Indian Journal of Chemical Technology
Vol. II , July 2004, pp 575-5 8 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 , Indi a
Received 12 jilly 2003; revised received 26 March 2004; accepted 3 May 2004
Cyc lotetrailleth ylene-tetranit ramine (HM X) is prepared by continuous nitrol ys is of hexamine using nitric acid and am monium
nitrate in the presence of acetic anhydride and aceti c acid. HMX obtained alter nitrol ys is 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 tern ary system
co nsistin g of HMX as the solute, acetone as solve nt and water as an ti -so lvent was studied in order to obtai n solubilit y data and
intluence of diffe rent parameters viz. percentage recovery of so lve nt , stirrer speed, cooling rate, and rate of recovery of so lve nt
on the crys tal size dis tributi on (CS D) of HMX . The crystal morphology was also studied by usi ng scanning electron mi croscope
(S EM ). The CS D of HMX obtained by usin g aceto ne-water mixlUre is co mpared with the CS D obtained by using acetone as
so lvent. It has been observed th at 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 powe rful and thermally s tabl e
ex plosive manufactured in bulk. It is used in propellant
formulati o ns, plasti c bonded e xplosives and in differe nt
warheads. HMX is mi xed in the propellants to in crease
the e nergy and in plastic bonded ex pl os ives to attain
supe ri o r performance .
In th e production of HMX , the ma in pro blem
besides its low y ie ld a nd hi g h cost is the require ment of
differe nt cry stal sizes for differe nt e nd uses. For in stan ce,
Octol needs HMX of c rysta l size 90 %< 1680 /1, 40 %<
207 /1 and 20 % < 149 /1. RX-08 cast hi g h ex plos ive
co mpos iti o n requires HMX of c ry stal s ize 100 %<
207 p and 75 % < 44~l. Differe nt s izes of HMX c ry stal s
a re re quire d to achieve desired lo adin g d e nsit y in
diffe rent warhead fillings. Therefore, to obt a in required
c rystal s ize of HMX , se lec ti o n of proper co mbination of
operatin g parame ters is impo rta nt. He nce th e present
study is tak e n up .
Ba s ic re quire 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
C rysta l S ize Di stributi o n (CSD ). Th e re are number of
reports o n refined HMX and cry stal tra nsformati o n, 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. Wrig ht and Evan s 2 studi ed g rowth of HMX
seed crystals usin g acetone as so lvent and water as no nso lvent. Svensson et af.-1 studi ed the e ffec t of coolin g
rate on the CSD o f HMX from y-butyro lacton e . Krober
el . ({1.4 studi ed the effect of coo lin g prog ram and stirre r
speed on c rystalli zati o n of HMX from cyc lohexa no ne,
N-methylpyridine, dime thylfo rmamide and propy le ne
carbonate. Jingca i et aU studied re-crystallizati o n and
CSD of HMX using dimethyl sulfox ide as so lvent and
wate r as non- so lv e nt. H e ijd e n a nd Duv a lo is {'
rec rys talli sed HMX usin g I : I mo lar mi xture of aceto ne
and y-butyro lac to ne. H o rs t et a l. 7 s tudi ed th e rec rys ta lli zation and CSD for RDX w ith th e so lve nts
cyclohex anone, cyc lo hexa no ne 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 o us pa ram e te rs viz. percentage recovery , sti rre r
speed , coo l ing rate and recovery rate of sol vent o n CS D
of HMX usin g aceto ne as so lvent and water as ant iso lvent and its compari so n w ith data ob ta in ed us in g
acetone as so lve nt.
