 (1967) 675/ 16d.

3 Svensson LeiI', Nyqu ist Jan-Olof & Westling Lars, Ha:wd Mater, 13 ( 1986) 103.

4 Krober H, Tiepel U, Leisin ger K & Krau se H, FO/'llwtioll 0/ HM X C rrsta ls with High Ill te mal Q ua/ill' hI' Coolill g Cn'stallizatioll , 29'h Intern ati onal Ann ual Cunfe rence ur ICT, Karl sruhe Germany, 30,h June - 2'''' Jul y I l)l)~ , 66.1.

5 Jingcai Cheng, Xi noqing Wu & Ja in wen Guo, S/I{(lv of' IiMX Craill size Classijicati(J/I hr DMSO Method, 26'" In ternationa l Annual Conrerenee or ICT, Karl sruhe Germany, 4-7 .I ul y 1l)l)S. 50. 1.

6 Van der Heijden A E & Du va luis W, Clw/'{{Cll'ri.wtioll of'lhe

In tem al Quality of HMX Crvstals, 27'h Internati unal Annual Conference of ICT, Karl sruhe Germany, 25-2~ June 1996, 32. 1.

7 Horst J H, Geertman R M, Vander Heijden A E & Rosmalen G M, Ben ch Scale Coo lin g CrVSfa l/ iwtioll of' RDX. 27 'h Intern ati onal Annual Conference of ICT, Karl sruche German y, 25-28 June (1996) , 126. 1.

8 Myerson All an S, Halldbook (~lIl1duslrill / Crvsw I/iW lion (B utterworth-Heinemann , USA), 1952.

9 Bedard M, Hu ber H, Myers J L & Wright G F, COllad .l Chelll . 40 (1962) 2278.

10 Achutan C P & .Jose C I, Prollellall l.l· Exp losives Pyrotechnics, 15 (1990) 27 1.