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
0.4
0 .6
Mass fraction HMX
Fig. 1 -
Tern ary solubility di agram of I-lM X in acetone-wa ter syste m
Theoretical background of crystallization
The kineti c processes of nu c leat ion and crystal
g rowth require supersaturati o n, which ca n gene rall y be
o bta in ed by ( i) coolin g in case of a pos iti ve g radi e nt o f
the so lubility curve, (ii) re movi ng th e so lvent (usuall y
by eva porati o n) , ( iii ) add in g drownin g-out agent o r
re acti o n partn e rs. S up e rsat ur a te d so luti o n s ar e
metastab le. Se veral nuc le i are bo rn in the supersaturated
s olution du e to lo ca l co nc e ntra ti o n , pre se nce of
impuriti es and crysta ls. Nucl e i thu s generated w ill grow
if th e ir diam e te rs are g reate r than cr iti ca l diam e te r
re quire d fo r g rowth . Nucl e i s m a ll e r than c riti ca l
di a meter get di sso l ved becau se net c hange in free ene rgy
(free e ne rgy c han ge du e to c hange in surface area and
vo lume) is pos iti ve . Solubility be ha vio ur and pha se
re lat io ns p rovide useful guide lin es fo r the meth od of
operati on and c ho ice of crysta lli ser. Syste ms whic h have
la rge pos iti ve te mpe rature so lubili ty coeffi c ient are
no rmall y c rys ta llis e d by th e coo lin g m ode. F o r
substances ha vin g flat so lubility profil e , c rys ta lli za ti o n
is mos tly ca rri ed o ut by evapora ti o n of solv e n t.
Solub ility be hav iour of HMX in diffe re nt co m bin ati o ns
of aceto ne- wate r syste m was de te rmined in lab sca le at
40 and sOD
e and te rn ary diag ram of the sa me is show n
in Fig. I. Du e to flat so lubility profil e in acetone and
aceton e-wate r mi xture, the combinati o n of e vapo rati ve
and coo lin g c rystalli zation meth od was ad opted.
Experimental Procedure
Ex perimental set-up
Th e expe rim e nt a l se t-up (as show n in F ig. 2)
co nsists of jacketed reacto r with provisio n for c irculati on
of ho t/co ld water. The reac tor is prov id e d w ith a
conde nser and a magn eti c reflu x divid e r. Th e re is a lso
provi sio n fo r th e di splay a nd control of stirre r speed.
Method
Aceto ne-wate r mi xture (90: 10 V/ V) was used
fo r study o f c rystalli za tion by vary in g d iffe re nt process
parame te rs. Durin g the ex pe rime nts, firs t th e acetonewate r mi xture was made saturated w ith HMX at :'iODe
and the n required a mou nt of acetone was recovered w ith
predete rmined evapo rat io n rate to ac hi eve th e parti c ula r
percentage recovery. Rate of evaporat io ll can be adjusted
b y c hangin g th e h e at in p u t to th e sys te m. T he
supe rsat urated solution was coo led up to 30 De w it h
different coo ling rates. Des ired coo lin g rate was attained
by adju stin g the te mpe rature of coo li ng wate r in th e
jacket. Th e p rod uct was th e n filtered , was hed and dri ed
in the oven at 60 ± sD
e and c harac te rised to its eSD,
BASU
e/
577
al .: CRYSTALLI ZATION OF HMX
FV\ME PROOF
,.CMOTOIl
WIIH
GEARBOX
It
INVERTEIl
JACKETED
VESSEL
t:11Il.l.r:O
\II! ,\T Ef;.
CIJ1CU IATION
UNIT
V\l'\TER (IiOT)
Fig.2 - L"bora tory crysta lliser
shape and surface structure. The experime nts were
carried out in four different sets. In the first set, the
recove ry of so lvent 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 carri ed out by varying coo ling rate at 0.2, 0.4 and
0.6°C/min respectively. In the fourth set, the ex periments
were ca rri ed out by chan gi ng recovery rate to 1.8, 1.2
and 0.6 Llh respective ly.
Results and Discussion
During th e crystallization, there are two competing
phenomenon for new so lute as it co mes out of th e
so luti 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 nucl eation rate and growth rate
depend on deg ree of supersa turat ion , temperature,
solution hyd rodynam ics and presen ce 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 solven t
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 show n in Fi g. 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 so lvent, 14% crystals
are >250 fl; 3 % c rystal s are >500 fl and 0.4 % crys tals
are >850 fl. For 40% recove ry, 9 % c rystal s are >250 fl;
1.5% c rystal s 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 co mpared to crystal size obtained
in acetone-water mixture as so lvent. It is observed that
as the percentage recovery increases, pe rcentage of large
c rystals increases in both cases. This is due to increase
in net driving force for crystal g rowth. Al so, crystals
remain in sol ution for a lo nger time as pe rcent age
80
70
60
u
. '"
~
50
W
a: 40
~
30
20
10
0
0.1
0 .3
0 .5
0.7
0 .9
P a rti c le s ize (mm)
h g.3-E ttect o t percent recov ery on particle s ize dis tribution
~ 60%
r eco very in acetone-water
-e- 50% recovery in acetone -water
----6---- 40 %
recovery in acetone- water
____ 60% recovery in acetone
____ 50%
recovery in acetone
--6--
Table I - Erfec t or % recovery or sol vent on the y ield of crystalline
HMX
Recovery
Yi eld (Acetone )
Yi eld (Acetone- water)
%
%
%
40
49
45 .8
50
57
65.4
(iO
65. 6
77.5
recove ry in c reases and recovery rate re mains constant.
Prod uc t y ie ld in c reases w:th increase in pe rce ntage
recovery which is sho wn in Tabl e I.
40%
recovery in acetone
on CSD for acetone-water system is co mpared with the
data reported for acetone I and are shown in Fig. 4 . For
acetone-water sys te m 38% crysta ls are >250 11 ; 14%
crysta ls are >500 11 and 2 .5% crystals are >85 0 p for
stirrer speed of 30 rpm . In case of 50 rpm , 14% crystals
are >250 fJ.; 3 % crystals are >500 fJ. a nd 0 .3% crystals
are >850 fJ. . For 90 rpm , 22 % crystals a re >25 0 1-' , 4 %
c ry sta 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 highe r 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 aceto ne
as solvent. The results show that the crystal si ze is larger
in acetone than that in aceton e-water mixture .
Effed (If stilTer speed
Ge ne rall y c rys ta l size in batch crystall izer
dec reases with in c rease in stirre r speed. For acetone
wate r sys te m, it is observed that crystal size dec reases
as stir,·e r speed in c reases from 30 to 50 rpm. Howe ve r,
crys tal s ize in c reases for furth e r increase in stirre r speed
from 50 to 90 rpm. Th is can be ex plained by "diffusion
la yer mode l" of c rystal g rowth and " terminal settling
ve loc it y" of crysta ls. Acco rdin g to this mod e l th e
di ffusion of solute through boundary lay er pl ays a
s ig ni ficant role in c rystal g rowth . As the stirrer speed
increases, th e bo undary laye r thickn ess dec reases and
in turn increases c rystal g rowth rate . Also , as ag itation
speed inc reases from 50 to 90 rpm ve locity of crystal is
marc than te rminal settling ve locity. As a res ult, c rystals
reill ain in suspens io n ro r a lo nger time and prov ide a
1 ~II ·gcr surface area to g ro w. T he e ffects of st irrer speed
Effect of cooling rate
My e rson X has d esc rib e d th e b atch cooling
c rystallization . For a batch coo ling crys ta lli zation , if a
co nstant cooling rate is maintain ed throu g ho ut th e
c rystallization , a significantly hi g h supersaturati o n is
generated at the beg inning o f the run , w hich res ults in
high nucleation and low growth ratc. So th e re is an
opt imum cooling rate which c an ,dance both th e
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. Fo r aceton e-water
system coo ling rate of 0 .6°Clmin g i ve~; 30% c rysta ls
> 125 p; 6 % crystal s >300 I-' and all th e c rysta ls ha ve th e
s izes smaller than 850 11 . For coo lin g rate of OAoC/min ,
59 % crysta ls are> 125 1-' , 10% crysta ls are >3 00 I-' and
0 .3% c rystal s are >8 50 11 and for coo lin g rate 0.2°CI
BASU
el
579
al.: CRYSTALLIZATION OF HMX
80
90
80
70
70
60
60
-g 50
c
~
50
40
a:
40
"if!. 30
'" 30
20
20
10
0. 1
0. 1
0.3
0.5
0.7
50 '1:1T1 (acelone-waler) --s-- 90 rfnl (acelone-waler)
---6-
--+- 30 rpm ( acelare)
-.- 50 rfnl (acelmo)
0.7
0 .9
1.1
Parti cle si ze (mm)
---Q-O . 2"C I min (acetone-water)
-e- 0.4 "C/ min (acetone-w ater)
--6--0.6-C/min (acetone-water)
______.O. 2 "C/ min (acetone)
_ O. 4· C / m in (ac etone)
-r- O.G· C/ min
(aceto n e)
____ 90 rfnl (acelone)
Tab le 2 - Effect of sol ven t recovery rate on the yield of
crys talline HMX
70
60
50
u
0. 5
Fig.5- Effect of cooling rate on pa rticle s ize distribution
Particle size (mm )
Fig.4- Effect of s tirre r speed on particle s ize distribution
-<>-30 rpm (acelan<>-waler)
0.3
0 .9
Rate of
recovery (Llh)
Yield
(Acetone) %
Yi eld
(Acetone-water) %
20
0.65
54. 1
65.4
10
1. 2
48 .3
54. 1
1.8
57
65 .4
Q)
.~ 40
W
a: 30
0~
0
0. 1
0. 3
0 .5
0.7
0. 9
1.1
Pa nlcles!7.e (mm)
Fig.6-Effect of recove ry ra te on par ticl e s ize di str ib utio n
-e-- O.6Uhr (acelane-wale r)
---6- 1.8lJhr (acelone-wale r)
-.-1 .2Uhr (acelone)
-<>- 1.2Uhr (acetone-water)
--.-o.6Uhr (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 th at as the cooling rate decreases from 0.6 to OAoC/
min , th e formati on of large r crystal increases but furth er
decrease in th e coo ling rate from OA to O,2 DChnin the
fo rmati on of smaller crys tal increases. For both systems
coo ling rate of OA DC/min is optimum for reali sing large r
crys tal s. These results show that crystal size is larger in
acetone than in acetone-water mi xture.
Effect of sol vent l'ccover y 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 show n in Fi g. 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 recove ry 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 recove ry rate, 25% crystals are >200~ ; 1%
crystals are >600 ~ and 0.3 % cry stal s are >850 ~, The
resu 1ts show th at as th e rate of recove ry dec reases,
crystal size increases , This is due to the depend ence of
nu cleat ion process on th e rate of super saturati on. Also
at a low rate of recovery, crystals get more ti me to grow
than at hi gher rate of recovery as the percentage recove ry
is constant. It is observed that crys ta l size obtained in
acetone is larger as co mpared to the crystal size obtai ned
in acetone-water mi xture as solvent. It is observed that
crys tal yie ld of HMX dec reas es 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 show n
ill Table 2.
Crystal morphology
HMX ex ists in four p<i> lymorphi c forms kn ow n as
ex, ~ , y and 8. The rol e of so lvent on the nature of
polYlllorphs and its stability is not we llullclerstoocl . The
roo m temperature sta ble 13 form is used for mi I itary
applications due to its hi gh density and low sensiti vity.
The infrared differences are so strikin g for different
polymorphs of HMX that th ey se rve as we ll as th e Xray diffracti on patterns for identificat ion'} . In ge neral
the spec trum of I3-HMX differ large ly from ex, alld 8
HMX both in respect of several meth ylene modes and
al so of the strong NN01 absorption in the 1]00 C I11 ,I
range.
"
.'
580
IND IAN J . C HEM. TECHNOL. , J ULY 2004
112. 9
-r--~---
11 0 '
100
%T
70
\;
V
<to'--
I
!
?9."
~--hl
J I----.-----r------,-.---,---,-- - -,---16CO
1500
1400
1300
"tZOO
1100
--,·1000
-
--,, - - - -, -- -r------T··········---·, 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 fr o m ac e to ne-water
mi xture we re co nfirm ed by infrared spec tra in KBr
matri x as show n in Fi g . 7 . The results were compared
with the spec tra obta ined by Bedard ('I of.') and Achuthan
(' I af. JO It is co nfirmed that the crystal obtai ned from
th e acetone-water mixture is on Iy in f3 polymo rphic form.
Fig. 8 and Fi g. 9 (a,b,c) show the sca nnin g electron
mi crosco pe (SEM ) photog raphs o f HMX c ry stals in
acetone-wat er mixture and in acetone_It is obse rved that
all th e crysta ls do not have th e same shape, thou gh they
have the same internal stru cture _The relative areas of
the face s present arc different. Thi s is due to different
fa ces o f crystals grown at different rates, at different
mi cro mi xin g co nditi ons as ex ternal habits of th e crysta ls
arc controll ed by mi xin g co ndition s. In both th e cases
th e crys tal s ob tain ed are mo noc lini c. Th e c rys tal s
obtained from acetone have rough surfaces, cracks and
inclusions. wh ereas c rys tals from acetone-water mixture
have co mparati ve ly smooth surfaces and no cracks and
inclu sions. The crys tal s observed by both sys tems ha ve
terraces. steps and kink s. Crystalli zati on in both acetonewa te r mixtur e and ace to ne s how 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 th e desired crys tal size di ;;tribution ,
one ha s to co ntrol th e competi ng phenome non of
nu cleati on and cry stal growth. For crysta lli zation of
b) Crystal habit
c) Aggl omeration
Fig.8- SEM images or HMX crys tals rrom aceton e-wa te r mi xture
BASU et al .: CRYSTALLIZATION OF HMX
a) Different shapes
58 1
these co nditi ons CSD obtained are 64% crys tals> 200
Jl, 72% c rys tals> 150 Jl, 3% c rystals> 1700 ~L whi c h
ca n b e u sed in preparati o n of a hi g h ex pl osive
co mpos iti on oc to l. Th e optimum pa ram e te rs to get
small e r crystal size are so lvent recovery 50 %, sti rrer
speed 90 rpm , recovery rate 1.8 Llh and coolin g rate
0 .6°Chnin . Under these conditi ons CS D obta ined are
15% c rysta ls> 200 Jl, 32% c rystals> 150 ~L a nd a ll the
c rys tals have sizes lass tha n 850 Jl. Hence, by se lecti ng
prope r co mbin ati on of these ope ratin g para meters and
by co ntrolling th e m, it is poss ibl e to get th e des ired
crystal s ize di stributi on for diffe re nt e nd uses.
Acknowledgement
The auth ors are grateful to Dr. Hari dwa r S ingh,
Direc to r HE MRL fo r hi s e nco urage me nt a nd ki nd
pe rmi ss ion to pu bli sh th e paper.
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Crystallizat io ll. 54'h Annua l Ses sion ur In dia n Institut e ur
b) Crystal habit
2
3
4
C hem ica l E ngineers a nd Indian Che mi ca l En g ine er in g
Co ngress, Chennai, 19-22 Dec 200 1.
Wri ght Sam B & Evans Albert D, u.s. Pa t 3.297.68 / . 1%7;
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Svensson LeiI', Nyqu ist Jan-Olof & Westling Lars, Ha:wd
Ma te r, 13 ( 1986) 103.
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HM X C rrsta ls w ith Hi gh Ill te mal Q u a/ill' hI' Coolill g
C n'stalliza tioll, 29'h Intern ati onal Ann ual Cu nfe rence ur ICT,
5
c) Crystal defect
Fig.9- SEM images of H MX crys ta ls from aceto ne
HMX fro m ace to ne-wa te r mixture, th e paramete rs
affec tin g thi s phe nome non are pe rcentage recove ry of
so lvent , st irre r speed , coo lin g rate and rate of recove ry
of so lve nt. By optimi s ing th ese pa rame te rs requi red
crysta l s ize di stributi ons ca n be rea li zed . Optimi zed
parameters for getting large c ry stal size in acetone-wate r
mi xture are so lve nt recove ry 50%, stirrer speed 30 rpm ,
recovery rate 0.6 Llh and coo ling rate O.4°C/min . Under
6
7
8
9
10
Karl sruhe Germany, 30,h June - 2'''' Jul y I l)l)~ , 66.1.
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50. 1.
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In tem a l Qua lity of HM X Crvstals, 27'h Intern ati unal Annual
Conference of ICT, Karl sruh e Ge rmany, 25-2~ June 1996, 32. 1.
Horst J H, Geertman R M, Vande r Heijden A E & Rosmalen G
M, B e n c h Scale Coo lin g CrVSfa l/ iwtioll of' R DX . 27 'h
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25 -28 June ( 1996) , 126. 1.
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(B utterworth-Heinemann , USA), 1952.
Bedard M, Hu ber H, Mye rs J L & Wright G F, COllad .l
Chelll . 40 ( 1962) 2278.
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